A method for preparing a sandwich structure multilayer composite membrane for fluid separation

Abstract

The present invention relates to a method for preparing a sandwich structure composite membrane for fluid separation. The composite membrane structure is composed of a top microporous membrane protective layer, an intermediate dense ultra-thin active layer and a bottom porous support layer. The top microporous membrane protective layer plays a role in isolating the active layer from direct contact with the outside world, specifically protecting the intermediate dense ultra-thin active layer from the scouring shear force of the separation system, avoiding direct damage from external mechanical impact, not being decomposed by the complex components of the separation liquid, and fixing and limiting the swelling of the active layer; the intermediate dense ultra-thin active layer plays a role in bonding the top layer and the bottom layer and fluid separation; the bottom porous support layer itself has strong mechanical properties, which can enhance the ability of the entire composite membrane to resist external force damage and ensure service time. The preparation steps of the composite membrane are: preparing a certain concentration of the intermediate active layer membrane solution, applying the membrane solution to the bottom porous support layer occupied by the solvent, and when the membrane solution is in a gel state and has strong adhesion, covering the top microporous membrane protective layer whose pores have been partially occupied, pressing and drying, and finally washing the membrane with clean water to remove the occupying solvent, and obtaining the composite membrane after drying. The preparation process does not require strict conditions and is simple and easy to implement industrial production.

Description

Technical Field

[0001] The invention relates to a sandwich structure multilayer composite film with separation function, resistance to fluid scouring shear force, resistance to swelling and strong mechanical properties and a preparation method thereof. Background Art

[0002] Membrane technology is an emerging high-tech technology. Compared with traditional separation technologies such as distillation, adsorption and extraction, it has the advantages of low energy consumption, simple equipment, room temperature operation, strong adaptability and high reliability. It has been widely used in fluid separation, such as gas separation, reverse osmosis and pervaporation [1]. Common polymer materials for fluid separation membranes include polydimethylsiloxane (PDMS), polyamide (PA), polyvinyl alcohol (PVA), polyether block amide (PEBA) and polytrimethylsilyl propyne (PTMSP). When these materials are made into fluid separation membranes alone, there are problems such as insufficient strength if the thickness is too thin and poor separation performance if the thickness is too thick. Therefore, polyamide (PA), polypropylene (PP), polysulfone (PSF), polyacrylonitrile (PAN), polyetherimide (PEI), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) or other polymer materials are usually used as support layers to form double-layer or three-layer composite membranes with high strength. In order to ensure that the separation membrane has high separation performance, the thickness of the active layer is generally controlled at the nanometer to micrometer level.

[0003] In general two-layer or three-layer composite membrane structures, the ultra-thin active separation layer is at the top layer and directly contacts the separation liquid. However, due to its extreme fragility, when it is used for fluid separation, especially when separating liquid containing complex components such as solid particles, it is usually threatened by two aspects: first, the shear force on the membrane surface caused by fluid scouring or the external mechanical force during installation and use. During the separation process, a higher liquid flow rate is used to reduce the boundary layer on the membrane surface and enhance mass transfer. When the high-flow liquid flows through the membrane surface, it will produce a strong shear effect on it, causing membrane surface wear. In practical applications, during the transportation or installation of the membrane, when the flexible and fragile membrane surface encounters the mechanical force of extremely rigid membrane components, screws, etc., it is extremely easy to be scratched or directly punctured, thereby destroying the entire membrane; the second problem is that when the organic component or acid content in the separation liquid is high, the separation layer formed by the polymer material will swell and the structure will be destroyed, resulting in a decrease in membrane separation performance. In addition, when membrane separation technology is used to treat fermentation broth, the microbial cells therein may decompose the separation layer material, causing the separation layer to be partially or overall thinned. The shear force of the fluid on the membrane surface and the decomposition effect of the microorganisms may promote each other, further reducing the membrane life.

[0004] There is currently a gap in the research field regarding the shearing effect of fluids on the membrane surface and the mechanical force during use. Regarding the membrane swelling problem, current research focuses on post-treatment after membrane formation, adding inorganic ions, or increasing the cross-linking degree of the separation membrane. However, these methods still have shortcomings such as long heat treatment time, high treatment temperature, poor particle dispersion, complex membrane manufacturing process, and reduced separation performance [2-6]. Summary of the invention

[0005] In order to improve the durability of the membrane and increase the service life of the membrane, the present invention proposes a multi-layer composite membrane with a sandwich structure. In this structure, a dense active layer that plays a separation role serves as an intermediate layer, a top microporous membrane separates the fluid from the active layer, and a bottom non-woven fabric supports the entire composite membrane while protecting the other surface of the active layer. The composite membrane under this structure has high mechanical properties and separation performance.

