Gas Separation by Sorption: A Statistical Thermodynamic Fluctuation Theory

To understand how gas mixtures can be separated and purified by adsorption, we present a novel theory of selective sorption that (i) is applicable to any interfacial geometry and porosity, (ii) synthesizes sorption and activity with built-in thermodynamic consistency, and (iii) incorporates the inhomogeneous distribution of sorbates at the interface. Number correlations from the fluctuation theory provide a unified quantification of sorbate-surface and sorbate-sorbate interactions in the language common to the liquid theory. From the gradient of the isotherm alone, our theory can reveal the sorbate interactions that underlie selective sorption. The multicomponent ABC isotherm, derived from our theory, can successfully model gas-mixture adsorption without the need for implicit functions to maintain thermodynamic consistency between activity and adsorption. Our AB isotherm captures sorption selectivity while retaining the mathematical simplicity of the Extended Langmuir model. Handling attractive and repulsive interactions between sorbates was shown to be crucial, which was beyond the reach of previous isotherm models.