Analysis of the Stefan Tube at Supercritical Conditions and Diffusion Coefficient Measurements

Abstract

An analysis of the Stefan tube for binary diffusion coefficient measurement of a system comprised of a solid solute and a supercritical fluid has been carried out. The prototype system used to evaluate the approach is naphthalene−supercritical carbon dioxide. An initial theoretical calculation with the available data shows that the diffusion process would be in the unsteady-state region for a practical duration of the experiment. The development of a theoretical model was carried out for systems where the transport of the species is dominated only by diffusion as well as for systems involving convective transport along with diffusion. Tractable analytical solutions are developed for finite control volume, relaxing the semi-unbounded boundary assumption often used. The binary diffusion coefficients were determined at temperatures of 35, 50, and 65 °C for the pressures 80, 100, and 120 atm. The quantification of the convective term showed little effect on the diffusion coefficient for the supercritical naph...