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Our F&S International Edition is now 20 years old. The German-language F&S-Filtrieren und Separieren magazine has now been in existence for 33 years. It quickly established itself as an important source of information for those who use filtration and separation technologies on account of its trend-setting and exclusive reporting. The idea of making the contents of the German-language F&S accessible to readers on a global basis was born around 20 years ago. This is when the idea of a special English-language edition, i.e. the F&S International Edition, was born. It will be published for the 20th time this year – a small anniversary. 20 years of the F&S International Edition is impressive proof that this special edition, like its German-language counterpart, has also become well established. The new F&S International Edition will include a small selection of the best technical articles from the 2019 German edition – translated into English for you. We hope you enjoy reading our International Edition and would be pleased to read your feedback. If you would like to find more about the German-language F&S, please do not hesitate to contact us at the address given below (also see imprint: page 5). Please visit our internet site at: www.fs-journal.de. Our two-language 2020 - 2022 F&S Global Guide to the Filtration and Separation Industry can be found at www.issuu.com/fs-journal.
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F & S International Edition No. 20/2020
PLATE AND FRAME FILTER PRESS
t Highlights 2019 Separation processes used in the food and beverage industry S. Ripperger
High yields and drip losses are of utmost importance for the economic efficiency of processes in the pharmaceutical sector. The highest demands are also placed on cleaning and hygiene. The HERMETIX filter press was designed with a hermetically sealed filter plate package to meet these requirements. Without drip losses and with maximum yield. The special design allows the HERMETIX filtration system to be completely emptied. This is used for the fractionation of blood plasma, the separation of precipitates and at every clarification stage. Filter presses and plate and frame filters play an important role in basic fractionation and the production of albumin and immunoglobulin. Other areas of application are the cell harvesting of proteins and the clarification filtration of vaccines and antibiotics. CIP cleaning processes are necessary and possible with HERMETIX to automate cleaning to the greatest extent.
Special features Hermetically sealed with internal filter sheets / CIP-/SIP-compatible Complete draining / venting due to CIP design Turbidity frames without any dead spaces Flexibility due to various turbidity frame widths Easy to clean thanks to hygienic GMP-compliant design Highest surface quality and processing
Separation processes for beverage production: process options and new developments Report from the BrauBeviale 2018 exhibition J. Barth
High-end blood plasma fractionation J. Thomas
Analytical photo-centrifugal filtration (ACF): Membrane resistance and filterability S. Boldt , D. Lerche , M. Loginov
Membrane processes used as the basis for sustainable and cost-optimized water management in the mining industry Th. Peters
Developments in the membrane technology sector Report about the “2019 Aachen membrane course for water technologies” St. Schütze
From waste water to medical technology Report from the Aachener Membran Kolloquium 2018 J. Barth
Using reverse osmosis as the 4th clarification stage M. Schröder
Sludge drying reduces cost – efficient drying technology P. Schlachter, R. Specht , R. Weber
Drying apparatuses Report from the Powtech 2019 exhibition J. Barth
Reverse flow adsorption with integrated regeneration for retaining critical trace substances in aqueous media F. Blauth, B. Schiemann, J. Schiemann
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F & S International Edition No. 20/2020
Contents Highlights 2019
Reducing the pressure drop rate increase in surface filters used for dust separation through additive dosing or recirculation E. Schmidt
The testing of air filters for general ventilation in accordance with EN ISO 16890 F. Schmidt, E. Däuber, T. Schuldt, T. Engelke
tS eparation processes for beverage production: process options and new developments page 10
The practical implementation of DIN EN ISO 16891 “Test methods for evaluating the degradation of characteristics of cleanable filter media” F. Schmidt, J. Weimann, C. König 63 This issue contains a supplement of the company W. Köpp GmbH & Co. KG. We ask for your attention.
t Developments in the membrane technology sector page 28
tT he testing of air filters for general ventilation in accordance with EN ISO 16890 page 59
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F & S International Edition No. 20/2020
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Separation processes used in the food and beverage industry S. Ripperger* Separation processes are used in many different ways within food and beverage technologies. The following article takes a closer look at the importance of the food industry and it also examines the requirements applied to food processing technology and in particular, the separation technology used with it. Separation processes are used in many different ways within food and beverage technologies. The following article takes a closer look at the importance of the food industry and it also examines the requirements applied to food processing technology and in particular, the separation technology used with it. 1. Introduction The food and beverage industry will have to make optimum use of available resources during these times of volatile commodity prices for food, increasingly scarcer natural resources and a global population expected to reach 8.2 billion by 2030. The highest hygiene and environmental standards will have to be implemented and at the same time the yield will have to be increased and the generation of waste will have to be minimised. This means that continuous improvements must be implemented in the food and beverage industry. Water, energy and the processed commodities needed for production should be used as efficiently as possible. Food and beverage producers have always worked very closely with the commodity producers. Separation processes are essential when it comes to producing food and beverages. Filtration and centrifugation as well as extraction are just a few of the known processes used during the production of food and beverages. Membrane processes now play a key role in many sectors within the industry. Food and beverage preparation is based on biological materials that mostly have a high natural liquid content. These liquid contents are often present as complex suspensions during processing. These suspensions are used during the production of juices, alcoholic beverages, dairy products as well as starches and sugar. During processing, solid / liquid separation is frequently seen as being a decisive stage and it is often used several times. Different processes have to be used for this as they depend on the current task. A closer look at the food industry should be taken before going into these processes in more detail.
* Prof. Dr.-Ing. Siegfried Ripperger Information and Engineering Services (IES) GmbH Luxstr. 1 67655 Kaiserslautern, Germany Tel.: +49 6302-5707 E-mail: firstname.lastname@example.org
2. Information about the food industry The food and beverage industry is one of the world’s most important economic sectors and it is also a significant employer. The industry is characterised by the diversity of its activity sectors and the end products that it produces. Products can range from bulk products such as sugar, starch, vegetable oil and fats to different types of beverages and millions of special end products for the consumers. Separation processes for solid/liquid separation are essential stages in food processing, especially in the starch, sugar, fruit juice, wine, beer, vegetable oil and fat sectors. The food industry in the EU is one of the largest industries involving more than 289,000 companies, 4.24 million employees and has an annual turnover of 1,098 billion euros (figures for 2017 according to ). It is characterised by low but always stable growth, which is often below one per cent. The food and beverage industry
is the largest manufacturing sector within the EU and it accounts for 15.4% of total manufacturing. The food and drink industry is a very fragmented industrial sector that is dominated by small and medium-sized enterprises (SMEs - 99.1% in total) and they account for approxi mately 50% of the turnover. These companies generate 48.3% of food and beverages sales and employ 62.1% of the sectoral workforce. The USA is by far the EU’s most important trading partner (both for exports and imports), followed by China, Switzerland, Japan and Russia. Domestic exports within the EU reached 254.6 billion euros in 2017. This amount is considerably higher than exports from the EU that totalled 102 billion euros. The EU is the number one exporter and the second biggest importer of food and beverages in the world. 5.8 million people are employed in over 700,000 companies in Germany according details released by the German Food Association . This means that 13 per
Agriculture1 Agricultural wholesaling Food craft industry5 Food industry8 Food wholesaling Food retailing Hotels and restaurants Total food industry9 Comments: Figures for 2017 All of the figures have been rounded for clarity. 1) Includes forestry and fisheries 2) Production value 3) 2016 value 4) Estimated 5) Craft companies include the companies registered in the 2016 crafts census 6) Estimate based on the 2016 crafts census, excluding VAT 7) Estimate based on the 2016 crafts census 8) Companies and employees from companies with at least 20 employees 9) The totals have been adjusted for double counting due to different sector definitions (industry - crafts; crafts - outsourced market).
Fig. 1: Companies in the German food industry
F & S International Edition No. 20/2020
CUT Membrane Technology cent of all employed people are employed in agriculture, agricultural wholesaling, the food trade, the food industry, food wholesaling and retailing as well as the hotel and restaurant industry. They form the food chain, which includes all of the stages from initial production right up to the consumer. Whilst agriculture produces the plant and animal commodities, around 84 per cent of this is processed into food by the food processors and the food industry. The products reach the commercial customers and end consumers through the wholesale and retail trade and often directly through craft businesses as well. Many of these companies are family businesses that help to ensure that the approximately 170,000 different products are sold on. Further details about the German food industry can be found in the information graphic (Fig. 1). The importance of industrial food processing can be seen from the fact that it generates 22.8% of total turnover with only 6,050 companies (8.6% of all companies) and 580,000 employees (1.3% of all employees) involved. The majority of its products are exported within the European domestic market. The European domestic market is a marketplace with over 500 million consumers and opportunities for all. Germany is now the largest food producer in the EU. Because of the EU domestic market, the same rules and standards apply throughout Europe, i.e. with regard to food labelling, maximum residue limits and hygiene regulations.
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3. Importance of the food industry to German mechanical engineering The importance of the food industry to the German mechanical engineering industry is documented through the figures released by the VDMA Food Processing and Packaging Machinery Association . This trade association represents around 300 companies, which are mainly medium-sized companies and they generate over 80% of the industry’s turnover. According to the association, the member companies generated nearly 15.2 billion euros in 2018, which was an increase of 8 per cent over the previous year. The packaging machinery sector also grew by 8 per cent to 7.1 billion euros. Growth rates in the specific food processing machinery sub-sectors varied, but were positive without any exceptions. Production of beverage-producing machinery also grew by 7 per cent to 552 million euros. Exports of food processing and packaging machinery rose by 6.1 per cent to over 9 billion euros in 2018. Deliveries within the most important sales region, the EU-28, increased by 9 per cent. Demand from the USA, the most important foreign market, continued to remain high. Exports to China and Russia showed double-digit growth rates. There was also significant momentum in many other markets, including Brazil, India, Japan and the Republic of Korea. Domestic business was also an important pillar of positive business development during 2018. Some food sectors invested heavily in capacity expansions and modernisation projects. The lack of personnel in the processing companies resulted in further investments being made in machinery and systems. The association assesses the prospects as good because the industry continues to benefit from the increasing global demand for processed and packaged food and beverages as well as pharmaceutical products. Uncertainties caused by ongoing trade disputes as well as regional political crises and the corona pandemic are also making investors reluctant to place new orders. 4. S eparation technology applications used in food production Concentrating solids and/or clarifying liquids during the production of food and beverages is often necessary. The resulting liquid or solid or even both can be valuable products. Some will be food, whereas others can also be used as animal feed or fertilizer. Separation processes are used here to separate the substance F & S International Edition No. 20/2020
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mixtures into fractions with specific properties. When mechanical processes are used, separation is realised through the effect of the forces (e.g. compressive, inertial, centrifugal, frictional or surface forces) that are applied to disperse the solids. Separation occurs with low-energy consumption when compared to thermal separation processes. At the forefront here are (depending on the task): - separating solid substances (clarification), - concentrating solids (concentration, dewatering) - separating particles of a specific size (classification) - separating according to another specific characteristic (sorting). Many of the substance systems that have to be processed are available as complex colloid-dispersed suspensions. The density difference between the solid and the liquid is usually quite low. These properties significantly degrade a mechanical separation process. Small particles and the low density difference result in a very low settling speed being needed for the dispersal phase. Therefore economical use of the sedimentation can usually only be realised in conjunction with sedimentation centrifuges operated at high speed. Sedimentation in the gravitational field is often only considered with regard to the storage of liquid products. The large specific surfaces of the substance systems working in conjunction with a surface charge in the same direction and the resulting repelling surface forces counteract agglomeration as well as the de-watering of the formed sediment. Furthermore, consideration must also be given to the fact that a compact and compressible filter cake will be formed during filtration and this will cause a correspondingly reduced filtrate flow. Therefore filtering aids, e.g. kieselguhr or cellulose based, frequently have to be used to ensure economical suspension filtration. The advantage of cellulose-based filtering aids is that the cloudy matter removed from the food can be used with the cellulose as animal feed. Energy-intensive dynamic filtration processes used in conjunction with a microporous membrane can also be used to clarify the liquids. The most frequently used dynamic filtration processes are cross-flow filtration using membrane modules. The filter cake is often washed as well during a cake filtration process. This means using another suitable liquid (washing agent) to remove the original liquid and its components from the filter cake. The original liquid will adhere to the particles (crystals, precipitation products, micro-organisms) and the majority
of the substances dissolved in them will be removed. The dissolved substances might be undesirable substances or even valuable products, which, in the latter case, are recovered from separate stages. The transition from washing to extracting the solids is a fluid process. Therefore, consideration must be given to the possibility of extracting wanted or unwanted substances from the particles when you select the washing liquid that will be used. The objective in many cases is to recover the solid as a dry product in a pure and concentrated form. This is why filtration or cake washing is usually followed by a mechanical liquid separation process. Details covering this are described in . Thorough drying of the solid is then carried out in a thermal drying process. The drying methods that can be used here are described in [5-7]. It must also be noted that high temperatures are to be avoided here due to the temperature sensitivities of the products. Filtration is mainly used for separating dusts and aerosols. If the dusts are valuable products, then they will be recovered if possible. Cleaning is more difficult with fatty-products such as milk, coffee and cocoa powder. The devices and machines used in food and beverage technology must meet the highest standards. Detailed explanations of these standards are given in . One problem here is the tendency of the food to decompose, i.e. to change negatively due to physical, thermal, chemical or microbiological processes. These possible negative changes must be averted during the processing. This is why filtration often strives for sterilisation as well, which significantly increases the separation requirements. A multi-stage process with pre-filters and final filters based on filter layers or filter membranes is often used for this purpose. Consideration should also be given to the fact that batch processes are widely used in food companies. At the same time, there might be a requirement to process different products in one plant or even in a single processing line. As in other industries, there is also a trend here towards automated plant operation. These high requirements and the specific properties of the substance systems mean that efforts that far exceed those of other industrial sectors are needed. There is usually a high demand for process water for use in food processing. More and more companies are trying to recycle water and aqueous solutions as much as possible at their place of origin in order to prevent the production of waste water. Different separation techniques, such as centrifuges, membrane
processes or flotation processes, are being used. Membrane units are often used for improving the quality of raw water. Small and medium-sized food companies are mainly indirect dischargers as they have their own equalisation, neutralisation and buffer tanks for treating and storing waste water. Larger companies usually have their own waste water treatment plants for economic reasons. They are often direct dischargers. We now have a few examples that show that process water can be economically recycled through biological treatment processes that use MBR (Membrane Bio-Reactors) in conjunction with RO (Reverse Osmosis). The sludge produced in various stations is concentrated or else press filters or decanters are used to concentrate it. 5. Development trends Competitive pressures and increasing consumer demands on food products are forcing companies in the industry to optimise their products and processes to produce ever shorter production cycles and reduce their costs. The requirements placed on the processing systems used in food and beverage technology show that progress in various sectors also contributes to improvements (see  as well). Great efforts have been made in recent years to improve the design and construction of machinery and equipment so that the risk of contamination by micro-organisms is minimised and cleaning the plants is made easier. The “hygienic design” guidelines drawn up by various working groups and organisations have proved to be trendsetters. Digitalisation has also become a key issue within the food industry. Real-time information from the production processes enables timely and resource-saving production to be realised and it also improves the documentation and traceability. Networking the systems means that it is often possible to optimise production and batch tracking as well as the use of raw materials and energy, and to better adapt the supply produced to meet the demand from the respective customer. The digitalisation of laboratories is also very important to the food industry. Networkcompatible laboratory equipment with smart functions and efficient interface solutions are becoming indispensable. In future laboratories, analysis and measuring equipment will often be networked with the production processes and the sensors that are integrated in them Automation, information and management systems will control and regulate this network. The resulting flood of data will become a huge challenge to the companies. Increased integration of the laboratories
F & S International Edition No. 20/2020
The PACO + HETA-Formula: into a company’s structure will also be beneficial with regard to product development. However, it is important here that formula development, product and process development and quality assurance are already being supported by software. The resulting manufacturing process and material and processing cost transparencies will be of great benefit in conjunction with a fast market launch or customised production. The separation processes being used must be optimised for the relevant application. It is important here that the operating parameters are selected so that optimum separation and the wanted product properties can be attained. Even today, selecting suitable separation process and the associated operating parameters is still predominantly based on complex experimental studies. Therefore it will be worthwhile to use the possibilities that will be provided by digitalisation here as well. The optimum operating parameters can also be determined if suitable software is used and they should be based on automatic recording of the relevant data from the separation process as well as the defined product data. These can be adjusted and monitored later on during the process so that a product with optimum properties will be produced at all times. For the providers of separation processes, there will be the possibility to theoretically determine the optimal operating parameters in advance when setting up the corresponding databases and working in conjunction with the software for specified products and requirements. This will enable the experimental effort involved in product development to be significantly reduced, development times will be shortened and development costs will be saved. As in other sectors, the basic condition for this development is that product safety is never jeopardised and data security is always guaranteed. The digital infrastructure needed for this must also be available. Reference literature:  Data & Trends – EU Food and Drink Industrie 2017 Hrsg.: FOODDRINK Europe, Brüssel, Oktober 2017 www.fooddrinkeurope.eu  Lebensmittelverband Deutschland, Pressemitteilung vom 18.03.2019 www.lebensmittelverband.de  Nahrungsmittel- und Verpackungsmaschinen legen kräftig zu Pressemitteilung des VDMA vom 19.06.2019, www.vdma.org  S. Ripperger: Trocknung disperser Feststoffe, Teil 1: Mechanische Entfeuchtung als Vorstufe der thermischen Trocknung Filtrieren und Separieren 2018, 32, Nr. 4, S. 256-253  S. Ripperger: Trocknung disperser Feststoffe Teil 2: Trocknungsverfahren und Flüssigkeitsbindung Filtrieren und Separieren 2018, 32, Nr. 5, S. 343-347  S. Ripperger: Trocknung disperser Feststoffe Teil 3: Bauarten und Betrieb von Konvektionstrocknern Filtrieren und Separieren 2018, 32, Nr. 6, S. 418-420  S. Ripperger: Trocknung disperser Feststoffe Teil 4: Bauarten und Betrieb von Kontakttrocknern Filtrieren und Separieren 2019, 33, Nr. 2, S. 103-106  S. Ripperger: Anforderungen an verfahrenstechnische Anlagen für die Lebensmittel- und Getränketechnik Filtrieren und Separieren 2017, 31, Nr. 4, S. 258-262
F & S International Edition
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Separation processes for beverage production: process options and new developments
Report from the BrauBeviale 2018 exhibition J. Barth* Products for the beverage industry covering different fields, e.g. raw materials, process technology, packaging, or marketing, were presented at the BrauBeviale exhibition at Nuremberg in 2018. Apparatuses and products for separation processes form a major part in the field of process technology. Focusing on the brewing process, different, competing separation processes for the different separation steps involved will be presented with their advantages and disadvantages. For the different separation processes single products will be presented as examples together with selected new developments. Introduction The BrauBeviale exhibition was held in the period 13–15 November 2018 at Nuremberg. In 7 exhibition halls, 697 exhibitors presented their products for the beverage industry covering different fields, e.g. raw materials and sensory refinement, process technology, components, packaging, or accessories and marketing. 176 apparatuses and products for filtration were presented in the field of process technology. These included proven and tested apparatuses and products, e.g. sheet filters, sieve filters, cartridge filters, rotary filters, or cross-flow filters with membranes, as well as new developments, e.g. cartridge filters from borosilicate that allow for faster sterilisation due to their higher stability. This article presents a survey of process options for separation in the beverage industry giving the advantages of the different process options. Focusing on the brewing process, different apparatuses and products for these process options as well as new developments will be presented. The brewing process mainly consists of the process steps mashing, boiling of wort, fermenting, and packaging. Every step includes specific separation processes: separation of the wort from the grains (lautering), clarification of the beer, and sterile filtration of the process gases involved (cf. Donaldson ).
In apparatuses for homebrewing or microbreweries, lautering is almost always realised using slotted sieves: The brew kettle contains a mobile sieve plate. At the beginning of the mashing process the sieve plate rests on the bottom of the kettle. After the mashing process is finished, the sieve plate is moved upwards in the suspension. It collects the solids while the liquid is drained. The advantages of slotted sieves are the minimal energy demand and the technically simple implementation. The disadvantages are the high residual liquid content of the separated solids (product loss) and the low throughput that is decreased even further with finely milled malt (cf. Schneider ). For these
reasons, slotted sieves are almost never used in industrial brewing processes. In industrial breweries, lautering is almost always realised using separate apparatuses. Static surface filters (filter presses) or dynamic surface filters (membrane filters with membrane tubes or rotating membrane discs) are used as well as centrifuges: After the mashing process is finished, the suspension is pumped from the mash tun into the separator and the separated liquid into the brew kettle. All the separators used in industrial breweries allow for the use of malt that is milled more finely than would be possible with slotted sieves. The finely milled malt facilitates a higher yield at shorter mashing times.
Lautering The milled malt is mixed with hot water in the mash tun (mashing) to extract the sugars. The resulting liquid (wort) must be separated from the washed-out grains (draff). * Dr.-Ing. Jakob Barth Backnang E-Mail: Jakob.Barth@outlook.com
Fig. 1: Disc centrifuge Seital SE30 from SPX Flow for mash separation and/or clarification. Source: SPX Flow Technology Germany GmbH
F & S International Edition No. 20/2020
The advantages of centrifuges are a high throughput and their flexible applicability in the brewing process. The same centrifuge can be used for lautering after mashing as well as for clarification after fermentation (see below). The disadvantage of centrifuges is their comparatively high specific energy demand. The advantage of dynamic surface filters is a high filtrate flow rate even in case of suspensions with poor filterability, especially a suspension of finely milled malt. The disadvantage of dynamic surface filters is their comparatively high specific energy demand in case of suspensions with good filterability, especially a suspension of coarsely milled malt. The advantages of filter presses are a comparatively low specific energy demand in case of suspensions with good filterability, especially a suspension of coarsely milled malt, and the low residual liquid content of the separated solids. The disadvantages are a low filtrate flowrate in case of suspensions with poor filterability and a high setup time to open the filter, discharge the filter cake, and close the filter again. A new development that avoids some of these disadvantages is the Steinecker Platos filter press from Krones: Due to a new design of the filter frames the draff can be discharged without the necessity to open and close the filter axially. This reduces the setup time significantly. Additionally, a multistage extraction of the sugars by filtration and subsequent addition of fresh water can be realised directly in the filter. This enables the processing of up to twelve brews per day. The design of the filter plates, filter media, and membrane plates improves the discharge of the filter cake and the cleanability of the filter. Thus, it improves the hygienics of the filtration process (cf. Krones ).
Sefar – Filter Solutions for Process Filtration Process Filtration Broad selection of innovative solutions for solid / liquid filtration, screening and drying processes.
Clarification If a bright beer is desired, it must be clarified after fermentation and before packaging to remove yeast and particulate matter from the liquid. The clarification of the beer is a much more challenging separation process than the lautering of the wort after mashing, as the solids exhibit a much smaller particle size. Therefore, in apparatuses for homebrewing or microbreweries clarification is usually omitted as the equipment and energy cost is very high. Yeast and particulate matter remain in the beer, therefore it is naturally cloudy. In industrial breweries, the same separation processes that are used for lautering can be utilised for clarification: centrifuges, static surface filtration with filter aids, or dynamic surface filtration with membranes. If necessary, the process can be realised with multiple stages for primary and fine clarification (cf. Donaldson ). The advantages of centrifuges (e.g. Seital SE 451 from SPX Flow, see fig. 1) are a high throughput and their flexible applicability in the brewing process. The same centrifuge can be used for clarification after fermentation as well as for lautering after mashing (see above). The separation efficiency can be adjusted by the process parameters: To produce a naturally cloudy beer, only part of the yeast and particulate matter are separated, the rest remains in the beer. To produce a bright beer, most of the yeast and particulate matter are separated in the primary clarification stage. Thus, the amount of solids that has to be separated in the fine clarification stage via filtration is reduced. Some brewers do not employ filtration processes for clarification at all because they claim that they remove flavouring substances as well and thus compromise the taste. The disadvantage of centrifuges is their high specific energy demand. The advantages of static surface filtration with filter aids are a high throughput and a low specific energy demand. The application of filter aids like kieselguhr is necessitated by the small particle size of the solids. Typical filter apparatuses are filter presses. The advantage of filter presses is the low residual liquid content of the separated solids. The disadvantage is the high setup time to 11
F & S International Edition No. 20/2020 www.sefar.com
Fig. 2: NDCF 1-5 dynamic cross-flow filter (left) with rotating ceramic filter discs (right) from Novoflow for clarification. Source: Novoflow GmbH
open the filter, discharge the filter cake, and close the filter again (see above). The general disadvantage with the use of filter aids are the increasing costs for the disposal of the separated solids with the filter aid due to the increasingly strict regulations for the disposal of the filter aids. The advantages of dynamic surface filtration with membranes are the absence of filter aids and a sharp separation. Despite
Fig. 3: Sterile filter (P)-SRF C from Donaldson for process gases. Source: Donaldson Filtration Deutschland GmbH
the absence of filter aids a relatively high throughput is achieved by the flow of the suspension across the filter medium. Furthermore, the problematic disposal of the filter aids is avoided. Additionally, the separated yeast can even be recirculated in the brewing process to reduce the amount of solids for disposal even further. The sharp separation produces a filtrate with a very low concentration of solids (bright beer). The general disadvantage of dynamic surface filtration is its high specific energy demand. Typical filter designs are tubular or capillary filter modules and rotating disc filters. The advantage of tubular or capillary filter modules is their simple design without any moving parts. While capillary modules offer a greater filter area in relation to the installation space, tubular modules (e.g. Cerinox BF from Bucher Unipektin) are less prone to blockage due to their greater diameter. Therefore, they are especially well suited for the clarification of e.g. fruit or vegetable juice (cf. Bucher ). The disadvantage of tubular or capillary filter modules is that the achievable degree of concentration is limited by the flowability. The relatively high liquid content of the separated solids causes a correspondingly high product loss. By contrast, the advantage of rotating disc filters (e.g. NDCF 1-5 dynamic cross-flow filter from Novoflow, see fig. 2) is their capability of concentrating the separated solids to a paste-like consistency. The relatively low liquid content of the separated
solids causes a correspondingly low product loss. The disadvantage of rotating disc filters is their more complex design with rotating shafts and dynamic seals. Sterile filtratrion of process gases To avoid contamination of the product, all gases that are employed in the brewing and filling process must be sterilised by sterile filtration: air for aeration of the mash tun, carbon dioxide (CO2) for buffering of the fermentation and clarification vessels, for liquid removal in (static) filtration, and for filling under pressure. Commonly, for the sterilisation of the vessels, apparatuses, storage tanks, and the sterile filters steam is employed. A new development is the sterile filter (P)-SRF C from Donaldson (see fig. 3): The use of borosilicate for the filter material allows hydrogen peroxide (VPHP) and ozone to be employed for sterilisation. Thus, the sterilisation time can be substantially reduced. Reference literature:  Donaldson: Brewery Filtration Applications. Company publication, 2018.  Schneider, J.: Dynamische Mikrofiltration von Feinstschrotmaische mit oszillierenden Membranen. TU München: Diss., 2001.  Krones: Steinecker Platos – Entwicklungsprojekt für eine innovative Maischeseparation [online]. Company publication, 2018 [accessed 09.01.2019]. Available under: https://www.krones.com/media/downloads/ platos_de.pdf  Bucher: Let’s be clear! – Innovative filtration. Company publication, 2018.
