Process for the preparation of 2,4-dichlorophenoxyacetic acid and sodium hydroxide

Abstract

The present application relates to fine chemical product separation and recovery and bipolar membrane electrodialysis separation technical field, disclose a kind of preparation 2,4-dichlorophenoxyacetic acid and sodium hydroxide method.The present application is with the help of the same ion effect and temperature difference to improve 2,4-dichlorophenoxyacetic acid precipitation quantity, utilize osmotic membrane to realize system water balance, without adding exogenous inorganic acid in whole process, from the root, high-salinity wastewater is generated, realize clean production.The method has the advantages of high product purity, resource recycling efficient, energy consumption economic, process easy control, etc., 2,4-dichlorophenoxyacetic acid purity can reach more than 99.5%, sodium hydroxide recovery rate is not less than 99.9%, current efficiency is not less than 85%, unit product energy consumption is controlled in 800-1200kWh / t, suitable for 2,4-dichlorophenoxyacetic acid industrialized clean production.

Description

Technical Field

[0001] This invention relates to the field of fine chemical product separation and recovery and bipolar membrane electrodialysis separation technology, specifically to a method for preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide. Background Technology

[0002] Since American scholar R. Poconi published the synthesis method of 2,4-dichlorophenoxyacetic acid (2,4-dichlorophenoxyacetic acid) in 1941, numerous synthetic methods have been publicly reported. Currently, the main industrial methods for producing 2,4-dichlorophenoxyacetic acid use phenol and chloroacetic acid as raw materials, which can be divided into two categories: one involves a chlorination reaction followed by a condensation reaction; the other involves a condensation reaction followed by a chlorination reaction.

[0003] Chlorination followed by condensation is a mature production method adopted by manufacturers in many countries. However, this method has several drawbacks. The chlorination process easily generates various chlorophenol byproducts, the condensation process produces large amounts of difficult-to-treat phenol-containing wastewater, and the production process can easily generate highly toxic dioxins, causing serious environmental pollution. Therefore, this production method is currently restricted. In contrast, the condensation followed by chlorination method generates relatively less wastewater, excess phenol can be recovered through solvent extraction, and the chlorination process has higher selectivity. Therefore, it is receiving increasing attention, and there are numerous reports on this synthetic method.

[0004] CN101066915A discloses a method for synthesizing 2,4-dichlorophenoxyacetic acid. This method uses phenol as a raw material, first undergoing a condensation reaction with chloroacetate under saturated brine conditions, followed by a chlorination reaction in an organic solvent.

[0005] CN101062893A discloses a method for preparing 2,4-dichlorophenoxyacetic acid. First, 90% of 2,4-dichlorophenol and chloroacetic acid are separately prepared into sodium salt solutions. Then, a condensation reaction is carried out in a condensation reactor. Simultaneously, an oil phase is continuously separated through an oil-water separation operation to complete the condensation process. Finally, under specific conditions, the 2,4-dichlorophenoxyacetic acid product is obtained.

[0006] CN101857544A discloses a method for synthesizing 2,4-dichlorophenoxyacetic acid. This method uses 2,4-dichlorophenol as a raw material, and reacts it with monochloroacetone via a Williamson condensation reaction in the presence of a weak base, sodium iodide, and a phase transfer catalyst to obtain 2,4-dichlorophenoxyacetone. Subsequently, a chloroform reaction is carried out in the presence of sodium hypochlorite solution, followed by acidification treatment to obtain the final 2,4-dichlorophenoxyacetic acid product. This method can control the free phenol content (0.3%) in the product to below the national standard (0.5%).

[0007] In addition, CN102180788A, CN102659571A, CN103058855A, CN103159610A, CN104402707A, CN103274925A, CN106167453A, CN108424362A, CN108503544A, CN108424347A, and CN108503536A have also disclosed different methods for preparing 2,4-dichlorophenoxyacetic acid. Although these methods have achieved relatively ideal results in improving product yield, reducing product phenol content, and reducing the amount of phenol-containing wastewater discharged, they still cannot fundamentally solve the problem of waste salt and wastewater generated by the reaction, and require the use of large amounts of alkali and acid, ultimately resulting in waste salt and wastewater.

[0008] The reason for this is that the condensation reaction involves chloroacetic acid reacting with phenol or 2,4-dichlorophenol in an alkaline solution. This requires alkali to produce sodium phenolate or sodium 2,4-dichlorophenolate, while chloroacetic acid also needs to be converted to sodium chloroacetate. The total amount of alkali required is more than twice the molar amount. Finally, more than one molar amount of acid is needed to adjust the pH, ultimately producing more than twice the molar amount of waste salt. Traditional chemical acidification methods also suffer from problems such as consuming chemicals (alkali and acid), generating wastewater, producing waste salt, and limiting product purity.

[0009] Therefore, there is an urgent need to develop a clean, efficient, and resource-recoverable method for preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide. Summary of the Invention

[0010] Given the shortcomings of existing 2,4-dichlorophenoxyacetic acid preparation technologies, such as the consumption of large amounts of acid and alkali, the generation of large amounts of wastewater and waste salt, and limited product purity, this invention provides a method for simultaneously preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide using a bipolar membrane electrodialysis and osmotic membrane combined technology, with an aqueous solution of 2,4-dichlorophenoxyacetic acid as the raw material. The prepared sodium hydroxide can be used to synthesize sodium 2,4-dichlorophenoxyacetic acid, thus achieving recycling.

[0011] The method provided by this invention solves the problem of decreased salt concentration in the later stages of simple bipolar membrane electrodialysis, which affects efficiency. Throughout the process, the water concentrations in the salt, acid, and alkali chambers are in equilibrium, eliminating the need to replenish water to the acid and alkali chambers. Water dissociates under an electric field through the bipolar membrane to generate H₂. + and OH - H + It enters the acid chamber and combines with the 2,4-dichlorophenoxyacetic acid ion to form 2,4-dichlorophenoxyacetic acid; Na + It enters the alkaline chamber through the cation exchange membrane and reacts with OH-. -Sodium hydroxide is generated. High-temperature operation in the acid chamber prevents membrane crystallization. After discharge, sodium salt is added to generate a common ion effect. Combined with cooling, the solubility is greatly reduced, allowing 2,4-dichlorophenoxyacetic acid to precipitate completely. Water in the salt chamber automatically migrates to the acid and alkali chambers through the permeation membrane, achieving a water balance that requires no external water replenishment and has no wastewater discharge throughout the process.

