Process for removing nitric oxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream

Abstract

The present invention relates to a method for removing nitric oxide, nitrogen dioxide, and nitrous oxide from an exhaust gas stream obtained during nitration, the method comprising: (a) removing nitric oxide and nitrogen dioxide from the exhaust gas stream by adding an oxygen-containing gas to the exhaust gas stream to oxidize the nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-deficient gas stream; (b) washing the nitric oxide-deficient gas stream with water to obtain a nitric oxide- and nitrogen dioxide-deficient gas stream; and (c) converting the nitrous oxide in the nitric oxide- and nitrogen dioxide-deficient gas stream into nitrogen and carbon dioxide, and optionally oxygen, in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide.

Description

[0001] This invention relates to a method for removing nitric oxide, nitrogen dioxide, and nitrous oxide from an exhaust gas stream obtained during nitration, the method comprising:

[0002] (a) Remove nitric oxide and nitrogen dioxide from the exhaust gas stream by adding oxygen-containing gas to the exhaust gas stream to oxidize the nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream;

[0003] (b) Wash the nitrogen monoxide-deficient gas stream with water to obtain a nitrogen monoxide- and nitrogen dioxide-deficient gas stream;

[0004] (c) Convert nitrous oxide in a gas stream that is deficient in nitric oxide and nitrogen dioxide into nitrogen, carbon dioxide and, optionally, oxygen.

[0005] Exhaust gases from nitrification processes typically contain harmful nitric oxide and nitrogen dioxide, nitrous oxide (a greenhouse gas), and at least trace amounts of hydrogen cyanide. Additionally, the exhaust gases may contain carbon monoxide, which is also harmful. Due to the harmful properties of these gases, it is necessary to reduce or preferably completely remove them from the exhaust gas stream.

[0006] Currently, the exhaust gas from the nitration process is treated, for example, by thermal oxidation (burning the exhaust gas with a high-heat substance such as natural gas) or by catalytic oxidation (specifically for removing carbon monoxide).

[0007] Several elimination techniques are known to reduce nitrous oxide in exhaust gases. Elimination of nitrous oxide from diacid production processes is preferred. Typically, nitrous oxide is removed from the gas stream by thermal decomposition or catalytic decomposition. Catalytic decomposition is usually carried out in the presence of a suitable nitrous oxide decomposition catalyst at a temperature between 300°C and 1000°C, while thermal decomposition is carried out at a temperature above 1000°C. Nitrous oxide is decomposed into nitrogen and oxygen through thermal decomposition and non-reductive catalytic decomposition. In addition to non-reductive catalytic decomposition, the amount of nitrous oxide in the gas stream can also be reduced by reductive catalytic decomposition. For this purpose, nitrous oxide is reacted with methane, for example, to form nitrogen, carbon dioxide, and water. Non-reductive catalytic decomposition is typically carried out at a temperature between 430°C and 1000°C, while reductive catalytic decomposition is carried out at a temperature between 300°C and 600°C.

[0008] Methods for removing nitric oxide, nitrogen dioxide and / or nitrous oxide from a gas stream are disclosed, for example, in DE-A10 2010 048 040, EP-A 1 022 047, EP-A 0 514 739, WO-A 02 / 072244, WO-A 03 / 084646 or WO-A 2013 / 118064.

[0009] The zeolite catalyst is used to eliminate nitrous oxide according to the methods disclosed in WO-A 02 / 072244, WO-A 03 / 084646 or WO-A 2013 / 118064.

[0010] Both DE-A 10 2010 048 040 and EP-A 1 022 047 describe the thermal decomposition of nitrous oxide, which produces oxygen and nitrogen. To remove nitrous oxide and nitrogen dioxide from the gas stream that may be generated according to the method in DE-A 10 2010 048 040, a reducing agent can be added to the method to selectively reduce nitrous oxide and nitrogen dioxide.

[0011] EP-A 0 514 739 describes a method for removing nitric oxide and nitrogen dioxide from a gas stream obtained through combustion. To create a reducing environment, steam is introduced in the first step. In the second step, oxygen is added to convert all carbonaceous matter into carbon dioxide. In the SCR unit, residual nitric oxide and nitrogen dioxide are converted into nitrogen and oxygen.

[0012] Methods involving gas streams containing nitrogen oxides include, for example, nitration, such as the production of dinitrotoluene or adipic acid. Methods for producing dinitrotoluene are described, for example, in WO-A 2015 / 059185, WO-A 2016 / 005070, WO-A 2016 / 050759, WO-A 2011 / 082977 or US 5,963,878.

[0013] Specifically, WO-A 2016 / 050759, WO-A 2011 / 082977, and US 5,963,878 also relate to the treatment of tail gases containing nitric oxide and nitrogen dioxide generated during the process. In the methods described in WO-A 2011 / 082977 and US 5,963,878, the waste gas containing nitric oxide and nitrogen dioxide is removed from the process, and the process for producing nitric acid is carried out by absorbing nitric oxide and nitrogen dioxide in water. According to the method of WO-A 2016 / 050759, the waste gas containing nitric oxide and nitrogen dioxide is incinerated. WO-A 2016 / 005070 only mentions that the waste gas can be treated in a scrubbing unit and a subsequent thermal waste gas treatment plant, or only in a thermal waste gas treatment plant.

