Method for reducing NOx in nitric acid plant during transition event

Abstract

A method for reducing NOx emissions during start-up and shut-down events of a nitric acid plant, where the nitric acid plant comprises a tail gas treatment section (10) comprising a first catalytic bed (2) for removal of N2O followed by a second catalytic bed (3) for removal of NOx, where during start-up or shut-down, the first catalytic bed (2) is separated from the second catalytic bed (3). The operation of the treatment section (10) is temporarily altered by adding a NOx reductant (4) upstream of the first catalytic bed (2) such that the first catalytic bed (2) provides reduction of NOx.

Description

[0001] manual Technical Field

[0002] The present invention belongs to the field of nitric acid production. In particular, the present invention relates to a method for reducing NO during a transition event in a nitric acid plant. x Method of emission. Background Art

[0003] The industrial production of nitric acid uses well-known methods described in the literature, for example, as described in Ullmann's Encyclopedia of Industrial Chemistry, 2012 edition, Volume 24, "Nitric Acid, Nitrous Acid and Nitrogen Oxides".

[0004] Briefly, the process involves the catalytic oxidation of ammonia with an oxidant over a suitable catalyst, typically a platinum / rhodium catalytic mesh. The product of the ammonia oxidation is a hot nitrogen oxide-containing gas which is absorbed into water in a dedicated tower, producing a nitric acid solution and tail gas.

[0005] Exhaust gas contains nitrogen oxides NO x (NO and NO2) and nitrous oxide N2O, which are sources of air pollution, must be removed before exhaust gases are released into the atmosphere.

[0006] In the prior art, there are several abatement systems that can be used to remove nitrogen oxides and nitrous oxide from the exhaust gas during normal or steady-state operation of the plant. x and N2O are removed catalytically. Some installations use a so-called three-stage abatement system, with multiple catalytic beds arranged in series downstream of the absorber. The term "three-stage" refers to the location downstream of the absorber.

[0007] In more detail, the three-stage emission reduction system generally includes a first catalyst bed and a second catalyst bed arranged in series, wherein: the first catalyst bed is used to catalytically decompose N2O into nitrogen and oxygen at a temperature range of 300°C to 600°C; in the second catalyst bed, nitrogen oxides NO x In the presence of a suitable reducing agent, it is reduced to nitrogen and water. Commonly used reducing agents are ammonia and hydrocarbons.

[0008] In general, the three-stage abatement system described above is capable of achieving greater than 95% N2O and NO reduction during normal plant operation. x Emission reduction efficiency. For example, NO in exhaust gas x The content may be reduced to values ​​below 40 ppmv (eg 20 ppmv), which is considered satisfactory in most cases.

[0009] Unfortunately, during transient events, such as plant startup or shutdown, the tail gas temperature is insufficient to maintain adequate conversion of N2O and NO x The catalyst has poor catalytic activity at low temperatures. For example, the exhaust gas temperature during the startup transition may be in the range of 180°C to 230°C.

[0010] During the transition event, low conversion of N2O is generally not considered a problem because the amount of N2O formed during the transition event is limited; in contrast, NO x In addition to this pollution, NO x The emission of nitric acid may result in the formation of an undesirable yellow to reddish brown smoke column. The formation of such a visible smoke column is undesirable because it may attract attention to persons in the vicinity of the plant. Therefore, efforts are made to provide a smokeless start-up of a nitric acid plant.

[0011] Prior art solutions suggest adding equipment, such as additional heaters, to heat the process gas more quickly, especially during startup. However, these solutions have the disadvantage of requiring the installation of expensive equipment.

[0012] It is therefore highly desirable to find a method that allows for the reduction of nitrogen oxides (NO) during the transition events of startup and shutdown of nitric acid plants. x Released into the atmosphere. Summary of the invention

[0013] The object of the present invention is to overcome the above-mentioned disadvantages of the prior art. Specifically, the present invention solves the problem of NO emissions during the transition period between startup and shutdown of a nitric acid plant due to deviations from normal operating conditions. x The present invention also aims to solve the problem of how to reduce NO during the transition period without adding expensive equipment. x Emissions problem.

