Method for improving yield and purity of regenerated bhets

Abstract

The present application relates to polyester recycling, and provides a method for improving the yield and purity of regenerated BHET. After glycolysis of waste polyester in the presence of a metal catalyst, most of the free glycol is removed by evaporation under reduced pressure. A chelating agent composed of a polycarboxylic acid chelating agent and an organic phosphonic acid compound is added to the glycolysis solution, and a fixed bed circulation contact of a solid-supported chelating agent with polycarboxylic acid groups supported on a porous carrier is used to remove metal ions. Subsequently, the glycol is further removed by thin film evaporation to obtain a BHET-rich glycolysis solution. Finally, the distillate is collected in a short path distiller according to the set switching conditions of BHET content and acid value to obtain a high-purity BHET product. This method can inhibit re-polymerization and thermal degradation, reduce metal residues and oligomer content, and improve the yield and purity of BHET.

Description

Technical Field

[0001] This invention relates to the technical field of polyester recycling, and in particular to a method for improving the yield and purity of recycled BHET. Background Technology

[0002] The extensive use of waste polyester materials in textiles, packaging, and engineering plastics has made recycling a key direction for industrial development. Ethylene glycol hydrolysis of waste polyester yields reusable monomers such as bis(2-hydroxyethyl) terephthalate (BHET), making it one of the most widely used chemical recycling methods. However, the hydrolysis solution typically contains high levels of residual metal catalysts, oligomers, degradation products, and excess ethylene glycol. These impurities can easily trigger repolymerization, thermal degradation, and increased color during subsequent high-temperature evaporation and refining processes, leading to reduced product purity, decreased yield, and even scaling or blockage of evaporation and distillation equipment, severely impacting the stability of continuous production. Existing processes still have significant shortcomings in removing metal ions, stabilizing the high-temperature evaporation process, and improving the purity of BHET products, making it difficult to balance high purification efficiency with reliable industrial operation. Therefore, effectively controlling metal impurities and accompanying side reactions in the hydrolysis solution to stably achieve the separation of high-purity BHET is a critical technical problem that urgently needs to be solved in the field of chemical recycling. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and to provide a method for improving the yield and purity of regenerated BHET.

[0004] To achieve the above objectives, the first aspect of the present invention provides a method for improving the yield and purity of regenerated BHET, comprising the following steps:

[0005] S1: Waste polyester and ethylene glycol are subjected to alcoholysis in the presence of a metal catalyst to obtain alcoholysis solution;

[0006] S2: The alcoholysis solution is subjected to first-stage reduced pressure evaporation at 180-220℃ and 0.5-30 kPa to remove most of the free ethylene glycol in the alcoholysis solution, resulting in an alcoholysis solution with partial ethylene glycol removal. At this time, the mass ratio of ethylene glycol to BHET in the alcoholysis solution is 1.5-4:1.

[0007] S3: Cool the alcoholysis solution obtained in step S2 to 100-140°C, add a metal chelating agent to the alcoholysis solution within this temperature range, stir the reaction, and allow the metal chelating agent to complex with the metal ions in the alcoholysis solution.

[0008] S4: The alcoholysis obtained in step S3 is treated with a supported chelating agent. The supported chelating agent is packed in the contactor in the form of particles. The alcoholysis is circulated through the contactor at 100-140 °C for 0.2-1 h. The supported chelating agent is removed by solid-liquid separation to obtain an alcoholysis treated with metal removal.

[0009] S5: The alcoholysis solution obtained in step S4 is subjected to a second stage of reduced pressure evaporation through a thin-film evaporator at 140-190 °C and 0.2-5 kPa to remove the remaining free ethylene glycol, so that the mass fraction of free ethylene glycol in the alcoholysis solution is reduced to below 5 wt%, and BHET enriched alcoholysis solution is obtained.

[0010] S6: The BHET enriched alcoholysis solution obtained in step S5 is subjected to short-path distillation in a short-path still at 190–230 °C and 0.05–1 kPa, and the distillate is collected as the BHET product.

