Preparation method and device of double-layer photo-thermal membrane distillation material

Abstract

The invention relates to a preparation method of a double-layer photo-thermal membrane distillation material, and belongs to the technical field of membrane distillation. According to the double-layer film, a polyvinylidene fluoride super-hydrophobic film serves as a hydrophobic base layer, a carbon nano tube / nano semiconductor material composite nano film serves as a photo-thermal layer, and the photo-thermal layer is attached to the hydrophobic base layer through polyurethane. The preparation method is characterized by comprising the following steps: (1) preparing an electrostatic spinning polyvinylidene fluoride super-hydrophobic membrane; and (2) preparing the electrostatic spinning carbon nano tube / nano semiconductor material composite nano film. Meanwhile, the invention provides a photo-thermal driving membrane distillation device. The double-layer photo-thermal membrane distillation material prepared by adopting the method has good photo-thermal conversion capacity, relatively high water vapor flux and long-term stability, and has a wide application prospect in the aspect of 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 light by steam. Background Art

[0002] One of the main challenges that society has always faced is obtaining sufficient quality and quantity of fresh water. Although water treatment technology has made great progress in the past few years, climate change, population growth, increased agricultural activities, and rapid industrialization and urbanization have exacerbated the water crisis. Therefore, membrane distillation technology is widely used in seawater desalination, especially direct contact membrane distillation and vacuum membrane distillation. Compared with traditional membrane separation technology, photothermal membrane distillation technology has simple equipment, is suitable for a wide range of water quality, and can treat high-concentration salt water that is difficult to treat with membrane technologies such as reverse osmosis.

[0003] Solar-driven interfacial water evaporation, which can localize the heat generated by solar energy to achieve efficient water distillation, has become a promising and sustainable process to obtain fresh water from seawater or wastewater. Photothermal membrane distillation uses a hydrophobic microporous membrane and uses the vapor pressure difference on both sides of the membrane as the driving force for mass transfer. It is a new type of membrane separation process with the advantages of low operating temperature, relatively insensitive to the salinity of the feed water, and the ability to remove almost all non-volatile solutes.

[0004] At present, photothermal membrane distillation has been widely used in the fields of seawater desalination, industrial wastewater treatment, etc. However, the existing photothermal membrane has problems such as low light conversion rate and high manufacturing cost. For example, the three-layer membrane with the prior art Chinese patent application number CN202110993248.2 is complicated to manufacture, has many consumables, and has a voltage of up to 45kV. In addition, the photothermal material is single, which makes its photothermal conversion efficiency low. Therefore, it is necessary to design a photothermal membrane material to enhance the absorption conversion rate and cost of solar energy.

[0005] At the same time, the existing solar membrane distillation system has a complex structure, high manufacturing cost, and serious heat loss. For example, the solar membrane distillation device with the prior art Chinese patent application number CN201910412866.6 is semi-open, resulting in serious heat loss, large volume, high system cost and other problems. Therefore, it is necessary to design a membrane distillation device to reduce heat loss and improve economic efficiency and practicality. Summary of the invention

[0006] The purpose of the present invention is to address the problem of low membrane light conversion rate and the current situation that membrane separation requirements cannot be met well. 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., and 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 an electrostatic spinning technology, and the specific steps are as follows: (1) Preparation of superhydrophobic film The substances involved in the synthesis steps of the super-hydrophobic membrane are composed of the following mass percentages: 8-15% of the substrate, 3-8% of the modified 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 drop N,N-dimethylformamide, and finally add the modified 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 liquid. Add the obtained casting liquid to the syringe and connect the electrospinning machine to spin for 3-7 hours to obtain a polyvinylidene fluoride super-hydrophobic membrane; (2) Preparation of photothermal film material casting solution The casting liquid 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 form a uniformly dispersed solution after ultrasonic treatment for 5-20 minutes. Continue to heat and stir at 60-90℃ for 10-24 hours, with a stirring speed of 150-500r / min, so that it is evenly mixed to obtain a black casting liquid; (3) Preparation of electrospun carbon nanotube / nano-semiconductor composite nanofilm The polyvinylidene fluoride superhydrophobic base membrane prepared in (1) is fixed on the electrospinning drum, and the photothermal membrane material casting liquid prepared in (2) is added into the syringe. The electrospinning machine is connected and the syringe pump spinning parameters are set. The spinning volume is 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. A voltage is applied to the needle to make the casting liquid filamentous. Electrospin for 3-7 hours to obtain a photothermal membrane.

[0008] Preferably, the substrate of the synthetic membrane in steps (1) and (2) is one of polyvinylidene fluoride, polytetrafluoroethylene, polypropylene and polyether sulfone; and the modified material is one of thermoplastic polyurethane, polyvinyl alcohol and polyvinyl pyrrolidone.

