A method for preparing a hollow fiber nanofiltration composite membrane with a stable and flexible structure

Abstract

The present invention belongs to the field of polymer materials, and in particular to a method for preparing a hollow fiber composite membrane. The present invention uses a hollow fiber ultrafiltration membrane as a base membrane, immerses it in an isopropyl alcohol aqueous solution for cleaning; then immerses the outer surface of the membrane in an alcohol solution of polyether block polyamide; then puts it into a hot drying oven for drying; again immerses the outer surface of the base membrane in an aqueous solution of polyether amine, takes it out and then dries it; then immerses the base membrane in a polyacyl chloride monomer organic phase solution; finally, the base membrane is extracted from the organic phase, placed in a 60-120°C forced air drying oven for 10-30 minutes, and after washing, a polyamide hollow fiber nanofiltration composite membrane is obtained. The advantage of the present invention is that the structure is controlled from two aspects: the surface properties of the base membrane and the separation layer material. The polymer with both ether bonds and amide bonds is used as a modified material, which not only increases the material flexibility, but also strengthens the binding force between the separation layer and the base membrane surface, significantly improving the structural stability of the polyamide nanofiltration separation layer on the curved surface of the hollow fiber.

Description

Technical Field

[0001] The invention belongs to the field of polymer materials, and particularly relates to a method for preparing a hollow fiber composite membrane. Background Art

[0002] The current mainstream commercial nanofiltration membrane products are in the form of flat membranes, which are rolled together with a grid to form a roll element for use. The water channel structure filled with the grid will lead to large pressure losses in the element and reduce the recovery efficiency of hydraulic flushing and chemical cleaning. In the actual nanofiltration process, the roll element has poor pollution resistance when treating complex water bodies. Existing research shows that the accumulation of pollutants in the grid area is more serious than the pollution on the membrane surface. To mitigate the impact of membrane fouling, it is usually necessary to strengthen the pretreatment process at the front end of the membrane process to reduce the pollutant components of the nanofiltration influent, but this will increase the operating cost of the entire treatment.

[0003] Hollow fiber nanofiltration membrane is a nanofiltration membrane material based on hollow fibers and possessing nanofiltration separation properties. Compared to traditional flat-plate rolled nanofiltration membrane elements, it has the characteristics of high flux, low pressure, and pollution resistance. The self-supporting structure of the hollow fiber can save unnecessary auxiliary components, thus having a higher packing density. Therefore, the water flux per unit volume of this configuration element can generally be more than doubled compared to that of a rolled element. In addition, the internal water inlet flow channel of the hollow fiber membrane element is highly open, making it easier for pollutants to be discharged with the concentrated water. It has better pollution resistance and cleaning efficiency, making its pretreatment and subsequent maintenance simpler. The characteristics of high flux, low pressure, and pollution resistance give hollow fiber nanofiltration membranes a clear advantage in the field of large-scale municipal water supply deep treatment.

[0004] Although the separation membrane in the form of hollow fiber has the above advantages, the separation layer and the hollow fiber base membrane are prone to separation after long-term operation. This is because the interface curvature between the base membrane and the separation layer is large and the stress is not released. The traditional interfacial polymerization membrane preparation method anchors the separation layer to the surface of the base membrane through weak physical action. The bonding force between the two is insufficient, which causes the separation layer to easily fall off from the base membrane. Summary of the Invention

[0005] In order to overcome the problems existing in hollow fiber nanofiltration membranes, the present application provides a polyamide film material with a stable and flexible structure and a preparation method thereof.

[0006] The technical solution adopted to achieve the purpose of the present invention is:

[0007] (1) A hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 40,000-80,000 Da) is used as a base membrane, and the hollow fiber ultrafiltration membrane is immersed in a cleaning solution (30% by mass isopropyl alcohol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane, and then immersed in pure water for cleaning and allowed to stand for 6 hours before use. The ultrafiltration membrane can be made of one of polysulfone, polyethersulfone, polyethylene, polypropylene, and polyvinylidene fluoride;

