World Fertilizer September Issue 2021

MAGAZINE | SEPTEMBER 2021

PRILLING. MORE CONTROL, MORE PROFIT.

CONTENTS 03 05 09

Comment World News Coming Up To The Boil

32

Chris Lawson, CRU, UK, examines an Asian fertilizer market going haywire on sharp price increases.

16

22

Anna Bashuk, Stamicarbon, the Netherlands, considers how the recycling of precious alloys at urea plants is helping the European fertilizer and steelmaking industries contribute together to the EU’s target of climate neutrality by 2050.

38

43

STRENGTH Christina M. Konecki, Arkema-ArrMaz, USA, examines the additives that can be deployed by producers to maintain uniform granule integrity across the fertilizer supply chain.

49

53

57

63

29

Granular fertilizer is produced through the agglomeration of layers of liquid or slurry material to form a particle. Each layer solidifies from crystallisation or drying, producing relatively spherical particles that are not as smooth as prills. A typical granular fertilizer is diammonium phosphate (DAP), as shown in Figure 1b.

67

Vibration Control In Prilling

An Unfinished Chapter Philip A. Henry, E2G | The Equity Engineering Group, Inc., USA, examines ongoing efforts to update and improve the current industry codes and standards on pressure relief valve instability.

Granules

22

Energy-Efficient Sulfuric Acid Cooling Nelson Clark, Breno Avancini and Vitor Sturm, Clark Solutions, Brazil, examine a new technology designed to safely cool strong sulfuric acid and reduce heat loss.

Types of granular fertilizer

Prills

A Welcome Adaptation Gordon Woolf and Jose R. Ferrer, Tecnicas Reunidas Group, Spain, consider how the design of heat exchangers for use in nitric acid units can be adapted to realise energy savings.

n the fertilizer industry, producers strive to make a high-quality fertilizer product that maintains granule integrity from manufacturing to application on the farm. Unlike some commodity products, fertilizers are often transported and handled multiple times before arriving in the hands of the end user. There are many stops along the supply chain where fertilizer must continue to maintain its granule size and integrity. Some of the physical properties of fertilizer that are monitored and used to assess performance by the industry include granule size, density, porosity, moisture uptake, nutrient release profile, dust, caking susceptibility and granule strength.

Prills are formed via solidified droplets of molten fertilizer solution. During this process the molten fertilizer is dropped from a height in a tower with upward air flow to facilitate the drying process.1 This method is common for urea, ammonium nitrate (Figure 1a) and some NPK grades, and produces smooth spherical particles.

Improving Precious Metal Collection Chris Brown, K.A. Rasmussen, Norway, David Horbury, URSG, Europe, and Michael Schriner, URSG, USA, examine the available methods for precious metal recovery and recycling in nitric acid plants.

I

Granulation is a process that helps produce a high-quality granular product that has less dust and tighter particle size distribution in the final fertilizer. Depending on the method of granulation used, the resulting product may be defined as one of the following:

Re-thinking Ammonia Oxidation Brad Cook and Juergen Neumann, Sabin Metal Corp., USA, demonstrate the benefits of taking a holistic approach when considering the gauze catalyst for ammonia oxidation.

Strength In Uniformity Christina M. Konecki, Arkema-ArrMaz, USA, examines the additives that can be deployed by producers to maintain uniform granule integrity across the fertilizer supply chain.

An Industry Co-Production Julie Ashcroft, Johnson Matthey, UK, explores the technologies that have been jointly developed to enable ammonia and methanol co-production as well as retrofits of ammonia plants.

Particle Engineering That Profits The Planet Toon Nieboer, Kreber, the Netherlands, demonstrates how particle engineering can be employed to improve nitrogen use efficiency and help counter the negative environmental effects of nitrogen fertilizer use.

Steel Goes Sustainable

Relying On Rugged Flame Detection Mike Spalding, Reuter-Stokes, USA, outlines the importance of flame detectors in the fertilizer industry.

Gabriele Marcon, Casale, Switzerland, charts the development of a new vibrating prilling bucket for urea producers.

72

Beyond The Fence Line Mike Schmidt, Bluefield Process Safety, USA, argues that the industry is responsible for process safety even after chemical products leave the plant, and shares some ideas on how to meet that responsibility.

76

Fingerprinting Ammonia Leaks René Braun, Grandperspective GmbH, Germany, demonstrates how the use of infrared imaging to give early warning of dangerous gas leaks can improve plant safety in ammonia complexes.

WORLD FERTILIZER SEPTEMBER 2021

MAGAZINE | SEPTEMBER 2021

ON THE COVER WWW WORLDFERTILIZER COM

The tangible proof to convince your customer – over the years, Kreber has built an impressive track record in manufacturing prilling equipment and is now ready to take the next step. The Kreber Pilot Facility is a modular and transportable prilling facility capable of generating industrial representative samples of new products and acquiring key data for prilling process optimisation.

PRILLING. MORE CONTROL, MORE PROFIT.

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NICHOLAS WOODROOF, DEPUTY EDITOR

I

t has been over a year now since the devastating explosion of ammonium nitrate (AN) at the port of Beirut in Lebanon. Physical reconstruction may have been completed, yet many key questions remain unanswered, such as how the AN ignited and who exactly in the Lebanese political class holds responsibility for the disaster. The prospect of imminent closure for the victims seems remote, given the strength of official obfuscation and the country’s socio-economic collapse over the past few years.

The global repercussions of the blast are reflected in the fact that the UN and Organisation for Economic Co-operation and Development (OECD) have jointly organised a seminar for December 2021 that will consider the lessons learned from Beirut. Part of the justification for the seminar is that ‘action is still needed to address risks management of AN…’ Despite this ominous note, the explosion and its effects thankfully do not seem to have tarnished the fertilizer industry’s reputation. A tragic, but avoidable, failure to follow basic storage procedures – the AN had been left inside a warehouse for almost 7 years, and was reportedly stored next to fireworks and jugs of kerosene and acid – as well as a negligent government were key causes of what was clearly an accident waiting to happen.

Without these (or sheer bad luck), it is eminently possible to stockpile AN safely, as it has been for decades in countries around the world. In a presentation at World Fertilizer’s Ammonia 2021 conference this month, Ron Peddie (a consultant with 42 years’ experience in the chemical industry, including advising on legislation and regulation) makes the point that there is no feasible substitute for AN and that the industry must continue to assert that it can handle and use AN safely. Failure to articulate that position convincingly may lead to overly restrictive regulations.

In a section of this issue of World Fertilizer dedicated to plant safety that starts from pg. 67, articles by Mike Schmidt and René Braun of Bluefield Process Safety and Grandperspective GmbH respectively are a reminder of the importance of being a good neighbour, whether it be ensuring the safe manufacture, storage or transport of chemical products. The former makes the case that the industry must be responsible for chemical products ‘beyond the fence line’, while Grandperspective GmbH’s contribution demonstrates how remote sensing technology is being used at the Chemelot industrial park in the Netherlands to reduce the risk of a gas leak and protect local students and entrepreneurs. The fertilizer industry must not rest on its laurels when it comes to its social licence to operate, and any corporate ESG policy worthy of the name should entail a sincere commitment to both local and global stewardship.

Before I finish, if you are attending the CRU Sustainable Fertilizer Production Technology Forum online between 20 – 23 September then do pop by our booth, download a copy of our 2022 media pack and say hello. Fingers crossed it won’t be too long until we’re doing this face to face again!

SEPTEMBER 2021 | WORLD FERTILIZER | 3

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WORLD NEWS USA Mosaic Co. expects disruption to phosphate operations following Hurricane Ida

M

osaic Co. has said that its North American phosphate operations are expected to be negatively impacted by damage caused by Hurricane Ida. Wind damage to the Faustina and Uncle Sam facilities from the storm is expected to result in reduced production as repairs are completed over the next 8 to 9 weeks. The following expectations also include estimates of production loss from an August equipment failure at the company’s New Wales facility in Florida.

In 3Q21, relative to historical averages, production is expected to be down by approximately 300 000 t. 4Q21 operating rates are expected to improve sequentially, but production may still be down from historical averages.

Mosaic plans to provide an update, including estimated financial impacts of the hurricane, when it reports 3Q21 results.

The hurricane also caused navigational issues on the Mississippi River, which could cause congestion during the busy autumn application season and create logistical risks for Mosaic’s production.

As Mosaic completes repairs to operations, the company is also supporting its employees and communities through a US$100 000 disaster relief grant to the Capital Area United Way and by providing affected employees with access to funds through the company’s employee-to-employee assistance plan.

AUSTRALIA PWR Hybrid given Preferred Proponent status to build Lake Wells

renewable microgrid

A

ustralian Potash has awarded PWR Hybrid Preferred Proponent status to build, own and operate a circa 35 MW high renewable energy fraction microgrid at the Lake Wells sulfate of potash (LSOP) project in Western Australia.

The microgrid will integrate a gas-fuelled power station with solar PV, wind and battery energy storage technology that is expected to achieve a renewable energy fraction (REF) above 65%.

Australian Potash also commissioned an assessment of the Lake Wells project’s greenhouse gas (GHG) footprint as part of its preparation for compliance with the Sustainable Finance Disclosure Regulation (SFDR) regime, effective in Europe from 10 March 2021. This assessment, taking into consideration the power balance across the project and energy usage through to ports of loading in Western Australia, concludes that the project will produce a CO2-e GHG that is materially lower than either a comparable Mannheim operation (LSOP<Mannheim by 69%) or solar-salt/brine operation (LSOP<other by 49%).

The project’s microgrid will be developed in a staged approach, with the thermal component to be completed within approximately 15 months of the company making a final investment decision. This timeline ensures power supply preparedness for steady state operations.

Australian Potash is aiming to make Lake Wells the lowest carbon dioxide emitting SOP project in Australia.

Lake Wells, located in the north-eastern Goldfields region of Western Australia, is the company’s flagship project

and contains the largest JORC compliant Measured SOP resource in Australia at approximately 18.1 million t.

SOP production from Lake Wells is expected to start in mid-2023, with a projected production of 170 000 tpy.

Australian Potash started construction of a village for the project in July 2021, with accommodation units onsite and in use.

Borefield development drill rigs, which were contracted from Pentium Hydro in April 2021, are also onsite and development of the Western borefield is underway. The drilling contract is for an estimated 13 months. The mobilisation of a civil earthmoving fleet has also commenced, with surveys to construct the initial crystalliser ponds having been completed.

Western Australia’s Environmental Protection Agency (EPA) approved the project’s Cultural Heritage Management Plan (CHMP) in July, completing the environmental permitting requirements for the development of the LSOP. The CHMP provides a framework for understanding the cultural context within which the LSOP will be developed.

It provides for processes that directly mitigate risks of impacts on sites and objects of cultural value to the Traditional Owners of this region.

The commitments given by Australian Potash in the CHMP are binding, and the CHMP outlines a process and timetable for the on-going consultative process with Aboriginal and other stakeholders over the several stages of the LSOP’s development and operations.

SEPTEMBER 2021 | WORLD FERTILIZER | 5

WORLD NEWS DIARY DATES Ammonia 2021 15 September 2021 Online worldfertilizer.com/ammonia2021/

CRU Sustainable Fertilizer Production Technology Forum 2021 20 – 23 September 2021 Online events.crugroup.com/ sustainableferttech/

CRU Sulphur + Sulphuric Acid 2021 01 – 04 November 2021 Online events.crugroup.com/sulphur/ home

SPAIN Highfield Resources signs MoU with Port of Bilbao

A

s part of Highfield Resources’ transport and logistics strategy for the sale of muriate of potash (MOP) and salt from its Muga Potash Mine, Highfield Resources has signed a non-binding memorandum of understanding (MoU) with the Port Authority of Bilbao.

The MoU complements a similar agreement with the Port of Pasajes, announced on 2 August 2021.

Under the terms of the MoU, the Port of Bilbao confirms the availability of sufficient port capacity for up to

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700 000 tpy of MOP and salt. The port will facilitate the implementation of the requisite logistics infrastructure and services at port to meet the project’s needs. The port also commits to have all facilities for product handling and shipping operational from Muga’s Phase 1.

The mining concessions for the Muga potash project were approved and signed by the administrations in Madrid, Aragón and Navarra in July. The company is now focusing on the preparation of construction at Muga.

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6 | WORLD FERTILIZER | SEPTEMBER 2021

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COMING UP

TO THE BOIL Chris Lawson, CRU, UK, examines an Asian fertilizer market going haywire on sharp price increases.

A

fter bottoming out in 2020, fertilizer prices have increased sharply through 2021. Downstream markets such as urea, phosphate fertilizers and potash have all bounced to levels not seen in a decade, and the raw materials ammonia, phosphate rock and sulfur have all followed. This article explains the specific reasons for the price rises and the outlook for the coming year. It will draw on developments in the Asian market, which has been at the epicentre of the market volatility.

9

Agricultural commodity markets support strong demand The key driver for fertilizer prices has been rising grains and oilseeds markets. Despite some weakness over June (typically a volatile month), agricultural commodity prices have boomed throughout 2H20 and 2021. This is mainly owing to import demand from China, where exceptionally high pork prices – triggered by the lesser acknowledged pandemic of the African swine flu – has pushed grain prices sharply higher. Global stocks have been drawn down, unfavourable weather has spurred on supply concerns and demand has been insatiable. Fertilizer markets have long been able to ride the tailwinds of agricultural markets, and 2021 is no exception.

Nitrogen market remains hot despite new supply

constraints have seen the market ratchet higher. China remains the marginal exporter, and its own domestic demand pull and rising coal prices have seen its market tighten significantly, elaborated on further in this piece.

Furthermore, India has been slow to ramp up its new capacity, while other new projects have also been delayed.

The February deep freeze in the US further hindered global supply in an already delicate market. Indian and Chinese market dynamics, again elaborated upon later in this article, will support the market over the coming months, but a new wave of supply should see the price bubble burst in 2022. This new supply includes Asia, with new plants in China, India and Brunei all set to commission over 2021 and 2022, significantly altering the balance of the market.

Phosphate price spread a gaping chasm

The world has ample nitrogen supply, and this supply is being added to, mainly through West Africa and the Commonwealth of Independent States (CIS). But demand has been exceptionally strong and short-term supply

Phosphate prices plunged to new decade low levels through 2019, and producers started 2020 with extremely tight margins. But markets have rebounded beyond expectations and are now tracking beyond decade highs.

Like nitrogen, demand has been extraordinary. But trade flows have been rerouted following successful countervailing duties issued by the US government against Morocco and Russia last year. Supply has also been constrained by low 2020 prices, COVID-19 lockdowns, technical issues and a lack of raw materials – all issues evident throughout Asia.

The perfect storm of fundamentals has seen prices spring higher. However, the end may be near, with China ramping up production and Figure 1. Phosphate price spreads have opened to new extremes. Source: CRU Fertilizer Week. the market seemingly loosening. Price spreads between east and west markets have never been so wide. This may cause some short-term pain for the Asian markets – but the producers are slowly but surely responding, and prices will correct.

Potash markets subject to geopolitical tensions

Figure 2. Belaruskali alone accounts for around a fifth of global MOP production and exports.

Data from CRU Potassium Chloride Market Outlook. Note: all data refers to 2020.

10 | WORLD FERTILIZER | SEPTEMBER 2021

Potash markets are famed for being controlled and unsurprising. But 2021 has been the ultimate exception to this. Geopolitics, strong demand, tight supply and

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producer strategy have all played their part in sharp price rises.

Belarus, which is responsible for close to 25% of global muriate of potash (MOP) exports and a key supplier to Asia, is now subject to sanctions following the grounding of a Ryanair flight earlier this year. However, these sanctions excluded the most important grade of potash exported from the country.

Elsewhere, Mosaic has been forced to curtail production, while Nutrien is belatedly lifting its. India has signed two different annual potash contracts, but suppliers have been unsuccessful in attempting to draw the hand of China’s buyer consortium in reagreeing its annual prices.

Despite Belarusian sanctions being limper than anticipated, prices are expected to continue appreciating over 2021. But potash buyers should not fear tight longer-term supply, given the number of major projects in the pipeline over the medium and long term.

Sulfur supply tight on reduced refinery activity Sulfur markets have been characterised by remarkable rises throughout 1Q21. But 2Q21 price changes have been remarkably unremarkable, despite volatility across all other fertilizer markets.

Refinery curtailments throughout 2020 tightened sulfur supply, and this carried over to 2021 as demand picked up.

Industrial demand, another key sink for the sulfur market, has also increased more than anticipated.

While further increases in phosphate prices and a drawdown in sulfur stocks hold some hope for producers looking to push prices higher, new supply on the horizon is set to spoil the party. A wave of new refinery projects is set to arrive throughout 2H21, with the full price impact set to hit the market in 2022.

Indian government slow to respond to dynamic markets Indian farmers benefited from favourable weather and subsidy support in 2020. And, despite the COVID-19 gloom, 2020 was a much-improved year for fertilizer companies.

However, 2021 is a different story. Demand has remained strong, but the government has been slow to respond to higher international prices. Importers and producers have faced unfavourable economics and have been unable to respond to these market signals due to rigid sourcing and subsidy structures. As a result, the market is entering 2H21 short of material, with inventories drawn down.

Producers in India have suffered from sharp rises in raw material prices. The Indian market is now in a precarious position – if subsidy or pricing mechanisms are not changed soon, some states will risk exhausting fertilizer inventory and supply. The drama will undoubtedly continue over the coming year.

Chinese government looks to influence exports as domestic market tightens China has arguably been the catalyst behind the fertilizer price rebound. Its insatiable demand for corn and soybeans has underpinned agricultural commodity prices. This has also boosted domestic fertilizer demand, which for nitrogen and phosphate fertilizer has decreased over the past 5 years.

The strong domestic demand in China has corresponded with supply issues across nitrogen and phosphate (China is the largest producer of both commodities). This has tightened the market significantly, and domestic prices have risen beyond decade highs as a result. With a tight market and appreciating prices, the

Figure 3. Delayed sulfur supply growth enters market in 2H21. Data from CRU Sulphur Market Outlook.

12 | WORLD FERTILIZER | SEPTEMBER 2021

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Chinese government has stepped in to improve availability.

Rumours of the reimposition of export taxes – a mechanism designed to improve local supply – have swirled around the Chinese fertilizer market since February. However, China’s National Development and Reform Commission (NDRC) and the Ministry of Finance have not formally discussed the matter. No tax changes will be imposed without serious discussion within government beforehand.

