A method and system for acetonitrile-water azeotrope dehydration

Abstract

The application provides a method and system for acetonitrile-water azeotrope rectification dehydration. The method is carried out in a rectification column kettle unit containing a rectification heating kettle and a packed rectification column connected with each other; and comprises the following steps: performing normal pressure initial distillation on acetonitrile waste liquid to obtain an initial distillation fraction; mixing the initial distillation fraction with dichloromethane to obtain a liquid to be rectified; feeding the liquid to be rectified into the rectification column kettle unit, heating the liquid to be rectified, and firstly performing top full reflux on the top condensate, and then performing kettle reflux on the dichloromethane phase obtained by layering the top condensate, while the water phase obtained by layering the top condensate is collected to perform azeotrope rectification dehydration. Through the innovative operation of the temperature-enhanced azeotrope, the control of the top temperature interval, and the enhancement of the kettle reflux vaporization, the application breaks the traditional binary azeotrope composition limit, optimizes the top gas phase composition, greatly improves the dehydration efficiency, and significantly reduces the volatilization loss of the dichloromethane azeotrope.

Description

Technical Field

[0001] This invention relates to the field of azeotropic separation technology, specifically to a method and system for distilling and dehydrating acetonitrile-water azeotropes. Background Technology

[0002] Acetonitrile (CH3CN) is a high-performance polar organic solvent widely used in pharmaceutical intermediate synthesis, pesticide production, high-performance liquid chromatography (HPLC) mobile phases, and electronic material cleaning. Industrial acetonitrile is primarily a byproduct of the ammoxidation of propylene to acrylonitrile, and can also be synthesized via routes such as acetic acid ammoxidation. Industrial production generates a large amount of acetonitrile waste liquid, typically composed of approximately 35.2 wt% acetonitrile, 61.3 wt% water, 1.7 wt% methanol, and 1.8 wt% ethanol. The acetonitrile in this waste liquid needs to be recovered and purified to achieve resource recycling.

[0003] Acetonitrile and water form an azeotrope under normal pressure. This azeotrope consists of approximately 84 wt% acetonitrile and 16 wt% water, with an azeotropic point of 76.5°C. Therefore, high-purity acetonitrile with extremely low water content cannot be directly obtained using ordinary distillation. Currently, industrial methods for acetonitrile dehydration and purification include pressure swing distillation, extractive distillation, salting out, pervaporation membrane separation, and azeotropic distillation. Azeotropic distillation involves adding a third azeotropic agent to the system, forming a new azeotrope with a lower boiling point, which is then collected from the top of the column, thus achieving acetonitrile dehydration. Dichloromethane (CH₂Cl₂) is a commonly used azeotropic agent for acetonitrile azeotropic distillation dehydration due to its low toxicity, non-flammability, low cost, and ability to form a low-boiling-point azeotrope (azeotropic point 38.6°C) with water (98.6 wt% dichloromethane and 1.4 wt% water).

[0004] However, the existing acetonitrile azeotropic distillation dehydration process using dichloromethane still has many drawbacks: First, the dehydration efficiency is low. The water content of dichloromethane-water binary azeotrope is only about 1.4%, and the amount of water carried by a unit azeotropic agent is limited, resulting in a slow dehydration rate and a long production cycle. The dehydration time for a single batch in laboratory small-scale tests can be more than 20 hours, and the energy consumption and time costs for industrial production remain high.

[0005] Secondly, the azeotropic agent is severely depleted. Dichloromethane has a boiling point of only 39.8℃ at atmospheric pressure and is extremely volatile. In traditional overhead reflux processes, the temperature of the gas phase at the top of the column is close to the azeotropic point, and dichloromethane easily escapes from the tail gas along with the non-condensable gas. The loss rate can reach more than 32% during small-scale testing, and the loss is even higher when the condensation effect is insufficient, which increases operating costs and brings environmental risks.

[0006] Third, it has poor adaptability to impurities. Light component impurities such as methanol and ethanol contained in acetonitrile waste liquid can form a multi-component azeotropic system with acetonitrile, water, and dichloromethane, interfering with the composition at the top of the column. Traditional processes lack targeted control over this, which can easily lead to substandard product purity.

[0007] Fourth, the degree of automation is low. Traditional top reflux separation processes require frequent manual separation and addition of azeotropic agents, which is cumbersome, has high labor costs, and poor process stability.

[0008] Existing improvement approaches all revolve around enhancing condensation efficiency, optimizing packing structure, and optimizing azeotropic agent ratios. They all remain within the conventional operational logic of "azeotropic agent reflux at the top of the column to maintain the binary azeotropic point," thus failing to overcome the bottleneck in dehydration efficiency. To address the shortcomings of existing technologies, there is an urgent need to develop a new azeotropic distillation dehydration process and supporting equipment that features high dehydration efficiency, low azeotropic agent loss, simple operation, and suitability for acetonitrile-containing waste liquids. Summary of the Invention

[0009] To overcome the aforementioned deficiencies of the prior art, this invention provides a method and system for the distillation and dehydration of acetonitrile-water azeotropes. This invention, through innovative operation of temperature-enhanced azeotropic agents, combined with control of the column top temperature range and enhanced vaporization via reflux in the reactor, breaks through the compositional limitations of traditional binary azeotropes, optimizes the composition of the top gas phase, and significantly improves dehydration efficiency. Simultaneously, it significantly reduces the volatilization loss of dichloromethane azeotropic agents, and also improves process stability and ease of operation.

