Urea (carbamide), readily produced from ammonia and carbon dioxide, is a very important chemical in the agricultural and the polymer industries.
Uses of urea
Urea is the world's most commonly used nitrogen fertilizer and indeed more urea is manufactured by mass than any other organic chemical. Containing 46% N, it is the most concentrated nitrogen fertilizer, and is readily available as free-flowing prills (granules). It is the cheapest form of nitrogen fertilizer to transport and it is also the least likely to 'cake'. It is therefore favoured in developing countries.
While over 90% of urea produced is used as a fertilizer, it has other uses, which include the manufacture of the melamine, used in melamine-methanal resins. Urea itself also forms important resins.
An increasingly important use of urea is in reducing air pollution from diesel engines in cars, buses and lorries. Diesel engines run at high temperatures and nitrogen and oxygen, from the air, are able to react together under these conditions to produce high concentrations of nitric oxide. One way to remove this pollutant is to allow it to react with ammonia to form nitrogen.
However it is not possible to use ammonia directly as it is too volatile and is poisonous. Instead a solution of urea in water is injected into the hot gases emerging from the engine in the exhaust. Urea is thermally decomposed to ammonia and carbon dioxide. This is the reverse of the process used to make ammonia:
Unlike ammonia, urea is safe and easy to handle.
The products, ammonia and carbon dioxide, together with the exhaust gases, are passed immediately over a catalyst in the exhaust system. Ammonia reduces the oxides of nitrogen (mainly nitric oxide), formed in the combustion processes, to nitrogen. The process is complex but the overall reaction can be represented thus:
The system is known as Urea SCR (urea-based selective catalytic reduction) and can reduce pollution by nitrogen oxides to almost zero.
Several catalysts have been used. One series is based on transition metal oxides (for example those of vanadium and tungsten) on a carrier, titanium dioxide. Another series is based on zeolites, in which some of the cations have been exchanged with a metal such copper.
Annual production of urea
Ammonia reacts with carbon dioxide to produce urea. Urea is always manufactured close to an ammonia plant (Figure 5).
Ammonia and carbon dioxide are heated together at 450 K and 200 atm pressure. First ammonium carbamate is formed, which rapidly decomposes to form urea:
Figure 5 An aerial view of a large plant in Alberta, Canada, in which ammonia is synthesized
Much of the urea is prilled (Figure 6) and sold in that form.
Date last amended: 18th March 2013