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High-end blood plasma fractionation J. Thomas* Today, the global pharmaceutical industry is demanding new answers for producing innovative, highly specific and lifesaving medicinal products. Human blood plasma, which is an extremely valuable raw material, is of special interest here. Well over 100 different proteins are found in human plasma, many of which can be used as the starting material for new medicinal products. Best-known agents: Immunoglobulins for passive immunisation or isolating specific vaccines and albumin for treating blood coagulation disorders or for treating shock conditions caused by severe blood loss. During fractionation, human plasma is broken down into its specific components in industrial-scale batches of several thousand litres as part of a complex process. The requirements covering all of the equipment that is used here are subject to strict regulatory guidelines and extreme quality standards. Ethanol fractionation Ethanol fractionation is one of the fundamental processing steps. Strassburger Filter is a leading international manufacturer of high-quality filtration systems. The precipitation properties of the separate protein fractions are used under specific temperatures, ion concentrations and pH * Jens Thomas, Anwendungstechniker STRASSBURGER Filter GmbH + Co.KG Osthofener Landstraße 14 67593 Westhofen/Rheinhessen Tel.: +49 (0) 6244 90800 – 0 Fax: +49 (0) 6244 90800 – 8 www.strassburger-filter.de
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values during ethanol fractionation. An increasing amount of ethanol will be added to the plasma here for each fraction that has to be separated and, depending on the process being used, the temperature will also be lowered below 0°C. Adsorbents can also be used to some extent to bind the corresponding protein and this will cause precipitation. The resulting stable intermediates are separated during the filtration process by a plate and frame filter, such as the Hermetix made by Strassburger Filter (Figs 1 and 2). The Hermetix filter is a system that has been especially developed for the
pharmaceutical industry with vertically arranged filter elements, whose special design hermetically seals them against the environment. The solids are retained by an appropriate internal filter layer, which is installed inside the hermetically designed filter elements. The inlet and outlet channels are also sealed. GMP conformity means that there is no need for seals on the collecting channels. The resulting filter cake’s (paste) porosity is implemented by adding appropriate filter aids as body feed, but this depends on the actual fraction. The optimised flow control system in the filter’s inlet openings
Fig. 1: A Hermetix filter, which is made by Strassburger Filter, with polypropylene filter elements
proteins. It can also be used to stabilise the filter cake for cake washing. Being a genuine new development, the system is fully CIP-compatible and reusable filter cloths can be used in compliance with the GMP guidelines. The process is made more economical through eliminating the single-use filter media costs and no abrasions from the filter media can enter the product. The Hermetix is fitted with a mechanical interlock as a special processing safety feature in addition to the hydraulic closing system that prevents the filter from being opened unintentionally whilst the processes are running. Laboratory and pilot plant scale
Fig. 2: A Hermetix filter, which is made by Strassburger Filter, with stainless steel filter elements
guarantees a homogeneous and stable cake structure – a basic requirement for reproducible, validated and largely automated application processes. Two target products: Permeate or intermediate The focus here can be on both the permeate or the precipitated intermediate being the target product, but this will depend on the process stage. Therefore the filtration system has to be multifunctional and it must also ensure the homogeneity and purity of the filter cake and simultaneously guarantee a high yield and minimal permeate losses. Polypropylene or stainless steel The Hermetix is available with either polypropylene or stainless steel filter elements. Both systems are characterised by a product-specific design and special processing characteristics. - polypropylene filter elements can be integrated in temperatures ranging from 14
-12°C to +80°C, which is a very broad spread for this type of material. In particular, the demand for ever higher temperatures in the CIP sector has advanced the development of a special manufacturing process and the choice of materials used in it. - stainless steel filter elements have a drainage profile that has been developed for the pharmaceutical industry and it has a special surface finish and cleanability, which was especially designed for use in high temperature ranges. Both systems are available with membrane plates for increasing the yield and making post-processing operations, such as cake washing, more effective. The membrane’s function here is to mechanically dehumidify the filter cake. Special feature: The membrane plate has two functions. On the one hand it functions as a normal filter plate and on the other hand it also provides a cake re-pressing option. This eliminates the need to use compressed air to blow out the filter cake, a process that causes a sensitive reaction in some
The pharmaceutical industry is continuously striving to produce new developments and to optimise current and well-established processes. The initial stages of any development have their origins on a laboratory and pilot plant scale and appropriately designed filtration systems will be needed for production later on. Strassburger Filter has the 200 series (20 cm plate format) in its portfolio for laboratory testing and later scaling-up and systems up to the 400 series (40 cm plate format) are available for pilot plant applications and small production runs. The portfolio includes sizes up to 1,200 mm plate size with variable device designs and frame widths. This means that customer requirements can be implemented individually. The Hermetix filter system can be summarised as follows: - sealed filtration system that meets the special requirements of the pharmaceutical industry – sealed multi-layer filter with depth filter sheets (filter cloths or paper can be used as an alternative) together with vertically arranged filter plates and filter frames - movable cover for pressing and sealing the plate packs - filter plates are sealed by the circumferential O-ring seals fitted on the filter plates and by the internal filter layers; alternatively: the seals can be integrated in the filter frames - 100% sealing without any leaks, no contact with the environment - easy handling and CIP cleaning possibility - filtering surface can be flexibly adjusted through the size and number of filter plates being used - large selection of filter aids for ensuring safe separation of solids or micro-organisms - hydraulic closing with mechanical locking option
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Analytical photo-centrifugal filtration (ACF): Membrane resistance and filterability S. Boldt *, D. Lerche *, M. Loginov ** Filtration has broad applications in academic and industrial research as well as in product development. In general, Nutsche filter or filter press are applied for filtration at lab scale. Recently, an analytical photo-centrifugal set-up was introduced as a new laboratory technique to characterize filter media and membranes, or filterability of solutions and liquid dispersions (e.g., suspensions, protein solutions, extracts, etc.) [1 - 3]. We describe a new analytical centrifugation technique for determining filter media or membrane resistance as well as filterability. Analytical centrifugal filtration (ACF) by a multi-sample photo-centrifuge is based on continuous in-situ measurement of space resolved light transmission through a specially designed filtration measuring module (FMM) monitoring the volume decrease of the sample at top or the filtrate increase at bottom by STEP-Technology® . The developed FMM hold the sample that has to be filtered as well as the resulting filtrate, and enables different filter media to be used. Up to 12 FMM can be placed on the rotor. The filtration pressure is applied by the centrifugal force acting on the sample, which can easily be set by programming the rotor speed. A maximum pressure of up to 7·105 Pa can be realised in this way. Typical sample volumes vary from 0.2 ml to 1 ml. Filter media with thicknesses ranging from a few micrometres up to 6 mm and a diameter of 7 mm can be employed. Experimental data are presented and discussed regarding filter media and membrane resistance in dependence on different pure liquids filtered as well as, exemplarily, filterability of protein solutions and suspensions. ACF does not need any externally-generated pressure and only needs a minimal cleaning effort. Due to the small sample volumes and filter media size, it is beneficial when used in development projects or if very valuable product samples have to be analysed. In principle, the process is comparable with a classic Nutsche filter, but in the form of a “micro-Nutsche”. 1. Introduction Filter media and membranes enable extremely efficient and economical separating processes to be implemented and materials can be separated from solutions or dispersions. Filtration process is an essential processing stage used during the production of food or active ingredients, for ensuring the functionalisation of the dispersed phases, de-watering processes and the recycling of valuable materials, to name just a few. Filter media usually consist of a porous carrier material, which has a thinner functional layer that forms the actual separating membrane and it also has a crucial role with regard to overall functionality. The separating performance depends both on this selective layer (membrane) as well as the composition and properties of the feed sample . This is why academic and industrial research is now involved in studies that are focused on developing and optimising filter media, e.g., for the pharmaceutical and recycling industries, as well as developing effective quality control processes. The characterisation of filter media and membranes as well as the filterability of dispersions (suspensions, emulsions) and macromolecule solutions is often carried out using complex and time-consuming analytical methods such as gas adsorption, mercury porosimetry and imaging processes such as electron microscopy or micro-computer tomography or even complex filter media test benches for running pressure-driven processes . Classic pressure and vacuum filtration processes are frequently used to characterise filter media or the filterability of dispersions and colloidal solutions [7 - 8]. * Sebastian Boldt (M.Sc.) Prof. Dr Dr Dietmar Lerche LUM GmbH Justus-von-Liebig-Str. 3 12458 Berlin, Germany Tel.: +49 3067 806030 E-mail: firstname.lastname@example.org
** Dr Maksym Loginov STLO, UMR 1253, INRAE, Agrocampus Ouest 35000 Rennes France
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Fig. 1: STEP-Technology® and measuring principle of an analytical multi-sample photo-centrifuge (LUMiSizer®) for quantifying the local change in the concentration of dispersed phase  or tracking the growth of the sediment or supernatant (clear phase) of suspension or emulsion during centrifugation, a) Transmission profile for an unseparated sample at t0. b) Transmission profile at the time of centrifugation t1 and t2 (green) of the sample being separated. Legend: 1. Light source, 2. Parallel light beams, 3. Cell 4. Multi-sample rotor 5. Line detector
Fig. 2: Diagram of the experimental filtration measuring module (FMM) setup
Fig. 3: Components of the filtration measuring module
New challenges arise when developing bio- and nano-technologies, e.g., the need for smaller sample quantities as well as a reduced cleaning effort and smaller disposal volumes. There are also models that simulate separation properties in addition to the experimental approaches. Simulating the transport of a liquid through a membrane is not a trivial process. The basic models are based either on the Hagen-Poiseuille law (parallel cylindrical pores) or the Carman-Kozeny law that applies to the porosity of a densely packed bed of spherical particles. Advanced models and excellent know-how are needed to interpret experimental filtration data . This article describes an innovative technique that is based on analytical photo-centrifugation and using a “micro-Nutsche” for the filtration studies [1 - 3] and filter media characterisations . Measuring principle of ACF, designed FMM as well as experimental data analysis are described. 2. M easuring principle of analytical (photo-) centrifugal filtration (ACF) The laboratory technique of analytical photo-centrifugation is based on in-situ visualization of the concentration of solutes and dispersed particles and its alterations during centrifugation. The analytical filter centrifuge uses the STEP-Technology® measuring principle . As can be seen on Fig. 1, parallel light (2) traverses the cell, which contains the sample (3). Up to 12 cells are placed on the rotor (4) and a line sensor (5) records space- and time-resolved transmitted light intensity during centrifugation (known as STEPTechnology®). Light transmission profiles at different programmable times t0 (red), t1 and t2 (green) are schematically shown in Fig. 1). The extinction is obtained using (ln Ir,0/Ir,t) and this enables the changes in the concentration to be tracked. This measurement principle is widely used for particle characterization, like size and density distribution , [11 - 13] as well as for particle surface properties [14 - 16] and, on the other hand, for stability analysis according to ISO 13097 and ISO 18811 [17 - 18] and consolidation studies [19 - 25] of liquid-liquid and solid-liquid dispersions. This study reports on the use of STEP-Technology® in filtration experiments. An FMM was developed (Fig. 2) for use with LUMiFuge® or LUMiSizer® analytical centrifuges (both from LUM GmbH, Berlin) and it was placed on the rotor instead of the cell as depicted on Fig. 1. Up to 12 modules can be used on the rotor. This module basically consists of a widened standard PC-cell (10 mm, LUM GmbH, Berlin) that also holds the filter cartridge with the filter medium as well as the filtrate. It is a 2-part module, consisting of a support (holds the membrane) and a sleeve (holds the feed volume) and it is sealed by an O-ring (Fig. 2 & 3). Filtration pressure p is calculated from centrifugal force Fc relative to surface A of the filter medium. The pressure in the liquid layer increases with distance r from the rotation centre. The pressure on the surface of the filter medium can be calculated by eq. 1:
Fig. 4: Filtration of pure water through a micro-porous PP film (Treopore) (rotor speed 4,000 rpm and temperature 25 °C). The spatially resolved transmission profiles track the filtrate volume from red (t = 0 s) to green (t = 225 s) over time. The indent (minimum) transmission corresponds to the meniscus position (filtrate volume) during centrifugal filtration. Only 7 profiles are shown for reasons of clarity. Filtrate height hf results from the difference between the starting meniscus / cell bottom and the meniscus position at time t. The graph inserted in Fig. 4 shows the increase of filtrate volume versus the measuring time
With typical filling quantities of approx. 1 ml feed volume and rotation speeds ranging from 200 - 5,300 rpm (special LUMiSizer rotor), filtration pressures of (5·102 - 7·105) Pa can be realised (in case of using water with a density of approx. 1 g/m-3). The filtrate volume is collected in the transparent cell (Fig. 2 & 3) and its height is recorded during centrifugation by detecting the position of the meniscus rf in the lower part of the cell as shown in Fig. 4. The position of the meniscus no longer changes when all of the sample liquid passed the filter medium. Measuring the space- and time-resolved transmissions across the modified cell enables the amount of filtrate to be quantified
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The following eq. 2 applies if we assume that no liquid remains in the filter medium: (2) Am is the inner cross-section of the filter module, V0 is the starting feed volume and Af is the cross-sectional area of the collecting cell. The flow velocity u through the membrane is given by the time derived from the filtered volume Vf relative to the flow-relevant Am membrane cross-section. The following applies: Fig. 5: Relative membrane pressure p/pmax (pmax = 5 bar, at 4,000 rpm and 1 ml aqueous sample) during centrifugal filtration in dependence on relative centrifugation time
Furthermore, the Darcy equation can be used to determine the resistance of the filter medium as it depends on the pressure. The pressure p (Eq. 1) divided by the flow u (Eq. 3) and viscosity η applies for pure liquids (no cake formation, no membrane fouling): (4)
by the increase in the liquid filtrate column, accordingly, as the centrifugation / filtration time progresses (Fig. 4 inlet). According to Eq. 1 the pressure is dependent on the liquid height hm of the feed sample. hm can be recalculated from the filtrate volume obtained from Fig. 4. Another, equivalent method is to record the decrease of the feed sample over time like described in . When compared to the dead-end filtration of a classic Nutsche filter, the pressure used in centrifugal filtration is not constant as it depends on liquid column hm. The pressure reduction during centrifugal filtration was calculated according to Eq. 1 and it is shown graphically in Fig. 5. The increase of filtrate height hf reflects the decrease of the sample height (feed volume) hm above the membrane accordingly.
3. F ilter media / Membrane resistance determination by ACF Experimental results from selected filter media are presented below in order to demonstrate the principles discussed in the previous section. In a first series of experiments, membrane resistance for pure water of a ceramic carrier medium (average pore size d50 = 5 nm) coated with TiO2 layer (provided by IKTS, Hermsdorf) was determined in dependence on the maximum initial pressure. It was generated according to Eq. 1 by different rotor speeds (1,500 up to 3,500 rpm).
Fig. 6. Time increase for the relative filtrate volume at constant speeds ranging from 1,500 rpm up to 3,500 rpm
Fig. 7. Filtration pressure p dependent on the relative filtrate quantity for the applied speeds
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Fig. 8: Calculated transmembrane filter medium resistances dependent on the relative pressure and pressure values of the applied speeds, see Fig. 7
Fig. 9: Pressure dependence of the specific ﬂow k of the ceramic filter material
Fig. 6 shows the continuous increase in the relative filtrate volume V/V0 versus filtration time for different centrifugal accelerations. As expected, the flow rate (V/t) decreases for all speeds due to the reduction in the feed volume and consequently correspondingly the hydrodynamic filtration pressure (Fig. 7). Membrane resistances were calculated using Eq. 4 and results are shown in Fig. 8. In general, it was found that the calculated resistances were largely independent from the centrifugal forces that were used. The deviations could be explained by the run-up time of the rotor to reach the target speed of 4000 rpm (typically < 10s) as well as dead-volume effects. The deviations seen at the end of the experiment were caused by the small remaining amount of feed and its imprecise determination (low pressure, see Eq. 1).
Furthermore, the filtrate volume inside the membrane (thickness of ceramic membrane was 1 mm) was not taken into account in Eq.1 and Eq. 2. This results in the continual decreasing resistance value seen at the end of the experiment. A quasi-constant resistance of R = (3.8 ± 0.3) × 1012 m-1 was calculated for the average relative pressures p/p0 ranging from 0.3 to 0.8 (see Fig. 8). The pressure independence of the specific flow k (flow through the membrane standardised for time, pressure and filter area) through the filter medium (Fig. 9) substantiated the expected pressure stability of the ceramic filter medium. The new measurement technique (ACF) described here also enabled to study structural and pressure stability of permeable filter media based on the calculated pressure-dependent resistance.
Fig. 10: Specific ﬂow k of a pressure-stable ceramic medium (blue line) and a compressible polymer membrane (red dotted line)
Fig. 11: Water mean membrane resistance and standard deviation of ceramic supported membranes in dependence on the mean pore diameter
Fig. 12: Filter medium resistance for microporous PP film measured with water after pre-wetting with different surfactant solutions (blue symbols, as explained in the text) and pre-wetted with ethanol and measured with water (red symbol) or ethanol (black symbol).
Fig. 13: Filter medium resistance of PVDF membrane for glycerine/ water mixtures with different glycerine concentrations. 2,000 rpm for all measurements (maximum pressure = 1.2 bar)
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The performance of a recently developed ultrafiltration polymer membrane (IPF Dresden) is shown in Fig. 10. In contrast to Fig. 09, the flow depends on pressure applied to the membrane indicating some structure changes. Fig. 11 documents the mean membrane resistance for pure water of different ceramic membranes in dependence on nominal pore size over about four orders of magnitude. As expected, the resistance of membranes increases with decreasing pore size. Both axes are in logarithmic scale. There is an almost double logarithmic relationship between mean pore size and membrane resistance. The interaction between the membrane hydrophobicity / wettability and the surface tension of liquid phases was studied, exemplarily in another experiment. A micro-porous PP film (‘Supor’) from the TreoPore product family (TreoFan, Neunkirchen) was employed here. The effects of the pre-wetting of the filter medium as well as the surface tension of the liquids were studied. The determined resistances are summarised in Fig. 12. In a first series (blue left symbols), the filter medium was generally pre-wetted with ultrapure water and the resistance was determined for ultrapure water (circle, σWater = 73 × 10-3 Nm-1), ultrapure water containing 0.01 % m/m wetting agent ABS-Na (delta sign, σ = 42 × 10-3 Nm-1) or 0.001 % m/m ABS-Na (nabla sign, σ = 31 × 10-3 Nm-1). In dependence on the surfactant concentration, resistance was lowered by more than two orders of magnitude. In two additional experiments, the filter medium was prewetted with ethanol and filtered with ultrapure water
Fig. 14: Filtration experiments with BSA solutions of different concentrations realised by ACF filtration at 4,000 RPM (left) and by dead-end filtration at 5 bar (right). A 5 kDa PES membrane (Microdyn-Nadir) was used.
(centre red square) or with ethanol (black diamond), respectively. The membrane wetting with ethanol resulted in a more significant reduction of the membrane resistance. Membrane resistance was reduced by more than 3 or 4 orders of magnitude. Surface tension of ethanol equals to σEthanol = 23 × 10-3 Nm-1, which is lower than the tension of water with 0.001 % m/m ABS-Na. Membrane resistance should not depend on liquid viscosity according to eq. 4. Feed samples of different viscosities with otherwise constant properties should therefore result in a comparable value of membrane resistance. This was studied using solutions of different glycerine concentrations (viscosities) for a hydrophilic filter medium with a nominal pore size of 100 nm (PVDF membrane, Durapore, Ireland). The resistances for this membrane for glycerine solutions of different concentrations are shown in Fig. 13, using viscosity data from . A systematic moderate reduction in the resistance of up to three times was observed increasing glycerine concentration up to 90 wt. %. In
the light of Fig 12, it could be explained by the decrease of the surface tension increasing the glycerine ratio (pure glycierin, σGlycerine = 63 × 10-3 Nm-1). A generally similar dependency for differently concentrated saccharose solutions was also found (results not shown). 4. Filterability and cake resistance determination by ACF Applicability of ACF for filterability studies is exemplarily presented below. ACF can be classified as a batch dead-end operation where the maximal transmembrane pressure is generated by centrifugal force (Eq. 1) and experimentally set by RPM of the rotor. Principally, ACF can be operated by constant RPM or dynamic RPM and centrifugal filtration kinetics modelled based on filtration-consolidation theory and accounting for particle settling and pressure kinetics during centrifugation . Fortunately, filtration can be described and behaviour of different samples characterized in terms of filterability with no strict modelling.
