Hollow fiber composite membrane with double-gradient pore structure and method for preparing the same

Abstract

This invention provides a hollow fiber composite membrane with a dual-gradient pore structure and its preparation method, relating to the field of membrane separation technology. The hollow fiber composite membrane with a dual-gradient pore structure provided by this invention is composed of a dense layer with a low-gradient pore structure and a support layer with a high-gradient pore structure. The dense layer has a smaller average pore size and pore size gradient, improving the separation accuracy of the membrane material; the support layer has a larger pore size gradient, minimizing water permeation resistance. The dual-gradient pore structure truly solves the contradiction between membrane material separation accuracy and water permeation resistance. The preparation method of the hollow fiber composite membrane with a dual-gradient pore structure provided by this invention uses a three-layer spinneret to extrude and coat two casting solutions A and B with different thermodynamic states onto a reinforcing liner. By combining casting solutions with different thermodynamic states, a dual-gradient pore structure can be easily prepared, avoiding large-pore defects and providing membrane material developers with a wide adjustment window.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a hollow fiber composite membrane with a dual gradient pore structure and its preparation method. Background Technology

[0002] Polymer separation membranes inevitably experience a decrease in water flux during use due to the adsorption of pollutants on the membrane surface. Therefore, aeration and backwashing are necessary to clean the membrane surface. However, high-speed water or airflow can cause significant damage to the membrane fibers, frequently resulting in fiber breakage, leading to decreased water quality, shortened equipment lifespan, and increased replacement costs. The introduction of composite technology has greatly improved the mechanical strength of membrane materials, especially reinforced hollow fiber membranes, which virtually eliminate fiber breakage and have achieved a dominant position in global MBR engineering applications.

[0003] Membrane pore size and distribution, as well as pore structure shape, are all important factors affecting membrane flux and membrane fouling. From the mass transfer mechanism perspective of porous models, increasing pore size significantly improves membrane flux. For MBR processes, increasing membrane flux means reducing engineering costs and floor space. However, in practical applications, it has been found that microporous membranes with large pore sizes are more prone to pore closure and fouling than those with small pore sizes. An ideal membrane should have an asymmetric structure, meaning that the pore size of the surface membrane, which performs the separation function, is much smaller than the micropore size of the support layer. Contaminants are blocked outside the separation layer and cannot enter the interior of the membrane material, thus reducing irreversible fouling. The microporous structure of the support layer should have high pore size and porosity to reduce filtration resistance, while avoiding large pore defects such as finger-like structures to enhance the mechanical strength and pressure resistance of the membrane material.

[0004] Chinese patent (CN1099309C) discloses a method for preparing integral asymmetric polyethersulfone hollow membranes using a dry-jet-wet spinning method. The hollow fiber polyethersulfone membrane prepared by this method has a symmetrical polygonal cavity structure, and the cavity size gradually and stably changes from one side of the surface to the other side. Simply put, a controllable pore gradient is achieved through the composition of the casting solution and the film-forming process, so as to achieve the purpose of high porosity of the separation layer, no large pores in the support layer, and negligible filtration resistance.

[0005] Chinese patent (CN201410081610.9) discloses a superhydrophilic gradient pore hollow fiber membrane and its preparation method. The hollow fiber membrane prepared by this method has a micro-nano bead-like network structure with the pore size increasing from the outer surface layer to the inner surface layer along the radial cross section of the hollow fiber membrane, which further eliminates the inner skin layer and achieves minimal filtration resistance.

