HRP20000063A2 - Method for reducing nitrous oxide in gases and corresponding catalysts - Google Patents

Description

The present invention relates to processes for treating gases with the aim of removing nitrous oxide from them before they are discharged into the atmosphere.

The invention is within the general field of reducing the level of greenhouse gases in waste industrial gases discharged into the atmosphere. It is now recognized the significant contribution of nitrous oxide (N2O) in increasing the greenhouse effect, the dangers of which lead to climate change with an uncontrolled effect, and also its possible participation in the destruction of the ozone layer. Its removal therefore becomes the main concern of public and industrial authorities.

Nitric oxide, or dinitrogen oxide, with the formula N2O, is formed especially during the synthesis of nitric acid. In principle, it is formed with platinum strips on which ammonia is oxidized with oxygen in the air at high temperature. In addition to the desired formation of nitrogen oxide NO, which is formed according to the reaction

4 NH3 + 5 O2 → 4 NO + 6 H2O,

nitrous oxide N2O is formed due to a side reaction

NH3 + 3 NO → N2O + N2 + 3 H2O,

which, unless special treatment is carried out, passes through the system without conversion and is discharged into the atmosphere with residual gases.

State of the art

Various zeolite catalysts have been proposed for the reduction of nitrous oxide, for example based on ZSM5-Cu or ZSM5-Rh (Y. Li and J.N. Armor, Appl. Catal. B.l, 1992, 21), or based on ferrierite/iron (according to the French application No. 97 16803). However, the low activity of the catalyst thus obtained below 300°C and the lack of stability of zeolites at elevated temperatures make their use possible only within a relatively narrow temperature range (350-600°C). In addition to these zeolite preparations, compounds based on cobalt and nickel oxides deposited on zirconium grains (US 5,314,673) or amorphous mixtures of magnesium and cobalt oxides (R.S. Drago and others, Appl. Catal. B. 13, 1997, 69). However, like the zeolite-based catalysts mentioned above, these preparations are only effective at moderate temperatures (400-600°C). Therefore, in the case of treating gases from nitric acid plants, their use downstream of the recovery boiler can be considered. However, it is virtually impossible, given the prevailing temperature conditions between the platinum strips and the boiler (800-900°C), to place them upstream of the latter.

However, in most existing plants, installing a catalytic reactor downstream of a recovery boiler involves difficulties and costly adjustments. Conversely, a catalyst for the selective cracking of N2O, effective between 800°C and 900°C, in the presence of high concentrations of NO and H2O, can very well be placed in the space generally available in fact within the combustor between the platinum strips and the boiler, and will allow substantial and inexpensive reduction of NaO discharged from most nitric acid plants currently in use.

Refractory oxides have already been used to destroy N2O, for example γ-Al2O3 powder injected into the fluidized bed of some oil-fired furnaces to prevent the burnt gases from becoming laden with nitrous oxide (JP-A-06123406). US 5,478,549 also refers to the use of zirconium aggregates to convert N2O produced by the combustion of ammonia on platinum gases.

Invention

It has just been discovered that the destruction of NaO is greatly improved when the gases containing it pass over a catalyst consisting of aggregates (clusters), which show negligible intergranular porosity, of refractory metal oxides taken from the group consisting of Al2O3 and zirconium, when the latter is impregnated with zirconium salt. Impregnation of an aluminum substrate assisted with zirconium salt has already been recommended earlier (FR-A-2,546,769) to improve the hydrothermal resistance of the catalyst, without recognizing the possibility of N2O destruction.

The way this type of porosity can be built into a refractory solid is to produce it by aggregating refractory metal oxide powder with a particle size of several micrometers and solidify it by heating at temperatures that do not remove this accumulated porosity. In the case of zirconia, the solidification temperature should remain below the temperature (1200-1500°C) that can cause sintering that removes this porosity.

Significantly improved results were obtained if refractory oxides of aluminum or zirconia with intergranular porosity, impregnated with zirconium salts, were used as catalysts. Impregnation can be achieved very simply by immersing the refractory aggregates in an aqueous solution of a zirconium salt, for example zirconium oxychloride, and drying after draining. The amount of zirconium salt, which, expressed in terms of zirconium, can range from 0.2 to 5% by weight, is therefore constant on untreated granules. Impregnation of refractory substrates without intergranular porosity, for example alveolar zirconia or cordierite honeycomb, does not lead to any significant activity in relation to N2O under the conditions of the invention. Catalysts made of impregnated refractory oxides with intergranular porosity are new products and form the subject of the present invention.

