Production process of acid dye with low salt precipitation

Abstract

The invention discloses an acid dye production process with low salt precipitation, and relates to the technical field of acid dyes, a core raw material for acid dye synthesis is selected, inorganic salt impurities in the raw material are removed, and a salt-free cosolvent is selected when a reaction solvent and an auxiliary agent are prepared; putting the refined raw materials into a reaction kettle, introducing compressed air, stirring for reaction, tracking the reaction progress by an online concentration monitor, and stopping the reaction when the concentration of the dye intermediate reaches a set concentration threshold value to obtain a reaction solution; according to the method, the salt-free cosolvent is selected as a source, traditional salting-out is replaced with the ultrafiltration and nanofiltration grading membrane separation technology, atomization spraying washing is matched, the salt content of the final wet dye crystal is controlled within 0.3%, the solubility and coloring uniformity of the dye are effectively improved, and the color light stability and color fastness are improved. The molecularly imprinted material prepared with the assistance of false template molecules can accurately capture organic by-products and homologues in a nanofiltration concentrated solution, the purity of the dye is greatly improved, and stable product performance is ensured.

Description

Technical Field

[0001] This invention relates to the field of acid dye technology, and in particular to a process for producing acid dyes with low salt precipitation. Background Technology

[0002] Acid dyes are important coloring materials in textiles, leather, and paper industries. The environmental friendliness of their production process and the purity of the product directly affect their application effects and market competitiveness. Traditional acid dye production often uses salting-out to precipitate the dye. This method requires adding a large amount of inorganic salt to the reaction system, which not only leads to a high salt content in the final product, affecting the dye's solubility, color uniformity, and color fastness, but also generates high-salinity industrial wastewater. This type of wastewater is difficult and costly to treat, easily causing water pollution, and does not meet the current requirements of green manufacturing development.

[0003] To address the drawbacks of salting out, some processes have begun to use membrane separation technology to replace salting out, but many shortcomings still exist.

[0004] On the one hand, concentration polarization and membrane fouling are prone to occur during membrane separation, leading to a decrease in membrane flux, a shortened service life, and an impact on production efficiency; On the other hand, existing processes have limited effectiveness in removing organic byproducts and homologues from dyes, making it difficult to guarantee high product purity. Furthermore, the water resources generated during production are not fully recycled, resulting in water waste and further increasing the environmental and economic costs of production.

[0005] Therefore, developing a stable and efficient acid dye production process that can effectively reduce product salt content, improve dye purity, enhance water resource utilization, and achieve high efficiency has become an urgent need in the industry. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a process for producing acid dyes with low salt precipitation. The technical solution is as follows: A process for producing acid dyes with low salt precipitation includes the following steps: Step 1: Select the core raw materials for the synthesis of acid dyes, remove the inorganic salt impurities inherent in the raw materials, and select salt-free cosolvents when preparing reaction solvents and auxiliaries. Step 2: Put the refined raw materials into the reaction vessel, introduce compressed air to stir and react, track the reaction progress with an online concentration monitor, and stop the reaction when the concentration of the dye intermediate reaches the set concentration threshold to obtain the reaction solution. Step 3: Pump the reaction solution into a precision filter to remove insoluble impurities, and then adjust the temperature and pH of the reaction solution to obtain a pretreated reaction solution. Step 4: First, the pretreated reaction solution is sent to an ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities. The ultrafiltration permeate is then sent to a nanofiltration membrane system to retain acidic dye molecules and concentrate to obtain nanofiltration concentrate. Step 5: Selectively purify the residual organic byproducts and homologues in the nanofiltration concentrate using molecular imprinting to obtain a high-purity dye concentrate. Step 6: Pump the high-purity dye concentrate into a low-temperature crystallization kettle, cool it to the set temperature to precipitate the dye crystals, separate them by centrifugation and filtration, and wash the residual salt on the surface of the crystals with circulating water to obtain wet dye crystals. Step 7: The wet dye crystals are sent to a low-temperature spray dryer for drying, then crushed and standardized, and mixed evenly with a low-salt dispersant.

[0007] Optionally, the following steps may also be included: Step 8: Collect the permeate from the membrane separation system, the dye crystal washing water, and the equipment rinsing water, and store them after pH adjustment and activated carbon adsorption. Step 9: Test the salt content, color, intensity and solubility of the final dye product, and adjust the process parameters according to the test results.

[0008] Optionally, step 5 includes the following sub-steps: Step 51: Use the target acid dye product as the template molecule and the structural analogue of the target acid dye as the pseudo template molecule. Step 52: The template molecule, functional monomer, and crosslinking agent are polymerized in a porogen to form a block polymer; Step 53: Elute the template molecules with an elution buffer to obtain MIPs materials with imprinted holes that are complementary to the shape, size, and functional groups of the target dye molecules. Step 54: Fill the stainless steel adsorption column with MIPs material to form a MIPs adsorption and purification system. Step 55: Pump the nanofiltration concentrate obtained in step 4 into the MIPs adsorption purification system and control the flow rate and temperature of the nanofiltration concentrate through the adsorption column. Step 56: Collect the permeate to obtain a high-purity dye concentrate.

[0009] Optionally, after the adsorption column is saturated in step 55, the MIPs adsorption column is regenerated using an eluent to elute and regenerate the captured impurity molecules. The regenerated MIPs can then be reused.

[0010] Optionally, in step 1, high-purity p-aminobenzenesulfonic acid, naphthol compounds, and coupling agents are selected as core raw materials, and vacuum distillation is used to deeply remove inorganic salt impurities from the raw materials to ensure that the initial salt content of the raw materials is less than or equal to 0.1%.

