Some Facts about Urea Stripper By Prem Baboo.pdf

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DOI: 10.13140/RG.2.2.16894.66880
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“Some Fact about
T The Fellow of Institution of Engineers (India)
Abstract
If we talk about the costly and
exchanger in urea plants, then the name of the
stripper comes first. Nowadays the competition
of stripper is running for more and more
capacity. In this regards some urea plant
licensers have gone ahead some are left behind
some are very much behind and some are
dragging.Urea stripper is a vertical in tube
falling film decomposer in which the liquid,
distributed on the heating surface as a film,
flows by gravity to the bottom.
stripper is the heart of urea plants.
is a vertical shell-and-tube exchanger with the
heating medium on the shell side, and an upper
head on tube side sheet specially designed to
permit the uniform distribution of urea and
carbamate solution. In fact, each tube has an
insert-type distributor (ferrule or male type
Fig-Ammonia & CO2 Stripping
28 th
Sept, 2022
Research Gate is an academic social networking
Some Fact about Urea Stripper”
By
Prem Baboo
Fellow of Institution of Engineers (India)
costly and biggest heat
, then the name of the
Nowadays the competition
of stripper is running for more and more
capacity. In this regards some urea plant
licensers have gone ahead some are left behind
behind and some are
is a vertical in tube
falling film decomposer in which the liquid,
distributed on the heating surface as a film,
flows by gravity to the bottom. The Urea
In practice, it
e exchanger with the
heating medium on the shell side, and an upper
head on tube side sheet specially designed to
permit the uniform distribution of urea and
carbamate solution. In fact, each tube has an
or male type)
designed to put the feed uniformly around the
tube wall in film form. The holes of the ferrule
act as orifices and their diameter and liquid head
control the flow rate. As the liquid film flows, it
is heated and decomposition of carbamate and
surface evaporation occur. The carbon dioxide
content of the solution is reduced by the
stripping action of the ammonia as it boils out of
the solution. Generated vapors (essentially
ammonia and carbon dioxide) are removed by
flowing to the top of the tube. The carbamate
decomposition heat is supplied by means of
condensing saturated steam at 219 °C.
paper described the procedure for calculation
NH3 & CO2 Stripper material & Energy balance
and differences of NH3& CO2
Keywords-Stripper, CO2, Ammonia,
differences, material,
Research Gate is an academic social networking site
1
”
ned to put the feed uniformly around the
tube wall in film form. The holes of the ferrule
act as orifices and their diameter and liquid head
control the flow rate. As the liquid film flows, it
is heated and decomposition of carbamate and
n occur. The carbon dioxide
content of the solution is reduced by the
stripping action of the ammonia as it boils out of
the solution. Generated vapors (essentially
ammonia and carbon dioxide) are removed by
flowing to the top of the tube. The carbamate
composition heat is supplied by means of
condensing saturated steam at 219 °C. In this
procedure for calculation
Stripper material & Energy balance
Stripper.
, Ammonia, Energy.

www.researchgate.net 28 th
Sept, 2022
DOI: 10.13140/RG.2.2.16894.66880 Research Gate is an academic social networking site
2
Fig- Ammonia stripping process
Description
Efforts to find some additional driving force
beyond the usual addition of heat and reduction
in pressure to decompose carbamate have met
with a great success of totally new route. In this
process, NH3 or CO2 is used to strip urea
reactor effluent for decomposition of carbamate.
The stripper and the reactor are operating
essentially at the same pressure of about 140 to
158 kg/cm2
g. The synthesis mixture from the
reactor, consisting of urea, unconverted
ammonium carbamate, excess ammonia and
water is fed to the top of the stripper. The
stripper has two functions i.e. its upper part is
equipped with trays where excess ammonia is
partly separated from the stripper feed by direct
counter current contact of the feed solution with
the gas coming from the lower part of the
stripper. This pre stripping in the top is said to
be required to achieve effective CO2 stripping in
the lower part. In the lower part of the stripper (a
falling film heater), ammonium carbamate is
decomposed and the resulting CO2 and NH3 as
well as the excess ammonia are evaporated by
CO2 stripping and steam heating. The overhead
gaseous mixture from the top of the stripper is
introduced into the carbamate condensers. Here,
two units in parallel are installed, where the
gaseous mixture is condensed and absorbed by
the carbamate solution coming from the medium
pressure recovery stage. Heat liberated in the
high pressure carbamate condensers is used to
generate low pressure steam in one of the
condensers and to heat the urea solution from the
stripper after the pressure is reduced to about 18-
24 kg/cm2 g in the shell side of the second
carbamate condenser. The gas and liquid from
the carbamate condensers are recycled to the
reactor by gravity flow.
Theory of stripping
The theory of stripping is based on Henry’s law
which states that the concentration of
components in a solution while in equilibrium
with vapour phase is directly proportional to the
partial pressure of the components in vapour
phase. If the partial pressure of the one
component is altered, while keeping the total
pressure constant, the concentration of that
component in solution will vary accordingly.
This is illustrated as below .
Let us assume that A and B are two components.

