Single porous crystal membranes: mixtures in steady flow

Abstract

A treatment, which uses the Langmuir adsorption isotherm as the simplest representation of the type 1 contours typical of most isotherms in porous crystals, has been developed, from the case of steady-state diffusive flow of a single gas through a porous crystal to that of flow of a mixture containing any number of components. Quantitative expressions have been obtained for the departures from sorption equilibrium for each component not only across the gas/solid interfaces but also for adsorption on the external surfaces. These departures must arise whenever the flow density, J, is finite. They are found by taking into account the mass balance equations at and across the gas/solid interfaces at entry to and departure from the crystal membrane.

When J_id is the flow density which would arise for the usual but incorrect assumption of exact sorption equilibrium at and across the gas/crystal interfaces, J_id > J and the treatment leads to a simple expression for the ratio J_id/J for a single pure gas flowing alone. For the mixture of gases each component affects the flow of each of the other components and the same is true of the separation factor for any pair of gases in the mixture. In accord with this the irreversible thermodynamic formulation of mixture flow has been developed in a way that takes account of non-equilibrium at and across interfaces.