DE2148707A1 - Ammonia oxidation catalyst - by decomposing cobalt carbonate pellets and sintering the resulting oxide - Google Patents

Abstract

Catalyst consists mainly of cobalt oxide of specific surface 0.1 - 7m2/g and porosity 1-15% vol/wt. Cylindrical particles of basic cobalt carbonate are decomposed to the oxide and heated to 700-900 degrees C to sinter the product which has length equal to dia. (2-8mm) and vertical crushing strength >=35kg/cm2. Ammonia (9-11 vol. % in air) is oxidised at 650-800 degrees C with a catalyst ped 8-30 pellets thick. The catalyst has much reduced tendency to disintegrate in use and is also used for liq. phase hydrogenation reactions.

Description

Catalyst, process for its preparation and its use The invention relates to a cobalt oxide catalyst used for oxidation from ammonia to nitrogen oxides and for other reactions.

Although it has long been known that cobalt oxide is involved in the oxidation of ammonia is effective, technical processes for the production of Nitric oxides or nitric acid using a catalyst based on Platinum, such as platinum-rhodium, usually in the form of metal gauze. This is expensive in terms of usage costs as well as running costs because it is metal is lost from the catalyst and not fully recovered from the reaction gases will. Such a loss of metal is all the more serious the the higher the pressure at which the oxidation is carried out and is therefore an acute one This has become a problem with modern single-flow, high-throughput nitric acid plants.

One problem with using cobalt oxide is getting it adequate Resistance to thickening of granules (pellets) from this at the high Ensure temperatures of oxidation. There has now been a method of manufacture of cobalt oxide pellets which have a much lower incidence of disintegration during the ammonia oxidation and which in other reactions, such as liquid-phase hydrogenation, are useful, found.

The catalyst according to the invention consisting mainly of Cobalt oxide, has a specific surface area in the range from 0.1 to 7 m² / g (especially 0.2 to 6 m² / g) and a volume / weight porosity in the range from 1 to 15% and if so it in the form of cylinders of a length and a diameter which are approximately the same and in the range of 2 to 8 runs, there is a medium vertical crush resistance of at least 35 kg / cm2. Suitably the density of the catalyst is in the range from 3.0 to 5.0 g / cm3.

The invention in particular provides such a catalyst for liquid-phase ilysis reactions with a specific surface area in the range from 2 to 7 m2 / g and one medium vertical crush resistance in the range from 70 to 200 kg / cm2 and also a catalyst for ammonia oxidation with a specific surface area in the range from 0.1 to 3 / ground an average vertical crush strength of 140 to 1050, e.g. 140 to 560 kg / cm2.

According to a further feature of the invention, a catalyst is produced, by forming particles of a cobalt compound that is easily decomposable to oxide the particulate compound is decomposed to the oxide and the particles are heated to trigger sintering.

Preferably, the compound becomes theirs upon first decomposition treated with partial decomposition to the oxide, then converted into particles, then into treated to a second decomposition stage to largely remove the rest of the compound to decompose to the oxide, whereupon the particles are heated to initiate sintering will.

The catalyst preferably contains at least 95% cobalt oxide, calculated as Co304. If other ingredients are present, these can be, for example Nickel oxides or e.g. chromium and aluminum, which reduce the effectiveness of the catalyst do not affect or promoters and stabilizers, such as rare earth oxides, e.g. cerium oxide. It is preferred that no more than traces of alkali metal compounds available.

The cobalt compound is preferably a basic carbonate however, if desired, be an oxalate or hydroxide. To the extent of the decomposition To maintain control, the compound is preferably maintained prior to the formation of the particles dried at a temperature not exceeding 1200C.

When the first decomposition of the cobalt compound to the oxide is carried out this takes place preferably at a temperature below 2000C. Preferably the extent of the first decomposition of the cobalt compound to the oxide is such that yields a product containing at least 10 and preferably 13 to 25% by weight volatile material at 7500C, i.e. mainly carbon dioxide and water. Here it concerns values measured as gross loss, i. e. which not for the Fact that 'during the decomposition Robalt compounds, which are initially divalent, are converted to Co304.

The formation of particles can take place by any suitable method, such as shell granulation, roller compaction, extrusion, wet pelleting or dry pelleting.

If desired, two or more methods of particle formation can be used can be used in conjunction, for example, small particles passing through Spray drying, extruded or dry pelletized, the two stages of dry pelletization can be used.

For dry pelletization, a pelletizing pressure of 1 to 12 to / cm2 preferred, since higher pressures reduce the active catalytic surface and are not needed because the subsequent sintering increases the mechanical strength elevated.

If, as preferred, two stage dry pelletization is used suitably the first stage is at a pressure in the range 0.1 to 2.0 tons / cm².

Whichever method of particle formation is used, the The size of the particles formed should preferably be 15 to 30% in each linear dimension be larger than the particles that are ultimately required to shrink in the final heat treatment.

