CN118122141B - A membrane separation device and method - Google Patents

Abstract

The present invention relates to a membrane separation device and method. The membrane separation device comprises: a high-frequency alternating current power supply and at least one pair of relatively spaced conductive porous functional membranes, wherein the space inside the pair of conductive porous functional membranes forms a concentrated water channel for concentrated water, and the space outside each of the conductive porous functional membranes forms a fresh water channel for fresh water, each of the conductive porous functional membranes is electrically connected to the high-frequency alternating current power supply, and high-frequency alternating current is applied to the conductive porous functional membranes through the high-frequency alternating current power supply, so that the concentrated water generates heat and water vapor by itself, and the water vapor passes through the conductive porous functional membranes and enters the fresh water channel to obtain fresh water.

Description

A membrane separation device and method

Technical Field

The present invention relates to the technical field of separation or water treatment, in particular to the technical field of brine separation.

Background technique

The shortage of fresh water resources has become an important factor restricting the sustainable development of human society. Using desalination and desalination technology to obtain fresh water resources from seawater or wastewater is considered a feasible strategy. This puts higher requirements on desalination and desalination devices.

Traditional desalination technology is mainly based on reverse osmosis and nanofiltration processes. However, the high pressure required to overcome the osmotic pressure of highly concentrated brine can easily lead to damage of membrane components, limiting their application in key industrial wastewater. In addition, conventional thermal desalination methods, such as multi-effect distillation and multi-stage flash evaporation, although they have improved the desalination and concentration capabilities of highly concentrated brine, require bulky treatment equipment, high operating energy consumption and investment costs.

Although the membrane distillation technology developed in recent years can be used for seawater or wastewater desalination, it is affected by small distillation flux, easy membrane scaling and poor stability in actual application. Specifically, 1) there is temperature polarization on the surface of the hydrophobic membrane, that is, the membrane surface temperature on one side of the concentrate chamber is lower than the main body temperature of the brine, resulting in low heat utilization efficiency and low water production flux when the brine flows through the concentrate chamber in a single pass; 2) there is concentration polarization on the surface of the hydrophobic membrane, that is, the salt concentration on the membrane surface is higher than the main body concentration of the brine during the desalination process, resulting in the accumulation of scaling ions in the brine such as calcium, magnesium, sulfate, etc. on the surface of the hydrophobic membrane, and the formation of mineral scale to block the membrane pores, resulting in reduced membrane flux and even hydrophobic membrane wetting failure; 3) in traditional membrane distillation, the brine needs to be heated externally by conduction, which usually requires a rapid circulation reflux process, which not only increases the system complexity and construction cost, but also increases heat loss and brine circulation energy consumption; 4) with the increase of feed channel length and membrane assembly size, the average transmembrane temperature difference on both sides of the hydrophobic membrane gradually decreases, resulting in a decrease in the mass transfer driving force of water vapor, which is not conducive to the large-scale device.

Summary of the invention

In order to overcome the problems of temperature difference polarization, concentration difference polarization and ion crystallization on the membrane surface, the present invention proposes a membrane separation device and method.

One of the present inventions provides a membrane separation device, including: a high-frequency AC power supply and at least one pair of relatively spaced conductive porous functional membranes, the space inside the pair of conductive porous functional membranes forms a concentrated water channel for concentrated water, the space outside each conductive porous functional membrane forms a fresh water channel for fresh water, each conductive porous functional membrane is electrically connected to the high-frequency AC power supply, and high-frequency AC is applied to the conductive porous functional membrane by the high-frequency AC power supply to make the concentrated water self-heat and generate water vapor, and the water vapor passes through the conductive porous functional membrane into the fresh water channel to obtain fresh water. Wherein, by applying high-frequency AC to the conductive porous functional membrane by the high-frequency AC power supply, the heat in the concentrated water channel does not come from conductive heating, but the concentrated water itself generates heat uniformly. Wherein, the concentrated water in the concentrated water channel may not flow or may flow. When the concentrated water in the concentrated water channel is connected to the outside and flows, continuous desalination of the concentrated water can be achieved.

