Kinetic Analysis on Initial Concentration Effect in Supercritical Water Oxidation of Alcohols by Elementary Reaction Model

摘要

Supercritical water oxidation (SCWO) has received attention as an innovative technology for waste treatment process. A number of experiments have been reported which explore the oxidation of methanol in supercritical water (SCW), however, there has been considerable disagreement among the experimentally determined SCWO reaction rates. Difference of initial concentration of methanol was considered as one of the reason of this disagreement, but detailed analysis has not been conducted. In this study, the effect of initial alcohols concentration on the SCWO rate in methanol and methanol/ethanol system was investigated, both experimentally and by elementary reaction model simulation. Experiments for different initial methanol concentrations (from 6.48 × 10{sup}(-6) mol/l to 1.15 × 10{sup}(-3) mol/l) at 520°C and 24.7 MPa showed that the initial concentration of methanol affected methanol conversion. Methanol conversion decreased with increase of initial methanol concentration at lower methanol concentrations, whereas it increased with increasing initial methanol concentration at higher methanol concentrations. Simulation based on the elementary reaction model revealed the same trend, and at the low methanol concentrations, H{sub}2O + HO{sub}2 = H{sub}2O{sub}2 + OH was identified as a key reaction that caused these phenomena. The characteristic dependence of methanol conversion on initial methanol concentration was due to the very high concentration of H{sub}2O in SCWO. From the calculation of co-oxidation of methanol and ethanol at 530°C and 24.7 MPa, the effect of acceleration of methanol conversion was observed. The effect of initial alcohol concentration was well characterized by HO{sub}2 radical, which accumulated by the reaction of organic radicals with O{sub}2, whereas the effect of the presence of alcohol co-existence was well characterized by OH radical.