[0009] 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.

[0010] 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.

[0011] Preferably, the electrospinning voltage in step (3) is 9-18V.

[0012] The present invention provides a photothermal driven membrane distillation fresh water treatment device, the main components of which are as follows: 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), an upright 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.

[0013] 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 photothermal conversion of the photothermal evaporation composite membrane to form steam and then enters the steam chamber. The steam is then sent to the upright double-layer serpentine coil condenser by the action of the vacuum pump at the right end for condensation and collection.

[0014] Preferably, the photothermal membrane distillation chamber is provided with a light-transmitting glass plate.

[0015] 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.

[0016] Preferably, the electrospinning film-making mechanism is to start the electrospinning machine, and under the action of the high voltage electric field, the polymer solution forms a Taylor cone from the nozzle and generates a charged jet flow. In the process of the jet flow flying to the receiving device, the solvent evaporates rapidly or the melt cools and solidifies, and finally forms fibers and deposits on the receiving device to finally form a film. Beneficial Effects

[0017] Compared with the prior art, the present invention has significant advantages: 1. This patent uses electrospinning technology to prepare a double-layer membrane for photothermal membrane distillation. The hydrophobic base membrane has a smooth surface and uniform fiber pores, which improves water vapor flux and hydrophobic properties. The micro-nano structure and low surface energy of the membrane combine to give the surface an extremely high contact angle. The membrane prepared by electrospinning technology does not require a pore-forming agent, which reduces the cost of membrane making. The polyvinylidene fluoride composite membrane has excellent photothermal conversion and water vapor transmission channels.

[0018] 2. This patent prepares a composite double-layer photothermal film, and the photothermal layer is made of a composite of carbon-based materials and nano-semiconductor materials. Carbon nanotubes absorb light energy and convert it into electron excitation energy, interact with TiO2 to improve light absorption, and the rough porous surface improves the photothermal conversion performance. Carbon nanotubes form a multi-dimensional structural design, which increases the solar energy absorption surface area, reduces reflection and heat loss, and improves thermal efficiency.

[0019] 3. This patent prepares a composite double-layer photothermal film with antibacterial properties, long-term operation stability and corrosion resistance. The double-layer design enhances the rigidity of the film, and graphene improves the mechanical strength and toughness. Nano-titanium dioxide improves the thermal stability and corrosion resistance of the film, ensuring the long-term stability of the film. Water vapor flux ≥ 0.68 kg m −2 h −1 , photothermal conversion rate ≥90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The picture of the casting solution and super hydrophobic film in Example 1 Figure 2 The picture of the casting solution and photothermal film in Example 2 Figure 3 The cross-sectional scanning electron microscope image of the double-layer membrane of Example 6 is Figure 4 Schematic diagram of the connection of the photothermal driven membrane distillation fresh water treatment device provided DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with specific embodiments. Example

[0022] (1) Add a stirring bar to the reagent bottle, add 1.1g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.3g of N,N-dimethylformamide, and finally add 0.6g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 280r / min, so that it is mixed evenly to obtain a light yellow casting liquid. Add the obtained casting liquid to the syringe and connect it to the electrospinning machine for spinning for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0023] (2) Dissolve 1.1g of polyvinylidene fluoride and 0.5g of thermoplastic polyurethane in 8.3g of N,N-dimethylformamide, heat to 80°C, and stir continuously for 30min in an airtight condition. After dissolution, add 0.1g of multi-walled carbon nanotubes to the mixed solution, and ultrasonically treat for 5min to form a uniformly dispersed solution. Continue to heat and stir at 80°C for 24h, with a stirring speed of 280r / min, so that the mixture is evenly mixed to obtain a black casting solution.

[0024] (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 1.2 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.

[0025] The performance of the double-layer photothermal membrane distillation material prepared in Example 1 was tested, and the water vapor flux of the membrane was 0.74 kgm −2 h −1 The photothermal conversion rate is 90%. After a continuous 48-hour photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 95%. Example

[0026] (1) Add a stirring bar to the reagent bottle, add 1.1g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.3g of N,N-dimethylformamide, and finally add 0.6g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 280r / min, so that it is mixed evenly to obtain a light yellow casting liquid. Add the obtained casting liquid to the syringe and connect it to the electrospinning machine for spinning for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0027] (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 under air-tight conditions. After dissolution, add 0.02g of multi-walled carbon nanotubes to the mixed solution, and ultrasonically treat for 5min to form a uniformly dispersed solution. Continue to heat and stir at 80°C for 24h, with a stirring speed of 280r / min, so that the mixture is evenly mixed to obtain a black casting solution.