In a specific embodiment, each conductive porous functional membrane includes a porous hydrophobic substrate and a porous conductive layer composited with the porous hydrophobic substrate, the porous hydrophobic substrate is located on one side of the fresh water channel, and the porous conductive layer is located on one side of the concentrated water channel. When high-frequency alternating current is applied to the conductive porous functional membrane, high-frequency ion adsorption and desorption occur in the porous conductive layer, thereby suppressing the occurrence of Faraday electrochemical reactions, avoiding the deposition and crystallization of ions on the membrane surface, reducing scaling and wetting on the surface of the conductive porous functional membrane, and improving the long-term operation stability of the conductive porous functional membrane; due to the high-frequency vibration migration of salt ions under the alternating current field, the accumulation of ion concentration on the surface of the porous conductive layer caused by water vapor passing through the membrane is reduced, and the concentration polarization on the surface of the conductive porous functional membrane is eliminated.

In a specific embodiment, the material for preparing the porous hydrophobic substrate includes at least one of polytetrafluoroethylene, polyvinylidene fluoride and ceramic. For example, the material for preparing the porous hydrophobic substrate includes polytetrafluoroethylene, polyvinylidene fluoride or ceramic. In addition, the material for preparing the porous hydrophobic substrate can also be a mixture or composite material of polytetrafluoroethylene and polyvinylidene fluoride. Wherein, the porous hydrophobic substrate is insulated or non-conductive.

In a specific embodiment, the material for preparing the porous conductive layer includes at least one of a carbon-based material, a metal-based material and a conductive polymer. For example, the material for preparing the porous conductive layer includes a carbon-based material, a metal-based material or a conductive polymer. Among them, the carbon-based material may include at least one of activated carbon, carbon black, carbon nanotubes, graphene and carbon fiber. For example, the carbon-based material may include activated carbon, carbon black, carbon nanotubes, graphene or carbon fiber.

In a specific embodiment, the material for preparing the porous conductive layer includes at least one of carbon nanofiber interpenetrating graphene anchored molybdenum disulfide, biochar, and electrospun carbon nanofiber. For example, the material for preparing the porous conductive layer includes carbon nanofiber interpenetrating graphene anchored molybdenum disulfide, biochar, or electrospun carbon nanofiber.

In a specific embodiment, an electrode patch is disposed on the porous conductive layer.

In a specific embodiment, the porous conductive layer is constructed onto the porous hydrophobic substrate by spin coating, suction filtration, spray coating, dipping or vapor deposition.

In a specific embodiment, the membrane separation device further comprises a shell, a pair of conductive porous functional membranes are fixed in parallel in the shell, and a fresh water channel is formed between the shell and the conductive porous functional membranes.

In one embodiment, the membrane separation device includes a pair of conductive porous functional membranes.

In a specific embodiment, a first insulating screen and a second insulating screen for supporting the conductive porous functional membrane are independently arranged in the concentrated water channel and the dilute water channel. The insulating screen can be used to support the conductive porous functional membrane on the one hand, and can also prevent a pair of conductive porous functional membranes from contacting on the other hand.

In a specific embodiment, cold water or cooling gas is introduced into the fresh water channel, or the fresh water channel is evacuated to achieve condensation and transportation of the water vapor entering therein.

In a specific embodiment, the membrane separation device also includes an online monitoring module, which includes a temperature monitoring device for monitoring the temperature of the inlet and outlet of the concentrated water channel and/or the fresh water channel, a quality monitoring device for monitoring the quality of the concentrated water and/or the fresh water, a conductivity monitoring device for monitoring the conductivity of the fresh water, and an energy intensity monitoring device for monitoring the energy input intensity of the high-frequency AC power supply.

In a specific embodiment, the frequency of the high-frequency alternating current is greater than or equal to 1 kHz.

In a specific embodiment, the frequency of the high-frequency alternating current is between 1 kHz and 1 MHz.

In a specific embodiment, the voltage of the high frequency alternating current is greater than 0V.

In a specific embodiment, the voltage of the high-frequency alternating current is greater than 0 V and less than 1.2 V.

In a specific embodiment, the voltage of the high-frequency alternating current is greater than or equal to 1.2 V.

In a specific embodiment, the voltage of the high frequency alternating current is between 1.2 V and 3 V.

In a specific embodiment, the voltage of the high-frequency alternating current is greater than or equal to 3 V.

In one specific embodiment, the salt ion concentration in the concentrated water is greater than or equal to 1 g/L.

In a specific embodiment, the salt ion concentration in the concentrated water is greater than or equal to 5 g/L.

In a specific embodiment, the salt ion concentration in the concentrated water is greater than or equal to 10 g/L.

