Recovery method and system of organic solvent

Abstract

To reduce a use amount of pure water in a scrubber and reduce an amount of organic components contained in treatment gas from the scrubber when recovering an organic solvent by bringing gas containing the organic solvent into contact with water in the scrubber.SOLUTION: A mixture of an organic solvent and water is obtained by bringing gas containing the organic solvent into contact with water in a scrubber 1, and the organic solvent and the water are separated from the mixture by a separation device such as a pervaporation device 3. The separated water is processed with a reverse osmosis membrane 6 and the water permeating through the reverse osmosis membrane 6 is fed to the scrubber 1.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and system for recovering an organic solvent, comprising a step of contacting organic solvent vapor with water. [Background technology]

[0002] When an organic solvent is used, it is necessary to recover the organic solvent after use. When an organic solvent is discharged in the form of a gas from a process using the organic solvent, and the organic solvent is easily dissolved in water, a method is often used in which the organic solvent vapor is brought into contact with water to dissolve the organic solvent in water to obtain a mixture of the organic solvent and water, and then the water and the organic solvent are separated from the mixture. A scrubber is generally used for contacting the organic solvent vapor with water. For example, distillation or pervaporation (PV) is used to separate the water and the organic solvent from the mixture. In particular, the method using pervaporation makes it possible to recover a high-purity organic solvent with a small energy consumption when recovering the organic solvent from a mixture of an organic solvent and water whose boiling point at 1 atmospheric pressure exceeds 100°C.

[0003] An example of the recovery of an organic solvent is the recovery of N-methyl-2-pyrrolidone (hereinafter also referred to as NMP) from the manufacturing process of a lithium ion secondary battery. NMP has a property of being easily dissolved in water. In the manufacturing process of a lithium secondary ion battery, particles of an electrode active material are dispersed in NMP to form a slurry, and this slurry is applied to an electrode current collector and dried to form an electrode. When the slurry is dried, the NMP is vaporized, and the vaporized NMP is recovered using a water scrubber. The recovered NMP is in the form of a mixed liquid of water and NMP. Patent Document 1 discloses that the mixed liquid of water and NMP is supplied to a pervaporation membrane module to separate the water and NMP, and the water obtained at that time is supplied to a water scrubber and reused for recovering NMP. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-141793 A Summary of the Invention [Problem to be solved by the invention]

[0005] When organic solvent vapor is brought into contact with water in a scrubber to recover an organic solvent, the purity of the water supplied to the scrubber becomes an issue. As described in Patent Document 1, when a mixture of organic solvent and water obtained by recovering an organic solvent in a scrubber is treated with a pervaporation membrane to separate the organic solvent and water, the water separated by the pervaporation membrane contains low-boiling organic acids and amines in addition to the organic solvent that leaked through the pervaporation membrane. When such water is reused in a scrubber, the concentration of organic acids and amines as impurities increases in the water circulating between the scrubber and the pervaporation membrane module, and as a result, the treated gas discharged from the scrubber contains organic acids and amines, or the recovery rate of the organic solvent in the scrubber decreases and the treated gas contains organic solvents. When an organic solvent and water are separated by distillation from a mixture of organic solvent and water recovered in a scrubber, if the boiling point of the organic solvent is higher than the boiling point of water, water is discharged as overhead gas from the distillation column, but the overhead gas contains low-boiling organic acids and amines in addition to a small amount of organic solvent. Therefore, the same problem occurs when the water obtained by condensing the overhead gas is reused in a scrubber.

[0006] Organic solvents, organic acids, and amines are collectively referred to as organic components, and in order to avoid releasing organic components into the environment, it is possible to use pure water produced by a water purification system in a scrubber, rather than reusing the water separated by a pervaporation membrane or a distillation column. However, if the pure water produced by the water purification system is used to cover the entire amount of water used in the scrubber, a large amount of consumables and energy will be required, and a process will be required to remove the organic components from the large amount of water generated by the pervaporation membrane module and the distillation column.

[0007] An object of the present invention is to provide a recovery method and recovery system which, when recovering an organic solvent by contacting a gas containing an organic solvent with water in a scrubber, can reduce the amount of pure water used in the scrubber while also reducing the amount of organic components contained in the treated gas from the scrubber. [Means for solving the problem]

[0008] The recovery method of the present invention is a method for recovering an organic solvent from a gas containing an organic solvent, and includes a contacting step of contacting the gas with water to obtain a mixed liquid of the organic solvent and water, a separation step of separating the organic solvent and water from the mixed liquid, and a reverse osmosis step of treating the water separated in the separation step with a reverse osmosis membrane, and the water that has permeated the reverse osmosis membrane is supplied to the contacting step.

[0009] The recovery system of the present invention is a recovery system for recovering an organic solvent from a gas containing an organic solvent, and includes a scrubber that brings the gas into contact with water and discharges a mixed liquid of the organic solvent and water, a separation device that separates the organic solvent and water from the mixed liquid, and a reverse osmosis membrane, in which the water separated in the separation device is supplied to the reverse osmosis membrane, and the water that has permeated the reverse osmosis membrane is supplied to the scrubber for contact with the organic solvent. Effect of the Invention

[0010] According to the present invention, when an organic solvent is recovered by contacting a gas containing an organic solvent with water in a scrubber, it is possible to reduce the amount of pure water used in the scrubber while also reducing the amount of organic components contained in the treated gas from the scrubber. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing a recovery system according to an embodiment of the present invention. [Diagram 2] FIG. 13 illustrates another example of a recovery system. [Diagram 3] FIG. 13 illustrates another example of a recovery system. [Figure 4] FIG. 13 illustrates another example of a recovery system. [Diagram 5] FIG. 13 shows a recovery system according to another embodiment of the present invention. [Figure 6] FIG. 13 illustrates another example of a recovery system. [Figure 7] 1 is a graph showing the relationship between NMP concentration and NMP leakage rate in a reverse osmosis membrane. [Figure 8] FIG. 1 is a diagram showing an example of the configuration of a reverse osmosis membrane device that can be used in the recovery system. [Figure 9] FIG. 11 is a diagram showing another example of the configuration of a reverse osmosis membrane device. [Figure 10] FIG. 11 is a diagram showing another example of the configuration of a reverse osmosis membrane device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Next, an embodiment of the present invention will be described with reference to the drawings. The recovery system of the present invention recovers an organic solvent having a property of dissolving in water, and includes a scrubber that contacts a gas containing the organic solvent with water and discharges a mixed liquid of the organic solvent and water, and a separator that separates the organic solvent and water from the mixed liquid discharged from the scrubber, and is intended to reuse the water separated by the separator in the scrubber for contact with gas. In the following description, the organic solvent having a property of dissolving in water is, for example, NMP (N-methyl-2-pyrrolidone), but the organic solvent to which the present invention can be applied is not limited to NMP. The present invention can be applied to any organic solvent as long as it is an organic solvent that is easily vaporized, i.e., easily becomes organic solvent vapor, and can be absorbed by water in a scrubber.

