Hollow fiber membrane

Abstract

To provide a hollow fiber membrane excellent in both of water permeability and a maintenance factor of the water permeability.SOLUTION: A hollow fiber membrane includes cellulose ester and cellulose type nanofibers.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a hollow fiber membrane.

Background Art

[0002] Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2012-81533), Patent Document 2 (International Publication No. 2016 / 136294), and Patent Document 3 (Japanese Patent Application Laid-Open No. 2018-506161) disclose hollow fiber membranes containing a cellulose-based resin other than cellulose acetate and cellulose-based nanofibers.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a hollow fiber membrane is used in a membrane separation process such as the reverse osmosis (RO) method, there is a problem that the water permeation rate of the hollow fiber membrane decreases over time. When the water permeation rate of the hollow fiber membrane decreases over time, problems such as an increase in the operating energy required for the membrane separation process over time occur.

[0005] By adding cellulose-based nanofibers to the polymer material constituting the hollow fiber membrane, generally, the strength of the hollow fiber membrane is improved, physical changes over time are suppressed, and an effect of suppressing a decrease in the water permeation rate over time is expected.

[0006] An object of the present invention is to provide a hollow fiber membrane excellent in both water permeation performance and the maintenance rate of the water permeation performance.

Means for Solving the Problems

[0007] (1) A hollow fiber membrane containing a cellulose ester and cellulose-based nanofibers. (2) The hollow fiber membrane according to (1), wherein the ratio of the amount of the cellulose-based nanofibers to the total amount of the cellulose ester and the cellulose-based nanofibers is 0.01 to 10% by mass. (3) The hollow fiber membrane according to (1) or (2), wherein the fiber width (fiber diameter) of the cellulose-based nanofibers is 1 to 200 nm. (4) A method for producing a hollow fiber membrane containing a cellulose ester and cellulose-based nanofibers, comprising: a spinning step of discharging a spinning dope from a nozzle through an air traveling section into a coagulation liquid and pulling out a coagulum of the spinning dope from the coagulation liquid to obtain a hollow fiber membrane which is a semipermeable membrane of a hollow fiber type; the spinning dope contains a cellulose ester, cellulose-based nanofibers, a solvent and a non-solvent; a production method in which the spinning dope is kneaded before the spinning step. (5) Whether the spinning dope obtained by mixing the cellulose ester, the powder of the cellulose-based nanofibers, the solvent and the non-solvent is kneaded, whether the spinning dope obtained by mixing a slurry obtained by dispersing the powder of the cellulose-based nanofibers in the solvent with the cellulose ester and the non-solvent is kneaded, or the production method according to (4), wherein the spinning dope obtained by mixing a slurry obtained by dispersing the powder of the cellulose-based nanofibers in the non-solvent with the cellulose ester and the solvent is kneaded. (6) The production method according to (4) or (5), wherein in the spinning dope, the ratio of the amount of the cellulose-based nanofibers to the total amount of the cellulose ester and the cellulose-based nanofibers is 0.01 to 10% by mass. (7) The production method according to any one of (4) to (6), wherein the concentration of the cellulose ester in the spinning dope is 20 to 60% by mass. (8) The temperature when kneading the spinning dope is 150 to 200 °C, and the production method according to any one of (4) to (7). (9) The shear rate when kneading the spinning dope is 500 to 3500 sec -1 and the production method according to any one of (4) to (8). (10) In the spinning dope, the fiber width (fiber diameter) of the cellulose nanofiber is 1 to 200 nm, and the production method according to any one of (4) to (9). (11) A hollow fiber membrane containing a cellulose ester and a cellulose nanofiber, produced by the production method according to any one of (4) to (10).

Advantages of the Invention

[0008] According to the present invention, it is possible to provide a hollow fiber membrane excellent in both water permeability and the maintenance rate of the water permeability.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

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Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0010] Embodiments of the present invention will be described with reference to the drawings. Also, dimensional relationships such as length, width, thickness, depth, etc. have been appropriately changed for clarity and simplification of the drawings and do not represent actual dimensional relationships.

[0011] <Hollow fiber membrane> The hollow fiber membrane of this embodiment is a hollow fiber type semipermeable membrane. The hollow fiber membrane contains a cellulose ester and cellulose-based nanofibers.

