Method for purifying monomer obtained by depolymerizing waste polyester through ethylene glycol

Abstract

The invention relates to a method for purifying a monomer bis (2-hydroxyethyl) terephthalate (BHET) obtained by depolymerizing waste polyester through ethylene glycol (EG), and belongs to the technical field of polyester recycling. The BHET microwave radiation induced film evaporation separation and molecular distillation refining continuous process provided by the invention solves the problems of long process flow, poor continuity, more side reactions and low purity and yield in the direct separation and refining process of BHET in alcoholysis liquid based on a gas-liquid equilibrium separation method at present. The whole technological parameter requirement is mild, the process is short, continuity is good, the BHET purity of the product is larger than or equal to 99.0 wt%, the hue is good, and the BHET yield is larger than or equal to 80.0%. And meanwhile, the method has the advantages that EG is conveniently recycled in a system, the production process is cleaner and more environment-friendly, and the comprehensive efficiency advantage is obvious.

Description

Technical Field

[0001] The present invention relates to a method for purifying monomers obtained by glycolysis of waste polyester, belonging to the technical field of polyester recycling and reuse. Background Art

[0002] Polyethylene terephthalate (PET) is the dominant variety of polyester polymers, with an annual waste volume of over 20 million tons. Its recycling technology has always been the focus of the industry. Among them, glycolysis is the most widely used in the chemical recycling technology of PET. After the monomers bis(2-hydroxyethyl) terephthalate (BHET) obtained by glycolysis are purified, they can be used as standard raw materials or synthetic intermediates for the synthesis of PET and related structural polymers to achieve recycling. Therefore, the purification technology of monomers obtained by glycolysis of waste polyester has attracted much attention in the industry.

[0003] When the EG glycolysis of waste PET is completed, the composition of the system components includes: EG, BHET, oligomers (mainly dimers) inevitably formed due to the glycolysis equilibrium, the side reaction product of EG self-polymerization, diethylene glycol (DEG), coloring and modifier fragments (mainly small molecules containing phthalocyanine, azo, and anthraquinone structures), anions and cations (mainly all catalysts for polymerization and glycolysis), and filterable impurities (inorganic powders, metal oxides, and other miscellaneous polymers that do not undergo glycolysis). It can be seen that the composition of this system is complex, and the purification of the monomer BHET obtained by EG glycolysis of PET is to separate and refine BHET in the above-mentioned system. Around this purpose, the currently published patented technologies have solved some problems to a certain extent from different angles and different focuses, which are respectively: The separation and refinement of BHET mentioned in the invention patents US4609608, US6630601, and CN104710601B are mainly achieved based on the control of the solubility of BHET. That is, after the BHET in the system is cooled and crystallized for rough separation, water is introduced for recrystallization refinement. Although this method can improve the purity of BHET, the temperature required for the aqueous phase recrystallization of BHET is much lower than room temperature, the process takes a long time and consumes a lot of energy, and it is also difficult to recycle the solvent. Moreover, for systems with high contents of coloring and modifier fragments and anion and cation impurities, it is very difficult to obtain BHET with ideal purity only through aqueous phase recrystallization, even if it is repeated many times.

[0004] In the invention patent WO2002 / 010117, filterable impurities are first removed by filtration, and then activated carbon is used to adsorb and remove coloring and modifier fragments in the system. Ion exchange resins are used to remove anionic and cationic impurities in the system. After that, the temperature of BHET in the system is lowered for crystallization and separation to obtain crude BHET. For the crude BHET, a three-stage step-by-step refining process is adopted, that is, first, part of the EG is removed by vacuum distillation, then the low-boiling substances are more thoroughly removed by thin-film evaporation under a higher vacuum state, and finally, BHET is collected by high-vacuum molecular distillation condensation to complete the refining. The three-stage step-by-step refining process can effectively inhibit the occurrence of side reactions, and this method has good effects in terms of BHET yield, BHET purity and chromaticity. However, the overall process is too long, the efficiency is low, the overall optimization control complexity is high, and it is difficult to recycle EG.

[0005] In order to further inhibit side reactions, based on WO2002 / 010117, in the three-stage step-by-step refining link of the invention patent WO2003 / 101929, the evaporation process is further classified, and the control of the ratio of distillate to kettle residue is also increased in the molecular distillation link. The BHET purity can reach 99.0%, and the BHET yield is about 75%. However, the disadvantages of too long overall process, low efficiency, high control complexity and difficult EG recycling still exist.

