Fritz Haber demonstrated the ammonia synthesis in Karlsruhe in the presence of Carl Bosch and Alwin Mittasch on July 2, 1909. Haber's apparatus produced 100 milliliters of liquid ammonia that day, operated at a pressure of 200 atmospheres and a temperature of 500œ-600œ Celsius, and involved a 0.75-meter-tall reactor.

Bosch and his coworkers at Badische Anilin- und Soda- Fabrik (BASF), having acquired the rights to develop the process, immediately tried to scale up to an industrial-sized apparatus. They needed to solve at least three problems: First, they needed a supply of pure hydrogen (nitrogen was readily available from liquid air); second, they had to find a suitable catalyst; third, they needed a technique for handling hydrogen-nitrogen mixtures at high temperatures and pressures. In the succeeding three years, these difficulties were overcome, in part, through inventions by Bosch. The hydrogen needed was initially obtained by electrolysis, but later it proved less expensive to use the water-gas reaction. Steam reacted with red-hot coke, giving a mixture ("water gas") of carbon monoxide and hydrogen. A catalyzed "shift reaction " allowed the carbon monoxide to react with more steam to form additional hydrogen and carbon dioxide. To purify the hydrogen, the carbon dioxide and residual carbon monoxide were removed by scrubbing with water and copper formate solution.

The catalysts used by Haber for ammonia synthesis were rare metals (osmium and uranium) too costly to use on an industrial scale. The BASF chemists, led by Mittasch, conducted thousands of experiments to discover a cheap, effective catalyst. Their eventual choice was an iron catalyst containing small amounts of iron oxide and other metal oxides.

The early attempts to scale up the ammonia synthesis resulted in catastrophic failure of the steel reactors after about eighty hours of operation. Metallographic examination showed cracking of the metal caused by exposure to high-pressure hydrogen. The carbon steel used in the reactor was vulnerable because of its carbon content. The carbon in the metal could slowly react with high-pressure hydrogen, forming hydrocarbon gases that forced the metal apart, creating cracks. Pure iron was unaffected.

Bosch solved the hydrogen embrittlement problem by an ingenious invention. By lining the inside of the reactor with soft iron, it was possible to circulate the high-pressure gases over the catalyst in such a manner as to prevent them from contacting the carbon steel containment vessel. Cool gas was circulated between the walls and the inner liner. Bosch also supervised the design of pressure-, flow-, and temperature-recording instruments that enabled close control of the reaction conditions. These instruments are the ancestors of those used in all modern high-pressure industrial processes.

The reactor size was steadily increased over the period 1909-1915. The largest reactor was 12 meters long, 1.08 meters in diameter, and weighed 75 tons. The ammonia plant at Oppau began production late in 1913, and soon was fixing 20 tons of nitrogen per day. Ammonia became a major source of profits for BASF. Converted to ammonium sulfate fertilizer, ammonia relieved Germany from dependence on imported nitrates. Oxidation of ammonia (Ostwald process) afforded a route to nitric acid needed for making explosives.

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