[0014] This problem is solved by the method according to claim 1 .

[0015] The method reduces NO in a nitric acid plant during a transient event of startup or shutdown of the plant. x Emissions, of which:

[0016] The nitric acid plant comprises a synthesis part and an absorption part, wherein in the synthesis part ammonia is catalytically oxidized to obtain a gas containing nitrogen oxides, and wherein in the absorption part the gas containing nitrogen oxides is absorbed into water to produce concentrated nitric acid, NO x and N2O exhaust.

[0017] The nitric acid plant comprises a nitric acid plant adapted to remove N2O and NO from the tail gas before the tail gas is discharged into the atmosphere. x The treatment part comprises a first catalytic bed and a second catalytic bed, the first catalytic bed and the second catalytic bed are arranged to be passed through by the tail gas in sequence, wherein during normal operation of the device, one of the two beds is used to remove N2O, and the other of the two beds is used to reduce NO in the presence of a reducing agent x .

[0018] During the transition event, the exhaust gas passes through the catalyst bed for removing NO in the presence of an additional amount of reducing agent, the additional amount of reducing agent being added only during the transition event so that the catalyst bed temporarily acts as a catalyst for reducing NO in the exhaust gas. x There is an extra bed available.

[0019] The method is applicable to a nitric acid plant comprising a tail gas treatment section comprising a catalytic bed for the subsequent removal of N2O and a catalyst for the reduction of NO in the presence of a reducing agent such as ammonia. x The method modifies the operation of the treatment section by introducing an additional amount of reducing agent so that the catalyst bed originally designed for removing N2O during normal operation temporarily serves as a catalyst for reducing NO x Additional catalyst beds are in operation.

[0020] The tail gas passes through the first catalytic bed and the second catalytic bed in sequence. In a first general embodiment of the present invention, the first bed is used to remove N2O, followed by the second bed for removing NO x Thus, in this first embodiment, the first bed is temporarily used to remove NO x In a second general embodiment, the arrangement is reversed compared to the first embodiment, i.e. the first bed is used to remove NO x , followed by a second bed for removing NO. In this second embodiment, the second bed provides temporary additional NO removal during the transition event. x ability.

[0021] DeNO x and de-N2O represent NO x and removal of N2O.

[0022] In a broad sense, the present invention is based on the sensible recognition that in the above-mentioned apparatus, a catalytic bed originally designed for the removal of N2O in the absence of a reducing agent can be used to supplement the function of another bed, namely to remove NO by providing a suitable additional amount of reducing agent. x , so that the bed for removing N2O is also operated in the presence of a reducing agent during the transition event. This can be achieved if the catalyst of the bed is also suitable for reducing NO in the presence of the reducing agentx This is the case for many N2O decomposition catalysts, including the widely used iron zeolite catalysts.

[0023] Designed to remove NO generated during normal operation x The catalyst bed may not be able to remove the larger amount of NO generated during the transition x , especially during startup. The present invention provides additional NO removal x The invention is also based on the understanding that the removal of N2O is not important in this transition process and that in any case the relevant catalytic bed has a poor catalytic activity at low temperatures, whereas the same bed can be used temporarily in an advantageous manner for the removal of NO x During the transition period, compared with normal operation, the present invention can remove NO x Capacity increased by approximately 50%.

[0024] In summary, during the transition event, a catalytic bed originally designed for N2O decomposition is advantageously used for NO x of restoration.

[0025] During the transition event, a reducing agent may be added to the gas upstream of the catalytic bed to remove N2O or directly to the catalytic bed. The addition of the reducing agent may be carried out in a suitable mixer upstream of the catalytic bed.

[0026] The reducing agent added between beds and the reducing agent temporarily added during the transition event can be the same reducing agent or a different reducing agent. For both, a highly preferred reducing agent is ammonia.

