Aqueous UNIFAC

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)]. 

Arbuckle, W. B. (1986) Using UNIFAC to calculate aqueous solubilities. Environ. Sci. Technol. 20, 1060-1064. 

Kan, A. T., and M. B. Tomson, UNIFAC prediction of aqueous and nonaqueous solubilities of chemicals with enviromental interest , Environ. Sci. Technol., 30,1367-1376 (1996). 

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

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. 

Estimation of Koc Using Aqueous Phase and Soil Organic Matter (SOM) Phase Activity Coefficients Calculated from UNIFAC and ELBRO-FV... 

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. 

A test set of 6 to 13 aroma compound partition coefficients between different food contact polymers (low density polyethylene (LDPE), high density polyethylene (HDPE) polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA)) and different food simulant phases (water, ethanol, aqueous ethanol/water mixtures, methanol, 1-propanol) were taken from the literature (Koszinowski and Piringer, 1989, Baner, 1992, Franz, 1990, Koszinowski, 1986, Franz, 1991, Baner, 1993, Piringer, 1992). Table 4-2 shows the test set of 13 different aroma compounds, with their properties and their structures. The experimental data were compared to estimations using different estimation methods of UNIFAC-FV, GCFLORY (1990), GCFLORY (1994) and ELBRO-FV. 

Baner, A.L., Partition coefficients of aroma compounds between polyethylene and aqueous ethanol and their estimation using UNIFAC and GCFEOS. Ph.D. Dissertation, Michigan State University, E. Lansing, 1992. 

Crittenden, Jr. E.D., Hixon, A.W. (1954) Extraction of hydrogen chloride from aqueous solutions. Ind. Eng. Chem. 46, 265-168. Dallos, A., Wienke, G., Ilchman, A., Gmehlong, J. (1993) Vorausberechnung von octanol/wasser-verteilungskoeffizienten mit hilfe der UNIFAC-method. Chem.-Ing.-Tech. 65(2), 201-203. 

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 application of UNIFAC to the solubility of naphthalene in nonaqueous mixed solvents provided satisfactory results when compared to experimental data (Acree, 1984). However, the UNIFAC was inaccurate in predicting the solubilities of solids in aqueous solutions (Fan and Jafvert, 1997). Furthermore, the application of the traditional UNIFAC to mixtures containing a polymer or another large molecule, such as a drug, and low molecular weight solvents is debatable (Fredenslund and Sprensen, 1994). The reason is that the UNIFAC parameters were determined mostly... 

Li, A. Doucette, W. J. Andren, A. W. Estimation of aqueous solubility, octanol/water partition-coefficient, and henrys law constant for polychlorinated biphenyls using UNIFAC. Chemo-sphere 1994, 29, 657-669. 

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. 

Liquid Mixtures Compositions at the liquid-vapor interface are not the same as in the bulk liquid, and so simple (bulk) composition-weighted averages of the pure-fluid values do not provide quantitative estimates of the surface tension at the vapor-liquid interface of a mixture. The behavior of aqueous mixtures is more difficult to correlate and estimate than that of nonpolar mixtures because small amounts of organic material can have a pronounced effect upon the surface concentrations and the resultant surface tension. These effects are usually modeled with thermodynamic methods that account for the activity coefficients. For example, a UNIFAC method [Suarez, J. T. C. Torres-Marchal, and P. Rasmussen, Chem. Eng. Set, 44 (1989) 782] is recommended and illustrated in PGL5. For nonaqueous systems the extension of the parachor method, used above for pure fluids, is a simple and reasonably effective method for estimating a for mixtures. 

A modest data base for aqueous systems has beSen obtained by the use of these techniques. The data are reasonably reliable for systems with y values less then a couple thousand and not measured by the liquid-liquid chromatography technique. A reliable data base is required in the development of predictive techniques for y. Several predictive techniques are currently available the MOSCED (45) model has not yet been extended to aqueous systems. UNIFAC (46-48), which is really an outgrowth of ASOG (21,49) does include water, but with mixed results at best. Linear solvation energy relationships (LSER s) have been used to correlate ratios of y values for aqueous systems (50) and may be capable of some prediction. Nonetheless, a more extensive and accurate data base is what is really needed for correlation development... 

Banneijee (h) determined the solubilities of mixtures of several chlorobenzenes with one another and with toluene and benzyl alcohol. The results agreed satisfactorily with predictions from equation (8). The water phase activity coefficients were obtained from the pure component solubilities via equation (6). The organic phase activity coefficients for mixtures of chlorobenzenes with hydrocarbons were predicted by the UNIFAC group contribution method. Mixtures containing only chlorobenzenes were essentially ideal, Yi = When benzyl alcohol was used as a cosolute, the agreement with equation (8) was improved by using UNIFAC to predict the activity coefficient in the aqueous phase. 

Gupta and Heidemann (1990) used a modified UNIFAC model to predict the effects of temperature and pH on the solubility of amino acids in water. They also made a similar approach to the modelling of the solubility of several antibiotic substances in mixed non-aqueous solvents. Macedo, Skovborg and Rasmussen (1990) used a modified UNIFAC model to calculate phase equilibria for aqueous solutions of strong electrolytes. 

This effluent is cooled to 38°C and enters a flash-decanter vessel at 278 kPa. Three phases leave that vessel. The vapor phase (hydrogen rich) is sent to the vapor separation system. The aqueous phase (mostly water, with some methanol) is sent to the aqueous stream separation system. The organic-rich phase is sent to the organic stream separation system, which you will design. To obtain the composition of the feed to your section, use a simulator with the UNIFAC method to perform a three-phase flash for the above conditions. If the resulting organic liquid stream contains small amounts of hydrogen and water, assume they can be completely removed at no cost before your stream enters your separation section. 

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