CN114832629B - Composite membrane for membrane distillation and preparation method thereof - Google Patents

Composite membrane for membrane distillation and preparation method thereof Download PDF

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CN114832629B
CN114832629B CN202210319919.1A CN202210319919A CN114832629B CN 114832629 B CN114832629 B CN 114832629B CN 202210319919 A CN202210319919 A CN 202210319919A CN 114832629 B CN114832629 B CN 114832629B
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polymer
porous
layer
membrane
solvent
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CN114832629A (en
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贾伟
王佳冕
张红斌
胡楠
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Shenzhen National Research Institute of High Performance Medical Devices Co Ltd
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Shenzhen National Research Institute of High Performance Medical Devices Co Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/36—Pervaporation; Membrane distillation; Liquid permeation
    • B01D61/364—Membrane distillation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/08—Polysaccharides
    • B01D71/12—Cellulose derivatives
    • B01D71/14—Esters of organic acids
    • B01D71/16—Cellulose acetate
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/26—Polyalkenes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B01D71/34—Polyvinylidene fluoride
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/38—Polyalkenylalcohols; Polyalkenylesters; Polyalkenylethers; Polyalkenylaldehydes; Polyalkenylketones; Polyalkenylacetals; Polyalkenylketals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/40—Polymers of unsaturated acids or derivatives thereof, e.g. salts, amides, imides, nitriles, anhydrides, esters
    • B01D71/42—Polymers of nitriles, e.g. polyacrylonitrile
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/52—Polyethers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/56—Polyamides, e.g. polyester-amides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/66—Polymers having sulfur in the main chain, with or without nitrogen, oxygen or carbon only
    • B01D71/68—Polysulfones; Polyethersulfones
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/38—Hydrophobic membranes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A20/00—Water conservation; Efficient water supply; Efficient water use
    • Y02A20/124—Water desalination
    • Y02A20/131—Reverse-osmosis

Abstract

The invention discloses a composite membrane for membrane distillation and a preparation method thereof, wherein the preparation method comprises the following steps: providing a first porous polymer layer; dissolving a second polymer in a second solvent to obtain a second polymer solution; forming a second polymer solution on the first porous polymer layer to obtain a second coating layer, arranging a porous pattern mold on the second coating layer, and placing the second coating layer in a gas-phase extraction agent; and continuously placing the second coating in a second liquid-phase extracting agent, standing, removing the porous pattern mould, continuously standing until the second liquid-phase extracting agent completely extracts the second solvent from the second coating, and solidifying the second polymer to form a second porous polymer layer, thereby obtaining the composite membrane. According to the invention, the porous pattern die is selected, and the two processes of gas-induced phase separation and non-solvent-induced phase separation are simultaneously combined to prepare the double-layer composite film, wherein the first layer of film is used for ensuring the basic separation effect, and the second layer of film is used for forming the surface pattern, so that the hydrophobicity is enhanced, and the anti-pollution capability is improved.

Description

Composite membrane for membrane distillation and preparation method thereof
Technical Field
The invention relates to the technical field of membrane distillation, in particular to a composite membrane for membrane distillation and a preparation method thereof.
Background
Membrane Distillation (MD) technology has been widely used in recent years for the desalination of challenging high salinity wastewater and desalination of seawater due to its tolerance to high salinity feed solutions. MD uses a porous hydrophobic membrane as the interface between the liquid feed phase and the gas phase (air filling the gap) to induce water evaporation. The hydrophobicity of the membrane allows vapor to pass through its pores, but not liquid feed.
Excellent MD membranes need to have certain properties including high hydrophobicity, controlled pore size, narrow pore distribution, and high resistance to fouling and scaling. The fouling problem of membranes is a major challenge facing MD. Fouling can lead to membrane pore plugging, thereby reducing the effective vapor transport area, ultimately reducing permeate flux, and also leading to higher membrane pressure drop and temperature differential polarization. Gryta (M.Gryta, long-term performance of membrane distillation process, J.Membr.Sci.265 (2005) 153-159, http:// dx.doi.org/10.1016/j.memsci.2005.04.049.) it was also noted that scale deposits formed on the membrane surface also resulted in partial membrane wetting during MD, resulting in a reduction in contaminant rejection. In addition, the fouling layer also creates additional thermal resistance, changing the overall heat transfer coefficient, increasing energy consumption. Therefore, research and development of a novel anti-scaling and anti-pollution MD membrane has great scientific research and application values.