[0027] Once the transition event is over, the catalytic bed for removing N2O can be restored to its original function, i.e., N2O decomposition, by interrupting the supply of additional amounts of reducing agent. The end of the transition event can be determined based on one or more operating parameters of the process section, such as the gas temperature at a selected location (e.g., the outlet of the first catalytic bed), or the NO content in the input gas. x Preferably, when the tail gas temperature at the inlet of the bed for removing N2O is equal to or greater than the set minimum temperature (preferably at least 250°C), the system resumes normal operation.

[0028] Another advantage is that the downtime of the catalyst in the bed for N2O removal is minimized. In a typical three-stage abatement configuration, the catalyst is active for N2O decomposition at temperatures above 380°C, preferably above 400°C, and more preferably above 430°C. During transition periods, especially during startup or shutdown, the gas temperature entering the bed for N2O removal is kept low. During this period, the catalyst is inactive for N2O decomposition. Using the method of the present invention, the bed is used to reduce NO x , in fact, even at low temperatures using catalysts for NO x reduction.

[0029] A preferred reducing agent is ammonia. Another advantage is that during startup, the exothermic reaction of the reduction of nitrogen oxides with ammonia helps to heat the catalyst bed and shorten the startup duration. Another advantage is that when ammonia is added upstream of the first catalyst bed, the ammonia passes through the two catalyst beds in series, which reduces the risk of ammonia slip. It must be noted that ammonia slip during startup is particularly undesirable because the low temperature of the gas may favor the formation and deposition of explosive ammonium nitrate, which is very undesirable. The risk is particularly high downstream of the exhaust gas expander, where the temperature of the gas during startup may drop to below 100°C. The present invention reduces this risk. DETAILED DESCRIPTION

[0030] The present invention can be applied to newly designed nitric acid plants, but can also be applied to existing nitric acid plants comprising a three-stage abatement system, wherein the system comprises a first catalytic bed and a second catalytic bed arranged in series, wherein one catalytic bed is used to decompose N2O and the other is used to reduce NO x .

[0031] In a first embodiment, the first catalytic bed is a bed configured to remove N2O, followed by a second catalytic bed configured to remove NO x During normal operation, a reducing agent is added between the first catalyst bed and the second catalyst bed to act as a reducing agent for NO in the second catalyst bed. x During the transition event, the method includes temporarily adding a certain amount of reducing agent upstream of the first catalyst bed, so that the first catalyst bed acts as a catalyst for reducing NO in the presence of the reducing agent during the transition event. x Additional catalyst beds are in operation.

[0032] In a second embodiment, the first catalytic bed is configured to remove NO x A bed is provided, followed by a second catalytic bed, which is a bed configured to remove N2O. During normal operation, a reducing agent is added to the tail gas upstream of the first catalytic bed to act as a NO in the first catalytic bed. xDuring the transition event, the method includes temporarily adding an additional amount of the reducing agent upstream of the first catalytic bed or between the first catalytic bed and the second catalytic bed, so that the second catalytic bed acts as a catalyst for reducing NO in the presence of the reducing agent during the transition event. x Additional catalyst beds are in operation.

[0033] Therefore, the catalytic bed designed and used to remove N2O during normal operation can be the first bed or the second bed in the sequence.

[0034] The following description applies to both general embodiments described above.

[0035] The catalytic bed designed for the removal of N2O comprises a catalyst suitable for decomposing N2O and also suitable for reducing NO in the presence of a suitable reducing agent. x .

[0036] The method comprises changing the operation of the treatment section during a startup or shutdown transition event by adding a reducing agent at a suitable location upstream of the catalyst bed for removing NO so that the catalyst bed temporarily serves as a catalyst for reducing NO. x When the event is over, the addition of the reducing agent is interrupted, thereby resuming the normal operation of the treatment part. In addition, the temporarily added reducing agent is preferably ammonia.

[0037] The invention is applicable to transition events when the nitric acid synthesis process deviates significantly from normal operating conditions. Particularly preferably, the invention is applicable to start-up events.

