Double-polymer modified composite SSZ-13 molecular sieve membrane, and preparation method and application thereof

Abstract

The invention discloses a double-polymer modified composite SSZ-13 molecular sieve membrane as well as a preparation method and application thereof, and belongs to the technical field of gas separation. The SSZ-13 molecular sieve membrane is firstly covered with a layer of 6FDA-APAF0.5-BIA0.5 polymer membrane, and then the SSZ-13 molecular sieve membrane is covered with a layer of PDMS membrane, so that the preparation of the composite membrane is realized, and compared with a membrane material which is not modified or only modified by one polymer, the helium / methane separation performance of the composite membrane material co-modified by two polymers is obviously improved; as the Young modulus of the inorganic molecular sieve membrane is higher than that of a polymer membrane, the composite SSZ-13 molecular sieve membrane has higher mechanical stability than the polymer membrane, has excellent performance under different pressures, especially under high pressure, can keep good operation stability, and can be applied to extraction of helium from low-concentration helium-containing natural gas in pipelines, such as high-concentration helium-containing natural gas, high-concentration helium-containing natural gas and high-concentration helium-containing natural gas. The invention not only provides a new way for successful separation and recovery of helium, but also provides a new idea for performance optimization of other separation membranes.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas separation, and specifically relates to a double polymer modified SSZ-13 molecular sieve membrane, a preparation method and its application in separation of He / CH 4 Application in mixed gas. Background Art

[0002] Helium is a colorless, non-toxic, low-boiling monatomic inert gas that plays an important role in magnetic resonance imaging, semiconductor manufacturing, aerospace, and nuclear power plants. In recent years, the demand for helium has increased year by year. Helium originates from the decay of radioactive metals in the earth's crust and was initially emitted as a byproduct of natural gas along with other waste gases. Although helium is also distributed in the atmosphere and rock formations, it is difficult to enrich helium due to its extremely low concentration, and helium-containing natural gas is the only commercial source of helium.

[0003] The main methods for separating and recovering helium from natural gas include cryogenic separation, pressure swing adsorption (PSA) and membrane separation technology. Cryogenic separation is a traditional separation process. The cryogenic liquefaction separation process is energy-intensive and expensive. PSA uses the difference in adsorption force of adsorbents on gas components for separation and purification. It occupies a large area and is difficult to regenerate the adsorbent, with a high loss rate.

[0004] As a new separation method developed in the 1950s, membrane separation has the characteristics of low energy consumption, easy operation, flexible device design, small footprint, etc., and has a wide range of applications. The membrane separation method uses the difference in the transmission speed of helium and methane in the membrane to selectively screen the two gases, and has obvious effects on the enrichment and concentration of low-concentration helium. The core of membrane separation technology is the separation membrane material, among which polymer membrane is a material widely used in gas separation, liquid filtration, ion exchange and other fields. They are usually made of polymer materials, have a certain pore size and selectivity, and can effectively separate molecules or ions of different sizes. The advantages of polymer membranes include low cost, easy processing and good mechanical properties. However, existing polymer membrane materials generally show low helium permeability and the selectivity of helium / methane is difficult to meet the separation requirements. In response to the corresponding problems, Chinese patent CN113337095A proposes a self-polymerized microporous polymer fluorinated membrane, which is obtained by replacing the hydrogen element on the surface of the self-polymer microporous polymer membrane with the fluorine element in the fluorine-nitrogen mixed gas. The obtained membrane material has an ideal helium / methane selectivity of more than 3000, but it should be noted that although the ideal performance of this membrane is superior, it is difficult to prepare, and the mechanical stability and thermal conductivity of the obtained membrane material are poor, which limits its practical application. Chinese patent CN115414793A discloses an STT molecular sieve membrane suitable for helium purification, which provides a new method for the separation of helium, but the separation performance of the membrane under specific environments (such as high pressure conditions) remains to be investigated.

[0005] SSZ-13 molecular sieve membrane is a molecular sieve with chabazite (CHA) structure, which is composed of AlO 4 and SiO 4 The tetrahedrons are connected by oxygen atoms to form an ellipsoidal cage structure with an eight-membered ring structure. The pore size of SSZ-13 is 0.38 nm×0.38 nm, which is comparable to the kinetic diameter of many small molecule gases, and has a high specific surface area (up to 700 m² / g). SSZ-13 molecular sieve membranes have shown great potential in the field of gas separation due to their efficient catalytic reaction performance, excellent selectivity, outstanding sulfur resistance and unique three-dimensional pore structure, and are expected to be mass-produced to reduce production costs. However, SSZ-13 molecular sieve membranes have shortcomings such as limited hydrothermal stability, limited number of acid sites, pore size limitations and aging problems. In the prior art, the performance of SSZ-13 molecular sieve membranes is often improved by optimizing the synthesis process, loading metal ions and introducing modifiers.

[0006] According to relevant reports, the introduction of specific polymers into the molecular sieve membrane can change the surface properties and pore structure of the molecular sieve membrane, such as changing the hydrophilic or hydrophobic properties of the molecular sieve membrane, thereby improving the CO 2 / CH 4 , H 2 / CH 4 In addition, polymer modification can also enhance the mechanical strength and stability of the zeolite membrane and reduce the defects of the zeolite membrane. Compared with polymer membranes, organic-inorganic composite membranes can enhance mechanical strength and stability, making them more suitable for industrial applications. However, there are few reports on polymer-modified SSZ-13 zeolite membranes. For example, it is not possible to successfully prepare He / CH by polymer modification on SSZ-13 zeolite membranes. 4 New membrane material with excellent selectivity will provide a new method for separating He / CH 4 Provide new possibilities. Summary of the invention

[0007] In view of the above problems, the present invention aims to provide a double polymer modified composite SSZ-13 molecular sieve membrane, by sequentially modifying 6FDA-APAF on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 The He / CH 4 The separation performance is excellent even under high pressure conditions, and the membrane preparation process is simple, the preparation cost is low, and the obtained composite membrane material has a stable structure, which is suitable for promotion and use in the field of gas separation.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows: a method for preparing a double polymer modified composite SSZ-13 molecular sieve membrane, comprising the following steps: 1) Polymer 6FDA-APAF 0.5 -BIA 0.5 Dissolve in solvent, mix, filter, defoam, and obtain 6FDA-APAF 0.5 -BIA 0.5 Solution; 2) Mix PDMS, curing agent and n-heptane, mix well, filter, defoam, and prepare PDMS solution; 3) First coat the SSZ-13 molecular sieve membrane and carrier with the 6FDA-APAF prepared in step 1) 0.5 -BIA 0.5 Solution; 4) Covered with 6FDA-APAF 0.5 -BIA 0.5 The zeolite membrane and the carrier are coated with PDMS solution to obtain a double polymer modified composite SSZ-13 zeolite membrane.

