Method for producing refined gas

Abstract

To provide a method for producing refined gas which suppresses permeation of sweep gas from a permeation side to a non-permeation side, and can obtain a high-purity component as gas, even if the sweep gas is used, in mixed gas membrane separation.SOLUTION: There is provided a method for producing refined gas which is separated by supplying mixed gas containing carbon dioxide to a separation membrane, and permeating the carbon dioxide in the separation membrane, wherein the separation membrane is a zeolite membrane, and the permeation side of the separation membrane is swept by nitrogen or air.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing purified gas. [Background technology]

[0002] Cryogenic separation and adsorption have been used as methods for concentrating and purifying one component from a gas mixture containing multiple components, but in recent years membrane separation using polymer separation membranes has also become popular. In the case of separation by membrane separation, a known method is to increase the partial pressure difference of the target component permeating the membrane by flowing a sweep gas on the permeation side, thereby increasing the separation efficiency.

[0003] Non-Patent Document 1 describes a separation method using an inorganic porous membrane, in which nitrogen gas or water vapor is used as a sweep gas. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Separation Technology Series 31 "Fundamentals and Applications of Gas Separation Membrane Processes" Published May 30, 2015, Authors: Kenji Haratani and Naotsugu Ito Summary of the Invention [Problem to be solved by the invention]

[0005] In the method described in Non-Patent Document 1, the sweep gas permeates from the permeate side to the non-permeate side, and each simulation is performed on this assumption. In this case, the sweep gas is mixed into the non-permeate side, which makes it difficult to increase the purity of the non-permeate side components.

[0006] Therefore, an object of the present invention is to provide a method for producing a purified gas that, even when a sweep gas is used in mixed gas membrane separation, prevents the sweep gas from permeating from the permeate side to the non-permeate side, thereby enabling high-purity components to be obtained as gas. [Means for solving the problem]

[0007] As a result of extensive research into the above-mentioned problems, the present inventors have completed the following invention. [1] A method for producing a purified gas by supplying a mixed gas containing carbon dioxide to a separation membrane and separating the carbon dioxide by permeating the separation membrane, the separation membrane is a zeolite membrane, Sweeping the permeate side of the separation membrane with nitrogen or air; A method for producing purified gas.

[0008] [2] The method for producing a purified gas according to [1], wherein the zeolite constituting the zeolite membrane has a pore structure with an oxygen ring of eight or less members.

[0009] [3] The method for producing a purified gas according to [1] or [2], wherein the zeolite membrane contains a CHA-type aluminosilicate zeolite, and in an X-ray diffraction pattern obtained by irradiating the membrane surface with X-rays, the peak intensity around 2θ=17.9° is less than 0.5 times the peak intensity around 2θ=20.8°, and the peak intensity around 2θ=9.6° is 2.0 times or more but less than 4.0 times the peak intensity around 2θ=20.8°.

[0010] [4] The method for producing a purified gas according to any one of [1] to [3], wherein the SiO2 / Al2O3 of the zeolite is 20 or more and 500 or less. [Effects of the Invention]

[0011] According to the present invention, by using a zeolite membrane as a separation membrane, even if a sweep gas is used in membrane separation of a mixed gas, the sweep gas is prevented from permeating from the permeate side to the non-permeate side, making it possible to obtain a high-purity gas. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of a separation membrane module using a sweep gas. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following describes in more detail the embodiments of the present invention. However, the description of the constituent elements described below is an example of an embodiment of the present invention, and the present invention is not limited to these contents, and can be implemented in various modifications within the scope of its gist.

[0014] <Method of producing purified gas> The method for producing a purified gas of the present invention includes supplying a mixed gas containing carbon dioxide to a separation membrane and separating the carbon dioxide by allowing the carbon dioxide to permeate through the separation membrane, and the separation membrane is a zeolite membrane, Sweeping the permeate side of the separation membrane with nitrogen or air; A method for producing purified gas.

[0015] (mixed gas) The mixed gas in the present invention is sufficient as long as it contains at least two components, namely, carbon dioxide, which is the target for removal, in addition to one gas, which is the component to be obtained as a purified gas, and the number of components to be contained and the specific composition are not limited.

[0016] Examples of the gas include hydrocarbons having 2 to 4 carbon atoms, such as methane, ethane, and propane, rare gases, and hydrogen. In addition to the gas 1 and carbon dioxide, the gas may contain non-hydrocarbon gases such as heavy hydrocarbons having five or more carbon atoms, water vapor, hydrogen sulfide, and nitrogen. Examples of mixed gases include biogas and natural gas. The method for producing a purified gas of the present invention is capable of more efficiently removing carbon dioxide from a mixed gas such as the above-mentioned biogas by supplying the mixed gas, such as the biogas, to a separation membrane and sweeping the permeate side of the separation membrane with a predetermined gas.

[0017] The amount of carbon dioxide that can be contained in the mixed gas is not particularly limited, and is usually 10 mol% or more, preferably 20 mol% or more, more preferably 30 mol% or more, and is usually 80 mol% or less, preferably 70 mol% or less, more preferably 60 mol% or less. The amount of the gas to be purified in the mixed gas is not particularly limited, but is usually 30 mol% or more, preferably 40 mol% or more, more preferably 50 mol% or more, and is usually 90 mol% or less, preferably 80 mol% or less, more preferably 70 mol% or more.

[0018] The mixed gas to be separated in the present invention may be one from which some components have been removed by pretreatment.

