Method and system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation

Abstract

The present invention relates to the technical field of helium purification, and specifically to a method and system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation. The method provided by the present invention comprises: subjecting the raw gas to cryogenic separation, first-stage polymer membrane separation, palladium membrane separation, and second-stage polymer membrane separation in sequence to obtain ultrapure helium. The present invention specifically and effectively combines processes such as cryogenic separation, polymer membrane separation (one-stage membrane separation and two-stage membrane separation), and palladium membrane dehydrogenation, and uses natural gas, shale gas, multi-stage flash gas, etc. as raw gas. Among them, the core and key is the flexible two-stage polymer membrane separation and the intermediate combination of palladium (alloy) membrane separation technology to prepare high-purity helium (above 5N grade), and the conditions of each process are stable and controllable, and it has a very broad application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of helium purification, and in particular to a method and system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation. Background Art

[0002] Helium, due to its low density, low boiling point, and inert properties, is widely used in aerospace, refrigeration, medical, fiber optics, leak detection, deep-sea diving, and high-precision welding production. While high-purity helium offers greater application value and promising prospects, the corresponding separation and production technologies and processes are more complex, resulting in poor economic efficiency and low production yields. Currently, the more commonly used technologies include cryogenics and polymer separation membranes. Cryogenics utilizes the low boiling point of helium compared to other gaseous components to separate gases such as nitrogen, methane, and carbon dioxide from helium. Polymer separation membranes utilize the membrane's selective permeability to separate helium. However, when hydrogen, with its extremely low boiling point and small size, is present in the gas, conventional cryogenics and membrane separation technologies struggle to achieve efficient separation. Efficient dehydrogenation is a key step in the production of ultrapure helium. Most helium refineries utilize catalytic dehydrogenation, but its efficiency and energy consumption are primarily limited by the high cost of the catalysts, which are susceptible to aging and poisoning. Furthermore, the reaction is associated with certain risks, and its effectiveness is limited for removing trace amounts of hydrogen. How to economically and efficiently extract ultrapure helium has not yet been effectively solved. Summary of the Invention

[0003] The purpose of the present invention is to overcome the technical problems existing in the prior art and to provide a method and system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation.

[0004] The first aspect of the present invention provides a method for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation. The method comprises: subjecting the raw gas to cryogenic separation, first-stage polymer membrane separation, palladium membrane separation, and second-stage polymer membrane separation in sequence to obtain ultrapure helium.

[0005] The second aspect of the present invention provides a system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation, characterized in that the system comprises a cryogenic separation unit, a first polymer membrane separation unit, a palladium membrane separation unit and a second polymer membrane separation unit connected in sequence.

[0006] Compared with the prior art, the present invention has the following beneficial effects:

[0007] The present invention effectively combines cryogenic separation, polymer membrane separation (one-stage membrane separation and two-stage membrane separation), palladium membrane dehydrogenation and other processes, and uses natural gas, shale gas, multi-stage flash gas, etc. as raw gas to prepare high-purity helium (5N grade or above), and the conditions of each process are stable and controllable. Among them, the cryogenic process can effectively increase the concentration of hydrogen and helium, and the polymer membrane separation process has the characteristics of compact equipment, low energy consumption, and good separation effect, which is a further purification of cryogenic separation gas. Compared with conventional catalytic dehydrogenation, the palladium membrane dehydrogenation technology used in this process can be carried out under mild chemical conditions, does not involve redox reaction, and the operating conditions are more stable and controllable. The core and key of the process of the present invention is the flexible two-stage polymer membrane separation combined with palladium (alloy) membrane separation technology in the middle, which can efficiently purify and refine helium to above 5N level, reducing the energy consumption and equipment requirements of the helium extraction process. By combining the advantages of various process technologies, the helium preparation process is simple, economical, continuous and stable, solving the problem of ultra-pure helium preparation, and can achieve efficient utilization of helium resources including natural gas, oilfield associated gas, chemical waste gas, etc. The prepared helium is of very high value and has very broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is an infrared spectrum of the polyimide random copolymer prepared in Preparation Example 1;

[0009] Figure 2 is a scanning electron micrograph of a cross section of the polyimide-based hollow fiber membrane prepared in Preparation Example 1; DETAILED DESCRIPTION

[0010] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0011] The first aspect of the present invention provides a method for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation. The method comprises: subjecting the raw gas to cryogenic separation, first-stage polymer membrane separation, palladium membrane separation, and second-stage polymer membrane separation in sequence to obtain ultrapure helium.

[0012] According to some embodiments of the present invention, the feed gas may be selected from at least one of natural gas, shale gas, helium-rich hydrogen-containing gas, and liquefied natural gas flash gas (BOG).

[0013] In order to obtain better results, the present invention performs cryogenic separation on the raw gas before the first polymer membrane separation, wherein the conditions of the cryogenic separation may include: a temperature of -250°C to -50°C, preferably -220°C to -100°C; a pressure of 0.1MPa to 20MPa, preferably 0.1MPa to 10MPa.

[0014] In the present invention, in order to obtain better effects, before the cryogenic separation, the pressure of the feed gas is controlled below 50°C, preferably 10-35°C, and the pressure is controlled to 0.01-10 MPa.

[0015] According to some embodiments of the present invention, the polymers used in the first polymer membrane separation and the second polymer membrane separation are each independently selected from at least one of a hollow fiber membrane, a flat sheet membrane and a tubular membrane.

[0016] In the present invention, there is no particular limitation on the polymer membranes used for the first-stage polymer membrane separation and the second-stage polymer membrane separation. As long as they can meet the requirements of the present invention, they can be commercially available or prepared by conventional methods in the art such as phase separation, solution phase separation, melt stretching, interfacial polymerization, coating polymerization, and in situ polymerization.

[0017] According to some embodiments of the present invention, the materials of the polymer membranes used in the first stage polymer membrane separation and the second stage polymer membrane separation are the same or different, and can be independently selected from at least one of polyimide, polybenzimidazole, polycellulose acetate and polysulfone, more preferably polyimide.

[0018] According to some embodiments of the present invention, the first stage polymer membrane separation adopts a one-stage or multi-stage (two to five-stage) separation method. Among them, two-stage membrane separation refers to the polymer membrane separation in which the permeate side gas is pressurized and then used as the inlet gas of the membrane again. Tertiary membrane separation, four-stage membrane separation and five-stage membrane separation have similar meanings. Among them, the process of polymer membrane separation can be one to five stages. Preferably, multi-stage membrane separation is separation through multiple membranes or membrane components. Among them, the first-stage membrane separation, the second-stage membrane separation, the third-stage membrane separation, the fourth-stage membrane separation and the fifth-stage membrane separation are respectively carried out in the first-stage membrane separation unit, the second-stage membrane separation unit, the third-stage membrane separation unit, the third-stage membrane separation unit and the fourth-stage membrane separation unit.

[0019] According to some embodiments of the present invention, the conditions for the first stage polymer membrane separation may include: before performing the first stage polymer membrane separation, controlling the gas pressure obtained by cryogenic separation to 0.01-10 MPa, preferably 0.1-8 MPa, and controlling the temperature to less than 50°C, preferably 0-25°C.

