NMP (N-Methyl Pyrrolidone) purification method and purification equipment

Abstract

The invention relates to the technical field of solution purification, in particular to an NMP (N-Methyl Pyrrolidone) purification method and purification equipment. The device comprises a purification device, the purification device comprises an NMP waste liquid tank, an impurity removal distillation kettle group, a water removal distillation kettle group and an NMP new liquid tank which are sequentially connected on a discharge pipeline in the NMP solution conveying direction, the impurity removal distillation kettle group comprises at least two impurity removal distillation kettles which are sequentially connected, and the water removal distillation kettle group comprises a water removal distillation kettle and a water removal distillation kettle which are sequentially connected. A liquid discharging and impurity removing pipeline for discharging precipitated impurities at the bottom of the kettle in real time is arranged at the bottom of the impurity removing distillation kettle, and a concentration sensor is mounted in the impurity removing distillation kettle. According to the method, a filter does not need to be arranged, and the impurities in the NMP solution can be efficiently removed.

Description

Technical Field

[0001] The invention relates to the technical field of solution purification, in particular to a NMP purification method and purification equipment. Background Art

[0002] The chemical name of NMP is N-methylpyrrolidone. It is a high-boiling point, environmentally friendly and excellent solvent with low viscosity, chemical stability, good resistance and thermal stability, high polarity, low volatility, and infinite miscibility with water and many organic solvents. Its market application areas are mainly concentrated in lithium batteries, circuit boards, insulation materials, petrochemicals, medicines, pesticides, cleaning, polymers and other industries.

[0003] The existing operation of extracting high-purity NMP solution from NMP waste liquid mainly involves the removal of solid impurities and water. As described in the text of NMP distillation purification method and NMP distillation purification process with Chinese patent publication number CN117357919B, in the waste liquid treatment process, solid impurities are filtered by a filtering component, and in the distillation treatment process, the NMP solution is purified by heating distillation.

[0004] However, in actual implementation, when using filters to filter impurities for large-scale processing needs, there are often the following problems:

[0005] First, during the distillation process of NMP, the properties of the solution change, and the solubility of some impurities at a specific concentration decreases, causing some impurities to precipitate out of NMP during the distillation process, resulting in some impurities still existing in the NMP solution after distillation.

[0006] Second, NMP waste liquid may contain impurities of various properties, some of which may change in form, particle size, etc. over time and under different operating conditions; it is difficult to find a universal filter that can effectively remove all impurities.

[0007] Third, the filter element may wear or break, causing impurities to leak into the filtered NMP.

[0008] Fourth, during the filtration process, the filter element will need to be frequently replaced or cleaned, thus affecting the continuity of production.

[0009] Fifth, the filter element is usually a consumable item that needs to be purchased and replaced regularly, which will increase long-term operating costs. Therefore, it is urgent to solve this problem. Summary of the invention

[0010] In order to avoid and overcome the technical problems existing in the prior art, the present invention provides a NMP purification method and purification equipment, which do not require the provision of a filter and can achieve efficient removal of impurities in an NMP solution.

[0011] To achieve the above object, the present invention provides the following technical solutions:

[0012] A method for purifying NMP comprises a purification device, wherein the purification device comprises an NMP waste liquid tank, an impurity removal distillation kettle group, a water removal distillation kettle group and an NMP new liquid tank connected in sequence on a discharge pipeline along the conveying direction of the NMP solution, wherein the impurity removal distillation kettle group comprises at least two impurity removal distillation kettles connected in sequence, and a discharge impurity removal pipeline for discharging the impurities precipitated at the bottom of the kettle in real time is arranged at the bottom of the impurity removal distillation kettle, and a concentration sensor is installed in the impurity removal distillation kettle; the purification method comprises the following steps:

[0013] S1, obtaining the concentration of NMP in the solution in each impurity removal distillation kettle in real time by a concentration sensor;

[0014] S2, according to the concentration of NMP in the solution in the impurity removal distillation kettle obtained in real time, the concentration of NMP in the solution in each impurity removal distillation kettle is deployed to the target concentration, wherein, along the flow direction from the NMP waste liquid tank to the NMP new liquid tank, the target concentration of NMP in the solution in the adjacent impurity removal distillation kettle increases successively;