In a specific embodiment, the salt can be any inorganic salt, such as at least one of sodium chloride, potassium chloride, sodium sulfate and potassium sulfate.

The second aspect of the present invention provides a membrane separation method, comprising: using at least one pair of relatively spaced conductive porous functional membranes to form a concentrated water channel for concentrated water, and forming fresh water channels for fresh water on both sides of the concentrated water channel; applying high-frequency alternating current to the conductive porous functional membrane to cause the concentrated water to self-generate heat and produce water vapor, and the water vapor passes through the conductive porous functional membrane into the fresh water channel to obtain fresh water.

In a specific embodiment, the membrane separation method further includes: making the concentrated water in the concentrated water channel flow in the opposite direction to the fresh water in the fresh water channel.

In a specific embodiment, the salt can be any inorganic salt, such as at least one of sodium chloride, potassium chloride, sodium sulfate and potassium sulfate.

The beneficial effects of the present invention are as follows: 1) The present invention discovers for the first time that high-frequency alternating current can make brine self-heating and make brine generate heat evenly, which can avoid energy transfer based on the heat conduction mechanism and solve the problem of uneven heating of concentrated water (brine), thus being beneficial to eliminating the temperature polarization phenomenon of the membrane and enhancing the heat transfer efficiency and water production rate; 2) Under the action of high-frequency alternating current, the ions on the surface and nearby of the conductive porous functional membrane undergo high-frequency oscillation, which is beneficial to reducing the accumulation of ion concentration caused by large-scale evaporation of water, eliminating concentration polarization on the membrane surface, and enhancing the water production rate and membrane surface scale inhibition; 3) Under the action of high-frequency alternating current, the ions on the surface of the conductive porous functional membrane undergo high-frequency adsorption and desorption, which is beneficial to preventing the ions from adhering, nucleating and crystallizing on the membrane surface and enhancing the anti-pollution of the membrane surface; 4) Under the action of high-frequency alternating current, the ions on the surface of the conductive porous functional membrane undergo a non-Faraday process of high-frequency adsorption and desorption, and no Faraday chemical reactions such as water electrolysis and material corrosion occur, which is beneficial to maintaining the stability of the electrode material and the heat production efficiency; 5) By increasing the frequency of alternating current, high energy density can be achieved at a lower voltage input, promoting high-intensity heat production and high-flux water production of brine; 6) The membrane separation device based on high-frequency alternating current has high heat production intensity and can adopt a one-way continuous flow operation mode, which is beneficial to reduce energy loss and operating costs and simplify the process flow; 7) The membrane separation device has the advantages of compact modular structure, small footprint, and high water production flux, and can realize the desalination and concentration treatment of a variety of salty water such as seawater, municipal sewage concentrate, and industrial wastewater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic structural diagram of a membrane separation device according to an embodiment of the present invention.

FIG. 2 shows a graph showing the relationship between the frequency of alternating current and the power density at different voltages for saline (100 g/L NaCl) according to an embodiment of the present invention.

Detailed ways

The present invention will be further described below in conjunction with examples, but the examples of the present invention are only exemplary descriptions, and the implementation methods do not constitute limitations of the present invention under any circumstances.

FIG1 shows the structure of a membrane separation device 100 according to an embodiment of the present invention. As shown in FIG1 , the membrane separation device 100 may include: a high-frequency AC power supply 1 and at least one pair of relatively spaced conductive porous functional membranes 2. A concentrated water channel 5 for concentrated water to flow is formed between the inner sides of a pair of conductive porous functional membranes 2. The outer sides of a pair of conductive porous functional membranes 2 are respectively formed as fresh water channels 6 for fresh water to flow. Each conductive porous functional membrane 2 is connected to the high-frequency AC power supply 1. During the operation of the high-frequency AC power supply 1, when high-frequency AC is applied to the conductive porous functional membrane 2, the salt ions in the concentrated water undergo high-frequency oscillation and self-heating, so that the concentrated water can directly and uniformly generate heat. The water vapor generated by the heat in the concentrated water passes through the conductive porous functional membrane 2 and enters the fresh water channel 6 to condense to obtain fresh water, and at the same time, the concentrated water in the concentrated water channel 5 is concentrated. In the present invention, concentrated water refers to water with a relatively high salt ion concentration, which is particularly relative to the fresh water in the fresh water channel. The membrane separation device 100 is particularly suitable for salt water with a salt ion concentration greater than or equal to 1 g/L to ensure a certain electrical conductivity; and the salt ion concentration of fresh water is relatively lower than that of concentrated water. For example, the salt ion concentration of fresh water can be 0.01 g/L.