It is believed that the anti-fouling, anti-wetting capabilities of the membrane can be improved by forming corrugations or other patterns in the membrane surface to build up a highly roughened surface, which is achieved primarily by two mechanisms: (1) They increase the effective surface area of the membrane, thereby reducing local flux and reducing fouling; (2) They promote turbulence at the membrane surface, increasing the shear rate, and can make it more difficult for fouling to adhere to the membrane surface.
Many studies have attempted to form corrugations or other patterns in the surface of the film by various means. Maruf et al (S.H.Maruf, A.R.Greenberg, J.Pellegrino, YDing, publication and characterization of a surface-patterned thin film composite membrane, J.Membr.Sci.452 (2014) 11-19, http:// dx.doi.org/10.1016/j.memsci.2013.10.017.) succeeded in imprinting submicron-level patterns on the surface of a commercial polysulfone ultrafiltration membrane by nanoimprint lithography, and the patterns significantly improve the anti-staining performance of the membrane during the filtration of colloidal silica particle suspension. However, the nanoimprint lithography is complex in operation and high in cost, and large-scale industrial production is difficult to realize.
Peters et al (A. Peters, micro-patterned interfaces processing transport and impact membranes, 2008. Http:// doc. Utwente. Nl/60339/>) used silicone rubber as a patterned mold to form a gas separation membrane over silicone rubber, but the silicone rubber mold was not openworked, resulting in a patterned surface immediately adjacent to the silicone rubber mold susceptible to the formation of an undesirable densified layer.
In addition, the MD film is coated in a single layer, and the patterning process on the surface of the MD film tends to cause a difference in pore structure at different positions on the film surface, and if a macro-pore defect exists, the performance tends to be degraded in the precision separation requirement.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides a composite membrane for membrane distillation and a preparation method thereof.
In order to realize the purpose, the technical scheme of the invention is as follows:
a preparation method of a composite membrane for membrane distillation comprises the following processes:
providing a first porous polymer layer for membrane separation, the first porous polymer layer comprising a first polymer;
dissolving a second polymer in a second solvent to obtain a second polymer solution;
forming the second polymer solution on the first porous polymer layer to obtain a second coating layer, arranging a porous pattern mold on the second coating layer, and placing the second coating layer and the porous pattern mold in a gas-phase extracting agent, wherein the gas-phase extracting agent extracts the second solvent on the surface of the second coating layer from the second coating layer; and continuously placing the second coating layer and the porous pattern mould into a second liquid-phase extracting agent, standing, removing the porous pattern mould, and continuously standing until the second liquid-phase extracting agent completely extracts the second solvent from the second coating layer, wherein the speed of extracting the second solvent by the second liquid-phase extracting agent is greater than the speed of extracting the second solvent by the gas-phase extracting agent, and solidifying the second polymer to form a second porous polymer layer, thereby obtaining the composite membrane.
The invention also discloses a composite membrane for membrane distillation prepared by the method.
The embodiment of the invention has the following beneficial effects:
according to the embodiment of the invention, the first porous polymer layer for membrane separation is formed, and the second porous polymer layer for forming patterns such as surface ripples and enhancing hydrophobicity is formed above the first porous polymer layer, so that the first porous polymer layer is prevented from being damaged during surface patterning, and the membrane separation function is prevented from being influenced.
