Purified gas production method

Abstract

To provide a purified gas production method that allows reduction of compressor capacity and improvement of the recovery rate of the purified gas, and can solve issues with polymer membranes.SOLUTION: The purified gas production method is a method for producing a purified gas containing one gas at high purity as a permeate gas by treating a mixed gas comprising multiple components using a plurality of separation membrane modules, where a predetermined non-permeate gas is returned to a predetermined position in an upstream stage.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 a gas contained in a mixed gas, but in recent years, membrane separation using a separation membrane has also become popular. In the case of separation by membrane separation, multiple separation membranes may be used in multiple stages to obtain a highly pure gas.

[0003] Patent Document 1 describes a method for producing high-purity hydrogen or helium using multiple separation membrane modules, in which permeate gas that has permeated through a polymer separation membrane module in a first stage is supplied to a polymer separation membrane module in a second stage without being pressurized between the polymer separation membrane modules.

[0004] US Patent No. 5,949,999 describes a method for separating a raw feed gas stream using multiple membrane separation stages, where a third non-permeate stream is combined with the raw feed gas stream to form a combined feed stream. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Publication No. 63-296820 [Patent Document 2] Patent Publication No. 2021-159913 Summary of the Invention [Problem to be solved by the invention]

[0006] The method of Patent Document 1 involves supplying gas to the next separation membrane module without increasing its pressure between the separation membrane modules, which has the problems of making it difficult to improve the gas recovery rate and requiring a large membrane area, which is economically disadvantageous.

[0007] The method of Patent Document 2 combines the third non-permeate stream with the raw feed gas stream to form a combined feed stream, which requires a large compressor capacity. Furthermore, because the composition of the third non-permeate stream differs significantly from that of the feed gas stream, the concentration of the permeating gas is diluted, resulting in a low partial pressure, which increases the first membrane area, which is economically disadvantageous. Furthermore, the dilution of the purified third non-permeate stream results in poor purification efficiency.

[0008] Furthermore, while polymer membranes are characterized by their excellent processability, they have the problem of degrading due to heat, chemicals, and pressure, resulting in a decrease in performance.

[0009] Based on the above, an object of the present invention is to provide a method and system for producing purified gas that can reduce the compressor capacity, improve the recovery rate of purified gas, and solve the problems associated with polymer membranes. [Means for solving the problem]

[0010] 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 containing one gas at high purity as a permeable gas by treating a mixed gas containing multiple components using multiple separation membrane modules, comprising: the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module, a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, A method for producing a purified gas, wherein the second pressure is equal to or greater than the first pressure, and the third pressure is equal to or greater than the second pressure.

[0011] [2] The method for producing a purified gas according to [1], wherein the mixed gas is supplied to the separation membrane module located most upstream of the preceding stage, and the non-permeated gas obtained is supplied to another separation membrane module without being pressurized, and the permeated gas having an increased concentration of the first gas is returned to the side further upstream of the first compressor.

[0012] [3] The method for producing a purified gas according to [1] or [2], wherein the pressure of the pressurized mixed gas supplied to the upstream separation membrane module is 0.1 MPaG or more and 1 MPaG or less.

[0013] [4] A method for producing a purified gas according to any one of [1] to [3], 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°.

[0014] [5] The method for producing a purified gas according to any one of [1] to [4], wherein the SiO2 / Al2O3 of the zeolite is 20 or more and 500 or less.

[0015] [6] A purified gas production system that produces a purified gas containing one gas at high purity as a permeate gas by treating a mixed gas containing multiple components using multiple separation membrane modules, the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module, a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, A system for producing a purified gas, wherein the second pressure is controlled to be equal to or higher than the first pressure, and the third pressure is controlled to be equal to or higher than the second pressure. [Effects of the Invention]

