An asymmetric gas separation membrane, its preparation method and application

Abstract

The application relates to the technical field of gas separation, in particular to an asymmetric copolyimide membrane and a preparation method and application thereof. The asymmetric copolyimide helium separation membrane with a selection layer thickness of 300 nm is prepared by optimizing casting solution conditions and using a non-solvent induced phase separation technology. The membrane is dense in the outside and porous in the inside, the helium permeability reaches 87 GPU, the membrane thickness is reduced by one order of magnitude compared with a self-supporting membrane, the membrane also has good gas separation selectivity, the membrane also has good gas separation performance when the helium feed composition is changed, and the membrane also has excellent impurity resistance and stable separation performance when impurities C2H6 and CO2 are introduced.

Description

Technical Field

[0001] This invention relates to the field of gas separation technology, specifically to an asymmetric copolyimide membrane, its preparation method, and its application. Background Technology

[0002] Helium (He), a non-renewable resource, plays an irreplaceable role in aerospace, semiconductors, medicine, welding, and other fields. The concentration of helium in the air is only 5.2 ppm, and commercial helium primarily comes from natural gas reservoirs.

[0003] Currently, cryogenic distillation (deep cryogenic technology) is the main process technology for helium recovery from natural gas. However, it is costly and difficult to extract, and in most natural gas plants, helium is usually released into the atmosphere as a byproduct. Membrane separation is considered a more efficient and energy-saving method for helium production due to its room-temperature operating conditions and lack of phase change. Combining membrane separation technology with other processes can economically and efficiently concentrate and prepare high-purity helium from natural gas. Most current industrial gas separation membrane materials are organic membranes, which have simple preparation processes and relatively low costs. Polymer membranes have been widely studied for separating helium from N2 and CH4; however, the helium permeability of CA membranes is only 14 Barrer, and that of PSF membranes is only 13 Barrer. Since the 1980s, when soluble and easily processed polyimides were reported, they have been increasingly used for gas separation, and commercially available... The He permeability of the membrane is 22 Barrer, and the He / CH4 selectivity is 129; the helium permeability of the P84 copolyimide membrane is 25 Barrer, and the He / CH4 selectivity is only 7.

[0004] It is well known that adding bulk groups to the polymer backbone can significantly improve the separation performance of polymers. Soo et al. added a large 4,4'-methylene-di-(3-chloro-2,6-diethylaniline) group to 6FDA-APAF (6FDA = 4,4'-(hexafluoroisopropyl)phthalic anhydride, APAF = 2,2-di(3-amino-4-hydroxyphenyl)-hexafluoropropane chain), resulting in an increase in He permeability from 49 Barrer to 66 Barrer. However, due to the excessive increase in free fraction volume (FFV), the He / CH4 selectivity decreased by 37%. In our inter-group work, we controlled the microporous structure by modulating the Cardo component in the 6FDA-APAF-Cardo chain, preparing a self-supported membrane with 20% Cardo. This improved the He / CH4 selectivity by 125, but the He permeability was only 113 Barrer. When the Cardo ratio increased to 80%, the He permeability further increased to 2128 Barrer, but the He / CH4 selectivity was only 38. Therefore, how to improve the selectivity of copolyimide membranes in He separation is an urgent problem to be solved. Summary of the Invention

[0005] This invention utilizes a solvent-induced phase separation technique to prepare an asymmetric copolyimide helium separation membrane with a selective layer thickness of 300 nm by optimizing the comonomer structure and casting solution conditions. The membrane has a dense outer layer and a porous inner layer, achieving a helium permeability of 87 GPU. Compared to self-supporting membranes, the membrane thickness is reduced by an order of magnitude, while also exhibiting excellent gas separation selectivity. Furthermore, the membrane maintains good gas separation performance even when the helium feed composition is changed. When impurities such as C2H6 and CO2 are introduced, the membrane also demonstrates excellent impurity resistance and stable separation performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an asymmetric copolyimide membrane, wherein the membrane consists of an outer dense skin layer and an inner porous layer, the thickness of the dense skin layer being 200 nm to 1 μm; the film-forming polymer of the copolyimide membrane is 6FDA-APAF. 0.5 -BIA 0.5 The structural formula of the polymer is as follows:

[0007]

[0008] The above-mentioned method for preparing an asymmetric copolyimide film includes the following steps:

[0009] S1: Synthesis of 6FDA-APAF using a two-step method 0.5 -BIA 0.5 polymer;

[0010] S2: Dissolve the obtained polymer in N-methylpyrrolidone, add ethanol and tetrahydrofuran in sequence, stir to dissolve, and obtain casting solution;

[0011] S3: Pour the obtained casting solution onto a glass plate, allow it to evaporate in the air, then transfer the glass plate to a coagulation bath for phase separation to obtain an asymmetric copolyimide membrane.

