Estimation of Binary Diffusion Coefficients in Supercritical Water: Mini Review

Diffusion coefficients of various solutes in supercritical water, which were either measured or retrieved from Molecular Dynamics simulations, were reviewed. 'Diffusion coefficients of molecules relevant for supercritical water processes were calculated with correlations reported in the literature and compared to the values of reference data. For conditions well above the critical point of water, the simple Stokes Einstein equation predicts the diffusion coefficients with an accuracy better than 20%. For conditions near the critical point, the Wilke-Chang correlation gives the most accurate results. Diffusion coefficients for typical molecules occurring in supercritical water processes such as O-2, N-2, CO, CO2, or CH4 are estimated to be in the range of 60 x 10-(9) m(2)/s at 673 K and 30 MPa. For H-2, for which no experimental data are available, much higher diffusion coefficients in the range of 250 x 10-(9) m(2)/s seem plausible. The data set of binary diffusion coefficients in supercritical water, either determined experimentally or by Molecular Dynamics simulations, should be extended significantly to include more solutes, as well as higher temperatures and pressures.