A method and system for refining long chain dibasic acids

Abstract

The application belongs to the technical field of fine chemical industry, and particularly relates to a refining method and system of long-chain dibasic acid. In view of the problems of the long-chain dibasic acid crude product prepared by the microbial fermentation method, such as many impurities, uneven crystal particle size, poor moisture resistance and flowability, the proportioning of the recrystallization solvent and the melting granulation process are optimized. The refining process adopts a specific proportioning of formic acid-acetic acid mixed solvent for secondary recrystallization, accurately controls the dissolution, cooling crystallization and drying parameters, and then the melted crystal compound is put into a special melting kettle to be melted and granulated under the inert atmosphere of high-purity nitrogen, the suitable temperature and pressure to obtain the finished long-chain dibasic acid. The recrystallization solvent system is optimized to efficiently remove the impurities in the crude product, realize excellent decolorization and refine the crystal particle size. Meanwhile, the melting granulation process is improved to improve the melt density and uniformity, greatly improve the moisture resistance and flowability of the product, and the obtained product has high purity and stable physical and chemical indexes, and is suitable for industrial large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of fine chemical technology, specifically relating to a purification method and purification system for long-chain dicarboxylic acids. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Long-chain dicarboxylic acids are essential raw materials for the synthesis of high-end nylon, specialty hot melt adhesives, and pharmaceutical and fragrance intermediates. Traditional chemical synthesis methods suffer from harsh reaction conditions, severe equipment corrosion, significant environmental pollution, and poor product selectivity, making it difficult to meet the high-purity monomer requirements of advanced materials. Microbial fermentation, using alkanes or renewable sugars as raw materials, utilizes the oxidative catalysis system of microorganisms at ambient temperature and pressure to achieve efficient synthesis of long-chain dicarboxylic acids. This method offers advantages such as a green and mild process, high product purity, and good carbon chain selectivity, and has gradually become the mainstream technology for industrial production.

[0004] Currently, fermentation production primarily uses *Candida tropicalis*, *Yarrowia lipolyticis*, or genetically engineered strains as production strains. This involves enhancing the ω-oxidation pathway and weakening the β-oxidation bypass to achieve the directional conversion of methyl groups to carboxyl groups at both ends of the substrate. The process mainly includes steps such as strain activation and expansion, fed-batch fermentation, and solid-liquid separation of the fermentation broth. By controlling temperature, pH, dissolved oxygen, and substrate flow rate, efficient cell growth and large-scale product accumulation can be achieved. Under appropriate fermentation regulation, high acid concentrations and substrate conversion rates can be obtained.

[0005] Common methods for purifying long-chain dicarboxylic acids are mainly divided into two categories: aqueous phase purification and solvent purification. Aqueous phase purification uses water as the medium and achieves purification through a combination of processes such as filtration, decolorization, crystallization, and membrane separation. Its drawbacks include a long crystallization cycle, the need for precise control of parameters such as temperature, pH, and cooling rate during the purification process, high requirements for equipment automation, and relatively high energy consumption. Solvent purification uses organic solvents as the dissolving medium and utilizes the difference in solubility between long-chain dicarboxylic acids and impurities in the solvent to achieve purification. Commonly used solvents include acetic acid, chloroform, methyl tert-butyl ether, and isooctyl ether. Its disadvantages include the risk of solvent residue and the need for decolorization during the purification stage, adding an extra processing step.

[0006] Therefore, providing a refining process for long-chain dicarboxylic acids, effectively controlling solvent risks, and providing high-quality pure long-chain dicarboxylic acids has good prospects for industrial application. Summary of the Invention

[0007] In view of the above-mentioned research status, the present invention provides the following technical content: In a first aspect, a method for purifying a long-chain dicarboxylic acid is provided, the method comprising the following steps: (1) Using crude long-chain dicarboxylic acid as raw material, a mixed solution of formic acid and acetic acid is added and recrystallized to obtain a crystalline compound; (2) After the crystal compound is fully melted, it is transferred to a melting vessel, and after removing the residual solvent under nitrogen conditions, it is granulated.

[0008] The purification method provided in the first aspect above is applicable to crude long-chain dicarboxylic acids with a purity of 85-95%. In processes for preparing long-chain dicarboxylic acids based on microbial fermentation, this purification method is suitable for crude products obtained after column chromatography or membrane chromatography of the fermentation products. The purified product is a high-purity, granular long-chain dicarboxylic acid product, which can be directly sold or used in downstream formulation processing. The long-chain dicarboxylic acids applicable to this purification method are saturated aliphatic dicarboxylic acids with a carbon chain length of C10-C18, and more specifically, sebacic acid (DC10), dodecanoic acid (DC12), tridecanoic acid (DC13), tetradecanoic acid (DC14), and hexadecanoic acid (DC16).

[0009] One improvement of this refining method compared to existing technologies lies in the optimization of the recrystallization solvent. Long-chain dicarboxylic acids prepared by microbial fermentation processes often contain microbial metabolic byproducts or soluble proteins produced by the cells. Industrial-grade products require high-purity white or off-white powders / crystals, thus requiring decolorization and impurity removal during the refining stage. Common methods include activated carbon decolorization and mother liquor recycling, which can lead to some product loss. This invention discovers that a mixture of formic acid and acetic acid in an appropriate ratio, used as a recrystallization solvent, can directly dissolve impurities and pigments in the crude product upon heating. Upon cooling, long-chain dicarboxylic acid crystals precipitate, while impurities and pigments remain in the mixed solvent. This achieves impurity removal and decolorization during the recrystallization stage. Furthermore, the introduction of formic acid alters the crystallization characteristics of long-chain dicarboxylic acids. Experience has shown that, compared to using pure formic acid or pure acetic acid as crystallizing agents, long-chain dicarboxylic acid crystals obtained using a mixed solvent are less likely to encapsulate impurities and solvents.

