(223f) Bed Density in a Grating Internals Fluidized Bed Stripper

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 main industrial applications of fluidized bed strippers are in fluid catalytic cracking (FCC) and fluid coking processes where residual hydrocarbons are stripped from solids particles. The stripper facilitates 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. State of the art knowledge suggests that the counter-current contacting between the down-flowing emulsion phase stream and the up-flowing bubbles stream is enhanced using an assembly of horizontal or inclined baffles. Also, previous studies have shown that in unbaffled fluidized beds the bubbles movement is unrestricted so, as they rise, interact, coalesce and grow they tend to move laterally toward the center. Those conditions result in too low of mass transfer to be an effective stripper.

As is typical for two-phase countercurrent flow unit operations, flooding can be an issue with strippers. Flooding can occur at a given gas velocity if the solids flow is too high or at a given solids flow rate if the gas velocity is too high. Installing internals reduces the risk of flooding because they promote cross-current contacting between gas bubbles and the catalyst. Therefore, as has been previously observed, usage of internals increases not only the stripping efficiency but also the flow through capacity of the stripper.

Test were conducted in a 0.9-m-diameter, 7.5 m tall column with a 3.7 m height baffled with horizontal grating trays, spaced 0.6 m apart. Pressure drops were measured across the baffled height to estimate the stripper bed density. Tests were conducted using equilibrium fluid catalytic cracking (FCC) catalyst at a superficial gas velocity of 0.3 m/s at solids fluxes of up to 150 kg/s-m². In this presentation, we will report stripper density variation with solids flux.