Method and apparatus for decomposing nitrous oxide in gas

Abstract

To provide a method for efficiently decomposing N2O in gas even at a low temperature and in the presence of O2.SOLUTION: A method for decomposing N2O in gas containing N2O includes: a step of bringing the gas containing N2O into contact with an adsorbent to adsorb N2O in the gas onto the adsorbent; a step of desorbing N2O from the adsorbent having adsorbed N2O; and a step of reducing and decomposing the desorbed N2O in the presence of a reduction catalyst.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for decomposing N2O (dinitrogen monoxide) in a gas. [Background technology]

[0002] N2O (nitrous oxide) is contained in exhaust gases from fossil fuel-burning power plants, automobile exhaust gases, exhaust gases from biomass combustion, exhaust gases from chemical plants that produce nitric acid and adipic acid, and exhaust gases from farm fields that have received excessive nitrogen-based fertilizers. N2O is one of the greenhouse gases, and its impact on global warming is known to be approximately 300 times greater than that of CO2 (carbon dioxide). Therefore, from the perspective of preventing global warming, there is a strong demand to reduce N2O emissions into the atmosphere, and methods for breaking down N2O into harmless N2 are being investigated.

[0003] Known methods for decomposing NO in exhaust gas into harmless N include direct decomposition (e.g., Non-Patent Document 1) and selective catalytic reduction (SCR) (e.g., Non-Patent Document 2). The direct decomposition method is a method in which NO is decomposed by contact with a catalyst without using a reducing agent. The selective catalytic reduction method is a method in which NO is decomposed by a reducing agent in the presence of a catalyst. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] J. Oi, A. Obuchi, et al., "Decomposition of nitrous oxide over supported rhodium catalysts and dependency on feed gas composition", Applied Catalysis B: Environmental, 1997, Vol. 12, pp. 277-286. [Non-patent document 2] M. Mauvezin, et al., "N2O decomposition in the presence of ammonia on faujasite-supported metal catalysts", Applied Catalysis B: Environmental, 1999, Vol. 23, pp. L79-L82. Summary of the Invention [Problem to be solved by the invention]

[0005] However, decomposing N2O in the presence of O2 (oxygen) typically requires high temperatures of over 300°C. From the perspective of energy conservation, it is desirable to be able to decompose and remove N2O at as low a temperature as possible.

[0006] On the other hand, at low temperatures, there is a problem that the decomposition reaction of N2O does not proceed sufficiently in the presence of O2. For example, the gas to be treated, such as exhaust gas, usually contains a relatively large amount of O2 because combustion is carried out under an excess of oxygen. This O2 inhibits the decomposition performance of the catalyst, making it difficult for the decomposition of N2O to proceed smoothly.

[0007] Furthermore, when exhaust gas or the like is used as the gas to be treated, the concentration of N2O is small compared to other components (for example, 1000 ppm or less), so in order to treat it efficiently, it is desirable to use some method to efficiently concentrate and decompose N2O.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method and apparatus for efficiently decomposing N2O in gas even at low temperatures and in the presence of O2. [Means for solving the problem]

[0009] The present invention relates to the following method and apparatus for decomposing N2O:

[0010] [1] A method for decomposing N2O in a gas containing N2O, comprising the steps of: bringing the gas containing N2O into contact with an adsorbent to adsorb the N2O in the gas onto the adsorbent; desorbing the N2O from the adsorbent; and reducing the desorbed N2O in the presence of a reduction catalyst to decompose it. [2] The method according to [1], wherein the gas contains O2, and the desorption step is carried out in an atmosphere that is substantially free of O2. [3] The method according to [1] or [2], wherein the adsorbent is heated to a temperature of 80 to 200°C in the desorption step. [4] An apparatus for decomposing N2O in a gas containing N2O, the apparatus comprising: an adsorption / desorption unit having an adsorbent capable of adsorbing and desorbing N2O in a gas containing N2O; a heating unit for heating the adsorbent that has adsorbed N2O to desorb the N2O adsorbed by the adsorbent; and a decomposition unit having a reduction catalyst that reduces the desorbed N2O discharged from the adsorption / desorption unit. [5] The apparatus according to [4], further comprising: a first switching unit that selectively switches between the N2O-containing gas (A) and the gas (B) that is substantially free of O2 and supplies the selected gas to the adsorption / desorption unit; and a second switching unit that is disposed between the adsorption / desorption unit and the decomposition unit, wherein the second switching unit discharges the gas (A) after the N2O has been adsorbed by the adsorbent to an exhaust path when the first switching unit supplies the N2O-containing gas (A) to the adsorption / desorption unit; and supplies the gas (B) containing the desorbed N2O to the decomposition unit when the first switching unit supplies the gas (B) to the adsorption / desorption unit. [6] The system further includes a second adsorption / desorption unit having an adsorbent that adsorbs and desorbs N2O, wherein the first switching unit is switchable between a first switching state in which the gas (A) containing N2O is supplied to the adsorption / desorption unit and the gas (B) substantially not containing O2 is supplied to the second adsorption / desorption unit, and a second switching state in which the gas (B) substantially not containing O2 is supplied to the adsorption / desorption unit and the gas (A) containing N2O is supplied to the second adsorption / desorption unit, and the second switching unit is The apparatus described in [5] is switchable between a first switching state in which the gas (A) after being adsorbed by the adsorbent is discharged to an exhaust path, and the gas (B) substantially not containing O2 discharged from the second adsorption / desorption unit is supplied to the decomposition unit, and a second switching state in which the gas (B) substantially not containing O2 discharged from the adsorption / desorption unit is supplied to the decomposition unit, and the gas (A) after the N2O discharged from the second adsorption / desorption unit has been adsorbed by the adsorbent is discharged to an exhaust path. [Effects of the Invention]

