Mar-2023

Optimising FCC economics against changing market dynamics

CO promoters are a frequently overlooked parameter for better control of afterburning, as demonstrated in a case study amidst a changing platinum group metals marketplace.

Heather Blair, Xunhua Mo, Marie Goret-Rana, Todd Hochheiser, Rick Fisher and Paul Diddams
Johnson Matthey

Article Summary

CO promoters are used in the majority of fluid catalytic cracking (FCC) operations. They are generally used in low quantities and, as a result, are often overlooked as an important point for optimisation. CO promoter optimisation and an understanding of coke burning fundamentals, afterburning, and methods for its control can lead to significant cost savings. Better control of afterburning helps limit downtime and allows for higher feed rates or the processing of more challenging feeds.

In addition to elaboration on the development of the new COP-NP HD promoter, the fundamentals of coke combustion, afterburn, CO promoter usage, and optimisation will be discussed, including a case study demonstrating the successful use of COP-NP HD in a commercial application. Reviewing the current status of the platinum group metals marketplace and drivers for innovation are also forthcoming.

FCC regenerator and heat balance
The FCC regenerator restores catalyst activity by removing coke from spent catalyst. This coke combustion provides the main source of heat necessary to drive the FCC process. Additional heat is also supplied from fresh and recycle feed preheat, steam, compression of air at the main air blower, and other minor sources. The total heat from these sources, especially coke combustion, provides the heat necessary for the FCC operation and keeps the system in energy balance (energy in equals energy out).

The FCC regenerator restores catalyst activity by removing coke from spent catalyst. This coke combustion provides the main source of heat necessary to drive the FCC process. Additional heat is also supplied from fresh and recycle feed preheat, steam, compression of air at the main air blower, and other minor sources. The total heat from these sources, especially coke combustion, provides the heat necessary for the FCC operation and keeps the system in energy balance (energy in equals energy out).

The FCC requires energy for the following: to vaporise feed and recycles and raise their temperature to the reactor temperature; to supply the energy for the endothermic cracking reaction (heat required to break the bonds); to heat steam to process temperatures in the riser, stripper, and standpipes; to heat the air from the blower discharge to the flue gas temperature; and to account for heat losses (such as catalyst cooler) in the system.

Coke combustion in regenerator and heat generation
Carbon and hydrogen in coke are combusted in the FCC regenerator by reaction with oxygen to form carbon dioxide, carbon monoxide, water, and heat:

Coke + O2 ® CO2 + CO + H2O+ Heat
Coke combustion also yields SOx and NOx since portions of feed nitrogen and sulphur also convert to coke which is also exothermic, but the heat produced is small and usually not included in heat balance calculations.

S in Coke + O2 ® SO2 + SO3 + COS + H2S + Heat

N in Coke + O2 ®  N2 + NOx + HCN + Heat

Coke combustion is kinetically limited; increasing the rate of coke combustion helps limit afterburn by keeping this reaction in the dense bed where there is more catalyst available to absorb the heat produced. Higher excess oxygen increases oxygen partial pressure, which in turn increases the coke burning rate. Higher regenerator pressure also increases oxygen partial pressure and reduces the superficial velocity, thereby increasing residence time. Higher regenerator dense bed temperature increases the rate of coke combustion, and the even distribution of catalyst and air improves coke combustion.
Incomplete combustion in the regenerator dense phase can result in CO and O2 breaking through to the dilute phase. Combustion of CO in the dilute phase is called afterburning.

Dealing with different types of afterburn
When CO is combusted to CO2 in the dilute phase above the regenerator dense bed, it causes a large temperature rise because there is insufficient catalyst to absorb the heat released. In order to minimise afterburn in full burn units, CO is minimised leaving the dense bed, and in partial burn units O2 is minimised leaving the dense bed.

In many cases, afterburn can be controlled or minimised using a CO promoter or methods that increase the rate of coke combustion outlined previously. It is much easier to control afterburn that is uniform across the regenerator, as indicated by consistent dilute phase and cyclone temperatures.

Afterburn caused by poor catalyst and/or air distribution is more difficult to manage. Usually, this type of afterburning results from damage to the regenerator internals, such as the air grid or catalyst distributor. It can also occur due to maldistribution resulting from poor regenerator operation or design that does not allow for good catalyst and air mixing throughout the bed.

To mitigate this type of afterburn, air distribution to the individual grids or rings can be adjusted or lift air relative to the main air blower, and regenerator bed levels can be optimised. CO promoter can be added and will be effective if there is enough CO and O2 present. However, sometimes the only method of resolution for this type of afterburn is a shutdown with modification to the air grid or catalyst distribution.

Importance and mitigation of afterburning
Afterburning has two major consequences in the FCC. First, the dilute phase temperature limits the feed rate and flexibility to run opportunity feeds. Second is potential serious damage to cyclones and flue gas systems from operating at excessively high temperatures, which can lead to premature shutdowns and costly repairs.

Higher dense bed temperature increases the rate of combustion of CO to CO2, thereby avoiding CO breakthrough to the dilute phase. Figure 1 shows a common example of afterburn decreasing with increasing dense bed temperature.

Regenerator bed level can have a major impact on CO breakthrough and afterburn. Higher bed levels often help minimise CO breakthrough because of increased residence time. Understanding how different regenerator levels impact CO, NOx, and O2 can help refiners define their optimum operating window. It is also important to observe CO, NOx, and O2 when undertaking catalyst withdrawals. If CO or afterburn are increasing significantly at lower bed levels or during catalyst withdrawals, it is generally advised to operate at a higher bed level and decrease time intervals between withdrawals. Higher bed levels can also cause increases in catalyst carryover, especially if transport disengaging height (TDH), where the catalyst concentration in the flue gas stays constant, is not maintained.¹