Method for manufacturing inorganic separation membranes

Abstract

The present invention provides a manufacturing method that allows for the easy production of at least two types of inorganic separation membranes with different gas separation performance characteristics. [Solution] A method for manufacturing at least two types of inorganic separation membranes with different separation performance for mixed gases, comprising: a hydrothermal synthesis step of forming a zeolite precursor layer on the surface of a plurality of porous supports by hydrothermal synthesis to obtain a plurality of membrane intermediates comprising a porous support and a zeolite precursor layer; a firing step of firing the plurality of membrane intermediates at a first firing temperature to form a plurality of inorganic separation membranes comprising a zeolite layer which is a fired body of the zeolite precursor layer and a porous support; a selection step of selecting a plurality of first separation membranes having separation performance of a predetermined level or higher from the plurality of inorganic separation membranes; and a re-firing step of firing a portion of the plurality of first separation membranes at a second firing temperature to obtain a second separation membrane having lower separation performance than the first separation membrane.

Description

[Technical Field]

[0001] This invention relates to a method for producing inorganic separation membranes. [Background technology]

[0002] In recent years, membrane separation technology has attracted attention for its use of zeolite membranes that selectively permeate carbon dioxide, removing carbon dioxide from natural gas and biogas to concentrate methane.

[0003] Patent Document 1 proposes a gas separation system for recovering unpermeated gas from a mixed gas of carbon dioxide gas and methane gas using a gas separation membrane, wherein a first gas separation membrane unit and a second gas separation membrane unit are connected in series, and the system includes a line for supplying unpermeated gas discharged from the first gas separation membrane unit to the second gas separation membrane unit, and a line for supplying permeated gas discharged from the second gas separation membrane unit to the first gas separation membrane unit, wherein the gas separation selectivity of the first gas separation membrane unit (expressed as the permeation rate of carbon dioxide gas / permeation rate of methane gas) is higher than the gas separation selectivity of the second gas separation membrane unit, and the permeation rate of carbon dioxide gas discharged from the second gas separation membrane unit is greater than the permeation rate of carbon dioxide gas discharged from the first gas separation membrane unit.

[0004] Patent documents 2 and 3 propose that when using a zeolite membrane composite, which is used to separate highly permeable components from a gaseous or liquid mixture consisting of multiple components, membranes with different performance characteristics should be installed at each stage. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 5948853 [Patent Document 2] Patent No. 6107808 [Patent Document 3] Patent No. 6167489 [Overview of the project] [Problems that the invention aims to solve]

[0006] Patent Document 1 proposes making the gas separation selectivity of the first gas separation membrane unit higher than that of the second gas separation membrane unit. Patent Documents 2 and 3 propose installing membranes with different performance at each stage in a multi-stage separation membrane system. Furthermore, Patent Documents 2 and 3 propose controlling the separation performance by changing the SiO2 / Al2O3 molar ratio of the zeolite raw material, or by controlling the effective pore size of the zeolite through the type of metal introduced into the zeolite, ion exchange, acid treatment, silylation treatment, etc. However, significantly changing the manufacturing conditions not only increases manufacturing costs, but also makes it difficult to manufacture separation membranes with different separation performance. [Means for solving the problem]

[0007] One aspect of this disclosure relates to a method for manufacturing at least two types of inorganic separation membranes with different mixed gas separation performance, comprising: a hydrothermal synthesis step of forming a zeolite precursor layer on the surface of a plurality of porous supports by hydrothermal synthesis to obtain a plurality of membrane intermediates comprising the porous supports and the zeolite precursor layer; a firing step of firing the plurality of membrane intermediates at a first firing temperature to form a plurality of inorganic separation membranes comprising a zeolite layer which is a fired body of the zeolite precursor layer and the porous supports; a selection step of selecting a plurality of first separation membranes having separation performance of a predetermined level or higher from the plurality of inorganic separation membranes; and a re-firing step of firing a portion of the plurality of first separation membranes at a second firing temperature to obtain a second separation membrane having lower separation performance than the first separation membrane. [Effects of the Invention]

[0008] According to this disclosure, at least two types of inorganic separation membranes with different gas separation performance can be easily manufactured. [Modes for carrying out the invention]

[0009] Examples of embodiments relating to this disclosure are described below. While the following description provides examples of embodiments of this disclosure, this disclosure is not limited to the examples described below. The following description may include specific numerical values ​​and materials as examples, but other numerical values ​​and materials may be used as long as the effects of this disclosure are achieved. In this specification, the range described as "numerical value A to numerical value B" includes numerical value A and numerical value B.

