Effect of regeneration conditions on the emission of HCN in FCC regeneration process

Abstract

Stringent environmental regulations have made HCN emission from fluid catalytic cracking (FCC) unit attracted more attention. HCN is always considered as a volatile intermediate, which is transformed into NOx after oxidation. However, the on-site monitoring data from a typical FCC unit in this work showed that HCN emissions were much higher than expected. The spent FCC catalysts were analyzed by different methods to identify HCN precursors, and found that pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and quaternary nitrogen (N-Q) are the main precursors of HCN. In the regeneration process, most of pyridinic nitrogen was transformed into NH3 and HCN, while the rest were partially oxidized to N-5, and the degree of oxidization conversion is related to regeneration temperature and oxygen concentration. A series of experiments were conducted on a fixed-bed regenerator to evaluate the effects of different regeneration conditions on the nitrogen-containing gas emission. Experiment results demonstrated that low oxygen concentration is beneficial to the reduction of HCN emission during the partial combustion regeneration stage. Under complete combustion condition, with excess oxygen content, the contents of NO, HCN and NO2 are much higher than these under partial combustion conditions, indicating that the oxygen content affects the emission of nitrogen-containing gas significantly.