[0019] (purified gas) In the present invention, a purified gas containing a high concentration of the gas (1) is produced by treating a mixed gas. As described above, examples of gas 1 include gases with a carbon number such as methane, and the concentration of gas 1 in the purified gas is preferably 70 mol% or more, more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more.

[0020] When a sweep gas is used in membrane separation of mixed gases using a polymer membrane, the sweep gas permeates from the permeate side to the non-permeate side, making it difficult to obtain a gas containing the target gas (1) at a high purity as the permeate gas. The present inventors have discovered that when a zeolite membrane is used as a separation membrane, the permeation of the sweep gas from the permeate side to the non-permeate side is suppressed. Zeolite membranes separate gases by the difference between adsorption and diffusion, whereas polymer membranes separate gases only by the difference in diffusion. The present inventors believe that this difference in mechanism is the cause of the above phenomenon.

[0021] (Separation membrane module) The method for producing purified gas of the present invention uses a zeolite membrane as a separation membrane, and a sweep gas is passed through the permeate side of the separation membrane. As the separation membrane module, for example, the one described in JXTG Technical Review, Vol. 60, No. 3 (November 2018) can be used. FIG. 1 shows a schematic diagram of a method for producing purified gas using a separation membrane module that uses a sweep gas. In the illustrated embodiment, a mixed gas is supplied to the outside of a separation membrane 53 as a feed gas, and carbon dioxide selectively permeates through the separation membrane 53 to become a permeated gas. The mixed gas and the permeated gas are separated by the separation membrane 53. Note that while FIG. 1 shows the mixed gas and the sweep gas flowing countercurrently, they may also flow cocurrently. Countercurrent flow is preferable because it allows for a larger average partial pressure difference than cocurrent flow, which is advantageous for separation. The advantages of countercurrent flow are particularly evident when purifying to a high concentration. When actually manufacturing and using a separation membrane module, the separation membrane 53 is a cylindrical zeolite membrane composite, and is preferably housed in a stainless steel pressure vessel and placed in a thermostatic chamber (not shown). The thermostatic chamber is equipped with a temperature control device so that the temperature of the sample gas can be adjusted.

[0022] A mixed gas containing gas No. 1 and carbon dioxide is supplied at a constant flow rate to separation membrane 53. The pressure on the supply side is constant. Carbon dioxide permeates separation membrane 53 according to the partial pressure difference between the inside and outside of separation membrane 53 and is discharged together with the sweep gas. The separation membrane module includes a zeolite membrane that selectively permeates carbon dioxide and can separate it from the gas 1 as a separation membrane 53. The separation membrane 53 may be a zeolite membrane composite including a support.

[0023] The mixed gas is supplied to the non-permeation side. By making the partial pressure of carbon dioxide on the non-permeation side of separation membrane 53 higher than the partial pressure of carbon dioxide on the permeation side of separation membrane 53, the carbon dioxide in the mixed gas selectively permeates separation membrane 53. This allows one gas in the mixed gas to be purified as a non-permeation gas.

[0024] The pressure of the mixed gas supplied to the separation membrane module is preferably 0.01 MPaG or higher, more preferably 0.03 MPaG or higher, and even more preferably 0.05 MPaG or higher. Furthermore, from the viewpoint of reducing the capacity of the compressor that compresses the gas to be supplied to the separation membrane module, the pressure is preferably 1.0 MPaG or less, and more preferably 0.9 MPaG or less.

[0025] The linear velocity of the mixed gas supplied to the separation membrane module is not particularly limited, as it varies depending on the size of the separation membrane module and the gas processing rate, but is usually 0.01 m / s or more, preferably 0.05 m / s or more, and more preferably 0.1 m / s or more. There is no particular upper limit, and the linear velocity is usually 10 m / s or less, preferably 8 m / s or less.

[0026] The separation membrane module may be of a flat membrane type, spiral type, hollow fiber type, cylindrical type, honeycomb type, etc., and the optimum type is selected depending on the application.

[0027] (Zeolite membrane) In the method for producing a purified gas of the present invention, a zeolite membrane formed on an inorganic porous support can be used as the zeolite membrane.

[0028] ·Porous support The porous support may be any inorganic porous support (inorganic porous support) that has chemical stability such that zeolite can be crystallized into a membrane on its surface, etc. Examples include sintered ceramics (ceramic supports) such as silica, α-alumina, γ-alumina, mullite, zirconia, titania, yttria, silicon nitride, and silicon carbide; sintered metals such as iron, bronze, and stainless steel; glass; and molded carbon materials.

[0029] Among these porous supports, inorganic porous supports (ceramic supports) containing sintered ceramics, which are solid materials whose basic components or most of which are composed of inorganic non-metallic substances, are preferred. The use of such supports has the effect of enhancing the adhesion at the interface by converting a portion of them into zeolite during the synthesis of the zeolite membrane.

[0030] Specific examples include ceramic sintered bodies (ceramic supports) containing silica, α-alumina, γ-alumina, mullite, zirconia, titania, yttria, silicon nitride, silicon carbide, etc. Among these, inorganic porous supports containing at least one of alumina, silica, and mullite are preferred. If these supports are used, partial zeolite formation is easy, and the bond between the support and zeolite is strengthened, making it easier to form a dense membrane with high separation performance.

