Method and system for preparing ethylene by membrane separation combined with distillation

Abstract

The present invention relates to the technical field of cracking gas separation, and specifically discloses a method and system for preparing ethylene by combining membrane separation with distillation. The method provided by the present invention comprises the following steps: (1) subjecting the raw gas to a first polymer membrane separation to obtain a first hydrogen-rich gas and a first methane-rich gas; (2) subjecting the first hydrogen-rich gas to a second polymer membrane separation to obtain high-purity hydrogen and a second methane-rich gas; (3) subjecting the first methane-rich gas and the second methane-rich gas to a third polymer membrane separation after mixing them to obtain a second hydrogen-rich gas and a third methane-rich gas; optionally, returning the second hydrogen-rich gas to step (1) and subjecting it to the first polymer membrane separation together with the raw gas; (4) subjecting the third methane-rich gas to distillation to obtain a fourth methane-rich gas and a product gas rich in ethylene. The method provided by the present invention can greatly improve the yield of ethylene, and also has the advantages of low energy consumption and the ability to produce a large amount of crude hydrogen and high-purity hydrogen. It has broad prospects.

Description

Technical Field

[0001] The invention relates to the technical field of cracking gas separation, and in particular to a method and system for preparing ethylene by combining membrane separation with rectification. Background Art

[0002] Ethylene is one of the chemical products with the largest output and consumption in the world, and is known as the "mother of the petrochemical industry." Ethylene production capacity is one of the important indicators to measure the level of development of a country's petrochemical industry. my country's ethylene production capacity has long ranked second in the world, second only to the United States. The production of ethylene from petroleum mainly relies on cracking technology, which leads to the generation of a large number of by-products other than the target product in the process, such as methane, hydrogen and other fuel gases, which require effective removal and recovery methods. In the process of purifying and separating ethylene, a corresponding distillation demethanizer is set up to remove methane. Hydrogen removal mainly relies on deep cooling technology to separate hydrogen from other groups, which can be set at the front or back end of the demethanizer. Since hydrogen consumes additional cooling capacity and affects the methane separation process, pre-dehydrogenation is widely used. Therefore, it is necessary to seek a better method for purifying and separating ethylene. Summary of the invention

[0003] In order to solve the technical problems of high energy consumption and poor separation effect in the prior art, the present invention provides a method and system for preparing ethylene by combining membrane separation with distillation.

[0004] The first aspect of the present invention provides a method for preparing ethylene by membrane separation combined with rectification, the method comprising the following steps:

[0005] (1) subjecting the raw gas to a first polymer membrane separation to obtain a first hydrogen-rich gas and a first methane-rich gas;

[0006] (2) subjecting the first hydrogen-rich gas to a second polymer membrane separation to obtain high-purity hydrogen gas and a second methane-rich gas;

[0007] (3) mixing the first methane-rich gas and the second methane-rich gas and performing a third polymer membrane separation to obtain a second hydrogen-rich gas and a third methane-rich gas; optionally, returning the second hydrogen-rich gas to step (1) and performing a first polymer membrane separation together with the feed gas;

[0008] (4) distilling the third methane-rich gas to obtain a fourth methane-rich gas and an ethylene-rich product gas.

[0009] The second aspect of the present invention provides a system for preparing ethylene by membrane separation combined with distillation, the system comprising: a first polymer membrane separation unit, a second polymer membrane separation unit, a third polymer membrane separation unit and a distillation tower, wherein:

[0010] The first polymer membrane separation unit is used to perform a first polymer membrane separation on the feed gas to obtain a first hydrogen-rich gas and a first methane-rich gas;

[0011] The second polymer membrane separation unit is connected to the first polymer membrane separation unit, and is used for performing a second polymer membrane separation on the first hydrogen-rich gas from the first polymer membrane separation unit to obtain high-purity hydrogen gas and a second methane-rich gas;

[0012] The third polymer membrane separation unit is connected to the first polymer membrane separation unit and the second polymer membrane separation unit, and is used for mixing the first methane-rich gas from the first polymer membrane separation unit and the second methane-rich gas from the second polymer membrane separation unit, and then performing third polymer membrane separation to obtain a second hydrogen-rich gas and a third methane-rich gas;

[0013] The distillation tower is connected to the third polymer membrane separation unit and is used for distilling the third methane-rich gas from the third polymer membrane separation unit to obtain a fourth methane-rich gas and an ethylene-rich product gas.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1) The method provided by the present invention optimizes the methane-hydrogen separation process of the ethylene demethanizer, and can realize the expansion and capacity expansion of the ethylene plant without rebuilding or building a new cold box and demethanizer;

[0016] 2) The method provided by the present invention can reduce the energy consumption of methane / hydrogen separation;

[0017] 3) The method provided by the present invention can prepare crude hydrogen, directly supply the catalytic hydrogenation process, and optimize the hydrogen balance of the refinery;

[0018] 4) The method provided by the present invention can prepare high-purity hydrogen and increase the technical added value of the product; under the operating conditions of normal temperature and low pressure, helium is concentrated, avoiding the extremely high requirements of cryogenic technology on equipment and energy consumption, making the helium preparation process simple and economical;

[0019] 5) The method provided by the present invention improves the quality of the gas entering the demethanizer and can greatly increase the ethylene yield in the ethylene demethanizer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is an infrared spectrum of the polyimide random copolymer prepared in Preparation Example 1;

[0021] Figure 2 is a scanning electron microscope image of a cross section of the polyimide-based hollow fiber membrane prepared in Preparation Example 1;

[0022] Figure 3 This is a schematic diagram of a process for preparing ethylene by membrane separation combined with distillation according to a specific embodiment of the present invention;

[0023] Figure 4 This is a schematic diagram of the process of separating cracking gas to produce ethylene in Comparative Example 1.

[0024] Description of Reference Numerals

[0025] MEM1-first polymer membrane separation unit; MEM2-second polymer membrane separation unit; MEM3-third polymer membrane separation unit; M1, M2-gas mixing device; C1, C2, C1', C2', C3', C4', C5'-heat exchange device (cooler); CP1-pressurizing device; E1, E2-rectification tower; 1-first separator; 2-second separator; 3-third separator; 4-fourth separator; 5-fifth separator; S1-raw gas; S2-mixed gas of raw gas and the second hydrogen-rich gas after compression and heat exchange; S3-first hydrogen-rich gas; S4-first methane-rich gas; S5-high-purity hydrogen; S6-second methane-rich gas; S7-mixed gas of the first methane-rich gas and the second methane-rich gas; S8-second hydrogen-rich gas; 9-third methane-rich gas; S10-second hydrogen-rich gas after compression; S11-second hydrogen-rich gas after compression and heat exchange; S12-first methane-rich gas after heat exchange The third methane-rich gas; S13-the fourth methane-rich gas; S14-the product gas rich in ethylene; S2'-the raw gas after heat exchange; S3'-the first mixed gas rich in methane and hydrogen; S4'-the first hydrocarbon condensate; S5'-the first mixed gas rich in methane and hydrogen after the first cooling; S6'-the second mixed gas rich in methane and hydrogen; S7'-the second hydrocarbon condensate; S8'-the second mixed gas rich in methane and hydrogen after the second cooling; S9 '- the third mixed gas rich in methane and hydrogen; S10'- the third hydrocarbon condensate; S11'- the third mixed gas rich in methane and hydrogen after the third cooling; S12'- the fourth mixed gas rich in methane and hydrogen; S13'- the fourth hydrocarbon condensate; S14'- the fourth mixed gas rich in methane and hydrogen after the fourth cooling; S15'- hydrogen-rich gas; S16'- methane condensate; S17'- methane-rich gas; S18'- olefin-containing product. DETAILED DESCRIPTION

[0026] The endpoints and any values ​​of the ranges disclosed in this article 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 endpoint values ​​of each range, the endpoint values ​​of each range and the 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 as specifically disclosed in this article.

