A method for purifying high purity dimethyl terephthalate

Abstract

The application discloses a kind of purification methods of high-purity dimethyl terephthalate, it is related to high polymer material technical field;The purification method of the application uses industrial-grade crude DMT as raw material, not only sequentially through the synergistic effect of six core steps of melting, solution washing, distillation, recrystallization, drying and filtration, effectively remove the residual terephthalic acid, methanol, methyl benzoate, colored impurities, mechanical impurities and trace moisture in crude DMT, significantly improve the purity of DMT product (purity ≥99.95%);Moreover, by optimizing the solvent ratio, distillation pressure temperature gradient, crystallization cooling rate, while ensuring the purification effect, effectively reduce the loss of DMT in the purification process, significantly improve the overall yield of product (yield ≥92%);The process operation of the purification method of the application is simple, the condition is controllable, raw material loss is small, energy consumption is low, suitable for industrialized continuous batch production, with very high popularization and application value.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, mainly to the field of polymer material purification technology, and specifically to a method for purifying high-purity dimethyl terephthalate. Background Technology

[0002] Dimethyl terephthalate (DMT) is an important organic chemical intermediate widely used in polyester chips, high-end chemical fibers, optical films, engineering plastics, and coating resins. It is a core raw material for the preparation of polymer materials such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). With the rapid development of the high-end polyester materials and optical-grade film industries, the market has placed extremely high demands on the purity, color, and impurity content of DMT products. Ordinary industrial-grade DMT, due to its high impurity content, cannot meet the needs of high-end polymerization production, easily leading to problems such as uneven molecular weight of polymerization products, yellowing color, and decreased mechanical properties.

[0003] Currently, the main impurities in crude DMT produced industrially include unreacted terephthalic acid, methanol, dimethyl methyl benzoate, isomer impurities, colored tar impurities, trace amounts of moisture, and mechanical impurities. Existing DMT purification processes mostly employ single distillation or single recrystallization methods, which have significant drawbacks: single vacuum distillation is insufficient to completely remove isomers with similar boiling points and trace colored impurities, typically achieving a product purity of only around 99.5%; single solvent recrystallization suffers from high solvent loss, severe crystal encapsulation of impurities, low product yield, and the tendency for residual solvent to affect product quality; furthermore, the long high-temperature residence time during traditional purification processes easily leads to the oxidative decomposition of DMT, generating new impurities and resulting in poor product color, failing to meet the requirements of high-end applications.

[0004] Most existing purification technologies suffer from limitations such as the inability to achieve graded removal of impurities, poor simultaneous treatment of light components, heavy components, and polar and non-polar impurities, insufficient purification precision, high energy consumption, and poor industrial adaptability, hindering industrial applications. Therefore, developing a high-purity DMT purification method that achieves thorough impurity removal, high product purity, stable yield, low energy consumption, and suitability for industrial production has become a pressing technical problem for those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of low product purity and low yield in existing DMT purification processes, and to propose a purification method for high-purity dimethyl terephthalate.

[0006] To achieve the above objectives, the present invention provides a method for purifying high-purity dimethyl terephthalate, comprising the following steps: (1) Pretreatment: The crude DMT raw material to be purified is melted and kept at 160-175℃ for 30-60 minutes under a protective atmosphere to obtain molten crude DMT liquid; (2) Washing: Under a protective atmosphere, a composite organic solvent is added to the molten crude DMT liquid, and after stirring and mixing evenly, the temperature is lowered to 80-95℃ and stirred and washed for 20-40 minutes; after standing and separating into layers, the organic phase is obtained; the composite organic solvent is a mixture of n-hexane and ethyl acetate with a mass ratio of 4-9:1. (3) Distillation: Under a protective atmosphere, the organic phase is processed by a three-stage gradient heating and vacuum distillation process to obtain DMT fraction; the vacuum degree of the three-stage gradient heating and vacuum distillation process is 0.085~0.098MPa; (4) Recrystallization: Under a protective atmosphere, the DMT fraction is first cooled to 110-120℃ at 8-10℃ / min and kept at a constant temperature for 15-25 min; then cooled to 30-40℃ at 2-4℃ / min and allowed to stand for crystallization for 2-4 h, and filtered to obtain crude DMT crystals. (5) Drying: Under a protective atmosphere, the crude DMT crystal is dried for 4 to 6 hours at a vacuum of 0.09 to 0.095 MPa and a temperature of 70 to 85°C to obtain dried DMT crystal; (6) Filtration: Under a protective atmosphere, dry DMT crystals are melted, filtered, and cooled to solidify to obtain high-purity DMT products.