[0006] In the present structure, the top microporous membrane protective layer serves to isolate the middle dense ultra-thin active layer from direct contact with the outside world, specifically to protect the middle dense ultra-thin active layer from the scouring shear force of the separation system, avoid direct damage from external mechanical impact, and prevent decomposition by the complex components of the separation liquid, as well as to fix and limit the swelling of the active layer; the middle dense ultra-thin active layer serves to bond the top layer and the bottom layer and to separate the fluid; the bottom porous support layer is used to support the top structure and the active layer and enhance the mechanical properties of the composite membrane.

[0007] The preparation of the sandwich composite membrane includes the following steps: first, coating the first solvent on the substrate; laying the top protective layer microporous membrane flat on the first solvent so that the pores of the microporous membrane are partially occupied by the first solvent; dissolving another polymer and the corresponding cross-linking agent, additives and compounding agents in the second solvent to prepare a membrane-making solution for the active layer; using a third solvent to occupy the pores of the bottom porous support layer; pouring the membrane-making solution of the active layer on the bottom support layer occupied by the third solvent; when the active layer solution is in a gel state, flipping the top protective layer microporous membrane occupied by the first solvent and covering it on the bottom support layer, applying a pressing force; then performing a heat treatment to cross-link and solidify the polymer membrane-making solution to form an active layer; after the active layer is solidified into a membrane and stabilized, placing the formed membrane under high temperature and vacuum to dry; the dried composite membrane is placed in pure water for a period of time to remove the occupying solvent (the first solvent) in the pores of the protective layer microporous membrane, and then continuing to dry to finally obtain a dry sandwich structure multilayer composite membrane.

[0008] In the preparation process of sandwich composite membranes, when the top microporous membrane is occupied, the entire pore is not filled with solvent, and a certain amount of space must be left. The active layer membrane solution is a dilute solution of the polymer used, and its concentration is between 2% and 20% in terms of its mass ratio to the solvent. The pores of the bottom support layer can be occupied in such a way that the support layer floats on the surface of the third solvent or covers the substrate coated with the third solvent. The occupation time is based on the micropores of the support layer being partially occupied rather than fully occupied.

[0009] The solvent for the polymer microporous membrane occupying the top protective layer (the first solvent) can be glycerol or other solvents with a volatile vacuum degree higher than 90 kPa. The solvent for dissolving another polymer and the corresponding cross-linking agent, additives and compounding agents (the second solvent) can be water, benzene, n-heptane or other solvents that can evaporate cleanly at 60-100°C. The solvent for occupying the holes in the bottom porous supporting layer (the third solvent) can be benzene, water or other solvents that can evaporate cleanly at 60-100°C. The first solvent is immiscible with the second solvent, and the third solvent is immiscible with the second solvent. The relative volatility of the three solvents should be the first solvent> the third solvent> the second solvent.

[0010] The microporous membrane of the top protective layer is polypropylene (PP), polysulfone (PSF), polyetherimide (PEI), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) or other polymer materials, and the micropore size of the microporous membrane is between 0.05 and 10 microns, and the thickness is between 20 and 1000 microns; the polymer used in the polymer membrane solution of the middle dense ultra-thin active layer is polydimethylsiloxane (PDMS), polyamide (PA), polyvinyl alcohol (PVA), polyether block amide (PEBA), polytrimethylsilyl propyne (PTMSP) or other polymer materials, and the thickness is between several nanometers and several microns; the material of the bottom porous supporting layer is polypropylene (PP), polyterephthalic acid (PET), polyacrylonitrile (PAN), polysulfone (PSF), polyamide (PA) or other microporous polymer materials with high mechanical properties and can be bonded, and the pore size is between 0.01 and 1000 microns.

[0011] The sandwich structure multilayer composite membrane can be made into various types of membrane components, including but not limited to plate-and-frame type, roll type, folding type, disc type, etc.

[0012] The composite membrane can be applied to gas separation, pervaporation or reverse osmosis, and the separation system can be a gas mixture, a liquid homogeneous mixture, a liquid heterogeneous mixture or a gas-liquid two-phase mixture.