[0014] R.A. Reimer et al., “Adipic Acid Industry - N2O Abatement, Implementation of Technologies for Abatement of N2O Emissions Associated with Adipic Acid Manufacture,” J. Van Ham et al. (eds.), Non-CO2 Greenhouse Gases: Scientific Understanding, Control and Implementation, pp. 347–358, Kluwer Academic Publishers, 2000; or A. Shimizu et al., “Abatement technologies for N2O emissions in the adipic acid industry,” Chemosphere - Global Change Science 2 (2000), pp. 425–434, describe methods for removing nitrous oxide from exhaust gases obtained in the production of adipic acid.

[0015] Methods for oxidizing carbon monoxide from an airflow to carbon dioxide are disclosed, for example, in WO-A 2006 / 098914, WO-A 2014 / 138397, EP-B 1558367, US-A 2020 / 0368727, WO-A 2010 / 077843 or JP-A2012-126616.

[0016] Typically, the gas mixtures used in these methods also contain reducible nitrogen (NO). x and / or carbon monoxide, wherein the catalyst is used for NO x It purifies and oxidizes carbon monoxide into carbon dioxide.

[0017] Catalytic decomposition of hydrogen cyanide via hydrolysis or oxidation is described in, for example, Zhongxian Song et al., “Catalytic hydrolysis of HCN on ZSM-5 modified by Fe or Nb for HCN removal: surfacespecies and performance, RSC Advances, 2016, 6, pp. 111389-111397”; O. Kröcher et al., “Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts”, Applied Catalysis B: Environmental 92 (2009), pp. 75-89; Ning Liu et al., “Selective catalytic combustion of hydrogen cyanide over metal modified zeolitic catalysts: From experiment to theory, Catalysis Today 297 (2017), pp. 201-210”; or Irene OYLiu et al., “The formation and reactions of hydrogen cyanide during isobutane-SCR over Fe-MFI catalysts”, Catalysis Surveys. From Asia, Volume 7, Issue 4, December 2003, pp. 191-202. However, these articles specifically address the decomposition of hydrocyanic acid and further demonstrate that the decomposition can be improved by reacting it with nitrogen dioxide.

[0018] On the other hand, it is known from the following literature that hydrogen cyanide poisons catalysts: Thomas N. Mashapa et al., “Catalytic Performance and Deactivation of Precipitated Iron Catalyst for Selective Oxidation of Hydrogen sulfide to Elemental Sulfur in the Waste GasStreams from Coal Gasification”, Ind. Eng. Chem. Res 2007, 46, pp. 6338-6344. An additional problem may be the polymerization of hydrogen cyanide, and the resulting polymers may clog the pores in the catalyst. For these reasons, nitration processes are currently generally carried out in a manner that minimizes the formation of hydrogen cyanide.

[0019] The object of this invention is to provide a method for removing nitric oxide, nitrogen dioxide and nitrous oxide from the exhaust gas stream obtained during nitration with minimal supply of reducing agent, heated gas or electrical energy.

[0020] This objective is achieved by a method for removing nitric oxide, nitrogen dioxide, and nitrous oxide from the exhaust gas stream obtained during nitrification, the method comprising:

[0021] (a) Remove nitric oxide and nitrogen dioxide from the exhaust gas stream by adding oxygen-containing gas to the exhaust gas stream to oxidize the nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream;

[0022] (b) Wash the nitrogen monoxide-deficient gas stream with water to obtain a nitrogen monoxide- and nitrogen dioxide-deficient gas stream;

[0023] (c) In the presence of a nitrous oxide reduction catalyst and hydrogen cyanide, nitrous oxide in a gas stream leaning towards nitrous oxide and nitrogen dioxide is converted into nitrogen and carbon dioxide, and optionally oxygen, to obtain a purified gas stream.

[0024] The term "nitrogen oxides" refers to nitric oxide, nitrogen dioxide, and nitrous oxide, and, where appropriate, also to other oxides of nitrogen, such as, for example, N2O3, N2O4, and N2O5.

[0025] Steps (a) and (b) also include removing nitric oxide formed during airflow scrubbing by absorption of nitrogen dioxide in water and reaction of nitrogen dioxide with water.

[0026] Due to the absorption of nitrogen dioxide in water, a nitric acid solution is also formed in step (b) in addition to the gas streams of nitrogen monoxide and nitrogen dioxide. The nitric acid content in the solution is in the range of 20% to 62% by weight, and preferably in the range of 40% to 60% by weight.

[0027] The gas stream fed into a method for removing nitric oxide, nitrogen dioxide, nitrous oxide, and carbon monoxide can be obtained in any nitration process, such as the production of organic nitro compounds, for example, the nitration of benzene, toluene, xylene, phenol, benzoic acid, monochlorobenzene or polychlorobenzene, monobromobenzene or polybromobenzene, imidazole, and 5-ethyl-2-methyl-pyridine. Nitration can be carried out as mononitration, dinitration, or trinitration. Preferably, the gas stream originates from mononitration or dinitration, and particularly from the production of dinitrotoluene.

