Kinetics of nitrogen oxides (NOx) formation and carbon oxidation: Relevance to catalyst regeneration in Fluid Catalytic Cracking (FCC) units

  • Sree Vidya Chaparala

Student thesis: Master's Thesis

Abstract

The aim of this thesis is to examine the possible mechanisms for the reaction between coke-bound Nitrogen and Carbon with Oxygen in the regenerator of Fluid Catalytic Cracking (FCC) units. This information is required for the reliable modelling of the FCC regenerator section to predict the noxious emissions during catalyst regeneration, which in turn, may help in developing techniques to reduce emissions of NOx, CO and CO2. The kinetics involved in their formation are related, as they are formed simultaneously in regenerators. The chemically accurate energetics and molecular parameters for various species involved in the reactions between the N and C complexes distributed on coke and O2 are determined through quantum calculations (density functional theory), and the rates of the elementary reactions are evaluated using transition state theory. Based on these calculations, the most probable pathways for NO, NO2, CO and CO2 formation and their rate constants are provided. The reaction mechanisms for NOx formation from three types of nitrogen-containing functional groups (pyridinic, anilinic and pyrrolic) were studied using the model molecules with free-edge and zig-zag sites. From this study, it was found that the addition of O2 on the C radicals was exothermic and barrierless, while the addition of O2 on the N radicals was endothermic and had positive energy barriers for all the model molecules considered here. The rate constants of O2 addition on N radicals were significantly lower than the rate constants for its addition on C radicals. This indicates that NOx formation results mainly through the transfer of an O atom from a neighboring carbon site to the N atom, and not through the direct attack of O2 on N atoms on coke. A large PAH model molecule, rich in zigzag sites and with only one radical on a C atom, was used as a realistic representation of coke surface to study the kinetics of CO and CO2 formation. The reaction pathways were studied using density functional theory. The addition of O2 on the radical site of the model PAH molecule was found to be exothermic and barrierless. Four routes for the formation of CO and CO2 have been proposed in this study involving desorption, rearrangement and surface migration reactions with barriers less than 500 kJ/mol. The detailed reaction mechanisms presented in this study on NOx formation and carbon oxidation provide a deep insight into the reactions taking place inside FCC regenerators, and may help in developing and improving the predictive capability of regenerator models.
Date of Award2014
Original languageAmerican English
SupervisorAbhijeet Raj (Supervisor)

Keywords

  • Pure sciences
  • Applied sciences
  • Analytical chemistry
  • Inorganic chemistry
  • Organic chemistry
  • Chemical engineering
  • 0486:Analytical chemistry
  • 0542:Chemical engineering
  • 0488:Inorganic chemistry
  • 0490:Organic chemistry

Cite this

'