A polyester chemical regeneration method based on multi-parameter crystal form regulation

Abstract

The application discloses a polyester chemical regeneration method based on multi-parameter crystal form regulation, and belongs to the technical field of high polymer material purification.The polyester chemical regeneration method based on multi-parameter crystal form regulation comprises the following steps: after pretreatment of a depolymerization solution of polyethylene terephthalate, a good solvent is added to obtain a crystallization system; the crystallization system is put into a first-stage crystallizer, the crystallization system is heated to a first-stage initial temperature, the crystallization system is cooled to a first-stage crystallization temperature at a first-stage cooling rate and is kept warm, slurry is obtained, first solid-liquid separation is carried out, and a solution rich in polyethylene terephthalate monomers is obtained; the solution is put into a second-stage crystallizer, initial heating, second-stage monomer nucleation and second-stage monomer growth are sequentially carried out while stirring, and after the completion, second solid-liquid separation, washing and drying are carried out, and polyethylene terephthalate monomers are obtained.The application realizes directional induction and stable generation of alpha crystal form through crystallization process design and multi-parameter regulation.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering crystallization and polymer material purification technology, specifically relating to a polyester chemical regeneration method based on multi-parameter crystal form control. Background Technology

[0002] Polyester chemical regeneration is a key link in achieving closed-loop and sustainable development of polyester materials. Its goal is to remanufacture waste polyester (such as PET) into high-value products with original quality through chemical depolymerization, purification and repolymerization. However, despite the promising prospects of this technology route, current industrial practice still faces significant technical bottlenecks in the core purification and crystallization process, especially in the precise control of crystal morphology and crystal form. For example: (1) Purity requirements: During the regeneration process, the monomer purity is required to be ≥98% and the L value is required to be ≥95 for application in high-value fields. (2) Separation efficiency requirements: The solvent residue is required to be <20%, while metastable crystal forms seriously affect solvent separation. Therefore, it is necessary to obtain regular crystals with low solid-liquid separation resistance, and the crystal size needs to be uniformly controlled at >100μm to enhance solid-liquid separation. (3) Limitations of existing technology: Existing crystallization processes generally lack precise control of crystal form, resulting in products that cannot meet the standards in terms of purity, stability and processing performance. These technical bottlenecks seriously restrict the development of recycled products towards high-end and functional fields.

[0003] Patent application CN115894223A discloses a method for chemically recycling waste PET products using the phase change properties of BHET crystals, and prepares BHET monomers using a sublimation method. However, the low saturated vapor pressure of BHET limits its large-scale production. Patent application CN120289291A discloses a purification method for monomers obtained from the depolymerization of waste polyester with ethylene glycol, which improves the production efficiency and quality of refined BHET through a continuous process of microwave-induced thin-film evaporation separation and molecular distillation purification. However, this method suffers from poor uniformity of the microwave field and complex process control. Patent application CN114630815A discloses a method for recovering polyethylene terephthalate (PET), which uses a multi-stage depolymerization crystallization adsorption method with a series of depolymerization reactors. The resulting depolymerization mixture can contain a very high proportion of BHET monomers and relatively low amounts of dimers and trimers, thus eliminating the need for conventional purification steps to remove dimers and trimers. However, this method is lengthy and has poor economic efficiency in material recycling. Patent application CN109574835A discloses a method for decolorizing BHET, a polyester alcoholysis product, using ion-modified activated carbon. This method employs ion-modified activated carbon for adsorption and decolorization, enhancing the specific surface area of ​​the activated carbon and thus improving adsorption and decolorization efficiency. However, this technology is simplistic and lacks integration.

[0004] BHET monomer is obtained from waste PET bottle flakes through ethylene glycol glycolysis, and its subsequent crystallization process naturally occurs in an ethylene glycol solvent environment. The difference in the microscopic growth mechanism between the α and β crystal forms directly leads to different macroscopic morphologies—the α crystal form is a needle-like or rod-like structure with high strength, easily forming a porous structure when stacked, facilitating solvent separation; the β crystal form is scaly, tightly stacked with narrow channels, increasing the resistance to solid-liquid separation and resulting in a higher solvent residue in the crystals after separation. Given that existing research mainly focuses on aqueous recrystallization systems, this invention addresses the practical problem that BHET monomer is more likely to form the β crystal form in ethylene glycol systems by developing a recrystallization process.

[0005] To achieve precise control of crystal morphology and crystal form, the following technical challenges exist: (1) Crystal inclusions and purity limits: Before crystallization, both the product and impurities are dissolved in the solvent. When the product crystals precipitate, most of the impurities are still dissolved in the solution. The β crystals generated during the crystallization process have difficulties in solid-liquid separation, which leads to the impurities dissolved in the residual solvent eventually mixing into the product, resulting in low crystal purity. There is an insurmountable upper limit to the purity of the product, which restricts its high-value applications. (2) Lack of polymorph control (core pain point): Polymer monomers (BHET) exhibit polymorphism. Existing processes cannot precisely control the crystal form, mainly forming metastable or mixed crystal forms. Due to their scaly stacking, metastable crystals have tight stacking and narrow channels, which increases the resistance to solid-liquid separation, resulting in a high amount of solvent residue in the crystals after separation. Its inherent physicochemical defects lead to poor downstream processing stability, which may cause fluctuations in product performance indicators. (3) Uncontrolled crystal size and morphology: Uncontrolled crystallization process produces a large number of small crystals, resulting in a huge specific surface area that makes filtration difficult and leaves high solvent residue.