[0010] In a preferred embodiment, the above-mentioned mixed solution is composed of formic acid and acetic acid, wherein the mass fraction of formic acid is 15-50%, and the purity of formic acid is ≥85%, and the purity of acetic acid is ≥99%. The recrystallization based on this mixed solution is as follows: the crude long-chain dicarboxylic acid is mixed with the above mixed solution at a mass ratio of 1:2-1:3, heated to 75-85°C, stirred and dissolved for 60-90 minutes until all the dicarboxylic acid is dissolved, cooled to 10-30°C at a rate of 2-4°C / h, and crystallized at this temperature for 2-4 hours. Solid-liquid separation yields the crystalline compound. In actual production, depending on factors such as the purity of the crude product, the state of the crystalline compound, or formulation requirements, this recrystallization step can be repeated. That is, the obtained crystalline compound can be redissolved in the mixed solution and crystallized again. In one refining process, the recrystallization can be performed 1-5 times. If the obtained crystalline compound is not to be repeatedly subjected to the crystallization process, it is dried at 60-90°C for later use.

[0011] In some embodiments verified by this invention, a double recrystallization process is used. In this case, step (1) above is specifically as follows: Formic acid and acetic acid are mixed in a ratio of 15-50 wt% to 50-85 wt% to obtain a mixed solution. The crude long-chain dicarboxylic acid is mixed with the mixed solvent in a mass ratio of 1:2 to 1:3. The mixture is heated to 75-85°C and stirred for 60-90 minutes until the solid is fully dissolved. Then, it is cooled to 25-30°C at a rate of 2-4°C / h and kept at this temperature for 2-4 hours to crystallize. After solid-liquid separation, the solid part is retained. Another portion of the mixed solution is taken and the above operation is repeated to obtain a solid part, which is then dried under vacuum at 60-90°C to obtain the crystalline compound.

[0012] In step (2) above, the obtained crystalline compound is transferred to the purification system described in the second aspect for solvent removal and granulation.

[0013] In a second aspect, the present invention provides a purification system for long-chain dicarboxylic acids, wherein the main technical improvement of the purification system is that it has a melting unit, including a devolatilization melter and a melting vessel connected to the devolatilization melter.

[0014] The melting vessel includes a vessel body with a feed inlet at the bottom, forming a bottom-feed method. The melting vessel is equipped with a stirring assembly and a nitrogen inlet. The vessel body also includes a compartmentalization assembly, which includes a horizontally arranged bottom plate fixedly connected to the circumferential side wall of the vessel body. A partition is fixedly connected to one side of the bottom plate, and the top of the partition is fixedly connected to the inner wall of the vessel top. The partition and the bottom plate divide the space within the vessel body into two parts, forming an isolation chamber enclosed by the partition, the bottom plate, and part of the inner wall of the vessel body. An air passage is formed on the other side of the partition. At least one guide pipe is connected to the bottom plate to achieve communication between the internal space of the vessel body and the internal space of the isolation chamber. A discharge port is located above the bottom plate, fixedly connected to the vessel body, and the isolation chamber is connected to the discharge port. A downwardly bent recess is provided on the side of the bottom plate near the discharge port, forming a flow guide structure towards the discharge port. An inner coil is also provided inside the isolation chamber.

[0015] In existing processes, the crystalline compounds obtained by recrystallization still contain a large amount of solvent residue, which usually requires high-temperature water washing and drying to remove. Then, the high-purity long-chain dicarboxylic acid is transferred to a melting reactor for granulation, making the post-processing steps cumbersome. Furthermore, existing melting reactors cannot continuously feed materials; after each feeding, all materials must be transferred and granulated before the next feeding can be carried out.

[0016] In the optimized scheme of this invention, the recrystallized crystalline compound contains only a small amount of solvent residue, eliminating the need for high-temperature water washing. After complete melting, it is fed into the aforementioned melting unit. Upon entering the melting unit, nitrogen gas at the bottom of the vessel rises, allowing the residual solvent to exit the melting unit along with the nitrogen gas through the gas channel at a set temperature. The completely melted crystalline compound at the bottom overflows into an isolation chamber, where it is further insulated before entering the granulation stage. The isolation chamber allows for continuous feeding of the melting unit according to production needs, eliminating the need to wait for the previous batch to empty before feeding again. The benefits of this adjustment include simplified post-processing steps, continuous production capability, and improved quality of the resulting long-chain dicarboxylic acid.

[0017] Furthermore, the above-mentioned purification system can also recover nitrogen and residual solvents, that is, the purification system also includes: The primary condensation and gas-liquid separation unit includes a shell-and-tube condenser, a condenser tank, and a collection tank; the gas phase outlet of the melting vessel is connected in sequence to the gas phase inlet of the shell-and-tube condenser and the upper gas phase inlet of the condenser tank via pipelines; the bottom liquid phase outlet of the condenser tank is connected to the top inlet of the collection tank via pipelines. The secondary condensation and vacuum unit includes a plate condenser. The top gas phase outlet of the condenser is connected to the gas phase inlet of the plate condenser through a pipeline, and the gas phase outlet of the plate condenser is connected to the gas inlet of the vacuum pump through a pipeline. The exhaust gas purification unit includes a water film absorption device, and the outlet of the vacuum pump is connected to the inlet of the water film absorption device through a pipeline. The nitrogen supply unit includes a nitrogen storage tank, the outlet of which is connected to a nitrogen pipe on the melting vessel via a pipeline.

[0018] This invention optimizes the recrystallization and melt granulation processes of long-chain dicarboxylic acids, and compared with traditional refining processes, it has the following significant advantages: 1. Excellent decolorization and impurity removal effect, significantly improved product purity: Secondary recrystallization is carried out using a specific ratio of formic acid-acetic acid mixed solvent. Compared with single acetic acid solvent and unbalanced mixed solvent, it can efficiently remove heteroalkyl acids, soluble proteins and pigment impurities from fermentation crude products. The resulting crystals have significantly reduced platinum-cobalt color number and greatly improved light transmittance. The purity of the final product can reach over 99.6%, with single impurity content ≤0.09%, total metal ion residue ≤0.4ppm, and moisture content ≤0.08%. All purity indicators are far superior to products from traditional processes, meeting the quality requirements of high-end application scenarios.

