Calculation of adsorption equilibria using CALPHAD models and software

Abstract The utility of CALPHAD (CALculation of PHAse Diagrams) models and software for calculation of gas-on-solid adsorption equilibria is demonstrated. Thermodynamic models formulated in CEF (Compound Energy Formalism) are constructed that account for adsorption on single site, several sites as well as for adsorption in pores and multilayers. Site blocking (the case where the adsorption of a single molecule on a single site also blocks some adjacent sites) may be emulated by the ionic-liquid model within CEF. The parameters of these thermodynamic models may be determined by fitting to experimental results or to results of ab-initio calculations. Alternatively, the parameters may be calculated from already-fitted adsorption isotherm equations such as Langmuir, dual site Langmuir, Nitta and Ruthven's model for adsorption in pores. Pure-gas adsorption models may be extrapolated to mixed-gas adsorption. The thermodynamic consistency of such extrapolation is ensured by the CALPHAD method of modeling and the CEF formalism. Examples include adsorption of various gases on carbon as well as on zeolites and MOF (Metal Organic Framework).