Ken & Bob: We are here at the Duesseldorf CatCracking.com Conference. We’ll be opening your question for discussion with the delegates on Friday. Here’s our response until then:
The greatest period of time for hot/dry catalyst circulation we have personally observed was 7-8 days (Ken), 10-11 days (Bob). Sour water, catalyst attrition/fines generation, maintaining adequate heat in the reactor are concerns but are easily managed.
What is more difficult is balancing the regenerator temperature…too hot and slide valve position, and fluidization/circulateability (catalyst flow control). There is an increased chance of damaging the reactor internals if the reactor temperature gets away from you (it would be good to know is you have carbon steel (good to 1050 degF) or 1.25 Cr/0.5 Mo (good to 1150 degF)). A lower regenerator temperature (<1275 degF) would help but there would a tendency toward higher CO emissions and cyclone hanger temperatures (from afterburn). The “best” is to strike a balance for minimum regenerator bed temperature for catalyst circulation stability/control and a maximum for CO emissions control. You can operate at low air rates and low regenerator pressure to keep regenerator cyclone velocities elevated. If available, you can pressure up your overhead accumulator with fuel gas to establish the reactor and main fractionator pressures. This may help to establish better slide valve deltaP for stabilizing your catalyst circulation. Keep reactor/stripper hot (at least 500 degF) to avoid steam condensation and catalyst wetting.
We would expect losses in catalyst activity (MAT) and pore volume (PV) as well as increasing bulk density (ABD) due to lengthy exposure to a very hot flame front near each of the torch oil injection nozzles. You can add fresh catalyst at a rate to make up for losses and perhaps somewhat higher to achieve a slight build in bed level to counter the activity and physical property shifts. Send out equilibrium catalyst samples frequently to track the Ecat properties and assess the potential for having to replace a portion of the inventory when you want to return to normal operations.