Method and system for repeated recycling of aromatic solvents

Abstract

The present application relates to fine chemical clean production, organic solvent resource recycling, industrial hazardous waste reduction and emission reduction technical field, especially relates to a kind of aromatic hydrocarbon solvent repeated recycling method and system.A kind of aromatic hydrocarbon solvent repeated recycling method, comprising the following steps: S1, after UV-P diazotization-coupling alkaline reduction reaction is completed, the mixed organic phase of xylene and / or toluene enriched with azo colored impurities is separated out;S2, sulfur dioxide is added to the organic phase as decolorizing reducing agent, and stirring reaction is carried out under strong alkaline condition and 55-65 DEG C temperature, so that azo conjugated N=N double bond is broken and degraded into water-soluble aromatic amine;S3, after decolorization is completed, suspended solids are filtered out, and the regenerated solvent obtained is directly conveyed to the reduction reaction kettle for next batch production without distillation or rectification purification;S4, S1-S3 are executed in a cycle, and fresh aromatic hydrocarbon is supplemented to solvent storage tank liquid level due to volatilization loss.The regenerated solvent treated by the method of the present application can be directly reused without distillation or rectification.

Description

Technical Field

[0001] This invention relates to the fields of clean production of fine chemicals, resource recycling of organic solvents, and reduction and emission reduction of industrial hazardous waste, and in particular to a method and system for the repeated recycling of aromatic solvents. Background Technology

[0002] The industrial synthesis route of benzotriazole ultraviolet absorbers (such as UV-P) typically includes three core processes: diazotization, coupling, and alkaline reduction. The coupling process inevitably generates aromatic azo intermediates containing N=N conjugated double bonds. These substances are highly soluble in aromatic solvents such as toluene and xylene, and enter the subsequent system along with the solvent from the reduction stage. If the used solvent is not decolorized and purified and is directly reused, azo impurities will continue to accumulate, causing the solvent to gradually change from colorless and transparent to yellow and orange-red. This directly results in a decrease in the 440nm and 500nm transmittance of the finished UV-P product, excessive colorimetry, and the product failing to meet industry shipping standards.

[0003] Currently, there are two main approaches to treating such contaminated solvents in industrial production: The first is physical adsorption, which uses activated carbon, activated clay, or diatomaceous earth to adsorb colored impurities. However, these materials rely solely on intermolecular forces to capture colored molecules and cannot break the N=N conjugated chemical bonds of the azo chromophores. The adsorption active sites typically reach saturation limits after only 1-3 batches, and each batch produces a large amount of hazardous filter residue containing aromatics. The loss rate of aromatic materials due to the filtration process exceeds 5%, and subsequent deep regeneration via distillation is necessary. Distillation consumes a large amount of steam and generates high-concentration hazardous organic waste. The second approach is direct distillation, which requires shutdown for solvent regeneration every 2-3 batches. This method involves high equipment investment, high energy consumption, and high disposal costs, and the residue at the bottom of the distillation vessel is also a high-concentration hazardous organic waste.

[0004] Therefore, those skilled in the art are dedicated to developing a method and system for the repeated recycling of aromatic solvents that can irreversibly eliminate azo colorimetric impurities at the chemical bond level, have a wide operating window, and can be stably recycled. Summary of the Invention

[0005] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is to provide a method and system for the repeated recycling of aromatic solvents.

[0006] To achieve the above objectives, the present invention provides a method for repeated recycling of aromatic solvents, comprising the following steps: S1, after the alkaline reduction reaction following UV-P diazotization-coupling is completed, a mixed organic phase of xylene and / or toluene enriched with azo colored impurities is separated; S2, thiourea dioxide is added to the organic phase as a decolorizing reducing agent, and the reaction is stirred under strongly alkaline conditions and at a temperature of 55-65°C to break the azo conjugated N=N double bonds and degrade them into water-soluble aromatic amines; S3, after decolorization is completed, the suspended solids are filtered out, and the resulting regenerated solvent is directly transported to the reduction reactor for the next batch of production without distillation or rectification purification; S4, S1-S3 are repeated, and fresh aromatics lost due to volatilization are replenished according to the solvent storage tank level.