[0011] According to the present invention, a method and apparatus for efficiently decomposing N2O in a gas even at low temperatures and in the presence of O2 are provided. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing an overview of an apparatus according to one embodiment of the present invention. [Figure 2] 2A and 2B are schematic diagrams showing the operation of the device shown in FIG. [Figure 3] FIG. 3 is a graph showing the N2O-TPD profile in the example. [Figure 4] FIG. 4 is a graph showing the breakthrough characteristics of Ca ion-exchanged zeolites in the examples. [Figure 5] FIG. 5 is a graph showing the results of a decomposition test of N2O in a mixed gas using the apparatus shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] As a result of extensive research, the inventors discovered an approach to neutralize NO by adsorbing NO in a gas to be treated containing NO using an adsorbent, desorbing the adsorbed NO, and reducing it with a reducing agent such as H in the presence of a reduction catalyst. This makes it possible to selectively separate NO from O even when the gas to be treated contains a large amount of O, and then reduce the separated NO with a reduction catalyst. This makes it possible to reduce and decompose NO even at low temperatures and in the presence of O.

[0014] Furthermore, it is preferable to combine, for example, Ca ion-exchanged zeolite as the adsorbent with a catalyst in which a noble metal is supported on a La-containing alumina carrier as the reduction catalyst, which enables reduction and decomposition of NO even at low temperatures, for example, below 200°C, in the presence of O.

[0015] A method for decomposing NO in a gas to be treated according to one embodiment of the present invention will be described in detail below. In this embodiment, the method for desorbing NO will be described by way of example, in which an adsorbent that has adsorbed NO is heated to desorb the NO. However, the present invention is not limited to this embodiment. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0016] 1. How to decompose N2O A method for decomposing NO in a gas according to one embodiment of the present invention includes the steps of: 1) bringing a gas to be treated containing NO into contact with an adsorbent to selectively adsorb the NO in the gas onto the adsorbent (adsorption step), 2) heating the adsorbent to adsorb NO and selectively desorb the adsorbed NO (desorption step), and 3) reducing and decomposing the desorbed NO in the presence of a reduction catalyst (decomposition step). Each step will be described below.

[0017] 1-1.Adsorption process In this step, the gas to be treated (gas (A)) containing N2O is brought into contact with the adsorbent, and the N2O in the gas to be treated is adsorbed onto the adsorbent.

[0018] (adsorbent) The adsorbent is not particularly limited as long as it can selectively adsorb and desorb NO, but is preferably a porous material. Examples of porous materials include zeolite, activated carbon, and silicates, with zeolite being preferred. That is, the adsorbent preferably contains zeolite.

[0019] The type of zeolite framework is not particularly limited, and examples include MOR (mordenite), MFI (ZSM-5), CHA (CHA type), BEA (beta type), etc. Among these, MOR is preferred from the viewpoint of its relatively large pore size and higher NO adsorption capacity.

[0020] The Si / Al ratio of the zeolite is not particularly limited, but from the viewpoint of further enhancing the NO adsorption capacity, a low ratio is preferable. For example, the Si / Al ratio is preferably 3 to 100 (molar ratio), more preferably 3 to 50 (molar ratio), and even more preferably 5 to 20 (molar ratio). When the Si / Al ratio is 100 (molar ratio) or less, a larger amount of cations, which will be described later, can be supported, thereby further enhancing the NO adsorption capacity. When the Si / Al ratio is 3 (molar ratio) or more, durability can be further enhanced. The Si / Al ratio of the zeolite can be measured, for example, by X-ray fluorescence spectroscopy (XRF: X-ray Fluorescence Spectrometer).

[0021] Furthermore, from the viewpoint of further enhancing the NO adsorption capacity, the zeolite is preferably an ion-exchanged zeolite. The type of ion is not particularly limited, and examples include monovalent cations such as Li, K, and Na; divalent cations such as Ca, Ba, Mg, and Sr; and transition metal cations such as Ce. The type of ion depends on the zeolite skeleton, but from the viewpoint of achieving a higher NO adsorption capacity, Ca and Sr are preferred, with Ca being more preferred. That is, the adsorbent preferably contains Ca ion-exchanged zeolite (see FIG. 3 described below).

[0022] From the viewpoint of desorbing the adsorbed N2O at a lower temperature (for example, a temperature of 200°C or lower), CHA or MFI may be used as the zeolite carrier, and Ba may be used as the ion type (see FIG. 3 described later).

[0023] The amount of ion exchange is not particularly limited, but is preferably 1 mol% or more, more preferably 10 mol% or more, relative to the number of moles of Al in the zeolite. If the amount of ion exchange is equal to or greater than the lower limit, the NO adsorption capacity can be further increased. The upper limit of the amount of ion exchange is not particularly limited, but from the viewpoint of effective Ca utilization, it can be set to, for example, 100 mol% or less. The amount of ion exchange can be confirmed by X-ray fluorescence analysis (XRF: X-ray Fluorescence Spectrometer).

[0024] (gas to be treated) Examples of gases to be treated that contain NO include gases emitted from power plants, factories, boilers, and burners, and exhaust gases from internal combustion engines of automobiles, agricultural machinery, construction machinery, ships, and the like. These exhaust gases may be exhaust gases resulting from the combustion of fossil fuels as well as alternative fuels to fossil fuels, such as biomass, hydrogen, and ammonia. The gas to be treated may further contain O. The O concentration of the gas to be treated is not particularly limited, but may be, for example, 0.1 to 25% by volume, or 3 to 15% by volume. Alternatively, all components of the gas to be treated other than NO may be O.