[0013] Compared with the existing technical solutions, this study has the following advantages.

[0014] (1) Long service life: In the sandwich structure composite membrane, the separation active layer is located in the middle. The microporous membrane on the top separates the active layer from the separation fluid, effectively avoiding the shear force of the fluid flow on the active layer, and reducing the swelling and decomposition of the complex components in the separation fluid on the active layer; at the same time, the protective layer on the top prevents the active layer from being directly subjected to external mechanical forces during packaging, transportation, installation and disassembly and reassembly, thereby increasing the service life of the membrane.

[0015] (2) The preparation method is simple and easy to realize industrial production: The film-making process has no strict requirements on temperature and humidity, and there is no complicated reaction process. The film-making process mainly includes position taking, film liquid coating and support layer adhesion. The position taking can be completed by floating on the surface of a flat plate or solvent. Film liquid coating technologies such as scraping and spin coating are mature and have been applied in industry. The adhesion of the support layer is mainly achieved by pressure, which can be squeezed by gravity pressure, roller pressure and other forces. The technology is mature and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the production process of the PP / PDMS / PA composite film in Example 1 of the present invention.

[0017] Figure 2 This is the SEM cross-sectional characterization of the PP / PDMS / PA composite film prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0018] The present invention is described in detail below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments. Without departing from the content and scope of the present invention, changes in implementation should be included in the technical scope of the present invention.

[0019] Example 1: Separation of 5% ethanol aqueous solution by pervaporation using PP / PDMS / PA flat composite membrane with a membrane solution concentration of 2%

[0020] In this embodiment, a polyamide (PA) porous membrane with a thickness of 100 μm and a thickness of 0.22 μm is selected as the bottom support layer, polydimethylsiloxane (PDMS) is the active layer material, and a microporous polypropylene (PP) membrane with a pore size of 50 nm and a thickness of 29 μm is used as the top protective layer. The placeholder solvent for the porous membrane of the top protective layer is glycerol, the placeholder solvent for the porous membrane of the bottom support layer is water, the solvent for dissolving PDMS is n-heptane, and the concentration of the PDMS active layer solution is 2%. In order to test the separation performance of the composite membrane, an ethanol aqueous solution is selected as the separation system, and the pervaporation method is used to separate ethanol. The specific implementation steps are as follows:

[0021] The solvent glycerol was coated on the glass plate of the scraper, and a PP film with a length of 24 cm and a width of 20 cm was cut and laid flat on the glycerol; PDMS, cross-linking agent ethyl orthosilicate and catalyst dibutyltin dilaurate were dissolved in n-heptane to prepare a film-making solution for the active layer; the PA membrane was floated on the water surface for hole occupation; the PDMS film-making solution was cast on the PA membrane; after the film-making solution was in a gel state, it was taken out and placed on a flat plate; the top layer of PP porous membrane after the hole occupation was completed was turned over and buckled on the PDMS in a gel state; a glass rod was used to remove bubbles and a pressing force was applied to the PP / PDMS / PA membrane; it was placed at room temperature for 4 hours, and then placed in a 60 ℃ drying oven overnight, and washed and dried with clean water to obtain a PP / PDMS / PA composite membrane; it was used to separate a 5% ethanol aqueous solution, and the flux at 40 ℃ was 956 gm -2 h -1 , the separation factor is 8.4.

[0022] Example 2: Separation of 5% ethanol aqueous solution by pervaporation using PP / PDMS / PP flat composite membrane with a membrane solution concentration of 10%

[0023] In this embodiment, polypropylene (PP) non-woven fabric is selected as the bottom support layer, polydimethylsiloxane (PDMS) is the active layer material, and a microporous polypropylene (PP) membrane with a pore size of 50 nm and a thickness of 29 μm is used as the top protective layer. The placeholder solvent for the top porous membrane is glycerol, and the PDMS concentration of the active layer solution is 10%. In order to test the separation performance of the composite membrane, an ethanol aqueous solution is selected as the separation system, and the pervaporation method is used to separate ethanol. The specific implementation steps are as follows:

[0024] The solvent glycerol was coated on the glass plate of the scraper, and a polypropylene porous membrane with a length of 24 cm and a width of 20 cm was cut and laid flat on the glycerol for 5-10 minutes. PDMS, tetraethyl orthosilicate and dibutyltin dilaurate were dissolved in the solvent n-heptane to prepare a membrane solution for the PDMS active skin layer, and the concentration of PDMS was 10% (W / W). The polyPDMS membrane solution was coated on the polypropylene porous membrane with partially occupied micropores using a scraper. After 5-10 minutes, when the PDMS membrane liquid on the surface of the polypropylene membrane was in a semi-gel state, a polypropylene non-woven fabric with a length of 24 cm and a width of 20 cm was cut and covered on it. A flat plate of a certain weight was selected as a pressure source and placed on top of the non-woven fabric. After drying at room temperature for 2 hours, it was sent to a 60 ℃ vacuum drying oven for heat treatment for 12 hours. The dried film was soaked in pure water for 2 hours to remove glycerol and other uncross-linked substances, and continued to dry at 60 ℃ for 2 hours to obtain a sandwich structure PP-PDMS-PP composite membrane. The performance of the composite membrane was evaluated by pervaporation with a 5% ethanol aqueous solution. At 38 °C, the membrane permeation flux was 900 gm -2 h -1 , the separation factor is 9.4.

[0025] Example 3: Separation of 5% ethanol aqueous solution by pervaporation using PP / PDMS / PA flat composite membrane with a membrane solution concentration of 20%

[0026] In this embodiment, a polyamide (PA) porous membrane with a thickness of 100 μm and a thickness of 0.22 μm is selected as the bottom support layer, polydimethylsiloxane (PDMS) is the active layer material, and a microporous polypropylene (PP) membrane with a pore size of 50 nm and a thickness of 29 μm is used as the top protective layer. The placeholder solvent for the top protective layer porous membrane is glycerol, the placeholder solvent for the bottom support layer porous membrane is water, the solvent for dissolving PDMS is n-heptane, and the concentration of the PDMS active layer solution is 20%. In order to test the separation performance of the composite membrane, an ethanol aqueous solution is selected as the separation system, and the pervaporation method is used to separate ethanol. The specific implementation steps are as follows:

[0027] The solvent glycerol was coated on the glass plate of the scraper, and a PP film with a length of 24 cm and a width of 20 cm was cut and laid flat on the glycerol; PDMS, tetraethyl orthosilicate and dibutyltin dilaurate were dissolved in n-heptane to prepare a film-making solution for the active layer; the PA membrane was floated on the water surface for hole occupation; the PDMS film-making solution was cast on the PA membrane; after the film-making solution was in a gel state, it was taken out and placed on a flat plate; the top layer of PP porous membrane after the hole occupation was completed was turned over and buckled on the PDMS in a gel state; a glass rod was used to remove bubbles and a pressing force was applied to the PP / PDMS / PA membrane; it was placed at room temperature for 4 hours, and then placed in a 60 ℃ drying oven overnight, and washed and dried with clean water to obtain a PP / PDMS / PA composite membrane; it was used to separate a 5% ethanol aqueous solution, and the flux at 40 ℃ was 215 gm -2 h -1 , the separation factor is 6.9.

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[0030] [3] Cesaria M, Arima V, Manera MG, Rella R. Protocol of thermal agingagainst the swelling of poly(dimethylsiloxane) and physical insight inswelling regimes. Polymer. 2018;139:145-54.

[0031] [4] Zhou Z, Wang Y, Lin J, Zhang Y, Qu L, Wu W, et al. In-situmolecular-level hybridization enabling high-sulfonation-degree sulfonatedpoly(ether ether ketone) membrane with excellent anti-swelling ability andproton conduction. Int J Hydrogen Energ. 2021;46(61):31312-23.

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Claims

1. A method for preparing a sandwich structure multilayer composite membrane for fluid separation, It is characterized in that The method comprises the following steps: firstly, coating a solvent (4) on a substrate; laying a top microporous membrane protective layer (1) flat on the solvent (4) so ​​that the pores of the top microporous membrane protective layer (1) are partially occupied by the solvent (4); dissolving another polymer and corresponding crosslinking agent, additive and compounding agent in solvent (5) to prepare a membrane-making solution (6) of an intermediate dense ultra-thin active layer (2); occupying the pores of a bottom porous support layer (3) with solvent (3); pouring the membrane-making solution (6) of the intermediate dense ultra-thin active layer (2) on the bottom porous support layer (3) occupied by solvent (3) ); when the membrane-forming solution (6) is in a gel state, the top microporous membrane protective layer (1) occupied by the solvent (4) is turned over and covered on the bottom porous support layer (3), and a pressing force is applied; then a heat treatment is performed to make the membrane-forming solution cross-linked and solidified to form an intermediate dense ultra-thin active layer (2); after the intermediate dense ultra-thin active layer (2) is solidified into a membrane and stabilized, the formed membrane is placed at high temperature and vacuum for drying; the dried composite membrane is placed in pure water for a certain period of time to remove the occupying solvent (4) in the micropores of the top microporous membrane protective layer (1), and then the drying is continued to obtain a dry sandwich structure multi-layer composite membrane.

