A method for pet chemical recycling into monomers

Abstract

This invention belongs to the field of chemical recycling technology and discloses a method for chemically recycling polyethylene terephthalate (PET) into monomers. The steps are as follows: crushing, washing, drying, depolymerization, filtration, distillation, filtration, acidification, and filtration. This invention utilizes a mixed anhydrous solution of 1,2-dimethyl-2-imidazolium ketone (DMI) or other solvents that can act as hydrogen bond acceptors but not hydrogen bond donors, along with ethylene glycol (EG) and sodium hydroxide (NaOH), as the degradation solution. While ensuring the solubility of NaOH, this significantly reduces the solvent effect on hydroxide ions, resulting in excellent PET degradation efficiency.

Description

Technical Field

[0001] This invention belongs to the field of chemical recycling technology, specifically relating to a method for chemically recycling ethylene glycol terephthalate (PET) into monomers. Background Technology

[0002] Currently, most PET products are single-use consumer goods, resulting in a large amount of PET waste being generated and leaked into nature, placing a huge burden on the environment. In China, the main method of PET recycling is mechanical recycling. However, during mechanical recycling, the molecular structure of PET is significantly degraded, leading to a decrease in the viscosity and mechanical strength of the molten material. Chemical recycling is considered a more promising option because it can degrade the polymer into monomers, thereby producing products with quality comparable to the original PET. Depending on the nucleophile used, chemical recycling of PET can be divided into ethylene glycololysis, methanololysis, and hydrolysis. Ethylene glycololysis and alcohololysis are transesterification reactions, which have many side reactions and difficulties in product purification. PET hydrolysis can be divided into acidic hydrolysis, neutral hydrolysis, and alkaline hydrolysis. Acidic hydrolysis usually yields high yields of terephthalic acid (TPA) monomers, but the use of large amounts of acid makes the process very expensive and can also adversely affect the purity of ethylene glycol (EG). Environmentally friendly neutral hydrolysis usually requires high temperature and pressure, placing high demands on equipment and materials, and making continuous operation difficult. Alkaline hydrolysis is a two-step process. First, PET is depolymerized into alkali metal salts of EG and TPA. Then, the reaction solution is acidified to obtain TPA. The main advantages of alkaline hydrolysis are its ability to degrade highly polluting waste PET, such as magnetic tapes, metallized PET films, or photographic film, and its relatively simple process and lower cost compared to methanol hydrolysis. Traditional alkaline hydrolysis is carried out in alkaline aqueous solutions, typically requiring temperatures above 200°C, and complete depolymerization of PET takes a long time.

[0003] Many researchers have reported that PET can rapidly depolymerize under mild conditions in alkaline mixed solutions. S. Ugduler et al. [GREEN CHEMISTRY, 22(2020)5376-5394] reported that hydrolyzing PET in an alkaline ethanol-water binary solution resulted in a TPA yield of 95% after 20 min of degradation at 80°C. However, this process requires a large amount of acid to neutralize the alkaline reaction solution in order to precipitate the alkali metal salts of TPA. Furthermore, the water added during this process significantly increases the energy consumption of EG separation. Recently, S. Zhang et al. [GREEN CHEMISTRY, 24(2022)3284-3292] developed a non-aqueous alkaline PET hydrolysis method using an EG-tetrahydrofuran (THF)-potassium hydroxide (KOH) system, which allows terephthalate to precipitate automatically from the degradation solution. Compared to traditional aqueous alkaline hydrolysis of PET, acid consumption and wastewater generation can be reduced by at least five times. However, the KOH used in this system is too expensive, and separating the EG generated during the depolymerization process from the EG-THF-KOH solution system is relatively troublesome. Furthermore, there is currently no literature reporting the relationship between solvent properties and degradation efficiency, and little is known about suitable solvent systems for alkaline hydrolysis of PET. Therefore, there is an urgent need in this field for a method that offers clear solvent selection and is economically viable. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for the chemical recovery of PET into monomers. A mixed anhydrous system consisting of 1,2-dimethyl-2-imidazolium ketone (DMI) or other solvents that can act as hydrogen bond acceptors but not hydrogen bond donors, and EG and sodium hydroxide (NaOH) can rapidly depolymerize PET into EG and disodium terephthalate (TPA-Na2). The product is then filtered, and the filtrate is a mixed solution of DMI-EG and remaining NaOH. The EG added during PET depolymerization is separated by distillation, and the NaOH consumed in the depolymerization process can be recycled for PET depolymerization. A small amount of water is added to dissolve the filter residue TPA-Na2, and then hydrochloric acid or sulfuric acid is added for acidification. The filtered solution yields TPA, and the remaining sodium chloride or sodium sulfate solution is the only byproduct of the entire process.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution: a method for chemically recycling PET into monomers, comprising the following steps:

