Most of us think about plastic recycling at least every day. We regularly sort our paper, glass, and plastic; choose a waste container; or make purchase decisions based on recycling concerns. But we seldom think about the processes used to recycle the products that regularly pass through our hands. This article concerns itself with chemical recycling or advanced recycling of plastics.

There are two common forms of plastic recycling: chemical and mechanical. It is yet to be determined if chemical recycling competes with mechanical recycling, which is the most dominant form of plastic recycling today. Mechanical recycling collects plastic bottles, grinds them up, washes them, and separates them to ensure there isn’t contamination. The plastic is then typically recycled back into plastic bottles and other items. Chemical recycling can’t compete with that from a cost standpoint because simply sorting, grinding, and washing is inexpensive.

What are the different types of chemical recycling?

It’s important to be aware that instead of chemical recycling, many industry players prefer the term advanced recycling. But no matter what they’re calling it, the processes are the same. There are three primary types of chemical recycling: pyrolysis, gasification, and solvolysis.

Pyrolysis primarily targets polyethylene and polypropylene, or olefins, as they’re commonly called. The pyrolysis process heats up the collected recycled plastic without oxygen in a reactor. This rips apart the molecules. There are many different configurations and types of reactors, but most of them involve a catalyst that helps speed up the reaction and provide the desired output. You end up with a naphtha-like liquid called pyrolysis oil along with gases that are often circulated back to create any heat you need, along with waxes that can be sold on the open market. Depending on your feedstock, you may have some ash created, but this is typically a minor percentage.

If the pyrolysis oil is of high enough quality, it goes to an industrial steam cracker at a polymer production facility, where it is cracked into monomers like ethylene and propylene, which are then made into polyethylene, thus making them circular.

One of the advantages of pyrolysis is it’s a relatively small footprint. Typically, you see facilities that process between 20,000 and 40,000 tons per year. If you want more capacity, you build more units of roughly the same size instead of building bigger units.

Pyrolysis has been around for a long time. I saw my first pyrolysis unit 30 years ago. It turned ground-up car tires into diesel oil and carbon black. What’s new is the use of pyrolysis for plastics as consumers, regulators, and consumer product companies seek to improve the circularity of plastic, especially plastic packaging.

The most critical thing is the plastic that comes out of pyrolysis is essentially virgin. Because you’ve gone all the way back to the monomer and then recreated the plastic, it can be used in any application, especially food contact, cosmetic, or even medical packaging. Pyrolysis processed plastic won’t have any contaminants, where mechanical recycling might have some colour or odour contamination. Some of the disadvantages is that contaminants outside of polyethylene and polypropylene interfere with the process somewhat. Things like polyethylene terephthalate (PET) have oxygen in the backbone. And since you don’t want oxygen in this process, it causes inefficiencies.

Gasification is where you take the polymers and treat them with high temperature in a controlled oxygen environment. Essentially, this is a molecular scrambler. It reverts the plastics to base components of hydrogen and carbon monoxide — referred to as syngas — which are then typically converted into methanol. The methanol value chain would more likely end up in something like fertilizer, but it could be turned back into polymers and made circular.

The major advantage of gasification is less sorting and the wide availability of feedstocks. Because it goes back to base hydrogen and carbon monoxide, it’s much more forgiving, and if you don’t have enough plastics to feed it, you could feed it with food waste, wood, or just about any carbon source.

Gasification is not widely practiced today because it requires very large capital outlays to build all the equipment to get back to plastics. Gasification itself is not very capital intensive. It’s all the backend to move it into methanol and then methanol into olefins, for example. It has a fairly low conversion rate, and thus its carbon footprint is pretty high.

Solvolysis is a solvent-based process. The current main focus of solvolysis-based plastic recycling is PET. This is what comprises today’s water and soda bottles as well as many disposables and much clear plastic packaging. It’s also called glycolysis or methanolysis or hydrolysis. The big PET polymer companies have talked about investing billions of dollars in recycling and are investing in both mechanical and solvolysis recycling of PET packaging. The typical process might lead to monomers used to make virgin PET.

Procter & Gamble developed a solvent-based recycling platform for recycling thin polypropylene films, such as the little sachets used in the developing world where you might get your shampoo or detergents. PureCycle Technologies is scaling up this solvolysis-based recycling technology to help improve the circularity of polypropylene.

The advantages of all these chemical recycling technologies are that the output can be used in food contact and other critical applications. The U.S. Food and Drug Administration, the European Food Safety Authority, and similar authorities around the globe have given approval for PET bottles to be recycled mechanically back into food contact applications, given certain requirements are met. For other resins and applications, like non-bottle applications, particularly films, there are no current good options for circularity into high-value and food contact applications without some form of chemical recycling.

The biggest disadvantage for these chemical recycling technologies is cost versus mechanical recycling and the need for extra sorting to make sure you’re getting just what you want to have the process work properly.

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About Tony Kingsbury

Tony Kingsbury is the Founder and President at TKingsbury (2021 to present), his own consultancy focused on working to integrate sustainability, innovation, and business value throughout supply chains that use chemicals and plastics. Before this, Tony was Director of Sustainability, EMEA at Dow (2019 to 2021) and Vice President, Sustainability at Cardno ChemRisk (2012 to 2013).

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Follow-Up Questions Asked During the Webcast

• Can you elaborate on best practices for structuring agreements between pyrolysis operators and the cracker operators?
• Do you think regulations and incentives will become more globally harmonized, or will they remain regionally specific?
• Would you say that these chemical recycling processes are really sensitive to quality relating to feedstock quality?
• Would you view mechanical recycling as perhaps like a competing process, or would you view it as more of a complementary process?
• Would you say that LDPE chemicals have any dynamics that are unique or different from other substrates, such as PET and HDP?
• What would your main observations be regarding halogens in the feedstocks?
• Can we expect permanent or semi-permanent divergence from virgin resin costs independent from what energy is doing? (Away from convention-based pricing.)