New research reports on an approach that could finally usher in energy-efficient plastic recycling, with massive implications for the industry and the environment both.
Plastics are, chemically speaking, long molecules made up mostly of carbon atoms strung together. This structure is what makes them so useful, as it imparts both good physical properties and outstanding chemical resilience to the material. But that last trait is also what makes plastics very resistant to being broken back down into carbon that can be used to make more plastic, or another product entirely.
Given that simply melting the plastic down to reuse it eventually degrades it so much it’s not really viable as a material, the high energy cost of transforming plastic back into carbon is, effectively, the death knell of our efforts to recycle this material and solve the plastic waste problem. But a new study could fix that.
Reverse refining
“It’s difficult to build a house and it’s easy to smash it apart,” said Dionisios Vlachos, a professor of physics at the University of Delaware and lead author of the paper, for Inverse. “This is the reverse. Plastic is very easy to make and difficult to break apart.”
Millions upon millions of tons of plastic waste are generated, globally, every year. This ranges from materials used in containers or packaging to electronics and a huge range of consumable products. The problem is compounded by the fact that virtually all of that plastic was freshly produced from crude oil instead of from recycled plastics, since the processes we have of doing so are slow, inefficient, and thus, expensive. This high cost is why most recyclable plastics today are not recycled, and end up in the landfill.
The current study describes an approach that can make recycling processes cost-efficient. This would revert plastic to its chemical building blocks which can then be used to produce fresh plastics or items such as fuel. The approach involves undergoing the refining process ‘in reverse’, according to Vlachos. It relies on zeolite and platinum as catalysts (both of these are already heavily used in the plastic industry to produce it from crude oil). Both platinum and zeolite can help break down the long chemical chains that make up plastic, but neither can carry the process to completion by themselves. Put together at high pressure, however, the team found that the catalysts can completely degrade the plastic molecule.
The process effectively ‘cracks’ (a term used in the oil industry) the long polymer chains into shorter, ‘short-C’ chains, that are much easier to process. In essence, the process does exactly what you want plastic to not do normally: break down, fast. Increasing pressure during this process allows for the plastic to be broken down efficiently even at low temperatures, the team explains, which helps further bring costs down.
“This is the first technology that’s able to take the most difficult plastics and recycle them into something really useful,” Vlachos added. “It’s the best way to recycle single-use plastics and packaging like polyethylene and polypropylene.”
In effect, the platinum catalyst starts the cracking reaction, which is then completed by the zeolite. This results in high yields of liquid hydrocarbons (oil) and a small quantity of solid byproducts. Currently, the process has a yield of around 85% of the original material by weight. Virtually all the major types of plastic in use today can be recycled using this approach, the team explains, including plastic bags and bottles (PET), HDPE, PP, polystyrene (PS), even layered (PP-PE-PS) plastic composites.
Different ratios of the two catalysts can be used to change the type of product that is output. This essentially would allow engineers to produce raw materials for a wide range of products simply by adding more of either compound.
Currently, however, the process does require quite a lot of water. Around 150 liters of water are required to make a gallon (3.8 liters) of gasoline. This will probably be improved upon in the future.
Right now, the technology has been patented, and Vlachos says we could expect its successful commercialization within 5 to 10 years. One of the main hurdles before that happens is developing a failproof method of eliminating impurities like food waste from the plastic before recycling it. However, once that is done, we have a decent shot at actually removing all the plastic waste clogging up landfills and natural landscapes the world over — in a nice, clean, efficient manner.
The paper “Plastic waste to fuels by hydrocracking at mild conditions” has been published in the journal Science Advances.