[0011] A continuous process pathway was developed to synergistically control ethylene glycol content, BHET content, and metal ion state by sequentially passing the alcoholysis solution through a high-temperature first-stage vacuum desulfurization process, the addition of a small-molecule chelating agent at a mid-temperature window, contact demetallization with a supported chelating agent, a second-stage thin-film evaporation process, and short-path distillation. This pathway thoroughly passivates and removes metal ions before thin-film evaporation and short-path distillation, significantly inhibiting the repolymerization and thermal degradation of BHET under high-temperature conditions, resulting in a marked improvement in the yield and purity of the BHET product.

[0012] Meanwhile, the process of the present invention can avoid the deposition of metals and their complexes on the evaporator wall, improve the stability of equipment operation, and make the final product have lower color, less metal residue, and lower oligomer content.

[0013] As a further improvement of the present invention, the metal catalyst includes one or more of cobalt glycol, manganese glycol, and zinc glycol.

[0014] When the metals contained in the alcoholysis system are cobalt glycolate, manganese glycolate, or zinc glycolate, the coordination ability of small molecule chelating agents and supported chelating agents matches the coordination characteristics of these three commonly used metal catalysts, resulting in higher metal complexation efficiency. This selection can more effectively eliminate the residual catalytic effect of the catalyst during high-temperature evaporation and short-path distillation stages, thereby further reducing the polycondensation tendency and color of BHET, and improving the metal purity and thermal stability of the final product.

[0015] As a further improvement of the present invention, the metal chelating agent is composed of a polycarboxylic acid chelating agent containing at least 4 carboxyl groups and 2 or more nitrogen atoms and an organophosphonic acid compound containing –PO3H2 groups.

[0016] By simultaneously introducing polycarboxylic acid-based and phosphate-containing organophosphonic acid chelating agents, a dual complexing system with multiple sites, strong coordination, and high thermal stability can be formed. This system possesses both rapid and stable complexing capabilities, exhibiting higher capture efficiency for polyvalent metal ions in the alcoholysis solution, enabling complete passivation of metal activity in a short time. Compared to single-type chelating agents, this combination method can achieve more complete metal complexation at lower dosages, thereby significantly reducing side reactions during the high-temperature evaporation stage and improving the yield and purity of BHET.

[0017] As a further improvement of the present invention, the polycarboxylic acid chelating agent includes at least one of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, or diethylenetriaminepentaacetic acid;

[0018] The organophosphonic acid compound includes at least one of aminotrimethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid or its sodium salt;

[0019] The molar ratio of the polycarboxylic acid chelating agent to the organophosphonic acid compound is 0.7:1 to 2:1.

[0020] By specifying the exact types of polycarboxylic acid chelating agents and organophosphonic acid compounds, and ensuring their synergistic presence in a molar ratio of 0.7:1 to 2:1, the system achieves optimal matching in terms of complexation strength, metal ion steric hindrance control, and dissolution equilibrium. This molar ratio range ensures sufficient metal ion complexation while avoiding acidity increases due to excess polycarboxylic acids or precipitation caused by excess phosphonic acids. This makes the chelation reaction more stable and facilitates smoother subsequent immobilization, capture, and evaporation processes, thereby improving the purity of BHET and reducing impurity formation.

[0021] As a further improvement of the present invention, the total amount of the two components, polycarboxylic acid chelating agent and organophosphonic acid compound, in the metal chelating agent is 0.05 to 3 wt% of the waste PET mass.

[0022] By setting the total amount of the two-component chelating agent within the range of 0.05–3 wt% of the waste PET mass, it is possible to ensure complete metal complexation while avoiding system contamination and subsequent refining burden caused by excessive chelating agent. This dosage range achieves a balance between metal removal efficiency, system viscosity control, and process economy, resulting in more stable operation without increasing distillation load, thereby improving BHET purity and reducing process costs.

[0023] As a further improvement of the present invention, the supported chelating agent in step S4 is a particle obtained by grafting a polycarboxylic acid chelating group onto the surface of an inorganic porous support via a silane coupling bond. The inorganic porous support is selected from silica, alumina, or a mixture of the two. The supported chelating agent has a BET specific surface area of ​​30 to 300 m² / g and an average particle size of 50 to 2000 μm.

