A molecular sieve membrane element for natural gas denitrification, a preparation method thereof and a membrane separator for natural gas denitrification

Abstract

The present application relates to the technical field of separation of nitrogen and methane in nitrogen-containing natural gas, and discloses a molecular sieve membrane element for natural gas denitrification, a preparation method thereof and a membrane separator for natural gas denitrification. The problems of incompatibility between a porous silicon carbide support layer and aluminum phosphate molecular sieve seeds and a film forming system, a large number of additional transition layer coating processes and difficult-to-guarantee interfacial bonding stability are solved. The molecular sieve membrane element for natural gas denitrification comprises a porous silicon carbide support layer, a porous transition layer and a microporous molecular sieve selection layer. The porous transition layer is formed in situ by surface area oxidation and heat treatment of the porous silicon carbide support layer and contains a silicon oxide phase distributed in the pore wall, so that the surface pore size is smaller than the pore size of the bulk of the porous silicon carbide support layer and a fine pore film forming surface is formed. The microporous molecular sieve selection layer is composed of mutually connected aluminum phosphate molecular sieve crystals with AEI topology.

Description

Technical Field

[0001] This invention relates to the field of nitrogen and methane separation technology in nitrogen-containing natural gas, specifically to a molecular sieve membrane element for natural gas denitrification, its preparation method, and a membrane separator for natural gas denitrification. Background Technology

[0002] Natural gas denitrification is an important way to improve the calorific value and quality of natural gas. Methane and nitrogen are both non-polar gases with similar molecular dynamic diameters and some similar physical properties. Specifically, the molecular dynamic diameter of nitrogen is approximately 3.64 Å, while that of methane is approximately 3.80 Å, a difference of only about 0.16 Å, making the separation of methane and nitrogen quite challenging. Existing separation methods include cryogenic separation, which mainly utilizes the difference in boiling points between methane and nitrogen through low-temperature liquefaction and distillation, typically requiring high equipment investment and operating energy consumption; and adsorption separation, which utilizes differences in adsorption capacity, adsorption equilibrium, or diffusion rate between methane and nitrogen, potentially leading to problems such as methane loss, adsorbent regeneration, and adaptability to different operating conditions. In contrast, membrane separation methods offer advantages such as compact equipment, shorter process flow, and ease of modular arrangement, making it a promising technological route for natural gas denitrification.

[0003] Molecular sieve membranes possess uniformly sized micropores, enabling selective separation by utilizing the differences in diffusion rates of different gas molecules within these micropores. AlPO-18 type aluminum phosphate molecular sieves exhibit an AEI topology, where their framework tetrahedrons are interconnected in an AEI-type relationship to form a three-dimensional microporous structure with interconnected eight-membered ring pores. The effective size of these eight-membered ring pores is approximately 3.8 Å, close to the molecular dynamics size of nitrogen and methane. Compared to methane, smaller nitrogen gas experiences less diffusion restriction when entering and passing through these micropores, while methane faces more significant spatial confinement at the pore openings, resulting in a higher diffusion rate for nitrogen. A continuous membrane layer composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals can primarily utilize the diffusion rate difference between nitrogen and methane, combined with molecular size sieving, to preferentially allow nitrogen to permeate relative to methane, thus demonstrating promising applications for natural gas denitrification.

[0004] Patent document CN104785125A discloses an AlPO-18 molecular sieve membrane and its preparation method. The method first prepares AlPO-18 type aluminum phosphate molecular sieve seed crystals, which are then placed on the inner or outer surface of a porous tubular support. The tubular support with seed crystals is then placed in a film-forming system containing an aluminum source, a phosphorus source, an organic structure-directing agent, and water for hydrothermal crystallization, allowing the AlPO-18 type aluminum phosphate molecular sieve crystals to grow and interconnect. Finally, the organic structure-directing agent is removed by calcination. The tubular support used in this document is made of alumina, mullite, or stainless steel.

[0005] It is worth noting that after the AlPO-18 type aluminum phosphate molecular sieve membrane completes hydrothermal crystallization, it needs to be calcined at high temperature to remove the organic structural guiding agent in the molecular sieve channels. During the heating and cooling processes, the differences in thermal expansion behavior between the molecular sieve membrane layer and the support due to their different coefficients of thermal expansion, as well as the temperature gradient between different locations in the tube, will induce thermal stress in the membrane layer, thereby increasing the risk of membrane cracking or defects. Specifically, it is difficult to achieve completely uniform heating in the furnace during the calcination process—whether it is the radiative heating of the resistance furnace or the convective heating of the atmosphere furnace, the temperature and heat flow distribution in different areas of the furnace are inherently different. In addition, the radial and inner / outer, and axial ends and middle of the tubular membrane element are not uniform in terms of heating and heat dissipation, making it difficult for different parts of a single tube to be heated or cooled synchronously and uniformly, thus forming an uneven distribution of thermal stress in the membrane layer. To suppress the aforementioned thermal stress, CN104785125A preferably employs a relatively low heating and cooling rate of 0.5℃ / min to 1℃ / min for calcination, so as to make the temperature of each part of the tube as uniform as possible and reduce the possibility of defects in the film layer. However, while a lower heating and cooling rate is beneficial to reduce the risk of film layer cracking, it will significantly prolong the calcination cycle of a single batch of film elements and reduce production efficiency.

[0006] Furthermore, CN104785125A uses a 100mm long tubular alumina support. For single short-tube membrane elements under laboratory conditions, a slower heating and cooling process can control the calcination process. However, the engineering application of natural gas denitrification does not only require the fabrication of a single membrane tube capable of separating nitrogen and methane, but also necessitates the arrangement of multiple membrane elements with sufficient effective membrane area within a limited membrane separator volume, and the stable batch production, sealing assembly, and long-term operation of these membrane elements. Therefore, the engineering of membrane elements involves not only the molecular sieve material itself, but also the dimensions, thermal conductivity, mechanical strength, thermal shock resistance, pore structure stability, and fabrication repeatability of the tubular support structure. To increase the effective membrane area of ​​a single membrane element and reduce the number of tube end seals per unit membrane area, the tubular membrane element needs to have a large length and aspect ratio. At the same time, to increase the packing density within the membrane separator, the outer diameter of the membrane element should not be too large. Based on factors such as membrane module packing density, effective membrane area per tube, tube mechanical strength, internal gas flow, and the number of end seals, the applicant designed the tubular membrane element as a slender tube structure with an outer diameter of 8mm–20mm, a length of 500mm–1500mm, and a length-to-outer diameter ratio of not less than 40 during the research and development process. One specific specification is an outer diameter of 13mm and a length of 800mm. The above dimensions and proportions are designs proposed by the applicant based on the engineering requirements of natural gas denitrification membrane separators and do not constitute an admission of prior art.

[0007] When tubular membrane elements have a large length and aspect ratio, their fabrication process presents more significant engineering challenges compared to shorter prototypes. During the calcination heating and cooling processes, temperature differences are more easily established between different positions along the axial and radial directions of the tube. When multiple membrane tubes are placed simultaneously in the calcination equipment, temperature uniformity between different tubes and between different parts of the same tube becomes more difficult to control. The superposition of these temperature differences with the differences in thermal expansion behavior between the molecular sieve membrane layer and the support structure can lead to cracks, localized peeling, or non-selective through-hole defects in the membrane layer, thus affecting the consistency and yield of batch production. If thermal stress is still primarily controlled by reducing the heating and cooling rates, the calcination cycle will be further prolonged, reducing the utilization rate of the heat treatment equipment and hindering the large-scale manufacturing of membrane elements. Furthermore, slender tubular membrane elements must withstand bending loads, end constraints, pressure differences, and repeated temperature changes during handling, assembly, sealing, and operation. Deformation, microcracks, or changes in the pore structure of the support structure can all be transmitted to the microporous molecular sieve selective layer, thereby affecting the separation performance of the membrane element. Therefore, from an engineering perspective, the support structure of natural gas denitrification membrane elements should not only meet the requirements of laboratory film formation, but also have high thermal conductivity, mechanical strength, thermal shock resistance and long-term structural stability.

[0008] Silicon carbide possesses high thermal conductivity, mechanical strength, and thermal shock resistance. Employing a porous silicon carbide support layer improves temperature uniformity during heating and cooling of slender tubular membrane elements, reduces temperature gradients between different locations within the tube, and provides high-strength mechanical support and continuous gas transport channels for the microporous molecular sieve selective layer. Therefore, using a porous silicon carbide support layer meets the comprehensive requirements for heat transfer, thermal shock resistance, mechanical strength, and reproducibility in the development of AlPO-18 type aluminum phosphate molecular sieve membranes from single-piece preparation to long-tube, batch, and modular applications.

[0009] When applying the existing AlPO-18 type aluminum phosphate molecular sieve membrane seed setting and hydrothermal crystallization process to a porous silicon carbide support layer, the applicant discovered during the trial production process that the untreated porous silicon carbide support layer has low surface polarity and few surface hydroxyl groups, making it difficult for the aqueous molecular sieve seed suspension to spread evenly and adhere stably on the intended film-forming surface. During subsequent drying, the seeds are also prone to migrate with the liquid and accumulate in local depressions or larger pores, resulting in insufficient seed coverage in some areas. Simultaneously, when some surface pores of the porous silicon carbide support layer are larger than the particle size of the aluminum phosphate molecular sieve seeds, the seeds and film-forming system may enter the surface pores and promote the growth of molecular sieve crystals into the porous silicon carbide support layer during hydrothermal crystallization. The combined effect of these factors easily leads to uneven seed distribution on the intended film-forming surface, insufficient local crystal growth, or excessive aggregation, affecting the full connection of adjacent molecular sieve crystals and the continuity of the microporous molecular sieve selective layer.

[0010] To address the aforementioned issues, one possible solution is to separately coat the predetermined film-forming surface of the porous silicon carbide support layer with a transition layer composed of fine particles. This layer covers some of the larger pores and provides an adhesion surface for the molecular sieve seed crystals that differs from the untreated silicon carbide surface. This method typically involves steps such as slurry preparation, coating, drying, and sintering. For slender tubular membrane elements, it is also necessary to control the thickness, pore size, and compositional uniformity of the transition layer along the axial and circumferential directions of the tube, making the preparation process complex and increasing the manufacturing and usage costs of the porous silicon carbide support. Furthermore, an independently formed interlayer interface exists between the separately coated transition layer and the porous silicon carbide support layer. During subsequent hydrothermal crystallization, calcination to remove the organic structure-directing agent, and temperature cycling during membrane element operation, this interlayer interface must withstand the differences in shrinkage and thermal deformation between the transition layer and the porous silicon carbide support layer. If the thickness, pore structure, or interface bonding state of the transition layer is not properly controlled, it may affect the bonding stability between the transition layer and the porous silicon carbide support layer and increase the risk of defects in the microporous molecular sieve selective layer. Summary of the Invention

[0011] The purpose of this invention is to provide a molecular sieve membrane element for natural gas denitrification, its preparation method, and a membrane separator for natural gas denitrification, in order to solve the problems of incompatibility between the surface chemical properties and surface pore structure of the porous silicon carbide support layer and the aluminum phosphate molecular sieve seed crystals and film formation system, which makes it difficult for the aluminum phosphate molecular sieve seed crystals to be uniformly distributed and stably attached, and the problem that the method of separately coating a transition layer has many preparation steps, and the consistency of the transition layer along the axial and circumferential directions of the slender tubular membrane element is difficult to control and form an independent interlayer interface.

[0012] In a first aspect, the present invention provides a molecular sieve membrane element for natural gas denitrification, comprising a porous silicon carbide support layer, a porous transition layer, and a microporous molecular sieve selective layer.

[0013] The porous silicon carbide support layer provides mechanical support for the porous transition layer and the microporous molecular sieve selective layer, forming a porous structure for permeate gas transport. Compared to alumina or mullite support structures, the porous silicon carbide support layer has higher thermal conductivity, mechanical strength, and thermal shock resistance, which helps to reduce the temperature gradient of slender tubular membrane elements during heating and cooling processes, and improves the structural stability of the membrane elements during handling, assembly, sealing, and operation.

