Gas-Solids Fluidized Bed Strippers - a Review

Fluidized bed strippers are essentially flowing fluidized beds where an up-flowing gas removes (strips out) product entrained in a down-flowing stream of the emulsion phase (denser solid phase). The stripper has to facilitate the transfer of the gas in the interstices of the emulsion phase and that adsorbed on the particles into the bubbles of the stripping gas. The counter-current contacting between the down-flowing emulsion phase stream and the up-flowing bubbles stream is enhanced by the use of an assembly of horizontal or inclined baffles. In unbaffled fluidized beds, the bubbles movement is unrestricted so, as they rise, interact, coalesce and grow in size they tend to move laterally toward the center. This would result in too low of a mass transfer to be an effective stripper.

Fluidized bed strippers play a major role in hydrocarbon processing with respect to fluid catalytic cracking (FCC) and fluid coking. In the FCC process, the catalyst collected by the reactor cyclones contains significant amounts of the product hydrocarbon vapors. Failure to remove these vapors from the catalyst results in a loss of a valuable product and excessively high temperatures in the regenerator. Thus, entrained and adsorbed hydrocarbon vapors are typically removed from the catalyst in a fluidized bed stripper using steam. In a fluid coker stripper, steam is used to remove entrained and adsorbed hydrocarbons from the coke particles descending from the fluid coker reactor thus minimizing carry-under of valuable hydrocarbon product. For both unit operations, a variety of proprietary and standard baffles are used with these fluidized bed strippers. The most common types are disk and donut trays, horizontal sheds and structured packings. Horizontal grating trays, sometimes referred to as subway gratings, are also used but to a much lesser extent. This paper reviews studies available in the open literature on fluidized bed strippers as applied to the FCC and fluid coker unit operations. The review has shown that experimental work has dealt with stripping efficiencies, stripper flooding and other flow dynamics issues. Fundamental studies on mass transfer in fluidized bed strippers are few and the application of computational fluid dynamic techniques to model fluid bed strippers appears to be a fairly recent addition. Overall, fluid catalytic cracking strippers, in particular disk and donut strippers, have received the most attention, undoubtedly, due to their much wider industrial usage.