Method for removing nitric oxides, nitrogen oxides, and carbon monoxide from a gas stream

Abstract

The present invention relates to a method for removing nitric oxides, nitrogen oxides, and carbon monoxide from a gas stream (1), the method comprising: (a) removing nitric oxide from said gas stream (1) by adding an oxygen-containing gas (3) to said gas stream (1) to oxidize nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream, and scrubbing said nitric oxide-depleted gas stream with water (7) to obtain a nitric oxide-depleted gas stream (11); (b) oxidizing at least a portion of the carbon monoxide in said nitric oxide-depleted gas stream (11) to produce carbon dioxide, thereby obtaining a carbon monoxide-depleted gas stream (19); (c) converting the nitrogen oxides in said carbon monoxide-depleted gas stream (19) to nitrogen and oxygen if no carbon monoxide is present in said carbon monoxide-depleted gas stream (19), or to nitrogen and oxygen and / or carbon dioxide if carbon monoxide is present in said carbon monoxide-depleted gas stream (19), to obtain a purified off-gas stream (25); Includes.

Description

[Technical Field]

[0001] The present invention relates to a process for removing nitric oxide, nitrogen oxides, and carbon monoxide from a gas stream. [Background technology]

[0002] Gaseous streams containing nitric oxide, nitrogen oxides, and carbon monoxide are generated as off-gases, for example, in nitration processes. Because nitric oxide and carbon monoxide are harmful and nitrogen oxides are greenhouse gases, there is a need to reduce or, preferably, completely remove these gases from the gas stream.

[0003] Currently, for example, off-gases from nitration processes are treated by thermal oxidation, in which the off-gases are combusted with a high-calorie substance such as natural gas, or by catalytic oxidation, particularly to remove carbon monoxide.

[0004] Several techniques are known for reducing nitrogen oxides in off-gases. Nitrogen oxide removal is primarily known from the adipic acid production process. Nitrogen oxides are typically removed from gas streams by thermal or catalytic decomposition. Catalytic decomposition is typically carried out at temperatures between 300 and 1000°C in the presence of a suitable nitrogen oxide decomposition catalyst, while thermal decomposition is carried out at temperatures above 1000°C. Thermal and non-reductive catalytic decomposition decompose nitrogen oxides into nitrogen and oxygen. In addition to non-reductive catalytic decomposition, it is also possible to reduce the amount of nitrogen oxides in a gas stream by reductive catalytic decomposition. For this purpose, for example, nitrogen oxides react with methane to produce nitrogen, carbon dioxide, and water. Non-reductive catalytic decomposition is typically carried out at temperatures between 430 and 1000°C, while reductive catalytic decomposition is carried out at temperatures between 300 and 600°C.

[0005] Processes for removing nitric oxide and / or nitrogen oxides from gas streams 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.

[0006] According to the processes disclosed in WO-A 02 / 072244, WO-A 03 / 084646 or WO-A 2013 / 118064, zeolite catalysts are used for the abatement of nitrogen oxides.

[0007] DE-A 10 2010 048 040 and EP-A 1 022 047 describe the thermal decomposition of nitrogen oxides to produce oxygen and nitrogen. To remove nitrogen oxide from the gas stream that may be generated in the process according to DE-A 10 2010 048 040, a reducing agent that selectively reduces nitrogen oxide can be added to the process.

[0008] EP-A 0 514 739 describes a process for removing nitrogen oxides from gas streams obtained by combustion. In the first stage, steam is added to create a reducing environment. In the second stage, oxygen is added to convert all carbon species to carbon dioxide. In the SCR unit, the remaining nitrogen oxides are converted to nitrogen and oxygen.

[0009] Processes in which gas streams containing nitrogen oxides are generated include, for example, nitration processes, such as the production of dinitrotoluene, which processes 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.

[0010] In particular, WO-A 2016 / 050759, WO-A 2011 / 082977, and US 5,963,878 also deal with the treatment of nitrogen oxides, which constitute the off-gas generated during the process. In the processes described in WO-A 2011 / 082977 and US 5,963,878, the nitrogen oxides that constitute the waste gas are removed from the process and subjected to a process for producing nitric acid by absorbing the nitrogen oxides in water. According to the process of WO-A 2016 / 050759, the nitrogen oxides that constitute the waste gas are incinerated. WO-A 2016 / 005070 only mentions that the waste gas may be treated in a scrubber and a subsequent thermal waste gas treatment plant, or may be treated only in a thermal waste gas treatment plant.

[0011] WO-A 2022 / 152608 describes a process for removing nitrogen oxides from a gas stream, and US-B 8,012,446 and US-A 2013 / 315810 describe processes for removing nitrogen dioxide from a stream containing carbon dioxide.

[0012] Processes for the 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 JP-A 2012-126616.

[0013] A further process for oxidizing carbon monoxide is described in US-B 8,323,602. Furthermore, nitric oxide contained in the gas stream is oxidized to form nitrogen dioxide, which is then removed or reduced to nitrogen and oxygen.