[0014] The porous transition layer is formed in situ by an oxidation heat treatment of the surface region of the porous silicon carbide support layer. The porous transition layer is not formed by coating the surface of the porous silicon carbide support layer with separately prepared particles or coating materials, but rather by an oxidation reaction occurring on the surface region of the porous silicon carbide support layer itself. Thus, the porous transition layer extends continuously from the body of the porous silicon carbide support layer and has gas transport channels that communicate with the pores of the porous silicon carbide support layer body.

[0015] During the oxidation heat treatment, the silicon carbide in the surface region of the porous silicon carbide support layer is oxidized, and a silicon-containing oxide phase is generated on the pore walls of the original porous framework in the surface region. The silicon-containing oxide phase is distributed along the pore walls of the porous transition layer and occupies at least part of the pore space, making the surface pore diameter of the porous transition layer smaller than the pore diameter of the porous silicon carbide support layer body.

[0016] The porous transition layer forms a fine-pore film-forming surface on the side opposite to the porous silicon carbide support layer. This fine-pore film-forming surface possesses surface chemistry properties more suitable for wetting with aqueous seed suspensions and seed attachment compared to unoxidized silicon carbide surfaces, and has a surface pore structure suitable for supporting molecular sieve seeds. This fine-pore film-forming surface constitutes a nucleation and bonding surface for the attachment, growth, and interconnection of microporous molecular sieve crystals.

[0017] The microporous molecular sieve selective layer is disposed on the surface of the fine-pore film formation and is composed of interconnected aluminum phosphate molecular sieve crystals with an AEI topology. The aluminum phosphate molecular sieve crystals grow and connect to each other to form a continuous microporous molecular sieve selective layer. The microporous molecular sieve selective layer has micropores that allow nitrogen and methane to have different permeation rates, enabling the selective separation of nitrogen and methane from nitrogen-containing natural gas.

[0018] In some embodiments, the composite consisting of the porous silicon carbide support layer and the porous transition layer has a maximum through-pore diameter of no more than 2 micrometers, a porosity of no less than 45%, and an average pore diameter of 1.5 micrometers to 1.8 micrometers.

[0019] Controlling the maximum through-pore diameter of the composite to no more than 2 micrometers helps reduce the size of large through-pore throats in the composite, thus decreasing the likelihood of aluminum phosphate molecular sieve seeds and film-forming systems entering the porous silicon carbide support layer through these large through-pore throats. Maintaining the porosity of the composite to no less than 45% helps to retain interconnected gas transport channels while reducing the size of the throats controlling through-flow, thereby reducing the transport resistance of permeated gas as it passes through the porous transition layer and the porous silicon carbide support layer. The average pore diameter of the composite, measured by mercury intrusion porosimetry, is used to characterize the equivalent pore diameter level of the overall measurable porosity of the composite.

[0020] In some embodiments, the body of the porous silicon carbide support layer is a symmetrical porous structure with pore sizes that are substantially consistent along the thickness direction, and the porous transition layer is formed in situ by oxidation heat treatment of the surface region of the symmetrical porous structure.

[0021] In some embodiments, the molecular sieve membrane element for natural gas denitrification includes a porous silicon carbide support tube. The porous silicon carbide support tube is a single-channel tubular structure, and the tube wall of the single-channel tubular structure forms a single axially continuous inner cavity, the tube wall constituting the porous silicon carbide support layer.

[0022] The porous transition layer is formed on the outer peripheral surface of the porous silicon carbide support tube, and the microporous molecular sieve selective layer is disposed on the fine-pore film-forming surface outside the porous transition layer. The feed gas can flow outside the microporous molecular sieve selective layer, and the preferentially permeating components sequentially pass through the microporous molecular sieve selective layer, the porous transition layer, and the porous silicon carbide support layer, and then enter the inner cavity of the porous silicon carbide support tube.

[0023] In some embodiments, the molecular sieve membrane element for natural gas denitrification has an outer diameter of 8 mm to 20 mm and a length of 500 mm to 1500 mm, and the ratio of the length to the outer diameter is not less than 40.

[0024] Setting the outer diameter to 8mm to 20mm helps to balance the mechanical strength of the tube, the gas flow capacity inside the tube, and the packing density in the membrane separator. Setting the length to 500mm to 1500mm, and ensuring that the ratio of the length to the outer diameter is not less than 40, helps to increase the effective membrane area of ​​a single membrane element, reduce the number of end seals per unit membrane area, and form a slender tubular membrane element suitable for modular arrangement.

[0025] In some embodiments, the thickness of the porous transition layer is 5 micrometers to 30 micrometers. Controlling the thickness of the porous transition layer to 5 micrometers to 30 micrometers is beneficial for ensuring that the porous transition layer has sufficient oxidation conversion depth to form a fine porous film surface suitable for the attachment and growth of aluminum phosphate molecular sieve crystals, while also avoiding excessive pore shrinkage and a significant increase in gas transport resistance caused by an excessively thick porous transition layer.

[0026] In some embodiments, the silicon-containing oxide phase comprises silicon dioxide. The silicon dioxide adheres to the pore walls of the porous transition layer and is capable of forming a surface region containing silanol groups on the porous film surface during cooling and subsequent contact with moisture.

[0027] The surface regions containing silanol groups can improve the affinity of the microporous film-forming surface for aqueous aluminum phosphate molecular sieve seed suspensions and aluminum- and phosphorus-containing film-forming systems, which is beneficial for the spreading of the seed suspension on the microporous film-forming surface and improves the adhesion stability of the seed crystals. In some embodiments, the silicon-containing oxide phase comprises amorphous silicon dioxide. The amorphous silicon dioxide has a relatively disordered network structure, which can provide sites suitable for the formation of surface hydroxyl groups or further surface reactions.

[0028] In some embodiments, the surface of the silicon oxide phase can be treated with a phosphorus-containing compound to form a phosphorus-containing surface-modified phase, which contains phosphorus and phosphorus groups and constitutes a nucleating and binding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

[0029] The phosphorus-containing surface-modified phase can change the chemical environment of the porous transition layer surface, improve the compositional compatibility between the porous transition layer surface and the phosphorus-containing film-forming system and aluminum phosphate molecular sieve crystals, and form a nucleating and binding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

[0030] In other embodiments, at least a portion of the surface of the silicon oxide phase may be treated with aluminum-containing and phosphorus-containing compounds to form an aluminum-phosphorus surface-modified phase. The aluminum-phosphorus surface-modified phase contains aluminum and phosphorus elements and constitutes a nucleating and binding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

[0031] The aluminum-phosphorus surface-modifying phase can alter the chemical environment of the microporous film-forming surface, giving it an aluminum, phosphorus, and silicon-oxygen composition closer to that of aluminum phosphate molecular sieve crystals, and forming a nucleating and binding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

[0032] In some embodiments, the microporous molecular sieve selective layer is an AlPO-18 type aluminum phosphate molecular sieve selective layer, and is composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals. The AlPO-18 type aluminum phosphate molecular sieve has an AEI topology, and its micropores allow nitrogen and methane to have different permeation rates, allowing nitrogen to permeate preferentially over methane.

[0033] Secondly, the present invention also provides a method for preparing a molecular sieve membrane element for natural gas denitrification, comprising the following steps.

[0034] First, the surface region of the porous silicon carbide support layer is subjected to an oxidation heat treatment, which causes the silicon carbide in the surface region to be oxidized in situ, forming a porous transition layer that extends continuously from the body of the porous silicon carbide support layer on the surface of the porous silicon carbide support layer.

[0035] The porous transition layer contains a silicon-containing oxide phase generated by in-situ oxidation of the silicon carbide and distributed on the pore walls of the porous transition layer. This silicon-containing oxide phase causes the surface pore size of the porous transition layer to be smaller than the pore size of the porous silicon carbide support layer body, forming a fine-pore film-forming surface on the side of the porous transition layer opposite to the body. During cooling and subsequent contact with moisture, the silicon-containing oxide phase can form surface hydroxyl groups on the fine-pore film-forming surface.

[0036] Subsequently, aluminum phosphate molecular sieve seed crystals with an AEI topology are provided, and the aluminum phosphate molecular sieve seed crystals are disposed on the surface of the fine-pore film to form a seed layer.

[0037] The porous silicon carbide support layer with the seed layer is then placed in a film-forming system containing an aluminum source, a phosphorus source, an organic structure directing agent, and water for hydrothermal crystallization, so that the aluminum phosphate molecular sieve seed crystals grow on the surface of the fine-pore film and connect with each other to form an aluminum phosphate molecular sieve film layer.

[0038] Finally, the aluminum phosphate molecular sieve membrane layer is calcined to remove the organic structure directing agent, thereby opening the micropores of the aluminum phosphate molecular sieve membrane layer and forming a microporous molecular sieve selective layer.

[0039] In some embodiments, the oxidative heat treatment is carried out in an oxidizing atmosphere containing oxygen and water vapor, wherein the dew point of the water vapor in the oxidizing atmosphere is 55°C to 80°C, and the temperature of the oxidative heat treatment is 1050°C to 1150°C.

[0040] When the porous silicon carbide support layer is formed by the tube wall of a single-channel porous silicon carbide support tube, the oxidizing atmosphere flows through the outer periphery of the porous silicon carbide support tube, while sealing both ends of the porous silicon carbide support tube.

[0041] By adjusting the temperature, processing time, and processing atmosphere of the oxidation heat treatment, the degree of oxidation on the surface of the porous silicon carbide support layer can be adjusted, thereby controlling the thickness, silicon oxide phase content, and surface pore size of the porous transition layer.

[0042] In some embodiments, by controlling the oxidation heat treatment, the thickness of the porous transition layer is 5 micrometers to 30 micrometers, and the maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer is not greater than 2 micrometers, the porosity is not less than 45%, and the average pore diameter is 1.5 micrometers to 1.8 micrometers.

[0043] Controlling the maximum through-pore diameter of the composite to no more than 2 micrometers helps reduce the size of large through-pore throats in the composite, thus decreasing the likelihood of aluminum phosphate molecular sieve seeds and film-forming systems entering the porous silicon carbide support layer through these large through-pore throats. Maintaining the porosity of the composite to no less than 45% helps to retain interconnected gas transport channels while reducing the size of the throats controlling through-flow. The average pore diameter of the composite, measured by mercury intrusion porosimetry, is used to characterize the equivalent pore diameter level of the overall measurable porosity of the composite.

[0044] In some embodiments, after the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is subjected to liquid-phase treatment or gas-phase treatment with a phosphorus-containing compound, and the porous transition layer treated with the phosphorus-containing compound is subjected to heat treatment to form a phosphorus-containing surface-modified phase on at least a portion of the surface of the silicon oxide phase.

[0045] In other embodiments, after the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is subjected to liquid-phase treatment or gas-phase treatment with aluminum-containing compounds and phosphorus-containing compounds, and the porous transition layer treated with the aluminum-containing compounds and phosphorus-containing compounds is subjected to heat treatment to form an aluminum-phosphorus surface-modified phase on at least a portion of the surface of the silicon oxide phase.

[0046] In some embodiments, the aluminum phosphate molecular sieve seed crystals are AlPO-18 type aluminum phosphate molecular sieve seed crystals, and the aluminum phosphate molecular sieve membrane layer is an AlPO-18 type aluminum phosphate molecular sieve membrane layer. After the hydrothermal crystallization and calcination, an AlPO-18 type aluminum phosphate molecular sieve selective layer is formed on the surface of the fine-pore film.

[0047] Thirdly, the present invention also provides a membrane separator for natural gas denitrification, comprising a housing and a plurality of the aforementioned molecular sieve membrane elements for natural gas denitrification disposed within the housing.

[0048] The housing forms a feed-side space and a permeation-side space. The microporous molecular sieve selective layer of each of the natural gas denitrification molecular sieve membrane elements faces the feed-side space, and the pores of the porous silicon carbide support layer of each of the natural gas denitrification molecular sieve membrane elements are connected to the permeation-side space.

