Molecular Dynamics Simulations and a Quintic Equation of State for Nitrogen in a Wide P–T Range, with Validation of a Reference Model up to Ultrahigh P–T Conditions

Abstract

Pressure−density−temperature data of nitrogen (N 2 ) at 589 state points are obtained by molecular dynamics simulations at 1573.15−3273.15 K and 500−3500 MPa (using NPT ensemble) or 0.2−0.5 g•cm −3 (using NVT ensemble). The results agree well with those of experiments at high temperatures and pressures, and the Helmholtz free energy model of Span et al. [J. Phys. Chem. Ref. Data 2000, 29, 1361−1433. The model of Span et al. is confirmed to be valid to at least 6000 K and 30000 MPa (or 2.0 g•cm −3 ) using the new and previous data, as well as thermodynamic analysis. Using this model as a reference, a quintic equation of state (EOS) is developed for supercritical N 2 . Compared to the reference model, the volume deviations of the EOS are within 0.21% at 273−4273 K, 0−4300 MPa, and 0−1.035 g•cm −3 . The EOS compares well with the new simulation results and previous experimental and simulated data at high pressures and temperatures. It is capable of accurate prediction of fugacity coefficient, residual enthalpy and entropy, and other properties. It can also be extrapolated to lower temperatures or higher temperatures and densities, but with lowering of accuracy.