MXPA00001211A - Method for reducing nitrous oxide in gases and corresponding catalysts - Google Patents

Abstract

The invention concerns a method which consists in selectively destroying nitrous oxide at high temperature on catalysts consisting of refractory oxide agglomerates (alumina or zirconium) having intergranular porosity impregnated with a zirconium salt. The method is useful for treating gases in workshops producing nitric acid. The invention is also useful, using specific initiators, for treating gases rich inN2O produced in nitric oxidation of organic compounds.

Description

PROCEDURE FOR. DOWNLOAD THE NITROGEN PROTOXIDE IN GASES AND CORRESPONDING CATALYSTS The present invention relates to gas treatment processes for removing nitrogen protoxide therefrom before disposal to the atmosphere. The invention falls within the general framework of reducing the gas content with greenhouse effect in gaseous effluents of industrial origin, which are thrown into the atmosphere. Currently the world has realized the remarkable contribution of nitrous oxide (N20) to increase this greenhouse effect, which causes the risk of leading to climatic changes with uncontrolled effects and perhaps also of their participation in the destruction of the ozone layer. Thus, its elimination becomes a concern for the public authorities and for the industries. The nitrous oxide or also called nitrous oxide, of the formula N20, is produced especially during the synthesis of nitric acid. It is formed mainly at the level of the platinum layers in which the oxidation of the ammonia takes place by the oxide of the air at high temperature. In addition to the desired formation of nitric oxide NO, which is produced according to the reaction 4NH3 + 502 = > 4NO + 6H20, nitrous oxide N20 is formed because of the parasitic reaction NH3 + 3N0 = > N20 + N2 + 3H20, which in the absence of a specific treatment, passes through the installation without being transformed and then is thrown into the atmosphere in the exhaust gases. PREVIOUS TECHNIQUE Various zeolitic catalysts have been proposed for lowering the nitrogen protoxide, for example based on ZSM5-Cu or ZSM5-Rh (Y. Li and JN Armor, Appl. Catal. Bl, 1992, 21), or on the basis of ferrite / iron (according to French application number 97 16803). On the other hand, the reduced activity of the catalysts thus obtained below 300 ° C and the lack of stability of the zeolites at high temperature only authorize the use of the latter within a field of relatively narrow temperatures (from 350 to 600 ° C). In addition to these zeolitic formulations, there are also mentioned as N20 destruction catalysts having an activity compatible with industrial applications, certain compositions based on cobalt and nickel oxides deposited in zirconium granules (US 5,314,673) or also amorphous compositions. of magnesium oxide and cobalt (RS Drago et al., Appl. Catal. B.13, 1997, 69). However, these formulations, such as the zeolite based catalysts noted above, are only active at an average temperature (from .400 to 600 ° C). It is also possible to contemplate, where appropriate, and as regards the case of the treatment of gases from nitric acid factories, their use downstream of the recovery boiler. On the other hand, it is practically excluded, considering the prevailing temperature conditions between the layers of platinum and the boiler (800 to 900 ° C), being able to install them upstream of the latter. However, in most of the existing factories, the implementation of a catalytic reactor downstream of the recovery boiler implies serious modifications, which are quite onerous. On the other hand, a catalyst for the selective destruction of N2O, which would be active between 800 and 900 ° C, in the presence of high concentrations of NO and H20 could be placed perfectly well in the space generally available inside the burners, between the layers of platinum and the boiler, thus allowing to reduce significantly and at a lower cost, the waste of N20 from most of the set of nitric acid factories, currently in service. Refractory oxides have already been used for the destruction of N2O, for example the use of gamma-alumina powder that is injected in a fluidized bed of certain fuel furnaces has been used to avoid loading the burnt gases with nitrous oxide (JP-A-06123406). The North American memory US 5, 478,549 also reports on the use of zirconium agglomerates to convert the N 2 O formed in the combustion of ammonia in the platinum layers. THE INVENTION It has now been found that it is possible to considerably improve the destruction of N20 when the gases containing this material are allowed to pass through a catalyst constituted by agglomerates endowed with a non-negligible intergranular porosity of refractory metal oxides taken from the group consisting of alumina or zirconium oxide, when the latter have been impregnated with a zirconium salt. The impregnation of the aluminous support with a zirconium salt has already been previously recommended (FR-A-2 546 769) to improve the hydrothermal resistance of the catalysts without this ability to destroy the N20 being recognized. The means for imparting such a porosity to a refractory solid body is to produce it by agglomeration of refracted metal oxide powders with a particle size of a few microns and to consolidate the material with a thermal treatment at a temperature that does not obliterate or eliminate this porosity of agglomeration. In the case of zirconium oxide, the consolidation temperature must be lower than the temperatures (1200 to 1500 ° C) at which an obliterating frit will be produced. Clearly improved results are obtained using alumina refractory oxides or zirconia with an intergranular porosity impregnated with zirconium ales as catalysts. This impregnation can be carried out very simply by immersing the bodies of refractory agglomerates in an aqueous solution of a zirconium salt, for example the oxychloride, and drying after the run-off. In this way, amounts of zirconium salt which, expressed in zirconium, can be between 0.2 to 5% by weight are fixed in the refractory granulate. Impregnation of refractory supports without intergranular porosity, such as alveolar zirconia or cordierite honey nests, does not lead to any significant activity with respect to N20 under the conditions according to the invention. Refractory oxide catalysts with impregnated intergranular porosity are new products and are listed as objects of the present invention. The invention is applied to the treatment of gases generated by oxidation of ammonia in fabrics or platinum layers in nitric acid production plants. Besides the N20, present in contents generally comprised between 500 and 2000 ppmv, these gases contain from 10 to 12% of NO and of the order of 20% by way of H2O. The transformation into nitrogen of the N20 contained in a gas mixture, as is known, is carried out according to the main reaction: 2N20 = 1 >; 2N2 + 02 On the other hand it is noted that the NO content of the treated gas is slightly higher after passing through the catalyst of the invention. It is a side effect, but very appreciated, since it contributes to the increase in the overall performance of the nitric acid factory. This was unexpected. Other applications can be contemplated, such as the treatment of the gases produced by the nitric oxidation processes of organic compounds, especially the synthesis of adipic acid and glyoxal. In these latter cases, the gases of which one has, are present at a relatively low temperature. It is necessary to foresee in the installation, a device for bringing these materials to a temperature sufficient to cause the destruction reaction of N20, whose exothermicity allows the process according to the conditions of the invention, together with a device intended for the extraction and recovery of calories thus generated.

