Title of Invention	A PROCESS AND AN APPARATUS FOR REMOVING NITROGEN OXIDES FROM A GAS STREAM
Abstract	"A process and an apparatus for removing nitrogen oxides from a gas stream" The invention relates to a process for removing nitrogen oxides from a gas stream containing same, which comprises passing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of 1.5 to 20 bar, and (B) through a stage for reducing the amount of N2O, preferably using the pressure level present in stage A, and to apparatus therefor and the use thereof.

Title of Invention

A PROCESS AND AN APPARATUS FOR REMOVING NITROGEN OXIDES FROM A GAS STREAM

Abstract

"A process and an apparatus for removing nitrogen oxides from a gas stream" The invention relates to a process for removing nitrogen oxides from a gas stream containing same, which comprises passing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of 1.5 to 20 bar, and (B) through a stage for reducing the amount of N2O, preferably using the pressure level present in stage A, and to apparatus therefor and the use thereof.

Full Text	The present invention relates to a process and an apparatus for removing nitrogen oxides such as NO, NO2 and N2O from a gas stream containing same. Nitrogen oxides are formed as by-products in many processes in which HNO3 is used as oxidizing agent in liquid phase. Especially the conversion of alcohols, aldehydes and ketones, for example the conversion of cyclohexanol and cyclohexanone into adipic acid, of acetaldehyde into glyoxal or of glyoxal into glyoxylic acid, and also the production of nicotinic acid and hydroxylamines liberate for example appreciable amounts of N2O as well as other nitrogen oxides. In Science 251 (1991), 932, Thiemens and Trogler show that N2O has a certain destructive potential for the Earth's atmosphere. N2O serves as the major stratospheric source of NO, which in turn has an essential influence on the depletion of ozone in the stratosphere. In addition, N2O is considered a greenhouse gas whose global warming potential is said to be about 290 times greater than that of CO2. Recent years have witnessed the publication of a multiplicity of patent and non¬patent documents concerned with reducing the N2O emissions due to anthropogenic sources. A multiplicity of patents describe catalysts for reducing or decomposing N2O, for example DE 43 01 470, DE 42 24 881, DE 41 28 629, W093/15824, EP 625369, WO94/27709, US 5,171,553. US 5 200 162 discloses that the exothermic reaction of the decomposition of N2O into nitrogen and oxygen can lead to a multiplicity of process problems associated with high process temperatures. It describes a process for decomposing N2O in a gas stream by contacting an N20-contai ning gas stream under N2O decomposition conditions with a catalyst for decomposing N2O into nitrogen and oxygen by first cooling part of the exit gas whose N2O content is reduced and then recycling it into the N2O decomposition zone. In the case of N2O-containing waste gas streams containing additional NO^ it is stated to be frequently very desirable to pretreat the gas stream to remove NO^ upstream of the N2O decomposition zone by selective reduction of NO^ with ammonia in the presence of oxygen. In Abatement of N2O emissions produced in the adipic acid indu¬stry, Environmental Progress 13 (1994), No. 2, May, 134 - 137, Reimer, Slaten, Seapan, Lower and Tomlinson describe a boiler gas rebum system coupled with selective non-catalytic reduction (SNCR) for destroying N2O. A flow diagram of the catalytic decomposition of N2O shows an N2O decomposition catalyst stage coupled with an NO^ abatement SCR catalyst stage. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A17, 1991, pages 293-339, describes the production of HNO3 by burning ammonia and absorbing the combustion products in water. Nonselec¬tive catalytic reduction (NSCR) and selective catalytic reduction (SCR) processes can be used for treating the waste gases from the HNO3 produc¬tion process. It is an object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same. It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing major quantities of N2O as well as other nitrogen oxides. It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same to produce nitric acid (HNO3). It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same under simple conditions. It is a further object of the present invention to provide an apparatus for the aforementioned processes. Accordingly, the present invention provides a process for removing nitrogen oxides from a gas stream comprising same, which comprises directing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of from 1.5 to 20 bar, and (B) through a stage for lowering the N2O content, preferably by utilizing the pressure level at a temperature of 200 - 700°C prevailing in process stage A, and, after stages A and B, through (C) a stage for catalytically reducing nitrogen oxides other than N2O at a temperature of 150 - 500'C, wherein the gas stream is directed first through stage A and then through stage B, wherein the catalyst used in stage C is one selected of the group comprising platinium, vanadium pentoxide, iron oxide, titanium, rhodium, ruthenium, palladium, iron, cobalt, nickel, tungsten oxide and molybdenum oxide. Accordingly, the present invention also provides an apparatus for removing nitrogen oxides from a gas stream comprising same, comprising (a) an absorption unit for absorbing the nitrogen oxides other than N2O in an absorbent or for reacting the nitrogen oxides other than N2O with an absorbent, (b) a process unit for lowering the N2O content and (c) a reduction unit for reducing nitrogen oxides other than N2O, wherein the gas stream is directed successively through the stages (a), (b) and (c). The term "nitrogen oxides" as used in the description and the claims designates the oxides of nitrogen, especially dinitrogen oxide (NoO), nitrogen monoxide (NO), dinitrogen trioxide (N2O3), nitrogen dioxide (NO2), dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N2O5), nitrogen peroxide (NO3). The present invention provides in particular a process for removing nitrogen oxides from gas streams as obtained for example as waste gas streams in processes for producing adipic acid, nitric acid, hydroxylamine derivatives, caprolactam, glyoxal, methyl glyoxal, glyoxylic acid, or in processes for burning nitrogenous materials. The aforementioned processes as well as other processes for oxidizing organic compounds with nitric acid give rise to reaction products containing nitrogen oxides. For instance, tlie production of adipic acid by oxidation of a cyclohexanone/cyclohexanol mixture gives rise to a 'waste gas having, for example, the following composition: NO2 20% by