Upcycling of Mixed Plastic Waste to Oil and Multi-Walled Carbon Nanotubes: Techno-Economic Feasibility and Environmental Impacts

Large amounts of mixed plastic waste are generated worldwide, including in Singapore. The available technologies, such as mechanical sorting, chemical treatment and pyrolysis of mixed plastics to fuels have low economic benefits. Owing to low profitability, the recycling rates for mixed plastic waste are low. One of the possible solutions is the conversion of mixed plastics into carbon nanotubes (CNTs) that have high value (> 80 USD/kg) and thus, create an additional stream of revenue for pyrolysis plant operators encouraging waste recycling. The objectives of this study were to investigate the techno-economic feasibility and environmental impacts of the integrated process that upcycles mixed plastic waste with high PET content (11.8 and 27.5 wt.% of PET) into oil and CNTs. In the proposed process, mixed plastics were initially pyrolyzed and the produced vapours were subjected to catalytic treatment to improve oil quality. After oil condensation, the remaining non-condensable pyrolysis gases were used for CNTs synthesis. Cost-benefit analysis indicated 3-10 times more revenue for the plant operators with the addition of CNTs synthesis unit. The life cycle assessment (LCA) study indicated that fossil depletion and climate change potentials for the feedstock with 27.5 wt.% PET are 30% and 73% higher, respectively, compared to the feedstock with 11.8 wt.% PET. Integration of CNTs synthesis with the pyrolysis process showed significant improvements in the climate change, fossil depletion, eco-toxicity and acidification potentials yielding positive effects on the environment. Overall, the pyrolysis process integrated with CNTs synthesis showed environmental benefits while incineration resulted in net negative impacts.