[0020] In the present invention, according to a preferred embodiment, the gas obtained from cryogenic separation enters the first polymer membrane separation stage, wherein the permeate gas of the first membrane (first-stage membrane separation unit) is pressurized to 0.01-5 MPa by a 1^ compressor and enters the next-stage membrane after heat exchange in a heat exchanger, and the retentate gas enters the inlet of the first membrane or is discharged and collected;

[0021] The permeate gas of the secondary membrane (second-stage membrane separation unit) is pressurized to 0.01-5Mpa by a 1^^ compressor and then enters the tertiary membrane or directly enters the next unit after heat exchange. The residual gas of the secondary membrane enters the inlet of the secondary membrane or the primary membrane or is discharged for collection;

[0022] The permeate gas of the tertiary membrane (third-stage membrane separation unit) is pressurized to 0.01-5Mpa by a 1^^^ compressor and then enters the quaternary membrane or directly enters the next unit after heat exchange. The residual gas of the tertiary membrane enters the inlet of the first or second membrane or is discharged for collection;

[0023] The permeate gas of the quaternary membrane (fourth-stage membrane separation unit) is pressurized to 0.01-5Mpa by a 1^^^^ compressor and then enters the fifth-stage membrane or directly enters the next unit after heat exchange. The residual gas of the quaternary membrane enters the inlet of the first, second or third membrane or is discharged for collection;

[0024] The permeate gas of the fifth-stage membrane (fifth-stage membrane separation unit) is pressurized to 0.01-5Mpa by a 1^^^^^ compressor and then enters the fourth-stage membrane or directly enters the next unit after heat exchange. The residual gas of the third-stage membrane enters the inlet of the first, second, third or fourth membrane or is discharged for collection.

[0025] Among them, "1^^" stands for "one-stage membrane and two-stage", "1" is a stage, and "^^" is a level; "1^^^", "1^^^^" and "1^^^^^" have similar meanings.

[0026] According to some embodiments of the present invention, the polymer membranes used in the first stage polymer membrane separation and the second stage polymer membrane separation are polyimide-based hollow fiber membranes.

[0027] Preferably, the polyimide-based hollow fiber membrane comprises a support layer and a dense layer attached to the outer surface of the support layer, the thickness of the dense layer is less than 1000 nm, and the porosity of the hollow fiber membrane is 40-80%.

[0028] Preferably, the thickness of the dense layer is 100-500 nm, and the porosity of the hollow fiber membrane is 50-70%.

[0029] Preferably, the hollow fiber membrane is made of polyimide random copolymer.

[0030] According to some embodiments of the present invention, the polyimide copolymer has a structure shown in formula (I):

[0031]

[0032] In formula (I), m and n are each independently an integer of 10-2000;

[0033] X has a structure represented by any one of formula (X1) to formula (X3);

[0034]

[0035] In formula (X1) to formula (X3), R 1 、R 2 、R 3 、R 4 、R 5 and R 6 Each is independently H, C1-C4 alkyl, C6-C10 aryl, amino, hydroxyl or carboxyl;

[0036] Y has a structure represented by any one of formulas (Y1) to (Y5);

[0037]

[0038] In formula (Y1) to formula (Y5), R 7 、R 8 、R 9 、R 10 、R 11 、R 12 、R 13 、R 14 、R 15 and R 16 Each is independently H, C1-C4 alkyl, C6-C10 aryl, amino, hydroxyl or carboxyl;

[0039] Z and Z' each independently have a structure represented by formula (Z1) or formula (Z2);

[0040]

[0041] In formula (Z2), Ra and Rb are each independently H, a C1-C4 alkyl group, or a C1-C4 haloalkyl group.

[0042] Preferably, m and n are each independently an integer of 50-1000.

[0043] Preferably, 0.9≥n / (m+n)≥0.3, preferably, 0.7≥n / (m+n)≥0.5.

[0044] In the present invention, X has one of the following structures:

[0045]

[0046] In the present invention, Y has one of the following structures:

[0047]

[0048]

[0049] In the present invention, Z and Z' both have the structure shown by Z1 or Z3,

[0050]

[0051] Preferably, X is Xa, Y is Ya, and Z and Z' are both Z1;

[0052] Alternatively, X is Xa, Y is Yb, and Z and Z' are both Z1;

[0053] Alternatively, X is Xa, Y is Yd, and Z and Z' are both Z1;

[0054] Alternatively, X is Xb, Y is Ya, and Z and Z' are both Z1;

[0055] Alternatively, X is Xb, Y is Yb, and Z and Z' are both Z1;

[0056] Alternatively, X is Xb, Y is Yd, and Z and Z' are both Z1;

[0057] Alternatively, X is Xc, Y is Ya, and Z and Z' are both Z1;

[0058] Alternatively, X is Xc, Y is Yb, and Z and Z' are both Z1;

[0059] Alternatively, X is Xc, Y is Yc, and Z and Z' are both Z1;

[0060] Alternatively, X is Xc, Y is Y4, and Z and Z' are both Z1;

[0061] Alternatively, X is Xc, Y is Yd, and Z and Z' are both Z1;

[0062] Alternatively, X is Xb, Y is Ya, and Z and Z' are both Z3;

[0063] Alternatively, X is Xb, Y is Yb, and Z and Z' are both Z3;

[0064] Alternatively, X is Xb, Y is Yd, and Z and Z' are both Z3;

[0065] Alternatively, X is Xc, Y is Ya, and Z and Z' are both Z3;

[0066] Alternatively, X is Xc, Y is Yb, and Z and Z' are both Z3;

[0067] Alternatively, X is Xc, Y is Yd, and Z and Z' are both Z3.

[0068] Although according to a preferred embodiment of the present invention, X, Y, and Z have a specific structure, the present invention does not exclude the situation where "X is taken from two or three different structures, Y is taken from two, three, four or five different structures, and Z is taken from two different structures."

[0069] In the present invention, based on first dianhydride monomer (dianhydride shown in the formula (II) and the dianhydride shown in the formula (III)) and diamine monomer being carried out polycondensation reaction and obtaining polyamic acid, then polyamic acid is carried out to the principle of imidization (intramolecular dehydration), dianhydride monomer and diamine monomer can be carried out polycondensation reaction and obtain polyamic acid by one pot process, also can by first dianhydride monomer (also being the dianhydride shown in the formula (II) and the dianhydride shown in the formula (III)) after mixing, then carry out polycondensation reaction with diamine monomer.But, for better controlling the carrying out of reaction, preferably react in the latter mode.Therefore, the present invention also provides a kind of method for preparing polyimide random copolymer, described method comprises the following steps:

[0070] (S1) in the presence of a first solvent, mixing a mixture of a dianhydride monomer represented by formula (II) and a dianhydride monomer represented by formula (III) with a diamine monomer, and performing a polycondensation reaction to obtain a material containing polyamic acid,

[0071]

[0072] (S2) imidizing the polyamic acid-containing material obtained in step (S1) to cause intramolecular dehydration of the polyamic acid to obtain a polyimide random copolymer;

[0073] In formula (II) and formula (III), X and Y have the same meanings as described above.

[0074] Wherein, the diamine monomer is selected from at least one compound having a structure such as H2N-Zp-NH2, wherein Zp has a structure shown in formula (Z1) or (Z2),

[0075]

[0076] In formula (Z2), Ra and Rb are each independently H, a C1-C4 alkyl group, or a C1-C4 haloalkyl group.

[0077] Preferably, X is Xa, Y is Ya, and Zp is Z1;

[0078] Alternatively, X is Xa, Y is Yb, and Zp is Z1;

[0079] Alternatively, X is Xa, Y is Yd, and Zp is Z1;

[0080] Alternatively, X is Xb, Y is Ya, and Zp is Z1;

[0081] Alternatively, X is Xb, Y is Yb, and Zp is Z1;

[0082] Alternatively, X is Xb, Y is Yd, and Zp is Z1;

[0083] Alternatively, X is Xc, Y is Ya, and Zp is Z1;

[0084] Alternatively, X is Xc, Y is Yb, and Zp is Z1;

[0085] Alternatively, X is Xc, Y is Yc, and Zp is Z1;

[0086] Alternatively, X is Xc, Y is Y4, and Zp is Z1;

[0087] Alternatively, X is Xc, Y is Yd, and Zp is Z1;

[0088] Alternatively, X is Xb, Y is Ya, and Zp is Z3;

[0089] Alternatively, X is Xb, Y is Yb, and Zp is Z3;

[0090] Alternatively, X is Xb, Y is Yd, and Zp is Z3;

[0091] Alternatively, X is Xc, Y is Ya, and Zp is Z3;

[0092] Alternatively, X is Xc, Y is Yb, and Zp is Z3;

[0093] Alternatively, X is Xc, Y is Yd, and Zp is Z3.

