A molecular-based model for normal fluid mixtures: Perturbed Lennard-Jones chain equation of state

The perturbed Lennard-Jones chain (PLJC) equation of state, previously used for pure fluids, is extended to normal fluid mixtures. The PLJC equation utilizes the hard-sphere chain mixture as the reference system, while the perturbation term is derived based on the first-order variational theory. Binary vapor-liquid equilibrium data of a wide range of fluids were well described by this model through the use of one binary parameter k(ij). We showed that the PLJC equation of state was able to predict vapor-liquid equilibrium fairly accurately using binary data. Also, the PLJC equation is able to model retrograde condensation and, liquid-liquid-vapor three-phase equilibrium of ternary mixtures without the need to reevaluate binary parameters. Theoretical phase equilibrium compositions were determined through an isothermal-isobaric flash calculation based on global minimization of the Gibbs free energy using a genetic algorithm.