[0007] This invention discloses a method for purifying high-purity dimethyl terephthalate (DMT). Using industrial-grade crude DMT as raw material, the method employs a synergistic effect of six core steps: melting, washing, distillation, recrystallization, drying, and filtration. Washing, as a pretreatment unit, can selectively remove polar organic acids, residual alcoholysis aids, and trace metal ions, thus preventing these impurities from entering the distillation step and causing DMT thermal decomposition and coking. It also eliminates the problem of impurities adsorbing and encapsulating crystal nuclei during the crystallization stage. Distillation significantly reduces the impurity load in subsequent recrystallization, eliminating the need for repeated crystallization. Recrystallization can separate isomers and trace conjugated colored impurities that cannot be removed in the first two steps, and relies on differentiated crystallization rates for precise separation. Slow, temperature-controlled crystal growth yields regular, large-particle crystals, reducing mother liquor entrainment losses. The three purification steps (washing, distillation, etc.) further enhance the purification process. This invention employs a method that combines ordered coupling and synergistic processes (including recrystallization) to remove different types of impurities step by step, addressing the limitations of single purification methods and the inability of simple combined processes to simultaneously achieve both high purity and high yield. It effectively removes residual terephthalic acid, methanol, methyl parabens, colored impurities, mechanical impurities, and trace amounts of moisture from crude DMT, significantly improving the purity of the DMT product (purity ≥ 99.95%). Furthermore, by specifically optimizing solvent ratios, distillation pressure and temperature gradients, and crystallization cooling rates, it effectively reduces DMT loss during purification while ensuring purification efficiency, significantly improving the overall product yield (yield ≥ 92%). This purification method is simple to operate, has controllable conditions, low raw material loss, and low energy consumption, making it suitable for continuous industrial-scale production and possessing high application value.

[0008] This invention uses a compound solvent of n-hexane and ethyl acetate, which balances the ability to dissolve non-polar impurities and extract polar impurities. Compared with a single solvent, it can remove polar organic acids and alcohols to the maximum extent without damaging the DMT raw material. The three-stage gradient distillation accurately distinguishes impurities with different boiling points, avoiding the azeotropic residue of impurities in traditional constant temperature distillation. The stepwise crystal growth process can effectively reduce impurities encapsulated in crystals and improve crystal purity and crystal regularity. The nitrogen protection throughout the process completely solves the problem of high-temperature oxidation and discoloration of DMT, ensuring the high whiteness quality of the product.

[0009] In step (1), preferably, the purity of the crude DMT raw material is 95.0% to 98.5%.

[0010] Preferably, the heating rate during the melting process is 5–8 °C / min.

[0011] In step (2), preferably, the mass ratio of the composite organic solvent to the crude DMT raw material is 0.4 to 0.6:1; the preferred mass ratio can better dissolve the polar impurities in DMT, which is beneficial to improving the purity of DMT products.

[0012] Preferably, the stirring speed for the stirring and washing process is 200-300 r / min.

[0013] In step (3), preferably, in the three-stage gradient heating and vacuum distillation process, the first stage distillation temperature is 120-135℃ and the holding time is 15-20min; the second stage distillation temperature is 140-155℃ and the holding time is 20-25min; the third stage distillation temperature is 165-172℃ and the holding time is 30-40min; using this preferred distillation condition can better remove impurity components from DMT.

[0014] Preferably, in the three-stage gradient heating and vacuum distillation process, the heating rate throughout the process is 5-8℃ / min.

[0015] In step (5), preferably, the drying is carried out by a segmented heating drying method, specifically: first, preheat drying at 55-65℃ for 0.5-1.5h, and then heating to 70-85℃ for constant temperature drying for 4-6h; the preferred drying method can effectively avoid crystal cracking and solvent residue.