[0006] The methods described above all involve controlling the pore structure of homogeneous membranes to eliminate macropore defects. However, for reinforced hollow fiber membranes, structural control is much more challenging. This is because homogeneous membranes have internal and external coagulation baths, allowing for precise control of phase separation rates in different parts of the membrane wall. Reinforced hollow fiber membranes, on the other hand, only have an external coagulation bath, with a reinforcing liner on the inside. During membrane fabrication, as non-solvents penetrate, phase separation gradually progresses from the outside in, with the inner membrane wall solidifying last. Simultaneously, due to the denser outer separation layer and slower solvent / non-solvent exchange rate, the polymer dilute phase on the inner membrane wall continuously develops, generating macropore defects. Consequently, most commercially available hollow fiber composite membranes contain numerous finger-like pores. Under conventional process conditions, this problem is difficult to overcome.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The primary objective of this invention is to provide a hollow fiber composite membrane with a dual-gradient pore structure, exhibiting excellent water permeability and antifouling properties, in which the pore size increases slowly and then rapidly along the radial cross-section of the hollow fiber membrane from the outer surface to the inner surface.

[0009] A second objective of this invention is to provide a method for preparing a hollow fiber composite membrane with a dual gradient pore structure, in order to solve at least one of the above-mentioned problems.

[0010] In a first aspect, the present invention provides a hollow fiber composite membrane with a dual gradient pore structure, wherein the hollow fiber composite membrane comprises, from the outside to the inside, a low gradient pore structure dense layer, a high gradient pore structure support layer, and a reinforcing liner.

[0011] The pore size gradient Δd / Δl of the low-gradient pore structure dense layer is 0.02 to 0.05 μm / 10 μm, and the pore size gradient Δd / Δl of the high-gradient pore structure support layer is 0.1 to 1.0 μm / 10 μm.

[0012] The average pore size of the low-gradient pore structure dense layer is 0.02–0.45 μm, and the average pore size of the high-gradient pore structure support layer is 1–25 μm.

[0013] As a further technical solution, the thickness of the low-gradient porous structure dense layer is 10–50 μm;

[0014] The thickness of the high gradient pore structure support layer is 20–100 μm;

[0015] The thickness of the high gradient pore structure support layer is 2 to 5 times the thickness of the low gradient pore structure dense layer.

[0016] As a further technical solution, the porosity of the low-gradient pore structure dense layer is 55-65%;

[0017] The porosity of the high gradient pore structure support layer is 65-80%.

[0018] As a further technical solution, the reinforcing liner is a braided tube or a hook-and-loop tube;

[0019] The reinforcing lining material is nylon, acrylic, polypropylene, polyester, chlorofiber, vinylon, spandex, or aramid.

[0020] Secondly, the present invention provides a method for preparing a hollow fiber composite membrane, comprising the following steps:

[0021] Following the order from the outside to the inside, casting solution A, casting solution B, and reinforcing liner are co-extruded from three spinnerets, and then passed through an external coagulation bath and washed in sequence to prepare a hollow fiber composite membrane.

[0022] The casting solution A and casting solution B are mainly composed of polymer resin, pore-forming agent, non-solvent and solvent;

[0023] The three-dimensional solubility parameter δ1 of the casting solution A is 21–30 MPa. 1 / 2 The three-dimensional solubility parameter δ2 of the casting solution B is 25–34 MPa. 1 / 2 The difference between the three-dimensional solubility parameter δ2 of the casting solution B and the three-dimensional solubility parameter δ1 of the casting solution A is greater than or equal to 4 MPa. 1 / 2 .

[0024] As a further technical solution, the polymer resin includes polyvinylidene fluoride, polysulfone, polyethersulfone, polyacrylonitrile, or polyvinyl chloride;

[0025] The pore-forming agent includes PVP-K17, PVP-K30, PVP-K90, polyvinyl alcohol, polyethylene glycol, or cellulose acetate;

[0026] The non-solvent includes at least one of water, ethanol, ethylene glycol, glycerol, isopropanol, n-butanol, diethylene glycol, PEG200, or PEG400;

[0027] The solvent includes at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, or dimethyl sulfoxide.

[0028] As a further technical solution, in the casting solution A, the mass ratio of polymer resin, pore-forming agent, non-solvent and solvent is (14-22):(1-18):(5-18):(55-70);

[0029] In the casting solution B, the mass ratio of polymer resin, pore-forming agent, non-solvent and solvent is (12-18):(2-20):(10-20):(50-65).

