Some Properties of Equation of State Mixing Rules Derived from Excess Gibbs Energy Expressions

Several recent proposals for using excess free energy models in constructing mixing rules for cubic equations of state have been analyzed. The reproduction of the base model behavior at the correlating temperature and the nature of the extrapolation to higher temperatures have been examined. The Wong−Sandler, LCVM, and MHV-1 mixing rules in an equation of state result in excess Gibbs free energies with contributions in addition to those from the base activity coefficient model. As a result, considerable degradation of calculated equilibrium behavior can occur. The excess enthalpies produced by these mixing rules are generally more positive than would be found from the temperature derivative of the G excess models, with only the MHV-2 mixing rule giving a reasonably quantitative reproduction. The success of the Wong−Sandler mixing rule in extrapolating to higher temperatures appears to derive in part from this positive addition to the excess enthalpy.