Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
Module: 9
Lecture: 37
UREA
INTRODUCTION
Urea (NH2CONH2) or carbamide is an organic compound has two —NH2
groups joined by a carbonyl (C=O) functional group. Urea serves an important role
in the metabolism of nitrogen containing compounds by animals and is the main
nitrogen containing substance in the urine of mammals.
Urea was first discovered in urine in 1727 by Herman Boerhaave, though this
discovery is often credited to Hilaire Rouelle.
Friedrich Wöhler synthesized urea from an inorganic precursor in 1828. It was
the first time that the molecule found in living organisms could be synthesized in the
laboratory without biological starting materials. Due to this discovery, Wöhler is
considered as the father of organic chemistry by many scientists.
Urea has the highest nitrogen content ava*ilable in a solid fertilizer (46%). It is
easy to produce as prills or granules and easily transported in bulk or bags with no
explosive hazard. It dissolves readily in water. It leaves no salt residue after use on
crops and can often be used for foliar feeding.
Urea is an acceptable fertilizer for rice and preferable to nitrates for flooded
rice because of the reduction of nitrates to N2O and/or nitrogen (in anaerobic
conditions) which is lost to the atmosphere. Also, rice can utilize the ammonium form
of nitrogen efficiently. Hydrolysis and nitrification (in aerobic conditions) are rapid in
tropical, sub-tropical and warm climates
Urea can be sprayed on leaves and can also be mixed with insecticides or
herbicides for soil application. A urea ammonium nitrate mixture with herbicide is
also used for weed control.
Disadvantages
 When applied to a bare soil surface, urea hydrolyzes rapidly result into loss of
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 solution form.
NPTEL
239
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
 It is phytotoxic due to rapid hydrolysis of urea in soils can cause injury to the
seedlings by ammonia,
 The fertilizer grade urea may contain toxic biuret which is formed during urea
manufacture by an excessive temperature rise. Above 2% concentration of
biuret in urea is harmful to plants.
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.
MANUFACTURE
Raw materials
Basis: 1000kg prilled urea
Item
NH3
CO2
Power
Steam
Cooling water
Once Through
1150kg
1470kg
210kWH
1800kg
120000kg
Partial recycle
880kg
910kg
165kWH
2000kg
70000kg
Total Recycle
600kg
770kg
145kWH
2400kg
110000kg
Sources of raw material
Ammonia can be synthesized by Haber – Bosch or Modern process as
described in Module: 2, Lecture: 6.
CO2 shall be obtained from any one source as described in Module: 2,
Lecture: 2
Reaction
CO2 + 2NH3
NH2COONH4
NH2COONH4 ΔH = - 37,021 Kcal
NH2CONH2 + H2O ΔH = + 6.3 kcals
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
NPTEL
240
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
To Vaccum
Pump
Compressor
CO2
Inner Cup
(reaction chamber)
Evaporator
Synthesis
tower
Distillation
Tower
Molten Mass
Liquid
Ammonia
Air
Steam
Evaporator
(NH3 + CO2 + H2O)
Prilling
Tower
Condensate
Air
Steam
Urea
Condensate
Condensate
Conveyor
Tank for the
molten mass
Figure: Manufacturing of Urea
Urea is always made in an ammonia plant because it produces CO2 as by
product, which can be used directly without further treatment.
Two reactions are involved in the manufacture of urea. First ammonium
carbonate is formed under pressure by highly exothermic reaction between carbon
dioxide and ammonia followed by the endothermic decomposition reaction. While
the former reaction under pressure, reaches to almost completion and the
decomposition reaction incomplete. Unconverted carbon dioxide and ammonia,
along with un decomposed carbamate, must be recovered and reused. The
synthesis is further complicated by the formation of a dimer called biuret,
NH2CONHCONH2.H2O which must be kept low because it adversely affects the
growth of some plants.
Liquid ammonia, gaseous carbon dioxide and recycle materials charged in
the heat exchanger-reactor at the pressure of 14MPs at 170 - 1900C to form
carbamate, with most of the heat of reaction carried away as useful process steam.
The carbamate decomposition reaction is both slow and endothermic. The mixture
of unreacted reactants and carbamate flows to the decomposer. The
stoichiometric ratio of CO2/NH3 conversion to urea is essentially about 55%, but by
using an excess of CO2 (or NH3) the equilibrium can be driven as high as 85%. The
reactor must be heated to force the reaction to proceed. CO2 is introduced at
process pressure followed by stripper. All the unreacted gases and undecomposed
carbamate to be removed from the product, the urea must be heated at lower
pressure (400kPa). The reagents are reacted and pumped back into the system.
Evaporation and prilling or granulating produces the final product. Overall, over 99%
of both CO2 and NH3 are converted to urea, making environmental problems to
minimum. Carbamate is highly corrosive to both ordinary and stainless steel, but with
oxygen present, 300 series stainless steel resist it very well, so some air is introduced
along with CO2 reagent to reduce system corrosion.
NPTEL
241
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
Developments in urea process technologies
Item
Unit
Process
B
570
740
Ammonia
CO2
Kg
Kg
A
570
740
c
570
740
D
570
740
Steam
Kg
900
800
660
790
Power
KWH
140
140
16
21
Water
m3
3.1
2.4
3.1
3
The raw material and utilities requirement for different processes for synthesis
and purification of urea are tabulated as earlier.
a) Montedison's IDR process
Montedison's process employing two specially designed stripping columns.
