Composite nanofiber membrane and preparation method and application thereof

Abstract

The application discloses a composite nanofiber membrane and a preparation method and application thereof, and relates to the technical field of regeneration and purification of transformer insulating oil. The application provides a composite nanofiber membrane, wherein the composite nanofiber membrane is in a sandwich structure, and the sandwich structure comprises a nanofiber membrane and an adsorbing fiber layer; the nanofiber membrane is located on the upper and lower sides of the adsorbing fiber layer; raw materials of the adsorbing fiber layer comprise paper pulp and an adsorbing material, the adsorbing material is a mixture of activated carbon and Y molecular sieve, the activated carbon and the Y molecular sieve are in a ratio of 3-4:6, and the particle size D90 of the activated carbon is 0.15-0.25 mm. The application can realize deep removal of organic impurities in aged insulating oil, and has good environmental tolerance. The composite adsorbing material has excellent comprehensive performance and is suitable for deep removal of impurities in aged insulating oil.

Description

Technical Field

[0001] This invention relates to the field of transformer insulating oil regeneration and purification technology, and in particular to a composite nanofiber membrane, its preparation method and application. Background Technology

[0002] According to statistics from the China Electricity Council, in 2022, the total electricity consumption in China reached 8,636.9 billion kilowatt-hours, an increase of 3.6% over the previous year. Transformers, as core components of power transmission and distribution systems across the country, are crucial for the operation of the entire power system, undertaking the tasks of voltage conversion, transmission, and distribution. Insulating oil, as an important insulating medium in electrical equipment, mainly plays a role in insulation, heat dissipation, cooling, and arc extinguishing; its performance directly affects the safe operation of the power grid. However, insulating oil used for a long time will continuously age under the influence of electromagnetic fields, temperature, oxygen, electric arcs, and copper and iron catalysts, undergoing oxidation, cracking, and carbonization reactions, generating various impurities, reducing its insulation and cooling performance, and even causing electrical equipment failures, threatening the safe operation of the power grid system. Therefore, a large amount of waste transformer insulating oil is replaced and returned every year. Waste transformer oil contains a large number of toxic substances; direct discharge would cause serious environmental pollution. It is listed as HW08 hazardous waste in the "National Hazardous Waste List," requiring safe disposal or recycling. Recycling and reusing waste transformer oil can reduce hazardous waste emissions and achieve resource utilization of waste.

[0003] Currently, commonly used purification methods include centrifugal separation, vacuum filtration, pressure filtration, adsorption filtration, and electrostatic oil purification. These methods can effectively filter out gases and larger particulate solid impurities from insulating oil, restoring its insulation performance to some extent. However, these physical filtration methods are not effective at removing soluble impurities such as corrosive sulfur, nor are they effective at removing colloidal impurities, trace amounts of moisture, and submicron-sized impurities generated by oxidation and emulsification in insulating oil. Filtration methods can only remove moisture and mechanical impurities from transformer oil, but are difficult to remove soluble polar organic impurities. For the removal of soluble impurities, adsorption methods have the advantages of high removal rates and ease of industrialization. A filtration-adsorption composite method is used to purify transformer oil, that is, filling powdered adsorbent material between two layers of filter paper to create a combined adsorption filter plate, which has the dual function of filtering moisture and mechanical impurities and adsorbing polar organic matter. Summary of the Invention

[0004] Based on this, the purpose of the present invention is to overcome the shortcomings of the prior art and provide a composite nanofiber membrane, its preparation method and application.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a composite nanofiber membrane, wherein the composite nanofiber membrane has a sandwich structure, the sandwich structure includes a nanofiber membrane and an adsorption fiber layer; the nanofiber membrane is located on the upper and lower sides of the adsorption fiber layer; the raw materials of the adsorption fiber layer include pulp and adsorption material, wherein the adsorption material is a mixture of activated carbon and Y molecular sieve, the ratio of activated carbon to Y molecular sieve is (3-4):6, and the particle size D90 of the activated carbon is 0.15-0.25 mm.

[0006] The inventors discovered that adsorption methods offer advantages such as high removal rates and ease of industrialization for the removal of soluble impurities. When using a composite nanofiber membrane with a specific sandwich structure, filtration and adsorption can be combined to purify transformer oil through a specific filtration-adsorption composite method, giving it the dual function of filtering moisture and mechanical impurities while adsorbing polar organic matter. This invention composites an adsorption fiber layer with adsorption function onto a simple nanofiber membrane. This adsorption fiber layer has high adsorption capacity and selective adsorption, enabling deep removal of organic impurities from aged insulating oil and exhibiting good environmental tolerance. The composite adsorption material boasts excellent overall performance and is suitable for the deep removal of impurities from aged insulating oil.

