Improving phase equilibrium calculation with the Peng-Robinson EOS for fats and oils related compounds/supercritical CO2 systems

Several predictive methods for normal boiling point, critical temperature, critical pressure, and acentric factor, applied to fatty acids and fatty acid esters of high molecular weight were evaluated. The properties estimated by the selected methods were used to analyze the ability of the Peng-Robinson equation of state (EOS) to predict the vapor-liquid equilibria of binary and ternary systems of fats and oils related compounds with CO2. Van der Waals mixing rules with the combining rules proposed by Kwak and Mansoori and Park et al. were used. For fatty acids, the method of Constantinou and Gani was selected for the prediction of the critical temperature and that of Somayajulu for the prediction of the critical pressure when experimental boiling temperatures were available. In the absence of experimental data, the method of Constantinou and Gani was selected for the prediction of critical and boiling temperatures, as well as for the critical pressure. For fatty acid esters, the method of Joback and Reid was chosen for the prediction of the critical and boiling temperatures, and that of Constantinou and Gani was selected for the prediction of the critical pressure. The indirect method of Tu was preferred for the prediction of the acentric factors of high molecular weight fatty acids. The correlation of Vetere was chosen for the prediction of the acentric factors of low molecular weight fatty acid esters, and the indirect method of Tu was selected for the prediction of the acentric factors for the methyl ester family. The results for the phase equilibria suggest that the Peng-Robinson equation, with the quadratic mixing rules for the three combining rules tested, is capable of predicting vapor-liquid equilibrium with quality comparable to the results obtained with the mixing rules for more complex models like MHV1 and LCVM. (C) 2000 Elsevier Science B.V. All rights reserved.