[0016] According to the present invention, by returning the non-permeate gas from the middle separation membrane module between the first compressor and the previous separation membrane module, and returning the non-permeate gas from the subsequent separation membrane module between the second compressor and the middle separation membrane module, the capacity of each compressor can be reduced, the recovery rate of the purified gas can be improved, and the problems associated with polymer membranes can be solved. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram showing the arrangement of a plurality of separation membrane modules and a plurality of compressors. [Figure 2] FIG. 1 is a schematic diagram of an example of a separation membrane module used in the present invention. [Figure 3] FIG. 2 is a schematic diagram showing the composition and pressure of non-permeable gas and permeable gas in each separation membrane module in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] [Method of producing purified gas] The method for producing a purified gas of the present invention is a method for producing a purified gas containing one gas at high purity as a permeable gas by treating a mixed gas containing a plurality of components using a plurality of separation membrane modules, comprising: the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module, a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, The method for producing a purified gas is such that the second pressure is equal to or higher than the first pressure, and the third pressure is equal to or higher than the second pressure.

[0020] <Mixed gas> In the present invention, the mixed gas may contain one gas component that is desired to be obtained as a purified gas and at least two other gas components that are desired to be removed, and the number of components to be contained and the specific composition are not limited.

[0021] However, a preferred embodiment includes a gas mixture containing carbon dioxide as one gas and nitrogen as the other gas. The mixed gas may also include other gases such as methane, oxygen, and argon. An example of the mixed gas is a gas used in semiconductor manufacturing. The method for producing a purified gas of the present invention makes it possible to reuse carbon dioxide, which has conventionally been discarded after a single use.

[0022] The amount of carbon dioxide that can be contained in the mixed gas is not particularly limited, but is usually 30 mol % or more, preferably 40 mol % or more, and usually 99 mol % or less, preferably 95 mol % or less. The amount of nitrogen that can be contained in the mixed gas is not particularly limited, but is usually 20 mol % or more, preferably 30 mol % or more, and usually 60 mol % or less, preferably 50 mol % or less. The amount of oxygen that can be contained in the mixed gas is not particularly limited, but is usually 3 mol % or more, preferably 5 mol % or more, and usually 20 mol % or less, preferably 15 mol % or less. The amount of methane that can be contained in the mixed gas is not particularly limited, but is usually 1 mol % or more, preferably 3 mol % or more, and usually 20 mol % or less, preferably 15 mol % or less.

[0023] The mixed gas to be separated in the present invention may be one from which some components have been removed by pretreatment.

[0024] (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 mentioned above, the gas 1 can be, for example, carbon dioxide, and the concentration of the gas 1 in the purified gas is preferably 80 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, and particularly preferably 99 mol% or more.

[0025] <Separation membrane module> In the method for producing purified gas of the present invention, the mixed gas described above is treated in a plurality of separation membrane modules. The separation membrane modules are equipped with zeolite membranes that selectively permeate one gas and can separate it from other gases. In a preferred embodiment, the zeolite membranes selectively permeate carbon dioxide.

[0026] An example of a separation membrane module is shown in Figure 2. In the illustrated embodiment, a feed gas is supplied to the outside of a separation membrane 50, and one gas selectively permeates through the separation membrane 50 to become a permeate gas. The gas on the feed side and the gas on the permeate side are separated by the separation membrane 50. The mixed gas is supplied to the supply side. By making the partial pressure of the first gas (e.g., the partial pressure of carbon dioxide) on the supply side of the separation membrane 50 higher than the partial pressure of the first gas on the permeation side of the separation membrane 50, the first gas in the mixed gas selectively permeates the separation membrane 50. This allows the first gas in the mixed gas to be purified.

[0027] The pressure of the mixed gas supplied to the separation membrane module (the pressure of the mixed gas supplied to the upstream separation membrane module described below) is preferably 0.1 MPaG or more, more preferably 0.2 MPaG or more, even more preferably 0.3 MPaG or more, and particularly preferably 0.5 MPaG or more. 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.

[0028] 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. The configuration shown in FIG. 2 is one such cylindrical separation module.

[0029] The separation module has a separation membrane 50 including a zeolite membrane, and the separation membrane 50 is placed in a thermostatic bath (not shown) while being housed in a stainless steel pressure-resistant container 55. The thermostatic bath is equipped with a temperature control device so that the temperature of the sample gas can be adjusted.

