Application of the Cubic-Plus-Association (CPA) Equation of State to Cross-Associating Systems

Abstract

The cubic-plus-association (CPA) equation of state (EoS) is applied, using different combining rules, to vapor−liquid equilibria (VLE) and liquid−liquid equilibria (LLE) of alcohol−water systems. It is demonstrated that the Elliott combining rule (ECR) with a common temperature-independent interaction parameter provides very adequate VLE correlations over extended temperature and pressure ranges, yielding also a very satisfactory description of the azeotropic behavior. LLE of heavy alcohol−water systems is best described with the CR-1 combining rule and a single interaction parameter. Satisfactory predictions of multicomponent, multiphase equilibria of water−alcohol−alkane systems at various conditions are achieved using solely one interaction parameter per binary. A study of the dominant binary systems for the prediction of the multicomponent systems demonstrates that both the binary alcohol−water and alcohol−hydrocarbon systems are crucial for the prediction of the partition coefficients of alcohols. Finally, the CPA EoS combined with a model for the solid-complex formation can successfully describe solid−liquid equilibria of glycol/methanol−water systems including the description of the solid-complex phase, which is known to exist at intermediate concentrations.