Preparation and application of high-throughput flat ceramic-based carbon membranes

Abstract

The preparation and application of high-throughput flat ceramic-based carbon membranes belong to the field of membrane water treatment technology. The high-throughput flat ceramic membranes are prepared by phase transformation and high-temperature sintering technology, the nanocatalysts are loaded by the impregnation-calcination-reduction method, and the synthesis conditions of carbon nanotubes are optimized. The carbon nanotube membrane formed on the flat ceramic membrane carrier has a higher network porosity and a more uniform distribution. The formed carbon nanotube network structure membrane not only has a high porosity, but also ensures its good electrical conductivity and light absorption properties. With the help of an external light source or power supply, through the interface heating membrane distillation process, the composite membrane is used to treat salt water of different concentrations. Compared with the traditional membrane distillation process, it has a higher water flux, smaller temperature polarization and lighter membrane pollution under the condition of ensuring a high salt rejection rate. The present invention provides an efficient and energy-saving membrane treatment process for saline wastewater.

Description

Technical Field

[0001] The invention relates to a process for preparing a flat ceramic-based carbon membrane with high flux, high conductivity and light absorption performance and treating saline wastewater by using an interface heating membrane distillation, and belongs to the technical field of membrane water treatment. Background Art

[0002] Water shortage is one of the global issues that restricts the sustainable development of mankind. With the desalination of seawater, a lot of high-salinity wastewater will be produced. If this kind of wastewater is discharged directly, it will not only endanger the ecological environment and cause water pollution, but also waste the water and salt resources. At the same time, with the improvement of industrialization level, the discharge of industrial saline wastewater is also increasing, and the efficient treatment of saline wastewater is particularly urgent.

[0003] Compared with other water treatment processes, membrane distillation is more suitable for treating saline wastewater, especially high-salt wastewater. During the membrane distillation process, under the driving force of the vapor pressure difference, the steam generated by the evaporation of water on the feed side enters the condensation side through the membrane pores. The heat required for evaporation is provided by the feed side. The required latent heat of vaporization and the heat conducted through the membrane will cause a temperature gradient between the raw liquid and the membrane surface, making the transmembrane temperature difference smaller than the temperature difference between the feed side and the condensation side, and the driving force for mass transfer is reduced. This phenomenon is called temperature polarization. Traditional membrane distillation also has the problem of membrane pollution. The existence of temperature polarization and membrane pollution will reduce the driving force for transmembrane mass transfer and reduce the flux. Therefore, to increase the membrane flux, it is necessary to weaken temperature polarization and membrane pollution, strengthen the temperature boundary layer heat transfer, and the interface heating method can effectively alleviate the problems of temperature polarization and membrane pollution, and significantly improve the performance of membrane distillation.

[0004] As an emerging carbon material, carbon nanotubes have a wide range of applications. With an absorption rate of up to 98% under visible light irradiation conditions and good Joule heating effect, they are a promising interfacial photoelectric heating material. There are many problems that need to be solved in the existing membrane distillation process, such as the limited illumination area of ​​the commonly used tubular membrane configuration, and the low porosity of the traditional flat ceramic membrane and its flat ceramic-based carbon membrane. Summary of the invention

[0005] Taking the above problems into consideration, the present invention improves the porosity and permeability of the ceramic substrate, changes the loading state of carbon nanotubes, and makes full use of the photoelectric interface heating effect in the membrane distillation process to increase the flux, slow down the temperature polarization and membrane pollution. We use the phase transformation-high temperature sintering method to prepare the flat ceramic membrane, use nickel titanate as a catalyst, and deposit and grow a flat ceramic-based carbon membrane, which shows better hydrophobicity and higher gas flux; at the same time, the ceramic-based carbon membrane has higher light absorption and conductivity.

