Preparation method of one-step formed hollow fiber nanofiltration membrane

Abstract

The invention belongs to the field of nanofiltration membranes, and provides a preparation method of a one-step formed hollow fiber nanofiltration membrane, which comprises the following steps: respectively extruding a membrane casting solution prepared by mixing polyether sulfone, an additive and a solvent and a core solution prepared by water and the solvent from a membrane casting solution port and a core solution port of a spinning nozzle to form a fiber tubular liquid membrane; the liquid membrane passes through an air gap and then enters a gel tank to be subjected to phase inversion curing to form a membrane, after rinsing is completed, shrinkage and sizing are conducted on line, rolling is conducted, and the hollow fiber nanofiltration membrane is obtained. According to the invention, a shrinkage process is synchronously carried out in the spinning process of the hollow fiber membrane, and shrinkage is directly carried out on the surface of the polyether sulfone hollow fiber membrane to form a nanofiltration separation layer, so that the stripping problem is fundamentally avoided, the performance stability of a membrane product is enhanced, and the membrane product has good acid and alkali resistance, can be recoiled, has a thin separation layer and high flux, and is suitable for large-scale production. The method is simple to operate, easy to control, simple in equipment requirement, small in investment, easy for industrial production and suitable for large-scale popularization.

Description

Technical Field

[0001] The invention belongs to the field of nanofiltration membranes, and in particular relates to a method for preparing a one-step formed hollow fiber nanofiltration membrane. Background Art

[0002] Nanofiltration membranes are a new type of membrane separation technology that has developed rapidly in recent years. The current mainstream preparation methods include phase inversion and interfacial polymerization. Among them, the phase inversion method is to immerse the polymer homogeneous casting liquid in a non-solvent to induce thermodynamically unstable state solidification to form a membrane structure. The operation is simple, but it is difficult to accurately control the dense cortex with a pore size of 2-10nm. If it is improved by adjusting the solid content or additives, it is easy to form an overly thick cortex, resulting in a decrease in flux, and it is difficult to balance the process and performance. The interfacial polymerization method uses highly active monomers to undergo polymerization reactions at the interface of mutually incompatible solvents to form a dense layer with a pore size of about 2-10nm and a thickness of only 50-200nm on a porous support. It is currently the most effective and widely used preparation method; however, this method relies on repeated immersion operations in multi-property solutions. The process is cumbersome, the equipment requirements are high, and the investment cost is high, which restricts its industrial promotion. The separation layer is prone to peeling at the interface between the two phases, affecting the stability and separation efficiency of the membrane. These preparation methods each have their own limitations, which have promoted the exploration of other technologies (such as layer-by-layer self-assembly), but phase transformation and interfacial polymerization still dominate.

[0003] Chinese invention patent CN112915804A discloses a one-step method for preparing a hollow fiber nanofiltration membrane, comprising the following steps: 1) preparing a casting solution comprising a mixture of 19.3-25.6% polyacrylonitrile, 0.5-15.5% pore size regulator, and the balance solvent; 2) preparing a core solution comprising a mixture of an alkali solution, polyethyleneimine, or ethylenediamine solution; and 3) preparing the hollow fiber nanofiltration membrane by spraying the casting solution prepared in step 1) and the core solution prepared in step 2) from the core solution port and the casting solution port, respectively, to form a fibrous tubular liquid film. The liquid film then passes through an air gap and enters a gel device for phase inversion and solidification to form a membrane. However, this preparation method has some drawbacks in practical applications. For example, the base membrane uses polyacrylonitrile, which has poor acid and alkali resistance, limiting its application range. Furthermore, the chemical reaction involved makes it difficult to simultaneously control the chemical reaction and phase inversion process, resulting in high operational complexity, which restricts the application of interfacial polymerization methods.

[0004] In summary, there is an urgent need to develop a controllable preparation method for hollow fiber nanofiltration membranes with simple process and scalable production, and to develop high-performance hollow fiber nanofiltration membranes. Summary of the Invention

[0005] In response to the technical problems existing in the preparation of the above-mentioned nanofiltration membranes, the present invention proposes a one-step method for preparing hollow fiber nanofiltration membranes. The preparation process is simple, the skin is thin, the skin pore size is small, and the equipment requirements are low. It is easy to industrialize and produce. The hollow fiber nanofiltration membrane produced has the advantages of good acid and alkali resistance, good uniformity of membrane surface pore size and good mechanical properties.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is to provide a one-step method for preparing a hollow fiber nanofiltration membrane, comprising the following specific steps: S1. Casting solution: Made from a mixture of polyethersulfone, additives and solvents; S2. Configuration of core liquid: made of water and solvent; S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 are respectively extruded from the casting liquid outlet and the core liquid outlet of the spinneret to form a fiber tubular liquid film. After passing through the air gap, the liquid film enters the gel tank to undergo phase transformation and solidify into a film. After rinsing, the pores are shrunken and shaped online, and then rolled up to obtain a hollow fiber nanofiltration membrane.

[0007] Preferably, the air gap height is 3-30 cm, and the air gap temperature is controlled at 30-65°C.

