Predicting self-diffusion coefficients of light gases in supercritical carbon dioxide: A molecular dynamics method

Abstract

The methanation of CO₂ and supercritical CO₂ (scCO₂) fracturing represent effective approaches for CO₂ utilization and greenhouse effect mitigation. In these process, light gases like H₂, O₂, CO, CH₄ are commonly involved. However, limited studies have focused on the diffusion behavior of these gases in scCO₂ under high-pressure conditions, as well as a shortage of self-diffusion coefficient data and prediction equations specifically designed for scCO₂ solvents. To address this gap, molecular dynamics (MD) simulations were employed to compute the self-diffusion coefficients of several light gases in scCO₂ across a wide temperature range (313 K–713 K) and at high pressures (75.994 bar, 101.325 bar and 126.656 bar) under infinitely dilute conditions. The effects of temperature, density, viscosity, molecular mass, and solute–solvent interaction energy on the self-diffusion behavior were systematically analyzed. Two empirical equations (a Speedy–Angell-type power-law equation and a newly developed equation incorporating temperature, density, and viscosity) were refitted based on simulation data. The overall relative errors are 4.52 % and 4.44 %, respectively, indicating an improved accuracy compared to conventional experimental and MD-based empirical equations.