The invention can be applied to the treatment of gases produced by the oxidation of ammonia on platinum strips in plants for the production of nitric acid. Apart from N2O, present at levels generally between 500 and 2000 ppmv, these gases contain 10 to 12% NO and more than 20% H2O.

The conversion of N2O contained in the gas mixture into nitrogen is said to occur according to the main reaction:

2N2O → 2N2 + O2

It was found, however, that the level of NO in the treated gas was slightly higher after passing over the catalyst of the invention. It's a side effect. but very valuable, because it improves the overall utilization of the plant in terms of nitric acid. That was unexpected.

Other applications may be seen from, for example, the treatment of gases resulting from processes involving nitrogen oxidation of organic compounds, particularly the synthesis of adipic acid and glyoxal. In the latter case, the respective gases are at relatively low temperatures. The system must be equipped with a device for heating them to a temperature high enough to start the reaction with which N2O breaks down, exothermicity that makes further continuation under the conditions of the invention possible, and a device for removing and recovering the heat generated in this way.

EXAMPLES

In the following examples, the catalyst was tested in a test unit with a stationary surface through which reactants pass (catatest) and which is surrounded by heating shields whose temperature is monitored in PID mode (which means "Proportional Integral Origin"),

Unless otherwise indicated, the test conditions are as follows:

The reactor is 2.54 cm in diameter. The volume of the applied catalyst is 25 cm, that is a 50 mm high substrate.

The reaction gas is prepared from compressed air, nitrogen and normal gas, N2O in N2 at 2%, NO in N2 at 2%. The level of water evaporation is adjusted using a saturator according to the laws of gas pressure. Their mixture is adjusted at:

NO = 1400 ppm

N2O = 700-1000 ppm

O2 = 3%

H2O = 15%

Hourly rate (HSV) is fixed at 10,000 h-1 (gas flow rate of 250 L/h).

The analysis of N2O was carried out using infrared, and the analysis of NO was carried out by chemiluminescence.

The term nitrous oxide conversion is used to denote the factor by which it is removed from the reactor exit gases, or crude conversion, as follows

[image]

where N2O input and N2O output respectively represent N2O concentrations in the gas before and after its passage through the catalyst.

In the case of NO it is the other way around, their increase factor that is recorded (for this reason it is written with a - sign). In the same way, changes in NO level, or crude conversion, are written as follows

[image]

This presentation fits into the interpretation of the disappearance of N2O, on the one hand through the process by which it dissociates into nitrogen and oxygen, and on the other hand through its conversion into NO, if the results are interpreted as N2O → NO conversion with

[image]

and as N2O -> N2 conversion

[image]

The numbers given below are those obtained in each case after the system reached an equilibrium state (achieved after about 3 hours after each parameter change).

EXAMPLE 1

Magnesium

The catalyst used is magnesium present in the form of grains of 0.5 - 1 mm obtained by the accumulation of magnesium powder with a binder consisting of silicon salt (binder content expressed as SiO2 == 10% by weight of the aggregate), pellet formation, calcination and then re-breaking and testing to the intended particle size.

The following was obtained:

[image]

The conversion rates, for both N2O and NO, observed at 800°C are virtually constant over a continuous 24-hour operating time.

These experiments were repeated under slightly different conditions

NO = 1400 ppm

N2O = 700-1000 ppm

O2 = 3%

H2O = 15% HSV = 30,000 h-1

The following was obtained:

[image]

This was repeated one after the other for 24 hours at an HSV of 10,000 h-1. Initial conversion is 99% and is still at 93-94% after 24 hours.

These are very favorable results. The industrial utility of magnesium is however reduced by the fact that it is impossible to maintain the composition of the granular body subjected to this temperature. All experimentally investigated samples were reduced to dust after testing.

EXAMPLE 2 and 2A

Zirconium

These examples make it possible to evaluate the influence of the intergranular porosity factor on the effectiveness of the catalyst.

EXAMPLE 2

Grains

The catalyst is commercial zirconia (ZR-0404T 1/8 from Engelhard) in the form of pellets approximately 3 cm (1/8 inch) in diameter, a specific surface area between 30 and 40 m 2 /g and a pore volume between 0.19 and 0, 22 cm3/year. It is used, under the general conditions of the example, with gases whose composition is therefore adjusted:

NO = 1000 ppm

N2O = 1000 ppm

O2 = 3%

H2O = 15%

The following was obtained with HSV 10,000 h-1:

[image]

The following was obtained with HSV 30,000 h-1:

[image]

These results demonstrate the confirmed effectiveness of granular zirconia.

