The study looked at mechanical recycling and solvent dissolution of PE, PET and PP, as well as enzymatic hydrolysis, glycolysis and vapor methanolysis of PET. | By tezzstock/Shutterstock

A study by the National Renewable Energy Laboratory evaluated various PET recycling methods from technical, economic, and environmental standpoints to provide an overview of current options.

The research, published by the American Chemical Society, found that mechanical recycling outperformed all other technologies as well as virgin plastic production on economic and environmental considerations, but ranked lower on technical metrics.

Of several chemical recycling methods available for PET, glycolysis “offered the best economic and environmental returns,” the study found, with areas for future improvement including process efficiencies, consumable reduction or replacement, decarbonization of utilities, and waste recycling. closed cycle for polyolefins.

“Chemical recycling” generally refers to a wide range of processes that use heat, pressure, and solvents to break down the molecular chains of polymers into liquids or gases that can then be processed into fuels, oils, waxes, new plastics, or other chemicals.

“This work quantitatively characterized the performance of plastic recycling technologies and established a robust methodology for benchmarking new recycling processes as they emerge in the future,” the report noted, in addition to providing a decision tree to assist recyclers. to choose the best method for each material. .

The study looked at mechanical recycling and solvent-based dissolution of PE, PET and PP, as well as enzymatic hydrolysis, glycolysis and vapor methanolysis of PET, using virgin production as a reference.

Enzymatic hydrolysis uses hydrolase enzymes to depolymerize PET, glycolysis uses ethylene glycol, and methanolysis uses vaporized methanol.

The metrics evaluated were material quality, material retention, circularity, tolerance to contamination, minimum sale price, greenhouse gas emissions, energy use, land use, toxicity, waste generation, and water use.

Overall, mechanical recycling and glycolysis of PET outperformed virgin PET on all metrics, and methanolysis and dissolution scored above virgin levels in the economic and technical sections, but scored lower in the economic and technical sections. environmental and resource issues due to high GHG emissions, toxicity, land use, and water use. .

PET enzymatic hydrolysis did not exceed naive baseline in any modeled scenario, the study noted.

As for other materials, mechanical recycling of HDPE outperformed virgin production in the environmental and resource categories, but scored low technically and economically.

HDPE dissolution did not meet the virgin threshold for any section, nor did LDPE or PP dissolution, although LDPE and PP mechanical recycling performed better in the economic and technical sections due to lower bale prices, he reported. the study.

However, lower scores should not mean abandoning the technologies, the study noted, because “these techniques may not be advantageous across all metrics, but could provide benefits in specific priority areas, especially with continued improvements as technologies are developed.” technologies mature”.

The study also highlighted improvements for the different methods. Mechanical recycling could benefit from higher bale quality and polymer yields, as well as the addition of advanced sorting technologies such as fluorescent markers or robotic sorting along with artificial intelligence, the study noted.

For dissolution, switching to biobased solvents could improve your environmental score. The enzymatic hydrolysis of PET needs higher yields of monomers and uses less water to become viable, the study indicated.

Glycolysis could also benefit from better monomer yields, as well as new catalysts. PET methanolysis also needs new catalysts, the study indicated, and using less water.

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