Highly compressible materials filtration

F.M. Tiller and T.C. Green, The role of porosity in Filtration IX Skin effect with highly compressible materials. AIChE, 7 (1973) 1266-1269. 

The tubular or monolith geometry of ceramic membranes proceeds from inherent constraints in ceramic material processing. Due to the mechanical properties of sintered ceramic materials which can be summed up in a high compressive resistance and a brittle character, cylindrical shapes offer the best compromise between pressure resistance and cross-flow filtration adaptability. Typical geometries for ceramic membranes are shown in Fig. 12.7. 

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

Non-woven media in the form of felts and compressed cellulose pulps, are used for clarification by depth filtration. Unless carefully prepared, they have the disadvantage of losing fibrous material from the downstream side of the filter. The application of sheet media has been discussed earlier. High wet strength is conferred on paper sheets by resin impregnation. An alternative technique employs asbestos fibers supported in a cellulose framework. 

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