Advanced chemical recycling of plastic waste

Abstract

The increasing production of plastic waste and its related environmental impact have driven the need to research advanced chemical recycling strategies. Such methods seek to transform non-recyclable, rejected, multi-layer, and contaminated plastic waste streams into high-value chemicals, ultimately promoting a circular economy. This thesis presents a comprehensive investigation into the chemical recycling of plastic waste, encompassing multiple recycling technologies such as pyrolysis, gasification, and hydrothermal liquefaction. The study began with an extensive critical review of existing chemical recycling technologies and their challenges, opportunities, and future development. Following, kinetic modelling of mixed plastic waste was conducted to identify the dependencies of activation energy on conversion. The derived kinetic models were then applied to the experimental data, proving to be effective at predicting thermal decomposition of single and mixed plastics. The activation energies of the studied plastics i.e., PP, PET, PS, LDPE, and HDPE, were found to be 176, 196, 215, 244, and 258 kJ mol-1, respectively. Subsequently, the study evaluated the performance of affordable catalysts derived from waste biomass for the pyrolysis of mixed plastic waste. A two-stage fixed-bed reactor setup was employed to investigate the product yield distribution, composition, and carbon nanotube (CNT) formation across different temperature ranges. The nickel- and iron-based biochar catalysts showed promising catalytic activity and CNT production with maximum H2 yields of 4.2 and 2 wt%, and CNT yields of 34.5 and 12.4 wt%, respectively. HZSM-5 zeolite was identified as the most effective catalyst for converting heavy fractions into lighter hydrocarbons and monomer recovery, particularly at low temperatures. This was evidenced by the high gas yield of 48.4 wt% at 500 °C using zeolite compared to the low yield of the thermal run (28 wt%). In addition, the potential of catalytic pyrolysis for converting mixed plastic waste into valuable products, such as syngas and monomers, was investigated. The optimal processing conditions for maximizing CO2 conversion (0.389 gCO2 g-1) while minimizing carbon deposition (4.65 wt%) were found to be a temperature of 700 °C, a CO2 concentration of 60 mol%, and a catalyst-toplastic ratio of 16 wt%. These conditions resulted in a syngas yield of 34.25 wt%. Additionally, the study attained a 28 wt% recovery of ethylene and propylene monomers. These results contribute to the development of efficient and sustainable processes for the conversion of ii mixed plastic waste and CO2 into valuable products, providing insights into process optimization and catalyst reusability. Lastly, the thesis examined the technical and economic aspects of industrial-scale hydrothermal liquefaction and pyrolysis processes, as well as the outlook and impact of chemical recycling technologies. This thesis, overall, provides a comprehensive understanding of various chemical recycling technologies and their potential applications for transforming plastic waste into high-value materials such as monomers and chemicals. The research findings contribute to the development of sustainable solutions for plastic waste management and the transition to a circular economy.