[0028] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning drum. 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, 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.

[0029] The performance of the double-layer photothermal membrane distillation material prepared in Example 2 was tested, and the water vapor flux of the membrane was 0.75 kgm −2 h −1 The photothermal conversion rate is 92%. After 48 hours of continuous photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 96%. Example

[0030] (1) Add a stirring bar to the reagent bottle, add 1.1g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.3g of N,N-dimethylformamide, and finally add 0.6g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 260r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to a syringe and connect it to the electrospinning machine for spinning for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0031] (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 under air-tight conditions. After dissolution, add 0.035g of multi-walled carbon nanotubes and 0.1g of polyvinyl pyrrolidone to the mixed solution, and ultrasonically treat for 5min to form a uniformly dispersed solution. Continue to heat and stir at 80°C for 24h, with a stirring speed of 260r / min, so that the mixture is evenly mixed to obtain a black casting solution.

[0032] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning drum. 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, 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.

[0033] The performance of the double-layer photothermal membrane distillation material prepared in Example 3 was tested, and the water vapor flux of the membrane was 0.79 kgm −2 h −1 The photothermal conversion rate is 95%. After 48 hours of continuous photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 94%. Example

[0034] (1) Add a stirring bar to the reagent bottle, add 1.1g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.3g of N,N-dimethylformamide, and finally add 0.6g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 280r / min, so that it is mixed evenly to obtain a light yellow casting liquid. Add the obtained casting liquid to the syringe and connect it to the electrospinning machine for spinning for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0035] (2) Dissolve 1.1g polyvinylidene fluoride and 0.6g thermoplastic polyurethane in 8.3g N,N-dimethylformamide, heat to 80°C, and stir continuously for 30min in an airtight condition. After dissolution, add 0.05g multi-walled carbon nanotubes and 0.05g nano-titanium dioxide to the mixed solution, and ultrasonically treat for 5min to form a uniformly dispersed solution. Continue to heat and stir at 80°C for 24h, with a stirring speed of 280r / min, so that the mixture is evenly mixed to obtain a black casting solution.

[0036] (3) Attach the polyvinylidene fluoride superhydrophobic base film prepared in (1) to aluminum foil, and then fix it flat on the electrospinning drum. 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, 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.

[0037] The performance of the double-layer photothermal membrane distillation material prepared in Example 4 was tested, 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

[0038] (1) Add a stirring bar to the reagent bottle, add 1.2g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.1g of N,N-dimethylformamide, and finally add 0.7g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 280r / min to mix evenly to obtain a light yellow casting solution. Add the obtained casting solution to the syringe and connect the electrospinning machine to spin for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0039] (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.

[0040] (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 1.2 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.

[0041] The performance of the double-layer photothermal membrane distillation material prepared in Example 5 was tested, and the water vapor flux of the membrane was 0.76 kgm −2 h −1 The photothermal conversion rate is 94%. After 48 hours of continuous photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 93%. Example

[0042] (1) Add a stirring bar to the reagent bottle, add 1.2g of polyvinylidene fluoride to the reagent bottle, then slowly add 8.1g of N,N-dimethylformamide, and finally add 0.7g of thermoplastic polyurethane. Heat to 80℃ and stir continuously for 12 hours under air-tight conditions, with the stirring speed of 290r / min, so that it is mixed evenly to obtain a light yellow casting liquid. Add the obtained casting liquid to the syringe and connect it to the electrospinning machine for spinning for 4.3 hours to obtain a polyvinylidene fluoride superhydrophobic film.

[0043] (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.

[0044] (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 1.2 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.

[0045] The performance of the double-layer photothermal membrane distillation material prepared in Example 6 was tested, and the water vapor flux of the membrane was 0.78 kgm −2 h −1 The photothermal conversion rate is 93%. After 48 hours of continuous photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 95%. Example

[0046] (1) Add a stirring bar to a reagent bottle, add 1.1 g of polyvinylidene fluoride to the reagent bottle, then slowly drop 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 condition, with a stirring speed of 280 r / min, to mix evenly to obtain a light yellow casting solution.

[0047] (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. Dissolve 1.3 g of polyvinylidene fluoride in 48 g of N-methyl pyrrolidone, then add 1 g of polyvinyl pyrrolidone and 0.2 g of multi-walled carbon nanotubes and heat and stir. After dissolution, continue to heat and stir at 60°C for 12 h until a transparent casting solution is formed, and let it stand for 12 h to degas. Then use an automatic coating machine to evenly coat the casting solution on a smooth glass at a speed of 30 mm / s, and control the thickness of the liquid film to be 250 μm. Wash the formed film with pure water until there is no obvious foam in the washing liquid to obtain a base film. Store in pure water.

