(472b) Disk and Donut Stripper Vent Holes

Strippers are commonly used in fluidized catalytic cracking (FCC) and fluid coking unit operations for the removal of valuable hydrocarbon products from the circulating catalyst.  Failure to remove these vapors from the catalyst results in a loss of valuable products and result in excessively high temperatures from the regenerator. Thus, entrained and adsorbed hydrocarbon vapors in these unit operations are typically removed from the catalyst in fluidized bed strippers using steam.  One of the most common types of strippers is the disk and donut tray stripper.  Alternating trays of annular rings and conical hats are used to enhance the mass transfer between the solids and stripping gas by (i) breaking up and slowing down the bubbles or voids, (ii) dividing the stripper into several axial stages for more plug flow behavior, and (iii) limiting gas mal-distribution due to gas-bypassing [Issangya, et al., 2006, 2008].

However, disk and donut strippers are not without their problems.  Although these types of strippers can achieve high stripper efficiencies, their effectiveness is limited to flooding.  In many cases, this flooding limits the productivity of the entire fluidized catalytic cracking or coker units.  Adding vent holes to the disk and donut stripper can minimize this problem without sacrificing stripper efficiencies [Revault, et al.].  Disk and donut strippers with these vent holes can operate at approximately 20% higher solids fluxes than disk and donut strippers without these holes.

In order to better understand the hydrodynamic differences in disk and donut strippers with and without vent holes, computational fluid dynamic (CFD) models using CPFD-Software’s Barracuda® were used to study the differences in the gas and solid hydrodynamics for disk and donut strippers with and without vent holes.  According to the model, disk and donut elements without vent holes results in the catalyst to be “squeezed” by the gas in the region between the internals.  As a result, the solids are consolidated into streamers of fast moving particles.  This allows the gas to consolidate and form channels of gas, a prelude to flooding in the unit.  The addition of vent holes appears to reduce this effect.  The better distribution of gas, even with only 10% of the gas going through the vent holes, results in a better distribution of both the gas and solids.  Gas voidages are not consolidated and the stripper operates with more stable hydrodynamics.

References

Issangya, A., Karri, S.B.R., Knowlton, T.M., “Why Gas Bypassing Occurs in Deep Fluidized Beds of Geldart Group A Particles and How to Prevent It,” 10th Int. Conf. on Multiphase Flow in Industrial Plant, vol. 10, pp. 1–21, Sep. 2006.

Issangya, A., Karri, S.B.R., Knowlton, T.M., “Effect of Baffles on Jet Streaming in Deep Fluidized Beds of Group A Particles,” Circulating Fluidized Bed 9, vol. 9, pp. 1–6, Jan. 2008.