There are a number of scenarios that can lead to a FCC unit shutdown due to excessive catalyst losses. Generally, inability to maintain regenerator bed level leads to an imminent unit shutdown. Since the stripper bed level is controlled with the spent catalyst valve any catalyst loss from the reactor or the regenerator is reflected in a loss of regenerator level. Refiners will face the decision to shut down whenever the catalyst loss is so high that:
– Not enough make-up catalyst (fresh or equilibrium) is available to restore the catalyst lost
– Catalyst loading via loader or pressuring from the hopper falls short of the required make-up rate
– Cost of replacing the lost catalyst becomes prohibitive
– Slurry oil ash or BS&W specifications cannot be met, leading to severe product discounts and loss of sales
– Wet gas scrubber (WGS) purge solids separation/containment inventory is inadequate
– Electrostatic precipitator fines collection rate exceeds the number of rolloff bins available
– Stack opacity/particulate emission limit compliance is difficult to achieve
– Slurry oil pump reliability and mechanical availability is unacceptable leading to significant feedrate reductions.
There may be opportunities to mitigate the catalyst loss and delaying the need for shutdown. The delay would allow the refiner to troubleshoot and develop corrective action plans for staying on-line and keeping operating costs down.
The refiner could consider:
– Pressure bumping to possibly dislodge an obstruction or aerate a de-fluidized zone
– Adjusting the fresh catalyst attrition resistance or fines content
– Making up with equilibrium catalyst versus fresh catalyst
– Adjusting air/steam rates, as well as the distribution if multiple grids/rings are available
– Lowering/raising bed levels to check impact on cyclone operation
– Lowering/raising operating temperature to lower/raise velocities
– Lowering/raising operating pressure to raise/lower velocities
– Conducting diagnostic studies to help identify what equipment needs repair.
posted for B.L.