SOME PROPERTIES OF EQUATION OF STATE MIXING RULES DERIVED FROM EXCESSGIBBS ENERGY EXPRESSIONS

Several recent proposals for using excess free energy models in constr ucting mixing rules for cubic equations of state have been analyzed. T he reproduction of the base model behavior at the correlating temperat ure and the nature of the extrapolation to higher temperatures have be en examined. The Wong-Sandler, LCVM, and MHV-1 mixing rules in an equa tion of state result in excess Gibbs free energies with contributions in addition to those from the base activity coefficient model. As a re sult, considerable degradation of calculated equilibrium behavior can occur. The excess enthalpies produced by these mixing rules are genera lly more positive than would be found from the temperature derivative of the G excess models, with only the MHV-2 mixing rule giving a reaso nably quantitative reproduction. The success of the Wong-Sandler mixin g rule in extrapolating to higher temperatures appears to derive in pa rt from this positive addition to the excess enthalpy.