Self-heating hydrophobic microporous membrane based on high-air-permeability porous carbon fiber support layer and preparation method and application thereof

Abstract

The application discloses a self-heating hydrophobic microporous membrane and a preparation method and application thereof. The self-heating hydrophobic microporous membrane comprises a porous carbon fiber support layer and a hydrophobic microporous membrane on the porous carbon fiber support layer. The porous carbon fiber support layer is selected from porous carbon fibers or porous carbon fiber non-woven fabric. The porous carbon fiber paper has a thickness of 20-150 microns and air permeability of 2-10 L.cm ‑2 ·min ‑1 ·kPa ‑1 The porous carbon fiber non-woven fabric has a thickness of 15-180 microns and air permeability of 8-15 L.cm ‑2 ·min ‑1 ·kPa ‑1 The self-heating hydrophobic microporous membrane prepared by combining the porous carbon fiber support layer and the hydrophobic microporous membrane can generate heat under the action of an external current. The heat can effectively reduce the heat energy lost due to heat conduction of the feed liquid and can compensate the heat energy consumed due to vaporization of the feed liquid on the membrane surface, thereby weakening the negative effect caused by the temperature difference polarization phenomenon and significantly improving the heat efficiency and membrane flux of the membrane distillation process.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of membrane distillation, and relates to a self-heating hydrophobic microporous membrane and a preparation method and application thereof. BACKGROUND

[0002] Membrane distillation (MD) is a new membrane separation process with unique advantages, which is based on the combination of diffusion and convection of vapor molecules. The main principle is to use hydrophobic microporous membranes as the separation interface, and the temperature gradient between the two ends of the membrane forms a saturated vapor pressure difference, which serves as the driving force of the process. The volatile components pass through the dry membrane pores into the membrane permeation side, and different methods are used to collect or condense the vapor. Compared with multi-stage vacuum evaporation, reverse osmosis, distillation and other separation technologies, the rejection rate of non-volatile solutes in membrane distillation can theoretically reach 100%, and the process can be operated at low temperature and normal pressure. The equipment process is simple, and can use solar energy, low-grade industrial waste heat and alternative energy for separation.

[0003] Although membrane distillation can effectively utilize low-cost heat sources, the thermal efficiency is much lower than other thermal concentration processes. The energy consumed in the membrane distillation process is composed of heat energy and electric energy (fluid flow), and the heat energy consumed accounts for more than 90% of the total energy consumption. The heat energy consumption is divided into two parts: one part is the heat energy consumed by the vaporization of volatile components in the membrane surface, which is the effective heat energy (referred to as effective heat load); the other part is the heat energy consumed by the heat conduction across the membrane, which is the ineffective heat energy (referred to as ineffective heat load). The ratio of effective heat energy to total heat energy consumed in the process is the thermal efficiency of the membrane distillation process. In order to obtain high flux, the temperature difference between the two sides of the membrane must be maintained, so the main part of the ineffective heat energy is the heat loss caused by the heat conduction across the membrane. In order to pursue high flux and improve the treatment efficiency, the temperature difference between the two sides of the membrane is increased, which usually accompanies the decrease of the thermal efficiency. Therefore, there is a dilemma between improving the thermal efficiency and improving the membrane flux.

[0004] On the other hand, the hot feed solution preferentially vaporizes from the position in contact with the membrane surface, and the heat energy consumed by the vaporization of the feed solution reduces the temperature of the feed solution in contact with the membrane surface, resulting in the phenomenon of temperature polarization, i.e. the temperature of the feed solution is positively correlated with the distance from the membrane surface, and the vaporization rate of the feed solution in contact with the membrane surface decreases with the decrease of the temperature. The phenomenon of temperature polarization will seriously inhibit the improvement of the membrane flux.

[0005] Therefore, it is desirable to simultaneously improve the thermal efficiency and the membrane flux by a simple method.

[0006] The present application aims to solve the above problems. SUMMARY

[0007] The present application aims to overcome the deficiencies of the prior art, and aims to improve the thermal efficiency and membrane flux, and adopts a porous carbon fiber paper (or a porous carbon fiber non-woven fabric) as a base material to prepare an organic polymer hydrophobic microfiltration membrane applied to a membrane distillation process.

[0008] On the other hand, the support layer of the hydrophobic membrane for membrane distillation needs good air permeability to meet the requirements of high flux performance of membrane distillation, however, the carbon fiber paper or non-woven fabric has poor air permeability and is not suitable as the support layer of the hydrophobic microporous membrane applied to the membrane distillation process. In view of the poor air permeability of the carbon fiber support layer, the present application adds an organic salt pore-forming agent during preparation to obtain a porous carbon fiber support layer, and introduces polylactic acid material, which is relatively easy to decompose, so that the addition of polylactic acid can improve the porosity of the polyacrylonitrile-based porous carbon fiber support layer and increase the air permeability. In addition, the micro defects of the porous carbon fiber support layer caused by the decomposition of polylactic acid can enhance the bonding force between the separation layer and the carbon fiber support layer, so that the separation layer and the support layer are difficult to peel off, thereby ensuring the stability of the self-heating hydrophobic microporous membrane. Therefore, a high-air-permeability porous carbon fiber material suitable for the support layer of the hydrophobic membrane for membrane distillation is prepared.

