A wear-resistant and anti-pollution double-layer hollow fiber membrane, a preparation method and application thereof

Abstract

The application provides a preparation method of a double-layer hollow fiber membrane, which comprises adding inorganic nanoparticles and foaming particles in an outer layer spinning solution, adding nanosheets in an inner layer spinning solution, and then extruding simultaneously through wet spinning to obtain the double-layer hollow fiber membrane after coagulation. In the application, the inorganic nanoparticles are uniformly dispersed in the outer layer spinning solution to improve the wear resistance of the membrane, the foaming particles, especially the nanoparticles, are added to cooperate with an acidic gel bath to generate nanobubbles, so that the open porosity of the outer layer surface of the membrane is increased, the inner layer membrane is effectively shortened in the transmission path by inserting two-dimensional nanosheets, the mass transfer resistance is reduced, the flux of the double-layer hollow fiber membrane itself is ensured to be high, and the bonding force between the inner layer and the outer layer is improved. The double-layer hollow fiber membrane prepared by the application has the advantages of wear resistance, anti-pollution and difficulty in delamination, while the membrane flux is suitable, and the operation is simple and easy to scale up.

Description

Technical Field

[0001] This invention relates to hollow fiber membranes, and particularly to a wear-resistant and pollution-resistant double-layer hollow fiber membrane, as well as a method for preparing the hollow fiber membrane and its application. Background Technology

[0002] MBR technology boasts advantages such as high solid-liquid separation efficiency, small footprint, and low sludge treatment costs, making it widely used in water supply, industrial wastewater, domestic sewage treatment, and wastewater resource recovery. The membrane is the core component of an MBR, and the properties of the membrane material play a crucial role in its operation. Improving membrane lifespan and reducing replacement frequency are key drivers of MBR technology development. Among these factors, physical wear and membrane fouling significantly impact membrane lifespan.

[0003] Numerous studies have focused on improving membrane fouling by optimizing the properties of the membrane material itself. Blending multifunctional nanomaterials is a promising approach. Chinese invention patent application CN 105080363A discloses a high-strength, high-toughness, and high-flux PVDF ultrafiltration membrane modified with nanoparticles. This method utilizes TiO2 and SiO2 nanoparticles to synergistically strengthen and toughen PVDF, while simultaneously improving hydrophilicity and increasing membrane flux. Chinese invention patent application CN 113244778A discloses a method for preparing a high-performance ultrafiltration membrane. This method involves blending nanoparticles that generate nanobubbles in a casting solution. During membrane formation, these nanobubbles overflow from the membrane surface, creating more openings and increasing surface porosity. Simultaneously, the hydrophobic and oleophobic properties of the nanobubbles control the phase transformation exchange rate, achieving a membrane surface with small pore size and high porosity.

[0004] On the other hand, compared with single-layer hollow fiber membranes, bilayer hollow fiber membranes prepared by one-step co-extrusion can retain the structure and properties of each layer within the membrane, thus achieving the advantages of both single-layer and composite membranes. However, delamination is a common defect in bilayer membranes. Chinese invention patent application CN 110180404 A discloses a novel bilayer hollow fiber membrane with a hydrophilic inner layer and a hydrophobic outer layer, using PVDF and polyvinylidene fluoride-hexafluoropropylene, which have similar unit structures, as the polymers for the inner and outer layers, respectively, in an effort to prevent delamination. However, the wear resistance of the bilayer membrane has not been investigated.

[0005] Currently, there is a lack of double-layer hollow fiber membranes in this field that can simultaneously possess wear resistance, pollution resistance, and high flux properties while being less prone to delamination. Summary of the Invention

[0006] The purpose of this invention is to provide a double-layer hollow fiber membrane that is not easily delaminated and has wear-resistant and anti-fouling properties.

[0007] The present invention involves adding inorganic nanoparticles and foaming particles to a high-concentration polymer as the outer spinning solution to obtain an outer membrane with a uniform microporous structure and high porosity, which inhibits contaminant clogging of the membrane pores while exhibiting good wear resistance. The inner layer uses a low-concentration polymer with inserted two-dimensional nanosheets to prepare an inner membrane with a loose, porous structure. The introduction of two-dimensional nanosheets effectively shortens the transport path, reduces mass transfer resistance, and solves the problem of low flux in bilayer membranes. Simultaneously, the non-solvent permeated from the outer layer can rapidly diffuse through the inner layer, preventing accumulation at the interface between the inner and outer layers, improving the bonding force between them, thus reducing the likelihood of delamination and achieving good membrane flux.

