A practical perspective of fluid (gas or liquid) - Solid adsorption equilibrium

Abstract

The fluid (gas or liquid) – solid adsorption equilibrium is a core thermodynamic property of an adsorbate-adsorbent system for establishing the efficiency of separation of a fluid mixture by an adsorptive process and for selection of an optimum adsorbent for a specified separation duty. The shape of an equilibrium adsorption isotherm of a pure gas or those for the components of a gas mixture are determined by the nature of the adsorbate-adsorbent pair, the system temperature, the porosity of the adsorbent, and the energetic heterogeneity of the adsorbent. The same factors also determine the shape of an equilibrium isotherm of a binary liquid mixture. The published literature on this subject is vast. The purpose of this article is not to review the literature but to provide a systematic practical perspective of the subject.Analytical models for fluid-solid adsorption isotherms facilitate numerical simulation of separation process performance using mathematical process models. However, the practically viable isotherm models must obey several physical and thermodynamic constraints. A selection of such isotherm models for pure and multi-component gas and binary liquid mixtures is presented. It may not, however, be possible to a priori chose a reliable isotherm model for a system of interest without extensively testing the model using experimental isotherm data for that system. The same conclusion applies to theoretical concepts for (a) predicting multi-component gas-solid adsorption equilibria from the corresponding pure gas adsorption equilibria, (b) coalescing adsorption isotherms of different pure gases at different temperatures and (c) coalescing isotherms for adsorption of trace components from a mixture (gas or a liquid) into a single characteristic curve on a given adsorbent for predictive purposes.