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 determination of hydrodynamic and transfer parameters of multiphase reactors has been used with electrolytic liquids. The response to an oxygen pulse injected in the gas phase is detected in the liquid phase at two levels in the column. The gas holdup, Peclet numbers for each fluid phase, and volumetric mass transfer coefficient can be optimized in the frequency domain. Gas Peclet numbers are always much higher than liquid Peclet 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 dispersion has not been neglected. Very good agreement has been obtained with gas holdup measurements by simultaneous shut-off of liquid and gas feeds. In a fluidized bed, gas holdup increases slightly with the particle diameter but is still lower at d(P) = 3 x 10(-3) m than in the bubble column. The similar behaviour of k(L)a has clearly shown the coalescing effect of small solid particles in this experimental range. The axial dispersion coefficient of the liquid phase is greater in the fluidized bed than in the bubble column and increases at a higher rate with gas velocity. The axial dispersion coefficient of the gas phase is derived with less precision but is of the same order of magnitude, the large difference in the two Peclet numbers is mainly due to different interstitial velocities.