[0024] By employing particles with polycarboxylic acid chelating groups fixed to the surface of an inorganic porous support via silane coupling, the solid-phase capture capacity for metals and metal complexes can be significantly improved. The particles possess a suitable specific surface area and particle size range, ensuring sufficient mass transfer area for the alcoholysis solution during cyclic contact while maintaining good mechanical strength and stability, preventing pulverization or equipment blockage. This design allows for the true removal of metals and their complexes from the system without them entering the thin-film evaporation or short-path distillation steps, effectively reducing metal residue in the final BHET and improving product color and quality stability.

[0025] As a further improvement of the present invention, after adding the metal chelating agent in step S3, the mass ratio of ethylene glycol to BHET in the alcoholysis solution is maintained at 1.5:1 to 3:1, and the mass ratio is controlled by adding ethylene glycol or adjusting the degree of decompression.

[0026] By controlling the mass ratio of ethylene glycol to BHET within the range of 1.5:1 to 3:1 after adding a chelating agent, the system viscosity, solubility, and mass transfer conditions are placed within a process window favorable for complexation reactions and immobilized capture. This mass ratio not only ensures that the chelating agent can disperse rapidly and make sufficient contact with metal ions, but also prevents crystallization or condensation caused by localized high concentrations of BHET. Controlling this range allows for more thorough metal removal, more stable subsequent evaporation behavior, and improved BHET distillation yield and product purity.

[0027] As a further improvement of the present invention, when the film evaporation is completed in step S5, the mass fraction of BHET in the alcoholysis solution is 70-90 wt%, and the acid value is less than or equal to 1.0 mgKOH / g.

[0028] By controlling the BHET mass fraction to 70–90 wt% and the acid value to below 1.0 mg KOH / g at the end of thin-film evaporation, the feed entering short-path distillation is ensured to be in a state of high target component concentration and low free carboxyl content. This state significantly reduces the risk of re-polymerization under the high-temperature conditions of short-path distillation, improving the purity and stability of the distillate. Furthermore, the high-concentration feed reduces distillation energy consumption and equipment load, making the separation process more efficient.

[0029] As a further improvement of the present invention, in the short-path distillation process of step S6, the switching condition for starting to collect the distillate BHET product is that the mass fraction of BHET in the BHET enrichment alcoholysis solution reaches not less than 70 wt% and the acid value is not higher than 1.0 mgKOH / g, and the switching condition for stopping the collection of the distillate BHET product is that the mass fraction of BHET in the distillate drops to less than 92 wt%.

[0030] By setting the starting conditions for collecting the distillate as a BHET content ≥70 wt% and an acid value ≤1.0 mgKOH / g on the feed side, and the ending condition as a BHET content <92 wt% in the distillate, precise fractionation control based on component changes can be achieved. This control method removes the fractions with higher impurity proportions from the initial and final distillates, collecting only the most stable and pure fraction. This significantly improves the batch stability of BHET products, reduces color and oligomer content, and decreases the need for rework and redistillation, further enhancing overall process efficiency and product quality.

[0031] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0032] This invention achieves synergistic control over ethylene glycol content, BHET concentration, and metal ion state by sequentially subjecting waste polyester alcoholysis solution to a series of steps: high-temperature first-stage vacuum removal of ethylene glycol, addition of a two-component chelating agent at a medium-temperature window, contact treatment with a supported chelating agent, second-stage thin-film evaporation, and short-path distillation. The first-stage vacuum evaporation under high-temperature conditions rapidly reduces the free ethylene glycol content in the system, placing the alcoholysis solution within a composition range favorable for subsequent chelation reactions. The addition of a two-component chelating agent composed of a polycarboxylic acid chelating agent and an organophosphonic acid compound at medium temperature enables rapid complexation and passivation of metal ions in the alcoholysis solution. Subsequently, a fixed-bed configuration ensures sufficient contact between the alcoholysis solution and the supported chelating agent particles, and solid-liquid separation removes the metal complexes from the system in solid form, further reducing metal residue. The second-stage thin-film evaporation further enriches the system composition and suppresses the presence of unreacted carboxyl groups, thereby improving stability under high-temperature distillation conditions. Ultimately, high-purity BHET fractions are obtained through short-path distillation, avoiding common problems in traditional processes such as repolymerization, coking, and increased color.