volume N2O 23% by volume O2 10% by volume CO+CO2 2% by volume N2+Ar 45% by volume According to the present invention, the removal of nitrogen oxides as present for example in the aforementioned composition is effected by passing the gas stream A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent, and B) through a stage for reducing N2O. Preferably, the gas stream passes first through stage A and then through stage B. Stage A The absorption of the nitrogen oxides other than N2O in an absorbent and the reaction of the nitrogen oxides other than N2O with an absorbent, as the case may be, can be carried out with any desired suitable absorbents. The preferred absorbent is water or an aqueous solution, e.g. of nitric acid, in which case the absorption is preferably carried out in the presence of free oxygen and the nitrogen oxides other than N2O are prefe¬rably converted into HNO3. In particular, for example, nitrogen monoxide is oxidized to nitro¬gen dioxide and nitrogen dioxide is absorbed in water to form HNO3. Such a process is described in Ulknann's Encyclopedia of Industrial Chemistry, 5th edition, volume A17, 1991, pages 293-339. The process for the conversion into nitric acid can be characterized by two exothermic reaction steps: oxidation of nitrogen monoxide with atmospheric oxygen to nitrogen dioxide according to: The reactions are promoted by high pressures and low temperatures. Pressu¬res of from 1,5 to 20 bar, preferably from 3 to 12 bar, particularly preferably from 5 to 10 bar are employed. The gas inlet temperature on entry into stage A is preferably from 10 to 100°C, particularly preferably 20-60°C, in particular from 30 to 40°C. The gas streams from the oxidation of alcohols, aldehydes and ketones often contain NO2 in a concentration of more than 1% by volume, so that the NO2 can be considered not an impurity but a material of value and therefore can be converted into nitric acid by reaction with water. The reaction can take place in absorption columns and is described for example in Ulhnann's, loc. cit. The heat produced in the exothermic reaction can be utilized for generating process steam and/or for heating the gas streams containing nitrogen oxides, for example in a gas/gas heat exchanger. Stage B Stage B is a stage for reducing the amount of N2O. The reduction of the amount of N2O can be effected by tiiermal decomposition and/or by catalytic decomposition. The process can be carried out adiabatically or isothermally, preferably using the pressure level of process step A. The removal of N2O can be carried out in various ways, for example by heterogeneous catalysis. In the adiabatic reaction regime, where the heat evolved by the exotherm of the decomposition reaction is utilized for heatmg the catalyst bed, the gas inlet temperamre on entry into stage B is 200-700°C, preferably 300-600°C, more preferably 400-550°C, particular¬ly preferably 430-550°C, in particular 450-500°C. The gas inlet temperamre can depend on the activity of the catalyst. To minimize the thermal formation of NO^ and to protect the catalyst used from destruction due to excessive temperamres (e.g. by sinte- ring), the temperature of the gas stream on exit from the reactor (stage B) should not significantly exceed 800°C. This can be achieved for example by the concentration of N2O in the gas stream on entry mto stage B not being more than 40% by volume, preferably within the range from 0.1 to 20% by volume, particularly preferably within the range from 0.5 to 15% by volume, in particular within the range from 1 to 13% by volume. Gas streams often comprise N2O in amounts of more than 20% by volume. A reduction of the N2O concentration can be achieved for example by admixmg the gas stream with an essentially N20-free gas stream up¬stream of stage B. The admixing can also be carried out upstream of stage A, if the gas stream first passes through stage A. The essentially N20-free gas stream can be the gas stream leaving stage B or, as explained below, optionally the gas stream leaving stage C and/or a gas stream containing free oxygen, and/or a process gas. The N2O removal can also be carried out isothermally. This is possible for example in a tube bundle reactor with salt bath or metal bath cooling. This process is characterized in that the temperatore of the gas stream on exit from the reactor (stage B) corresponds to the temperature of the salt or metal batii and the molten salt or metal absorbs the heat relea¬sed by the N2O decomposition reaction. The salt or metal bath temperamre is preferably 400-650 °C or corresponds to the temperature of the adiabatic reaction regime. The gas stream can be heated up either upstream of stage B by a heat exchanger, such as a gas/gas heat exchanger, or directly in the salt or metal bath reactor of stage B. Another possibility is the removal of N2O (decomposition) in a fluidized bed. Catalysts Examples of catalysts suitable for N2O removal by catalytic decom¬position are the catalysts described in DE 43 01 470, DE 42 24 881, DE 41 28 629, W093/15824, EP 625369, WO94/27709, US 5,171,553. Suitable catalysts may consist for example of CuO, ZnO and AI2O3 or addi¬tionally include Ag. It is possible to use catalysts with Ag as active compo¬nent applied to a gamma-Al203 support. Further examples of usable cata¬lysts are those having CoO and/or NiO on a Zr02 support. The use of zeolitic catalysts, for example mordenites, which are present in the H+ or NH4"'" form and may be exchanged with V, Cr, Fe, Co, Ni, Cu and/or Bi is likewise possible. Also suitable are catalysts consisting of zeolites having an Si02/Al203 ratio of at least 550, for example beta zeolite, ZSM-5, 4 zeoli¬te, mordenite or chabazite and are present in the H"" or NH4'' form and optionally exchanged with alkali, alkaline earth, transition metals or rare earth elements, in which case cobalt can be preferred as particularly suit¬able. Likewise usable are catalysts based on zeolite which have been exchanged with Cu, Co, Rh, Pd or Ir, for example. Other catalysts which make possible the reduction or decomposition of N2O are likewise usable. As well as catalytic reduction or decomposition of N2O, thermal decomposition is also possible, for example in a regenerative heat exchanger (thermoreactor). Stage C In a preferred embodiment of the present invention, the gas stream from stages A and B can be passed through a stage C for reducing nitro¬gen oxides other than N2O. The decomposition of N2O in stage B may in certain circumstances lead to the formation of nitrogen oxides NO^. These newly formed nitrogen oxides can preferably be removed in stage C. Stage C is for the reduction of nitrogen oxides other than N2O. In stage C the gas stream can be reacted by means of