[0013] FIG. 1 is a diagram showing a recovery system according to an embodiment of the present invention. The recovery system shown in FIG. 1 uses a pervaporation device 3 equipped with a pervaporation membrane 4 as a separation device, and can be suitably used for organic solvents (NMP, for example) having a boiling point higher than 100° C. at 1 atmospheric pressure. The recovery system includes a scrubber 1 that brings a gas containing an organic solvent into contact with water to absorb the organic solvent into the water, and a pure water device 2 that produces pure water by receiving raw water such as tap water. Inside the scrubber 1, a gas containing an organic solvent is blown in from the bottom, and water to be brought into contact with the gas is sprayed from the top. In the scrubber 1, the organic solvent is dissolved in water, and a mixture of water and the organic solvent is discharged from the bottom of the scrubber 1. The gas from which the organic solvent has been removed is discharged from the top of the scrubber 1 as a treated gas.

[0014] The mixed liquid discharged from the scrubber 1 contains impurities in addition to the organic solvent and water. This mixed liquid is supplied to the pervaporation device 3. Since water easily permeates the pervaporation membrane 4, whereas the organic solvent does not easily permeate the pervaporation membrane 4, the organic solvent is discharged from the concentration side of the pervaporation device 3. The component that permeated the pervaporation membrane 4, i.e., the permeate, is discharged from the permeation side of the pervaporation device 3. The permeate is mainly composed of water, and contains the organic solvent that leaked through the pervaporation membrane 4 and other impurities. The concentration of the organic solvent contained in the permeate of the pervaporation device 3 is, for example, less than 20% by mass, and preferably less than 8% by mass. If the organic solvent is NMP, the impurities contained in the permeate are, for example, low-boiling organic acids and amines. Usually, the permeate does not contain non-volatile impurities such as salts. If the permeate containing such impurities is reused as it is in the scrubber 1 as water for absorbing the organic solvent, the impurity concentration in the water circulating between the scrubber 1 and the pervaporation device 3 will increase, resulting in the treated gas discharged from the scrubber containing organic acids or amines, or a decrease in the recovery rate of the organic solvent in the scrubber causing the organic solvent to be contained in the treated gas.

[0015] In order to prevent water containing impurities such as organic acids and amines from being used in the scrubber 1, a reverse osmosis membrane device 5 is provided to treat the permeate from the pervaporation device 3. The reverse osmosis membrane device 5 is provided with a reverse osmosis membrane 6, and the organic solvents and impurities contained in the permeate from the pervaporation device 3 cannot permeate through the reverse osmosis membrane 6, so they are discharged as a concentrate from the concentrate side of the reverse osmosis membrane device 5. Water can permeate through the reverse osmosis membrane 6, so they are discharged as permeated water from the permeation side of the reverse osmosis membrane device 5. This permeated water is mixed with pure water from the pure water device 2 and used to absorb organic solvent vapor in the scrubber 1.

[0016] 1 recovers and reuses the water used in scrubber 1 to absorb the organic solvent, reducing the amount of pure water to be produced in water purification system 2 and significantly reducing the consumables used in producing pure water and the energy required for producing pure water. In addition, since the permeated water from reverse osmosis membrane 6 contains almost no organic solvent or impurities, the organic solvent concentration and impurity concentration in the treated gas discharged from scrubber 1 can be kept low, and the organic solvent can be separated with high accuracy by the entire system.

[0017] Incidentally, the pure water apparatus that generates the pure water to be supplied to the scrubber 1 is required to be able to remove inorganic ions, particularly calcium ions and magnesium ions, in order to prevent the generation of scale in the scrubber 1. For this reason, it is often configured to have a reverse osmosis membrane after a filter or an activated carbon device. If a pure water apparatus equipped with a reverse osmosis membrane is used, the permeate from the pervaporation device 3 can be treated in the pure water apparatus itself without providing a reverse osmosis membrane device 5 that treats the permeate from the pervaporation device 3. FIG. 2 shows a recovery system configured in this way. The recovery system shown in FIG. 2 includes a pure water apparatus 7 having a reverse osmosis membrane as a pure water apparatus that generates pure water from raw water, and the permeate from the pervaporation device 3 is sent to the pure water apparatus 7. In this case, the water separated in the separation device, i.e., the permeate from the pervaporation device 3, is supplied to the pure water apparatus 7 at a position before the reverse osmosis membrane. If necessary, a relay tank may be provided, or the permeate from the pervaporation device 3 may be supplied to a raw water tank in the pure water apparatus 7. Even with the configuration shown in FIG. 2, the permeate from the pervaporation device 3 contains almost no ionic impurities, so there is no increase in the consumption of consumables in the pure water device, and no increase in costs is brought about.

[0018] In the recovery system shown in FIG. 1 or FIG. 2, organic components (organic solvents, organic acids, amines) contained in the permeate of the pervaporation device 3 are removed by the reverse osmosis membrane, and these organic components, especially organic acids and amines, are prevented from circulating to the scrubber 1. However, in order to prevent the organic components from circulating in the system and increasing in concentration, it is useful to measure the quality of the permeate of the reverse osmosis membrane and confirm that the organic components are not contained. The recovery system shown in FIG. 3 is the recovery system shown in FIG. 1, in which an impurity concentration measuring device 9 is provided for measuring the concentration of organic components contained in the permeate of the reverse osmosis membrane device 5. The recovery system shown in FIG. 4 is the recovery system shown in FIG. 2, in which an impurity concentration measuring device 9 is provided for measuring the concentration of organic components contained in the permeate of the reverse osmosis membrane in the pure water device 7 by connecting it to the pure water device 7. In both the recovery systems shown in FIG. 3 and FIG. 4, the impurity concentration measuring device 9 can be configured by, for example, a resistivity meter, a TOC (total organic carbon) concentration meter, a refractometer, or the like. It is preferable that the impurity concentration measuring device 9 is capable of measuring the concentration of at least one of the organic solvents that have leaked through the reverse osmosis membrane and become contained in the permeated water, and the organic substances that have a lower boiling point than the organic solvents and are contained in the permeated water.