In 2008, China imposed a combined export tax on fertilizers as high as 185%, including urea and phosphates, and it failed to put a brake on price increases. Fertilizers climbed to new records. To reimpose an export duty on

fertilizers would likely have the opposite of the desired impact, triggering further price rises outside China amid a perceived scarcity of product.

Rather than taxes, the government has looked to influence the export volumes of state-owned enterprises (SOEs). The NDRC held a meeting on 29 June with large SOEs and related associations at which participants verbally agreed to guarantee fertilizer supply to the domestic market as a priority and constrain export volumes.

It remains to be seen how effective this strategy will be, but CRU does expect a pull-back in exports of fertilizer through 3Q21 because of this action. While it may provide some relief to the Chinese domestic market, it will keep international markets tight.

Other Asian countries pay up to serve strong demand The narrative of strong demand has been commonplace throughout other Asian markets. Demand in Indonesia and Malaysia has been bolstered by high palm oil prices. Thailand and Vietnam have gained from favourable rice markets. Bangladesh and Pakistan have been scrambling for imports to cover increasingly low inventory levels.

The strength of demand in Asian markets will become increasingly important in determining the global fertilizer balance, and 2021 has been the perfect reminder of that.

When will the Asian fertilizer price bubble burst?

Asia is going to have to ‘fork out’ for fertilizer for at least another few months. While some domestic actions in India (government support through higher subsidies) and China (farmer subsidies and directives to keep exports low) may provide some relief to farmers, international markets are likely to remain tight. Asia is not immune to this.

Note Figure 4. Most of China’s domestic fertilizer prices have reached highs not seen in over a decade. Data from CRU.

14 | WORLD FERTILIZER | SEPTEMBER 2021

This article was written in July 2021.

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PARTICLE

16

ENGINEERING that profits the planet Toon Nieboer, Kreber, the Netherlands, demonstrates how particle engineering can be employed to improve nitrogen use efficiency and help counter the negative environmental effects of nitrogen fertilizer use.

A

t the turn of the twentieth century, humanity was confronted with its biggest challenge up until that point: a global food crisis and subsequent mass starvation that loomed over the world. The global consumption of nitrogen compounds, be it in the form of fertilizers or explosives, threatened to surpass its natural supply, in the form of natural niter and guano deposits. This challenge was solved by Fritz Haber and Carl Bosch with their novel process, capable of capturing atmospheric dinitrogen with hydrogen at high temperatures and pressures. Through this process, humanity became able to nourish the crops they were growing with the nutrient it needed the most: reactive nitrogen. In his Nobel lecture in 1920, Haber himself predicted that in the coming future his process would be needed to produce millions of tonnes of fixed nitrogen.1 He turned out to be right, as the world today produces over 100 Tg of nitrogen.2 What Haber could not have foreseen was the environmental impact the mass consumption of such a high amount of nitrogen compounds would have.

As of now, the world is facing the opposite of the challenge it did at the end of the nineteenth century: battling the effects of nitrogen pollution on earth.

However, this time there seems to be no one-stop solution to this looming crisis, and global collaboration is needed to change the way the world produces and consumes nitrogen-based fertilizers globally.

17

Kreber is active in the field of particle engineering, a science that can potentially help with addressing the coming challenges. This article will reflect on the possible ways particle engineering can improve the shaping and production of nitrogen fertilizers, and how that can help in overcoming this global challenge.

Global challenge While the application of nitrogen-based fertilizers is still vital to sustaining the current population, the surplus of nitrogen leaking into the environment is leading to numerous concerning developments. The current method of using and producing nitrogen fertilizers has led to reductions in biodiversity and the acceleration of climate change through the production of nitrous oxide. The global challenge the world is currently facing can be described as the problem of maximising the net positive outcomes of all available nitrogen fertilizers.

As suggested by Houlton et al, making the most of these benefits lies in the development and maximising of the co-benefits of the following five strategic imperatives.3

Increasing the efficiency of nitrogen use The nitrogen use efficiency is defined as the nitrogen that will end up absorbed into the crops, over the total nitrogen applied in the form of fertilizers.

Counter-intuitively, research has shown that over the last century the efficiency has actually decreased, from 60% down to approximately 46%.3 Efficiency improvement can be achieved by focusing on the 4Rs of good nutrient stewardship: right source, right rate, right time, right place.4 The ‘right source’ means that the fertilizer type is matched to the specific needs of the crop to which it will be applied, while the ‘right rate’ focuses on the ideal amount of nutrients that is tailored to the crops’ exact needs. The ‘right time’ or timing means that the relevant nutrients are made available at the ideal moment that the crop needs them. Finally, the ‘right place’ or placement means that the nutrient is placed where the crop can easily consume it.

Figure 1. Case examples of different prilled products.

18 | WORLD FERTILIZER | SEPTEMBER 2021

Getting nitrogen to where it is needed While there is an abundance of affordable nitrogen fertilizers in wealthier nations, there are still countries with only limited access to them. Improved accessibility will increase resilience to negative weather effects and lower the likelihood of worldwide famine, as long as the nitrogen is applied in an efficient and low-polluting manner.

Removing pollution from the environment In order to reduce the unwanted effects of nitrogen pollution, leaked nitrogen can be removed from the environment by either agro-ecological or technological methods. These two removal methods also bring the possibility of potentially recycling the nitrogen and increasing the nitrogen use efficiency, thus lowering the environmental impact in the long-run.

Reducing food waste At a governmental, industrial, social and individual level, spillage and overbuying both lead to the fact that approximately a quarter of all food is wasted along its lifecycle, rendering a significant quantity of applied nitrogen fertilizer unused in terms of nutrition.

Encouraging diets with low nitrogen footprints Lastly, dietary choices have consequences for both the environment and individual health. For instance, approximately 10% of all nitrogen used for the production of cow feed for beef production is lost in the process.

Lifecycle analysis and consumer education will be required to guide consumers to a smaller footprint.

Particle engineering Particle engineering is the science of altering or forming solids into a desired shape, size distribution or composition, and is also concerned with other aspects of the particle’s morphology and surface characteristics. This type of engineering is mainly applied in the fields of pharmaceuticals, food, cosmetics and paints, where the morphology of particles is handled with the upmost care.

Prilling, the core technology of Kreber, is a method of particle engineering for spherical solids in the range of approximately 0.5 to 3 mm, as shown in Figure 1. In-depth particle engineering can have an effect on several physical aspects, such as: Solubility and/or dissolution behaviour.

Flowability (free flowing).

Homogeneity and stability against segregation.

Dose uniformity.

Bulk density.

Stabilisation of mixtures.

Hygroscopicity.

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Solutions

For instance, in milk powder production – which is achieved by spray drying – particle engineering is widely used. For the consumers, food powder dissolution has a direct impact on their perception of the overall product quality. By altering the size, shape and porosity – whilst maintaining the important flavour characteristics – the solubility is greatly increased. When adding milk powder to water in the past, it would float on the top of the liquid.

Powdered milks of today readily fall to the bottom of the water, resulting in a fast-dissolving powder.

Use in practice for fertilizers The science of particle engineering can, in terms of the five solution directions discussed in the previous section, play a role in the improvement of the efficiency of nitrogen usage. More specifically, when applied to the 4Rs, it helps to improve the right time and right place solutions. A couple of examples will be discussed in this section.

For the first solution (the right time), the timing of consistent nitrogen release can be coupled, among other variables, to the dissolving rate of the nitrogen source.

Nitrogen release is achieved either by water dissolving the nitrogen, or by bacterial life transforming the fertilizers’ nitrogen into nitrites that are essential to the crop. The available surface area can be adjusted through particle engineering by ensuring a specific particle size or porosity, or by applying a coating or encapsulation of the nitrogen source.

Currently, particle engineering solutions are already being explored in order to achieve a controlled release rate. For instance, ammonium nitrate in its more porous low-density form has an accessible surface area that is approximately five times greater than ammonium nitrate in its high-density form.5 As surface area can be coupled to the dissolving rate, a dissolution rate decrease of up to five times can be achieved when porosity is minimised.

Subsequently, when the high-density form of ammonium nitrate is encapsulated, the nitrogen release rate can be altered even further. In urea fertilizers, the application of coating urea prills with slowly dissolvable sulfur or a polymer has led to a clearly slower release rate, with the added benefit of sulfur as an additional nutrient for the crops.

In pharmaceutical applications, when the control rate is of upmost importance, encapsulation is usually used. A carrier melt is used in which the active compound is suspended. The melt is subsequently either spray-chilled or prilled in order to arrive at a solid material with the active compound encapsulated. The carrier melt dissolution characteristics, coupled with the active compound concentration and compound size relative to the final product, can be altered accordingly to accurately obtain the desired release rate of the active compound. A similar type of process can be explored for nitrogen fertilizers as well, where either urea or ammonium nitrate powder can be suspended in either a wax or another slowly dissolving, bio-friendly compound in order to gain control of the release characteristics.

20 | WORLD FERTILIZER | SEPTEMBER 2021

As for the second solution, the right place, it is important that the nutrients are delivered at the place where the crops need them. Currently, a consistent particle size is of importance for the traditional mechanical method of distributing fertilizers over farmland.

Newer methods of fertilizer distribution have shown the potential of using liquid fertilizers in bulk to obtain a better control of the dose uniformity over the crop fields.

This liquid fertilizer would consist of either slurries (solids suspended in a liquid) or a solution of nutrients in an environmentally-friendly liquid. The liquid nature of these fertilizers means that the nutrient can be sprayed evenly over the land, leading to a high degree of control over the dosage per square metre and therefore over the placement of the nutrients.

The way slurries or solutions are created will yield a need for different and more flexible shaping technologies for application to nitrogen fertilizers. When directly supplied to the crops, the nitrogen release rate needs to match the needs of the crop and the land it grows upon.

When used to create a liquid fertilizer however, where the solid will be dissolved or suspended in a liquid, the particle size has to be much smaller to be able to accurately dose and quickly dissolve the fertilizer in the carrier liquid.

Conclusion Fighting the negative effects of nitrogen fertilizer use, while still having the means to help feed all of humanity, is as daunting a challenge now as it was in the late 1800s.

This time, however, it is not an option to wait for a singular, brilliant solution such as the Haber-Bosch process.

Industry-wide collaboration is essential, coupled with a level of decisiveness from governments all over the world to tackle these problems. Smarter ways of offering nitrogen fertilizers to crops is one of the main issues on which a better method has to be found in order to increase nitrogen use efficiency. In that light, particle engineering can prove to be a piece of the puzzle, which has to be solved together.

References 1.

2.

3.

4.

5.

HABER, F., ‘The synthesis of ammonia from its elements’, Nobel Lecture (2 June 1920), https://www.nobelprize.org/ uploads/2018/06/haber-lecture.pdf HEFFER, P., and PRUD’HOMME, M., ‘Global nitrogen fertiliser demand and supply: trend, current level and outlook’, paper given at the 7th International Nitrogen Initiative Conference, Melbourne, Australia (December 2016), https://www.fertilizer. org/images/Library_Downloads/2016%20Global%20nitrogen%20 fertiliser%20demand%20and%20supply.pdf HOULTON, B.Z., ALMARAZ, M., ANEJA, V., AUSTIN, A.T., BAI, E., CASSMAN, K.G., COMPTON, J.E., DAVIDSON, E.A., ERISMAN, J.W., GALLOWAY, J.N., GU, B., YAO, G., MARTINELLI, L.A., SCOW, K., SCHLESIGNER, W.H., TOMICH, T.P., WANG, C., and ZHANG, X., ‘A World of Cobenefits: Solving the Global Nitrogen Challenge’, Earth’s Future, Vol. 7, No. 8 (August 2019), pp. 865 – 872.

FLIS, S., ‘4R history and recent phosphorus research’, Crops & Soils, Vol. 51, No. 2 (March – April 2018), pp. 36 – 47.

ZYGMUNT, B., and BUCZKOWSKI, D., ‘Infuence of Ammonium Nitrate Prills’ Properties on Detonation Velocity of ANFO’, Propellants, Explosives, Pyrotechnics, Vol. 32, No. 5 (October 2007), pp. 411 – 414.

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22

STRENGTH Christina M. Konecki, Arkema-ArrMaz, USA, examines the additives that can be deployed by producers to maintain uniform granule integrity across the fertilizer supply chain.

I

n the fertilizer industry, producers strive to make a high-quality fertilizer product that maintains granule integrity from manufacturing to application on the farm. Unlike some commodity products, fertilizers are often transported and handled multiple times before arriving in the hands of the end user. There are many stops along the supply chain where fertilizer must continue to maintain its granule size and integrity. Some of the physical properties of fertilizer that are monitored and used to assess performance by the industry include granule size, density, porosity, moisture uptake, nutrient release profile, dust, caking susceptibility and granule strength.

Types of granular fertilizer Granulation is a process that helps produce a high-quality granular product that has less dust and tighter particle size distribution in the final fertilizer. Depending on the method of granulation used, the resulting product may be defined as one of the following:

Prills Prills are formed via solidified droplets of molten fertilizer solution. During this process the molten fertilizer is dropped from a height in a tower with upward air flow to facilitate the drying process.1 This method is common for urea, ammonium nitrate (Figure 1a) and some NPK grades, and produces smooth spherical particles.

Granules Granular fertilizer is produced through the agglomeration of layers of liquid or slurry material to form a particle. Each layer solidifies from crystallisation or drying, producing relatively spherical particles that are not as smooth as prills. A typical granular fertilizer is diammonium phosphate (DAP), as shown in Figure 1b.

23

Crystals Crystalline fertilizer is not spherical but resembles the shape of salt or sugar crystals that have sharp edges and flat sides. The crystals are formed from a saturated solution where water is removed. Larger particles formed during this process are screened out and referred to as granules.1 Typical crystallised fertilizer is ammonium sulfate (Figure 1c) and some muriate of potash (MOP).

Compacted particles Compacted fertilizer is formed from crushing down on a pressed sheet of material, resulting in irregular sized particles.1 Typical compacted fertilizer includes some MOP (Figure 1d).

Granulation processes The most common granulation processes are rotating drum, fluidised bed and high shear mixing.2,3

Rotating drum A rotary drum mixer can be used with either dry feed or a slurry of fertilizer material. In the case of dry feed, water is sprayed onto the material to produce wet, granular material that is tumbled in a rotating drum, where a binder may be added.

As the particles become tacky they agglomerate, resulting in larger granules.4

Fluidised bed Fluidised bed granulation is a process whereby an air stream is used to suspend particles while spraying liquid onto the particles. Once the particles have reached the proper level of moisture, the agglomeration of particles begins to form granules.5,6

High shear mixing High shear mixing is a process whereby a liquid binder is applied to dry material that is exposed to aggressive mixing, either from the top or bottom. Similar to other techniques, the tacky particles agglomerate as the material is mixed to form granules.7

Granulation aids

Figure 1. Arkema-ArrMaz laboratory microscope images at 25x of (a) prilled ammonium nitrate, (b) granulated DAP, (c) crystalline ammonium sulfate and (d) compacted MOP.

24 | WORLD FERTILIZER | SEPTEMBER 2021

While optimising the fertilizer manufacturing process can improve some or all granule performance metrics, it is often necessary to use granulation aids or coating agents to further enhance and maintain granule integrity throughout the entire fertilizer supply chain.

Granulation aids, such as the additives (binders) in Arkema-ArrMaz’s Granulaid product line, are designed to promote rapid and uniform granulation formation, which is critical for efficient, high-quality fertilizer production. They can improve the hardness and crush strength of granules, and can also provide the added benefits of dust and moisture reduction, enabling granules to maintain their shape and size throughout the fertilizer supply chain. Today, biodegradable options are also available to meet sustainability requirements.

During granulation, binders can be used to promote better agglomeration of the fertilizer substrate. Binders can be incorporated during both wet and dry granulation. Wet granulation incorporates water and the binding agent to alter the viscosity of the fertilizer slurry and promote coalescence of the substrate. In wet granulation there are three key stages of granule/pellet growth.6,8

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Stage 1 – wetting and nucleation

Stage 3 – breakage and attrition

This stage describes the start of the granulation process, where seed granules are used to initiate the process. In continuous granulation the undersized and crushed oversized granules are recycled in the process and used as seed granules.2 During this process, the fertilizer substrate is typically misted with water and/or a binder solution to create an environment that promotes fertilizer-liquid interactions. The resulting viscosity, surface tension and mixing method altogether impact binder distribution.9

This stage encompasses the breakage of wet or dry granules through some form of impact.8

Stage 2 – consolidation and growth Once the granule begins to form, the process moves to stage 2 – consolidation and growth. The growth of the granule occurs with the addition of layers onto the seed or by coalescence (agglomeration of particles).10 The residence time in the mixing vessel also impacts the final size and size distribution of the fertilizer.

Classes of binders During granulation, binders promote adhesion through spreading in the mobile phase, creating liquid bridges that strengthen during the drying or cooling process.11,12 There are several classes of binders that are typically used in the industry today, including starches, sugars, lignosulfonates and polymers. Granulaid binders were developed to help fertilizer producers improve the starting integrity of their product. They are custom-tailored to provide different levels of granule hardness – even following exposure to high humidity – and are customised to the inorganic salt combination present. But how can the performance of binders, such as those included in the Granulaid product line, be effectively measured?

Measuring performance

Figure 2. SSP with 0.5 wt% binder.

Figure 3. SSP/TSP/Calcite 0.9 wt% loading.

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One way to evaluate the efficacy of the granulation process is to measure granule hardness. If the granule is strong enough to resist breakage during transportation and handling, there will be less dust and hence reduced losses of the finished product.

To evaluate the effectiveness of Granulaid binders, Arkema-ArrMaz evaluated three fertilizer types – SSP, an SSP/TSP/Calcite Blend and a urea-based NPK – made with experimental binders. Pellets were produced by combining fertilizer powder with water and the binder agent, and then extruding into 4 mm thick pellets. Once the pellets were dry, an initial hardness value was collected (Figures 2 – 5).

To evaluate the performance of the binder further, the pellets were exposed to 30˚C and 65% relative humidity (RH) for 3 hours and then allowed to dry; the results are represented in Figures 2 – 5 as ‘Post Aging’. The control group for all fertilizers was water. Overall, binders 1 – 7 were evaluated and the efficacy was dependent on the substrate to be bound. Granule hardness is presented as the average force required to break 30 granules, measured as kg of force. It should also be stated that the laboratory method typically produces a higher break strength than what is experienced in the field, although performance trends have been proven to scale up into plants.

