Gas separation method and apparatus

Abstract

Provided are a gas separation method and a device with which, upon occurrence of mixing of a combustible component into a permeable gas, the operation of a membrane module is immediately stopped. This gas separation method includes: supplying a raw material gas including a combustible component to a primary side of a membrane module by a pressurizing pump; depressurizing a secondary side of the membrane module by a depressurizing pump to a pressure less than an atmospheric pressure; and performing membrane separation into a primary-side gas having a high combustible component concentration and a secondary-side gas having a low combustible component concentration. The gas separation method is characterized by: analyzing the compositional makeup of the secondary-side gas; and stopping the pressurizing pump and the depressurizing pump when said compositional makeup falls within a specified range.

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for separating combustible components from a gas containing combustible components using a selective gas permeable membrane (hereinafter sometimes referred to as a membrane).

Background Art

[0002] Membrane separation of combustible components and other gas components from a gas containing combustible components such as methane has been widely performed (for example, Patent Documents 1 and 2).

[0003] Conventionally, organic membranes that have been widely used for gas separation have low separation performance (selectivity between combustible gas and non-combustible gas), so the permeate side contains a large amount of combustible gas. When air mixes in from the outside due to leakage when the concentration of combustible gas is high, it easily enters the explosive range.

[0004] Patent Document 2 describes that in a membrane separation method for exhaust gas containing combustible organic vapor, an inert gas is flowed through the permeate side through which the combustible organic vapor permeates in order to avoid the explosive range.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the case of inorganic membranes, separation performance is usually high, so the content of selectively separated substances decreases in the later stages of separation. As a result, the partial pressure that drives the separation decreases, and the amount of permeable gas decreases drastically. Therefore, it is usually necessary to increase the partial pressure difference between the permeable and non-permeable sides of the membrane, either by increasing the pressure on the non-permeable side or by decreasing the pressure on the permeable side. Increasing the pressure requires a lot of power from compressors, etc., which is economically disadvantageous. On the other hand, decreasing the pressure on the permeable side may cause a small amount of air to mix with the permeable gas due to leakage. However, when the separation performance is high, the concentration of flammable components is extremely low and usually does not fall within the explosive range. The inventors noticed that the explosive range is only entered when a leak occurs and then a rapid increase in the concentration of flammable components occurs due to membrane damage, etc., and thus developed the present invention.

[0007] In other words, the present invention aims to provide a gas separation method and apparatus for separating gases containing flammable components using a membrane module, which immediately stops the operation of the membrane module if flammable components are introduced into the permeate gas. [Means for solving the problem]

[0008] The gist of this invention is as follows:

[0009] [1] A gas separation method comprising a gas separation operation in which a raw material gas containing flammable components is pressurized and supplied to the primary side of a membrane module, the secondary side of the membrane module is depressurized to a pressure lower than atmospheric pressure, and membrane-separated into a primary side gas with a high concentration of flammable components and a secondary side gas with a low concentration of flammable components, characterized in that the composition of the secondary side gas is detected and the gas separation operation is stopped when the composition falls within a specified range.

[0010] [2] A gas separation method comprising a gas separation operation in which a raw material gas containing flammable components is pressurized and supplied to the primary side of a membrane module, the secondary side of the membrane module is depressurized to a pressure lower than atmospheric pressure, and membrane-separated into a primary side gas with a high concentration of flammable components and a secondary side gas with a low concentration of flammable components, characterized in that the pressure or flow rate of the secondary side is detected, and the gas separation operation is stopped when the detected pressure or flow rate exceeds a reference value.

[0011] [3] The gas separation method according to [2], wherein the reference value is 10 kPaG or more higher than the initial value in the case of detection pressure, and 50% or more higher than the initial value in the case of flow rate.

[0012] [4] A gas separation method consisting of a combination of [1] and [2] or [3].

[0013] [5] A gas separation method according to [1] or [4], wherein valves are provided in the supply line of the raw material gas to the membrane module and in the pressure reduction line of the membrane module, and these valves are closed when the composition falls within a specified range.

