A method for purifying crude dihydroxyethyl terephthalate, a method for preparing dihydroxyethyl terephthalate, and a product of dihydroxyethyl terephthalate.

Abstract

This application provides a method for purifying crude dihydroxyethyl terephthalate (BHET), a method for preparing BHET, and a BHET product. The method includes: mixing crude BHET with a solvent to obtain a first mixture; wherein the solvent includes a first solvent and a second solvent; subjecting the first mixture to stepwise cooling, during which the BHET in the first mixture crystallizes to obtain purified BHET. This method optimizes crystallization kinetics, shortens crystallization time, reduces solution viscosity, and thus improves solid-liquid separation performance, ultimately obtaining a high-purity, uniformly sized BHET product.

Description

Technical Field

[0001] This application relates to the field of polyester recycling technology, and in particular to a purification method for crude dihydroxyethyl terephthalate, a preparation method for dihydroxyethyl terephthalate, and a dihydroxyethyl terephthalate product. Background Technology

[0002] The recycling of waste textiles is a crucial issue in the global resource recycling field. my country generates hundreds of millions of tons of waste textiles annually, with polyester textiles (such as polyethylene terephthalate, PET) accounting for a significant portion. Due to the chemical inertness and long natural degradation cycle of polyester materials, traditional landfill or incineration methods not only waste resources but also cause severe environmental pollution. Therefore, converting waste polyester into renewable monomers (such as polyethylene terephthalate, BHET) through chemical depolymerization has become a key technological pathway for achieving a circular economy.

[0003] In recycled polyester production, BHET is the core monomer for synthesizing PET. Its purity directly affects the performance of recycled polyester products (such as color, mechanical properties, and thermal stability). However, waste polyester is often treated with additives such as flame retardants, colorants, and antistatic agents during production, resulting in a large number of impurities (such as oligomers, additive residues, and metal ions) in the depolymerized BHET solution. If these impurities are not effectively removed, recycled polyester products will exhibit problems such as poor color and decreased mechanical properties, making it difficult to meet the requirements of industrial or medical applications.

[0004] Therefore, developing an efficient and low-cost BHET purification technology that can optimize crystallization kinetics, shorten crystallization time, reduce solution viscosity, and thus improve solid-liquid separation performance, ultimately obtaining BHET products with high purity and uniform particle size, is the core link in realizing the high-value recycling of waste polyester. Summary of the Invention

[0005] This application provides a purification method for crude dihydroxyethyl terephthalate (BHET), a preparation method for BHET, and a BHET product. This method can optimize crystallization kinetics, shorten crystallization time, and reduce solution viscosity, thereby improving solid-liquid separation performance and ultimately obtaining a BHET product with high purity and uniform particle size.

[0006] In a first aspect, embodiments of this application provide a method for purifying crude dihydroxyethyl terephthalate, comprising: mixing crude dihydroxyethyl terephthalate with a solvent to obtain a first mixture; wherein the solvent includes a first solvent and a second solvent; subjecting the first mixture to stepwise cooling, wherein the dihydroxyethyl terephthalate in the first mixture crystallizes during the stepwise cooling process to obtain purified dihydroxyethyl terephthalate; wherein the stepwise cooling process of the first mixture includes: cooling the first mixture to a first temperature, performing a first crystallization treatment at the first temperature, then cooling to a second temperature, performing a second crystallization treatment at the second temperature, then cooling to a third temperature, and performing a third crystallization treatment at the third temperature to obtain purified dihydroxyethyl terephthalate.

[0007] In one possible implementation, the first solvent comprises water and / or isopropanol.

[0008] In one possible implementation, the second solvent includes at least one of acetonitrile, ethyl acetate, propyl acetate, acetone, methanol, ethanol, propylene glycol, diethylene glycol, glycerol, etc.

[0009] In one possible implementation, the mass ratio of the solvent to the crude dihydroxyethyl terephthalate is (0.5-18):1.

[0010] In one possible implementation, the mass ratio of the first solvent to the second solvent is 1:(0.5-2).