[0011] Optionally, in step 2, the raw materials from step 1 are added to the reactor in a precise ratio. Based on the characteristics of the target dye, the reaction temperature is controlled between 40℃ and 80℃, and the pH value is between 2 and 5. An online concentration monitor is used to track the concentration changes of key intermediates in real time. The reaction is terminated when the concentration of the target dye intermediate reaches 80%.

[0012] Optionally, in step 3, the reaction solution obtained in step 2 is pumped into a precision filter with a pore size of 5μm-10μm to remove trace amounts of insoluble raw material residues or polymer colloids generated during the reaction. The filtrate is then cooled to a temperature of 25-35℃ and the pH value is adjusted to 4-5 so that the filtrate meets the optimal feed requirements for the membrane separation and adsorption purification process.

[0013] Optionally, step 4 includes the following sub-steps: Step 41: The pretreated reaction solution is sent into the ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities from the reaction solution. The permeate enters the ultrafiltration membrane system, which retains molecular weights of 1000 Da-5000 Da. In cross-flow mode, macromolecular polymers and colloidal impurities are retained to obtain a pure permeate. Step 42: The permeate is pumped into the nanofiltration membrane system to retain molecular weights of 200 Da-500 Da. Dye molecules are retained under operating pressure of 1.5 MPa-2.5 MPa and temperature of 25℃-35℃, and the concentration is concentrated to 25%-30%. At the same time, small molecule inorganic salts and water are allowed to permeate, thereby achieving dye desalination and concentration.

[0014] Optionally, in step 4, the ultrafiltration membrane system and nanofiltration membrane system adopt an online pulse backwashing process with a backwashing cycle of 30-60 minutes. The backwashing solution is the circulating water treated in step 8, the backwashing pressure is 1.2-1.5 times the operating pressure, and each backwashing time is 1-3 minutes. The ultrafiltration membrane is made of polyethersulfone, and the nanofiltration membrane is made of composite aromatic polyamide. The membrane module operates at a cross-flow rate of 2m / s-3m / s to reduce concentration polarization on the membrane surface and maintain membrane flux at 15LMH-25LMH.

[0015] Optionally, in step 6, the cooling process of the low-temperature crystallization vessel adopts a gradient cooling mode, first cooling from 25-35℃ to 15-20℃ at a rate of 3℃ / h-5℃ / h, holding for 1-2 hours, and then cooling to 5-10℃ at a rate of 1-2℃ / h; during crystallization, the stirring rate is controlled at 100r / min-200r / min, the centrifugal filtration speed is 3000r / min-4000r / min, and the filtration time is 5-10 minutes; the circulating water washing adopts an atomized spray method, the spray volume is 10-15% of the mass of the wet dye crystals, and the salt content of the wet dye crystals after washing is less than or equal to 0.3%.

[0016] In summary, the present invention has at least one of the following beneficial technical effects: This invention provides a low-salt-precipitation acid dye production process. By using a salt-free cosolvent at the source and combining ultrafiltration and nanofiltration fractional membrane separation technology to replace traditional salt precipitation, along with atomized spray washing, the salt content of the final wet dye crystals is controlled to within 0.3%, effectively improving the solubility and color uniformity of the dye, and enhancing color stability and color fastness.

[0017] Molecularly imprinted materials prepared with the assistance of pseudo-template molecules can accurately capture organic byproducts and homologues in nanofiltration concentrates, significantly improving dye purity, ensuring stable product performance, and meeting the needs of high-end applications.

[0018] By combining cross-flow operation mode with online pulse backwashing process, the concentration polarization and fouling of membrane surface are effectively reduced, maintaining membrane flux at 15-25 LMH, significantly extending the service life of ultrafiltration and nanofiltration membranes, and reducing equipment maintenance costs.

[0019] By constructing a full-process water recycling system, membrane separation permeate, washing water, and other wastewater can be recycled and reused, reducing fresh water consumption, lowering wastewater discharge, complying with environmental protection policies, and reducing water resource costs in production.

[0020] By optimizing processes such as online concentration monitoring and gradient cooling crystallization, precise control of the reaction and crystallization processes can be achieved, improving the batch qualification rate of products, ensuring the stable and efficient operation of the production process, and enhancing overall production efficiency. Attached Figure Description

[0021] Figure 1 This is a schematic flow diagram of a low-salt precipitation acid dye production process according to the present invention. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings.

[0023] This invention discloses a process for producing acid dyes with low salt precipitation.

[0024] Reference Figure 1 Example 1: A process for producing acid dyes with low salt precipitation, comprising the following steps: Step 1: Select the core raw materials for the synthesis of acid dyes, remove the inorganic salt impurities inherent in the raw materials, and select salt-free cosolvents when preparing reaction solvents and auxiliaries. Step 2: Put the refined raw materials into the reaction vessel, introduce compressed air to stir and react, track the reaction progress with an online concentration monitor, and stop the reaction when the concentration of the dye intermediate reaches the set concentration threshold to obtain the reaction solution. Step 3: Pump the reaction solution into a precision filter to remove insoluble impurities, and then adjust the temperature and pH of the reaction solution to obtain a pretreated reaction solution. Step 4: First, the pretreated reaction solution is sent to an ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities. The ultrafiltration permeate is then sent to a nanofiltration membrane system to retain acidic dye molecules and concentrate to obtain nanofiltration concentrate. Step 5: Selectively purify the residual organic byproducts and homologues in the nanofiltration concentrate using molecular imprinting to obtain a high-purity dye concentrate. Step 6: Pump the high-purity dye concentrate into a low-temperature crystallization kettle, cool it to the set temperature to precipitate the dye crystals, separate them by centrifugation and filtration, and wash the residual salt on the surface of the crystals with circulating water to obtain wet dye crystals. Step 7: The wet dye crystals are sent to a low-temperature spray dryer for drying, then crushed and standardized, and mixed evenly with a low-salt dispersant.

