Predictive UNIFAC method

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]). 

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. 

A model is needed to calculate liquid-liquid equilibrium for the activity coefficient from Equation 4.67. Both the NRTL and UNIQUAC equations can be used to predict liquid-liquid equilibrium. Note that the Wilson equation is not applicable to liquid-liquid equilibrium and, therefore, also not applicable to vapor-liquid-liquid equilibrium. Parameters from the NRTL and UNIQUAC equations can be correlated from vapor-liquid equilibrium data6 or liquid-liquid equilibrium data9,10. The UNIFAC method can be used to predict liquid-liquid equilibrium from the molecular structures of the components in the mixture3. 

The vapour pressure of BHET is approximately three orders of magnitude lower than that of EG. Nevertheless, evaporation of BHET still occurs in significant amounts under vacuum. In Figure 2.26, the experimentally determined vapour pressure of BHET is compared to the vapour pressure predicted by the Unifac group contribution method [95], The agreement between the measured and calculated values is quite good. In the open literature, no data are available for the vapour pressure of dimer or trimer and so a prediction by the Unifac method is shown in Figure 2.26. The correspondence between measured and predicted data for BHET indicates that the calculated data for dimer and trimer... 

The Oishi-Prausnitz modification, UNIFAC-FV, is currently the most accurate method available to predict solvent activities in polymers. Required for the Oishi-Prausnitz method are the densities of the pure solvent and pure polymer at the temperature of the mixture and the structure of the solvent and polymer. Molecules that can be constructed from the groups available in the UNIFAC method can be treated. At the present, groups are available to construct alkanes, alkenes, alkynes, aromatics, water, alcohols, ketones, aldehydes, esters, ethers, amines, carboxylic acids, chlorinated compounds, brominated compounds, and a few other groups for specific molecules. However, the Oishi-Prausnitz method has been tested only for the simplest of these structures, and these groups should be used with care. The procedure is described in more detail in Procedure 3C of this Handbook. 

The application of UNIFAC to the solid-liquid equilibrium of sohds, such as naphthalene and anthracene, in nonaqueous mixed solvents provided quite accurate results [11]. Unfortunately, the accuracy of UNIFAC regarding the solubility of solids in aqueous solutions is low [7-9]. Large deviations from the experimental activity coefficients at infinite dilution and the experimental octanol/water partition coefficients have been reported [8,9] when the classical old version of UNIFAC interaction parameters [4] was used. To improve the prediction of the activity coefficients at infinite dilution and of the octanol/water partition coefficients of environmentally significant substances, special ad hoc sets of parameters were introduced [7-9]. The reason is that the UNIFAC parameters were determined mostly using the equihbrium properties of mixtures composed of low molecular weight molecules. Also, the UNIFAC method cannot be applied to the phase equilibrium in systems containing... 

The PERVAP simulator (tubular module) was developed by Alvarez (2005), using FORTRAN language (Compaq Visual Fortran Professional Edition 6.6.a). The mathematical model applied is based on the solution-diffusion mechanism. Activity coefficients of the components in the feed phase (jj) were determined using the UNIFAC method (Magnussen et al, 1981). The prediction of diffusion coefficient (Z) ) was carried out using the free-volume theory. 

Cheng, H., Kontogeorgis, G.M. and Stenby, E.H. (2005) Correlation and prediction of environmental properties of alcohol ethoxylate surfactants using the UNIFAC method. Ind. Eng. Chem. Res., 44, 7255-7261. 

The UNIFAC-FV and Entropic-FV models are not the only extensions of UNIFAC to polymers. Similar models have been presented by Iwai and Arai ° ° and by Choi etal. ° Choi s model is not based on UNIFAC but on ASOG. ASOG (Analytical Solution of Groups) is a predictive GC method for calculating activities, similar to UNIFAC, but which has not experienced the widespread use of UNIFAC. It is mostly employed in Japan. Besides this difference, the models of Aral and Choi contain an FV, which is different from that of Equation 16.49. These two models have been applied with success for some polymer-solvent systems but not for LEE. 

Rocklin and Bonner (65) developed a computer method that predicts solvent balance and evaporation times of water-solvent blends at any humidity with any number of water-soluble organic solvents. The method also can be used for regular water-free solvent blends but Ignores humidity. Key considerations of the method are the following it uses the UNIFAC method for calculating activity coefficients it computes the actual evaporation temperature on the filter paper substrate it calculates evaporation rates at the calculated temperature by using the activity coefficients at that temperature humidity is accommodated by applying a correction factor to the water evaporation rate. Experimental data on several systems verified the computer calculations. 

Alternatively, the van Laar, NRTL. and UNIQUAC activity coefficient models could be used, yielding more accurate results. (The UNIFAC method can also be used to predict liquid-liquid equilibrium, but only with different main group interaction parameters than are used to predict vapor-liquid equilibrium.)... 

The UNIFAC method for predicting liquid-phase activity coefficients is based on the UNIQUAC equation (5-72), wherein the molecular volume and area parameters in the combinatorial terms are replaced by... 

While not as quantitative for predicting liquid-liquid equilibrium as the UNIQUAC equation, the UNIFAC method can be used for order-of-magnitude estimates in the absence of experimental data. The UNIFAC method is almost always successful in predicting whether or not two liquid phases will form. Comparisons of experimental data with predictions by the UNIFAC method are given for a number of ternary systems by Fredenslund et al. Two such comparisons are shown in Fig. 5.11. 

Figure 4. Phase diagrams for temaiy system (1-hexene + n-hexane + [BMIMJpsrOs] + AgNOs) at high concentration of AgNOs (a 0.20 mol salt /mol IL, b 0.67 mol salt /mol IL) (Blue solid lines for experimental data Red short dashed hnes for prediction results using the UNIFAC method).

Chapter 5 gives a comprehensive overview on the most important models and routes for phase equilibrium calculation, including sophisticated phenomena like the pressure dependence of liquid-liquid equilibria. The abilities and weaknesses of both models and equations of state are thoroughly discussed. A special focus is dedicated to the predictive methods for the calculation of phase equilibria, applying the UNIFAC group contribution method and its derivatives, that is, the Mod. UNIFAC method and the PSRK and VTPR group contribution equations of state. Furthermore, in Chapter 6 the calculation of caloric properties and the way they are treated in process simulation programs are explained. 

Activity coefficients can be determined by specific methods for a defined thermodynamic system or for general purposes by a predictive method. The best known method is the UNIFAC method [35-37], which spHts the activity coefficient into two parts, a combinatorial and a residual part ... 

The UNIFAC method can be applied to predict the evaporation behavior of fuels such as gasoline, kerosene, and diesel. In the following example, the evaporation behavior of gasoline is analyzed. Applying a boihng method according to EN ISO... 

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. 

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... 

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