[0006] Therefore, it is urgent to develop a method for controlling the crystal form of waste polyester depolymerization into BHET monomers. Summary of the Invention

[0007] To address the shortcomings of existing technologies and to solve the problems of low purity of crystal inclusions, high content of non-dominant crystal forms such as metastable or mixed crystal forms, and difficulty in controlling crystal size and morphology, this invention aims to design and provide a polyester chemical regeneration method based on multi-parameter crystal form control. This invention, through multi-parameter control, achieves higher strength in α-crystal products and facilitates the formation of porous structures. Such structures allow for better separation of impurity-containing solvents from the product, ultimately improving the purity of the final product.

[0008] The principle of this invention is as follows: In order to achieve the goal of not introducing other solvents, in the intrinsic process environment of ethylene glycol system, by systematically controlling the crystallization kinetic parameters, BHET is directionally induced and promoted to precipitate stably into α-crystals with larger size, uniform particle size distribution and easy separation, thereby improving crystallization efficiency, reducing solvent residue and ensuring product quality.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] On the one hand, the present invention provides a method for the chemical regeneration of polyester based on multi-parameter crystal form control, comprising the following steps:

[0011] After pretreatment of the depolymerization solution of polyethylene terephthalate, a good solvent is added to obtain a crystallization system;

[0012] The above crystallization system is fed into a primary crystallizer and heated to the primary initial temperature (to ensure the system is completely dissolved). It is then cooled to the primary crystallization temperature at a primary cooling rate and held at that temperature (so that oligomers, including but not limited to BHET dimers and trimers, preferentially precipitate out, while the majority of BHET monomers remain in the solution). A slurry is obtained, and the first solid-liquid separation is performed (this step yields a solid filter cake rich in oligomers and an EG solution rich in monomers; the solid filter cake rich in oligomers can be further processed, such as inputting it into the polymerization process, or refluxed to the depolymerization process), resulting in a solution rich in ethylene terephthalate monomers. The purpose of the treatment in the primary crystallizer is to separate the oligomers.

[0013] The solution rich in ethylene terephthalate monomer was placed into a secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period were carried out sequentially. After completion, a second solid-liquid separation was performed, followed by washing and drying to obtain ethylene terephthalate monomer. The purpose of the secondary crystallizer is to purify the monomer and control the crystal form, and to carry out monomer crystallization by using multi-parameter synergistic control.

[0014] The initial heating period is as follows: stirring and heating to the secondary initial temperature;

[0015] The nucleation period for secondary crystallization monomers is as follows: stir and cool to the seed crystal addition point temperature, and add the seed crystals at this temperature;

[0016] The growth period for secondary crystallization monomers is as follows: stir and slowly cool to the endpoint temperature, then hold at that temperature.

[0017] The pretreatment method for the chemical regeneration of polyester based on multi-parameter crystal form control is as follows: insoluble impurities are removed by sedimentation and filtration.

[0018] The polyester chemical regeneration method based on multi-parameter crystal form control, wherein the good solvent is at least one of water, acetonitrile, ethylene glycol, methanol or acetone;

[0019] The good solvent accounts for 25%-100% of the mass fraction of the pretreated depolymerization solution;

[0020] The described method for chemical regeneration of polyester based on multi-parameter crystal form control, wherein the initial temperature of the first stage is 65-85℃;

[0021] The first-stage cooling rate is 0.8-3.0℃ / min;

[0022] The primary crystallization temperature is 40-55℃;

[0023] The heat preservation time is 30-90 minutes.

[0024] The polyester chemical regeneration method based on multi-parameter crystal form control, wherein the first solid-liquid separation is carried out by sedimentation decantation, filtration or centrifugation.

[0025] When using sedimentation and decantation, the sedimentation time is 5-45 minutes;

[0026] When using vacuum filtration, the filtration pressure is 10-100 kPa and the filtration time is 120-300 min.

[0027] When using centrifugal separation, the rotation speed is 4000-12000 rpm and the time is 5-15 minutes.

[0028] Preferably, the first solid-liquid separation method is either sedimentation and decantation followed by filtration, or sedimentation and decantation followed by centrifugation.

[0029] The method for chemical regeneration of polyester based on multi-parameter crystal form control, wherein during the initial heating period, the stirring speed is 200-600 rpm, the secondary initial temperature is 55-75℃, and the secondary initial temperature is lower than the primary initial temperature but higher than the primary crystallization temperature;

[0030] During the secondary crystallization monomer nucleation period, the stirring speed is 50-300 rpm, the cooling rate is 0.8-3.0℃ / min, and the seed crystal addition point temperature is 45-55℃.

