Separation method of organic solvent and water and system

Abstract

To maintain the high separation performance between water and an organic solvent in the entire system and reduce the operation time and energy consumption when separating the organic solvent and water from a mixed liquid of the organic solvent and water by using a reverse osmosis membrane.SOLUTION: A separation method includes the steps of: supplying a mixed liquid to a reservoir 21 as a stock solution; processing the liquid stored in a reservoir 1 by reverse osmosis membrane apparatuses 41-44, separating it into permeable water and concentrated water, and circulating the concentrated water from the reverse osmosis membrane apparatuses 41-44 into the reservoir 21; transferring at least a portion of the liquid in the reservoir 21 to a concentrated liquid reservoir 20 when the concentration of the organic solvent in the liquid in the reservoir 21 is equal to or greater than the threshold concentration; processing the liquid stored in the concentrated liquid reservoir 21 by the reverse osmosis membrane apparatus 40 for concentration, separating it into the permeable water and the concentrated water, circulating the concentrated water in the concentrated liquid reservoir 20, and supplying the permeable water to the reservoir 21; and intermittently discharging the concentrated liquid in which the organic solvent is concentrated from the concentrated liquid reservoir 20.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method and system for separating an organic solvent and water from a mixture of the organic solvent and water using a reverse osmosis membrane. [Background technology]

[0002] There is a demand for separating the organic solvent and water from a mixture of the organic solvent and water. For example, when an organic solvent is recovered, purified, and reused after use, water may be mixed into the organic solvent during the process of using and recovering the organic solvent. In such a case, the mixed water must be separated from the organic solvent to obtain an organic solvent that does not contain water. Even if the organic solvent is not reused, it is generally not permitted to release water containing a certain concentration or more of organic solvent into the environment, so it is necessary to separate the organic solvent and water. As an example, in the manufacturing process of lithium secondary ion batteries, particles of an electrode active material are dispersed in N-methyl-2-pyrrolidone (hereinafter also referred to as NMP), which is one of the organic solvents, to form a slurry, and the 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 or the like. The recovered NMP is in the form of a mixture of water and NMP, and whether the recovered NMP is to be reused or discarded, it is necessary to separate the NMP and water from the mixture.

[0003] A method using a reverse osmosis membrane is known as a method for separating an organic solvent and water from a mixture of an organic solvent and water. For example, Patent Document 1 discloses that when isopropyl alcohol is separated and recovered from wastewater containing isopropyl alcohol, a reverse osmosis membrane (RO membrane) is used to obtain water containing almost no isopropyl alcohol, and a vapor permeable membrane (VP membrane) is used to concentrate the isopropyl alcohol. When a mixture of an organic solvent and water is supplied to a reverse osmosis membrane, the organic solvent is blocked by the reverse osmosis membrane, while water permeates the reverse osmosis membrane. Since a small amount of organic solvent leaks (leaks) from the reverse osmosis membrane, in order to make the organic solvent concentration in the finally obtained water equal to or lower than a desired value, it is considered to connect multiple reverse osmosis membrane devices in series so that the permeated water from the reverse osmosis membrane device in the previous stage is supplied to the reverse osmosis membrane device in the subsequent stage. When multiple reverse osmosis membrane devices are connected in series, a storage tank is provided to temporarily store the mixed liquid supplied from the outside, and the mixed liquid is supplied from the storage tank to the reverse osmosis membrane device in the first stage. In this case, if the water from the outlet of the concentrating side of each reverse osmosis membrane device (i.e., concentrated water) is returned to the storage tank, the organic solvent concentration in the mixed liquid in the storage tank will also gradually increase, making it possible to obtain from this storage tank a liquid with a higher organic solvent concentration than the mixed liquid initially supplied from the outside. In other words, the entire separation system formed by directly connecting a plurality of reverse osmosis membrane devices makes it possible to obtain both a concentrated liquid with an increased organic solvent concentration and water from which the organic solvent has been removed. If the obtained concentrated liquid is further treated, an organic solvent of a purity that can be reused can be obtained. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-532542 Summary of the Invention [Problem to be solved by the invention]