[0012] Preferably, the two-step method specifically involves: firstly, monomer polymerization is carried out at a low temperature of 5–10°C to form polyamic acid; then, thermal amidation is performed at a high temperature of 170–190°C to form polyimide.

[0013] Preferably, the casting solution contains 25 wt.% polymer, 55 wt.% N-methylpyrrolidone, and a total content of tetrahydrofuran and ethanol between 0 wt.% and 20 wt.%.

[0014] Preferably, the relative humidity during evaporation in the air is controlled between 35% and 45%, and the temperature is controlled between 20°C and 25°C.

[0015] Preferably, the evaporation time in the air is 5 to 10 seconds.

[0016] Preferably, the coagulation bath time is 45 to 55 hours.

[0017] Preferably, after phase separation, the membrane is further immersed in methanol and dried at room temperature.

[0018] Preferably, the soaking time is 8 to 12 hours and the drying time is 5 to 8 hours.

[0019] The above-mentioned asymmetric copolyimide membrane is used in helium separation.

[0020] A method for helium separation evaluates membrane performance by testing the separation selectivity of He / CH4 and He / N2 mixtures.

[0021] Preferably, the test temperature range of the membrane is 30℃~180℃.

[0022] Preferably, C2H6 or CO2 is introduced into the feed gas to further evaluate the membrane performance in the presence of impurities.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) This invention improves membrane selectivity by optimizing the comonomer structure and utilizing longer BIA chains to increase chain packing density. Combined with optimized casting solution conditions, an asymmetric copolyimide helium separation membrane with a selectivity layer thickness of 300 nm was prepared using a solvent-induced phase separation technique. The ideal selectivity for He / CH4 was 124, for He / N2 it was 73, and for He / CH4 and He / N2 binary mixtures it was 80 and 60, respectively. This separation performance exceeds and approaches the 2019 upper limit for He / CH4 separation. Furthermore, the membrane has a porous interior with helium permeability reaching 87 GPU, and the membrane thickness is reduced by an order of magnitude compared to self-supporting membranes.

[0025] (2) When the helium feed composition is changed, this membrane also has good gas separation performance. Even when the He content is reduced to 0.2%, the permeability of this membrane to He is 73 GPU, the He / CH4 selectivity is 75, and the He / N2 selectivity is 60.

[0026] (3) When impurities C2H6 and CO2 are introduced, this membrane exhibits excellent resistance to impurities. Once the impurities are removed, the membrane performance can be restored, indicating that no structural change has occurred. Furthermore, after 160 hours of continuous testing, the asymmetric 6FDA-APAF membrane showed excellent resistance to impurities. 0.5 -BIA 0.5The membrane's separation performance remains stable, and this membrane has great potential in the process of extracting helium from natural gas. Attached Figure Description

[0027] Figure 1 (a) is 6FDA-APAF 0.5 -BIA 0.5 (a) Ternary phase diagram of polymer, NMP, and ethanol; (b) Viscosity of polymer dissolved in NMP solution;

[0028] Figure 2 The effect of the ratio of THF to ethanol on membrane morphology is shown in the following cases: (a) M1, 0:1; (b) M2, 1:3; (c, d) M3 surface and cross-section, 1:1; (e) M4, 3:1; (f) M5, 1:0.

[0029] Figure 3 The effect of the ratio of THF to ethanol on the performance of equimolar He / CH4 and He / N2 separators at 298 K and 0.1 MPa;

[0030] Figure 4 Asymmetric 6FDA-APAF 0.5 -BIA 0.5 The separation performance of the membrane at 0.1 MPa as a function of temperature is shown in the graphs, where (a) represents He / N2 separation; (b) represents He / CH4 separation; (c) represents the apparent activation energies of He, N2, and CH4 as individual components fitted according to the Arrhenius formula; and (d) represents the apparent activation energies of the mixed components of He, N2, and CH4 fitted according to the Arrhenius formula.

[0031] Figure 5 For different feed components, the effect of asymmetric 6FDA-APAF 0.5 -BIA 0.5 The influence of membrane separation performance is shown, where the dashed line represents He / N2 separation and the solid line represents He / CH4 separation.

