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

Abstract

The invention relates to a process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream obtained in a nitration process, comprising: (a) Removing nitrogen monoxide and nitrogen dioxide from the exhaust gas stream by adding an oxygen comprising gas to the exhaust gas stream, oxidizing the nitrogen monoxide in the gas stream to form nitrogen dioxide, thereby obtaining a gas stream depleted in nitrogen monoxide; (b) Washing the gas stream depleted in nitrogen monoxide with water to obtain a gas stream depleted in nitrogen monoxide and nitrogen dioxide; (c) Converting nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide 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] Process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream

[0002] Specification

[0003] The invention relates to a process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream obtained in a nitration process, comprising:

[0004] (a) Removing nitrogen monoxide and nitrogen dioxide from the exhaust gas stream by adding an oxygen comprising gas to the exhaust gas stream, oxidizing the nitrogen monoxide in the gas stream to form nitrogen dioxide, thereby obtaining a gas stream depleted in nitrogen monoxide;

[0005] (b) Washing the gas stream depleted in nitrogen monoxide with water to obtain a gas stream depleted in nitrogen monoxide and nitrogen dioxide;

[0006] (c) Converting nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide into nitrogen and carbon dioxide and optionally oxygen.

[0007] Exhaust gases from nitration processes usually contain nitrogen monoxide and nitrogen dioxide, which are harmful, nitrous oxide, which is a greenhouse gas and at least traces of hydrogen cyanide. Additionally, the exhaust gas may contain carbon monoxide, which also is harmful. Due to the detrimental properties of these gases, it is necessary to reduce or, preferably, completely remove these gases from the exhaust gas stream.

[0008] Presently, off-gases of nitration processes for example are treated by thermal oxidation by burning the off-gas with high caloric substances like natural gas, or by catalytic oxidation, particularly for removing carbon monoxide.

[0009] For reducing nitrous oxide in the off-gas, several abatement technologies are known. Abatement of nitrous oxide preferably is known from production processes of adipic acid. Usually, nitrous oxide is removed from a gas stream by thermal decomposition or by catalytic decomposition. Usually, catalytic decomposition is carried out at a temperature in a range between 300 and 1000 °C in the presence of a suitable nitrous oxide decomposition catalyst, and thermal decomposition is carried out at a temperature above 1000 °C. By thermal decomposition and non- reductive catalytic decomposition, the nitrous oxide is decomposed into nitrogen and oxygen. Besides non-reductive catalytic decomposition, it is further possible to reduce the amount of nitrous oxide in a gas stream by reductive catalytic decomposition. For this purpose, the nitrous oxide for example reacts with methane, forming nitrogen, carbon dioxide and water. Non-reduc- tive catalytic decomposition usually is carried out at a temperature in a range between 430 and 1000 °C and reductive catalytic decomposition at a temperature in a range between 300 and 600 °C. Processes for removing nitrogen monoxide, nitrogen dioxide and / or nitrous oxide from a gas stream are disclosed for example in DE-A 10 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.

[0010] According to the processes disclosed in WO-A 02 / 072244, WO-A 03 / 084646 or WO-A 2013 / 118064, a zeolite catalyst is used for the abatement of nitrous oxide.

[0011] DE-A 10 2010 048 040 and EP-A 1 022 047 both describe thermal decomposition of nitrous oxide forming oxygen and nitrogen. To remove nitrogen monoxide and nitrogen dioxide from the gas stream which may be generated in the process according to DE-A 10 2010 048 040, a reduction agent may be added to the process to selectively reduce the nitrogen monoxide and nitrogen dioxide.

[0012] EP-A 0 514 739 describes a process for removing nitrogen monoxide and nitrogen dioxide from a gas stream obtained by combustion. For creating a reductive environment, in a first step steam is added. In a second step oxygen is added to convert all carbon species into carbon dioxide. In a SCR-unit, residual nitrogen monoxide and nitrogen dioxide are converted into nitrogen and oxygen.

[0013] Processes in which gas streams containing nitrogen oxides occur, are for example nitrations like the production of dinitrotoluene or of adipic acid. Processes 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.

[0014] Particularly WO-A 2016 / 050759, WO-A 2011 / 082977 and US 5,963,878 also deal with the treatment of nitrogen monoxide and nitrogen dioxide comprising off-gases which are generated during the process. In the processes as described in WO-A 2011 / 082977 and US 5,963,878, the nitrogen monoxide and nitrogen dioxide comprising waste gas is removed from the process and subjected to a process for producing nitric acid by absorbing the nitrogen monoxide and nitrogen dioxide in water. According to the process of WO-A 2016 / 050759, the nitrogen monoxide and nitrogen dioxide comprising waste gas is incinerated. WO-A 2016 / 005070 only mentions that the waste gas may be treated in a washing device and a following thermal waste gas treatment plant or only in a thermal waste gas treatment plant.

