Low-pressure hydrothermal processing for conversion of polystyrene into oils

Abstract

Global polystyrene (PS) waste accumulates at a rate of 28 million tons annually, while recycling rates stay at 1.3 %. PS degrades into microplastics, releasing various chemicals that affect ecosystems and human health. Conventional waste treatment methods are ineffective in reducing PS waste accumulation. This study developed batch low-pressure hydrothermal processing (LP-HTP) methods to convert PS to oils. Oil yields of 96–99 % were obtained with 1–2 % char and up to 2 % gas at average temperatures of 341–424°C for 19–75 minutes. Reversible reactions between monomers (C6-C9) and poly-aromatics (C10-C20+) were found to limit monomer yields. A two-step kinetic model accounting for the reversible reactions was developed. Temperature histories of the batch experiments were considered such that the estimated kinetic parameters were independent of reactor heating or cooling rates. The predicted yields of monomers and poly-aromatics agreed with experimental yields to within 6 %. The model predicted increasing monomer yields with decreasing PS loadings. This predictive model can aid future process optimization and scale-up. PS and polyolefins were co-processed to produce oils with 87 % yields and higher aromatic contents than oils produced from polyolefins alone. LP-HTP methods required no catalyst, had higher oil yields and less char formation than pyrolysis, and used much lower operating pressures and energy than supercritical water liquefaction. The methods also potentially have lower environmental impacts and 4.7 times higher energy recovery than incineration. The oils from LP-HTP, if separated into pure monomers, can be used as chemical feedstocks to achieve a circular use of hydrocarbons.