[0005] In general, it is believed that the rejection rate of a solute in a reverse osmosis membrane when a solution of a solute and water is supplied to the reverse osmosis membrane increases as the solute concentration in the solution increases. However, the present inventors have found that, for a certain type of organic solvent, when the organic solvent concentration in the mixture of the organic solvent and water exceeds a certain value, the rejection rate of the organic solvent in the reverse osmosis membrane decreases. The present inventors have also found that a reverse osmosis membrane in which the rejection rate of an organic solvent has once decreased will continue to decrease even if a solution with a low organic solvent concentration is subsequently supplied. If the rejection rate of an organic solvent in a reverse osmosis membrane decreases when the organic solvent concentration is high, the separation performance of water and organic solvent in the entire system decreases, and the number of stages of a reverse osmosis membrane device required to make the organic solvent concentration in the finally obtained water below a predetermined value increases, or the processing time becomes longer. As a result, the energy required for the process of separating the organic solvent from water also increases.

[0006] An object of the present invention is to provide a separation method and a separation system that can maintain high separation performance between water and an organic solvent in the entire system and reduce operating time and energy consumption when separating an organic solvent and water from a mixed liquid of an organic solvent and water using a reverse osmosis membrane. [Means for solving the problem]

[0007] The separation method of the present invention is a method for separating water and a concentrate in which the organic solvent is concentrated from a mixture of an organic solvent and water, and includes the steps of: supplying the mixture to a first-stage storage tank arranged at an inlet of a first-stage reverse osmosis membrane device equipped with a reverse osmosis membrane; a first separation step of treating the liquid stored in the first-stage storage tank with the first-stage reverse osmosis membrane device to separate it into permeate and concentrate, and supplying at least a portion of the concentrate from the first-stage reverse osmosis membrane device to the first-stage storage tank; a step of transferring at least a portion of the liquid in the first-stage storage tank to a concentrate storage tank when the concentration of the organic solvent in the liquid in the first-stage storage tank becomes equal to or higher than a threshold concentration; and a second separation step of treating the liquid stored in the concentrate storage tank with a concentration reverse osmosis membrane device equipped with a reverse osmosis membrane to separate it into permeate and concentrate, circulating the concentrate from the concentration reverse osmosis membrane device to the concentrate storage tank, and supplying the permeate from the concentration reverse osmosis membrane device to the first-stage storage tank.

[0008] The separation system of the present invention is a separation device that separates water and a concentrated liquid in which the organic solvent is concentrated from a mixed liquid of an organic solvent and water, and includes a low-concentration treatment section having a first-stage storage tank to which the mixed liquid is supplied, a concentrated liquid storage tank, and a reverse osmosis membrane device for concentration that is equipped with a reverse osmosis membrane and is supplied with the liquid in the concentrated liquid storage tank. The low-concentration treatment section includes at least a first-stage reverse osmosis membrane device that is equipped with a reverse osmosis membrane and is supplied with the liquid in the first-stage storage tank. The first-stage storage tank is disposed at an inlet of the first-stage reverse osmosis membrane device. At least a portion of the concentrated water from the first-stage reverse osmosis membrane device 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 and supplied to the reverse osmosis membrane device for concentration. The concentrated water from the reverse osmosis membrane device for concentration is circulated to the concentrated liquid storage tank, and the permeated water from the osmosis membrane device for concentration is supplied to the first-stage storage tank. Effect of the Invention

[0009] According to the present invention, when an organic solvent and water are separated from a mixed liquid of an organic solvent and water using a reverse osmosis membrane, the separation performance of the water and the organic solvent can be maintained at a high level in the entire system, and the operating time and energy consumption can be reduced. [Brief description of the drawings]

[0010] [Figure 1] 1 is a graph showing the relationship between NMP concentration and NMP leakage rate in a reverse osmosis membrane. [Diagram 2] FIG. 1 illustrates a separation system according to an embodiment of the present invention. [Diagram 3] FIG. 1 illustrates another embodiment of a separation system. [Figure 4] FIG. 1 illustrates another embodiment of a separation system. [Diagram 5] FIG. 1 is a diagram showing a separation system of a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Next, an embodiment of the present invention will be described with reference to the drawings. Before describing a separation system based on the present invention, the findings of the present inventors regarding the treatment of a mixed liquid of an organic solvent and water using a reverse osmosis membrane will be described.