[0015] 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, pages 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), pages 425 to 434, describe processes for removing nitrous oxide from exhaust gases obtained in the production of adipic acid. Processes for oxidation of carbon monoxide to carbon dioxide from a gas stream 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 J P-A 2012- 126616.

[0016] Typically, the gas mixture of these processes contains reducible nitrogen species, NOXand / or carbon monoxide simultaneously where the catalyst is used for NOXpurification and carbon monoxide oxidation to carbon dioxide.

[0017] Catalytic decomposition of hydrogen cyanide by hydrolysis or oxidation is described for example in Zhongxian Song et aL, “Catalytic hydrolysis of HCN on ZSM-5 modified by Fe or Nb for HCN removal: surface species and performance, RSC Advances, 2016, 6, pages 111389 to 111397, in O. Krocher et aL; “Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts”, Applied Catalysis B: Environmental 92 (2009), pages 75 to 89, in Ning Liu et aL, “Selective catalytic combustion of hydrogen cyanide over metal modified zeolitic catalysts: From experiment to theory, Catalysis Today 297 (2017), pages 201 to 210, or Irene O.Y. Liu et aL, “The formation and reactions of hydrogen cyanide during isobutane-SCR over Fe-MFI catalysts”, Catalysis Surveys from Asia, Vol. 7, No. 4, December 2003, pages 191 to 202. However, these articles particularly deal with the decomposition of hydrocyanic acid and further show, that the decomposition may be improved by reaction with nitrogen dioxide.

[0018] On the other hand, it is known from 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 Gas Streams from Coal Gasification”, Ind. Eng. Chem. Res 2007, 46, pages 6338 to 6344, that hydrogen cyanide poisons catalysts. An additional problem may be that hydrogen cyanide polymerizes and the thus produced polymers may block the pores in the catalyst. For these reasons, presently the nitration processes usually are carried out in such a way that the formation of hydrogen cyanide is minimized.

[0019] It was an objective of the present invention to provide a process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream obtained in a nitration process with a minimum supply of reducing agents, heating gases or electrical energy.

[0020] This objective is achieved by a process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream obtained in a nitration process, comprising:

[0021] (a) Removing nitrogen monoxide and nitrogen dioxide from the exhaust gas stream by adding an oxygen comprising gas to the exhaust gas stream, oxidizing the nitrogen monoxide in the gas stream to form nitrogen dioxide, thereby obtaining a gas stream depleted in nitrogen monoxide; (b) Washing the gas stream depleted in nitrogen monoxide with water to obtain a gas stream depleted in nitrogen monoxide and nitrogen dioxide;

[0022] (c) Converting nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide into nitrogen and carbon dioxide and optionally oxygen in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide to obtain a purified gas stream.

[0023] The term “nitrogen oxides” means nitrogen monoxide, nitrogen dioxide and nitrous oxide and, if appropriate, also further oxides of nitrogen like e.g. N2O3, N2O4 and N2O5.

[0024] Steps (a) and (b) cover also the removal of nitrogen monoxide formed during the washing of the gas stream by absorption of nitrogen dioxide in water and reaction of nitrogen dioxide with water.

[0025] Due to the absorption of nitrogen dioxide in water, besides the gas stream depleted in nitrogen monoxide and nitrogen dioxide, a solution of nitric acid is formed in step (b). The content of the nitric acid in the solution is in the range from 20 to 62 weight-% and preferably in the range from 40 to 60 weight-%.

[0026] The gas stream fed into the process for removing nitrogen monoxide, nitrogen dioxide, nitrous oxide and carbon monoxide may be obtained in any nitration process like the production of organic nitro compounds, for example the nitration of benzene, toluene, xylene, phenol, benzoic acid, mono or multiple chlorobenzenes, mono or multiple bromobenzenes, imidazole, 5-ethyl-2- methyl-pyridine. The nitration may be carried out as mononitration, dinitration, or trinitration. Preferably, the gas stream emanates from a mononitration or dinitration and particularly from the production of dinitrotoluene.

[0027] The gas stream fed into the process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide may be treated in advance by washing with acidic water, preferably with a mixture of water and nitric acid or water and sulfuric acid.

[0028] For purifying the gas stream, in stages (a) and (b) nitrogen monoxide and nitrogen dioxide are removed from the gas stream by adding an oxygen comprising gas to the gas stream, oxidizing the nitrogen monoxide in the gas stream to form nitrogen dioxide, thereby obtaining a gas stream depleted in nitrogen monoxide and washing the gas stream depleted in nitrogen monoxide with water to obtain the gas stream depleted in nitrogen monoxide and nitrogen dioxide.

[0029] For removing the nitrogen monoxide and nitrogen dioxide from the gas stream, any process known to a skilled person for removing nitrogen monoxide and nitrogen dioxide can be used. Preferably, in a first step the nitrogen monoxide reacts with the oxygen of the oxygen compris- ing gas to form nitrogen dioxide and subsequently subject the thus produced gas stream depleted in nitrogen monoxide to a washing stage in which the nitrogen dioxide in the gas stream is absorbed in a suitable washing liquid, for example water.