[0006] Based on WO2003 / 101929, in the invention patent WO2022 / 003990, for the removal of nitrogen-containing disperse dyes, ethylene glycol monoethers or diethers with 4-12 carbon atoms are introduced for extraction and decolorization. Although the homogeneous phase can improve the decolorization efficiency, the decolorizing agents with such chemical structures will inevitably increase the separation difficulty from the alcoholysis agent ethylene glycol, which will hinder the recycling of ethylene glycol and decolorizing agents in the system. At the same time, the problems of long overall process and low efficiency still exist.

[0007] Compared with WO2002 / 010117, WO2003 / 101929, and WO2022 / 003990, the invention patent CN1135846441B adopts a more general purification route to improve efficiency. The purification of crude BHET is divided into two steps: removing low-boiling substances under low vacuum and collecting BHET by high-temperature flash evaporation and condensation under high vacuum. However, this method does not mention the control of side reactions during the process and the actual purity of BHET, and only shows that it can be used for spinning polymerization production. Similarly, the invention patent CN107189044A only mentions rectification for the refining of BHET and then directly polymerizes, but it also points out that a toner needs to be added. Therefore, it can be judged that the efficiency improvement obtained by such streamlined measures comes at the cost of quality, and there are still difficulties in inhibiting side reactions.

[0008] Patent inventions CN115894223A and CN116284710 propose a method to create the sublimation conditions of BHET crystals for purification. Although the process flow of this method is short, due to the strong intermolecular hydrogen bonding of BHET molecules, its saturated vapor pressure is very low. Therefore, the process window for the sublimation purification of BHET in a strict thermodynamic sense is very narrow, and it is difficult to achieve large-scale production. For large-scale sublimation purification, a more practical operation route is to first convert crude BHET into dimethyl terephthalate (DMT) through transesterification reaction with methanol. Because DMT has no intermolecular hydrogen bonding and its saturated vapor pressure is much higher than that of BHET, large-scale purification can be relatively smoothly completed. Then, the purified DMT is reconverted into BHET through transesterification reaction with ethylene glycol, and then polymerized to prepare recycled PET. Patent inventions WO / 2003 / 033581A1, CN109503818A, CN110964188B, CN114437328A, CN114044892B, and CN114149574B all contain monomer separation and purification technologies developed based on this idea. However, these methods also have a most significant common defect, that is, two additional transesterification reactions are introduced. This not only increases the use of catalysts, but also involves the management and reuse of highly flammable and volatile methanol during the process. The process flow is long and the production cost is high. Summary of the Invention

[0009] The purpose of the present invention is to provide a purification method for the monomer BHET obtained by glycolysis of waste polyester, aiming at the deficiencies of the above-mentioned existing technologies. Through the continuous process of microwave radiation-induced thin-film evaporation separation and molecular distillation purification of BHET, this method can solve the problems of long process flow, many side reactions, low purity and yield in the direct separation and purification of BHET in the alcoholysis solution based on the gas-liquid equilibrium separation method. It can significantly improve the production efficiency and quality of purified BHET, and at the same time has the advantages of easy recycling of EG in the system, short production process flow, and low energy consumption.

[0010] The present invention realizes the above purpose through the following technical solutions: A purification method for monomers obtained by glycolysis of waste polyester, characterized by comprising the following steps: 1) Uniformly mix waste polyester materials, EG, catalysts, stabilizers, and activated carbon in a certain proportion, then feed them into an alcoholysis device. Under the protection of inert gas, stir evenly at a temperature of 185 - 200 °C and a pressure of 100 - 200 kPa for 2 - 5 h for the alcoholysis reaction. Then, at a temperature of 60 - 100 °C, filter to remove activated carbon and other filterable impurities, and maintain the filtrate at 60 - 100 °C. Flow through cation exchange resin and anion exchange resin in sequence for ion reduction until the total concentration of cations and anions ≤ 50 ppm. The resulting liquid is denoted as the alcoholysis solution.

[0011] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 110 - 120 °C, transfer it into a wiped film evaporator with microwave radiation, and conduct microwave radiation-induced thin film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET; then control the air pressure in the thin film evaporation chamber to be 100 - 300 Pa through a vacuum system, control the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 20 - 100 s by the feed flow rate and the scraper rotation speed, and control the temperature of the completed solution at the concentration outlet to be maintained at 170 - 180 °C through the microwave power to obtain a concentrated completed solution, denoted as the separation solution; at the same time, condense and collect the low-boiling substances removed in this step, transfer the low-boiling substances to a conventional distillation separation device, and separate EG with a purity ≥ 99.0 wt% from the conventional distillation separation device. The obtained EG is denoted as recycled EG; it can be realized that the total content of EG and DEG in the concentrated completed solution is ≤ 0.2 wt%.

