Modeling the Critical and Phase Equilibrium Properties of Pure Fluids and Mixtures with the Crossover Cubic-Plus-Association Equation of State

Abstract

The cubic-plus-association (CPA) equation of state (EoS) is unable to correctly describe the vapor–liquid critical behavior of pure fluids and mixtures because of its classical behavior. In fact, the traditional parametrization procedure (i.e., matching the saturated pressure and liquid density curves far from the critical point) causes an overprediction of the critical pressures and temperatures of pure components. Besides, the deviations with respect to experimental data are even larger for systems containing hydrogen-bonding species, in comparison to systems composed of nonassociating molecules. To improve the representation of the thermodynamic properties of fluids in near-critical regions with CPA, we have applied White’s recursive procedure to introduce density fluctuations in the classical model, allowing the correct description of the nonanalytical behavior of real fluids close to the critical point. The resulting model (i.e., the crossover CPA (CCPA) EoS) is capable of accurately representing the phase behavior of fluids, specifically normal alkanes and alcohols, carbon dioxide, and water as well as some of their binary mixtures, far away from and close to the critical region. This is shown by the comparison of the results obtained from CCPA and classical EoS with the experimental data.