Arc Processes

The combination of nitrogen and oxygen by electric sparks was first shown by Cavendish in 1784. Rayleigh in 1897, describing his experiments which isolated argon from the atmosphere, pointed out the possibilities of utilising the electric arc for the industrial fixation of nitrogen.

The conditions for production of a good yield of nitric oxide are:

1. The maintenance of a high-tension arc.
2. The exposure of a large quantity of air to electrical action.
3. The rapid chilling of the products.

The first technical attempt to utilise this reaction was that of Bradley and Lovejoy in 1902 at Niagara. The arcs were struck between platinum points fixed in an outer casing and similar points which were rapidly rotated on a concentric axle, resulting in the making and breaking of over 400,000 arcs per minute. Although the efficiency was good, the yield being 86 grams of nitric acid per K.W.H., it was found that the mechanism was too complicated for technical use.

The Birkeland-Eyde Arc

The first commercially successful process was that of Birkeland and Eyde, which was started in 1903 at Notodden in Norway. An alternating current arc struck between water-cooled copper electrodes is deflected by a powerful magnetic field applied in a direction at right angles to that containing the electrodes. The arcs formed across the shortest air space are driven outwards in the form of a semicircle until their resistance becomes so great that they are extinguished, or until the direction of the current changes so that the arcs are directed to the other side. The result is a disc of electric flame or an electric " sun," two yards in diameter, formed in a firebrick chamber through which the air, preheated in the outer part of the furnace, is blown. The percentage of nitric oxide in the issuing gases is about 1.25, and the yield of nitric acid 67 grams per K.W.H., or 580 to 590 kilograms per K.W.Y. (reckoned as acid and nitrate). Originally about 700 K.W. were employed in the furnaces, but 4000 K.W. are now used. The voltage employed is about 5000, of which up to 4000 are across the arc, the remainder being used in the inductances, which are necessary to steady the arc. The power consumed is great, so that the process is only applicable where power is cheap, e.g. derived from water, as at Notodden. In these localities, the simple nature of the process and the robustness of the plant, combined with the fact that the raw material - air - costs nothing, have allowed it to retain its position in the manufacturing world.

Birkeland Eyde Furnace

The gases escaping from the furnace, which have rapidly cooled to a temperature of about 800° to 1000° C., are used for steam-raising. At 600° C. the reaction

2NO + O₂ → 2NO₂

begins, and is complete at 140° C. The process of cooling from 300° to 175° C. is used to preheat the entering air. The oxides of nitrogen, after cooling to 50° C. by passing through water-cooled aluminium pipes, pass into a cylinder lined with acid-proof stone in order to carry further the (slow) conversion into NO₂. These oxides are then absorbed by water trickling over broken quartz in granite towers 70 feet high and 20 feet in diameter. The acid produced in the first tower reaches a concentration of 30 to 40 per cent, of nitric acid, while the strengths in the succeeding towers are 20, 10, and 5 per cent, respectively. The maximum production of nitric acid corresponds to the reaction

2NO₂ + H₂O → HNO₂ + HNO₃,

which only takes place in cold dilute solution. In the case of hot solutions containing more than 46 per cent. HNO₃, and gases containing about 1 per cent. NO, the nitrous acid is decomposed:

3HNO₂ → HNO₃ + 2NO + H₂O.

Hence in practice some NO will escape, and this after further oxidation is absorbed by solutions of sodium hydroxide or sodium carbonate, or a mixture of both, when about 95 per cent, of the NO appears as sodium nitrite and 5 per cent, as the nitrate. Evaporation of the solution produces the crystalline mixture, which is used in the chamber process for sulphuric-acid manufacture, or else the nitrite is recovered for the manufacture of dyes.

The normal calcium nitrate, CaNO₃)₂, is converted into a basic salt, containing 13 per cent, of nitrogen, which is not deliquescent and is used as a fertiliser. Calcium nitrite, Ca(NO₂)₂, containing 21.2 per cent, of nitrogen, may easily be made from the gases containing excess of nitric oxide.

Pauling furnace

The Pauling Furnace

The first technical installation of the Pauling process was erected at Innsbruck in 1909, following experiments which began in 1906. Steel water-cooled electrodes, bent away from each other in the shape of a V, are employed, and the arc is kept at high tension by a blast of air. The alternating arcs, struck between ignition knives of iron which pass through the electrodes, are carried upwards by the blast of air, giving a fan-shaped electric flame of a high temperature. The gases leaving at about 1000° C. contain about 0.8 per cent, of nitric oxide, which corresponds to 60 grams of HNO₃ per K.W.H. or 525 kilograms K.W.Y.

The modification of Rossi, who substituted electrodes of aluminium alloyed with barium and lead, greatly improved the yield, as particles of iron are found to decompose the nitric oxide catalytically.

Schonherr-Hessberger furnace

The Schonherr Furnace

The arc process devised by Schonherr and Hessberger in 1905 also depends upon the fact that an arc may be carried by a current of heated air. This is supplied not across the discharge, giving a series of rapidly extinguished arcs, but along it, giving a single high-tension arc which is in contact with the air for over a long distance. The furnace is in the form of a cylinder, 20 to 25 feet long, made of iron lined with firebrick, in the centre of which is one electrode consisting of an iron tube, a few inches in diameter, water-cooled at its top end. At the lower end another insulated electrode is enclosed in a water-cooled copper block. The arc, first struck at the bottom, is carried up spirally by the air, which is supplied through holes bored tangentially to the furnace walls. The air is regulated so that the upper end of the arc is made on the water-cooled end of the tube electrode.

The furnace, supplied by a 3-phase alternating current, may take from 700 to 1000 K.W. and about 3500 volts and 290 amperes. The air is preheated to about 500° C. by the issuing gases, and leaves the furnace at about 850° C. with a content of about 1.8 per cent, of nitric oxide. The yield is 68 grams of HNO₃ per K.W.H., or 500 to 600 kilograms per K.W.Y.

Absolute and Comparative Efficiency of Electrical Processes for the Combustion of Nitric Oxide

The proportions of the total electrical energy supplied which are used in the actual production of nitric oxide are small. If all the electrical energy were so used in the form of heat to combine nitrogen and oxygen, which have a heat of combination equal to - 21-600 Cals., it can easily be shown that 1 K.W.H. should give as a maximum 2500 grams HNO₃. The amounts given above are about 3 per cent, of this. But the electrical energy is also required to heat up a large excess of air (with 1.25 per cent, of NO) to, say, 900° C., and if this heat is included, the theoretical yield is only 102 grams HNO₃ per K.W.H., or the efficiency is over 60.