A kinetic study of methanol oxidation in supercritical water

The oxidation rate of methanol in supercritical water at 253 bar and temperatures between 673 and 773 K is investigated using an isothermal, isobaric plug-flow tubular reactor and GC/FID and GC/TCD chromatographic methods. Experiments are conducted at a nominal methanol feed concentration of 0.88 mol % (1.53 wt %) using H₂O₂ as an initial oxidant. In some experiments, the O₂MeOH molar ratios are varied from 1.5 to 3.0 and show that the rate of methanol oxidation is independent of the oxygen initial feed concentration. Overall first-order rate constants calculated from the data lead to Arrhenius parameters of A = 10^11.8 s^-1 and E_a = 178 kJ/mol (42.5 kcal/mol). The identified reaction products are mainly CO and CO₂. The temporal variation of the CO yield exhibits a maximum at temperatures of 723 and 748 K, whereas the CO₂ yield increases monotonically over the experimental range of residence time (3-50 s). The experimental data are consistent with a set of consecutive first-order reactions CH₃OH → CO → CO₂. The global rate-controlling step in the complete oxidation of methanol is the conversion of CO to CO₂. The first-order rate constants calculated for CO oxidation to CO₂ lead to A = 10^10.8 s^-1 and E_a = 172 kJ/mol (41.0 kcal/mol). Kinetics of this system may be useful to study supercritical water oxidation (SCWO) of polychlorinated biphenyls (PCBs) dissolved in methanol.