[0012] The present inventors planned to treat a mixed solution of NMP (i.e., N-methyl-2-pyrrolidone) and water with a reverse osmosis membrane to obtain a concentrated solution with an increased NMP concentration and permeate from which NMP has been removed. Therefore, the NMP concentration in the mixed solution supplied to the reverse osmosis membrane 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 solution to the reverse osmosis membrane was set to 6 MPa. The results are shown in FIG. 1. In FIG. 1, 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. 1, in the concentration range of the NMP concentration in the mixed solution 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%.

[0013] 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. In other words, 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 separation system in which a plurality of reverse osmosis membrane devices are connected in series and concentrated water from the reverse osmosis membrane devices of each stage is returned to the inlet side of the reverse osmosis membrane device of the first stage to obtain permeated water from which the organic solvent has been removed and a concentrated liquid with an increased organic solvent concentration, if such a reduction in NMP rejection occurs, the separation performance between water and organic solvent of the entire separation system is reduced, the number of stages of reverse osmosis membrane devices required to make the NMP concentration in the finally obtained water below a predetermined value increases, the processing time is prolonged, and the energy required for processing also increases. In particular, the deterioration of the reverse osmosis membrane in the reverse osmosis membrane device of the first stage becomes significant.

[0014] The separation system according to the present invention is configured by connecting a plurality of reverse osmosis membrane devices in series, and is configured to prevent the influence of a decrease in the organic solvent rejection rate at the reverse osmosis membrane caused by a liquid containing an organic solvent at a high concentration from reaching the downstream reverse osmosis membrane device, thereby increasing the separation efficiency between water and an organic solvent in the entire separation system. Fig. 2 is a diagram showing a separation system according to an embodiment of the present invention. In the following description, the organic solvent in the mixture of an organic solvent and water is NMP, but the organic solvent constituting the mixture together with water in the present invention is not limited to NMP.

[0015] The separation system shown in Fig. 2 includes a low-concentration treatment section 10 in which reverse osmosis membrane devices are connected in series in multiple stages so that permeated water from a reverse osmosis membrane device in a preceding stage is supplied to a reverse osmosis membrane device in a succeeding stage, and a reverse osmosis membrane device 40 for concentration that is provided separately from the low-concentration treatment section 10. In the example shown in the figure, the low-concentration treatment section 10 includes reverse osmosis membrane devices 41-44 connected in series in four stages. Each of the reverse osmosis membrane devices 40-44 includes a reverse osmosis membrane 45 therein. First, the configuration of the low-concentration treatment section 10 will be described.

[0016] 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, and the liquid in the storage tank 21 is supplied to the first-stage reverse osmosis membrane device 41 by a pump 32. The storage tank 21 is referred to as the first-stage storage tank. The liquid that has permeated the reverse osmosis membrane 45 in the reverse osmosis membrane device 41, i.e., the permeated water, is temporarily stored in the storage tank 22 and supplied to the second-stage reverse osmosis membrane device 42 via the pump 32. The storage tank 22 is referred to as the second-stage storage tank. Similarly, the permeated water from the reverse osmosis membrane device 42 is temporarily stored in the storage tank 23 and supplied to the third-stage reverse osmosis membrane device 43 via the pump 33, and the permeated water from the reverse osmosis membrane device 43 is temporarily stored in the storage tank 24 and supplied to the fourth-stage reverse osmosis membrane device 44 via the pump 34. The storage tanks 23 and 24 are the third-stage and fourth-stage storage tanks, respectively. The permeated water from the fourth-stage reverse osmosis membrane device 44 contains almost no NMP and can be used as, for example, pure water, and is discharged outside the separation system. In the first-stage reverse osmosis membrane device 41, the liquid discharged from the outlet on the concentration side of the reverse osmosis membrane device 41 without permeating the reverse osmosis membrane 45, i.e., the concentrated water, is returned to the storage tank 21 via piping. Similarly, the concentrated water from the reverse osmosis membrane devices 42 to 44 is also returned to the storage tank 21 via piping.

[0017] On the other hand, the reverse osmosis membrane device 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 device 40 for concentration. A pipe 51 is provided for supplying the liquid in the storage 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 device 40 for concentration by the pump 30, and the liquid that has permeated the reverse osmosis membrane 45 of the reverse osmosis membrane device 40, i.e., the permeated water, is supplied to the storage tank 21 of the low concentration treatment unit 10. The concentrated water from the reverse osmosis membrane device 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.