selective catalytic reduction (SCR), for example. In SCR, the nitrogen oxides are reacted with ammonia as reducing agent over catalysts. DENOX catalysts can be used for example. The products are nitrogen and water. Stage C may also be run as a nonselective catalytic reduction (NSCR). NSCR involves the use of hydrocarbons to reduce the nitrogen oxides and catalysts containing noble metals. SCR and NSCR processes are described for example in UUmann's Encyclopedia of Chemical Technology, loc. cit. The catalysts used in this process can be any desired suitable catalysts. For example, catalysts for nonselective reduction processes can be based on platinum, vanadium pentoxide, iron oxide or titanium. Selective catalytic reduction catalysts may contain for example noble metals, such as Pt, Rh, Ru, Pd and/or metals of the iron group, such as Fe, Co, Ni. It is also possible to use, for example, vanadiirai pentoxide, tungsten oxide or molybdenum oxide. A further suitable catalyst is vanadium pentoxide on an alumina support. The nonselective reduction process may involve the use of suitable hydrocarbons, such as natural gas, propane, butane, naphtha, but also hydrogen. The temperature of the gas stream on entry into stage C can be for example 150-500°C, preferably 200-350°C, particularly preferably 260-300°C. It was found accordmg to the present invention that the reactions of stages A, B and, if employed, C can preferably be carried out on one pressure level. This means that the pressure of the gas stream is not additionally significantly increased or reduced between the individual stages. The pressure is at least 3 bar, preferably within the range from 3 to 20 bar, more preferably 3 to 12 bar, particularly preferably within the range from 5 to 10 bar. Stages A, B and, if employed, C can thus be accommodated in an integrated pressure apparatus consisting of die two or three, as the case may be, reactors, ie. as an integrated unit in which the gas stream is brought to the starting pressure prior to entry into one of the stages, for example by compression, and between the individual stages tiiere are no further means whereby the pressure of the gas stream is significantly increased or reduced. As the gas stream passes through the stages, the pressure in the gas can vary as a function of the stages used. Preferably, however, the pressure of the gas stream is not varied beyond that. On exit from the last stage the gas stream can be brought to atmospheric pressure, for example by means of a decompression turbine. Conducting the entire process at one pressure level allows simple process control and a simplified construction of the entire apparatus for removing nitrogen oxides. Process control can be greatly simplified as a result. In a preferred embodiment, the gas stream passes through stages A, B, C, preferably in that order, and before entry into stage A is admi¬xed with air and/or a gas stream leaving B or C and/or a process gas so that the N2O content is preferably not more than 20% by volume. The gas stream is contacted in stage A with water or an aqueous solution of e.g. nitric acid in an absorption column in countercurrent to form HNO3 and the product HNO3 is removed at the base of the column, then the remaining gas stream is brought to a temperature of 200-700°C, preferably 450-500°C and contacted in stage B in a fixed bed with a catalyst for catalytic decomposition of N2O, the remaining gas stream is then brought to a temperature of 150-500°C, preferably 260-300°C and subjected in stage C to a catalytic reduc¬tion. The heat of reaction evolved in the individual stages can be utilized for generating steam and mechanical drive energy. For example, the gas stream can be brought upstream of stage A to a pressure of from 1,5 to 20 bar abs. by means of a compressor (VI) and downstream of stage C to ambient pressure by means of an expansion turbine (Tl), in which case the energy released in the expansion turbine (Tl), as can be provided for example by a motor or engine, is supplied to the compressor (VI) with or without further energy (M), The energy released in the individual reaction stages can also be used for preheating the gas stream. For example, the gas stream, before entry into stage A, can be cooled in a heat exchanger (WTl) with the gas stream emerging from stage A. Similarly, the gas stream, before entry mto stage B, can be heated in a heat exchanger (WT3) with the gas stream emerging from stage B. In addition, the gas stream, downstream of the heat exchanger (WTl) and before entry into stage A, can be additionally further cooled to the desked temperature with a further heat exchanger (WT2). Furthermore, the gas stream, downstream of the heat exchanger (WT3) and before entry into stage C, can be additionally further cooled with a heat exchanger (WT4). As well as the process for removing nitrogen oxides from a gas stream containing same, the present invention also provides an apparatus therefor. The apparatus comprises the above-described stages A, B and preferably the above-described stages A, B and C preferably in that order. Other orders of the stages, e.g. B A C or A C B or similar are possible according to one embodiment of the invention. The individual stages in the apparams are preferably interconnected using suitable lines in such a way that the gas stream can pass through the stages in succession. Preferably, the apparatus for removing nitrogen oxides from a gas stream containing same includes, upstream of the first stage, an apparams whereby the gas stream can be brought to a desired pressure and no further apparams for additionally significantly increasing or reducing the pressure of the gas stream between the individual stages. In a preferred embodiment, the apparatus comprises the above-described compressor (VI) and expansion turbine (Tl) and also a motor/-engine (M), as described above. In a further preferred embodiment of the apparatus, it comprises the heat exchangers (WTl) and (WT3) arranged as described above. In a further preferred embodiment of the apparatus, it comprises the heat exchangers (WT2) and (WT4) arranged as described above. The present invention also relates to the use of the above-described apparatus for removing nitrogen oxides from a gas stream containing same. The gas stream in question preferably comprises a waste gas stream from processes for producing adipic acid, nitric acid, hydroxylamine derivatives or caprolactam or from processes for burning nitrogenous materials. The present invention further provides for the use of the above-described apparatus for producing HNO3. A preferred apparatus according to the present mvention and a preferred process according to the present invention will now be described