Patent Application
20240342690
The present invention relates to a process for the catalytic decomposition of N2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia. The invention further relates to a catalyst for the catalytic decomposition of N2O, its use and a reactor which is suitable for carrying out the process.
In the industrial preparation of nitric acid by the Ostwald process, ammonia is reacted with oxygen over a noble metal catalyst to form oxides of nitrogen which are subsequently absorbed in water. In this process, ammonia and oxygen or air are reacted at from 800 to 955° C. over a catalyst gauze comprising noble metals in a reactor. The catalyst gauze generally comprises platinum and rhodium as active metals. In the catalytic reaction, ammonia is firstly oxidized to nitrogen monoxide which is subsequently further oxidized by oxygen to give nitrogen dioxide or dinitrogen tetroxide. The gas mixture obtained is cooled and then passed to an absorption tower in which nitrogen dioxide is absorbed in water and converted into nitric acid. The reactor for the catalytic combustion of ammonia also contains, downstream of the catalyst gauze, a recovery gauze for depositing and thus recovering catalyst metals which have been vaporized at the high reaction temperatures. A heat exchanger is located downstream of the recovery gauze to cool the gas mixture obtained. Absorption is carried out outside the actual reactor in a separate absorption column.
The combustion and the absorption can be carried out at the same pressure level. It is possible to employ an intermediate pressure of from about 230 to 600 kPa or a high pressure of from about 700 to 1100 kPa. In the case of a process with two pressure stages, the absorption is carried out at a higher pressure than the combustion. The pressure in the combustion is then from about 400 to 600 kPa and the pressure in the absorption is from about 900 to 1400 kPa. An overview of the Ostwald process may be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Volume A 17, pages 293 to 339 (1991).
The combustion of ammonia forms not only nitrogen monoxide and nitrogen dioxide or dinitrogen tetroxide but generally also N2O (nitrous oxide, dinitrogen monoxide) as by-product. In contrast to the other oxides of nitrogen formed, N2O is not absorbed by the water during the absorption step. If no further step for removing N2O is provided, N2O can be emitted into the environment in a concentration of from about 500 to 3000 ppm in the waste gas.
Since N2O is a greenhouse gas, very substantial removal from the waste gas is desirable. Although N2O is not the major contributor to global warming (˜6%), it is much more potent than either of the other two most common anthropogenic greenhouse gases, CO2 and CH4. Due to its long lifetime of approximately 150 years in the atmosphere, N2O has 310 and 21 times the Global Warming Potential of CO2 and CH4, respectively.
A number of methods of removing N2O from waste gas streams have been described. Development of N2O abatement systems aims at the achievement of high efficiency (>90% N2O conversion) and selectivity (<0.2% NO loss). The approaches followed by industry, research institutes, and universities can be classified in four groups, according to the position in the process (J. Perez-Ramirez et al., Applied Catalysis B: Environmental 44 (2003) 117-151):
DE-A-195 33 715 describes a process for removing nitrogen oxides from a gas stream, in which the nitrogen oxides apart from N2O are absorbed in an absorption medium and remaining N2O is subsequently decomposed catalytically at from 700 to 800° C. in a decomposition reactor. Since nitrogen oxides can be formed in this decomposition, a selective catalytic reduction (SCR) can follow.
U.S. Pat. No. 5,478,549 describes a process for preparing nitric acid by the Ostwald method, in which the N2O content is reduced by passing the gas stream after the oxidation over a catalyst bed of zirconium oxide in the form of cylindrical pellets at a temperature of at least 600° C.
EP-B 0 359 286 describes a process for the reduction of N2O. For this purpose, a reactor for carrying out the Ostwald process is modified in such a way that the gases obtained after the catalytic combustion are subjected to a retention time of from 0.1 to 3 seconds before cooling by means of the heat exchanger. If desired, a catalyst for the selective decomposition of N2O can be additionally provided.