[0007] During actual experiments, the inventors discovered that when the adsorption material of the adsorption fiber layer is a mixture of activated carbon and Y molecular sieve, it can better synergize with the nanofiber membrane. Furthermore, the inventors found that when the ratio of activated carbon to Y molecular sieve is (3-4):6, and the particle size D90 of the activated carbon is 0.15-0.25 mm, the prepared adsorption fiber layer exhibits better adsorption performance and can better synergize with the nanofiber membrane to jointly purify aged insulating oil.

[0008] Preferably, the nanofiber membrane is prepared by electrospinning, wherein the pre-spinning solution of the electrospinning method comprises the following components: PVDF-HFP powder, tetrahydrofuran solution, N,N-dimethylformamide solution, and nano-oxide particles; the nano-oxide particles are a mixture of nano-BaTiO3 and nano-ZnO, wherein the particle size D90 of the nano-BaTiO3 is 40-50 nm, the particle size D90 of the nano-ZnO is 80-90 nm, and the weight ratio of BaTiO3 to ZnO is (3-4):1.

[0009] The nanofiber membrane used in this invention uses PVDF-HFP nanoparticles, which are more flexible and bendable than commonly used PVDF nanoparticles on the market, and PVDF-HFP also has higher thermal stability. The nano-oxide particles added to the spinning solution in this invention are a mixture of BaTiO3 and ZnO. Both BaTiO3 and ZnO nanoparticles are good electrets. Doping with BaTiO3 is beneficial for forming more β-phase PVDF with piezoelectric effect during electrospinning, which helps the filter membrane electrostatically adsorb small polar impurities in the oil. Doping with ZnO is beneficial for improving adsorption efficiency, achieving dual optimization of acid value and dielectric loss in aged insulating oil. In actual experiments, the inventors found that the weight ratio of BaTiO3 to ZnO affects the purification effect provided by the two nano-oxides. Because there is a mutually reinforcing coupling effect between the purification effects provided by BaTiO3 and ZnO, the weight ratio of BaTiO3 to ZnO affects the final synergistic coupling. When the weight ratio of BaTiO3 to ZnO is (3-4):1, the prepared nanofiber membrane has a better purification effect. During actual experiments, the inventors discovered that the particle size D90 of BaTiO3 and ZnO affects the final purification effect. This is because the particle size D90 of BaTiO3 nanoparticles is 40-50 nm, and the particle size D90 of ZnO is 80-90 nm. At this particle size D90, the number of nano-oxides and the specific surface area both increase, that is, the number of interfaces inside the composite material increases, the interfacial polarization is enhanced, which is conducive to the electrostatic adsorption of small polar impurities in the oil. In addition, it can also make the formed pore size smaller, and the interception effect of small impurities is better.

[0010] Preferably, the method for preparing the nanofiber membrane includes the following steps:

[0011] (1) The dried PVDF-HFP powder was added to a mixed solution of tetrahydrofuran and N,N-dimethylformamide and dissolved completely. Nano-oxide particles were added, ultrasonically dispersed, and allowed to stand to remove bubbles to obtain a spinning precursor solution.

[0012] (2) The spinning precursor solution obtained in step (1) is subjected to electrospinning and dried to obtain a composite nanofiber membrane.

[0013] Preferably, in step (1), the specific method for drying the PVDF-HFP powder can be as follows: placing the PVDF-HFP powder in an open beaker and drying it in a vacuum drying oven at a temperature of 50-60℃ for 12-24 hours; the complete dissolution is achieved by sealing and stirring for 30-60 minutes; the water bath ultrasonic dispersion temperature is 20-30℃ and the water bath ultrasonic dispersion time is 30-60 minutes; the condition for static degassing is to place it in a room temperature sealed drying oven.

[0014] Preferably, in step (1), the mass of the PVDF-HFP powder is 9-11 wt.% of the total mass of the spinning precursor solution, the mass of the nano-oxide particles is 8-10 wt.% of the total mass of the spinning precursor solution, and the volume ratio of the tetrahydrofuran solution to the N,N-dimethylformamide solution is 2:3.

[0015] Preferably, the mass of PVDF-HFP powder and the mass fraction of nano-oxide particles will affect the final result. In actual production, it is necessary to select an appropriate mass fraction according to the requirements. Specifically, the mass of PVDF-HFP powder can be any one of 9 wt.%, 10 wt.%, and 11 wt.% of the total mass of the spinning precursor liquid, or a range of both. The mass of nano-oxide particles can be any one of 8 wt.%, 9 wt.%, and 10 wt.% of the total mass of the spinning precursor liquid, or a range of both.

[0016] Preferably, in step (2), the electrospinning operation comprises the following steps:

[0017] (a) Use a syringe to draw 7-9 mL of spinning precursor solution, invert the needle hole to remove all air bubbles, attach the metal needle, and make sure the piston tail is in full contact with the peristaltic pump push rod.