[0014] According to the processes disclosed in WO2006 / 098914 or US20200368727, a platinum group metal or a mixture of two or more platinum group metals is used to oxidize nitrogen oxide to carbon dioxide. In the described processes, typically, the effluent gas of a fluid catalytic cracking unit (FCC) is treated to remove NO xand CO. High concentrations of carbon monoxide usually remain in the exhaust gas from an FCC regeneration unit.

[0015] EP-B 1558367 describes a carbon monoxide to carbon dioxide combustion promoter for use in FCC, comprising aluminum oxide, cerium oxide, and a noble metal such as platinum and / or rhodium and / or other noble metal components or mixtures thereof.

[0016] Typically, the gas mixtures in these processes contain NO, a reducible nitrogen species. x and carbon monoxide are also contained, and the catalyst x It is used in the purification of CO and the oxidation of carbon monoxide to carbon dioxide.

[0017] A further application for the oxidation of carbon monoxide to carbon dioxide can be found in the treatment of exhaust gases from automobile engines, as disclosed in US-A 2020 / 0368727, US 11,248,505, WO-A 2010 / 077843, WO-A 2020 / 188518 or EP-B 832688.

[0018] US-A 2020 / 0368727 describes a four-way catalyst for treating exhaust gases from gasoline engines. It is intended to remove particulate matter, hydrocarbons and / or NO. x In addition to the removal of CO, the removal of carbon monoxide by oxidation to carbon dioxide is also described. A porous wall-flow filter substrate containing an on-coat comprising a platinum group metal supported on a refractory metal oxide is used as a catalyst.

[0019] The oxidation of carbon monoxide to carbon dioxide is x It is well known in the literature that this oxidation can be carried out without the presence of HCl under typical oxidation conditions such as those described by Haruta et al., "Low-Temperature Oxidation of CO on Gold Supported on TiO2, Fe2O3, and Co3O4," Journal of Catalysis, 1993, Vol. 144, pp. 177-195.

[0020] According to the processes disclosed in WO-A 2014 / 138397 or JP-A 2012-126616, metal oxide catalysts can also be used to oxidize carbon monoxide to carbon dioxide, and therefore this process is not limited to noble metal elements, in particular platinum group metals.

[0021] WO-A 2014 / 138397 describes a catalyst for oxidizing, among other components, carbon monoxide and volatile organic compounds (VOCs), which comprises CuO and / or MnO supported on ceria and / or zirconia.

[0022] A disadvantage of all known processes is that sufficient amounts of reducing agent, heating gas or energy must be provided in order to remove nitrogen oxides and / or carbon monoxide from the gas stream. [Prior art documents] [Patent documents]

[0023] [Patent Document 1] DE-A 10 2010 048 040 [Patent Document 2] EP-A 1 022 047 [Patent Document 3] EP-A 0 514 739 [Patent Document 4] WO-A 02 / 072244 [Patent Document 5] WO-A 03 / 084646 [Patent Document 6] WO-A 2013 / 118064 [Patent Document 7] WO-A 2015 / 059185 [Patent Document 8] WO-A 2016 / 005070 [Patent Document 9] WO-A 2016 / 050759 [Patent Document 10] WO-A 2011 / 082977 [Patent Document 11] US 5,963,878 [Patent Document 12] WO-A 2022 / 152608 [Patent Document 13] US-B 8,012,446 [Patent Document 14] US-A 2013 / 315810 [Patent Document 15] WO-A 2006 / 098914 [Patent Document 16] WO-A 2014 / 138397 [Patent Document 17] EP-B 1558367 [Patent Document 18] US-A 2020 / 0368727 [Patent Document 19] WO-A 2010 / 077843 [Patent Document 20] JP-A 2012-126616 [Patent Document 21] US-B 8,323,602 [Patent Document 22] US-A 2020 / 0368727 [Patent Document 23] US 11,248,505 [Patent Document 24] WO-A 2010 / 077843 [Patent Document 25] WO-A 2020 / 188518 [Patent Document 26] EP-B 832688 [Non-patent literature]

[0024] [Non-Patent Document 1] Haruta et al., "Low-Temperature Oxidation of CO on Gold Supported on TiO2, Fe2O3, and Co3O4," Journal of Catalysis, 1993, Vol. 144, pp. 177-195 Summary of the Invention [Problem to be solved by the invention]

[0025] It is therefore an object of the present invention to provide a method for removing nitric oxides, nitrogen oxides and carbon monoxide from a gas stream with minimal supply of reducing agent, heated gas or electrical energy. [Means for solving the problem]

