A distillation device and N,N-dimethylformamide purification process based on the distillation device

Abstract

The present invention discloses a distillation apparatus and a N,N-dimethylformamide purification process based on the distillation apparatus, relating to the field of DMF purification technology, in order to solve the problem that the existing process only performs a distillation impurity removal process during purification, so that the overall process purity is not high, and the problem of the high purity demand of DMF in certain fields cannot be met. The distillation apparatus also includes a dryer, which is arranged at the rear of the upper end of the distillation reactor; a second electrically controlled three-way valve, which is arranged at the upper end of the condenser, and the second electrically controlled three-way valve is connected to the distillation gas outlet through a distillation delivery pipe; a third electrically controlled three-way valve, which is arranged at the lower end of the condenser, and the condenser is connected to a storage tank through the third electrically controlled three-way valve, and the lower end of the storage tank is provided with a feed valve; a pressure reducing port, which is arranged between the raw material addition port and the distillation gas outlet; a reflux pipe outlet joint, which is arranged at the front end of the storage tank.

Description

Technical Field

[0001] The invention belongs to the technical field of DMF purification, and in particular relates to a distillation device and an N,N-dimethylformamide purification process based on the distillation device. Background Art

[0002] N,N-Dimethylformamide (DMF), known as a "universal organic solvent" or "all-purpose solvent," has a wide range of applications, including leather, synthetic fibers, pharmaceuticals, petrochemicals, electronics, dyes, coatings, and metalworking. DMF products contain various impurities, such as water, trace amounts of ethanol, and primary and secondary amines. Effectively removing these impurities without compromising DMF purity is challenging. Some DMF applications require high purity levels, exceeding 99.8% or even higher. Meeting these high purity requirements requires sophisticated purification techniques and strict control.

[0003] The current DMF purification process is to first pass the raw material N,N-dimethylformamide through a calcium hydride adsorption column; then the adsorbed N,N-dimethylformamide is distilled under reduced pressure with alumina at 5-10 mmHg; finally, the distillate is filtered through a 0.45 μm microporous filter membrane, bottled, and stored in a nitrogen atmosphere to obtain chromatographic grade N,N-dimethylformamide.

[0004] However, the existing process only performs one distillation and impurity removal treatment during purification, resulting in a low degree of purification of the overall process and being unable to meet the high purity requirements of DMF. Summary of the Invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a device that can realize cyclic distillation and impurity removal to improve the purity.

[0006] Technical solution: The distillation apparatus of the present invention comprises a distillation reactor, a condenser and a storage tank. A reflux pipe inlet joint and a raw material addition port are installed above one side of the distillation reactor. A gas outlet is installed at the upper end of the other side of the distillation reactor. The reflux pipe inlet joint and the raw material addition port are both externally connected to a valve seat. A residual material discharge valve is installed at the lower end of the distillation reactor. The distillation apparatus also includes:

[0007] A dryer is provided at the rear of the upper end of the distillation reactor; a first electrically controlled three-way valve is installed inside the distillation delivery pipe, and a front distillation gas outlet is installed at the upper end of the first electrically controlled three-way valve;

[0008] a second electrically controlled three-way valve, which is installed at the upper end of the condenser and is connected to the distillation gas outlet through a distillation delivery pipe;

[0009] a third electrically controlled three-way valve, which is installed at the lower end of the condenser, the condenser is connected to the storage tank through the third electrically controlled three-way valve, and a discharge valve is installed at the lower end of the storage tank;

[0010] a pressure reducing port, installed between the raw material adding port and the distilled gas outlet;

[0011] The reflux pipe outlet joint is arranged at the front end of the storage tank, and the reflux pipe outlet joint on the storage tank is connected to the reflux pipe inlet joint on the distillation reactor through the reflux pipe.

[0012] Furthermore, the dryer of the distillation device includes a feed hopper, a feed section, a discharge section and an electrically controlled feed valve. The feed section is installed at the lower end of the feed hopper, and the lower end of the feed section and the upper end of the discharge section are both installed with connecting plates. The connecting plates are connected by a flexible connecting section, and an isolation net is installed at the bottom end of the discharge section.

[0013] Furthermore, the four sides of the connecting plate of the distillation device are connected by springs, a damping rod is installed inside the spring, and a vibration motor is installed on one side of the outer wall of the feeding section.