Example 2A

Alveolar zirconium

The catalyst used here is alveolar zirconium containing 94.2% ZrO2, 2.9% CaO and 0.425% MgO. This form was obtained by impregnating polyurethane foam with zirconium, calcining the semi-erethane support and sintering the zirconium structure. Use in the form of 1 cm in diameter and 2 cm in height.

The following was obtained with HSV 10,000 h-1 :

[image]

This alveolar substance, without micropores, has the advantage of selectivity, but with a very low level of nitric oxide-reducing activity, and therefore of no practical use.

EXAMPLE 3

Aluminate

The catalyst used in this case is aluminate with 93.5% Al2O3, in grains 2-5 mm in diameter, whose porosity is about 0.42 cm3/g for pores less than 8 μm, and specific surface area of 280-360 m2/g ( Procatalyse AA 2-5 Grade P alumina).

The obtained results are given below:

[image]

The conversion rates for N2O and NO are stable but moderate over time.

EXAMPLE 4 according to the invention

zirconium treated aluminate

The catalyst used in this case is P grade aluminate as in Example 3, but changed as follows: 100 cm3 of grains are covered with an aqueous solution of zirconium oxychloride ZrOCl2. 8H2O at a density of 0.2 mol/liter. The system was left without stirring at 60°C for 3 hours. After cooling, the beads were recovered by filtration on a filter funnel, washed very gently with demineralized water and dried in a room oven at 100°C. The zirconium content in grains treated in this way is 0.61%, measured with ICP (plasma lamp).

Under the general test conditions described above, the following was obtained:

[image]

The crude conversion rates for N2O and NO observed at 800°C are remarkably stable. In the case of N2O, they stabilize at speeds close to 100% and maintain that speed for at least 24 hours of continuous operation.

An increase in the concentration of NO in the delivered gases does not significantly affect the total conversion of N2O. Therefore, the following was obtained when NO changed from 1400 to 5000 ppm:

[image]

and when 8,000 ppm NO was delivered,

[image]

Due to sensitivity to the HSV factor, the procedure was also carried out under the following conditions:

NO = 1440 ppm

N2O = 700-1000 ppm

O2 = 3%

H2O = 15%

with established HSV at 50,000 h-1.

The following was obtained:

[image]

EXAMPLE 5

Cordierite covered with zirconium salt (an example that counts)

The catalyst used in this case is cordierite in a honeycomb structure with 620,000 cells per square meter (produced in Coming), covered with zirconium oxide bonded to silica gel. The deposition (ZrO2 in the form of a powder of 2 μm + 10% SiO2) was carried out at a rate of 122 g/L of the structure.

The obtained results are given below:

[image]

A dense support, even in open honeycomb form, when simply covered with zirconium oxide offers no practical capacity to reduce nitrous oxide.

Claims (8)

1. A process for reducing the level of nitrous oxide N2O in gases, which, in addition to N2O, contain nitrogen oxide NO and water, which consists of passing these gases through a catalyst substrate consisting of refractory oxides selected from the group consisting of aluminate and zirconium at temperature between 800 and 900°C, indicated by the fact that the catalyst in the form of aluminate or zirconium grains with intergranular porosity is impregnated with zirconium salt. 2. The method according to claim 1, characterized in that the catalyst is granular aluminate impregnated with zirconium salt. 3. The method according to claim 1, characterized in that the catalyst is granular zirconium impregnated with zirconium salt. 4. The method according to any one of claims 1 to 3, characterized in that aluminate or zirconium grains with intergranular porosity are impregnated with zirconium salt at a ratio of 0.2 to 5% zirconium by weight relative to the grains. 5. Catalyst, characterized by the fact that it consists of zirconium aggregates with intergranular porosity impregnated with zirconium salt. 6. Catalyst according to claim 5, characterized in that zirconium represents 0.2 to 5% of zirconium by weight relative to the aggregate. 7. Application of the process according to claims 1 to 4, indicated by the fact that it reduces N2O in the gases produced by the oxidation of ammonia on platinum strips in nitric acid production facilities. 8. Application of the procedure according to claims 1 to 4, indicated by the fact that it reduces N2O in gases created by nitrogen oxidation of organic compounds, in a system equipped with a device for heating them to a temperature of 800 to 900°C with the aim of starting the reaction with which N2O decomposes.