Estimation of relevant mass transfer parameters for the extraction of packed substrate beds using supercritical fluids

Simulations were performed on supercritical fluid extraction (SFE) of solid substrates in packed beds using a general model (Model 1) that considers both diffusion within the solid particle and convection in a supercritical fluid (SF) film surrounding each particle. Two simplified models were also used that neglected either the internal (diffusion) (Model 2) or the external (convection) resistance to mass transfer between the phases (Model 3). Results of simulations showed that all models could predict similar, smooth integral extraction plots (of recovered solute, Y, versus extraction time, t). However, some differences in residual concentration profiles (solute concentration in the solid substrate, C-s, versus axial position, z) resulted when they were compared. As an example, typical S-shaped C-s(z) curves became slightly steeper when the contribution of the 1/k(f)alpha (p) term (external resistance to mass transfer) was neglected (Model 3) or flattened when that of R-2/15D(m) (internal resistance to mass transfer) was neglected (Model 2). As a result of our analysis, a multi-objective fit of experimental Y versus t curves as well as C-s versus z profiles to proposed models is suggested, so as to estimate relevant-mass transfer parameters for the SFE of packed substrate beds.