Low-concentration DMF-DMAC wastewater purification method based on MVR thermal coupling

Abstract

The invention discloses a low-concentration DMF-DMAC (Dimethyl Formamide-Dimethylacetamide) wastewater purification method based on MVR (Mechanical Vapor Recompression) thermal coupling. Belongs to the technical field of wastewater treatment processes. 2, metering and preheating the wastewater; step 3, MVR (mechanical vapor recompression) concentration circulation; step 4, rectification treatment; 5, deacidification refining: under the combined action of a deacidification tower reboiler and tower top reflux, extracting a DMF finished product from a side line; and 6, heat recycling: performing thermal coupling by a thermal coupling heater, cooling the steam which is not subjected to thermal coupling by a condenser, enabling one part of the steam to flow back into a corresponding tower, and feeding the other part of the steam to sewage treatment. Optimization is carried out on the basis of an MVR process, available heat in the system is coupled, waste heat in the system is recycled by preheating waste water feeding through steam condensate, comprehensive energy consumption is reduced, impurities in a solution are effectively separated, and most separated tower top water can be returned to a workshop to be repeatedly used.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment processes, and in particular to a method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling. Background Art

[0002] In many industrial production fields such as chemical engineering, pharmaceuticals, and textiles, DMF and DMAC are widely used as excellent organic solvents. However, this has also led to the generation of a large amount of low-concentration wastewater containing DMF and DMAC. If these wastewaters are directly discharged without effective treatment, it will not only cause serious environmental pollution but also lead to a large waste of resources. Therefore, purifying and treating low-concentration wastewater containing DMF and DMAC to achieve recycling and reuse has great environmental and economic value.

[0003] Traditional DMF and DMAC wastewater purification technologies have significant defects in several key aspects. From the perspective of the quality of the finished product, the previous processes did not adopt the side-line extraction method. During long-term operation, it was difficult to stably guarantee the quality of the finished product. After testing, the water content in the finished product often exceeded 100 ppm, the formic acid / acetic acid content was greater than 15 ppm, and the dimethylamine content was higher than 5 ppm, which severely limited the application of the recycled product in high-end fields. In the wastewater concentration stage, conventional concentration equipment mostly uses steam or heat-conducting oil as the heating medium and adopts traditional evaporation technology, resulting in high energy consumption. Even if a steam compressor is used to compress and heat up the secondary steam, the overall energy consumption is still far beyond the actual demand. Traditional evaporators not only consume a large amount of energy and have high operating costs, but in actual use, their operating costs are twice that of MVR evaporators, and they also occupy a large area. Compared with multi-effect evaporation, the occupied space is more than 50% larger, and a large amount of cooling water is required, resulting in serious waste of water resources. In addition, the existing ordinary MVR processes lack systematic planning in heat utilization and do not fully exploit the utilization potential of system waste heat, resulting in relatively high overall energy consumption. Most processes only rely on a single energy input and fail to effectively recover and couple the heat generated by steam condensate, rectification towers, and refining towers, causing a large amount of energy to be wasted. In terms of the operating stability of the equipment, traditional processes often set large-flow circulation pumps, which not only increase energy consumption but also cause uneven fluid flow rates in the heat exchanger, resulting in low heat transfer efficiency of the heat exchanger, easy blockage of the tube bundle, and a significant reduction in the operating cycle of the equipment. Frequent equipment maintenance and cleaning further increase the operating costs of enterprises. For the above reasons, this application proposes a method for purifying low-concentration DMF-DMAC wastewater with low energy consumption and high heat recovery utilization rate based on the MVR thermal coupling treatment process. Summary of the Invention

[0004] Aiming at the problems existing in the background art, the object of the present invention is to propose a method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling with low energy consumption and high heat recovery utilization rate.

