Equations for the SRKH method
- Last UpdatedAug 21, 2025
- 2 minute read
The general equation relating excess Gibbs free energy to fugacity coefficients is given by:
where
gE is the excess Gibbs free energy per mole
f is the fugacity coefficient of the mixture
fi is the fugacity coefficient of component i
We calculate the excess Gibbs free energy at infinite pressure by using the Redlich-Kwong equation of state (EOS) and the linear mixing rules for the parameter b from the general cubic EOS. At infinite pressure, the preceding equation becomes:
where
gE∞ is the excess Gibbs free energy at infinite pressure
We can rewrite this equation to produce a new mixing rule for parameter a from the general cubic EOS:
We can then use any liquid activity coefficient (LACT) method to calculate the excess Gibbs free energy. Huron and Vidal chose to use the NRTL method to calculate the excess Gibbs free energy.
where
gji and gii are experimentally determined parameters taken from the SRKH data bank
The only difference between the classical Non-Random Two-Liquid (NRTL) equation and the preceding equation is the definition of the local composition as corrected volume fractions, which leads to the introduction of the volume parameter bj in the calculation of Gji. Substituting for the excess Gibbs free energy in the preceding equation yields:
