A method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET

Abstract

The present invention belongs to the technical field of green recycling of waste bottle flakes and relates to a method for producing high-purity and high-yield BHET by depolymerizing waste PET. First, waste PET bottle flakes are subjected to an ethylene glycol alcoholysis reaction to obtain an alcoholysis solution. The alcoholysis solution is then decolorized with activated carbon, cooled and crystallized (the crystals in the material after cooling and crystallization are short columnar), and then solid-liquid separated to obtain a crystalline mixture containing BHET, a small amount of oligomers, and a small amount of ethylene glycol. The crystalline mixture containing BHET, a small amount of oligomers, and a small amount of ethylene glycol is then placed in a thin-film evaporator to remove residual ethylene glycol under vacuum. Finally, the crystals containing BHET and oligomers are melt-crystallized, and the melt is collected to obtain BHET. The purity of BHET is greater than 99%, and the yield is greater than 90%. The method of the present invention can produce BHET with high purity and high yield. The obtained BHET monomer can be directly used as a raw material for recycled PET polyester, providing a new path for high-quality recycling of waste PET bottle flakes.

Description

Technical Field

[0001] The invention belongs to the technical field of polyester recycling and relates to a method for preparing BHET with high purity and high yield by depolymerizing waste PET. Background Art

[0002] In recent years, the consumption of polyester (PET) has steadily increased due to its high strength, good wear resistance, and low cost. However, discarded polyester generally does not degrade in nature. Long-term accumulation not only results in a massive waste of resources but also poses a significant threat to the ecological environment. Therefore, developing technologies to recycle waste PET products, especially waste PET bottle flakes, and producing high-quality, high-purity recycled PET polyester raw materials, can reduce the consumption of fossil resources in PET production and promote plastic recycling.

[0003] Currently, mechanical and chemical depolymerization have become the most widely used technologies for recycling waste PET products. While mechanical recycling of recycled PET (r-PET) is simpler and more economical than chemical recycling, it suffers from drawbacks such as polymer molecular weight loss, which leads to decreased product performance, and an inability to remove impurities during the recycling process. This weakens the overall performance of r-PET, making true high-quality recycling difficult. Among chemical recycling methods, ethylene glycol alcoholysis is considered a highly promising method for recycling waste PET due to its relatively mild reaction conditions, the absence of other chemical solvents, and the fact that the depolymerization product, bis(hydroxyethyl) terephthalate (BHET), can be directly used as a raw material for recycled PET.

[0004] However, the core challenge of the glycol hydrolysis method lies in the efficient separation and purification of the depolymerization products. Traditionally, to separate BHET from oligomers and improve purity, a process using water as the recrystallization solvent is often used. This type of water-based recrystallization method, for example, patent CN 115806485 B discloses a method for improving the recovery rate of recovered ethylene terephthalate. The method involves distilling most of the ethylene glycol from the glycolysate in a first separator, then adding water to crystallize the BHET dimer from the solution; the crystallized BHET dimer is separated from the first mother liquor, and then crystallized from the first mother liquor to form BHET crystals. This process uses a large amount of water as the crystallization solvent, requiring significant water resources for crystallization and washing. The subsequent separation of water and ethylene glycol consumes extremely high energy consumption, making it an energy-intensive operation. It also produces a large amount of ethylene glycol-containing wastewater, increasing the environmental burden and treatment costs.

[0005] Patent CN 114269714 B discloses a method for increasing the recovery rate of ethylene terephthalate by dissolving the alcoholysis solution in water and then performing secondary crystallization at a second crystallization temperature. Patent CN 118878419 A employs a process that involves flash distillation to remove ethylene glycol, followed by melt crystallization and then recrystallization with water. Both methods use water as the crystallization solvent, which cannot avoid the introduction of water.

[0006] To overcome the drawbacks of water-based processes, researchers have begun exploring separation and purification techniques using the reaction medium, ethylene glycol, as the crystallization solvent. For example, patent CN 114437329 A discloses a method for purifying BHET by dehydrating the alcoholysis solution through a decolorizer, removing metal ions with an ion exchange resin, cooling and crystallizing under vacuum, and then using thin-film evaporation. The depolymerization product is a mixture of BHET and its oligomers. However, during the cooling and crystallization process, BHET oligomers also crystallize along with the BHET monomers, making it impossible to separate the BHET oligomers and ultimately achieve high-purity BHET production.

[0007] Therefore, developing an efficient and green method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, which can simultaneously achieve high purity and high recovery rate (yield) of BHET in an environment with ethylene glycol as the main medium and effectively solve the problem of catalyst residues, is of vital importance to promoting the industrial application of PET chemical recycling technology. Summary of the Invention

[0008] The purpose of the present invention is to solve the problems existing in the prior art and provide a method for high-purity recovery of BHET by utilizing a cooling crystallization and melt crystallization coupling process.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing BHET with high purity and high yield by depolymerizing waste PET comprises the following steps:

[0011] (1) subjecting waste PET bottle flakes to an ethylene glycol alcoholysis reaction to obtain an alcoholysis solution;

[0012] (2) decolorizing the alcoholysis solution obtained in step (1) with activated carbon;

[0013] (3) Cooling and crystallizing the decolorized alcoholysis solution, including: first, preliminarily cooling the decolorized alcoholysis solution to allow most of the oligomers to crystallize out, and then adding a co-solvent, a non-ionic surfactant, and an organic additive in sequence after solid-liquid separation to obtain a mixed solution; then cooling the mixed solution to the crystallization temperature, and aging it at a constant temperature for 2 to 4 hours at the crystallization temperature. This time range ensures the uniformity of the cooling process and the crystallization effect; finally, filtering and washing with ethylene glycol to obtain the cooled and crystallized material;