[0008] Based on this, the present invention provides a method for preparing a double-layer hollow fiber membrane, the method comprising the following steps:

[0009] (1) Prepare the outer spinning solution

[0010] Weigh 18-22 parts of polymer, 5-15 parts of pore-forming agent, 0.2-1.0 parts of inorganic nanoparticles, 0.1-0.7 parts of foaming particles and 61-76 parts of organic solvent by weight, mix thoroughly until uniform, degas and set aside to obtain the outer spinning solution;

[0011] (2) Preparation of inner spinning solution

[0012] Weigh 14-18 parts of polymer, 5-15 parts of pore-forming agent, 0.2-1.0 parts of nanosheets and 66-78 parts of organic solvent by weight, mix thoroughly until uniform, degas and set aside to obtain inner layer spinning solution;

[0013] (3) Spinning

[0014] By wet spinning, the outer spinning solution, the inner spinning solution, and the inner liner are simultaneously extruded from the outer channel, middle channel, and inner channel of a three-channel nozzle, respectively, and then enter an acidic gel bath. After the membrane fibers solidify, they are washed to obtain a double-layer hollow fiber membrane.

[0015] In this invention, the polymer is selected from polyvinylidene fluoride, polysulfone, polyethersulfone, or polyvinyl chloride.

[0016] If the amount of polymer used in step (1) is less than 18 parts, it will cause the outer layer of the bilayer membrane to form a macroporous structure, and pollutants will easily enter the membrane pores; if it is more than 22 parts, the viscosity of the liquid is too high and it is not suitable for spinning.

[0017] If the amount of polymer used in step (2) is less than 14 parts, it will result in low bilayer membrane strength and bubble point; if it is more than 18 parts, it will affect membrane flux.

[0018] In this invention, the pore-forming agent is selected from polyvinylpyrrolidone and / or polyethylene glycol.

[0019] If the amount of pore-forming agent is less than 5 parts, it will affect the membrane flux; if it is more than 15 parts, it will increase the viscosity of the feed solution, which will also affect the flux.

[0020] In this invention, the inorganic nanoparticles are selected from hydrophilic nano-silica, nano-titanium dioxide, nano-clay, nano-iron oxide, nano-alumina, nano-zirconia, or nano-cuprous oxide; the particle size of the inorganic nanoparticles is 1-50 nm.

[0021] In this invention, the foamed particles are micron-sized particles or nano-sized particles. The micron-sized particles are selected from calcium carbonate, magnesium carbonate, barium carbonate, sodium carbonate, or sodium bicarbonate particles with an average particle size of 1 μm, and the nano-sized particles are selected from nano-calcium carbonate, nano-magnesium carbonate, or nano-barium carbonate particles with an average particle size of 1-50 nm.

[0022] The organic solvent used in this invention is one or a mixture of two of the following: dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and triethyl phosphate.

[0023] Those skilled in the art can also choose to form a mixed solvent from two organic solvents based on their existing knowledge. In this case, the two organic solvents can usually be used in any proportion or adjusted according to common knowledge.

[0024] According to a preferred embodiment, the nanosheet is a two-dimensional ZSM-5 molecular sieve nanosheet or a graphene oxide nanosheet.

[0025] The two-dimensional ZSM-5 nanosheets can be prepared by the following method: 1.224 g NaOH, 3.7 g C... 22-6-6 The surfactant and 36.7 mL of deionized water were stirred evenly at room temperature to form solution A. 0.34 g of aluminum sulfate octahydrate was dissolved in sulfuric acid solution and stirred evenly at room temperature to form solution B. Solution B was added dropwise to solution A under rapid stirring and stirred in a 60 °C water bath for 1 h. After the solution cooled to room temperature, 10.625 g of tetraethyl orthosilicate was added and stirred for another 1 h in a 60 °C water bath. The resulting gel was crystallized in a 150 °C reactor for 5 days, and after centrifugation, washing, and drying, it was calcined in a muffle furnace at 550 °C for 4 h to obtain two-dimensional ZSM-5 nanosheets.

[0026] The graphene nanosheets are commercially available, such as XF021 sold by Nanjing Xianfeng Nanomaterials Technology Co., Ltd., with a thickness of 3-10 nm.

[0027] In this invention, the acidic gel bath is an aqueous solution of hydrochloric acid, sulfuric acid, or citric acid with a pH of 1-5.

