Analysis of Filtration Mechanism of Crossflow Upward and Downward Ultrafiltration

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The filtration characteristics of crossflow ultrafiltration are studied for two types of solutes, a protein (BSA) and a colloid (silica sol), under constant-pressure conditions. It is shown that the time variation of the filtration rate coincides with that of dead-end ultrafiltration until the filtration rate drops to a certain value, and that the gel-cake is easily swept away at a relatively small crossfiow velocity. Effects of the crossflow velocity, the filtration pressure and the solute concentration on the dynamically balanced filtration rate are explained by considering the balance between the "particulate" solutes accumulating on the gel-cake surface and those to be swept away by the shear force due to crossflow at steady state. It was also found that the dynamically balanced filtration rate in crossfiow upward ultrafiltration coincides with that in dead-end upward ultrafiltration under conditions below the critical shear stress τw·c, whereas it is in accord with that in crossflow downward ultrafiltration above τw·c.
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