[0025] From the viewpoint of maintaining the NO adsorption activity of the adsorbent for a longer period of time, it is preferable that the concentration of other adsorbable components other than NO, such as H2O, in the gas to be treated be as low as possible (see FIG. 4 described later). The H2O concentration in the gas to be treated is, for example, preferably less than 3 vol%, more preferably less than 1 vol%, even more preferably less than 0.1 vol%, and particularly preferably less than 0.01 vol%. Similarly, it is preferable that the CO2 concentration in the gas to be treated is low, for example, preferably less than 5 vol%, more preferably less than 1 vol%, even more preferably less than 0.1 vol%, and particularly preferably less than 0.01 vol%.

[0026] (Regeneration of adsorbent materials) Furthermore, the NO adsorption activity of the NO adsorbent, which has been inhibited by H2O and CO2 during operation, can be more effectively restored by further regeneration, such as by heating. Therefore, by appropriately inserting this regeneration process during continuous operation, more stable continuous operation is possible.

[0027] (Analysis of components in gas) The O2 concentration, H2O concentration, and CO2 concentration in the gas to be treated can be measured using an online FT-IR spectrometer (for example, FT / IR-4600 manufactured by JASCO).

[0028] (adsorption conditions) The temperature (adsorption temperature) when the gas to be treated is brought into contact with the adsorbent is not particularly limited, but from the viewpoint of facilitating the adsorption of NO, it is preferable that the temperature is low. For example, the adsorption temperature is preferably room temperature to 100°C, and more preferably 30 to 50°C. The adsorption temperature is measured, for example, by the atmospheric temperature in the adsorption / desorption layer.

[0029] 1-2. Desorption process In this step, the adsorbent that has adsorbed N2O is heated to selectively desorb the adsorbed N2O.

[0030] The desorption of NO adsorbed on the adsorbent is preferably carried out in an atmosphere substantially free of O. This is because, when the desorption step and the decomposition step are carried out continuously (flow-type), the O concentration in the atmosphere of the decomposition step is reduced, making it less likely that the reaction activity of the reduction catalyst will decrease. In this specification, "substantially free of O" means that the O concentration is 0.1% by volume or less, preferably 0.05% by volume or less. In this embodiment, an atmosphere substantially free of O can be created by flowing a gas (B) substantially free of O through a space filled with an adsorbent that has adsorbed NO.

[0031] The gas (B) substantially free of O2 contains, for example, an inert gas (e.g., a rare gas such as helium or argon, or nitrogen gas) as a main component. When the desorption step and the decomposition step are performed continuously, the gas (B) substantially free of O2 may further contain a reducing agent, which will be described later.

[0032] The heating temperature (desorption temperature) of the adsorbent may be any temperature at which most of the NO adsorbed by the adsorbent is desorbed. For example, it is preferably a temperature higher than the peak temperature of the desorption amount in the NO-TPD profile of the adsorbent. For example, the desorption temperature, although it depends on the type of adsorbent, is preferably 80°C or higher, more preferably 100°C or higher, and even more preferably 130°C or higher. On the other hand, from the viewpoint of lowering the heating temperature in the decomposition step, the desorption temperature is preferably 200°C or lower, more preferably 160°C or lower, and even more preferably 150°C or lower. That is, the desorption temperature is preferably 80 to 200°C, and more preferably 130 to 150°C. As above, the desorption temperature refers to the ambient temperature in the adsorption / desorption layer.

[0033] The adsorbent may be heated by a heater or the like disposed in an adsorption / desorption layer (described later) filled with the adsorbent, or by supplying a heated gas to the adsorption / desorption layer. In this embodiment, the adsorbent is preferably heated by supplying a heated gas (B) to the adsorption / desorption layer.

[0034] 1-3. Decomposition process In this step, the desorbed NO is reduced and decomposed in the presence of a reduction catalyst. In this embodiment, the gas (B) containing the NO desorbed in the desorption step is brought into contact with a reducing agent in the presence of a reduction catalyst to reduce the NO to N.

[0035] (reduction catalyst) The reduction catalyst is not particularly limited as long as it can promote the reduction and decomposition of N2O by the reducing agent. Such a reduction catalyst is preferably a catalyst in which a noble metal is supported on a carrier.

[0036] The type of support is not particularly limited, and may be a metal oxide or a composite oxide. The support is used for the purpose of stably supporting catalytically active transition metals in a reaction environment, and also for the purpose of carrying out the target reduction reaction in a favorable environment by utilizing its porosity and crystalline structure. Specific examples of the carrier that can be preferably used include metal oxides such as silica (SiO), alumina (AlO), ceria (CeO), titania (TiO), zirconia (ZrO), and magnesia (MgO), and composite oxides thereof such as zeolite and clay. From the viewpoints of reactivity and durability, an alumina carrier is particularly preferably used.

[0037] The alumina support is preferably an alumina support containing a metal element such as lanthanum (La), copper (Cu), or cerium (Ce). Alumina supports containing these metal elements can further enhance the NO decomposition reaction activity. Among these, an alumina support containing lanthanum (La) is most preferred from the viewpoint of maintaining good activity even at low temperatures, for example, below 200°C. The content of the metal element in the alumina carrier containing the metal element is preferably, for example, 3 to 15 mass % relative to the total amount of the metal element and alumina, which can further promote the decomposition of NO by the reduction catalyst and enable decomposition at a lower temperature.

[0038] Examples of catalytically active noble metals include platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), and iridium (Ir). These can be used alone or in combination with several transition metals. Among these, Pd and Rh are preferred, and Pd is more preferred. The amount of the noble metal supported is not particularly limited, but can be, for example, 0.1 to 3 mass % relative to the support.

[0039] The form of the reduction catalyst is not particularly limited, and it can be used in the form of a powder, or in the form of a so-called pellet or honeycomb.