[0011] In one possible implementation, the cooling rate to the first temperature is 50°C / h - 150°C / h.

[0012] In one possible implementation, the rate of cooling to the second temperature is 2°C / h - 10°C / h.

[0013] In one possible implementation, the rate of cooling to the third temperature is 10°C / h - 30°C / h.

[0014] In one possible implementation, the first temperature is 45°C-60°C.

[0015] In one possible implementation, the second temperature is 35°C-50°C.

[0016] In one possible implementation, the third temperature is 5°C-30°C.

[0017] In one possible implementation, the mass concentration of dihydroxyethyl terephthalate in the crude dihydroxyethyl terephthalate is 10%-50%.

[0018] In one possible implementation, the crude dihydroxyethyl terephthalate comprises the dihydroxyethyl terephthalate product obtained by alcoholysis of textile materials.

[0019] In one possible implementation, the process of mixing crude dihydroxyethyl terephthalate with a solvent includes: performing alcoholysis on the crude dihydroxyethyl terephthalate using an alcohol solvent to obtain a second mixture containing the crude dihydroxyethyl terephthalate and the alcohol solvent; mixing the second mixture with a solvent to obtain a first mixture; wherein the alcohol solvent includes ethylene glycol.

[0020] Secondly, embodiments of this application provide a method for preparing dihydroxyethyl terephthalate, comprising: subjecting textile materials to alcoholysis to obtain crude dihydroxyethyl terephthalate; and purifying the crude dihydroxyethyl terephthalate according to the purification method described above to obtain the dihydroxyethyl terephthalate product.

[0021] Thirdly, this application provides a dihydroxyethyl terephthalate product, which is prepared by the purification method of crude dihydroxyethyl terephthalate described above or by the preparation method of dihydroxyethyl terephthalate described above; the purity of the dihydroxyethyl terephthalate product is above 99%; the color value L of the dihydroxyethyl terephthalate product is 80-97, the color value a is -1-1, and the color value b is 0-3; the median particle size of the dihydroxyethyl terephthalate product is 80 μm-200 μm.

[0022] This application provides a purification method for crude dihydroxyethyl terephthalate (BHET), a preparation method for BHET, and a BHET product. The method involves mixing the crude BHET with a solvent comprising a first solvent and a second solvent, followed by crystallization during a stepped cooling process. The stepped cooling process includes three crystallization treatments at three different temperatures to obtain purified BHET. This method optimizes crystallization kinetics, shortens crystallization time, reduces solution viscosity, and thus improves solid-liquid separation performance, ultimately yielding a high-purity BHET product with uniform particle size. Detailed Implementation

[0023] To enable those skilled in the art to better understand the solutions of this invention, the following provides a more detailed description of this application. The specific embodiments listed below are merely descriptions of the principles and features of this invention; the examples are only for explaining the invention and are not intended to limit its scope. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0024] This application provides a method for purifying crude dihydroxyethyl terephthalate (BHET), comprising: mixing crude BHET with a solvent to obtain a first mixture; wherein the solvent includes a first solvent and a second solvent; subjecting the first mixture to stepwise cooling, wherein the BHET in the first mixture crystallizes during the stepwise cooling process to obtain purified BHET; wherein the stepwise cooling process of the first mixture includes: cooling the first mixture to a first temperature, performing a first crystallization treatment at the first temperature, then cooling to a second temperature, performing a second crystallization treatment at the second temperature, then cooling to a third temperature, and performing a third crystallization treatment at the third temperature to obtain purified BHET.

[0025] This application introduces a solvent system in which a first solvent and a second solvent work together to form a mixed solvent system with the ethylene glycol in the crude dihydroxyethyl terephthalate obtained by alcoholysis of textile materials. On the one hand, the introduction of the low-viscosity first solvent can reduce the viscosity of the first mixture and improve the solid-liquid separation efficiency; on the other hand, the simultaneous introduction of the second solvent utilizes the differences in solubility of different solvents for different impurities to enhance the impurity removal capability.

