Activity coefficient group contribution model

The UNIFAC (Unified quasi chemical theory of liquid mixtures Functional-group Activity Coefficients) group-contribution method for the prediction of activity coefficients in non-electrolyte liquid mixtures was first introduced by Fredenslund et al. (1975). It is based on the Unified Quasi Chemical theory of liquid mixtures (UNIQUAC) (Abrams and Prausnitz, 1975), which is a statistical mechanical treatment derived from the quasi chemical lattice model (Guggenheim, 1952). UNIFAC has been extended to polymer solutions by Oishi and Prausnitz (1978) who added a free volume contribution term (UNIFAC-FV) taken from the polymer equation-of-state of Flory (1970). 

It is very satisfying and useful that the COSMO-RS model—in contrast to empirical group contribution models—is able to access the gas phase in addition to the liquid state. This allows for the prediction of vapor pressures and solvation free energies. Also, the large amount of accurate, temperature-dependent vapor pressure data can be used for the parameterization of COSMO-RS. On the other hand, the fundamental difference between the liquid state and gas phase limits the accuracy of vapor pressure prediction, while accurate, pure compound vapor pressure data are available for most chemical compounds. Therefore, it is preferable to use experimental vapor pressures in combination with calculated activity coefficients for vapor-liquid equilibria predictions in most practical applications. 

Repeat Illustration 12.1-1 using the UNLF.AC group contribution model to estimate the naphthalene activity coefficient. 

When there is no built-in binary parameter in Aspen, and also when we cannot find any experimental data in the open literature, another option is to use the group contribution model in Aspen to predict liquid activity coefficient for this binary pair or the entire component system. UNIFAC is an activity coefficient model, like NRTL or UNIQUAC, but it... 

Prediction of Infinite Dilution Activity Coefficients for Polyisoprene (PIP) Systems with Two Predictive Group Contribution Models... 

Traditional activity coefficient based thermodynamic models have been successfully used to describe several LLE systems. The nonrandom two-liquid (NRTL) model of Renon and Prausnitz (1968) and the universal quasi-chemical (UNIQUAC) method of Abrams and Prausnitz (1975) models have been used to correlate LLE data for the many multi-component mixtures (Ghanadzadeh et al., 2009 Se and Aznar, 2002), while a group contribution method (UNIFAC) (Fredenslund et. al., 1977) has been widely used to predict the LLE systems. 

The Achard model combines the UNIFAC group contribution model modified by Larsen et al. [LAR 87], the Pitzer-Debye-Hiickel equation [PIT 73a, PIT 73b] and solvation equations (Figure 2.1). The latter are based on the definition of the number of hydration for each ion, which corresponds to the assumed number of water molecules chemically related to the charged species. It divides the activity coefficient into two terms ... 

BEN 10] Ben Gaida L., Dussap C.G., Gros J.B., Activity coefficients of concentrated strong and weak electrolytes by a hydration equilibrium and group contribution model . Fluid Phase Equilibria, vol. 289, no. 1, pp. 40-48, 2010. 

It took a little over half a century for the next major development to occur. G. Wilson in 1964 introduced the idea of local compositions which led to an expression for y,- bearing his name and later to the NRTL and UNIQUAC expressions by J. Prausnitz and his co-workers (1967 and 1975). The expressions have two main advantages over the van Laar model first, their parameters have a built-in temperature dependency making them very useful in distillation applications (why ) second, they provide successful prediction of multicomponent activity coefficients from binary ones. Finally, based on the UNIQUAC model, Fredenslund and his co-workers (1977) have developed a group-contribution model, UNIFAC, that can be used for the estimation of activity coefficients for a large variety of systems. 

The second group of studies tries to explain the solvent effects on enantioselectivity by means of the contribution of substrate solvation to the energetics of the reaction [38], For instance, a theoretical model based on the thermodynamics of substrate solvation was developed [39]. However, this model, based on the determination of the desolvated portion of the substrate transition state by molecular modeling and on the calculation of the activity coefficient by UNIFAC, gave contradictory results. In fact, it was successful in predicting solvent effects on the enantio- and prochiral selectivity of y-chymotrypsin with racemic 3-hydroxy-2-phenylpropionate and 2-substituted 1,3-propanediols [39], whereas it failed in the case of subtilisin and racemic sec-phenetyl alcohol and traws-sobrerol [40]. That substrate solvation by the solvent can contribute to enzyme enantioselectivity was also claimed in the case of subtilisin-catalyzed resolution of secondary alcohols [41]. 

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