[0009] Furthermore, in step 1), the solvent is N-methylpyrrolidone, and the stirring is carried out by stirring at a speed of 600 rpm-800 rpm and a stirring time of 6-12 h; the prepared 6FDA-APAF 0.5 -BIA 0.5 6FDA-APAF in solution 0.5 -BIA 0.5 The mass percentage is 1-30 wt.%.

[0010] Furthermore, in step 2), the mass percentage of PDMS in the prepared PDMS solution is 0.5-25 wt.%, and the solution is stirred and mixed at a speed of 600 rpm-800 rpm and a stirring time of 6-12 h.

[0011] Furthermore, the coating process in steps 3) and 4) is completed by methods including pulling and dipping, spraying, brushing, spin coating, and vacuum coating.

[0012] Preferably, 6FDA-APAF is coated on the SSZ-13 molecular sieve membrane using a pull-up impregnation method. 0.5 -BIA 0.5 and PDMS, the specific process is as follows: S1. First, immerse the SSZ-13 molecular sieve membrane and carrier with both ends sealed into the prepared 6FDA-APAF 0.5 -BIA 0.5The solution was immersed in water for 15-20 s, and the immersion and withdrawal rates were both 40-60 mm / min. After being taken out, it was dried in a 60-70 °C oven for 1-2 h, and then the temperature was raised to 180-200 °C at a rate of 1-5 °C / min in a vacuum oven to completely remove the solvent, and then the temperature was lowered to 303 K at a rate of 1-5 °C / min to coat 6FDA-APAF on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 ; S2, coated with 6FDA-APAF 0.5 -BIA 0.5 The SSZ-13 molecular sieve membrane and the carrier are immersed in the PDMS solution for 1-30 min at an immersion and withdrawal rate of 1-3 cm / s. After drying at room temperature for 1-3 h, they are placed in an oven with a heating rate of 1-5°C / min. After drying at 90-110°C for 1-3 h, the temperature is lowered to 333 K at a rate of 1-3°C / min to coat PDMS on the SSZ-13 molecular sieve membrane.

[0013] Furthermore, the 6FDA-APAF 0.5 -BIA 0.5 6FDA-APAF in solution 0.5 -BIA 0.5 The mass percentage of is 10wt.%, the mass percentage of PDMS in the PDMS solution is 5 wt.%; and the immersion time in the PDMS solution is 5~25 min.

[0014] The present application also provides a dual polymer modified composite SSZ-13 molecular sieve membrane, wherein 6FDA-APAF is modified on the SSZ-13 molecular sieve membrane successively. 0.5 -BIA 0.5 Membrane layer and PDMS membrane layer.

[0015] The dual polymer modified composite SSZ-13 molecular sieve membrane can be used in the field of gas separation, especially for the separation of He / CH 4 Mixed gas, the He / CH 4 The mixed gas is a simulated natural gas component, which contains 0.097% helium, a certain concentration of carbon dioxide and ethane. The feed pressure during the gas separation process is 0.1~10 MPa, preferably 0.2~5 MPa; the feed flow rate is 1~300 mL•min -1 , preferably 5-200 mL•min -1 .

[0016] The present application also proposes a three-stage natural gas helium extraction device, comprising: Composite SSZ-13 molecular sieve membrane separation unit, a first-stage membrane separation component based on composite SSZ-13 molecular sieve membrane, used to separate helium from high-pressure pipeline natural gas; The natural gas liquefaction unit, the feed side of which is connected to the permeate side of the composite SSZ-13 molecular sieve membrane separation unit, is used to liquefy the helium-rich natural gas; SSZ-13 molecular sieve membrane separation unit, a 1-3 stage membrane separation component constructed based on SSZ-13 molecular sieve membrane, has its feed side connected to the gas outlet of the natural gas liquefaction unit and is used for multi-stage extraction of helium from non-condensable gas to obtain crude helium.

[0017] The method for extracting helium from natural gas using the above three-stage natural gas helium extraction device is as follows: 1) The high-pressure pipeline natural gas is passed into the composite SSZ-13 molecular sieve membrane separation unit to carry out in-situ helium concentration of the high-pressure pipeline natural gas, and the helium-rich natural gas is output on the permeate side, and the residual gas is returned to the natural gas pipeline; 2) The helium-rich natural gas is liquefied by the natural gas liquefaction unit, the liquefied gas is directly output, and the non-condensable gas obtained is passed into the SSZ-13 molecular sieve membrane separation unit; 3) Utilize the SSZ-13 molecular sieve membrane separation unit to perform multi-stage extraction of helium from non-condensable gas, obtain crude helium on the permeate side, and return the retentate gas to the natural gas liquefaction unit.

