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

The primary objective of this article is to gain fundamental insights into the dependences of component permeances Π i in microporous membranes on the operating conditions (upstream partial pressures, temperature, and feed composition). It is argued that the permeances Π i for unary systems and mixtures need to be compared on the basis of the adsorption potential π A / RT , a convenient and practical proxy for the spreading pressure π that is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The use of π A / RT as a yardstick serves to elucidate and rationalize a wide variety of published experimental Π i data on unary and mixture permeances in microporous membranes. For cage-type host structures such as SAPO-34, DDR, and ZIF-8, the Π i values are uniquely dictated by the magnitude of π A / 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 π A / RT .