[0014] (4) performing solid-liquid separation on the material after cooling and crystallization in step (3) to obtain a crystalline mixture containing BHET, oligomers and ethylene glycol;

[0015] (5) placing the crystal mixture obtained in step (4) in a thin film evaporator and removing the residual ethylene glycol under vacuum to obtain a material containing BHET and oligomers;

[0016] (6) The material obtained in step (5) is subjected to melt crystallization treatment; in order to save energy consumption, the crude BHET liquid (containing BHET and a small amount of oligomers) from which the residual ethylene glycol has been removed is further purified directly in a melt crystallization device; the crude BHET liquid enters the crystallizer (in the melt crystallization device) and flows through the wall of the device in the form of a liquid film. By controlling the temperature of the heat medium, the liquid temperature is reduced to below the oligomer crystallization temperature and above the melting point of BHET, and oligomer crystals appear;

[0017] (7) collecting the melt after the melt crystallization in step (6) to obtain high-purity BHET;

[0018] The co-solvent in step (3) is an organic solvent with a dielectric constant (ε) <30 and a dipole moment <3.0D. The crystal morphology of the material after cooling and crystallization is short columnar with a length (L) of 0.5-5 mm and an aspect ratio (L / D) of 3-8:1;

[0019] The co-solvents function as follows:

[0020] 1. Weaken the hydrogen bond competition between ethylene glycol and BHET, and enhance the adsorption efficiency of surfactants such as Tween-80.

[0021] 2. Reduce solution viscosity, promote molecular diffusion, and facilitate crystal growth.

[0022] The nonionic surfactant plays a role in morphology regulation: it adsorbs on the hydroxyl / ester crystal surface of BHET, forcing the crystals to grow along the low polarity direction to form short columnar crystals.

[0023] Organic additives play a role in size regulation: slightly inhibiting the nucleation rate, reducing the number of crystal nuclei, and promoting single crystal growth. Through steric hindrance, they homogenize the growth interface, reduce defects, and increase crystal size.

[0024] The above-mentioned co-solvent, non-ionic surfactant and organic additive are used to control the formation of short columnar morphology of BHET, improve the BHET crystal separation effect and BHET crystal yield, greatly reduce the residual ethylene glycol in the crystals, reduce the number of washing times, significantly reduce the energy consumption of subsequent ethylene glycol removal, and achieve the effect of easier separation of ethylene glycol; at the same time, by controlling the cooling temperature and time, the catalyst and most of the ethylene glycol are effectively removed, creating favorable conditions for subsequent melt crystallization, thereby significantly improving the purity of the final product.

[0025] The purity of BHET in step (7) is greater than 99%, and the yield is greater than 90%.

[0026] As the preferred technical solution:

[0027] In the method for preparing BHET with high purity and high yield by depolymerizing waste PET as described above, the waste PET bottle flakes in step (1) are at least one of white transparent bottle flakes, bluish-white bottle flakes, green bottle flakes and multicolored bottle flakes, and the waste PET bottle flakes are subjected to pretreatment steps of impurity removal, crushing and washing in sequence before alcoholysis.

[0028] The above-mentioned method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, the specific process of step (1) is: putting waste PET bottle flakes into a mixture of catalyst and ethylene glycol, and performing alcoholysis reaction at a temperature of 200-230°C for 2-5 hours to obtain an alcoholysis solution.

[0029] In the above-mentioned method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, the mass ratio of waste PET bottle flakes to ethylene glycol is 1:2-10, and the mass ratio of waste PET bottle flakes to catalyst is 1:0.001-0.02.

[0030] As described above, a method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, the catalyst is a homogeneous polyester exchange catalyst, and the homogeneous polyester exchange catalyst is one or more of a transition metal acetate, a halogen-containing ionic liquid and a zinc-containing metal salt.

[0031] In the method for preparing BHET with high purity and high yield by depolymerizing waste PET as described above, in step (2), the alcoholysis solution is decolorized by activated carbon until it becomes colorless and transparent, the decolorization temperature is 80-150° C., and the mass ratio of waste PET bottle flakes to activated carbon is 1:0.03-0.15.

[0032] The method for preparing BHET with high purity and high yield by depolymerizing waste PET as described above, wherein the co-solvent in step (3) is one or more of acetone, isopropanol and tetrahydrofuran, the non-ionic surfactant is a Tween series or a Span series, and the organic additive is polyethylene glycol, specifically one or more of PEG 200-600;

[0033] The mass ratio of the solution after the oligomers are initially separated by cooling crystallization to the co-solvent is 6:4~8:2, the amount of the non-ionic surfactant added is 0.1~1.0wt% of the solution after the oligomers are initially separated by cooling crystallization, the amount of the organic additive added is 0.5~1.5wt% of the solution after the oligomers are initially separated by cooling crystallization, and the mass ratio of the amount of ethylene glycol used for washing to the filter cake (crude BHET crystals) obtained by filtration after the crystallization step is 1:1~3:1.

[0034] In the method for preparing BHET with high purity and high yield by depolymerizing waste PET as described above, the initial cooling temperature in step (3) is 45-67°C, the initial cooling time (the time required to cool from the decolorization temperature to the initial cooling temperature) is 2-5 hours, the crystallization temperature is 20-45°C, and the cooling rate to the crystallization temperature is 0.2-0.5°C / min.

[0035] In the above-mentioned method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, the temperature of the thin film evaporator in step (5) is 120~180℃.

[0036] In the above-described method for preparing high-purity and high-yield BHET by depolymerizing waste PET, the temperature for melt crystallization in step (6) is 110-130°C; under these conditions, BHET does not crystallize and is collected in a solution state. Based on the differences in physical properties between BHET and oligomers, BHET of higher purity is ultimately obtained.

