Dipl.-Ing. Jörg Alexander
POLYMER FILTERS WITH (PRELIMINARY) CLEANING
New generation of CVF filters
Filtration is an essential process step in many polymer-processing procedures. The finer the final product (filaments, films), the more frequently largearea filters equipped with discs or filter candles are used.
To minimize the technical drawbacks, Barmag has further developed the proven CVF series in a targeted manner and implemented the following technical solutions. 1. Drainage at the filter insert
As these filter elements are top-quality components, they are cleaned and reused after utilization. This cleaning is cost-intensive and hence has a considerable impact on the filter’s operating costs. The polymer filter described in the following was developed to reduce these costs. Starting situation
To achieve complete ventilation of the filter inserts during the switchover process, all Barmag filters are designed with vertical filter inserts. The CVF series filters, designed specifically for extruder spinning systems, are also equipped with vertical filter inserts. As a result of the filter inserts’ design, it was simple to integrate a drainage option in the base of the insert (Fig. 1).
Filters with a filter surface of 0.4 - 5 m2 are standard components in extruder spinning systems. These are primarily duplex systems, i.e. the melt is conveyed using integrated switchover valves to one of the two selectable filter inserts. In this market segment, Barmag offers the CVF series of filters. Upon reaching the dirt accumulation threshold, the system is switched to a clean filter insert. The, thus far, installed filter insert is removed while still hot. Subsequently, the filter insert – while still hot, if possible – is disassembled so that the candles can be exposed and cleaned. In the event of incorrect handling, melt released from the filter insert can result in damage to surroundings as well as injuries to the operating personnel. In contrast, disassembling a filter insert that has been allowed to cool can be extremely difficult and time-consuming. In the first cleaning step, the candles coated with PETP or PA are placed in a heated TEG (triethylene glycol) bath for the purpose of removing the polymer. This method is standard practice for candles equipped with sintered random metal fibers, whereas candles equipped with woven wire cloth can also undergo an initial cleaning process using a pyrolysis furnace. Any polymer stuck to the filter parts is removed in the TEG bath. Because one kg of TEG can only absorb 250 g/kg, the TEG baths must be sufficiently proportioned for complete removal of the polymer. Depending on the amount of polymer stuck to the filter parts, a correspondingly large amount of TEG will subsequently have to be disposed of or recycled. 1
Fig. 1: Drainage option in the base of the insert In principle, it comprises a drain pipe that is externally (through the cover and heating jacket) screwed to the filter insert. Through the drain pipe and using a special key, the sealing plug is removed from the base of the filter insert, which allows the polymer to flow out of the filter insert. 2. Preliminary cleaning with steam In the case of PETP, operators can take advantage of the hydrolytic degradation of the molecule chain Developments • Trends • Technologies Corporate Communications
Polymer filters with (preliminary) cleaning
for cleaning by adding water. Hence, steam in a filter both decomposes and propels. There are also cleaning facilities and filters available whereby the cleaning is carried out using hydrolysis. However, these rely on overheated steam being generated and supplied externally.
steam directly to the condensation basin. The melt particles released during the steam cleaning process are caught in a steam-proof collection chamber. The steam is then forwarded and recondensed in a condensation basin (Fig. 3).
The special feature of the Barmag design is that the steam is generated in the heating jacket integral to the filter. Heat exchangers are integrated into the heating jacket (heated with dowtherm vapour), in which the added water vaporizes. The operator need only provide a tap water feed with volume flow restriction. The enclosed photograph shows the heating jacket with water inlet and steam outlet (Fig. 2).
Fig. 3: Collection chamber and condensation basin
Function tests To examine the influence of the technical changes on the operating behavior more accurately, various cleaning processes were carried out using a CVF304 with a filter area of 0.4 mÂ˛. Fig 2: Heating jacket with water inlet and steam outlet
Subsequently, the steam is fed through the filter insert via a pressure- and temperature-stable hose counter to the melt flow direction. For this, the filter insert is turned by 180 degrees within the heating jacket, which ensures that the steam does not have a negative impact of the switchover valves or the production process. To minimize the risk potential of overpressure in the event of blocked filter inserts, the steam zone is equipped with an overpressure valve, which is activated at 10 bar and feeds the 2
In the case of PETP, the filter insert was extensively cleaned using the drainage system. Subsequently, disassembly of the filter insert in a cold state proved unproblematic. Following this test, the candles (diameter 36 x 200) were still coated with an average of 43 g of melt each. The internal surfaces of the filter insert also retained a thin layer of polymer (Fig. 4+5). A further test looked into preliminary cleaning solely on the basis of steam cleaning. Here, it transpired that the siphon-like design of the filter insert retained a large amount of residual polymer in the base of the unit. Developments â€˘ Trends â€˘ Technologies Corporate Communications
Polymer filters with (preliminary) cleaning
Fig. 4: Filter element bundle after drainage Fig. 6: Filter element bundle â€“ firstly drained and secondly cleaned using steam Outlook The additional, above-mentioned installations reduce both the risk potential and cleaning costs. As the housing requires no further cleaning, considerable costs savings (TEG, staff costs) can be expected for each filter change.
Fig 5: Filter insert after drainage
In a third test, the used filter insert was first drained and subsequently cleaned using steam. Using this method, the residual polymer stuck to the candles was reduced to 35 g. Following this cleaning process, disassembling the filter insert in a cold state proved unproblematic. After being wiped with a cloth, it could be reused without any further cleaning. Photograph 6 depicts the candle bundle following this cleaning process.
The cleaning costs for the six candles are reduced as a result of the extensive preliminary cleaning. These overall savings for each filter change can only be achieved with additional investments, which in turn means that the additional installations at the filter require approx. 1 - 2 years before reaching break-even point. In the longer term, these installations will also be available for larger filters for other applications.
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