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Fig. 15: Schematic representation of total filter pressure pT and its distribution between a fully retentive membrane pm and a thin, reversible filter cake layer pc building up during filtration
pressures from 0.5 kPa to 500 kPa can be applied for a typical sample volume of 1 ml. Up to 12 filtration samples can be measured simultaneously in a single experiment. A special commercially available filtration measuring cell (“microNutsche filter”) was developed carrying the filtration module with the membrane holder and the cartouche for the feed sample as well as collecting in a transparent part the filtrate. STEP-Technology® records automatically by space resolved transmission profiles the increase of the filtrate level (volume) during the centrifugation (filtration). Filtrate volume versus filtration time data at constant or changing rotor speeds allow to comprehensively quantify resistance and permeability of filter-media and membranes, hydraulic resistance of fouled membranes, specific filter cake resistance, solids accumulated in filter cake, filter cake compressibility and filter cake dryness. Acknowledgements
Fig. 16: Pressure dependence of average specific cake resistance α(pc ) for BSA solution (left; concentration 0.3 %, MW 66 kDa) and a Lapolite suspension (right; 0.3 %, particle size 30 nm). Filtration experiments were performed by conventional constant pressure dead-end filtration (open symbols)
Fig. 14 compares filterability data of BSA solutions of different concentrations performed by ACF at 4,000 RPM and by dead-end ultrafiltration at 5 bar. Filtrate volume increases versus time, in both cases it is very similar. For further analysis, it should be reminded that total pressure in accordance with Eq. 1 decreases with centrifugation time (Fig. 5) if rotor speed is constant. Secondly, in contrast to filtration experiments with pure liquids as discussed in the above chapter, in case of ultrafiltration of colloid solutions or dispersions, membrane resistance is not constant anymore but the total hydraulic resistance increases due to membrane fouling/cake formation (for details [2 - 3]). The total pressure drop is distributed between the filter media/ membrane and the filter cake in relation to the resistance of the membrane Rm and the cake Rc. Fig. 15 sketches these dependencies if we assume that the membrane itself is not altered during the filtration experiment and the cake compression is fully reversible. The average specific cake resistance ac,av which can vary with filtration pressure is given by Eq. 5. (5) The retained amount of dispersed phase in the filter cake is wc. The latter depends on the initial concentration in the sample,
filtrate volume and the cross-sectional membrane area Am. Average specific cake resistance was determined for a BSA solution and a Laponite suspension in dependence on the pressure pc for analytical centrifugal filtration and conventional dead-end filtration according to Eq. 5. Filtration pressure was applied by rotor speeds of 1,000 RPM, 2,000 RPM and 4,000 RPM, respectively, in case of ACF and of 1 bar to 5 bar for conventional dead-end filter tests. Same PES membrane with MWCO = 5 kDa (Microdyn-Nadir) was used in both studies. As Fig. 16 reveals, obtained specific cake resistance is very similar for both experimental set ups. 5. Summary Analytical centrifugal filtration (ACF) is an efficient alternative approach to constant pressure dead-end (ultra)-filtration and is based on photocentrifuges (LUMiFuge®, LUMiSizer®) combined with STEP-Technology®. It is applicable for filter media / membrane testing using pure liquids and characterization of filterability of macromolecule solutions, nanoand micro-suspensions and -emulsions, extracts, juices, slurries etc. The filtration pressure is applied by centrifugal force programable by the rotor speed with no additional means. Constant or dynamic
The authors would like to thank the Federal Ministry of Education and Research (Research project 03XP0104B) for the funding and Professor Dr Eugene Vorobiev (Université de Technologie de Compiègne, UTC) for his support. Glossary ACF … Analytical centrifugal filtration FMM … Filtration measering module RPM … revs per minute Af … Cuvette cross-section (m2) Am … Filter module cross-section (m2) Fc … Centrifugal force (kg m s-2) h0 … liquid’s height above the membrane at t = 0 s (m) hm … Liquid column above the membrane (m) … Filtrate liquid column (m) hf k … Specified flow (l m-2h-1bar-1) m … Infeed mass (kg) p … Transmembrane pressure (Pa) R … hydraulic membrane resistance (m-1) r … Distance from the centre of rotation to a given position (m) rf,o … Radial position of the bottom of the cuvette (m) rf … Meniscus position in the filtrate (m) rm,o … Filter medium position (m) rm … Infeed meniscus position (m) u … Flow velocity (m s -1) V0 … Feed volume in the filter module at t = 0 s (m3) Vf … Filtrate volume (m3) η … dynamic viscosity (Pa s) ρ … Density (kg m-3) w … Annular velocity (rad s-1) s … Surface tension (N/m)
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Reference literature:  M. Loginov, N. Lebovka, E. Vorobiev. Multistage centrifugation method for determination of filtration and consolidation properties of mineral and biological suspensions using the analytical photocentrifuge. Chemical Engineering Science. 107 (2014) 277 – 289  M. Loginov, F. Samper, G. Gesan-Guiziou, T. Sobisch, D. Lerche, E. Vorobiev. Centrifugal ultrafiltration for determination of filter cake properties of colloids. Journal of Membrane Science. 536 (2017) 59 - 75  M Loginov, F. Samper, G. Gésan-Guiziou, T. Sobisch, D. Lerche, E. Vorobiev. Characterization of membrane fouling via single centrifugal ultrafiltration. J. Taiwan Inst. Chem. Engineers. 94 (2019) 18 - 23  D. Lerche. Comprehensive characterization of nanoand microparticles by in-situ visualization of particle movement using advanced sedimentation techniques. Kona Powder Particle J. 36 (2019) 156 - 186  J. Pinnekamp. Membrantechnik für die Abwasserreinigung. (2006). FiW Verlag ISBN: 9783939377009  A. B. Abell, K. L. Willis, D. A. Lange, Mercury Intrusion Porosimetry and Image Analysis of Cement-Based Materials. Journal of Colloid and Interface Science. 211.1. (1999). 39-44. 10.1006/ jcis.1998.5986  VDI Richtlinie 2762. Filtrierbarkeit von Suspensionen  K. Luckert. Handbuch zur mechanischen Fest-FlüssigTrennung (2004). Vulkan Verlag ISBN: 978-3-80272196-0  R. W. Baker. Membrane Technology and Applications, 2nd Edition (2004), WILEY ISBN: 0071354409  S. Boldt, D. Lerche, M. Loginov. Filtrationsexperimente mittels analytischer Photozentrifugation 1. Filtermediumwiderstand, Filtrieren und Separieren 33 Nr. 5. (2019), 282 - 287  T. Detloff, T. Sobisch, D. Lerche. Particle size distribution by space or time dependent extinction profiles obtained by analytical centrifugation (concentrated systems). PowderTechnology. 174 (2007) 50 - 55
 D. J. Growney, P. W. Fowler, O. O. Mykhaylyk, L. A. Fielding, M. J. Derry, N. Aragrag, G. D. Lamb and S. P. Armes. Determination of effective particle density for sterically stabilized carbon black particles: Effect of diblock copolymer stabilizer composition. Langmuir. 31 (2015) 8764 - 8773  J. Walter, T. Thajudeen, S. Süß, D. Segets, W. Peukert. New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation,Nanoscale. 7 (2015) 6574 - 6587  D. Lerche, S. Horvat and T. Sobisch. Efficient instrument-based determination of the Hansen Solubility Parameters for talc-based pigment particles by multisample analytical centrifugation: Zero to One Scoring. Dispersion Letters. 6 (2015) 13 - 18  S. Süss, T. Sobisch, W. Peukert, D. Lerche, D. Segets, Determination of Hansen parameters for particles: A standardized routine based on analytical centrifugation. Advanced Powder Technology. 7 (2018) 1550 - 1561  D. Lerche and T. Sobisch. Evaluation of particle interactions by in situ visualization of separation behavior. Colloids and Surfaces A. 440 (2014) 122 130  D. Lerche and T. Sobisch. Direct and accelerated characterization of formulation stability. J. Dispersion Sci. Technol. 32 (2011) 1799 - 1811  A. Brunelli, A. Zabeo, E. Semenzin, D. Hristozov, A. Marcomini, Extrapolated long-term stability of titanium dioxide nanoparticles and multi-walled carbon nanotubes in artificial freshwater, J. Nanoparticle Res. 18, (2016) 113  T. Sobisch, D. Lerche, T. Detloff, M. Beiser and A. Erk. Tracing the centrifugal separation of fine-particle slurries by analytical centrifugation. Filtration. 6 (2006) 313 - 321  D. Lerche and T. Sobisch. Consolidation of concentrated dispersions of nano- and microparticles determine by analytical centrifugation. Powder Technology 174 (2007) 46 - 49
 S. P. Usher, L. J. Studer, R. C. Wall, P. J. Scales. Characterisation of dewaterability from equilibrium and transient centrifugation test data. Chemical Engineering Science. 93 (2013) 277 - 291  M. Loginov, M. Citeau, N. Lebovka, E. Vorobiev. Evaluation of low-pressure compressibility and permeability of bentonite sediment from centrifugal consolidation data. Separation and Purification Technology. 92 (2012) 168 - 173  M. Loginov, A. Zierau, D. Kavianpour, D. Lerche, E. Vorobiev, G. Gesan-Guiziou, S. Mahnic-Kalamiza, T. Sobisch. Multistep centrifugal consolidation method for characterization of filterability of aggregated concentrated suspensions. Separation and Purification Technology. 183 (2017) 304 - 317  E. Iritani, N. Katagiri, K. Aoki, M. Shimamoto, K.-M. Yoo. Determination of permeability characteristics from centrifugal flotation velocity of deformable oil droplets in O/W emulsions. Separation and Purification Technology. 58 (2007) 247 - 255  U. Schuldt, H. Woehlecke, D. Lerche. Characterization of mechanical parameters of microbeads by means of analytical centrifugation. Food hydrocolloids. 86 (2019) 201 - 209  B. Radel, M. Funcka, T. H. Nguyena, H. Nirschl, Determination of filtration and consolidation properties of protein crystal suspensions using analytical photocentrifuges with low volume samples. Chemical Engineering Science. 196. (2019) 72–81.  Taka Koichi Takamura, Herbert Fischer, Norman R. Morrow, Physical properties of aqueous glycerol solutions, Journal of Petroleum Science and Engineering, Volumes 98–99 (2012), 50-60  E. Iritani, N. Katagiri, R. M. Tsukamoto, K.-J. Hwang. Determination of cake properties determined from step-up pressure filtration test. AICHE J. 61 (2015) 4426 - 4436
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Membrane processes used as the basis for sustainable and cost-optimized water management in the mining industry Th. Peters * No other industrial activity sector shows such diverse and multifaceted water-related areas of conflict as the mining industry currently does. The contributing factors range from the procurement of process water to the environmental damage caused by process wastewater, which still cannot be assessed from a long-term perspective, and the acidic saline mine water (Acid Mine Drainage, AMD) produced by leaching processes. The resulting water treatment requirements in mining have led to developments in which sustainable and cost-optimized processes for treating water or wastewater, which were not normally used in the mining industry beforehand, but have been tried and tested in water management elsewhere, have been successfully adapted to the related tasks. They increasingly include pressure-driven membrane processes, whose effective mechanisms and the possible applications derived from them are discussed here using examples. 1. Introduction No other industrial activity sector on earth shows such diverse and multifaceted water-related areas of conflict, which are partly location-related, as the mining industry currently does. The effecting factors range from procuring the absolutely necessary process water of drinking water quality in areas of water shortage to the environmental damage that cannot yet be assessed from a long-term perspective as it includes the increase in the life-threatening potential of wastewater caused by the treatment processes as well as its large-volume storage . Aside from the sometimes critical and growth-limiting water supply situation in arid areas and the disposal problems that involve tailings, for which solutions that are difficult to comprehend from an environmental point of view are currently being studied as well , also the increasingly more stringent environmental legal requirements – that are intended to increase the protection of human health and the ecological balance of water bodies – are leading to significant international changes in water management  and, according to renowned economic experts, are already forcing shortterm changes to corporate strategy within the mining industry . This problem area also includes the acidic saline mine waters produced by leaching processes . * Dr.-Ing. Thomas Peters Dr.-Ing. Peters Consulting for Membrane Technology and Environmental Engineering Rheinfährstr. 201 41468 Neuss, Germany Tel.: +49 2131 228963 Mob.: +49 172 2441442 E-mail: firstname.lastname@example.org
The resulting water treatment requirements in mining have led to developments in which sustainable and cost-optimized processes for treating water or wastewater, which are not yet common in the mining industry, but were tried and tested in water management elsewhere, have been successfully adapted to the related tasks. In addition to the optimization of the conventional processes there is also the increasing use of pressure-driven membrane processes, the benefits of which can also be derived from the “Interdependencies between processes for treating or processing wastewater, separable constituents and remaining residues” shown in Fig. 1 .
It should be noted that, depending on the type of substances and the required discharge limits for the remaining liquids, specific processes or process combinations can be used for the further treatment or reprocessing of the residues that are generated by the specific processes, but are often not taken into consideration appropriately, in order to attain legally compliant disposal-compatible fractions. The range of options that have to be aligned to economic and ecological standards extends from the reuse and usage, as far as possible, of all of the substances still retained in the system up to operation without wastewater (Zero Liquid Discharge, ZLD) or residue-free operation.
Fig.1: Interdependencies between processes for treating or processing wastewater, separable substances and remaining residual substances (Source: Peters)
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It should also be mentioned that the details and interrelationships shown in Fig. 1 are intended to provide a basic phenomenology-based understanding of suitable processes to be used for treating water, wastewater and process fluids, and their usage can then be studied, planned and implemented in consultation with experts for successful large-scale realization . 2. Membrane processes Even though membrane processes appear to be, or are, sufficiently well-known and have been explained in countless articles and technical papers, descriptions of some technological details and technical terms differ in the various publications. Consequently, specific definitions and technical details commonly used by professionals are described below, together with some considerations based on many years of experience gained in this sector . The membrane processes considered here are all pressure-driven separation processes in which the driving force is a differential pressure across the membrane. The membrane is used to separate the water that is being treated into a filtrate (the term used for the product water in microfiltration [MF] or ultrafiltration [UF]) or permeate (the term used for the product in nanofiltration [NF] and reverse osmosis [RO]) and a remaining amount of retentate, which is also referred to as residual water or concentrate (Fig. 2). The valuable substances, pollutants and dissolved or undissolved components contained in the feed and retained by the membrane are collected and concentrated accordingly in the retentate. These processes are usually operated in the so-called cross-flow mode, which allows the forming of deposits on the membrane’s surface to be controlled and also reduces scaling, fouling and
Fig. 2: Flow diagram used as an example for pressure-driven membrane processes
bio-fouling to a certain extent. Here, the negative effects of unavoidable biofouling during long-term operation can be positively controlled through the use of optimized pre-treatment processes and/or the use of innovative cleaning processes. With regard to highly polluted wastewater, this particularly includes the integration of constructive measures such as using modules with “open channel technology” on the raw water side, which is now used very successfully throughout the world for treating landfill leachate and seawater desalination without the need for chemicals in the pre-treatment . Similar positive effects can be realized in connection with wastewater from mining, but the risks of calcium sulphate scaling or gypsum crystallization on the membrane must always be taken into consideration here [9 & 10]. The liquid to be treated in cross-flow mode is pumped across the membrane(s) and divided into the two above-mentioned partial streams; the operating pressure is realized by adjusting the inflow pressure generated by the feed pump accordingly through
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membranes usually have an average pore diameter of 0.2 µm. The transmembrane pressure ranges from 10 to 500 kPa (0.1 to 5 bar) and is normally around 100 kPa. A vacuum is applied, e.g. in the range of 0.1 bar, for special applications, such as MBR plants that use submerged modules.
Fig. 3: Difference between conventional processes and membrane processes (Source: Peters)
Fig. 4: Basic working principles of membranes (Source: Peters)
reducing the cross-section in the pressure control valve behind the module unit. This pressure control valve must be fully closed at precisely specified intervals for working in dead-end mode (which is only possible with MF and UF). It is important that flushing (back-flushing and chemically-supported back-flushing (only for MF and UF) should be used wherever possible) and other membrane cleaning procedures are to be selected accordingly in all cases and should also be optimized during long-term operation. It should be remembered that the most important prerequisite for the successful operation of membrane systems is the possibility of regular, highly effective cleaning of the membranes and all components in contact with the media in the corresponding membrane system, which can also be influenced by design. The membranes used in these processes can be regarded as clearly defined barriers, as shown in Fig. 3 . This enables continuous and reproducible function controlling through the use of robust measuring instruments. The membrane’s barrier function simultaneously ensures a consistently high filtrate or permeate quality, which, with the exception of NF, is virtually independent with regard to any changes in the pollutant concentration in the inflow . 24
Plants fitted with membranes or membrane modules have high operational stability as the process itself is controlled electrically and the functions are monitored electronically. Furthermore, no special attention is needed during start up or shut down, which only takes a few minutes if the continuously slow control speed is strictly maintained. The modular design of the plants forms the basis for high flexibility with regard to volumetric changes in the water to be treated and a small installation area. These features are the result of the membrane’s properties and their combination with a suitable module configuration and plant design, all of which must be adapted to precisely meet the requirements of the relevant application. The basis for selecting the specific membrane process is the performance range , whose basic operating principles are shown in Fig. 4 : 2.1 Microfiltration (MF): This is the membrane process with the lowest requirements placed on the membranes. MF is used to separate bacteria, pigments and other particles that have particle diameters in the sub-micron range. Porous membranes with pore diameters in the range of around 0.1 to 1.0 µm (1 mm = 1,000 µm) are used; commercial
2.2 Ultrafiltration (UF): These membranes can retain bacteria and viruses and separate macromolecules such as proteins as well as colloidal silicium and pyrogens. Typical separation limits for molecular weights range from 5,000 to 200,000 g/mol. The pore diameter normally covers the 0.02 up to 0.05 µm range. The transmembrane pressure ranges between 20 and 1,000 kPa (0.2 to 10 bar) and is normally between 100 and 300 kPa. A vacuum is applied in the range up to 0.2 bar for special applications, such as MBR plants that use submerged modules. 2.3 Nanofiltration (NF): NF membranes work according to the solution diffusion principle. Monovalent ions can diffuse through the membrane to a high extent, whereas multivalent ions are largely retained. The separation principle used here differs from that used in MF and UF where the retention of particles or other water constituents is determined by the diameter of the pores in the membranes. NF is suitable for removing pigments, sugar and trihalomethane (THM) precursors as well as for removing hardness or sulphate from a water inflow. As it can be used with low pH values, NF is also extremely well suited for cleaning acidic mine wastewater (Acid Mine Drainage, AMD). When used in conjunction with seeding technology (seed crystal dosing) and hydro-cyclone classification, it is possible to realize a permeate yield of up to 95% from highly concentrated landfill leachate . The transmembrane pressure can be up to 5,000 kPa (50 bar) and normally lies in the 1,500 to 2,000 kPa range. 2.4 Reverse osmosis (RO): The most-compact membrane types are used in reverse osmosis. A solution diffusion process with a retention rate of up to 99% is used to separate organic and inorganic molecules from the inflow. RO membranes are used to separate the salts dissolved in water and ions with less than 200 D, whereby one Dalton (Da) numerically corresponds to the molecular weight in g/mol. Applications range from ultra-pure water for the semiconductor and pharmaceutical industries to the desalination of seawater to produce drinking water and the purification of industrial wastewater, such as landfill leachate. The RO operating pressure is usually up to 7,000 kPa (70 bar), up to 1,500 kPa for low-pressure RO and up to 15,000 kPa (150 bar) for high-pressure RO . F & S International Edition No. 20/2020
3. W orking clusters in membrane plants While proven technical solutions are generally available for designing and manufacturing a membrane-based plant, the pre-treatment of the water to be treated and the treatment of the retentate as well as the wastewater streams generated during the operation of the membrane plant must be adapted on a case-by-case basis to meet the specific conditions at the site where the plant is constructed. These conditions can be very different because, apart from the factors that determine the quality of the raw water, the details regarding the on-site infrastructure and logistics usually differ and must therefore be taken into account accordingly when planning, constructing and operating a membrane plant . The basic principle is: “Each case is different!” The considerations listed above also include systems needed for: - dosing and handling of chemicals used for the pre-treatment of the raw water - dosing and handling of chemicals to ensure compliance with the process design during operation - dosing and handling of chemicals for the cleaning of membranes and plant components in contact with media
Fig. 5: Operation cluster of a membrane plant used for specification and evaluation of the process flow (Source: Peters)
- treating and discharging the individual wastewater streams generated during operation To specify and evaluate these interrelationships, a flow chart was developed (Fig. 5) which summarizes the main operating clusters of a membrane plant, shown here by way of example for a seawater desalination plant that uses reverse osmosis . The absolutely necessary components of “Information and Communication Technology” (ICT) are also the basis for an indispensable efficient
SCADA (Supervisory Control and Data Acquisition) system that includes modules for remote monitoring and control as well as comprehensive data processing and evaluation. Today this also includes precautions to protect against cyberattacks. 4. W ater management in the mining industry – requirements and solutions The special challenges facing water management in mining are caused on the one hand by the available water resources
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Acidic water for reuse in the mine Pre-treatment (without chemicals)
Treatment with pH-stable nanofiltration membrane
pH value increase/ treatment
and/ or 70–80% Permeate (clean water)
20–30% Retentate (concentrate)
Water for reuse in the mine
Retentate treatment e.g. using highpressure RO, crystallization or ion exchange
Sludge Rinsing water with metals
Returned to the natural water cycle
Extracting the metals
Fig. 6: Process diagram for mine water treatment by nanofiltration (Peters/Haertel-Prozess – PeHa process) ~85% Permeate (treated water) for reuse
AMD / Abwasser / Tailings Fließrate 1.000 m3/h TDS 6.500 mg/L Cu 490 mg/L SO4 2.600 mg/L pH ~2,0
Fließrate 850 m3/h TDS 130 mg/L Cu 10 mg/L SO4 52 mg/L pH ~2,0
pH-stable NF membrane Retentate Fließrate 150 m3/h TDS 42.600 mg/L Cu 3.200 mg/L SO4 17.000 mg/L pH ~2,0
10–15% liquid flow for feeding into the process
Residue minimization process
1–5% sludge for landfilling
Fig. 7: Process flow chart with analysis results from a nanofiltration-based process used for treating mine water (AMD), (Source: BPT/Peters)
with different quality characteristics or the associated need for further treatment under certain circumstances and on the other hand, by site-specific infrastructural, logistical and technological requirements up to and including the local legal requirements. The available water-related resources are known to include surface water, including brackish water of various origins as well as seawater, groundwater and spring water, sewage plant discharges, mine water, leachate and dump seepage water as well as tailings, for example. Pollutants can be present in these types of water in dissolved and/or undissolved form and cover the entire range of possible organic and inorganic components as well as the full range of size distribution for particulate water constituents. Solids are usually separated in settling or clarification tanks through the use of sedimentation and downstream microfiltration systems or disposable filter solutions. Processes such as oxidation, coagulation / flocculation or precipitation and/or downstream processes such as ultrafiltration, nanofiltration, reverse osmosis or ion exchange are used to remove dissolved pollutants [3 & 13]. Just as diverse as the types of water resources available are the objectives pur26
sued with water treatment. Examples of these include (as per ): - measures to overcome water scarcity through water reuse - improving the water quality for process optimization and increasing efficiency - reducing operating costs and increasing plant reliability - complying with the discharge values in order to abide by the guidelines and avoid fines - protecting health, both internally and externally - using water for dust suppression - reducing the water consumption index (ratio between fresh and recycled water) Examples of the successful trend-setting and large-scale use of membrane processes in mining include: - treating open-cast mining groundwater through the use of ultrafiltration and twofold concentrate-staged reverse osmosis to obtain 3,840 m³/day at the Cobre Las Cruces copper mine near Seville in Spain  - treating wastewater generated during the extraction of methane from coal seams using microfiltration and reverse osmosis for 12,000 m³/day in Queensland, Australia 
- producing 50,000 m³/day of drinking water from mine water (AMD) using a combination of reactors, clarifiers, ultrafiltration and reverse osmosis at the Emalahleni water reclamation plant in South Africa  - producing 54,000 m³/day of process water from sea water using reverse osmosis with upstream ultrafiltration at the AREVA Orongo desalination plant in Namibia  - producing approx. 89,000 m³/day of process water from sea water using reverse osmosis with upstream ultrafiltration at the Sierra Gorda mine in Chile  In addition to these large-scale membrane process applications that have already been implemented in the mining industry, other highly promising applications for these separation processes also exist. The process diagram for a mine water treatment version that uses nanofiltration (Peters/Haertel process, i.e. PeHa process) is shown in Fig. 6, and the results of further studies with this nanofiltration based process are exemplarily shown in Fig. 7 . More recent approaches to the extraction of indium and germanium through bio-leaching are also based on the use of membrane processes. In this microbial leaching process, microfiltration is used to separate bacteria from the solution and nanofiltration to concentrate the valuable substances . In general, it can be determined that the successful use of membrane processes requires the corresponding pretreatment of the raw water to be fed into a plant, but on a strictly case-by-case basis. In the mining industry this also includes using these processes in conjunction with other operating principles, such as chemical-free micro-bubble flotation  or innovative adsorption. With these processes, individual water constituents that interfere with efficient operation can be selectively separated and thus contribute to the optimization of the function or long-term operation of membrane technology plants. For an adsorption-based single step, for example, the use of ready-made manganese-oxide-based filter material in sand filter-like function for the separation of iron, manganese, hydrogen sulphide and individual heavy metals is conceivable .The components enriched in the sand filter backwash water can then be treated by other means. 5. Conclusion Water management in the mining industry is increasingly faced with two challenges: F & S International Edition No. 20/2020
a) process water supply b) wastewater treatment Modern process engineering solutions are available for both areas, with membrane processes increasingly playing a key role. The background here is that the need for clean and affordable water for a growing population on the one hand, and rising costs, water scarcity, overload and/or the pollution of natural resources in many areas of the world on the other, are the main driving forces behind the growing demand for improved water and wastewater treatment technology. Membrane processes can still be used successfully in this regard and they are also being increasingly used for different purposes and for a wide range of applications in the water and wastewater treatment sectors . This type of development is based on features such as the compactness of the plants, short construction times and a clean, simple, economical operation with long-term reliability as well as high retention rates for the components or pollutants that have to be separated. High production efficiency is mainly realized through the membrane’s barrier function. However, it is also based on the experience gained during the previous decades and continu-
ous optimization with regard to selecting the materials used to produce membranes and plants as well as being based on the increasing optimization of module designs and operational aspects, which also include operational training and preventive maintenance. Reference literature:  Peters, Th.: Nachhaltiges und kostenoptimiertes Wassermanagement in der Bergbauindustrie durch Nutzung von Membran-Verfahren. Konferenzband, Vodamin II Workshop „Möglichkeiten und Herausforderungen bei der Nutzung von Grubenwässern“, Freiberg, 4.04.2019  Cervantes-Guerra, Y. M. et al.: The deep-sea tailings placement (DSTP) as alternative for the residuals management in the mining industry. Minería y Geología, January 2019. https://www.researchgate. net/publication/330398092  N.N.: Wasseraufbereitung im Bergbau. Broschüre, Pall Corporation, Dreieich 2012  Jamasmie, C.: Rethinking mining strategies among the top-10 global trends to shape 2019. http://www. mining.com/author/cecilia/ visited 30.01.2019  N.N.: VODAMIN II, Projektflyer 20180605  Peters, Th.: Druckgetriebene Membranverfahren zur Wasser- und Abwasseraufbereitung. Seminarhandbuch, VDI-Wissensforum, Dresden, 18.19.09.2018  Peters, Th.: Membrantechnologie - Vergangenheit, Gegenwart & Zukunft in der Wasserindustrie. Filtrieren und Separieren, Jahrgang 29 (2015) Nr. 3  Peters, Th.: Reinigungs-Effizienz von Verfahren zur Reinigung von Deponie-Sickerwasser. Tagungsband, 14. Leipziger Deponiefachtagung, Leipzig, 6.-7.03.2018
 Steinberger, P., Rieger, A., Haseneder, R., Härtel, G., Pelz, W.: Scalingproblematik bei der Aufbereitung von AMD mittels Membranverfahren. Tagungsband, 59. Berg- und Hüttenmännischer Tag, Freiberg, 12.13.06.2008  Koch, R., Preuss, V., Koch, Th., Schöpke, R.: Verminderung der Sulfatbelastung neutralisierter Grubenwässer mittels Nanofiltration – Laborversuche zur Verfahrensentwicklung. Heft 18, Schriftenreihe Siedlungswasserwirtschaft und Umwelt, Cottbus, 2009  Peters, Th.: Deponie-Sickerwasser - Herkunft, Inhaltsstoffe, Reinigungsmöglichkeiten. Vortrag, Umwelttechnisches Fachseminar am iTN, Hochschule Zittau/Görlitz, Zittau, 3.04.2019  Peters, Th.: Membrane technology - past, present and future within the water industry. Filtrieren und Separieren, International Edition No. 16/2016  Andrade, L. H. et al.: NANOFILTRATION AND REVERSE OSMOSIS APPLIED TO GOLD MINING EFFLUENT TREATMENT AND REUSE. Brazilian Journal of Chemical Engineering, Vol. 34, Jan.-Mar. 2017  N.N.: Contact Water Treatment at the “Cobre Las Cruces” Copper mine in Spain. FirmenVeröffentlichung, ROTREAT Abwasserreinigung GmbH, Graz, Österreich, 2010  N.N.: WATER STRATEGIES IN THE MINING INDUSTRY. White Paper, PENTAIR – X-FLOW, Enschede, Netherlands  Peters, Th.: Purification of Acid Mine Drainage (AMD) with a Multi-staged Process Based on Nanofiltration - Avoiding environmental harmful discharge and enabling reuse of valuable components: Water, Acid, Metals. Vortrag, II International Congress on Water Management in the Mining Industry, Water in Mining WIM 2010, Santiago de Chile, Chile, 9.-11.06.2010  Werner, A., Haseneder, R., Repke, J.-U.: Design and Conception of a Membrane Pilot Plant for the In-Situ Treatment of Bioleaching Solutions. Chem. Ing. Tech., 19, No. 1-2, 2019  N.N.: Interne Untersuchungen. Studiengruppe AMD Peters + Spiegl, Neuss/Nufringen, 2018
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Developments in the membrane technology sector
Report about the “2019 Aachen membrane course for water technologies” St. Schütze * AVT’s “Membrane course for water technologies” held at RWTH Aachen university and directed by Prof. Dr Matthias Wesseling, was presented as an international event once again this year: the event has been held alternately in German under the “Hochschulkurs Membrantechnik” title or, as in 2019, in English under the “Aachen membrane course for water technologies” title. Its international orientation was also reflected in the participants: 34 participants from 7 countries and different industrial sectors made their way to Aachen. A wide range of topics covering membrane separating technology were presented to the group of predominantly young participants. The selected topics provided an overview of the multitude of different membrane processes, separation models, module designs and membrane materials used in a wide variety of applications such as treating drinking water, treating municipal waste water as well as the treatment of industrial waste water flows. A carefully organised basic lecture helped to orient and structure the complex and extensive topics. The basic difference between pore flow membranes and solution-diffusion membranes is still expedient and it was clearly explained here. Transferring the different substance transporting models into the designing and structuring of membrane processes for use in applications went very well. The participants were given fascinating insights into the architecture and structuring that has to be used with different membrane materials and types through the presentation of high-resolution SEM (Scanning Electron Microscope) images in the “Materials & Structures” section. Fig. 1 shows a polymer membrane with circular pores and a pore size of approx. 5 µm, to which another membrane layer with a *Stephan Schütze Dipl.-Ing. Verfahrenstechnik, Dipl.-Wirt.-Ing. M.A. Organisation studies Publicly appointed and certified by the Chamber of Commerce as being an expert in pressure-driven membrane separation processes and filtration Klausur 10 47839 Krefeld, Germany Tel.: +49 170 7542945 E-mail: email@example.com
Fig. 1: Membrane architecture / Image: AVT
pore size of approx. 0.2 µm was added. Inorganic as well as ceramic membranes, their manufacture, configuration and application options were also discussed and explained in detail during a guest lecture given by P. Bolduan, from atech innovations GmbH. Fig. 2 shows a cross-section of the different geometries used in ceramic tubular membranes. Going from so-called “single-bore” membranes, which are just “simple” tubes, up to “multi-bore” modules with different diameters and sizes and round, oval or even angular flow channels means that (virtually) everything is now possible. A new thinking direction here is to use helically-profiled flow channels to improve the fouling properties of ceramic membranes.
