Peng-Robinson

The Peng-Robinson equation of state (PR) is a modification of the Redlich-Kwong equation of state and was published by Peng and Robinson in 1976. It is similar to the Soave-Redlich-Kwong equation in many respects and was designed to improve the poor liquid density predictions for the SRK method. As with the SRK equation, the $image\THRM10.gif$ term in the Redlich-Kwong equation was replaced with a more general temperature dependent term, a(T). The expression is as follows:

P = RT / (V-b) - a(T) / [ V(V+b) + b(V-b)]

where:

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Introduction of the alpha term was an attempt to improve the vapor pressure prediction for the pure components. The combination formula for calculation of a(T) with the introduction of the term kij was intended to improve prediction of the mixture properties.

Using the Peng-Robinson formulation for prediction of mixture properties involves two steps. First, the component acentric factor, $image\THRM21.gif$ , is "tuned" for each component such that the component vapor pressure is accurately predicted. Secondly, the term kij is determined from experimental data for the binary system with components i and j such that the phase equilibria is matched.

This equation has been found to give accurate predictions for non-polar mixtures of hydrocarbons. It does not give accurate predictions for polar components. PRO/II contains correlations for kij 's for systems with hydrocarbons and N2, H2S and CO2. Some kij's are also provided for hydrocarbon splits such as ethane-ethylene and propane-propylene. Hydrogen phase behavior is approximated by Peng-Robinson by modification of the acentric factor.

In addition to K-values, the PR equation may be used to predict the enthalpies, entropies and densities for the liquid and vapor phases. The predicted liquid phase densities are not very accurate and the API method is selected by default when the PR system is chosen.

See "Peng-Robinson (PR)" under "Equations of State" in Chapter 2, Volume 1, of the PRO/II Reference Manual for more information.

Application Guidelines

All light hydrocarbon processes such as natural gas processing and light ends processing in refinery gas plants. For hydrogen-rich systems the accuracy for the hydrogen solubility may be improved by using a different alpha formulation or one of the more advanced mixing rules.