Using Equations of State EOS for VLE Calculations

As mentioned above, the use of equations of state for VLE calculations has distinct advantages compared to the alternative route. However, an equation of state has to be used that is able to describe the PVT behavior for both the liquid and the vapor phase. 

Most commonly used are further developments of the cubic van der Waals EOS. This EOS for the first time allowed the description of different phenomena, such as condensation, vaporization, occurrence of the two-phase region and critical phenomena, using only two parameters a and b which take into account the interaction forces between the molecules and the volume of the molecules. The introduction of a further parameter, the acentric factor m, which can be derived from vapor pressure data, leads to a more reliable description of the saturation vapor pressures. 

In practice, the most commonly used EOSs are the Soave-Redlich-Kwong equation and the Peng-Robinson equation. These equations were developed for pure components only. Applying these models to multicomponent systems requires mixing rules for the calculation of the parameters a and b in the mixture. These parameters have to be calculated from the pure component parameters and b.  

Simple empirical mixing and combination rules often used are given in Eqs. 

The required parameters k j are fitted to experimental binary VLE data. However, problems with this empirical mixing rules arise for highly polar or associating mixtures. g mixing rules as introduced by Huron and Vidal (1979) lead to an improved description of these systems. These types of mixing rules include g , which can be calculated from a g model like UNIQUAC. 

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