[0094] In the present invention, the molar amounts of the dianhydride monomer represented by formula (II) and the dianhydride monomer represented by formula (III) are defined as M and N, respectively, and the ratio of M to N is (10-2000):(10-2000), more preferably 1:(0.5-15), and further preferably 1:(1-9).

[0095] In the present invention, M and N satisfy 0.9≥N / (M+N)≥0.3, preferably, 0.7≥N / (M+N)≥0.5.

[0096] In the present invention, the molar ratio of the total molar amount of the dianhydride monomer represented by formula (II) and the dianhydride monomer represented by formula (III) to the diamine monomer is 1:(0.6-1.5), preferably 1:(0.8-1.2).

[0097] In the present invention, in step (S1), the polycondensation reaction conditions may include: reaction temperature of -20°C to 60°C, preferably -10°C to 40°C; reaction time of 5-30h, preferably 8-24h.

[0098] In the present invention, the polycondensation reaction is carried out under an inert atmosphere, preferably provided by nitrogen.

[0099] In the present invention, the first solvent can be selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone (NMP), preferably selected from N-methylpyrrolidone and / or N,N-dimethylformamide.

[0100] In the present invention, the amount of the first solvent used is 1000-3000 mL relative to 1 mmol of the diamine monomer.

[0101] In the present invention, the dianhydride monomer represented by formula (II) and the dianhydride monomer represented by formula (III) can be mixed to obtain a mixture by mechanical stirring, shaking, or ultrasound. The mechanical stirring conditions may include: 20-40° C., 2000-15000 rpm, for 2-12 hours; the ultrasound conditions may include: 20-40° C., 0.5-2.0 hours; and the shaking conditions may include: 20-40° C., 260-800 rpm, for 12-36 hours.

[0102] In the present invention, the imidization treatment is carried out by adding a dehydrating agent and a catalyst to the amic acid-containing material obtained in step (S1), and reacting at 170-200° C. for 12-24 hours.

[0103] In the present invention, the dehydrating agent is at least one selected from dichlorobenzene, toluene, acetic anhydride and xylene.

[0104] In the present invention, the catalyst is selected from pyridine and / or diquinoline.

[0105] In the present invention, the amount of the dehydrating agent used may be 2-15 mol, preferably 3-8 mol, relative to 1 mol of the diamine monomer.

[0106] In the present invention, the amount of the catalyst used may be 2-15 mol, preferably 3-8 mol, relative to 1 mol of the diamine monomer.

[0107] In the present invention, the method further comprises: before obtaining the polyimide copolymer, diluting the material after the imidization treatment in step (S2) and contacting it with a precipitant to obtain the polyimide copolymer. Wherein, the precipitant can be a poor solvent for polyimide, selected from at least one of ethanol, acetone and water, and more preferably selected from at least two of ethanol, acetone and water. The total amount of the precipitant can be 10-50L equivalent to 1mol of diamine monomer. Wherein, the solvent for dilution can be N-methylpyrrolidone. Preferably, the amount of the solvent for dilution can be 5-8L relative to 1mol of diamine monomer.

[0108] In the present invention, there is no particular limitation on the manner in which the imidized material is contacted with the precipitant in step (S2), as long as the requirements of the present invention are met. For example, the method can be as follows: the imidized material (after dilution) in step (S2) is added to the precipitant to precipitate the polyimide, and the precipitated polyimide is then rinsed with the precipitant (the precipitated polyimide can be rinsed 3-5 times), and finally filtered and dried (70-150° C., 24-48 hours) to obtain a polyimide random copolymer.

[0109] In the present invention, the polyimide-based hollow fiber membrane is prepared according to a method comprising the following steps:

[0110] (1) preparing a casting solution containing a polyimide, a diluent, and an additive, wherein the diluent contains a good solvent for the polyimide, a poor solvent for the first polyimide, and a poor solvent for the second polyimide, wherein the boiling point B1 of the poor solvent for the first polyimide is higher than the boiling point B2 of the poor solvent for the second polyimide;

[0111] (2) extruding the core liquid and the casting liquid at a temperature T, and then curing to obtain a hollow fiber membrane precursor, wherein B2≤T<B1;

[0112] (3) The hollow fiber membrane precursor is rolled up and extracted to obtain the polyimide-based hollow fiber membrane.

[0113] In the present invention, in step (1), based on the total weight of the casting solution, the content of the polyimide is 20-40wt%, the content of the diluent is 50-75wt%, and the content of the additive is 0.5-10wt%.

[0114] Preferably, based on the total weight of the casting solution, the content of the polyimide is 25-35 wt %, the content of the diluent is 60-70 wt %, and the content of the additive is 1-5 wt %.

[0115] In the present invention, in order to facilitate the formation of a dense layer of the hollow fiber membrane, the boiling point B1 of the first polyimide poor solvent is 5-200°C higher than the boiling point B2 of the second polyimide poor solvent, preferably 10-20°C higher. Unless otherwise specified, the boiling point refers to the boiling point at atmospheric pressure.

[0116] In the present invention, the poor solvent for the first polyimide is at least one selected from the group consisting of a C2-C4 saturated monohydric alcohol, γ-butyrolactone and water.

[0117] In the present invention, the poor solvent for the second polyimide is at least one selected from the group consisting of C3-C5 alkanes, tetrahydrofuran, acetone and chloroform.

[0118] In the present invention, the good solvent for polyimide is at least one selected from N-methylpyrrolidone, N,N-dimethylacetamide and N,N-dimethylacetamide.

[0119] In the present invention, in order to obtain better effects, the present invention has certain requirements on the usage ratio of the good solvent of the polyimide, the poor solvent of the first polyimide and the second polyimide. Preferably, the weight ratio of the good solvent of the polyimide, the poor solvent of the first polyimide and the poor solvent of the second polyimide is 1:(0.1-0.5):(0.1-0.5), preferably 1:(0.15-0.3):(0.15-0.3).

[0120] In the present invention, the additive may be a lithium salt, preferably selected from lithium nitrate and / or lithium chloride.

[0121] In the present invention, in step (1), the casting solution is prepared according to a method comprising the following steps: stirring polyimide, a diluent and an additive at 20-50°C and 100-1200 r / min for 12-48 hours, and then removing impurities by vacuum degassing and filtering (20-50°C).

[0122] In the present invention, the vacuum degassing conditions include: pressure of -0.1 MPa to -0.095 MPa, temperature of 20-30° C., rotation speed of 10-50 r / min, and time of 12-24 h.

[0123] In the present invention, in step (2), the inner core liquid includes solvent A and solvent B, wherein the solvent A is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide and N,N-dimethylacetamide, and the solvent B is selected from at least one of C1-C4 saturated monohydric alcohol, γ-butyrolactone and water.

[0124] In the present invention, the solvent A accounts for 50-99 wt %, preferably 60-95 wt %, of the total weight of the inner core liquid.

[0125] According to some embodiments of the present invention, the extrusion is performed in a spinneret, wherein the extrusion temperature (temperature of the spinneret) is 40-75°C, preferably 60-70°C.

[0126] In the present invention, during the extrusion process, the flow rate of the casting solution is 6-30 mL / min.