[0005] Technical solution of the present invention: A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling, comprising the following steps:

[0006] Step 1, wastewater collection and transportation: Collect the low-concentration wastewater containing DMF generated in the production workshop into the wastewater collection tank, and transport it to the DMF recovery device area through the waste liquid feed pump;

[0007] Step 2, wastewater metering and preheating: Meter the wastewater entering the recovery device area, and then send it to the plate heat exchanger for preheating;

[0008] Step 3, MVR concentration cycle: The preheated wastewater enters the MVR falling film evaporator, the generated gas phase enters the MVR concentration tower, the DMF in the tower kettle is continuously concentrated, the water vapor at the top of the tower enters the compressor for compression, and the compressed secondary steam returns to the MVR falling film evaporator to heat the wastewater. During the circulation process, primary steam is supplemented to make up for heat loss and heat loss of the discharged material. Part of the water separated at the top of the tower is returned to the concentration tower through the reflux pump, and part is sent to the workshop for reuse through the top water pump, and the wastewater is concentrated to 85%-90%;

[0009] Step 4, rectification treatment: The concentrated DMF solution after being cooled by the condensation cooler is sent to the rectification tower for rectification. Under the combined action of the rectification tower reboiler and the top reflux, water rises plate by plate and DMF drops plate by plate. When the water content in the rectification tower kettle ≤ 300 ppm, the vapor phase at the tower kettle is taken out and sent to the deacidification tower;

[0010] Step 5, deacidification and refining: Under the combined action of the deacidification tower reboiler and the top reflux, DMF vaporizes and rises plate by plate, and the DMF finished product is taken out from the side line. After being cooled by the pure DMF cooler, it exchanges heat with the concentrated solution discharged from the concentration tower, and then is stored after being cooled by circulating water;

[0011] Step 6, heat recovery and utilization: The steam at the top of the rectification tower and the wastewater feed are thermally coupled in the thermal coupling heater, and the steam at the top of the deacidification tower and the liquid at the bottom of the concentration tower are thermally coupled in the thermal coupling reboiler to realize heat recovery. The steam not thermally coupled is cooled by the condenser, and part of it is returned to the corresponding tower, and part is sent to the sewage treatment.

[0012] Optionally, a flow sensor and a pressure sensor are arranged on the conveying pipeline of the intermediate buffer tank to monitor the flow and pressure of the wastewater in real time. The intelligent control module combines the wastewater parameters fed back by the on-line water quality monitor and the data of the flow and pressure sensors to automatically adjust the operating frequency of the waste liquid feed pump and the opening degree of the valve to achieve stable conveying flow and pressure. When the initial concentration of DMF in the wastewater is relatively high, the conveying flow is reduced and the residence time in the subsequent treatment equipment is increased; when the flow and pressure show abnormal fluctuations, the intelligent control module issues an alarm and automatically adjusts the state of the pump and the valve;

[0013] Optionally, the intelligent control module includes a data acquisition unit, a data analysis and processing unit, a decision-making unit, a control execution unit, and a communication unit;

[0014] The data acquisition unit is responsible for collecting data from various sensors, including on-line water quality monitors, flow sensors, and pressure sensors, etc. The on-line water quality monitor can monitor parameters such as the pH value, initial DMF concentration, and suspended solid content of the wastewater in real time; the flow sensor can obtain the flow data of the wastewater in the conveying pipeline; the pressure sensor is used to monitor the pressure in the conveying pipeline; the data acquisition unit is connected to each sensor in a wired or wireless manner, continuously collects the electrical signals output by the sensors at a certain sampling frequency, and converts them into digital signals for subsequent processing.

[0015] The data analysis and processing unit analyzes and processes the digital signals transmitted by the data acquisition unit. It pre-stores the standard ranges of various wastewater parameters and the correlation models between different parameters; for example, knowing what the appropriate conveying flow rate and pressure range are at a specific initial DMF concentration, by comparing the actually collected data with the standard range, it judges whether the current wastewater treatment process is normal;

[0016] The decision-making unit makes decisions based on the results of the data analysis and processing unit; if it judges that the wastewater treatment process is normal, it will maintain the current operating states of the pump and valve; if an abnormality is found, it will decide what adjustment measures to take according to the preset strategy. The decision-making unit makes decisions based on a rule base and a machine learning model.

