A New Equation of State and Tables of Thermodynamic Properties for Methane Covering the Range from the Melting Line to 625 K at Pressures up to 1000 MPa

Abstract

This work reviews the data on thermodynamic properties of methane which were available up to the middle of 1991 and presents a new equation of state in the form of a fundamental equation explicit in the Helmholtz free energy. A new strategy for optimizing the structure of empirical thermodynamic correlation equations was used to determine the functional form of the equation. The Helmholtz function containing 40 fitted coefficients was fitted to selected experimental data of the following properties: (a) thermal properties of the single phase (pρT) and (b) of the liquid-vapor saturation curve (psρ′ρ″) including the Maxwell criterion, (c) speed of sound w, (d) isochoric heat capacity cv, (e) isobaric heat capacity cp, (f) difference of enthalpy Δh, and (g) second virial coefficient B. Independent equations are also included for the vapor pressure, the saturated liquid and vapor densities, the isobaric ideal gas heat capacity and the melting pressure as functions of temperature. Tables for the thermodynamic properties of methane from 90 K to 620 K for pressures up to 1000 MPa are presented. For the density, uncertainties of ±0.03% for pressures below 12 MPa and temperatures below 350 K and ±0.03% to ±0.15% for higher pressures and temperatures are estimated. For the speed of sound, the uncertainty ranges from ±0.03% to ±0.3% depending on temperature and pressure. Heat capacities may be generally calculated within an uncertainty of ±1%. To verify the accuracy of the new formulation, the calculated property values are compared with selected experimental results and existing equations of state for methane. The new equation of state corresponds to the new International Temperature Scale of 1990 (ITS-90) and is extrapolable up to pressures of 20000 MPa.