[0127] According to some embodiments of the present invention, during the extrusion process, the flow rate of the core liquid is 2-10 mL / min.

[0128] In the present invention, before solidification, the hollow fibers obtained by extrusion are passed through an air gap to promote the formation of a dense layer and better control the thickness of the dense layer.

[0129] In the present invention, the height of the air gap is 5-30 cm.

[0130] In the present invention, the air gap is heated by an annular sleeve, and the temperature is preferably controlled at 70-150°C.

[0131] In the present invention, the solidification is carried out in a coagulation bath. Preferably, the bath liquid used in the coagulation bath is solvent C and / or water, and the temperature of the coagulation bath is 40-70°C.

[0132] In the present invention, the solvent C is selected from at least one of a C1-C4 saturated monohydric alcohol, γ-butyrolactone and water.

[0133] In the present invention, in step (3), the winding rate is 0.5-2 m / s.

[0134] In the present invention, the purpose of the extraction is to remove the diluent and additives in the hollow fiber membrane precursor.

[0135] In the present invention, the extraction agent is selected from at least one of water, a C1-C4 saturated monohydric alcohol, and a C5-C7 alkane. The amount of the extraction agent is not particularly limited, as long as it can meet the requirements of the present invention.

[0136] In the present invention, the extraction conditions include: a temperature of 20-35° C. and a time of 3-48 hours. The extraction time refers to the time the membrane fibers (hollow fiber membrane precursors) are immersed.

[0137] In the present invention, preferably, the extraction method is preferably: extracting in water, C1-C4 saturated monohydric alcohol and C5-C7 alkane in sequence for 2-5 times respectively.

[0138] In the present invention, the extraction further includes a drying step.

[0139] In the present invention, the drying conditions include: temperature of 20-35° C. and time of 2-15 hours.

[0140] According to some embodiments of the present invention, the conditions for palladium membrane dehydrogenation separation include: before performing palladium membrane dehydrogenation separation, the temperature of the gas obtained by polymer membrane separation is controlled to 200-500°C (the temperature of palladium membrane dehydrogenation); and the gas pressure is controlled to 1-50MPa (the pressure of palladium membrane dehydrogenation).

[0141] According to some embodiments of the present invention, the thickness of the palladium membrane used in the palladium membrane dehydrogenation separation is 5-100 μm.

[0142] Preferably, the palladium membrane is a tubular membrane or a porous support composite membrane.

[0143] Preferably, the porous carrier in the porous carrier composite membrane is selected from one of porous ceramics, porous Vickers glass and porous stainless steel.

[0144] Preferably, the palladium film is a pure palladium film or a palladium-based alloy film.

[0145] Preferably, the palladium-based alloy film is selected from at least one of a palladium-yttrium alloy film, a palladium-cerium alloy film, a palladium-copper alloy film, a palladium-gold alloy film, a palladium-nickel alloy film and a palladium-silver alloy film.

[0146] According to some embodiments of the present invention, the second polymer membrane separation adopts a one-stage or multi-stage separation method. Preferably, the second polymer membrane separation adopts a one-stage separation method.

[0147] According to some embodiments of the present invention, the conditions for the second polymer membrane separation include: before performing the second stage polymer membrane separation, the gas pressure obtained by palladium membrane separation is controlled to 0-5MPa, and the temperature is controlled to be less than or equal to 50°C, preferably 0-25°C.

[0148] The second aspect of the present invention provides a system for purifying helium by combining two-stage polymer membrane separation with palladium membrane separation, characterized in that the system comprises a cryogenic separation unit, a first polymer membrane separation unit, a palladium membrane separation unit and a second polymer membrane separation unit connected in sequence.

[0149] The present invention will be described in detail below through examples.

[0150] In the following preparation examples, unless otherwise specified, all examples were commercially available; the hollow spinneret was purchased from Shanghai Zhanxin; the porosity of the hollow fiber membrane support layer was measured by mercury intrusion porosimetry; and the thickness of the dense layer was measured by scanning electron microscopy.

[0151] In the following examples, the volume fraction of each gas is measured using gas chromatography. In the examples, primary membrane separation represents a single polymer membrane separation, and secondary membrane separation represents a re-polymer membrane separation (using fresh membrane modules) with the gas after the primary polymer separation as the feed gas. Tertiary, quaternary, and quintuple membrane separations are similar.

[0152] The following preparation examples are used to illustrate the preparation of polyimide random copolymers or polyimide flat films.

[0153] Preparation Example 1

[0154] (1) Under nitrogen protection, 200 mL of anhydrous N-methylpyrrolidone and m-phenylenediamine (10.81 g, 0.1 mmol) were added to a 1 L three-necked flask in sequence and stirred until the materials were completely dissolved; 4,4-diphenyl ether dianhydride (ODPA) (12.4138 g, 0.04 mmol) and 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA) (26.6544 g, 0.06 mmol) were mixed under mechanical stirring, and then added to the above system at 20°C and polycondensed for 12 h to obtain a material containing polyimide acid;

[0155] (2) A mixture of acetic anhydride (36.7524 g, 0.36 mmol) and pyridine (28.4760 g, 0.36 mmol) was added to the polyimide acid material obtained in step (1), and intramolecular dehydration was performed at 180° C. for 18 h to obtain a material containing polyimide; then 600 mL of N-methylpyrrolidone (NMP) was added to the polyimide-containing material for dilution, and the diluted material was poured into a mixed solvent of water and ethanol (500 mL:500 mL) under stirring to precipitate polyimide to obtain polyimide, and then rinsed with a mixed solution of water and ethanol (1500 mL:1500 mL) (3 times), filtered and dried to obtain a polyimide random copolymer, and the polyimide random copolymer was subjected to infrared testing using a Fourier transform infrared spectrometer (Thermo Nicolet 380), and the infrared spectrum is shown in FIG. Figure 1 Infrared testing showed that the polyimide random copolymer had a structure represented by formula (I), wherein X was Xc, Y was Ya, and Z and Z' were both Z1. Furthermore, no raw materials were detected in the remaining liquid phase after the polyimide was precipitated, indicating that all raw materials participated in the reaction.

[0156] Figure 1 Medium, 1784cm -1 and 1730cm -1 The peaks at 1357 cm-1 are the symmetric stretching vibration peaks and asymmetric stretching vibration peaks of the two carbonyl groups on the five-membered imine ring in polyimide. -1 is the stretching vibration of CN in polyimide, 721 cm-1 is the deformation vibration peak of the imine ring, 1255cm -1 The stretching vibration of the ether bond in aromatic ether is 1144 cm -1 The stretching vibration peak of CF is located at , that is, the appearance of the above characteristic peak indicates the successful synthesis of the above random copolymer.

[0157] The polyimide obtained above was used to prepare a gas separation membrane:

[0158] The above-mentioned polyimide random copolymer was added to NMP at a solid content of 15 wt%, and stirred in a shaker at 50°C until PI-4 was completely dissolved to obtain a uniform and stable casting solution; the casting solution was cooled to room temperature, ultrasonically degassed, and then evenly coated on the surface of a clean glass plate. After being dried in a 70°C forced air oven for 6 hours to remove a large amount of solvent, the solution was transferred to a vacuum oven (dried at 120°C for 12 hours) to further remove the solvent; after cooling to room temperature, the glass plate with the membrane was immersed in deionized water until the membrane fell off the glass plate surface to obtain a gas separation membrane (flat membrane).

