Composite polyimide film as well as preparation method and application thereof in helium separation

Abstract

The invention discloses a composite polyimide film for separating and purifying helium. The composite polyimide film is obtained by wrapping a layer of polyimide film on the periphery of a carbon molecular sieve film. Compared with a polyimide film and a carbon molecular sieve film, the composite polyimide film has the advantages that the selectivity of helium to nitrogen and / or methane is remarkably improved, the permeability is high, the composite polyimide film is used for separating and purifying helium and nitrogen and / or helium in methane, helium can be effectively and quickly separated out, and meanwhile, the composite polyimide film has better reusability. The invention discloses application of the composite polyimide film to separation and purification of helium from helium and mixed gas of at least one of nitrogen and methane.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas separation membranes, and relates to a composite polyimide membrane and a preparation method thereof and an application in helium separation, and specifically relates to a composite polyimide membrane for selectively separating helium (He) from a mixed gas of helium (He) and at least one gas selected from nitrogen (N2) and methane (CH4), and specifically relates to a staggered pore size composite membrane prepared by wrapping a carbon molecular sieve membrane with a polyimide membrane solution, wherein the composite membrane can effectively separate and purify helium. Background Art

[0002] He is an important resource. It plays an irreplaceable and important role in the fields of national defense, biomedicine, nuclear facilities, electrical industry, semiconductor manufacturing and cryogenic industry. He is mainly distributed in the mantle, rocks, air and natural gas. He in the air is difficult to be utilized as a resource due to its low concentration. Therefore, extracting He from natural gas, especially from non-condensable tail gas during the natural gas liquefaction process, is the only way to utilize He as a resource.

[0003] At present, the main technologies for extracting helium from natural gas include cryogenic method, pressure swing adsorption method, absorption method and membrane osmosis separation method. The advantages and disadvantages of natural gas helium extraction technology are shown in Table 1.

[0004] Table 1. Advantages and disadvantages of natural gas helium extraction technology

[0005]

[0006] Membrane permeation separation can economically and efficiently extract and prepare high-purity helium from natural gas. It is generally operated at room temperature and does not undergo phase change. It is considered to be a relatively efficient and energy-saving helium purification technology. The membrane permeation separation method is based on the different dissolution and diffusion rates of each gas component in the membrane material. Under the action of the partial pressure difference on both sides of the membrane, different gases have different permeation rates through the membrane wall, thereby achieving separation. Compared with other processes, gas membrane permeation separation technology has the advantages of simple process, easy operation and maintenance, low investment cost and operating cost, no use of chemicals and no wastewater discharge.

[0007] In recent years, due to the rapid development of materials science, He separation membranes can be divided into inorganic membranes, polymer membranes and modified polymer membranes according to the different materials of separation membranes. The above-mentioned He separation membranes are widely used in the fields of hydrogen separation, He purification and natural gas purification. In the natural gas He extraction industry, most of them are concentrated on polysulfone and polyimide membrane methods for natural gas helium extraction. However, the polyimide membrane (PI) method for natural gas helium extraction has a long chain relaxation time and poor mobility. The channels for molecules to pass through are tortuous, and gas molecules pass through the gaps inside, so small molecules are easy to pass through, with high selectivity, but low permeability. Therefore, in the case of high selectivity, in order to increase the permeation rate, the polyimide membrane must be modified.

[0008] After BOG, dehydrogenation and primary separation, LNG gas with a content of about 30% helium, about 65% nitrogen and a small amount of methane can be separated by secondary membrane separation technology to obtain high-purity helium. Many well-known universities and research institutions have conducted in-depth research on the construction of molecular sieve membranes and prepared a series of molecular sieve membrane materials with high permeability and high selectivity. The industrial membranes of the United States Prism membrane and Japan Ube Industries both use Sen membranes, and their He / CH4 selectivity coefficients can reach about 120, and the He / N2 selectivity coefficients can reach about 50. Hossain et al. cross-linked PEG with different chain lengths and ionic groups with biimidazole with polyimide to prepare membrane materials for gas separation. Liu Bing of Harbin Institute of Technology used modified PI / UiO-66-PEI and PI / UiO-66-PEI-pSBMA to separate gases and achieved good results. Although the selectivity of polyimide membranes for separating gases is high, the permeability of the material needs to be further improved. There is still a lack of corresponding high-performance polyimide membrane materials in China, and there are currently no commercialized polyimide-based helium extraction membrane components. Summary of the invention