[0038] According to a particularly preferred embodiment, during the transition event, ammonia is added to obtain a target NH3 / NO2 in the tail gas. x Molar Ratio. According to various embodiments, the target ratio may be about 1.0 or greater than 1.0 or less than 1.0.

[0039] Preferably, during a transition event, the NO in the tail gas passing through the first catalytic bed x The NO content in the tail gas passing through the second catalyst bed is reduced by at least 80%. x The content can be further reduced by at least 80%. The reduction percentage in the second catalytic bed refers to the NO contained in the gas at the inlet of the second catalytic bed. x .

[0040] During the transition event, the catalytic bed designed for N2O removal is primarily used to remove NO x However, depending on the operating conditions, the catalytic bed may still decompose some N2O.

[0041] In a preferred application, during the transition event, the catalytic bed originally designed for N2O removal is operated at a temperature not higher than 250°C, preferably at a temperature between 180°C and 250°C.

[0042] In a more preferred application, during a transient event, it is used to remove NO x The catalyst bed is operated at a temperature of 180°C to 250°C, preferably at a temperature of 180°C to 220°C.

[0043] The first catalytic bed and the second catalytic bed may contain the same catalyst or different catalysts. Preferably, in the catalytic bed for removing N2O, an iron-loaded zeolite catalyst is used, and more preferably a ferriesite catalyst Fe-FER is used. x In the catalyst bed, a zeolite catalyst may be used. The zeolite catalyst preferably comprises a metal and / or a metal oxide, the metal comprising copper, iron, vanadium, molybdenum, tungsten or a mixture thereof.

[0044] The method is applicable during any kind of transition event, in particular during startup or shutdown. A person skilled in the art is able to distinguish normal operation of a device from startup or shutdown or other transition events.

[0045] Typically during startup, the exhaust gas temperature has not yet reached its final operating temperature, whereas during normal operation, the exhaust gas has reached its operating temperature of about 300°C to about 600°C, after which, except for typical fluctuations, the exhaust gas temperature no longer changes. Startup of the unit precedes normal operation of the unit, and normal operation follows startup.

[0046] Transition events can be identified by one or more of the following parameters: NO in the incoming exhaust gas x Content, such as NO at the inlet of the first catalyst bed x content; exhaust gas temperature detected at the outlet of the catalyst bed for removing N2O; detected or estimated temperature of the catalyst bed for removing N2O; flow rate of exhaust gas. All of these parameters deviate from normal values ​​during the transition event, so the beginning and end of the transition event can be identified based on this deviation.

[0047] The temperature of the catalyst bed can be estimated based on the inlet and outlet temperatures of the tail gas.

[0048] The exhaust gas temperature at the outlet of the catalytic bed for removing N2O is particularly preferred as a parameter for distinguishing transition events from normal operation. A minimum value can be defined, for example 250°C, below which the decomposition of N2O in the catalytic bed is greatly reduced or negligible, which makes it possible to use the catalytic bed according to the invention for the reduction of NO x Attractive. In addition, the temperature can be easily and practically detected.

[0049] A system suitable for implementing the method of the present invention may, for example, identify the end of the startup event by one or more of the following: NO in the tail gas at the inlet of the first catalytic bed; x The content drops below the target value; the temperature of the exhaust gas leaving the catalytic bed for removing N2O becomes equal to or greater than the minimum target value; the temperature of the catalytic bed for removing N2O becomes equal to or greater than the minimum target value.

[0050] In some embodiments, the temperature of the catalytic bed refers to the temperature of the catalyst in the bed, which may be an average temperature.

[0051] In a preferred embodiment, a transition event, particularly a start-up event, can be identified by one or more of the following conditions: the exhaust gas contains more than 600 ppm of NO x ; The tail gas temperature at the outlet of the catalyst bed for removing N2O is not higher than 250°C, especially between 180°C and 230°C; the flow rate of the tail gas does not exceed 80% or 70% of the nominal flow rate.