According to the invention, the second porous polymer layer is formed by combining the VTPS process and the NTPS process, and the gas-phase induced phase separation has a slower separation speed, so that relatively dense pores can be formed by the gas-phase induced phase separation, and the dense pores are positioned right below the porous pattern mold, so that the damage of the gravity action of the porous pattern mold to the pores on the surface of the film can be avoided, and the local pores are enlarged or cracks are formed; the rest pores of the second porous polymer layer close to one side of the first porous polymer layer are continuously formed by a non-solvent induced phase separation method, the solvent replacement speed of the non-solvent induced phase separation method is high and sufficient, relatively large pores can be formed, and the pores of the first porous polymer layer are prevented from being blocked.
According to the invention, the porous pattern die is adopted and arranged above the second coating, and the solvent replacement on the surface of the second coating is completed through the pores of the porous pattern die, so that an undesirable dense layer formed when silicon rubber is adopted as the die in the prior art is avoided, and the influence of the die on the pore structure in the phase conversion film forming process is reduced as much as possible.
The surface patterning method is simple and efficient, can be combined with a common phase transformation process in a nested manner, and is suitable for large-scale production.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the prior art descriptions will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
Wherein:
FIG. 1 is a schematic flow diagram of a method for preparing a composite membrane for membrane distillation according to an embodiment of the present invention; wherein (a) shows the PP support layer on the glass plate, (b) shows the casting of the first 12wt.% PVDF casting solution, (c) shows the soaking in a deionized water coagulation bath for 2 hours, (d) shows the casting of the second 10wt.% PVDF casting solution, (e) shows the placement of the diamond-shaped porous gasket on the second coating, (f) shows the exposure to a humid environment (80 rh) for 5 minutes, (g) shows the deionized water coagulation bath, (h) shows the gasket removal after 1 minute, (i) shows the final PVDF corrugated bilayer membrane after drying.
FIG. 2 is a top view of a structure of a porous pattern mold (left) and dimensional parameters of a single prism (right) according to an embodiment of the present invention.
FIG. 3 is a photograph of a surface pattern formed using the porous pattern mold of FIG. 2 in accordance with one embodiment of the present invention.
FIG. 4 is a scanning electron microscope image of the surface of the composite film before and after MD operation, wherein a and d are non-pad covered areas of the composite film prepared in example 1, b and e are pad indentation areas of the composite film prepared in example 1, c and f are the surface of the composite film prepared in comparative example 1, and g is the surface image of the composite film prepared in example 1 after MD operation; h is the surface image of the film of comparative example 1 after the MD run.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.
The invention discloses a simple and effective preparation method of a composite membrane for membrane distillation, which selects a porous pattern mould, and simultaneously combines and applies two processes of gas-induced phase separation (VIPS) and non-solvent-induced phase separation (NIPS) to prepare a double-layer membrane, wherein the first layer of membrane is mainly used for controlling a pore structure to ensure a basic separation effect, and the second layer of membrane is specially used for forming patterns such as surface ripples and the like and enhancing hydrophobicity to improve the anti-pollution capability of the membrane. Specifically, the preparation method of the composite membrane for membrane distillation of the present invention, referring to fig. 1, comprises the following steps:
step 1: providing a first porous polymer layer, wherein the first porous polymer layer is used for membrane separation and mainly used for controlling a pore structure and ensuring a basic separation effect, and comprises a first polymer.
In this step, in a specific embodiment, a first porous polymer layer is formed by a NIPS process, in which a polymer is dissolved in a solvent to form a homogeneous solution, then a reagent (referred to as an extractant) having a higher intersolubility with the solvent is slowly added to extract the solvent, so as to form a two-phase structure in which the polymer is used as a continuous phase and the solvent is used as a dispersed phase, and then the solvent is removed, thereby obtaining a polymer having a certain pore structure. Specifically, the method comprises the following steps:
step 11: the first polymer is dissolved in a first solvent to obtain a first polymer solution.
In this step, the first polymer may be sufficiently and uniformly dissolved in the first solvent by stirring, magnetic stirring, ultrasonic stirring or heating, etc. to form a homogeneous first polymer solution.
Step 12: forming or depositing a first polymer solution on the fiber support layer, placing the first coating layer in a first liquid phase extractant, extracting the first solvent from the first coating layer by the first liquid phase extractant, completing the NIPS process, and curing the first polymer to form a first porous polymer layer.