[0009] The purpose of the present invention is to address the problems of the prior art and provide a composite polyimide membrane. By wrapping a layer of polyimide membrane on a carbon molecular sieve membrane, the pore size of the carbon molecular sieve membrane and the pore size of the polyimide membrane are staggered, the pore size is reduced, and the selectivity of helium to nitrogen and / or methane is increased, thereby improving the overall screening capacity of the membrane and achieving effective separation and purification of helium (He).

[0010] To achieve the above object, the technical solution adopted by the present invention is:

[0011] The invention discloses a composite polyimide membrane for separating and purifying helium (He), which is a composite polyimide membrane obtained by wrapping a layer of polyimide membrane around a carbon molecular sieve membrane.

[0012] Preferably, the composite polyimide membrane is obtained by immersing a carbon molecular sieve membrane in a polyimide membrane solution, adsorbing the carbon molecular sieve surface to wrap a layer of polyimide membrane, and then drying the composite polyimide membrane. First, prepare a carbon molecular sieve membrane.

[0013] Preferably, the polyimide film solution is an acetone solution of polyimide.

[0014] Preferably, the polyimide is prepared using 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA), 4,4'-(hexafluoroisopropylene) diphthalide (HPDD), 3-methylpyridine (3-MPD) and acetic anhydride as raw materials.

[0015] Preferably, the carbon molecular sieve membrane is prepared by using 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA), 4,4'-(hexafluoroisopropylene) diphthalic acid glycoside (HPDD), 3-methylpyridine (3-MPD) and acetic anhydride as raw materials to prepare polyimide, and then using acetone as solvent to prepare polyimide membrane solution, and using casting method to prepare polyimide membrane, and then calcining the polyimide membrane under Ar or N2 atmosphere.

[0016] Another object of the present invention is to provide a method for preparing the composite polyimide membrane for separating and purifying He, comprising the following steps:

[0017] Step (1), under N2 protection, using N-methylpyrrolidone (NMPD) as a solvent, preparing a 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) solution, considering that if the polymerization temperature is too high, it will lead to a violent reaction or even explosion, so at a temperature of -20 to -5°C, 4,4'-(hexafluoroisopropylene) diphthaloside (HPDD) is added to the 2-(4-aminophenyl)-5-aminobenzimidazole solution, and stirred at room temperature for 12 to 24 hours; then 3-methylpyridine (3-MPD) and acetic anhydride are added, and stirring is continued at room temperature for 12 to 24 hours, and the reaction solution is mixed with methanol for precipitation, and the precipitate is washed with methanol and vacuum dried to obtain a polyimide;

[0018] Step (2), taking polyimide, using acetone as solvent, preparing a polyimide film solution with a concentration of about 5 to 20 wt.%, preparing a film by a casting method, soaking the film in methanol to replace the residual solvent in the film, and then vacuum drying to obtain a polyimide film;

[0019] Step (3), placing the polyimide membrane in a tube furnace, and calcining it at 500-600° C. for 1-3 hours in an Ar and / or N2 atmosphere to obtain a carbon molecular sieve membrane;

[0020] Step (4), taking polyimide, using acetone as solvent, preparing a polyimide membrane solution with a concentration of about 5 to 20 wt.%; soaking the carbon molecular sieve membrane in the polyimide membrane solution for 3 to 10 minutes, taking it out, drying it, and vacuum drying it at about 60 to 100° C. for 12 to 24 hours to obtain a composite polyimide membrane.

