JP7704186B2 - Hollow fiber membrane - Google Patents

Description

The present invention relates to a hollow fiber membrane.

Patent Document 1 (JP 2012-81533 A), Patent Document 2 (WO 2016/136294 A) and Patent Document 3 (JP 2018-506161 A) disclose hollow fiber membranes containing cellulose-based resins other than cellulose acetate and cellulose-based nanofibers.

JP 2012-81533 A International Publication No. 2016/136294 Special table 2018-506161 publication

When hollow fiber membranes are used in membrane separation processes such as reverse osmosis (RO), there is a problem in that the amount of water permeating the hollow fiber membrane decreases over time. When the amount of water permeating the hollow fiber membrane decreases over time, the operating energy required for the membrane separation process increases over time.

By adding cellulose nanofibers to the polymer material that makes up the hollow fiber membrane, it is expected that the strength of the hollow fiber membrane will generally be improved, physical changes over time will be suppressed, and the decrease in the amount of permeated water over time will be suppressed.

The present invention aims to provide a hollow fiber membrane that is excellent in both water permeability and water permeability retention rate.

(1) A hollow fiber membrane comprising a cellulose ester and a cellulosic nanofiber.
(2) The hollow fiber membrane according to (1), wherein a ratio of an amount of the cellulose nanofiber to a total amount of the cellulose ester and the cellulose nanofiber is 0.01 to 10 mass%.
(3) The hollow fiber membrane according to (1) or (2), wherein the cellulose nanofiber has a fiber width (fiber diameter) of 1 to 200 nm.
(4) A method for producing a hollow fiber membrane comprising cellulose ester and cellulose nanofibers, comprising the steps of:
The method includes a spinning process in which a spinning dope is discharged from a nozzle through an air gap into a coagulating liquid, and a coagulated product of the spinning dope is pulled out of the coagulating liquid to obtain a hollow fiber membrane, which is a hollow fiber type semipermeable membrane;
The spinning dope comprises a cellulose ester, a cellulosic nanofiber, a solvent, and a non-solvent;
The method comprises kneading the spinning dope prior to the spinning step.
(5) The spinning dope obtained by mixing the cellulose ester, the cellulose nanofiber powder, the solvent, and the non-solvent is kneaded;
The spinning dope obtained by mixing the slurry obtained by dispersing the cellulose nanofiber powder in the solvent with the cellulose ester and the non-solvent is kneaded, or
The manufacturing method according to (4), wherein the spinning dope is prepared by mixing the cellulose ester and the solvent with a slurry obtained by dispersing the cellulose nanofiber powder in the non-solvent, and kneading the slurry.
(6) The method according to (4) or (5), wherein in the spinning dope, a ratio of the amount of the cellulose nanofibers to the total amount of the cellulose ester and the cellulose nanofibers is 0.01 to 10 mass%.
(7) The 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 method according to any one of (4) to (7), wherein the temperature during kneading of the spinning dope is 150 to 200° C.
(9) The method according to any one of (4) to (8), wherein the shear rate during kneading of the spinning dope is 500 to 3500 sec ⁻¹ .
(10) The method according to any one of (4) to (9), wherein in the spinning dope, the cellulose nanofibers have a fiber width (fiber diameter) of 1 to 200 nm.
(11) A hollow fiber membrane comprising a cellulose ester and a cellulose nanofiber, produced by the production method according to any one of (4) to (10).

The present invention provides a hollow fiber membrane that is excellent in both water permeability and water permeability retention.

FIG. 2 is a schematic diagram for explaining an example of a method for producing a hollow fiber membrane. 1 is a microscope photograph of the hollow fiber membrane of Example 3 after a dyeing test. 1 is a microscope photograph of the hollow fiber membrane of Comparative Example 2 after a dyeing test. 1 is a microscope photograph of the hollow fiber membrane of Example 3 after breakage in the measurement of strength and elongation. 1 is a microscope photograph of the hollow fiber membrane of Comparative Example 2 after breakage in the measurement of strength and elongation. FIG. 2 is a schematic diagram for explaining an example of a membrane performance test device. 1 is a graph showing the relationship between breaking strength and _FR4 for Examples and Comparative Examples.