[0049] The housing is provided with a raw material gas inlet and a residual gas outlet communicating with the feed side space, and a permeate gas outlet communicating with the permeate side space.

[0050] Nitrogen-containing natural gas enters the feed-side space through the feed gas inlet. The preferentially permeable components in the nitrogen-containing natural gas sequentially pass through the microporous molecular sieve selective layer, the porous transition layer, and the porous silicon carbide support layer into the permeable-side space, and are then led out through the permeate gas outlet; the unpermeated components form residual permeate gas, which is also led out through the residual permeate gas outlet.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects: First, the present invention uses a porous silicon carbide support layer as the support structure for the microporous molecular sieve selective layer. The porous silicon carbide support layer has high thermal conductivity, mechanical strength, and thermal shock resistance, which helps to reduce the temperature gradient of slender tubular membrane elements during calcination heating and cooling processes, and improves the structural stability and reproducibility of membrane elements during batch preparation, handling, assembly, sealing, and operation, thereby meeting the requirements of long tubular, batch, and modular applications of molecular sieve membrane elements for natural gas denitrification.

[0052] Secondly, this invention regulates the surface chemical properties and surface pore structure of the porous silicon carbide support layer by in-situ transforming the surface region of the porous silicon carbide support layer into a porous transition layer. The porous transition layer extends continuously from the body of the porous silicon carbide support layer, eliminating the need for a separate transition layer composed of fine particles to be coated on the surface of the porous silicon carbide support layer. This reduces additional processes such as slurry preparation, coating, drying, and sintering, and lowers the difficulty of controlling the thickness, pore size, and compositional uniformity of the transition layer along the axial and circumferential directions in slender tubular film elements.

[0053] Third, the porous transition layer is formed in situ by the surface region of the porous silicon carbide support layer itself, thereby avoiding the formation of an independent interlayer interface between the separately coated material and the porous silicon carbide support layer. This helps to improve the bonding stability between the porous transition layer and the porous silicon carbide support layer, and reduces the risk of interface defects caused by differences in interlayer shrinkage and thermal deformation during subsequent hydrothermal crystallization, calcination and temperature cycling.

[0054] Fourth, the silicon-containing oxide phase is generated by in-situ oxidation of silicon carbide on the surface region of the porous silicon carbide support layer and distributed on the pore walls of the porous transition layer. This makes the surface pore size of the porous transition layer smaller than the pore size of the porous silicon carbide support layer body, and forms a fine-pore film-forming surface on the side of the porous transition layer away from the body. This fine-pore film-forming surface reduces the entry of aluminum phosphate molecular sieve seeds and film-forming systems into the interior of the porous silicon carbide support layer, which helps to retain and uniformly distribute the aluminum phosphate molecular sieve seeds in the predetermined film-forming area, thereby improving the continuity of the microporous molecular sieve selective layer.

[0055] Fifth, during cooling and subsequent contact with water, the silicon-containing oxide phase can form a surface region containing silanol groups on the microporous film-forming surface, thereby improving the wetting and adhesion of the aqueous aluminum phosphate molecular sieve seed suspension and film-forming system on the microporous film-forming surface. The silicon-containing oxide phase also provides a bonding surface for the adhesion, growth, and interconnection of the aluminum phosphate molecular sieve crystals, which is beneficial for improving the adhesion stability of the aluminum phosphate molecular sieve seed crystals.

[0056] Sixth, the porous transition layer has gas transport channels that communicate with the pores of the porous silicon carbide support layer body, and can form a structure with smaller pore diameters near the microporous film-forming surface and larger pore diameters near the porous silicon carbide support layer body. Therefore, the porous transition layer can provide a microporous film-forming surface for the aluminum phosphate molecular sieve seed crystals while maintaining a channel for permeated gas to transport from the microporous molecular sieve selective layer to the porous silicon carbide support layer, thus balancing film-forming performance and gas transport performance.

[0057] Seventh, by treating at least a portion of the surface of the silicon oxide phase with a phosphorus-containing compound or simultaneously with an aluminum-containing compound, a phosphorus-containing surface-modified phase or an aluminum-phosphorus-containing surface-modified phase can be formed. The phosphorus-containing surface-modified phase or the aluminum-phosphorus-containing surface-modified phase can alter the chemical environment of the porous transition layer surface, further improving the compatibility between the porous transition layer and the aluminum phosphate molecular sieve seed crystals and film-forming system, and providing a bonding surface for the adhesion and growth of the aluminum phosphate molecular sieve crystals.

[0058] Eighth, the microporous molecular sieve selective layer is composed of interconnected aluminum phosphate molecular sieve crystals with an AEI topology, and has microporous channels that allow nitrogen and methane to have different permeation rates, enabling nitrogen to permeate preferentially relative to methane, thereby achieving selective separation of nitrogen and methane in nitrogen-containing natural gas.

[0059] Ninth, the molecular sieve membrane element for natural gas denitrification can be made into a slender tubular structure with an outer diameter of 8mm to 20mm, a length of 500mm to 1500mm, and a length-to-outer diameter ratio of not less than 40. This is beneficial to increase the effective membrane area of ​​a single membrane element, reduce the number of pipe end seals corresponding to a unit membrane area, and increase the packing density of multiple molecular sieve membrane elements for natural gas denitrification in the membrane separator, thereby meeting the engineering application requirements of the membrane separator for natural gas denitrification.

[0060] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments. Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice. Attached Figure Description

[0061] Figure 1 A schematic diagram of the overall structure of a molecular sieve membrane element for natural gas denitrification.

[0062] Figure 2 A photograph of an experimental sample of a molecular sieve membrane element for natural gas denitrification.

[0063] Figure 3 This is a structural diagram of a membrane separator used for natural gas denitrification.

[0064] Figure 4 for Figure 3 The image shows a side view of a membrane separator for natural gas denitrification.

[0065] Figure 5 for Figure 3 A magnified view of a section at point I (with key dimensions marked).

[0066] Figure 6 for Figure 3 Enlarged view of section II (with key dimensions marked).

[0067] Figure 7 for Figure 3 Sectional view along axis AA (with key dimensions marked).

[0068] Figure 8 Scanning electron microscope image of a cross-section of a molecular sieve membrane element used for natural gas denitrification.

[0069] Figure 9 X-ray diffraction patterns and phase reference peaks of molecular sieve membrane elements for natural gas denitrification in Example 1 at different preparation stages. Figure 9 (a) is the X-ray diffraction pattern of the outer surface of the molecular sieve membrane element for natural gas denitrification, which forms a microporous molecular sieve selective layer after hydrothermal crystallization and calcination. Figure 9 (b) is the X-ray diffraction pattern of the composite consisting of a porous silicon carbide support layer and a porous transition layer formed in situ by the in-situ transformation of its surface region after oxidative heat treatment. Figure 9 (c) is the X-ray diffraction pattern of the porous silicon carbide support layer before oxidation heat treatment; Figure 9 (d) shows the phase reference diffraction peak positions of AlPO-18 (AEI) and silicon carbide, used for comparison with... Figure 9 (a) to Figure 9 (c) Conduct object comparison. Figure 9 Each experimental spectral line is normalized to 100 with its own maximum peak intensity and is used only to compare the position and shape of diffraction peaks, not to compare the absolute diffraction intensity between different samples.

[0070] Figure 10 The diagram shows the clamping, rotation, and spraying states of the porous silicon carbide support tube when setting aluminum phosphate molecular sieve seed crystals using the spraying method.

[0071] Figure 11 The graph shows a comparison of nitrogen permeation rate and nitrogen-to-methane separation coefficient of the molecular sieve membrane elements for natural gas denitrification obtained in Examples 1 to 4 and the comparative example.

[0072] The following are labeled in the figure: 10 molecular sieve membrane element for natural gas denitrification; 11 porous silicon carbide support layer; 12 porous transition layer; 13 microporous molecular sieve selective layer; 21 shell; 22 head; 23 tube sheet; 24 baffle plate; 25 support; 26 tie rod; N1 feed gas inlet; V1 vent; N2 residual gas outlet; N3 permeate gas outlet; V2 vent; D1 drain; D2 drain. Detailed Implementation

[0073] The present invention will now be clearly and completely described in conjunction with the accompanying drawings. Those skilled in the art will be able to implement the present invention based on these descriptions. The following embodiments are only some embodiments of the present invention and do not constitute an undue limitation on the scope of protection of the present invention. Features in the various embodiments can be combined with each other without conflict.

[0074] like Figure 1 and Figure 2 As shown, the molecular sieve membrane element 10 for natural gas denitrification of the present invention has an overall elongated single-channel tubular structure. The molecular sieve membrane element 10 for natural gas denitrification uses a porous silicon carbide support tube as its body. The tube wall of the porous silicon carbide support tube forms a single axially penetrating inner cavity, which constitutes a central channel for the flow of permeable gas. The tube wall forms a porous silicon carbide support layer 11. Figure 1 As shown, the outer diameter of the molecular sieve membrane element 10 for natural gas denitrification is 8mm to 20mm, and the length is 500mm to 1500mm. The ratio of length to outer diameter is not less than 40, thus presenting a long tubular configuration that is advantageous for providing a larger effective membrane area in a limited space.

[0075] like Figure 1 As shown, along the thickness direction of the porous silicon carbide support tube wall, from the inner cavity side to the outer peripheral surface side, there are sequentially a porous silicon carbide support layer 11, a porous transition layer 12, and a microporous molecular sieve selective layer 13. The porous silicon carbide support layer 11 provides mechanical strength, thermal conductivity, and thermal shock resistance for the molecular sieve membrane element. The porous transition layer 12 extends continuously from the body of the porous silicon carbide support layer 11, with a surface pore size smaller than that of the body of the porous silicon carbide support layer 11, and forms a fine-pore film-forming surface on the side facing away from the body. The microporous molecular sieve selective layer 13 is disposed above the fine-pore film-forming surface and is composed of interconnected aluminum phosphate molecular sieve crystals, used to achieve selective permeation separation of nitrogen relative to methane.

[0076] Figure 2This is a photograph of an experimental sample of a molecular sieve membrane element for natural gas denitrification. It is a slender tube with a uniform surface and no macroscopic defects. The specific specifications of the molecular sieve membrane element 10 for natural gas denitrification are: an outer diameter of approximately 13 mm, a length of approximately 800 mm, a wall thickness of approximately 2 mm, and correspondingly, an inner diameter of approximately 9 mm for each single channel, resulting in an aspect ratio of approximately 62. These dimensional parameters are the result of a compromise optimization among multiple factors, including packing density, mechanical strength, mass transfer resistance, and flow pressure drop.

[0077] Specifically: First, regarding the aspect ratio, the aspect ratio of approximately 62 is significantly greater than the lower limit required by this invention (not less than 40), allowing a single membrane element to provide a larger effective membrane area within a limited axial length. This enables a higher membrane area packing density in the membrane separator, which helps reduce the equipment volume and footprint per unit throughput. Second, regarding the matching of outer diameter and wall thickness, the approximately 13mm outer diameter combined with the approximately 2mm wall thickness provides the porous silicon carbide support layer with sufficient mechanical strength and stiffness to withstand assembly preload, transmembrane pressure differential, and loads generated by start-up, shutdown, and operating temperature changes, ensuring that the membrane element is not prone to bending deformation or end cracking. On the other hand, the 2mm wall thickness is not excessive, avoiding a significant increase in the mass transfer path length during gas diffusion due to excessive support thickness. This keeps the mass transfer resistance of the support layer itself at a low level, which is beneficial for maintaining a high nitrogen permeation flux. Thirdly, regarding the internal flow cross-section, the approximately 9mm inner diameter ensures sufficient flow cross-sectional area in the central channel, resulting in a smaller pressure drop when the permeate flows axially along the central channel. This helps suppress the accumulation of back pressure on the permeate side and ensures a more uniform distribution of transmembrane pressure differential along the membrane element axis, thus contributing to stable separation performance. Fourthly, regarding the length selection, the approximately 800mm length ensures a sufficiently effective membrane length while also addressing the challenges of consistent forming and straightness control in the preparation, drying, calcination, and membrane formation processes of the porous silicon carbide support tube. This facilitates the mass production of membrane elements with regular shapes that are easy to align with the membrane separator tube sheet. In summary, the combination of an outer diameter of approximately 13mm, a length of approximately 800mm, and a wall thickness of approximately 2mm, while ensuring mechanical reliability and assembly compatibility, also achieves a balance between high packing density and low mass transfer resistance, making it a preferred configuration for natural gas denitrification.

