Thermal Expansivity of Near- and Supercritical Fluids: Equation-of-State Models and Calculations for Carbon Dioxide and C1 to C4 Normal Alkanes

A model for thermal expansivity (β) of gases at high pressure was developed based on the Peng–Robinson equation of state (EoS). A previously developed model based on the van der Waals EoS was enhanced by proposing a correction procedure. Values at near-critical and supercritical conditions were calculated. Since no direct experimental data for expansivities were found, calculated expansivities were compared to values computed from PVT data extracted from the NIST database. This comparison was done for carbon dioxide and four n-alkanes (C1 through C4). The graphical comparison consisted of thermal expansivity versus temperature plots at various reduced pressures (0.8, 0.9, 1.0, 1.1, and 1.2). The Peng–Robinson EoS provided results that demonstrated significant improvement from the van der Waals EoS when compared to NIST-based values. This was expected since it was originally designed to describe hydrocarbons behavior. Although the van der Waals model shows larger deviations from the reference (NIST-based) values, the simple correction procedure proposed dramatically improves its predictions near the critical point. The proposed procedure provides a good balance between accuracy and simplicity for design purposes.