Predicting the phase equilibria of mixtures of hydrogen fluoride with water, difluoromethane (HFC-32), and 1,1,1,2-Tetrafluoroethane (HFC-134a) using a simplified SAFT approach

Abstract

A simplified version of the statistical associating fluid theory (SAFT) is used to describe the thermodynamic properties and hence predict the phase equilibrium in associating mixtures containing hydrogen fluoride (HF). In the SAFT-HS approach the molecules are treated as hard spheres or chains of hard spherical segments with square-well association sites to mediate the hydrogen-bonding interactions and with van der Waals dispersion forces treated at the mean field level. Hydrogen fluoride is represented by a two association site model, as are the refrigerant molecules difluoromethane (HFC-32) and 1,1,1,2-tetrafluoroethane (HFC-134a), while water (H2O) is represented by a four-site model. The intermolecular parameters of the pure components are determined in order to give the best representation of the vapor pressures and saturated liquid densities. The unlike intermolecular parameters of the mixtures are adjusted to specific features of the system: in the case of the H2O + HF system the azeotropic temperature at atmospheric pressure is used, while for the HFC-32 + HF and HFC-134a + HF systems bubble-point compositions are used. The SAFT-HS approach provides an excellent representation of the H2O + HF mixture over a large temperature and pressure range. The predicted phase equilibria of hydrogen fluoride mixtures with refrigerants are also in good agreement with experimental data; the presence of liquid-liquid equilibria in the mixture of HFC-134a + HF is predicted by the theory. It is hoped that the SAFT approach will be used as a predictive tool to describe these systems which are of importance in the manufacture of replacement refrigerants.