[0010] The present invention achieves the above objectives through the following technical solutions: The present invention provides a method for distilling and dehydrating acetonitrile-water azeotrope. The method is carried out in a distillation column reboiler unit containing a distillation heating kettle and a packed distillation column connected to each other. The top of the packed distillation column is also connected to a liquid separator for settling and stratifying the condensate at the top of the column. The method includes the following steps: S1. The acetonitrile waste liquid is subjected to atmospheric pressure primary distillation to obtain a primary distillate with a water content of 18%-22%; S2. The initial distillate is mixed with dichloromethane to obtain the distillate to be purified; S3. The liquid to be distilled is fed into the distillation column reboiler unit. The liquid to be distilled is heated using the distillation heating reboiler. The top condensate is first subjected to total reflux at the top of the column, and then the dichloromethane phase obtained from the separation of the top condensate is refluxed in the reboiler. At the same time, the aqueous phase obtained from the separation of the top condensate is collected for azeotropic distillation and dehydration. During the reflux process in the reboiler, the top temperature is controlled at 48-58℃. S4. After dehydration is completed, stop the reflux operation in the reactor, and then control the temperature at the top of the column to 39-41℃ to collect the dichloromethane component. S5. Then raise the temperature at the top of the tower to 80-82℃ and extract the acetonitrile component.

[0011] The core principle of this invention lies in the following: In step S3, the low-temperature dichloromethane phase is refluxed into the high-temperature liquid phase in the reboiler, where it instantly vaporizes to generate a large number of microbubbles. This enhances gas-liquid mass transfer and the vaporization and carrying effect of water, resulting in a dichloromethane-water-acetonitrile multi-component mixed gas phase at the top of the column. This breaks the traditional upper limit of 1.4% water content for dichloromethane-water binary azeotropes. Simultaneously, by controlling the column top temperature within the 48-58°C range during the refluxing process, the water content at the top of the column can be increased to 7 wt%-18 wt% while maintaining acetonitrile yield, achieving a significant increase in dehydration efficiency. During the refluxing process, when the column top temperature is below 48°C, the column top approaches a dichloromethane-water binary azeotropic composition, limiting the increase in water carrying capacity. When the column top temperature is above 58°C, a large amount of acetonitrile vaporizes and enters the column top, significantly reducing product yield and drastically increasing separation energy consumption.

[0012] In step S1 of this invention, the acetonitrile waste liquid is pretreated by atmospheric distillation to remove some free water and light component impurities, thereby reducing the water content in the initial distillate. The crude distillation of the acetonitrile waste liquid utilizes the fact that acetonitrile can form an azeotrope with water, achieving initial dehydration and concentration of the acetonitrile waste liquid at an azeotropic temperature below the boiling point of water. If atmospheric distillation is not performed on the acetonitrile waste liquid, or if the water content of the acetonitrile waste liquid is greater than 22%, the subsequent dehydration load will be significantly increased.

[0013] In some embodiments, during step S1, the temperature at the top of the column is controlled to be 88~92℃ during the atmospheric distillation process.

[0014] In step S1 of the present invention, those skilled in the art will understand that the acetonitrile waste liquid is subjected to atmospheric pressure primary distillation to obtain primary distillate and column bottom residue; the primary distillate is used as the distillation object in the subsequent steps of the present invention, while the column bottom residue is mainly composed of water and high-boiling-point residues, which will be discharged into the wastewater treatment system.

[0015] In step S2 of the present invention, the dichloromethane acts as an azeotropic agent.

[0016] In some embodiments, in step S2, the mass ratio of the dichloromethane to the initial distillate is (0.5~1):1.

[0017] In some embodiments, in step S2, the mixture of the initial distillate and dichloromethane is used as the distillate to be refined for subsequent azeotropic distillation and dehydration steps.

[0018] In some embodiments, in step S2, the initial distillate is mixed with dichloromethane and allowed to stand to separate into layers. The upper raffinate phase is then removed, and the lower extract phase is used as the distillate to be further purified by azeotropic distillation. This step is a pretreatment step for extraction. By incorporating this step, most of the water can be removed in advance, further shortening the distillation and dehydration cycle. The lower extract phase contains acetonitrile, dichloromethane, and a small amount of alcohol impurities.

[0019] In step S3 of the present invention, heating the liquid to be distilled allows for total reflux operation at the top of the column after a stable condensate appears. The purpose of total reflux at the top of the column is to fully wet the packing material inside the column and achieve a stable mass transfer state between the gas and liquid phases.

[0020] In some implementations, the total reflux operation at the top of the tower in step S3 lasts for 0.8-1.2 hours.

[0021] In step S3 of the present invention, those skilled in the art should understand that the top total reflux operation is performed first, and the top total reflux is stopped after a specific operation time is reached, and then the in-bottle reflux mode is started; and during the in-bottle reflux process, the top condensate is allowed to stand and separate into layers in the liquid separator at the top of the column to form an upper aqueous phase and a lower dichloromethane phase, and then the upper aqueous phase and the lower dichloromethane phase are collected and refluxed respectively.

[0022] In some embodiments, in step S3, when the total reflux at the top of the column ends and the reflux in the reactor begins (i.e., in the initial stage of reflux in the reactor), the reflux ratio of the dichloromethane phase is controlled to be 1:1-1:4, wherein the reflux ratio is the ratio of the reflux flow rate at the top of the column to the reflux flow rate in the reactor.

[0023] In some embodiments, step S3, the reflux in the reboiler includes: continuously or intermittently refluxing the dichloromethane phase obtained by stratifying the condensate at the top of the column to below the working liquid level of the reboiler (distillation heating vessel).

[0024] In step S3 of the present invention, as the water in the column bottom is continuously removed, the proportion of light components decreases and the temperature at the top of the column gradually increases. At this time, the temperature at the top of the column can be brought back and maintained within the dehydration temperature range required by the present application by increasing the reflux ratio and reducing the output. Increasing the reflux ratio means increasing the proportion of reflux flow at the top of the column and / or decreasing the proportion of reflux flow in the column bottom.