[0027] The first aspect of the present invention provides a method for preparing ethylene by membrane separation combined with rectification, the method comprising the following steps:

[0028] (1) subjecting the raw gas to a first polymer membrane separation to obtain a first hydrogen-rich gas and a first methane-rich gas;

[0029] (2) subjecting the first hydrogen-rich gas to a second polymer membrane separation to obtain high-purity hydrogen gas and a second methane-rich gas;

[0030] (3) mixing the first methane-rich gas and the second methane-rich gas and performing a third polymer membrane separation to obtain a second hydrogen-rich gas and a third methane-rich gas; optionally, returning the second hydrogen-rich gas to step (1) and performing a first polymer membrane separation together with the feed gas;

[0031] (4) distilling the third methane-rich gas to obtain a fourth methane-rich gas and an ethylene-rich product gas. The first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are performed in a cross-flow separation manner. "Cross-flow" means that the flow direction of the gas to be separated is parallel to the membrane surface.

[0032] In the present invention, preferably, the second methane-rich gas is compressed and heat-exchanged before being returned to step (1) for first polymer membrane separation together with the cracked gas. There is no particular limitation on the conditions for compression and heat exchange, as long as they can meet the requirements of the present invention.

[0033] The present invention does not impose any particular restriction on the type of raw gas, as long as the ethylene product gas can be prepared according to the method of the present invention. Preferably, the raw gas is cracked gas, in which the volume fraction of ethylene is 30-40 mol%, the volume fraction of methane is 25-35 mol%, and the volume fraction of hydrogen is 10-20 mol%. Preferably, the volume fraction of ethylene is 32-37 mol%, the volume fraction of methane is 30-33 mol%, and the volume fraction of hydrogen is 12-20 mol%.

[0034] According to some embodiments of the present invention, the permeation pressure in the first polymer membrane separation process, the permeation pressure in the second polymer membrane separation process, and the permeation pressure in the third polymer membrane separation process can each independently be 100 kPa-3600 kPa. Wherein, "permeation pressure" refers to the transmembrane pressure to which the gas is subjected when passing through the membrane.

[0035] Preferably, the permeation pressure of the first polymer membrane during separation is 1200 kPa-3600 kPa.

[0036] Preferably, the permeation pressure of the second polymer membrane during separation is 500 kPa-1000 kPa.

[0037] Preferably, the permeation pressure during the separation process of the third polymer membrane is 500 kPa-1500 kPa.

[0038] According to some embodiments of the present invention, the membranes used for the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation can each independently have a flux of 10-120 GPU for hydrogen, a flux of 0.1-3 GPU for methane and a flux of 0.1-2.5 GPU for ethylene. The unit of "GPU" is 10 -6 cm 3 (STP) / cm 2 ·s·cmHg.

[0039] According to some embodiments of the present invention, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are each independently selected from at least one of a hollow fiber membrane, a flat membrane and a tubular membrane.

[0040] According to some embodiments of the present invention, the materials of the polymer membranes used for the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same or different, and are independently selected from at least one of polysulfone, polyethersulfone, polyimide, polypropylene, polyethylene, synthetic resin, polyvinylidene fluoride, polytetrafluoroethylene, polyetheretherketone, polybenzimidazole, polydimethylsiloxane and metal organic framework materials.

[0041] In the present invention, the polymer membranes used for the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation can be commercially available, or prepared by thermally induced phase separation, solution induced phase separation, melt stretching, interfacial polymerization, coating polymerization, in situ polymerization and the like.

[0042] In the present invention, the permeation pressure refers to the transmembrane pressure to which the gas is subjected when passing through the membrane.

[0043] According to some embodiments of the present invention, the polymer membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are polyimide-based hollow fiber membranes.

[0044] 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%.

[0045] Preferably, the thickness of the dense layer is 100-500 nm, and the porosity of the hollow fiber membrane is 50-70%.

[0046] Preferably, the hollow fiber membrane is made of polyimide random copolymer.

[0047] According to some embodiments of the present invention, the polyimide copolymer has a structure shown in formula (I):

[0048]

[0049] In formula (I), m and n are each independently an integer of 10-2000;

[0050] X has a structure represented by any one of formula (X1) to formula (X3);

[0051]

[0052] 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;

[0053] Y has a structure represented by any one of formula (Y1) to formula (Y5);

[0054]

[0055] 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;

[0056] Z and Z' each independently have a structure represented by formula (Z1) or formula (Z2);

[0057]

[0058] In formula (Z2), Ra and Rb are each independently H, C1-C4 alkyl or C1-C4 halogenated alkyl.

[0059] Preferably, m and n are each independently an integer of 50-1000.

[0060] Preferably, 0.9≥n / (m+n)≥0.3, preferably, 0.7≥n / (m+n)≥0.5.

[0061] In the present invention, X has one of the following structures:

[0062]

[0063] In the present invention, Y has one of the following structures,

[0064]

[0065] In the present invention, Z and Z' both have the structure shown by Z1 or Z3,

[0066]

[0067] Preferably, X is Xa, Y is Ya, and Z and Z' are both Z1;

[0068] Alternatively, X is Xa, Y is Yb, and Z and Z' are both Z1;

[0069] Or, X is Xa, Y is Yd, and Z and Z' are both Z1;

[0070] Or, X is Xb, Y is Ya, and Z and Z' are both Z1;

[0071] Or, X is Xb, Y is Yb, and Z and Z' are both Z1;

[0072] Or, X is Xb, Y is Yd, and Z and Z' are both Z1;

[0073] Or, X is Xc, Y is Ya, and Z and Z' are both Z1;

[0074] Alternatively, X is Xc, Y is Yb, and Z and Z' are both Z1;

[0075] Or, X is Xc, Y is Yc, and Z and Z' are both Z1;

[0076] Alternatively, X is Xc, Y is Y4, and Z and Z' are both Z1;

[0077] Or, X is Xc, Y is Yd, and Z and Z' are both Z1;

[0078] Or, X is Xb, Y is Ya, and Z and Z' are both Z3;

[0079] Or, X is Xb, Y is Yb, and Z and Z' are both Z3;

[0080] Or, X is Xb, Y is Yd, and Z and Z' are both Z3;

[0081] Or, X is Xc, Y is Ya, and Z and Z' are both Z3;

[0082] Or, X is Xc, Y is Yb, and Z and Z' are both Z3;

[0083] Alternatively, X is Xc, Y is Yd, and Z and Z' are both Z3.

[0084] 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 that "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".