[0014] Furthermore, a jacket is provided on the outer wall of the distillation reactor of the distillation device, a heat transfer liquid inlet is installed at the upper end of the jacket side wall, a heat transfer liquid outlet is installed at the lower end of the jacket side wall, and an electric heating component is installed inside the jacket.

[0015] Furthermore, a stirring motor is provided on the distillation reactor of the distillation apparatus, an output end of the stirring motor penetrates and extends into the interior of the distillation reactor, and is equipped with a stirring shaft.

[0016] Furthermore, the condenser of the distillation device includes a first condensation chamber and a second condensation chamber, and the first condensation chamber and the second condensation chamber are respectively connected to different branches on the second electrically controlled three-way valve and the third electrically controlled three-way valve; a separation section is provided at the lower end of the second condensation chamber, a discharge section is provided below one end of the separation section, an electrically controlled discharge valve is installed at the lower end of the discharge section, an inclined mesh plate is installed inside the separation section, and a blocking support block is installed below one end of the inclined mesh plate.

[0017] Furthermore, a deflector tube is installed inside the first condensing chamber of the distillation device, a cooling water inlet is installed at the upper end of one side of the first condensing chamber, and a cooling water outlet is installed at the lower end of one side of the first condensing chamber.

[0018] Furthermore, a straight-through pipe is installed inside the second condensing chamber of the distillation device, a partition is installed at the lower end of the straight-through pipe, and the straight-through pipe passes through and extends to the outside of the partition, a liquid nitrogen inlet is installed at the upper end of one side of the second condensing chamber, and a liquid nitrogen outlet is provided at the lower end of one side of the second condensing chamber.

[0019] The purification process of N,N-dimethylformamide of the present invention is based on the above-mentioned distillation apparatus and comprises the following steps:

[0020] (1) Industrial-grade N,N-dimethylformamide is passed through a dryer to initially absorb the water therein and then enters a distillation reactor;

[0021] (2) adding 3-5% by weight of alumina powder to the distillation reactor from the raw material addition port to carry out the reaction. After the reaction is completed, the pressure in the reactor is reduced from the pressure reducing port and distillation is carried out;

[0022] (3) During distillation, the first electrically controlled three-way valve on the distillation delivery pipe is switched to the front distillation gas outlet to discharge the front distillation gas. After the temperature in the kettle reaches the preset temperature, it is switched back to the condenser passage, and the second electrically controlled three-way valve controls the gas to enter the first condensation chamber, so that the gas is condensed and then enters the storage tank;

[0023] (4) After the primary distillation is completed, the liquid in the storage tank is pumped back to the distillation reactor from the reflux pipe outlet joint along the reflux pipe by a pump, and alumina powder with a weight ratio of 8-10% of the reflux liquid is further added to the reactor, and a secondary distillation process is performed after stirring;

[0024] (5) During the distillation process, the front distillation gas is also discharged, and then the condenser path is switched back. The second electrically controlled three-way valve controls the gas to enter the second condensation chamber, and liquid nitrogen is supplied to the second condensation chamber to quickly cool the gas. At this time, the DMF in the gas is cooled to liquid and returns to the storage tank through the third electrically controlled three-way valve.

[0025] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: the distillation device is combined with a dryer to achieve preliminary moisture absorption and can achieve secondary distillation purification; based on the DMF purification process of the distillation device, the distillate is refluxed into the distillation reactor on the basis of the first purification, and is further distilled again after adding alumina powder, and at the same time, conventional condensation treatment is adopted in the first distillation, and the distillation gas is quickly cooled based on liquid nitrogen in the second distillation, and based on the different freezing points of water and DMF, DMF is kept in liquid state while water is allowed to crystallize into ice, so as to achieve the purpose of further separation and purification, thereby solving the problem that the existing process only performs one distillation and impurity removal treatment during purification, resulting in low purification degree of the overall process and failure to meet high purity requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0027] Figure 2 It is a schematic diagram of the back structure of the present invention;

[0028] Figure 3 Schematic diagram of the condenser structure of the present invention;

[0029] Figure 4 Schematic diagram of the internal structure of the dryer of the present invention;

[0030] Figure 5 Schematic diagram of the internal structure of the condenser of the present invention;