[0012] To achieve the above objectives, the present invention provides a method for preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide, wherein the method includes the following steps: (1) Assemble a three-chamber bipolar membrane electrodialysis device; The device contains a cation exchange membrane and an anion exchange membrane connected in series with a permeation membrane; the acid chamber outlet of the device is connected to a cooling system and a solid-liquid separation system. (2) Establish a cyclical system; Deionized water is pumped into the acid and alkali chambers to achieve a closed-loop circulation; sodium 2,4-dichlorophenoxyacetic acid aqueous solution is pumped into the salt chamber to achieve a closed-loop circulation between the two chambers (acid and alkali chambers). (3) Process monitoring and product separation; During operation, when the temperature of the material in the acid chamber rises to 50-60℃, sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution is added to the effluent from the acid chamber, and the temperature is lowered to promote the crystallization of 2,4-dichlorophenoxyacetic acid. After solid-liquid separation, the product is obtained by washing with water and drying. When the temperature of the acid chamber drops to 0-20℃, the system switches to circulation mode. The mother liquor from solid-liquid separation, the dried water, and the washing water are combined and returned to the salt chamber for recycling. Water is supplied to the acid and alkali chambers through a permeation membrane to maintain a stable liquid level.

[0013] The beneficial technical effects achieved by the present invention through the above technical solution are as follows: (1) Green and environmentally friendly: No foreign inorganic acids are added in the whole process, which avoids the generation of high-salt wastewater from the source. There is no wastewater discharge, which achieves clean production and meets the requirements of environmental protection policies.

[0014] (2) High efficiency in resource recycling: Sodium ions are simultaneously converted into sodium hydroxide solution with a high value, and the recovery rate is over 99.9%. It can be directly reused in the production of sodium 2,4-dichlorophenoxyacetic acid, realizing the closed-loop utilization of sodium resources.

[0015] (3) High-efficiency product production: The near-saturated 2,4-dichlorophenoxyacetic acid is crystallized out by utilizing the common ion effect and cooling. After filtration and separation, solid 2,4-dichlorophenoxyacetic acid is obtained. After washing with water and drying, a high-content 2,4-dichlorophenoxyacetic acid product is obtained.

[0016] (4) Simplified process: The concentration of sodium 2,4-dichlorophenoxyacetate in the salt chamber gradually decreases as bipolar membrane electrodialysis progresses. At this time, water will enter the acid and alkali chambers through the permeation membrane to maintain the water balance of the system. There is no need to add water to the acid and alkali chambers separately, but it is achieved through permeation of the permeation membrane.

[0017] (5) High product purity: The electrochemically driven process has strong selectivity, avoiding the inorganic acid radicals (such as SO42-) in the traditional acidification method. 2- Cl - The contamination of the product can be reduced by post-treatment, and the purity of 2,4-dichlorophenoxyacetic acid can reach over 99.5%. The 2,4-dichlorophenoxyacetic acid produced is washed with water, which reduces the content of sodium 2,4-dichlorophenoxyacetic acid in the product to less than 0.1%.

[0018] (6) The process is easy to control and stable: By optimizing key parameters such as current density, circulation flow rate and temperature, the reaction rate and product concentration can be flexibly controlled, which facilitates automated and continuous operation; at the same time, the long-term stable operation of the membrane stack is ensured by monitoring the conductivity of the salt chamber and replenishing the feed liquid.

[0019] (7) Energy consumption economy: The process parameters of the 2,4-dichlorophenoxyacetic acid sodium salt system were optimized, the current efficiency reached more than 85%, and the energy consumption per unit product was controlled at 800-1200kWh / t, which is within the economically reasonable range. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the bipolar membrane electrodialysis membrane stack unit structure used in an embodiment of the present invention.

[0021] In the diagram: 1-Sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution; 2-Alkali chamber circulation pump; 3-Pipeline mixer; 4-Cooling system; 5-Solid-liquid separation system; 6-Dried 2,4-dichlorophenoxyacetic acid; 7-Salt chamber circulation pump; 8-Acid chamber circulation pump; 9-Sodium hydroxide solution; 10-First bipolar membrane; 11-First osmotic membrane; 12-Second osmotic membrane; 13-Second bipolar membrane; 14-Cathode; 15-Cathode chamber; 16-Alkali chamber; 17-Cathode membrane; 18-Anion membrane; 19-Acid chamber; 20-Anode chamber; 21-Anode; 22-Washing water; 23-Drying equipment; 24-Condensate. Detailed Implementation

[0022] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] Sodium 2,4-dichlorophenoxyacetate has high solubility in water, increasing from 100 g / L to approximately 500 g / L within a temperature range of 20-50°C. The solubility of the resulting 2,4-dichlorophenoxyacetic acid increases from 3 g / L to approximately 25 g / L. When the generated 2,4-dichlorophenoxyacetic acid is supersaturated, it easily clogs the membrane. Furthermore, the resulting 2,4-dichlorophenoxyacetic acid mother liquor contains a small amount of sodium 2,4-dichlorophenoxyacetate, which needs to be recovered and reused to reduce raw material waste.

