UNIFAC equation

The UNIFAC equation is the preferred equation for use in design, and it is included as a sub-routine in most simulation and design programs. 

The UNIQUAC and UNIFAC equations can be used to estimate liquid-liquid equilibria, see Chapter 8. 

The group interaction parameter anm is found from the large sets of VLE and LLE data in the literature, which are tabulated for many subgroups. It is worth noting that a m a. There are some modifications to the original UNIFAC equation in order to make the model robust for some complex systems. In the UNIFAC-DM method, the modification is made on the combinatorial part ... 

The UNIQUAC equation is the basis for the development of the group contribution method, the UNIFAC equation, which predicts liquid activity coefficients from component structures on the basis of interactions between chemical functional groups. 

The effect of phase compositions is often considerable on K-values (distribution coefficients) for liquid-liquid extraction. Therefore, it is best also to provide initial estimates of Xy and y,-, from which initial values of K-,j are computed. Initial values of Xy are obtained by linear interpolation with stage of the compositions of the known entering and assumed exit streams. Corresponding values of y, are computed by material balance from (15-1). Values of yiLj and yiv., are determined from an appropriate correlation—for example, the van Laar, NRTL, UNIQUAC, or UNIFAC equations discussed in Chapter 5. Corresponding K-values are obtained from the following equation, which is equivalent to (4-31). 

Most of the empirical and semitheoretical equations for liquid-phase activity coefficient listed in Table 5.3 apply to liquid-liquid systems. The Wilson equation is a notable exception. As examples, the van Laar equation will be discussed next, followed briefly by the NRTL, UNIQUAC, and UNIFAC equations. 

Eor mixtures containing polar substances, more complex predictive equations for that involve binary-interaction parameters for each pair of components in the mixture are required for use in Eq. (13-4), as discussed in Sec. 4. Six popular expressions are the Mar-gules, van Laar, Wilson, NRTL, UNIFAC, and UNIQUAC equations. Extensive listings of binary-interaction parameters for use in all but the UNIFAC equation are given by Gmehling and Onken (op. cit.). They obtained the parameters for binary systems at 101.3 kPa (1 atm) from best fits of the experimental T-y-x equilibrium data by setting and Of to their ideal-gas, ideal-solution limits of 1.0 and P VP respectively, with the vapor pressure P given by a three-constant Antoine equation, whose values they tabulate. Table 13-2 lists their parameters for some of the binary systems included in... 

This method deserves special mention because, unlike all of the previous methods, it allows the prediction of activity coefficients based entirely on tabulated parameters i.e., no fitting of parameters is necessary. It builds on UNIQUAC and is based on the premise that a solution maybe regarded as a mixture of structural units rather than of chemical species. For example, a mixture of n-pentane and n-heptane is considered as a mixture of CHa and CH3 subgroups and so is a mixture of cyclohexane and ethane. In this approach, interaction parameters are determined between a finite number of subgroups and are tabulated. It is then possible to calculate activity coefficients for any solution, binary or multicomponent, from a relatively small number of tabulated values. This is the main advantage of the method. Its applicability is limited to components that are liquid at 25 C. Parameters for the UNIFAC equation have been... 

Problem 12.24 Use the UNIFAC equation to estimate the enthalpy of mixing of a solution that contains 37% (mol) acetic acid in water at 20 °C, 1 bar. 

Problem 12.27 Use the UNIFAC equation to do the following calculations for the system methanol(i)-acetic acid(2) ... 

Problem 12.29 Use the UNIFAC equation to calculate the Pxy graph for the system water/acetic acid at 100 °C. The saturation pressure of acetic acid at 100 °C is 0.57 bar. 

Table D-l Parameters for the UNIFAC Equation. Main refers to the index of the main group k refers to the index of the subgroup.

Unifac equation (uniquac functional group activity coefficient) Contributions of individual functional groups prediction of the interaction between the activity coefficients of functional groups by interaction parameters, only valid for small and medium operating pressures Fredenslund, a., Gmehling, j., and Rasmussen, P., Vapor-Liquid Equilibria using Unifac. Elsevier Publ. 1977. 

Xj. = Adjustable parameter in the UNIFAC equation = Pure-component constant. 

C = Combinatorial part of the activity coefficient Q = UNIFAC equation R = Residual part of the activity coefficient... 

Formic acid, cumene are used to estimate the activity coefficients from the UNIFAC. Equations 1 and 2 are solved for the mole fraction (x) of component i in the two liquid phases. This method of calculation gives a single tie line. 

Appendix 1 gives the equations for calculating the combinational part and the residual part of the activity coefficient for the UNIFAC equation modified by Larsen et al. [LAR 87]. The volume (q) and surface groups (qi)... 

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