Correlation for self-diffusion coefficients of H2, CH4, CO, O2 and CO2 in supercritical water from molecular dynamics simulation

Molecular dynamics simulations were employed for the calculation of self-diffusion coefficients of H-2, CH4, CO, O-2 and CO2 in supercritical water over a wide range of temperature (673-973 K) and pressure (250-280 atm). Based on our results, H-2 always diffuses the fastest while CO2 diffuses the slowest, and self-diffusion coefficients of CH4, CO and O-2 are between H-2 and CO2. Temperature, density and viscosity of supercritical water are the main factors determining self-diffusion coefficient. Self-diffusion coefficient has an Arrhenius behavior in a certain temperature range. The term Dp is independent of temperature and ln(D eta/T) has a linear relation with temperature. A new empirical equation is proposed as D = A(0)F(sow) = A0 T-e/rho(a)eta(b) to predict self-diffusion coefficient in supercritical water. The influence of the solute gas and solution supercritical water is divided into A(0) and F-sow respectively. A(0) and exponential indexes of a, b, c are fitted from our calculation results. For all 75 data points, the average relative error between the results from simulation and this equation is only 4.40%.