Soave alpha function at supercritical temperatures

• Soave alpha function and its pitfalls at supercritical temperatures are studied.
• Different with other compounds Soave alpha function for Helium is monotonically increasing.
• Abnormal behaviors of Soave alpha function do not have any adverse effect on calculation of the thermodynamic properties.

Soave–Redlich–Kwong (SRK) cubic equations of state (CEoS), Peng–Robinson (PR) and four modifications of these equations with Soave type alpha functions at supercritical temperatures are investigated. For pure compounds two different typical behaviors of Soave type alpha function at supercritical temperature are introduced. Then criteria for occurring of these two kinds of behavior for each one of compounds are established. It is exhibited that CEoSs with Soave type alpha function after temperatures that alpha function has minimum are reasonably precise. Furthermore theoretical reason for accuracy of these equations after the abnormal behaviors of alpha functions is presented. Derivatives of soave type alpha functions are calculated and their behaviors at supercritical temperatures are explained. Finally, accuracy of CEoSs with Soave alpha function in prediction of compressibility factor, fugacity coefficient, enthalpy and constant volume heat capacity of 25 pure compounds at supercritical region are studied.

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