A critical assessment on two predictive models of binary vapor–liquid equilibrium

A critical assessment on two predictive theoretical models for vapor–liquid equilibrium (VLE) is presented. The first model utilizes molecular dynamics to evaluate interaction energies in liquids. The energy terms are then used as input parameters in activity coefficient models to calculate the vapor–liquid equilibrium and activity coefficients. The second model is based on the recently proposed COSMO-RS to derive VLE diagrams. The assessment was made with 16 binary mixture systems. We show that the hybrid model that combines a predictive method based on molecular dynamics with a correlated method based on activity coefficient methods in general gives qualitatively incorrect results due to the fact that the energy terms used in activity coefficient models do not represent the true intermolecular interaction energies. COSMO-RS on the other hand often predicts VLE behavior with qualitative or semi-quantitative accuracy. However, the current implementations of the method do not allow for a description of non-ideal vapor phases or partially miscible mixtures.