[0030] As a further technical solution, the extrusion pressure is 0.1–0.3 MPa;

[0031] The temperature of the casting solution A is 40–70°C;

[0032] The temperature of the casting solution B is 40–90°C;

[0033] The temperature of the external coagulation bath is 30–80°C;

[0034] The cleaning process involves water washing.

[0035] As a further technical solution, the external coagulation bath is a mixture of solvent and water or water;

[0036] The solvent includes N-methylpyrrolidone, dimethylformamide, dimethylacetamide, or dimethyl sulfoxide.

[0037] As a further technical solution, the mass ratio of solvent to water in the mixed solution is less than or equal to 7:3.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. The dual-gradient pore structure hollow fiber composite membrane provided by this invention is composed of a dense layer with a low-gradient pore structure and a support layer with a high-gradient pore structure. The dense layer has a smaller average pore size and pore size gradient, which improves the separation accuracy of the membrane material; the support layer has a larger pore size gradient, which minimizes water permeation resistance. The dual-gradient pore structure truly solves the contradiction between membrane material separation accuracy and water permeation resistance.

[0040] 2. The method for preparing a hollow fiber composite membrane with a dual gradient pore structure provided by the present invention uses a three-layer spinneret to extrude and coat two casting solutions A and B with different thermodynamic states onto a reinforcing liner. By combining casting solutions with different thermodynamic states, a dual gradient pore structure can be easily prepared, avoiding large pore defects and providing a wide adjustment window for membrane material developers.

[0041] 3. The polymer resins of the low-gradient porous structure dense layer and the high-gradient porous structure support layer can be the same or different polymer resins. This allows for the convenient combination of polymer resins with different performance advantages (such as structural tunability, mechanical strength, chemical resistance, hydrophilicity, etc.) to prepare hollow fiber composite membranes without the need for double coating. The latter is not only cumbersome and time-consuming, but also prone to product defects. Attached Figure Description

[0042] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0043] Figure 1 Scanning electron microscope image of a hollow fiber composite membrane with a dual gradient pore structure;

[0044] Figure 2 This is a schematic diagram of a three-layer spinneret structure;

[0045] Figure 3 Scanning electron microscope image of the hollow fiber membrane provided for Comparative Example 1.

[0046] Icons: 1-Low gradient dense layer; 2-High gradient support layer; 3-Reinforced liner channel; 4-Casting solution B channel; 5-Casting solution A channel. Detailed Implementation

[0047] The embodiments and examples of the present invention will be described in detail below. However, those skilled in the art will understand that the following embodiments and examples are for illustrative purposes only and should not be considered as limiting the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0048] In a first aspect, the present invention provides a hollow fiber composite membrane with a dual gradient pore structure, wherein the hollow fiber composite membrane comprises, from the outside to the inside, a low gradient pore structure dense layer, a high gradient pore structure support layer, and a reinforcing liner.

[0049] The pore size of the hollow fiber membrane increases slowly and then rapidly along the radial cross-section from the outer surface to the inner surface. The pore size gradient Δd / Δl of the low-gradient pore structure dense layer can be, for example, but is not limited to, 0.02 μm / 10 μm, 0.03 μm / 10 μm, 0.04 μm / 10 μm, or 0.05 μm / 10 μm. The pore size gradient Δd / Δl of the high-gradient pore structure support layer can be, for example, but is not limited to, 0.1 μm / 10 μm, 0.2 μm / 10 μm, 0.4 μm / 10 μm, 0.6 μm / 10 μm, 0.8 μm / 10 μm, or 1.0 μm / 10 μm. The thickness of the high-gradient pore structure support layer is 2 to 50 times the pore size gradient of the low-gradient pore structure dense layer. Pore size gradient refers to the increase in membrane pore size per unit depth. The pore size gradient of the support layer in a high-gradient pore structure is less than twice that of the pore size gradient of the dense layer in a low-gradient pore structure. The overall structure of the membrane material is approximately a single-gradient pore structure, resulting in greater water permeation resistance. The pore size gradient of the support layer in a high-gradient pore structure is more than 50 times that of the dense layer in a low-gradient pore structure. The support layer has insufficient pressure resistance and cannot effectively provide support.

