Continuous low-pressure hydrothermal processing methods for polystyrene conversion to oils†

Polystyrene (PS) waste is generated at an annual rate of 28 million tons, yet less than 1% is recycled. PS in landfills and ecosystems degrades into microplastics and releases harmful chemicals. This study introduces a continuous low-pressure hydrothermal processing (LP-HTP) method for converting PS into valuable oils without a catalyst. Two continuous reactors were designed, built, and tested at average temperatures from 394 to 538 °C, PS feed rates up to 1.2 kg h⁻¹, and residence times from 0.7 to 3.4 minutes. The process achieved 99 wt% oil yields with minimal gas formation. Water in LP-HTP suppressed char formation to <1 wt%. Lower temperatures and shorter residence times favored styrene production. At 397 °C and 2.0 minutes, the oil contained 65 wt% styrene monomer, and 88 wt% combined styrene monomer, dimer, and trimer. Reaction pathways were proposed based on hydrocarbon species identified via GC-MS/FID analyses. A detailed kinetic model was developed using oil composition data from 28 oil samples across three experiments. The model-predicted oil compositions agreed within 6% standard error of the values measured in an independent fourth experiment. The model was used to determine optimal conditions for maximizing styrene product yields. This atmospheric-pressure continuous LP-HTP process is simpler and more cost-effective than batch LP-HTP (2–3 MPa) or supercritical water liquefaction (>22 MPa). It offers a scalable and efficient route to recover valuable monomers from PS waste. It has the potential to reduce PS waste consigned to landfills and mitigate its impact on human health and the environment.