[0018] (b) Cut a 20cm×30cm piece of aluminum foil as the receiving substrate for spinning nanofibers, and adjust the distance between the metal needle and the receiving surface of the aluminum foil;

[0019] (c) Connect the metal needle of the syringe and the aluminum foil substrate to positive and negative high voltage DC power respectively, turn on the temperature control lamp, and perform electrospinning; the parameters of the electrospinning operation are: electrospinning voltage of 15-20kV, liquid flow rate of 0.5-0.75mL / h, needle tip distance from receiving surface of 10-15cm, and control the temperature inside the electrospinning machine chamber at 35±0.5℃.

[0020] Preferably, in step (2), the drying conditions are vacuum drying at 50-60°C for 3-4 hours.

[0021] Preferably, the Y-type molecular sieve is one of AgY-modified molecular sieve, ZnY-modified molecular sieve, and CrY-modified molecular sieve. More preferably, the Y-type molecular sieve is AgY-modified molecular sieve.

[0022] In actual experiments, the inventors found that compared with AgY molecular sieve, the adsorption capacity of Cu by ZnY and CrY modified molecular sieves was reduced. This may be because AgY and ZnY have more micropores, larger surface area and pore volume, and AgY has a larger adsorption capacity, thus having a stronger adsorption capacity for Cu under the same volume.

[0023] Preferably, the adsorbent fiber layer is prepared by the following method: pulp, adsorbent material and binder are mixed and mixed, and paper is made to obtain the adsorbent fiber layer.

[0024] Preferably, the pulp is at least one of softwood pulp, hardwood pulp, straw pulp, and bamboo pulp, and the binder is at least one of cationic polyacrylamide (CPAM) and polyamide epichlorohydrin resin (PAE).

[0025] Preferably, the dispersive mixing is achieved by rotating the propeller at high speed to disperse the fiber bundle into individual fibers in the water, thereby ensuring uniform dispersion of the raw materials.

[0026] Furthermore, the present invention provides a method for preparing the aforementioned composite nanofiber membrane, comprising the following steps:

[0027] S1. Place the nanofiber membrane on the upper and lower sides of the adsorption fiber layer to obtain a three-layer composite structure;

[0028] S2. The three-layer composite structure is hot-pressed and dried to obtain the sandwich structure composite nanofiber membrane.

[0029] Preferably, in step S1, the hot pressing temperature is 80-100℃ and the hot pressing time is 10-20 min; in step S2, the drying temperature is 100-110℃ and the drying time is 4-6 h.

[0030] Furthermore, the present invention provides the application of the aforementioned composite nanofiber membrane in the purification of transformer insulating oil.

[0031] Compared to existing technologies, the advantages of this invention are as follows: This invention incorporates an adsorption fiber layer with adsorption function into a simple nanofiber membrane. The adsorption fiber layer has high adsorption capacity and selective adsorption, enabling deep removal of organic impurities from aged insulating oil and exhibiting good environmental tolerance. The composite adsorption material has excellent overall performance and is suitable for deep removal of impurities from aged insulating oil. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. The purpose of this description is to provide a detailed understanding of the invention, not to limit its scope. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this invention. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this invention are all commonly used reagents and instruments.

[0033] The raw materials used in the embodiments and comparative examples of this invention are as follows:

[0034] Activated carbon-1: Particle size D90 is 0.2mm, manufacturer is Guangdong Hanyan, model HY-010;

[0035] Activated carbon-2: Particle size D90 is 0.15mm, manufacturer is Guangdong Hanyan, model HY-014;

[0036] Activated carbon-3: Particle size D90 is 0.25mm, manufacturer is Guangdong Hanyan, model HY-008;

[0037] Activated carbon-4: Particle size D90 is 0.1mm, manufacturer is Guangdong Hanyan, model HY-018;

[0038] Activated carbon-5: Particle size D90 is 0.3mm, manufacturer is Guangdong Hanyan, model HY-007;

[0039] AgY modified molecular sieve, manufacturer: Jiangxi Xintao, model: XT-04;

[0040] ZnY modified molecular sieve, manufacturer: Jiangxi Xintao, model: XT-016;

[0041] Nano BaTiO3-1: Particle size D90 is 60nm, manufacturer is Zhongke Yannuo, model number is 2021123456;

[0042] Nano BaTiO3-2: Particle size D90 is 50nm, manufacturer is Zhongke Yannuo, model number 2021123386;

[0043] Nano BaTiO3-3: Particle size D90 is 40nm, manufacturer is Zhongke Yannuo, model number 2021123475;

[0044] Nano ZnO-1: Particle size D90 is 80nm, manufacturer is Lanxess, Germany, model 1080;

[0045] Nano ZnO-2: Particle size D90 is 90nm, manufacturer is Lanxess, Germany, model 1090;

[0046] Nano ZnO-3: Particle size D90 is 70nm, manufacturer is Lanxess, Germany, model 1070;

[0047] Nano ZnO-4: Particle size D90 is 100nm, manufacturer: Lanxess, Germany, model 1100;

[0048] This invention provides a method for preparing the aforementioned composite nanofiber membrane, which includes the preparation of the nanofiber membrane, the preparation of the adsorption fiber layer, and the composite nanofiber membrane, as detailed below:

[0049] The preparation method of the nanofiber membranes in the embodiments and comparative examples of the present invention includes the following steps:

[0050] (1) The dried PVDF-HFP powder was added to a mixed solution of tetrahydrofuran and N,N-dimethylformamide and dissolved completely. Nano-oxide particles were added, ultrasonically dispersed, and allowed to stand to remove bubbles to obtain a spinning precursor solution.