[0018] As described with reference to FIG. 1, the NMP rejection rate of the reverse osmosis membrane 45 increases as the NMP concentration increases until the NMP concentration reaches a certain value when the NMP concentration is low, and rapidly decreases when the NMP concentration exceeds that value. And the reverse osmosis membrane 45 that has come into contact with a high concentration of NMP then has a reduced 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. 1, the maximum rejection rate concentration A is about 8% by mass. In the separation system of the present embodiment, based on a threshold concentration B set lower than the maximum rejection rate concentration A in view of safety (that is, 0 < B < A), a liquid containing NMP at a concentration lower than the threshold concentration B is treated in the low-concentration treatment unit 10 configured by connecting the reverse osmosis membrane devices 41 to 44 in series in four stages, and in a reverse osmosis membrane device 40 for concentration provided separately therefrom, a liquid containing NMP at a concentration equal to or higher than the threshold concentration B is treated. Here, corresponding to the maximum rejection rate concentration A of about 8% by mass, the threshold concentration B is set to 5% by mass.

[0019] Next, the operation method of the separation system shown in Fig. 2 will be described. In the separation system, the raw solution, which is a mixture of NMP and water, is supplied to a storage tank 21 arranged on the inlet side of a first-stage reverse osmosis membrane device 41 in the low-concentration treatment section 10. A valve 52 provided on a pipe 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 device 41 by a pump 31, and the permeate of the first-stage reverse osmosis membrane device 41 is supplied to the second-stage reverse osmosis membrane device 42 by a pump 32, and is similarly supplied up to a fourth-stage reverse osmosis membrane device 44, and water containing almost no NMP is discharged as wastewater from the fourth-stage reverse osmosis membrane device 44. Since the concentrated water from these reverse osmosis membrane devices 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 device 40 for concentration. The permeated water of this reverse osmosis membrane device 40 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 concentrated liquid 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 %.

[0020] By repeating the above operations, a liquid having 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 having 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 device 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 device 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 device 41 can be ensured. The maximum value of the NMP concentration in the liquid supplied to the reverse osmosis membrane device 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 device 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 device 41 in the low-concentration treatment unit 10 is sufficiently high, the separation system of this embodiment makes it possible to obtain an NMP concentrate with a high NMP concentration while suppressing the amount of NMP leakage to the subsequent stage to a low value, and the separation performance of water and organic solvents in the entire separation system 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 separation system 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 unit 10. However, even if the NMP concentration of the raw liquid is equal to or higher than the threshold concentration B, the raw liquid may be supplied to the storage tank 21 of the low concentration treatment unit 10. In addition, in the separation system shown in FIG. 2, it is not necessary to return all of the concentrated water from the reverse osmosis membrane devices 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 inlet of the reverse osmosis membrane device. For example, in the case of the third stage reverse osmosis membrane device 43, the concentrated water discharged from the reverse osmosis membrane device 43 may be circulated to at least one of the storage tanks 21-23 of the first stage to the third stage. In addition, if there is no problem with the operation of the reverse osmosis membrane device 40 for concentration, it is also possible to supply a part of the concentrated water from the reverse osmosis membrane devices 41-44 of each stage to the concentrated liquid storage tank 20.

[0021] In the separation system shown in FIG. 2, concentrated water from the second and subsequent reverse osmosis membrane devices 42-44 is returned to the first reverse osmosis membrane device 41 in the low concentration treatment section 10, so the amount of treated liquid in the first reverse osmosis membrane device 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 device 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 device 41 while maintaining the required separation performance. The separation system of another embodiment of the present invention shown in FIG. 3 is configured to equalize the amount of treated liquid among the multiple reverse osmosis membrane devices 41-44 provided in the low concentration treatment section 10 as much as possible. The separation system shown in FIG. 3 will be described below.

[0022] The separation system shown in FIG. 3 is different from the separation system shown in FIG. 2 only in the configuration of the low-concentration processing unit 10. The low-concentration processing unit 10 is provided with reverse osmosis membrane devices 41-44 connected in series in four stages via storage tanks 21-24 and pumps 31-34, as in the case shown in FIG. 2, but differs from the system shown in FIG. 2 in that the liquid discharged from the outlet on the concentration side of the reverse osmosis membrane devices 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.

