Accurate correlations for the self-diffusion coefficients of CO2, CH4, C2H4, H2O, and D2O over wide ranges of temperature and pressure

In this work, two models are proposed for the correlation of self-diffusivities over the entire ranges of gas and liquid phases. The first model is an empirical equation containing 12 adjustable parameters and is based on the density expansion for self-diffusivities presented by Kawasaki and Oppenheim, The second model is a modification of the free-volume expression by Macedo and Litovitz, with seven adjustable parameters. Self-diffusion coefficient data for CO2, CH4, C2H4, H2O, and D2O, which are known to be very important supercritical fluids employed in extraction processes, were correlated with the proposed equations, The average absolute deviations obtained are smaller than 5% for wide ranges of both temperature and pressure. Therefore, these equations can be recommended for accurate predictions of self-diffusivities for these substances in the covered density ranges.