[0031] Figure: 1, distillation reactor; 2, support plate; 3, support leg; 4, residual material discharge valve; 5, stirring motor; 6, reflux pipe inlet connector; 7, raw material addition port; 8, distillation gas outlet; 9, dryer; 901, feed hopper; 902, feed section; 903, discharge section; 904, electric control feed valve; 905, threaded joint; 906, connecting plate; 907, spring; 908, vibration motor; 909, barium oxide particles; 910, flexible connecting section; 911, isolation net; 10, distillation delivery pipe; 11, first electric control three-way valve; 12, front distillation gas outlet; 13, second electric control three-way valve Valve; 14. Condenser; 141. First condensing chamber; 142. Second condensing chamber; 143. Separation section; 144. Discharge section; 145. Electric discharge valve; 146. Cooling water inlet; 147. Cooling water outlet; 148. Liquid nitrogen inlet; 149. Liquid nitrogen outlet; 150. Baffle; 151. Straight-through pipe; 152. Partition; 153. Inclined mesh plate; 154. Blocking support block; 15. Third electric three-way valve; 16. Storage tank; 17. Reflux pipe outlet connector; 18. Discharge valve; 19. Retaining rod; 20. Thermal fluid inlet; 21. Thermal fluid outlet; 22. Pressure reducing port. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0033] like Figures 1 to 5 As shown, the distillation apparatus of the present invention includes a distillation reactor 1, a condenser 14, and a storage tank 16. A reflux pipe inlet connector 6 is installed above one side of the distillation reactor 1, a raw material addition port 7 is installed above the front end of the distillation reactor 1, and a distillation gas outlet 8 is installed at the upper end of the other side of the distillation reactor 1. The reflux pipe inlet connector 6 and the raw material addition port 7 are both externally connected to a valve seat. A support plate 2 is installed on the outer wall of the distillation reactor 1. Support legs 3 are installed at the four corners of the support plate 2 and the lower end of the storage tank 16. A residual material discharge valve 4 is installed at the lower end of the distillation reactor 1. A stirring motor 5 is installed in the middle position of the distillation reactor 1. The output end of the stirring motor 5 passes through and extends into the interior of the distillation reactor 1 and is equipped with a stirring shaft. The distillation apparatus also includes:

[0034] A dryer 9 is installed behind the upper end of the distillation reactor 1;

[0035] A second electrically controlled three-way valve 13 is installed at the upper end of the condenser 14 and is connected to the distillation gas outlet 8 through the distillation delivery pipe 10;

[0036] A third electrically controlled three-way valve 15 is installed at the lower end of the condenser 14. The condenser 14 is connected to the storage tank 16 through the third electrically controlled three-way valve 15. A discharge valve 18 is installed at the lower end of the storage tank 16.

[0037] a pressure reducing port 22, which is installed between the raw material addition port 7 and the distillation gas outlet 8;

[0038] The reflux pipe outlet joint 17 is installed at the front end of the storage tank 16 , and the reflux pipe outlet joint 17 on the storage tank 16 is connected to the reflux pipe inlet joint 6 on the distillation reactor 1 through a reflux pipe.

[0039] like Figure 4 As shown, the dryer 9 includes a feed hopper 901, a feed section 902, a discharge section 903 and an electrically controlled feed valve 904. The feed section 902 is installed at the lower end of the feed hopper 901. The lower end of the feed section 902 and the upper end of the discharge section 903 are both installed with connecting plates 906. The connecting plates 906 are connected by a flexible connecting section 910. Barium oxide particles 909 are arranged inside the feed section 902, the flexible connecting section 910 and the discharge section 903. An isolation net 911 is installed at the bottom end of the discharge section 903. The isolation net 911 can isolate the barium oxide particles 909 to facilitate the water absorption and drying treatment of industrial-grade DMF through the barium oxide particles 909.

[0040] like Figure 4 As shown, the four sides of the connecting disk 906 are connected by springs 907, a damping rod is installed inside the spring 907, and a vibration motor 908 is installed on one side of the outer wall of the feeding section 902. Since the feeding section 902 and the unloading section 903 of the dryer 9 are connected by a flexible connecting section 910, when the vibration motor 908 is running, it can drive the feeding section 902 to vibrate along the direction of the spring 907 to speed up the unloading speed. The damping rod inside the spring 907 not only improves the bearing capacity of the spring 907, but also plays an auxiliary guiding role to prevent the spring 907 from deflecting.

