Diffusion and mass transfer in supercritical fluids

The different behavior of fluid density and viscosity in going from the dilute gas to the dense fluid state gives rise to kinematic viscosities which, in the near supercritical region, are lower than for liquid metals. As a consequence, the relative importance of natural convection (as measured by the ratio of buyoant to inertial forces) is two orders of magnitude higher in a supercritical fluid (at constant Reynolds number) than in normal liquids. Binary diffusion coefficients of nonvolatile solutes in supercritical fluids were measured with a technique that involved laminar flow and diffusion in a rectangular channel. The solution to this hydrodynamic problem is presented. By varying the inclination of the solute source plane with respect to the horizontal position, apparent diffusion coefficients were measured that were up to six times higher than the “true” coefficients. Experimental binary diffusion coefficients, including published literature data, were analyzed in the light of hydrodynamic (Stokes‐Einstein) theory. The analysis suggests that under the high‐density, low‐viscosity conditions that characterize supercritical fluids, hydrodynamic behavior at the molecular level is approached, and can be used as a basis for data extrapolation.