[0052] Thus, in the above example, when the input tail gas (at the inlet of the first bed) is detected to contain less than 600 ppm NO x The startup event can be considered to be over when the temperature of the tail gas leaving the bed for N2O removal reaches at least 250°C, and / or the flow rate of the tail gas reaches at least 70% of the design flow rate.

[0053] In a preferred application of the present invention, NO in the tail gas extracted from the absorption tower is x The content of 600 ppm to 1500 ppm, more preferably 600 ppm to 800 ppm, is used to identify the start.

[0054] The symbol ppm stands for parts per million by volume.

[0055] According to various embodiments, during the transition event, most or all of the amount of reductant added to the catalytic bed may be added upstream of the first catalytic bed. x When the first bed is the second bed, the addition of reductant after the first bed and upstream of the second bed may be reduced or interrupted during the transition event and restored to the operating value when the transition event (eg, startup) is complete.

[0056] When ammonia is used as the reducing agent, adding most or all of the ammonia before the first catalytic bed is advantageous to reduce the risk of ammonia slip under adverse start-up conditions, when low temperatures increase the risk of ammonium nitrate formation. In one embodiment, the amount usually introduced between the catalytic beds is also temporarily introduced before the first catalytic bed.

[0057] The present invention can be applied to the modification of existing devices. In this case, the method of modification may include providing a mixer, which is arranged to mix the exhaust gas with the reducing agent during the transition event. The method may also include providing a control system suitable for performing the method of the present invention as described above.

[0058] The above treatment section is part of a three-stage abatement system of a nitric acid plant. The nitric acid plant can be a single pressure plant or a dual pressure plant. Dual pressure plants known in the prior art are plants that perform absorption at a higher pressure than ammonia oxidation.

[0059] Another aspect of the invention is a method for starting a nitric acid synthesis process, wherein:

[0060] The nitric acid synthesis method comprises the following steps: catalytically oxidizing ammonia to obtain a gas containing nitrogen oxides, and absorbing the gas containing nitrogen oxides in water to produce nitric acid, a gas containing NO x and N2O exhaust.

[0061] The nitric acid method also involves treating the tail gas to remove N2O and NO before it is released to the atmosphere. x The tail gas treatment comprises passing the tail gas through a first catalytic bed and a second catalytic bed, wherein one of the two beds is suitable for removing N2O and the other is suitable for removing NO x , and the exhaust gas passes through to remove NO x A reducing agent is added to the exhaust gas before the catalyst bed.

[0062] The startup method includes temporarily adding an additional amount of a reducing agent to the exhaust gas at a suitable location so that the catalytic bed configured to remove N2O during normal operation temporarily provides NO in the presence of the reducing agent. x The bed comprises a catalyst suitable for decomposing N2O and also suitable for reducing NO in the presence of the reducing agent. x catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 According to the prior art, N2O and NO in a nitric acid plant under normal operation x Schematic diagram of the abatement system.

[0064] Figure 2 The present invention is shown during a transition such as a startup event or a shutdown event. Figure 1 NO x Emission reduction system.

[0065] Figure 3 It shows the N2O and NO x Another embodiment of an abatement system.

[0066] Figure 4 The present invention shows the operation during the transition period. Figure 3 system.

[0067] Figure 1 The emission reduction system comprises a first catalyst bed 2 and a second catalyst bed 3 arranged in series, wherein the first catalyst bed 2 comprises a catalyst suitable for decomposing N2O, and the second catalyst bed 3 comprises a catalyst suitable for decomposing NO x Ammonia 6 is supplied to the second catalyst bed 3 as NO x of reducing agent.

[0068] The abatement system basically works as follows: x The tail gas 1 containing N2O, which is the effluent of the absorption tower of the nitric acid plant (not shown in the figure), is supplied to the first catalytic bed 2, where the N2O is decomposed in the temperature range of 300° C. to 600° C. Typically, during normal operation of the plant, a 98% N2O reduction efficiency can be achieved in the first catalytic bed 2. Note that no reducing agent is injected into the first catalytic bed.

