Molecular Dynamics Simulation And Thermodynamic Modeling Of CH₄,N₂ And H₂O

So far,the pressure–molar volume–temperature（PVT）data of methane（CH₄）,nitrogen（N₂）and water（H₂O）in the literature only cover a limited range of temperature and pressure,where accurate data at high temperatures and high pressures are much fewer.This situation is quite inadequate for the research of the deep interiors of the Earth and planets and other scientific or technologic fields.In this work,the equations of state（or other thermodynamic models）for supercritical N₂in geological fluids are tested by using the calculated results of highly accurate reference model and its extrapolated results at high temperatures and high pressures.The results show that the equations tested have obvious or large systematic deviations;furthermore,their accuracies and applicable ranges are often significantly different from those reported in the original articles.The recent studies of our group also show that the equations of state（or other thermodynamic models）for CH₄and H₂O in geological fluids have similar problems.In this work,molecular dynamics simulations of the PVT properties of CH₄,N₂and H₂O are made using COMPASSⅡfields.The simulations are as follows:922 state points of CH₄at 200–3000 K,0.3–3.0 GPa and 0.22–0.668 g·cm^–3are simulated using NPT ensemble（i.e.isothermal and isobaric ensemble）and NVT ensemble（i.e.isothermal and isochoric ensemble）;72 state points of N₂on the isochors of 0.2,0.3,0.4 and 0.5 g·cm^–3are simulated using NVT ensemble at 1573.15–3273.15 K and 0.12–0.88 GPa;225 H₂O state points are simulated using NPT ensemble at 673.15–2073.15 K and 1.6–3.0 GPa,whose density range is 0.773–1.353 g·cm^-3.These results are in good agreement with the corresponding high-precision reference models and experimental and simulated results in literature.The simulation results in this work provide basic thermodynamic data for the study of geology and other related fields.A four-parameter cubic equation of state for supercritical CH₄is established by using a high-precision reference model of CH₄and the simulation results in this work calibrated by the reference model.The cubic equation of state with the same form for supercritical N₂is also established by using a high precision reference model of N₂and its extrapolated results at high temperatures and high pressures.For CH₄,the new equation is accurate at about300–3000 K and 0–3 GPa(or 0–0.53 g·cm^–3).For N₂,the new equation is accurate at about273.15–4273.15 K and 0–2.9 GPa(or 0–0.85 g·cm^–3).The new equation agrees well with experimental data,simulation data and high-precision reference models at high temperatures and pressures.The fugacity coefficients,residual enthalpies,residual entropies and other thermodynamic properties calculated with the new equation agree very well with the calculated results of the corresponding reference models.The new equations are significantly superior to other existing equations of state（or other thermodynamic models）for geological fluids,which provides a good model of pure components for the development of equations of state for related geological fluid mixtures.