[0019] In the recovery system based on the present invention, a distillation apparatus can also be used as a separation apparatus for separating the organic solvent and water from the mixture of the organic solvent and water. FIG. 5 shows the recovery system shown in FIG. 1, in which a distillation apparatus 8 is provided instead of a pervaporation apparatus. Here, a distillation apparatus consisting of two distillation columns 8A and 8B is used. If the boiling point of the organic solvent is higher than that of water, when the mixture from the scrubber 1 is supplied to the distillation apparatus and distilled, the water is discharged as the top gas of the first distillation column 8A. By supplying the bottom component of the first distillation column 8A to the second distillation column 8B, an organic solvent is obtained from the top of the second distillation column 8B, and impurities consisting of high boiling point components are discharged from the bottom. The top liquid obtained by condensing the top gas of the first distillation column 8A contains water as the main component, as well as a small amount of organic solvent and low boiling point organic acids and amines. The top liquid does not contain non-volatile impurities such as salts. The concentration of the organic solvent contained in the overhead liquid obtained by condensing the overhead gas of the first-stage distillation column 8A is, for example, less than 20% by mass, and preferably less than 8% by mass. If the overhead liquid containing such impurities is reused as water for absorbing the organic solvent in the scrubber 1 as it is, the impurity concentration in the water circulating between the scrubber 1 and the distillation device 8 increases, and as a result, the treated gas discharged from the scrubber may contain organic acids or amines, or the recovery rate of the organic solvent in the scrubber may decrease, causing the treated gas to contain the organic solvent. Therefore, the recovery system shown in FIG. 5 is also provided with a reverse osmosis membrane device 5, and the permeate obtained by treating the overhead liquid with the reverse osmosis membrane device 5 is supplied to the scrubber 1. In this recovery system, as in the recovery system shown in FIG. 1, it is possible to significantly reduce the consumables used in pure water production and the energy required for pure water production, and the organic solvent concentration and impurity concentration in the treated gas discharged from the scrubber 1 can be kept low, and the organic solvent can be separated accurately as a whole system.

[0020] In the recovery system shown in Fig. 5, when a purifier equipped with a reverse osmosis membrane is used to generate the pure water to be supplied to the scrubber 1, the overhead liquid from the distillation apparatus 8 can be treated in the purifier itself without the need to separately provide a reverse osmosis membrane device 5. Fig. 6 shows such a recovery system, in which a purifier 7 equipped with a reverse osmosis membrane is used as the purifier, and the overhead liquid obtained by condensing the overhead gas is sent to the purifier 7.

[0021] Next, an example of the configuration of the reverse osmosis membrane device 5 used in each of the above-mentioned recovery systems will be described. When a single-stage pervaporation device is provided as a separation device, the permeate from the pervaporation device may contain an organic solvent at several mass % to several tens of mass %. Even when a distillation device is used, the water obtained from the distillation device may contain an organic solvent at about several hundred ppm. Therefore, as the reverse osmosis membrane device 5, a plurality of reverse osmosis membrane units each having a reverse osmosis membrane are connected in series so that the permeate from the reverse osmosis membrane unit in the previous stage is supplied to the reverse osmosis membrane unit in the subsequent stage, and concentrated water from the reverse osmosis membrane unit in each stage is returned to the inlet side of the reverse osmosis membrane unit in the first stage can be used. By using the reverse osmosis membrane device 5 of such a configuration, water with an extremely low concentration of organic solvent and impurities can be obtained by permeating through the reverse osmosis membranes in multiple stages, and a liquid with an increased organic solvent concentration, i.e., a concentrated liquid, can be obtained from the inlet side of the reverse osmosis membrane unit in the first stage.

[0022] Here, the findings obtained by the present inventors will be described assuming that the organic solvent is NMP. It was planned to treat the permeate of the pervaporation device 3 with a reverse osmosis membrane to obtain concentrated water with an increased NMP concentration and permeate from which NMP has been removed. Therefore, a mixed liquid of NMP and water was shared with the reverse osmosis membrane, and the NMP concentration in the mixed liquid and the rejection rate of NMP in the reverse osmosis membrane were obtained. As the reverse osmosis membrane, a high-pressure reverse osmosis membrane SWC5-LD-4040 (manufactured by Hydranautics) made of polyamide was used, and the supply pressure of the mixed liquid to the reverse osmosis membrane was set to 6 MPa. The results are shown in FIG. 7. In FIG. 7, instead of the rejection rate of NMP, the results are shown by the leakage rate, which is a value obtained by subtracting the rejection rate from 1. As shown in FIG. 7, in the concentration range of NMP in the mixed liquid from the dilute concentration side to about 80 g / kg (i.e., 8 mass%), the NMP leakage rate gradually decreased as the NMP concentration increased. In other words, the NMP rejection rate increased. In contrast, when the NMP concentration exceeded about 80 g / kg, the NMP leakage rate increased rapidly. This means that the NMP rejection rate decreased. When the NMP concentration was 80 g / kg, the NMP rejection rate was 99.2%, whereas when the NMP concentration was 200 g / kg (i.e., 20% by mass), the NMP rejection rate was 81.6%.