[0085] In the present invention, based on firstly carrying out polycondensation reaction of dianhydride monomer (dianhydride shown in formula (II) and dianhydride shown in formula (III)) and diamine monomer to obtain polyamic acid, then imidization (intramolecular dehydration) is carried out to polyamic acid, dianhydride monomer and diamine monomer can be carried out polycondensation reaction to obtain polyamic acid by one-pot method, or dianhydride monomer (i.e. dianhydride shown in formula (II) and dianhydride shown in formula (III)) can be mixed uniformly before carrying out polycondensation reaction with diamine monomer. However, in order to better control the reaction, it is preferred to react in the latter mode. Therefore, the present invention also provides a method for preparing polyimide random copolymer, and the method comprises the following steps:

[0086] (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 to carry out a polycondensation reaction to obtain a material containing polyamic acid,

[0087]

[0088] (S2) imidizing the material containing polyamic acid obtained in step (S1) to cause intramolecular dehydration of the polyamic acid to obtain a polyimide random copolymer;

[0089] In formula (II) and formula (III), X and Y have the same meanings as described above.

[0090] The diamine monomer is selected from at least one of the compounds having a structure such as H2N-Zp-NH2, wherein Zp has a structure as shown in formula (Z1) or (Z2),

[0091]

[0092] In formula (Z2), Ra and Rb are each independently H, C1-C4 alkyl or C1-C4 halogenated alkyl.

[0093] Preferably, X is Xa, Y is Ya, and Zp is Z1;

[0094] Alternatively, X is Xa, Y is Yb, and Zp is Z1;

[0095] Or, X is Xa, Y is Yd, and Zp is Z1;

[0096] Or, X is Xb, Y is Ya, and Zp is Z1;

[0097] Or, X is Xb, Y is Yb, and Zp is Z1;

[0098] Or, X is Xb, Y is Yd, and Zp is Z1;

[0099] Or, X is Xc, Y is Ya, and Zp is Z1;

[0100] Or, X is Xc, Y is Yb, and Zp is Z1;

[0101] Or, X is Xc, Y is Yc, and Zp is Z1;

[0102] Or, X is Xc, Y is Y4, and Zp is Z1;

[0103] Or, X is Xc, Y is Yd, and Zp is Z1;

[0104] Or, X is Xb, Y is Ya, and Zp is Z3;

[0105] Or, X is Xb, Y is Yb, and Zp is Z3;

[0106] Or, X is Xb, Y is Yd, and Zp is Z3;

[0107] Or, X is Xc, Y is Ya, and Zp is Z3;

[0108] Or, X is Xc, Y is Yb, and Zp is Z3;

[0109] Alternatively, X is Xc, Y is Yd, and Zp is Z3.

[0110] 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).

[0111] In the present invention, M and N satisfy 0.9≥N / (M+N)≥0.3, preferably, 0.7≥N / (M+N)≥0.5.

[0112] In the present invention, the molar ratio of the total 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).

[0113] 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.

[0114] In the present invention, the polycondensation reaction is carried out under an inert atmosphere, which is preferably provided by nitrogen.

[0115] 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.

[0116] In the present invention, the usage of the first solvent is 1000-3000 mL relative to 1 mmol of the diamine monomer.

[0117] 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 the following methods: mechanical stirring, shaking or ultrasound. The mechanical stirring conditions may include: 20-40°C, 2000-15000rpm, 2-12h; the ultrasound conditions may include: 20-40°C, 0.5-2.0h; the shaking conditions may include: 20-40°C, 260-800rpm, 12-36h.

[0118] 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.

[0119] In the present invention, the dehydrating agent is selected from at least one of dichlorobenzene, toluene, acetic anhydride and xylene.

[0120] In the present invention, the catalyst is selected from pyridine and / or diquinoline.

[0121] 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.

[0122] 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.

[0123] In the present invention, the method further comprises: before obtaining the polyimide copolymer, the material after the imidization treatment in step (S2) is diluted and contacted 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. Equivalent to 1 mol of diamine monomer, the total amount of the precipitant can be 10-50L. Wherein, the solvent for dilution can be N-methylpyrrolidone. Preferably, relative to 1 mol of diamine monomer, the amount of the solvent for dilution can be 5-8L.

[0124] In the present invention, there is no particular limitation on the manner in which the imidization-treated material in step (S2) is contacted with the precipitant, as long as the requirements of the present invention are met. For example, the following manner may be used: the imidization-treated material in step (S2) (after dilution) is added to the precipitant to precipitate the polyimide, and then the precipitated polyimide is eluted with the precipitant (the elution may be 3-5 times), and finally filtered and dried (70-150° C., 24-48 hours) to obtain a polyimide random copolymer.

[0125] In the present invention, the polyimide-based hollow fiber membrane is prepared according to a method comprising the following steps:

[0126] (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;

[0127] (2) extruding the inner core liquid and the casting liquid at a temperature T, and then curing to obtain a hollow fiber membrane precursor, wherein B2≤T<B1;

[0128] (3) The hollow fiber membrane precursor is rolled up and extracted to obtain the polyimide-based hollow fiber membrane.

[0129] 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%.

[0130] Preferably, 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%.

[0131] In the present invention, in order to facilitate the formation of the 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. Wherein, the boiling point refers to the normal pressure boiling point unless otherwise specified.

[0132] 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.

[0133] In the present invention, the poor solvent for the second polyimide is at least one selected from C3-C5 alkanes, tetrahydrofuran, acetone and chloroform.

[0134] In the present invention, the good solvent for the polyimide is at least one selected from N-methylpyrrolidone, N,N-dimethylacetamide and N,N-dimethylacetamide.

[0135] 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).

[0136] In the present invention, the additive may be a lithium salt, preferably selected from lithium nitrate and / or lithium chloride.

[0137] 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).

[0138] In the present invention, the vacuum degassing conditions include: pressure of -0.1MPa to -0.095MPa, temperature of 20-30°C, rotation speed of 10-50r / min, and time of 12-24h.

[0139] 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 a saturated monohydric alcohol of C1-C4, γ-butyrolactone and water.

[0140] In the present invention, the solvent A accounts for 50-99wt%, preferably 60-95wt%, of the total weight of the inner core liquid.

[0141] According to some embodiments of the present invention, the extrusion is carried out in a spinneret, wherein the extrusion temperature (temperature of the spinneret) is 40-75°C, preferably 60-70°C.

[0142] In the present invention, during the extrusion process, the flow rate of the casting solution is 6-30 mL / min.

[0143] According to some embodiments of the present invention, during the extrusion process, the flow rate of the core liquid is 2-10 mL / min.

[0144] 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.

[0145] In the present invention, the height of the air gap is 5-30 cm.

[0146] In the present invention, the air gap is heated by an annular sleeve, and the temperature is preferably controlled to be 70-150°C.

[0147] 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.

[0148] In the present invention, the solvent C is selected from at least one of a C1-C4 saturated monohydric alcohol, γ-butyrolactone and water.

[0149] In the present invention, in step (3), the winding rate is 0.5-2 m / s.

[0150] In the present invention, the purpose of the extraction is to remove the diluent and additives in the hollow fiber membrane precursor.

[0151] In the present invention, the extractant used for extraction is selected from at least one of water, C1-C4 saturated monohydric alcohol and C5-C7 alkane. There is no particular limitation on the amount of the extractant, as long as it can meet the requirements of the present invention.