[0024] This invention employs a combined bipolar membrane electrodialysis and permeation membrane technique to prepare low-solubility organic acids, simplifying the operation process and achieving the following effects: A sodium 2,4-dichlorophenoxyacetic acid aqueous solution is input into the salt chamber, and a 2,4-dichlorophenoxyacetic acid solution is produced in the acid chamber. The mother liquor, after separation by common ion effect and cooling crystallization, is combined with the water used to wash the 2,4-dichlorophenoxyacetic acid and then enters the salt chamber. A sodium hydroxide solution is produced in the alkali chamber for the synthesis of sodium 2,4-dichlorophenoxyacetic acid. Water enters the acid and alkali chambers from the salt chamber via the permeation membrane. Anions enter the acid chamber through the anion membrane, and cations enter the alkali chamber through the cation membrane.

[0025] This invention uses a low concentration of sodium 2,4-dichlorophenoxyacetate and performs bipolar membrane electrodialysis and permeate membrane operations at higher temperatures to ensure that the generated 2,4-dichlorophenoxyacetic acid is in an unsaturated state at higher temperatures. Due to the common ion effect, the solubility of 2,4-dichlorophenoxyacetic acid in solution is limited by the concentration of sodium 2,4-dichlorophenoxyacetate in the solution. As the concentration of sodium 2,4-dichlorophenoxyacetate increases, the common ion effect leads to a decrease in the solubility of 2,4-dichlorophenoxyacetic acid. Therefore, by adding sodium 2,4-dichlorophenoxyacetate to the outside of the acid chamber system and using an external condenser to lower the temperature, the 2,4-dichlorophenoxyacetic acid is induced to reach a supersaturated state, thus crystallizing out as much of the 2,4-dichlorophenoxyacetic acid product as possible. The 2,4-dichlorophenoxyacetic acid product obtained after water washing has a higher quality.

[0026] The first aspect of the present invention provides a method for preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide, wherein the method includes the following steps: (1) Assemble a three-chamber bipolar membrane electrodialysis device; The device contains a cation exchange membrane and an anion exchange membrane connected in series with a permeation membrane; the acid chamber outlet of the device is connected to a cooling system and a solid-liquid separation system. (2) Establish a cyclical system; Deionized water is pumped into the acid and alkali chambers to achieve a closed loop; sodium 2,4-dichlorophenoxyacetic acid aqueous solution is pumped into the salt chamber to achieve a dual-chamber linkage closed loop. (3) Process monitoring and product separation; During operation, when the temperature of the material in the acid chamber rises to 50-60℃, sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution is added to the effluent from the acid chamber, and the temperature is lowered to promote the crystallization of 2,4-dichlorophenoxyacetic acid. After solid-liquid separation, the product is obtained by washing with water and drying. When the temperature of the acid chamber drops to 0-20℃, the system switches to circulation mode. The mother liquor from solid-liquid separation, the dried water, and the washing water are combined and returned to the salt chamber for recycling. Water is supplied to the acid and alkali chambers through a permeation membrane to maintain a stable liquid level.

[0027] The working principle of this invention can be summarized as follows: Driven by a DC electric field, water molecules in the bipolar film interface layer dissociate, generating H+. + and OH - H + It enters the acid chamber and reacts with R-COO in sodium 2,4-dichlorophenoxyacetate (R-COONa). - This combination produces free 2,4-dichlorophenoxyacetic acid (R-COOH). Since 2,4-dichlorophenoxyacetic acid has extremely low solubility in water, it will precipitate from the solution when its concentration exceeds its solubility. Simultaneously, Na... + Under the influence of an electric field, it migrates through the cation exchange membrane to the alkaline chamber, where it reacts with OH- generated by the bipolar membrane. - NaOH is generated through ion transfer. As ions transfer, the salt concentration in the salt chamber gradually decreases, while water molecules transfer through the permeation membrane to the acid and alkali chambers, respectively. The electrolytic reaction in the electrode chambers is balanced by a circulating electrolyte solution. The entire process requires no addition of exogenous inorganic acids, enabling simultaneous clean conversion and resource recovery.

[0028] The bipolar membrane electrodialysis device assembled in this invention includes at least an anode, a cathode, a membrane stack, a permeate membrane, and supporting circulation piping, a circulation pump, a DC power supply, a temperature monitoring unit, a cooling and filtration unit (cooling system + solid-liquid separation system), and a control unit (maintaining system water balance). The membrane stack is defined by the anode and cathode and contains several repeating unit pairs. Each unit pair is assembled sequentially in the order of "cathode → first bipolar membrane → cation membrane → anion membrane → second bipolar membrane → anode". The anion exchange layer of the bipolar membrane faces the alkaline chamber, and the cation exchange layer faces the acidic chamber, thereby ensuring H₂O₂ concentration. + OH -Directional migration is achieved. An anode chamber is formed between the anode and the adjacent cation exchange membrane, and a cathode chamber is formed between the cathode and the adjacent cation exchange membrane. Each chamber partition has pre-installed inlet and outlet ports compatible with the circulation pipeline. The bipolar membrane and the permeate membrane are installed together in series. The bipolar membrane is used for ion exchange, and the permeate membrane is used to facilitate water flow, thereby achieving water balance. The circulation pumps for the acid and alkali chambers each have two outlets: one for circulation and the other for discharge. After discharge from the acid chamber circulation pump, sodium 2,4-dichlorophenoxyacetate (solid salt or aqueous solution) is added, then cooled, filtered, washed, and dried before being returned to the salt chamber. The outlet of the alkali chamber removes alkali of a certain concentration from the system for the synthesis of sodium 2,4-dichlorophenoxyacetate.

[0029] After establishing the circulation system, this invention utilizes bipolar membrane electrodialysis technology to prepare 2,4-dichlorophenoxyacetic acid and sodium hydroxide from sodium 2,4-dichlorophenoxyacetic acid. Water is transferred from the salt chamber to the acid and alkali chambers via a permeation membrane. The resulting solid 2,4-dichlorophenoxyacetic acid is washed with water, and the washing water, drying recovery water, and separation mother liquor are combined and recycled back into the salt chamber. This achieves the goal of removing 2,4-dichlorophenoxyacetic acid from the system and allowing new sodium 2,4-dichlorophenoxyacetic acid to enter. Furthermore, the resulting 2,4-dichlorophenoxyacetic acid product, after being washed with water, has a higher quality.

