spent catalyst will enter the bottom of the combustion riser where it is mixed with a regeneration gas the regeneration gas is an oxygen containing gas which is typically air that is injected through a distributor into the bottom of the combustion riser the distributor provides a uniform injection of the regeneration gas across the entire cross section of the combustor in order to increase the combustion rate of coke from the spent catalyst regenerated catalyst may be mixed with the spent catalyst at the bottom of the combustion riser and or anywhere along the vertical length of the combustion riser it has been found that combustion of coke within the riser is more effective than previously believed and that an essentially complete combustion of coke can be obtained by contacting of catalyst in the relatively dilute phase in the combustion riser the dilute phase of the combustion riser will have a catalyst density of from 3 to 20 lbs ft 3 and more preferably from between 3 to 15 lbs ft 3 superficial gas velocity within the combustion riser will usually be at least 7 ft sec and more typically will be in a range of from 7 to 20 ft sec normally at these conditions complete combustion of coke from the catalyst can be obtained with a residence time of at least 30 seconds and more typically from 30 to 60 seconds when the catalyst entering the regenerator has a coke content of from.