[0152] 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 for the membrane fibers (hollow fiber membrane precursors) to be immersed.

[0153] 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.

[0154] In the present invention, the extraction further includes a drying step.

[0155] In the present invention, the drying conditions include: temperature of 20-35° C. and time of 2-15 h.

[0156] According to some embodiments of the present invention, the distillation is performed in a distillation tower, and the distillation tower is a plate tower or a packed tower.

[0157] Preferably, the height of the distillation tower is 5-20m.

[0158] Preferably, the feed position of the raw gas is 2-15m of the distillation tower.

[0159] According to some embodiments of the present invention, the distillation conditions may include: a feed temperature of -150°C to -50°C, and a feed pressure of 1000-4000 kPa.

[0160] The second aspect of the present invention provides a system for preparing ethylene by membrane separation combined with distillation, the system comprising: a first polymer membrane separation unit MEM1, a second polymer membrane separation unit MEM2, a third polymer membrane separation unit MEM3 and a distillation tower E1, wherein:

[0161] The first polymer membrane separation unit MEM1 is used to perform a first polymer membrane separation on the raw gas S1 to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4;

[0162] The second polymer membrane separation unit MEM2 is connected to the first polymer membrane separation unit MEM1, and is used for performing a second polymer membrane separation on the first hydrogen-rich gas S3 from the first polymer membrane separation unit MEM1 to obtain high-purity hydrogen gas S5 and a second methane-rich gas S6;

[0163] The third polymer membrane separation unit MEM3 is connected to the first polymer membrane separation unit MEM1 and the second polymer membrane separation unit MEM2, and is used for mixing the first methane-rich gas S4 from the first polymer membrane separation unit MEM1 and the second methane-rich gas S6 from the second polymer membrane separation unit MEM2, and then performing third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9;

[0164] The distillation tower E1 is connected to the third polymer membrane separation unit MEM3 and is used for distilling the third methane-rich gas S9 from the third polymer membrane separation unit MEM3 to obtain a fourth methane-rich gas and an ethylene-rich product gas S14.

[0165] According to some embodiments of the present invention, a gas mixing device is respectively provided before the first polymer membrane separation unit and before the third polymer membrane separation unit.

[0166] According to some embodiments of the present invention, a heat exchange device is provided between the third polymer membrane separation unit and the distillation tower.

[0167] In the present invention, a compression device and a heat exchange device are further provided between the third polymer membrane separation unit and the first polymer membrane separation unit.

[0168] According to a preferred embodiment, Figure 3 The use of the method for preparing ethylene by membrane separation combined with distillation of the present invention in the system for preparing ethylene by membrane separation combined with distillation of the present invention specifically includes the following process:

[0169] (a) mixing the feed gas S1 with the reflux gas S11 to obtain S2, and passing the mixture into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4;

[0170] (b) sending the first hydrogen-rich gas S3 obtained in step (a) into a second polymer membrane separation unit for second polymer membrane separation to obtain high-purity hydrogen S5 and a second methane-rich gas S6;

[0171] (c) mixing the first methane-rich gas S4 obtained in step (a) and the second methane-rich gas S6 obtained in step (b) to obtain S7, and sending the mixture to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the second hydrogen-rich gas S8 is compressed to obtain S10, and is heat exchanged to obtain S11, which is returned to step (a) and subjected to the first polymer membrane separation together with the cracked gas;

[0172] (d) The third methane-rich gas S9 obtained in step (c) is heat exchanged to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 (fourth methane-rich gas) is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower.

[0173] The present invention will be described in detail below through examples.

[0174] The following embodiments will be combined with Figure 3 The method for preparing ethylene by membrane separation combined with distillation of the present invention is described below. Unless otherwise specified, the specific operations of the process are as described above. The first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are performed by cross-flow separation.

[0175] In the following examples, the volume fraction of each gas is measured by gas chromatography.

[0176] 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; and the thickness of the dense layer was measured by scanning electron microscopy.

[0177] The following preparation examples are used to illustrate the preparation of polyimide random copolymers and polyimide flat films.

[0178] Preparation Example 1

[0179] (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;

[0180] (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 the mixture was rinsed with a mixed solution of water and ethanol (1500 mL: 1500 mL) (3 times), and a polyimide random copolymer was obtained after suction filtration and drying. 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 shows that the polyimide random copolymer has a structure shown in formula (I), wherein X is Xc, Y is Ya, and Z and Z' are both Z1. In addition, no raw materials are detected in the remaining liquid phase after the polyimide is precipitated, indicating that all raw materials participate in the reaction.

[0181] 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 peak at is the stretching vibration peak of CF, that is, the appearance of the above characteristic peaks indicates the successful synthesis of PI-1.

[0182] The polyimide obtained above is used to prepare a gas separation membrane:

[0183] The above-mentioned polyimide random copolymer is added to NMP at a solid content of 15wt%, and stirred in a shaking table at 50°C until the above-mentioned polyimide random copolymer is completely dissolved to obtain a uniform and stable casting liquid; the casting liquid is cooled to room temperature, ultrasonically degassed, and then uniformly coated on the surface of a clean glass plate, placed in a blast oven at 70°C for 6 hours to remove a large amount of solvent, and then moved 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 is immersed in deionized water until the membrane falls off the surface of the glass plate to obtain a gas separation membrane (flat membrane).

[0184] Preparation Example 2

[0185] (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) (0.01 mmol) and 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA) (0.09 mmol) were mixed under mechanical stirring, and then added to the above system at 0°C and polycondensed for 12 hours to obtain a material containing polyimide acid;

[0186] (2) Add a mixture of acetic anhydride (0.36 mmol) and pyridine (0.36 mmol) to the polyimide acid material obtained in step (1), and perform intramolecular dehydration at 200°C for 24 hours to obtain a material containing polyimide; then add 600 mL of N-methylpyrrolidone (NMP) to the polyimide-containing material for dilution, and pour the diluted material into a mixed solvent of water and ethanol (500 mL: 500 mL) under stirring to precipitate polyimide to obtain polyimide, and then rinse with a mixed solution of water and ethanol (1500 mL: 1500 mL) (3 times), and obtain a polyimide random copolymer after suction filtration and drying. Infrared testing shows that the polyimide random copolymer has a structure shown in formula (I), wherein X is Xc, Y is Ya, and Z and Z' are both Z1. In addition, no raw materials can be detected in the remaining liquid phase after the polyimide is precipitated, indicating that all raw materials participate in the reaction.

[0187] The polyimide obtained above is used to prepare a gas separation membrane:

[0188] The above-mentioned polyimide random copolymer is added to NMP at a solid content of 40wt%, and stirred in a shaking table at 50°C until PI-2 is completely dissolved to obtain a uniform and stable casting liquid; the casting liquid is cooled to room temperature, ultrasonically degassed and evenly coated on the surface of a clean glass plate, placed in a 70°C forced air oven for 6 hours to remove a large amount of solvent, and then moved 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 is immersed in deionized water until the membrane falls off the surface of the glass plate to obtain a gas separation membrane (flat membrane).

[0189] The following preparation examples are used to illustrate the preparation of polyimide random copolymers or polyimide flat films.