U.S. Pat. No. 6,743,404 B1 discloses a process for the catalytic decomposition of nitrous oxide N2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger over a catalyst for the decomposition of N2O which is installed between the noble metal catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N2O before subsequent cooling. The catalyst for the decomposition of N2O contains copper aluminate CuAl2O4 and preferably a further metal oxide, in particular ZnO. It is generally said that the catalyst is preferably used in the star extrudate form. However, a specific catalyst form is not disclosed.
The object underlying the present invention is to provide an improved catalyst for the decomposition of N2O which has an improved geometry, combining high geometric outer surface area (GSA) with low pressure drop and preferably also with high mechanical stability, specifically high side and/or bulk crushing strength, under practical conditions in a packed catalyst bed.
The invention is based thereon that the inventors have now been able to provide a star-shaped extrudate, having an optimum GSA and pressure drop, preferably combined with a high side and/or bulk crushing strength.
The object is achieved by a process for the catalytic decomposition of nitrous oxide N2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger, over a catalyst for the decomposition of N2O which is installed between the noble metal gauze catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N2O before subsequent cooling, characterized in that the catalyst is in the form of a star-shaped body having six lobes, wherein the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range from 1.9 to 3.61, the ratio of the area F1 inside this circle to the summed area F2 of the lobes outside this circle is in the range of from 0.54 to 0.90, the ratio of the distance x2 between the two intersections I of one lobe with neighboring lobes and the radius r1 of the circle is in the range of from 0.67 to 1.11.
Preferably, in the star-shaped catalyst body having a cross-section having six lobes, used according to the present invention, the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range of from 1.9 to 3.0 or 2.17 to 3.61, more preferably from 1.9 to 2.5.
Preferably, the cross-section of the body has six axes of mirror symmetry, so that the six lobes have an identical shape.
The mirror symmetry allows for a slight deviation from mirror symmetry. Preferably, at most one or two lobes deviate from the six axes of mirror symmetry. Thus, one or two lobes might be of different size or slightly inclined with regard to the other lobes, while still fulfilling the above geometrical requirements for the star-shaped catalyst body.
The mirror symmetry allows for a maximum of 10% deviation from ideal mirror symmetry, more preferably of 5% or less deviation from ideal mirror symmetry. Most preferably, the cross-section of the body has six axes of mirror symmetry without deviation from mirror symmetry. The mirror symmetry can be seen in
According to the present invention, N2O can be reacted directly in the reactor for the catalytic oxidation of ammonia when a suitable catalyst is located between the noble metal gauze catalyst and the heat exchanger. In this way, N2O formed as by-product is decomposed immediately after it is formed. The decomposition occurs at the temperature prevailing in the catalytic oxidation of ammonia. Heating or cooling of the gaseous reaction mixture is thus unnecessary. The catalyst for the decomposition of N2O which is used according to the present invention is located directly in the reactor, preferably between the position of a noble metal recovery gauze located downstream of the noble metal catalyst and the position of the heat exchanger. Reactors for the Ostwald process are usually provided with inserts for accommodating the noble metal catalyst and the noble metal recovery gauze. These reactors can easily be modified by additionally providing a holder for the N2O decomposition catalyst.
The low catalyst bed height required according to the present invention allows installation in existing reactors without great rebuilding of the reactors. Thus, existing reactors can be modified to enable the process of the present invention to be carried out, without replacement of the reactor being necessary. The Ostwald process can be carried out at one pressure level or at two pressure levels, as described above. The height of the catalyst bed is preferably from 2 to 50 cm, particularly preferably from 5 to 25 cm. In production, the residence time over the catalyst is preferably less than 0.1 s. The pressure drop caused by installation of the catalyst is therefore very low, a small amount of catalyst can be employed, and the gas has to be held at a high temperature level for only a short time after the oxidation, so that secondary reactions can largely be suppressed.