In this step, as shown in fig. 1, in a specific embodiment, a first polymer solution is cast on a fiber support layer, as shown in (a) and (b) of fig. 1, and then a coating process is completed by using a blade coating machine to form a first coating layer, although in other embodiments, the first coating layer can be formed by spraying, coating, casting, etc. After the first coating layer is formed, the first coating layer and the fiber support layer are immersed into the first liquid-phase extraction agent, as shown in fig. 1 (c), the mutual solubility between the first liquid-phase extraction agent and the first solvent is stronger than the mutual solubility between the first solvent and the first polymer, the first liquid-phase extraction agent extracts the first solvent from the first coating layer, after the first solvent is fully extracted, the first coating layer and the fiber support layer are taken out, and the first polymer is dried at room temperature to be completely cured or semi-cured, so that the first porous polymer layer is formed.
In one embodiment, the first polymer may be selected from one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, polyetheretherketone, polyamide, and the like; the first solvent may include one or more of N, N-dimethylacetamide, N-methylpyrrolidone, N-dimethylformamide, triethyl phosphate, and dimethylsulfoxide; the first liquid phase extractant is deionized water.
In a specific embodiment, the first polymer solution contains 10% to 14% by mass of the first polymer, and after the first solvent is completely extracted, a separation membrane (i.e., a first porous polymer layer) having a function of separating a gas phase and a liquid phase can be obtained.
The extraction time of the first coating in the first liquid-phase extractant may be 45min to 120min.
Step 2: and dissolving the second polymer in a second solvent to obtain a homogeneous second polymer solution.
In one embodiment, the second polymer may be selected from one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, polyetheretherketone, polyamide, and the like; the second solvent may include one or more of N, N-dimethylacetamide, N-methylpyrrolidone, N-dimethylformamide, triethyl phosphate, and dimethylsulfoxide; the second polymer may be the same as or different from the first polymer, and the second solvent may be the same as or different from the first solvent.
In a specific embodiment, the mass percentage of the second polymer in the second polymer solution is 8% to 12%, and the mass percentage of the second polymer in the second polymer solution is less than the mass percentage of the first polymer in the first polymer solution, so that the pores of the second porous polymer layer are larger than those of the first porous polymer layer, which is beneficial to outward diffusion of gas and avoids blockage.
And step 3: forming a second polymer solution on the first porous polymer layer to obtain a second coating layer, as shown in fig. 1 (d), and then disposing a porous pattern mold on the second coating layer, as shown in fig. 1 (e), and placing the second coating layer together with the porous pattern mold in a gas phase extracting agent, as shown in fig. 1 (f), which extracts the second solvent on the surface of the second coating layer from the second coating layer, thereby completing the VIPS process. And (e) continuing to place the second coating layer and the porous pattern mold in a second liquid-phase extraction agent, standing as shown in (g) in fig. 1, removing the porous pattern mold, standing as shown in (h) in fig. 1, continuing to stand until the second liquid-phase extraction agent completely extracts the second solvent from the second coating layer, completing the NIPS process, taking the extracted film out of the second solvent, and solidifying the second polymer to form a second porous polymer layer, thus obtaining the composite film as shown in (i) in fig. 1. Wherein the second liquid-phase extractant extracts the second solvent at a rate greater than the rate at which the gas-phase extractant extracts the second solvent.
In the step, the second porous polymer layer is formed by combining the VIPS process and the NIPS process, and the gas-phase induced phase separation has a slower separation speed, so that relatively dense pores can be formed by the gas-phase induced phase separation, and the dense pores are positioned right below the porous pattern mold, so that the damage of the gravity action of the porous pattern mold to the pores on the surface of the film can be avoided, and the local pores are enlarged or form cracks; the rest pores of the second porous polymer layer close to one side of the first porous polymer layer are continuously formed by a non-solvent induced phase separation method, the solvent replacement speed of the non-solvent induced phase separation method is high and sufficient, relatively large pores can be formed, and the pores of the first porous polymer layer are prevented from being blocked.
