Studies on steady state and post-steady state ultrafiltration of different solutes.

Abstract

Concentration polarization in ultrafiltration with fully retentive membrane under unstirred conditions has been studied using three different types of solutes: (a) solute which exerts osmotic pressure and also forms a gel (polyethylene glycol), (b) solute which exerts osmotic pressure but does not form a gel (dextran) and (c) solute which does not exert osmotic pressure but forms a gel (silica). An unequivocal determination of whether pressure independent flux regime is osmotically controlled or gel layer dominated, is still open for discussion in the membrane literature. The present work reports a method that could be used to address this issue. It is shown that analysis of post steady state transient filtration data leads to clear demarcation of osmotically limited and gel layer controlled filtration. A method for determining additional filtration resistance offered by the polarization layer to permeate flow in macromolecular ultrafiltration has been proposed and verified experimentally. It has also been shown that the polarization layer thickness is not sensitive to the feed pressure but varies as a function of the bulk solute concentration; higher the bulk concentration thicker is the polarization layer. A fundamental analysis of deposited gel layer has been presented based on constant pressure filtration theory for the solute, which only forms a gel layer (silica). This was used to calculate specific resistance of the solute particles. The concentration and pressure dependence of specific resistance was also reported while silica gel porosity was determined by the Kozeny-Carman equation.