[0051] (2) The spinning precursor solution obtained in step (1) is subjected to electrospinning operation, and after vacuum drying at 50-60℃ for 3-4h, a composite nanofiber membrane is obtained.

[0052] In step (1), the specific method for drying the PVDF-HFP powder can be as follows: place the PVDF-HFP powder in an open beaker and dry it in a vacuum drying oven at a temperature of 50-60℃ for 12-24 hours; the complete dissolution is achieved by sealing and stirring for 30-60 minutes; the water bath ultrasonic dispersion temperature is 20-30℃ and the water bath ultrasonic dispersion time is 30-60 minutes; the condition for static degassing is to place it in a room temperature sealed drying oven.

[0053] In step (1), the mass of the PVDF-HFP powder is 10 wt.% of the total mass of the spinning precursor solution, the mass of the nano-oxide particles is 10 wt.% of the total mass of the spinning precursor solution, and the volume ratio of the tetrahydrofuran solution to the N,N-dimethylformamide solution is 2:3.

[0054] The nano-oxide particles are a mixture of BaTiO3 and ZnO, wherein the particle size D90 of the nano-BaTiO3 is 40-50 nm, the particle size D90 of the nano-ZnO is 80-90 nm, and the weight ratio of BaTiO3 to ZnO is (3-4):1.

[0055] In step (2), the electrospinning operation consists of the following steps:

[0056] (a) Use a syringe to draw 7-9 mL of spinning precursor solution, invert the needle hole to remove all air bubbles, attach the metal needle, and make sure the piston tail is in full contact with the peristaltic pump push rod.

[0057] (b) Cut a 20cm×30cm piece of aluminum foil as the receiving substrate for spinning nanofibers, and adjust the distance between the metal needle and the receiving surface of the aluminum foil;

[0058] (c) Connect the metal needle of the syringe and the aluminum foil substrate to positive and negative high voltage DC power respectively, turn on the temperature control lamp, and perform electrospinning; the parameters of the electrospinning operation are: electrospinning voltage of 15-20kV, liquid flow rate of 0.5-0.75mL / h, needle tip distance from receiving surface of 10-15cm, and control the temperature inside the electrospinning machine chamber at 35±0.5℃.

[0059] The preparation method of the adsorbent fiber layer in the embodiments and comparative examples of the present invention is as follows: pulp, adsorbent material and binder are loosely mixed (by high-speed rotation of a propeller, the fiber bundles are dispersed into single fibers in water, so that the raw materials are evenly dispersed), paper is made, and the adsorbent fiber layer is obtained.

[0060] The pulp is at least one of softwood pulp, hardwood pulp, straw pulp, and bamboo pulp, and the binder is at least one of cationic polyacrylamide (CPAM) and PAE.

[0061] The adsorbent material is a mixture of activated carbon and AgY modified molecular sieve, with a ratio of (3-4):6 between the activated carbon and AgY modified molecular sieve, and the particle size D90 of the activated carbon is 0.15-0.25 mm.

[0062] The amount of pure pulp fiber used in the sample was 200-210 g / m³. 2 Adsorbent dosage: 370-375 g / m³ 2 The amount of cationic polyacrylamide (CPAM) or PAE added is 1 wt% of fiber.

[0063] The preparation method of the composite nanofiber membrane described in the embodiments and comparative examples of the present invention includes the following steps:

[0064] S1. Place the nanofiber membrane on the upper and lower sides of the adsorption fiber layer to obtain a three-layer composite structure;

[0065] S2. The three-layer composite structure is hot-pressed and dried to obtain the sandwich structure composite nanofiber membrane.

[0066] The hot pressing temperature is 80-100℃, and the hot pressing time is 10-20 min; in step S2, the drying temperature is 100-110℃, and the drying time is 4-6 h.

[0067] Examples 1-8

[0068] Example 1

[0069] This invention provides a method for preparing the aforementioned composite nanofiber membrane, which includes the preparation of the nanofiber membrane, the preparation of the adsorption fiber layer, and the composite nanofiber membrane, as detailed below:

[0070] The preparation method of the nanofiber membrane in Example 1 includes the following steps:

[0071] (1) The dried PVDF-HFP powder was added to a mixed solution of tetrahydrofuran and N,N-dimethylformamide and dissolved completely. Nano-oxide particles were added, ultrasonically dispersed, and allowed to stand to remove bubbles to obtain a spinning precursor solution.

[0072] (2) The spinning precursor liquid obtained in step (1) was subjected to electrospinning operation, and the composite nanofiber membrane was obtained after vacuum drying at 50°C for 4 hours.