The embodiments of the present invention will be described with reference to the drawings. Furthermore, dimensional relationships such as length, width, thickness, and depth have been appropriately changed to clarify and simplify the drawings, and do not represent actual dimensional relationships.

<Hollow fiber membrane>
The hollow fiber membrane of this embodiment is a hollow fiber type semipermeable membrane.
The hollow fiber membrane comprises cellulose esters and cellulosic nanofibers.

(Cellulose ester)
Examples of cellulose esters include cellulose acetate (such as cellulose triacetate, cellulose monoacetate, cellulose diacetate, cellulose acetate butyrate, and cellulose acetate propionate), cellulose phthalate, and cellulose succinate.
The cellulose ester is preferably cellulose acetate. Cellulose acetate is resistant to chlorine, a bactericide, and has the characteristic of being able to inhibit the growth of microorganisms. From the viewpoint of durability, the cellulose acetate is preferably cellulose triacetate.

(Cellulose-based nanofiber)
Cellulosic nanofibers (CNF) can be obtained by, for example, breaking down (defibrating) naturally occurring cellulose derived from wood, plants, etc., into nanometer-order sizes.
The fiberization method may be, for example, at least one of mechanical crushing and chemical fragmentation. Examples of chemical fragmentation methods include TEPMO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation, phosphoric acid esterification, carboxymethylation, sulfonation, xanthation, and enzyme treatment.
As commercially available CNF products, products such as CNF powder and gel in which CNF is dispersed in water or an organic solvent can be purchased.

Cellulosic nanofibers (hereinafter sometimes abbreviated as "CNF") contain a cellulose resin (cellulose or a cellulose derivative).
Examples of cellulose derivatives include cellulose esters, cellulose ethers, and mixtures thereof.
Examples of cellulose esters include cellulose acetate (cellulose triacetate, cellulose monoacetate, and cellulose diacetate), cellulose phthalate, cellulose succinate, and the like.
Examples of cellulose ethers include methyl cellulose, ethyl cellulose, and hydroxypropyl methyl cellulose.

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, even 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 (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, whereas if the CNF ratio is too high, the fractionation ability (salt rejection rate) of the hollow fiber membrane will be low.

The fiber width (fiber diameter) of CNF is preferably 1 to 200 nm, and more preferably 1 to 50 nm. The fiber width of CNF can be measured using a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning probe electron microscope (SPM), an environmental scanning electron microscope (E-SEM), a cryo-SEM, etc.

The fiber length of the CNF is preferably 100 μm or less, and more preferably 20 nm to 10 μm. The fiber length of the CNF can be measured using a SEM (scanning electron microscope), a TEM (transmission electron microscope), an SPM (scanning probe electron microscope), an E-SEM (environmental scanning electron microscope), a cryo-SEM, etc.

(Shape of hollow fiber membrane, etc.)
The inner diameter of the hollow fiber membrane is preferably 30 μm or more and 300 μm or less, and more preferably 35 μm or more and 260 μm or less.
The thickness of the hollow fiber membrane (whole membrane) is preferably 20 to 200 μm, more preferably 30 to 150 μm. The membrane thickness can be calculated by (outer diameter-inner diameter)/2.
The hollow fiber membrane preferably has a hollow ratio of 10 to 65%, and more preferably 12 to 55%. The hollow ratio is the ratio of the area of the hollow portion in the cross section of the hollow fiber membrane and is expressed as "hollow portion cross-sectional area/(membrane portion cross-sectional area+hollow portion cross-sectional area)×100(%)".
The average pore size of the hollow fiber membrane (average pore size of the micropores in the entire membrane) is preferably 2 nm or less. Examples of a method for measuring the average pore size include differential scanning calorimetry (DSC).

The hollow fiber membrane of this embodiment exhibits the effect of suppressing the decrease over time in the amount of water permeating the hollow fiber membrane when used in membrane separation processes, particularly in processes such as reverse osmosis (RO) and brine concentration (BC), in which the hollow fiber membrane is exposed to high pressure. This suppresses the increase over time in the operating energy required for membrane separation processes using the hollow fiber membrane.

