Mountain of plastic trash
Our system of plastic recycling doesn’t work very well. Fortunately, advanced plastic recycling technology exists. It just hasn’t yet become economically feasible.
Recycling post-consumer plastic in the US today starts with putting mixed recyclables out to the curb. Businesses have their own recycling procedures. The truck that collects everything takes it to a material recovery facility (MRF). There, a combination of automated and manual processes sorts it.
MRFs typically sort plastics into PET (no. 1 in the recycling triangle), HDPE (no. 2), and everything else. They sell it to companies that chop it up, wash it, and turn it into flake that can be used to make new products.
Unfortunately, MRFs can’t handle every kind of plastic. Plastic bags, films, Styrofoam
, and almost everything that’s not some kind of container can either damage equipment or contaminate finished bales.
It’s possible to take plastic bags and films to grocery stores for recycling. Most waste plastic, however, is fit only for the landfill.
Why is recycling plastic so difficult? And what are some solutions to the problems?
Polymers
Technically, this illustration compares a monomer and a trimer (chain of three monomers). For a polymer, imagine the chain extended to ten or more of the same monomer.
Plastics are various polymers. Substitute “of something” for “mer,” and polymer means a lot of something. A monomer is one of something. Dimer is two of something. Oligomer is a few of something.
Not all polymers are man-made. Sugar makes a good illustration of a natural polymer. Glucose and fructose are among the monosaccharides, “saccharide” being a “something” that means sugar. Table sugar combines those two monosaccharides into a disaccharide. Cellulose is a polysaccharide, a chain of ten or more monosaccharides, in this case, glucose.
In the same way, ethylene is a monomer used to make a polymer called polyethylene. Every other plastic you can think of that starts with “poly” is a chain of at least ten molecules of the corresponding monomer.
But there’s a big difference between natural polymers and plastics. Microorganisms can consume even the toughest plant fiber. In part, they break the bonds that link the monomers together. In a short time, that plant fiber composts and returns to the soil.
Plastics have an especially strong carbon bond. Few microorganisms take any interest. With these few exceptions, no natural process changes plastic’s molecular structure.
At least in part for that reason, it’s more difficult to recycle plastic than anything else in the waste stream. In principle, everything is recyclable. In practice, the current recycling infrastructure can only deal economically with a few kinds of plastic.
The need for advanced plastic recycling technology
Bales of recycled plastic ready for sale
Current recycling practices in the US and Canada recover less than 10% of all the plastics consumers discard. California has recently passed legislation requiring bottlers to use 50% recycled content to make bottles by 2030.
Even without such legislation, many large companies have committed to using more recycled plastic. But the current supply meets only 6% of demand. Supply of PET, the most commonly recycled plastic, meets less than 20% of demand.
Making plastic uses about 3-4% of the crude oil and natural gas liquid produced in North America, not counting the energy required in manufacture.
Standard sorting machinery at a material recovery facility (MRF) can’t deal with any of the various plastic bags and films. So much of our packaging requires thin layers of different kinds of plastic in one product.
The standard “chop and wash” technology works well enough for the PET and HDPE used to make bottles and jars. It can’t deal with the complexity of the entire plastic waste stream. Even under the best of circumstances, degradation and contamination make it impossible to keep all that plastic suitable for its most valuable uses.
It’s also expensive. From the time a MRF bales plastic to the time recycling company makes it into pellets, it can cost more than $1,000 per ton. Until costs come down drastically, recycled plastic can’t compete with virgin plastic from fossil fuels.
Another part of the problem, current recycling processes degrade the plastic so that it recycled plastic cannot be remade for its original use. Recycled PET bottles cannot become new bottles, for example.
As a society, we can stop using plastic straws or shopping bags. We can’t stop using films to protect meat or any number of disposable plastic products in the health-care system.
Currently available advanced technologies
Albatross carcass after it ate a lot of plastic
Besides mechanical processing, three basic kinds of advanced plastic recycling technology exist:
- Purification involves using a solvent to extract dyes, additives, and other contaminants to produce a pure polymer. This process does not change the molecular structure of the polymer.
- Conversion also breaks molecular bonds, but instead of producing monomers, it produces gas or liquid hydrocarbons comparable to crude oil. These products can become combustible fuels or various other petrochemicals.
- Depolymerization uses a thermal, biological, or chemical process—or some combination—to break a polymer down into simple monomers. These monomers were the building blocks of plastics in the first place. Therefore, it becomes a simple matter to use them instead of fossil fuels to make new plastics.
These processes are not new, but dozens of companies have devised innovative ways of using them.
Plastic purification
Styrofoam packaging
Procter & Gamble, like many other companies, seeks to use more recycled plastic in its packaging. It relies heavily on polypropylene (PP, no. 5 in the recycling triangle), one of the more difficult plastics to recycle.
MRFs can separate polypropylene from other plastics, although many don’t bother. The problem with recycling it comes from the color added to bottles and the odor of the product it contained. No one had been able to purify it enough to reuse it.
So the company found a way to use solvents to remove the contaminants. It is currently working to find solvents that can work with other kinds of plastic.
It licensed the technology to a new company called PureCycle. PureCycle expects its manufacturing plant to open in 2020. It will buy post-consumer and post-industrial waste and purify it. It will sell PP pellets not only to Procter & Gamble but also to other companies including Procter & Gamble’s competitors.
