Forward osmosis concentration system

Abstract

To improve an object recovery rate while maintaining concentration efficiency even when the ion radius of the object is small in a forward osmosis concentration system.SOLUTION: This forward osmosis concentration system comprises a plurality of semipermeable membrane modules for concentrating an object having an ion radius of 600 pm or smaller. Each of the semipermeable membrane modules includes first and second chambers partitioned by a semipermeable membrane, In each of the semipermeable membrane modules, a draw solution flows into the second chamber to transit water contained in a target liquid in the first chamber through the semipermeable membrane to the draw solution in the second chamber, and the target liquid is concentrated. The first chambers of the respective semipermeable membrane modules are connected in series. In the last semipermeable membrane module connected to the most downstream side of the first chamber among the semipermeable membrane modules, a transmission coefficient of the semipermeable membrane is 25% or less of those of the semipermeable membranes of the semipermeable membrane modules other than the last module among the semipermeable membrane modules.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a forward osmosis concentration system. [Background technology]

[0002] The separation and concentration of liquid mixtures by membrane separation is an energy-saving method compared to conventional separation techniques such as distillation because it does not involve a phase change, and because it does not involve a change in the state of the substance, it is widely used in many fields, such as the food industry, where fruit juice is concentrated and beer yeast is separated, and the recovery of organic matter from industrial wastewater. Among membrane separation methods, the forward osmosis (FO) method is a more energy-saving method than reverse osmosis and other methods.

[0003] As a forward osmosis concentration system using such a forward osmosis method, a forward osmosis concentration system is known in which a plurality of semipermeable membrane modules are connected to each other by piping to increase the concentration rate, etc. For example, Patent Document 1 (JP 2014-100624 A), Patent Document 2 (JP 2020-199430 A), and Patent Document 3 (JP 2021-146297 A) disclose forward osmosis concentration systems equipped with a plurality of semipermeable membrane modules. Here, the semipermeable membranes used in each of the plurality of semipermeable membrane modules are basically the same. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-100624 A [Patent Document 2] JP 2020-199430 A [Patent Document 3] Patent Publication No. 2021-146297 Summary of the Invention [Problem to be solved by the invention]

[0005] When the ionic radius of the target substance in the target liquid is small, it easily permeates the semipermeable membrane, resulting in increased leakage into the draw solution and a decrease in the recovery rate of the target substance.

[0006] The present invention provides a forward osmosis concentration system for concentrating a target substance in a target liquid, which is excellent in both the concentration processing efficiency and the recovery rate of the target substance even when the ionic radius of the target substance in the target liquid is small. [Means for solving the problem]

[0007] (1) A forward osmosis concentration system having a plurality of semipermeable membrane modules for concentrating a target substance in a target liquid, The ionic radius of the target object in the target liquid is 600 pm or less; Each of the plurality of semipermeable membrane modules has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane; In each of the plurality of semipermeable membrane modules, the target liquid is caused to flow into the first chamber, and a draw solution having a higher osmotic pressure than the target liquid is caused to flow into the second chamber, so that water contained in the target liquid in the first chamber is transferred to the draw solution in the second chamber through the semipermeable membrane, and the target liquid is concentrated; The first chambers of each of the plurality of semipermeable membrane modules are connected in series, In a final module, which is a semipermeable membrane module connected to the most downstream side of the flow of the first chamber among the plurality of semipermeable membrane modules, the ratio of the solute permeability coefficient of the semipermeable membrane to the solute permeability coefficient of the semipermeable membrane of a semipermeable membrane module other than the final module among the plurality of semipermeable membrane modules is 25% or less. Forward osmosis concentration system. (2) The forward osmosis concentration system described in (1), wherein the draw solution may be supplied to pass through the second chambers of two or more of the plurality of semipermeable membrane modules in series, or may be supplied independently to the second chambers of each of the plurality of semipermeable membrane modules. (3) The forward osmosis concentration system according to (1) or (2), wherein the draw solution is seawater. (4) The forward osmosis concentration system according to any one of (1) to (3), wherein the semipermeable membrane is a hollow fiber membrane. (5) The forward osmosis concentration system according to any one of (1) to (4), wherein the semipermeable membrane contains cellulose triacetate. Effect of the Invention