[0010] (Inorganic separation membrane) An inorganic separation membrane includes a porous support and a separation layer disposed on the surface of the porous support. The separation layer of the inorganic separation membrane according to this disclosure includes a zeolite layer. The zeolite layer is a membrane composed of zeolite.

[0011] <Zeolite layer> Zeolite layers have tiny, uniform pores that allow molecules smaller than the pore diameter to pass through while inhibiting the passage of larger molecules, making them suitable for use as molecular sieve films. Typical zeolites are crystalline aluminosilicates, mainly composed of silica (silicon dioxide) and alumina (aluminum oxide). Zeolites have a silicon dioxide (SiO2) skeleton, and some of the silicon is replaced by aluminum, causing a portion of the skeleton to be negatively charged. Therefore, the charge balance is maintained by containing cations such as alkali metals and alkaline earth metals within the pores.

[0012] The structure of the zeolite constituting the zeolite layer may be an 8-membered ring structure or a structure having an 8-membered oxygen ring (8-membered oxygen ring pore). Examples of zeolite structures having an 8-membered oxygen ring include DDR type, CHA type, LTA type, AEI type, and AFX type. By using a zeolite layer having an 8-membered oxygen ring, an inorganic separation membrane suitable for separating carbon dioxide from natural gas and biogas can be obtained. A zeolite layer having an 8-membered oxygen ring can be formed, for example, by the method described in the examples.

[0013] The structure of the zeolite constituting the zeolite layer may be a CHA-type structure. By using a zeolite layer composed of zeolites having a CHA-type structure (CHA-type zeolite), an inorganic separation membrane with a large maximum pore volume and high carbon dioxide permeability can be obtained. A zeolite layer having a CHA-type structure can be formed, for example, by the method described in the examples. "CHA" is a code defined by the International Zeolite Association (IZA) for classifying zeolites by structure.

[0014] <Porous support> The average pore size of the porous support is preferably selected to satisfy the conditions that it can firmly support the zeolite layer, have low pressure loss, and possess high mechanical strength.

[0015] There are no particular limitations on the porous support; any porous support used in known inorganic separation membranes containing a zeolite layer may be used. The porous support only needs to be capable of forming a zeolite layer on it. Examples of porous supports include those made of alumina, silica, zirconia, titania, silicon carbide, and stainless steel. Typically, molecules that pass through the zeolite layer then pass through the pores of the porous support, effectively acting as a molecular sieve. Therefore, the porous support usually has pores larger than those of the zeolite layer.

[0016] The shape and size of the porous support are not particularly limited. Examples of porous shapes include tubular, plate-shaped, honeycomb-shaped, hollow fiber-shaped, and pellet-shaped. For example, an example of a tubular porous support has a length in the range of 2 cm to 200 cm, an inner diameter in the range of 0.5 cm to 2.0 cm, and a thickness in the range of 0.5 mm to 4.0 mm.

[0017] The porous support may have a cylindrical shape with an outer surface. In this case, the zeolite layer is placed on the outer and / or inner surface of the cylindrical porous support. For example, the zeolite layer may be placed on the outer surface.

[0018] In an example of a separation method using a cylindrical inorganic separation membrane, a mixed gas containing a gas to be separated is passed, for example, to the outer peripheral surface side (supply side) of the cylindrical inorganic separation membrane. A part of the mixed gas passes through the zeolite layer and the porous support and flows through the hollow part (permeation side) of the porous support. In this way, the gas to be separated can be separated from the mixed gas.

[0019] (Method for manufacturing inorganic separation membrane)

[0018] In general, it is preferable that the inorganic separation membrane has high separation performance, but there are cases where it is preferable to use a separation membrane with low separation performance depending on the application. An example of this is the case of concentrating CH4 using a separation membrane that selectively permeates CO2 in the purification of biogas mainly composed of CO2 and CH4. With only a separation membrane having high separation performance, the pressure difference of CO2 between the supply side and the permeation side of the separation membrane becomes smaller toward the downstream side (CH4 concentration side) of the gas. As a result, it becomes difficult for CO2 to permeate on the downstream side of the gas, and thus the concentration of CH4 does not progress. In such a case, it is necessary to use a combination of an inorganic separation membrane with high separation performance and an inorganic separation membrane with low separation performance.

[0020] When two or more types of inorganic separation membranes having different separation performances of the mixed gas are required as in the case of biogas purification, it is desirable to manufacture two or more types of inorganic separation membranes having desired separation performance as simply as possible. On the other hand, it is also important to reliably obtain a high-performance inorganic separation membrane having sufficiently high separation performance.