[0019] 2. Effectively improve crystallinity: A reasonable ratio of formic acid and acetic acid mixed solvents can control the crystal growth rate and crystal morphology. The resulting crystal compound is plate-like, with improved crystallinity. Compared with existing acetic acid crystals, it has higher purity, fewer impurities, and more uniform particle size.

[0020] 3. Significantly improves product moisture-proof performance and reduces moisture absorption loss: After recrystallization, the crystals have fewer defects and a denser structure. Combined with the high-purity nitrogen inert protection of a specially designed melting kettle and a zoned temperature-controlled melting process, the melt is free of bubbles and micropores. After granulation, the particle surface is smooth with very few water vapor adsorption sites. The static moisture absorption rate of the finished product is as low as 0.12% after 72 hours, which is far lower than that of the comparative product of traditional processes. This effectively avoids moisture absorption, clumping, and deterioration loss during product storage and transportation, and extends the product's shelf life.

[0021] 4. Optimize product flowability and adapt to industrial production: The refined finished product has an angle of repose of ≤30°, excellent flowability, smooth particle flow without bridging or sticking, and can be directly adapted to automated packaging and continuous production equipment without the need for additional flow aids, greatly improving production efficiency; compared with traditional melt-crushing granulation products, the flowability is significantly improved, solving the pain points of poor flowability and unstable material supply of traditional products.

[0022] 5. High process safety and wide applicability: The use of a formic acid-acetic acid mixed solvent avoids the strong irritation and equipment corrosion caused by the use of pure formic acid, as well as the characteristic that pure acetic acid is prone to crystallization and clogging of pipelines below 16 degrees Celsius, ensuring production in winter and making the process safer and more environmentally friendly. Moreover, this process can be adapted to the refining of crude products of various long-chain dicarboxylic acids such as dodecanedicarboxylic acid, tridecanedicarboxylic acid, and sebacic acid. Only minor adjustments to the melting temperature are needed to adapt to the melting points of different products. The process is highly versatile and easy to promote and apply industrially.

[0023] 6. Improved economic efficiency: 6-1. Reduced energy consumption: This process eliminates the decolorization and high-temperature water steps in the traditional process, significantly reducing the proportion of water entering the solvent, lowering solvent recovery costs, increasing production efficiency, and reducing steam energy consumption by 30%.

[0024] 6-2. Elimination of hazardous waste generation: This process does not require the addition of activated carbon or bleaching clay for decolorization, and no hazardous waste is generated, which reduces the consumption of raw and auxiliary materials and saves the cost of hazardous waste treatment.

[0025] 6-3. Reduced environmental protection costs: The solvents and nitrogen used in the refining process can be recycled and reused, and the reduction of refining steps significantly reduces wastewater generation and lowers environmental treatment costs.

[0026] 7. Improved on-site environment: The entire process operates in a fully enclosed manner, which greatly reduces solvent spillage, and the on-site odor is controllable, creating a good production environment. Attached Figure Description

[0027] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0028] Figure 1 This is a schematic diagram of the melting device described in Example 1; Figure 2 for Figure 1 Enlarged view of the structure of the intermediate melting vessel; Figure 3 This is a three-dimensional view of the connection structure between the partition plate 7 and the cross brace 8 in the compartment component.

[0029] Figure 4 This is a top view of the compartmentalized component structure; Figure 5 The above are schematic diagrams of the existing melting tanks described in Comparative Example 1 and Comparative Example 2. Explanation of reference numerals in the attached figures: 1. Melting vessel; 2. Stirring shaft; 3. Drive motor; 4. Feed pipe; 5. Discharge port; 6. Bottom plate; 61. Recessed part; 7. Baffle plate; 8. Cross brace; 9. Guide pipe; 10. Nitrogen pipe; 11. Gas passage; 12. Isolation chamber; 13. Safety valve; 14. Pressure gauge; 15. Shell and tube condenser; 16. Condenser; 17. Plate condenser; 18. Vacuum pump; 19. Water film absorption device; 20. Collection tank; 21. Nitrogen storage tank; 22. Deviation melting machine; 23. Inner coil; 24. Negative pressure pipe; 25. Vacuum generator.

[0030] Figure 6 This is a state diagram of the recrystallized product in step (1) of Example 1; Figure 6 In section A, the crude product is in the state after being mixed with the mixed solution and heated. Figure 6 B represents the bottom state of the reactor. Figure 6 C represents the state of the crystalline compound obtained in step (1) of Example 1; Figure 7 The state of the crude product after dissolving it in a 20% formic acid mixed solution; Figure 7 In section A, the crude product is in the state after being mixed with the mixed solution and heated. Figure 7 B represents the bottom state of the reactor; Figure 8 This is a state diagram of the recrystallized product in step (1) of Comparative Example 1; Figure 8 In section A, the crude product is in the state after being mixed with the mixed solution and heated. Figure 8 B represents the precipitate remaining at the bottom. Figure 8 In the middle, C represents the state of the filtrate obtained after decolorization. Figure 8 D represents the state of the crystalline compound obtained in step (1) of Comparative Example 1; Figure 9 This is a state diagram of the recrystallized product in step (1) of Comparative Example 2; Figure 9 In section A, the crude product is in the state after being mixed with the mixed solution and heated. Figure 9 B represents the bottom state of the reactor. Figure 9 C represents the precipitate remaining at the bottom. Figure 9 D represents the state of the crystalline compound obtained in step (1) of Comparative Example 2; Figure 10 The image shows the high-performance liquid chromatogram of the dodecanedicarboxylic acid product obtained in Example 2; the peak corresponding to dodecanedicarboxylic acid in the image has a retention time of 19.275 min. Figure 11 The image shows the high-performance liquid chromatogram of the tridecanoic acid obtained in Example 3; the peak corresponding to the tridecanoic acid in the image has a retention time of 22.046 min. Figure 12 The image shows the high-performance liquid chromatogram of sebacic acid obtained in Example 4; the peak corresponding to sebacic acid in the image has a retention time of 14.706 min. Detailed Implementation

[0031] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0033] In the context of this specification, the word "comprising" is considered to mean "especially including". It should not be interpreted as "consisting of only".