[0012] (Cellulose ester) Examples of the cellulose ester include cellulose acetate (triacetate cellulose, monoacetate cellulose, diacetate cellulose, cellulose acetate butyrate, cellulose acetate propionate, etc.), cellulose phthalate, cellulose succinate, and the like. The cellulose ester is preferably cellulose acetate. Cellulose acetate has the characteristics of being resistant to chlorine, which is a bactericide, and can suppress the growth of microorganisms. From the viewpoint of durability, cellulose acetate is preferably triacetate cellulose.

[0013] (Cellulose-based nanofibers) Cellulose-based nanofibers (CNF) can be obtained, for example, by subdividing (defibrating) natural cellulose derived from wood, plants, etc. into a size on the nanometer order. As a method for performing defibrillation, for example, at least one of a mechanical pulverization method and a chemical subdivision method can be used. Examples of the chemical subdivision method include TEPMO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation, phosphoric acid esterification, carboxymethylation, sulfonation, xanthation, enzyme treatment, and the like. As commercially available products of CNF, products such as CNF powder and gels in which CNF is dispersed in water or an organic solvent can be purchased.

[0014] Cellulose-based nanofibers (hereinafter sometimes abbreviated as "CNF") contain a cellulose-based resin (cellulose or a cellulose derivative). Examples of the cellulose derivative include cellulose ester, cellulose ether, and a mixture thereof. Examples of the cellulose ester include cellulose acetate (triacetate cellulose, monoacetate cellulose, and diacetate cellulose), cellulose phthalate, and cellulose succinate. Examples of the cellulose ether include methyl cellulose, ethyl cellulose, and hydroxypropyl methyl cellulose.

[0015] The ratio of the amount of CNF to the total amount of cellulose ester and CNF (CNF ratio) is preferably 0.01 to 10% by mass, more preferably 0.01 to 5% by mass, still more preferably 0.01 to 3% by mass, and most preferably 0.01 to 1% by mass. The CNF content is calculated by the following formula. CNF ratio (% by mass) = [amount of CNF] / ([amount of cellulose ester] + [amount of CNF]) × 100 If the CNF ratio is too low, the strength of the hollow fiber membrane will be low. On the other hand, if the CNF ratio is too high, the fractionation property (salt rejection rate) of the hollow fiber membrane will be low.

[0016] The fiber width (fiber diameter) of CNF is preferably 1 to 200 nm, more preferably 1 to 50 nm. The fiber width of CNF can be measured by SEM (scanning electron microscope), TEM (transmission electron microscope), SPM (scanning probe electron microscope), E-SEM (environmental control type scanning electron microscope), cryo-SEM, etc.

[0017] The fiber length of CNF is preferably 100 μm or less, more preferably 20 nm to 10 μm. The fiber length of CNF can be measured by SEM (scanning electron microscope), TEM (transmission electron microscope), SPM (scanning probe electron microscope), E-SEM (environmental control type scanning electron microscope), cryo-SEM, etc.

[0018] (Shape of the hollow fiber membrane, etc.) The inner diameter of the hollow fiber membrane is preferably 30 μm or more and 300 μm or less, more preferably 35 μm or more and 260 μm or less. The thickness of the hollow fiber membrane (the entire membrane) is preferably 20 to 200 μm, more preferably 30 to 150 μm. Note that the membrane thickness can be calculated as (outer diameter - inner diameter) / 2. The hollow ratio of the hollow fiber membrane is preferably 10 to 65%, more preferably 12 to 55%. Note that the hollow ratio is the ratio of the area of the hollow part in the cross-section of the hollow fiber membrane, and is expressed as "hollow part cross-sectional area / (membrane part cross-sectional area + hollow part cross-sectional area) × 100 (%)". The average pore diameter of the hollow fiber membrane (the average pore diameter of the fine pores of the entire membrane) is preferably 2 nm or less. As a method for measuring the average pore diameter, for example, the differential scanning calorimetry (DSC) method can be mentioned.

[0019] The hollow fiber membrane of the present embodiment exhibits an effect of suppressing a decrease in the water permeation amount of the hollow fiber membrane over time when used in membrane separation processes such as the reverse osmosis (RO) method and the brine concentration (BC) method, particularly when the hollow fiber membrane is exposed to high pressure. As a result, an increase in the operating energy required for the membrane separation process using the hollow fiber membrane over time is suppressed.

