Molecular models for the vapor-liquid equilibrium of simple binary mixtures

Simple analytical expressions are proposed for the calculation of the equilibrium pressure, as well as (for a given temperature and pressure) the mole fractions of both liquid and vapor phases at the vapor-liquid equilibrium of binary mixtures. They are based on a recently proposed molecular model for the vapor pressure of pure nonpolar fluids, which, for a given temperature, only requires as input the values of the two Lennard-jones (LJ) molecular parameters and the acentric factor, which are parameters related to the molecular shape of each substance and whose values are readily available. The model for the equilibrium pressure of a binary mixture (which also permits one to obtain the liquid phase mole fraction) is similar to that derived from Raoult's law, where a properly modified Lorentz-Berthelot mixing rule is used, the interaction parameters being given as simple functions of the temperature and composition with eight appropriate constants for each binary mixture. A different model is needed to calculate the vapor mole fraction in which five appropriate constants are needed for each mixture. Here, we show how the models reproduce accurately and straightforwardly the vapor liquid equilibrium properties (pressure, liquid mole fraction, and vapor mole fraction) of eight binary systems over a broad temperature range, including some data at or near the critical locus.