VAPOR-LIQUID-EQUILIBRIA OF MIXTURES FROM THE NPT PLUS TEST PARTICLE METHOD

The NpT + test particle method for the calculation of vapour-liquid ph ase equilibria by molecular simulation is extended to binary mixtures. The independent thermodynamic variables are the temperature T and the liquid concentration x. On the liquid side, one NpT simulation is per formed at a prescribed pressure which determines the chemical potentia ls and their derivatives with respect to the pressure, i.e., the parti al molar volumes. On the vapour side, two NpT simulations are performe d at two different concentrations and at a prescribed pressure yieldin g again the chemical potentials and their derivatives with respect to the pressure. Using first-order Taylor expansions for the chemical pot entials, the vapour-liquid phase equilibria are obtained. As an exampl e, the vapour-liquid phase equilibria for the mixture argon + methane are predicted at four temperatures and compared with experimental resu lts. For that purpose, an optimized redetermination of the molecular p arameters was made using only excess volumes and enthalpies for the un like interaction.