[0009] The technical scheme of the present application is as follows:

[0010] The present application provides a self-heating hydrophobic microporous membrane based on a high-air-permeability porous carbon fiber support layer, which comprises a porous carbon fiber support layer and a hydrophobic microporous membrane on the porous carbon fiber support layer, wherein the porous carbon fiber support layer is selected from a porous carbon fiber or a porous carbon fiber non-woven fabric, the porous carbon fiber paper has a thickness of 20-150 μm and an air permeability of 2-10 L·cm -2 ·min -1 ·kPa -1 ; the porous carbon fiber non-woven fabric has a thickness of 15-180 μm and an air permeability of 8-15 L·cm -2 ·min -1 ·kPa -1 .

[0011] Preferably, the porous carbon fiber paper has a tensile strength of 10-20 MPa and a parallel resistivity of 8-25 mΩ·cm; and the porous carbon fiber non-woven fabric has a tensile strength of 18-30 MPa and a parallel resistivity of 6-18 mΩ·cm.

[0012] The second aspect of the present application provides a method for preparing the self-heating hydrophobic microporous membrane according to the first aspect of the present application, comprising the following steps:

[0013] (1) preparing a porous carbon fiber support layer;

[0014] (2) preparing a hydrophobic microporous membrane compounded in the porous carbon fiber support layer.

[0015] Preferably, in step (1), the porous carbon fiber support layer is selected from a porous carbon fiber paper, and the preparation method is as follows:

[0016] (a11) performing a pulp refining treatment on the polylactic acid fibers by a paper pulp refiner, and then mixing the refined polylactic acid pulp with the polyacrylonitrile fibers;

[0017] (a12) adding water, polysorbate, polyvinyl alcohol resin and organic salt pore-forming agent to the mixed fibers to perform defibration to obtain a carbon fiber mixed slurry, and then performing wet forming on the mixed slurry by a former and drying to obtain a carbon fiber paper precursor;

[0018] (a13) immersing the carbon fiber paper precursor in a phenol-formaldehyde resin-ethanol solution, pre-curing by oven heating, and then performing hot pressing treatment on the precursor by a flat vulcanizing machine;

[0019] (a14) carbonizing the carbon fiber paper precursor after the hot pressing treatment to obtain a carbonized sample;

[0020] (a15) performing graphitization treatment on the carbonized sample to prepare a porous carbon fiber paper.

[0021] Preferably, in step (a11), the polyacrylonitrile fiber has a length of 3-10 mm and a diameter of 5-8 μm; the mixing mass ratio of the polylactic acid pulp and the polyacrylonitrile fiber is 1:19-6:14; in step (a12), the pore-forming agent is one of zinc lactate and zinc oxalate, and the mass ratio of the pore-forming agent and the polyacrylonitrile fiber is 1:80-1:10; the beating revolution is 5000-15000 revolutions, and the beating time is 5-30 min; the polyacrylonitrile fiber content in the mixed slurry is 0.003-0.05 wt%; the drying time is 1-5 h, and the drying temperature is 60-100℃; in step (a13), the mass fraction of the phenolic resin-ethanol solution is 10%-25%, the impregnation time is 0.5-2 h, the pre-curing temperature is 50-90℃, and the pre-curing time is 0.5-1 h; the hot-pressing treatment conditions are: pressure 3.5-4.5 MPa, temperature 120-160℃, and time 0.5-1 h; in step (a14), the carbonization conditions are: in an inert gas atmosphere, the temperature is raised to 300-400℃ at a rate of 2-5℃ / min and maintained for 1-2 h, then the temperature is further raised to 1000-1500℃ at a rate of 5-10℃ / min and maintained for 1-4 h, and then the temperature is lowered to room temperature; in step (a15), the graphitization treatment conditions are: graphitization treatment in a graphitization furnace at a pressure of 5-120 kPa and a temperature of 2500-3000℃, and pressure maintaining time 0.1-0.5 h.

[0022] Preferably, in step (1), the porous carbon fiber support layer is selected from a porous carbon fiber non-woven fabric, and the preparation method is as follows:

[0023] (b11) dissolving polyacrylonitrile, polylactic acid and an organic salt pore-forming agent in an N,N-dimethylacetamide solution to obtain a spinning dope;

[0024] (b12) electrospinning the spinning dope to obtain a nascent non-woven fabric;

[0025] (b13) pre-oxidizing the nascent non-woven fabric;

[0026] (b14) carbonizing the pre-oxidized non-woven fabric to obtain a porous carbon fiber non-woven fabric.

