NO122755B - - Google Patents

Description

Process for reacting carbon monoxide with water vapour.

This invention relates to the conversion of carbon monoxide

with steam over catalysts with high activity at low temperature. The catalysts in preferred forms have the advantage that they remain active for a longer time in use, or they are less dense without serious loss of mechanical strength, than many previously known related catalysts.

According to the invention, a process is provided

method for reacting carbon monoxide with water vapour, in particular for the production of ammonia synthesis gas or hydrogen or for making fuel gas non-toxic, at temperatures below 300°C over a catalyst containing copper and zinc. The method is characterized by the use of a catalyst

comprising the product from the partial reduction of the combined oxides of copper and zinc and at least one additional metal selected from the group consisting of aluminium, magnesium and manganese, whose oxides are usable as catalyst supports, and optionally a non-combined catalyst support material.

The quantity ratio of the oxide of aluminium, magnesium or manganese is preferably in the range 4 to 20, in particular 8 to 20 atomic percent of the total quantity of the said oxides.

If manganese oxide is present, its valency is preferably not higher than 4 in the catalyst as it is normally handled,

ie before reduction prior to use or in use.

The copper content of the catalyst or composition

is preferably at least 10%, for example in the range 10 to 70%, especially 10 to 50%. The zinc content is preferably at least 20%, for example between 20 and 80%. Suitable catalysts thus contain copper, zinc and the additional metal or metals in the ratios 30:60:10, 40:40:20 and 60:30:10 and ratios between these ratios. These percentages and ratios are according to atoms.

In the production of the catalyst used according to the invention, a mixture of non-halide, sulphur-free salts of the metals is reacted with a carbonate or bicarbonate of an alkali metal or (less preferred) ammonium, after which the resulting precipitate is washed approximately free of alkali, dried and calcined. The mixing of the solutions in the precipitation step is preferably carried out continuously. The temperature of the mixing step is preferably in the range

65 - 85°C, and preferably it is raised to 80 to 100°C after

mixture and held there until the pH value stops rising. Preferably, a relatively low but constant degree of alkalinity (corresponding to a pH value at 90°C in the range up to 9.8, especially up to 8.0) is maintained during and after the precipitation.

(In the "provisional specification" the lower limit of 7.0 was stated in the sense of "neutrality", although at the temperatures used here, neutrality can be present at a pH value as low as about 6.0). The washing of the precipitate, if prepared using an alkali metal compound, should preferably be such that less than 0.5% alkali metal oxide (calculated as Na 2 O) remains, more preferably less than

0.2%.

In the above-mentioned method, it is preferred that at least some and preferably all of the content of each metallic component which is introduced by precipitation is introduced as a compound in which the metal is present as cations, especially as nitrate or acetate. However, it is also possible to introduce some of one or more metallic constituents such as anions. The calcination of the catalyst is preferably carried out at temperatures up to 350°C, for example approx. 300°C.

If the catalyst is to be used in a static layer,

it can be shaped, for example by granulation, pelletisation or extrusion. These operations are preferably carried out after charcoal sintering of the precipitate. The pellet density in the composition is preferably in the range 1.0 to 2.0, especially 1.4 to 1.8. Before it is put to use, the catalyst composition is reduced to give it its active form; and this is usually carried out by the user of the catalyst in the converter where it is to be used. Reducing gases such as hydrogen or carbon monoxide, preferably substantially diluted with a gas which is inert to the catalyst, such as nitrogen or steam, may be used for the reduction provided they are substantially free of sulfur and other catalyst poisons. A suitable reduction temperature is 230°C,

and too high temperatures should of course be avoided.

The catalyst preferably contains a proportion of catalyst carrier material in addition to that introduced by co-precipitation. This material is conveniently the same as that introduced by co-precipitation, although this is not necessary. Preferably, the main part of the catalyst support material is introduced by co-elution. A particularly good catalyst contains 2 to 8 percent by weight of added alumina, and this is preferably added in the form of an alumina hydrate or a hydratable alumina to one or more of the solutions that take part in the co-precipitation, so that the precipitation takes place in its presence . The catalyst used according to the invention may optionally contain a proportion of chromium oxide, preferably up to approx. 25% of the total composition, and this is preferably introduced by co-definement. Both of the above percentages are calculated by weight of the oxides present in the catalyst composition

in oxide form.

