UNIFAC method functional group activity coefficient

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. 

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. 

A wide-spread and widely applicable procedure for the calculation of the activity or the activity coefficient of non-electrolytic liquid mixtures is the so-called UNIFAC (Universal Functional Group Activity Coefficient) method (Reid et al. 1977). T. Oihsi and J. M. Prausnitz reported about an extension of the procedure which can be used to determine the activity of substances dissolved in amorphous polymers (Oishi and Prausnitz... 

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). 

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. 

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. 

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). 

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). 

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 (the universal qrrasi-cherrrical function group activity coefficient) is one of the best methods in estimating activity coefficient that has been established to date (Fredenslrmd et al., 1975 Fredenslrmd et al., 1977 Magnussen et al., 1981) has been successfully applied for the prediction of several LLE systems. This model depends on interaction parameters between each pair of components in the systerrr, which can be obtained by between each of the main groups. 

More reliable phase behaviour predictions for binary ionic liquid systems with carbon dioxide or organics come from group-contribution equations of state, such as the universal functional activity coefficient (UNIFAC) method, the group-contribution nonrandom lattice ffuid equation of... 

The liquid phase activity coefficient, which is a function of the subgroups, composition and temperature, can be evaluated using the UNIFAC group contribution method (Freedunslund et al., 1975). 

The activity coefficients of nonideal mixtures can be calculated using the molecular models of NRTL, UNIQUAC, or the group contribution method of UNIFAC with temperature-dependent parameters, since nonideality may be a strong function of temperature and composition. The Maxwell-Stefan diffusivity for a binary mixture of water-ethanol can be considered independent of the concentration of the mixture at around 40°C. However, for temperatures above 60°C, deviation from the ideal behavior increases, and the Maxwell-Stefan diffusivity can no longer be approximated as concentration independent. For highly nonideal mixtures, one should consider the concentration dependence of the diffusivities. 

When it is necessary to estimate activity coefficients where no data or very limited data are available, estimates may be made by using a group contribution method. In this case, a molecule is divided into fimctional groups, or subgroups of the molecule. These subgroups are assumed to act independently of the molecule in which they appear. Molecular interactions are accounted for by properly weighted sums of group interactions. Fredenslund, Jones, and Prausnitz developed the method for UNIQUAC and named it as universal functional activity coefficient (UNIFAC). Smith, van Ness, and Abbott report the equations for the activity coefficients of multicomponent solutions and their parameters. These equations are very... 

When using the UNIFAC model one needs to identify the functional subgroups present in each molecule by means of the UNIFAC group table. Next, similar to the UNIQUAC model, the activity coefficient for each species is written as eqn. (2.4.14), except for the the residual term, which is evaluated by a group contribution method in UNIFAC, The residual contribution of the logarithm of the activity coefficient of group k in the mixture. In F., is obtained from... 

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. 

The UNIFAC (universal functional activity coefficient) method [16] is similar to the ASOG method and is based on the four postulates of Wilson and Deal [13] regarding solution of groups. In UNIFAC, the activity coefficient is made of two parts. 

As an alternative, group contribution methods can be applied to predict the required activity coefficients. Methods like UNIFAC or modified UNIFAC are based on the functional groups of the components and group interaction parameters, which are fitted to a large number of experimental data. The calculated activity coefficients therefore have a high accuracy (Gmehling et ah, 2002) and can be used for reactive systems. Especially for very fast reactions, group contribution methods are often recommended. 

Perhaps the most important term in Eq. (5.2-3) is the liquid-phase activity coefficient, and methods for its prediction have been developed in maiiy forms and by many workers. For binary systems the Van Laar (Eq. (1.4-18)], Wilson [Eq. (1.4-23)], NRTL (Eq. (1.4-27)], and UmQUAC [Eq. (t.4-3ti)] relationships are useful for predicting liqnid-iffiase nonidealities, but they require some experimental data. When no dim 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-Hildebrand method provides liquid-phase activity coefficients based on easily obtained pure-component properties. 

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