[0023] When the separation system 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 system shown in FIG. 2, 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 that 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.

[0024] In this separation system, too, a liquid containing NMP at a concentration lower than the threshold concentration B is treated in the low concentration treatment unit 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 device 40 for concentration provided separately from the low concentration treatment unit. Therefore, when the NMP concentration of the raw liquid supplied to this separation system is less than the threshold concentration B, it is supplied to the first-stage storage tank 21 of the low concentration treatment unit 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.

[0025] In the separation system shown in Fig. 3, concentrated water from the second and subsequent reverse osmosis membrane devices 42-44 is not continuously supplied to the first-stage reverse osmosis membrane device 41 of the low-concentration treatment unit 10, so the amount of treated liquid in the first-stage reverse osmosis membrane device 41 can be reduced compared to the separation system shown in Fig. 2. Also, the second and subsequent reverse osmosis membrane devices 42-44 re-treat at least their own concentrated water. As a result, in the separation system shown in Fig. 3, the amounts of treated liquid can be made as equal as possible among the multiple reverse osmosis membrane devices 41-44 provided in the low-concentration treatment unit 10.

[0026] In the separation system shown in Fig. 2 and Fig. 3, in the low concentration treatment section 10, the permeated water of the first-stage reverse osmosis membrane device 41 is supplied to the second-stage reverse osmosis membrane device 42 via the second-stage storage tank 22, and the permeated water of the second-stage reverse osmosis membrane device 42 is supplied to the second-stage reverse osmosis membrane device 43 via the third-stage storage tank 23, and so on. In this manner, a plurality of reverse osmosis membrane devices 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 devices each equipped with a reverse osmosis membrane, where i is an integer between 1 and n, and the permeated water of the i-th stage reverse osmosis membrane device is supplied to the i+1-th stage reverse osmosis membrane device via the i+1-th stage storage tank. In the separation system shown in Fig. 2 and Fig. 3, at least a portion of the permeated water of the reverse osmosis membrane device of each stage is supplied to one or more storage tanks from the first stage to the stage. However, in the separation system based on the present invention, in the low concentration treatment section 10, a reverse osmosis membrane device may be provided in a system separate from the total of n+1 reverse osmosis membrane devices connected in series, and at least a portion of the concentrated water from any one or more of the reverse osmosis membrane devices in the second stage or later of the total of n+1 reverse osmosis membrane devices connected in series may be treated in the reverse osmosis membrane device in the separate system. Figure 4 shows an example of a separation system in which concentrated water from any one or more reverse osmosis membrane devices in the second stage or later is treated in the reverse osmosis membrane device in the separate system.

[0027] In a separation system according to another embodiment of the present invention shown in FIG. 4, in the low-concentration treatment section 10, the first-stage reverse osmosis membrane device 41 to the n+1-stage reverse osmosis membrane device 49 are connected in series, and a reverse osmosis membrane device 82 is provided that is separated from the series connection, i.e., a separate system. Focusing on the n-th stage reverse osmosis membrane device 48 in this separation system, the n-th stage storage tank 28 is provided on the inlet side of the n-th stage reverse osmosis membrane device 48, and the liquid in the storage tank 28 is supplied to the n-th stage reverse osmosis membrane device 48 via the pump 38. The permeated water of the n-th stage reverse osmosis membrane device 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 device 49 via the pump 39. Meanwhile, the concentrated water of the n-th and n+1-stage reverse osmosis membrane devices 48, 49 is sent to a storage tank 80 provided at the inlet of the reverse osmosis membrane device 82 of the separate system. The liquid in this storage tank 80 is supplied to a reverse osmosis membrane device 82 via a pump 81, and concentrated water from the reverse osmosis membrane device 82 is returned to the storage tank 80. The permeated water from the reverse osmosis membrane device 82 may be discharged as wastewater, for example, together with the permeated water from the (n+1)-th stage reverse osmosis membrane device 49. A separate reverse osmosis membrane device to which the permeated water from the reverse osmosis membrane device 82 is supplied may be provided downstream of the reverse osmosis membrane device 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.

