Downstream process modeling, design, and optimization of plastic (PET) chemical recycling

Nowadays, plastics have been widely used in every aspect of our lives. Its widespread application in every corner of today's economy creates a massive demand for its production. This increment in plastic manufacturing escalates the concern about the growing amount of waste around the world. Plastic wastes are mainly either incinerated in the waste handling industry or dumped into nature. The increasing amount of plastic residues hurts the environment as well as the economy. It damages the environment since the plastic materials are considered as noxious material remaining in a substantial volume in the waste streams. From the economic point of view, it can lead up to 50-60% of capital loss due to not recycling and not coming back to the life cycle. The 12th UN sustainable development goal asks for urgent action to ensure that plastic products do not lead to the overexploitation of resources or harm the environment. 

This thesis aims to contribute to the elimination of the plastic waste issue on a global basis. The contribution is on offering plastic producers and waste recyclers a profitable way to treat and recycle plastic residues. I work on developing a sustainable and beneficial chemical recycling process to close the plastic life cycle. Specifically, I focus on one of the most used plastics, Poly-Ethylene Terephthalate, abbreviated to “PET”. I explore experimentally the thermodynamics of a chemical system widely used in the plastic recycling industry. This chemical system, which is mainly the product of depolymerization reactions, contains terephthalic acid, disodium terephthalate, sodium hydroxide, sodium chloride, ethylene glycol, and water. The studied chemical system is an electrolyte solution involving various ions and molecules which are 𝑇𝑃^2-, 𝐻𝑇𝑃^-, 𝑁𝑎⁺, 𝐶𝑙^-, 𝐻⁺, 𝑂𝐻^-, 𝑀𝐸𝐺, and 𝐻₂𝑂. This experimental study clearly draws a picture of properties and thermodynamic characteristics, including solid-liquid equilibrium, vapor-liquid equilibrium, density, and viscosity examinations. 

The thermodynamics of the defined chemical complex is mathematically modelled in order to predict its behaviour at various conditions. The Extended UNIQUAC model, which is widely applicable for electrolyte systems, is adapted for this purpose. The model adaption is carried out based on a data bank consisting of almost all of the existing experimental works in addition to the experimental results obtained through this Ph.D. study. The Extended UNIQUAC model has some parameters related to each of the species and their interactions; thus, for adopting this model, I tune those parameters

based on the experimental results. The developed thermodynamics model is critical for the further process design and simulation. 

Furthermore, this thesis presents a closed chemical process to recycle PET polymers. The design and simulation of the proposed process are carried out in ASPEN Plus software. In addition to the base case design, several improvements to save up to 15% energy consumption are suggested.