[0034] In the description of this invention, it should be understood that the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. In this document, the symbol " / " indicates that the related objects are in an "or" relationship; for example, A / B means A or B.

[0035] In the description of embodiments of the present invention, the words "exemplary" or "for example" are used to indicate that they are examples, illustrations, or descriptions. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of the present invention should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the words "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0036] In the description of the embodiments of the present invention, unless otherwise stated, "a plurality of" means two or more.

[0037] As described in the background section, the purification of long-chain dicarboxylic acids in the prior art usually involves recrystallization, which also includes multiple steps of decolorization and washing, making the process quite cumbersome. In order to solve the above-mentioned technical problems, the present invention provides a purification method for long-chain dicarboxylic acids, which includes recrystallization using a mixed solution of formic acid and acetic acid, eliminating the decolorization step and the high-temperature water washing step, and then combining it with a nitrogen blowing process for melt granulation, which effectively improves the purity of long-chain dicarboxylic acids and the performance of granulated acids.

[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0039] Example 1 In this embodiment, a refining system is provided, the structure of which is as follows: Figures 1-4 As shown, the refining system includes a melting unit, a primary condensation and gas-liquid separation unit, a secondary condensation and vacuum unit, an exhaust gas purification unit, and a nitrogen supply unit.

[0040] The melting unit includes a devolatilization melter 22 and a melting vessel 1, which are the core melting reaction containers. The melting vessel 1 includes a vessel body, the outer wall of which is covered with a heating jacket. The heating jacket can be heated by steam or electricity to provide the heat required for melting the material inside the vessel. A feed pipe 4 is provided on the outside of the melting vessel 1, which is connected to the feed port at the bottom of the vessel body to form a bottom feeding method. A pressure gauge 14, a safety valve 13, and a gas phase outlet are provided on the top of the melting vessel 1. A nitrogen inlet is opened on the side wall of the melting vessel 1 near the bottom, and a nitrogen pipe 10 is sealed and connected through the nitrogen inlet, extending into the interior of the vessel body. A stirring assembly is provided in the melting vessel 1. The stirring assembly includes a vertical stirring shaft 2 located inside the melting vessel 1. Stirring blades are fixedly connected to the stirring shaft 2. One end of the stirring shaft 2 passes through the bottom of the vessel and is driven and connected to the output end of the drive motor 3.

[0041] The feed inlet of feed pipe 4 is connected to the discharge outlet of devolatilization melt 22. Devolatilization melt 22 is used to melt, efficiently devolatilize and purify materials. The continuously melted material from devolatilization melt 22 is introduced into melting kettle 1 through feed pipe 4.

[0042] The vessel body is equipped with a compartmentalization assembly, which is located above the stirring blades. This assembly is used to rationally divide the internal space of the vessel body, achieving functions such as isolated storage and gas flow guidance. The compartmentalization assembly includes a horizontally arranged base plate 6, which is fixedly connected to the circumferential side wall of the vessel body. A partition plate 7 is fixedly connected to one side of the base plate 6, and the top of the partition plate 7 is fixedly connected to the inner wall of the vessel top. The area of ​​the base plate 6 is smaller than the cross-sectional area of ​​the vessel body. The partition plate 7 and the base plate 6 divide the space inside the vessel body into two parts. The partition plate 7, the base plate 6, and part of the inner wall of the vessel body enclose an isolation compartment 12. A gas passage 11 is formed on the other side of the partition plate 7. The function of the gas passage 11 is to guide and circulate the gaseous medium inside the vessel body, preventing the gaseous medium from accumulating inside the vessel body. It can also work with the vessel body's exhaust structure to achieve the circulating discharge of the medium.

[0043] To improve overall stability, several cross braces 8 are welded between the baffle 7 and the inner wall of the vessel for reinforcement. The number of cross braces 8 is set according to actual needs to avoid occupying too much space in the gas passage 11 and affecting the flow of gaseous media.

[0044] A discharge port 5 is provided above the bottom plate 6. The discharge port 5 is fixedly connected to the vessel body. The isolation chamber 12 is connected to the discharge port 5, and the material can be discharged through the discharge port 5.

[0045] At least one guide pipe 9 is also connected to the bottom plate 6. The guide pipe 9 is set vertically through the bottom plate 6 to realize the mutual communication between the internal space of the vessel body and the internal space of the isolation chamber 12, so as to ensure the normal flow of materials between the two spaces.

[0046] In some embodiments, the guide pipe 9 is equipped with a filter screen to filter the passing material and prevent impurities or lumpy materials from entering the guide pipe 9 and causing pipeline blockage.

[0047] In some embodiments, in order to facilitate material discharge and reduce material residue accumulation on the bottom plate 6, a downwardly bent recess 61 is provided on the side of the bottom plate 6 near the discharge port. The recess 61 forms a flow guiding structure in the direction of the discharge port, which allows the material to converge towards the discharge port along the recess 61, thereby improving the smoothness and thoroughness of the discharge.

[0048] Two pressure gauges 14 are provided, which are used to monitor the pressure in the isolation chamber 12 and the vessel body respectively. Correspondingly, two safety valves 13 are also provided to automatically release air and pressure when the pressure rises abnormally, so as to regulate and stabilize the working pressure and ensure that the melting device operates stably within the safe pressure range.

[0049] To ensure vacuum, a negative pressure system is also provided on the vessel body. The negative pressure system includes a negative pressure pipe 24, one end of which is connected to a negative pressure generator 25. The negative pressure pipe 24 passes through the partition 7 and enters the isolation chamber 12. The negative pressure pipe 24 is provided with at least one negative pressure suction port in the isolation chamber 12. At least one other negative pressure suction port is provided in the vessel body area other than the isolation chamber 12 to maintain the vacuum in the entire vessel body.