[0030] The oxygen comprising gas may be any gas mixture, which comprises oxygen or pure oxygen. Preferably, the oxygen comprising gas is air or oxygen enriched air. If a gas mixture different from air is used, it is preferred to use a mixture comprising oxygen and inert gases, for example nitrogen or noble gases. However, particularly preferably, the oxygen comprising gas is air.

[0031] For absorbing the nitrogen dioxide, the gas stream depleted in nitrogen monoxide preferably is fed into a washing column. If water is used as washing liquid, during washing nitrogen monoxide and nitric acid are formed. The nitrogen monoxide usually oxidizes again and can be absorbed by the washing liquid.

[0032] The washing column used for absorbing the nitrogen dioxide may be a tray column or a packed column. Preferably a tray column is used. In the tray column, the trays preferably are cooled, for example by providing cooling coils on the trays. For cooling, a cooling medium, particularly water, flows through the cooling coils. The number of trays in the tray column preferably is in a range from 2 to 50, particularly from 3 to 30. The trays used for absorbing the nitrogen dioxide in the washing liquid may be any trays known to the skilled person. Suitable trays for example are sieve trays, perforated trays, valve trays or bubble trays.

[0033] The washing column usually is operated at a pressure in a range from ambient pressure to 10 bar (abs), preferably at a pressure in a range from 3 bar(abs) to 8 bar(abs). The temperature in the washing column preferably is in a range from 5 to 45 °C, preferably from 10 to 30 °C.

[0034] Depending on the process from which the gas stream to be purified originated, the gas stream depleted in nitrogen monoxide and nitrogen dioxide obtained in the washing column usually comprises nitrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen monoxide, nitrogen dioxide, and nitrous oxide. If for oxidizing the nitrogen monoxide air is used, the gas stream depleted in nitrogen monoxide and nitrogen dioxide further may contain airborne noble gases with the main component argon. Further, particularly if the gas stream to be purified originates from a nitration process, for example from the production of dinitrotoluene, the gas stream depleted in nitrogen monoxide and nitrogen dioxide further may contain traces sulfur dioxide, and mononitromethane, dinitromethane, and trinitromethane. Further, the gas stream depleted in nitrogen monoxide and nitrogen dioxide may contain non-methane-hydrocarbons.

[0035] Generally, the oxygen content in the gas stream depleted in nitrogen monoxide and nitrogen dioxide is in a range from 5 to 18 volume-%, preferably 6 to 13 volume-%, the carbon monoxide content is in a range from 0.5 to 7 volume-%, preferably between 1 to 5 volume-%, the carbon dioxide content is in a range from 2 to 10 volume-%, preferably in a range from 3 to 7 volume- %, the nitrous oxide content is in a range from 0.2 to 4 volume-%, preferably in a range from 0.3 to 2.5 volume-%, the nitrogen content is in a range from 50 to 90 volume-%, preferably in a range from 60 to 80 volume-% and the content in nitrogen monoxide and nitrogen dioxide is in a range from 40 to 800 volume-ppm, preferably in a range from 80 to 400 volume-ppm. If air or oxygen enriched air is used as oxygen comprising gas, the amount of argon in the gas stream depleted in nitrogen monoxide and nitrogen dioxide usually is in a range from 0.5 to 0.95 vol- ume-%, particularly in a range between 0.7 and 0.94 volume-%. If the gas stream depleted in nitrogen monoxide and nitrogen dioxide contains non-methane-hydrocarbons, the amount of non-methane-hydrocarbons preferably is in a range from 50 to 600 volume-ppm, more preferred in a range from 100 to 500 volume-ppm.

[0036] After removing the nitrogen monoxide and the nitrogen dioxide, the gas stream depleted in nitrogen monoxide and nitrogen dioxide is fed into step (c) for converting nitrous oxide into nitrogen and carbon dioxide and optionally oxygen.

[0037] Converting the nitrous oxide into nitrogen and carbon dioxide and optionally oxygen is carried out in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide.

[0038] Surprisingly it has shown that the conversion of nitrous oxide into nitrogen and carbon dioxide and optionally oxygen is faster, if hydrogen cyanide is contained in the reaction mixture. It is assumed that the hydrogen cyanide acts as a reducing agent according to following equation:

[0039] 5 N2O + 2 HCN -» 6 N2+ 2 CO2+ H2O.

[0040] Thus, on the one hand, hydrogen cyanide supports the decomposition reaction of nitrous oxide and reduces the addition of other reducing agents, and, on the other hand, the hydrogen cyanide emissions are simultaneously reduced.

[0041] The amount of hydrogen cyanide in the exhaust gas stream preferably is in a range from 0.5 to 100 vol-ppm based on the total volume of the exhaust gas stream. More preferred, the amount of hydrogen cyanide in the exhaust gas stream is in a range from 1 to 30 vol-ppm and particularly in a range from 2 to 15 vol-ppm, each based on the total volume of the exhaust gas stream.

