Proper design of mass transfer internals in the FCC flue gas scrubber can help reduce PM emissions

The EPA’s New Source Performance Standards (40 C.F.R. §60.100-1-0, subpart Ja) regulates refinery particulate emissions, including the discharge of catalyst fines from the FCCU flue gas scrubber stack. Because refiners have traditionally correlated particulate matter (PM) emissions with FCCU cokeburn, high flue gas stack PM can result in reducing severity or throughput in the FCCU at a potentially huge economic cost. The proper selection of mass transfer internals in the scrubber can contribute to its performance in controlling PM emissions, and can improve the refinery’s bottom line.

A unit turnaround is a prime opportunity for the refiner to address such issues as de-entrainment section fouling, chimney tray plugging, and overall poor performance contributing to stack PM.

Removing Solids with a Flue Gas Scrubber
Flue gas scrubbing is one method to control particulate and SO2 in FCCU flue gas vents. In scrubbers with external venturis, the flue gas is mixed with water and caustic to neutralize SOX. The combined stream enters a disengaging drum through large venturis,

where centrifugal force is used to separate the liquid from the flue gas. The flue gas then travels upward toward the stack. A bed of structured grid packing is used to eliminate entrained droplets that contain particles of catalyst or salt. The condensate is collected in a chimney tray and drained to the bottom of the disengaging drum. The scrubbed and de-entrained vapor is allowed to exit the scrubber stack.

Design of the Chimney Tray
A Sulzer chimney tray design for flue gas scrubbing service features a sloping floor to prevent solids accumulation and multiple small chimneys. The open area is sized to minimize pressure drop, while the riser arrangement allows for the best distribution into the packed bed.

Selection of Packing
Grid-type structured packing is used in direct-contact heat transfer, scrubbing, and de-entraining services such as the FCC flue gas scrubber. Due to its high open area, grid has a very low pressure drop and high capacity. Grids have low wetting rates compared with structured packing, and can therefore achieve low turndowns. An excellent option for de-entrainment in the flue gas stack is Sulzer Mellagrid™, which has a smooth surface to provide the maximum fouling resistance and is designed to be self-draining to avoid any solids trap-out problem. The Mellagrid smooth angles and transitions minimize shearing of liquid droplets, aiding in droplet settling. The Sulzer F-Grid™ or Nutter Grid™ can be utilized as drop-in replacements for an existing flue gas scrubber de-entrainment bed during a turnaround, or Sulzer can customize the grid design for optimum capacity, pressure drop, efficiency, and fouling resistance with a combination bed.

Design of the Grid Wash Sprays
Scrubber packing is subject to plugging from the residual salts and catalyst fines that are removed from the flue gas. A set of wash sprays is positioned above the bed for a periodic solids removal water wash. High stack velocities can entrain the spraying wash water overhead, not allowing adequate washing of the grid. Sulzer takes this into consideration in our design of the spray header, with the selection of nozzle type and nozzle pressure drop keeping the droplet sizes large and preventing re-entrainment. The sprays would be designed to fully cover the cross sectional grid area with sufficient overlap to account for some droplet carry-up.

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