[0031] The shape of the porous support is not particularly limited as long as it can effectively separate mixed gases. Specific examples include tubular supports such as flat plates and cylindrical tubes, honeycomb structures with a large number of cylindrical, columnar, or prismatic holes, and monoliths. Forming a zeolite membrane on such a porous support, i.e., on the surface of the support, results in a separation membrane containing a zeolite membrane. The surface of the support may be any surface, or multiple surfaces, depending on the shape of the support. For example, in the case of a cylindrical support, the surface may be either the outer surface or the inner surface, or in some cases, both the outer and inner surfaces.

[0032] The average pore size of the porous support surface is not particularly limited, but is preferably controlled. The average pore size of the support surface is usually 0.02 μm or more, preferably 0.05 μm or more, more preferably 0.1 μm or more, and usually 20 μm or less, preferably 10 μm or less, more preferably 5 μm or less. If the average pore size is too small, the permeation rate tends to be low, and if it is too large, the strength of the support itself becomes insufficient, and the proportion of pores on the support surface increases, making it difficult to form a dense zeolite membrane.

[0033] The average thickness (wall thickness) of the porous support is usually 0.1 mm or more, preferably 0.3 mm or more, more preferably 0.5 mm or more, and even more preferably 0.8 mm or more, and is usually 7 mm or less, preferably 5 mm or less, and more preferably 3 mm or less. The support is used to impart mechanical strength to the zeolite membrane, but if the average thickness of the support is too thin, the separation membrane containing the zeolite membrane will not have sufficient strength and will tend to be vulnerable to impact, vibration, etc. If the average thickness of the support is too thick, diffusion of the permeated substance will be poor, and permeability will tend to be low.

[0034] The porosity of the porous support is usually 20% or more, preferably 25% or more, more preferably 30% or more, and usually 70% or less, preferably 60% or less, more preferably 50% or less. The porosity of the support affects the permeation flow rate during gas separation; if the porosity is below the lower limit, the diffusion of the permeate tends to be hindered, and if the porosity is above the upper limit, the strength of the support tends to decrease.

[0035] The surface of the porous support may be polished with a file or the like as needed. The surface of the porous support refers to the surface portion of the inorganic porous support on which the zeolite is crystallized, and may be any surface of the respective shapes, or may be multiple surfaces. For example, in the case of a cylindrical support, it may be either the outer surface or the inner surface, or in some cases, both the outer and inner surfaces.

[0036] Zeolite membrane The components constituting the zeolite membrane may optionally include, in addition to zeolite, inorganic binders such as silica and alumina, organic compounds such as polymers, or Si compounds or their reaction products that modify the zeolite surface as described in detail below. Although the zeolite membrane may also contain a portion of amorphous components, a zeolite membrane that is substantially composed of zeolite alone is preferred.

[0037] The thickness of the zeolite membrane is not particularly limited, but is usually 0.1 μm or more, preferably 0.6 μm or more, more preferably 1 μm or more, and still more preferably 2 μm or more, and is usually 100 μm or less, preferably 60 μm or less, and more preferably 20 μm or less. If the membrane thickness is too large, the permeation amount tends to decrease, and if it is too small, the selectivity and membrane strength tend to decrease.

[0038] The particle size of the zeolite is not particularly limited, but if it is too small, the grain boundaries will become large, which tends to reduce the permeability selectivity. Therefore, it is usually 30 nm or more, preferably 50 nm or more, and more preferably 100 nm or more, with the upper limit being equal to or less than the membrane thickness. Furthermore, it is particularly preferable that the particle size of the zeolite is the same as the membrane thickness. When the particle size of the zeolite is the same as the membrane thickness, the grain boundaries of the zeolite are smallest. Zeolite membranes obtained by hydrothermal synthesis, which will be described later, are particularly preferred because the particle size of the zeolite and the thickness of the membrane may be the same.

[0039] The shape of the separation membrane, including the zeolite membrane, is not particularly limited, and any shape can be used, such as tubular, hollow fiber, monolith, honeycomb, etc. The size is also not particularly limited, and for example, in the case of a tubular membrane, a length of typically 2 cm or more, preferably 20 cm or more and typically 200 cm or less, an inner diameter of 0.05 cm to 2 cm, and a thickness of 0.5 mm to 4 mm are practical and preferred.

[0040] One of the separation functions of zeolite membranes is that they act as molecular sieves, and can effectively separate gas molecules with sizes equal to or larger than the effective pore size of the zeolite used from gas molecules with sizes smaller than that. Although there is no upper limit to the molecules that can be separated, the size of the molecules is usually about 100 Å or less.

[0041] The zeolite constituting the zeolite membrane is preferably an aluminosilicate. The SiO2 / Al2O3 molar ratio (SAR) of the zeolite membrane is preferably 5 or more, more preferably 8 or more, even more preferably 10 or more, particularly preferably 12 or more, and most preferably 20 or more. It is preferably 2000 or less, more preferably 1000 or less, even more preferably 500 or less, even more preferably 100 or less, and particularly preferably 50 or less. When the SAR of the membrane is at or above the lower limit, durability tends to improve, while when it is at or below the upper limit, there is the advantage that the permeability is not reduced in terms of adsorption.

[0042] The SAR of a zeolite membrane is a value obtained by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). In SEM-EDX, measurements are taken at an X-ray acceleration voltage of about 10 kV, which allows information on a membrane of only a few microns in size to be obtained. Because zeolite membranes are formed uniformly, the SAR of the membrane can be determined from this measurement.