[0007] Preferably, in S1, the reaction system is heated to 65-70°C and allowed to stand for 20-40 minutes before separation, so that the organic phase and the aqueous phase are separated into layers, and the upper layer of toluene and / or xylene mixed organic phase is separated.

[0008] Preferably, in S2, the mass concentration of the thiourea dioxide in the organic phase is 0.01% to 1.0%.

[0009] Preferably, in S2, the alkaline conditions are provided by sodium hydroxide or potassium hydroxide, and the pH of the system is ≥12.

[0010] Preferably, in S2, the stirring reaction time is 20–40 min; the stirring rate is such that thiourea dioxide is uniformly dispersed in the organic phase.

[0011] Preferably, in S2, the thiourea dioxide is added in a single metered dose for each batch.

[0012] Preferably, in S2, the thiourea dioxide is added continuously online via a metering pump in the solvent circulation pipeline.

[0013] The present invention also provides a system for repeatedly recycling aromatic solvents in any of the above methods, comprising an alkaline reduction reactor, an oil-water separation settling tank, a decolorization reaction unit, and a solvent circulation conveying main pipe connected in sequence by pipelines. The decolorization reaction unit is connected to a metering feeding device and is equipped with a constant temperature control jacket and an automatic alkali replenishment pipeline. A filter device is provided on the solvent circulation conveying main pipe.

[0014] Preferably, the thiourea dioxide metering and feeding device includes an intermittent solid feeding port and a continuous micro-metering pump; the constant temperature control jacket maintains the temperature in the decolorization reaction unit at a constant 55-65°C; and the automatic alkali replenishment pipeline maintains the pH value of the system above 12 in real time.

[0015] Preferably, it also includes an aqueous phase trace aromatic hydrocarbon extraction and recovery branch located after the oil-water separation settling tank for recovering residual aromatic hydrocarbons dissolved in the aqueous phase; it also includes a fresh aromatic hydrocarbon replenishment storage tank connected in parallel to the solvent circulation and delivery main pipe.

[0016] The beneficial effects of this invention are as follows: This invention utilizes the hyposulfate ions released by thiourea dioxide under strongly alkaline conditions to break and degrade the N=N double bonds of azo color-producing impurities in the aromatic phase into water-soluble aromatic amines. These amines are then removed from the system through oil-water separation. The regenerated solvent is directly reused without distillation or rectification, thus eliminating the high-energy-consuming distillation step of traditional processes. This significantly reduces energy consumption and operating costs while achieving ultra-long-term stable recycling of aromatic solvents. Furthermore, it avoids the problem of generating large amounts of hazardous waste by adsorption methods, thus possessing both significant economic and environmental benefits. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the steps of a method for repeatedly recycling aromatic solvents according to a specific embodiment of the present invention.

[0018] Figure 2 This is a process flow diagram of a system for the repeated recycling of aromatic solvents according to a specific embodiment of the present invention.

[0019] 1. Alkaline reduction reactor; 2. Oil-water separation settling tank; 21. Aromatic hydrocarbon extraction and recovery branch; 22. Wastewater collection tank; 3. Decolorization reaction unit; 31. Metering and feeding device; 31a. Intermittent solid feed port; 31b. Continuous micro metering pump; 32. Constant temperature control jacket; 33. Automatic alkali replenishment pipeline; 4. Solvent circulation and conveying main pipeline; 41. Filtration device; 42. Fresh aromatic hydrocarbon replenishment storage tank; 5. Emergency bypass valve group. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise specified, all raw materials used in the embodiments of the present invention are commercially available products.

[0021] like Figure 1 As shown, the present invention provides a method for the closed-loop recycling of aromatic solvents, specifically including the following steps: S1. After diazotization and coupling reactions, the ultraviolet absorber UV-P is fed into alkaline reduction reactor 1 to complete the alkaline reduction reaction. The aromatic solvent used in the diazotization-coupling reaction stage is a mixture of xylene and / or toluene in any proportion; the alkalinization medium is selected from one or more of 30% liquid alkali, potassium hydroxide solution, and sodium methoxide / sodium ethoxide low-carbon alcohol bases.