[0042] The hydrogen cyanide usually is a by-product which is produced during the nitration process. However, if the amount of hydrogen cyanide in the exhaust gas stream obtained in the nitration process is not sufficient, it may be necessary to add additional reducing agent for reducing the nitrous oxide.

[0043] If the gas stream depleted in nitrogen monoxide and nitrogen dioxide is treated in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide, the amount of ox- ygen, which is optionally generated, depends on the temperature and / or the molar stoichiometric ratio of the available reducing agent to nitrous oxide. A higher temperature and / or a lower molar stoichiometric ratio of the available reducing agent to nitrous oxide generally results in more oxygen. Generally, nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide is converted into nitrogen and carbon dioxide and oxygen.

[0044] The nitrous oxide reduction catalyst may be either a catalyst bed or a monolithic catalyst, a catalyst bed being preferred. The particles used in the catalyst bed preferably are in the shape of solid cylinders, hollow cylinders or strands. Preferably, the particles used in the catalyst bed are star strands. The strands used in the catalyst bed preferably have an outer diameter of 1 .5 to 10 mm, preferably of 2 to 6 mm and a length from 3 to 20 mm, preferably from 4 to 10 mm.

[0045] The catalyst may be any commercially available catalyst, which can be used for the decomposition of nitrous oxide, for example catalysts comprising copper oxide and / or zinc oxide as catalytic active material on a support made of silicon oxide and / or aluminum oxide. Preferably, the catalyst is a zeolitic catalyst, particularly an Fe-beta zeolite, for example an Fe-beta zeolite of the type ZSM5 or BEA, preferably BEA. Particularly preferably, the catalyst is an organotem- plate-free produced catalyst as described in EP-B 2 812 283. Using an Fe-beta zeolite of the type ZSM5 or BEA, particularly of the type BEA, has the additional advantage that these catalysts surprisingly are not poisoned by the hydrogen cyanide in the gas stream.

[0046] Further suitable catalysts are for example Fe / Cu-OFF-ERI-zeolites as described for example in CN-A 113198525. The catalyst (Mgo.o25Ceo.o5Coo.925)Co204-Fei-Cu4-OFF-ERI described in CN-A 113198525 is a composite of three separate compounds and contains about 20 wt% (Mgo.o25Ceo.o5Coo.925)Co204 spinel, about. 35 wt% of an Fe and Cu exchanged OFF-ERI zeolite and is bound by about 45 wt% Al / Si mixed metal oxide that is presumably also OFF-ERI zeolite.

[0047] The decomposition of the nitrous oxide in the presence of the catalyst takes place at a temperature in a range from 350 to 600 °C, preferably at a range from 420 to 560 °C, and a pressure in a range from 800 mbar(abs) to 10 bar(abs), preferably in a range from 900 mbar(abs) to 8 bar(abs) and particularly in a range from 1000 mbar(abs) to 1200 mbar(abs). Particularly preferably, the decomposition of the nitrous oxide is carried out at an excess pressure in a range from 5 to 300 mbar relative to the atmosphere.

[0048] The gas hourly space velocity (GHSV) for decomposing the nitrous oxide may be in a range from 1000 to 40000 standard m3 / (m3catalyst ■ h), preferably from 2000 to 30000 standard m3 / (m3catalyst ■ h) and particularly in a range from 4000 to 10000 standard m3 / (m3catalyst ■ h).

[0049] If the gas stream depleted in nitrogen monoxide and nitrogen dioxide contains carbon monoxide, at least a part of the carbon monoxide reacts with the nitrous oxide, thereby forming carbon dioxide and nitrogen. Depending on the amounts of carbon monoxide and nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide and the reaction conditions, particularly the GHSV, either all of the carbon monoxide comprised in this gas stream reacts with the nitrous oxide or only a part of the carbon monoxide reacts with the nitrous oxide.

[0050] Typically, carbon monoxide contained in the gas stream depleted in nitrogen monoxide and nitrogen dioxide is in molar excess to nitrous oxide.

[0051] Both reactions, the conversion of nitrous oxide with carbon monoxide forming carbon dioxide and nitrogen and the decomposing of nitrous oxide forming nitrogen and oxygen usually are carried out in the same reactor at the same conditions. For this reason, if the gas stream depleted in nitrogen monoxide and nitrogen dioxide contains carbon monoxide, in the reaction in stage (c), a part of the nitrous oxide reacts with the carbon monoxide forming carbon dioxide and nitrogen and simultaneously, nitrous oxide is decomposed into nitrogen and oxygen.

[0052] If the amount of carbon monoxide is such that not all of the carbon monoxide reacts with nitrous oxide, the purified gas stream obtained in stage (c) still contains carbon monoxide. To remove the remaining carbon monoxide, the gas stream is subject to a second oxidation of carbon monoxide in which the remaining carbon monoxide is oxidized to carbon dioxide.

