PET Recycling via Segmented Depolymerisation Reactors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional PET recycling methods produce low yields of bis(2-hydroxyethyl) terephthalate (BHET) monomers due to significant amounts of dimers and trimers, leading to reduced quality and efficiency, and often result in the entrainment of isophthalic acid (IPA), requiring additional purification steps and energy for high-quality applications.
Innovation Solution
A method involving a series of depolymerisation reactors with ethylene glycol and a catalyst system, followed by evaporation crystallisation, protic solvent recrystallisation, and impurity removal to produce a high-purity BHET product with minimal IPA, allowing for simplified polymerisation processes and improved visual grade materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional depolymerisation methods are used to recycle PET, then the process can be simplified, but the yield of BHET monomer is reduced due to significant amounts of dimers and trimers
Solution Approach 1:
The single depolymerisation reactor is divided into multiple reactors in series. Each reactor performs a specific function: the first reactor depolymerises PET to BHET, the second reactor removes dimers and trimers through selective precipitation, and the third reactor completes the purification. This segmentation allows each stage to optimize for its specific function, achieving high BHET yield while maintaining process simplicity.
Solution Approach 2:
The harmful components (dimers and trimers) are extracted from the BHET solution through selective precipitation in the second reactor. By removing these impurities at this specific stage, the process achieves high BHET monomer yield without requiring complex purification steps later, thus resolving the contradiction between simplicity and productivity.
2Reliability
If conventional depolymerisation methods are used, then the process can be completed, but additional purification steps are required to remove dimers, trimers and IPA, increasing energy consumption
Solution Approach 1:
The removal of dimers and trimers is performed as a preliminary action in the second reactor through selective precipitation, before the final purification stage. This preliminary removal of major impurities significantly reduces the energy required for subsequent purification steps, as the solution entering the third reactor already has reduced impurity content.
Solution Approach 2:
A solvent is introduced as an intermediary medium in the second reactor to facilitate the selective precipitation and removal of dimers and trimers. This intermediary allows for efficient separation of impurities from BHET at a lower energy cost compared to direct thermal or chemical purification methods.
3Ease of manufacture
If conventional methods are used to produce recycled PET, then the process can be completed, but the visual grade is reduced due to yellow hue from impurities
Solution Approach 1:
The yellow-hue causing impurities (dimers, trimers, and IPA) are extracted and removed through the multi-stage purification process. The second reactor removes dimers and trimers through precipitation, while the third reactor completes the purification to achieve high visual grade. This extraction of harmful components enables production of high-quality transparent PET bottles.
Solution Approach 2:
The process utilizes changes in physical parameters (temperature, pressure, solvent composition) to control the solubility and precipitation behavior of different components. By adjusting these parameters in each reactor stage, the process selectively removes impurities while preserving BHET, achieving high visual grade product.
4Loss of substance
If significant amounts of dimers and trimers are produced during depolymerisation, then the raw material utilization is reduced, but recycling the dimers and trimers requires additional time and energy
Solution Approach 1:
The dimers and trimers, which are normally considered harmful by-products representing raw material loss, are converted into a beneficial separation stage. In the second reactor, these impurities are selectively precipitated and removed, converting what was previously a loss into a controlled purification step that actually improves the final product quality without requiring time-consuming re-recycling operations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method achieves a high proportion of BHET monomer with low dimer and trimer content, eliminating the need for additional purification steps and reducing energy consumption, enabling the production of high-quality recycled PET suitable for transparent and colour-free applications.
Implementation Method 1
depolymerising PET in the presence of ethylene glycol and a catalyst system in a series of depolymerisation reactors to form a depolymerised mixture comprising bis(2-hydroxyethyl) terephthalate (BHET)
Implementation Method 2
crystallising a precipitate comprising BHET by removing a volatiles stream comprising ethylene glycol from the depolymerised mixture using evaporation crystallisation
Implementation Method 3
dissolving the precipitate in a protic solvent to form a solution comprising BHET
Implementation Method 4
removing impurities from the solution to form a purified solution comprising BHET
Implementation Method 5
crystallising a purified product comprising BHET from the purified solution
Data Source
AI summary
The present invention relates to a method and apparatus for recycling polymers, in particular to a method for recycling polyethylene terephthalate (PET) to produce bis(2-hydroxyethyl) terephthalate (BHET). The BHET produced using the method and apparatus of the present invention may be of a quality which allows plastic preparation methods in which the BHET is used to be simplified.