Conversion of polyethylene to oil using supercritical water and donation of hydrogen in supercritical water

To clarify the characteristics of cracking polyethylene in supercritical water, thermal cracking and supercritical water cracking experiments were conducted. In comparing the species, yields and structures of the cracked products, consideration was given to the characteristics of the degradation mechanism. How supercritical water contributes to the degradation of polyethylene was also considered. It was found that supercritical water cracking, as compared to thermal cracking, is characterized by high oil yields and suppressed coke production. The result of an experiment using D2O as a tracer to crack polyethylene indicated that the hydrogen in the supercritical water is captured into the oil (a cracking product) and the donation of hydrogen from the supercritical water to the oil increases with a higher water fill rate for the reactor volume. The hydrogen donation mechanism from the supercritical water to the oil was found to include some prominent reaction stages : lower alkenes (propylene, etc. produced from the cracking of polyethylene are converted into secondary alcohols (2-propanol, etc.); and the hydrogen released in the oxidation of secondary alcohols into ketones (2-propanone, etc.) is donated to the cracked product. Additionally, in the hydrogenation degradation reaction, it was clarified that supercritical water donates hydrogen to the active ends of the molecule produced by the cracking of polyethylene and such donation allows the molecule produced to be stable; hydrogen donation is limited to a specific site within the methylene chain; and the higher fill rate has a more suppressive effect on the formation of gaseous cracked products. This is because the hydrogen donation capability of supercritical water with the higher fill rate is superior to that of the lower fill rate and the active ends of the molecule are more easily stabilized, suppressing the progress of the reactions.