Method for recovering acetonitrile-containing waste liquid

Abstract

The invention discloses a method for recovering acetonitrile-containing waste liquid, and relates to the technical field of separation and purification, the waste liquid containing water, ethanol and methanol is purified step by step through a continuous process of concentration rectification, two-stage light component removal, extractive rectification and solvent recovery, and recyclable acetonitrile is obtained. The process comprises the following steps: recovering most of acetonitrile by low-temperature concentration, removing light components by two stages through a light component removal tower, then introducing a composite extraction agent compounded by ethylene glycol, 1, 2-propylene glycol and dimethyl carbonate according to a specific ratio for extractive distillation, breaking acetonitrile-water azeotropy by utilizing polarity difference, and deeply removing trace moisture and organic impurities; the extracting agent is recycled after reduced pressure distillation and dehydration in a subsequent solvent recovery tower. According to the recovery method, the reflux ratio and the number of tower plates are optimally matched, and the effects of low energy consumption and high yield can be achieved under mild conditions.

Description

Technical Field

[0001] This application relates to the technical field of separation and purification, and in particular to a method for recovering acetonitrile-containing waste liquid. Background Technology

[0002] In the production and processing of biopharmaceuticals, acetonitrile, as a high-performance organic solvent, is widely used in key stages such as drug synthesis, natural product extraction, and product purification due to its excellent solubility and chemical stability, becoming an indispensable basic material in biopharmaceutical production. However, after actual production operations, a large amount of acetonitrile waste liquid is generated. This waste liquid has a complex composition, containing not only acetonitrile and water, but also various complex organic substances, including unreacted raw materials and excipients, byproducts generated during the reaction process, and degradation products generated during drug storage or reaction. If this acetonitrile waste liquid is discarded directly without treatment, it will not only cause a serious waste of this valuable resource and increase the production costs of biopharmaceutical companies, but also cause serious pollution to the surrounding water bodies, soil, and atmospheric environment, disrupting the ecological balance. Furthermore, it contradicts the national development concept of green environmental protection and resource recycling. Therefore, developing an efficient and feasible acetonitrile waste liquid recycling process to achieve the recycling and reuse of acetonitrile resources and reduce environmental pollution is of significant economic and environmental value for the sustainable development of the biopharmaceutical industry.

[0003] Currently, various processes have been developed in the industry for the separation and purification of acetonitrile and water. Each method has its own characteristics and limitations. Pressure swing distillation separates acetonitrile and water by changing the system pressure to disrupt the azeotropic system. However, this method consumes a large amount of energy, resulting in high operating costs and hindering large-scale application. Azeotropic distillation requires the addition of a third component as an entrainer to the waste liquid to break the original azeotropic equilibrium. However, the addition of the entrainer may introduce new impurities, affecting the purity of the recovered acetonitrile. Additional processes are needed to remove these impurities, increasing the complexity of the production process. Salting out is relatively simple to operate and requires no complex equipment investment, but it suffers from difficulties in salt recovery and the potential for secondary pollution, increasing environmental protection pressure in the long term. Dehydrating agents offer high separation efficiency and can quickly separate acetonitrile and water. However, dehydrating agents themselves are expensive, and some are corrosive, making subsequent treatment difficult and limiting their application in large-scale production. Extractive distillation, with its excellent selectivity, can effectively separate acetonitrile and water. However, its core lies in the selection of the extractant. A suitable extractant not only needs to have good separation performance but also needs to be easy to recover and reuse; otherwise, it will increase process costs. The screening and recovery of high-quality extractants has always been a key challenge for this process. Adsorption and ion exchange methods are simple to operate and require less equipment investment, making them suitable for small-scale acetonitrile wastewater treatment. However, the adsorbents and ion exchange resins used in these two methods are difficult to regenerate, and the regeneration process may generate new pollutants. Long-term use will increase the operating costs and environmental burden of enterprises. Membrane separation, as a novel separation technology, has advantages such as energy saving, environmental protection, high separation efficiency, and no secondary pollution, which aligns with the development trend of green chemistry. However, this method is limited by the performance of membrane materials. The solvent resistance, stability, and service life of membranes are relatively short, and the preparation cost of membranes is high, making it difficult to meet the needs of large-scale acetonitrile wastewater recovery in the biopharmaceutical field.

