Performance Improvement by Mitigating Gas Bypassing in Fluidized Beds of Group A Materials

Gas-solids fluidized beds have been extensively studied. Most of the gas in bubbling beds flows up the bed as bubbles with the remaining gas going into the emulsion phase. The rising bubbles cause rigorous solids mixing which leads to a uniform temperature in the bed and improves solids-gas contacting. Turbulent beds operate at higher gas velocities and have much fewer discrete bubbles than the bubbling beds resulting in higher heat and mass transfer.

This picture of a fluidized bed is, however, not always correct. Deep beds of Group A materials can fluidize poorly, even though all the “criteria” for good fluidization are met. Fluidization tests of FCC catalyst particles by Knowlton [1] in a 0.3-m-diameter column and a static bed height of 1.83 m and by Wells [2] in a 2.44-m-diameter semicircular column and a static bed height of 4.88 m, both units being transparent, showed a significant bypassing of the fluidizing gas occurring in the beds when at superficial gas velocities that are typical of the bubbling bed fluidization regime. Knowlton [1] observed that the fluidizing gas preferentially flowed through one side of the bed whereas Wells [2] found that the streaming gas moved about the center near the faceplate, occasionally splitting and passing up the sides of the unit. In both studies, gas-solids contacting was extremely poor. The remainder of the bed was a mass of nearly stagnant, poorly fluidized solids.

The gas bypassing phenomenon has rarely been described in the literature perhaps because most laboratory beds are often not deep enough to cause bypassing, or because the steel construction of large commercial beds makes visual observation of gas bypassing impossible. It appears that the reason for the bypassing is that the pressure head generated by a deep fluidized bed causes gas compression significant enough to cause defluidization of the solids. If gas bypassing occurs in industrial beds it can result in poor yields, afterburning in the freeboards of combustion reactors, poorly fluidized entrances to standpipes, and poorly fluidized discharge regions for cyclone diplegs and poor solids flows around CFB loop seals. It can also compromise scale-up work. A small-scale unit might not have gas bypassing while the larger unit can have it. Over the last decade, PSRI conducted considerable work on gas bypassing in in 0.6-m, 0.9-m and 1.52-m-diameter FCC fluidized beds. This paper reviews that work and presents measures to mitigate gas bypassing in deep fluidized beds of Group A materials.