[0053] Generally, the exhaust gas stream also contains carbon monoxide. As the carbon monoxide usually is not oxidized in stage (a), in a preferred embodiment, at least a part of the carbon monoxide which still is contained in the gas stream depleted in nitrogen monoxide and nitrogen dioxide is oxidized before converting the nitrous oxide into nitrogen and carbon dioxide and optionally oxygen.

[0054] Before feeding the gas stream depleted in nitrogen monoxide and nitrogen dioxide into the oxidizing of carbon monoxide, it is preferred to preheat the gas stream. Preferably, the gas stream depleted in nitrogen monoxide and nitrogen dioxide is preheated by indirect heat exchange with the hot purified off-gas stream, which simultaneously is cooled. For this purpose, any suitable heat exchanger may be used, for example a tube bundle heat exchanger, U-tube-bundle heat exchanger or a plate heat exchanger. Preferably, a tube bundle heat exchanger is used.

[0055] If preheating the gas stream depleted in nitrogen monoxide and nitrogen dioxide by heat exchange with the purified off-gas stream is not sufficient, an additional heater can be used, for example an electric heater or a burner, for example a gas burner. Preferably, the heater used for additional heating is an electric heater. The additional heater further is used during start-up of the process to heat the gas stream depleted in nitrogen monoxide and nitrogen dioxide to the temperature at which oxidation of carbon monoxide is carried out.

[0056] The temperature to which the gas stream depleted in nitrogen monoxide and nitrogen dioxide is heated by indirect heat exchange with the hot purified off-gas stream and / or in the additional heater preferably is in a range from 200 to 500 °C, more preferred in a range from 220 to 450 °C and particularly in a range from 230 to 400 °C.

[0057] In a first alternative, in the stage of oxidizing the carbon monoxide, the whole carbon monoxide is oxidized from the gas stream depleted in nitrogen monoxide and nitrogen dioxide. However, alternatively and preferably, only a part of the carbon monoxide is oxidized so that after leaving the oxidizing stage the gas stream depleted in nitrogen monoxide and nitrogen dioxide still contains carbon monoxide.

[0058] For oxidizing the carbon monoxide to carbon dioxide, any process known to a skilled person can be used.

[0059] For oxidizing only a part of the carbon monoxide contained in the gas stream depleted in nitrogen monoxide and nitrogen dioxide, it is possible to select the oxidation conditions, e.g. the oxidation temperature or the GHSV, and / or the catalyst volume such that only a part of the carbon monoxide is oxidized. As an alternative, the gas stream depleted in nitrogen monoxide and nitrogen dioxide is split into a first partial stream and a second partial stream and the carbon monoxide in the first partial stream is oxidized to obtain a partial stream depleted in carbon monoxide.

[0060] Independently of whether the whole carbon monoxide or only a part of the carbon monoxide is oxidized, the oxidation usually is carried out in a reactor in the presence of a carbon monoxide oxidizing catalyst. The reactor for example may be a vessel with a catalyst bed or a monolithic molded body containing the carbon monoxide oxidizing catalyst. Preferably, a reactor is used which contains at least one monolithic molded body. The monolithic shaped body preferably is designed as a straight prism with a round base or a 4 or 6 sided base, e.g. cylinders or cuboids. The monolithic molded body may be made of the catalytic active material or may be made of a ceramic or metal body which is coated with the catalytic active material.

[0061] The monolithic shaped body containing the catalytic active material may be mounted in direct contact into the vessel forming the reactor or may be incorporated into a supporting framework.

[0062] In the context of the present invention, the term “catalyst bed” is used for fluidized beds or packed beds. In a catalyst bed, particles or packings of any shape can be used. The particles or packings may either be made of the catalytic active material or may be made of a support material, for example a polymer or a metal, which onto which the catalytic active material is applied.

[0063] The carbon monoxide oxidizing catalyst used in the reactor for oxidizing the carbon monoxide preferably is a 3-way catalyst as used for the treatment of exhaust gases for simultaneous destruction of carbon monoxide, hydrocarbons and nitrogen oxides from engine combustion. In this case, residual traces of nitrogen monoxide and nitrogen dioxide, which still may be obtained in the gas stream depleted in nitrogen monoxide and nitrogen dioxide also are reduced at least partly during the oxidation of carbon monoxide. Alternatively, but less preferred the catalyst may be a 2-way catalyst or a so-called VOC catalyst for the conversion of hydrocarbons and carbon monoxide to carbon dioxide and water by reaction with oxygen. Further catalytic active materials which may be used for the oxidation of the carbon monoxide to from carbon dioxide may be for example mixed oxides like aluminum and / or silicon and / or copper oxides and / or magnesium oxides.

[0064] Also suitable as catalytic material for the oxidation of carbon monoxide are precious metals like platinum, ruthenium or palladium.

[0065] The gas hourly space velocity (GHSV) for the carbon monoxide oxidizing catalyst usually is in a range from 4000 to 200000 standard m3 / (m3catalyst ■ h), preferably from 8000 to 150000 standard m3 / (m3catalyst ■ h).

