PREDICTING THE PHASE-EQUILIBRIA OF MIXTURES OF HYDROGEN-FLUORIDE WITHWATER, DIFLUOROMETHANE (HFC-32), AND 1,1,1,2-TETRAFLUOROETHANE (HFC-134A) USING A SIMPLIFIED SAFT APPROACH

A simplified version of the statistical associating fluid theory (SAFT ) is used to describe the thermodynamic properties and hence predict t he phase equilibrium in associating mixtures containing hydrogen fluor ide (HF). In the SAFT-HS approach the molecules are treated as hard sp heres or chains of hard spherical segments with square-well associatio n sites to mediate the hydrogen-bonding interactions and with van der Waals dispersion forces treated at the mean field level. Hydrogen fluo ride is represented by a two association site model, as are the refrig erant molecules difluoromethane (HFC-32) and 1,1,1,2-tetrafluoroethane (HFC-134a), while water (H2O) is represented by a four-site model. Th e intermolecular parameters of the pure components are determined in o rder to give the best representation of the vapor pressures and satura ted liquid densities. The unlike intermolecular parameters of the mixt ures are adjusted to specific features of the system: in the case of t he H2O + HF system the azeotropic temperature at atmospheric pressure is used, while for the HFC-32 + HF and HFC-134a + HF systems bubble-po int compositions are used. The SAFT-HS approach provides an excellent representation of the H2O + HF mixture over a large temperature and pr essure range. The predicted phase equilibria of hydrogen fluoride mixt ures with refrigerants an also in good agreement with experimental dat a; 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 w ill be used as a predictive tool to describe these systems which are o f importance in the manufacture of replacement refrigerants.