[0032] Figure 6 Asymmetric 6FDA-APAF 0.5 -BIA 0.5 Long-term stability of the membrane at 0.1 MPa and 298 K and the effect of impurities on separation performance, where (a) is He / N2 separation; (b) is He / CH4 separation; blue area: 5 mol% C2H6 added; purple area: 5 mol% CO2 added. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] (1) Synthesis of 6FDA-APAF-BIA copolyimides in different proportions:

[0035] A two-step method was used to synthesize o-hydroxyl copolyimide, detailed as follows: Step 1: In a three-necked round-bottom flask, different ratios of diamine monomers (APAF:BIA = 1:3, APAF:BIA = 1:1, APAF:BIA = 3:1) were dissolved in 35 mL of NMP (N-methylpyrrolidone) solution. After thorough stirring, 10 mmol of acid anhydride 6FDA was added. Under an inert argon atmosphere, the polymer mixture was reacted at 278 K for 16 h to synthesize polyamic acid. Step 2: The synthesized polyamic acid was reacted at 450 K for 8 h. During the reaction, 10 mL of o-xylene was added as an azeotropic agent, resulting in intramolecular dehydration and ring closure to form polyimide. The polymer was washed three times with a methanol:water aqueous solution of 1:3 to remove residual solvent. Finally, the washed polymer was dried in a vacuum oven at 150 °C for 24 h to obtain the desired polymer, named 6FDA-APAF. 0.25 -BIA 0.75 6FDA-APAF 0.5 -BIA 0.5 6FDA-APAF 0.75 -BIA 0.25 .

[0036] (2) Preparation of gas separation membrane

[0037] With 6FDA-APAF 0.5 -BIA 0.5 Taking the preparation of polymeric membranes as an example, polymer 6FDA-APAF is used. 0.5 -BIA 0.5 The solution composition consisted of NMP, THF (tetrahydrofuran), and ethanol. Tetrahydrofuran was chosen as a highly volatile solvent to promote skin formation, while ethanol was chosen as a non-solvent due to its excellent pore-forming properties. The polymer 6FDA-APAF was then used. 0.5 -BIA 0.5 Dissolve in NMP, then add ethanol and THF in sequence, and stir thoroughly to dissolve; the formula is shown in Table 1. The casting solution should be left to stand for 12 hours before use.

[0038] The prepared casting solution was poured onto a clean glass plate. Using an adjustable preparation device, the doctor blade thickness was set to 60 μm. The cast solution was then placed in an environment of 25°C and 35-40% relative humidity for 5 seconds to induce the formation of the epidermal layer. The prepared membrane was then immersed in deionized water for 48 hours to allow for sufficient solvent-non-solvent exchange. Finally, it was soaked in methanol for 10 hours to remove residual solvent. The prepared membrane was then dried at room temperature for 6 hours to obtain the asymmetric 6FDA-APAF. 0.5 -BIA 0.5 Membrane (i.e., asymmetric copolyimide membrane).

[0039] Table 1 Casting solution formulation

[0040] No. Polymer / wt.% NMP / wt.% THF / wt.% Ethanol / wt.% M1 25 55 0 20 M2 25 55 5 15 M3 25 55 10 10 M4 25 55 15 5 M5 25 55 20 0

[0041] Use 6FDA-APAF 0.5 -BIA 0.5 Phase separation behavior observed in a ternary system composed of NMP and ethanol (Phase Diagram) Figure 1 The casting solution composition is located in the homogeneous region and far from the double-nodal line, indicating a low enthalpy of mixing; when the THF to ethanol ratio increases from 0:1 to 1:0, the casting solution composition approaches the double-nodal line. (6FDA-APAF) 0.5 -BIA 0.5 The χ² value for ethanol was 0.63 (Table 2), which is much higher than that for 6FDA-APAF. 0.5 -BIA 0.5 The interaction parameter with THF is 0.11; the χ value with water is the highest, 5.10 (>0.5, critical solubility value), so the polymer is insoluble in water; NMP is readily soluble in water, therefore, water is chosen as the coagulation bath.

[0042] Table 2 6 FDA-APAF 0.5 -BIA 0.5 Interaction parameters between solvent and coagulation bath, physical parameters of solvent.