[0073] In step (1), the specific method for drying the PVDF-HFP powder can be as follows: place the PVDF-HFP powder in an open beaker and dry it in a vacuum drying oven at 50°C for 24 hours; the complete dissolution is achieved by sealing and stirring for 30 minutes; the water bath ultrasonic dispersion temperature is 30°C and the water bath ultrasonic dispersion time is 30 minutes; the static degassing condition is to place it in a room temperature sealed drying oven.

[0074] In step (1), the mass of the PVDF-HFP powder is 10 wt.% of the total mass of the spinning precursor solution, the mass of the nano-oxide particles is 10 wt.% of the total mass of the spinning precursor solution, and the volume ratio of the tetrahydrofuran solution to the N,N-dimethylformamide solution is 2:3.

[0075] The nano-oxide particles are a mixture of BaTiO3 and ZnO, wherein the particle size D90 of the nano-BaTiO3 is 40 nm, the particle size D90 of the nano-ZnO is 80 nm, and the weight ratio of BaTiO3 to ZnO is 4:1.

[0076] In step (2), the electrospinning operation consists of the following steps:

[0077] (a) Use a syringe to draw 8 mL of spinning precursor solution, invert the needle hole to remove all air bubbles, attach the metal needle, and make the piston tail fully contact the peristaltic pump push rod.

[0078] (b) Cut a 20cm×30cm piece of aluminum foil as the receiving substrate for spinning nanofibers, and adjust the distance between the metal needle and the receiving surface of the aluminum foil;

[0079] (c) Connect the metal needle of the syringe and the aluminum foil substrate to positive and negative high voltage DC power respectively, turn on the temperature control lamp, and perform electrospinning; the parameters of the electrospinning operation are: electrospinning voltage is 20kV, liquid flow rate is 0.75mL / h, needle tip distance from receiving surface is 15cm, and the temperature inside the electrospinning machine is controlled at 35±0.5℃.

[0080] The preparation method of the adsorbent fiber layer in Example 1 is as follows: pulp, adsorbent material, and binder are loosely mixed (by high-speed rotation of a propeller, the fiber bundles are dispersed into single fibers in water, so that the raw materials are evenly dispersed), and paper is made to obtain the adsorbent fiber layer.

[0081] The amount of pure pulp fiber sample used was 200g / m³. 2 Adsorbent dosage 370g / m³ 2The CPAM addition amount is 1 wt% of fiber.

[0082] The pulp is softwood pulp, and the binder is cationic polyacrylamide (CPAM).

[0083] The adsorbent material is a mixture of activated carbon and AgY modified molecular sieve, with a ratio of 3:6 between the activated carbon and AgY modified molecular sieve, and the particle size D90 of the activated carbon is 0.2 mm.

[0084] The preparation method of the composite nanofiber membrane in Example 1 includes the following steps:

[0085] S1. Place the prepared nanofiber membrane on the upper and lower sides of the prepared adsorption fiber layer to obtain a three-layer composite structure.

[0086] S2. The three-layer composite structure is hot-pressed and dried to obtain the sandwich structure composite nanofiber membrane.

[0087] The hot pressing temperature is 100℃ and the hot pressing time is 10 min; in step S2, the drying temperature is 100℃ and the drying time is 6 h.

[0088] Example 2

[0089] This invention provides a method for preparing the aforementioned composite nanofiber membrane, which includes the preparation of the nanofiber membrane, the preparation of the adsorption fiber layer, and the composite nanofiber membrane, as detailed below:

[0090] The preparation method of the nanofiber membrane in Example 2 includes the following steps:

[0091] (1) The dried PVDF-HFP powder was added to a mixed solution of tetrahydrofuran and N,N-dimethylformamide and dissolved completely. Nano-oxide particles were added, ultrasonically dispersed, and allowed to stand to remove bubbles to obtain a spinning precursor solution.

[0092] (2) The spinning precursor liquid obtained in step (1) is subjected to electrospinning operation, and after vacuum drying at 50°C for 3 hours, a composite nanofiber membrane is obtained.

[0093] In step (1), the specific method for drying the PVDF-HFP powder can be as follows: place the PVDF-HFP powder in an open beaker and dry it in a vacuum drying oven at 50°C for 24 hours; the complete dissolution is achieved by sealing and stirring for 50 minutes; the water bath ultrasonic dispersion temperature is 30°C and the water bath ultrasonic dispersion time is 40 minutes; the condition for static degassing is to place it in a room temperature sealed drying oven.

[0094] In step (1), the mass of the PVDF-HFP powder is 10 wt.% of the total mass of the spinning precursor solution, the mass of the nano-oxide particles is 10 wt.% of the total mass of the spinning precursor solution, and the volume ratio of the tetrahydrofuran solution to the N,N-dimethylformamide solution is 2:3.

