Initiation of carbon monoxide oxidn. - in fluid catalytic cracking regenerators by increase of coke content of regenerated catalyst and then of air flow rate

Initiating the oxidn. of CO to CO2 in a dense-phase catalyst bed in the regeneration zone of a fluid catalytic cracking (FCC) unit comprises (a) contacting the spent catalyst with fresh regeneration gas (pref. air) to reduce the coke content to a 1st value (pref. 0.1-0.6 wt.%) and produce a CO-contg. regeneration gas; (b) increasing the coke content of the catalyst to a value 0.1-0.5 wt. % higher than the 1st value (pref. 0.2-0.8 wt. %) and maintaining this coke content until a constant dense-bed temp. is reached; and (c) increasing the flow rate of the fresh regeneration gas to reduce the coke content of the catalyst to a value below the 1st value (pref. 0.01-0.09 wt. %) and to oxidise CO to CO2. Increasing the coke content in step (b) increases the supply of fuel which, when the air flow rate is increased, increases the temp. in the dense phase to a level (677-760 degrees C) at which CO oxidn. is initiated; once initiated, the exothermic oxidn. proceeds spontaneously. This prevents post-combustion in the dilute phase, allows part of the heat of combustion to be recovered, and prevents CO emission into the atmosphere.