[0008] According to the present invention, it is possible to provide a forward osmosis concentration system for concentrating a target substance in a target liquid, which is excellent in both the concentration processing efficiency and the recovery rate of the target substance even when the ionic radius of the target substance in the target liquid is small. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of a forward osmosis concentration system according to an embodiment. [Diagram 2] FIG. 1 is a schematic diagram showing a modified example of a forward osmosis concentration system according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The forward osmosis concentration system of this embodiment will be described below with reference to the drawings. In the drawings, the same reference numerals represent the same or corresponding parts. In addition, the dimensional relationships of the length, width, thickness, depth, etc. are appropriately changed for clarity and simplification of the drawings, and do not represent the actual dimensional relationships.

[0011] <Forward osmosis concentration system> Referring to FIG. 1, the forward osmosis concentration system of this embodiment includes a plurality of semipermeable membrane modules 1, 2, and 3 for concentrating a target substance in a target liquid.

[0012] Each of the plurality of semipermeable membrane modules 1, 2, 3 has a semipermeable membrane 10, 20, 30, and a first chamber 11, 21, 31 and a second chamber 12, 22, 32 separated by the semipermeable membrane.

[0013] In each of the multiple semipermeable membrane modules 1, 2, 3, the target liquid is flowed into the first chamber 11, 21, 31, and a draw solution having a higher osmotic pressure than the target liquid is flowed into the second chamber 12, 22, 32, so that the water contained in the target liquid in the first chamber 11, 21, 31 transfers to the draw solution in the second chamber 12, 22, 32 through the semipermeable membrane, the target liquid is concentrated, and the concentrated target liquid (concentrated liquid) is discharged from the forward osmosis concentration system.

[0014] At the same time, the draw solution in the second chambers 12, 22, 32 is diluted and the diluted draw solution (diluent) is discharged from the forward osmosis concentration system.

[0015] In this embodiment, the first chambers 11, 21, 31 of the multiple semipermeable membrane modules 1, 2, 3 are connected (communicated) in series.

[0016] That is, the target liquid is caused to flow by the pump 41 through the first chamber 11, the first chamber 21 and the first chamber 31 in this order.

[0017] The second chambers 12, 22, 32 of each of the multiple semipermeable membrane modules 1, 2, 3 may be connected (communicated) in series (see FIG. 1), may be connected in parallel (see FIG. 2), or may not be connected to each other (may be independent of each other).

[0018] When the second chambers 12, 22, 32 of each of the multiple semipermeable membrane modules 1, 2, 3 are connected in series (see FIG. 1), the draw solution may be supplied to pass in series through the second chambers 12, 22, 32 of two or more semipermeable membrane modules among the multiple semipermeable membrane modules 1, 2, 3.

[0019] The draw solution may flow through the first chamber 11, the first chamber 21, and the first chamber 31 in this order (the flow of the target liquid and the flow of the draw solution may be parallel), or may flow through the first chamber 31, the first chamber 21, and the first chamber 11 in this order (the flow of the target liquid and the flow of the draw solution may be counter-flowing). As shown in FIG. 1, the draw solution is preferably caused to flow through the first chamber 31, the first chamber 21, and the first chamber 11 in this order by a pump 42.

[0020] In addition, when the second chambers 12, 22, 32 of each of the multiple semipermeable membrane modules 1, 2, 3 are connected in parallel (see FIG. 2) or are not connected to each other, the draw solution may be supplied independently to the second chambers 12, 22, 32 of each of the multiple semipermeable membrane modules 1, 2, 3.

[0021] In addition, in each semipermeable membrane module 1, 2, 3, the flow directions of the liquids on either side of the semipermeable membranes 10, 20, 30 (the target liquid in the first chambers 11, 21, 31 and the draw solution in the second chambers 12, 22, 32) may be in any direction, and may be opposite directions (counterflow method) or parallel directions (parallel flow method).