[0021] For example, consider the case where a first gas separation unit and a second gas separation unit are connected in series, 50 inorganic separation membranes with high separation performance are installed in the first gas separation unit, and 50 inorganic separation membranes with low separation performance are installed in the second gas separation unit, and the mixed gas is supplied from the upstream first gas separation unit. The most important thing to note when manufacturing a total of 100 inorganic separation membranes is to reliably manufacture 50 high-performance separation membranes. In that case, it is necessary to select manufacturing conditions so that all 100 inorganic separation membranes can exhibit the highest possible separation performance.

[0022] On the other hand, the separation performance of the manufactured inorganic separation membranes is not identical, and there are differences in separation performance. In addition, inorganic separation membranes with low separation performance may be obtained that are unsuitable for use in the first gas separation unit. However, since the manufacturing conditions are selected to obtain inorganic separation membranes with high separation performance, an excess of high-performance inorganic separation membranes will be produced. For example, when manufacturing 100 inorganic separation membranes, it is possible that 80 will have high separation performance above a certain level, and 20 will have low separation performance. In that case, the 20 low-performance inorganic separation membranes that cannot be used in the first gas separation unit can be used as is in the second gas separation unit.

[0023] In the method for manufacturing inorganic separation membranes according to this disclosure, the insufficient amount of low-separation-performance inorganic separation membranes is manufactured by reducing the separation performance of high-separation-performance inorganic separation membranes. In this case, the separation performance of the high-separation-performance inorganic separation membranes may be determined, and the required number of inorganic separation membranes (for example, 30 out of 80 in the above example) may be selected in order of lowest separation performance, thereby reducing their separation performance.

[0024] One easy way to reduce the separation performance of a high-performance inorganic separation membrane is to calcine it. For example, calcining an inorganic separation membrane at a predetermined temperature causes cracks to form, reducing its separation performance. The calcination conditions should be appropriately selected to obtain the desired separation performance. By appropriately selecting the calcination conditions, the separation performance of the inorganic separation membrane can be moderately reduced. In other words, situations where the separation performance of the inorganic separation membrane is reduced too much can be eliminated. If the separation performance of the inorganic separation membrane is too low, the loss of the gas to be separated increases. Also, if the separation performance of the inorganic separation membrane is too low, when the permeated gas is circulated upstream of the separation process, the amount of circulating gas increases, increasing the load on the separation equipment.

[0025] According to the above manufacturing method, the manufactured inorganic separation membrane can be applied to the gas separation process without loss. Therefore, compared to methods that use different manufacturing methods to produce inorganic separation membranes with different separation performances, it is possible to efficiently produce at least two types of inorganic separation membranes with different separation performances.

[0026] In view of the foregoing, the method for manufacturing an inorganic separation membrane according to the present disclosure (hereinafter also referred to as "manufacturing method (M)") is a method for manufacturing at least two types of inorganic separation membranes with different gas separation performance, and includes the following steps in this order: (A) hydrothermal synthesis step, (B) calcination step, (C) sorting step, and (D) re-calcination step.

[0027] (A) A hydrothermal synthesis step to obtain a plurality of membrane intermediates comprising porous supports and zeolite precursor layers by hydrothermal synthesis on the surfaces of a plurality of porous supports.

[0028] (B) A firing step in which multiple membrane intermediates are fired at a first firing temperature to form multiple inorganic separation membranes comprising a zeolite layer which is a fired zeolite precursor layer and a porous support.

[0029] (C) A sorting step in which multiple first separation membranes having separation performance of a predetermined level or higher are selected from multiple inorganic separation membranes.

[0030] (D) A re-calcination step in which a portion of the multiple first separation membranes is calcined at a second calcination temperature to obtain a second separation membrane with lower separation performance than the first separation membrane.

[0031] <Hydrothermal synthesis process> In the hydrothermal synthesis step, a porous support is prepared, and a zeolite precursor layer is formed on the surface of the porous support by hydrothermal synthesis. This yields a membrane intermediate comprising the porous support and the zeolite precursor layer. However, multiple porous supports (for example, 20 or more) are prepared. That is, the number of membrane intermediates and inorganic separation membranes formed using them per lot may be 20 or more.

[0032] (1) Preparation of porous support It is preferable to remove impurities such as dust adhering to the surface of the porous support by methods such as washing with water or ultrasonic cleaning. For example, the surface of the support may be cleaned by ultrasonic cleaning with water for 1 to 10 minutes.