Fig. 2: An example of ceramic membrane technology
Fig. 3: Ceramic 6-8 µm carrier structure with an MF coating (Image: atech innovations GmbH) F & S International Edition No. 20/2020
Fig. 4: Flow performance in a tubular membrane, longitudinal section
Fig. 5: Flow performance in a multi-bore tubular membrane, cross-section
The structuring and morphology of ceramic membranes are shown in Fig. 3. A carrier material (right side of the image) with a pore width of 6 to 8 µm was coated with a finer suspension here so that a micro-filtration membrane (left side of the image) could be created. Beside the pressure-driven reverse osmosis, nano-filtration, micro- and ultra-filtration membrane processes, two examples for electro-membrane processes were also discussed: well-established electro-dialysis and the relatively new capacitive membrane desalination or “Capacitive De-Ionisation” (CDI) process. Many interesting developments can be expected here, especially with the relatively new CDI process. A processing version was presented in which, to simplify matters, a “liquid electrode” was used in an activated carbon suspension to ensure that the ions were continuously removed. This provides continuous desalination without any process interruptions. One of the seminar highlights was the AVT-lab-tour. Excitingly innovative and surprising discoveries are being made here almost every day! An imaging method able to display the flow patterns in technical devices was presented this year. As opposed to CFD simulations, the actual flow patters are displayed in real time. The technology used for this, i.e. CT (Computer Tomography), has been used in medical technology for a long time. With its help, a membrane can actually be seen “working” in real time. Fig. 4 shows the flow distribution with direction and velocity of a flow going through a tubular membrane piece with small increases on the wall inside the tube. Also clearly visible here are the effects of the increases on the return transport into the core flow and the flow performance on the membrane’s permeate side. Currrently there are spatial limitions due to the dimensions of the CT's measuring chamber. The to be examined structure needs to functionable inside a small void. Nonetheless, the described measuring process provides deeper insights into the mass-transport-processes and this allows to verify the fluid-dynamic models and improve simulations. Fig. 5 shows the fluid-dynamic performance of a multi-bore ultra-filtration module. An unexpectedly inhomogeneous and non-symmetrical velocity distribution can be seen on the permeate side. A spirally-curled hollow-fibre membrane caused much amusement amongst the presentation’s participants – more so because it is not yet known whether and if an advantage would result from using this special geometry. Given the huge amount of content that was imparted during the relatively short time available, it is all the more pleasing to find that all of the course documents and presentations are still available to the participants online after the event. The so-called “Moodle” (Modular Object-Oriented Dynamic Learning Environment) platform was used for this. It provides both the relevant presentations
as well as extensive reference and literature lists for further study. The contents can be specifically post-edited and used optimally. The user will also receive feedback about the specific “processing statuses”. To summarise, we are talking here of the successful continuation of an already legendary event. Organisation, course contents, didactic preparation and supporting programmes all help to create a positive learning atmosphere and make it easy for us to digest the “heavy stuff” involved in membrane technology. Wether the younger engineers are taking their first steps into the world of membranes or if the “older hands” are refreshing their knowledge: either way, the ATV’s MCW is worth the while.
F & S International Edition No. 20/2020
From waste water to medical technology Report from the Aachener Membran Kolloquium 2018 J. Barth * Up-to-date results, both from industry and academia, concerning new materials, apparatuses, plants, and processes in the field of membrane technology were presented at the 17th Aachener Membran Kolloquium. The focus of that conference was the treatment of municipal and industrial waste water as well as the treatment of process water to achieve a reduction of the demand for fresh water and of the amount of waste water down to Zero Liquid Discharge. Selected results will be presented from the fields of membrane reactors, osmosis, membrane distillation, micro- and nanofiltration, new membranes and modules, and new processes. Introduction The 17th Aachener Memban Kollo quium was held in the period 14–15 November at Aachen. During the two days, participants both from industry and academia presented up-to-date results in 35 talks and 36 posters. They covered the fields of membrane reactors, osmosis, membrane distillation, micro- and nanofiltration, new membranes and modules, and new processes. The focus of the conference was the treatment of municipal and industrial waste water as well as the treatment of process water. Many of the investigations were aimed at water reuse to achieve a reduction of the demand for fresh water and of the amount of waste water down to Zero Liquid Discharge. In the following, a review of selected results is given. The results are structured according to the different types of established processes. Additionally, new developments of membranes and modules as well as processes are presented.
Titanium dioxide (TiO2) nanoparticles are immobilised on a polyethersulfone (PES) membrane Millipore Express Plus GPWP from Millipore with a pore size of dPr = 0.22 µm to act as a photocatalyst. The advantages of titanium dioxide as a photocatalyst are its high photocatalytic activity, chemical stability, and low cost. The advantages of the polyethersulfone membrane used are its large specific surface combined with a good mass transfer between the flow channel and the pore volume. The membrane with the immobilised photocatalyst is placed on one half of the shell surface of pipes through which the test solution flows. To ensure good mixing of the test solution, the pipes are fitted with
static mixing elements. When exposed to ultraviolet (UV) radiation, hydroxide ions form in the water through photocatalysis that oxidise organic molecules. Through this Advanced Oxidative Process (AOP) the pharmaceutical residues are not just separated but degraded completely. In experiments lasting up to 7 days, a reduction of the diclofenac concentration by approximately 70 % was observed. The relative reduction is slower at the higher concentration of diclofenac in ultrapure water or with the use of pretreated waste water. The reason for this is the higher concentration of (foreign) molecules. At the lower concentration of diclofenac in ultrapure water, a photocatalytic degrada-
Membrane reactors Amira Abdul Latif from the Leibniz Institute of Surface Engineering (IOM) has reported on the oxidative photocatalytic degradation of pharmaceutical residues in a membrane reactor on the laboratory scale. The reduction of pharmaceutical residues is a challenge faced in both the production of drinking water and the treatment of waste water. Therefore, as a test material diclofenac was used, firstly, with a concentration of cm,0 = 25 mg/L or cm,0 = 100 mg/L in ultrapure water, and secondly, with a concentration of cm,0 = 100 mg/L in municipal waste water that had been pre-treated by ultrafiltration. * Dr.-Ing. Jakob Barth Backnang E-mail: Jakob.Barth@outlook.com
Fig. 1: Different tubular modules for forward osmosis (FO) processes (pilot series design). Image courtesy of the Berghof Membrane Technology GmbH. All rights reserved.
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tion of the support membrane is observed. At the higher concentration of diclofenac in ultrapure water or with the use of pretreated waste water, this degradation is not observed any longer. Fouling of the membrane with the immobilised photocatalyst is not observed. Osmosis Kimball Roelofs from the Berghof Membrane Technology GmbH has reported on the development of tubular membranes and modules for forward osmosis (FO) processes. Common types of membrane modules for osmosis processes are hollow fibre, capillary, spiral wound, and plate modules. These module types are prone to failures when the feed exhibits a high viscosity and/or particle concentration. Therefore, tubular membranes and modules were developed for osmosis processes of feeds with a high viscosity and/ or particle concentration. Tubular ultrafiltration membranes from Berghof Membrane Technology GmbH with an inner diameter of di = 5 mm are used as the support membrane. They consist of a support from a polyester non-woven material and a polyethersulfone (PES) covering layer on the inside. Modules with different length and diameter are used: P1 (membrane area AM = 0.008 m2), D50 (AM = 0.04 m2), and D90 (AM = 2 m2) (see fig. 1). The Aquaporin Inside® membrane from Aquaporin Asia Pte. Ltd. is used as the semi-permeable covering layer for the osmosis process. It consists of natural aquaporin proteins forming a biomimetic matrix. The aquaporin matrix is immobilised on the inside of the tubular membrane.
The water permeability of the tubular ultrafiltration membrane is aUF = 150– 200 L/m2 h bar. The water permeability of the tubular membrane with the semi-permeable covering layer for the osmosis process is decreased by a factor of approximately 100. The forward osmosis process was investigated experimentally using a 1 mol/L solution of sodium chloride (NaCl) in water at room temperature. Applying a pressure retarded osmosis (PRO) mode of operation for the P1 modules at a typical operating pressure of ∆p = 11–15 bar, a specific volumetric flow rate of vP = 15.4 L/m2 h of water and a specific mass flow rate of m· NaCl < 2 g/m2 h of salt are observed for the permeate. After scaleup to the D50 modules, the same specific permeate flow rates are realised. Membrane distillation Georg Eisenmann from the University of Reutlingen and Gerhard Schories from the Technologie-Transferzentrum (TTZ) Bremerhaven have reported on the comparison of different process variants for membrane distillation. The prevalent membrane distillation process is Direct Contact Membrane Distillation (DCMD), where both the feed and the permeate are in direct contact with a hydrophobic, porous membrane. Alternative process variants employ different conditions on the permeate side of the membrane and/or different types of membrane. The different process variants have been compared with respect to the specific energy demand, the specific permeate flow rate, and the permeate quality (electrical conductivity). The examined process variants are: - Direct Contact Membrane Distillation (DCMD): hydrophobic, porous mem-
brane, both the feed and the permeate are in direct contact with the membrane - Vacuum Membrane Distillation (VMD): hydrophobic, porous membrane, the feed is in direct contact with the membrane, on the permeate side vacuum is applied - Air Gap Membrane Distillation (AGMD): hydrophobic, porous membrane, the feed is in direct contact with the membrane, on the permeate side are an air gap, internal condenser and condensate outlet - Sweeping Gas Membrane Distillation (SGMD): hydrophobic, porous membrane, the feed is in direct contact with the membrane, on the permeate side are an air gap with a continuous gas flow and an external condenser - Pervaporation (PV): non-porous, waterselective membrane, mass transfer by the solution-diffusion mechanism, the feed temperature is significantly below the boiling point, the feed is in direct contact with the membrane, on the permeate side vacuum is applied - Thermopervaporation (TPV): non-porous, water-selective membrane, mass transfer by the solution-diffusion mechanism, the feed temperature is close to the boiling point, the feed is in direct contact with the membrane, on the permeate side vacuum is applied All experiments are performed with plate modules in counter flow. As the porous membrane, a teflon (PTFE) membrane from Lydall or a polypropylene (PP) membrane from the Solar Spring GmbH are used. As the non-porous membrane, a polyvinylidene fluoride (PVDF) membrane, that has been coated with polyelectrolytes according to Bell , is used. As the feed, waste water from membrane production with dimethylacetamide (DMAc),
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Tab. 1: Retention of dissolved organic carbon (DOC), sulphate, hardness components, and organic micropollutants by the nanofiltration pollutant
feed concentration cm,feed/µg/L retention 1 – cm,F / cm,feed /%
dissolved organic carbon (DOC)
sulphate (SO4) 130∙103
ethylenediamine tetraacetic acid (EDTA) 11.7
methyl-tert-butyl ether (MTBE)
tert-butyl alcohol (TBA)
Fig. 2: Progression of the transmembrane pressure (TMP) over the filtration time at different process parameters (specific filtrate flow rate (flux), ratio of filtrate flow rate to feed flow rate (recovery, rec), cross-flow velocity (CFV)), and with different cleaning concepts. Source: Jeannette Jaehrig, Kompetenzzentrum Wasser Berlin
polyvinylpyrrolidone (PVP), and K30 as well as sodium chloride (NaCl) solution with a mass concentration of cm = 150 g/L are used. The influence of the process parameters feed temperature, cross-flow velocity, and permeate pressure are studied. For the estimation of the specific energy demand, it is assumed that waste heat can be utilised to cover the thermal energy demand, so that only the mechanical energy demand is considered. DCMD yields the greatest specific permeate flow rate but also the least permeate quality (greatest electrical conductivity), i.e. the least retention of soluble impurities, compared to VMD, AGMD, and SGMD. TPV offers the least specific energy demand if waste heat can be utilised, as the driving force for the mass transfer is greater with pervaporation due to the higher feed temperature. The Technologie Transferzentrum Bremerhaven provides its test facilities to conduct experiments with different membrane materials and modes of operation. Micro- and nanofiltration Sandra Motta Cabrera from the University of Twente has reported on the
reuse of process water in a Canadian oil sand mine employing nanofiltration. In the conventional production process of oil sand, large amounts of fresh water are required and correspondingly large amounts of waste water are produced. The waste water is usually passed into tailings ponds. To be able to reuse the waste water as process water, the concentration of suspended solids, dissolved organic carbon, and dissolved ions must be reduced. Results were presented of the treatment of water from a tailings pond employing a semi-commercial nanofiltration pilot plant · with a capacity of V = 20 m3/h using titanium dioxide (TiO2) membranes. During the experimental operation over a period of almost two years, the composition of the waste water was varied over a wide range. During the operation at constant filtrate flow rate, the formation of an increasing covering layer is observed. The formation of the covering layer causes an increase of the filtration pressure. The suspended solids are retained almost completely by the employed membrane. The concentration of dissolved organic matter is reduced by 1 – cout/cin = 75–90 %. The formation of the covering layer improves the retention of dissolved ions by electrostatic and size
cut-off mechanisms. The resulting quality of the filtrate is suitable for its reuse as process water that would reduce significantly both the demand for fresh water and the amount of waste water produced. Han van de Griek from Evides Industriewater has reported on the treatment of municipal waste water employing micro- and nanofiltration with ceramic membranes. The aim of the investigation was the production of pure water and the testing of ceramic membranes as an alternative to polymer membranes. The tested ceramic microfiltration membranes are multichannel membranes from Coorstec, comprising 4 channels with a diameter of dK = 7.8 mm each. They consist of an aluminium oxide (Al2O3) support material with an active filtration layer of aluminium oxide (Al2O3) as well. They have a nominal pore size in the range of dPr = 0.15–0.20 µm and a nominal molecular cut-off in the range of dM > 1,000,000 Da. The tested ceramic nanofiltration membranes are multichannel membranes M37-19-25-L from Inopor, comprising 19 channels with a diameter of dK = 3.5 mm each. They consist of an aluminium oxide (Al2O3) support material with an active filtration layer of aluminium oxide (Al2O3) and titanium dioxide (TiO2). They have a nominal pore size of dPr = 0.9 nm and a nominal molecular cut-off in the range of dM = 450 Da. Both membranes are installed in tubular modules with a length of lK = 1200 mm comprising 37 membranes. The complete process is a physico-chemical sewage treatment process. Two process variants were investigated: In the first variant, the waste water is pre-treated employing a drum sieve, a belt sieve, and a flotation stage. The main process stage consists of the micro-/ nanofiltration. In the second variant, the waste water is pre-treated employing flocculation and subsequent separation of the solids using a belt sieve. The main process stage consists of the micro-/nanofiltration and subsequent reverse osmosis (RO) of the filtrate. During operation, automatic, periodic cleaning of the feed side of the membrane modules is performed to remove the covering layer formed by the separated precipitated solids. At the beginning of the experimental operation, an automatic CIP procedure (flushing) of the concentrate side of the membrane module with acidic and alkaline cleaning agents was performed. Later during the experimental operation, additional back-flushing with pure filtrate and back-flushing with acidic and alkaline cleaning agents were tested. The process variant with the microfiltration membranes was operated over a period of toperation = 500 d. The best mode of operation employs a cleaning interval
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of tclean = 15 min for the back-flushing with acidic and alkaline cleaning agents. The resulting filtrate flow rate is vF = 27 L/m2 h. The first membranes used were found to be damaged after a period of toperation = 140 d, the next membranes were still intact at the end of the experimental operation. The process variant with the nanofiltration membranes was operated over a period of toperation = 197 d. The average cleaning interval was tclean = 28 h for the automatic CIP procedure (flushing) of the concentrate side of the membrane module with acidic and alkaline cleaning agents. This method of cleaning succeeded in restoring the filtrate flow rate of the unused membrane. The molecular cut-off of the unused membrane was determined experimentally as dM = 479 Da, while the molecular cutoff of the membrane at the end of the experimental operation was determined as dM = 600 Da. Thus, with the microfiltration membranes, frequent back-flushing with cleaning agents is necessary to achieve a reliable operation. On the other hand, with the nanofiltration membranes, the active filtration layer is damaged during longterm use. Therefore, both the tested microfiltration and nanofiltration membranes seem to be unsuitable for long-term use
in the treatment of municipal waste water. Jeannette Jaehrig from the Kompetenzzentrum Wasser Berlin has reported on the application of nanofiltration after conventional natural bank filtration in the production of drinking water. Additionally, a suitable cleaning concept employing back-flushing and chemical cleaning was investigated. Sulphates and organic micropollutants (e.g. ethylenediamine tetraacetic acid (EDTA)) are a growing problem in drinking water production. Nanofiltration is a suitable process to (further) reduce the concentration of sulphates, organic micropollutants, hardness components, and dissolved organic carbon (DOC). A common problem in nanofiltration is biofouling. Bank filtration reliably reduces the concentration of organic carbon, biopolymers, algae, and particles. Thus, biofoulig of the nanofiltration membrane is significantly reduced compared to direct nanofiltration of surface water. Another problem in nanofiltration is the precipitation of iron (Fe2O3) and manganese (MnO2) oxide in the case of contact with oxygen. Especially when precipitates form inside the nanofiltration membrane, the membrane can be blocked irreversibly. Therefore, iron and manganese are largely removed upstream of the nanofiltration by
air sparging and a sand filter. Additionally, the nanofiltration process including the filtrate storage are operated anoxicly. A pilot plant was designed and built by Pentair Water Process Technology BV. It was used to perform experiments, firstly, with ground water drawn from wells of the Tiefwerder waterworks near Berlin and secondly, with bank filtered river water of the river Havel drawn from wells with a depth of hW = 30–100 m and a · capacity of up to VW < 80,000 m3/d. In this pilot plant, a newly developed membrane is used. This membrane has special retention properties for sulphates and organic micropollutants and is mounted in a capillary module with a filter area of AF = 40 m2. The experiments were always performed at constant specific filtrate flow rate while the different process parameters were varied: specific filtrate flow rate (vF = 15.0/22.5/27.5 L/m2 h, long-term experiments at vF = 22.5 L/m2 h), crossflow velocity (vCF = 0.2/0.5/1.0 m/s, longterm experiments at vCF = 0.5 m/s, ratio of filtrate flow rate to feed flow rate (recov· · ery) ( VF /Vfeed = 0.5/0.75/0.85, long-term · · experiments at VF /Vfeed = 0.75). The retention of dissolved organic carbon (DOC), sulphate, hardness components, and organic micropollutants was
Ceramic tubular membranes for crossflow micro-, ultra- and nanofiltration
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Fig. 3: WMC200 module with hollow fibre nanofiltration membranes MexFil from NXFiltration for waste water treatment, processing of process solutions, and drinking water production employing a single-stage process (Direct NanoFiltration). Source: Erik Roesink, NXFiltration
Fig. 4: Spiral wound membrane module with a hydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membrane for waste water treatment in the production of mineral oil and natural gas. Source: Tobias Steube, Microdyn-Nadir GmbH
measured (see table 1). The retention of dissolved organic carbon, sulphate, hardness components, and organic micropollutants with high molar mass (M = 266–292 g/mol) (e.g. ethylenediamine tetraacetic acid (EDTA)) is high. The retention of organic micropollutants with low molar mass (M = 63–88 g/mol) (e.g. methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), or vinyl chloride) is low, as expected. Cleaning is achieved by back-flushing with anoxic filtrate and forward-flushing with chemical cleaning agents. To determine a suitable cleaning concept different cleaning intervals, concentrations of the cleaning agents, cleaning temperatures, flushing durations, and cleaning agents were investigated (see fig. 2). Back-flushing with anoxic filtrate alone at a back-flushing interval of tclean = 60 min achieves no cleaning effect. Additional cleaning with hydrochloric acid and sodium hydroxide solution achieves no cleaning effect either while the concentration of the chemical cleaning agents exhibits no influence. An increase of the cleaning temperature to
θclean = 30 °C achieves a better, but no complete, cleaning effect. Therefore, other cleaning agents were tested, including hydrogen peroxide, citric acid, oxalic acid, and ascorbic acid. Cleaning with ascorbic acid restores the filtration properties of the unused membrane. A stable operation is achieved by back-flushing with anoxic filtrate at a back-flushing interval of tclean = 60 min and chemical cleaning twice a week, once with ascorbic acid and once with sodium hydroxide solution first and ascorbic acid afterwards. Erik Roesink from NXFiltration has presented the newly developed hollow fibre nanofiltration membranes MexFil for waste water treatment, the treatment of process solutions, and the production of drinking water. The newly developed membranes allow for a single-stage process (Direct NanoFiltration) without complex pre-treatment. Especially in waste water treatment, they exhibit a high retention of organic micropollutants, e.g. hormones, pesticides, and pharmaceutica. The nanofiltration membranes have a nominal cut-off of either dM = 800 Da or dM = 400 Da. They are manufactured by
applying the active nanoscale filter material in layers to the support material. The number of active filter layers determines the flow resistance and the retention of the membrane. The modified polyethersulfone (PES) material allows for continuous operation with periodic forward-flushing, back-flushing, and occasional cleaning. The material is resistant even at high and low pH values as well as against oxidative cleaning agents, e.g. chlorine compounds. The hollow fibre design offers the advantage of an open flow channel with a low fouling tendency compared to spiral wound membrane modules with spacers. The hollow fibre membranes are arranged in modules with a diameter of either dF =110 mm or dF = 200 mm (see fig. 3). The membranes can be used for waste water treatment, treatment of process solutions, or drinking water production: Waste water or process solutions (e.g. sodium hydroxide cleaning solutions) can be treated to be reused in the process. Both measures reduce the amount of waste water and the demand for fresh water of the process. Surface water can be purified to achieve drinking water quality. With the newly developed membranes, all these applications can be realised as single-stage processes. Thus, the investment costs, the footprint, and the operating costs of the installations are reduced compared to e.g. combined microfiltration and reverse osmosis (RO) processes for seawater desalination. Tobias Steube from the Microdyn-Nadir GmbH has reported on new ultrafiltration membranes for waste water treatment in the production of mineral oil and natural gas as well as experimental operation of the new membranes employing different pre-treatment processes. The production of mineral oil and natural gas has a high demand for fresh water and produces correspondingly large amounts of waste water. The treatment of the waste water for reuse as process water can reduce both the demand for fresh water and the amount of waste water. The composition of the waste water varies strongly over time and is very complex with different pollutants, e.g. mineral oil residue, suspended solids, solute materials, heavy metals, and alkyl phenols. Despite the great variation of the composition of the waste water, a constant filtrate quality is required for its reuse as process water. Ultrafiltration is often employed to remove suspended solids, bacteria and other micro-organisms, and mineral oil residue and produces filtrate with a constant quality. However, conventional ultrafiltration membranes made from polymers are usually hydrophobic and oleophilic. Therefore, they are prone to fouling when employed for the treatment of waste water from the F & S International Edition No. 20/2020
production of mineral oil and natural gas that contains mineral oil residue. Thus, a new hydrophilic polyvinylidene fluoride (PVDF) membrane with a pore size of dPr = 0.03 µm is used. The membrane is already used successfully for the treatment of suspensions with solids concentrations up to cm,sus < 3000 mg/L. The membrane is designed as a submerged spiral wound membrane module (see fig. 4) in suction operation mode. The new membrane module was operated experimentally in three different production plants in the USA. In each, feed with different concentration of mineral oil residue was used with different pre-treatment: coalescer (average concentration of mineral oil residue in the feed cm,oil = 22 mg/L, experimental operation for two weeks), depth filter from walnut shell granulate (average concentration of mineral oil residue in the feed cm,oil = 35 mg/L, maximum concentration of mineral oil residue in the feed cm,oil = 166 mg/L, experimental operation for one month), and multi-stage elctrocoagulation (average concentration of mineral oil residue in the feed cm,oil = 20 mg/L, experimental operation for two weeks). In all experiments, the achieved filtrate quality exhibits a concentration of mineral oil residue in the filtrate below cm,oil < 1.0 mg/L and a concentration of suspended solids in the filtrate below cm,s < 10 mg/L. To determine the maximum concentration of mineral oil residue possible in the feed, experiments without any pre-treatment were performed as well (average concentration of mineral oil residue in the feed cm,oil = 72 mg/L, maximum concentration of mineral oil residue in the feed cm,oil = 288 mg/L). Even without any pre-treatment the same filtrate quality is achieved. Denis Vial from the inge GmbH has reported on the treatment of municipal waste water employing ultrafiltration. The treated municipal waste water can be reused for industrial or agricultural applications. Thus, the demand for fresh water is reduced. Alternatively, it can be used to replenish groundwater reservoirs (groundwater recharge). Different individual process stages and combinations thereof were investigated in a pilot plant attached to the communal sewage treatment plant at Nordenham: pre-treatment via inline flocculation with subsequent ultrafiltration and reverse osmosis (RO). The pilot plant was designed and constructed by DeEnCon. As the flocculation agent for the inline flocculation polyaluminium chloride Feralco FD ACH with a concentration of cm,Floc = 3 mg/L and iron chloride sulphate Ferrifloc with a concentration of cm,Floc = 5–10 mg/L were compared. The installed ultrafiltration modules were dizzer XL 0.9 MB WT from the inge GmbH and reverse osmosis (RO) modules from the IAB Ionenaustauscher GmbH. During the experimental operation over a period of 8 months the temperature of the waste water from the sewage treatment plant was in the range of θW = 7–20 °C and its turbidity in the range of ΦW = 2.5–18.5 NTU with a maximum value of ΦW = 80 NTU. The process parameters were a constant filtrate flow rate of vF = 70 L/m2 h at a filtration time of tF = 30 min with subsequent back-flushing with pure filtrate. Back-flushing with chemical cleaning agents (alkaline at pH = 12, sulphuric acid at pH = 2.3, soaking time tsoak = 15 min each) was employed once per day. Within the investigated range, the turbidity of the waste water exhibits no influence on the performance of the ultrafiltration process. The use of polyaluminium chloride as the flocculation agent results in a permeability of the ultrafiltration membrane of approximately aM ≈ 300 L/m2 h bar. The use of iron chloride sulphate as the flocculation agent results in a significantly lower permeability, irrespective of its concentration. The replacement of iron chloride sulphate by polyaluminium chloride results in no increase of the permeability. However, the initial conditions can be restored by intensive chemical cleaning (CIP). This result suggests the formation of a less permeable, irreversible covering layer with the use of iron chloride sulphate as the flocculation agent. A retention of organic carbon of approximately 1 – cm,F/cm,feed ≈ 30 % is achieved by the ultrafiltration, irrespective of the flocculation agent used. F & S International Edition No. 20/2020