[0021] In step (1), the dosage ratio of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) and N-methylpyrrolidone (NMPD) is 0.03:60 to 0.03:80 mol / mL.

[0022] The molar ratio of the APBI to 4,4'-(hexafluoroisopropylene) diphthaloside (HPDD) is 1:1; the molar ratio of the 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) to 3-methylpyridine (3-MPD) is 1:1; and the molar ratio of the 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) to acetic anhydride is 1:10 to 1:12.

[0023] The vacuum drying temperature is 100-130° C., and the vacuum drying time is 12 hours.

[0024] The vacuum drying equipment is a vacuum drying box.

[0025] In step (2), preferably, the concentration of the polyimide membrane solution is 8 to 12 wt.%.

[0026] More preferably, the concentration of the polyimide membrane solution is 10 wt.%.

[0027] The casting method is as follows: slowly pour the polyimide film solution onto a clean glass plate preheated to 60° C., and use a scraper to scrape out a film at a uniform speed.

[0028] The vacuum drying temperature is about 60 to 80° C., preferably 70 to 80° C., and the vacuum drying time is 10 to 18 hours.

[0029] The vacuum drying is carried out in a vacuum drying oven.

[0030] In step (3), in order to ensure that the calcination process is not oxidized, the calcination atmosphere is nitrogen (N2), argon (Ar), or a mixed gas of nitrogen and argon.

[0031] Preferably, the calcination temperature is 500-580°C.

[0032] Preferably, the polyimide membrane is placed in a tubular furnace, heated from room temperature to 500°C to 580°C at a rate of 5°C / min in an Ar and / or N2 atmosphere, and calcined at 500°C to 580°C for 1 to 3 hours to obtain a carbon molecular sieve membrane.

[0033] Specifically, the polyimide membrane is placed in a tubular furnace, and in an atmosphere of Ar, N2 or a mixed gas with a volume ratio of N2 and Ar of 70 / 30, the temperature is increased from room temperature to 500°C-580°C at a rate of 5°C / min, and calcined at a temperature of 500°C-580°C for 1-3h to obtain a carbon molecular sieve membrane.

[0034] In step (4), preferably, the carbon molecular sieve membrane is immersed in a 8-12 wt. % polyimide membrane solution.

[0035] More preferably, the carbon molecular sieve membrane is immersed in a 10 wt. % polyimide membrane solution.

[0036] Preferably, the soaking time is 5 minutes.

[0037] The concentration of the polyimide membrane solution for soaking the carbon molecular sieve membrane may be the same as or different from the concentration of the polyimide membrane solution for preparing the polyimide membrane.

[0038] Preferably, the vacuum drying temperature is 80° C. and the vacuum drying time is 15 to 18 hours. In order to prevent the membrane material from being oxidized during drying, the drying equipment is preferably a vacuum drying oven.

[0039] Another object of the present invention is to provide the use of the composite polyimide membrane in separating and purifying helium.

[0040] Preferably, the application is the use of the composite polyimide membrane to separate and purify helium (He) from a mixed gas of helium (He) and at least one gas selected from nitrogen (N2) and methane (CH4).

[0041] A method for selectively separating and purifying helium comprises: using the composite polyimide membrane to perform membrane separation on a mixed gas of helium (He) and at least one selected from nitrogen (N2) and methane (CH4), thereby separating and purifying the helium; wherein the feed pressure of the membrane separation is 0.1-0.5 MPa, preferably 0.2 MPa, and the temperature of the membrane separation is 10-50°C, preferably 20-30°C.

[0042] When helium (He) is mixed with nitrogen (N2) and methane (CH4), it is processed natural gas, in which the volume fraction of helium is 25-32%, the volume fraction of nitrogen is 60-65%, and the volume fraction of methane is 8-10%.

[0043] The beneficial effects of the present invention are:

[0044] The preparation method of the composite polyimide film of the invention is simple and the raw materials are easily available.