[0017] The control execution unit is connected to devices such as pumps and valves, and controls the wastewater conveying process by adjusting the operating frequency of the pump and the opening degree of the valve. At the same time, it is also responsible for controlling the alarm device to send out alarm signals; specifically, the control execution unit receives the digital instructions from the decision-making unit, converts them into corresponding control signals, such as voltage signals or current signals, and sends them to the driving devices of the pump and valve, thereby changing the rotational speed of the pump and the opening degree of the valve. When an alarm needs to be issued, it will control the alarm to emit light and sound signals.

[0018] The communication unit can upload the operating status, detection data, and decision-making results of the intelligent control module to the remote monitoring center for convenient remote monitoring and management by operators; at the same time, it can also receive instructions from the remote monitoring center to adjust the parameters and strategies of the intelligent control module; the communication unit supports multiple communication protocols, such as Modbus, TCP / IP, etc., and conducts data transmission with other devices or systems through a wired network or a wireless network.

[0019] The intelligent control module is used to obtain the data of the water quality on-line monitor, flow sensor and pressure sensor in real time, monitor the water quality parameters, conveying flow rate and pressure of the wastewater in real time, and provide intelligent control strategies.

[0020] Optionally, the plate heat exchanger preheater is E101, which is used to preheat the wastewater entering the recovery device area.

[0021] Optionally, the steam at the top of the rectifying column is thermally coupled with the wastewater feed in the E201 heat-coupling heater, and then exchanged heat in the E204 dehydration condenser to become liquid overhead water.

[0022] Optionally, the DMF finished product taken out from the side line in the fifth step has a water content of ≤100 ppm, formic acid / acetic acid ≤15 ppm, and dimethylamine ≤5 ppm.

[0023] Optionally, in the MVR concentration cycle, the steam compressor raises the temperature of the secondary steam by 15°C.

[0024] Optionally, the fourth step adopts micro-pressure rectification, and the top temperature of the rectifying column is 95°C.

[0025] Optionally, the steam not thermally coupled and utilized in the sixth step is cooled by a condenser and then pumped to the gas-liquid separation tank by the T201 water pump, and then pumped to the sewage treatment by the water pump of the gas-liquid separation tank.

[0026] Optionally, the top temperature of the deacidification tower in the sixth step is 115°C and the temperature of the bottom material of the concentration tower is 95°C for thermal coupling.

[0027] Optionally, the MVR concentration tower, rectifying column, and deacidification tower are under vacuum operation, and their vacuum is automatically adjusted by vacuum pumps.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention uses the MVR steam compression technology to initially concentrate, which solves the problem of high consumption of primary steam and high cost in multi-effect evaporation concentration. The comprehensive treatment cost is reduced from the original 140-150 yuan / ton to 80-90 yuan / ton, greatly reducing the treatment cost of enterprises;

[0030] 2. The present invention ensures that the quality of the finished product can reach a water content of ≤100 ppm, formic acid / acetic acid ≤15 ppm, and dimethylamine ≤5 ppm by taking out the finished product from the side line;

[0031] 3. The present invention has low energy consumption and low operating costs. Theoretically, using an MVR evaporator saves more than 75% of energy compared with a traditional evaporator, and the operating cost of the MVR evaporator is reduced by half in actual use;

[0032] 4. The evaporation equipment of the present invention is compact, with a small floor area and a small required space. It only requires a small amount of cooling water, can save more than 90% of the cooling water, has less supporting utilities, operates stably, has a high degree of automation, uses clean energy, is clean and environmentally friendly. The MVR evaporator mainly uses electricity and has a low labor cost. Only a few operators are needed to ensure the normal operation of the equipment.