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New membranes and modules
Cube pattern diff. heights
Hills and valleys pattern
rn ys va
e av W
e er ph
i-c m Se
tte pa le
ff. di rn tte
s ht ig he
pa e ub C
Wall shear rate [1/s]
Pressure drop [mbar]
Fig. 5: Different spacer geometries for use in a channel module with flat membranes for screening experiments. Source: Alexander Helling, Sartorius Stedim Biotech GmbH
Filtrate flow rate [mL/min]
Fig. 6: Average wall shear rate at the filter medium and pressure drop of the loop flow with the different spacer geometries at a loop flow rate of V̇sus = 6.7 mL/min, determined using computational fluid dynamics (CFD) simulations. Source: Alexander Helling, Sartorius Stedim Biotech GmbH
Thin conventional woven spacer
Herringbone pattern spacer
Hills and valleys pattern spacer
Empty flow channel
Fig. 7: Filtrate flow rate determined experimentally with bovine serum albumin as a model protein employing a thin conventional woven spacer, the new spacer geometries herringbone pattern and hills and valleys pattern, and the empty flow channel. Source: Alexander Helling, Sartorius Stedim Biotech GmbH
Alexander Helling from the Sartorius Stedim Biotech GmbH has reported on the development of channel modules for dynamic surface filtration (cross-flow filtration) with integrated spacer geometries. In the pharmaceutical industry, dynamic surface filtration is a common process for the concentration and diafiltration of protein-based agents. The stability of the proteins is influenced significantly by the process parameters, e.g. composition of the liquid phase and concentration, conductivity, and pH value of the suspension. Therefore, it must be investigated, at an early stage of the process development already, whether the production process is feasible with respect to these process parameters. At this early stage, usually only very small samples are available. Therefore, correspondingly small test filters are required for screening. In the typical production scale filter modules with flat membranes woven spacers are employed. Transfering the geometry of these woven spacers to the small test filters required for screening constitutes a difficult problem. Therefore, new spacer geometries have been developed that are integrated directly into a channel module with flat membranes for screening. The modules with the integrated spacer geometries can be produced employing additive manufacturing or injection moulding. The geometries can be specially designed to achieve a high filtrate flow rate, a low flow resistance of the loop flow, or easy manufacturing. A number of different geometries have been designed (see fig. 5). To determine a preselection of the geometries that suit the process demands best, Computational Fluid Dynamics (CFD) simulations were employed rather than expensive filtration experiments. As a qualitative measure of the expected filtrate flow rate the shear rate at the filter medium at a loop flow · rate of VSus= 6.7 mL/min and the pressure drop of the loop flow are calculated. The average wall shear rate and the pressure drop of the loop flow rate are compared for all the different geometries (see fig. 6). The hills and valleys pattern is determined to suit the process demands best. While the herringbone pattern and especially the cuboid pattern exhibit a significantly higher average wall shear rate, these patterns are much more difficult to manufacture and the pressure drop of the loop flow is too high. Therefore, protein damage is more likely. After the preselection had been determined by flow simulation, filtration experiments were performed with the herringbone pattern and the hills and valleys pattern using bovine serum albumin as a
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model protein. Although the wall shear rate is significantly lower for the hills and valleys pattern compared to the herringbone pattern, the filtrate flow rate is similar to that achieved with a thin conventional woven spacer (see fig. 7). Thus, the filtrate flow rates that are achieved with the test filters in screening experiments can be used as a reference for the expected filtrate flow rates with bigger filter modules in the production process. Soraya Laghmari from the University of Duisburg and Essen has reported on the investigation and reduction of the fouling tendency of reverse osmosis (RO) membranes that are employed in the treatment of cooling water in the steel industry. Due to the closed loop operation and additional losses through evaporation, the concentration of salts, particles, oil, biocides, corrosion inhibitors, dispersion agents, and anti-scaling agents is increased over time. With the conventional mode of operation, the cooling water is diluted with fresh water continuously to limit the concentration of the salt. By employing continuous desalination of a part of the loop flow the demand for fresh water could be reduced significantly. Reverse osmosis (RO) is a suitable desalination process. Due to their high retention, the membranes in reverse osmosis (RO) processes are highly prone to fouling. Therefore, the fouling tendency of different membanres was investigated and compared. Furthermore, an additional zwitterionic anti-fouling coating with suitable interactions was developed. The compared reverse osmosis (RO) membranes are from AlfaLaval, GE Osmonics, Dow FilmTec, and Lanxess. Actual cooling water from a direct contact cooling process in the steel industry is used as the test material. The fouling tendency of the membranes is investigated in both static (dead end) and dynamic (cross-flow) surface filtration experiments. With the membrane from AlfaLaval, a permanent fouling layer occurs only after the concentration exceeds 20 times its initial value. With all the other membranes, the concentration limit is five times the initial value. Thus, the membrane from AlfaLaval is least prone to fouling with the cooling water used. The newly developed anti-fouling layer can be applied to the membrane. It decreases the fouling tendency significantly further but, at the same time, significantly decreases the permeability as well. Mina Ahsani and Hossein Alinia from the Sahand University of Technology (Iran) have reported on the development of membranes with anti-fouling and anti-biofouling properties for membrane bioreactors (MBR). Membrane bioreactor processes are a proven means of treatF & S International Edition No. 20/2020
ing municipal and industrial waste water. They achieve a higher quality of the treated waste water and require a smaller footprint compared to activated sludge processes. A drawback of membrane bioreactor processes is their fouling, especially biofouling, tendency. A proven method of providing anti-fouling and anti-biofouling properties for a membrane is the embedding of silver nano particles in the membrane material. To avoid agglomeration of the silver nanoparticles, nanocomposite nanoparticles from silver (Ag) and silicon dioxide (SiO2) are produced for the newly developed membrane. These are then embedded in either a polyvinylidene fluoride (PVDF) membrane or a polyethylene (PE) membrane. Different polyvinylidene fluoride (PVDF) membranes with mass fractions of nanoparticles of wP = 0/0.3/0.6/0.9 wt% were produced. They are compared with respect to their contact angle with water, tensile strength, and flow resistance for pure water. Additionally, different polyethylene (PE) membranes with mass fractions of nanoparticles of wP = 0/0.25/0.75/1 wt% were produced. They, too, are compared with respect to their contact angle with water, tensile strength, flow resistance for pure water, and, in addition, their limiting filtrate flow rate. The unmodified polyvinylidene fluoride (PVDF) membrane without any nanoparticles exhibits the following material properties: contact angle with water ϕW = 100°, tensile strength σm = 5.8 MPa, flow resistance for pure water RW = 1.80·1011 1/m. The membranes with embedded nanoparticles exhibit a decrease of the contact angle with water by approximately δϕW ≈ 10 % compared to the unmodified membrane without any nanoparticles. The different content of nanoparticles causes only minor differences. The reason for the decrease of the contact angle with water are the hydrophilic properties of silicon dioxide and silver. The tensile strength of the membrane with a mass fraction of nanoparticles of wP = 0.3 wt % is increased by approximately δϕw ≈ 10 % compared to the unmodified membrane, with a mass fraction of nanoparticles of wP = 0.6/0.9 wt % it is decreased by approximately δσm ≈ 15 %. The reason for this behaviour is twofold: On the one hand, the inorganic filler material causes an increase in stability. On the other hand, at higher solid content the nanoparticles are poorly dispersed in and wetted by the membrane material. The flow resistance for pure water of the membrane with a mass fraction of nanoparticles of wP = 0.3/0.9 wt % is decreased by approximately δRW ≈ 20 % compared to the unmodified membrane, with a mass fraction of nanoparticles of wP = 0.6 wt % it is decreased by approximately δRW ≈ 40 %. 37
Fig. 9: Convective and diffusive mass transfer with a short separation distance for diffusive mass transfer, as it occurs in the pores with a hydrogel layer inside the membrane of a membrane adsorber. Source: Franziska Hagemann, Sartorius Stedim Biotech GmbH
Fig. 8: Convective and diffusive mass transfer with a great separation distance for diffusive mass transfer, as it occurs in the pores inside the particles of a conventional chromatography column. Source: Franziska Hagemann, Sartorius Stedim Biotech GmbH
The properties of the membrane with a mass fraction of nanoparticles of wP = 0.6 wt% are considered best: contact angle with water ϕW = 90°, tensile strength σm = 5.0 MPa, flow resistance for pure water RW = 1.13·1011 1/m. Actual waste water from a pharmaceutical production plant was filtered with a filtration time of tF = 6 h, and the membrane was backflushed with pure water afterwards to investigate the fouling and biofouling behaviour of the membrane: After back-flushing, the unmodified polyvinylidene fluoride (PVDF) membrane without any nanoparticles exhibits a relative filtrate flow rate of vF/vF,0 = 62 % compared to the initial filtrate flow rate of the unused membrane. The membrane with nanoparticles exhibits a relative filtrate flow rate of vF/vF,0 = 73 %. This result suggests a reduced fouling and biofouling tendency of the membrane with the embedded nanoparticles compared to the unmodified membrane without any nanoparticles. The unmodified polyethylene (PE) membrane without any nanoparticles exhibits the following material properties: contact angle with water ϕW = 123°, tensile strength σm = 0.85 MPa, flow resistance for pure water RW = 2.64·1012 1/m. The membranes with embedded nano particles exhibit a decrease of the contact angle with water. The tensile strength of the membrane increases with the mass fraction of nanoparticles at a low content of nanoparticles and decreases significantly at a mass fraction of nanoparticles of wP = 1 wt %. The flow resistance for pure water decreases with the mass fraction of nanoparticles at a low content of nanoparticles and increases significantly at a mass fraction of nanoparticles of wP = 1 wt %. 38
The properties of the membrane with a mass fraction of nanoparticles of wP = 0.75 wt % are considered best: contact angle with water ϕW = 95°, equivalent to a relative decrease by δϕW ≈ 23 % compared to the unmodified polyethylene (PE) membrane without any nanoparticles, tensile strength σm = 1.2 MPa, equivalent to a relative increase by δσm ≈ 45 %, flow resistance for pure water RW = 1.86·1011 1/m, equivalent to a relative decrease by δRW ≈ 66 %. In a filtration experiment, the membrane was operated as a submerged membrane bioreactor (sMBR). With the unmodified polyethylene membrane (PE) without any nanoparticles, a limiting filtrate flow rate of vlim,0 = 13 L/m2 h is observed. With the membrane with nanoparticles a limiting filtrate flow rate of vlim,0 = 30 L/m2 h is observed, equivalent to a relative increase by δvlim = 131 %. This result suggests a reduced fouling and biofouling tendency of the membrane with the embedded nanoparticles compared to the unmodified membrane without any nanoparticles. The polyvinylidene fluoride (PVDF) membrane exhibits a higher tensile strength and lower flow resistance for pure water than the polyethylene (PE) membrane. On the other hand, the price of the polyethylene (PE) membrane is lower. Franziska Hagemann from Sartorius Stedim Biotech has reported on the development of membrane adsorbers. For the processing of fermentation broths in biotechnology, membrane adsorbers offer an alternative to conventional diffusion limited chromatography columns with a fixed bed of particles. In conventional chromatography columns, the stationary phase is formed by a fixed bed of porous particles. In the volume between the particles convective mass transfer occurs, in the pores of the particles diffusive mass transfer occurs (see fig. 8). At high volumetric flow rates, the process is limited by diffusion due to the great separation distance in the pores of the particles. Thus, the binding capacity of conventional chromatography columns
strongly depends on the volumetric flow rate and the corresponding residence time employed in the process. Typical values of the residence time and the specific binding capacity are thyd = 5 min and mads = 70 mg/ mL, respectively. One approach to avoid the limitation by diffusion is to have the loop flow pass through the open pores of the support material, e.g. a porous membrane. The active chromatographic layer is applied to the walls of the membrane pores as a thin hydrogel. The separation distance inside the pores of the hydrogel is significantly shorter (see fig. 9). Thus, the binding capacity of a membrane adsorber is almost independent of the volumetric flow rate and the corresponding residence time employed in the process. This results in a higher possible volumetric flow rate and productivity compared to a conventional chromatographic column. However, the specific surface area of porous membanes is less than that of porous particles. This results in a smaller specific binding capacity of membrane adsorbers. Typical values of the residence time and the specific binding capacity are thyd = 10 s and mads = 30 mg/mL, respectively. The geometric material parameters are the diameter of the membrane pores and the thickness of the hydrogel layer. With increasing diameter of the membrane pores the convective mass transfer increases, too. On the other hand, the specific surface area and the binding capacity decrease. With increasing thickness of the hydrogel layer both the binding capacity and the separation distance increase, necessitating an increase of the residence time. Thus, optimum values for both material parameters must be determined together to realise a given volumetric flow rate and binding capacity for a membrane adsorber. New processes Bernd Bauer from the Fumatech BWT GmbH has reported on new fields of application for ion exchange membranes. To date, ion exchange membranes are used for niche applications only. The F & S International Edition No. 20/2020
only established industrial application is the chloralkali process. New process goals and fields of application result from the efforts to reduce the amount of waste (Zero Liquid Discharge), the increased utilisation of regenerative energy, and the advance of electromobility: To reduce the amount of waste, process solutions are treated and reused to an increasing degree, employing e.g. acid dialysis, Membrane Capacitive Deionisation (MCDI) of highly concentrated saline solutions, salt separation by membrane dialysis, or electrolysis with bipolar membranes. The increased utilisation of regenerative energy and the advance of electromobility require the application of electrochemical processes for energy conversion and energy storage, e.g. production of hydrogen by electrolysis, use of hydrogen in fuel cells, energy storage in redox flow batteries, or electrochemical conversion and use of carbon dioxide (CO2). To date, widespread utilisation of these processes is impeded by both high investment costs due to the high price of the membranes and high operating costs due to the high energy demand of the processes. New developments to reduce the investment costs include e.g. counter flow spiral wound membane modules for Donnan dialysis processes. New developments to reduce the operating costs include e.g. thin film ion exchange membranes with a lower mass transfer resistance. Christian Linnartz and Alexandra Rommerskirchen from the RWTH Aachen have reported on water treatment employing a novel flow electrode process for Capacitive Deionisation (FCDI). The Capacitive Deionisation (CDI) process is advantageous due to its low energy demand. However, with conventional static electrodes the process has the disadvantage of being limited to saline solutions with a low concentration.
This limitation can be overcome by the flow electrode process: A suspension of activated charcoal particles in saline solution is employed as a flow electrode to realise a convective charge transfer. A salt metathesis process was presented as a sample application. This process produces a concentrated solution of magnesium chloride (MgCl2) in water and additional pure water from diluted solutions of sodium chloride (NaCl) and magnesium sulfate (MgSO4) in water. The water yield is greater than ηW > 90 %. The principle can be transfered to any other salt metathesis process. Jens Potreck from Pentair has reported on the increase of the average filtrate flow rate in a cross-flow microfiltration process for the clarification of beer by the patented +Flux process. The clarification of beer to remove yeast and particulate matter after fermentation is the final filtration stage. In the past years, static filtration with filter aids (kieselguhr) in filter presses has widely been superseded by cross-flow microfiltration in membrane modules. To increase the capacity of the filtration apparatuses, the installation of a greater membrane area per floor space is desired. To achieve this, an increase in the length of the modules is necessary. In many cases, cross-flow microfiltration is operated at constant filtrate flow rate. Over the filtration time, a growing covering layer forms on the membrane increasing the filter resistance. To realise a constant filtrate flow rate the pressure at the module inlet is adjusted (increased) over the filtration time. The flow of the suspension across the membrane causes a pressure drop along the module on the feed side. At the beginning of the filtration process, the height of the covering layer is very small, leading to a low pressure at the module inlet. Thus, the pressure drop along the module on the feed side leads to a negative local filtration pressure (transmembrane pressure (TMP)) causing a back-
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tion pressure ∆pF according to equation (1) is negative. However, the filtration effectively occurs only in the lower part of the module (see fig. 10). Therefore, the effective average filtration pressure is higher.
Fig. 10: Schematic diagram of the +Flux process. Source: Jens Potreck, Pentair
Fig. 11: Progression of the transmembrane pressure over time in the +Flux process and a conventional filtration process. Source: Jens Potreck, Pentair
flow of filtrate to the feed side near the module outlet (see fig. 10). This backflow reduces the average filtrate flow rate for the entire module. With progressing filtration time, the height of the covering layer increases, too. To realise a constant filtrate flow rate, the pressure at the module inlet is adjusted (increased) accordingly, and the backflow vanishes. The backflow can be prevented entirely by lowering the fluid level on the filtrate side of the upright modules by applying a pressurised gas buffer at the beginning of the filtration process (see fig. 10). With progressing filtration time and increasing pressure at the module inlet, the fluid level on the filtrate side is adjusted (increased).
The average filtrate flow rate for the entire module is increased by the greater effective membrane area. The working principle of the +Flux process has been confirmed in laboratory tests using modules with a length of lM = 750 mm (see fig. 11). At the same controlled, constant filtrate flow rate, the average filtration pressure ∆pF according to equation (1) increases more slowly, before the maximum pressure at the module inlet is reached. Then the filtration process is interrupted and back-flushing is applied to the module. Thus, the +Flux process enables longer filtration times and back-flushing intervals. At the beginning of the filtration process the average filtra-
(1) The +Flux process enables longer filtration times and back-flushing intervals. When larger modules with a length of lM = 1500 mm are used, the increase of the filtration time is even greater. The process is already employed by several breweries of different sizes around the world since 2015. Daniel Janowitz from the STEP Consulting GmbH has reported on the recovery of magnesium oxide (MgO) from the concentrate of seawater desalination plants. The waste water (concentrate) of seawater desalination plants pollutes the environment. On the other hand, the contained salts constitute valuable reusable materials, e.g. magnesium as a material commonly employed in lightweight construction. The following recovery process is suggested for magnesium oxide: concentration of the saline solution, precipitation with sodium hydroxide (NaOH), filtration and washing of the precipitated magnesium hydroxide (Mg(OH)2), sintering to produce magnesium oxide (MgO). Early experiments of concentration by nanofiltration have shown the precipitation of a considerable amount of gypsum together with the magnesium hydroxide. The gypsum constitutes a serious impurity of the magnesium hydroxide. As an alternative, concentration in evaporation ponds has been suggested: In this process variant, the gypsum precipitates in the concentration stage already. Thus, in the subsequent precipitation stage, only small amounts of gypsum are produced together with the magnesium hydroxide and a higher purity is achieved. The operating costs are caused mainly by the sodium hydroxide employed for precipitation and the energy required for sintering. A feasibility study of the process has been conducted with actual sea water concentrate from the sea water desalination plant Shuwaikh (Kuwait). Reference literature:  Bell, C.M.: Comparison of polyelectrolyte coated PVDF membranes in thermopervaporation with porous hydrophobic membranes in membrane distillation using plate-and-frame modules Chemical Engineering and Processing: Process Intensification. 2016, 104, pp. 58–65.
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Using reverse osmosis as the 4th clarification stage M. Schröder * Clarifying waste water involves major challenges. Residues from drugs and hormones are not sufficiently retained in conventional clarification plants that using three clarification stages. Furthermore, micro-pollutants such as plastic particles as well as microbes are now the subject of public discussions. The need for a further clarification stage is being widely discussed in this context. In addition to adsorptive processes that use activated carbon and treating with ozone, reverse osmosis presents an option that can be used as the 4th clarification stage. Whereas adsorption using activated carbon and ozonisation have already been implemented in several clarification plants (see German Federal Environment Agency ), this is not yet the case with reverse osmosis. The first promising results from experimental studies are now available. 1. Introduction
Tab. 1: Substances studied during the project
Purifying waste water is a necessity to ensure that the environment can be preserved for humans. There are also more extensive demands from society besides the legal purifying requirements. It is rather alarming that modern analysis has revealed that conventional clarification plants are incapable of removing specific micro-pollutants from waste water. Examples of this are: drugs, hormones, contrast agents, micro-plastics and bacteria (including multi-resistant pathogens). Today, the majority of clarification plants consist of three clarification stages: mechanical, biochemical and chemical. Margot et al.  report that in a typical clarification plant the purification performance is only 23% for selected micro-pollutants from the pharmaceuticals, pesticides and endocrine classes. Just 43% can be realised if an additional nitrifying fluidised bed is used. These values are insuffi* Dr.-Ing. M. Schröder Freiherr-vom-Stein-Str. 36 63322 Rödermark, Germany Tel.: +49 176 5680 5864 E-mail: firstname.lastname@example.org
Molecular weight [Da]
cient with regard to sustainably improving or guaranteeing our water quality. Different procedures are available for a 4th clarification stage. Huge technical experience has already been gained from ozonisation and adsorptive purification processes that use PAC (Powdered Activated Carbon) or GAC (Granulated Activated Carbon). A cleaning performance of around 80% can be realised using these processes, but the values for specific substances can vary widely (see ). Reverse osmosis is another purifying process that is well known for its high purifying performance (>99%). Reverse osmosis has been used successfully for a long time in seawater desalination, treating process water as well as in groundwater enrichment (e.g. in Orange County, California ).
There are concerns about the economics of using reverse osmosis as the fourth purification stage in clarification plants (see ). 2. Reverse osmosis The osmosis process is reversed in a reverse osmosis process. The water, which would diffuse from the clean side into the dirty side through osmosis, is forced to diffuse in the opposite direction by a pressure that is applied on the dirty side. As the membranes are non-porous, reverse osmosis is suitable for allowing substances with a molecular weight of less than 200 g/mol (Dalton or Da) to pass through. This property ensures that reverse osmosis is well suited to removing pharmaceuticals such as diclofenac (296.148 Da) and larger
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impurities such as micro-plastics or even bacteria from the discharge. It is technologically ensured that reverse osmosis can undertake the role of being used as a fourth cleaning stage, even if the requirements still have to be precisely defined. Why hasn’t reverse osmosis been installed in a clarification plant to date? The main reason is the high cost of reverse osmosis. The German Federal Environment Agency  has not included reverse osmosis in its fourth clarification stage evaluations due to the high costs involved in the process.
stages, which were integrated into the process after the biological clarification stage. The reverse osmosis stage consisted of two lines for studying different processing parameters as well as different membranes. More specifically, two different membrane types were used. One should have been especially resistant to fouling (FR) and the other should have been suitable for use with particularly low pressures (ULP). The experiments resulted in the expected: The FR type actually needed around 30% more pressure. However, this type of membrane also showed better retention in relation to the TOC (Total Organic Content) and the measured conductivity of around 1.5%. However and in general, both membranes attained retention rates of over 95% (TOC) and 97% for salts. Two substances were also explicitly measured during the experiments. A 99.8% retention rate for diclofenac and an 88% retention rate for benzotriazole were measured at the ULP membrane. These test values are higher than measurements made under laboratory conditions. Whereas the experimental value for diclofenac was only 0.2% higher, the value for benzotriazole was significantly higher, namely 23%. Nevertheless, these values prove that the process has the technological capability to remove these micro-pollutants.
3. Using reverse osmosis as the 4th cleaning stage
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A pilot plant for studying the use of reverse osmosis as the fourth purification stage was realised as part of the “MultiReUse” project funded by the German Federal Ministry of Education and Research. First results from this pilot plant are now available (see Lehmann et al ). The main focus of the project was on the micro-pollutants that are listed in Table 1. The molecular weights listed in Table 1 show that the reverse osmosis retention tasks examined in this project are quite demanding. The implemented pilot plant consisted of ultra-filtration and reverse osmosis
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Ziel des vorliegenden Buches war die Zusammenstellung möglichst umfassender Informationen über die Filtrationsund Separationsindustrie und deren Perspektiven. Der erste Teil des Nachschlagewerkes gibt Unternehmen der Branche die Gelegenheit, sich und ihr Leistungsspektrum vorzustellen. Der Mittelteil enthält redaktionelle Beiträge von Autoren mit internationalem Ruf. Des weiteren ﬁndet der Leser in diesem Buch ein Wörterbuch ausgesuchter Fachbegriffe, ein Stichwortverzeichnis und eine Auswahl nützlicher Adressen.
