Process for the preparation of terephthalic acid bis-hydroxyethyl ester from waste textiles and products thereof

Abstract

The application provides a method for preparing bis-hydroxyethyl terephthalate from waste textiles and a product thereof, and the method comprises the following steps: S1: waste textiles are depolymerized by ethylene glycol to obtain an alcoholysis solution containing bis-hydroxyethyl terephthalate, and the alcoholysis solution is crystallized to obtain a bis-hydroxyethyl terephthalate crude product; S2: the bis-hydroxyethyl terephthalate crude product is dissolved in a mixed solvent to form an adsorption original solution; the adsorption original solution is decolorized by a fixed bed containing an adsorbent to obtain a bis-hydroxyethyl terephthalate solution; the mixed solvent comprises a hydrolysis inhibitor; the hydrolysis inhibitor has a thermal decomposition temperature higher than 100 DEG C; the pH of the adsorption original solution is 4-7; and S3: the bis-hydroxyethyl terephthalate solution is crystallized to obtain the bis-hydroxyethyl terephthalate. The method can reduce the hydrolysis reaction of BHET in a water system and improve the purity of BHET crystals.

Description

Technical Field

[0001] This application relates to the field of chemical fiber technology, and in particular to a method for preparing dihydroxyethyl terephthalate from waste textiles and the product thereof. Background Technology

[0002] Currently, the main recycling methods for waste textiles include physical recycling, mechanical recycling, chemical recycling, and energy recovery. Chemical recycling involves processing waste polyester into monomer compounds using chemical methods, and then manufacturing new polyester products. This mainly includes hydrolysis, alcoholysis, and ammonolysis. Alcoholysis is further divided into methanol alcoholysis and ethylene glycol alcoholysis. Methanol alcoholysis often requires supercritical conditions (250-280℃, 8-15 MPa), placing extremely high demands on equipment for industrialization. Catalyst recovery is also difficult, and the yield of DMT monomer, a product of methanol alcoholysis, is at most only 95%, limiting its industrialization. Ethylene glycol alcoholysis is currently the most promising method for industrialization. Unlike methanol alcoholysis, ethylene glycol alcoholysis typically occurs at 190-230℃ under normal pressure. The product of ethylene glycol alcoholysis is diethyl terephthalate (BHET). BHET has a high boiling point (317℃), making it difficult to purify, and it readily polymerizes at high temperatures, producing dimers and oligomers.

[0003] Waste textiles are complex in composition, typically containing a mixture of polyester, cotton, cellulose, wool, silk, spandex, nylon, acrylic, oils, matting agents, and polyester catalysts. The dye composition is particularly complex, with nearly a thousand dyes available on the market, each with varying structures and separation difficulties, posing a significant challenge to the purification and refining process of BHET. Current technologies primarily focus on purifying high-purity PET bottle flakes or foam materials, making them unsuitable for handling the complex composition of waste textiles. Secondly, existing adsorption treatments are all based on aqueous systems. However, at conventional adsorption temperatures (80-90℃), BHET readily hydrolyzes to form PTA (terephthalic acid) and MHET (methyl terephthalate). Furthermore, PTA and MHET have low solubility in water, making them difficult to separate from adsorbents (such as activated carbon) during filtration, resulting in significant loss of BHET products.

[0004] Therefore, there is an urgent need to develop a method for preparing dihydroxyethyl terephthalate (BHET) from waste textiles and to reduce the hydrolysis reaction of BHET in the aqueous system to obtain high-purity BHET. Summary of the Invention

[0005] This application provides a method for preparing dihydroxyethyl terephthalate (BHET) from waste textiles and the product thereof, which can reduce the hydrolysis reaction of BHET in an aqueous system and obtain high-purity BHET.

[0006] In a first aspect, this application provides a method for preparing dihydroxyethyl terephthalate from waste textiles, the method comprising:

[0007] S1: Waste textiles are depolymerized with ethylene glycol to obtain an alcoholysis solution containing dihydroxyethyl terephthalate, and the alcoholysis solution is crystallized to obtain crude dihydroxyethyl terephthalate.

[0008] S2: Crude dihydroxyethyl terephthalate is dissolved in a mixed solvent to form the adsorption stock solution; the adsorption stock solution is decolorized through a fixed bed containing adsorbent to obtain a dihydroxyethyl terephthalate solution.

