BUBBLE-COLUMN AND 3-PHASE FLUIDIZED-BEDS - A COMPARISON OF AXIAL-DISPERSION AND GAS-LIQUID MASS-TRANSFER BY DYNAMIC ABSORPTION

A powerful transferable tracer analysis for the simultaneous determina tion of hydrodynamic and transfer parameters of multiphase reactors ha s been used with electrolytic liquids. The response to an oxygen pulse injected in the gas phase is detected in the liquid phase at two leve ls in the column. The gas holdup, Peclet numbers for each fluid phase, and volumetric mass transfer coefficient can be optimized in the freq uency domain. Gas Peclet numbers are always much higher than liquid Pe clet numbers. Nevertheless by comparing optimizations with and without gas dispersion it has been shown that differences of up to 30% in Pe( L) and k(L)a can occur and thus, contrary to previous works, gas dispe rsion has not been neglected. Very good agreement has been obtained wi th gas holdup measurements by simultaneous shut-off of liquid and gas feeds. In a fluidized bed, gas holdup increases slightly with the part icle diameter but is still lower at d(P) = 3 x 10(-3) m than in the bu bble column. The similar behaviour of k(L)a has clearly shown the coal escing effect of small solid particles in this experimental range. The axial dispersion coefficient of the liquid phase is greater in the fl uidized bed than in the bubble column and increases at a higher rate w ith gas velocity. The axial dispersion coefficient of the gas phase is derived with less precision but is of the same order of magnitude, th e large difference in the two Peclet numbers is mainly due to differen t interstitial velocities.