Title of Invention

"METHOD FOR THE HIGH- TEMPERATURE CATALYTIC DECOMPOSITION OF N2O INTO N2 AND O2

Abstract The present invention relates to a method for the decomposition of N2O to N2 and O2 at a temperature between 700 and 1000°C and at HSV between 50000 h-1 and 80000 h-1, characterized in that it is put into practice in the presence of a catalyst consisting of a zirconium cerium mixed oxide mainly in the form of a solid solution.
Full Text The invention relates to a method for decomposition of N20 to N2 and 0; at high temperature, put into practice in the presence of a catalyst consisting of a zirconium cerium nixed oxide.
Global warming is a widely acknowledged phenomenon today, its principal cause being the greenhouse effect caused by the increase of the C02 content of the atmosphere, associated with the even increasing combustion of fossil materials.
In addition to CO? emissions, other gases emitted in much smaller quantities also participate in this greenhouse effect, due to their high radiative power. These include in particular methane (CH4) and nitrous oxide (N20) of which the radiative powers compared with that of C02 are 60 and 310 respectively. It is therefore important to reduce the emissions generated by industrial development. A significant part of this nitrous oxide of industrial origin results from the production of nitric acid by the oxidation of ammonia (nitric acid plant).
In a nitric acid plant, ammonia (NH3) is oxidized to NO by air with catalysts consisting of platinum gauze. The reaction is carried out under a pressure of a few bar, variable according to the type of plant. This reaction is highly exothermic, and the temperature after passage over the gauze is very high. For a temperature of about 2 00°C of the air/NH3 mixture entering the reactor, it generally leaves the reactor at a temperature between 300 and 900-c.
The oxidation to NO also produces small quantities of nitrous oxide N20. The gaseous effluent from the reactor accordingly contains NO, N2, H20 and residual 02, as well as small quantities of N20. The energy in the gases is recovered during cooling by boilers with the production of steam. The NO, which becomes a

NO/NO2 mixture at equilibrium, with a majority of N02, is then converted to nitric acid by absorption in water in a pressurized column, according to well known mechanisms, giving rice to the presence of various nitrogen oxides NO, N02. generally called N0X and N20, in the gaseous effluent from the column. The N0X content depends on the dimensions of the column and hence on its efficiency.
In a subsequent step, this effluent is treated to remove the N0X, generally by catalytic reduction by NH3. This step is well known and widely applied today on the tail gases of nitric acid plants before the expansion turbine. The temperature of these gases is usually between 200 and 250°C.
N2O is very slightly soluble in water and nearly all of it is therefore found at the outlet of the column in what is called the tail gases. To remove the N20, the N20 can be treaced with a specific catalyst on these tail gases. This is, for example, the subject matter of the French parent filed on 31 December 1997 in the names of the applicants and published under No. 2 773 144, using a zeolite-based catalyst of the ferrierite type exchanged with iron. However, this catalyst is only active above about 400°C, therefore requiring heating of the gases, and thereby incurring large investments and high energy consumption.
It is known that N20 is formed in the first step of the method on the platinum gauze. One solution, which is economical in principle, is to treat the N20 directly in the burner, hence in the absence of reducing compounds, upon leaving the platinum gauze, by installing an appropriate specific catalyst, thereby avoiding costly arrangements and not incurring any significant additional energy cost.
Recent investigations have been conducted on this subject and have been the subject matter of patents. These include in particular the use of mixed oxides based on cobalt and magnesium (international application WO 01/58570 m the name of Krupp Uhde), the

use of Cu, Zn, Al spinels (international application WO 99/55661 in the name of BASF) and catalysts based on zirconia (patent US No. 5 478 549 in the name of DuPont). Also used are mixed oxides based on cobalt and iron on cerine (Ce02) support, possibly doped with zirconia (ZrO2) (international application WO 02/022230 in the name of Norsk Hydro).
A catalyst suitable for the method must therefore be active for the decomposition of N2O to N2 and O2 In this type of nitric acid synthesis, the volume available in the burner after the platinum gauze is reduced, implying the need to operate at high hourly space velocities (HSV) . This HSV is generally higher than 30 000 h-1 and rather between 50 000 and 80 000 h-1. In consequence, the catalyst must be able to operate at these high space velocities. This catalyst must also be stable over time, despite the high temperature, posing an essential problem.
Many oxides are available, for example MgO and CaO, which are suitable for decomposing N20 with good efficiency, that is a high yield for an HSV of about 10 000 h-1. However, these types of oxides do not meet the requirements set, that is, very good activity at HSV of about 50 000 h-1 or more.
Catalysts that may be effective, such as, for example, zirconia, under the temperature and HSV conditions considered, nevertheless present the drawback of not being stable over time. The drop in activity is probably due to textural and/or structural changes.
To solve this problem, it is necessary to preserve the structure and texture of the oxides making up the catalyst, which implies in particular the preservation of a high specific surface area.
The present invention relates to catalysts based on oxides of zirconium (Zr) and cerium (Ce) in the form of a solid solution.
This is the subject matter of the present invention.

