Modified α Function for the Peng-Robinson Equation of State To Improve the Vapor Pressure Prediction of Non-hydrocarbon and Hydrocarbon Compounds

On the basis of the available vapor pressures for 59 non-hydrocarbon and hydrocarbon compounds, including heavy alkanes up to n-tritetracontane (n-C43H88), a modified a function for the Peng-Robinson equation of state (PR-EOS) has been developed to more accurately determine the vapor pressure for pure non-hydrocarbon and hydrocarbon compounds, especially heavy components. To balance the characterization of both light and heavy compounds, the Pitzer acentric factor is first redefined in terms of reduced vapor pressure at a reduced temperature of 0.6. In comparison to the evaluated a functions used for the PR-EOS, it is found that the newly developed alpha function with the redefined acentric factor provides a more accurate prediction of vapor pressures with a percentage average absolute deviation of 1.90% and a percentage maximum absolute deviation of 21.22% for the 59 chemical species. In addition, the newly developed alpha function results in the best prediction of the vaporization enthalpy data with an average absolute deviation of 3.92% in comparison to the other existing alpha functions evaluated.