The Role of Material in Energy Saving Fertilizer Industries

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The Role of Material in Fertilizers Industries with Energy Saving
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DOI: 10.13140/RG.2.2.21615.36002
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The Role of Material in Fertilizers Industries
with
Energy Saving
By
Prem Baboo
Former Sr. Manager (Production)
National Fertilizers Ltd., Vijaipur, Guna (M.P.), India
Abstract
In Fertilizers industries the material plays a
very important role. Right material can save
energy as well as plant life and wrong
selection of material may lead to
catastrophic failures and outage of plants &
even loss of Human lives, Right selection of
material leads to long life of plant. In urea
plant very corrosive chemicals are used for
urea production. The raw material of urea
plant is Ammonia and Carbon Dioxide gas
which makes very corrosive chemical
ammonium Carbamate. For reduced
corrosion passivation air is used in reactor as
well as stripper according to material of
selection different quaintly of air is used.
Controlled and less quantity of passivation
air is required for Titanium and Zirconium
material and hence saving of energy because
the huge amount of ammonia is waste with
venting of inerts, i.e. saving of energy and
environments also. Passivation oxygen
given in CO2 is directly proportional to
ammonia losses in MP section. This paper
intended how to increase life of urea reactor
liner and energy saving with low passivation
air and high N/C ratio, Ammonia is the
noncorrosive and ammonia to CO2 Ratio is
an important parameter for process
optimization occurring less losses and less
explosion probability in urea reactor because
it affects the amount of produced urea and
corrosion to the material in the reactor.
Generally we are using urea reactor liner
316L (urea grade), now adopted 2RE69 for
large capacity plant (world largest) and
Duplex stainless steel.
Keywords
Corrosion, Duplex stainless steel, energy,
2RE-69, Zirconium, Liner
Introduction
For Urea production number of Process are
used there are two types of Process
Conventional and nonconventional process
or stripping process. Stripping process first
introduced by M/S. Saipem and called
ammonia stripping process and then by M/S
Stamicarbon Process called CO2 stripping
Process. Urea is formed according to the
following reactions: -
2NH3 + CO2 = NH2 COO
NH4 + 37.64 K Cal/Mole ....... (1)
Ammonium Carbamate.
NH2 COO NH4 = NH2 CO NH2
+ H2O - 6.32 K.Cal/Mole. (2)
Urea

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Reaction No. (1) Given
instantaneous and completes very fast. It is
exothermic in nature and 37.64 K.Cal of
heat is produced in the formation
Ammonium Carbamate. Reaction No.2,
however, is very slow and controls the Urea
formation. It is endothermic and 6.32
K.Cal/Mole is consumed in the dehydration
of Ammonium Carbamate. Actual available
heat in an ideal Urea Synthesis reaction will,
however, is only approx. 5.74 Kcal/mole
because of the heat lost in evaporation of
liquid NH3, evaporation of water
melting of Urea. If the material of liner
changed with 2RE 69 or Duplex material
instead of SS316(urea grade), then
passivation air can be reduced, resulting the
energy saving because the inerts vented
from M.P section and loss of ammonia and
problem of pollution as shown in the table
above is
instantaneous and completes very fast. It is
exothermic in nature and 37.64 K.Cal of
heat is produced in the formation
Ammonium Carbamate. Reaction No.2,
however, is very slow and controls the Urea
ic and 6.32
K.Cal/Mole is consumed in the dehydration
of Ammonium Carbamate. Actual available
heat in an ideal Urea Synthesis reaction will,
however, is only approx. 5.74 Kcal/mole
because of the heat lost in evaporation of
, evaporation of water formed and
If the material of liner
changed with 2RE 69 or Duplex material
instead of SS316(urea grade), then
passivation air can be reduced, resulting the
energy saving because the inerts vented
from M.P section and loss of ammonia and
on as shown in the table-2.
Material Used for Urea Process
In Urea reactor following material are used
for liner
1. Titanium
2. Zirconium
3. 2RE-69
4. SS-316L(Urea grade)
5. Duplex stainless steel
For stripper following material are
used
1. Titanium
2. 2RE-69
3. Duplex stainless
4. Bimetallic stripper( the two metal
are used inside Zirconium and
outside 2RE-69.
In MP section SS
LP/Vacuum and section SS304
are used.
30th
Oct 2021
te for scientists and researchers.
Material Used for Urea Process
In Urea reactor following material are used
316L(Urea grade)
Duplex stainless steel
For stripper following material are
steel
Bimetallic stripper( the two metal
are used inside Zirconium and
In MP section SS-316 L and for
LP/Vacuum and section SS304