[0009] The mixed solvent includes a hydrolysis inhibitor; the hydrolysis inhibitor has a thermal decomposition temperature exceeding 100°C; the pH of the original solution to be adsorbed is 4-7;

[0010] S3: Diethyl terephthalate is obtained by crystallization of a dihydroxyethyl terephthalate solution.

[0011] In one possible implementation, the hydrolysis inhibitor comprises at least one or more of 1,3-dimethylurea, triethanolamine, N,N-diisopropylethylamine, 2,6-dimethylpyridine, 3-morpholinopropanesulfonic acid, 2,4,6-trimethylpyridine, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, acetic acid-triethanolamine, and acetic acid-sodium acetate.

[0012] In one possible implementation, the hydrolysis inhibitor comprises a combination of triethanolamine and a phosphate; the phosphate comprises at least one of dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0013] In one possible implementation, the mass percentage of the hydrolysis inhibitor in the mixed solvent is 1%-5%.

[0014] In one possible implementation, the mixed solvent further includes a co-solvent, which includes at least one or more of ethylene glycol, propylene glycol, butanediol, methanol, ethanol, butyl acetate, ethyl acetate, petroleum ether, polyethylene glycol, and glycerol.

[0015] In one possible implementation, the co-solvent comprises at least one or more of polyethylene glycol and glycerin.

[0016] In one possible implementation, the mass percentage of the co-solvent in the mixed solvent is 1%-10%.

[0017] In one possible implementation, the mixed solvent further includes water; in the mixed solvent, the water accounts for 85-98% by mass.

[0018] In one possible implementation, the adsorbent comprises acid-modified activated carbon and / or alkali-modified activated carbon;

[0019] And / or; the acid-modified activated carbon includes at least one of nitric acid-modified activated carbon, phosphoric acid-modified activated carbon, and hydrochloric acid-modified activated carbon;

[0020] And / or; the shape of the adsorbent includes at least one of the following: irregular granular, clover-shaped, cylindrical, and spherical;

[0021] And / or; the adsorbent includes at least one of granular activated carbon, kaolin, diatomaceous earth, and molecular sieve;

[0022] And / or; the granular activated carbon includes at least one of coal-based activated carbon, petroleum coke activated carbon, fruit shell activated carbon, and wood-based activated carbon.

[0023] In one possible implementation, the crystallization in steps S1 and S3 independently includes vacuum crystallization or cooling crystallization, respectively.

[0024] The termination temperature of the cooling crystallization is -5℃ to 30℃; the cooling rate of the cooling crystallization is 1-25℃ / min, or the cooling rate of the cooling crystallization is 5-15℃ / min.

[0025] And / or; in step S2, the temperature of the fixed bed decolorization is 40-100℃, or the temperature of the fixed bed decolorization is 40-60℃.

[0026] Secondly, this application provides a dihydroxyethyl terephthalate prepared according to the above method.

[0027] This application provides a method for preparing dihydroxyethyl terephthalate (BHET) from waste textiles and the resulting product. The method involves ethylene glycol alcoholysis and crystallization to obtain crude BHET, fixed-bed adsorption, and recrystallization. The fixed-bed adsorption process replaces traditional activated carbon adsorbents, achieving continuous and efficient decolorization. Furthermore, the process is compatible with complex impurities in waste textiles and has a wide range of applicable raw materials, providing high-quality monomer raw materials for the production of recycled polyester fibers and high-end polyester new materials. Additionally, a hydrolysis inhibitor is introduced into the crude BHET solution. This inhibitor can regulate the pH value of the crude BHET solution, solving the hydrolysis problem of BHET in aqueous systems, inhibiting the hydrolysis reaction of BHET, reducing the formation of PTA and MHET, and improving the purity of BHET crystals. Attached Figure Description

[0028] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0029] Figure 1High-performance liquid chromatography chromatogram of BHET crystals from Example 1 provided in this application;

[0030] Figure 2 High-performance liquid chromatogram of BHET crystals from Comparative Example 2 provided for this application;

[0031] Figure 3 This is a schematic diagram of the process for preparing BHET crystals according to Example 1 provided in this application.

[0032] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0033] To enable those skilled in the art to better understand the present invention, this application will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the invention and are not intended to limit its scope. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. When the following description relates to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements.