According to the present invention, the decomposition of N2O is carried out in the reactor for the oxidation of ammonia at the oxidation temperature, generally at a temperature in the range from 600 to 950° C., preferably from 800 to 930° C., in particular from 850 to 920° C. The pressure is, depending on the pressure level at which the Ostwald process is carried out, generally from 1 to 15 bar.
As noble metal gauze catalyst, it is possible to use any noble metal gauze catalyst suitable for the catalytic oxidation of ammonia. The catalyst preferably comprises platinum and possibly rhodium and/or palladium as catalytically active metals. The noble metal recovery gauze is preferably made of palladium.
The catalyst used according to the present invention for the decomposition of N2O is selected from among catalysts which still have sufficient activity at above 900° C. to decompose N2O at this temperature in the presence of NO and/or NO2. Catalysts which are suitable for the purposes of the present invention are, for example, binary oxides such as MgO, NiO, ZnO, Cr2O3, TiO2, WOx, SrO, CuO/Cu2O, Al2O3, Se2O3, MnO2 or V2O5, if desired doped with metal oxides, lanthanide complexes such as La2NiO4, La2CuO4, Nd2CuO4 and multinary oxide compounds thereof, spinels, ternary perovskites, and also oxidic systems such as CuO-ZuO-Al2O3CoO—MgO, CoO—La2O3, CO-ZuO, NiO—MoO3 or metals such as Ni, Pd, Pt, Cu, Ag.
The catalyst is preferably a copper-containing catalyst, containing a compound of the formula MxAl2O4, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, preferably Zn, Mg, Ca, Sr and/or Ba, more preferably Zn and/or Mg, and x is from 0.6 to 1.5.
The preferred catalysts consists essentially of an essentially spinel phase which may still contain small amounts of free oxides in crystalline form, such as MO (where M is Cu, Zn or Mg) and M2O3 (where M is Al). The presence of a spinel phase can be detected by recording XRD spectra. The amount of the oxides in the catalyst is in general from 0 to 5, preferably from 0 to 3.5% by weight.
The amount of Cu and any Zn and/or Mg should be chosen such that a filled or virtually filled spinel is obtained. This means x in the formula MxAl2O4 is from 0.6 to 1.5, preferably from 0.8 to 1.5, more preferably from 0.9 to 1.2, particularly preferably from 0.95 to 1.1. For x values below 0.6, the thermal stability is substantially lost. x values above 1.5 likewise lead to a deterioration in the catalyst activity and catalyst stability. The catalyst having an x value of from 0.6 to 1.5, preferably from 0.8 to 1.5, more preferably from 0.9 to 1.2, particularly preferably from 0.95 to 1.1, in the formula MxAl2O4 is thus a high temperature-stable catalyst for decomposition of N2O. The catalyst has advantageous aging behavior, i.e. the catalyst remains active for a long time without being thermally deactivated.
The catalyst contains copper in oxide form, calculated as copper oxide CuO, in an amount of in general from 1 to 54, preferably from 5 to 40, particularly preferably from 10 to 30, % by weight, based on the total catalyst.
The catalyst may additionally contain further dopants, in particular Zr and/or La, in oxide form. Doping with Zr and/or La further increases thermal stability of the catalysts, but the initial activity is slightly reduced. It is particularly advantageous to introduce Zr and/or La dopants via corresponding element-doped aluminum oxides. The content of the dopant compounds in the novel catalyst is in general from 0.01 to 5.0% by weight, preferably from 0.05 to 2% by weight.
The catalyst may contain further metallic active components. Such metallic active components are preferably metals of the 8th subgroup of the Periodic Table of the Elements, particularly preferably Pd, Pt, Ru or Rh. As a result, it is possible to obtain catalysts which not only are very active at high temperatures but have a very high activity at temperatures as low as below 400° C. The catalysts can therefore be used in a wide temperature range, which is a major advantage in the case of adiabatically operated N2O decomposition processes. The amount of the metals of the 8th subgroup in the novel catalyst is in general from 0.01 to 5% by weight, preferably from 0.1 to 2% by weight.
