(155a) Gassed Power, Hold-up and Mass Transfer Under High Intensity Sparging and Agitation; Issues with Heterogeneous Gas Flows

With high intensity agitation and sparging, heterogeneous flow is found in which much of the gas passes through a reactor in the form of large bubbles (> ~ 30 mm). This condition has been studied with mean specific energy dissipation rates up to 100 W/kg and gas superficial velocities, vs, up to 0.13 m/s (~30 vvm) with a wide range of impeller types. The transition from homogeneous flow occurs at vs between ~ 0.02 and 0.05 m/s and depends mainly on the coalescence characteristics of the fluid and less on scale, the agitator type and mean specific energy dissipation rate. The phenomenon has rarely been studied because it is difficult to do so at pilot scale, though it often occurs at plant scale because with constant vvm (fixed stoichiometry) scale-up and geometric-similarity, vs is proportional to linear scale. Compared to homogeneous conditions, the minimum ratio of gassed to ungassed power is much lower, e.g., Pg/P = ~0.2 with Rushton turbines, ~0.6 with Scaba 6SRGT and ~0.4 with down- or up-pumping high solidity hydrofoils. Hold-up increases much more slowly with vs and may even fall; it is dependent on impeller type, especially with non-coalescing fluids with values up to 0.7. With this hold-up, Pg/Pdisp > 1 for 6SRGT (where Pdisp is based on the dispersion density). Mass transfer coefficients are difficult to measure accurately, especially at low vs and high mean specific energy dissipation rate, because of intense ?surface' gassing. This makes it difficult to determine the relationship between mass transfer and agitation and aeration conditions. Nevertheless, it is suggested that an equation of the usual type can describe the link beteen kLa, vs and mean specific energy dissipation rate where the empirical constants differ from those found for the homogeneous regime, though they are still independent of impeller type and the exponent on vs is less. In addition, the values of these empirical constants are very sensitive to composition, with kLa generally much higher in non-coalescing fluids.