[0030] In this invention, when the acid chamber temperature rises to 60°C (more preferably 50°C), the outlet of the acid chamber circulation pump is immediately switched to the outside of the system, allowing the acid chamber to discharge. Subsequently, the common ion effect and cooling are used to induce the near-saturated 2,4-dichlorophenoxyacetic acid to crystallize out. After solid-liquid separation, a high-content solid acid is obtained, which is then washed with water and dried to obtain a high-content 2,4-dichlorophenoxyacetic acid product. The closed-loop process of "naturally heating to 60°C (more preferably 50°C) to start the crystallization system → adding sodium 2,4-dichlorophenoxyacetic acid → cooling to 0°C (more preferably 20°C) for solid-liquid separation → a new round of electrodialysis" is repeated.

[0031] Alternatively, when the concentration of sodium 2,4-dichlorophenoxyacetate in the salt chamber approaches 50 g / L, the nearly saturated 2,4-dichlorophenoxyacetic acid solution in the acid chamber can be transferred out, and a high concentration of sodium 2,4-dichlorophenoxyacetate solution can be added. The crystallization and washing operations can be repeated, and the mother liquor and washing liquid can be returned to the salt chamber for recycling.

[0032] In some embodiments of the present invention, in step (1), the membrane stack of the three-chamber bipolar membrane electrodialysis device adopts a homogeneous cation exchange membrane and a homogeneous bipolar membrane.

[0033] In some embodiments of the present invention, the effective membrane area of ​​the membrane stack in the three-chamber bipolar membrane electrodialysis device is 0.1-0.5 m². 2 Preferably 0.3m2 .

[0034] In some embodiments of the present invention, the thickness of the permeation membrane is 0.10-0.15 mm, preferably 0.12 mm.

[0035] In some embodiments of the present invention, the three-chamber bipolar membrane electrodialysis device includes an anode, a cathode, and a membrane stack disposed between the two stages; the membrane stack includes a first bipolar membrane, a cation membrane, an anion membrane, and a second bipolar membrane, wherein an alkali chamber is formed between the first bipolar membrane and the cation membrane, a salt chamber is formed between the cation membrane and the anion membrane, and an acid chamber is formed between the anion membrane and the second bipolar membrane; the cation membrane is connected in series with the first permeation membrane, and the anion membrane is connected in series with the second permeation membrane; the outlet of the acid chamber is connected to a cooling system and a solid-liquid separation system.

[0036] In some embodiments of the present invention, in step (2), the aqueous solution of sodium 2,4-dichlorophenoxyacetate is obtained by one of the following methods: (1) Sodium 2,4-dichlorophenoxyacetate was directly dissolved in water to obtain an aqueous solution of sodium 2,4-dichlorophenoxyacetate; (2) Add the solid salt or aqueous solution of sodium 2,4-dichlorophenoxyacetate to the nearly saturated 2,4-dichlorophenoxyacetic acid solution (i.e., the acid chamber discharge liquid) flowing out from the acid chamber. After cooling, solid-liquid separation and washing, combine the solid-liquid separation mother liquor and washing water to obtain an aqueous solution of sodium 2,4-dichlorophenoxyacetate.

[0037] In this invention, when the system is first started, the first method is needed to supply the raw material solution to the salt chamber; after the system is running stably and can produce acid products, the second method can be used to carry out a new round of electrodialysis to achieve recycling.

[0038] In some embodiments of the present invention, the concentration of the sodium 2,4-dichlorophenoxyacetic acid aqueous solution is 50-300 g / L, preferably 100-250 g / L, more preferably 100-150 g / L, and most preferably 120 g / L. In this invention, if the concentration is too low, the electrodialysis efficiency will be reduced; if the concentration is too high, the 2,4-dichlorophenoxyacetic acid produced in the acid chamber will clog the membrane, and the 2,4-dichlorophenoxyacetic acid produced during cooling and separation will also have this effect.

[0039] In some embodiments of the present invention, an aqueous solution of sodium 2,4-dichlorophenoxyacetate is pretreated by filtration through a filter membrane, wherein the pore size of the filter membrane is preferably 0.22-1.0 μm. This filtration operation can remove mechanical impurities and trace amounts of insoluble particles from the raw material, prevent clogging of the membrane module flow channels and contamination of the membrane surface, and ensure the stability of the membrane stack operation.

[0040] In some embodiments of the present invention, in step (2), the circulation flow rate of the acid chamber and the alkali chamber is 10-30 L / h, preferably 18-22 L / h, and more preferably 20 L / h, to provide a basic system for the generation of 2,4-dichlorophenoxyacetic acid and sodium hydroxide.

[0041] In some embodiments of the present invention, the circulation flow rate of the salt chamber is 8-25 L / h, preferably 15-18 L / h, and more preferably 16 L / h, to balance the electrode reaction ions and avoid electrode polarization. A small flow rate will cause uneven system concentration, while a large flow rate will waste electricity and be uneconomical.

[0042] In some embodiments of the present invention, in step (3), a direct current is applied between the anode and the cathode, and the circuit is powered on in either a constant current density mode or a constant voltage mode, with the constant current density mode being preferred.

[0043] In some embodiments of the present invention, the constant current density mode is 50-800 A / m. 2 Preferably 200-350A / m 2 Further preferred is 280A / m 2 Too low a current density will affect operating efficiency, while too high a current density will easily lead to reduced utilization.

[0044] In some embodiments of the present invention, under the constant voltage mode, the membrane stack voltage is controlled at 10-50V, preferably 15-40V, to avoid membrane damage due to excessive voltage.

[0045] In some embodiments of the present invention, in step (3), the temperature of the acid chamber material is controlled at 0-60℃, preferably 10-50℃, and more preferably 20-50℃. In the present invention, a temperature monitoring unit is activated to track the temperature of the acid chamber material in real time. In the initial stage, the system temperature naturally rises by utilizing the heat generated by the electrodialysis process itself (Joule heating, exothermic electrode reaction), maintaining the temperature of the acid chamber material in the range of 0-60℃ throughout the process, with a preferred range of 10-50℃ and an optimal range of 20-50℃. Too low a temperature requires high external cooling, which is not economical; too high a temperature will affect the service life of the membrane.