[0028] In the separation system shown in FIG. 4, in the reverse osmosis membrane devices 41, ..., 48, 49 connected in series in n+1 stages, concentrated water from the reverse osmosis membrane devices in the second stage to the n-1th stage may be supplied to the storage tank 21 of the first stage as in the separation system shown in FIG. 2, or may be circulated to the storage tank of the stage as in the separation system shown in FIG. 3. In addition, when the reverse osmosis membrane devices 41, ..., 48, 49 of the n+1 stages are connected in series, the reverse osmosis membrane devices 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 devices of 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 device of at least one stage is supplied to one or more storage tanks from the first stage to that stage. In other words, it is sufficient that at least a portion of the concentrated water from the reverse osmosis membrane device of the jth stage is supplied to at least one storage tank from the first stage to the jth stage for at least one integer j that satisfies 2≦j≦n+1.

[0029] Although the separation system based on the present invention has been described above, the organic solvent to be separated from water in the present invention 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 the above description, the threshold concentration B is set to 5 mass %, but an appropriate value is selected for the threshold concentration B depending on the type of organic solvent to be separated and the type of reverse osmosis membrane used in the reverse osmosis membrane device. The number of stages of reverse osmosis membrane devices connected in series in the low concentration treatment unit 10 is not limited to 4, and the number of stages is appropriately determined depending on the water quality requirements of the finally obtained water. In some cases, the low concentration treatment unit 10 may be configured to have only one reverse osmosis membrane unit. EXAMPLES

[0030] Next, the present invention will be described in more detail with reference to examples and comparative examples.

[0031] [Comparative Example] A separation system shown in FIG. 5 was assembled. This separation system is the one in which the reverse osmosis membrane device 40 for concentration and the concentrated liquid storage tank 20 in the preceding stage are removed from the separation system shown in FIG. 2, that is, the separation system is composed of the low concentration treatment section 10 in the separation system shown in FIG. 2. The liquid in the storage tank 21 provided in the preceding stage of the first reverse osmosis membrane device 41 was made to be intermittently discharged via a valve 55. As the reverse osmosis membrane 45 in each of the reverse osmosis membrane devices 41 to 44, a high-pressure reverse osmosis membrane SWC5-LD-4040 (manufactured by Hydranautics) made of polyamide was used, and the supply pressure of the liquid to each reverse osmosis membrane 45 was set to 6 MPa. As the stock solution to be supplied to the storage tank 21, a mixture of water and NMP with an NMP concentration of 1 mass% was used. As the first batch, 100 L of the stock solution was supplied to the storage tank 21, and the separation system was operated until the NMP concentration in the liquid in the storage tank 21 reached 20 mass%. Thereafter, the entire amount of liquid in the storage tank 21 was discharged as an NMP concentrate, and the permeate from the first batch was supplied to the storage tank 21 as a raw liquid, and the separation system was operated until the NMP concentration in the liquid in the storage tank 21 reached 20 mass%. This was the second batch. Similarly, the third batch was also carried out. The amount of raw liquid and its NMP concentration, the amount of concentrated liquid and its NMP concentration, the amount of permeate and its NMP concentration, and the theoretical concentration factor were determined for each batch. The theoretical concentration factor is a value indicating how much of a concentrated liquid of a desired concentration can theoretically be produced from a raw liquid of a given concentration and a given concentration. The results are shown in Table 1.

[0032] [Table 1]

[0033] As can be seen from Table 1, in the comparative example, even when batch concentration was performed three times, only 3.8 L of a concentrated solution with an NMP concentration of 20 mass % could be obtained.

[0034] [Example] The separation system shown in FIG. 2 was assembled. As the reverse osmosis membrane 45 in each of the reverse osmosis membrane devices 40-44, a high-pressure reverse osmosis membrane SWC5-LD-4040 (manufactured by Hydranautics) made of polyamide was used as in the comparative example, and the supply pressure of the liquid to each reverse osmosis membrane 45 was set to 6 MPa. As the raw liquid to be supplied to the storage tank 21, a mixed liquid of water and NMP with an NMP concentration of 1 mass% was used. 100 L of the raw liquid was supplied to the storage tank 21, and the low-concentration treatment unit 10 was operated until the NMP concentration of the liquid in the storage tank 21 became 5 mass%. Thereafter, the entire amount of the liquid in the storage tank 21 (i.e., 5% concentrated liquid) was transferred to the concentrated liquid storage tank 20 to make it the raw liquid, and the reverse osmosis membrane device 40 for concentration was operated until the NMP concentration of the liquid in the concentrated liquid storage tank 20 became 20 mass%. Then, the entire amount of the concentrated liquid with an NMP concentration of 20 mass% was extracted from the concentrated liquid storage tank 20. This is the first batch. The permeated water from the reverse osmosis membrane device 40 for concentration produced in the first batch was supplied to the storage tank 21 of the low-concentration treatment section 10.