[0045] The composite polyimide membrane of the present invention combines the high permeability of the carbon molecular sieve membrane and the high selectivity of the polyimide membrane. Compared with the polyimide membrane and the carbon molecular sieve membrane, the composite polyimide membrane of the present invention has a higher selective permeability to helium, significantly increases the selectivity of helium to nitrogen and / or methane, has a high permeability, is used for the separation and purification of helium from nitrogen and / or methane, can effectively and quickly separate helium, and has good reusability.

[0046] The composite polyimide membrane of the invention uses the same raw materials to prepare the carbon molecular sieve membrane and the polyimide membrane, so that the carbon molecular sieve membrane and the polyimide membrane have good compatibility and the material is not easy to be fractured. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 FT-IR images of polyimide film and composite polyimide film.

[0048] Figure 2 XPS graphs of O1s and C1s of polyimide film and composite polyimide film.

[0049] Figure 3 This is the SEM image of the polyimide film.

[0050] Figure 4 This is the SEM image of the composite polyimide membrane. DETAILED DESCRIPTION

[0051] The technical solution of the present invention will be described in more detail below through specific embodiments, but these embodiments do not limit the protection scope of the present invention.

[0052] In the present invention, during the vacuum drying process, the vacuum pump is used to pump the pressure in the vacuum drying box to -1 MPa.

[0053] Example 1

[0054] Step (1), under N2 protection, APBIA (6.7278 g, 0.03 mol) and N-methylpyrrolidone (70 mL) are added to a 200 mL beaker, stirred to completely dissolve APBIA, and then HPDD (13.3272 g, 0.03 mol) is added at -12°C. After the addition is completed, the mixture is stirred at room temperature for 20 h; then a mixed solution of 3-methylpyridine (3 mL, 0.03 mol) and acetic anhydride (28.3 mL, 0.3 mol) is slowly added to the mixed solution. After the addition is completed, the mixture is stirred at room temperature for 24 h to generate the corresponding polyimide; the reaction solution is added to methanol for precipitation, the precipitate is separated, the precipitate is washed with methanol several times, and then vacuum dried in a vacuum drying oven at 110°C for 12 h to obtain a polyimide.

[0055] Step (2), taking the polyimide prepared in step (1), using acetone as a solvent, preparing a polyimide film solution with a concentration of about 12wt.%, standing, and waiting for the polyimide to be completely dissolved, slowly pouring the polyimide film solution onto a clean glass plate preheated to 60°C to allow the film solution to quickly solidify into a film, scraping out a film at a uniform speed with a scraper, soaking it in methanol for 12 hours to replace the residual solvent in the film, and finally placing it in a vacuum drying oven at about 70°C for 12 hours to obtain a polyimide film.

[0056] Step (3), transferring the polyimide membrane to a tubular furnace, heating the tubular furnace from room temperature to 550°C at a rate of 5°C / min under a N2 atmosphere, and calcining at 550°C for 2h to obtain a carbon molecular sieve membrane.

[0057] Step (4), taking the polyimide prepared in step (1), using acetone as a solvent, preparing a polyimide membrane solution with a concentration of 12wt.%, and letting it stand until the polyimide is completely dissolved; placing the carbon molecular sieve membrane prepared in step (3) in the 12wt% polyimide membrane solution and soaking it for 5 minutes, taking it out, drying it, and placing it in a vacuum drying oven at about 80°C for vacuum drying for 18 hours to obtain a composite polyimide membrane.

[0058] The FT-IR images of the prepared polyimide film and composite polyimide film are shown in Figure 1 As shown, from Figure 1 It can be seen that 2700-3100cm -1 is the stretching vibration peak of CH, 1700cm -1 It is the characteristic peak of C=O, 1200-1300cm -1 are the characteristic peaks of CN, which are all characteristic peaks of polyimide, indicating that the polyimide membrane and composite polyimide membrane were successfully synthesized.

