Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: Plastic Waste to Chemicals: A Study of Thermochemical Processes to Produce Monomers and Other Valuable Products

Plastic waste is a major environmental challenge that endangers human and ecological well-being. Most plastic waste is either landfilled or ocean-dumped, where it can last for centuries. Thus, alternative solutions that transform plastic waste into valuable products, such as chemicals, fuels, or materials, are needed. Thermochemical conversion, which uses heat and catalysts to cleave the polymer chains of plastic waste into smaller units, is a promising technique. This thesis examines thermochemical processes, namely pyrolysis and hydrothermal liquefaction (HTL), for producing monomers and other valuable products from plastic waste. The objectives are: (1) To survey the current state-of-the-art of thermochemical conversion of plastic waste; (2) To investigate the effect of co-HTL of polyvinyl chloride (PVC) on ethylene-vinyl acetate (EVA) and nylon; (3) To compare the effect of HTL and pyrolysis on the monomer selectivity and polymer degradation of polystyrene; (4) To study the effect of ethanol co-solvent on the monomer or precursor recovery from polyurethane polymers during HTL; (5) To assess the feasibility and potential of HTL for converting non-recyclable mixed plastic waste into bio-oil and pure polyolefins as a feedstock for pyrolysis; (6) To evaluate the effect of HTL for chemical recovery from cellulose acetate wastes. The main results and findings of this thesis are: (1) Co-HTL of PVC with other polymers can speed up the hydrolysis of other plastics; (2) Pyrolysis of polystyrene waste can yield styrene with high efficiency while HTL can depolymerize polystyrene at a lower temperature; (3) Ethanol co-solvent can improve the polyurethane depolymerization; (4) HTL of mixed plastic waste can generate bio-oil and pretreated solid olefins which can potentially enhance the quality of the oil derived from its pyrolysis; and (5) HTL can recover acetic acid and diethyl phthalate with high yield and concentration from cellulose acetate wastes. This thesis advances the knowledge in thermochemical conversion, particularly HTL and pyrolysis of plastic waste, by offering novel insights into the mechanisms, kinetics, and optimization of the HTL and pyrolysis processes. It also provides valuable information for developing and implementing sustainable and circular solutions for plastic waste management.