[0026] This application employs a stepped cooling process, employing a three-gradient cooling crystallization treatment and controlling the cooling rate in stages. Combined with crystallization kinetics principles, this balances nucleation and growth rates within the metastable region, preventing explosive nucleation that could lead to impurity inclusion, ensuring crystal purity and particle size uniformity, and simultaneously shortening the crystallization time. Specifically, the crystallization process involves cooling to a first temperature at a first cooling rate and performing a first crystallization treatment at that temperature; then cooling to a second temperature at a second cooling rate and performing a second crystallization treatment at that temperature; finally, cooling to a third temperature at a third cooling rate and performing a third crystallization treatment at that temperature. Before the stepped cooling, the dihydroxyethyl terephthalate in the first mixture is in an undersaturated state. At this point, the mixture is cooled to a first temperature at a relatively high first cooling rate for the first crystallization treatment, which rapidly brings the dihydroxyethyl terephthalate in the first mixture to a near-saturated state. Then, the mixture is cooled to a second temperature at a lower second cooling rate for the second crystallization treatment, and then cooled to a third temperature at a third cooling rate between the first and second cooling rates for the third crystallization treatment. During the second and third crystallization treatments, the dihydroxyethyl terephthalate in the first mixture is in a saturated state. The synergistic effect of the second and third crystallization treatments allows the nucleation and growth rates of dihydroxyethyl terephthalate to be balanced within the metastable region, ensuring crystal purity and particle size uniformity.

[0027] Therefore, the purification method for crude dihydroxyethyl terephthalate provided in this application can optimize crystallization kinetics, shorten crystallization time, reduce solution viscosity, and thus improve solid-liquid separation performance, ultimately obtaining a high-purity BHET product with uniform particle size. It does not rely on adsorbents or complex distillation steps, simplifies the process, reduces energy consumption and cost, and can adapt to the characteristics of complex types of impurities and large concentration fluctuations in raw materials.

[0028] In some embodiments, the first solvent includes water and / or isopropanol. Solvents such as water and isopropanol can reduce the viscosity of ethylene glycol in the first mixture, which helps molecules to diffuse and contact fully, thereby promoting crystal growth and increasing the crystal grain size.

[0029] In some embodiments, the second solvent includes at least one selected from acetonitrile, ethyl acetate, propyl acetate, acetone, methanol, ethanol, propylene glycol, diethylene glycol, and glycerol. The aforementioned second solvents, such as acetonitrile, ethyl acetate, propyl acetate, acetone, methanol, ethanol, propylene glycol, diethylene glycol, and glycerol, help to increase the solubility of impurities in the system, allowing colored impurities to fully dissolve in the solvent phase and be effectively removed, thereby significantly improving the color value of the product.

[0030] In some embodiments, the mass ratio of the solvent to dihydroxyethyl terephthalate in the crude dihydroxyethyl terephthalate is (0.5-18):1. For example, a range consisting of 0.5:1, 5:1, 10:1, 15:1, 18:1, or any two of these.

[0031] The solvent system, consisting of the first and second solvents working synergistically, forms a mixed solvent system with the ethylene glycol in the crude dihydroxyethyl terephthalate obtained from the alcoholysis of textile materials using ethylene glycol. This improves solid-liquid separation efficiency. Maintaining the mass ratio of solvent to crude dihydroxyethyl terephthalate within the aforementioned range helps to balance good solid-liquid separation with crystallization performance. If the mass ratio is too high, the product yield will decrease; if the mass ratio is too low, the crystallization effect will be poor, easily triggering explosive nucleation, ultimately leading to a decline in product quality.

[0032] In some embodiments, the mass ratio of the first solvent to the second solvent is 1:(0.5-2). For example, it is a range consisting of 1:0.5, 1:0.8, 1:1.2, 1:1.6, 1:2, or any two of these.