[0004] Particularly in the API (Active Pharmaceutical Ingredient) production workshops of small and medium-sized biopharmaceutical companies, these workshops primarily engage in the synthesis and purification of small molecule APIs. During daily production, acetonitrile wastewater is continuously generated, and the production process requires high purity of the recovered acetonitrile. The recovered acetonitrile must be directly recycled for subsequent drug synthesis and purification processes without additional advanced treatment. In such workshops, the production scale is moderate, wastewater generation is stable, and space is limited, making it impossible to accommodate large, complex recycling equipment. Furthermore, these companies prioritize cost control and environmental compliance, requiring recycling processes that are low-energy, easy to operate, free of secondary pollution, and highly efficient, enabling the recycling of acetonitrile resources while reducing the company's environmental treatment costs. Summary of the Invention

[0005] The purpose of this application is to provide a method for recovering acetonitrile-containing waste liquid, which can improve the recovery rate of acetonitrile and is less likely to introduce new impurities, thereby achieving efficient recovery and reuse of acetonitrile.

[0006] Firstly, the method for recovering acetonitrile-containing waste liquid provided in this application adopts the following technical solution: A method for recovering acetonitrile-containing waste liquid includes the following steps: S1. Pump the acetonitrile-containing waste liquid into a concentrating distillation column at a temperature of 80-105℃ to obtain a concentrated liquid. S2. Pump the concentrated liquid from step S1 into the first light-weight removal tower at a temperature of 75-82℃ to obtain the first light-weight removal liquid. S3. Pump the first light-removed liquid from step S2 into the second light-removed tower at a temperature of 62-68℃ to obtain the second light-removed liquid. S4. Pump the second light liquid from step S3 into the extractive distillation column, add extractant, and maintain the temperature at 70-90℃ to obtain the extract. S5. Pump the extract from step S4 into a light-light-removal distillation column to obtain purified acetonitrile; S6. Recover and extract the extractant for recycling.

[0007] By adopting the above technical solution, this method for recovering acetonitrile-containing waste liquid can effectively achieve the recycling of acetonitrile resources and significantly reduce the environmental pollution caused by waste liquid discharge. The entire recovery process combines stepwise distillation and extraction to gradually remove light impurities and other harmful components from the waste liquid, ensuring that the final purified acetonitrile has stable quality and can be reused in the production process. This reduces the company's dependence on new acetonitrile raw materials and also reduces raw material procurement costs. The rational design of the stepwise light impurity removal process can target the separation of impurities with different boiling points, avoiding the accumulation of impurities that affect the recovery effect, improving the overall recovery efficiency, and reducing the operational difficulty of individual separation steps. The application of extractive distillation further improves the separation purity of acetonitrile, effectively solving the problem of components that are difficult to separate by conventional distillation, and ensuring the practicality of the recovered product. In addition, the extractant can be recycled, which not only reduces the consumption of extractant and lowers the processing cost, but also avoids secondary pollution caused by extractant discharge, achieving the dual goals of environmental protection and energy conservation.

[0008] Optionally, the extractant is one or more selected from ethylene glycol, 1,2-propanediol, dimethyl carbonate, and isopropanol.

