Oxidation kinetics of model compounds of metabolic waste in supercritical water

Oxidation of human metabolic waste in a supercritical water environment is an effective method for the recycling of aqueous wastes in a closed or partially enclosed environmental life support system. Partial oxidation of human wastes leads to formation of smaller compounds such as low molecular weight carbohydrates and urea which in turn oxidize to alcohols and ammonia. The complete oxidation of alcohols and ammonia to carbon dioxide and nitrogen is frequently the rate limiting step in the overall oxidation mechanism. In this NASA-funded study (grant no. NAG9-252), the oxidation kinetics of methanol and ammonia in supercritical water have been experimentally determined in an isothermal plug flow reactor. Theoretical studies have also been carried out to characterize key reaction pathways. Methanol oxidation rates were found to be proportional to the first power of methanol concentration and independent of oxygen concentration and were highly activated with an activation energy of approximately 98 kcal/mole over the temperature range 480 to 540°C at 246 bar. The oxidation of ammonia was found to be catalytic with an activation energy of 38 kcal/mole over temperatures ranging from 640 to 700°C. An elementary reaction model for methanol oxidation was applied after correction for the effect of high pressure on the rate constants. The conversion of methanol predicted by the model was in good agreement with experimental data although the concentration of hydrogen was underpredicted and the formation of formaldehyde (not detected in the experiments) was predicted.