[0012] 3) Transfer the separation solution obtained in step 2) into a molecular distillation device. Under the conditions that the heating surface temperature is 205 - 210 °C, the pressure is ≤ 25 Pa, and the condensing surface temperature is 110 - 120 °C, by controlling the residence time of the separation solution on the heating surface, make the mass ratio of the distillate to the residue maintain at 5:5 - 6:4; the main component of the obtained residue is oligomer, denoted as alcoholysis refined residue. After collecting the alcoholysis refined residue, transfer it into the alcoholysis device in step 1) to participate in the depolymerization reaction of the next production cycle; the obtained distillate is refined BHET. Cool the refined BHET solution to room temperature and solidify it under dry inert gas, and crush it into powder form, which is the obtained refined BHET product.

[0013] As described above, a method for purifying monomers obtained by glycolysis of waste polyester. In step 1), the waste polyester material is a used PET product and / or production waste with a PET component content ≥ 85 wt%, preferably a used PET product and / or production waste with a PET component content of 95 wt%; the EG is a mixture of polyester polymerization-grade EG and the recycled EG obtained in step 2) of the present invention in a ratio of 1:0.1 - 1:1; the catalyst is one of sodium carbonate, zinc acetate, tetra-n-butyl titanate, and tetra-isopropyl titanate; the stabilizer is one of sodium acetate and potassium acetate; in step 1), according to a certain ratio, based on the mass of PET, the addition amount of EG is 200 - 400 wt% of the mass of PET, the addition amount of the catalyst is 0.5 - 1.5 wt% of the mass of PET, the addition amount of the stabilizer is 0.5 - 5 × 10 -2 wt%, and the addition amount of activated carbon is 1 - 5 wt% of the mass of PET.

[0014] As described above, a method for purifying monomers obtained by glycolysis of waste polyester. The structural characteristics of the scraper thin-film evaporator with microwave radiation in step 2) are as follows: Microwaves are generated by a microwave generator with adjustable power and transmitted to the evaporator cavity through a waveguide connected to the inner wall of the evaporator. The materials of the inner wall of the evaporator and the rotor scraper need to be microwave-transparent materials. The gap between the rotor scraper and the inner wall of the evaporator is 0.5 - 2 mm. Microwave reflection blocking and heat insulation treatments are applied to each interface on the inner wall and the outer wall. The evaporation gas condensation system and the concentrated liquid collection system connected through the interfaces are both located outside the cavity where the liquid film is formed and are not affected by microwave radiation.

[0015] As described above, a method for purifying monomers obtained by glycolysis of waste polyester. The calculation of the BHET yield in step (3) is obtained from the production data by formula 1 after the process runs stably and experiences three complete cycles.

[0016] 。

[0017] The purification principle and substantial features of the present invention will be described below: A method for purifying BHET monomers obtained by EG depolymerization of waste polyester provided by the present invention. Its purification principle and substantial features are mainly reflected as follows: As described above, in the purification method of monomers obtained by glycolysis of waste polyester, the microwave radiation-induced thin-film evaporation treatment described in step 2) is the core of the present invention. The theoretical basis for the enhanced separation under microwave radiation is as follows: i) Molecules with different polarities have different microwave absorption capabilities and kinetic energy conversion characteristics. Therefore, in a certain microwave radiation field, especially in the thin-film evaporation mode where diffusion limitations can be ignored, the separation characteristics differences between different polar molecules can be significantly amplified, effectively suppressing separation entrainment and improving separation efficiency at the same time; ii) Under the action of microwave radiation, the relaxation and breakage of intermolecular hydrogen bonds will be intensified, weakening the separation entrainment and side reactions caused by intermolecular hydrogen bonds, and improving the separation accuracy. Based on this principle and combined with systematic practice, it is determined that by only undergoing the said step 2), the glycolysis solution can be concentrated to a total content of EG and DEG ≤ 0.2 wt%. Moreover, due to the weakening of intermolecular hydrogen bond interaction and the extremely short residence time of high-temperature treatment, side reactions in both the gas phase and the liquid phase can be significantly suppressed. Therefore, there is no need to undergo the complex and time-consuming steps such as cooling crystallization and multi-stage evaporation and gradual concentration of crude BHET as mentioned in the above background technology to avoid problems such as evaporation separation entrainment and side reactions. Then, on this basis, by undergoing molecular distillation in step (3) of the present invention, a BHET purity ≥ 99.0 wt%, good hue, and a BHET yield ≥ 80.0% can be finally achieved, with a shorter and more efficient process compared to the background technology. At the same time, due to the concentrated distillation of low-boiling substances and small side reactions during the process, it is easier to recycle EG in the system compared to the background technology, with less raw material consumption. In addition, transferring the glycolysis refined residue generated in step 3) into the glycolysis device to participate in the depolymerization reaction of the next production cycle has two advantages. On the one hand, due to the principle of similar compatibility, the addition of the glycolysis refined residue can improve the solubility of waste polyester materials in the depolymerization system, enabling the reaction to enter the homogeneous state more quickly, significantly shortening the reaction time required for depolymerization in step 1), reducing side reactions, and at the same time effectively improving the overall BHET yield.

