A preparation method and device for double-layer photothermal membrane distillation material

Abstract

The present invention relates to a method for preparing a double-layer photothermal membrane distillation material, and belongs to the field of membrane distillation technology. The double-layer membrane is composed of a polyvinylidene fluoride super-hydrophobic membrane as a hydrophobic base layer and a carbon nanotube / nano-semiconductor material composite nano-membrane as a photothermal layer, and the photothermal layer is adhered to the hydrophobic base layer using polyurethane. Its characteristics include: (1) preparation of electrostatically spun polyvinylidene fluoride super-hydrophobic membrane; (2) preparation of electrostatically spun carbon nanotube / nano-semiconductor material composite nano-membrane. At the same time, a photothermal-driven membrane distillation device is provided. The double-layer photothermal membrane distillation material prepared by the present invention has good photothermal conversion ability, high water vapor flux and long-term stability, and has broad application prospects in fresh water treatment.

Description

Technical Field

[0001] The present invention belongs to the technical field of photothermal membrane distillation, and specifically relates to a method and device for preparing a double-layer membrane for photothermal membrane distillation. The photothermal membrane has the advantages of high photothermal conversion efficiency, high steam flux, and good anti-fouling effect. The device can efficiently perform photothermal conversion and effectively reduce the obstruction of steam to light. Background Art

[0002] One of the major challenges society continues to face is access to sufficient quality and quantity of fresh water. Despite significant advances in water treatment technology over the past few years, climate change, population growth, increased agricultural activity, and rapid industrialization and urbanization have exacerbated the water crisis. Consequently, membrane distillation technology has been widely used in seawater desalination, with direct contact membrane distillation and vacuum membrane distillation being particularly popular. Compared to traditional membrane separation technologies, photothermal membrane distillation technology offers simpler equipment, adaptability to a wide range of water qualities, and the ability to treat high-concentration saltwater that is difficult to treat with other membrane technologies, such as reverse osmosis.

[0003] Solar-driven interfacial water evaporation, which localizes solar heat for efficient water distillation, has emerged as a promising and sustainable process for obtaining freshwater from seawater or wastewater. Photothermal membrane distillation, which utilizes a hydrophobic microporous membrane with the vapor pressure difference across the membrane as the driving force for mass transfer, is a novel membrane separation process with advantages such as low operating temperatures, relative insensitivity to feedwater salinity, and the ability to remove nearly all non-volatile solutes.

[0004] Photothermal membrane distillation is currently widely used in fields such as seawater desalination and industrial wastewater treatment. However, existing photothermal membranes suffer from low light conversion efficiency and high manufacturing costs. For example, the three-layer membrane in the prior art (Chinese patent application number CN202110993248.2) is complex to manufacture, requires many consumable materials, and requires a high voltage of 45kV. Furthermore, the single photothermal material results in low photothermal conversion efficiency. Therefore, it is necessary to design a photothermal membrane material to enhance the solar energy absorption conversion efficiency and reduce costs.

[0005] At the same time, existing solar membrane distillation systems are complex in structure, expensive to manufacture, and suffer from significant heat loss. For example, the existing solar membrane distillation device in Chinese Patent Application No. CN201910412866.6 is semi-open, resulting in significant heat loss, bulky design, and high system costs. Therefore, it is necessary to design a membrane distillation device that reduces heat loss and improves economic efficiency and practicality. Summary of the Invention

[0006] The purpose of the present invention is to address the current problem of low membrane light conversion rate and the inability to meet the requirements of membrane separation. A method and device for preparing a double-layer photothermal membrane distillation material are proposed. The double-layer photothermal membrane prepared by this method has the advantages of high water vapor flux, good anti-fouling performance, good hydrophobic effect, etc., is suitable for membrane distillation separation process, and can ensure excellent separation effect and long-term stable operation. At the same time, the present invention provides a membrane distillation device with low heat loss and low equipment cost. Through the innovative method and device of the present invention, the light conversion rate of the photothermal membrane is improved, the problems existing in the membrane separation process are solved, and new breakthroughs are brought to the development of membrane distillation technology. The double-layer photothermal membrane distillation material of the present invention has excellent performance and stability, will play an important role in the field of water treatment, and provide reliable technical support for the acquisition and utilization of fresh water resources.

