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When a fluidizing gas is switched from a low-density gas to a high-density gas, particles in the emulsion phase agglomerate and bubbles disappear. At the same time, channeling occurs and a decrease in the pressure drop over the bed is observed. The disturbance of fluidization is temporary, and normal fluidization is restored after several minutes. The study of mechanisms of the transient phenomena shows that the difference in diffusion rates of the two gases in the emulsion phase caused the agglomeration of particles. Because the mobility of particles was reduced, channels were formed and the fluidizing gas bypassed through them. Effects of the properties of the fluidizing gases and particles on the intensity of the transient phenomena were also studied. The intensity increased with increasing difference in the densities of the two gases. The transient phenomena were considerable for small- and light-particle systems.

A significant defluidization occurs when carrying out reactions involving a decrease in gas volume in a fluidized catalyst bed. The cause of this phenomenon is a decrease in the gas velocity in the emulsion phase below the minimum fluidization velocity. Fluidization quality is improved by a staged gas feed when hydrogenation of CO2 is carried out. To evaluate the experimental results, two parameters are introduced; gas-volume reduction rate and gas-volume ratio. Fluidization quality and defluidization zone are indicated as a map using these parameters. The vertical distributions of these parameters are calculated using a reactor model to obtain operating lines. The calculation shows that fluidization quality can be improved by operating the reactor by avoiding the operating lines of the defluidization zone in the map. For this purpose, it is required to control the gas-volume ratio at a level near unity and maintain the gas-volume reduction rate below 0.01/s. © 2010 American Institute of Chemical Engineers AIChE J, 2010

The fluidization quality significantly decreased when the reaction involving a decrease in the gas volume was carried out in a fluidized catalyst bed. In the present study, we carried out the hydrogenation of carbon dioxide in a transparent column to observe the bed behavior. When the reaction rate was increased, the fluidity of the emulsion phase decreased and channeling occurred. When the fluidity of the emulsion phase decreased even further, the upper part of the bed was defluidized and this part was lifted up through the column. This phenomenon caused the entrainment of a large amount of catalyst particles from the reactor. We studied the effects of the reduction rate of the gas volume and the maximum gas contraction ratio on the fluidization behavior. The criterion for a stable operation, which could be successfully given by the gas-volume reduction rate, is useful for determining the operating conditions needed to establish good fluidization when the reactions involving a decrease in the gas volume are carried out in a fluidized catalyst bed.