A Canadian company, Polystyvert, specializes in recycling polystyrene, the kind of plastic used to make Styrofoam. MRFs can’t handle any kind of polystyrene foam. It breaks into little pieces and contaminates everything else.
Polystyvert uses an essential oil that quickly dissolves the polystyrene—and doesn’t dissolve anything else. The company can reuse the oil after separating the plastic from it. It pelletizes the polystyrene. Companies can use these pellets the same way they use virgin plastic.
Conversion with catalytic pyrolysis
Pyrolysis
Pyrolysis uses extremely high temperatures in the absence of oxygen to convert most plastics to valuable and useful lubricant oils or waxes(It is not suitable for all plastics.) These products, in turn, become raw materials for consumer products, such as detergents or cosmetics. Pyrolysis is a well-established technology with more than two dozen companies operating worldwide.
Pyrolysis can take place at lower temperatures in the presence of a catalyst. Catalysts enhance the efficiency of pyrolysis but increase its cost.
A study in Saudi Arabia using two different modified natural zeolite catalysts. Zeolite is less expensive than many other catalysts. The team worked with the four plastics that make up most of the plastic waste stream, both individually and in various mixtures. Each batch produced somewhat different oils, all potentially valuable resources.
A research team at the Department of Energy’s Ames Laboratory has begun to investigate a new catalyst to convert waste polyethylene.
The process of making the catalyst requires depositing platinum nanoparticles on slightly larger perovskite nanoparticles.
In testing the catalyst, the Ames team began with research-grade polyethylene. It yielded a high volume of high-quality liquid oil. Then they tried the same process with a typical commercial plastic bag. The catalyst worked just as well on that. It also produces less methane and other gasses than pyrolysis.
This research, published in March and October 2019 respectively, is too recent for any commercial application to come of it any time soon.
Commercial depolymerization
Plastic bag in a garden
If polymerization means putting monomers together to make a polymer, then depolymerization means taking polymers apart to return them to the original monomers. It stands to reason that depolymerization is more difficult to scale up.
A French company, Carbios, has developed an enzymatic depolymerization process. Enzymes act like chemical scissors that cut polymers. The process doesn’t require presorting the plastic. Certain enzymes target particular plastics, but it is possible to apply different enzymes successively to the same batch of mixed plastics until all the various monomers have been separated.
The ability to process mixed plastics also makes it possible to depolymerize plastic products made by layering different kinds of plastic. It has never been possible to recycle them before except, perhaps, with pyrolysis.
After purification, these monomers can become the building blocks of now polymers. They have all the characteristics of virgin polymers.
Carbios has made partnerships with some major corporations to market its process on an industrial scale, beginning with Limagrain Céréales Ingrédients. More recently, it has begun to work with L’Oréal, PepsiCo, Nestlé Waters, and Sunstory Beverage & Food.
BP announced the opening of a depolymerization plant in Naperville, Illinois, which will open in late 2020. I haven’t been able to determine if it uses Carbios’ process, but it claims it will convert PET to monomers that will be interchangeable with traditional feedstocks.
It seems significant that an oil company has invested in a plant that can eventually make oil unnecessary.
The most serious remaining problem
Plastic flake from PET bottles
All these advanced plastic recycling technologies show great promise. Certainly, if it becomes possible to recycle all the waste plastic that now exists, it will not be necessary to use petrochemical feedstocks. Already, the demand for recycled plastic far exceeds the supply.
But most of these processes are still in the experimental stage. The companies that have commercialized some of them are still small.
Typically, it takes a company 17 years to work from concept to commercial-scale production. Part of the problem? Potential investors don’t understand either the technology or the business models. The time companies must invest in fundraising is time they can’t spend on improving or scaling up production.
As recently as ten years ago, we lacked the technology to deal with mountains of plastic waste. Once the technology develops, we will still lack the business models, infrastructure, and market incentives to scale them up to where they can begin to make a dent in our problems.
Sources:
The 5 things you need to know about chemical recycling / Lauren Phipps, GreenBiz. April 15, 2019
Accelerating circular supply chains for plastics: a landscape of transformational technologies that stop plastic waste, keep materials in play and grow markets / Closed Loop Partners. April 11, 2019
BP opening $25M chemical recycling plant in Illinois / Jim Johnson, Plastics News. October 25, 2019
Catalytic pyrolysis of plastic waste: moving toward pyrolysis based biorefineries / Rashid Miandad et al., Frontiers in Energy Research. March 19, 2019
Procter & Gamble, PureCycle introduce virgin-like recycled plastic / Arlene Karidis, Waste 360. August 16, 2018
Rethinking the science of plastic recycling / Argonne National Laboratory. October 23, 2019
Image credits:
Mountain of plastic trash. Some rights reserved by Shafiu Hussein.
Monomer, polymer. Some rights reserved by Zappys Technology Solutions
Plastic bales. bogdanwanko / 123RF Stock Photo
Albatross carcass. Some rights reserved by Sea Studios Foundation
Styrofoam packaging. Public domain from Wikimedia Commons
Pyrolysis. Source unknown
Plastic bag in a garden. Some rights reserved by Julian Stallabrass.
Plastic flake. Source unknown
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