Lactic acid cooling crystallization system and method

Abstract

This invention discloses a lactic acid cooling crystallization system and method. The system includes a precooler, a primary crystallizer, a secondary crystallizer, a solid-liquid separation mechanism, and a mother liquor tank connected in sequence, and is equipped with a mother liquor circulation loop. The primary crystallizer integrates a dynamic anti-scaling and high-efficiency heat exchange device composed of a flexible wall scraper and a dual heat exchange structure. A distributor is provided at the end of the raw material solution outlet pipe to achieve multi-point feeding. A three-way diversion valve and a return pipe connected to the raw material pipe are provided on the mother liquor inlet pipe to directly mix part of the cold mother liquor into the raw material to precisely control the supersaturation. The method includes raw material precooling and multi-point feeding, primary programmed cooling crystallization combined with online fine-tuning of the mother liquor, secondary low-temperature ripening, and solid-liquid separation. Through the above structure and method, this invention solves the problems of easy scaling of crystallizers, decreased heat transfer efficiency, wide crystal particle size distribution, and difficulty in balancing product purity and yield in the prior art, and achieves efficient, energy-saving, continuous and stable production of high-purity lactic acid crystals.

Description

Technical Field

[0001] This invention relates to the field of industrial crystallization technology, specifically to a lactic acid cooling crystallization system and method. Background Technology

[0002] Lactic acid, as an important organic acid, is widely used in food, medicine, cosmetics, and biodegradable materials (such as polylactic acid PLA). Industrially, it is mainly produced through microbial fermentation. The resulting crude lactic acid solution contains various impurities such as residual sugar, protein, pigments, and inorganic salts. Therefore, it must undergo efficient purification to obtain a high-purity product that meets the requirements of high-end applications.

[0003] Currently, the mainstream industrial technology pathways for achieving deep purification of lactic acid mainly include the following three, but each has significant limitations: 1. Esterification hydrolysis method: This method first esterifies lactic acid with alcohols, then separates and purifies the lactate ester, and finally hydrolyzes it to obtain pure lactic acid. Its main disadvantages are: long process flow, many steps, need to use and recover large amounts of flammable alcohol solvents, posing safety hazards and environmental pressures, high overall energy consumption, and large equipment investment.

[0004] 2. Molecular distillation method: This method separates lactic acid and impurity molecules under high vacuum by utilizing the difference in volatility. Its main drawbacks are: extremely high vacuum requirements for the equipment, high manufacturing and maintenance costs, high refrigeration load under high vacuum, high energy consumption, and the risk of degradation for heat-sensitive lactic acid due to the heating of the material during distillation.

[0005] 3. Conventional cooling crystallization method: This method utilizes the difference in solubility of lactic acid and impurities in solution with temperature changes. By cooling, lactic acid precipitates out in crystal form, thus achieving purification. Compared with the methods mentioned above, crystallization has potential advantages such as mild operation, good selectivity, and high product purity. However, existing conventional cooling crystallization technologies have long faced the following prominent technical bottlenecks in terms of industrial scale-up and continuous stable operation: First, there are issues with heat transfer efficiency degradation and scale buildup. On cooling surfaces, precipitated lactic acid crystals readily adhere and accumulate, forming a dense scale layer. This scale layer has extremely high thermal resistance, causing a sharp drop in the heat transfer coefficient during the later stages of operation. This results in insufficient cooling capacity, a significantly prolonged crystallization cycle, increased energy consumption per unit of product, and the need for frequent shutdowns for scale removal, impacting continuous production.

[0006] Secondly, the crystallization process is poorly controlled, leading to conflicting product specifications. Traditional processes rely heavily on linear cooling or empirical control, making it difficult to precisely regulate the core driving force of crystallization (i.e., supersaturation) in real time. This directly results in the formation of excessive fine crystals, leading to a wide crystal size distribution (CSD) in the final product, affecting filtration, washing efficiency, and product flowability. Furthermore, rapid cooling to achieve high purity can easily form fine crystals that encapsulate impurities, and severe mother liquor entrainment during solid-liquid separation results in low yields. Conversely, slow cooling to pursue high yields may cause crystals to grow excessively large, but at the cost of decreased purity.