In the process for reacting carbon monoxide with steam, the catalyst is deactivated at elevated temperatures, and the process is therefore carried out at temperatures up to 300°C. Pressures over a wide range, for example 1 to 50 atmospheres and higher, can be used. The inlet gas should be virtually sulphur-free, i.e. contain less than 5, especially less than 1 part of free or bound sulfur per million, and it is therefore advantageous to protect the catalyst by means of a sulphur-absorbing layer or other protective cover, suitably one which acts at the inlet temperature of the catalyst. The method according to the invention is suitably used for treating a gas mixture whose C0 content is reduced (for example to less than 5%) over a catalyst (such as iron oxide or a molybdate) which is effective at higher temperatures. Such two-stage carbon monoxide conversion systems are previously well known. The method according to the invention can be used in connection with known method steps for adjusting the steam concentration and temperature and for removing carbon dioxide.

The method according to the invention is particularly suitable

for the production of ammonia synthesis gas or hydrogen, or to remove carbon monoxide from fuel gas to, for example, reduce its toxicity.

The invention is illustrated by the following examples.

EXAMPLE 1

A solution (4 liters) of copper(II) nitrate trihydrate (0.694 kg), zinc nitrate hexahydrate (1.323 kg) and aluminum nitrate nonahydrate (0.853 kg) in water was mixed continuously at 74°C with a solution (20 litres) of sodium carbonate (3.4 kg as Na2C0g). The flow rates during mixing were such that a small excess of alkali was obtained, corresponding to a pH for the slurry of 7.0 to 7.5. The entire slurry was then diluted with 10 liters of water, heated to 90°C and held there for 1 hour. It was observed that as a result of this heat treatment the pH value of the slurry rose to 8.4. The precipitate was collected on a filter, reslurried with 20 liters of water, then collected again and washed with 30 liters

water. The filter cake was dried at 120°C, calcined at 300°C

for 8 hours, and then finely powdered. A sample of this was pelletized with 2% graphite to obtain 5 mm x 5 mm cylindrical pellets. The percentage composition of the pellets by weight was as follows:

This corresponds to an atomic ratio of 35% copper, 53% zinc and 12% aluminium. The pellets had an average vertical crushing strength of 167 kg, pellet density of 1.191 and fill weight of 1.2.

A sample of the composition was tested as pellets by reducing them with dilute hydrogen (1.5% H 2 , 98.5% N 2 ) at 230°C. A catalyst containing only copper and zinc (molar ratio 34:66) and with an average vertical crushing strength of 68 kg, a pellet density of 2.46 and a fill weight of 1.5

was tested in the same way. The activity of the catalysts at 240°C, expressed as rate constants based on the degree of decomposition of carbon monoxide, is given in the table. The values in parentheses are percentages of original activity.

Another sample of the filter cake was pelletized with 2% graphite, but using a lower pelletizing pressure. The average vertical crushing strength of these pellets was 68 kg and their density was 1.66. When tested in the same way as the higher density catalysts, the following activities were obtained:

EXAMPLE 2

The procedure from Example 1 was followed with the exception that 0.256 kg of magnesium nitrate hexahydrate was used instead of the aluminum nitrate, and minor changes were made when washing the precipitate. Under the conditions used, magnesium is not completely precipitated, and as a result the magnesium content of the catalyst was less than the aluminum content of the catalyst in Example 1. The percentage composition of the catalyst was

This corresponds to an atomic ratio of 34.9% copper, 58.4% zinc

and 6.7% magnesium. The pellets had an average vertical crushing strength of 73 kg, a pellet density of 1.66 and a fill weight of 1.11.

To examine this catalyst, the pellets were crushed to pass a 17 B.S.S, sieve, but retained on a 25 B.S.S, sieve, and then reduced using a mixture of hydrogen (1.5% by volume) and a nitrogen at 230 °C. A mixture of water vapor (50%), carbon monoxide (25%), and hydrogen (25%) was then passed over the reduced catalyst at a dry gas volume rate of 18,000 hours.

The activity of the catalyst in reciprocal second units at 240°C was as indicated in Table 3.