[0031] During the growth period of the secondary crystallized monomer, the stirring speed is 100-600 rpm, the cooling rate is 0.05-1.0℃ / min, the final temperature is 5-25℃, and the holding time is 30-90 min (to allow the monomer to grow fully).

[0032] The method for chemical regeneration of polyester based on multi-parameter crystal form control, wherein the amount of seed crystal added is 0.5-5.0 wt% of the theoretical mass of the solution monomer rich in ethylene terephthalate monomer;

[0033] The seed crystals are high-purity α-crystalline polyethylene terephthalate with a D50 of 45-150 μm.

[0034] The polyester chemical regeneration method based on multi-parameter crystal form control, wherein the second solid-liquid separation is performed by vacuum filtration or centrifugation.

[0035] When using vacuum filtration, the pressure is 10-100 kPa and the time is 120-240 min;

[0036] When using centrifugal separation, the rotation speed is 5000-8000 rpm and the time is 5-10 minutes.

[0037] Preferably, the washing method involves washing the crystal surface with a pre-cooled saturated EG solution of BHET.

[0038] Preferably, the drying method is as follows: the moist crystals are dried under a vacuum degree ≤ -0.09MPa and a temperature T5 = 30-60℃ for t8 = 60-240min.

[0039] The polyester chemical regeneration method based on multi-parameter crystal form control is described above, wherein the primary crystallizer and the secondary crystallizer are equipped with a guide tube and an apple-shaped bottom structure.

[0040] Secondly, the present invention provides the use of any of the polyester chemical regeneration methods described in the preparation of high-purity polyethylene terephthalate monomer products with a specific α-crystal form.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1. Precise crystal form control to stably obtain α crystal form: This invention, through innovative crystallization process design and multi-parameter control, directionally induces and stably generates α crystal form with superior processing characteristics, fundamentally solving a series of problems caused by β crystal form.

[0043] 2. This invention can significantly solve the problem of crystal inclusions by controlling the crystal form, and improve the product purity to over 98.5% and the L value to ≥96.

[0044] 3. The large size and low specific surface area of ​​the α-crystalline particles of this invention improve the filtration efficiency by more than 50% and the solvent residue is less than 20%. Attached Figure Description

[0045] Figure 1The XRD patterns of the α and β crystal forms of the polymer monomer (BHET) in Example 1 of the present invention are shown.

[0046] Figure 2 The above are DSC diagrams of the α and β crystal forms of the polymer monomer (BHET) in Example 1 of the present invention.

[0047] Figure 3 The infrared spectra of the polymer monomer (BHET) α-crystal form and β-crystal form in Example 1 of the present invention are shown below.

[0048] Figure 4 The images show microscopic images of the α and β crystal forms of the polymer monomer (BHET) in Example 1 of the present invention. The scale bar in the images is 250 μm in length. Detailed Implementation

[0049] The present application is described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to these embodiments.

[0050] Example 1:

[0051] 1. Obtaining a depolymerization solution for polyethylene terephthalate (PET)

[0052] Weigh out waste polyester, ethylene glycol, and depolymerization catalyst in a mass ratio of 1:3:0.01, stir at 220°C for 3 hours to obtain a polymer depolymerization solution.

[0053] 2. Pretreatment

[0054] The polymer depolymerization solution was pretreated by sedimentation, filtration and other methods to remove insoluble impurities.

[0055] 3. Configure the crystallization system

[0056] The pretreated depolymerization solution was added to the crystallizer, and ethylene glycol was added to prepare a suitable crystallization system. A DTB crystallizer with a flow guide tube and an apple-shaped bottom structure was selected.

[0057] The added ethylene glycol accounts for 75% of the mass fraction of the pretreated depolymerization solution.

[0058] 4. Crystallization steps

[0059] (1) Primary crystallization (separation of oligomers)

[0060] The filtered and prepared crystallization system was fed into a primary crystallizer. The system was heated to an initial temperature T0 = 80℃ to ensure complete dissolution; the system was then cooled at a primary cooling rate R1 = 2.5℃ / min to the primary crystallization temperature T1 = 52℃; the system was held at T1 temperature for t1 = 55 min to allow oligomers (including but not limited to BHET dimers and trimers) to precipitate preferentially, while the majority of BHET monomers remained in the solution.

[0061] (2) First solid-liquid separation

[0062] The solid-liquid mixture that has completed primary crystallization undergoes solid-liquid separation, using methods including but not limited to sedimentation decantation, filtration, or centrifugation. A sedimentation decantation process is employed for 25 minutes, followed by filtration at a pressure of P1 = 20 kPa and a time of t3 = 260 minutes. This step yields a solid filter cake rich in oligomers and an EG solution rich in monomers. The solid filter cake rich in oligomers can be further processed (e.g., fed into the polymerization process) or recycled to the depolymerization process.

[0063] (3) Secondary crystallization (purifying monomers and controlling crystal form)

[0064] The monomer-rich EG solution obtained from the primary solid-liquid separation is fed into the secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period are carried out sequentially. After completion, a solid-liquid mixture of secondary crystals is obtained.