[0026] The objectives are: (a) removing nitric oxide from the gas stream by adding an oxygen-containing gas to the gas stream to oxidize nitrogen monide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream, and scrubbing the nitric oxide-depleted gas stream with water to obtain a nitric oxide-depleted gas stream; (b) oxidizing at least a portion of the carbon monoxide in the nitric oxide-depleted gas stream to produce carbon dioxide, thereby obtaining a carbon monoxide-depleted gas stream; (c) converting the nitrogen oxides in the carbon monoxide-depleted gas stream to nitrogen and oxygen if the carbon monoxide-depleted gas stream does not contain any carbon monoxide, or to nitrogen and oxygen and / or carbon dioxide if the carbon monoxide-depleted gas stream contains carbon monoxide, to obtain a purified off-gas stream; This is accomplished by a method for removing nitric oxides, nitrogen oxides, and carbon monoxide from a gas stream, comprising: [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 shows a flow chart of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] In the context of the present invention, the term "nitric oxide" means nitric oxide and / or nitrogen dioxide, and is defined as "NO x" where x is 1 or 2. The term "nitrogen oxides" refers to nitric oxide and nitrous oxide, and, where appropriate, further nitrogen oxides such as N2O3, N2O4, N2O5 and NO3.

[0029] Step (a) also involves removing nitric oxide produced during scrubbing of the gas stream by absorption of nitrogen dioxide in water and reaction of the nitrogen dioxide with water.

[0030] Absorption of nitrogen dioxide in water results in the formation of a nitric acid solution in step (a) in addition to a nitric acid-depleted gas stream, the nitric acid content of which is in the range of 20 to 65% by weight, preferably in the range of 40 to 60% by weight.

[0031] The gas stream fed to the process for removing nitric oxide, nitrogen oxides, and carbon monoxide can be obtained from any process in which nitric oxide, nitrogen oxides, and carbon monoxide are produced. Such processes include, for example, nitration processes, such as the production of organic nitro compounds, for example, the nitration of benzene, toluene, xylene, phenol, benzoic acid, mono- or polychlorobenzenes, mono- or polybromobenzenes, imidazole, and 5-ethyl-2-methyl-pyridine. The nitration can be carried out as mono-, di-, or tri-nitration. Preferably, the gas stream results from mono- or dinitration, particularly the production of dinitrotoluene.

[0032] The gas stream fed to the process for removing nitric oxides, nitrogen oxides and carbon monoxide can be pretreated by washing with acidic water, preferably a mixture of water and nitric acid or water and sulfuric acid.

[0033] The gas stream to be treated to remove nitric oxides, nitrogen oxides and carbon monoxide will typically contain carbon monoxide and nitrogen oxides, with the ratio of carbon monoxide to nitrogen oxides being in the range of 0.25:1 to 4:1, preferably in the range of 0.33:1 to 3:1, in particular in the range of 0.5:1 to 2:1, based on the respective contents in volume percent.

[0034] To purify the gas stream, in a first step (a), nitric oxide is removed from the gas stream by adding a gas containing oxygen to the gas stream to oxidize nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream, and scrubbing the nitric oxide-depleted gas stream with water to obtain a nitric oxide-depleted gas stream.

[0035] To remove nitric oxide from a gas stream, any process known to those skilled in the art for removing nitric oxide can be used. Preferably, in a first step, nitric oxide is reacted with oxygen from an oxygen-containing gas to form nitrogen dioxide, and the resulting gas stream, depleted of nitric oxide, is then subjected to a scrubbing step in which the nitrogen dioxide in the gas stream is absorbed in a suitable scrubbing liquid, e.g., water. Preferably, the reaction is carried out without a catalyst, and the reaction conditions are such that carbon monoxide is not oxidized. The reaction of nitric oxide with oxygen is generally carried out at temperatures ranging from 5 to 280°C, more preferably from 8 to 160°C, and particularly preferably from 10 to 50°C, and at pressures ranging from 1 to 12 bar (abs), more preferably from 3 to 10 bar (abs). These reaction conditions ensure that carbon monoxide does not begin to react with oxygen in step (a).

[0036] The oxygen-containing gas may be a gas mixture containing oxygen or pure oxygen. Preferably, the oxygen-containing gas is air or oxygen-enriched air. When a gas mixture other than air is used, it is preferred to use a gas mixture containing oxygen and an inert gas, such as nitrogen or a noble gas. However, it is particularly preferred that the oxygen-containing gas is air.

[0037] The nitric oxide-depleted gas stream is preferably fed to a scrubbing column to absorb the nitrogen dioxide. If water is used as the scrubbing liquid, nitric oxide and nitric acid are produced during scrubbing. The nitric oxide is usually reoxidized and absorbed in the scrubbing liquid.

[0038] The wash column used to absorb nitrogen dioxide may be a tray column or a packed column. Preferably, a tray column is used. In a tray column, the trays are preferably cooled, for example, by providing a cooling coil on the tray. For cooling, a cooling medium, particularly water, flows through the cooling coil. The number of trays in the tray column is preferably in the range of 2 to 50, more preferably 3 to 15. The trays used to absorb nitrogen dioxide in the wash liquid may be any tray known to those skilled in the art. For example, suitable trays are sieve trays, perforated trays, valve trays, or bubble trays.