The decomposition of the remainder of the compound to the oxide (second decomposition) is preferably used at a. Temperature in the range of 3500 to 6000C carried out. It is preferred to have decomposition down to 5% volatile matter or less to ensure at this stage before the final heating stage is carried out, because such a procedure has a greater mechanical strength after sintering he brings. The second decomposition and final heating can occur in a single The stage in which the particles are gradually heated can be carried out.

The final heating stage is preferably at a temperature in the range from 700 to 9000C and in any case higher than the temperature, where it is intended to use the catalyst. Preferably the product from the second decomposition to the final heating stage without immediate cooling convicted. This final heat treatment should preferably be such that causes the particles to shrink by 15 to 30% in each linear dimension will.

The time and temperature of the final heating stage will be like this selected that the desired specific surface area, the desired porosity and the desired crush resistance can be produced.

The invention also provides a method for the oxidation of ammonia provided in nitrogen oxides in the presence of a catalyst according to the invention, in particular in the presence of such a catalyst produced by the process of the invention became. For this process, the temperature of the catalyst is preferably in Range from 650 ° to 800 ° C, i.e. on the order of 150 ° C below the temperature of the previously used metal catalyst, since the method is preferably at a Inlet ammonia / air preheating temperature of below 1500C is carried out, which is lower than normally used for a previous metal catalyst. For this process, the catalyst is mixed with an ammonia / air mixture that is suitable about 10% Vol./5 Asl. Ammorty OF2 holds, i.e. in the range of preferably 9 to 11% Vol./Vol., introduced. The catalyst loading is preferably in the range from 200 to 600, in particular 250 to 350 liters of ammonia / air mixture per hour per cm2 of catalyst bed area when working at atmospheric pressure will. At higher pressures, the catalyst loadings are correspondingly higher. To Oxidation cools the gases and a treatment for the extraction of nitrogen oxides or subjected to their absorption in water to nitric acid or in alkaline Medium to give nitrates or nitrites or to reduce them to iydroxylamine. The examples relate to the oxidation of ammonia as the first stage in the Production of nitric acid.

The air and ammonia should be essentially free of catalyst poisons using protective absorbents if necessary.

In the case of a catalyst for the oxidation of ammonia, the is The porosity range of the catalyst is preferably 4 to 12%. This is true regardless whether the catalyst is in cylindrical form or not, provided that that the specific surface is in the defined size and the mechanical Strength is in a corresponding order of magnitude. By the use of It is possible for catalysts with this preferred porosity to have a more stable type to be achieved, i.e. the catalyst is more easily ignited at the beginning of the reaction " and during the reaction less easily "blown out" than when a lower porosity catalyst was used. When using a catalyst which is more porous than the preferred area, the selectivity is lower, apparently due to side reactions within the pores of the catalyst.

It has been found that the depth of the catalyst bed is preferred in the range from 8 to 30 particles, e.g. in the range from 3 to 5 cm, if the Particles have a diameter and a height of 2 to 2.5 m, and within this range is preferably smaller, the lower the porosity of the catalyst is. So it is possible to avoid bed depths of 2 cm or less where there is a risk of the occasional formation of holes in the catalyst bed is.

The catalyst can also after reduction to an essentially metallic one Cobalt can be used in hydrogenation reactions, especially those under Application of reaction components can be carried out in the liquid phase.

Example 1 To a solution of cobalt nitrate (248 g cobalt per loo ml) became a solution in the course of 20 to 25 minutes of ammonium carbonate (9.9 g NtI3 and 13.1 g CO2 per 100 ml) were added. That amount was ammonium carbonate in excess of the cobalt nitrate, and consequently further tobalt nitrate solution was made added to bring the entire suspension to low alkalinity (20 ml normal HCl per loo ml suspension).

The precipitate was collected on a filter with water added Washed 65 ° C to 75 ° C, then slurried again in water and put on the filter collected again. It was dried at 1050C to 120 ° C for 12 hours, then at 140 ° C until the volatile content was 24%. The resulting Product mainly consisted of basic cobalt carbonate. The product was made with 2% graphite mixed and made into cylindrical pellets with a diameter of and compressed to a height of 4 mm. These were calcined at Li5o0C for 6 hours, then calcined at 800.degree. C. to 900.degree. C. for 1 hour. The resulting sintered Pellets were 4 mm in diameter and 3 mm in height and medium vertical crush strength of 72.6 kg (580 kg / cm2). The specific surface was 0.4 m2 / g, the density 3.9 g / cm3 and the porosity 7%.

The cobalt oxide pellets were placed in a 7 cm deep bed in a catalyst basket in a small ammonia oxidation plant in which a platinum-rhodium-gauze catalyst was used in a parallel experiment used at the same time. The inlet ammonia content was 10% by volume, the flow rate 320 l / h / cm2, the pressure atmospheric and the catalyst temperature 79o0C. After three weeks of implementation, during which minor deviations the Process conditions applied was the effectiveness of the cobalt oxide catalyst undiminished, and its vertical crush strength was 63 kg. The sales efficiency, based on NH3 to NO was 91.6 to 94.1% compared to 94% for a platinum-rhodium catalyst under the same conditions.

