Evaluation of three solvent-based recycling pathways for circular polypropylene†

Abstract

Solvent-based methods for recycling polyolefin plastic waste have caught increasing attention, as they can produce recycled plastic of significantly higher quality than currently employed techniques. In this study, to demonstrate the development of plastic recycling systems for a circular economy, three solvent-based processes used for recycling polypropylene (PP), one of the most widely used plastic materials in Japan, were rigorously modeled and analyzed in terms of economic performance and CO₂ emissions. A cradle-to-gate life cycle assessment-based method was applied to quantify all sources of CO₂ emissions comprehensively. The most common solvent-based recycling method, in which the polymer is dissolved in a solvent and precipitated with an antisolvent, had the lowest economic performance and produced the highest CO₂ emissions: 1.30 kg of CO₂-equivalent per kg rPP. A more recently developed process in which the temperature of the solvent is manipulated to effect dissolution and precipitation had lower CO₂ emissions, at 0.92 kg kg⁻¹ rPP, and the most promising economic performance. A novel process using supercritical propane as the solvent achieved the lowest emissions of 0.32 kg kg⁻¹ rPP with similar economic performance to the temperature-dependent separation process. The environmental competitiveness (in terms of CO₂ emissions) of these recycling processes is further investigated by comparison with alternative state-of-the-art methods of plastic waste disposal, including mechanical recycling, gasification, and incineration with thermal recovery. Sensitivity studies were carried out to explore the effect of the waste plastic feed composition resulting from different preparation (sorting) methods on the economic and environmental performance of the three solvent-based recycling processes. The results obtained from this study are expected to provide valuable insights for constructing a green and cost-effective PP recycling process toward a circular economy.