Method for efficiently degrading PET and recycling TPA

Abstract

The invention relates to the technical field of plastic degradation, in particular to a method for efficiently degrading PET and recycling TPA. The invention provides a method for efficiently degrading PET and recovering TPA, which comprises the following steps: adding PET waste to be treated into a polyethylene glycol aqueous solution in which an alkaline reagent is dissolved, carrying out depolymerization reaction at 70-110 DEG C and normal pressure, and carrying out solid-liquid separation while the solution is hot after the reaction; wherein the volume ratio of the water to the polyethylene glycol is (1: 1.5)-(1: 9); adding water to dissolve a solid phase obtained by solid-liquid separation, filtering to remove residues, adjusting the pH value of the obtained solution, and separating out a TPA monomer. The method is simple in step, low in required energy consumption, high in degradation efficiency and easy for large-scale production, and the degradation liquid can be recycled, so that the method has a good industrial application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of plastic degradation, and in particular to a method for efficiently degrading PET to recover TPA. Background Art

[0002] Polyethylene terephthalate (PET) is a highly used thermoplastic polyester material. Due to its excellent transparency, strength and chemical resistance, it is widely used in plastic bottles, food packaging and textiles. However, with the rapid increase in the use of PET materials, the waste problem has become increasingly serious, seriously affecting the sustainability of the ecological environment. Therefore, the effective recycling of PET is of great significance to achieving green environmental protection and sustainable development.

[0003] At present, the recycling of PET mainly adopts incineration, physical recycling, chemical recycling and biological recycling. Among them, incineration is to directly burn the waste PET materials to recover heat energy. Although this method is fast and occupies less space, the harmful gases and ash produced may have irreversible effects on the environment. Physical recycling uses mechanical means to clean, crush and reprocess the waste PET materials. This method is effective, but its recycling efficiency is significantly reduced when dealing with waste containing other types of impurities or mixed materials. Biological recycling is to use microorganisms to degrade PET. Although this method has broad prospects, its actual large-scale application is still in the research and development stage and cannot be achieved in large-scale commercial application in the short term. In contrast, chemical recycling relies on chemical reactions to degrade PET into lower molecular weight compounds or terephthalic acid (TPA), which can achieve a higher level of resource recovery and is currently the most suitable recycling method for industrial application.

[0004] However, the degradation process of PET by chemical methods often involves complex and harsh process conditions, and requires a long reaction time to completely degrade PET into TPA. Therefore, it is of great significance to explore more economical and environmentally friendly methods for PET degradation. Summary of the invention

[0005] In view of this, the present invention provides a method for efficiently degrading PET to recover TPA. The method has simple steps, low energy consumption, high degradation efficiency, is easy to scale up production, and the degradation liquid can be recycled, thus having good industrial application prospects.

[0006] In order to solve the above technical problems, the present invention provides a method for efficiently degrading PET to recover TPA, the steps comprising: S1. Add the PET waste to be treated into a polyethylene glycol aqueous solution containing an alkaline reagent, carry out a depolymerization reaction at 70-110° C. and normal pressure, and separate the solid and liquid while hot after the reaction; wherein the volume ratio of water to polyethylene glycol is 1:1.5-1:9; S2. Dissolve the solid phase obtained by solid-liquid separation in water, filter out the residue, adjust the pH of the obtained solution, and precipitate TPA monomer.

[0007] The present invention uses "alkaline reagent-polyethylene glycol-water" as a degradation solution, which can rapidly degrade PET waste in a short time and greatly improve the degradation efficiency. The inventor analyzed that it may be that the hydroxide ions ionized by the alkaline reagent and the polyethylene glycol produce hydrogen bonds, so that under the "leadership" of the polyethylene glycol, the hydroxide ions can "get close" to the PET waste faster, accelerating the degradation of the PET waste. A small amount of water in the polyethylene glycol aqueous solution can accelerate the complete dissolution of the alkaline reagent, thereby accelerating the degradation rate; at the same time, the presence of a small amount of water can also enrich the terephthalate generated by the degradation and deposit it at the bottom of the reaction vessel, which is more conducive to solid-liquid separation. The solid obtained by solid-liquid separation is re-dissolved and the pH of the solution is adjusted by adding acid to precipitate the TPA monomer, and the TPA monomer has high purity and high recovery rate.

