(150b) Bubbles Distribution in a 0.6-m-Diameter Disk and Donut Fluidized Bed Stripper
The disk and donut stripper has been widely used in the fluid catalytic cracking (FCC) process to strip out product hydrocarbon vapors entrained by the catalyst before regeneration. The stripper consists of a number trays of central conical hats and inclined annular rings. Steam introduced at the bottom of the stripper displaces hydrocarbon vapors from the downwardly flowing emulsion phase into the rising steam bubbles or voids. Disk and donut strippers haveexcellent stripping efficiencies, but they are very prone to flooding. The objective of this study was to determine the cause of flooding through the measurement of stripper density and local bubble void fraction. Tests were conducted in a cold-flow 0.6-m-diameter, 7.6 m tallunit that had seven trays of disk and donut spaced 0.46 m apart. Bubble void fraction profiles were measured with optical fiber probes at five axial locations for stripper solids fluxes of zero to about 70 kg/s.m2. Bubble probes were traversed in the 133 mm space between the apex of the cone and the bottom of the donut above it, and in the 31.8 mm space between the top of the donut and bottom of the cone. The stripper bed density was found to decrease linearly with increasing solids flux until a solids flux was reached at which it dropped precipitously. The solids flux at the sudden density drop was taken as the stripper flooding point. The flooding solids flux was determined for fluidizing gas velocities of 0.15, 0.3 and 0.46 m/s. The geometry of the disk and donut trays had a significant impact on the radial distribution of bubbles in the stripper. The radial bubble void fraction profiles had a characteristic, M-shape, profile. The bubble void fraction increased outward from a low value at the center reaching a peak at the radius corresponding to the edge of the cone and then decreased to low values close to the wall. The void fraction at the wall increased more with increasing superficial gas velocity than that in the core causing the M-shape profile to nearly become an inverted-V shape at the highest gas velocity. Similar profiles were observed at no solids outflow (Gs = 0). The bubble void fraction both in the core region and near the wall for solids outflow conditions at any given superficial gas velocity was, however, higher than at zero solids flux. It appears that, large pockets of gas accumulated momentarily underneath the conical hats and donuts before escaping, while the donuts âfunneledâ solids through the core of the stripper causing the core to have fewer bubbles. The only escape route of the rising gas bubbles was through the annular gap between the cone edge and the wall. As the solids flux increased it became increasingly more difficult for the gas voids to rise against the downflowing solids, which eventually led to flooding. It is likely that the width of the annular gap and, therefore, the column diameter play a role in the flooding of gas fluidized bed strippers.