The formation of nitrogen oxides (NO x ) during the regeneration of fluid catalytic cracking (FCC) catalyst can be attributed to oxidation of nitrogen in coke. Thermal NO x does not occur to any significant extent at FCC regenerator temperature. NO x is primarily comprised of nitric oxide (NO) with only a small concentration of NO2 detected. Under typical FCC regenerator conditions, only 10% of the nitrogen in coke evolves from the regenerator as nitric oxide with flue gases. Our findings show that ∼90% of nitrogen in coke is converted to dinitrogen (N2) in the regenerator. The authors suggest that dinitrogen is a secondary product produced by the reduction of NO with carbon and/or carbon monoxide in the regenerator. NO x emissions are lower in controlled burn regenerators where exposure to excess oxygen is minimized. Two-stage regeneration was found to lower NO x emissions by conversion to N2 during recontacting of flue gases and catalyst in the dense bed.

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Sieve analysis is used to obtain the particle size distribution of a solid material by determining the amount of powder retained on a series of sieves with different sized apertures. A sample is added to the top of a nest of sieves arranged in decreasing size from top to bottom. As the sieves are vibrated, the sample is segregated onto the different sized sieves. The weight of sample retained on each sieve is then used to determine the particle size distribution as well as the mean diameter of the sample.