[0025] Example 2 further includes the following steps: Step 8: Collect the permeate from the membrane separation system, the dye crystal washing water, and the equipment rinsing water, and store them after pH adjustment and activated carbon adsorption. Step 9: Test the salt content, color, intensity and solubility of the final dye product, and adjust the process parameters according to the test results.

[0026] Through the above technical solution, in step 1, removing the inorganic salts inherent in the raw materials and selecting salt-free co-solvents can reduce the initial salt content of the system from the source, avoid the contamination of the final product by raw material impurities and salt-containing additives in traditional processes, and lay the foundation for subsequent low-salt production.

[0027] Precise control of the reaction process is key to reducing by-product salts. In step 2, by monitoring the concentration of dye intermediates online and controlling the timing of reaction termination, the amount of by-products (including by-product inorganic salts) generated by excessive reaction of raw materials can be reduced. At the same time, compressed air stirring can enhance the mass transfer of the reaction system, improve the conversion rate of raw materials, and reduce the amount of impurities generated.

[0028] In step 3, precision filtration removes insoluble impurities to prevent them from clogging subsequent membrane pores or contaminating the adsorption material; adjusting the temperature and pH value is because the separation performance of the membrane and the adsorption activity of the molecularly imprinted material are closely related to the system temperature and pH, and suitable parameters can maximize the efficiency of subsequent separation and purification.

[0029] In step 4, the ultrafiltration membrane retains large polymer molecules and colloids based on their molecular weight differences, thus purifying the reaction solution; the nanofiltration membrane, on the other hand, utilizes pore size sieving and charge action to selectively retain dye molecules (with larger molecular weights) while allowing small inorganic salt molecules and water molecules to pass through, thereby achieving dye concentration and desalination without adding salt, replacing the traditional salting-out precipitation mechanism.

[0030] In step 5, the imprinted cavities inside the molecularly imprinted materials (MIPs) are complementary to the shape, size, and functional groups of the target dye molecules. This allows them to specifically recognize and adsorb organic byproducts and homologues (impurities) in the nanofiltration concentrate without adsorbing the target dye. This process effectively removes impurities while retaining the dye, thereby improving product purity.

[0031] In step 6, the low temperature environment can reduce the solubility of the dye in water, causing it to precipitate in crystal form; gradient cooling helps to form crystals with uniform particle size and improves separation efficiency; circulating water atomization spray washing utilizes the solubility of water for residual salts, and a small amount of water can wash away the trace amounts of salt attached to the crystal surface, further reducing the salt content of the product.

[0032] In step 7, low-temperature spray drying can avoid the damage to the dye structure caused by high temperature; pulverization and standardization treatment, by controlling the particle size and adding low-salt dispersants, ensures the dispersibility and color uniformity of the dye in application, without introducing additional salt load.

[0033] Water recycling and monitoring control achieve a closed-loop process. Step 8, water recycling, reduces fresh water consumption and lowers discharge pressure by treating and recycling wastewater; step 9, monitoring and control, optimizes parameters at each stage based on product indicators to ensure process stability and consistent product quality.

[0034] Each step is designed with specific principles to form a low-salt control system from source to end, enabling the low-salt, high-purity, and efficient production of acid dyes.

[0035] Example 3, step 5 includes the following sub-steps: Step 51: Use the target acid dye product as the template molecule and the structural analogue of the target acid dye as the pseudo template molecule. Step 52: The template molecule, functional monomer, and crosslinking agent are polymerized in a porogen to form a block polymer; Step 53: Elute the template molecules with an elution buffer to obtain MIPs materials with imprinted holes that are complementary to the shape, size, and functional groups of the target dye molecules. Step 54: Fill the stainless steel adsorption column with MIPs material to form a MIPs adsorption and purification system. Step 55: Pump the nanofiltration concentrate obtained in step 4 into the MIPs adsorption purification system and control the flow rate and temperature of the nanofiltration concentrate through the adsorption column. Step 56: Collect the permeate to obtain a high-purity dye concentrate.

[0036] In Example 4, after the adsorption column is saturated in step 55, the MIPs adsorption column is regenerated using an eluent to elute and regenerate the captured impurity molecules. The regenerated MIPs can be reused.

[0037] By adopting the above technical solution, in step 51, the target acid dye is selected as the template molecule so that the polymer formed subsequently can accurately match the structural characteristics of the target dye. At the same time, the addition of the structural analog of the target dye as a pseudo template molecule can reduce the residue of the template molecule in the subsequent elution process, and can further optimize the specificity of the imprinted hole, avoid the non-specific adsorption of the hole on the target dye, and ensure that only impurities are captured without loss of the target dye.

[0038] The structural analogue and the target acid dye must have slight and distinguishable structural differences, such as the type of substituent (e.g., the difference between methyl and ethyl) or the position of substitution. This difference allows the structural analogue to be more easily removed from the polymer during subsequent elution, reducing template molecule residues, while not destroying the specificity of the imprinted holes. This ensures that the final prepared MIPs material can accurately identify and adsorb organic byproducts and homologues in the nanofiltration concentrate, without adsorbing the target dye.

[0039] In the polymerization reaction of step 52, the functional monomers will be oriented around the template molecules through chemical bonds. The crosslinking agent will connect these oriented functional monomers to form a stable blocky polymer skeleton. The porogen will form a porous structure inside the polymer, providing channels for subsequent molecular diffusion and adsorption, and laying the structural basis for specific adsorption.

[0040] After eluting the template molecules in step 53, imprinted cavities that are completely complementary to the shape, size, and functional groups of the target dye molecules will be left inside the polymer. These cavities are the core structure for MIPs materials to achieve specific recognition, enabling MIPs materials to have the ability to strongly adsorb only organic byproducts and homologues (impurities) that match the cavities.

