COMPARISON OF EXACT AND APPROXIMATE SOLUTIONS OF TRANSIENT RESPONSE FOR LEACHING BY SCF IN CSTR

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Abstract
Leaching or extraction by supercritical fluids (SCF) is a relatively new technology which offers several key advantages over traditional methods, especially enhanced solubility and rapid diffusion. Because the molecular properties are somewhere in between those of either liquid or gas, the design of SCF systems does not fit the classical mold based on intuitive or well-known rules of thumb. In the present work, transient transport for dissolution of solutes filling granulated porous media is studied for SCF leaching in spinning basket or slurry reactors. We present an approximate model based on taking the intraparticle composition profile to be always parabolic. This leads to a simple one-term expression (transportable on the back of a postage stamp) which is shown to compare very favorably with the more tedious exact solution. The approximate solution thus allows designers to quickly delineate those conditions, such as sampling times, which are realizable in the experimental sense. Moreover, it is shown that parameters such as diffusivity can be estimated directly, using only time-to-peak information. The specific results presented are directly applicable to SCF systems, but the analysis can be used for any extraction system