[0033] (2) Formation of seed crystals The seed crystal is not particularly limited, but for example, a seed crystal having the same structure as the zeolite layer to be formed is prepared. The particle size of the seed crystal may be in the range of 10 nm to 2 μm (preferably in the range of 100 nm to 1 μm). By setting the particle size of the seed crystal to 2 μm or less, it becomes easier to form a dense zeolite layer. If the particle size of the seed crystal is too large, the seed crystal may be crushed.

[0034] (3) Formation of seed crystal layer Next, a seed crystal layer is formed on the surface of the porous support. There are no particular limitations on the method of forming the seed crystal layer on the surface of the porous support. For example, a dispersion can be prepared by dispersing seed crystals in a solvent such as water, and the seed crystal layer can be attached to the support by immersing the porous support in the dispersion (dip method). Alternatively, a slurry can be prepared by mixing a solvent such as water with seed crystals, and the seed crystal layer can be formed by applying the slurry to the surface of the porous support. The amount of seed crystal applied is, for example, 1 × 10⁻⁶ of the mass of the porous support. -4 ~1 × 10 -3 It may also be expressed as %. The porous support coated with seed crystals may be heat-treated at a temperature in the range of 450°C to 700°C to improve the adhesion between the porous support and the seed crystals.

[0035] (4) Formation of zeolite precursor layer The zeolite precursor layer is formed on a porous support with seed crystals arranged on its surface. Specifically, the zeolite precursor layer is synthesized hydrothermally while the porous support is immersed in a raw material composition (gel) for synthesizing the zeolite precursor layer.

[0036] The raw material composition for synthesizing the zeolite precursor layer includes, for example, a silica source, an aluminum source, a metal element (M) source, an organic structure modifier, hydrogen fluoride (HF), and water.

[0037] Examples of silica sources include amorphous silica, colloidal silica, silica gel, sodium ketate, tetraethyl orthosilicate (TEOS), and trimethylethoxysilane. Examples of alumina sources include alumina oxide, sodium aluminate, and aluminum hydroxide. Alternatively, zeolites may be used as materials that function as both silica and alumina sources. Examples of such zeolites include FAU-type zeolites and MOR-type zeolites.

[0038] There are no particular limitations on the source of the metal element (M); for example, a hydroxide of the metal element (M) may be used. Specifically, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, etc., may be used as the source of the metal element (M).

[0039] Organic structure-determining agents may include those having an N,N,N-trialkyl-1-adamantanamine cation derived from 1-adamantanamine, for example, their hydroxides are preferably used. Among these, N,N,N-trimethyladamantanamine ammonium hydroxide (TMAdaOH) is preferably used. Other examples include N,N,N-trialkylbenzylammonium hydroxide.

[0040] Bases may be added to the raw material composition to enhance structural selectivity. Examples of bases include alkali hydroxides (such as NaOH and KOH). These also function as sources of metal elements (M).

[0041] The raw material composition (gel) is aged before hydrothermal synthesis. The temperature and time for aging are, for example, 1 to 24 hours at room temperature (e.g., 15-25°C). Specifically, the mixed raw materials are reacted by stirring while heating, and the resulting water and ethanol are removed by evaporation. Then, ion-exchanged water is added to the remaining solid and stirred while heating. Note that ethanol is produced by the hydrolysis and condensation of alkoxysilane (such as TEOS) when alkoxysilane is used as the silica source.

[0042] The zeolite precursor layer is formed by hydrothermal synthesis. Specifically, first, the raw material composition and a porous support coated with seed crystals are placed in a designated container. In one example, the raw material composition and the porous support are placed in a container, and the container is placed in a sealed space. Then, hydrothermal synthesis is performed with the porous support immersed in the raw material composition. This forms a zeolite layer on the porous support coated with seed crystals.

[0043] The temperature and time for hydrothermal synthesis may be in the range of 140 to 180°C and 6 to 48 hours. The pressure for hydrothermal synthesis may be pressurized or natural pressure. For example, the pressure for hydrothermal synthesis may be the natural pressure inside a sealed container when hydrothermal synthesis is performed at the above temperature.

[0044] <Firing Process> After hydrothermal synthesis, the generated zeolite precursor layer is washed with deionized water and dried. Subsequently, a calcination process is performed on multiple membrane intermediates to remove any remaining organic structure-regulating agents within the zeolite precursor layer. In the calcination process, the multiple membrane intermediates are calcined at a first calcination temperature. During this process, the zeolite precursor layer is calcined, and multiple inorganic separation membranes are formed, each comprising a zeolite layer (the calcined zeolite precursor layer) and a porous support.