In order to achieve large-scale production, the water in the concentrated water channel 5 and the fresh water channel 6 can flow. In a specific embodiment, the flow direction of water vapor can refer to the direction of the dotted arrow shown in Figure 1, the flow direction of concentrated water can refer to the direction of the black arrow shown in Figure 1, and the flow direction of fresh water can refer to the direction of the gray arrow shown in Figure 1. Power devices such as peristaltic pumps and centrifugal pumps can be used in the concentrated water channel 5 and the fresh water channel 6 for fluid transportation.

During the operation of the membrane separation device 100 of the embodiment of the present invention, a pair of conductive porous functional membranes 2 are connected to a high-frequency AC power supply 1. The pair of conductive porous functional membranes 2 serve as the surface of the concentrated water channel. When a high-frequency current is applied, on the one hand, high-frequency ion adsorption and desorption occur on the surface, which inhibits the occurrence of Faraday electrochemical reactions; on the other hand, the salt ions in the concentrated water between the pair of conductive porous functional membranes 2 generate heat by themselves due to high-frequency oscillation, which realizes direct and uniform heat generation of the concentrated water. This process avoids the energy transfer of traditional membrane distillation by the mechanism of indirect heat conduction, which is conducive to eliminating the temperature difference polarization phenomenon on the membrane surface, enhancing the heat transfer efficiency and water production flux. In addition, since the ions in the concentrated water undergo high-frequency vibration migration under the AC electric field, the accumulation of ion concentration on the surface of the conductive porous functional membrane 2 caused by water evaporating through the membrane is reduced, which is conducive to eliminating the concentration difference polarization phenomenon on the membrane surface, enhancing the water production flux and reducing the crystallization of the inorganic salt membrane surface, inhibiting membrane wetting, and improving the long-term operation stability of the conductive porous functional membrane 2. The membrane separation device 100 of the embodiment of the present invention can be widely used in the treatment of seawater and various salty waters, such as seawater, coking wastewater, industrial wastewater such as reverse osmosis concentrated water, municipal sewage and other different salty waters.

According to the present invention, in a preferred embodiment as shown in FIG1 , the conductive porous functional membrane 2 may include a porous hydrophobic substrate 21 and a porous conductive layer 22 formed by combining the porous hydrophobic substrate 21, wherein the porous conductive layer 22 faces the concentrated water channel 5, that is, the porous conductive layer 22 is located on one side of the concentrated water channel 5; the porous hydrophobic substrate 21 faces the fresh water channel 6, and the porous hydrophobic substrate 21 is located on one side of the fresh water channel 6. The porous conductive layer 22 is used to be connected to the high-frequency AC power source 1.

Preferably, the porous hydrophobic substrate 21 can be made of polytetrafluoroethylene, polyvinylidene fluoride or ceramic-based materials; the porous conductive layer 22 can be made of carbon-based, metal-based or conductive polymer. Further, the carbon-based material can be porous activated carbon, carbon black, carbon nanotubes, graphene or carbon fiber.

Also preferably, the porous conductive layer 22 can be constructed on the porous hydrophobic substrate 21 by spin coating, suction filtration, spray coating, dipping or vapor deposition.

In a preferred embodiment, the porous conductive layer 22 can be made of carbon nanotube material, and the porous hydrophobic substrate 21 is made of polytetrafluoroethylene. The preparation method of the conductive porous functional membrane 2 can be as follows: first, a carbon nanotube solution with a mass fraction of 0.2% is prepared, and sodium dodecylbenzene sulfonate with a mass fraction of 0.1% is added as a surfactant, and ultrasonic dispersion is performed for 1 hour; then the uniformly dispersed carbon nanotube solution is sprayed onto the surface of the polytetrafluoroethylene hydrophobic membrane, and the spraying density is 2 mg/ ^cm2 ; at the same time, a polyvinyl alcohol solution with a volume ratio of 20:1 (carbon nanotubes: polyvinyl alcohol) is sprayed as a binder, and the composite membrane after spraying is washed with deionized water for 30 minutes to remove the surfactant; then the composite membrane is immersed in a glutaraldehyde solution with a volume fraction of 4.4% (the solvent is deionized water), the temperature of the immersion solution is 70°C, the immersion time is 1 hour, and after the immersion is completed, it is washed with deionized water for 30 minutes, and the conductive porous functional membrane 2 is obtained after vacuum drying.

