WO2011004239A2

WO2011004239A2 - Catalyst for high temperature decomposition of nitrous oxide

Info

Publication number: WO2011004239A2
Application number: PCT/IB2010/001656
Authority: WO
Inventors: Marek Inger; Andrzej Kotarba; Monika Ruszak; Zbigniew Sojka; Stefan Witkowski; Marcin Wilk
Original assignee: Instytut Nawozow Sztucznych; Uniwersytet Jagiellonski
Current assignee: Instytut Nawozow Sztucznych; Uniwersytet Jagiellonski
Priority date: 2009-07-10
Filing date: 2010-07-06
Publication date: 2011-01-13
Anticipated expiration: 2012-01-10
Also published as: PL388518A1; WO2011004239A3

Abstract

The present invention provides the catalyst for high temperature decomposition of nitrous oxide in process gas mixture, for nitric acid plants, which is based on calcium aluminates. The catalyst according to the invention is composed of CaO from 25 to 49 % by weight and Al₂O₃ from 26 to 51% by weight and SiO₂up to 26% by weight, MgO up to 10% by weight, SrO - up to 32% by weight and BaO up to 12% by weight and exists in the form of active catalytically main phase in the minimum amount of 80% of the Mayenite structure CIF 62040-ICSD, described by the chemical formula as: Ca₁₂-_kX_kAl_14-nSi_nO_33+(n/2), where k has numerical value from O to 6, and n has numerical value from O to 7, and X stands for the following chemical elements: magnesium or strontium or barium or the mixture of them, and the remaining phase having at least one of the auxiliaries components: Ca₃AI₂O₆, CaAI₂O₄ and Ca₂Al₄O₇, with the catalyst grains size 0.5 - 10 μm.

Description

CATALYST FOR HIGH TEMPERATURE DECOMPOSITION OF NITROUS OXIDE

The present invention provides the catalyst for high temperature decomposition of nitrous oxide in process gas mixture in ammonia- oxidizing reactor, particularly in nitric acid installation.

Nitrous oxide, N₂O, appears as the superfluous by-product in catalytic ammonia- oxidizing reactions in installations designed to produce nitric acid, it is not absorbed in water and is emitted with tail gases into atmosphere. N₂O emission can contribute significantly to the greenhouse effect. It can as well have the influence on the destruction of the ozone layer of the stratosphere. Due to the fact that the number of industrial installations in which the waste N₂O is produced is relatively small, its removal is technically much easier than the reduction of CO₂ emission which comes from scattered sources.

There are possible two different locations of catalysts for N₂O removal in installations designed to produce nitric acid. The first one is based on the high temperature decomposition of nitrous oxide, in ammonium- oxidation reactor in the temperature range of 800-950⁰C, while the second one - is the low temperature decomposition, in the stream of tail gases, within the temperature of 200-450⁰C.

The place to apply the high temperature catalyst in an ammonium-oxidation reactor is in the space immediately below the catalytic R-Rh gauze. Implementation of the catalyst being the subject matter of the patent does not require any significant modifications of the existing nitric acid installations, therefore its implementation does not generate substantial costs.

Catalyst for high temperature decomposition of nitrous oxide shall exhibit the high activity and selectivity towards N₂O (decomposition level above 90%), the lack of activity in NO decomposition and high mechanical stability, including catalyst sintering resistance.

The academic literature provides little examples of high temperature catalytic decomposition of nitrous oxide in installation designed to produce nitric acid. However, many catalytic systems which may be successfully adopted or are adopted already have been studied. Johnson Matthey (S. A. Axon et al., Patent WO04096702A2) developed the perovskite-based catalyst

that was applied in nitric acid pressure systems. It is however supposed that in the temperature inside the reactor exceeding 800⁰C, the structure and activity of the said catalyst may show insufficient stability.

BASF (V. Schumacher et al., Patent WO9955621) proposed oxide catalyst of which cupric oxide constituted the active phase. Such a system being successfully tested in industrial installation reached the N₂O conversion of 70-90% with the simultaneous NOx conversion below 0,5%. The most essential problem of its implementation may be the risk of cupric ions leaching from the catalyst and being accommodated in the final product, that is ammonium nitrate. In turn, the cupric ions can catalyze the ammonium nitrate spontaneous decomposition and therefore increase the risk of explosion.