[0045] The calcination process is carried out under conditions that generate multiple first separation membranes having separation performance at or above a predetermined level from multiple membrane intermediates. Alternatively, all of the multiple membrane intermediates may be converted into first separation membranes having separation performance at or above a predetermined level.

[0046] To obtain a sufficient number of first separation membranes having separation performance above a predetermined level, it is preferable to set the firing temperature to a relatively low, mild temperature. This minimizes the probability of crack formation in the multiple inorganic separation membranes produced.

[0047] The first firing temperature may be, for example, in the range of 400 to 600°C, and the firing time may be in the range of 3 to 24 hours. The first firing temperature may also be, for example, 500°C or lower. The firing atmosphere may be air.

[0048] Since the first firing is performed at a relatively low, mild temperature, some of the structurally defined material may remain in the pores of the inorganic separation membrane. In that case, the surface of the inorganic separation membrane will appear yellowish-brown. Therefore, the surface of the first separation membrane used as is may appear yellowish-brown.

[0049] <Sorting Process> Next, a selection process is performed to select multiple first separation membranes from the multiple inorganic separation membranes that have separation performance at or above a predetermined level. Separation performance at or above a predetermined level means, for example, the highest level of separation performance required for the inorganic separation membrane installed in the upstream first gas separation unit when multiple gas separation units, including a first gas separation unit and a second gas separation unit, are connected in series and a mixed gas is supplied to the upstream first gas separation unit to separate the gases.

[0050] For example, the superiority or inferiority of the separation performance of multiple inorganic separation membranes may be determined, and a ranking of the separation performance may be assigned to the multiple inorganic separation membranes. Then, the inorganic separation membranes may be selected in order of the ranking of separation performance, and the inorganic separation membrane having a separation performance of a predetermined level or higher may be selected as the first separation membrane. The inorganic separation membranes other than the selected one do not have a separation performance of a predetermined level or higher, and can be used as the second separation membrane, which has a lower separation performance than the first separation membrane.

[0051] From the first separation membranes having separation performance above a predetermined level, the required number to be used as the first separation membrane is further selected. At this time, the required number of inorganic separation membranes may be selected in order of decreasing separation performance. The first separation membranes further selected from the first separation membranes are used, for example, as the first separation membranes installed in the upstream first gas separation unit described above. The first separation membranes that were not selected for use as the first separation membrane are first separation membranes with relatively low separation performance and are converted into second separation membranes by the next re-calcination process.

[0052] <Re-firing process> In the re-calcination process, the remaining first separation membranes that were not sorted for use as the first separation membrane are calcined at the second calcination temperature. Through the re-calcination process, minute cracks are formed in the first separation membrane, and these first separation membranes are converted into second separation membranes.

[0053] The second firing temperature should be any temperature that can convert the first separation membrane into a second separation membrane, but it is preferable that it be higher than or equal to the first temperature. For example, the second firing temperature is usually higher than or equal to the first firing temperature, and no more than 300°C higher than the first firing temperature. More preferably, the second firing temperature is no more than 200°C higher than the first firing temperature, and even more preferably no more than 100°C higher than the first firing temperature. By setting the second firing temperature to no more than 300°C higher than the first firing temperature, the formation of excessive cracks is limited, and a second separation membrane with appropriate separation performance can be obtained. The firing atmosphere for re-firing may be air.

[0054] During the re-firing process, the organic structural guide is further combusted and decomposed, causing the surface of the inorganic separation membrane to become whiter. Typically, since the zeolite layer is a thin film, the surface of the inorganic separation membrane becomes closer in color to that of the porous support. If the porous support is made of white alumina, the surface of the inorganic separation membrane will also become closer to white.

[0055] (Applications of inorganic separation membranes) The following describes, as an example, the case where an inorganic separation membrane separates carbon dioxide from a mixed gas containing methane and carbon dioxide. Such an inorganic separation membrane selectively permeates carbon dioxide. In particular, an inorganic separation membrane having a zeolite layer composed of zeolites with an 8-membered ring structure is suitable for separating carbon dioxide from natural gas and biogas. Among zeolite layers composed of zeolites with an 8-membered ring structure, a zeolite layer composed of CHA-type zeolite has particularly high carbon dioxide permeability.

[0056] The zeolite layer may be formed, for example, on the outer surface of a tubular porous support. This allows for the creation of a cylindrical inorganic separation membrane. In this case, when a mixed gas containing methane and carbon dioxide is passed through the outer surface side (supply side) of the cylindrical inorganic separation membrane, carbon dioxide is concentrated in the hollow portion of the porous support (permeate side).

