Mass transfer in sparged and stirred reactors: influence of carbon particles and electrolyte

Mass transfer in multiphase systems is one of the most studied topics in chem. engineering. However, in three-phase systems contg. small particles, the mechanisms playing a role in the increased rate of mass transfer compared to two-phase systems without particles, are still not clear. Therefore, mass transfer measurements were carried out in a 2D slurry bubble column reactor (0.015*0.30*2.00 m3), a stirred tank reactor with a flat gas-liq. interface, and in a stirred tank reactor with a gas inducing impeller. The rate of mass transfer in these reactors was investigated with various concns. of active carbon particles (av. particle size of 30 mm), with electrolyte (sodium gluconate), and with combinations of these. In the bubble column, high-speed video recordings were captured from which the bubble size distribution and the specific bubble area were detd. In this way, the specific mass transfer area agl was detd. sep. from the mass transfer coeff. kl. Mechanisms proposed in literature to describe mass transfer and mass transfer enhancement in stirred tank reactors and bubble columns are compared. It is shown that the increased rates of mass transfer in the 2D bubble column and in the stirred tank reactor with the gas inducing impeller are completely caused by an increased gas-liq. interfacial area upon addn. of carbon particles and electrolyte. It is suggested that an increased level of turbulence at the gas-liq. interface caused by carbon particles accounts for a smaller effective boundary layer thickness and an enhancement of mass transfer in the flat gas-liq. surface stirred tank reactor. However, for the carbon particles used, it is rather unlikely that mass transfer enhancement takes place due to the well-known shuttle or grazing effect. [on SciFinder (R)