[0016] In step (6), preferably, the cooling and curing adopts a gradient cooling curing method, specifically: first cool down to 110-130℃ and keep at a constant temperature for 8-12 minutes, and then cool down to room temperature; the preferred cooling method can obtain DMT solid products with more uniform particle size.

[0017] Preferably, in the purification method, the protective atmosphere is nitrogen.

[0018] Preferably, the airflow rate of the protective atmosphere is 10-15 L / min.

[0019] Beneficial effects of the present invention

[0020] 1. The purification method of this invention effectively removes residual terephthalic acid, methanol, methyl parabens, colored impurities, mechanical impurities and trace amounts of moisture from crude DMT through the synergistic effect of six core steps: melting, washing, distillation, recrystallization, drying and filtration, thereby significantly improving the purity of DMT products (purity ≥ 99.95%).

[0021] 2. The purification method of this invention optimizes the solvent ratio, distillation pressure and temperature gradient, and crystallization cooling rate to ensure purification effect while effectively reducing DMT loss during purification and significantly improving the overall product yield (yield ≥ 92%).

[0022] 3. The purification method of this invention is protected by inert gas throughout the process, which effectively prevents DMT from being oxidized, yellowed and decomposed at high temperature, and avoids the generation of new impurities; the solvents used have moderate boiling point differences, are easy to recycle and reuse, have low solvent loss, and no large amount of wastewater or waste residue is discharged, making it green and environmentally friendly.

[0023] 4. The purification method of this invention has simple operation, controllable conditions, low raw material loss, and low energy consumption. It is suitable for continuous industrial batch production and has extremely high application value. Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0025] Example 1

[0026] A method for purifying high-purity DMT, the specific steps of which are as follows: (1) Pretreatment: Take 100 kg of industrial grade crude DMT with a purity of 98.2% and put it into a closed melting vessel. Protect it with nitrogen flow rate of 12 L / min and heat it to 168℃ at a rate of 6℃ / min. Keep it at a constant temperature for 45 min to remove free water and low boiling point volatile impurities and obtain molten crude DMT. (2) Washing: Add 50 kg of composite organic solvent (mass ratio of n-hexane: ethyl acetate = 8.5: 1.5) according to the mass ratio of raw material to composite organic solvent 1:0.5, stir and mix at 250 r / min, cool down to 90℃ and wash at a constant temperature for 30 min, let stand for 20 min to separate the layers, separate the lower layer to obtain the organic phase, and discard the upper layer of polar impurity waste liquid. (3) Distillation: The organic phase was transferred into a vacuum distillation apparatus and heated at a rate of 6℃ / min. The vacuum degree of the system was maintained at -0.092 MPa (gauge pressure). The first stage was distilled at 130℃ for 18 min to remove light component impurities; the second stage was distilled at 148℃ for 22 min to remove intermediate impurity components; and the third stage was distilled at 168℃ for 35 min to obtain DMT fraction. (4) Recrystallization: The fraction was transferred to a crystallization vessel, protected by nitrogen with a slight positive pressure, and cooled to 115°C at a rate of 9°C / min for 20 min to grow crystals. Then, the temperature was lowered to 35°C at a rate of 3°C / min and allowed to stand for crystallization for 3 h. The crystals were then filtered under negative pressure of 0.075 MPa to obtain crude DMT crystals. (5) Drying: Place the crude crystal product into a vacuum drying oven, preheat at 60°C for 1 hour under a vacuum of 0.092 MPa, and then heat to 78°C and dry for 5 hours to remove residual solvent and trace moisture. (6) Filtration: After drying, the crystal is heated to melt and filtered through a 0.22μm precision microporous membrane. After being kept at a constant temperature of 120℃ for 10 minutes, it is naturally cooled and solidified to obtain a high-purity DMT product.

[0027] Testing showed that the DMT product prepared in this embodiment had a purity of 99.97%, a moisture content of 0.008%, an ash content of 0.003%, and a product yield of 93.2%.

