Catalysis 2023 Issue

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

C O 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 opti - misation and an understanding of coke burning fundamen - tals, 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 pro - cessing of more challenging feeds. In addition to elaboration on the development of the new COP-NP HD promoter, the fundamentals of coke combus - tion, afterburn, CO promoter usage, and optimisation will be discussed, including a case study demonstrating the successful use of COP-NP HD in a commercial applica - tion. 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 bal - ance (energy in equals energy out). The FCC requires energy for the following: to vaporise feed and recycles and raise their temperature to the reac - tor 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 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 avail - able 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 super - ficial 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 O 2 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 con - trol afterburn that is uniform across the regenerator, as indi - cated 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 afterburn - ing 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

Coke + O2 → CO2 + CO + H2O+ Heat

Catalysis 2023