Universal quasi chemical

UNIFAC was built on the framework of a contemporary model for correlating the properties of solutions in terms of pure-component molecular properties and fitting parameters, viz. UNIQUAC (the universal quasi-chemical) model... 

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. 

Here y,1 and y,2 are the corresponding activity coefficients of component i in phase 1 and 2, Xj1, and x,2 are the mole fraction of components i in the system and in phase 1 and 2 respectively. The interaction parameters between methylcyclohexane, methanol and ethyl benzene are used to estimate the activity coefficients from the UNIQUAC groups. Eqs. (1) and (2) are solved for the mole fraction (x) of component i in the two liquid phase.The UNIQUAC model (universal quasi -chemical model) is given by Abrams and prausnitz [8] as... 

UNIQUAC functional group activity coefficient universal quasi-chemical [15]... 

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. 

Uniquac Functional-group Activity Coefficient (where UNIQUAC = Universal Quasi-Chemical)... 

Modern theoretical developments in the molecular thermodynamics of liquid-solution behavior are based on the concept of local composition. Within a liquid solution, local compositions, different from the overall mixture composition, are presumed to account for the short-range order and nonrandom molecular orientations that result from differences in molecular size and intermolecular forces. The concept was introduced by G. M. Wilson in 1964 with the publication of a model of solution behavior since known as the Wilson equation. The success of this equation in the correlation of VLE data prompted the development of alternative local-composition models, most notably the NRTL (Non-Random-Two Liquid) equation of Renon and Prausnitz and the UNIQUAC (UNIversal QUAsi-Chemical) equation of Abrams and Prausnitz. A further significant development, based on the UNIQUAC equation, is the UNIFAC method,tt in which activity coefficients are calculated from contributions of the various groups making up the molecules of a solution. 

There are many other equations, which have been proposed, that do not result from Wohl s method. Two of the most popular equations are the Wilson and the universal quasi-chemical theory (UNIQUAC) by Abrams and Prausnitz.These equations are based on the concept of local composition models, which was proposed by Wilson in his paper. It is presumed in a solution that there are local compositions that differ... 

The Universal Quasi-chemical Theory or UNIQUAC method of Abrams and Prausnitz divides the excess Gibbs free energy into two parts. The dominant entropic contribution is described by a combinatorial part ( ). Intermolecular forces responsible for the enthalpy of mixing are described by a residual part ( ). The sizes and shapes of the molecule determine the combinatorial part, which is thus dependent on the compositions and requires only pure component data. Since the residual part depends on the intermolecular forces, two adjustable binary parameters are used to better describe the intermolecular forces. As the UNIQUAC equations are about as simple for multi-component solutions as for binary solutions, the UNIQUAC equations for multicomponent solutions are given below. Species are identified by subscript i, subscript j is a dummy index. Here, is a relative molecular surface area and r, is a relative molecular volume. Both of these quantities are pure-species parameters. 

Laar Margules Wilson nonrandom, two liquid phases (NRTL), or Renon-Prausnitz and Universal Quasi-Chemical Activity Coefficients (UNIQUAC). All of these equations have two constants except for the NRTL, which has three. 

Liquid activity models must be used in vapor-liquid equilibria calculations, with the appropriate model tested against available data. Models often used include Margules, Van Laar, Wilson, nonrandom two-liquid (NRTL), and universal quasi-chemical (UNIQUAC). For mixtures, mixing rules are used to combine pure component parameters. Table 16.28 suggests regions of applicability for different models. 

The universal quasi-chemical UNIQUAC and the modihed Guggenheim quasi-chemical (MCQ) models were compared to ht and predict the heat of mixing of alkanes, esters, and chlorinated hydrocarbons. The MCQ model was found to yield a better ht and accurately predicted PVC miscibility with aliphatic polyesters. The MCQ model was also shown by the same investigators to predict aliphatic polyester miscibility with poly-... 

UNIQUAC stands for UNIversal QUAsi-Chemical model, and has been developed by Abrams and Prausnitz (1978). Unlike Wilson and NRTL, where loeal volume fraction is used, in UNIQUAC the primary variable is the local surface area fraction O j. Each molecule is characterised by two structural parameters r, the relative number of segments of the molecule (volume parameter) and q, the relative surface area (surface parameter). Values of these parameters have been obtained in some cases by statistical mechanics. There is also a special form of UNIQUAC for systems containing alcohols, where a third surface parameter q can increase significantly the accuracy (Prausnitz et al., 1980). 

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). 

Uniquac equation (universal quasi chemical) Method based on the principle of the local compositions, similar to the Wilson and NRTL equations, which are derived as special cases also describes real liquid phases only valid for small and medium operating pressures Gmehung, j., Anderson, T.F., and Prausnitz, J.M., Ind. Eng. Chem. Fund. 17 (1978) 269. 

It would be desirable to apply analytical expressions for the activity coefficient, which are not only able to describe the concentration dependence, but also the temperature dependence correctly. Presently, there is no approach completely fulfilling this task. But the newer approaches, as for example, the Wilson [13], NRTL (nonrandom two liquid theory) [14], and UNIQUAC (universal quasi-chemical theory) equation [15] allow for an improved description of the real behavior of multicomponent systems from the information of the binary systems. These approaches are based on the concept of local composition, introduced by Wilson [13]. This concept assumes that the local composition is different from the overall composition because of the interacting forces. For this approach, different boundary cases can be distinguished ... 

Wilson, NRTL (Non-Random Two Liquidd), UNIQUAC (UNIversal QUAsi-Chemical)... 

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. 

T = Absolute temperature (Kelvin) m = Interaction energy Uniquac = Universal quasi chemical X = Mole fraction... 

---