[0065] Initial heating period: (Heating to T2 after adding to the secondary crystallizer) The stirring speed is N1=350rpm, and the monomer-rich EG solution is heated to the secondary starting temperature T2=63℃ (lower than T0 and higher than T1 to ensure complete dissolution of monomers).

[0066] Secondary crystallization monomer nucleation period: (cooling from T2 to the seed addition point) Rotation speed is N2=200rpm. Starting from T2, the temperature is reduced at a relatively fast cooling rate R2=1.2℃ / min to the seed addition temperature T3=50℃. At temperature T3, the seed crystal is added. Seed addition strategy: The amount of seed crystal added is w=1.8wt% of the theoretical mass of the monomer in the secondary crystallization system; the seed crystal specification is a high-purity target α-crystal form (needle-like) crystal with D50,D1=75μm.

[0067] Secondary crystallization monomer growth period (from seed addition to crystallization completion): Rotation speed N3 = 400 rpm. After seed addition, a slow cooling rate R3 = 0.25℃ / min is used to cool to the final temperature T4 = 5℃. The temperature is then maintained at T4 for t5 = 50 min to allow for sufficient monomer growth.

[0068] (4) Second solid-liquid separation

[0069] The above-mentioned secondary crystallized solid-liquid mixture was subjected to solid-liquid separation, including but not limited to vacuum filtration or centrifugation. Vacuum filtration was used, with a filtration pressure P2 = 18 kPa and a time t6 = 200 min; the crystal surface was washed with a pre-cooled saturated EG solution of BHET. The moist crystals were then dried under a vacuum of -0.09 MPa and a temperature T5 = 55 °C for t8 = 180 min to obtain a high-purity monomer product with a specific crystal form. Figures 1-4 The figures shown are the XRD patterns, DSC patterns, infrared spectra, and microscopic images of the α- and β-crystalline forms of the polymer monomer (BHET). Figure 1 It is evident that the diffraction peak positions of the two crystal forms differ significantly, directly reflecting their different lattice parameters. This demonstrates that different molecular stacking arrangements and tilt angles result in different crystal structures. Figure 2 It is evident that there are significant differences in thermodynamic properties between the two crystal forms; the α-crystal form exhibits a higher and sharper melting peak, indicating its greater thermal stability. Figure 3 It is evident that due to the different intermolecular forces between the two crystal forms, the vibrational absorption peaks of certain chemical bonds shift. This indicates that there are differences in the microenvironment, conformation, and intermolecular force network of the molecules in the two crystal forms. Figure 4 It visually demonstrates the significant differences between the two crystal forms, which can be used for quick and rough identification and separation of crystal forms.

[0070] The results showed that the purity of BHET monomer was 98.82%, the yield of BHET monomer was 82.1%, the solvent residue was 8.5%, the α-crystal ratio of the product was 94.5%, the D50 was 1200 μm, and the L value was 97.68.

[0071] Example 2:

[0072] 1. Obtaining a depolymerization solution for polyethylene terephthalate (PET)

[0073] Obtained in the same manner as in Example 1 above.

[0074] 2. Pretreatment

[0075] The polymer depolymerization solution was pretreated by sedimentation, filtration and other methods to remove insoluble impurities.

[0076] 3. Configure the crystallization system

[0077] The pretreated depolymerization solution was added to the crystallizer, along with acetonitrile, to prepare a suitable crystallization system. A DTB crystallizer with a flow guide and an apple-shaped bottom structure was selected.

[0078] The added acetonitrile accounts for 50% of the mass fraction of the pretreated depolymerization solution.

[0079] 4. Crystallization steps

[0080] (1) Primary crystallization (separation of oligomers)

[0081] The filtered and prepared crystallization system was fed into a primary crystallizer. The system was heated to an initial temperature T0 = 80℃ to ensure complete dissolution; the system was then cooled at a primary cooling rate R1 = 2.0℃ / min to the primary crystallization temperature T1 = 45℃; the system was held at T1 temperature for t1 = 90 min to allow oligomers (including but not limited to BHET dimers and trimers) to precipitate preferentially, while the majority of BHET monomers remained in the solution.

[0082] (2) First solid-liquid separation

[0083] The solid-liquid mixture that has completed primary crystallization undergoes solid-liquid separation, including but not limited to sedimentation decantation, filtration, or centrifugation. Separation is performed using sedimentation decantation with a settling time of t2 = 40 min. Following this, centrifugation is used with a rotation speed controlled at r1 = 10000 rpm for t4 = 15 min. This step yields a solid filter cake rich in oligomers and an EG solution rich in monomers. The solid filter cake rich in oligomers can be further processed (e.g., fed into the polymerization process) or recycled to the depolymerization process.

[0084] (3) Secondary crystallization (purifying monomers and controlling crystal form)

[0085] The monomer-rich EG solution obtained from the primary solid-liquid separation is fed into the secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period are carried out sequentially. After completion, a solid-liquid mixture of secondary crystals is obtained.

[0086] Initial heating period: (Heating to T2 after adding to the secondary crystallizer) The stirring speed is N1=500rpm, and the monomer-rich EG solution is heated to the secondary starting temperature T2=55℃ (lower than T0 and higher than T1 to ensure complete dissolution of monomer).

