Internal mass transfer in sintered metallic pellets filled with supercritical fluid

The shrinking core model has been used in some cases to describe kinetics of extraction of seeds with a supercritical fluid as solvent, despite the fact that seed geometry may be quite irregular and internal walls may strongly affect diffusion and the liquid-fluid-solid equilibria. Consequently, the values of model parameters (effective diffusivity, particle-fluid mass-transfer coefficient) hardly, if ever, prove meaningful or have a limited value. In the present work, the extraction of benzene, toluene, ethylbenzene, and 1, 2-dichlorobenzene from shallow packed beds of macroporous pellets of well-defined geometries, are studied both experimentally and theoretically using the shrinking core model. Sintered metallic pellets, formed by powder metallurgy in two sizes, were used. These were bronze cylinders with either sealed ends or open ends. For a bed a few particles thick, the differential reactor approximation is acceptable. With this assumption, the extraction models contain only three parameters-a liquid-gas partition coefficient, an external mass-transfer coefficient, and an intraparticle or effective diffusivity. The former two were determined in separate experiments (solubility) or taken from previous studies (mass transfer coefficient), so that the effective diffusion coefficient is the only model parameter to be fitted. The procedure adopted eliminates the usual uncertainties regarding geometry, conditions at the boundaries, and axial mass dispersion in the bed. A careful look at experimental results allows to distinguish between equilibrium-limited and rate-limited extraction process. By fitting extraction data for the rate-limited runs to model solutions, values of the effective diffusivity were calculated. Abnormally low (less than unity) tortuosity factors which decrease with temperature were obtained. (C) 1997 Elsevier Science Ltd.