[0025] In some embodiments, during step S3, the reflux process in the reactor is carried out by adjusting the reflux ratio of the dichloromethane phase and / or the heating power of the distillation heating reactor to control the top temperature of the column within the dehydration range of 48~58°C until dehydration is completed.

[0026] In a specific implementation, during the reflux process in the reactor, when the temperature at the top of the column exceeds 58°C, the reflux ratio of the dichloromethane phase is increased to 3:1-4:1, thereby stabilizing the composition at the top of the column and improving stability by increasing the proportion of reflux flow at the top.

[0027] In step S3 of the present invention, during the reflux process in the reactor, the temperature of the refluxed dichloromethane phase is about 25-30°C. The temperature inside the reactor is greater than the temperature at the top of the reactor and is also much greater than the temperature of the dichloromethane phase. This causes the dichloromethane phase to vaporize instantly when it refluxes to the reactor, generating a large number of microbubbles, thereby forming a multi-component mixed gas phase.

[0028] In some embodiments, the liquid separation device includes an intelligent liquid separation device equipped with interface detection and automatic control elements, which are used to automatically control the opening and closing of the valve for the aqueous phase extraction and the valve for the dichloromethane phase reflux based on the interface height between the aqueous phase and the dichloromethane phase, thereby realizing the automated operation of the liquid separation and reflux process without frequent manual operation.

[0029] In some embodiments, during step S3, in the azeotropic distillation dehydration stage, the top exhaust valve is closed to maintain the gauge pressure inside the packed distillation column below 5 kPa until dehydration is complete. Since the top temperature is maintained at 48-58°C, the proportion of dichloromethane in the top vapor phase decreases while the proportion of high-boiling-point components increases. By maintaining the gauge pressure inside the packed distillation column below 5 kPa, the vapor phase volatilization loss of dichloromethane can be reduced to below 1%. The top exhaust valve is only interlocked and opened to release pressure when the system is overpressured.

[0030] In some implementations, the criteria for determining the completion of dehydration in step S4 include: the condensate at the top of the column no longer separates into layers, or the water content in the liquid in the distillation heating vessel is ≤0.3% when sampled and tested.

[0031] In some embodiments, in step S5, the acetonitrile component extracted has a purity of ≥99.5% and a moisture content of ≤0.2%.

[0032] In this invention, the acetonitrile waste liquid may include typical industrial acetonitrile waste liquid.

[0033] In some embodiments, the acetonitrile waste liquid comprises 30 wt%-40 wt% acetonitrile, 55 wt%-65 wt% water, 1 wt%-2 wt% methanol, and 1 wt%-2 wt% ethanol, where wt% is the mass percentage of each component in the acetonitrile waste liquid.

[0034] In this invention, step S4 is the recovery step of dichloromethane (azeotropic agent), and the temperature at the top of the column is controlled by adjusting the heating power of the column bottom and the reflux ratio; the dichloromethane recovered in this way can be recycled for the next batch of processes; step S5 is the purification and extraction step of acetonitrile, sampling and testing the composition of the fraction, and when the purity of acetonitrile in the acetonitrile component reaches the target value, the extraction pipeline is switched to collect qualified acetonitrile product.

[0035] The present invention also provides a system for the distillation and dehydration of acetonitrile-water azeotrope, the system being used to implement the method described above, and comprising: a distillation column reboiler unit, a column top condenser unit, a reflux distributor, an intelligent liquid separator, a column top reflux pipeline, a reboiler reflux pipeline, an aqueous phase collection pipeline, a detection unit, and a control unit; The distillation column reboiler unit includes a distillation heating reboiler and a packed distillation column that are connected to each other. The top condensation unit is located at the top of the packed distillation column, and the condensate outlet of the top condensation unit is connected to the inlet of the reflux distributor; the outlet of the reflux distributor is connected to the feed inlet of the intelligent liquid separator. The intelligent liquid separator has an upper aqueous phase outlet and a lower oil phase outlet; the aqueous phase outlet is connected to the aqueous phase collection pipeline, the oil phase outlet is connected to the inlet of the top reflux pipeline and the inlet of the inlet reflux pipeline, respectively, the outlet of the top reflux pipeline is connected to the top liquid distributor of the packed distillation column, and the outlet of the inlet reflux pipeline extends below the working liquid level of the distillation heating vessel; the top reflux pipeline and the inlet reflux pipeline are switched by valves to achieve independent operation or proportional flow distribution; The detection unit includes at least a temperature detection element and a pressure detection element located at the top of the packed distillation column; The control unit is connected to the detection unit, the heating element of the distillation kettle, and the reflux distributor, respectively, and is used to adjust the reflux ratio of the material and / or the heating power of the distillation kettle according to the top temperature of the column.

[0036] In this invention, preferably, the distillation heating kettle is equipped with an electric heating or steam heating element; the packed distillation column is filled with structured packing to enhance gas-liquid two-phase mass transfer; the column top condensation unit uses circulating water to cool the gas phase fraction; the intelligent liquid separator is a vertical intelligent liquid separator, which, through structural and intelligent interface monitoring design, ensures that the aqueous phase outlet is oil-free, and the oil phase is stably and controlledly discharged from the oil phase outlet of the intelligent liquid separator. The column top condensate enters the intelligent liquid separator through a reflux distributor and is allowed to stand and separate into layers, forming an upper aqueous phase and a lower dichloromethane oil phase.

[0037] In this invention, the control unit can be used to adjust the reflux ratio of the material and / or the heating power of the distillation kettle according to the top temperature of the column, so as to stably control the top temperature of the column at 48~58℃.

[0038] In some embodiments, the intelligent liquid separation device is equipped with a float-type interface detector, which is matched with a density of 1.10~1.20 g / cm³. 3 The float; the float-type boundary detector is signal-connected to the control unit and is used to output a real-time detection signal of the two-phase boundary.