[0087] Secondary crystallization monomer nucleation period: (cooling from T2 to the seed addition point) Rotation speed is N2=300rpm. Starting from T2, cool at a relatively fast cooling rate R2=2.5℃ / min to the seed addition temperature T3=45℃. At temperature T3, add the seed crystal. Seed addition strategy: The amount of seed crystal added is w=3.0wt% of the theoretical mass of the monomer in the secondary crystallization system; the seed crystal specification is a high-purity target α-crystal (needle-like) crystal with D50, D1=45μm.

[0088] Secondary crystallization monomer growth period: (from the addition of seed crystals to the end of crystallization) Rotation speed is N3 = 500 rpm. After adding seed crystals, a slow cooling rate R3 = 1.0℃ / min is used to cool to the final temperature T4 = 5℃. The temperature is held at T4 for t5 = 385 min to allow the monomers to grow fully.

[0089] (4) Second solid-liquid separation

[0090] The above-mentioned secondary crystallized solid-liquid mixture was subjected to solid-liquid separation, including but not limited to filtration or centrifugation. Centrifugation was used, with a rotation speed controlled at r2 = 7000 rpm and a time t7 = 10 min; the crystal surface was washed with a pre-cooled saturated EG solution of BHET. The moist crystals were then dried under a vacuum of -0.09 MPa and a temperature of T5 = 50 °C for t8 = 120 min to obtain a high-purity monomer product with a specific crystal form.

[0091] Test results: BHET monomer purity 98.75%, BHET monomer yield 76.31%, solvent residue 18wt%, α-crystal ratio of product 85%, D50 800μm, L value 96.33.

[0092] Example 3:

[0093] 1. Obtaining a depolymerization solution for polyethylene terephthalate (PET)

[0094] Obtained in the same manner as in Example 1 above.

[0095] 2. Pretreatment

[0096] The polymer depolymerization solution was pretreated by sedimentation, filtration and other methods to remove insoluble impurities.

[0097] 3. Configure the crystallization system

[0098] The pretreated depolymerization solution was fed into the crystallizer, and methanol was added to prepare a suitable crystallization system. A DTB crystallizer with a flow guide and an apple-shaped bottom structure was selected.

[0099] The added methanol accounts for 60% of the mass fraction of the pretreated depolymerization solution.

[0100] 4. Crystallization steps

[0101] (1) Primary crystallization (separation of oligomers)

[0102] The filtered and prepared crystallization system was fed into a primary crystallizer. The system was heated to an initial temperature T0 = 65℃ to ensure complete dissolution; the system was then cooled at a primary cooling rate R1 = 1.5℃ / min to the primary crystallization temperature T1 = 40℃; the system was held at T1 temperature for t1 = 90 min to allow oligomers (including but not limited to BHET dimers and trimers) to precipitate preferentially, while the majority of BHET monomers remained in the solution.

[0103] (2) First solid-liquid separation

[0104] The solid-liquid mixture that has completed primary crystallization undergoes solid-liquid separation, using methods including but not limited to sedimentation decantation, filtration, or centrifugation. When using sedimentation decantation, the sedimentation time is t2 = 5 min; subsequently, filtration is used with a filtration pressure P1 = 100 kPa and a time t3 = 120 min. This step yields a solid filter cake rich in oligomers and an EG solution rich in monomers. The solid filter cake rich in oligomers can be further processed (e.g., fed into the polymerization process) or recycled to the depolymerization process.

[0105] (3) Secondary crystallization (purifying monomers and controlling crystal form)

[0106] The monomer-rich EG solution obtained from the primary solid-liquid separation is fed into the secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period are carried out sequentially. After completion, a solid-liquid mixture of secondary crystals is obtained.

[0107] Initial heating period: (Heating to T2 after adding to the secondary crystallizer) The stirring speed is N1=250rpm, and the monomer-rich EG solution is heated to the secondary starting temperature T2=60℃ (lower than T0 and higher than T1 to ensure complete monomer dissolution).

[0108] Secondary crystallization monomer nucleation period: (cooling from T2 to the seed addition point) Rotation speed is N2=100rpm. Starting from T2, cool at a relatively fast cooling rate R2=1.5℃ / min to the seed addition temperature T3=50℃. At temperature T3, add the seed crystal. Seed addition strategy: The amount of seed crystal added is w=1.5wt% of the theoretical mass of the monomer in the secondary crystallization system; the seed crystal specification is a high-purity target α-crystal (needle-like) crystal with D50, D1=75μm.

[0109] Secondary crystallization monomer growth period: (from the addition of seed crystals to the end of crystallization) Rotation speed is N3 = 400 rpm. After adding seed crystals, a slow cooling rate R3 = 0.75℃ / min is used to cool to the final temperature T4 = 15℃. The temperature is held at T4 for t5 = 30 min to allow the monomers to grow fully.