[0190] Preparation Example 1

[0191] (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;

[0192] (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 the mixture was rinsed with a mixed solution of water and ethanol (1500 mL: 1500 mL) (3 times), and a polyimide random copolymer was obtained after suction filtration and drying. 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 shows that the polyimide random copolymer has a structure shown in formula (I), wherein X is Xc, Y is Ya, and Z and Z' are both Z1. In addition, no raw materials are detected in the remaining liquid phase after the polyimide is precipitated, indicating that all raw materials participate in the reaction.

[0193] Figure 1 Medium, 1784cm -1 and 1730cm -1The 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 shown in FIG. 4 , and the appearance of the characteristic peak indicates the successful synthesis of the random copolymer.

[0194] The polyimide obtained above is used to prepare a gas separation membrane:

[0195] The above-mentioned polyimide random copolymer is added to NMP at a solid content of 15wt%, and stirred in a shaking table at 50°C until PI-4 is completely dissolved to obtain a uniform and stable casting liquid; the casting liquid is cooled to room temperature, ultrasonically degassed and evenly coated on the surface of a clean glass plate, placed in a blast oven at 70°C for 6 hours to remove a large amount of solvent, and then moved 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 is immersed in deionized water until the membrane falls off the surface of the glass plate to obtain a gas separation membrane (flat membrane).

[0196] Preparation Example 2

[0197] (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 tripterycene 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;

[0198] (2) Add a mixture of acetic anhydride (0.36 mmol) and pyridine (0.36 mmol) to the polyimide acid material obtained in step (1), and perform intramolecular dehydration at 180°C for 18 hours to obtain a material containing polyimide; then add 600 mL of N-methylpyrrolidone (NMP) to the polyimide-containing material for dilution, and pour the diluted material into a mixed solvent of water and ethanol (500 mL: 500 mL) under stirring to precipitate polyimide to obtain polyimide, and then rinse with a mixed solution of water and ethanol (1500 mL: 1500 mL) (3 times), and obtain a polyimide random copolymer after suction filtration and drying. Infrared testing shows that the polyimide random copolymer has 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 can be detected in the remaining liquid phase after the polyimide is precipitated, indicating that all raw materials participate in the reaction.

[0199] The polyimide obtained above is used to prepare a gas separation membrane:

[0200] The polyimide random copolymer prepared as above is added into NMP at a solid content of 15 wt%, and stirred in a shaking table at 50° C. until the polyimide random copolymer is completely dissolved to obtain a uniform and stable casting liquid; the casting liquid is cooled to room temperature, ultrasonically degassed, and then uniformly coated on the surface of a clean glass plate, placed in a blast oven at 70° C. for 6 hours to remove a large amount of solvent, and then moved to a vacuum oven (dried at 120° C. for 12 hours) to further remove the solvent; thereafter, after cooling to room temperature, the glass plate with the membrane adhered thereto is immersed in deionized water until the membrane falls off the surface of the glass plate to obtain a gas separation membrane (flat membrane).

[0201] The following preparation examples are used to illustrate the preparation of polyimide-based hollow fiber membranes.

[0202] Preparation Example 1

[0203] (1) Add 27.5 wt % of the random copolymer of polyimide obtained in the above-mentioned 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 into a kettle with a stirring device, heat to 50° C., and stir (speed 1200 r / min) for 36 hours under nitrogen protection. After stopping stirring, degas at 25° C., -0.1 MPa, and speed 50 r / min for 24 hours, and then filter through a filter screen (pore size 100 mesh) at 50° C. to obtain a casting solution;

[0204] (2) using a metering pump to deliver the casting liquid and the inner core liquid (NMP: water = 95wt%: 5wt%) to the hollow spinneret, respectively, the inner core liquid and the casting liquid are extruded through the spinneret together to obtain a hollow fiber, the hollow fiber is 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 is 70°C; the flow rates of the casting liquid and the inner core liquid entering the hollow spinneret are 8mL / min and 2.5mL / min, respectively;

[0205] (3) The polyimide hollow fiber membrane precursor obtained in step (2) is wound up by a winding machine, and then extracted twice in water, ethanol, and n-hexane in sequence, and the extraction time is 3 hours; then the extracted hollow fiber membrane is 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 is 1 m / s.

[0206] The obtained hollow fiber membrane was characterized by mercury intrusion method, and the porosity of the support layer was 67.5%. The thickness of the dense layer was 400nm. The electron microscope scanning image of the cross section of the hollow fiber membrane is shown in Figure 2.

[0207] Preparation Example 2

[0208] (1) Add 30 wt % of the random copolymer of polyimide 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 into a kettle with a stirring device, heat to 50° C., and stir (speed 600 r / min) for 36 hours under nitrogen protection. After stopping stirring, degas at 25° C., -0.1 MPa, and speed 30 r / min for 24 hours, and then filter through a filter screen (pore size 100 mesh) at 50° C. to obtain a casting solution;

[0209] (2) using a metering pump to deliver the casting liquid and the inner core liquid (NMP: water = 95wt%: 5wt%) to the hollow spinneret, respectively, the inner core liquid and the casting liquid are extruded through the spinneret together to obtain a hollow fiber, the hollow fiber is 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) is 75°C; the flow rates of the casting liquid and the inner core liquid entering the hollow spinneret are 6mL / min and 2mL / min, respectively;

[0210] (3) The polyimide hollow fiber membrane precursor obtained in step (2) is wound up by a winding machine, and then extracted twice in water, ethanol, and n-hexane in sequence, and the extraction time is 3 hours; then the extracted hollow fiber membrane is 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 is 1 m / s.

[0211] 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.

[0212] Example 1

[0213] The cracking gas (ethylene cracking gas) of a certain refinery contains 15% hydrogen, 30% methane, 35% ethylene, 8% ethane, 10% propylene and 2% other alkane components by volume.

[0214] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; wherein the permeation pressure during the first polymer membrane separation process is 1600 kPa;

[0215] (2) sending the first hydrogen-rich gas S3 obtained in step (1) into a second polymer membrane separation unit for second polymer membrane separation to obtain high-purity hydrogen S5 and second methane-rich gas S6; wherein the permeation pressure during the second polymer membrane separation process is 800 kPa;

[0216] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 500 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) for first polymer membrane separation together with the cracked gas;

[0217] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 10m, the position where the third methane-rich gas S9 enters the distillation tower is 8.6m, the feed temperature of the third methane-rich gas S9 is -90°C, and the feed pressure is 2500kPa.

[0218] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, which are all polyimide hollow fiber membranes prepared in the above-mentioned Preparation Example 1, wherein the flux of the hollow fiber membrane to hydrogen is 100GPU, the flux to methane is about 3GPU, the flux to ethylene is about 2.5GPU, and the flux to other alkane components is about 2GPU.

[0219] The gas composition of each flow is shown in Table 1.

[0220] Table 1

[0221] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 35.0 8.0 10.0 S2 60.0 14.6 16.4 3.6 4.6 S3 85.0 6.3 6.0 1.1 1.4 S4 58.0 15.2 17.2 3.8 4.8 S5 99.0 0.5 0.4 0.1 0 S6 49.1 20.7 20.6 3.9 4.8 S7 57.8 15.3 17.3 3.8 4.8 S8 88.9 4.6 4.5 0.8 1.0 S9 2.3 34.4 40.2 9.2 11.5 S10 88.9 4.6 4.5 0.8 1.0 S11 88.9 4.6 4.5 0.8 1.0 S12 2.3 34.4 40.2 9.2 11.5 S13 6.1 90.0 3.8 0.1 0 S14 0 0 62.8 14.8 18.6

[0222] Example 2

[0223] The cracking gas (ethylene cracking gas) of a certain refinery contains 17% hydrogen, 31% methane, 33% ethylene, 7% ethane, 10% propylene and 2% other alkane components by volume.