EXAMPLES In the following examples, the catalytic test was carried out in a test unit with a fixed bed traversed ("catatest"), surrounded by heating shells regulated in their temperature by means of the PID derivative (international abbreviation from the English "Proportional Integral Derive"). "or Proportional Integral Derivation). Unless otherwise indicated, the conditions of the tests are as follows: The reactor has a diameter of 2.54 cm. The volume of the catalyst used is 25 cm, which corresponds to a bed with a height of 50 mm. The reaction gas is prepared from compressed air, nitrogen and sample gas, N20 at 2% N2 and NO at 2% N2. The water vapor content is adjusted by a saturator, according to the laws of vapor tension.

Its composition was established as follows NO = 1400 ppm N20 = 700 to 1000 ppm 02 = 3% H20 = 15% The volumetric velocity per hour (VVH) was set at 10 000 h "1 (gas flow of 250 1 / h) The N2O analyzes were carried out by light - "**» infrared and NO analyzes by chemo-lumenscence. The rate of disappearance in the gases was recorded under the term of conversion for nitrogen protoxide. ] to reactor output or raw conversion as Conv. N20 = N20 input - N20 output x 100 N20 input in which N2O inlet and N20 outlet respectively represent the concentrations of N20 in the gas ahead and after passage through the catalyst. For the NO, this is contrary to its rate of appearance that is noted (and that is symbolized by a sign of -). In the same way, the variation of the rate of NO or gross conversion, Var. NO = NO entry - NO exit x 100 No entry This representation lends itself to interpreting the disappearance of the N20 according to, on the one hand, the process of its dissociation in nitrogen and oxygen and on the other hand of its transformation into NO if we interpret the results of the conversion of N2O = > Not for SZ-áß! -? ..

Conv. N20 = > NO = NO exit - NO entry x 100 NO entry x 2 and as for the conversion of N20 = > N2 by Conv. M20 = > N2 = N20 entry - N20 exit xlOO NO exit - NO entry x 100 N20 entry N20 entry x 2 The figures reported below are those that are obtained after each technical implementation of the system (system that is reached approximately 3 hours after each modification of the parameters). EXAMPLE 1: Magnesia The catalyst used is a magnesia presented in the form of granules of 0.5-1 mm, which are obtained by agglomeration of a magnesia powder with a binder constituted by the silica sol (expressed binder content Si02 = 10% in weight of agglomerate) formation of pellets, calcination and then re-treatment and sieving at the granulometry contemplated.