[0028] The feed gas stream used in methods for removing nitric oxide, nitrogen dioxide, and nitrous oxide can be pretreated by washing with acidic water, preferably a mixture of water and nitric acid or water and sulfuric acid.

[0029] In order to purify the gas stream, in stages (a) and (b), nitric oxide and nitrogen dioxide are removed from the gas stream by adding oxygen-containing gas to oxidize the nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-deficient gas stream, and washing the nitric oxide-deficient gas stream with water to obtain a gas stream deficient in both nitric oxide and nitrogen dioxide.

[0030] To remove nitric oxide and nitrogen dioxide from the gas stream, any method known to those skilled in the art for removing nitric oxide and nitrogen dioxide may be used. Preferably, in the first step, nitric oxide is reacted with oxygen in an oxygen-containing gas to form nitrogen dioxide, and the resulting nitrogen-depleted gas stream is then subjected to a scrubbing stage in which nitrogen dioxide in the gas stream is absorbed in a suitable scrubbing liquid (e.g., water).

[0031] The oxygen-containing gas can be any gas mixture containing oxygen or pure oxygen. Preferably, the oxygen-containing gas is air or oxygen-enriched air. If a gas mixture other than air is used, a mixture containing oxygen and an inert gas (such as nitrogen or a rare gas) is preferred. However, it is particularly preferred that the oxygen-containing gas is air.

[0032] To absorb nitrogen dioxide, it is preferable to feed a nitrogen monoxide-lean gas stream into the scrubbing tower. If water is used as the scrubbing liquid, nitric oxide and nitric acid are formed during scrubbing. Nitric oxide is usually re-oxidized and can be absorbed by the scrubbing liquid.

[0033] The scrubbing tower for absorbing nitrogen dioxide can be a plate tower or a packed tower. A plate tower is preferred. In a plate tower, the plates are preferably cooled, for example, by providing cooling coils on the plates. For cooling, a cooling medium, particularly water, flows through the cooling coils. The number of plates in a plate tower is preferably in the range of 2 to 50, and more preferably in the range of 3 to 30. The plates used for absorbing nitrogen dioxide from the scrubbing liquid can be any plates known to those skilled in the art. Suitable plates are, for example, sieve plates, porous plates, valve trays, or bubble cap trays.

[0034] The scrubbing tower typically operates at pressures ranging from ambient pressure to 10 bar (absolute pressure), preferably from 3 bar (absolute pressure) to 8 bar (absolute pressure). The temperature within the scrubbing tower is preferably in the range of 5°C to 45°C, and more preferably in the range of 10°C to 30°C.

[0035] Depending on the method from which the gas stream to be purified originates, the nitric oxide- and nitrogen dioxide-lean gas stream obtained in the scrubbing tower typically contains nitrogen, oxygen, carbon dioxide, carbon monoxide, nitric oxide, nitrogen dioxide, and nitrous oxide. If air is used to oxidize nitric oxide, the nitric oxide- and nitrogen dioxide-lean gas stream may also contain rare gases from the air, primarily argon. Furthermore, particularly if the gas stream to be purified originates from a nitration process, such as from the production of dinitrotoluene, the nitric oxide- and nitrogen dioxide-lean gas stream may also contain trace amounts of sulfur dioxide, as well as mononitromethane, dinitromethane, and trinitromethane. Additionally, the nitric oxide- and nitrogen dioxide-lean gas stream may contain non-methane hydrocarbons.

[0036] Typically, the oxygen content in a nitrogen monoxide- and nitrogen dioxide-deficient gas stream is in the range of 5 vol% to 18 vol%, preferably 6 vol% to 13 vol%, the carbon monoxide content is in the range of 0.5 vol% to 7 vol%, preferably between 1 vol% and 5 vol%, the carbon dioxide content is in the range of 2 vol% to 10 vol%, preferably between 3 vol% and 7 vol%, the nitrous oxide content is in the range of 0.2 vol% to 4 vol%, preferably between 0.3 vol% and 2.5 vol%, the nitrogen content is in the range of 50 vol% to 90 vol%, preferably between 60 vol% and 80 vol%, and the nitrogen monoxide and nitrogen dioxide content is in the range of 40 vol ppm to 800 vol ppm, preferably between 80 vol ppm and 400 vol ppm. If air or oxygen-enriched air is used as the oxygen-containing gas, the amount of argon in the nitrogen monoxide- and nitrogen dioxide-lean gas stream is typically in the range of 0.5 vol% to 0.95 vol%, particularly in the range of 0.7 vol% to 0.94 vol%. If the nitrogen monoxide- and nitrogen dioxide-lean gas stream contains non-methane hydrocarbons, the amount of non-methane hydrocarbons is preferably in the range of 50 vol ppm to 600 vol ppm, more preferably in the range of 100 vol ppm to 500 vol ppm.

[0037] After removing nitric oxide and nitrogen dioxide, a gas stream leaning from nitric oxide and nitrogen dioxide is fed into step (c) to convert nitrous oxide into nitrogen and carbon dioxide, and optionally oxygen.

[0038] The conversion of nitrous oxide into nitrogen, carbon dioxide, and optionally oxygen is carried out in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide.