[0034] In a first aspect, this application provides a method for preparing dihydroxyethyl terephthalate from waste textiles, comprising:

[0035] S1: Waste textiles are depolymerized with ethylene glycol to obtain an alcoholysis solution containing dihydroxyethyl terephthalate, and the alcoholysis solution is crystallized to obtain crude dihydroxyethyl terephthalate.

[0036] S2: Crude dihydroxyethyl terephthalate is dissolved in a mixed solvent to form the adsorption stock solution; the adsorption stock solution is decolorized through a fixed bed containing adsorbent to obtain a dihydroxyethyl terephthalate solution.

[0037] The mixed solvent includes a hydrolysis inhibitor; the hydrolysis inhibitor has a thermal decomposition temperature exceeding 100°C; the pH of the original solution to be adsorbed is 4-7;

[0038] S3: Dihydroxyethyl terephthalate is obtained by crystallization of a solution of dihydroxyethyl terephthalate.

[0039] The method of this application uses alcoholysis followed by crystallization to obtain crude BHET. The crystallization after alcoholysis initially removes oligomers, reducing the non-specific adsorption of macromolecular impurities by the adsorbent in the subsequent fixed-bed adsorption, thus reducing the amount of adsorbent used and improving the adsorption efficiency. Then, fixed-bed adsorption is used to further remove dyes, improving the purity of BHET. By performing two crystallization operations (crude product crystallization and crystallization after fixed-bed adsorption) to remove oligomers and dyes stepwise, the loading pressure of the adsorption steps is reduced, and the staged crystallization extends the adsorbent lifespan and reduces adsorption costs.

[0040] Secondly, the raw materials used in this application have a wide range of applications. Existing technologies are limited to relatively pure PET products / bottle flakes, while the raw materials used in this application are unrestricted in terms of both color and type, and can include waste clothing, quilts, blankets, curtains, and other types of waste textiles provided by waste recycling companies. Compared with the DMT route, this scheme has a shorter process route, does not involve two-step transesterification reactions and high-temperature distillation, has lower energy consumption, and lower production costs. Compared with the existing BHET process, this scheme solves the problem of poor product quality, low purity, and difficulty in purification caused by the easy polymerization of BHET during molecular distillation through an adsorption-crystallization process. Compared with the existing BHET decolorization process that directly adds adsorbent to the alcoholysis solution, this scheme uses fixed-bed adsorption after crystallizing crude BHET, effectively solving the problem of large adsorbent consumption and significantly reducing the cost of the preparation process.

[0041] Furthermore, the pH value of BHET aqueous solution is typically 2-3 at the conventional adsorption temperature (80-90℃). However, the applicant discovered that under these acidic conditions, BHET readily hydrolyzes to form PTA (terephthalic acid) and MHET (monohydroxyethyl terephthalate). Therefore, this application introduces a hydrolysis inhibitor into the crude BHET solution. This inhibitor can regulate the pH value of the crude BHET solution, solving the hydrolysis problem of BHET in an aqueous system, inhibiting the hydrolysis reaction of BHET, reducing the formation of PTA and MHET, and improving the purity of BHET crystals.

[0042] It is understandable that the pH of the stock solution to be adsorbed is 4-7, such as 4, 4.5, 5, 5.5, 6, 6.5, 7 or any two of the above values.

[0043] In one possible implementation, the hydrolysis inhibitor includes at least one or more of 1,3-dimethylurea, triethanolamine, N,N-diisopropylethylamine, 2,6-dimethylpyridine, 3-morpholinopropanesulfonic acid, 2,4,6-trimethylpyridine, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, acetic acid-triethanolamine, and acetic acid-sodium acetate.

[0044] In this application, the hydrolysis inhibitor reduces the hydrolysis rate of BHET molecules by neutralizing the acidic environment, thereby decreasing the formation of PTA and MHET. Simultaneously, the inhibitor's sustained-release properties maintain pH stability, preventing hydrolysis reactions caused by localized acidity fluctuations. The final product purity is significantly improved, and the residual amount of hydrolysis products is reduced.

[0045] In one possible implementation, the hydrolysis inhibitor comprises a combination of triethanolamine and a phosphate; the phosphate comprises at least one of dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0046] This application combines triethanolamine with inorganic buffer phosphate to construct a dynamic pH control system. Through the ion exchange capacity of the buffer salt, acidic substances generated by hydrolysis are neutralized in real time, maintaining the solution pH within the neutral range of 4-7.

