Status and results of group contribution methods

Group contribution methods, such as UNIFAC or ASOG can be successfully applied to the prediction of the real behavior of non electrolyte mixtures. Therefore these methods are used worldwide in the different process simulators for the synthesis and design of separation processes. Since the well known group contribution methods still show some weaknesses a modified version of the UNIFAC method has been developed. Furthermore the solution of groups concept has been applied to equations of state. This allows the simultaneous description of sub and supercritical compounds and at the same time the prediction of other important properties, such as densities, enthalpies, etc. . INTRODUCTION The knowledge of the real behavior of fluid mixtures plays an important role in different fields of application, such as for the synthesis, design and optimization of separation processes, the selection of selective solvents for extractive distillation, extraction and absorption, the calculation of chemical equilibrium compositions, the estimation of flash points of flammable liquid mixtures, etc. . For the description of the real behavior of non electrolyte mixtures equations of state or gE models can be used. Both permit the behavior of multicomponent systems to be calculated using binary data alone. When no experimental data are available, group contribution methods can be applied. For fitting reliable group interaction parameters a comprehensive data base is required. This was the reason for which the Dortmund Data Bank ( DDB ) was started in 1973. Today the Dortmund Data Bank contains, besides the required pure component properties, the most important mixture data in unified, evaluated and computer readable form. With the help of this data bank different well known group contribution methods, such as UNIFAC (ref. 1 ), ASOG ( ref. 2 ), modified UNIFAC ( ref. 3, 4 ) and PSRK ( Predictive Soave Redlich Kwong equation of state ) ( ref. 5 ) have been developed in our group, partly in collaboration with other research groups. MODIFIED UNIFAC METHOD Although the UNIFAC or ASOG method is used worldwide, both methods still show some weaknesses such as poor results for activity coefficients at infinite dilution ( ym ) or excess enthalpies ( h ) and systems with compounds very different in size. This is not surprising, since the data base ( nearly only VLE data ) used for fitting the interaction parameters of these methods only covered a limited concentration range ( 5 95 % ) and mostly compounds of similar size. Furthermore no quantitative information about the temperature dependence was used. To eliminate most of the above mentioned weaknesses a modified UNIFAC method has been developed ( ref. 3, 4 ). In the modified UNIFAC method the combinatorial part has been slighti 1 permanent address: Tsinghua University, Department of Chemical Engineering, Peking, China 919 920 J. GMEHLING eta/ ly changed, new main groups have been defined and at the same time temperature dependent parameters were introduced : Anm anm + bnmT + cnmT2 To obtain reliable results the parameters ( anm, b m, cn ) have been fitted simultaneously to different thermodynamic properties using a very large 8ata base. Besides experimental vapor liquid equilibrium ( VLE ) data heats of mixing data and activity coefficients at infinite dilution were also included in the data base. In a few cases liquid liquid equilibrium data ( LLE ) and excess heat capacities cz data have also been taken into account. All the required pure component and mixture data were taken directly from the Dortmund Data Bank ( DDB )( ref. 6 ). The present status of the Dortmund Data Bank is given in Table 1. TABLE 1. Current status of the Dortmund Data Bank ( DDB ) Pure component properties for approximately 3200 compounds Mixture data from approximately 8000 references Vapor Liquid Equilibria Liquid Liquid Equilibria Heats of Mixing Activity Coefficients at Infinite Dilution Gas Solubilities Excess Heat Capacities Azeotropic Data Solid Liquid Equilibria Number of isotherms or isobars 14700 6600 8500