[0035] Next, for the second batch, the permeated water supplied from the reverse osmosis membrane device 40 for concentration to the storage tank 21 in the first batch was used as the stock solution, and the low concentration treatment unit 10 was operated until the NMP concentration of the liquid in the storage tank 21 reached approximately 5%. Thereafter, the entire amount of the liquid in the storage tank 21 was transferred to the concentrated liquid storage tank 20 to be used as the stock solution, and the reverse osmosis membrane device 40 for concentration was operated until the NMP concentration of the liquid in the concentrated liquid storage tank 20 reached 20% by mass, and the concentrated liquid was extracted from the concentrated liquid storage tank 20. The permeated water from the reverse osmosis membrane device 40 for concentration generated in the second batch was also supplied to the storage tank 21 of the low concentration treatment unit 10 in the same manner. Furthermore, for the third batch, the permeate supplied from the concentration reverse osmosis membrane device 40 to the storage tank 21 in the second batch was used as the raw liquid, and the low concentration treatment unit 10 was operated until the NMP concentration of the liquid in the storage tank 21 reached approximately 5%, and then the entire amount of the liquid in the storage tank 21 was transferred to the concentrated liquid storage tank 20, and the concentration reverse osmosis membrane device 40 was operated until the NMP concentration of the liquid in the concentrated liquid storage tank 20 reached 20 mass%, and the concentrated liquid was extracted from the concentrated liquid storage tank 20.

[0036] Table 2 shows the amount of the stock solution treated in the first-stage reverse osmosis membrane device 41 of the low-concentration treatment section 10, the amount of the concentrated solution generated, the theoretical concentration factor, the amount of the permeated water generated, the NMP concentration in the stock solution, the NMP concentration in the concentrated solution, and the NMP concentration in the permeated water in each of the three batches described above. Similarly, Table 3 shows the amount of the stock solution treated in the reverse osmosis membrane device for concentration, the amount of the concentrated solution generated, the theoretical concentration factor, the amount of the permeated water generated, the NMP concentration in the stock solution, the NMP concentration in the concentrated solution, and the NMP concentration in the permeated water. As is clear from the above explanation, in each batch, the amount of the concentrated solution generated in the first-stage reverse osmosis membrane device 41 of the low-concentration treatment section 10 and its NMP concentration correspond to the amount of the stock solution treated in the reverse osmosis membrane device for concentration 40 and its NMP concentration, respectively. In addition, the amount of permeate generated in the concentration reverse osmosis membrane device 41 in a given batch and its NMP concentration correspond to the amount of raw liquid treated in the first stage reverse osmosis membrane device 41 of the low concentration treatment unit 10 in the next batch and its NMP concentration, respectively.

[0037] [Table 2]

[0038] [Table 3]

[0039] As can be seen from Table 3, a total of 4.7 L of concentrated solution having an NMP concentration of 20 mass % was obtained from the concentrated solution storage tank 20 in the upstream of the concentration reverse osmosis membrane device 40 by three batches. This is close to the theoretical concentrated amount of 5 L, and it was found that the separation system based on the present invention can separate water and NMP with high efficiency. [Explanation of symbols]

[0040] 10 Low concentration processing section 20 Concentrate storage tank 21~24,28,29,80 Storage tank 30~34,38,39,81 Pump 40~44,48,49,82 Reverse osmosis membrane equipment 45 Reverse osmosis membrane 51, 53, 62~64 Piping 52, 54, 55, 72-74, 83 Valves

Claims

1. 1. A method for separating water and a concentrated solution of an organic solvent from a mixture of the organic solvent and water, comprising the steps of: supplying the mixed liquid to a first-stage storage tank disposed at an inlet of a first-stage reverse osmosis membrane device having a reverse osmosis membrane; a first separation step of treating the liquid stored in the first-stage storage tank by the first-stage 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 first-stage reverse osmosis membrane device to the first-stage storage tank; transferring at least a portion of the liquid in the first-stage storage tank to a concentrated liquid 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; a second separation step of treating the liquid stored in the concentrate storage tank with a reverse osmosis membrane device for concentration having a reverse osmosis membrane to separate the liquid into permeate and concentrated water, circulating the concentrated water from the reverse osmosis membrane device for concentration to the concentrate storage tank, and supplying the permeate from the reverse osmosis membrane device for concentration to the first-stage storage tank; The separation method comprising the steps of:

2. The method according to claim 1 , wherein when the concentration of the organic solvent in the mixture is equal to or greater than the threshold concentration, the mixture is directly supplied to the concentrate storage tank.

