Mutual Diffusivities of Mixtures of Carbon Dioxide and Hydrogen and Their Solubilities in Brine: Insight from Molecular Simulations.

H-CO mixtures find wide-ranging applications, including their growing significance as synthetic fuels in the transportation industry, relevance in capture technologies for carbon capture and storage, occurrence in subsurface storage of hydrogen, and hydrogenation of carbon dioxide to form hydrocarbons and alcohols. Here, we focus on the thermodynamic properties of H-CO mixtures pertinent to underground hydrogen storage in depleted gas reservoirs. Molecular dynamics simulations are used to compute mutual (Fick) diffusivities for a wide range of pressures (5 to 50 MPa), temperatures (323.15 to 423.15 K), and mixture compositions (hydrogen mole fraction from 0 to 1). At 5 MPa, the computed mutual diffusivities agree within 5% with the kinetic theory of Chapman and Enskog at 423.15 K, albeit exhibiting deviations of up to 25% between 323.15 and 373.15 K. Even at 50 MPa, kinetic theory predictions match computed diffusivities within 15% for mixtures comprising over 80% H due to the ideal-gas-like behavior. In mixtures with higher concentrations of CO, the Moggridge correlation emerges as a dependable substitute for the kinetic theory. Specifically, when the CO content reaches 50%, the Moggridge correlation achieves predictions within 10% of the computed Fick diffusivities. Phase equilibria of ternary mixtures involving CO-H-NaCl were explored using Gibbs Ensemble (GE) simulations with the Continuous Fractional Component Monte Carlo (CFCMC) technique. The computed solubilities of CO and H in NaCl brine increased with the fugacity of the respective component but decreased with NaCl concentration (salting out effect). While the solubility of CO in NaCl brine decreased in the ternary system compared to the binary CO-NaCl brine system, the solubility of H in NaCl brine increased less in the ternary system compared to the binary H-NaCl brine system. The cooperative effect of H-CO enhances the H solubility while suppressing the CO solubility. The water content in the gas phase was found to be intermediate between H-NaCl brine and CO-NaCl brine systems. Our findings have implications for hydrogen storage and chemical technologies dealing with CO-H mixtures, particularly where experimental data are lacking, emphasizing the need for reliable thermodynamic data on H-CO mixtures.