[0018] The beneficial effects of the present invention are: 1. This application first coats the polymer 6FDA-APAF on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 SSZ-13 molecular sieve membrane was modified with polymer PDMS, 6FDA-APAF 0.5 -BIA 0.5 The coverage of the membrane layer can optimize the gas separation performance of the base membrane, and improve the membrane selectivity and permeability by forming a dense selective layer. After covering the PDMS membrane layer again, the gas separation performance of the membrane can be further improved while protecting the polyimide from water vapor corrosion and maintaining a good He / CH 4 Separation performance: Compared with the unmodified molecular sieve membrane and the molecular sieve membrane modified with only a single polymer, the helium / methane separation performance of the composite SSZ-13 molecular sieve membrane modified with two polymers prepared in the present application is significantly improved; 2. The composite SSZ-13 molecular sieve membrane prepared in this application has a good molecular sieving effect and shows good helium / methane separation performance under different feed pressures. Even in the simulated gas (helium concentration is 0.097%) helium extraction test under a high pressure of 5 MPa, the separation performance is still excellent, and the He recovery rate is R HeIt can even reach 11.5%; 3. The composite SSZ-13 molecular sieve membrane disclosed in the present application was tested for helium extraction using simulated gas (helium concentration of 0.097%), and it was found that it can effectively increase the helium concentration multiple, and with the increase of feed pressure, the helium / methane gas separation selectivity of this membrane decreases slightly, and the He recovery rate gradually increases. Therefore, the molecular sieve membrane can be efficiently used to extract helium from low-concentration helium-containing natural gas in pipelines, and has the advantages of convenient operation, energy saving and environmental protection, and significant economic benefits; 4. This application uses 6FDA-APAF 0.5 -BIA 0.5 After the SSZ-13 molecular sieve membrane was modified with these two polymers, PDMS, the separation performance of the membrane was improved, and good operation stability was maintained. After a long time, the membrane structure was not significantly damaged, and the He gas permeability and He / CH 4 Gas separation selectivity and He recovery rate are maintained at a high level; 5. This application uses two polymers to co-modify the SSZ-13 molecular sieve membrane to improve the gas permeability and separation performance of the membrane, which not only provides a new method for the successful separation and recovery of helium, but also provides a new idea for the performance optimization of other separation membranes; 6. The present application discloses a three-stage natural gas helium extraction process, which first uses a highly selective composite SSZ-13 molecular sieve membrane to separate high-pressure pipeline natural gas to obtain highly concentrated helium-rich natural gas, and then uses a high-throughput SSZ-13 molecular sieve membrane to separate the non-condensable gas obtained after liquefying the helium-rich natural gas. The SSZ-13 molecular sieve membrane separation unit is a multi-stage membrane component, which can increase the membrane processing capacity, reduce the floor space and save investment costs. Extracting helium through the SSZ-13 molecular sieve membrane component greatly reduces energy consumption compared to traditional cryogenic separation and pressure swing adsorption, providing a strong reference for obtaining high-purity helium. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the characterization of SSZ-13 molecular sieve membrane; wherein, (a) inset is the SEM image of the surface of SSZ-13 molecular sieve membrane, (b) inset is the SEM image of the cross section of SSZ-13 molecular sieve membrane, (c) inset is the XRD image of SSZ-13 molecular sieve membrane (a is SSZ-13 molecular sieve, b is the standard peak of SSZ-13 molecular sieve); Figure 2are XRD patterns of SSZ-13 molecular sieves modified with different concentrations of PDMS, wherein (a) is the XRD pattern of SSZ-13 molecular sieve, (b)-(g) are the XRD patterns of SSZ-13 molecular sieves modified with 0 wt.%, 0.5 wt.%, 1 wt.%, 2 wt.%, 5 wt.%, and 16 wt.% PDMS solutions, respectively; Figure 3 Figures 2 and 3 are SEM images of SSZ-13 molecular sieve membranes modified with PDMS at different concentrations, wherein (a)-(b) are surface SEM images of SSZ-13 molecular sieve membranes modified with 0.5 wt.% PDMS solution at different magnifications, (c) is a cross-sectional SEM image of SSZ-13 molecular sieve membranes modified with 0.5 wt.% PDMS solution; (d)-(e) are surface SEM images of SSZ-13 molecular sieve membranes modified with 1 wt.% PDMS solution at different magnifications, (f) is a cross-sectional SEM image of SSZ-13 molecular sieve membranes modified with 1 wt.% PDMS solution; (g)-(h) are surface SEM images of SSZ-13 molecular sieve membranes modified with 2 wt.% PDMS solution at different magnifications, (i) is a cross-sectional SEM image of SSZ-13 molecular sieve membranes modified with 2 wt.% PDMS solution. wt.% PDMS solution modified SSZ-13 molecular sieve membrane cross-sectional SEM images; (j)-(k) panels are surface SEM images of SSZ-13 molecular sieve membrane modified with 5 wt.% PDMS solution at different magnifications, (l) panel is a cross-sectional SEM image of SSZ-13 molecular sieve membrane modified with 5 wt.% PDMS solution; (m)-(n) panels are surface SEM images of SSZ-13 molecular sieve membrane modified with 16 wt.% PDMS solution at different magnifications, (o) panel is a cross-sectional SEM image of SSZ-13 molecular sieve membrane modified with 16 wt.% PDMS solution; Figure 4 To use 6FDA-APAF 0.5 -BIA 0.5 TG curves of SSZ-13 molecular sieve before and after modification. The modified SSZ-13 molecular sieve is recorded as SSZ-13-10 wt.%BIA; Figure 5 The preparation process of the composite SSZ-13 molecular sieve membrane and the schematic diagram of the morphology change of the molecular sieve membrane after the corresponding membrane is used to separate the gas, wherein 1-SSZ-13 molecular sieve membrane, 2-coated 6FDA-APAF 0.5 -BIA 0.5 SSZ-13 molecular sieve membrane after coating, 3- composite SSZ-13 molecular sieve membrane coated with PDMS again, 4- pipeline natural gas (5MPa), 5- four-channel a -Al2 O 3 Hollow fiber carrier, 6-6FDA-APAF with obvious deformation after gas separation 0.5 -BIA 0.5 Polymer membrane, 7-slightly deformed after separation of gases coated with 6FDA-APAF 0.5 -BIA 0.5 and a molecular sieve membrane with a carrier, 8-a composite SSZ-13 molecular sieve membrane that is not deformed after separating the gas; Figure 6 The single-component gas permeation test results of the composite SSZ-13 molecular sieve membrane M12, where (a) the small figure shows the permeability of gases with different molecular kinetic diameters, and (b) the small figure shows the ideal selectivity of M12 for different mixed gases at 0.2 MPaG; Figure 7 The effect of feed pressure on He / CH 4 The effect of modification on the separation performance, where (a) shows the He / CH ratio of the molecular sieve membrane before and after modification at different pressures. 4 Gas separation selectivity and gas permeability (the orange curves represent He / CH 4 Gas separation selectivity, the purple curves represent He gas permeability); (b) The small figure shows the recovery rate of the molecular sieve membrane before and after modification under different pressures; Figure 8 Figure 2 shows the effect of feed pressure on the helium extraction performance of the composite SSZ-13 molecular sieve membrane M12 from natural gas. (a) shows the He permeability and He / CH 4 Gas separation selectivity, (b) the small figure shows the He recovery rate and He concentration multiple of M12 at different pressures; Fig. 9 Figure 2 shows the effect of feed flow rate on the natural gas helium extraction performance of composite SSZ-13 molecular sieve membrane M12. (a) shows the He permeability and He / CH 4 Changes in gas separation selectivity; (b) The small figure shows the changes in He recovery and He concentration of M12 with the increase of feed flow rate; Fig.10 The results of long-term stability test of composite SSZ-13 molecular sieve membrane M12 at 5 MPa and 298 K, where (a) shows the He permeability and He / CH2 of M12 at different times. 4 Changes in gas separation selectivity; (b) The small figure shows the changes in He recovery rate and He concentration multiple of M12 at different times; Fig.11It is a schematic diagram of the structure of a three-stage natural gas helium extraction device, wherein 1-high-pressure pipeline natural gas, 2-composite SSZ-13 molecular sieve membrane separation unit, 3-natural gas liquefaction unit, 4-boosting device, 5-SSZ-13 molecular sieve membrane separation unit, 6-crude helium, 7-liquefied gas; Fig.12 Helium extraction performance data for the first-grade SSZ-13 molecular sieve membrane BOG (helium content of the raw material is 2%); Fig.13 Provide helium performance data for secondary SSZ-13 molecular sieve membrane BOG (raw material helium content 15%).

