Molecular Dynamics Simulation and a Cubic Equation of State of Supercritical Methane Up to 3000 K and 3000 MPa

Molecular dynamics simulation of the pressure-density-temperature properties of supercritical methane (CH 4 ) are made with the COMPASS II force field model in the range of 200–3000 K, 0.1–3.0 GPa, and 0.22–0.668 g·cm −3 , where 710 states are simulated using NPT ensemble, and 212 states are simulated using NVT ensemble. These results are in good agreement with experimental data and the calculated results from highly accurate reference model of Setzmann and Wagner (J Phys Chem Ref Data 20:1061–1155, 1991) and its extrapolation in the region where the reference model can be validated. The simulation results are calibrated with the reference model. The calibrated simulations results and the reference model are used simultaneously to develop an accurate cubic equation of state for supercritical CH 4 in the range of about 300–3000 K and 0–3 GPa (0–0.53 g·cm −3 ). The equation are tested against experimental and simulated data at high temperatures and pressures. Compared with the overwhelming majority of experimental results, the volume deviations are within 0.4 % to 1.1 %, with averages of about 0.1 % to 0.4 %; Compared with the molecular simulation results in literature and this work, the volume deviations are within 0.6 % to 3.7 %, with averages of about 0.1 % to 1.2 %. The equation can accurately predict the fugacity coefficients, residual enthalpies, and entropies and other thermodynamic properties.