[0066] The oxidation of the carbon monoxide to form carbon dioxide in the presence of the carbon monoxide oxidizing catalyst usually takes place at a reaction temperature in a range from 230 to 600 °C, preferably in a range from 250 to 540 °C. The pressure at which the oxidation of the carbon dioxide is carried out, usually is in a range from 800 mbar (abs) to 10 bar (abs), preferably in a range from 900 mbar (abs) to 8 bar (abs). Particularly preferably, the oxidation of the carbon monoxide is carried out at an excess pressure in a range from 5 to 300 mbar relative to the atmospheric pressure.

[0067] If the gas stream depleted in nitrogen monoxide and nitrogen dioxide is split into the first partial stream and the second partial stream and the carbon monoxide in the first partial stream is oxidized to obtain a partial stream depleted in carbon monoxide, after oxidizing the carbon monoxide in the first partial stream, the partial stream depleted in carbon monoxide is mixed with the second partial stream, thereby obtaining a mixed stream depleted in nitrogen monoxide and nitrogen dioxide but still containing carbon monoxide.

[0068] Mixing of the partial stream depleted in carbon monoxide and the second partial stream may be carried out in any mixing unit for gas streams known to a skilled person. Preferably, the partial stream depleted in carbon monoxide and the second partial stream are combined directly by either introducing the partial stream depleted in carbon monoxide into the second partial stream or by introducing the second partial into the partial stream depleted in carbon monoxide, or by using a static mixer. If the partial stream depleted in carbon monoxide is introduced into the second partial stream or the second partial stream is introduced into the partial stream depleted in carbon monoxide, it is for example possible to use a bypass for the second partial stream bypassing the reactor in which the carbon monoxide of the first stream is oxidized and to open the bypass into a gas line leaving the reactor or open the gas line leaving the reactor into the bypass. As a further alternative, a Y-connector can be used, the bypass being connected to one leg of the Y, the line leaving the reactor being connected to the second leg of the Y and the combined partial streams, forming the gas stream depleted in carbon monoxide, leaves the Y at the base.

[0069] If a static mixer is used for mixing the partial stream depleted in carbon monoxide and the second partial stream, any static mixer known to a skilled person may be used. Usually such static mixers comprise inserts which divert the flow, thereby inducing a turbulent flow by which the partial streams are mixed.

[0070] Preferably, for mixing the partial stream depleted in carbon monoxide and the second partial stream, a static mixer is used.

[0071] The molar ratio of carbon monoxide to nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide after oxidizing a part of the carbon monoxide or, if the gas stream depleted in nitrogen monoxide and nitrogen dioxide is split into the first partial stream and the second partial stream, after mixing the partial stream depleted in carbon monoxide and the second stream preferably is in a range from 0.1 to 1 to 2 to 1 , particularly in a range from 0.3 to 1 to 1 .5 to 1 .

[0072] If at least a part of the carbon monoxide contained in the gas stream depleted in nitrogen monoxide and nitrogen dioxide is oxidized, the gas stream depleted in nitrogen monoxide and nitrogen dioxide obtained thereby, or, if the gas stream depleted in nitrogen monoxide and nitrogen dioxide is split in partial streams, the mixed stream after remixing the partial streams is fed into stage (c) in which the nitrous oxide is removed from the gas stream depleted in carbon monoxide.

[0073] For removing the nitrous oxide in stage (c), the nitrous oxide is decomposed into nitrogen and oxygen and / or may react with hydrogen cyanide forming nitrogen, carbon dioxide and water. If the gas stream depleted in carbon monoxide still contains carbon monoxide, at least a part of the carbon monoxide still contained in the gas stream depleted in carbon monoxide reacts with the nitrous oxide, thereby forming carbon dioxide and nitrogen.

[0074] The purified gas stream obtained in (c), if the complete carbon monoxide is oxidized before converting the nitrous oxide in step (c) or if the remaining carbon monoxide has reacted completely with the nitrous oxide, or, after oxidizing the remaining carbon monoxide if the gas stream obtained in (c) still contains carbon monoxide, has usually less than 400 mg nitrogen monoxide and nitrogen dioxide per cubic meter dry gas at normalized conditions preferably less than 200 mg nitrogen monoxide and nitrogen dioxide per cubic meter dry gas at normalized conditions and more preferred less than 100 mg nitrogen monoxide and nitrogen dioxide per cubic meter dry gas at normalized condition, where the nitrogen monoxide and nitrogen dioxide are assumed as nitrogen dioxide, less than 400 weight-ppm preferably less than 100 weight-ppm carbon monoxide and less than 2000, preferably less than 1000 and more preferably less than 500 volume-ppm nitrous oxide and particularly preferably less than 100 volume-ppm nitrous oxide. Particularly preferably, if carbon monoxide is oxidized before feeding the gas stream depleted in nitrogen monoxide and nitrogen dioxide into step (c) for converting the nitrous oxide, the process is carried out in such a way that the gas stream depleted in nitrogen monoxide and nitrogen dioxide which is obtained after oxidizing carbon monoxide still contains carbon monoxide and that only a part of the remaining carbon monoxide reacts with the nitrous oxide, so that the purified gas stream obtained in stage (c) still contains carbon monoxide and that this remaining carbon monoxide is oxidized in a second oxidation step to form carbon dioxide. For oxidizing the carbon monoxide in the second oxidation step preferably the residual oxygen in the gas stream is used so that no additional oxygen comprising gas needs to be added.