[0018] As can also be seen from the above background technology, in the disclosed methods related to the purification of monomers obtained by glycolysis of waste polyester with ethylene glycol, no technologies and processes related to microwave radiation-induced separation are mentioned. In addition, among the related polyester glycolysis methods, only patents CN108602974B and CN112940344A mention the use of microwave radiation means, and it is clearly pointed out that microwave radiation only acts on the depolymerization process, and the goal is only to speed up the depolymerization process, without any microwave-related operations in the depolymerized monomer purification link, which is essentially different from the technical solution of the present invention. Therefore, there is no technical inspiration for the technical solution of the present invention in the prior art.

[0019] The beneficial effects of the present invention are: (1) The continuous process for the separation of BHET by microwave radiation-induced thin-film evaporation and molecular distillation refining provided by the present invention can solve the problems of long process flow, poor continuity, many side reactions, low purity and low yield in the direct separation and refining of BHET in the alcoholysis solution based on the gas-liquid equilibrium separation method. The purity of the product BHET is ≥99.0 wt%, the hue is good, the yield of BHET is ≥80.0%, and the production process is significantly shorter than the prior art. It can significantly improve the production efficiency and reduce the energy consumption while ensuring the high-quality output of the product.

[0020] (2) The method for the purification and refining of BHET provided by the present invention is easier to achieve the efficient and sufficient reuse of EG and oligomer residues during the production process compared with the prior art, can significantly reduce the raw material and waste treatment costs, and the production process is cleaner and more environmentally friendly.

[0021] (3) The method for the purification and refining of BHET provided by the present invention has mild overall process parameter requirements, a short and continuous process, is more easily adaptable to the quality fluctuations of waste polyester raw materials, the investment cost of production equipment is moderate, the comprehensive efficiency advantage is obvious, and the industrial application prospect is good. Detailed implementation manners

[0022] The technical solutions of the present invention will be further described below in conjunction with specific embodiments. Example 1

[0023] 1) After uniformly mixing waste polyester materials, EG, catalyst, stabilizer and activated carbon in a certain proportion, they are fed into an alcoholysis device. Under the protection of nitrogen gas at a temperature of 195 °C and a pressure of 110 kPa, they are evenly stirred and reacted for 3.5 h. Then, at a temperature of 80 °C, the activated carbon and other filterable impurities are removed by filtration, and the filtrate is maintained at 80 °C and flows through a cation exchange resin and an anion exchange resin in sequence for ion removal. The total concentration of cations and anions in the obtained liquid is 22 ppm, which is recorded as the alcoholysis solution; Among them, the waste polyester material is a film dense material with a PET component content of 98.4 wt%; the EG is a mixture of polyester polymerization-grade EG and the EG recovered in step 2) at a ratio of 1:0.5; the catalyst is sodium carbonate; the stabilizer is sodium acetate; the ratio of the waste polyester material, EG, catalyst, stabilizer and activated carbon is based on the mass of PET, the addition amount of EG is 300 wt% of PET, the addition amount of the catalyst is 1.0 wt% of PET, the addition amount of the stabilizer is 1×10 -2 wt% of PET, and the addition amount of activated carbon is 3 wt% of PET.

[0024] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 115 °C, transfer it to a wiped film evaporator with microwave radiation, and perform microwave radiation-induced thin film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET. The gap between the rotor scraper and the inner wall of the evaporator is 1 mm. This treatment controls the air pressure in the thin film evaporation cavity to be 150 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 50 s through the feed flow rate and the scraper rotation speed, and controls the temperature of the concentrated product at the outlet to be maintained at 175 °C through the microwave power. The total content of EG and DEG in the concentrated product is 0.14 wt%, which is recorded as the separated liquid; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.1 wt%, which is recorded as the recovered EG.