[0039] The wash column is usually operated at a pressure ranging from atmospheric pressure to 10 bar (abs), preferably from 3 bar (abs) to 8 bar (abs). The temperature in the wash column is preferably in the range of 5 to 45°C, preferably 10 to 30°C.

[0040] Depending on the process from which the gas stream to be purified originates, the nitric oxide-depleted gas stream obtained in the scrubbing column typically contains nitrogen, oxygen, carbon dioxide, carbon monoxide, nitric oxide, and nitrogen oxides. When air is used for the oxidation of nitric oxide, the nitric oxide-depleted gas stream may further contain noble gases in the air, primarily argon. Furthermore, particularly when the gas stream to be purified originates from a nitration process, such as the production of dinitrotoluene, the nitric oxide-depleted gas stream may further contain traces of sulfur dioxide and mono-, di-, and trinitromethane. Furthermore, the nitric oxide-depleted gas stream may also contain non-methane hydrocarbons.

[0041] Typically, the oxygen content in the nitric oxide-depleted gas stream is 3-18% by volume, preferably 10-17% by volume; the carbon monoxide content is 0.3-7% by volume, preferably 0.8-5% by volume; the carbon dioxide content is 2-10% by volume, preferably 3-7% by volume; the nitrogen oxide content is 0.2-4% by volume, preferably 0.3-2.5% by volume; the nitrogen content is 50-90% by volume, preferably 60-80% by volume; and the nitric oxide content is 20-800 ppm by weight, preferably 60-400 ppm by weight. When air or oxygen-enriched air is used as the oxygen-containing gas, the argon content in the nitric oxide-depleted gas stream is typically 0.5-0.95% by volume, in particular 0.7-0.94% by volume.

[0042] After removing nitric oxide from the gas stream, the resulting nitric oxide-depleted gas stream is fed to step (b) for oxidizing at least a portion of the carbon monoxide.

[0043] It is preferred to preheat the nitric oxide-depleted gas stream before feeding it to step (b). Preferably, the nitric oxide-depleted gas stream is preheated by indirect heat exchange with the simultaneously cooled high-temperature purified off-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.

[0044] If preheating the nitric oxide-depleted gas stream by heat exchange with the purified off-gas stream is not sufficient, an additional heater, such as an electric heater or a burner, e.g., a gas burner, can be used. Preferably, the heater used for additional heating is an electric heater. The additional heater is further used at the start-up of the process to heat the nitric oxide-depleted gas stream to the temperature at which step (b) is carried out.

[0045] The temperature to which the nitric oxide-depleted gas stream is heated by indirect heat exchange with the hot purified off-gas stream and / or in an additional heater is preferably in the range of 200 to 400°C, more preferably in the range of 220 to 350°C, and particularly preferably in the range of 230 to 345°C.

[0046] In a first alternative, in step (b) the entire carbon monoxide is oxidized from the nitric oxide-depleted gas stream, however, alternatively and preferably only a portion of the carbon monoxide is oxidized in step (b), the carbon monoxide-depleted gas stream still comprising carbon monoxide.

[0047] Total carbon monoxide reaction in the context of the present invention means that the amount of unreacted carbon monoxide is less than 2000 ppm by volume, preferably less than 1000 ppm by volume, more preferably less than 400 ppm by volume, and in particular less than 200 ppm by volume. Thus, "carbon monoxide-free" means that the amount of carbon monoxide is less than 2000 ppm by volume, preferably less than 1000 ppm by volume, more preferably less than 400 ppm by volume, and in particular less than 200 ppm by volume.

[0048] If only a portion of the carbon monoxide reacts, the carbon monoxide-depleted gas stream, which still contains carbon monoxide, preferably contains from more than 2000 ppm by volume to a maximum of 6% by volume of carbon monoxide, preferably from 4000 ppm by volume to 5% by volume or less of carbon monoxide, more preferably from 6000 ppm by volume to 4% by volume of carbon monoxide, and particularly preferably from 8000 ppm by volume to 3% by volume of carbon monoxide.

[0049] Any process known to those skilled in the art can be used to oxidize carbon monoxide to carbon dioxide. Regardless of the process used to oxidize carbon monoxide, traces of carbon monoxide may react with nitrogen oxides. Furthermore, if carbon monoxide oxidation is performed at temperatures above 350°C, unintended reactions of traces of carbon monoxide with nitrogen oxides may occur.

[0050] In order to oxidize only a portion of the carbon monoxide contained in the nitric oxide-depleted gas stream, the oxidation conditions, such as the oxidation temperature or GHSV, and / or the amount of catalyst, can be selected so that only a portion of the carbon monoxide is oxidized. Alternatively, the nitric oxide-depleted gas stream can be divided into a first and a second partial stream, and the carbon monoxide in the first partial stream can be oxidized in (b) to obtain a carbon monoxide-depleted partial stream.