[0040] (reducing agent) Examples of reducing agents include hydrogen gas, ammonia gas, urea that derives ammonia, carbon monoxide (CO), and lower hydrocarbons such as propylene. Among these, hydrogen gas is preferred. The reducing agent may be added separately from the gas (B) or may be contained in the gas (B). The amount of reducing agent relative to gas (B) may be at least the amount necessary to reduce the NO contained in the gas, and may be, for example, 2 to 5 volume % relative to gas (B). Alternatively, the amount of reducing agent may be 1 to 2 equivalents, more preferably 1.1 to 1.3 equivalents, relative to the amount of NO contained in gas (B).

[0041] (Reaction conditions) The reduction reaction is preferably carried out in an atmosphere substantially free of O2, from the viewpoint of efficiently progressing the reaction and preventing the reaction activity of the reduction catalyst from being impaired. In this embodiment, the atmosphere can be made substantially free of O2 by circulating a gas (B) containing NO desorbed in the selective adsorption / desorption step and substantially free of O2 through a space filled with the reduction catalyst.

[0042] Furthermore, from the viewpoint of further increasing the reaction activity of the reduction catalyst, the heating temperature of the reduction catalyst is preferably high, for example, preferably 100°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher. On the other hand, from the viewpoint of further reducing energy consumption, the heating temperature of the reduction catalyst is preferably low, for example, preferably 300°C or lower, more preferably 200°C or lower, and even more preferably 160°C or lower. For these reasons, the heating temperature of the reduction catalyst suitable for decomposing NO in the presence of the reduction catalyst is particularly preferably 130 to 160°C, although it varies depending on the amount and concentration of NO to be treated, the amount of heat that can be supplied, the allowable volume of the device, and the like.

[0043] The heating temperature of the reduction catalyst may be the same as or different from the desorption temperature. For example, the difference between the heating temperature of the reduction catalyst and the desorption temperature (heating temperature of the reduction catalyst - desorption temperature) can be, for example, 150°C or less, preferably 50°C or less, and more preferably 30°C or less.

[0044] In this embodiment, an adsorbent capable of adsorption and desorption at low temperatures is used as the adsorbent used in the adsorption step and the desorption step, so that the reduction reaction can be carried out at a low temperature, for example, at or below 200° C. Note that the heating temperature of the reduction catalyst means the ambient temperature in the catalyst layer, as described above.

[0045] The reduction catalyst may be heated by a heater or the like arranged in a catalyst layer (described later) filled with the reduction catalyst, or by supplying a heated gas to the catalyst layer. In this embodiment, it is preferable to heat the reduction catalyst by supplying heated gas (B) that has undergone the desorption step to the catalyst layer. This is because the heat of the heated gas (B) can be utilized.

[0046] Hereinafter, an example of a preferable apparatus for realizing the method for decomposing N2O according to this embodiment will be described with reference to the drawings.

[0047] 2.N2O decomposition device [composition] FIG. 1 is a schematic diagram showing an overview of an apparatus according to one embodiment of the present invention. As shown in FIG. 1, the N2O decomposition device 10 according to this embodiment includes a first switching unit 11, a first adsorption / desorption unit 12, a second adsorption / desorption unit 13, a heating unit 14, a second switching unit 15, and a decomposition unit 16.

[0048] The first switching unit 11 switches the gas supplied to each of the first adsorption / desorption unit 12 and the second adsorption / desorption unit 13. In this embodiment, the first switching unit 11 selectively switches between a gas (A) containing N2O and O2 (gas to be treated) and a gas (B) containing a reducing agent and substantially not containing O2, and supplies the gas to the first adsorption / desorption unit 12 and the second adsorption / desorption unit 13.

[0049] The O2 concentration in gas (A), the O2 concentration in gas (B), and the amount of reducing agent can be set as described above.

[0050] In this embodiment, four pipes a, b, c, and d are connected to the first switching unit 11. The pipe a is a path through which gas (A) is supplied from a supply source S of gas (A) (for example, a combustion chamber in a power plant or factory, or an internal combustion engine of an automobile). The pipe b is a path through which gas (B) is supplied after passing through the heating unit 14. The pipe c is a path through which gas (A) or gas (B) is supplied to the first adsorption / desorption unit 12. The pipe d is a path through which gas (A) or gas (B) is supplied to the second adsorption / desorption unit 13. The first switching unit 11 switches the connections of these pipes under the control of, for example, the control unit 17.

[0051] For example, in the first switching state, the first switching unit 11 connects the pipe a to the pipe c and the pipe b to the pipe e. As a result, the gas (A) containing N2O and O2 is supplied to the first adsorption / desorption unit 12, and the gas (B) substantially not containing O2 is supplied to the second adsorption / desorption unit 13. In the second switching state, the first switching unit 11 connects the pipe a to the pipe d and the pipe b to the pipe c. As a result, the first switching unit 11 supplies the gas (B) containing a reducing agent and substantially no O to the first adsorption / desorption unit 12, and supplies the gas (A) containing N2O to the second adsorption / desorption unit 13.

[0052] The first adsorption / desorption unit 12 and the second adsorption / desorption unit 13 each have the adsorbent capable of adsorbing and desorbing NO. In this embodiment, the first adsorption / desorption unit 12 and the second adsorption / desorption unit 13 are adsorption / desorption beds (adsorption / desorption material packed beds) in which an adsorbent is packed in a container, and are arranged in parallel. These adsorption / desorption beds may have the same configuration. The first adsorption / desorption unit 12 and the second adsorption / desorption unit 13 perform the adsorption step or desorption step by bringing the gas supplied from the first switching unit 11 into contact with the adsorbent.