[0007] Finally, the system suffers from low energy efficiency and automation. Most units operate intermittently, heat is not effectively recovered, process parameters rely on manual experience for adjustment, and product quality exhibits poor batch-to-batch stability, making it unsuitable for large-scale continuous production. Summary of the Invention

[0008] The technical problem to be solved by this invention is to provide a lactic acid cooling crystallization system and method. By integrating technologies such as wall scraping and scale prevention, programmed nonlinear cooling and precise control of mother liquor microcirculation, it solves the prominent problems in the prior art, such as easy scaling of crystallizers, rapid decline in heat transfer efficiency, difficulty in balancing product purity and yield, and uneven crystal particle size distribution. It achieves high-efficiency, high-purity, low-energy consumption lactic acid crystallization and separation suitable for continuous production.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a lactic acid cooling crystallization system, including a precooler, one of the output ports of the precooler is connected to a primary crystallizer, the bottom of the primary crystallizer is connected to a secondary crystallizer through a crystal slurry conveying pipe, the bottom of the secondary crystallizer is connected to a solid-liquid separation mechanism through a solid-liquid mixture conveying pipe, the liquid phase outlet of the solid-liquid separation mechanism is connected to a mother liquor tank through a mother liquor discharge pipe, and the mother liquor tank is provided with a mother liquor inlet pipe connected to one of the input ports of the precooler.

[0010] In a preferred embodiment, another inlet of the precooler is connected to the raw material solution inlet pipe, and the raw material solution inlet pipe is connected to the raw material solution outlet pipe; The other output port of the precooler is connected to the mother liquor outlet pipe, which is connected to the mother liquor inlet pipe.

[0011] In a preferred embodiment, the primary crystallizer is provided with a horizontal rotating shaft, on which are mounted a spiral stirring blade and a flexible wall scraper, the flexible wall scraper being in contact with the inner wall of the primary crystallizer.

[0012] In a preferred embodiment, a heat exchange jacket is provided on the outer wall of the primary crystallizer, and a heat exchange coil is also provided on the wall of the primary crystallizer.

[0013] In a preferred embodiment, the secondary crystallizer is equipped with a stirring mechanism.

[0014] In a preferred embodiment, the solid-liquid separation mechanism is a centrifugal separation device, and the bottom of the solid-liquid separation mechanism is provided with a crystal discharge pipe for outputting the finished crystal; The mother liquor discharge pipe is installed on the side wall of the solid-liquid separation mechanism.

[0015] In a preferred embodiment, the raw material solution outlet pipe is connected to a distributor, and multiple output branches on the distributor are connected to different positions on the primary crystallizer.

[0016] In a preferred embodiment, the mother liquor inlet pipe is equipped with a three-way diverter valve, and the three-way diverter valve is equipped with a return pipe connected to the raw material solution outlet pipe; The reflux pipe is equipped with a check valve near the connection point with the raw material solution outlet pipe.

[0017] The lactic acid cooling crystallization method based on the above-mentioned lactic acid cooling crystallization system includes the following steps: S1. A raw material lactic acid solution with a concentration of 85%-95% is pumped into the precooler through the raw material solution inlet pipe. It exchanges heat with the cold mother liquor from the mother liquor tank at a temperature of 10-15°C, so that the temperature of the raw material solution drops to 30-40°C. Then, it is distributed through the raw material solution outlet pipe and the distributor to enter the primary crystallizer from multiple locations. S2. Inside the primary crystallizer, the rotating shaft drives the spiral stirring blades and flexible scraper blades to operate, while the liquid is cooled non-linearly through the heat exchange jacket and heat exchange coil. In the above process, by adjusting the three-way diverter valve, part of the cold mother liquor from the mother liquor tank is injected into the raw material solution outlet pipe through the mother liquor inlet pipe and the return pipe. After mixing with the pre-cooled raw material solution, it enters the first-stage crystallizer together, so as to precisely control the supersaturation of the crystallizer feed area. S3. When the solid content of the crystal slurry in the primary crystallizer reaches 35-45%, it is pumped into the secondary crystallizer through the crystal slurry delivery pipe. Under the action of the stirring mechanism, it is matured at 0-10℃ for 2-4 hours. After that, the matured crystal slurry is sent to the solid-liquid separation mechanism for separation to obtain lactic acid crystals and mother liquor. The mother liquor is stored in the mother liquor tank.

