Extension of the SAFT-VR-Mie equation of state for adsorption

Abstract

An empirical extension of the Statistical Associating Fluid Theory (SAFT-VR-Mie) is presented to take into account the effect of confinement of fluids within cylindrical nanopores. The modification of the equation of state retains the bulk phase limit presented in the original formulation and adds a term corresponding to the contribution to the Helmholtz energy of the confined fluid. The resulting expression employs the fluid-fluid parameters obtained from fitting bulk fluid behaviour and adds two additional adjustable parameters reflecting the strength of the solid-fluid energy and the range of the surface attraction. The capability of the theoretical model is showcased by fitting adsorption isotherms of methane and n −nonane on activated carbons; ethane, n −hexane and benzene on MCM-41, and methane and carbon dioxide on carbon surrogate models of shale rocks; providing for an accurate correlation of the data with parameters that are temperature-independent and robust. The physical nature of the underlying model allows it to be mapped to fluid-solid molecular models which can then be resolved employing classical molecular simulation methods, providing for an avenue into probing not only the adsorption behaviour but also the transport and interfacial properties.