Abstract
Polystyrene (PS) recycling remains limited despite large waste volumes, largely because many routes struggle to recover high-value chemicals at scale. This work develops a full process concept that upgrades PS through low-pressure hydrothermal processing (HTP) and directs the resulting aromatic oil to separation and downstream conversion. The superstructure includes HTP at 30 bar and 350°C, three distillation columns for toluene/ethylbenzene/styrene splits, ethylbenzene dehydrogenation to boost styrene yield, steam reforming of the heavier C9+ fraction to syngas, and an ICI-type (Imperial Chemical Industries) methanol loop with inter-bed quench. The integrated flowsheet was simulated in Aspen Plus V14 using a consistent NRTL-RK (Non-Random Two-Liquid-Redlich-Kwong) property framework. Deep-vacuum operation (˜100-400 mbar) was applied where needed to limit styrene polymerization. Sensitivity and optimization focused on meeting a syngas stoichiometric number near 2; the best case occurred at ~998.8°C and 20.2 bar with ~37.8 t/h steam. Normalized to the polystyrene feed, the model predicts ~0.13 kg toluene, 0.16 kg styrene, and 1.27 kg methanol per kg PS, with only trace benzene/ethanol, at a water-to-PS ratio of ~3.77 kg/kg. When heat recovery is excluded, the overall heat demand corresponds to ~84 MJ per kg of PS processed. The flowsheet is technically consistent, but heat integration and a full TEA/LCA (Techno-economic Assessment/Life-cycle Assessment) will ultimately determine its viability.