SSP was evaluated with a dry binder content of 0.5 wt% (Figure 2). On initial pellet hardness, binders 5 and 6 showed improvement over water. After ageing, binders 1, 4, 5 and 6 all yielded pellets with increased granule hardness. Binder 6 showed an initial hardness improvement of 20% and, after ageing, continued to improve pellet strength by 115% compared to the control.

When looking at the SSP/TSP/Calcite blend, binder 6 was not effective at improving pellet strength post-ageing (Figure 3). However, binders 5 and 7 improved initial and post-ageing strength at both 0.9 and 1.8 wt% binder loading (Figures 3 and 4). While the improved strength is good at 0.9 wt%, the strength improvement of the blend significantly

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increased when binder loading was increased. The initial strength was greater than 700% and post-ageing strength was greater than 100% with the binder content of 1.8 wt%. This fertilizer blend illustrates that while one binder may work well on an individual component, when substrates are blended into one pellet/granule a different binding agent could prove more effective. In addition, binder loading can have a significant impact on granule strength.

When considering a binder that is already in use or when evaluating new binders, binder type and loading will be critical in determining the true efficacy of the product.

The last fertilizer type evaluated was a urea-based NPK (25-00-16-0.67), which had a binder loading of 2.25 wt% (Figure 5).

The NPK trended the same as SSP, where binders 5 and 6 were very effective granule-strengthening agents. Both binders improved the pellet strength by over 600% compared to the control, regardless of condition.

It is important to work closely with a granulation aid formulator, such as Arkema-ArrMaz, to receive specific product recommendations, screen all available product options to identify the best performing binder for a given substrate and benefit from technical support to further optimise the granulation process in the manufacturing facility.

Summary

References

Fertilizer producers are being challenged to manufacture high-quality fertilizer products that maintain granule integrity

1.

Figure 4. SSP/TSP/Calcite 1.8 wt% loading.

Figure 5. Urea-based NPK 2.25 wt% loading.

28 | WORLD FERTILIZER | SEPTEMBER 2021

across the fertilizer supply chain. Granulation aids, such as binders, are often needed to enhance and maintain granule integrity from production to field application. There are multiple factors that must be considered when selecting a binder for the granulation process, including: Type of fertilizer substrate.

Loading level.

Binder type.

Manufacturing capabilities.

United Nations Industrial Development Organization (UNIDO) and International Fertilizer Development Center (IFDC), Fertilizer Manual (1998).

2. RAMACHANDRAN, R., POON, J. M-H., SANDERS, C.F.W., GLASER, T., IMMANUEL, C.D., DOYLE, F.J., LITSTER, J.D., STEPANEK, F., WANG, F-Y., and CAMERON, I.T., ‘Experimental studies on distributions of granule size, binder content and porosity in batch drum granulation: Inferences on process modeling requirements and process sensitivities’, Powder Technology, Vol. 188, No. 2 (December 2008), pp. 89 – 101.

3. RODRIGUES, R.F., LEITE, S.R., SANTOS, D.A., and BARROZO, M.A.S., ‘Drum granulation of single super phosphate fertilizer: Effect of process variables and optimization’, Powder Technology, Vol. 321 (November 2017), pp. 251 – 258.

4. FEECO International, ‘FEECO Granulation Drums’ https://feeco.com/granulation-drums/ (Accessed 8 June 2021).

5. Spraying Systems Co., ‘Fluid Bed Processing’ https:// www.spray.com/-/media/dam/industrial/usa/salesmaterial/catalog/c12b_pharm_fluid-bed-processing. pdf (Accessed 8 June 2021).

6. BURGGRAEVE, A., MONTEYNE, T., VERVAET, C., REMON, J.P., and DE BEER, T., ‘Process analytical tools for monitoring, understanding, and control of pharmaceutical fluidized bed granulation: A review’, European Journal of Pharmaceutics and Biopharmaceutics, Vol. 83 (January 2013), pp. 2 – 15.

7. Glatt GmbH, ‘High shear granulation’, https://www. glatt.com/en/processes/granulation/high-sheargranulation/ (Accessed 8 June 2021).

8. IVESON, S.M., LITSTER, J.D., HAPGOOD, K., and ENNIS, B.J., ‘Nucleation, growth and breakage phenomena in agitated wet granulation process: a review’, Powder Technology, Vol. 117, No. 1 – 2 (June 2001), pp. 3 – 39.

9. XUE, B.C., LIU, T., HUANG, H., and LIU, E.B., ‘The effect of the intimate structure of the solid binder on material viscosity during drum granulation’, Powder Technology, Vol. 253 (February 2014), pp. 584 – 589.

10. ADETAYO, A.A., LITSTER, J.D., and DESAI, M., ‘The effect of process parameters on drum granulation of fertilizers with borad size distributions’, Chemical Engineering Science, Vol. 48, No. 23 (1993), pp. 3951 – 3961.

11. TARDOS, G.I., IRFAN KHAN, M., and MORT, P.R., ‘Critical parameters and limiting conditions in binder granulation of fine powders’, Powder Technology, Vol. 94, No. 3 (December 1997), pp. 245 – 258.

12. SIMONS, S.J.R., ROSSETTI, D., PAGLIAI, P., WARD, R., and FITZPATRICK, S., ‘The relationship between surface properties and binder performance in granulation’, Chemical Engineering Science, Vol. 60, No. 14 (July 2005), pp. 4055 – 4060.

VIBRATION CONTROL IN PRILLING Gabriele Marcon, Casale, Switzerland, charts the development of a new vibrating prilling bucket for urea producers.

C

oupling efficiency with product quality is nowadays the main goal pursued by any urea producer, whilst at the same time fulfilling increasingly stringent sustainability requirements. With that in mind, Casale has recently developed a solution aimed at improving product quality in the prilling finishing section, before dispatching the product to users.

The R&D project, carried out with the support of Donald Snyder (founder of Tuttle Prilling Systems), focused on the development of a vibrating prilling bucket – the Vibrating Skin Bucket (VSB) – that is now available on the market.

From R&D programme to first industrial application The key features of the bucket (Figure 1) are:

Suitable for prilling towers with a diameter of up to 28 m.

Uniform prill size distribution.

Low dust emissions.

Low prill temperature.

Able to function in a wide range of operating conditions.

The R&D programme led, firstly, to the design of the bucket and then to the execution of experimental tests at pilot scale to validate the theoretical calculations.

The testing activities provided promising results, so the decision was taken to prove the technology at the industrial scale. The first industrial application has now been successfully completed and the technology is available on the market for both small and large capacity urea plants.

29

Description of the technology

Figure 1. First industrial application of the VSB.

The VSB rotates on the vertical axis, like a conventional prilling bucket. As a result of the design, the external conical wall of the bucket vibrates in a vertical direction, while the liquid contained inside the bucket is kept substantially free from vibrations. The vibrations are induced by an electronic actuator installed above the prilling bucket. Figure 2 shows the main components of the bucket.

The bucket’s operating mechanism is based on Rayleigh’s principle of controlled break-up of liquid jets, which guarantees the generation of liquid droplets with a predictable diameter.

As a result of its peculiar ridged shape and optimised hole size distribution, the ‘vibrating skin’ enhances the uniformity of the generated droplets and the degree of occupancy of the prilling tower cross area.

In comparison with a conventional rotating bucket, the VSB’s configuration makes it possible to increase plant production as well as improve the uniformity (in terms of size and shape) of the urea prills. Moreover, a reduction in the fine particle content in the product and a decrease in the amount of dust emitted from the top of the prilling tower are obtained.

Table 1 shows the product size distribution achieved by the first industrial application of the VSB compared to that obtained with a conventional bucket.

In contrast to other vibropriller technologies currently available on the market, in the VSB the vibration is transferred only to the external wall of the bucket and not to the liquid inside.

Therefore, a better control of liquid droplet generation can be achieved. Figure 3 depicts the general assembly of this technology.

The main components of the technology are: Vibrating bucket: the complete bucket (not including the urea melt distributor) rotates on its axis, driven by the electric motor. The electronic actuator, installed above the prilling bucket, transmits the vibration to the bottom cover and consequently to the conical perforated wall of the bucket.

The bucket is fed with urea melt through a pipe distributor.

Shaft: the driver shaft has a hollow-tube design so that the electronic actuator can be installed inside of it.

Electronic actuator: the electronic actuator, located inside the rotating shaft, allows transmission of the vibrations to the external skin of the bucket. A sinusoidal current of controlled frequency and voltage must be used for this purpose. The frequency of the vibration generated by the actuator is the same as that of the supplied current, while the amplitude is proportional to the voltage.

VSB control system Figure 2. 3D model of Vibropriller. Orange and purple denote the rotating and vibrating parts. Red denotes the static part – liquid melt distributor.

Table 1. Prills size distribution Standard rotating bucket Load

100%

50 – 75%

<1.25 mm

3%

1.8%

1.25 – 1.5 mm

13%

6.4%

1.5 – 2 mm

54%

73.5%

2 – 4.5 mm

30.3%

18.3%

30 | WORLD FERTILIZER | SEPTEMBER 2021

The rotation speed of the bucket is controlled by the inverter installed on the electric motor.

The vibration, in terms of frequency and displacement, is controlled by means of a signal generator (powered at 220 Vac) that is able to regulate the frequency and the voltage of the signal supplied to the electronic actuator. This signal generator VSB can be operated manually from a local 75 – 100% 100 – 120% panel or integrated in the distributed 1.5% 1.6% control system (DCS).

Casale has developed a control 4% 3.9% system for the VSB that can be easily 73.2% 74% integrated with the existing DCS and 21.3% 20.5% supplied as an option.

This system allows the remote and automatic control of vibrations, in terms of frequency and amplitude, by measuring the urea melt flow rate fed to the bucket itself.

Therefore, a simple function implemented in the DCS automatically adjusts both the frequency and the amplitude according to the plant load, without the need for any manual input. The rotating speed of the bucket is also automatically controlled from the DCS, in accordance with the urea melt feed flow.

Performance The outputs of the VSB are: Product size distribution with uniformity index improved by 20%.

The VSB can be specifically designed to meet the desired average prill diameter.

Reduction of dust emissions by 25%.

Steam saving due to reduction of the urea solution recycle from the scrubbing.

Significant reduction in prill temperature.

High quality of final product between 50 – 120% of urea plant load.

Figure 3. VSB general assembly: mechanical drawing (left) and 3D model (right).

Conclusion New solutions in urea technology, with the aim of further strengthening plant efficiency and providing the end user with a

high standard of product, are being proposed and successfully tested and operated at the industrial scale.

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sustainable I

ntegration of circular systems and processes is one of the main directions for achieving industrial sustainability. As the world’s societies are accelerating their transition to a decarbonised future, circularity becomes even more important. The more materials fertilizer producers can recover, reuse and recycle, the more they can reduce their carbon footprint, while also helping other industries such as steel – which relies heavily on fossil fuels such as coal and natural gas – to contribute to the EU’s goals of climate neutrality by 2050 and meeting other climate commitments.

To address the challenge, Stamicarbon, the innovation and licence company of Maire Tecnimont Group, and Sandvik have created a buy-back programme for urea customers that allows recycling of precious alloys used in the manufacturing of urea equipment and piping. This is especially important for achieving a higher percentage of recycled raw material in the production of high-alloyed duplex steels that are able to withstand the challenging environments of the urea process, such as Safurex®.

Closing the loop The direct buy-back programme was introduced in late 2019; Stamicarbon decommissions old equipment in plants and Sandvik then uses the recycled precious alloys in the production of new stainless steels. The programme represents a new type of collaboration between the industries, not only closing the loop in the lifecycle of Safurex material, but also lowering the need for virgin raw material in the production of equipment and piping for the urea industry. This is an important step, as reducing virgin material input leads to greater sustainability, less environmental impact and more efficient use of resources.

32

Anna Bashuk, Stamicarbon, the Netherlands, considers how the recycling of precious alloys at urea plants is helping the European fertilizer and steelmaking industries contribute together to the EU’s target of climate neutrality by 2050.

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The programme is designed to make it easier for urea customers to dispose of old equipment and piping, which is otherwise a complicated and costly procedure. Instead of making their way to the scrapyard, precious alloys are collected and recycled in the manufacturing of steels that meet the demands of urea processes. To achieve the right material composition in a new melt, virgin alloys must be added. Naturally, the percentage of virgin raw material depends on the composition of the scrap, and lowering this percentage will contribute to building circularity and reducing the carbon footprint of both steel manufacturers and fertilizer producers.

When the two companies considered the process of designing this ecosystem, or circular system, they looked at the best steel that they could obtain: steel that they already knew the composition of. Consequently, the next time the same steel is made, the right composition is almost ready

Figure 1. Decommissioning of a Safurex high-pressure stripper.

Figure 2. Dismantling a Safurex high-pressure stripper.

Figure 3. Safurex STAR heat exchanger tubes.

34 | WORLD FERTILIZER | SEPTEMBER 2021

from the start in that secondary raw material. An ecosystem, or a closed loop, is thus created – not very common in these types of industries.

The programme is an addition to their full lifecycle services, and started with the purchase of the Safurex high-pressure stripper heat exchanger tubes from a urea plant in the Netherlands in 2019 (Figures 1 and 2). The buy-back offer is now included in revamping packages for Stamicarbon’s customers when replacing old equipment in their fertilizer plants. Facilitating the shift towards a more sustainable fertilizer production is a key aim of Stamicarbon’s, and the programme has contributed significantly to improving circularity in both the steel and fertilizer industries.

Sustainable materials for urea plants Urea industry processes are demanding in terms of temperature and pressure, which means that the equipment used has to be manufactured from strong, light and corrosion-resistant materials. Stamicarbon partnered with Sandvik in the 1990s on the development of Safurex, a duplex stainless steel designed to withstand the adverse conditions of the high-pressure synthesis section of the urea plant and the high corrosiveness of ammonium carbamate. Keeping in mind that different pieces of equipment in the high-pressure synthesis section require dedicated material applications to ensure optimal performance, three grades were developed: Safurex INFINITY∞ is the standard super duplex stainless steel used in the high-pressure synthesis section of urea plants, because of its mechanical properties and corrosion resistance.

Safurex DEGREE° is a super duplex stainless steel fabricated via the Hot Isostatic Pressing (HIP) method, which enables the enhancement of the material’s mechanical properties at low temperatures (-35˚C). It is used in liquid dividers in the high-pressure stripper to eliminate the risk of cross-cut end attack, as well as the housing of the radar level measurement equipment to eliminate cross-cut end attack in combination with condensation corrosion.

Safurex STAR* (Figure 3) is specifically designed for application in demanding equipment, such as heat exchanger tubes in the high-pressure stripper for all stripping processes. It helps to reduce the passive corrosion rate below detectable limits, extend the lifetime of the stripper, lower the need for maintenance and reduce the frequency of eddy current inspections.

As of now, Stamicarbon has built over 400 pieces of equipment with Safurex and has used it as a standard material for all high-pressure equipment and piping in urea plants designed by the company since 2003. During this time, active corrosion or rupture of Safurex heat exchanger tubes has never been observed.

Apart from its durability and corrosion resistance – which are both crucial for ensuring safe and efficient plant operations – the manufacturing of Safurex is also more sustainable than that of other steels used in urea

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plants, due to a more environmentally friendly heating process and a high percentage of recycled materials used in the melt. In 2020, Sandvik Materials Technology produced 16 heats of Safurex with an average scrap content of 74.6% (72.2 – 79.4%). In 2021, seven heats with an average scrap content of 73.9% (72 – 77%) have been manufactured so far.

Through the use of equipment made of Safurex, it is possible to achieve a 7% reduction in energy consumption for the tube system’s lifetime, or 265 000 t of carbon dioxide (CO2), compared to standard tube materials in urea plants.

However, Safurex requires the presence of special alloys such as nickel, chromium and molybdenum in the right composition, which makes it challenging to find enough scrap as feed. Although the percentage of secondary raw material that goes into production is approximately the same as for other duplex steels at Sandvik, more virgin chromium needs to be added due to stricter requirements concerning the purity of the final product. This highlights the importance of the buy-back programme and the need for more collaboration between the fertilizer and steel industries, in order to recover and recycle precious materials.

Sandvik selects secondary raw materials with a chemical composition as close as possible to the final product, and adds raw materials to achieve the desired composition. All materials rejected during production go back into the feed.

For high alloy steels, the recycled component could be lower than the average 82%. For low alloy steels, scrap can represent as much as 95 – 97% of the final product.

Once secondary raw material (purchased recycled steel and internal recycled materials rejected in production) is collected, it is charged into an electric arc furnace (EAF) in its metallic form and melted using electricity (Figure 4). Then, its chemistry is checked and virgin metals and alloys are added to reach the required material composition. Unlike the traditional blast furnace-basic oxygen furnace (BF-BOF) steelmaking process – which occupies more than 70% of industrial steel manufacturing2 – EAF is a more carbon-efficient and thus more future-proof method. As part of the move forward towards a carbon-free society, this process combined with 100% steel scrap feed could lead to a 70% reduction in energy consumption and a 74% reduction in CO2 emissions from the steel industry.3

Sustainable steelmaking

Why this matters

Steel products have a long lifecycle and can be recycled indefinitely without losing their mechanical properties, so they can be designed for reuse or remanufacture. Sandvik’s use of steel has a recycled component of, on average, 82%, which – while aiming to continuously increase the use of secondary raw material – represents a high sustainability number for the steel industry as a whole; typically the amount of recycled steel in new products averages 37%.1

Traditionally, neither steelmaking nor the fertilizer industry have been considered sustainable, and both sectors need to reduce their CO2 emissions significantly to make their processes more future-proof. To help accelerate the change, Stamicarbon is also participating in an EU-funded project called ‘INITIATE’ (Innovative industrial transformation of the steel and chemical industries of Europe), a novel symbiotic and circular process that transforms residual steel gases into feedstock for urea production. Carbon-rich off-gases from steel mills will be captured using modular carbon capture utilisation and storage (CCUS) technology, which integrates the flexible conditioning of steel gases with ammonia synthesis.