[0039] The density is 1.10~1.20 g / cm³. 3 The float has a density between that of water and dichloromethane, and can be suspended at the interface between the two phases.

[0040] In some embodiments, the top of the intelligent liquid separation device is provided with an exhaust gas valve, which is signal-connected to the control unit and used to control the exhaust gas valve to remain normally closed during the dehydration stage. When the pressure of the system exceeds a safety threshold, the exhaust gas valve is interlocked to open to release pressure.

[0041] In some embodiments, the outlet of the reflux pipeline inside the vessel is provided with a liquid distribution structure, which is located below the working liquid level of the distillation heating vessel (or, the liquid distribution structure is immersed in the liquid phase of the distillation heating vessel). The refluxed dichloromethane phase is dispersed and discharged through the liquid distribution structure, forming a dispersion vaporization center in the liquid phase of the bottom of the column.

[0042] In some embodiments, the top reflux line and the bottom reflux line are directly connected to the oil phase outlet of the intelligent liquid separator. The dichloromethane oil phase after separation can be switched by a valve to distribute its flow direction: a portion is sent to the bottom of the column via the bottom reflux line, and the remaining portion is returned to the top of the distillation column via the top reflux line. The flow ratio of the two phases is automatically adjusted by the bottom reflux control valve. The outlet of the bottom reflux line extends downward to below the normal working liquid level inside the bottom of the column. A liquid distribution structure is configured at the end of the bottom reflux line. The refluxed dichloromethane phase is dispersed and sprayed into the liquid phase inside the bottom through the liquid distribution structure, forming dispersed vaporization centers and enhancing gas-liquid contact and mass transfer.

[0043] In this invention, the aqueous phase extraction pipeline is connected to the aqueous phase outlet of the intelligent liquid separation device for discharging the separated wastewater, which can be sent to the plant's wastewater treatment system for unified treatment.

[0044] In this invention, by setting up a control unit, the reflux and separation process in the reactor can be fully automated through temperature and interface detection and control valves, eliminating the need for frequent manual separation and addition of azeotropic agents, reducing labor intensity, and improving process stability and batch reproducibility; the system is simple to operate and easy to automate.

[0045] In some embodiments, the control unit includes a DCS control unit, which has a built-in phased control program for automatically switching the operating parameters of the full reflux stage, dehydration stage, azeotropic agent recovery stage and product extraction stage, so as to realize fully automated operation of the entire process.

[0046] The total reflux stage corresponds to the total reflux at the top of the column in step S3 above, the dehydration stage corresponds to the reflux in the kettle or azeotropic distillation dehydration in step S3 above, the azeotropic agent recovery stage corresponds to step S4 above, and the product output stage corresponds to step S5 above.

[0047] The core function of the DCS control unit is to automatically adjust the flow ratio of the top and bottom of the column during reflux and / or the heating power of the bottom of the column based on the feedback signal of the top temperature of the column, so as to stably maintain the top temperature of the column within the target range of 48~58℃; at the same time, it can automatically control the opening and closing of the reflux and the outflow valves based on the interface signal of the intelligent liquid separator, so as to realize the fully automated operation of the process.

[0048] In some embodiments, the detection unit includes a temperature detection element located at the bottom of the column, a temperature detection element located at the top of the column, a pressure detection element located at the top of the column, a boundary detection element in the intelligent liquid separator, a liquid level detection element at the bottom of the column, and a reflux flow detection element at the top of the column and inside the bottom.

[0049] In some embodiments, the system further includes a primary distillation column and an extraction tank, with the fraction outlet of the primary distillation column connected to the inlet of the extraction tank, and the lower liquid phase outlet of the extraction tank connected to the rectification heating vessel, thus forming a complete acetonitrile waste liquid recovery and refining production line.

[0050] Compared with the prior art, the present invention has the following significant advantages: 1. Significantly Improved Dehydration Efficiency: This invention, by adding an in-bottle reflux dichloromethane operation mode and controlling the tower top temperature within the range of 48~58℃, breaks the compositional limitation of the binary azeotropic dichloromethane-water process in traditional processes, increasing the water content in the tower top condensate to 7~18wt%, far exceeding the 1.4wt% of the traditional process. Actual operation shows that the dehydration rate in the method of this application embodiment is more than 2.6 times higher than the traditional tower top reflux process, and the single batch dehydration time is shortened from more than 20 hours to about 7.5 hours, significantly shortening the production cycle and reducing unit energy consumption.

[0051] 2. Significantly reduced azeotropic agent loss: During the azeotropic distillation dehydration stage, the rise in column top temperature and changes in gas phase composition maintain a slight positive pressure inside the column, which can significantly reduce the escape of dichloromethane with the tail gas. According to actual measurements, the dichloromethane loss rate in the method of this application can be reduced to below 1%, which is far lower than the loss level of 6% to 32% in traditional processes, effectively reducing operating costs and organic waste gas emissions, and meeting the requirements of clean production.

[0052] 3. Strong adaptability to impurities: This process can effectively treat acetonitrile waste liquid containing light components such as methanol and ethanol. Alcohol impurities can be removed together with the aqueous phase during the dehydration stage, and will not accumulate in the system to affect product purity. It is suitable for industrial acetonitrile waste liquid with complex composition and has a wider range of applications.

[0053] 4. High product quality and yield: The acetonitrile product obtained by the method of this invention has a purity of over 99.5% and a moisture content of ≤0.2%, meeting the requirements of the standard "SH / T 1627.1-2014 Industrial Acetonitrile"; in addition, the effective yield of acetonitrile can reach 93.97%, which has excellent industrial application value.