[0053] For example, when the first switching unit 11 is in the first switching state (when gas (A) is supplied to the first adsorption / desorption unit 12 and gas (B) is supplied to the second adsorption / desorption unit 13), the first adsorption / desorption unit 12 performs the adsorption process and the second adsorption / desorption unit 13 performs the desorption process. On the other hand, when the first switching unit 11 is in the second switching state (when gas (B) is supplied to the first adsorption / desorption unit 12 and gas (A) is supplied to the second adsorption / desorption unit 13), the first adsorption / desorption unit 12 performs the desorption process and the second adsorption / desorption unit 13 performs the adsorption process.

[0054] The heating unit 14 heats the adsorbent that has adsorbed N2O, thereby desorbing the N2O adsorbed by the adsorbent.

[0055] The heating unit 14 is not particularly limited as long as it is capable of heating the adsorbent, and may be a heated gas supply unit that supplies heated gas to the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13, or a heater or the like arranged in the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13. In this embodiment, the heating unit 14 is a heated gas supply unit that supplies heated gas (B) to the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13 (see FIG. 1 ). In this way, by supplying heated gas (B), the adsorbent in the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13 can be heated. The heating temperature by the heating unit 14 can be adjusted under the control of, for example, the control unit 17.

[0056] The second switching unit 15 is disposed between the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13 and the decomposition unit 16. The second switching unit 15 selectively switches the gas discharged from the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13 between the discharge path and the decomposition unit 16.

[0057] In this embodiment, four pipes e, f, g, and h are connected to the second switching unit 15. Pipe e connects the first adsorption / desorption unit 12 and the second switching unit 15, and is a path through which gas discharged from the first adsorption / desorption unit 12 is supplied. Pipe f connects the second adsorption / desorption unit 13 and the second switching unit 15, and is a path through which gas discharged from the second adsorption / desorption unit 13 is supplied. Pipe g is a discharge path through which gas discharged from the second switching unit 15 is discharged, for example, to the outside. Pipe h connects the second switching unit 15 and the decomposition unit 16, and is a path through which gas is supplied from the second switching unit 15 to the decomposition unit 16. The second switching unit 15 switches the connections of these pipes under the control of, for example, the control unit 17.

[0058] For example, in the first switching state, the second switching unit 15 connects the pipe e to the pipe g and the pipe f to the pipe h. As a result, the gas (A) that is supplied from the first switching unit 11 to the first adsorption / desorption unit 12 and from which NO is adsorbed by the adsorbent is discharged to the discharge path. In the second switching state, the second switching unit 15 connects the pipe e to the pipe h and the pipe f to the pipe g. As a result, the gas (B) that is supplied from the first switching unit 11 to the first adsorption / desorption unit 12 and contains desorbed NO is supplied to the decomposition unit 16.

[0059] The decomposition unit 16 has the reduction catalyst that reduces N2O. In this embodiment, the decomposition unit 16 is a catalyst packed bed in which the reduction catalyst is packed in a container. The decomposition unit 16 performs the decomposition step by bringing the gas supplied from the second switching unit 15 into contact with the reduction catalyst.

[0060] [Operation] Next, the operation of the device in FIG. 1 will be described.

[0061] 2A and 2B are schematic diagrams showing the operation of the device 10. Note that in this embodiment, an example will be described in which the adsorbent of the first adsorption / desorption unit 12 does not adsorb N2O, and the adsorbent of the second adsorption / desorption unit 13 has adsorbed N2O in advance.

[0062] (First operating mode) The first operation mode is a mode in which the adsorption process is carried out in the first adsorption / desorption section 12 and the desorption process is carried out in the second adsorption / desorption section 13 (see FIG. 2A).

[0063] That is, the first switching unit 11 is set to the first switching state, connecting the pipes a and c, and the pipes b and e. The second switching unit 15 is set to the first switching state, connecting the pipes e and g, and connecting the pipes f and h. As a result, the first switching unit 11 supplies gas (A) containing N2O and O2 to the first adsorption / desorption unit 12, and supplies gas (B) containing a reducing agent and substantially no O2 to the second adsorption / desorption unit 13. In addition, the second switching unit 15 discharges the gas discharged from the first adsorption / desorption unit 12 to the discharge path, and supplies the gas discharged from the second adsorption / desorption unit 13 to the decomposition unit 16 (see FIG. 2A).

[0064] In this embodiment, the temperature of the gas (A) corresponds to the above-mentioned adsorption temperature, and is, for example, room temperature to 50° C. The supply flow rate of the gas (A) can be, for example, 10 to 100 mL / min. The temperature of gas (B) corresponds to the desorption temperature described above and is, for example, 100 to 300° C., preferably 130 to 160° C. The supply flow rate of gas (B) can be, for example, 10 to 100 mL / min.

[0065] In the first adsorption / desorption unit 12, the gas (A) containing NO and O is brought into contact with the adsorbent to selectively adsorb NO. This performs the adsorption process described above. After NO has been adsorbed by the adsorbent, the gas (A) containing O is discharged from the first adsorption / desorption unit 12, passes through the second switching unit 15, and is then discharged to the outside via the pipe g (see FIG. 2A).

[0066] In the second adsorption / desorption unit 13, gas (B) containing a reducing agent and substantially no O2 is brought into contact with an adsorbent that has previously adsorbed N2O, thereby desorbing the N2O. This performs the desorption process described above. The gas (B) containing the desorbed N2O and the reducing agent and substantially no O2 is discharged from the second adsorption / desorption unit 13. In this embodiment, the adsorbent of the second adsorption / desorption unit 13 can be heated by supplying gas (B) heated by the heating unit 14. This allows the N2O adsorbed on the adsorbent to be desorbed. The gas (B) discharged from the second adsorption / desorption unit 13, containing the desorbed N2O and the reducing agent and substantially no O2, is supplied to the decomposition unit 16 via the second switching unit 15 (see FIG. 2A).