[0037] Principle of the invention:

[0038] In existing technology, BHET crystallization is induced by adding water to the alcoholysis solution, which tends to form needle- or plate-like crystals. These crystals, due to their high aspect ratio and irregular stacking, form a dense filter cake, resulting in high filtration resistance and high residual mother liquor. This residual mother liquor not only carries catalyst and oligomers, but also requires more than four washes to achieve acceptable results. The high specific surface area and mechanical fragility of these crystals lead to the following problems:

[0039] (1) Inefficient solid-liquid separation: Crystals interlace to form a dense filter cake with low porosity, resulting in high filtration resistance and prolonged filtration time. In addition, crystals are easily broken into microcrystals, leading to mass loss and reduced yield during the separation process.

[0040] (2) Quality degradation and low washing efficiency: The capillary action of the micropores formed by the interlaced crystals, the high specific surface area and high surface energy of such crystals induce strong impurity adsorption, entrainment of catalysts and oligomers, resulting in quality degradation, and it is still difficult to meet the standards after more than 4 washings;

[0041] (3) Yield loss: In addition to the crystals being easily broken and lost during the solid-liquid separation stage, BHET crystals have a slight dissolution behavior in the washing solvent. Inefficient filtration separation process and multiple washings lead to BHET dissolution loss, reducing the product yield.

[0042] The present invention effectively solves the above problems by regulating crystal habits to obtain short columnar crystals. Its advantages are:

[0043] (1) High solid-liquid separation efficiency: The filter cake structure is optimized and arranged more regularly into a large-porosity filter cake, the filtration resistance is significantly reduced, and the filtration time is greatly shortened;

[0044] (2) Filter cake permeability: Regular arrangement forms a filter cake with a porosity of >40%, and the filtration speed is increased by 2 times;

[0045] (3) Improved washing efficiency: The smooth surface and low specific surface area reduce the amount of mother liquor adsorbed, and only two washes are required to meet the requirements;

[0046] (4) Improved purity: Low surface energy crystal surfaces reduce impurity adsorption, oligomers and catalyst residues, and the product purity is higher;

[0047] (5) Improved yield: The anti-crushing property reduces the loss of microcrystals, and combined with the reduction in washing times and the reduction in dissolution loss of crystal products, the yield is improved.

[0048] While the prior art melt crystallization process can separate BHET from polymers, the presence of residual ethylene glycol and catalyst in the alcoholysis solution limits the purity of the final product. Specifically, prior art processes typically remove ethylene glycol by flash distillation before melt crystallization. This flash distillation process primarily relies on the volatility of ethylene glycol to remove it from the alcoholysis solution. However, catalysts are generally nonvolatile substances, making flash distillation ineffective in removing them. Because flash distillation cannot remove the catalyst and the removal of ethylene glycol is not thorough enough, residual ethylene glycol and catalyst in the alcoholysis solution enter the subsequent melt crystallization process. These residual impurities can interfere with the crystallization process and affect the purity of BHET. Furthermore, the presence of the catalyst during the flash distillation process can cause BHET to repolymerize, affecting product yield. Therefore, even with prior art melt crystallization, the presence of residual impurities still results in low BHET purity and low product yield.

[0049] The present invention performs melt crystallization on the basis of cooling crystallization, thereby significantly improving the purity of BHET and reaching a purity level higher than 99%.

[0050] Based on the existing process of melt crystallization to purify BHET, the present invention creatively proposes to introduce a cooling crystallization step before melt crystallization, forming a multi-stage separation process through the coupling process of cooling crystallization and melt crystallization:

[0051] 1. Cooling crystallization: After the alcoholysis reaction, the alcoholysis solution is first cooled and crystallized to control the crystal morphology, achieve efficient separation of BHET crystals and crystallization mother liquor, and improve the purity of BHET, while also removing the catalyst and most of the ethylene glycol;

[0052] 2. Thin film evaporation: further removes residual ethylene glycol to ensure that the material entering the melt crystallization stage is purer;

[0053] 3. Melt crystallization: Separate BHET and oligomers by utilizing the difference in crystallization properties between BHET and oligomers.

[0054] Each stage is optimized for specific impurities or separation targets. After initial impurity removal through cooling crystallization, the melt crystallization process allows for more efficient separation of BHET and oligomers. This synergistic effect makes the overall process far more efficient and effective than the combined effects of each individual stage, ultimately achieving BHET recovery with a purity exceeding 99%. Furthermore, by controlling the crystal morphology to a short columnar shape during cooling crystallization, efficient separation of BHET crystals from the crystallization mother liquor and improved purity of the BHET crystals are achieved using ethylene glycol as the crystallization solvent, avoiding the energy-intensive water recovery and water-ethylene glycol separation steps required when using large amounts of water as the crystallization solvent in existing technologies.

[0055] Beneficial effects:

[0056] (1) The present invention provides a method for preparing BHET with high purity and high yield by depolymerizing waste PET. It proposes for the first time to couple the cooling crystallization process with the melt crystallization process. After the depolymerization reaction, the alcoholysis liquid is cooled and crystallized to remove most of the ethylene glycol and the catalyst therein. Then, the residual ethylene glycol in the crystallized material is removed by a thin film evaporator. Finally, the difference in the crystallization properties of BHET and oligomers is utilized to control the crystallization precipitation of oligomers. BHET does not crystallize and is in liquid state, thereby effectively separating BHET and oligomers.

[0057] (2) The present invention provides a method for preparing BHET with high purity and high yield by depolymerizing waste PET. By controlling the cooling temperature and time, adding a co-solvent, a nonionic surfactant and an organic additive, controlling the crystallization cooling rate and constant temperature aging, short columnar BHET crystals are prepared, the BHET crystal separation effect is improved, the BHET crystal yield is improved, the ethylene glycol residue in the crystals is greatly reduced, the number of washing times is reduced, and the energy consumption for subsequent ethylene glycol removal is significantly reduced.