[0043] The main zeolite constituting the zeolite membrane preferably contains zeolite having a pore structure with an oxygen ring of 8 or less members, and more preferably contains zeolite having a pore structure with an oxygen ring of 6 to 8 members. The value of n in the zeolite having n-membered oxygen rings refers to the zeolite with the largest number of oxygen atoms among the pores composed of oxygen and T elements (elements other than oxygen that make up the framework) that form the zeolite framework. For example, when pores with 12-membered oxygen rings and 8-membered oxygen rings exist, as in MOR-type zeolites, the zeolite is considered to have 12-membered oxygen rings.

[0044] Examples of zeolites having a pore structure with eight or fewer oxygen rings include AEI, AFG, AFX, ANA, BRE, CAS, CDO, CHA, DDR, DOH, EAB, EPI, ERI, ESV, FAR, FRA, GIS, GIU, GOO, ITE, KFI, LEV, LIO, LOS, LTN, MAR, MEP, MER, MEL, MON, MSO, MTF, MTN, MWF, NON, PAU, PHI, RHO, RTE, RTH, RUT, SGT, SOD, TOL, TSC, UFI, VNI, and YUG.

[0045] Examples of zeolites having a 6- to 8-membered oxygen ring structure include AEI, AFG, AFX, ANA, CHA, DDR, EAB, ERI, ESV, FAR, FRA, GIS, ITE, KFI, LEV, LIO, LOS, LTN, MAR, MWF, PAU, RHO, RTH, SOD, TOL, and UFI. In this specification, the structure of zeolite is shown by the code defining the structure of zeolite established by the International Zeolite Association (IZA), as described above.

[0046] The n-membered oxygen ring structure determines the pore size of the zeolite, and in zeolites with oxygen rings smaller than six, the pore diameter is smaller than the kinetic diameter of a carbon dioxide molecule, resulting in low permeability and making them impractical. In addition, in zeolites with oxygen rings larger than eight, the pore diameter is large, which can reduce separation performance for small gas components and limit their applications.

[0047] Zeolite framework density (T / 1000Å 3 ) is not particularly limited, but is usually 18 or less, preferably 17 or less, more preferably 16 or less, particularly preferably 15.5 or less, and most preferably 15.1 or less, and is usually 10 or more, preferably 11 or more, and more preferably 12 or more. The framework density is the density of 1000Å of zeolite. 3This refers to the number of elements (T elements) other than oxygen that make up the framework per unit area, and this value is determined by the structure of the zeolite. The relationship between framework density and zeolite structure is shown on the website of the International Zeolite Association (IZA).

[0048] Preferred zeolite structures are AEI, AFG, AFX, CHA, DDR, EAB, ERI, ESV, FAR, FRA, GIS, ITE, KFI, LEV, LIO, LOS, LTN, MAR, MWF, PAU, RHO, RTH, SOD, TOL, and UFI, more preferred structures are AEI, AFX, CHA, DDR, ERI, KFI, MWF, LEV, PAU, RHO, RTH, and UFI, even more preferred structures are CHA and DDR, and most preferred structure is CHA.

[0049] CHA-type zeolite is a code for zeolite structure established by the International Zeolite Association (IZA) that indicates a CHA structure. This is a zeolite with a crystalline structure equivalent to that of naturally occurring chabazite. CHA-type zeolite has a structure characterized by three-dimensional pores consisting of eight-membered oxygen rings with a diameter of 3.8 x 3.8 Å, and its structure is characterized by X-ray diffraction data. Framework density (T / 1000Å) of CHA-type zeolite 3 ) is 15.1. The SiO2 / Al2O3 molar ratio is the same as above.

[0050] When the zeolite membrane is a CHA-type zeolite membrane, the zeolite membrane composite, in which the zeolite membrane is formed on the porous body, preferably has an X-ray diffraction pattern obtained by irradiating the membrane surface with X-rays, in which the peak intensity around 2θ=17.9° is less than 0.5 times the peak intensity around 2θ=20.8°. By keeping the intensity within this range, both good permeation performance and separation performance can be achieved.

[0051] Here, peak intensity refers to the measured value minus the background value. The peak intensity ratio (hereinafter sometimes referred to as "peak intensity ratio A"), expressed as (peak intensity around 2θ = 17.9°) / (peak intensity around 2θ = 20.8°), is usually less than 0.5, preferably 0.45 or less. There is no particular lower limit, but it is usually 0.001 or more.

[0052] In the zeolite membrane composite of the present invention, the intensity of the peak near 2θ=9.6° in the X-ray diffraction pattern is preferably 2.0 to less than 4.0 times the intensity of the peak near 2θ=20.8°.

[0053] The peak intensity ratio (hereinafter sometimes referred to as "peak intensity ratio B"), expressed as (peak intensity around 2θ=9.6°) / (peak intensity around 2θ=20.8°), is usually 2.0 or more, preferably 2.1 or more, more preferably 2.3 or more, and particularly preferably 2.5 or more. The upper limit is usually less than 4.0, preferably 3.9 or less, more preferably 3.7 or less, and particularly preferably 3.5 or less.

[0054] The X-ray diffraction pattern referred to here is obtained by irradiating the surface on which the zeolite is mainly attached with X-rays using a CuKα radiation source, with the scanning axis set to θ / 2θ. The shape of the sample to be measured may be any shape that allows X-rays to be irradiated onto the surface of the membrane composite on which the zeolite is mainly attached. In order to clearly represent the characteristics of the membrane composite, it is preferable to use the prepared membrane composite as is, or one cut to an appropriate size limited by the apparatus.

