THE UREA MANUFACTURING PROCESS
Urea is produced from ammonia and carbon dioxide in two equilibrium reactions:
2NH3 + CO2 ! NH2COONH4
ammonium carbamate
NH2COONH4 ! NH2CONH2 + H2O
urea
The urea manufacturing process, shown schematically in Figure 2, is designed to maximise
these reactions while inhibiting biuret formation:
2NH2CONH2 ! NH2CONHCONH2 + NH3
biuret
This reaction is undesirable, not only because it lowers the yield of urea, but because biuret
burns the leaves of plants. This means that urea which contains high levels of biuret is
unsuitable for use as a fertiliser. The structure of these compounds is shown in Figure 3.
Step 1 – Synthesis
A mixture of compressed CO2 and ammonia at 240 barg is reacted to form ammonium
carbamate. This is an exothermic reaction, and heat is recovered by a boiler which produces
steam. The first reactor acheives 78% conversion of the carbon dioxide to urea and the liquid
is then purified. The second reactor recieves the gas from the first reactor and recycle
solution.
from the decomposition and concentration sections. Conversion of carbon dioxide to urea is
approximately 60% at a pressure of 50 barg. The solution is then purified in the same
process as was used for the liquid from the first reactor.
Step 2 – Purification
The major impurities in the mixture at this stage are water from the urea production reaction
and unconsumed reactants (ammonia, carbon dioxide and ammonium carbamate). The
unconsumed reactants are removed in three stages3
. Firstly, the pressure is reduced from 240
to 17 barg and the solution is heated, which causes the ammonium carbamate to decompose
to ammonia and carbon dioxide:
NH2COONH4 ! 2NH3 + CO2
At the same time, some of the ammonia and carbon dioxide flash off. The pressure is then
reduced to 2.0 barg and finally to -0.35 barg, with more ammonia and carbon dioxide being
lost at each stage. By the time the mixture is at -0.35 barg a solution of urea dissolved in
water and free of other impurities remains.
At each stage the unconsumed reactants are absorbed into a water solution which is recycled
to the secondary reactor. The excess ammonia is purified and used as feedstock to the
primary reactor.
Step 3 – Concentration
75% of the urea solution is heated under vacuum, which evaporates off some of the water,
increasing the urea concentration from 68% w/w to 80% w/w. At this stage some urea
crystals also form. The solution is then heated from 80 to 110o
C to redissolve these crystals
prior to evaporation. In the evaporation stage molten urea (99% w/w) is produced at 140o
C.
The remaining 25% of the 68% w/w urea solution is processed under vacuum at 135o
C in a
two series evaporator-separator arrangement.
Step 4 – Granulation
Urea is sold for fertiliser as 2 – 4 mm diameter granules. These granules are formed by
spraying molten urea onto seed granules which are supported on a bed of air. This occurs in
a granulator which receives the seed gransules at one end and discharges enlarged granules at
the other as molten urea is sprayed through nozzles. Dry, cool granules are classified using
screens. Oversized granules are crushed and combined with undersized ones for use as seed.
All dust and air from the granulator is removed by a fan into a dust scrubber, which removes
the urea with a water solution then discharges the air to the atmosphere. The final product is
cooled in air, weighed and conveyed to bulk storage ready for sale.om the decomposition and concentration sections. Conversion of carbon dioxide to urea is
approximately 60% at a pressure of 50 barg. The solution is then purified in the same
process as was used for the liquid from the first reactor.
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