Thermodynamic Insights into the Characteristics of Unary and Mixture Permeances in Microporous Membranes

Abstract

The primary objective of this article is to gain fundamental insights into the dependences of component permeances Πi in microporous membranes on the operating conditions (upstream partial pressures, temperature, and feed composition). It is argued that the permeances Πi for unary systems and mixtures need to be compared on the basis of the adsorption potential πA/RT, a convenient and practical proxy for the spreading pressure πthat is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The use of πA/RT as a yardstick serves to elucidate and rationalize a wide variety of published experimental Πi data on unary and mixture permeances in microporous membranes. For cage-type host structures such as SAPO-34, DDR, and ZIF-8, the Πi values are uniquely dictated by the magnitude of πA/RT, irrespective of the partner species in the mixture. For MFI membranes, the tardier species slows down the more mobile partners due to correlated molecular motion within the channels; the degree of correlation is also a function of πA/RT.