In other preferred embodiments, the porous conductive layer 22 can also be made of other materials, such as carbon nanofiber interpenetrating graphene anchored molybdenum disulfide, biochar or electrospun carbon nanofiber. By adjusting the technical parameters and performance of the above materials, the porous conductive layer 22 can exhibit high porosity, good conductivity, good cycle stability, excellent electrochemical performance and other characteristics; by using the above spin coating, filtration, spraying, dipping or vapor deposition method to construct the prepared porous conductive layer 22 on the porous hydrophobic substrate 21, the heat generation, water flux, stability and other performance indicators of high-salinity concentrated water can be further enhanced.

According to the present invention, in a preferred embodiment as shown in FIG. 1 , the membrane separation device 100 may include a pair of conductive porous functional membranes 2, and the membrane separation device 100 further includes a housing 4, wherein the pair of conductive porous functional membranes 2 are fixed in parallel in the housing 4, and a fresh water channel 6 is formed between the housing 4 and the conductive porous functional membrane 2. In this embodiment, the pair of conductive porous functional membranes 2 divide the housing 4 into three spaces, namely, a concentrated water channel 5 and a fresh water channel 6 located on both sides of the concentrated water channel 5, and the housing 4 directly participates in the formation of the fresh water channel 6. This arrangement can make the structure of the membrane separation device 100 simpler and more compact, modularly produced, smaller in footprint, and higher in energy utilization efficiency and water production flux.

Preferably, the housing 4 can be made into various configurations, such as a flat plate, a rolled type, and a tubular type, and the material can be selected from various materials such as acrylic plate, polytetrafluoroethylene, etc.

Preferably, the membrane separation device 100 may also include a plurality of pairs of conductive porous membranes 2. The plurality of pairs of conductive porous membranes 2 may be arranged at intervals in a direction substantially perpendicular to the flow of water, and the fresh water channel 6 may be shared between two adjacent pairs of conductive porous membranes 2. Thus, a multi-stage stacked membrane distillation device having an alternating structure of concentrated water channel 5-fresh water channel 6 similar to "...fresh water-concentrated water-fresh water-concentrated water-fresh water..." may be formed. In such a membrane separation device 100, a concentrated water channel 5 is formed between the porous conductive layers 22 in a pair of conductive porous functional membranes 2, and a fresh water channel is formed between the porous hydrophobic substrates 21 in two adjacent pairs of conductive porous functional membranes 2. By arranging a plurality of pairs of conductive porous functional membranes 2 at intervals, the membrane filling density may be increased, the amount of shell material used may be reduced, heat loss may be reduced, and the overall performance may be improved.

Further, in a preferred embodiment as shown in FIG. 1 , the membrane separation device 100 may further include a first insulating screen 3. The first insulating screen 3 is disposed in the concentrated water channel 5. The first insulating screen 3 is mainly used to prevent a pair of porous conductive layers 22 from contacting each other to maintain the physical structure of the porous conductive layer 22 stable. Structurally, the two sides of the first insulating screen 3 are respectively in contact with a pair of porous conductive layers 22, that is, the thickness of the first insulating screen 3 can be equivalent to the width of the concentrated water channel. It can be seen that the first insulating screen 3 can support and fix the conductive multifunctional membrane 2, and can also limit the relative position of a pair of conductive porous functional membranes 2 to ensure normal operation. The first insulating screen 3 needs to have high temperature resistance and corrosion resistance, and can be set in a grid shape; or the first insulating screen 3 can also have a porous characteristic. For example, the first insulating screen 3 can be made of polypropylene material or polyvinyl chloride material, and its thickness can be selected as needed, such as 1 mm. Among them, setting it in a grid shape or using a porous material can avoid blocking the concentrated water from flowing in the concentrated water channel.

According to the present invention, the fresh water channel 6 can achieve condensation and transportation of water vapor entering therein by passing cold water, cooling gas or vacuuming, so that the fresh water channel 6 can be operated in different modes such as direct contact membrane distillation, air gap membrane distillation, gas sweep membrane distillation, vacuum membrane distillation, etc.