[0033] By combining the above steps, this invention can significantly suppress side reactions of metal catalysts during high-temperature evaporation, resulting in a significantly higher yield and purity of BHET compared to traditional methods. It also reduces the impact of oligomers, residual ethylene glycol, and metal impurities on product performance. The entire process operates stably, is less prone to evaporator scaling or metal deposition, extends equipment cleaning cycles, and is suitable for continuous and industrial-scale production. In summary, the process of this invention achieves excellent results in metal ion removal efficiency, BHET purity improvement, color control, and stable equipment operation, demonstrating significant industrial application value. Attached Figure Description

[0034] Figure 1 This is the liquid chromatogram of the final product of Example 1.

[0035] Figure 2 The original ICP-OES emission spectrum of the metal residue in the final product of Example 1 is shown. Detailed Implementation

[0036] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0037] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0040] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0041] Example 1

[0042] This embodiment provides a method for improving the yield and purity of regenerated BHET.

[0043] The waste polyester raw material used in this embodiment is PET fragments derived from recycled polyester bottle flakes, with a particle size of 5-10 mm and a moisture content of less than 0.2 wt%.

[0044] Ethylene glycol is industrial grade (purity ≥99.5 wt%).

[0045] The alcoholysis catalyst was a cobalt glycolate solution (10 wt% by mass, based on metallic cobalt).

[0046] The polycarboxylic acid chelating agent used is disodium ethylenediaminetetraacetate (analytical grade);

[0047] The organophosphonic acid compound selected was a tetrasodium aminotrimethylenephosphonate solution (solid content 40 wt%).

[0048] The supported chelating agent is EDTA-functionalized silica particles with a BET specific surface area of ​​approximately 150 m² / g and an average particle size of approximately 500 μm.

[0049] The specific steps of the preparation method of regenerated BHET in this embodiment are as follows:

[0050] S1: Alcohololysis reaction

[0051] Add 100 kg of waste polyester bottle flakes and 250 kg of ethylene glycol to a stainless steel reactor. Start mechanical stirring (80 rpm), purge the air with nitrogen, and then add 3 kg of cobalt glycolate solution (0.3 wt% of PET by weight of metallic cobalt). Heat to 200 °C and maintain for 3 h to allow the waste polyester to fully alcoholyze.

[0052] After the reaction was completed, the solution was cooled and diluted, and the BHET mass fraction was found to be approximately 82 wt%, along with a certain amount of oligomers and unreacted ethylene glycol.

[0053] S2: First-stage reduced-pressure evaporation

[0054] The alcoholysis solution was heated to 190 °C in a reactor, and the vacuum system was adjusted to maintain the absolute pressure inside the reactor at 10 kPa. Ethylene glycol was continuously recovered through a top condenser. Vacuum evaporation was continued for 1.5 h, reducing the mass ratio of ethylene glycol to BHET in the system to approximately 2:1, corresponding to the removal of approximately 65% ​​of the initial ethylene glycol. After evaporation, the alcoholysis solution was cooled to 120 °C for later use.

[0055] S3: Add a metal chelating agent and stir to react.

[0056] When the temperature of the alcoholysis solution drops to 120 °C, a pre-prepared metal chelating agent mixture is added to the reactor. This mixture consists of disodium ethylenediaminetetraacetate (EDTA) and tetrasodium aminotrimethylenephosphonate (MTP) in a molar ratio of 1:1, with the amount of disodium EDTA being 0.3 wt% of the waste polyester mass and the amount of MTP being 0.2 wt% of the waste polyester mass. After addition, the mixture is stirred at 120 °C for 0.5 h to allow the metal ions to fully complex with the chelating agent. At this point, the mass ratio of ethylene glycol to BHET in the system remains approximately 2:1.

[0057] S4: Immobilized chelating agent contact treatment

[0058] The alcoholysis solution was introduced from the bottom of the reactor to a fixed-bed contactor filled with 15 kg of EDTA-functionalized silica particles. A circulation pump was started, circulating the alcoholysis solution through the fixed bed at approximately 300 L / h at 120 °C. The empty column linear velocity was approximately 0.3 m / h, and the average residence time per pass through the bed was approximately 15 min. After 0.5 h of continuous circulation contact, all the material at the bottom of the bed was returned to the reactor, and any small entrained particles were removed through a filter in the pipeline. ICP-OES analysis showed that the residual cobalt ion content in the alcoholysis solution was below 5 ppm at this point.