[0020] Note that the BC method is, for example, a membrane separation method in which a part of the target solution is passed through one first chamber of a hollow fiber membrane module and another part of the target solution is passed through the other second chamber, and the target solution in the first chamber is pressurized, so that the solvent (such as water) contained in the target solution in the first chamber migrates through the hollow fiber membrane into the second chamber, the target solution in the first chamber is concentrated, and the target solution in the second chamber is diluted, as described in, for example, Japanese Patent Application Laid-Open No. 2018-65114.

[0021] In addition, since the treatments using the RO method and the BC method are often used as part of a system combined with other treatments, if the water permeation amount of the hollow fiber membrane decreases over time, it may become difficult to control the entire system. Therefore, by using the hollow fiber membrane of the present embodiment having a high water permeation amount retention rate in a system in which the RO method or the BC method is combined with other treatments, it is possible to particularly facilitate the control of the entire system.

[0022] Further, the hollow fiber membrane of the present embodiment can suppress a decrease in the amount of water permeated through the hollow fiber membrane over time, particularly when pressurized from the outside of the hollow fiber membrane (that is, when the pressure of the liquid outside the hollow fiber membrane is higher than that inside the hollow fiber membrane).

[0023] <Method for manufacturing hollow fiber membrane> The present invention also relates to a method for manufacturing a hollow fiber membrane for obtaining a hollow fiber membrane containing the above cellulose ester and cellulose-based nanofibers. The method for manufacturing the hollow fiber membrane of the present embodiment includes at least the spinning process described later.

[0024] Note that the spinning method used in the method for manufacturing the hollow fiber membrane of the present embodiment is a method called "solution spinning", which is different from the (melt spinning) as disclosed in Patent Document 2 (International Publication No. 2016 / 136294).

[0025] 〔Spinning process〕 Referring to FIG. 1, in the spinning process, the spinning dope 10 is discharged from the nozzle 11 through the air traveling section into the coagulating liquid 21, and the solidified product of the spinning dope is drawn out from the coagulating liquid, whereby a hollow fiber membrane, which is a semipermeable membrane of a hollow fiber type, is obtained. The drawing out of the hollow fiber membrane and the like are performed by, for example, rollers 12, 13, 14, and 15. Note that the drawing out speed is the surface speed of the roller 13.

[0026] (Spinning dope) The spinning dope (spinning dope) contains a raw material for the hollow fiber membrane (a material containing the cellulose ester and cellulose-based nanofibers constituting the above hollow fiber membrane), a solvent, and a non-solvent. The solvent is a liquid capable of dissolving the cellulose ester, and the non-solvent is a liquid (excluding water) that does not dissolve the cellulose ester. Note that the spinning dope may further contain water in addition to the solvent and the non-solvent.

[0027] The concentration of the cellulose ester in the spinning dope is preferably 20 to 60% by mass, more preferably 30 to 50% by mass. If the concentration of the cellulose ester is too low, the fractionation property and the strength of the membrane will be low. On the other hand, if the concentration of the cellulose ester is too high, the water permeability will be low. Further, if the concentration of the cellulose ester is too high, the viscosity of the spinning dope will become too high, and it may be difficult to carry out the spinning.

[0028] The ratio (CNF ratio) of the amount of CNF to the total amount of the cellulose ester and CNF is preferably 0.01 to 10% by mass, more preferably 0.01 to 5% by mass, still more preferably 0.01 to 3% by mass, and most preferably 0.01 to 1% by mass. The CNF content is calculated by the following formula. CNF ratio (% by mass) = [Charge amount of CNF] / ([Charge amount of cellulose ester] + [Charge amount of CNF]) × 100 If the CNF ratio is too low, the strength of the hollow fiber membrane will be low. On the other hand, if the CNF ratio is too high, the fractionation property of the hollow fiber membrane will be low. Further, if the CNF ratio is too high, the viscosity of the spinning dope will become too high, and it may be difficult to carry out the spinning.

[0029] The mass ratio (S / NS ratio) of the solvent (S) / non-solvent (NS) in the spinning dope is preferably 40 / 60 to 70 / 30. If the mass ratio of the solvent / non-solvent in the spinning dope becomes too small (the ratio of NS is too large), the homogeneity of the structure of the membrane cross-section will increase, but the spinning stability may decrease. Therefore, the S / NS ratio is more preferably 50 / 50 to 70 / 30.

