A method and device for purifying crude bis-hydroxyethyl terephthalate and a method for preparing bis-hydroxyethyl terephthalate

Abstract

The application provides a terephthalic acid bis-hydroxyethyl ester crude product purification method and device and a terephthalic acid bis-hydroxyethyl ester preparation method. The method comprises the following steps: allowing a first mixed solution containing terephthalic acid bis-hydroxyethyl ester crude product to enter a first adsorption unit, performing first adsorption treatment in the presence of a first adsorbent, and obtaining a second mixed solution; allowing at least part of the second mixed solution to enter a cross-flow filtration unit, performing cross-flow filtration through a cross-flow membrane, and obtaining a first filtrate containing terephthalic acid bis-hydroxyethyl ester and a first slurry containing activated carbon; allowing the first filtrate to enter a second adsorption unit, performing second adsorption treatment in the presence of a second adsorbent, and obtaining a purified solution; and allowing the purified solution to enter a crystallization unit to perform crystallization treatment, and obtaining purified terephthalic acid bis-hydroxyethyl ester. The method can avoid filter membrane pollution and blockage, avoid single adsorption effect, maintain stable adsorption efficiency, and realize recycling and recovery of the adsorbent.

Description

Technical Field

[0001] This application relates to the fields of green chemistry and polymer recycling technology, and in particular to a purification method and apparatus for crude dihydroxyethyl terephthalate and a method for preparing dihydroxyethyl terephthalate. Background Technology

[0002] The textile industry generates a massive amount of waste textiles (waste textiles) during production. The main component of waste textiles is polyester fiber (such as polyester), whose chemical structure is polyethylene terephthalate (PET). High-value recycling of waste textiles has become a key research focus. Among the methods, converting waste textiles into dihydroxyethyl terephthalate (BHET) via ethylene glycol alcoholysis, followed by repolymerization to generate high-quality recycled polyester, is considered the optimal path for resource utilization of waste textiles. However, the sources of waste textiles are complex, and impurities such as dyes, pigments, and oils carried within them enter the crude BHET system after alcoholysis, resulting in products with high color values, low purity, and poor thermal stability, making it difficult to meet the production requirements of high-end recycled polyester. Therefore, the refining and purification of crude BHET has become the core bottleneck of this technical route.

[0003] In existing technologies, the refining and purification of crude BHET involves two main approaches. First, multi-stage filtration combined with flash evaporation can initially remove impurities and undepolymerized substances. While this method achieves partial decolorization, its effectiveness in removing dyes, trace heavy metals, and small organic molecule pollutants is limited. Furthermore, the multiple flash evaporation processes are energy-intensive and pose a risk of degradation of heat-sensitive components, potentially leading to melt viscosity fluctuations and increased terminal carboxyl group content during subsequent polycondensation, thus affecting the quality of regenerated fibers. Second, activated carbon can be used for adsorption and decolorization via oscillation or ultrasound-assisted adsorption. However, this technology also has drawbacks: while powdered activated carbon possesses a high specific surface area and broad-spectrum adsorption capacity, its separation is challenging, and traditional filtration methods easily result in carbon powder residue or clogging, affecting product purity and process continuity. Additionally, the lack of effective regeneration and reuse pathways leads to low adsorbent utilization and high operating costs.

[0004] Therefore, developing a method that can avoid filter membrane fouling and clogging, avoid single adsorption effect, maintain stable adsorption efficiency without reduction, and realize the recycling and recovery of adsorbent, thereby achieving deep purification of BHET, significantly improving product purity and color, ensuring product quality stability, while improving resource utilization and reducing costs, is of great significance for promoting the development of waste textile recycling. Summary of the Invention

[0005] This application provides a purification method, apparatus, and preparation method for crude dihydroxyethyl terephthalate (BHET). This method avoids filter membrane contamination and clogging, prevents single adsorption effect, maintains stable adsorption efficiency, and enables the recycling and recovery of the adsorbent. This achieves deep purification of BHET, significantly improves product purity and color, ensures product quality stability, and simultaneously increases resource utilization and reduces costs.

[0006] In a first aspect, embodiments of this application provide a method for purifying crude dihydroxyethyl terephthalate, comprising: introducing a first mixture containing crude dihydroxyethyl terephthalate into a first adsorption unit, performing a first adsorption treatment in the presence of a first adsorbent to obtain a second mixture; wherein the first adsorbent includes activated carbon; introducing at least a portion of the second mixture into a cross-flow filtration unit, performing cross-flow filtration through a cross-flow membrane to obtain a first filtrate containing dihydroxyethyl terephthalate and a first slurry containing the activated carbon; returning the first slurry to the first adsorption unit; and introducing a portion of the second mixture into a separation unit. Solid-liquid separation is performed to obtain a second filtrate and a carbon-containing solid product. The second filtrate is then returned entirely to the first adsorption unit. The ratio of the sum of the flow rates of the first slurry and the second filtrate returned to the first adsorption unit to the flow rate of the first filtrate is 1:(2~11). The first filtrate is then introduced into the second adsorption unit for a second adsorption treatment in the presence of a second adsorbent to obtain a purified liquid. The second adsorbent includes activated carbon and / or resin adsorbent materials. The purified liquid is then introduced into a crystallization unit for crystallization treatment to obtain purified dihydroxyethyl terephthalate.

[0007] In one possible implementation, the first slurry is returned to the first adsorption unit.

[0008] In one possible implementation, during the cross-flow filtration process, the sum of the flow rates of the first slurry returning to the first adsorption unit and the second filtrate returning to the first adsorption unit is in the ratio of the flow rate of the first filtrate to 1:(2~11).

[0009] In one possible implementation, the purification method further includes: allowing a portion of the second mixture to enter a separation unit for solid-liquid separation to obtain a second filtrate and a carbon-containing solid phase product, respectively; and returning all of the second filtrate to the first adsorption unit.

[0010] In one possible implementation, the mass ratio of the second mixture entering the cross-flow filtration unit to the second mixture entering the separation unit is (85~97):(3~15).

[0011] In one possible implementation, during the cross-flow filtration process, the flow rate ratio of the second mixture entering the cross-flow filtration unit to the flow rate of the first slurry is (5~15):1.

[0012] In one possible implementation, the cross-flow filtration temperature is 60°C to 90°C.

[0013] In one possible implementation, the transmembrane pressure difference of the cross-flow filter is 0.1 MPa ~ 0.3 MPa.

[0014] In one possible implementation, the operating pressure of the cross-flow filter is 0.3 MPa to 0.6 MPa.

[0015] In one possible implementation, the flow velocity across the membrane surface of the cross-flow filter is 1 m / s to 5 m / s.

[0016] In one possible implementation, the crossflow membrane has a pore size of 0.05 μm to 0.5 μm.

[0017] In one possible implementation, the crossflow membrane includes one or more of a metal membrane, a ceramic membrane, and an organic polymer membrane.

[0018] In one possible implementation, the particle size of the first adsorbent is 45 μm to 150 μm.

[0019] In one possible implementation, the temperature of the first adsorption is 60°C to 95°C, and the time of the first adsorption is 20 min to 40 min.

[0020] In one possible implementation, the mass of the first adsorbent accounts for 1% to 5% of the mass of dihydroxyethyl terephthalate in the first mixture.

[0021] In one possible implementation, the particle size of the second adsorbent is 0.3 mm to 2 mm.

[0022] In one possible implementation, the temperature of the second adsorption is 60°C to 80°C, and the time of the second adsorption is 90 min to 180 min.

[0023] In one possible implementation, the crude dihydroxyethyl terephthalate comprises the dihydroxyethyl terephthalate product obtained by alcoholysis of textile materials.

[0024] Secondly, embodiments of this application provide a purification apparatus for crude dihydroxyethyl terephthalate, comprising a first adsorption unit, a cross-flow filtration unit, a second adsorption unit, a crystallization unit, a separation unit, and a regeneration unit;

[0025] The first adsorption unit includes a first liquid phase outlet, a second liquid phase outlet, a first material inlet, and a fifth liquid phase inlet;

[0026] The cross-flow filtration unit includes a feed end, a first discharge end, and a second discharge end. The first liquid phase outlet of the first adsorption unit is connected to the feed end of the cross-flow filtration unit, and the second discharge end of the cross-flow filtration unit is connected to the first material inlet of the first adsorption unit.

[0027] The second adsorption unit includes a second liquid inlet and a purified liquid outlet; the first discharge end of the cross-flow filtration unit is connected to the second liquid inlet of the second adsorption unit.

[0028] The crystallization unit includes a third liquid phase inlet; the purified liquid outlet of the second adsorption unit is connected to the third liquid phase inlet of the crystallization unit.

[0029] The separation unit includes a fourth liquid phase inlet, a third liquid phase outlet, and a solid phase outlet; the second liquid phase outlet of the first adsorption unit is connected to the fourth liquid phase inlet of the separation unit, and the third liquid phase outlet of the separation unit is connected to the fifth liquid phase inlet of the first adsorption unit.

[0030] The regeneration unit includes a material inlet and a material outlet; the material inlet of the regeneration unit is connected to the solid phase outlet of the separation unit, and the material outlet of the regeneration unit is connected to the first material inlet of the first adsorption unit.

[0031] Thirdly, embodiments of this application provide a method for preparing dihydroxyethyl terephthalate, comprising the following steps: subjecting textile materials to alcoholysis to obtain dihydroxyethyl terephthalate product; purifying the dihydroxyethyl terephthalate product according to the purification method described above for crude dihydroxyethyl terephthalate to obtain dihydroxyethyl terephthalate product.

[0032] This application provides a purification method, apparatus, and preparation method for crude dihydroxyethyl terephthalate (BHET). The method involves first adsorbing a first mixture containing BHET using a first adsorbent including activated carbon to obtain a second mixture; then, cross-flow filtration of the second mixture using a cross-flow membrane to obtain a first filtrate containing BHET and a first slurry containing activated carbon; next, a second adsorbent including activated carbon and / or resin adsorption is used to perform a second adsorption treatment on the first filtrate to obtain a purified solution; finally, the purified solution is crystallized to obtain purified BHET. This coupled process of first adsorption treatment, cross-flow filtration, and second adsorption treatment avoids membrane fouling and clogging, prevents single adsorption effect, maintains stable adsorption efficiency, and enables adsorbent recycling and recovery. This achieves deep purification of BHET, significantly improves product purity and color, ensures product quality stability, improves resource utilization, and reduces costs. Attached Figure Description

[0033] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0034] Figure 1 This is a schematic diagram of the purification apparatus for the crude dihydroxyethyl terephthalate provided in this application.

[0035] Explanation of reference numerals in the attached figures:

[0036] 1: First adsorption unit;

[0037] 2: Cross-flow filtering unit;

[0038] 3: Second adsorption unit;

[0039] 4: Separation unit;

[0040] 5: Crystallization unit;

[0041] 6: Regeneration unit.

[0042] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0043] To enable those skilled in the art to better understand the present invention, this application will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the invention and are not intended to limit its scope. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. When the following description relates to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements.

[0044] In existing technologies, the refining and purification of crude BHET involves two main approaches. First, multi-stage filtration combined with flash evaporation can initially remove impurities and undepolymerized substances. While this method achieves partial decolorization, its effectiveness in removing dyes, trace heavy metals, and small organic molecule pollutants is limited. Furthermore, the multiple flash evaporation processes are energy-intensive and pose a risk of degradation of heat-sensitive components, potentially leading to melt viscosity fluctuations and increased terminal carboxyl group content during subsequent polycondensation, thus affecting the quality of regenerated fibers. Second, activated carbon can be used for adsorption and decolorization via oscillation or ultrasound-assisted adsorption. However, this technology also has drawbacks: while powdered activated carbon possesses a high specific surface area and broad-spectrum adsorption capacity, its separation is challenging, and traditional filtration methods easily result in carbon powder residue or clogging, affecting product purity and process continuity. Additionally, the lack of effective regeneration and reuse pathways leads to low adsorbent utilization and high operating costs.

[0045] To address the aforementioned technical issues, this solution provides a purification method and apparatus for crude dihydroxyethyl terephthalate, as well as a method for preparing dihydroxyethyl terephthalate.

[0046] This invention provides a method for purifying crude dihydroxyethyl terephthalate (DHT), comprising: introducing a first mixture containing DHT into a first adsorption unit, performing a first adsorption treatment in the presence of a first adsorbent to obtain a second mixture; wherein the first adsorbent includes activated carbon; introducing at least a portion of the second mixture into a cross-flow filtration unit, performing cross-flow filtration through a cross-flow membrane to obtain a first filtrate containing DHT and a first slurry containing the activated carbon; returning the first slurry to the first adsorption unit; and introducing a portion of the second mixture into a separation unit for further purification. Solid-liquid separation yields a second filtrate and a carbon-containing solid product. The second filtrate is then returned entirely to the first adsorption unit. The ratio of the sum of the flow rates of the first slurry and the second filtrate returned to the first adsorption unit to the flow rate of the first filtrate is 1:(2~11). The first filtrate is then introduced into the second adsorption unit for a second adsorption treatment in the presence of a second adsorbent to obtain a purified liquid. The second adsorbent includes activated carbon and / or resin adsorbent materials. The purified liquid is then introduced into a crystallization unit for crystallization treatment to obtain purified dihydroxyethyl terephthalate.

[0047] Firstly, the cross-flow filtration process effectively solves the technical problems of adsorbent separation difficulty and filter membrane fouling and clogging, while ensuring product quality stability and reducing costs. Specifically, replacing the energy-intensive flash evaporation process in existing technologies with cross-flow filtration has several advantages. On the one hand, the high shear force of cross-flow filtration prevents activated carbon from depositing on the filter membrane surface, greatly reducing membrane fouling, simplifying adsorbent separation, and avoiding filter membrane fouling and clogging. On the other hand, the partial entry of the second mixture into the cross-flow filtration unit helps to achieve a reasonable distribution of the filtration load between the two processes, further reducing the operating pressure of the cross-flow membrane and reducing the risk of membrane fouling from the source. In addition, cross-flow filtration is a physical process that does not require phase change, and it is lower in temperature than flash evaporation. This not only effectively suppresses the side reactions that are prone to occur in the alcoholysis system at high temperatures, fundamentally ensuring product quality stability, but also significantly reduces energy consumption and costs.

[0048] Furthermore, the process employing a dual adsorption unit consisting of a first adsorption treatment and a second adsorption treatment in series effectively solves the technical problems of limited efficiency or lack of specificity of single adsorption technologies, achieving highly efficient removal of molecular-level impurities, thereby achieving deep purification of BHET and significantly improving product purity and color. Specifically, the first adsorption treatment utilizes the advantages of activated carbon's large specific surface area and strong adsorption capacity to selectively adsorb dyes, small organic molecule impurities, trace heavy metal ions, and some oligomers in the first mixed solution containing crude dihydroxyethyl terephthalate, achieving broad-spectrum adsorption and performing preliminary decolorization and purification of the first mixed solution containing crude dihydroxyethyl terephthalate; the second adsorption treatment utilizes activated carbon and / or resin adsorbent materials arranged in a regular and dense manner in a fixed bed to further remove residual trace dissolved organic matter, extremely fine color components, and small molecule impurities that were not completely adsorbed by the first adsorbent in the first filtrate. Therefore, the dual adsorption unit consisting of the first adsorption treatment and the second adsorption treatment connected in series can simultaneously solve the technical problems that may exist in single activated carbon adsorption, such as incomplete adsorption due to adsorption blind zones, and the technical problems that may exist in single fixed bed adsorption, such as the need to stop the machine for replacement and regeneration after the adsorbent is saturated, increased system pressure drop, and low adsorbent utilization rate, thereby avoiding the single adsorption effect.

[0049] Therefore, the embodiments of the present invention, through the coupled process of first adsorption treatment, cross-flow filtration, and second adsorption treatment, can avoid filter membrane fouling and clogging, avoid single adsorption effect, maintain stable adsorption efficiency without reduction, and realize the recycling and recovery of adsorbent, thereby achieving deep purification of BHET, significantly improving product purity and color, ensuring product quality stability, while improving resource utilization and reducing costs.

[0050] In some embodiments, the first slurry is returned to the first adsorption unit. By recycling the first slurry back to the first adsorption unit, the concentration of the first adsorbent, including activated carbon, in the first adsorption unit is maintained, the utilization rate of the first adsorbent is improved, the adsorbent is recycled, operating costs are reduced, and the economic efficiency and environmental friendliness of the entire process are enhanced.

[0051] In some embodiments, in the first mixture containing crude dihydroxyethyl terephthalate (BHET), BHET is dissolved in the first mixture as a solute, and the mass of the BHET solute accounts for 10 wt% to 25 wt% of the total mass of the first mixture. For example, it is a range of 10 wt%, 12 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, 25 wt%, or any two of these.

[0052] When the mass of BHET solute in the first mixed solution containing crude dihydroxyethyl terephthalate is controlled within the above-mentioned range, both mass transfer driving force and process economy are considered. This avoids the problem of low batch throughput and insufficient production efficiency caused by excessively low BHET solute mass, while also preventing the situation where excessively high BHET solute content leads to increased feed viscosity, increased filtration resistance, and decreased impurity removal efficiency. This facilitates the efficient advancement of subsequent adsorption and cross-flow filtration processes, while also reasonably controlling the state of the feed system to balance decolorization and purification effects with product recovery rate.

[0053] In some embodiments, the particle size of the first adsorbent is 45 μm to 150 μm, for example, a range of 45 μm, 50 μm, 70 μm, 100 μm, 120 μm, 150 μm, or any combination thereof. When the particle size of the first adsorbent is within the above range, the first adsorbent can maintain a large specific surface area and adsorption capacity in the form of powdered activated carbon, which can quickly and efficiently adsorb most of the main pollutant components such as pigments and macromolecular organic impurities in the crude product, achieving primary broad-spectrum purification; at the same time, this particle size range can be well adapted to cross-flow filtration process, ensuring sufficient adsorption and facilitating subsequent efficient solid-liquid separation through membrane separation, forming a synergistic system of staged treatment with the second adsorption unit, taking into account both adsorption efficiency and process feasibility.

[0054] In one possible implementation, the temperature of the first adsorption is 60°C to 95°C, for example, a range of 60°C, 70°C, 80°C, 90°C, 95°C, or any combination thereof. The time of the first adsorption is 20 min to 40 min, for example, a range of 20 min, 25 min, 30 min, 35 min, 40 min, or any combination thereof. When the temperature and time of the first adsorption are within the above range, it helps to accelerate the liquid-phase mass transfer rate, allowing the powdered activated carbon to fully contact and rapidly adsorb pigments and macromolecular organic impurities in the crude product, thereby improving the efficiency and effect of primary purification. At the same time, it can avoid side reactions caused by excessively high temperatures in the BHET aqueous solution, ensuring the stability of BHET and the purity of the product. Meanwhile, the moderate adsorption time can achieve a balance between sufficient adsorption and production efficiency and energy consumption control, making it suitable for continuous purification processes and laying a stable material foundation for subsequent cross-flow separation and second adsorption.

[0055] In one possible implementation, the mass of the first adsorbent accounts for 1% to 5% of the mass of dihydroxyethyl terephthalate in the first mixture. For example, it is a range of 1%, 2%, 3%, 4%, 5%, or any combination thereof. When the mass ratio of the first adsorbent to dihydroxyethyl terephthalate in the first mixture is within the above range, it helps to provide sufficient adsorption sites for impurities such as pigments, small organic molecules, heavy metals, and oligomers in the crude product, efficiently completing the primary broad-spectrum purification and significantly reducing the processing load of the subsequent second adsorption unit. It also avoids problems such as increased raw material costs and increased separation load caused by excessive adsorbent, thus balancing adsorption purification effect, process economy, and system continuous operation stability.

[0056] In some embodiments, the purification method further includes: allowing a portion of the second mixture to enter a separation unit for solid-liquid separation (fine filtration) to obtain a second filtrate and a carbon-containing solid phase product; and returning all of the second filtrate to the first adsorption unit. This step, through fine filtration to separate a portion of the second mixture into solid and liquid components, effectively separates the adsorbed saturated deactivated activated carbon and adsorbed impurities, discharging them from the system. Simultaneously, the second filtrate is returned to the first adsorption unit, achieving full system recovery of the BHET components, avoiding waste of effective components, maintaining stable system adsorption efficiency, balancing process continuity and resource utilization, and improving the overall economic and environmental benefits of the purification process.

[0057] In some embodiments, the sum of the flow rate of the first slurry returning to the first adsorption unit and the flow rate of the second filtrate returning to the first adsorption unit is in the ratio of the flow rate of the first filtrate to 1:(2~11), for example, a range consisting of 1:2, 1:5, 1:8, 1:10, 1:11 or any two of these.

[0058] In this invention, the ratio of the sum of the flow rates of the first slurry returned to the first adsorption unit (cross-flow membrane concentrate return flow rate) and the second filtrate returned to the first adsorption unit (fine filtration filtrate return flow rate) to the flow rate of the first filtrate (cross-flow membrane permeate flow rate) can be defined as the circulation ratio. Controlling the circulation ratio within the aforementioned range helps to improve adsorbent utilization, enhance the system's resistance to raw material fluctuations, balance process operating energy consumption, ensure sufficient circulating contact between activated carbon and materials, strengthen adsorption and purification effects, guarantee continuous and stable system operation, and balance purification quality and production economy.

[0059] In this embodiment of the invention, a first mixture containing crude dihydroxyethyl terephthalate undergoes a first adsorption treatment to obtain a second mixture. Further, the second mixture can be divided into two parts: one part enters a cross-flow filtration unit for cross-flow filtration through a cross-flow membrane; the other part enters a separation unit for solid-liquid separation, yielding a second filtrate and a carbon-containing solid phase product. The second filtrate contains crude dihydroxyethyl terephthalate, therefore it is returned to the first adsorption unit for further first adsorption treatment and subsequent purification. This arrangement allows for the continuous discharge of deactivated activated carbon during the operation of the first adsorption unit, thereby ensuring the continuous and efficient operation of the first adsorption process and maintaining stable overall adsorption efficiency.

[0060] In some embodiments, the solid-liquid separation temperature is 60°C to 90°C, for example, a range of 50°C, 60°C, 70°C, 80°C, 90°C, or any combination thereof.

[0061] In some embodiments, the separation unit includes a plate and frame filter press, a candle filter, a honeycomb cross-flow membrane filtration system, etc., which can achieve solid-liquid separation. This application does not further limit the specific selection of the separation unit. For example, the plate and frame filter press is equipped with a fine filter bag or filter screen, and the filtration accuracy can reach 0.5μm~10μm; the candle filter can be a filter bag filter or a filter cartridge filter, etc., and the filtration accuracy can reach 0.5μm~10μm; the honeycomb cross-flow membrane filtration system is a fine filtration device with stronger anti-fouling ability, and the filtration accuracy can reach 0.005μm~0.1μm, that is, the filtration accuracy can reach the nanofiltration level, and it can remove smaller impurity molecules at the same time; the honeycomb cross-flow membrane filtration system adopts a honeycomb cross-flow membrane assembly.

[0062] In some embodiments, the mass ratio of the second mixture entering the cross-flow filtration unit to the second mixture entering the separation unit is (85~97):(3~15). For example, it is a range of 85:15, 88:12, 90:10, 93:7, 95:5, 97:3 or any two of these.

[0063] The mass ratio of the second mixture entering the cross-flow filtration unit to the second mixture entering the separation unit is controlled within the above-mentioned range. This ratio is determined based on three principles: material balance, adsorbent activity management, and economic optimization. This helps to regulate the average residence time and renewal rate of activated carbon in the system. It ensures that most of the material is circulated through cross-flow filtration to maintain high-efficiency adsorption, while a small portion of the material enters the separation unit to discharge saturated activated carbon. The saturated activated carbon is then regenerated and returned to the first adsorption unit as the first adsorbent. This ensures the continuous and efficient operation of the first adsorption process and maintains stable overall adsorption efficiency. Controlling the mass ratio within the above-mentioned range helps to balance the operating load of the cross-flow membrane and the regeneration efficiency of the adsorbent, taking into account both continuous and stable system operation and process economy.

[0064] In some embodiments, during cross-flow filtration, the flow rate ratio of the second mixture entering the cross-flow filtration unit to the flow rate of the first slurry is (5~15):1. For example, 5:1, 7:1, 9:1, 11:1, 13:1, 15:1 are ranges or combinations of any two of these.

[0065] In this embodiment of the invention, the ratio of the flow rate of the second mixed liquid entering the cross-flow filtration unit (the flow rate into the cross-flow membrane) to the flow rate of the first slurry can be defined as the concentration factor. If the concentration factor is too high, the rate of activated carbon deposition and adsorption on the surface of the cross-flow membrane will increase non-linearly, leading to increased membrane fouling and a significant reduction in the flux per unit time. Even if the operating pressure of the cross-flow filtration is increased to maintain the flux per unit time, a vicious cycle will still occur, where excessively high operating pressure leads to increased membrane fouling, further reducing the flux per unit time. Ultimately, this results in a significant decrease in the efficiency of the cross-flow filtration, failing to achieve the desired permeate flux. Therefore, the cross-flow filtration effect is best when the concentration factor is within the aforementioned range.

[0066] In some embodiments, the cross-flow filtration temperature is 60°C to 90°C. For example, it can be a range of 60°C, 70°C, 75°C, 80°C, 90°C, or any combination thereof. When the cross-flow filtration temperature is within the above range, the temperature should not be lower than 60°C to prevent BHET from crystallizing and being separated as a solid impurity, thus preventing product loss. The temperature should not exceed 90°C to match the upper temperature resistance limit of the polyvinylidene fluoride (PVDF) cross-flow membrane, preventing swelling damage to the membrane material. It also matches the operating temperature requirements of precision filtration equipment, contributing to stable membrane separation operation, avoiding side reactions in the BHET aqueous solution, and balancing separation efficiency with membrane module lifespan.

[0067] In some embodiments, the transmembrane pressure difference of cross-flow filtration is 0.1 MPa to 0.3 MPa, for example, a range of 0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, or any combination thereof. Transmembrane pressure difference refers to the pressure difference between the upstream feed side and the downstream permeate side of the membrane during cross-flow filtration. When the transmembrane pressure difference of cross-flow filtration is within the above range, it helps the feed liquid to have a suitable permeate flux, improving the solid-liquid separation efficiency of powdered activated carbon and feed liquid, and avoiding slow separation rate and insufficient treatment efficiency due to excessively low pressure difference. It also prevents excessively high pressure difference from aggravating membrane surface contaminant deposition and worsening membrane fouling, while avoiding damage to the membrane module due to excessive pressure, balancing operating energy consumption, maintaining long-term stable operation of cross-flow filtration, and extending membrane lifespan.

[0068] In some embodiments, the operating pressure of the cross-flow filtration is 0.3 MPa to 0.6 MPa, for example, a range of 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.6 MPa, or any combination thereof. The operating pressure refers to the feed pressure on the liquid side during cross-flow filtration. When the operating pressure of the cross-flow filtration is within the above range, it provides sufficient separation power to ensure the liquid permeate flux and the solid-liquid separation efficiency of the powdered activated carbon, meeting the needs of continuous production. It also avoids excessive pressure that could exacerbate membrane fouling and damage the membrane module, while simultaneously considering energy consumption and ensuring long-term stable and efficient operation of the cross-flow filtration system.

[0069] In some embodiments, the membrane surface velocity of the cross-flow filtration is 1 m / s to 5 m / s, for example, a range of 1 m / s, 2 m / s, 3 m / s, 4 m / s, 5 m / s, or any combination thereof. When the membrane surface velocity of the cross-flow filtration is within the above range, it helps to provide sufficient shear force to the membrane surface, flushing away powdered activated carbon and impurities deposited on the membrane surface, significantly reducing membrane fouling and concentration polarization, maintaining a stable membrane permeate flux, and ensuring solid-liquid separation efficiency. Simultaneously, it avoids the separation effect decreasing due to excessively low flow rates, and the system energy consumption and equipment wear increasing due to excessively high flow rates, balancing separation performance and operating economy, thus enabling continuous and stable operation of the cross-flow filtration system.

[0070] In some embodiments, the pore size of the cross-flow membrane is 0.05 μm to 0.5 μm, for example, a range of 0.05 μm, 0.1 μm, 0.2 μm, 0.4 μm, 0.5 μm, or any combination thereof. When the pore size of the cross-flow membrane is within the above range, it helps to retain powdered activated carbon in the system, achieving efficient solid-liquid separation and preventing activated carbon from entering subsequent processes and affecting product purity. It also ensures that effective components such as BHET can pass through the membrane layer smoothly, ensuring material yield. At the same time, it can balance membrane flux and separation accuracy, avoiding low flux and reduced processing efficiency due to excessively small pore size, and preventing activated carbon leakage due to excessively large pore size. It adapts to the separation requirements of powdered activated carbon and ensures stable and efficient operation of cross-flow filtration.

[0071] In some embodiments, the crossflow membrane includes one or more of a metal membrane, a ceramic membrane, and an organic polymer membrane.

[0072] In some embodiments, the metal film includes a stainless steel-ceramic composite film and / or a titanium alloy film.

[0073] In some embodiments, the ceramic membrane includes any one or more of alumina ceramic membrane, titanium dioxide ceramic membrane, zirconia ceramic membrane, and silicon dioxide ceramic membrane.

[0074] In some embodiments, the organic polymer membrane includes a fluoropolymer membrane, such as one or more of polyvinylidene fluoride membranes or polytetrafluoroethylene membranes.

[0075] In one possible implementation, the second adsorbent comprises activated carbon and / or resin adsorbent material, wherein the activated carbon comprises activated carbon fiber and / or refined granular carbon; and the resin adsorbent material comprises at least one of ion exchange resin, macroporous adsorption resin, and chelating resin.

[0076] In one possible embodiment, the particle size of the second adsorbent is 0.3 mm to 2 mm, for example, a range consisting of 0.3 mm, 0.5 mm, 1 mm, 1.5 mm, 2 mm, or any two of these. Specifically, when the second adsorbent is selected from ion exchange resins, the particle size of the ion exchange resins is 0.3 mm to 1.2 mm, for example, a range consisting of 0.3 mm, 0.6 mm, 0.9 mm, 1.0 mm, 1.2 mm, or any two of these; when the second adsorbent is selected from macroporous adsorption resins, the particle size of the macroporous adsorption resins is 0.3 mm to 1.0 mm, for example, a range consisting of 0.3 mm, 0.5 mm, 0.7 mm, 0.9 mm, 1.0 mm, or any two of these; when the second adsorbent is selected from special granular activated carbon, the particle size of the special granular activated carbons is 0.5 mm to 2 mm, for example, a range consisting of 0.5 mm, 0.9 mm, 1.3 mm, 1.7 mm, 2 mm, or any two of these.

[0077] When the particle size of the second adsorbent is within the above-mentioned range, it can be specifically adapted to the adsorption characteristics and process compatibility of different types of adsorbents, forming a uniform and stable adsorption bed in the fixed bed. This ensures smooth flow of the feed liquid, moderate pressure drop in the bed, avoids flow deviation, channeling, or bed blockage, and guarantees long-term continuous and stable operation of the device. It also maintains sufficient adsorption contact area, which helps to achieve deep adsorption of residual trace impurities in the feed liquid. Together with the first-stage powdered activated carbon, it forms a synergistic purification effect of graded treatment, taking into account both adsorption efficiency and final product purity.

[0078] In one possible implementation, the temperature of the second adsorption is 60°C to 80°C, for example, a range of 60°C, 65°C, 70°C, 75°C, 80°C, or any combination thereof. The time of the second adsorption is 90 min to 180 min, for example, a range of 90 min, 100 min, 120 min, 140 min, 160 min, 180 min, or any combination thereof. When the temperature and time of the second adsorption are within the above range, they can be well matched with the temperature of the feed liquid in the preceding cross-flow filtration process, eliminating the need for additional large-scale heating and cooling, saving energy and ensuring the stability of the feed liquid without precipitation; at the same time, it can ensure sufficient contact between the second adsorbent and the feed liquid, deeply removing residual trace pigments, small molecules and ionic impurities, achieving purification, balancing adsorption depth and processing efficiency, and ensuring the purity and quality stability of the final product; in addition, if the temperature of the second adsorption exceeds 80°C, it will cause the resin to soften and deform.

[0079] In one possible implementation, the crude dihydroxyethyl terephthalate comprises the dihydroxyethyl terephthalate product obtained by alcoholysis of textile materials.

[0080] In specific implementation, a first mixture containing crude dihydroxyethyl terephthalate is introduced into a first adsorption unit, and a first adsorbent is added for first adsorption treatment to obtain a second mixture. At least a portion of the second mixture is introduced into a cross-flow filtration unit, where it undergoes cross-flow filtration through a cross-flow membrane to obtain a first filtrate containing dihydroxyethyl terephthalate and a first slurry containing activated carbon. The first slurry is returned to the first adsorption unit for recycling. A portion of the second mixture is introduced into a separation unit for solid-liquid separation (fine filtration) to obtain a second filtrate and a carbon-containing solid phase product. All of the second filtrate is returned to the first adsorption unit, and the carbon-containing solid phase product is regenerated and recycled. The first filtrate is introduced into a second adsorption unit, and a second adsorbent is added for second adsorption treatment to obtain a purified liquid. The purified liquid is then subjected to crystallization, separation, and drying to obtain purified dihydroxyethyl terephthalate.

[0081] The present invention also provides a purification device for crude dihydroxyethyl terephthalate, comprising a first adsorption unit 1, a cross-flow filtration unit 2, a second adsorption unit 3, a crystallization unit 5, a separation unit 4, and a regeneration unit 6.

[0082] like Figure 1 As shown, the purification apparatus for crude dihydroxyethyl terephthalate provided in this embodiment of the invention includes a first adsorption unit, a cross-flow filtration unit, a second adsorption unit, a crystallization unit, a separation unit, and a regeneration unit. The first adsorption unit performs a first adsorption treatment on a first mixture containing crude dihydroxyethyl terephthalate. The resulting second mixture is output from the first adsorption unit and enters the cross-flow filtration unit for cross-flow filtration. The first filtrate obtained after cross-flow filtration enters the second adsorption unit for a second adsorption treatment, and the resulting purified liquid enters the crystallization unit for crystallization.

[0083] The first adsorption unit includes a first liquid phase outlet 24, a second liquid phase outlet 10, a first material inlet 16 / 17, and a fifth liquid phase inlet 9. The first adsorption unit also includes a first liquid phase inlet 25, through which a first mixture containing crude diethyl terephthalate enters the first adsorption unit. The first adsorption unit is used to perform a first adsorption treatment on the first mixture containing crude diethyl terephthalate to obtain a second mixture. The first liquid phase outlet 24 is used to output a portion of the second mixture, which enters a cross-flow filtration unit under the action of pump 15. The second liquid phase outlet is used to output another portion of the second mixture, which enters a separation unit under the action of pump 7.

[0084] The cross-flow filtration unit includes an inlet end 22, a first outlet end 20, and a second outlet end 23. The first liquid phase outlet 24 of the first adsorption unit is connected to the inlet end 22 of the cross-flow filtration unit, and the second outlet end 23 of the cross-flow filtration unit is connected to the first material inlet 17 of the first adsorption unit. A portion of the second mixture flows out of the first liquid phase outlet of the first adsorption unit and enters the cross-flow filtration unit through the inlet end 22. It undergoes cross-flow filtration through the cross-flow membrane to obtain a first filtrate containing dihydroxyethyl terephthalate and a first slurry containing activated carbon. The first outlet end 20 is used to output the first filtrate, which enters the second adsorption unit. The second outlet end 23 is used to output the first slurry, which enters the first adsorption unit. The first slurry flows out of the second outlet end 23 of the cross-flow filtration unit and enters the first adsorption unit through the first material inlet 17 of the first adsorption unit.

[0085] The second adsorption unit includes a second liquid phase inlet 21 and a purified liquid outlet 19; the first discharge end 20 of the cross-flow filtration unit is connected to the second liquid phase inlet 21 of the second adsorption unit. The first filtrate flows out through the first discharge end of the cross-flow filtration unit and enters the second adsorption unit through the second liquid phase inlet 21 for second adsorption treatment to obtain purified liquid. The purified liquid outlet 19 is used to output the purified liquid, which then enters the crystallization unit.

[0086] Crystallization unit 5 includes a third liquid phase inlet 18; the purified liquid outlet 19 of the second adsorption unit is connected to the third liquid phase inlet 18 of the crystallization unit. After the purified liquid flows out of the purified liquid outlet 19 of the second adsorption unit, it enters the crystallization unit through the third liquid phase inlet 18 of the crystallization unit for crystallization treatment to obtain purified dihydroxyethyl terephthalate.

[0087] Separation unit 4 includes a fourth liquid phase inlet 11, a third liquid phase outlet 8, and a solid phase outlet 12. The second liquid phase outlet 10 of the first adsorption unit is connected to the fourth liquid phase inlet 11 of the separation unit, and the third liquid phase outlet 8 of the separation unit is connected to the fifth liquid phase inlet 9 of the first adsorption unit. Another portion of the second mixture flows out through the second liquid phase outlet 10 of the first adsorption unit and then enters the separation unit through the fourth liquid phase inlet 11 for solid-liquid separation, yielding a second filtrate and a carbon-containing solid phase product. The third liquid phase outlet 8 is used to output the second filtrate, which enters the first adsorption unit. The second filtrate flows out through the third liquid phase outlet 8 of the separation unit and then enters the first adsorption unit through the fifth liquid phase inlet 9. The solid phase outlet 12 is used to output the carbon-containing solid phase product, which enters the regeneration unit 6.

[0088] The regeneration unit includes a material inlet 13 and a material outlet 14. The material inlet 13 of the regeneration unit is connected to the solid phase outlet 12 of the separation unit, and the material outlet 14 of the regeneration unit is connected to one of the first material inlets 16 of the first adsorption unit. The carbonaceous solid phase product flows out through the solid phase outlet of the separation unit and then enters the regeneration unit through the material inlet for regeneration treatment to obtain regenerated adsorbent. The material outlet is used to output the regenerated adsorbent, which flows out through the material outlet of the regeneration unit and then returns to the first adsorption unit through the first material inlet of the first adsorption unit.

[0089] It should be noted that the first adsorption unit may include multiple first material inlets, for example, two first material inlets. The second discharge end of the cross-flow filtration unit is connected to one of the first material inlets 17, and the material outlet of the regeneration unit is connected to the other first material inlet 16.

[0090] This invention also provides a method for preparing dihydroxyethyl terephthalate, comprising the following steps: subjecting textile materials to alcoholysis to obtain dihydroxyethyl terephthalate product; purifying the dihydroxyethyl terephthalate product according to the above-described purification method for crude dihydroxyethyl terephthalate to obtain dihydroxyethyl terephthalate product.

[0091] The preparation method of dihydroxyethyl terephthalate in this invention can avoid filter membrane fouling and clogging, avoid single adsorption effect, maintain stable adsorption efficiency without reduction, and realize the recycling and recovery of adsorbent, thereby achieving deep purification of BHET, significantly improving product purity and color, ensuring product quality stability, while improving resource utilization and reducing costs.

[0092] Example 1

[0093] (1) Preparation of the first mixture containing crude dihydroxyethyl terephthalate

[0094] The shredded waste textiles were subjected to ethylene glycol alcoholysis. The resulting crude BHET depolymerization solution was washed, separated, crystallized, and centrifuged to obtain 55 kg of wet crude BHET product (15% water content, equivalent to 46.75 kg of BHET). Of the mother liquor produced, 80% by mass was returned to the alcoholysis process as a solvent for reuse, while the remainder was discharged from the system to prevent impurity accumulation. The above wet crude BHET product was added to a 400 L dissolving vessel, along with 150 kg of deionized water by mass. The mixture was heated to 70°C using a jacketed steam heater at a stirring speed of 50 rpm and maintained at this temperature for 30 min to completely dissolve the BHET, yielding a dark brown first mixed solution containing crude dihydroxyethyl terephthalate. The total mass of this solution was approximately 205 kg, and the temperature was 70°C.

[0095] The mass of BHET solute in the first mixture containing crude dihydroxyethyl terephthalate was 22.8 wt%.

[0096] (2) First adsorption treatment

[0097] The first mixture obtained in step (1) was pumped into a stirred tank reactor (as the first adsorption unit), and 0.6 kg of powdered activated carbon with a particle size of 50 μm (i.e., the first adsorbent) was added. The adsorption reaction was carried out for 30 min under continuous stirring at 70°C to complete the first adsorption treatment, and a second mixture containing activated carbon and the adsorbed product was obtained. The mass of the first adsorbent accounted for 1.3% of the mass of diethyl terephthalate in the first mixture containing crude diethyl terephthalate.

[0098] (3) Cross-flow filtration

[0099] 95% of the total mass of the second mixture obtained in step (2) is sent to the cross-flow membrane separation unit. This unit uses a ceramic microfiltration membrane with a pore size of 0.2 µm, and the operating conditions are: temperature 70℃, transmembrane pressure difference 0.2 MPa, operating pressure 0.3 MPa, and membrane surface velocity 2 m / s.

[0100] After separation, a purified BHET solution (i.e., the first filtrate) is obtained on the permeate side, and a concentrated slurry rich in activated carbon (i.e., the first slurry) is obtained on the retentate side. The concentration factor of the control unit (defined as the ratio of the flow rate of the crossflow membrane to the flow rate of the first slurry) is 10. The first slurry is returned entirely to the stirred tank reactor (first adsorption unit) of step (2) for recycling.

[0101] (4) Solid-liquid separation

[0102] Five percent of the total mass of the second mixture obtained in step (2) is sent to a fine filtration unit for solid-liquid separation (i.e., fine filtration). The solid-liquid separation is carried out at 70°C. In this embodiment, a plate and frame filter press is used as the fine filtration unit, with a filtration accuracy of 1 μm. Of course, other solid-liquid separation equipment such as candle filters or honeycomb cross-flow membrane modules can also be selected according to actual needs. After separation, a clear second filtrate and a filter cake rich in saturated activated carbon (carbon-containing solid phase product) are obtained.

[0103] The carbon-containing solid product was thermally regenerated at 600°C under a nitrogen atmosphere to obtain a regenerated adsorbent. Both the regenerated adsorbent and the second filtrate were returned to the stirred tank reactor of step (2) for recycling.

[0104] (5) Second adsorption treatment

[0105] The first filtrate obtained in step (3) (i.e., the BHET solution purified by the cross-flow membrane) is transported to a fixed-bed adsorption tower (as the second adsorption unit). The adsorption tower is filled with granular activated carbon with a particle size of 1.5 mm (as the second adsorbent). At 70°C, the contact time of the liquid in the empty bed in the tower is controlled to be 100 min for deep adsorption treatment to obtain a high-purity purified liquid.

[0106] (6) Crystallization treatment

[0107] The purified liquid obtained in step (5) is crystallized, separated and dried to obtain a solid product of dihydroxyethyl terephthalate with a purity of 99%, an L value of 98, an a value of 0.1 and a b value of 0.5.

[0108] Referring to Example 1, purified dihydroxyethyl terephthalate of Examples 2 to 11 was prepared, as detailed in Tables 1 and 2.

[0109] Comparative Example 1

[0110] The second mixture was prepared according to steps (1) and (2) of Example 1. The total mass of the second mixture was 205 kg, the temperature was 70°C, and it contained approximately 0.6 kg of powdered activated carbon with a particle size of approximately 50 μm. All of the above second mixture was pumped into an industrial plate and frame filter press. The filter press had a filter cloth with a filtration accuracy of 1 μm and a filtration area of ​​1 m². 2 Start the feed pump, control the constant pressure feed pressure at 0.5 MPa, and maintain the system temperature at 70℃.

[0111] Initial stage (0-5 minutes): The filtrate flows out smoothly and becomes clear.

[0112] Intermediate stage (5-15 minutes): A significant decrease in filtrate flow rate was observed. A dense activated carbon filter cake had formed inside the filter press plates and frames. To maintain the flow rate, the system inlet pressure was continuously increased to 0.7 MPa.

[0113] Later stage (15-25 minutes): The filtrate flow rate drops to less than 20% of the initial value, and the system pressure reaches the equipment's safe limit (0.85 MPa). At this point, only about 30% of the batch material (about 60 kg) has been processed, and the remaining about 70% of the slurry (about 145 kg) remains in the feed line and the first adsorption unit, unable to continue filtration.

[0114] Since the filtration pressure differential has reached its limit, the feed pump must be stopped to prevent equipment damage and the current filtration operation terminated. After disassembly, the moisture content of the filter cake is as high as 56%. At this time, the operator must perform the following cleaning procedures:

[0115] ① Disassemble the plate and frame, manually remove and collect the filter cake.

[0116] ② Rinse the filter cloth with a high-pressure water gun to restore the flow rate.

[0117] ③The entire cleaning and preparation process takes about 60 minutes.

[0118] This filtration operation was intermittent, with an effective filtration time of only 25 minutes, followed by a 60-minute cleaning and preparation period, making continuous production impossible. Each run processed only about 30% of the batch of material. The trapped activated carbon mixed with adsorbed impurities to form a filter cake with high moisture content, which was directly disposed of as solid waste, failing to achieve the recycling of the adsorbent.

[0119] Comparative Example 2

[0120] Compared with Example 1, the second adsorption treatment in step (5) is omitted, and everything else is the same as in Example 1.

[0121] Table 1

[0122]

[0123] In Table 1, “ / ” indicates that Comparative Example 1 did not use the corresponding process step, so the parameter does not exist.

[0124] Table 2

[0125]

[0126] In Table 2, “ / ” indicates that Comparative Example 1 and Comparative Example 2 did not use the corresponding process steps, so this parameter does not exist.

[0127] Test methods

[0128] (1) The purity of purified diethyl terephthalate was determined by high performance liquid chromatography (HPLC) using a Waters e-2695 HPLC system. Before the determination, 0.01 g of purified diethyl terephthalate solid sample was weighed, dissolved and diluted with chromatographic methanol, and filtered through a 0.22 μm organic phase filter membrane to obtain the sample solution to be tested.

[0129] The chromatographic conditions were as follows: mobile phase was chromatographically pure methanol, flow rate was 1.0 mL / min; column was a C18 reversed-phase column (4.6 mm × 250 mm, 5 μm); column temperature was 45℃; detection wavelength was 254 nm; injection volume was 20 μL.

[0130] After the instrument baseline stabilizes, blank methanol, diethyl terephthalate standard solution and the sample solution to be tested are injected sequentially. The chromatogram is recorded, and the content of diethyl terephthalate in the sample is calculated by peak area using the external standard method.

[0131] (2) The colorimetric index of purified diethyl terephthalate was measured using a ColorFlex EZ colorimeter. Before measurement, the instrument was calibrated. An appropriate amount of dried diethyl terephthalate solid sample was placed in the sample chamber and detected under a standard light source. The L value, a value, and b value were recorded. The measurement was repeated 3 times and the average value was taken as the final result.

[0132] Table 3

[0133]

[0134] As shown in Table 3, the purification method for crude dihydroxyethyl terephthalate (BHET) provided by this invention, through a coupled process of first adsorption treatment, cross-flow filtration, and second adsorption treatment, yields BHET products with a purity ≥80%, L value ≥89, a value ≤1.6, and b value ≤2.5, exhibiting excellent overall performance and enabling adsorbent recycling and continuous production. In contrast to Examples 1-11, Comparative Example 1, which uses traditional plate and frame filter press instead of cross-flow filtration, not only fails to achieve continuous production but also suffers from filter membrane clogging, resulting in a product purity of only 76%, an L value of 81, an a value of 2, and a b value as high as 14. The impurity removal effect is extremely poor, and the adsorbent cannot be recycled. Comparative Example 2, which omits the second adsorption treatment, sees the product purity drop to 72%, with an L value of 75, an a value of 2.5, and a b value as high as 17, and the color is severely substandard. As can be seen from the comparison, the present invention can effectively avoid the problems of filter membrane clogging and single adsorption effect by limiting the coupling of the first adsorption treatment, cross-flow filtration and the second adsorption treatment, significantly improve product purity and color, ensure production continuity and reduce operating costs.

[0135] Furthermore, by further limiting the process parameters in Examples 1 to 3 of this invention, such as the first adsorbent particle size of 45μm to 150μm, the first adsorption temperature of 60℃ to 95℃, the cross-flow filtration temperature of 60℃ to 90℃, the pore size of the cross-flow filtration membrane of 0.05μm to 0.5μm, the second adsorbent particle size of 0.3 mm to 2 mm, and the second adsorption temperature of 60℃ to 80℃, the resulting BHET product exhibits a purity ≥96%, an L value ≥96, an a value ≤0.5, and a b value ≤1.4, demonstrating optimal performance. When the examples deviate from the above parameter ranges, the product performance significantly decreases. Experiments show that the reasonable limitation of each process parameter in this invention can further improve adsorption efficiency and process stability, optimize product purity and color, and achieve a balance between purification effect and production economy.

[0136] In summary, this invention addresses the pain points of existing technologies through a coupled process of first adsorption, cross-flow filtration, and second adsorption, achieving efficient purification of crude BHET. Furthermore, by precisely defining each process parameter, it further enhances product performance stability and process adaptability, balancing product quality, production continuity, and economic efficiency, thus providing a reliable technical path for preparing high-quality BHET from waste textile recycling.

[0137] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for purifying crude dihydroxyethyl terephthalate, characterized in that, include: A first mixture containing crude dihydroxyethyl terephthalate is introduced into a first adsorption unit and subjected to a first adsorption treatment in the presence of a first adsorbent to obtain a second mixture; wherein the first adsorbent includes activated carbon. At least a portion of the second mixture is fed into a cross-flow filtration unit and filtered through a cross-flow membrane to obtain a first filtrate containing dihydroxyethyl terephthalate and a first slurry containing the activated carbon, respectively. The first slurry is returned to the first adsorption unit; A portion of the second mixture is introduced into a separation unit for solid-liquid separation to obtain a second filtrate and a carbon-containing solid product; the second filtrate is then returned entirely to the first adsorption unit; wherein the ratio of the sum of the flow rates of the first slurry returned to the first adsorption unit and the second filtrate returned to the first adsorption unit to the flow rate of the first filtrate is 1:(2~11). The first filtrate is introduced into the second adsorption unit, where it undergoes a second adsorption treatment in the presence of a second adsorbent to obtain a purified liquid; wherein the second adsorbent includes activated carbon and / or resin adsorption material. The purified liquid is then introduced into a crystallization unit for crystallization to obtain purified dihydroxyethyl terephthalate.

2. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The mass ratio of the second mixture entering the cross-flow filtration unit to the second mixture entering the separation unit is (85~97):(3~15).

3. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, During the cross-flow filtration process, the flow rate ratio of the second mixed liquid entering the cross-flow filtration unit to the flow rate of the first slurry is (5~15):

1. And / or, the temperature of the cross-flow filter is 60℃~90℃; And / or, the transmembrane pressure difference of the cross-flow filter is 0.1 MPa ~ 0.3 MPa; And / or, the operating pressure of the cross-flow filter is 0.3 MPa to 0.6 MPa; And / or, the membrane surface velocity of the cross-flow filtration is 1 m / s to 5 m / s; And / or, the pore size of the crossflow membrane is 0.05 μm to 0.5 μm; And / or, the crossflow membrane includes one or more of the following: metal membrane, ceramic membrane, and organic polymer membrane.

4. The purification method for crude dihydroxyethyl terephthalate according to claim 1 or 2, characterized in that, The particle size of the first adsorbent is 45 μm to 150 μm.

5. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The temperature of the first adsorption is 60℃~95℃, and the time of the first adsorption is 20min~40min; And / or, the mass of the first adsorbent accounts for 1% to 5% of the mass of dihydroxyethyl terephthalate in the first mixture.

6. The purification method for crude dihydroxyethyl terephthalate according to claim 1 or 2, characterized in that, The particle size of the second adsorbent is 0.3 mm to 2 mm.

7. The purification method for crude dihydroxyethyl terephthalate according to claim 1, characterized in that, The second adsorption temperature is 60℃~80℃, and the second adsorption time is 90min~180min.

8. The purification method for crude dihydroxyethyl terephthalate according to claim 1 or 2, characterized in that, The crude dihydroxyethyl terephthalate includes dihydroxyethyl terephthalate products obtained from the alcoholysis of textile materials.

9. A purification apparatus for crude dihydroxyethyl terephthalate, characterized in that, It includes a first adsorption unit, a cross-flow filtration unit, a second adsorption unit, a crystallization unit, a separation unit, and a regeneration unit; The first adsorption unit includes a first liquid phase outlet, a second liquid phase outlet, a first material inlet, and a fifth liquid phase inlet; A cross-flow filtration unit includes an inlet end, a first outlet end, and a second outlet end. The first liquid phase outlet of the first adsorption unit is connected to the inlet end of the cross-flow filtration unit, and the second outlet end of the cross-flow filtration unit is connected to the first material inlet of the first adsorption unit. The second adsorption unit includes a second liquid phase inlet and a purified liquid outlet; The first discharge end of the cross-flow filtration unit is connected to the second liquid phase inlet of the second adsorption unit; The crystallization unit includes a third liquid phase inlet; the purified liquid outlet of the second adsorption unit is connected to the third liquid phase inlet of the crystallization unit. The separation unit includes a fourth liquid phase inlet, a third liquid phase outlet, and a solid phase outlet; The second liquid phase outlet of the first adsorption unit is connected to the fourth liquid phase inlet of the separation unit, and the third liquid phase outlet of the separation unit is connected to the fifth liquid phase inlet of the first adsorption unit. The regeneration unit includes a material inlet and a material outlet; the material inlet of the regeneration unit is connected to the solid phase outlet of the separation unit, and the material outlet of the regeneration unit is connected to the first material inlet of the first adsorption unit.

10. A method for preparing dihydroxyethyl terephthalate, characterized in that, Includes the following steps: Textile materials were subjected to alcoholysis to obtain dihydroxyethyl terephthalate product. The crude dihydroxyethyl terephthalate product is purified according to the purification method of any one of claims 1-8 to obtain the dihydroxyethyl terephthalate product.

Images