[0009] By adopting the above technical solutions, ethylene glycol and 1,2-propanediol have similar properties, exhibiting strong extraction selectivity and efficient separation of acetonitrile from difficult-to-separate components. They also demonstrate good stability, low volatility, and long-term recyclability. However, compared to the other two extractants, their viscosity is slightly higher, which has a minor impact on operational smoothness. Dimethyl carbonate and isopropanol have lower viscosity and better flowability, accelerating the extraction and separation rate and reducing operational energy consumption. Dimethyl carbonate is more environmentally friendly, with good degradability, reducing the risk of secondary pollution. Isopropanol's advantage lies in its easy separation from acetonitrile and low recovery difficulty, further reducing extractant recovery costs. These four extractants can be used individually or in combination, meeting different purity requirements for acetonitrile recovery while balancing operational convenience and cost control. They are suitable for large-scale recovery processes, helping to improve overall recovery efficiency and economics.

[0010] Optionally, the extractant is a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate.

[0011] By adopting the above technical solution, and selecting ethylene glycol, 1,2-propanediol, and dimethyl carbonate as extractants, the synergistic complementarity of the properties of each component can be achieved. Ethylene glycol and 1,2-propanediol have good polarity and solubility, which can efficiently extract the target component. They also have good stability and good compatibility with the system. Dimethyl carbonate can adjust the overall polarity and viscosity of the extractant, improve the mass transfer efficiency during the extraction process, and also improve the separation and recovery performance of the extractant. The compounded extractant has the characteristics of excellent extraction effect, strong operational adaptability, and recyclability, which can ensure the efficient and stable operation of the extraction process.

[0012] Optionally, the extractant is a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in a mass ratio of (5-6):(3-4):1.

[0013] By adopting the above technical solution, using a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in a specific mass ratio as the extractant, the advantages of the three substances can be complemented, avoiding the shortcomings of a single extractant, and significantly improving the extraction effect and overall process rationality of acetonitrile recovery. The ratio of the three substances fully leverages their respective advantages. Ethylene glycol and 1,2-propanediol ensure strong extraction selectivity, efficiently separating acetonitrile from difficult-to-separate impurities in the system. Simultaneously, their good stability ensures long-term recycling of the extractant, reducing waste. Dimethyl carbonate effectively improves the flowability of the mixed extractant, reduces overall viscosity, solves the problem of insufficient operational smoothness when using ethylene glycol alone, accelerates the extraction and separation rate, and reduces energy consumption during operation. The reasonable ratio of the mixed extractant takes into account high selectivity, good flowability and environmental friendliness. It avoids the impact of impurity residue on acetonitrile purification effect, reduces the difficulty and cost of extractant recovery, adapts to the original large-scale recovery process, can steadily improve the quality of purified acetonitrile, and reduce secondary pollution. It further takes into account the economic and environmental benefits of the recovery process, making the entire acetonitrile recovery process more efficient, stable and sustainable.

[0014] Optionally, the mass ratio of the second light-removing liquid to the extractant is 1:(1-4).

[0015] By adopting the above technical solution, the mass ratio of the second light-removed liquid to the extractant fully meets the needs of the extractive distillation process, ensuring the stability and efficiency of acetonitrile recovery. This ratio range has been scientifically optimized to ensure that the extractant fully contacts and reacts with the acetonitrile in the second light-removed liquid, efficiently separating residual trace impurities and avoiding incomplete separation and substandard acetonitrile purity due to insufficient extractant. Simultaneously, reasonable control of the extractant dosage avoids waste caused by excessive use, reduces energy consumption and costs in extractant recovery, and prevents excessive extractant from affecting subsequent distillation separation efficiency. This ratio has good adaptability, matching the characteristics of the mixed extractant and the overall recovery process described above, ensuring a smooth extraction process and thus guaranteeing the stable quality of the final purified acetonitrile, making the entire recovery process more rational and practical.

[0016] Optionally, the theoretical plate number of the extractive distillation column is 42-46, the second light liquid is at feed position 38-42 of the extractive distillation column, and the extractant is at feed position 3-4 of the extractive distillation column.

