Prediction of Vapor−Liquid Equilibria of Associating Mixtures with UNIFAC Models That Include Association

Abstract

Two UNIFAC association models are developed using Wertheim's theory of association rather than a chemical theory. In both models the activity coefficient is the sum of combinatorial, residual, and association contributions. The UNIFAC group-contribution model is used for the combinatorial and residual terms, and two different types of association models are considered. The UNIFAC-AG model uses functional-group-based association, while the UNIFAC-AM model considers association to occur between molecules. For associating mixtures containing acids, alcohols, or water, both activity coefficient models provide better predictions of binary vapor−liquid equilibria than the original UNIFAC model. Of those models, the UNIFAC-AM model led to the best predictions. The association term was also added to the more recent, modified UNIFAC model and briefly tested with vapor−liquid equilibrium data for the acetic acid + heptane and acetic acid + butanol mixtures. It was found that, for vapor−liquid equilibria, the modified UNIFAC + association model is only slightly better than the UNIFAC-AM model. However, the modified UNIFAC + association model has four more adjustable parameters than the UNIFAC-AM model. Therefore, use of the UNIFAC-AG or UNIFAC-AM models is recommended.

Associating lattice cluster theory and application to modeling oleic acid + formic acid + formoxystearic acid

“…Expressions for the Wertheim association term to the Gibbs energy or free energy based on lattice models have been reported previously in the literature . The main difference of these expressions and the ones presented here is the introduction of the total number of sites on species i M i in the MAE.…”

“…For applying it to lattice models, basically only the terms describing the composition have to be adapted. This adaption is not unambiguous, so that different proposals have been made in the literature . A new consistent expression for an arbitrary number of association sites is presented here and used for the development of ALCT.…”

The Statistical Associating Fluid Theory (SAFT)

“…Fu et al 111 have presented such a GC association term (essentially that of Wertheim's theory) coupled with UNIFAC leading to an 'association UNIFAC model'. Fu et al 111 have presented such a GC association term (essentially that of Wertheim's theory) coupled with UNIFAC leading to an 'association UNIFAC model'.…”

A Hydration Shell‐Based Thermodynamic Model for Aqueous Two‐Phase Systems

“…The most promising work in this fi eld is that by Fu et al (1996), who developed a group contribution method based on a previous model (Fu et al, 1995) in which the UNIQUAC equation was modifi ed by the introduction of a perturbation term. The expression suggested in these developments is a logical extension of the consideration that the contribution of each site is independent of the other:…”