[0039] Surprisingly, it has been shown that the conversion of nitrous oxide to nitrogen and carbon dioxide, and optionally oxygen, is faster if hydrogen cyanide is present in the reaction mixture. This is assumed to be based on the following equation:

[0040] 5 N2O + 2 HCN → 6 N2 + 2 CO2 + H2O.

[0041] Therefore, on the one hand, hydrogen cyanide supports the decomposition reaction of nitrous oxide and reduces the need for other reducing agents, and on the other hand, hydrogen cyanide emissions are reduced simultaneously.

[0042] Based on the total volume of the exhaust gas stream, the amount of hydrogen cyanide in the exhaust gas stream is preferably in the range of 0.5 volume ppm to 100 volume ppm. More preferably, the amount of hydrogen cyanide in the exhaust gas stream is in the range of 1 volume ppm to 30 volume ppm, and particularly in the range of 2 volume ppm to 15 volume ppm, each based on the total volume of the exhaust gas stream.

[0043] Hydrogen cyanide is typically a byproduct produced during the nitration process. However, if the amount of hydrogen cyanide in the exhaust stream obtained during nitration is insufficient, additional reducing agents may be needed to reduce nitrous oxide.

[0044] When a gas stream leaning towards nitric oxide and nitrogen dioxide is treated in the presence of a nitrous oxide reduction catalyst and hydrogen cyanide, the amount of oxygen optionally produced depends on the temperature and / or the molar stoichiometric ratio of the available reducing agent to nitrous oxide. Higher temperatures and / or lower molar stoichiometric ratios of the available reducing agent to nitrous oxide generally produce more oxygen. Typically, nitrous oxide in a gas stream leaning towards nitric oxide and nitrogen dioxide is converted into nitrogen, carbon dioxide, and oxygen.

[0045] The nitrous oxide reduction catalyst can be a catalyst bed or a monolithic catalyst, with a catalyst bed being preferred. The particles used in the catalyst bed are preferably solid cylinders, hollow cylinders, or chains. Preferably, the particles used in the catalyst bed are star-shaped chains. The chains used in the catalyst bed preferably have an outer diameter of 1.5 mm to 10 mm, more preferably 2 mm to 6 mm, and a length of 3 mm to 20 mm, more preferably 4 mm to 10 mm.

[0046] The catalyst can be any commercially available catalyst suitable for the decomposition of nitrous oxide, such as a catalyst containing copper oxide and / or zinc oxide as the catalytically active material on a support made of silica and / or alumina. Preferably, the catalyst is a zeolite catalyst, particularly Fe-β zeolite, such as ZSM5 or BEA type Fe-β zeolite, preferably BEA. Particularly preferably, the catalyst is a template-free catalyst prepared as described in EP-B 2 812 283. Using ZSM5 or BEA type, especially BEA type, Fe-β zeolite has the added advantage that these catalysts are surprisingly not poisoned by hydrogen cyanide in the gas stream.

[0047] Other suitable catalysts are, for example, Fe / Cu-OFF-ERI zeolites, as described, for example, in CN-A 113198525. The catalyst described in CN-A 113198525 (Mg...) 0.025 Ce 0.05 Co 0.925 Co₂O₄-Fe₁-Cu₄-OFF-ERI is a complex of three individual compounds and contains approximately 20% by weight of (Mg) 0.025 Ce 0.05 Co 0.925Co2O4 spinel. 35 wt% Fe and Cu exchange OFF-ERI zeolite, and bonded with about 45 wt% Al / Si mixed metal oxide, which is presumably also OFF-ERI zeolite.

[0048] The decomposition of nitrous oxide in the presence of a catalyst is carried out at temperatures ranging from 350°C to 600°C, preferably from 420°C to 560°C, and at pressures ranging from 800 mbar (absolute pressure) to 10 bar (absolute pressure), preferably from 900 mbar (absolute pressure) to 8 bar (absolute pressure), and particularly at pressures ranging from 1000 mbar (absolute pressure) to 1200 mbar (absolute pressure). Particularly preferably, the decomposition of nitrous oxide is carried out at an overpressure ranging from 5 mbar to 300 mbar relative to atmosphere.

[0049] The gas hourly space velocity (GHSV) used for decomposing nitrous oxide can reach 1000 standard m. 3 / (m 3 Catalyst (h) up to 40000 standard m 3 / (m 3 Within the range of catalyst (h), preferably within 2000 standard m 3 / (m 3 Catalyst (h) up to 30000 standard m 3 / (m 3 Within the range of catalyst (h), and particularly within 4000 standard m 3 / (m 3 Catalyst (h) to 10000 standard m 3 / (m 3 Within the range of catalyst (h).

[0050] If the nitrogen monoxide- and nitrogen dioxide-lean gas stream contains carbon monoxide, at least a portion of the carbon monoxide reacts with nitrous oxide to form carbon dioxide and nitrogen. Depending on the amount of carbon monoxide and nitrous oxide in the nitrogen monoxide- and nitrogen dioxide-lean gas stream and the reaction conditions, particularly GHSV, all the carbon monoxide in the gas stream may react with nitrous oxide, or only a portion of the carbon monoxide may react with nitrous oxide.

[0051] Typically, gas streams leaning towards nitric oxide and nitrogen dioxide contain carbon monoxide in molar excess of nitrous oxide.