The global sustainability shift means that the steel industry will need to embrace sustainable practices where possible in the future, and building more circularity through reuse and recycling of materials is an important step on this journey. Moving forward towards a carbon-free society, the shift away from conventional industrial steelmaking processes and increased use of recycled materials can help reduce CO2 and NOX emissions by the industry, while also positively affecting water and energy use indicators. As international commitments and legislation on climate drive the need for the industry to catch up, these factors will play a critical role in addressing the sustainability challenge and moving closer to achieving the EU target of net zero emissions by 2050.

References 1.

2.

3.

Figure 4. EAF at Sandvik Materials Technology’s Sandviken site.

36 | WORLD FERTILIZER | SEPTEMBER 2021

World Steel Association, ‘Steel Recycling’, https://www.worldsteel. org/steel-by-topic/sustainability/materiality-assessment/recycling. html International Energy Agency, ‘Global crude steel production by process route and scenario, 2019-2050’, https://www.iea.org/dataand-statistics/charts/global-crude-steel-production-by-processroute-and-scenario-2019-2050 MADEDDU, S., UECKERDT, F., PEHL, M., PETERSEIM, J., LORD, M., KUMAR, K.A., KRÜGER, C., and LUDERER, G., ‘The CO2 reduction potential for the European industry via direct electrification of heat supply (power-to-heat)’, Environmental Research Letters, Vol. 15, No. 12, https://doi.org/10.1088/1748-9326/abbd02

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AN INDUSTRY CO-PRODUCTION Julie Ashcroft, Johnson Matthey, UK, explores the technologies that have been jointly developed to enable ammonia and methanol co-production as well as retrofits of ammonia plants.

T

he production processes for the manufacture of ammonia (NH3) and methanol (CH3OH) are similar in the goal to manufacture syngas as an intermediate prior to the final product, with the processes sharing key upstream unit operations. To develop these synergies, Johnson Matthey (JM) and Kellogg Brown Root (KBR) have formed an alliance to offer co-production standalone flowsheets and retrofits of ammonia plants to incorporate a methanol production section.

Both standalone methanol and ammonia plants contain natural gas feedstock purification units and steam methane reformers (SMRs). The gas stream exiting the SMR contains

38

syngas, a mixture of hydrogen (H2) and carbon oxides (CO and CO2). In conventional methanol plants there is excess hydrogen in this gas stream, and in ammonia plants the carbon oxides present are not required for ammonia production and can act as poisons to downstream catalysts.

After leaving the SMR in a methanol plant, the syngas mixture passes through a methanol synthesis converter loop prior to a separation stage.

For ammonia production, the gas stream exiting the reformer section passes through a secondary reformer, where air is added. The gas stream then undergoes water-gas shift to reduce the level of carbon oxides, before these are removed in the carbon dioxide removal section, with residual carbon oxides being converted to methane (CH4) in the methanator before passing into the ammonia synthesis loop.

Benefits of co-production The benefits of co-production stem from the different requirements of syngas from each process. In methanol

production, the reaction can be summarised as one mole of carbon monoxide reacting with two moles of hydrogen to give one mole of methanol (Equation 1).

CO + 2H2 l CH3OH

Equation 1

However, steam methane reforming generates hydrogen and carbon monoxide in a ratio of 3:1 (Equation 2), resulting in one mole of hydrogen in excess. This is typically used as a fuel source for the SMR in standalone methanol plants.

CH4 + H2O l CO + 3H2

Equation 2

In ammonia production, the presence of carbon oxides is a concern. The species are not required for ammonia generation, and carbon oxide and carbon dioxide are poisons to ammonia synthesis catalysts. As such, a significant section of the ammonia production process is dedicated to removing carbon oxides from the gas stream.

The water-gas shift reaction (Equation 3), taking place in the high- and low-temperature shift sections, is beneficial in generating more hydrogen as it converts carbon oxide to carbon dioxide; however, the methanation section uses up hydrogen in removing the residual carbon oxides from the gas stream (Equations 4 and 5). The dedicated carbon dioxide removal section is energy-intensive.

CO + H2O l CO2 + H2

Equation 3

CO + 3H2 l CH4 + H2O

Equation 4

CO2 + 4H2 l CH4 + 2H2O

Equation 5

Methanol production, on the other hand, requires the presence of carbon oxides in the gas stream, and has been used in coal-based ammonia synthesis as a carbon dioxide removal step. Combining ammonia and methanol production into one plant can allow the excess carbon dioxide to be used as a methanol feedstock.

39

There are numerous approaches to ammonia-methanol co-production that utilise the different hydrogen and carbon oxide requirements for each section; new projects can look to design a standalone process that can vary production between ammonia and methanol (depending on demand),

whilst retrofits to ammonia plants will be driven by the desired methanol production rates.

Standalone co-production unit

For companies considering a new-build ammonia or methanol plant, a co-production unit – as shown in Figure 1 – gives flexibility of production and better utilisation of the syngas generated within the plant.

Compared to a standalone ammonia unit, the ammonia section of the co-production unit does not contain a primary reformer, with hydrogen being provided from secondary reforming of natural gas and from the hydrogen-rich methanol synthesis loop purge.

There is a reduction in size of the carbon dioxide removal system and Figure 1. Ammonia methanol co-production flowsheet, showing the hydrogen-rich recycle the energy demands are reduced, as purge from the methanol section being used as feedstock for the ammonia section. the use of the methanol section purge gas to provide hydrogen reduces the carbon dioxide content of the syngas in the ammonia section.

The methanol section remains similar to a standalone unit, with the exception of some shared/integrated equipment and the possibility to utilise carbon dioxide removed from the ammonia unit as additional feedstock.

Within the ammonia section, the use of the KBR PurifierTM process is key to operating the secondary reformer with excess of air. This provides additional reaction heat, removing methane and other inerts from the syngas Figure 2. Methanol co-production retrofitted to an ammonia flowsheet, with the carbon before entering the synthesis loop oxides provided by the gas stream inlet the HTS reactor. and maintaining the 3:1 H:N ratio required by the ammonia synthesis loop, which in a standalone ammonia plant would be controlled by limiting the airflow into the secondary reformer. In this application, a cryogenic unit is used to purify syngas and achieve the 3:1 ratio before entering the ammonia synthesis loop. The removal of excess nitrogen and the high purity of stream are important for the co-production unit.

There is sufficient flexibility in the flowsheet to manufacture a wide range of ammonia:methanol production ratios, with a maximum production rate of 6800 tpd. The Figure 3. Methanol co-production retrofitted to an ammonia flowsheet, with the carbon process is designed in such a way oxides provided by the gas stream exit the CO2 removal stage.

40 | WORLD FERTILIZER | SEPTEMBER 2021

that any production issues affecting one product do not impact the other.

In addition to the inherent flexibility of the co-production process, there are further benefits over standalone ammonia and methanol plants. There are financial benefits – CAPEX for a co-production unit is reduced by 15 – 25% when compared to separate standalone ammonia and methanol plants, while OPEX is also reduced due to the improvement of feed and energy efficiencies within the integrated process streams when compared to separate standalone plants.

There is also a significant reduction in carbon dioxide generated per tonne of ammonia, due to purge gas from the methanol section providing a large proportion of hydrogen feedstock for ammonia production, rather than generating further hydrogen via the SMR. Additionally, the co-production unit does not require pure oxygen, due to JM’s knowledge in developing large-scale SMRs. This removes the requirement for an air separation unit (ASU) which, in turn, improves safety and reliability.

Ammonia plant retrofits Retrofits to ammonia plants utilise the levels of carbon oxides in process gas to produce a desired level of methanol. The level of methanol production required will determine where the gas stream will be redirected from the ammonia plant.

For high levels of methanol production, the gas stream inlet to the high-temperature shift (HTS) reactor will be directed to the methanol section, as shown in Figure 2. For lower levels of methanol production, the use of syngas exiting the carbon dioxide removal system provides sufficient carbon oxide levels, outlined in Figure 3.

Retrofit using syngas inlet-HTS reactor The recommended case for higher production rates of methanol, using syngas from the HTS-inlet stream, has the benefit of a relatively high fraction of carbon oxides. However, due to a relatively high level of water in the gas stream, the syngas must first pass through a desaturation step to reduce the water content. The gas stream then passes through a once-through converter to produce methanol, which is then separated from the gas stream in a recovery step, giving crude methanol that can be sent to a distillation stage to achieve the required product quality.

As there is a single connection point between the ammonia plant and the retrofit, the ammonia plant can continue operations whilst the methanol unit is shut down.

Due to the nature of the retrofit, the syngas will enter the methanol section with a very high flowrate due to a high concentration of nitrogen. As a result, a once-through methanol reactor is preferred to a synthesis loop in order to minimise the capital cost of the unit.

The addition of a retrofit at this point in the plant will result in a reduction in ammonia production rates, as a proportion of hydrogen is used in the methanol section rather than in the ammonia synthesis loop. If this is a concern, there is the potential to uprate the front-end section of the ammonia plant to offset some or all of this loss in production.

The methanol unit will change to H:N ratio from the required 3:1 ratio required by the ammonia synthesis loop. This can be addressed through optimisation of the reforming section or through use of the PURIFIER process, as discussed in the standalone co-production section.

Retrofit using syngas exit-CO2 removal

The process is similar to the inlet-HTS retrofit described earlier; however, the concentration of water in the syngas at this stage is low enough to remove the saturator and desaturator unit operations. The gas passes through a once-through methanol reactor, before crude methanol is isolated in the methanol recovery stage. The remaining syngas is then returned to the ammonia plant and enters the interchangers prior to methanation. As with the inlet-HTS retrofit, there is a single connection point between the ammonia plant and methanol section.

There are substantial benefits to a retrofit of this nature, which include a potential boost in ammonia production. In a standalone ammonia plant, carbon oxides at this stage enter the methanation section where they react with hydrogen to produce methane and water.

This is required, as carbon oxides are poisons to the ammonia synthesis catalyst. However, each mole of carbon oxide and carbon dioxide uses up three and four moles of hydrogen respectively (see reactions 4 and 5).

When these carbon oxides are instead used for methanol production, each carbon oxide reacts with two and three moles of hydrogen respectively, resulting in an increase in hydrogen entering the ammonia synthesis loop.

Methanol applications

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Depending on the end use of the crude methanol, it can be sent to a distillation stage to reach the required quality level for selling or the crude methanol can be used as a feedstock for formaldehyde production. This would be suitable for sites manufacturing granular and certain types of prilled urea products, where urea formaldehyde concentration (UFC) is used to condition these products.

In the integrated UFC (iUFCTM) process, the methanol section retrofit is combined with a downstream JM FORMOXTM plant that uses the crude methanol as a feedstock for formaldehyde production, allowing the plant to manufacture UFC to the desired concentration.

The rate of methanol production is typically sufficient to support a production rate of UFC that matches the site requirements, allowing the site to be self-sufficient in UFC supply and no longer have to import UFC from a third party. The key benefit of in-house production of UFC is a reduction in cost versus imported UFC, which is priced at a premium to cover the cost of feedstock, transportation and margins for the producer.

The potential savings are dependent on the circumstances of each individual producer, but can be in the region of the current purchase cost of UFC.

Conclusion The flexibility of production of a wide range of ammonia and methanol ratios can be advantageous, as it allows plants to respond quickly to any changes in market dynamics and product demand. With various options available, from a standalone co-production unit or a retrofit to an existing ammonia plant, the technologies outlined in this article are applicable to most ammonia producers.

In addition to the general benefits outlined, site specific benefits and potential savings can be assessed for individual plants. Large-scale co-production in a single train can be achieved for new plants and retrofits can allow for significant methanol co-production, providing a high degree of flexibility and operational savings. Retrofits can also allow a plant to become self-sufficient in UFC production and reduce the costs associated with UFC procurement.

T

Brad Cook and Juergen Neumann, Sabin Metal Corp., USA, demonstrate the benefits of taking a holistic approach when considering the gauze catalyst for ammonia oxidation.

he gauze catalyst for ammonia (NH3) oxidation has a unique position among the catalysts of the large-scale chemical industry since, in contrast to the supported catalysts usually used, it consists entirely of noble metal.

Despite this circumstance and its long history, there have been hardly any developments and advances for this catalyst.

From a scientific point of view, this may be because only partial aspects have so far been in focus, and only in the rarest cases has an attempt been made to create a holistic picture of the whole process.

Despite early recognition that the process takes place under a mass transfer regime, this approach is rarely reflected in scientific literature or considered in the design of

the catalyst. Instead, for several decades, research and development has devoted itself to kinetic investigations of the surface reaction on the catalyst, and the interpretation of recrystallisation processes and precious metal losses of the catalyst.

Little attention has so far been paid to the correlation with the plant design which, with its specific reaction environment, places fundamentally different demands on the catalyst. Most of the articles on this topic deal primarily with the effect and not the cause.

This article will try to bring together the interconnected influencing factors of mass transfer limitation and the kinetics of the reaction, as well as the effects of recrystallisation and precious metal losses, into a holistic context.

43

A brief glance at history

Figure 1. NH3 diffusion rate as a function of the pressure.

The Ostwald process of ammonia oxidation for the production of nitric acid was made possible by the large-scale industrial availability of ammonia, according to the Haber-Bosch process. In the patent, Ostwald describes the use of platinum as a catalyst in the form of wire nettings in a multi-layer arrangement.1 As early as the 1920s, Bodenstein recognised the mass transfer limitation of the catalysed reaction. However, few academic publications are devoted to this topic. The overwhelming number of research activities focused on mechanistic and kinetic investigations of the processes on the catalyst surface, which have only been comprehensively clarified in the last 20 years.

The use of fabrics was a logical consequence of the requirements of the catalyst being able to flow through, as well as generating the lowest possible flow resistance. Up until the beginning of the 1990s, all catalyst gauzes were woven. This changed with the introduction of knitting technology for the production of catalyst gauzes, and knitted gauzes now differ from one another according to the different technology used.

The alloy of the catalyst was initially adapted to the requirements of the process. In later years, the price and availability of the different precious metals increasingly influenced the alloy used. Only two alloys have been specifically developed out of necessity. These include the platinum-rhodium-palladium (Pt-Rh-Pd) ternary alloy, with increasing palladium content to reduce nitrous oxide (N2O) emissions in the process, as well as the four-component alloy that additionally contains tungsten in order to reduce the precious metal losses occurring in the process.

Mass transfer limitation

Figure 2. NH3 plant load as a function of the pressure.

Figure 3. Course of N2, N2O and NO formation selectives as a function of the temperature.

44 | WORLD FERTILIZER | SEPTEMBER 2021

As stated previously, it was in the 1920s that the ammonia oxidation reaction was discovered to proceed under a mass transfer limited regime, which means that the transfer of ammonia from the bulk gas phase to the surface of the catalyst is the slowest step in the course of the reaction.

Two conclusions can be drawn from this finding. On the one hand, the extent of the mass transfer limitation influences the number of necessary gauze layers in the catalyst package. On the other hand, the extent can be influenced by the structure of the catalyst gauze.

From the knowledge of mass transfer limitation, the number of necessary catalyst layers in the catalyst package can be calculated with relative accuracy by means of the ratio of the mass transfer rate and the gas velocity in the catalyst package.

Two parameters are in the foreground here: the plant load, which determines the flow rate; and the pressure, which is found as the decisive variable in the diffusion rate for the ammonia. Figures 1 and 2 show the change in the NH3 diffusion rate respective to the course of the NH3 plant load as a function of the pressure.

With regard to the mass transfer limitation, switching from woven to knitted catalyst gauzes has been a counterproductive step. While woven gauzes have a height of twice the wire diameter, in knitted gauzes the mesh protrudes out of the plane, meaning that they reach a height of approximately seven times the wire diameter and consequently have a significantly higher porosity. The porosity can also be translated as the permeability for ammonia, which makes it clear that a knitted structure is not necessarily the first choice for a mass transfer limited process.

The reason for the change to knitting technology is the inflexibility of weaving, with long wire fabrics being made in stock.

High precious metal stocks with rising precious metal prices lead

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to uneconomically high costs, which are solely due to the precious metal. The knitting technology offers the possibility of just-in-time production of the catalyst gauzes, whereby the costs based on the precious metal inventories are reduced to a minimum.

Engelhard-Clal, with its optimised mass transfer (OMT) concept, developed a gauze structure with the Bispin® catalyst gauze that promised improved mass transfer.2 This Bispin gauze, in principle, consists of parallel wires around which a second wire is wound in a spiral shape.

The basic structure of a woven fabric, in which the spiral-shaped wire protrudes into the mesh of the fabric, thus explains an improved mass transfer compared to a standard woven gauze. Although this solution takes on the problem of the mass transfer limitation, the ratio of the relatively high production costs for such a catalyst structure to the benefits must be questioned.

Reaction kinetics and process yield While the mass transfer limitation is expressed in the reaction rate and ultimately the number of necessary gauze layers in the catalyst pack, the kinetics of the reaction have a direct influence

on the selectivity of the reaction and thus on the yield of the process.

In the catalytic ammonia oxidation, three different reactions forming nitrogen (N2), N2O and nitric oxide (NO) take place in parallel.

NH3 + 3 02

1

N2 +

3

H20

ΔHR = 319.9 kJ/mol

NH3 + 4 02

1

N20 +

3

H20

ΔHR = 275.9 kJ/mol

N0 +

3

H20

ΔHR = 226.6 kJ/mol

NH3 + 5 02

The kinetics provide information about the conditions under which each reaction takes place and at what rate, and thus the expected ratio of the three different reaction products according to the reaction conditions. Using the kinetic parameters from Kraehnert, Figure 3 shows how the product distribution over the temperature is obtained.4 N2 and NO are the main products of ammonia oxidation over platinum. N2 formation prevails between 150 and 400˚C, while NO formation is favoured at higher temperatures.

Linkage of mass transfer limitation and kinetics Only by linking the mass transfer process with the kinetics of the catalysed reaction will a holistic picture of the reaction process be realised. Figure 4 shows the NO selectivity over the temperature for the individual gauze layers in the catalyst package for the various types of plants, from low-pressure to high-pressure operations.

The figure clearly shows the temperature gradient in the catalyst package, which increases with the operating pressure of the reaction and which ultimately determines the NO selectivity and thus the product yield of the process. From the figure, it can also be seen that only the reduction of this temperature gradient creates both the possibility of a reduction in the number of gauze layers in the catalyst package and an increase in the NO product yield.

