AU3833199A

AU3833199A - Method for reducing nitrous oxide in gases and corresponding catalysts

Info

Publication number: AU3833199A
Application number: AU38331/99A
Authority: AU
Inventors: Christian Hamon
Original assignee: Institut Regional des Materiaux Avances IRMA; Grande Paroisse SA
Current assignee: Grande Paroisse SA
Priority date: 1998-06-05
Filing date: 1999-05-31
Publication date: 1999-12-30
Also published as: IL134307A0; HUP0100827A3; BR9906483A; TR200000336T1; CA2299562A1; FR2779360A1; CN1274297A; HUP0100827A2; PL338216A1; BG104214A; HRP20000063A2; ZA200000838B; EP1017478A1; FR2779360B1; WO1999064139A1

Description

WO 99/64139 PCT/FR99/01271 PROCESS FOR THE ABATEMENT OF NITROUS OXIDE IN GASES, AND CORRESPONDING CATALYSTS The present invention relates to processes for treating gases in order to remove nitrous oxide 5 from them before they are discharged to the atmosphere. The invention lies within the general scope of reducing the level of gases with a greenhouse effect in industrial waste gases discharged to the atmosphere. There is now considerable awareness of the significant 10 contribution of nitrous oxide (N 2 0) to amplification of the greenhouse effect, which risks leading to climatic changes with uncontrolled effects, and possibly also its participation in the destruction of the ozone layer. Its elimination has thus become a major 15 preoccupation of public and industrial authorities. Nitrous oxide, or dinitrogen oxide, of formula N 2 0, is produced in particular during the synthesis of nitric acid. It is principally formed at the platinum sheets on which ammonia is oxidized by 20 oxygen in air at high temperature. Besides the desired formation of nitric oxide NO, which takes place according to the reaction 4NH 3 + 502 - 4NO + 6H 2 0, nitrous oxide N 2 0 is formed because of the side reaction 25

NH

3 + 3NO -4 N 2 0 + N 2 + 3H 2 0, REPLACEMENT SHEET (RULE 26) 2 which, unless specific treatment is carried out, passes through the system without being converted and becomes discharged to the atmosphere in the tail gases. PRIOR ART 5 A variety of zeolite catalysts have been proposed for abating nitrous oxide, for example ones based on ZSM5-Cu or ZSM5-Rh (Y. Li and J.N. Armor, Appl. Catal. B.1, 1992, 21), or based on ferrierite/iron (according to French application 10 No. 97 16803). However, the low activity of the catalysts obtained in this way below 3000C and the lack of stability of zeolites at elevated temperature make it possible to use them only within a relatively narrow temperature range (350-6004C) . Besides these zeolite 15 formulations, mention is also made, by way of catalysts for breaking down N 2 0 which have activity compatible with industrial applications, of compounds based on cobalt and nickel oxides deposited on zirconia granules (US 5,314,673) or amorphous compositions of magnesium 20 and cobalt oxides (R.S. Drago et al., Appl. Catal. B.13, 1997, 69). However, like the zeolite-based catalysts mentioned above, these formulations are active only at moderate temperature (400-600 0 C). It may thus be possible to envisage, in the case of treating 25 gases from nitric acid plants, their use downstream of the recovery boiler. It is, however, virtually impossible, in view of the temperature REPLACEMENT SHEET (RULE 26) 3 conditions prevalent between the platinum sheets and the boiler (800-900 0 C), to install them upstream of the latter. However, in the majority of existing plants, 5 the installation of a catalytic reactor downstream of the recovery boiler involves difficult and expensive modifications. Conversely, a catalyst for the selective breakdown of N 2 0, active between 800 0 C and 900 0 C, in the presence of high concentrations of NO and H 2 0, could 10 very well be put in the space generally available actually inside the furnace between the platinum sheets and the boiler, and would permit substantial and inexpensive reduction of the N 2 0 discharges from most of the range of nitric acid plants currently in service. 15 Refractory oxides have already been used to destroy N 2 0, for example y-alumina powder injected into the fluidized bed in certain fuel-oil-burning furnaces in order to prevent the burnt gases from becoming laden with nitrous oxide (JP-A-06123406). US 5,478,549 also 20 refers to the use of zirconia aggregates to convert the