[0030] (Kneading) In the present embodiment, the spinning dope is kneaded before the spinning process. "Kneading" means that after mixing the materials of the spinning dope, a specific range of shear force as described later is further applied, and CNF is forcibly dispersed in the spinning dope under high temperature and / or pressure conditions. By carrying out such kneading, the dispersibility (dispersion uniformity) of CNF in the spinning dope is improved, the occurrence of defects in the hollow fiber membrane due to aggregation of CNF and the like is suppressed, the strength of the hollow fiber membrane is improved, and an improvement in the maintenance rate of the water permeation performance is expected.

[0031] The kneading process is carried out using, for example, a planetary mixer, an extruder, a kneader (such as a pressure kneader, a two-arm kneader, etc.). When kneading the spinning dope, the temperature is preferably 150 to 200 °C, more preferably 160 to 190 °C, considering the solubility of the cellulose ester and heat-induced denaturation. The shear rate during kneading is preferably 500 to 3500 sec -1 considering the improvement of the kneading degree and heat deterioration due to shear, and more preferably 500 to 2000 sec -1 is.

[0032] The order of addition and mixing method of the polymer containing the cellulose ester as the constituent material of the hollow fiber membrane, the cellulose-based nanofiber (CNF), the solvent, and the non-solvent are not particularly limited. For example, CNF may be added to the kneaded product of the polymer, the solvent, and the non-solvent, or a slurry obtained by dispersing CNF in the solvent or the non-solvent may be added to the kneaded product containing the polymer.

[0033] It is also preferable to remove CNF with a large fiber length or large clusters from the spinning dope by filtering after the final kneading of the spinning dope. Thereby, the dispersibility (dispersion uniformity) of CNF in the spinning dope is further improved, the occurrence of defects in the hollow fiber membrane due to aggregation of CNF, etc. is suppressed, the strength of the hollow fiber membrane is further improved, and a further improvement in the maintenance rate of the water permeation performance is expected.

[0034] (Coagulating liquid) The coagulating liquid preferably contains a solvent and a non-solvent (excluding water). In this case, the coagulating liquid may further contain water in addition to the solvent and the non-solvent. The ratio of the total amount of the solvent and the non-solvent in the coagulating liquid (hereinafter, may be referred to as "the concentration of the coagulating liquid") is preferably 30 to 70% by mass, more preferably 33 to 50% by mass.

[0035] Also, the temperature of the coagulating liquid is preferably 10 to 30 °C. In this case, the structural homogeneity in the membrane thickness direction of the hollow fiber membrane can be enhanced.

Example

[0036] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

[0037] [Example 1] The hollow fiber membrane of Example 1 was produced under the following conditions by the method for producing a hollow fiber membrane described in the embodiment.

[0038] (Composition of spinning dope) Cellulose ester: cellulose triacetate (CTA) (LT35, manufactured by Daicel Corporation) Cellulose ester concentration: 41.2 mass% (in the spinning dope) CNF: commercially available CNF powder (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., I-2SX) CNF ratio (ratio of the amount of CNF to the total amount of CTA and CNF): 1.0 mass% Solvent: N-methylpyrrolidone (NMP) Non-solvent: ethylene glycol (EG) [Solvent / non-solvent (S / NS) ratio = 55 / 45] Benzoic acid [0.3 mass%]

[0039] (Preparation of spinning dope) The above CNF powder was mixed with other materials, and the mixed materials (spinning dope) were kneaded to prepare a spinning dope for use in the spinning process. The kneading was carried out under the conditions of temperature: 185 °C, shear rate: 1500 - 1700 [1 / sec], and the kneading residence time of the dope was adjusted to 30 minutes.

[0040] (Conditions of spinning process) Dissolution temperature of spinning dope: 185 °C Extrusion temperature of spinning dope: 158 °C Nozzle for extrusion: three-split nozzle (cross-sectional area of nozzle: 0.05 mm 2 ) [The cross-sectional area of the nozzle is the cross-sectional area of the spinning dope discharge hole at the tip of the nozzle.] Residence time of the air traveling section (AG): 0.06 seconds Temperature of the coagulating liquid: 18 °C Drawing speed: 40 m / min

[0041] (Composition of the coagulating liquid) Solvent (S): NMP Non-solvent (NS): EG Water Concentration of the coagulating liquid [(mass of S + mass of NS) / mass of the coagulating liquid]: 45% The S / NS ratio is the same as that of the spinning dope.

