Most of us consider recycling plastic at least once every day. We frequently separate our paper, glass, and plastic; pick a trash can; or decide what to buy based on recycling considerations. However, we rarely consider the procedures employed to recycle the goods that frequently come into our possession. The topic of this essay is advanced or chemical recycling of plastics.
Chemical and mechanical recycling of plastic are the two most prevalent types. It is yet unclear if chemical recycling will challenge mechanical recycling, which is now the most popular method of recycling plastic. To prevent contamination, mechanical recycling gathers plastic bottles, grinds them up, washes them, and separates them. Typically, the plastic is then recycled again to make plastic bottles and other products. Chemical recycling cannot compete with that in terms of cost because the processes of sorting, grinding, and washing alone are low-cost.
What are the different types of chemical recycling?
It is important to note that instead of chemical recycling, many industry players prefer the term advanced recycling. But no matter what you call it, the processes are the same. There are three main types of chemical recycling: pyrolysis, gasification, and solvolysis.
Pyrolysis primarily targets polyethylene and polypropylene, or olefins as they are commonly called. The pyrolysis process heats the collected recycled plastic in a reactor without oxygen. Separate the molecules. There are many different configurations and types of reactors, but most include a catalyst that helps speed up the reaction and provides the desired yield. You end up with a naphtha-like liquid called pyrolysis oil with gases often recirculated to generate the heat you need, along with waxes that can be sold on the open market. Depending on your raw material, some ash may accumulate, but it is usually a minor percentage.
If the pyrolysis oil is of sufficient quality, it goes to an industrial steam cracker at a polymer production facility, where it is cracked into monomers such as ethylene and propylene, which are then converted to polyethylene, thus making them spherical.
One of the advantages of pyrolysis is that it takes up a relatively small space. Typically, you will see facilities that process between 20,000 and 40,000 tons per year. If you want more capacity, build more drives of roughly the same size instead of larger drives.
Pyrolysis has been around for a long time. I saw my first pyrolysis unit 30 years ago. He changed car tires to diesel oil and carbon black. New is the use of pyrolysis for plastics as consumers, regulators and consumer product companies seek to improve plastics, particularly plastic packaging.
Crucially, the plastic that comes out of pyrolysis is essentially virgin. Because it has reverted to the monomer and then regenerated the plastic, it can be used in any application, especially in contact with food, cosmetics, or even medical packaging. Plastics processed by pyrolysis will have no contamination, while mechanical recycling may have some color or odor contamination. Some disadvantages are that foreign impurities in the polyethylene and polypropylene interfere with the process to some extent. Materials like polyethylene terephthalate (PET) have an oxygen backbone. And because you don’t want oxygen in the process, it leads to inefficiency.
Gasification consists of taking polymers and treating them with high temperatures in a controlled oxygen environment. Basically, it’s a molecular encoder. This converts the plastic into base components of hydrogen and carbon monoxide, called syngas, which are then typically converted to methanol. The methanol value chain is likely to end up in something akin to fertilizer, but it can be made into polymers and become circular.
The main advantage of gasification is the low classification and wide availability of raw materials. Because it reverts to the hydrogen and carbon monoxide base, it’s much more forgiving, and if you don’t have enough plastic to eat, you can use it as food waste, wood, or any other carbon source.
Gasification is not widely practiced today because it requires large capital expenditures to build all the equipment to recover the plastic. Gasification itself does not require a lot of capital. For example, it’s all backend to move it to methanol and then methanol to olefins. Its conversion rate is quite low and therefore its carbon footprint is very high.
Solvolysis is a solvent-based process. The current main focus of solvolysis-based plastic recycling is PET. This is what goes into today’s water and soda bottles, as well as many highly transparent, disposable plastic containers. It is also called glycolysis or methanolysis or hydrolysis. Major PET polymer companies have talked about investing billions of dollars in recycling and are investing in both mechanical recycling and solvolysis of PET containers. The general process can give rise to the monomers that are used to make virgin PET.
Procter & Gamble has developed a solvent-based recycling platform to recycle polypropylene thin films, like the little sachets used in the developing world where you can get your shampoo or detergent. PureCycle Technologies is extending this solvolysis-based recycling technology to help improve the circularity of polypropylene.
The advantages of all these chemical recycling techniques are that the output can be used in food contact and other important applications. The US Food and Drug Administration, the European Food Safety Authority, and similar authorities worldwide have approved the mechanical recycling of PET bottles in food contact applications as long as certain requirements are met. There are currently no effective options for circularity without some form of chemical recycling in other resins and applications, such as non-bottle products, especially films, high-value and food contact applications.
The main downside of these chemical recycling technologies is the cost compared to mechanical recycling and the additional need for sorting and using the process properly to ensure you get what you want.