OXIDATION-KINETICS OF AMMONIA AND AMMONIA-METHANOL MIXTURES IN SUPERCRITICAL WATER IN THE TEMPERATURE-RANGE 530-DEGREES-C 700-DEGREES-C AT 246 BAR

Oxidation of industrial chemical and metabolic wastes in a supercritical water medium is an effective method for the treatment and disposal of these materials. Partial oxidation of nitrogen-containing organics in supercritical water leads to the formation of ammonia, which is the rate-limiting step in the overall oxidation to nitrogen. In this study, the oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water were experimentally determined in a packed and unpacked tubular plug flow reactor. The oxidation of ammonia was found to be partially catalyzed by the Inconel 625 (a nickel-chromium alloy) reactor walls. In the unpacked reactor, the activation energy was 38 kcal/mol over temperatures ranging from 640 to 700-degrees-C. Oxidation of ammonia in the packed reactor gave an activation energy of 7.1 kcal/mol over the temperature range 530-680-degrees-C and a reaction rate approximately 4 times larger than the tubular reactor data. A power law model and a catalytic model were tested, and the catalytic model was found to represent the data well. In experiments with ammonia-methanol mixtures, the oxidation of ammonia was unaffected by the presence of methanol for experiments conducted in the tubular reactor and retarded by the presence of methanol for packed bed experiments. These observations are consistent with the hypothesis that ammonia oxidation proceeds predominantly as a catalytic reaction in the packed bed reactor at the temperatures investigated. Competitive adsorption of methanol and ammonia reduces the oxidation rate of ammonia in the packed bed reactor.