The conversion rates, both of N20 and NO, observed at 800 ° C are practically constant during a period of 24-hour continuous operation. These tests were repeated under slightly different conditions: NO = 1400 ppm N20 = 700 - 1000 ppm 02 = 3% H20 = 15% VVH = 30000 h "1 * '% 11 This product was then treated for 24 hours at a VHV value of 10 000 hf1. The initial conversion is 99% and is still maintained at 93 to 94% after 24 hours. These are very interesting results. The industrial interest for magnesia, on the other hand, is reduced by the impossibility of preserving its consistency with a granular body submitted to said temperature regime. All the samples tested are returned to powder after the agreement has been made. EXAMPLES 2 AND 2 BIS: zircomo These examples allow to appreciate the influence of the granular porosity factor on the efficiency of the catalyst. AND? 2: Granulate The catalyst is a commercial zirconia (ZR-0404T 1/8 Engelhard) presented in tablets of approximately 3 cm (1/8 inch) diameter, whose specific surface area is between 30 and 40 m / g and the porous volume is between 0.19 and 0.22 cm / g. It was carried out under the general conditions of the examples, with gases whose composition was established in the following way: NO = 1000 ppm N20 = 1000 ppm 02 = 3% H20 = 15% ^ rfc 13 The following data were obtained with a value of The following results were obtained with a value These results bear witness to a verified efficacy of granular zirconia. Example 2bis: alveolar zirconia The catalyst used here is an alveolar zirconia with trituration of 94.2% Zr02 ,. 2.9% CaO and 0.425% MgO. This form is obtained by impregnation with zirconia of a polyurethane foam, calcination of the polyurethane support and sintering of the zirconia structure. It is used in the form of a carrot-type plug with an ife diameter. fifteen of 1 cm and a height of 2 cm. The following results were obtained with a value This alveolar material, without microporosity, exhibits an interesting selectivity but at a very low level of nitrogen protoxide reduction activity and therefore no practical interest. EXAMPLE 3: alumina The catalyst used is in this case an alumina with 93.5% AI2O3, in pellets with a diameter of 2 to 5 mm, whose porosity is of the order of 0.42 cm3 / g for pores smaller than 8 μ, and with a specific surface from 280 to 360 pv / g (Alumina AA 2-5 Grade P from Procatalyse). The results obtained are summarized below: The conversion rates of N20 are stable but modest with this duration of operation. EXAMPLE 4 ACCORDING TO THE INVENTION: alumina doped with zirconium. The catalyst used is in this case an alumina of grade P of Example 3 modified in the following manner: 100 cm3 of the pellets are coated with an aqueous solution of zirconium oxychloride ZrOCl2, 8H2O at a level of 0.2 mol per liter. The system is discontinued without agitation at 60 ° C with a time of 3 hours. After cooling, the • and 17 are recovered pellets by filtration in a filter funnel, the material is washed very lightly with demineralized water and dried at 100 ° C in the oven. The zirconium content of the pellets thus treated is 0.61%, which is measured by the ICP system (plasma torch). In the general conditions of the test described above, the following results were obtained: The crude conversion rates of N2O and NO, observed at 800 ° C are remarkably stable. They are set for N20 at rates close to 100% and are maintained at this level for at least 24 hours of continuous operation. The increase in the NO content in the charge gases does not significantly modify the overall conversion of N20. if, when passing from 1400 ppm of NO to 5000 ppm, the following results are obtained: Tempera Conv. Brute Conv. Brute Conv. N2 = > Conv. N20 = > N20 (%) NO (%) NO N2 700 ° C 50.5 -27.9 800 ° C 93.9 -25.1 61.5 32.4 850 ° C 99.1 -24.4 60.6 38.5 For sensitivity to the WH factor, the following conditions were also followed: NO = 1400 ppm N20 = 700-1000 ppm 02 = 3% H20 = 15% with a WH value set at 50,000 h-i The following results were obtained Tempera Conv. Brute Conv. Brute Conv. N2 = Conv. N20 = * 1 ura N20 (%) NO (%) NO N2 700 ° C 19.4 -30.2 15.8 3.6 800 ° C 58.7 -31.1 16.3 42.4 850 ° C 77.6 -27.4 14.3 63.3 EXAMPLE 5: Cordierite coated with zirconium salt (opposite example). The catalyst used is in this case a cordierite formed in a structure of honeycomb or honeycomb at a rate of 620,000 cells per square meter (construction manufactured by Corning), coated with zirconium oxide linked to the silica. The deposit (ZrÜ2 powder 2 μ + 10% S ?02) was carried out in a proportion of 122 g / 1 of structure.

The dense support, still under the open form of the honeycomb and simply coated with zirconium oxide, does not offer any practical attitude for lowering the nitrous oxide.

Claims (8)

1. CLAIMS 1. Procedure to reduce the content of nitrous oxide N2O in gases that contain, in addition to NO, nitrogen oxides NO and water, which consists of circulating these gases through a catalyst bed constituted by a refractory oxide taken from the group consisting of alumina and zirconia at temperatures between 800 and 900 ° C, characterized in that the catalyst is present in the form of granules of alumina or zirconia with an intergranular porosity impregnated with a zirconium salt.
2. 2. Process according to claim 1, characterized in that the catalyst is a granulated alumina impregnated with a zirconium salt.
3. 3. The process according to claim 1, characterized by the catalyst is a granulated zirconia impregnated with a zirconium salt.
4. 4. The process according to claims 1 to 3, characterized in that the granules of alumina or zirconia with an intergranular porosity are impregnated with a zirconium salt at a rate of 0.2 to 5% zirconium, weight by weight, with respect to the granulate. .
5. 5. A catalyst consisting of an agglomerate of zirconia with an intergranular porosity, impregnated with a Zirconium salt.
6. 6. The catalyst according to claim 5, characterized in that the zirconium represents from 0.2% to 5% zirconium, weight by weight, with respect to the agglomerate.
7. 7. Application of the process according to claims 1 to 4, to lower the N20 in the gases generated by oxidation of the ammonia in layers of platinum in the nitric acid production factories.
8. 8. The application of the method according to claims 1 to 4 to lower the N20 in the gases generated by nitric oxidation of organic compounds in an installation provided with a device to bring them to a temperature of 800 to 900 ° C to cause the destruction reaction of N2O.