[0110] (4) Second solid-liquid separation

[0111] The above-mentioned secondary crystallized solid-liquid mixture was subjected to solid-liquid separation, including but not limited to vacuum filtration or centrifugation. Vacuum filtration was used for separation, with a filtration pressure of P2 = 20 kPa and a time of t6 = 200 min. The crystal surface was then washed with a pre-cooled saturated EG solution of BHET. The moist crystals were then dried under a vacuum of -0.09 MPa and a temperature of T5 = 50 °C for t8 = 200 min to obtain a high-purity monomer product with a specific crystal form.

[0112] Test results: BHET monomer purity 98.94%, BHET monomer yield 81.26%, solvent residue 12wt%, α crystal form ratio 82%, D50 750μm, L value 96.03.

[0113] Example 4:

[0114] 1. Obtaining a depolymerization solution for polyethylene terephthalate (PET)

[0115] Obtained in the same manner as the above embodiments.

[0116] 2. Pretreatment

[0117] The polymer depolymerization solution was pretreated by sedimentation, filtration and other methods to remove insoluble impurities.

[0118] 3. Configure the crystallization system

[0119] The pretreated depolymerization solution was added to the crystallizer, along with acetone, to prepare a suitable crystallization system. A DTB crystallizer with a flow guide and an apple-shaped bottom structure was selected.

[0120] The added acetone accounts for 25% of the mass fraction of the pretreated depolymerization solution.

[0121] 4. Crystallization steps

[0122] (1) Primary crystallization (separation of oligomers)

[0123] The filtered and prepared crystallization system was fed into a primary crystallizer. The system was heated to an initial temperature T0 = 85℃ to ensure complete dissolution; the system was then cooled at a primary cooling rate R1 = 1.5℃ / min to the primary crystallization temperature T1 = 2℃; the system was held at T1 temperature for t1 = 35 min to allow oligomers (including but not limited to BHET dimers and trimers) to precipitate preferentially, while the majority of BHET monomers remained in the solution.

[0124] (2) First solid-liquid separation

[0125] The solid-liquid mixture that has completed primary crystallization undergoes solid-liquid separation, using methods including but not limited to sedimentation decantation, filtration, or centrifugation. If sedimentation decantation is used, the sedimentation time is t2 = 10 min; subsequently, filtration is used, with a filtration pressure P1 = 50 kPa and a time t3 = 165 min. This step yields a solid filter cake rich in oligomers and an EG solution rich in monomers. The solid filter cake rich in oligomers can be further processed (e.g., fed into the polymerization process) or recycled to the depolymerization process.

[0126] (3) Secondary crystallization (purifying monomers and controlling crystal form)

[0127] The monomer-rich EG solution obtained from the primary solid-liquid separation is fed into the secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period are carried out sequentially. After completion, a solid-liquid mixture of secondary crystals is obtained.

[0128] Initial heating period: (Heating to T2 after adding to the secondary crystallizer) The stirring speed is N1=450rpm, and the monomer-rich EG solution is heated to the secondary starting temperature T2=65℃ (lower than T0 and higher than T1 to ensure complete dissolution of monomers).

[0129] Secondary crystallization monomer nucleation period: (cooling from T2 to the seed addition point) Rotation speed is N2=100rpm. Starting from T2, the temperature is reduced at a relatively fast cooling rate R2=2.2℃ / min to the seed addition temperature T3=51℃. At temperature T3, the seed crystal is added. Seed addition strategy: The amount of seed crystal added is w=0.8wt% of the theoretical mass of the monomer in the secondary crystallization system; the seed crystal specification is a high-purity target α-crystal (needle-like) crystal with D50, D1=100μm.

[0130] Secondary crystallization monomer growth period: (from the addition of seed crystals to the end of crystallization) Rotation speed is N3 = 350 rpm. After adding seed crystals, a slow cooling rate R3 = 0.15℃ / min is used to cool to the final temperature T4 = 7.5℃. The temperature is then maintained at T4 for t5 = 45 min to allow the monomers to grow fully.

[0131] (4) Second solid-liquid separation

[0132] The above-mentioned secondary crystallized solid-liquid mixture was subjected to solid-liquid separation, including but not limited to vacuum filtration or centrifugation. Vacuum filtration was used for separation, with a filtration pressure of P2 = 15 kPa and a time of t6 = 150 min. The crystal surface was then washed with a pre-cooled saturated EG solution of BHET. The moist crystals were then dried under a vacuum of -0.09 MPa and a temperature of T5 = 55 °C for t8 = 180 min to obtain a high-purity monomer product with a specific crystal form.

[0133] Test results: BHET monomer purity 98.54%, BHET monomer yield 83.44%, solvent residue 18.6%, α-crystal form ratio 85%, D50 630, L value 96.56.

[0134] Example 5:

[0135] 1. Obtaining a depolymerization solution for polyethylene terephthalate (PET)

[0136] Obtained in the same manner as in Example 1 above.

[0137] 2. Pretreatment

[0138] The polymer depolymerization solution was pretreated by sedimentation, filtration and other methods to remove insoluble impurities.

[0139] 3. Configure the crystallization system

[0140] The pretreated depolymerization solution was added to the crystallizer, and ethylene glycol was added to prepare a suitable crystallization system. A DTB crystallizer with a flow guide tube and an apple-shaped bottom structure was selected.