[0047] In one possible implementation, the hydrolysis inhibitor in the mixed solvent comprises 1%-5% by mass, for example, 1%, 2%, 3%, 4%, 5%, or any combination of two of the above values. By selecting a hydrolysis inhibitor within the above range, this application can achieve hydrolysis inhibition without increasing the ion concentration in the system, thus maintaining the quality of the final BHET product.

[0048] In one possible implementation, the mixed solvent further includes a co-solvent, which includes at least one or more of ethylene glycol, propylene glycol, butanediol, methanol, ethanol, butyl acetate, ethyl acetate, petroleum ether, polyethylene glycol, and glycerol.

[0049] BHET has low solubility in water at low temperatures. This application incorporates a co-solvent to enhance BHET's solubility through hydrogen bonding or polar interactions, thereby lowering the adsorption temperature. Specifically, the co-solvent forms a stable solvated structure with the BHET molecule, improving its solubility at low temperatures. Secondly, lowering the adsorption temperature further reduces hydrolysis reactions caused by high-temperature adsorption. Simultaneously, the addition of the co-solvent reduces the energy requirement of the adsorption process, improves process continuity, and ensures product purity.

[0050] In one possible implementation, the cosolvent includes at least one or more of polyethylene glycol and glycerol. Polyethylene glycol and glycerol possess hydrogen bond donor and acceptor properties, which can further enhance the solubility of BHET through intermolecular hydrogen bonding.

[0051] In one possible implementation, the mass percentage of the co-solvent in the mixed solvent is 1%-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of the above values. This application selects a co-solvent within the above range to minimize subsequent separation energy consumption while ensuring suitable solubility.

[0052] In one possible implementation, the mixed solvent further includes water; in the mixed solvent, the water mass percentage is 85-98%, for example 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or any range of two of the above values.

[0053] In one possible implementation, the adsorbent comprises acid-modified activated carbon and / or alkali-modified activated carbon.

[0054] And / or; Acid-modified activated carbon includes at least one of nitric acid-modified activated carbon, phosphoric acid-modified activated carbon, and hydrochloric acid-modified activated carbon;

[0055] And / or; the shape of the adsorbent includes at least one of the following: irregular granular, clover-shaped, cylindrical, and spherical;

[0056] And / or; the adsorbent includes at least one of granular activated carbon, kaolin, diatomaceous earth and molecular sieve;

[0057] And / or; granular activated carbon includes at least one of coal-based activated carbon, petroleum coke activated carbon, fruit shell activated carbon, and wood-based activated carbon.

[0058] In one possible implementation, the adsorbent is acid-modified granular activated carbon and / or alkali-modified granular activated carbon. This application uses granular activated carbon to replace traditional powdered activated carbon. Combined with acid and alkali modification treatment, the adsorption capacity and mechanical strength can be improved. At the same time, the adsorbent can be recycled through regeneration treatment, reducing adsorption costs.

[0059] In one possible implementation, the crystallization in steps S1 and S3 independently includes vacuum crystallization or cooling crystallization, respectively.

[0060] The termination temperature for cooling crystallization is -5℃ to 30℃; the cooling rate for cooling crystallization is 1-25℃ / min, or 5-15℃ / min.

[0061] In one possible implementation, the termination temperature for cooling crystallization is -5°C to 30°C, for example -5°C, 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, or any combination of two of the above values.

[0062] In one possible implementation, the cooling rate for cooling crystallization is 1-25°C / min, for example, 1°C / min, 5°C / min, 10°C / min, 15°C / min, 20°C / min, 25°C / min, or any combination of two of the above values.

[0063] In one possible implementation, the mass concentration of crude dihydroxyethyl terephthalate in the adsorption stock solution is 1-50%, for example, 1%, 10%, 20%, 30%, 40%, 50%, or any combination of two of the above values.

[0064] In one possible implementation, the mass concentration of crude dihydroxyethyl terephthalate in the adsorption stock solution is 5-20%.

[0065] In one possible implementation, the temperature for fixed-bed decolorization is 40-100℃, and the adsorption residence time is 0.5-3h.

[0066] It is understandable that the temperature for fixed-bed decolorization is 40-100℃, such as 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃ or any combination of two of the above values.

[0067] It is understandable that the adsorption residence time for fixed-bed decolorization is 0.5-3h, for example, 0.5h, 1h, 2h, 3h or any two of the above values.

