UNIFAC Limitations

Figure 13.16 UNIFAC limitations isomers. Experimental and calculated y s for alcohols(l) in n-heptane(2) at. Xj = 0.1.

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. 

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)]. 

Another estimation method of mixture flashpointe was sugg ed by Gmehling (note p.63). The method uses the forecast technique of activity coefficients of iiquid mixtures called UNIFAC that would therefore enable calculation of the vapour pressure of the mixtures and, thanks to Le Chdtelier equation, calculate the temperature to which the mixture has to be heated so that its equilibrium concentration reaches the lower explosive limit. 

Care must be exercised in applying the UNIFAC method. The specific limitations of the method are ... 

The application of thermodynamic models to the correlation and prediction of pharmaceutical solubility behaviour is an underutilized technique in today s process research and development environment. This is due to the relatively poor accuracy and limited predictive ability of the previous generation of models. Recent advances in computational chemistry and an increased focus on the life science sectors has led to the development of more appropriate models with significantly improved predictive capabilities. The NRTL-SAC and Local UNIFAC approaches will be discussed here with additional examples given in section 8. 

The group contribution method UNIFAC [18] is based on the UNIQUAC thermodynamic model as well. It thus suffers from the same thermodynamic approximations as UNIQUAC, especially for strong interactions in the infinite dilution limit. 

When gas solubility data are lacking or are unavailable at the desired temperature, they can be estimated using available models. The method of Prausnitz and Shair (1961), which is based on regular solution theory and thus has the limitations of that theory. The applicability of regular solution theory is covered in detail by Hildebrand et al. (1970). A more recent model, now widely used, is UNIFAC, which is based on structural contributions of the solute and solvent molecular species. This model is described by Fredenslund et al. (1977) and extensive tabulations of equilibrium data, based on UNIFAC, have been published by Hwang et al. (1992) for aqueous systems where the solute concentrations are low and the solutions depart markedly from thermodynamic equilibrium. 

Thomas, E.R., Eckert, C.A. Prediction of limiting activity coefficients by a modified separation of cohesive energy density model and UNIFAC. End. Eng. Chem. Process Des. Dev., 1984,23 194-209. 

The compositions of the vapor and liquid phases in equilibrium for partially miscible systems are calculated in the same way as for miscible systems. In the regions where a single liquid is in equilibrium with its vapor, the general nature of Fig. 13.17 is not different in any essential way from that of Fig. I2.9< Since limited miscibility implies highly nonideal behavior, any general assumption of liquid-phase ideality is excluded. Even a combination of Henry s law, valid for a species at infinite dilution, and Raoult s law, valid for a species as it approaches purity, is not very useful, because each approximates real behavior only for a very small composition range. Thus GE is large, and its composition dependence is often not adequately represented by simple equations. However, the UNIFAC method (App. D) is suitable for estimation of activity coefficients. 

The importance of a suitable G.C. method for evaluating the pure solid properties was evidenced. The limitations of the different G.C. methods are due to the limited experimental data available in the literature for heavy multifunctional compounds. The PR EOS with classical mixing rules gives the same results than the most complex UNIFAC approach. The importance of the sublimation pressure for correlating solubility data was underlined. 

Thomas, E.R, and Eckert, C.A. (1984). Prediction of Limiting Activity Coefficients by a Modified Separation of Cohesive Energy Density Model and UNIFAC. I EC.Process Des.Dev., 23, 194-209. 

In the exploratory phases of product and process research and development, generally little or limited data are available for model parameterization. However, chemists and engineers need to evaluate a multitude of possible molecular or process variations. Rather than high accuracy, successful evaluation depends on the ability to discard the least viable options and select better options for further detailed studies. Estimation methods that give reliable results for new or unknown species are required. The well-known group-contribution methods, like UNIFAC, have demonstrated their value in cases where the molecules can be decomposed to functional groups for which parameters are already available. 

Estimation of liquid mixture viscosity without any mixture data is difficult because the viscosity is strongly affected by large molecular size differences and strong cross interactions between the different types of molecules. Viscosity-composition plots for aqueous mixtures can have maxima or minima, and viscosities for these mixtures are particularly difficult to estimate. The UNIFAC-VISCO method described below can be used to predict liquid viscosity of organic mixtures without any mixture data. It is relatively successful even for large differences in molecular size, but it is currently limited in scope by the small number of group contributions available. 

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... 

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 DOS BASIC programs were provided with the third edition, and are also supplied in stand-alone executible and BASIC code forms. They are a similar suite of VISUAL BASIC programs, except that there is no PROPERTY database. The stand-alone versions run in a DOS window, and have less functionality and are not as user-friendly as the VISUAL BASIC programs. Also, the DOS BASIC UNIFAC program uses the original version of UNIFAC (as a result of DOS BASIC memory limitations). The BASIC code programs can be modified if a BASIC or QUICKBASIC compiler is available. 

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