[0027] Preferably, in step (b11), the polyacrylonitrile solute mass fraction is 5-30%, the pore-forming agent is one of zinc lactate and zinc oxalate, the mass ratio of polyacrylonitrile to polylactic acid is 30:1-1:1, and the mass ratio of polyacrylonitrile to the pore-forming agent is 80:1-10:1; in step (b12), the spinning conditions are as follows: a spinning voltage of 8-20 Kv, a humidity of 10-60%, a receiving distance of 12-25 cm, and a pushing speed of 0.5-3 ml / h; in step (b13), the pre-oxidation treatment conditions are as follows: under an air atmosphere, a gas flow rate of 20-80 ml / min, a temperature rising speed of 0.5-5 ℃ / min, a temperature of 100-160 ℃ for 2-8 h, and then a temperature rising speed of 0.5-5 ℃ / min, a temperature of 200-250 ℃ for 2-8 h; and in step (b14), the carbonization treatment conditions are as follows: under an inert gas atmosphere, a temperature rising speed of 2-5 ℃ / min, a temperature of 300-400 ℃ for 1-2 h, and then a temperature rising speed of 5-10 ℃ / min, a temperature of 800-1500 ℃ for 2-8 h, and then a temperature reduction to room temperature.

[0028] Preferably, in step (2), the hydrophobic microporous membrane is a polypropylene (PP) and polypropylene / ethylene octene copolymer (POE) blend, a polypropylene / ethylene propylene rubber (EPR) blend, a polypropylene / polyethylene (PE) blend, a polypropylene / polyvinyl chloride (PVC) blend, or a polypropylene / polyethylene terephthalate (PET) blend hydrophobic microporous membrane.

[0029] The method for preparing the hydrophobic microporous membrane is as follows:

[0030] (21) stirring and mixing polypropylene or polypropylene and a corresponding blend with a diluent in a reactor, introducing nitrogen protection, heating, stirring, and defoaming to obtain a homogeneous casting solution;

[0031] (22) coating the casting solution on the surface of the porous carbon fiber support layer to form a flat plate, and then cooling and solidifying in a water bath / oil bath to obtain a preliminarily formed membrane blank;

[0032] (23) immersing the membrane blank in an extraction agent to complete extraction of the diluent, and then drying to obtain a self-heating hydrophobic microporous membrane compounded on the porous carbon fiber support layer.

[0033] Preferably, in step (21), the mass fraction of polypropylene is 20-60%, the mass fraction of the blend is 2-30%, and the mass fraction of the diluent is 38-78%; the diluent is selected from one or more of dimethyl phthalate, diphenyl ether, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, soybean oil and other vegetable oils; the heating temperature is 160-300 DEG C, and the heating time is 2-6 h; in step (22), the temperature of the water bath / oil bath is 0-130 DEG C; in step (23), the extractant is one or a combination of methanol, ethanol, propanol, n-hexane, cyclohexane and acetone; the drying temperature is 60-100 DEG C, and the drying time is 1-5 h.

[0034] The third aspect of the present application provides an application of the self-heating hydrophobic microporous membrane based on the high-air-permeability porous carbon fiber support layer according to the first aspect of the present application as a membrane for membrane distillation.

[0035] The present application adds a pore-forming agent (one of zinc lactate and zinc oxalate) to the slurry, and the pore-forming agent is first reduced to zinc oxide, then to zinc single element, and finally sublimated at a temperature higher than its boiling point during the heat treatment of the carbon fiber paper precursor, thereby forming pores in the carbon fiber.

[0036] The present application mixes polylactic acid fibers into the carbon fiber papermaking slurry, and the polylactic acid fibers and the carbon fibers are randomly and staggeredly arranged, and when the heat treatment temperature of the carbon fiber paper precursor reaches 300-400 DEG C, the polylactic acid fibers are decomposed into carbon dioxide and water, and further generate micro defects on the surface of the carbon fiber paper.

[0037] The present application adds a pore-forming agent (one of zinc lactate and zinc oxalate) to the spinning dope, and the pore-forming agent is first reduced to zinc oxide, then to zinc single element, and finally sublimated at a temperature higher than its boiling point during the heat treatment of the electrospun primary non-woven fabric, thereby forming pores in the carbon fiber.

[0038] The application blends polylactic acid and polyacrylonitrile, and prepares polyacrylonitrile / polylactic acid blended nascent non-woven fabric through electrospinning. When the carbonization treatment temperature of the nascent non-woven fabric reaches 300-400 DEG C, the polylactic acid in the non-woven fabric fiber partially decomposes into carbon dioxide and water, leaving only polyacrylonitrile, and the temperature continues to rise, and polyacrylonitrile-based carbon fiber is further generated. The carbon fiber presents a fiber defect state, which is caused by the decomposition of polylactic acid. Since polylactic acid and polyacrylonitrile are uniformly mixed, the decomposition of polylactic acid will not cause the fiber to break, and has less influence on the mechanical properties of the non-woven fabric, and at the same time, the roughness of the fiber and the air permeability of the non-woven fabric are increased. When used as a hydrophobic microfiltration membrane support layer, the rough fiber structure can greatly increase the bonding force between the separation layer and the support layer, so that the separation layer and the support layer are difficult to peel off. At the same time, the decomposition of polylactic acid increases the porosity of the non-woven fabric, greatly increases the air permeability of the porous carbon fiber non-woven fabric, and does not have a negative impact on the flux performance of the separation layer.