[0067] In the decomposition section 16, the gas (B) containing the desorbed N2O and the reducing agent but substantially no O2 is brought into contact with a reduction catalyst to convert the N2O to N2, thereby completing the decomposition step. The gas (B) containing N2 is discharged from the decomposition section 16 (see FIG. 2A).

[0068] (Second operating mode) Next, the first switching unit 11 and the second switching unit 15 are switched to switch to the second operation mode. The second operation mode is a mode in which the first adsorption / desorption unit 12 performs the desorption process and the first adsorption / desorption unit 13 performs the adsorption process (see FIG. 2B).

[0069] That is, the first switching unit 11 is set to the second switching state, so that the pipes a and d are connected, and the pipes b and c are connected. Also, the second switching unit 15 is set to the second switching state, so that the pipes e and h are connected, and the pipes f and g are connected. As a result, the first switching unit 11 supplies gas (A) containing N2O and O2 to the second adsorption / desorption unit 13, and supplies gas (B) containing a reducing agent and substantially no O2 to the first adsorption / desorption unit 12. In addition, the second switching unit 15 discharges the gas discharged from the second adsorption / desorption unit 13 to the discharge path (piping g), and supplies the gas discharged from the first adsorption / desorption unit 12 to the decomposition unit 16 (see FIG. 2B).

[0070] The temperatures and supply flow rates of the gas (A) and the gas (B) can be the same as in the first operation mode.

[0071] The above-mentioned desorption step is carried out in the first adsorption / desorption section 12. The gas (B) containing the desorbed N2O and reducing agent and substantially no O2 is discharged from the first adsorption / desorption section 12 and supplied to the decomposition section 16 via the second switching section 15 (see FIG. 2B).

[0072] The above-mentioned adsorption process is carried out in the second adsorption / desorption unit 13. The gas (A) containing O after N2O has been adsorbed by the adsorbent is discharged from the second adsorption / desorption unit 13 and then passed through the second switching unit 15 to the discharge path (piping g) (see FIG. 2B).

[0073] The decomposition step described above is carried out in the decomposition section 16. Then, the gas (B) containing N2 is discharged from the decomposition section 16 (see FIG. 2B).

[0074] (action) According to the above embodiment, by alternately repeating the first operation mode and the second operation mode, N2O in the gas (A) containing N2O and O2 can be continuously decomposed into N2 and removed even at low temperatures.

[0075] In particular, by combining an adsorbent and a reduction catalyst, after adsorbing N2O in the presence of O2, for example, in the first adsorption / desorption unit 12 or the second adsorption / desorption unit 13, N2O can be desorbed, preferably at a low temperature of, for example, 200°C or less, in an atmosphere substantially free of O2. Then, by treating the desorbed N2O-containing gas (B) in the decomposition unit 16, N2O can be reduced to N2 even at low temperatures. As a result, even in the case of gas (A) containing N2O and O2, N2O can be reduced to N2 and decomposed at a low temperature of, for example, 200°C or less.

[0076] [Variations] The configuration of the N2O decomposition device of the present invention is not limited to the above embodiment.

[0077] For example, in the above embodiment, the N2O decomposition device has two adsorption / desorption units, but is not limited to this, and may have one adsorption / desorption unit.

[0078] Furthermore, in the above embodiment, the gas (B) containing a reducing agent is supplied to the first adsorption / desorption section 12 and the second adsorption / desorption section 13, but this is not limiting. For example, a gas containing a reducing agent may be supplied separately from the gas (B), for example, at a position between the first adsorption / desorption section 12 or the second adsorption / desorption section 13 and the decomposition section 16. That is, the above device may further include a reducing agent supply section that supplies a gas containing a reducing agent, separate from the supply section that supplies the gas (B).

[0079] In the above embodiment, the heating unit 14, the first switching unit 11, and the second switching unit 15 are controlled by the control unit 18, but this is not limitative and they may be controlled manually. [Example]

[0080] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0081] 1. Consideration of adsorbent materials 1-1. Preparation of adsorbent In preparing the ion-exchanged zeolite, the following zeolite and ion-exchange material were used.

[0082] (Zeolite) MOR (Si / Al ratio 9.1, manufactured by Tosoh) CHA (Si / Al ratio 11.15, manufactured by Tosoh) MFI (Si / Al ratio 11.15, manufactured by Tosoh) BEA (Si / Al ratio 8.75, manufactured by Tosoh)

[0083] (ion exchange material) Mg(NO3)2 (99.2% or more, manufactured by Kanto Chemical Co., Ltd.) Ca(NO3)2 (98.5% or more, manufactured by Wako Pure Chemical Industries, Ltd.) Sr(NO3)2 (98.0% or more, manufactured by Kanto Chemical Co., Ltd.) Ba(NO3)2 (99.0% or more, manufactured by Wako Pure Chemical Industries, Ltd.)

[0084] (Preparation of ion-exchanged zeolite Z1) For ion exchange, 1 g of zeolite powder (MOR) was mixed with 50 mL of an aqueous solution containing an ion-exchange material, Ca(NO3)2, in a molar amount five times the molar amount of Al in the zeolite. The slurry was heated and stirred at 60 °C for 4 hours and then centrifuged at 4000 rpm to recover the ion-exchanged zeolite. The resulting sample was dried at 110 °C for 12 hours and then calcined at 500 °C for 3 hours to obtain the ion-exchanged zeolite. The Ca ion-exchange content of ion-exchanged zeolite Z1 was measured by XRF and found to be 28 mol% relative to the number of Al moles in the zeolite.

