Polystyrene recycling pioneers claim energy efficiency and cost effectiveness

Senior lecturer in chemical engineering at the University of Bath Dr Bernardo Castro-Dominguez said the recently-modelled method used in collaboration with colleagues from the US Worcester Polytechnic Institute in Massachusetts has the potential to work on an industrial scale.

Castro-Dominguez, who is also co-director of the Centre for Digital, Manufacturing & Design said: “Less than 5% of polystyrene is recycled at present – our work shows that as much as 60% of all polystyrene used today could be replaced by chemically recycled styrene.”

While polystyrene can be chemically recycled using heat, successive treatments provide diminishing returns as it is deprived of strength and flexibility. The majority of recycling centres do not accept polystyrene because they lack the necessary treatment facilities, while its bulk entails high transport costs, providing a further deterrent.

“Chemical recycling techniques are a major focus within chemical engineering right now, and cost- and energy-efficient ways to breakdown plastics to their primary building blocks such as polystyrene are urgently needed,” said Castro-Dominguez.

In a paper published in Chemical Engineering Journal, the researchers outline their new approach which employs pyrolysis to break down polystyrene into parts that can be subsequently reformed into new pieces of the material.

The process exposes material to temperatures of more than 450°C in an oxygen-free chamber, which prevents it from igniting. The polystyrene breaks down into monomers, which can then be reformed into virgin polystyrene.

This method offers a more sustainable approach, point out the scientists, because 1kg of the new material uses fewer than 10 megajoules of energy; equivalent, they say, to the power required for a microwave operating for half an hour.

Professor of chemical engineering at Worcester Michael Timko emphasised that the process could address the need for energy efficient sustainable practice but also fulfil the need for methods to be cost effective for scale up.

He explained: “Our analysis finds polystyrene to be an ideal candidate for a chemical recycling process.

“Surprisingly, the process is energetically efficient and potentially economically competitive. In terms of emissions, investing in this process has the potential to be equivalent to simple measures such as energy conservation in terms of the amount of emissions reduction that can be achieved for a given investment.”

For every kilogram of used polystyrene processed, 600 grams of 99% pure monomer grade styrene would be left available to generate new polystyrene, giving a yield of 60%, says the research team. In addition to reducing fossil fuel use, the cost to decrease the amount of carbon emissions through process is cheaper than many rival recycling methods, they state, amounting to c.US $1.5 per ton of CO₂.