Prediction Of Vapor-Liquid Equilibrium By HVOS-PR-MLUNIFAC Group Contribution Model

The first successful mixing rule of combining an equation of state and a free energy model had been proposed by Huron and Vidal(1979). The idea was to require the excess Gibbs free energy from an equation of state to be equal to that from an activity coefficient model at infinite pressure. Since is known experimentally to be a function of pressure, the parameters obtained at low pressures and reported in compilations such as the DECHEMA Chemistry Data Series cannot be used and the model has little predictive capability.Orbey and Sandler(1995) developed the HVOS mixing rule using an infinite pressure reference state. In this model it is assumed that there is a universal linear algebraic function that relates the liquid molar volumes to their hard core volumes as V=ub, where u is a positive constant larger than unity. The model contains only the one parameter C*, C*= -0.62323, that is well defined once an equation of state has been selected. Orbey et al. believe if one wishes to tune parameters, it is easier to adjust the parameter C*. The excess Helmholtz free energy in the mixing rule is approximatively calculated by activity coefficient model. So, prediction of vapor-liquid equilibria is improved by adjusting the group interaction parameters that the prediction accuracy in original MLUNIFAC model is poor.The model for predicting vapor-liquid equilibria of mixtures was presented using the mixing rule of HVOS, PR equation of state combined with MDUNIFAC and MLUNIFAC models. Vapor-liquid equilibria for 191 binary systems were predicted and the results were compared with experimental data in this work, and results of HVOS-PR-MLUNIFAC model was better than those of HVOS-PR-MDUNIFAC model. Further, the group interaction parameters for 12 group combinations were refitted by Nelder-Mead simplex method. Additionally, new group interaction parameters for 21 group combinations were fitted by experimental data of vapor-liquid equilibria.The model was applied to predict the vapor-liquid equilibria for 86 binary systems and 18 ternary systems at different temperatures and pressures by using the modified and supplemented group interaction parameters. The results calculated by using modified parameters were much better than those obtained by using original parameters for alcohol and acid systems except water. The mean absolute deviation of calculation of vapor-composition and the mean relative deviation of calculation of bubble-pressure were less than 0.04 and 5%, respectively. Because there exist hydrogen-bonding and association effect for polar systems with water, the model cannot describe interaction between groups and the results of calculation of vapor-composition and bubble-pressure were improved slightly. The results of prediction by using supplementary group interaction parameters were good agreements with experimental data of vapor-liquid equilibria for binary systems. To extend the range of application of the new group interaction parameters, satisfactory accuracy was also obtained in calculating the vapor-composition and bubble-pressure of vapor-liquid equilibria for ternary systems. The group interaction parameters in this work were proved to have wide applications in industry.