[0028] Example 2

[0029] A method for purifying high-purity DMT, the specific steps of which are as follows: (1) Pretreatment: Take 100 kg of industrial grade crude DMT with a purity of 97.5%, put it into a closed melting vessel, protect it with nitrogen flow rate of 10 L / min, heat it to 165℃ at a rate of 5℃ / min, keep it at a constant temperature for 50 min, remove free water and low boiling point volatile impurities, and obtain molten crude DMT. (2) Washing: Add 45 kg of composite organic solvent (mass ratio of n-hexane: ethyl acetate = 4:1) according to the mass ratio of raw material to composite organic solvent 1:0.45, stir and mix at 220 r / min, cool down to 85℃ and wash at a constant temperature for 35 min, let stand for 30 min to separate the layers, separate the lower layer to obtain the organic phase, and discard the upper layer of polar impurity waste liquid. (3) Distillation: The organic phase was transferred into a vacuum distillation apparatus and heated at a rate of 5℃ / min. The vacuum degree of the system was maintained at -0.09MPa (gauge pressure). The first stage was distilled at 125℃ for 20min to remove light component impurities; the second stage was distilled at 145℃ for 25min to remove intermediate impurity components; the third stage was distilled at 166℃ for 38min to obtain DMT fraction. (4) Recrystallization: The fraction was transferred to a crystallization vessel, protected by nitrogen with a slight positive pressure, and cooled to 112℃ at a rate of 8℃ / min for 22 min to grow crystals. Then, the temperature was lowered to 32℃ at a rate of 2.5℃ / min, and the crystals were allowed to stand for 3.5 h to crystallize. The product was then filtered under negative pressure of 0.07 MPa to obtain crude DMT crystals. (5) Drying: Place the crude crystal product into a vacuum drying oven, preheat at 60°C for 1 hour under a vacuum of 0.09 MPa, and then heat to 75°C and dry for 6 hours to remove residual solvent and trace moisture. (6) Filtration: After drying, the crystal is heated to melt and filtered through a 0.22μm precision microporous membrane. After being kept at a constant temperature of 110℃ for 12 minutes, it is naturally cooled and solidified to obtain a high-purity DMT product.

[0030] The DMT product prepared in this embodiment has a purity of 99.96%, a moisture content of 0.009%, an ash content of 0.004%, and a yield of 92.5%, according to the test results.

[0031] Example 3

[0032] A method for purifying high-purity DMT, the specific steps of which are as follows: (1) Pretreatment: Take 100 kg of industrial grade crude DMT with a purity of 95.1%, put it into a closed melting vessel, protect it with nitrogen flow rate of 15 L / min, heat it to 175℃ at a rate of 8℃ / min, keep it at a constant temperature for 30 min, remove free water and low boiling point volatile impurities, and obtain molten crude DMT. (2) Washing: Add 40 kg of composite organic solvent (mass ratio of n-hexane: ethyl acetate = 9:1) according to the mass ratio of raw material to composite organic solvent 1:0.4, stir and mix at 300 r / min, cool to 95℃ and wash at a constant temperature for 20 min, let stand for 30 min to separate the layers, separate the lower layer to obtain the organic phase, and discard the upper layer of polar impurity waste liquid. (3) Distillation: The organic phase was transferred into a vacuum distillation apparatus and heated at a rate of 8°C / min. The vacuum degree of the system was maintained at -0.085 MPa (gauge pressure). The first stage was distilled at 135°C for 15 min to remove light component impurities; the second stage was distilled at 155°C for 20 min to remove intermediate impurity components; and the third stage was distilled at 172°C for 30 min to obtain DMT fraction. (4) Recrystallization: The fraction was transferred to a crystallization vessel, protected by nitrogen with a slight positive pressure, and cooled to 120°C at a rate of 10°C / min for 15 min to grow crystals. Then, the temperature was lowered to 40°C at a rate of 4°C / min, and the crystals were allowed to stand for 4 h. The product was then filtered under negative pressure of 0.08 MPa to obtain crude DMT crystals. (5) Drying: Place the crude crystal product into a vacuum drying oven, heat it to 85°C under a vacuum of 0.095MPa and dry it for 4 hours to remove residual solvent and trace moisture; (6) Filtration: After drying, the crystal is heated to melt, filtered through a 0.22μm precision microporous membrane, and then naturally cooled and solidified to obtain a high-purity DMT product.

[0033] The DMT product prepared in this embodiment has a purity of 99.95%, a moisture content of 0.011%, an ash content of 0.005%, and a yield of 92.1%, according to the test results.