[0015] S3, within one iteration time, according to the concentration of NMP in the solution in the impurity removal distillation kettle obtained in real time, the concentration growth rate of the impurity removal distillation kettle within one iteration time is obtained, and the concentration growth rate is compared with the set threshold value;

[0016] S4. When the concentration growth rate is greater than the set threshold, the inlet flow rate of the impurity removal distillation kettle is increased and the discharge flow rate is reduced so that the concentration of NMP in the solution in the impurity removal distillation kettle approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle reaches the target concentration, return to step S3; when the concentration growth rate is less than the set threshold, the inlet flow rate of the impurity removal distillation kettle is reduced and the discharge flow rate is increased so that the concentration of NMP in the solution in the impurity removal distillation kettle approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle reaches the target concentration, return to step S3.

[0017] As a further solution of the present invention: the target concentration of the solution in the lowest level impurity removal distillation kettle in step S1 is 90%.

[0018] As a further solution of the present invention: the threshold value set in step S3 is 0.

[0019] A purification device, which applies the NMP purification method described above. In the purification device, the middle part of the adjacent upper impurity removal distillation kettle is connected to the adjacent lower impurity removal distillation kettle through a discharge pipeline, and the bottom of the adjacent lower impurity removal distillation kettle is connected to the adjacent upper impurity removal distillation kettle through a reflux pipeline, and the reflux pipeline constitutes the liquid discharge impurity removal pipeline.

[0020] As a further solution of the present invention: the number of impurity removal distillation kettles is three.

[0021] As a further solution of the present invention: the number of the water removal distillation kettles is two.

[0022] As a further solution of the present invention: the top of the impurity removal distillation kettle and the top of the water removal distillation kettle are both connected to condensers, and the condenser connected to the adjacent upper impurity removal distillation kettle is connected to the adjacent lower impurity removal distillation kettle, and the condenser connected to the lowest impurity removal distillation kettle and the condenser connected to the water removal distillation kettle are both connected to the drainage tank.

[0023] As a further solution of the present invention: the heating temperature of the impurity removal distillation kettle group and the water removal distillation kettle group is 120°C.

[0024] As a further solution of the present invention: the condensation temperature of the condenser is 7°C.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The purification method of the present invention can realize continuous distillation under the target concentration that increases successively between adjacent impurity removal stills, and for those impurities that may form precipitates, it can prompt them to separate out from the solution more easily at a higher NMP concentration to form precipitation. Meanwhile, at a higher concentration, due to the change of properties such as the viscosity of the solution, the interaction between the solid particles is enhanced, and it is easier to gather and settle. In addition, the liquid discharge impurity removal pipeline that is arranged at the bottom of the still of the impurity removal still is utilized to discharge the precipitated impurities at the bottom of the still in real time, thereby it is not necessary to arrange a filter, and the impurities produced by the NMP solution in the purification process can be efficiently removed.

[0027] 2. The present invention processes impurities during the distillation process of NMP, so the impurities precipitated by the distillation of NMP solution at a specific concentration during the distillation process can be processed simultaneously. In addition, the treatment method of precipitation and reflux of particulate impurities does not require the installation of a filtering mechanism, and there is no need to worry about leakage problems caused by filter element rupture, thereby ensuring the stability and reliability of the purification process. In addition, the precipitation and reflux method does not require frequent cleaning or replacement of the filter element. It only requires regular cleaning of the particulate impurities in the top-level impurity removal distillation kettle, which has the advantages of low cleaning cycle and maintenance cost; in addition, it also has the advantages of efficient cleaning process and no need to interrupt the purification operation, so that impurities can be continuously removed without interrupting production, which is more suitable for large-scale industrial production.

[0028] 3. In the present invention, it is preferred that three impurity removal stills are set, which can be applied to most working conditions. Among the three impurity removal stills, the concentrations of the NMP solutions in the three impurity removal stills can be set to be different, and the particulate impurities can be basically precipitated under the distillation of different concentrations. In addition, by precipitating the particulate impurities by the second-stage impurity removal still, and returning them to the first-stage impurity removal still, the impurity content in the second-stage impurity removal still can be at a lower level, so that the content of impurities in the third-stage impurity removal still in the middle of the second-stage impurity removal still is lower; thereafter, by the impurity reflux again in the third-stage impurity removal still, the impurities precipitated in the third-stage impurity removal still and the impurities sent to the third-stage impurity removal still can be returned to the second-stage impurity removal still, so that the third-stage impurity removal still is in a state of being almost free of impurities.