[0046] In some embodiments of the present invention, during the energized operation, when the sodium hydroxide concentration in the alkali chamber reaches 50-200 g / L, preferably 80-150 g / L, and more preferably 120 g / L, part of the sodium hydroxide solution produced in the alkali chamber is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetate, and the remainder continues to be recycled; if the alkali concentration is too low, it will affect subsequent recycling, and if it is too high, it will affect the electrodialysis effect.

[0047] In some embodiments of the present invention, during energized operation, the conductivity of the salt chamber is controlled at 100-500 μS / cm, preferably 150-250 μS / cm. This operation ensures full utilization of electrical energy and improves efficiency.

[0048] In some embodiments of the present invention, in step (3), the total concentration of the sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution added to the acid chamber discharge liquid, after mixing with the solid-liquid separation mother liquor, drying recovery water, and washing water, is equal to the molar concentration of the produced sodium hydroxide.

[0049] In some embodiments of the present invention, the cooling medium of the cooling system is low-temperature deionized water, and the temperature control accuracy is ±1℃.

[0050] In some embodiments of the present invention, the separation method of the solid-liquid separation system is selected from at least one of centrifugal separation, pressure filtration and sedimentation separation.

[0051] In some embodiments of the present invention, the separation pressure is 0.1-0.3 MPa and the separation time is 10-30 min. If the pressure is too low, the separation speed will be slow; if the pressure is too high, it will affect the service life of the equipment.

[0052] In some embodiments of the present invention, the amount of washing water used is equal to the amount of water carried out from the alkali chamber, thereby achieving a balance in the entire process.

[0053] This invention maximizes the yield of 2,4-dichlorophenoxyacetic acid by leveraging the combined effects of the common ion effect and temperature difference. It utilizes bipolar membrane electrodialysis to prepare 2,4-dichlorophenoxyacetic acid and sodium hydroxide from sodium 2,4-dichlorophenoxyacetic acid. A permeation membrane is used to transfer water from the salt chamber to the acid and alkali chambers, achieving water transfer. The resulting solid 2,4-dichlorophenoxyacetic acid is washed with water; the wash water and separation mother liquor are combined and recycled back into the salt chamber. This process removes 2,4-dichlorophenoxyacetic acid from the system, allowing new sodium 2,4-dichlorophenoxyacetic acid to enter. Simultaneously, the resulting 2,4-dichlorophenoxyacetic acid product, after washing with water, is of higher quality. The supplementary wash water precisely matches the amount of water carried out from the alkali chamber, achieving a balanced process. The entire process involves no material loss, thus achieving multiple goals: clean production, resource recycling, and improved product purity.

[0054] The present invention will be described in detail below through embodiments.

[0055] Unless otherwise specified in the following examples and comparative examples, all conditions were performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available products.

[0056] The following examples all use Figure 1The bipolar membrane electrodialysis membrane stack unit structure shown is implemented.

[0057] Figure 1 The bipolar membrane electrodialysis unit shown is a three-chamber structure, including: 1-2,4-dichlorophenoxyacetic acid sodium solid salt or aqueous solution; 2-alkali chamber circulation pump; 3-pipeline mixer; 4-cooling system; 5-solid-liquid separation system; 6-2,4-dichlorophenoxyacetic acid dry product; 7-salt chamber circulation pump; 8-acid chamber circulation pump; 9-sodium hydroxide solution discharge; 10-first bipolar membrane; 11-first permeation membrane; 12-second permeation membrane; 13-second bipolar membrane; 14-cathode; 15-cathode chamber; 16-alkali chamber; 17-cation membrane; 18-anion membrane; 19-acid chamber; 20-anode chamber; 21-anode; 22-wash water; 23-drying equipment; 24-condensate; and a control unit, etc. The first bipolar membrane 10 separates the cathode chamber 15 and the alkali chamber 16; the second bipolar membrane 13 separates the acid chamber 19 and the anode chamber 20; and the salt chamber is located between the cation membrane 17 and the anion membrane 18.

[0058] The function of the pipeline mixer 3 is to mix the solid salt or aqueous solution of sodium 2,4-dichlorophenoxyacetate 1 with the output liquid from the acid chamber; after the 2,4-dichlorophenoxyacetic acid crystallizes out, it is separated into solid and liquid, washed with water 22, and dried by the drying equipment 23 to obtain the dried product 6 of 2,4-dichlorophenoxyacetic acid; the sodium hydroxide solution 9 produced in the alkali chamber is used to synthesize sodium 2,4-dichlorophenoxyacetate.

[0059] Example 1 This example illustrates how to achieve efficient preparation and recycling of 2,4-dichlorophenoxyacetic acid and sodium hydroxide by selecting optimal process parameters.

[0060] (1) Preparation of raw material solution: During the first run, dissolve sodium 2,4-dichlorophenoxyacetic acid in deionized water and stir continuously until completely dissolved to prepare a raw material solution with a concentration of 120 g / L. Pre-treat by filtration using a 0.45 μm filter membrane. After the operation is stable, dissolve sodium 2,4-dichlorophenoxyacetic acid in the nearly saturated 2,4-dichlorophenoxyacetic acid solution flowing out of the acid chamber, combine the mother liquor separated by crystallization with the washing water, and prepare the raw material solution. (2) Assemble a three-chamber bipolar membrane electrodialysis device: the effective membrane area of ​​the membrane stack is 0.3 m². 2 The thickness of the permeation membrane is 0.12 mm.