[0058] (3) The method of the present invention for preparing BHET with high purity and high yield by depolymerizing waste PET can remove the catalyst in the alcoholysis and depolymerization products, effectively solving the technical problem that BHET is easily polymerized under heating and vacuum conditions. In addition, the process technology of the present invention can save a lot of energy consumption, reduce equipment investment, and facilitate the continuous separation process, which can meet the requirements of industrial large-scale production.

[0059] (4) The present invention provides a method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, which can obtain BHET with a purity higher than 99%, and a BHET yield greater than 90%. The obtained BHET monomer can be directly used as a raw material for recycled PET polyester, providing a new path for high-quality recycling of waste PET bottle flakes.

[0060] (5) The method of the present invention for preparing BHET with high purity and high yield by depolymerizing waste PET is simple, easy to operate, green and environmentally friendly, suitable for industrialization, and has good social and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 This is a photograph of the short columnar BHET crystal obtained in Example 1;

[0062] Figure 2 HPLC analysis spectrum of BHET prepared in Example 1;

[0063] Figure 3 HPLC analysis spectrum of BHET prepared in Comparative Example 1;

[0064] Figure 4 HPLC analysis spectrum of BHET prepared in Comparative Example 3;

[0065] Figure 5 HPLC analysis spectrum of BHET prepared in Comparative Example 4;

[0066] Figure 6 This is a photo of a flake-like BHET crystal;

[0067] Figure 7 HPLC analysis spectrum of BHET prepared in Example 2;

[0068] Figure 8 HPLC analysis spectrum of BHET prepared in Example 3;

[0069] Figure 9 HPLC analysis spectrum of BHET prepared in Example 4;

[0070] Figure 10 This is the HPLC analysis spectrum of BHET prepared in Example 5. DETAILED DESCRIPTION

[0071] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0072] The test methods involved in the performance indicators in the embodiments and comparative examples of the present invention are as follows:

[0073] BHET purity: BHET purity was analyzed using an Agilent 1260 Infinity II HPLC. Samples were dissolved in acetonitrile before analysis. A Poroshell 120 EC-C18 column with 4 μm filler diameter and 4.6 mm × 150 mm (D × L) was used. The column oven temperature was 35°C. The mobile phase used was an acetonitrile-water mixture (6:4 mass ratio). The detection wavelength was 254 nm, the flow rate was 1 mL / min, the run time was 10 minutes, and the injection volume was 2 μL.

[0074] Yield Y of BHET BHET (%): Among them, Y BHET is the yield of BHET; M PET and M BHET is the molecular weight of the PET repeating unit (192 g / mol) and BHET (254 g / mol); W PET is the mass of the input PET, W BHET is the quality of the output BHET.

[0075] Metal ion content: Metal ion content was measured using an inductively coupled plasma optical emission spectrometer (Avio 200). Samples were digested with a mixture of concentrated nitric acid and concentrated hydrochloric acid in a 3:1 volume ratio. 0.15 g of sample was dissolved in 4 mL of the mixture to obtain an ionic liquid containing the metal elements. Before testing, the sample was filtered through a 0.22 μm filter. The Avio 200 was turned on and the optical chamber temperature was stabilized at 36°C. Argon was used as the carrier gas, maintaining a partial pressure of no less than 0.7 mPa, for 15 minutes. Subsequently, the cooling water was turned on and the TEC temperature reached -44°C before ignition. After successful ignition, the instrument was allowed to stabilize for 20 minutes before testing began. A calibration curve was prepared using commercially available metal ion standard solutions (Merck, Germany, Part No.: ICP Multielement Standard IV). Initial calibration curve concentrations were set at 10, 20, 30, 40, and 50 mg / L. Based on the test results, the sample is diluted or a new standard curve is reconfigured to ensure that the metal ion concentration in the sample is within the standard curve concentration range. The mass of the metal in the corresponding sample is calculated. The metal ions tested include: Zn, Sb, Co, Mn, and Ti. The metal ion content is the sum of the contents of these ions.

[0076] Example 1

[0077] A method for high-purity recovery of BHET using a coupled cooling crystallization and melt crystallization process, the specific steps are as follows:

[0078] (1) Waste PET blue and white bottle flakes (manufacturer: Zhejiang Jiali Recycling Resources Co., Ltd.) were pre-treated by impurity removal, cleaning and crushing before alcoholysis;

[0079] (2) adding the waste PET blue and white bottle flakes pretreated in step (1) into a mixture of zinc acetate and ethylene glycol, and performing an alcoholysis reaction at a temperature of 200° C. for 3 h to obtain an alcoholysis solution;

[0080] Among them, the mass ratio of waste PET blue and white bottle flakes to ethylene glycol is 1:5, and the mass ratio of waste PET blue and white bottle flakes to zinc acetate is 1:0.001;

[0081] (3) The alcoholysis solution was decolorized to colorless and transparent by activated carbon at a decolorization temperature of 90°C, and the mass ratio of waste PET blue and white bottle flakes to activated carbon was 1:0.08;

[0082] (4) Cooling and crystallizing the decolorized alcoholysis solution;

[0083] (4.1) The decolorized alcoholysis solution was initially cooled to 50°C for 4 h. After solid-liquid separation, acetone, Tween-80, and PEG 200 were added to the filtrate in sequence to obtain a mixed solution.

[0084] The mass ratio of the filtrate obtained by separation in step (4.1) to acetone is 8:2, the amount of Tween-80 added is 0.3wt% of the filtrate obtained by separation in step (4.1), and the amount of PEG200 added is 0.8wt% of the filtrate obtained by separation in step (4.1);

[0085] (4.2) Cool the mixture to 25°C at a rate of 0.2°C / min and age at 25°C for 3.5 h.

