WO1999064352A1

WO1999064352A1 - Catalytic process for the oxidation of ammonia and catalyst assembly

Info

Publication number: WO1999064352A1
Application number: PCT/GB1999/001478
Authority: WO
Inventors: Bernard John Crewdson; Andrew Mark Ward
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1998-06-09
Filing date: 1999-05-11
Publication date: 1999-12-16
Anticipated expiration: 2000-12-09
Also published as: GB9812276D0; AU3838799A

Abstract

Ammonia oxidation by passing a mixture of ammonia and air at an elevated temperature through a catalyst comprising one or more gauzes of at least one noble metal in elemental filamentary form, and then passing the resultant gas mixture through a bed of a particulate oxidic cobalt-containing catalyst.

Description

CATALYTIC PROCESS FOR THE OXIDATION OF AMMONIA AND CATALYST ASSEMBLY

This invention relates to a catalytic process and in particular to a process for the oxidation of ammonia Ammonia oxidation is widely employed in the manufacture of nitric acid and hydrogen cyanide In the manufacture of nitric acid ammonia is oxidised with air to nitric oxide, while in the manufacture of hydrogen cyanide a mixture of ammonia and methane (often as natural gas) is oxidised with air In both processes, the gas mixture is passed at an elevated temperature over a catalyst to effect the oxidation

However the oxidation of ammonia gives rise to nitrous oxide N₂0 and nitrogen, as well as nitric oxide The formation of nitrogen and nitrous oxide represents undesirable side reactions Whereas discharge of any nitrogen produced into the atmosphere is acceptable, discharge of nitrous oxide is becoming environmentally unacceptable Thus it is often necessary to treat the exhaust gas, after absorption of the nitric oxide in an aqueous medium to give nitric acid, before discharge to the atmosphere

It has been proposed in our co-pending PCT patent application PCT/GB/97/03193 (now published as WO 9828073) to effect the oxidation of ammonia using a catalyst consisting of an oxidic composition comprising the oxides of cobalt and yttrium and/or at least one rare earth metal wherein the oxides are combined so that less than 30% by weight of the cobalt is present as free cobalt oxides

We have found that such catalysts give less nitrous oxide than the conventional catalysts employed for ammonia oxidation, viz a wad of gauzes of a noble metal in elemental filament form such as gauzes formed from platinum wire, although the overall selectivity to nitric oxide may be similar or less than that of conventional catalysts

One disadvantage of the aforesaid oxidic cobalt-containing catalyst is that they appear to be more sensitive to poisoning by sulphur compounds than the conventional noble metal catalysts As a result of the poisoning, the oxidic cobalt-containing catalysts lose their activity Sulphur compounds, such as sulphur dioxide or hydrogen sulphide, may be present in small proportions in the air employed for the ammonia oxidation Under the conditions prevailing in ammonia oxidation processes, such sulphur compounds, if not already in the form of sulphur dioxide, are oxidised to sulphur dioxide We have now found that the reduction in the formation of nitrous oxide is also observed if the aforesaid oxidic cobalt-containing catalysts are employed in combination with conventional noble metal gauze catalysts and the combination is less susceptible to de-activation by sulphur poisoning Accordingly the present invention provides a process for the oxidation of ammonia comprising passing a mixture of ammonia and air at an elevated temperature through a catalyst comprising one or more gauzes of at least one noble metal in elemental filamentary form, and then passing the resultant gas mixture through a bed of a particulate oxidic cobalt-containing catalyst We also provide a catalyst assembly comprising one or more gauzes formed from at least one noble metal in elemental filamentary form and a bed of a particulate oxidic cobalt-containing catalyst As described in our co-pending PCT application GB 98/03386, the oxidic cobalt-containing bed may be in the form of a thin layer disposed between two metal meshes Accordingly the present invention also provides a catalyst assembly as aforesaid wherein the oxidic cobalt-containing bed is in the form of a thin layer disposed between two metal meshes, at least one of which comprises a gauze formed from at least one noble metal in elemental filamentary form In use the assembly is disposed with a noble metal gauze on the upstream side of the bed of the oxidic cobalt-containing bed In a conventional nitric acid plant, the number of gauzes employed depends on the pressure at which the process is operated For example in a plant operating at low pressure, e g up to about 5 bar abs , 3 to 6 gauzes may be employed, while at higher pressures, e g up to 20 bar abs , a greater number of gauzes, typically 35-45, may be employed In the present invention a substantial proportion of the gauzes may be replaced by the oxidic cobalt-containing catalyst bed Thus in a low pressure plant, in the present invention there may be only 1 or 2 gauzes followed by the oxidic cobalt-containing catalyst bed Likewise in a high pressure plant, there may be less than 10 gauzes followed by the bed of the oxidic cobalt-containing catalyst