[0020] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0021] This application chooses to use 6FDA-APAF 0.5 -BIA 0.5 The SSZ-13 molecular sieve membrane was modified with PDMS. First, the effect of modifying SSZ-13 with a single polymer on the molecular sieve membrane was explored. The specific exploration process is as follows: 1. Preparation of PDMS-modified SSZ-13 molecular sieve membrane 1) Prepare dense SSZ-13 molecular sieve membrane according to existing technology First, NaOH and Al(OH) 3 and deionized water were placed in a polytetrafluoroethylene bottle and stirred for 2 h. Finally, colloidal silica was gradually dropped into the solution. 2 、Al 2 O 3 , NaOH, TMAdaOH and H 2 The molar ratio of O is 100:2.5: 20.5: 16.9: 4436.5, and the mixture is stirred at room temperature for another 12 hours. The synthetic solution is placed in a reactor for hydrothermal synthesis. After the reaction is completed, the SSZ-13 molecular sieve is centrifuged and washed several times with a high-speed centrifuge to make the molecular sieve neutral, and then placed in a 60°C oven to dry overnight. Finally, the SSZ-13 molecular sieve is calcined at 550°C for 24 h to remove the template, and the heating and cooling rate is 1°C·min -1 . The 7 cm commercial four-channel a -Al 2 O 3The hollow fiber carrier was placed in a 0.1 M NaOH solution and allowed to stand for 24 hours, then washed with deionized water and dried in a 60°C oven. The above-mentioned SSZ-13 molecular sieve was then used to prepare a 0.5 wt.% seed solution, and the seed solution was coated on the surface of the hollow fiber carrier by the immersion and pulling method. After two immersion and pulling steps, the carrier was placed in a 60°C oven and allowed to stand overnight. The preparation process of the synthetic solution was the same as that of the above-mentioned SSZ-13 molecular sieve synthetic solution, and the molar composition of the synthetic solution was 20 TMAdaOH: 20.4 NaOH: 0.45 Al 2 O 3 : 105 SiO 2 : 4400 H 2 O. After the synthesis is completed, the molecular sieve membrane is taken out, the surface is cleaned and dried, and finally the template agent is removed in an ozone atmosphere. This method successfully synthesizes a well-grown SSZ-13 molecular sieve membrane without other impurities. The membrane is characterized, and the results are as follows Figure 1 As shown, from Figure 1 As can be seen in the middle (c) panel, SSZ-13 molecular sieve has been successfully produced.

[0022] 2) Preparation of polydimethylsiloxane (PDMS) solution: Weigh polydimethylsiloxane, curing agent (tetramethyltetravinylcyclotetrasiloxane) and n-heptane in a mass ratio of 1:0.1:x (x=199, 99, 49, 19, 5.25) to prepare 0.5 wt.%, 1 wt.%, 2 wt.%, 5 wt.% and 16 wt.% PDMS n-heptane mixed solutions. Pour the prepared solutions into reagent bottles respectively, stir vigorously at 600 rpm for 12 h to make the solution uniform, filter, defoam by ultrasonic treatment, and set aside.

[0023] 3) The SSZ-13 molecular sieve membrane and hollow fiber carrier sealed with raw tape at both ends were immersed in PDMS n-heptane mixed solutions with different mass percentages, respectively. The immersion was 40 s in 16 wt.% PDMS solution and 15 min in other concentrations of PDMS solution. The immersion and withdrawal rate was 2 cm / s. After drying at room temperature for 1 h, they were placed in an oven with a controlled heating rate of 1°C / min, heated to 100°C and dried for 1 h, and then cooled to 333 K at a rate of 1°C / min to successfully coat PDMS on the SSZ-13 molecular sieve membrane.

[0024] In order to investigate whether PDMS modification of SSZ-13 molecular sieve membrane will damage the CHA crystal form, XRD analysis of SSZ-13 molecular sieve before and after modification was performed ( Figure 2 ) and SEM characterization of SSZ-13 molecular sieves modified with different concentrations of PDMS ( Figure 3 ).

[0025] XRD analysis results are shown in Figure 2 ,From the figure, it can be seen that the SSZ-13 molecular sieve still retains the CHA crystal form after PDMS modification, and with the increase of PDMS concentration, the CHA standard peak area of ​​SSZ-13 molecular sieve gradually decreases.

