Evaluation of association schemes in the CPA and PC-SAFT equations of state in modeling VLE of organic acids + water systems

The performance of various association schemes was evaluated in modeling isothermal and isobaric vapor–liquid equilibria of mixtures of organic acids and water with the CPA and the PC-SAFT equations of state. The organic acids considered were formic, acetic, propanoic and butanoic acid. Combinations of the 1A and 2B association schemes for the acids and the 2B, 3B, and 4C for water were tested. Polar contributions were also studied in PC-SAFT. The case in which no association contribution is included in the thermodynamic model was also assessed. It is concluded that the chosen association scheme greatly affects the performance of the equation of state. It was not possible to identify a single association scheme combination that would work well for all the systems and conditions studied. On average, PC-SAFT with the organic acid modeled as 1A and water as 4C showed the greatest accuracy. Interestingly, for some of the mixtures the nonassociation case gave considerably better representations than when the association term was used. When a binary interaction parameter is used the performance of the equations studied is comparable to the classic PR.