Such a catalyst can be prepared, for example, by combining CuAl2O4 with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba as oxide or salt or in elemental form, and subsequently calcining the mixture at from 300 to 1300° C. and a pressure in the range from 0.1 to 200 bar.
In one embodiment, the catalyst is preferably prepared using zinc, magnesium, calcium, strontium and/or barium as oxide or salt or in elemental form in addition to CuAl2O4. The catalyst is preferably free of noble metals.
The present invention also relates to a catalyst body for the decomposition of N2O as described herein, wherein the catalyst contains a compound of the formula MxAl2O4, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, preferably Zn, Mg, Ca, Sr and/or Ba, more preferably Zn and/or Mg, and x is from 0.6 to 1.5.
To prepare the catalyst, use is made of CuAl2O4 of which from 1 to 100% by weight, preferably from 10 to 100% by weight, particularly preferably from 80 to 100% by weight, is present as spinel. It is particularly preferably completely in the form of spinel. Mixing with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, is preferably carried out at from 500 to 1200° C., particularly preferably from 600 to 1100° C., and preferably at pressures of from 0.5 to 10 bar, particularly preferably at atmospheric pressure. Mixing can be carried out, for example, by spraying, mechanical mixing, stirring or kneading of the milled solid of the composition CuAl2O4. Particular preference is given to impregnation of the unmilled solid. During the calcination after the mixing with the additive, the copper is preferably replaced at least partly by the additional metal. The finished catalyst preferably comprises at least 70%, particularly preferably at least 80%, in particular at least 90%, of a spinel phase.
It is possible to use not only oxides of the metals or the metals in metallic form, but also their salts. Examples are carbonates, hydroxides, carboxylates, halides and oxidic anions such as nitrites, nitrates, sulfides, sulfates, phosphites, phosphates, pyrophosphates, halites, halates and basic carbonates. Preference is given to carbonates, hydroxides, carboxylates, nitrites, nitrates, sulfates, phosphates and basic carbonates, particularly preferably carbonates, hydroxides, basic carbonates and nitrates. The additional metal is particularly preferably in the oxidation state +2. Preference is given to using Zn, Mg, Ca, Sr and/or Ba, in particular Zn and/or Mg.
The preparation of the starting oxide of the composition CuAl2O4, preferably in the form of a spinel, is known from, for example, Z. Phys. Chem., 141 (1984), pages 101 to 103. Preference is given to impregnating an Al2O3 support with a solution of an appropriate salt. The anion is then preferably decomposed thermally to form the oxide. It is also possible to mix the salt with the aluminum compound (for example in suspension with subsequent spray drying), compact it and then bring it into the desired shape, followed by calcination.
The catalyst preferably comprises from 0.1 to 30% by weight of CuO, from 0.1 to 40% by weight of the further metal oxide M(II)O of the one or more of the further metals, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, in particular ZnO, and from 50 to 80% by weight of Al2O3.
Apart from the spinel, preferably small amounts of CuO and further metal oxide are also present. Preferably, not more than 3.5% by weight of CuO and not more than 10% by weight of ZnO are present.
Suitable catalysts are also described in DE-A-43 01 469 and EP-A-0 687 499. Further examples of preparations of catalysts which can be used according to the present invention may be taken from the documents cited.
The catalyst preferably has a BET surface area of from 1 to 350 m2/g. The porosity is preferably in the range from 0.01 to 0.8 l/g.
According to the present invention, the catalyst is preferably used in the star extrudate form described as a fixed bed. The thickness of the fixed bed is preferably from 2 to 50 cm, particularly preferably from 5 to 25 cm. The residence time over the catalyst for the decomposition of N2O is preferably less than 0.1 s.