[0033] When the mass ratio of the first solvent to the second solvent is within the above range, the solid-liquid separation efficiency can be improved to the greatest extent. When the mass ratio of the first solvent to the second solvent is too high, the impurity removal capacity of the system will decrease, thus affecting the overall quality of the product; when the mass ratio of the first solvent to the second solvent is too low, the product particle size is likely to be too small, failing to meet the expected particle size control requirements.

[0034] In some embodiments, the rate of cooling to the first temperature is 50°C / h - 150°C / h. For example, it is a range of 50°C / h, 80°C / h, 100°C / h, 120°C / h, 150°C / h, or any combination thereof.

[0035] In some embodiments, the rate of cooling to the second temperature is 2°C / h - 10°C / h. For example, it is a range of 2°C / h, 4°C / h, 6°C / h, 8°C / h, 10°C / h, or any combination thereof.

[0036] In some embodiments, the rate of cooling to the third temperature is 10°C / h - 30°C / h. For example, it is a range of 10°C / h, 15°C / h, 20°C / h, 25°C / h, 30°C / h, or any combination thereof.

[0037] In some embodiments, the first temperature is 45°C-60°C. For example, it is a range of 45°C, 48°C, 50°C, 53°C, 55°C, 58°C, 60°C, or any combination thereof.

[0038] In some embodiments, the second temperature is 35°C-50°C. For example, it is a range of 35°C, 38°C, 40°C, 43°C, 45°C, 48°C, 50°C, or any combination thereof.

[0039] In some embodiments, the third temperature is 5°C-30°C. For example, it is a range of 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, or any combination thereof.

[0040] When the cooling rates to the first temperature, the second temperature, and the third temperature, as well as the first, second, and third temperatures, are within the aforementioned ranges, the nucleation and growth rates can be balanced to the greatest extent within the metastable region, avoiding explosive nucleation that could lead to impurity inclusion, ensuring crystal purity and particle size uniformity, and simultaneously shortening the crystallization time.

[0041] If the cooling rate to the first temperature is too high, it is not conducive to accurately controlling the endpoint temperature of the first cooling process, i.e., the first temperature, and it is easy for the first mixture to reach saturation, which is not conducive to the subsequent crystallization nucleation process; if the cooling rate to the first temperature is too low, it will lead to an increase in crystallization time.

[0042] When the cooling rate to the second temperature and the cooling rate to the third temperature are too high, the crystallization process is prone to nucleation explosion, limiting crystal growth and leading to a decrease in the quality of dihydroxyethyl terephthalate products. When the cooling rate to the second temperature is too low, on the one hand, it will lead to an increase in crystallization time, and on the other hand, it will make the crystals more prone to breakage, resulting in a decrease in the quality of dihydroxyethyl terephthalate products.

[0043] When the first temperature is above 60℃, the crystallization time will be prolonged; when the first temperature is below 45℃, it will exceed the metastable region of the system, making the crystallization process difficult to control effectively, which will result in smaller crystal grain size.

[0044] When the second temperature is higher than 50℃ or lower than 35℃, the crystallization time becomes longer, and prolonged stirring makes the crystals more prone to breakage.

[0045] When the third temperature is above 30℃, it is easy to trigger explosive nucleation, resulting in smaller crystal grain size; when the third temperature is below 5℃, it will exceed the metastable region of the system, making it impossible to control the crystallization process well, which in turn will result in smaller crystal grain size.

[0046] In some embodiments, the mass concentration of dihydroxyethyl terephthalate (BHET) in the crude dihydroxyethyl terephthalate is 10%-50%. For example, it is a range of 10%, 20%, 30%, 40%, 50%, or any combination thereof.

[0047] By controlling the BHET concentration in crude dihydroxyethyl terephthalate within the aforementioned range and adjusting it with a mixed solvent, the BHET concentration in the system can fall within a suitable crystallization range. If the concentration exceeds 50%, the crystallization process is prone to explosive nucleation, which not only leads to smaller crystal size but also causes a decrease in product color value, affecting the overall product quality. If the concentration is below 10%, the system is prone to no crystal precipitation within the aforementioned temperature range, or the product yield will also decrease significantly.