[0059] Figure 2 a. Figure 2b are the C1s XPS and O1s XPS graphs of the polyimide film, respectively. Figure 2 In a, 284.3 eV and 287.6 eV are attributed to CC and NC=N, respectively. Figure 2 In b, oxygen species on the film surface were observed at three positions with binding energies of 532.6 eV, 531.2 eV, and 530.1 eV, respectively. 532.6 eV corresponds to oxygen adsorbed by surface water or carbonate, 531.2 eV corresponds to oxygen chemically adsorbed on the surface, and 531.8 eV corresponds to lattice oxygen in the film. Figure 2 c. Figure 2 d are the C1sXPS and O1sXPS graphs of the composite polyimide membrane, respectively. It can be seen from the figures that the C1s and O1s peaks of the composite polyimide membrane both move toward low bond energy. This is because the electrons on the polyimide membrane move to the surface of the carbon molecular sieve membrane.

[0060] The SEM image of the prepared polyimide film is shown in Figure 3 shown; from Figure 3 a. Figure 3 b It can be seen that the surface of the polyimide film is relatively smooth. Figure 3 c shows that the carbon content of the membrane is about 82.33%. The SEM image of the prepared composite polyimide membrane is as follows Figure 4 As shown. Figure 4 It can be seen that the surface of the composite polyimide membrane is smoother, indicating that the interface compatibility between the carbon molecular sieve membrane and the polyimide membrane is good, and the carbon content of the composite polyimide membrane is slightly increased.

[0061] Separation performance evaluation: The gas separation performance of the composite polyimide membrane and the polyimide membrane was tested respectively, and the test conditions were: temperature 25°C, feed pressure 0.2MPa, He, CH4 and N2 mixed system (the volume ratio of He, CH4 and N2 was 30:8:62). The helium permeability of the composite polyimide membrane was 4.33×10 -7 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 196, and the separation factor of He / CH4 gas is 203; the helium permeability of the polyimide membrane is 3.18×10 -8 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 86, and the separation factor of He / CH4 gas is 98. It can be seen that compared with the polyimide membrane, under the same test conditions, the permeability, separation factor of He / N2 gas, and separation factor of He / CH4 gas of the composite polyimide membrane are significantly improved.

[0062] Example 2

[0063] The preparation method of Example 1 was referred to, except that the concentrations of the polyimide membrane solutions for preparing the polyimide membrane and soaking the carbon molecular sieve membrane were both adjusted to 10 wt %, thereby preparing a composite polyimide membrane.

[0064] The separation performance evaluation method of the composite polyimide membrane was the same as that of Example 1, and the helium permeability was 5.0×10 - 7 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 178, and the separation factor of He / CH4 gas is 185.

[0065] Example 3

[0066] The preparation method of Example 1 was referred to, except that the concentrations of the polyimide membrane solutions for preparing the polyimide membrane and soaking the carbon molecular sieve membrane were both adjusted to 8 wt %, thereby preparing a composite polyimide membrane.

[0067] The separation performance evaluation method of the composite polyimide membrane was the same as that of Example 1, and the helium permeability was 5.46×10 - 7 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 156, and the separation factor of He / CH4 gas is 161.

[0068] Example 4

[0069] Step (1), under N2 protection, APBIA (6.7278 g, 0.03 mol) and N-methylpyrrolidone (80 mL) are added to a 200 mL beaker, stirred to completely dissolve APBIA, and then HPDD (13.3272 g, 0.03 mol) is added at -12 ° C. After the addition is completed, the mixture is stirred at room temperature for 20 h; then a mixed solution of 3-methylpyridine (3 mL, 0.03 mol) and acetic anhydride (33 mL, 0.35 mol) is slowly added to the mixed solution, and after the addition is completed, the mixture is stirred at room temperature for 24 h. The reaction solution is added to methanol for precipitation, and the precipitate is separated. The precipitate is washed with methanol several times, and then vacuum dried in a vacuum drying oven at 105 ° C. for 12 h to obtain polyimide.

