Vapor-liquid (azeotropic systems) and liquid-liquid equilibrium calculations using UNIFAC and NRTL-SAC activity coefficient models

Abstract

The aim of present study was to investigate some of the most important maximum and minimum azeotrope boiling temperature systems using UNIFAC and NRTL-SAC models as the predictive models. Also, the predictive power of UNIFAC, UNIFAC-LLE and NRTL-SAC models were compared in liquid-liquid equilibrium calculations. For maximum azeotrope temperature systems, except for formic acid-water, the UNIFAC model had higher degree of accuracy than the NRTL-SAC model. The NRTL-SAC model could not appropriately predict the maximum azeotrope temperature systems, in particular for acetone-chloroform, chloroform-methyl acetate, and chloroform-diethyl amine systems. For these systems, the NRTL-SAC model gave a minimum azeotrope temperature. For the examined minimum azeotrope boiling temperature systems, the results of both models were in consistent with the experimental data, but in most cases, the results of the UNIFAC model were in good agreement with the experimental data. On the other hand, as the pressure deviated from 101.3 kPa, the deviation of both models increased. In the case of ternary VLE systems under study, UNIFAC model indicated better results than NRTL-SAC model. Totally, in most VLE systems, UNIFAC was much better than NRTL-SAC and we preferred UNIFAC model as a good candidate for VLE calculations of the studied systems. Based on the overall deviation of models for LLE calculations at all temperatures, the best representation of the system was in the following order: UNIFAC-LLE > UNIFAC > NRTL-SAC. There would appear to be a major difficulty in selecting one thermodynamic model as the best, as the choice might be system-dependent.