Filtration properties and skin formation of micro-crystalline cellulose

Abstract

Solid–liquid filtration is an important unit operation used in several applications in the pulp and paper industry. It can be challenging especially for materials that forms filter cakes that has a tendency to be highly compressible. The formation of a skin, i.e. an initial cake with a considerably higher filtration resistance, can further increase the filtration resistance for these filter cakes. The separation process in these cases requires considerably larger filter areas.This study investigates skin formation during the filtration of micro-crystalline cellulose, examining the effect of the filter medium, filtration pressure and pH.Depending on the choice of filter medium, the average specific filtration resistance was found to vary by more than one order of magnitude. A decrease in the suspension pH from 6.3 to 2.9 lowered both the filtration resistance and the differences between the filtration media. Using local pressure profiles, a region of high filtration resistance for some of the filter media and pH values was identified close to the bottom of the filter cell. Characterisation of the media and micro-crystalline cellulose indicated that internal clogging of the filter medium was unlikely. This suggests that the filter cake closest to the filter medium had a considerably higher filtration resistance compared to the cake formed immediately after, i.e. a skin had been formed under certain conditions.