PCP-SAFT Density Functional Theory as a much-improved approach to obtain confined fluid isotherm data applied to sub and supercritical conditions

Gravimetric measurements are often used to generate excess adsorption isotherms. However, adsorbed density and volume should be known to transform excess isotherms into absolute ones by considering buoyance effects. Depending on the strategy used, the results may present artifacts, especially at high pressures. A Density Functional Theory (DFT) augmented with the PCP-SAFT equation of state is applied to describe the experimental data of excess and absolute adsorption isotherms of CH4 and CO2 fluids on activated carbon slit pores. The representative pore size distribution (PSD) adequately predicted the excess adsorption isotherms at different temperatures. The absolute adsorption isotherms obtained using our approach are compared with the classic approaches as the triple point density of the liquid and the Ozawa expression (and extrapolated to the high-pressure regions). The classical methodologies underestimate the amount adsorbed while the proposed methodology provides thermodynamically consistent results even when condensation of CO2 occurs in the bulk phase. (c) 2021 Elsevier Ltd. All rights reserved.