[0039] Compared with the prior art, the application has the following beneficial effects:

[0040] 1. The application first combines a porous carbon fiber support layer and a hydrophobic microporous membrane to prepare a hydrophobic microfiltration membrane which can generate heat under the action of an applied current. This part of heat can effectively reduce the heat energy lost by heat conduction of the feed liquid, and can make up for the heat energy consumed by the evaporation of the feed liquid on the membrane surface, thereby weakening the negative effects caused by temperature difference polarization, and significantly improving the thermal efficiency and membrane flux of the membrane distillation process.

[0041] 2. The application faces a new problem when combining the carbon fiber support layer and the hydrophobic microporous membrane. The support layer of the hydrophobic membrane used for membrane distillation needs good air permeability to meet the requirements of high flux performance of the membrane distillation. However, the carbon fiber paper or non-woven fabric has poor air permeability and is not suitable as a support layer for the hydrophobic microporous membrane used in the membrane distillation process. In view of the problem of poor air permeability of the carbon fiber support layer, the application adds an organic salt pore former during preparation. However, the application finds that the addition of the pore former does not significantly improve the air permeability of the membrane. The reason for this phenomenon is that although the pore former makes the carbon fiber support layer have a uniform microporous structure, only when these pores coincide with the microporous structure of the separation layer can through holes be formed between the separation layer and the support layer. After the separation layer is combined with the support layer, a part of the micropores will become dead holes, which is not conducive to the improvement of the air permeability of the membrane by the pore former. Therefore, the introduction of the pore former alone cannot significantly improve the air permeability of the membrane. The application further introduces polylactic acid material during preparation. Polylactic acid is relatively easy to decompose, and the micro defects of the porous carbon fiber support layer caused by the decomposition of polylactic acid, combined with the microporous structure formed by the addition of the pore former, can significantly improve the porosity of the polyacrylonitrile-based porous carbon fiber support layer and increase the air permeability.

[0042] In addition, it is crucial to improve the comprehensive performance of the self-heating hydrophobic microporous membrane to improve the air permeability of the support layer while ensuring that its mechanical performance is not affected. The addition of the organic salt pore-forming agent makes the porous carbon fiber support layer porous. In the case of less impact on the mechanical properties of the porous carbon fiber support layer, a high-air-permeability porous material suitable for the support layer of the hydrophobic membrane for membrane distillation is prepared. The porous carbon fiber paper has a thickness of 20-150 μm, an air permeability of 2-10 L·cm -2 ·min -1 ·kPa -1 , a tensile strength of 10-20 MPa, and a parallel resistance of 8-25 mΩ·cm. The porous carbon fiber non-woven fabric has a thickness of 15-180 μm, an air permeability of 8-15

[0043] L·cm -2 ·min -1 ·kPa -1 , a tensile strength of 18-30 MPa, and a parallel resistance of 6-18 mΩ·cm.

[0044] 3. The micro-defects of the porous carbon fiber support layer caused by the decomposition of polylactic acid can improve the porosity of the carbon fiber material and increase the air permeability. In addition, the micro-defects of the porous carbon fiber support layer caused by the decomposition of polylactic acid help the combination of the separation layer and the support layer, enhance the bonding force, make the separation layer and the support layer difficult to peel off, and ensure the stability of the self-heating hydrophobic microporous membrane. BRIEF DESCRIPTION OF DRAWINGS

[0045] None. DETAILED DESCRIPTION

[0046] The present application is further illustrated by the following examples, which are not intended to limit the present application. In the examples, the experimental methods not specified in the examples are generally carried out according to the conventional conditions and the conditions described in the manual, or using the general equipment, materials, reagents, etc. recommended by the manufacturer, unless otherwise specified.

[0047] The specific steps of the method described in the present application are as follows:

[0048] Example 1

[0049] Preparation of a high-air-permeability porous carbon fiber paper suitable for the support layer of a hydrophobic membrane for membrane distillation:

[0050] 5 g of absolutely dry polylactic acid fibers (length 5 mm, fineness 3 dtex) were added to 50 g of deionized water and placed in a PFI machine for beating treatment. The beating revolution was 8000 r, and the beating degree was 43°SR. After dewatering and drying (drying temperature 100°C), a mechanically treated polylactic acid pulp was prepared.

[0051] Take 1.5 g of polylactic acid pulp and 5 g of polyacrylonitrile fiber (length 6 mm, diameter 5 μm) mixed and placed in a defibrator, 10 L of deionized water, 0.2 g of polysorbate, 0.2 g of polyvinyl alcohol resin and 0.5 g of zinc lactate were added, the defibrator was set to 10000 revolutions, and the defibration was carried out for 20 min to obtain a mixed slurry with a polyacrylonitrile fiber content of 0.05 wt%. The mixed slurry was wet-formed on a paper former, dried at 80℃ for 2 h to obtain a carbon fiber paper precursor.