[0078] like Figures 3 to 7 As shown, the membrane separator for natural gas denitrification using the above-mentioned molecular sieve membrane element 10 for natural gas denitrification will be described below. Figure 3 This is a structural diagram of a membrane separator used for natural gas denitrification. Figure 4 for Figure 3 The image shows a side view of a membrane separator for natural gas denitrification. Figure 5 for Figure 3 A magnified view of a section at point I (with key dimensions marked). Figure 6 for Figure 3 Enlarged view of section II (with key dimensions marked). Figure 7 for Figure 3 Sectional view along axis AA (with key dimensions marked).

[0079] The core objective of this membrane separator for natural gas denitrification is to modularly bundle and package several of the aforementioned slender tubular molecular sieve membrane elements 10 for natural gas denitrification. While ensuring reliable isolation and sealing between the feed gas side (permeate side) and the permeate side, the goal is to maximize the membrane area per unit volume of the shell and ensure uniform flow and transmembrane pressure distribution of the feed gas on the surface of each molecular sieve membrane element, thereby achieving a stable nitrogen permeation rate and separation coefficient under natural gas denitrification conditions. Therefore, the design of membrane separators for natural gas denitrification needs to take into account the following aspects: First, fully utilize the characteristics of long tubes and large aspect ratio of molecular sieve membrane elements, and adopt a tube bundle arrangement to improve the packing density; Second, considering the characteristics of porous silicon carbide support layer with high rigidity and good thermal shock resistance but brittle material sensitive to local stress, stress mitigation and flexible sealing measures should be taken at the sealing of both ends of the molecular sieve membrane element 10 for natural gas denitrification and at the tube sheet fixing points to avoid cracking at the ends of the membrane element due to assembly stress or thermal stress concentration; Third, rationally organize the flow channels between the feed gas inlet, the residual gas outlet and the permeate collection chamber to reduce flow dead zones and concentration polarization.

[0080] like Figures 3 to 7 As shown, the natural gas denitrification membrane separator using the above-mentioned natural gas denitrification molecular sieve membrane element 10 is a horizontal tube bundle pressure vessel, mainly composed of shell 21 (which forms a permeate collection chamber), end cap 22 (which forms a permeate collection chamber), tube sheet 23, natural gas denitrification molecular sieve membrane element bundle (containing 177 natural gas denitrification molecular sieve membrane elements 10), elastic seals (which are respectively set at both ends of each natural gas denitrification molecular sieve membrane element 10), baffle 24, several connecting pipes, support 25 and tie rod 26, etc.

[0081] Specifically, such as Figure 3 As shown, the shell 21 of the membrane separator for natural gas denitrification is a DN400 cylinder. Its left end is detachably connected to a flanged end cap via a DN400 flange, and its right end is connected to an elliptical end cap 22. The detachable flange connection structure at the left end facilitates the opening and closing of the shell 21, improving the maintainability of the equipment.

[0082] like Figure 3As shown, multiple baffles 24 are spaced apart along the axial direction of the shell 21. Each baffle 24 is positioned and fixed in the permeate collection chamber between the two tube sheets 23 by a tie rod 26, so as to form multiple spaced support positions along the length direction of the molecular sieve membrane element 10 for natural gas denitrification, providing central support for the molecular sieve membrane element 10 for natural gas denitrification with a large length-to-diameter ratio, and forming a multi-stage baffle channel on the shell side.

[0083] like Figure 7 As shown, the baffle plate 24 has several membrane element holes arranged in a triangular close-packed pattern on its surface. Multiple molecular sieve membrane elements 10 for natural gas denitrification are respectively inserted into the corresponding membrane element holes to form... Figure 4 and Figure 7 The diagram shows a closely spaced tube arrangement. This triangular close-packed arrangement allows for the placement of more membrane elements 10 within a given baffle plate 24 area, increasing the effective membrane area and membrane area packing density per unit shell volume. Simultaneously, the close-packed arrangement creates a relatively uniform shell-side flow gap between adjacent membrane elements 10, which is beneficial for improving the flow field distribution around the membrane elements 10 in conjunction with the baffle structure.

[0084] like Figure 7 As shown, each baffle 24 has a straight edge on its side. The straight edge is formed by cutting off an arc-shaped portion from one side of a circular plate surface, and an arc-shaped notch is formed between the straight edge and the inner wall of the housing 21. The arc-shaped notch forms a continuous flow area by utilizing the edge region of the baffle 24 near the inner wall of the housing 21, providing a flow cross section for the gas on the housing side to pass through the baffle 24 without reducing the number of membrane element holes.

[0085] Along the axial direction of the housing 21, the straight edges of two adjacent baffles 24 are arranged radially offset from each other. For example, the straight edge of one baffle 24 is located on the left side of the cross-section of the housing 21, and the straight edge of the next adjacent baffle 24 is located on the right side of the cross-section of the housing 21, and subsequent baffles 24 are arranged alternately on the left and right sides in the above manner. Correspondingly, the arcuate notches are alternately distributed along the axial direction of the housing 21.

[0086] By staggering the bow-shaped notches as described above, the flow channels of multiple baffles 24 can be prevented from forming a continuous straight path along the axial direction. This would force the shell-side gas to change its flow direction between adjacent baffles 24 and flow around the outer circumference of the membrane element 10 through the flow area between the membrane elements 10. If the bow-shaped notches are arranged on the same side along the axial direction, the shell-side gas will tend to flow preferentially along the continuous notch area, resulting in insufficient gas renewal in some areas around the membrane element 10. The deflected flow path formed by the staggered bow-shaped notches can improve the contact between the shell-side gas and the outer surface of the membrane element 10, promote gas renewal near the membrane surface, and reduce concentration polarization caused by methane enrichment.

[0087] The two ends of the multi-strand natural gas denitrification molecular sieve membrane element 10 are respectively... Figure 5 and Figure 6 The tube sheets 23 on both the left and right sides are sealed and fixed as shown. The shell-side space between the two tube sheets 23, surrounding the outer periphery of each molecular sieve membrane element 10 for natural gas denitrification, forms a permeate collection chamber. The tube sheets 23 and the end sealing structure reliably isolate the feed gas side (i.e., the permeate collection chamber on the shell side) from the permeate side (i.e., the central channel side of the molecular sieve membrane element 10 for natural gas denitrification), preventing internal leakage of permeate and permeate, which would reduce the methane purity of the separated product gas.

[0088] like Figure 5 , Figure 6 As shown, the molecular sieve membrane element 10 for natural gas denitrification has a sealing section at each end. Its outer circumferential surface is machined to a size that matches the tube sheet bore and inserted into the corresponding tube sheet bore, forming a seal with a compressed elastic sealing element. Wherein, as... Figure 5 At point I shown, after the sealing section of the molecular sieve membrane element 10 for natural gas denitrification is inserted into the tube sheet hole, the elastic seal is axially pressed by an M16×1.5 threaded fastener, thereby forming an elastic seal between the outer circumferential surface of the sealing section and the tube sheet hole wall. For example... Figure 6 At point II shown, the sealing section of the molecular sieve membrane element 10 for natural gas denitrification is inserted into a Φ15mm / Φ13mm stepped hole with a length of 40mm. The elastic seal is pressed against the end of the sealing section. The bottom of the stepped hole is provided with a Φ7mm channel. The central channel of the molecular sieve membrane element 10 for natural gas denitrification is connected to the corresponding flow channel of the permeate collection chamber through the Φ7mm channel.

[0089] The elastic seal serves two purposes: firstly, it ensures reliable isolation between the feed side and the permeation side; secondly, it compensates for the relative deformation caused by the difference in thermal expansion coefficients between the ceramic molecular sieve membrane element 10 for natural gas denitrification and the metal tube sheet. This reduces the risk of end damage or membrane cracking of the molecular sieve membrane element 10 due to rigid constraints during start-up, shutdown, and temperature changes. Therefore, as mentioned earlier, the mechanical strength and rigidity provided by the approximately 13mm outer diameter and approximately 2mm wall thickness of the molecular sieve membrane element 10 for natural gas denitrification, along with the thermal shock resistance of the porous silicon carbide support layer 11, are prerequisites for working in conjunction with the elastic seal structure to ensure end reliability.

[0090] like Figure 3As shown, the membrane separator for natural gas denitrification has a raw gas inlet N1, a vent V1, a residual gas outlet N2, a permeate outlet N3, a vent V2, and drain ports D1 and D2 located at the bottom of the shell. The raw gas inlet N1, vent V1, residual gas outlet N2, and vent V2 are distributed sequentially from left to right along the upper part of the shell. The permeate outlet N3 is located on the right end cap and is connected to the permeate collection chamber inside the end cap. The drain ports D1 and D2 are located on the left and right sides of the lower part of the shell, respectively.

[0091] The functions of the above-mentioned pipes are as follows: the feed gas inlet N1 is used to introduce the natural gas feed gas to be denitrified into the permeate gas collection chamber on the shell side (feed gas side); the permeate gas outlet N2 is used to draw out the product gas (permeate gas) that has not permeated the membrane wall and is enriched with methane from the shell side; the permeate gas outlet N3 is used to draw out the permeate gas enriched with nitrogen that is collected in the permeate gas collection chamber of the head by the central channels of each membrane element; the venting ports V1 and V2 are located on the upper part of the shell 21 and are used to discharge the gas accumulated in the shell 21, perform gas phase venting and pressure relief during start-up, shutdown or maintenance; the drain ports D1 and D2 are located on the lower part of the shell 21 and are used to discharge the condensate and impurities accumulated at the bottom of the shell 21 and the bottom of the head 22 during operation or shutdown.

[0092] During operation, the natural gas feedstock gas to be denitrified enters the shell-side permeate collection chamber through the feedstock gas inlet N1. Driven by the transmembrane pressure difference, the nitrogen in the feedstock gas preferentially passes through the microporous molecular sieve selective layer 13, the porous transition layer 12, and the porous silicon carbide support layer 11 in sequence due to the selectivity of nitrogen relative to methane, and enters the central channel. It is collected in the central channel to the head permeate collection chamber, forming nitrogen-enriched permeate gas, which is then led out through the permeate outlet N3. The permeate gas that does not pass through the membrane wall and is enriched with methane flows axially along the shell side and is led out through the permeate outlet N2, thereby realizing the denitrification and separation of natural gas and obtaining product gas with increased methane content. The ample flow cross-section provided by the central channel with an inner diameter of approximately 9 mm serves to reduce the pressure drop when the permeate flows out of the central channel, suppress the accumulation of back pressure on the permeate side, prevent the increase of the partial pressure on the permeate side from weakening the effective driving force across the membrane, and make the distribution of the transmembrane pressure difference along the element axis more uniform. Together with the aforementioned baffle plate's improvement of the shell-side flow field, they jointly ensure stable separation performance.

[0093] like Figure 3As shown, the membrane separator for natural gas denitrification is horizontally supported by two saddle supports 25, one of which is a fixed support and the other is a movable support. The movable support is designed to accommodate the axial thermal expansion of the shell during temperature changes, allowing the shell to expand and contract freely without generating additional thermal stress. This protects the shell, tube sheet, and the densely packed molecular sieve membrane elements 10 for natural gas denitrification from deformation and damage caused by temperature loads.