According to the invention, the porous pattern die is adopted and arranged above the second coating, and the solvent replacement on the surface of the second coating is completed through the pores of the porous pattern die, so that an undesirable dense layer formed when silicon rubber is adopted as the die in the prior art is avoided.
In a specific embodiment, the gas-phase extraction agent is room-temperature air with a relative humidity of 70% to 90%, water vapor in the air is used for extracting the second solvent, and the room-temperature air is also used for semi-curing the surface of the second coating, so that on one hand, separation of the porous pattern mold in the subsequent steps is facilitated, and on the other hand, the supporting strength of the surface of the second coating to the porous pattern mold is improved, and formation of large cracks or holes is avoided.
In one embodiment, the second solvent comprises one or more of N, N-dimethylacetamide, N-methylpyrrolidone, N-dimethylformamide, triethyl phosphate, and dimethylsulfoxide; the second liquid phase extractant is deionized water.
In a preferred embodiment, the extraction time of the second coating in the gas phase extracting agent is 2min to 10min; the extraction time of the second coating in the second liquid-phase extracting agent is 60-120 min, so that the second coating has high and stable gas flux and strong anti-fouling capability, and the surface does not have undesirable macroporous defects such as macropores or crack bands and the like.
Referring to fig. 2, in one embodiment, the cellular pattern mold comprises a plurality of first threads sequentially parallel and extending in a first direction and a plurality of second threads sequentially parallel and extending in a second direction, the first threads and the second threads being crossed at an angle to form a prismatic lattice, the first threads being in the form of waves, the peaks and valleys of the first threads being oriented perpendicular to the surface of the second cellular polymer layer, the first threads of the waves forming a pattern on the surface of the second cellular polymer layer when the cellular pattern mold is placed on the second coating layer, the pattern comprising a plurality of corrugations sequentially parallel and extending in the first direction, the peaks and valleys of the corrugations being oriented perpendicular to the surface of the second cellular polymer layer, as shown in fig. 3.
Referring to fig. 2, in a specific embodiment, the first and second filaments are perpendicularly crossed, the distance between adjacent first filaments is 3mm to 4mm, the distance between adjacent second filaments is 3mm to 4mm, the span between adjacent peaks of the first filaments is 3mm to 4mm, the diameter of the first filaments is 0.3mm to 0.8mm, the distance between the peaks and valleys of the first filaments is 1.2mm to 2.0mm, and the surface pattern formed after embossing can promote turbulence, enhance the hydrophobicity of the surface of the membrane, and improve the anti-fouling and anti-scaling capability of the membrane.
The surface pattern formed after imprinting undergoes a thickness reduction (vertical shrinkage) in a deionized water coagulation bath, enabling the porous pattern mold to be separated, leaving a patterned surface. The surface pattern that forms after the impression includes that many parallel arrangement's ripple is sunken in proper order, and the sunken crest of ripple and trough direction perpendicular to second porous polymer layer surface, the distance between the adjacent ripple is 3mm ~ 4mm, and the span between the sunken adjacent crest of ripple is 0.3mm ~ 0.8mm, and the sunken crest of ripple and the distance of trough are 1.2mm ~ 2.0mm.
In one embodiment, the first porous polymer layer has a thickness of 200 μm to 260 μm; the thickness of the second porous polymer layer is 200-260 mu m, the first porous polymer layer and the second porous polymer layer are both thin, when a non-solvent induced phase separation method is adopted to extract a solvent in a polymer, the extraction along the film thickness direction can be sufficient and uniform, the pore size in the film is uniform, the pore distribution is narrow, and the problem that the bottom layer extraction is insufficient and the pores are too small due to too thick film thickness is avoided.
The invention also discloses a composite membrane for membrane distillation prepared by the method.
The following are specific examples.
Example 1
1) Polyvinylidene fluoride (PVDF) high polymer is dissolved in N, N-dimethylacetamide (DMAc) solvent under the strong stirring of magnetons, the heating temperature is 65 ℃ until the polymer is completely dissolved, and a first polymer solution is obtained, wherein the mass percent of the polyvinylidene fluoride in the first polymer solution is 12%.