[0141] The added ethylene glycol accounts for 100% of the mass fraction of the pretreated depolymerization solution.

[0142] 4. Crystallization steps

[0143] (1) Primary crystallization (separation of oligomers)

[0144] The filtered and prepared crystallization system was fed into a primary crystallizer. The system was heated to an initial temperature T0 = 85℃ to ensure complete dissolution; the system was then cooled at a primary cooling rate R1 = 3.0℃ / min to the primary crystallization temperature T1 = 55℃; the system was held at T1 temperature for t1 = 90 min to allow oligomers (including but not limited to BHET dimers and trimers) to precipitate preferentially, while the majority of BHET monomers remained in the solution.

[0145] (2) First solid-liquid separation

[0146] The solid-liquid mixture that has completed primary crystallization undergoes solid-liquid separation, including but not limited to sedimentation decantation, filtration, or centrifugation. Separation is performed using sedimentation decantation with a settling time of t2 = 45 min, followed by centrifugation at a speed of r1 = 12000 rpm for 15 min. This step yields a solid filter cake rich in oligomers and an EG solution rich in monomers. The solid filter cake rich in oligomers can be further processed (e.g., fed into the polymerization process) or recycled to the depolymerization process.

[0147] (3) Secondary crystallization (purifying monomers and controlling crystal form)

[0148] The monomer-rich EG solution obtained from the primary solid-liquid separation is fed into the secondary crystallizer. While stirring, the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period are carried out sequentially. After completion, a solid-liquid mixture of secondary crystals is obtained.

[0149] Initial heating period: (Heating to T2 after adding to the secondary crystallizer) The stirring speed is N1=200rpm, and the monomer-rich EG solution is heated to the secondary starting temperature T2=55℃ (lower than T0 and higher than T1 to ensure complete dissolution of monomers).

[0150] Secondary crystallization monomer nucleation period: (cooling from T2 to the seed addition point) Rotation speed is N2=50rpm. Starting from T2, cool at a relatively fast cooling rate R2=3.0℃ / min to the seed addition temperature T3=45℃. At temperature T3, add the seed crystal. Seed addition strategy: The amount of seed crystal added is w=5.0wt% of the theoretical mass of the monomer in the secondary crystallization system; the seed crystal specification is a high-purity target α-crystal (needle-like) crystal with D50, D1=45μm.

[0151] Secondary crystallization monomer growth period: (from the addition of seed crystals to the end of crystallization) Rotation speed is N3 = 100 rpm. After adding seed crystals, a slow cooling rate R3 = 1.0℃ / min is used to cool to the final temperature T4 = 5℃. The temperature is held at T4 for t5 = 90 min to allow the monomers to grow fully.

[0152] (4) Second solid-liquid separation

[0153] The above-mentioned secondary crystallized solid-liquid mixture was subjected to solid-liquid separation, including but not limited to vacuum filtration or centrifugation. If vacuum filtration was used, the filtration pressure P2 = 20 kPa and the time t6 = 240 min; if centrifugation was used, the rotation speed was controlled at r2 = 5000-8000 rpm and the time t7 = 5-10 min. The crystal surface was washed with a pre-cooled saturated EG solution of BHET. The moist crystals were dried under a vacuum of -0.09 MPa and a temperature T5 = 60 °C for t8 = 240 min to obtain a high-purity monomer product with a specific crystal form.

[0154] Test results: BHET monomer purity 99.01%, monomer yield 79.33%, solvent residue 9.4wt%, α-crystal form ratio 89%, D50 760, L value 96.12.

[0155] Comparative Example 1:

[0156] Compared with Example 1, only the following were changed: (1) the amount of good solvent added X = 0 wt%; (2) the initial temperature T0 = 60℃; (3) the first-stage cooling rate R1 = 5.0℃ / min; (4) the first-stage crystallization temperature T1 = 60℃; (5) the holding time t1 = 0 min, the settling time = 0 min; (6) the second-stage starting temperature T2 = 60℃; (7) the stirring speed N1 = 200 rpm; N2 = 50 rpm; N3 = 100 rpm; 8) Secondary cooling rate R2 = 5.0℃ / min; (9) Seed addition = 55℃; (10) Secondary crystallization cooling rate = 2.0℃ / min; (11) Final crystallization temperature = 30℃; (12) Seed addition amount = 0wt%; (13) Secondary heat preservation time t2 = 0min; The conditions for the second solid-liquid separation are: secondary separation filtration pressure = 100KPa, time = 200min; drying temperature = 70℃, drying time = 30min.

[0157] Results: Monomer purity: 85.43%, monomer yield: 75.52%, solvent residue: 55.4%, α-crystal ratio of product: 13%, D50: 200, L value: 92.56.