[0070] Step (2), taking the polyimide prepared in step (1), using acetone as solvent, preparing a polyimide film solution with a concentration of about 12wt.%, letting it stand until the polymer is completely dissolved, slowly pouring it onto a clean glass plate preheated to 60°C, scraping out a film at a uniform speed with a scraper, soaking the film in methanol for 12 hours to replace the residual solvent in the film, and then placing it in a vacuum drying oven at about 80°C for vacuum drying for 12 hours to obtain a polyimide film.

[0071] Step (3), transferring the polyimide membrane to a tubular furnace, heating the tubular furnace from room temperature to 500° C. at a rate of 5° C. / min under an Ar atmosphere, and calcining the membrane at 500° C. for 2 h to obtain a carbon molecular sieve membrane.

[0072] Step (4), take the polyimide prepared in step (1), use acetone as solvent, prepare a polyimide membrane solution with a concentration of about 12wt.%, let it stand until the polymer is completely dissolved, place the carbon molecular sieve membrane prepared in step (3) in the 12wt% polyimide membrane solution and soak it for 5 minutes, take it out, dry it, and place it in a vacuum drying oven at about 80°C for 18 hours to obtain a composite polyimide membrane.

[0073] Separation performance evaluation: The composite polyimide membrane was tested for gas separation performance under the following conditions: temperature 30°C, feed pressure 0.2 MPa, He, CH4 and N2 mixed system (volume ratio of He, CH4 and N2 mixed gas is 35:5:60). The permeability of helium is 4.39×10 -7 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 187, and the separation factor of He / CH4 gas is 194.

[0074] Example 5

[0075] Step (1), under N2 protection, APBIA (6.7278 g, 0.03 mol) and N-methylpyrrolidone (80 mL) are added to a 200 mL beaker, stirred to completely dissolve APBIA, then HPDD (13.3272 g, 0.03 mol) is added at -12 ° C. After the addition is completed, the mixture is stirred at room temperature for 20 h; then a mixed solution of 3-methylpyridine (3 mL, 0.03 mol) and acetic anhydride (33 mL, 0.35 mol) is slowly added to the mixed solution, after the addition is completed, the mixture is stirred at room temperature for 24 h, the reaction solution is added to methanol for precipitation, the precipitate is separated, the precipitate is washed with methanol several times, and then vacuum dried in a vacuum drying oven at 115 ° C. for 12 h to obtain polyimide.

[0076] Step (2), taking the polyimide prepared in step (1), using acetone as solvent, preparing a polyimide film solution with a concentration of about 10wt.%, letting it stand until the polymer is completely dissolved, slowly pouring it onto a clean glass plate preheated to 60°C, scraping out a film at a uniform speed with a scraper, soaking the film in methanol for 12 hours to replace the residual solvent in the film, and finally placing it in a vacuum drying oven at about 70°C for 12 hours to obtain a polyimide film.

[0077] Step (3), transferring the polyimide membrane to a tubular furnace, heating the tubular furnace from room temperature to 530°C at a rate of 5°C / min under a N2 atmosphere, and calcining at 530°C for 2.5h to obtain a carbon molecular sieve membrane.

[0078] Step (4), take the polyimide prepared in step (1), use acetone as solvent, prepare a polyimide membrane solution with a concentration of about 10wt.%, let it stand until the polymer is completely dissolved, place the carbon molecular sieve membrane prepared in step (3) in the 10wt% polyimide membrane solution and soak it for 5 minutes, take it out, dry it, and place it in a vacuum drying oven at about 80°C to dry it for 15 hours to obtain a composite polyimide membrane.

[0079] Separation performance evaluation: The gas separation performance of the composite polyimide membrane was tested under the following conditions: temperature 30°C, feed pressure 0.25 MPa, He, CH4 and N2 mixed system (volume ratio of He, CH4 and N2 mixed gas is 32:6:62). The permeability of helium is 3.62×10 -7 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 153, and the separation factor of He / CH4 gas is 161.

[0080] Comparative Example 1

[0081] The preparation method of the polyimide film is the same as that of patent CN118079681 A.

