DK170529B1 - Ammonia oxidation catalyst - Google Patents

Description

- X - DK 170529 B1

The present invention relates to a catalyst of the kind set out in the preamble of claim 1.

Ammonia oxidation catalysts are mainly used in the production of nitric acid by air oxidation of ammonia. In industrial nitric acid production processes, ammonia is mixed in a first step with preheated air and catalytically oxidized to nitric oxide at temperatures of 800-950 ° C and pressures from atmospheric pressure to approx. 10 bar after the following reactions: 10 4NH3 + 502 4NO + 6 H2O (1) 2NO + O2 ^ 2NO2 (2)

Nitric oxide-rich exhaust gas from the oxidation step is converted in a later process step to nitric acid by absorption in water after the reaction: 3NO 2 + H 2 O 2HNO 3 + NO (3) 20 In addition to the oxidation reactions (1) and (2), ammonia also reacts with oxygen to nitrogen after the reaction: 4NH3 + 302 - = »2N2 + 6H2O (4) 25

In order to obtain high nitric acid yields, the process depends to a large extent on the selectivity of the ammonia oxidation catalyst to form nitric oxide as the main oxidation product at the expense of the more stable nitrogen.

At present, the only catalysts which show good efficiency in industrial nitric acid systems are platinum catalysts in the form of a wire mesh gauze stabilized with small amounts of rhodium and palladium, or catalysts of platinum deposited on a large-mesh steel mesh with a high chromium content.

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Despite the efficiency of these catalysts, the main disadvantage of the commercial platinum catalysts is the high cost and reduced service life caused by platinum loss, especially when the catalysts are used at elevated pressure during the ammonia oxidation step.

In order to reduce catalyst costs and to improve the overall process economy, attempts have been made to use other ammonia oxidation catalysts which are based on significantly cheaper base metals.

Of these, so far only iron (II, III) and cobalt (II, III) based catalysts are commercially produced. Although these catalysts are significantly cheaper than platinum catalysts, these oxidation catalysts are only to a limited extent used in industrial nitric acid plants. This is likely due to a rapid loss of selectivity, which is thought to be caused by loss of activity by a reduction in the available catalytically active surface, as suggested by studies by S.P.S.

Andrew and G.C. Chinchen (Catalyst Deactivation, page 141,

Elsevier, Amsterdam, 1980).

These studies have shown that a packed bed of porous Co304 tablets loses their selectivity in a short time due to sintering and poisoning by accumulation of metal oxide dust on the catalyst surface.

Ammonia oxidation catalysts with noble metal or noble metal alloys deposited on a monolithic support are known from U.S. Patent No. 3,428,424. Monolithically structured carriers exhibit a large surface area and at the same time a large cavity volume, thereby reducing sintering and accumulation of dust particles on the surface of catalyst bodies. ^

It has now been found that the known catalysts for use in the oxidation of ammonia can be improved when iron (II, III) and / or cobalt dl, III) oxide catalytically active material is deposited on a monolithic support prepared from silicate fibers of a certain length and diameter. With these materials a catalyst with greater mechanical stability is obtained than the known catalysts on a ceramic 5 support. Furthermore, the fibrous structure of the silicate fibrous material increases the adhesion strength of the active material to the support.

Accordingly, the invention provides a catalyst with improved mechanical stability and catalytic selectivity in the oxidation of ammonia to nitrogen oxides, consisting of cobalt (II, III) oxide and / or iron (II, III) oxide as catalytically active constituents deposited on a monolithic structured carrier of a heat-resistant material. The catalyst is characterized in that the heat-resistant material comprises silicate fibers with an average diameter of 2-50 micrometers and an average length of 2-60 mm.

The monolithic support may be present, either in the form of a straight channel body through the body, such as in the known honeycomb structured type, or may be made from corrugated sheets of silicate fiber rolled or stacked into a straight channel monolith.