[0007] The double-layer photothermal membrane distillation material prepared by the present invention is first modified by a blending modification method, and then the casting solution is prepared by electrostatic spinning technology. The specific steps are as follows:

[0008] (1) Preparation of superhydrophobic film

[0009] The substances involved in the synthesis step of the superhydrophobic membrane are composed of the following mass percentages: 8-15% of the substrate, 3-8% of the modifying material, and the remainder is N,N-dimethylformamide. Add a stirring bar to the reagent bottle, add the substrate to the reagent bottle, then slowly add N,N-dimethylformamide dropwise, and finally add the modifying material. Heat to 60-90°C and stir continuously for 12-24 hours under air-tight conditions, wherein the stirring speed is 150-500r / min, so that it is evenly mixed to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect the electrospinning machine to spin for 3-7 hours to obtain a polyvinylidene fluoride superhydrophobic membrane;

[0010] (2) Preparation of photothermal film casting solution

[0011] The casting solution of the photothermal film is prepared by the following components in mass percentage: 0.1-2% carbon-based material, 0.1-2% nano-semiconductor material, 7-16% substrate, 3-8% modified material, and the remainder is organic solvent. Dissolve polyvinylidene fluoride and adhesive in an organic solvent and keep heating and stirring for 5-30 minutes. After dissolution, add different weights of carbon-based material / nano-semiconductor material and dispersant to the above mixture and ultrasonically treat for 5-20 minutes to form a uniformly dispersed solution. Continue heating and stirring at 60-90℃ for 10-24 hours, with a stirring speed of 150-500r / min, to ensure uniform mixing and obtain a black casting solution.

[0012] (3) Preparation of electrospun carbon nanotube / nano-semiconductor composite nanofilms

[0013] The polyvinylidene fluoride superhydrophobic base film prepared in (1) is fixed on the electrospinning drum, and the photothermal film material casting liquid prepared in (2) is added to the syringe. The electrospinning machine is connected, and the injection pump spinning parameters are set to 5-8 ml and the spinning rate is 0.9-1.7 ml / h. The injection pump is pushed to the bottom of the syringe so that the casting liquid just flows out of the syringe needle. Voltage is applied to the needle to make the casting liquid into filaments. Electrospinning is carried out for 3-7 hours to obtain a photothermal film.

[0014] Preferably, the base material of the synthetic membrane in steps (1) and (2) is one of polyvinylidene fluoride, polytetrafluoroethylene, polypropylene and polyethersulfone; and the modified material is one of thermoplastic polyurethane, polyvinyl alcohol and polyvinyl pyrrolidone.

[0015] Preferably, the carbon-based material in step (2) is one of graphitized hydroxyl multi-walled carbon nanotubes, multi-walled carbon nanotubes, and nano-carbon powder.

[0016] Preferably, in step (2), the nano-semiconductor material is one of nano-silicon dioxide, nano-titanium dioxide, and nano-zirconium dioxide; and the organic solvent is one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0017] Preferably, the electrospinning voltage in step (3) is 9-18V.

[0018] The present invention provides a photothermal driven membrane distillation fresh water treatment device, the main components of which are as follows:

[0019] The photothermal driven membrane distillation device comprises, from left to right and from top to bottom, a water storage chamber (1), a sunlight simulation device (4), a photothermal membrane distillation chamber (2), a vertical double-layer serpentine coil condenser (3), a fresh water collector (5), an electronic balance (6), and a vacuum pump (7); the photothermal membrane distillation chamber is provided with an independent steam chamber, a photothermal evaporation composite membrane, and a water supply chamber, from top to bottom.

[0020] Preferably, the bottom of the photothermal evaporation composite membrane is close to the water surface of the water supply chamber. The water in the water supply chamber is evaporated by the photothermal conversion of the photothermal evaporation composite membrane to form steam and then enters the steam chamber. The steam is then sent into the upright double-layer serpentine coil condenser by the action of the vacuum pump at the right end for condensation and collection.

[0021] Preferably, the photothermal membrane distillation chamber is provided with a light-transmitting glass plate.

