(7a) A Hydrodynamic Study of Subway Grating and Disk and Donut Trays in a 0.6-m Diameter Fluidized Bed Stripper
The counter-current contacting between the down-flowing emulsion phase stream and the up-flowing bubbles stream in fluidized bed strippers is enhanced by the use of an assembly of horizontal or inclined baffles. The baffles are important because in un-baffled fluidized beds, the movement of the bubbles is unrestricted. So, as they rise, interact, coalesce and grow in size they tend to move laterally toward the center or, in some instances, the bubbles can consolidate in a jet stream that bypasses significant portions of the bed in what has been termed gas bypassing. Therefore, open beds would give too low of a mass transfer to be effective strippers. Installing baffles enhances gas-solids interaction by controlling the bubble size and improving bubble distribution over the stripper cross section. As is typical for two-phase countercurrent flow unit operations, fluidized bed strippers can also suffer from flooding at relatively high solids mass fluxes.
Literature shows that disk and donut trays give excellent stripping efficiencies. They are among the most widely-used fluidized bed stripper internals in the fluid catalytic cracking (FCC) process. Disk and donut internals are, however, expensive and prone to flooding. This study compares the hydrodynamics of simple and cheaper subway grating trays with disk and donut trays in a 0.6-m-diameter fluidized-bed stripper. The stripper column was 7.6 m tall and the test solids were FCC catalyst particles containing 8% fines, less than 44 microns. Solids flowing out of the stripper were transferred by a 0.25-m-diameter, 3.4-m long vertical standpipe to a 15-m tall, 0.3-m-diameter riser that conveyed the solids back to the stripper via primary and secondary cyclones. The measurements performed in the study included differential pressure and local bubble properties.
Results showed that the subway grating stripper built a higher-pressure head, operated more smoothly without flooding over a wider range of operating gas velocities and solids fluxes compared to the disk and donut stripper. Literature studies showed, however, that the stripping efficiency of subway grating tray strippers is significantly lower than that of disk and donut strippers. Therefore, while hydrodynamics suggests a superior subway grating stripper, the future challenge will be to find ways of raising its stripping efficiency. Installing more subway grating trays in the same vertical height will most likely improve stripping.