Mass transfer in supercritical fluid extraction of a binary aromatic mixture, naphthalene and phenanthrene, from porous rods

Abstract

Supercritical extraction of naphthalene—phenanthrene mixtures from porous rods has been studied at temperatures above and below their melting points. Despite the different solubilities of both components in the solvent (carbon dioxide), the selectivity of the process is very low. This is due to the eutectic phase behaviour of the aromatic mixture which effects a phase separation within the pores. The more volatile component, naphthalene, becomes impoverished in an outer zone and retreats, leaving a porous layer of crystallized phenanthrene behind. The formation of this layer increases the diffusional resistance of the naphthalene. A model has been proposed which is based on the assumption that the dissolution of both components takes place at different extraction fronts. Solubilities of mixtures and their diffusion coefficients in carbon dioxide were measured for mass transfer calculations. The diffusional resistance of the outer boundary layer and the resistance of the pores were determined. If the mixture dissolves near the surface of the cylinder, the mass transfer coefficient is strongly enhanced by free convection within the annular gap which is formed by the cylinder and the extraction vessel. This was observed when liquefied mixtures were extracted. During a first extraction period, liquefied mixtures dissolved from the surface of the cylinder because of the capillary transport of liquid and an increase in liquid volume by dissolved gas.