Recrystallisation and loss of precious metals Recrystallisation and precious metal losses from the catalyst are the third element in the holistic view. Looking at the temperature gradient in the catalyst package, the lowest NO selectivity (and the individual gauze layers in the catalyst package thus the highest N2 formation rate) is found in the uppermost (low-pressure to high-pressure operations). catalyst layers. Relative to the higher enthalpy of reaction of the N2 formation, a correspondingly larger amount of reaction heat is released, which results in higher surface temperatures than in the lower catalyst gauze layers.

This temperature is sufficient for precious metal atoms to detach from the metal lattice, migrate over the surface and form larger surface agglomerates on colder surface regions or energetically exposed places. This leads to extensive surface reconstruction through the growth of agglomerates that form steps and terrace surfaces. Figure 5 shows such stepped and terrace-shaped agglomerates on the recrystallised Figure 5. Surface of a used catalyst gauze (left) and detail enlargement with catalyst surface in 30 000-fold magnification. terraced agglomerates (right).

Figure 4. NO selectivity over the temperature for

46 | WORLD FERTILIZER | SEPTEMBER 2021

Due to the high surface temperature, the highest levels of precious metal losses due to the formation of gaseous platinum oxide (PtO2) are also found on the uppermost gauze layers. PtO2 is stable down to a temperature of 350 – 450˚C. Below this temperature it decomposes into elemental platinum and oxygen.

Such a decomposition of PtO2 is also responsible for the formation of cauliflower-like shaped growths on the catalyst surface.5 If agglomerates grow far enough along the surface, they can come into contact with the relatively cold process gas. Here, the platinum from the decomposition of the PtO2 will condense on these growths, forming the cauliflower-like structure as a result.

The growths are therefore nothing more than an expression for the temperature difference between the catalyst surface and this

surrounding process gas temperature. The size, number and formation of these structures decrease with increasing process gas temperature and correspondingly increasing gauze position in the catalyst package.

Figure 6 illustrates this relationship between the position of the gauze layer in the catalyst package and the resulting surface morphology for increasing gauze layer positions in the flow direction of the process gas flow.

The precious metal losses are mainly on the part of platinum, since rhodium does not form any volatile oxides and its losses are primarily of mechanical origin. The resulting enrichment of rhodium on the catalyst surface leads to a decreasing NO selectivity and an increase in N2 formation. In general, this process is also referred to as the ageing of the catalyst.

While its NO selectivity increases at the beginning of the use of the catalyst due to the recrystallisation of its surface, it decreases continuously with the onset of ageing. As a result of the increasing formation of N2, more reaction heat is released which, in a temperature-controlled regime, leads to a reduction in the ammonia ratio in the mixed gas. This in turn has a negative effect on the NH3 mass transfer rate.

With regard to rhodium enrichment, two contrary approaches are generally followed. The first approach pursues the use of a small ratio of rhodium in the starting alloy, which delays the enrichment of rhodium. Although this guarantees a relatively high NO selectivity over a longer period of time, this is at the expense of precious metal losses, which are comparably high. In the second approach, a relatively high proportion of rhodium is used in the alloy. Rhodium stabilises platinum in the alloy and the precious metal losses are comparatively low, but at the expense of the activity and NO selectivity of the catalyst.

Regardless of this knowledge, the main driver for the choice of catalyst alloy is inevitably the relative cost of the precious metals.

Summary From an economic point of view, the dilemma of the catalyst for ammonia oxidation is its binding of precious metals and the precious metal losses that occur during operation. On the scientific side, the dilemma lies in the one-sided focus on the modelling of the reaction and its kinetics, the findings of which cannot be implemented in an improved catalyst design.

Only a holistic approach – which includes the mass transfer limitation as the slowest step in the course of the reaction, its surface kinetics and the relationship with the ageing of the catalyst – creates a sound basis on which a target-oriented development of catalyst design and individually adapted catalyst systems to the specific reaction environment can be implemented successfully.

Bibliography 1.

2.

3.

4.

Figure 6. Faster electron microscopic images of the surface of used catalyst gauzes: top layer, layer from the middle part and bottom layer of the pack.

48 | WORLD FERTILIZER | SEPTEMBER 2021

5.

OSTWALD, W., ‘Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides’, GB190200698A (20 March 1902).

GUERLET, J., and LAMBERT, C., ‘Wires incorporating a helical component, assemblies thereof, and use of said assemblies as catalyzer and/or to recover precious metals’ (23 December 1997), US 5 699 680.

DU CHATELIER, L., ‘Optimizing catalyst pack design for ammonia oxidation’, Nitrogen & Methanol, No. 239 (May/June 1999).

KRAEHNERT, R., ‘Ammonia Oxidation over Polycrystalline Platinum: Surface Morphology and Kinetics at Atmospheric Pressure’, (Doctoral thesis), Technical University of Berlin, Doktor der Ingenieurwissenschaften (2005).

HANNEVOLD, L., NILSEN, O., KJEKSHUS, A., and FJELLVAG, H., ‘Chemical vapor transport of platinum and rhodium with oxygen as transport agent’, Journal of Crystal Growth, Vol. 279, No. 1 – 2 (2005), pp. 206 – 212.

Chris Brown, K.A. Rasmussen, Norway, David Horbury, URSG, Europe, and Michael Schriner, URSG, USA, examine the available methods for precious metal recovery and recycling in nitric acid plants.

P

recious metals, by their nature, are rare and there are a limited number of accessible areas around the world where they can be viably extracted. Precious metals are important for various reasons, including their array of special properties essential for many industrial processes, emissions reduction catalysts, bullion, investments and high-value jewellery.

The properties of precious metals allow them to be used as inert, long lasting materials, often in harsh industrial

environments as well as precision measuring instruments, sensors and high-tech catalysts.

Over recent years, supply and demand dynamics have highlighted the sensitivity of precious metals, reflected most noticeably in their market pricing. The importance of recycling as a prerequisite for securing raw materials has always been a strong part of working with precious metals, and events in recent years have reiterated the importance now and for the future from around the world.

49

Recycling

Figure 1. Chart highlighting the recent divergence of platinum versus palladium market prices.

Figure 2. Chart indicating the recent rhodium market price volatility.

The recycling journey for precious metals after they have been mined encompasses all the key aspects of good recycling practices. The use of precious metals catalysts in the production of nitrogen-based fertilizers via nitric acid production (Ostwald process) is a prime example of where full-circle recycling can be applied to precious metals, reducing the quantity of new metals required from mines around the world.

Precious metal gauzes are high-efficiency catalysts used to ensure the efficient running of the industrial plants in which they are installed, enabling companies to minimise the use of ammonia, a valuable ingredient in the production of nitrogen-based fertilizers. During the production process, precious metals volatilise (evaporate) from the gauzes and can be deposited through the heat-train and equipment in the plant.

There are a number of approaches to recovering the deposited precious metals, and innovations have enabled safer, sustainable and more environmentally-friendly processes to be used.

Once materials have been collected from the plants, they need to be transported to a precious metals refinery, fulfilling any appropriate legislation where the next stages of recycling begin. In order to refine, separate and purify the precious metals from the recovered materials, a number of technology steps are needed.

Once the processes are complete, elemental precious metals can be used to create the pure metal alloys needed to manufacture new gauzes and start the cycle again.

Optimising collection methods for precious metals

Figure 3. A precious metal catalyst gauze.

Figure 4. Precious metal recycling.

50 | WORLD FERTILIZER | SEPTEMBER 2021

Nitric acid plants operate with different ammonia oxidation pressures, temperatures and nitrogen loading, depending on the type, size and production rate of the plant. All of these variables affect the quantity of precious metals evaporating from the gauzes during plant operation, and optimising the recovery of these metals is therefore specific to the plant’s operating conditions.

Platinum can be recovered using palladium gauzes below the catalyst as part of the platinum group metals system, with the advantage that captured platinum can be recycled regularly and the quantity of platinum progressing downstream into the heat-train of the plant is therefore reduced. In plants where getters are less frequently used, other technologies exist which – for high-pressure/high-loading plants – may include the use of hot-gas filters to collect precious metals lost from the catalyst gauzes during operation and also provide a method of regularly recovering precious metals.

Some precious metals are known to continue further downstream and recoveries can be achieved using in-line liquid filter systems. However, over time precious metals can also build up in storage tanks or other vessels in the plant. Recovery and recycling of precious metals from storage tanks requires careful planning to ensure the environment of the tank is carefully controlled to allow tasks to be executed safely using appropriate methods and safety equipment.

There are a variety of designs of storage tank and, over time, some tanks can accumulate good quantities of precious metals which can be a valuable source for recycling.

Plant cleaning operations – what, why and how often Traditionally, plant cleaning operations focused primarily on precious metals recovery from the heat-train equipment and were planned relatively infrequently. However, with the focus of recycling and sustainability at the forefront of business culture, it is important to consider the other effects of plant cleaning and how the results are achieved.

By cleaning the plant, precious metals are recovered from the pipes and exposed surfaces of heat exchangers, which can allow the plant to increase its thermal efficiency.

Thermal efficiency is important because not only does it help capture energy; in certain circumstances it may prolong the life of equipment or potentially reduce the quantity or frequency of tube failures in exchangers due to operational temperatures. This is an area which would need careful analysis, and should include a review of heat generation and thermal efficiency of heat exchangers that considers design capacities and production outputs to ensure the heat exchangers are operating effectively. An interesting area for discussion is whether plants should be cleaned for either recovery of precious metals or for thermal efficiency, or for both.

Further to onsite plant cleaning and precious metals recovery operations, end of life heat-train equipment can yield significant quantities of precious metals by destructive recovery processes, beneficial for recycling. In addition, steels are also recycled, yielding value to clients as well as maximising the recycling of materials.

A key component of a nitric acid plant is the absorber, which dissolves the nitrogen dioxide (NO2) gas with water to produce nitric acid of various strengths, depending on the plant and downstream requirements. It is possible to recover precious metals from the absorber, although quantities and accessibility are plant-dependent, and it would be recommended to review this as part of a plant overhaul where other precious metals recovery activities are taking place.

Maintaining the good condition of absorption columns can achieve a number of positive results, which include improved production volumes but also reduced emissions. Aside from the engineering aspects of maintaining the absorber, benefits can be achieved by taking actions to ensure good water flow is achieved; significant improvements are possible, especially in warmer conditions.

In addition to the cost of precious metals and the available benefits of recycling, as we look to a future where the cost of energy and emissions may increase, the topic of maximising energy efficiency is likely to be a strong consideration along with available

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Figure 5. Precious metal laboratory analysis. methods to sustainably achieve the desired results, whilst minimising risks to the plant, personnel, equipment and the environment.

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best practice metals management strategies should be employed at all times.

Best practice strategies for precious metal management apply in a wide range of scenarios, for example: Handling and packaging during the removal of gauzes and hot-gas filters.

End-of-life equipment, during removal from plant, transport, dismantling and processing.

Handling and refining operations, including batch separation and traceability at the precious metals refinery.

To ensure metals are assayed accurately and maximum recovery is achieved, materials should be evaluated prior to the refining process and treated with great care and attention.

The results achieved by planning, developing and integrating best practice strategies can make a valuable difference to the recovery of precious metals, increasing the proportion of recycled metals used in manufacturing and working towards achieving the long-term sustainability goals of the industry.

Mee t us at SFP T Fo rum 20 23 S ept .

A WELCOME ADAPTATION Gordon Woolf and Jose R. Ferrer, Tecnicas Reunidas Group, Spain, consider how the design of heat exchangers for use in nitric acid units can be adapted to realise energy savings.

I

t is well known that nitric acid plants are powered by means of ammonia combustion with nitric acid production.

The self-generated power of the typical dual nitric acid plant reduces the overall energy consumption. However, with increasing focus on the worldwide energy transition, maximising the utilisation of energy resources other than fossils fuels is taking on even greater relevance for plant owners. The perceived trend in the coming decade is that there will be an increase in ammonia production from hydrogen coming from water electrolysis – using zero carbon electricity from renewable sources, such as solar panels and wind turbines, or even from natural gas with full carbon capture. The energy produced from this ammonia in a nitric acid plant would be sustainable: a welcome adaptation for future generations.

53

consequently, substantial operational cost savings.

This needs to be balanced whilst considering a unit’s CAPEX, especially in the low-pressure section which operates at approximately 4 barg and normally features equipment with a larger diameter to achieve the pressure requirements.

Here, any design that helps to minimise the size of the equipment will significantly influence the overall investment cost.

One of the company’s heat exchanger designs is the DDS exchanger (Figures 2 and 3), which was originally developed by Tecnicas Reunidas’ heat transfer division for nitric acid exchanger trains (in collaboration with ESPINDESA) and power generation applications, specifically for nuclear power plant safety cooling systems.

Its key characteristic is the achievement of Figure 1. Simplified process flow diagram of a nitric acid dual pressure unit. low shell side pressure drop for high flowrates, whilst maintaining a shell side heat exchanger coefficient that enhances its performance and heat transfer.

Even though the exchanger’s design concept is primarily intended for applications in new build plants, it is also particularly of use during revampings or re-rating applications.

In both cases, the design can be tailored to meet the specific needs of various processes and plants. Additionally, the mechanical design is adaptable to and compliant with all major international codes, such as TEMA, ASME, GOST and PED. The system is a variation of the TEMA triple segmented baffle, in which – due to the special DDS baffle configuration – the main process stream crossing the shell is divided into several streams in parallel, with the following advantages: The cross flow velocity of the shell side fluid through the bundle is reduced, as is the length of each shellpass between baffles. Consequently, the pressure drop can be up to 30 times less when compared against a conventional single segmental baffle for the same baffle distance.

The design minimises vibration problems, as the distance between baffles can be reduced while maintaining low-pressure drop. This minimises the tube’s unsupported length or span, contributing to a design that is robust in the face of fluid-induced vibration.

The configuration combines cross flow (perpendicular to the bundle) with longitudinal flow (parallel to the tubes), both of which contribute to and enhance heat transfer coefficient.

The design permits the development of very compact designs, while maintaining acceptable pressure drop limits.

Figure 2. Vertical DDS unit in nitric acid plant.

It is therefore well suited for gas systems with high gas flows and liquid systems with large volumetric flows. In Nitric acid unit heat exchangers both instances absorbed power (for compressors and/or Looking at a typical dual process nitric acid unit designed by pumps) can be reduced, improving operational cost.

ESPINDESA (the branch of Tecnicas Reunidas specialising in Design constraints fertilizer technologies) approximately 30% of the main When considering design constraints for a new plant, equipment present in the unit are process heat exchangers low-pressure drop on its own can be resolved in a number of (Figure 1). With regards to energy, for a typical dual nitric acid ways by an able designer. Increasing the size of the equipment process where the total pressure drop in the system is in the – i.e. a larger tube pitch, tube outer diameter (OD) or larger range of 1.5 – 1.7 bar (depending on the absorption tower shell diameter – is the most common solution for standard design), approximately half of the overall unit pressure drop is exchangers. The key is to find the right balance between the generated in the heat exchangers. It is therefore obvious that constraints of pressure drop, exchanger duty and cost. any reduction in pressure drop through those heat exchangers, However, a larger diameter leads to large shells and increased via an optimised design, will translate into energy savings and, 54 | WORLD FERTILIZER | SEPTEMBER 2021

Your process licensor counterpart for nitric acid, ammonium nitrate and fertilizer technologies

Process Technology Licensor

Arapiles, 13. Madrid 28015 - Spain Tel.: (34) 914 483 100 Fax: (34) 914 480 456 www.espindesa.es

Engineering and Design

EPC / EPCM Projects

Commissioning & Start-Up Assistance

Plant Operation Assistance

tubesheet thickness, while an increased number of tubes (yielding a higher exchange area) leads to a larger installed weight; together, these all lead to a higher investment cost.

As part of the plant design constraints, the particularities of the nitric acid process must also be taken into account, and specifically the acidic condensation phenomenon that occurs during normal operation and particularly during start-up and shutdown procedures. The acceptable temperature profiles and manufacturing material selections are limited and metallurgical experience is critical for nitric acid units. During material selection, an equilibrium is sought for each heat exchanger – based on their individual or specific service within the whole system – by using different specialised grades of steel, titanium and/or zirconium materials, depending on the specific temperature, pressure and acidic conditions.

It is also critical to study different exchanger configurations to minimise constraints imposed by piping design, and consider different options for heat exchangers in vertical-horizontal or interconnection between heat exchangers.

Therefore, as pressure drop is often a key design factor in industrial plants, the ability to design for lower pressure loss – without necessarily increasing length and diameter – provides clear cost benefits. This is particularly of interest for gas applications, owing to compression costs. However, there are other advantages that can be obtained, depending on the configuration required for the client’s operation.

Reduced operating costs By utilising a heat exchanger design that features lower pressure losses, the power required to move fluids through the system is thereby reduced. The pump/compressor will therefore draw lower amperages, resulting in lower operational expenditure. This technology is highly recommended in gas plants for reducing compressor size.

Saving space Implementing a heat exchanger design with the same pressure loss as an existing system, but with smaller exterior dimensions, yields an overall reduction in outer shell diameter and thickness. This can be of benefit in an existing

plant, where conforming to space restrictions when substituting an exchanger unit is critical and the reduced envelope and weight can make the installation process easier without sacrificing process capacity. Other potential benefits might include improved access or maintenance space in older congested plants, or a reduction in installed weight in existing structures where weight limitations need to be observed.

Increased capacity By producing a heat exchanger design with the same exterior dimensions, yet capable of higher flow rates for a similar pressure loss, an increased capacity is achieved. This is especially advantageous for revamped or upgraded plants where power consumption, space or other existing limitations need to be considered, such as an existing nuclear power plant where key exchangers are the emergency cooling systems. These units are installed in areas which – due to radiation protection, seismic and other specific conditions of such plants – are basically concrete bunkers, where the option to expand or modify the available space is out of the question.

When plants of this type undergo a re-rating to increase their capacity, the exchangers can be required to process larger flows of cooling water; thus a solution that meets the new capacity with the existing or near to existing dimensions is required.

The company’s DDS baffle arrangement is specifically developed to permit a lower pressure resistance for large flowrates. This design is based on fundamental engineering principles and complies with the code requirements for heat exchangers. The calculation formulas, together with design methodology, are incorporated into proprietary software that has been developed and refined – based on empirical data gathered over more than 30 years – to achieve a repeatable, fully validated design method that has been proven in installations worldwide.

The low-pressure DDS baffle arrangement is, as mentioned, essentially a multi-segmental baffle and a variation on existing TEMA standards. It is, therefore, sufficiently similar to standard single or double segmental designs that it benefits from well-proven fabrication and installation procedures, requiring no additional or specialist tooling (unlike other high flow baffle technologies).