[0054] 5. Outstanding benefits: The combined effects of shortened production cycle, reduced azeotropic agent loss, and decreased energy consumption can significantly reduce the unit cost of acetonitrile recovery, thereby enhancing the economic benefits and market competitiveness of the enterprise's resource utilization process. Attached Figure Description

[0055] Figure 1 This is a schematic diagram of the acetonitrile-water azeotropic distillation and dehydration system of Embodiment 3 of the present invention; Figure 2 This is a schematic diagram of the intelligent liquid separation device in the system of Embodiment 3 of the present invention; Figure 3 This is a schematic diagram of the liquid distribution structure at the end of the reflux pipe inside the vessel, which is submerged below the liquid surface in the system of Embodiment 3 of the present invention.

[0056] Explanation of reference numerals in the attached figures: Distillation heating vessel 1; residual discharge line 1-1; feed line 1-2; reflux pipe inside the vessel 1-3; 2. Packed distillation column; 2-1. Top liquid distributor; Tower top condensation unit 3; circulating water inlet 3-1; circulating water outlet 3-2; Return distributor 4; Return distributor inlet 4-1; Return distributor outlet 4-2; Return distributor return interface 4-3; Intelligent liquid separation device 5; exhaust gas discharge valve 5-1; aqueous phase outlet 5-2; oil phase outlet 5-3; intelligent interface monitoring meter 5-4; Feed valve 61; Distributor outlet valve 62; Waste gas discharge valve 63; Aqueous phase outlet valve 64; Oil phase outlet valve 65; Oil phase main valve 66; Reactor reflux control valve 67; Aqueous phase zone 71; Oil phase zone 72; Mixed phase zone 73; Aqueous phase overflow weir 74; Drain outlet 75. Detailed Implementation

[0057] The present invention will be further described in detail below through preferred embodiments, but the scope of protection of the present invention is not limited thereto.

[0058] Example 1 This embodiment discloses a method for dehydrating acetonitrile-water azeotrope by distillation. This embodiment uses a small-scale laboratory setup to verify the basic dehydration effect of the method. The specific steps are as follows: (1) Raw material pretreatment: Take 1000g of acetonitrile waste liquid, which is composed of 35.2wt% acetonitrile, 61.3wt% water, 1.7wt% methanol and 1.8wt% ethanol; add it to an atmospheric distillation apparatus for initial distillation, control the temperature at the top of the column to about 90℃, collect the initial distillate, and obtain about 450g of distillate with a water content of about 20wt%.

[0059] (2) Azeotropic agent mixing: Add 400g of dichloromethane to the initial distillate, stir evenly, and then add it all to the distillation column bottom unit. The distillation column bottom unit includes a distillation heating kettle and a packed distillation column connected to each other. The packed distillation column is a glass packed distillation column with an inner diameter of 25mm and a packing height of 1.2m.

[0060] (3) Azeotropic distillation dehydration: Turn on the electric heating of the distillation kettle. After the top condensate of the glass-packed distillation column is stably present, perform total reflux operation for 1 hour. Then, stop the total reflux and turn on the reflux in the kettle. Introduce all the top condensate into the pear-shaped intelligent separating funnel for stratification. Adjust the heating power of the distillation kettle and the reflux ratio of the dichloromethane phase to control the top temperature to be stable at 50~55℃. The intelligent separating funnel is equipped with interface detection and automatic control elements to automatically control the opening and closing of the valve for the aqueous phase and the valve for the dichloromethane phase reflux according to the interface height between the aqueous phase and the dichloromethane phase. The lower dichloromethane phase and the upper aqueous phase are obtained through the intelligent separating funnel. The dichloromethane phase is refluxed to below the liquid surface of the bottom of the column through the polytetrafluoroethylene pipeline, and the aqueous phase is metered intermittently. During the above process, as the temperature at the top of the column increases, the reflux ratio is gradually increased from the initial 1:1 to 4:1 to maintain the top temperature within the target range. During the azeotropic distillation dehydration stage, the tail gas valve at the top of the column is closed to maintain the gauge pressure inside the packed distillation column at 2-3 kPa.

[0061] (4) Recovery and purification: After dehydration until the condensate at the top of the column no longer separates into layers, stop the reflux in the reactor; adjust the heating power to control the temperature at the top of the column at 39~41℃ and the reflux ratio at 4:1, collect the recovered dichloromethane and weigh it; when the temperature at the top of the column rises to about 81℃, take a sample for testing. The acetonitrile purity is 99.62% and the moisture content is 0.16%. The qualified acetonitrile product is then collected.

[0062] In this embodiment, the total dehydration time was 7.2 hours, and the total dichloromethane loss rate was 0.87%. Based on the total amount of acetonitrile in the raw material waste liquid, the acetonitrile recovery rate was 92.8%, and the product quality met the first-class standard for industrial acetonitrile.

[0063] Comparative Example 1 This comparative example discloses a traditional azeotropic distillation method.

[0064] This comparative example uses the exact same raw materials, feed rate, and distillation column reboiler unit as Example 1; however, it only uses the traditional top reflux process in the reflux process. The specific difference from Example 1 is that after the top condensate is separated, the dichloromethane phase is returned to the packed distillation column from the top reflux port, and the top temperature is controlled to be stable at 38~40℃. All other operating conditions are the same as in Example 1.

[0065] According to the test and calculation, the total dehydration time of this comparative example was 20.5 hours, the dichloromethane loss rate was 33.6%, and the acetonitrile recovery rate was 87.2%.

[0066] Comparing Example 1 with this comparative example, Example 1 showed a dehydration rate that was increased by about 2.8 times and an azeotropic agent loss that was reduced by more than 97%, demonstrating significant advantages.