N

2 0 formed in the combustion of ammonia on platinum gauzes. THE INVENTION 25 It has just been discovered that the destruction of N 2 0 is very substantially improved when the gases that contain it are made to pass over a 4 catalyst consisting of aggregates, exhibiting non negligible intergranular porosity, of refractory metal oxides taken from the group consisting of alumina and zirconia, when the latter are impregnated with a 5 zirconium salt. The impregnation of an aluminous support with a zirconium salt has previously been recommended (FR-A-2,546,769) for improving the hydrothermal resistance of catalysts, without this capability of destroying N 2 0 having been recognized. 10 The way in which this kind of porosity can be imparted to a refractory solid body is to produce it by aggregating refractory metal oxide powders with a particle size of a few micrometres and to consolidate it by a heat treatment at a temperature which does not 15 eliminate this aggregation porosity. In the case of zirconia, the consolidation temperature should remain below the temperatures (1200-1500 0 C) which would cause sintering which eliminates this porosity. Substantially improved results are obtained 20 if use is made, as catalysts, of the refractory oxides alumina or zirconia with intergranular porosity, impregnated with zirconium salts. This impregnation can be carried out very simply by immersing the refractory aggregate bodies in an aqueous solution of a zirconium 25 salt, for example zirconium oxychloride, and drying after drainage. Amounts of zirconium salt which, expressed in terms of zirconium, can range from 0.2 to 5 5 % weight for weight, are thus fixed on the refractory granules. The impregnation of refractory supports without intergranular porosity, for example alveolar zirconias or cordierite honeycombs, does not lead to 5 any significant activity with respect to N 2 0 under the conditions of the invention. Catalysts made up of impregnated refractory oxides with intergranular porosity are novel products and form subjects of the present invention. 10 The invention can be applied to the treatment of gases generated by oxidation of ammonia on platinum sheets in nitric acid production plants. Besides N 2 0, present at levels which are generally between 500 and 2000 ppmv, these gases contain from 10 to 12 % NO and 15 of the order of 20 % H20. The conversion of the N 2 0 contained in a gas mixture into nitrogen is said to take place according to the main reaction: 2N 2 0 -> 2N 2 + 02 20 It is found, however, that the NO level in the gas which is treated is slightly higher after it has passed over the catalyst of the invention. This is a side effect, but a very appreciable one, since it promotes the overall yield of the plant in terms of 25 nitric acid. It was unexpected. Other applications may be envisaged, for example the treatment of gases resulting from processes 6 involving the nitric oxidation of organic compounds, in particular the synthesis of adipic acid and glyoxal. In the latter cases, the relevant gases are at relatively low temperature. The system needs to be provided with a 5 device for heating them to a temperature high enough to initiate the reaction by which N 2 0 is broken down, the exothermicity of which makes it possible to proceed further under the conditions of the invention, and a device for removing and recovering the heat produced in 10 this way. EXAMPLES In the following examples, the catalyst test was carried out in a test unit with a stationary bed 15 through which the reactants passed (catatest) and which is surrounded by heating shells whose temperature is controlled in PID mode (standing for "Proportional Integral Derive"). Unless otherwise indicated, the test 20 conditions are as follows: The reactor has a diameter of 2.54 cm. The volume of catalyst employed is 25 cm 3 , i.e. a 50 mm high bed. The reaction gas is prepared from compressed 25 air, nitrogen and standard gas, N 2 0 in N 2 at 2 %, NO in

N

2 at 2 %. The water vapour level is adjusted using a 7 saturator, according to the vapour pressure laws. Its composition was set at: NO = 1400 ppm