[0014] [6] The specified range is the explosion range of any of the gas separation methods [1], [4] and [5].

[0015] [7] A gas separation method according to any of [1] to [6], wherein the flammable component is methane.

[0016] [8] A gas separation method of any of the following [1] to [7], wherein the pressure on the secondary side is 30 kPa or more lower than atmospheric pressure.

[0017] [9] A gas separation method of any of [1] to [8] wherein the separation coefficient of the membrane of the membrane module is 80 or more.

[0018]

[10] A gas separation method of any of [1] to [9] wherein the composition falls within the specified range due to the mixing of the raw material gas.

[0019]

[11] The gas separation method according to any one of [1] to

[10] , wherein the raw material gas is biogas.

[0020]

[12] A membrane separation device having a membrane module that separates a raw material gas containing a combustible component into a primary-side gas with a high concentration of the combustible component and a secondary-side gas with a low concentration of the combustible component, a supply line having a pressure pump that pressurizes and supplies the raw material gas to the primary side of the membrane module, and a permeation line having a pressure reduction pump for reducing the pressure on the secondary side of the membrane module, the gas separation device comprising: detection means for detecting the composition of the secondary-side gas; and control means for stopping the pressure pump and the pressure reduction pump when the composition reaches a specified range.

[0021]

[13] A membrane separation device having a membrane module that separates a raw material gas containing a combustible component into a primary-side gas with a high concentration of the combustible component and a secondary-side gas with a low concentration of the combustible component, a supply line having a pressure pump that pressurizes and supplies the raw material gas to the primary side of the membrane module, and a permeation line having a pressure reduction pump for reducing the pressure on the secondary side of the membrane module, the gas separation device comprising: detection means for detecting the pressure or flow rate on the secondary side; and control means for stopping the pressure pump and the pressure reduction pump when the pressure or flow rate on the secondary side becomes greater than a specified value.

[0022]

[14] The gas separation device according to [第13項], wherein the reference value is a value 10 kPaG or more higher than the initial value in the case of the detected pressure, and a value 50% or more greater than the initial value in the case of the flow rate.

[0023]

[15] A gas separation device comprising a combination of

[12] and

[13] or

[14] .

[0024]

[16] The gas separation device according to

[12] or

[15] , wherein valves are provided in the supply line of the raw material gas to the membrane module and the pressure reduction line of the membrane module, respectively, and valve control means for closing these valves when the composition reaches a specified range.

[0025]

[17] The specified range is an explosion range, and any of the gas separation devices

[12] ,

[15] , and

[16] .

[0026]

[18] Any of the gas separation devices

[12] to

[17] in which the combustible component is methane.

[0027]

[19] Any of the gas separation devices

[12] to

[18] in which the pressure on the secondary side is set to -30 kPaG or less by the vacuum pump.

[0028]

[20] Any of the gas separation devices

[12] to

[19] in which the separation coefficient of the membrane of the membrane module is 80 or more.

[0029]

[21] Any of the gas separation devices

[12] to

[20] in which the composition enters the specified range due to the mixing of the raw material gas.

[0030]

[22] Any of the gas separation devices

[12] to

[21] in which the raw material gas is biogas. [Advantages of the Invention]

[0031] According to the gas separation method and apparatus of the present invention, when the mixing of combustible components into the permeated gas occurs, the operation of the membrane module can be immediately stopped. [Brief Description of the Drawings] <0000,118> [Figure 1] It is a configuration diagram of a gas separation device. [Figure 2] It is a ternary composition diagram of a gas. [Embodiments for Carrying Out the Invention]

[0033] Hereinafter, the present invention will be described in detail.

[0034] The gas (raw material gas) to be processed in this invention contains flammable and non-flammable components. Examples of flammable components include hydrocarbon gases such as methane, ethane, and propane, as well as hydrogen and ammonia. Examples of non-flammable components include, but are not limited to, carbon dioxide (CO2), nitrogen, and halogen gases.