[0085] (Preparation of ion-exchanged zeolites Z2 to Z8) Ion-exchanged zeolites Z2 to Z8 were prepared in the same manner as for ion-exchanged zeolite Z1, except that the combinations of zeolite and ion-exchange material were changed as shown in Table 1.

[0086] The prepared ion-exchanged zeolites Z1 to Z8 are shown below.

[0087] [Table 1]

[0088] 1-2. Measurement of NO-TPD profile For the samples of ion-exchanged zeolites Z1 to Z8 prepared above, N2O-TPD (temperature programmed desorption) profiles were measured by the following method.

[0089] (test) The measurement device used was a BELCAT II (MicrotracBEL). After pretreatment under He gas flow at 500 °C for 30 min, the sample was exposed to 10% NO / He gas (20 mL / min) at 50 °C for 30 min, followed by a 10-min He gas purge. Next, the sample was heated from 50 °C to 250 °C at a heating rate of 10 °C / min, and the NO-TPD profile was measured. The evaluation results are shown in Figure 3.

[0090] (Consideration) As shown in Figure 3, when the amount of NO desorption was calculated in the temperature range of 50 to 250°C, Z1 and Z2, especially ion-exchanged zeolite Z1, showed high amounts of NO desorption. This result indicates that Ca-MOR (Si / Al ratio = 9.1) is optimal for NO adsorption at 50°C.

[0091] In addition, ion-exchanged zeolites Z5 (Ba-CHA) and Z6 (Ba-MFI) are considered preferable because they desorb most of the adsorbed NO at low temperatures below 200°C, and therefore have excellent desorption properties at low temperatures.

[0092] 1-3. Measurement of breakthrough characteristics For the ion-exchanged zeolite Z1 prepared above, an N2O breakthrough experiment was carried out in the following manner.

[0093] (test) The measurement equipment was a TGS detector (resolution 4cm -1 An online FT-IR spectrometer (JASCO, FT / IR-4600) equipped with a custom-made gas cell (CaF window) was used. Measurements were performed at 50 °C, with a Ca-MOR weight of 100 mg, a total flow rate of 50 mL / min, and a He balance. The distribution conditions were as follows: (a) 500 ppm by volume NO (b) 500 ppm by volume N2O + 10% by volume O2 (c) 500 ppm by volume N2O + 3% by volume H2O The evaluation results are shown in Figure 4.

[0094] (Consideration) As shown in Figure 4, the presence of O2 has little effect on the N2O adsorption capacity of Ca-MOR (see condition (b)). On the other hand, the presence of 3 vol% H2O slightly reduces the amount of N2O adsorbed onto Ca-MOR (see condition (c)).

[0095] These results indicate that, in order to more easily maintain the high efficiency of Ca-MOR for N2O adsorption, it is preferable to reduce the concentration of H2O in the N2O-containing gas as much as possible.

[0096] 2. NO decomposition test 2-1. Preparation of reduction catalyst A Pd-supported catalyst (Pd / La / Al2O3) on an Al2O3 support loaded with La (La / Al2O3: La = 15 mass%) was prepared with reference to the following paper: Y. Jing, Z. Cai, C. Liu, T. Toyao, Z. Maeno, H. Asakura, S. Hiwasa, S. Nagaoka, H. Kondoh, and K. Shimizu, "Promotional Effect of La in the Three-Way Catalysis of La-Loaded Al2O3-Supported Pd Catalysts (Pd / La / Al2O3)," ACS Catalysis, 2020, Vol. 10, pp. 1010-1023.

[0097] Specifically, the support La / Al2O3 was prepared by impregnation using an aqueous solution of γ-Al2O3 and La(NO3)3 (98.0 mass% or more, manufactured by Wako Pure Chemical Industries, Ltd.). Pd was supported by impregnation using an aqueous solution of Pd(NH3)2(NO3)2 nitric acid (Furuya Metals). After calcining at a predetermined temperature, reduction was carried out in a H2 stream at 500°C for 0.5 hours to obtain Pd / La / Al2O3 (Pd = 1 mass%).

[0098] 2-2.N2O decomposition test Using the device shown in Figure 1, an N2O decomposition test was conducted. Specifically, two adsorption / desorption layers, each filled with an adsorbent (the above-mentioned ion-exchanged zeolite Z1 (Ca-MOR, Si / Al ratio 9.1)), were arranged in parallel to form the first and second adsorption / desorption sections. A decomposition layer filled with a reduction catalyst (Pd / La / Al2O3) served as the decomposition section. Gas (A) (NO / O2 / He gas) and gas (B) (H2 / He gas) were alternately passed through the first and second adsorption / desorption sections at normal pressure, and the outlet gas during the passage of gas (B) was passed through the decomposition section. The operating conditions were as follows: (Operating conditions) Adsorbent: Ca-MOR (Si / Al ratio 9.1), filling amount 300mg Reduction catalyst: the reduction catalyst prepared above (Pd / La / Al2O3, Pd=1 mass% / (La / Al2O3)), loading amount 30 mg Gas (A): 500 volume ppm N2O / 10 volume% O2 / He gas (gas to be treated containing N2O and substantially no CO2 or H2O) Gas (B): 4% by volume H2 / He gas (gas containing a reducing agent and substantially no O2) Flow rate: 50mL / min Operating time per mode: 40 min

[0099] (First operating mode) Specifically, gas (A) (NO / O / He gas, 50°C) was passed through the first adsorption / desorption section at a flow rate of 50 mL / min to selectively adsorb NO onto the adsorbent, while oxygen-containing gas (O / He gas) was discharged from the outlet. At this time, gas (B) (H / He gas, 150°C) was passed through the second adsorption / desorption section at a flow rate of 50 mL / min, and the temperature was raised to 150°C at a temperature increase rate of 20°C / min to desorb the NO adsorbed by the adsorbent in the previous step. At this time, gas containing the desorbed NO but not containing oxygen (NO / H / He gas) was discharged from the outlet side and supplied to the decomposition section. The N2O in the gas supplied to the decomposition section was converted to N2, and a gas containing N2 (N2 / H2 / He gas) was discharged from the outlet side of the decomposition section. The operation was carried out in the above operating mode for 40 minutes.