[0041] like Figure 2 and Figure 4 As shown, retaining rods 19 are provided on both sides of the unloading section 903, and the retaining rods 19 are fixedly connected to the distillation reactor 1. The retaining rods 19 can assist in supporting the dryer 9 to ensure its stability.

[0042] like Figure 4 As shown, the lower end of the unloading section 903 is threadedly connected to the electric-controlled feed valve 904 via a threaded joint 905 , and the dryer 9 is a detachable structure for easy inspection and maintenance.

[0043] Furthermore, a jacket is provided on the outer wall of the distillation reactor 1, a heat transfer liquid inlet 20 is installed at the upper end of the jacket side wall of the distillation reactor 1, a heat transfer liquid outlet 21 is installed at the lower end of the jacket side wall of the distillation reactor 1, and an electric heating component is installed inside the jacket of the distillation reactor 1. The distillation reactor 1 adopts a jacketed electric heating structure.

[0044] like Figure 3 and Figure 5 As shown, the condenser 14 includes a first condensing chamber 141 and a second condensing chamber 142, and the first condensing chamber 141 and the second condensing chamber 142 are respectively connected to different branches on the second electrically controlled three-way valve 13 and the third electrically controlled three-way valve 15. A baffle 150 is installed inside the first condensing chamber 141, a cooling water inlet 146 is installed at the upper end of one side of the first condensing chamber 141, a cooling water outlet 147 is installed at the lower end of one side of the first condensing chamber 141, a straight pipe 151 is installed inside the second condensing chamber 142, and a Partition 152, and the straight-through pipe 151 passes through and extends to the outside of the partition 152, a separation section 143 is provided at the lower end of the second condensing chamber 142, a discharge section 144 is provided below one end of the separation section 143, an electric-controlled discharge valve 145 is installed at the lower end of the discharge section 144, an inclined mesh plate 153 is installed inside the separation section 143, and a blocking support block 154 is installed below one end of the inclined mesh plate 153, a liquid nitrogen inlet 148 is installed at the upper end of one side of the second condensing chamber 142, and a liquid nitrogen outlet 149 is provided at the lower end of one side of the second condensing chamber 142.

[0045] like Figure 1 As shown, a first electrically controlled three-way valve 11 is installed inside the distillation delivery pipe 10, and a pre-distillate gas outlet 12 is installed at the upper end of the first electrically controlled three-way valve 11. Since the gas pre-generated during distillation is impurities and water, the pre-distillate gas needs to be discharged.

[0046] The present invention adopts the above-mentioned distillation apparatus to carry out the purification process of N,N-dimethylformamide, comprising the following steps:

[0047] (1) Industrial-grade N,N-dimethylformamide is passed through a dryer 9 to initially absorb the water therein and then enters a distillation reactor 1;

[0048] (2) Alumina powder accounting for 3-5% by weight of the precipitate is added to the distillation reactor 1 through the raw material addition port 7 to carry out the reaction. After the reaction is completed, the pressure in the reactor is reduced through the pressure reducing port 22 and the distillation process is carried out;

[0049] (3) During distillation, the first electrically controlled three-way valve 11 on the distillation delivery pipe 10 is switched to the front distillate gas outlet 12 to discharge the front distillate gas. After the temperature in the kettle reaches a preset temperature, the passage is switched back to the condenser 14. The second electrically controlled three-way valve 13 controls the gas to enter the first condensing chamber 141, and the gas is condensed and then enters the storage tank 16;

[0050] (4) After the primary distillation is completed, the liquid in the storage tank 16 is pumped back to the distillation reactor 1 through the reflux pipe outlet connector 17 along the reflux pipe by a pump. Alumina powder with a weight ratio of 8-10% of the reflux liquid is further added to the reactor, and a secondary distillation process is performed after stirring;

[0051] (5) During the distillation process, the front distillation gas is also discharged, and the passage is switched back to the condenser 14. The second electrically controlled three-way valve 13 controls the gas to enter the second condensation chamber 142, and liquid nitrogen is supplied to the second condensation chamber 142 to quickly cool the gas. At this time, the DMF in the gas is cooled to liquid and returns to the storage tank 16 through the third electrically controlled three-way valve 15.