Consequently, the equipment can be fabricated at standard workshop rates almost anywhere in the world.

Conclusion

Figure 3. DDS exchanger during SS hydrotest.

56 | WORLD FERTILIZER | SEPTEMBER 2021

Whether the objective is to optimise the initial design of a new plant from the outset or upgrade an existing plant within tight constraints, both investment and energy cost savings are always major factors. The ability to balance the right combination of these costs versus other key factors – such as space, weight, capacity and so on – is therefore highly beneficial to the end user, as is selecting the ideal combination that best adapts to the application and unique conditions of the final plant location.

E F F Y I C G I R E E N N T E Nelson Clark, Breno Avancini and Vitor Sturm, Clark Solutions, Brazil, examine a new technology designed to safely cool strong sulfuric acid and reduce heat loss.

SULFURIC Acid Cooling S

ulfuric acid plants must utilise acid cooling and energy recovery effectively.

Strong acid cooling, paired with energy recovery, is not yet a widely accepted practice, though interest has been growing since the 1980s. There have been some proven technologies employed, but the associated risks – hydrogen corrosion, heat generation or high temperatures – have

limited their acceptance and spread. To eliminate these risks, Clark Solutions has developed the SAFEHR® heat recovery system for use in 150 tpd acid plants; it can replace conventional heat exchanger systems, providing protection against corrosion and hydrogen incidents while recovering energy.

The system has been supplied to a new plant being constructed that will process 3200 tpd.

57

The subsequent development of the SAFEHX® heat exchanger system resulted from the company’s interest in enhancing process capacity and efficiency through cost-effective approaches. This article discusses a novel

Figure 1. Basic SAFEHX operating scheme.

technology that switches from sensible heat to latent heat while merging both heat exchangers into one. In addition to increasing cost-effectiveness, SAFEHX has shown a number of benefits. As well as sulfuric acid plants, this technology can be used in other industries due to its innate temperature control and strong resistance to fouling. A pressure dominant system results in this temperature control, since the shell buffer fluid operates in a boiling-condensing regime. This means that cold fluid will not exceed the set buffer temperature.

SAFEHR has a wide range of potential applications (boiler feed water [BFW] heating, steam production, service water heating, conventional cooling, etc.) while SAFEHX offers a compact, precise and robust approach to heat transfer. In this heat exchanger design, an inert fluid is enclosed in the middle of a three-fluid heat exchanger. Bottom tubes carry hot fluid while top tubes carry cold fluid, with buffer inert fluid constantly boiling and condensing to transfer heat between them. Shell side inert fluids serve as temperature buffers, reducing the potential risks of top and bottom fluid mixing.

In this article, prototype results are shown that demonstrate a safe and cost-effective technology. The benefits of SAFEHX are applicable to virtually any heat exchanger system where corrosion hazards, mixture risks, fouling, process liquid loss (or contamination) and temperature control are key issues.

Design

Figure 2. Conventional acid cooling (rejecting heat to cooling water).

Figure 3. SAFEHX acid cooling (heat can be transferred to cooling water, BFW or elsewhere).

SAFEHX uses a mutually inert buffer fluid as part of the cooling of hot acid or another fluid, inside a three-fluid heat exchanger.

A shell and multiple bundles of tubes make up the design. In contrast to the upper tubes – which reclaim heat and condense the buffer fluid – the bottom, submerged tubes provide heat to boil the buffer fluid.

The boiling-condensing regime generates buffer fluid flow within the shell. Utilising a compact device based on latent heat, it maximises heat transfer safety and reliability, minimises damage due to hot and cold fluid mixing and eliminates hydrogen production and accumulation in the event of a leak.

Figure 1 illustrates a basic SAFEHX operating scheme.

In this system, an inert fluid is used as a thermal buffer inside a single heat exchanger. Sulfuric acid plants cool hot acid in the bottom tubes by boiling an inert fluid, the vapours of which are then condensed when BFW or another fluid is heated.

Tube bundles do not require attention when it comes to flow direction, so connection and piping interferences are reduced. By eliminating shell baffles, the equipment is easier to manufacture and maintain as well as being cost-effective.

SAFEHX can be used in a conventional acid cooling system (Figure 2) or be attached to a heat recovery tower as a safety and reliability aid.

Figure 3 shows how the technology would be integrated within the absorption tower; almost no change is required to install the equipment.

Inert fluid Figure 4. Immiscibility and inert aspect (left: sulfuric acid and fluid; middle: fluid and water; right: sulfuric acid, fluid and water).

58 | WORLD FERTILIZER | SEPTEMBER 2021

Clark Solutions has performed extensive testing and evaluation of direct cooling of sulfuric acid by mixing with inert fluids

since the early 2000s (Figure 4). A few fluid families with the proper properties were identified from this research: Stability: fluid can withstand processing temperature and pressures.

Inertness: fluid must be inert in both hot and cold fluids, e.g. acid and water.

Flammability: fluid had to be non-flammable.

Toxicity: entirely non-toxic.

Corrosivity: fluid cannot be corrosive to steel and other alloys.

Safe to handle: minimal or no handling or special storage Figure 5. 316L plate in acid/inert fluid system (left); 316L plate in acid/water system requirements. (right) – note that a hole was used to mix fluids and represent a mild leak in initial Density: fluid density that is better in ambient temperature. between water and acid densities; in Table 1. Sulfuric acid corrosion progression on leaking the event of a leakage of both water and acid, the fluid hole would be between both and would minimise the contact Fluid pair Conditions Results of the fluids, preventing heat generation and weak concentration.

Water 12 hours duration Corrosion rate: 98% sulfuric acid Start at ambient 145 mm/yr Heat transfer: good heat transfer properties.

The direct contact approach failed; the amount of fluid required would be substantially higher than desirable, the fluid’s final temperature would be much lower than the acid temperatures – impeding any heat recovery – and the potential loss of fluid due to acid carryover on the acid stream could be costly.

The direct cooling approach proved to be unattractive from a commercial standpoint, but the inert fluid was deemed to be suitable for a trim cooling system, which led to the development of SAFEHR. Similar characteristics were relevant for SAFEHX, but different vapour pressures were needed. However, different vapour pressures would simply require operating with different pressures. The benefit of effectively controlling temperature was evident from this.

A cooling process may utilise different buffer fluids that interact better with hot or cold fluids specific to that process.

Safety and hydrogen incidents When an acid cooling system is used in a plant without a recovery system, the flow of acid through the acid coolers is accomplished by a high acid pressure over the cooling water pressure. If an acid leak occurs, it will usually be detected by measuring the pH of the contaminated water downstream from coolers instead of dilute acids forming inside the plant equipment. Heat transfer equipment and cooling water systems will be severely damaged by leaks, and any subsequent plant shutdown may last between days and weeks depending on the leak extent.

In a heat recovery system, acid is always under a lower pressure than boiling water, which is under a pressure of up to 10 bar, according to steam quality requirements. The large pressure difference between acid and water is a stronger driving force than the pressure difference in a conventional system. As a result, leaks resulting from corrosion, poor materials and workmanship quality, or even stress issues, will occur at flow rates that are substantially higher than those

temperature 316L stainless steel apparatus CS270 inert fluid 72 hours duration 98% sulfuric acid Start at ambient temperature 316L stainless steel apparatus

Mass lost: 0.1% per hour Corrosion rate: nil Mass lost: nil

experienced in a conventional system. The results will be an increase in acid dilution and temperature rise and, ultimately, critical failure.

Aside from hot water, the leak area receives hot acid at temperatures in excess of 180˚C, which is mixed with water to produce diluted acid at even higher temperatures. The local environment is extremely aggressive.

A study by the company of dilution-induced corrosion progression demonstrated that a 1 mm hole in an acid/water system can corrode 150 mm per year at an ambient temperature and pressure, with the hole diameter growing by as much as 0.1% per hour under these mild conditions (Figure 5).

Using an inert fluid greatly reduces corrosion rates, even if a leak occurs. Table 1 shows the results measured from these corrosion tests.

When a water leak into the acid occurs in a heat recovery system, the strong acid in the system will be diluted. The corrosion effects will occur inside acid plant equipment and hydrogen will accumulate in high spots. Several hydrogen explosions have been reported by the industry in the recent past.

Fe0(s) → yields Fe2+ (aq.) +2e2H+ (aq.) +2e- → yields H2(g)↑

(1)

A number of actions and measures have been taken to minimise the risk of a disastrous failure in a heat recovery system due to leaks of hot high-pressure water into acid.

SEPTEMBER 2021 | WORLD FERTILIZER | 59

Modern, sophisticated and redundant concentration, corrosion and temperature digital monitoring and control, as well as segregated heat recovery towers, have been employed to reduce the probability of emergency shutdown. Advances in instrumentation have also substantially reduced risks through monitoring and safety measures. However, none of these safety schemes eliminate the root cause of the failure: acid-water contact. SAFEHR and SAFEHX do not allow the mixing of acid and water.

Prototype Using heat and mass transfer models, a rigorous numerical analysis was conducted to size the prototype of SAFEHX. Using the results obtained from this numerical study, Clark Solutions’ engineering team designed a mechanical project, an experimental bench set-up and an automation system to validate the results quantitatively.

Figure 6 shows the experiment bench. It had two pumps, two volumetric flowmeters, five thermistors and three pressure transmitters. Data acquisition and bench control were handled by the PLC and supervisor system. The supervisor system was operated by a computer, which was used for bench control.

Looking at the basic flow diagram of the bench in Figure 7, it can be seen that it has two closed-loop water cycles.

One is called the cold water loop, which has a tank, a pump, a flow control valve, one pressure transducer, two temperature transducers and one air radiator to decrease water temperature. The other is the hot water loop. This other loop also has a tank, a pump, a flow control valve, one pressure transducer and two temperature transducers, but it has a heat resistance system instead of an air radiator to increase water temperature.

Experimental results Figure 6. Physical experimentation bench.

Figure 7. Experimental bench basic flow diagram.

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The company conducted a variety of bench tests to verify that the instrumentation and heat transfer system worked as expected, before performing some prototype runs to verify its efficiency. First, there is the visual evidence of both boiling and condensing processes inside the heat exchanger shell.

Condensation is verified by the dripping that occurs uniformly from the upper tubes in the boiling process.

Boiling is verified by the froth that covers the submerged tubes.

In addition, the buffering effect of inert fluids is important to mention. The temperature control of the hot loop causes oscillations in the temperature of the hot fluid. In Figure 8, it is observable that no similar oscillations are present in the cold loop temperature. This shows the high capacity of damping eventual temperature changes of a hot loop circuit and keeping the heat transfer coefficient steady for a cold loop circuit. For processes that need to maintain heat exchanger performance and offer the safety of not crossing a minimum or maximum temperature, this feature is crucial.

In Figure 8, it can be seen that the buffer liquid maintains a constant temperature as a result of the SAFEHX shell pressure setting point. Despite oscillations in the hot fluid inlet, cold fluid temperatures remain stable. In addition to providing a safe means of heat transfer, the system also provides strong temperature control.

Figure 9 illustrates heat transfer by further demonstrating the dampening effect caused by buffer fluid-controlled temperature.

The system was tested with varying heater output to change the inlet temperature of the hot fluid over time.

After the excess heat was transferred to the buffer fluid, some heat was lost to the surroundings (not a perfectly adiabatic shell, as observed from the gap between lines). In Figure 9, the red lines indicate the transfer of heat from the hot liquid to SAFEHX. But the erratic buffer liquid energy intake from the hot loop did not necessarily cause the cold fluid to behave as erratically; this may seem contradictory at first, but this occurs due to internal energy variations of the buffer liquid that dampen system temperature variations.

The blue lines have plateaus caused by changes in cold liquid flowrates, rather than differences in cold fluid temperatures.

Fouling resistance Shell interiors and tube fins are typically difficult to maintain and clean.

Fouling is an important design and operation consideration in several heat transfer applications, either because of temperature-induced precipitation or a particle-rich feed stream. The buffer liquid in the SAFEHX system has no fouling characteristics, and the shell is a closed circuit; no material can precipitate or get into the shell side and cause fouling.

The finned tubes can also be used to increase area on the shell side, where fouling cannot occur. Fouling can only occur

inside pipes, and cleaning inside pipes is a easy and fast solution for the SAFEHX, even for high fouling systems.

Fouling inside pipes is also mitigated against for applications where denaturation and precipitation concerns are present, as the system exhibits innate high precision temperature control for both buffer and cold fluid as shown above, which reduces precipitation due to process oscillations.

Product loss and contamination With a closed buffer fluid on the shell side, any leaks of process fluid will not be carried away. In conventional heat exchanger systems, a leak will cause the fluid with more pressure to leak into the current with lower pressure. Wide systems may not have the same capability of retrieving specialty fluids.

Although leaks are unlikely – considering the inert fluid properties – SAFEHX detects variations in the shell side to determine if any leaks have occurred. If it does happen, the closed buffer circuit can be carefully drained to remove any process fluids introduced into the shell.

The risk of contamination is even lower, since this would require a leak in both the submerged and upper tubes as well as an existing gradient in pressure from one tube to the other.

Contaminating cold fluid with hot fluid, and vice-versa, is near impossible.

Materials of construction The materials of construction for SAFEHX pipes depend on hot and cold fluids within tubes, but a carbon steel shell can

always be employed, as the buffer fluid is inert. Because the system works under inert conditions, the shell can be made of common carbon steel alloys since it only contacts the inert fluid.

Despite using a carbon steel shell, piping must be compatible with process fluids. To be able to supply its

technology for a wide variety of process, Clark Solutions can manufacture the SAFEHX piping with almost any material.

Several materials have proven effective for handling strong acids at normal absorption temperatures. High silicon austenitic stainless steels, such as the company’s CSXTM, 310 Stainless Steel, Alloy 20 and others, have all proved resistant to different ranges of acid conditions.

Water tubes can be made of carbon steel or, if necessary, stainless steel.

While proper material selection is very important to minimise failure risks, corrosion is a matter of both time and operational control – leaks will eventually happen. In the event of a leak, sensors can identify any liquid entering the shell side to properly drain in a timely manner, if necessary. Even when corrosive liquids enter the shell side, the absence of oxygen inside will hinder oxidation and corrosion. SAFEHX systems can be constructed in the materials most suitable to specific operating conditions.

Performance

Figure 8. SAFEHX experimental data (red line – hot inlet temperature; blue line – hot outlet temperature; black line – buffer liquid temperature; orange line – cold liquid outlet temperature; green line – cold liquid inlet temperature).

Figure 9. Heat transfer for hot and cold fluid (red line – hot fluid heat transfer to the system; blue line – cold fluid heat transfer from the system).

Table 2. SAFEHX preliminary test results

Conclusion

Fluid pair

Conditions

Results

Cold side heat transfer coefficient

3 hours duration Inert fluid condensation to warm cold water

500 to 1000 W/(m2.K)

Hot side heat transfer coefficient

3 hours duration Inert fluid evaporation to cool hot water

500 to 1000 W/(m2.K)

Overall heat transfer 3 hours duration coefficient Continuous evaporation and condensation of buffer fluid

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Using data from the sensors, it was possible to determine that the heat exchanger, operating at a volumetric flowrate of approximately 1000 – 1200 litres/hr in the hot water loop, is capable of transferring approximately 4 kW. Figures 8 and 9 show temperature and heat transfer results. In the cold water loop, approximately 3 kW of heat was retrieved from the exchanger at a flow rate of 1000 – 1200 litres/hr. As a result of the absence of insulation, shell convection heat loss contributed to the difference between those two heat rates. Hot water loop temperature oscillations are caused by the on-off control of the hot water loop. In Table 2, the overall heat transfer coefficients calculated from prototype measured parameters are close to those calculated from numerical models. Further prototype improvements and different sets of operation conditions were already issued and, when just installing shell insulation, an increase of at least 15% on the overall heat transfer coefficient is expected. As a starting point of the technology, there is already a prototype with heat transfer performance for liquids compatible with commercial products and with all the safety and process control improvements already discussed.

250 to 500 W/(m2.K)

SAFEHX adds a substantial safety feature to existing or new sulfuric acid plants. The prototype achieved its original objective of safe heat transfer in a compact device. But it also benefits heat transfer operations with high fouling rates, operations where cold liquid temperature control is important, and for cases where process liquid loss is costly. The prototype is at an advanced stage and a commercial version of the product should be available in 2021.

An unfinished chapter

Philip A. Henry, E2G | The Equity Engineering Group, Inc., USA, examines ongoing efforts to update and improve the current industry codes and standards on pressure relief valve instability. he increased emphasis on safety in the ammonia and fertilizer industries has produced greater sophistication in the analysis of overpressure concerns. One area where this is particularly evident is in the study of the stability of pressure relief valves (PRVs) during activation. This affects all industries where PRVs are used to provide overpressure protection, including the ammonia and fertilizer industries.

The current guidance published in industry codes and standards is not sufficient to ensure that any company is

T

fully protected from PRV instability issues. Limiting the inlet pressure losses to 3% of set pressure is a requirement laid out in ASME Section VIII, Appendix M that relates to preventing valve instability during an overpressure event.1 This 3% rule also appears in other industry standards, such as API 520 Part II and ISO 4126-9.2,3 However, the former states that limiting the inlet pressure drop to a specific value may not be the primary factor that guarantees PRV stability. PRV stability is an evolving issue, and there are additional factors that impact whether the valve operates in a stable manner.

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Additional research is needed to fully understand all the parameters that affect PRV stability. Current flow testing programmes being conducted by the American Petroleum Institute (API) and the European Center of Safety Excellence (CSE) recognise the need to better understand the phenomena of PRV instability, with the goal of improving the guidance given to the PRV user community.

3% inlet pressure drop rule The basic premise of the 3% rule is that during relief events, if the inlet pressure to the PRV fell below the

blowdown pressure (or closing pressure) of the PRV due to high inlet pressure loss, the valve would close. Once the valve closes, static pressure at the inlet again builds rapidly and the valve re-opens. This repeats itself, often at high frequencies, and can produce chattering. Chattering creates serious vibrational disturbances, and can result in damage to the relief valve parts as well as possible failures of piping connections and loss of containment. For many years, limiting the inlet pressure loss to 3% of the set pressure was the only Recognized and Generally Accepted Good Engineering Practices (RAGAGEPs) guidance provided for designers to mitigate concerns regarding valve instability.