[0022] (Target liquid) The target liquid is a liquid (such as an aqueous solution) that contains a target substance.

[0023] The ionic radius of the target object in the target liquid is 600 pm or less.

[0024] In addition, the "ionic radius of the target object in the target liquid" refers to the ionic radius when the target object in the target liquid (such as an aqueous solution) is an ion, and refers to the radius of the hydrate ion when the target object is a hydrate ion. In addition, when the target object exists in the target liquid dissociated into multiple ions (e.g., anions and cations) or multiple hydrate ions, the sum of the ionic radii (or hydrate ion radii) of each ion (or hydrate ion) is regarded as the "ionic radius of the target object."

[0025] When the ionic radius of the target substance in the target liquid is within this range, the amount of the target substance leaking into the draw solution increases (resulting in a lower recovery rate of the target substance) (see the recovery rate of Comparative Example 1 described below), so the forward osmosis concentration system of this embodiment is particularly useful.

[0026] The ionic radius of the target substance in the target liquid is preferably 50 to 600 pm, more preferably 130 to 600 pm. If the ionic radius of the target substance in the target liquid is too small, recovery by the semipermeable membrane module may become difficult.

[0027] The target substance is not particularly limited, but examples thereof include ammonium ions, nitrate ions, nitrite ions, urea, ammonium chloride, and other nitrogen compounds.

[0028] In addition, the ammonium ion (NH4 + ) has an ionic radius of 246 pm. The ionic radius of ammonium chloride (NH4Cl) in the target liquid is 564 pm. For reference, the ionic radius of NaCl hydrate (the sum of the hydrated ionic radii of the Na hydrate ion and the Cl hydrate ion) is 621 pm.

[0029] The target liquid is not particularly limited, but examples thereof include wastewater containing nitrogen compounds.

[0030] The flow rate of the target liquid supplied to the forward osmosis concentration system (first chamber 11 of semipermeable membrane module 1 in Figures 1 and 2) is preferably 0.01 to 50 L / min, more preferably 0.05 to 25 L / min, and even more preferably 0.1 to 15.0 L / min.

[0031] The concentration of the target substance in the target liquid is preferably 10 to 10,000 mg / L, more preferably 100 to 5,000 mg / L, and further preferably 200 to 3,000 mg / L.

[0032] (Drawing solution) The draw solution is not particularly limited as long as it has a higher osmotic pressure than the target liquid, and various known draw solutions used in forward osmosis treatment can be used.

[0033] As the draw solution, it is preferable to use, for example, seawater, which is easily available.

[0034] The flow rate of the draw solution supplied to the forward osmosis concentration system (the second chamber 32 of the semipermeable membrane module 3 in Figures 1 and 2) is preferably 0.01 to 40.0 L / min, more preferably 0.05 to 20 L / min, and even more preferably 0.1 to 10.0 L / min.

[0035] The concentration of the draw solution is preferably 1000 to 200000 mg / L, more preferably 5000 to 100000 mg / L, and further preferably 10000 to 50000 mg / L.

[0036] (semi-permeable membrane) In the forward osmosis concentration system of the present embodiment, in the final module 3, which is a semipermeable membrane module connected to the most downstream side in the flow of the first chambers 11, 21, 31 among the multiple semipermeable membrane modules 1, 2, 3, the ratio of the solute permeability coefficient of the semipermeable membrane 30 is 25% or less to the solute permeability coefficients of the semipermeable membranes 10, 20 of the semipermeable membrane modules 1, 2 (the first module 1 and the second module 2) other than the final module 3 among the multiple semipermeable membrane modules 1, 2, 3. The said ratio of the semipermeable membrane 30 of the final module 3 is preferably 5 to 30%, more preferably 10 to 25%, and even more preferably more than 10% and 25% or less.

[0037] In addition, when there are multiple semipermeable membrane modules other than the final module 3 (when the total number of semipermeable membrane modules is three or more), the solute permeability coefficients of the semipermeable membranes of each of the semipermeable membrane modules other than the final module 3 may be the same or different.

