Sheet filtration

SHEET FILTRATION. Sheet filters consist of a tubular steel chassis or a freestanding filter chassis in which square metal or plastic filter plates are vertically arranged. The filter plates are fluted plates made of ribbed material or tubular frames with perforated plates. 

The frames are normally about 40 mm wide. Re-usable carrier sheets, which are used as carriers for the filter cake, can be used here to reduce the weight. The advantage of this type of equipment is that the frame is also suitable for filter plates for sheet filtration. 

Fig. 11.5. Effect of glucane from Botrytis cinerea on flat-sheet filtration (Dubourdieu, 1982) 1. Dry white wine made from healthy grapes. II. The same wine +200 mg/1 of glncane...

The wine should be properly clarified prior to flat-sheet filtration at the time of bottling to ensure a satisfactory flow rate. This preliminary clarification may involve spontaneous settling, fining, centrifugation (Section 11.11) or filtration through a diatomaceous earth precoat (Section 11.6). 

Flat-sheet filtration is subject to the law of gradual clogging of pores under well-defined conditions. A test (Serrano, 1981) to check a wine s aptitude for clarification by flat-sheet filtration may be carried out using the apparatus shown in Fignre 11.12. 

If these criteria are not satisfied, this means that the wine has not been sufficiently clarified in advance to ensure that flat-sheet filtration will be efficient and cost-effective. 

The heterogeneity of the filter sheets makes it impossible to obtain a direct result for sheet filtration, so the fouling index measurement on membranes (Section 11.8.3) may be used. Wines... 

These criteria are necessary to ensure that flat-sheet filtration will provide proper clarification and a satisfactory elimination of microorganisms, combined with an adequate flow rate (Table 11.7). 

Vmax (Gaillard, 1984) is calculated using the same formnla as that used for flat-sheet filtration (Section 11.7.2), althongh the experimental method is different. The volnme thronghpnt of the membrane at a pressure of 1 bar is noted after 2 and 5 mn. The formula (Section 11.2.3) is the following ... 

As in flat-sheet filtration, the constant flow rate of the bottle filler dictates the nnmber of cartridges to nse. It has been calculated that three 30-inch cartridges (Alter surface of 1.8 m each, flow rate 720 Fh) are required to supply a bottling line operating at 3000 bottles/h or 2250 1/h. Filter membranes must be used for several weeks, or even months, before they become completely blocked in order to make this system cost-effective. 

When all the preceding operations are efficiently carried out, simultaneous filtration with prefilter cartridges and membranes gives good results in terms of clarification quality (Table 11.11). Quality remains high for several days with satisfactory flow rates. It has, however, been observed that this technique, combined with the necessary preliminary clarification processes, may have a greater effect on the polysaccharide concentration than flat-sheet filtration (Section 11.10.2). It shonld therefore be used very carefully. Proper operating conditions are essential and filtration resnlts shonld be carefnlly monitored. 

It is not advisable, nor is it useful, to filter wines on a fine diatomaceous earth precoat (0.35 Darcy) prior to flat-sheet filtration. 

Membrane filtration (0.65 xm) caused a more marked reduction in polysaccharides, phenols and esters than flat-sheet filtration. Muscat aromas were not affected. However, no significant differences were fonnd when the wines were left to rest for one month after filtration and then tasted. 

The standard filter sheet in breweries is 60 x 62 cm (approx. 23 6 X 24 4 in) and the largest is normally 100 x 100 cm (39 4 x 39-4 in). Some large single-ended filter presses have 240 filter-plates which permit an hourly flow rate of about 120 hl/hr (73 brl/hr). Such small throughput capacity means that sheet filtration has lost popularity except for second stage filtration in which they are designed to polish and in some cases sterile-filter. Considerable... 

Over the past thirty years, powder filters have become increasingly popular, either as a complete filtration system or as the first step of a two-stage filtration in which sheet filtration follows. The most popular powder for this purpose is kieselguhr or diatomaceous earth which is mined from Miocene deposits in a number of areas in Europe and the Americas. An alternative is the volcanic material called Perlite obtained from certain Greek islands. Very recently it has been shown that silica hydrogel can substitute for kieselguhr and at the same time stabilize the beer against chill haze [13]. 

Filtration processes may be classified as either depth filtration or surface filtration. Depth filtration relies on a layer of porous media in which suspended particles in the beer are trapped within the media. Examples in brewing include filter aid filtration, sheet filtration and some forms of filter cartridge. Surface filtration normally refers to membrane technology. A thin layer of membrane has pores throughout the structure. This means that it is possible to achieve very exact filtration, perhaps enabling sterilisation, but typically the quantity of suspended beer particles that may be removed is less than for depth filtration. 

Brenner H., 1961. Three-dimensional filtration on a circular leaf, AIChEJ, 7, 666-671. Brewer R, 1981. Sheet filtration apphed to the pharmaceutical and food industries, FUtr. Sep., 18, 242-246. 

Outside the food and beverage industries, there are many other applications for sheet filtration. Some of the more important include antibiotics, medicinal syrups, toilet and cosmetic preparations, gelatine and vinegar. 

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