[0051] Regardless of whether all or only a portion of the carbon monoxide is oxidized in step (b), the oxidation is usually carried out in a reactor in the presence of a catalyst. The reactor is, for example, a vessel with a catalyst bed or a monolithic body containing a catalyst. Preferably, a reactor containing at least one monolithic body is used. The monolithic body is preferably designed as a rectangular parallelepiped, such as a cylinder or cube, with a circular or four- or six-sided bottom. The monolithic body may be made of a catalytically active material or a ceramic or metal body coated with a catalytically active material.

[0052] The monolithic body containing the catalytically active material may be mounted in direct contact with the vessel forming the reactor or may be incorporated into a supporting frame.

[0053] In the context of the present invention, the term "catalyst bed" is used to refer to a fluidized bed or a packed bed. In a catalyst bed, particles or packings of any shape can be used. The particles or packings can be made of catalytically active material or of a support material, such as a polymer or metal, to which a catalytically active material is applied.

[0054] The catalyst used in the reactor for oxidizing carbon monoxide is preferably a three-way catalyst, such as those used in the treatment of exhaust gases from engine combustion to simultaneously destroy carbon monoxide, hydrocarbons, and nitrogen oxides. In this case, traces of nitrogen oxide remaining in the nitrogen oxide-depleted gas stream are also at least partially reduced in step (b). Alternatively, but less preferably, the catalyst may be a two-way catalyst or so-called VOC catalyst for converting hydrocarbons and carbon monoxide to carbon dioxide and water by reaction with oxygen. The additional catalytically active material used to oxidize carbon monoxide to form carbon dioxide may be, for example, a mixed oxide such as aluminum and / or silicon and / or copper oxide and / or magnesium oxide.

[0055] Precious metals such as platinum, ruthenium, or palladium are also suitable as catalytic materials for oxidizing carbon monoxide. However, these precious metal catalysts are typically sulfur-sensitive. Because gas streams may contain sulfur-containing components such as sulfur dioxide, the use of mixed oxide catalysts is preferred, especially when the gas stream being treated originates from a nitration process.

[0056] The gas hourly space velocity (GHSV) of the catalyst is usually 4,000 to 200,000 standard m 3 / (m 3 Catalyst·h), preferably 8000 to 150,000 standard m 3 / (m 3 catalyst·h).

[0057] The production of carbon dioxide by oxidation of carbon monoxide in the presence of a catalyst is usually carried out at a reaction temperature in the range of 230 to 600°C, preferably in the range of 250 to 540°C. The pressure at which the oxidation of carbon dioxide is carried out is usually in the range of 800 mbar (abs) to 10 bar (abs), preferably in the range of 900 mbar (abs) to 8 bar (abs). Particularly preferably, the oxidation of carbon monoxide is carried out at an excess pressure in the range of 5 to 300 mbar above atmospheric pressure.

[0058] When the nitric oxide-depleted gas stream is divided into a first partial stream and a second partial stream, and the carbon monoxide in the first partial stream is oxidized in (b) to obtain a carbon monoxide-depleted partial stream, after the carbon monoxide in the first partial stream is oxidized, the carbon monoxide-depleted partial stream is mixed with the second partial stream, thereby obtaining a mixed stream that is depleted in nitric oxide but still contains carbon monoxide.

[0059] The mixing of the carbon monoxide-depleted partial stream and the second partial stream can be carried out in any mixing device for gas streams known to those skilled in the art. Preferably, the carbon monoxide-depleted partial stream and the second partial stream are directly combined by introducing the carbon monoxide-depleted partial stream into the second partial stream, or by introducing the second partial stream into the carbon monoxide-depleted partial stream, or by using a static mixer. When the carbon monoxide-depleted partial stream is introduced into the second partial stream, or when the second partial stream is introduced into the carbon monoxide-depleted partial stream, it is possible, for example, to use a bypass for the second partial stream that bypasses the reactor in which the carbon monoxide in the first stream is oxidized, and to open the bypass to the gas line exiting the reactor, or to open the gas line exiting the reactor to the bypass. As a further alternative, a Y connector can be used, where the bypass is connected to one leg of the Y and the line exiting the reactor is connected to the second leg of the Y, and the combined partial streams forming the carbon monoxide-depleted gas stream leave the Y at the bottom.

[0060] When a static mixer is used to mix the carbon monoxide-depleted partial stream with the second partial stream, any static mixer known to those skilled in the art can be used. Typically, such a static mixer is comprised of an insert that diverts the flow, thereby inducing turbulence in which the partial streams are mixed.

[0061] Preferably, a static mixer is used to mix the carbon monoxide-depleted partial stream with the second partial stream.

[0062] The ratio of carbon monoxide to nitrogen oxides in the carbon monoxide-depleted gas stream obtained in (b), or, if the nitric oxide-depleted gas stream is divided into a first and a second partial stream, in the gas stream after mixing the carbon monoxide-depleted partial stream with the second partial stream, is in the range of 0.1:1 to 2:1, preferably 0.3:1 to 1.5:1.