[0038] In the latter case, it is preferable that the ratio of the solute permeability coefficient of the semipermeable membrane 30 of the final module 3 to the solute permeability coefficient of each of the semipermeable membranes of all semipermeable membrane modules other than the final module 3 is all 25% or less (preferably 1 to 25%, more preferably 5 to 20%).

[0039] In the latter case, the difference (deviation) between the solute permeability coefficients of the semipermeable membranes of all semipermeable membrane modules other than the final module 3 and their average values ​​is preferably 20% or less, and more preferably 10% or less.

[0040] The solute permeability coefficient of a semipermeable membrane is the salt transfer coefficient (B value) [units: m / h or m] obtained by dividing the amount of salt passing through a semipermeable membrane such as an RO membrane or FO membrane per unit time by the membrane area and the intermembrane concentration difference. 3 / (m 2 ·h)] and is expressed by the following formula: B=Js / ΔC(=DSM / l) Js: Solute flux ΔC: Concentration difference on both sides of the membrane DSM: Solute diffusion coefficient in membrane l: effective membrane thickness The Js (solute permeation flux) is a value obtained when operating according to the method described in 3-3.1-(1) of JIS K3805_1990 "Performance test method for reverse osmosis elements and modules."

[0041] The solute permeability coefficient of a semipermeable membrane can be calculated from the measured value by performing concentration by reverse osmosis as described in the above-mentioned JIS K3805_1990 using a semipermeable membrane module equipped with the semipermeable membrane, and measuring the amount of solute that has permeated the semipermeable membrane.

[0042] By using a semipermeable membrane with a small solute permeability coefficient (i.e., the target substance does not easily permeate), the leakage of the target substance into the draw solution is reduced. However, the amount of permeated water is reduced, which reduces the concentration efficiency.

[0043] Therefore, if semipermeable membranes with a small solute permeability coefficient are used in all of the multiple semipermeable membrane modules, the concentration efficiency of the forward osmosis concentration system will decrease, and the target liquid will not be able to be concentrated until the concentration of the target substance reaches the desired concentration.

[0044] In this embodiment, a semipermeable membrane with a small solute permeability coefficient (i.e., through which the target substance does not easily permeate) is used as the semipermeable membrane 30 of the final module 3, thereby suppressing leakage of the target substance into the draw solution in the final module 3 (permeation from the first chamber 31 to the second chamber 32), while not reducing the concentration processing efficiency in modules other than the final module. Therefore, even when the target substance is small in the target liquid (for example, when the ionic radius of the target substance in the target liquid is 600 pm or less), the concentration processing efficiency can be maintained while improving the recovery rate of the target substance.

[0045] The semipermeable membranes 10, 20, and 30 are not particularly limited, and various known semipermeable membranes that can be used for forward osmosis can be used.

[0046] The material constituting the semipermeable membrane is not particularly limited, but examples thereof include cellulose-based resins, polysulfone-based resins, polyamide-based resins, etc. The semipermeable membrane is preferably composed of a material containing at least one of a cellulose-based resin and a polysulfone-based resin.

[0047] The cellulose-based resin is preferably a cellulose acetate-based resin. The cellulose acetate-based resin is resistant to chlorine, which is a bactericide, and can suppress the growth of microorganisms. The cellulose acetate-based resin is preferably cellulose acetate, and from the viewpoint of durability, more preferably cellulose triacetate.

[0048] The polysulfone-based resin is preferably a polyethersulfone-based resin.The polyethersulfone-based resin is preferably a sulfonated polyethersulfone.

[0049] The shape of the semipermeable membrane is not particularly limited, but may be, for example, a flat membrane, a spiral membrane, or a hollow fiber membrane. In the drawings, the semipermeable membranes 10, 20, and 30 are simplified flat membranes, but the shape is not limited thereto.

[0050] The semipermeable membrane is preferably a hollow fiber membrane.

[0051] When the semipermeable membrane is a hollow fiber membrane, the water permeability per unit membrane area is not large compared to a spiral membrane (flat membrane), but the membrane area per volume of the semipermeable membrane module can be made large. This allows the water permeability of the entire module to be increased. In other words, it has the advantage of very high volumetric efficiency and excellent compactness.