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Lehmann et al.  also provided insights into the expected economic efficiency of reverse osmosis if it is used as the 4th purification stage. Based on the experimental results, the authors forecast that an energy requirement of 0.47 kWh/m3 will be needed for a clarification plant with a discharge of 8,500 m3/d. This means energy costs of around 0.09 €/m3 based on the 2019 industrial electricity price of 0.19 €/kWh . This means that it will be a challenge to run reverse osmosis in a financially attractive way. The costs of running a 4th purification stage have already been estimated by the German Federal Environment Agency  to be 0.05 to 0.19 €/m3. Reverse osmosis will find it difficult to be competitive given the above-mentioned energy costs. However, in addition to the costs, there is also a question about the cleaning performance that a 4th purification stage should provide. 4. Conclusion Reverse osmosis is an interesting technology for use as the 4th clarification stage. The first available results show promising cleaning performances for specific micro-pollutants. Bacteria and parasites were also retained. However, this was also the case when other membrane processes were used. The question as to whether reverse osmosis can be used as a technology in a 4th clarification stage will depend on two essential factors: 1. The cleaning performance required by the company 2. Whether the reverse osmosis operating costs can be reduced any further. Reference literature:  Umweltbundesamt, Hrsg: Organische Mikroverunreinigungen in Gewässern, Vierte Reinigungsstufe für weniger Einträge. Dessau-Roßlau, UBA Positionspapier März 2015. ISSN 2363-829X  Margot, J., Magnet, A., Thonney, D., Chèvre, N., de Alencastro, F., Rossi, L.: Traitement des micropolluants dans les eaux usées – Rapport final sur les essais pilotes à la STEP de Vidy (Lausanne). Ed. Ville de Lausanne. 2011.[UBA26/2015] Umweltbundesamt, Hrsg: Mikroverunreinigungen und Abwasserabgabe. Dessau-Roßlau, Texte 2015, 26. ISSN 1862-4804  Umweltbundesamt, Hrsg: Empfehlungen zur Reduzierung von Mikroverunreinigungen in den Gewässern. Dessau-Roßlau, Hintergrung April 2018  Burris, D.: Groundwater Replenishment System: 2018 Annual Report. Juni 2019  Umweltbundesamt, Hrsg: Mikroverunreinigungen und Abwasserabgabe. Dessau-Roßlau, Texte 2015, 26. ISSN 1862-4804  Lehmann, S., Ogier, J., Lipnizki J.: Erfahrungen mit Umkehrosmose als vierte Reinigungsstufe. WWT – Wasserwirtschaft Wassertechnik. 2018, 68 (6) S. 10-13. ISSN 1438-5716  https://de.statista.com/statistik/daten/studie/252029/ umfrage/industriestrompreise-inkl-stromsteuer-indeutschland/ Stand 02.09.2019
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Sludge drying reduces cost – efficient drying technology P. Schlachter, R. Specht *, R. Weber ** Sludge treatment and disposal regulations are strict. Not only for producing companies but also for waste management companies. What applies to either of them – Low energy drying of pre-dewatered sludge from industrial waste water plants can considerably reduce transport and disposal cost. Introduction Chiresa AG, a Swiss waste management company, found a low energy way of drying their customers’ pre-dewatered sludge. And they could substantially reduce running cost by investing in such a sludge drying system. When dried, sludge is drastically reduced in weight and volume, sometimes by as much as 60 percent. Less sludge means less volume to be transported and to be disposed of, regard* Petra Schlachter, Reinhold Specht Harter GmbH Harbatshofen 50 88167 Stiefenhofen, Germany Tel.: +49 8383 9223-15 E-mail: firstname.lastname@example.org www.harter-gmbh.de ** Ralf Weber Chiresa AG Landstr. 2 5300 Turgi, Switzerland Tel.: +41 56 20170-80 E-mail: email@example.com www.chiresa.ch
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less of whether the sludge is transported to the disposal site straight away or to a recycling company first, where materials such as zinc, cadmium or copper can be recovered from the dried sludge.
Heat Pump Based Condensation Drying
Chiresa AG, a specialist in integrated waste management, had first learned about heat pump based condensation drying at the Niederurnen, Switzerland, based KVA Linth waste incineration plant. The plant uses such a drying system after flue gas cleaning and fly ash washing. The technology is capable of drying aqueous sludge and substrates of any kind. It uses extremely dry air and operates at low temperatures in an energetically closed system. The method is used for filter-pressed sludge with a water content of some 60 to 75 percent after pressing. If such sludge is dried subsequently, its weight and volume may be reduced by as much as 60 percent.
SEAMLESS SOLUTIONS Fig. 1: The drying container attached to the dehumidification module (left) also serves as a transport container. The process air is conditioned in the dehumidification module.
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midification module through air ducts. The drying process starts. The drying temperature ranges between 40 °C and 45 °C. Drying is completed after about 48 hours. A humidity sensor senses and reports the residual moisture, and the system is shut down automatically when the desired dry matter content is achieved. The target dry matter content is 85 percent maximum, as applicable to the type of sludge being dried. Some 8,000 to 12,000 litres of water need to be extracted to reach the target content. The sludge drying results vary with the various types of sludge Chiresa processes for its customers. The amount of water which may actually be extracted from the sludge depends on the properties of the sludge as well as the substances and the amount of water contained in the sludge.
Fig 2: The humidity laden air is cooled in the Drymex dehumidification module (front), the moisture condenses and is drained off. Subsequently, the extremely dry air is reheated and returned to the drying container.
Fig. 3: Extremely dry air combined with appropriate air routeing ensures uniform drying of the sludge. Drying can reduce sludge weight and volume by as much as 60 percent.
The decisive factor for investing in a drying system is the prospect of reducing cost considerably. This applies to both producing companies and waste management companies. Harter GmbH of Stiefenhofen, Germany, developed the heat pump based condensation drying technology more than 25 years ago and has been optimizing it ever since. The owner-managed company has established its reputation on various markets with its low temperature drying systems. Sludge drying is one of Harter’s several drying applications. It has been experiencing a boost again in recent years, mainly as a result of rising disposal cost, stricter regulations and recycling issues. Drying in the Transport Container Sludge must first be dewatered mechanically to be dried afterwards. More and 44
more companies turn to using diaphragm or chamber filter presses – a precondition for subsequent drying. The drying system installed at Chiresa AG (established in 1977) is an in-container type. This is to say that drying takes place in two combined drying and transport containers with a useful volume of 22 m? each. Attached to either of these containers is one Drymex® S9 dehumidification module (ref. fig. 1). This module features a water extraction rate of an ideal 200 to 240 litres per hour, depending, of course, on the type of sludge being dried. The system processes 1,200 tons of hydroxide sludge a year. The drying procedure is as follows. Filter cake is loaded in a container by a wheeled loader. The dry matter content of the cake is 25 to 30 percent. When full, the container is moved to the drying station. There, it is connected to the Drymex dehu-
A Perfect Match – Air Dehumidification and Air Routeing Two components are essential for Harter’s heat pump based condensation drying to be fully effective and for reliable drying large volumes of filter cake in a container. First, the core of each drying system – the Drymex dehumidification module. It supplies extremely dry and thus unsaturated air to the container. In a physical process, the air quickly absorbs humidity from the filter cake, thus drying the cake. The humidity laden air is the cooled, the moisture condenses and is drained off. Subsequently, the air is reheated and returned to the container in a closed air circuit (ref. fig. 2). This alone, however, does not suffice to ensure adequate drying. The second critical factor is air routeing. The dry air must be routed exactly to the place where it is supposed to absorb humidity. In the case of sludge drying, this place is everywhere in the filter cake. The air must be passed uniformly through the filter cake and out again. To ensure this, each container features a purpose-developed perforated bottom and customized air routeing provisions. The powerful fans used for in-container drying are also purpose-made. The containers also feature a hydraulic hinged lid system. The lids are open for filling and closed for drying. Only in this way can air pass evenly through 1,600 mm high filter cake such that uniform and reliable drying is ensured (ref. fig. 3). The cost for investment in the sludge drying system is more than offset by a considerable reduction in running transport and disposal cost resulting from as much as 60 percent sludge weight and volume decrease. Heat pump based condensation drying is an energy-saving and sustainable technology. It is thus currently eligible for government subsidy. F & S International Edition No. 20/2020
Report from the Powtech 2019 exhibition J. Barth * Drying is an important unit operation in many production processes. It is necessary for the removal of liquids from solids in any process producing solids from suspensions or solutions. In many cases, these production processes are realised as multi-stage processes comprising a mechanical concentration and a subsequent thermal drying stage. The multi-stage processes with their mechanical  and thermal  stages have been described in several articles by Ripperger. Furthermore, different apparatus und process designs of driers employing convective  and contact  energy transfer have been discussed. The following article describes special apparatuses that were presented at the Powtech exhibition at Nuremberg last year. Introduction Due to different process and product specific requirements, several different design variants of apparatuses are available for the drying of solids. They evaporate the liquid from the solids and then remove it from the apparatus. The different design variants can be categorised with regard to the operating mode, the realisation of the product transport, and the implementation of the energy transfer and the drying method. The operating mode can be divided into continuous and batch processes. Tpyical methods for the product transport are oscillating conveyors, fluidised beds, and fixed beds. The energy transfer and drying method can be divided into convective, contact, and radiation heat transfer. By employing convective or contact energy transfer energy can be conveyed to the product for drying * Dr.-Ing. Jakob Barth Backnang E-mail: Jakob.Barth@outlook.com
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as well as removed from the product for cooling. With some design variants, drying and cooling can be realised in different zones of the same apparatus. In the case of convective energy transfer and drying, the process gas primarily acts as the medium for energy transfer. In the case of contact energy transfer, the process gas has a secondary role in creating a protective atmosphere and removing gaseous decomposition products from the apparatus. Suitable treatment may be necessary for the exhaust gas from the apparatus to remove gaseous decomposition products or particulate solids. Oscillating conveyor driers Oscillating conveyor driers (see fig. 1) combine product transport via oscillating conveyors with convective energy transfer/ drying. The operating mode in continuous. The oscillating conveyors realise the transport as well as a certain amount of mixing of the product. By adjusting the amplitude, frequency, and angle of the oscil-
lation the rate of transport can be varied. The height of the product layer in the drier is regulated by the exit weir. The combination of the height of the product layer and the rate of transport determines the residence time of the product in the drier. The convective energy transfer is achieved by feeding a gas flow through a sieve plate and passing it uniformly through the product layer. The gas flow through the product layer together with the oscillation of the conveyor realises a certain amount of disaggregation of the product, but no complete fluidisation. The gas used for drying can be heated e.g. via steam heat exchangers or natural gas burners. With the product and its intended use permitting, exhaust gas can be used as the process gas directly to utilise its waste heat. Alternatively, waste heat can be utilised via heat exchangers. In certain cases, partial or complete recirculation of the process gas may be employed, if necessary with reheating (cf. Jöst ). The flow velocity and temperature of the process gas and the residence time of the product
Examples for the use of oscillating conveyor driers are the drying and cooling of styrene-butadiene rubber (SBR) before packaging, the drying and cooling of granulated fertilisers before packaging, or the cooling of PVC graulate after hot-cutting (cf. Jöst ). Bowl feeders with drying
Fig. 1: Oscillating conveyor drier from Jöst. Source: Jöst GmbH + Co. KG.
in the drier must be chosen with respect to the properties of the product. As the product is conveyed through the drier continuously, a division into different zones is possible: The flow velocity and temperature of the process gas can be adjusted in different zones. If the density of the product is significantly changed by the reduction of the liquid content the flow velocity must be adjusted to avoid the discharge of the product. Adjusting the temperature of the process gas if often employed to cool the product after drying (cf. Jöst ). If the drier comprises just one single zone the product and the process gas
are operated in counter flow. If the drier comprises more than just one zone and the process gas is recirculated, if necessary with reheating, the product and the process gas are operated in cross counter flow. Appropriate measures for dust removal (e.g. cyclon or filter) must be provided for the exhaust gas from the drier to remove the fine fraction of the product which is discharged with the gas. Even if the abrasive wear of the product is rather small in an oscillating conveyor drier compared to e.g. a fluidised bed drier, the particle-particle and particle-wall collisions are likely to create a fine fraction through abrasive wear of the product.
Fig. 2: Bowl feeder with drying from AViTEQ. Source: AViTEQ Vibrationstechnik GmbH. 46
Bowl feeders (see fig. 2) combine product transport via oscillating conveyors with different methods of energy transfer/ drying. Convective, contact, or radiation energy transfer/drying are possible. The operating mode is continuous. The product is transported through a spiral-shaped chute, that is designed as an open channel or a closed pipe, by an oscillating conveyor alone. By adjusting the amplitude, frequency, and angle of the oscillation the rate of transport and hence the residence time of the product in the drier can be varied. The abrasive wear of the product due to the transport by the oscillating conveyor is much less than in a fluidised bed. Only a very small amount of (additional) fine particles are generated and fine-grained, coarse-grained, smallsized, and medium-sized products, even with broad particle size distributions, can be conveyed. For abrasive products the chute can be equiped with a plastic coating (cf. AViTEQ ). Convective energy transfer (heating or cooling) is achieved by a gas flow inside the chute which is preferably designed as
Fig. 3: Fluidised bed drier from Neuhaus Neotec. Source: Neuhaus Neotec Maschinen- und Anlagenbau GmbH. F & S International Edition No. 20/2020
a closed pipe. With the closed pipe design, the application of a protective atmosphere and/or the removal of gaseous decomposition products are possible. Contact energy transfer (heating or cooling) is achieved by controlling the wall temperature of the chute. Electrically heated chutes are provided by e.g. Revtech . Chutes with a double-walled bottom for heating or cooling by means of a heat exchanger medium (water or oil) are provided by e.g. AViTEQ . Radiation energy transfer is achieved by radiators along the chute. Chutes with infrared radiators are provided by e.g. AViTEQ. Bowl feeders are used for heating or cooling in the chemical, pharmaceutical, food, and basic materials industry. Examples for the use of bowl feeders are drying of sand, salt, or iron oxide, ultra-drying (water content w < 0.1 wt%) of silica or zeolite, calcination of metal oxides or bentonite, pyrolysis of wood or agricultural waste products, thermal desorption of catalysts, synthetic rubber, or cork, and sterilisation of pharmaceutical products (cf. AViTEQ , Revtech ). Fluidised bed driers Fluidised bed driers (see fig. 3) employ convective energy transfer/drying. The apparatuses can be designed to operate continuously or batch-wise. Continuous operation allows for high throughput and short process time. Batch-wise operation allows for simpler design and is especially convenient for frequent change of the product. The process gas is fed at the bottom of the apparatus through a sieve plate and passes through the product. The gas flow fluidises the product in the fluidised bed and causes intensive mixing. Thus, a good energy and mass transfer are realised and the progress of the drying process is uniform and well-controlled. Well-defined temperatures or temperature profiles can be realised over the process time. For even better energy transfer, Andritz  provides apparatuses with a heat exchanger submerged in the product layer. They greatly reduce the energy demand compared to apparatuses where the energy transfer is realised via the process gas alone. Fluidised bed driers for continuous operation, like oscillating conveyor driers, allow for a division of the drier into different zones by internal weirs: Different processes can be realised in different zones of a single apparatus, e.g. spray agglomeration, drying, and cooling (cf. Andritz ). In fluidised bed driers for batch-wise operation different processes for each batch can be realised in succession. To achieve fluidisation the process gas flow is greater than with other drier variants. Therefore, the exhaust gas flow F & S International Edition No. 20/2020
is economically relevant. Firstly, when a protective atmosphere is applied and secondly, due its thermal engergy. To utilise the thermal energy recirculation of the process gas may be employed, alternatively Andritz  provides options for waste heat recovery. Due to the different sedimentation velocity of particles with different sizes, the fine fraction of the product is discharged from the fluidised bed with the process gas. Depending on the process, e.g. in the case of agglomeration, the fine fraction can be recirculated with a part of or the complete process gas, if necessary with reheating. Generally, appropriate measures for dust removal must be provided for the exhaust gas from the drier. Depending on the field of application, fluidised bed driers are provided with a CIP (cleaning in place) feature as well. To provide explosion protection the apparatuses are designed pressure resistant or shock pressure resistant and are equiped with pressure relief devices and optionally with explosion suppression devices (cf. Andritz ). Fluidised bed driers are used e.g. in the food, fine chemicals, chemical, or pharmaceutical industry as well as in biotechnology. Examples for the use of fluidised bed driers are drying of granulated plastic materials or other extruded materials (e.g. caffeine), spraying of liquid active agents (e.g. enzymes) on carrier materials with subsequent drying for good processing properties, agglomeration or granulation for dust-free products (e.g. plant protection products) or stable non-demixable powder mixtures (e.g. instant coffee mixtures), and spray drying of malt extract or vitamins for extended shelf life (cf. Andritz , Neuhaus Neotec ). Paddle driers Paddle driers (see fig. 4) combine axial transport of the product through the drier with radial mixing by mixing elements (paddle shafts). They employ contact energy transfer/drying. The operating mode is continuous. The axial rate of transport of the product is controlled by the discharge of product at the outlet of the drier. It determines the residence time of the product in the drier. The mixing elements provide a good, controlled, and gentle radial mixing. Thus, the abrasive wear of the product is smaller compared to e.g. fluidised bed driers. Additionally, the uniform axial transport of the product exhibits minimal axial backmixing so that a narrow residence time distribution is achieved (cf. Andritz ). The contact energy transfer is realised by utilising the entire mixing elements (paddle shafts) and the wall of the drier as heat exchangers. Due to the great heat 47
Fig. 4: Paddle drier from Andritz for drying granulated plastic materials. Source: Andritz AG.
exchanger area the driers have a small footprint. Water, oil, or steam can be used as heat exchanger media. The energy demand is much smaller compared to apparatuses where the energy transfer is realised via the process gas (cf. Andritz ). Normally, an additional flow of flushing gas is not necessary. As the product is conveyed through the drier continuously, a division into different zones is possible: The temperature can be adjusted in different zones. Thus, drying and subsequent cooling of the product can be realised. A modular design allows for an adjustment of the drier to different products. The mixing elements of paddle driers from Andritz  are mounted in an overlapping arrangement (see fig. 4). Thus, they are self-cleaning and the driers are suitable for the handling of non-free-flowing products. For abrasive products Andritz provides mixing elements with a highstrength coating that is created by HighVelocity Oxygen Fuel (HVOF). With a closed, pressure-resistant design, the application of a protective atmosphere or vacuum and/or the removal of gaseous decomposition products are possible. Thus, the processing of products that are toxic, flammable, or sensitive to oxygen or heat is possible. The exhaust gas can be recirculated or treated in a suitable way with optional solvent or heat recovery (cf. Andritz ). Paddle driers are used for the processing of powdery, granulated, or paste-like products. They are used e.g. in the chemical, environmental protection, or food industry. Examples for the use of paddle driers are crystallisation and drying of polyethylene terephthalate (PET), calcination of gypsum, removal of solvents, (partial) drying of mechanically dewatered sludge, fermentation residue from biogas production, or residue from pulp production, crystallisation of sugar, granulation of powdered drinks, roasting of cocoa, melting, or sterilisation (cf. Andritz ). 48
Fig. 5: Drying hurdles from Harter. Source: Harter GmbH.
Drying hurdles Drying hurdles (see fig. 5) generally process the products in batch-wise operation. The products are introduced into the drying chamber using hurdle trolleys or transport containers. Drying hurdles employ convective energy transfer/drying. While medium-sized products are processed in monolayers, bulk materials are processed in packed beds of suitable height. The typical height of the packed bed in the hurdles is h = 200 mm. Products with a high throughput (e.g. sewage sludge) can be processed in transport containers with a greater height of the packed bed (cf. Harter ). The transport of the product in hurdles or containers is very gentle and causes very little abrasive wear. Harter  provides hurdles or transport containers with integrated load cells to monitor the residual moisture content of the product gravimetrically. To realise the convective energy transfer and achieve homogeneous drying conditions the process gas is circulated within the drying chamber. If the product is processed in monolayers the process gas is directed horizontally to the individual hurdles. If the product is processed in packed beds the process gas is directed vertically through the packed beds to achieve uniform drying. Part of the gas is conducted through the dehumidification unit. Due to the closed loop operation, the process conditions are independent of the environmental conditions, and stable and reproducible process results are ensured. The profiles of the flow velocity, temperature, and humidity of the process gas over the process time must be chosen with respect to the properties of the product. By increasing the temperature at the end of the process the product can be sterilised (cf. Harter ). In the dehumidification unit the gas is first dehumidified by cooling and corresponding condensation and subsequently reheated. Harter  provides options for waste heat recovery through a heat pump process that utilises the heat released dur-
ing dehumidification for reheating. Due to the increased energy efficiency, condensation driers with this heat pump process currently receive public funding. The capacity of the installation can be adjusted easily due to the modular design of the drying chamber(s) and the dehumidification unit(s). The separated condensate can be processed further to utilise the contained reusable materials (cf. Harter ). Due to the gentle product handling and low gas velocity, the discharge of product with the process gas is minimal. Thus, the filters required for dust removal from the exhaust gas are much smaller compared to e.g. fluidised bed driers. Drying hurdles are used e.g. in the food, pharmaceutical, environmental protection, basic materials, chemical, and metal working industry. They are used for drying fruit, vegetable, and meat products (e.g. apple chips, beef jerky), spice and medical plants, medicine, sewage and paint sludge, grinding debris, or granulated plastic materials (cf. Harter ). Reference literature:  Ripperger, S.: Trocknung disperser Feststoffe – Teil 1: Mechanische Entfeuchtung als Vorstufe der thermischen Trocknung. F&S – Filtrieren und Separieren. 2018, 32 (4), pp. 256–263  Ripperger, S.: Trocknung disperser Feststoffe – Teil 2: Trocknungsverfahren und Flüssigkeitsbindung. F&S – Filtrieren und Separieren. 2018, 32 (5), pp. 343–347  Ripperger, S.: Trocknung disperser Feststoffe – Teil 3: Bauarten und Betrieb von Konvektionstrocknern. F&S – Filtrieren und Separieren. 2018, 32 (6), pp. 418–420  Ripperger, S.: Trocknung disperser Feststoffe – Teil 4: Bauarten und Betrieb von Kontakttrocknern. F&S – Filtrieren und Separieren. 2019, 33 (2), pp. 103–106  Jöst: Vibrationsfließbetttrockner. Company publication, 2017.  AViTEQ: Vertikal Fördern – Wendelförderer. Company publication, 2019.  Revtech: Continuous Heat Treatment for Chemical Engineering. Company publication, 2019.  Andritz: Kompetenz in Sonder- und Massenkunst stoffen – Wirbelschichttrocknung / Kühlung für Poly mere und Kunststoffe. Company publication, 2018.  Neuhaus Neotec: Wirbelschicht-Technologie – Bringing Ideas in Motion. Company publication, 2018.  Andritz: Versatile Thermal Processing – Andritz Gouda Paddle Dryer. Company publication, 2018.  Harter: Harter-Hordentrocknungssysteme – Lebens mittel schonend und sicher trocknen. Company publication, 2019.
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Reverse flow adsorption with integrated regeneration for retaining critical trace substances in aqueous media F. Blauth*, B. Schiemann**, J. Schiemann*** Continuous adsorption processes for treating aqueous media are still not being widely used. Powdered activated carbon is added into the biological cleaning stage in wastewater treatment plants into the biological or the clarification step. Complete separation of the activated carbon as well as its regeneration or disposal must be ensured. The paper presents a continuous process that uses the moving bed principle together with granulated, abrasionresistant adsorbents. This process combines the benefits of fixed-bed adsorption with those of a continuous process. The process concept and the effective forces are described, the results from the studies involving component development are presented and the results from treating the discharge from a wastewater treatment plant that was loaded with trace elements are also described. Process design and dimensioning conclusions are described afterwards. 1. Introduction Approximately 5,000 anthropogenic pollutants from medicines, hormone preparations, pesticides and organophosphorous flame retardants are presently classified as environmentally hazardous . The detection of these substances in discharges from sewage treatment plants is increasing as are the concentrations found in surface waters in concentrations above the quality standards currently defined in Europe. It is particularly necessary to prevent critical substances from entering waters in metropolitan areas where the river courses are used to receive discharge from the sewage treatment plant and as drinking water production source. Treating the discharges from sewage treatment plants could prevent the accumulation and spreading of trace substances in surface waters and this would also improve the long-term chemical status of the waters . Adsorption through the use of activated carbon to remove trace substances in aqueous media has already proven to be a cost-effective and efficient process for * Dipl.-Ing. Franziska Blauth E-mail: firstname.lastname@example.org Tel.: +49 2065 418-217 Fax: +49 2065 418-211 ** Bettina Schiemann E-mail: email@example.com Tel.: +49 2065 418-158 Fax: +49 2065 418 -211 *** Jochen Schiemann E-mail: firstname.lastname@example.org Tel.: +49 2065 418-259 Fax: +49 2065 418 -200 Institut für Energie- und Umwelttechnik e.V. Bliersheimer Straße 58 - 60 47229 Duisburg, Germany
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treating the discharges from clarification plants. Adsorption can occur in a fixedbed stage downstream from the secondary settling tank as a batch process or as a continuous process through adding powdered activated carbon to the discharges from the biological cleaning stage or the secondary settling tank . As part of a project funded by the Federal Ministry for Economic Affairs and Energy through their “Central innovation programme for medium-sized enterprises” programme, a continuous process for the eliminating trace substances using granulated activated carbon was developed in conjunction with Blücher GmbH. This process is based on creating a so-called moving bed and the batch releasing of loaded activated carbon.
Wetting the reactive adsorbents
Cycled Infeeding of the wetted adsorbents
The process includes purifying the waste water as well as discontinuing the on-site adsorbent cycle by using the regeneration part of the process. The separate processes were considered, designed, optimised and then integrated into an overall process during the two-and-a-half year project. Blücher GmbH developed special, highly-activated polymer-based adsorbents that are characterised by their especially high abrasion resistance as well as an adsorption surface that is consistently large, even after repeated regeneration processes. 2. Processing concept The processing principle shown in Fig. 1 consists of an adsorber in which the
De-watering Desorption gas Drying / feeding
Thermal drying, regeneration / reactivation
Inertisation Impure water
Cycled discharging of the loaded adsorbents
Intermittent steam supply
Fig. 1: Principle sketch of processing concept
Fig. 2: Pilot adsorber plant
Fig. 3: Effective forces in an adsorber
water passes through the adsorber bed as a reverse flow. The loaded adsorbents are discharged from the sump in the adsorber at defined intervals, de-watered, dried and sent on for regeneration afterwards. The reactivated adsorbents are then added into the reverse flow to the water flow through a wetting stage. The centrepiece of this process development is the generation, maintenance and movement of the moving-bed in the adsorber.
3. Fluid bed versus a moving bed A continuous adsorption process can only be realised in a single-stage or multi-stage fluidised bed as well as by a crossflow or a reverse flow in a moving-bed. This process is a long-established process used in gas cleaning, e.g. for treating exhaust gases loaded with VOC. Fixedbed reactors are predominantly used in water treatment processes .