[0094] In summary, the membrane separator for natural gas denitrification of the present invention uses the aforementioned molecular sieve membrane element 10 for natural gas denitrification with an outer diameter of about 13 mm, a length of about 800 mm, a wall thickness of about 2 mm, and an aspect ratio of about 62 as the separation element. Through the tube bundle-type triangular close-packing, the baffle plate supporting the middle and the shell side baffle, the elastic end sealing, and the horizontal shell arrangement, a high membrane area packing density and a low mass transfer resistance and flow pressure drop are achieved while ensuring mechanical reliability and thermal adaptability. Thus, a membrane separation device with a compact structure, stable separation performance, and easy installation, disassembly and maintenance is provided for natural gas denitrification.

[0095] The present invention will be further described below with reference to specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments can be obtained through conventional commercial means or prepared according to conventional methods in the art. Unless otherwise stated, the aluminum phosphate molecular sieve seed crystals, film-forming systems, seed crystal settings, hydrothermal crystallization, and calcination procedures in each embodiment all use the same baseline conditions. Examples 1 to 3 use a first type of porous silicon carbide support tube and the same oxidation heat treatment conditions, the difference being whether further surface treatment is performed after the oxidation heat treatment and the surface treatment method used; Example 4 uses a second type of porous silicon carbide support tube, and its oxidation heat treatment conditions will be described separately.

[0096] The main characterization methods involved in each embodiment are as follows: X-ray diffraction (XRD) was used to quantitatively analyze the silicon carbide mass fraction of the porous silicon carbide support layer, and the silicon oxide phase of the porous transition layer and the microporous molecular sieve selective layer were identified; scanning electron microscopy (SEM) was used to observe the layered structure and thickness of each layer of the molecular sieve membrane element cross-section for natural gas denitrification; the bubble point method was used to determine the maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer; and mercury intrusion porosimetry was used to determine its porosity and average pore diameter.

[0097] Natural gas denitrification separation performance test. The natural gas denitrification separation performance was measured using a 50 / 50 N2 / CH4 volume ratio mixture as the feed gas. During the test, a molecular sieve membrane element for natural gas denitrification was installed in the test membrane separator and sealed, with the microporous molecular sieve selective layer on the outer periphery of the membrane element facing the feed side. Gas permeating through the microporous molecular sieve selective layer, porous transition layer, and porous silicon carbide support layer entered the inner cavity of the membrane element and was led out from the permeate side. The test temperature was controlled at 25°C, and the N2 / CH4 mixture was continuously introduced into the feed side. The feed side pressure and permeate side pressure were adjusted to maintain a total pressure difference of 0.3 MPa across the membrane. After the permeate flow rate and composition stabilized, the permeate was collected, and the molar fractions of nitrogen and methane were determined using gas chromatography, while the total permeate flow rate was also measured.

[0098] Based on the total molar flow rate of the permeate and the molar fraction of each component in the permeate, the molar flow rates of nitrogen and methane on the permeate side are calculated respectively. The permeation rate of any component is calculated by dividing the molar flow rate of that component on the permeate side by the product of the effective membrane area and the partial pressure difference of that component between the feed and permeate sides. The effective membrane area is calculated as the product of the effective perimeter of the membrane element and the effective length of the membrane element. The unit of the permeation rate is mol·m⁻². -2 ·s -1 ·Pa -1 The nitrogen-to-methane separation coefficient described in this specification refers to the ratio of nitrogen permeation rate to methane permeation rate. A higher nitrogen permeation rate indicates a stronger nitrogen permeation capacity of the membrane element; a higher nitrogen-to-methane separation coefficient indicates a higher selectivity of the membrane element in allowing nitrogen to permeate preferentially relative to methane.

[0099] <Standard Materials and General Preparation Conditions> Porous silicon carbide support layer. All embodiments use porous silicon carbide support tubes of the same shape and size. The porous silicon carbide support tube is a single-channel tubular structure, with its wall forming a single axially continuous inner cavity. The wall constitutes the porous silicon carbide support layer. The porous silicon carbide support tube has an outer diameter of approximately 13 mm, an inner diameter of 9 mm, and a length of 800 mm. These porous silicon carbide support tubes are ultrasonically cleaned with deionized water and then dried at 110°C for 12 hours before use.

[0100] Based on differences in composition and pore structure, porous silicon carbide support tubes are divided into two types. The first type of porous silicon carbide support tube is a sintered porous silicon carbide support tube without the addition of Al2O3-Y2O3 sintering aid. Quantitative phase analysis by X-ray diffraction showed that the silicon carbide mass fraction of the porous silicon carbide support layer was 98.7%; chemical analysis showed that the mass fraction of elemental silicon in free state was 0.3%, and the total oxygen content was 0.8% by mass fraction. The maximum through-pore diameter of the porous silicon carbide support layer before oxidation heat treatment, measured by the bubble point method, was approximately 3.2 μm; the porosity measured by mercury porosimetry was approximately 50%. Examples 1-3 all used the first type of porous silicon carbide support tube.

[0101] The second type of porous silicon carbide support tube is a sintered porous silicon carbide support tube with added Al2O3-Y2O3 sintering aid. Specifically, in preparing the second type of porous silicon carbide support tube, silicon carbide powder is used as the main raw material, and Al2O3-Y2O3 sintering aid is added; based on the total mass of the silicon carbide powder and the Al2O3-Y2O3 sintering aid, the amount of Al2O3-Y2O3 sintering aid added is 10 wt.%; the molar ratio of Al2O3 to Y2O3 in the Al2O3-Y2O3 sintering aid is 5:3. After sintering, the second type of porous silicon carbide support tube contains a yttrium aluminum oxide bound phase. The maximum through-pore diameter of the second type of porous silicon carbide support tube measured before oxidation heat treatment is approximately 5.5 μm, and the porosity is approximately 50%. Example 4 uses the second type of porous silicon carbide support tube.

[0102] Two types of porous silicon carbide support tubes were designed primarily because porous silicon carbide support tubes with different compositions exhibit different oxidation kinetic characteristics under humid air oxidation heat treatment. The first type of porous silicon carbide support tube, without the addition of Al2O3-Y2O3 sintering aid, has a relatively low wet oxidation rate; therefore, a support tube with a smaller maximum through-pore diameter before oxidation heat treatment was selected. The second type of porous silicon carbide support tube, with the addition of Al2O3-Y2O3 sintering aid, has a relatively high wet oxidation rate; therefore, a support tube with a larger maximum through-pore diameter before oxidation heat treatment was selected. By matching the initial pore structure of the porous silicon carbide support tubes with different compositions to their oxidation kinetic characteristics, a porous transition layer meeting the requirements for subsequent film formation can be formed after oxidation heat treatment.

[0103] It should be noted that in the second type of porous silicon carbide support tube with added Al2O3-Y2O3 sintering aid, the Al2O3-Y2O3 sintering aid forms a yttrium aluminum oxide bonding phase during sintering. This bonding phase is distributed in the bonding region between silicon carbide particles. During the humid air oxidation heat treatment, the yttrium aluminum oxide bonding phase can react with the generated silicon oxide phase to form an aluminum- and yttrium-containing silicon oxide phase, thereby altering the oxide layer composition and the transport behavior of oxidizing components. Therefore, compared with the porous silicon carbide support tube without added sintering aid, the second type of porous silicon carbide support tube exhibits a relatively higher oxidation response under humid air conditions, which is beneficial for forming a porous transition layer that meets the requirements of subsequent film formation in a shorter oxidation time.

[0104] All of the aforementioned porous silicon carbide support tubes are purchased products. These porous silicon carbide support tubes have a symmetrical structure along their wall thickness direction. That is, the porous silicon carbide support layer consists of a symmetrical porous silicon carbide structure with basically consistent pore size and porosity throughout the entire wall thickness. The tube wall does not contain a gradient transition layer with progressively decreasing pore size, nor does it contain a fine-pore film layer with a significantly smaller pore size pre-loaded on the tube wall surface. This distinguishes it from asymmetric ceramic films that have a fine-pore ceramic film layer pre-composite on the surface of the porous support substrate. Compared to the asymmetric ceramic film, the symmetrical porous silicon carbide support tubes do not require the additional preparation and sintering of the fine-pore film layer during tube manufacturing. Their manufacturing process is simpler, requires fewer sintering cycles, and therefore has lower raw material and manufacturing costs.

[0105] Before performing anodizing heat treatment on the porous silicon carbide support tube with the symmetrical structure, high-temperature resistant plugs made of high-purity alumina ceramic fibers can be pressed into the inner cavity openings at both ends of the porous silicon carbide support tube, so that the high-temperature resistant plugs fit against the corresponding inner cavity walls, thereby removably sealing the inner cavity openings at both ends of the porous silicon carbide support tube. This sealing is used to prevent oxidizing atmosphere from directly entering the inner cavity axially through the inner cavity openings. In all embodiments, the above method is used to seal the inner cavity openings at both ends of the porous silicon carbide support tube before the anodizing heat treatment.

[0106] Aluminum phosphate molecular sieve seed crystals were prepared by mixing aluminum isopropoxide as the aluminum source, orthophosphoric acid (phosphate) as the phosphorus source, and tetraethylammonium hydroxide (TEAOH) as the organic structure directing agent with deionized water to form a stable sol. The molar ratio of each component was P2O5∶Al2O3∶TEAOH∶H2O = 3.16∶1∶5.32∶186. After aging for 2 hours, the sol was transferred to a high-pressure reactor lined with polytetrafluoroethylene and hydrothermally synthesized at 150°C for 20 hours. The reaction was quenched with cold water, and the product was centrifuged, washed with deionized water until neutral, and dried to obtain AlPO-18 type aluminum phosphate molecular sieve seed crystals with an AEI topology. XRD analysis confirmed a pure-phase AEI topology, and the average particle size after ball milling was approximately 0.3 μm. The aluminum phosphate molecular sieve seed crystals were prepared into an aqueous seed crystal suspension with a mass fraction of 0.5% for later use.

[0107] Film-forming system. Aluminum isopropoxide was used as the aluminum source, orthophosphoric acid (phosphate) as the phosphorus source, and tetraethylammonium hydroxide (TEAOH) as the organic structure directing agent. The above raw materials were mixed with deionized water to form a homogeneous and stable sol. The molar ratio of each component was P2O5∶Al2O3∶TEAOH∶H2O = 1.0∶1∶1.8∶120. The sol was aged at room temperature for 8 hours before use.

[0108] Seed crystal placement. The aqueous seed crystal suspension is placed on the porous film-forming surface of the porous transition layer on the side opposite to the bulk material using a spraying method. Before spraying, the aqueous seed crystal suspension is fully dispersed and filtered, and low-speed stirring or intermittent dispersion is maintained during spraying to reduce the sedimentation and agglomeration of aluminum phosphate molecular sieve seed crystals. During spraying, the porous silicon carbide support tube is clamped at at least one end, keeping the porous silicon carbide support tube vertical or nearly vertical and rotating it at a low, uniform speed around its axis (see...). Figure 10 Simultaneously, the nozzle is moved back and forth at a uniform speed along the axial direction of the porous silicon carbide support tube to ensure that the aqueous seed suspension is evenly distributed along the axial and circumferential directions of the microporous film-forming surface. Spraying is performed in thin layers and multiple passes. After each pass, the porous silicon carbide support tube continues to rotate and is flash-dried at a low temperature using clean hot air until the formed wet layer does not exhibit significant sagging or form a continuous liquid film before proceeding to the next pass. This spraying and flash-drying process is repeated once, followed by drying at 60°C to form a seed layer with uniform seed distribution and no significant sagging.

[0109] Hydrothermal crystallization. A porous silicon carbide support tube with the seed layer formed is vertically placed in a reactor, ensuring the effective outer surface of the film-forming layer is completely submerged in the film-forming system. Air bubbles should be prevented from adhering to the effective outer surface of the film-forming layer, and the support structure within the reactor should be prevented from obstructing the effective outer surface. Hydrothermal crystallization is performed at 200°C for 20 hours, allowing the aluminum phosphate molecular sieve seed crystals to grow and interconnect on the surface of the fine-pore film, forming an aluminum phosphate molecular sieve membrane layer. After hydrothermal crystallization, the resulting membrane element is removed, washed with deionized water, and dried at 100°C.

