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 b oth diffusion within the solid particle and convection in a supercritical f luid (SF) film surrounding each particle. Two simplified models were also u sed that neglected either the internal (diffusion) (Model 2) or the externa l (convection) resistance to mass transfer between the phases (Model 3). Re sults of simulations showed that all models could predict similar, smooth i ntegral extraction plots (of recovered solute, Y, versus extraction time, t ). However, some differences in residual concentration profiles (solute con centration 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 beca me slightly steeper when the contribution of the 1/k(f)alpha (p) term (exte rnal resistance to mass transfer) was neglected (Model 3) or flattened when that of R-2/15D(m) (internal resistance to mass transfer) was neglected (M odel 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 s uggested, so as to estimate relevant-mass transfer parameters for the SFE o f packed substrate beds.