[0018] In the preferred embodiment, the nonlinear programmed cooling and control in step S2 includes an induced nucleation stage and a crystal growth and control stage; During the induced nucleation stage, the cooling system is controlled to rapidly cool the liquid in the primary crystallizer from the feed temperature to 25-28°C at a rate of 3-5°C / h. During the crystal growth and control stage, the cooling rate is reduced to 0.5-1.5℃ / h, allowing the feed solution to be slowly cooled from 25-28℃ to 10-15℃. During this stage, the three-way diversion valve is dynamically adjusted according to the supersaturation signal to change the flow rate of the cold mother liquor mixed into the raw material through the reflux pipe, thereby maintaining the supersaturation in the primary crystallizer within the metastable range.

[0019] The lactic acid cooling crystallization system and method provided by the present invention have the following beneficial effects by adopting the above-described structure and method: (1) A flexible scraping device is integrated in the primary crystallizer, which can scrape off the nascent crystals on the heat exchange surface in real time during operation, preventing the formation and accumulation of hard crystal scale from the source. This makes the heat transfer coefficient remain stable throughout the crystallization cycle, the cooling efficiency does not decrease, the crystallization cycle is greatly shortened, and the production capacity loss and energy consumption increase caused by frequent shutdowns for cleaning are avoided. (2) By adopting nonlinear program cooling combined with online reflux control of mother liquor, the supersaturation can be precisely controlled in the optimal metastable region, which effectively inhibits harmful secondary nucleation and promotes uniform crystal growth. This allows for the one-time acquisition of high-purity, large-particle-size, concentrated, and highly fluid crystals, while improving the yield of single crystallization. This solves the core contradiction in traditional processes where quality and yield are difficult to balance. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the overall system structure of the present invention.

[0021] In the diagram: 1. Precooler; 2. Primary crystallizer; 3. Secondary crystallizer; 4. Solid-liquid separation mechanism; 5. Mother liquor tank; 6. Raw material solution inlet pipe; 7. Raw material solution outlet pipe; 8. Mother liquor inlet pipe; 9. Mother liquor outlet pipe; 10. Diverter; 11. Rotary shaft; 12. Spiral stirring blade; 13. Flexible scraper; 14. Heat exchange jacket; 15. Heat exchange coil; 16. Crystal slurry conveying pipe; 17. Stirring mechanism; 18. Solid-liquid mixture conveying pipe; 19. Crystal discharge pipe; 20. Mother liquor discharge pipe; 21. Three-way diverter valve; 22. Reflux pipe; 23. Check valve. Detailed Implementation

[0022] Example 1: Please see Figure 1 The lactic acid cooling crystallization system provided by this invention comprises, in sequence along the material flow path, a precooler 1, a primary crystallizer 2, a secondary crystallizer 3, a solid-liquid separation mechanism 4, and a mother liquor tank 5. All components are connected by pipelines, forming a closed-loop system integrating reaction, separation, energy recovery, and circulation control.

[0023] in: The precooler 1 employs a high-efficiency plate heat exchanger with two independent flow channels. The inlet of the first flow channel is connected to the raw material supply system via the raw material solution inlet pipe 6, and the outlet is the raw material solution outlet pipe 7, which outputs the precooled raw material. The inlet of the second flow channel is connected to the mother liquor tank 5 via the mother liquor inlet pipe 8, and the outlet is the mother liquor outlet pipe 9, which transports the heated mother liquor to subsequent processing units or for recycling. The core function of the precooler 1 is to achieve countercurrent heat exchange between the crude lactic acid raw material and the low-temperature mother liquor in the system, thereby recovering cold energy and reducing the total cooling load of the system.

[0024] A distributor 10 is connected to the end of the raw material solution outlet pipe 7. The distributor 10 has one inlet and multiple outlet branches, which are respectively connected to different positions on the side wall of the primary crystallizer 2. The purpose is to introduce the pre-cooled raw material solution into the crystallization body in a uniform and dispersed manner, avoiding excessively high local concentrations and temperatures, and creating conditions for the formation of a uniform initial supersaturation field within the crystallizer.

[0025] The primary crystallizer 2 is a horizontal or vertical tank with a jacket, and is the core reaction unit of this invention.

[0026] It contains a horizontal rotating shaft 11 driven by a variable frequency motor. Spiral stirring blades 12 and flexible scraper blades 13 are installed at intervals on the shaft 11. The spiral stirring blades 12 are used to create a strong axial and radial composite flow field within the tank, ensuring uniform crystal suspension and solution temperature and concentration. The flexible scraper blades 13 are typically made of polytetrafluoroethylene (PTFE) or similar flexible wear-resistant materials, and their cutting edges maintain elastic contact with the inner wall of the crystallizer and the surface of the inner heat exchange coil 15.