[0061] (3) Establishing a circulation system: During the initial circulation, deionized water is pumped into the acid chamber and the alkali chamber, using a closed circulation mode, with the circulation flow rate controlled at 20 L / h. The prepared 120 g / L sodium 2,4-dichlorophenoxyacetate solution is pumped into the salt chamber, with the circulation flow rate controlled at 16 L / h, achieving a closed-loop circulation between the two chambers. During normal circulation, the 2,4-dichlorophenoxyacetic acid produced in the acid chamber undergoes ion exchange, cooling, crystallization, filtration, and water washing. The mother liquor and washing water are then combined and returned to the salt chamber. Part of the sodium hydroxide solution produced in the alkali chamber is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetate, while the remaining part continues to circulate. Water is replenished through a permeation membrane to maintain a stable liquid level.

[0062] (4) Power-on operation: A constant current density mode is adopted, with the current density set at 280A / m². The temperature of the acid chamber material is maintained at 20-50℃ throughout the process. Natural heating is achieved by utilizing the Joule heat generated by electrodialysis itself and the exothermic reaction of the electrodes, without the need for additional heating equipment. The conductivity of the salt chamber is monitored in real time and controlled between 150-250μS / cm.

[0063] (5) Process monitoring and product separation: When the temperature of the material in the acid chamber rises to 50℃, the outlet of the acid chamber circulation pump is switched to the outside of the system, and a higher concentration of sodium 2,4-dichlorophenoxyacetic acid aqueous solution is added (the total concentration after addition and washing water is equal to the molar concentration of the produced sodium hydroxide). The solution is then introduced into the cooling system (the cooling medium is low-temperature deionized water, with a temperature control accuracy of ±1℃). Centrifugal separation is used (separation pressure 0.2MPa, separation time 20min) to promote the crystallization of 2,4-dichlorophenoxyacetic acid. After filtration and separation, the solution is washed with deionized water, and the amount of washing water used is equal to the amount of water carried out from the alkali chamber. When the temperature of the acid chamber drops to 20℃, the outlet is switched to the circulation state, the heat exchanger is turned off, and the closed-loop process of "heating-crystallization-separation-circulation" is repeated. The concentration of sodium hydroxide in the alkali chamber is monitored simultaneously. When the concentration reaches 120g / L, part of the solution is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetic acid salt, and the remaining alkali solution continues to circulate.

[0064] Results analysis: After 8 hours of continuous operation, a total of 120 L of sodium 2,4-dichlorophenoxyacetic acid feedstock solution was processed, yielding 11.2 kg of 2,4-dichlorophenoxyacetic acid product with a purity of 99.7%, of which the residual sodium 2,4-dichlorophenoxyacetic acid was 0.08%. Simultaneously, 89 L of sodium hydroxide solution (concentration 120 g / L) was recovered, with a sodium hydroxide recovery rate of 99.92%, a current efficiency of 88.3%, and an energy consumption of 950 kWh / t per unit product. The entire process involved no wastewater discharge, and the materials in the salt chamber were recycled in a closed loop, resulting in no raw material waste.

[0065] Example 2 This embodiment illustrates the efficient preparation and recycling of 2,4-dichlorophenoxyacetic acid and sodium hydroxide by selecting optimal process parameters, and verifies the stability and applicability of the method.

[0066] (1) Preparation of raw material solution: During the first run, dissolve sodium 2,4-dichlorophenoxyacetic acid in deionized water and stir continuously until completely dissolved to prepare a raw material solution with a concentration of 200 g / L. Pre-treat by filtration using a 0.22 μm filter membrane. After the operation is stable, dissolve sodium 2,4-dichlorophenoxyacetic acid in the nearly saturated 2,4-dichlorophenoxyacetic acid solution flowing out of the acid chamber, combine the mother liquor separated by crystallization with the washing water, and prepare the raw material solution. (2) Assemble a three-chamber bipolar membrane electrodialysis device: the effective membrane area of ​​the membrane stack is 0.2 m². 2 The thickness of the permeation membrane is 0.10 mm.

[0067] (3) Establishing a circulation system: During the first circulation, the circulation flow rate of both the acid chamber and the alkali chamber is controlled at 18L / h, and the circulation flow rate of the salt chamber is controlled at 15L / h; the normal circulation process is the same as in Example 1.

[0068] (4) Power-on operation: The constant current density mode is adopted, the current density is set to 350A / m², the acid chamber material temperature is maintained at 15-45℃, and the salt chamber conductivity is monitored in real time and controlled between 180-250μS / cm.

[0069] (5) Process monitoring and product separation: When the temperature of the material in the acid chamber rises to 45°C, a high-concentration aqueous solution of sodium 2,4-dichlorophenoxyacetate is added, and solid-liquid separation is carried out by pressure filtration (separation pressure 0.3 MPa, separation time 15 min). After washing with water, the 2,4-dichlorophenoxyacetic acid product is obtained. When the sodium hydroxide concentration in the alkali chamber reaches 100 g / L, part of it is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetate. Other operations are the same as in Example 1.

[0070] Results analysis: After 10 hours of continuous operation, a total of 150 L of sodium 2,4-dichlorophenoxyacetic acid feedstock solution was processed, yielding 17.8 kg of 2,4-dichlorophenoxyacetic acid product with a purity of 99.6%, of which the residual sodium 2,4-dichlorophenoxyacetic acid was 0.09%. Simultaneously, 132 L of sodium hydroxide solution (concentration 100 g / L) was recovered, with a sodium hydroxide recovery rate of 99.91%, a current efficiency of 86.7%, and an energy consumption of 1080 kWh / t per unit product. The process operated stably, with no membrane clogging and smooth material circulation.

[0071] Example 3 This embodiment illustrates the efficient preparation and recycling of 2,4-dichlorophenoxyacetic acid and sodium hydroxide by selecting basic process parameters, and verifies the feasibility of the method.

[0072] (1) Preparation of raw material solution: Dissolve sodium 2,4-dichlorophenoxyacetate in deionized water and stir continuously until completely dissolved to prepare a raw material solution with a concentration of 80 g / L. Pre-treatment is carried out by filtration through a 1.0 μm filter membrane.