with reference to the drawing which is a diagram of an apparatus according to the present invention. The reference symbols in the drawing have the following meanings: Kl: absorption column (stage A) CI: N20-cracking reactor (stage B) C2: reactor for catalytic NO^ reduction (stage C) WTl: heat exchanger 1 WT2: heat exchanger 2 WT3: heat exchanger 3 WT4: heat exchanger 4 VI: compressor Tl: expansion turbine [: motor/engine Example In an apparatus constructed according to the accompanying drawing, process and waste gases containing nitrogen oxides (line 1) are mixed via line 2 with air and/or via line 3 with N20-lean or NO- and N02-containing process gases. The admixture of air and N2O lean or -free (process) gas limits the temperature increase due to the adiabatically operated N2O decom¬position in the downstream reactor CI to 350°C maximum. In addition, air is admitted to support the oxidation of NO according to the above-recited equation (I) and thus the formation of nitric acid according to equation (II) in the absorption column Kl. The production of nitric acid (HNO3) in the absorption column Kl can be additionally increased by the addition of NO-and/or N02-containing gases. In a preferred embodiment, process gases from ammonia oxidation reactors can be fed m via line 3. The gas mixture (the gas stream containing nitrogen oxides) is then compressed by means of the compressor (V). The resulting increased pressu¬re of the gas mixture considerably improves the effectiveness of the downst¬ream absorption column Kl (stage A) of the N20-cracking reactor CI (stage B) and of the reactor for catalytic NOx reduction C2 (stage C) in a prefer¬red embodiment. The evolved heat of compression and the simultaneous oxidation of NO to NO2 increases the temperature of the gas stream in line 4 to 250-350°C. The gas stream is cooled down to 30-40°C in a gas/gas heat exchanger (WTl) with cold gas stream from the absorption subsequent¬ly in the heat exchanger (cooler) (WT2) with a suitable cooling medium such as an- or cooling water. The NO2 absorption and reaction with water to form nitric acid is carried out in the downstream absorption column Kl (stage A), where the gas stream and the absorbent (e.g. water or aqueous nitric acid) are passed countercurrently over suitable internal fitments and the resulting nitric acid is withdrawn from the base of the column. The gas stream (line 6) freed from the bulk of the NO2 and NO is then heated in a gas/gas heat exchanger (WTl) to 200-300°C (line 7) and in the downstream gas/gas heat exchanger CWT3) to 450-5(X)°C (line 8). The removal of the N2O takes place in reactor C1 (stage B), the temperature rismg to up to 825°C (Ime 9), The gas stream is then cooled down in gas/gas heat exchanger (WT3) and subsequently in the steam generator (heat exchanger WT4) to 260-300°C (line 10). Then the gas stream is freed by catalytic reduction from remaining nitrogen oxide traces in reactor C2 (stage C) by catalytic reduction. In the case of NOx contents of the waste gas of 1(XX) ppm the adiabatic temperature increase is about 10°C. The gas stream is then fed via line 11 at a temperature of 265-310°C to an expansion turbine (Tl), where it is decompressed to atmosphe¬ric pressure and released into the atmosphere at about 100°C via line 12. The drive energy generated in turbine (Tl) can be utilized, via a common shaft, for driving the compressor (VI). The missing drive energy is then additionally supplied via an additional motor/engine (M). WE CLAIM; 1. A process for removing nitrogen oxides from a gas stream comprising same, which comprises directing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of from 1.5 to 20 bar, and (B) through a stage for lowering the N2O content, preferably by utilizing the pressure level at a temperature of 200 - 700°C prevailing in process stage A, and, after stages A and B, through (C) a stage for catalytically reducing nitrogen oxides other than N2O at a temperature of 150 - 500°C, wherein the gas stream is directed first through stage A and then through stage B, wherein the catalyst used in stage C is one selected of the group comprising platinium, vanadium pentoxide, iron oxide, titanium, rhodium, ruthenium, palladium, iron, cobalt, nickel, tungsten oxide and molybdenum oxide. The process as claimed in claim 1, wherein, in stage A, the absorbent used is water or an aqueous solution of nitric acid and the nitrogen oxides other than N2O are converted into HNO3 in the presence or absence of free oxygen. The process as claimed in claim 1 or 2, wherein, in stage B, the lowering in the N2O content is effected by thermal decomposition and/or by catalytic decomposition. The process as claimed in any one of claims 1 to 3, wherein the gas stream is brought upstream of stage A to a pressure of from 1.5 to 20 bar by means of a compressor (VI) and downstream of stage C to ambient pressure by means of is supplied to the compressor (VI) with or without further energy (M), wherein the gas stream, before entry into stage A, is preferably cooled in a heat exchanger (WTl) with the gas stream emerging from stage A and, before entry into stage B, heated in a heat exchanger (WT3) with the gas stream emerging from stage B, wherein the gas stream, downstream of the heat exchanger (WTl) and before entry into stage A, is preferably additionally further cooled with a heat exchanger (WT2) and the gas stream, downstream of the heat exchanger (WT3) and before entry into stage C, is additionally further cooled with a heat exchanger (WT4). An apparatus for removing nitrogen oxides from a gas stream comprising same, comprising (a) an absorption unit for absorbing the nitrogen oxides other than N2O in an absorbent or for reacting the nitrogen oxides other than N2O with an absorbent, (b) a process unit for lowering the N2O content and (c) a reduction unit for reducing nitrogen oxides other than N2O, wherein the gas stream is directed successively through the stages (a), (b) and (c). The apparatus as claimed in claim 5, wherein, in stage A, the absorbent used is water or an aqueous solution of nitric acid and the nitrogen oxides other than N2O are converted into HNO3 in the presence or absence of free oxygen. The apparatus as claimed in claim 5 or 6, wherem, in stage B, the lowering in the N2O content is effected by thermal and/or catalytic decomposition. The apparatus as claimed in any one of claims 5 to 7 further comprising the apparatus of claim 4. 9. A process for removing nitrogen oxides from a gas stream substantially as herein described with reference to the accompanying drawing. 10. An Apparatus for removing nitrogen oxides from a gas stream substantially as herein described with reference to the accompanying drawing.