[0017] By adopting the above technical solution, selecting an appropriate range of theoretical plates for the extractive distillation column, and matching it with specific feed locations for the second light-removal liquid and the extractant, the extractive distillation effect can be significantly improved, ensuring the purity and efficiency of acetonitrile separation and adapting to the overall recovery process. A reasonable number of theoretical plates can fully meet the separation requirements of acetonitrile and impurities, ensuring a thorough separation process and avoiding problems such as insufficient separation or excessive energy consumption caused by an inappropriate number of theoretical plates. The two feed locations are scientifically matched, allowing the second light-removal liquid and the extractant to achieve sufficient contact and efficient reaction within the column, enabling the extractant to better exert its selective separation effect and maximizing the removal of trace impurities remaining in the light-removal liquid. This design balances separation effect and operational rationality, ensuring the stable quality of subsequent purified acetonitrile while reducing operating energy consumption and costs. It aligns with the aforementioned proportions and process steps, ensuring the smooth and efficient progress of the entire extractive distillation process, further enhancing the overall practicality and economy of acetonitrile recovery.

[0018] Optionally, the pressure of the extractive distillation column is 0.4-0.5 atm, and the reflux ratio is 1.5-2.5.

[0019] By adopting the above technical solution, the extractive distillation column, with its appropriate pressure and reflux ratio range, can effectively adapt to the operational requirements of the extractive distillation process, ensuring the high efficiency and stability of acetonitrile separation and aligning with the overall recovery process. A reasonable pressure setting within the column optimizes the separation conditions between acetonitrile and impurities, preventing decreased separation efficiency or increased operational difficulty due to improper pressure, while also reducing energy consumption. An appropriate reflux ratio precisely controls the component separation effect within the column, ensuring that acetonitrile purity meets standards, reducing impurity residue, and balancing separation quality with operational economy.

[0020] Optionally, the top temperature of the extractive distillation column is 62-66℃, and the bottom temperature is 120-130℃.

[0021] By adopting the above technical solution, the top and bottom temperatures of the extractive distillation column are set within an appropriate range, precisely matching the separation requirements within the column and ensuring the high efficiency and stability of acetonitrile recovery. This optimal combination with process parameters such as column pressure, reflux ratio, and theoretical plate number forms the best balance. The rational design of the top temperature ensures smooth condensation and collection of the separated acetonitrile, avoiding component volatilization or incomplete condensation due to improper temperature, thus guaranteeing product recovery. Optimized control of the bottom temperature effectively separates high-boiling-point impurities from the system while providing sufficient power for the distillation process within the column, preventing impurity residues from affecting the purity of acetonitrile.

[0022] Optionally, the reflux ratio of the first light tower stripping tower is 1.0-1.6; the reflux ratio of the second light tower stripping tower is 1.4-2.2.

[0023] By adopting the above technical solution, the appropriate reflux ratio range for the first and second light-weight component removal towers precisely matches the core requirements of the two-step light-weight component removal process. This ensures a smooth transition with the light-weight component removal temperature and subsequent extraction and distillation stages, guaranteeing the continuity and efficiency of the overall recovery process. The reflux ratio of the two-step light-weight component removal process can specifically separate impurities at different stages of light-weight component removal, ensuring that each step can fully remove the corresponding light components and preventing impurities from being carried into subsequent stages and affecting the purity of acetonitrile. A reasonable reflux range can guarantee the light-weight component removal effect while avoiding excessive energy consumption or incomplete separation caused by improper reflux ratios, balancing separation quality and economy. The reasonable combination of the two reflux ratios provides high-quality feedstock for subsequent extraction and distillation, further improving the overall efficiency of acetonitrile recovery and the stability of the final product quality.

[0024] Optionally, the acetonitrile waste liquid contains 60-80 wt% water, 10-20 wt% acetonitrile, 3-10 wt% ethanol, and the balance is methanol.