[0041] Step 54 fills the stainless steel adsorption column with MIPs material to form a stable continuous purification system, ensuring that the nanofiltration concentrate can fully contact the MIPs material, avoiding material loss, and facilitating the control of the flow state of the feed solution.

[0042] Step 55 controls the flow rate and temperature of the nanofiltration concentrate because a flow rate that is too slow will reduce production efficiency, while a flow rate that is too fast will result in insufficient contact between the feed liquid and the MIPs material, affecting the adsorption effect of impurities; temperature affects the molecular motion rate and the strength of the adsorption force, and a suitable temperature can maximize the adsorption efficiency of the MIPs material for impurities, ensuring that impurities are fully captured.

[0043] In step 56, since the MIPs material only specifically adsorbs organic byproducts and homologues, the target dye molecules cannot match the imprinted holes and directly penetrate the adsorption column. Therefore, the collected permeate is a high-purity dye concentrate with impurities removed.

[0044] By selectively using eluents, the interaction forces between MIPs materials and adsorbed impurities are disrupted, enabling the regeneration and recycling of MIPs materials, reducing production costs, and ensuring process continuity.

[0045] Once the MIPs adsorption column is saturated, impurity molecules are tightly bound to the imprinted holes in the MIPs material through hydrogen bonds, hydrophobic interactions, van der Waals forces, and other forces. At this point, the added eluent will compete with the impurity molecules for binding sites in the imprinted holes, or directly disrupt the interactions between the MIPs material and the impurity molecules, causing the impurity molecules to detach from the imprinted holes and dissolve in the eluent.

[0046] As the eluent flows, the eluted impurity molecules are carried out of the adsorption column, and the imprinted cavities inside the MIPs material are re-exposed, restoring their specific adsorption capacity. The regenerated MIPs material can be reused for the purification of nanofiltration concentrate, realizing the recycling of materials. This not only reduces the consumption of MIPs material and lowers the material cost of production, but also ensures the continuous and stable operation of the entire purification process, improving the economy and practicality of the process.

[0047] In Example 5, in step 1, high-purity p-aminobenzenesulfonic acid, naphthol compounds, and coupling agents are selected as core raw materials. Vacuum distillation process is used to deeply remove inorganic salt impurities from the raw materials, ensuring that the initial salt content of the raw materials is less than or equal to 0.1%.

[0048] By adopting the above technical solution, the selection of raw materials focuses on the core requirements of acid dye synthesis. P-aminobenzenesulfonic acid and naphthol compounds are key building blocks of the acid dye molecule structure, while coupling agents ensure the efficient execution of the coupling reaction in dye synthesis. Selecting high-purity raw materials directly reduces the amount of inorganic salt impurities inherent in the raw materials, reducing the pressure of subsequent impurity removal, while ensuring the directionality of the synthesis reaction and minimizing side reactions caused by insufficient raw material purity.

[0049] Vacuum distillation utilizes the difference in boiling points to achieve deep separation of salt impurities. In a vacuum environment, the reduced system pressure significantly lowers the boiling point of organic feedstocks (such as p-aminobenzenesulfonic acid and naphthol compounds), allowing them to be vaporized and separated under relatively mild temperature conditions. In contrast, inorganic salts (such as sodium chloride and sodium sulfate) have much higher boiling points than the organic feedstocks and do not vaporize under these conditions, remaining as residue at the bottom of the distillation vessel. This achieves highly efficient separation of organic feedstocks from salt impurities.

[0050] Controlling the initial salt content of raw materials to ≤0.1% is crucial because salt impurities in the raw materials accumulate continuously during subsequent synthesis and separation processes, directly affecting the salt content of the final dye product. This combined approach reduces salt impurities at the source, preventing increased desalination difficulties in subsequent processes due to excessive salt load. It also minimizes the adverse effects of salt impurities on the activity of the synthesis catalyst and membrane separation performance, ensuring the successful achievement of the low-salt control target throughout the entire production process.

[0051] In Example 6, in step 2, the raw materials from step 1 are added to the reactor in a precise ratio. Based on the characteristics of the target dye, the reaction temperature is controlled between 40℃ and 80℃, and the pH value is between 2 and 5. An online concentration monitor is used to track the concentration changes of key intermediates in real time. The reaction is terminated when the concentration of the target dye intermediate reaches 80%.

[0052] By employing the above technical solution, precise raw material proportioning is fundamental to ensuring the targeted conduction of the reaction. The synthesis of acid dyes largely relies on specific reactions such as coupling and condensation, and the proportion of raw materials directly determines the reaction pathway. Precise material proportioning avoids side reactions caused by excess of any single raw material, reduces impurities (including byproduct inorganic salts) resulting from raw material waste, and simultaneously ensures the efficient formation of the target dye intermediate, thereby improving reaction selectivity.

[0053] Maintaining the temperature between 40℃ and 80℃ is crucial for balancing the reaction rate and product stability. Too low a temperature leads to a slow reaction rate and low production efficiency; too high a temperature may damage the chemical structure of raw materials or intermediates, triggering side reactions such as decomposition and polymerization, and increasing the total amount of impurities. This temperature range is suitable for the reaction characteristics of acid dye synthesis, ensuring efficient reaction progress while maintaining the structural stability of the target intermediate.

[0054] Maintaining a pH between 2 and 5 is essential for ensuring the proper functioning of the reaction mechanism. The core reactions in the synthesis of acidic dyes (such as the coupling of aromatic compounds) require an acidic environment to initiate. A suitable pH value can activate the reactive sites of the starting materials, promoting the directed occurrence of the reaction. If the pH value exceeds this range, it may lead to changes in the reaction pathway, generating non-target intermediates or byproducts, which not only reduces the yield of the target product but also increases the difficulty of subsequent separation and purification.