[0023] Furthermore, even if a mixed solution with a low NMP concentration (for example, a mixed solution with an NMP concentration of 10 g / kg) was supplied to the reverse osmosis membrane with such a reduced NMP rejection, the NMP rejection did not recover. It was found that once a reverse osmosis membrane comes into contact with high-concentration NMP, the reverse osmosis membrane is irreversibly deteriorated and the NMP rejection is reduced. In a reverse osmosis membrane device in which a plurality of reverse osmosis membrane units are connected in series and concentrated water from the reverse osmosis membrane units of each stage is returned to the inlet side of the reverse osmosis membrane unit of the first stage to obtain permeated water from which the organic solvent has been removed and a liquid with an increased organic solvent concentration, if such a reduction in NMP rejection occurs, the separation performance of the reverse osmosis membrane device as a whole between water and organic solvent is reduced, the number of reverse osmosis membrane units required to make the impurity concentration in the finally obtained water below a predetermined value increases, the processing time is prolonged, and the energy required for processing is also increased. In particular, the deterioration of the reverse osmosis membrane in the reverse osmosis membrane unit of the first stage becomes significant.

[0024] The reverse osmosis membrane device shown in FIG. 8 is a reverse osmosis membrane device configured by connecting a plurality of reverse osmosis membrane units in series, and is configured to increase the separation efficiency of water and organic solvents in the reverse osmosis membrane device as a whole by preventing the influence of the decrease in the organic solvent rejection rate in the reverse osmosis membrane caused by a liquid containing an organic solvent at a high concentration from reaching the reverse osmosis membrane units at the subsequent stage, and is suitably used as the reverse osmosis membrane device 5 in the recovery system of the present invention. The reverse osmosis membrane device shown in FIG. 8 includes a low-concentration processing section 10 in which reverse osmosis membrane units are connected in series in a plurality of stages so that permeated water from the reverse osmosis membrane unit at the preceding stage is supplied to the reverse osmosis membrane unit at the subsequent stage, and a reverse osmosis membrane unit 40 for concentration provided separately from the low-concentration processing section 10. In the illustrated example, the low-concentration processing section 10 includes reverse osmosis membrane units 41 to 44 connected in series in four stages. Each of the reverse osmosis membrane units 40 to 44 includes a reverse osmosis membrane 45 therein. First, the configuration of the low-concentration processing section 10 will be described.

[0025] In the low-concentration treatment section 10, a stock solution, which is a mixture of NMP and water, is supplied from the outside and a storage tank 21 is provided for temporarily storing the stock solution. The liquid in the storage tank 21 is supplied to the first-stage reverse osmosis membrane unit 41 by a pump 32. The storage tank 21 is the first-stage storage tank. The stock solution is the permeated liquid from the pervaporation apparatus 3, or the overhead liquid obtained by condensing the overhead gas of the first-stage distillation column 8A of the distillation apparatus 8. The liquid that has permeated the reverse osmosis membrane 45 in the reverse osmosis membrane unit 41, i.e., the permeated water, is temporarily stored in the second-stage storage tank, storage tank 22, and is supplied to the second-stage reverse osmosis membrane unit 42 via the pump 32. Similarly, the permeated water from the reverse osmosis membrane unit 42 is temporarily stored in the third-stage storage tank 23 and is supplied to the third-stage reverse osmosis membrane unit 43 via the pump 33, and the permeated water from the reverse osmosis membrane unit 43 is temporarily stored in the fourth-stage storage tank 24 and is supplied to the fourth-stage reverse osmosis membrane unit 44 via the pump 34. The permeated water from the fourth-stage reverse osmosis membrane unit 44 is water that contains almost no NMP and can be used as, for example, pure water, and is supplied to the scrubber 1. In the first-stage reverse osmosis membrane unit 41, the liquid discharged from the outlet on the concentration side of the reverse osmosis membrane unit 41 without permeating the reverse osmosis membrane 45, i.e., concentrated water, is returned to the storage tank 21 via piping. Similarly, the concentrated water from the reverse osmosis membrane units 42 to 44 is also returned to the storage tank 21 via piping.

[0026] On the other hand, the reverse osmosis membrane unit 40 for concentration is used to generate a concentrated liquid with an increased NMP concentration. A concentrated liquid storage tank 20 is provided in front of the reverse osmosis membrane unit 40 for concentration. A pipe 51 is provided for supplying the liquid in the tank 21 of the low concentration treatment unit 10 to the concentrated liquid storage tank 20, and a valve 52 is provided on the pipe 51. The liquid in the concentrated liquid storage tank 20 is supplied to the reverse osmosis membrane unit 40 for concentration by the pump 30, and the water that has permeated the reverse osmosis membrane 45 of the reverse osmosis membrane unit 40, i.e., the permeated liquid, is supplied to the tank 21 of the low concentration treatment unit 10. The concentrated water from the reverse osmosis membrane unit 40 for concentration is returned to the concentrated liquid storage tank 20. A pipe 53 for discharging the liquid in the concentrated liquid storage tank 20 is connected to the concentrated liquid storage tank 20, and a valve 54 is provided on the pipe 53.

[0027] As described with reference to FIG. 7, the NMP rejection rate of the reverse osmosis membrane 45 increases with an increase in the NMP concentration until the NMP concentration reaches a certain value when the NMP concentration is low, and then rapidly decreases when the NMP concentration exceeds that value. The reverse osmosis membrane 45 that has come into contact with a high concentration of NMP then maintains a decreased rejection rate even when treating a liquid with a low NMP concentration. If the NMP concentration at which the NMP rejection rate is maximized is defined as the maximum rejection rate concentration A, in the example shown in FIG. 7, the maximum rejection rate concentration A is approximately 8% by mass. In the reverse osmosis membrane device described herein, based on a threshold concentration B set lower than the maximum rejection rate concentration A for safety reasons (i.e., 0 < B < A), in the low-concentration treatment section 10 configured by connecting the reverse osmosis membrane units 41 to 44 in series in four stages, a liquid containing NMP at a concentration lower than the threshold concentration B is treated, and separately, in the reverse osmosis membrane unit 40 for concentration provided separately, a liquid containing NMP at a concentration higher than the threshold concentration B is treated. Here, corresponding to the maximum rejection rate concentration A of approximately 8% by mass, the threshold concentration B is set to 5% by mass.