[0008] The method for efficiently degrading PET to recover TPA provided by the present invention can achieve rapid degradation of PET only under normal pressure conditions, without high temperature and high pressure depolymerization conditions, thereby improving the safety of the reaction. The degradation method has high specificity for PET components. When the waste to be degraded contains other components in addition to PET, the method can only degrade the PET component in the waste, while other non-PET components and impurities will be distributed in the upper solution, so that only terephthalate is deposited at the bottom of the container.

[0009] Preferably, after the depolymerization reaction is completed, the generated terephthalate solid will be deposited at the bottom of the reaction container, and the degradation solution can be separated from the terephthalate solid at the bottom of the container by pouring. For large-scale treatment processes, the upper layer solution can be directly extracted by external power, and solid-liquid separation can be achieved without using a filtering device, which simplifies the treatment steps of PET waste.

[0010] Preferably, the pH of the solution may be adjusted to less than 3 using an aqueous solution of hydrochloric acid, sulfuric acid or phosphoric acid.

[0011] Preferably, the alkaline agent includes at least one of sodium hydroxide or potassium hydroxide.

[0012] Preferably, the concentration of the alkaline agent contained in the polyethylene glycol aqueous solution is 0.04-0.2 g / mL, more preferably 0.12 g / mL.

[0013] Preferably, the molecular weight of the polyethylene glycol is 200-600, preferably polyethylene glycol 200, polyethylene glycol 400 or polyethylene glycol 600.

[0014] Preferably, the amount of the PET waste added to each liter of the polyethylene glycol aqueous solution containing the alkaline agent is 10-130 g, more preferably 40-100 g / L, and more preferably 70 g / L.

[0015] Preferably, the temperature of the depolymerization reaction is 90-100° C., and the reaction time is 2-30 min.

[0016] The method for efficiently degrading PET and recovering TPA provided by the present invention has different depolymerization times when degrading different forms of PET waste. However, according to the method provided by the present invention, the PET waste can be completely degraded in a maximum of 30 minutes, and the degradation treatment efficiency is significantly improved.

[0017] Further preferably, the reaction time of the depolymerization reaction is 10 to 18 minutes.

[0018] Preferably, the PET waste includes at least one of plastic bottles, polyester fibers, textiles, strapping tapes or engineering plastic parts containing PET components. The method provided by the present invention can also be used to degrade PET in other forms of PET waste.

[0019] Preferably, in order to further reduce the processing cost, the liquid phase obtained from the solid-liquid separation in step S1 is recovered, the ratio of the polyethylene glycol aqueous solution to the alkaline reagent in the liquid phase is adjusted, and then recycled for the depolymerization reaction to achieve the recycling of the degradation liquid.

[0020] Preferably, the liquid phase is enriched with ethylene glycol monomers. When the volume concentration of ethylene glycol monomers in the liquid phase exceeds 10%, the ethylene glycol monomers are recovered by distillation, and the liquid phase after distillation of the ethylene glycol monomers is recycled for depolymerization reaction.

