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FIGURE 1.7 Effect of feed characteristics and process parameters on filtration profile under constant pressure filtration mode. (a) Effect of cake compressibility (s), (b) effect of differential pressure (AP), and (c) effect of solids content in the feedstock.

1.2.2.1.3 Filter Aid Assisted Depth Filtration

Various types of filter aids have been used for enhancing capacity of a filtration step [49-51]. These include those based on diatomaceous earth, perlite, and cellulose. Figure 1.8 shows images of the variety of diatoms that are found in some of the filter aids [51]. The mechanism of filter aid assisted filtration is illustrated in Figure 1.9 [18]. The filter aid is added to the feed material and when depth filtration is performed, the filter aid particles maintain separation of the otherwise impermeable solids in the feed and thus can cause significant improvement in permeability of the filter cake, yielding higher flux and filter capacity.

The cake filtration theory as described in Equation 1.24, Equation 1.28, and Equation 1.31 still applies to filter aid-assisted filtration. With filter aid addition, r'c is the combined specific resistance from the mixture of filter aid and original solids in the feed; Cp is the total mass of filter aid and original

FIGURE 1.8 Variety of diatoms that are found in some of the commercially available filter aids. (From Hunt T. In Encyclopedia of Bioprocess Technology: Fermentation, Biocatalysis andBioseparation. New York: John Wiley & Sons, 1999. With permission.)

solids in the suspension; and s is the combined compressibility of filter aid and original solids.

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