UNIFAC

Significant effort has been devoted to activity coefficient estimation methods in the fields of chemical engineering, environmental and pharmaceutical research because the necessary experimental data for many substances are not available and are difficult to measure. While there are numerous activity coefficient estimation methods for 

One of the central problems with estimating activity coefficients in polymer systems is that general observations made for low molecular weight component systems are no longer valid for polymers. It is observed that solution dependent properties are no longer directly proportional to the mole fraction of solute in the polymer at dilute concentrations. For example the solute partial pressure in a system containing a polymer is no longer directly proportional to its mole fraction, which is an apparent deviation from Raoult s law. 

All current activity coefficient estimation models are by necessity semi-empirical in nature, because too little is known about solution theory for outright estimation. Chemical modeling is not readily available and is not far enough developed to make this type of calculation. The constants required by these models must be estimated using either experimental data (e.g. an infinite dilution activity coefficient or a molar volume) or group contributions derived from experimental data (e.g. interaction constants, molecular volumes and surface areas). 

The activity coefficient is described in UNIFAC-FV and several other estimation models as being roughly composed of two or three different components. These components represent combinatorial contributions (yf) which are essentially due to differences in size and shape of the molecules in the mixture, residual contributions (yt) which are essentially due to energy interactions between molecules in the solution, and free volume (y[v) contributions which take into consideration differences between the free volumes of the mixture s components  

The concept of free volume varies on how it is defined and used, but is generally acknowledged to be related to the degree of thermal expansion of the molecules. When liquids with different free volumes are mixed, that difference contributes to the excess functions (Prausnitz et al., 1986). The definition of free volume used by Bondi (1968) is the difference between the hard sphere or hard core volume of the molecule (Vw= van der Waals volume) and the molar volume, V  

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Fredenslund, A., J. Gmehling, and P. Rasmussen "Vapor-Liquid Equilibria using UNIFAC," Elsevier, New York, 1977. 

Detailed and extensive information on the UNIFAC method for estimating activity coefficients with application to vapor-liquid equilibria at moderate pressures. 

When no experimental data at all are available, activity coefficients can sometimes be estimated using the UNIFAC method (Fredenslund et al., 1977a, b). However, for many real engineering problems it is often necessary to obtain new experimental data. 

Solubihties of 1,3-butadiene and many other organic compounds in water have been extensively studied to gauge the impact of discharge of these materials into aquatic systems. Estimates have been advanced by using the UNIFAC derived method (19,20). Similarly, a mathematical model has been developed to calculate the vapor—Hquid equiUbrium (VLE) for 1,3-butadiene in the presence of steam (21). 

A.queous Solubility. SolubiHty of a chemical in water can be calculated rigorously from equiHbrium thermodynamic equations. Because activity coefficient data are often not available from the Hterature or direct experiments, models such as UNIFAC can be used for stmcture—activity estimations (24). Phase-equiHbrium relationships can then be appHed to predict miscibility. Simplified calculations are possible for low miscibiHty however, when there is a high degree of miscibility, the phase-equiHbrium relationships must be solved rigorously. 

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). 

Fig. 4. UNIFAC group interaction parameter matrix, the ISi represents parameters fit and parameters not available (168). A represents an aromatic...

The basic equational form of UNIFAC and many other QSARs is... 

Both UNIFAC and ASOG are typically generalized to multicomponent systems in commercial software packages. An important feature of these methods is that only binary interaction information is used to generate multicomponent predictions. 

A.ssessmentofUNIFy C. UNIFAC is a method to predict the activity of binary Hquid solutions in the absence of all data except stmctural information. Because state-of-the-art real fluid estimation methods are empirical or semi-empirical, the use of more data results in improved activity estimation. 

UNIFAC was developed for situations where experimental data are scarce, and its appHcation should generally be that of last resort (3,7,162,178). That is, UNIFAC is not a method for comparative testing of methods based on experimental data even though the method is sometimes Hsted in commercial computer simulator menus without indication of its limitations. 

Numerous assessments of the rehabiUty of UNIFAC for various appHcations can be found in the Hterature. Extrapolating a confidence level for some new problem is ill-advised because accuracy is estimated by comparing UNIFAC predictions to experimental data. In some cases, the data are the same as that used to generate the UNIFAC interaction parameters in the first place. Extrapolating a confidence level for a new problem requires an assumption that the nature of the new problem is similar to that of the UNIEAC test systems previously considered. With no more than stmctural information, such an assumption may not be vaHd. 

The use of UNIFAC for estimating activity coefficients in binary and multicomponent organic and organic—water systems is recommended for those systems composed of nonelectrolyte, nonpolymer substances for which only stmctural information is known. UNIFAC is not recommended for systems for which some reUable experimental data are available. The method, including revisions through 1987 (39), is available in commercial software packages such as AspenPlus (174). 

Octano/—Water Partition Coefficient. The Fragment approach (234—236) has been reviewed (227) and another method similar to the UNIFAC refit for Henry s constant has been proposed. Improved accuracy for many species and the abiUty to correct for temperature effects have been claimed for the newer method. 

A. Fredenslund, J. Gmehting, and P. Rasmussen, Uapor—Eiquid Using UNIFAC, a Group Contribution Method, Elsevier Scientific Publishing,... 

Data Reduction Correlations for G and the activity coefficients are based on X T.E data taken at low to moderate pressures. The ASOG and UNIFAC group-contribution methods depend for validity on parameters evaluated from a large base of such data. The process... 

For most LLE applications, the effect of pressure on the Yi < an be ignored, and thus Eq. (4-327) constitutes a set of N equations relating equilibrium compositions to each other and to temperature. For a given temperature, solution of these equations requires a single expression for the composition dependence of suitable for both liquid phases. Not all expressions for suffice, even in principle, because some cannot represent liquid/liquid phase splitting. The UNIQUAC equation is suitable, and therefore prediction is possible by the UNIFAC method. A special table of parameters for LLE calculations is given by Magnussen, et al. (Jnd E/ig Chem Process Des Dev, 20, pp. 331-339 [1981]). 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

One problem limiting the consideration of salt extractive distillation is the fact that the performance and solubility of a salt in a particiilar system is difficult to predict without experimental data. Some recent advances have been made in modeling the X T.E behavior of organic-aqueous-salt solutions using modified UNIFAC, NRTL, UNIQUAC, and other approaches [Kumar, Sep. Sci. Tech., 28(1), 799 (1993)]. 

Fredenslund, A., Gmehling, J., and Rasmussen, P., Vapor-Liquid Equilibria using UNIFAC, Elsevier Scientific Publishing Co., 1977. 

Examples of chemical functional groups used in UNIFAC... 

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... 

In UNIFAC, one eonstruets the moleeular parameters fj and q from the appropriate group eontributions for eomputing the eombinatorial and eomputes the residual contribution to G in terms of group fractions. The latter requires the energeties of the interaetion between main groups m and n in the form ... 

Tseng et al. [164] suecessfully used UNIFAC to optimize polymer-solvent interactions in three-solvent systems, determining polymer activity as a function of the solvent eomposition. The composition yielding the minimum in polymer aetivity was taken as the eriterion for optimum interaetion, and it eompared well with experimental measurements of dissolution rate and solution clarity. Better agreement was obtained using UNIFAC than using solubility parameter methods. 

Recent unpublished results from the author s laboratory, shown in Fig. 26, confirm the good correlation obtainable between mechanically measured interfacial strengths, as detailed above, and UNIFAC calculations of ( —AG )o5 for... 

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