[0050] The average pore size of the low-gradient pore structure dense layer can be, for example, but not limited to, 0.02μm, 0.05μm, 0.1μm, 0.2μm, 0.3μm, 0.4μm or 0.45μm, and the average pore size of the high-gradient pore structure support layer can be 1μm, 5μm, 10μm, 15μm, 20μm or 25μm.

[0051] Gradient pore structures have become a research hotspot in the field of membrane materials in recent years. Current gradient pore polymer separation membranes are all single-gradient, meaning the pore size increases linearly from the outermost to the innermost surface along a single gradient in the radial cross-section of the hollow fiber membrane. Theoretically, this structure can resolve the contradiction between membrane material separation precision and water permeation resistance, maximizing pure water permeability. However, the inventors discovered during their research that adjusting the single-gradient membrane pore structure has significant limitations. Smaller pore sizes on the membrane material surface result in higher filtration precision, but also a smaller pore size gradient, and the adjustable range is limited, making it difficult to obtain membrane materials with both high filtration precision and a large pore size gradient. In other words, for high-precision polymer separation membranes, single-gradient pore structures cannot significantly reduce water permeation resistance.

[0052] This invention employs a dual-gradient pore structure with inner and outer layers. The outer dense layer has a smaller average pore size and pore size gradient, improving the separation accuracy of the membrane material; the middle support layer has a larger pore size gradient, minimizing water permeation resistance. This dual-gradient pore structure not only truly resolves the contradiction between membrane material separation accuracy and water permeation resistance, but also provides membrane material developers with a wide adjustment window.

[0053] In some preferred embodiments, the thickness of the low-gradient porous structure dense layer can be, for example, but not limited to, 10 μm, 20 μm, 30 μm, 40 μm or 50 μm;

[0054] The thickness of the high gradient pore structure support layer can be, for example, but not limited to, 20μm, 40μm, 60μm, 80μm or 100μm.

[0055] Low-gradient pore structure dense layer thickness is less than 10 μm, making it difficult to prepare a uniform and complete coating; thickness greater than 50 μm results in greater water permeation resistance. Preferably, the thickness of the high-gradient pore structure support layer is 2 to 5 times the thickness of the low-gradient pore structure dense layer.

[0056] In some preferred embodiments, the porosity of the low-gradient pore structure dense layer is 55-65%; and the porosity of the high-gradient pore structure support layer is 65-80%.

[0057] In some preferred embodiments, the reinforcing liner is a braided tube or a hook-and-loop tube;

[0058] The reinforcing lining material is nylon, acrylic, polypropylene, polyester, chlorofiber, vinylon, spandex, or aramid.

[0059] Secondly, the present invention provides a method for preparing a hollow fiber composite membrane, comprising the following steps:

[0060] Following the order from the outside to the inside, casting solution A, casting solution B, and reinforcing liner are co-extruded from three spinnerets, and then passed through an external coagulation bath and washed in sequence to prepare a hollow fiber composite membrane.

[0061] The casting solution A and casting solution B are mainly composed of polymer resin, pore-forming agent, non-solvent and solvent.

[0062] The three-dimensional solubility parameter δ1 of the casting solution A can be, for example, but not limited to, 21 MPa. 1 / 2 22MPa 1 / 2 24MPa 1 / 2 26MPa 1 / 2 28MPa 1 / 2 or 30MPa 1 / 2 The three-dimensional solubility parameter δ2 of the casting solution B can be, for example, but not limited to, 25 MPa. 1 / 2 27MPa 1 / 2 29MPa 1 / 2 31MPa 1 / 2 33MPa 1 / 2 or 34MPa 1 / 2 The difference between the three-dimensional solubility parameter δ2 of the casting solution B and the three-dimensional solubility parameter δ1 of the casting solution A is greater than or equal to 4 MPa.1 / 2 .