[0159] Preparation Example 2

[0160] (1) Under nitrogen protection, 200 mL of anhydrous N-methylpyrrolidone and m-phenylenediamine (10.81 g, 0.1 mmol) were added to a 1 L three-necked flask in sequence and stirred until the materials were completely dissolved; 4,4'-biphenyl ether dianhydride (0.05 mmol) and tripterylenyl dianhydride (0.05 mmol) were mixed under mechanical stirring, and then added to the above system at 20°C and polycondensed for 12 h to obtain a material containing polyimide acid;

[0161] (2) A mixture of acetic anhydride (0.36 mmol) and pyridine (0.36 mmol) was added to the polyimide acid material obtained in step (1), and intramolecular dehydration was performed at 180°C for 18 hours to obtain a material containing polyimide; then 600 mL of N-methylpyrrolidone (NMP) was added to the polyimide-containing material for dilution, and the diluted material was poured into a mixed solvent of water and ethanol (500 mL:500 mL) under stirring to precipitate polyimide to obtain polyimide, and then rinsed with a mixed solution of water and ethanol (1500 mL:1500 mL) (3 times), filtered and dried to obtain a polyimide random copolymer. Infrared testing showed that the polyimide random copolymer had a structure shown in formula (I), wherein X is Xc, Y is Yd, and Z and Z' are both Z1. In addition, no raw materials were detected in the remaining liquid phase after the polyimide was precipitated, indicating that all raw materials participated in the reaction.

[0162] The polyimide obtained above was used to prepare a gas separation membrane:

[0163] The polyimide random copolymer prepared above was added to NMP at a solid content of 15 wt%, and stirred in a shaking table at 50° C. until the polyimide random copolymer was completely dissolved to obtain a uniform and stable casting solution. The casting solution was cooled to room temperature, ultrasonically degassed, and then uniformly coated on the surface of a clean glass plate. After being dried in a 70° C. forced air oven for 6 hours to remove a large amount of solvent, the solution was transferred to a vacuum oven (dried at 120° C. for 12 hours) to further remove the solvent. After cooling to room temperature, the glass plate with the membrane was immersed in deionized water until the membrane fell off the glass plate surface to obtain a gas separation membrane (flat membrane).

[0164] The following preparation examples are used to illustrate the preparation of polyimide-based hollow fiber membranes.

[0165] Preparation Example 1

[0166] (1) 27.5 wt % of the polyimide random copolymer obtained in Preparation Example 1, 48.5 wt % of DMF, 10 wt % of ethanol (boiling point 78° C.), 10 wt % of THF (boiling point 68.28° C.), and 4 wt % of lithium nitrate were added to a kettle equipped with a stirring device, heated to 50° C., and stirred under nitrogen protection (speed of 1200 r / min) for 36 hours. After stopping stirring, degassing was carried out at 25° C., -0.1 MPa, and speed of 50 r / min for 24 hours, and then filtered through a filter (pore size 100 mesh) at 50° C. to obtain a casting solution;

[0167] (2) Using a metering pump, the casting solution and the inner core solution (NMP: water = 95 wt%: 5 wt%) were respectively delivered to a hollow spinneret, and the inner core solution and the casting solution were extruded through the spinneret to obtain a hollow fiber. The hollow fiber was passed through an air gap of 5 cm and then placed in 50°C water for solidification to obtain a polyimide-based hollow fiber membrane precursor; wherein the spinneret temperature was 70°C; the flow rates of the casting solution and the inner core solution entering the hollow spinneret were 8 mL / min and 2.5 mL / min, respectively;

[0168] (3) The polyimide hollow fiber membrane precursor obtained in step (2) was wound up by a winder and then extracted twice in water, ethanol, and n-hexane, respectively, for 3 hours. The extracted hollow fiber membrane was then placed in a fume hood at room temperature and air-dried for 12 hours to obtain a polyimide-based hollow fiber membrane. The winding speed was 1 m / s.

[0169] The obtained hollow fiber membrane was characterized by mercury intrusion porosimetry. The porosity of the support layer was 67.5%. The thickness of the dense layer was 400 nm. The electron microscope scanning image of the cross section of the hollow fiber membrane is shown in Figure 2 .

[0170] Preparation Example 2

[0171] (1) 30 wt % of the polyimide random copolymer obtained in Preparation Example 2, 45 wt % of NMP, 10 wt % of ethanol (boiling point 78° C.), 10 wt % of THF (boiling point 68.28° C.), and 5 wt % of lithium nitrate were added to a kettle equipped with a stirring device, heated to 50° C., and stirred under nitrogen protection (speed of 600 r / min) for 36 hours. After stopping stirring, degassing was carried out at 25° C., -0.1 MPa, and speed of 30 r / min for 24 hours, and then filtered through a filter (pore size 100 mesh) at 50° C. to obtain a casting solution;

[0172] (2) Using a metering pump, the casting solution and the inner core solution (NMP: water = 95 wt%: 5 wt%) were respectively delivered to a hollow spinneret, and the inner core solution and the casting solution were extruded through the spinneret to obtain a hollow fiber. The hollow fiber was passed through an air gap of 10 cm and then placed in 50°C water for solidification to obtain a polyimide-based hollow fiber membrane precursor; wherein the spinneret temperature (extrusion temperature) was 75°C; the flow rates of the casting solution and the inner core solution entering the hollow spinneret were 6 mL / min and 2 mL / min, respectively;

[0173] (3) The polyimide hollow fiber membrane precursor obtained in step (2) was wound up by a winder and then extracted twice in water, ethanol, and n-hexane, respectively, for 3 hours. The extracted hollow fiber membrane was then placed in a fume hood at room temperature and air-dried for 12 hours to obtain a polyimide-based hollow fiber membrane. The winding speed was 1 m / s.

[0174] The obtained hollow fiber membrane was characterized by mercury intrusion porosimetry, and the porosity was 63.5% and the thickness of the dense layer was 140 nm.

[0175] The following examples are used to illustrate the process of purifying helium using the method of the present invention.

[0176] Example 1

[0177] Raw gas 1# is obtained by multi-stage flash distillation of natural gas extracted from a gas field. The volume fraction of helium in the raw gas is 8.5%. The composition of other gases includes: 35% by volume of methane, 37.3% by volume of nitrogen, 2.1% by volume of hydrogen, 7.5% by volume of carbon dioxide, and 9.5% by volume of oxygen.

[0178] The raw gas is pressurized to 5MPa and sent to the cryogenic separation unit after heat exchange to 25°C, wherein the operating temperature of the cryogenic separation unit is -150°C; the gas 2# after cryogenic separation is heat exchanged to 20°C and enters the polyimide-based hollow fiber membrane assembly prepared in the above-mentioned Preparation Example 1 for the first stage of polymer membrane separation (the first-stage membrane separation and the second-stage membrane separation operation are performed in sequence (the first-stage membrane separation is pressurized to 5MPa, and the second-stage membrane separation is pressurized to 2MPa)), and the permeate gas 3# after the first stage of polymer membrane separation is passed into the palladium-copper alloy membrane (the thickness of the alloy membrane is 20μm, purchased from Yiwu Ruisheng New Materials Technology Co., Ltd.) separation unit for dehydrogenation, wherein the operating temperature of the palladium-copper alloy membrane separation unit is 300°C and the pressure is 5MPa; the permeate gas 4# after dehydrogenation by the palladium-copper alloy membrane separation unit is pressurized to 3MPa and then subjected to the second stage of polymer membrane separation by the polyimide hollow fiber membrane prepared in the above-mentioned Preparation Example 1 to obtain ultrapure helium. The volume fractions of the gas components after separation in each stage are shown in Table 1.

[0179] In Table 1, “one-stage membrane separation” refers to “first-stage polymer membrane separation”, and “two-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0180] Table 1

[0181]

[0182]

[0183] Example 2

[0184] Flash gas from a liquefied natural gas station is used as feed gas #1. The volume fraction of helium in the feed gas is 15.73%. The composition of other gases includes: methane with a volume fraction of 19.9%, nitrogen with a volume fraction of 57.7%, hydrogen with a volume fraction of 6.62%, and oxygen with a volume fraction of 0.05%.

