Use of Supercritical Water for the Liquefaction of Polypropylene into Oil

TLDR

Five billion tons of plastic waste have accumulated, with polypropylene making up about 23 % of this stock; converting PP into useful products can reduce environmental and health risks. The study aims to convert model polypropylene into oil via supercritical water treatment at 380–500 °C and 23 MPa over 0.5–6 h. Supercritical water at 380–500 °C, 23 MPa, and 0.5–6 h was used, achieving up to 91 wt % conversion at 425 °C for 2–4 h or 450 °C for 0.5–1 h. Oil products—rich in olefins, paraffins, cyclics, and aromatics—contained 80–90 wt % naphtha‑range hydrocarbons with 48–49 MJ/kg heating values; higher temperatures or longer times increased gas yields, and the process is net‑energy positive with lower GHG emissions than incineration, offering potential as gasoline blendstocks or chemical feedstocks.

Abstract

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 h. Up to 91 wt % of model PP was converted into oil at 425 °C with a 2–4 h reaction time or at 450 °C with a 0.5–1 h reaction time. Higher reaction temperatures (>450 °C) or longer reaction times (>4 h) 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 the potential to be used as gasoline blendstocks or feedstocks for other chemicals.