[0052] The two reactions—the conversion of nitrous oxide with carbon monoxide to form carbon dioxide and nitrogen, and the decomposition of nitrous oxide to form nitrogen and oxygen—are typically carried out in the same reactor under the same conditions. For this reason, if the gas stream leaning towards nitrous oxide and nitrogen dioxide contains carbon monoxide, then in the reaction of stage (c), a portion of the nitrous oxide reacts with carbon monoxide to form carbon dioxide and nitrogen, while simultaneously the nitrous oxide decomposes into nitrogen and oxygen.

[0053] If the amount of carbon monoxide is such that not all of it reacts with nitrous oxide, the purified gas stream obtained in stage (c) will still contain carbon monoxide. To remove the remaining carbon monoxide, the gas stream is subjected to a second oxidation of carbon monoxide, in which the remaining carbon monoxide is oxidized to carbon dioxide.

[0054] Typically, the exhaust gas stream also contains carbon monoxide. Since carbon monoxide is not usually oxidized in stage (a), in a preferred embodiment, at least a portion of the carbon monoxide still contained in the gas stream leaning towards nitric oxide and nitrogen dioxide is oxidized before nitrous oxide is converted into nitrogen and carbon dioxide, and optionally oxygen.

[0055] Preheating the gas stream is preferable before feeding it into the oxidation of carbon monoxide. Preferably, the gas stream is preheated by indirect heat exchange (simultaneous cooling) with a hot, purified tail gas stream. For this purpose, any suitable heat exchanger can be used, such as a tube bundle heat exchanger, a U-tube bundle heat exchanger, or a plate heat exchanger. Preferably, a tube bundle heat exchanger is used.

[0056] If preheating the lean nitric oxide and nitrogen dioxide gas stream by heat exchange with the purified exhaust gas stream is insufficient, an additional heater, such as an electric heater or a burner, such as a gas burner, can be used. Preferably, the heater used for additional heating is an electric heater. During method startup, the additional heater is further used to heat the lean nitric oxide and nitrogen dioxide gas stream to the temperature at which carbon monoxide oxidation takes place.

[0057] The temperature at which the gas stream leaning towards nitric oxide and nitrogen dioxide is heated by indirect heat exchange with the thermally purified tail gas stream and / or in an additional heater is preferably in the range of 200°C to 500°C, more preferably in the range of 220°C to 450°C, and particularly in the range of 230°C to 400°C.

[0058] In the first alternative, during the carbon monoxide oxidation stage, all carbon monoxide is oxidized from a nitrogen monoxide- and nitrogen dioxide-lean gas stream. However, alternatively and preferably, only a portion of the carbon monoxide is oxidized, such that the nitrogen monoxide- and nitrogen dioxide-lean gas stream still contains carbon monoxide after leaving the oxidation stage.

[0059] To oxidize carbon monoxide into carbon dioxide, any method known to those skilled in the art can be used.

[0060] To oxidize only a portion of the carbon monoxide contained in a gas stream leaning towards nitric oxide and nitrogen dioxide, oxidation conditions, such as oxidation temperature or GHSV, and / or catalyst volume, can be selected such that only a portion of the carbon monoxide is oxidized. Alternatively, the gas stream leaning towards nitric oxide and nitrogen dioxide can be split into a first part stream and a second part stream, and the carbon monoxide in the first part stream can be oxidized to obtain a part stream leaning towards carbon monoxide.

[0061] Regardless of whether all or only a portion of the carbon monoxide is oxidized, oxidation typically takes place in a reactor in the presence of a carbon monoxide oxidation catalyst. The reactor can be, for example, a vessel with a catalyst bed or a monolithic molded body containing a carbon monoxide oxidation catalyst. Preferably, a reactor comprising at least one monolithic molded body is used. The monolithic molded body is preferably designed as a right prism with a circular base or four or six side bases, such as a cylinder or cuboid. The monolithic molded body can be made of a catalytically active material, or it can be made of a ceramic or metallic body coated with a catalytically active material.

[0062] The monolithic molded body containing catalytically active materials can be directly installed into the container that forms the reactor, or it can be integrated into the support frame.

[0063] In the context of this invention, the term "catalyst bed" is used for fluidized beds or packed beds. In a catalyst bed, particles or packing materials of any shape can be used. The particles or packing materials can be made of catalytically active materials, or can be made of a support material (e.g., a polymer or metal) on which the catalytically active material is applied.

[0064] The carbon monoxide oxidation catalyst used in the reactor for oxidizing carbon monoxide is preferably a three-way catalyst for treating exhaust gases to simultaneously destroy carbon monoxide, hydrocarbons, and nitrogen oxides from engine combustion. In this case, during the oxidation of carbon monoxide, residual trace amounts of nitric oxide and nitrogen dioxide, still available in a lean nitric oxide and nitrogen dioxide gas stream, are also at least partially reduced. Alternatively, but less preferably, the catalyst can be a two-way catalyst or a so-called VOC catalyst for converting hydrocarbons and carbon monoxide into carbon dioxide and water by reacting with oxygen. Other catalytically active materials that can be used to oxidize carbon monoxide to form carbon dioxide can be, for example, mixed oxides such as oxides of aluminum and / or silicon and / or copper and / or magnesium.