[0042] 〔Conditions of the post-treatment process〕 Conditions of the hot water treatment Temperature 98 °C Time 20 minutes Conditions of the salt pickling (salt annealing) treatment Concentration of the brine 4.5 mass% Temperature of the brine 82 °C Time 20 minutes

[0043] 〔Examples 2 to 11〕 As shown in Table 1, the CNF addition concentration, cellulose ester type, S / NS ratio of the raw material solution, kneading temperature, shear rate during kneading, and coagulation bath temperature were changed. In Example 7, the ratio of LT35 / LT75 was set to 88 / 12. Except for this, the hollow fiber membranes of Examples 2 to 11 were produced in the same manner as in Example 1. (Note that in Table 1, the left arrow means the same as the left column.)

[0044] 〔Comparative Example 1〕 In the spinning dope, CNF was not added. Except for this point, the hollow fiber membrane of Comparative Example 1 was obtained in the same manner as in Example 1.

[0045] 〔Comparative Examples 2 to 4〕 As shown in Table 1, the CNF addition concentration, S / NS ratio of the raw material solution, and kneading temperature were changed. Except for this, the hollow fiber membranes of Comparative Examples 2 to 4 were produced in the same manner as in Comparative Example 1.

[0046] <Measurement of outer diameter and inner diameter> For the hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4, the inner diameter and outer diameter were measured by the following method.

[0047] The outer diameter and inner diameter of the hollow fiber membrane were obtained by passing an appropriate number of hollow fiber membranes through a 3-mm diameter hole opened in the center of a slide glass so that the hollow fiber membranes did not fall out, cutting the hollow fiber membranes with a razor along the upper and lower surfaces of the slide glass to obtain a hollow fiber membrane cross-sectional sample, and then measuring the minor axis and major axis of the hollow fiber membrane cross-section using a projector (Nikon PROFILE PROJECTOR V-12).

[0048] For the outer diameter, the dimensions in the X-X direction and Y-Y direction of the outer surface of the hollow fiber membrane were measured for each single hollow fiber membrane cross-section, and the arithmetic mean value of these values was taken as the outer diameter of one hollow fiber membrane cross-section. For the inner diameter, the dimensions in the X-X direction and Y-Y direction of the hollow part were measured for each single hollow fiber membrane cross-section, and the arithmetic mean value was taken as the inner diameter of one hollow fiber membrane cross-section. In addition, the same measurement was performed for 10 cross-sections including the maximum and minimum, and the average values were taken as the inner diameter and outer diameter.

[0049] The measurement results of the outer diameter and inner diameter of the hollow fiber membrane are shown in Table 1.

[0050] <Measurement of Tensile Strength> For the hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4, the tensile strength (yield strength, breaking strength, yield elongation, and breaking elongation) was measured by the following method.

[0051] The measurement of the tensile strength was carried out using a yarn tensile tester (Tensilon (Model No. RTC1210A) manufactured by A&D Company, Ltd.). A cell with a full scale of 5000 g (200 g in the condition setting) was used. A single fiber with a total length of about 15 cm was fixed to the chuck (distance between chucks: 5 cm), and the lower chuck was lowered at a speed of 50 mm / min.

[0052] From the S-S curve printed on the chart paper, the load per single filament (breaking strength), elongation (breaking elongation), the load per single filament (yield strength), and elongation (yield elongation) at the yield point of the hollow fiber membrane were read. Specifically, the load and elongation were obtained using the method described in

[0061] of JP-A-2011-212638.

[0053] The measurement of the strength and elongation was carried out using a wet hollow fiber membrane under the conditions of a temperature of 20 °C and a humidity of 65%. The measurement results are shown in Table 1. For each of the examples and comparative examples, the measurement was carried out five times, and the average value is shown in Table 1.

[0054] [Table 1]

[0055] As shown in Table 1, in the examples, the yield elongation was suppressed and the yield strength was improved as compared with the comparative examples, the ratio of the yield strength (gf / strand) / yield elongation (%) (that is, the required stress per unit elongation) increased, indicating that the strength in the reversible yield region was improved. Also, the breaking elongation was suppressed and the breaking strength was improved, the ratio of the breaking strength (gf / strand) / breaking elongation (%) (that is, the stress / elongation ratio at the breaking point) increased, indicating that the hollow fiber membrane had a small shape change and was more difficult to break.