The BC method is a membrane separation method, as described in JP 2018-65114 A, in which a part of the target solution is passed through a first chamber on one side of a hollow fiber membrane module, and another part of the target solution is passed through a second chamber on the other side, and the target solution in the first chamber is pressurized, causing the solvent (such as water) contained in the target solution in the first chamber to migrate through the hollow fiber membrane to the second chamber, concentrating the target solution in the first chamber and diluting the target solution in the second chamber.

In addition, because processes using the RO method or the BC method are often used as part of a system in combination with other processes, if the amount of water permeating through the hollow fiber membrane decreases over time, it may become difficult to control the entire system. For this reason, by using the hollow fiber membrane of this embodiment, which has a high rate of water permeation volume maintenance, in a system in which the RO method or the BC method is combined with other processes, it is possible to easily control the entire system.

The hollow fiber membrane of this embodiment can also suppress the decrease over time in the amount of water passing through the hollow fiber membrane, particularly when pressure is applied from the outside of the hollow fiber membrane (i.e., when the pressure of the liquid outside the hollow fiber membrane is higher than that inside the hollow fiber membrane).

<Method of manufacturing hollow fiber membrane>
The present invention also relates to a method for producing a hollow fiber membrane comprising the above-mentioned cellulose ester and cellulose-based nanofibers.
The method for producing the hollow fiber membrane of this embodiment includes at least a spinning step described below.

The spinning method used in the hollow fiber membrane manufacturing method of this embodiment is a method called "solution spinning," which is a different spinning method from the (melt spinning) method disclosed in Patent Document 2 (WO 2016/136294).

[Spinning process]
1, in the spinning process, a spinning dope 10 is discharged from a nozzle 11 through an airborne passage into a coagulation liquid 21, and a coagulated product of the spinning dope is pulled out of the coagulation liquid to obtain a hollow fiber membrane, which is a hollow fiber type semipermeable membrane. The hollow fiber membrane is pulled out by, for example, rollers 12, 13, 14, and 15. The pulling speed is the surface speed of the roller 13.

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

The concentration of the cellulose ester in the spinning dope is preferably 20 to 60% by mass, and more preferably 30 to 50% by mass.
If the concentration of the cellulose ester is too low, the fractionation ability 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. Also, if the concentration of the cellulose ester is too high, the viscosity of the spinning solution will be too high, which may make it difficult to carry out spinning.

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, even 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 (mass%)=[CNF charge amount]/([cellulose ester charge amount]+[CNF charge amount])×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 of the hollow fiber membrane will be low. Also, if the CNF ratio is too high, the viscosity of the spinning dope will be too high, which may make it difficult to perform spinning.

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

(Kneading)
In this 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 is applied as described below, and the CNF is forcibly dispersed in the spinning dope at a high temperature and/or under pressure. 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, etc. is suppressed, the strength of the hollow fiber membrane is improved, and an improvement in the maintenance rate of water permeability is expected.

The kneading step is carried out using, for example, a planetary mixer, an extruder, a kneader (such as a pressure kneader or a twin-arm kneader), or the like.
The temperature at which the spinning dope is kneaded is preferably 150 to 200° C., more preferably 160 to 190° C., taking into consideration the solubility of the cellulose ester and its thermal modification.
The shear rate during kneading is preferably 500 to 3500 sec ^-1 , more preferably 500 to 2000 sec ^-1 , taking into consideration the improvement of the degree of kneading and thermal deterioration due to shear.

When mixing a polymer containing cellulose ester, which is a constituent material of the hollow fiber membrane, cellulose nanofibers (CNF), a solvent and a non-solvent, the order of addition of the polymer, CNF, etc. and the mixing method are not particularly limited.
For example, CNF may be added to a kneaded mixture of a polymer, a solvent, and a non-solvent, or a slurry in which CNF is dispersed in a solvent or non-solvent may be added to a kneaded mixture containing a polymer.

After the final mixing of the spinning solution, it is also preferable to remove CNF with long fiber length or large clusters from the spinning solution by filtering. This further improves the dispersibility (dispersion uniformity) of CNF in the spinning solution, suppresses the occurrence of defects in the hollow fiber membrane due to CNF aggregation, etc., further improves the strength of the hollow fiber membrane, and is expected to further improve the maintenance rate of water permeability.