[0021] The beneficial effects obtained by the present invention are as follows: compared with the prior art, the present invention uses green and environmentally friendly non-toxic reagents to rapidly degrade PET waste, the conversion rate of PET is as high as 93% or more, and the highest can reach 99.7%, the yield of the obtained TPA is maintained at more than 85%, and the highest is 95.86%, and the purity of the obtained TPA is maintained at more than 98.5%. The method not only shortens the degradation reaction time and reduces energy consumption, but also the intermediate products obtained by degradation can be automatically enriched at the bottom of the container, actively realizing the effective separation of the degradation products and the degradation liquid, omitting the cumbersome filtering and separation steps, and the separated degradation products (terephthalate) can be dissolved by adding only a small amount of water. Compared with the traditional PET hydrolysis process that produces a large amount of salt-containing wastewater, the degradation method provided by the present invention greatly reduces the generation of salt-containing wastewater, and the liquid phase after solid-liquid separation can still be reused for depolymerization reaction after component adjustment, and a higher PET conversion rate and TPA yield can still be obtained; at the same time, the ethylene glycol in the liquid phase obtained by solid-liquid separation can also be recovered when it is enriched to a certain concentration. The degradation method provided by the method significantly improves the economy of the entire degradation process and has great application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The infrared spectra of TPA obtained in Examples 1 to 3 and Comparative Examples 1 to 3 are shown. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] The molecular weights of the polyethylene glycols used in the following examples and comparative examples are all weight average molecular weights.

[0025] Example 1 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is PET polyester fiber, and the mass percentage w of PET is 100%. The method specifically includes the following steps: 6 g of potassium hydroxide was added to a round-bottom flask containing 50 mL of an aqueous solution of polyethylene glycol 600 (the volume ratio of water to polyethylene glycol 600 was 1:4), heated and stirred to dissolve and mix evenly, 3.5 g of PET polyester fiber (denoted as X) was added, and the reaction was stirred at 400 rpm at 90 °C under normal pressure for 10 min, and the PET polyester fiber was basically completely degraded.

[0026] After the reaction is completed, the potassium terephthalate generated by the degradation of PET polyester fiber is firmly deposited at the bottom of the round-bottom flask through enrichment, and the upper solution is poured into another beaker. 50 mL of water is added to the round-bottom flask to dissolve the potassium terephthalate deposited at the bottom of the flask, and the undissolved residue (including undegraded PET raw materials and impurities) (0.01 g, recorded as Y) is filtered to remove the undissolved residue, and the pH of the obtained filtrate is adjusted to less than 3 with an acidic solution to precipitate TPA. The TPA filter cake is collected by filtration again, washed with water, and dried in an oven at 65°C for 24 h to obtain 2.90 g of the monomer product TPA.

[0027] In addition, the separated upper solution was adjusted in composition (polyethylene glycol 600 and potassium hydroxide were appropriately added) and then used for the next PET degradation.

[0028] The conversion rate of PET and the yield of TPA were calculated according to the following formula, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0029] PET conversion rate (%) = (XY) / (X×w)×100. Wherein, X represents the initial mass of the PET raw material added to the reactor, and the unit in this example is grams; Y represents the mass of the insoluble matter obtained by filtering out the undegraded PET raw material and impurities; and w represents the mass percentage of the PET component in the PET raw material.

[0030] TPA yield (%) = mass of TPA actually produced / mass of TPA that should be produced theoretically × 100.

[0031] Among them, "the mass of TPA actually generated" refers to the mass of the monomer product TPA finally obtained, and "the mass of TPA that should be generated theoretically" refers to the mass of TPA that should be generated theoretically based on the mass of the PET component in the added PET raw material (i.e. the mass percentage of X×PET accounting for w).

[0032] Example 2 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is PET polyester fiber, and the mass percentage w of PET is 100%. The method specifically includes the following steps: 5 g of potassium hydroxide was added to a round-bottom flask containing 50 mL of an aqueous solution of polyethylene glycol 400 (the volume ratio of water to polyethylene glycol 400 was 1:9), heated and stirred to dissolve and mix evenly, 3.0 g of PET polyester fiber was added, and the reaction was stirred at 400 rpm at 90 °C under normal pressure for 8 min, and the PET polyester fiber was basically completely degraded.

[0033] The steps after the reaction are completed are the same as those in Example 1.