[0068] In one possible implementation, the temperature for fixed-bed decolorization is 40-60°C, and the adsorption residence time is 0.5-2 hours.

[0069] In this application, the addition of a co-solvent can lower the fixed-bed decolorization temperature, further reducing the hydrolysis reaction caused by excessively high temperatures and improving the purity of BHET.

[0070] Secondly, this application provides a dihydroxyethyl terephthalate prepared according to the above method.

[0071] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0072] Example 1

[0073] like Figure 3 As illustrated, the method for preparing dihydroxyethyl terephthalate using waste textiles in this embodiment is as follows:

[0074] S1: 5 kg of waste textiles are added to the continuous alcoholysis reactor (continuous stirred tank reactor) through a solid feeding device.

[0075] The waste textiles contain approximately 82% polyester, 10% cotton, 1% wool and silk (wool and silk in a 1:1 ratio), 2% nylon, 2% spandex and acrylic (spandex and acrylic in a 1:1 ratio), 2% dyeing agents (of which approximately 70 wt% are azo dyes, approximately 20 wt% are anthraquinone dyes, and approximately 10 wt% are other dyes), and 1 wt% other substances.

[0076] S2: Add 15 kg of ethylene glycol and 10 g of zinc acetate as an alcoholysis catalyst to a continuous alcoholysis reactor, raise the reaction temperature to 235 °C, and react for 2 hours.

[0077] S3: Cool the liquid temperature to 100℃ for primary filtration, and then reduce the temperature to 20℃ by cooling crystallization at a rate of 5℃ / min to obtain crude BHET.

[0078] S4: Dissolve crude BHET in a mixed solvent to form the adsorption stock solution. The pH of the adsorption stock solution is 5.5. The mixed solvent is water, the co-solvent is ethylene glycol, and the hydrolysis inhibitor is a hydrolysate. The concentration of crude BHET in the adsorption stock solution is controlled at 20 wt%. Pass the adsorption stock solution into a fixed-bed adsorption device containing adsorbent for adsorption and decolorization to obtain a BHET solution. The adsorbent is granular activated carbon.

[0079] In the mixed solvent, the co-solvent ethylene glycol accounts for 5% by mass. The hydrolysis inhibitor in the mixed solvent is 2.5 wt%, which is a mixture of 1,3-dimethylurea and triethanolamine in a 1:1 mass ratio. The fixed-bed adsorption temperature is 45℃, and the adsorption residence time is 2 h.

[0080] S5: BHET solution flows into the crystallization reactor, where the temperature is reduced from 45℃ to 10℃ at a rate of 5℃ / min. After filtration, washing, and drying, 3 kg of BHET crystals are obtained. Analysis shows that the liquid phase purity of the BHET crystals is greater than 99%, and the BHET crystals exhibit excellent color values: L value 96, b value 0.31, and melting point 110.5℃.

[0081] Liquid chromatography data showed that the total content of MHET and PTA was approximately 0.31%, the content of dimers and oligomers was approximately 0.51%, and the purity of BHET monomer was greater than 99%. It can be seen that the addition of solubilizer and hydrolysis inhibitor effectively solved the problem of BHET hydrolysis into PTA and MHET, effectively improved the purity of BHET monomer, and significantly reduced the cost of purification process.

[0082] Examples 2-5

[0083] The only difference between Examples 2-5 and Example 1 is the specific type of co-solvent, as shown in Table 1.

[0084] Test example:

[0085] High-performance liquid chromatography (HPLC) testing: The BHET prepared in Examples 1-5 was tested by HPLC. The selected chromatographic column was a ZORBAX Eclipse Plus C18 with dimensions of 4.6 (inner diameter) * 250 mm (length) and a column particle size of 5 micrometers. The mobile phase was methanol and phosphoric acid water (the volume percentage of phosphoric acid in the phosphoric acid water was 0.05%). The MHET content, PTA content, and BHET purity were obtained by HPLC testing.

[0086] Table 1

[0087]

[0088] As demonstrated in Examples 1-5, using different co-solvents such as ethyl acetate, ethylene glycol, ethanol, propylene glycol, or petroleum ether can effectively improve the solubility of crude BHET, reduce the adsorption temperature, and thus stabilize its interaction with hydrolysis inhibitors, inhibiting BHET hydrolysis and improving the purity of BHET monomers.