[0086] (4.3) The product of step (4.2) was filtered and washed twice with ethylene glycol, with the mass ratio of ethylene glycol used for washing to the filter cake obtained by filtration being 1.5:1; the ethylene glycol content in the filter cake was 12 wt%;

[0087] like Figure 1 As shown, the crystal morphology of the material after cooling crystallization is short columnar, with a length (L) of 2.4±1.5mm and an aspect ratio (L / D) of 3~4.5:1;

[0088] (5) subjecting the material after cooling and crystallization in step (4) to solid-liquid separation for 12 minutes to obtain a crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol;

[0089] (6) The crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol was placed in a thin film evaporator at 150°C, and the residual ethylene glycol was removed under vacuum for 0.8 hours to obtain a mixture containing BHET and a small amount of oligomers;

[0090] (7) A mixture containing BHET and a small amount of oligomers was melt crystallized at 110°C. The melt was collected and cooled to obtain high-purity BHET.

[0091] like Figure 2 As shown, the purity of the finally prepared BHET is 99.03%, the yield of BHET is 91.34%, and the metal ion content of BHET is 0 ppm.

[0092] Table 1 shows the specific HPLC test data of the product BHET in Example 1. The peak of the product BHET has a retention time of 1.655 min, and the purity of BHET is 99.03% as calculated by the peak area ratio.

[0093] Table 1 HPLC test data of the product BHET in Example 1

[0094]

[0095] Comparative Example 1

[0096] A method for recovering BHET is basically the same as that in Example 1, except that step (4) is omitted.

[0097] The removal of residual ethylene glycol in step (6) takes 5.4 hours; Figure 3 As shown, the purity of the finally prepared BHET is 95.23%, the total metal ion content of BHET is 473.2 ppm, and the yield of BHET is 78.35%.

[0098] Comparing Comparative Example 1 with Example 1, it can be found that the thin film evaporation step for removing ethylene glycol in Comparative Example 1 took 5.4 hours, which was significantly longer. The yield of the melt crystallization step was reduced, and the purity of BHET decreased, resulting in a decrease in yield. This is because the cooling crystallization step plays the following key roles in the process:

[0099] (1) Initial removal of ethylene glycol:

[0100] The cooling crystallization step effectively controls the crystal shape to short, columnar crystals, effectively removing the majority of the ethylene glycol and reducing the amount of ethylene glycol entering subsequent steps. If the cooling crystallization step is omitted, the amount of ethylene glycol remaining in the alcoholysis solution will increase significantly, requiring the thin-film evaporation step to process more ethylene glycol, thus extending the evaporation time.

[0101] (2) Removal of catalyst:

[0102] The cooling crystallization step not only removes a large amount of ethylene glycol but also effectively removes the catalyst from the alcoholysis solution. The presence of the catalyst can cause some BHET to repolymerize during the thin-film evaporation and melt crystallization processes, affecting the purity and yield of BHET.

[0103] (3) Reduce oligomer interference:

[0104] The cooling crystallization step controls the temperature and time to allow most of the oligomers to crystallize and be filtered out. If this step is omitted, more oligomers will enter the subsequent melt crystallization step, reducing the efficiency of this step.

[0105] Table 2 shows the specific HPLC test data of the product BHET in Comparative Example 1. The peak of the product BHET has a retention time of 1.654 min, and the purity of BHET is 95.23% as calculated by calculating the peak area ratio.

[0106] Table 2 HPLC test data of the product BHET in Comparative Example 1

[0107]

[0108] Comparative Example 2

[0109] A method for recovering BHET is basically the same as that in Example 1, except that step (6) is omitted.

[0110] Under these conditions, BHET products cannot be produced by melt crystallization. This is because without removing ethylene glycol through thin film evaporation, ethylene glycol (boiling point 197.3°C) remains in a liquid state within the temperature range of melt crystallization and mixes with the BHET product and cannot be separated, so BHET products cannot be produced.

[0111] Comparative Example 3

[0112] A method for recovering BHET is basically the same as that in Example 1, except that step (7) is omitted.

[0113] like Figure 4 As shown in FIG, the purity of the finally prepared BHET is 97.91%, the metal ion content of BHET is 0 ppm, and the yield of BHET is 90.65%.

[0114] Comparing Comparative Example 3 with Example 1 reveals a decrease in purity and an increase in yield in Comparative Example 3. This is because even after omitting the melt crystallization step, a very small amount of oligomers remains in the crystals. These oligomers cannot be completely removed and ultimately mix into the BHET product, resulting in a decrease in purity. Furthermore, this inclusion of oligomers causes the BHET yield to over-calculate the mass of these oligomers, resulting in a slight false increase in the yield value.

[0115] Table 3 shows the specific HPLC test data of the product BHET in Comparative Example 3. The peak of the product BHET has a retention time of 1.622 min, and the purity of BHET is 97.91% as calculated by calculating the peak area ratio.

[0116] Table 3 HPLC test data of the product BHET in Comparative Example 3

[0117]

[0118] Comparative Example 4

[0119] A method for recovering BHET is basically the same as Example 1, except that the cooling crystallization step in step (4) is simplified and the step (4.1) of adding the co-solvent acetone, the nonionic surfactant Tween-80 and the organic additive PEG200 is omitted.

[0120] like Figure 5 As shown, the purity of the finally prepared BHET is 98.38%, the metal ion content of BHET is 0 ppm, and the yield of BHET is 82.21%.

[0121] Comparing Comparative Example 4 with Example 1, it can be found that the crystallized product mixture of Comparative Example 4 needs to be washed with ethylene glycol at least 5 times to completely remove the catalyst. The solid-liquid separation time after crystallization is 95 minutes, the ethylene glycol content in the filter cake is 53%, and the obtained BHET crystals are flaky (such as Figure 6 The thin film evaporation step for removing ethylene glycol took 2.8 hours, which was significantly longer. This was because the addition of a co-solvent, a nonionic surfactant, and an organic additive in the cooling crystallization step controlled the formation of a short columnar morphology of BHET, making it easier to separate ethylene glycol.