[0052] The carbon fiber paper precursor was immersed in a phenol-formaldehyde resin-ethanol solution (15 wt%) for 1 h, then pre-cured in an oven (temperature 60℃, time 1 h), and then hot-pressed in a flat vulcanizing machine (pressure 4.0 MPa, temperature 150℃, time 0.5 h). The hot-pressed carbon fiber paper precursor was carbonized by heating to 300℃ at a rate of 2℃ / min and holding for 2 h, then heating to 1200℃ at a rate of 10℃ / min and holding for 4 h, and then cooling to room temperature. Finally, the carbonized sample was graphitized in a graphitization furnace at a pressure of 100 kPa and a temperature of 2500℃, and held for 0.5 h. A carbon fiber paper with a thickness of 85 μm, a porosity of 88%, a gas permeability of 8.3 L·cm -2 ·min -1 ·kPa -1 , a tensile strength of 13.8 MPa, and a parallel resistivity of 12.4 mΩ·cm was finally prepared.

[0053] Example 2

[0054] Preparation of a high gas permeability porous carbon fiber non-woven fabric suitable for use as a support layer for hydrophobic membranes in membrane distillation:

[0055] Weigh 4 g of polyacrylonitrile powder, 0.08 g of dry zinc lactate powder, 1 g of polylactic acid powder and 25 g of N, N-dimethylacetamide liquid in a flask, stir for 12 h, and ultrasonic defoaming for 0.5 h to obtain a zinc lactate / polylactic acid / polyacrylonitrile spinning dope. The non-woven fabric was prepared by electrospinning under the following conditions: spinning voltage 18 Kv, humidity 60%, receiving distance 17 cm, and pushing speed 0.8 ml / h. The nascent non-woven fabric was obtained.

[0056] The as-spun nonwoven fabric was subjected to pre-oxidation treatment, the pre-oxidation treatment conditions were: under air atmosphere, air flow rate was 30 ml / min, the temperature was raised to 120℃ at a rate of 1℃ / min and maintained for 2h, then the temperature was raised to 200℃ at a rate of 2℃ / min and maintained for 5h; the pre-oxidized nonwoven fabric was subjected to carbonization treatment, under N2 atmosphere, the temperature was raised to 300℃ at a rate of 2℃ / min and maintained for 2h, then the temperature was continuously raised to 1200℃ at a rate of 10℃ / min and maintained for 8h before cooling to room temperature. A carbon fiber nonwoven fabric with a thickness of 110μm was prepared, the porosity was 84%, the air permeability was 8.9L·cm -2 ·min -1 ·kPa -1 , the tensile strength was 20.4MPa, the parallel resistivity was 10.1mΩ·cm.

[0057] Example 3

[0058] Preparation of a self-heating hydrophobic microporous membrane with carbon fiber paper as support layer and application of the membrane in membrane distillation process:

[0059] Polypropylene 24g, ethylene octene copolymer 6g, soybean oil 35g, dibutyl phthalate 35g were weighed into a three-necked flask, protected by nitrogen, stirred at 180℃ for 4h, and then allowed to stand to remove bubbles, to obtain a homogeneous casting solution. The casting solution was coated on the surface of the carbon fiber paper prepared in Example 1 at 180℃ to form a flat plate, and then placed in a 25℃ water bath for cooling and solidification to obtain a preliminarily shaped membrane blank. The membrane blank was immersed in n-hexane for 12h, then taken out and immersed in ethanol for 12h, and the process was repeated twice. The extracted membrane was dried in an oven at 60℃ for 2h to obtain a microporous hydrophobic membrane with self-heating capability which can be used in membrane distillation process.

[0060] The obtained product had a porosity of 75%, an average pore size of 0.2μm, and an air permeability of 3.1L·cm -2 ·min -1 ·kPa -1 , water contact angle 125.5°, tensile strength 14MPa, and separation layer and support layer peeling strength 65N / m. When applied in vacuum membrane distillation process of 5% NaCl aqueous solution, the feed flow rate was 20L·h -1 , the feed temperature was 50℃, when no voltage was applied to the separation membrane, the membrane flux was 5.81kg·m -2 ·h -1 , the rejection rate was 99.98%, when a voltage of 12v was applied to the separation membrane, the membrane flux was 7.45kg·m -2 ·h -1 , the rejection rate was 99.98%. The self-heating performance of the membrane under the action of current increased the membrane distillation flux by 28% under the same process conditions, and the rejection rate did not decrease.

[0061] Example 4

[0062] Preparation of a self-heating hydrophobic microporous membrane with carbon fiber non-woven fabric as a support layer and application in membrane distillation process

[0063] The preparation method of a self-heating hydrophobic microporous membrane with carbon fiber non-woven fabric as a support layer is the same as that in Example 3, except that the casting solution is coated on the surface of the carbon fiber non-woven fabric prepared in Example 2.