[0029] 4. In the present invention, the condensed water of the upper impurity removal still kettle is connected to the adjacent lower impurity removal still kettle, rather than being directly sent to the drainage tank for collection, which can play a role in stabilizing the operation of the system to a certain extent; the condensed water helps maintain the pressure balance between the still kettles at each level, promotes the flow of gas and the transfer of substances, thereby helping to improve the purification effect and the sequential progression of concentration. In addition, due to the pressure balance effect brought by the condensed water, the reflux process can be made more stable, which is convenient for the further precipitation of particulate impurities, thereby further ensuring the removal effect of particulate impurities. At the same time, the water entering the lower impurity removal still kettle may also play a certain auxiliary separation role in the entire purification process. To a certain extent, the presence of water can affect the boiling point and vapor pressure of different substances, thereby promoting the separation of NMP from other impurities. If the condensed water does not enter the adjacent lower impurity removal still kettle, it may interfere with this separation mechanism, making it impossible to effectively remove some impurities, reducing the purity of the purified NMP.

[0030] 5. The present invention adopts a multi-stage distillation tower design, and the structure of five-stage distillation towers connected in series in sequence enables the NMP waste liquid to be gradually purified. Each stage of the distillation tower processes specific impurities and concentration ranges, improves the overall purification effect, and reduces the dependence on high-temperature steam. For example, in the first stage distillation tower, some impurities can be initially separated, reducing the processing burden of the subsequent distillation tower, so that effective purification can be achieved at a relatively low temperature. Therefore, compared with the traditional 150°C distillation, the present invention uses 120°C, which reduces the energy consumption during the distillation process. At the same time, due to the reduction in distillation temperature, the condensation temperature of the condenser 50 does not need to be the traditional 5°C, and 7°C can be used, which effectively reduces the energy consumption of condensation after distillation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flowchart of the purification method of the present invention.

[0032] Figure 2It is a schematic diagram of the structure of the purification device in the present invention.

[0033] In the figure: 10, NMP waste liquid tank; 20, impurity removal distillation kettle group; 21, impurity removal distillation kettle; 30, discharge pipeline; 40, reflux pipeline; 50, condenser; 60, water removal distillation kettle group; 61, water removal distillation kettle; 70, NMP new liquid tank; 80, drainage tank. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] For ease of understanding, the specific structure and working mode of the present invention are further described below in conjunction with the accompanying drawings:

[0036] The purification method of the present invention refers to Figure 1 As shown, a method for purifying NMP includes a purification device, which includes an NMP waste liquid tank 10, an impurity removal distillation kettle group 20, a water removal distillation kettle group 60 and an NMP new liquid tank 70 connected to a discharge pipeline 30 in sequence along the NMP solution conveying direction, wherein the impurity removal distillation kettle group 20 includes at least two impurity removal distillation kettles 21 connected in sequence, and the bottom of the impurity removal distillation kettle 21 is provided with a discharge impurity removal pipeline for discharging the impurities precipitated at the bottom of the kettle in real time, and a concentration sensor is installed in the impurity removal distillation kettle 21; the purification method includes the following steps:

[0037] S1, obtaining the concentration of NMP in the solution in each impurity removal distillation kettle 21 in real time by a concentration sensor;

[0038] S2, according to the concentration of NMP in the solution in the impurity removal distillation kettle 21 obtained in real time, the concentration of NMP in the solution in each impurity removal distillation kettle 21 is adjusted to the target concentration, wherein, along the flow direction from the NMP waste liquid tank 10 to the NMP new liquid tank 70, the target concentration of NMP in the solution in the adjacent impurity removal distillation kettles 21 increases successively;

[0039] S3, within one iteration time, according to the concentration of NMP in the solution in the impurity removal distillation kettle 21 obtained in real time, the concentration growth rate of the impurity removal distillation kettle 21 within one iteration time is obtained, and the concentration growth rate is compared with the set threshold value;

[0040] S4. When the concentration growth rate is greater than the set threshold, the inlet flow rate of the impurity removal distillation kettle 21 is increased and the discharge flow rate is reduced so that the concentration of NMP in the solution in the impurity removal distillation kettle 21 approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle 21 reaches the target concentration, return to step S3; when the concentration growth rate is less than the set threshold, the inlet flow rate of the impurity removal distillation kettle 21 is reduced and the discharge flow rate is increased so that the concentration of NMP in the solution in the impurity removal distillation kettle 21 approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle 21 reaches the target concentration, return to step S3.

