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Chemical and Process Engineering Resources

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Category: Refining
Question: What could be a possible cause for 35% Nitrogen being in our FCC dry gas?
Keywords: nitrogen,dry,gas,fluidized,catalytic,cracker,fcc
Answer: There will always be some nitrogen in the dry gas. Your primary source is the N2 that is entrained in the catalyst from the Regenerator. Specific ratios will vary, based on catalyst circulation rates and moles of dry gas produced. 35% is on the high side. The first thing I would check is the possibility of air contamination of the sample. Once that possibility is eliminated, I would look for other areas where air or nitrogen might be getting into the system. For example, bleeds and purges in the FCC reactor system. Some units use air for instrument bleeds. Some also use air to fluidize regenerated catalyst standpipes. If either air or nitrogen is used in any of these services, I would make sure that all of the restriction orifices are in place. Finally, I would check to make sure that the design of the regenerator air distributor and spent catalyst withdrawal system is not causing air/flue gas entrainment into the spend catalyst standpipe. Have these high nitrogen levels occurred recently, or are these typical values for your unit? Have you made any modifications recently that would be expected to lower the dry gas yield (for example, an improved riser termination device or better feed nozzles). These same modifications would also be expected to increase the catalyst circulation and this would increase the amount of nitrogen entrained from the regenerator. The combination of lower dry gas and increased entrainment would be expected to raise the nitrogen level in the dry gas.