[0055] Online monitoring and precise reaction termination are key measures to reduce by-product salts. Online concentration monitors can capture changes in intermediate concentration in real time, allowing for timely monitoring of reaction progress. Terminating the reaction when the intermediate concentration reaches 80% avoids over-reaction; over-reaction can lead to the further conversion of intermediates into by-products (including by-product inorganic salts), increasing the system's salt load and impurity content. Timely termination ensures a high feed conversion rate while minimizing ineffective side reactions, reducing pressure on subsequent membrane separation and purification processes, and guaranteeing the low-salt characteristics and high purity of the final product.

[0056] In Example 7, in step 3, the reaction solution obtained in step 2 is pumped into a precision filter with a pore size of 5μm-10μm to remove trace amounts of insoluble raw material residues or polymer colloids generated during the reaction. The filtrate is cooled to a temperature of 25-35℃ and the pH value is adjusted to 4-5 so that the filtrate meets the optimal feed requirements for the membrane separation and adsorption purification process.

[0057] By adopting the above technical solutions, contamination can reduce membrane flux and lifespan. If impurities come into contact with molecularly imprinted adsorption materials, they can occupy adsorption sites, affecting the specific adsorption effect on organic impurities. Pre-removing impurities reduces the operational burden on subsequent processes from the source, ensuring smooth separation and purification.

[0058] Cooling the temperature to 25℃-35℃ is a key control measure to adapt to subsequent processes. The core performance of membrane separation (such as flux and rejection rate) is closely related to the feed temperature. This temperature range can balance the physical stability of the membrane and the molecular diffusion rate, avoiding excessively high temperatures that accelerate membrane aging and degradation, or excessively low temperatures that lead to sluggish molecular motion and decreased membrane flux. At the same time, the adsorption activity of molecularly imprinted materials depends on a suitable temperature environment. This range can ensure the stability of the interaction forces between imprinted holes and impurity molecules, thereby improving the efficiency of adsorption and purification.

[0059] Adjusting the pH to 4-5 is essential for ensuring the proper functioning of subsequent processes. The surface charge state of the membrane changes with pH; this pH range allows the nanofiltration membrane to maintain its selective retention of dye molecules while reducing non-specific adsorption of molecules from the feed solution onto the membrane surface. For molecularly imprinted materials, a suitable acidic environment maintains the activity of their functional groups, ensuring that imprinted holes can accurately recognize and bind organic byproducts and homologues. By adjusting the pH, the chemical environment of the filtrate can be perfectly matched to the process requirements of membrane separation and adsorption purification, maximizing the treatment effect of subsequent processes and laying the foundation for obtaining low-salt, high-purity dyes.

[0060] Example 8, step 4 includes the following sub-steps: Step 41: The pretreated reaction solution is sent into the ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities from the reaction solution. The permeate enters the ultrafiltration membrane system, which retains molecular weights of 1000 Da-5000 Da. In cross-flow mode, macromolecular polymers and colloidal impurities are retained to obtain a pure permeate. Step 42: The permeate is pumped into the nanofiltration membrane system to retain molecular weights of 200 Da-500 Da. Dye molecules are retained under operating pressure of 1.5 MPa-2.5 MPa and temperature of 25℃-35℃, and the concentration is concentrated to 25%-30%. At the same time, small molecule inorganic salts and water are allowed to permeate, thereby achieving dye desalination and concentration.

[0061] Example 9: In step 4, the ultrafiltration membrane system and nanofiltration membrane system adopt an online pulse backwashing process. The backwashing cycle is 30-60 minutes. The backwashing solution is the circulating water treated in step 8. The backwashing pressure is 1.2-1.5 times the operating pressure. Each backwashing time is 1-3 minutes. The ultrafiltration membrane is made of polyethersulfone, and the nanofiltration membrane is made of composite aromatic polyamide. The membrane module operates at a cross-flow rate of 2m / s-3m / s to reduce concentration polarization on the membrane surface and maintain membrane flux at 15LMH-25LMH.

[0062] By adopting the above technical solution, the ultrafiltration process in step 41 utilizes the sieving effect of the membrane to achieve primary purification. The molecular weight cutoff of the ultrafiltration membrane is set to 1000Da-5000Da, which can accurately retain large molecular polymers and colloidal impurities (whose molecular weight is usually greater than 5000Da) in the reaction solution, while dye molecules and small molecular inorganic salts (molecular weight less than 1000Da) permeate through the membrane with the water, forming a pure permeate. When operating in cross-flow mode, the feed solution flows at high speed along the membrane surface, which can wash away the impurities retained on the membrane surface, reduce the deposition of impurities on the membrane surface, reduce the risk of membrane fouling, and ensure the stability and efficiency of the ultrafiltration process.

[0063] Step 42, the nanofiltration process, achieves dye desalination and concentration through selective retention. The nanofiltration membrane's molecular weight cutoff is set at 200 Da-500 Da, matching the molecular weight of acidic dye molecules (typically 300 Da-800 Da), effectively retaining dye molecules. Small-molecule inorganic salts (such as sodium chloride and sodium sulfate, with molecular weights less than 200 Da) permeate the membrane along with water molecules, achieving dye-salt separation. The operating pressure is controlled at 1.5 MPa-2.5 MPa to provide sufficient power to drive small-molecule substances through the membrane while avoiding damage to the membrane structure due to high pressure. The temperature is maintained at 25℃-35℃ to balance membrane permeability and stability, ensuring efficient dye molecule retention while maintaining the permeation efficiency of water and salt. Concentration to 25%-30% meets the feed concentration requirements of subsequent crystallization processes while avoiding a decrease in membrane flux due to excessive concentration, achieving synergistic efficiency in desalination and concentration.

