UNIFAC UNIquac Functional-group

The UNIFAC (UNIQUAC functional group activity coefficient) method is an extension of the UNIQUAC (Universal quasi chemical) method, which has been used widely in chemical process engineering to describe partitioning in organic systems as occur in petroleum and chemical processing (Fredenslund et al., 1975,1977). It has been applied less frequently to aqueous systems. It expresses the activity coefficient as the sum of a "combinational" component, which quantifies the nature of the area "seen" by the solute molecule, and a "residual" component, which is deduced from group contributions. Arbuckle (1983,1986), Banerjee (1985), Banerjee and Howard (1988), and Campbell and Luthy (1985) have tested the applicability of the method to water solubility. 

Kojima and Togichi, 1979)] model and UNIFAC [UNIQUAC functional-group activity coefficient (Fredenslund et al., 1977)] model discussed earlier, have been developed. However, these models are limited to the range of classes of compounds and conditions of the regressed experimental data used in their development. 

The two most developed group contribution methods are the ASOG (Analytical Solution Of Groups) and UNIFAC (UNIquac Functional-group Activity Coefficient) " models, both of which are the subjects of books. We will consider only the UNIFAC model here. UNIFAC is based on the UNIQUAC model of Sec. 9.5. This model, you will remember, has a combinatorial term that depends on the volume and surface area of each molecule, and a residual term that is a result of the energies of interaction between the molecules. In UNIQUAC the combinatorial term was evaluated using group contributions to compute the size parameters, whereas the residual term had two adjustable parameters for each binary system that were to be fit to experimental data. 

UNIFAC andASOG Development. Pertinent equations of the UNIQUAC functional-group activity coefficient (UNIFAC) model for prediction of activity coefficients including example calculations are available (162). Much of the background of UNIFAC involves another QSAR technique, the analytical solution of groups (ASOG) method (163). 

Another group contribution method that has been applied to the prediction of soil sorption is the UNIquac Functional-group Activity Coefficient (UNIFAC, where UNIQUAC = Universal Quasichemical) approach (Fredenslund et al., 1977). Ames and Grulke (1995) applied the method to a small diverse set of chemicals, with rather poor results. They did not report any correlations, but from their results it can be shown that the correlation of observed and predicted log values using the Bondi method was n = 17, R2 = 0.571, 5 = 0.524, and F = 20.0 eight chemicals were predicted with an error of < 0.5 log units, 7 chemicals were predicted with an error between 0.5 and 1.0 log units, and 2 chemicals were predicted with an error of > 1.0 log units. 

UNIQUAC Functional-group Activity Coefficients proposed by Aa.Fredenslund, R. L. Jones, and J.M. Prausnitz, AIChE J.,vol. 21, p. 1086—1099,1975 given detailed treatment in the monograph Aa. Fredenslund, J. Gmehling, and P. Rasmussen, Vapor-LiquidEquilihrium using UNIFAC, Elsevier, Amsterdam, 1977. 

A group contribution method called UNIFAC, an acronym which stands for the UNIQUAC Functional Group Activity Coefficient (UNIQUAC stands for the Universal Quasi-chemical Activity Coefficient), has been developed for estimating liquid-phase activity coefficients in non-electrolyte mixtures. The UNIFAC method is fully described by Fredenslund, Jones and Prausnitz (1975) and Skold-Jorgensen, Rasmussen and Fredenslund (1982). 

Unifac equation (uniquac functional group activity coefficient) Contributions of individual functional groups prediction of the interaction between the activity coefficients of functional groups by interaction parameters, only valid for small and medium operating pressures Fredenslund, a., Gmehling, j., and Rasmussen, P., Vapor-Liquid Equilibria using Unifac. Elsevier Publ. 1977. 

For binary pairs without measured phase equilibrium, use the predictive UNIQUAC functional-group activity coefficients (UNIFAC) [19] property method to estimate the BIPs. There are two UNIFAC methods one for VLE and another for LEE. A predictive model requires that group interaction parameters be available for the various subgroups of a chemical s structure. 

The UNIFAC model (UNIQUAC Functional Group Activity Coefficient), which was put forward by Fredenslund, Jones and Prausnitz in 1975, is a group model where the idea is to use the existing data on equilibrium states to predict the properties of systems for which no experimental data are available. Thus, it is an entirely predictive model which, unlike the models discussed above, does not require us to determine parameters by calibrating the experimental results found for partial systems... 

UNIFAC A semi-empirical thermodynamic model used to predict the behaviour of components in complex mixtures, which uses structural groups to estimate component interactions. It is an abbreviation for UNIQUAC Functional-group Activity Coefficients and is used to predict non-electrolyte activity in non-ideal mixtures. It is used to predlctthe activity coefficients as a function of composition and temperature. It is useful when experimental data is not available. 

The UNIquac Functional group Activity Coefficient (UNIFAC) model modified by Larsen et al. [LAR 87] does not take into account the sulfur groups among the common amino acids, methionine and cysteine cannot be decomposed into functional groups (Table 3.3). 

These models are semiempirical and are based on the concept that intermolecular forces will cause nonrandom arrangement of molecules in the mixture. The models account for the arrangement of molecules of different sizes and the preferred orientation of molecules. In each case, the models are fitted to experimental binary vapor-liquid equilibrium data. This gives binary interaction parameters that can be used to predict multicomponent vapor-liquid equilibrium. In the case of the UNIQUAC equation, if experimentally determined vapor-liquid equilibrium data are not available, the Universal Quasi-chemical Functional Group Activity Coefficients (UNIFAC) method can be used to estimate UNIQUAC parameters from the molecular structures of the components in the mixture3. 

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). 

The UNIQUAC equation is the basis for the development of the group contribution method, the UNIFAC equation, which predicts liquid activity coefficients from component structures on the basis of interactions between chemical functional groups. 

Perhaps the most important term in Eq. (5.2-3) is the liquid-phase activity coefficient, and mathods for its prediction have been developed in many forms and by many workers. For binery systems die Van Laar [Eq. (1.4-18)]. Wilson [Eq. (1.4-23)]. NRTL (Eq. (1.4-27)], and UNIQUAC [Eq. (1.4-3 )] relationships are useful for predicting liquid-phase nonidealities, but they require some experimental data. When no data are available, and an approximate nonideality correction will suffice, the UNiFAC approach Eq-(1.4-31)], which utilizes functional group contributions, may be used. For special cases Involving regular solutions (no excess entropy of mixing), the Scatchard-Hiidebmod mathod provides liquid-phase activity coefficients based on easily obtained pane-component properties. 

UNIFAC is an extension of UNIQUAC, in which the interaction parameters are estimated by means of contributions of groups. Firstly, the molecules are decomposed in characteristic structures, as functional groups and subgroups. Some small molecules are considered separately for more accuracy. Then the parameters involved in UNIQUAC-type equations are determined, as follows ... 

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