[0224] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; wherein the permeation pressure during the first polymer membrane separation process is 3600 kPa;

[0225] (2) sending the first hydrogen-rich gas S3 obtained in step (1) into a second polymer membrane separation unit for second polymer membrane separation to obtain high-purity hydrogen S5 and second methane-rich gas S6; the permeation pressure during the second polymer membrane separation process is 600 kPa;

[0226] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 500 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) for first polymer membrane separation together with the cracked gas;

[0227] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 20m, the position where the third methane-rich gas S9 enters the distillation tower is 15m, the feed temperature of the third methane-rich gas S9 is -150°C, and the feed pressure is 4000kPa.

[0228] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, which are all polyimide hollow fiber membranes prepared in the above-mentioned Preparation Example 1, wherein the flux of the hollow fiber membrane to hydrogen is 100GPU, the flux to methane is about 3GPU, the flux to ethylene is about 2.5GPU, and the flux to other alkane components is about 2GPU.

[0229] The gas composition of each flow is shown in Table 2.

[0230] Table 2

[0231] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 17.0 31.0 33.0 7.0 10.0 S2 61.2 15.2 15.3 3.1 4.4 S3 79.3 8.9 8.0 1.4 2.0 S4 58.0 16.3 16.6 3.4 4.8 S5 99.1 0.4 0.3 0.1 0.1 S6 69.5 13.0 11.9 2.1 3.0 S7 59.2 16.0 16.1 3.2 4.6 S8 84.8 6.7 5.8 1.0 1.4 S9 3.5 36.0 38.4 8.1 11.6 S10 84.8 6.7 5.8 1.0 1.4 S11 84.8 6.7 5.8 1.0 1.4 S12 3.5 36.0 38.4 8.1 11.6 S13 8.3 86.8 4.7 0.2 0 S14 0 0 62.3 13.8 19.9

[0232] Example 3

[0233] The cracking gas (ethylene cracking gas) of a certain refinery contains 15% hydrogen, 30% methane, 34% ethylene, 8% ethane, 10% propylene and 3% other alkane components by volume.

[0234] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; wherein the permeation pressure during the first polymer membrane separation process is 1800 kPa;

[0235] (2) sending the first hydrogen-rich gas S3 obtained in step (1) into a second polymer membrane separation unit for second polymer membrane separation to obtain high-purity hydrogen S5 and second methane-rich gas S6; wherein the permeation pressure during the second polymer membrane separation process is 500 kPa;

[0236] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 900 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) for first polymer membrane separation together with the cracked gas;

[0237] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 5m, the position where the third methane-rich gas S9 enters the distillation tower is 2m, the feed temperature of the third methane-rich gas S9 is -50°C, and the feed pressure is 1000kPa.

[0238] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, all of which are polybenzimidazole hollow fiber membrane components (China Science Energy Materials Technology (Dalian) Co., Ltd., PBI membrane). Among them, the flux of the hollow fiber membrane component to hydrogen is 50GPU, the flux to methane is about 3GPU, the flux to ethylene is about 2.5GPU, and the flux to other alkane components is about 2GPU.

[0239] The gas composition of each flow is shown in Table 3.

[0240] Table 3

[0241] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 34.0 8.0 10.0 S2 21.2 35.5 26.7 6.3 7.9 S3 27.8 33.8 23.8 5.6 7.0 S4 19.6 35.9 27.5 6.5 8.1 S5 86.1 7.3 4.1 1.0 1.2 S6 22.9 36.0 25.4 6.0 7.5 S7 20.2 36.0 27.1 6.4 8.0 S8 21.8 36.0 26.0 6.1 7.7 S9 0 34.9 40.3 9.5 11.8 S10 21.8 36.0 26.0 6.1 7.7 S11 21.8 36.0 26.0 6.1 7.7 S12 0 34.9 40.3 9.5 11.8 S13 0 80.1 18.9 1.0 0 S14 0 0 56.7 16.0 21.0

[0242] Example 4

[0243] The cracking gas (ethylene cracking gas) of a certain refinery contains 15% hydrogen, 30% methane, 35% ethylene, 8% ethane, 10% propylene and 2% other alkane components by volume.

[0244] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; wherein the permeation pressure during the first polymer membrane separation process is 1500 kPa;

[0245] (2) sending the first hydrogen-rich gas S3 obtained in step (1) to a second polymer membrane separation unit (second polymer membrane (MEM2) for second polymer membrane separation to obtain high-purity hydrogen S5 and second methane-rich gas S6; wherein the permeation pressure during the second polymer membrane separation process is 990 kPa;

[0246] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 1200 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) to be separated by the first polymer membrane together with the cracked gas;

[0247] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 8m, the position where the third methane-rich gas S9 enters the distillation tower is 6m, the feed temperature of the third methane-rich gas S9 is -90°C, and the feed pressure is 2500kPa.

[0248] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, all of which are hollow fiber membrane components made of polysulfone (PRISM○R), wherein the flux of the hollow fiber membrane component to hydrogen is 20GPU, the flux to methane is about 0.15GPU, the flux to ethylene is about 0.1GPU, and the flux to other alkane components is about 0.1GPU.

[0249] The gas composition of each flow is shown in Table 4.

[0250] Table 4

[0251] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 35.0 8.0 10.0 S2 28.3 25.9 29.1 6.7 8.3 S3 39.7 24.2 23.0 5.3 6.6 S4 11.7 28.5 38.0 8.7 10.9 S5 99.0 0.5 0.3 0.1 0.1 S6 28.2 28.8 27.4 6.3 7.8 S7 20.8 28.6 32.2 7.4 9.2 S8 72.2 12.5 9.8 2.2 2.8 S9 2.9 34.2 40.0 9.1 11.4 S10 72.2 12.5 9.8 2.2 2.8 S11 72.2 12.5 9.8 2.2 2.8 S12 2.9 34.2 40.0 9.1 11.4 S13 7.5 88.4 4.0 0.1 0 S14 0 0 62.8 14.8 18.7

[0252] Example 5

[0253] The cracking gas (ethylene cracking gas) of a certain refinery contains 15% hydrogen, 30% methane, 35% ethylene, 8% ethane, 10% propylene and 2% other alkane components by volume.

[0254] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; the permeation pressure during the first polymer membrane separation process is 3500 kPa;

[0255] (2) sending the first hydrogen-rich gas S3 obtained in step (1) to a second polymer membrane separation unit (second polymer membrane (MEM2) for second polymer membrane separation to obtain high-purity hydrogen S5 and second methane-rich gas S6; wherein the permeation pressure during the second polymer membrane separation process is 700 kPa;

[0256] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 500 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) for first polymer membrane separation together with the cracked gas;

[0257] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 10m, the position where the third methane-rich gas S9 enters the distillation tower is 9m, the feed temperature of the third methane-rich gas S9 is -90°C, and the feed pressure is 2500kPa.