[0050] In some embodiments, an inner coil 23 is also provided inside the isolation chamber 12. The inner coil 23 can be an electric heating coil, used to heat and keep the material in the isolation chamber 12 warm. The inner coil 23 is connected to a controller, which can flexibly adjust the working parameters, thereby realizing the adjustable and controllable temperature inside the isolation chamber 12.

[0051] In some embodiments, to improve the impurity removal efficiency, the nitrogen pipe 10 is also connected to a gas coil. The gas coil is installed in the vessel body and reliably fixed to the inner wall of the vessel body. The gas coil has several spaced-apart gas outlets along its body, with the gas outlets facing downwards. The advantages of this arrangement are: nitrogen rises from the bottom area and stays in the melt for a significantly longer time, which can more fully remove volatiles, moisture, small molecule impurities, and residual monomers from the melt, significantly improving the devolatilization and impurity removal efficiency; the gas coil is arranged along the vessel body, and with multiple downward-facing gas outlets, a large-area, uniform microbubble field can be formed in the vessel, avoiding short circuits and flow deviations caused by local concentrated jetting, so that all parts of the melt can be purged with nitrogen, eliminating material dead zones.

[0052] The specific structure of the primary condensation and gas-liquid separation unit is as follows: the gas phase outlet of the melting vessel 1 is connected in sequence to the gas phase inlet of the shell-and-tube condenser 15 and the upper gas phase inlet of the condenser 16 via pipelines; the outer wall of the condenser 16 is surrounded by cooling coils, and the two ends of the cooling coils are respectively provided with cold water inlet and cold water outlet, which are respectively connected to cold water inlet pipe and cold water outlet pipe to maintain the low temperature environment inside the tank; the liquid phase outlet at the bottom of the condenser 16 is connected to the top inlet of the collection tank 20 via pipelines to collect the liquid phase product obtained by condensation.

[0053] The specific structure of the secondary condensation and vacuum unit is as follows: the top gas phase outlet of the condenser 16 is connected to the gas phase inlet of the plate condenser 17 through a pipeline, and the tube side of the plate condenser 17 is connected to the cooling water inlet pipe and the cooling water outlet pipe respectively to achieve deep condensation of the uncondensed gas phase; the gas phase outlet of the plate condenser 17 is connected to the gas inlet of the vacuum pump 18 through a pipeline to provide a vacuum environment for the system.

[0054] The specific structure of the exhaust gas purification unit is as follows: the outlet of the vacuum pump 18 is connected to the inlet of the water film absorption device 19 through a pipeline. The water film absorption device 19 absorbs, washes and purifies the residual impurities entrained in the gas phase, and clean nitrogen is obtained after treatment.

[0055] The nitrogen supply unit includes a nitrogen storage tank 21. The outlet of the nitrogen storage tank 21 is connected to the nitrogen pipe 10 on the melting vessel 1 through a pipeline, and nitrogen is introduced into the melting vessel 1 to form an inert protective atmosphere.

[0056] It should be noted that the purified nitrogen outlet of the water film absorption device 19 can also be connected back to the nitrogen storage tank 21 through a pipeline to recover the purified nitrogen into the storage tank, thereby realizing the closed-loop recycling and reuse of nitrogen.

[0057] The above-mentioned melting device is operated as follows: the melting vessel 1 is pre-evacuated to a vacuum degree of 98 kPa, the molten crystalline compound is put into the vessel, stirring is started, and then nitrogen is introduced. The melting vessel temperature is set to be 6~10℃ higher than the melting point of the long-chain dicarboxylic acid to be treated.

[0058] At the set temperature, the residual solvent impurities are fully vaporized and leave the melting vessel 1 through the gas passage 11 along with the nitrogen gas, and enter the condensation and recovery unit to recover high-purity nitrogen gas and a mixed solution of methylacetic acid. The nitrogen gas can be stored in the nitrogen storage tank 21 or repeatedly enter the melting vessel 1. The mixed solution of methylacetic acid can also be reused in the recrystallization process after the proportion is adjusted.

[0059] Once the liquid level is above the bottom plate 6, the long-chain dicarboxylic acid, heated to a molten state, enters the isolation chamber 12 through the guide pipe 9. The temperature of the isolation chamber is set to be the same as or slightly higher than the set temperature of the melting vessel 1 by 1~3°C. The long-chain dicarboxylic acid continues to be kept warm in the isolation chamber 12 until it is fully melted before being transferred to the granulation stage. The advantage of setting up the isolation chamber 12 is that, as materials are continuously fed in, residual solvent impurities can be fully vaporized under the action of nitrogen blowing and stirring, while the impurity-removed molten long-chain dicarboxylic acid can enter the isolation chamber 12 for full melting. Even if continuous feeding occurs at this time, it will not cause temperature fluctuations in the isolation chamber 12.

[0060] Example 2 This embodiment provides a method for purifying long-chain dicarboxylic acids. The raw material is crude dodecanoic acid produced by microbial fermentation, with a purity of 92.5% and a moisture content of 1.2%. The main impurities are undecanoic acid, protein, and pigment. The purification method includes the following steps: Specifically, the steps include the following: (1) Weigh 300g of formic acid (88.5% purity) and 700g of acetic acid (99.8% purity), mix and stir evenly to obtain a mixed solution; mix the above crude dodecanedicarboxylic acid with the mixed solution at a mass ratio of 1:2, heat to 80~82℃, stir and dissolve for 80min to fully dissolve the solid dicarboxylic acid, then cool to 25℃ at a rate of 4℃ / h, keep warm to crystallize for 3 hours, separate the solid part, take another mixed solution and repeat the above operation for secondary crystallization, separate the product of secondary crystallization, and vacuum dry at 75℃ to obtain a crystalline compound (denoted as S2); (2) The crystalline compound obtained in step (1) is fully melted by the devolatilization melter 22 and then flows into the melting kettle 1. The vacuum degree in the melting kettle 1 is set in advance, and the temperature of the melting kettle 1 is set to 135~140℃. Nitrogen gas is continuously introduced into the melting kettle 1 during continuous feeding. The residual solvent is vaporized into gas at this time and leaves the melting kettle with the blowing of nitrogen gas to enter the subsequent nitrogen recovery. The impurity-removed molten dodecane dicarboxylic acid enters the isolation chamber 12. After further heat preservation in the isolation chamber 12, it enters the subsequent granulation.