2. The method for preparing the sandwich structure multilayer composite membrane for fluid separation according to claim 1, It is characterized in that The material of the top microporous membrane protective layer (1) is polypropylene (PP), polysulfone (PSF), polyetherimide (PEI), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) or other hydrophobic polymer materials, and the micropore size of the microporous membrane is between 0.05 and 10 microns, and the thickness is between 20 and 1000 microns; the polymer used in the polymer membrane solution (6) of the middle dense ultra-thin active layer (2) is polydimethylsiloxane (PDMS), polyamide (PA), polyvinyl alcohol (PVA), polyether block amide (PEBA), polytrimethylsilyl propyne (PTMSP) or other polymer materials, and the thickness is between several nanometers and several microns; the material of the bottom porous support layer (3) is polypropylene (PP), polyterephthalic acid (PET), polyacrylonitrile (PAN), polysulfone (PSF), polyamide (PA) or other microporous polymer materials with high mechanical properties and can be bonded, and the pore size is between 0.01 and 1000 microns.

3. The method for preparing the sandwich structure multilayer composite membrane for fluid separation according to claim 1, It is characterized in that The amount of solvent (4) coated on the substrate and the pore occupancy time of the top microporous membrane protective layer (1) are based on the partial occupation of the micropore volume of the top microporous membrane protective layer (1).

4. The method for preparing the sandwich structure multilayer composite membrane for fluid separation according to claim 1, It is characterized in that The active layer membrane-forming solution (6) is a dilute solution of the polymer used, and its concentration is between 2% and 20% based on the mass ratio of the polymer to the solvent.

5. The method for preparing the sandwich structure multilayer composite membrane for fluid separation according to claim 1, It is characterized in that The hole occupation mode of the bottom porous support layer (3) is that the bottom porous support layer (3) floats on the liquid surface of the solvent three (7) or covers the substrate coated with the solvent three (7), and the occupation time is based on the pores in the support layer being partially occupied rather than fully occupied.

6. The method for preparing the sandwich structure multilayer composite membrane for fluid separation according to claim 1, It is characterized in that The solvent one (4) used to occupy the top microporous membrane protective layer (1) is glycerol or other solvents with a volatile vacuum degree higher than 90 kPa. The solvent two (5) used to dissolve another polymer and the corresponding cross-linking agent, additives and compounding agents is water, benzene, n-heptane or other solvents that can evaporate cleanly at 60-100°C. The solvent three (7) used to occupy the holes of the bottom porous support layer (3) is benzene, water or other solvents that can evaporate cleanly at 60-100°C. The solvent one (4) and the solvent two (5) are not miscible, and the solvent three (7) and the solvent two (5) are not miscible. The relative volatility of the three solvents should be solvent one (4)> solvent three (7)> ​​solvent two (5).

7. A sandwich structure multilayer composite membrane for fluid separation, wherein the multilayer composite membrane is obtained according to the preparation method of a sandwich structure multilayer composite membrane for fluid separation according to any one of claims 1 to 6, and the multilayer composite membrane is composed of a top microporous membrane protective layer (1), a middle dense ultra-thin active layer (2) and a bottom porous support layer (3).

8. The sandwich structure multilayer composite membrane for fluid separation according to claim 7, It is characterized in that The prepared composite membrane is used to make various types of membrane components, including plate-and-frame type, roll type, folding type or disc type.

9. The sandwich structure multilayer composite membrane for fluid separation according to claim 7, It is characterized in that The composite membrane is applied to gas separation, pervaporation or reverse osmosis, and the separation system is a gas mixture, a liquid homogeneous mixture, a liquid heterogeneous mixture or a gas-liquid two-phase mixture.

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