[0025] 3) Transfer the separated liquid obtained in step 2) to a molecular distillation device. At a heating surface temperature of 205 °C, a pressure of 15 Pa, and a condensing surface temperature of 120 °C, control the residence time of the separated liquid on the heating surface to be 80 s, so that the mass ratio of the distillate to the residue can be stabilized at 6:4; the main component of the obtained residue is oligomer, which is recorded as the alcoholysis refined residue, collected and transferred to the alcoholysis device to participate in the depolymerization reaction in the next production cycle; the obtained distillate is refined BHET. The refined BHET solution is cooled to room temperature and solidified under dry inert gas and crushed into powder to obtain the refined BHET product. The purity of BHET is 99.3 wt%, the hue L / a / b value is 99.04 / -1.53 / 1.78, and the BHET yield is 82.7% Example 2

[0026] 1) Uniformly mix waste polyester materials, EG, catalyst, stabilizer, and activated carbon in a certain proportion, then feed them into the alcoholysis device. Under the protection of nitrogen gas at a temperature of 200 °C and a pressure of 200 kPa, stir and react evenly for 2 h. Then, at 60 °C, filter out the activated carbon and other filterable impurities, and maintain the filtrate at 60 °C. Flow through the cation exchange resin and the anion exchange resin in sequence for ion reduction. The total concentration of cations and anions in the obtained liquid is 50 ppm, which is recorded as the alcoholysis solution.

[0027] Among them, the waste polyester material is a colored cloth foam material with a PET component content of 85.0 wt%; the EG is a mixture of polyester polymerization-grade EG and the recovered EG in step 2) at a ratio of 1:0.1; the catalyst is zinc acetate; the stabilizer is potassium acetate; the certain proportion means that based on the mass of PET, the addition amount of EG is 400 wt%, the addition amount of the catalyst is 1.5 wt%, the addition amount of the stabilizer is 5×10 -2 wt%, and the addition amount of activated carbon is 5 wt%.

[0028] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 110 °C, transfer it to a wiped-film evaporator with microwave radiation, and perform microwave radiation-induced thin-film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET. The gap between the rotor scraper and the inner wall of the evaporator is 2 mm. This treatment controls the air pressure in the thin-film evaporation chamber to be 300 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 100 s through the feed flow rate and the scraper rotation speed, controls the temperature of the concentrated finished solution at the concentration outlet to be maintained at 180 °C through the microwave power. The total content of EG and DEG in the concentrated finished solution is 0.19 wt%, which is recorded as the separated solution; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.0 wt%, which is recorded as the recovered EG.

[0029] 3) Transfer the separated solution obtained in step 2) to a molecular distillation device. Under the conditions that the heating surface temperature is 210 °C, the pressure is 25 Pa, and the condensation surface temperature is 110 °C, control the residence time of the separated solution on the heating surface to be 60 s, so that the mass ratio of the distillate to the residue can be stabilized at 5:5; the main component of the obtained residue is oligomer, which is recorded as the alcoholysis refined residue, collected and transferred to the alcoholysis device to participate in the depolymerization reaction in the next production cycle; the obtained distillate is refined BHET. The refined BHET solution is cooled to room temperature and solidified under dry inert gas and pulverized into powder, and the obtained refined BHET product has a BHET purity of 99.1 wt%, a hue L / a / b value of 99.12 / 1.96 / 1.43, and a BHET yield of 80.5%. Example 3

[0030] 1) Uniformly mix waste polyester materials, EG, catalyst, stabilizer, and activated carbon in a certain proportion, then feed them into an alcoholysis device. Under the protection of nitrogen gas at a temperature of 185 °C and a pressure of 100 kPa, stir and react evenly for 5 h. Then, at 100 °C, filter out the activated carbon and other filterable impurities, and maintain the filtrate at 100 °C. Flow through a cation exchange resin and an anion exchange resin in sequence for ion removal. The total concentration of cations and anions in the obtained liquid is 11 ppm, which is recorded as the alcoholysis solution.

[0031] Among them, the PET component content of the waste polyester material is 98.8 wt% of polyester bottle chips; the EG is a 1:1 mixture of polyester polymerization-grade EG and the recovered EG in step 2); the catalyst is tetrabutyl titanate; the stabilizer is sodium acetate; the so-called "certain proportion" means that based on the mass of PET, the addition amount of EG is 200 wt%, the addition amount of the catalyst is 0.5 wt%, the addition amount of the stabilizer is 0.5×10 -2 wt%, and the addition amount of activated carbon is 1 wt%.

