FREE LIQUID-TO-SUPERCRITICAL FLUID MASS-TRANSFER IN PACKED-BEDS

In the supercritical fluid extraction of seeds and other naturally occ urring porous solids, as much as 35-90% of the solute is extracted by evaporation of the free liquid into the dense gas, during the so-calle d fast regime. In this work we characterize this fast extraction perio d by evaporating 1,2-dichlorobenzene deposited on a shallow bed of non -porous, non-adsorbing glass spheres arranged in a packed bed with thr ough circulation of supercritical carbon dioxide. Measurements perform ed over the ranges of pressure and temperature of 8-25.5 MPa and 310-3 20 K, respectively, provided information on the rate of mass transfer and on solubility. Mass conservation equations for solute were solved using the mixed-flow and the plug-flow assumptions. The two models inv olve two parameters: a vapour-liquid partition constant and a free liq uid-to-fluid mass transfer coefficient. These were simultaneously dete rmined using time-domain curve-fitting of the extraction curves with t he solution to the model equations. The measured global mass transfer coefficients are put into perspective and compared with published tran sfer data for packed columns and fixed beds of solids. Copyright (C) 1 996 Elsevier Science Ltd.