[0258] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, all of which are hollow fiber membrane components made of polysulfone (PRISM○R), wherein the flux of the hollow fiber membrane component to hydrogen is 20GPU, the flux to methane is about 0.15GPU, the flux to ethylene is about 0.1GPU, and the flux to other alkane components is about 0.1GPU.

[0259] Among them, the composition of the gas of each logistics is shown in Table 5.

[0260] Table 5

[0261] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 35.0 8.0 10.0 S2 28.6 26.2 28.8 6.6 8.2 S3 40.0 24.3 22.7 5.2 6.5 S4 22.8 27.2 31.8 7.3 8.1 S5 97.7 1.2 0.7 0.2 0.2 S6 13.0 35.1 33.0 7.5 9.4 S7 20.3 29.2 32.1 7.3 9.2 S8 63.5 16.5 12.7 3.0 3.6 S9 0.5 35.1 41.0 9.4 11.7 S10 63.5 16.5 12.7 3.0 3.6 S11 63.5 16.5 12.7 3.0 3.6 S12 0.5 35.1 41.0 9.4 11.7 S13 1.3 95.2 3.4 0.1 0 S14 0 0 63.0 14.8 18.5

[0262] Example 6

[0263] The cracking gas (ethylene cracking gas) of a certain refinery contains 15% hydrogen, 30% methane, 35% ethylene, 8% ethane, 10% propylene and 2% other alkane components by volume.

[0264] (1) The above cracked gas S1 is mixed with S11 to obtain S2, and S2 is passed into a first polymer membrane separation unit for first polymer membrane separation to obtain a first hydrogen-rich gas S3 and a first methane-rich gas S4; wherein the permeation pressure during the first polymer membrane separation process is 1600 kPa;

[0265] (2) sending the first hydrogen-rich gas S3 obtained in step (1) to a second polymer membrane separation unit (the second polymer membrane (MEM2) is used for second polymer membrane separation to obtain high-purity hydrogen gas S5 and second methane-rich gas S6; wherein the permeation pressure during the second polymer membrane separation process is 100 kPa;

[0266] (3) The first methane-rich gas S4 obtained in step (1) and the second methane-rich gas S6 obtained in step (2) are mixed to obtain S7, and sent to a third polymer membrane separation unit for third polymer membrane separation to obtain a second hydrogen-rich gas S8 and a third methane-rich gas S9; wherein the permeation pressure during the third polymer membrane separation process is 500 kPa; the obtained second hydrogen-rich gas S8 is compressed to obtain S10, and the obtained second hydrogen-rich gas S11 is returned to step (1) for first polymer membrane separation together with the cracked gas;

[0267] (4) The third methane-rich gas S9 obtained in step (3) is subjected to heat exchange to obtain S12, and then S12 is sent to a distillation tower for distillation, and a methane-rich product gas S13 is obtained at the top of the tower, and an ethylene-rich product gas S14 is obtained at the bottom of the tower; wherein the height of the distillation tower is 5m, the position where the third methane-rich gas S9 enters the distillation tower is 3m, the feed temperature of the third methane-rich gas S9 is -90°C, and the feed pressure is 2500kPa.

[0268] Among them, the membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same, all of which are polyimide hollow fiber membranes prepared in the above-mentioned Preparation Example 2, wherein the flux of MEM1 to hydrogen is 100GPU, the flux to methane is about 3GPU, the flux to ethylene is about 2.5GPU, and the flux to other alkane components is about 2GPU, and the permeation pressure of the hollow fiber membrane is 1600kPa.

[0269] Among them, the gas composition of each logistics is shown in Table 6.

[0270] Table 6

[0271] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 35.0 8.0 10.0 S2 76.1 8.7 9.8 2.2 2.7 S3 96.3 1.5 1.5 0.3 0.3 S4 63.4 13.1 15.0 3.4 4.2 S5 99.9 0.1 0 0 0 S6 96.1 1.6 1.6 0.3 0.4 S7 75.5 8.9 10.1 2.2 2.8 S8 95.6 1.8 1.7 0.3 0.4 S9 5.7 33.3 38.8 8.9 11.1 S10 95.6 1.8 1.7 0.3 0.4 S11 95.6 1.8 1.7 0.3 0.4 S12 5.7 33.3 38.8 8.9 11.1 S13 13.9 81.5 4.5 0.1 0 S14 0 0 62.6 14.9 18.8

[0272] Comparative Example 1

[0273] In this embodiment, the feed gas is the same as that in Example 1. The process of this comparative example is a traditional pre-dehydrogenation process (the purpose is to remove and recover as much hydrogen as possible before entering the distillation tower, and remove most of the methane), and the process is a multi-stage cooling process, which is carried out in a system including 5 tower separators 1-5 connected in sequence and a distillation tower E-2, and heat exchangers C1'-C5' are respectively arranged between the 5 separators.

[0274] Among them, after each stage of cooling, the light components methane and hydrogen will be enriched at the top of the separator and enter the next stage of cooling and separator. The bottom of the separator is hydrocarbon condensate, which is discharged through the bottom and enters the distillation tower to further remove the remaining methane and hydrogen. Finally, the top of the fifth separator obtains hydrogen with a high concentration, and the liquid at the bottom of the tower is methane condensate with a high concentration, which is discharged and collected. The bottom liquid in the distillation tower is a mixed liquid of hydrocarbons such as ethylene and propylene with a high concentration, which is discharged into the next separation unit, and the top is a small amount of methane gas with a high concentration. Among them, the composition of the gas of each logistics is shown in Table 7.

[0275] Table 7

[0276] No.\composition mol% hydrogen Methane Ethylene Ethane Propylene S1 15.0 30.0 35.0 8.0 10.0 S2’ 15.0 30.0 35.0 8.0 10.0 S3’ 25.1 41.0 26.8 4.7 2.2 S4’ 1.2 14.9 46.2 12.5 20.7 S5’ 25.1 41.0 26.8 4.7 2.2 S6’ 34.5 47.4 15.7 2.0 0.3 S7’ 1.5 25.0 54.5 11.6 7.1 S8’ 34.5 47.4 15.7 2.0 0.3 S9’ 47.8 47.6 4.3 0.3 0.1 S10’ 1.9 47.1 43.9 6.2 0.9 S11’ 47.8 47.6 4.3 0.3 0.1 S12’ 73.0 26.8 0.2 0 0 S13’ 3.2 84.3 11.6 0.8 0.1 S14’ 73.0 26.8 0.2 0 0 S15’ 94.3 5.7 0 0 0 S16’ 3.0 96.1 0.9 0 0 S17’ 6.0 93.2 0.8 0 0

[0277] Comparative Example 2

[0278] The process was carried out in the same manner as in Example 1, except that the third polymer membrane was not provided. The composition of the gas of each flow is shown in Table 8.