[0025] By adopting the above technical solution, the acetonitrile waste liquid with this component ratio is well adapted to each stage of the existing recovery process, ensuring a stable and efficient recovery process. The differences in properties between the various impurity components in the waste liquid and acetonitrile are suitable for the process characteristics of stepwise light component removal and extractive distillation, facilitating precise separation through the existing process without the need for additional adjustments to process parameters. This approach can efficiently recover acetonitrile while reasonably treating impurities, balancing recovery efficiency and product quality.

[0026] In summary, this application includes at least one of the following beneficial technical effects: 1. Compared with existing technologies, the process route of multiple light component removal and extractive distillation coupling removes light components from acetonitrile waste liquid step by step according to the boiling point gradient, avoiding the accumulation of impurities in the later stage; extractive distillation breaks the azeotropic reaction between acetonitrile and water under negative pressure and mild conditions, achieving deep dehydration and de-alcoholization. The whole flow equipment has low load and low energy consumption. The recovery of acetonitrile can significantly improve the resource recycling rate and reduce the cost of environmental treatment. 2. Compared with existing technologies, the extractant is a ternary compound of ethylene glycol, 1,2-propanediol and dimethyl carbonate, and the polarity balance is controlled by mass ratio. The feed mass ratio of the second light liquid to the extractant and the theoretical plate number of the extractive distillation column are synergistically optimized to ensure that the two phases are fully contacted in the column and maintain an appropriate residence time. This prevents impurity entrainment due to insufficient extractant and avoids the energy consumption and separation efficiency reduction caused by excessive extractant, thereby stably obtaining high-purity acetonitrile. Detailed Implementation

[0027] Unless otherwise specified, all raw materials used in the embodiments and comparative examples of this application are commercially available products. Example 1

[0028] A method for recovering acetonitrile-containing waste liquid includes the following steps: 10 tons of acetonitrile waste liquid containing 78.32 wt% water and 15.43 wt% acetonitrile was preheated to 80°C and then pumped steadily at 500 kg / h into a concentrator distillation column. The concentrator distillation column has 22 theoretical plates, with the feed inlet located on the 10th plate. Distillation was carried out at an absolute pressure of 0.95 atm, a top temperature of 82°C, a bottom temperature of 105°C, and a reflux ratio of 0.8. The top distillate of the concentrator distillation column contained a stream of crude acetonitrile containing water, with 45 wt% acetonitrile. This stream was condensed in a condenser to obtain the reflux product.

[0029] The reflux product was fed to the first light-light product removal column at a rate of 165 kg / h. Wastewater containing 96 wt% water was discharged from the bottom of the first light-light product removal column. The first light-light product removal column has a theoretical number of 35 plates, with the feed inlet located on the 20th plate. Distillation was carried out at an absolute pressure of 0.65 atm, a top temperature of 69°C, a bottom temperature of 80°C, and a reflux ratio of 1.3, at a rate of 4 kg / h. The liquid removed from the top of the first light-light product removal column contained 40 wt% methanol, 55 wt% ethanol, and 5 wt% water. The liquid was discharged from the bottom of the tower at a rate of 160 kg / h to obtain a first light removal liquid containing 95.7 wt% acetonitrile and 1.1 wt% water.

[0030] The first light-removed liquid was pumped into the second light-removed column at a rate of 160 kg / h. The second light-removed column has a theoretical number of 50 plates, with the feed inlet located on the 32nd plate. Distillation was carried out at an absolute pressure of 0.30 atm, a top temperature of 64°C, a bottom temperature of 71°C, and a reflux ratio of 1.7. Light impurities were removed from the top of the second light-removed column at a rate of 159.5 kg / h, and the bottom of the second light-removed column yielded a secondary light-removed liquid containing 99.9 wt% acetonitrile.

