SUPERCRITICAL-FLUID EXTRACTION OF PACKED-BEDS - EXTERNAL MASS-TRANSFER IN UPFLOW AND DOWNFLOW OPERATION

The effects of flow direction of solvent (CO2) on the fluid-to-particl e mass transfer were studied for packed beds of sintered porous pellet s of two sizes (diameters of 1 and 2 cm) at conditions supercritical w ith respect to carbon dioxide using toluene and 1,2-dichlorobenzene as the impregnating solutes. Dynamic extraction experiments were perform ed in the laminar flow regime (Re = 8-90) where both free and forced c onvection modes of mass transfer were found to be significant. Measure d mass transfer coefficients showed an almost linear dependence on the Reynolds number (Re-approximate to 0.9), Downflow of fluid had a stro ng effect on accelerating extraction rates, in particular at lower Rey nolds numbers and for conditions near the critical point of CO2, where natural convection is dominant. Experimental mass-transfer coefficien ts were well correlated using a single general equation that accounts for both modes of mass transfer (free and forced convection) as well a s for upflow or downflow operation (i.e,, with gravity opposing or ass isting forced convection).