[0059] S5: Second-stage film evaporation

[0060] The alcoholysis solution processed in step S4 was fed into a thin-film evaporator. The evaporator jacket temperature was set to 160 °C, the absolute pressure in the evaporation zone was controlled at 1.0 kPa, and the scraper speed was adjusted to 250 rpm to ensure that the material formed a uniform film on the inner wall and flowed downwards along the wall. After 1 hour of film evaporation, the free ethylene glycol mass fraction in the material decreased to approximately 2 wt%, the BHET mass fraction was approximately 80 wt%, and the acid value was 0.8 mg KOH / g, yielding a BHET-enriched alcoholysis solution.

[0061] S6: Short-path distillation refining BHET

[0062] The BHET-enriched alcoholysis solution was fed into a short-path still. The still jacket temperature was set to 210 °C, the internal condenser temperature was controlled at 120 °C, and the absolute pressure in the distillation space was approximately 0.2 kPa. The scraping film system was activated to allow the material to form a uniform thin film for evaporation. Collection was initiated when the BHET mass fraction in the BHET-enriched alcoholysis solution was ≥70 wt% and the acid value was ≤1.0 mgKOH / g. The distillate obtained after distillation and condensation was collected as the BHET product. Collection was stopped when the BHET mass fraction in the distillate dropped below 92 wt%. After 2 hours of distillation, approximately 70 kg of BHET product was obtained.

[0063] Combination Figure 1 and Figure 2 High performance liquid chromatography analysis showed that the BHET mass fraction was 96.8 wt%, oligomers were about 2.5 wt%, and free ethylene glycol was about 0.7 wt%. ICP-OES analysis showed that the total metal content was not higher than 3 ppm. The acid value was 0.5 mg KOH / g, and the product was a white crystalline solid.

[0064] Example 2

[0065] This embodiment follows the same overall process as Example 1, and uses the same experimental materials and equipment. The only difference is that the total amount of chelating agent is increased to 2.5 wt% of the waste PET mass. The chelating agent solution is prepared as follows:

[0066] Disodium ethylenediaminetetraacetate: 1.8 wt%

[0067] Tetrasodium aminotrimethylenephosphonate: 0.7 wt%

[0068] The molar ratio between the two remains approximately 1:1.

[0069] In this embodiment, approximately 73 kg of BHET was produced. High performance liquid chromatography analysis showed that the BHET mass fraction was 97.3 wt%, the oligomer content was approximately 2.0 wt%, and the residual ethylene glycol was approximately 0.7 wt%. ICP-OES analysis showed that the total metal content was ≤2 ppm and the acid value was 0.45 mgKOH / g.

[0070] Example 3

[0071] This embodiment is consistent with Embodiment 1 in overall process, and the experimental materials and equipment are the same as in Embodiment 1. The only difference is that the total amount of chelating agent is reduced to 0.05 wt% of the waste PET mass, wherein:

[0072] Disodium ethylenediaminetetraacetate: 0.03 wt%

[0073] Tetrasodium aminotrimethylenephosphonate: 0.02 wt%

[0074] The molar ratio between the two remains approximately 1:1.

[0075] This embodiment ultimately yielded approximately 64 kg of BHET product. High-performance liquid chromatography (HPLC) analysis showed a BHET mass fraction of 94.1 wt%, an oligomer content of approximately 4.1 wt%, and residual ethylene glycol of approximately 1.0 wt%. ICP-OES analysis revealed a total metal content of approximately 15 ppm, and an acid value of 1.0 mg KOH / g. Compared to Example 1, the product exhibited a significantly darker color, decreased purity, and a significantly increased residual metal content.

[0076] Example 4

[0077] The overall process of this embodiment is the same as that of Embodiment 1, and the experimental materials and equipment are the same as those of Embodiment 1. The only difference is the ratio between the components in the metal chelating agent, as follows:

[0078] The total amount of chelating agent used is 0.5 wt% of the waste PET mass, of which:

[0079] Disodium ethylenediaminetetraacetate: 0.33 wt%

[0080] Tetrasodium aminotrimethylenephosphonate: 0.17 wt%

[0081] The molar ratio between the two remains approximately 2:1.

