Mass transfer rates, supercritical Mechanism

The rate of extraction is due, firstly, to the solubility of the analyte in the supercritical fluid and, secondly, to the transfer of the mass of the solute outside of the matrix. As indicated in Figure 5, we can therefore conveniently consider four mechanisms of mass transfer ... 

Because of their tunable properties, supercritical solvents provide a useful medium for enzyme-catalyzed reactions.f The mechanism of enzyme-catalyzed reactions is similar to the mechanism described for solid-catalyzed reactions. External as well as internal transport effects may limit the reaction rate. Utilizing supercritical fluids enhances external transport rate due to increase in the diffusivity and therefore mass transfer coefficient. Internal transport rate depends on the fluid medium as well as the morphology of the enzyme. Supercritical fluids can alter both. 

The viscosity of CO2 is quite low, leading to weak resistance to mass transfer. Thus, the diffusion of solutes is better in supercritical fluids than in liquids. The diffusion rate is critical to the simultaneous heat and mass transfer occurring in drying mechanisms. The typical coefficient of diffusion ranges from 0.1 to 3.3 x 10 cmVs. 

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