[0022] Preferably, a backflow prevention ball is provided at the bottom of the upright double-layer serpentine coil condenser, and the condensing agent in the condenser is acetone.

[0023] Preferably, the electrospinning membrane formation mechanism involves activating an electrospinning machine. Under the action of a high-voltage electric field, the polymer solution forms a Taylor cone from the nozzle, generating a charged jet. As the jet travels toward a receiving device, the solvent rapidly evaporates or the melt cools and solidifies, ultimately forming fibers that are deposited on the receiving device and ultimately form a membrane. Beneficial effects

[0024] Compared with the prior art, the present invention has significant advantages:

[0025] 1. This patent describes a double-layer membrane fabricated using electrospinning technology for photothermal membrane distillation. The hydrophobic base membrane features a smooth surface and uniform fiber pores, enhancing water vapor flux and hydrophobic properties. The membrane's micro-nanostructure and low surface energy combine to create an extremely high contact angle. Electrospinning eliminates the need for pore-forming agents, reducing production costs. The polyvinylidene fluoride composite membrane exhibits excellent photothermal conversion and water vapor transport pathways.

[0026] 2. This patented method utilizes a composite double-layer photothermal film, the photothermal layer of which is composed of a carbon-based material and a nano-semiconductor material. Carbon nanotubes absorb light energy and convert it into electron excitation energy. This interaction with TiO2 enhances light absorption, while the rough, porous surface improves photothermal conversion performance. The multi-dimensional structure of carbon nanotubes increases the solar absorption surface area, reduces reflection and heat loss, and improves thermal efficiency.

[0027] 3. This patented composite double-layer photothermal membrane exhibits antibacterial properties, long-term operational stability, and corrosion resistance. The double-layer design enhances membrane rigidity, while graphene improves mechanical strength and toughness. Nano-titanium dioxide enhances the membrane's thermal stability and corrosion resistance, ensuring long-term membrane stability. Water vapor flux ≥ 0.68 kg m −2 h −1 , photothermal conversion rate ≥90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Pictures of the casting solution and super-hydrophobic film in Example 1

[0029] Figure 2 Pictures of the casting solution and photothermal film in Example 2

[0030] Figure 3 This is a cross-sectional scanning electron microscope image of the double-layer membrane of Example 6

[0031] Figure 4 Schematic diagram of the connection of the photothermal driven membrane distillation fresh water treatment device provided DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to specific embodiments. Example

[0033] (1) Add a stirring bar to a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 280 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic membrane.

[0034] (2) Dissolve 1.1 g of polyvinylidene fluoride and 0.5 g of thermoplastic polyurethane in 8.3 g of N,N-dimethylformamide. Heat to 80°C and stir continuously for 30 min in an airtight environment. After dissolution, add 0.1 g of multi-walled carbon nanotubes to the mixed solution and ultrasonicate for 5 min to form a uniformly dispersed solution. Continue heating and stirring at 80°C for 24 h at a stirring speed of 280 r / min to ensure uniform mixing and obtain a black casting solution.

[0035] (3) The polyvinylidene fluoride superhydrophobic base film prepared in (1) was attached to aluminum foil, and then fixed flat on the electrospinning roller. The photothermal film material casting liquid prepared in (2) was added to the syringe, and the electrospinning machine was connected. The syringe pump spinning parameters were set to 6 ml and the spinning rate was 1.2 ml / h. The syringe pump was pushed to the bottom of the syringe so that the casting liquid just flowed out of the syringe needle. Voltage was applied to the needle to make the casting liquid into filaments. Electrospinning was performed for 5 hours to obtain the photothermal film.

[0036] The performance test of the double-layer photothermal membrane distillation material prepared in Example 1 was carried out, and the water vapor flux of the membrane was 0.74 kgm −2 h −1 The photothermal conversion rate is 90%, and the performance of the double-layer photothermal membrane distillation material can still reach 95% after a continuous 48-hour photothermal membrane distillation experiment. Example

[0037] (1) Add a stirring bar to a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 280 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic membrane.