[0030] One end of the cylindrical separation membrane 50 is sealed with a circular end piece 57. The other end is connected by a connector 58, and the other end of the connector 58 is connected to a pressure-resistant vessel 55. The inside of the cylindrical separation membrane 50 is connected to a pipe 54 that discharges permeated gas containing a high concentration of gas 1 via the connector 58, and the pipe 54 extends to the outside of the pressure-resistant vessel 55. Furthermore, a pressure gauge 53 that measures the pressure on the supply side of the sample gas (gas mixture) and a back pressure valve 59 that adjusts the pressure on the supply side are connected to somewhere leading to the pressure-resistant vessel 55. Each connection is airtight.

[0031] A mixed gas containing the gas No. 1 is supplied at a constant flow rate between pressure vessel 55 and separation membrane 50, and the pressure on the supply side is kept constant by back pressure valve 59. The gas permeates the zeolite membrane in accordance with the partial pressure difference between the inside and outside of the zeolite membrane of separation membrane 50, and is discharged through pipe 54.

[0032] (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.

[0033] ·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.

[0034] 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.

[0035] 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.

[0036] The shape of the porous support is not particularly limited as long as it can effectively separate mixed gases. Specific examples include tubular shapes such as flat plates and cylindrical tubes, honeycomb shapes with a large number of cylindrical, columnar, or prismatic holes, and monolithic shapes. 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The surface of the porous support may be polished with a file or the like as needed. The surface of the porous support means 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.

[0041] 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.

[0042] 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, more preferably 20 μm or less, and still more preferably 15 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 6 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.

[0048] 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.

[0049] 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.

[0050] Examples of zeolites having a 6- to 8-membered oxygen ring structure include AEI, AFG, AFX, ANA, CHA, 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.

[0051] 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.

[0052] 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).

[0053] 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, CHA, ERI, KFI, MWF, LEV, PAU, RHO, RTH, and UFI, even more preferred structures are CHA and DDR, and most preferred structure is CHA.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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°.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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

[0065]

number

[0066] When the peak is (No. 166), it is a peak derived from the plane with index (1,0,0) in the CHA structure.

[0067] 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.

[0068]

number

[0069] When the peak is (No. 166), it is a peak derived from the plane with index (1,1,1) in the CHA structure.

[0070] 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

[0071]

number

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] -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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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, even more preferably 15 or more, particularly preferably 30 or more, and most preferably 35 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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, the heat treatment means drying the separation membrane containing the zeolite membrane by applying heat, or calcining the template when a template is used.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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 washing with water. If necessary, the membrane may be further calcined at 200°C to 500°C.

[0108] 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), 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), 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)].

[0109] <Layout of each separation membrane module and each compressor> The method for producing purified gas of the present invention uses a plurality of separation membrane modules. Figure 1 shows the layout of a plurality of separation membrane modules and a plurality of compressors. In the present invention, among the multiple separation membrane modules, one or more upstream separation membrane modules are referred to as front-stage separation membrane modules 10, one or more midstream separation membrane modules are referred to as middle-stage separation membrane modules 20, and one or more downstream separation membrane modules are referred to as rear-stage separation membrane modules 30.

[0110] In FIG. 1, the front-stage, middle-stage, and rear-stage separation membrane modules 10, 20, and 30 are each illustrated as a single separation membrane module, but each may be composed of a plurality of separation membrane modules.

[0111] (Compressor) In the method for producing purified gas of the present invention, a first compressor 11 is provided upstream of the front-stage separation membrane module 10, a second compressor 21 is provided upstream of the middle-stage separation membrane module 20, and a third compressor 31 is provided upstream of the rear-stage separation membrane module 30. The separation membrane modules 10, 20, 30 each include a separation membrane 13, 23, 33, which separates spaces 12, 22, 32 on the gas supply side from spaces 14, 24, 34 on the gas permeation side.