[0031] The secondary light-removed liquid was mixed with an extractant composed of ethylene glycol, 1,2-propanediol, and dimethyl carbonate at a mass ratio of 1:1 to obtain a mixture. The ethylene glycol, 1,2-propanediol, and dimethyl carbonate were mixed at a mass ratio of 5:4:1. The mixture was heated to 75°C. The secondary light-removed liquid was simultaneously introduced into an extractive distillation column from the 40th plate and the extractant from the 3rd plate. The extractive distillation column had a theoretical number of 45 plates. Distillation was carried out at an absolute pressure of 0.45 atm, a column top temperature of 65°C, a column bottom temperature of 127°C, and a reflux ratio of 2. High-purity acetonitrile (99.87 wt%) was collected from the top of the extractive distillation column at a rate of 157.5 kg / h, and a water-rich extractant product containing the extractant was obtained from the bottom of the column. The water-rich extractant product was fed into a solvent recovery tower at 0.2 atm. The solvent recovery tower had 13 theoretical plates, a top temperature of 100°C, a bottom temperature of 153°C, and a reflux ratio of 0.8. Wastewater containing ethylene glycol and water was removed from the top of the solvent recovery tower at a rate of 18 kg / h, and the water-containing composite extractant was recovered from the bottom at a rate of 461.5 kg / h. After being cooled to 25°C by a cooler, it was combined with the added extractant and recycled back to the extractive distillation tower for reuse. Example 2

[0032] The difference between Example 2 and Example 1 is that the mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in the mass ratio of 5:4:1 in Example 1 is replaced by a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in the mass ratio of 5:3:1. Example 3

[0033] The difference between Example 3 and Example 1 is that the ethylene glycol, 1,2-propanediol and dimethyl carbonate mixed in the mass ratio of 5:4:1 in Example 1 are replaced with 5.5:3.5:1. Example 4

[0034] The difference between Example 4 and Example 1 is that the ethylene glycol, 1,2-propanediol and dimethyl carbonate mixed in the mass ratio of 5:4:1 in Example 1 are replaced with 6:3:1. Example 5

[0035] The difference between Example 5 and Example 3 is that the secondary delighting liquid from Example 3 is mixed with the extractant composed of ethylene glycol, 1,2-propanediol and dimethyl carbonate at a mass ratio of 1:2.5 to obtain a mixture. Example 6

[0036] The difference between Example 6 and Example 3 is that the secondary delighting liquid from Example 3 is mixed with an extractant composed of ethylene glycol, 1,2-propanediol, and dimethyl carbonate at a mass ratio of 1:4 to obtain a mixture. Example 7

[0037] The difference between Example 7 and Example 5 is that the theoretical number of plates in the extractive distillation column in Example 5, which was 45, is replaced with 42. Example 8

[0038] The difference between Example 8 and Example 5 is that the theoretical number of plates in the extractive distillation column in Example 5, which was 45, is replaced with 46. Comparative Example 1

[0039] The difference between Comparative Example 1 and Example 1 is that the mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in the mass ratio of 5:4:1 in Example 1 is replaced by the mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in the mass ratio of 3:5:1. Comparative Example 2

[0040] The difference between Comparative Example 2 and Example 3 is that the secondary delighting liquid from Example 3 was mixed with an extractant composed of ethylene glycol, 1,2-propanediol, and dimethyl carbonate at a mass ratio of 1:6 to obtain a mixture. Comparative Example 3

[0041] The difference between Comparative Example 3 and Example 5 is that the theoretical number of plates in the extractive distillation column in Example 5, which was 45, is replaced with 37. Test case

[0042] Recovery performance test: The acetonitrile produced after the recovery of acetonitrile waste liquid in Examples 1-8 and Comparative Examples 1-3 was detected by gas chromatography. The test results are shown in Table 1.

[0043]

[0044] By observing the test data in Table 1, it was found that the recovery rate of acetonitrile was the highest using the recovery method in Example 5.

