Monte Carlo simulations of vapor-liquid-liquid equilibrium of some ternary petrochemical mixtures

The aim of this study was to determine the capability and accuracy of Monte Carlo simulations to predict ternary vapor-liquid-liquid equilibrium (VLLE) for some industrial systems. Hence. Gibbs ensemble Monte Carlo simulations in the isobaric-isothermal (NpT) and isochoric-isothermal (NVT) ensembles were performed to determine vapor-liquid-liquid equilibrium state points for three ternary petrochemical mixtures: methane/n-heptane/water (1), n-butane/l-butene/water (2) and n-hexane/ethanol/water (3). Since mixture (1) exhibits a high degree of mutual insolubility amongst its components, and hence has a large three-phase composition region, simulations in the NpT ensemble were successful in yielding three distinct and stable phases at equilibrium. The results were in very good agreement with experimental data at 120 kPa. but minor deviations were observed at 2000 kPa. Obtaining three phases for mixture (2) with the NpT ensemble is very difficult since it has an extremely narrow three-phase region at equilibrium, and hence the NVT ensemble was used to simulate this mixture. The simulated results were, once again, in excellent agreement with experimental data. We succeeded in obtaining three-phase equilibrium in the NpT ensemble only after knowing, a priori, the correct three-phase pressure (corresponding to the force fields that were implemented) from NVT simulations. The success of the NVT simulation, compared to NpT, is due to the fact that the total volume can spontaneously partition itself favorably amongst the three boxes and only one intensive variable (T) is fixed, whereas the pressure and the temperature are fixed in an NpT simulation. NpT simulations yielded three distinct phases for mixture (3), but quantitative agreement with experimental data was obtained at very low ethanol concentrations only. (C) 2010 Elsevier B.V. All rights reserved.