[0064] The online pulse backwashing process removes contaminants from the membrane surface through periodic high-pressure rinsing. The backwash cycle is set at 30-60 minutes to remove contaminants before they form a stable deposit. The backwash pressure is 1.2-1.5 times the operating pressure, generating sufficient impact force to peel off impurities adhering to the membrane surface while avoiding damage to the membrane structure due to excessive pressure. Each backwash lasts 1-3 minutes, minimizing disruption to normal production while ensuring efficient cleaning. The circulating water treated in step 8 is used as the backwash solution, achieving water resource recycling and preventing the introduction of new contaminants during the backwashing process, as most impurities have already been removed.

[0065] The choice of membrane material is adapted to the process environment and separation requirements. Ultrafiltration membranes are made of polyethersulfone, which has good acid resistance, mechanical strength and chemical stability, and can withstand the acidic environment of the reaction solution and the corrosion of cleaning agents; nanofiltration membranes are made of composite aromatic polyamide, which has a high rejection rate of dye molecules and strong resistance to fouling, and is suitable for dye desalination and separation under acidic conditions.

[0066] Maintaining a cross-flow velocity of 2-3 m / s enhances fluid turbulence on the membrane surface, reducing solute accumulation (concentration polarization) on the membrane surface. Concentration polarization reduces the membrane's effective separation efficiency and accelerates membrane fouling. A suitable cross-flow velocity can promptly remove solutes from the membrane surface, maintaining a stable membrane flux of 15 LMH-25 LMH, ensuring long-term efficient operation of the membrane system, and providing a stable guarantee for the continuous desalination and concentration of dyes.

[0067] In Example 10, the cooling process of the low-temperature crystallization vessel in step 6 adopts a gradient cooling mode. First, the temperature is reduced from 25-35℃ to 15-20℃ at a rate of 3℃ / h-5℃ / h, and held for 1-2 hours. Then, the temperature is reduced to 5-10℃ at a rate of 1-2℃ / h. During crystallization, the stirring rate is controlled at 100r / min-200r / min, the centrifugal filtration speed is 3000r / min-4000r / min, and the filtration time is 5-10 minutes. The circulating water washing adopts an atomized spray method, and the spray volume is 10-15% of the mass of the wet dye crystals. After washing, the salt content of the wet dye crystals is less than or equal to 0.3%.

[0068] By adopting the above technical solution, the gradient cooling mode is the key to ensuring crystal quality. Initially, cooling at a rate of 3℃ / h-5℃ / h to 15-20℃ can quickly build up a moderate supersaturation, inducing the generation of a large number of uniform crystal nuclei and avoiding crystal agglomeration or uneven particle size caused by excessive local supersaturation. Holding at this temperature for 1-2 hours in between allows the generated crystal nuclei to grow sufficiently and stably, reducing the dissolution loss of small crystal nuclei. Subsequently, cooling at a slow rate of 1℃ / h-2℃ / h to 5-10℃ can promote the slow growth of crystals, forming dense and uniformly sized dye crystals, which not only improves the separation performance of the crystals but also reduces the impurity salts adsorbed on the surface.

[0069] The stirring rate is controlled at 100r / min-200r / min, which can ensure that the temperature and concentration fields in the crystallizer are uniform, avoid inconsistent crystallization rates caused by local temperature differences, and prevent crystals from being broken by stirring too fast or crystals from settling and agglomerating by stirring too slow, thus ensuring a stable crystal growth environment.

[0070] Centrifugal filtration parameters are set to focus on efficient separation. A rotation speed of 3000-4000 rpm generates sufficient centrifugal force to quickly separate crystals from the mother liquor, shortening the residence time of crystals in the saline mother liquor and reducing salt re-adsorption. A filtration time of 5-10 minutes can improve production efficiency while ensuring sufficient separation and avoiding over-filtration that could lead to crystal drying and clumping.

[0071] Atomized spray washing enhances desalination by optimizing the contact method. The atomized circulating water evenly covers the crystal surface in the form of fine droplets, increasing the contact area between the washing water and the crystal. Compared with traditional soaking washing, it can more efficiently dissolve and remove residual salts from the crystal surface. The spray volume is controlled at 10-15% of the wet crystal mass, which ensures the desalination effect while avoiding crystal dissolution and loss due to excessive washing water. Ultimately, the salt content of the wet dye crystals is precisely controlled within 0.3%, meeting the requirements of low-salt products.

[0072] The following specific embodiments illustrate the implementation principle of the present invention: Step 1: Raw material refining and pretreatment 100 kg of high-purity p-aminobenzenesulfonic acid, 85 kg of naphthol AS-TR, and 20 kg of naphthylethylenediamine hydrochloride were selected as the core raw materials. Vacuum distillation (vacuum degree -0.095 MPa, temperature 130℃) was used to deeply remove inorganic salt impurities, controlling the initial salt content of the raw materials to 0.08%. When preparing the reaction solvent, 50 kg of a polyethylene glycol-propylene glycol compound co-solvent (mass ratio 3:1) was used to replace the traditional salt-containing co-solvent.

[0073] Step 2: Dye Synthesis Reaction The refined raw materials were added to a 500L reactor equipped with an online concentration monitor according to the above proportions. The reaction temperature was controlled at 55℃, and the pH value was adjusted to 3.0 with dilute sulfuric acid. Compressed air was introduced to stir (300 r / min) for coupling reaction. The concentration of the dye intermediate was monitored in real time. When the concentration reached 80%, the reaction was immediately terminated, yielding approximately 300L of reaction solution.