[0045] A comparative analysis of Examples 1-4 and Comparative Example 1 revealed that the method in Example 3 showed the best acetonitrile recovery. The difference in performance may be due to variations in the mass ratio of ethylene glycol, 1,2-propanediol, and dimethyl carbonate. Ethylene glycol and 1,2-propanediol are both highly polar components with good hydrophilic properties. Their main function is to enhance the removal of water from the system during extractive distillation, while also reducing the relative volatility of acetonitrile and impurities, thus facilitating the separation of acetonitrile from lighter components such as ethanol. Dimethyl carbonate, as a polarity modifier, optimizes the overall polarity and boiling point of the extractant, improves the miscibility between the extractant and the secondary light-removal liquid, and prevents stratification from affecting distillation efficiency. When the mass ratio of the three components is within a reasonable range, the highly polar component can fully exert its hydrophilic effect, efficiently removing water and ethanol impurities from the system, while the polarity modifier ensures the stability of the extractive distillation process, thereby improving the purity of the recovered acetonitrile. If the proportion of the highly polar component is too low, its hydrophilicity and separation enhancement effect will be weakened, making it unable to effectively remove water and ethanol, resulting in a decrease in the purity of the recovered acetonitrile and an increase in the impurity content. If the proportion of the highly polar component is too high, the extractant will be too polar, increasing the difficulty of separation from acetonitrile, which is also detrimental to improving the purity of acetonitrile. In Example 3, the mass ratio of the three components was adjusted to the optimal reasonable range, so that the hydrophilicity, polarity, and miscibility of the extractant reached the best matching state. Therefore, its acetonitrile recovery effect was better than that of other ratios in Examples and Comparative Example 1.

[0046] A comparative analysis of Examples 3, 5-6, and Comparative Example 2 revealed that the method in Example 5 showed the best acetonitrile recovery effect. The performance difference in this aspect may be due to the different mass ratios of the second light-removal liquid to the extractant. The mass ratio of the second light-removal liquid to the composite extractant is a key process parameter affecting the purity of acetonitrile recovery and the removal of impurities. Differences in its value directly lead to significant fluctuations in the acetonitrile, water, and ethanol content of the recovered product. The composite extractant is composed of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in a specific ratio. Ethylene glycol and 1,2-propanediol significantly increase the relative volatility of water and acetonitrile, while dimethyl carbonate enhances the dissolution and removal of light impurities such as ethanol. The synergistic effect of these three components achieves efficient separation of acetonitrile from impurities. When the mass ratio of the secondary light-removed liquid to the extractant is within a suitable range, the extractant can fully encapsulate the acetonitrile component in the secondary light-removed liquid, forming a stable extraction system. This effectively inhibits the entrainment of water and ethanol during the distillation process and provides a good mass transfer environment for the distillation separation of acetonitrile, promoting the full enrichment of acetonitrile at the top of the distillation column. This, in turn, improves the purity of the recovered acetonitrile and reduces the content of impurities such as water and ethanol. If this mass ratio is too low, the amount of extractant is insufficient, making it impossible to achieve sufficient extraction of acetonitrile and effective removal of impurities from the secondary light-removed liquid. Insufficient mass transfer results in some water and ethanol impurities precipitating out with the acetonitrile during distillation, leading to a decrease in the purity of the recovered acetonitrile and an increase in the impurity content. If the mass ratio is too high, the excessive extractant will increase the load on the distillation system. This will not only lead to increased energy consumption during the extraction and distillation process, but may also reduce the mass transfer efficiency of the system components due to the excessive extractant. This will weaken the separation effect between acetonitrile and impurities. Although impurities can be removed to a certain extent, the purity of acetonitrile cannot be further improved. In fact, the imbalance of system components may lead to incomplete removal of impurities, ultimately affecting the overall quality of the recovered acetonitrile.