2) Pouring the first polymer solution prepared in the step 1) on a macroporous polypropylene (PP) non-woven fabric, completing the coating process by using a scraper film coating machine with the thickness of 230 mu m, and immediately immersing in a coagulation bath of deionized water to complete the NIPS process. The soaked membrane was left to stand for at least 45 minutes to form a cured membrane, and then the membrane was dried at room temperature for 48 hours to form a first porous polymer layer.
3) Polyvinylidene fluoride (PVDF) high polymer is dissolved in N, N-dimethylacetamide (DMAc) solvent under the strong stirring of magnetons, the heating temperature is 65 ℃ until the polymer is completely dissolved, and a first polymer solution is obtained, wherein the mass percent of the polyvinylidene fluoride in the first polymer solution is 10%.
4) The second polymer solution prepared in step 3) was poured onto the first porous polymer layer at the same thickness, immediately after which the diamond-shaped corrugated mat shown in fig. 2 was fixed on the cast second coating layer and placed together in an environment of 80% Relative Humidity (RH) at room temperature for 5 minutes to complete the VIPS process.
5) The membrane together with the gasket was immersed in a coagulation bath of deionized water and after 1 minute the gasket was removed. The solidification in the coagulating bath is carried out for 1 hour, and the double-layer corrugated flat plate composite PVDF film is obtained after the film is continuously dried for 48 hours at room temperature.
Comparative example 1
The single-layer polymer film obtained in step 2) of example 1 was taken as comparative example 1.
Comparative example 2
Comparative example 2 is different from example 1 only in that step 4) of example 1 is not included, i.e., only the NIPS process is used when forming the second porous polymer layer. Because the porous pattern mold is not semi-cured in humid air and is directly transferred into deionized water, the second coating has no supporting strength for the porous pattern mold, the porous pattern mold is embedded into the second coating, the surface of the porous pattern mold is excessively pressed by the mold, no pore exists right below the mold, a large crack zone or hole is formed beside the mold, the mold cannot be disassembled, and the surface of the porous pattern mold cannot form an expected regular pattern as shown in fig. 3.
Test example 1
In order to verify the anti-pollution and anti-scaling performance of the prepared composite membrane, seawater is selected as a material liquid to test the MD performance. The initial flux of the membrane of comparative example 1 was 18L/m during 100 hours of operation 2 H and the flux decreased throughout the experiment, reaching a final value of 6L/m after 100 hours 2 H, 67% reduction from its initial flux. However, the final flux of the bilayer membrane prepared in example 1 was 11.2L/m compared to the initial flux 2 The initial flux of h is reduced by only 10.7%. The degree of flux reduction can be used as a criterion for membrane fouling. The influence of the film thickness shows that although the double-layer film has lower initial flux, the flux is reduced by far less than that of a single-layer film, and the introduction of the ripple morphology is favorable for improving the anti-pollution performance of the PVDF film。
Test example 2
In order to further evaluate the anti-fouling performance of the membrane material, the present invention performs scanning electron microscope image acquisition on the surface of the membrane material before and after MD operation, as shown in fig. 4. It can be clearly seen that at the end of the MD operation, there was a significant deposition of salt on the surface of the monolayer film of comparative example 1, almost completely blocking the pores of the film (as shown in the f-plot in fig. 4), which is also responsible for a large reduction in its flux; on the other hand, on the bilayer membrane surface of example 1, showing only relatively little salt crystal deposition, either in the gasket-free coverage area or in the gasket indentation area (as shown in fig. 4, panels d and e), it is still clear that most of the pores are open, which also ensures a relatively constant state of MD flux. The photographs of the membrane surfaces after MD operation also revealed a large difference in fouling propensity between the two, with the monolayer membrane of comparative example 1 having a surface that was highly covered with fouling material, with visible large salt crystals attached, while the corrugated bilayer membrane of example 1 had a relatively clean membrane surface, with much less fouling, and no significant fouling phenomena.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the claims. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent should be subject to the appended claims.

