KINETICS AND REACTION PATHWAYS OF PYRIDINE OXIDATION IN SUPERCRITICAL WATER

There is growing interest in applying supercritical water oxidation (SCWO) to the treatment of wastewaters and sludges. Existing mechanistic and kinetic data relate primarily to SCWO of simple compounds, hydrocarbons, and oxygenated hydrocarbons. Since many organic pollutants contain heteroatoms, the knowledge of SCWO reaction pathways, transition products, and kinetics for heteroatom-containing organic compounds is of critical design importance. This study focused on the kinetic and mechanistic aspects of pyridine oxidation in supercritical water using high-pressure oxygen gas. A laboratory-scale, continuous-flow reactor system was used. The experimental variables included temperatures varying from 426 to 525-degrees-C, reactor residence times varying from 2.1 to 10.7 s, and oxygen/pyridine molar feed ratios varying from 0 to 2.64. Pressure and feed flow rate, respectively, were maintained at a nominal value of 27.6 +/- 0.4 MPa and 35 +/- 0.1 g/min. The rate (mol/(L.s)) of pyridine oxidation in supercritical water was found to be 10(13.1+/-1.65) exp([-209.5 +/- 22.4 (kJ/gmol)]/RT)[Pyr]1.0+/-0.30[O2]0.20+/-0.16. Below 500-degrees-C, the extent of pyridine hydrolysis was less than 1%. The largest hydrolysis impact corresponded to 4.2% of pyridine conversion, which occurred at 521-degrees-C in less than 7 s. A number of transition products in both liquid and gaseous effluents from the pyridine oxidation experiments were identified. On the basis of these identified compounds, a network of simplified reaction pathways for pyridine oxidation in supercritical water was constructed.