PHASE-EQUILIBRIA OF BINARY LENNARD-JONES MIXTURES WITH CUBIC EQUATIONS OF STATE

The ability of cubic equations of state to predict the phase behavior of the Lennard-Jones pure fluid and asymmetric binary mixtures obeying the Lorentz-Berthelot combining rules is examined. It is shown that t he commonly used Soave-Redlich-Kwong and Peng-Robinson equations of st ate give good vapor pressures but incorrect saturated liquid densities for the pure fluid. A modification of the Peng-Robinson equation of s tate that uses a volume translation term to describe accurately the sa turated liquid densities is proposed. For the mixture calculations, we use the van der Waals one-fluid conformal solution mixing rules. All the cubic equations of state tested give excellent pressure-compositio n phase diagrams for the binary mixtures. Predictions of pressure-dens ity phase envelopes are less satisfactory, but equivalent to those of other more complex equations of state previously developed for the Len nard-Jones fluid. Our results suggest that cubic equations of state ca n be used to model mixtures of significantly different compounds provi ded that appropriate combining and mixing rules are used.