[0061] The purity of the dodecanedicarboxylic acid product was determined to be 99.7%, with 0.08% single impurities, 0.08% moisture, a melting point of 129.8℃ (standard value 128.5-131℃), and a total residual metal ion content of 0.3ppm.

[0062] Example 3 In this embodiment, another method for purifying long-chain dicarboxylic acids is provided. The raw material is crude tridecanoic acid produced by microbial fermentation (purity 91.8%, moisture 1.5%, main impurities are dodecanoic acid, soluble protein and carotenoid pigments). The purification method includes the following steps: (1) Weigh 150g of formic acid (purity 88.5%) and 850g of acetic acid (purity 99.8%), mix and stir evenly to obtain a mixed solution; mix the above crude tridecanoic dicarboxylic acid with the mixed solution at a mass ratio of 1:3, heat to 78~80℃, stir and dissolve for 80~90min to fully dissolve the solid dicarboxylic acid, then cool to 25℃ at a rate of 2℃ / h, keep warm and crystallize for 4 hours, separate the solid part, take another mixed solution and repeat the above operation for secondary crystallization, separate the product of secondary crystallization, and vacuum dry at 90℃ to obtain crystalline compound.

[0063] (2) The setup is the same as in Example 2, except that the temperature of the melting vessel 1 is set to 116~125℃ (5~10℃ higher than the melting point of tridecanoic acid).

[0064] The purity of the obtained tridecanoic acid was determined to be 99.6%, with 0.09% impurities, 0.07% moisture, a melting point of 113.7℃ (standard value 112-115℃), and a total residual metal ion content of 0.4ppm.

[0065] Example 4 In this embodiment, another method for purifying long-chain dicarboxylic acids is provided. The raw material is crude sebacic acid produced by microbial fermentation (purity 92.2%, moisture 1.3%, main impurities are nonanic acid, fermentation residue protein and melanin). The purification method includes the following steps: (1) Weigh 300g of formic acid (purity 88.5%) and 300g of acetic acid (purity 99.8%), mix and stir evenly to obtain a mixed solution; mix the above crude sebacic acid with the mixed solution at a mass ratio of 1:3, heat to 86~88℃, stir and dissolve for 60~70min to fully dissolve the solid dicarboxylic acid, then cool to 25℃ at a rate of 2℃ / h, keep warm to crystallize for 4 hours, separate the solid part, take another mixed solution and repeat the above operation for secondary crystallization, separate the product of secondary crystallization, and vacuum dry at 90℃ to obtain the crystalline compound.

[0066] (2) The setup is the same as in Example 2, except that the temperature of the melting vessel 1 is set to 135~145℃ (6~13℃ higher than the melting point of sebacic acid).

[0067] The obtained sebacic acid product has a purity of 99.8%, a single impurity of 0.07%, a moisture content of 0.07%, a melting point of 133.2℃ (standard value 131.0-135.0℃), and a total residual metal ion content of 0.3ppm.

[0068] Comparative Example 1 This comparative example was set up to compare the effects of different solvents on the crystallization effect of long-chain dicarboxylic acids during the recrystallization stage. The same batch of crude dodecanedicarboxylic acid raw material as in Example 2 was used. The difference from step (1) in Example 1 was that acetic acid was used as the solvent, and the mass ratio of crude raw material to acetic acid was 1:3. The mixture was heated to 90°C and stirred to dissolve it completely. The rest of the settings were the same as step (1) in Example 2. A crystalline compound was obtained and denoted as D1.

[0069] Comparative Example 2 To compare the effects of different solvents on the crystallization of long-chain dicarboxylic acids during the recrystallization stage, this comparative example was set up. The same batch of crude dodecanedicarboxylic acid raw material as in Example 2 was used. The difference from step (1) in Example 1 was that formic acid and acetic acid were mixed in a ratio of 10wt%:90wt%, and the rest of the settings were the same as in step (1) in Example 2. A crystalline compound was obtained and denoted as D2.

[0070] Comparative Example 3 To investigate the effect of the melting device on the granulation effect, this comparative example was set up. Taking the crystalline compound S2 described in step (1) of Example 2 as an example, a traditional melting vessel (structure as shown in the figure) was used. Figure 5 As shown, without introducing nitrogen blowing, the long-chain dicarboxylic acid product obtained by melt granulation is denoted as D3.

[0071] Comparative Example 4 To investigate the effect of the melting device on the granulation effect, this comparative example was set up. Taking the crystalline compound S2 described in step (1) of Example 2 as an example, a traditional melting vessel (structure as shown in the figure) was used. Figure 5 As shown, nitrogen gas is continuously introduced during the melting process to carry out melt granulation, and the resulting long-chain dicarboxylic acid product is denoted as D4.

[0072] Performance testing 1. Recrystallization effect The preparation was carried out according to the method of Example 1. The crude raw material was added to a 30 wt.% formic acid mixed solution and heated to dissolve. The solution state was as follows: Figure 6 As shown, the solution is clear and transparent, with almost no visible impurities at the bottom. After heating and dissolving, the subsequent cooling and recrystallization steps can be performed without filtration or additional decolorization. The resulting crystalline compound is as follows: Figure 6As shown in C, the crystalline compound is pure white, flaky, and has relatively large particles, proving that the crystalline compound has a better crystallization effect.

[0073] During the research process of this invention, a 20 wt.% formic acid mixed solution was also tested, and the results are as follows: Figure 7 As shown, the solution becomes almost transparent after heating, and there is no solid residue at the bottom of the container.

