.The previous SRK and PR mixing rule modifications included composition-dependence for applying Equations Of State (EOS) to complex mixtures. A more rigorous way to represent the phase behavior of strong non-ideal systems is to develop the relationship between the mixing rule and excess free energy model such that the zero-pressure Gibbs free energy could be expressed by a NRTL-like method (see Section 1.2.6, Liquid Activity Methods). Such approach had been extended by Twu, Bluck and Coon in 1998 with a newly developed zero-pressure-based mixing rule which would accurately reproduce the excess Gibbs model and allow the avail able activity coefficient models at low pressures to be used directly.
At zero pressure, the value of the excess Helmholtz energy is identical to the excess Gibbs energy model. Therefore, any activity model such as the NRTL equation can be used directly for
.
By regressing experimental data to obtain the parameters in the NRTL expression, excellent representation of vapor-liquid equilibria can be made. The prediction of equilibria at low densities is reasonable, and the EOS can be expected to yield better results at higher pressures, because the mixing rules have been derived at the zero pressure limit of the excess Gibbs free energy. Twu-Bluck-Coon EOS overcomes the limitation of infinite-pressure based models, such as Huron-Vidal (SRKH, PRH…) by directly utilizing parameters in the NRTL method correlated from low temperature data.
See "Twu-Bluck-Coon (TBC)" under "Equations of State" in PRO/II Reference Manual, Chapter 2, Volume 1 for more information.
Twu, C. H.; Coon, J. E.; Bluck, D. "Comparison of the Peng-Robinson and Soave-Redlich-Kwong Equations of State Using a New Zero-Pressure-Based Mixing Rule for the Prediction of High-Pressure and High-Temperature Phase Equilibria". Ind. Eng. Chem. Res. 1998, 37, 1580-1585.