3. 3. The separation method according to claim 1 or 2, wherein in the first separation step, a total of n+1 stages of reverse osmosis membrane devices are used, where i is an integer between 1 and n, inclusive, such that permeate from an i-th stage reverse osmosis membrane device is supplied to an i+1-stage reverse osmosis membrane device via an i+1-stage storage tank, to supply liquid in the first-stage reverse osmosis membrane device to the first-stage reverse osmosis membrane device, and for at least one integer j satisfying 2≦j≦n+1, at least a portion of concentrated water from a j-th stage reverse osmosis membrane device is supplied to at least one of the storage tanks from the first stage to the j-th stage.

4. 4. The method according to claim 3, further comprising the step of transferring at least a part of the liquid in a second or subsequent storage tank to a preceding storage tank when the organic solvent is concentrated to a predetermined concentration or more in the first separation step.

5. In the first separation step, a reverse osmosis membrane device of another system is used in addition to the first to n+1th stage reverse osmosis membrane devices, 3. The separation method according to claim 1 or 2, wherein at least a portion of the concentrated water from at least one of the second to n+1-th stage reverse osmosis membrane devices is supplied to the reverse osmosis membrane device of the other system via a storage tank provided at an inlet of the reverse osmosis membrane device of the other system.

6. 1. A separation system for separating water and a concentrated solution of an organic solvent from a mixture of the organic solvent and water, comprising: a low concentration treatment section including a first stage storage tank to which the mixed liquid is supplied; A concentrate storage tank; a reverse osmosis membrane device for concentration, the reverse osmosis membrane device being provided with a reverse osmosis membrane and receiving the liquid in the concentrated liquid storage tank; Equipped with The low-concentration treatment unit includes at least a first-stage reverse osmosis membrane device having a reverse osmosis membrane and receiving the liquid in the first-stage storage tank, the first-stage storage tank is disposed at an inlet of the first-stage reverse osmosis membrane device, and at least a portion of the concentrated water from the first-stage reverse osmosis membrane device is supplied to the first-stage storage tank, a separation system in which, when a 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 and supplied to the concentration reverse osmosis membrane device, concentrated water from the concentration reverse osmosis membrane device is circulated to the concentrated liquid storage tank, and permeated water from the concentration osmosis membrane device is supplied to the first-stage storage tank.

7. The separation system of claim 6 , wherein when the concentration of the organic solvent in the mixture is equal to or greater than the threshold concentration, the mixture is fed directly to the concentrate reservoir.

8. 8. The separation system according to claim 6 or 7, wherein i is an integer between 1 and n, and the low-concentration treatment section includes a total of n+1 stages of reverse osmosis membrane devices each equipped with a reverse osmosis membrane, and connected so that permeate from an i-th stage reverse osmosis membrane device is supplied to an i+1-stage reverse osmosis membrane device via an i+1-stage storage tank, and for at least one integer j satisfying 2≦j≦n+1, at least a portion of concentrated water from a j-th stage reverse osmosis membrane device is supplied to at least one of the storage tanks from the first stage to the j-th stage.

9. The low-concentration treatment unit further includes a separate reverse osmosis membrane device and a separate storage tank provided at an inlet of the separate reverse osmosis membrane device, 9. The separation system according to claim 8, wherein at least a portion of the concentrated water from at least one of the reverse osmosis membrane devices of the first stage to the (n+1)th stage is supplied to the reverse osmosis membrane device of the other system via the separate system storage tank.

10. 8. The separation system according to claim 6, wherein when the organic solvent is concentrated to a predetermined concentration or more in a second or subsequent storage tank, at least a portion of the liquid in the storage tank is transferred to a previous storage tank.

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