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

The temperature of the coagulation liquid is preferably 10 to 30°C. In this case, the structural homogeneity of the hollow fiber membrane in the thickness direction can be improved.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

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.

(Composition of spinning dope)
Cellulose ester: cellulose triacetate (CTA) (LT35, manufactured by Daicel Corporation)
Cellulose ester concentration: 41.2% by mass (in the spinning dope)
CNF: Commercially available CNF powder (I-2SX, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
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% by mass]

(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 to be used in the spinning process.
The kneading was carried out under conditions of a temperature of 185° C. and a shear rate of 1500 to 1700 [1/sec], with the kneading residence time of the raw liquid being adjusted to 30 minutes.

(Spinning process conditions)
Dissolution temperature of spinning dope: 185°C
Discharge temperature of spinning dope: 158°C
Discharge nozzle: three-part nozzle (cross-sectional area of nozzle: 0.05 mm ² )
(The cross-sectional area of the nozzle is the cross-sectional area of the fiberizable liquid discharge hole at the tip of the nozzle.)
Air gap (AG) residence time: 0.06 seconds Temperature of coagulation liquid: 18°C
Pulling speed: 40m/min

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

[Conditions for post-treatment process]
Hot water treatment conditions Temperature: 98℃
Time: 20 minutes Conditions for salt annealing treatment Salt concentration: 4.5% by mass
Salt water temperature: 82℃
Duration: 20 minutes

[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 LT35/LT75 ratio was 88/12. Other than that, the hollow fiber membranes of Examples 2 to 11 were produced in the same manner as in Example 1. (In Table 1, the left-pointing arrow means the same as the left column.)

Comparative Example 1
The hollow fiber membrane of Comparative Example 1 was obtained in the same manner as in Example 1, except that CNF was not added to the spinning dope.

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

<Measurement of outer and inner diameter>
The inner and outer diameters of the hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4 were measured by the following method.

The outer and inner diameters of the hollow fiber membrane are obtained by passing an appropriate number of hollow fiber membranes through a 3 mm diameter hole drilled in the center of a slide glass so that the hollow fiber membranes do not fall out, cutting the hollow fiber membranes with a razor along the top and bottom surfaces of the slide glass, obtaining a cross-sectional sample of the hollow fiber membrane, and then measuring the short and long diameters of the cross-section of the hollow fiber membrane using a projector (Nikon PROFILE PROJECTOR V-12).

For the outer diameter, the dimensions in the X-X and Y-Y directions of the outer surface of each hollow fiber membrane cross section were measured, and the arithmetic mean of these values was taken as the outer diameter of each hollow fiber membrane cross section. For the inner diameter, the dimensions in the X-X and Y-Y directions of the hollow part of each hollow fiber membrane cross section were measured, and the arithmetic mean was taken as the inner diameter of each hollow fiber membrane cross section. Similar measurements were made for 10 cross sections, including the maximum and minimum, and the average values were taken as the inner and outer diameters.

The measurement results of the outer and inner diameters of the hollow fiber membrane are shown in Table 1.

<Measurement of strength and elongation>
The hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4 were measured for strength and elongation (yield strength, breaking strength, yield elongation, and breaking elongation) by the following method.

The strength and elongation were measured using a yarn tensile tester (Tensilon (Model No. RTC1210A) manufactured by A&D Co., Ltd.).
A full-scale 5000 g cell (200 g under the conditions) was used, and a single yarn with a total length of about 15 cm was fixed to a chuck (distance between chucks: 5 cm), and the lower chuck was lowered at a speed of 50 mm/min.

From the S-S curve printed on the chart paper, the load (breaking strength) and elongation (breaking elongation) per single fiber at the breaking point of the hollow fiber membrane, and the load (yield strength) and elongation (yield elongation) per single fiber at the yield point were read. Specifically, the load and elongation were obtained using the method shown in [0061] of JP 2011-212638 A.

The strength and elongation were measured using a wet hollow fiber membrane at 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 values thereof are shown in Table 1.