[0035] Examples of raw material gases containing such combustible and non-combustible components include, but are not limited to, biogas, landfill gas, hot spring gas, coal mine gas, and natural gas. Biogas is a gas obtained by methane fermentation of biomass, and is sometimes further subdivided into landfill gas when the biomass originates from landfills, digester gas when it originates from sewage sludge, and biogas when it originates from food waste or livestock waste. The concentration of combustible components in the raw material gas, excluding saturated water vapor, is 30-70% (by volume; the same applies below) for biogas, 20-70% for landfill gas (depending on the extraction method), and approximately 10-90% methane for hot spring gas. From the viewpoint of economic efficiency and explosion avoidance, the concentration of flammable components in the raw material gas is preferably 20-95%.

[0036] The membrane module preferably consists of multiple tubular separation membranes arranged in a vessel, with the raw material gas flowing outside the tubular separation membranes and the non-combustible components permeating through to the inside of the tubular separation membranes, or the raw material gas flowing inside the tubular separation membranes and the non-combustible components permeating through to the outside of the tubular separation membranes. However, it can also be applied to honeycomb structures, hollow fiber structures, sheet structures, etc., and is not limited to these.

[0037] As a tubular separation membrane, one having a molecular sieve membrane as a selective gas permeable membrane on the inner and / or outer surfaces of a tubular porous substrate is preferred.

[0038] <Porous substrate> The porous substrate functions as a support for the molecular sieve membrane. The material constituting the porous substrate is not particularly limited, and various materials such as glass, ceramics, metals, carbon molded bodies, or resins can be applied. In the case of a ceramic support, any porous inorganic material with chemical stability that allows zeolite to crystallize into a film on its surface may be used. Specifically, examples include ceramic sintered bodies of silica, α-alumina, γ-alumina, mullite, zirconia, titania, yttria, silicon nitride, silicon carbide, etc. Among these, alumina such as α-alumina and γ-alumina, and mullite are preferred, and alumina is particularly preferred. Using these supports facilitates partial zeolization, resulting in a strong bond between the support and the zeolite, making it easier to form a dense film with high separation performance.

[0039] In the present invention, the porous substrate itself does not need to have molecular sieving ability. The porous substrate has fine pores (voids, air spaces) that connect the outer wall side (outer surface) and the inner wall side (inner surface).

[0040] The porous substrate typically has a porosity of 20% or more, preferably 25% or more, more preferably 30% or more, and typically 80% or less, preferably 60% or less, and more preferably 50% or less. Its average pore diameter is typically 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.1 μm or more, and the upper limit is typically 20 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. A porous substrate having such pores can adequately support a molecular sieve membrane with sufficient strength, and can allow molecules that have permeated the molecular sieve membrane to pass through at a sufficient speed, or can allow molecules to reach the molecular sieve membrane at a sufficient speed. The porosity and pore diameter of the porous substrate can be easily determined by methods such as mercury intrusion or observation of the cross-section with SEM. The average pore diameter can also be calculated from volume and mass using true specific gravity.

[0041] When forming a zeolite film as a molecular sieve, the porous substrate typically has a maximum diameter of 10 μm or less for the through-pores extending from the outer surface to the inner surface, preferably 8 μm or less. The lower limit of the maximum diameter is preferably 0.05 μm or more. Furthermore, the through-pores extending from the outer surface to the inner surface may all have the same pore diameter, or, for example, as described in Japanese Patent Application Publication No. 2005-270887, a substrate with partially or staggeredly different pore diameters may be used.

[0042] The tubular porous substrate preferably has a circular cross-section perpendicular to the axial direction of the tube, both on its outer and inner surfaces. The thickness of the tubular porous substrate (the difference between the radius of the outer surface and the radius of the inner surface) is not particularly limited. It varies depending on the material and porosity, but for example, the thickness is preferably 0.5 mm or more, more preferably 0.8 mm or more, and even more preferably 1.0 mm or more.