[0048] Preferably, the mass concentration of dihydroxyethyl terephthalate (BHET) in the crude dihydroxyethyl terephthalate is 15%-30%. For example, it is a range of 15%, 20%, 23%, 28%, 30%, or any combination thereof.

[0049] In some embodiments, crude dihydroxyethyl terephthalate includes dihydroxyethyl terephthalate products obtained by alcoholysis of textile materials.

[0050] Specifically, the aforementioned textile materials include waste mixed textiles, such as at least one of polyester, nylon, cellulose, spandex, polypropylene, silk, wool, dyes, matting agents, oils, etc.

[0051] In some embodiments, the process of mixing crude dihydroxyethyl terephthalate with a solvent includes: performing alcoholysis on the crude dihydroxyethyl terephthalate using an alcohol solvent to obtain a second mixture containing crude dihydroxyethyl terephthalate and an alcohol solvent; mixing the second mixture with a solvent to obtain a first mixture; wherein the alcohol solvent includes ethylene glycol.

[0052] In some embodiments, before mixing the second mixture with the solvent, the method further includes: concentrating the second mixture to obtain a third mixture; and mixing the third mixture with the solvent to obtain a first mixture.

[0053] In some embodiments, concentration is carried out using a rotary evaporator.

[0054] In this application, the third mixture is concentrated using a rotary evaporator to achieve a dihydroxyethyl terephthalate mass concentration of 10%-50% in the crude dihydroxyethyl terephthalate product.

[0055] In some embodiments, after the third crystallization treatment at the third temperature, the method further includes: performing solid-liquid separation on the first mixture after the third crystallization treatment to obtain a crystalline product, drying the obtained crystalline product, and obtaining purified dihydroxyethyl terephthalate.

[0056] In some embodiments, the solid-liquid separation temperature is 5-30°C, for example, a range of 5, 10, 15, 20, 25, 30°C or any combination thereof.

[0057] In some embodiments, the solid-liquid separation time is 2-10 min, for example, a range of 2, 5, 8, 10 min or any combination thereof.

[0058] In some embodiments, the drying temperature is 40-60°C, for example, a range of 40, 50, 60°C or any combination thereof.

[0059] In some embodiments, the drying time is 6-24 hours, for example, a range of 6, 12, 18, 24 hours or any combination thereof.

[0060] Secondly, embodiments of this application provide a method for preparing dihydroxyethyl terephthalate, comprising: subjecting textile materials to alcoholysis to obtain crude dihydroxyethyl terephthalate; and purifying the crude dihydroxyethyl terephthalate according to the purification method described above to obtain the dihydroxyethyl terephthalate product.

[0061] The preparation method of dihydroxyethyl terephthalate provided in this application can optimize crystallization kinetics, shorten crystallization time, reduce solution viscosity, and thus improve solid-liquid separation performance, ultimately obtaining a BHET product with high purity and uniform particle size.

[0062] Thirdly, embodiments of this application provide a dihydroxyethyl terephthalate product, which is prepared by the purification method of crude dihydroxyethyl terephthalate described above or by the preparation method of dihydroxyethyl terephthalate described above.

[0063] The purity of dihydroxyethyl terephthalate (DHT) products is above 99%. The color value (L) of DHT products is 80-97, for example, a range of 80, 84, 88, 92, 95, 97, or any two of these. The color value (a) is -1 to 1, for example, a range of -1, 0, 1, or any two of these. The color value (b) is 0-3, for example, a range of 0, 1, 2, 3, or any two of these.

[0064] The volumetric median particle size (Dv50) of the dihydroxyethyl terephthalate product is 80 μm-200 μm. For example, a range of 80 μm, 100 μm, 140 μm, 180 μm, 200 μm, or any combination thereof.

[0065] The dihydroxyethyl terephthalate product provided in this application embodiment is prepared by the purification method of crude dihydroxyethyl terephthalate described above or by the preparation method of dihydroxyethyl terephthalate described above. It has high purity, low color value, and uniform particle size.