[0082] The o-hydroxy copolymer polyimide was synthesized by a two-step method, and the detailed steps are as follows:

[0083] Step 1: According to the molar ratio of APAF (4,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane) and BIA (N,N-dimethylformamide) = 1:3, weigh 1mmol APAF) and 3mmol BIA, place them in a three-necked round-bottom flask, add 35mL NMP (N-methylpyrrolidone), stir and dissolve them thoroughly, add 6FDA (4,4'-(hexafluoroisopropyl) diphthalic anhydride, 10mmol), and react at 5°C for 16h under argon protection to synthesize polyamic acid. The polyamic acid reacts at a temperature of 178°C for 8h. During the reaction, 10mL of o-xylene is added as an azeotropic agent to generate polyimide by intramolecular dehydration and ring closure; wash the polymer three times with a mixed solution of methanol and water in a volume ratio of 1:3 to remove residual solvent; and dry the washed polymer in a vacuum oven at 150°C for 24h to obtain a polymer. The polymer is prepared into a casting solution with a concentration of 25% using tetrahydrofuran, and the casting solution is poured onto a clean glass plate. The scraper thickness is set to 60 μm using an adjustable preparation device, and the scraped casting solution is placed in an environment with a temperature of 25°C and a relative humidity of 35-40% for 5 seconds to induce the formation of a skin layer; the prepared membrane is immersed in deionized water for 48 hours, and then immersed in methanol for 10 hours to remove residual solvent, dried at room temperature for 6 hours, and baked in a vacuum drying oven at 60°C for 16 hours to obtain a polyimide membrane.

[0084] Separation performance evaluation: The gas separation performance of the polyimide membrane was tested under the following conditions: temperature 25°C, feed pressure 0.2 MPa, He, CH4 and N2 mixed system (volume ratio of He, CH4 and N2 was 30:8:62). The permeability of helium was 4.92×10 -8 mol / (m 2 ·s·Pa), the separation factor of He / N2 gas is 101, and the separation factor of He / CH4 gas is 114.

[0085] Compared with the polyimide membrane of comparative example 1, under the same test conditions, the permeability of the composite polyimide membranes of examples 1 to 3 of the present invention is significantly improved, and the separation factors of He / N2 gas and He / CH4 gas are also significantly improved.

[0086] Comparative Example 2

[0087] According to the preparation method of Example 1, a carbon molecular sieve membrane was prepared.

[0088] Separation performance evaluation: The gas separation performance of the carbon molecular sieve membrane was tested under the following conditions: temperature 25°C, feed pressure 0.2 MPa, He, CH4 and N2 mixed system (volume ratio of He, CH4 and N2 was 30:8:62), and the helium permeability was 5.62×10 -7 mol / (m 2·s·Pa), the separation factor of He / N2 gas is 79, and the separation factor of He / CH4 gas is 101.

[0089] Compared with the carbon molecular sieve membrane, under the same test conditions, the composite polyimide membrane of the present invention not only maintains a permeability close to that of the carbon molecular sieve membrane, but also significantly improves the selectivity for helium.

Claims

1. A composite polyimide membrane for separating and purifying helium, characterized in that: It is a composite polyimide membrane obtained by wrapping a layer of polyimide membrane around a carbon molecular sieve membrane.

2. The composite polyimide film according to claim 1, characterized in that: The composite polyimide membrane is obtained by immersing a carbon molecular sieve membrane in a polyimide membrane solution and then drying the solution. Wherein, the polyimide film solution is an acetone solution of polyimide; The polyimide is prepared from 2-(4-aminophenyl)-5-aminobenzimidazole, 4,4'-(hexafluoroisopropylene) diphthalic acid glycoside, 3-methylpyridine and acetic anhydride as raw materials; The carbon molecular sieve membrane is prepared by using 2-(4-aminophenyl)-5-aminobenzimidazole, 4,4'-(hexafluoroisopropylene) diphthalic acid glycoside, 3-methylpyridine and acetic anhydride as raw materials to prepare polyimide, and then using acetone as solvent to prepare polyimide into polyimide membrane solution, and adopting casting method to prepare polyimide membrane, and then calcining the polyimide membrane in Ar and / or N2 atmosphere.

