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February 2001
Vol. 10, No. 02, pp. 117–122.
 
 
 
Chemistry Chronicles
Capturing Nitrogen Out of the Air

Fritz Haber’s high-pressure process for combining nitrogen with hydrogen was a major milestone.

Illustration: Edgar Fahs Smith collection, University of Pennsylvania LibraryAbout a century ago, the eminent British chemist Sir William Crookes gloomily warned that the world would soon face starvation. Nitrogen, needed for use as fertilizer, would be in increasingly short supply, sharply curtailing plant growth and food production. “It is the chemist who must come to the rescue,” Crookes asserted. “It is through the laboratory that starvation may ultimately be turned to plenty.”

Within only a decade, however, the brilliant German chemist Fritz Haber devised an industrial method by which the nitrogen gas abundant in the atmosphere could be combined with hydrogen, under high pressure and at high temperature, to form ammonia. With this inexhaustible supply for the essential plant nutrient available, the world need never again fear a scarcity.

Haber’s method, moreover, had a more far-reaching significance for industrial chemistry. By pioneering high-pressure synthesis, this development laid the groundwork for modern methods of producing methanol and polyethylene, hydrogenating coal, and cracking petroleum. It contributed to the rapid growth of chemical engineering during the rest of the 20th century. It was a milestone in the progress of chemical technology.

A Vital Ingredient
Nitrogen is not uncommon. It is a vital ingredient of all plant and animal life. It is found in scattered mineral deposits. It constitutes 78% of the air we breathe. But its availability as a plant food is problematic. Certain bacteria in the soil, including some that live on peas and similar plants, can fix atmospheric nitrogen. Spreading manure on a field can replenish the soil. Ammonium sulfate can be recovered from gas-works and coke-oven gases obtained when coal or coke is processed. Yields from these sources, however, are relatively small. And atmospheric nitrogen is relatively inert.

During the 19th century, guano (bird droppings) from islands off the west coast of South America were a major source of fertilizer, but by 1900, these supplies were largely depleted. At that time, the principle source of nitrogen was deposits of saltpeter (sodium nitrate) found in desert areas of Chile. They furnished two-thirds of the world’s nitrogen requirements. Unfortunately, they were far across the ocean from their major consumer, industrialized Europe. And it was their potential exhaustion that prompted Crookes’s doleful prediction of worldwide food shortage. Nitrogen also had other key uses. Nitric acid was vital in making explosives and munitions. It was used in manufacturing dyestuffs. And ammonia had growing applications as a refrigerant.

Far from the First
The method devised by Haber was not the first attempt to fix nitrogen from the air. Early experimenters had observed that when an electric arc is passed through air, a tiny amount of nitric oxide is formed. Crookes and other scientists investigated this process in the 1890s. By then, high-voltage electricity and supplies of nitrogen from air liquefaction had become available.

In 1902, Atmospheric Products Co. built a plant at Niagara Falls, NY, that produced dilute nitric acid by using an electric arc, but two years later, the company failed financially. Two Norwegian scientists, Kristian Birkeland and Samuel Eyde, also designed arc furnaces to make nitric acid. Yields were slight, however, and power consumption very high, so the process was practical only at a site where electricity was available in huge amounts at cheap rates.

The only successful plants were operated in Norway by Norsk Hydro. The company’s furnaces at Notodden were making 7000 tons of fixed nitrogen a year by 1908, and a later one at Rjuken had an annual capacity of 28,000 tons. By the late 1920s, though, the high-cost arc process was largely obsolete.

In the late 1890s, two German scientists, Adolf Frank and Nikodem Caro, began studying ways for making cyanides needed in gold recovery. They found that when nitrogen was passed over powdered calcium carbide in a retort heated to 1000 °C, calcium cyanamide is formed (CaC2 + N2 = CaCN2 + C). Calcium carbide had become commercially available in the 1890s by reacting lime (calcium carbonate) with coke in an electric-arc furnace (see “Quest for Aluminum Revealed Route to Acetylene” TCAW, Sept 1999, p 81). Calcium cyanamide could be spread on fields as a source of plant nutrient. Or when treated with steam, it evolved ammonia. The first full-scale plant for making cyanamide was completed at Westeregeln in central Germany in 1905; a second German plant started up in 1910. By 1912, 10 plants were in place worldwide; the first plant in North America was underway in 1909 by the newly formed American Cyanamid Co. at Niagara Falls, Ontario, with a capacity of 4500 tons per year.

Calcium cyanamide dominated the nitrogen fertilizer business well into the 1920s. The process was relatively simple, and capital costs were low. But it did consume large amounts of electrical energy—to make carbide—although not as much as the arc process. Meanwhile, as Frank–Caro cyanamide production was being developed, Fritz Haber turned his chemical prowess to the synthesis of ammonia directly from nitrogen and hydrogen.

