A team of chemical engineers based at the University of Bath in the UK and Worcester Polytechnic Institute in Massachusetts, US, has shown that as much as 60% of all PS used today could be replaced by chemically recycled styrene.
They shared their findings in ‘Thermodynamic and economic analysis of a deployable and scalable process to recover Monomer-Grade styrene from waste polystyrene’, recently published in Chemical Engineering Journal.
The scientists analysed the thermodynamic and economic feasibility of using a pyrolysis reactor for thermal depolymerisation and two distillation columns to obtain over 99% pure styrene from waste polystyrene. The study assumed that a feed of segregated polystyrene is available with negligible contamination.
A major challenge preventing a wide use of chemical recycling by pyrolysis of segregated polystyrene is that the styrene in the product stream is typically contaminated with side products including α-methyl styrene, toluene, ethyl benzene, and styrene dimer. Whilst catalytic pyrolysis can increase liquid oil yields, it produces contaminants that diminish styrene yield since monomer purity of over 99% is required for polymerisation.
Distillation can help recover monomer-grade styrene from the polystyrene pyrolysis mixture. The study modelled the use of two distillation columns. The first column separates styrene from the light fraction consisting primarily of benzene, toluene, ethylbenzene and the second separates styrene from α-methyl styrene and the remaining heavy fraction. The scientists then analysed the yield and cost of the entire process.
Results show the process has a yield of 60% - meaning that if 1kg of used polystyrene were used, 600 grams of 99% pure monomer grade styrene would be left available to generate new polystyrene, thus reducing the use of fossil fuels. Assuming that all the recovered monomer is polymerised back into PS, a maximum recycle rate of 60% can be achieved.
Cost
The team analysed two cost scenarios: one where feedstock costs are equal to 15% ±5% of total product cost as well as a scenario in which feedstock costs are set to zero.
The analysis assumed a 30-year payoff period; a 10% internal rate of return; 3-year plant build; 50% annual sales in year 1; and 100% annual sales in year 2. The total cost of the system for a 30 tonnes/day plant was estimated at $1,331,000, with the distillation columns accounting for 95% of the cost ($1,266,300) and the pyrolysis reactor for less than 5% ($53,500).
Under the 15%-cost-feedstock scenario, the levelised styrene production cost is $2.09 ± $0.21 per kg for a plant processing 120 tonnes/day of PS – the maximum ‘realist’ scale for a plant receiving feedstock from a single municipal recycling facility in the US, according to the scientists.
That is roughly twice the cost of the 2023 selling price of commodity styrene.
“While doubling the cost of styrene – and hence polystyrene – is not a favourable economic outcome, some context is required,” the researchers noted. “The primary application for polystyrene is in packaging, especially for markets such as electronics, automotive parts, and food. In these cases, packaging accounts for a small fraction of the total product cost, meaning that doubling the cost of packaging might easily be absorbed. Of course, further cost reductions might be realised by further increases in process scale by aggregating feeds from multiple recycling facilities, up to the limits that are feasible in the presence of transportation costs.”
Under the zero-cost feedstock scenario, however, the projected levelised production cost is less than $1/kg of styrene, substantially less than its average historical selling price.
The team said the zero-cost feedstock scenario is a ‘reasonable scenario to consider on its own merits’, adding that ‘real negative feedstock costs can often be realised for waste feeds, even when accounting for transportation costs’.
Nonetheless, such a scenario would require substantial changes to the current plastic recycling value chain. To avoid polystyrene waste ending up in landfill due to it being valued at $0, waste management companies would have to be compensated in other ways.
PS chemical recycling ventures often struggle to prove profitable. Regenyx, a PS pyrolysis joint-venture between Agilyx and AmSty, for example, lost $4.5 million between 2021 and April 2024, when it closed its doors.