[0112] The main flow of the mixed gas will be described. The mixed gas to be purified is first pressurized to a first pressure by a first compressor 11. The mixed gas pressurized to the first pressure is supplied to the supply side space 12 of the upstream separation membrane module 10, and one gas contained in the mixed gas selectively permeates through a separation membrane 13 and moves to the permeation side space 14. The permeation side space 14 is maintained at a lower pressure than the supply side space 12, for example, 0 MPaG. The first pressure is the "pressure of the mixed gas supplied to the upstream separation membrane module" described above.

[0113] The permeate gas obtained in the first separation membrane module 10 is pressurized to a second pressure by the second pressure booster 12, and the pressurized permeate gas is supplied to the supply side space 22 of the middle separation membrane module 20. Similarly, in the second separation membrane module 20, gas 1 selectively permeates the separation membrane 23 and moves to the permeate side space 24, where the concentration of gas 1 is increased. The pressure in the permeate side space 24 is the same as in the first separation membrane module 10.

[0114] The permeate gas obtained in the second separation membrane module 20 is pressurized to a third pressure by a third pressure booster 31, and the pressurized permeate gas is supplied to a feed space 32 of a subsequent separation membrane module 30. Similarly, in the third separation membrane module 30, gas 1 selectively permeates a separation membrane 33 and moves to a permeate space 34, where the concentration of gas 1 is further increased. The pressure in the permeate space 34 is the same as that in the first separation membrane module 10. In the present invention, as described above, by connecting multiple separation membrane modules 10, 20, and 30 in series, it is possible to gradually increase the concentration of gas 1 in the permeable gas, and ultimately produce a purified gas containing gas 1 at a high concentration.

[0115] (Return of non-permeable gas) In the method for producing purified gas of the present invention, the non-permeated gas from the middle-stage separation membrane module 20 is returned between the first compressor 11 and the previous-stage separation membrane module 10, and the non-permeated gas from the subsequent-stage separation membrane module 30 is returned between the second compressor 21 and the middle-stage separation membrane module 20. In this way, by returning and mixing non-permeable gas in which the concentration of gas 1 is higher than the concentration of gas 1 in the returned gas, the partial pressure of gas 1 is increased, which makes it possible to increase the recovery rate of gas 1 and reduce the energy required for the compressor.

[0116] Furthermore, in the method for producing purified gas of the present invention, in order to efficiently return the non-permeable gas to the preceding stage, the second pressure must be equal to or higher than the first pressure, and the third pressure must be equal to or higher than the second pressure. The pressure in each stage may be approximately 0 MPaG to 0.02 MPaG higher than that in the previous stage. If the non-permeate gas in the middle / rear stage is not pressurized to a higher pressure than that in the front stage, a separate booster will be installed, but this will require the non-permeate gas to be connected to the booster pump suction in the front stage, which will increase the amount of gas to be compressed, making it economically disadvantageous.

[0117] The non-permeate gas at the outlet side of the upstream separation membrane module 10 has a lower concentration of gas 1 than the concentration of gas 1 in the mixed gas. Therefore, the non-permeate gas at the outlet side of the upstream separation membrane module 10 may be discarded without being recovered. Alternatively, the non-permeate gas at the outlet side obtained by supplying it to the upstream separation membrane module 10 may be supplied to another separation membrane module (not shown) without being pressurized, and the permeate gas with an increased concentration of gas 1 may be returned further upstream of the first compressor 11. In this case, the same improvement in recovery rate and energy reduction effects as those described above can be obtained.

[0118] [Purified gas production system] The purified gas production system of the present invention is a system for producing a purified gas containing one gas at high purity as a permeate gas by treating a mixed gas containing a plurality of components using a plurality of separation membrane modules, comprising: the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module, a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, The system for producing a purified gas controls the second pressure to be equal to or higher than the first pressure, and the third pressure to be equal to or higher than the second pressure.

[0119] The respective configurations and preferred ranges of the purified gas production system of the present invention are the same as those for the production of purified gas described above, and therefore will not be repeated. There are no particular limitations on the method for controlling the purified gas production system of the present invention, and any known method can be used. [Example]

[0120] 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.