[0047] A comparative analysis of Examples 5, 7-8, and Comparative Example 3 revealed that Example 5 exhibited the best acetonitrile recovery performance. The reason for this performance difference might be the more optimal number of theoretical plates (45) in the extractive distillation column used in Example 5. The number of theoretical plates in the extractive distillation column is a key process parameter affecting the acetonitrile recovery and purification effect. Its optimal number directly determines the gas-liquid mass transfer separation efficiency within the column, thus influencing the purity and impurity content of the final product. Example 5 employed 45 theoretical plates, a number within the optimal range. This provides sufficient space and time for adequate contact and mass transfer separation between the secondary light liquid and the composite extractant, ensuring that the gas and liquid phases reach near-equilibrium on each plate, effectively achieving precise separation of acetonitrile from impurities such as water and ethanol. Compared to Example 5, Example 7 reduced the number of theoretical plates to 42, while Example 8 increased it to 46. In Example 7, the insufficient number of theoretical plates resulted in inadequate mass transfer between the secondary light liquid and the extractant, leading to incomplete impurity separation and a decrease in acetonitrile purity and an increase in impurity content. Example 8, while having an excessive number of theoretical plates, did improve separation to some extent, but this excessive increase in plate number increased gas-liquid flow resistance within the column, limiting the improvement in mass transfer efficiency and failing to further optimize impurity separation. Instead, it increased energy consumption, failing to achieve the optimal balance between separation efficiency and energy consumption. Comparative Example 3, with a significantly reduced number of theoretical plates to 37, far below the reasonable range, exhibited severely insufficient mass transfer separation, resulting in ineffective impurity removal and a significant decrease in acetonitrile purity and a substantial increase in impurity content. This further confirms that controlling the number of theoretical plates in the extractive distillation column within the reasonable range of 45 is a key factor in ensuring optimal acetonitrile recovery.

[0048] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made to the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for recovering acetonitrile-containing waste liquid, characterized in that, This includes the following steps: S1. Pump the acetonitrile-containing waste liquid into a concentrating distillation column at a temperature of 80-105℃ to obtain a concentrated liquid. S2. Pump the concentrated liquid from step S1 into the first light-weight removal tower at a temperature of 75-82℃ to obtain the first light-weight removal liquid. S3. Pump the first light-removed liquid from step S2 into the second light-removed tower at a temperature of 62-68℃ to obtain the second light-removed liquid. S4. Pump the second light liquid from step S3 into the extractive distillation column, add extractant, and maintain the temperature at 70-90℃ to obtain the extract. S5. Pump the extract from step S4 into a light-light-removal distillation column to obtain purified acetonitrile; S6. Recover and extract the extractant for recycling.

2. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The extractant is one or more of ethylene glycol, 1,2-propanediol, dimethyl carbonate, and isopropanol.

3. The method for recovering acetonitrile-containing waste liquid according to claim 2, characterized in that, The extractant is a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate.

4. The method for recovering acetonitrile-containing waste liquid according to claim 3, characterized in that, The extractant is a mixture of ethylene glycol, 1,2-propanediol, and dimethyl carbonate in a mass ratio of (5-6):(3-4):

1.

5. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The mass ratio of the second light-removing liquid to the extractant is 1:(1-4).

6. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The theoretical plate number of the extractive distillation column is 42-46, the second light liquid is at feed position 38-42 of the extractive distillation column, and the extractant is at feed position 3-4 of the extractive distillation column.

7. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The pressure of the extractive distillation column is 0.4-0.5 atm, and the reflux ratio is 1.5-2.

5.

8. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The temperature at the top of the extractive distillation column is 62-66℃, and the temperature at the bottom is 120-130℃.

9. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The reflux ratio for the first light tower removal is 1.0-1.6; the reflux ratio for the second light tower removal is 1.4-2.

2.

10. The method for recovering acetonitrile-containing waste liquid according to claim 1, characterized in that, The acetonitrile waste liquid contains 60-80 wt% water, 10-20 wt% acetonitrile, 3-10 wt% ethanol, and the balance is methanol.

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