[0063] After at least a portion of the carbon monoxide has been oxidized, the resulting carbon monoxide-depleted gas stream, or, if the nitric oxide-depleted gas stream has been divided into partial streams, the partial streams are recombined and the combined stream is fed to step (c) to remove nitrogen oxides from the carbon monoxide-depleted gas stream.

[0064] Once all of the carbon monoxide has been oxidized in step (b), the nitrogen oxides are decomposed into nitrogen and oxygen. If the carbon monoxide-depleted gas stream still contains carbon monoxide, at least a portion of the carbon monoxide still contained in the carbon monoxide-depleted gas stream reacts with the nitrogen oxides, thereby forming carbon dioxide and nitrogen. Subsequently or simultaneously, the remaining nitrogen oxides are converted to nitrogen and oxygen in step (c).

[0065] Processes known to those skilled in the art can be used to decompose nitrogen oxides into nitrogen and oxygen and / or carbon dioxide.

[0066] Regardless of whether the carbon monoxide-depleted gas stream still contains carbon monoxide or whether all of the carbon monoxide is oxidized in step (b), the decomposition of nitrogen oxides is usually carried out in the presence of a suitable catalyst. The catalyst may be a catalyst bed or a monolithic catalyst, with a catalyst bed being preferred. The particles used in the catalyst bed are preferably in the form 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 2 to 6 mm, and a length of 3 to 20 mm, preferably 4 to 10 mm.

[0067] The catalyst may be any commercially available catalyst suitable for the decomposition of nitrogen oxides, such as a catalyst containing copper oxide and / or zinc oxide as catalytically active material on a support made of silicon oxide and / or aluminum oxide. Preferably, the catalyst is a zeolite catalyst, such as ZSM5 or BEA, preferably Fe-β zeolite of the BEA type. Particularly preferably, the catalyst is an organic template-free catalyst, such as that described in EP-B 2 812 283. Alternatively, a noble metal-based catalyst may be used for the decomposition of nitrogen oxides, preferably a rhodium-based catalyst, more preferably a rhodium-based catalyst, such as that described in EP-B 3 227 019.

[0068] Further suitable catalysts are Fe / Cu-OFF-ERI-zeolites, as described for example in CN-A 113198525. The catalysts described in CN-A 113198525 (Mg 0.025 Ce 0.05 Co 0.925 )Co2O4-Fe1-Cu4-OFF-ERI is a composite of three separate compounds, containing approximately 20% by mass of (Mg 0.025 Ce 0.05 Co 0.925 ) Co2O4 spinel, approximately 35 wt% Fe and Cu exchanged OFF-ERI-zeolite, bound by approximately 45 wt% Al / Si mixed metal oxide, also estimated as OFF-ERI-zeolite.

[0069] The decomposition of nitrogen oxides in the presence of a catalyst is carried out at a temperature in the range of 300 to 600°C, preferably in the range of 420 to 560°C, and at a pressure in the range of 800 mbar (abs) to 10 bar (abs), preferably in the range of 900 mbar (abs) to 8 bar (abs), in particular in the range of 1000 mbar (abs) to 1200 mbar (abs). Particularly preferably, the decomposition of nitrogen oxides is carried out at an excess pressure in the range of 5 to 300 mbar relative to atmosphere.

[0070] The gas load for decomposing nitrogen oxides is 2000-30000 standard m3 / (m 3 Catalyst·h), preferably 4000 to 20000 standard m 3 / (m 3 Catalyst·h), especially 8000~15000 standard m 3 / (m 3 The catalyst may range from 0.1 to 0.25.

[0071] If the carbon monoxide-depleted gas stream still contains carbon monoxide, at least a portion of the carbon monoxide will react with nitrogen oxides to form carbon dioxide and nitrogen. Depending on the amount of carbon monoxide and nitrogen oxides in the carbon monoxide-depleted gas stream and the reaction conditions (particularly the GHSV), either all of the carbon monoxide in this gas stream will react with nitrogen oxides or only a portion of the carbon monoxide will react with nitrogen oxides.

[0072] Once all the carbon monoxide in a carbon monoxide-depleted gas stream has reacted with nitrogen oxides, the amount of carbon monoxide will usually be in excess of nitrogen oxides, and the remaining nitrogen oxides must be decomposed to produce nitrogen and oxygen.

[0073] Both the reaction of nitrogen oxides with carbon monoxide to form carbon dioxide and nitrogen, and the reaction of nitrogen oxides decomposing to form nitrogen and oxygen, typically take place in the same reactor and under the same conditions. Thus, if the carbon monoxide-depleted gas stream still contains carbon monoxide, in the reaction of step (c) some of the nitrogen oxides will react with carbon monoxide to form carbon dioxide and nitrogen, while simultaneously decomposing the nitrogen oxides to nitrogen and oxygen.