[0074] Prepared according to the method of Comparative Example 1, after adding acetic acid and heating to dissolve the crude raw material, there were still many insoluble impurities at the bottom of the container (see Comparative Example 1). Figure 8 (A and B), and the solution was turbid. To prevent the recrystallized crystalline compound from mixing again with impurities at the bottom of the container, activated carbon was added to the container for decolorization and filtration. The filtered state is as follows. Figure 8 As shown in Figure C. The state of the obtained crystalline compound is as follows. Figure 8 Medium D particles are not concentrated in size distribution, mostly small-sized powders, with some agglomerated large particles mixed in.

[0075] The crude raw material was prepared according to the method of Comparative Example 2. After adding a 10 wt.% formic acid mixed solution, the mixture was heated to 90°C. The solution state was as follows: Figure 9 As shown in Figure A, the solution is clear, but there is still a significant amount of impurities precipitated at the bottom of the container. Figure 9 (B) needs to be filtered out.

[0076] The crystalline compound obtained by recrystallization ( Figure 9 Compared to the crystalline compound obtained by recrystallization of acetic acid (C), (C) Figure 8 The particle size of D increases, approaching a flaky shape.

[0077] 2. Decolorization effect The decolorization effect of the crude raw material used in Example 2 (denoted as CK), the crystalline compounds described in Example 2, and Comparative Examples 1-2 was determined. The test methods included the following two aspects: Platinum-cobalt colorimetric method: Each group of crystalline compounds is prepared into a 20wt% solution with dimethyl sulfoxide, and compared with the platinum-cobalt standard colorimetric solution to determine the platinum-cobalt color number.

[0078] Transmittance measurement: The transmittance of the above solution at 440 nm and 550 nm was measured using a UV-Vis spectrophotometer. The results are shown in Table 1 below: Table 1. Results of decolorization effect measurement of crystalline compounds in the examples and comparative examples. 3. Crystal particle size determination The crystal particle size of the crude raw material used in Example 2 (denoted as CK), the crystalline compounds described in Example 2, and Comparative Examples 1-2 was determined using the following method: the volume average particle size (D50) of each group of crystalline compounds was measured using a laser particle size analyzer, and the crystal morphology was observed under a microscope. The particle size distribution range (10%~90% quantile) was statistically analyzed. The measurement results are shown in Table 2 below: Table 2. Crystal particle size determination results of crystalline compounds in the examples and comparative examples. Decolorization effect: The platinum cobalt color number (12±1) of Example 2 (D2) was significantly lower than that of Comparative Example 1 (28±1), Comparative Example 2 (20±1) and the blank group (45±2), and the transmittance at 440nm and 550nm (96.8%±0.5, 98.5%±0.4) was significantly higher than that of other groups, indicating that the formic acid-acetic acid mixed solvent with appropriate ratio can remove pigment impurities in the crude product more efficiently, and the decolorization effect is optimal.

[0079] Crystal particle size: The volume average particle size D50 (72±4μm) of Example 2 (D2) is much smaller than that of Comparative Example 1 (168±8μm), Comparative Example 2 (115±6μm) and the blank group (285±12μm), and the particle size distribution range is narrower (30~120μm). Microscopic observation shows that the crystals are uniform fine particles without obvious agglomeration, indicating that the mixed solvent ratio can effectively refine the crystal particle size and improve the crystal dispersibility.

[0080] In summary, recrystallization using a properly proportioned formic acid-acetic acid mixed solvent not only has a superior decolorization effect compared to using a single acetic acid solvent, but also significantly improves the purity of long-chain dicarboxylic acid crystalline compounds, achieving a dual optimization of purification and crystal modification.

[0081] In actual production, using pure formic acid as a crystallization solvent can cause significant irritation to production personnel and is highly corrosive to the crystallization vessel; therefore, using pure formic acid is not advisable. This invention has found that a properly proportioned mixture of formic acid and acetic acid can significantly reduce the particle size of the crystalline compound. However, excessively high (greater than 50%) or excessively low (less than 15%) proportions of formic acid will not produce the aforementioned effect.

[0082] 4. Moisture resistance The melt-granulated product was dried under vacuum at 100℃ and -0.09MPa for 4 hours. After cooling, three 5.000g parallel samples were weighed and placed in constant weight weighing bottles (recorded as m0). The bottles were then placed open in a constant temperature and humidity chamber at 25℃ and 60% relative humidity. The samples were rapidly weighed at 24h, 48h, and 72h respectively (recorded as m). n ), according to moisture absorption rate (%) = (m nThe average value is calculated as (-m0) / m0×100%, and the moisture absorption performance is evaluated based on the 72-hour moisture absorption rate. The lower the moisture absorption rate, the better the performance.

[0083] Table 3. Moisture absorption properties of the products obtained in the examples and comparative examples According to the moisture-proof performance prediction data, the 72-hour static moisture absorption rate of Examples 2-4 is much lower than that of the comparative examples. This is due to the optimal ratio of formic acid-acetic acid mixed solvent recrystallization, which results in uniform crystal particle size and fewer defects. Combined with the partitioned heat preservation effect of the melting kettle, the melt is dense and free of micropores. After granulation, the particle surface is smooth, which greatly reduces the water vapor adsorption sites.

[0084] Comparative Example 3 involved directly melting and granulating the recrystallized crystalline compound, resulting in the product with the highest moisture absorption rate. This demonstrates that a small amount of solvent residue remains in the crystalline compound, and failure to remove it will significantly affect the final product performance. The solvents used in Comparative Examples 1 and 2 resulted in incomplete recrystallization and impurity removal, leading to loose crystal agglomeration and high moisture absorption. Comparative Example 4 employed a traditional melting vessel without zoning, combined with a nitrogen blowing process. While significantly better than Comparative Example 3, it was still significantly inferior to the examples, demonstrating that the present invention, by improving the structure of the melting vessel and implementing zoning for the molten product, effectively improves the melt performance and achieves better granulation results.

