Plastics present significant environmental challenges in landfills due to their persistence over extended timescales, often spanning thousands to millions of years [1]. As a result, increasing attention has been directed toward plastic depolymerization technologies to address the dual issues of plastic waste management and the growing global demand for plastic materials [2], [3]. This study proposes a novel chemical recycling process for the recovery of olefins—specifically ethylene (C2H4) and propylene (C3H6)—from waste polyethylene (PE) and polypropylene (PP). The process integrates low-pressure hydrothermal liquefaction (LP-HTL) and microwave steam pyrolysis (MSP). Initially, mixed PE and PP waste undergo LP-HTL to generate gaseous and liquid (oil) products. The resulting oil is then subjected to MSP for catalytic cracking, thereby enhancing olefin recovery. Distillation is employed to isolate olefinic compounds, which are subsequently utilized as refrigerants. The experimental results demonstrated the production of 39.75 wt% ethylene and 13.32 wt% propylene, corresponding to a total monomer recovery of 53.07 wt%. The levelized cost of ethylene (LCOE) for the proposed process was calculated to be 0.89 USD/kg C2H4, representing a 72.86% cost reduction compared to conventional flash pyrolysis. Furthermore, life cycle assessment (LCA) outcomes indicated significant environmental benefits, with 100-year global temperature potential (GTP100) and global warming potential (GWP100) emissions reduced to 2.46 and 2.55 kg CO2-eq per kg of ethylene, respectively—approximately 90% lower than those associated with the flash process. These findings highlight the proposed process as a promising benchmark for future plastic depolymerization technologies, offering improved energy efficiency, economic viability, and environmental sustainability within the context of a circular carbon economy.