[0089] Examples 6-9

[0090] The only difference between Examples 6-9 and Example 1 is the specific type of hydrolysis inhibitor, as shown in Table 2.

[0091] Test case

[0092] High-performance liquid chromatography (HPLC) testing: The BHET prepared in Examples 6-9 was tested by HPLC. The selected chromatographic column was a ZORBAX Eclipse Plus C18 with dimensions of 4.6 (inner diameter) * 250 mm (length) and a column particle size of 5 micrometers. The mobile phase was methanol and phosphoric acid water (the volume percentage of phosphoric acid in the phosphoric acid water was 0.05%). The MHET content, PTA content, and BHET purity were obtained by HPLC testing.

[0093] Table 2

[0094]

[0095] Analysis of Examples 6-9 and Example 1 shows that the use of different organic and inorganic hydrolysis inhibitors, either alone or in combination, can effectively regulate the pH value of the solution, increasing the pH from 2-3 to 4-7. This effectively reduces the rate of BHET hydrolysis to produce MHET and PTA, thereby reducing the content of MHET and PTA and increasing the purity of BHET monomer.

[0096] Examples 10-11

[0097] The only difference between Examples 10-11 and Example 1 is the proportion of the cosolvent. The proportion of the hydrolysis inhibitor is the same as in Example 1, as shown in Table 3.

[0098] Examples 12-14

[0099] The only difference between Examples 12-14 and Example 1 is the proportion of hydrolysis inhibitor, while the proportion of solvent is the same as in Example 1. See Table 4 for details.

[0100] Example 15

[0101] The only difference between this embodiment and Embodiment 1 is that no cosolvent is added, and the proportion of hydrolysis inhibitor is the same as in Embodiment 1.

[0102] Example 16

[0103] The only difference between this embodiment and Embodiment 1 is that the mass percentage of the co-solvent ethylene glycol in the mixed solvent is 20%, and the percentage of the hydrolysis inhibitor is the same as in Embodiment 1.

[0104] Example 17

[0105] The only difference between Example 17 and Example 1 is that:

[0106] In S3, the temperature is reduced to -5℃ by cooling crystallization at a rate of 10℃ / min.

[0107] In S4, the fixed-bed adsorption temperature is 40℃.

[0108] In S5, the BHET solution flows into the crystallization reactor, and the temperature drops from 40℃ to -5℃ at a rate of 10℃ / min.

[0109] Example 18

[0110] The only difference between Example 18 and Example 1 is that:

[0111] In S3, the temperature is reduced to 30°C by cooling crystallization at a rate of 15°C / min.

[0112] In S4, the fixed-bed adsorption temperature is 60℃.

[0113] In S5, the BHET solution flows into the crystallization reactor, and the temperature drops from 60°C to 30°C at a rate of 15°C / min.

[0114] Example 19

[0115] The only difference from Example 1 is that the fixed bed adsorption temperature is 85°C.

[0116] Comparative Example 1

[0117] The only difference between this comparative example and Example 1 is that no hydrolysis inhibitor is added, and the proportion of the cosolvent is the same as in Example 1.

[0118] Comparative Example 2

[0119] The only difference between this comparative example and Example 1 is that no hydrolysis inhibitors and cosolvents are added.

[0120] Test example:

[0121] BHET crystal color value: 45 / 0 under the optical geometry conditions of the colorimeter, specifically, the light source is at a 45° angle to the normal of the sample surface. ° Illumination at an angle, detector at vertical (0) ° The reflected light is received in the direction of the light source, and the L value and b value of the BHET crystal are measured under the illumination of a light source with a spectral range of 400nm~700nm.

[0122] Table 3

[0123]

[0124] Analysis of Examples 10 and 11, compared with Example 1, shows that within a reasonable range, different proportions of co-solvent can effectively reduce the content of MHET and PTA, and improve the purity of BHET monomer. However, Examples 15 and 16 reveal that when no co-solvent is used, at an adsorption temperature of 45°C, the solubility of BHET is low, and BHET precipitates during the adsorption process, leading to blockage of the adsorption column bed, a lower concentration of BHET in the effluent, and an increase in the content of MHET. This indicates that the co-solvent not only improves the solubility of BHET but also inhibits its hydrolysis. When the co-solvent content is increased to 20 wt%, although BHET hydrolysis is inhibited, the color value of the BHET product deteriorates, especially the b-value, which increases from 0.31 to 2.3, causing the BHET product to turn slightly yellow.