[0057] As described above, in the selection process, multiple first separation membranes having separation performance above a predetermined level are selected from multiple inorganic separation membranes. As the first separation membranes having separation performance above a predetermined level, for example, inorganic separation membranes with a carbon dioxide selectivity S1 of 100 or more may be selected, or even higher-performing inorganic separation membranes with a carbon dioxide selectivity S1 of 125 or more may be selected.

[0058] As previously described, in the re-calcination process, among the first separation membranes, the one with relatively low separation performance is converted into a second separation membrane with lower separation performance than the first separation membrane through re-calcination. At this time, the carbon dioxide selectivity S2 of the second separation membrane may be set to 30% to 80% of the carbon dioxide selectivity S1. In this case, for example, if carbon dioxide selectivity S1 = 100 is set to achieve a separation performance above a predetermined level, the carbon dioxide selectivity S2 of the second separation membrane is re-calcined to 30 to 80.

[0059] Here, carbon dioxide selectivity S1 and S2 are expressed as the ratio (PCO2 / PCH4) of the permeance of carbon dioxide (PCO2) to the permeance of methane (PCH4) in the first and second separation membranes, respectively.

[0060] In the case of a cylindrical inorganic separation membrane, the permeance of each gas can be measured by placing the inorganic separation membrane in a membrane module in a room temperature (15-30°C) atmosphere and supplying a gas with a dew point of -60°C or lower to the supply side (outer surface side) of the inorganic separation membrane from a predetermined gas supply source. The pressure on the supply side is 0.2 MPa, the pressure on the permeate side (hollow portion) is atmospheric pressure, and the gas flow rate on the permeate side is measured using a flow meter such as a soap film flow meter.

[0061] The permeance of a gas can be calculated using the following equation: Q = A / {(P1-P2)·S·t}. Q: Gas permeance (mol / m³) 2 ·s·Pa) A: Gas permeation rate (mol) P1: Supply pressure (Pa) P2: Permeation pressure (Pa) S: Apparent area of ​​the zeolite film (m²) 2 ) t: Time (seconds) for gas with permeation rate A to pass through.

[0062] The permeance of carbon dioxide in the first separation membrane is, for example, 1 × 10⁻⁶ -7 mol / m 2 Preferably, s·Pa or higher, 1 × 10 ―4 mol / m 2 It is preferable that the value is less than or equal to s·Pa. [Examples]

[0063] Embodiments of this disclosure will be described in more detail by reference to examples. In these examples, multiple separation members were manufactured and evaluated. Their manufacturing methods and evaluation methods are described below.

[0064] Examples 1-3 (A) Hydrothermal synthesis process (1) Preparation of porous support Sixty cylindrical porous supports made of alumina were prepared. Each cylindrical porous support had a length of 1 m, a thickness of 2 mm, an inner diameter of 12 mm, and a surface roughness (arithmetic mean roughness Ra as described above) of 1.2 to 2.0 μm.

[0065] (2) Formation of seed crystals (Seed crystal preparation example 1) In Preparation Example 1, silica seed crystals having a CHA-type structure were prepared by the following method. First, TMAdaOH (sometimes referred to as "organic structure-determining agent") and colloidal silica (silica source) were mixed in a polytetrafluoroethylene (PTFE) container to form a mixture, and hydrofluoric acid was added to this mixture. Then, the mixture was heated while stirring to completely remove water and obtain a solid.

[0066] Next, the obtained solid was crushed in an agate mortar and pestle, and deionized water was added. The final molar ratio of the mixed gel was SiO2:TMAdaOH:HF:H2O = 1:1.4:1.4:6.0. Then, the obtained gel was transferred to a polytetrafluoroethylene inner cylinder in an autoclave, and hydrothermal synthesis was carried out at 150°C for 24 days. After that, the inside of the autoclave was cooled. The gel in the inner cylinder was then washed with deionized water, filtered, and dried under reduced pressure. In this way, silica seed crystals having a CHA-type structure were obtained.

