Estimation of liquid-liquid-vapor equilibria using predictive EOS models. 1. Carbon dioxide-n-alkanes

This study looks for the first time at the possibility of predicting liquid-liquid-vapor equilibria without preliminary resource to experimental data. For this purpose, the two most successful G^E-based semipredictive models (PSRK and LCVM) and the global phase diagram (klGPD)-based semi-predictive approach (GPDA) have been implemented and compared. Although G^E-based models require a significant amount of experimental data for evaluation of their parameters and prediction of the equilibrium, GPDA needs no more that 2-3 key experimental points. The superiority of GPDA, which considers the entire thermodynamic phase space, over the G^E-based models, which are based on the fit of VLE data, is clearly demonstrated. Although the G^E-based models fail to predict global phase behavior and to describe three-phase data accurately, GPDA yields exact predictions of the global phase behavior and accurate estimations of the composition and density of the vapor and the CO₂-rich phases. In addition, it is accurate in describing the P-T projection of three-phase lines. All of the models considered are unable to predict the hydrocarbon-rich phase accurately. Presently, this problem cannot be solved without violating the predictive character of the model.