FCC Regenerator Afterburn Causes and Cures

Description:

FCC Regenerator afterburn is defined as any increase in temperature above the regenerator bed. Afterburn can occur in the regenerator dilute phase, cyclones, or the flue gas line. Regenerator afterburn problems can also be categorized into one of two main types and within each type two subgroups. Type I afterburn is generalized. The temperature increase is uniform, or nearly uniform across the dilute phase cross section and/or the cyclone outlets. Type Ia afterburn is intermittent in nature. The unit may operate for a period with normal temperatures and then, for no apparent reason, the dilute phase, or cyclone temperatures will increase to an unacceptable level for some period after which, the temperatures will return to normal. Type Ib afterburn, on the other hand, is continuous and temperatures remain high as long as operating conditions are not changed. Type II afterburn is localized with high temperatures occurring across only a part of the regenerator cross section or in only some of the regenerator cyclones. Type IIa afterburn is intermittent or changes location – appearing in different parts of the regenerator or in different cyclones. Type IIb afterbur n remains in the same location as long as there are no significant changes in regenerator operation. Afterburning that is confined to the regenerator dilute phase does not generally present a problem. In most units, the dilute phase contains sufficient entrained catalyst to absorb the heat released by CO combustion with only a small increase in flue gas temperature. Once this entrained catalyst has been removed in the first stage cyclones, however, the oxidation of even small amounts of carbon monoxide will cause a large increase in flue gas temperature. This in turn can lead to equipment damage, unplanned shutdowns or extended turnarounds. Eliminating unwanted afterburn will improve unit reliability and increase the operating flexibility of the FCCU.