[0122] Table 4 HPLC test data of the product BHET of Comparative Example 4

[0123]

[0124] Example 2

[0125] A method for high-purity recovery of BHET using a coupled cooling crystallization and melt crystallization process, the specific steps are as follows:

[0126] (1) Waste PET blue and white bottle flakes (manufacturer: Zhejiang Jiali Recycling Resources Co., Ltd.) were pre-treated by impurity removal, cleaning and crushing before alcoholysis;

[0127] (2) adding the waste PET blue and white bottle flakes pretreated in step (1) into a mixture of zinc acetate and ethylene glycol, and performing an alcoholysis reaction at a temperature of 200° C. for 3 h to obtain an alcoholysis solution;

[0128] Among them, the mass ratio of waste PET blue and white bottle flakes to ethylene glycol is 1:5, and the mass ratio of waste PET blue and white bottle flakes to zinc acetate is 1:0.003;

[0129] (3) The alcoholysis solution was decolorized to colorless and transparent by activated carbon at a decolorization temperature of 90°C, and the mass ratio of waste PET blue and white bottle flakes to activated carbon was 1:0.08;

[0130] (4) Cooling and crystallizing the decolorized alcoholysis solution;

[0131] (4.1) The decolorized alcoholysis solution was initially cooled to 50°C for 4 h. After solid-liquid separation, isopropanol, Tween-80, and PEG-600 were added to the filtrate in sequence to obtain a mixed solution.

[0132] The mass ratio of the filtrate obtained by separation in step (4.1) to isopropanol is 7:3, the amount of Tween-20 added is 0.5wt% of the filtrate obtained by separation in step (4.1), and the amount of PEG600 added is 0.7wt% of the filtrate obtained by separation in step (4.1);

[0133] (4.2) Cool the mixture to 25°C at a rate of 0.3°C / min and age at 25°C for 3.5 h.

[0134] (4.3) The product of step (4.2) was filtered and washed twice with ethylene glycol to obtain a cooled and crystallized material, wherein the mass ratio of ethylene glycol used for washing to the filter cake obtained by filtration was 1.5:1; the ethylene glycol content in the filter cake was 15.5 wt%;

[0135] The crystal morphology of the material after cooling crystallization is short columnar, the crystal length L=1.7±1.1mm, and the crystal aspect ratio L / D=3.5~5.2:1;

[0136] (5) The material after cooling and crystallization in step (4) was subjected to solid-liquid separation for 18 minutes to obtain a crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol;

[0137] (6) The crystalline mixture containing BHET, a small amount of oligomers, and a small amount of ethylene glycol was placed in a thin film evaporator at 150°C, and the residual ethylene glycol was removed under vacuum for 1.1 hours to obtain a mixture containing BHET and a small amount of oligomers;

[0138] (7) A mixture containing BHET and a small amount of oligomers was melt crystallized at 115°C. The melt was collected and cooled to obtain high-purity BHET.

[0139] like Figure 7 As shown, the purity of the finally prepared BHET is 99.08%, the yield of BHET is 90.17%, and the metal ion content of BHET is 0 ppm.

[0140] Table 5 shows the specific HPLC test data of the product BHET in Example 2. The peak of the product BHET has a retention time of 1.656 min, and the purity of BHET is 99.08% as calculated by the peak area ratio.

[0141] Table 5 HPLC test data of the product BHET in Example 2

[0142]

[0143] Example 3

[0144] A method for high-purity recovery of BHET using a coupled cooling crystallization and melt crystallization process, the specific steps are as follows:

[0145] (1) Waste PET bottle flakes (manufacturer: Zhejiang Jiali Recycling Resources Co., Ltd.) were pre-treated by impurity removal, cleaning, and crushing before alcoholysis;

[0146] (2) adding the waste PET white bottle flakes pretreated in step (1) into a mixture of zinc octoate and ethylene glycol, and performing an alcoholysis reaction at a temperature of 230° C. for 5 h to obtain an alcoholysis solution;

[0147] Among them, the mass ratio of waste PET white bottle flakes to ethylene glycol is 1:10, and the mass ratio of waste PET white bottle flakes to zinc octoate is 1:0.02;

[0148] (3) The alcoholysis solution was decolorized to colorless and transparent by activated carbon at a decolorization temperature of 90°C, and the mass ratio of waste PET bottle flakes to activated carbon was 1:0.08;

[0149] (4) Cooling and crystallizing the decolorized alcoholysis solution;

[0150] (4.1) The decolorized alcoholysis solution was initially cooled to 45°C for 5 h. After solid-liquid separation, tetrahydrofuran, Tween-60, and PEG 200 were added to the filtrate in sequence to obtain a mixed solution.

[0151] The mass ratio of the filtrate obtained by separation in step (4.1) to tetrahydrofuran is 6:4, the amount of Tween-60 added is 0.1wt% of the filtrate obtained by separation in step (4.1), and the amount of PEG200 added is 0.5wt% of the filtrate obtained by separation in step (4.1);

[0152] (4.2) Cool the mixture to 20°C at a rate of 0.5°C / min and age at 20°C for 3.8 h.

[0153] (4.3) The product of step (4.2) was filtered and washed twice with ethylene glycol, with the mass ratio of ethylene glycol used for washing to the filter cake obtained by filtration being 1.5:1; the ethylene glycol content in the filter cake was 14 wt%;

[0154] The crystal morphology of the material after cooling crystallization is short columnar, with a length (L) of 2.5±1.4mm and an aspect ratio (L / D) of 3.3~4.5:1;

[0155] (5) The material after cooling and crystallization in step (4) was subjected to solid-liquid separation for 16 minutes to obtain a crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol;

[0156] (6) The crystalline mixture containing BHET, a small amount of oligomers, and a small amount of ethylene glycol was placed in a thin film evaporator at 120°C, and the residual ethylene glycol was removed under vacuum for 1.2 hours to obtain a mixture containing BHET and a small amount of oligomers;

[0157] (7) A mixture containing BHET and a small amount of oligomers was melt crystallized at 130°C. The melt was collected and cooled to obtain high-purity BHET.

