CO2 Diffusivity in H2O for Supercritical Conditions: A Molecular Dynamics Study

Supercritical H2O/CO2 mixtures (650 K-973 K and 25 MPa, not far away from the critical temperature and pressure of water) have recently been used as the working fluid for a novel power generation system with higher efficiency and zero pollutant emission compared to conventional coal-fired power plants. The knowledge of the diffusion of these mixtures is important for efficient design of the system process. However, both experimental and theoretical data are scarce for typical working conditions in the system. Here, we investigated both the self-diffusion and the mutual diffusion of the supercritical H2O/CO2 mixtures using molecular dynamics (MD) simulations. Simulations show that the mixture diffusion coefficients for supercritical conditions are more than one order of magnitude larger than those in the sub-critical region. MD results reveal that H2O molecules prefer to gather with themselves rather than with CO2 molecules in the near-critical region, while there is no featured molecular structure for higher temperatures in the supercritical region. Therefore, the self-diffusion of CO2 in H2O in the infinite-dilution regime features Arrhenius temperature dependent behavior in the supercritical region excluding the near-critical-temperature segment. These molecular interaction mechanisms also provide insights into the mutual diffusion behavior of CO2 in H2O, which is well described by the Darken equation in the supercritical region. Furthermore, engineering equations, namely the Speedy-Angell power-law equation and the Vignes equation, can reproduce MD simulated infinite-dilution self-diffusion coefficients and Maxwell-Stefan mutual diffusion coefficients, respectively, of the supercritical H2O/CO2 mixtures.