Takehiko Moriya

Kinetics of oxidation of food wastes with H2O2 in supercritical water

Abstract In this study, supercritical water oxidation (SCWO) of carrots and beef suet was carried out in a batch reactor system with an H 2 O 2 oxidant, at a temperature between 400 and 450°C and reaction times from 10 s to 10 min. The results showed that the oxidative decomposition of carrots and beef suet proceeded rapidly and a high total organic carbon (TOC) decomposition of up to 97.5% was obtained within 3 min at 420°C for carrots and within 5 min at 450°C for beef suet when there was a sufficient supply of oxygen. It was also found that the oxidation reaction for both carrots and beef suet might be separated into a fast reaction at the early stage and a slow reaction at the later stage. In the later stage following the early stage reaction, acetic acid, which is a fairly stable product of the early stage reaction, is the reactant and the rate of overall oxidation reaction for complete decomposition is dominated by the later stage reaction. Global kinetic analysis based on the model described above showed that the early stage oxidative reaction of beef suet could be considered as a first-order reaction with respect to the concentration of organic carbon. The activation energy was 37.3 kJ mol −1 . Oxidation of acetic acid could also be expressed as a first-order reaction, and the activation energy was 106.5 kJ mol −1 . The early stage oxidation reaction of carrots was too fast to be analyzed. On the basis of intermediate products identified, reaction pathways were discussed. For carrots, polysaccharides may first be hydrolyzed to glucose and then oxidation of the glucose may take place. For beef suet, glyceride is first hydrolyzed to glycerin and carboxylic acids corresponding to the components of glyceride, followed by consecutive reactions for oxidative decomposition.

Characteristics of polyethylene cracking in supercritical water compared to thermal cracking

Polyethylene (PE) cracking obtained from hydrothermal experiment with supercritical water was compared to the water-free thermal cracking. Degradation of polymers was slow in supercritical water cracking and the yield of oil, because coke production was suppressed, was higher. In a comparison of the intermediate products during degradation between supercritical water and thermal crackings, the results of the experiments using n-dodecane (model intermediate of n-alkanes) and 1-dodecene (model intermediate of l-alkenes) showed significant differences in product species and cracking rate for l-dodecene but the results were not significant for n-dodecane. In supercritical water cracking, the conversion of l-dodecene was slow and the intramolecular transfer of double bonds occurred easily compared to thermal cracking. A difference in the reaction mechanism is supposed to explain the different cracking rates and products. The main products in the aqueous phase after supercritical water cracking were 2-propanol and 2-butanol (secondary alcohols) and 2-propanone and 2-butanone (ketones). It was confirmed that, in supercritical water, lower l-alkenes are hydrated into secondary alcohols, as shown by the hydration of propylene into 2-propanol. This 2-propanol is further oxidized into 2-propanone. It is plausible that the hydrogen, which is liberated into supercritical water, participates in PE cracking.