[0075] For starting the process, firstly, the catalysts for the oxidation of carbon monoxide and decomposing the nitrous oxide must be brought to operating temperature. This can be achieved by passing a gaseous medium like air, nitrogen or exhaust gas through a heater and then passing the gaseous medium over the catalysts for oxidizing the carbon monoxide and decomposing the nitrous oxide. The gaseous medium used for heating may either be pressurized by a blower so that it flows through the heater and the catalysts, or it can be taken, for example, from the plants operating network. For heating the gaseous medium for example an electric heater may be used. Alternatively, the gaseous medium may be heated by direct or indirect heat exchange with exhaust gases from natural gas combustion or by a regenerative heat exchanger, which is operated with the hot exhaust gas from the catalysts. Preferably, a combination of electric heating and regenerative heating is used, the heating can also take place in several stages simultaneously, e.g. regenerative and electrical heating at the same time.

[0076] The heating of the catalysts can be carried out in a straight pass or in a cycle. If the heating is carried out in a cycle, the gaseous medium after having passed the catalysts to be heated is passed again to the input side of the heater by using a suitable blower.

[0077] Examples

[0078] The catalytic tests were performed in a tube reactor with an inner diameter of 20 mm and a total length of 1300 mm, equipped with four thermocouples inside a 6 mm thermo sleeve. The tube reactor was heated with an electrical tube furnace having a ceramic lining which was in direct contact with the wall of the tube reactor to ensure good heat transfer. The fresh and spent catalyst was tested as 3 mm strands within the isothermal zone of the reactor. The position of the catalyst bed was adjusted by a steatite inert packing before and after the catalyst bed. The performance was monitored by online FT-IR spectroscopy of the reactor exhaust gas. Prior to each catalytic measurement, the concentration of the feed gas was analyzed via the I R spectrometer and taken as basis to calculate the conversion of N2O. Example 1 (Catalyst activation procedure):

[0079] The fresh and spent catalyst were activated before each catalyst test to ensure a comparable catalytic activity. In general, the catalyst does not need the here described activation procedure to catalyze the investigated reaction, but it was chosen to activate it, nevertheless. Prior to each experiment the fresh and spent catalysts were activated by the following procedure:

[0080] 250 NL / h N2, 150 NL / h air and 0.25 g / h H2O were dosed through the catalyst bed while heating the reactor in 25 °C / 2 h steps from 325 to 500 °C. This procedure primes the zeolite pores for the subsequent reaction and ensures that the initial performance stays stable and is not increasing or decreasing during the measurement. After the activation period, the furnace was cooled down to 325 °C and the catalytic test was started.

[0081] Example 2:

[0082] After the activation procedure according to example 1 , 11 .5 g of the fresh catalyst, which corresponds to a catalyst bed volume of 20 mL, were tested without HCN co-dosage from 325 °C to 375 °C in 25 °C / 2 h steps with a total volume flow of 400 NL / h at a pressure of 4 bar (GHSV: 20000 h’1). A gas stream containing 7.9 vol.% O2, 750 ppm N2O, 1500 ppm CO, 750 ppm H2O and N2 balance was dosed. The CO used for this experiment was taken from a gas bottle containing 4 vol.% CO in N2. O2 used for this experiment was taken from pressurized air. H2O was dosed as liquid via a vaporizer. N2O and N2 were added as pure gases. The N2O conversion X(N2O) at 325 °C was determined to 16 % based on the online FT-IR measurement of the reactor exhaust gas.

[0083] Example 3:

[0084] A catalytic test of the same catalyst bed as tested in example 2 was conducted after the test described in example 2. The test was identical to that described in example 2 but 200 ppm HCN were co-dosed in addition and the test was performed from 325 to 500 °C. The HCN used for this experiment was taken from a gas bottle containing 5000 ppm in N2. The N2O conversion X(N2O) at 325 °C without HCN dosage was determined to 24.4% and to 67.4% with HCN codosage based on the online FT-IR measurement of the reactor exhaust gas as the N2O concentration was below the detection limit of the FT-IR spectrometer.

[0085] Example 4:

[0086] A catalytic test of the same catalyst bed as tested in example 3 was conducted after the test described in example 3. The test was identical to that described in example 2 but was performed from 325 to 500 °C. The N2O conversion X(N2O) at 325 °C was determined to 18.8 % based on the online FT-IR measurement of the reactor exhaust gas. Example 5:

[0087] A catalytic test of the same catalyst bed as tested in examples 2 to 4 was conducted after the test described in example 4. The test was identical to that described in example 3 but 20 ppm HCN were co-dosed, and the test was performed from 325 to 425 °C. The N2O conversion X(N2O) at 325 °C without HCN dosage was determined to 18.2% and to 31 .1 % with 20 ppm HCN co-dosage. At 425 °C the test was continued for seven days in total. Based on the online FT-IR measurement of the reactor exhaust gas no decline in N2O conversion was observed during the duration of the experiment.

