THE CHALLENGE:
Poly(ethylene terephthalate) (PET) is extensively used in packaging, textiles, and various consumer goods. The recycling of PET reduces the consumption of materials to produce the virgin PET and processes recycling PET have the potential to provide economic value. Supply chains and the economic incentive to collect and recycle waste PET reduces the consumption of fossil fuel derived feedstock and reduces the rate of mismanaged PET waste contributing to the growing global plastic waste problem. Chemical recycling methods, such as glycolysis producing BHET from PET, aid in this effort, by breaking down PET into its original monomers that can be conveniently purified, enabling the production of high-quality recycled materials from low-purity PET waste, drastically expands the scope of recoverable PET waste.
Current PET chemical recycling technologies face challenges that hinder their effectiveness and efficiency. One problem is the formation of volatile materials during depolymerization that lower the yield and yellows the product. Glycolysis methods also struggle with purification of the product BHET, which does not crystallize well in ethylene glycol, requiring energy expensive addition of water co-solvent. Further, the presence of oligomers complicates crystallization leading to limited ability to purify by this method. This problem can be solved by evaporation of BHET products to separate from oligomers, but this process is very technically challenging and existing processes have not addressed operational challenges or utilized available heat integrations.
OUR SOLUTION:
The method enables continuous depolymerization of polyethylene terephthalate (PET) using ethylene glycol (EG) to produce bis(hydroxyethyl) terephthalate (BHET). The process operates under controlled conditions, reacting PET with EG to break down polymers into BHET and oligomers. It incorporates an advanced purification system utilizing short-path distillation to evaporate BHET, which operates at low pressures (10–100 Pa) and high temperatures (180–250°C). We introduce reaction stripping where volatile components such as water and acetaldehyde are stripped from solvent as they form, which improves selectivity and reduces coloration. The energy from this configuration is mitigated by using the condensing reaction vapor to supply energy to vaporized EG in downstream separation. The system also leverages heat integration where energy to condense ethylene glycol is used to generate steam supplied downstream BHET crystallization. Finally, this method introduces technical innovations to the operation of the short path distillation unit to maintain precise control of the condensing unit temperature, utilize the condensing energy, and mitigate BHET fouling in the vacuum system.
This technology stands out due to its integrated system design, which combines the depolymerization reaction with a sophisticated purification and separation system. It utilizes a reaction vaporization system with a rectifying column to minimize degradation impurities in the reaction product. Extensive heat integration and precise operational controls ensure high product quality and operational stability. Additionally, the method addresses common challenges in PET recycling, including impurity management, oligomer control, and improved energy efficiency, making it more advantageous compared to conventional recycling techniques. The ability to optimize product energy efficiency, while maintaining energy efficiency sets it apart as a PET chemical recycling technology.
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