Oct 2021
Fig-1(Urea Plant Process flow diagram with
material)
In Conventional process Titanium liner is
used in reactor, very small quantity of air is
required for Titanium material , CPI allied
process Zirconium liner is used .In
conventional process high pressure and high
temperature are required but in stripping
process less pressure and temperature.
Zirconium has a very tenacious, naturally
occurring, passive oxide layer that is
virtually impervious to conditions inside a
urea process plant. As compared to stainless
steel, zirconium does not require any
additional oxygen to be added to the process
stream to remain resistant to corrosion. In
ammonia stripping process 158 bar pressure
and 190 0C in Urea reactor while CO2
stripping 140, 150 & 175 bar pressure are
used in Stamicarbon, ACES-21 and ACES
Process respectively. Following material
detail.
Sr. No. MOC Advantages Disadvantages
1 Titanium Good passivation properties with
less air
Susceptible to Erosion Difficult
to Weld Costly.
2 Zirconium Zirconium is extremely well suited
to the urea processing environment.
No passivation required
Costly
4 Stainless steel Immune to Erosion Good Weld
ability
Large amount of passivation
required hence more losses
5 SS-316-L(urea
grade)
Excellent weld ability Large amount of passivation
6 2-RE-
69(Sandvik)
Excellent weld ability Better
Corrosion resistance than
316L(urea grade)
susceptible to SCC by chloride
7 Duplex Stainless
Steel (SAFUREX)
Good Weld ability Excellent for
stress corrosion cracking Corrosion
Resistance Passivation air is not
required
Costly
Table-1 (Material detail)
Additionally, Zirconium has a much higher
temperature capability in the urea process
solution compared to other materials.
Zirconium is extremely well suited to the
urea processing environment; it has a very
high tolerance for variable plant conditions
and compositions. Very little formal
corrosion data has been generated for
Zirconium in this application beyond the
numerous successful applications in
operating plants. The material of liner in
urea reactor essentially affects the reliability,
operability and maintainability of urea plant.
Zirconium is highly resistant to corrosion in
typical urea processing conditions. Sandvik
2 RE 69 is a fully austenitic stainless steel
with extra low carbon and impurity contents.
Excellent resistance to corrosion in
ammonium Carbamate excellent resistance
to inter granular corrosion, high resistance to
pitting and crevice corrosion and good weld
ability. Zirconium has a long successful
track record in solving very difficult
corrosion problems in the urea industry. In
an era where urea plants are expanding and
“mega” plants with capacities approaching
5000 TPD in Saipem plants are designed and
built, plant downtime and corrosion issues in

Oct 2021
general are greatly magnified in their
significance regarding process reliability and
operation profitability. The excellent
corrosion resistant properties of the Safurex
material eliminate the need of using
passivation air in the urea processes.
Because of the absence of oxygen in the
synthesis section, hydrogen or any other
combustibles present in the feed no longer
poses any risk of explosion for the urea
plant. The ammonia emissions are also kept
to an absolute minimum. Increasing, process
designers and operators are looking to
Zirconium to provide the materials
performance necessary for these mega-
plants. By using Zirconium, existing urea
processes can perform at a higher level.
Increasingly capacity, energy savings,
reduction of corrosion products in the
process stream, and elimination of the
requirement for additional passivation air,
and other environmental benefits in addition
to longer equipment life may be possible
with the proper application of Zirconium.
The Zirconium also used in Omega Bond
Tubing in urea applications developed by
Saipem Technology. However, certain
impurities are known to be deleterious to
Zirconium even in relatively low
concentrations. Fluoride, in particular can
cause rapid general attack on zirconium,
especially in an acidic environment.
Additionally, some metal ions (i.e., Cu or
Fe3+) may promote the initiation of pitting
in certain conditions. In MTC process the
reactor liner is used Titanium. Titanium is
used frequently in urea processing
applications due to its passivity in the urea
processing environment. However the oxide
layer of titanium is not nearly as hard and
tenacious as that of Zirconium, and it
therefore suffers from localized erosion /
corrosion on the inside of heat exchanger
and stripper tubes, specifically where the
fluid velocity is high. Titanium is used in the
relatively pure state .It has excellent
corrosion resistance but is one of the more
costly and difficult alloy to weld. Titanium
corrosion resistance is due to the impervious
oxide film i.e.100% TiO2 film on surface.
Titanium is resistant to stress corrosion
cracking and erosion corrosion, but it is
susceptible to crevice corrosion in stagnant
chloride solutions. Titanium is not
maintenance friendly the difficult in welding
Titanium is due to high affinity for
Hydrogen, Nitrogen and oxygen in the
molten state. Therefore, it must be welded
by such inert-gas welding method as the TIG
or MIG process. After a weld has been
made, the inert-gas protection must be
maintained until the welding joint cool
below 6500
C, otherwise the Titanium will
react with the oxygen, Nitrogen and
moisture in the air, resulting in weld
embrittlement. The conventional process is
generally not economical to maximize the
percentage conversion in the reactor since
this would require an excessive retention
time. The aim therefore is to attain
maximum quantity of urea production per
unit of time with due regard to the cost of
recycling un reacted CO2 and NH3 as well as
the cost of increased reactor size and
corrosion difficulties which increase with
temperature and reduction of N/C ratio.
Typical operating conditions are:
temperature, 180°-210°C; pressure, 140-250
kg/cm2
; NH3 CO2 mole ratio, 3:1 4:1; and
retention time, 20-30 minutes. The MTC

