In the fluid catalytic cracking (FCC) process, heavy fuel oil or petroleum residue feedstock is broken down into lower molecular-weight products in a riser reactor, then the exiting catalyst particles are stripped using steam to recover product hydrocarbon vapors in the fluidized bed stripper before the catalyst is then transferred to the regenerator to burn off a carbonaceous coke layer that builds up on the catalyst surface during the reaction. The hot regenerated catalyst is then recirculated to the riser via a standpipe. The fluidized bed stripper operates such that an up-flowing steam strips out the gaseous hydrocarbons entrained by the catalyst particles in the down-flowing emulsion phase. Ideally, both the gas in the interstices of the emulsion phase and that adsorbed on the solid particles need to be transferred into the bubbles of the stripping gas. This study was targeted at understanding the local flow and flooding behaviors of fluidized bed strippers. Bed density, pressure fluctuations and local bubble properties were measured in a 0.6-m diameter fluidized bed stripper for two types of internals: disk and donut, and grating trays operating with FCC catalyst particles. The grating stripper operated smoothly without flooding over a wider range of gas and solids flows than the disk and donut stripper. The disk and donut stripper flooding was a sudden rather than a gradual occurrence when a limiting solids flux was exceeded. Increasing fines composition and adding holes to the disk and donut trays mitigated flooding to some extent. The gratings stripper had flattened parabolic radial bubble void fraction profiles whereas the disk and donut stripper had characteristic M-shape profiles. The grating stripper had a higher bed density than the disk and donut stripper. These findings are valuable for the operation and design of gas-solids fluidized bed strippers.