[0055] The X-ray diffraction pattern referred to here may be measured by fixing the irradiation width using an automatic variable slit when the surface of the zeolite membrane composite is curved. The X-ray diffraction pattern when an automatic variable slit is used refers to a pattern after variable-to-fixed slit correction.

[0056] Here, the peak near 2θ=17.9° refers to the maximum peak present in the range of 17.9°±0.6° among peaks not attributable to the substrate.

[0057] The peak near 2θ=20.8° refers to the largest peak present in the range of 20.8°±0.6° among peaks not attributable to the substrate.

[0058] The peak near 2θ=9.6° refers to the maximum peak present in the range of 9.6°±0.6° among peaks not attributable to the substrate.

[0059] According to the COLLECTION OF SIMULATED XRD POWDER PATTERNS FOR ZEOLITE Third Revised Edition 1996 ELSEVIER, the peak at 2θ=9.6° in the X-ray diffraction pattern is in the rhombohedral setting and has the space group

[0060]

number

[0061] When the peak is (No. 166), it is a peak derived from the plane with index (1,0,0) in the CHA structure.

[0062] In addition, the peak at 2θ=17.9° in the X-ray diffraction pattern is in the rhombohedral setting according to the COLLECTION OF SIMULATED XRD POWDER PATTERNS FOR ZEOLITE Third Revised Edition 1996 ELSEVIER.

[0063]

number

[0064] When the peak is (No. 166), it is a peak derived from the plane with index (1,1,1) in the CHA structure.

[0065] According to the COLLECTION OF SIMULATED XRD POWDER PATTERNS FOR ZEOLITE Third Revised Edition 1996 ELSEVIER, the peak at 2θ=20.8° in the X-ray diffraction pattern is in the rhombohedral setting and has the space group

[0066]

number

[0067] When the peak is taken as (No. 166), it is a peak derived from the plane with index (2,0,-1) in the CHA structure.

[0068] According to Non-Patent Document 2, the typical ratio (peak intensity ratio B) of the peak intensity derived from the (1,0,0) plane to the peak intensity derived from the (2,0,-1) plane in a CHA-type aluminosilicate zeolite membrane is less than 2.

[0069] Therefore, a ratio of 2.0 or more and less than 4.0 is considered to mean that the zeolite crystals grow with a moderate orientation, for example, such that the (1,0,0) plane of the CHA structure in the rhombohedral setting is nearly parallel to the surface of the membrane composite. The growth of zeolite crystals with an orientation in the zeolite membrane composite is advantageous in that it allows for the production of a dense membrane with high separation performance.

[0070] The term "moderate orientation" here means that there are a moderate proportion of crystallites with the (1,0,0) plane oriented nearly parallel to the surface of the membrane composite relative to the total crystallites. This proportion is higher than that of powdered CHA-type aluminosilicate, in which the crystallites are randomly oriented, and is lower than that of CHA-type aluminosilicate zeolite membranes in which the (1,0,0) planes of many crystallites are oriented nearly parallel to the surface, such as those with a peak intensity ratio B of 4 or more.

[0071] According to Non-Patent Document 2, the typical ratio (peak intensity ratio A) of the peak intensity derived from the (1,1,1) plane to the peak intensity derived from the (2,0,-1) plane in a CHA-type aluminosilicate zeolite membrane is less than 0.5.

[0072] Therefore, a ratio of less than 0.5 is thought to mean that, for example, when the CHA structure is in a rhombohedral setting, the zeolite crystals are not oriented to a large extent in their growth so that the (1,1,1) plane is nearly parallel to the surface of the membrane composite.

[0073] Here, the low degree of orientation of the zeolite crystals means that the proportion of zeolite crystallites with their (1,1,1) planes oriented nearly parallel to the surface of the membrane composite is low relative to the total crystallites, and that the orientation of the (1,1,1) planes of the crystallites is almost random.

[0074] In this way, the peak intensity ratios A and B being within the above-mentioned specific ranges indicates that the zeolite crystals have grown with a moderate orientation, and a dense membrane with high separation performance has been formed.

[0075] A dense zeolite membrane in which CHA-type zeolite crystals grow in a moderately oriented state can be formed by, for example, using a specific organic template and adding K to the aqueous reaction mixture when forming the zeolite membrane by hydrothermal synthesis, as described below. + This can be achieved by the coexistence of ions.

[0076] -Method for manufacturing separation membranes including zeolite membranes The method for producing a separation membrane including a zeolite membrane is not particularly limited, but a method in which zeolite is formed on a porous support by hydrothermal synthesis, for example, is preferred. Specifically, for example, a separation membrane including a zeolite membrane can be prepared by placing a reaction mixture for hydrothermal synthesis (hereinafter sometimes referred to as an "aqueous reaction mixture"), the composition of which has been adjusted and made uniform, into a heat-resistant and pressure-resistant container such as an autoclave, with a porous support loosely fixed inside, sealing it, and heating it for a certain period of time.

[0077] The aqueous reaction mixture preferably contains a Si element source, an Al element source, an alkali source, and water, and further contains an organic template as needed. Examples of the Si element source that can be used in the aqueous reaction mixture include amorphous silica, colloidal silica, silica gel, sodium silicate, amorphous aluminum silicate gel, tetraethoxysilane (TEOS), and trimethylethoxysilane.