[0088] Example 6:

[0089] After the activation procedure according to example 1 , 11 .5 g of the spent catalyst, which corresponds to a catalyst bed volume of 15.8 mL, were tested without HCN co-dosage from 325 °C to 500 °C in 25 °C / 2 h steps with a total volume flow of 400 NL / h at a pressure of 4 bar (GHSV: 25000 h’1). A gas stream containing 7.9 vol.% O2, 750 ppm N2O, 1500 ppm CO, 750 ppm H2O, and N2 balance was dosed. The CO used for this experiment was taken from a gas bottle containing 4 vol.% CO in N2. O2 used for this experiment was taken from pressurized air. H2O was dosed as liquid via a vaporizer. N2O and N2 were added as pure gases. The N2O conversion X(N2O) at 325 °C was determined to 4.9 % based on the online FT-IR measurement of the reactor exhaust gas.

[0090] Example 7:

[0091] A catalytic test of the same catalyst bed as tested in example 6 was conducted after the test described in example 6. The test was identical to that described in example 6 but 20 ppm HCN were co-dosed in addition, and the test was performed from 325 to 425 °C. The N2O conversion X(N2O) at 325 °C without HCN dosage was determined to 4.9% and to 12.4% with 20 ppm HCN co-dosage. At 425 °C the test was continued for seven days in total. Based on the online FT-IR measurement of the reactor exhaust gas no decline in N2O conversion was observed during the duration of the experiment.

[0092] The N2O conversion X(N2O) at different temperatures is shown in table 1. The temperatures relate to the set furnace temperatures. As can be seen in table 1 , the N2O conversion increases remarkably, when the gas stream contains HCN. Thus, when using HCN as a reduction agent, the N2O conversion can be carried out at remarkably lower temperatures compared to the catalytic conversion without HCN. Table 1

[0093] HCN

[0094] Cone. X(N2O) [%]

[0095] [PPm]

[0096] Exp. Name 325 °C 350 °C 375 °C 400 °C 425 °C 450 °C 475 °C 500 °C

[0097] Example 2 0 16 39 62 -

[0098] Example 3 200 67.4 100 100 100 100

[0099] Example 4 0 18.8 37.6 57.0 73.5 87.2 94.5 98.2 100

[0100] Example s 20 31.1 52.2 69.2 81.9 89.8

[0101] Example 6 0 4.9 11.7 21.0 31.3 41.6 51.1 60.4 68.7

[0102] Example 7 20 12.4 22.9 32.4 41.4 49.0

Claims

Claims1 . A process for removing nitrogen monoxide, nitrogen dioxide and nitrous oxide from an exhaust gas stream obtained in a nitration process, comprising:(a) Removing nitrogen monoxide and nitrogen dioxide from the exhaust gas stream by adding an oxygen comprising gas to the exhaust gas stream, oxidizing the nitrogen monoxide in the gas stream to form nitrogen dioxide, thereby obtaining a gas stream depleted in nitrogen monoxide;(b) Washing the gas stream depleted in nitrogen monoxide with water to obtain a gas stream depleted in nitrogen monoxide and nitrogen dioxide;(c) Converting nitrous oxide in the gas stream depleted in nitrogen monoxide and nitrogen dioxide into nitrogen and carbon dioxide and optionally oxygen in the presence of a nitrous oxide reduction catalyst and in the presence of hydrogen cyanide.

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

3. The process according to claim 1 or 2, wherein the hydrogen cyanide is contained in the exhaust gas stream and the amount of hydrogen cyanide in the exhaust gas stream is in a range from 0.5 to 100 vol-ppm based on the total volume of the exhaust gas stream.

4. The process according to any of claims 1 to 3, wherein the nitrous oxide reduction catalyst is a zeolitic catalyst.

5. The process according to claim 4, wherein the zeolitic catalyst is a Fe-beta zeolite.

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

7. The process according to any of claims 1 to 6, wherein the gas hourly space velocity for converting the nitrous oxide in step (c) is in a range from 1000 to 40000 standard m3 / (h-m3catalyst)8. The process according to any of claims 1 to 7, wherein the oxygen comprising gas added in step (a) is air.

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

10. The process according to claim 9, wherein oxidizing the carbon monoxide is carried out at a temperature in a range from 230 to 600 °C and a pressure in a range from800 mbar(abs) to 10 bar(abs) in the presence of a carbon monoxide oxidizing catalyst.11 . The process according to any of claims 1 to 10, wherein the nitration process is a nitration of benzene, toluene, xylene, phenol, benzoic acid, mono or multiple chlorobenzene, mono or multiple bromobenzene, imidazole, or 5-ethyl-2-methyl pyridine.