[0043] The inner diameter of the tubular porous substrate is not particularly limited. It varies depending on the material and porosity, but for example, the ratio of the inner diameter (diameter) to the thickness of the porous substrate (inner diameter (mm) / thickness (mm)) is preferably 20 or less, more preferably 17 or less, even more preferably 13 or less, and particularly preferably 9 or less. In the case of a porous substrate made of ceramic sintered body, the inner diameter is preferably 3 mm or more, particularly 5 mm or more, 20 mm or less, and particularly 15 mm or less.

[0044] The length (axial length) of the porous substrate is not particularly limited.

[0045] <Molecular sieve membrane> Molecular sieve membranes are formed on the outer and / or inner surfaces of a porous substrate. The morphology of the molecular sieve membrane is not particularly limited, as long as it can adequately perform its molecular sieving function.

[0046] The molecular sieve membrane may be either an organic or inorganic membrane, but an inorganic membrane is preferred. The inorganic membrane is preferably a zeolite membrane, a silica membrane, or a carbon membrane, or a combination thereof, with zeolite membranes or silica membranes being preferred, and zeolite membranes being particularly preferred from the viewpoint of separation performance, water resistance, and durability.

[0047] Here, when a zeolite membrane is used as the molecular sieve membrane, the zeolite is preferably an aluminosilicate, but metal elements such as Ga, Fe, B, Ti, Zr, Sn, and Zn may be used instead of Al as long as the performance of the membrane is not significantly impaired, and elements such as Ga, Fe, B, Ti, Zr, Sn, Zn, and P may be included along with Al.

[0048] Furthermore, it is preferable that the crystalline zeolite skeleton that forms the pores of the zeolite film has an oxygen ring of 8 members or less, and more preferably an oxygen ring of 6 to 8 members.

[0049] Examples of zeolite structures include AEI, AFG, ANA, CHA, DDR, EAB, ERI, ESV, FAR, FRA, GIS, ITE, KFI, LEV, LIO, LOS, LTA, LTN, MAR, MWF, PAU, RHO, RTH, SOD, STI, TOL, and UFI. Of these, it is preferable to use a membrane composed of AEI, CHA, DDR, ERI, KFI, LEV, MWF, PAU, RHO, RTH, SOD, LTA, and UFI type zeolites, and more preferable to use a membrane composed of CHA, DDR, MWF, RHO, and SOD type zeolites. The value of n in zeolites having an n-membered oxygen ring refers to the pore with the largest number of oxygen atoms among the pores composed of the zeolite skeleton and T elements (elements other than oxygen that constitute the skeleton).

[0050] When synthesizing zeolite membranes as molecular sieve membranes, organic templates (structure-controlling agents) can be used as needed. However, there are generally no particular restrictions as long as the template can create the desired zeolite structure, and if synthesis is possible without a template, it does not need to be used.

[0051] When a silica membrane is used as the molecular sieve membrane, the silica content in the silica membrane is not particularly limited as long as it does not significantly impair the effects of the present invention. For example, in the silicon oxide composition, the ratio of silicon to all positive elements, including silicon, is usually 50 mol% or more.

[0052] Silica films are fabricated on inorganic porous substrates by methods such as the sol-gel method, CVD method, and polymer precursor method. In the sol-gel method, a silica film can be fabricated by reacting a metal alkoxide with water on an inorganic porous substrate to form a gel through hydrolysis and dehydration condensation. In the counter-diffusion CVD method, for example, if the inorganic porous substrate is a porous tubular substrate, an amorphous silica layer can be deposited into the substrate pores by circulating oxygen inside and a silica source outside, thereby fabricating a silica film. In the polymer precursor method, a silica precursor such as alkoxysilane or polysilazane can be coated onto an inorganic porous substrate, and then heat-treated to fabricate a silica film.