[0078] Examples of Al element sources that can be used include sodium aluminate, aluminum hydroxide, aluminum sulfate, aluminum nitrate, aluminum oxide, amorphous aluminosilicate gel, etc. In addition to the Al element source, other element sources such as Ga, Fe, B, Ti, Zr, Sn, and Zn may also be contained.

[0079] When crystallizing zeolite, an organic template (structure-directing agent) can be used as needed. By synthesizing using an organic template, the ratio of silicon atoms to aluminum atoms in the crystallized zeolite increases, improving its acid resistance and steam resistance. The organic template may be of any type as long as it can form the desired zeolite membrane, and one type of template may be used, or two or more types may be used in combination.

[0080] When the zeolite is a CHA type, the organic template is usually an amine or a quaternary ammonium salt. For example, preferred examples of the organic templates include those described in U.S. Pat. No. 4,544,538 and U.S. Patent Publication No. 2008 / 0075656.

[0081] Examples of alkali sources that can be used in the aqueous reaction mixture include hydroxide ions of the counter anions of the organic template, alkali metal hydroxides such as NaOH and KOH, and alkaline earth metal hydroxides such as Ca(OH)2. There are no particular limitations on the type of alkali, and typically, Na, K, Li, Rb, Cs, Ca, Mg, Sr, Ba, and the like are used. Among these, Li, Na, and K are preferred, with K being more preferred. Two or more types of alkali may be used in combination; specifically, Na and K, or Li and K are preferred in combination. It is particularly preferred to use only Na and / or K as the alkali source.

[0082] The ratio of the Si element source to the Al element source in the aqueous reaction mixture is usually expressed as the molar ratio of the oxides of each element, i.e., the SiO2 / Al2O3 molar ratio. The SiO2 / Al2O3 molar ratio is not particularly limited, but is usually 5 or more, preferably 8 or more, more preferably 10 or more, and even more preferably 15 or more. It is also usually 10,000 or less, preferably 1,000 or less, more preferably 300 or less, and even more preferably 100 or less.

[0083] When the SiO2 / Al2O3 molar ratio is within this range, the zeolite membrane is dense and has high separation performance. Furthermore, since the zeolite contains a moderate amount of Al atoms, the separation ability of gas components that are adsorbed by Al is improved. Furthermore, when Al is within this range, the zeolite membrane has high resistance to acid and water vapor.

[0084] The ratio of the silica source to the organic template in the aqueous reaction mixture, expressed as the molar ratio of the organic template to SiO (organic template / SiO molar ratio), is usually 0.005 or more, preferably 0.01 or more, more preferably 0.02 or more, and is usually 1 or less, preferably 0.4 or less, more preferably 0.2 or less. When the organic template / SiO2 molar ratio is within the above range, a dense zeolite membrane can be produced, and the produced zeolite has high resistance to acid and water vapor.

[0085] The ratio of the Si element source to the alkali source is M (2 / n) The O / SiO (where M represents an alkali metal or alkaline earth metal, and n represents its valence of 1 or 2) molar ratio is usually 0.02 or more, preferably 0.04 or more, more preferably 0.05 or more, and usually 0.5 or less, preferably 0.4 or less, more preferably 0.3 or less.

[0086] When forming a CHA-type zeolite membrane, it is preferable to include K among the alkali metals in order to form a denser and more crystalline membrane. In this case, the molar ratio of K to all alkali metals and / or alkaline earth metals including K is usually 0.01 or more and 1 or less, preferably 0.1 or more and 1 or less, and more preferably 0.3 or more and 1 or less.

[0087] The ratio of the Si element source to water, in terms of the molar ratio of water to SiO (H2O / SiO2 molar ratio), is usually 10 or more, preferably 30 or more, more preferably 40 or more, and particularly preferably 50 or more, and is usually 1000 or less, preferably 500 or less, more preferably 200 or less, and particularly preferably 150 or less. When the molar ratio of the materials in the aqueous reaction mixture is within these ranges, dense zeolite membranes can be produced. The amount of water is particularly important in producing dense zeolite membranes, and dense membranes tend to be produced under conditions where the water content is higher relative to the silica content than under conditions typically used in powder synthesis.

[0088] Generally, the amount of water used when synthesizing powdered CHA-type zeolite is an HO / SiO molar ratio of about 15 to 50. By using a high HO / SiO molar ratio (50 or more and 1000 or less), that is, by using conditions with a high water content, it is possible to obtain a separation membrane that includes a zeolite membrane with high separation performance, in which CHA-type zeolite is crystallized in the form of a dense membrane on a support.

[0089] Furthermore, during hydrothermal synthesis, it is not necessary to have seed crystals present in the reaction system, but adding seed crystals can promote the crystallization of zeolite on the support. The method for adding seed crystals is not particularly limited, and methods such as adding seed crystals to an aqueous reaction mixture, as in the synthesis of powdered zeolite, or attaching seed crystals to a support can be used.

[0090] When producing a separation membrane including a zeolite membrane, it is preferable to attach seed crystals to a support in advance. By attaching seed crystals to a support in advance, a dense zeolite membrane with good separation performance can be easily produced. The seed crystals to be used may be of any type as long as they are zeolite that promotes crystallization, but in order to achieve efficient crystallization, it is preferable that they have the same crystal type as the zeolite membrane to be formed. When forming a CHA-type zeolite membrane, it is preferable to use seed crystals of CHA-type zeolite.