[0034] Comparative Example 1: Industrial-grade crude DMT was purified using a conventional solvent-free, single-stage vacuum distillation process. The process conditions were fixed: constant temperature 170℃, vacuum 0.09 MPa, continuous distillation for 90 min, and the collected fraction was allowed to cool naturally for crystallization. Testing showed that the purified DMT product had a purity of 99.42%, a moisture content of 0.025%, and a product yield of 84.1%. The product had a slightly yellowish appearance and contained high levels of polar impurities and light component impurities that are difficult to remove at high temperatures. Overall, the purification performance was significantly lower than that of the process described in this invention.

[0035] Comparative Example 2

[0036] A method for purifying DMT is based on Example 1, with only the following adjustments: step (3) is constant temperature distillation at 148°C for 75 minutes, while the remaining process parameters remain the same as in Example 1.

[0037] Testing revealed that the purified DMT product had a purity of 99.51% and a moisture content of 0.022%, with a product yield of only 78.3%. The reasons for this were analyzed as follows: the low-temperature isothermal distillation mode was insufficient in temperature, failing to vaporize and distill off the majority of the DMT. Large molecular association impurities and encapsulated impurities in the crude DMT were difficult to fully dissociate, resulting in significant losses of the effective DMT components trapped in the bottom residue, drastically reducing the product yield. Furthermore, the process failed to effectively remove light component impurities, leaving a large amount of intermediate impurities and excessive heavy component impurities in the distillate. Incomplete separation of the system ultimately led to a decrease in DMT product purity and a poor overall purification effect.

[0038] Comparative Example 3

[0039] A method for purifying DMT, the specific steps of which are based on Example 1, with only the following adjustment: Step (3) adopts constant temperature distillation at a temperature of 168°C and a time of 75 min.

[0040] Testing revealed that the purified DMT product had a purity of 99.38%, a moisture content of 0.028%, and a product yield of 82.5%. Simple high-temperature, constant-temperature, long-duration distillation caused slight thermal decomposition and oxidation side reactions of the DMT monomer, generating trace amounts of colored impurities and high-boiling-point byproducts. Simultaneously, some light, beneficial components were lost due to excessive vaporization. The product's yellowing phenomenon was significantly aggravated, and the residual amount of oxidized impurities increased markedly. Excessive temperature not only reduced product purity and yield but also directly deteriorated the product's appearance quality, failing to meet the standards for high-purity DMT.

[0041] Comparative Example 4

[0042] A method for purifying DMT is based on Example 1, with the only adjustment being that the distillation temperature of the third stage in step (3) is 175°C, while the remaining process parameters remain the same as in Example 1.

[0043] After testing, the purity of the purified DMT product was 99.72%, the moisture content was 0.014%, and the product yield was 87.4%.

[0044] Comparative Example 5

[0045] A method for purifying DMT is based on Example 1, with the only adjustment being that the distillation temperature of the second stage of step (3) is 160°C, while the remaining process parameters remain the same as in Example 1.

[0046] Testing revealed that the purified DMT product had a purity of 99.63%, a moisture content of 0.016%, and a product yield of 88.2%. (Comparative Example 6)

[0047] A method for purifying DMT is based on Example 1, with the only adjustment being that the distillation temperature of the first stage in step (3) is 140°C, while the remaining process parameters remain the same as in Example 1.

[0048] After testing, the purity of the purified DMT product was 99.68%, the moisture content was 0.017%, and the product yield was 88.7%.

[0049] Comparative Examples 4-6 were performed with the three-stage distillation temperatures adjusted outside the preferred range of this invention. The results showed that the temperature parameters of each stage of the three-stage gradient distillation significantly affected the DMT purification effect. Excessively high temperatures in the third stage caused thermal decomposition of DMT, producing thermally degraded impurities; excessively high temperatures in the second stage caused a large amount of intermediate-boiling-point impurities to be distilled into the main fraction; excessively high temperatures in the first stage prevented the complete removal of light component impurities, which then entered subsequent processes with the DMT. All of these situations resulted in a significant decrease in the purity of the finished product. This demonstrates that the three-stage distillation temperature range defined in this invention can achieve the orderly separation of light impurities, intermediate impurities, and the main product, yielding a high-purity DMT product.