[0038] (2) Dissolve 1.1 g of polyvinylidene fluoride and 0.6 g of thermoplastic polyurethane in 8.3 g of N,N-dimethylformamide. Heat to 80°C and stir continuously for 30 min in an airtight environment. After dissolution, add 0.02 g of multi-walled carbon nanotubes to the mixed solution and ultrasonicate for 5 min to form a uniformly dispersed solution. Continue heating and stirring at 80°C for 24 h at a stirring speed of 280 r / min to ensure uniform mixing and obtain a black casting solution.

[0039] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning roller. Add the photothermal film material casting liquid prepared in (2) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters, the spinning volume is 6 ml, the spinning rate is 1.3 ml / h, push the injection pump to the bottom of the syringe so that the casting liquid just flows out of the syringe needle, apply voltage to the needle to make the casting liquid into filaments, and electrospin for 5 hours to obtain the photothermal film.

[0040] The performance test of the double-layer photothermal membrane distillation material prepared in Example 2 was carried out, and the water vapor flux of the membrane was 0.75 kgm −2 h −1 The photothermal conversion rate is 92%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 96%. Example

[0041] (1) Add a stirring bar to a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 260 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic film.

[0042] (2) Dissolve 1.1 g of polyvinylidene fluoride and 0.6 g of thermoplastic polyurethane in 8.3 g of N,N-dimethylformamide. Heat to 80°C and stir continuously for 30 min in an airtight state. After dissolution, add 0.035 g of multi-walled carbon nanotubes and 0.1 g of polyvinyl pyrrolidone to the mixed solution. Ultrasonicate for 5 min to form a uniformly dispersed solution. Continue heating and stirring at 80°C for 24 h at a stirring speed of 260 r / min to ensure uniform mixing and obtain a black casting solution.

[0043] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning roller. Add the photothermal film material casting liquid prepared in (2) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters, the spinning volume is 6 ml, the spinning rate is 1.3 ml / h, push the injection pump to the bottom of the syringe so that the casting liquid just flows out of the syringe needle, apply voltage to the needle to make the casting liquid into filaments, and electrospin for 5 hours to obtain the photothermal film.

[0044] The performance test of the double-layer photothermal membrane distillation material prepared in Example 3 showed that the water vapor flux of the membrane was 0.79 kgm −2 h −1 The photothermal conversion rate is 95%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 94%. Example

[0045] (1) Add a stirring bar to a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 280 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic membrane.

[0046] (2) Dissolve 1.1g of polyvinylidene fluoride and 0.6g of thermoplastic polyurethane in 8.3g of N,N-dimethylformamide. Heat to 80°C and stir continuously for 30min in an airtight state. After dissolution, add 0.05g of multi-walled carbon nanotubes and 0.05g of nano-titanium dioxide to the mixed solution. Ultrasonicate for 5min to form a uniformly dispersed solution. Continue heating and stirring at 80°C for 24h at a stirring speed of 280r / min to ensure uniform mixing and obtain a black casting solution.

[0047] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning roller. Add the photothermal film material casting liquid prepared in (2) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters, the spinning volume is 6 ml, the spinning rate is 1.3 ml / h, push the injection pump to the bottom of the syringe so that the casting liquid just flows out of the syringe needle, apply voltage to the needle to make the casting liquid into filaments, and electrospin for 5 hours to obtain the photothermal film.

[0048] The performance test of the double-layer photothermal membrane distillation material prepared in Example 4 was carried out, and the water vapor flux of the membrane was 0.78 kgm −2 h−1 The photothermal conversion rate is 96%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 94%. Example

[0049] (1) Add a stirring bar to a reagent bottle and add 1.2 g of polyvinylidene fluoride to the bottle. Then slowly add 8.1 g of N,N-dimethylformamide and finally add 0.7 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 280 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic membrane.

[0050] (2) Dissolve 1.1 g of polyvinylidene fluoride in 8.2 g of N,N-dimethylformamide, then add 0.05 g of multi-walled carbon nanotubes and 0.2 g of nano-titanium dioxide to the mixed solution, and finally add 0.45 g of polyvinyl alcohol. Heat and stir at 80°C for 24 h to form a uniformly dispersed solution, with the stirring speed at 340 r / min to ensure uniform mixing and obtain a black casting solution.