N

2 0 = 700-1000 ppm 5 02= 3% H20= 15% The hourly space velocity (HSV) was fixed at 10,000 h1 (gas flow rate of 250 1/h) . The N 2 0 analysis was carried out using 10 infrared, and the NO analysis was carried out by chemiluminescence. The term conversion for nitrous oxide has been used to denote the factor by which it is removed from the gases at the reactor outlet, or raw 15 conversion, as follows N2Oinlet - N20outlet Cony. N 2 O= 22x100 N20inlet where N 2 0inlet and N 2 Ooutlet respectively represent the 20 N 2 0 concentrations in the gas before and after it passes over the catalyst. In the case of NO it is, conversely, their increase factor which is recorded (for this reason, it is written with a - sign). In the same way, the 25 variation in the NO level, or raw conversion, is written as follows 8 Var. NO = NOinlet - NOoutletx100 NOinlet This representation fits the interpretation of the 5 disappearance of the N 2 0, 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 N 2 0 -+ NO conversion by 10 NOoutlet -NOinlet Conv. N 2 0 -+ NO = x100 NOinlet x 2 and as N 2 0 -+ N2 conversion by N20inlet - N2Ooutlet NOoutlet - NOinlet 15 Conv.N 2 0 -> N2 = 2x100 - x100

N

2 Oinlet N 2 0inletx2 The numbers given below are those obtained in each case after the system has reached a steady state (achieved about 3 hours after each parameter 20 modification). EXAMPLE 1: Magnesia The catalyst used is a magnesia present in the form of 0.5 - 1 mm granules obtained by aggregating 25 a magnesia powder with a binder consisting of silica sol (binder content expressed as SiO 2 = 10 % by weight 9 of the aggregate), pelleting, calcining then crushing again and screening to the intended particle size. The following were obtained: T*C Concentration Inlet Outlet Raw Conv. Conv. of (ppm) (ppm) conversion N 2 0->NO N 2 0->N2 (%) 700 0 C N 2 0 810 50 93.8 24.7 69.1 NO 1060 1460 -37.7 800 0 C N 2 0 810 15 98.2 23.4 74.8 NO 1060 1440 -35.9 850*C N 2 0 810 15 98.2 22.8 75.4 NO 1060 1430 -34.9 1 1 5 The conversion rates, both for N 2 0 and NO, which were observed at 800 0 C are virtually constant over a continuous operating period lasting 24 hours. These tests were repeated under slightly 10 different conditions NO = 1400 ppm

N

2 0 = 700-1000 ppm 02 = 3%

H

2 0 = 15% 15 HSV = 30,000 h1 The following were obtained: 10 T*C Concentration Raw conv. (%) Conv. N 2 0 Conv. of -+NO N 2 0->N 2 700*C N 2 0 66.9 18.5 48.4 NO -28.3 800*C N 2 0 98.9 15.4 83.5 NO -23.6 850 0 C N 2 0 99.6 16.7 82.9 I _ NO -25.5 This was subsequently repeated for 24 hours at an HSV of 10,000 h 1 . The initial conversion is 99 % 5 and is still at 93-94 % after 24 hours. These are very advantageous results. The industrial benefit of magnesia is, however, reduced by the fact that it is impossible to keep the consistency of a granular body subjected to a temperature regime of 10 this type. All the samples investigated experimentally were reduced to dust after the test. EXAMPLES 2 AND 2A: Zirconia These examples make it possible to assess the 15 influence of the intergranular porosity factor on the efficiency of the catalyst. EXAMPLE 2 : Granulate. The catalyst is a commercial zirconia 20 (ZR-0404T 1/8 from Engelhard) in the form of pellets 11 approximately 3 cm (1/8 of an inch) in diameter, the specific surface area of which is between 30 and 40 m 2 /g and the pore volume of which is between 0.19 and 0.22 cm 3 /g. It was used, under the general conditions of 5 the examples, with gases whose composition was set thus: NO = 1000 ppm