[0028] Next, a method of operating the reverse osmosis membrane device will be described. In the reverse osmosis membrane device, the raw solution, which is a mixture of NMP and water, is supplied to the storage tank 21 arranged on the inlet side of the first-stage reverse osmosis membrane unit 41 in the low-concentration treatment section 10. The valve 52 provided on the piping 51 connected to the storage tank 21 is kept closed. The NMP concentration in the raw solution is less than the threshold concentration B, for example, 1 mass %. The raw solution in the storage tank 21 is supplied to the first-stage reverse osmosis membrane unit 41 by the pump 31, and the permeate of the first-stage reverse osmosis membrane unit 41 is supplied to the second-stage reverse osmosis membrane unit 42 by the pump 32, and is similarly supplied up to the fourth-stage reverse osmosis membrane unit 44, and water containing almost no NMP or other impurities is discharged from the fourth-stage reverse osmosis membrane unit 44 as wastewater. This water can be supplied to the scrubber 1. Since the concentrated water from the reverse osmosis membrane units 41-44 is returned to the storage tank 21, the NMP concentration of the liquid in the storage tank 21 increases as the operation of the low concentration treatment unit 10 continues, and eventually reaches 5 mass % which is the threshold concentration B. When the NMP concentration of the liquid in the storage tank 21 reaches the threshold concentration B, the valve 52 is opened to discharge the liquid in the storage tank 21 to the concentrated liquid storage tank 20. The valve 54 provided on the pipe 53 connected to the concentrated liquid storage tank 20 is kept closed. Then, the pump 30 is started to supply the liquid in the concentrated liquid storage tank 20 to the reverse osmosis membrane unit 40 for concentration. The permeate of the reverse osmosis membrane unit 40 for concentration is supplied to the storage tank 21 of the low concentration treatment unit 10, and the concentrated water is returned to the concentrated liquid storage tank 20. As a result, the NMP concentration of the liquid in the concentration storage tank 20 is initially 5 mass % which is the threshold concentration B described above, but gradually increases and eventually reaches a predetermined concentration, for example, 20 mass %. When a predetermined concentration is reached, the valve 54 provided on the pipe 53 is opened, and the liquid in the concentrated liquid storage tank 20 is discharged to the outside as an NMP concentrated liquid having an NMP concentration of 20 mass %.

[0029] By repeating the above operations, a liquid with an NMP concentration of 5 mass% is intermittently discharged from the storage tank 21 of the low concentration treatment section 10 to the concentrated liquid storage tank 20, and an NMP concentrated liquid with an NMP concentration of 20 mass% is intermittently discharged from the concentrated liquid storage tank 20. During this time, pure water is discharged from the fourth-stage reverse osmosis membrane unit 44 of the low concentration treatment section 10. In the low concentration treatment section 10, the NMP concentration in the first-stage reverse osmosis membrane unit 41 is the maximum NMP concentration in the low concentration treatment section 10, but since the threshold concentration B is set to 5 mass%, an NMP rejection rate of about 99% in the first-stage reverse osmosis membrane unit 41 can be ensured. The maximum value of the NMP concentration in the liquid supplied to the reverse osmosis membrane unit 40 for concentration is 20 mass %, and at that time, the NMP rejection rate of the reverse osmosis membrane 45 in the reverse osmosis membrane unit 40 for concentration is, for example, 80%, and the NMP concentration in the permeate is about 4%. However, since the NMP rejection rate in the first-stage reverse osmosis membrane unit 41 of the low-concentration treatment section 10 is sufficiently high, the reverse osmosis membrane device described here makes it possible to obtain an NMP concentrate with a high NMP concentration while suppressing the amount of NMP leaking to the subsequent stage to a low value, and the separation performance of water and organic solvents in the entire reverse osmosis membrane device can be maintained at a high level. As a result, the operating time required to separate the mixed liquid into pure water and an NMP concentrate of a desired concentration can be shortened, and the energy consumption required for driving the pumps 30 to 34 can be reduced. Note that, when the NMP concentration in the raw liquid supplied to the reverse osmosis membrane device is equal to or higher than the threshold concentration B, it is preferable to supply the raw liquid to the concentrated liquid storage tank 20 as shown by the broken line in the figure, rather than to the storage tank 21 of the low-concentration treatment section 10. However, even if the NMP concentration of the raw solution is equal to or higher than the threshold concentration B, the raw solution may be supplied to the storage tank 21 of the low concentration treatment unit 10. In the reverse osmosis membrane apparatus shown in Fig. 8, it is not necessary to return all of the concentrated water from the reverse osmosis membrane units 41-44 of each stage to the first-stage storage tank 21, and the concentrated water may be returned to any of the storage tanks located before the inlets of the reverse osmosis membrane units. For example, in the case of the third-stage reverse osmosis membrane unit 43, the concentrated water discharged from the reverse osmosis membrane unit 43 may be circulated to at least one of the first-stage to third-stage storage tanks 21-23.When the operation of the reverse osmosis membrane device 40 for concentration is not impaired, a portion of the concentrated water from the reverse osmosis membrane units 41 to 44 at each stage can be supplied to the concentrated liquid storage tank 20.

[0030] In the reverse osmosis membrane device shown in FIG. 8, concentrated water from the second and subsequent reverse osmosis membrane units 42-44 is returned to the first reverse osmosis membrane unit 41 in the low concentration treatment section 10, so the amount of treated liquid in the first reverse osmosis membrane unit 41 becomes large, and the amount of permeated liquid must also be increased. However, since the amount of permeated liquid in a reverse osmosis membrane unit is generally determined by the membrane area and permeation flux of the reverse osmosis membrane, there is a limit in terms of the size and cost of the device to increasing the amount of permeated liquid in the first reverse osmosis membrane unit 41 while maintaining the required separation performance. The reverse osmosis membrane device shown in FIG. 9 is preferably used as the reverse osmosis membrane device 5 in the recovery system of the present invention, and is configured to equalize the amount of treated liquid among the multiple reverse osmosis membrane units 41-44 provided in the low concentration treatment section 10 as much as possible. The reverse osmosis membrane device shown in FIG. 9 will be described below.