[0006] The technical solution of the present invention is: a method for preparing a high-throughput flat ceramic-based carbon membrane, comprising the following steps:

[0007] (1) Preparation of porous flat ceramic membrane

[0008] Polyethersulfone:N-methyl-pyrrolidone are mixed in a mass ratio of 1:2-8, and then polyvinylpyrrolidone is added. The three organic substances are stirred in a planetary ball mill to form a stable and uniform polymer mixture, and then ceramic powder is added and ball milling is continued to obtain a ceramic slurry; the mass fractions of polyethersulfone and N-methyl-pyrrolidone in the slurry are 30-60wt.%, the mass fraction of polyvinylpyrrolidone is 0.5-10wt.%, and the balance is ceramic powder, and the above ceramic powder is made of aluminum oxide, zirconium oxide, silicon carbide, titanium oxide, mullite, and spinel;

[0009] After degassing, pour the ceramic slurry onto a clean glass plate, apply a film so that the slurry is evenly spread on the surface of the glass plate, and then transfer it to deionized water for full exchange and solidification to form a green body. After natural drying, place it in a muffle furnace and heat it to 1100-1800°C, and calcine it to obtain a porous flat ceramic membrane;

[0010] (2) Preparation of catalyst solution

[0011] Nickel nitrate and tetrabutyl titanate are used as raw materials, ethanol is used as a solvent, and citric acid is used as a complexing agent; the molar ratio of nickel nitrate:tetrabutyl titanate:citric acid is (0.5-5):(0.5-5):(0.5-5), mixed and stirred evenly, deionized water is added, ammonia water is used to adjust the pH to neutral, and the solution is diluted to a nickel ion concentration of 1-5 mol / L to prepare a catalyst solution;

[0012] (3) Preparation of flat ceramic-based carbon films

[0013] The porous flat ceramic membrane is immersed in the catalyst solution for 60-240 minutes, placed in an oven for drying for 1-5 hours, and after drying, placed in a box furnace and heated to 600-1000°C at a rate of 1-5°C / min, and calcined for 1-5 hours; after calcination, it is placed in a tubular furnace and reacted at 700-800°C for 10-60 minutes in an atmosphere of H2, N2 and C2H4 with a flow ratio of (0.5-5):(0.5-5):(1-10) to obtain a flat ceramic-based carbon membrane.

[0014] The flat ceramic-based carbon membrane is applied to the photothermal membrane distillation process.

[0015] The flat ceramic-based carbon membrane is applied to the electrothermal membrane distillation process.

[0016] The optical power density used in the photothermal membrane distillation process is 1-3 kW·m -2 .

[0017] The voltage used in the electrothermal membrane distillation process is 1-10V.

[0018] Furthermore, combined with its comprehensive capabilities, the present invention can combine electrical energy and light energy in water treatment applications, allowing applications to be expanded to dark nights or remote mountainous areas where electricity resources are scarce.

[0019] The alumina flat ceramic carrier was prepared by phase inversion-high temperature sintering method, the synthesis conditions of carbon nanotube membrane were optimized, and the prepared flat ceramic-based carbon membrane was applied to the interface heating membrane distillation process. The specific steps are as follows:

[0020] (1) Preparation of alumina flat ceramic carrier

[0021] (1.1) Raw material processing

[0022] The average particle size of the alumina powder is 0.825 μm, and the alumina powder is dried in an oven at 120° C. for 24 h.

[0023] (1.2) Preparation of casting solution

[0024] In this experiment, a single-layer phase transformation method was used to prepare a porous alumina flat ceramic carrier. The average particle size (D50) of commercial α-Al2O3 powder was 0.825 μm, and the mass fraction was 50 wt.%. Polyethersulfone, polyvinylpyrrolidone and N-methyl-pyrrolidone were mixed in a planetary ball mill in a mass ratio of (8:2:40) and stirred for 2 hours to form a stable and uniform polymer mixture. Then, 50 g of alumina powder was added and ball milling was continued for 48 hours to obtain a uniform and stable alumina slurry.

[0025] (1.3) Formation of alumina flat ceramic carrier

[0026] Pour the stable slurry after ball milling into a beaker and place it in a vacuum drying oven for degassing for 30 minutes until the bubbles are completely removed. Adjust the height of the adjustable applicator and pour the slurry evenly on a clean glass plate. Manually drag the applicator slowly and evenly to spread the slurry evenly on the surface of the glass plate. Then quickly transfer it to deionized water for solidification. After standing for 24 hours, fully exchange and solidify to form a green body. After taking out the green body, dry it naturally at room temperature for 48 hours, and then cut it into different shapes and sizes as needed. Place the sample in a muffle furnace for calcination. The specific heating program is: from room temperature 2℃·min -1 Raise the temperature to 600℃ and keep it for 2h to make the solvent in the green body burn completely. -1 The temperature was raised to 1550°C, kept at that temperature for 4 hours, and finally cooled to room temperature at a rate of 5°C / min. Finally, the calcined alumina ceramic membrane was obtained.

