Distribution of porosity of emulsion phase and its effect on conversion in a fluidized bed.

Abstract

Spatial distribution of emulsion-phase porosity and bubble frequency in a fluidized bed of cracking catalyst are determined by use of a seven-channel light transmission method developed for this purpose. The bed was operated at moderate gas velocity at atmospheric pressure or at low gas velocity in slightly pressurized conditions. Conversion was calculated by taking into account the physical properties of the emulsion phase such as porosity, interstitial gas velocity and volume fraction. Experiments revealed in general the non-uniform features within a bed; therefore reactant concentrations and conversions calculated numerically were compared with those from the analytical solutions for an isothermal first-order reversible reaction.The conversion in the emulsion phase was almost completed at 0.1 m height above the distributor. Hence, the overall conversion was controlled in the bubble phase. Increasing bed pressure was accompanied by a decrease in the volumefraction of the emulsion phase, and the pressure effect on the overall conversion at the exit of the dense phase under 300 kPa, for example, amounted to approximately seven percent gain in conversion in comparison with the case of atmosphericpressure, whenother factors such as reaction scheme,reaction rate, gas exchangerate and volume fraction of catalyst in bubble phase were assumed to be independent of pressure.It was also suggested that the conventional two-phase theory seemed to give approximately 1 % higher conversion than the numerically calculated figure under the same assumptions.