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

Simple analytical expressions are proposed for the calculation of the equil ibrium 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 f or the vapor pressure of pure nonpolar fluids, which, for a given temperatu re, only requires as input the values of the two Lennard-jones (LJ) molecul ar parameters and the acentric factor, which are parameters related to the molecular shape of each substance and whose values are readily available. T he model for the equilibrium pressure of a binary mixture (which also permi ts 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 bina ry mixture. A different model is needed to calculate the vapor mole fractio n in which five appropriate constants are needed for each mixture. Here, we show how the models reproduce accurately and straightforwardly the vapor l iquid equilibrium properties (pressure, liquid mole fraction, and vapor mol e fraction) of eight binary systems over a broad temperature range, includi ng some data at or near the critical locus.