[0008] Preferably, the casting solution comprises, by mass percentage, polyethersulfone: 16-24%, additives: 0-20%, and solvent: 56-84%.

[0009] Preferably, the molecular weight of the polyethersulfone is 50,000-100,000.

[0010] Preferably, the additive is at least one of polyvinyl pyrrolidone, polyethylene glycol and acetone; and the solvent is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, dimethylpyrrolidone and dimethyl sulfoxide.

[0011] Preferably, the pore shrinkage treatment in step S3 is as follows: above the glass transition temperature, the polymer chain segments move, and the micropores gradually shrink to reach the nanofiltration level.

[0012] Preferably, the ratio of water to solvent in the core liquid is 100:0-50:50.

[0013] Compared with the prior art, the advantages and positive effects of the present invention are: 1. The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane. By synchronously performing a pore shrinkage process during the spinning process of the hollow fiber membrane, pore shrinkage is directly performed on the surface of the polyethersulfone hollow fiber membrane to form a nanofiltration separation layer, thereby fundamentally avoiding the peeling problem, enhancing the performance stability of the membrane product, and overcoming the shortcomings of traditional hollow fiber nanofiltration membranes in which the separation layer and the support layer are made by interfacial polymerization, which makes it easy for the separation layer to peel off at the interface between the two phases and affects the stability and separation efficiency of the membrane.

[0014] 2. The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane, which is a polyethersulfone integrated hollow fiber nanofiltration membrane with good acid and alkali resistance, backwashability, a thin separation layer, and high flux. It overcomes the shortcomings of traditional hollow fiber nanofiltration membranes, such as poor acid and alkali resistance, inability to backwash, and poor pollution resistance.

[0015] 3. The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane. Through the pore shrinkage process, the polyethersulfone hollow fiber membrane undergoes segment movement at a temperature above the glass transition temperature. Since there is no other force around the micropores on the membrane surface, the segments gradually move toward the micropores, causing the micropores to gradually shrink and reach the nanofiltration level.

[0016] 4. The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane. The method is simple to operate, easy to control, requires simple equipment, requires little investment, is easy to industrialize, and is suitable for large-scale promotion. DETAILED DESCRIPTION

[0017] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described below in conjunction with the embodiments. It should be noted that, in the absence of conflict, the embodiments of the present application and the features therein can be combined with each other.

[0018] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0019] DMAC mentioned in the embodiments of the present invention refers to dimethylacetamide; NMP is N-methylpyrrolidone; DMF is dimethylformamide; DMSO is dimethyl sulfoxide; PVP is polyvinylpyrrolidone; PEG is polyethylene glycol, and PES is polyethersulfone. Example 1:

[0020] S1. Configuration of casting solution: The casting solution is 18% PES with a molecular weight of 78,000, the additives are 5% PVP and 2% acetone, and the solvent is 75% DMAC.

[0021] S2. Configuration of core liquid: The core liquid is 30% DMAC aqueous solution.

[0022] S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 were extruded from the casting liquid outlet and the core liquid outlet of the spinneret respectively to form a fiber tubular liquid film. After passing through an air gap of 25 cm, the liquid film entered the gel tank for phase inversion and solidification to form a film. After rinsing, it was heated in a 230°C oil bath for 5 minutes to shrink the pores, and then fixed in a 50°C water bath. Finally, it was rolled up to obtain a hollow fiber nanofiltration membrane. Example 2:

[0023] S1. Configuration of casting solution: The casting solution is 20% PES with a molecular weight of 50,000, additives are 7% PVP and 3% acetone, and the solvent is 70% DMF.

[0024] S2. Configuration of core liquid: The core liquid is 40% DMF aqueous solution.

[0025] S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 were extruded from the casting liquid outlet and the core liquid outlet of the spinneret respectively to form a fiber tubular liquid film. After passing through an air gap of 30 cm, the liquid film entered the gel tank for phase inversion and solidification to form a film. After rinsing, it was heated in a 240°C oil bath for 3 minutes to shrink the pores, and then fixed in a 50°C water bath. Finally, it was rolled up to obtain a hollow fiber nanofiltration membrane. Example 3:

[0026] S1. Configuration of casting solution: The casting solution is 22% PES with a molecular weight of 50,000, additives are 10% PEG and 3% acetone, and the solvent is 60% DMAC.

[0027] S2. Configuration of core liquid: The core liquid is 50% DMAC aqueous solution.

[0028] S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 were extruded from the casting liquid outlet and the core liquid outlet of the spinneret respectively to form a fiber tubular liquid film. After passing through an air gap of 30 cm, the liquid film entered the gel tank for phase inversion and solidification to form a film. After rinsing, it was heated in a 245°C oil bath for 3 minutes to shrink the pores, and then fixed in a 50°C water bath. Finally, it was rolled up to obtain a hollow fiber nanofiltration membrane.

[0029] Comparative Example: S1. Configuration of casting solution: The casting solution is the same as that in Example 1.