[0091] The particle size of the seed crystal is usually 0.5 nm or more, preferably 1 nm or more, more preferably 2 nm or more, and usually 20 μm or less, preferably 15 μm or less, more preferably 10 μm or less. The method for attaching the seed crystals to the support is not particularly limited, and examples that can be used include a dipping method in which the seed crystals are dispersed in a solvent such as water and the support is immersed in the dispersion to attach the seed crystals, or a method in which the seed crystals are mixed with a solvent such as water to form a slurry, which is then applied to the support. The dipping method is desirable for controlling the amount of attached seed crystals and producing a membrane composite with good reproducibility.

[0092] The solvent in which the seed crystals are dispersed is not particularly limited, but water is particularly preferred. When forming a zeolite membrane on a support by hydrothermal synthesis, there is no particular limitation on the method for immobilizing the support, and any form such as vertical placement, horizontal placement, etc. In this case, the zeolite membrane may be formed by a static method, or by stirring the aqueous reaction mixture.

[0093] The temperature for forming the zeolite membrane is not particularly limited, but is usually 100°C or higher, preferably 120°C or higher, and more preferably 150°C or higher, and usually 200°C or lower, preferably 190°C or lower, and more preferably 180°C or lower. If the reaction temperature is too low, the zeolite may be difficult to crystallize. If the reaction temperature is too high, a type of zeolite different from the zeolite of the present invention may be easily produced.

[0094] The heating time is not particularly limited, but is usually 1 hour or more, preferably 5 hours or more, and more preferably 10 hours or more, and usually 10 days or less, preferably 5 days or less, more preferably 3 days or less, and even more preferably 2 days or less. If the heating time is too short, the zeolite may be difficult to crystallize. If the heating time is too long, a type of zeolite different from the desired zeolite may be easily produced.

[0095] The pressure during zeolite membrane formation is not particularly limited, and the autogenous pressure generated when an aqueous reaction mixture placed in a sealed container is heated to this temperature range is sufficient. If necessary, an inert gas such as nitrogen may be added. The separation membrane containing the zeolite membrane obtained by hydrothermal synthesis is washed with water, then heat-treated, and dried. Here, heat treatment means drying the separation membrane containing the zeolite membrane by applying heat, or calcining the template when a template is used.

[0096] When the purpose of the heat treatment is drying, the temperature is usually 50° C. or higher, preferably 80° C. or higher, and more preferably 100° C. or higher, and usually 200° C. or lower, and preferably 150° C. or lower. When the purpose is calcining the template, the temperature is usually 350° C. or higher, preferably 400° C. or higher, more preferably 430° C. or higher, and even more preferably 480° C. or higher, and usually 900° C. or lower, preferably 850° C. or lower, more preferably 800° C. or lower, and even more preferably 750° C. or lower.

[0097] If the purpose is to calcinate the template, if the heat treatment temperature is too low, a large proportion of the organic template will remain, resulting in fewer pores in the zeolite, which may reduce the permeation flux during separation and concentration.If the heat treatment temperature is too high, the difference in thermal expansion coefficients between the support and the zeolite will be large, which may lead to cracks in the zeolite membrane, causing the zeolite membrane to lose its density and reduce its separation performance.

[0098] The heating time is not particularly limited as long as it is sufficient to sufficiently dry the zeolite membrane or calcinate the template, and is preferably 0.5 hours or more, more preferably 1 hour or more. There is no particular upper limit, and it is usually within 200 hours, preferably within 150 hours, and more preferably within 100 hours. When the purpose is to calcinate the template, the heat treatment may be carried out in an air atmosphere, but it may also be carried out in an atmosphere containing an inert gas such as N2 or oxygen.

[0099] When hydrothermal synthesis is carried out in the presence of an organic template, it is appropriate to wash the resulting separation membrane containing the zeolite membrane with water and then remove the organic template, for example, by heat treatment or extraction, preferably by heat treatment, i.e., calcination. The rate of temperature rise during heat treatment for calcining the template is desirably as slow as possible to prevent cracks in the zeolite membrane due to the difference in thermal expansion coefficient between the support and the zeolite. The rate of temperature rise is usually 5°C / min or less, preferably 2°C / min or less, more preferably 1°C / min or less, and particularly preferably 0.5°C / min or less. In consideration of workability, the rate is usually 0.1°C / min or more.

[0100] The rate of temperature drop after firing must also be controlled to avoid cracking of the zeolite membrane. As with the rate of temperature rise, the slower the rate, the better. The rate of temperature drop is usually 5°C / min or less, preferably 2°C / min or less, more preferably 1°C / min or less, and particularly preferably 0.5°C / min or less. In consideration of workability, the rate is usually 0.1°C / min or more.

[0101] The zeolite membrane may be ion-exchanged as needed. When the membrane is synthesized using a template, the ion exchange is usually carried out after removing the template. The ions to be exchanged include protons, Na, and the like. + , K. + , Li + Alkali metal ions such as Ca 2+ , Mg 2+ , Sr 2+ , Ba 2+ ions of group 2 elements such as Fe, Cu, and Zn, and ions of transition metals such as Fe, Cu, and Zn. + , K. + , Li + Alkali metal ions such as are preferred.

[0102] Ion exchange can be performed by treating the zeolite membrane after calcination (when a template is used, for example) with an aqueous solution containing an ammonium salt such as NH4NO3 or NaNO3 or the ions to be exchanged, or in some cases with an acid such as hydrochloric acid, usually at a temperature between room temperature and 100°C, followed by rinsing with water. If necessary, the membrane may be further calcined at 200°C to 500°C.