[0034] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0035] Example 3 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is PET polyester fiber, and the mass percentage w of PET is 100%. The method specifically includes the following steps: 8 g of potassium hydroxide was added into a round-bottom flask containing 50 mL of polyethylene glycol 400 aqueous solution (the volume ratio of water to polyethylene glycol 400 was 1:1.5), heated and stirred to dissolve and mix evenly, 0.5 g of PET polyester fiber was added, and the reaction was stirred at 400 rpm at 100 °C under normal pressure for 30 min. The PET polyester fiber was basically completely degraded.

[0036] The steps after the reaction are completed are the same as those in Example 1.

[0037] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0038] Example 4 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is a plastic bottle made of PET, and the mass percentage w of PET is 100%. The method specifically includes the following steps: Add 10 g of sodium hydroxide into a round-bottom flask containing 50 mL of polyethylene glycol 600 aqueous solution (the volume ratio of water to polyethylene glycol 600 is 1:4), heat and stir to dissolve and mix evenly, add 6.5 g of PET polyester bottle flakes, stir and react at 400 rpm at 70 °C under normal pressure for 30 min, and the PET plastic bottle is basically completely degraded.

[0039] The steps after the reaction are completed are the same as those in Example 1.

[0040] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0041] Example 5 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is an engineering plastic part made of PET, and the mass percentage w of PET is 96%. The method specifically includes the following steps: Add 2 g of sodium hydroxide into a round-bottom flask containing 50 mL of polyethylene glycol 600 aqueous solution (the volume ratio of water to polyethylene glycol 600 is 1:4), heat and stir to dissolve and mix evenly, add 0.5 g of PET engineering plastic parts, stir and react at 400 rpm at 110 °C under normal pressure for 20 min, and the PET in the engineering plastic parts is basically completely degraded.

[0042] The steps after the reaction are completed are the same as those in Example 1.

[0043] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0044] Example 6 This embodiment provides a method for efficiently degrading PET to recover TPA. The PET waste used in this embodiment is PET polyester fiber, and the mass percentage w of PET is 100%. The method specifically includes the following steps: Add 6.5 g of potassium hydroxide into a round-bottom flask containing 50 mL of an aqueous solution of polyethylene glycol 600 (the volume ratio of water to polyethylene glycol 600 is 1:4), heat and stir to dissolve it and mix it evenly, add 3 g of PET polyester fiber, and stir and react at 400 rpm at 100 °C under normal pressure for 2 min. The PET polyester fiber is basically completely degraded.

[0045] The steps after the reaction are completed are the same as those in Example 1.

[0046] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0047] Comparative Example 1 This comparative example provides a method for recovering TPA by degrading PET. The PET-containing waste used in this comparative example is the same as the PET waste in Example 1, that is, PET polyester fiber, and the mass percentage w of PET is 100%. The specific steps include: 6 g of potassium hydroxide was added to a round-bottom flask containing 50 mL of water, heated and stirred to dissolve and mix evenly, 3.5 g of PET polyester fiber was added, and the reaction was stirred at 400 rpm at 90 °C under normal pressure for 10 min. The PET polyester fiber was not completely degraded.

[0048] The reaction solution was filtered to remove the undegraded PET polyester fiber, and the pH of the filtrate was adjusted to less than 3 with an acidic solution to precipitate TPA. After filtration, the solution was washed with water and dried in an oven at 65° C. for 24 h to obtain the monomer product TPA.

[0049] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0050] Comparative Example 2 This comparative example provides a method for recovering TPA by degrading PET. The PET-containing waste used in this comparative example is the same as the PET waste in Example 1, that is, PET polyester fiber, and the mass percentage w of PET is 100%. The specific steps include: 6 g of potassium hydroxide was added to a round-bottom flask containing 50 mL of ethylene glycol aqueous solution, wherein the volume ratio of water to ethylene glycol was 1:4, and heated and stirred to dissolve and mix evenly. 3.5 g of PET polyester fiber was added, and the reaction was stirred at 400 rpm at 90 °C under normal pressure for 10 minutes. The PET polyester fiber was not completely degraded.

