Alan R. English
I am familiar with several units that typically operate in partial combustion and occasionally operate without the CO boiler. How this is accomplished depends on the local environmental restrictions. Local regulations regarding the operation of the CO boiler vary but are generally targeted to result in only minimal CO emissions. It the regulations specify a CO level at the boiler stack, you can adjust the degree of CO combustion in the regenerator to achieve it. The addition of CO combustion promoter will likely be required to maintain control of the regenerator temperature and achieve total combustion. Keep in mind that this will increase regenerator temperature and additional steps (lowering preheat and removing heavy feeds) may be required. Since this is intended to be a temporary operation; only for the duration of CO boiler maintanence, a catalyst change is not an option. The addition of inert particles, to act as a heat sink, might be an option to lower regenerator temperature if sufficient solid circulation capacity exists.
If local regulations do not limit CO emissions at the stack, you might be able to bypass the CO boiler and continue to operate in partial combustion. In this case, you must consider the impact on the surrounding area air quality to assure that no unsafe conditions are created. A computer dispersion model should be used to determine the maximum flue gas CO content that would result in acceptable ground level CO concentrations given prevailing atmospheric conditions and stack height.
Assuming a safe and legal operation without the CO boiler can be maintained, the decision to keep the FCC running or shutdown with the CO boiler becomes an economic one. Changing feeds, adjusting conditions and adding additives will have a cost. The can be compared to the benefit of a 4 year versus 5 year FCC turnaround cycle to make the final decision.