[0066] Example 1

[0067] Waste mixed textiles were subjected to ethylene glycol alcoholysis to obtain a second mixture containing crude dihydroxyethyl terephthalate (DHT) and ethylene glycol. The mass of the second mixture was 500 g, and the mass concentration of DHT in the second mixture was 10%. The second mixture was concentrated using a rotary evaporator with a vacuum of 3 kPa, a temperature of 130 °C, a rotation speed of 40 r / min, and a time of 40 min to obtain a third mixture. The mass of the third mixture was 210 g, and the mass concentration of DHT in the third mixture was 24%. The mass of ethylene glycol in the third mixture was 160 g.

[0068] The third mixture is mixed with a solvent, which includes water and ethanol; wherein the mass of water is 20g and the mass of ethanol is 20g; the mass ratio of water to ethanol is 1 / 1, and the mass ratio of the total mass of water and ethanol to the mass of dihydroxyethyl terephthalate in the crude dihydroxyethyl terephthalate is 1:1.25 (the mass ratio of the total mass of water and ethanol to the mass of ethylene glycol in the crude dihydroxyethyl terephthalate is 1:4), yielding 250g of the first mixture.

[0069] The first mixture was cooled to 55°C at a cooling rate of 100°C / h, then to 45°C at a cooling rate of 5°C / h, and then to 20°C at a cooling rate of 20°C / h. Solid-liquid separation was performed by vacuum filtration at 20°C for 10 min. The mixture was washed with a solvent (water to ethanol in a mass ratio of 1:1), vacuum filtered again, and then dried at 50°C for 24 h to obtain purified dihydroxyethyl terephthalate.

[0070] The bis(hydroxyethyl) terephthalate prepared in this embodiment has a purity of 99.9% and color values ​​of L (97.0), a (-0.1), and b (0.5).

[0071] Referring to Example 1, purified dihydroxyethyl terephthalate of Examples 2 to 11 was prepared. The detailed reaction conditions are shown in Table 1.

[0072] Comparative Example 1

[0073] Compared with Example 1, the method does not include "mixing the third mixture with a solvent, which includes water and ethanol", that is, no solvent is added, and the other conditions are the same as in Example 1.

[0074] Comparative Example 2

[0075] Compared with Example 1, the phrase "mix the third mixture with a solvent, the solvent including water and ethanol; wherein the mass of water is 20g and the mass of ethanol is 20g" is changed to "mix the third mixture with 40g of water", that is, the solvent is changed to include only the first solvent, and other conditions are the same as in Example 1.

[0076] Comparative Example 3

[0077] Compared with Example 1, the phrase "mix the third mixture with a solvent, the solvent including water and ethanol; wherein the mass of water is 20g and the mass of ethanol is 20g" is changed to "mix the third mixture with 40g of ethanol", that is, the solvent is changed to include only the second solvent, and other conditions are the same as in Example 1.

[0078] Comparative Example 4

[0079] Compared with Example 1, the phrase "the first mixture is cooled to 55°C at a cooling rate of 100°C / h, then to 45°C at a cooling rate of 2°C / h, and then to 20°C at a cooling rate of 20°C / h" is changed to "the first mixture is cooled to 20°C at a cooling rate of 100°C / h". In other words, the step-wise cooling is changed to linear cooling. All other conditions are the same as in Example 1.

[0080] Comparative Example 5

[0081] Compared with Example 1, the phrase "the first mixture is cooled to 55°C at a cooling rate of 100°C / h, then to 45°C at a cooling rate of 2°C / h, and then to 20°C at a cooling rate of 20°C / h" is changed to "the first mixture is cooled to 20°C at a cooling rate of 5°C / h". In other words, the stepped cooling is changed to linear cooling. All other conditions are the same as in Example 1.