[0028] In this invention, an acidic gel bath is used to allow the foaming particles in the outer spinning solution to react with the acid and generate carbon dioxide bubbles. The overflow of these bubbles creates more pores on the membrane surface and within the membrane itself, effectively increasing the porosity. If the pH value of the acidic gel bath is below 1, the excessive acidity is detrimental to the membrane material and the equipment. If it is above 5, insufficient acidity leads to a slow bubble generation rate, affecting the achievement of a good porous structure on the membrane surface and within the membrane.

[0029] In this invention, the lining is a polyester fiber crocheted rope or braided rope with an outer diameter of 1.5-1.8 mm.

[0030] According to a preferred embodiment, in step (3), the temperature of the outer and inner spinning solutions of the three-channel nozzle is 60-80°C, the temperature of the acidic gel bath is 60-80°C, the cleaning is performed by using room temperature pure water, and the winding wheel linear speed of the wet spinning equipment is 15-35 m / min.

[0031] Based on this, the present invention also provides the application of hollow fiber membranes obtained by the above preparation method in water purification.

[0032] This invention uniformly disperses inorganic nanoparticles in the outer spinning solution to improve the membrane's wear resistance. Adding foaming particles, particularly nanoparticles, in conjunction with an acidic gel bath generates nanobubbles, thereby increasing the porosity of the outer membrane surface. Simultaneously, these nanobubbles control the phase transformation exchange rate, resulting in an outer layer pore size below 100 nm with a uniform pore size distribution. This pore structure and distribution effectively inhibit contaminant clogging of the membrane pores. The inner membrane, through the insertion of two-dimensional nanosheets, effectively shortens the transport path, reduces mass transfer resistance, and ensures the high flux of the double-layer hollow fiber membrane itself. Furthermore, non-solvents permeating from the outer layer can rapidly diffuse through the inner layer, preventing accumulation at the interface between the inner and outer layers, improving the bonding force between them, and preventing delamination. The compatibility between the inner and outer layer materials also prevents delamination.

[0033] Therefore, the double-layer hollow fiber membrane prepared by this invention has the advantages of being wear-resistant, pollution-resistant, and not easily delaminated, while ensuring a suitable membrane flux and being simple to operate and easy to prepare on a large scale. Attached Figure Description

[0034] Figure 1 This is an electron microscope image of the outer surface of the double-layer hollow fiber membrane in Example 1.

[0035] Figure 2 This is an electron microscope image of the outer surface of the double-layer hollow fiber membrane in Comparative Example 2. Detailed Implementation

[0036] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0037] In this invention, unless otherwise specified, “%” for concentration refers to mass percentage, “parts” refers to weight parts, and “:” refers to mass ratio.

[0038] In this invention, the pore size of the outer surface of the hollow fiber membrane is obtained by processing the surface electron micrograph using Nano Measurer software.

[0039] The pure water flux was obtained by testing the permeate flow rate per unit time per unit membrane area under conditions of 0.1 MPa and 25℃ (using the average value of test results from three or more membrane fibers with an effective length of 30 cm).

[0040] The initial bubble point is obtained by recording the minimum pressure required for compressed air to pass through the membrane surface wetted by the liquid; the initial bubble point indirectly reflects the maximum pore size of the membrane fibers.

[0041] Example 1

[0042] Add 1.224g NaOH and 3.7g C 22-6-6 The surfactant and 36.7 mL of deionized water were stirred evenly at room temperature to form solution A. 0.34 g of aluminum sulfate octahydrate was dissolved in sulfuric acid solution and stirred evenly at room temperature to form solution B. Solution B was added dropwise to solution A under rapid stirring and stirred in a 60 °C water bath for 1 h. After the solution cooled to room temperature, 10.625 g of tetraethyl orthosilicate was added and stirred for another 1 h in a 60 °C water bath. The resulting gel was crystallized in a 150 °C reactor for 5 days, and after centrifugation, washing, and drying, it was calcined in a muffle furnace at 550 °C for 4 h to obtain two-dimensional ZSM-5 nanosheets.

[0043] Take 20 parts PVDF, 15 parts PVP K30, 0.5 parts silica, 0.3 parts nano calcium carbonate and 64.2 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0044] Measure 15 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 69.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0045] The outer and inner spinning solutions are deaerated. The outer and inner spinning solutions, along with a 1.8mm outer diameter crocheted rope liner, enter the three-channel nozzle of the spinning equipment through independent channels: the outer channel, middle channel, and inner channel. They are simultaneously extruded at 70°C. The resulting membrane fibers are then placed in a 70°C, pH 3 hydrochloric acid aqueous solution gel bath. After gel separation, the membrane fibers are cleaned in a room-temperature pure water washing tank. During the spinning process, the winding wheel's linear speed is 25 m / min.

