Mass transfer in gas-sparged ultrafiltration: upward slug flow in tubular membranes

Gas sparging is recognized as an effective way to increase permeate flux in ultrafiltration processes. This paper discusses the mechanism of flux enhancement in ultrafiltration processes by gas sparging, in the special case of upward slug flow in tubular membrane module. An attempt is made to model gas sparged ultrafiltration with a view to predicting permeate flux in such processes. The model is based on dividing the region near the gas slug into three different zones depending on the nature of flow in the vicinity of the membrane. These different zones are (a) the film zone, where there is falling him flow, (b) wake zone, which is a region of free turbulence, and (c) liquid slug zone, where the flow could be either streamline or turbulent depending on the bulk fluid flow. The mass transfer coefficients for each of these zones can be determined for a particular feed solution, and from these, the averaged permeate flux for gas sparged ultrafiltration can be calculated. The model was tested for ultrafiltration of 167 kDa dextran. A reasonably good agreement between experimental and theoretically predicted data was obtained. The discrepancies observed are discussed. Simulations were run in order to examine the effects of operational parameters on permeate flux. The results suggest that gas sparging is more effective at higher transmembrane pressure, and increasing the liquid flow rate has opposite effects in single phase flow and gas-sparged ultrafiltration. (C) 1999 Elsevier Science B.V. All rights reserved.