[0032] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 120 °C, transfer it to a wiped film evaporator with microwave radiation, and perform microwave radiation-induced thin film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET. The gap between the rotor scraper and the inner wall of the evaporator is 0.5 mm. This treatment controls the air pressure in the thin film evaporation chamber to be 100 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 20 s through the feed flow rate and the scraper rotation speed, controls the temperature of the concentrated product at the outlet of the concentration to be maintained at 170 °C through the microwave power, and the total content of EG and DEG in the concentrated product is 0.16 wt%, which is recorded as the separated liquid; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.5 wt%, which is recorded as the recovered EG.

[0033] 3) Transfer the separated liquid obtained in step 2) to a molecular distillation device. At a heating surface temperature of 208 °C, a pressure of 20 Pa, and a condensing surface temperature of 115 °C, control the residence time of the separated liquid on the heating surface to be 70 s, so that the mass ratio of the distillate to the residue can be stabilized at 5.5:4.5; the main component of the obtained residue is oligomer, which is recorded as the alcoholysis refined residue, collected and transferred to the alcoholysis device to participate in the depolymerization reaction in the next production cycle; the obtained distillate is refined BHET. The refined BHET liquid is cooled to room temperature and solidified under dry inert gas and crushed into powder, that is, the obtained refined BHET product, the purity of BHET is 99.4 wt%, the hue L / a / b value is 99.13 / -1.09 / 1.25, and the yield of BHET is 83.8%. Example 4

[0034] 1) Uniformly mix waste polyester materials, EG, catalyst, stabilizer, and activated carbon in a certain proportion, feed them into an alcoholysis device, and stir and react uniformly for 2.5 h under the protection of nitrogen gas at a temperature of 198 °C and a pressure of 160 kPa. Then, at 80 °C, filter to remove activated carbon and other filterable impurities, and maintain the filtrate at 80 °C. Flow through a cation exchange resin and an anion exchange resin in sequence for ion removal. The total concentration of cations and anions in the obtained liquid is 22 ppm, which is recorded as the alcoholysis solution.

[0035] Among them, the PET component content of the waste polyester material is 96.3 wt% of colored cloth bubble material; the EG is a mixture of polyester polymerization-grade EG and the recovered EG in step 2) at a ratio of 1:0.3; the catalyst is tetra-isopropyl titanate; the stabilizer is sodium acetate; the so-called certain proportion means that based on the mass of PET, the addition amount of EG is 320 wt%, the addition amount of the catalyst is 1.2 wt%, the addition amount of the stabilizer is 1.5 × 10 -2 wt%, and the addition amount of activated carbon is 4 wt%.

[0036] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 118 °C, transfer it to a wiped-film evaporator with microwave radiation, and perform microwave radiation-induced thin-film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET. The gap between the rotor scraper and the inner wall of the evaporator is 1.5 mm. This treatment controls the air pressure in the thin-film evaporation chamber to be 150 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 85 s through the feed flow rate and the scraper rotation speed, controls the temperature of the completed solution at the concentration outlet to be maintained at 178 °C through the microwave power, and the total content of EG and DEG in the concentrated completed solution is 0.11 wt%, denoted as the separated liquid; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.3 wt%, denoted as the recovered EG.

[0037] 3) Transfer the separated liquid obtained in step 2) to a molecular distillation device. At a heating surface temperature of 207 °C, a pressure of 16 Pa, and a condensing surface temperature of 110 °C, control the residence time of the separated liquid on the heating surface to be 65 s, so that the mass ratio of the distillate to the residue can be stabilized at 5.2:4.8; the main component of the obtained residue is oligomer, denoted as the alcoholysis refined residue, collect it and transfer it to the alcoholysis device to participate in the depolymerization reaction in the next production cycle; the obtained distillate is refined BHET, and the refined BHET solution is cooled to room temperature and solidified and crushed into powder under dry inert gas, and the obtained refined BHET product has a BHET purity of 99.2 wt%, a hue L / a / b value of 99.04 / -1.53 / 1.78, and a BHET yield of 81.7%. Example 5

[0038] 1) Uniformly mix waste polyester materials, EG, catalyst, stabilizer, and activated carbon in a certain proportion, then feed them into the alcoholysis device. Under the protection of nitrogen gas at a temperature of 190 °C and a pressure of 101 kPa, stir and react evenly for 4 h. Then, at 95 °C, filter to remove activated carbon and other filterable impurities, and maintain the filtrate at 95 °C. Flow through cation exchange resin and anion exchange resin in sequence for ion reduction. The total concentration of cations and anions in the obtained liquid is 22 ppm, denoted as the alcoholysis solution.