[0050] Comparative Example 7

[0051] A method for purifying DMT, the specific steps of which are based on Example 1, with only the following adjustment: the vacuum degree in step (3) of the distillation process is adjusted to 0.08 MPa.

[0052] Testing revealed that the purified DMT product had a purity of 99.45%, a moisture content of 0.026%, and a yield of 83.2%. Insufficient vacuum led to an increase in the system's boiling point and a decrease in the precision of component separation. Mixed impurities with similar boiling points in the crude DMT could not be effectively separated from the DMT, resulting in severe impurity entrainment. The product exhibited slight turbidity, and the residual amount of azeotropic impurities was significantly increased. The purification process was not refined enough, making it difficult to achieve high-quality purification results.

[0053] Comparative Example 8

[0054] A method for purifying DMT is based on Example 1, with the only adjustment being that the vacuum degree in step (3) of the distillation process is adjusted to 0.13 MPa, while the other process parameters remain the same as in Example 1.

[0055] Testing revealed that the purified DMT product had a purity of 99.42%, a moisture content of 0.024%, and a product yield of 76.5%. (Comparative Example 9)

[0056] A method for purifying DMT, the specific steps of which are based on Example 1, only the following adjustment is made: in step (2), the mass ratio of the composite organic solvent to the crude DMT raw material is 0.35:1, that is, the amount added is 35kg.

[0057] Testing revealed that the purified DMT product had a purity of 99.53%, a moisture content of 0.024%, and a yield of 81.6%. Excessive organic solvent increases the system's vaporization load and prolongs component separation time. Furthermore, excess solvent easily forms azeotropes with trace impurities, leaving some solvent residue in the distillate and carrying small amounts of impurities into the product system. Incomplete impurity removal resulted in the solid residue containing a large amount of effective DMT components, completely negating the advantages of solvent-assisted purification, and rendering the purification effect close to that of traditional single distillation.

[0058] Comparative Example 10

[0059] A method for purifying DMT, the specific steps of which are based on Example 1, only the following adjustment is made: in step (2), the mass ratio of the composite organic solvent to the crude DMT raw material is 0.65:1, that is, the amount added is 65kg.

[0060] Testing revealed that the purified DMT product had a purity of 99.47%, a moisture content of 0.031%, and a yield of 83.5%. Excessive organic solvent increases the vaporization load of the system, prolongs the component separation time, and easily forms azeotropes with trace impurities. Some solvent remains in the distillate and carries a small amount of impurities into the product system. The product's moisture content increased significantly, and trace solvent residues were present, resulting in new soluble impurities and a decrease in product purity and cleanliness.

[0061] Comparative Example 11

[0062] A method for purifying DMT, the specific steps of which are based on Example 1, with only the following adjustment: in step (3), during the segmented distillation, the heating rate is 10℃ / min.

[0063] Testing revealed that the purified DMT product had a purity of 99.44%, a moisture content of 0.027%, and a yield of 82.8%. The excessively rapid heating rate resulted in uneven heating of the crude DMT system, with the surface material rapidly vaporizing while the internal material was not fully heated and dissociated, leading to a phenomenon of "surface vaporization and internal inclusions." A large number of encapsulated impurities were not removed in time and were carried into the distillate. The product had uneven impurity distribution, poor batch stability, and an extremely high risk of hidden impurity residues.

[0064] Comparative Example 12

[0065] A method for purifying DMT is based on Example 1, with the following adjustments: in step (3), the heating rate during segmented distillation is 3℃ / min, and the remaining process parameters are consistent with those in Example 1.

[0066] Testing revealed that the purified DMT product had a purity of 99.75%, a moisture content of 0.013%, and a product yield of 88.0%.

[0067] This invention specifies an optimal heating rate of 5–8 °C / min. Comparative Example 12 reduces the heating rate to 3 °C / min; an excessively slow heating rate significantly prolongs the high-temperature heating time of the material. During the three-stage distillation process, the DMT material is under a prolonged high-temperature negative pressure environment, which easily leads to slight thermal oxidation and thermal degradation reactions, generating trace amounts of high-boiling-point byproducts. Simultaneously, the excessively long heating time causes some light component impurities to fully fuse with the material, making rapid removal impossible within the corresponding temperature range. Compared to the optimal process of this invention, the product purity decreases slightly.