Full Text

The present invention relates to a process and an apparatus for removing nitrogen oxides such as NO, NO2 and N2O from a gas stream containing same. Nitrogen oxides are formed as by-products in many processes in which HNO3 is used as oxidizing agent in liquid phase. Especially the conversion of alcohols, aldehydes and ketones, for example the conversion of cyclohexanol and cyclohexanone into adipic acid, of acetaldehyde into glyoxal or of glyoxal into glyoxylic acid, and also the production of nicotinic acid and hydroxylamines liberate for example appreciable amounts of N2O as well as other nitrogen oxides.
In Science 251 (1991), 932, Thiemens and Trogler show that N2O has a certain destructive potential for the Earth's atmosphere. N2O serves as the major stratospheric source of NO, which in turn has an essential influence on the depletion of ozone in the stratosphere. In addition, N2O is considered a greenhouse gas whose global warming potential is said to be about 290 times greater than that of CO2.
Recent years have witnessed the publication of a multiplicity of patent and non¬patent documents concerned with reducing the N2O emissions due to anthropogenic sources.
A multiplicity of patents describe catalysts for reducing or decomposing N2O, for example DE 43 01 470, DE 42 24 881, DE 41 28 629, W093/15824, EP 625369, WO94/27709, US 5,171,553.
US 5 200 162 discloses that the exothermic reaction of the decomposition of
N2O into nitrogen and oxygen can lead to a multiplicity of process problems
associated with high process temperatures. It describes a process for decomposing
N2O in a gas stream by contacting an N20-contai

ning gas stream under N2O decomposition conditions with a catalyst for decomposing N2O into nitrogen and oxygen by first cooling part of the exit gas whose N2O content is reduced and then recycling it into the N2O decomposition zone. In the case of N2O-containing waste gas streams containing additional NO^ it is stated to be frequently very desirable to pretreat the gas stream to remove NO^ upstream of the N2O decomposition zone by selective reduction of NO^ with ammonia in the presence of oxygen.
In Abatement of N2O emissions produced in the adipic acid indu¬stry, Environmental Progress 13 (1994), No. 2, May, 134 - 137, Reimer, Slaten, Seapan, Lower and Tomlinson describe a boiler gas rebum system coupled with selective non-catalytic reduction (SNCR) for destroying N2O. A flow diagram of the catalytic decomposition of N2O shows an N2O decomposition catalyst stage coupled with an NO^ abatement SCR catalyst stage.
Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A17, 1991, pages 293-339, describes the production of HNO3 by burning ammonia and absorbing the combustion products in water. Nonselec¬tive catalytic reduction (NSCR) and selective catalytic reduction (SCR) processes can be used for treating the waste gases from the HNO3 produc¬tion process.
It is an object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same.
It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing major quantities of N2O as well as other nitrogen oxides.
It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same to produce nitric acid (HNO3).

It is a further object of the present invention to provide a process for removing nitrogen oxides from a gas stream containing same under simple conditions.
It is a further object of the present invention to provide an apparatus for the aforementioned processes.
Accordingly, the present invention provides a process for removing nitrogen oxides from a gas stream comprising same, which comprises directing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of from 1.5 to 20 bar, and (B) through a stage for lowering the N2O content, preferably by utilizing the pressure level at a temperature of 200 - 700°C prevailing in process stage A, and, after stages A and B, through (C) a stage for catalytically reducing nitrogen oxides other than N2O at a temperature of 150 - 500'C, wherein the gas stream is directed first through stage A and then through stage B, wherein the catalyst used in stage C is one selected of the group comprising platinium, vanadium pentoxide, iron oxide, titanium, rhodium, ruthenium, palladium, iron, cobalt, nickel, tungsten oxide and molybdenum oxide.
Accordingly, the present invention also provides an apparatus for removing nitrogen oxides from a gas stream comprising same, comprising (a) an absorption unit for absorbing the nitrogen oxides other than N2O in an absorbent or for reacting the nitrogen oxides other than N2O with an absorbent, (b) a process unit for lowering the N2O content and (c) a reduction unit for reducing nitrogen oxides other than N2O, wherein the gas stream is directed successively through the stages (a), (b) and (c).

The term "nitrogen oxides" as used in the description and the claims designates the oxides of nitrogen, especially dinitrogen oxide (NoO), nitrogen monoxide (NO), dinitrogen trioxide (N2O3), nitrogen dioxide (NO2), dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N2O5), nitrogen peroxide (NO3).
The present invention provides in particular a process for removing nitrogen oxides from gas streams as obtained for example as waste gas streams in processes for producing adipic acid, nitric acid, hydroxylamine derivatives, caprolactam, glyoxal, methyl glyoxal, glyoxylic acid, or in processes for burning nitrogenous materials.
The aforementioned processes as well as other processes for
oxidizing organic compounds with nitric acid give rise to reaction products
containing nitrogen oxides. For instance, tlie production of adipic acid by
oxidation of a cyclohexanone/cyclohexanol mixture gives rise to a 'waste gas
having, for example, the following composition:
NO2 20% by volume
N2O 23% by volume
O2 10% by volume
CO+CO2 2% by volume
N2+Ar 45% by volume
According to the present invention, the removal of nitrogen oxides as present for example in the aforementioned composition is effected by passing the gas stream

A) through a stage for absorbing the nitrogen oxides other than N2O in an
absorbent or reacting the nitrogen oxides other than N2O with an
absorbent, and
B) through a stage for reducing N2O.
Preferably, the gas stream passes first through stage A and then through stage B. Stage A
The absorption of the nitrogen oxides other than N2O in an absorbent and the reaction of the nitrogen oxides other than N2O with an absorbent, as the case may be, can be carried out with any desired suitable absorbents. The preferred absorbent is water or an aqueous solution, e.g. of nitric acid, in which case the absorption is preferably carried out in the presence of free oxygen and the nitrogen oxides other than N2O are prefe¬rably converted into HNO3.
In particular, for example, nitrogen monoxide is oxidized to nitro¬gen dioxide and nitrogen dioxide is absorbed in water to form HNO3. Such a process is described in Ulknann's Encyclopedia of Industrial Chemistry, 5th edition, volume A17, 1991, pages 293-339.
The process for the conversion into nitric acid can be characterized by two exothermic reaction steps:
oxidation of nitrogen monoxide with atmospheric oxygen to nitrogen dioxide according to:

The reactions are promoted by high pressures and low temperatures. Pressu¬res of from 1,5 to 20 bar, preferably from 3 to 12 bar, particularly preferably from 5 to 10 bar are employed.
The gas inlet temperature on entry into stage A is preferably from 10 to 100°C, particularly preferably 20-60°C, in particular from 30 to 40°C.
The gas streams from the oxidation of alcohols, aldehydes and ketones often contain NO2 in a concentration of more than 1% by volume, so that the NO2 can be considered not an impurity but a material of value and therefore can be converted into nitric acid by reaction with water.
The reaction can take place in absorption columns and is described for example in Ulhnann's, loc. cit.
The heat produced in the exothermic reaction can be utilized for generating process steam and/or for heating the gas streams containing nitrogen oxides, for example in a gas/gas heat exchanger. Stage B
Stage B is a stage for reducing the amount of N2O.
The reduction of the amount of N2O can be effected by tiiermal decomposition and/or by catalytic decomposition. The process can be carried out adiabatically or isothermally, preferably using the pressure level of process step A.
The removal of N2O can be carried out in various ways, for example by heterogeneous catalysis. In the adiabatic reaction regime, where the heat evolved by the exotherm of the decomposition reaction is utilized for heatmg the catalyst bed, the gas inlet temperamre on entry into stage B is 200-700°C, preferably 300-600°C, more preferably 400-550°C, particular¬ly preferably 430-550°C, in particular 450-500°C. The gas inlet temperamre can depend on the activity of the catalyst.
To minimize the thermal formation of NO^ and to protect the catalyst used from destruction due to excessive temperamres (e.g. by sinte-

ring), the temperature of the gas stream on exit from the reactor (stage B) should not significantly exceed 800°C. This can be achieved for example by the concentration of N2O in the gas stream on entry mto stage B not being more than 40% by volume, preferably within the range from 0.1 to 20% by volume, particularly preferably within the range from 0.5 to 15% by volume, in particular within the range from 1 to 13% by volume. Gas streams often comprise N2O in amounts of more than 20% by volume.
A reduction of the N2O concentration can be achieved for example by admixmg the gas stream with an essentially N20-free gas stream up¬stream of stage B. The admixing can also be carried out upstream of stage A, if the gas stream first passes through stage A. The essentially N20-free gas stream can be the gas stream leaving stage B or, as explained below, optionally the gas stream leaving stage C and/or a gas stream containing free oxygen, and/or a process gas.
The N2O removal can also be carried out isothermally. This is possible for example in a tube bundle reactor with salt bath or metal bath cooling. This process is characterized in that the temperatore of the gas stream on exit from the reactor (stage B) corresponds to the temperature of the salt or metal batii and the molten salt or metal absorbs the heat relea¬sed by the N2O decomposition reaction. The salt or metal bath temperamre is preferably 400-650 °C or corresponds to the temperature of the adiabatic reaction regime. The gas stream can be heated up either upstream of stage B by a heat exchanger, such as a gas/gas heat exchanger, or directly in the salt or metal bath reactor of stage B.
Another possibility is the removal of N2O (decomposition) in a fluidized bed. Catalysts
Examples of catalysts suitable for N2O removal by catalytic decom¬position are the catalysts described in DE 43 01 470, DE 42 24 881, DE 41 28 629, W093/15824, EP 625369, WO94/27709, US 5,171,553.

Suitable catalysts may consist for example of CuO, ZnO and AI2O3 or addi¬tionally include Ag. It is possible to use catalysts with Ag as active compo¬nent applied to a gamma-Al203 support. Further examples of usable cata¬lysts are those having CoO and/or NiO on a Zr02 support. The use of zeolitic catalysts, for example mordenites, which are present in the H+ or NH4"'" form and may be exchanged with V, Cr, Fe, Co, Ni, Cu and/or Bi is likewise possible.
Also suitable are catalysts consisting of zeolites having an Si02/Al203 ratio of at least 550, for example beta zeolite, ZSM-5, 4 zeoli¬te, mordenite or chabazite and are present in the H"*" or NH4'*' form and optionally exchanged with alkali, alkaline earth, transition metals or rare earth elements, in which case cobalt can be preferred as particularly suit¬able.
Likewise usable are catalysts based on zeolite which have been exchanged with Cu, Co, Rh, Pd or Ir, for example.
Other catalysts which make possible the reduction or decomposition of N2O are likewise usable.
As well as catalytic reduction or decomposition of N2O, thermal decomposition is also possible, for example in a regenerative heat exchanger (thermoreactor). Stage C
In a preferred embodiment of the present invention, the gas stream from stages A and B can be passed through a stage C for reducing nitro¬gen oxides other than N2O.
The decomposition of N2O in stage B may in certain circumstances lead to the formation of nitrogen oxides NO^. These newly formed nitrogen oxides can preferably be removed in stage C.
Stage C is for the reduction of nitrogen oxides other than N2O.
In stage C the gas stream can be reacted by means of selective catalytic reduction (SCR), for example. In SCR, the nitrogen oxides are