[0069] Subsequently, the effluent of the first catalytic bed 2 is mixed with ammonia 6 and then supplied to the second catalytic bed 3, wherein nitrogen oxides NO x In the second catalytic bed, at least 95% of NO can be achieved during normal operation of the device. x Emission reduction efficiency. The effluent of the second catalytic bed 3 is a purified gas 7 which can be discharged into the atmosphere. Typically, the gas 7 is expanded in a tail gas expander to recover energy before discharge.

[0070] Known Figure 1 This abatement system performs well during normal operation of the plant. However, during transient events, such as during startup, the temperature of the tail gas 1 and therefore the temperatures of the two catalytic beds 2 and 3 are too low to maintain sufficient decomposition of N2O and NO x full restoration.

[0071] Figure 2 Involving transition events, such as when the tail gas temperature extracted from the absorber is below 300°C or even below 250°C, and with the generated NO x Compared with the nitric acid method, the formation of N2O is negligible.

[0072] During the transition event, NO x The tail gas 1 containing N2O is temporarily mixed with ammonia 4 to achieve a suitable molar ratio in the gas, such as NH3 / NO x The ratio is greater than 1.0. The gas to which ammonia 4 is added is fed to the first catalyst bed 2, which is temporarily used to reduce NO xThe extra bed is Figure 2 The first bed 2 is marked as "NO removal x ” to emphasize that since ammonia is added upstream, it essentially acts as a nitrogen oxide NO x The bed is running.

[0073] The tail gas effluent of the first catalytic bed 2 may still be mixed with the ammonia stream 6 before entering the second catalytic bed 3. In some embodiments, stream 6 may be temporarily reduced or shut off during a transition event.

[0074] The effluent from the second catalytic bed is a purified gas which can be discharged into the atmosphere without the risk of forming a smoke plume.

[0075] At the end of the transition event, for example when startup is complete, the temporary flow 4 is interrupted, allowing the first catalytic bed 2 to resume normal operation.

[0076] exist Figure 3 In the embodiment of the present invention, the positions of the catalyst beds 2 and 3 are Figure 1 In contrast, i.e. under normal conditions, the first bed 2 reduces NO in the presence of ammonia. x , the second bed 3 removes N2O. In this case, ammonia is usually added before the first bed, and during the transition event, additional amounts of ammonia are added upstream of the first bed 2 and / or at the same location between the two beds, such as Figure 4 shown.

Claims

1. A method for reducing NO in a nitric acid plant during a transient event of startup or shutdown. x The method of emission, where: The nitric acid plant comprises a synthesis section and an absorption section, wherein in the synthesis section ammonia is catalytically oxidized to obtain a nitrogen oxide-containing gas, and wherein in the absorption section the nitrogen oxide-containing gas is absorbed into water to produce concentrated nitric acid, NO-containing x and N2O tail gas (1), The nitric acid plant comprises a device adapted to remove N2O and NO from the tail gas (1) before the tail gas (1) is discharged into the atmosphere. x The treatment part (10) comprises a first catalytic bed (2) and a second catalytic bed (3), wherein the first catalytic bed (2) and the second catalytic bed (3) are arranged to be passed through by the tail gas in sequence, wherein during normal operation of the device, one of the two beds is used to remove N2O, and the other of the two beds is used to remove NO in the presence of a reducing agent (6). x , wherein during the transition event, the exhaust gas passes through a catalytic bed for removing N2O in the presence of an additional amount of a reducing agent, which is added only during the transition event, so that the catalytic bed temporarily acts as a catalyst for removing NO from the exhaust gas x There is an extra bed available.

2. The method according to claim 1, wherein: During normal operation, the first catalyst bed is a bed for removing N2O, followed by the second catalyst bed, which is a bed for removing NO x bed; During normal operation, a reducing agent (6) is added between the first catalyst bed (2) and the second catalyst bed (3) to act as a reducing agent for NO in the second catalyst bed (3). x and no reducing agent is added to the tail gas upstream of the first catalytic bed (2), During the transition event, the method comprises temporarily adding a certain amount of reducing agent (4) upstream of the first catalytic bed (2), so that the first catalytic bed (2) acts as a catalyst for reducing NO in the presence of the reducing agent during the transition event. x Additional catalyst beds are in operation.