[0110] Calcination. Subsequently, the aluminum phosphate molecular sieve membrane layer is calcined to remove the organic structure-directing agent. During calcination, the porous silicon carbide support tube is supported by the non-effective film-forming area to prevent the outer peripheral surface of the effective film-forming layer from directly contacting the support structure. The temperature is raised to 450°C in air at a heating rate of 3°C / min, held for 8 hours, and then cooled to room temperature at a cooling rate of 3°C / min. This removes the organic structure-directing agent from the aluminum phosphate molecular sieve membrane layer and opens its micropores, forming a microporous molecular sieve selective layer. It should be noted that the above calcination uses a relatively high heating and cooling rate of 3°C / min. Because this invention uses a porous silicon carbide support layer with high thermal conductivity, mechanical strength, and thermal shock resistance, and the porous transition layer is formed in situ from the surface area of ​​the porous silicon carbide support layer and extends continuously with the body without an independent interlayer interface, the continuity of the microporous molecular sieve selective layer can be maintained even at a relatively fast heating and cooling rate. This is beneficial for shortening the calcination cycle and improving the utilization rate of the heat treatment equipment.

[0111] <Example 1> This embodiment uses the first type of porous silicon carbide support tube. The first type of porous silicon carbide support tube does not contain Al2O3-Y2O3 sintering aid. X-ray diffraction quantitative phase analysis shows that the silicon carbide mass fraction of the porous silicon carbide support layer is 98.7%. Chemical analysis shows that the mass fraction of elemental silicon in free state is 0.3%, and the total oxygen content by mass fraction is 0.8%. The maximum through-pore diameter of the porous silicon carbide support layer before oxidation heat treatment is approximately 3.2 μm, and the porosity is approximately 50%.

[0112] Oxidation heat treatment. A cleaned and dried porous silicon carbide support tube is placed in a tube furnace, and moist air containing water vapor with a dew point of approximately 75°C is introduced as an oxidizing atmosphere. The temperature is increased to 1100°C at a heating rate of 3°C / min, held for 8 hours, and then cooled to room temperature at a cooling rate of 3°C / min. During this oxidation heat treatment, the silicon carbide in the surface region of the outer peripheral surface of the porous silicon carbide support layer undergoes in-situ oxidation. The oxidation reaction can be represented as SiC + (3 / 2)O₂ → SiO₂ + CO↑. A silicon-containing oxide phase is generated at the pore walls and throats of the original porous framework in the surface region, thus forming a porous transition layer continuously extending from the porous silicon carbide support layer body. Each mole of silicon carbide, after oxidation, generates one mole of silicon dioxide. The solid molar volume of silicon dioxide is greater than the corresponding solid molar volume of the consumed silicon carbide, with a volume ratio of approximately 2.0 to 2.2. Therefore, during the gradual oxidation of the silicon carbide pore walls, the generated silicon-containing oxide phase, in addition to occupying the space formed after the original silicon carbide is consumed, at least partially extends towards the pore space. This silicon-containing oxide phase is constrained by the surrounding solid-phase framework and distributed along the pore walls and throats of the porous transition layer, thereby occupying part of the pore space and reducing the effective flow cross-section of the throats that at least partially control the through-flow. Consequently, the surface pore size of the porous transition layer near its outer peripheral surface is smaller than the pore size of the porous silicon carbide support layer body.

[0113] Furthermore, since the oxidizing atmosphere permeates from the outer periphery of the porous silicon carbide support layer into the body, the oxygen partial pressure is higher closer to the outer periphery, the degree of silicon carbide oxidation is deeper, and the more silicon-containing oxide phase is deposited on the pore walls. The water vapor in the humid air can promote the in-situ oxidation of silicon carbide; the silicon-containing oxide phase formed by oxidation can form surface hydroxyl groups on the surface of the fine-pore film during cooling and subsequent contact with moisture.

[0114] The degree of pore size reduction was verified by the bubble point method: the maximum through-pore diameter of the porous silicon carbide support layer before the oxidation heat treatment was approximately 3.2 μm, while after the formation of the porous transition layer by the aforementioned oxidation heat treatment, its maximum through-pore diameter decreased to approximately 1.8 μm, indicating that the pore size of the fine-pore film surface is significantly smaller than that of the porous silicon carbide support layer body. Simultaneously, the silicon-containing oxide phase only occupies a portion of the pore space and does not completely fill the pores; therefore, the porous transition layer still possesses gas transport channels connected to the pores of the porous silicon carbide support layer body, ensuring unobstructed transport channels during subsequent gas separation.

[0115] Film formation. Seed setting, hydrothermal crystallization and calcination are carried out in sequence according to the aforementioned general conditions to form a microporous molecular sieve selective layer composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals with AEI topology on the surface of the fine-pore film, thereby obtaining a molecular sieve membrane element for natural gas denitrification.

[0116] Structural and performance characterization. For example... Figure 8 As shown, the cross-section of the obtained molecular sieve membrane element for natural gas denitrification exhibits a layered structure consisting of a porous silicon carbide support layer, a porous transition layer, and a microporous molecular sieve selective layer stacked sequentially. The thickness of the porous transition layer is approximately 10 μm, and the thickness of the microporous molecular sieve selective layer is approximately 6 μm, and it is continuous and dense. The maximum through-pore diameter of the composite material, determined by the bubble point method, is approximately 1.8 μm. The measurable open porosity of the composite material, determined by mercury porosimetry, is approximately 48%, and the average pore diameter is approximately 1.7 μm.

[0117] like Figure 9 As shown in (b), XRD analysis revealed that no obvious crystalline silicon dioxide characteristic diffraction peaks were detected in the silicon oxide phase of the porous transition layer, and a weak broad peak was present near 20° to 25°. This result is consistent with the presence of an amorphous silicon oxide phase in the porous transition layer. The sample after film formation exhibited characteristic diffraction peaks of the AEI topology.

[0118] Performance tests on natural gas denitrification separation showed that the nitrogen permeation rate of the molecular sieve membrane element used for natural gas denitrification was approximately 3.5 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The separation coefficient of nitrogen relative to methane is approximately 4.7, indicating that the microporous molecular sieve selective layer can utilize the difference in permeation rates between nitrogen and methane in the microporous channels to allow nitrogen to permeate preferentially relative to methane.

[0119] <Example 2> The difference between this embodiment and Embodiment 1 is that, after the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is treated with a phosphorus-containing compound to form a phosphorus-containing surface-modified phase on at least a portion of the surface of the silicon oxide phase. This phosphorus-containing surface-modified phase is then used as the surface for setting and growing the aluminum phosphate molecular sieve seed crystals. The remaining raw materials and steps are the same as in Embodiment 1.

[0120] Oxidation heat treatment. Similar to Example 1, an oxidation heat treatment was performed at 1100°C for 8 hours in humid air containing water vapor with a dew point of approximately 75°C to form a porous transition layer that extends continuously from the bulk on the outer peripheral surface of the porous silicon carbide support layer. The silicon oxide in the porous transition layer contains amorphous silicon dioxide, and a surface region containing silanol groups is formed on the surface of the fine-pore film.

[0121] Phosphorylation treatment. The porous silicon carbide support tube, after oxidation heat treatment, is immersed in a 0.5 mol / L aqueous solution of orthophosphoric acid for 30 min for liquid phase treatment. After being pulled out, it is dried at 80 °C, followed by heat treatment at 350 °C for 2 h. During this heat treatment, the phosphorus-containing compound adsorbs, condenses, or bonds with the hydroxyl groups on the surface of the silicon oxide phase, forming a phosphorus-containing surface-modified phase on at least a portion of the surface of the silicon oxide phase. The phosphorus-containing surface-modified phase contains phosphorus and phosphoroyl groups.

[0122] Film formation. Seed setting, hydrothermal crystallization and calcination are carried out in sequence according to the aforementioned general conditions to form a microporous molecular sieve selective layer composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals on the surface of the fine-pore film, thereby obtaining a molecular sieve membrane element for natural gas denitrification.

[0123] Structural and performance characterization. The maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer was approximately 1.8 μm, determined by the bubble point method. The measurable open porosity of the composite was approximately 46%, and the average pore diameter was approximately 1.6 μm, determined by the mercury porosimetry method.

[0124] Performance tests on natural gas denitrification separation showed that the nitrogen permeation rate of the molecular sieve membrane element used for natural gas denitrification was approximately 3.2 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The separation coefficient of nitrogen relative to methane was approximately 5.1, higher than that in Example 1. This difference may be related to changes in the chemical environment of the porous film surface after phosphorus-containing surface modification, as well as changes in the adhesion and growth state of the seed crystals.

[0125] <Example 3> The difference between this embodiment and Embodiment 1 is that, after the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is treated with an aluminum-containing compound and a phosphorus-containing compound to form an aluminum-phosphorus surface-modified phase containing aluminum and phosphorus elements on at least a portion of the surface of the silicon oxide phase. This aluminum-phosphorus surface-modified phase is then used as the surface for setting and growing the aluminum phosphate molecular sieve seed crystals. The remaining raw materials and steps are the same as in Embodiment 1.

[0126] Oxidation heat treatment. Similar to Example 1, an oxidation heat treatment was performed at 1100°C for 8 hours in humid air containing water vapor with a dew point of approximately 75°C to form a porous transition layer that extends continuously from the bulk in the outer peripheral region of the porous silicon carbide support layer. The porous transition layer contains an amorphous silicon oxide phase.

[0127] Aluminum-phosphorus compound treatment. An ethanol-water mixed solution containing aluminum nitrate and orthophosphoric acid is prepared, wherein the molar ratio of aluminum to phosphorus is approximately 1:1, and the total concentration of aluminum and phosphorus is approximately 0.3 mol / L. A porous silicon carbide support tube, after undergoing oxidation heat treatment, is immersed in the mixed solution for 30 min for liquid-phase treatment. After being pulled out, it is dried at 80 °C, followed by heat treatment at 450 °C for 2 h. During the liquid-phase treatment and heat treatment, aluminum-containing and phosphorus-containing compounds adsorb, deposit, condense, or bond on at least a portion of the surface of the silicon oxide phase, forming an aluminum-phosphorus surface-modified phase containing aluminum and phosphorus elements on at least a portion of the surface of the silicon oxide phase. This creates an aluminum- and phosphorus-containing silicon oxide chemical environment on the surface of the microporous film.

[0128] Film formation. Seed setting, hydrothermal crystallization and calcination are carried out in sequence according to the aforementioned general conditions to form a microporous molecular sieve selective layer composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals on the surface of the fine-pore film, thereby obtaining a molecular sieve membrane element for natural gas denitrification.

[0129] Structure and performance characterization. The maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer was approximately 1.8 μm, as determined by the bubble point method. The measurable open porosity of the composite was approximately 45%, and the average pore diameter was approximately 1.6 μm, as determined by the mercury porosimetry method.

[0130] Performance tests on natural gas denitrification separation showed that the nitrogen permeation rate of the molecular sieve membrane element used for natural gas denitrification was approximately 3.0 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The nitrogen-to-methane separation coefficient was approximately 5.3, the highest in this set of examples. Compared to Examples 1 and 2, Example 3 underwent treatment with an aluminum-phosphorus compound, resulting in a membrane element exhibiting a higher nitrogen-to-methane separation coefficient. This difference may be related to changes in the chemical environment of the microporous film surface after aluminum-phosphorus surface modification, as well as changes in the adhesion and growth state of the aluminum phosphate molecular sieve seeds.

[0131] <Example 4> This embodiment employs a second type of porous silicon carbide support tube. This second type of porous silicon carbide support tube is a sintered porous silicon carbide support tube with added Al2O3-Y2O3 sintering aid, having an outer diameter of approximately 13 mm, an inner diameter of approximately 9 mm, and a length of approximately 800 mm. In preparing this second type of porous silicon carbide support tube, silicon carbide powder is used as the main raw material, and Al2O3-Y2O3 sintering aid is added. Based on the total mass of the silicon carbide powder and the Al2O3-Y2O3 sintering aid, the amount of Al2O3-Y2O3 sintering aid added is 10 wt.%, and the molar ratio of Al2O3 to Y2O3 in the Al2O3-Y2O3 sintering aid is 5:3. After sintering, the second type of porous silicon carbide support tube contains a yttrium aluminum oxide bound phase.