Oct 2021
Process having pressure 240-250 kg/cm2
pressure, Liquid NH3 and gaseous CO2 are
pumped to the urea reactor at 200 atm. The
temperature of the reactor is maintained at
about 185°C by proper balance of excess
NH3 and Carbamate solution recycles feed.
About 100%-110% excess ~NH3 is used;
about 70% of the NH3 and 87% of the C02
are converted to urea. The remaining 30% of
the NH3 will be recycled back to reactor.
Shortly after Stamicarbon introduction of the
C02 stripping process, Saipem launched its
ammonia stripping technology. In this
process stripping gas is ammonia and carbon
dioxide is introduced directly into the
synthesis reactor. The route is also
distinguished by the use of two stage
Carbamate decomposition, the first
operating pressure~ at 17 kg/cm2 g and the
second at 3.5 kg/cm2 g. The synthesis
conditions are maintained at 185-190°C and
155-166 kg/cm2 . Ammonia corning from
M.P section is raised to 230 kg/cm2 g and is
used as driving fluid in Carbamate ejector
where the recycled Carbamate is compressed
up to synthesis pressure. The reaction
products leaving the reactor flow to the
steam heated falling film stripper which
operates at 150kg/cm2 g.
Role of oxygen and stainless steel
The practical was done in November 2018 in
National Fertilizers Ltd, Vijaipur Oxygen Vs
ammonia losses in Urea Plant line- and
tabulated in table number-2 and it is found
the supplied passivation air is the function of
ammonia losses. In every urea plant where
no special precautions have been taken to
avoid the presence of hydrogen & Oxygen
explosive gas mixtures will show up if the
non condensable gases are purified
thoroughly to prevent ammonia losses or to
minimize emission of ammonia into the
atmosphere. The optimum passivation is
required if more than optimum the chances
of explosive mixture and also ammonia
losses, i.e. danger for environment also.
Flammable gas mixtures and the consequences
there of on urea plant safety have been important
issues in the entire history of the urea process
industry. High chromium stainless steel owe
their high resistance to corrosion under
oxidizing condition due to the formation of a
surface-oxide film which is very adherent
and highly impervious; thus the metal is
protected from attack or we say it is passive.
However, if the oxidizing conditions are
lost, the metal is rapidly attacked. The
protective film once formed is not damaged
in normal course. The presence of sulphur
compounds damages this protective film. It
is essential that the protective film is not
damaged during operation and as such
continuous feeding of air has been
incorporated in our process. It was found
from experiments that SS 316L stainless
steel requires 5 ppm of 02 for passivation
and 2RE-69 stainless steel requires 3 ppm.
Although this value is low and in actual
practice as high as 6000 ppm of 02 is being
maintained. Material of higher chromium
content requires less oxygen to remain
passive than low chromium steels. This fact
also points to the suitability of using
stainless steel with a chromium content of
24.5% in HP stripper. Chromium slows
down and nickel accelerates the corrosion of
materials in the active state .This is best
illustrated by corrosion rates of a number of
materials in relation to their Ni contents. At
higher temperature and pressure and where
aggressive condition of urea Carbamate