N

2 0 = 1000 ppm 02 = 3% 10 H20= 15% The following were obtained with an HSV of 10,000 h 1 : T*C Gas Inlet Outlet Raw Conv. Conv. (ppm) (ppm) conv. N 2 0->NO N 2 0->N 2 (%) 700 N 2 0 1030 120 88.1 13.6 74.5 NO 1140 1420 -24.6 800 N 2 0 1030 22 97.9 14.8 83.1 NO 1095 1400 -27.8 850 N 2 0 1030 14 98.6 15.3 83.3 NO 1095 1410 -28.7 15 The following were obtained with an HSV of 30,000 h'; 12 T*C Gas Inlet Outlet Raw Conv. Conv. (ppm) (ppm) conv. N 2 0-+NO N 2 0-+N 2 (%) 700 N 2 0 1030 560 45.6 15.5 30.1 NO 1170 1490 -27.4 800 N 2 0 1045 25 72.7 20.6 52.1 NO 1100 1530 -39 850 N 2 0 1045 185 82.3 16.3 66 NO 1100 1440 -30.9 These results demonstrate a verified efficiency of the granular zirconia. 5 Example 2A: Alveolar zirconia The catalyst used here is an alveolar zirconia containing 94.2 % ZrO 2 , 2.9 % CaO and 0.425 % MgO. This form is obtained by impregnating a 10 polyurethane foam with zirconia, calcining the polyurethane support and sintering the zirconia structure. It is used in the form of a slug measuring 1 cm in diameter and 2 cm in height. The following were obtained with an HSV of 15 10,000 h- 1

:

13 T*C Concentrations Inlet Outlet Raw conv. Conv. Conv. of (ppm) (ppm) (%) N 2 0-+NO N 2 0-*N 2 700 0 C N 2 0 1047 1039 0.76 0.85 0.19 NO 1310 1330 -1.5 800 0 C N 2 0 1042 1003 3.7 3.8 0.6 NO 1200 1280 -6.7 850 0 C N 2 0 1044 942 9.8 10.0 0.3 NO 1020 1229 -20.5 This alveolar material, with no micropores, has advantageous selectivity, but with a very low level 5 of activity for the abatement of nitrous oxide, and therefore without practical interest. EXAMPLE 3: Alumina The catalyst used in this case is an alumina 10 with 93.5 % A1 2 0 3 , in 2-5 mm diameter beads, the porosity of which is about 0.42 cm 3 /g for pores smaller than 8 pm, and a specific surface of 280-360 m 2 /g (Procatalyse A.A. 2-5 Grade P alumina). 15 The results obtained are given below: 14 Temperature N 2 0 - NO Inlet (ppm) Outlet (ppm) Raw conv. (%) 700 0 C N 2 0 1006 761 24.4 NO 1352 1679 -24.2 800*C N 2 0 1006 364 63.8 NO 1352 1673 -23.7 850*C N 2 0 1006 192 80.9 NO 1352 1675 -23.9 The conversion rates for N 2 0 and NO are stable but modest with the operating time.

15 EXAMPLE 4 ACCORDING TO THE INVENTION: Zirconium-doped alumina The catalyst used is in this case a grade P alumina as in Example 3, but modified in the following 5 way: 100 cm3 of beads are covered with an aqueous solution of zirconium oxychloride ZrOCl 2 .8H 2 0 at a strength of 0.2 mol/litre. The system is left without stirring at 60 0 C for 3 hours. After cooling, the beads are recovered by filtering on a filter funnel, washed 10 very gently with demineralized water and stove dried at 100 0 C. The zirconium content of the beads treated in this way is 0.61 %, measured by ICP (plasma torch). Under the general test conditions described above, the following were obtained: 15 Temperature Raw conv. Raw conv. Conv. Conv.

N

2 0 (%) NO (%) N 2 0-+NO N 2 0-+N 2 700 0 C 61.3 -29.8 14.9 46.4 800 0 C 96.6 -25.0 12.5 84.1 850 0 C 99.3 -28.7 13.8 85.8 The raw conversion rates for N 2 0 and NO which were observed at 800 0 C are remarkably stable. In the case of N 2 0, they stabilize at rates close to 100 % and 20 keep to this rate for at least 24 hours of continuous operation. The increase in the NO concentration in the charge gases does not substantially alter the overall 16

N

2 0 conversion. Thus, the following are obtained when changing from 1400 ppm of NO to 5000 ppm: Temperature Raw conv. Raw conv. of Conv. Conv.