[0008] (2) After removing the ultrafiltration base membrane, hang it vertically for 10 minutes and wipe off the excess solution with a paper towel. Then, immerse the outer surface of the membrane in an alcohol solution containing 0.1-0.5% (mass concentration) polyether block polyamide (PEBAX) for 1-5 minutes, wherein the molecular weight of PEBAX can be selected from 8000, 10000, and 30000 Da, and the alcohol solvent can be selected from isopropanol, n-pentanol, and n-hexanol;

[0009] (3) Take the base film out of the alcohol solution, hang it vertically in the air for 1-10 minutes, wipe off the excess solution with a paper towel, and then place it in a hot drying oven at 30-60°C for 5-10 minutes;

[0010] (4) Re-immerse the outer surface of the basement membrane in a 1-4% (mass concentration) aqueous solution of polyetheramine (PEA) for 1-5 minutes, wherein the molecular weight of PEA can be selected from 200, 400, and 2000 Da; then remove the membrane and hang it vertically in the air for 1-10 minutes, and wipe off the excess solution with a paper towel;

[0011] (5) immersing the base film again in a 0.1-1% (mass concentration) polyacyl chloride monomer organic phase solution for 1-10 minutes, wherein the polyacyl chloride monomer can be one of succinyl chloride, diphenyl diacetyl chloride, 4,4'-oxybis(benzoyl chloride), trimesoyl chloride or a mixture thereof, and the organic solvent can be one of pentane, hexane, cyclohexane, heptane or a mixture thereof;

[0012] (6) The base membrane is extracted from the organic phase, placed in a forced air drying oven at 60-120° C. for 10-30 minutes, and washed with water to obtain a polyamide hollow fiber nanofiltration composite membrane.

[0013] Preferably, the molecular weight cut-off of the hollow fiber ultrafiltration base membrane in the above step (1) is 50,000-60,000 Da;

[0014] Preferably, the mass concentration of PEBAX in the above step (2) is 0.2-0.4%.

[0015] Preferably, the immersion time of the base film in the alcohol solution in the above step (2) is 2-4 minutes.

[0016] Preferably, the molecular weight of PEBAX in the above step (2) is 10000 Da;

[0017] Preferably, the heat treatment temperature in the above step (3) is 40-50° C., and the heat treatment time is 6-8 minutes;

[0018] Preferably, the mass concentration of PEA in the above step (4) is 2%-3% and the molecular weight is 400Da;

[0019] Preferably, the immersion time of the membrane in the aqueous phase in the above step (4) is 2-4 minutes;

[0020] Preferably, the concentration of the polyacyl chloride monomer in the above step (5) is 0.2%-0.5%;

[0021] Preferably, in the above step (5), the immersion time of the basement membrane in the organic phase is 2-5 minutes;

[0022] Preferably, in the above step (6), the oven temperature is 80-100°C and the heat treatment time is 15-20 minutes;

[0023] In this application, the poor bonding and easy dissociation of existing "base membrane + separation layer" composite membrane materials are addressed by structurally modifying the base membrane surface properties and the separation layer material. From the base membrane perspective, the PEBAX polymer is first used to modify the base membrane surface. Because PEBAX contains both ether and amide bonds, with the ether bonds being flexible, this material possesses excellent flexibility and stable adhesion to the hollow fiber base membrane surface. Secondly, the amide bonds rich in PEBAX can form hydrogen bonds with the separation layer, also made of a polyamide, thereby enhancing the bonding strength between the base membrane and the separation layer. From the separation layer perspective, a polyamine such as PEA, also rich in ether bonds, is used as a polymerization monomer. Through interfacial polymerization with polyacyl chlorides, a highly flexible polyamide separation layer is obtained. Therefore, the hollow fiber nanofiltration membrane material prepared according to the method described in this application, due to its rich ether bond-rich flexible structure and the hydrogen bonding between the separation layer and the base membrane surface, possesses excellent stability, enabling long-term operation in high-pressure and high-flow hydraulic environments.