[0053] When using a carbon film as a molecular sieve, a carbon film precursor solution is dip-coated onto a porous substrate, heat-treated at approximately 600-800°C, and dried to form a carbon film. Examples of carbon film precursors include aromatic polyimides, polypyrrolone, polyfurfuryl alcohol, polyvinylidene chloride, phenolic resins, lignin derivatives, wood tar, and bamboo tar. Suitable solvents include organic solvents such as tetrahydrofuran, acetone, methanol, ethanol, and N-methylpyrrolidone.

[0054] The thickness of the molecular sieve film is not particularly limited, but for zeolite films, the lower limit of the thickness is usually 0.01 μm or more. The upper limit of the thickness is preferably 30 μm or less, and more preferably 10 μm or less. For silica films, it may be a single layer or a film consisting of two or more layers, and the thickness is preferably 1 nm or more. The thickness is preferably 10 μm or less, and more preferably 1 μm or less. For carbon films, the thickness is preferably 0.05 μm or more, and more preferably 0.1 μm or less. The upper limit of the thickness is preferably 5 mm or less, and more preferably 500 μm or less, but a thinner film thickness is preferable as long as it does not significantly impair the film performance.

[0055] The method for forming a molecular sieve film on the surface of a porous substrate is not limited to the method described above. For example, (1) a method of fixing a substance that can constitute a molecular sieve film to the surface of a porous substrate with a binder or the like, (2) a method of impregnating a porous substrate with a slurry or solution in which a substance that can constitute a molecular sieve film is dispersed, thereby fixing the substance to the surface of the porous substrate, and (3) a method of crystallizing a substance that can constitute a molecular sieve film (especially zeolite) in a film form on the surface of a porous substrate (see, for example, International Publication No. 2013 / 125660).

[0056] The molecular sieve film may be formed only on the outer surface of the porous substrate, only on the inner surface of the porous substrate, or on both the outer and inner surfaces. Generally, from the viewpoint that the coating material has an inward shrinking force, it is desirable that the molecular sieve film be formed on the outer surface of the porous substrate.

[0057] The membrane separation coefficient α is preferably 80 or higher, more preferably 100 or higher, and particularly preferably 150 or higher. Here, the separation coefficient α is expressed by the following formula.

[0058]

number

[0059] The separation coefficient α can vary depending on the separation conditions, but the separation coefficient α used here is the value under the separation conditions in the separation operation of each membrane module.

[0060] In one aspect of the present invention, the secondary side (permeate side) of the membrane module is depressurized to a pressure lower than atmospheric pressure using a depressurizing pump. The gas composition, pressure, flow rate, etc. of the secondary side are then analyzed by detecting them online or inline. When the gas composition of the secondary side enters a specified range (for example, a range where the concentration of flammable components exceeds 20 vol%, or the explosive range) due to damage such as membrane breakage, or when the oxygen concentration of the secondary side enters a specified range (for example, a range where it exceeds 1 vol%, or the explosive range), and / or when at least one of the pressure or flow rate of the gas on the secondary side exceeds a standard value, the raw gas supply and the depressurizing pump are stopped. Furthermore, valves installed on the raw gas inlet side and the permeate gas outlet side of the membrane module may be closed. Any cause within the specified range is acceptable, but contamination by the source gas is considered the most likely. Contamination by the source gas can occur if the separation membrane is damaged during the separation operation, or if the seal of the separation membrane becomes insufficient.

[0061] The reference value for the secondary gas pressure is, for example, 10 kPa higher than the initial value, or the value of the specified pressure reduction level for secondary side 4b. The reference value for the secondary gas flow rate is, for example, 50% higher than the initial value.

[0062] Here, stopping the pump means stopping the supply of raw material gas and depressurization, and also includes states such as idling, where the pump does not supply, pressurize, or depressurize any substances.

[0063] Figure 1 shows an example of a gas separation apparatus equipped with a membrane module.

[0064] The raw material gas containing flammable components is introduced into the primary side 4a of the membrane module 4 via piping 1, which has a pressurizing pump 2 and a valve (emergency shut-off valve) 3. Inside the membrane module 4, the primary side 4a and the secondary side 4b are separated by a membrane 4m.