[0027] (2) Improved preparation of flat ceramic-based carbon films

[0028] (2.1) Add 0.01 mol Ni(NO3)2·6H2O to 30 mL ethanol and stir until a transparent solution is obtained. Then add 0.01 mol Ti(OC2H9)4 to make n(Ni 2+ ):n(Ti 4+ )=1:1, when a certain amount of citric acid is added, n(citric acid):n(Ni 2 + ):n(Ti 4+ )=1:1:1, stir evenly, finally add 150mL of water, stir at room temperature for 3h to form a green uniform solution, and adjust the pH to neutral with ammonia water.

[0029] (2.2) Impregnation and calcination: Take 5 mL of nickel titanate solution, add deionized water to dilute it 10 times, and use the vacuum impregnation method to immerse the alumina flat membrane in the diluted nickel titanate for 120 minutes so that the nickel titanate is evenly attached to the surface of the carrier. Put it in a 60°C oven and dry it for 3 hours. After drying, put the carrier in a box furnace and heat it to 700°C at a rate of 1°C / min, and keep it at this temperature and calcine it for 3 hours.

[0030] (2.3) Preparation of flat ceramic-based carbon membrane: The calcined carrier was taken out and placed in a tubular furnace. In an atmosphere of H2 and N2 in a ratio of 1:1, the temperature was raised to 500°C at a rate of 1°C / min, and kept at this temperature for 1 h. The temperature was then raised to 700°C at a rate of 1°C / min, and reacted for 25 min in an atmosphere of H2, N2 and C2H4 in a ratio of 1:1:2. The carrier was then cooled to room temperature to obtain a flat ceramic-based carbon membrane.

[0031] (3) Application of photothermal membrane distillation process of flat ceramic-based carbon membrane

[0032] The prepared composite membrane was fixed on a membrane distillation device component. The feed liquid was 0-7.0wt.% sodium chloride brine, the vacuum degree was 0.8bar, and the operation interval was 0.5 hours. The composite membrane was able to maintain a stable flux by continuous operation with an external light source. The salt rejection rate was above 99%, and the flux enhancement rate could reach 29.38%. There was no obvious damage to the separation layer before and after the operation, indicating that the flat ceramic-based carbon membrane has good stability.

[0033] (4) Application of electrothermal membrane distillation process using flat ceramic-based carbon membrane

[0034] The prepared composite membrane was fixed on a membrane distillation device component, the feed liquid was 0-7.0wt.% sodium chloride brine, the vacuum degree was 0.8bar, the operation interval was 0.5 hours, and the composite membrane was able to maintain a stable flux by continuous operation with an external light source, the salt rejection rate was above 99%, and the flux improvement rate could reach 25.19%.

[0035] Beneficial effects of the present invention:

[0036] The ceramic membrane carrier prepared by the phase transformation-high temperature sintering preparation process has a long finger-like pore structure, high porosity and high flux. The carbon nanotube layer structure is controlled by the catalyst, dosage and growth time, thereby changing the membrane porosity. After optimizing the preparation conditions, a highly stable and high-throughput flat ceramic-based carbon membrane is prepared. At the same time, the membrane distillation process with interfacial heating is introduced. Compared with the traditional process, it has higher flux, smaller temperature polarization and lower membrane pollution.

[0037] (1) In the present invention, the carrier is a phase-transformation flat membrane, and the flat membrane structure is divided into a skin layer, a finger-like pore layer (accounting for about 96.45% of the total thickness) and a sponge layer. The skin layer serves as a separation layer, the sponge layer provides a support layer, and the finger-like pore layer provides an efficient transmission channel. At the same time, due to the macroscopic planar structure of the flat membrane, it can better absorb light energy, and the absorption rate in a wide range of wavelengths (400-1600nm) is significantly improved.