[0074] Step 3: Pretreatment of the reaction solution The reaction solution was pumped into a precision filter with an 8μm pore size to remove insoluble residues. The filtrate was then cooled to 30°C and the pH was adjusted to 4.5 with dilute sulfuric acid to obtain the pretreated reaction solution.

[0075] Step 4: Fractionated membrane separation and concentration Ultrafiltration: The pretreated reaction solution is fed into a polyethersulfone ultrafiltration membrane system (molecular weight cutoff 3000 Da) and operated in cross-flow mode (flow rate 2.5 m / s) to retain macromolecular polymers and colloidal impurities, yielding 280 L of permeate.

[0076] Nanofiltration concentration: The permeate is pumped into a nanofiltration membrane system made of composite aromatic polyamide material (molecular weight cutoff 300 Da), and the operating pressure is controlled at 2.0 MPa and the temperature at 30°C. The solution is concentrated to a dye concentration of 28% to obtain 80 L of nanofiltration concentrate. The permeate is sent to the water circulation system.

[0077] Membrane system backwashing: Online pulse backwashing is performed every 45 minutes using the circulating water treated in step 8. The backwashing pressure is 2.5 MPa, and each backwash lasts for 2 minutes to maintain the membrane flux at a stable 20 LMH.

[0078] Step 5: Molecular Imprinting and Selective Purification Preparation of MIPs materials: Using Acid Red 3R as the template molecule and anthranilic acid as the pseudo template molecule, the polymers were polymerized in a porogen at a molar ratio of template molecule: functional monomer: crosslinking agent = 1:4:20 to form a block polymer, which was then eluted with an elution solution to obtain the MIPs materials.

[0079] Adsorption purification: MIPs material was packed into a stainless steel adsorption column, and nanofiltration concentrate was pumped into the adsorption column at a flow rate of 1.5 BV / h and a temperature of 30°C. The permeate was collected to obtain 78 L of high-purity dye concentrate.

[0080] Material regeneration: After the adsorption column is saturated, impurities are eluted with an ethanol-water solution, and the regenerated MIPs material is recycled.

[0081] Step 6: Low-temperature crystallization and separation washing The high-purity dye concentrate was pumped into a low-temperature crystallization vessel, and a gradient cooling mode was adopted: first, the temperature was reduced from 30°C to 18°C ​​at a rate of 4°C / h, and held at that temperature for 1.5 hours, and then reduced to 8°C at a rate of 1.5°C / h. During the crystallization process, the stirring speed was controlled at 150 r / min. After the crystals precipitated, they were filtered by a centrifuge at 4000 r / min for 8 minutes, and then washed with circulating water (12% of the mass of the wet crystals) by atomized spraying to obtain 65 kg of wet dye crystals.

[0082] Step 7: Dye post-treatment The wet dye crystals were fed into a low-temperature spray dryer (inlet temperature 130℃, outlet temperature 65℃) for drying, and then pulverized to a particle size of 15μm by an air jet mill. 1.2% of a low-salt composite dispersant (sodium lignosulfonate-polycarboxylate mass ratio 2:1) was added and mixed evenly to obtain 42kg of finished Acid Red 3R powder.

[0083] Step 8: Water Recycling A total of 220L of membrane separation permeate, crystallization washing water, and equipment flushing water was collected. After adjusting the pH value to 7.0 and treating it with activated carbon adsorption, the water was stored in a circulating water tank. Of this, 190L was reused for reaction dilution and crystal washing, and the remaining 30L was sent to the wastewater treatment system.

[0084] Step 9: Product Quality Inspection The finished dyes were subjected to multi-dimensional testing, and the test results are shown in Table 1.

[0085] Table 1

[0086] Table 2 shows a comparison of the product test results obtained with those obtained through the traditional salting-out process: Table 2

[0087] Therefore, it is evident that the product of this technical solution has a salt content of only 0.25% and an organic impurity content as low as 0.0008%, exhibiting significantly superior color stability and solubility compared to traditional salting-out processes. Traditional salting-out processes typically produce products with a salt content ranging from 3.5% to 5.0%, and a higher content of organic impurities, which can easily affect the uniformity of dye application and color fastness.

[0088] This solution achieves a water recycling rate of 90.5%, reducing fresh water consumption and wastewater discharge by 85% and 83% respectively, significantly alleviating environmental protection pressure. Traditional salting-out processes have a water resource utilization rate of only 30%-40%, and treating high-salinity wastewater is difficult, costly, and prone to causing water pollution.

[0089] This solution shortens the production cycle to 8 hours per batch, increasing capacity by 33% compared to traditional processes. The membrane equipment undergoes optimized maintenance, resulting in a longer service life and lower maintenance costs. Traditional processes have production cycles as long as 12 hours per batch, and the reactors are susceptible to corrosion from high-salt environments, leading to severe equipment wear and tear.

[0090] Overall Cost: This solution reduces the overall cost per unit product by 20% through water conservation, reduced consumption, and improved production efficiency. Traditional processes suffer from high overall costs due to high water consumption, high wastewater treatment costs, and limited raw material conversion rates.

[0091] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A process for producing acid dyes with low salt precipitation, characterized in that, Includes the following steps: Step 1: Select the core raw materials for the synthesis of acid dyes, remove the inorganic salt impurities inherent in the raw materials, and select salt-free cosolvents when preparing reaction solvents and auxiliaries. Step 2: Put the refined raw materials into the reaction vessel, introduce compressed air to stir and react, track the reaction progress with an online concentration monitor, and stop the reaction when the concentration of the dye intermediate reaches the set concentration threshold to obtain the reaction solution. Step 3: Pump the reaction solution into a precision filter to remove insoluble impurities, and then adjust the temperature and pH of the reaction solution to obtain a pretreated reaction solution. Step 4: First, the pretreated reaction solution is sent to an ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities. The ultrafiltration permeate is then sent to a nanofiltration membrane system to retain acidic dye molecules and concentrate to obtain nanofiltration concentrate. Step 5: Selectively purify the residual organic byproducts and homologues in the nanofiltration concentrate using molecular imprinting to obtain a high-purity dye concentrate. Step 6: Pump the high-purity dye concentrate into a low-temperature crystallization kettle, cool it to the set temperature to precipitate the dye crystals, separate them by centrifugation and filtration, and wash the residual salt on the surface of the crystals with circulating water to obtain wet dye crystals. Step 7: The wet dye crystals are sent to a low-temperature spray dryer for drying, then crushed and standardized, and mixed evenly with a low-salt dispersant.

