Ind. Eng. Chem. Res. 2001, 40, 957-964
957
From UNIFAC to Modified UNIFAC (Dortmund)† Ju 1 rgen Lohmann,‡ Ralph Joh,§ and Ju 1 rgen Gmehling*,‡ Lehrstuhl fu¨ r Technische Chemie (FB9), Carl von Ossietzky Universita¨ t Oldenburg, Postfach 2503, D-26111 Oldenburg, Germany, and Axiva GmbH, Industriepark Ho¨ chst, D-65926 Frankfurt/Main, Germany
The group contribution method UNIFAC has become very popular because of its availability via commercial process simulators, its reliably predicted vapor-liquid equilibrium (VLE) results, its simple usage, and, above all, its large range of applicability. Nevertheless, the original UNIFAC shows a few weaknesses. For example, there is no chance to describe simultaneously VLE data and excess enthalpies with the required accuracy. This means that the temperature dependence of the activity coefficient following the Gibbs-Helmholtz equation cannot be described correctly. Furthermore, predictions of the real phase behavior in the dilute region and for asymmetric systems can lead to poor results. The aim of this paper is to show how the mentioned weaknesses were overcome by using modified UNIFAC (Dortmund) instead of the original UNIFAC method, in which the required temperature-dependent parameters are fitted simultaneously to a large database covering various types of reliable phase equilibrium and excess property data. Furthermore, 10 new modified UNIFAC (Dortmund) parameter pairs for the new main group “epoxides“ and the interactions between aromatics and perfluorinated compounds are given. Introduction Group contribution methods are of major interest for the synthesis, design, and optimization of thermal separation processes and a large number of other applications of industrial interest. In particular, they can be successfully applied to predict phase equilibria and excess properties which are necessary requirements for a reliable synthesis and design of chemical processes, in particular separation processes. Especially during the development of chemical processes, group contribution methods are most useful.1 Different group contribution methods have been suggested, in which UNIFAC2,3 has become one of the most popular models. The main objective during the development of the original UNIFAC was the prediction of vapor-liquid equilibrium (VLE) data for a limited temperature range from 290 to 400 K. With the help of UNIFAC, the VLE behavior of a large number of mixtures (binary and higher) can be predicted using van der Waals volumes (Rk) and surface areas (Qk) as well as the fitted binary group interaction parameters anm and amn. Extensive tables for the mentioned van der Waals properties and group interaction parameters have been published.4 Nevertheless, weaknesses of the original UNIFAC method are also well-known. Predictions of the real phase behavior for mixtures at high (>400 K) and low (