[0022] After the alkaline reduction reaction, the resulting oil-water mixture is subjected to a phase separation process. During phase separation, the system is heated to 65-70℃ and maintained within this temperature window. If the system temperature is below 65℃, the viscosity of the oil and water phases is high, the density difference is reduced, the phase separation effect is poor, the phase interface is blurred, and the purity of the upper organic phase is affected. If the system temperature exceeds 70℃, it will exacerbate the volatilization loss of aromatics.

[0023] After heating to the target temperature, let it stand for 20-40 minutes to allow the oil and water to fully separate.

[0024] After the stratification is completed, the lower layer is a salt-containing alkaline aqueous phase, and the upper layer is an aromatic organic phase containing dissolved azo colored impurities (i.e., the aforementioned xylene and / or toluene mixed organic phase). This organic phase is the target for subsequent decolorization and regeneration.

[0025] S2. Thiourea dioxide is added to the organic phase obtained from S1 as a decolorizing and reducing agent. The mass concentration of thiourea dioxide in the aforementioned organic phase is 0.01%-1.0%.

[0026] After adding thiourea dioxide, sodium hydroxide or potassium hydroxide is added to the system to provide strongly alkaline conditions, making the pH ≥ 12. It should be noted that when pH < 11, the decomposition rate of thiourea dioxide is insufficient, and a sufficient amount of hyposulfate cannot be generated. The decolorization reaction is difficult to carry out effectively.

[0027] While providing strong alkaline conditions, the system temperature also needs to be controlled within 55-65℃. If the reaction temperature is below 55℃, the activation reaction of the reducing agent will be insufficient, and the decolorization efficiency will decrease by more than 40%. If the temperature is above 65℃, the reducing agent will undergo ineffective decomposition, which will not only waste the reagent, but also exacerbate the volatilization of aromatics due to the high temperature.

[0028] Under the aforementioned conditions, stir the reaction for 20-40 minutes, with the stirring rate determined by ensuring that thiourea dioxide is uniformly dispersed in the organic phase.

[0029] Under these conditions, thiourea dioxide decomposes in the presence of a strong alkali to produce hyposulfate, which has extremely strong reducing properties. This process breaks the azo conjugated N=N double bond, degrading it into water-soluble aromatic amines, which are then separated and removed by subsequent water washing into the aqueous phase. The reaction principle is as follows: Reaction 1 (Thiourea dioxide activated and decomposed by strong base): H2N-C (=NH)-SO2H + OH - →[55~65℃] →H2NCONH2 + HSO 2- Under strongly alkaline conditions, thiourea dioxide molecules decompose to produce urea (a water-soluble, harmless byproduct) and hyposulfate (…). (A highly reducing intermediate, which is the core active substance in this decolorization process). This decomposition reaction mainly produces urea and a reducing intermediate, without generating sulfides or sulfur dioxide gas, making it environmentally friendly.

[0030] Reaction 2 (reduction and bond breaking of azo chromophores): Ar-N=N-Ar' + 4 HSO 2- + 2 H2O →[OH-] → Ar-NH2+ Ar'-NH2 + 4 SO 2- 3 The hyposulfite ion attacks the conjugated N=N double bond of the azo chromophore molecule, causing the chromophore structure to break and the colored macromolecular organic compound to degrade into water-soluble colorless aromatic amine compounds; the sulfite ion generated in the reaction... It dissolves in the lower aqueous phase and automatically separates during subsequent oil-water separation. In this way, colored impurities will not continuously accumulate in the aromatic hydrocarbon system, and the solvent can be recycled stably over a long period of time.

[0031] Furthermore, there are two methods for adding thiourea dioxide in this step, which companies can choose according to their production scale and level of automation. The first method is to add it in a single metering manner for each batch, which is suitable for small production lines with 50-300 batches / year. The second method is to add it continuously online through a metering pump on the solvent circulation pipeline, which is suitable for continuous production lines with thousands of batches per year. This method has a higher utilization rate of thiourea dioxide and more uniform decolorization.

