Notation of the Wilson, NRTL, and UNIQUAC Equations in Process Simulation Programs

Notation of the Wilson, NRTL, and UNIQUAC Equations in Process Simulation Programs 

For the use in a process simulator. Eq. (C.242) is not appropriate as an expression for the Wilson equation, as the specific volumes have an influence on the activity coefficients. When the binary parameters Akij are stored, they are related to the specific volumes that were used in the parameter regression run. If the pure component values are changed, maybe due to an improved data situation, the binary parameters would have to be refitted, which is usually not considered by the user. In process simulation programs, the Wilson equation is therefore written in a different way which avoids these disadvantages. Starting with Eq. (C.242), we can write 

A further advantage is that the gas constant R is no more involved in the parameters therefore, the parameters do not depend on the units or the value used for R. As well, the parameters are always written with a positive sign in the equation. It is worth mentioning that In vj/v, represents the linear temperature dependence of the binary interaction parameter. 

In case temperature-dependent parameters are necessary, the A parameters are fitted to experimental data as well, making the equation completely independent of the specific volume. 

In case temperature-independent parameters are used, only the 6 parameters are fitted. As worked out in Eq. (C.246). the A parameters can be set to A,j = In(v /Vj) and stored in this way. Without loss of quality, the A,j can also be set to 0, before the B parameters are adjusted. 

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