It is thought that the initial gauze or gauzes effect a substantial proportion, typically 75% or more, of the reaction and the remaining gauzes (in a conventional process) or the oxidic cobalt-containing catalyst bed (in the present invention) act to increase the conversion to a commercially acceptable level It is therefore surprising that, despite the bulk of the reaction occurring over the conventional noble metal gauzes in the present invention substantially lower proportions of nitrous oxide are produced Since the bulk of the reaction is effected by the initial noble metal gauze or gauzes, contamination of the air by sulphur compounds has little effect on the activity of the catalyst

The noble metal gauzes may be formed by weaving or knitting or otherwise forming noble metal wires or filaments into a mesh like structure The noble metal is preferably platinum, particularly alloyed with rhodium

The oxidic cobalt-containing catalyst is preferably a particulate composition containing oxides of cobalt and other metals, particularly rare earths, especially cerium and/or lanthanum, and as described in the aforesaid WO 9828073 the oxides are preferably in combined form so that less than 30% by weight of the cobalt is present as free cobalt oxides The cobalt-containing catalyst thus preferably contains at least one mixed oxide phase containing cobalt and yttrium and/or at least one rare earth element Preferably the catalyst comprises oxides of cobalt and two or more rare earth elements In addition to the mixed oxide phase, the catalyst may also contain free rare earth oxides and/or one or more mixed oxide phases containing two or more rare earth elements The rare earth to cobalt atomic ratio is 0 8 to 1 2, particularly 1 0 to 1 2 Preferably less than 25% (by atoms) of the cobalt is present as free cobalt oxides, and in particular it is preferred that less than 15% (by atoms) of the cobalt is present as the cobalt monoxide, CoO The proportion of the various phases may be determined by X-ray diffraction (XRD) or by thermogravimetπc analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co₃0₄ which occurs at approximately 930°C in air Preferably less than 10%, particularly less than 5%, by weight of the composition is free cobalto-cobaltic oxide and less than 2% by weight is free cobalt monoxide

Preferably the catalyst contains, in addition to cobalt, oxides of at least one element selected from yttrium, cerium, lanthanum, neodymium, and praseodymium Preferably a mixture of at least one variable valency element Vv selected from cerium and praseodymium and at least one non-variable valency element Vn selected from yttrium and the non-variable valency rare earth elements such as lanthanum or neodymium is employed In particular it is preferred that the atomic proportions of variable valency element Vv to non-variable valency element Vn is in the range 0 to 1 , particularly 0 to 0 3 It is preferred that most of the cobalt is present as a Perovskite phase MCo0₃, where M represents yttrium and/or a rare earth Where there are two or more elements M, e g Vv and Vn, it is not necessary that there is a mixed Perovskite phase, e g VvNn, _xCo0₃ where x is between 0 and 1 Thus there may be a Perovskite phase, e g VnCo0₃ or VvCo0₃, mixed with other phases such as Vv₂0₃, Vn₂0₃, (VvNn, _x)₂θ₃ or VvN^{^}n, _x0₂

As indicated above, the catalyst may be in a form wherein the amount of oxygen is non-stoichiometπc This arises from the variable valency of cobalt and also of any variable valency rare earth present as part, or all, of the yttrium and/or rare earth

The process of the invention may be operated at temperatures of 800-1000°C, particularly 850-950^cC, pressures of 1 to 20 bar abs , with ammonia in air concentrations of 5-15%, often about 10%, by volume The invention is illustrated by the following examples

Example 1 (comparative) 5 layers of new un-worked Pt / Rh gauze (95%/5% by weight) were cut to give a stack of 25mm diameter catalyst discs which were placed on top of a fine stainless steel mesh, in turn supported on an alumina monolith, in a 25mm ID quartz reactor tube Thermocouples were provided to measure the temperature of the feed to the reactor and the catalyst temperature A mixture of 10% by volume oxygen, 1% by volume argon and 89% by volume helium were fed, at a rate of 35 litres (at STP) per minute together with 1 84 litres (at STP) per minute of gaseous ammonia, to a preheater and then passed over the catalyst bed The temperature of the feed gas was increased from ambient at a rate of about 20°C per minute to a maximum of about 425°C The temperature was then held at this maximum for 20 minutes and then decreased at about 20°C per minute until the reaction ceased A mass spectrometer was used to analyse the composition of the product gases, with Ar used as an internal standard