[0095] The nano-oxide particles are a mixture of BaTiO3 and ZnO, wherein the particle size D90 of the nano-BaTiO3 is 40 nm, the particle size D90 of the nano-ZnO is 80 nm, and the weight ratio of BaTiO3 to ZnO is 4:1.

[0096] In step (2), the electrospinning operation consists of the following steps:

[0097] (a) Use a syringe to draw 7 mL of spinning precursor solution, invert the needle hole to remove all air bubbles, attach the metal needle, and make the piston tail fully contact the peristaltic pump push rod.

[0098] (b) Cut a 20cm×30cm piece of aluminum foil as the receiving substrate for spinning nanofibers, and adjust the distance between the metal needle and the receiving surface of the aluminum foil;

[0099] (c) Connect the metal needle of the syringe and the aluminum foil substrate to positive and negative high voltage DC power respectively, turn on the temperature control lamp, and perform electrospinning; the parameters of the electrospinning operation are: electrospinning voltage is 15kV, liquid flow rate is 0.5mL / h, needle tip distance from receiving surface is 10cm, and the temperature inside the electrospinning machine is controlled at 35±0.5℃.

[0100] The preparation method of the adsorbent fiber layer in Example 2 is as follows: pulp, adsorbent material, and binder are loosely mixed (by high-speed rotation of a propeller, the fiber bundles are dispersed into single fibers in water, so that the raw materials are evenly dispersed), and paper is made to obtain the adsorbent fiber layer.

[0101] The amount of pure pulp fiber sample used was 210 g / m³. 2 Adsorbent dosage: 375 g / m³ 2 The CPAM addition amount is 1 wt% of fiber.

[0102] The pulp is hardwood pulp, and the binder is cationic polyacrylamide (CPAM).

[0103] The adsorbent material is a mixture of activated carbon and AgY modified molecular sieve, with a ratio of 3:6 between the activated carbon and AgY modified molecular sieve, and the particle size D90 of the activated carbon is 0.2 mm.

[0104] The preparation method of the composite nanofiber membrane in Example 2 includes the following steps:

[0105] S1. Place the prepared nanofiber membrane on the upper and lower sides of the prepared adsorption fiber layer to obtain a three-layer composite structure.

[0106] S2. The three-layer composite structure is hot-pressed and dried to obtain the sandwich structure composite nanofiber membrane.

[0107] The hot pressing temperature is 80℃ and the hot pressing time is 20min; in step S2, the drying temperature is 110℃ and the drying time is 4h.

[0108] Example 3

[0109] The only difference between the preparation method of the composite nanofiber membrane described in Example 3 and Example 1 is that the weight ratio of BaTiO3 and ZnO in step (1) of the preparation method of the nanofiber membrane is different, and the weight ratio of BaTiO3 and ZnO is 3:1.

[0110] Example 4

[0111] The only difference between the preparation method of the composite nanofiber membrane in Example 4 and that in Example 1 is that the ratio of activated carbon to AgY modified molecular sieve is different; the ratio of activated carbon to AgY modified molecular sieve is 4:6.

[0112] Example 5

[0113] The only difference between the preparation method of the composite nanofiber membrane in Example 5 and that in Example 1 is that the particle size D90 of the activated carbon is different in the preparation of the adsorption fiber layer, and the particle size D90 of the activated carbon is 0.15 mm.

[0114] Example 6

[0115] The only difference between the preparation method of the composite nanofiber membrane described in Example 6 and Example 1 is that the particle size D90 of the activated carbon is different in the preparation of the adsorption fiber layer. The particle size D90 of the activated carbon is 0.25 mm.

[0116] Example 7

[0117] The only difference between the preparation method of the composite nanofiber membrane in Example 7 and Example 1 is that the molecular sieve used in the preparation of the adsorption fiber layer is different; the molecular sieve used is a ZnY modified molecular sieve.

[0118] Example 8

[0119] The only difference between the preparation method of the composite nanofiber membrane described in Example 8 and Example 1 is that the particle size D90 of nano BaTiO3 and nano ZnO is different. The particle size D90 of nano BaTiO3 is 50 nm, and the particle size D90 of nano ZnO is 90 nm.

[0120] Comparative Examples 1-12

[0121] Comparative Example 1

[0122] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 1 and Example 1 is that in step (1) of the preparation method of the nanofiber membrane of the present invention, PVDF-HFP powder is replaced with PVDF particles.

[0123] Comparative Example 2

[0124] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 2 and Example 1 is:

[0125] The adsorption fiber layer of this invention is PP filter paper.

[0126] Comparative Example 3

[0127] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 3 and Example 1 is that the weight ratio of BaTiO3 and ZnO in step (1) of the preparation method of the nanofiber membrane is different, and the weight ratio of BaTiO3 and ZnO is 2:1.

[0128] Comparative Example 4

[0129] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 4 and Example 1 is that the weight ratio of BaTiO3 and ZnO in step (1) of the preparation method of the nanofiber membrane is different, and the weight ratio of BaTiO3 and ZnO is 1:1.