The channels in the monolithic support preferably have a hydraulic diameter of between 0.8 and 30 mm, and a void volume of between 60-85%, which provides a large surface / volume ratio and thus a significantly reduced tendency for dust accumulation on the catalytic surface.

With a reduced dust accumulation, poisoning of the catalytic surface in the monolithic catalyst is avoided and retains its catalytic activity for a much longer operating time than with the known particulate metal oxide catalysts.

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The monolithic support is preferably made by corrugating sheets of fibrous silicate paper with a through-hole.

section fiber diameter and an average fiber length, as indicated above, in a conventional corrugating machine and assembled into a monolithic carrier by winding or stacking.

The catalytically active component is deposited on the support by impregnation with a solution of soluble salts of the desired metals which, after heating, decomposes to the oxides, or preferably by painting or washing-coating the support with a slurry of the metal oxides.

Before depositing the catalytic component or a precursor thereof on the support, it may be advantageous to provide the support with one or more layers of binder material of ceramic oxides, such as silica, alumina, titanium oxide, cerium oxide, lanthanum oxide, or mixtures thereof deposited on the support. the carrier for washing or painting.

Thus, a typical procedure for preparing the ammonia oxidation catalyst of the invention will involve corrugating sheets of the support material, if desired, pre-coated with a binder material; assembling the corrugated sheets into a monolithic body and calcining the body at temperatures between 400 ° C and 800 ° C; then coating or impregnating with the catalytically active ingredient or a precursor thereof; and finally drying and calcining the impregnated or coated body at temperatures between 400 ° C and 700 ° C.

Alternatively, the binder material may be mixed with a slurry or solution containing the catalytically active ingredient before depositing it on the corrugated sheet. ;

The catalytically active constituents consist of Co (II, III) oxides and / or Fe (II, III) oxide.

The amount of the catalytically active component deposited on the support can vary from 5 to 60% by weight and preferably between 15 and 25% by weight, based on the finished catalyst.

When using the catalyst according to the invention, it is placed in a cylindrical oxidation reactor which is kept under a pressure of from atmospheric pressure to a pressure of approx. 10 bar. A feed gas consisting of 0.5-11% ammonia in air is preheated to approx. 300 ° C and introduced into the reactor. After a short contact time of the feed gas over the catalyst, the temperature in the reactor rises to approx. 800 ° C by oxidation of ammonia with air to nitric oxide. The exhaust gas from the reactor is then conventionally converted to nitric acid by absorption in water as previously discussed.

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Example 1

Preparation of a cobalt (II, III) oxide ammonia oxidation catalyst deposited on a monolithic support according to the invention.

A sheet (500 mm x 500 mm) of commercially sieved heat-resistant paper with a thickness of 0.25 mm consisting of silica fibers with an average diameter of approx. 6 μχα and an average length of 20 mm, as supplied by Crane & Company Inc., U.S.A., were corrugated to a corrugation height of approx. 2.5 mm in a conventional corrugation machine. The corrugated sheet was then fitted with a liner made of the same material as the corrugated sheet and rolled up into a monolith with straight channels and with an outer diameter of 50 mm and a height of 30 500 mm.

The monolithic support thus prepared was coated with a slurry containing a cobalt compound as a catalytic material together with a binder as specified below.

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The slurry was prepared by mixing 1200 g of Co (NO 3) 2 * 6H 2 O with 250 g of demineralized water and 845% g of an ammonia-stabilized SiO 2 binder, supplied by Monsanto Co., United Kingdom, under the trade name "Syton T40". tjt 5 The slurry was ball milled at room temperature for 12 hours, after which the slurry could be used to coat the carrier.

The support was washed coated with the slurry by repeated immersions in the slurry and drying at room temperature until the support was coated with 25% by weight of cobalt nitrate.

The coated support was finally activated by calcination at 450 ° C in air for approx. 2 hours, during which cobalt nitrate was cleaved to the catalytically active Co 3 O 4 corresponding to 15 to an amount of approx. 19% by weight of Co304 on the support.