[0038] (2) The catalyst is nickel titanate. After high-temperature calcination, the catalyst becomes a mixture of nickel titanate, nickel oxide and titanium oxide. It is reduced to a titanium-nickel alloy under a reducing atmosphere. The pure nickel catalyst is a single metal catalyst with small metal particles. After adding titanium, it becomes a titanium-nickel alloy particle, and the metal particles increase. The carbon nanotube synthesis mechanism is based on the growth of catalyst particles. The increase in particle size leads to more sparse carbon nanotubes. By optimizing the carbon nanotube synthesis conditions, the carbon nanotube film formed on the flat ceramic membrane carrier has a higher network porosity and a more uniform distribution. The formed carbon nanotube network structure film not only has a high porosity, but also ensures its good electrical conductivity and light absorption properties.

[0039] (3) The network structure formed by nanotubes provides a very efficient membrane structure. The network structure has a high membrane flux because the apparent porosity can exceed 70% and no dead-end pores are formed. At the same time, the thickness is only about 0.75 μm. The surface of the nano-network structure usually exhibits good hydrophobicity, which can further ensure its flux stability.

[0040] (4) By introducing interface heating, the temperature polarization problem and membrane pollution problem of traditional membrane distillation can be effectively improved. After the introduction of light source, the flux improvement rate can reach 29.38%. After the introduction of power supply, the flux improvement rate can reach 25.19%, which effectively improves the flux of membrane distillation and improves its long-term operation capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a scanning electron microscope photograph of the cross-section of the alumina ceramic membrane in Example 1.

[0042] Figure 2 This is a scanning electron microscope photograph of the surface of the alumina ceramic membrane in Example 1.

[0043] Figure 3 This is a scanning electron microscope photograph of the cross-section of the flat ceramic-based carbon film in Example 2.

[0044] Figure 4 This is a scanning electron microscope photograph of the surface of the flat ceramic-based carbon film in Example 2.

[0045] Figure 5 This is the diffuse reflection spectrum of the flat ceramic-based carbon film in Example 3.

[0046] Figure 6 This is a performance diagram of the flat ceramic-based carbon membrane photothermal membrane distillation process for treating a 3.5 wt. % sodium chloride solution in Example 3.

[0047] Figure 7 This is the CV curve of the flat ceramic-based carbon film in Example 4.

[0048] Figure 8 This is a performance diagram of the flat ceramic-based carbon membrane electrothermal membrane distillation process in Example 4 for treating a 3.5 wt. % sodium chloride solution.

[0049] Fig. 9 This is a long-term performance diagram of the membrane distillation process for treating a 3.5 wt. % sodium chloride solution when the flat ceramic-based carbon membrane in Example 5 is equipped with an external light source and the feed liquid temperature is 45°C.

[0050] Fig.10 This is a long-term performance diagram of the flat ceramic-based carbon membrane in Example 5, in which a 3.5 wt. % sodium chloride solution is treated by membrane distillation process when no light source is added and the feed liquid temperature is 55°C. DETAILED DESCRIPTION

[0051] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0052] Example 1: Preparation of alumina flat ceramic carrier

[0053] (1) Material pretreatment: Commercial Al2O3 is used as the raw material of the present invention, with an average particle size of about 0.825 μm and low price. Since the raw material powder has a high water content, it is first dried at 120° C. for 24 hours to remove most of the water.

[0054] (2) Phase transformation process: The solvent used for the polymer slurry is N-methylpyrrolidone (NMP), and polyethersulfone (PES) is dissolved in NMP to play a bonding role and also as a dispersant. PES is adsorbed on the surface of the particles, increasing the steric effect between alumina, overcoming the van der Waals force to prevent alumina from agglomerating, and adding an appropriate amount of polyvinylpyrrolidone (PVP) as a dispersant to play a role in uniform slurry. At the same time, PVP is a water-soluble polymer. The addition of PVP can enhance the rate of exchange of the slurry with the non-solvent in the coagulation bath. 50g of alumina powder, 40g of N-methylpyrrolidone, 8g of polyethersulfone, and 2g of polyvinylpyrrolidone. The four mixtures are continuously ball-milled to obtain a uniformly mixed slurry. Manually operate the scraper coating, the coagulation bath is water, and it is immersed at room temperature. After sufficient exchange of solvent (polymer slurry) and non-solvent (water), gelation and solidification form an alumina ceramic green body. After natural drying, it is placed in a high-temperature box furnace and sintered at 1550℃ to finally obtain an asymmetric alumina ceramic carrier. The cross section shows an asymmetric structure, with a skin layer with separation function, a finger-like pore layer with fast penetration, and a sponge layer with support function. The finger-like pore layer is longer and accounts for 96.45% of the overall structure. Figure 1 The surface skin layer shows a loose and porous structure, such as Figure 2 shown.

