(503c) The Effect of Horizontal Baffles on Gas Bypassing in Deep Fluidized Beds of Group a Particles

Solids mixing and bubble characteristics studies in smaller laboratory-scale test units have shown a fluidized bed to be well mixed. Recent work has shown that deep fluidized beds of Group A particles in the bubbling and near-turbulent bed modes can exhibit severe gas bypassing. The gas bypassing (also called jet streaming) occurs in the form of a high velocity air jet or rapidly moving series of bubbles that preferentially flow up near the column wall. The rest of the bed stays defluidized or poorly fluidized. The jet generally rotates around the column wall, but there are instances when it anchors at one position (i.e. at cyclone dipleg return locations). Multiple gas bypass streams can also be present. If gas bypassing occurs in industrial beds it can result in afterburning, poor gas/solids contacting and poorly fluidized entrances to standpipes or discharge regions of cyclone diplegs.

Baffles or tubes have traditionally been added to fluidized beds to improve gas-solid contact and/or to enhance heat and mass transfer. The objective of this work was to study how horizontal baffles can be used to reduce or eliminate gas bypassing in deep beds of Group A particles. Tests were conducted in a 0.3-m-diameter test unit using 76-micron FCC catalyst particles. Bed heights from 0.76 to 3.05 m and gas velocities of up to 0.9 m/s were investigated. Visual observation, measurement of differential pressure fluctuations and radial bubble void fraction profiles were used to determine how effective steel grating baffles and horizontal rods were in reducing gas bypassing. It was found that for a given operating condition, the ability of the internals to eliminate gas bypassing depended on vertical baffle spacing, open area, and grid (or mesh) spacing.