[0041] According to the above purification method, continuous distillation can be achieved between adjacent impurity removal stills 21 at successively increasing target concentrations, and for those impurities that may form precipitates, a higher NMP concentration will make them more easily precipitate out of the solution and form precipitates. At the same time, at higher concentrations, due to changes in the properties of the solution such as the viscosity, the interaction between the solid particles is enhanced, making it easier to aggregate and precipitate. If the concentration is not increased layer by layer, the precipitate may be mixed with NMP and other impurities, making it difficult to effectively separate. In addition, by utilizing the drainage and impurity removal pipeline provided at the bottom of the still 21, the precipitated impurities at the bottom of the still are discharged in real time, thereby eliminating the need to provide a filter, and effectively removing the impurities generated by the NMP solution during the purification process.

[0042] Purification equipment using the above purification method is as follows Figure 2 As shown, its main structure includes an NMP waste liquid tank 10, an impurity removal distillation kettle group 20, a water removal distillation kettle group 60 and an NMP new liquid tank 70 which are sequentially connected to the discharge pipeline 30 along the solution conveying direction. The NMP solution in the NMP waste liquid tank 10 first enters the impurity removal distillation kettle group 20 for distillation and removal of particulate impurities. Thereafter, the NMP solution after the particulate impurities are removed enters the water removal distillation kettle group 60 for further distillation to evaporate the residual water, and finally is conveyed to the NMP new liquid tank 70 to achieve the final solution purification.

[0043] Specifically, Figure 2As shown, the impurity removal still kettle group 20 includes at least two impurity removal still kettles 21 connected in sequence. The middle of the impurity removal still kettle 21 of the adjacent upper level is connected to the impurity removal still kettle 21 of the adjacent lower level through a discharge pipeline 30, and the bottom of the impurity removal still kettle 21 of the adjacent lower level is connected to the impurity removal still kettle 21 of the adjacent upper level through a reflux pipeline 40, and the reflux pipeline 40 constitutes the liquid discharge impurity removal pipeline. During the distillation process of the impurity removal still kettle 21, the concentration of the impurity removal still kettle 21 will continue to change, so that the impurities dissolved in the NMP solution in the impurity removal still kettle 21 gradually precipitate and settle at the bottom of the impurity removal still kettle 21. Since the middle of the impurity removal still kettle 21 of the adjacent upper level is connected to the impurity removal still kettle 21 of the adjacent lower level through a discharge pipeline 30, the solution entering the impurity removal still kettle 21 of the adjacent upper level and entering the impurity removal still kettle 21 of the adjacent lower level is mostly clear liquid after precipitation. In addition, the bottom of the adjacent lower-level impurity removal still kettle 21 is connected with the adjacent upper-level impurity removal still kettle 21 through the reflux line 40, so that the impurity turbid liquid at the bottom of the lower-level impurity removal still kettle 21 is transported to the adjacent upper-level impurity removal still kettle 21 in sequence. Not only the impurities in the adjacent lower-level impurity removal still kettle 21 are made less and less, and then the impurities discharged into the adjacent lower-level impurity removal still kettle 21 are also less and less, in addition, the particulate impurities in all the lower-level impurity removal still kettles 21 eventually converge in the uppermost impurity removal still kettle 21, so, when the final particulate impurities are cleaned up, only the impurities in the uppermost impurity removal still kettle 21 need to be cleaned up. In summary, the present invention processes impurities in NMP during distillation, and can realize the synchronous processing of NMP solution being separated out impurities again at a specific concentration. In addition, the processing method of the particulate impurity precipitation reflux is adopted, and there is no need to set a filtering mechanism, and there is no need to worry about the leakage problem caused by the rupture of the filter element, so as to ensure the stability and reliability of the purification process. Furthermore, the sedimentation reflux method does not require frequent cleaning or replacement of the filter element. It only requires regular cleaning of the particulate impurities in the top-level impurity removal distillation kettle 21, which has the advantages of low cleaning cycle and maintenance cost. In addition, it also has the advantages of efficient cleaning process and no need to interrupt the purification operation, so that impurities can be continuously removed without interrupting production, which is more suitable for large-scale industrial production.