[0032] S3. After decolorization, the reaction system is filtered to remove suspended matter (mainly a small amount of insoluble byproducts and mechanical impurities). The resulting regenerated solvent is not purified by distillation or rectification, but is directly transported to the alkaline reduction reactor through the solvent circulation pipeline for use in the alkaline reduction section of the next batch of UV-P production.

[0033] In this application, the return path of the regenerated solvent from the decolorization reaction unit to the reduction reactor completely bypasses distillation and rectification equipment. In traditional processes, aromatic solvents must undergo high-temperature distillation / rectification after decolorization to remove residual impurities. This process is not only energy-intensive, but also prone to oxidative condensation of aromatics at high distillation temperatures, generating larger colored molecules that are more difficult to remove, leading to a deterioration in solvent quality with each batch. This invention, through a combination of "thiourea dioxide strong alkali decolorization + water washing to separate water-soluble degradation products," ensures that the color and purity of the regenerated solvent meet the requirements of the next batch of alkaline reduction reaction, thereby eliminating the energy-intensive distillation / rectification steps and significantly reducing steam consumption per ton of UV-P production.

[0034] S4. Repeat steps S1-S3 to form a closed-loop recycling of aromatic solvents.

[0035] Due to the volatilization loss of aromatics during stratification, decolorization, filtration and pipeline transportation, as well as the discharge of a small amount of aromatics with aqueous phase extraction, fresh toluene and / or xylene are replenished periodically or continuously according to the solvent storage tank level (or according to the total volume / mass monitoring data of the regenerated solvent) to maintain the total solvent volume balance of the system.

[0036] like Figure 2 As shown, the present invention also provides a system for repeatedly recycling aromatic solvents to achieve the above-described method. The system includes an alkaline reduction reactor 1, an oil-water separation settling tank 2, a decolorization reaction unit 3, and a solvent circulation and delivery main pipe 4, which are connected sequentially by pipelines.

[0037] The decolorization reaction unit 3 is connected to a thiourea dioxide metering and feeding device 31, and is equipped with a constant temperature control jacket 32 ​​and an automatic alkali replenishment pipeline 33. The thiourea dioxide metering and feeding device 31 includes an intermittent solid feed port 31a and a continuous micro-metering pump 31b, corresponding to two feeding methods: batch-by-batch metering addition of thiourea dioxide and continuous online addition via the solvent circulation pipeline, respectively. The constant temperature control jacket 32 ​​is responsible for maintaining a constant temperature of 55-65℃ within the decolorization reaction unit to ensure that thiourea dioxide fully decomposes in a strongly alkaline environment to produce highly reducing hyposulfite. Studies have shown that when the alkalinity increases and the temperature rises, thiourea dioxide rearranges from stable isomer A to unstable isomer B. B decomposes to produce urea and highly reducing hyposulfite. The reduction potential of hyposulfite can reach -800mV or even below -1000mV, far exceeding that of the traditional reducing agent sodium dithionite (sodium hydrosulfite).

[0038] The automatic alkali replenishment pipeline 33 maintains the pH value of the system above 12 in real time to ensure the continuous and stable decomposition of thiourea dioxide.

[0039] A filter device 41 is installed on the solvent circulation transport main pipe 4 to filter out suspended matter in the reaction system after decolorization.

[0040] Furthermore, the system of the present invention also includes an aqueous phase trace aromatic hydrocarbon extraction and recovery branch 21 located after the oil-water separation settling tank, used to recover residual aromatic hydrocarbons dissolved in the aqueous phase to reduce the total loss of aromatic hydrocarbons. The recovered wastewater is directly discharged into the wastewater collection tank 22. In addition, it also includes a fresh aromatic hydrocarbon replenishment storage tank 42, which is connected in parallel to the solvent circulation and delivery main pipe 4, used to replenish fresh aromatic hydrocarbons lost due to evaporation according to the solvent storage tank level, maintaining the total solvent volume balance of the system.

[0041] Furthermore, the system also includes an emergency bypass valve group 5; when equipment maintenance is required or solvent color exceeds the standard, the system can switch to the bypass to handle the abnormal solvent separately, without requiring the main production line to be shut down. The entire circulation system does not include a distillation column or high-temperature distillation equipment, eliminating the solvent high-temperature regeneration process entirely.