[0056] Fig. 7 shows, in a graph, the relationship between the breaking strength and FR4 (the water permeation amount after 4 years under the pressure resistance acceleration conditions) for the examples and comparative examples. The range above the line that divides the examples and comparative examples shown in Fig. 7 is a range that satisfies the following relational expression (see the bottom two lines of Table 1). That is, the examples satisfy the following relational expression.

[0057] Breaking strength [gf / strand] / 3 + FR 4 [L / m 2 / D] ≧ 4.58×10 -3 ×(AN post OD) 2 [μm]

[0058] Thus, in the hollow fiber membrane of the present invention, even when the breaking strength is increased, the permeation performance of a predetermined level or higher can be maintained.

[0059] <Measurement of water permeation amount> Regarding the hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4, a confirmation test of RO performance with highly concentrated brine was conducted.

[0060] Specifically, first, the hollow fiber membranes were bundled in a U shape and inserted into a plastic sleeve. Then, a thermosetting resin was injected into the sleeve and cured to seal it. By cutting the ends of the hollow fiber membranes cured with the thermosetting resin, an opening surface of the hollow fiber membranes was obtained, and an evaluation module 30 with a membrane area of 0.16 m 2 based on the outer diameter was produced (Fig. 6).

[0061] Using a membrane performance test apparatus including an evaluation liquid tank 40, a supply pump 42, a shell 31, a flow rate adjustment valve 43, a pressure adjustment valve 44, etc. as shown in Fig. 6, the RO performance of the evaluation module 30 was evaluated.

[0062] 〔Standard condition evaluation〕 Specifically, an aqueous solution of sodium chloride (NaCl) with a concentration of 35000 ppm was passed from the outside to the inside of the hollow fiber membrane under the condition (standard condition) of flowing on the outside of the hollow fiber membrane at 25 °C and a pressure of 5.4 MPa to permeate water. This RO treatment was performed for 1 hour. Then, the membrane permeated water was collected from the opening surface of the hollow fiber membrane, and the water permeation amount was measured.

[0063] Based on this water permeation amount, the water permeation amount per unit membrane area per day (standard condition permeation flux: FRs) under the above standard conditions was calculated from the following formula. FRs [L / m 2 / day] = water permeation amount [L] / membrane area [m 2 / sampling time [min] × (60 [min] × 24 [h])

[0064] 〔Evaluation under pressure resistance acceleration conditions〕 Next, the conditions were changed from the standard conditions to the pressure resistance acceleration test conditions, and water was permeated from the outside to the inside of the hollow fiber membrane under the conditions of flowing a sodium chloride (NaCl) aqueous solution with a concentration of 47,300 ppm at 35°C and a pressure of 6.76 MPa on the outside of the hollow fiber membrane. This RO treatment was performed for 2 hours, and the membrane permeated water was collected from the opening surface of the hollow fiber membrane, the permeated water volume was measured, and the permeated water volume per unit membrane area per day (pressure resistance acceleration test condition permeation flux: FR0) was calculated from the following formula. FR0 [L / m 2 / day] = permeated water volume [L] / membrane area [m 2 / sampling time [min] × (60 [min] × 24 [h])

[0065] 〔Calculation of coefficient of change in permeated water volume (-m value)〕 The m value was determined as follows. Furthermore, the permeated water volume was continuously measured up to 100 hours to confirm the change in the permeated water volume. The coefficient of change in the permeated water volume (-m value) indicates the slope of the change in the permeated water volume according to the elapsed time. The -m value was calculated from the slope of the regression line equation of the logarithm of time and the permeated water volume, x = log (elapsed time), y = log (permeated water volume) (the following formula).

[0066]

Equation

[0067] Next, from the above formula, the permeated water volume retention rate (MF) after 4 years under the pressure resistance acceleration conditions and the permeated water volume after 4 years under the pressure resistance acceleration conditions were calculated. Permeated water volume retention rate (MF) after 4 years under the pressure resistance acceleration conditions = (4 years × 365 days × 24 h / 2 h) (-m) = 17520 (-m) Permeated water volume after 4 years under the pressure resistance acceleration conditions (FR4) = FR0 × MF

[0068] The results of the above measurements and the like are shown in Table 1. As shown in Table 1, in Examples 1 to 11, a hollow fiber membrane excellent in both water permeability and the maintenance rate of water permeability after aging was obtained as compared with Comparative Examples 1 to 4.