[0033] In summary, the present invention is optimized based on the MVR process, couples the available heat in the system, preheats the wastewater feed with steam condensate to recover and utilize the waste heat in the system, reduces the comprehensive energy consumption, effectively separates the impurities in the solution, and most of the separated overhead water can be recycled and reused in the workshop. Brief Description of the Drawings

[0034] Figure 1 The flow chart of the proposed treatment method of the present invention is given. Detailed Embodiments

[0035] The technical solutions of the present application will be further described in detail below in conjunction with the drawings and specific embodiments.

[0036] Generally, the components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the present application claimed, but merely represents the selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts fall within the scope of protection of the present application.

[0037] Embodiment 1

[0038] As Figure 1 shown, a method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling proposed by the present invention. This embodiment is for the treatment of DMF, and the treatment method of DMAC is the same as that of DMF;

[0039] DMF decomposition reaction: HCON(CH3)2 + H2O → (CH3)2NH + HCOOH;

[0040] DMAC decomposition reaction: CH3CON(CH3)2 + H2O → (CH3)2NH + (CH3)COOH;

[0041] Formic acid neutralization reaction: HCOOH + NaOH → HCOONa + H2O;

[0042] Acetic acid neutralization reaction: (CH3)COOH + NaOH → (CH3)COONa + H2O;

[0043] It includes the following steps:

[0044] Step 1: Wastewater collection and transportation: The low-concentration wastewater containing DMF generated in the production workshop is collected into the wastewater collection tank after being buffered by the intermediate buffer tank. Flow sensors and pressure sensors are installed on the transportation pipeline of the intermediate buffer tank to monitor the flow rate and pressure of the wastewater in real time. The intelligent control module combines the wastewater parameters feedback by the on-line water quality monitor and the data of the flow rate and pressure sensors to automatically adjust the operation frequency of the waste liquid feed pump and the opening degree of the valve, so as to achieve stable transportation flow rate and pressure. When the initial concentration of DMF in the wastewater is relatively high, the transportation flow rate is reduced to increase the residence time in the subsequent treatment equipment; when abnormal fluctuations occur in the flow rate and pressure, the intelligent control module issues an alarm and automatically adjusts the states of the pump and the valve.

[0045] The intelligent control module includes a data acquisition unit, a data analysis and processing unit, a decision-making unit, a control execution unit and a communication unit; the intelligent control module is used to obtain the data of the on-line water quality monitor, the flow sensor and the pressure sensor in real time, monitor the water quality parameters, transportation flow rate and pressure of the wastewater in real time, and provide an intelligent control strategy;

[0046] Step 2: Wastewater metering and preheating: The wastewater entering the recovery device area is metered and then sent to the plate heat exchanger for preheating; the plate heat exchanger is E101 and is used to preheat the wastewater entering the recovery device area.

[0047] Step 3: MVR concentration cycle: The preheated wastewater enters the MVR falling film evaporator, and the generated gas phase enters the MVR concentration tower. The DMF in the tower bottom is continuously concentrated, and the water vapor at the tower top enters the compressor for compression. The compressed secondary steam returns to the MVR falling film evaporator to heat the wastewater. The steam compressor raises the temperature of the secondary steam by 15 °C. During the circulation process, primary steam is supplemented to make up for the heat loss and the heat loss of the discharged material. Part of the water separated at the tower top is refluxed to the concentration tower through the reflux pump, and part is sent to the workshop for reuse through the tower top water pump, and the wastewater is concentrated to 85%-90%; the evaporation system adopts a large-flow circulating pump forced circulation evaporation system, which has the advantages of less system dead corners, easy cleaning and long continuous operation time. At the same time, due to the improvement of the heat transfer efficiency of the heat exchanger, the heat energy is fully utilized;

