Efficiency of H 2O 2 and O 2 in supercritical water oxidation of 2,4-dichlorophenol and acetic acid

Abstract

The efficiencies of hydrogen peroxide and oxygen as oxidants for the destruction of acetic acid and 2,4-dichlorophenol were compared under supercritical water conditions using a batch reactor system. The effects of temperature, water density, and oxidant concentration on the destruction of both compounds were studied. Diluted oxygen (21% O 2 and 79% N 2) and premixed hydrogen peroxide (32 % aqueous solution) were used as the oxidants. The reaction temperatures were 400, 450, and 500°C. Water densities were fixed at 0.15 g/mL and 0.35 g/mL. The destruction efficiency of hydrogen peroxide was significantly higher than that of oxygen. For acetic acid, the highest conversion of 97.7% was achieved with hydrogen peroxide at 500°C for a reaction time of 10 min. while 64.3% conversion was achieved with oxygen at the same temperature but for a reaction time of 30 min. For 2,4-dichlorophenol, conversions higher than 99.995% were obtained with hydrogen peroxide at 450°C for a reaction time of 2 min as compared to 87.6% conversion with oxygen at 500°C for the same reaction time. The change in supercritical water density from 0.15 g/mL to 0.35 g/mL at 400°C only slightly increased the conversion of both compounds. The conversion of acetic acid was enhanced by 10% at 450°C as hydrogen peroxide supply was increased from 100 to 200% of stoichiometric demand. However, concentration of hydrogen peroxide higher than 300% of stoichiometric demand did not yield higher conversions of acetic acid. The effect of excess oxygen was not conclusive.