[0065] Precious metals such as platinum, ruthenium, or palladium are also suitable as catalysts for the oxidation of carbon monoxide.

[0066] The gas hourly space velocity (GHSV) of carbon monoxide oxidation catalysts is typically around 4000 standard m.3 / (m 3 Catalyst (h) to 200,000 standard m 3 / (m 3 Within the range of catalyst (h), preferably within 8000 standard m 3 / (m 3 Catalyst (h) up to 150,000 standard m 3 / (m 3 Within the range of catalyst (h).

[0067] The oxidation of carbon monoxide to form carbon dioxide in the presence of a carbon monoxide oxidation catalyst is typically carried out at a reaction temperature in the range of 230°C to 600°C, preferably in the range of 250°C to 540°C. The pressure at which the oxidation of carbon dioxide takes place is typically in the range of 800 mbar (absolute pressure) to 10 bar (absolute pressure), preferably in the range of 900 mbar (absolute pressure) to 8 bar (absolute pressure). Particularly preferably, the oxidation of carbon monoxide is carried out at an overpressure in the range of 5 mbar to 300 mbar relative to atmospheric pressure.

[0068] If the gas streams of nitrogen monoxide and nitrogen dioxide are divided into a first part stream and a second part stream, and carbon monoxide in the first part stream is oxidized to obtain a carbon monoxide-depleted part stream, then after the carbon monoxide in the first part stream is oxidized, the carbon monoxide-depleted part stream is mixed with the second part stream to obtain a mixed stream that is carbon monoxide-depleted but still contains carbon monoxide.

[0069] The mixing of the carbon monoxide-lean partial stream and the second partial stream can be carried out in any mixing unit known to those skilled in the art for gas flow. Preferably, the carbon monoxide-lean partial stream and the second partial stream are directly combined by introducing the carbon monoxide-lean partial stream into the second partial stream or by introducing the second partial stream into the carbon monoxide-lean partial stream, or by using a static mixer. If the carbon monoxide-lean partial stream is introduced into the second partial stream or the second partial stream is introduced into the carbon monoxide-lean partial stream, a bypass of the second partial stream that bypasses the reactor can be used, for example, where the carbon monoxide in the first stream is oxidized, and the bypass can be opened to the gas line leaving the reactor or the gas line leaving the reactor can be opened to the bypass. As another alternative, a Y-connector can be used, with the bypass connected to one branch of the Y, the line leaving the reactor connected to the second branch of the Y, and the combined partial stream of the carbon monoxide-lean gas flow exiting the Y at the base.

[0070] If a static mixer is used to mix the carbon monoxide-lean partial flow and the second partial flow, any static mixer known to those skilled in the art can be used. Typically, such static mixers include inserts that redirect the flow, thereby inducing turbulence, and mixing the partial flows by turbulence.

[0071] Preferably, a static mixer is used to mix the carbon monoxide-lean partial stream and the second partial stream.

[0072] After a portion of the carbon monoxide is oxidized, or if the gas streams of lean nitric oxide and nitrogen dioxide are divided into a first part stream and a second part stream, after mixing the carbon monoxide-lean part stream and the second part stream, the molar ratio of carbon monoxide to nitrous oxide in the gas streams of lean nitric oxide and nitrogen dioxide is preferably in the range of 0.1:1 to 2:1, particularly in the range of 0.3:1 to 1.5:1.

[0073] If at least a portion of the carbon monoxide contained in the nitrogen monoxide-lean gas stream is oxidized, the resulting nitrogen monoxide-lean gas stream is fed into stage (c); or if the nitrogen monoxide-lean gas stream is split into partial streams, the mixed stream after remixing the partial streams is fed into stage (c), in which nitrous oxide is removed from the carbon monoxide-lean gas stream.

[0074] To remove nitrous oxide in stage (c), nitrous oxide is decomposed into nitrogen and oxygen and / or reacts with hydrogen cyanide to form nitrogen, carbon dioxide, and water. If the carbon monoxide-lean gas stream still contains carbon monoxide, at least a portion of the carbon monoxide still contained in the carbon monoxide-lean gas stream is reacted with nitrous oxide to form carbon dioxide and nitrogen.

[0075] If all carbon monoxide is oxidized before the conversion of nitrous oxide in step (c), or if the remaining carbon monoxide has already reacted completely with nitrous oxide, or if the gas stream obtained in (c) still contains carbon monoxide, then after oxidizing the remaining carbon monoxide, the purified gas stream obtained in (c) typically has less than 400 mg of nitric oxide and nitrogen dioxide per cubic meter of dry gas under standardized conditions, preferably less than 200 mg of nitric oxide and nitrogen dioxide per cubic meter of dry gas under standardized conditions, more preferably less than 100 mg of nitric oxide and nitrogen dioxide per cubic meter of dry gas under standardized conditions, wherein nitric oxide and nitrogen dioxide are assumed to be nitrogen dioxide, less than 400 ppm by weight, preferably less than 100 ppm by weight of carbon monoxide, and less than 2000 ppm by volume, preferably less than 1000 ppm by volume of nitrous oxide, more preferably less than 500 ppm by volume of nitrous oxide, and particularly preferably less than 100 ppm by volume of nitrous oxide.

