Multiphase Equilibrium Analysis via a Generalized Equation of State for Associating Mixtures

A detailed analysis of thermodynamic models for multiphase equilibria is presented with special emphasis on hydrocarbon + water systems. Evaluation of representing association sites in water by four-site, three-site, and two-site models by Wertheim's thermodynamic perturbation theory shows that any of the models may be accurately applied, but the four-site model is difficult to apply for generalized analysis of multiphase equilibria. The two-site model is recommended because its accuracy is at least equivalent to that of the three-site model or four-site model, but it is more convenient to apply in general. A predictive correlation is proposed for hydrocarbon -I- water phase equilibria which offers improvements in accuracy and general applicability over the best available models. An analytical solution for the thermodynamics of mixtures containing two associating species is derived. The proposed model is shown to provide accurate representation of liquid-liquid, vapor-liquid, and vapor-liquid-liquid equilibria of binary mixtures that include water, hydrocarans, alcohols, ketones and H₂S. A single binary interaction parameter was adjusted to provide the best fit of vapor-liquid and liquid-liquid equilibria for all binary mixtures.