[0024] Beneficial effects: Compared with the existing invention patents for modifying the base membrane surface (such as CN202010135374.X, CN202010690672.5, CN202110853756.0), the difference of this application is that a polymer with both ether bonds and amide bonds is used as the base membrane surface modification material, which not only increases the flexibility of the nanofiltration membrane material, but also strengthens the binding force between the separation layer and the base membrane surface, significantly improving the structural stability of the polyamide nanofiltration separation layer on the curved surface of the hollow fiber. In addition, the present application also uses a polyetheramine monomer with an ether bond structure to increase the structural flexibility of the polyamide separation layer itself. In short, the present application introduces ether bond flexibility from both the base membrane modification and the separation layer material structure, and specifically solves the problem of long-term stability of the hollow fiber nanofiltration membrane. DETAILED DESCRIPTION

[0025] In order to evaluate the effectiveness of the method, the present application also prepared three different hollow fiber nanofiltration membranes as comparative examples. The specific preparation process is shown below.

[0026] The test method for the flux and salt rejection of the hollow fiber nanofiltration membrane of the present invention is as follows:

[0027] Membrane separation performance test: The prepared hollow fiber nanofiltration membrane was pre-pressed with an electrolyte solution with a mass concentration of 0.2% MgSO4 at 0.4 MPa for half an hour to test the desalination performance and water flux of the hollow fiber nanofiltration membrane.

[0028] The calculation formula of water flux is shown in (1):

[0029]

[0030] Where A is the effective membrane area, m 2 ; t - the time required to collect Q volume of produced liquid, h; Q - the volume of produced liquid collected within t time, L.

[0031] The calculation method of membrane desalination performance is shown in (2):

[0032] (2)

[0034] Among them, R is the desalination rate, C f - Conductivity of the stock solution, μS / cm; C p - Conductivity of produced water, μS / cm.

[0035] Bonding Strength Test: To verify the bonding strength between the separation layer and the basement membrane, the membranes tested above were immersed in an ethanol solution for 24 hours. After rinsing, the membranes were subjected to the same separation performance test using the same method. The ethanol immersion method works by assuming that due to the varying swelling coefficients of various materials, the basement membrane and the separation layer swell to varying degrees after treatment with the ethanol solvent, weakening the stability of the interface. Therefore, this method can be used to test interfacial bonding strength. The test results are shown in Table 2.

[0036] Example 1:

[0037] (1) A polysulfone hollow fiber ultrafiltration membrane (molecular weight cut-off ≈ 50,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0038] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in an isopropanol solution containing 0.2% (mass concentration) PEBAX for 2 minutes, where the molecular weight of PEBAX is selected to be 10,000 Da;

[0039] (3) The basement membrane was removed from the alcohol solution, hung vertically in the air for 1 minute, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 40°C for 6 minutes;

[0040] (4) The outer surface of the basement membrane was re-immersed in a 2% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 200 Da, for 2 minutes; then, the membrane was removed and hung vertically in the air for 1 minute, and the excess solution was wiped off with a paper towel;

[0041] (5) The base membrane was immersed in a 0.2% (mass concentration) pentane solution of trimesoyl chloride again and kept for 2 minutes. The base membrane was taken out from the solution and placed in an 80°C forced air drying oven for 15 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0042] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 93.4%; the water flux was 30.4L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 93.1% and the water flux was 31.9 L / m 2 .h.

[0043] Example 2:

[0044] (1) A polyethersulfone hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 40,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0045] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in a n-pentanol solution containing 0.3% (mass concentration) PEBAX for 3 minutes, where the molecular weight of PEBAX is selected to be 8000 Da;

[0046] (3) The basement membrane was removed from the alcohol solution, hung vertically in the air for 5 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 45°C for 7 minutes;

[0047] (4) The outer surface of the basement membrane was re-immersed in a 3% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 400 Da, for 3 minutes; then, the membrane was removed and hung vertically in the air for 5 minutes, and the excess solution was wiped off with a paper towel;

[0048] (5) The base membrane was immersed in a 0.3% (mass concentration) hexane solution of succinyl chloride again and kept for 5 minutes. The base membrane was taken out from the solution and placed in a 90°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0049] Performance test: The desalination rate of the hollow fiber nanofiltration membrane is 94.1%; the water flux is 33.2L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 94.2% and the water flux was 34.6L / m 2 .h.