[0052] Specifically, the following steps are included:

[0053] (1) Pour industrial-grade DMF into the feed hopper 901 of the dryer 9. A plurality of barium oxide particles 909 are provided in the dryer 9. After entering, the industrial-grade DMF can flow down along the gaps between the barium oxide particles 909 and initially absorb the moisture therein through the barium oxide particles 909. During the downflow process, the vibration motor 908 drives the dryer 9 to vibrate in the direction of the spring 907 to accelerate the speed of liquid precipitation;

[0054] (2) The precipitated liquid enters the distillation reactor 1, and alumina powder is added to the reactor from the raw material addition port 7 at a weight ratio of 3-5% of the precipitated liquid. The alumina powder is pre-calcined at 500-600°C overnight. After the addition, the stirring shaft is controlled by the stirring motor 5 to rotate, so that the alumina powder and the precipitated liquid repeatedly react. After the reaction is completed, the pressure in the reactor is reduced by the pressure reducing port 22, and a heat transfer liquid is injected into the jacket of the distillation reactor 1. The temperature of the heat transfer liquid is controlled by the electric heating component to distill the precipitated liquid;

[0055] (3) During distillation, the first electrically controlled three-way valve 11 on the distillation delivery pipe 10 is switched to the front distillate gas outlet 12 to discharge the front distillate gas. After the temperature in the kettle reaches 150°C, the passage is switched back to the condenser 14. The second electrically controlled three-way valve 13 controls the gas to enter the first condensing chamber 141. The gas is condensed by the cooling water and the deflection effect of the deflection pipe 150 and then enters the storage tank 16.

[0056] (4) After the primary distillation is completed, the liquid in the storage tank 16 is pumped back to the distillation reactor 1 through the reflux pipe outlet connector 17 along the reflux pipe by a pump. Alumina powder with a weight ratio of 8-10% of the reflux liquid is further added to the reactor, and a secondary distillation process is performed after stirring;

[0057] (5) During the distillation process, the front distillate gas is also discharged, and the passage is switched back to the condenser 14. The second electrically controlled three-way valve 13 controls the gas to enter the second condensing chamber 142. Liquid nitrogen is supplied to the second condensing chamber 142 to rapidly cool the gas. At this time, the DMF in the gas is cooled to liquid, and the residual water reaches the freezing point and freezes into ice crystals. It is isolated by the inclined mesh plate 153 below the second condensing chamber 142 to the discharge section 144, and the DMF returns to the storage tank 16 through the third electrically controlled three-way valve 15.

[0058] (6) The DMF in the storage tank 16 is stored with a 3A or 4A molecular sieve and is finally separated by a membrane to obtain the finished product.

[0059] The components of the DMF purified by distillation of the present invention were detected, and the results obtained are shown in Table 1 below.

[0060] Table 1 DMF composition table purified by the present invention

[0061]

[0062]

Claims

1. A distillation apparatus comprising a distillation reactor (1), a condenser (14) and a storage tank (16), wherein a reflux pipe inlet joint (6) and a raw material addition port (7) are installed above one side of the distillation reactor (1), a gas outlet (8) is installed at the upper end of the other side of the distillation reactor (1), the reflux pipe inlet joint (6) and the raw material addition port (7) are both externally connected to a valve seat, and a residual material discharge valve (4) is installed at the lower end of the distillation reactor (1); characterized in that: The distillation apparatus also includes: A dryer (9) is provided at the rear of the upper end of the distillation reactor (1); a distillation delivery pipe (10) is connected to the gas outlet (8); a first electrically controlled three-way valve (11) is installed inside the distillation delivery pipe (10); and a front distillation gas outlet (12) is installed at the upper end of the first electrically controlled three-way valve (11); a second electrically controlled three-way valve (13) installed at the upper end of the condenser (14), the second electrically controlled three-way valve (13) being connected to the distillation gas outlet (8) via the distillation delivery pipe (10); a third electrically controlled three-way valve (15) installed at the lower end of the condenser (14); the condenser (14) is connected to the storage tank (16) via the third electrically controlled three-way valve (15); a discharge valve (18) is installed at the lower end of the storage tank (16); a pressure reducing port (22) installed between the raw material adding port (7) and the distillation gas outlet (8); A reflux pipe outlet connector (17) is provided at the front end of the storage tank (16), and the reflux pipe outlet connector (17) on the storage tank (16) is connected to the reflux pipe inlet connector (6) on the distillation reactor (1) through a reflux pipe; The condenser (14) includes a first condensation chamber (141) and a second condensation chamber (142), and the first condensation chamber (141) and the second condensation chamber (142) are respectively connected to different branches on the second electrically controlled three-way valve (13) and the third electrically controlled three-way valve (15); a separation section (143) is provided at the lower end of the second condensation chamber (142), a discharge section (144) is provided below one end of the separation section (143), an electrically controlled discharge valve (145) is installed at the lower end of the discharge section (144), an inclined mesh plate (153) is installed inside the separation section (143), and a blocking support block (154) is installed below one end of the inclined mesh plate (153).