[0048] The concentration enhancement circulation equipment uses steam (or heat transfer oil) as the heating medium. The secondary steam generated is compressed by a steam compressor and the temperature of the secondary steam is increased by about 15°C, with the enthalpy value increasing. In this way, it can re-enter the heating chamber to heat the material continuously in a cycle, and at the same time ensure low-temperature concentration to concentrate the original low-concentration (5-20%) wastewater to 80%-90%, and then pump it to the rectification equipment for rectification. It has the characteristics of low energy consumption and low operating cost. Theoretically, using an MVR evaporator can save more than 75% of the energy compared with traditional evaporators; in actual use, the operating cost of the MVR evaporator is only 50% of that of the traditional evaporator, and the energy consumption changes when the materials are different.

[0049] Step 4, rectification treatment: The concentrated DMF solution after concentration enhancement is cooled by a condensation cooler and then sent to a rectification tower for rectification. The steam at the top of the rectification tower is thermally coupled with the wastewater feed in the E201 heat-coupling heater. The top temperature of the rectification tower is 95°C, and then it is heat-exchanged by the E204 dehydration condenser to form liquid top water; under the combined action of the reboiler of the rectification tower and the top reflux, water rises plate by plate and DMF descends plate by plate. When the water content in the bottom of the rectification tower ≤ 300 ppm, the vapor phase at the bottom of the tower is taken out and sent to the deacidification tower; the heat of the rectification tower is used for thermal coupling with the material in the concentration enhancement tower to recycle the heat of the rectification tower and save about 12% of the energy;

[0050] Step 5, deacidification and refining: Under the combined action of the reboiler of the deacidification tower and the top reflux, DMF vaporizes and rises plate by plate, and the DMF finished product is taken out from the side line. The water content of the DMF finished product taken out from the side line is ≤ 100 ppm, formic acid / acetic acid ≤ 15 ppm, and dimethylamine ≤ 5 ppm. After being cooled by the pure DMF cooler, it is heat-exchanged with the concentrated solution discharged from the concentration enhancement tower, and then stored after being cooled by circulating water;

[0051] Step 6, heat recovery and utilization: The steam at the top of the rectification tower is thermally coupled with the wastewater feed in the heat-coupling heater, and the steam at the top of the deacidification tower is thermally coupled with the bottom liquid of the concentration enhancement tower in the heat-coupling reboiler to achieve heat recovery. The top temperature of the deacidification tower is 115°C and the temperature of the bottom material of the concentration enhancement tower is 95°C for thermal coupling. The steam not utilized by thermal coupling is cooled by a condenser. Part of it is refluxed to the corresponding tower, and part of it is sent to sewage treatment. Specifically, the steam not utilized by thermal coupling is cooled by a condenser and pumped to the gas-liquid separation tank by the water pump T201, and then pumped to sewage treatment by the water pump of the gas-liquid separation tank;

[0052] In this embodiment, the rectification tower is T201, the deacidification tower is T301, and the T101 concentration enhancement tower, T201 rectification tower, and T301 deacidification tower are under vacuum operation, and are suctioned by a vacuum pump and the vacuum degree is automatically adjusted.

[0053] In this embodiment, a large-flow circulation pump is not provided, which improves the heat transfer efficiency of the heat exchanger, makes the tube bundle in the heat exchanger not easily blocked, and extends the operation cycle of the equipment.

[0054] Specific application: A large chemical fiber manufacturing factory uses DMF as a solvent in the production process, and about 100 cubic meters of low-concentration wastewater containing DMF is generated every day, with a DMF content of about 12%. To respond to the policy requirements of energy conservation, emission reduction, and resource recovery and utilization, the factory adopts the wastewater purification method proposed by the present invention and constructs a DMF wastewater recovery and treatment system.