[0031] The reverse osmosis membrane device shown in FIG. 9 is different from the reverse osmosis membrane device shown in FIG. 8 only in the configuration of the low-concentration treatment section 10. The low-concentration treatment section 10 is provided with reverse osmosis membrane units 41-44 connected in series in four stages via storage tanks 21-24 and pumps 31-34, as in the device shown in FIG. 8, but differs from the device shown in FIG. 8 in that the liquid discharged from the outlet on the concentrated side of the reverse osmosis membrane units 41-44 in each stage, i.e., concentrated water, is returned to the storage tanks 21-24 of that stage. Furthermore, a pipe 62 is provided for supplying the liquid in the second-stage storage tank 22 to the first-stage storage tank, and a valve 72 is provided on the pipe 62. Similarly, a pipe 63 is provided for supplying the liquid in the third-stage storage tank 23 to the second-stage storage tank, which is the previous stage, and a valve 73 is provided on the pipe 63. A pipe 64 is provided for supplying the liquid in the fourth-stage storage tank 24 to the third-stage storage tank, and a valve 74 is provided on the pipe 64.

[0032] When the reverse osmosis membrane device is operated with the valves 52, 72-74 closed, the NMP concentration of the liquid in the storage tanks 21-24 of each stage gradually increases. As for the first stage storage tank 21, as in the case of the reverse osmosis membrane device shown in FIG. 8, when the NMP concentration of the liquid in the storage tank 21 reaches the threshold concentration B (for example, 5%), the valve 52 is opened and the liquid in the storage tank 21 is discharged to the concentrated liquid storage tank 20. As for the second and subsequent storage tanks 22-24, when the NMP concentration of the liquid in the storage tanks 22-24 is concentrated to a predetermined value, the valves 62-64 are opened, respectively, and the liquid is discharged to the storage tanks 21-23 of the previous stage. That is, intermittent discharge to the storage tanks of the previous stage is performed. Concentrated to a predetermined value here means that, for a storage tank of a certain stage, the current NMP concentration is a predetermined value compared to the NMP concentration of the liquid in the storage tank immediately after the previous discharge of the liquid to the storage tank of the previous stage. The predetermined multiple is, for example, 10 times, but may be different depending on which stage the storage tank is in. Of course, when intermittent discharge has not yet been performed after the start of operation, whether or not to perform intermittent discharge is determined depending on how much the current NMP concentration has increased compared to the NMP concentration in the storage tank immediately after the start of operation.

[0033] In this reverse osmosis membrane apparatus, too, a liquid containing NMP at a concentration lower than the threshold concentration B is treated in the low concentration treatment section 10, and a liquid containing NMP at a concentration equal to or higher than the threshold concentration B is treated in a reverse osmosis membrane unit 40 for concentration provided separately from the low concentration treatment section. Therefore, when the NMP concentration of the raw liquid supplied to this reverse osmosis membrane apparatus is less than the threshold concentration B, it is supplied to the first-stage storage tank 21 of the low concentration treatment section 10, and when the NMP concentration is equal to or higher than the threshold concentration B, it is supplied to the concentrated liquid storage tank 20.

[0034] In the reverse osmosis membrane device shown in Fig. 9, concentrated water from the second and subsequent reverse osmosis membrane units 42-44 is not continuously supplied to the first reverse osmosis membrane unit 41 of the low-concentration treatment unit 10, so the amount of treated liquid in the first reverse osmosis membrane unit 41 can be reduced compared to the reverse osmosis membrane device shown in Fig. 8. Furthermore, the second and subsequent reverse osmosis membrane units 42-44 re-treat at least their own concentrated water. As a result, in the reverse osmosis membrane device shown in Fig. 9, the amounts of treated liquid can be made as equal as possible among the multiple reverse osmosis membrane units 41-44 provided in the low-concentration treatment unit 10.

[0035] In the reverse osmosis membrane device shown in Fig. 8 and Fig. 9, in the low-concentration treatment section 10, the permeated water from the first-stage reverse osmosis membrane unit 41 is supplied to the second-stage reverse osmosis membrane unit 42 via the second-stage storage tank 22, and the permeated water from the second-stage reverse osmosis membrane unit 42 is supplied to the third-stage reverse osmosis membrane unit 43 via the third-stage storage tank 23, and so on. In this manner, a plurality of reverse osmosis membrane units are connected in series with the first-stage reverse osmosis membrane device 41 at the head. This configuration can be said to be a configuration including a total of n+1 stages of reverse osmosis membrane units each having a reverse osmosis membrane, where i is an integer between 1 and n, and the permeated water from the i-th stage reverse osmosis membrane unit is connected to the i+1-stage reverse osmosis membrane unit via the i+1-stage storage tank. In the reverse osmosis membrane device shown in Fig. 8 and Fig. 9, at least a portion of the permeated water from the reverse osmosis membrane unit of each stage is supplied to one or more storage tanks from the first stage to the stage. However, in the reverse osmosis membrane device 5 used in the recovery system of the present invention, a reverse osmosis membrane unit may be provided in a system separate from the total of n+1 reverse osmosis membrane units connected in series in the low concentration treatment section 10, and at least a portion of the concentrated water from any one or more of the reverse osmosis membrane units from the second stage onwards among the total of n+1 reverse osmosis membrane units connected in series may be treated in the reverse osmosis membrane unit of the separate system. Figure 10 shows an example of a reverse osmosis membrane device in which concentrated water from any one or more reverse osmosis membrane units from the second stage onwards is treated in the reverse osmosis membrane unit of the separate system.

[0036] In the reverse osmosis membrane device shown in FIG. 10, the first-stage reverse osmosis membrane unit 41 to the n+1-stage reverse osmosis membrane unit 49 are connected in series in the low-concentration treatment section 10, and a reverse osmosis membrane unit 82 is provided that is separated from the series connection, i.e., a separate system. Focusing on the n-th stage reverse osmosis membrane unit 48 in this reverse osmosis membrane device, the n-th stage storage tank 28 is provided at the inlet side of the n-th stage reverse osmosis membrane unit 48, and the liquid in this storage tank 28 is supplied to the n-th stage reverse osmosis membrane unit 48 via the pump 38. The permeated water of the n-th stage reverse osmosis membrane unit 48 is sent to the n+1-stage storage tank 29. Similarly, the liquid in the n+1-stage storage tank 29 is supplied to the n+1-stage reverse osmosis membrane unit 49 via the pump 39. Meanwhile, the concentrated water of the n-th stage and the n+1-stage reverse osmosis membrane units 48, 49 is sent to the storage tank 80 provided at the inlet of the reverse osmosis membrane unit 82 of the separate system. The liquid in this storage tank 80 is supplied to the reverse osmosis membrane unit 82 via a pump 81, and concentrated water from the reverse osmosis membrane unit 82 is returned to the storage tank 80. The permeated water from the reverse osmosis membrane unit 82 may be discharged as wastewater together with the permeated water from the (n+1)-th stage reverse osmosis membrane unit 49, for example. A separate reverse osmosis membrane unit to which the permeated water from the reverse osmosis membrane unit 82 is supplied may be provided downstream of the reverse osmosis membrane unit 82 in a separate system. When the amount of liquid in the storage tank 80 increases or when the NMP concentration in the liquid in the storage tank 80 increases, the liquid in the storage tank 80 may be intermittently discharged via a valve 83 to the first-stage storage tank 21.