[0030] S2. Configuration of core liquid: The core liquid is the same as that in Example 1.

[0031] S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 are extruded from the casting liquid outlet and the core liquid outlet of the spinneret respectively to form a fiber tubular liquid membrane. After passing through an air gap of 30 cm, the liquid membrane enters the gel tank for phase inversion and solidification into a membrane, and is finally wound up to obtain a hollow fiber membrane.

[0032] Flux and desalination rate detection method: Rinse the sample to be tested with deionized water and soak it for at least 12 hours. Prepare 250 ppm magnesium sulfate solution and sodium chloride solution as the test solution, adjust the test solution temperature to 25±2°C and the pH to approximately 7. Use a test pressure of 0.4 MPa. Run the test solution at a constant temperature and pressure for 30 minutes, and measure the volume of the permeate within a certain period of time using a stopwatch and a graduated cylinder.

[0033]

[0034] It can be seen from Example 1, Example 2 and Example 3 as well as the comparative example that the hollow fiber membrane produced by rinsing-online shrinkage-water bath shaping, since the polymer chain segments move above the glass transition temperature during the shrinkage process, there are no other forces around the micropores on the membrane surface, the segments gradually move toward the micropores, and the micropores gradually shrink to reach the nanofiltration level, and its retention rate for magnesium sulfate is significantly improved, reaching more than 97%, and the nanofiltration performance is good.

[0035] The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane. By simultaneously performing a pore shrinkage process during the hollow fiber membrane spinning process, pores are directly shrunk on the surface of the polyethersulfone hollow fiber membrane to form a nanofiltration separation layer. This fundamentally avoids the peeling problem and enhances the performance stability of the membrane product. The membrane product prepared by this method is a one-piece polyethersulfone hollow fiber nanofiltration membrane with excellent acid and alkali resistance, backwashability, a thin separation layer, and high flux. This overcomes the shortcomings of traditional hollow fiber nanofiltration membranes, such as poor acid and alkali resistance, inability to backwash, and poor pollution resistance.

[0036] The present invention provides a one-step method for preparing a hollow fiber nanofiltration membrane, wherein the air gap height is 3-30 cm and the air gap temperature is controlled between 30°C and 65°C. This extends the air gap height range and expands the air gap temperature downward, broadening the range and making it easier to control. This helps reduce heat leakage and ensures uniform membrane product quality. This method is simple to operate and control, requires minimal equipment, requires little investment, and is amenable to industrial production and large-scale promotion.

[0037] In a one-step method for preparing a hollow fiber nanofiltration membrane provided by the present invention, the additive is at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and acetone; the solvent is at least one of N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), dimethylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO). The reagents are selected with specificity, are not scarce, and are easily available, which is conducive to controlling the large-scale production cost of the product.

[0038] Furthermore, the ratio of water to solvent in the core liquid provided by the present invention is 100:0-50:50, that is, the proportion of water is not less than 50%, eliminating the disadvantages of membrane preparation with alkaline solution, and the pure water flux can be above 25LMH, which is beneficial to improving the acid and alkali resistance of the membrane product after it is finally offline, and is beneficial to the membrane product to overcome the shortcomings of existing hollow fiber nanofiltration membranes such as poor acid and alkali resistance, inability to backwash, and poor pollution resistance.

[0039] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any person skilled in the art may utilize the technical contents disclosed above to change or modify them into equivalent embodiments with equivalent changes for application in other fields. However, any simple modification, equivalent change, and modification of the above embodiments made in accordance with the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a one-step hollow fiber nanofiltration membrane, characterized in that: The specific steps include: S1. Casting solution: Made from a mixture of polyethersulfone, additives and solvents; S2. Configuration of core liquid: made of water and solvent; S3. Preparation of hollow fiber nanofiltration membrane: The casting liquid prepared in step S1 and the core liquid prepared in step S2 are respectively extruded from the casting liquid outlet and the core liquid outlet of the spinneret to form a fiber tubular liquid film. After passing through the air gap, the liquid film enters the gel tank to undergo phase transformation and solidify into a film. After rinsing, the pores are shrunken and shaped online, and then rolled up to obtain a hollow fiber nanofiltration membrane.

2. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 1, characterized in that: The height of the air gap is 3-30 cm, and the temperature of the air gap is controlled at 30-65°C.

3. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 1, wherein: The casting solution comprises, by mass percentage, polyethersulfone: 16-24%, additives: 0-20%, and solvent: 56-84%.

4. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 3, characterized in that: The molecular weight of the polyethersulfone is 50,000-100,000.

5. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 3, wherein: The additive is at least one of polyvinyl pyrrolidone, polyethylene glycol and acetone; the solvent is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl pyrrolidone and dimethyl sulfoxide.

6. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 1, wherein: The pore shrinkage treatment in step S3 is as follows: above the glass transition temperature, the polymer chain segments move, and the micropores gradually shrink to reach the nanofiltration level.

7. The method for preparing a one-step hollow fiber nanofiltration membrane according to claim 1, wherein: The ratio of water to solvent in the core liquid is 100:0-50:50.

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