[0082] In this embodiment, approximately 69 kg of BHET product was finally obtained. High performance liquid chromatography analysis showed that the mass fraction of BHET was 95.8 wt%, oligomers were approximately 3.2 wt%, and residual ethylene glycol was 0.9 wt%. ICP-OES analysis showed that the total metal content was approximately 6 ppm, the acid value was approximately 0.8 mgKOH / g, and the product was slightly yellow.

[0083] Example 5

[0084] This embodiment is consistent with Embodiment 1 in overall process, and the experimental materials and equipment are the same as in Embodiment 1. The only difference is that in step S4, the BET specific surface area of ​​the immobilized chelating agent is controlled to be approximately 300 m². 2 / g.

[0085] In this embodiment, approximately 74 kg of BHET product was obtained. High-performance liquid chromatography (HPLC) analysis showed that the BHET mass fraction was 97.5 wt%, the oligomer content was approximately 1.9 wt%, and the residual ethylene glycol was 0.6 wt%. ICP-OES analysis showed that the total metal content was ≤1 ppm and the acid value was approximately 0.45 mg KOH / g.

[0086] The product crystals are white, and their color is significantly better than that of Example 1.

[0087] Example 6

[0088] This embodiment is consistent with the overall process of embodiment 1, and the experimental materials and equipment are the same as those in embodiment 1. The only difference is that the particle size of the immobilized chelating agent is controlled to be about 2000 μm in step S4.

[0089] Approximately 66 kg of BHET product was ultimately obtained. High-performance liquid chromatography (HPLC) analysis showed that the BHET mass fraction was 94.7 wt%, the oligomer content was approximately 3.7 wt%, and the residual ethylene glycol was approximately 1.0 wt%. ICP-OES analysis revealed a total metal content of approximately 9 ppm and an acid value of approximately 0.9 mg KOH / g.

[0090] The product was light yellow but darker than usual, and was significantly less desirable than in Examples 1, 2, and 5.

[0091] Example 7

[0092] This embodiment is consistent with the overall process of embodiment 1, and the experimental materials and equipment are the same as those in embodiment 1. The only difference is that the contact time of the fixed bed of the immobilized chelating agent in step S4 is extended from 0.5 h to 1 h.

[0093] This embodiment ultimately yielded approximately 75 kg of BHET product. High-performance liquid chromatography (HPLC) analysis revealed the following: BHET mass fraction: 97.8 wt%, oligomer content: approximately 1.8 wt%, residual ethylene glycol: approximately 0.6 wt%. ICP-OES analysis showed: total metal residue: ≤1 ppm, acid value: 0.45 mg KOH / g, product color: bright white with no obvious yellowing.

[0094] Example 8

[0095] This embodiment is consistent with the overall process of embodiment 1. The experimental materials and equipment are the same as those in embodiment 1. The only difference is that the alcoholysis solution in stage S2 is cooled to 140°C for later use, and a metal chelating agent is added at 140°C in stage S3.

[0096] Approximately 65 kg of BHET product was ultimately obtained. High-performance liquid chromatography (HPLC) analysis revealed the following BHET mass fraction: 94.5 wt%, oligomers: approximately 3.8 wt%, and residual ethylene glycol: approximately 1.1 wt%. ICP-OES analysis showed a total metal content of approximately 8 ppm, an acid value of approximately 0.9 mg KOH / g, and a slightly dark, yellowish color.

[0097] The purity and color of the product are inferior to those of Examples 1, 2, 5, and 7.

[0098] Comparative Example 1

[0099] This comparative example follows the same overall process as Example 1, and uses the same experimental materials and equipment. The only difference is that a fixed-load chelating agent is not used, and the fixed-bed contactor step is omitted. The specific steps are as follows:

[0100] S1: Alcohololysis reaction

[0101] 100 kg of waste polyester and 250 kg of ethylene glycol were added to a 500 L reactor. Stirring was started (80 rpm), and nitrogen was introduced to purge the air. Then, 3 kg of cobalt glycolate solution (0.3 wt% of the waste PET mass based on metallic cobalt) was added. The temperature was raised to 200 °C and maintained for 3 h to allow complete alcoholysis of the waste polyester. The BHET mass fraction in the resulting alcoholysis solution was approximately 81 wt%.