[0042] The following analysis and explanation are based on specific embodiments and comparative examples: Example 1 (Small-scale test of intermittent feeding mode) Take 5L of xylene (containing aromatic hydrocarbons recovered from the UV-P alkaline reduction section enriched with azo color-producing impurities), add 12.9g of thiourea dioxide (equivalent to 0.3% of the solvent mass), adjust the pH of the system to 13, and stir at 60℃ for 30min.

[0043] The results of 20 consecutive batches of recycling showed that the regenerated solvent in each batch was transparent, with a visible light transmittance (440nm) that remained stable between 98% and 99%, and no yellowing occurred. This result verifies the technical feasibility of the "thiourea dioxide strong alkali decolorization + direct return without distillation" process of this invention.

[0044] Example 2 (Industrial continuous verification under intermittent feeding mode) Continuous validation was conducted on an industrial production line. Specifically, 4.3 tons of xylene were fed into a single reactor, with 12.9 kg of thiourea dioxide added per batch. 115 batches were run continuously without the distillation unit being activated. Only approximately 0.5% fresh xylene was added per batch to compensate for evaporation losses. The continuous monitoring data for the 115 batches are shown in Table 1.

[0045] Table 1

[0046] The above data shows that during the hundred-batch cycle, the transmittance of the UV-P finished product at 440nm and 500nm remained stable at over 98%, which is better than the industry standard. Although the solvent color gradually increased from 15 to 23, it was still at a low color level, which fully met the solvent quality requirements for continuous production of UV-P. Moreover, there were no significant fluctuations between batches, indicating good production stability.

[0047] Example 3 (Continuous Micro-metering Dosing Mode) For a large-scale continuous production line with an annual output of 1,000 class reactors, a 10% thiourea dioxide alkaline suspension is pre-prepared and continuously fed online through a continuous micro-metering pump in the solvent circulation pipeline, maintaining a steady-state thiourea dioxide mass concentration of 0.3% in the system. The layered organic phase is directly reused after online isothermal decolorization, without a distillation process. Monthly product sampling inspections all passed, and only about 0.5% xylene volatilization loss needs to be replenished per reactor.

[0048] Example 4 (Concentration Gradient Verification) Three comparative experiments were conducted with thiourea dioxide dosages of 0.05%, 0.3%, and 0.8% of the solvent mass. At 0.05%, the solvent began to turn slightly yellow after the 8th batch, and the transmittance at 440nm dropped below 90%, indicating insufficient decolorization ability. At 0.3%, 115 batches consistently met the required standards, and the cost of the reducing agent was moderate. At 0.8%, the decolorization effect was not significantly different from the 0.3% group, but the cost of the reducing agent increased to approximately 2.7 times that of the 0.3% group. Therefore, the optimal process range was determined to be 0.2%-0.4%, with 0.3% being the preferred value for industrial application.

[0049] Comparative Example 1 (Activated Carbon Adsorption) Add 3% activated carbon to the separated organic phase, stir at 60℃ for 30 min, and then filter. The transmittance of the first batch of solvent recovered to over 95% after treatment, but by the third batch, the solvent showed a distinct orange-red color, and the transmittance at 440 nm dropped below 78%, necessitating shutdown for distillation regeneration. The aromatic-containing waste activated carbon produced during filtration must be disposed of as hazardous waste, generating a large amount of hazardous solid waste per batch. Activated carbon relies solely on physical adsorption to capture colored molecules and cannot break the N=N conjugated chemical bonds of the azo chromophores; the adsorption sites quickly saturate, lacking continuous recycling capability.

[0050] Comparative Example 2 (a mixture of activated clay and diatomaceous earth) An adsorption treatment was performed using a mixture of 2% activated clay and 1% diatomaceous earth. Color exceeded the standard after only two batches, and the amount of hazardous solid waste residue generated per batch was approximately 12 times that of this invention, resulting in high solid waste disposal costs.

[0051] Comparative Example 3 (Traditional Distillation Process) After every 2-3 batches, all solvents are regenerated by distillation during a shutdown. Considering the material loss, steam energy consumption, and hazardous waste disposal costs, the overall operating cost of this process is significantly higher than that of the method of this invention.