EXAMPLE 3

A solution (4 liters) of copper (II) nitrate trihydrate (0.694 kg), zinc nitrate hexahydrate (1.488 kg) and manganese nitrate hexahydrate (0.143 kg) in water was mixed continuously at 70°C with a solution (20 liters) of sodium carbonate (3.4 kg as Na^O^) . The flow rates during mixing were such that a small excess of alkali was obtained, corresponding to a pH

value for the slurry of 7.0 to 7.5. The entire slurry was then diluted with 6 liters of water, heated to 90°C

and held there for 1/2 hour. It was observed that as a result of this treatment the pH of the slurry rose to 8.7. The precipitate was collected on a filter, reslurried with

20 liters of water, then collected again and washed with 20

liters of water. The filter cake was dried at 120°C, calcined at 300°C for 8 hours, and then finely pulverized. A sample of this was pelletized with 2% graphite to give 5 mm x 5 mm cylindrical pellets. The percentage composition of the pellets by weight was: CuO 29.0, ZnO 54.7, MnO 5.3, loss at 900°C

10.5 (includes volatile substances and graphite). This corresponds to an atomic ratio of 32.2% copper, 59.4% zinc and 8.4% manganese. The pellets had an average vertical crushing strength of 52kg, a pellet density of 1.55 and a filling weight of 1.0.

A sample of the pelletized composition was examined after it had been reduced with dilute hydrogen (1.5%

H2, 98.5% N2) at 230°C. A catalyst containing only

copper and zinc (atomic ratio 34:66) and with an average vertical crushing strength of 68 kg and a pellet density of 2.46 was under-

applied in the same way.

When using an inlet gas mixture consisting of

of CO 3.3%, C0210%, H2 53.3% and H20 33.4% at atmospheric

pressure, ) at a volume rate of 22,500 hour -1, the conversion at 230°C was 32.5% for the copper-zinc-manganese catalyst, and 30% for the copper-zinc catalyst. These results were obtained using the same weight of each catalyst, although the copper-zinc catalyst is considerably denser. The activity of the catalysts, expressed as rate constants based on the degree of decomposition of carbon monoxide at 240°C, was 9.6 sec.<->"<*>"for the copper-zinc-manganese catalyst and 12.5 sec.<->'< *>' for the copper-zinc catalyst. Although the activity of the copper-zinc-manganese catalyst is less than that of the copper-zinc catalyst, this is the original activity, and after a few weeks of use the copper-zinc-manganese catalyst is the most active.

EXAMPLE 4

The procedure according to Example 1 was followed with the exception that 0.286 kg of manganese nitrate hexahydrate was used instead of 0.143 kg and 1.323 kg of zinc nitrate hexahydrate instead of 1.488 kg. The pellets had an average vertical crushing strength of 61 kg, a pellet density of 1.68 and a fill weight of 1.07.

The percentage composition of these pellets by weight was CuO 28.7, ZnO 47.6, MnO 10.6, loss at 900°C 12.0. This corresponds to an atomic ratio of 32.8% copper, 53.5% zinc

and 13.6% manganese.

The catalyst was investigated as described in Example

3 and the results are shown in table 4.

Although the manganese-containing catalyst is somewhat inferior in terms of initial activity, it is obviously the most active after long-term use.

EXAMPLE 5

To a solution of copper nitrate trihydrate (1.04 kg), zinc nitrate hexahydrate (2.56 kg) and concentrated nitric acid (400 ml) in 3 liters of water was added 0.19 kg of sodium aluminate in 00 ml of water. Alumina was precipitated, but redissolved upon stirring. Then 0.075 kg of finely divided, solid alumina trihydrate was added. The resulting suspension was mixed at its flow junction with a sodium carbonate solution sufficient to give a pH of 7.0 to 8.0 at the precipitation temperature of 70°C. The slurry was diluted with 6 liters of water, heated at 90°C for 1 hour (final pH 8.5), and then filtered. The washing, calcining and pelletizing process was as described in Example 1. The pellets (5.4 mm diameter, 3.6 mm long) had an average vertical crushing strength of 85 kg, a pellet density

of 1.56 and a filling weight of 0.99. The percentage composition of the pellets was

This corresponds to an atomic ratio of 29% copper, 60% zinc, 11% aluminium.

The pellets were crushed and examined as described in example 2. At 200°C the conversion was 32% and at 250°C 51%.

Claims (2)

1. Process for reacting carbon monoxide with water vapour, in particular for the production of ammonia synthesis gas or hydrogen or for making fuel gas non-toxic, at temperatures below 300°C over a catalyst containing copper and zinc, characterized in that the a catalyst is used comprising the product from the partial reduction of the combined oxides of copper and zinc and at least one additional metal selected from the group consisting of aluminium, magnesium and manganese, whose oxides are usable as catalyst carriers, and optionally a non-combined catalyst support material . 2. Process as stated in claim 1, characterized in that the additional metal used in the catalyst is aluminium.