[0006] 1. The waste PET material is subjected to a series of processes including crushing, washing, and drying.

[0007] 2. The waste PET material treated in step 1 is added to a reactor containing a mixed solution of DMI, EG, and NaOH for depolymerization;

[0008] 3. The product after step 2 is filtered for the first time. The filtrate enters the distillation column. The EG added during the depolymerization process is separated and recovered as a product. The remaining solution is recycled into the depolymerization reactor in step 2.

[0009] 4. Add a small amount of water to the filter residue produced after filtration in step 3 to dissolve TPA-Na2, and then perform a second filtration;

[0010] 5. Acidify the filtrate from step 4 and filter it a third time.

[0011] The filter residue produced in the second filtration in step 4 consists of other types of waste material carried in the PET waste.

[0012] The filter cake produced after the third filtration in step 5 consists of TPA, and the filtrate consists of an acidic solution containing sodium ions.

[0013] Furthermore, the depolymerization reaction temperature in step 2 is 50-80℃.

[0014] Furthermore, in step 2, the volume percentage of DMI in the mixed solution of DMI, EG, and NaOH is 60%-98%.

[0015] The advantages of this invention compared to existing technologies are as follows: This invention operates under normal pressure and low temperature conditions, enabling the recovery of PET from multilayered PET waste containing materials like polyethylene (PE) and PET films, as well as polyester blended fabrics, without altering the properties of other materials. In this invention, TPA-Na2 self-precipitates in the degradation solution, avoiding acidification of the reaction solution and saving a significant amount of acid and alkali. Under ideal conditions, this process consumes 10.4 mol (0.416 kg) of NaOH, the acid required for acidifying TPA-Na2, and a small amount of water per kg of depolymerized PET, without consuming DMI. After the reaction, the degradation solution enters the distillation section, where the EG added during the depolymerization process can be separated. The remaining degradation solution can be recycled after replenishing the consumed NaOH. Using a DMI-EG binary solution as a solvent significantly reduces the solvent effect on hydroxide ions while ensuring NaOH solubility; therefore, this method exhibits excellent PET degradation efficiency. Attached Figure Description

[0016] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0017] Figure 1 This is a flowchart of the waste PET recycling process;

[0018] Figure 2 This is a graph showing the depolymerization rate and yield of PET after alkaline hydrolysis at 60℃ for 1 hour in different solvent systems;

[0019] Figure 3This is a schematic diagram illustrating the effect of co-solvent volume fraction on degradation;

[0020] Figure 4 This is a schematic diagram illustrating the effect of reaction time on PET degradation;

[0021] Figure 5 This is a schematic diagram of the circulation performance of the degradation solution.

[0022] In the diagram: 1. PET waste; 2. Pretreatment area; 3. PET fragments; 4. Depolymerization reactor a; 5. Depolymerization product; 6. Filtration device; 7. Filtrate a; 8. Distillation device; 9. EG; 10. Degradation liquid; 11. Filter residue a; 12. Water; 13. Filter a; 14. Filter residue b; 15. Filtrate b; 16. Acid solution; 17. Acidification reactor b; 18. Acidification product; 19. Filter b; 20. Filter cake; 21. Filtrate c. Detailed Implementation

[0023] The present invention is described in detail below through specific embodiments, but this does not limit the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be obtained commercially.

