Critical points with the Wong-Sandler mixing rule—II. Calculations with a modified Peng-Robinson equation of state

Abstract

This is the second of two papers in which critical point calculations in binary systems were performed utilizing cubic equations of state (EOS) combined with excess energy models using the Wong-Sandler mixing rule. In the first paper, a qualitative study of critical phase diagrams calculated using the simple van der Waals EOS combined with the NRTL model was made. In this paper, the Stryjek and Vera version of the Peng-Robinson EOS was also combined with the NRTL model and the resulting model was used in the computation of the critical loci of real systems. The binary interaction parameters of the model were estimated by correlating vapor-liquid equilibrium (VLE) data and, for some systems, successful predictions of the critical loci were obtained even when VLE data far from the critical point were used. To estimate parameters in systems for which the equation of state model may incorrectly predict a false liquid-liquid split, we used a penalty function approach based on the results of global stability tests. While the model studied here has been able to quantitatively predict the critical behavior of some non-ideal systems, involving compounds such as water, acetone and alkanols, only qualitatively correct behavior could be predicted for some highly asymmetric and non-ideal mixtures, such as water + n-dodecane.