Haber was born in Breslau, Germany, in 1868. He received his Ph.D. with a dissertation in organic chemistry from the Charlottenburg Technische Hochschule in 1891. He held various posts until he joined the staff of the Karlsruhe Technische Hochschule in 1894. There, his interests turned to physical chemistry and electrochemistry, fields in which he was largely self-taught.

One subject that intrigued him, because of its practical nature, was the thermodynamics of the chemical equilibrium of ammonia with nitrogen and hydrogen (N2 + 3H2 left - right arrow 2NH3). Other scientists also were working on this problem at the time. In 1900, the French chemist Henri Le Chatalier showed that ammonia could be formed from nitrogen and hydrogen in the presence of a catalyst, but when an explosion destroyed his apparatus, he lost interest in the experiment.

The German physical chemist Walther Nernst likewise investigated the equilibrium simultaneously with Haber around 1905. The experimental data of the two did not agree, resulting in contentious exchanges between them. Nernst managed to produce small amounts of ammonia, but only at pressures and temperatures that he considered impractical for industrial use. Haber persisted, and his more favorable data proved correct.

Haber repeated his experiments at increased pressures, with the help of his English student Robert Le Rossignol. They concluded that adequate amounts of ammonia would be produced at temperatures of 500–600 °C, and pressures of about 200 atmospheres (the highest they could reach with their laboratory equipment), with constant circulation of the gases and using a suitable catalyst. Further experiments demonstrated that both osmium and uranium were efficient catalysts. Haber patented his process.

BASF Buys Patents
In 1909, Karl Bosch, an engineer, and Alwin Mittasch, an expert on catalysis, at the big German chemical firm Badische Anilin- und Soda Fabrik (BASF), watched a demonstration of Haber’s experiments. After several miscues, the apparatus produced 100 cc of ammonia. BASF, which had been experimenting with the arc ammonia process, acquired Haber’s patents and agreed to pay him 1 pfennig for every kilogram of ammonia turned out. The arrangement netted Haber millions of dollars.

Now the process had to be translated into full-scale production. The task was daunting. No chemical had ever been manufactured at such high pressures. Bosch—who later became head of BASF—was assigned the job. He and his team skillfully designed a large lined steel reactor vessel to withstand the high pressures without being imbrittled by hydrogen. They also came up with low-cost methods for producing pure nitrogen—by air liquefaction—and hydrogen—by liquefying coke-oven or water gas. Meanwhile, Mittasch had developed an improved activated iron catalyst.

BASF started up its first large-scale plant based on the Haber –Bosch process at Oppau, near Ludwigshafen in 1913. Its annual capacity of 8700 tons was increased to 60,000 tons by the end of 1915. A second, much larger plant was completed at Leona, west of Leipzig, in 1918. Together, the two plants were supplying about half of Germany’s need for nitrogen compounds. Without them, the country, cut off by blockade from Chilean nitrate, probably would not have had enough munitions to maintain its war effort; Germany’s cyanamide output largely was funneled to fertilizers.

Several cyanamide plants were built outside Germany during World War I to supply nitrogen for both munitions and fertilizers. The Haber–Bosch process clearly had the economic advantage, however, and its use spread rapidly during the 1920s. In 1929, when world consumption of nitrogen was 2.1 million tons—about triple what it had been in 1913—the Haber–Bosch process (or modifications of it) accounted for 43% of the total; 24% came from Chilean saltpeter, 20% from coke-oven gas, 12% from cyanamide, and only 1% from the arc process.

At the start of World War I, Haber volunteered his services to the German government. In 1915, he was in charge of the army’s chemical warfare programs, which centered on chlorine and mustard gas. His wife Clara, who had trained as a chemist, considered the use of poison gas a barbaric perversion of science. One day in 1915, when Haber left home for the eastern front to supervise the installation of poison gas cylinders, Clara killed herself. Haber won the Nobel Prize in Chemistry in 1918 for “the synthesis of ammonia from its elements”. Many scientists protested the award because of Haber’s role in promoting the use of poison gas in war.

Gold from the Sea
After the war, Haber resumed his chemical research. One major project that he undertook was extracting gold from the oceans. He saw this as a means of paying off the heavy war reparations imposed on Germany. The effort failed, though, when seawater proved to contain less gold than he had expected.

When the Nazi government came to power, Haber, as director of the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry, was ordered, in 1933, to dismiss all the Jewish scientists working there. Instead, he resigned. (Haber was of Jewish descent, although he was not a practicing Jew.) He left for England, where he worked for a brief time at Cambridge University. Then he accepted a position at a new research institute in Palestine. In 1934, on his way there, he died in Switzerland.


David M. Kiefer, assistant managing editor of Chemical & Engineering News until his retirement in 1991, is a consulting editor for Today’s Chemist at Work. Send your comments and questions for the author can be addressed to the Editorial Office by e-mail at tcaw@acs.org, by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.

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