[0085] 5. Liquidity The dried granulated product was sieved through a 3mm standard sieve. Using the fixed funnel method (the lower end of the funnel is 10cm from the table), the sample was slowly poured into the funnel and naturally piled into a cone shape. The diameter (d) and height (h) of the bottom surface of the pile were measured. The angle of repose was calculated by tanθ=h / (d / 2). The calculation was repeated 3 times and the average value was taken. An angle of repose ≤35° is considered to be of good flowability. The smaller the angle, the better the flowability.

[0086] Table 4. Flowability of Products Obtained from Examples and Comparative Examples The mobile phase results showed a similar trend to the aforementioned moisture absorption and desorption properties. The products obtained in Examples 2-4 had smaller angles of repose, looser particles, smoother flow, and almost no bridging. The recrystallization solvents used in Comparative Examples 1 and 2 could not effectively separate crystalline compounds and impurities, resulting in decreased product purity and increased angles of repose. The product obtained by melt granulation in Comparative Example 3 using a traditional melting kettle had a poor flowability rating. In Comparative Example 4, the introduction of nitrogen blowing effectively removed solvent residue and improved product flowability, but it still had significant shortcomings compared to Example 2. This demonstrates that the melting kettle provided in Example 1 of this invention effectively improves granulation performance by continuously separating and storing molten long-chain dicarboxylic acids in separate compartments.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for purifying a long-chain dicarboxylic acid, characterized in that, The refining method includes the following steps: (1) Using crude long-chain dicarboxylic acid as raw material, recrystallize it in a mixed solution of formic acid and acetic acid to obtain a crystalline compound; (2) After the crystal compound is fully melted, nitrogen gas is introduced to remove the residual solvent and then granulation is performed.

2. The purification method for long-chain dicarboxylic acids as described in claim 1, characterized in that, The crude long-chain dicarboxylic acid has a purity of 85-95%.

3. The purification method for long-chain dicarboxylic acids as described in claim 1, characterized in that, The long-chain dicarboxylic acid is selected from sebacic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, and hexadecanoic acid.

4. The purification method for long-chain dicarboxylic acids as described in claim 1, characterized in that, The mixed solution consists of 15-50 wt% formic acid and 50-85 wt% acetic acid, wherein the purity of formic acid is ≥85% and the purity of acetic acid is ≥99%.

5. The purification method for long-chain dicarboxylic acids as described in claim 4, characterized in that, The recrystallization based on the mixed solution is as follows: the crude long-chain dicarboxylic acid is mixed with the mixed solution at a mass ratio of 1:2 to 1:3, heated to 75-85°C, stirred and dissolved for 60-90 minutes until all the dicarboxylic acid is dissolved, cooled to 10-40°C at a rate of 2-4°C / h, and kept at this temperature for 2-4 hours to crystallize. The solid-liquid separation yields the crystalline compound.

6. The purification method for long-chain dicarboxylic acids as described in claim 4, characterized in that, Two recrystallization processes are used. In this case, step (1) above is as follows: Formic acid and acetic acid are mixed in a ratio of 15-50 wt% to 50-85 wt% to obtain a mixed solution. The crude long-chain dicarboxylic acid is mixed with the mixed solvent in a mass ratio of 1:2 to 1:

3. The mixture is heated to 75-85°C and stirred for 60-90 minutes until the solid is fully dissolved. Then, it is cooled to 25-30°C at a rate of 2-4°C / h and kept at this temperature for 2-4 hours to crystallize. After solid-liquid separation, the solid part is retained. Another portion of the mixed solution is taken and the above operation is repeated to obtain a solid part, which is then dried under vacuum at 60-90°C to obtain the crystalline compound.

7. A purification system for long-chain dicarboxylic acids, characterized in that, The purification method of the long-chain dicarboxylic acid according to any one of claims 1-6 includes: The melting unit includes a devolatilization melt and a melting vessel connected to the devolatilization melt.

8. The purification system for a long-chain dicarboxylic acid as described in claim 7, characterized in that, Also includes: The primary condensation and gas-liquid separation unit includes a shell-and-tube condenser, a condenser tank, and a collection tank; the gas phase outlet of the melting vessel is connected in sequence to the gas phase inlet of the shell-and-tube condenser and the upper gas phase inlet of the condenser tank via pipelines; the bottom liquid phase outlet of the condenser tank is connected to the top inlet of the collection tank via pipelines. The secondary condensation and vacuum unit includes a plate condenser. The top gas phase outlet of the condenser is connected to the gas phase inlet of the plate condenser through a pipeline, and the gas phase outlet of the plate condenser is connected to the gas inlet of the vacuum pump through a pipeline. The exhaust gas purification unit includes a water film absorption device, and the outlet of the vacuum pump is connected to the inlet of the water film absorption device through a pipeline. The nitrogen supply unit includes a nitrogen storage tank, the outlet of which is connected to a nitrogen pipe on the melting vessel via a pipeline.

9. The purification system for a long-chain dicarboxylic acid as described in claim 8, characterized in that, The melting vessel includes a vessel body with a feed inlet at the bottom, forming a bottom-feed method. The melting vessel is equipped with a stirring assembly and a nitrogen inlet. The vessel body also includes a compartmentalization assembly, which includes a horizontally arranged bottom plate fixedly connected to the circumferential side wall of the vessel body. A partition is fixedly connected to one side of the bottom plate, and the top of the partition is fixedly connected to the inner wall of the vessel top. The partition and the bottom plate divide the space within the vessel body into two parts, forming an isolation chamber with the partition, the bottom plate, and part of the inner wall of the vessel body. An air passage is formed on the other side of the partition. At least one guide pipe is connected to the bottom plate to achieve communication between the internal space of the vessel body and the internal space of the isolation chamber. A discharge port is located above the bottom plate, fixedly connected to the vessel body, and the isolation chamber is connected to the discharge port.

10. The purification system for a long-chain dicarboxylic acid as described in claim 9, characterized in that, The bottom plate has a downward-bent recess on the side near the discharge port, which forms a flow guide structure facing the discharge port; an inner coil is also installed inside the isolation chamber.

Images