[0065] In this embodiment, the raw material gas contains methane (CH4) as a flammable component and carbon dioxide (CO2) as a non-flammable component, and the membrane 4m is a zeolite membrane. Carbon dioxide permeates through the zeolite membrane 4m, while methane hardly permeates through the zeolite membrane 4m. Therefore, methane-rich primary side outlet gas flows out from the primary side outlet of the membrane module 4 into the piping 5.

[0066] A piping 10 containing a pressure sensor 6, a valve (emergency shut-off valve) 7, a pressure reducing pump 8, and a methane sensor 9 for detecting methane concentration is connected to the secondary side 4b of the membrane module 4. The pressure reducing pump 8 reduces the pressure on the secondary side 4b to a specified degree of reduced pressure, preferably -30 kPaG or less, and particularly preferably -50 kPaG or less. Any means capable of detecting composition other than a methane sensor may be used, and known types of gas sensors may also be used. From the viewpoint of detecting flammable gases, it is preferable to use a methane sensor.

[0067] Carbon dioxide permeates through a 4m zeolite membrane and is extracted from pipe 10 as CO2-rich gas.

[0068] The detection signals from the pressure sensor 6 and the methane sensor 9 are input to the control device 11, and in an emergency, the control device 11 is configured to stop the pumps 2 and 8 and close the valves 3 and 7. Regarding the emergency stop operation, any operation that stops the separation operation is acceptable, even if it is not stopping the pumps.

[0069] An example of the steady-state operating conditions (mass balance, etc.) of the membrane separation apparatus shown in Figure 1 is described below.

[0070] The raw material gas supply rate per unit time is 40 moles of CO2 and 60 moles of CH4. The raw material gas is pressurized to 600 kPaG by the pressure pump 2 and supplied to the membrane module 4. The absolute pressure on the permeate side of the membrane is reduced to -30 kPaG, and the CO2 permeation is 2.0E-06 mol / m³. 2 / Pa / s, membrane area 2.6 m² 2When the membrane separation coefficient α is 83, the amount of methane-rich gas, which is the primary side outflow gas, per unit time is 3 moles of CO2 and 8 moles of CH4.

[0071] The secondary side outflow gas, which is CO2-rich gas, has an outflow rate of 37 moles of CO2 and 2 moles of CH4 per unit time.

[0072] Generally, zeolite separation membranes have high separation performance. Therefore, when the supply pressure of the raw material gas is low, the partial pressure of the substance to be separated, which is the driving force for separation, becomes low on the outlet side of the primary chamber 4a, resulting in reduced separation performance. For this reason, a vacuum pump 8 is used in this embodiment.

[0073] In a configuration where the secondary side 4b is depressurized, if air leaks into the secondary side 4b due to a leak, methane may flow into the secondary side 4b if the separation membrane is damaged or otherwise damaged, potentially causing the gas composition of the secondary side 4b to fall within the explosive range.

[0074] Therefore, in this embodiment, the inflow of methane into the secondary side 4b is detected by the methane sensor 9, and if the methane concentration enters a specified range (for example, a range where the methane concentration exceeds 20 vol%, or an explosion range), it is determined that an abnormality such as membrane damage has occurred, and the pressurizing pump 2 and the depressurizing pump 8 are stopped. Valves 3 and 7 may also be closed. Alternatively, a pressure sensor 6 and / or a flow meter may be installed in the piping 10, and the separation operation may be stopped by stopping pumps 2 and 8 when the pressure exceeds a reference value (for example, a value 10 kPa higher than the initial value, or a specified degree of depressurization on the secondary side 4b) and / or when the detected flow rate exceeds a reference value (for example, a value 50% higher than the initial value). Valves 3 and 7 may also be closed. The reference value can be set in advance according to the separation conditions of the gas to be separated.