[0067] Example 2 This embodiment discloses a method for dehydrating acetonitrile-water azeotropic compounds by distillation. Compared with Embodiment 1, this embodiment employs an extraction pretreatment process before the azeotropic distillation dehydration stage to further shorten the dehydration cycle. The specific steps are as follows: (1) Raw material pretreatment: Same as in Example 1, to obtain 450g of initial distillate with a water content of 20wt%.

[0068] (2) Extraction pretreatment: Add 400g of dichloromethane to the initial distillate, stir thoroughly for 10 minutes, then transfer to a separatory funnel and let stand for 30 minutes to separate the layers. Remove about 120g of the upper aqueous phase and send the lower extract phase to the distillation column bottom unit.

[0069] (3) Azeotropic distillation dehydration: Turn on the electric heating of the distillation kettle. After the top condensate of the glass-packed distillation column stabilizes, perform total reflux for 1 hour. Then, stop the total reflux and turn on the reflux in the kettle, controlling the top temperature of the column at 52~56℃. The reflux ratio is initially 1:1 and gradually increased to 3:1. The lower dichloromethane phase and the upper aqueous phase are obtained through the intelligent separating funnel. The dichloromethane phase is refluxed to the bottom of the column kettle, and the aqueous phase is continuously collected. During the azeotropic distillation dehydration stage, close the tail gas valve to maintain the gauge pressure in the packed distillation column at 2~4 kPa.

[0070] (4) Recovery and purification: After dehydration until the condensate at the top of the column no longer separates into layers, stop the reflux in the reactor; adjust the heating power to control the temperature at the top of the column to 39~41℃ and the reflux ratio to 4:1, collect the recovered dichloromethane and weigh it; when the temperature at the top of the column rises to about 81℃, take a sample for testing, and finally collect the qualified acetonitrile product.

[0071] In this embodiment, the total dehydration time was 5.8 hours (compared to the traditional overhead reflux process, the dehydration rate of this embodiment is increased by approximately 3.5 times), the dichloromethane loss rate was 0.92%, and the acetonitrile recovery rate was 91.5%. It is evident that by removing some free water through extraction pretreatment, the dehydration cycle is further shortened, making it suitable for processing raw materials with high water content.

[0072] Example 3 This embodiment is a pilot-scale industrialization verification test. Figure 1 This is a schematic diagram of the acetonitrile-water azeotropic distillation and dehydration system in this embodiment; Figure 2 This is a schematic diagram of the intelligent liquid separation device in the system of this embodiment.

[0073] The industrial-scale pilot test was conducted on the production-level equipment shown in Figure 1 to verify the industrial feasibility, operational stability, and automation adaptability of the process of this invention. The equipment configuration and operation process are as follows: 1. Experimental apparatus The acetonitrile-water azeotropic distillation and dehydration system includes a distillation column reboiler unit consisting of an interconnected distillation heating kettle 1 and a packed distillation column 2, a column top condenser unit 3, a reflux distributor 4, an intelligent liquid separator 5, and a DCS automatic control system. The supporting piping and valve structure is shown in Figures 1 and 2, and the specific parameters are as follows: Distillation heating vessel 1: effective volume 5000L, made of 304 stainless steel, jacketed steam heating, steam pressure adjustable range 0.1~0.3MPa; the vessel body is equipped with a venting pipe 1-1, a feeding pipe 1-2 and an internal reflux pipe 1-3, and a feed valve 61 is installed on the feeding pipe 1-2; a platinum resistance temperature detection element is installed inside the vessel, and the signal is connected to the DCS system. Figure 3This is a schematic diagram of the liquid distribution structure at the end of the reflux pipe inside the vessel, which is submerged below the liquid surface in this embodiment of the system. (See diagram below.) Figure 3 As shown, the outlet of the reflux pipe inside the vessel is equipped with a liquid distribution structure, which is located below the working liquid level of the distillation heating vessel.

[0074] Packed distillation column 2: The column diameter is φ500mm and the total height is 16m. The column is filled with metal CY type structured corrugated packing. Each packing section is 2.5m high and a liquid redistributor is installed between sections. A top liquid distributor 2-1 is provided at the top of the column to ensure uniform distribution of reflux liquid.

[0075] Top condensing unit 3: Horizontal shell and tube condenser, heat exchange area 20 m² 2 304 stainless steel pipe bundle, equipped with circulating water inlet 3-1 and circulating water outlet 3-2, with circulating cooling water flow rate controlled at 8~12m³ / h during operation. 3 / h, the liquid phase temperature after condensation is ≤35℃.

[0076] Reflux distributor 4: It is equipped with a reflux distributor inlet 4-1, a reflux distributor outlet 4-2 and a reflux interface 4-3. A distributor outlet valve 62 is installed on the pipeline connected to the reflux distributor outlet 4-2. The reflux ratio can be remotely and continuously adjusted through the DCS system, supporting the switching between top reflux and in-bottle reflux modes.

[0077] Intelligent liquid separator 5: The main body is a vertical 304 stainless steel intelligent liquid separator with a diameter of φ400mm and a total height of 1.8m. Its internal structure is as follows: Figure 2 As shown: The tank is divided into an aqueous phase zone 71, a mixed phase zone 73, and an oil phase zone 72 from top to bottom; an aqueous phase overflow weir 74 is installed in the tank to ensure stable overflow discharge of the aqueous phase; an exhaust gas discharge valve 5-1 is installed at the top of the tank, and an aqueous phase outlet 5-2, an oil phase outlet 5-3, and an intelligent interface monitoring meter 5-4 are installed on the side wall; an intelligent liquid separation device vent 75 is installed at the bottom of the tank.