[0050] Example 3:

[0051] (1) A polyethylene hollow fiber ultrafiltration membrane (molecular weight cut-off ≈ 80,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0052] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in a n-pentanol solution containing 0.4% (mass concentration) PEBAX for 4 minutes, where the molecular weight of PEBAX is selected to be 30,000 Da;

[0053] (3) The base film was removed from the alcohol solution, hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 50°C for 8 minutes;

[0054] (4) The outer surface of the basement membrane was re-immersed in a 4% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 2000 Da, for 4 minutes; then, the membrane was removed and hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel;

[0055] (5) The base membrane was immersed in a 0.5% (mass concentration) cyclohexane solution of diphenyl diacetyl chloride again and kept for 5 minutes. The base membrane was taken out from the solution and placed in a 100°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0056] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 93.7%; the water flux was 35.9 L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 92.9% and the water flux was 36.8L / m 2 .h.

[0057] Example 4:

[0058] (1) A polypropylene hollow fiber ultrafiltration membrane (molecular weight cut-off ≈ 60,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0059] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in a n-hexanol solution containing 0.5% (mass concentration) PEBAX for 3 minutes, where the molecular weight of PEBAX is selected to be 10,000 Da;

[0060] (3) The base film was removed from the alcohol solution, hung vertically in the air for 8 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 50°C for 8 minutes;

[0061] (4) The outer surface of the basement membrane was re-immersed in a 4% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 400 Da, for 5 minutes; then, the membrane was removed and hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel;

[0062] (5) The base membrane was immersed in a 1.5% (mass concentration) heptane solution of 4,4'-oxybis(benzoyl chloride) for 10 minutes. The base membrane was taken out of the solution and placed in a blast drying oven at 120°C for 30 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0063] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 94.1%; the water flux was 28.8L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 93.8% and the water flux was 30.5L / m 2 .h.

[0064] Example 5:

[0065] (1) Using a polyvinylidene fluoride hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 70,000 Da) as a base membrane, the hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass fraction isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane, and then immersed in pure water for cleaning and allowed to stand for 6 hours before use;

[0066] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in an isopropanol solution containing 0.1% (mass concentration) PEBAX for 2 minutes, where the molecular weight of PEBAX is selected to be 8000 Da;

[0067] (3) The basement membrane was removed from the alcohol solution, hung vertically in the air for 5 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 45°C for 6 minutes;

[0068] (4) The outer surface of the basement membrane was re-immersed in a 1% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 400 Da, for 2 minutes; then, the membrane was removed and hung vertically in the air for 1 minute, and the excess solution was wiped off with a paper towel;

[0069] (5) The base membrane was immersed in a pentane solution of trimesoyl chloride (0.2% by mass concentration) again for 2 minutes. The base membrane was taken out of the solution and placed in a forced air drying oven at 80°C for 15 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0070] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 94.7%; the water flux was 35.8L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 93.9% and the water flux was 37.3L / m 2 .h.

[0071] Example 6:

[0072] (1) A polysulfone hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 80,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0073] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in an isopropanol solution containing 0.5% (mass concentration) PEBAX for 5 minutes, where the molecular weight of PEBAX is selected to be 10,000 Da;

[0074] (3) The basement membrane was removed from the alcohol solution, hung vertically in the air for 6 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 40°C for 10 minutes;

[0075] (4) The outer surface of the basement membrane was re-immersed in a 4% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 2000 Da, for 5 minutes; then, the membrane was removed and hung vertically in the air for 7 minutes, and the excess solution was wiped off with a paper towel;

[0076] (5) The base membrane was immersed in a 1% (mass concentration) cyclohexane solution of 4,4'-oxybis(benzoyl chloride) for 10 minutes. The base membrane was taken out of the solution and placed in a blast drying oven at 70°C for 30 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0077] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 94.3%; the water flux was 30.7L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 93.6% and the water flux was 35.2L / m 2 .h.

