Range of Particle Properties Inducing Defluidization after Gas Switching in a Fluidized Bed

Abstract

Gas channeling and temporary defluidization occur during the operation of fluidized beds when the fluidizing gas is switched to one of higher density. These effects arise because the gas velocity in the emulsion phase falls below the minimum fluidizing velocity due to non-equimolar counter-diffusion between the emulsion phase and the bubble phase, a major factor in which is the difference between the average molecular velocities of the two gases. During defluidization, the pressure-drop decreases due to the channeling and recovers after reaching a minimum value. In this study, this change was divided into three processes, and a parameter for each process was correlated with particle size and density. Based on the correlations obtained, the range of particle properties that has the potential to induce defluidization was mapped. It was found that defluidization is inevitable for a fluidized catalyst bed with particles of less than 100 µm. Further, the effect of defluidization could be reduced by increasing the gas velocity after the switching, and defluidization could be avoided by gradually changing the gas composition when the fluidizing gas was switched.