Experience and testing have shown that some PRV installations with inlet pressure losses greater than 3% have relieved and operated in a stable manner. Conversely, testing has shown that some PRVs may become unstable with inlet pressure drops that are less than 3% of set pressure. A joint industry project conducted by API demonstrated that there was no apparent correlation between valve stability and inlet pressure drop.4 Research is ongoing.

API historical guidance

Figure 1. Crosby JOS spring-loaded PRV.

Figure 2. PRV installation showing 3% inlet loss criteria.

64 | WORLD FERTILIZER | SEPTEMBER 2021

API is the leader in the development of petroleum and petrochemical equipment engineering standards that provide proven, sound engineering and operating practices and safe, interchangeable equipment and materials. Many of its standards have been incorporated into US state and federal regulations and adopted by the International Organization for Standardization (ISO) for worldwide acceptance. The API Subcommittee on Pressure Relieving Systems (SPRS) produces several standards related to pressure relief systems, and these are used worldwide as the de facto RAGAGEPs for PRV design and installation. The API standard providing installation requirements for pressure relief devices is API 520 Part II. This standard has gone through many changes over the years, but only recently has the guidance related to PRV instability been appended to address some of the recent developments in understanding the phenomena.

From 1963 until March 2015, the only guidance provided to industry through the API SPRS standards concerning PRV instability was the 3% rule related to excessive pressure drop in the inlet piping to the PRV. In 1994, the fourth edition of API 520 Part II was published, and it permitted owner-users to increase the allowable inlet pressure drop above the 3% criteria, provided an engineering analysis was completed.

Modern relief valves provided by valve manufacturers have blowdowns in the range of 7 to 10% of set pressure, and the API SPRS determined that inlet losses up to 5% still gave a suitable safety margin between the inlet pressure loss and blowdown. Many operating companies were already accepting inlet losses above the 3% guidance for existing installations without experiencing any instability problems. Although no additional guidance was given as to what the engineering analysis should entail, conventional wisdom was that the engineer should work with the valve manufacturer to understand the blowdown characteristics

Relax, of the installed PRV, and assess the potential for valve instability with increased inlet losses.

it’s Venti.

API PERF results In 1999, API initiated a joint industry project to further investigate PRV instability. This effort was conducted under the Petroleum Environmental Research Forum (PERF) and is generally referred to as PERF I. The PERF I project was completed in 2013 and the results from this effort were mixed. Testing showed that, in many cases, PRVs did not chatter when inlet losses exceeded 3% of set pressure, while in some tests PRVs chattered when inlet pressure loss was less than 3%. Some PRVs even became unstable during closing.

This was the first time that flow testing provided any indication that PRVs can become unstable upon closing.

The industry has only concentrated on instability during opening because, once the valve is fully open, inlet losses become less of a concern, since the valve now has much more overpressure at the inlet to keep it open. This further supports the conclusion that the whole concept of valve instability is ever evolving.

One thing that became clear from the PERF I testing programme was that there was no real correlation between inlet pressure drop and valve instability. As stated in API 520 Part II: “Limited testing has shown that in many cases PRVs did not chatter when inlet losses exceeded 3% of set pressure while in some tests PRVs chattered when inlet pressure loss were less than 3%.”

API current guidance Results of the PERF I research and more recent analytical approaches were used to enhance the current edition of API 520 Part II, which was published in 2020.

In addition to providing more details on the engineering analysis required to exceed the 3% rule, additional information was given pertaining to the current state of the technology available for predicting PRV instability.

API 520 Part II describes several phenomena that can contribute to valve instability, including excessive PRV inlet pressure loss, excessive built-up back pressure, acoustic interaction, retrograde condensation, improper valve selection and oversized PRVs. It also makes it clear that there may be other phenomena not included in API 520 Part II that can lead to valve instability: “Research and experience show that PRV instability is complex and cannot be attributed to just one issue...There may be other phenomena that can lead to valve instability”.2 API 520 Part II now has guidance on performing an engineering analysis to check for PRV stability in applications where the 3% pressure drop criterion is not met. As conventional wisdom moves away from the 3% inlet pressure drop criterion, the engineering analysis should be performed for all new and existing installations.

API suggests that the engineering analysis should include the following tasks: Verify that the applicable flowrate is used in inlet and outlet pressure drop calculations.

Verify valve sizing using corrected capacity based on inlet and outlet pressure drop.

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Review available history to identify evidence of chatter.

Perform a force balance assessment.

Perform checks for acoustic interaction.

Overall, the goal of the API engineering analysis is to validate that the relief system installation has adequate relieving capacity and that it conforms to current design practice regarding relief valve stability.

A force balance method has also been developed that appears to have much more promise at predicting PRV instability than the 3% inlet loss rule. Whereas the 3% rule looks at the steady state conditions of flow through a PRV, the force balance method includes the dynamic effects of the pressure wave as it travels back and forth from the PRV. This technique was tested against the flow test data collected during the PERF I and found to have very promising results. API is developing a technical publication that will provide designers with example problems that show how to perform these calculations, but this has not been published yet.

The force balance is essentially a comparison between the forces acting to keep the valve open and the forces acting to close the valve. It is dependent on the PRV characteristics (opening time, blowdown, etc.), the speed of sound in the fluid and pressure drops caused by the flowing fluid and pressure waves. It may be used both for conventional and balanced bellows spring-operated PRVs.

The API SPRS is moving away from steady state inlet pressure drop as the major contributor of PRV instability and is focusing more on dynamic effects, such as those addressed with the force balance and acoustic interaction methods. Annex C was added to API 520 Part II to address one form of instability caused by acoustic effects: acoustic interaction. The methodology presented in Annex C compares the acoustic frequency of the inlet piping to the opening and closing times of the PRV to establish a critical line length. Using this method, piping designers can show that the length of the inlet piping does not approach this critical line length, thereby avoiding resonant interaction between the piping and the PRV.

Options to mitigate deficiencies associated with the 3% inlet loss rule API discussed several options in which the relief system designer has to mitigate deficiencies identified where inlet pressure drops are calculated to exceed 3% of set pressure.

The first option to consider would be to complete the engineering analysis as defined in API 520. Making use of the force balance and acoustic interaction checks may eliminate any need to make costly physical modifications to the relief valve installation. Where the PRV has modulation tendencies – such as with modulating pilot-operated relief valves – using the required flow instead of the actual rated capacity of the valve can significantly reduce the calculated inlet pressure drop. Some PRV manufacturers are willing to provide assurance in writing that their spring-loaded PRVs have modulating characteristics, in which case use of the required flow could certainly be justified.

Physical modifications to the pressure relief installation may inevitably be required. Replacing the existing PRV with a modulating pilot, restricting the lift of the PRV or making use of remote sensed pilot valves are relatively simple modifications that can be made to mitigate an inlet pressure drop deficiency. As a last resort, the inlet piping may need to be rerouted or increased in size to mitigate the situation.

Conclusion This is an active area of research, and updates to the guidance given in API 520 are expected to change periodically as more research is completed and knowledge is gained. It is expected that, in the near future, API guidance will continue to move away from an inlet pressure drop criteria towards methods that address the dynamic effects of PRV operation.

References 1.

2.

3.

4.

Figure 3. PRV instability.

66 | WORLD FERTILIZER | SEPTEMBER 2021

American Society of Mechanical Engineers, ‘Boiler and Pressure Vessel Code, Section VIII, Division I, Rules for the Construction of Pressure Vessels’ (2019).

American Petroleum Institute, ‘API Standard 520, Sizing, Selection, and Installation of Pressure-Relieving Devices, Part II – Installation’, 7th edition (2020).

ISO 4126-9, ‘Safety devices for protection against excessive pressure – Part 9: Application and installation of safety devices excluding stand-alone bursting disc safety devices’, DIN Deutsches Institut für Normung e. V. (2008).

ALDEEB, A., DARBY, R., and ARNDT, S., ‘The dynamic response of pressure relief valves in vapor or gas service. Part II: Experimental investigation’, Journal of Loss Prevention in the Process Industries, Vol. 31 (September 2014), pp. 127 – 132.

RELYING ON

Mike Spalding, Reuter-Stokes, USA, outlines the importance of flame detectors in the fertilizer industry.

F

lame detectors, while comparatively small in size in the world of industrial equipment, play a large role behind the scenes in fertilizer production.

These detectors are used to ensure that the steam methane reformers (SMRs) – a key piece of equipment in fertilizer and methanol production – have started up properly. The detectors sense that there is a lack of flame and transmit signals to the burner management system (BMS). The BMS responds to the lack of flame by shutting the fuel valves that feed the burners.

An improper or failed start-up can result in an explosion, as the Yara Tertre plant in St. Ghislain, Belgium, learned in 2009.

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Older equipment and safety considerations During the fertilizer production process, these SMRs produce a combination of hydrogen and carbon monoxide, known together as synthesis gas (or syngas), which is used in the industrial synthesis of ammonia and other chemicals.

This gas is produced by using steam and methane or other hydrocarbons in the presence of a catalyst, at temperatures ranging between 950˚C and 1100˚C. In addition to being the feedstock, natural gas is also used as the heating fuel for the production material in the catalyst tubes.

Using high volumes of flammable materials makes steam methane reforming a high-risk process. Start-up is a critical event and poses the most risks for a fertilizer or methanol plant. If an SMR does not start properly, natural gas that has not been combusted can accumulate in the firebox and explosively ignite.

Outdated safety systems have been used for decades, with many plants incorrectly assuming they are sufficient. Older systems, however, rely on human visual examination and checklists at start-up to bring the SMR up to the minimum operating temperature.

When humans are involved, there are increased risks of unsafe conditions and even explosions.

The aftermath of an SMR explosion

Figure 1. Yara Tertre SMR.

Designed and built in the 1960s, Yara Tertre’s SMR unit featured nine rows of 20 Hamworthy burners per row.

Like many of the SMRs from these early years, the start-up was not automated and relied on human intervention.

Over the years, this method worked. The Yara Tertre SMR consistently started up and ran smoothly. In 2009, however, an explosion rocked the plant, injuring two and destroying the SMR. Investigation revealed that erroneous accumulation of uncombusted gas in the firebox had caused the explosive combustion.

In the aftermath, Yara rebuilt the equipment and implemented an improved safety system, including the installation of flame sensors. Engineers installed 36 of these sensors on the firebox’s arch burners and connected them to the BMS to lessen the human element risk. The flame sensor outputs were used as inputs for the SMR start-up process to ensure safe operation.

Addressing flame detection challenges

Figure 2. Burner top view.

68 | WORLD FERTILIZER | SEPTEMBER 2021

After these important safety upgrades, the Yara Tertre plant went back online in 2011. When plant engineers tested SMR flame sensor positioning, they discovered that the sensors did not consistently detect flames due to the burner configuration. The sensor-position location offered an indirect view of the flame, which delivers lower light intensity.

Engineers reinstalled the sensors in the igniter port, which offers a direct view of the flame. However, during start-up the detectors needed to be removed from the burner to allow an igniter to be inserted in the port. Sensors were then reinstalled after ignition.

The specified flame sensors were not up to this task. The top-fired design of the Yara Tertre SMR houses the arch burners and sensors in a penthouse above the firebox. This location posed temperature issues with the sensors.

Ambient temperatures on the burner fronts could easily exceed 150˚C, a level that is well beyond the temperature capability of most modern flame detectors. The sensors installed during the post-explosion rebuild had a maximum operating temperature of 93˚C. The high temperatures in the penthouse caused these sensors to fail.

The technology was not robust enough to handle the harsh environment in Yara Tertre’s SMR application. As a result, the detectors were bypassed and ignored during start-up. Engineers realised they needed to find a more rugged and reliable sensor.

Technology built for harsh environments Beyond needing a flame detector that could deliver reliable performance in the SMR penthouse, the company knew it also wanted one that was sensitive enough to detect flames with an indirect view. After reviewing the many options on the market, the Reuter-Stokes Flame Tracker Dry 325 (FTD 325), a sensitive UV sensor built for harsh environments, was selected.

A single-piece assembly that is hermetically sealed from end to end, the sensor features a hot end that attaches to the

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Figure 3. FTD 325 hot end connected to the sensor position of the burner.

burner sight tube, a 30 ft mineral-insulated cable and a cool end that is used for amplification and signal conditioning. The hot end has a maximum operating temperature of 325˚C, which reaches well beyond the ambient temperature near the SMR penthouse’s burner fronts.

Yara chose to test one of the FTD 325 sensors on the SMR before committing to a complete installation. Engineers took initial temperature measurements on four burners over a 24-hour period and tracked infrared temperature

High-temperature sensing success

Figure 4. FTD 325 cool ends with excess MI cable in a loop.

Based on this successful, 6-month test with a single FTD 325 sensor, Yara installed 17 additional flame detectors on arch burners. The team placed two sensors on each of the nine burner rows and easily connected the sensors’ hot ends to the burners’ scanner positions. They are not installed in the igniter ports and do not interfere with the ignition process. The output from the cool ends was connected to the BMS. Yara performs several permissive checks during use, including a firebox purge, before any start. The sensors are used during the light-on sequence to confirm one row of burners is lit before lighting the next row. This occurs for each of the rows.

Since the initial installation in 2018, there have been at least 10 starts, and the sensors were used in each start. The sensors have performed well in the penthouse’s harsh environment with zero failures, and now play an important role in making this SMR safe in Yara Tertre’s harsh environment.

Conclusion measurements on these burners near the scanner position on the burner front. Measured temperatures ranged from 108˚C to 159˚C. The plant’s team then installed the sensor on one of the burners in the sensor position. The sensor performed flame supervision using an indirect view for 6 months with ‘very satisfactory’ performance and zero reliability issues. Yara realised the system’s temperature capability was the missing link to installing a successful detection solution in the penthouse.

With start-ups in fertilizer and methanol plants posing clear hazards due to human error or poor sensing conditions, it is important to ensure that flame detectors are designed for extreme environments and have high sensitivity for low-light environments.

High maximum operating temperatures are key for top-fired reformers. Modern sensing technology has eliminated the human element in flame detection and restores confidence in high-risk applications at fertilizer and methanol processing plants.

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BEYOND THE

FENCE LINE Mike Schmidt, Bluefield Process Safety, USA, argues that the industry is responsible for process safety even after chemical products leave the plant, and shares some ideas on how to meet that responsibility.

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here is not much talk about process safety once hazardous chemicals are loaded and shipped. But process safety – the concern with fires, explosions and toxic releases – does not end at the fence line. Wikipedia lists 35 incidents in its article on ammonium nitrate (AN) disasters. Of those, over a third involved the transport of AN; four were explosions of cargo ships loaded with AN; three were explosions of freight trains pulling cars loaded with AN; and six were explosions of trucks hauling loads of AN. They follow a common pattern: an incident – sometimes a wreck, sometimes an onboard failure – ignites a fire. The fire burns and attempts are made to bring the fire under control. Then, without warning, the AN explodes, with dramatic consequences. By way of example, an AN truck explosion on a highway in Arkansas, US, in 2019 was heard as far away as Little Rock, almost 100 miles (160 km) to the north (Figure 1).

Stability of AN AN is not combustible. It is an oxidiser, however, and will accelerate the combustion of flammable and combustible materials. It is stable at ambient temperatures and pressures, and is reported as having very low shock sensitivity as a pure compound. Prior to the 1921 explosion at Oppau in Germany, BASF reportedly used small charges of dynamite to disaggregate solid masses of AN/ammonium sulfate mixtures as many as 20 000 times without incident.

However, AN is sensitised when contaminated with organic materials. In fact, ANFO (the bulk industrial blasting agent) typically consists of a 94% AN/6% fuel oil mixture. Spilled diesel fuel at truck accidents has been suspected as a sensitiser in some cases of AN

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truck explosions. Occasionally, reporters or the public will seize on terms such as ‘fertilizer grade’ versus ‘explosive grade’ to explain different stabilities or energies of explosion between various inventories of AN. The difference is in the porosity of the prill. Agricultural grade is not very porous, so it ships a little denser.

Explosive grade is porous, so it can absorb fuel oil more readily to make ANFO. Without fuel oil, though, explosive grade AN is just AN, and no more or less likely to explode.

The Institute of Makers of Explosives (IME, based in Washington, D.C., US) recommends that trailers be dedicated to hauling AN, and if not, that they be washed out and dried before being used to haul AN. Contaminants from previous loads that the IME particularly warns about include: Animal fats.

Baled rags, burlap or cotton bags.

Charcoal.

Foam rubber.

Hay. Metal powders.

Sawdust.

Straw.

Vegetable oil.

Wood, wood chips or wood shavings.

Other impurities are known as sensitisers for AN, including chlorates, mineral acids and metal sulfides.

That does not mean that uncontaminated AN will not explode. It is the sudden and catastrophic decomposition of AN that results in explosions. The melting point of AN is 169.6˚C (337.3˚F), and uncontaminated AN starts decomposing at 210˚C (410˚F).

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There are two decomposition reactions. The first dominates at lower temperatures, in the range of 200 ~ 300˚C (390 ~ 570˚F): NH4NO3 l N2O + 2 H2O(g) This means that for each volume of AN prill, decomposition produces 780 volumes of gas at standard conditions. At 210˚C, that same volume of AN prill will decompose into 1280 volumes of gas when released to the open atmosphere. On the other hand, if that volume of AN prill were enclosed (say by being encased in a fused mass of AN), the volume would remain constant and the pressure after decomposition would increase to almost 1300 bar (almost 19 000 psi). An explosive release would happen long before reaching that pressure.

Then, at around 300˚C (approximately 570˚F), the reaction shifts:

At this higher temperature and higher amount of gas generated, the decomposition of a volume of AN prill releases 1770 volumes of gas to the open atmosphere. Again, if the volume of AN prill decomposes with no way of venting, it will build to a pressure of almost 1800 bar (almost 26 000 psi).

AN decomposition does not require the presence of air; suppression efforts have to be directed to cooling the mass, not on smothering it.

Truck fires

There are thousands of truck fires each year involving large, over-the-road trucks. Still, the probability of any one truck catching fire is less than 0.1% in any given year. Obviously, the probability of a specific truck catching fire during a specific trip depends on the age of the truck, its maintenance history and the experience and skill of the driver.

When trucks do catch fire, however, trailer fires are invariably caused by one of four reasons (Figure 2).1 The two causes that get the most attention are tyre fires, 2 NH4NO3 l 2 N2 + O2 + 4 H2O(g) because the other causes often set tyres on fire as well, and brake fires, perhaps because they are so difficult to put out.