Ammonia and CO2 are used as the stripping agent in 1st and 2nd column
respectively. The reactor constructed in two sections having perforated trays and
also a down comer meant for circulation solution. High NH3 to CO2 ratio results in
increased conversion efficiency and lower carbamate recycle duty of the plant.
Excess NH3 is removed by CO2 stripping instead of distillation as practiced in
conventional total recycle processes, minimizing the energy requirement.
b) TEC-ACES process
This is typically CO2 stripping process employing higher ratio (4:1) of NH3 to
CO2, and higher synthesis pressure leading to high conversion efficiencies as
compare to total recycle process. Stripping is carried out in a two stage stripper
constructed of special steel. The upper part of the stripper is a tray column for the
removal of excess ammonia whereas the lower part is a falling film exchanger for
the stripping action.
c) Stamicarbon stripping process
Consumption of steam is decrease by employing a pool condenser of new
design featuring high resistance time and direct heat exchange between
condensing vapours from stripper and the stripped urea solution; and an evaporator
of improved design which allows better utilization of multiple effect principle in heat
transfer.
d) Ammonia casale's SRR process
Split reaction recycle (SRR) process of ammonia casale is specifically
developed for revamping plants based on stripping technology of either
snamprogetti or stamicarbon and includes installation of secondary high pressure
section consisting of feed pump, reactor, supplementary decomposer and
NPTEL
242
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
separator which extend the urea formation reaction. The operating conditions are
same as traditional ones. The new secondary section added to the synthesis loop
can be prefabricated on skid mounted units and can be erected at site without any
modification on the layout of the existing synthesis section.
Granulation
Now a day, granular urea has gained importance since it minimize air
pollution and granules has higher strength larger sizes and is more compatible with
other granular fertilizers.
Following commercial processes are available for granulation of urea:
 Pan granulation and falling curtain granulation process of Tennessee Valley
Authority (TVA)
 High temperature pan granulation (GTPG) process of Norsk Hydro.
 Fluidized bed granulation process of Hydro Agri Licensing & Engineering.
 Fluidized bed granulation process of TEC.
Major Engineering problems
Autoclave variables
The objective of autoclave reaction is to produce the optimum economic
yield. The conditions which affects rate of reactions are temperature, pressure,
NH3/CO2 ratio and feed rate. The urea production rate can be varied as follows
 Increase with increasing pressure
 Increase with temperature to maximum at 175-1800C, then falls of sharply. The
operating pressure should be above the dissociation pressure (dissociation
pressure is 180atm at 190°C) for the carbamate.
 Use no excess ammonia.
Reasons for not operating at maximum temperature and pressure without
excess ammonia
 Increased pressure increases capital and operating cost of compression and
reaction equipment.
 At higher temperature urea decomposed to biuret, which is detrimental to
germinating seeds and toxic to animals.
 The above process conditions enhance corrosion rates to machinery
Carbamate decomposition and recycle
It is optimized by short residence times in a stripping column operating at low
pressure and high temperature. Later should be below 1100C if hold up time
NPTEL
243
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
exceeds 1-2 seconds to avoid biuret formation. Use of millisecond contact time in a
flash evaporator allows 1400C operating temperatures in the high recycle design.
Main difference in competing processes is in the recycle design. Since
conversion is only 40-50% per pass, the unreacted off gases must be recirculated or
used economically elsewhere. Recompression of off gases is virtually impossible
because of corrosion and formation of solid carbamate in compressors. A solution is
formed and pumped into the autoclave.
Production of granular urea (Prilling)
Problem again is biuret formation. Vacuum drying of 80% urea to > 99% and
spraying to air cooled and solidify must be done just above the melting point of
urea and with a minimum residence time.
Heat dissipation in the autoclave
The exothermic heat of reaction can be removed by coils, wall cooling, or by
adding excess reactant to provide sensible heat pick up.
Corrosion
It can be minimized by use of the corrosion resistant metals and maintaining
the proper reaction conditions. High cost silver or tantalum liners are used in the
autoclaves with titanium, stainless (321SS) and aluminum alloys used in other parts of
the plant. Minimum temperature and pressure with excess NH3 are desirable to
reduce the severe corrosion rates.
PROPERTIES










Molecular formula
Molecular weight
Appearance
Odour
Bulk density
Angle of repose
Melting point
Density
Solubility
Moisture
: CH4N2O
: 60.06gm/mole
: White granules
: Odourless
: 673-721kg/m3
: 300
: 132-1350C
: 1.32gm/ml
: Solubility in water, ethanol, glycerol
: 1% by wt. (Max.)
It is highly soluble in water and practically non-toxic (LD50 is 15 gm/kg for rat).
Dissolved in water, it is neither acidic nor alkaline. As soon as urea dissolves in the soil,
it forms around it a zoning layer of high pH and ammonia concentration turning the
soil to be acidic and toxic at the same level. Urea is high moisture absorbent
therefore it should be stored in sealed and well enclosed bags.
NPTEL
244
Module: 9
Lecture: 37 Urea
Dr. N. K. Patel
USES






NPTEL
As a fertilizer
As a protein food supplements for ruminant
As an ingredient in the manufacture of resins, plastics, adhesive, coatings
Textiles anti-shrink agents and ion exchange resins
In melamine production
It is an intermediate in the manufacture of ammonium sulfamate, sulfamic
acid and pthalocyanines.
245