reacted with ammonia as reducing agent over catalysts. DENOX catalysts can be used for example. The products are nitrogen and water.
Stage C may also be run as a nonselective catalytic reduction (NSCR). NSCR involves the use of hydrocarbons to reduce the nitrogen oxides and catalysts containing noble metals.
SCR and NSCR processes are described for example in UUmann's Encyclopedia of Chemical Technology, loc. cit.
The catalysts used in this process can be any desired suitable catalysts. For example, catalysts for nonselective reduction processes can be based on platinum, vanadium pentoxide, iron oxide or titanium. Selective catalytic reduction catalysts may contain for example noble metals, such as Pt, Rh, Ru, Pd and/or metals of the iron group, such as Fe, Co, Ni. It is also possible to use, for example, vanadiirai pentoxide, tungsten oxide or molybdenum oxide. A further suitable catalyst is vanadium pentoxide on an alumina support.
The nonselective reduction process may involve the use of suitable hydrocarbons, such as natural gas, propane, butane, naphtha, but also hydrogen.
The temperature of the gas stream on entry into stage C can be for example 150-500°C, preferably 200-350°C, particularly preferably 260-300°C.
It was found accordmg to the present invention that the reactions of stages A, B and, if employed, C can preferably be carried out on one pressure level. This means that the pressure of the gas stream is not additionally significantly increased or reduced between the individual stages. The pressure is at least 3 bar, preferably within the range from 3 to 20 bar, more preferably 3 to 12 bar, particularly preferably within the range from 5 to 10 bar.
Stages A, B and, if employed, C can thus be accommodated in an integrated pressure apparatus consisting of die two or three, as the case

may be, reactors, ie. as an integrated unit in which the gas stream is brought to the starting pressure prior to entry into one of the stages, for example by compression, and between the individual stages tiiere are no further means whereby the pressure of the gas stream is significantly increased or reduced. As the gas stream passes through the stages, the pressure in the gas can vary as a function of the stages used. Preferably, however, the pressure of the gas stream is not varied beyond that. On exit from the last stage the gas stream can be brought to atmospheric pressure, for example by means of a decompression turbine.
Conducting the entire process at one pressure level allows simple process control and a simplified construction of the entire apparatus for removing nitrogen oxides. Process control can be greatly simplified as a result.
In a preferred embodiment, the gas stream passes through stages A, B, C, preferably in that order, and before entry into stage A is admi¬xed with air and/or a gas stream leaving B or C and/or a process gas so that the N2O content is preferably not more than 20% by volume.
The gas stream is contacted in stage A with water or an aqueous solution of e.g. nitric acid in an absorption column in countercurrent to form HNO3 and the product HNO3 is removed at the base of the column,
then the remaining gas stream is brought to a temperature of 200-700°C, preferably 450-500°C and contacted in stage B in a fixed bed with a catalyst for catalytic decomposition of N2O,
the remaining gas stream is then brought to a temperature of 150-500°C, preferably 260-300°C and subjected in stage C to a catalytic reduc¬tion.
The heat of reaction evolved in the individual stages can be utilized for generating steam and mechanical drive energy. For example, the gas stream can be brought upstream of stage A to a pressure of from 1,5 to 20 bar abs. by means of a compressor (VI) and downstream of stage C

to ambient pressure by means of an expansion turbine (Tl), in which case the energy released in the expansion turbine (Tl), as can be provided for example by a motor or engine, is supplied to the compressor (VI) with or without further energy (M),
The energy released in the individual reaction stages can also be used for preheating the gas stream.
For example, the gas stream, before entry into stage A, can be cooled in a heat exchanger (WTl) with the gas stream emerging from stage A. Similarly, the gas stream, before entry mto stage B, can be heated in a heat exchanger (WT3) with the gas stream emerging from stage B. In addition, the gas stream, downstream of the heat exchanger (WTl) and before entry into stage A, can be additionally further cooled to the desked temperature with a further heat exchanger (WT2). Furthermore, the gas stream, downstream of the heat exchanger (WT3) and before entry into stage C, can be additionally further cooled with a heat exchanger (WT4).
As well as the process for removing nitrogen oxides from a gas stream containing same, the present invention also provides an apparatus therefor. The apparatus comprises the above-described stages A, B and preferably the above-described stages A, B and C preferably in that order. Other orders of the stages, e.g. B A C or A C B or similar are possible according to one embodiment of the invention.
The individual stages in the apparams are preferably interconnected using suitable lines in such a way that the gas stream can pass through the stages in succession.
Preferably, the apparatus for removing nitrogen oxides from a gas stream containing same includes, upstream of the first stage, an apparams whereby the gas stream can be brought to a desired pressure and no further apparams for additionally significantly increasing or reducing the pressure of the gas stream between the individual stages.

In a preferred embodiment, the apparatus comprises the above-described compressor (VI) and expansion turbine (Tl) and also a motor/-engine (M), as described above.
In a further preferred embodiment of the apparatus, it comprises the heat exchangers (WTl) and (WT3) arranged as described above.
In a further preferred embodiment of the apparatus, it comprises the heat exchangers (WT2) and (WT4) arranged as described above.
The present invention also relates to the use of the above-described apparatus for removing nitrogen oxides from a gas stream containing same. The gas stream in question preferably comprises a waste gas stream from processes for producing adipic acid, nitric acid, hydroxylamine derivatives or caprolactam or from processes for burning nitrogenous materials.
The present invention further provides for the use of the above-described apparatus for producing HNO3.
A preferred apparatus according to the present mvention and a preferred process according to the present invention will now be described with reference to the drawing which is a diagram of an apparatus according to the present invention.
The reference symbols in the drawing have the following meanings:
Kl: absorption column (stage A)
CI: N20-cracking reactor (stage B)
C2: reactor for catalytic NO^ reduction (stage C)
WTl: heat exchanger 1
WT2: heat exchanger 2
WT3: heat exchanger 3
WT4: heat exchanger 4
VI: compressor
Tl: expansion turbine
[: motor/engine

