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Oxidation of Ammonia

The direct oxidation of ammonia to nitric acid has been known for a considerable time, as shown by the experiments of Milner, who, in 1788, passed a mixture of ammonia and air over heated manganese dioxide with the production of oxides of nitrogen. Apparently saltpetre was made on the large scale in France during the Napoleonic wars by Milner's method.

Kuhlmann in 1839 brought about a similar reaction by passing a mixture of ammonia and air over platinum sponge heated to 300° C. The well-known lecture experiment4 consists in introducing a hot platinum spiral into a flask containing ammonia solution through which oxygen is passing, when a series of explosions occurs with the formation of red nitrous fumes.

Many other catalysts have been proposed from time to time, some of which have been patented; these include platinum in different forms, manganates, permanganates, chromates, bichromates, iron, iron oxides, plumbites, platinised glazed porcelain, ceria, and thoria.

In 1900 an exhaustive study was made of the necessary conditions for the oxidation of ammonia by means of platinum, and patents were taken out in 1902 for the production of nitric acid on the technical scale.

The final reaction is probably summed up in the equation
  1. NH3 + 2O2 = HNO3 + H2O,

    but actually in practice at least four steps have to be realised.

    Thus the first product of oxidation is nitric oxide:
  2. 4NH3 + 5O2 = 4NO + 6H2O.

    This nitric oxide is then converted into the dioxide by means of oxygen (air):
  3. 2NO + O2 = 2NO2.

    Thirdly, the conversion of nitrogen peroxide into nitric acid occurs under the combined action of oxygen (air) and water:
  4. 2NO2 + O + H2O = 2HNO3.

    Finally, the dilute nitric acid thus produced has to be concentrated.

    Ostwald's experiments were first conducted with ordinary smooth platinum and also the finely divided metal.

    It was found that while smooth platinum favoured the reaction (1), the use of platinum black caused the oxidation of the ammonia to occur largely in the sense of the equation
  5. 4NH3 + 3O2 = 2N2 + 6H2O,

    although at the same time the oxidation, according to equation (1), was accelerated.
It was found, however, that on the technical scale platinum black was unsuitable - even in small amounts. Ordinary platinum, in the form of spirals or gauze, was found to be quite efficient, provided that correct conditions were used as regards volumes of ammonia and air, pressure and the temperature of the catalyst. The required oxygen is furnished by using 10 volumes of air to 1 volume of ammonia, and this mixture is passed at the rate of 1.5 metres per second through 1 to 2 cm. of contact substance maintained at 300° C. The nitric acid is condensed in vitreosil receivers cooled by water, while uncondensed nitric acid vapours are passed into the absorption towers, where they meet nitric acid.

As with other processes, there are numerous modifications of the catalyst vessel - the so-called "converter."

The original Ostwald type is used by the Nitrogen Products Company at Dagenham Dock. Crinkled platinum foil is used in an inner nickel tube surrounded by an outer iron tube. The initial heating of the catalyst by a hydrogen flame starts the reaction, and the heat evolved from the reaction serves to maintain the incoming gases at the right temperature (800° to 830° C.), so that once started no external heating is required.

Ammonia oxidation converter
Ammonia oxidation converter
The Frank-Caro converter uses a platinum gauze heated electrically, and this type is found to be very efficient, especially for supplying nitrogen oxides to the chambers in sulphuric acid manufacture. The Kaiser patent was the first to utilise multiple super-imposed gauzes of platinum in close contact, and the preheating of the gases to 300° - 400° C. This new feature is the basis of all modern developments in the oxidation converter. The Partington Al Converter makes use of a modified Frank-Caro type, and consists of a top and bottom cone of enamelled iron, containing a vertical double layer of platinum gauze with an exposed surface 6 inches by 4 inches, made with wire 0.0025 inch diameter and 80 mesh. The gaseous mixture is obtained by blowing air through pure 25 per cent, ammonia liquor, and the proportion of constituents is 7.5 volumes of air to 1 volume of ammonia, which is that required theoretically to give nitrogen trioxide, N2O3. The platinum gauze is heated externally to 650°- 700° C. initially, after which the temperature is maintained by the heat of the reaction (220 Cals.); preheating of the gases to about 400°-600° C. improves the yield. The conversion efficiency is from 92 to 95 per cent., and the output is 1.5 tons of nitric acid per square foot of converter cross-section (2 square feet total gauze area) per twenty-four hours.

A converter of the above dimensions is capable of supplying a large chamber sulphuric acid plant with all the necessary oxides of nitrogen, thus completely dispensing with nitre pots. The above work by Partington was carried out for the Munitions Inventions Department, and an interesting feature was the separation of water from the oxidation product of the converter by rapid cooling. The equation expressing the reaction

4NH3 + 5O2 = 4NO + 6H2O

shows that removal of steam will result in a considerable concentration of the nitric oxide, although of course the condensed steam will contain nitric acid which is worth concentrating or absorbing with alkali. The further oxidation of the oxides to nitric acid is brought about by addition of secondary air after the coolers.

Partington has much further improved the efficiency of the above type of converter by the substitution of oxygen for air, whereby an almost quantitative conversion is obtained. Two gauzes in contact are used, and, in order to damp down the explosive mixture of ammonia and oxygen, steam is mixed with the gases, which are preheated to 500° C. The correct mixture is obtained by volatilising ammonia liquor (sp. gr. 0.88) in sufficient oxygen, i.e. that required to completely oxidise the ammonia to nitric acid:

NH3 + 2O2 = HNO3 + H2O.

Direct cooling after conversion results in the production of a fairly concentrated acid, and absorption is dispensed with.

Partington has determined experimentally the various conversion efficiencies obtained by various types of converters, and his results are tabulated below:

Type of ConverterCatalystMethod of Heating.Conversion Efficiency. Per cent.Output kgm. HNO3 per Gram Pt per 24 Hours.
OstwaldPlatinum foilRegenerative90-951.7-2
Frank-CaroSingle platinum gauzeElectrical90-9311
Kaiser4 superposed platinum gauzesPreheating of gases9512
Partington2 platinum gauzes slightly separatedElectrical or preheating of gases9516
Partington (with oxygen)2 platinum gauzes in contactGases and steam preheated to 500°-600° C.98-9921

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