Claims (10)

1. A method for preparing a composite membrane for membrane distillation, characterized by comprising the following steps:
providing a first porous polymer layer for membrane separation, the first porous polymer layer comprising a first polymer;
dissolving a second polymer in a second solvent to obtain a second polymer solution;
forming the second polymer solution on the first porous polymer layer to obtain a second coating layer, arranging a porous pattern mold on the second coating layer, and placing the second coating layer and the porous pattern mold in a gas-phase extracting agent, wherein the gas-phase extracting agent extracts the second solvent on the surface of the second coating layer from the second coating layer; continuing to place the second coating layer together with the porous pattern mold in a second liquid-phase extractant, standing, removing the porous pattern mold, continuing to stand until the second liquid-phase extractant completely extracts the second solvent from the second coating layer, the second liquid-phase extractant extracting the second solvent at a rate higher than the gas-phase extractant extracting the second solvent, and solidifying the second polymer to form a second porous polymer layer, thereby obtaining the composite membrane;
the gas-phase extracting agent is room-temperature air with the relative humidity of 70-90%;
the pores of the porous pattern mold are used for completing solvent replacement of the surface of the second porous polymer layer;
the second porous polymer layer is used to form a surface pattern and enhance hydrophobicity.
2. The method for preparing a composite membrane for membrane distillation according to claim 1,
the second solvent comprises one or more than two of N, N-dimethylacetamide, N-methylpyrrolidone, N-dimethylformamide, triethyl phosphate and dimethyl sulfoxide;
the second liquid-phase extractant is deionized water.
3. The method for preparing a composite membrane for membrane distillation according to claim 2,
the extraction time of the second coating in the gas-phase extracting agent is from 2min to 10min;
the extraction time of the second coating in the second liquid-phase extracting agent is 60min to 120min;
in the second polymer solution, the mass percent of the second polymer is 8% -12%;
the second polymer is one or more selected from polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, polyetheretherketone and polyamide.
4. The method for preparing the composite membrane for membrane distillation according to any one of claims 1 to 3, wherein the method for preparing the first porous polymer layer comprises the following steps of:
dissolving the first polymer in a first solvent to obtain a first polymer solution;
forming the first polymer solution on a fiber support layer to obtain a first coating layer, placing the first coating layer in a first liquid phase extracting agent, extracting the first solvent from the first coating layer by the first liquid phase extracting agent, and solidifying the first polymer to form the first porous polymer layer.
5. The method for preparing a composite membrane for membrane distillation according to claim 4,
the first solvent comprises one or more than two of N, N-dimethylacetamide, N-methylpyrrolidone, N-dimethylformamide, triethyl phosphate and dimethyl sulfoxide;
the first liquid-phase extracting agent is deionized water;
in the first polymer solution, the mass percent of the first polymer is 10% -14%.
6. The method for preparing a composite membrane for membrane distillation according to claim 5,
the mass percentage of the first polymer in the first polymer solution is greater than the mass percentage of the second polymer in the second polymer solution.
7. The preparation method of the composite membrane for membrane distillation as claimed in claim 5, wherein the extraction time of the first coating in the first liquid phase extractant is 45min to 120min;
the first polymer is one or more selected from polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, polyether ether ketone and polyamide.
8. The preparation method of the composite membrane for membrane distillation according to claim 1, wherein the thickness of the first porous polymer layer is 200 μm to 260 μm;
the thickness of the second porous polymer layer is 200-260 mu m.
9. The method for preparing a composite membrane for membrane distillation according to claim 1, wherein the porous pattern mold comprises a plurality of first filaments arranged in parallel and extending in a first direction and a plurality of second filaments arranged in parallel and extending in a second direction, the first filaments and the second filaments are crossed to form a prismatic grid, the first filaments are in a wave shape, the directions of peaks and valleys of the first filaments are perpendicular to the surface of the second porous polymer layer, and the first filaments are used for forming a pattern on the surface of the second porous polymer layer.
10. A composite membrane for membrane distillation prepared by the preparation method of any one of claims 1 to 9.
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