[0074] If the amount of carbon monoxide is such that not all of the carbon monoxide reacts with the nitrogen oxides, the purified gas stream obtained in step (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. This second oxidation of carbon monoxide is carried out in the same manner as the oxidation of carbon monoxide in step (b) described above.

[0075] The purified gas stream obtained in (c), when the carbon monoxide has been completely oxidized in (b), or when the remaining carbon monoxide has been completely reacted with nitrogen oxides, or after oxidizing the remaining carbon monoxide if the gas stream obtained in (c) still contains carbon monoxide, will typically have less than 800 ppm by mass, preferably less than 400 ppm by mass, nitrogen oxides, less than 400 ppm by mass, preferably less than 200 ppm by mass, carbon monoxide, and less than 2000, preferably less than 1000, more preferably less than 500 ppm by volume, nitrogen oxides, and especially preferably less than 100 ppm by volume.

[0076] Particularly preferably, the process is carried out so that the carbon monoxide-depleted gas stream obtained in step (b) still contains carbon monoxide, with only a portion of the remaining carbon monoxide reacting with nitrogen oxides, and the purified gas stream obtained in step (c) still contains carbon monoxide, which remaining carbon monoxide is oxidized to carbon dioxide in a second oxidation step. For the oxidation of carbon monoxide in the second oxidation step, preferably the residual oxygen in the gas stream is used, so that no additional oxygen-containing gas is required.

[0077] The ability to minimize or completely eliminate the addition of external reducing agents is a particularly significant advantage of the process of the present invention. A further advantage is that the process is highly effective and environmentally friendly, due to two mechanisms that reduce emissions of carbon monoxide, volatile organic compounds, and non-methane hydrocarbons.

[0078] A further advantage of the process of the present invention is that by separating the conversion of nitrogen oxides in step (a) and the oxidation of carbon monoxide in step (b) into two distinct steps, the temperature in step (a) is not additionally increased by the heat of reaction of the exothermic oxidation of carbon monoxide, because nitrogen dioxide absorption works better at lower temperatures and the additional heat generated by the oxidation of carbon monoxide would have to be removed. Furthermore, the heat generated in the oxidation of carbon monoxide in step (b) increases the temperature of the carbon monoxide-depleted gas stream, which is favorable for the conversion of nitrogen oxides. This additional heating of the gas stream has the advantage that the amount of catalyst or the amount of heat added can be kept low.

[0079] To begin the process, the catalyst for carbon monoxide oxidation and nitrogen oxide decomposition 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 over the catalyst for carbon monoxide oxidation and nitrogen oxide decomposition. The gaseous medium used for heating can be pressurized by a blower and passed through the heater and catalyst, or it can be taken from the plant's operating network, for example. An electric heater, for example, is used to heat the gaseous medium. Alternatively, the gaseous medium can be heated by direct or indirect heat exchange with exhaust gases from natural gas combustion or by a regenerative heat exchanger operated with hot exhaust gases from the catalyst. Preferably, a combination of electrical and regenerative heating is used. Heating can also be performed in multiple stages simultaneously, for example, by simultaneous regenerative and electrical heating.

[0080] The catalyst can be heated either in a straight path or in a cycle. In the cycle heating mode, the gaseous medium, after passing through the catalyst to be heated, is again directed to the input side of the heater by means of a suitable blower.

[0081] Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

[0082] In the diagram: FIG. 1 shows a flow chart of the method of the present invention.

[0083] The only figure shows a schematic flow chart of the method of the invention.

[0084] To remove nitric oxide, nitrogen oxides, and carbon monoxide from gas stream 1, oxygen-containing gas 3 is added to gas stream 1. The oxygen in the oxygen-containing gas stream begins to react with the nitric oxide in gas stream 1 to form nitrogen dioxide. The mixture obtained by adding oxygen-containing gas 3 to gas stream 1 is then fed to absorption step 5, where the gas stream is mixed with water 7, thereby producing a nitric acid-containing aqueous stream 9 and a nitric oxide-depleted gas stream 11.

[0085] The nitric oxide-depleted gas stream 11 is then preheated in the first heat exchanger 13 and optionally in the additional heater 15. Preferably, however, during steady-state operation of the method, the additional heater 15 is switched off and only the nitric oxide-depleted gas stream 11 is preheated in the first heat exchanger 13. However, if the nitric oxide-depleted gas stream 11 is not sufficiently heated in the first heat exchanger 13 or during start-up of the method, the nitric oxide-depleted gas stream 11 is (additionally) heated in the additional heat exchanger 15.

[0086] The preheated nitric oxide-depleted gas stream is then passed to an oxidation step 17 where at least a portion of the carbon monoxide is oxidized to form carbon dioxide, resulting in a carbon monoxide-depleted gas stream 19.

