Characteristics of polyethylene cracking in supercritical water compared to thermal cracking

Abstract

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.