[0078] Example 7:

[0079] (1) Using a polyvinylidene fluoride hollow fiber ultrafiltration membrane (molecular weight cut-off ≈ 60,000 Da) as a base membrane, the hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% mass fraction isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane, and then immersed in pure water for cleaning and allowed to stand for 6 hours before use;

[0080] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in a n-hexanol solution containing 0.2% (mass concentration) PEBAX for 3 minutes, where the molecular weight of PEBAX is selected to be 8000 Da;

[0081] (3) The base film was removed from the alcohol solution, hung vertically in the air for 8 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 55°C for 7 minutes;

[0082] (4) The outer surface of the basement membrane was re-immersed in a 4% (mass concentration) aqueous solution of PEA, where the molecular weight of PEA was selected to be 200 Da, for 3 minutes; then, the membrane was removed and hung vertically in the air for 9 minutes, and the excess solution was wiped off with a paper towel;

[0083] (5) The base membrane was immersed in a 0.3% (mass concentration) hexane solution of trimesoyl chloride again for 6 minutes, and the base membrane was taken out from the solution and placed in a 90°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0084] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 92.4%; the water flux was 33.7L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 92.1% and the water flux was 36.3L / m 2 .h.

[0085] In order to further investigate the strengthening effect of the present invention on the stability of the hollow fiber nanofiltration membrane, the following comparative examples are added:

[0086] Comparative Example 1:

[0087] (1) A polysulfone hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 60,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0088] (2) After taking out the ultrafiltration base membrane, hang it vertically for 10 minutes and wipe off the excess solution with a paper towel. Then, immerse the outer surface of the membrane in pure hexanol solution for 3 minutes;

[0089] (3) The base film was removed from the alcohol solution, hung vertically in the air for 8 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 40°C for 20 minutes;

[0090] (4) The outer surface of the basement membrane was re-immersed in a 0.5% (mass concentration) piperazine aqueous solution for 3 minutes, then taken out and hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel;

[0091] (5) The base membrane was immersed in a 0.3% (mass concentration) hexane solution of trimesoyl chloride again and kept for 6 minutes. The base membrane was taken out from the solution and placed in a 100°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0092] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 98.6%; the water flux was 22.8L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The desalination rate was measured to be 84.4% and the water flux was 45.4L / m 2 .h

[0093] Comparative Example 2:

[0094] (1) A polysulfone hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 60,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0095] (2) After taking out the ultrafiltration base membrane, hang it vertically for 10 minutes and wipe off the excess solution with a paper towel. Then, immerse the outer surface of the membrane in pure hexanol solution for 3 minutes;

[0096] (3) The base film was removed from the alcohol solution, hung vertically in the air for 8 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 40°C for 20 minutes;

[0097] (4) The outer surface of the basement membrane was re-immersed in a 0.5% (mass concentration) aqueous solution of PEA for 3 minutes, wherein the molecular weight of PEA was selected to be 400 Da, and then taken out and hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel;

[0098] (5) The base membrane was immersed in a 0.3% (mass concentration) hexane solution of trimesoyl chloride again and kept for 6 minutes. The base membrane was taken out from the solution and placed in a 100°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0099] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 94.7%; the water flux was 25.6L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The measured desalination rate was 90.5% and the water flux was 31.4 L / m 2 .h.

[0100] Comparative Example 3:

[0101] (1) A polysulfone hollow fiber ultrafiltration membrane (molecular weight cutoff ≈ 60,000 Da) was used as the base membrane. The hollow fiber ultrafiltration membrane was immersed in a cleaning solution (30% by mass isopropanol aqueous solution) for 6 hours to clean the organic solvent remaining in the base membrane. The membrane was then immersed in pure water for cleaning and allowed to stand for 6 hours before use.

[0102] (2) After the ultrafiltration base membrane is taken out, it is hung vertically for 10 minutes and the excess solution is wiped off with a paper towel. Then, the outer surface of the membrane is immersed in a n-hexanol solution containing 0.3% (mass concentration) PEBAX for 5 minutes, where the molecular weight of PEBAX is selected to be 10,000 Da;

[0103] (3) The base film was removed from the alcohol solution, hung vertically in the air for 8 minutes, and the excess solution was wiped off with a paper towel, and then placed in a hot drying oven at 40°C for 20 minutes;

[0104] (4) The outer surface of the basement membrane was re-immersed in a 0.5% (mass concentration) piperazine aqueous solution for 3 minutes, then taken out and hung vertically in the air for 10 minutes, and the excess solution was wiped off with a paper towel;

[0105] (5) The base membrane was immersed in a 0.3% (mass concentration) hexane solution of trimesoyl chloride again and kept for 6 minutes. The base membrane was taken out from the solution and placed in a 100°C forced air drying oven for 20 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane was obtained.