[0130] Comparative Example 5

[0131] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 5 and Example 1 is that the weight ratio of BaTiO3 and ZnO in step (1) of the preparation method of the nanofiber membrane is different, and the weight ratio of BaTiO3 and ZnO is 1:3.

[0132] Comparative Example 6

[0133] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 6 and Example 1 is that the ratio of activated carbon to AgY modified molecular sieve is different in the preparation of the adsorption fiber layer. The ratio of activated carbon to AgY modified molecular sieve is 1:1.

[0134] Comparative Example 7

[0135] The only difference between the preparation method of the composite nanofiber membrane in Comparative Example 7 and Example 1 is that the ratio of activated carbon to AgY modified molecular sieve is different in the preparation of the adsorption fiber layer. The ratio of activated carbon to AgY modified molecular sieve is 1:3.

[0136] Comparative Example 8

[0137] The only difference between the preparation method of the composite nanofiber membrane in Comparative Example 8 and Example 1 is that the ratio of activated carbon to AgY modified molecular sieve is different in the preparation of the adsorption fiber layer. The ratio of activated carbon to AgY modified molecular sieve is 3:1.

[0138] Comparative Example 9

[0139] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 9 and Example 1 is that the particle size D90 of the activated carbon is different in the preparation of the adsorption fiber layer, and the particle size D90 of the activated carbon is 0.1 mm.

[0140] Comparative Example 10

[0141] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 10 and Example 1 is that the particle size D90 of the activated carbon is different in the preparation of the adsorption fiber layer, and the particle size D90 of the activated carbon is 0.3 mm.

[0142] Comparative Example 11

[0143] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 11 and Example 1 is that the particle size D90 of nano BaTiO3 and nano ZnO is different in the preparation method of the nanofiber membrane. The particle size D90 of nano BaTiO3 is 60 nm and the particle size D90 of nano ZnO is 70 nm.

[0144] Comparative Example 12

[0145] The only difference between the preparation method of the composite nanofiber membrane described in Comparative Example 12 and Example 1 is that the particle size D90 of nano BaTiO3 and nano ZnO is different in the preparation method of the nanofiber membrane. The particle size D90 of nano BaTiO3 is 60 nm and the particle size D90 of nano ZnO is 100 nm.

[0146] Performance testing

[0147] Experimental Example 1: Purification Performance Test

[0148] The composite nanofiber membranes prepared in the examples and comparative examples were cut into 50mm×50mm rectangular membranes, and the composite membranes were used in a dedicated vacuum filtration device for the purification of synthetic ester insulating oil.

[0149] The acid value, dielectric loss, and trace water content of the synthetic ester insulating oil were measured before and after purification. According to the standard DL / T 1811-2018, qualified natural ester insulation has a breakdown strength higher than 40kV, a dielectric loss not higher than 0.04, and a trace water content not higher than 200mg / L.

[0150] In this invention, the filter membrane purification standard is set as follows: the natural ester insulation breakdown strength is above 49kV, the dielectric loss is below 0.03, the trace water content is less than 12.5mg / L, and the Cu content is below 0.05 to be considered a qualified filter membrane.

[0151] The breakdown strength, dielectric loss, trace water content, particle size, and Cu content of the aged insulating oil before purification are shown in Table 1. The breakdown strength, dielectric loss, trace water content, particle size, and Cu content of the comparative examples after purification are also shown in Table 1.

[0152] Table 1

[0153]

[0154] 1. The impact of particle size D90 on oil purification

[0155] Table 1 shows the various indicators of purifying 100 mL of aged natural ester insulating oil using filter membranes prepared with different nano-oxide particle sizes D90 and activated carbon particle sizes D90 under a negative pressure of 0.05 MPa. It can be seen that different particle sizes D90 have a certain impact on the purification effect of nanofiber membranes. When the nanoparticle size D90 is small, it will cause the nanofiber membrane to agglomerate during the spinning process, resulting in larger gaps. The retention effect of impurities and moisture becomes worse, and its acid value and moisture content both increase. When the activated carbon particle size D90 is small, the filtration speed will be relatively increased. However, activated carbon with small particle size D90 is easy to reach adsorption saturation during adsorption and is not suitable for long-term use.

[0156] 2. The impact of purification materials on oil purification

[0157] Table 1 shows the various indicators of 100 mL of aged natural ester insulating oil purified by filter membranes made of different purification materials under a negative pressure of 0.05 MPa. It can be seen that the pure PVDF-HFP nanofiber membrane combined with the PP membrane substrate still has a certain degree of purification effect on the natural ester insulating oil; however, the purified natural ester insulating oil still does not meet the breakdown strength standard. In Comparative Example 1, replacing PVDF-HFP with PVDF had no significant effect on the purification effect, but increased its tensile strength. Compared with AgY molecular sieves, ZnY and CrY molecular sieves showed reduced adsorption of Cu. This may be because AgY has a larger number of micropores, a larger surface area and pore volume, and a larger adsorption capacity, resulting in stronger Cu adsorption capacity for the same volume.

