Filter aids block

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

The transition from pore-blocked filtration to more favorable cake filtration can therefore be achieved with a suspension of low settling particles by initially feeding it to the filter medium at a low rate for a time period sufficient to allow surface accumulation. This is essentially the practice that is performed with filter aids. 

During filtration, body feed, or the continuous addition of filter aid, can help prevenr blinding of the suspended solids on the precoat. The body feed surrounds the suspended solids to provide flow around them. The body feed slurry of filter aid and oil is injected into the system prior to the filter. The suspended solids are ridged or deformable and can elongate under pressure to extrude through the filter cake and slow or block the product flow. Body feed coats the deformed solids, allowing them to be retained on the filter cake. 

Two operating periods lead to a reduction in permeability. During the first period, filtration primarily proceeds with pore blocking on the outer surface of the filter aid. During the second period, the filter aid pores undergo blocking within the filter aid cake. 

Fig. 4.3. (A) Diagram of the amnion invasion assay. The invasion chamber represents a cylindrical well produced by a Teflon ring (a) to which epithelium-free amnion (b) is fastened with the aid of a viton ring (c), to face the BM side up and stromal side down. A smaller lower chamber is created by a silicone rubber ring support attached to the bottom of a 35-mm tissue culture well (d) with silicone grease, and filled with medium. The (upper) invasion chamber is placed on this support, and medium with or without additives (to be tested for invasion-blocking or stimulating ability) is added to this chamber 1 h prior to the addition of labeled cells to be tested for invasive ability. Medium is then added to the tissue culture well (d) outside these chambers to bring the fiuids inside and outside the Teflon ring to the same level (e) represents a well that includes the complete invasion chamber seeded with cells on the BM. (Reproduced from Yagel et al., 1989.) (B) (a) Human amnion. Epithelium (EP), basement membrane (BM), connective tissue stroma (ST). Haematoxylin-eosin PAS stain, (b) Denuded human amnion membrane. Basement membrane (BM), connective tissue stroma (ST), Milfipore filter (F). Haematoxylin-eosin, PAS stain. (Reproduced from Russo, 1986.)...

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