[0067] (Seed crystal preparation example 2) In Preparation Example 2, zeolite seed crystals having a CHA-type structure containing aluminosilicate were prepared according to the method described in "Separation and Purification Technology 199 (2018), 298-303". Specifically, TMAdaOH, sodium hydroxide, FAU-type zeolite (manufactured by Tosoh Corporation, product numbers: HSZ-360, HSZ-390), and deionized water were first added to a PTFE inner cylinder in an autoclave. The chemical composition of HSZ-360 is SiO2 / Al2O3 = 14 (molar ratio). The chemical composition of HSZ-390 is SiO2 / Al2O3 = 400 (molar ratio). The zeolite functions as a silica source and an aluminum source. The molar ratio of the materials was SiO2:TMAdaOH:NaOH:H2O:Al2O3 = 1:0.2:0.2:7.0:20. Subsequently, the autoclave was sealed, and hydrothermal synthesis was carried out at 160°C for 40 hours. After that, the inside of the autoclave was cooled. Then, the gel in the inner cylinder was washed with deionized water and dried at 100°C. In this way, zeolite seed crystals containing aluminosilicate and having a CHA-type structure were obtained.

[0068] The above method for producing seed crystals is just one example. In the above production method, a preferred molar ratio of materials is, for example, SiO2:TMAdaOH:NaOH:H2O:Al2O3 = 1:0.05~0.5:0.1~0.3:2.5~8.0:5~100. Since aluminosilicate-containing zeolites can use less organic structure-determining agent than in Preparation Example 1, a more preferred molar ratio of materials is, for example, SiO2:TMAdaOH:NaOH:H2O:Al2O3 = 1:0.05~0.2:0.1~0.3:2.5~8.0:10~30.

[0069] (3) Formation of seed crystal layer The seed crystals obtained in Preparation Example 1 and the seed crystals obtained in Preparation Example 2 were mixed in a mass ratio of 4:1, and deionized water was added to the resulting mixture to prepare a seed crystal dispersion. The seed crystal content in the dispersion was 0.05% by mass. Using this dispersion, the seed crystals were coated onto the outer surface of a cylindrical porous support made of alumina by the dip method.

[0070] (4) Formation of zeolite precursor layer A raw material composition (secondary growth solution) for forming a zeolite precursor layer was prepared by the following method. First, TMAdaOH, ion-exchanged water, NaOH, and FAU-type zeolites (HSZ-360 and HSZ-390, both manufactured by Tosoh Corporation) were added to a beaker and stirred for 3 hours to obtain the target secondary growth solution. The molar ratios of the substances in this secondary growth solution were set as TMAdaOH / SiO2 = 0.1, NaOH / SiO2 = 0.167, H2O / SiO2 = 150, and SiO2 / Al2O3 = 30.

[0071] Next, the porous support with seed crystals attached was placed inside the PTFE inner cylinder in the autoclave, and the inner cylinder was filled with the above secondary growth solution. Then, the autoclave was sealed and hydrothermal synthesis was carried out at 160 °C for 24 hours. By this hydrothermal synthesis, a zeolite precursor layer containing aluminosilicate and having a CHA-type structure was formed on the surface of the porous support, and 60 membrane intermediates were obtained.

[0072] Next, the inside of the autoclave was cooled. Next, the membrane intermediates were taken out of the inner cylinder and washed with ion-exchanged water.

[0073] (B) Firing process Next, in order to remove the organic structure-directing agent, 60 membrane intermediates were fired in an electric furnace at 500 °C for 10 hours in air. In this way, 60 inorganic separation membranes A with a zeolite layer formed on the outer peripheral surface of the porous support were obtained. When the carbon dioxide permeability of the 60 inorganic separation membranes A was measured, all of them were in the range of 1×10 ―6 ~4×10 ―6 mol / m 2 ·s·Pa. Also, the outer peripheral surface of the inorganic separation membrane A showed a yellowish-brown color.

[0074] XRD analysis (X-ray diffraction analysis) was performed to confirm whether the formed zeolite layer had a CHA-type structure. The analysis was carried out using a Rigaku Corporation instrument (model: Ultima IV). Similar to the FE-SEM measurement, 15 analyses were performed at 5 arbitrary points across 3 locations. X-ray diffraction peaks originating from the CHA-type structure (diffraction angles 2θ = approximately 9.6°, 17.9°, and 20.8°) were obtained at all measurement locations, and no X-ray diffraction peaks of other zeolite structures were observed. Therefore, it was confirmed that the zeolite layer of inorganic separation membrane A1 has a CHA-type structure.