mixture exists, stainless steel of higher
chromium and nickel contents are found to
be more effective against corrosion. At this
condition, the influence of Nickel
apparent. However, the corrosion rate of
2RE-69 stainless steel is about 5
low as that of SS-316L which indicates that
the favorable influence of Chromium
much larger than the unfavorable
of Nickel.
Since the liquid phase in urea synthesis
behaves as an electrolyte, the corrosion is of
an electrochemical nature. Stainless steel in
a corrosive medium owes its corrosion
resistance to the presence of a protective
oxide layer on the metal. As long as this
layer is intact, the metal corrodes at a very
low rate. Passive corrosion rates of
austenitic urea grade stainless steels are
Fig-2(Oxygen Vs Corrosion rate in Duplex
mixture exists, stainless steel of higher
chromium and nickel contents are found to
be more effective against corrosion. At this
ckel is also
the corrosion rate of
stainless steel is about 5 times as
which indicates that
influence of Chromium is
unfavorable influence
Since the liquid phase in urea synthesis
n electrolyte, the corrosion is of
an electrochemical nature. Stainless steel in
a corrosive medium owes its corrosion
resistance to the presence of a protective
oxide layer on the metal. As long as this
layer is intact, the metal corrodes at a very
te. Passive corrosion rates of
austenitic urea grade stainless steels are
generally between <0.01and (max.) 0.10
mm/a. Upon removal of the oxide layer,
activation and consequently, corrosion set in
unless the medium contains sufficient
oxygen or oxidation agent to build a new
layer. Active corrosion rates can reach
values of 50 mm/a. Stainless steel exposed
to Carbamate containing solutions involved
in urea synthesis can be kept in a passivating
(non corroding) state by a given quantity of
oxygen. If the oxygen content drops below
this limit, corrosion starts after some time
depending upon process conditions and the
quality of the passive layer. Hence,
introduction of oxygen and maintenance of
sufficiently high oxygen content in the
various process streams are prerequisites to
preventing corrosion of the equipment.
Material)
2(Oxygen Vs Corrosion rate in Duplex
30th
Oct 2021
generally between <0.01and (max.) 0.10
mm/a. Upon removal of the oxide layer,
activation and consequently, corrosion set in
unless the medium contains sufficient
agent to build a new
layer. Active corrosion rates can reach
/a. Stainless steel exposed
arbamate containing solutions involved
in urea synthesis can be kept in a passivating
(non corroding) state by a given quantity of
ygen content drops below
this limit, corrosion starts after some time
depending upon process conditions and the
quality of the passive layer. Hence,
introduction of oxygen and maintenance of
high oxygen content in the
re prerequisites to
preventing corrosion of the equipment.

Oct 2021
Table-2(Oxygen Vs ammonia losses with inerts)
From the point of view of corrosion
prevention, the condensation of NH3-CO2-
H2O gas mixtures to Carbamate solutions
deserves great attention. This is necessary
because an oxygen deficient corrosive
condensate is initially formed on
condensation. In this condensate the oxygen
is absorbed only slowly. The Oxygen Vs
Material shows in the figure-2.This accounts
for the severe corrosion sometimes observed
in cold spots inside gas lines. The trouble
can be remedied by adequate isolation and
tracing of the lines. When the condensation
constitutes an essential process step - for
example, in high pressure and low pressure
Carbamate condensers-special technological
measures must be taken. These measures can
involve ensuring that an oxygen rich liquid

phase is introduced into the condenser, while
appropriate liquid gas distribution devices
Fig-3(Passivation Oxygen Vs Corrosion
Not only condensing but also stagnant
conditions are dangerous, especially where
narrow crevices are present, into which
hardly any oxygen can penetrate and oxygen
depletion may occur. As an attractive
alternative to austenitic stainless steel the use of
phase is introduced into the condenser, while
distribution devices
ensure that no dry spots exist on condensing
surfaces.
Corrosion rate)
Not only condensing but also stagnant
conditions are dangerous, especially where
are present, into which
hardly any oxygen can penetrate and oxygen
As an attractive
alternative to austenitic stainless steel the use of
duplex stainless steel in the chemical industry is
being experimented over the past few years.
They are now used not only in chloride
environment, where they are more resistant to
SCC than austenitic stainless steel, but also in a
wide variety of other demanding applications.
Due to the frequent tube failure, SS 304L tubes
30th
Oct 2021
ensure that no dry spots exist on condensing
duplex stainless steel in the chemical industry is
being experimented over the past few years.
y are now used not only in chloride
environment, where they are more resistant to
SCC than austenitic stainless steel, but also in a
wide variety of other demanding applications.
Due to the frequent tube failure, SS 304L tubes