[0082] Comparative Example 6

[0083] Compared with Example 1, the phrase "the first mixture is cooled to 55°C at a cooling rate of 100°C / h, then to 45°C at a cooling rate of 2°C / h, and then to 20°C at a cooling rate of 20°C / h" is changed to "the first mixture is cooled to 20°C at a cooling rate of 20°C / h". In other words, the stepped cooling is changed to linear cooling. All other conditions are the same as in Example 1.

[0084] Comparative Example 7

[0085] Compared with Example 1, the phrase "the first mixture is cooled to 55°C at a cooling rate of 100°C / h, then to 45°C at a cooling rate of 2°C / h, and then to 20°C at a cooling rate of 20°C / h" is changed to "the first mixture is cooled to 35°C at a cooling rate of 1°C / h, and then to 25°C at a cooling rate of 2°C / h". In other words, the three-stage stepped cooling is changed to a two-stage stepped cooling, and other conditions are the same as in Example 1.

[0086] The purity and color value data of dihydroxyethyl terephthalate prepared in each example and comparative example were tested according to the following procedure.

[0087] 1. Liquid chromatography determination of the purity of dihydroxyethyl terephthalate

[0088] 10 mg of bis(hydroxyethyl) terephthalate (BHET) prepared in the examples and comparative examples was dissolved in methanol to obtain a 0.2% (w / w) solution of BHET, which was used as the sample for liquid chromatography (LC). The LC instrument was an Agilent 1260, with a Poroshell 120EC-C18 column (4 μm, 4.6 x 150 mm), a column temperature of 35°C, and a UV detector. The mobile phase was 70% methanol and 30% water, the flow rate was 0.25 mL / min, and the detection wavelength was 243 nm. LC chromatograms of the BHET crystals prepared in the examples and comparative examples were obtained, and the purity of the BHET crystals was calculated.

[0089] 2. Colorimeter test of dihydroxyethyl terephthalate color value

[0090] Take 10g of the dihydroxyethyl terephthalate prepared in the examples and comparative examples as a sample, place it in a glass dish, make its surface flat, and test it in the setup3 mode of the colorimeter (ColorFlex EZ) to obtain the color values ​​L, a, and b of the BHET crystals prepared in the examples and comparative examples.

[0091] 3. Malvern particle size analyzer was used to test the particle size (Dv50) of dihydroxyethyl terephthalate.

[0092] Take 3 g of the dihydroxyethyl terephthalate prepared in the examples and comparative examples, dissolve it in pure water, add the sample to obtain a light-blocking range of 10%, click "Start", the data is automatically recorded, and the particle size Dv50 of the BHET crystals prepared in the examples and comparative examples is obtained.

[0093] Table 1

[0094]

[0095] Table 2

[0096]

[0097] As shown in Table 2, the BHET products purified by the preparation methods (mixed solvent system + three-stage stepped cooling crystallization) provided in Examples 1-11 all exhibited significantly higher purity and color values ​​than Comparative Examples 1-7. Among them, Comparative Example 1, which did not use any solvent, had the lowest product purity (87.2%), the worst color (L value 67.1, b value 5.24), and the smallest particle size (23 μm). The superior performance of the products from Examples 1-11 compared to Comparative Examples 1-7 indicates that the purification method described above can significantly improve the quality of BHET products. The inferior performance of the product from Comparative Example 1 compared to Examples 1-11 and Comparative Examples 2-3 demonstrates that the purification process without adding a mixed solvent cannot achieve efficient purification of BHET, resulting in extremely poor technical performance.

[0098] Comparing the product performance differences under different process conditions reveals that the purification process using a single solvent (Comparative Examples 2-3) resulted in lower product purity and color value compared to Examples 1-11. This indicates that a single solvent cannot achieve the synergistic effect of reducing system viscosity, optimizing crystal growth, and improving impurity separation efficiency. The purification process using a non-three-stage stepped cooling process (Comparative Examples 4-7) also resulted in lower product purity and color value compared to Example 1. This result demonstrates that the lack of a three-stage stepped cooling process makes it impossible to precisely control crystallization kinetics, easily leading to explosive nucleation, resulting in finer crystal particles, increased impurity residue, and decreased product quality.