[0279] Table 8

[0280]

[0281]

[0282] According to the data of the above embodiments and comparative examples, it can be known that the method of the present invention can effectively separate the two small molecules of hydrogen and methane that are difficult to separate from ethylene in the raw gas through the membrane separation process through the selective separation of the membrane, and greatly reduce the hydrogen in the cracking gas through the primary membrane separation operation. After the secondary membrane and tertiary membrane separation, the hydrogen concentration can reach 99% purity. For example, compared with Example 1, the hydrogen concentration in the dehydrogenated gas S15' in Comparative Example 1 is significantly lower, and the hydrogen loss in its output demethanized gas is relatively large, and the methane concentration is also much lower than the methane concentration of S13 in Example 1. It can be seen from Comparative Example 2 that the hydrogen concentration in S13 and S5 (S13: demethanized gas, S5: helium purified gas) is much lower than the hydrogen concentration in Example 1 of the present invention, and the methane concentration of the output gas S13 is lower than the methane concentration of the output gas S13 of the three membranes (the system provided by the present invention). The membrane process unit in the present invention has a significant effect on the purification and recovery of hydrogen and methane.

[0283] 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).

[0284] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing ethylene by membrane separation combined with distillation, characterized in that: The method comprises the following steps: (1) subjecting the raw gas to a first polymer membrane separation to obtain a first hydrogen-rich gas and a first methane-rich gas; (2) subjecting the first hydrogen-rich gas to a second polymer membrane separation to obtain high-purity hydrogen and a second methane-rich gas; (3) mixing the first methane-rich gas and the second methane-rich gas and performing a third polymer membrane separation to obtain a second hydrogen-rich gas and a third methane-rich gas; optionally, returning the second hydrogen-rich gas to step (1) and performing a first polymer membrane separation together with the feed gas; (4) distilling the third methane-rich gas to obtain a fourth methane-rich gas and an ethylene-rich product gas; The materials of the polymer membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are the same or different; the material of the polymer membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation is a polyimide random copolymer, and 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 formula (X1) to formula (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 formula (Y1) to formula (Y5); <h2 style=";text-align:left;direction:ltr"> <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) (Y4) (Y5), 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; 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, C1-C4 alkyl or C1-C4 halogenated alkyl.

2. The method according to claim 1, wherein: The raw material gas is cracked gas, in which the volume fraction of ethylene is 30-40 mol%, the volume fraction of methane is 25-35 mol%, and the volume fraction of hydrogen is 10-20 mol%.

3. The method according to claim 1 or 2, wherein: The permeation pressure during the first polymer membrane separation process, the permeation pressure during the second polymer membrane separation process, and the permeation pressure during the third polymer membrane separation process are each independently 100 kPa-3600 kPa; and / or, the permeation pressure during the separation process of the first polymer membrane is 1200 kPa-3600 kPa; and / or, the permeation pressure of the second polymer membrane during separation is 500 kPa-1000 kPa; And / or, the permeation pressure during the separation process of the third polymer membrane is 500 kPa-1500 kPa.

4. The method according to claim 1 or 2, wherein: The membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation each independently have a flux of 10-120 GPU for hydrogen, a flux of 0.1-3 GPU for methane and a flux of 0.1-2.5 GPU for ethylene.

5. The method according to claim 1 or 2, wherein: The membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are each independently selected from at least one of a hollow fiber membrane, a flat membrane and a tubular membrane.

6. The method according to claim 5, wherein: The polymer membranes used in the first polymer membrane separation, the second polymer membrane separation and the third polymer membrane separation are polyimide-based hollow fiber membranes.

7. The method according to claim 6, 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 1000nm, and 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, wherein: m and n are each independently an integer of 50-1000.

10. The method according to claim 1 or 9, 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, 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; Or, X is Xa, Y is Yd, and Z and Z' are both Z1; Or, X is Xb, Y is Ya, and Z and Z' are both Z1; Or, X is Xb, Y is Yb, and Z and Z' are both Z1; Or, X is Xb, Y is Yd, and Z and Z' are both Z1; Or, 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; Or, 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; Or, X is Xc, Y is Yd, and Z and Z' are both Z1; Or, X is Xb, Y is Ya, and Z and Z' are both Z3; Or, X is Xb, Y is Yb, and Z and Z' are both Z3; Or, X is Xb, Y is Yd, and Z and Z' are both Z3; Or, X is Xc, Y is Ya, and Z and Z' are both Z3; Or, 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 any one of claims 6 to 13, wherein: The polyimide-based hollow fiber membrane is prepared according to a method comprising the following steps: (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; (2) extruding the inner 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 good solvent of 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; And / or, the casting solution is prepared according to a method comprising the following steps: stirring the polyimide, diluent and additive at 20-50° C. and 100-1200 r / min for 12-48 hours, and then removing impurities by vacuum degassing and filtering at 20-50° C.; And / or, in step (2), the inner 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 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 inner core liquid is 2-10 mL / min; and / or, prior to solidification, passing the extruded hollow fibers through an air gap; and / or, the solidification is carried out in a coagulation bath; And / or, in step (3), the winding speed is 0.5-2 m / s; And / or, the extractant used for extraction is selected from at least one of water, C1-C4 saturated monohydric alcohol and C5-C7 alkane; And / or, the extraction conditions include: temperature of 20-35°C and time of 3-48h; And / or, the extraction further includes a drying step.

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 vacuum degassing conditions include: pressure of -0.1MPa to -0.095MPa, temperature of 20-30°C, rotation speed of 10-50r / min, and time of 12-24h; and / or, the solvent A accounts for 50-99wt% of the total weight of the inner core liquid; And / or, the height of the air gap is 5-30 cm; And / or, the air gap is heated by an annular sleeve, and the temperature is controlled to be 70-150°C; And / or, the solvent C is selected from at least one of a C1-C4 saturated monohydric alcohol, γ-butyrolactone and water; And / or, the drying conditions include: temperature of 20-35°C and time of 2-15h; 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; And / or, the extrusion temperature is 60-70°C; 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.

17. The method according to claim 16, wherein: The poor solvent of 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 solvent A accounts for 60-95% of the total weight of the inner core liquid.

18. The method according to claim 1, wherein: The distillation is carried out in a distillation tower, which is a plate tower or a packed tower; And / or, the distillation conditions include: a feed temperature of -150°C to -50°C, and a feed pressure of 1000-4000 kPa.

19. The method according to claim 18, wherein: The height of the distillation tower is 5-20m; And / or, the feed position of the raw gas is 2-15m of the distillation tower.

20. The method according to claim 1, wherein: The method is carried out in a system for preparing ethylene by membrane separation combined with rectification, the system comprising: a first polymer membrane separation unit, a second polymer membrane separation unit, a third polymer membrane separation unit and a rectification tower, wherein: The first polymer membrane separation unit is used to perform a first polymer membrane separation on the feed gas to obtain a first hydrogen-rich gas and a first methane-rich gas; The second polymer membrane separation unit is connected to the first polymer membrane separation unit, and is used for performing a second polymer membrane separation on the first hydrogen-rich gas from the first polymer membrane separation unit to obtain high-purity hydrogen gas and a second methane-rich gas; The third polymer membrane separation unit is connected to the first polymer membrane separation unit and the second polymer membrane separation unit, and is used for mixing the first methane-rich gas from the first polymer membrane separation unit and the second methane-rich gas from the second polymer membrane separation unit, and then performing third polymer membrane separation to obtain a second hydrogen-rich gas and a third methane-rich gas; The distillation tower is connected to the third polymer membrane separation unit and is used for distilling the third methane-rich gas from the third polymer membrane separation unit to obtain a fourth methane-rich gas and an ethylene-rich product gas.

Images