It can be understood that a second insulating screen 7 is provided in the fresh water channel 6. The second insulating screen 7 abuts against the conductive multifunctional membrane 2 to support the conductive multifunctional membrane 2, thereby maintaining the structural stability of the conductive multifunctional membrane 2. At the same time, the second insulating screen 7 can also define a fresh water flow space as the fresh water channel 6. The second insulating screen 7 needs to have high temperature resistance and corrosion resistance, and can be set in a grid shape; or the second insulating screen 7 can also have a porous property. For example, the second insulating screen 7 can be made of polypropylene material or polyvinyl chloride material, and its thickness can be selected as needed, such as 5 mm. Among them, setting it in a grid shape or using a porous material can avoid blocking the flow of fresh water in the fresh water channel.

In a preferred embodiment, the membrane separation device 100 may also include a fresh water pipeline connected to the fresh water channel 6 for collecting desalinated fresh water (not shown in the figure, for example, it may be located on the left and right sides of the fresh water channel 6 shown in Figure 1) and a concentrated water pipeline connected to the concentrated water channel 5 for transporting concentrated water (not shown in the figure, for example, it may be located on the left and right sides of the concentrated water channel 5 shown in Figure 1). The concentrated water pipeline may be connected to an external concentrated water storage tank, and the concentrated water in the concentrated water storage tank enters the concentrated water channel 5 through the concentrated water pipeline and is then discharged; the fresh water pipeline may be connected to an external fresh water collection tank, and the desalinated fresh water generated by condensation entering the fresh water channel 6 enters the fresh water collection tank through the fresh water pipeline for collection.

Preferably, the materials of the concentrated water pipeline and the fresh water pipeline need to have properties such as corrosion resistance, and materials such as polyvinyl chloride and polystyrene may be used; also preferably, a cooling device may be added to the fresh water pipeline at the upstream end of the membrane separation device to increase the steam pressure difference between the concentrated water channel 5 and the fresh water channel 6. For example, an intelligent low-temperature constant temperature tank may be provided at the upstream end of the fresh water pipeline (the end where cooling water or cooling gas flows in or where a vacuum is drawn).

In addition, according to the present invention, the membrane separation device 100 may also include an online monitoring module (not shown in the figure), which includes a temperature monitoring device for monitoring the inlet and outlet temperatures of the concentrated water channel and the fresh water channel, a quality monitoring device for monitoring the quality of concentrated water and fresh water, a conductivity monitoring device for monitoring the conductivity of fresh water, and an energy intensity monitoring device for monitoring the energy input intensity of the high-frequency AC power supply. The system gain-to-output ratio and water production energy consumption (kWh/m ³ ) can be calculated based on the inlet and outlet temperatures of the concentrated water channel 5 and the fresh water channel 6, the water production flux (obtained by the quality monitoring device) and the input energy intensity.

Preferably, the temperature monitoring device can be a thermocouple thermometer; the quality monitoring device can be preferably installed in the concentrated water storage tank and the fresh water collection tank respectively to record the reduction of concentrated water and the increase of fresh water, and it can use an electronic analytical balance; the fresh water conductivity monitoring device can use a conductivity meter; the energy intensity monitoring device can use an ammeter, a voltmeter, a power meter, etc., which can be connected to the high-frequency AC power supply 1 to monitor the system operating voltage, current, and power density in real time; the terminals of all the above monitoring devices can be connected to a computer for data recording and storage.

According to the present invention, it has been verified through research that the impedance values of concentrated water with different salt contents are all related to the frequency provided by the high-frequency AC power supply 1. As the frequency provided by the high-frequency AC power supply 1 increases, the impedance value gradually decreases, which indicates that high-frequency AC can increase the current of concentrated water and strengthen the input power density of the system. Therefore, the input energy intensity of the high-frequency AC power supply 1 can be increased by increasing the frequency of AC within a reasonable range, thereby promoting high-intensity heat production and high-flux water production of concentrated water, thereby improving the separation efficiency of the membrane separation device 100. In order to ensure the separation quality, the frequency of the high-frequency AC power supply in the present invention is not less than 1kHz.