[0158] like Figure 8 As shown, the purity of the finally prepared BHET is 99.24%, the yield of BHET is 92.35%, and the metal ion content of BHET is 0 ppm.

[0159] Table 6 shows the specific HPLC test data of the product BHET in Example 3. The peak of the product BHET has a retention time of 1.656 min, and the purity of BHET is 99.24% as calculated by the peak area ratio.

[0160] Table 6 HPLC test data of the product BHET in Example 3

[0161]

[0162] Example 4

[0163] A method for high-purity recovery of BHET using a coupled cooling crystallization and melt crystallization process, the specific steps are as follows:

[0164] (1) Waste PET green bottle flakes (manufacturer: Zhejiang Jiali Recycling Resources Co., Ltd.) were pre-treated by impurity removal, cleaning and crushing before alcoholysis;

[0165] (2) adding the waste PET green bottle flakes pretreated in step (1) into a mixture of cobalt acetate and ethylene glycol, and performing an alcoholysis reaction at a temperature of 210° C. for 4 h to obtain an alcoholysis solution;

[0166] Among them, the mass ratio of waste PET green bottle flakes to ethylene glycol is 1:8, and the mass ratio of waste PET green bottle flakes to cobalt acetate is 1:0.01;

[0167] (3) The alcoholysis solution was decolorized to colorless and transparent by activated carbon at a decolorization temperature of 90°C, and the mass ratio of waste PET green bottle flakes to activated carbon was 1:0.08;

[0168] (4) Cooling and crystallizing the decolorized alcoholysis solution;

[0169] (4.1) The decolorized alcoholysis solution was initially cooled to 60°C for 3 h. After solid-liquid separation, acetone, Span-40, and PEG-400 were added to the filtrate in sequence to obtain a mixed solution.

[0170] The mass ratio of the filtrate obtained by separation in step (4.1) to acetone is 7:3, the amount of Span-40 added is 1.0 wt % of the filtrate obtained by separation in step (4.1), and the amount of PEG400 added is 1.5 wt % of the filtrate obtained by separation in step (4.1);

[0171] (4.2) Cool the mixture to 30°C at a rate of 0.3°C / min and age at 30°C for 3 h.

[0172] (4.3) The product of step (4.2) was filtered and washed twice with ethylene glycol, with the mass ratio of ethylene glycol used for washing to the filter cake obtained by filtration being 1.5:1; the ethylene glycol content in the filter cake was 10 wt%;

[0173] The crystal morphology of the material after cooling crystallization is short columnar, with a length (L) of 2.8±1.3mm and an aspect ratio (L / D) of 3.1~4.7:1;

[0174] (5) The material after cooling and crystallization in step (4) was subjected to solid-liquid separation for 11 minutes to obtain a crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol;

[0175] (6) The crystalline mixture containing BHET, a small amount of oligomers, and a small amount of ethylene glycol was placed in a thin film evaporator at 160°C, and the residual ethylene glycol was removed under vacuum for 0.7 hours to obtain a mixture containing BHET and a small amount of oligomers;

[0176] (7) A mixture containing BHET and a small amount of oligomers is melt-crystallized at 110°C, and the melt is collected and cooled to obtain the BHET product.

[0177] like Figure 9 As shown, the purity of the finally prepared BHET is 99.15%, the yield of BHET is 91.03%, and the metal ion content of BHET is 0 ppm.

[0178] Table 7 shows the specific data of the HPLC test of the product BHET in Example 4. The peak of the product BHET is at a retention time of 1.648 min, and the purity of BHET is 99.15% as calculated by the peak area ratio.

[0179] Table 7 HPLC test data of the product BHET in Example 4

[0180]

[0181] Example 5

[0182] A method for high-purity recovery of BHET using a coupled cooling crystallization and melt crystallization process, the specific steps are as follows:

[0183] (1) Waste PET bottle flakes (manufacturer: Zhejiang Jiali Recycling Resources Co., Ltd.) were pre-treated by impurity removal, cleaning and crushing before alcoholysis;

[0184] (2) putting the waste PET variegated bottle flakes pretreated in step (1) into a mixture of manganese acetate and ethylene glycol, and performing an alcoholysis reaction at a temperature of 210° C. for 2 h to obtain an alcoholysis solution;

[0185] The mass ratio of waste PET variegated bottle flakes to ethylene glycol is 1:2, and the mass ratio of waste PET variegated bottle flakes to manganese acetate is 1:0.01;

[0186] (3) The alcoholysis solution was decolorized to colorless and transparent by activated carbon at a decolorization temperature of 90°C, and the mass ratio of waste PET bottle flakes to activated carbon was 1:0.08;

[0187] (4) Cooling and crystallizing the decolorized alcoholysis solution;

[0188] (4.1) The decolorized alcoholysis solution was initially cooled for 2 h to 67°C. After solid-liquid separation, isopropyl alcohol, Span-60, and PEG-400 were added to the filtrate in sequence to obtain a mixed solution.

[0189] The mass ratio of the filtrate obtained by separation in step (4.1) to isopropanol is 6:4, the amount of Span-60 added is 0.8wt% of the filtrate obtained by separation in step (4.1), and the amount of PEG400 added is 1wt% of the filtrate obtained by separation in step (4.1);

[0190] (4.2) Cool the mixture to 45°C at a rate of 0.2°C / min and age at 45°C for 4 h.

