Ind. Eng. Chem. Res. 1998, 37, 4631-4636
4631
NOx Emissions in Fluid Catalytic Cracking Catalyst Regeneration Karla L. Dishman, Patricia K. Doolin,* and Larry D. Tullock Ashland Petroleum Company, P.O. Box 391, Ashland, Kentucky 41114
The formation of nitrogen oxides (NOx) during the regeneration of fluid catalytic cracking (FCC) catalyst can be attributed to oxidation of nitrogen in coke. Thermal NOx does not occur to any significant extent at FCC regenerator temperature. NOx 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. NOx emissions are lower in controlled burn regenerators where exposure to excess oxygen is minimized. Twostage regeneration was found to lower NOx emissions by conversion to N2 during recontacting of flue gases and catalyst in the dense bed. Introduction In recent years, fluid catalytic cracking (FCC) units have shifted toward processing heavier feeds containing higher levels of aromatic compounds, metal contaminants, sulfur, and nitrogen. Nitrogen compounds can poison active sites on the catalyst and lead to reduced cracking activity. Organic nitrogen in the feed is also deposited in the coke on catalyst during the cracking reaction and is especially predominant with heavy feeds. During catalyst regeneration, nitrogen oxides may form and be emitted to the atmosphere. Nitrogen oxides have been linked with smog formation, and therefore, regulations concerning NOx emissions are expected to become more stringent in the future. A fundamental understanding of how nitrogen oxides form in the regenerator is just now evolving. The terms nitrogen oxide(s) and NOx are used in a generic sense and include various oxides such as nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2). Many researchers have studied NOx control strategies in the FCC regenerator. These include feed hydrotreating to reduce the nitrogen content entering the FCC unit, catalytic approaches that suppress NOx formation, stack gas cleanup methods down stream of the FCC unit, and process approaches which reduce the amount of NOx formed in a regenerator via regenerator modification. This paper discusses the chemistry of NOx formation and reduction in an FCC regenerator. The impact of novel two-stage regenerator design on emission of nitrogen oxides is explored. Fluid Catalytic Cracking (FCC). An inventory of catalytic cracking catalyst is circulated between the cracking reactor and a catalyst regenerator. Hydrocarbon feed contacts the catalyst at reactor temperatures of 460 °C to 560 °C. The hydrocarbons crack and deposit carbonaceous material (coke) on the catalyst. Cracked hydrocarbon products are separated from the coked catalyst. The spent (coked) catalyst is then stripped of volatiles by steam and regenerated. In the catalyst * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 606-921-6541.
regenerator, coke is burned from the catalyst with an oxygen-containing gas. The hot regenerated catalyst is circulated back to the reactor where the cycle again begins. Flue gases formed from the combustion of coke in the catalyst regenerator may be treated to remove particulates and to complete the conversion of carbon monoxide, before being discharged into the atmosphere. NOx levels in the regenerator flue gas from an FCC typically range from 50 to 300 ppmn, depending upon feed nitrogen levels and regenerator conditions. Ashland Reduced Crude Conversion (RCC) Unit. The Ashland Reduced Crude Conversion (RCC) process is detailed in U.S. Patent 4,332,6731 and, unlike the typical FCC, is designed to handle residuum oils which have at least 70% greater than 343 °C boiling range material and high carbon residue. Residuum is also known to possess high metals, sulfur, and nitrogen contents. These oils produce during processing unusually high amounts of coke, which deposits on the cracking catalyst. Problems arise in the regenerator because reactions which convert coke to water, carbon monoxide, and carbon dioxide are highly exothermic. The RCC unit’s unique two-stage regenerator is designed to produce a complete catalyst burn without reaching excessive temperatures much above 732 °C (Figure 1). Catalyst coolers in the first stage provide additional temperature control. Stripped spent catalyst from the reactor is introduced into the first stage, i.e., the top section of the regenerator, and is partially regenerated (nominally two-thirds of the carbon) with regenerator flue gas from the second stage or lower section of the regenerator and additional fresh air as necessary. The partial combustion minimizes the heat release from the coke. Air added to the second stage provides an excess of the oxygen required to remove the remaining coke. This produces a clean regenerated catalyst (