[0075] The booster pump 2 and depressurizing pump 8 may be stopped if the methane concentration of the gas on the secondary side 4b falls within a specified range (for example, a range where the methane concentration exceeds 20 vol%, or the explosive range), and the pressure also exceeds a standard value (for example, a value 10 kPa higher than the initial value, or a value of the specified degree of depressurization on the secondary side 4b), and / or the detected flow rate exceeds a standard value (for example, a value 50% higher than the initial value). In addition, valves 3 and 7 may be closed.

[0076] <Procedure for restarting the device after it has been shut down> If the device is stopped, restart it by following the steps (1) to (7) below. (1) Discharge the gas. On the primary side, valves 3 and 7 are closed, then residual gas is released from piping 5, returning the pressure inside the module to atmospheric pressure. The secondary side may be restored to pressure using gas that has permeated through the membrane, or it may be restored to pressure using air from a separately provided line. Next, an inert gas such as nitrogen or air is introduced through piping 1 to replace the gas inside the module. At this time, the pressure reducing pump 8 may be started. (2) Release the module. (3) Identify the damaged separation membrane. For example, the method described in Japanese Patent Application Publication No. 2020-192482 (a method using a Pitot tube for identification) can be applied. (4) Replace the damaged separation membrane. (5) Close the module. (6) Perform a backup run. After activating the depressurizing pump 8, the raw material gas is introduced from piping 1, and the separation operation is resumed once the permeation rate or composition on the secondary side has stabilized. (7) Restart the separation.

[0077] Generally, in the case of a methane (CH4), carbon dioxide (CO2), and oxygen (O2) ternary system, if the separation performance of the separation membrane is high, there is almost no CH4 in the zeolite membrane permeate gas. Therefore, even if air is mixed into the permeate gas due to leakage during depressurization, the composition changes from point a to point X in Figure 2, so it does not fall within the explosive range. Even when the CH4 concentration is approximately 20% or less, if air is mixed into the permeate gas due to leakage during depressurization, the composition changes from point b to point X, so it hardly falls within the explosive range. However, if methane and oxygen originating from the source gas are mixed into the permeate gas due to leakage during depressurization, in addition to membrane damage or scratches, the composition changes from point c to point X, so there is a risk of it falling within the explosive range. Therefore, when the raw material gas mainly consists of methane or carbon dioxide, it is highly effective to detect the inflow of methane into the secondary side 4b using a methane sensor, pressure sensor, or flow meter, as in the embodiment described above, and to stop the pressurizing pump 2 and depressurizing pump 8 when the methane concentration falls within the standard range or when the pressure or flow rate exceeds the standard value. Furthermore, the effect is further enhanced by configuring the system to close valves 3 and 7.

[0078] In this embodiment, only one module is used, but multiple modules may be connected in series, in parallel, or in combination. When multiple modules are connected, the composition, flow rate, or pressure may be detected for each module, or a specific number of modules may be detected together.

[0079] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible without departing from the intent and scope of the invention. This application is based on Japanese Patent Application No. 2020-181559, filed on 29 October 2020, which is incorporated herein by reference in its entirety. [Explanation of symbols]

[0080] 2. Pressure pump 3,7 valves 4 membrane modules 6. Pressure Sensor 8. Pressure reducing pump 9. Methane sensor

Claims

1. A gas separation method comprising a gas separation operation in which a raw material gas containing flammable components is pressurized and supplied to the primary side of a membrane module, the secondary side of the membrane module is depressurized to a pressure lower than atmospheric pressure, and membrane-separated into a primary side gas with a high concentration of flammable components and a secondary side gas with a low concentration of flammable components, The separation coefficient of the membrane in the aforementioned membrane module is 80 or higher. A gas separation method characterized by detecting the composition of the secondary gas and stopping the gas separation operation when the composition falls within a specified range where the concentration of flammable components exceeds 20 vol% or is within the explosive range.

