Benedict, Webb, and Rubin Equation of State

The virial equation of state given by eq 5.1 applies to gases and has been discussed in Chapter 3. The composition dependence for the second and third virial coefficients are obtained from statistical mechanics and given by eqs 5.3 and 5.4. Consequently, the virial equation has formed the basis for the development of other semi-empirical equations of state capable of correlating both (p, V, T) and phase behaviour some approaches are discussed in Chapter 12. One example of this form of equation is the Benedict, Webb and Rubin (known by the acronym BWR) equation of state given by  

The BWR equation has been used to calculate the thermodynamic properties of mixtures based on the idea that both the mixture and the pure-fluid equations should satisfy the same equation of state and provide the correct composition dependence of as many virial coefficients as possible. This was achieved with mixing rules similar to those obtained from the van der Waals one-fluid theory that are as follows  

Equation 5.114 has been, depending on the requirements, replaced by the linear form 

Equation 5.125 was used by Orye for hydroearbon mixtures and by Nohka et al for mixtures of refrigerants with nitrogen, argon and methane. 

Many modifications of the BWR equation have been proposed, most by increasing the number of terms and thus increasing the number of adjustable parameters. One of the first such modifications was by Strobridge  

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