[0064] The obtained product has a porosity of 74%, an average pore size of 0.2 μm, an air permeability of 3.3 L·cm -2 ·min -1 ·kPa -1 , a water contact angle of 125.7°, a tensile strength of 20.7 MPa, and a separation layer and support layer peeling strength of 62 N / m. In the vacuum membrane distillation process of a 5% NaCl aqueous solution, the feed flow rate is 20 L·h -1 , the feed temperature is 50℃, when no voltage is applied to the separation membrane, the membrane flux is 5.66 kg·m -2 ·h -1 , the rejection rate is 99.98%, when a voltage of 12 V is applied to the separation membrane, the membrane flux is 7.53 kg·m -2 ·h -1 , and the rejection rate is 99.98%. The self-heating property of the membrane under the action of current makes the membrane distillation flux increase by 33% under the same process conditions, and the rejection rate does not decrease.

[0065] Comparative Example 1

[0066] The preparation method of a carbon fiber paper is the same as that in Example 1, except that the polyacrylonitrile fibers are not mixed with polylactic acid pulp, and no zinc lactate is added. Finally, a carbon fiber paper with a thickness of 83 μm is prepared, having a porosity of 42%, an air permeability of 1.7 L·cm -2 ·min -1 ·kPa -1 , a tensile strength of 15.1 MPa, and a parallel resistance of 10.5 mΩ·cm.

[0067] Comparative Example 2

[0068] The preparation method of a carbon fiber paper is the same as that in Example 1, except that the polyacrylonitrile fibers are not mixed with polylactic acid pulp. Finally, a carbon fiber paper with a thickness of 83 μm is prepared, having a porosity of 51%, an air permeability of 2.9 L·cm -2 ·min -1 ·kPa -1 , a tensile strength of 15 MPa, and a parallel resistance of 10.4 mΩ·cm.

[0069] Comparative Example 3

[0070] A carbon fiber nonwoven fabric was prepared in the same manner as in Example 2, except that no polylactic acid and zinc lactate were added to the spinning dope. A carbon fiber nonwoven fabric having a thickness of 108 μm, a porosity of 40%, and a gas permeability of 2.0 L-cm -2 • min -1 • kPa -1 , a tensile strength of 23.4 MPa, and a parallel-direction resistivity of 9.4 mΩ-cm.

[0071] Comparative Example 4

[0072] A carbon fiber nonwoven fabric was prepared in the same manner as in Example 2, except that no polylactic acid was added to the spinning dope. A carbon fiber nonwoven fabric having a thickness of 110 μm, a porosity of 55%, and a gas permeability of 3.1 L-cm -2 • min -1 • kPa -1 , a tensile strength of 23.2 MPa, and a parallel-direction resistivity of 9.7 mΩ-cm.

[0073] Comparative Example 5

[0074] A hydrophobic microporous membrane having a carbon fiber paper as a support layer was prepared in the same manner as in Example 3, except that the casting solution was applied to the surface of the carbon fiber paper prepared in Comparative Example 1. The resulting product membrane had a porosity of 70%, a gas permeability of 0.9 L-cm -2 • min -1 • kPa -1 , a tensile strength of 14.1 MPa, and a separation layer / support layer peeling strength of 46 N / m.

[0075] Comparative Example 6

[0076] A hydrophobic microporous membrane having a carbon fiber paper as a support layer was prepared in the same manner as in Example 3, except that the casting solution was applied to the surface of the carbon fiber paper prepared in Comparative Example 2. The resulting product membrane had a porosity of 70%, a gas permeability of 1.2 L-cm -2 • min -1 • kPa -1 , a tensile strength of 14.1 MPa, and a separation layer / support layer peeling strength of 46.5 N / m.

[0077] Comparative Example 7

[0078] A hydrophobic microporous membrane having a carbon fiber nonwoven fabric as a support layer was prepared in the same manner as in Example 4, except that the casting solution was applied to the surface of the carbon fiber nonwoven fabric prepared in Comparative Example 3. The resulting product membrane had a porosity of 71%, a gas permeability of 1.6 L-cm -2 • min -1 • kPa -1Tensile strength 23.5 MPa, peel strength between separation layer and support layer 42 N / m.

[0079] Comparative Example 8

[0080] A hydrophobic microporous membrane with carbon fiber nonwoven fabric as support layer was prepared in the same way as in Example 4, except that the casting solution was coated on the surface of the carbon fiber nonwoven fabric prepared in Comparative Example 4. The obtained product membrane had a porosity of 72%, a gas permeability of 2.8 L-cm -2 ·min -1 ·kPa -1 Tensile strength 23.3 MPa, peel strength between separation layer and support layer 42.6 N / m.

