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oai:digital.library.adelaide.edu.au:2440/1513892026-07-09T00:59:10Zcom_2440_14759col_2440_14760

van Eyk, Philip Lewis, David Leow, Chye Yi School of Chemical Engineering 2024 https://hdl.handle.net/2440/151389 As plastic pollution continues to escalate globally, the need for innovative recycling technologies becomes increasingly urgent. HTL is a thermochemical process that decomposes plastics feedstocks using water as a medium under near-critical conditions to produce valuable liquid products. This work aims to investigate the valorisation of plastic waste using HTL, both as an isolated feedstock and in combination with biomass. To achieve this, polyethylene terephthalate (PET), polyethylene (PE), and polystyrene (PS) was processed independently at 350 °C for 30 minutes using a batch reactor system. The product distribution of the experiments were quantified using mass balances. Gas chromatography-mass spectrometry (GC-MS) was then used to characterise the compounds formed in the liquid product phases. To investigate the feasibility of the HTL for contaminated waste, the plastics were then mixed with lignin and cellulose respectively under varying feedstock composition. The same experimental and analytical techniques were used for these experiments to ensure comparability of data across the studies. The studies revealed that when processed independently, each of the plastics showed large discrepancies in their liquid product yields. PS was observed to produce the highest yields of liquid products, whereas the decomposition products of PE largely remained in the solid phase. From the product characterisation, it was deduced that HTL successfully depolymerised PET and PS into an amalgamation of monomers, oligomers, and secondary decomposition product. In contrast, PE showed limited decomposition, but the detection of a homologous series of alkanes suggests that PE is not completely inert. When processed in combination with lignin and cellulose, experiments found that in most combinations regardless of the feedstock mass ratio, the organic yield of the HTL process observed synergistic effects, with the exception of PS-cellulose mixtures which observed an antagonistic effects instead. In contrast, the solid yields revealed a strong dependency on the plastic-to-biomass ratio which significantly impacts if synergistic or antagonistic effects are observed. From the feedstock blending of PET with cellulose or lignin, it was found that the decomposition products of the feedstocks strongly reacts with one another. The release of methanol from the hydrolysis of lignin, enabled PET to depolymerise via methanolysis producing 1,4-benzenedicarboxylic acid, dimethyl ester. This reaction pathway is in competition with the hydrolysis and glycolysis pathways that produce ethylene glycol, benzoic acid, and bis(2-hydroxyethyl) terephthalate; thus, resulting a lower concentration of these compounds instead. Additionally, when processed with cellulose, the ethylene glycol production resulted in an esterification processes which significantly reduced the production levulinic acid. Furthermore, the co-liquefaction of PE and PET with biomass did not have significant changes in the type of compounds produced. However, the concentration of the alkanes produced when co-processing PE showed notable increase. Similarly, co-liquefaction of PS with biomass showed significant increase in short-chain alkylbenzenes and notable changes in the ratios of oligomer produced depending on the feedstock composition. These findings highlight the potential of hydrothermal liquefaction as a promising plastic recycling technology, capable of recovering feedstock for the repolymerisation of plastics and producing biocrude that could serve as an alternative to fossil fuels. Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2024 en monomer circular economy biocrude synergy alternative fuel Hydrothermal Liquefaction of Plastic as an Alternative Recycling Method Thesis This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legalsopen.access