[0068] Meanwhile, the slower heating rate allows for smoother vaporization of materials at each temperature range, resulting in a slight reduction in material loss. Therefore, the yield is slightly higher than that of the comparative process of the extreme difference process. However, the overall purification effect is significantly worse than the heating rate range specified in this invention, proving that the heating rate of 5-8℃ / min in this invention has a significant optimizing effect on the purification effect of DMT.

[0069] Comparative Example 13

[0070] A method for purifying DMT, the specific steps of which are based on Example 1, with only the following adjustments: in step (3), the first distillation period is only 12 min; the second period is only 15 min; and the third period is only 25 min.

[0071] Testing revealed that the purified DMT product had a purity of 99.50%, a moisture content of 0.023%, and a yield of 80.2%. However, the heat treatment time was insufficient, the separation equilibrium between impurities and DMT components was not fully established, the removal of light and heavy component impurities was inadequate, and the effective DMT components were not completely vaporized and collected. Consequently, the yield was significantly reduced, impurities were not completely removed, and both purification efficiency and product qualification rate were low.

[0072] Comparative Example 14

[0073] A method for purifying DMT, referring to the raw materials and process parameters of Example 1, only adjusts the process operation sequence, changing the order of distillation and recrystallization, that is, recrystallizing crude DMT first, and then distilling for purification, while the remaining process conditions are completely consistent with those of Example 1.

[0074] The specific operating steps are as follows: Take the same crude DMT raw material as in Example 1, put it into the recrystallization solvent for recrystallization, cool and crystallize, filter to obtain crude DMT crystals; send the obtained crude DMT crystals into a distillation apparatus for vacuum distillation, collect the main DMT fraction, and obtain the purified DMT product.

[0075] The obtained DMT product was tested and found to have a purity of 98.2%, a yield of 76.3%, and an APHA color number of 110.

[0076] Effect Analysis: The comparison between Example 1 and Comparative Example 14 shows that, with the raw materials and process parameters of each unit remaining unchanged, simply changing the order of distillation and recrystallization resulted in a decrease in product purity from 99.97% to 98.2%, with a significant deterioration in yield and product color. The reason for this is that some light-boiling-point impurities in crude DMT are easily trapped inside the recrystallized crystals, making it difficult to effectively remove these trapped impurities during subsequent distillation. Simultaneously, the recrystallized crystals still carry some colloidal impurities, which are prone to thermal decomposition under the high-temperature conditions of subsequent distillation, generating new colored impurities and further worsening product indicators. This demonstrates that the "distillation first, recrystallization later" process of this invention, which removes most light and heavy impurities through distillation and then achieves deep purification through recrystallization, exhibits a significant synergistic effect between the processes, making it an indispensable technical feature for obtaining high-purity DMT products.

[0077] Comparative Example 15

[0078] A method for purifying DMT is based on Example 1, except that the order of the processes is adjusted: the order of distillation and washing is changed, and the operation order of distillation and washing is changed to washing first and then distillation, while the other operation steps remain unchanged.

[0079] The specific process flow is as follows: after pretreatment and melting to remove impurities from industrial-grade crude DMT, solvent washing and layering to remove impurities are carried out directly. Then, the obtained organic phase is subjected to three-stage vacuum distillation, followed by recrystallization, vacuum drying and precision filtration processes to finally obtain the finished DMT product.

[0080] The DMT product prepared in this comparative example had a purity of 98.60%, a moisture content of 0.028%, an ash content of 0.015%, and a product yield of 78.1%.

[0081] Effect Analysis: Compared to Example 1 (finished product purity 99.97%), which uses a core process of distillation followed by washing, the comparative example's process arrangement of washing followed by distillation has significant defects. A large amount of gum and difficult-to-remove impurities in the crude DMT are not removed before distillation; washing alone can only remove surface impurities, leaving significant residue of internally encapsulated impurities. During subsequent high-temperature distillation, these residual impurities are easily pyrolyzed to generate new impurities, which cannot be completely separated and removed, ultimately resulting in a significant decrease in product purity and yield, and an increase in impurity content. This fully verifies the synergistic effect of the distillation-washing process in this invention, which is a key core process for preparing high-purity DMT.

