Sources And Production Of Ammonia
Ammonia is manufactured by the reaction of hydrogen with nitrogen in the presence of an iron catalyst, which is known as the Haber-Bosch process. The reaction is exothermic and is accompanied by a concentration in volume. (The ammonia occupies less volume than the gases from which it is made.) High pressure conditions (150-250 bar) are used, and temperatures range from 752–932°F (400–500°C). The mixed gases circulate through the catalyst, ammonia is formed and removed, and the unconverted reactants are recirculated. Large ammonia plants can produce over 1,000 tons per day. Each ton of ammonia requires 3,100 cu yd (2,400 cu m) of hydrogen and 1,050 cu yd (800 cu m) of nitrogen, as well as 60 gigajoules of energy. Much of the energy is consumed in the compressors needed to attain the high pressure used in the synthesis, and in heating the reactants. Further energy is needed to produce the hydrogen from hydrocarbon feedstocks, and to separate nitrogen from air. The synthesis reaction itself produces some heat, and great attention is given to heat efficiency, and use of waste heat. The gases that enter the catalytic converter must be highly purified and free of sulfur compounds, which adversely affect the catalyst. The catalyst is prepared in place by hydrogen treatment of magnetite, an iron oxide containing potassium hydroxide and other oxides in small amounts as promoters. A large ammonia plant might have as much as 100 tons of catalyst.
Since the hydrogen is usually derived from a natural gas called methane, the price of ammonia is very sensitive to the availability or price of fuels. United States production of ammonia reached 17 million tons in 1991, and demand was even larger than U.S. production, leading to about two million tons of imports. World ammonia production is about 100 million tons per year, which amounts to about 40 lbs (18 kg) for each person on earth.
Ammonia is formed from nitrogen in air by the action of nitrogen-fixing bacteria that exist in the soil on the roots of certain plants like alfalfa. Nitrogen fixation can also be accomplished by blue-green algae in the sea. These bacteria and algae possess an enzyme called nitrogenase that permits them to convert nitrogen to ammonia at 77°F (25°C) and 1 bar of pressure, much milder conditions than those of the Haber-Bosch process. Nitrogenase is known to be a complex protein containing metal atoms, such as iron and molybdenum, and sulfide ions, but its structure and mode of action are imperfectly understood, even after decades of research. Recent research indicates that the nitrogen molecule may bind to iron atoms in the enzyme as a reaction step.
Ammonia can be formed in the human body, and may build up abnormally during serious illnesses such as Reye's syndrome. Much nitrogen is normally excreted by humans (and other mammals) as urea, a water soluble solid, but fish can excrete ammonia directly.
Urea eventually reacts with water to form ammonia, which therefore is usually present to some extent in waste water. Low concentrations of ammonia in water can be detected and measured using a solution called Nessler's reagent, which develops a strong color in the presence of ammonia. A recent toxic substance inventory done by the United States government estimated that in 1989, 200,000 tons of ammonia were released into the environment. This figure does not include fertilizer applications of ammonia.
Although Earth's atmosphere is free of ammonia, liquid and solid ammonia exist on other planets, such as Jupiter, where it may have originally formed from metal nitrides reacting with water. Ammonia has also been detected in interstellar space by radioastronomy.
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