Measurements and correlation of infinite dilution binary diffusion coefficients for Cr(acac)3 in high temperature supercritical carbon dioxide

Abstract

The infinite dilution binary diffusion coefficient (D12) for chromium (III) acetylacetonate (Cr(acac)3) in supercritical carbon dioxide (scCO2) was measured at temperatures from 343 K to 423 K and pressures from 10 MPa to 22 MPa by the chromatographic impulse response (CIR) method. We previously reported D12 data at relatively low temperatures below 343 K; the D12 data were accurately represented by the hydrodynamic equation where the D12/T vs. CO2 viscosity were expressed by a straight line on a logarithmic plot. However, the newly measured D12 values in the present study showed greater deviations at higher temperatures or lower viscosities (lower pressures/densities) from the correlation determined for low – temperature D12 data. The maximum deviation was 68% at the lowest measured CO2 viscosity of 22 μPa s; thus, the hydrodynamic equation could be no longer applied in this region. The D12 data that deviate from the hydrodynamic equation fully correspond to the gas-like scCO2 region, whereas those fitted by the equation correspond to the liquid –like scCO2 region. The gas–like and liquid–like regions can be separated on the basis of density fluctuations and a ridge line in a pressure vs. temperature diagram, claimed by Nishikawa and Morita [Chem. Phys. Lett., 316 (2000) 238]. Compared with the accuracy in predicting the D12 data by the original Schmidt number correlation, the accuracy was improved by the modified Schmidt number correlation, with an average relative deviation of 6.20 % for 136 conditions, including both gas–like and liquid–like scCO2 regions.