PREDICTION OF PHASE-EQUILIBRIA IN BINARY AQUEOUS SYSTEMS CONTAINING ALKANES, CYCLOALKANES, AND ALKENES WITH THE CUBIC-PLUS-ASSOCIATION EQUATION OF STATE

The cubic-plus-association (CPA) equation of state (EoS) is applied in this study to binary aqueous mixtures containing hydrocarbons. The CP A EoS combines the Soave-Redlich-Kwong (SRK) cubic equation of state f or the physical part and perturbation theory for the chemical (associa tion) part. Rigorous expressions for the contribution of the associati on term to the pressure and to the chemical potential, which do not in clude any derivatives of the mole fraction of molecules i not bonded a t site A (X-Ai), are presented. Three different association models for water have been considered depending on the number of hydrogen bondin g sites per water molecule: the two-, three-, and four-site models. Su ccessful correlation of both vapor pressures and saturated liquid volu mes is obtained with all three models. However, satisfactory correlati on results of the mutual solubilities of water/aliphatic hydrocarbon s ystems are obtained only with the four-site model using a single inter action parameter (k(ij)) in the attractive term of the EoS. A generali zed expression of k(ij) as a function of the molecular weight of the m embers of the homologous series is presented, something that allows CP A to be used as a predictive tool. Very satisfactory prediction result s are obtained, comparable to the correlation ones of the SAFT EoS for the water solubility in the hydrocarbon-rich phase and orders of magn itude better for the hydrocarbon solubility in the water-rich phase. S atisfactory predictions are also obtained for the vapor-phase composit ions and the three-phase equilibrium pressures.