[0087] The carbon monoxide-depleted gas stream 19 is then fed to a nitrogen oxides conversion step 21 in which the nitrogen oxides are converted to nitrogen and oxygen if the carbon monoxide-depleted gas stream does not contain any carbon monoxide, or to nitrogen and oxygen and / or carbon dioxide if the carbon monoxide-depleted gas stream still contains carbon monoxide. If only a portion of the carbon monoxide is oxidized in the oxidation step 17 and the gas stream still contains carbon monoxide after the nitrogen oxides conversion step 21, the nitrogen oxides conversion step 21 is followed by a second oxidation step 23 in which the remaining carbon monoxide is oxidized to carbon dioxide.

[0088] A purified off-gas stream 25 is obtained by converting nitric oxide to nitrogen dioxide, absorbing the nitrogen dioxide in water, oxidizing the carbon monoxide, converting the nitrogen oxides to nitrogen and oxygen and / or carbon dioxide, and optionally secondary oxidation of the carbon monoxide.

[0089] To operate the process energy efficiently, it is preferred to use heat from the purified off-gas stream 25 to heat the nitric oxide-depleted gas stream 11 in the first heat exchanger 13. The first heat exchanger may be any heat exchanger suitable for transferring heat from the purified off-gas stream 25 to the nitric oxide-depleted gas stream 11 by indirect heat transfer.

[0090] During steady-state operation of the process, the heat transferred from the purified off-gas stream 25 to the nitric oxide-depleted gas stream 11 is sufficient to preheat the nitric oxide-depleted gas stream 11, so that the additional heater 15 can be switched off and the nitric oxide-depleted gas stream 11 simply passes through the additional heater 15 without being further heated. In this case, the additional heater 15 is only required to heat the nitric oxide-depleted gas stream 11 during start-up or when the process is operated at part load and the volumetric flow of the purified off-gas stream is not sufficient to heat the nitric oxide-depleted gas stream 11.

Claims

1. 1. A method for removing nitric oxide, nitrogen oxides, and carbon monoxide from a gas stream (1), comprising: (a) removing nitric oxide from said gas stream (1) by adding an oxygen-containing gas (3) to said gas stream (1) to oxidize nitric oxide in the gas stream to form nitrogen dioxide, thereby obtaining a nitric oxide-depleted gas stream, and scrubbing said nitric oxide-depleted gas stream with water (7) to obtain a nitric oxide-depleted gas stream (11); (b) oxidizing at least a portion of the carbon monoxide in said nitric oxide-depleted gas stream (11) to produce carbon dioxide, thereby obtaining a carbon monoxide-depleted gas stream (19); (c) converting the nitrogen oxides in said carbon monoxide-depleted gas stream (19) to nitrogen and oxygen if said carbon monoxide-depleted gas stream (19) does not contain any carbon monoxide, or to nitrogen and oxygen and / or carbon dioxide if said carbon monoxide-depleted gas stream (19) contains any carbon monoxide, to obtain a purified off-gas stream (25); A method comprising:

2. The method of claim 1 wherein the oxygen-containing gas is air.

3. 3. The method of claim 1, wherein the volume ratio of carbon monoxide to nitrogen oxides in the gas stream is in the range of 0.25:1 to 4:

1.

4. 3. The method of claim 1 or 2, wherein a portion of the carbon monoxide in the nitric oxide-depleted gas stream (11) is oxidized in (b) so that the carbon monoxide-depleted gas stream (19) still contains carbon monoxide and at least a portion of the remaining carbon monoxide is reacted with nitrogen oxides.

5. 3. The method according to claim 1, wherein the nitrogen oxide-depleted gas stream (11) is divided into a first partial stream and a second partial stream in order to oxidize only a portion of the carbon monoxide, the carbon monoxide in the first partial stream is oxidized in (b) to obtain a carbon monoxide-depleted partial stream, and then the carbon monoxide-depleted partial stream and the second partial stream are mixed to obtain a combined stream, which is fed to (c).

6. 3. The method of claim 1 or 2, wherein a portion of the remaining carbon monoxide reacts with nitrogen oxides, and if only a portion of the carbon monoxide reacts with nitrogen oxides, after decomposition of the nitrogen oxides in (c), the remaining carbon monoxide is oxidized to form carbon dioxide.

7. 3. The process according to claim 1 or 2, wherein the oxidation of carbon monoxide is carried out in the presence of a catalyst at a temperature in the range of 230 to 600° C. and a pressure in the range of 800 mbar (abs) to 10 bar (abs).

8. 3. The method of claim 1, wherein the ratio of carbon monoxide to nitrogen oxides in the carbon monoxide-depleted gas stream (19) obtained in (b), or, if the nitric oxide-depleted gas stream (11) is divided into a first and a second partial stream, the ratio of carbon monoxide to nitrogen oxides in the carbon monoxide-depleted gas stream (19) after mixing the carbon monoxide-depleted partial stream with the second partial stream, is in the range of 0.1:1 to 2:1.

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