(446g) NH3 Interactions with O2 and NO Over Pd-Based FCC CO Emission Control Additives
AIChE Annual Meeting
2010
2010 Annual Meeting
Catalysis and Reaction Engineering Division
Applied Environmental Catalysis I
Wednesday, November 10, 2010 - 2:36pm to 2:57pm
Fluid Catalytic Cracking (FCC) is the primary process used to convert crude oil into a variety of higher-value products in modern petroleum refineries. Some portion of the nitrogen present in the oil feed is trapped in the coke formed during the cracking process and is eventually released in the FCC regenerator unit in different forms, including NOx and NH3. The levels of these pollutants depend primarily on the content and the type of nitrogen compounds being present in the feed, as well as on the regeneration mode used. Among different emission control technologies, the use of catalytic additives is the most attractive one, as it is simple, cost-effective, and applicable in existing FCC units. Most of the modern refineries for example, use promoters to accelerate the oxidation of CO. The goal of this study is to understand if and how the presence of these additives affects the reactions of NH3 with other gaseous components present (i.e., O2 and NO) under conditions approaching those existing in FCC regenerators, and to assess their effect on the control of N-containing gaseous species. FTIR and kinetic measurements were used to investigate the possible reactions of NH3 with O2 and NO over a Pdn+/Cen+/Na+/g-Al2O3 model additive. The FTIR results show that NHx intermediates can be formed upon adsorption of NH3 on the Pdn+/Cen+/Na+/g-Al2O3 surface in a wide range of temperatures. All components of this material (i.e., Pdn+, Cen+, and Na+) interact with NOx at elevated temperatures and promote the formation of nitrate/nitrite species to various degrees, essentially "trapping" NOx species on the surface. Nitrite/nitrate species can also be formed on the surface due to NH3 oxidation. A combination of FTIR and kinetic measurements indicates that NH3 can react efficiently with surface nitrites/nitrates to produce N2. These results suggest that some type of SCR chemistry is taking place on the Pdn+/Cen+/Na+/g-Al2O3 surface, involving the formation of nitrites/nitrates that are capable of interacting with NHx to form N2. However, at higher temperatures (i.e., 700oC) the oxidation of NH3 becomes dominant and leads to the formation of NOx, which quickly desorbs from the surface. In the absence of O2 in the feed, NH3 continues to interact with NOx at high rates yielding N2 and N2O. Given the dynamic nature and the significant variation of conditions in commercial FCC regenerators, it is possible that both schemes described above contribute to overall NOx reduction mechanisms.