[0024] Figure 1 This demonstrates the recycling process of waste PET. PET waste 1 is fed into pretreatment zone 2 for crushing, washing, and drying. Then, PET fragments 3 are fed into depolymerization reactor a4 for depolymerization, along with NaOH. The reaction product 5 is fed into filtration device 6. Filtrate a7 enters distillation device 8, where EG9 produced from PET depolymerization is separated, and the remaining degradation liquid 10 is recycled back into depolymerization reactor a4. Water 12 is added to dissolve TPA-Na2 in filter residue a11, and then fed into filter a13. Filter residue b14 contains other types of waste, and filtrate b15 and acid solution 16 are fed into acidification reactor b17 for acidification and precipitation. Acidification product 18 is fed into filter b19; filter cake 20 is the product TPA, and filtrate c21 is the byproduct sodium chloride (or sodium sulfate) salt solution.

[0025] Example 1

[0026] Selection of suitable alkaline anhydrous solvent system for PET depolymerization

[0027] PET was depolymerized in different solvent systems. The reaction conditions were: 2g PET powder; 2g NaOH; 60℃; 1h; 30ml solvent, where EG:cosolvent = 1:4 (volume ratio). The eight cosolvents used were: EG, water, toluene, carbon tetrachloride (CCl4), dimethyl sulfoxide (DMSO), DMI, THF, and 1,4-dioxane (DX). The results are attached. Figure 2As shown in the figure, PET exhibits better depolymerization in the solvent-EG-NaOH system, which can act as a hydrogen bond acceptor but not a hydrogen bond donor. This is because, during hydrogen bond formation, this type of co-solvent competes with hydroxide ions for protons in EG, effectively weakening the solvation effect on the hydroxide ions.

[0028] Example 2

[0029] Selection of the optimal solvent ratio

[0030] PET was depolymerized in solutions with different solvent ratios. The reaction conditions were: 2g PET powder; 2g NaOH; 60℃; 1h; 30ml solvent. The results are attached. Figure 3 As shown in the figure, the optimal co-solvent volume ratio is approximately 80%.

[0031] Example 3

[0032] The depolymerization of post-consumer PET bottle flakes was performed using a DMI-EG-NaOH solvent system. The reaction conditions were: 2g PET bottle flakes; 1.41g NaOH; 60℃; 30ml solvent, where EG:DMI = 1:4 (volume ratio). The results are attached. Figure 4 As shown in the figure, the degradation rate of PET reaches 70% within 10 minutes and increases to 98% after 20 minutes, demonstrating the excellent depolymerization ability of this system for PET.

[0033] Example 4

[0034] The cyclic reaction performance of the DMI-EG-NaOH solution system was tested. After the reaction was completed, the degradation solution was filtered, and a quantitative amount of NaOH (10.4 mmol / g PET) was added for the next round of degradation. The reaction conditions were: 2g PET flakes; 1.41g NaOH; 60℃; 25min; 30ml solvent, where EG:DMI = 1:4 (volume ratio). The results are attached. Figure 5 As shown in the figure, the degradation rate fluctuated slightly during the five rounds of degradation, but remained above 95% throughout, demonstrating good cyclic reaction performance of the system.

[0035] The embodiments described above are merely preferred embodiments of the present invention, and not all feasible embodiments of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for chemically recycling PET into monomers, characterized in that... Using waste PET material that has been crushed, washed, and dried, follow these steps: S1. The treated waste PET material is added to a reactor containing a mixed solution of DMI, EG, and NaOH for depolymerization; S2. The product after step S1 is filtered for the first time. The filtrate enters the distillation column. The EG added during the depolymerization process is separated and recovered as a product. The remaining solution enters the depolymerization reactor in step S1 for recycling. S3. Add a small amount of water to the filter residue produced after step S2 to dissolve the sodium terephthalate, and then perform a second filtration; S4. Acidify the filtrate after filtering in step S3 and filter it for a third time; The depolymerization reaction temperature in step S1 is 50-80℃; In step S1, the volume percentage of DMI in the mixed solution of DMI, EG, and NaOH is 60%-98%.

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