Implementation of solvation free energy framework to predict the vapor-liquid equilibrium behaviors for the water–hydrazine and ethanol–water azeotropic systems

Abstract

An azeotrope is a special vapor–liquid equilibrium in which the composition of two coexisting phases is equal. Due to its strong deviation from the Raoult’s law, it poses a great challenge to the simple distillation process and the understanding of fundamental molecular mechanisms. In this study, we calculated the coupling work of solute species based on the solvation free energy framework to predict the vapor–liquid equilibrium (VLE) behavior and azeotrope point of mixtures. Two prototypical binary mixtures: the negative water-hydrazine and positive ethanol–water azeotrope were investigated using molecular dynamic simulations and the OPLS-AA force field. All the simulations were performed with the TIP3P water model, because it has a good capability to represent enthalpy properties. The molecular models of hydrazine and ethanol have been re-parameterized to reproduce the density and solvation free energy properties of pure substance. Using free energy perturbation simulation, the coupling work of each substance was studied against liquid mixture composition. The coupling work is also translated to x-y or T-x-y phase diagrams for comparison with available experimental data. The tendency of two azeotropes is that the more volatile components experience the less pronounced coupling work changes in their local environments as the azeotropes form. The coupling work is also divided into van der Waals and Coulomb contributions to investigate the interactions between particles. Lithium Chloride was investigated as an entrainer for the separation of ethanol–water system. The coupling works of binary mixtures were calculated at different salt concentrations to study the salt-out effect for the violate species and salt-in effect for the less violate species. By adding LiCl, it was found that the azeotrope point was efficiently eliminated at a salinity 0.05 mol fraction at 1 bar. All the results show that the simulation methodology of coupling work are reliable for calculating the liquid–vapor equilibrium behavior. This method is also valuable for selecting and designing new extractants for distillation process.