[0185] The raw gas is pressurized to 0.1MPa and sent to the cryogenic separation unit after heat exchange to 20℃, wherein the operating temperature of the cryogenic separation unit is -220℃; the gas 2# after cryogenic separation is heat exchanged to 20℃ and then enters the hollow fiber membrane component made of polybenzimidazole (Zhongke Energy Materials Technology (Dalian) Co., Ltd.) for the first stage polymer membrane separation (first stage membrane separation and second stage membrane separation operations are carried out in sequence (first stage membrane separation is pressurized to 0.3MPa, and second stage membrane separation is pressurized to 0.2MPa)). After the first stage polymer membrane separation After dehydrogenation, permeate gas #3 passed through a palladium-copper alloy membrane (20 μm thick, purchased from Yiwu Ruisheng New Materials Technology Co., Ltd.) separation unit for dehydrogenation. The operating temperature (dehydrogenation temperature of the palladium membrane) of the palladium-copper alloy membrane separation unit was 200°C and the pressure was 10 MPa. After dehydrogenation through the palladium-copper alloy membrane separation unit, permeate gas #4 was pressurized to 0.5 MPa and then passed through a polybenzimidazole hollow fiber membrane module (Zhongke Energy Materials Technology (Dalian) Co., Ltd.) for a second polymer membrane separation, yielding ultrapure helium. The volume fractions of the gas components after each separation stage are shown in Table 2.

[0186] In Table 2, “first-stage membrane separation” refers to “first-stage polymer membrane separation”, and “second-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0187] Table 2

[0188] No.\composition mol% Helium methane Nitrogen hydrogen oxygen Raw gas 1# 15.73 19.9 57.7 6.62 0.05 Cryogenic separation gas 2# 69.7 0 1.2 29.2 0 One-stage membrane separation (first stage) permeate gas 77.2 0 0.04 22.76 0 First stage membrane separation (secondary) permeate gas 3# 81.978 0 0.002 18.02 0 Palladium membrane retentate gas 99.778 0 0.002 0.22 0 Palladium membrane permeate gas 4# 0.01 0 0 99.99 0 Second stage membrane separation permeate gas 5# 99.999 0 0 0.001 0

[0189] Example 3

[0190] Raw gas 1# is obtained by multi-stage flash distillation of natural gas extracted from a gas field. The volume fraction of helium in the raw gas is 19.7%. The composition of other gases includes: 15.9% by volume of methane, 53.7% by volume of nitrogen, 0.05% by volume of hydrogen, 10.65% by volume of carbon dioxide, and 9.5% by volume of oxygen.

[0191] The raw gas is pressurized to 10 MPa and sent to the cryogenic separation unit after heat exchange at 25°C, wherein the operating temperature of the cryogenic separation unit is -160°C; the gas 2# after cryogenic separation is heat exchanged to 20°C and enters the polyimide-based hollow fiber membrane assembly prepared in the above-mentioned Preparation Example 1 for the first stage polymer membrane separation (primary membrane separation and secondary membrane separation operations are performed in sequence (primary membrane separation is pressurized to 1 MPa, and secondary membrane separation is pressurized to 0.5 MPa)), and the permeate gas 3# after the first stage polymer membrane separation is passed into the palladium-copper alloy membrane (the thickness of the alloy membrane is 20 μm, purchased from Yiwu Ruisheng New Materials Technology Co., Ltd.) separation unit for dehydrogenation, wherein the operating temperature of the palladium-yttrium alloy membrane separation unit is 389°C and the pressure is 50 MPa; the permeate gas 4# after dehydrogenation by the palladium-copper alloy membrane separation unit is pressurized to 1 MPa and then passed through the (polyimide hollow fiber membrane prepared in the above-mentioned Preparation Example 1) for the second stage polymer membrane separation to obtain ultrapure helium. The volume fractions of the gas components after separation at each stage are shown in Table 3.

[0192] In Table 3, “first-stage membrane separation” refers to “first-stage polymer membrane separation”, and “second-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0193] Table 3

[0194] No.\composition mol% Helium methane Nitrogen <![CDATA[CO2]]> hydrogen Raw gas 1# 19.7 15.9 53.7 10.65 0.05 Cryogenic separation gas 2# 78.8 0.9 20.2 0 0.1 One-stage membrane separation (first stage) permeate gas 97.25 0.11 2.47 0 0.17 First stage membrane separation (secondary) permeate gas 3# 99.771 0.004 0.093 0 0.132 Palladium membrane retentate gas 99.902 0.004 0.093 0 0.001 Palladium membrane permeate gas 4# 0.01 0 0 0 99.99 Second stage membrane separation permeate gas 5# 99.999 0 0 0 0.001

[0195] Example 4

[0196] Flash gas from a liquefied natural gas station is used as feed gas #1. The volume fraction of helium in the feed gas is 22.32%. The composition of other gases includes: 17.8% by volume of methane, 52.5% by volume of nitrogen, 7.32% by volume of hydrogen, and 0.06% by volume of oxygen. Other light hydrocarbons are less than 0.001% (negligible).

[0197] The raw gas is pressurized to 2MPa and sent to the cryogenic separation unit after heat exchange to 20°C, wherein the operating temperature of the cryogenic separation unit is -200°C and the pressure is 2MPa; the gas 2# after cryogenic separation is heat exchanged to 20°C and enters the polyimide-based hollow fiber membrane assembly prepared in the above-mentioned Preparation Example 1 for the first stage polymer membrane separation (primary membrane separation (pressurized to 5MPa)), and the permeate 3# after the first stage polymer membrane separation is passed into the palladium-copper alloy membrane (the thickness of the alloy membrane is 20μm, purchased from Yiwu Ruisheng New Materials Technology Co., Ltd.) separation unit for dehydrogenation, wherein the operating temperature of the palladium-copper alloy membrane separation unit is 260°C and the pressure is 32MPa; the permeate 4# after dehydrogenation by the palladium-nickel alloy membrane separation unit is pressurized to 3.6MPa and then separated by the second stage polymer membrane (the polyimide hollow fiber membrane prepared in the above-mentioned Preparation Example 1) to obtain ultrapure helium. Wherein, the volume fractions of the gas components after separation in each stage are shown in Table 4.

[0198] In Table 4, “one-stage membrane separation” refers to “first-stage polymer membrane separation”, and “two-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0199] Table 4

[0200] No.\composition mol% Helium methane Nitrogen hydrogen oxygen Raw gas 1# 22.32 17.8 52.5 7.32 0.06 Cryogenic separation gas 2# 74.04 0.02 3.1 22.84 0 One-stage membrane separation (first stage) permeate gas 83.6 0 0.08 16.32 0 Palladium membrane retentate gas 99.7 0 0.1 0.2 0 Palladium membrane permeate gas 4# 0.01 0 0 99.99 0 Second stage membrane separation permeate gas 5# 99.999 0 0 0.001 0

[0201] Example 5

[0202] Natural gas extracted from a shale gas field is used as feed gas #1. The volume fraction of helium in the feed gas is 0.02%. The composition of other gases includes: 75% by volume of methane, 15.3% by volume of nitrogen, 2.1% by volume of hydrogen, 5.5% by volume of carbon dioxide, and 1.5% by volume of oxygen.