[0051] (3) The polyvinylidene fluoride superhydrophobic base film prepared in (1) was attached to aluminum foil, and then fixed flat on the electrospinning roller. The photothermal film material casting liquid prepared in (2) was added to the syringe, and the electrospinning machine was connected. The syringe pump spinning parameters were set to 6 ml and the spinning rate was 1.2 ml / h. The syringe pump was pushed to the bottom of the syringe so that the casting liquid just flowed out of the syringe needle. Voltage was applied to the needle to make the casting liquid into filaments. Electrospinning was performed for 5 hours to obtain the photothermal film.

[0052] The performance test of the double-layer photothermal membrane distillation material prepared in Example 5 showed that the water vapor flux of the membrane was 0.76 kgm −2 h −1 The photothermal conversion rate is 94%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 93%. Example

[0053] (1) Add a stirring bar to a reagent bottle and add 1.2 g of polyvinylidene fluoride to the bottle. Then slowly add 8.1 g of N,N-dimethylformamide and finally add 0.7 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 290 r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to an electrospinning machine for spinning for 4.3 h to obtain a polyvinylidene fluoride superhydrophobic film.

[0054] (2) Dissolve 1.1 g of polyvinylidene fluoride in 8.2 g of N,N-dimethylformamide, then add 0.1 g of multi-walled carbon nanotubes and 0.1 g of nano-titanium dioxide to the mixed solution, and finally add 0.5 g of thermoplastic polyurethane. Heat and stir at 80°C for 24 h to form a uniformly dispersed solution, with the stirring speed at 340 r / min to ensure uniform mixing and obtain a black casting solution.

[0055] (3) The polyvinylidene fluoride superhydrophobic base film prepared in (1) was attached to aluminum foil, and then fixed flat on the electrospinning roller. The photothermal film material casting liquid prepared in (2) was added to the syringe, and the electrospinning machine was connected. The syringe pump spinning parameters were set to 6 ml and the spinning rate was 1.2 ml / h. The syringe pump was pushed to the bottom of the syringe so that the casting liquid just flowed out of the syringe needle. Voltage was applied to the needle to make the casting liquid into filaments. Electrospinning was performed for 5 hours to obtain the photothermal film.

[0056] The performance test of the double-layer photothermal membrane distillation material prepared in Example 6 was carried out, and the water vapor flux of the membrane was 0.78 kgm −2 h −1 The photothermal conversion rate is 93%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 95%. Example

[0057] (1) Place a stirring bar in a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly add 8.3 g of N,N-dimethylformamide dropwise, and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 280 r / min to obtain a light yellow casting solution.

[0058] (2) The casting solution of the photothermal film is prepared by the following components in mass percentage: 1% multi-walled carbon nanotubes, 1% polyvinyl pyrrolidone, and 95.4% N-methyl pyrrolidone. 1.3 g of polyvinylidene fluoride is dissolved in 48 g of N-methyl pyrrolidone, and then 1 g of polyvinyl pyrrolidone and 0.2 g of multi-walled carbon nanotubes are added and heated and stirred. After dissolution, the solution is heated and stirred at 60°C for 12 h until a transparent casting solution is formed. The solution is then allowed to stand for 12 h to degas. The casting solution is then evenly coated on a smooth glass at a speed of 30 mm / s using an automatic coating machine, and the film thickness is controlled to be 250 μm. The formed film is washed with pure water until there is no obvious foam in the washing solution to obtain a base film. The film is stored in pure water.

[0059] (3) Attach the photothermal base film prepared in (2) to the aluminum foil, and then fix it flatly on the electrospinning roller. Add the photothermal film material casting liquid prepared in (1) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters. The spinning volume is 6 ml, the spinning rate is 1.2 ml / h, and the injection pump is pushed to the bottom of the syringe so that the casting liquid just flows out of the syringe needle. Apply voltage to the needle to make the casting liquid into filaments. Electrospin for 5 hours to obtain the photothermal film.

[0060] The performance test of the double-layer photothermal membrane distillation material prepared in Example 7 was carried out, and the water vapor flux of the membrane was 0.68 kgm −2 h −1 The photothermal conversion rate is 94%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 89%. Example

[0061] (1) Place a stirring bar in a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly dropwise add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 290 r / min to obtain a light yellow casting solution.