2. A gas separation method comprising a gas separation operation in which a raw material gas containing flammable components is pressurized and supplied to the primary side of a membrane module, the secondary side of the membrane module is depressurized to a pressure lower than atmospheric pressure, and membrane-separated into a primary side gas with a high concentration of flammable components and a secondary side gas with a low concentration of flammable components, The separation coefficient of the membrane in the aforementioned membrane module is 80 or higher. A gas separation method characterized by detecting the pressure or flow rate on the secondary side, and stopping the gas separation operation when the detected pressure exceeds a reference value which is 10 kPaG or more higher than the initial value, or when the detected flow rate exceeds a reference value which is 50% or more higher than the initial value.

3. Furthermore, the gas separation method according to claim 2, wherein the composition of the secondary gas is detected, and the gas separation operation is stopped when the composition falls within a specified range where the concentration of flammable components exceeds 20 vol% or is within the explosive range.

4. Valves are provided in the supply line for the raw material gas to the membrane module and in the pressure reduction line of the membrane module, respectively. The gas separation method according to claim 1 or 3, wherein these valves are closed when the composition falls within a specified range.

5. The gas separation method according to any one of claims 1 to 4, wherein the flammable component is methane.

6. The gas separation method according to any one of claims 1 to 5, wherein the pressure on the secondary side is set to a pressure 30 kPa or more lower than atmospheric pressure.

7. The gas separation method according to claim 1 or 3, wherein the composition falls within the specified range due to the mixing of the raw material gas.

8. The gas separation method according to any one of claims 1 to 7, wherein the raw material gas is biogas.

9. A membrane module that separates a raw material gas containing flammable components into a primary gas with a high concentration of flammable components and a secondary gas with a low concentration of flammable components by membrane separation, A supply line having a pressurized pump that supplies pressurized raw material gas to the primary side of the membrane module, A permeation line having a vacuum pump for reducing the pressure on the secondary side of the membrane module In a membrane separation apparatus having, The separation coefficient of the membrane in the aforementioned membrane module is 80 or higher. Means for detecting the composition of the secondary gas, A control means for stopping the pressurizing pump and depressurizing pump when the composition falls within a specified range where the concentration of flammable components exceeds 20 vol% or is within the explosive range. A gas separation apparatus having the following features.

10. A membrane module that separates a raw material gas containing flammable components into a primary gas with a high concentration of flammable components and a secondary gas with a low concentration of flammable components by membrane separation, A supply line having a pressurized pump that supplies pressurized raw material gas to the primary side of the membrane module, A permeation line having a vacuum pump for reducing the pressure on the secondary side of the membrane module In a membrane separation apparatus having, The separation coefficient of the membrane in the aforementioned membrane module is 80 or higher. The means for detecting the pressure or flow rate on the secondary side, A control means that stops the pressurizing pump and depressurizing pump when the detected secondary pressure exceeds a reference value which is 10 kPaG or more higher than the initial value, or when the detected secondary flow rate becomes greater than a reference value which is 50% or more greater than the initial value. A gas separation apparatus having the following features.

11. Furthermore, the means for detecting the composition of the secondary gas, The gas separation apparatus according to claim 10, further comprising control means for stopping the pressurizing pump and the depressurizing pump when the composition falls within a specified range where the concentration of flammable components exceeds 20 vol% or is within the explosive range.

12. Valves are provided in the supply line for the raw material gas to the membrane module and in the pressure reduction line of the membrane module, respectively. The gas separation apparatus according to claim 9 or 11, further comprising valve control means for closing these valves when the composition falls within a specified range.

13. The gas separation apparatus according to any one of claims 9 to 12, wherein the flammable component is methane.

14. The gas separation apparatus according to any one of claims 9 to 13, wherein the pressure on the secondary side is reduced to -30 kPaG or less by the aforementioned pressure reducing pump.

15. The gas separation apparatus according to claim 9 or 11, wherein the composition falls within the specified range due to the mixing of the raw material gas.

16. The gas separation apparatus according to any one of claims 9 to 15, wherein the raw material gas is biogas.

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