Kinetic models of supercritical fluid extraction

The scope of this paper is to review the theoretical basis for modeling various applications of supercritical fluid extractions (SFE), including industrial processes and analytical tests. Most of the models for SFE are concerned mainly with one stage SFE in packed beds. However, in SFE tests, an initial static stage (with no flow) is usually implemented for the purpose of achieving higher yields with pre-extractions. The importance of accounting for the initial static stage is illustrated and a basis for modeling such a stage is presented. Theoretically, SFE models are similar to those for conventional transport processes in that they are based on differential mass balance for solute in both phases and invoke a rate equation (based on desorption kinetics or mass transfer) as well as a thermodynamic equation to describe the interactions at the interface. Local equilibrium theory can be implemented using linear or non-linear isotherms. A conceptual scheme for developing SFE models from these general equations is suggested. The similarity between modeling SFE processes and reversible adsorption/desorption processes is demonstrated and shown to provide a useful basis for developing new models for the SFE process. The concept of analogy is presented and used to develop a new model for SFE in packed beds from a previous deposition model. Recent models for SFE tests with CSTR flow field are presented, including a mass transfer model with local linear equilibrium and a model based on the Langmuir isotherm. A new equilibrium CSTR model is also presented and compared to these two models.