[0044] In specific implementation, Figure 2As shown, the number of impurity removal distillation kettles 21 can be increased or decreased accordingly according to the content of particulate impurities in the specific NMP waste liquid. In the present invention, it is preferred that the impurity removal distillation kettle 21 is set to three, which can be applicable to most working conditions. Among the three impurity removal distillation kettles 21, the concentrations of the NMP solutions in the three impurity removal distillation kettles 21 can be set to be different, and the particulate impurities can be basically precipitated under distillation of different concentrations. In addition, by precipitating the particulate impurities through the second-stage impurity removal distillation kettle 21 and refluxing them to the first-stage impurity removal distillation kettle 21, the impurity content in the second-stage impurity removal distillation kettle 21 can be at a lower level, so that the content of impurities in the third-stage impurity removal distillation kettle 21 sent to the middle of the second-stage impurity removal distillation kettle 21 is lower; thereafter, by refluxing the impurities in the third-stage impurity removal distillation kettle 21 again, the impurities precipitated in the third-stage impurity removal distillation kettle 21 and the impurities sent to the third-stage impurity removal distillation kettle 21 can be refluxed to the second-stage impurity removal distillation kettle 21, so that the third-stage impurity removal distillation kettle 21 is in a state of being almost free of impurities.

[0045] Further, such as Figure 2 As shown, two water removal distillation kettles 61 are provided, which can not only further improve the efficiency of distillation water removal, but also can re-precipitate the impurities that are not completely filtered through the first-stage water removal distillation kettle 61 to ensure that the NMP solution sent to the NMP new liquid tank 70 by the second-stage water removal distillation kettle 61 is of higher purity.

[0046] In addition, if Figure 2 As shown, the top of the impurity removal still 21 and the top of the water removal still 61 are both connected with a condenser 50, and the condenser 50 connected to the impurity removal still 21 of the adjacent upper level is connected to the impurity removal still 21 of the adjacent lower level. In the present invention, the condensed water of the upper impurity removal still 21 is connected to the adjacent lower level impurity removal still 21, rather than being directly sent to the drainage tank 80 for collection, which can play a role in stabilizing the operation of the system to a certain extent, and helps to maintain the pressure balance between the stills at various levels by the condensed water, promotes the flow of gas and the transfer of material, thereby helping to improve the purification effect and the concentration of the progressive. In addition, due to the pressure balance effect brought by the condensed water, the reflux process may become more stable, which is convenient for the further precipitation of particulate impurities, thereby further ensuring the removal effect of particulate impurities. At the same time, the water entering the lower level impurity removal still 21 may also play a certain auxiliary separation role in the whole purification process. To a certain extent, the presence of water can affect the boiling point and vapor pressure of different substances, thereby promoting the separation of NMP from other impurities. If the condensed water does not enter the adjacent lower-stage impurity removal distillation kettle 21, this separation mechanism may be disturbed, making it impossible to effectively remove some impurities, thereby reducing the purity of the purified NMP.

[0047] The condenser 50 connected to the lowest level impurity removal distillation kettle 21 and the condenser 50 connected to the water removal distillation kettle 61 are both connected to the drainage tank 80, which can ensure that the distillation and water removal process proceeds quickly and improves the purification efficiency of the equipment.

[0048] On the basis of the above, in the present invention, the heating temperature of the impurity removal distillation kettle group 20 and the water removal distillation kettle group 60 is 120°C, and the condensation temperature of the condenser 50 is 7°C. The present invention adopts a multi-stage distillation tower design, and the structure of the five-stage distillation towers connected in series in sequence enables the NMP waste liquid to be gradually purified. Each stage of the distillation tower processes specific impurities and concentration ranges, improves the overall purification effect, and reduces the dependence on high-temperature steam. For example, in the first stage distillation tower, some impurities can be initially separated, reducing the processing burden of the subsequent distillation tower, so that effective purification can be achieved at a relatively low temperature. Therefore, compared with the traditional 150°C distillation, the present invention uses 120°C, which reduces the energy consumption in the distillation process. At the same time, due to the reduction in the distillation temperature, the condensation temperature of the condenser 50 does not need to be the traditional 5°C, and 7°C can be used, which effectively reduces the energy consumption of condensation after distillation.