[0039] Among them, the PET component content of the waste polyester material is 96.5 wt% of the tightly packed belt material; the EG is a mixture of polyester polymerization-grade EG and the recovered EG in step 2) at a ratio of 1:0.2; the catalyst is tetra-n-butyl titanate; the stabilizer is sodium acetate; the so-called certain proportion means that based on the mass of PET, the addition amount of EG is 380 wt%, the addition amount of the catalyst is 0.8 wt%, the addition amount of the stabilizer is 4 × 10 -2 wt%, and the addition amount of activated carbon is 1.5 wt%.

[0040] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 118 °C, transfer it into a wiped film evaporator with microwave radiation, and perform microwave radiation-induced thin film evaporation treatment to remove substances with boiling points lower than BHET in the alcoholysis solution. The gap between the rotor scraper and the inner wall of the evaporator is 1 mm. This treatment controls the air pressure in the thin film evaporation chamber to be 200 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 90 s through the feed flow rate and the scraper rotation speed, and controls the temperature of the completed solution at the concentration outlet to be maintained at 180 °C through the microwave power. The total content of EG and DEG in the concentrated completed solution is 0.09 wt%, which is recorded as the separated solution; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.2 wt%, which is recorded as the recovered EG.

[0041] 3) Transfer the separated solution obtained in step 2) into a molecular distillation device. At a heating surface temperature of 206 °C, a pressure of 18 Pa, and a condensing surface temperature of 112 °C, by controlling the residence time of the separated solution on the heating surface to be 68 s, the mass ratio of the distillate to the residue can be stabilized at 5.3:4.7; the main component of the obtained residue is oligomer, which is recorded as the alcoholysis refined residue, collected and transferred to the alcoholysis device to participate in the depolymerization reaction in the next production cycle; the obtained distillate is refined BHET. The refined BHET solution is cooled to room temperature and solidified under dry inert gas and pulverized into powder form, that is, the obtained refined BHET product, the purity of BHET is 99.4 wt%, the hue L / a / b value is 98.91 / -1.19 / 1.62, and the BHET yield is 82.7%. Example 6

[0042] 1) Uniformly mix waste polyester materials, EG, catalyst, stabilizer, and activated carbon in a certain proportion, then feed them into an alcoholysis device. Under the protection of nitrogen gas at a temperature of 196 °C and a pressure of 130 kPa, stir and react evenly for 3.5 h. Then, at 70 °C, filter to remove activated carbon and other filterable impurities, and maintain the filtrate at 70 °C. Flow through cation exchange resin and anion exchange resin in sequence for ion reduction. The total concentration of cations and anions in the obtained liquid is 22 ppm, which is recorded as the alcoholysis solution.

[0043] Among them, the PET component content of the waste polyester material is 92.5 wt% of cloth bubble material; the EG is a mixture of polyester polymerization grade EG and the recovered EG in step 2) at a ratio of 1:0.2; the catalyst is tetra-isopropyl titanate; the stabilizer is potassium acetate; the so-called "in a certain proportion" means based on the mass of PET, the addition amount of EG is 380 wt%, the addition amount of catalyst is 1.0 wt%, the addition amount of stabilizer is 1×10 -2 wt%, and the addition amount of activated carbon is 3 wt%.

[0044] 2) Under the protection of inert gas, preheat the alcoholysis solution obtained in step 1) to 120 °C, transfer it into a wiped film evaporator with microwave radiation, and carry out microwave radiation-induced thin film evaporation treatment to remove substances with boiling points lower than BHET in the alcoholysis solution. The gap between the rotor scraper and the inner wall of the evaporator is 0.5 mm. This treatment controls the air pressure in the thin film evaporation cavity to be 100 Pa through the vacuum system, controls the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 60 s through the feed flow rate and the scraper rotation speed, controls the temperature of the completed solution at the concentration outlet to be maintained at 180 °C through the microwave power. The total content of EG and DEG in the concentrated completed solution is 0.16 wt%, which is recorded as the separated solution; at the same time, condense and collect the low-boiling substances removed in this step, and transfer them to a conventional distillation separation device to obtain EG with a purity of 99.3 wt%, which is recorded as the recovered EG.

[0045] 3) Transfer the separated solution obtained in step 2) into a molecular distillation device. Under the conditions that the heating surface temperature is 209 °C, the pressure is 23 Pa, and the condensation surface temperature is 118 °C, control the residence time of the separated solution on the heating surface to be 75 s, so that the mass ratio of the distillate to the residue can be stabilized at 5.7:4.3; the main component of the obtained residue is oligomer, which is recorded as the alcoholysis refined residue, collected and transferred into the alcoholysis device to participate in the depolymerization reaction of the next production cycle; the obtained distillate is refined BHET. The refined BHET solution is cooled to room temperature and solidified under dry inert gas and pulverized into powder, that is, the obtained refined BHET product. The purity of BHET is 99.1 wt%, the hue L / a / b value is 99.21 / -2.12 / 0.98, and the BHET yield is 81.2%.