[0099] In summary, the purification method of "mixed solvent system + three-stage stepped cooling crystallization" adopted in this invention can effectively optimize crystallization kinetics, shorten crystallization time, and reduce solution viscosity, thereby improving solid-liquid separation performance. The prepared BHET product has better solubility (less impurity residue), excellent product quality, and can effectively improve product purity, optimize product color, and increase crystal particle size. It can be used as a high-purity BHET raw material in polyester recycling and other fields. At the same time, this method can avoid problems such as nucleation and impurity encapsulation during crystallization. The product has uniform particle size, is not prone to impurity residue, and has good purity stability. The BHET product purified by this method has better performance indicators than products prepared by existing conventional purification processes. Even when deviating from the optimal value limit, it can still maintain good product quality and has a long-term stable purification effect, providing a feasible technical solution for the high-value recycling of waste polyester.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for purifying crude dihydroxyethyl terephthalate, characterized in that, include: Crude dihydroxyethyl terephthalate is mixed with a solvent to obtain a first mixture; wherein the solvent includes a first solvent and a second solvent; The first mixture is subjected to step cooling, during which dihydroxyethyl terephthalate in the first mixture crystallizes to obtain purified dihydroxyethyl terephthalate. The process of stepwise cooling of the first mixture includes: cooling the first mixture to a first temperature, performing a first crystallization treatment at the first temperature, cooling it to a second temperature, performing a second crystallization treatment at the second temperature, cooling it to a third temperature, and performing a third crystallization treatment at the third temperature to obtain the purified dihydroxyethyl terephthalate.

2. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The first solvent includes water and / or isopropanol; And / or, the second solvent includes at least one of acetonitrile, ethyl acetate, propyl acetate, acetone, methanol, ethanol, propylene glycol, diethylene glycol, glycerol, etc.

3. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The mass ratio of the solvent to the crude dihydroxyethyl terephthalate is (0.5-18):

1.

4. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The mass ratio of the first solvent to the second solvent is 1:(0.5-2).

5. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The rate of cooling to the first temperature is 50℃ / h - 150℃ / h; And / or, the rate of cooling to the second temperature is 2°C / h - 10°C / h; And / or, the rate of cooling to the third temperature is 10℃ / h - 30℃ / h; And / or, the first temperature is 45°C-60°C; And / or, the second temperature is 35°C-50°C; And / or, the third temperature is 5℃-30℃.

6. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The crude dihydroxyethyl terephthalate contains 10%-50% by mass.

7. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The crude dihydroxyethyl terephthalate includes dihydroxyethyl terephthalate products obtained from the alcoholysis of textile materials.

8. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The process of mixing crude dihydroxyethyl terephthalate with a solvent includes: The crude dihydroxyethyl terephthalate was subjected to alcoholysis using an alcohol solvent to obtain a second mixture containing the crude dihydroxyethyl terephthalate and the alcohol solvent; The second mixture is mixed with the solvent to obtain the first mixture; The alcohol solvents include ethylene glycol.

9. A method for preparing dihydroxyethyl terephthalate, characterized in that, include: Textile materials were subjected to alcoholysis to obtain crude dihydroxyethyl terephthalate. The crude dihydroxyethyl terephthalate was purified according to any one of claims 1-8 to obtain the dihydroxyethyl terephthalate product.

10. A dihydroxyethyl terephthalate product, characterized in that, The dihydroxyethyl terephthalate product is prepared by the purification method of crude dihydroxyethyl terephthalate according to any one of claims 1-8 or by the preparation method of dihydroxyethyl terephthalate according to claim 9. The purity of the dihydroxyethyl terephthalate product is above 99%; The color value L of the dihydroxyethyl terephthalate product is 80-97, the color value a is -1-1, and the color value b is 0-3. The median particle size of the dihydroxyethyl terephthalate product is 80 μm-200 μm.

Images