[0026] Depend on Figure 3 It can be seen that when the SSZ-13 molecular sieve membrane is modified with 0.5 wt.%, 1 wt.% and 2 wt.% PDMS solution, obvious SSZ-13 molecular sieve cubic crystals and some larger cracks can be seen on the surface of the SSZ-13 molecular sieve membrane ( Figure 2 In the (a), (b), (d), (e), (g), and (h) panels, PDMS failed to effectively modify these defects; the cross section of the SSZ-13 molecular sieve membrane modified with 0.5 wt.% PDMS solution ( Figure 2 The thickness of the PDMS coating is not visible in the middle (c) panel. When the PDMS solution concentration is 1 wt.%, the cross section of the modified SSZ-13 molecular sieve membrane shows that there is a clear PDMS polymer part inside the membrane layer ( Figure 2 (f) This is because the SSZ-13 molecular sieve membrane has large defects, and PDMS penetrates into the carrier through the defects, increasing the gas mass transfer resistance. When the PDMS solution concentration is 5 wt.%, a clear PDMS coating can be seen on the surface of the SSZ-13 molecular sieve membrane, but it still does not completely cover the SSZ-13 molecular sieve cubic crystals ( Figure 2 In the middle (j) and (k) panels, the polymer thickness cannot be clearly seen in the cross section of the film ( Figure 2 When the SSZ-13 zeolite membrane was modified with 16 wt.% PDMS solution, the surface of the zeolite membrane was completely covered by PDMS, and the SSZ-13 zeolite crystal particles were almost invisible ( Figure 2 In the middle (m) and (n) panels, a certain thickness of PDMS layer can be seen in the cross section of the SSZ-13 molecular sieve membrane ( Figure 2 (o) in the middle panel) is about 6 μm.

[0027] The above results show that modifying the SSZ-13 zeolite membrane with PDMS will not damage the CHA crystal form of the zeolite membrane, and low concentration of PDMS fails to modify the SSZ-13 zeolite membrane.

[0028] 2. Preparation of 6FDA-APAF 0.5 -BIA 0.5 Individually modified SSZ-13 molecular sieve membrane 1) Preparation of dense SSZ-13 molecular sieve membrane: the method is the same as above.

[0029] 2) Prepare 6FDA-APAF with a mass fraction of 10% 0.5 -BIA 0.5 The solution can be found in the prior art literature (LI Y, WANG L, WANG XR, et al., Asymmetric copolyimide membranes fabricated by nonsolvent-induced phase separation for He / CH 4 and He / N 2 separation, Frontiersof Chemical Science and Engineering, 2024, 18, 44.): A certain amount of 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF, >98%) and 5-amino-2-(4-aminophenyl)benzimidazole (BIA, 99%) were dissolved in 20 ml of N-methylpyrrolidone (NMP) in a four-necked round-bottom flask, and then 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 98%) monomer was added. The molar ratio was controlled at 6FDA:APAF:BIA =1:0.5:0.5. The mixture was mechanically stirred under a flowing nitrogen atmosphere. The polymerization temperature was maintained at 278 K by an ice bath for 12 hours. The temperature was then raised to 453 K, and the reaction was carried out for 8 hours using a Dean-Stark reflux apparatus, in which 10 mL of o-xylene was used as an azeotropic agent to remove water. The resulting polymer was collected and washed with an aqueous 25% methanol solution and dried in a vacuum oven at 180 °C. The obtained 6FDA-APAF 0.5 -BIA 0.5 Dissolved in N-methylpyrrolidone solvent (mass ratio 1:9) to obtain 6FDA-APAF with a mass fraction of 10% 0.5 -BIA 0. The solution was poured into a reagent bottle and stirred vigorously at 600 rpm for 12 h to make the solution uniform, filtered, and subjected to ultrasonic defoaming treatment for standby use.

[0030] 3) Immerse the SSZ-13 molecular sieve membrane and hollow fiber carrier sealed with raw tape at both ends into the 6FDA-APAF 0.5 -BIA 0.5 The solution was immersed in water for 15 s at a rate of 50 mm / min for immersion and withdrawal. After removal, the solution was heated to dryness at a rate of 1°C / min. After drying, the temperature was lowered to 303 K at a rate of 1°C / min.

[0031] In order to determine whether the polymer has entered the molecular sieve pores, thermogravimetric analysis was performed on the SSZ-13 molecular sieve before and after polymer modification. The results are as follows: Figure 4 As shown. Figure 4 When the thermogravimetric test temperature is between 30 and 200 °C, the weight loss of SSZ-13 molecular sieve is mainly caused by the desorption of water and the residual solvent that is not completely removed from the molecular sieve; when it is between 200 and 500 °C, the weight loss is caused by the CO2 reaction between NMP and polymer in the SSZ-13 molecular sieve cage during the thermal rearrangement. 2 After 500℃, the weight loss is caused by the gradual decomposition of the polymer backbone. 0.5 -BIA 0.5 When modifying the SSZ-13 molecular sieve membrane, the polymer and the solvent N-methylpyrrolidone will block the molecular sieve pores and reduce the effective pore size of the molecular sieve.

[0032] Based on the above research, we continued to study 10 wt.% 6FDA-APAF 0.5 -BIA 0.5 The effect of the solution on the modification of SSZ-13 molecular sieve membrane and carrier, and the effect of 10 wt.% 6FDA-APAF 0.5 -BIA 0.5 He / CH 4 Separation performance was tested (n He :n CH4 =1:1), and the results are shown in Table 1.

[0033] Table 1, 6FDA-APAF 0.5 -BIA 0.5 Effect of Modification on the Performance of SSZ-13 Molecular Sieve Membrane

[0034] a: A polymer film is coated on the SSZ-13 molecular sieve membrane or carrier, and the polymer 6FDA-APAF 0.5 -BIA 0.5 The concentration is 10 wt.%, and 4 replicates are set to prepare membranes M1-M4, b: ×10 -8 mol·m -2 ·s -1 ·Pa -1 ; c:×10 -9 mol·m -2 ·s -1 ·Pa -1 , test conditions: 0.2 MPaG, n He :n CH4 =1:1, feed flow rate: 200 mL min -1 , 50 mL·min -1 Purge with argon gas, test temperature: 298 K.

[0035] In addition, the effects of different mass fractions of PDMS solution on the modification of SSZ-13 zeolite membrane and carrier were investigated, and the He / CH 4 Separation performance (n He :n CH4 =1:1), and the results are shown in Table 2.

[0036] Table 2. Effect of different concentrations of PDMS modification on the performance of SSZ-13 molecular sieve membrane

[0037] a: A layer of polymer PDMS membrane is coated on the SSZ-13 molecular sieve membrane or carrier. The concentration of PDMS solution is wt.%. When the concentration of PDMS solution is 16 wt.%, the immersion time is 40 s, and the immersion time of other concentrations is 15 min. b: ×10 -8 mol·m -2 ·s -1 ·Pa -1 ; c:×10 -9 mol·m -2 ·s -1 ·Pa -1 , test conditions: 0.2 MPaG, n He :n CH4 =1:1, feed flow rate: 200 mL min -1 , 50 mL·min -1 Purge with argon gas, test temperature: 298 K.