[0069] <Dyeing test> For Example 3 and Comparative Example 2, a dyeing test was carried out. Specifically, in the membrane performance test apparatus of FIG. 6, a fluorescent dye (molecular weight 570) manufactured by Nacalai Tesque was added to the evaluation liquid 41 in the evaluation liquid tank 40, and an RO evaluation operation was carried out for 1 hour under the above standard conditions. In addition, by this dyeing test, the portion where partial defects of the membrane occurred was dyed.

[0070] Microscope photographs of the hollow fiber membranes of Example 3 and Comparative Example 2 after the dyeing test are shown in FIGS. 2 and 3, respectively. Note that FIG. 2 is the hollow fiber membrane obtained in Example 3, and FIG. 3 is the hollow fiber membrane obtained in Comparative Example 2.

[0071] In addition, for Example 3 and Comparative Example 2, microscope photographs of the hollow fiber membranes of Example 3 and Comparative Example 2 after the measurement of the above-mentioned tensile strength and elongation at break after the dyeing test are shown in FIGS. 4 and 5, respectively. Note that FIGS. 4(a) and (b) are the corresponding photographs to FIGS. 2(a) and (b), and FIGS. 5(a) to (c) are the corresponding photographs to FIGS. 3(a) to (c). From the photograph shown in FIG. 5, it can be seen that the hollow fiber membrane is broken at the portion dyed by the dyeing test.

[0072] <Measurement of salt rejection rate> For Example 3 and Comparative Example 2, the salt rejection rate was measured. Specifically, for the feed aqueous solution with a sodium chloride (NaCl) concentration of 35000 ppm used in the measurement of the above permeated water volume and the membrane permeated water collected in the measurement of the above permeated water volume, the NaCl concentration (salt concentration) was measured using a conductivity meter (CM-25R, manufactured by Toa DKK Corporation). Based on the measurement results, the salt rejection rate was calculated from the following formula. Salt rejection rate [%] = (1 - salt concentration of membrane permeated water [mg / L] / salt concentration of feed aqueous solution [mg / L]) × 100

[0073] As a result, the salt removal rate of Example 3 was 99.9% (salt permeability of 0.1%), and the salt removal rate of Comparative Example 2 was 95.0% (salt permeability of 5%). From these results, in Comparative Example 2, the amount of salt permeation was large, suggesting that partial defects may have occurred due to the poor dispersibility of CNF.

[0074] The embodiments and examples disclosed herein should be considered illustrative in all respects and not restrictive. The scope of the present invention is indicated by the claims rather than the above description, and it is intended that all modifications within the meaning and scope equivalent to the claims be included.

Explanation of Reference Numerals

[0075] 10 Spinning dope, 11 Nozzle, 12, 13, 14, 15 Roller, 16 Hollow fiber membrane, 21 Coagulating liquid, 30 Evaluation module, 31 Shell, 40 Evaluation liquid tank, 41 Evaluation liquid, 42 Supply pump, 43 Flow rate adjustment valve, 44 Pressure adjustment valve.

Claims

1. A hollow fiber membrane comprising a cellulose ester and a cellulose nanofiber, wherein the ratio of the amount of the cellulose nanofiber to the total amount of the cellulose ester and the cellulose nanofiber is 0.01 to 10% by mass, and the cellulose ester is cellulose acetate.

2. The hollow fiber membrane according to claim 1, wherein the fiber width (fiber diameter) of the cellulose nanofiber is 1 to 200 nm.

3. A method for producing a hollow fiber membrane comprising a cellulose ester and a cellulose nanofiber, the method including a spinning step of discharging a spinning dope from a nozzle through an air traveling section into a coagulation liquid and pulling out a coagulum of the spinning dope from the coagulation liquid to obtain a hollow fiber membrane which is a hollow fiber type semipermeable membrane, wherein the spinning dope contains a cellulose ester, a cellulose nanofiber, a solvent and a non-solvent, the spinning dope is kneaded and filtered before the spinning step, and in the spinning dope, the ratio of the amount of the cellulose nanofiber to the total amount of the cellulose ester and the cellulose nanofiber is 0.01 to 10% by mass.

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