[0082] As can be seen from the above embodiments and comparative examples, adjusting any one of the core parameters—temperature, vacuum, solvent ratio, heating rate, or holding time—in the purification method of this invention will lead to significant defects in at least one of the following indicators of the DMT purified product: purity, yield, moisture content, appearance quality, impurity residue, and batch stability. Traditional single vacuum distillation processes rely solely on temperature and vacuum for single-dimensional control, failing to achieve a balance between precise impurity removal and high yield. In contrast, this invention achieves synergistic effects by limiting optimal distillation temperature, vacuum, low-polarity solvent ratio, heating rate, holding time, and process sequence. This multi-dimensional synergy solves the problems of low yield, high impurity residue, product yellowing, and unstable quality in existing purification processes.

[0083] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for purifying high-purity dimethyl terephthalate, characterized in that, Includes the following steps: (1) Pretreatment: The crude DMT raw material to be purified is melted and kept at 160-175℃ for 30-60 minutes under a protective atmosphere to obtain molten crude DMT liquid; (2) Washing: Under a protective atmosphere, a composite organic solvent is added to the molten crude DMT liquid, and after stirring and mixing evenly, the temperature is lowered to 80-95℃ and stirred and washed for 20-40 minutes; after standing and separating into layers, the organic phase is obtained; the composite organic solvent is a mixture of n-hexane and ethyl acetate with a mass ratio of 4-9:

1. (3) Distillation: Under a protective atmosphere, the organic phase is processed by a three-stage gradient heating and vacuum distillation process to obtain DMT fraction; the vacuum degree of the three-stage gradient heating and vacuum distillation process is 0.085~0.098MPa; (4) Recrystallization: Under a protective atmosphere, the DMT fraction is first cooled to 110-120℃ at 8-10℃ / min and kept at a constant temperature for 15-25 min; then cooled to 30-40℃ at 2-4℃ / min and allowed to stand for crystallization for 2-4 h. The crude DMT crystals are obtained by filtration. (5) Drying: Under a protective atmosphere, the crude DMT crystal is dried for 4 to 6 hours at a vacuum of 0.09 to 0.095 MPa and a temperature of 70 to 85°C to obtain dried DMT crystal; (6) Filtration: Under a protective atmosphere, dry DMT crystals are melted, filtered, and cooled to solidify to obtain high-purity DMT products.

2. The purification method according to claim 1, characterized in that, In step (1), preferably, the purity of the crude DMT raw material is 95.0% to 98.5%.

3. The purification method according to claim 1, characterized in that, In step (1), the heating rate during the melting process is 5 to 8 °C / min.

4. The purification method according to claim 1, characterized in that, In step (2), the mass ratio of the composite organic solvent to the crude DMT raw material is 0.4 to 0.6:

1.

5. The purification method according to claim 1, characterized in that, In step (2), the stirring speed for stirring and washing is 200-300 r / min.

6. The purification method according to claim 1, characterized in that, In step (3), in the three-stage gradient heating and vacuum distillation process, the first stage distillation temperature is 120-135℃ and the time is 15-20min; the second stage distillation temperature is 140-155℃ and the time is 20-25min; the third stage distillation temperature is 165-172℃ and the time is 30-40min.

7. The purification method according to claim 1, characterized in that, In step (3), the heating rate in the three-stage gradient heating and vacuum distillation process is 5-8℃ / min.

8. The purification method according to claim 1, characterized in that, In step (5), the drying process adopts a segmented heating drying method. Specifically, the process involves preheating and drying at 55-65℃ for 0.5-1.5 hours, followed by heating to 70-85℃ for 4-6 hours of constant temperature drying.

9. The purification method according to claim 1, characterized in that, In step (6), the cooling and curing adopts a gradient cooling curing method, specifically: first cool down to 110-130℃ and keep at a constant temperature for 8-12 minutes, then cool down to room temperature.

10. The purification method according to any one of claims 1-9, characterized in that, In the purification method, the protective atmosphere is nitrogen.