[0037] In the reverse osmosis membrane device shown in Fig. 10, in the reverse osmosis membrane units 41, ..., 48, 49 connected in series in n+1 stages, concentrated water from the reverse osmosis membrane units in the second stage to the n-1th stage may be supplied to the storage tank 21 of the first stage as in the reverse osmosis membrane device shown in Fig. 8, or may be circulated to the storage tank of the stage as in the reverse osmosis membrane device shown in Fig. 9. In addition, when the reverse osmosis membrane units 41, ..., 48, 49 in the n+1 stages are connected in series, the reverse osmosis membrane units that supply concentrated water to the storage tank 80 are not limited to the nth stage and the n+1th stage, and concentrated water from the reverse osmosis membrane units of the other stages may be supplied to the storage tank 80. In this separation system, it is sufficient that at least a portion of the concentrated water from the reverse osmosis membrane unit of at least one stage is supplied to one or more storage tanks from the first stage to the stage. In other words, for at least one integer j satisfying 2≦j≦n+1, at least a portion of the concentrated water from the j-th reverse osmosis membrane unit is supplied to at least one of the storage tanks from the first stage to the j-th stage.

[0038] Although several examples of reverse osmosis membrane devices that can be used in the recovery system of the present invention have been described above, the organic solvent to be separated from water in the reverse osmosis membrane device is not limited to NMP. The reverse osmosis membrane constituting the reverse osmosis membrane device may be made of polyamide, cellulose acetate, polysulfone, or polyimide. In particular, the reverse osmosis membrane used in the present invention is a high-pressure membrane that is used for desalination of seawater and has a permeation flux of 0.1 to 3.0 m at a primary inlet pressure of 5.5 MPa, an effective pressure of 2.7 MPa, a NaCl concentration of 32,000 mg / L, and a temperature of 25°C. 3 / m 2 / day range, preferably 0.3 to 2.0 m 3 / m 2 / day, more preferably 0.5 to 1.7 m 3 / m 2 / day range, and a NaCl rejection rate of 90% or more, preferably 99% or more, and more preferably 99.6% or more is exemplified. Here, "effective pressure" refers to the effective pressure acting on the membrane, which is the average operating pressure minus the osmotic pressure difference and the secondary pressure, as described in JIS K3802:2015 "Membrane Terminology." The average operating pressure is the average value of the pressure of the membrane feed water on the primary side of the membrane (operating pressure: primary side inlet pressure) and the pressure of the concentrated water (concentrated water outlet pressure), Average operating pressure = (operating pressure + concentrated water outlet pressure) / 2 It is represented by:

[0039] The permeation flux at the primary inlet pressure of 5.5 MPa, effective pressure of 2.7 MPa, NaCl concentration of 32000 mg / L, and temperature of 25°C is 0.1 to 3.0 m 3 / m 2 / day range and have a NaCl rejection rate of 90% or more, and are known as high-pressure membranes, examples of which include the SW30 series, SW30HR series, SW30HRLE series, SW30ULE series, SW30XLE series, SW30XHR series, and SEAMAXX series manufactured by FILMTEC; the SWC2 series, SWC3 series, SWC4 series, SWC5 series, and SWC6 series manufactured by Nitto Denko; and the TM800C series, TM800M series, TM800E series, TM800R series, TM800V series, TM800K series, and SU-800 series manufactured by Toray.

[0040] In the above explanation, the threshold concentration B is set to 5% by mass, but an appropriate value is selected for the threshold concentration B depending on the type of organic solvent to be separated, the type of reverse osmosis membrane used in the reverse osmosis membrane device, etc. The number of stages of reverse osmosis membrane units connected in series in the low concentration treatment unit 10 is not limited to four, and the number of stages is appropriately determined depending on the required water quality value of the finally obtained water, etc. In some cases, the low concentration treatment unit 10 may be configured to have only one reverse osmosis membrane unit.

[0041] The recovery system according to the present invention is preferably used when recovering an organic solvent that dissolves in water. For example, it can be preferably used when recovering NMP by treating NMP vapor generated by evaporating NMP in the manufacturing process of a lithium ion secondary battery. EXAMPLES

[0042] Next, the present invention will be described in more detail with reference to examples.

[0043] [Example 1] The pervaporation device 3 and the reverse osmosis membrane device 5 of the recovery system shown in FIG. 1 were assembled, and an organic solvent aqueous solution containing an organic acid and an amine was supplied to the pervaporation device 3. A single-stage pervaporation device was used as the pervaporation device 3, and a single-stage reverse osmosis membrane device 5 was also used. A type A zeolite membrane and a CHA type zeolite were used as the pervaporation membrane 4, and a high-pressure type SWC5 manufactured by Nitto Denko was used as the reverse osmosis membrane 6. In each case, the composition of the inlet liquid (PV inlet liquid) of the pervaporation device 3, the composition of the permeate liquid (PV permeate liquid) of the pervaporation device 3 (i.e., the inlet liquid of the reverse osmosis membrane device 5), and the composition of the permeate water (RO permeate water) of the reverse osmosis membrane device 5 were investigated. The results when the pervaporation membrane 4 was an A type zeolite membrane are shown in Table 1, and the results when the pervaporation membrane 4 was a CHA type zeolite membrane are shown in Table 2.

