A modified mixing model for vapor-liquid equilibrium calculations

Vapor-liquid equilibria (VLE) on binary and ternary mixtures are calculated using the Peng-Robinson equation of state (EOS). The mixture parameters of the EOS are evaluated in a similar approach to the Wong-Sandler model. Besides the Helmholtz free energy and the second virial coefficient equations in the Wong-Sandler model, the EOS mixture parameters are determined in this study by also using a coordination number model. The EOS parameters satisfy the theoretical requirements at low and high density limits. At an intermediate density condition, the coordination number expressed by the EOS is equal to that determined from a theoretically-based model. The group contribution UNIFAC activity coefficient model and a modified coordination number equation are employed in this work. Without any empirically adjusted parameter, we obtain comparably good VLE calculation results, as those from other existing EOS + GE models, from low to high pressures. With this modified mixing model, the Henry's law constants are predicted with significantly better results than those from other approaches.