After pulse 1
After pulse 2
Fixed-bed Fluid-bed Fluid-bed additive
Fig. 4: Principle sketch of the fixed-bed movement in a moving-bed
After pulse 3
The advantages of a fluidised-bed when compared to a fixed-bed reactor are a balanced concentration profile and the pressure drop not being dependent on the fluid load. The disadvantages of a single-stage fluidised-bed are a large residence time distribution of the solids in the fluid and the resulting inefficient adsorption capacity utilisation . A fixed-bed process can be integrated into a continuous process if a so-called moving-bed is used. The use of different effective forces on the adsorbent transport were studied in a pilot adsorber plant with an overall height of 1.80 m and a diameter of 150 mm, see Fig. 2. The phase boundary values for the water outlets that are affected by the forces acting on the flow are shown in Fig. 3. The lifting force caused by the waterâ€™s volumetric flow acts on the flow from below. A weight can be generated on the flow by using dry adsorbents as the receiver at the adsorber head. A stirrer can also be used to create an additional downward force. During the studies it was found that the weight force of dry adsorbents was insufficient for generating an adsorbent additive at the chosen empty tube speeds. Blockages, bridging and gap formations were seen in the fixed-bed flow.
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Water discharge [ml]
Carbon discharge [ml]
Discharge volume [ml]
Empty tube speed [m/h] Volumetric air flow at 0 m3/h
Opening duration [s]
Volumetric air flow at 0.1 m3/h
Volumetric air flow at 0.2 m3/h
Fig. 5: Evaluation of the adsorbent discharge through the volumetric water flow
A >2 factor was determined for the flow height that was needed above the water outlets. This would need very high working heights and large quantities of unloaded adsorbents would always have to be available for use as a buffer. The added value when compared to a batch process with two fixed-bed reactors operating alternately would be low. Another operating principle that was studied for transporting the adsorbent was a brief interruption to the water flow. This eliminates the buoyancy force at the phase boundary and allows the unloaded adsorbents to move through. A defined, brief interruption to the water supply was implemented via a pinch valve fitted in the water supply. This causes a local loosening of the fixed-bed and detaches adsorbents from the fixedbed. Dry or already wetted adsorbents are simultaneously added from the receiver tank above the water level. The adsorbents detached in the adsorber sump will be present in a fluidised-bed. The bed will solidify again when the water supply is restored. The fluidised-bed remains in the sump. Repeating the interruptions will result in other adsorbents being detached until the fluidised-bed and the fixed-bed combine again and the fixed-
Fig. 6: Discharges depending on the opening duration
bed grows. Fig. 4 shows the separate stages for repeated interruptions to the water supply. A moving-bed can be created through repeated, brief interruptions to the water supply. The required adsorbent receiver can be adapted to the relevant process and minimised. 4. Using an air lift pump to discharge the adsorbents Another basic component of the overall process is the discharging of loaded adsorbents and their de-watering. The adsorbent discharging was realised by using an airlift pump, which is also known as a mammoth pump. The adsorbents were discharged with the volumetric water flow through a tube that was centrally installed in the adsorber, as the resistance inside the tube is lower than the resistance in the flow. A ceramic air vent can also be used to supply extra air to support the discharging process. A pinch valve was used to control the discharging. The effects of the empty tube speeds, the volumetric air flow and the valve opening times on the carbon discharges were also studied.
It can be seen in Fig. 5 that the volumetric air flow is hugely affected at low empty tube speeds. Conversely, the affect on the volumetric air flow is low at high empty tube speeds. This means that, if necessary, discharging can occur without any additional air injections. On one hand, this will avoid the need to install additional peripheral equipment and on the other hand, it reduces the processâ€™s power requirements. The discharged adsorbents must be passed through a sieve or water separator in order to make the subsequent drying process as efficient as possible. Discharging the adsorbents as well as discharging the water when using different parameters and settings for both were also studied in order to be able to design this process. It can be seen in Fig. 6 that an almost linear increase in the discharged water occurs when extended valve opening times were used. Discharged water contains water that has to be separated by a sieve as well as the pore- and tapered-water that remains in the adsorbents flow. However, discharging the adsorbents during prolonged valve opening shows an asymptotic curve.
The Original Filter Papers since 1883 Protect what matters HaMuNaÂŽ Care mouth and nose protection
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Tab. 1: Parameter matrix for the factors that affect the adsorption process Discharge volumes
Feeding of dry adsorbents
Interruption Fludized bed (rotary kiln) to the water (FB) setting RK setting supply
Discharge volumes Superficial velocity Opening duration Feeding of dry adsorbents Wetting duration
5. D esigning the processes and developing the operating modes
Interruption to the water supply Fludized bed (FB) setting (rotary kiln) RK setting
Am en z id otr ot i riz azo l o C ic e an A de cid s 1H Di art a Be clo n 10 nz fen ,1 1 ot ac D ria ih yd Io zo pr le ro o xy -C Me mid ar to e ba pr m olo az l G ep ua in ny e lu re V Er als a a yt r t h C ar ro an ba m y m c az in e Tr pin e a G ma ab d ap ol Ph en en tin az Io one m ep Su ro lfa m So l et t ho alo xa l B zo Te iso le bu pro co lo na l zo le
Average concentration [ngl]
Pharmaceutical residues in the experimental receiver
Fig. 7: Concentrations of the detected substances in the clarification plant discharge
Bed height 117cm Contact time: approx. 2.5 min
Bed height 37cm Contact time: approx. 0.8 min
Bed height 15cm Contact time: approx. 0.3 min
Experiment duration [h]
Fig. 8: Trace substance elimination during the experiment but depending on the adsorber bed height
This performance must be taken into consideration during the subsequent detailed designing of the process. Repeated discharging using brief opening times as compared to a long valve opening can be useful in reducing the water content already present during the discharging of the adsorbent and to make the subsequent water separation process cost-efficient, but this will depend on the discharging quantity that is required.
In addition to the factors affecting the process that were previously presented, the inlet quantity and the wetting duration of the unloaded adsorbent as well as the settings and parameters used for the subsequent drying of the loaded adsorbents in a swirl tray (ST) and regeneration in a rotary kiln (RK) will also affect the separate processing stages. An effects matrix is shown in Table 1. Most of the factors will affect the volume of adsorbents that has to be discharged. This is to be determined for the relevant application, whereby the trace substance concentrations occurring as well as the potential adsorber installation height must also be determined during the preliminary design stages. Experiments on laboratory or pilot plant scales with defined contaminated waters will be suitable for this purpose in order to estimate the mass transfer zones that will be needed and the loading progress. 6. E xperiments with clarification plant discharges The process-engineering functionality of the process was studied and verified using water taken from a secondary clarifier installed at a municipal wastewater treatment plant. The measured overall concentration of the trace substances and the pharmaceutical residues in the water used for the experiment was 44 µg/l. The concentrations shown in Fig.7 were determined for specific indicator substances. The adsorber bed was flown through with an empty tube speed of 28 m/h. This is above the usual speeds between 5 - 15 m/h that are realised and it should indicate the highest adsorber loading that is possible . The trace substance concentration was analysed at two sampling points within the adsorber bed and in the water discharge in order to determine the loading progress and the complete elimination rate. The DOC (Dissolved Organic Carbon) sum parameter, TOC (Total Carbon) and SAK 254 were also determined in addition to the pharmaceutical residues. F & S International Edition No. 20/2020
Outlet ci/c0 [%]
Complete elimination [%]
Sampling point discharge
Experiment duration [h] SAK254
Fig. 9: Relative concentration and total elimination in the adsorber discharge
The complete elimination of pharmaceutical residues over the test duration at different sampling points is shown in Fig. 8. It can be seen at the lowest sampling point that complete elimination of only 65% was realised at the first sampling when the experiment was started. One explanation for this is the high empty tube speed and the resulting short contact time. It is probable that the adsorbents were not fully loaded in the first layer. A continuous increase in the sum parameters and the concentration of pharmaceutical residues was determined in the outlet over an experimental period of 14 h. A 91% total elimination of trace substances had been realised by the end of the experiment. This is clearly well above the 80% value that was specified as the cleaning target . The results are shown in Fig. 9. The results show that the required elimination rates in the adsorber outlet can be realised using this process. The adsorbent volume to be discharged and the cycle time can be determined using the breakthrough curve and the loading progress in the adsorber-bed. The other components needed for wetting, water separation, drying and reactivation can be selected and dimensioned based on this. Furthermore, the results also show that the adsorption capacity in the lower layers is not fully utilised at high empty tube speeds. Frequent adsorbent discharging, even with high empty tube speeds and large volumetric flows, means that the treatment process can take place with reduced area requirements. However, this form of process controlling would reduce the regeneration frequency as well as the overall lifetime of the adsorbents and the process efficiency. The parameters can be varied and optimisation between the area F & S International Edition No. 20/2020
being used and process efficiency can be realised, but this will depend on the general conditions of the relevant application. 7. Summary and conclusions An adsorption process was developed to combine the advantages of a fixed-bed process with those of a continuous process by implementing a so-called moving-bed. The relevant process parameters, especially the adsorbent volume to be discharged and the discharge cycles, must be specifically determined for each application in order to ensure energy-efficiency and economical process utilisation. By increasing the discharge frequency, high empty tube speeds can also be achieved, thus reducing space requirements. This is particularly important in applications where area requirements are a limiting factor in selecting the process to be used. Reference literature:  Türk, et.al.; Volkswirtschaftlicher Nutzen der Ertüchtigung kommunaler Kläranlagen zur Elimination von organischen Spurenstoffen, Arzneimitteln, Industriechemikalien, bakteriologisch relevanten Keimen und Viren, Abschlussbericht im Auftrag des Ministeriums für Klimaschutz, Umwelt, Landwirtschaft, Natur- und Verbraucherschutz Nordrhein-Westfalen (MKULNV), Duisburg; 2013  UBA, Positionspapier organische Mikroverunreinigungen von Gewässern, vierte Reinigungsstufe für weniger Einträge, 2015  Bornemann, „Der Einsatz von Pulveraktivkohle in Flockungsfiltrationsanlagen von Kläranlagen zur Elimination von Spurenstoffen“, 5. Symposium Flussgebietsmanagement beim Wupperverband und Gebietsforum „Wupper“ der Bezirksregierung Düsseldorf, 2012  Bathen, Dieter; Gasphasen-Adsorption in der Umwelttechnik – Stand der Technik und Perspektiven, 2002  Sielemann, Heinrich; Adsorption in flüssigfluidisierten mehrstufigen Rieselbodenwirbelschichten, Dissertation, Dortmund, 1996  Kompetenzzentrum Mikroschadstoffe.NWR, Anleitung zur Planung und Dimensionierung von Anlagen zur Mikroschadstoffelimination, Stand 20.03.2015, Köln
Reducing the pressure drop rate increase in surface filters used for dust separation through additive dosing or recirculation E. Schmidt * Surface filters are widely used throughout industry to separate particles from gases. The increase in the pressure drop over time is a decisive parameter with regard to the operating performance. Dosing with a coarse-grained additive or recirculating coarse materials from the dust bunker may reduce the pressure drop increase by more than one order of magnitude. This can be shown by using an example on a technically relevant scale. Initial model calculations confirm the experimentally obtained result that showed that the quantity of additive or recirculates must be approximately equal to the quantity of dust introduced with the raw gas in order to realise a minimum increase in the pressure drop. Introduction Surface filters are frequently used to separate particles from gases for the purpose of reducing emissions or recovering product. The most common design is characterised by its filter elements (bags, cartridges, rigid bodies, etc.), which are arranged in parallel and the flow direction is from the outside to the inside. The separation process is based on the gas passing through the porous filter medium, whereby the particles are held on the particle layer that forms and then thickens. This layer must be removed periodically in order to limit the pressure drop increase that it causes. The first regeneration stage consists of separating the particle layer from the filter medium, e.g. by briefly increasing the pressure inside the bag so that the layer disintegrates into more or less large fragments that depend on adhesiveness, cohesiveness and regeneration intensity . The second stage consists of using sedimentation to transport the fragments past the filter elements through which the gas flows around and through, often as counter-flows, and into the dust collecting bunker. Reattachment to the filter medium, which is mostly unwanted, cannot always be prevented. Finely dispersed particles will form a dust cake with a high flow resistance; this in turn will cause a rapid increase in the pressure drop and shorten the cycle time until the next regeneration. Frequent * Univ.-Prof. Dr.-Ing. habil. Eberhard Schmidt Bergische Universität Wuppertal Institute of Particle Technology Rainer-Gruenter-Straße Geb. FF 42119 Wuppertal, Germany Tel.: +49 202 439 2389 Fax: +49 202 439 3957 E-mail: email@example.com
regeneration causes high operating costs, copious fine dust emissions and an increase in the irreversible particle deposits formed inside the filter medium. Conditioning the raw gas with the aim of moving the particle size distribution into the coarser area can help here. Agglomeration through electrical effects and the dosing of coarsely dispersed additives are well known . These methods can be important for the various circuits involved in the dust separation process: manufacturers and suppliers of filter systems can design them to be more efficient. Operators of these plants can choose the operating sizes in a functional or cost-optimised way, especially when the products are changed. Manufacturers of dust dosing and dust dispersing devices can develop and supply them in a targeted way. Improving the operating performance (e.g. slowing down the pressure drop
increase, improving the regenerability) is usually the aim here and an attempt to realise this is made by permanently dosing in additives during surface filtration. The need for permanent dosing of an additive or a recirculate may also result from the fact that an existing filter system might not be able to be operated economically any longer under certain circumstances, e.g. during changes in preceding technical processes caused by changed particle properties. The cohesiveness of particles has been recognised as being defining with regard to the separating and regenerating performances of surface filters. In principle, the adhesive properties [3 & 4] of the particles to be separated or those of the filter medium should be changed through the addition of additives so that the above-mentioned objectives can be realised. This normally involves intervening in either the
Fig. 1:· Schematic diagram of the material flows in the particle separation system (volumetric · flow V, mass flow m, particle mass transport concentration c, average particle size x)
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particle size distributions of the substances that have to be separated by adding larger porous particles or intervening directly in the interactions between the particles and the filter medium by adding substances that will change the surface properties. Whether an increase or decrease of the interaction forces will have a positive effect on the operating performance will depend on the properties of the primary and the dosed particles as well as those of the filter medium. Two different procedures can be used for the permanent dosing of solid additives. On one hand particles with dimensions larger than those to be separated should be used to attain non-homogenisation of the cake structure. This will broaden the pore size distribution and move it towards larger pores and will reduce the cake’s specific outer surface. This will also reduce the resistance as well as the pressure drop in the flow passing through the cake. On the other hand, particles that are considerably smaller than those to be separated can be used as additives. This procedure can also be used if any problems occur during the filter media regeneration process. The physical principle that this type of use is based on is that of weakening the adhesive forces that act between the particles and the filter medium by increasing the distance inbetween the adhesive partners. The interactions that occur are mainly determined by the Van der Waals forces. However, they only have a short range as their effect decreases rapidly as the distance increases. If much smaller particles are now added through dosing, they will be deposited on the surfaces of the large particles. They will then act as so-called spacers and reduce the interactions between the particles that have to be separated. Their very large specific
surface areas (up to 450 m2/g) make them able to adsorb considerable amounts of moisture and organic vapours, which will further reduce the cake’s viscidity. These substances are mainly used as so-called flow aids or to prevent agglomeration from occurring in other processing stages. Finally, dry scrubbers used for flue gas cleaning should also be mentioned here as they are used in large-scale technical processes in which large amounts of separated solids are recirculated . Basic knowledge about the handling of recirculates can be brought in here, even though the objective will be different, i.e. utilising the capacity of the adsorbent.
the dust cake as well as the intensity of the regeneration. In the first stage it is assumed that the separated dust is divided into the discharge and the recirculate; xrec and xout are therefore identical but both are unknown initially. Classification could be included in a further stage in order to have specifically large particles in the recirculate. Additive dosing could be omitted if the recirculates’ properties are sufficiently favourable with regard to pressure drop development, as this is the wanted state. A two-substance mixture has been assumed in the following. Full particle size distributions have to be used to characterise the flow through the dust layers formed from the relevant particle size fractions i on the filter elements and the resulting pressure drops ∆p as a function of time t. In the following, a modification of the model originally presented by Rudnick and First  is used for the two component mixture; the solid introduced with the raw gas is indicated with an “s” and the agglomerated solid that will be added as an additive or recirculate is indicated with an “a”. The pressure drop rate increase ∆p/t, which depends on an average particle size xin, the filter face velocity v and structure function k, which characterises the dust cake and is subsequently assumed to be constant, is generally calculated using Equation (1). The structure function k is also affected by the resulting porosity and, if a mass concentration is being used here, by the densities of the specific dust fractions as well. According to Equation (2), the inlet concentration cges is the result of summing the incoming dust mass flows.
Substance flow and pressure drop calculation Flow switching within a filter system using additional, separately controlled dosing of solids is shown in simplified form in Fig. 1. When a raw gas with a · given gas volumetric flow of Vg and a dust mass flow are fed into a separator, this will result in mass transport concentration cs. Both an additive (m· add) and part of the separated amount of dust can be added to this flow as recirculate (m· rec). Any other gas quantities recorded here will be negligible when compared to the raw gas or purified gas volumetric flows. The dust mass flow discharged with the purified gas will also be negligible as the mass transport concentration cpure will be zero. The particle size distributions for the specific dust flows are shown in simplified form in the illustration by using only one size x with the relevant indexing according to Fig. 1; xrec and xadd are generally much larger than xs and xout can be significantly larger than xin here as it depends on the tendency of the particles to agglomerate during the formation of
GAS TURBINE FILTER TESTING - GTS 114
Testing according to ISO 29461-1, -5, -7; DIN EN ISO 16890; ASHRAE 52.2
gas turbine filter
differential pressure, degree of separation, water efficiency, dust loading, salt and water deluge challenge, humidity control
Topas GmbH | Gasanstaltstraße 47 | 01237 Dresden | +49 (351) 216643-0 | www.topas-gmbh.de | office firstname.lastname@example.org
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Results from the simulation calculations used to obtain the pressure drop increase Pressure drop increase / (Pa/h)
“Cn“ variant “Cm/x“ variant “Cm“ variant
Product concentration = 2.5 g/m3
In the case of spherical particles and if the Cunningham correction is disregarded, then the average particle size xin that will be relevant for the pressure drop can be calculated according to  and by using Equation (3). The two “s” and “a” components are weighted here with the number of particles and this variant is called “Cn” in the following. When the equations are summarised and after a few conversions, the pressure drop increase rate ∆p/t can be found using Equation (4).
Additive concentration / (g/m ) 3
Fig. 2: Diagram of the pressure drop rate increase ∆p/t as a function of the additive concentration ca, for three calculation variants of the mean particle size xin
Pressure drop increase / (Pa/h)
“Cm/x“ variant Additive plus basic value Pure additive
Additive concentration / (g/m3)
Pressure drop increase / (Pa/h)
Fig. 3: Diagram of the pressure drop rate increase ∆p/t as a function of the additive concentration ca, for a chosen calculation variant (see Fig. 2) and two additive dosing limit cases.
(5) If we assume that the characteristic particle size of the additive xa is ten times larger than that of the substrate xs introduced with the raw gas, then Equation (6) will be obtained through conversion. A graphical illustration of this equation is shown in Fig. 2 as the “Cn” variant. The calculation was made using v = 30 m/h, cs = 2.5 g/m3, xs = 1 µm, xa = 10 µm and k = 3.2 10-6 m2/s. (6) Weighting the two “s” and “a” components by using the particle masses instead of the particle numbers is called the “Cm” variant in the following. How the average particle size is calculated in this way is shown in Equation (7).
Product plus additive (measured value) Additive (calculated from the measured value) Additive (extrapolated from the measured value)
Product mass flow ≈ 10 kg/h
Additive mass flow / (kg/h)
Fig. 4: Effect of an additive (stone dust) on the pressure loss development ∆p/t with existing product (ammonium nitrate and ammonium sulphate) and a comparison against pure additive dosing.
(4) If this quantity is derived over the concentration ca of dosed-in substances, then a value independent of this concentration (see Equation (5)) will be found; accordingly there is no relative minimum as the pressure drop rate will increase linearly with the increase in the additive concentration used in the “Cn” variant.
(7) If pressure drop rate increase ∆p/t is calculated analogously to the calculation used above, if it is derived over the additive concentration ca and if the result is set to zero, a relative minimum can be found at point ca = 0.3135 cs, regardless of the absolute values of the physical quantities involved. The pressure drop increase over time is lowest when the additive mass flow is 31.35% of the substrate mass flow into the raw gas. If the numerical values used above are also used here, then the curve shown in Fig. 2 as the “Cm” variant will be obtained. With the third variant, called “Cm/x”, the fractions were weighted with the mass concentration related to the relevant parti-
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cle size. The xin average particle size can be found by using Equation (8). (8) The extreme value consideration produces a minimum at point ca = 0.9800 cs. The pressure drop increase over time is lowest when the additive mass flow is 98% of the substrate mass flow into the raw gas. If the numerical values used above are also used here, then the curve shown in Fig. 2 as the â€œCm/xâ€? variant will be obtained. The weightings that deviate from  are based on the assumption that the separate particles do not influence the flow independently of one another but rather that they affect each other. Smaller particles are virtually unable to develop their potential flow resistances when they are in the â€œslipstreamsâ€? from the larger particles. This theory and its quantification will need more detailed scientific studying in the future. The â€œCm/xâ€? variant was subjected to extended studying as shown in Fig. 3. The minimum can be clearly seen at ca â‰ˆ cs = 2.5â€‰g/m3. Calculating the pressure drop rate increase âˆ†p/t with â€œpure additiveâ€? dosing results in a curve that follows
the former curve for very high additive concentrations. This is plausible as the fine particles no longer play a role with the substrate concentration multiples and the pressure drop is caused solely by the coarse particles. In real plants the pressure drop is not only caused by the dust cake, but also by the filter medium that changes due to the irreversible particle deposits on it and the plant itself. This effect is symbolically illustrated in Fig. 3 by the middle curve; whereby a basic value of 20 Pa/h, which was added more or less arbitrarily and was assumed to be constant in order to illustrate the effect and enable later comparisons to be made against the experimentally determined values. Experimentally determined values for the pressure drop increase The effect of adding coarse inert stone dust (a mixture of different oxides produced through grinding) as an additive to the raw gas from a filter system, which contains ammonium nitrate sub-microns and ammonium sulphate particles from a flue gas cleaning process, is shown as an example in Fig. 4 . 64 bags, each
being 5 m long, were installed in the plant and the exhaust gas volumetric flow was approx. 4,000 m3/h during the operating state. A very low filter face velocity of 30 m/h was chosen due to the high pressure drop involved when running without any additive dosing. An extreme reduction in the pressure drop rate increase can be clearly seen here, i.e. as the pressure drop time gradient, resulting from additive dosing. In this case the optimum occurs with the dosed additiveâ€™s mass flow, which roughly corresponds to that of the productâ€™s mass flow, which is approx. 10 kg/h. A comparison against Fig. 2 shows that the simulationâ€™s â€œCm/xâ€? variant corresponds extremely well qualitatively with the experimentally determined values and the minimum pressure drop increase occurs at the same point: product mass flow rate â‰ˆ additive mass flow rate. Measurements that used a pure additive without any product , which are not described in detail here, enabled a grid point to be calculated and it is shown as a diamond at ca = 10 kg/h in Fig. 4. The extended curve was obtained through linear extrapolation in both directions. The qualitative gradient can be
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Fig. 5: SEM image of a compact dust cake formed from fine product particles (ammonium nitrate and ammonium sulphate).
Fig. 6: SEM image of a porous dust cake created by the addition of coarse additive particles (stone dust) to fine product particles (cf. Fig. 5).
described as being identical when compared to the lower curves shown in Fig. 3. The differences are probably due to the pressure drops in the other plant components, which cannot be quantified afterwards. The reason for the beneficial effects of an additive dosage on the pressure drop is probably due to the fact that the coarse-grained stone dust builds up into a loose framework with large pores and the fine-grained product is deposited on it.