[0044] [Table 1]

[0045] [Table 2]

[0046] As can be seen from Tables 1 and 2, the organic acid and amine components all migrated to the permeate in the pervaporation device 3, but were hardly contained in the permeate from the reverse osmosis membrane device 5. NMP was also hardly contained in the permeate from the reverse osmosis membrane device 5. This shows that by treating the permeate from the pervaporation device with a reverse osmosis membrane, water of a quality usable for contact with an organic solvent in a scrubber can be obtained.

[0047] [Example 2] Of the recovery system shown in Figure 5, the distillation apparatus 8 and reverse osmosis membrane apparatus 5 were assembled, and an organic solvent aqueous solution containing an organic acid and an amine was supplied to the distillation apparatus 8. A single-stage reverse osmosis membrane apparatus 5 was used. The composition of the water (top liquid) obtained by condensing the overhead gas of the first-stage distillation column 8A of the distillation apparatus 8, and the composition of the permeated water (RO permeated water) from the reverse osmosis membrane apparatus 5 were investigated. The results are shown in Table 3.

[0048] [Table 3]

[0049] As can be seen from Table 3, the overhead liquid contained NMP, organic acids and amines, but almost none of them were contained in the permeate from the reverse osmosis membrane device 5. This shows that by treating the water (overhead liquid) separated in the distillation device with a reverse osmosis membrane, it is possible to obtain water of a quality that can be used when contacted with an organic solvent in the scrubber. [Explanation of symbols]

[0050] 1 Scrubber 2,7 Pure water equipment 3. Pervaporation device 4. Pervaporation membrane 5 Reverse osmosis membrane equipment 6,45 Reverse osmosis membrane 8. Distillation Apparatus 8A, 8B Distillation tower 9 Impurity concentration measuring device 10 Low concentration processing section 20 Concentrate storage tank 21~24 Storage tank 30~34 Pump 40~44 Reverse osmosis membrane unit 51,53 Piping 52,54,55 Valve

Claims

1. 1. A method for recovering an organic solvent from a gas containing the organic solvent, comprising the steps of: a contacting step of contacting the gas with water to obtain a mixed liquid of the organic solvent and water; a separation step of separating the organic solvent and water from the mixture; a water treatment step of treating the water separated in the separation step with a reverse osmosis membrane; having The recovery method includes supplying the water that has permeated the reverse osmosis membrane to the contact step.

2. The method according to claim 1 , wherein the separation step is a step of separating the organic solvent and water by pervaporation or distillation.

3. The water treatment step includes: supplying water separated in the separation step and having a concentration of the organic solvent less than a threshold value to a storage tank; treating the liquid stored in the storage tank with a reverse osmosis membrane device to separate the liquid into permeate and concentrated water, and supplying at least a portion of the concentrated water from the reverse osmosis membrane device to the storage tank; When the concentration of the organic solvent in the liquid in the storage tank becomes equal to or greater than a threshold concentration, transferring at least a portion of the liquid in the storage tank to a concentrated liquid storage tank; a step of treating the liquid stored in the concentrated liquid storage tank with a reverse osmosis membrane device for concentration to separate the liquid into permeate and concentrated water, circulating the concentrated water from the reverse osmosis membrane device for concentration to the concentrated liquid storage tank, and supplying the permeate from the reverse osmosis membrane device for concentration to the storage tank; The recovery method according to claim 1 or 2, comprising:

4. The method according to claim 3 , wherein when the concentration of the organic solvent in the water separated in the separation step is equal to or higher than the threshold concentration, the separated water is directly supplied to the concentrated liquid storage tank.

5. The water separated in the separation step contains an organic substance having a lower boiling point than the organic solvent and the organic solvent mixed in the water in the separation step, 3. The method according to claim 1, wherein the organic matter and the organic solvent are removed by the reverse osmosis membrane, and the water that has permeated the reverse osmosis membrane is supplied to the contact step.

6. 1. A recovery system for recovering an organic solvent from a gas containing the organic solvent, comprising: a scrubber that contacts the gas with water and discharges a mixture of the organic solvent and water; a separation device for separating the organic solvent and water from the mixed liquid; A reverse osmosis membrane, having A recovery system wherein water separated in the separation device is supplied to the reverse osmosis membrane, and water permeating the reverse osmosis membrane is supplied to the scrubber for contact with an organic solvent.

7. The recovery system according to claim 6 , wherein the separation device is a pervaporation device having a pervaporation membrane for separating the organic solvent from the water, or a distillation device for separating the organic solvent from the water.

8. a reverse osmosis membrane device including the reverse osmosis membrane, The reverse osmosis membrane device includes a low-concentration treatment section including a first-stage storage tank to which the water separated in the separation device is supplied, a concentrated liquid storage tank, and a concentration reverse osmosis membrane unit to which the liquid in the concentrated liquid storage tank is supplied, the low-concentration treatment unit includes at least a first-stage reverse osmosis membrane unit to which the liquid in the first-stage storage tank is supplied, and at least a portion of the concentrated water from the first-stage reverse osmosis membrane unit is supplied to the first-stage storage tank; when the concentration of the organic solvent in the liquid in the first-stage storage tank becomes equal to or greater than a threshold concentration, at least a portion of the liquid in the first-stage storage tank is transferred to the concentrated liquid storage tank, the liquid in the concentrated liquid storage tank is supplied to the concentration reverse osmosis membrane unit, concentrated water from the concentration reverse osmosis membrane unit is circulated to the concentrated liquid storage tank, and permeated water from the concentration osmosis membrane unit is supplied to the first-stage storage tank, The recovery system according to claim 6 or 7, wherein permeate discharged from the low-concentration treated water is supplied to the scrubber.

9. The recovery system according to claim 8 , wherein when a concentration of the organic solvent in the water separated in the separation device is equal to or greater than the threshold concentration, the separated water is directly supplied to the concentrate storage tank.

10. 9. The recovery system according to claim 8, wherein the low concentration treatment unit includes a plurality of reverse osmosis membrane units connected such that permeate from an n-th stage reverse osmosis membrane unit is supplied to an n+1-th stage reverse osmosis membrane unit via an n+1-th stage storage tank, where n is an integer of 1 or greater, and at least a portion of the concentrated water from the n-th stage reverse osmosis membrane unit is supplied to at least one of the first stage to n-th stage storage tanks.

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