Synthetic Ammonia

The fixation of atmospheric nitrogen in the form of ammonia by direct combination with hydrogen is one of the most successful and economical of processes, as the amount of energy required is small. The fact that about 65 per cent, of the world's supply of fixed nitrogen is produced by this method (1925) is an indication of the importance of this process. The direct combination of nitrogen and hydrogen in the presence of a catalyst is shown by the equation

N₂ + 3H₂ ⇔ 2NH₃ + 23.800 Cals

and the equilibrium of this reversible reaction is given by the expression



It is obvious from the above equation that, as a contraction in volume from 4 vols, to 2 vols, occurs, increase of pressure will favour the formation of ammonia; also as the reaction is exothermic, as low a temperature consistent with other factors will give the best yield.

In the " Haber " process, the first to achieve technical success, the nitrogen and hydrogen are brought into contact with the catalyst at a pressure of 200 atmospheres and a temperature of 600° C. The catalyst most generally used is iron in some form or other, although this is not necessarily the best catalyst apart from the question of cost. In any case, whatever the catalytic material used, it is imperative in the Haber process that the reacting gases shall be as pure as possible. It is estimated that from 70 to 75 per cent, of the working cost of the plant is in the production of pure nitrogen and hydrogen.

In Germany there are two methods of obtaining the nitrogen- hydrogen mixture, and both processes involve the so-called " Bosch " purification methods. At Oppau, water-gas (hydrogen 50 per cent., carbon monoxide 40 per cent., carbon dioxide and nitrogen) is mixed with steam and air, and the mixture passed over a heated catalytic material of ferric oxide and nickel or chromium oxide. The chief reaction is that between the steam and carbon monoxide, resulting in the production of hydrogen and carbon dioxide:

CO + H₂O ⇔ CO₂ + H₂.

At comparatively low temperatures, namely, 400° to 600° C., the reaction proceeds to a considerable extent from left to right, and the resulting gas now contains nitrogen, hydrogen, carbon dioxide, and small amounts of carbon monoxide, hydrogen sulphide, argon, and hydrocarbons. Carbon dioxide is removed by washing with water at high pressure; carbon monoxide is absorbed by scrubbing with ammoniacal cuprous formate, followed by hot caustic soda solution, which also removes the hydrogen sulphide.

The nitrogen-hydrogen mixture is now brought up to the required ratio 1:3 by the addition of nitrogen from a Linde plant.

At Merseberg, the "Haber-Bosch " process differs from that in use at Oppau, in that the addition of pure nitrogen is dispensed with. Water- gas is mixed with producer gas (chiefly nitrogen and carbon monoxide) and steam, and the mixture passed through similar contact furnaces as at Oppau to oxidise the carbon monoxide to the dioxide:

CO + H₂O ⇔ CO₂ + H₂.

The carbon dioxide, carbon monoxide, and sulphuretted hydrogen are removed as before, and a final removal of impurities is effected by passing through a series of small contact furnaces. It is thus possible to obtain finally the right nitrogen and hydrogen mixture by suitable adjustment of the original amounts of water-gas, producer gas, and steam. Or nitrogen and steam may be passed over iron. The theoretical considerations underlying the combination of nitrogen and hydrogen have been dealt with in the section on the ammonia-nitrogen - hydrogen reaction and equilibrium, and it is only necessary here to indicate the technical working of the Haber process.

The purified mixture of nitrogen and hydrogen in the proportion of one to three is passed into the circulatory system through a soda- lime drier. The gases under a pressure of 200 atmospheres are preheated by electrical coils, and come into contact with the catalyst which is contained in steel bombs, 6 metres by 80 cm. internal dimensions. The catalyst material is maintained at 600° C. by means of a system of heat regenerators, and the ammonia is absorbed by water under pressure flowing down steel spirals. A solution containing about 25 per cent, of ammonia is obtained, and the uncombined nitrogen and hydrogen are passed back into the circulatory system after drying. This ammonia liquor is either used for oxidation direct to nitric acid, or else converted into ammonium sulphate by neutralisation with sulphuric acid. This latter substance became very scarce in Germany during the War owing to the blockade of the Allies, which cut off the supply of pyrites. This shortage of sulphuric acid was overcome to a large extent by the successful technical process of the Badische Company. Ammonia gas and carbon dioxide are passed simultaneously into water which contains freshly calcined gypsum in suspension. Calcium carbonate is precipitated and ammonium sulphate remains in solution:

2NH₃ + CO₂ + CaSO₄ + H₂O = CaCO₃ + 2(NH₄)₂SO₄.