The scanning electron microscope images shown in Fig. 5 (pure product) and in Fig. 6 (product with additive) support this theory. This means that it acts as a deep filtration process within a continuously thickening and renewing bulk layer made from stone dust particles . Outlook The application example used here makes it clear that a filter system’s operating performance can be considerably
improved by specifically manipulating the composition of the raw gas and in this case, it was even possible to economically use a specific design (bag filter) that was already installed. However, it has not yet been generally clarified which additives are particularly suitable for conditioning the different raw gases. The same applies to the required quality of the recirculated material that will be processed, especially with regard to particle and agglomerate size distributions. Both the properties of the particles as well as those of the carrier medium, such as temperature, pressure and chemical composition, play important roles here. Initial model calculations indicate that the amount of dosed dust should approximately correspond to the amount of dust introduced with the raw gas in order to realise the lowest possible pressure drop increase. However, a deeper understanding of the interactions between all of these factors is still needed. Urgent research is needed here. The objective of future work here is to develop a method for reducing the pressure drop rate increase and extending the cycle time of surface filters that are preferably regenerated through pulse jets, using specific recirculation processes involving the coarse dust from the filter system’s dust collecting container and having them controlled by model calculations. This should result in the particle emissions into the purified gas being reduced, an increase in the service life of the filter elements and a reduction in operating costs. Reference literature:  Zhang, Q.: Partikelschichtablösung unter Beachtung transienter kinetischer Effekte, Shaker Verlag, Aachen 2011. ISBN 978-3-8440-0705-3  Schmidt, E.: Rohgaskonditionierung und Partikelabscheidung, Shaker Verlag, Aachen 2000. ISBN 3-8265-8225-X  Hinrichsen, H., Wolf, E.: The Physics of Granular Media, Wiley-VCH-Verlag, Weinheim 2004. ISBN 3-527-40373-6  Mittal, K.L., R. Jaiswal, R.: Particle Adhesion and Removal, Scrivener Publishing, Beverly 2015. ISBN 978-1-118-83153-3  Stehmann, F.: Zur reaktiven Adsorption in der Abgasreinigung, Cuvillier Verlag, Göttingen 2018. ISBN 978-3-736-99779-0  Rudnick, SN, First, MW: Specific Resistance (K2) of Filter Dust Cakes: Comparison of Theory and Experiments, Third Symposium on Fabric Filters for Particulate Collection, EPA-600/7-78-087, 1978, S. 251-288  Schmidt, E.: Abscheidung von Partikeln aus Gasen mit Oberflächenfiltern, VDI Verlag, Düsseldorf 1998. ISBN 3-18-354603-5
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The testing of air filters for general ventilation in accordance with EN ISO 16890 F. Schmidt ¹, E. Däuber ², T. Schuldt ¹, T. Engelke ² The following article reports on the results of a round robin test in which a total of eight filter manufacturers and test laboratories participated. 1. Introduction The EN ISO 16890 series of standards, part 1 – 4 “Air filters for general ventilation” replaced the previously valid EN 779: 2012  in Europe on July 1, 2018 after a transition period . The most important changes are explained for example in . With the introduction of ISO 16890, determining the minimum fractional efficiency of filters based on the discharging of media samples with liquid isopropanol (isopropyl alcohol, IPA), which had been criticised up to now, is no longer necessary. According to part 4, entire filter elements must now be discharged in a conditioning cabinet with adjacent trays filled with isopropanol. This arrangement is intended to saturate the air with isopropanol vapour inside the chamber so that the filter element is uniformly loaded with IPA for 24 hours. First of all, this change in the test specification means that a higher safety-relevant effort will be needed as large quantities of isopropanol vapour could enter the laboratory whenever the chamber is opened (to remove the filter). This would have to be done in a separate explosion-protected zone. With the development of the TDC 584 discharge cabinet from Topas, an alternative is now available to ISO users, which has an integrated safety concept. Different test laboratories participated in a round robin test as part of a research project , which was intended to demonstrate in particular the comparability of the minimum fractional separation efficiencies after conditioning with isopropanol vapour to discharge the filter elements according to part 4 as well
as presenting the resulting differences in the classification of the filters. 2. Implementation of the round robin test and its results Testing and classification of fine dust filters with electret effect was carried out as part of the round robin test:
- Determining the initial pressure drop as a function of the volumetric flow rate - Determining the fractional efficiency curve of the new filters, using a DEHS test aerosol for particle sizes ranging from 0.3 µm to 1 µm as well as using a KCl test aerosol for particle sizes ranging from 1 µm to 10 µm
Fig. 1: Standard test rig at IUTA (Manufacturer: Topas, Dresden)
¹ Prof. Dr.-Ing. Frank Schmidt Tobias Schuldt (M.Sc.) Universität Duisburg-Essen Fachbereich Ingenieurwissenschaften Lehrstuhl: Nanopartikel-Prozesstechnik (NPPT) Lotharstr. 1, MF 148 47057 Duisburg Tel.: 0203-379 2780 E-mail: email@example.com ² Dipl.-Ing. Eckhard Däuber Dipl.-Ing. Thomas Engelke IUTA e.V., Duisburg
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Fig. 2: Pressure drop of the test filters (synthetic) as a function of the volumetric flow rate
Fig. 3: Arrangement of the KCl generator on the test rig (during operation)
- Conditioning with isopropanol vapour - Determining the fractional efficiency curve using a DEHS test aerosol for particle sizes ranging from 0.3 µm to 1 µm as well as using a KCl test aerosol for particle sizes ranging from 1 µm to 10 µm (both after isopropanol treatment in the conditioning cabinet according to ISO 16890-4) The procedure of the round robin test, the implementation of the measurement according to ISO 16890 and in particular the discharging of the filter elements are described in detail in the following:
The filter tests at IUTA were carried out on the test rig shown in Fig. 1 (Manufacturer: Topas, Dresden). First of all, the pressure drop of the 14 fine dust filters intended for the round robin test (8 pockets, filtering area: 5.4 m2, made of synthetic fibres) was determined, using a differential pressure sensor from Kalinsky Sensor Elektronik with a maximum linearity error of 1% of the final value (500 Pa). Fig. 2 shows the curves of the pressure drop for the specific filters as a function of the volumetric flow. The maximum deviation was 9 Pa with an average value of 166 Pa for the nominal volumetric flow rate of 3.400 m3/h. In the second step, the fractional separation efficiency was determined for each filter in its new condition with test aerosol DEHS for particle size range of 0.3 µm to 1.0 µm. The AGF 10, an atomizer made by Palas, and an optical particle counter (welas 2000 analog / sensor welas 2300) also from Palas, were used to generate or detect the particles, respectively. In a further working step, the fractional separation efficiencies for the test aerosol KCl were determined in the size range of 1.0 µm up to 10.0 µm. The aerosol was generated with the particle generator TSI 8108 (TSI, Aachen). An aqueous KCl solution (here: 125 g/l) was atomized in a Plexiglas tube with a diameter of 305 mm, the aerosol was dried using preheated air and brought into charge equilibrium by a corona charger. The supply line into the test rig was kept as short as possible by positioning the drying tower on the rig itself (see Fig. 3). The test aerosol was fed against the direction of flow in the symmetry axis of the test rig. Fig. 4 shows the good correlation between the determined fractional separation efficiencies as a function of the particle diameter. An average fractional efficiency of 85.1% ± 1.0% was determined for the 0.4 µm particle size when the test aerosol DEHS was used. The measured values for particle sizes larger than 1 µm (test aerosol KCl) also agree very well.
Fig. 4: Fractional separation efficiency of the fine dust filters in new condition as a function of particle diameter
In the next step, filters 4 to 13 were distributed to the participating laboratories, filters 1 to 3 remained for classification at IUTA, with filters 2 and 3 being used for a special question concerning the discharging process and filter 14 was kept as a reserve. The measurement results of the two laboratories that had received filters 6 and 8 were not provided, so they cannot be listed below. Fig. 5 compares the pressure drops measured by IUTA and the other laboratories. Overall, there was a good agreement. Only two measurements (those from the laboratories that received filters 4 and 9) differed significantly and were therefore not taken into account in the averaging process (nor was the non-existent measurement data from filters 6 and 8). The most important aspect to be validated within the scope of the round robin test was the question of the comparability of the test results from the different laboratories after discharging the filters in compliance with part 4. The problematic safety aspects arising from using the normative chamber have already been described (in  & ). The TDC 584 conditioning cabinet of the manufacturer Topas (see Fig. 6) was used by IUTA as well as in the participating laboratories (with one exception). The procedure used for standardised conditioning of a filter was as follows: 1. Filling the corresponding vessels with 1 l distilled water (for a possibly required humidification) and 1 l isopropanol (which corresponds to approx. 780 g; purity: min. 99.5 %) 2. Installation of the filter element in the chamber and 24-hour conditioning with IPA vapour (temperature: (23 ± 5)°C, relative humidity: (40 ± 20) %; switching points for the cold trap controller: 5 K min., 7 K max.) 3. Hourly recording of temperature and relative humidity 4. Weighing the IPA remainder in the bottle to quantify the consumption
Fig. 5: Comparison of the measured pressure drop of the fine dust filters
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For the special question of discharging, the conditioning cabinet used with filter 2 was operated with a temperature difference of 0 K at the cooler (i.e. with the cooler switched off) and temperature differences of 8 K (min.) and 10 K (max.) were set up for filter 3. Fig. 7 shows the good agreement when measuring the fractional separation efficiency after conditioning as a function of particle diameter. An average value of 56.6 % ± 1.5 % was determined for the 0.4 µm particle size when the test aerosol DEHS was used. KCl was used in accordance with the standard for the fractional efficiency measuring of particle sizes larger than 1 µm and the curves also match quite well. Even the measurements made with different cooler settings (see above, filters 1 – 3) did not show any significant differences. Fig. 8 compares the measured fractional separation efficiencies (in new condition and after being discharged) with the corresponding standard deviations. The result is a significant drop in fractional separation efficiency of 26 % points for the 0.4 µm particle size after discharging. The test procedure was also carried out on a fine dust filter made of glass fibres (8 pockets, filtering area: 5.8 m2; pressure drop: 168 Pa at nominal volumetric flow rate). The fractional separation efficiencies in the new condition and after discharging are identical within the possible measuring accuracy of an individual test (see Fig. 9). This had already been shown in previous investigations on media samples. In order to better classify the existing marginal differences between the individual laboratories in determining the fractional separation efficiencies, a comparison with the optical particle counters of different manufacturers was also carried out within the scope of the study: - LAP 340, company Topas, Dresden - OPS 3330, company TSI, Aachen - welas 2000 with sensor 2300, company Palas, Karlsruhe - Promo 3000 with sensor 2200, company Palas, Karlsruhe
Fig. 6: TDC 584 conditioning cabinet (Manufacturer: Topas, Dresden)
Very good agreement was seen when the fractional separation efficiencies of three identical pocket filters with electret effect (different filter class to the filters used in the round robin test) were determined for the test aerosols DEHS and KCl, using the different measuring instruments (see Fig. 10). The small differences in the fractional separation efficiencies determined by the individual laboratories could be caused by differences: - in the design of the test rigs used by the individual test laboratories - during particle generation and the feeding into the test rigs - between the optical particle counters of the respective laboratories - or also during the sampling process.
Fig. 7: Fractional separation efficiencies of the fine dust filters after conditioning
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More relevant are the ePM separation efficiencies according to ISO 16890 calculated from these fractional separation efficiencies after taking into consideration the standardised particle size distributions (see Fig. 11). The histogram shows the minimum separation efficiencies ePMx,min as well as the separation efficiencies ePMx. Only laboratories 4 and 12 determined a significantly lower ePM1,min value, which also affected the ISO ePM1 classification: ISO ePM1 70% instead of ISO ePM1 75%. A reminder: There was also a significant deviation in the pressure drop seen in laboratory 4. However, the laboratories predominately determined the identical ISO ePM1 as well as the ISO ePM2.5 classifications.
Fig. 8: Fractional separation efficiency of the fine dust filters before and after conditioning
3. Conclusion: The comparability of the filter test according to EN ISO 16890 by different laboratories can be rated as very good, as shown by the round robin test with fine dust filters. This applies even more as
the measurements were carried out using different test rigs, different optical particle counters as well as particle generators from different manufacturers. In particular, the experimental implementation of the new filter conditioning method recent-
Fig. 9: Fractional separation efficiency of a fine dust filter (glass fibres) before and after conditioning
ly introduced in part 4 led to comparable measurement results and, as a result, to determined ePMmin values of the participating laboratories. 4. Outlook The testing and classification procedure according to DIN EN ISO 16890 is a method that is used to characterise air filters for general ventilation under standardised conditions. The test results alone cannot be used to describe the filter performance in real operation. The extent to which the new ISO standard leads to better agreement between the measurement results from the laboratory and results under real operating conditions is currently being investigated in the research project mentioned above . Once again, the measurement results of air filters for general ventilation, which are tested with test particles in the laboratory in accordance with the standards are compared with those of filters that were aged in ventilation systems and then tested in the laboratory under defined conditions. Funding notes: The IGF project 19095 N of the Research Association for “Luft- und Trocknungstechnik (FLT) e.V.” was funded via the German Federation of Industrial Research Associations (AiF) as part of the program to support industrial collective research (IGF) by the German Federal Ministry for Economic Affairs and Energy (BMWi) based on a decision of the German Bundestag.
Fig. 10: Average fractional separation efficiency determined with four optical particle counters
Reference literature:  DIN EN ISO 16890: 2017-08 Blatt 1–4 „Luftfilter für die allgemeine Raumlufttechnik“; Deutsche Fassung der ISO 16890, Beuth- Verlag, Berlin  DIN EN 779:2012-10 Partikel-Luftfilter für die allgemeine Raumlufttechnik – Bestimmung der Filterleistung; Deutsche Fassung EN 779, BeuthVerlag, Berlin  Lyko, H.; Stoffel, T.: Die neue ISO 16890 zur Prüfung von Luftfiltern; F & S Filtrieren und Separieren 29 (2015) Nr. 6, S. 382–384  Untersuchung der Wirksamkeit von Filtern der allgemeinen Raumlufttechnik zur Reduzierung von Feinstaubkonzentrationen, insbesondere PM1, PM2,5 und PM10, IGF Forschungsvorhabennummer 19095 N  Schmidt, F.; Engelke, T.; Breidenbach, A.; Däuber, E.: Die Effizienz und der Druckverlust von Filtern für raumlufttechnische Anlagen – Labortests mit den Prüfstäuben ASHRAE und A2 im Vergleich mit Filtern aus realen Anlagen, F & S Filtrieren und Separieren (2017), 3, S.164–171  Schmidt, F.; Breidenbach, A.; Engelke, T.; Däuber, E.: Die Effizienz von Filtern für raumlufttechnische Anlagen bei zunehmender Beladung – Vergleich von Filtern aus dem Betrieb und Labortests, Gefahrstoffe – Reinhaltung der Luft 76 (2016), Nr. 3, S. 92–96
Fig. 11: ePMx separation efficiencies as determined by the participating laboratories
F & S International Edition No. 20/2020
The practical implementation of DIN EN ISO 16891 “Test methods for evaluating the degradation of characteristics of cleanable filter media” F. Schmidt, J. Weimann, C. König* EN ISO 16891: 2016 “Test methods for evaluating the degradation of characteristics of cleanable filter media” is the first standard in Germany that takes into account the thermal and chemical ageing of the filter media and stipulates how they are to be tested. These normative specifications were to be implemented as part of a research project. However, the boundary test conditions proved to be general conditions and many other details were not described in the standard. This is why, as well as there being many safety aspects, the filter testing has so far only been partially implemented. Uniform loading of several samples at the normal filter flow velocities used in practice could not be implemented. Doubt exists with regard to the comparability of the results of the tests that were based on the standard in its current form at different test institutes. 1. Introduction and motivation Standardized filter tests are carried out by manufacturers for internal quality control, but especially as proof of the performance and durability of the filter media or elements for the customer. The various air filter standards have now been agreed upon internationally. DIN EN ISO 16891 “Test methods for evaluating the degradation of characteristics of cleanable filter media”  represents the current state-of-the-art for testing the degradation of cleanable filter media due to thermal and chemical loads. The introduction of this new test method is motivated by the increasing importance of * Prof. Dr.- Ing. Frank Schmidt Johannes Weimann (M.Sc.) Claudia König (M.Sc.) Duisburg-Essen University Department of Engineering Chair: Nano-particle processing technology (NPPT) Lotharstr. 1, MF 148 47057 Duisburg, Germany Tel.: +49 203 379 2780 E-mail firstname.lastname@example.org
cleanable filters for exhaust air filter systems used in environmental technology. The further development of filter media in this sector has led to very good separation efficiencies with comparatively long service lives , so that surface filter systems can economically replace other basic gas cleaning operations. The establishment of a standardised test procedure should also simplify the selection of filter media, but this will depend on the later operating conditions. Until now, this selection has been based almost exclusively on the operating experience or by installing and subsequently evaluating test filters , as well as taking the investment costs into consideration. The mechanical stresses caused by the cleaning process and the pressure drop build-up caused by the filter cake as well as the effect of the stored dust were not taken into consideration here. The mechanical stress tests according to VDI 3926 or ISO 11057 are well established. The test procedure according to DIN EN ISO 16891: 2016 is intended to offer the possibility of comparative tests with regard to thermal and chemical ageing.
Fig. 1 Test structure with a temperature-controlled test chamber
F & S International Edition No. 20/2020
For this purpose, the standard specifies the test chamber structure (see Fig. 1) and the implementation of the durability tests. The test condition specifications are essentially based on industrial applications in which surface filters are exposed to aggressive gas components at increased temperatures (e.g. in flue gas dedusting or cleaning). In order to accelerate the degradation process in laboratory tests, gas concentrations were used that were far above the concentrations that occur in typical applications. Temperatures up to 20 % above the media’s maximum design temperature were also used. Only limited conclusions can be drawn about the service life of the respective filter in its later operation from these test results. The measurement results serve more to compare the media against each other. 2. E xperimental implementation of ISO 16891 In order to carry out tests according to ISO 16891 with test durations of up to 1,000 hours, highly technical efforts
Fig. 2: Test chamber (used here for simultaneous ageing of several media)
Fig. 3: Schematic diagram of the test setup for filter ageing according to standard
Fig. 4: Temperature performance in the ageing chamber at 160 °C
are required, especially with regard to the safety aspects. The harmful NOx and SO2 gases were supplied normally from compressed gas cylinders installed in gas cylinder cabinets. The safety concept included the rinsing device for the on-site piping system as well as an automatic pneumatic shut-off system connected to the relevant gas sensor. The mixture of harmful gases is cleaned together with resulting reaction products in a gas washer before being fed into the exhaust line from the test chamber. The ageing chamber had to be reworked several times to optimise
the design of the flange gasket in order to meet the safety concept’s tightness requirements (permissible leakage rate). Initially, the chemical ageing tests could only be carried out during the day. Further requirements had to be fulfilled for running the endurance tests overnight, i.e. without supervision. Many different problems, which are explained below, appeared during the practical implementation of the test specifications. It is not known whether these tests have already been fully implemented by a research institute or a filter media
manufacturer in Germany. The standard’s requirements are based solely on preliminary work by the University of Kanazawa (Japan) [5-8]. The chemical and thermal resistances of cleanable filter media were tested there in a continuous endurance test. In compliance with the standard, up to 3 filter samples were stacked on top of each other. However, these publications do not contain a complete set of boundary conditions, in particular, no tests with moisture were carried out. The samples were furled in a tube and the air flow mainly flowed through the empty centre. The volumetric flow rate was 40 ml/min, which is very low when compared to the inflow velocities in real operation. Some of the 500 h and 1000 h exposure times were clearly above the standard’s specifications. The test chamber used here was developed so that several samples could be stacked in the lower part of the chamber (with spacers in accordance with the standard) and several samples could also be suspended in the upper part of the chamber (see Fig. 2). The aim of this arrangement was to test whether the filter media could be homogeneously aged when stacked in layers. Due to the additional vertical arrangement of the samples, it is possible to distinguish between differences in the ageing of filter media through which the flow passes and those media where the harmful gases merely flowed across them. The question to be answered here was whether this extended concept would make it possible to age several filter samples at the same time in order to reduce the total effort. It was expected that because of the thermal and chemical stresses on the media in the test chamber, the polymer structure and physical properties of the individual fibres and therefore the entire filter medium (tensile strength, elongation at break) would change irreversibly as the test duration increased. The tensile strength measurements of the filter medium before and after ageing are regarded as being benchmarks of the damage caused to the fibres. For these tests the DIN EN 13934-1 standard states a minimum width of 50 ± 0.5 mm and a minimum length of 100 mm. The ageing chamber is surrounded by a heating jacket to achieve constant operating temperatures over the testing period (see Fig. 1). Using a Pt 100 sensor, the temperature was controlled and kept constant with an accuracy of ± 5 %. The sensor is located between the inner fabric of the heating sleeve and the outer wall of the ageing chamber and is approx. 130 mm above the chamber floor. The test setup was extended by a humidifier and a moisture detection sensor. Humidities between 6 % RH and 99.9 % RH (at 23 °C), which were measured in preliminary tests, can be realized. F & S International Edition No. 20/2020
Fig. 5: Supplied state (right) and after temperature ageing (left)
The carrier gas (N2, air) and the relevant noxious gas components (NOx, SO2) were fed to the gas mixing chamber (see Fig. 3) in PTFE lines with an internal diameter of 4 mm by mass flow controllers. The maximum volumetric flow was 1 l/min, whereby it could be adjusted via the control unit and different gas concentrations by varying the ratios. 3. Test results and discussion The normative material-dependent test temperatures are consistently higher than the maximum operating temperatures specified by the manufacturers. This aspect of the aging process was critically questioned by the manufacturers. It was initially necessary to ensure a largely homogeneous temperature distribution in the chamber in order to test the effects of the higher temperatures. In preliminary tests, the temperature in the middle of the ageing chamber was therefore determined with a sensor (NiCr-Ni) depending of the height of the chamber. The laboratory bench insulation was checked as the bottom of the chamber is not heated. Measuring was terminated after constant temperatures at the respective measuring points were realised. The initial results showed a significant heat loss in the bottom of the chamber and an inhomogeneous temperature distribution. The maximum deviation of 23.5 K was measured at the lowest measuring point. In the ageing test, the lower filters were subjected to less thermal stress than the filters positioned higher in the vessel. Therefore, the previously used PTFE was replaced with a glass foam plate with better insulation properties (thermal conductivity of λ = 0.05 W/mK). The test temperature could clearly be maintained more constantly over the entire chamber height; however, the minimum of 157.2 °C was determined at the lowest measuring point. The average temperature deviation was 1.3 % and this was within the minimum requirements of the ISO 16891 test specification of ±5 % (see Fig. 4).
F & S International Edition No. 20/2020
Fig. 6: Highest tensile strength of Polyamide (APA) after thermal ageing · (220 °C, 340 h, Vair = 1 l/min) 
Fine fibre needle felts were installed in the chamber for the first temperature-related ageing tests. Not only were the effects of the different boundary conditions studied, but also the time dependence with regard to the damage to the filter medium. On the one hand, the maximum operating temperature recommended by the manufacturer was set as the test temperature and on the other hand, the temperature level was set according to DIN 16891 (up to 20 % above the recommended maximum operating value). The standard compliant temperature resulted in a darker colour (see Fig. 5), a weight change of -6.3 %, a slight shrinkage and material solidification. The change in the maximum tensile strength remained within the significance range of the material fluctuations (see Fig. 6). However, as the maximum tensile force is the decisive criterion, this means that despite obvious visual and haptic changes, no media ageing occurred. During the endurance tests, the deposits in the exhaust system (see Fig. 7) proved to be extremely problematic, as the tests had to be interrupted time and time again in order to remove impurities in the PTFE tubes. The implementation of the chemical ageing test using SO2 at the specified
temperatures is described in the following. In addition to the change in the tensile strength, other non-destructive characterisation methods (weighing, determining the pressure drop, calculating the air permeability) were used. In order to study the effect of the test duration on the ageing, the test periods were set at 200 and 400 hours. A gas distribution plate was installed for the initial test in order to evenly distribute the flow profile. However, initial test results and simulations showed that the velocity profile was very inhomogeneous. Therefore, an “offering filter” of the same material was selected to equalise the differences in the inflow into the filter media. After a 200 h SO2 dosing, the filter samples showed a significant increase in pressure drop with a mass decrease of 3.6 %; the resulting air permeability decreased on average by 14.35 % as compared to the delivery condition. The first sample through which the flow passed showed stronger visual ageing than the samples above it. The damage to the adjacent stacked samples could not be compared against each other (see Fig. 8). The media that the flow passed over hanging in the upper part of the test chamber showed minor visual differences, but these tests did not conform the standard.
Fig. 7: Deposits during thermal ageing of APA media, 220 °C, inert and conditioned air
Fig. 8: Chemical ageing of aramid at T = 200 °C (at 23 °C), 99 % RH, 1,000 ppmv SO2 for 200 h (flow through)
Fig. 9: Chem. ageing of aramid at T = 200 °C, 99 % RH, 1,000 ppmv SO2 for 200 h (flowing over)
Fig. 10: Deposit at the gas outlet, thermal-chemical ageing using NO/NO2
Further chemical ageing tests were carried out using nitrogen oxide at 200 °C. A significant condensate formation could be seen at the chamber’s gas outlet after just one hour. The PTFE tubes showed bright yellow deposits, which indicate the formation of nitrous acid and the outgassing of highly volatile residues. This residue had already been seen during thermal ageing and had led to a yellowish deposit. After the plant underwent a short stoppage to clean the gas washer, a solid plug had formed in the gas outlet, which meant that the gas outlet had to be replaced. Even after increasing the cross-sectional area by a factor of 4, the gas outlet blocked again after a relatively short time. Salt-like crusts also formed on the chamber’s walls, so that tests using NO2 could not be carried out without interruptions. Chemical ageing of the aramid medium (NO2 concentration = 1,000 ppm) resulted in strong discolorations (see Fig. 11), as well as a significant difference in the haptics (softer and more fragile than before), a significant average weight loss of 54.3 % and an increase in the air permeability value of up to 262.7 % were also determined.
Fig. 11: Crust formation in the ageing chamber during thermalchemical ageing using NO/NO2 66
The existing normative specifications were to be implemented as part of a research project. However, it was found that the boundary conditions for the tests and many other details are not precisely defined. For this reason, but also due to many safety aspects, the implementation of the specifications has only been partially successful to date. Even damage to the samples when using flow velocities that are normal for the application seems hardly possible. Doubt exists with regard to the feasibility of comparable tests e.g. those run by the manufacturer and test institutes, based on the standard in its current form. Acknowledgements IGF project 18307 N of the DECHEMA (Frankfurt) research association is funded by the AiF within the framework of the programme for the promotion of industrial joint research (IGF) of the Federal Ministry of Economics and Energy on the basis of a resolution adopted by the German Bundestag. Reference literature:  DIN EN ISO 16891 „Prüfmethode zur Ermittlung der Abnahme der Wirksamkeit von abreinigbaren Filtermedien“ (2016) Beuth-Verlag, Berlin  Junker, J., & Fischer, R.: Vliese, Filze, Membranen, Textile Filtermedien in der industriellen Entstaubungstechnik für unterschiedliche Filtersysteme. Wasser, Luft, Boden (2008), p. 32-34  Gäng, P. Seminar: Prüfung und Auswahl von Filtermedien für Abreinigungsfilter. (27 28. September 2005) Düsseldorf, www.filteq.de  Weimann, J.; Schmidt, F.; Hugo, A.; Kreckel, S.; Mayer-Gall, T.: Tests for thermal, chemical and mechanical degradation characteristics of cleanable filter media, European Aerosol Conference (EAC), Zürich (2017)  Tanaka,S.; Kanakoka,C.: Durability validation of synthetic filter bags, Filtration 4(4) (2004), p.287-294  Tanthapanichakoon, W.; Furuuchi, M.; Nitta, K.-H.; Hata, M.; Otani,Y.: Degradation of bag-filter non-woven fabrics by nitric oxide at high temperatures, Advanced Powder Techn., 18, (2007), p. 349-354  Tanthapanichakoon, W.; Furuuchi, M.; Nitta, K.-H.; Hata, M.; Endoh, S.; Otani, Y.: Degradation of semi-crystalline PPS bag-filter materials by NO and O2 at high temperature, Polymer Degradation and Stability, 91, (2006), p. 1637-1644  Tanthapanichakoon,W.; Hata, M.; Endoh, S.; Furuuchi M.; Otani, Y.; Mechanical degradation of filter polymer materials: Polyphenoylene sulfide, Polymer Degradation and Stability, 91, (2006), p. 2614-2621 F & S International Edition No. 20/2020
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