[0203] The raw gas was pressurized to 10 MPa, heat-exchanged to 30°C, and then sent to the cryogenic separation unit, wherein the operating temperature of the cryogenic separation unit was -100°C and the pressure was 4 MPa; the gas 2# after cryogenic separation was heat-exchanged to 20°C and then entered the polyimide-based flat membrane assembly prepared in the above-mentioned Preparation Example 1 for the first stage polymer membrane separation (the first, second, third, fourth and fifth stage membrane separation operations were performed in sequence (the first, second, third, fourth and fifth stage membrane separations were pressurized to 4 MPa, 4 MPa, 3 MPa, 3 MPa, respectively). After the first polymer membrane separation, permeate gas 3# was passed through a palladium-cerium alloy membrane (10 μm thickness, Nanjing Gaoqian Functional Materials Technology Co., Ltd.) separation unit for dehydrogenation. The palladium-copper alloy membrane separation unit operated at a temperature of 410°C and a pressure of 27 MPa. After dehydrogenation in the palladium-copper alloy membrane separation unit, permeate gas 4# was pressurized to 4 MPa and then passed through a second polymer membrane separation unit (using the triptycene polyimide-based hollow fiber membrane module prepared in Preparation Example 2) to obtain ultrapure helium. The volume fractions of the gas components after each stage of separation are shown in Table 5.

[0204] In Table 5, “first-stage membrane separation” refers to “first-stage polymer membrane separation”, and “second-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0205] Table 5

[0206]

[0207]

[0208] Example 6

[0209] Flash steam 1# (BOG) from a liquefied natural gas station was used as feed gas 1#. The volume fraction of helium in the feed gas was 15.73%. The other gases included: methane (volume fraction 19.9%), nitrogen (volume fraction 57.7%), hydrogen (volume fraction 6.62%), and oxygen (volume fraction 0.05%).

[0210] The raw gas was pressurized to 2MPa, and sent to the cryogenic separation unit after heat exchange to 25°C, wherein the operating temperature of the cryogenic separation unit was -150°C; the gas 2# after cryogenic separation was heat exchanged to 20°C and then entered the polyimide-based hollow fiber membrane assembly prepared in Example 1 of the above embodiment for the first stage polymer membrane separation (first stage membrane separation, second stage membrane separation and third stage membrane separation operations were performed in sequence (first stage membrane separation pressurized to 2MPa, second stage membrane separation pressurized to 1.5MPa, second stage membrane separation pressurized to 1.5MPa), and the first stage polymer was separated. After membrane separation, permeate gas #3 passed through a palladium-copper alloy membrane (20 μm thick, purchased from Yiwu Ruisheng New Materials Technology Co., Ltd.) for dehydrogenation. The palladium-copper alloy membrane separation unit operated at a temperature of 256°C and a pressure of 13 MPa. After dehydrogenation, permeate gas #4 was pressurized to 4 MPa and then passed through a polysulfone hollow fiber polymer membrane (PRISM®) for a second polymer membrane separation, yielding ultrapure helium. The volume fractions of the gas components after each separation stage are shown in Table 6.

[0211] In Table 6, “one-stage membrane separation” refers to “first-stage polymer membrane separation”, and “two-stage membrane separation” refers to “second-stage polymer membrane separation”.

[0212] Table 6

[0213] No.\composition mol% Helium methane Nitrogen hydrogen water oxygen Raw gas 1# 15.73 19.9 57.7 6.62 0 0.05 Cryogenic separation 2# 18.47 12.52 61.24 7.72 0 0.05 One-stage membrane separation (first stage) permeate gas 38.72 7.73 37.83 15.69 0 0.03 First stage membrane separation (secondary) permeate gas 66.75 1.83 8.96 22.45 0 0.01 First stage membrane separation (three-stage) permeate gas 3# 77.48 0.12 0.56 21.84 0 0 Palladium membrane retentate gas 98.86 0.15 0.72 0.28 0 0 Palladium membrane permeate gas 4# 0.01 0 0 99.99 0 0 Second stage membrane separation permeate gas 5# 99.999 0 0 0.001 0 0

[0214] From the above data, it can be seen that the cryogenic separation unit can remove most of the impurity gases such as CO2, N2, and methane, and the content of helium and hydrogen can be greatly increased, which is conducive to further concentration by subsequent polymer membrane separation (one-stage membrane). The polymer membrane separation process further removes impurity gases and efficiently concentrates hydrogen and helium as a supplement to the cryogenic process. After greatly improving the purity of helium through palladium membrane dehydrogenation, ultra-pure helium with a concentration of 99.999% can be obtained through two-stage polymer membrane separation and enrichment.

[0215] The polymer films of the preferred embodiments used in the present invention can achieve better results. For details, please refer to the contents recorded in (Application No. 202110864549.5).

[0216] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for separating and purifying helium using a two-stage polymer membrane combined with a palladium membrane, characterized in that: The method comprises: sequentially subjecting the raw gas to cryogenic separation, first-stage polymer membrane separation, palladium membrane separation, and second-stage polymer membrane separation to obtain ultrapure helium; The polymer membranes used in the first stage polymer membrane separation and the second stage polymer membrane separation are polyimide-based hollow fiber membranes; the material of the hollow fiber membranes is polyimide random copolymer; Wherein, the polyimide random copolymer has a structure shown in formula (I): （I）， In formula (I), m and n are each independently an integer of 10-2000; X has a structure represented by any one of formulas (X1) to (X3); (X1)、 (X2)、(X3), In formula (X1)-formula (X3), R 1 、R 2 、R 3 、R 4 、R 5 and R 6 Each is independently H, C1-C4 alkyl, C6-C10 aryl, amino, hydroxyl or carboxyl; Y has a structure represented by any one of formulas (Y1) to (Y5); <h2 style=";text-align:left;direction:ltr">(Y1)、<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr"> (Y2)、<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr"> (Y3)、<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr"> (Y4)、<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr"> (Y5), In formula (Y1)-formula (Y5), R 7 、R 8 、R 9 、R 10 、R 11 、R 12 、R 13 、R 14 、R 15 and R 16 Each is independently H, C1-C4 alkyl, C6-C10 aryl, amino, hydroxyl or carboxyl; Z and Z' each independently have a structure represented by formula (Z1) or formula (Z2); (Z1)、 (Z2), In formula (Z2), Ra and Rb are each independently H, a C1-C4 alkyl group, or a C1-C4 halogenated alkyl group.

2. The method according to claim 1, wherein The raw gas is selected from at least one of natural gas, shale gas, helium-rich hydrogen-containing gas and liquefied natural gas flash steam (BOG).

3. The method according to claim 1 or 2, wherein: The conditions for the cryogenic separation include: a temperature of -250°C to -50°C.

4. The method according to claim 3, wherein: The conditions for the cryogenic separation include: a temperature of -220°C to -100°C.

5. The method according to claim 1 or 2, wherein: The first stage polymer membrane separation adopts a one-stage or multi-stage separation method; And / or, the conditions for the first stage polymer membrane separation include: before the first stage polymer membrane separation, controlling the pressure of the gas obtained by cryogenic separation to 0.01-10 MPa and the temperature to less than 50°C.

6. The method according to claim 5, wherein: The conditions for the first stage polymer membrane separation include: before the first stage polymer membrane separation, controlling the pressure of the gas obtained by cryogenic separation to 0.1-8 MPa and the temperature to 0-25°C.

7. The method according to claim 1, wherein The polyimide-based hollow fiber membrane comprises a support layer and a dense layer attached to the outer surface of the support layer. The thickness of the dense layer is less than 1000 nm. The porosity of the hollow fiber membrane is 40-80%.

8. The method according to claim 7, wherein: The thickness of the dense layer is 100-500 nm, and the porosity of the hollow fiber membrane is 50-70%.

9. The method according to claim 1 or 2, wherein: m and n are each independently an integer of 50-1000.

10. The method according to claim 1 or 2, wherein: 0.9≥n / (m+n)≥0.

3.