As shown in FIG2 , when the power frequency is converted from 0 Hz direct current to alternating current, its power density increases rapidly from 0 kW/m ^2. When the frequency of the alternating current is higher than 1 kHz, its heat generation power density remains stable and no longer changes with the increase in frequency. At the same time, with the increase in voltage, the heat generation power density increases significantly. When the voltage is 1.2 V, the power density at a high frequency above 1 kHz can reach 10 kW/m ² , which is far higher than the energy intensity of traditional solar energy (1 kW/m ² ), and theoretically can produce a water production flux of about 16 L/(m ² h) in membrane distillation. When the voltage is increased to 3 V, its power density at high frequency can reach more than 60 kW/m ^2. This power density is far higher than that of traditional electrothermal membrane distillation systems, and high-throughput water production of membrane distillation can be achieved. Therefore, in the actual production process, the frequency of the high-frequency alternating current power supply of the membrane separation device 100 is not less than 1 kHz, which is used to ensure the stability of the heat generation power density, and the voltage of the high-frequency alternating current power supply can be selected considering parameters such as water production flux.

In addition, the membrane separation method according to the embodiment of the present invention may include: using at least one pair of relatively arranged conductive porous functional membranes 2 to form a concentrated water channel 5 for circulating concentrated water, and forming a fresh water channel 6 for circulating fresh water on both sides of the concentrated water channel 5; applying high-frequency alternating current to the conductive porous functional membrane 2 so that the salt ions in the concentrated water oscillate at high frequency and evenly generate heat by themselves, wherein the generated water vapor passes through the conductive porous functional membrane 2 and enters the fresh water channel 6 to condense to form fresh water. At the same time, the concentrated water in the concentrated water channel 5 is desalinated and concentrated. In practical applications, the frequency of the high-frequency alternating current is not less than 1 kHz, and the concentration of salt ions in the concentrated water is greater than or equal to 1 g/L.

The membrane separation method of the embodiment of the present invention directly uses at least one pair of relatively arranged conductive porous functional membranes 2 to form a concentrated water channel 5 for circulating concentrated water, and forms a fresh water channel 6 on both sides of the concentrated water channel 5. High-frequency alternating current is applied to the conductive porous functional membrane 2 to oscillate the salt ions in the concentrated water uniformly to generate heat. This method avoids the indirect energy transfer by the heat conduction mechanism of traditional membrane distillation, which is conducive to eliminating the temperature difference polarization phenomenon on the membrane surface, enhancing the heat transfer efficiency and water production flux. At the same time, high-frequency ion adsorption and desorption behavior occur on the surface of the conductive porous functional membrane 2, which inhibits the occurrence of Faraday electrochemical reactions. In addition, since the ions in the concentrated water undergo high-frequency vibration migration under the alternating current field, the accumulation of ion concentration on the surface of the conductive porous functional membrane 2 due to water evaporation through the membrane is reduced, which is conducive to eliminating the concentration difference polarization phenomenon on the membrane surface, enhancing the water production flux and reducing the crystallization of the inorganic salt membrane surface, inhibiting membrane wetting, and improving the long-term operation stability of the conductive porous functional membrane 2. The membrane separation method of the embodiment of the present invention is simple and easy to implement, and has high separation efficiency, and can be widely used in the treatment of various salty waters, such as seawater, coking wastewater, reverse osmosis concentrated water and other industrial wastewater, municipal sewage, etc.

In a preferred embodiment, the membrane separation method of the embodiment of the present invention may further include: making the flow direction of the fluid in the concentrated water channel 5 opposite to the flow direction of the fluid in the desalted water channel 6. This method is conducive to increasing the vapor pressure difference between the concentrated water channel 5 and the desalted water channel 6, thereby achieving more complete and rapid condensation and transportation of the water vapor entering the desalted water channel 6.

Based on the membrane separation device and method of the present invention, in a specific embodiment, the following performance test experiments can be performed thereon.