[0158] Comparative Example 2:

[0159] Compared with Example 1, only the following changes were made: (1) the amount of good solvent added X = 200 wt%; (2) the initial temperature T0 = 60℃; (3) the first-stage cooling rate R1 = 0.5℃ / min; (4) the first-stage crystallization temperature T1 = 35℃; (5) the holding time t1 = 15 min, the settling time = 0 min; (6) the second-stage starting temperature T2 = 35℃; (7) the stirring speed N1 = 100 rpm; N2 = 100 rpm; N3 = 200 rpm; (8) the second-stage cooling rate R2 = 0.5℃ / min; (9) no seed crystals were added; (11) the final crystallization temperature = 25℃; (12) the amount of seed crystals added = 0 wt%; (13) the second-stage holding time = 0 min; the conditions for the second solid-liquid separation were: second-stage separation filtration pressure = 50 KPa; time = 100 min; drying temperature = 85℃, drying time = 300 min.

[0160] Results: Monomer purity: 89.69%, monomer yield: 89.69%, solvent residue: 22%, α-crystal ratio of product: 14%; D50: 450, L value: 93.55.

[0161] Comparative Example 3:

[0162] Compared to Example 1, only the cooling rate of secondary crystallization R3 was changed to 1.5℃ / min.

[0163] Results: Monomer purity: 89.41%; Monomer yield: 78.82%; Solvent residue: 27%; α-crystal ratio of product: 33%; D50: 660; L value: 92.73.

[0164] Comparative Example 4:

[0165] Compared to Example 1, only the amount of seed crystals added was changed to 0 wt%.

[0166] Results: Monomer purity: 81.86%; Monomer yield: 81.23%; Solvent residue: 31%; α-crystal ratio of product: 2%; D50: 340; L value: 88.04.

[0167] In summary, the data shows that the method of this invention can achieve a BHET monomer purity of over 98.5%, an L value of over 96, a solvent residue of less than 20%, and a product α-crystal ratio of over 82%.

Claims

1. A method for chemical regeneration of polyester based on multi-parameter crystal form control, characterized in that, Includes the following steps: After pretreatment of the depolymerization solution of polyethylene terephthalate, a good solvent is added to obtain a crystallization system; The above crystallization system is put into a primary crystallizer and heated to the initial primary temperature of 65-85℃. The system is then cooled to the primary crystallization temperature of 40-55℃ at a primary cooling rate of 0.8-3.0℃ / min and held for 30-90min to obtain a slurry. The first solid-liquid separation is then performed to obtain a solution rich in ethylene terephthalate monomer. The above solution rich in ethylene terephthalate monomer was put into a secondary crystallizer, and the initial heating period, the secondary crystallization monomer nucleation period, and the secondary crystallization monomer growth period were carried out in sequence while stirring. After completion, a second solid-liquid separation was performed, followed by washing and drying to obtain ethylene terephthalate monomer. The initial heating period is as follows: stirring and heating to the secondary initial temperature of 55-75℃, where the secondary initial temperature is lower than the primary initial temperature but higher than the primary crystallization temperature; The nucleation period for secondary crystallization monomers is as follows: stir and cool at a rate of 0.8-3.0℃ / min until the seed crystal addition point temperature is 45-55℃, and add the seed crystal at this temperature; The growth period of secondary crystallization monomers is as follows: stir and slowly cool down at a rate of 0.05-1.0℃ / min until the final temperature of 5-25℃ is reached, and then hold at that temperature for 30-90 min. The good solvent is at least one of acetonitrile, ethylene glycol, methanol, or acetone; the good solvent accounts for 25%-100% of the mass fraction of the pretreated depolymerization solution. During the initial heating period, the stirring speed is 200-600 rpm; during the secondary crystallization monomer nucleation period, the stirring speed is 50-300 rpm; during the secondary crystallization monomer growth period, the stirring speed is 100-600 rpm. The amount of seed crystals added is 0.5-5.0 wt% of the theoretical mass of the monomer in the solution rich in polyethylene terephthalate monomers; the specifications of the seed crystals are high-purity polyethylene terephthalate α-crystal crystals with a D50 particle size of 45-150 μm; The α-crystalline form of the ethylene terephthalate monomer accounts for more than 82%.

2. The polyester chemical regeneration method based on multi-parameter crystal form control as described in claim 1, characterized in that, The pretreatment method is to remove insoluble impurities by sedimentation and filtration.

3. The polyester chemical regeneration method based on multi-parameter crystal form control as described in claim 1, characterized in that, The first solid-liquid separation method is sedimentation decantation, filtration or centrifugation; When using sedimentation and decantation, the sedimentation time is 5-45 minutes; When using vacuum filtration, the filtration pressure is 10-100 kPa and the filtration time is 120-300 min. When using centrifugal separation, the rotation speed is 4000-12000 rpm and the time is 5-15 minutes.

4. The polyester chemical regeneration method based on multi-parameter crystal form control as described in claim 1, characterized in that, The second solid-liquid separation method is vacuum filtration or centrifugation. When using vacuum filtration, the pressure is 10-100 kPa and the time is 120-240 min; When using centrifugal separation, the rotation speed is 5000-8000 rpm and the time is 5-10 minutes.

5. The polyester chemical regeneration method based on multi-parameter crystal form control as described in claim 1, characterized in that, The primary and secondary crystallizers are equipped with guide tubes and apple-shaped bottom structures.

6. Use of the polyester chemical regeneration method according to any one of claims 1-5 in the preparation of high-purity α-crystalline polyethylene terephthalate monomer products.

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