[0049] Of course, it is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims involved.

[0050] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0051] The techniques, shapes, and structural parts not described in detail in the present invention are all well-known techniques.

Claims

1. A method for purifying NMP, characterized in that, The invention comprises a purification device, wherein the purification device comprises an NMP waste liquid tank (10), an impurity removal distillation kettle group (20), a water removal distillation kettle group (60) and an NMP new liquid tank (70) connected in sequence on a discharge pipeline (30) along the conveying direction of the NMP solution, wherein the impurity removal distillation kettle group (20) comprises at least two impurity removal distillation kettles (21) connected in sequence, and a liquid discharge impurity removal pipeline for real-time discharge of impurities precipitated at the bottom of the kettle is arranged at the bottom of the impurity removal distillation kettle (21), and a concentration sensor is installed in the impurity removal distillation kettle (21); the purification method comprises the following steps: S1, obtaining the concentration of NMP in the solution in each impurity removal distillation kettle (21) in real time by a concentration sensor; S2, according to the concentration of NMP in the solution in the impurity removal distillation kettle (21) obtained in real time, the concentration of NMP in the solution in each impurity removal distillation kettle (21) is adjusted to the target concentration, wherein the target concentration of NMP in the solution in adjacent impurity removal distillation kettles (21) increases in sequence along the flow direction from the NMP waste liquid tank (10) to the NMP new liquid tank (70); S3, within one iteration time, according to the concentration of NMP in the solution in the impurity removal distillation kettle (21) obtained in real time, the concentration growth rate of the impurity removal distillation kettle (21) within one iteration time is obtained, and the concentration growth rate is compared with a set threshold value; S4. When the concentration growth rate is greater than the set threshold, the inlet flow rate of the impurity removal distillation kettle (21) is increased and the discharge flow rate is reduced so that the concentration of NMP in the solution in the impurity removal distillation kettle (21) approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle (21) reaches the target concentration, return to step S3; when the concentration growth rate is less than the set threshold, the inlet flow rate of the impurity removal distillation kettle (21) is reduced and the discharge flow rate is increased so that the concentration of NMP in the solution in the impurity removal distillation kettle (21) approaches the target concentration, and within the next iteration time after the concentration of NMP in the solution in the impurity removal distillation kettle (21) reaches the target concentration, return to step S3.

2. A method for purifying NMP according to claim 1, characterized in that, The target concentration of the solution in the lowest stage impurity removal distillation kettle (21) in step S1 is 90%.

3. A method for purifying NMP according to claim 1, characterized in that, The threshold value set in step S3 is 0.

4. A purification device, which uses a NMP purification method as described in any one of claims 1 to 3, characterized in that: In the purification device, the middle of an adjacent upper impurity removal distillation kettle (21) is connected to an adjacent lower impurity removal distillation kettle (21) via a discharge pipeline (30), and the bottom of an adjacent lower impurity removal distillation kettle (21) is connected to an adjacent upper impurity removal distillation kettle (21) via a reflux pipeline (40), and the reflux pipeline (40) constitutes the liquid discharge impurity removal pipeline.

5. The purification device according to claim 4, characterized in that The number of impurity removal distillation kettles (21) is three.

6. The purification device according to claim 5, characterized in that The number of the water removal distillation kettles (61) is two.

7. The purification device according to claim 6, characterized in that The top of the impurity removal distillation kettle (21) and the top of the water removal distillation kettle (61) are both connected to a condenser (50), and the condenser (50) connected to the adjacent upper impurity removal distillation kettle (21) is connected to the adjacent lower impurity removal distillation kettle (21), and the condenser (50) connected to the lowest impurity removal distillation kettle (21) and the condenser (50) connected to the water removal distillation kettle (61) are both connected to a drainage tank (80).

8. The purification device according to claim 7, characterized in that The heating temperature of the impurity removal distillation kettle group (20) and the water removal distillation kettle group (60) is 120°C.

9. The purification device according to claim 7, characterized in that: The condensation temperature of the condenser (50) is 7°C.

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