Pressure-Independent Ultrafiltration—Is It Gel Limited or Osmotic Pressure Limited?

Abstract

In macromolecular ultrafiltration, as pressure is increased, permeate flux first increases and then in a large number of cases levels out and remains more or less pressure independent. This could be due to the increase in solute concentration at the membrane surface such that either gel formation occurs or the corresponding osmotic pressure approaches the applied pressure. Limiting flux for the gel polarized case was recently analyzed for cross flow and unstirred batch cell systems by Trettin and Doshi (1980,a, b). In this paper we have analyzed the osmotic pressure limited ultrafiltration for the two systems. Our unstirred batch cell data and the literature cross flow data agree quite well with the theory. We have further shown that an unstirred batch cell system can be used to determine whether pressure independent ultrafiltration of macromolecular solution is gel or osmotic pressure limited. Other causes for the observed pressure independence may be present but are not considered in this paper.