PREDICTION OF VAPOR-LIQUID-EQUILIBRIA BY USING EQUATIONS OF STATE WITH ZERO-PRESSURE EXCESS GIBBS ENERGY MIXING RULES - THE RKU2 MODEL FOR MODERATE TEMPERATURES

A zero-pressure mixing rule for the energy parameter in cubic equations of state has been developed. The composition dependence of the mixture volume u = v/b at zero pressure is approximated by a two-parameter Redlich-Kister expansion (RKU2). The RKU2 coefficients are determined from pure component parameters of a cubic equation of state and activity coefficients at infinite dilutions. The RKU2 mixing rule combined with the SRK equation of state has been tested for seven isothermal and nine isobaric binary systems. At moderate temperatures the model performs comparably to the treatment requiring an additional iterative procedure (Heidemann and Kokal, 1990). Extensions to ternary systems and the importance of correct temperature dependence of the activity coefficient models used in the mixing rules are discussed.