[0027] In addition, a heat exchange system is installed on the primary crystallizer 2. This system mainly includes a heat exchange jacket 14 mounted on the outer wall of the primary crystallizer 2, through which a cooling medium (such as chilled brine) is circulated for primary heat exchange. Furthermore, an internal heat exchange coil 15 is coiled and installed on the inner wall of the crystallizer, also circulated with a cooling medium. This dual heat exchange system, consisting of the jacket 14 and the coil 15, provides a larger and more uniformly distributed heat exchange area. During rotation, the flexible scraper 13 continuously scrapes the inner wall of the jacket and the surface of the coil 15, removing newly formed thin layers of crystal nuclei or tiny crystals in real time, preventing their accumulation into insulating scale, thus maintaining an extremely high heat transfer coefficient over a long period.

[0028] The top surface of the primary crystallizer 2 is equipped with multiple structures connecting to the distributor 10, through which the raw material from the distributor 10 enters. In addition, the system has a critical control path (see...). Figure 1A three-way diverter valve 21 is installed on the mother liquor inlet pipe 8. One outlet of this valve is connected to the precooler 1, and the other outlet is connected to the raw material solution outlet pipe 7 via a return pipe 22. A check valve 23 is installed at the connection between the return pipe 22 and the raw material solution outlet pipe 7 to prevent backflow of the raw material solution. This pipeline valve assembly is one of the core control methods of this invention. It allows the operator to inject a portion of the low-temperature mother liquor from the mother liquor tank 5 directly and precisely into the raw material solution that has been precooled to 30-40°C without passing through the precooler 1, completing rapid mixing and temperature adjustment before entering the primary crystallizer 2.

[0029] The secondary crystallizer 3 is a vertical storage tank with gentle stirring. It has an internal low-speed stirring mechanism 17. Its inlet is connected to the bottom of the primary crystallizer 2 through a crystal slurry delivery pipe 16, and its outlet is connected to the solid-liquid separation mechanism 4 through a solid-liquid mixture delivery pipe 18. Its main function is to provide a stable environment with low temperature, low shear, and controllable residence time.

[0030] The solid-liquid separation mechanism 4 is preferably a horizontal screw discharge centrifuge or a belt filter. It has a crystal discharge pipe 19 at its bottom for discharging wet lactic acid crystals; its side wall liquid phase outlet is connected to a mother liquor discharge pipe 20. The separated mother liquor enters a mother liquor tank 5 for temporary storage via the mother liquor discharge pipe 20. The mother liquor tank 5 serves as a buffer and homogenizer; the cold mother liquor within is pumped through a mother liquor inlet pipe 8 to two paths: the main stream enters the precooler 1 for energy recovery; the other branch can be used for precise online control via a three-way diversion valve 21 and a return pipe 22.

[0031] Example 2: The core workflow and underlying principles of the system described in Embodiment 1 are as follows: Step S1: Feeding, precooling and multi-point dispersion Warm crude lactic acid feedstock with a concentration of 85-95% enters the first flow channel of the precooler 1 through the feedstock solution inlet pipe 6, where it undergoes countercurrent heat exchange with the low-temperature cold mother liquor (10-15℃) flowing through the second flow channel from the mother liquor tank 5. This process recovers the cold energy contained in the mother liquor, reducing the feedstock temperature from approximately 50℃ to 30-40℃, close to its saturation point, significantly reducing the heat load on the subsequent crystallizer. The precooled feedstock is then transported to the distributor 10 through the feedstock solution outlet pipe 7 and evenly distributed to multiple different spatial locations in the primary crystallizer 2. The principle behind this design is to break the localized oversaturation hotspots caused by traditional single-point feeding, allowing fresh feedstock to quickly mix with the large amount of circulating crystal slurry within the tank. This facilitates the establishment of a more uniform initial temperature and concentration field throughout the crystallizer, laying the foundation for subsequent uniform nucleation and growth.

[0032] Step S2: Precise "feedforward-feedback" control of programmed cooling crystallization and supersaturation After the feed liquid enters the primary crystallizer 2, the rotating shaft 11 is started, and the flexible scraper 13 and the spiral stirring blade 12 operate synchronously. The cooling system (jacket 14 and coil 15) starts according to the preset nonlinear cooling program.

[0033] The nonlinear cooling process includes the following two stages: Phase 1: Induced Nucleation Stage (Rapid Cooling) The system operates at a high cooling rate (e.g., 3-5℃ / h), allowing the solution to quickly cross the saturation line and enter the upper part of the metastable region, generating a suitable amount of uniform primary crystal nuclei throughout the system. The continuous operation of the scraper blade 13 ensures that even during the rapid cooling phase, the microscopic supersaturation of the heat exchange surface can be broken in time, preventing the wall surface from becoming a "breeding ground" for heterogeneous nucleation.

[0034] Phase Two: Crystal Growth and Control Phase (Slow Cooling + Dynamic Fine-tuning) Once the crystal nuclei are formed, the system will significantly reduce the cooling rate (e.g., 0.5-1.5℃ / h) to maintain the overall supersaturation of the system within a low and stable "metastable region," allowing solute molecules sufficient time to grow in an orderly manner on the existing crystal nuclei, rather than forming new nuclei.

[0035] The "feedforward-feedback" control mechanism focuses on proactive intervention in local supersaturation. When online monitoring (e.g., through thermodynamic calculations or online refractometers) indicates a risk of supersaturation exceeding the optimal range in a certain area or overall, the control system dynamically adjusts the three-way diverter valve 21. Specifically, a stream of low-temperature (10-15℃) pure mother liquor is directly injected into the feed stream about to enter the crystallizer through the reflux pipe 22. This cold fluid acts as a fine-tuning agent, instantly and precisely reducing the temperature and concentration at the feed mixing point, thereby neutralizing the potentially excessive local supersaturation driving force. The check valve 23 ensures the unidirectional mixing.

[0036] The method described above, which uses the mother liquor as a physicochemical regulator to act directly on the feed end, has a faster response and more direct control compared to the traditional method of only adjusting the temperature of the cooling medium or the stirring speed. It can more effectively suppress secondary nucleation and is the key to achieving narrowing of the crystal size distribution (CSD) and increasing the average particle size.

[0037] Step S3: Crystal slurry maturation and optimized separation When the solid content of the crystals in the primary crystallizer 2 grows to 35-45%, the crystal slurry is transferred to the secondary crystallizer 3. At a constant low temperature of 3-5℃, the crystals are kept in uniform suspension by a stirring mechanism 17 for 2-4 hours for maturation. The principle is based on the "Ostwald maturation" effect; in this dynamic equilibrium environment, small crystals gradually dissolve while large crystals continue to grow. This process not only further increases the average particle size of the crystals but also reduces surface defects and increases strength, significantly optimizing subsequent solid-liquid separation performance and reducing crystal breakage and loss during washing.

[0038] Step S4: Separation, washing, and closed-loop circulation After maturation, the slurry is centrifuged to obtain high-purity lactic acid crystals, which are then washed with low-temperature pure water. The separated mother liquor is returned to mother liquor tank 5, completing a closed-loop cycle of materials and cooling. Some of the mother liquor enriched with impurities can be discharged periodically to prevent impurity accumulation.

[0039] In summary, this invention, through its unique equipment structure design and matching process flow, forms a complete, efficient, and controllable solution for lactic acid cooling and crystallization, encompassing energy recovery, uniform feeding, dynamic scale prevention, programmed cooling, precise control, crystal maturation, and efficient separation.

Claims

1. A lactic acid cooling crystallization system, characterized in that: The system includes a precooler (1), one of the output ports of which is connected to a primary crystallizer (2). The bottom of the primary crystallizer (2) is connected to a secondary crystallizer (3) via a crystal slurry conveying pipe (16). The bottom of the secondary crystallizer (3) is connected to a solid-liquid separation mechanism (4) via a solid-liquid mixture conveying pipe (18). The liquid phase outlet of the solid-liquid separation mechanism (4) is connected to a mother liquor tank (5) via a mother liquor discharge pipe (20). The mother liquor tank (5) is provided with a mother liquor inlet pipe (8) connected to one of the input ports of the precooler (1).

2. The lactic acid cooling crystallization system according to claim 1, characterized in that: The other inlet of the precooler (1) is connected to the raw material solution inlet pipe (6), and the raw material solution inlet pipe (6) is connected to the raw material solution outlet pipe (7); The other output port of the precooler (1) is connected to the mother liquor outlet pipe (9), and the mother liquor outlet pipe (9) is connected to the mother liquor inlet pipe (8).

3. The lactic acid cooling crystallization system according to claim 1, characterized in that: The primary crystallizer (2) is equipped with a horizontal rotating shaft (11), on which a spiral stirring blade (12) and a flexible scraper (13) are mounted. The flexible scraper (13) is in contact with the inner wall of the primary crystallizer (2).

4. The lactic acid cooling crystallization system according to claim 3, characterized in that: The outer wall of the primary crystallizer (2) is provided with a heat exchange jacket (14), and the wall of the primary crystallizer (2) is also provided with a heat exchange coil (15).

5. The lactic acid cooling crystallization system according to claim 1, characterized in that: The secondary crystallizer (3) is equipped with a stirring mechanism (17).

6. The lactic acid cooling crystallization system according to claim 1, characterized in that: The solid-liquid separation mechanism (4) is a centrifugal separation device, and the bottom of the solid-liquid separation mechanism (4) is provided with a crystal discharge pipe (19) for outputting finished crystals. The mother liquor discharge pipe (20) is installed on the side wall of the solid-liquid separation mechanism (4).

7. The lactic acid cooling crystallization system according to claim 1, characterized in that: The raw material solution outlet pipe (7) is connected to the distributor (10), and multiple output branches on the distributor (10) are connected to different positions on the primary crystallizer (2).

8. The lactic acid cooling crystallization system according to claim 1, characterized in that: The mother liquor inlet pipe (8) is equipped with a three-way diverter valve (21), and the three-way diverter valve (21) is equipped with a return pipe (22) connected to the raw material solution outlet pipe (7). The reflux pipe (22) is equipped with a check valve (23) near the connection with the raw material solution outlet pipe (7).

9. A method for cooling and crystallizing lactic acid based on the lactic acid cooling and crystallizing system according to any one of claims 1-8, characterized in that... Includes the following steps: S1. A raw material lactic acid solution with a concentration of 85%-95% is pumped into the precooler (1) through the raw material solution inlet pipe (6) and exchanged with the cold mother liquor from the mother liquor tank (5) at a temperature of 10-15℃. After the temperature of the raw material solution drops to 30-40℃, it is distributed through the raw material solution outlet pipe (7) and the distributor (10) and enters the primary crystallizer (2) from multiple locations. S2. Inside the primary crystallizer (2), start the rotating shaft (11) to drive the spiral stirring blade (12) and flexible scraper (13) to operate, and at the same time, perform non-linear program cooling of the liquid through the heat exchange jacket (14) and heat exchange coil (15). In the above process, by adjusting the three-way diverter valve (21), part of the cold mother liquor from the mother liquor tank (5) is injected into the raw material solution outlet pipe (7) through the mother liquor inlet pipe (8) and the return pipe (22), and mixed with the pre-cooled raw material solution before entering the first-stage crystallizer (2) to precisely control the supersaturation of the crystallizer feed area; S3. When the solid content of the crystal slurry in the primary crystallizer (2) reaches 35-45%, it is pumped into the secondary crystallizer (3) through the crystal slurry delivery pipe (16). Under the action of the stirring mechanism (17), it is matured at 0-10℃ for 2-4 hours. After that, the matured crystal slurry is sent to the solid-liquid separation mechanism (4) for separation to obtain lactic acid crystals and mother liquor. The mother liquor is stored in the mother liquor tank (5).

10. A method for cooling and crystallizing lactic acid according to claim 9, characterized in that: The nonlinear programmed cooling and control in step S2 includes the induced nucleation stage and the crystal growth and control stage; During the induced nucleation stage, the cooling system is controlled to rapidly cool the liquid in the primary crystallizer (2) from the feed temperature to 25-28℃ at a rate of 3-5℃ / h. During the crystal growth and control stage, the cooling rate is reduced to 0.5-1.5℃ / h, so that the feed liquid is slowly cooled from 25-28℃ to 10-15℃. During this stage, the three-way diversion valve (21) is dynamically adjusted according to the supersaturation signal to change the flow rate of the cold mother liquor mixed into the raw material through the return pipe (22) and maintain the supersaturation in the primary crystallizer (2) within the metastable range.

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