Example 2

Preparation of a cobalt (II, III) ammonia oxidation catalyst impregnated on a monolithic support according to the invention.

A monolithic support was prepared in the same manner as described above in Example 1. The support was coated with an alumina binder prepared by adding 2.3 liters of demineralized water to a stirred vessel, and by mixing 450 g of alumina powder ("Versal" supplied). of

Kaiser Chemicals, U.S.A.), 775 g alumina powder ("Versal") precalcined at 350 ° C for one hour in air, 45 g HNO 3 (62% by weight), 154 g polyethylene glycol binder ("PEG 20,000"),

Hoechst, Germany) and 4 g of a surfactant 30 ("Surfynol 104 E", Air Products & Chemical Inc., The Netherlands). The mixture was ball milled at room temperature for approx. 12 hours.

The binder layer was deposited on the support by repeated immersions in the alumina binder and drying until a final coating of 30-40 g of the binder layer per m2 of support surface.

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The coated support was then calcined at 600 ° C for 2 hours under air.

The coated support was finally impregnated at approx. 80 ° C with a Co (NO 3) 2 * 6H 2 O melting until a coating of approx. 19% by weight of Co (II, III) oxide on the finished monolithic catalyst after air calcination at 450 ° C for approx. 2 hours.

Example 3 The activity of the monolithic ammonia oxidation catalyst prepared as in Example 2 was tested in a laboratory reactor consisting of a quartz tube with an inner diameter of 3.8 mm and a length of 500 mm. The reactor was charged as further described below with fragments of the catalyst. The activity test was performed under isothermal conditions by placing the reactor tube in a thermostat furnace. In a first experiment, the reactor was filled with 1.7 g of catalyst fragments, resulting in a catalyst bed with a depth of 450 mm. A feed stream of an ammonia-20 / air mixture with increasing ammonia concentration was passed at 500-600 ° C and a flow rate of 14,700 Nl / kg cat.h through the bed. The nitric oxide (NOx) yields obtained in this experiment are shown in Table 1, where the yields are expressed in volume% NOx calculated on the amount of ammonia in the feed stream.

DK 170529 B1 - 8 -Table 1

Temp / ° C% NH3 Yield Operating feed --- time / h. % NO% NO,% NOy, current 2 xh 500 0.48 37.9 9.0 46.9 32 5 0.95 30.6 24.4 55.0 4 1.42 25.4 11.7 37.1 28 650 0.48 52.7 9.2 61.9 32 0.95 40.5 22.6 63.1 4 1.42 35.4 9.2 44.5 28 10

In another experiment, 0.6 g of catalyst fragments were placed in the reactor resulting in a 160 mm deep catalyst bed. A feed stream of an ammonia / air mixture with increasing ammonia concentration was sent at 500/650 ° C and a flow rate of 41,700 Nl / kg cat.h through the bed. The NOx yields obtained by this experiment are shown in Table 2.

Table 2

Temp / ° C% NH3 Yield Operating feed --- time / h. % NO% N0,% NOY.

current 2 x h 500 0.95 28.8 9.5 38.3 52 1.42 27.1 7.8 34.9 56 650 0.95 41.6 11.1 52.6 52? 1.42 35.0 10.8 45.8 56

Claims (3)

Catalyst with improved mechanical stability and catalytic selectivity by oxidation of ammonia to nitrogen oxides, consisting of cobalt (II, IIDoxide and / or iron (II, IIDoxide as catalytically active constituents deposited on a monolithically structured support of a heat-resistant material, characterized in, that the heat-resistant material comprises silicate fibers with an average diameter of 2-50 micrometers and an average length of 2-60 mm. The catalyst according to claim 1, characterized in that the monolithically structured support has channels with a hydraulic diameter of between 0.8 and 30 mm, and a void volume of between 60 and 85%. The catalyst according to claim 1, characterized in that the catalytically active component is deposited on the support in an amount of between 5 and 60% by weight, preferably of between 15 and 25% by weight, based on the finished catalyst.