Example 2 For a solution of cobalt nitrate (5 g cobalt per 100 ml) slowly, while stirring, enough ammonium bicarbonate powder to produce one final pH of 8 added. Stirring was continued for 0.5 hours. whereupon the mixture was filtered and washed thoroughly. The filter cake was dried at 140 ° C to give an essentially oxide-free basic cobalt carbonate (Volatile matter content 27.2%, w / w). This was with 2% (w / w) Graphite mixed and compressed into 3.2 mm cylindrical pellets. These were calcined at 4500C for 6 hours to produce essentially pure CO304. Four samples A through D were stripped and calcined to different degrees of sintering and thus to obtain different porosities. The pellets shrank during sintering each specimen to between 2.2 and 2.5 mm in diameter and height.

The catalysts were used in the oxidation of ammonia (lo% NH3, 90% air) at atmospheric pressure and a temperature of 750 ° C. the Results are given in the table in which the loading is expressed in Liters of ammonia + air, calculated at 2o0C, a pressure of 760 mm Hg per hour per cm2 of the area of the reactor cross-section.

Further tests were carried out using catalysts B through D with 11.5% ammonia in air and carried out with bed depths adapted to the porosity, the results listed in the second part of the table were obtained.

Table 1 ABCD catalyst Pellet density g / cm2 3.22 3.53 4.32 4.77 % Porosity cm3 / gx 100 14.6 10.9 6.6 4.6 Pore volume cm³ 0.47 0.38 0.29 0.22 Specific surface area m² / g 0.83 0.40 0.20 0.16 Medium Vertical Pound strength 1 kg per pellet 3.6 7.3 35.8 47.6 kg / cm² 91 182 900 1190 10% NH3 Optimal loading at over turning a bed of 150 300 300 300 4 cm, 1 / cm2 above Efficiency% 93 - 94 99 99 94 10% NH3 Optimal bed depth at 300 1 / cm² / h, load cm, 7 4 4 2 Efficiency% 95 99 99 99 11.5% NH3 Bed depth, cm. - 4 4 2 Optimal loading 1 / cm² / h - 300 320 300 Efficiency% - 95 95 96 Example 3 A catalyst batch, corresponding to catalyst B of Example 2, with the exception that it was in the form of 4 × 3 mm flat cylindrical pellets and had been produced on a larger scale, was in the same oxidation plant in a 7 cm bed at temperatures in Range from 730 ° C to 775 0C applied. It was found that with catalyst loadings in the range of 200 to 500 l / cm2 / hr, the efficiency was in the range of 94 to 97%. From a diagram of the efficiency against loading it can be seen that the efficiency in the tested area was largely independent of the loading.

Example 4 Catalyst B from Example 2 was tested in a small-scale pressure oxidation plant, the following results being obtained: Table 2 Pressure ammonia / air bed depth temperature loading efficiency ata ratio cm C l / cm2 / h 5 10.1 4 750 687 94.6 5 8.4 4 76o 722 94.0 4.7 8> o 7 | 775 712 91.5

Claims (9)

Claims 1. Catalyst, characterized in that it mainly consists of cobalt oxide and has a specific surface in the range from 0.1 to 7 m2 / g and a volume / weight porosity in the range of 1 to 15%. 2. Catalyst according to claim 1, characterized in that it is in Shape of cylinders, the length and diameter of which are approximately the same and in the area from 2 to 8 mm, and an average vertical crush strength of at least 35 kg / cm2. 3. Catalyst according to one of the preceding claims, characterized by a specific surface in the range from 0.1 to 3 m² / g and a medium one vertical crush strength in the range of 140 to 1,050 kg / cm². 4. Catalyst according to one of the preceding claims, characterized by a porosity in the range from 4 to 12%. 5. A method for producing a catalyst according to any one of the preceding Claims, characterized in that one consists of particles from an easily decomposable to the oxide Cobalt compound forms, decomposes the particle-shaped compound to the oxide and the Particle to trigger heated a sintering. 6. The method according to claim 5, characterized in that the Compound in a first decomposition stage with partial decomposition to the oxide treated, shaped into particles, the compound in a second decomposition stage extensive decomposition of the remainder of the compound to the oxide treated, whereupon the Particles are heated to initiate sintering. 7. The method according to any one of claims 5 or 6, characterized in that that the decomposition after the particle formation takes place at a temperature in the range from 35o0C to 600 ° C and the final heating stage at a temperature in Range from 7000C to 9bo0C. 8. Use of the catalyst according to one of the preceding claims in a process for the oxidation of ammonia to nitrogen oxides. 9. Use of the catalyst according to claim 8 with a catalyst bed, the depth of which is 8 to 30 particles.