Chemical Engineering

A team of researchers from WPI’s Department of Chemical Engineering and the University of Bath in the UK has analyzed a simple, scalable technology that has the potential to recycle up to 60% of the polystyrene used today, compared with less than 5% that is possible with current technology. Polystyrene that isn’t recycled ends up in landfills or worse – in our oceans, rivers, and beaches where it sheds microplastics for decades. 

Polystyrene has properties that make it nearly impossible to replace – it is a low-cost, impact resistant insulator that is perfect for shipping fragile parts and packaging food. These properties arise from polystyrene’s structure. Unfortunately, polystyrene’s structure also makes it difficult to recycle, as traditional mechanical recycling leads to loss of molecular weight and deterioration of the properties that make polystyrene so valuable. Now, the team of researchers has analyzed a system that can potentially solve these problems. 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 recently published in the Chemical Engineering Journal.

The core technology that the team analyzed is pyrolysis, a method that involves heating in the absence of air until the bonds of a molecule break apart. Polystyrene conveniently breaks primarily into a chemical called the monomer, that is the simplest unit that comprises the parent polymer. This has been known for decades and it opens the window for using pyrolysis to recover monomer followed by repolymerization to produce “good as new” polystyrene. However, monomer purity after pyrolysis is insufficient for repolymerization. And that was that, until the Bath-WPI team came along and analyzed a multi-step process to convert polystyrene into its monomer and then use distillation to purify the monomer. 

This multi-step process consists entirely of scalable technologies that are among the most reliable ones currently used in chemical processing. This means that the entire process should be readily deployable, whereas processes that require new, unproven technologies might require years – if they can ever be deployed at scale. 

The challenges are 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. Moreover, 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 the 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 CO2, comparable to costs associated with steps usually considered “low hanging fruit”, like installing energy efficient light bulbs. 

The researchers concluded that not only does the new process consist of scalable, proven technologies, it is realistic in terms of economics and energy use, and a net savings of emissions relative to combustion – all good things to keep polystyrene out of our water, out of our food, and ultimately out of our bodies. Continuing 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.

The research team consists of:

University of Bath

  • Bernardo Castro-Dominguez, senior lecturer in chemical engineering 

WPI Department of Chemical Engineering

Learn more about the research in this press release from the University of Bath.