Another system (O. Nirisen et. al., Patent WO0202230) is the spinel phase CeO₂- supported catalyst (NH-1) produced by Yara company. It was successfully implemented in a medium size installation designed to produce nitric acid in Porsgrunn and also in other similar installations. The said catalyst was characterized by high stability, high conversion of N₂O and low losses of NOx.

Hermsdorfer Institute (W. Burckhardt et al., Patent WO0013789) proposed perovskite and spinel active phases deposited on magnesium oxide. This kind of materials presented nearly 100% decomposition of N₂O at 800⁰C, however due to the presence of MgO the stability of the catalyst structure was doubtful for industrial purposes.

Steinmueler (M. Klein et al., Patent WO9907638) discovered a complex system in which the catalyst or combination of the catalysts had not only to remove N₂O but also to oxidize N₂O to NO. The tests carried out by BASF company that took out the patent, did not prove the effectiveness of the system.

Grande Paroisse company (B.Neveu et a., Patent WO9964139) reported the invention based on the catalytic system composed of mixed oxides such as ZrO₂ and Al₂θ₃ impregnated with zirconium salts. According to the inventors, at 85O⁰C the decomposition degree of N₂O amounted between 78 - 99% with 15% of N₂O being converted to NO. The catalyst was examined only in contact with tail gases but not with process gases of the same composition as in ammonia- oxidizing reactor; therefore the results can not be regarded as trustworthy.

Krupp Uhde company (M. Schwefer et al., Patent WO0151415) proposed the catalyst based on zeolites doped with iron, that in laboratory conditions reached N₂O conversion of 100%. Such systems were examined solely in tail gases stream; however they were not tested in process gases existing in ammonia-oxidizing reactor. In such reactor conditions there is the risk of zeolite dealumination and lost of its characteristic properties, as well as, the probability of NO decomposition activity.

M. Santiago and J. Perez - Ramirez (M. Santiago et al.Environ. Sci Technol., 41, (2007), 1704) examined systems based on the hexaaluminates ABAM 1019 (A=La₁ Ba, B=Mn, Fe, Ni), which show high stability and high activity toward N₂O decomposition. The said substances were tested in gas mixture similar to that as in ammonia-oxidizing reactor. The authors of the invention stated that the examined systems would be a promising alternative for commercial catalysts: 00₂AIOVCeO₂ and LaOeCe₀₂CoO₃.

The catalytic systems described above showed sufficient initial activity in N₂O decomposition reaction but due to the severe conditions of the process only a small amount of thecatalysts represented the required stability. A great part of the said catalysts s proved unstable because of leaching of the active component, or losing their activity as a result of the reactions between the component phases (J. Perez - Ramirez et ai, /Appl. Catal. B, 44, (2003), 117-151).

According to the literature, the catalysts' active components referring to the particular system can consist of the following ions: cobalt, iron, copper, magnesium, calcium, strontium, lanthanum, cerium and platinum. In the case of transition metal ions the vital issue is their leaching, that may cause serious problems with final product.

The aim of the invention was to develop the catalyst containing the smallest amount of transition metals ions and of high sintering resistance. The nature of the invention is the catalyst for high temperature decomposition of nitrous oxide in the process gas mixture, for installations designed for nitric acid production on the base of aluminates characterized in that the catalyst is composed of CaO from 25 to 49 % by weight and AI₂O₃ from 26 to 51% by weight and SiO₂ up to 26% by weight, MgO up to 10% by weight, SrO - up to 32% by weight and BaO up to 12% by weight and exists in the form of active catalytically main phase in the minimum amount of 80% of the Mayenite (calcium aluminate) structure CIF 62040- ICSD, described by the chemical formula as:

where k has numerical value from 0 to 6, and n has numerical value from 0 to 7, and X stands for the following chemical elements: magnesium or strontium or barium or the mixture of them, and the remaining phase having at least one of the auxiliaries components: Ca₃AI₂O₆, CaAI₂O₄ and Ca₂AI₄O₇, with the catalyst grains size 0.5 - 10 μm.

Stoichiometry of the main phase, that means weight ratios of the particular elements being the components of the active phase, is described by means of the chemical formula abovementioned. The said aluminate phases exist in solid form or as the coating on the support.