[0073] (2) Assemble the bipolar membrane electrodialysis device: the effective membrane area of ​​the membrane stack is 0.4 m². 2 The thickness of the permeation membrane is 0.15 mm.

[0074] (3) Establishing a circulation system: During the first circulation, the circulation flow rate of both the acid chamber and the alkali chamber is controlled at 25 L / h, and the circulation flow rate of the salt chamber is controlled at 18 L / h; the normal circulation process is the same as in Example 1.

[0075] (4) Power-on operation: The constant voltage mode is adopted, the membrane stack voltage is controlled at 30V, the acid chamber material temperature is maintained at 10-50℃, and the salt chamber conductivity is monitored in real time and controlled between 200-500μS / cm.

[0076] (5) Process monitoring and product separation: When the temperature of the material in the acid chamber rises to 50°C, a high-concentration aqueous solution of sodium 2,4-dichlorophenoxyacetic acid is added, and solid-liquid separation is carried out by sedimentation (separation pressure 0.1 MPa, separation time 30 min). After washing with water, the 2,4-dichlorophenoxyacetic acid product is obtained. When the sodium hydroxide concentration in the alkali chamber reaches 80 g / L, part of it is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetic acid. Other operations are the same as in Example 1.

[0077] Results analysis: After 12 hours of continuous operation, a total of 180 L of sodium 2,4-dichlorophenoxyacetic acid feed solution was processed, yielding 13.1 kg of 2,4-dichlorophenoxyacetic acid product with a purity of 99.5%, of which 0.10% sodium 2,4-dichlorophenoxyacetic acid residue was present. Simultaneously, 165 L of sodium hydroxide solution (concentration 80 g / L) was recovered, with a sodium hydroxide recovery rate of 99.90%, a current efficiency of 85.2%, and an energy consumption of 1150 kWh / t per unit product. The process generated no wastewater discharge, the membrane module operated stably, and there was no significant pollution.

[0078] Comparative Example 1 This comparative example uses a traditional chemical acidification method to prepare 2,4-dichlorophenoxyacetic acid.

[0079] 120 L of sodium 2,4-dichlorophenoxyacetic acid feed solution of the same concentration (120 g / L) as in Example 1 was placed in a reaction vessel. 98% concentrated sulfuric acid (1.2 times the molar amount of sodium 2,4-dichlorophenoxyacetic acid) was slowly added dropwise under stirring at a rate of 5 L / h. The system temperature was maintained at 25-30 °C during the addition. After the addition was complete, the reaction was continued with stirring for 2 h to promote the crystallization of 2,4-dichlorophenoxyacetic acid. Solid 2,4-dichlorophenoxyacetic acid was obtained by centrifugation (separation pressure 0.2 MPa, separation time 20 min), washed with deionized water until neutral, and dried to obtain the product. The sodium sulfate-containing wastewater generated during the reaction was neutralized before discharge.

[0080] Results analysis: 10.5 kg of 2,4-dichlorophenoxyacetic acid was obtained with a purity of 98.2%, containing 0.35% sulfate impurities and 0.21% sodium 2,4-dichlorophenoxyacetic acid residue. The reaction process consumed 8.7 kg of concentrated sulfuric acid and generated 135 L of wastewater containing sodium sulfate (COD 1860 mg / L, salt concentration 105 g / L), with no sodium hydroxide recovery. The product yield was 6.2% lower than in Example 1, and additional wastewater treatment costs were required. The overall energy consumption per unit product was 1520 kWh / t. This method has significant problems such as high acid and alkali consumption, large wastewater discharge, low product purity, and serious resource waste, and does not meet the requirements of clean production.

[0081] Comparative Example 2 This comparative example uses bipolar membrane electrodialysis, but the process parameters deviate from the limits defined in this invention, verifying the importance of parameter optimization.

[0082] The feed solution was prepared in the same manner as in Example 1 (concentration 120 g / L), and the apparatus was assembled in the same manner as in Example 1, but without the installation of a permeation membrane. In the circulation system, the circulation flow rate of the acid and alkali chambers was controlled at 8 L / h, and the circulation flow rate of the salt chamber was controlled at 30 L / h. During operation, the current density was set to 900 A / m², and the material temperature in the acid chamber was maintained at 65°C. During product separation, no 2,4-dichlorophenoxyacetic acid sodium salt solution was added; the temperature was directly lowered to 10°C for crystallization. Other operations were the same as in Example 1.

[0083] Results analysis: After 3 hours of operation, the membrane module showed significant blockage and was forced to stop. A total of 45 L of 2,4-dichlorophenoxyacetic acid sodium salt feed solution was processed, yielding 3.8 kg of 2,4-dichlorophenoxyacetic acid product with a purity of 98.5% and a residual sodium 2,4-dichlorophenoxyacetic acid content of 0.18%. 28 L of sodium hydroxide solution (concentration 65 g / L) was recovered, with a sodium hydroxide recovery rate of 97.2%. The current efficiency was only 68.5%, and the energy consumption per unit product was 1890 kWh / t. Due to the absence of a permeate membrane, the system water could not reach equilibrium, the acid chamber level continuously decreased, and the alkali chamber level increased. Excessive current density and temperature caused membrane damage, unreasonable circulation flow caused uneven system concentration, and the lack of replenishment of 2,4-dichlorophenoxyacetic acid sodium salt solution resulted in insufficient common ion effect and low product precipitation. Overall, the performance was far superior to that of the embodiment of the present invention, verifying the necessity of optimizing the process parameters of the present invention.

[0084] The comparison between the above embodiments and comparative examples shows that, compared with the traditional chemical acidification method, the method of the present invention has significant advantages such as high product purity (above 99.5% vs. 98.2%), no wastewater discharge, recovery of high-value sodium hydroxide (recovery rate above 99.9%), lower energy consumption (800-1200 kWh / t vs. 1520 kWh / t), and no raw material waste. Compared with the bipolar membrane electrodialysis method that deviates from the process parameters of the present invention, the present invention effectively avoids membrane blockage, improves current efficiency, reduces energy consumption, and ensures stable process operation by optimizing membrane module configuration, circulation flow rate, current density, temperature, and crystallization process, resulting in a significant improvement in product quality and recovery efficiency.