2. The process for producing acid dyes with low salt precipitation according to claim 1, characterized in that, It also includes the following steps: Step 8: Collect the permeate from the membrane separation system, the dye crystal washing water, and the equipment rinsing water, and store them after pH adjustment and activated carbon adsorption. Step 9: Test the salt content, color, intensity and solubility of the final dye product, and adjust the process parameters according to the test results.

3. The process for producing acid dyes with low salt precipitation according to claim 2, characterized in that, Step 5 includes the following sub-steps: Step 51: Use the target acid dye product as the template molecule and the structural analogue of the target acid dye as the pseudo template molecule. Step 52: The template molecule, functional monomer, and crosslinking agent are polymerized in a porogen to form a block polymer; Step 53: Elute the template molecules with an elution buffer to obtain MIPs materials with imprinted holes that are complementary to the shape, size, and functional groups of the target dye molecules. Step 54: Fill the stainless steel adsorption column with MIPs material to form a MIPs adsorption and purification system. Step 55: Pump the nanofiltration concentrate obtained in step 4 into the MIPs adsorption purification system and control the flow rate and temperature of the nanofiltration concentrate through the adsorption column. Step 56: Collect the permeate to obtain a high-purity dye concentrate.

4. The process for producing acid dyes with low salt precipitation according to claim 3, characterized in that, After the adsorption column is saturated in step 55, the MIPs adsorption column is regenerated using an eluent to elute the captured impurity molecules. The regenerated MIPs can then be reused.

5. The process for producing acid dyes with low salt precipitation according to claim 4, characterized in that, In step 1, high-purity p-aminobenzenesulfonic acid, naphthol compounds, and coupling agents are selected as core raw materials. Vacuum distillation process is used to deeply remove inorganic salt impurities from the raw materials, ensuring that the initial salt content of the raw materials is less than or equal to 0.1%.

6. The process for producing acid dyes with low salt precipitation according to claim 5, characterized in that, In step 2, the raw materials from step 1 are added to the reaction vessel in a precise ratio. Based on the characteristics of the target dye, the reaction temperature is controlled between 40℃ and 80℃, and the pH value is controlled between 2 and 5. An online concentration monitor is used to track the concentration changes of key intermediates in real time. The reaction is terminated when the concentration of the target dye intermediate reaches 80%.

7. The process for producing acid dyes with low salt precipitation according to claim 6, characterized in that, In step 3, the reaction solution obtained in step 2 is pumped into a precision filter with a pore size of 5μm-10μm to remove trace amounts of insoluble raw material residues or polymer colloids generated during the reaction. The filtrate is cooled to 25-35℃ and the pH value is adjusted to 4-5 so that the filtrate meets the optimal feed requirements for the membrane separation and adsorption purification process.

8. The process for producing acid dyes with low salt precipitation according to claim 7, characterized in that, Step 4 includes the following sub-steps: Step 41: The pretreated reaction solution is sent into the ultrafiltration membrane system to remove macromolecular polymers and colloidal impurities from the reaction solution. The permeate enters the ultrafiltration membrane system, which retains molecular weights of 1000 Da-5000 Da. In cross-flow mode, macromolecular polymers and colloidal impurities are retained to obtain a pure permeate. Step 42: The permeate is pumped into the nanofiltration membrane system to retain molecular weights of 200 Da-500 Da. Dye molecules are retained under operating pressure of 1.5 MPa-2.5 MPa and temperature of 25℃-35℃, and the concentration is concentrated to 25%-30%. At the same time, small molecule inorganic salts and water are allowed to permeate, thereby achieving dye desalination and concentration.

9. The process for producing acid dyes with low salt precipitation according to claim 8, characterized in that, In step 4, the ultrafiltration membrane system and nanofiltration membrane system adopt an online pulse backwashing process with a backwashing cycle of 30-60 minutes. The backwashing solution is the circulating water treated in step 8. The backwashing pressure is 1.2-1.5 times the operating pressure, and each backwashing time is 1-3 minutes. The ultrafiltration membrane is made of polyethersulfone, and the nanofiltration membrane is made of composite aromatic polyamide. The membrane module operates at a cross-flow rate of 2m / s-3m / s to reduce concentration polarization on the membrane surface and maintain membrane flux at 15LMH-25LMH.

10. The process for producing acid dyes with low salt precipitation according to claim 9, characterized in that, In step 6, the cooling process of the low-temperature crystallization vessel adopts a gradient cooling mode. First, the temperature is reduced from 25-35℃ to 15-20℃ at a rate of 3℃ / h-5℃ / h, and held for 1-2 hours. Then, the temperature is reduced to 5-10℃ at a rate of 1-2℃ / h. During crystallization, the stirring rate is controlled at 100r / min-200r / min, the centrifugal filtration speed is 3000r / min-4000r / min, and the filtration time is 5-10 minutes. The circulating water washing adopts the atomized spray method, and the spray volume is 10-15% of the mass of the wet dye crystals. After washing, the salt content of the wet dye crystals is less than or equal to 0.3%.

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