[0046] The outer surface pore diameter was measured to be approximately 60-70 nm, the pure water flux was 3200 LMH, and the initial bubble point was approximately 0.2-0.3 MPa.

[0047] The obtained membrane was placed in a friction machine and the membrane fibers were reciprocated and rubbed at a speed of 120 times / min for 30 minutes. Then, the wear on the surface of the membrane fibers was observed by electron microscopy. The results are as follows: Figure 1 As shown in the figure, the membrane fibers are basically intact, indicating good wear resistance.

[0048] Example 2

[0049] Take 20 parts PVDF, 15 parts PVP K30, 0.5 parts silica, 0.3 parts nano calcium carbonate and 64.2 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0050] Measure 15 parts PVDF, 15 parts PVP K30, 0.8 parts two-dimensional ZSM-5 nanosheets and 69.2 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0051] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0052] The outer surface pore diameter was measured to be approximately 60-70 nm, the pure water flux was 3500 LMH, and the initial bubble point was approximately 0.2-0.3 MPa.

[0053] Example 3

[0054] Take 20 parts PVDF, 15 parts PVP K30, 0.5 parts silica, 0.1 parts nano calcium carbonate and 64.4 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0055] Measure 15 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 69.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0056] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0057] The outer surface pore diameter was measured to be approximately 60-70 nm, the pure water flux was 3000 LMH, and the initial bubble point was approximately 0.2-0.3 MPa.

[0058] Example 4

[0059] Measure 22 parts PVDF, 15 parts PVP K30, 0.5 parts silica, 0.3 parts nano calcium carbonate and 62.2 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0060] Measure 15 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 69.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0061] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0062] The outer surface pore diameter was measured to be approximately 50-60 nm, the pure water flux was 2800 LMH, and the initial bubble point was approximately 0.26-0.33 MPa.

[0063] Example 5

[0064] Take 18 parts PVDF, 15 parts PVP K30, 0.5 parts silica, 0.3 parts nano calcium carbonate and 66.2 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0065] Measure 14 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 70.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0066] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0067] The outer surface pore diameter was measured to be approximately 100-120 nm, the pure water flux was 3700 LMH, and the initial bubble point was approximately 0.06-0.12 MPa.

[0068] Compare with Example 1

[0069] Take 20 parts PVDF, 15 parts PVP K30 and 65 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0070] Take 15 parts PVDF, 10 parts PVP K30 and 75 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the inner layer spinning solution.

[0071] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0072] The outer surface pore diameter was measured to be approximately 70-80 nm, the pure water flux was 1100 LMH, and the initial bubble point was approximately 0.1-0.2 MPa.

[0073] Compare with Example 2

[0074] Take 17 parts PVDF, 15 parts PVP K30 and 68 parts DMAC, mix them thoroughly and stir evenly, degas and prepare the outer spinning solution.

[0075] Take 20 parts PVDF, 15 parts PVP K30, 6 parts PEG8000 and 59 parts DMAC, mix them thoroughly and stir evenly, degas them, and prepare the inner layer spinning solution.

[0076] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0077] The obtained membrane was placed in a friction machine and the membrane fibers were reciprocated and rubbed at a speed of 120 times / min for 30 minutes. Then, the wear on the surface of the membrane fibers was observed by electron microscopy. The results are as follows: Figure 2 As shown.

[0078] As can be seen in the figure, there are a lot of cracks on the surface of the membrane fiber material, indicating that the membrane fiber material has poor wear resistance.

[0079] Furthermore, during the experiment, it was found that in order to prepare samples for electron microscopy observation, the inner liner of the membrane fibers that had undergone reciprocating friction treatment needed to be removed before subsequent observation. In the sample obtained in Example 1, the inner and outer layers were firmly bonded during the removal of the inner liner, and no material delamination occurred. However, in the sample obtained in this control example, when the inner liner was removed using the same method, obvious delamination was observed between the inner and outer layers, with the outer layer material detaching and exhibiting poor adhesion. The reason for this situation in the sample obtained in this control example may be that the compositions of the spinning solutions for the inner and outer layers are different, and the concentration of the spinning solution for the inner layer is greater than that for the outer layer. Therefore, a dense layer forms on the outer surface of the inner layer during preparation, increasing resistance and preventing the non-solvents permeating from the outer layer from quickly diffusing through the inner layer. This accumulation at the interface between the inner and outer layers ultimately prevents the two layers from bonding tightly.