Oct 2021
were replaced with Sandvik duplex tubes (SAF-
2205) in all inter stage coolers of CO2
Centrifugal Compressor of many urea plants
because the SCC occurred in number of the
plants. Figure- 2 shows the Oxygen Vs
Corrosion rate. And figure -3 shows the Oxygen
Vs Corrosion rate in different metal. It was
experienced that the tubes were failing due to
stress corrosion cracking in the presence of high
CO2 gas temperature (tube side) and chloride
presence in cooling water (shell side).
Sandvik SAF 2205 is a duplex (austenitic-
ferrite) stainless steel material. Advantages
of Sandvik duplex material (SAF 2205) over
304L / 316L are as follows.
1. High resistance to stress corrosion
cracking in chloride-bearing
environments and in environments
containing hydrogen supplied.
2. High resistance to general corrosion,
pitting and crevice corrosion.
3. High resistance to erosion corrosion
and corrosion fatigue. In oxygen free
Carbamate solutions, duplex has
proved to be more corrosion resistant
than much more costly materials
such as Titanium and high-nickel
alloys. In the high pressure synthesis
section of urea plants, duplex is an
excellent alternative to traditional
urea grades such as type AISI 316 L
UG ,2RE-69( 25 22 2). These are
particularly prone to SCC on the
steam side of heat exchangers when
the boiler feed water is contaminated
with chlorides.
Sr.
No.
Material Fluid Services
1. Titanium Urea, Carbamate Urea Reactor liner, Stripper
2 Zirconium Urea, Carbamate Stripper, Urea Reactors Liner
3 2-RE-69 Urea, Carbamate Urea Reactor liner, Strippers
Carbamate Condenser
4 SS-316L Urea Carbamate,
Carbonate
Medium Pressure section,
decomposer, ammonia receiver
absorber. Decomposers.
5 SS-316L(Urea
Grade)
Urea Reactor, heat exchangers
6 SS-304 Urea Carbonate Urea LP section, vacuum section and
waste water section
Table-3

Oct 2021
Grade Cr Ni Mo N PRE Microstructure
2 Re 60(UG) 18.5 4.9 2.7 0.07 28 Duplex
SAF 2304 23 4.5 - 0.1 24 Duplex
SAF 2205 22 5.5 3.2 0.18 35 Duplex
25 4 7 0.3 43
SAF2507 Duplex
18
AISI 304L 18.5 10 Austenite
Austenite
AISI 316L 17.5 13 2.1 24
Table-4
Conclusion
Based on the observation described above it
can be concluded that the energy can be
saved by choosing right material in
equipment like reactor liner and reducing the
amount of passivation air and hence losses
can be saved. The Duplex stainless steel makes
it possible to reduce the passivation air stream to
a urea plant more than tenfold .Oxygen is
normally added in the form of air to the
carbon dioxide flowing to the synthesis
section. Since Oxygen may form explosive
mixtures with Hydrogen the composition of
the gas phases in the various process steps
must be so controlled that explosion cannot
occur. With optimum addition of oxygen
and thus to realize an intrinsically safe urea
plant is only possible to Duplex stainless
steel and Sandvik (2RE69). Many issues
arise over material selection and corrosion
abatement in modern fertilizer plants. With
developments of metallurgy, and usage of
newer corrosion-prevention techniques, it is
possible to cost-effectively and, more
important, safely operate and maintain large-
scale plants.
References
1. Book the fertilizers Technology (pure
knowledge) by Prem Baboo published in
Notion Press January 2021.
2. Material Technology for Fertilizers
Industries by Prem Baboo Published
on Research Gate, August 2021.
3. Stress corrosion cracking in CO2
Compressor Intercooler By Prem
Baboo, published in International
Journal of Engineering Research &
Technology (IJERT) Vol-8 issue 04,
April 2019
4. Can Passivation Air in Urea Plants be
minimized through the Use of Safurex
and Consequently Improve Plant Safety
by Bart Gevers, Sr. and Will Lemmen,
published in Ammonia technical manual
year 2002.
Legends
SCC-Stress corrosion cracking, MP-medium
pressure. LP-Low pressure. PRE-Pitting
Resistance equivalent corrosion (%Cr + 3.3

Oct 2021
%Mo + 16 %N), MTC- Mitsue-Totsue “C”
improved Process. TIG-Tungsten Inerts Gas.
MIG-Metal Inerts gas.
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