[0051] The reaction solution was filtered to remove the undegraded PET polyester fiber, and the pH of the filtrate was adjusted to less than 3 with an acidic solution to precipitate TPA. After filtration, the solution was washed with water and dried in an oven at 65° C. for 24 h to obtain the monomer product TPA.

[0052] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0053] Comparative Example 3 This comparative example provides a method for recovering TPA by degrading PET. The PET-containing waste used in this comparative example is the same as the PET waste in Example 1, that is, PET polyester fiber, and the mass percentage w of PET is 100%. The specific steps include: 6 g of potassium hydroxide was added to a round-bottom flask containing 50 mL of ethylene glycol, heated and stirred to dissolve and mix evenly, 3.5 g of PET polyester fiber was added, and the reaction was stirred at 400 rpm at 90 °C under normal pressure for 10 min. The PET polyester fiber was not completely degraded.

[0054] The reaction solution was filtered, and the filter cake was collected (the filter cake was undegraded PET polyester fiber, impurities and potassium terephthalate generated by PET degradation), and the filter cake was redissolved in water to completely dissolve the potassium terephthalate, and then the undegraded PET polyester fiber and impurities were removed by filtration, and the pH of the obtained filtrate was adjusted to less than 3 with an acidic solution to precipitate TPA, which was then washed with water after filtration and dried in an oven at 65°C for 24 hours to obtain the monomer product TPA.

[0055] The conversion rate of PET and the yield of TPA were calculated by the same calculation method as in Example 1, and the purity of the obtained TPA was measured by liquid chromatography. The results are shown in Table 1.

[0056] Table 1 Measurement results of Examples 1 to 6 and Comparative Examples 1 to 3 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 PET conversion rate (%) 99.71 98.76 99.23 93.16 99.13 97.82 1.04 6.10 29.4 TPA yield (%) 95.86 95.68 94.06 85.86 94.35 92.21 0.70 3.13 24.50 TPA purity (%) 99.48 98.81 98.76 99.05 99.25 99.31 98.74 98.68 99.31

[0057] As can be seen from Table 1, by adopting the method for efficiently degrading PET provided by the present invention, the conversion rate of the PET component in the PET waste can be as high as 93% or more within 30 minutes, or as short as 2 minutes. At the same time, the yield of the obtained TPA is maintained at more than 85%, and the purity is maintained at more than 98.5%.

[0058] In Comparative Example 1, only water was used as the degradation solvent, and the obtained PET conversion rate and TPA yield were both low; in Comparative Example 2, a mixture of ethylene glycol and water was used as the degradation solvent. Compared with Comparative Example 1, although the PET conversion rate and TPA yield were increased, they were still at a low level and were not suitable for scale-up production; in Comparative Example 3, only ethylene glycol was used as the degradation solvent, and the PET conversion rate and TPA yield were further improved, but still did not exceed 30%, indicating that only using ethylene glycol as the degradation solvent could not completely degrade PET waste into TPA within 10 minutes.

[0059] Application Examples This application example is used to illustrate the reusability of the upper layer solution separated by the method provided by the present invention after filtration, as follows: An aqueous solution of polyethylene glycol 600 and potassium hydroxide were added to the upper solution separated in Example 1 to satisfy the component ratio required for degradation (the aqueous solution of polyethylene glycol 600 was added according to the volume loss, and the amount of potassium hydroxide added was determined by testing the potassium hydroxide content in the upper solution). The resulting solution was then used as a degradation solution again, and the PET polyester fiber was degraded using the same degradation steps and degradation conditions as in Example 1, and the conversion rate of PET and the yield of TPA were calculated. The aforementioned steps were repeated 4 times, and the conversion rate of PET and the yield of TPA were calculated after each repeated process. The results of each cycle experiment are shown in Table 2.