As shown in Table 1, the examples have reduced yield elongation and improved yield strength compared to the comparative examples, and the ratio of yield strength (gf/piece) to yield elongation (%) (i.e., the required stress per unit elongation) is larger, indicating that the strength in the reversible yield region is improved.
In addition, the breaking elongation is suppressed and the breaking strength is improved, and the breaking strength (gf/piece)/breaking elongation (%) (i.e., the stress/elongation ratio at the breaking point) is increased, indicating that the hollow fiber membrane undergoes little change in shape and is more resistant to breaking.

FIG. 7 is a graph showing the relationship between breaking strength and FR ₄ (permeated water amount after 4 years under accelerated pressure resistance conditions) for the examples and comparative examples.
7 is a range above the line dividing the Examples and Comparative Examples, which satisfies the following relational expression (see the bottom two rows of Table 1). That is, the Examples satisfy the following relational expression.

Breaking strength [gf/piece]/3+FR ⁴ [L/m ² /D] ≧ ^{4.58×10 −3} × (OD after AN) ² [μm]

In this way, the hollow fiber membrane of the present invention can maintain a certain level of permeability even when the breaking strength is increased.

<Measurement of Permeated Water Amount>
For the hollow fiber membranes of Examples 1 to 11 and Comparative Examples 1 to 4, a confirmation test of RO performance using highly concentrated salt water was carried out.

Specifically, the hollow fiber membranes were first bundled into a U-shape and inserted into a plastic sleeve, after which a thermosetting resin was injected into the sleeve and cured to seal it in. The ends of the hollow fiber membranes cured with the thermosetting resin were cut to obtain an opening surface of the hollow fiber membrane, and an evaluation module 30 with a membrane area of 0.16 ^m2 based on the outer diameter was produced (FIG. 6).

The RO performance of the evaluation module 30 was evaluated using a membrane performance test device equipped with an evaluation liquid tank 40, a supply pump 42, a shell 31, a flow control valve 43, a pressure control valve 44, etc., as shown in Figure 6.

[Standard condition evaluation]
Specifically, a sodium chloride (NaCl) aqueous solution with a concentration of 35,000 ppm was passed through the hollow fiber membrane at 25° C. and a pressure of 5.4 MPa (standard conditions), and water was allowed to permeate from the outside to the inside of the hollow fiber membrane. This RO treatment was carried out for 1 hour. Thereafter, the membrane permeated water was collected from the opening of the hollow fiber membrane, and the amount of permeated water was measured.

Based on this amount of permeated water, the amount of permeated water per unit membrane area per day under the above standard conditions (permeation flux under standard conditions: FRs) was calculated from the following formula.
FRs [L/m ² /day] = Permeated water amount [L] / Membrane area [m ² ] / Collection time [minutes] × (60 [minutes] × 24 [hours])

[Evaluation of accelerated pressure resistance conditions]
Next, the standard conditions were changed to the conditions of the accelerated pressure test, and water was permeated from the outside to the inside of the hollow fiber membrane under the conditions that an aqueous sodium chloride (NaCl) solution with a concentration of 47,300 ppm was passed through the outside of the hollow fiber membrane at 35° C. and a pressure of 6.76 MPa. This RO treatment was carried out for 2 hours, and the membrane permeated water was collected from the opening surface of the hollow fiber membrane, the amount of permeated water was measured, and the amount of permeated water per unit membrane area per day (permeation flux under accelerated pressure test conditions: FR0) was calculated from the following formula.
FR ₀ [L/m ² /day] = Permeated water amount [L] / Membrane area [m ² ] / Collection time [minutes] x (60 [minutes] x 24 [hours])

[Calculation of permeate volume change coefficient (-m value)]
The m value was determined as follows.
Furthermore, the amount of permeated water was continuously measured up to 100 hours to confirm the change in the amount of permeated water. The coefficient of change in the amount of permeated water (-m value) indicates the slope of the change in the amount of permeated water depending on the elapsed time.
The -m value was calculated from the slope of the regression line of the logarithm of the amount of permeated water versus time, x=log (elapsed time), y=log (amount of permeated water) (see the following formula).

