Extension of the GE Model FlexQUAC to Asymmetric Systems – NonlinearTransformation of the Surface Fraction in the Residual Part
Jürgen Rareya,b,c,*, Thishendren Naidooa and Deresh Ramjugernatha
aSchool of Chemical Engineering, University of Kwa-Zulu Natal, Durban 4041, RSA
b Industrial Chemistry, Institute for Pure and Applied Chemistry (F V), University of Oldenburg, 26111 Oldenburg, Germany
c DDBST GmbH, 26121 Oldenburg, Germany
*(corresponding author, tel.: +49 441 798 3846, fax: +49 441 798 3330, email: Juergen@Rarey.net)
Previously a method to increase the flexibility of GE-models and equations of state was proposed (Rarey, 2005). The formalism is based on a nonlinear but symmetrical transformation of the concentration space and was applied to the commonly used UNIQUAC equation. The resulting model (FlexQUAC) showed, besides significantly improved data correlation, the ability to describe VLE and LLE simultaneously. A symmetric transformation was chosen in order to minimize the intercorrelation of model parameters. It was already suggested in the previous publication, that this might not be adequate for systems with molecules of very different size. In order to extend the approach to asymmetric systems, a new model was derived in a similar manner. Instead of a transformation of the mole fraction, now only a flexibilisation of the surface fraction in the residual part of UNIQUAC was applied. The Guggenheim-Staverman expression in the combinatorial part was not transformed. Instead of the original combinatorial term the more suitable modification of Weidlich and Gmehling (1987) was used. The new model (FlexQUAC-Q) was applied to the same set of test-data as the previous model. It generally performs similar but shows definite advantages in case of asymmetric systems. Unlike the previous model, FlexQUAC-Q can be more easily incorporated into a group contribution method. The equations for a flexibilised functional group activity coefficient model (FlexFAC) are presented.