3. A method for preparing the composite polyimide film according to claim 1, characterized in that: The following steps are involved: Step (1), under N2 protection, using N-methylpyrrolidone as solvent, preparing a 2-(4-aminophenyl)-5-aminobenzimidazole solution, adding 4,4'-(hexafluoroisopropylene) diphthaloside to the 2-(4-aminophenyl)-5-aminobenzimidazole solution at a temperature of -20 to -5°C, stirring at room temperature for 12 to 24 hours; then adding 3-methylpyridine) and acetic anhydride, continuing to stir at room temperature for 12 to 24 hours, mixing the reaction solution with methanol for precipitation, washing the precipitate with methanol, and vacuum drying to obtain a polyimide; Step (2), taking polyimide, using acetone as solvent, preparing a polyimide film solution with a concentration of about 5 to 20 wt.%, preparing a film by a casting method, soaking the film in methanol to replace the residual solvent in the film, and then vacuum drying to obtain a polyimide film; Step (3), placing the polyimide membrane in a tube furnace, and calcining at 500-600° C. for 1-3 h in an Ar and / or N2 atmosphere to obtain a carbon molecular sieve membrane; Step (4), taking polyimide, using acetone as solvent, preparing a polyimide membrane solution with a concentration of about 5 to 20 wt.%; soaking the carbon molecular sieve membrane in the polyimide membrane solution for 3 to 10 minutes, taking it out, drying it, and vacuum drying it at 60 to 100° C. for 12 to 24 hours to obtain a composite polyimide membrane.

4. The method for preparing a composite polyimide film according to claim 3, characterized in that: In step (1), the dosage ratio of 2-(4-aminophenyl)-5-aminobenzimidazole to N-methylpyrrolidone is 0.03:60 to 0.03:80 mol / mL; The molar ratio of the 2-(4-aminophenyl)-5-aminobenzimidazole to 4,4'-(hexafluoroisopropylene) diphthaloside is 1:1; the molar ratio of the 2-(4-aminophenyl)-5-aminobenzimidazole to 3-methylpyridine is 1:1; and the molar ratio of the 2-(4-aminophenyl)-5-aminobenzimidazole to acetic anhydride is 1:10 to 1:

12.

5. The method for preparing a composite polyimide film according to claim 3, characterized in that: In step (2), the concentration of the polyimide membrane solution is 8-12 wt.%, preferably 10 wt.%.

6. The method for preparing a composite polyimide film according to claim 3, characterized in that: In step (3), the calcination temperature is 500-580°C.

7. The method for preparing a composite polyimide film according to claim 3, characterized in that: In step (3), the polyimide membrane is placed in a tubular furnace, heated from room temperature to 500°C-580°C at a rate of 5°C / min in an Ar and / or N2 atmosphere, and calcined at 500°C-580°C for 1-3h to obtain a carbon molecular sieve membrane.

8. The method for preparing a composite polyimide film according to claim 3, characterized in that: In step (4), the carbon molecular sieve membrane is immersed in a 8-12 wt. % polyimide membrane solution; preferably, the carbon molecular sieve membrane is immersed in a 10 wt. % polyimide membrane solution.

9. Use of the composite polyimide membrane according to claim 1 in separating and purifying helium.

10. A method for selectively separating and purifying helium, characterized in that: include: The composite polyimide membrane described in claim 1 is used to perform membrane separation on a mixed gas of helium and at least one selected from nitrogen and methane to separate and purify the helium; wherein the feed pressure of the membrane separation is 0.1 to 0.5 MPa, preferably 0.2 MPa, and the temperature of the membrane separation is 10 to 50°C, preferably 20 to 30°C.

Images