Free liquid-to-supercritical fluid mass transfer in packed beds

In the supercritical fluid extraction of seeds and other naturally occurring 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-called fast regime. In this work we characterize this fast extraction period by evaporating 1,2-dichlorobenzene deposited on a shallow bed of non-porous, non-adsorbing glass spheres arranged in a packed bed with through circulation of supercritical carbon dioxide. Measurements performed over the ranges of pressure and temperature of 8-25.5 MPa and 310-320 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 involve two parameters: a vapour-liquid partition constant and a free liquid-to-fluid mass transfer coefficient. These were simultaneously determined using time-domain curve-fitting of the extraction curves with the solution to the model equations. The measured global mass transfer coefficients are put into perspective and compared with published transfer data for packed columns and fixed beds of solids. Copyright (C) 1996 Elsevier Science Ltd.