[0052] In addition, when both high-concentration and low-concentration solutions are supplied to the module and brought into contact with each other through the semipermeable membrane, the module is less likely to experience drift in the solution, minimizing concentration polarization on the membrane surface. This allows for efficient concentration processing by forward osmosis.

[0053] When the semipermeable membrane is a hollow fiber membrane, it is preferable that the draw solution flows inside the hollow fiber membrane and the target liquid flows outside the hollow fiber membrane, i.e., the outside of the hollow fiber membrane is the first chamber and the inside of the hollow fiber membrane is the second chamber.

[0054] The form of the semipermeable membrane modules 1, 2, and 3 is not particularly limited, but in the case where hollow fiber membranes are used, examples include a module in which multiple hollow fiber membranes are arranged in a straight line, a cross-wound type module in which multiple hollow fiber membranes are wound around a core tube, etc. In the case where flat membranes are used, examples include a stacked type module in which multiple flat membranes are stacked, and a spiral type module in which multiple flat membranes are wrapped around a core tube in an envelope shape.

[0055] The outer diameter of the hollow fiber membrane is not particularly limited as long as it is used for permeation treatment, etc., but is, for example, 150 to 250 μm. If the outer diameter is smaller than the above range, the inner diameter will inevitably be small, which may cause problems such as increased flow pressure loss of the fluid flowing through the hollow part of the hollow fiber membrane. On the other hand, if the outer diameter is larger than the above range, it will be impossible to increase the membrane area per unit volume in the module, and the compactness, which is one of the advantages of the hollow fiber membrane module, will be lost.

[0056] The hollow fiber membrane has a hollow ratio that is not particularly limited as long as it is suitable for use in permeation treatment, and is, for example, 15 to 35%. If the hollow ratio is smaller than the above range, the flow pressure loss in the hollow portion increases, and the desired amount of permeated water may not be obtained. If the hollow ratio is larger than the above range, sufficient pressure resistance may not be ensured during permeation treatment. The hollow ratio (%) is calculated using the following formula: Hollowness ratio (%) = (inner diameter / outer diameter) 2 ×100 It can be calculated by: EXAMPLES

[0057] <Forward osmosis concentration test> For the forward osmosis concentration system as shown in FIG. 2, a simulation test was conducted to perform forward osmosis concentration by changing the ratio of the solute permeability coefficients of each semipermeable membrane module (first module 1, second module 2, and final module 3) as shown in Table 1.

[0058] Here, the subject liquid is an aqueous solution containing 0.1 mass % of a nitrogen compound (ammonium ion) as a subject.

[0059] The draw solution is an aqueous solution containing 3.5% by mass of salt (similar to the concentration in seawater).

[0060] The flow rates and concentrations of the target and draw solutions fed into the forward osmosis concentration system are shown in Figure 2 .

[0061] In addition, the "solute permeability coefficient ratio of each semipermeable membrane module" in the table is the ratio to the solute permeability coefficient ratio described in Comparative Example 1. In this test, the temperatures of the draw solution and the target liquid were both 25°C. The semipermeable membrane module was a hollow fiber membrane module (5-inch module), and the outer diameter of the hollow fiber membrane was 200 μm.

[0062] The recovery rate of the target substance (ammonium ion) was calculated using the following formula. Recovery rate (%) of the target substance=[amount of the target substance in the concentrated liquid / amount of the target substance in the target liquid supplied to the first chamber 11 of the first module 1]×100 In the above formula, "concentrate" means the target liquid discharged from the first chamber 31 of the final module 3 (see FIG. 1).

[0063] In addition, the "amount of the target substance in the concentrated liquid" is calculated by the following formula. "Amount of target substance in concentrated solution" = "Flow rate of concentrated solution" x "Concentration of target substance in concentrated solution" The "concentration of the target substance in the concentrated liquid" was measured using an automatic total nitrogen measuring device, a conductivity meter, etc.

