The Use of Supercritical Water for the Liquefaction of Polypropylene into Oil

About five billion tons of plastic waste have accumulated in landfills and the natural environment over the past 50 years. Polypropylene (PP) waste accounts for about 23% of the total plastic waste. Converting PP waste into useful products can reduce the accumulated waste and associated risks to the environment and human health. In this study, model PP was converted into oil using supercritical water at 380-500°C and 23 MPa over a reaction time of 0.5-6 hr. Up to 91 wt.% of model PP was converted into oil at 425°C with a 2-4 hr reaction time or at 450°C with a 0.5-1 hr reaction time. Higher reaction temperatures (>450°C) or longer reaction times (>4 hr) led to more gas products. The oil products consisted of olefins, paraffins, cyclics, and aromatics. About 80-90 wt.% of the oil components had the same boiling point range as naphtha (25-200°C) and heating values of 48-49 MJ/kg. Reaction pathways for converting model PP into oil under the tested conditions were proposed. Preliminary analyses indicate that this conversion process is net-energy positive and potentially has a higher energy efficiency and lower greenhouse gas emissions than incineration and mechanical recycling. The oil derived from PP has potential to be used as gasoline blendstocks or feedstocks for other chemicals.