A highly purified ammonium sulphate is obtained by concentrating the filtrate in vacuum pans.

An interesting modification of the Haber process was tried at Sheffield, Alabama, by the United States Government Nitrate Factory No. 1, under various patents of the General Chemical Company. The catalyst used was an activated sodamide made by heating to 550° C. pumice impregnated with nickel or ferric nitrate, and treating at 450° C. with sodium and ammonia, when sodamide was formed in the spongy metal. The nitrogen-hydrogen mixture, at the relatively low pressure of 70 to 100 atmospheres, was passed over the catalytic material maintained at a temperature of 500° C. This process was not a technical success owing to the faulty construction of the plant and the poisoning of the catalyst with water. Later the process was taken over by the Atmospheric Nitrogen Company in conjunction with the Solvay Company of New York, and a successful production of 10 tons of liquid ammonia per day for refrigerator purposes was attained.

M. Georges Claude carefully investigated the equilibrium percentages of ammonia produced at very much higher pressures than those used by the Badische Company in the Haber process, and in 1918 announced his invention by which the use of pressures of 1000 atmospheres very materially increased the yield of ammonia. Under a pressure of 1000 atmospheres and temperature of 600° C. the percentage of ammonia reaches the high figure of 25, of ammonia is given at the same temperature under 200 atmospheres. In addition to the enhanced yield there are important economies effected in the working of the plant, as in the Haber process the maintaining and restoring of pressure by stages is troublesome and expensive compared with the actual yield of 6 per cent, of ammonia. In the Claude process the work of compression is greater, but since it is proportional to the logarithm of the pressure ratio (a = RT log p₂/p₁), it is only greater in the proportion 2.3 to 3.

The pressure is obtained in stages of 100 - 300 - 1000 atmospheres, and the advantages of the process are:

1. A sufficient percentage conversion can be obtained in one operation, instead of a lengthy series of circulations as at 200 atmospheres.
Liquid ammonia is produced in bulk merely by releasing the pressure to that of the atmosphere and cooling with water.
The spontaneous evaporation of ammonia from the liquid state at ordinary temperatures obviates the expense of evaporation as required by the Haber process.
A useful source of refrigeration is provided in converting the liquefied ammonia into gas, which can be utilised for subsequent fixation of nitrogen as a fertiliser.

The second important feature of this process, due to Schreib, is the fixing of the ammonia in the form of ammonium chloride. It would appear that this salt is just as efficient a fertiliser as the sulphate, and could be obtained in larger bulk more cheaply. The ammonium chloride is obtained by a modification of the Solvay (ammonia-soda) process. Saturated sodium chloride solution is further saturated with ammonia gas under pressure and carbon dioxide is passed into the solution, with the formation of ammonium chloride and sodium bicarbonate

NaCl + NH₃ + CO₂ + H₂O = NH₄Cl + NaHCO₃.

The sodium bicarbonate, being practically insoluble in sodium chloride solution, is precipitated, and the filtrate is again saturated by the addition of solid sodium chloride. Further treatment with ammonia and carbon dioxide produces normal ammonium carbonate in solution and precipitates ammonium chloride. The ammonium carbonate in solution is converted by a further quantity of carbon dioxide into the bicarbonate,

(NH₄)₂CO₃ + CO₂ + H₂O = 2NH₄HCO₃;

and this ammonium bicarbonate precipitates sodium bicarbonate,

NH₄HCO₃ + NaCl = NH₄Cl + NaHCO₃,

giving a solution of ammonium chloride as in the first reaction.