[0076] Particularly preferably, if carbon monoxide is oxidized before feeding the gas stream of lean nitric oxide and nitrogen dioxide to step (c) for converting nitrous oxide, the method is carried out in such a way that the gas stream of lean nitric oxide and nitrogen dioxide obtained after the oxidation of carbon monoxide still contains carbon monoxide, and only a portion of the remaining carbon monoxide reacts with nitrous oxide, such that the purified gas stream obtained in stage (c) still contains carbon monoxide, and this remaining carbon monoxide is oxidized in the second oxidation step to form carbon dioxide. For the oxidation of carbon monoxide in the second oxidation step, it is preferable to use residual oxygen in the gas stream, so that no additional oxygen-containing gas needs to be added.

[0077] To initiate this method, the catalyst used for oxidizing carbon monoxide and decomposing nitrous oxide must first be brought to operating temperature. This can be achieved by passing a gaseous medium, such as air, nitrogen, or exhaust gas, through a heater, and then passing the gaseous medium through the catalyst to oxidize carbon monoxide and decompose nitrous oxide. The gaseous medium used for heating can be pressurized by a blower, causing it to flow through the heater and catalyst, or it can be obtained, for example, from the plant's operating network. For heating the gaseous medium, an electric heater can be used, for example. Alternatively, the gaseous medium can be heated by direct or indirect heat exchange with exhaust gas from natural gas combustion or by a regenerative heat exchanger operated with hot exhaust gas from the catalyst. Preferably, a combination of electric heating and regenerative heating is used. Heating can also be performed simultaneously in several stages, for example, by simultaneously performing regenerative heating and electric heating.

[0078] The catalyst can be heated in a single pass or in a cycle. If heated in a cycle, the gaseous medium that has passed through the catalyst to be heated is returned to the input side of the heater by using a suitable blower. Example

[0079] Catalytic testing was conducted in a tubular reactor with an inner diameter of 20 mm and a total length of 1300 mm, equipped with four thermocouples within a 6 mm heating sleeve. The tubular reactor was heated by an electric tubular furnace with a ceramic lining, which was in direct contact with the reactor wall to ensure good heat transfer. Fresh and spent catalysts were tested as 3 mm wire feed within the reactor's isothermal zone. The catalyst bed position was adjusted by talc inert packing before and after the catalyst bed. Performance was monitored by online FT-IR spectroscopy of the reactor exhaust gas. Prior to each catalytic measurement, the feed gas concentration was analyzed by IR spectroscopy and used as the basis for calculating the N2O conversion.

[0080] Example 1 (Catalyst Activation Procedure) :

[0081] Both fresh and spent catalysts are activated prior to each catalyst test to ensure comparable catalytic activity. Generally, the activation procedure described here is not required for the catalyst to catalyze the reaction under investigation; however, activation is still chosen nonetheless. Before each experiment, both fresh and spent catalysts are activated using the following procedure:

[0082] 250 NL / h N2, 150 NL / h air, and 0.25 g / h H2O were metered into the catalyst bed while the reactor was heated from 325 °C to 500 °C in 25 °C / 2 h increments. This procedure prepared the zeolite pores for subsequent reactions and ensured that the initial performance remained stable without increase or decrease during measurement. After the activation cycle, the furnace was cooled to 325 °C and catalytic testing began.

[0083] Example 2 :

[0084] Following the activation procedure according to Example 1, 11.5 g of fresh catalyst was tested at 4 bar at a step of 25 °C / 2 h from 325 °C to 375 °C, corresponding to a catalyst bed volume of 20 mL, with a total volumetric flow rate of 400 NL / h (GHSV: 20000 h). -1 A metered gas stream containing 7.9 vol% O2, 750 ppm N2O, 1500 ppm CO, 750 ppm H2O, and the balance N2 was added. CO used in this experiment was taken from a gas cylinder containing 4 vol% CO in N2. O2 used in this experiment was taken from pressurized air. H2O was added as a liquid via metered addition through the evaporator. N2O and N2 were added as pure gases. Based on online FT-IR measurements of the reactor exhaust gas, the N2O conversion X(N2O) at 325 °C was determined to be 16%.

[0085] Example 3 :

[0086] Following the tests described in Example 2, catalytic testing was performed on the same catalyst bed as in Example 2. This test was identical to that described in Example 2, but with an additional co-dosing of 200 ppm HCN, and the test was conducted at temperatures ranging from 325°C to 500°C. The HCN used in this experiment was obtained from a gas cylinder containing 5000 ppm N2. Based on online FT-IR measurements of the reactor exhaust gas, the N2O conversion X(N2O) at 325°C was determined to be 24.4% without HCN and 67.4% with co-dosing HCN, as the N2O concentration was below the detection limit of the FT-IR spectrometer.

[0087] Example 4 :

[0088] Following the tests described in Example 3, catalytic testing was performed on the same catalyst bed as in Example 3. This test was identical to that described in Example 2, but conducted at temperatures ranging from 325°C to 500°C. Based on online FT-IR measurements of the reactor exhaust gas, the N2O conversion X(N2O) at 325°C was determined to be 18.8%.