[0043]

[0044] Polymer viscosity is an important parameter affecting film morphology. NMP was selected as the solvent for 6FDA-APAF. 0.5 -BIA 0.5The primary solvent was NMP, as the interaction parameter (χ) between NMP and the polymer was only 0.08 (Table 2). The polymer viscosity in NMP was a function of polymer concentration, and solutions with polymer concentrations ranging from 2 wt.% to 32 wt.% were tested. Solution viscosity increased slowly with increasing polymer concentration from the initial 2 wt.% to 26 wt.%; when the concentration reached 26 wt.%, the viscosity increased exponentially. Connecting the two tangents in the high and low viscosity regions determined the critical polymer concentration to be 25 wt.%. Small amounts of ethanol and tetrahydrofuran were added to the solution as non-solvents and highly volatile solvents to control the microstructure of the membrane. By setting the polymer concentration to 25 wt.%, THF / ethanol solutions with different ratios (0:1, 1:3, 1:1, 3:1, and 1:0) were prepared, named M1-M5 (Table 1).

[0045] Table 3 Gas separation performance of asymmetric copolyimide membranes prepared with polymers of different diamine monomer ratios.

[0046]

[0047] The performance of asymmetric membranes varies considerably depending on the ratio of diamine monomers, as shown in Table 3. Because the AFAP diamine monomer contains a large -CF3 functional group, it can hinder chain stacking and increase gas permeability. Therefore, with the increase of APAF monomer content, He permeability increases monotonically, while He / CH4 and He / N2 selectivity decrease. Considering all factors, the asymmetric copolyimide membrane prepared with a diamine monomer ratio of 1:1 exhibits the best performance.

[0048] I. The Influence of Volatile Solvents and Non-solvents on Selective Layer Structure

[0049] Figure 2 SEM images show the effect of the ratio of volatile THF to non-solvent ethanol on the film morphology. The casting solution was placed in ambient air at a relative humidity of 35–40% and a temperature of 303 K for 5 s to induce the formation of a skin layer. In the absence of THF, a layer thickness of 220 nm was selected. Figure 2 ); as the THF concentration increases, the selective layer thickness increases monotonically. Figure 2 When the THF concentration is 5 wt.%, the selective layer thickness is increased to 300 nm. Figure 2 b), however, it is an order of magnitude thinner than the self-supporting membrane; when the THF concentration reaches 10 wt.% ( Figure 2 d) and 15wt.% Figure 2 e) When the selective layer thickness is further increased from 440 nm to 500 nm, the film thickness reaches as high as 1 μm when the THF concentration is 20 wt.%. This is because the high volatility of THF leads to the enrichment of polymer concentration in the top layer, resulting in a thicker skin layer.

[0050] II. Further study on the effect of THF concentration on separation performance ( Figure 3 When the THF concentration increased from 0 wt.% to 20 wt.%, the permeability monotonically decreased from 120 GPU to 14 GPU. This is because the high volatility of THF favors the formation of a thicker skin layer, thereby increasing mass transfer resistance. Figure 2 The increasing selectivity layer thickness observed by SEM was consistent with this. When the THF concentration reached 5 wt.%, the selectivity for He / CH4 and He / N2 decreased. Freeman et al. demonstrated that excessive THF concentration leads to extensive dry-phase separation. We observed that excessive THF volatility resulted in kinks and defects within the membrane. The maximum selectivity of the M2 membrane for He / CH4 and He / N2 was 80 and 60, respectively.

[0051] III. Temperature Effects

[0052] To gain a deeper understanding of the separation behavior, we conducted further tests on the membrane at different temperatures ranging from 298K to 453K. Figure 4 (ab) The permeability of He, N2, and CH4 increases monotonically with increasing operating temperature, exhibiting active diffusion. At 453 K, the permeability of He even reaches 235 GPU. Since the permeability of large molecules increases more significantly than that of small molecules, the selectivity of both He / N2 and He / CH4 decreases with increasing temperature. In single components, the apparent activation energy of N2 is 15.2 kJ mol. -1 The apparent activation energy of CH4 is 17.5 kJ mol. -1 ( Figure 4 (cd); The apparent activation energy of N2 in the mixed component is 14.3 kJ / mol. -1 The apparent activation energy of CH4 is 15.2 kJ / mol. -1 This indicates that He molecules promote the transport of N2 and CH4 molecules. The diffusion activation energy of He in the He / CH4 mixture is 8.1 kJ / mol. -1 Slightly lower than the 8.3 kJ mol in the He / N2 mixture. -1 This is because N2 is more permeable than CH4 and occupies more transport channel sites, so the N2 molecule has a greater effect on delaying the diffusion of He than CH4.