The use of the catalyst directly in the reactor for the catalytic oxidation of ammonia leads to complete degradation of N2O, with nitrogen oxides being formed. The nitrogen oxides formed in the oxidation of ammonia are not degraded over this catalyst. The catalyst has a high activity.
As a result of the low height of the catalyst bed and the star shape of the catalyst having six lobes, only a small pressure drop occurs in the reactor. No additional heating or cooling is required for the removal of N2O. Since the reactors are built for accommodating catalyst gauzes, rebuilding of a nitric acid plant is generally not necessary.
The catalyst body can be formed by various techniques including extrusion, additive manufacturing (like 3D printing) or tableting. Preferably, the ceramic bodies are prepared by extrusion.
Star-shaped bodies or extrudates can be defined as objects having some kind of central part or core, with three or more extensions on the circumference thereof. An advantageous property of the star-shaped extrudates is the fact that the ratio of geometric surface area to volume is more advantageous than in the case of conventional cylindrical extrudates or tablets.
According to the present invention, it has been found that by employing this specific six lobe geometry of the catalyst body, the geometric surface area GSA can be maximized while minimizing the pressure drop in a packed bed of the catalyst bodies, preferably extrudates, with regard to known star-shaped extrudates. Specifically, the gain in GSA can be higher than the penalty in the pressure drop experienced in such packed bed. Furthermore, a high side and/or bulk crushing strength can be obtained.
The gain in geometric surface area GSA in relation to a pressure drop is specifically achieved in a packed bed, so that not only the behavior of an individual extrudate is improved, but also the behavior of a packed bed of the extrudate.
The star-shaped catalyst bodies according to the present invention combine an advantageous property profile including high geometric surface area GSA and low pressure drop when in a packed bed. They preferably also are mechanically stable, and preferably have a high side crushing strength, high bulk crushing strength, and low attrition.
A high geometric surface area typically leads to a high activity of the catalyst bodies in particular in chemical reactions that are mass-transfer (diffusion) limited.
According to the present invention it has been found that a six-lobe star-shaped catalyst body, is superior to a five-lobe star-shaped extrudate.
Among other parameters, the size (diameter, length) of the catalyst body or catalyst extrudate, the slope of the lobes from intersection to top, the number of lobes, the sharpness of lobes, the depth of lobes and the size of the extrudates were varied, leading to the above improved star-shaped catalyst body or extrudate. The lobes can also be described as flutes or fingers of the stars.
The advantageous properties of the catalyst bodies, preferably extrudates, shall be maintained for a long time upon practical use, in which attrition of the catalyst bodies cannot be totally avoided. By employing the specific shape according to the present invention, however, a long-term stability of the properties of the catalyst bodies, can be achieved.
The geometry of the preferred star-shaped catalyst bodies used according to the present invention can be further illustrated with regard to
A circle connecting the intersections of the lobes, as shown in
The ratio x2 to r1 is 0.67 to 1.11, preferably 0.80 to 0.98, more preferably 0.85 to 0.93, for example 0.90.
The ratio of the area F1 inside this circle to the summed area F2 of the lobes outside the circle is in the range of from 0.54 to 0.90, preferably from 0.65 to 0.79, most preferably from 0.68 to 0.76, for example 0.72.
The ratio r2 to r3 is preferably from 0.80 to 1.33, more preferably from 0.95 to 1.17, most preferably from 1.01 to 1.11, for example 1.06.
