A Kinetic Study of Methanol Oxidation in Supercritical Water

Abstract

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 H 2 O 2 as an initial oxidant. In some experiments, the O 2 /MeOH 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 CO 2 . The temporal variation of the CO yield exhibits a maximum at temperatures of 723 and 748 K, whereas the CO 2 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 CH 3 OH → CO → CO 2 . The global rate-controlling step in the complete oxidation of methanol is the conversion of CO to CO 2 . The first-order rate constants calculated for CO oxidation to CO 2 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.