2. The distillation apparatus according to claim 1, wherein: The dryer (9) comprises a feed hopper (901), a feed section (902), a discharge section (903) and an electrically controlled feed valve (904), wherein the feed section (902) is installed at the lower end of the feed hopper (901), and the lower end of the feed section (902) and the upper end of the discharge section (903) are both installed with connecting plates (906), and the connecting plates (906) are connected by a flexible connecting section (910), and an isolation net (911) is installed at the bottom end of the discharge section (903).

3. The distillation apparatus according to claim 2, wherein: The four sides of the connecting disk (906) are connected via a spring (907), a damping rod is installed inside the spring (907), and a vibration motor (908) is installed on one side of the outer wall of the feeding section (902).

4. The distillation apparatus according to claim 1, wherein: The distillation reactor (1) is provided with a jacket on its outer wall, a heat transfer liquid inlet (20) is installed at the upper end of the jacket side wall, a heat transfer liquid outlet (21) is installed at the lower end of the jacket side wall, and an electric heating component is installed inside the jacket.

5. The distillation apparatus according to claim 1, wherein: The distillation reactor (1) is provided with a stirring motor (5), the output end of the stirring motor (5) passes through and extends into the interior of the distillation reactor (1), and is provided with a stirring shaft.

6. The distillation apparatus according to claim 1, wherein: A deflector tube (150) is installed inside the first condensation chamber (141), a cooling water inlet (146) is installed at the upper end of one side of the first condensation chamber (141), and a cooling water outlet (147) is installed at the lower end of one side of the first condensation chamber (141).

7. The distillation apparatus according to claim 1, wherein: A straight-through pipe (151) is installed inside the second condensing chamber (142), a partition (152) is installed at the lower end of the straight-through pipe (151), and the straight-through pipe (151) passes through and extends to the outside of the partition (152), a liquid nitrogen inlet (148) is installed at the upper end of one side of the second condensing chamber (142), and a liquid nitrogen outlet (149) is provided at the lower end of one side of the second condensing chamber (142).

8. A process for purifying N,N-dimethylformamide, characterized in that: The purification process is based on the distillation apparatus of claim 1 and comprises the following steps: (1) Industrial-grade N,N-dimethylformamide is passed through a dryer (9) to initially absorb the water therein and then enters a distillation reactor (1); (2) adding 3-5% by weight of aluminum oxide powder to the distillation reactor (1) from the raw material addition port (7) to carry out the reaction, and after the reaction is completed, the pressure in the reactor is reduced from the pressure reducing port (22) and the distillation process is carried out; (3) During distillation, the first electrically controlled three-way valve (11) on the distillation delivery pipe (10) is switched to the front distillation gas outlet (12) to discharge the front distillation gas. After the temperature in the kettle reaches a preset temperature, the passage is switched back to the condenser (14). The second electrically controlled three-way valve (13) controls the gas to enter the first condensation chamber (141), and the gas is condensed and then enters the storage tank (16); (4) After the primary distillation is completed, the liquid in the storage tank (16) is pumped back to the distillation reactor (1) through the reflux pipe outlet joint (17) along the reflux pipe by a pump, and alumina powder with a weight ratio of 8-10% of the reflux liquid is further added to the reactor, and a secondary distillation process is performed after stirring; (5) During the distillation process, the front distillation gas is also discharged and switched back to the condenser (14) passage. The second electrically controlled three-way valve (13) controls the gas to enter the second condensation chamber (142). Liquid nitrogen is supplied to the second condensation chamber (142) to quickly cool the gas. At this time, the DMF in the gas is cooled to liquid and returns to the storage tank (16) through the third electrically controlled three-way valve (15).

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