[0121] <Mixed gas purification simulation> The mixed gas purification simulation was performed as follows using three separation membrane modules as shown in Figure 2 and three compressors arranged as shown in Figure 3. The simulation was performed using Aspen manufactured by Aspen Tech. The separation performance of the separation membrane was determined by the single gas permeance ratio of CO2 / N2:21 and CO2 / O2:31. This corresponds to the performance of an inorganic porous support-CHA-type zeolite membrane composite, in which, 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 to 4.0 times the peak intensity around 2θ=20.8°.

[0122] The pressure at the non-permeate side inlet of the front-stage separation membrane module 10 was 0.60 MPaG, the pressure at the non-permeate side inlet of the middle-stage separation membrane module 20 was 0.60 MPaG, and the pressure at the non-permeate side inlet of the rear-stage separation membrane module 30 was 0.60 MPaG. At this time, the molar fraction at the non-permeate side outlet of the front-stage separation membrane module 10 was CO2:N2:O2 = 27.9:57.7:14.4, the molar fraction at the non-permeate side outlet of the middle-stage separation membrane module 20 was CO2:N2:O2 = 97.8:1.6:0.6, and the molar fraction at the non-permeate side outlet of the rear-stage separation membrane module 30 was CO2:N2:O2 = 99.8:0.1:0.1. The molar fractions on the permeate side of the front-stage separation membrane module 10 were CO2:N2:O2 = 97.9:1.5:0.6, the molar fractions on the permeate side of the middle-stage separation membrane module 20 were CO2:N2:O2 = 99.9:0.1:0.1, and the molar fractions on the permeate side of the rear-stage separation membrane module 30 were CO2:N2:O2 = 99.99:0.006:0.005. The CO2 recovery rate was 74.2%. The CO2 recovery rate can be further improved by adjusting the number of membranes and the amount of recycle. Thus, it was found that the purified gas production method of the present invention can achieve a high CO2 recovery rate. The compositions of the non-permeable and permeable gases obtained in the first, middle, and last stages are shown in Figure 3.

Claims

1. A method for producing a purified gas containing one gas at high purity as a permeate gas by treating a mixed gas containing a plurality of components using a plurality of separation membrane modules, comprising: the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module; a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, The method for producing a purified gas, wherein the second pressure is equal to or greater than the first pressure, and the third pressure is equal to or greater than the second pressure.

2. 2. The method for producing a purified gas according to claim 1, wherein the mixed gas is supplied to the separation membrane module located most upstream of the preceding stage, and the non-permeated gas obtained is supplied to another separation membrane module without being pressurized, and the permeated gas having an increased concentration of the first gas is returned to a further upstream side of the first compressor.

3. 3. The method for producing a purified gas according to claim 1, wherein the pressure of the pressurized mixed gas supplied to the upstream separation membrane module is 0.1 MPaG or more and 1 MPaG or less.

4. 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°.

5. 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.

6. A purified gas production system for producing a purified gas containing one gas at high purity as a permeate gas by treating a mixed gas containing a plurality of components using a plurality of separation membrane modules, comprising: the separation membrane module is provided with a zeolite membrane, When one or more upstream separation membrane modules among the plurality of separation membrane modules are defined as a front-stage separation membrane module, one or more midstream separation membrane modules are defined as a middle-stage separation membrane module, and one or more downstream separation membrane modules are defined as a rear-stage separation membrane module, a first compressor is provided upstream of the first-stage separation membrane module; a second compressor is provided upstream of the middle-stage separation membrane module; a third compressor is provided upstream of the subsequent separation membrane module; The gas pressurized to a first pressure by the first compressor is supplied to the separation membrane module in the previous stage, the permeated gas is pressurized to a second pressure by the second compressor and supplied to the separation membrane module in the middle stage, and the permeated gas is pressurized to a third pressure by the third compressor and supplied to the separation membrane module in the subsequent stage, The non-permeated gas from the middle-stage separation membrane module is returned between the first compressor and the front-stage separation membrane module, and the non-permeated gas from the rear-stage separation membrane module is returned between the second compressor and the middle-stage separation membrane module, A system for producing a purified gas, wherein the second pressure is controlled to be equal to or higher than the first pressure, and the third pressure is controlled to be equal to or higher than the second pressure.

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