Universal Mixing Rule for Cubic Equations of State Applicable to Symmetric and Asymmetric Systems: Results with the Peng−Robinson Equation of State

Abstract

A mixing rule for cubic equations of state (CEoS) applicable to all types of system asymmetriesreferred to hereafter as the universal mixing rule (UMR)is proposed. For the cohesion parameter of the CEoS, the mixing rule involves the Staverman−Guggenheim part of the combinatorial term and the residual term of the original UNIFAC Gibbs free energy expression. For the covolume parameter of the CEoS, the quadratic concentration-dependent mixing rule is used with the combining rule for the cross parameter bij = [1/2(bi1/2 + bj1/2)]2. This UMR is applied to the volume-translated and modified version of the Peng−Robinson equation of state of Magoulas and Tassios (Fluid Phase Equilib. 1990, 56, 119), leading to what is referred to as the UMR−PR model. Very satisfactory results are obtained using the existing interaction parameters of the original UNIFAC model for vapor−liquid equilibrium predictions at low and high pressures for a wide range of system asymmetries including mixtures containing polymers. Satisfactory liquid−liquid equilibrium predictions are also obtained with the UMR−PR model.