[0038] From the data in Table 1 and Table 2, it can be seen that the use of polymer 6FDA-APAF alone 0.5 -BIA 0.5 Or PDMS modified SSZ-13 molecular sieve membrane He / CH 4 The separation performance is only slightly improved. As can be seen from Table 2, the modification effect is best when the PDMS concentration is 5wt.%.

[0039] In order to improve the helium concentration efficiency of SSZ-13 molecular sieve membrane under high pressure, based on the above research, 10 wt.% polymer 6FDA-APAF was coated on the SSZ-13 molecular sieve membrane in the subsequent examples. 0.5 -BIA 0.5 After the solution was prepared, a layer of PDMS (concentration of 5 wt.%) was coated on it to prepare a composite SSZ-13 molecular sieve membrane. The preparation process is shown in Figure 5 ,from Figure 5It can also be seen that the composite SSZ-13 molecular sieve membrane has excellent mechanical stability compared to the polymer membrane, which can prevent it from breaking under high pressure. Example 1

[0040] 1) Preparation of dense SSZ-13 molecular sieve membrane: the method is the same as above.

[0041] 2) Prepare 6FDA-APAF with a mass fraction of 10 wt.% 0.5 -BIA 0.5 Solution: Same method as above.

[0042] 3) Preparation of polydimethylsiloxane (PDMS) solution: Weigh polydimethylsiloxane, tetramethyltetravinylcyclotetrasiloxane and n-heptane in a mass ratio of 1:0.1:19 to prepare a 5 wt.% PDMS n-heptane solution. Pour the prepared solution into a reagent bottle, stir vigorously at 600 rpm for 12 h to make the solution uniform, filter, and ultrasonically defoam for standby use.

[0043] 4) Coating 6FDA-APAF on SSZ-13 molecular sieve membrane using the pull-up impregnation method 0.5 -BIA 0.5 and PDMS to prepare composite SSZ-13 molecular sieve membrane: S1. First, immerse the SSZ-13 molecular sieve membrane and hollow fiber carrier sealed with raw tape at both ends into the 6FDA-APAF prepared in step 2). 0.5 -BIA 0.5 The solution was immersed in water for 15 s, and the immersion and withdrawal rates were both 50 mm / min. After being taken out, it was dried in a 60℃ oven for 1 h, placed in a vacuum oven and heated to 200℃ at a rate of 1℃ / min to completely remove the solvent, and then cooled to 303 K at a rate of 1℃ / min to successfully coat 6FDA-APAF on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 ; S2, continue to coat with 6FDA-APAF 0.5 -BIA 0.5 The SSZ-13 molecular sieve membrane and hollow fiber carrier were immersed in a 5wt.% PDMS solution for 1-30 min with an immersion and withdrawal rate of 2 cm / s. After drying at room temperature for 1 h, they were placed in an oven with a heating rate of 1°C / min. After drying at 100°C for 1 h, the temperature was lowered to 333 K at a rate of 1°C / min to successfully coat PDMS on the SSZ-13 molecular sieve membrane, completing the preparation of the composite membrane modified by the two polymers.

[0044] The effect of different PDMS solution immersion time on the He / CH 4 The results of separation performance are shown in Table 3.

[0045] Table 3. Effect of different PDMS immersion time on the performance of SSZ-13 molecular sieve membrane

[0046] a:min, first coat a layer of 6FDA-APAF on the SSZ-13 molecular sieve membrane or carrier 0.5 -BIA 0.5 The film was then coated with a polymer PDMS film, and the concentration of the PDMS solution was 5 wt.%, b:×10 -8 mol·m -2 ·s -1 ·Pa -1 ; c:×10 - 9 mol·m -2 ·s -1 ·Pa -1 , test conditions: 0.2 MPaG, n He :n CH4 =1:1, feed flow rate: 200 mL min -1 , 50 mL·min -1 Purge with argon gas, test temperature: 298 K.

[0047] From Table 3, it can be seen that the He / CH 4 The selectivity is greatly improved, especially at 15 min PDMS immersion time, the He / CH 4 The selectivity was increased from 14.1 to 80.9, and the modification effect was significant. Then the membrane M12 was used as the test object for the corresponding performance test.

[0048] 1. Test the single-component gas permeability of the composite SSZ-13 molecular sieve membrane M12, feed at 50 mL / min, add 20 mL / min argon purge (SF 6 Using helium purge), the results are as follows Figure 6 As shown in the figure, as the molecular dynamics diameter increases, the permeability of all gases decreases monotonically ( Figure 6 (a) inset), indicating that the membrane has a good molecular sieving effect. At 0.2 MPaG, the He / H 2 、He / N 2 、He / CH 4、He / C 2 H 6 With He / SF 6 The ideal selectivities are 1.1, 33, 101, 110 and 6430, respectively, all of which are much higher than the Knudsen diffusion selectivity ( Figure 6 (b) Inset).

[0049] 2. The effect of feed pressure on the separation performance of unmodified SSZ-13 zeolite membrane and composite SSZ-13 zeolite membrane M12 was investigated. The feed flow rate was 200 mL / min and 50 mL / min argon was added for purging. Figure 7 , as shown in the figure, compared with the unmodified SSZ-13 molecular sieve membrane, M12 has a lower He / CH 4 Gas separation selectivity a He / CH4 The ratio of He / CH increased from 15 to 89, an increase of 493%. Especially at 5 MPa feed pressure, the ratio of He / CH 4 Gas separation selectivity a He / CH4 From 4 to 57, an increase of 1325% ( Figure 7 (a) Inset); He recovery rate of M12 at 5 MPa R He Up to 11.5% ( Figure 7 (b) inset).

[0050] 3. Performance test of natural gas helium extraction using composite SSZ-13 molecular sieve membrane M12 The M12 was simulated with gas (helium concentration was 0.097%, containing a certain concentration of CO 2 , C 2 H 6 ) Helium extraction test (200 mL / min feed gas, 20 mL / min argon purge), the results are shown in Figure 8 .

