Pharos University
Faculty of Engineering
Petrochemical Department
‫جامعه فاروس‬
‫كلية اهلندسة‬
‫قسم البرتوكيماوايت‬
LECTURE (9)
Manufacture of Urea
1-Introduction
Urea (carbamide) is a high quality nitrogenous
fertilizer with 46% nitrogen content.
It is called carbamide because it has two groups of
NH2.
2-Uses of Urea
Making urea formaldehyde resins from which in turn
high quality plastics.
 Synthetic glues.
 Compositions for impregnating cloth to improve
cloth.
Pharmaceutics industry .
Manufacturing synthetic fibers
3-Properties of Urea
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Molecular weight: 60.06
Nitogen content: 46.6%
Color: White
Specific gravity: 1.335
Melting point: 132.7 C
Solubility in water (parts/100 part of water)
0 C: 66.7
40C: 167.0
80 C: 400.0
100 C: 733.0
4-Advantages of Fertilizer Urea:
 Urea can be applied to soil as a solid or solution or to
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certain crops as a foliar spray.
Urea usage involves little or no fire or explosion hazard.
Urea’s high analysis, 46% N, helps reduce handling,
storage and transportation costs over other dry N forms.
Urea manufacture releases few pollutants to the
environment.
Urea is highly water soluble (at normal atmospheric
temperatures, approximately 1 lb of urea can be dissolved
in 1 lb of water).
5-Steps of manufacturing Urea:
A-Chemical reaction or synthesis step.
B-Distillation of the synthesis products to remove un reacted
materials.
C-Processing as evaporation and prilling or granulation for
the urea solution obtained in the distillation operation into
the end product
A- Chemical reaction step:
 Urea is produced from ammonia and carbon dioxide in
two equilibrium reactions:
2NH3 +CO2 ↔ NH2COONH4 (Fast Reaction)
ammonium carbamate
NH2COONH4 ↔ NH2CONH2 +H2O (Slow reaction)
Urea
The first reaction is fast and exothermic and the second
one is slow and endothermic.
Synthesis step is considered:
Heterogeneous (more than one phase)
Kinetic limiting one with the rate determined
by that of dehydration of ammonium
carbamate.
Factors affecting this process:
1- Pressure:
 Since the vapor pressure of carbamate is very high and the
overall result of synthesis reaction is decrease in the gas
volume, the equilibrium urea yield grows with the
increase in pressure .
 Pressure must not increase above 200 atm, as after 200
atm, cost is increased and the production rate remains
constant or increase a little bit.
2-Temerature:
At 140º C: yield of urea is low and takes long time.
At 150ºC: Yield is better than 140C but also long time.
At 180ºC: Yield is improved and takes short time
At 200ºC: Is the best , because it is maximum yield at very
short time.
If T> 200ºC corrosion of equipments could occur.
3- Effect of Excess Ammonia in mole ratio of (NH3:CO2):
2NH3 +CO2 ↔ NH2COONH4 (Fast Reaction)
2
1
1
Excess NH3 above the stoichiometric mole ratio of 2 favors the
rate of the reaction because of:
 The reaction is shifted to the right.
 Excess Ammonia impedes formation of side products of
carbamate hydrolysis.
 Excess ammonia dissolve in water (from dehydration) so the
concentration of water decrease and rate of corrosion decrease.
Synthesis reactor
The suitable condition is:
P=180-200 atm
T= 180-200 ºC
No Catalyst
 Synthesis takes place in a tower of alloy steel, which
consists of two cylinders, inner and outer.
 The inner cylinder serves to protect the walls of the outer
one from high pressure and corrosion.
Reaction occurs inside inner cylinder.
 Ammonia is fed by a pump in the annular space
between the tower and the inner cylinder.
 Carbon dioxide is fed from the bottom of the inner
cylinder.
 The molten (urea, carbamate, and gases) is removed
from the upper part of the inner cylinder and
transferred to the distillation column.
B-Distillation of the synthesis products
 In to the distillation column, the excess ammonia and
the un reacted products of decomposition of
ammonium carbamate are stripped from the urea
solution.
C-Processing as evaporation and prilling
or granulation:
Urea solution is concentrated in an evaporator and then to a
tank for molten urea fed with steam to avoid caking the
urea is produced in either prilling or granulation towers.
Prilling
The concentrated (99.7%) urea melt is fed to the prilling
device (e.g. rotating bucket/shower type spray head)
located at the top of the prilling tower.
Liquid droplets are formed which solidify and cool on free
fall through the tower against a forced or natural up-draft
of ambient air.
The product is removed from the tower base to a conveyor
belt using a rotating rake, a fluidized bed or a conical
hopper.
Cooling to ambient temperature and screening may be
used before the product is finally transferred to storage.
 Normally mean prill diameters range from 1.6-2.0mm
Mechanical strength of prilled particles is weaker than
those granulated ones.
Modern plants produce granulated particles rather
than prilled particles because they are larger and
stronger.
Granulator (Fluid bed Urea
Granulation)
 Consist of 1 meter thick bed of grannules fluidized by air
and supported on a perforated plate.
Nozzles spraying urea solution upwards.
Fluidized bed is divided in several chambers.
Suspended solids move from one chamber to other
through the slots between the perforated plates and the
partionning walls.
Recycled fine and crushed new nuclei is first chamber
above bed level.
Atomization nozzels use low pressure to spray urea
solution.
Flow sheet for granulation process:
Aqueous urea solution typically from vaccum evaporation.
Fluidization air with start up heater.
Heated low pressure atomization air.
Exhaust air extracted by fan after scrubbing
Solids loop.
Granules cooled in cooler.
Elevator to a screen where the particles are divided into
one size and over size.
The one size is stored and the over size is recycled to
granulator as seeds.
Biuret Formation:
 During the urea formation, biuret is formed as a by-product,
according to the equation:
2NH 2CO NH2 +∆ H↔ NH2CONHCONH2+NH3
 This is a slow, endothermic equilibrium reaction.
 The equation shows that biuret formation will take place when
there is a high urea concentration, a low ammonia
concentration and a high temperature.
 As biuret is toxic to plants, the biuret content of fertilizer-grade
urea will have to be kept as low as possible. This means that at
least combinations of the above-mentioned factors will have to
be avoided, certainly in places where there is sufficient time for
a biuret concentration build-up.