Tyre fires are typically not the initiating cause of truck fires but are a source of fuel for fires once started by a brake fire or wheel bearing fire.

The normal operating temperature for truck tyres is 40 ~ 65˚C (100 ~ 150˚F). At approximately 120˚C (approximately 250˚F), tyres begin to deteriorate rapidly, which can lead to blowouts. At 260 ~ 290˚C (500 ~ 550˚F), tyre decomposition produces flammable vapours, which can ignite if there is an ignition source. The autoignition temperature for the decomposition products of tyre deterioration is around 450 ~ 480˚C (850 ~ 900˚F).

While brake fires and wheel bearings are most often the ignition sources of tyre fires, it is possible for truck tyres to catch fire even when not started by overheated brakes or bearings. The most likely cause then is a flat tyre. Flat tyres can overheat and catch fire by spinning on the rim, rubbing against Figure 1. The smouldering crater left after an AN truck explosion was 15 ft the other tyre or frame rail or becoming stuck and deep and stretched all the way across the highway (photo by David Archer, dragging on the road.

Arkansas Department of Transportation, used with permission).

The temperature of a tyre fire impinging on a load of AN prill is well above that necessary to cause the material to melt and then to decompose.

Brake fires are the most common cause of truck fires. Most brake fires occur within 30 to 50 miles (50 to 80 km) of the start of a trip. Brakes can catch fire if the driver rides the brakes. This is especially a problem with inexperienced drivers on highways with steep grades. More often, though, brake fires result when the brake drags. A driver will often not notice that brakes are dragging. Eight of the wheels on an 18-wheeler are on the trailer and each has a brake. Hopper trucks that are used to haul AN often have more wheels on the trailer. If the driver feels the drag at all, they may attribute it to a heavier than normal load or to headwinds.

Brett Aquila, a US-based truck driver and blogger, commented that: “...brake fires are Figure 2. The causes of truck trailer fires.

74 | WORLD FERTILIZER | SEPTEMBER 2021

incredibly difficult to put out. The brake shoes and drums can reach very high temperatures and take a long time to cool off!

Once they’ve reached high enough temperatures to ignite the shoes themselves and any grease that may be in the area, the fire continues to re-ignite even after you’ve tried putting it out several times. The fire won’t stay out until the brakes can be cooled down. So, it isn’t uncommon to hear about brake fires that end up burning the trailer to the ground because there wasn’t enough water or fire extinguishers around to put the fire completely out. If you ever happen to see a tractor trailer with a brake fire, make sure the fire department is called. A fire extinguisher will not often be able to put it out.”2

Preventing AN truck explosions Trucks hauling AN do not catch fire because they are hauling AN. They catch fire because they are trucks. Tyre fires and brake fires are not more likely because of the load. If the load is AN, however, the load can explode, something that will not happen if the load is furniture or potatoes.

What can those at the plant do to make shipping their products safer?

Ship in well-maintained trailers When the truck first arrives to pick up a load, it should be examined carefully. Does it appear to be in good repair? Are the tyres properly inflated? Brake fires typically occur within the first hour or two of a trip. Do the brakes smell hot?

Does the lubricant in the hubs smell burnt? Is the trailer clean and dry? While quality issues are typically the only consequence of cross-contamination between loads, cross-contamination with AN can result in more than simply quality issues for the customer – it may result in catastrophic incidents on the road.

Before the truck leaves, another examination should take place. Are valves securely closed? Are caps in place and secure? Are there fire extinguishers on board? The driver is responsible for the load, but a second set of eyes can help.

Confirm the driver’s understanding of the hazards of AN An experienced driver is more likely to handle unusual road conditions with greater skill, but experienced driving does not necessarily translate into experience of hauling AN. The hazards of AN should be explained.

It should be noted that a full truck-load of AN has the explosive energy of approximately 10 t of TNT. The detonation of a truck-load of AN is enough to blow a crater in the ground that stretches across the highway and to shatter windows nearly a third of a mile (half a kilometre) away. The driver should be reminded that fire extinguishers are just for incipient fires and that a tyre or brake fire is hot enough to cause the catastrophic decomposition of AN, whether it is contaminated or not. It should be emphasised that the emergency response guidance is to evacuate over half a mile (a kilometre) at least in all directions when a truck loaded with AN is involved in a fire. It is not reasonable to simply hand a driver a Safety Data Sheet and expect them to make sense of it.

The driver should be asked what they would do in the event of a brake fire, a jackknife or a collision that results in a

leak or spill. If they do not know the answers, they should be helped to figure out the best response.

Ensure that the load is correctly placarded Before a truck leaves a facility, it should be ensured that it is properly placarded. The placards are neither for the facility nor the driver: the placard is for any emergency responder that must react to an incident involving the truck.

Emergency responders are trained to approach truck incidents from upwind and uphill, and then scan the truck for placarding before deciding how to approach the incident. The few seconds they take to do this can make the difference between life and death. All too often, when there are fatalities or other injuries during an incident, it is the first responders that suffer them. There is not much chance of responding correctly if the placards are wrong.

What the future holds In a plant, any safety critical process condition is typically instrumented so that a Safety Instrumented System continuously monitors it and shuts down the process automatically when conditions are unsafe, or alarms to prompt an operator to shut down the process. Increasingly, it is also possible to continuously monitor conditions in a load that is on the road. Fleet tyre pressure monitoring systems allow for both drivers and fleets to stay aware of tyre pressures.

There are also fleet temperature monitoring systems, which are currently used primarily to ensure that food and medicine in cold chain logistics does not spoil. Monitoring the temperature of AN loads, especially directly above the rear axles, could also be useful in preventing future incidents.

Temperature monitoring should also reduce the number of truck fires in general, regardless of the load. It may be time to apply what has been learned about Safety Instrumented Systems in fixed locations to transportation.

Making a difference beyond the fence line now Process safety on the road is different from process safety in the plant. That does not mean that producers and process safety professionals do not have a role to play. Not only can they make plants safer, they can also make the road safer.

When the media reports on a highway chemical spill that leads to a fire, evacuation and shutdown of a road, the public are not thinking that the trucking industry is incapable of operating safety; the public are thinking that the chemical industry is sending dangerous products through their community.

As long as the industry must ship its products, the public are going to hold it accountable for how the products are shipped. Like it or not, the industry is expected to make a difference, both inside the fence and beyond the fence line.

References 1.

2.

FISHER, P.J., ‘Many trailer, truck fires can be prevented’, www. tirebusiness.com/opinion/fisher-many-trailer-truck-fires-can-beprevented (Accessed 17 February 2021).

AQUILA, B., ‘Tractor Trailer Brakes on Fire!’, www.truckingtruth.com/ trucking_blogs/Article-30/tractor-trailer-brakes-on-fire (Accessed 2 February 2021).

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René Braun, Grandperspective GmbH, Germany, demonstrates how the use of infrared imaging to give early warning of dangerous gas leaks can improve plant safety in ammonia complexes.

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FINGERPRINTING AMMONIA LEAKS D etecting ammonia leaks early, as well as emissions monitoring, has never been more important than today. The global production of fertilizers grows every year and the majority of nitrogenous fertilizers rely on ammonia as a base material, with an annual production output above 200 million t. The present trend towards lower carbon and zero carbon emission ammonia is putting pressure on producers to increase efficiency and reduce emissions. A growing awareness of safety, combined with higher population densities in urban areas, add additional demands for emission monitoring and rapid emission detection. At the same time, new markets such as ammonia as fuel and storage for hydrogen are emerging.

Implementing large area surveillance for gas leaks is the only viable means of ensuring the continued operation of such industrial sites near populated areas.

When production and storage areas become larger, stationary gas detectors prove ineffective at reliable early warning. Monitoring large areas for ammonia emissions can only be accomplished with stand-off measurement technologies: Fourier transform infrared spectroscopy (FTIR) is well known from routine laboratory chemical analysis, but it is also an established technology for remote sensing. First responders and security forces use FTIR remote sensing for the supervision of large areas in the event of chemical releases or terrorist attacks. The method is selective among hundreds of chemicals while being extremely sensitive to many organic compounds and, in particular, ammonia. The main advantage of FTIR remote sensing over other technologies is the fact that it can be used passively without lasers or the illumination of the target area. Like a highly sophisticated camera, it shows the gas even from miles away.

Challenges of gas detection Detecting gas leaks has been a challenge ever since engineers started making use of dangerous substances for producing fertilizers, plastics or energy. The famous canary birds warned miners of carbon monoxide accumulations deep in coal

mines. Burning sulfur sticks was a method of locating small ammonia leaks in refrigeration machines. The modern-day equivalents are gas sensors that will report even the smallest presence of a particular target gas in their vicinity. Obviously, their efficiency is limited outdoors, i.e. when wind spreads gases rapidly in the wrong direction and prevents timely detection, unless the density of installed sensors is extremely high (and therefore prohibitively expensive). Optical gas imaging, on the other hand, may provide larger area coverage but lacks sensitivity and, most importantly, specificity.

FTIR spectroscopy has the potential to tackle both issues: it is very sensitive and specific as well as capable of covering large areas and providing a broader image. The measurement principle is based on the spectroscopic analysis of infrared radiation. Due to their inherent temperature, all objects emit infrared radiation, which allows a chemical analysis to be made at distance. Within the infrared spectrum, one wavenumber range, called the fingerprint region (from 500 to 1500 cm-1), is most interesting for analysing the chemical composition of a gas remotely. In this spectral region, many chemicals interact with radiation in a compound specific manner. The effect of this interaction is a spectral absorption or emission feature in the infrared radiation that is unique to the chemical compound – the spectral fingerprint.

A modern-day solution for large installations Grandperspective’s scanfeld system is the first FTIR remote monitoring solution for the chemical industry that probes for suspicious spectral fingerprints. The sensors are entirely passive FTIR spectrometers that continuously scan the areas of interest. A sophisticated mechanical positioning system allows the imaging motion as well as the 360˚ coverage of many different scan areas. Each point (or Toxel, a very narrow cone) within the observed regions is represented by a complete infrared spectrum. The spectra are automatically analysed for substance and amount.

With a single installation ranging from a tall spot, multiple areas can be secured (e.g. several high-pressure vents, pipelines, reactors etc). The detection range can be as great as 4 km.

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Remote sensing early warning systems, in comparison to local sensing technology, provide both information on the location and concentration distribution of the gas cloud. They can be applied over wide areas within the overlapping fields of view of the scanfeld sensors. The propagation of a gas cloud can be tracked over several square kilometres, and the concentration distribution is measured in real-time. Due to the optical technology, the gas detection works in all three dimensions including height, a feature that is missing in ground-based gas sensors.

Effective early warning Time is of the essence whenever hazardous substances are airborne, especially in the case of toxic, flammable and/or combustible materials such as ammonia. If such events are not detected early, the consequences can be severe with respect to the environment, health and, obviously, cost caused by damages, loss of production and downtime. Once incidents occur, knowledge about the exact nature of the release is important but traditional methods fail to provide the level of detail that is desired by first responders and fire fighters.

The system combines the capability of early warning of even small releases with precise information about the gas emissions and their change over time. Finally, the sensors also help to de-escalate the situation once a release is stopped and the

atmospheric gas is dissipated by diffusion and wind, as confirmed by the distant sensors.

Mapping toxic gases Gas releases in the open are very different from gas emissions in confined spaces. In the latter, a gas detector can pick up the rising concentration no matter where the source is located. In the open, however, the gas coming from a release point becomes rapidly diluted, so tracing back the source with a gas sensor is next to impossible. Finding flammable or toxic gas leaks requires a near-field monitoring of the entire area around a potential point of release to ensure timely and accurate results. With respect to the dynamic gas distribution, it must operate in real-time.

Compared to field measurements with hand-held sensors or vehicles, remote sensing allows for near-field mapping without prior knowledge of the location and the wind situation.

Monitoring the gas distribution can be performed within seconds.

The scanfeld unit is therefore fast enough to map the dynamic propagation of gas releases for early warning of gas leakages.

Measures for mitigating the effects of an accidental gas release are most effective if they accurately represent the gas concentration across a wide radius of the incident. Without information on the location of a gas accumulation, wide areas of the compound need to be monitored manually. Assessment of the gas concentration becomes more difficult as the affected area grows larger, making reliable situation assessment with common gas sensors impossible. The concentration of the gas can be low at a certain location but rise dramatically when the wind sweeps the gas cloud in another direction. Therefore, the reading of a local gas sensor can create a false interpretation of safety when the location of the gas cloud is unknown.

Case study

Figure 1. Mounted at an elevated position, the scanfeld sensor unit continuously monitors large areas for dangerous emissions.

Figure 2. A single unit can cover many regions of interest within its direct field of view.

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The Chemelot industrial park is located in a densely populated region near Maastricht, the Netherlands, and Düsseldorf, across the border in Germany. It is one of the largest chemical parks in Europe, with numerous companies – such as OCI, Arlanxeo, DSM and SABIC – present. The two major production streams onsite process naphtha/gasoil to hydrocarbons or plastics and natural gas to ammonia, fertilizers and specialty chemicals. Within the Chemelot industrial park is the Brightlands Campus, a hub for start-ups, R&D facilities and education with more than 3900 researchers, entrepreneurs and students. The campus is situated close to the production facilities; for example, the OCI Nitrogen ammonia and urea complex is less than 400 m (1300 ft) away.

A project is being undertaken to reduce the risks from an unexpected airborne release by using the scanfeld early warning capability to protect the adjacent Brightlands Campus. The main requirements are swift detection at levels of 10 ppm and above, visualisation of the gas cloud in the control room in real-time, no false alarms, continuous operation for at least 99.9% of the time, at least 4 hours operating time in case of power outage, onsite data storage, internet independence and 24/7 support in case of outages.

The scope of the first phase of the project is focused on a production unit in the OCI complex, and in particular the reduction of risks due to unexpected airborne releases. A technical feasibility study for a future scale-up to monitor the entire Chemelot north side has also been carried out. There are four stacks within the plant that must be monitored for accidental

ammonia release. The early warning of such incidents is essential.

Once a release of ammonia is detected, the operator must be warned immediately. Situation assessment requires determination of the location of the release, the compound that has been found in the air, the total amount of gas and the dimensions and concentration of the gas cloud. In the event of a spontaneous, large-scale emission, the gas cloud will not be diluted within close range of the incident and it will propagate over the site.

Such clouds bear a large hazard potential, especially at low wind speeds.

The number and the positioning of the sensor units is driven by the scope of the monitoring plan. Each individual sensor unit can cover a radius of 4 km (2.5 miles). Monitoring of an entire facility can thus be accomplished by two or three sensor units.

The installation at Chemelot focuses on fast early warning within a minute and quick monitoring of emissions into the Brightlands Campus. For the 3D localisation of a moving gas cloud, two sensor units are installed, autonomously monitoring the production area.

The two units are coordinated in real-time to track the event.

The scanfeld sensors sequentially monitor the four stacks of the OCI site, including the urea plant, for unexpected gases. The two sensor units also oversee the Brightlands Campus to monitor any emissions onto the campus. The total monitoring area is approximately 350 000 m² (0.14 square mile).

In the normal operation mode, the scan pattern is optimised for a quick turnaround time to check the status of the plant. The scan pattern is specific to the scope of the installation, balancing the number of areas, scan speed and spatial resolution. Thus, monitoring areas for the potential release of highly toxic or flammable gases can be optimised for speed, whereas large area monitoring can be optimised for better spatial resolution. Once a gas release is detected, the scan pattern shifts to gas cloud distribution mapping. The operator is notified of all events through an indication on the user interface. As permitted emissions naturally occur in production facilities and fugitive emissions from other areas periodically come into view, an automatic situation assessment aids improved distinguishing between hazardous situations and harmless emissions.

passive, spectroscopic technology delivers early warning as well as detailed information about undesired gas emissions. In the context of the OCI Nitrogen plant in the Netherlands, it specifically helps with protection of the immediate neighbourhood of the facility. It is therefore in step with the growing demand for early warning systems, which are a necessity for the continued expansion of production and, more generally, the public acceptance of ammonia production, processing and handling.

Figure 3. Using two or more scanfeld sensors permits checking upon gas clouds for concentration, position and propagation by tomographic reconstruction.

Outlook The scanfeld monitoring system is a highly automated monitoring tool that continuously surveys predefined scan areas. At OCI Chemelot, a very short response time to unexpected releases is achieved by the careful choice of two installation sites in the near vicinity of both the production site and Brightlands. With a minimal time-to-alert, the system provides exact information about what, where and when as well as the severity of unexpected gas emission incidents. It thereby creates additional barriers to hazardous events and protects the Brightland Campus.

Furthermore, it allows the speedy post-incident release of affected areas as soon as the dispersion of the gas cloud is confirmed by the sensors. Compared to conventional gas sensor networks, it is cost-effective and provides more detailed information, both from the near and wide field point of view.

Since the system is highly scalable, just a few additional sensor installations can provide coverage of the entire Chemelot area and provide safety to the site and its neighbouring areas.

Conclusion Optical remote monitoring is proving to be a useful method for increasing operational safety in ammonia processing facilities. The

Figure 4. The sensors detect dangerous gases regardless of background – against sky, technical installations, buildings or ground.

SEPTEMBER 2021 | WORLD FERTILIZER | 79

AD INDEX 61 | Bagtech 47 | Boldrocchi Group 42 | Borsig IBC | Casale

25 | K.A. Rasmussen OFC & | Kreber 35 OBC | Messer Group 52 | Microtrac MRB

19 | Clark Solutions 69 | Comessa 71 | CRU Phosphates 2022

21 |.-Mitsubishi Heavy Industries Compressor International IFC | Neelam Aqua & Speciality Chem(P) Ltd.

08 | CRU Sulphur + Sulphuric Acid 2021

41 | Nel Hydrogen

45 | Elliott Group

70 | Palladian Publications

31 | EMT/Doyle Equipment Manufacturing

27 | REXA, Inc.

02 | Eurotecnica

07 | Sabin Metal Corp.

51 | GEA

13 | Solex Thermal Science

15 | Haldor Topsoe

55 | Tecnicas Reunidas Group/Espindesa

04 | Hydro, Inc.

11 | The Equity Engineering Group, Inc.

69 | ITT RHEINHÜTTE Pumpen GmbH

65 | Venti Oelde

37 | J&H Equipment Inc.

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