[0081] Key performance comparison:

[0082] Table 1 Comparison of properties of carbon fiber support layer

[0083]

[0084] As can be seen from Table 1, the addition of pore-forming agent can improve the gas permeability of carbon fiber material without affecting the mechanical properties. Polylactic acid greatly increases the porosity of carbon fiber material and also greatly increases the gas permeability of the material. Although it reduces the mechanical properties, compared to the good gas permeability, the impact on the mechanical properties is within an acceptable range.

[0085] Table 2 Comparison of key properties of hydrophobic membranes with carbon fiber material as support layer

[0086]

[0087] As can be seen from Table 2, the porosity of the membrane prepared by combining the separation layer and the support layer is mainly affected by the separation layer, so changing the support layer has little effect on the porosity of the membrane. When the gas permeability of the support layer is strong enough, the gas permeability of the membrane is mainly affected by the separation layer. When the gas permeability of the support layer is weak, the support layer will restrict the gas permeability of the membrane. The comparative examples belong to the latter. In addition, the reason why the pore-forming agent fails to significantly improve the gas permeability of the membrane is that although the pore-forming agent makes the carbon fiber support layer have a uniform microporous structure, only when these pores coincide with the microporous structure of the separation layer can through holes be formed through the separation layer and the support layer. When the separation layer is combined with the support layer, a part of the micropores will become dead holes, which is not conducive to the improvement of the gas permeability of the membrane by the pore-forming agent. The micro defects of the support layer caused by polylactic acid not only make the separation layer have high gas permeability, but also greatly increase the bonding force between the separation layer and the support layer, making it difficult to peel off the separation layer and the support layer, and the membrane has good stability.

[0088] Table 3 Comparison of membrane distillation application performance of self-heating hydrophobic microporous membrane

[0089]

[0090] Example 3 and 4 were applied to the vacuum membrane distillation process of 5% NaCl aqueous solution, the feed flow rate was 20 L·h -1 , and the feed temperature was 50℃. As can be seen in Table 3, under the same process conditions, the spontaneous heating performance of the membranes of Examples 3 and 4 under the action of electric current increased the membrane distillation flux by 28% and 33%, respectively, while maintaining a high retention rate. The spontaneous heating feature possessed by the hydrophobic microporous membrane of the present application effectively reduces the heat energy lost by the feed liquid due to heat conduction, while making up for the heat energy consumed by the feed liquid on the membrane surface due to vaporization, weakening the negative effects caused by temperature polarization, and significantly improving the thermal efficiency and membrane flux of the membrane distillation process. The present application will greatly promote the development of membrane distillation applications and play a positive role in the promotion of membrane distillation technology.

[0091] The above has exemplarily described the present application, it should be indicated that, without departing from the core of the present application, any simple transformation, modification or other equivalent replacement which can not cost creative labor of the person skilled in the art falls into the protection scope of the present application.

Claims

1. A self-heating hydrophobic microporous membrane based on a highly permeable porous carbon fiber support layer, characterized in that, It includes a porous carbon fiber support layer and a hydrophobic microporous membrane located on the porous carbon fiber support layer, wherein the porous carbon fiber support layer is selected from porous carbon fiber paper or porous carbon fiber nonwoven fabric, the porous carbon fiber paper has a thickness of 20~150μm and an air permeability of 2~10 L·cm -2 ·min -1 ·kPa -1 The porous carbon fiber nonwoven fabric has a thickness of 15~180μm and an air permeability of 8~15 L·cm. -2 ·min -1 ·kPa -1 ; The method for preparing the self-heating hydrophobic microporous membrane includes the following steps: (1) Preparation of porous carbon fiber support layer; (2) Preparation of a hydrophobic microporous membrane composited with a porous carbon fiber support layer; In step (1), the method for preparing the porous carbon fiber paper is as follows: (a11) Polylactic acid fibers are pulped using a pulp refiner, and then the refined polylactic acid pulp is mixed with polyacrylonitrile fibers; (a12) Water, polysorbate, polyvinyl alcohol resin and organic salt pore-forming agent are added to the mixed fibers to decompose and obtain carbon fiber mixed slurry. The mixed slurry is then wet-formed and dried through a forming machine to obtain carbon fiber paper precursor. (a13) The carbon fiber paper precursor was impregnated in a phenolic resin-ethanol solution, pre-cured by heating in an oven, and then hot-pressed by a flat vulcanizing machine. (a14) Carbonize the hot-pressed carbon fiber paper precursor to obtain a carbonized sample. (a15) The carbonized sample was graphitized to prepare porous carbon fiber paper. The method for preparing the porous carbon fiber nonwoven fabric is as follows: (b11) Polyacrylonitrile, polylactic acid and organic salt pore-forming agents are dissolved in N,N-dimethylacetamide solution to obtain spinning solution; (b12) Electrospinning the spinning solution yields nascent nonwoven fabric; (b13) Pre-oxidize the nascent nonwoven fabric; (b14) Carbonize the pre-oxidized nonwoven fabric to obtain porous carbon fiber nonwoven fabric.

