One of the most important industrial processes is the manufacture of ammonia, which is needed for fertilizers, for the manufacture of nitric acid and for use as a refrigerant and as a cleaning agent.
Ammonia is manufactured from the gases nitrogen and hydrogen by the Haber–Bosch process. In most modern plants, the nitrogen is obtained from air, of which it makes up 78 percent by volume, and the hydrogen from natural gas.
The impetus to make ammonia came after a prediction from a British scientist, Sir William Crookes, in 1898 that the world’s supplies of nitrogen compounds, which were in the form of Chile saltpeter, were being used up and unless an alternative source was found, the world would starve for lack of fertilizer. In 1909, a German chemist, Fritz Haber, managed successfully to synthesize ammonia under high pressure using a catalyst. Karl Bosch, a German chemical engineer, later developed the laboratory technique into a full-scale commercial process.
Theoretically, a low temperature combined with high pressure is required to produce ammonia, but in practice, temperatures of about 900°F (500°C) and pressures from 150 to 1,000 times atmospheric pressure are used with good results. Below 900°F (500°C) the yield of ammonia is high, but the rate of reaction is too slow. At higher temperatures, the reaction is faster, but the yield of ammonia is lower. Higher pressures improve the yield, around 20 percent at 250 atmospheres and around 50 percent at 800 atmospheres. In addition, by using a catalyst the rate of reaction can be speeded up.
The catalyst is usually iron oxide, which is often mixed with a small quantity of a promoter such as aluminum sesquioxide to increase its effectiveness. The iron oxide is reduced by the hydrogen to spongy pure iron when the process is started up. Over a period, the catalyst gradually loses its effectiveness as it becomes poisoned by traces of carbon dioxide, carbon monoxide, and sulfur compounds.
In the process itself, the catalyst is packed into catalyst beds inside the steel reaction vessel, which is designed to withstand very high pressure. The steel must also be resistant to attack from hydrogen, especially under the high temperature and pressure conditions. The nitrogen and hydrogen gases are purified, compressed, and passed through a warmup heater before entering the converter. The ammonia gas formed is liquefied by passing it through pipes cooled by cold water.
Prior to the invention of the Haber–Bosch process, the more energy-intensive cyanamide process was used to prepare ammonia, where calcium cyanamide is sprayed with water to remove traces of calcium carbide and is then treated with superheated steam, which causes it to decompose into ammonia and calcium carbonate.
