Researchers at a Swiss university have made an important advance in the use of a hydrogen-based process for the chemical recycling of plastics, which could ultimately enable the conversion of plastic waste into fuels.

Scientists at ETH Zurich have determined the optimal stirring conditions required when chemically recycling plastics using hydrogenolysis, moving closer to being able to turn plastic waste into fuels and lubricants.

Hydrogenolysis is an established process in the petrochemical industry but is still in the research stage with regards to plastic recycling.

If successful, it could provide an alternative chemical recycling process that is potentially faster and less energy-intensive than pyrolysis for certain plastics, said Javier Perez-Ramirez, professor of catalysis engineering at ETH Zurich.

This could have a large impact on reducing plastic waste contamination as the market for fuels is so large, he told Carbon Pulse.

Hundreds of millions of tonnes of plastic waste are generated worldwide every year, with the growing market for plastics credits hoping to support recycling efforts, to reduce the devastating impact of plastic pollution on biodiversity.

The production of plastics is also hugely CO2-intensive, as it is largely petrochemical-based and releases CO2 emissions during the production process.

Meanwhile, demand for less carbon-intensive fuel is on the rise, with growing demand for synthetic e-fuels (made from CO2 and hydrogen) to decarbonise the maritime and aviation sectors.

BETTER RECYCLING

Mechanical recycling of plastic is the most widespread form of plastics recycling used today but entails the deterioration of plastic over time, while chemical recycling, which uses pyrolysis to break down plastics to their ‘building block’ monomers, also has limitations.

Polyethylene and polypropylene, which account for 60% of all plastic waste, are much more difficult to break down into their building blocks using pyrolysis and the process produces many products that require separating out into useful fractions, said Perez-Ramirez.

It requires lots of energy to break these plastics down, to separate them out, and to make something useful out of the molecules, he explained.

Adding hydrogen to the reactor through the process of hydrogenolysis “can control the range of products produced much better, which hopefully will require fewer separation units” and will therefore be more efficient, he said.

Hydrogenolysis has been in research for the last 5-10 years and is not yet commercial, but the researchers say they have overcome an important hurdle in moving towards that goal.

STIRRING TECHNIQUE

Their discovery relates to how best to stir the molten plastic with gaseous hydrogen and the powdered catalyst inside the reactor operating at 200-300C.

“Stirring has always been important in chemical engineering, but up until now, there was no quantitative guidelines on how to stir so that your catalyst works to its best,” said Perez-Ramirez.

“We have developed guidelines so that everyone can apply these optimal stirring conditions. And now we can proceed to develop the catalyst.”

Through experiments and computer simulations, the research team demonstrated that the plastic is best stirred using an impeller with blades parallel to the axis, and that the ideal stirring speed is close to 1,000 revolutions per minute.

“The use of these optimal stirring conditions can improve the speed of reaction by up to 80% – so it is really important – you may lose up to 80% of the potential performance of your catalyst just by not stirring correctly,” said Perez-Ramirez.

Understanding the most optimal conditions, such as temperature, pressure, and stirring speed, is key to an effective chemical reaction.

Now researchers working on hydrogenolysis for plastics worldwide can turn their attention to developing an optimal catalyst, he said.

“So far, it’s been very difficult to know whether the lack of performance came from the catalyst, from the stirring, or from a combination of both. Now we have removed one of the variables.”

NEXT STEP – CATALYST

The next focus for chemical engineers will be to design the most optimum catalyst for the chemical reaction, which will ideally be made of widely available and affordable metals.

Current catalysts contain novel metals like gold, platinum, and ruthenium, but researchers need to develop catalysts using much more widely available metals for sustainability and feasibility reasons, said Perez-Ramirez.

The role of a catalyst in chemical recycling is to help accelerate the reaction and drive it forward.

He was unable to estimate when hydrogenolysis may become commercially available for plastics recycling.

The process needs to be sustainable and affordable for it to work, and also needs access to a ready supply of green hydrogen, which will require collaboration with other industries like energy.

However, the potential for hydrogenolysis to produce valuable fuels from waste plastics is significant, given its suitability for this and the scale of the fuel market globally, said Perez-Ramirez.

Other efforts to reduce the environmental impact of plastics include Dutch startup Avantium, which is developing CO2-derived plastics that it claims could reduce the sector’s greenhouse gas footprint and curb pressure on biomass resources.

By Bryony Collins – bryony@carbon-pulse.com