[0051] from Figure 8 As the feed pressure increases, the He permeability of M12 P He The He / CH 4 Gas separation selectivity a He / CH4 From 89 to 60, it only decreased by 33% ( Figure 8 (a) inset), He recovery rate of M12 R He Gradually increased from 1% to 8.6% ( Figure 8 (b) inset).

[0052] 4. Performance test of natural gas helium extraction using composite SSZ-13 molecular sieve membrane At 5 MPa, the feed flow rate (simulated gas helium concentration of 0.097%, containing a certain concentration of CO 2 , C 2 H 6 ) Effect of He / CH 4 The results are shown in Table 1. Fig. 9 As shown in Figure 2, the He gas permeability increases with the increase of feed flow rate. P He It is on the rise, from 3.27 × 10 -9 mol·m -2 ·s -1 ·Pa -1 Increased to 3.75 × 10 -9 mol·m -2 ·s -1 ·Pa -1 , He / CH 4 Gas separation selectivity a He / CH4 Increased from 54 to 67 ( Fig. 9 (a) inset). When the feed flow rate is increased from 10 mL min -1 Increased to 200 mL·min -1 When the permeate side He concentration multiple β He It is on an upward trend, from 22 to 47 ( Fig. 9 (b) In the middle panel). When the feed flow rate is 10 mL min -1 When the He recovery rate on the permeate side is R He Increased to 80%.

[0053] 5. Stability test of natural gas helium extraction performance of composite SSZ-13 molecular sieve membrane In the presence of CO 2 , C 2 H 6 The stability of the modified SSZ-13 molecular sieve membrane M12 in separating He and its effect on the concentration of helium under high pressure (5 MPa) were tested in a simulated natural gas. After a test of 96 hours, the membrane M12 remained stable, the structure was not damaged, and the He gas permeability was P He Maintained at 3.7 × 10 -9 mol·m -2 ·s -1 ·Pa -1 , He / CH 4 Gas separation selectivitya He / CH4 is 60 ( Fig.10 (a) inset). Long-term exposure to CO 2 , C 2 H 6 Test of impurities in natural gas simulation gas, He recovery rate R He Still maintained at 8.1%, He enrichment multiple β He Up to 43 ( Fig.10 (b) inset).

[0054] Application Examples This application example proposes a three-stage natural gas helium extraction device based on the above-mentioned composite SSZ-13 molecular sieve membrane, which combines the zeolite molecular sieve membrane separation and selection process with the natural gas liquefaction process, and successively separates the high-pressure pipeline natural gas and the non-condensable gas of the natural gas liquefaction unit to obtain high-purity helium.

[0055] Specifically, the device structure refers to Fig.11 The three-stage natural gas helium extraction device comprises a composite SSZ-13 molecular sieve membrane separation unit 2, a natural gas liquefaction unit 3 and a SSZ-13 molecular sieve membrane separation unit 5 which are connected in sequence.

[0056] The composite SSZ-13 molecular sieve membrane separation unit 2 is a first-stage membrane separation component constructed based on the composite SSZ-13 molecular sieve membrane (preferably M12) prepared in Example 1, and inputs high-pressure pipeline natural gas 1 (helium content is 0.02-0.3 mol%) from its inlet end, and is used to separate helium from the high-pressure pipeline natural gas 1. The gas pressure on the feed side is 3-10 MPa, and the permeate side is slightly positive pressure, and can also be vacuumed to increase the driving force. The permeate side obtains highly concentrated helium-rich natural gas, and the residual gas returns to the natural gas pipeline for helium extraction again.

[0057] The feed side of the natural gas liquefaction unit 3 is connected to the permeation side of the composite SSZ-13 molecular sieve membrane separation unit 2, and is used to liquefy the helium-rich natural gas. The obtained liquefied gas is directly output, and the non-condensable gas (BOG) is introduced into the SSZ-13 molecular sieve membrane separation unit 5.

[0058] Before the BOG is introduced into the SSZ-13 molecular sieve membrane separation unit 5, the non-condensable gas can be pressurized by using the pressurizing device 4.

[0059] The SSZ-13 molecular sieve membrane separation unit 5 is a 1-3 level membrane separation component constructed based on the SSZ-13 molecular sieve membrane prepared as described above, and is used for multi-stage extraction of helium from pressurized non-condensable gas. The gas pressure on the feed side is 0.5-3.0 MPa, and the permeate side is slightly positively pressurized. It can also be vacuumed to increase the driving force. Crude helium 6 is obtained on the permeate side, and the residual gas of the unit is returned to the natural gas liquefaction unit 3 for liquefaction treatment.

[0060] The specific process of extracting helium from natural gas based on the above device is as follows: pipeline natural gas has the characteristics of high pressure and helium deficiency, and is sent to the highly selective composite SSZ-13 molecular sieve membrane unit for preliminary helium extraction. The gas on the retentate side returns to the natural gas pipeline, and the helium in the gas on the permeate side is enriched (the helium concentration can be increased from the initial 0.075% to 4.4%). The helium-rich natural gas is passed into the natural gas liquefaction unit to obtain liquefied natural gas. The generated non-condensable gas BOG is compressed by the booster device and then purified again by SSZ-13 molecular sieve membrane separation unit at levels 1-3 to obtain crude helium.

[0061] When using the first-stage SSZ-13 molecular sieve membrane to extract helium from non-condensable gas, 2% helium-containing natural gas is used as the feed gas, and the feed flow rate is 0.75 L / min. The helium extraction performance of the first-stage membrane assembly under different feed pressures is tested. The results are as follows: Fig.12 As shown. At 0.2 MPa, He / CH 4 The selectivity is 14, the concentration factor is 10, and the helium recovery rate is 26.7%. The primary membrane separation can increase the helium concentration from 2% to 20%. With the increase of feed pressure, He / CH 4 The selectivity and concentration ratio showed a clear downward trend, but the helium recovery rate increased significantly. At 5.1 MPa, the helium recovery rate increased significantly to 83%, but He / CH 4 The selectivity dropped from 14 to 4, and the helium enrichment factor decreased from 10 to 2.