[0078] The intelligent boundary monitoring meter 5-4 is a float-type boundary detector, equipped with a dedicated detection float; the float density is set to 1.10~1.20 g / cm³. 3 The phase is between water (approximately 1.0 g / cm³) and dichloromethane (approximately 1.33 g / cm³), and can be stably suspended at the interface between the two phases, moving synchronously with the rise and fall of the interface height; the detector collects the position of the float through magnetic coupling sensing and outputs a 4-20mA standard remote transmission signal to the DCS control system to realize real-time continuous monitoring of the interface.

[0079] Reflux and Control System: An independent reflux pipe 1-3 is installed inside the vessel. The inlet of the reflux pipe 1-3 is connected to the oil phase outlet 5-3, and the outlet of the reflux pipe 1-3 extends to 200mm below the normal working liquid level of the distillation heating vessel 1. The reflux pipe 1-3 is equipped with an oil phase outlet valve 65, an oil phase main valve 66, and an inside reflux control valve 67. The aqueous phase outlet pipe is equipped with an aqueous phase outlet valve 64, and the waste gas pipe is equipped with a waste gas discharge valve 63. All regulating valves and measuring instruments are connected to the DCS control system, which can realize automatic interlocking of temperature and reflux ratio, automatic sampling of interface, and automatic mode switching.

[0080] 2. Operating Procedures (1) Raw material pretreatment: 4.4t of acetonitrile waste liquid was fed in. The composition of the acetonitrile waste liquid was 35.2wt% acetonitrile, 61.3wt% water, 1.7wt% methanol and 1.8wt% ethanol. After initial distillation in an atmospheric distillation column, the temperature at the top of the column was controlled at 90±2℃ to remove most of the free water and some light impurities, and about 2t of initial distillate with a water content of about 19.5wt% was obtained.

[0081] (2) Feeding and total reflux: 2t of the initial distillate and 2t of dichloromethane are added to the distillation heating vessel 1 through the feed pipeline 1-2, and the jacket steam is turned on to gradually raise the temperature; after the top condenser unit 3 stably produces top condensate, the total reflux operation is carried out for 1.5 hours to fully wet the packing in the column and achieve a stable gas-liquid two-phase mass transfer.

[0082] (3) Azeotropic distillation dehydration: Stop total reflux and switch to in-bottle reflux mode. The condensate at the top of the column is sent to the intelligent liquid separator 5 through the reflux distributor 4 for static stratification. The steam heating amount and reflux ratio are automatically adjusted by the DCS system to stabilize the temperature at the top of the column at 49~57℃. The lower layer of dichloromethane phase is continuously refluxed to below the liquid level of the distillation heating kettle 1 through the oil phase outlet 5-3, the oil phase outlet valve 65, the oil phase main valve 66, and the in-bottle reflux control valve 67. The upper layer of water phase is automatically collected by the water phase outlet valve 64 through the real-time interface signal interlock control of the float-type interface detector.

[0083] During the dehydration stage, the exhaust valve 63 is kept normally closed, and the gauge pressure in the packed distillation column is kept stable at 3~4 kPa. As the water in the vessel is continuously removed, the proportion of light components decreases, and the temperature at the top of the column gradually increases. The system automatically increases the reflux ratio to 3:1~4:1 to bring the temperature at the top of the column back and maintain it in the target range until the two-phase interface in the intelligent liquid separator 5 no longer changes and the condensate has no obvious stratification, at which point the dehydration endpoint is determined.

[0084] (4) Azeotropic agent recovery and product collection: After dehydration, close the reflux control valve 67 in the reactor and switch to the conventional top reflux mode; adjust the heating power and reflux ratio to 3:1~4:1, control the top temperature of the tower to 39~41℃, collect the dichloromethane component, which can be recycled for the next batch of production; when the top temperature of the tower rises to 80~82℃, take samples at intervals for testing, and when the acetonitrile purity is ≥99.5% and the moisture content is ≤0.2%, switch the collection pipeline to collect qualified acetonitrile products.

[0085] 3. Experimental Results In this embodiment, the distillation and dehydration stage takes about 23 hours, while the dehydration cycle of a traditional top reflux process of the same scale is about 62 hours. This process increases the dehydration rate by 2.7 times, achieving and exceeding the expected effect of "more than 2.6 times increase".

[0086] The final acetonitrile product had a purity of 99.71% and a moisture content of 0.12%, meeting the superior grade standard of "SH / T 1627.1-2014 Industrial Acetonitrile". Based on the total amount of acetonitrile in the raw material waste liquid, the effective yield of acetonitrile was 92.13%; the dichloromethane loss rate was 0.94%, stably controlled below 1%, consistent with the loss reduction effect of the pilot test.

[0087] The entire experiment was conducted automatically through a DCS system. The top temperature, internal pressure, two-phase interface, and reflux ratio were all controlled in a closed-loop interlocking manner. The operating parameters were stable, and there was no need for frequent manual separation, feeding, and adjustment operations. This verified the industrial applicability, operational reliability, and automation advantages of the process and supporting equipment of this invention.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for distilling and dehydrating acetonitrile-water azeotrope, characterized in that, The method is carried out in a distillation column reboiler unit containing an interconnected distillation heating vessel and a packed distillation column. The top of the packed distillation column is also connected to a liquid separator for settling and stratifying the condensate at the top of the column. The method includes the following steps: Acetonitrile waste liquid was subjected to atmospheric pressure primary distillation to obtain a primary distillate with a water content of 18%-22%; The initial distillate was mixed with dichloromethane to obtain the distillate to be purified. The liquid to be distilled is fed into the reboiler unit of the distillation column. The reboiler is used to heat the liquid to be distilled. The overhead condensate is first subjected to total reflux at the top of the column, and then the dichloromethane phase obtained from the separation of the overhead condensate is refluxed in the reboiler. At the same time, the aqueous phase obtained from the separation of the overhead condensate is collected for azeotropic distillation and dehydration. During the reflux process in the reboiler, the top temperature is controlled at 48-58°C. After dehydration is complete, stop the reflux operation in the reactor, and then control the temperature at the top of the column to 39-41°C to collect the dichloromethane component; The temperature at the top of the tower is then increased to 80-82°C to extract the acetonitrile component.

