EQUATION-OF-STATE MIXING RULES FOR MULTICOMPONENT MIXTURES - THE PROBLEM OF INVARIANCE

Several researchers have recently extended the usual quadratic mixing rules in equations of state by using higher-order polynomials in component mole fraction. Most of these higher-order polynomials exhibit a serious deficiency for multicomponent systems since they are not invariant when a component is divided into two or more identical subcomponents. In this paper we investigate the source of this deficiency and propose a formulation to overcome it. While our analysis here is illustrated with the Peng-Robinson equation of state, it is applicable to other equations of state and to excess-Gibbs-energy models for calculating activity coefficients.