[0085] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing 2,4-dichlorophenoxyacetic acid and sodium hydroxide, characterized in that, The method includes the following steps: (1) Assemble a three-chamber bipolar membrane electrodialysis device; The device contains a cation exchange membrane and an anion exchange membrane connected in series with a permeation membrane; the acid chamber outlet of the device is connected to a cooling system and a solid-liquid separation system. (2) Establish a cyclical system; Deionized water is pumped into the acid and alkali chambers to achieve a closed loop; sodium 2,4-dichlorophenoxyacetic acid aqueous solution is pumped into the salt chamber to achieve a dual-chamber linkage closed loop. (3) Process monitoring and product separation; During operation, when the temperature of the material in the acid chamber rises to 50-60℃, sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution is added to the effluent from the acid chamber, and the temperature is lowered to promote the crystallization of 2,4-dichlorophenoxyacetic acid. After solid-liquid separation, the product is obtained by washing with water and drying. When the temperature of the acid chamber drops to 0-20℃, the system switches to circulation mode. The mother liquor from solid-liquid separation, the dried water, and the washing water are combined and returned to the salt chamber for recycling. Water is supplied to the acid and alkali chambers through a permeation membrane to maintain a stable liquid level.

2. The method according to claim 1, wherein, In step (1), the membrane stack of the three-chamber bipolar membrane electrodialysis device adopts a homogeneous cation exchange membrane and a homogeneous bipolar membrane; Preferably, the effective membrane area of ​​the membrane stack in the three-chamber bipolar membrane electrodialysis device is 0.1-0.5 m². 2 Preferably 0.3m 2 ; Preferably, the thickness of the permeation membrane is 0.10-0.15 mm, and more preferably 0.12 mm; Preferably, the three-chamber bipolar membrane electrodialysis device includes an anode, a cathode, and a membrane stack disposed between the two stages; the membrane stack includes a first bipolar membrane, a cation membrane, an anion membrane, and a second bipolar membrane, wherein an alkali chamber is formed between the first bipolar membrane and the cation membrane, a salt chamber is formed between the cation membrane and the anion membrane, and an acid chamber is formed between the anion membrane and the second bipolar membrane; the cation membrane is connected in series with the first permeation membrane, and the anion membrane is connected in series with the second permeation membrane; the outlet of the acid chamber is connected to a cooling system and a solid-liquid separation system.

3. The method according to claim 1 or 2, wherein, In step (2), the aqueous solution of sodium 2,4-dichlorophenoxyacetate is obtained by one of the following methods: (1) Dissolve the solid salt of sodium 2,4-dichlorophenoxyacetate directly in water to obtain an aqueous solution of sodium 2,4-dichlorophenoxyacetate; (2) Add the solid salt or aqueous solution of sodium 2,4-dichlorophenoxyacetate to the nearly saturated 2,4-dichlorophenoxyacetic acid solution flowing out of the acid chamber. After cooling, solid-liquid separation, water washing and drying, the 2,4-dichlorophenoxyacetic acid product is obtained. Combine the solid-liquid separation mother liquor, the drying recovery water and the washing water to obtain the aqueous solution of sodium 2,4-dichlorophenoxyacetate.

4. The method according to any one of claims 1-3, wherein, The concentration of the 2,4-dichlorophenoxyacetic acid sodium aqueous solution is 50-300 g / L, preferably 100-250 g / L, more preferably 100-150 g / L, and most preferably 120 g / L; Preferably, the aqueous solution of sodium 2,4-dichlorophenoxyacetate is pretreated by filtration through a filter membrane, wherein the pore size of the filter membrane is preferably 0.22-1.0 μm.

5. The method according to any one of claims 1-4, wherein, In step (2), the circulation flow rate of the acid chamber and the alkali chamber is 10-30 L / h, preferably 18-22 L / h, and more preferably 20 L / h; Preferably, the circulation flow rate of the salt chamber is 8-25 L / h, more preferably 15-18 L / h, and even more preferably 16 L / h.

6. The method according to any one of claims 1-5, wherein, In step (3), the device is powered on and operated in constant current density mode or constant voltage mode; Preferably, in the constant current density mode, the constant current density is 50-800 A / m. 2 Preferably 200-350A / m 2 Further preferred is 280A / m 2 ; Preferably, in the constant voltage mode, the membrane stack voltage is controlled at 10-50V, more preferably 15-40V.

7. The method according to any one of claims 1-6, wherein, In step (3), the temperature of the acid chamber material is controlled at 0-60℃, preferably 10-50℃, and more preferably 20-50℃; Preferably, during the energized operation, when the sodium hydroxide concentration in the alkali chamber reaches 50-200 g / L, more preferably 80-150 g / L, and even more preferably 120 g / L, part of the sodium hydroxide solution produced in the alkali chamber is collected and reused for the synthesis of sodium 2,4-dichlorophenoxyacetate, and the remainder continues to be recycled. Preferably, during energized operation, the conductivity of the salt chamber is controlled at 100-500 μS / cm, and more preferably 150-250 μS / cm.

8. The method according to any one of claims 1-7, wherein, In step (3), the total concentration of the sodium 2,4-dichlorophenoxyacetic acid solid salt or aqueous solution added to the acid chamber discharge liquid, after mixing with the solid-liquid separation mother liquor, drying recovery water, and washing water, is equal to the molar concentration of the produced sodium hydroxide.

9. The method according to any one of claims 1-8, wherein, The cooling medium of the cooling system is low-temperature deionized water, and the temperature control accuracy is ±1℃. Preferably, the separation method of the solid-liquid separation system is selected from at least one of centrifugal separation, pressure filtration and sedimentation separation; Preferably, the separation pressure is 0.1-0.3 MPa and the separation time is 10-30 min; Preferably, the amount of washing water used is equal to the amount of water carried out from the alkali chamber.

Images