2. The method as described in claim 1, characterized in that, The method satisfies at least one of the following conditions: During the atmospheric pressure primary distillation process, the temperature at the top of the column is controlled at 88~92℃; The mass ratio of the dichloromethane to the initial distillate is (0.5~1):1; The mixture of the initial distillate and dichloromethane is used as the distillate to be refined; or, the initial distillate is mixed with dichloromethane, allowed to stand to separate into layers, the upper raffinate aqueous phase is removed, and the lower extract phase is used as the distillate to be refined.

3. The method as described in claim 1, characterized in that, The method satisfies at least one of the following conditions: The operation time for the total reflux at the top of the tower is 0.8-1.2 h; When the total reflux at the top of the column ends and the reflux in the reactor begins, the reflux ratio of the dichloromethane phase is controlled to be 1:1-1:4, wherein the reflux ratio is the ratio of the reflux flow rate at the top of the column to the reflux flow rate in the reactor. The in-bottle reflux includes: continuously or intermittently refluxing the dichloromethane phase obtained by stratifying the condensate at the top of the column to below the working liquid level of the distillation heating vessel; During the reflux process in the reactor, the top temperature of the column is controlled by adjusting the reflux ratio of the dichloromethane phase and / or the heating power of the distillation heating reactor.

4. The method as described in claim 3, characterized in that, During the reflux process in the reactor, when the temperature at the top of the column exceeds 58°C, the reflux ratio of the dichloromethane phase is increased to 3:1-4:1 to stabilize the composition at the top of the column and to ensure stability.

5. The method as described in claim 1, characterized in that, The liquid separation device includes an intelligent liquid separation device, which is equipped with interface detection and automatic control elements to automatically control the opening and closing of the valve for the aqueous phase extraction and the valve for the dichloromethane phase reflux based on the interface height between the aqueous phase and the dichloromethane phase.

6. The method as described in claim 1, characterized in that, The method satisfies at least one of the following conditions: During the azeotropic distillation dehydration stage, the exhaust valve at the top of the column is closed, and the gauge pressure inside the packed distillation column is maintained below 5 kPa until dehydration is complete. The criteria for determining whether dehydration is complete include: the condensate at the top of the column no longer separates into layers, or the water content in the liquid in the distillation heating vessel is ≤0.3% when sampled.

7. The method as described in claim 1, characterized in that, In the extracted acetonitrile component, the purity of acetonitrile is ≥99.5% and the moisture content is ≤0.2%.

8. The method as described in claim 1, characterized in that, The acetonitrile waste liquid includes 30 wt%-40 wt% acetonitrile, 55 wt%-65 wt% water, 1 wt%-2 wt% methanol, and 1 wt%-2 wt% ethanol, where wt% is the mass percentage of each component in the acetonitrile waste liquid.

9. A system for the distillation and dehydration of acetonitrile-water azeotrope, characterized in that, The system is used to implement the method as described in any one of claims 1-8, and includes: a distillation column reboiler unit, a column top condenser unit, a reflux distributor, an intelligent liquid separator, a column top reflux pipeline, a reboiler reflux pipeline, an aqueous phase extraction pipeline, a detection unit, and a control unit; The distillation column reboiler unit includes a distillation heating reboiler and a packed distillation column that are connected to each other. The top condensation unit is located at the top of the packed distillation column, and the condensate outlet of the top condensation unit is connected to the inlet of the reflux distributor; the outlet of the reflux distributor is connected to the feed inlet of the intelligent liquid separator. The intelligent liquid separator has an upper aqueous phase outlet and a lower oil phase outlet; the aqueous phase outlet is connected to the aqueous phase collection pipeline, the oil phase outlet is connected to the inlet of the top reflux pipeline and the inlet of the inlet reflux pipeline, respectively, the outlet of the top reflux pipeline is connected to the top liquid distributor of the packed distillation column, and the outlet of the inlet reflux pipeline extends below the working liquid level of the distillation heating vessel; the top reflux pipeline and the inlet reflux pipeline are switched by valves to achieve independent operation or proportional flow distribution; The detection unit includes at least a temperature detection element and a pressure detection element located at the top of the packed distillation column; The control unit is connected to the detection unit, the heating element of the distillation kettle, and the reflux distributor, respectively, and is used to adjust the reflux ratio of the material and / or the heating power of the distillation kettle according to the top temperature of the column.

10. The system as described in claim 9, characterized in that, The system satisfies at least one of the following conditions: The intelligent liquid separation device is equipped with a float-type interface detector, which is matched with a density of 1.10~1.20 g / cm³. 3 The float; the float-type boundary detector is signal-connected to the control unit and is used to output a real-time detection signal of the two-phase boundary; The top of the intelligent liquid separation device is equipped with an exhaust gas valve, which is signal-connected to the control unit. During the dehydration stage, the exhaust gas valve is kept normally closed. When the system pressure exceeds the safety threshold, the exhaust gas valve is interlocked to open for pressure relief. The outlet of the reflux pipeline inside the vessel is equipped with a liquid distribution structure, which is located below the working liquid level of the distillation heating vessel. The control unit includes a DCS control unit, which has a built-in phased control program. The phased control program is used to automatically switch the operating parameters of the full reflux stage, dehydration stage, azeotropic agent recovery stage and product extraction stage. The system also includes a primary distillation column and an extraction tank. The distillate outlet of the primary distillation column is connected to the inlet of the extraction tank, and the lower liquid phase outlet of the extraction tank is connected to the rectification heating vessel.

Images