[0103] The air permeability [L / (m 2 ·h)] is usually 1400L / (m 2 ·h) or less, preferably 1000 L / (m 2 ·h) or less, more preferably 700L / (m 2 ·h) or less, more preferably 600L / (m 2 ·h) or less, more preferably 500 L / (m 2 ·h) or less, particularly preferably 300 L / (m 2 ·h) or less, most preferably 200 L / (m 2 The lower limit of the permeation amount is not particularly limited, but is usually 0.01 L / (m 2 ·h) or more, preferably 0.1 L / (m 2 ·h) or more, more preferably 1 L / (m 2 ·h) or more. Here, the air permeability is the amount of air permeated [L / (m ] when a separation membrane containing a zeolite membrane is connected to a vacuum line with an absolute pressure of 5 kPa, as will be described later. 2 ·h)].

[0104] (sweep gas) In the method for producing a purified gas of the present invention, a sweep gas is used on the permeate side of the separation membrane, which increases the partial pressure difference of carbon dioxide between the non-permeate side and the permeate side of the separation membrane, thereby facilitating the permeation of carbon dioxide through the separation membrane. Examples of the sweep gas include nitrogen and air. When air is used as the sweep gas, if the separation membrane is damaged and, for example, methane flows into the permeation side as a component of the mixed gas, an explosion may occur. Therefore, nitrogen is preferred as the sweep gas. The purity of the sweep gas does not need to be high; 80 mol % or more is preferred, and 90 mol % or more is more preferred.

[0105] The pressure of the sweep gas is not particularly limited because it varies depending on the size of the separation membrane module and the gas processing rate, but is preferably 20 MPaG or less, more preferably 10 MPaG or less, and even more preferably 1 MPaG or less, and the lower limit is usually 0 MPaG or more, preferably 0.005 MPaG or more, and more preferably 0.01 MPaG or more. In some cases, the sweep gas may be used at reduced pressure. The flow rate of the sweep gas is not particularly limited as it is adjusted appropriately depending on the processing amount, but it is sufficient to sufficiently replace the permeating gas, and is usually 1 L / min or more, preferably 3 L / min or more, more preferably 5 L / min or more, with no particular upper limit, and is usually 30 L / min or less, preferably 20 L / min or less. [Example]

[0106] Examples of the present invention will be described below, but the present invention is not limited thereto. The methods for preparing and evaluating samples in the present invention are as follows.

[0107] <Gas permeation test> The gas permeation test was carried out using an apparatus 50 shown in Figure 1. A zeolite membrane composite 53 was disposed between a non-permeation side space 52 and a permeation side space 54 so as to separate these spaces. Carbon dioxide is supplied as a feed gas from an inlet 522 on the non-permeation side and discharged from an outlet 524. Nitrogen is supplied as a sweep gas from an inlet 542 on the permeation side and discharged from an outlet 544. In the device 50, the feed gas and the sweep gas flow in opposite directions in the non-permeation side space and the permeation side space, forming a countercurrent flow.

[0108] The purities of the carbon dioxide and nitrogen used were as follows: Carbon dioxide: 100% Nitrogen: 100%

[0109] The zeolite membrane was prepared by hydrothermal synthesis of CHA-type aluminosilicate zeolite (SAR: 65) directly on an inorganic porous support, to form a composite inorganic porous support-CHA-type zeolite membrane. During the purification test, the apparatus 50 is placed in the atmosphere and the temperature is set to room temperature.

[0110] Pressure (MPaG), gas flow rate (L / min), carbon dioxide concentration (mol%) at outlet 524 on the non-permeation side (outside the membrane), and Table 1 shows the pressure (MPaG), gas flow rate (L / min), and carbon dioxide concentration (mol%) at the outlet 544 on the permeation side (inside the membrane).

[0111] [Table 1]

[0112] From the above results, it was found that CO2 has permeated into the membrane based on the CO2 concentration at the sweep gas outlet, but the CO2 concentration at the supply gas outlet was 100%, so the sweep gas, N2, did not permeate to the supply gas side. This is a completely different result from the conventional idea that when a sweep gas is used, a partial pressure difference occurs between the inside and outside of the membrane, causing the sweep gas to permeate to the supply gas side and preventing the purity of the supply gas side from being increased, and is considered to be useful.

Claims

1. A method for producing a purified gas by supplying a mixed gas containing carbon dioxide to a separation membrane and separating the carbon dioxide by permeating the separation membrane, comprising: the separation membrane is a zeolite membrane, Sweeping the permeate side of the separation membrane with nitrogen or air; A method for producing purified gas.

2. 2. The method for producing a purified gas according to claim 1, wherein the zeolite constituting the zeolite membrane has a pore structure with an oxygen ring of eight or less members.

3. 3. The method for producing a purified gas according to claim 1, wherein the zeolite membrane comprises a CHA-type aluminosilicate zeolite, and in an X-ray diffraction pattern obtained by irradiating the membrane surface with X-rays, the peak intensity around 2θ = 17.9° has a value less than 0.5 times the peak intensity around 2θ = 20.8°, and the peak intensity around 2θ = 9.6° has a value between 2.0 times and 4.0 times the peak intensity around 2θ = 20.8°.

4. The SiO of the zeolite ２ / Al ２ O ３ The method for producing a purified gas according to claim 1 or 2, wherein the β-glucan content is 20 or more and 500 or less.

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