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.

Towards predictive association theories

“…CPA has been previously shown to perform very well in correlating with one adjustable parameter (per binary) LLE for water-alkanes [32] and glycol-alkanes [33] and with two adjustable parameters (as solvation is accounted for via the mCR-1 rule) LLE and VLE for water-aromatics [34] and glycol-aromatics [35]. Data have been until very recently published only for water-glycol-aromatic compounds and the successful predictive capabilities of the model have been illustrated [36], both in comparison to the few experimental data and also against other models such as the SRK equation of state combined with the Huron-Vidal mixing rules [37].…”

Extension of the new proposed association equation of state (AEOS) to associating fluid mixtures

“…The average absolute deviation percent of saturated pressure, saturated liquid molar volume, and saturated vapour molar volume calculated with proposed AEOS are represented in table 2 and are compared with those evaluated by other EOSs (Anderko [35,36], Shinta and Firoozabadi [43], Vafaei et al [44], CPA [30,48], PCP-SAFT [25], Huang and Radosz [16]). The obtained results indicate that the new AEOS have the least error.…”

“…Kontogeorgis [26] combined the Soav-Redlich-Kwong (SRK) EOS [27] and the theoretical background of the perturbation theory as the association part to develop cubic plus association (CPA) equation. Then, CPA was used frequently for systems containing associating compounds [28][29][30][31][32][33][34]. The most common form of compressibility factor, z, has been used by several authors to improve EOS for calculations of thermodynamic properties of hydrogen-bonding fluids, is in the following form [35][36][37][38][39][40][41][42][43]:…”

Pitfalls of using the geometric-mean combining rule in the density gradient theory

“…In other words, it is not affected by the influence parameters. Therefore this point becomes a natural choice for the middle point of the interface, and the spatial dimension can be calculated by Equation (24).…”