Example
In an apparatus constructed according to the accompanying drawing, process and waste gases containing nitrogen oxides (line 1) are mixed via line 2 with air and/or via line 3 with N20-lean or NO- and N02-containing process gases. The admixture of air and N2O lean or -free (process) gas limits the temperature increase due to the adiabatically operated N2O decom¬position in the downstream reactor CI to 350°C maximum. In addition, air is admitted to support the oxidation of NO according to the above-recited equation (I) and thus the formation of nitric acid according to equation (II) in the absorption column Kl. The production of nitric acid (HNO3) in the absorption column Kl can be additionally increased by the addition of NO-and/or N02-containing gases. In a preferred embodiment, process gases from ammonia oxidation reactors can be fed m via line 3.
The gas mixture (the gas stream containing nitrogen oxides) is then compressed by means of the compressor (V). The resulting increased pressu¬re of the gas mixture considerably improves the effectiveness of the downst¬ream absorption column Kl (stage A) of the N20-cracking reactor CI (stage B) and of the reactor for catalytic NOx reduction C2 (stage C) in a prefer¬red embodiment. The evolved heat of compression and the simultaneous oxidation of NO to NO2 increases the temperature of the gas stream in line 4 to 250-350°C. The gas stream is cooled down to 30-40°C in a gas/gas heat exchanger (WTl) with cold gas stream from the absorption subsequent¬ly in the heat exchanger (cooler) (WT2) with a suitable cooling medium such as an- or cooling water.
The NO2 absorption and reaction with water to form nitric acid is carried out in the downstream absorption column Kl (stage A), where the gas stream and the absorbent (e.g. water or aqueous nitric acid) are passed countercurrently over suitable internal fitments and the resulting nitric acid is withdrawn from the base of the column.

The gas stream (line 6) freed from the bulk of the NO2 and NO is then heated in a gas/gas heat exchanger (WTl) to 200-300°C (line 7) and in the downstream gas/gas heat exchanger CWT3) to 450-5(X)°C (line 8). The removal of the N2O takes place in reactor C1 (stage B), the temperature rismg to up to 825°C (Ime 9), The gas stream is then cooled down in gas/gas heat exchanger (WT3) and subsequently in the steam generator (heat exchanger WT4) to 260-300°C (line 10). Then the gas stream is freed by catalytic reduction from remaining nitrogen oxide traces in reactor C2 (stage C) by catalytic reduction. In the case of NOx contents of the waste gas of 1(XX) ppm the adiabatic temperature increase is about 10°C. The gas stream is then fed via line 11 at a temperature of 265-310°C to an expansion turbine (Tl), where it is decompressed to atmosphe¬ric pressure and released into the atmosphere at about 100°C via line 12.
The drive energy generated in turbine (Tl) can be utilized, via a common shaft, for driving the compressor (VI). The missing drive energy is then additionally supplied via an additional motor/engine (M).

WE CLAIM;
1. A process for removing nitrogen oxides from a gas stream comprising same, which comprises directing the gas stream (A) through a stage for absorbing the nitrogen oxides other than N2O in an absorbent or reacting the nitrogen oxides other than N2O with an absorbent at a pressure of from 1.5 to 20 bar, and (B) through a stage for lowering the N2O content, preferably by utilizing the pressure level at a temperature of 200 - 700°C prevailing in process stage A, and, after stages A and B, through (C) a stage for catalytically reducing nitrogen oxides other than N2O at a temperature of 150 - 500°C, wherein the gas stream is directed first through stage A and then through stage B, wherein the catalyst used in stage C is one selected of the group comprising platinium, vanadium pentoxide, iron oxide, titanium, rhodium, ruthenium, palladium, iron, cobalt, nickel, tungsten oxide and molybdenum oxide.
The process as claimed in claim 1, wherein, in stage A, the absorbent used is water or an aqueous solution of nitric acid and the nitrogen oxides other than N2O are converted into HNO3 in the presence or absence of free oxygen.
The process as claimed in claim 1 or 2, wherein, in stage B, the lowering in the N2O content is effected by thermal decomposition and/or by catalytic decomposition.
The process as claimed in any one of claims 1 to 3, wherein the gas stream is brought upstream of stage A to a pressure of from 1.5 to 20 bar by means of a compressor (VI) and downstream of stage C to ambient pressure by means of

is supplied to the compressor (VI) with or without further energy (M), wherein the gas stream, before entry into stage A, is preferably cooled in a heat exchanger (WTl) with the gas stream emerging from stage A and, before entry into stage B, heated in a heat exchanger (WT3) with the gas stream emerging from stage B, wherein the gas stream, downstream of the heat exchanger (WTl) and before entry into stage A, is preferably additionally further cooled with a heat exchanger (WT2) and the gas stream, downstream of the heat exchanger (WT3) and before entry into stage C, is additionally further cooled with a heat exchanger (WT4).
An apparatus for removing nitrogen oxides from a gas stream comprising same, comprising (a) an absorption unit for absorbing the nitrogen oxides other than N2O in an absorbent or for reacting the nitrogen oxides other than N2O with an absorbent, (b) a process unit for lowering the N2O content and (c) a reduction unit for reducing nitrogen oxides other than N2O, wherein the gas stream is directed successively through the stages (a), (b) and (c).
The apparatus as claimed in claim 5, wherein, in stage A, the absorbent used is water or an aqueous solution of nitric acid and the nitrogen oxides other than N2O are converted into HNO3 in the presence or absence of free oxygen.
The apparatus as claimed in claim 5 or 6, wherem, in stage B, the lowering in the N2O content is effected by thermal and/or catalytic decomposition.
The apparatus as claimed in any one of claims 5 to 7 further comprising the apparatus of claim 4.

9. A process for removing nitrogen oxides from a gas stream substantially as
herein described with reference to the accompanying drawing.
10. An Apparatus for removing nitrogen oxides from a gas stream substantially as
herein described with reference to the accompanying drawing.

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Patent Number

196362

Indian Patent Application Number

1572/MAS/1996

PG Journal Number

20/2006

Publication Date

19-May-2006

Grant Date

18-Jan-2006

Date of Filing

09-Sep-1996

Name of Patentee

BASF AKTIENGESELLSCHAFT

Applicant Address

67056 LUDWIGSHAFEN,

Inventors:

#	Inventor's Name	Inventor's Address
1	M/A. BASF AKTIENGESELLSCHAFT	67056 LUDWIGSHAFEN,

PCT International Classification Number

B01D53/26

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	195 33 715.8	1995-09-12	Germany