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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.
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Low-Pressure Hydrothermal Processing of Disposable Face Masks into Oils
A total of 5.4 million tons of face masks were generated worldwide annually in 2021. Most of these used masks went to landfills or entered the environment, posing serious risks to wildlife, humans, and ecosystems. In this study, batch low-pressure hydrothermal processing (LP-HTP) methods are developed to convert disposable face masks into oils. Three different materials from face masks were studied to find optimal processing conditions for converting full face masks into oil. The oil and gas yields, as well as oil compositions, depend on the feedstock composition, particle size, and reaction conditions. Yields of 82 wt.% oil, 17 wt.% gas, and minimal char (~1 wt.%) were obtained from full masks. LP-HTP methods for converting face masks have higher oil yields than pyrolysis methods in the literature and have lower operating pressures than supercritical water liquefaction. LP-HTP methods for face masks can increase net energy returns by 3.4 times and reduce GHG emissions by 95% compared to incineration. LP-HTP has the potential to divert 5.4 million tons of waste masks annually from landfills and the environment, producing approximately 4.4 million tons of oil.
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- Award ID(s):
- 2112554
- PAR ID:
- 10529174
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Processes
- Volume:
- 11
- Issue:
- 10
- ISSN:
- 2227-9717
- Page Range / eLocation ID:
- 2819
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/pr11102819