[0102] S2: First-stage reduced-pressure evaporation

[0103] The alcoholysis solution was heated to 190 °C, and the vacuum system was adjusted to maintain the absolute pressure at 10 kPa. The removed ethylene glycol was continuously recovered through a condenser. Evaporation was carried out for approximately 1.5 h to reduce the mass ratio of ethylene glycol to BHET in the alcoholysis solution to approximately 2:1, removing approximately 63% of the ethylene glycol. After completion, the alcoholysis solution was cooled to 120 °C for later use.

[0104] S3: Add a small molecule chelating agent and stir to react.

[0105] Prepare a double chelating agent mixture, the total amount of which is 0.5 wt% based on the mass of waste PET, of which:

[0106] Disodium ethylenediaminetetraacetate: 0.3 wt%

[0107] Tetrasodium aminotrimethylenephosphonate: 0.2 wt%

[0108] The molar ratio is approximately 1:1.

[0109] When the alcoholysis solution is cooled to 120 °C, all of the above-mentioned small molecule chelating agent is added to the reactor. Stirring is maintained and the temperature is kept at 120 °C for 0.5 h to allow for sufficient complexation of the metal ions in the system. The mass ratio of ethylene glycol to BHET in the system is maintained at approximately 2.0:1.

[0110] S4: Second-stage film evaporation

[0111] The alcoholysis solution was directly fed into a thin-film evaporator, with the jacket temperature set at 160 ℃, the absolute pressure in the evaporation zone at 1.0 kPa, and the scraper speed at 250 rpm, and evaporation was maintained for 1 h. At the end of the thin-film evaporation, the free ethylene glycol content in the material was approximately 3 wt%, the BHET content was approximately 77 wt%, and the acid value was approximately 1.3 mgKOH / g.

[0112] Because the metal complex was not removed, a slight increase in color intensity occurred during the film evaporation process.

[0113] S5: Short-path distillation

[0114] The BHET-enriched alcoholysis solution was added to a short-path distillation apparatus. The jacket temperature was set to 210 °C, the internal condenser temperature to 120 °C, and the distillation absolute pressure to 0.2 kPa. Distillation was initiated using a scraper. The conditions for starting fraction collection were a BHET mass fraction ≥ 70 wt% in the feed and an acid value ≤ 1.3 mg KOH / g. Collection was stopped when the BHET mass fraction in the distillate decreased to below 92 wt%.

[0115] Approximately 58 kg of BHET product was ultimately obtained. High-performance liquid chromatography (HPLC) analysis revealed the following: BHET mass fraction: 92.8 wt%; oligomer content: approximately 5.4 wt%; residual ethylene glycol: approximately 1.3 wt%. ICP-OES analysis showed: total metal content: approximately 26 ppm; acid value: 1.1 mgKOH / g; color significantly darker, ranging from light yellow to yellowish-brown.

[0116] Compared with the products in the example series, the metal residue, oligomer content and acid value of this comparative example are significantly higher, indicating a clear deterioration in quality.

[0117] Comparative Example 2

[0118] The overall process of this comparative example is the same as that of Example 1. The experimental materials and equipment are the same as those of Example 1. The only difference is that the two-component small molecule chelating agent is not added and step S3 is not performed, which limits the efficiency.

[0119] The final product yielded approximately 60 kg of BHET. High-performance liquid chromatography (HPLC) analysis showed that the BHET mass fraction was 93.1 wt%, the oligomer content was approximately 5.0 wt%, and the residual ethylene glycol was approximately 1.2 wt%. ICP-OES analysis revealed a total metal content of approximately 20 ppm, an acid value of 1.0 mg KOH / g, and a significantly darker color, ranging from yellow to deep yellow.

[0120] Comparative Example 3

[0121] The overall process of this comparative example is the same as that of Example 1. The experimental materials and equipment are the same as those of Example 1. The only difference is that the first stage of step S2, the removal of ethylene glycol, is not performed. After the alcoholysis reaction, the chelation step is performed directly.