[0100] (Second operating mode) Next, the first valve (first switching unit) and the second valve (second switching unit) were switched to switch to the second operation mode. That is, the first valve was switched to supply gas (B) (H2 / He gas) to the first adsorption / desorption section, and gas (A) (NO / H2 / He gas) to the second adsorption / desorption section, and the second valve was switched to supply the gas (NO / H2 / He gas) discharged from the first adsorption / desorption section, which contained desorbed NO but did not contain oxygen, to the decomposition section. The operation was carried out in the above operating mode for 40 minutes.

[0101] (Continuous operation) Continuous operation was performed by switching between the first and second operating modes every 40 minutes. The composition of the gas (B) discharged from the decomposition section was measured using a mass spectrometer (BELMass, manufactured by MicrotracBEL Corp.). The composition of the gas (A) discharged from the exhaust path (pipe g in Figure 1) was also measured using the online FT-IR spectrometer (FT / IR-4600, manufactured by JASCO). The evaluation results are shown in Figure 5.

[0102] 2-3.Results As shown in Figure 5, the introduced NO was not detected in the analysis system on the outlet side (both the outlet side of the first and second adsorption / desorption sections and the outlet side of the decomposition section). This can be seen from the fact that the NO concentration overlaps with the horizontal axis of the graph in Figure 5 and is barely detectable. Furthermore, while the adsorbent was heated from 50°C to 150°C, the formation of N with a maximum concentration of approximately 12,000 ppm was observed. During this period, almost no NO was detected.

[0103] These results show that all of the introduced N2O was adsorbed by the adsorbent, and that most of the N2O desorbed during the subsequent temperature increase process was reduced to N2 by H2 on the reduction catalyst. It was also confirmed that this process can continuously adsorb and reduce N2O for at least 15 hours (10 cycles, assuming that one cycle consists of a set of the first and second operating modes). [Industrial Applicability]

[0104] According to the present invention, a method and apparatus for efficiently decomposing N2O in a gas even at low temperatures and in the presence of O2 are provided. [Explanation of symbols]

[0105] 10 N2O decomposition equipment 11 First switching unit 12 1st suction / desorption section 13 Second suction / desorption section 14 Heating section 15 Second switching section 16 Disassembly section 17 Control Unit a, b, c, d, e, f, g, h piping S Gas supply source

Claims

1. N ２ N in O-containing gas ２ A method for decomposing O, comprising the steps of: N ２ A gas containing O is brought into contact with an adsorbent, and N in the gas is removed. ２ adsorbing O onto the adsorbent; The N ２ N from the adsorbent on which O is adsorbed ２ a step of eliminating O; The desorbed N ２ a step of reducing and decomposing O in the presence of a reduction catalyst; Including, method.

2. The gas is O ２ Including, In the desorption step, O ２ In an atmosphere substantially free of The method of claim 1.

3. In the desorption step, the adsorbent is heated to a temperature of 80 to 200°C. The method of claim 1.

4. N ２ N in O-containing gas ２ An apparatus for decomposing O, comprising: N ２ N in O-containing gas ２ an adsorption / desorption unit having an adsorbent capable of adsorbing and desorbing O; The N ２ The adsorbent on which O is adsorbed is heated to remove the N adsorbed on the adsorbent. ２ a heating unit for desorbing O; The desorbed N discharged from the adsorption / desorption unit ２ a decomposition unit having a reduction catalyst that reduces O; having Device.

5. The N ２ A gas containing O (A) and O ２ a first switching unit that selectively switches between the gas (A) and a gas (B) that is substantially free of the above and supplies the gas to the adsorption / desorption unit; a second switching unit disposed between the adsorption / desorption unit and the decomposition unit; further comprising The second switching unit is The first switching unit is ２ When the gas (A) containing O is supplied to the adsorption / desorption section, ２ Discharging the gas (A) after O has been adsorbed by the adsorbent into a discharge path; When the first switching unit supplies the gas (B) to the adsorption / desorption unit, the desorbed N ２ The gas (B) containing O is supplied to the decomposition section.

5. The apparatus of claim 4.

6. N ２ further including a second adsorption / desorption unit having an adsorbent that adsorbs and desorbs O; The first switching unit ２ A gas (A) containing O is introduced into the adsorption / desorption section, and the O ２ a first switching state in which a gas (B) substantially not containing O is supplied to the second adsorption / desorption unit; ２ A gas (B) substantially free of N is supplied to the adsorption / desorption section, and ２ a second switching state in which a gas (A) containing O is supplied to the second adsorption / desorption unit; The second switching unit is configured to ２ The gas (A) after O2 is adsorbed by the adsorbent is discharged to a discharge path, and the O2 discharged from the second adsorption / desorption unit is ２ a first switching state in which a gas (B) substantially not containing O is supplied to the decomposition unit; and a second switching state in which the O discharged from the adsorption / desorption unit is supplied to the decomposition unit. ２ A gas (B) substantially free of N is supplied to the decomposition section, and the N discharged from the second adsorption / desorption section is ２ a second switching state in which the gas (A) after O has been adsorbed by the adsorbent is discharged to the discharge path; 6. The apparatus of claim 5.

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