Friday, April 15, 2011

How Does the Earth Support over Six Billion People?


The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron or ruthenium catalyst, which is used to industrially produce ammonia.

Despite the fact that 78.1% of the air we breathe is nitrogen, the gas is relatively nonreactive because nitrogen molecules are held together by strong triple bonds. It was not until the early 20th century that the Haber process was developed to harness the atmospheric abundance of nitrogen to create ammonia, which can then be oxidized to make the nitrates and nitrites essential for the production of nitrate fertilizer and explosives.

Fritz Haber, 1918

The Haber process is important because previous to its discovery, ammonia had been difficult to produce on an industrial scale, and fertilizer generated from ammonia today is responsible for sustaining one-third of the Earth's population. It is estimated that half of the protein within human beings globally is made of nitrogen that was originally fixed by this process, the remainder was produced by nitrogen fixing bacteria.

History

Early in the twentieth century several chemists tried and failed to produce ammonia from atmospheric nitrogen. The enormous technical problems associated with the process were first solved by German chemist Fritz Haber (with the invaluable help of Robert Le Rossignol, who developed and built the necessary high-pressure devices). They first demonstrated their success in the summer of 1909, producing ammonia from air drop by drop, at the rate of about a cup every two hours. The process was purchased by the German chemical company BASF, which assigned Carl Bosch the difficult task of scaling up Haber's tabletop machine to industrial-level production. Haber and Bosch were later awarded Nobel prizes, in 1918 and 1931 respectively, for their work in overcoming the chemical and engineering problems posed by the use of large-scale, continuous-flow, high-pressure technology. Ammonia was first manufactured using the Haber process on an industrial scale in 1913 in BASF's Oppau plant in Germany. During World War I, production was shifted from fertilizer to explosives, particularly through the conversion of ammonia into a synthetic form of Chile saltpeter, which could then be changed into other substances for the production of gunpowder and high explosives (the Allies had access to large amounts of saltpeter from natural nitrate deposits in Chile that belonged almost totally to British industries; Germany had to produce its own). It has been suggested that without this process, Germany would not have fought in the war, or would have had to surrender years earlier.

The process

By far the major source of the hydrogen required for the Haber-Bosch process is methane from natural gas, obtained through a heterogeneous catalytic process, which requires far less external energy than the process used initially by Bosch at BASF, the electrolysis of water. Far less commonly, in some countries, coal is used as source of hydrogen through a process called coal gasification. However, the source of the hydrogen makes no difference to the Haber-Bosch process, which is only concerned with synthesizing ammonia from nitrogen and hydrogen.

Catalysts

The catalyst has no effect on the position of chemical equilibrium; rather, it provides an alternative pathway with lower activation energy and hence increases the reaction rate, while remaining chemically unchanged at the end of the reaction. The first Haber–Bosch reaction chambers used osmium and ruthenium as as catalysts. However, under Bosch's direction in 1909, the BASF researcher Alwin Mittach discovered a much less expensive iron-based catalyst that is still used today. Part of the industrial production now takes place with a ruthenium rather than an iron catalyst (the KAAP process), because this more active catalyst allows reduced operating pressures.

In industrial practice, the iron catalyst is prepared by exposing a mass of magnetite, an iron oxide, to the hot hydrogen feedstock. This reduces some of the magnetite to metallic iron, removing oxygen in the process. However, the catalyst maintains most of its bulk volume during the reduction, and so the result is a highly porous material whose large surface area aids its effectiveness as a catalyst. Other minor components of the catalyst include calcium and aluminium oxides, which support the porous iron catalyst and help it maintain its surface area over time, and potassium, which increases the electron density of the catalyst and so improves its activity.

Economic and environmental aspects

The Haber process now produces 100 million tons of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. 3–5% of world natural gas production is consumed in the Haber process (~1–2% of the world's annual energy supply). That fertilizer is responsible for sustaining one-third of the Earth's population, as well as various deleterious environmental consequences. Hydrogen production using electrolysis of water powered by renewable energy is not yet competitive cost-wise with hydrogen from fossil fuels, such as natural gas, and so has been responsible for only 4% of current hydrogen production (almost all as a byproduct of the chloralkali process). Notably, the rise of the Haber industrial process led to the "Nitrate Crisis" in Chile when the natural nitrate mines were no longer profitable and were closed, leaving a large unemployed Chilean population behind.

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