[0132] Oxidation heat treatment. The second type of porous silicon carbide support tube is placed in a tube furnace, and humid air containing water vapor with a dew point of approximately 60°C flows through the outer periphery of the second type of porous silicon carbide support tube as an oxidizing atmosphere. The temperature is increased to 1100°C at a heating rate of 3°C / min, held at this temperature for 2.5 hours, and then cooled to room temperature at a cooling rate of 3°C / min. During the oxidation heat treatment, the silicon carbide in the surface region of the outer periphery of the second type of porous silicon carbide support tube undergoes in-situ oxidation, and a silicon-containing oxide phase is generated at the pore walls and pore throats of the original porous framework in the surface region. The oxidation process can be simplified as SiC + (3 / 2)O2 → SiO2 + CO↑.

[0133] In the second type of porous silicon carbide support tube, the yttrium aluminum oxide phase formed by the Al2O3-Y2O3 sintering aid is distributed in the bonding area between silicon carbide particles and its adjacent region. Under the support tube composition, initial pore structure, and oxidation heat treatment conditions used in this embodiment, the second type of porous silicon carbide support tube exhibits a relatively high wet oxidation response. This phenomenon may be related to the influence of the yttrium aluminum oxide bonding on the oxidation reaction of adjacent regions and the transport process of oxidizing components.

[0134] As the oxidation heat treatment proceeds, the generated silicon-containing oxide phase distributes along the pore walls and throats of the original porous framework, occupying part of the pore space. This reduces the effective flow cross-section of the throats that control the through flow, thereby forming a porous transition layer on the outer peripheral surface of the second type of porous silicon carbide support tube. The porous transition layer retains gas transport channels that communicate with the pores of the porous silicon carbide support layer body, and forms a fine-pore film-forming surface on the side opposite to the porous silicon carbide support layer body.

[0135] Pore ​​structure characterization. The maximum through-pore diameter of the second type of porous silicon carbide support tube before oxidation heat treatment was approximately 5.5 μm, determined by the bubble point method. After the oxidation heat treatment, the maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer was approximately 1.9 μm. The measurable open porosity of the composite was approximately 47%, and the average pore diameter was approximately 1.8 μm, determined by mercury porosimetry.

[0136] The above pore structure test results show that the oxidation heat treatment significantly reduces the pore throat controlling maximum through-flow, but does not completely seal the original pores of the porous silicon carbide support layer. The formed porous transition layer can reduce the surface pore size while retaining interconnected gas transport channels, thus balancing the requirements of the fine pore surface needed for subsequent molecular sieve film formation and the transport requirements of permeate gas during membrane separation.

[0137] Film formation. Following the aforementioned general preparation conditions, AlPO-18 type aluminum phosphate molecular sieve seed crystals are placed on the surface of the fine-pore film formation. Subsequently, hydrothermal crystallization and calcination are performed sequentially, allowing the AlPO-18 type aluminum phosphate molecular sieve seed crystals to grow and interconnect on the surface of the fine-pore film formation, forming a microporous molecular sieve selective layer composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals with an AEI topology, thus obtaining a molecular sieve membrane element for natural gas denitrification.

[0138] Natural gas denitrification separation performance test. Following the aforementioned natural gas denitrification separation performance test method, a N2 / CH4 mixture with a volume ratio of 50 / 50 was used as the feed gas, and the test was conducted at a feed temperature of 25℃ and a transmembrane pressure difference of 0.3 MPa. After the permeation process reached steady state, the nitrogen permeation rate of the molecular sieve membrane element used for natural gas denitrification was measured to be approximately 3.7 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The separation coefficient of nitrogen relative to methane is approximately 5.0.

[0139] <Comparative> This comparative example uses commercially available asymmetric silicon carbide ceramic membrane tubes and does not undergo oxidation heat treatment. The remaining aluminum phosphate molecular sieve seed crystals, film formation system, seed crystal setting, hydrothermal crystallization and calcination procedures are the same as in Example 1.

[0140] Support body. The asymmetric silicon carbide ceramic membrane tube is a tubular structure with a layer of fine-pore silicon carbide membrane pre-composite and sintered on the outer peripheral surface of a coarse-pore silicon carbide support substrate. Its outer diameter, inner diameter, and length are comparable to the porous silicon carbide support tubes used in various embodiments. Since the fine-pore silicon carbide membrane is formed separately by composite and sintering after the coarse-pore silicon carbide support substrate is formed, the fine-pore silicon carbide membrane and the coarse-pore silicon carbide support substrate constitute an asymmetric layered structure and form independent interlayer interfaces.

[0141] The maximum through-pore diameter of the composite consisting of the coarse-porous silicon carbide support substrate and the fine-porous silicon carbide film layer was approximately 1.9 μm, as determined by the bubble point method. The measurable open porosity of the composite was approximately 48%, and the average pore diameter was approximately 1.7 μm, as determined by the mercury porosimetry method. These parameters are close to the corresponding parameters of the composite formed by oxidation heat treatment in Example 1. This comparative example did not undergo oxidation heat treatment, and therefore did not form the silicon-containing oxide phase and corresponding fine-porous film surface as described in Example 1 through in-situ oxidation of the silicon carbide surface region. The outer surface of the fine-porous silicon carbide film layer was directly used as the seed crystal setting and film formation surface for the aluminum phosphate molecular sieve. Film formation. Under the aforementioned general conditions, seed crystal setting, hydrothermal crystallization, and calcination were sequentially performed on the outer surface of the fine-porous silicon carbide film layer to form a microporous molecular sieve selective layer, resulting in the comparative example film element.

[0142] Natural gas denitrification and separation performance. Tested according to the aforementioned natural gas denitrification and separation performance test method, the nitrogen permeation rate of the comparative membrane element was approximately 3.3 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The nitrogen-to-methane separation coefficient was only about 4.1, lower than that of Examples 1 to 3. Under the experimental conditions of this group, the nitrogen-to-methane separation coefficient of the membrane element obtained in Example 1 was higher than that of the comparative example membrane element. This result indicates that there is a positive correlation between the fine-pore film-forming surface formed by in-situ oxidation and the separation performance of the resulting microporous molecular sieve selective layer.

[0143] Based on Examples 1 to 4 and the comparative examples, it can be seen that Examples 1 to 4 all involve oxidative heat treatment of the surface area of ​​the porous silicon carbide support layer, causing in-situ oxidation of the silicon carbide in the surface area, generating silicon oxide phases at the pore walls and pore throats of the original porous framework, thereby forming a porous transition layer that extends continuously from the porous silicon carbide support layer body and retains interconnected gas transport channels. Figure 8 The cross-sectional morphology shown is consistent with the structure of the porous transition layer, which is transformed in situ from the surface region of the porous silicon carbide support layer and continuously transitions to its body. Figure 9The phase characterization results shown are consistent with the presence of amorphous silicon oxide phase in the porous transition layer and the formation of an AlPO-18 type aluminum phosphate molecular sieve selective layer with AEI topology after film formation.

[0144] To facilitate comparison of the gas separation performance of the membrane elements obtained in Examples 1 to 4 and the comparative examples, Figure 11 The nitrogen permeation rate and nitrogen-to-methane separation coefficient for each membrane element are shown. Figure 11 The nitrogen permeation rate corresponds to the left ordinate, and the nitrogen-to-methane separation coefficient corresponds to the right ordinate. The broken line connecting the data points is only used to assist in comparing the performance differences between different samples and does not indicate a continuously changing process variable relationship between the various examples.

[0145] like Figure 11 As shown, the nitrogen permeation rates of the comparative examples and the membrane elements obtained in Examples 1 to 4 are approximately 3.3 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 3.5×10 -7 mol·m -2 ·s -1 ·Pa -1 3.2×10 -7 mol·m -2 ·s -1 ·Pa -1 3.0×10 -7 mol·m -2 ·s -1 ·Pa -1 and 3.7×10 -7 mol·m -2 ·s -1 ·Pa -1 The corresponding nitrogen-to-methane separation coefficients are approximately 4.1, 4.7, 5.1, 5.3, and 5.0, respectively. The nitrogen-to-methane separation coefficients of Examples 1 to 4 are all higher than those of the comparative example, and the nitrogen permeation rates of each example remain on a similar order of magnitude. This indicates that the porous transition layer formed by the present invention can maintain gas transport capacity while providing a fine-pore film-forming surface suitable for seed setting and subsequent growth of the microporous molecular sieve selective layer.

[0146] Specifically, the composite obtained in Example 1 has a maximum through-pore diameter of approximately 1.8 μm, a measurable open porosity of approximately 48%, and an average pore diameter of approximately 1.7 μm; the composite obtained in the comparative example has a maximum through-pore diameter of approximately 1.9 μm, a measurable open porosity of approximately 48%, and an average pore diameter of approximately 1.7 μm. The overall pore structure parameters of the two are quite similar. Meanwhile, the nitrogen permeation rates of the membrane elements obtained in Example 1 and the comparative example are also quite similar, approximately 3.5 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 and 3.3×10 -7 mol·m -2 ·s -1 ·Pa -1 The nitrogen-to-methane separation coefficient in Example 1 increased from approximately 4.1 in the comparative example to approximately 4.7. These results indicate that the improved separation performance of the membrane element obtained in Example 1 is not solely due to changes in the overall pore structure of the composite or gas transport resistance, but is related to the porous film-forming surface provided by the porous transition layer formed through in-situ oxidation.

[0147] The porous transition layer used in Example 1 was formed in situ by oxidative thermal treatment of the surface region of the outer peripheral surface of the porous silicon carbide support layer. The silicon-containing oxide phase in the porous transition layer is distributed along the pore walls and throats of the original porous framework, reducing the size of the throats that at least partially control the through-flow, and forming a fine-porous film-forming surface on the side of the porous transition layer away from the bulk, suitable for wetting and seed attachment of the aqueous aluminum phosphate molecular sieve seed suspension. This fine-porous film-forming surface helps reduce the deep penetration of aluminum phosphate molecular sieve seeds and the film-forming system into the porous silicon carbide support layer, retaining more seeds in the predetermined film-forming area, and facilitating the growth and interconnection of adjacent aluminum phosphate molecular sieve crystals during hydrothermal crystallization. Therefore, the improved separation coefficient shown in Example 1 compared to the comparative example is consistent with the effect of the fine-porous film-forming surface in improving the retention and growth state of seeds in the predetermined film-forming area and reducing non-selective transport channels in the resulting microporous molecular sieve selective layer.

[0148] Examples 2 and 3 respectively involved phosphorus-containing and aluminum-phosphorus surface modification on the porous transition layer formed in Example 1. The nitrogen permeation rates of the membrane elements obtained in Examples 2 and 3 were approximately 3.2 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 and 3.0×10 -7 mol·m -2 ·s -1 ·Pa -1The nitrogen-to-methane separation coefficients were slightly lower than those in Example 1, while the nitrogen-to-methane separation coefficients increased to approximately 5.1 and 5.3, respectively. Since the nitrogen-to-methane separation coefficient in this specification is the ratio of nitrogen permeation rate to methane permeation rate, the above results indicate that the decrease in methane permeation rate in Examples 2 and 3 was greater than the decrease in nitrogen permeation rate. This trend is consistent with the effect of phosphorus-containing or aluminum-phosphorus-containing surface-modified phases further improving the compatibility between the microporous film-forming surface and the aluminum phosphate molecular sieve crystals and film-forming system, promoting the interconnection of aluminum phosphate molecular sieve crystals, and reducing non-selective transport channels.