[0191] (4.3) The product of step (4.2) was filtered and washed twice with ethylene glycol to obtain a cooled and crystallized material, wherein the mass ratio of ethylene glycol used for washing to the filter cake obtained by filtration was 1.5:1; the ethylene glycol content in the filter cake was 12 wt%;

[0192] The crystal morphology of the material after cooling crystallization is short columnar, the crystal length L=2.9±1.4mm, and the crystal aspect ratio L / D=3.6~5.3:1;

[0193] (5) The material after cooling and crystallization in step (4) was subjected to solid-liquid separation for 14 minutes to obtain a crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol;

[0194] (6) The crystalline mixture containing BHET, a small amount of oligomers and a small amount of ethylene glycol was placed in a thin film evaporator at 180°C, and the residual ethylene glycol was removed under vacuum for 0.7 h to obtain a mixture containing BHET and a small amount of oligomers;

[0195] (7) A mixture containing BHET and a small amount of oligomers is melt-crystallized at 120°C, and the melt is collected and cooled to obtain the BHET product.

[0196] like Figure 10 As shown, the purity of the finally prepared BHET is 99.03%, the yield of BHET is 90.05%, and the metal ion content of BHET is 0 ppm.

[0197] Table 8 shows the specific HPLC test data of the product BHET in Example 5. The peak of the product BHET has a retention time of 1.648 min, and the purity of BHET is 99.03% as calculated by the peak area ratio.

[0198] Table 8 HPLC test data of the product BHET in Example 5

[0199] .

Claims

1. A method for depolymerizing waste PET to achieve high-purity and high-yield preparation of BHET, characterized in that The steps include: (1) subjecting waste PET bottle flakes to an ethylene glycol alcoholysis reaction to obtain an alcoholysis solution; (2) decolorizing the alcoholysis solution obtained in step (1); (3) cooling and crystallizing the decolorized alcoholysis solution, including: preliminarily cooling the decolorized alcoholysis solution, and sequentially adding a co-solvent, a nonionic surfactant, and an organic additive after solid-liquid separation to obtain a mixed solution; The mixed solution is cooled to the crystallization temperature and aged at a constant temperature for 2 to 4 hours at the crystallization temperature; the material after cooling and crystallization is obtained by filtering and washing with ethylene glycol in sequence; (4) performing solid-liquid separation on the material after cooling and crystallization in step (3) to obtain a crystalline mixture containing BHET, oligomers and ethylene glycol; (5) placing the crystal mixture obtained in step (4) in a thin film evaporator and removing the residual ethylene glycol under vacuum to obtain a material containing BHET and oligomers; (6) subjecting the material obtained in step (5) to melt crystallization; (7) collecting the melt after the melt crystallization in step (6) to obtain high-purity BHET; The co-solvent in step (3) is one or more of acetone, isopropanol and tetrahydrofuran, the non-ionic surfactant is a Tween series or a Span series, the organic additive is polyethylene glycol, and the crystal morphology of the material after cooling crystallization is short columnar, with a length of 0.5 to 5 mm and an aspect ratio of 3 to 8:1; The purity of BHET in step (7) is greater than 99%, and the yield is greater than 90%.

2. A method for preparing BHET in high purity and high yield by depolymerizing waste PET according to claim 1, characterized in that: In step (1), the waste PET bottle flakes are at least one of white transparent bottle flakes, blue-white bottle flakes, green bottle flakes and multicolored bottle flakes, and the waste PET bottle flakes are subjected to pretreatment steps of impurity removal, crushing and cleaning in sequence before alcoholysis.

3. A method for preparing BHET in high purity and high yield by depolymerizing waste PET according to claim 1, characterized in that: The specific process of step (1) is as follows: waste PET bottle flakes are put into a mixture of a catalyst and ethylene glycol, and alcoholysis reaction is carried out at a temperature of 200-230° C. for 2-5 hours to obtain alcoholysis solution.

4. A method for preparing BHET in high purity and high yield by depolymerizing waste PET according to claim 3, characterized in that: The mass ratio of waste PET bottle flakes to ethylene glycol is 1:2~10, and the mass ratio of waste PET bottle flakes to catalyst is 1:0.001~0.

02.

5. A method for preparing BHET in high purity and high yield by depolymerizing waste PET according to claim 4, characterized in that: The catalyst is a homogeneous polyester exchange catalyst, which is one or more of transition metal acetate, halogen element-containing ionic liquid and zinc metal salt.

6. A method for preparing BHET with high purity and high yield by depolymerizing waste PET according to claim 1, characterized in that: The decolorization temperature in step (2) is 80~150℃.

7. The method for preparing BHET with high purity and high yield by depolymerizing waste PET according to claim 1, wherein: In step (3), the mass ratio of the solution after the oligomers are initially separated by cooling crystallization to the co-solvent is 6:4~8:2, the amount of non-ionic surfactant added is 0.1~1.0wt% of the solution after the oligomers are initially separated by cooling crystallization, the amount of organic additive added is 0.5~1.5wt% of the solution after the oligomers are initially separated by cooling crystallization, and the mass ratio of the amount of ethylene glycol used for washing to the filter cake obtained by filtration after the crystallization step is 1~3:

1.

8. The method for preparing BHET with high purity and high yield by depolymerizing waste PET according to claim 1, wherein: In step (3), the initial cooling temperature is 45-67°C, the initial cooling time is 2-5h, the crystallization temperature is 20-45°C, and the cooling rate when cooling to the crystallization temperature is 0.2-0.5°C / min.

9. The method for preparing BHET with high purity and high yield by depolymerizing waste PET according to claim 1, wherein: The temperature of the thin film evaporator in step (5) is 120~180℃.

10. The method for preparing BHET with high purity and high yield by depolymerizing waste PET according to claim 1, wherein: The temperature of the melt crystallization in step (6) is 110-130°C.

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