Supervisor:
Thomas Helmer Pedersen

Co-Supervisor:
Simon Lennart Sahlin

Assessment Committee:
Christiano Varrone (Chair)
Professor Philip Savage, PennState University, USA
Professor Sascha Kersten, University of Twentw, NL

Moderator:
Reinhard Wimmer

Abstract:

This thesis aims to contribute to both the fundamental understanding and practical implementation of neutral hydrothermal processing (nHTP) of polyethylene terephthalate (PET) as a closed-loop chemical recycling solution. By providing a novel research framework, it seeks to advance the state-of-the-art in nHTP technology while offering industrially relevant results and insights. This project lays the groundwork for future studies in nHTP, which has the potential to enhance PET circularity and reduce the environmental impact of this essential material in our daily lives.

Previous nHTP studies have highlighted the benefits of using water as a green solvent for PET solvolysis. However, comparing results among these studies is challenging due to the variability in reactor types and sizes, heating times, and experimental conditions. Most research on nHTP has focused on recovering terephthalic acid, neglecting the degradation of ethylene glycol, which constitutes 20% of the carbon in PET. The existing literature is also limited to batch processing, with no examples of continuous nHTP.

This thesis introduces a comprehensive nHTP experimental framework that overcomes these limitations. The thermodynamic phenomena during PET solvolysis, including their onset temperatures and energetics, were investigated using differential scanning calorimetry with high-pressure crucibles. Potential chemical reactions in nHTP of PET were elucidated through micro-batch experiments and nuclear magnetic resonance spectroscopy. This analytical method was used for chemical compound identification and as a quantitative method to investigate monomer distribution among nHTP reaction products.

These fundamental insights into nHTP of PET were used to design a first-of-its-kind continuous flow system. Preliminary testing provided information about critical process challenges, and the results from the validation experiments confirmed the excellent performance of the Continuous Recycling Unit (CRU) and the benefits of transitioning to continuous processes. Finally, recycled PET was successfully synthesized from the heterogeneous solid nHTP product, showcasing the closed-loop recycling of PET.