3. The method according to claim 1, wherein: During normal operation, the first catalyst bed is used to remove NO x A bed, followed by the second catalytic bed, the second catalytic bed is a bed for removing N2O; During normal operation, a reducing agent (6) is added to the tail gas upstream of the first catalyst bed (2) to act as a reducing agent for NO in the first catalyst bed. x Reducing agent; During a transition event, the method comprises temporarily adding an additional amount of a reducing agent (4) upstream of the first catalytic bed or between the first catalytic bed (2) and the second catalytic bed (3), so that the second catalytic bed (3) acts as a catalyst for reducing NO in the presence of the reducing agent during the transition event. x Additional catalyst beds are in operation.

4. The method according to any one of the preceding claims, further comprising, when the transient event is over, interrupting the introduction of the additional amount of reducing agent (4), thereby resuming normal operation of the treatment portion (10).

5. A process according to any one of the preceding claims, wherein during a transient event, the tail gas at the outlet of the absorption section contains at least 600 ppm of NO x , for example 600 ppm to 1500 ppm or 600 ppm to 800 ppm of NO x .

6. The method according to any of the preceding claims, wherein the catalytic bed (2) for removing N2O during normal operation is operated at a temperature lower than or equal to 250°C, preferably at a temperature between 180°C and 250°C during a transient event.

7. A method according to any one of the preceding claims, wherein during the transition event the flow rate of the tail gas is 80% or less of the flow rate under normal operation.

8. A method according to any one of the preceding claims, wherein during normal operation the x The catalytic bed (3) is operated at a temperature of 180°C to 250°C, preferably 180°C to 220°C during the transition event.

9. The method according to any one of the preceding claims, wherein the end of the transition event is identified by one or more of the following: NO in the tail gas at the inlet of the first catalytic bed; x The content drops below the target value; the temperature of the exhaust gas leaving the catalytic bed usually used for removing N2O becomes equal to or greater than the minimum target value; the temperature of the catalytic bed usually used for removing N2O becomes equal to or greater than the minimum target value.

10. A method according to any one of the preceding claims, wherein the end of a start-up event is identified by one or more of the following: the input tail gas at the inlet of the first catalytic bed contains less than 600 ppm of NO x ; The tail gas leaving the catalytic bed usually used to remove N2O reaches a temperature of at least 250°C; the flow rate of the tail gas reaches at least 70% of the design flow rate.

11. A method according to any one of the preceding claims, wherein during normal operation the first bed is a bed for removing N2O and the second bed is a bed for removing NO x bed; during the transition event, the amount of reducing agent (6) added between the first catalytic bed (2) and the second catalytic bed (3) is reduced or zero compared to the amount added during normal operation.

12. The method according to any one of the preceding claims, wherein the reducing agent is ammonia and the reducing agent temporarily added during the transition event is also ammonia.

13. The method according to any one of the preceding claims, wherein the catalytic bed (2) typically used for removing N2O comprises an iron-supported zeolite catalyst, preferably a ferrierite catalyst Fe-FER.

14. A method according to any one of the preceding claims, wherein the x The catalytic bed (3) comprises a zeolite catalyst, which preferably comprises a metal and / or a metal oxide, wherein the metal comprises copper, iron, vanadium, molybdenum, tungsten or a mixture thereof.

15. A method according to any one of the preceding claims, wherein during normal operation the first bed is a bed for removing N2O and the second bed is a bed for reducing NO x wherein during the transition event, ammonia (4) is added upstream of the first catalytic bed (2), and the amount of ammonia added is determined to obtain a target NH3 / NO in the tail gas (1) at the inlet of the first catalytic bed x Molar ratio.

16. The method according to any of the preceding claims, wherein the treatment section (10) is part of a three-stage abatement system of a nitric acid plant and the plant is a single-pressure plant or a dual-pressure plant.

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