[0063] Structural control of reinforced hollow fiber membranes is more difficult than that of homogeneous membranes because reinforced hollow fiber membranes only have an external coagulation bath, with a reinforcing lining inside, lacking the technical means to control the phase separation behavior inside the casting solution. During their research, the inventors discovered that when two casting solutions with different thermodynamic states come into contact, their respective phase separation behaviors are influenced by the other casting solution. The non-solvent rapidly migrates from the side with higher concentration (the thermodynamically unstable casting solution) to the side with lower concentration (the thermodynamically stable casting solution). As a result, at the interface, the casting solution with a relatively stable initial thermodynamic state undergoes faster phase separation, resulting in larger membrane pores. Simultaneously, due to the rapid inflow of the non-solvent, the growth of the polymer dilute phase terminates promptly, preventing the formation of macropore defects. Conversely, the casting solution with a relatively unstable initial thermodynamic state undergoes slower phase separation, resulting in membrane pores much smaller than those of the same casting solution further from the interface. This leads to a membrane structure with a larger pore size gradient, and since its phase separation rate remains relatively high, macropore defects are also avoided.

[0064] The solubility parameter characterizes the polymer-solvent interaction. When the solubility parameters of the polymer and solvent are close, their compatibility is good, and the thermodynamic state of the casting solution tends to be stable. Conversely, when the solubility parameters of the polymer and solvent differ significantly, their compatibility is poor, the thermodynamic state of the casting solution tends to be unstable, and phase separation is more likely to occur. By adjusting the ratio of polymer, solvent, and non-solvent, casting solutions with different three-dimensional solubility parameters and different thermodynamic states can be obtained. By combining casting solutions with different thermodynamic states, dual-gradient pore structures can be easily prepared.

[0065] In some preferred embodiments, the polymer resin includes polyvinylidene fluoride (PVDF), polysulfone (PSF), polyethersulfone (PES), polyacrylonitrile (PAN), or polyvinyl chloride (PVC).

[0066] The pore-forming agent includes PVP-K17, PVP-K30, PVP-K90, polyvinyl alcohol (PVA), polyethylene glycol (PEG), or cellulose acetate (CA);

[0067] The non-solvent includes at least one of water, ethanol, ethylene glycol, glycerol, isopropanol, n-butanol, diethylene glycol, PEG200, or PEG400;

[0068] The solvent includes at least one of N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), or dimethyl sulfoxide (DMSO).

[0069] In some preferred embodiments, the mass ratio of polymer resin, pore-forming agent, non-solvent and solvent in the casting solution A can be, for example, but not limited to, 14:18:5:70, 16:12:10:65, 20:6:15:60 or 22:1:18:55.

[0070] In the casting solution B, the mass ratio of polymer resin, pore-forming agent, non-solvent and solvent is 12:20:10:65, 14:14:13:60, 16:8:16:55, 18:2:20:50.

[0071] In some preferred embodiments, the extrusion pressure is 0.1–0.3 MPa;

[0072] The temperature of the casting solution A is 40–70°C;

[0073] The temperature of the casting solution B is 40–90°C;

[0074] The temperature of the external coagulation bath is 30–80°C;

[0075] The cleaning is a water cleaning to remove residual solvent.

[0076] In some preferred embodiments, the external coagulation bath is a mixture of solvent and water or water;

[0077] The solvent includes N-methylpyrrolidone, dimethylformamide, dimethylacetamide, or dimethyl sulfoxide.

[0078] In some preferred embodiments, the mass ratio of solvent to water in the mixed solution is less than or equal to 7:3.

[0079] The present invention will be further illustrated below with specific embodiments and comparative examples. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0080] Example 1

[0081] (1) Preparation of casting solution: According to the ratio of PVDF resin: PVP-K30: PEG200: NMP = 15wt%: 14wt%: 10wt%: 61wt%, stir and dissolve at 90℃ for 12h, and then degas under vacuum for 12h to obtain casting solution A; According to the ratio of PVDF resin: PVP-K30: PEG200: NMP = 15wt%: 20wt%: 15wt%: 50wt%, stir and dissolve at 90℃ for 12h, and then degas under vacuum for 12h to obtain casting solution B.

