2. Use of BACK To Modify SAFT in Order To Enable Density and Phase Equilibrium Calculations Connected to Gas-Extraction Processes

Abstract

Statistical associating fluid theory (SAFT) has been extended to mixtures at temperatures slightly above the critical temperature of one of the compounds as often encountered in gas-extraction processes. The proposed extension is based on the contact values of the average correlation function for a pair in a mixture of hard convex bodies of different types by Boublik and allows a mixture of convex bodies described by the BACK equation of state with associating and nonassociating sphere chains from original SAFT. The supercritical solvent is modeled as a convex body, allowing an accurate description in the near-critical region, while all other compounds are modeled as sphere chains, utilizing the benefits of original SAFT. The proposed extension can be added to any existing SAFT software, quickly. Additionally, different two-parameter mixing rules are proposed that are defined on a segment basis instead of on a molecular or system basis like in original SAFT, promising a higher predictive capability for systems with chemically similar compounds. The SAFT equation extended this way has been used to correlate experimentally determined high-pressure phase equilibria from the gas-extraction processes of 40 systems, where original SAFT fails to give correct qualitative results, because the supercritical solvent's critical temperature is overpredicted.