Next, the permeate flow rate maintenance rate (MF) after four years under the accelerated pressure conditions and the permeate flow rate after four years under the accelerated pressure conditions were calculated from the above formula.
Permeation rate (MF) after 4 years under accelerated pressure resistance conditions
=(4 years x 365 days x 24 hours/2 hours) ^(-m) =17520 ^(-m)
Permeation rate after 4 years under accelerated pressure resistance conditions ( _FR4 )
= FR ₀ × MF

The results of the above measurements are shown in Table 1.
As shown in Table 1, in Examples 1 to 11, hollow fiber membranes superior to Comparative Examples 1 to 4 in both water permeability and water permeability retention rate after aging were obtained.

<Dyeing test>
A dyeing test was carried out for Example 3 and Comparative Example 2. Specifically, in the membrane performance test device 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 RO evaluation operation was carried out for 1 hour under the above-mentioned standard conditions. Note that this dyeing test dyes the parts of the membrane where partial defects have occurred.

Microscope photographs of the hollow fiber membranes of Example 3 and Comparative Example 2 after the dyeing test are shown in Figures 2 and 3, respectively. Note that Figure 2 shows the hollow fiber membrane obtained in Example 3, and Figure 3 shows the hollow fiber membrane obtained in Comparative Example 2.

In addition, for Example 3 and Comparative Example 2, microscope photographs of the hollow fiber membranes of Example 3 and Comparative Example 2 after breakage when the strength and elongation were measured after the dyeing test are shown in Figures 4 and 5, respectively. Note that Figures 4(a) and (b) are photographs corresponding to Figures 2(a) and (b), and Figures 5(a) to (c) are photographs corresponding to Figures 3(a) to (c).
From the photograph shown in FIG. 5, it can be seen that breakage of the hollow fiber membrane occurred at the portion stained by the staining test.

<Measurement of salt rejection rate>
The salt rejection rate was measured for Example 3 and Comparative Example 2. Specifically, the NaCl concentration (salt concentration) was measured using an electric conductivity meter (CM-25R, DKK-TOA Corporation) for the feed aqueous solution with a sodium chloride (NaCl) concentration of 35,000 ppm used in the measurement of the permeate volume and the membrane permeate collected in the measurement of the permeate volume. Based on the measurement results, the salt rejection rate was calculated by the following formula.
Salt rejection rate [%] = (1 - salt concentration in membrane permeate water [mg/L] / salt concentration in feed aqueous solution [mg/L]) x 100

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, the amount of salt permeated was large in Comparative Example 2, which suggests the possibility that partial defects occurred due to poor dispersibility of the CNF.

The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims.

10 spinning dope, 11 nozzle, 12, 13, 14, 15 roller, 16 hollow fiber membrane, 21 coagulation liquid, 30 evaluation module, 31 shell, 40 evaluation liquid tank, 41 evaluation liquid, 42 supply pump, 43 flow rate control valve, 44 pressure control valve.

Claims (3)

The present invention relates to a method for producing a cellulose ester and a method for producing a cellulose nanofiber.
a ratio of the amount of the cellulose nanofibers to the total amount of the cellulose ester and the cellulose nanofibers is 0.01 to 10% by mass;
The cellulose ester is cellulose acetate . The hollow fiber membrane according to claim 1, wherein the fiber width (fiber diameter) of the cellulose nanofiber is 1 to 200 nm. A method for producing a hollow fiber membrane comprising cellulose ester and cellulose-based nanofibers, comprising the steps of:
The method includes a spinning process in which a spinning dope is discharged from a nozzle through an air gap into a coagulating liquid, and a coagulated product of the spinning dope is pulled out of the coagulating liquid to obtain a hollow fiber membrane, which is a hollow fiber type semipermeable membrane;
The spinning dope comprises a cellulose ester, a cellulosic nanofiber, a solvent, and a non-solvent;
Prior to the spinning step, the spinning dope is kneaded and filtered;
In the spinning dope, the ratio of the amount of the cellulose nanofibers to the total amount of the cellulose ester and the cellulose nanofibers is 0.01 to 10 mass % .

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