[0048] (3) Attach the photothermal base film prepared in (2) to aluminum foil, and then fix it flat on the electrospinning drum. 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.

[0049] The performance of the double-layer photothermal membrane distillation material prepared in Example 7 was tested, 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

[0050] (1) Add a stirring bar to a reagent bottle, add 1.1 g of polyvinylidene fluoride to the reagent bottle, then slowly drop 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 condition, with a stirring speed of 290 r / min, to mix evenly to obtain a light yellow casting solution.

[0051] (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. Dissolve 6 g of polyvinylidene fluoride in 43 g of N-methyl pyrrolidone, then add 1 g of polyvinyl pyrrolidone and 0.25 g of multi-walled carbon nanotubes and heat and stir. After dissolution, continue to heat and stir at 60°C for 12 h until a transparent casting solution is formed, and let it stand for 12 h to degas. Then use an automatic coating machine to evenly coat the casting solution on a smooth glass at a speed of 30 mm / s, and control the thickness of the liquid film to be 250 μm. Wash the formed film with pure water until there is no obvious foam in the washing liquid to obtain a base film. Store in pure water.

[0052] (3) Attach the photothermal base film prepared in (2) to aluminum foil, and then fix it flat on the electrospinning drum. 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.

[0053] The performance of the double-layer photothermal membrane distillation material prepared in Example 8 was tested, and the water vapor flux of the membrane was 0.69 kgm −2h −1 The photothermal conversion rate is 95%. After 48 hours of continuous photothermal membrane distillation experiment on the double-layer photothermal membrane distillation material, its performance can still reach 92%. Example

[0054] (1) Add a stirring bar to a reagent bottle, add 1.1 g of polyvinylidene fluoride to the reagent bottle, then slowly drop 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 condition, with a stirring speed of 290 r / min, to mix evenly to obtain a light yellow casting solution.

[0055] (2) The casting solution of the photothermal film is prepared by the following components in mass percentage: 2% multi-walled carbon nanotubes, 1% polyvinyl pyrrolidone, and 94.4% N-methyl pyrrolidone. Dissolve 1.3 g of polyvinylidene fluoride in 47 g of N-methyl pyrrolidone, then add 0.5 g of polyvinyl pyrrolidone and 1.5 g of multi-walled carbon nanotubes and heat and stir. After dissolution, continue to heat and stir at 60°C for 24 hours to form a transparent casting solution, and let it stand for 12 hours to degas. Then use an automatic coating machine to evenly coat the casting solution on a smooth glass at a speed of 30 mm / s, and control the liquid film thickness to 250 μm. Wash the formed film with pure water until there is no obvious foam in the washing liquid to obtain a base film. Store in pure water.

[0056] (3) Attach the photothermal base film prepared in (2) to aluminum foil, and then fix it flat on the electrospinning drum. 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.

[0057] The performance of the double-layer photothermal membrane distillation material prepared in Example 9 was tested, and 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 super-hydrophobic membrane are composed of the following mass percentages: 8-15% of the substrate, 3-8% of the modified 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 drop N,N-dimethylformamide, and finally add the modified 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 liquid. Add the obtained casting liquid to the syringe and connect the electrospinning machine to spin for 3-7 hours to obtain a polyvinylidene fluoride super-hydrophobic membrane; (2) Preparation of photothermal film material casting solution The casting liquid 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 a stirring speed of 150-500r / min, so that it is evenly mixed to obtain a black casting liquid; (3) Preparation of electrospun carbon nanotube / nano-semiconductor composite nanofilm The polyvinylidene fluoride superhydrophobic base membrane prepared in (1) is fixed on the electrospinning drum, and the photothermal membrane material casting liquid prepared in (2) is added into the syringe. The electrospinning machine is connected and the syringe pump spinning parameters are set. The spinning volume is 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. A voltage is applied to the needle to make the casting liquid filamentous. Electrospinning is performed for 3-7 hours to obtain a photothermal membrane.

2. The preparation method according to claim 1, characterized in that 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, characterized in that 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 In step (2), the carbon-based material 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 9-18V.

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; an independent steam chamber, a photothermal evaporation composite membrane, and a water supply chamber are arranged in the photothermal membrane distillation chamber from top to bottom, and 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 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: A backflow prevention ball is provided at the bottom of the upright double-layer serpentine coil condenser, and the backflow prevention ball is connected to the steam inlet. The steam generated in the photothermal membrane distillation chamber enters the condenser from the steam inlet and condenses 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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