Prediction of thermodynamic properties of fluid mixtures by molecular dynamics simulations: methane-ethane

Abstract

NPT and NVT molecular dynamics simulation results are reported for the fluid methane-ethane mixture. Methane is modelled as Lennard-Jones fluid and ethane as a two-centre Lennard-Jones fluid with parameters obtained some time ago by fitting WCA-type perturbation theory results to two experimental vapour pressures and one bubble density for each substance. It has also already been shown that these effective pair potentials yield good predictions of the pressures and internal energies of the pure substances in the whole fluid region. In order to obtain an effective potential between the unlike molecules, previously derived fluctuation formulas for the dependence of the mixture excess properties on the unlike molecule interaction parameters are used. A test of these fluctuation formulas shows their statistical uncertainties to be small. Hence, the unlike interaction parameters can be determined from one mixture simulation and a fit to one experimental excess volume and one excess enthalpy. Then second virial coefficients, liquid excess properties, P, V, T data and enthalpy differences are calculated in the temperature range 100–400 K and pressures up to 30 MPa. Comparison is made directly with experimental data and partly also with the DDMIX equation of state showing mostly good to excellent agreement except in the critical region; for some state points in the gas the pressures are slightly too low. Technically, runs for several gas state points are NVT simulations whereas the other runs are NPT simulations part of which were checked again by NVT-simulations. The paper also compares pressures obtained by the Haar-Shenker-Kohler equation for the gas phase with NVT simulation results. All simulations were performed on a CYBER 205.