The present invention relates to a method for decomposition of N20 to N2 and 02 at a temperature between 700 and 1000°C, which is put into practice in the presence of a catalyst consisting of a zirconium cerium mixed oxide in the form of a solid solution.
Solid solution means a mixed oxide of two elements, cerium and zirconium here, in which one of the elements is inserted into the crystal structure of the other oxide and substitutes for the other element while preserving the structure of this oxide. This is reflected by identical X-ray diffraction spectra, the only minor changes, deriving from the different size of the Zr and Ce atoms, being the unit cell parameters and the intensity of certain diffraction lines.
These solid solutions can be prepared by methods described in French patents published under the following numbers: 2 584 388, 2 699 524, 2 701 471 and 2 748 740 in the name of Rhone Poulenc Chimie.
These documents, particularly patents 2 701 471 and 2 748 740, mention the use of such solid solutions for catalyzing various reactions. These include in particular the redox treatment of exhaust gases of internal combustion engines, which contain various compounds such as burnt hydrocarbons (HC) , CO and N0X, where N0X denotes the pair NO/NO2 of nitrogen oxides at equilibrium depending on the thermodynamic conditions of the system, and which are very different from nitrous oxide N20, in terms of both behavior and pollution.
The catalysts of the invention have an effective specific surface area above 2 5 m2/'g. It should be the highest possible and preferably more than 3 0 m2/g. "Effective specific surface area" means the specific surface area of the catalyst under steady state conditions, that is, after 100 h of operation in the reaction medium. This area is substantially different from that of the fresh catalyst. The specific surface areas of the fresh catalysts are generally between 6 0 and 150 m2/g. In the method of the invention, the

"effective" specific surface area is associated with the size of the grains and not their intrinsic porosity. In general, the particles tend to aggregate by sintering when the temperature is high during long periods. This causes a decrease in the specific surface area. In the method of the invention, this mechanism is limited and controlled.
To evaluate these catalysts and test their efficiency for the decomposition of N20, it is impossible to operate with powder because of the high pressure drop that this induces. Prior shaping, for example by palletizing, is therefore required. In the present case, for the tests conducted at laboratory scale, the powder was used to prepare pellets 5 mm in diameter, which are then crushed and screened between 0.5 and 1 mm.
The catalysts of the invention are based on zirconium oxide (ZrO2) and on cerium oxide (CeO2) in the form of solid solutions of these two oxides. The respective weight ratios of Zr02 and CeO2 in the solid solutions are generally between 80/2 0 and 20/80 and preferably between 70/3 0 and 3 0/70. If one of the oxides is in the majority, and the content of the minority element is below the solubility threshold, the solid solution is unique. A Zr02/CeO2 weight ratio close to 1 indicates the presence of two solid solutions.
The catalysts of the invention may contain other metals such as yttrium in low contents of about a few %, giving it certain additional properties, such as thermal stability, with better preservation of the structure and texture.
EXAMPLES 1 TO 4
Examples 1 to 4 below illustrate the use of these solid catalysts for the decomposition of N20 in experiments conducted at laboratory scale on a reconstituted gas having a composition close to that

found after the platinum gauze in the method followed in nitric acid plants.
The tests were conducted in a crossed fixed bed reactor 1" in diameter. The reactor, made of refractory-steel, is surrounded with heating shells regulated by PID.
The reaction mixture is prepared with dry air, nitrogen and N2O/N and NO/N2 cylinders.
The water vapor content is adjusted by a stainless steel saturator at a suitable temperature (60°C) to respect the moisture content in the gas after the platinum gauze, or about 15% by volume.
The flow rates are adjusted by mass flowmeters.
The standard composition of the reaction mixture is as follows:
■ N20: 1000 ppm
■ NO: 1400 ppm " 02: 3%
■ H20: 15%
■ N2: up to 100%.
The NO content does not correspond to that observed in the method followed in nitric acid plants after passage over the platinum gauze, which is about 12% by volume. It was set at this level for safety considerations. However, it was increased to 5000 ppm without observing any change in the results obtained, showing that the method of the invention does not depend on the NO content of the mixture to be treated.
The catalysts were supplied by Rhodia in powder form with various ZrO2/Ce02 ratios, one of them further comprising yttrium.
It is not possible to work with powder because of the pressure drop. Prior shaping (pellets 5 mm in diameter) is therefore required. The pellets are then crushed and screened to produce grains between 0.5 and 1 mm in size.
Their compositions are given below, together with their specific surface area. The contents indicated are those of the oxides Ce02, Zr02 and Y203 in weight %,

