Plastics waste to olefins in a circular economy: Techno-economic and life cycle comparison of hydrothermal liquefaction and pyrolysis
Roni Mallick, Prabu Vairakannu, N. Raveendran Shiju
Abstract
This study investigates pathways for the selective chemical upcycling of polyolefin waste, with a focus on two thermo-chemical conversion technologies: hydrothermal liquefaction (HTL-SC) and pyrolysis integrated with steam cracking (PYR-SC). A comparative techno-economic (TEA) and life cycle analysis (LCA) of both processes was conducted using Aspen Plus v12 and openLCA 2.0 software, respectively, to evaluate their potential for the production of light olefins (ethylene and propylene). The results indicate that for the HTL-SC plant produces 39.4 wt% ethylene and 19.9 wt% propylene at the optimal conditions of 450 °C, 230 bar and 30 min (HTL). For pyrolysis, the highest olefin yields were achieved at 600 °C. Under these optimized conditions, the HTL-SC process demonstrated an overall energy efficiency of 68.1 %, approximately 4.5 % higher than the PYR-SC process. The increased olefin yield also contributes to favorable production costs: 1.55 €/kg for ethylene and 3.07 €/kg for propylene. Notably, the PYR-SC plant achieved a return on investment (ROI) of 13.7 % and a payback period of 1.8 years, highlighting strong economic potential. From an environmental perspective, the LCA indicates that both the HTL-SC and PYR-SC routes produce lower CO 2 emissions relative to conventional petrochemical plants equipped with carbon capture systems. The dominant environmental impact categories identified were human toxicity, followed by acidification and global warming potential. These findings highlight the potential of advanced polyolefin upcycling pathways to enable integrated olefin recovery and decarbonization within the chemical industry.