Driving force for pervaporation through zeolite membranes

Separation factors for pervaporation of organic/water mixtures through zeolite membranes are shown to correlate with the fugacities of the feed components, and this correlation allows qualitative predictions of separation performance. For more than a 500-fold range of fugacities, organic/water separation factors for 5 wt.% organic feeds increased with organic feed fugacity for pervaporation through a hydrophobic Ge-ZSM-5 zeolite membrane at 303 and 333 K. The C1–C4 organics were carboxylic acids, alcohols, ketones, esters, propionaldehyde, and diethyl ether. The fluxes and separation factors also depended on organic heats of adsorption and size, but to a lesser extent than on fugacity. Competitive adsorption coverages, and therefore the driving forces for transport, appear to depend on the organic/water feed fugacity ratios. Separation factors for hydrophobic and hydrophilic zeolite membranes in the literature also correlated with feed fugacities. Separation factors for the hydrophobic membranes had trends with fugacity similar to those for the Ge-ZSM-5 membrane, but separation factors for the hydrophilic membrane increased as the water/organic feed fugacity ratio increased. Furthermore, fugacity ratios correctly predicted a selectivity inversion for 1-butanol/acetic acid separations from 1-butanol/acetic acid/water mixtures using the Ge-ZSM-5 zeolite membrane.