[0052] As can be seen from Examples 1-4 and Comparative Examples 1-3, adsorbent materials such as activated carbon rely solely on physical processes to capture colored molecules, failing to eliminate azo chromophores at the chemical bond level. This leads to rapid saturation of adsorption sites, short cycle life, and the generation of large amounts of hazardous solid waste. Distillation processes, on the other hand, face challenges such as high equipment investment, high energy consumption, and frequent shutdowns. This invention, however, utilizes a chemical bond-breaking decolorization mechanism of thiourea dioxide under specific conditions of 55-65℃ and a strong alkaline pH ≥ 12, combined with a closed-loop circulation system. This achieves a significant leap in solvent recycling batches, increasing from less than 3 batches to over 115 batches, without requiring distillation throughout the process. Furthermore, thiourea dioxide is safe and stable at room temperature. This method is applicable to UV-P production and can also be extended to the synthesis of benzotriazole ultraviolet absorbers such as UV-326, UV-328, and UV-329.

[0053] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for repeatedly recycling aromatic solvents, characterized in that: Includes the following steps: After the alkaline reduction reaction following S1 and UV-P diazotization-coupling is completed, a mixed organic phase of xylene and / or toluene enriched with azo colored impurities is separated. S2. Thiourea dioxide is added to the organic phase as a decolorizing and reducing agent, and the reaction is stirred under strongly alkaline conditions and at a temperature of 55-65°C to break the azo conjugated N=N double bond and degrade it into water-soluble aromatic amines. S3. After decolorization, the suspended solids are filtered out. The resulting regenerated solvent is directly transported to the reduction reactor for the next batch of production without distillation or rectification. S4. Repeat steps S1 to S3 to replenish fresh aromatics lost due to evaporation, depending on the solvent level in the storage tank.

2. The method as described in claim 1, characterized in that: In S1, before separation, the reaction system is heated to 65-70℃ and allowed to stand for 20-40 minutes to separate the organic phase and aqueous phase into layers, and then the upper layer of toluene and / or xylene mixed organic phase is separated.

3. The method as described in claim 1, characterized in that: In S2, the mass concentration of the thiourea dioxide in the organic phase is 0.01% to 1.0%.

4. The method as described in claim 1, characterized in that: In S2, the alkaline conditions are provided by sodium hydroxide or potassium hydroxide, and the pH of the system is ≥12.

5. The method as described in claim 4, characterized in that: In S2, the stirring reaction time is 20–40 min; the stirring rate is such that thiourea dioxide is uniformly dispersed in the organic phase.

6. The method as described in claim 1, characterized in that: In S2, the thiourea dioxide is added in a single metered dose for each batch.

7. The method as described in claim 1, characterized in that: In S2, the thiourea dioxide is added continuously online via a metering pump on the solvent circulation pipeline.

8. A system for repeatedly recycling aromatic solvents according to any one of claims 1-7, characterized in that: The system includes an alkaline reduction reactor (1), an oil-water separation settling tank (2), a decolorization reaction unit (3), and a solvent circulation conveying main pipe (4) connected in sequence by pipelines. The decolorization reaction unit (3) is connected to a metering feeding device (31) and is equipped with a constant temperature control jacket (32) and an automatic alkali replenishment pipeline. The solvent circulation conveying main pipe (4) is equipped with a filter device (41).

9. The system as described in claim 8, characterized in that: The thiourea dioxide metering and feeding device (31) includes an intermittent solid feeding port (31a) and a continuous micro metering pump (31b); the constant temperature control jacket (32) controls the temperature in the decolorization reaction unit (3) at a constant temperature of 55-65℃; the automatic alkali replenishment pipeline maintains the pH value of the system above 12 in real time.

10. The system as described in claim 8, characterized in that: It also includes an aqueous phase trace aromatic hydrocarbon extraction and recovery branch (21) located after the oil-water separation settling tank (2) for recovering residual aromatic hydrocarbons dissolved in the aqueous phase; it also includes a fresh aromatic hydrocarbon replenishment storage tank (42) connected in parallel to the solvent circulation transport main pipe (4).

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