[0106] Performance test: The desalination rate of the hollow fiber nanofiltration membrane was 98.5%; the water flux was 18.6L / m 2 .h; The hollow fiber nanofiltration membrane was soaked in ethanol solution for 1 hour and then taken out. The desalination rate was measured to be 90.5% and the water flux was 38.8L / m 2 .h.

[0107] Table 1 Preparation conditions of hollow fiber nanofiltration membrane material comparative group samples

[0108]

[0109] a: Use only pure solvent for processing

[0110] b: Polyamine monomer used in conventional nanofiltration membranes

[0111] Table 2 Performance changes and stability differences of the membranes of the examples and comparative examples

[0112]

[0113]

Claims

1. A method for preparing a hollow fiber nanofiltration composite membrane with a stable and flexible structure, characterized in that The steps include: (1) A hollow fiber ultrafiltration membrane is used as a base membrane, and the hollow fiber ultrafiltration membrane is immersed in a 30% by mass isopropanol aqueous solution for cleaning. The immersion treatment is carried out for 6 hours to clean the organic solvent remaining in the base membrane, and then immersed in pure water for cleaning and allowed to stand for 6 hours for standby use; wherein the material of the ultrafiltration membrane can be one of polysulfone, polyethersulfone, polyethylene, polypropylene, or polyvinylidene fluoride; the molecular weight cutoff of the base membrane is ≈40,000-80,000 Da; (2) After removing the ultrafiltration base membrane, hang it vertically for 10 minutes and wipe off the excess solution. Then, immerse the outer surface of the membrane in an alcohol solution containing a polyether block polyamide with a mass concentration of 0.1-0.5% for 1-5 minutes, wherein the molecular weight of the polyether block polyamide is one of 8000, 10000, or 30000 Da, and the alcohol solvent is one of isopropanol, n-pentanol, or n-hexanol; (3) Take the base film out of the alcohol solution, hang it vertically in the air for 1-10 minutes, wipe off the excess solution, and then place it in a hot drying oven at 30-60°C for 5-10 minutes; (4) Re-immerse the outer surface of the basement membrane in an aqueous solution of polyetheramine with a mass concentration of 1-4% for 1-5 minutes, wherein the polyetheramine molecular weight is selected from 200, 400, or 2000 Da; then remove the basement membrane and hang it vertically in the air for 1-10 minutes, and wipe off the excess solution; (5) Immersing the base film again in a polyacyl chloride monomer organic phase solution with a mass concentration of 0.1-1% for 1-10 minutes, wherein the polyacyl chloride monomer is one of succinyl chloride, diphenyl diacetyl chloride, 4,4'-oxybis(benzoyl chloride), trimesoyl chloride or a mixture thereof, and the organic solvent is one of pentane, hexane, cyclohexane, heptane or a mixture thereof; (6) The base membrane is extracted from the organic phase and placed in a forced air drying oven at 60-120°C for 10-30 minutes. After washing with water, a polyamide hollow fiber nanofiltration composite membrane is obtained.

2. A preparation method according to claim 1, characterized in that, The molecular weight cut-off of the hollow fiber ultrafiltration base membrane in step (1) is 50,000-60,000 Da.

3. A preparation method according to claim 1, characterized in that, In step (2), the mass concentration of the polyether block polyamide is 0.2-0.4% and the molecular weight is 10,000 Da; and the immersion time of the base film in the alcohol solution is 2-4 minutes.

4. A preparation method according to claim 1, characterized in that, In step (3), the heat treatment temperature is 40-50°C and the heat treatment time is 6-8 minutes.

5. A preparation method according to claim 1, characterized in that, In step (4), the mass concentration of the polyetheramine is 2%-3% and the molecular weight is 400 Da; the immersion time of the membrane in the aqueous phase is 2-4 minutes.

6. A preparation method according to claim 1, characterized in that, In step (5), the concentration of the polyacyl chloride monomer is 0.2%-0.5%; and the immersion time of the base film in the organic phase is 2-5 minutes.

7. A preparation method according to claim 1, characterized in that, In step (6), the temperature of the blast drying oven is 80-100° C., and the heat treatment time is 15-20 minutes.

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