The selectivity of the catalyst towards each of the detected products was calculated on the basis of the nitrogen balance using the following formulae

Selectivity to NO = [NO] / {2 * ([N₂] + [N₂0])}

Selectivity to N₂0 = 2 ^* [N₂0] / {(2 ^* [N₂]) + [NO]}

Selectivity to N₂ = 2 ^* [N₂] / {(2 ^* [N₂0]) + [NO]} where [NO], [N₂] and [N₂0] represent the concentrations of nitric oxide, nitrogen and nitrous oxide respectively in the product gas It is desirable that the selectivity to nitric oxide is high and that the selectivities to nitrogen and nitrous oxide are low.

The results are shown in the following table

As the performance of platinum/rhodium gauzes changes during an initial conditioning, the above procedure was repeated three times, using the same catalyst sample The results are shown in the following tables

First repeat

Second repeat

Third repeat

It is seen that the selectivity to nitric oxide improved with increased use, but a significant amount of nitrous oxide was still produced after such conditioning

Example 2 (comparative) A cobalt/lanthanum/ceπum catalyst was prepared by co-precipitation at a pH of 6-7 and at a temperature of 50-60°C from a solution of cobalt, lanthanum and cerium nitrates in the molar ratios of 5 cobalt to 4 lanthanum to 1 cerium, using ammonium carbonate as a precipitant in the presence of oxalic acid The resultant slurry was aged for 1 hour and then filtered and the filter cake washed to an ammonium nitrate concentration of below 0 06% by weight The filter cake was then dried at 120°C and then calcined at 500°C for 6 hours The calcined powder was then pelleted and the resultant pellets fired at 900^αC for 6 hours The fired pellets had a diameter and height of about 3 mm

The test procedure of Example 1 was repeated using a 31 mm deep bed of the pellets (mass 38 445g) in place of the Pt/Rh catalyst gauzes The results of this experiment are shown in the following tables

As in Example 1 , the test was repeated to simulate conditioning of the catalyst. The results are shown in the following tables.

First repeat

Second repeat

It is seen that the selectivity to nitric oxide was similar to that of the platinum/rhodium gauzes, but the selectivity to nitrous oxide was significantly better than that of the platinum/rhodium gauzes.

Example 3

The test procedure of Example 1 was repeated using a composite catalyst of a single platinum/rhodium gauze followed by a 31 mm depth bed of a fresh sample of the pellets as used in Example 2. The results are shown in the following tables.

It is seen that the composite catalyst has a good selectivity to nitric oxide and a low (good) selectivity to nitrous oxide

Since it is believed that the bulk of the reaction is effected by the single platinum/rhodium gauze, it is surprising that a good selectivity to nitrous oxide is obtained Also since the bulk of the reaction is effected by the platinum/rhodium gauze, it is expected that, since platinum/rhodium gauzes are relatively insensitive to sulphur poisoning while cobalt containing catalysts suffer from de-activation upon sulphur poisoning, the good selectivity to nitric oxide and low selectivity to nitrous oxide will be maintained upon sulphur poisoning

Claims

1. A process for the oxidation of ammonia comprising passing a mixture of ammonia and air at an elevated temperature through a catalyst comprising one or more gauzes of at least one noble metal in elemental filamentary form, and then passing the resultant gas mixture through a bed of a particulate oxidic cobalt-containing catalyst

2 A process according to claim 1 wherein the process is operated at a pressure of not more than 20 bar abs. and not more than 10 gauzes are employed.

3. A process according to claim 2 wherein the process is operated at a pressure of not more than 5 bar abs. and one or two gauzes are employed

4 A process according to any one of claims 1 to 3 wherein the cobalt-containing catalyst comprises oxides of cobalt and at least one rare earth

5. A process according to claim 4 wherein less than 30% by weight of the cobalt in the cobalt-containing catalyst is is present as free cobalt oxides

6 A catalyst assembly comprising one or more gauzes formed from at least one noble metal in elemental filamentary form and a bed of a particulate oxidic cobalt-containing catalyst.

7. A catalyst assembly according to claim 6 wherein the oxidic cobalt-containing bed is in the form of a thin layer disposed between two metal meshes and at least one of which comprises a gauze formed from at least one noble metal in elemental filamentary form