what is partial and post burning in FCC regenerator ?
Answers
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29/12/2016
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A:
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Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
I agree with William Miller's definitions and comments. Partial burn operation can be an effective way to relax a regenerator coke burn limit (blower capacity, regenerator superficial velocity or dilute phase temperature limit, etc) provided there is a CO Incinerator/Boiler downstream to convert the CO to CO2 and meet the regulator's flue gas CO emissions limit. However, it is important that the regenerator operation is carefully monitored and controlled to avoid high dilute phase temperatures (afterburn leading to possible cyclone and flue gas system damage) or high carbon on regenerated catalyst (poor yield selectivity and increased catalyst circulation rate leading to correspondingly low slide valve dPs). Damage to the spent catalyst or combustion air distributors could lead to poor mixing between coked catalyst and combustion air in the dense phase leading to oxygen breakthrough and afterburn. Controlling to a false low reading on the flue gas oxygen analyser would have a similar outcome. Carbon on regenerated catalyst can increase very rapidly if the combustion air flow is inadequate due to blower problems or a false high reading on the flue gas analyser.
Key factors affecting regeneration efficiency and carbon on regenerated catalyst are catalyst residence time in the regenerator bed, dense phase temperature, oxygen partial pressure, combustion promoter concentration, design/integrity of the spent catalyst and combustion air distributors and accuracy/reliability of the flue gas oxygen analyser. A typical temperature difference between dilute and dense phase in a well-operated regenerator would be 10 - 20 Deg. C.
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12/12/2016
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William Miller, WorleyParsons, william.miller@worleyparsons.com
Partial burn is where you burn some carbon only to CO, instead of full burn to CO2. It may be a more efficient use of air, but you would need facility downstream to complete the burn before releasing to atmosphere. This might be in a CO boiler or furnace.
Post burning, or after burn, is where the combustion is completed above the regen bed. It can lead to elevated temperatures at the cyclones and does not heat circulating catalyst. It may indicate that you are getting incomplete combustion in the bed, perhaps due to un-even distribution of air across the bed.
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