[0149] Example 4 uses a second type of porous silicon carbide support tube with an added Al2O3-Y2O3 sintering aid and a maximum through-pore diameter of approximately 5.5 μm before oxidation heat treatment. This second type of porous silicon carbide support tube exhibits a relatively high wet oxidation response, thus enabling the formation of a porous transition layer on its outer peripheral surface under conditions of a wet air dew point of approximately 60°C and a holding time of 1100°C for 2.5 hours. After oxidation heat treatment, the composite obtained in Example 4 has a maximum through-pore diameter of approximately 1.9 μm, a measurable open porosity of approximately 47%, and an average pore diameter of approximately 1.8 μm; the nitrogen permeation rate of the resulting membrane element is approximately 3.7 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The nitrogen-to-methane separation coefficient was approximately 5.0, which was slightly higher than that in Example 1.

[0150] The second type of porous silicon carbide support tube used in Example 4 has a larger through-pore diameter before the oxidation heat treatment, and the oxidation heat treatment time is shorter than that in Example 1. After forming a fine-pored film surface that meets the film formation requirements, its support layer body can still retain relatively large gas transport channels, which helps to reduce the overall transport resistance of the permeating gas when passing through the porous silicon carbide support layer and the porous transition layer. This is consistent with the higher nitrogen permeation rate shown in Example 4. At the same time, the yttrium aluminum oxide bonding phase formed by the Al2O3-Y2O3 sintering aid in the second type of porous silicon carbide support tube is distributed in the bonding part between silicon carbide particles and its adjacent area. This yttrium aluminum oxide bonding phase may affect the oxidation reaction of adjacent areas, the formation of silicon oxide phase, and the transport process of oxidizing components during the humid air oxidation process, and make the resulting fine-pored film surface form a chemical environment with good compatibility with aluminum-containing and phosphorus-containing film systems. The above factors may collectively improve the adhesion and growth of AlPO-18 type aluminum phosphate molecular sieve crystals on the microporous film surface, thereby enabling Example 4 to achieve a nitrogen-to-methane separation coefficient slightly higher than that of Example 1 while maintaining a high nitrogen permeation rate.

[0151] The separation coefficient of Example 4 was lower than that of Example 2, which underwent specific phosphorus-containing surface modification, and Example 3, which underwent specific aluminum-phosphorus-containing surface modification. This indicates that the interface regulation effect produced by the aluminum-containing phase in the support tube composition differs from the effect produced by directly forming a phosphorus-containing or aluminum-phosphorus-containing surface-modified phase on the surface of the silicon oxide phase in the porous transition layer. Examples 2 and 3, through targeted chemical modification of the microporous film-forming surface, were able to further regulate the surface chemical environment of the seed crystal setting and the hydrothermal film-forming region, exhibiting relatively high nitrogen-to-methane separation coefficients in this group of experiments.

[0152] In summary, the porous transition layer formed by this invention does not simply increase gas transport resistance, but rather provides a fine-porous film-forming surface suitable for the attachment, growth, and interconnection of aluminum phosphate molecular sieve seeds by simultaneously adjusting the pore structure and surface chemical state of the predetermined film-forming region. The membrane elements obtained in Examples 1 to 4 all achieved higher nitrogen-to-methane separation coefficients than the comparative examples while maintaining nitrogen permeation rates at similar orders of magnitude. Specifically, by modifying the porous transition layer with phosphorus or aluminum-phosphorus content, the separation selectivity of the resulting microporous molecular sieve selective layer can be further improved; by using a porous silicon carbide support tube with a large initial pore size and containing Al2O3-Y2O3 sintering aid, a low support gas transport resistance can be maintained while forming a fine-porous film-forming surface, thus balancing nitrogen permeation capacity and nitrogen-to-methane separation selectivity.

[0153] The foregoing has described the relevant content of the present invention. Those skilled in the art will be able to implement the present invention based on these descriptions. All other embodiments obtained by those skilled in the art based on the foregoing content of this specification without inventive effort should fall within the scope of the present invention.

Claims

1. A molecular sieve membrane element for natural gas denitrification, characterized by: It includes a porous silicon carbide support layer, a porous transition layer, and a microporous molecular sieve selective layer; The porous transition layer is formed in situ by oxidation heat treatment of the surface region of the porous silicon carbide support layer. The porous transition layer extends continuously from the body of the porous silicon carbide support layer and has gas transmission channels that are connected to the pores of the porous silicon carbide support layer body. The porous transition layer contains a silicon-containing oxide phase generated by in-situ oxidation of silicon carbide in the surface region and distributed on the pore walls of the porous transition layer. The silicon-containing oxide phase makes the surface pore size of the porous transition layer smaller than the pore size of the porous silicon carbide support layer body, and forms a fine-pore film-forming surface on the side of the porous transition layer away from the body. The fine-pore film-forming surface constitutes a nucleation and bonding surface for the attachment, growth and interconnection of microporous molecular sieve crystals. The microporous molecular sieve selective layer is disposed on the surface of the fine pore film formation. It is composed of interconnected aluminum phosphate molecular sieve crystals with an AEI topology and has microporous channels that allow nitrogen and methane to have different permeation rates, for selective separation of nitrogen and methane in nitrogen-containing natural gas.

2. The molecular sieve membrane element for natural gas denitrification according to claim 1, characterized by: The composite consisting of the porous silicon carbide support layer and the porous transition layer has a maximum through-pore diameter of no more than 2 micrometers, a porosity of no less than 45%, and an average pore diameter of 1.5 micrometers to 1.8 micrometers.

3. The molecular sieve membrane element for natural gas denitrification according to claim 2, characterized in that: The porous silicon carbide support layer has a symmetrical porous structure with pore sizes that are basically consistent along the thickness direction. The porous transition layer is formed by in-situ transformation of the surface region of the symmetrical porous structure through oxidation heat treatment.

4. The molecular sieve membrane element for natural gas denitrification according to claim 1, characterized in that: The device includes a porous silicon carbide support tube, which is a single-channel tubular structure. The tube wall of the single-channel tubular structure forms a single axially continuous inner cavity, and the tube wall constitutes the porous silicon carbide support layer. A porous transition layer is formed on the outer peripheral surface of the porous silicon carbide support tube, and a microporous molecular sieve selective layer is disposed on the fine pore film-forming surface outside the porous transition layer. The molecular sieve membrane element for natural gas denitrification has an outer diameter of 8 mm to 20 mm and a length of 500 mm to 1500 mm, and the ratio of the length to the outer diameter is not less than 40.

5. The molecular sieve membrane element for natural gas denitrification according to claim 1, characterized in that: The thickness of the porous transition layer is 5 micrometers to 30 micrometers.

6. The molecular sieve membrane element for natural gas denitrification according to claim 1, characterized in that: The silicon-containing oxide phase comprises silicon dioxide, which is attached to the pore walls of the porous transition layer.

7. The molecular sieve membrane element for natural gas denitrification according to claim 6, characterized in that: A phosphorus-containing surface-modified phase is formed on the surface of the silicon oxide phase. The phosphorus-containing surface-modified phase contains phosphorus element and phosphorus oxygen group, and constitutes a nucleating and binding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

8. The molecular sieve membrane element for natural gas denitrification according to claim 6, characterized in that: An aluminum-phosphorus surface-modified phase is formed on the surface of the silicon oxide phase. The aluminum-phosphorus surface-modified phase contains aluminum and phosphorus elements and constitutes a nucleating and bonding phase for the attachment and growth of the aluminum phosphate molecular sieve crystals.

9. The molecular sieve membrane element for natural gas denitrification according to claim 6, characterized in that: The silicon-containing oxide phase comprises amorphous silicon dioxide.

10. The molecular sieve membrane element for natural gas denitrification according to claim 1, characterized in that: The microporous molecular sieve selective layer is an AlPO-18 type aluminum phosphate molecular sieve selective layer, and is composed of interconnected AlPO-18 type aluminum phosphate molecular sieve crystals.

11. A method for preparing a molecular sieve membrane element for natural gas denitrification, characterized in that: include: The surface region of the porous silicon carbide support layer is subjected to an oxidation heat treatment to oxidize the silicon carbide in situ in the surface region, thereby forming a porous transition layer that extends continuously from the body of the porous silicon carbide support layer on the surface of the porous silicon carbide support layer. The porous transition layer contains a silicon-containing oxide phase generated by the in-situ oxidation of silicon carbide and distributed on the pore walls of the porous transition layer. The silicon-containing oxide phase makes the surface pore size of the porous transition layer smaller than the pore size of the porous silicon carbide support layer body, and forms a fine-pore film-forming surface on the side of the porous transition layer away from the body. Aluminum phosphate molecular sieve seed crystals with an AEI topology are provided, and the aluminum phosphate molecular sieve seed crystals are disposed on the surface of the fine-pore film to form a seed layer; The porous silicon carbide support layer with the seed layer is placed in a film-forming system containing an aluminum source, a phosphorus source, an organic structure directing agent and water for hydrothermal crystallization, so that the aluminum phosphate molecular sieve seed crystals grow on the surface of the fine pore film and connect with each other to form an aluminum phosphate molecular sieve film layer. as well as The aluminum phosphate molecular sieve membrane layer is calcined to remove the organic structure directing agent, forming a microporous molecular sieve selective layer.

12. The preparation method according to claim 11, characterized in that: The oxidation heat treatment is carried out in an oxidizing atmosphere containing oxygen and water vapor, wherein the dew point of the water vapor in the oxidizing atmosphere is 55℃~80℃, and the temperature of the oxidation heat treatment is 1050℃~1150℃. By adjusting the temperature, treatment time, and treatment atmosphere of the oxidation heat treatment, the thickness of the porous transition layer is made to be 5 micrometers~30 micrometers, and the maximum through-pore diameter of the composite consisting of the porous silicon carbide support layer and the porous transition layer is not greater than 2 micrometers, the porosity is not less than 45%, and the average pore diameter is 1.5 micrometers~1.8 micrometers.

13. The preparation method according to claim 11, characterized in that: After the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is subjected to liquid-phase treatment or gas-phase treatment with a phosphorus-containing compound, and the porous transition layer treated with the phosphorus-containing compound is subjected to heat treatment, so that at least a portion of the surface of the silicon oxide phase forms a phosphorus-containing surface-modified phase containing phosphorus elements and phosphorooxy groups.

14. The preparation method according to claim 11, characterized in that: After the oxidation heat treatment and before setting the aluminum phosphate molecular sieve seed crystals, the porous transition layer is subjected to liquid-phase treatment or gas-phase treatment with aluminum-containing compounds and phosphorus-containing compounds, and the porous transition layer treated with the aluminum-containing compounds and phosphorus-containing compounds is subjected to heat treatment, so that at least a portion of the surface of the silicon oxide phase forms an aluminum-phosphorus surface-modified phase containing aluminum and phosphorus elements.

15. The preparation method according to claim 11, characterized in that: The aluminum phosphate molecular sieve seed crystal is an AlPO-18 type aluminum phosphate molecular sieve seed crystal, and the aluminum phosphate molecular sieve membrane is an AlPO-18 type aluminum phosphate molecular sieve membrane. After hydrothermal crystallization and calcination, an AlPO-18 type aluminum phosphate molecular sieve selective layer is formed on the surface of the fine-pore film.

16. A membrane separator for natural gas denitrification, characterized in that: Includes a housing and a plurality of molecular sieve membrane elements for natural gas denitrification as described in any one of claims 1 to 10 disposed within the housing; The housing forms a feed-side space and a permeation-side space. The microporous molecular sieve selective layer of each molecular sieve membrane element for natural gas denitrification faces the feed-side space, and the pores of the porous silicon carbide support layer of each molecular sieve membrane element for natural gas denitrification are connected to the permeation-side space. The housing is provided with a raw material gas inlet and a residual gas outlet communicating with the feed side space, and a permeate gas outlet communicating with the permeate side space; Nitrogen-containing natural gas enters the feed-side space through the feed gas inlet. The preferential permeable components in the nitrogen-containing natural gas pass through the microporous molecular sieve selective layer, the porous transition layer and the porous silicon carbide support layer in sequence and enter the permeable-side space, and are led out through the permeable gas outlet. The unpermeable components form residual permeate gas, which is also led out through the residual permeate gas outlet.

Images