[0055] Example 2: Preparation of carbon nanotubes on alumina ceramic support

[0056] (1) The preparation method of nickel titanate catalyst is as follows: nickel nitrate and tetrabutyl titanate are used as raw materials, ethanol is used as solvent, citric acid is used as complexing agent, 0.01 mol Ni(NO3)2·6H2O is added to 30 mL ethanol, stirred to form a transparent solution, and then 0.01 mol Ti(OC2H9)4 is added to make n(Ni 2+ ):n(Ti 4+ )=1:1, then add citric acid, n(citric acid):n(Ni 2+ ):n(Ti 4+ )=1:1:1, stir evenly, and finally add 150mL of deionized water, with a ratio of 5:1 to ethanol, stir at room temperature for 3h to form a green uniform solution, and adjust the pH to about neutral with ammonia water.

[0057] (2) The preparation method of the flat ceramic-based carbon membrane is as follows: take 5 mL of nickel titanate solution, add deionized water to dilute it 10 times, use the vacuum impregnation method to immerse the aluminum oxide flat membrane in the diluted nickel titanate for 120 minutes, so that the nickel titanate is evenly attached to the surface of the carrier, put it in a 60°C oven to dry for 3 hours, put the dried carrier in a box furnace and heat it to 700°C at 1°C / min, and keep it warm for 3 hours. Take out the calcined carrier, which is light green, put it in a tube furnace, and heat it to 500°C at a rate of 1°C / min in an atmosphere of H2 and N2 at a ratio of 1:1, keep it warm for 1 hour, then heat it to 700°C at a rate of 1°C / min, react for 25 minutes in an atmosphere of H2, N2 and C2H4 at a ratio of 1:1:2, and then cool it to room temperature to obtain a flat ceramic-based carbon membrane. Under high-temperature reduction conditions of nickel titanate catalyst, the generated catalyst alloy particles are larger, the carbon nanotube layer is also looser, and the cross section shows a 0.75μm thick carbon nanotube layer, and most of it only exists on the surface of the alumina carrier, ensuring the high flux of its carrier, such as Figure 3 As shown, the surface presents a carbon nanotube network structure, such as Figure 4 shown.

[0058] Example 3: Photothermal membrane distillation process of flat ceramic-based carbon membrane

[0059] The test selected the flat ceramic-based carbon film with a functional layer thickness of 0.75 μm in Example 2. Figure 5 As shown, it shows a high light absorption rate of 98% within the wavelength range of 400nm to 1600nm. The simulated brine is a 3.5wt.% sodium chloride solution. The composite membrane is fixed on a highly transparent acrylic separation reactor assembly. The lateral flow rate is 4.5mL / min, the transmembrane pressure is 0.8bar, and a 500W xenon lamp (Beijing PerfectLight Technology Co., LTD) is used to simulate the sunlight source. The weight is counted every 30 minutes, and the treatment performance of the composite membrane is evaluated by analyzing the conductivity of the condensate. Figure 6 The results show that after treatment with photothermal membrane distillation process, the removal rate can reach 99%, and the flux improvement rate can reach 29.38%.

[0060] Example 4 Electrothermal membrane distillation process of flat ceramic-based carbon membrane

[0061] The test selected the flat ceramic-based carbon film with a functional layer thickness of 0.75 μm in Example 2. Figure 7It shows that after the growth of carbon nanotubes, the conductivity of the flat ceramic carrier is significantly improved. The simulated brine is a 3.5wt.% sodium chloride solution. The composite membrane is fixed on a highly transparent acrylic separation reactor assembly. The lateral flow rate is 4.5mL / min, the transmembrane pressure is 0.8bar, and a 10V external power supply is used. The weight is counted every 30 minutes, and the treatment performance of the composite membrane is evaluated by analyzing the conductivity of the condensate. Figure 8 The results show that after treatment with the electrothermal membrane distillation process, the removal rate can reach 99%, and the flux improvement rate can reach 25.19%.