[0053] IV. Effect of Feed Concentration

[0054] Natural gas fields can have He concentrations as high as 4 mol% (New Mexico, USA), but the average He concentration is 0.2-0.5 mol%. Therefore, the membrane performance was further evaluated within a He feed composition range of 0.2%-90%. Figure 5 Asymmetric 6FDA-APAF 0.5-BIA 0.5 The membrane exhibits constant He / CH4 and He / N2 selectivity. To achieve 99% He purity in both membrane grades, the He / CH4 selectivity must be at least 54. Even with a He content reduced to 0.2%, the membrane's He permeability is 73 GPU, with a He / CH4 selectivity of 75 and a He / N2 selectivity of 60. This He permeability is higher than that of commercially available membranes. The membrane is an order of magnitude higher than that of the DD3R molecular sieve membrane (He permeability of 9 GPU and He / CH4 selectivity of 44), and has higher separation selectivity for both He / N2 and He / CH4 separation.

[0055] V. Membrane resistance to impurities test

[0056] Physical aging is a common problem in polymer films. After 160 hours of continuous testing, the asymmetric 6FDA-APAF... 0.5 -BIA 0.5 The separation performance of the membrane remained stable. However, the permeability of the 140nm thick 6FDA-HAB polyimide membrane decreased by 75% after 100 hours of aging, mainly due to the asymmetric 6FDA-APAF. 0.5 -BIA 0.5 The rigid BIA portion of the membrane imparts a stable microporous structure. Considering impurities in natural gas, we investigated the effects of C2H6 and CO2. Figure 6 When 5 mol% ethane was introduced, the He permeability of the He / N2 / C2H6 mixture decreased by 16%, and the He permeability of the He / CH4 / C2H6 mixture decreased by 15%. In contrast, after adding 1 mol% ethane, the He permeability of the STT membrane decreased by 14% compared to the initial value, and after adding 3.6 mol% ethane, the permeability of the DD3R membrane decreased by 19%. After adding 5 mol% CO2, the He permeability decreased slightly, but the membrane performance recovered rapidly once the CO2 feed was stopped, indicating that no structural change occurred. However, in the He / CO2 / N2 / CH4 quaternary mixture, the He permeability of the Cardo / α-Al2O3 membrane decreased by 25%, and the He permeability of the commercial polyamide reverse osmosis membrane decreased by 45%.

[0057] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. An asymmetric copolyimide film, characterized in that, The membrane consists of an outer dense skin layer and an inner porous layer, the thickness of which is 200 nm to 1 μm; the film-forming polymer of the copolyimide membrane is 6FDA-APAF. 0.5 -BIA 0.5 The structural formula of the polymer is as follows:

2. A method for preparing an asymmetric copolyimide film according to claim 1, characterized in that, Includes the following steps: S1: Synthesis of 6FDA-APAF using a two-step method 0.5 -BIA 0.5 polymer; S2: Dissolve the obtained polymer in N-methylpyrrolidone, add ethanol and tetrahydrofuran in sequence, stir to dissolve, and obtain casting solution; S3: Pour the obtained casting solution onto a glass plate, allow it to evaporate in the air, then transfer the glass plate to a coagulation bath for phase separation to obtain an asymmetric copolyimide membrane.

3. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, The two-step method specifically involves: firstly, monomer polymerization is carried out at a low temperature of 5–10°C to form polyamic acid; then, thermal amidation is performed at a high temperature of 170–190°C to form polyimide.

4. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, The casting solution contains 25 wt.% polymer, 55 wt.% N-methylpyrrolidone, and a total content of tetrahydrofuran and ethanol between 0 wt.% and 20 wt.%.

5. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, The relative humidity during evaporation in the air is controlled between 35% and 45%, and the temperature is controlled between 20°C and 25°C.

6. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, It takes 5 to 10 seconds to evaporate in the air.

7. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, The coagulation bath time is 45–55 hours.

8. The method for preparing an asymmetric copolyimide film according to claim 2, characterized in that, After phase separation, the membrane needs to be immersed in methanol and dried at room temperature.

9. The method for preparing an asymmetric copolyimide film according to claim 8, characterized in that, The soaking time is 8 to 12 hours, and the drying time is 5 to 8 hours.

10. The application of the asymmetric copolyimide membrane according to claim 1 in helium separation.

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