Enhanced flux through double-sided zeolite membranes

Maxwell-Stefan models indicate that, under some conditions, fluxes through supported zeolite membranes can be significantly higher when zeolite films are on both sides of the porous support than for a single supported film. Partial pressure drops across the support make loadings higher on the downstream side of the zeolite in the presence of a support, and the lower driving forces result in lower fluxes. Depositing films on both sides of the support can reduce the effect of the support when the loading is high (not in the Henry regime). When a non-adsorbing sweep gas is used, the permeate-side zeolite layer can keep the sweep gas out of the support, and thus further reduce the support resistance. The permeance can be higher through a double-sided membrane (called FSF here, indicating the permeant passes through a film, then a support, then a film), even if the total thickness of the two films is greater than the thickness of a single film. For separations in which selectivity increases with increasing zeolite thickness, a FSF can yield higher flux and selectivity than a single supported film. The support resistance reduces the flux of stronger adsorbing components more than that of weaker adsorbing components, so a FSF can have higher flux and selectivity for binary mixtures in which the more strongly adsorbed component preferentially permeates. Calculations are presented here for butane in MFI-type zeolite films, though the concepts can be applied to different supported film types and permeating components. (c) 2007 Elsevier B.V. All rights reserved.