[0082] (2) Wet spinning: Under a pressure of 0.1 MPa, casting solution A at 70°C, casting solution B at 90°C, and nylon braided tubing are co-extruded from a three-layer spinneret in an order from the outside to the inside (e.g., Figure 2 As shown, from the outside to the inside, there are casting solution channels A (5), B (4), and reinforcing liner channel 3. The membrane is immersed in an external coagulation bath (pure water) through a 50cm air section, with the coagulation bath temperature controlled at 80℃ to generate nascent fibers. These fibers are then rinsed in pure water for 48 hours and dried to obtain a hollow fiber composite membrane with a dual-gradient pore structure. Figure 1 As shown, the hollow fiber membrane consists of a low-gradient dense layer 1 and a high-gradient support layer 2 from the outside to the inside.

[0083] Comparative Example 1

[0084] (1) Preparation of casting solution: PVDF resin: PVP-K30: PEG200: NMP = 15wt%: 14wt%: 10wt%: 61wt%, stir and dissolve at 90℃ for 12h, and then degas under vacuum for 12h to obtain casting solution.

[0085] (2) Wet spinning: Under a pressure of 0.1 MPa, the casting solution at 70°C and the nylon braided tube are co-extruded from the double-layer spinneret in an order from the outside to the inside. After passing through a 50 cm air section, the mixture is immersed in an external coagulation bath (pure water). The temperature of the coagulation bath is controlled at 80°C to generate nascent fibers. The fibers are then rinsed in pure water for 48 hours and dried to obtain a hollow fiber composite membrane. Figure 3 As shown, the cross-section of the hollow fiber membrane is full of large pore defects.

[0086] Example 2

[0087] (1) Preparation of casting solution: PSF resin: PVP-K30: PEG400: DMAc = 14wt%: 18wt%: 13wt%: 55wt% was stirred and dissolved at 70℃ for 8h, and then degassed under vacuum for 8h to obtain casting solution A; PSF resin: PVP-K30: PEG400: ethanol: DMAc = 13wt%: 18wt%: 13wt%: 2wt%: 54wt% was stirred and dissolved at 70℃ for 8h, and then degassed under vacuum for 8h to obtain casting solution B.

[0088] (2) Wet spinning: Under a pressure of 0.2 MPa, casting solution A at 50°C, casting solution B at 40°C and acrylic hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 30 cm air section, the fibers are immersed in an external coagulation bath (5 wt% DMAc aqueous solution). The temperature of the coagulation bath is controlled at 80°C to generate nascent fibers. The fibers are then rinsed in pure water for 48 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0089] Example 3

[0090] (1) Preparation of casting solution: PES resin: PVA: diethylene glycol: DMF = 15wt%: 14wt%: 11wt%: 60wt%, stir and dissolve at 70℃ for 12h, then degas under vacuum for 12h to obtain casting solution A; PES resin: PVA: diethylene glycol: water: DMF = 12wt%: 14wt%: 13wt%: 1wt%: 60wt%, stir and dissolve at 70℃ for 12h, then degas under vacuum for 12h to obtain casting solution B.

[0091] (2) Wet spinning: Under a pressure of 0.2 MPa, casting solution A at 40°C, casting solution B at 50°C and polypropylene braided tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 20 cm air section, the fibers are immersed in an external coagulation bath (15 wt% DMF aqueous solution). The temperature of the coagulation bath is controlled at 65°C to generate nascent fibers. The fibers are then rinsed in pure water for 24 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0092] Example 4

[0093] (1) Preparation of casting solution: According to the ratio of PAN resin:PVA:n-butanol:DMSO = 16wt%:12wt%:15wt%:57wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution A; According to the ratio of PAN resin:PVA:n-butanol:DMSO = 14wt%:16wt%:15wt%:55wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution B.

