Mass transfer to microparticles in agitated systems

Mass transfer to microparticles in agitated vessels was measured experimentally to establish both the turbulent and the asymptotic molecular diffusion contributions. Prior studies specifically focused on transport to microparticles, defined as those whose size is comparable to or smaller than the Kolmogoroff minimum eddy size, are limited. Further, in some cases, the earlier work that a value for the Sherwood number of less than 2 could be obtained. The asymptotic molecular diffusion contribution was experimentally measured from dissolution of 1.3- and 4-μm AgCl crystals in aqueous KCl solutions, and the exchange of Ag⁺ ions with 9-μm ion exchange particles. These results represent the first systematic experimental confirmation that the theoretical limit of Sherwood number=2 is valid for spherical particles suspended in stirred tanks. The turbulent contribution was determined from measurements of the transfer rate to ion exchange particles, 6 to 420 μm in diameter, in aqueous glycerol solutions. The mass transfer coefficient for microparticles can be well correlated by the expression {A figure is presented} where Re = (εd⁴/gs³) ^{1 3}. The turbulent contribution is compatible with a boundary layer model whose origin is in decaying microeddies. This equation also correlates well the reliable results on microparticles which could be found in the literature. A comparison of the proposed equation for microparticles with previous correlations for macroparticles, extrapolated into the microparticle regime, suggests that the use of macroparticle correlations for microparticles could significantly underestimate the turbulent contribution to mass transfer.