Preferably, each lobe has straight outer walls and a rounded top. Preferably, each lobe has straight outer walls with a rounded top, wherein the ratio of the length x1 from the intersection I of one lobe with neighboring lobes to the end of the straight walls to the distance x2 between two intersections I of one lobe with neighboring lobes is from 0.87 to 1.45, more preferably 1.04 to 1.28, most preferably 1.10 to 1.22, for example 1.16. The respective distances are shown in
Preferably, each lobe has straight outer walls with a rounded top, wherein the angle α between the straight wall and the straight line x2 between two intersections I of one lobe with neighboring lobes is from 70 to 140 degrees, preferably from 92 to 102 degrees, more preferably 94 to 100 degrees, most preferably 96 to 98 degrees, for example 97 degrees. This angle is also shown in
Preferably, the ratio of the length x2 between two intersections I of one lobe with neighboring lobes to the length x3 between the ends of the straight walls is from 0.9 to 1.8, preferably from 1.01 to 1.69, more preferably 1.22 to 1.49. The respective lengths x2 and x3 as well as the intersections I are shown in
Preferably, each lobe has straight outer walls with a rounded top, and the ratio of the lobe area of the trapeze confined by the straight walls of a lobe and the outer-lobe area outside this trapeze is from 2.5 to 14.35, preferably from 9.36 to 14.25, more preferably 10.33 to 12.63, most preferably 10.90 to 12.05, for example 11.48. The trapeze area F3 and the outer lobe area F4 are shown in
The rounded top has a radius of preferably 0.23 to 0.38 mm, more preferably 0.27 to 0.33 mm, most preferably 0.29 to 0.32 mm, for example 0.30 mm.
Preferably, the cross-section area of the extrudate is from 0.19 to 13.9 mm2, preferably from 5 to 11 mm2, preferred from 6 to 9 mm2.
Preferably, the maximum radius r2 is from 0.4 to 6 mm, preferably from 0.9 to 6 mm, preferred from 1.35 to 2.25 mm, more preferably 1.62 to 2.1 mm.
Preferably, the circle radius r1 is from 0.25 to 3.4 mm, preferably from 0.49 to 1.5 mm, most preferably 0.72 to 1.1 mm, for example 0.88 mm. The circle area F1 can be calculated therefrom.
As indicated above, the use of star-shaped extrudates is important in terms of pressure drop in relation to accessibility of the external surface of the extrudates. This also plays an important role in eliminating diffusion problems.
The present invention also relates to the use of a star shaped catalyst body as defined herein for the decomposition of N2O in N2O containing gas mixtures.
The present invention further relates to a reactor for the catalytic decomposition of nitrous oxide N2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia which contains in this order in the flow direction a noble metal gauze catalyst, a heat exchanger and a catalyst bed for the decomposition of N2O which is installed between the noble metal catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N2O before subsequent cooling, characterized in that the catalyst bed contains the catalyst for the decomposition of N2O in the form of a star-shaped body having six lobes as defined above.
The table shows an overview of the calculated properties of the prior art geometries (
The values for delta p are calculated based on CFD (Computational Fluid Dynamics). GSA is the surface area of the modeled catalyst geometry in the simulations and the volume respectively. Both are the numerically calculated values from the catalyst model and based on the real geometries.
The geometric surface area (GSA) and pressure drop for a packed bed of the extrudates of different shapes and sizes were obtained from a detailed numerical simulation.
First, a random packing is generated with a simulation using a representative geometry of a reactor tube and the real geometry of the catalyst. The packing is generated by virtually dropping the catalyst particles into the reactor tube and calculating the movement and impacts between particle-particle and particle-wall contacts according to Newton's second law of motion. A discrete element soft-sphere algorithm is used as numerical method. The pressure drop is the result of a detailed simulation applying computational fluid dynamics. The fluid volume is extracted from the numerically generated random packed bed. The fluid dynamics around each pellet as well as all interstitial flow phenomena are fully resolved. The pressure drop is then calculated for an assumed bed height of 84 mm and an assumed inner tube diameter of 56 mm. Compressed air is used as fluid. The pressure at the end of the packed bed is ambient pressure. The assumed temperature is ambient temperature. The assumed flow rate is 1.5 Nm3/h. The six-star extrudates according to the invention are compared to a five-star extrudate and a modified trilobe extrudate as references. All extrudates had an assumed length of 10 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21188199.0 | Jul 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/071176 | 7/28/2022 | WO |