[0125] Compared to Example 1, Example 19 increased the adsorption temperature to 85°C, which significantly increased the solubility of BHET. No BHET crystallization occurred, but the hydrolysis product content further increased from 0.31% to 4.55%. Figure 1 and Figure 2 The images show the high-performance liquid chromatograms of BHET products from Example 1 and Comparative Example 2, respectively.

[0126] By comparing Example 1 and Comparative Examples 1-2, the contents of PTA and MHET both increased when no hydrolysis inhibitor was added, or when neither hydrolysis inhibitor nor cosolvent was added.

[0127] Table 4

[0128]

[0129] Analysis of Examples 12-14 and Example 1 shows that the reasonable range for hydrolysis inhibitors is 1%-5%. When the content of hydrolysis inhibitors reaches 10%, the ion concentration of the system is high, the hydrolysis inhibition effect is slightly poor, and the color value b of BHET products increases to 2.8.

[0130] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for preparing dihydroxyethyl terephthalate from waste textiles, characterized in that, The method includes: S1: Waste textiles are depolymerized with ethylene glycol to obtain an alcoholysis solution containing dihydroxyethyl terephthalate, and the alcoholysis solution is crystallized to obtain crude dihydroxyethyl terephthalate. S2: Crude dihydroxyethyl terephthalate is dissolved in a mixed solvent to form the adsorption stock solution; the adsorption stock solution is decolorized through a fixed bed containing adsorbent to obtain a dihydroxyethyl terephthalate solution. The mixed solvent includes a hydrolysis inhibitor; the hydrolysis inhibitor has a thermal decomposition temperature exceeding 100°C; the pH of the original solution to be adsorbed is 4-7; S3: Diethyl terephthalate is obtained by crystallization of a dihydroxyethyl terephthalate solution.

2. The method according to claim 1, characterized in that, The hydrolysis inhibitors include at least one or more of 1,3-dimethylurea, triethanolamine, N,N-diisopropylethylamine, 2,6-dimethylpyridine, 3-morpholinopropanesulfonic acid, 2,4,6-trimethylpyridine, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, acetic acid-triethanolamine, and acetic acid-sodium acetate.

3. The method according to claim 1, characterized in that, The hydrolysis inhibitor comprises a combination of triethanolamine and phosphate; the phosphate comprises at least one of dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

4. The method according to claim 1, characterized in that, The mass percentage of the hydrolysis inhibitor in the mixed solvent is 1%-5%.

5. The method according to claim 1, characterized in that, The mixed solvent also includes a co-solvent, which includes at least one or more of ethylene glycol, propylene glycol, butanediol, methanol, ethanol, butyl acetate, ethyl acetate, petroleum ether, polyethylene glycol, and glycerol.

6. The method according to claim 5, characterized in that, The cosolvent includes at least one or more of polyethylene glycol and glycerin.

7. The method according to claim 5, characterized in that, In the mixed solvent, the mass percentage of the co-solvent is 1%-10%.

8. The method according to claim 1 or 5, characterized in that, The mixed solvent also includes water; in the mixed solvent, the water accounts for 85-98% by mass.

9. The method according to claim 1, characterized in that, The adsorbent includes acid-modified activated carbon and / or alkali-modified activated carbon; And / or; the acid-modified activated carbon includes at least one of nitric acid-modified activated carbon, phosphoric acid-modified activated carbon, and hydrochloric acid-modified activated carbon; And / or; the shape of the adsorbent includes at least one of the following: irregular granular, clover-shaped, cylindrical, and spherical; And / or; the adsorbent includes at least one of granular activated carbon, kaolin, diatomaceous earth, and molecular sieve; And / or; the granular activated carbon includes at least one of coal-based activated carbon, petroleum coke activated carbon, fruit shell activated carbon, and wood-based activated carbon; And / or; the crystallization in steps S1 and S3 independently includes vacuum crystallization or cooling crystallization, respectively; And / or; the termination temperature of the cooling crystallization is -5℃ to 30℃; the cooling rate of the cooling crystallization is 1-25℃ / min, or the cooling rate of the cooling crystallization is 5-15℃ / min; And / or; in step S2, the temperature of the fixed bed decolorization is 40-100℃, or the temperature of the fixed bed decolorization is 40-60℃.

10. A bis(hydroxyethyl) terephthalate prepared by the method according to any one of claims 1-9.

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