The membrane area of the conductive porous functional membrane 2 is designed to be 9 cm ² , and the conductive porous functional membrane 2 is made of carbon nanotube sprayed polytetrafluoroethylene material. The concentration of the prepared NaCl solution is 100 g/L, and the solution is poured into the concentrated water storage tank placed on the electronic analytical balance. The salt solution is transported to the concentrated chamber by the peristaltic pump. At the same time, the fresh water placed on the analytical balance enters the fresh water channel through the peristaltic pump and the intelligent low-temperature constant temperature bath in turn. The concentrated water and fresh water run in parallel countercurrent mode. The flow rates of the fluids in the concentrated water channel 5 and the fresh water channel 6 are 0.8 mL/min and 1.6 mL/min respectively. The high-frequency AC power supply 1 and the power meter connected to it are turned on, and the output power density is adjusted to 5 KW/m ² At the same time, the analytical balance, micro-thermocouple and conductivity meter were turned on to record the changes in water mass, inlet and outlet temperature and conductivity in the concentrated water channel 5 and the fresh water channel 6 during the operation of the system. The results are as follows: when the single-pass continuous flow operation is adopted, the inlet temperature of the concentrated water channel 5 is 20℃, the outlet temperature of the concentrated water channel 5 is 45℃, the inlet temperature of the fresh water channel 6 is 8℃, the outlet temperature of the fresh water channel 6 is 18℃, the system operating voltage is about 2.1V, the current is about 2.48A, and the water production flux is about 4.8 L/( ^m2h ).

Although the present invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that various changes may be made without departing from the true spirit and scope of the present invention. In addition, the subject matter, spirit and scope of the present invention may be varied to accommodate specific situations, structures, materials, material combinations and methods. All of these changes are included within the scope of the claims of the present invention.

Claims (10)

1. A membrane separation device, characterized in that it comprises: a high-frequency AC power supply and at least one pair of relatively spaced conductive porous functional membranes, wherein the spaces inside the pair of conductive porous functional membranes form concentrated water channels for concentrated water, and the spaces outside each of the conductive porous functional membranes form fresh water channels for fresh water, respectively; each of the conductive porous functional membranes is electrically connected to the high-frequency AC power supply, and high-frequency AC is applied to the conductive porous functional membranes by the high-frequency AC power supply so that the concentrated water generates heat by itself and produces water vapor, and the water vapor passes through the conductive porous functional membranes into the fresh water channels to obtain fresh water. 2. The membrane separation device according to claim 1 is characterized in that each of the conductive porous functional membranes includes a porous hydrophobic substrate and a porous conductive layer composited with the porous hydrophobic substrate, the porous hydrophobic substrate is located on one side of the fresh water channel, and the porous conductive layer is located on one side of the concentrated water channel. 3. The membrane separation device according to claim 2 is characterized in that the material used to prepare the porous hydrophobic substrate includes at least one of polytetrafluoroethylene, polyvinylidene fluoride and ceramics; and/or the material used to prepare the porous conductive layer includes at least one of carbon-based materials, metal-based materials and conductive polymers. 4 . The membrane separation device according to claim 3 , wherein the porous conductive layer is constructed on the porous hydrophobic substrate by spin coating, suction filtration, spray coating, dipping or vapor deposition process. 5. The membrane separation device according to any one of claims 1 to 4 is characterized in that the membrane separation device also includes a shell, a pair of the conductive porous functional membranes are fixed in parallel in the shell, and the fresh water channel is formed between the shell and the conductive porous functional membranes. 6 . The membrane separation device according to claim 5 , characterized in that a first insulating screen and a second insulating screen for supporting the conductive porous functional membrane are independently provided in the concentrated water channel and the dilute water channel. 7. The membrane separation device according to any one of claims 1 to 4, characterized in that cold water or cooling gas is introduced into the fresh water channel, or the fresh water channel is evacuated. To achieve the condensation and transportation of the water vapor entering it. 8. The membrane separation device according to any one of claims 1 to 4 is characterized in that the membrane separation device also includes an online monitoring module, which includes a temperature monitoring device for monitoring the temperature of the inlet and outlet of the concentrated water channel and/or the fresh water channel, a quality monitoring device for monitoring the quality of concentrated water and/or fresh water, a conductivity monitoring device for monitoring the conductivity of fresh water, and an energy intensity monitoring device for monitoring the energy input intensity of the high-frequency AC power supply. 9. The membrane separation device according to any one of claims 1 to 4, characterized in that the frequency of the high-frequency alternating current is greater than or equal to 1 kHz, and/or the salt ion concentration in the concentrated water is greater than or equal to 1 g/L. 10. A membrane separation method, comprising: At least one pair of relatively spaced conductive porous functional membranes is used to form a concentrated water channel for concentrated water, and fresh water channels for fresh water are formed on both sides of the concentrated water channel. High-frequency alternating current is applied to the conductive porous functional membrane to make the concentrated water self-heat and generate water vapor, and the water vapor passes through the conductive porous functional membrane and enters the fresh water channel to obtain fresh water.