[0062] (2) The casting solution of the photothermal film is prepared by the following components in mass percentage: 0.25% multi-walled carbon nanotubes, 2% polyvinyl pyrrolidone, and 86% N-methyl pyrrolidone. 6 g of polyvinylidene fluoride is dissolved in 43 g of N-methyl pyrrolidone, and then 1 g of polyvinyl pyrrolidone and 0.25 g of multi-walled carbon nanotubes are added and heated and stirred. After dissolution, the solution is heated and stirred at 60°C for 12 h until a transparent casting solution is formed. The solution is then allowed to stand for 12 h to degas. The casting solution is then evenly coated on a smooth glass at a speed of 30 mm / s using an automatic coating machine, and the film thickness is controlled to be 250 μm. The formed film is washed with pure water until there is no obvious foam in the washing solution to obtain a base film. The film is stored in pure water.

[0063] (3) Attach the photothermal base film prepared in (2) to the aluminum foil, and then fix it flatly on the electrospinning roller. Add the photothermal film material casting liquid prepared in (1) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters. The spinning volume is 6 ml, the spinning rate is 1.2 ml / h, and the injection pump is pushed to the bottom of the syringe so that the casting liquid just flows out of the syringe needle. Apply voltage to the needle to make the casting liquid into filaments. Electrospin for 5 hours to obtain the photothermal film.

[0064] The performance test of the double-layer photothermal membrane distillation material prepared in Example 8 was carried out, and the water vapor flux of the membrane was 0.69 kgm −2h −1 The photothermal conversion rate is 95%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 92%. Example

[0065] (1) Place a stirring bar in a reagent bottle and add 1.1 g of polyvinylidene fluoride to the bottle. Then slowly dropwise add 8.3 g of N,N-dimethylformamide and finally add 0.6 g of thermoplastic polyurethane. Heat to 80°C and stir continuously for 12 h in an airtight state at a stirring speed of 290 r / min to obtain a light yellow casting solution.

[0066] (2) The casting solution of the photothermal film is prepared by the following components in percentage by mass: 2% multi-walled carbon nanotubes, 1% polyvinyl pyrrolidone, and 94.4% N-methyl pyrrolidone. 1.3 g of polyvinylidene fluoride is dissolved in 47 g of N-methyl pyrrolidone, and then 0.5 g of polyvinyl pyrrolidone and 1.5 g of multi-walled carbon nanotubes are added and heated and stirred. After dissolution, the solution is heated and stirred at 60°C for 24 h until a transparent casting solution is formed, and the solution is allowed to stand for 12 h to degas. The casting solution is then evenly coated on a smooth glass at a speed of 30 mm / s using an automatic coating machine, and the film thickness is controlled to be 250 μm. The formed film is washed with pure water until there is no obvious foam in the washing solution to obtain a base film. Store in pure water.

[0067] (3) Attach the photothermal base film prepared in (2) to the aluminum foil, and then fix it flatly on the electrospinning roller. Add the photothermal film material casting liquid prepared in (1) into the syringe, connect the electrospinning machine, start the machine and set the injection pump spinning parameters. The spinning volume is 6 ml, the spinning rate is 1.2 ml / h, and the injection pump is pushed to the bottom of the syringe so that the casting liquid just flows out of the syringe needle. Apply voltage to the needle to make the casting liquid into filaments. Electrospin for 5 hours to obtain the photothermal film.

[0068] The performance test of the double-layer photothermal membrane distillation material prepared in Example 9 showed that the water vapor flux of the membrane was 0.69 kgm −2 h −1 The photothermal conversion rate is 96%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 93%.