N

2 0 (%) NO (%) N 2 0-+NO N20N2 700 0 C 50.5 -27.9 800 0 C 93.9 -25.1 61.5 32.4 850 0 C 99.1 -24.4 60.6 38.5 5 and when changing to 8000 ppm of NO, Temperature Raw conv. Raw conv. Conversion Conversion

N

2 0 (%) NO (%) N 2 0-+NO N 2 0-+N 2 700*C 51.6 -12.2 47.6 4 800 0 C 95.5 -13.7 53.5 42 850 0 C 99.1 -17.7 67.5 31.6 As regards sensitivity to the HSV factor, the procedure was also carried out under the following 10 conditions: NO = 1440 ppm

N

2 0 = 700-1000 ppm 02 = 3 %

H

2 0 = 15 % 15 with a the HSV fixed at 50,000 h 1 . The following were obtained: 17 Temperature Raw conv. Raw conv. Conv. Conv.

N

2 0 (%) NO (%) N 2 0-+NO N20-+N2 700 0 C 19.4 -30.2 15.8 3.6 800 0 C 58.7 -31.1 16.3 42.4 8500C 77.6 -27.4 14.3 63.3 EXAMPLE 5: Cordierite covered with zirconium salt (Counter-example) 5 The catalyst used is in this case a cordierite formed in a honeycomb structure in a ratio of 620,000 cells per square metre (manufactured by Corning), covered with zirconium oxide bound to silica. The deposition (ZrO 2 in 2 pm powder form + 10 % SiO 2 ) 10 was carried out at a rate of 122 g/l of structure. The results obtained are given below: T*C Concentration Inlet Outlet Raw conv. Conv. Conv. of (ppm) (ppm) (%) N 2 0-+NO N 2 0-+N 2 700 0 C N 2 0 1046 1029 1.6 24.7 69.1 NO 1285 1320 -2.7 800*C N 2 0 1054 980 7 23.4 74.8 NO 1100 1250 -13.6 850 0 C N 2 0 1054 941 10.7 22.8 75.4 NO 1100 1250 -13.6 18 The dense support, even in the open honeycomb form, when simply covered with zirconium oxide offers no practical capacity for abating nitrous oxide.

Claims

1. Process for reducing the level of nitrous oxide N 2 0 in gases which, further to N 2 0, 5 contain nitrogen oxides NO and water, which consists in passing these gases through a catalyst bed consisting of a refractory oxide selected from the group consisting of alumina and zirconia at temperatures of between 800 and 9000C, characterized in that the 10 catalyst is in the form of alumina or zirconia granules with intergranular porosity, impregnated with a zirconium salt.

2. Process according to Claim 1, characterized in that the catalyst is granulated 15 alumina impregnated with a zirconium salt.

3. Process according to Claim 1, characterized in that the catalyst is granulated zirconia impregnated with a zirconium salt.

4. Process according to Claims 1 to 3, 20 characterized in that the alumina or zirconia granules with intergranular porosity are impregnated with a zirconium salt at a ratio of from 0.2 to 5 % zirconium weight for weight relative to the granules.

5. Catalyst consisting of a zirconia 25 aggregate with intergranular porosity, impregnated with a zirconium salt. 20

6. Catalyst according to Claim 5, characterized in that the zirconium represents from 0.2 to 5 % zirconium weight for weight relative to the aggregate. 5

7. Application of the process according to Claims 1 to 4 to the abatement of N 2 0 in the gases generated by oxidation of ammonia on platinum sheets in nitric acid production plants.

8. Application of the process according to 10 Claims 1 to 4 to the abatement of N 2 0 in the gases generated by the nitric oxidation of organic compounds, in a system provided with a device for heating them to a temperature of from 800 to 900 0 C in order to initiate the reaction by which N 2 0 is decomposed.