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.

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