| Urea is a nitrogen-containing chemical product which is
produced on a scale of some 100,000,000 tonnes per year worldwide.
Applications
Fertilizer
More than 90% of world production is destined for use as
a fertilizer. Urea has the highest nitrogen content of all
solid nitrogeneous fertilizers in common use. (46.4%N.) It
therefore has the lowest transportation costs per unit of
nitrogen nutrient.
In the soil, urea is converted into the ammonium ion form
of nitrogen. For most flora, the ammonium form of nitrogen
is just as effective as the nitrate form. The ammonium form
is better retained in the soil by the clay materials than
the nitrate form and is therefore less subject to leaching.
Urea is highly soluble in water and is therefore also very
suitable for use in fertilizer solutions, e.g. in “foliar
feed’ fertilizers.
Solid urea is marketed as prills or granules. The advantage
of prills is that in general they can be produced more cheaply
than granules which, because of their narrower particle size
distribution have an advantage over prills if applied mechanically
to the soil. Properties such as impact strength, crushing
strength and free-flowing behaviour are particularly important
in product handling, storage and bulk transportation.
Urea contains a small percentage of biuret, which is normally
not a problem in soil fertilization. With foliar fertilization
however, this biuret content must not exceed 0.3%.
Urea can also be used as:
- A raw material for urea-formaldehyde resins production
in the adhesives and textile industries. A significant portion
of urea production is used in the preparation of urea-formaldehyde
resins. These synthetic resins are used in the manufacture
of adhesives, moulding powders, varnishes and foams. They
are also used for impregnating paper, textiles and leather.
- A raw material for melamine production More than 95%
of all melamine production is based on urea. Stamicarbon’s
parent company DSM is the largest melamine producer in the
world.
- A supplementary substitute protein source in feedstuffs
for cattle and other ruminants. Because of the activity
of micro-organisms in their cud, ruminants are able to metabolize
certain nitrogen containing compounds, including urea, as
protein substitutes. In the USA this capability is exploited
on a large scale. Western Europe, in contrast, uses little
urea in cattle feed.
- Feed for hydrolyzation into ammonia which in turn is used
to reduce emissions from powerplants and combustion engines.
- Other, miscellaneous products such as de-icing material
for airport runways. Although on a smaller scale than as
a fertilizer or as raw material for synthetic resins, urea
is also used as a raw material or auxiliary material in
the pharmaceutical industry, the fermenting and brewing
industries and in the petroleum industry.
Production
Urea is produced commercially from two raw materials, ammonia
and carbon dioxide. Large quantities of carbon dioxide are
produced during the manufacture of ammonia from coal or from
hydrocarbons such as natural gas and petroleum derived raw
materials. This allows direct synthesis of urea from these
raw materials.
The production of urea from ammonia and carbon dioxide takes
place in an equilibrium reaction, with incomplete conversion
of the reactants. The various urea processes are characterized
by the conditions under which urea formation takes place and
the way in which unconverted reactants are further processed.
Unconverted reactants can be used for the manufacture of other
products, for example ammonium nitrate or sulphate, or they
can be recycled for complete conversion to urea in a total-recycle
process.
Process reactions
The process reactions occurring in urea processes are illustrated
in the diagram of reaction sequences shown below. Two principal
reactions take place in the formation of urea from ammonia
and carbon dioxide.
The first reaction is exothermic and the second reaction is endothermic.
Both reactions combined are exothermic.
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UREA