[0122] The final yield was approximately 57 kg of BHET products.

[0123] High performance liquid chromatography (HPLC) analysis revealed the following: BHET mass fraction: 93.5 wt%, oligomers: approximately 4.9 wt%, residual ethylene glycol: approximately 1.4 wt%. ICP-OES analysis showed the following: total metal content: approximately 18 ppm, acid value: approximately 1.1 mgKOH / g, and a darker, yellowish-brown color.

[0124] Compared to the previous example, the product quality has significantly declined.

[0125] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the yield and purity of regenerated BHET, characterized in that, Includes the following steps: S1: Waste polyester and ethylene glycol are subjected to alcoholysis reaction in the presence of cobalt glycol catalyst to obtain alcoholysis liquid; the waste polyester raw material is PET fragments derived from recycled polyester bottle flakes; S2: The alcoholysis solution is subjected to first-stage reduced pressure evaporation at 180-220℃ and 0.5-30 kPa to remove most of the free ethylene glycol in the alcoholysis solution, resulting in an alcoholysis solution with partial ethylene glycol removal. At this time, the mass ratio of ethylene glycol to BHET in the alcoholysis solution is 1.5-4:

1. S3: Cool the alcoholysis solution obtained in step S2 to 100-140°C, add a metal chelating agent to the alcoholysis solution within this temperature range, and stir to react, so that the metal chelating agent complexes with the metal ions in the alcoholysis solution; the metal chelating agent is composed of disodium ethylenediaminetetraacetate and tetrasodium aminotrimethylenephosphonate, and the molar ratio of disodium ethylenediaminetetraacetate to tetrasodium aminotrimethylenephosphonate is 0.7-2:1; S4: The alcoholysis solution obtained in step S3 is treated with a supported chelating agent, wherein the supported chelating agent is EDTA-functionalized silica particles, the BET specific surface area of ​​the supported chelating agent is 30-300 m² / g, and the average particle size is 50-2000 μm. The supported chelating agent is packed in the contactor in granular form. The alcoholysis solution is circulated through the contactor at 100-140 °C for 0.2-1 h, and the supported chelating agent is removed by solid-liquid separation to obtain an alcoholysis solution with metal removal treatment. S5: The alcoholysis solution obtained in step S4 is subjected to a second stage of reduced pressure evaporation through a thin-film evaporator at 140-190 °C and 0.2-5 kPa to remove the remaining free ethylene glycol, so that the mass fraction of free ethylene glycol in the alcoholysis solution is reduced to below 5 wt%, and BHET enriched alcoholysis solution is obtained. S6: The BHET enriched alcoholysis solution obtained in step S5 is subjected to short-path distillation in a short-path still at 190–230 °C and 0.05–1 kPa, and the distillate is collected as the BHET product.

2. The method for improving the yield and purity of regenerated BHET according to claim 1, characterized in that, The total amount of disodium ethylenediaminetetraacetate and tetrasodium aminotrimethylenephosphonate in the metal chelating agent is 0.05 to 3 wt% of the waste PET mass.

3. The method for improving the yield and purity of regenerated BHET according to claim 1, characterized in that, After adding the metal chelating agent in step S3, the mass ratio of ethylene glycol to BHET in the alcoholysis solution is maintained at 1.5:1 to 3:

1. This mass ratio is controlled by adding ethylene glycol or adjusting the degree of depressurization.

4. The method for improving the yield and purity of regenerated BHET according to claim 1, characterized in that, When the film evaporation is completed in step S5, the mass fraction of BHET in the alcoholysis solution is 70-90 wt%, and the acid value is less than or equal to 1.0 mgKOH / g.

5. The method for improving the yield and purity of regenerated BHET according to claim 1, characterized in that, In step S6, during the short-path distillation process, the switching condition for starting to collect the distillate BHET product is that the mass fraction of BHET in the BHET enrichment alcoholysis solution reaches not less than 70 wt% and the acid value is not higher than 1.0 mgKOH / g. The switching condition for stopping the collection of the distillate BHET product is that the mass fraction of BHET in the distillate drops to less than 92 wt%.

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