There’s a Simple New Way to Recycle Polystyrene

Despite the best efforts of suppliers and industry groups, polystyrene is generally viewed as unrecyclable and therefore environmentally unfriendly.

In fact, less than 5% of polystyrene from packaging, packing “peanuts”, cushioning, and other sources is recycled.

Worse, polystyrene that isn’t recycled ends up in landfills, oceans, rivers, or beaches where it sheds microplastics for decades.

However, a new analysis from researchers at Worcester Polytechnic Institute and the University of Bath reveals a breakthrough using a new approach. They report that it’s now conceivable that up to 60% of the polystyrene used today could be replaced by recycled polystyrene, rPS.

As a sustainable bonus, increased recycling of PS can reduce the need for new polystyrene production, and thereby contribute to the decarbonization of plastics.

The approach involves breaking down and re-creating polystyrene. In a paper in the Chemical Engineering Journal, researchers analyzed a simple process combining pyrolysis and distillation that has the potential to be scalable, cost-competitive, and energy-efficient.

The team outlines its findings in a paper titled” Thermodynamic and economic analysis of a deployable and scalable process to recover Monomer-Grade styrene from waste polystyrene”, which was published in the Chemical Engineering Journal.

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The core technology that the team analyzed is pyrolysis, which heating materials in the absence of air until the bonds of a molecule break apart. Polystyrene conveniently breaks primarily into a monomer. This opens the window for using pyrolysis to recover monomer followed by repolymerization to produce “good as new” polystyrene.

However, there’s a catch: monomer purity after pyrolysis is insufficient for repolymerization.

The holdup was short lived when the Bath-WPI team analyzed a multi-step process to convert polystyrene into its monomer and then use distillation to purify the monomer.

This two-step process consists entirely of scalable technologies that are among the most reliable processes used in chemical processing. Because it relies on proven technologies, this means that the entire process should be readily deployable soon.

PS recycling clears energy and economic barriers.

The challenges are centered energy use and economics, which is where the team focused its efforts. Interestingly, the energy requirements for the entire process are much less than the value contained in the chemical bonds of polystyrene, meaning that the process is energy efficient.

As well, multiple pathways exist for styrene production at costs that are competitive with global historical averages over the past several years. That means that “good as new” post-consumer polystyrene is not just an idea in the lab.

To complete their analysis, the team considered greenhouse gas emissions. Technologies such as pyrolysis and distillation have faced criticism in recent years because of their association with greenhouse gas emissions. However, the research team found that the new process can reduce greenhouse gas emissions relative to polymer combustion at an investment cost of about $1.5/ton of carbon dioxide. This is comparable in costs associated with “low-hanging fruit” such as installing energy efficient light bulbs.

The researchers concluded that not only does the new combination process consist of scalable, proven technologies, it’s realistic in terms of economics and energy use, and a net savings of emissions relative to combustion. These are all positives that will help keep polystyrene out of water, food and, ultimately, out of people.

Continued work is focused on improving the polystyrene process by including additional steps for obtaining more products, scenarios for mixed plastic waste streams, and developing new, and even more efficient technologies for plastic recycling.

WPI’s role in this study was funded by the National Science Foundation.