Claims

1. A method for preparing a double-layer membrane for photothermal driven membrane distillation, the specific steps of which are as follows: (1) Preparation of superhydrophobic film The substances involved in the synthesis steps of the superhydrophobic membrane are composed of the following mass percentages: 8-15% of the substrate, 3-8% of the modifying material, and the remainder is N,N-dimethylformamide. Add a stirring bar to the reagent bottle, add the substrate to the reagent bottle, then slowly add N,N-dimethylformamide dropwise, and finally add the modifying material. Heat to 60-90°C and stir continuously for 12-24 hours under air-tight conditions, wherein the stirring speed is 150-500 r / min to mix it evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect the electrospinning machine to spin for 3-7 hours to obtain a polyvinylidene fluoride superhydrophobic membrane; (2) Preparation of photothermal film casting solution The casting solution of the photothermal film is prepared by the following components in mass percentage: 0.1-2% carbon-based material, 0.1-2% nano-semiconductor material, 7-16% polyvinylidene fluoride, 3-8% modified material, and the remainder is organic solvent. Dissolve polyvinylidene fluoride and adhesive in an organic solvent and keep heating and stirring for 5-30 minutes. After dissolution, add different weights of carbon-based material / nano-semiconductor material and dispersant to the above mixture, and ultrasonically treat for 5-20 minutes to form a uniformly dispersed solution. Continue to heat and stir at 60-90℃ for 12-24 hours, with the stirring speed of 150-500 r / min, so that the mixture is evenly mixed to obtain a black casting solution; (3) Preparation of electrospun carbon nanotube / nano-semiconductor composite nanofilms The polyvinylidene fluoride superhydrophobic base film prepared in (1) is fixed on the electrospinning drum, and the photothermal film material casting liquid prepared in (2) is added to the syringe. The electrospinning machine is connected, and the syringe pump spinning parameters are set to 5-8 ml and the spinning rate is 0.9-1.7 ml / h. The syringe pump is pushed to the bottom of the syringe so that the casting liquid just flows out of the syringe needle. Voltage is applied to the needle to make the casting liquid into filaments. Electrospinning is carried out for 3-7 hours to obtain the photothermal film.

2. The preparation method according to claim 1, wherein The substrate for synthesizing the super-hydrophobic film in step (1) is one of polyvinylidene fluoride, polytetrafluoroethylene, and polypropylene.

3. The preparation method according to claim 1, wherein The modified material in step (1) is one of thermoplastic polyurethane and polyvinyl alcohol.

4. The preparation method according to claim 1, characterized in that The carbon-based material in step (2) is one of graphitized hydroxyl multi-walled carbon nanotubes, multi-walled carbon nanotubes, and nano-carbon powder.

5. The preparation method according to claim 1, characterized in that The nano-semiconductor material in step (2) is one of nano-silicon dioxide, nano-titanium dioxide and nano-zirconium dioxide.

6. The preparation method according to claim 1, characterized in that The organic solvent in step (2) is one of N,N-dimethylacetamide, N,N-dimethylformamide and N-methylpyrrolidone.

7. The preparation method according to claim 1, characterized in that The voltage in step (3) is 9-18 kV.

8. A photothermal driven membrane distillation fresh water treatment device, characterized in that: The photothermal driven membrane distillation device includes a water storage chamber, a sunlight simulation device, a photothermal membrane distillation chamber, an upright double-layer serpentine coil condenser, a fresh water collector, an electronic balance, and a vacuum pump from left to right and from top to bottom; the photothermal membrane distillation chamber is provided with an independent steam chamber, a photothermal evaporation composite membrane, and a water supply chamber from top to bottom, the bottom of the photothermal evaporation composite membrane is close to the water surface of the water supply chamber, and the photothermal evaporation composite membrane is a photothermal driven membrane distillation double-layer membrane prepared by the preparation method according to any one of claims 1 to 7; the water in the water supply chamber is evaporated by photothermal conversion through the photothermal evaporation composite membrane to form steam and then enters the steam chamber, and then the steam is sent to the upright double-layer serpentine coil condenser by the action of the vacuum pump at the right end for condensation and collection.

9. The photothermal driven membrane distillation fresh water treatment device according to claim 8, characterized in that: The photothermal membrane distillation chamber is provided with a light-transmitting glass plate.

10. The photothermal driven membrane distillation fresh water treatment device according to claim 8, characterized in that: An anti-backflow ball is provided at the bottom of the upright double-layer serpentine coil condenser, which is connected to the steam inlet. The steam generated in the photothermal membrane distillation chamber enters the condenser from the steam inlet, is condensed, and then flows into the fresh water collector. The condensing agent in the upright double-layer serpentine coil condenser is acetone.

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