[0060] Table 2

[0062] It can be seen from Table 2 that after the reaction is completed and the solid-liquid separation is performed, the liquid obtained by the solid-liquid separation (i.e., the alkaline polyethylene glycol aqueous solution) can be reused for the degradation of PET after the proportion is adjusted to meet the component proportion required for degradation. Moreover, after repeated adjustment and use, a high PET conversion rate and TPA yield can still be guaranteed, indicating that the method for efficiently degrading PET provided by the present invention can also realize the reuse of the degradation liquid, further reducing the recovery cost of PET.

[0063] Test example The degradation products TPA obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were tested by infrared spectroscopy and compared with the infrared spectra of commercial TPA. The results are as follows: Figure 1 It can be seen that the degradation products obtained in Examples 1 to 3 and Comparative Examples 1 to 3 are all TPA.

[0064] In summary, when the solution system composed of "alkaline reagent-polyethylene glycol-water" provided by the present invention is not used to degrade PET waste, the efficient degradation of PET waste cannot be completed within 30 minutes, and a high PET conversion rate and TPA yield cannot be obtained, which shows that the method for efficiently degrading PET provided by the present invention can significantly improve the degradation efficiency of PET waste; and the liquid obtained after solid-liquid separation can also be adjusted in components and reused multiple times, reducing the recycling cost.

[0065] What is described above is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for efficiently degrading PET to recover TPA, characterized in that the steps include: S1. Add the PET waste to be treated into a polyethylene glycol aqueous solution containing an alkaline reagent, carry out a depolymerization reaction at 70-110° C. and normal pressure, and separate the solid and liquid while hot after the reaction; wherein the volume ratio of water to polyethylene glycol is 1:1.5-1:9; S2. Dissolve the solid phase obtained by solid-liquid separation in water, filter out the residue, adjust the pH of the obtained solution, and precipitate TPA monomer.

2. The method for efficiently degrading PET to recover TPA as claimed in claim 1, characterized in that: The alkaline agent includes at least one of sodium hydroxide or potassium hydroxide.

3. The method for efficiently degrading PET to recover TPA as claimed in claim 1, characterized in that: The concentration of the alkaline agent contained in the polyethylene glycol aqueous solution is 0.04-0.2 g / mL.

4. The method for efficiently degrading PET to recover TPA as claimed in claim 3, characterized in that: The molecular weight of the polyethylene glycol is 200-600.

5. The method for efficiently degrading PET to recover TPA as claimed in claim 1, characterized in that: The amount of the PET waste added to each liter of the polyethylene glycol aqueous solution containing the alkaline agent is 10-130 grams.

6. The method for efficiently degrading PET to recover TPA as claimed in claim 1, characterized in that: The temperature of the depolymerization reaction is 90-100° C., and the reaction time is 2-30 minutes.

7. The method for efficiently degrading PET to recover TPA according to any one of claims 1 to 6, characterized in that: The PET waste includes at least one of plastic bottles, polyester fibers, textiles, strapping tapes or engineering plastic parts containing PET components.

8. The method for efficiently degrading PET to recover TPA as claimed in claim 1, characterized in that: The liquid phase obtained by solid-liquid separation in step S1 is recovered, the ratio of the polyethylene glycol aqueous solution to the alkaline reagent in the liquid phase is adjusted, and then the liquid phase is recycled for depolymerization reaction.

9. The method for efficiently degrading PET to recover TPA as claimed in claim 8, characterized in that: The liquid phase is enriched with ethylene glycol monomers. When the volume concentration of the ethylene glycol monomers in the liquid phase exceeds 10%, the ethylene glycol monomers are recovered by distillation, and the liquid phase after the distillation of the ethylene glycol monomers is circulated for depolymerization reaction.

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