Ultrafiltration unstirred

Gel polarized ultrafiltration was recently analyzed for cross flow and unstirred batch cell systems by Trettin and Doshi (1980 a,b). We have shown in these papers that the widely used film theory does not predict the limiting flux accurately. The objective of this paper is to derive an expression for the permeate flux when the pressure independent ultrafiltration of macro-molecular solutions is osmotic pressure limited. We will also attempt to distinguish between gel and osmotic pressure limited ultrafiltration of macromolecular solutions. 

It is theoretically shown for the unstirred batch cell that, in limiting cases, the assumption of constant wall (membrane) concentration with respect to time may be made even in the absence of gel formation. Although the assumption of constant wall concentration is similar in both gel and osmotic pressure limited ultrafiltration, it is important to recognize that in gel polarized ultrafiltration, wall concentration is also pressure independent since it corresponds to the solute solubility limit. This is not the case in osmotic pressure limited ultrafiltration... 

When the assumption of constant wall concentration is justified, data for the unstirred batch cell and thin channel systems may be interpreted using models presented in Trettln and Doshi (1980a, 1980b). Such an analysis is performed where agreement is shown to be very good between theory and osmotic pressure limited ultrafiltration experiments. 

It is further shown that the current practice of plotting permeate flux vs. AP in macromolecular cross-flow ultrafiltration may lead to serious misinterpretation of gel polarization. It is therefore recommended that solutions be studied in the unstirred... 

In macromolecular ultrafiltration, 2is 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 Trettln 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. 

Chudacek M.W., Fane A.G. (1984), The dynamics of polarisation in unstirred and stirred ultrafiltration. Journal of Membrane Science, 21,145-160. 

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