[0062] When using a two-stage SSZ-13 molecular sieve membrane to extract helium from non-condensable gas, when the helium content in the raw natural gas is 15% and the feed flow rate is 0.3 L / min, the helium extraction performance of the two-stage membrane assembly under different feed pressures is tested. The results are as follows: Fig.13 At 3.0 MPa, the helium concentration ratio is reduced to 1.4 times, He / CH 4 The selectivity dropped to 3 and the helium recovery rate was 95%.

Claims

1. A method for preparing a dual polymer modified composite SSZ-13 molecular sieve membrane, characterized in that: The steps include: 1) Polymer 6FDA-APAF 0.5 -BIA 0.5 Dissolve in solvent, mix, filter, defoam, and obtain 6FDA-APAF 0.5 -BIA 0.5 Solution; 2) Mix PDMS, curing agent and n-heptane, mix well, filter, defoam, and prepare PDMS solution; 3) First coat the SSZ-13 molecular sieve membrane and carrier with the 6FDA-APAF prepared in step 1) 0.5 -BIA 0.5 Solution; 4) Covered with 6FDA-APAF 0.5 -BIA 0.5 The zeolite membrane and the carrier are coated with PDMS solution to obtain a double polymer modified composite SSZ-13 zeolite membrane.

2. The method for preparing a dual polymer modified composite SSZ-13 molecular sieve membrane according to claim 1, characterized in that: In step 1), the solvent is N-methylpyrrolidone, and the mixture is stirred at a speed of 600 rpm-800 rpm and a stirring time of 6-12 h; the prepared 6FDA-APAF 0.5 -BIA 0.5 6FDA-APAF in solution 0.5 -BIA 0.5 The mass percentage is 1-30 wt.%; In step 2), the mass percentage of PDMS in the prepared PDMS solution is 0.5-25 wt.%, and the solution is stirred and mixed at a speed of 600 rpm-800 rpm and a stirring time of 6-12 h.

3. The method for preparing a dual polymer modified composite SSZ-13 molecular sieve membrane according to claim 1, characterized in that: The coating process in steps 3) and 4) is completed by methods including pull-up dipping, spraying, brushing, spin coating, and vacuum coating.

4. The method for preparing a dual polymer modified composite SSZ-13 molecular sieve membrane according to claim 3, characterized in that: Coating 6FDA-APAF on SSZ-13 molecular sieve membrane using the Czochralski impregnation method 0.5 -BIA 0.5 and PDMS, the specific process is as follows: S1. First, immerse the SSZ-13 molecular sieve membrane and carrier with both ends sealed into the prepared 6FDA-APAF 0.5 -BIA 0.5 The solution was immersed in water for 15-20 s, and the immersion and withdrawal rates were both 40-60 mm / min. After being taken out, it was dried in a 60-70 °C oven for 1-2 h, and then the temperature was raised to 180-200 °C at a rate of 1-5 °C / min in a vacuum oven to completely remove the solvent, and then the temperature was lowered to 303 K at a rate of 1-5 °C / min to coat 6FDA-APAF on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 ; S2, coated with 6FDA-APAF 0.5 -BIA 0.5 The SSZ-13 molecular sieve membrane and the carrier are immersed in the PDMS solution for 1-30 minutes at a rate of 1-3 cm / s. After drying at room temperature for 1-3 hours, they are placed in an oven with a heating rate of 1-5°C / min. After drying at 90-110°C for 1-3 hours, the temperature is lowered to 333 K at a rate of 1-3°C / min to coat the SSZ-13 molecular sieve membrane with PDMS.

5. The method for preparing a dual polymer modified composite SSZ-13 molecular sieve membrane according to claim 4, characterized in that: The 6FDA-APAF 0.5 -BIA 0.5 6FDA-APAF in solution 0.5 -BIA 0.5 The mass percentage of is 10 wt.%, the mass percentage of PDMS in the PDMS solution is 5 wt.%; and the immersion time in the PDMS solution is 5~25 min.

6. A dual polymer modified composite SSZ-13 molecular sieve membrane, characterized in that: It is prepared based on the preparation method described in any one of claims 1 to 5, and 6FDA-APAF is modified on the SSZ-13 molecular sieve membrane. 0.5 -BIA 0.5 Membrane layer and PDMS membrane layer.

7. The use of the double polymer modified composite SSZ-13 molecular sieve membrane in gas separation as claimed in claim 6, characterized in that: The composite SSZ-13 molecular sieve membrane is used for separating He / CH4 mixed gas.

8. The use according to claim 7, characterized in that The He / CH4 mixed gas is a simulated natural gas component, containing 0.097% helium and a certain concentration of carbon dioxide and ethane. The feed pressure in the gas separation process is 0.1~10 MPa and the feed flow rate is 1~300 mL•min -1 .

9. A three-stage natural gas helium extraction device, characterized in that: It is constructed based on the double polymer modified composite SSZ-13 molecular sieve membrane according to claim 6, comprising: Composite SSZ-13 molecular sieve membrane separation unit, a first-stage membrane separation component based on composite SSZ-13 molecular sieve membrane, used to separate helium from high-pressure pipeline natural gas; The natural gas liquefaction unit, the feed side of which is connected to the permeate side of the composite SSZ-13 molecular sieve membrane separation unit, is used to liquefy the helium-rich natural gas; SSZ-13 molecular sieve membrane separation unit, a 1-3 stage membrane separation component constructed based on SSZ-13 molecular sieve membrane, has its feed side connected to the gas outlet of the natural gas liquefaction unit and is used for multi-stage extraction of helium from non-condensable gas to obtain crude helium.

10. The method for extracting helium from natural gas using the three-stage natural gas helium extraction device according to claim 9, characterized in that: The steps include: 1) The high-pressure pipeline natural gas is passed into the composite SSZ-13 molecular sieve membrane separation unit to carry out in-situ helium concentration of the high-pressure pipeline natural gas, and the helium-rich natural gas is output on the permeate side, and the residual gas is returned to the natural gas pipeline; 2) The helium-rich natural gas is liquefied by the natural gas liquefaction unit, the liquefied gas is directly output, and the non-condensable gas obtained is passed into the SSZ-13 molecular sieve membrane separation unit; 3) Utilize the SSZ-13 molecular sieve membrane separation unit to perform multi-stage extraction of helium from non-condensable gas, obtain crude helium on the permeate side, and return the retentate gas to the natural gas liquefaction unit.

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