[0094] (2) Wet spinning: Under a pressure of 0.2 MPa, casting solution A at 70°C, casting solution B at 80°C, and polyester hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 15 cm air section, the fibers are immersed in an external coagulation bath (25 wt% DMSO aqueous solution). The temperature of the coagulation bath is controlled at 60°C to generate nascent fibers. The fibers are then rinsed in pure water for 36 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0095] Example 5

[0096] (1) Preparation of casting solution: According to the ratio of PVC resin: CA: isopropanol: NMP = 17wt%: 1wt%: 18wt%: 64wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution A; according to the ratio of PVC resin: CA: isopropanol: NMP = 14wt%: 2wt%: 20wt%: 64wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution B.

[0097] (2) Wet spinning: Under a pressure of 0.2 MPa, casting solution A at 70°C, casting solution B at 80°C and chlorofiber braided tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 10 cm air section, the mixture is immersed in an external coagulation bath (40 wt% NMP aqueous solution). The temperature of the coagulation bath is controlled at 55°C to generate nascent fibers. The fibers are then rinsed in pure water for 36 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0098] Example 6

[0099] (1) Preparation of casting solution: PVDF resin: CA: glycerol: DMAc = 18wt%: 1wt%: 16wt%: 65wt%, stir and dissolve at 80℃ for 12h, then degas under vacuum for 12h to obtain casting solution A; PVDF resin: CA: glycerol: DMAc = 16.5wt%: 2wt%: 16.5wt%: 65wt%, stir and dissolve at 80℃ for 12h, then degas under vacuum for 12h to obtain casting solution B.

[0100] (2) Wet spinning: Under a pressure of 0.3 MPa, casting solution A at 70°C, casting solution B at 80°C and vinylon hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 10 cm air section, the mixture is immersed in an external coagulation bath (70 wt% DMAc aqueous solution). The temperature of the coagulation bath is controlled at 55°C to generate nascent fibers. The fibers are then rinsed in pure water for 36 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0101] Example 7

[0102] (1) Preparation of casting solution: According to the ratio of PVC resin:PEG:ethylene glycol:DMF = 17wt%:10wt%:10wt%:63wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution A; according to the ratio of PVC resin:PEG:ethylene glycol:DMF = 15wt%:14wt%:12wt%:59wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution B.

[0103] (2) Wet spinning: Under a pressure of 0.2 MPa, casting solution A at 50°C, casting solution B at 55°C and aramid hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 10 cm air section, the fibers are immersed in an external coagulation bath (15 wt% DMF aqueous solution). The temperature of the coagulation bath is controlled at 50°C to generate nascent fibers. The fibers are then rinsed in pure water for 48 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0104] Example 8

[0105] (1) Preparation of casting solution: PES resin: PVP-K17: PEG200: DMSO = 18wt%: 12wt%: 12wt%: 58wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution A; PES resin: PVP-K17: PEG200: DMSO = 15wt%: 10wt%: 12wt%: 63wt%, stir and dissolve at 70℃ for 12h, and then degas under vacuum for 12h to obtain casting solution B.

[0106] (2) Wet spinning: Under a pressure of 0.25 MPa, casting solution A at 50°C, casting solution B at 55°C and aramid hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 10 cm air section, the fibers are immersed in an external coagulation bath (20 wt% DMSO aqueous solution). The temperature of the coagulation bath is controlled at 50°C to generate nascent fibers. The fibers are then rinsed in pure water for 48 h and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0107] Example 9

[0108] (1) Preparation of casting solution: PAN resin: PVP-K17: PEG400: NMP = 20wt%: 9wt%: 8wt%: 63wt%, stir and dissolve at 70℃ for 12h, then degas under vacuum for 12h to obtain casting solution A; PAN resin: PVP-K17: PEG400: NMP = 18wt%: 14wt%: 10wt%: 58wt%, stir and dissolve at 70℃ for 12h, then degas under vacuum for 12h to obtain casting solution B.

