Estimation of liquid-liquid-vapor equilibria in binary mixtures of n-alkanes

This study tests the ability of the best presently available semipredictive models, namely, predictive Soave-Redlich-Kwong (PSRK), linear combination of Vidal mixing rules (LCVM), and the novel global phase diagram approach (GPDA), to estimate the three-phase liquid-liquid-vapor equilibrium (LLVE) in the homologous series methane-n-alkanes, ethane-n-alkanes, propane-n-alkanes, and n-butane-n-alkanes. It is demonstrated that at high pressures G^E mixing rules predict the wrong shape of the liquid-liquid critical curves, which involves fictitious closed loops of liquid-liquid equilibrium. Such a behavior is responsible for predicting nonrealistic three-phase LLVE behavior along the homologous series under consideration. In addition, both PSRK and LCVM fail in estimating the lower critical end-point curve. The results presented here for the G^E-based models under consideration are probably characteristic for them. In contrast, GPDA is consistent in the entire thermodynamic phase space and does not predict nonphysical results. Although GPDA tends to underestimate the temperatures of the end points and to overestimate the double critical end points, its results are at least qualitatively accurate.