Vapor-Liquid Phase Equilibrium Calculations with the PVDW Model

Vapor-Liquid Phase Equilibrium Calculations with the I PVDW Model 

The performance of two-parameter cubic equations of state with the conventional van der Waals mixing rules (1 PVDW model) is relatively well known and is presented here mainly for reference, but also to indicate certain misconceptions about this method. The results presented in this section were obtained using the computer program VDW provided on the accompanying diskette. The program details are presented in Section D.3 of Appendix D. 

When fitting VLE data with the IPVDW model, it is found that the binary interaction parameter kij is approximately zero for relatively simple mixtures, such as alkane mixtures, whereas for some other mixtures such as hydrocarbons with industrial gases like carbon dioxide and organic solvents, it is not only nonzero but will also change in value with temperature. For highly nonideal mixtures, which are our main concern here, accurate correlation of VLE is not possible by this method. 

The overall conclusion is that even though the conventional van der Waals mixing rules are simple to use and conform to the second virial coefficient boundary condition, they are very limited in their application and are not useful for either the correlation or the prediction of the VLE of complex mixtures. 

Why the van der Waals one-fluid mixing rules cannot describe highly nonideal mixtures can be understood by starting with the relation between the molar excess 

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