Urea Manufacturing 1

Urea Manufacture
Prof Dr. Abdullah Khan Durrani

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• Urea
(NH2)2 C=O
NH3 + CO2  Urea

• Urea also referred as carbamide, is a white,
crystalline, organic, water-soluble fertilizer. It
contains about 46 % nitrogen, the highest N
percentage any solid fertilizer can have.
• Apart from its major use as a fertilizer, urea is
also employed in the manufacture of paints,
glues, plastics, paper, textiles, feed and weed
control chemicals as wellas a source of non-
protein nitrogen.
Revision of
Ammonia
Manufacture
Revision of Ammonia
De-Cabonation
(CO2 Separation/Removal)
Methanator
Ammonia synthesis loop
Draw backs of Urea as fertilizer
• UREA
• Urea, although an excellent fertilizer, have
following drawbacks:
(i) When applied to a bare soil surface, urea
hydrolyzes rapidly and loses a significant quantity
of ammonia by volatilization. Such losses vary from
soil to soil and are greater for urea in a pellet form
rather than in a solution form. Burning residues on
the field is suggested as a practical means to control
the ammonia loss because the burning reduces the
concentration of the enzyme urease in plants.
• (ii) Rapid hydrolysis of urea in soils can cause
injury to the seedlings by ammonia, if large
quantities of the fertilizer are placed too close
to the seeds.
• (iii) The fertilizer grade urea may contain toxic
biuret which is formed during urea manufacture by an
excessive temperature rise.
• A large concentration of biuret in urea ( > 2 %)
causes injury Feed-grade urea is sometimes
referred to by the number 262 which is the
product of its nitrogen content (42%) multiplied
by 6.25, the latter being the factor used by
chemists to convert nitrogen to its protein
equivalent.
• Urea is sometimes phytotoxic when placed close
to seeds or seedlings. The phytotoxicity is caused
by high local concentrations of ammonia during
the hydrolysis stage or by accumulation of nitrite
during the nitrification
• step. Another possible cause is the presence of
biuret impurity in urea. to plants
Urea Reactors
• Chemistry of the Reaction
• Formation of Ammonium Carbamate
(Fast)
• Carbamate conversion to urea.
Urea Reaction
• NH3+CO2 NH4COONH2……….1
• NH4COONH2 NH2CONH2+H2O…2
• First reaction is instantaneous, and practically
complete, with evolving considerable amount of
heat.The major part of this heat is utilized in
raising the temperature of the reactants up to 375
–380F and the remaining smaller part of heat is
used to supply the heat of formation of Urea.
• High Pressure promotes production of
Urea because high pressure forces
gaseous Ammonia and CO2 to form
Carbamate. High Temperature, However
adversely affects the first reaction,
because it causes decomposition of
Carbamate back to Ammonia and CO2.
• A small excess of Ammonia in the reactor
promotes the first reaction, whereas the large
excess reduces the formation of CO2 into
Carbamate.
• The second reaction of Urea formation is
relatively slower, incomplete and requires
heat. It takes about 25-30 minutes to convert
75% of total Carbamate (Fresh feed+recycle)
into Urea.
• High Excess ammonia promotes the
conversion of Carbamate into Urea and vice
versa.
• During conventional Urea manufacturing
method, all stoichiometric CO2 required is
fed to reactor at high pressure.
• There may be different method to deliver

CO2 needed to produce Urea (i.e. 60-65% of
required CO2 directly to the reactor at high
pressure, The remaining 35-40 % of CO2 at
medium pressure ) thus absorbing excess
heat of Carbamate formation reaction.
CO2:NH3 1:4.5 Temp 270-280F P=3250psig
NH3
Reactor
CO2 Urea +Carbamate
NH3
• Urea solution from decomposition section is
then pumped to the prilling tower.
• Urea solution is concentrated by a multiple
effect heat exchanger (evaporators) up to
98.5%
• Concentrated Urea solution solidifies in form
of prills, when it is showered from top of
prilling tower in form of drops, and air is
blown counter current.
Factors affecting formation of Urea
1- Temperature (180-210 C)
2- Pressure (140-250 atm)
3- Mole ratio of (NH3/CO2 3to4:1)
4- Retention Time (25 -30 Minutes)
Processes
• Once Through
• Partial Recycle
• Total Recycle (New)
• Stripping Process
Once Through Process
Partial Recycle Process
Total Recycle Process
• Based on the recycle principle, the total-recycle processes
are classified into five types:
• (a) hot-gas mixture recycle,
• (b) separated-gas recycle,
• (c) slurry recycle,
• (d) carbamate solution recycle, and
• (e) stripping.
• All the first four types use carbamate decomposition
similar to the once-through or partial recycle processes.
The stripping process is, however, completely different
and will be treated separately.
UREA PLANT CO2
165 kg/cm2
CARBAMATE REACTER -1
Recovered T=170 -80 C , P=150 kg/cm2
32% urea
EJECTER STRIPPER
T=206 C , P=145 kg/cm2
46% urea
M P Decomposer
AMMONIA T=150 C, P=16 kg/cm2
63% urea
LP Decomposer
T=140 C, P=3.5 kg/cm2
70% urea
BAGGAGE/
1st Stage EVAPORATOR
/STORAGE
T=124 C , P=0.3 Ata
95.6% urea
2nd Stage EVAPORATOR

T=140 C , P=0.03 Ata
99.7% urea PRILLIUNG
Comparison of conditions

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Urea Manufacture

Prof Dr. Abdullah Khan Durrani

NH3 + CO2  Urea

• There may be different method to deliver

• Total Recycle (New)

2nd Stage EVAPORATOR

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