[0064] Moreover, "the amount of the target substance in the target liquid supplied to the first chamber 11 of the first module 1" is calculated by the following formula. "Amount of the target substance in the target liquid supplied to the first chamber 11 of the first module 1" = "Flow rate of the target liquid supplied to the first chamber 11 of the first module 1" x "Concentration of the target substance in the target liquid" The "concentration of the target substance in the target liquid" was measured using an automatic total nitrogen measuring device, a conductivity meter, or the like.

[0065] The results of "recovery rate of the target substance" and "concentration of the target substance in the concentrated solution" are shown in Table 1.

[0066] [Table 1]

[0067] From the results shown in Table 1, it can be seen that the forward osmosis concentration systems of Examples 1 and 2, in which the ratio of the solute permeability coefficient of the semipermeable membrane 30 of the final module 3 (the ratio to the solute permeability coefficient of the semipermeable membranes 10, 20 of the semipermeable membrane modules 1, 2 other than the final module 3) is 25% or less, are excellent in both concentration processing efficiency and recovery rate of the target substance even when the ionic radius of the target substance in the target liquid is small.

[0068] In addition, in the forward osmosis concentration system of Comparative Example 1 (a conventional forward osmosis concentration system) in which the ratio of the solute permeability coefficients of all semipermeable membrane modules is 1, it can be seen that the recovery rate of the target substance is significantly reduced when the target substance has a small ionic radius, such as ammonium ion.

[0069] In contrast, in the forward osmosis concentration systems of Examples 1 and 2, it is clear that even when the target object is small, the recovery rate of the target object can be significantly improved without significantly decreasing the concentration processing efficiency.

[0070] In addition, in the forward osmosis concentration system of Comparative Example 4, in which the ratio of the solute permeability coefficient of the semipermeable membrane of the final module is 0.81 (ratio is 81%), even when the target object is small, the concentration processing efficiency is hardly decreased, but the recovery rate of the target object cannot be improved much. For this reason, it is considered that it is desirable for the ratio of the solute permeability coefficient of the semipermeable membrane of the final module to be smaller. [Explanation of symbols]

[0071] 1 semipermeable membrane module (first module), 10 semipermeable membrane, 11 first chamber, 12 second chamber, 2 semipermeable membrane module (second module), 20 semipermeable membrane, 21 first chamber, 22 second chamber, 3 semipermeable membrane module (final module), 30 semipermeable membrane, 31 first chamber, 32 second chamber, 41, 42 pumps.

Claims

1. A forward osmosis concentration system for concentrating a target substance in a target liquid, the system comprising a plurality of semipermeable membrane modules, The ionic radius of the target object in the target liquid is 600 pm or less; Each of the plurality of semipermeable membrane modules has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane; In each of the plurality of semipermeable membrane modules, the target liquid is caused to flow into the first chamber, and a draw solution having a higher osmotic pressure than the target liquid is caused to flow into the second chamber, whereby water contained in the target liquid in the first chamber is transferred through the semipermeable membrane to the draw solution in the second chamber, and the target liquid is concentrated; The first chambers of each of the plurality of semipermeable membrane modules are connected in series, In a final module, which is a semipermeable membrane module connected to the most downstream side of the flow of the first chamber among the plurality of semipermeable membrane modules, the ratio of the solute permeability coefficient of the semipermeable membrane to the solute permeability coefficient of the semipermeable membrane of a semipermeable membrane module other than the final module among the plurality of semipermeable membrane modules is 25% or less. Forward osmosis concentration system.

2. 2. The forward osmosis concentration system of claim 1, wherein the draw solution may be supplied to pass through the second chambers of two or more of the plurality of semipermeable membrane modules in series, or may be supplied independently to the second chambers of each of the plurality of semipermeable membrane modules.

3. 10. The forward osmosis concentration system of claim 1, wherein the draw solution is seawater.

4. 2. The forward osmosis concentration system of claim 1, wherein the semipermeable membrane is a hollow fiber membrane.

5. 10. The forward osmosis concentration system of claim 1, wherein the semi-permeable membrane comprises cellulose triacetate.

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