A Combined Equation of State for Methane, Based on Bender’s Regular Equation with a Scaling Part in Explicit Form up to 50 MPa

• Published in: Russian Journal of Physical Chemistry A 2024, Vol.98 (1), p.14-24
• Main Authors: Bezverkhii, P. P., Matskevich, N. I.
• Format: Article
• Language: English
• Subjects: Chemistry; Chemistry and Materials Science; Crossovers; Equations of state; Ideal gas; Mathematical analysis; Methane; Physical Chemistry; Real variables; Thermodynamics and Materials Science; Thermophysical properties
• ISSN: 0036-0244; 1531-863X
• DOI: 10.1134/S0036024424010047

A combined equation of state of methane in real variables (pressure p , temperature T , density ρ) is proposed that allows calculations of heat capacities , C p , and speed of sound W in explicit form in single-phase states, including the critical region. The equation contains a regular part in the form of Bender’s equation for p (ρ, T ) with 19 coefficients, a scaling part with 6 coefficients, and a crossover function in explicit form (2 coefficients). The coefficients are determined from a CH 4 p , ρ, T data array up to 30 MPa. Data on , C p , and W are not involved, except for the ( T ) dependence in the ideal gas state and value at 100 K on the liquid branch of the boundary curve. In the regular region, the calculated values of , C p , and W are close to the experimental and tabulated values. Discrepancies with the tabular values are no more than 5% in the critical region. The RMS error for p is σ p = 0.8%, and error for is no more than 5%. Results from calculations are compared to crossover equations of state for CH 4 . It is concluded that the proposed model of the combined equation of state is preferable for calculating the thermophysical properties of methane.