the sum of these percentages naturally being 100. Also given are the specific surface areas in m2/g measured by the BET method for the fresh catalysts and the effective specific surface areas.
(Table Removed)

expressed by weight; ** in m2/g; *** measured after 24 h.
The catalysts are used in the form of 0.5 to 1 mm grains. The volume of catalyst used is 5 cm3, representing a height of 10 mm.
The tests were conducted at an HSV of 50 000 If1 or a total gas throughput of 250 Sl/h. The temperature was also set at 850°C.
The level of conversion of N20 as a function of time for the four catalysts is given below.
The N20 contents at the reactor exit are analyzed by infrared.
(Figure Removed)
EXAMPLE 5
This example illustrates the use of catalysts
according to the invention in a method for nitric acid
synthesis by oxidation of ammonia in air, carried out
in a pilot reactor with an effective inside diameter of
100 mm.
The operating conditions are as follows:
- a catalyst bed with a height of 50 mm consisting of the catalyst numeral III above is placed in a basket appropriate to the temperature conditions, disposed under the platinum gauze consisting of 0.060 mm diameter wires of a piatinum/rhodium alloy containing 5% rhodium,
- the operating conditions of the reactor are as
follows:
Ammonia content of the air-ammonia mixture: 10.6% by
volume
Temperature of air/ammonia mixture: 165°C
Temperature of platinum gauze: 870°C
Operating pressure: 5 bar absolute or 500 kilopascals
Reactor feed: HSV of 8 0 000 h-1.
To measure the removal rate (or reduction) of N20, a measurement is firstly taken of the quantity of N20 produced under the same operating conditions but in the absence of the catalyst bed. The quantity of N20 produced in these conditions is 1400 ppm.
In the presence of the catalyst bed, a measurement is taken of the quantity of N20 produced, which is 2 00 ppm, demonstrating a reduction of 85%.
The ammonia oxidation yield in the presence or absence of the catalyst is 96.2%, showing that the catalyst has no destructive effect on the NO produced at the platinum gauze.






We Claim:
1. A method for the decomposition of N2O to N2 and O2 at a temperature

between 700 and 1000°C and at high Hourly Space Velocity (HSV) between 50000 h and 80000 h"1, characterized in that it is carried out in the presence of a catalyst consisting of a zirconium cerium mixed oxide mainly in the form of a solid solution.
2. The method as claimed in claim 1, wherein the catalyst has an effective specific surface area above 25 m2/g and up to 150 m2/g.
3. The method as claimed in claim 1, wherein the ZrO2/CeO2 weight ratio in the catalyst is between 80/20 and 20/80 and preferably between 70/30 and 30/70.
4. The method as claimed in claim 1, wherein the catalyst optionally comprises yttrium.
5. The method as claimed in one of claims 1 to 4, wherein the specific surface area of the fresh catalyst is between 60 and 150 m2/g.
6. The method as claimed in one of claims 1 to 5, wherein the said N2O is present in the effluent of a nitric acid production unit and the said catalyst is disposed under the platinum gauze of the ammonium oxidation reactor.

Documents:

2684-delnp-2005-abstract.pdf

2684-delnp-2005-claims.pdf

2684-delnp-2005-complete specification (granted).pdf

2684-delnp-2005-correspondence-others.pdf

2684-delnp-2005-correspondence-po.pdf

2684-delnp-2005-description (complete).pdf

2684-delnp-2005-form-1.pdf

2684-delnp-2005-form-18.pdf

2684-delnp-2005-form-2.pdf

2684-delnp-2005-form-3.pdf

2684-delnp-2005-form-5.pdf

2684-delnp-2005-gpa.pdf

2684-delnp-2005-pct-210.pdf

2684-delnp-2005-petition-137.pdf

2684-delnp-2005-petition-138.pdf


Patent Number 245933
Indian Patent Application Number 2684/DELNP/2005
PG Journal Number 06/2011
Publication Date 11-Feb-2011
Grant Date 07-Feb-2011
Date of Filing 17-Jun-2005
Name of Patentee GRANDE-PAROISSE S.A.
Applicant Address 4/8 COURS MICHELET, F-92800 PUTEAUX (FR)
Inventors:
# Inventor's Name Inventor's Address
1 HAMON, CHRISTIAN 41, CHEMIN DE PORCE,F-44600 SAINT-NAZAIRE (FR)
2 DUCLOS, DELPHINE 1 PLACE DES GENETS, F-56520 GUIDEL (FR)
PCT International Classification Number B01D 53/86
PCT International Application Number PCT/FR2003/003506
PCT International Filing date 2003-11-27
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 02/15,135 2002-12-02 France