Scientists are improving the process of turning hard-to-recycle plastic waste into fuel

UNIVERSITY PARK, Pennsylvania — Turning plastic waste into useful products through chemical recycling is one strategy to address the growing problem of plastic pollution on Earth. A new study could improve the ability of a method, called pyrolysis, to process difficult-to-recycle mixed plastics – such as multi-layered food packaging – and generate fuel as a byproduct, the scientists said.

Pyrolysis involves heating plastic in an oxygen-free environment, causing the materials to break down and creating new liquid or gaseous fuels in the process. Current commercial applications, however, operate below the scale needed or can only handle certain types of plastics, the scientists said.

“We have a very limited understanding of pyrolysis of mixed plastics,” said Hilal Ezgi Toraman, assistant professor of energy engineering and chemical engineering at Penn State. “Understanding the interaction effects between different polymers during advanced recycling is very important as we try to develop technologies that can recycle real plastic waste.”

Scientists performed co-pyrolysis of two of the most common types of plastic, low-density polyethylene (LDPE) and polyethylene terephthalate (PET), along with different catalysts to study the interaction effects between plastics. They found that a catalyst could be a good candidate for converting mixed LDPE and PET waste into valuable liquid fuels. Catalysts are materials added to pyrolysis that can aid the process, such as inducing plastic to break down selectively and at lower temperatures.

“This type of work can allow us to provide guidance or suggestions to industry,” said Toraman, who is a Virginia S. and Philip L. Walker Jr. faculty member in the Department of Energy and Mineral Engineering. of the John and Willie Leone family in Penn. State. “It is important to find out what kind of synergies exist between these materials during advanced recycling and what kinds of applications they may be suitable for before scaling up.”

The plastics, LDPE and PET, are commonly found in food packaging, which often consists of layers of different plastic materials designed to keep produce fresh and safe, but are also difficult to recycle with traditional processes because the layers must be separated , which is an expensive process.

“If you want to recycle them, you basically have to separate those layers and maybe do something with the unique streams,” Toraman said. “But pyrolysis can handle it, so it’s a very important option. It is not easy to find such a technique that can accept this disorderly complexity of these different plastic materials.

The first step in developing new commercial pyrolysis processes relies on a better mechanistic understanding of how dynamic mixtures of plastic waste break down and interact, the scientists said.

The scientists performed pyrolysis on LDPE and PET separately and together and observed interaction effects between the two polymers when tested with each of the three catalysts they used. The scientists reported the findings in the journal Reaction Chemistry & Engineering.

“We’ve seen products that can be very good candidates for gasoline application,” Toraman said.

The team also developed a kinetic model capable of accurately modeling the interaction effects observed during the co-pyrolysis of LDPE and PET with each of the catalysts. Kinetic models attempt to predict the behavior of a system and are important for better understanding why reactions occur.

Toraman’s research group focuses on performing experiments under well-defined and well-controlled conditions to understand the interaction effects during advanced recycling of mixed plastics and the corresponding reaction mechanisms.

“Systematic and fundamental studies on understanding reaction pathways and developing kinetic models are the first steps towards process optimization,” Toraman said. “If we don’t have our kinetic models correct, our reaction mechanisms accurately, then if we scale up for pilot plants or full-scale operations, the results won’t be accurate.”

Toraman said she hopes the research will lead to greater environmental responsibility in the recovery, processing and use of Earth’s resources.

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