[0109] (2) Wet spinning: Under a pressure of 0.25 MPa, casting solution A at 70°C, casting solution B at 70°C, and acrylic braided tube are extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 10 cm air section, the mixture is immersed in an external coagulation bath (pure water). The temperature of the coagulation bath is controlled at 40°C to generate nascent fibers. Then, the fibers are rinsed in pure water for 48 hours and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0110] Example 10

[0111] (1) Preparation of casting solution: PSF resin: PVP-K90: diethylene glycol: DMAc = 22wt%: 3wt%: 5wt%: 70wt%, stir and dissolve at 70℃ for 24h, and then degas under vacuum for 4h to obtain casting solution A; PSF resin: PVP-K17: diethylene glycol: DMAc = 14wt%: 11wt%: 10wt%: 65wt%, stir and dissolve at 70℃ for 24h, and then degas under vacuum for 24h to obtain casting solution B.

[0112] (2) Wet spinning: Under a pressure of 0.3 MPa, casting solution A at 70°C, casting solution B at 90°C and polyester hook tube are co-extruded from the three-layer spinneret in the order from the outside to the inside. After passing through a 2 cm air section, the fibers are immersed in an external coagulation bath (pure water). The temperature of the coagulation bath is controlled at 30°C to generate nascent fibers. Then, the fibers are rinsed in pure water for 48 hours and dried to obtain a hollow fiber composite membrane with a double gradient pore structure.

[0113] The performance test results of the hollow fiber composite membranes with dual gradient pore structures obtained in the above embodiments are shown in Table 1.

[0114] Table 1. Performance test results of hollow fiber composite membranes with dual gradient pore structures in different embodiments.

[0115]

[0116]

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing a hollow fiber composite membrane, characterized by, The method comprises the following steps: In the order from outside to inside, the casting solution A, the casting solution B and the reinforced inner liner are co-extruded from a three-layer spinneret, and then sequentially pass through an outer coagulation bath and cleaning to prepare the hollow fiber composite membrane; The casting solution A and the casting solution B mainly consist of a polymer resin, a pore-forming agent, a non-solvent and a solvent; The three-dimensional solubility parameter δ1 of the casting solution A is 21-30 MPa 1 / 2 The three-dimensional solubility parameter δ2 of the casting solution B is 25-34 MPa 1 / 2 The difference between the three-dimensional solubility parameter δ2 of the casting solution B and the three-dimensional solubility parameter δ1 of the casting solution A is greater than or equal to 4 MPa 1 / 2 .

2. The production method according to claim 1, characterized by, The polymer resin comprises polyvinylidene fluoride, polysulfone, polyethersulfone, polyacrylonitrile or polyvinyl chloride; The pore-forming agent comprises PVP-K17, PVP-K30, PVP-K90, polyvinyl alcohol, polyethylene glycol or cellulose acetate; The non-solvent comprises at least one of water, ethanol, ethylene glycol, glycerol, isopropyl alcohol, n-butyl alcohol, diethylene glycol, PEG200 or PEG400; The solvent comprises at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide or dimethyl sulfoxide.

3. The preparation method according to claim 1, characterized in that, In the casting solution A, the mass ratio of the polymer resin, the pore-forming agent, the non-solvent and the solvent is (14-22):(1-18):(5-18):(55-70); In the casting solution B, the mass ratio of the polymer resin, the pore-forming agent, the non-solvent and the solvent is (12-18):(2-20):(10-20):(50-65).

4. The method of claim 1, wherein, The extrusion pressure is 0.1-0.3 MPa; The temperature of the casting solution A is 40-70℃; The temperature of the casting solution B is 40-90℃; The temperature of the outer coagulation bath is 30-80℃; The cleaning is water cleaning.

5. The preparation method according to claim 1, characterized in that, The outer coagulation bath is a mixed solution of solvent and water or water; The solvent comprises N-methylpyrrolidone, dimethylformamide, dimethylacetamide or dimethyl sulfoxide.

6. The preparation method according to claim 5, characterized in that, In the mixed solution, the mass ratio of the solvent to water is less than or equal to 7:3.

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