[0089] Example 5 :

[0090] Following the tests described in Example 4, catalytic testing was performed on the same catalyst bed as in Examples 2 through 4. This test was identical to that described in Example 3, but with a co-dosing of 20 ppm HCN, and the test was conducted at 325°C to 425°C. The N2O conversion X(N2O) at 325°C was determined to be 18.2% without HCN and 31.1% with a co-dosing of 20 ppm HCN. The tests lasted a total of seven days at 425°C. Based on online FT-IR measurements of the reactor exhaust gas, no decrease in N2O conversion was observed during the experimental duration.

[0091] Example 6 :

[0092] Following the activation procedure according to Example 1, 11.5 g of spent catalyst was tested at 4 bar at a pressure of 4 bar, from 325 °C to 500 °C in steps of 25 °C / 2 h, corresponding to a catalyst bed volume of 15.8 mL, with a total volumetric flow rate of 400 NL / h (GHSV: 25000 h). -1 A metered gas stream containing 7.9 vol% O2, 750 ppm N2O, 1500 ppm CO, 750 ppm H2O, and the balance N2 was added. CO used in this experiment was taken from a gas cylinder containing 4 vol% CO in N2. O2 used in this experiment was taken from pressurized air. H2O was added as a liquid via metered addition through the evaporator. N2O and N2 were added as pure gases. Based on online FT-IR measurements of the reactor exhaust gas, the N2O conversion X(N2O) at 325 °C was determined to be 4.9%.

[0093] Example 7 :

[0094] Following the tests described in Example 6, catalytic testing was performed on the same catalyst bed as in Example 6. This test was identical to that described in Example 6, but with an additional co-dosing of 20 ppm HCN, and the test was conducted at 325°C to 425°C. The N2O conversion X(N2O) at 325°C was determined to be 4.9% without HCN and 12.4% with a co-dosing of 20 ppm HCN. The tests lasted a total of seven days at 425°C. Based on online FT-IR measurements of the reactor exhaust gas, no decrease in N2O conversion was observed during the experimental duration.

[0095] The N2O conversion rate X(N2O) at different temperatures is shown in Table 1. The temperature is related to the set furnace temperature. As can be seen from Table 1, the N2O conversion rate increases significantly when the gas stream contains HCN. Therefore, when HCN is used as a reducing agent, N2O conversion can be carried out at significantly lower temperatures compared to catalytic conversion without HCN.

[0096] Table 1

[0097]

Claims

1. A method for removing nitric oxide, nitrogen dioxide, and nitrous oxide from an exhaust gas stream obtained during nitrification, the method comprising: (a) Nitric oxide and nitrogen dioxide are removed from the exhaust gas stream by adding oxygen-containing gas to the exhaust gas stream to oxidize the nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitrogen oxide-depleted gas stream; (b) Wash the nitrogen monoxide-poor gas stream with water to obtain a nitrogen monoxide-poor and nitrogen dioxide-poor gas stream; (c) In the presence of a nitrous oxide reduction catalyst and hydrogen cyanide, nitrous oxide in the gas streams of nitrous oxide and nitrogen dioxide is converted into nitrogen and carbon dioxide and optionally oxygen.

2. The method according to claim 1, wherein the nitrous oxide in step (c) is converted into nitrogen, carbon dioxide and oxygen.

3. The method according to claim 1 or 2, wherein the exhaust gas stream contains the hydrogen cyanide, and the amount of hydrogen cyanide in the exhaust gas stream is in the range of 0.5 ppm to 100 ppm based on the total volume of the exhaust gas stream.

4. The method according to any one of claims 1 to 3, wherein the nitrous oxide reduction catalyst is a zeolite catalyst.

5. The method according to claim 4, wherein the zeolite catalyst is Fe-β zeolite.

6. The method according to any one of claims 1 to 5, wherein the conversion of nitrous oxide in step (c) is carried out at a temperature in the range of 350°C to 600°C and a pressure in the range of 800 mbar (absolute pressure) to 10 bar (absolute pressure).

7. The method according to any one of claims 1 to 6, wherein the gas hourly space velocity of the nitrous oxide converted in step (c) is 1000 standard m / s. 3 / (h‧m 3 (catalyst) up to 40,000 standard m 3 / (h‧m 3 Within the range of catalysts.

8. The method according to any one of claims 1 to 7, wherein the oxygen-containing gas added in step (a) is air.

9. The method according to any one of claims 1 to 8, wherein the exhaust gas stream further contains carbon monoxide, and at least a portion of the carbon monoxide contained in the nitrogen monoxide- and nitrogen dioxide-depleted gas stream is oxidized before the conversion of the nitrous oxide in step (c).

10. The method of claim 9, wherein the oxidation of the carbon monoxide is carried out in the presence of a carbon monoxide oxidation catalyst at a temperature ranging from 230°C to 600°C and a pressure ranging from 800 mbar (absolute pressure) to 10 bar (absolute pressure).

11. The method according to any one of claims 1 to 10, wherein the nitration process is the nitration of benzene, toluene, xylene, phenol, benzoic acid, monochlorobenzene or polychlorobenzene, monobromobenzene or polybromobenzene, imidazole or 5-ethyl-2-methylpyridine.

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