[0158] 3. The impact of blending ratio on oil purification

[0159] Table 1 shows the various indicators of 100 mL of aged natural ester insulating oil purified by filter membranes prepared with different purification material ratios under a negative pressure of 0.05 MPa. Compared with Examples 1, 2, and 3, the proportion of nanoparticles in Comparative Examples 3, 4, and 5 changed, and the breakdown strength, dielectric loss, and trace water content of the purified natural ester insulating oil decreased compared with Example 1. This may be because as the proportion of BaTiO3 decreases, the formation of β-crystalline phase PVDF-HFP decreases, resulting in reduced charge stability and a weakened ability to electrostatically adsorb small polar impurities in the oil. This indicates that there is a mutually reinforcing coupling effect between the purification effects provided by the two nano-oxides in different proportions. Compared with Example 1, the proportion of intermediate adsorbents in Comparative Examples 6, 7, and 8 changed, and the trace water content, dielectric loss, and Cu content of the purified natural ester insulating oil increased compared with Example 1. This indicates that the optimal intermediate adsorbent is activated carbon and Y molecular sieve in a 1:2 ratio.

[0160] In summary, the composite fiber membrane prepared under the conditions of Example 1 has a good purification effect on natural ester insulating oil, and can effectively improve the acid value, dielectric loss performance, and trace water content of the purified natural ester insulating oil, so as to meet the standard requirements.

[0161] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A composite nanofiber membrane, characterized in that, The composite nanofiber membrane has a sandwich structure, which includes a nanofiber membrane and an adsorption fiber layer. The nanofiber membrane is located on the upper and lower sides of the adsorption fiber layer. The raw materials of the adsorption fiber layer include pulp and adsorption material. The adsorption material is a mixture of activated carbon and Y molecular sieve, and the ratio of activated carbon to Y molecular sieve is (3-4):

6. The particle size D90 of the activated carbon is 0.15-0.25 mm.

2. The composite nanofiber membrane as described in claim 1, characterized in that, The nanofiber membrane is prepared by electrospinning. The pre-spinning solution of the electrospinning method includes the following components: PVDF-HFP powder, tetrahydrofuran solution, N,N-dimethylformamide solution, and nano-oxide particles. The nano-oxide particles are a mixture of nano-BaTiO3 and nano-ZnO. The particle size D90 of the nano-BaTiO3 is 40-50 nm, the particle size D90 of the nano-ZnO is 80-90 nm, and the weight ratio of BaTiO3 to ZnO is (3-4):

1.

3. The composite nanofiber membrane as described in claim 2, characterized in that, The method for preparing the nanofiber membrane includes the following steps: (1) The dried PVDF-HFP powder was added to a mixed solution of tetrahydrofuran and N,N-dimethylformamide and dissolved completely. Nano-oxide particles were added, ultrasonically dispersed, and allowed to stand to remove bubbles to obtain a spinning precursor solution. (2) The spinning precursor solution obtained in step (1) is subjected to electrospinning and dried to obtain a composite nanofiber membrane.

4. The composite nanofiber membrane as described in claim 3, characterized in that, In step (1), the mass of the PVDF-HFP powder is 9-11 wt.% of the total mass of the spinning precursor solution, the mass of the nano-oxide particles is 8-10 wt.% of the total mass of the spinning precursor solution, and the volume ratio of the tetrahydrofuran solution to the N,N-dimethylformamide solution is 2:

3.

5. The composite nanofiber membrane as described in claim 1, characterized in that, The Y-type molecular sieve is one of AgY-modified molecular sieve, ZnY-modified molecular sieve, or CrY-modified molecular sieve.

6. The composite nanofiber membrane as described in claim 1, characterized in that, The preparation method of the adsorbent fiber layer is as follows: pulp, adsorbent material and adhesive are mixed and paper is made to obtain the adsorbent fiber layer.

7. The composite nanofiber membrane as described in claim 6, characterized in that, The pulp is at least one of softwood pulp, hardwood pulp, straw pulp, and bamboo pulp, and the binder is at least one of cationic polyacrylamide and polyamide epichlorohydrin resin.

8. A method for preparing a composite nanofiber membrane as described in claim 1, characterized in that, Includes the following steps: S1. Place the nanofiber membrane on the upper and lower sides of the adsorption fiber layer to obtain a three-layer composite structure; S2. The three-layer composite structure is hot-pressed and dried to obtain the sandwich structure composite nanofiber membrane.

9. The method for preparing the composite nanofiber membrane as described in claim 8, characterized in that, In step S1, the hot pressing temperature is 80-100℃ and the hot pressing time is 10-20 min; in step S2, the drying temperature is 100-110℃ and the drying time is 4-6 h.

10. The application of a composite nanofiber membrane as described in any one of claims 1-7 in the purification of transformer insulating oil.

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