[0055] Wastewater collection and transportation: A wastewater collection tank with a volume of 800 cubic meters is built to temporarily store the low-concentration DMF wastewater discharged from the production workshop. The waste liquid feed pump P103 with the model IS100-80-160 is selected, with a flow rate of 30 cubic meters per hour and a head of 40 meters, which can transport the wastewater from the collection tank to the DMF recovery device area.

[0056] Wastewater metering and preheating: An electromagnetic flowmeter is installed on the wastewater transportation pipeline to accurately measure the wastewater entering the recovery device area. The E101 plate heat exchanger is used for preheating the wastewater. Its heat transfer area is 80 square meters, and the steam condensate is used as the heat source to preheat the wastewater. The initial temperature of the steam condensate is 100 °C, and the initial temperature of the wastewater is 25 °C. After preheating, the wastewater temperature is raised to 55 °C.

[0057] MVR concentration cycle: The MVR falling film evaporator E104 with an evaporation area of 500 square meters is adopted, and the MVR concentration tower T101 with a tower diameter of 2.5 meters and a tower height of 18 meters is supporting. The centrifugal steam compressor C101 is selected, which can raise the temperature of the secondary steam by 15 °C to meet the circulating heating requirements. A reflux pump P105 with a flow rate of 15 cubic meters per hour and a top tower water pump P104 with a flow rate of 20 cubic meters per hour are set at the top of the tower.

[0058] Rectification treatment: The rectification tower T201 has a tower diameter of 2 meters and a tower height of 25 meters, and is equipped with a reboiler with a heat load of 800 kW. The steam at the top of the rectification tower is thermally coupled with the wastewater feed in the E201 thermal coupling heater with a heat transfer area of 150 square meters, and then cooled by the E204 dehydration condenser with a heat transfer area of 80 square meters.

[0059] Deacidification and refining: The deacidification tower T301 has a tower diameter of 1.5 meters and a tower height of 19 meters, and is equipped with a reboiler with a heat load of 400 kW. The DMF finished product taken out from the side line is cooled by the pure DMF cooler and exchanges heat with the concentrated liquid discharged from the concentration tower to further recover heat.

[0060] Heat recovery and vacuum system: The steam at the top of the deacidification tower and the bottom liquid of the concentration tower are thermally coupled in the E203 heat-coupled reboiler with a heat transfer area of 30 square meters. The steam not utilized by the thermal coupling is cooled by a condenser and then pumped by the water pump at the outlet of T201 to the gas-liquid separation tank. The 2BV6131 type water ring vacuum pump is selected to provide a vacuum environment for the T101 concentration tower, the T201 rectification tower, and the T301 deacidification tower and automatically adjust the vacuum degree.

[0061] Implementation effect: After 3 consecutive months of operation tests, this set of system is stable and reliable. It not only realizes the efficient recovery of DMF, and the recovered DMF finished product meets the high-standard quality requirements, but also greatly reduces the energy consumption through the heat coupling technology, significantly reduces the operation cost, bringing good economic and environmental benefits to the enterprise.

