Equation of state of the H 2O, CO 2, and H 2O–CO 2 systems up to 10 GPa and 2573.15 K: Molecular dynamics simulations with ab initio potential surface

Abstract

Based on our previous development of the molecular interaction potential for pure H 2O and CO 2 [Zhang, Z.G., Duan, Z.H. 2005a. Isothermal–isobaric molecular dynamics simulations of the PVT properties of water over wide range of temperatures and pressures. Phys. Earth Planet Interiors 149, 335–354; Zhang, Z.G., Duan, Z.H. 2005b. An optimized molecular potential for carbon dioxide. J. Chem. Phys. 122, 214507] and the ab initio potential surface across CO 2–H 2O molecules constructed in this study, we carried out more than one thousand molecular dynamics simulations of the PVTx properties of the CO 2–H 2O mixtures in the temperature–pressure range from 673.15 to 2573.15 K up to 10.0 GPa. Comparison with extensive experimental PVTx data indicates that the simulated results generally agree with experimental data within 2% in density, equivalent to experimental uncertainty. Even the data under the highest experimental temperature–pressure conditions (up to 1673 K and 1.94 GPa) are well predicted with the agreement within 1.0% in density, indicating that the high accuracy of the simulation is well retained as the temperature and pressure increase. The consistent and stable predictability of the simulation from low to high temperature–pressure and the fact that the molecular dynamics simulation resort to no experimental data but to ab initio molecular potential makes us convinced that the simulation results should be reliable up to at least 2573 K and 10 GPa with errors less than 2% in density. In order to integrate all the simulation results of this study and previous studies [Zhang and Duan, 2005a, 2005b] and the experimental data for the calculation of volumetric properties (volume, density, and excess volume), heat properties, and chemical properties (fugacity, activity, and possibly supercritical phase separation), an equation of state (EOS) is laboriously developed for the CO 2, H 2O, and CO 2–H 2O systems. This EOS reproduces all the experimental and simulated data covering a wide temperature and pressure range from 673.15 to 2573.15 K and from 0 to 10.0 GPa within experimental or simulation uncertainty.