[0075] The conditions for the XRD analysis were as follows: X-ray source: CuKα rays (output: 40kV, 40mA) Scan axis: θ / 2θ Scanning range (2θ): 5.0-50.0° Measurement mode: Continuous Scanning Divergence slit: 2 / 3° Divergence vertical limiting slit: 10mm Scattering slit: Open Light-receiving slit: Open

[0076] (C) Sorting process (1) First selection Next, 50 first separation membranes with separation performance above a predetermined level were selected from 60 inorganic separation membranes A. Specifically, the permeance of carbon dioxide (PCO2) and methane (PCH4) was measured for all 60 inorganic separation membranes A, and the PCO2 / PCH4 ratio was determined as the carbon dioxide selectivity. Inorganic separation membranes A with a carbon dioxide selectivity of 100 or more were selected as first separation membranes with separation performance above a predetermined level, and 50 were selected as first separation membranes. The remaining 10 had a carbon dioxide selectivity of less than 100. These 10 inorganic separation membranes A with a carbon dioxide selectivity of less than 100 can be used as second separation membranes.

[0077] (2) Second selection Next, 35 of the 50 first separation membranes were selected in order of their separation performance to be used as the first separation membranes. The first separation membranes with relatively low separation performance that were not selected in the second selection (a total of 15 membranes) were converted into second separation membranes in the subsequent process.

[0078] (D) Re-firing process Fifteen first separation membranes with a carbon dioxide selectivity of 100 or more that were not selected in the second selection were re-calcined in an electric furnace in air for 10 hours each: five at 500°C (Example 1), another five at 550°C (Example 2), and yet another five at 600°C (Example 3). As a result, the carbon dioxide selectivity (PCO2 / PCH4) of each first separation membrane decreased on average to the relative values ​​shown in Table 1, with the pre-calcination value set to 100, and the carbon dioxide permeance (PCO2) increased on average to the relative values ​​shown in Table 1, with the pre-calcination value set to 100. The inorganic separation membrane A after re-calcination has suitable performance as a second separation membrane.

[0079] [Table 1]

[0080] The reason for the average increase in carbon dioxide permeance (PCO2) is thought to be due to the formation of minute cracks when the first separation membrane was converted into the second separation membrane. However, since the carbon dioxide permeance (PCO2) of inorganic separation membrane A is within an appropriate range, the second separation membranes generated by the cracks all possess appropriate carbon dioxide selectivity and carbon dioxide permeance (PCO2) as second separation membranes with low separation performance. [Industrial applicability]

[0081] This disclosure can be used as a method for manufacturing at least two types of inorganic separation membranes with different gas separation performance.

Claims

1. A method for manufacturing at least two types of inorganic separation membranes with different gas separation performance, A hydrothermal synthesis step to form a zeolite precursor layer on the surface of multiple porous supports by hydrothermal synthesis to obtain multiple membrane intermediates comprising the porous supports and the zeolite precursor layer, A firing step of firing the plurality of membrane intermediates at a first firing temperature to form a plurality of inorganic separation membranes comprising a zeolite layer which is a fired body of the zeolite precursor layer and the porous support, A selection step of selecting a plurality of first separation membranes having separation performance of a predetermined level or higher from the plurality of inorganic separation membranes, A re-calcination step is performed by calcining a portion of the plurality of first separation membranes at a second calcination temperature to obtain a second separation membrane having lower separation performance than the first separation membrane. A method for producing an inorganic separation membrane, comprising the following:

2. The method for producing an inorganic separation membrane according to claim 1, wherein the zeolite constituting the zeolite layer has an eight-membered ring structure.

3. The method for producing an inorganic separation membrane according to claim 2, wherein the zeolite is a CHA-type zeolite.

4. The first firing temperature is 500°C or lower. The method for producing an inorganic separation membrane according to claim 1, wherein the second firing temperature is equal to or greater than the first firing temperature and less than or equal to 300°C higher than the first firing temperature.

5. The aforementioned mixed gas contains methane and carbon dioxide, The method for producing an inorganic separation membrane according to any one of claims 1 to 4, wherein the inorganic separation membrane is a separation membrane for separating carbon dioxide from the mixed gas.

6. The carbon dioxide selectivity S1 of the first separation membrane is 100 or more. The carbon dioxide selectivity S2 of the second separation membrane is 30% to 80% of the carbon dioxide selectivity S1. The method for producing an inorganic separation membrane according to claim 5, wherein the carbon dioxide selectivity S1 and S2 are expressed as the ratio (PCO2 / PCH4) of the carbon dioxide permeance (PCO2) to the methane permeance (PCH4) in the first and second separation membranes, respectively.

7. The permeance of carbon dioxide in the first separation membrane is 1 × 10⁻⁶ －７ mol / m 2 A method for producing an inorganic separation membrane according to claim 6, wherein the pressure is s·Pa or higher.

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