[0062] The above specific embodiments are only optional embodiments of the present invention. Based on the technical solution of the present invention and the relevant inspirations of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A purification method for low-concentration DMF-DMAC wastewater based on MVR thermal coupling, characterized in that It includes the following steps: Wastewater collection and transportation: The low-concentration wastewater containing DMF generated in the production workshop is buffered by the intermediate buffer tank and then collected in the wastewater collection tank, and is transported to the DMF recovery device area through the waste liquid feed pump; Wastewater metering and preheating: The wastewater entering the DMF recovery device area is metered and then sent to the plate heat exchanger for preheating; MVR concentration cycle: The preheated wastewater enters the MVR falling film evaporator, and the generated gas phase enters the MVR concentration tower. The DMF in the tower bottom is continuously concentrated, and the water vapor at the top of the tower enters the compressor for compression. The compressed secondary steam returns to the MVR falling film evaporator to heat the wastewater. During the cycle, primary steam is supplemented to make up for heat loss and heat loss of the discharged material. The water separated at the top of the tower, part of it is refluxed to the concentration tower through the reflux pump, and part of it is sent to the workshop for reuse through the top water pump, and the wastewater is concentrated to 80%-90%; Rectification treatment: The concentrated DMF solution after concentration is cooled by the condensation cooler and then sent to the rectification tower for rectification. Under the combined action of the rectification tower reboiler and the top reflux, water rises plate by plate and DMF drops plate by plate. When the water content in the rectification tower bottom ≤ 300 ppm, the vapor phase at the tower bottom is taken out and enters the deacidification tower; Deacidification and refining: Under the combined action of the deacidification tower reboiler and the top reflux, DMF vaporizes and rises plate by plate, and the DMF finished product is taken out from the side line. After being cooled by the pure DMF cooler, it exchanges heat with the concentrated solution discharged from the concentration tower, and then is stored after being cooled by circulating water; Heat recovery and utilization: The steam at the top of the rectification tower and the wastewater feed are thermally coupled in the thermal coupling heater, and the steam at the top of the deacidification tower and the liquid at the bottom of the concentration tower are thermally coupled in the thermal coupling reboiler to realize heat recovery. The steam not thermally coupled is cooled by the condenser, and part of it is refluxed to the corresponding tower, and part of it is sent to the sewage treatment; 2. The method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, wherein A flow sensor and a pressure sensor are arranged on the conveying pipeline of the intermediate buffer tank to monitor the flow and pressure of the wastewater in real time. The intelligent control module combines the wastewater parameters fed back by the on-line water quality monitor and the data of the flow and pressure sensors to automatically adjust the operating frequency of the waste liquid feed pump and the valve opening to achieve stable conveying flow and pressure. When the initial concentration of DMF in the wastewater is relatively high, the conveying flow is reduced and the residence time in the subsequent treatment equipment is increased; When the flow and pressure show abnormal fluctuations, the intelligent control module issues an alarm and automatically adjusts the states of the pump and the valve; The intelligent control module includes a data acquisition unit, a data analysis and processing unit, a decision-making unit, a control execution unit and a communication unit; the intelligent control module is used to obtain the data of the on-line water quality monitor, the flow sensor and the pressure sensor in real time, monitor the water quality parameters, conveying flow and pressure of the wastewater in real time, and provide an intelligent control strategy; The plate heat exchanger is E101 for preheating the wastewater entering the recovery device area.

3. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 2, characterized in that, The steam at the top of the rectification tower and the wastewater feed are thermally coupled in the E201 thermal coupling heater and then exchanged heat through the E204 dehydration condenser to form liquid top water.

4. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, characterized in that, In the deacidification and refining, the DMF finished product taken out from the side line has a water content ≤ 100 ppm, formic acid / acetic acid ≤ 15 ppm, and dimethylamine ≤ 5 ppm.

5. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, characterized in that, In the MVR concentration cycle, the steam compressor raises the temperature of the secondary steam by 15 °C.

6. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, characterized in that, In the rectification process, micro-negative pressure rectification is adopted, and the top temperature of the rectification column is 95 °C.

7. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, characterized in that, In the heat recovery and utilization, the steam that is not utilized by thermal coupling is cooled by a condenser and then pumped out by the pump T201 to the gas-liquid separation tank, and then pumped out by the pump of the gas-liquid separation tank to the sewage treatment.

8. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 7, characterized in that, In the heat recovery and utilization, the top temperature of the deacidification tower is 110 - 120 °C and the bottom temperature of the concentration tower is 90 - 100 °C for thermal coupling.

9. A method for purifying low-concentration DMF-DMAC wastewater based on MVR thermal coupling according to claim 1, characterized in that The rectification column is T201, the deacidification tower is T301, and the concentration tower T101, the rectification tower T201, and the deacidification tower T301 are under vacuum operation, and are sucked by a vacuum pump and the vacuum degree is automatically adjusted.

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