[0062] Example 5 Long-term operation of flat ceramic-based carbon membrane interface heating membrane distillation

[0063] The test selected the flat ceramic-based carbon membrane with a functional layer thickness of 0.75 μm in Example 2. The simulated brine was a 3.5 wt.% sodium chloride solution. The composite membrane was fixed on a highly transparent acrylic separation reactor assembly. The lateral flow rate was 4.5 mL / min, the transmembrane pressure was 0.8 bar, and a 500 W xenon lamp (Beijing PerfectLight Technology Co., LTD) was used to simulate the sunlight source. Fig. 9 and Fig.10 As shown in the figure, the flux when the external light source is added and the feed liquid temperature is 45°C is equivalent to the flux when the external light source is not added and the feed liquid temperature is 55°C, which is 2.01-2.2L·m -2 ·h -1 bar -1 Between 2.21L·m -2 ·h -1 bar -1 Reduced to 1.58L·m -2 ·h -1 bar - , with light at 45℃, the flux is still 1.98L·m -2 ·h -1 bar - Under the same flux, the situation with added light is significantly better than that without added light. It can be predicted that when an external light source or external power supply is added, the flux will be higher, and the long-term performance will be better than the traditional membrane distillation process. This is because the feed liquid of the interfacial heating membrane distillation process can operate at a lower temperature.

Claims

1. A method for preparing a high-throughput flat ceramic-based carbon membrane, characterized in that: The following steps are involved: (1) Preparation of porous flat ceramic membrane Polyethersulfone: N-methyl-pyrrolidone are mixed in a mass ratio of 1:2-8, and then polyvinylpyrrolidone is added. The three organic substances are stirred in a planetary ball mill to form a stable and uniform polymer mixture, and then ceramic powder is added and ball milling is continued to obtain a ceramic slurry; the mass fraction of polyethersulfone and N-methyl-pyrrolidone in the slurry is 30-60 wt.%, the mass fraction of polyvinylpyrrolidone is 0.5-10 wt.%, and the balance is ceramic powder, and the above ceramic powder is made of alumina, zirconium oxide, silicon carbide, titanium oxide, mullite, and spinel; After degassing, pour the ceramic slurry onto a clean glass plate, apply a film so that the slurry is evenly spread on the surface of the glass plate, then transfer it to deionized water for full exchange and solidification to form a green body, and after natural drying, place it in a muffle furnace and heat it to 1100-1800°C, and calcine it to obtain a porous flat ceramic membrane; (2) Preparation of catalyst solution Nickel nitrate and tetrabutyl titanate are used as raw materials, ethanol is used as a solvent, and citric acid is used as a complexing agent; the molar ratio of nickel nitrate:tetrabutyl titanate:citric acid is (0.5-5):(0.5-5):(0.5-5), mixed and stirred evenly, deionized water is added, and ammonia water is used to adjust the pH to neutral, and the solution is diluted to a nickel ion concentration of 1-5 mol / L to prepare a catalyst solution; (3) Preparation of flat ceramic-based carbon films The porous flat ceramic membrane is immersed in the catalyst solution for 60-240 minutes, placed in an oven for drying for 1-5 hours, and after drying, placed in a box furnace and heated to 600-1000°C at a rate of 1-5°C / min, and calcined for 1-5 hours; after calcination, it is placed in a tubular furnace and reacted at 700-800°C for 10-60 minutes in an atmosphere of H2, N2 and C2H4 with a flow ratio of (0.5-5):(0.5-5):(1-10) to obtain a flat ceramic-based carbon membrane.

2. Application of the flat ceramic-based carbon film prepared by the preparation method according to claim 1, characterized in that: The flat ceramic-based carbon membrane is applied to the photothermal membrane distillation process.

3. The use according to claim 2, characterized in that: The optical power density used in the photothermal membrane distillation process is 1-3 kw·m -2 。 4. Application of the flat ceramic-based carbon film prepared by the preparation method according to claim 1, characterized in that: The flat ceramic-based carbon membrane is applied to the electrothermal membrane distillation process.

5. The use according to claim 4, characterized in that: The voltage used in the electrothermal membrane distillation process is 1-10V.

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