2. The self-heating hydrophobic microporous membrane according to claim 1, characterized in that, The porous carbon fiber paper has a tensile strength of 10-20 MPa and a parallel resistivity of 8-25 mΩ·cm; the porous carbon fiber nonwoven fabric has a tensile strength of 18-30 MPa and a parallel resistivity of 6-18 mΩ·cm.

3. The self-heating hydrophobic microporous membrane according to claim 1, characterized in that, In step (a11), the polyacrylonitrile fiber has a length of 3-10 mm and a diameter of 5-8 μm; the mass ratio of polylactic acid pulp to polyacrylonitrile fiber is 1:19-6:14; in step (a12), the pore-forming agent is either zinc lactate or zinc oxalate, and the mass ratio of the pore-forming agent to polyacrylonitrile fiber is 1:80-1:10; the debonding rotation speed is 5000-15000 rpm, and the debonding time is 5-30 min; the polyacrylonitrile fiber content in the carbon fiber mixed slurry is 0.003-0.05 wt%; the drying time is 1-5 h, and the drying temperature is 60-100℃; in step (a13), the phenolic resin-ethanol solution has a mass fraction of 10%-25%, and the impregnation time is... 0.5~2h, pre-curing temperature 50~90℃, pre-curing time 0.5~1h; hot pressing conditions are: pressure 3.5~4.5MPa, temperature 120~160℃, time 0.5~1h; in step (a14), the carbonization conditions are: in an inert gas atmosphere, heat to 300~400℃ at a heating rate of 2~5℃ / min and hold for 1~2h, then continue heating to 1000~1500℃ at a heating rate of 5~10℃ / min and hold for 1~4h, then cool to room temperature; in step (a15), the graphitization conditions are: graphitize in a graphitization furnace at a pressure of 5~120kPa and a temperature of 2500~3000℃, and hold for 0.1~0.5h.

4. The self-heating hydrophobic microporous membrane according to claim 1, characterized in that, In step (b11), the mass fraction of polyacrylonitrile solute is 5-30%, and the pore-forming agent is either zinc lactate or zinc oxalate. The mass ratio of polyacrylonitrile to polylactic acid is 30:1 to 1:1, and the mass ratio of polyacrylonitrile to pore-forming agent is 80:1 to 10:

1. In step (b12), the spinning conditions are: spinning voltage 8-20 kV, humidity 10-60%, and receiving distance 12-25 km. cm, the propulsion speed is 0.5-3 ml / h; in step (b13), the pre-oxidation treatment conditions are: in an air atmosphere, the gas flow rate is 20-80 ml / min, the temperature is increased at a rate of 0.5-5℃ / min to 100-160℃ and held for 2-8h, then the temperature is increased at a rate of 0.5-5℃ / min to 200-250℃ and held for 2-8h; in step (b14), the carbonization treatment conditions are: in an inert gas atmosphere, the temperature is increased at a rate of 2-5℃ / min to 300-400℃ and held for 1-2h, then the temperature is increased at a rate of 5-10℃ / min to 800-1500℃ and held for 2-8h, and then the temperature is reduced to room temperature.

5. The self-heating hydrophobic microporous membrane according to claim 1, characterized in that, In step (2), the hydrophobic microporous membrane is a blend of polypropylene and polypropylene / ethylene octene copolymer, polypropylene / ethylene propylene rubber, polypropylene / polyethylene, polypropylene / polyvinyl chloride, or polypropylene / polyethylene terephthalate. The method for preparing the hydrophobic microporous membrane is as follows: (21) Polypropylene or polypropylene and its corresponding blends are mixed with diluent in a reactor, nitrogen gas is introduced for protection, heating is performed, stirring is carried out, and degassing is performed to obtain a homogeneous casting solution. (22) The casting liquid is coated on the surface of the porous carbon fiber support layer and scraped into a flat plate shape, and then cooled and solidified in a water bath / oil bath to obtain a pre-formed film blank. (23) Immerse the membrane blank in the extractant to complete the extraction of the diluent, and then dry it to obtain a self-heating hydrophobic microporous membrane composited with a porous carbon fiber support layer.

6. The self-heating hydrophobic microporous membrane according to claim 5, characterized in that, In step (21), the mass fraction of polypropylene is 20-60%, the mass fraction of the blend is 2-30%, and the mass fraction of the diluent is 38-78%. The diluent is selected from one or more of dimethyl phthalate, diphenyl ether, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, soybean oil, and other vegetable oils. The heating temperature is 160℃-300℃, and the heating time is 2-6h. In step (22), the temperature of the water bath / oil bath is 0℃-130℃. In step (23), the extractant is one or a combination of methanol, ethanol, propanol, n-hexane, cyclohexane, and acetone. The drying temperature is 60-100℃, and the drying time is 1-5h.

7. The application of a self-heating hydrophobic microporous membrane based on a highly permeable porous carbon fiber support layer as described in any one of claims 1-6 as a membrane for membrane distillation.

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