[0080] Compare with Example 3

[0081] Take 20 parts PVDF, 15 parts PVP K30, 0.5 parts silica and 64.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the outer spinning solution.

[0082] Measure 15 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 69.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0083] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0084] The outer surface pore diameter was measured to be approximately 70-80 nm, the pure water flux was 2300 LMH, and the initial bubble point was approximately 0.2-0.3 MPa.

[0085] Compare with Example 4

[0086] Take 20 parts PVDF, 15 parts PVP K30, 0.3 parts nano calcium carbonate and 64.7 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the outer layer spinning solution.

[0087] Measure 15 parts PVDF, 15 parts PVP K30, 0.5 parts two-dimensional ZSM-5 nanosheets and 69.5 parts DMAC, mix them thoroughly and stir evenly, degas, and prepare the inner layer spinning solution.

[0088] Spinning was performed using the same spinning method as in Example 1 to obtain a double-layer hollow fiber membrane.

[0089] The outer surface pore diameter was measured to be approximately 60-70 nm, the pure water flux was 2900 LMH, and the initial bubble point was approximately 0.2-0.3 MPa.

[0090] This invention improves the composition of the outer spinning solution to prepare an outer membrane material with good surface porosity and uniform pore size distribution, thereby inhibiting the clogging of membrane pores by contaminants. Meanwhile, the two-dimensional nanosheets in the inner membrane material effectively shorten the transport path, reduce mass transfer resistance, and ensure the high flux of the bilayer hollow fiber membrane itself. Furthermore, the non-solvent permeated from the outer layer can rapidly diffuse through the inner layer, preventing accumulation at the interface between the inner and outer layers, improving the bonding force between the inner and outer layers, enhancing the membrane's strength and wear resistance, and ensuring good membrane flux.

Claims

1. A method for preparing a double-layer hollow fiber membrane, the method comprising the following steps: (1) Prepare the outer spinning solution Weigh out 18-22 parts by weight of polymer, 5-15 parts by pore-forming agent, 0.2-1.0 parts by weight of inorganic nanoparticles, 0.1-0.7 parts by weight of foaming particles, and 61-76 parts by weight of organic solvent. Mix thoroughly until homogeneous, degas, and set aside to obtain the outer spinning solution. The foaming particles are nanoparticles selected from nano-calcium carbonate, nano-magnesium carbonate, or nano-barium carbonate particles with an average particle size of 1-50 nm. The inorganic nanoparticles are selected from hydrophilic nano-silica, nano-titanium dioxide, nano-clay, nano-iron oxide, nano-alumina, nano-zirconium oxide, or nano-cuprous oxide. (2) Prepare the inner spinning solution Weigh 14-18 parts of polymer, 5-15 parts of pore-forming agent, 0.2-1.0 parts of nanosheets and 66-78 parts of organic solvent by weight, mix thoroughly until uniform, degas and set aside to obtain inner layer spinning solution; the nanosheets are two-dimensional ZSM-5 molecular sieve nanosheets or graphene oxide nanosheets. (3) Spinning By wet spinning, the outer spinning solution, the inner spinning solution, and the inner liner are simultaneously extruded from the outer channel, middle channel, and inner channel of a three-channel nozzle, respectively, and then enter an acidic gel bath. After the membrane fibers solidify, they are washed to obtain a double-layer hollow fiber membrane.

2. The preparation method according to claim 1, characterized in that... The polymer is selected from polyvinylidene fluoride, polysulfone, polyethersulfone, or polyvinyl chloride; the porogen is selected from polyvinylpyrrolidone and / or polyethylene glycol.

3. The preparation method according to claim 1, characterized in that... The organic solvent is one or a mixture of two of the following: dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and triethyl phosphate.

4. The preparation method according to claim 1, characterized in that... The acidic gel bath is an aqueous solution of hydrochloric acid, sulfuric acid, or citric acid with a pH of 1-5.

5. The preparation method according to claim 1, characterized in that... The lining is made of polyester fiber crocheted or braided rope with an outer diameter of 1.5-1.8 mm.

6. The preparation method according to claim 1, characterized in that... In step (3), the temperature of the outer and inner spinning solutions of the three-channel nozzle is 60-80℃, the temperature of the acidic gel bath is 60-80℃, the cleaning is carried out by cleaning with room temperature pure water, and the winding wheel linear speed of the wet spinning equipment is 15-35m / min.

7. The application of the hollow fiber membrane obtained by the preparation method according to any one of claims 1-6 in water purification.

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