Mayenite is the calcium aluminate of nanoporous structure, composed of nanocells of formal charge +1/3 that is equilibrated by the charge of exchangeable anions (loosely bound to the lattice structure). The applied laboratory synthesis revealed that oxide ions (among other things O₂ ^', O^' and O^2") are generated in mayenite structure, the presence of which is the reason of the catalytic activity in high temperature decomposition reaction of nitrous oxide. Mayenite is the unique calcium aluminate of this type of structure among other calcium aluminates, such as: CaAI₂O₄, CaAI₄Cv, Ca₃AI₂Oe, CaAh₂Oig.

The catalyst is the ceramic material of high thermal strength and small specific surface area that makes its being sintering resistant. This material is characterized by the presence of extra-lattice exchangeable oxide ions which can contribute to the N₂O selective decomposition reaction with no change in the NO concentration.

The catalyst according to the invention can be applied to remove N₂O in ammonia- oxidizing reactor for production of nitric acid. The subject of the invention, is presented in more detail in preferable manufacturing examples:

Example 1

The mixture of 25g of CaCO₃ and 14.85 g of AI₂O₃ was placed in agate ball mill and grinded for 6 hours. The homogenous powder obtained consisted of mainly oblong aggregates of the diameter up to 2 micrometers and standard length of 5-10 micrometers, and was calcined in the platinum crucible at 135O⁰C. The value of the specific surface measured by N₂-BET method was 1 m²/g.

Due to the method described above the catalyst consisting of, 48.5 wt. % of CaO and 51.5 wt. % OfAI₂O₃ was obtained. Phase analysis by powder diffraction method of sample material showed the presence of the calcium aluminate of the mayenite structure in the amount of 80% and the remaining phase consisted of Ca₃AI₂Oe, CaAI₂O₄ and Ca₂AUO?. The catalyst grains size ranged 0.5 - 10 micrometers.

Example 2.

The mixture of 3.21 g of AI₂O₃, 1.51 g of SiO₂ and 7.57 g of CaCO₃ was prepared and grinded for 6 hours. Similarly to Example 1, the powder was calcined in the platinum crucible at 135O⁰C for 6 hours with the same temperature increase rate as in Example 1. The value of the specific surface measured by N₂-BET method was 1 m²/g. The preparation obtained consisted of the following: by weight, 47.3% of CaO, 35.8% Of AI₂O₃, 16.9 % of SiO₂. Phase analysis by powder diffraction method of sample material showed the presence of the calcium aluminate of the mayenite structure in the amount of 90% and the remaining phase consisted of Ca₃AI₂Oe, CaAI₂O₄ and Ca₂AI₄07. The catalyst grains size ranged 5 - 10 micrometers.

Example 3.

The three different mixtures were placed in separate agate balls mill and grinded for 6 hours. The first mixture consisted of: 14.85 g of AI₂O₃, 22g of CaCO₃ and 0.81g of MgO, the second consisted of: 14.85g of AI₂O₃, 22g of CaCO₃ and 2.95g of SrCO₃, the third one consisted of: 14.85g of AI₂O₃, 22g of CaCO₃ and 3.95g of BaCO₃ Similarly to Example 1, the powders were calcined in the platinum crucible at 1350⁰C for 6 hours with the same temperature increase rate as in Example 1. The value of the specific surface measured by N₂-BET method was 1 m²/g.

Phase analysis by powder diffraction method of sample material showed the presence of the calcium aluminate of the vnayenite structure in the amount of 95% and the remaining phase consisted of Ca₃AI₂O₆, CaAI₂O₄ and Ca₂AI₄O₇. The catalyst grains size ranged 0.5 - 10 micrometers.

Claims

Patent claims:

1. The catalyst for high temperature decomposition of nitrous oxide in process gas mixture of nitric acid plants, characterized in that the catalyst is based on aluminates wherein the catalyst is composed of CaO from 25 to 49 % by weight and AI₂O₃ from 26 to 51% by weight and SiO₂ up to 26% by weight, MgO up to 10% by weight, SrO - up to 32% by weight and BaO up to 12% by weight and exists in the form of active catalytically main phase in the minimum amount of 80% of the mayenite structure CIF 62040-ICSD, described by the chemical formula as:

where k has numerical value from 0 to 6, and n has numerical value from 0 to 7, and X stands for the following chemical elements: magnesium or strontium or barium or the mixture of them, and the remaining phase having at least one of the auxiliaries components: Ca₃AI₂O₆, CaAI₂O₄ and Ca₂AI₄Cv₁ with the catalyst grains size 0.5 - 10 μm.