11. The method according to claim 10, wherein: 0.7≥n / (m+n)≥0.

5.

12. The method according to claim 1 or 2, wherein: The X has one of the following structures, 、 、 ； And / or, Y has one of the following structures, 、 、 、 、 ； and / or, Z and Z' both have the structure shown by Z1 or Z3, 、 。 13. The method according to claim 12, wherein: X is Xa, Y is Ya, and Z and Z' are both Z1; Alternatively, X is Xa, Y is Yb, and Z and Z' are both Z1; Alternatively, X is Xa, Y is Yd, and Z and Z' are both Z1; Alternatively, X is Xb, Y is Ya, and Z and Z' are both Z1; Alternatively, X is Xb, Y is Yb, and Z and Z' are both Z1; Alternatively, X is Xb, Y is Yd, and Z and Z' are both Z1; Alternatively, X is Xc, Y is Ya, and Z and Z' are both Z1; Alternatively, X is Xc, Y is Yb, and Z and Z' are both Z1; Alternatively, X is Xc, Y is Yc, and Z and Z' are both Z1; Alternatively, X is Xc, Y is Y4, and Z and Z' are both Z1; Alternatively, X is Xc, Y is Yd, and Z and Z' are both Z1; Alternatively, X is Xb, Y is Ya, and Z and Z' are both Z3; Alternatively, X is Xb, Y is Yb, and Z and Z' are both Z3; Alternatively, X is Xb, Y is Yd, and Z and Z' are both Z3; Alternatively, X is Xc, Y is Ya, and Z and Z' are both Z3; Alternatively, X is Xc, Y is Yb, and Z and Z' are both Z3; Alternatively, X is Xc, Y is Yd, and Z and Z' are both Z3.

14. The method according to claim 1 or 2, wherein: The polyimide-based hollow fiber membrane is prepared according to a method comprising the following steps: (1) preparing a casting solution containing polyimide, a diluent and an additive, wherein the diluent contains a good solvent for polyimide, a poor solvent for the first polyimide and a poor solvent for the second polyimide, wherein the boiling point B1 of the poor solvent for the first polyimide is higher than the boiling point B2 of the poor solvent for the second polyimide; (2) Extruding the core liquid and the casting liquid at a temperature T, and then curing to obtain a hollow fiber membrane precursor, wherein B2≤T<B1; (3) The hollow fiber membrane precursor is rolled up and extracted to obtain the polyimide-based hollow fiber membrane.

15. The method according to claim 14, wherein In step (1), based on the total weight of the casting solution, the content of the polyimide is 20-40wt%, the content of the diluent is 50-75wt%, and the content of the additive is 0.5-10wt%; and / or the boiling point B1 of the poor solvent for the first polyimide is 5-200° C. higher than the boiling point B2 of the poor solvent for the second polyimide; and / or, the poor solvent for the first polyimide is selected from at least one of a C2-C4 saturated monohydric alcohol, γ-butyrolactone and water; and / or, the poor solvent for the second polyimide is selected from at least one of C3-C5 alkanes, tetrahydrofuran, acetone and chloroform; and / or, the good solvent for the polyimide is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide and N,N-dimethylacetamide; and / or, the weight ratio of the good solvent for the polyimide, the poor solvent for the first polyimide, and the poor solvent for the second polyimide is 1:(0.1-0.5):(0.1-0.5); And / or, the additive is a lithium salt.

16. The method according to claim 15, wherein Based on the total weight of the casting solution, the content of the polyimide is 25-35wt%, the content of the diluent is 60-70wt%, and the content of the additive is 1-5wt%; and / or, the boiling point B1 of the poor solvent for the first polyimide is 10-20° C. higher than the boiling point B2 of the poor solvent for the second polyimide; and / or, the weight ratio of the good solvent for the polyimide, the poor solvent for the first polyimide, and the poor solvent for the second polyimide is 1:(0.15-0.3):(0.15-0.3); And / or, the additive is selected from lithium nitrate and / or lithium chloride.

17. The method according to claim 14, wherein: The casting solution is prepared according to a method comprising the following steps: stirring polyimide, a diluent and an additive at 20-50° C. and 100-1200 r / min for 12-48 hours, and then performing vacuum degassing and filtering at 20-50° C. to remove impurities; And / or, the vacuum degassing conditions include: pressure of -0.1 MPa to -0.095 MPa, temperature of 20-30° C., rotation speed of 10-50 r / min, and time of 12-24 h.

18. The method according to claim 14, wherein In step (2), the core liquid comprises solvent A and solvent B, wherein the solvent A is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide and N,N-dimethylacetamide, and the solvent B is selected from at least one of C1-C4 saturated monohydric alcohol, γ-butyrolactone and water; and / or, the solvent A accounts for 50-99 wt % of the total weight of the inner core liquid; and / or, the extrusion is carried out in a spinneret, wherein the extrusion temperature is 40-75° C.; and / or, during the extrusion process, the flow rate of the casting solution is 6-30 mL / min; and / or, during the extrusion process, the flow rate of the core liquid is 2-10 mL / min; and / or, passing the extruded hollow fibers through an air gap prior to solidification; And / or, the height of the air gap is 5-30 cm; And / or, the air gap is heated by an annular sleeve to control the temperature to 70-150°C; and / or, the solidification is carried out in a coagulation bath; And / or, the bath liquid used in the coagulation bath is solvent C and / or water, and the temperature of the coagulation bath is 40-70°C; And / or, the solvent C is at least one selected from C1-C4 saturated monohydric alcohol, γ-butyrolactone and water.

19. The method according to claim 18, wherein The solvent A accounts for 60-95% of the total weight of the inner core liquid; And / or, the extrusion is carried out in a spinneret, wherein the extrusion temperature is 60-70°C.

20. The method according to claim 14, wherein In step (3), the winding speed is 0.5-2 m / s; And / or, the extraction agent used for the extraction is selected from at least one of water, a C1-C4 saturated monohydric alcohol and a C5-C7 alkane; And / or, the extraction conditions include: temperature of 20-35°C, time of 3-48h; And / or, the extraction further includes a drying step; And / or, the drying conditions include: temperature of 20-35° C. and time of 2-15 hours.

21. The method according to claim 1 or 2, wherein The conditions for the palladium membrane separation include: before the palladium membrane dehydrogenation separation, controlling the temperature of the gas obtained by polymer membrane separation to 200-500° C.; and controlling the gas pressure to 1-50 MPa.

22. The method according to claim 21, wherein The thickness of the palladium membrane used in the palladium membrane separation is 5-100 μm; and / or, the palladium membrane is a tubular membrane or a porous carrier composite membrane; And / or, the palladium film is a pure palladium film or a palladium-based alloy film.

23. The method according to claim 22, wherein The porous carrier in the porous carrier composite membrane is selected from one of porous ceramics, porous Vickers glass and porous stainless steel; And / or, the palladium-based alloy film is selected from at least one of a palladium-yttrium alloy film, a palladium-cerium alloy film, a palladium-copper alloy film, a palladium-gold alloy film, a palladium-nickel alloy film and a palladium-silver alloy film.

24. The method according to claim 1, wherein The second stage polymer membrane separation adopts a one-stage or multi-stage separation method; And / or, the conditions for the second polymer membrane separation include: before the second stage polymer membrane separation, controlling the pressure of the gas obtained by palladium membrane separation to 0-5 MPa and the temperature to be less than or equal to 50°C.

25. The method according to claim 24, wherein The conditions for the second polymer membrane separation include: temperature controlled at 0-25°C.

26. A system for use in the method according to any one of claims 1 to 25, characterized in that: The system comprises a cryogenic separation unit, a first polymer membrane separation unit, a palladium membrane separation unit and a second polymer membrane separation unit which are connected in sequence.

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