[0046] The above is only the preferred embodiment of the present invention. The above examples do not impose any formal restrictions on the substantive content of the present invention. Any simple modification or deformation made by those of ordinary skill in the technical field to the above specific embodiments according to the technical essence of the present invention after reading this specification, and any equivalent embodiments that may be changed or modified by using the technical content disclosed above into equivalent changes still fall within the scope of the technical solution of the present invention without departing from the essence and scope of the present invention.

Claims

1. A method for purifying monomers obtained by glycolysis of waste polyester, characterized in that, It includes the following steps: 1) After uniformly mixing waste polyester materials, EG, a catalyst, a stabilizer, and activated carbon in a certain proportion, feed them into an alcoholysis device. Under the protection of an inert gas, with a temperature of 185 - 200 °C and a pressure of 100 - 200 kPa, carry out a depolymerization reaction with uniform stirring for 2 - 5 h. Then, at a temperature of 60 - 100 °C, filter to remove the activated carbon and other filterable impurities, and maintain the filtrate at 60 - 100 °C. Flow through a cation exchange resin and an anion exchange resin in sequence for ion reduction until the total concentration of cations and anions ≤ 50 ppm. The resulting liquid is denoted as the alcoholysis solution; 2) Under the protection of an inert gas, preheat the alcoholysis solution obtained in step 1) to 110 - 120 °C, transfer it into a scraper - type thin - film evaporator with microwave radiation, and carry out microwave - radiation - induced thin - film evaporation treatment to remove substances in the alcoholysis solution with boiling points lower than BHET. Then, control the pressure in the thin - film evaporation chamber to be 100 - 300 Pa through a vacuum system, control the residence time of the alcoholysis solution on the evaporation surface under the action of microwave irradiation to be 20 - 100 s by controlling the feed flow rate and the scraper rotation speed, and control the temperature of the completed liquid at the concentration outlet to be maintained at 170 - 180 °C by controlling the microwave power to obtain a concentrated completed liquid denoted as the separation liquid. At the same time, condense and collect the low - boilers removed in this step, and transfer the low - boilers to a conventional distillation separation device. EG with a purity ≥ 99.0 wt% is separated from the conventional distillation separation device, and the obtained EG is denoted as recycled EG; 3) Transfer the separation liquid obtained in step 2) into a molecular distillation device. Under the conditions of a heating - surface temperature of 205 - 210 °C, a pressure of ≤ 25 Pa, and a condensing - surface temperature of 110 - 120 °C, by controlling the residence time of the separation liquid on the heating surface, maintain the mass ratio of the distillate to the residue at 5:5 - 6:

4. The main component of the obtained residue is oligomers, denoted as the alcoholysis - refined residue. After collecting the alcoholysis - refined residue, transfer it into the alcoholysis device in step 1) to participate in the depolymerization reaction in the next production cycle. The obtained distillate is refined BHET. Cool the refined BHET liquid to room temperature and solidify it under dry inert gas, and then crush it into a powder form to obtain the obtained refined BHET product.

2. The purification method of monomers obtained by glycolysis of waste polyester according to claim 1, characterized in that, The waste polyester material described in step 1) is a used polyester product and / or production waste with a PET component content ≥ 85 wt%; the ratio between the waste polyester material, EG, catalyst, stabilizer, and activated carbon is, based on the mass of PET contained in the waste polyester material, the addition amount of EG is 200 - 400 wt% of the mass of PET, the addition amount of the catalyst is 0.5 - 1.5 wt% of the mass of PET, the addition amount of the stabilizer is 0.5 - 5 × 10 -2 wt%, and the addition amount of activated carbon is 1 - 5 wt% of the mass of PET.

3. A purification method for monomers obtained by glycolysis of waste polyester, according to claim 1, characterized in that, The EG is a mixture of polyester - grade EG and the recycled EG obtained in step 2) of the present invention at a ratio of 1:0.1 - 1:

1.

4. A purification method for monomers obtained by glycolysis of waste polyester, according to claim 1, characterized in that The catalyst in step 1) is one of sodium carbonate, zinc acetate, tetra - n - butyl titanate, and tetra - isopropyl titanate; the stabilizer is one of sodium acetate and potassium acetate.