Hence a cycle of operations occurs with alternate precipitation of sodium bicarbonate and ammonium chloride. This latter salt is obtained 97 per cent, pure in two cycles, and as the impurity is chiefly sodium chloride, the ammonium chloride can be directly used as a fertiliser without injurious effects.

The chief source of hydrogen for the Claude process, as worked in France, is coke-oven gas, and one of the most promising methods for purification is by fractionation between -160° C. and -210° C., whereby hydrogen is obtained suitable for the ammonia synthesis. According to Cederberg, efficient purification of hydrogen from oxygen, water-vapour, carbon monoxide, and dioxide can be obtained by passing the gases through a solution of sodium in liquid ammonia. Of the various catalysts used, a material which will last for some hundreds of hours has been obtained by directing a jet of oxygen into a molten mass of ferrous oxide, iron, calcium oxide, and a small amount of alkaline oxide contained in a magnesia crucible.

The outstanding feature of the Casale process for synthetic ammonia is the large scale production of electrolytic hydrogen, part of which is burnt in air to give the requisite nitrogen. The nitrogen-hydrogen mixture is then passed into steel cylinders containing coaxial annular chambers, so that the walls of the vessel are protected from the hot hydrogen. The pressure employed is 300 atmospheres, and a special catalyst is claimed which functions in spite of relatively impure nitrogen and hydrogen. Mention has been made that 70 to 75 per cent, of the total costs of the Haber and Claude synthetic ammonia processes is the preparation of pure gases so that the catalyst will not be " poisoned." The catalytic material of the Casale process is made by bringing iron into violent ebullition with oxygen under pressure in the presence of alkaline earth oxides. About 10 per cent, of the oxidation product is allowed to vaporise so as to remove impurities detrimental to the catalysis. A certain predetermined amount of ammonia is mixed with the mixture of nitrogen and hydrogen; the ammonia is liquefied by refrigeration and an amount equal to that formed removed, and the gases again subjected to catalysis.

A later patent of Casale embodies the use of the oxygen obtained from the electrolysis of water. A gas approximating to the composition of water-gas is obtained by passing an oxygen-air mixture into a generator burning lignite or peat. This gas is then burnt in oxygen in a power generator, and the resulting gaseous mixture of hydrogen, nitrogen, and carbon dioxide is compressed by a multiple stage compressor varying from 100 to 1000 atmospheres. The carbon dioxide is removed by passing the compressed gases through a refrigerator cooled below 30° C. and is drawn off in the liquefied form. The last traces of carbon dioxide are removed by scrubbing with water, and the nitrogen-hydrogen mixture is enriched with electrolytic hydrogen to obtain the right proportions. Recirculation of the uncombined gases through the catalyst bombs occurs after removal of liquefied ammonia. A successful technical plant on the Casale principle has been installed at Terni in Italy, where the great waterfalls provide ample power for the electrolytic hydrogen (and oxygen).

The initial source of the nitrogen in the Fauser process is liquid air, and hydrogen of 99 per cent, purity is produced electrolytically. The special advantage of this process is that when used in conjunction with an ammonia oxidation plant, much nitrogen can be obtained from the residual gases after the oxidation to nitric acid and oxides of nitrogen. These gases, which contain relatively much nitrogen and little oxygen, are mixed with hydrogen and passed over platinum asbestos. The oxygen is removed as water, and practically pure nitrogen is now available for combination with hydrogen in the catalyst chamber. This latter takes the form of a bomb, and the nitrogen-hydrogen mixture at 300 atmospheres pressure is passed in. Preheating of the gases is effected by a system of heat interchangers which makes use of the heat of reaction, and the right temperature is secured finally by means of an electrically heated spiral. The heat of the gases from the catalyst chamber is used to volatilise ammonia from the solution obtained by absorbing the gas which has escaped liquefaction. The water serves as a lubricant for the compressors, and the ammonia liberated from the aqueous solution is obtained in the liquid form after cooling. The Fauser process offers a very economical method of nitrogen fixation, given an ample supply of water-power for producing electrolytic hydrogen, as a kilogram of nitrogen in the form of ammonia requires 17 K.W. hours direct current, and 19 K.W. hours if in the form of nitric acid.