Sept, 2022
3
CA = Concentration of A in solution. CB =
Concentration of B in solution. PA = Partial
pressure of A in vapour phase PB = Partial
pressure of B in vapour phase.
CA = k PA, CB =k PB
In the stripper which operates at around 140
kg/cm2
a pressure, the carbamate solution is split
into it’s components ammonia and CO2 by
adopting the above principle.
The reactor effluent is counter currently
contacted with fresh CO2 which increases the
partial pressure of CO2, thereby upsetting the
solution concentration and splitting the
carbamate into ammonia and CO2. The above is
achieved by providing external heat and without
change in operating pressure.
Technical realization of the stripping process
In this process, the formation of urea, the
stripping and the condensation of the carbamate
are accomplished at the same pressure level.
This gives rise to several advantages such as :-
Elimination of pumping concentrated carbamate
solution from various pressure levels.
Minimizing the quantity of water introduced to
the reaction system by means of recycle streams.
Optimum heat balance with the export of LP
steam. Reduced power consumption as reaction
pressure is comparatively low. Corrosion is the
function of Temperature as well as
Corrosiveness of process fluid is not constant
owing to effect of start-up/shut-down,
fluctuation of temperature and dissolved oxygen
content and geometrical flow distribution. For
stripper. Passivation air role is principal in this
issue The Passivation given in CO2 depends
upon the process to process in M/Saipem it is
0.25% and in M/S Stamicarbon it is 0.6%.This
passivation depends upon N/C ratio in Reactors.
Stripper top and bottom temperature, Design for
Saipem 205/190 but after losing of ferrules-
205/192-196.Mega bond stripper is the latest
stripper. Average corrosion rate of tubes. The
corrosion rate is depends upon Temperature and
passivation oxygen, No of shut down etc.
Generally Corrosion rate as follows as const.
Temperature Conditions. In Zr-0.005 mm/year,
Ti-0.05 mm/Year,2 RE-69-0.25 mm/year,
corrosion rate in duplex material is about 0.05
mm to 0.1 mm per year.* Stripper Tube
diameter-2.7 mm t0 3.5 mm.
Difference Between Stripper and Decomposer
Sr.
No.
Parameters Decomposer Stripper
1 Definition “The process based on the 1st
principal of decrease in
pressure and increase in
temperature” and then have a
series of decomposition stage
where the Reactor discharge is
treated in successively at lower
pressure.
“To reduce the partial pressure of product
by swamping the system by one of the
reactant which reduces the partial pressure
of other reactant considerable without
changing the total pressure either CO2 or
NH3 or both can be used as a stripping
agent.”
2 Delta P
between
Reactor and
stripper
Delta P is more than Stripper No Delta between Reactor and Stripper,
however in Saipem process there is small
delta P.
3 Base of
Decomposition
Differential pressure solution
flashed due to high difference
of pressure.
Stripping base hennery law of partial
pressure, Mass transfer also involve. Base
on partially pressure, In Stamicarbon CO2

Sept, 2022
4
introduced in stripper, is advantageous
because CO2 increase P1 CO2 to P2 CO2, so
P1 NH3 will reduced to P2 NH3 to maintain
total pressure constant as
PCO2+PNH3=Total Pressure. Now K-1 =
(X2NH3X XCO2)/X Carb. at particular temp
K1 is constant so when XNH3 is reduced to
keep K1 constant X Carbamate will be
reduced much faster by decomposition as
XNH3 appears in the equation with power
of 2
4 Theory Decomposition base only high
temp & low pressure,
decomposition due to let down
water recycle is more.
Decomposition is favoured by
low pressure but as
decomposition products are to
be recycled back to reactor it
will require energy for this
step. Also at low pressure more
water is evaporate during
decomposition and this water
would enter the urea reactor
along with recycle stream and
will adversely affect the
conversion. Thus if the
decomposition is carried out in
single stage near atmospheric
pressure the Carbamate formed
during recovery at the same
pressure carry a lot of water.
considering these factors,
decomposition is carried out
number of stages.
In stripping process the 1st stage
decomposition and recovery is done at the
reactor pressure which permit heat to be
recovered at high level and also results in
saving the power for returning the recycle
streams to the reactor .however in Saipem
process there is difference in pressure in
Reactor and Stripper, so additional
advantages of deferential decomposition.

DOI: 10.13140/RG.2.2.16894.66880
Fig-Stripper Vs decomposer water carryover
Comparison of Ammonia & CO2 Stripper
Sr.
No.
Parameters Ammonia stripping Process
1 Surface Area Less
2 Nos of Tubes Less
3 Cost Less(Except Mega bond)
4 In Case of
Tube or tube
sheet Leakage
In case of Leakage the pH of
Steam condensate is more
than 9.0
ammonia content is higher
Hence no problem of
corrosion, you can run the
plant at minor leakage by
separate draining of
condensate,
5 Steam
Requirement
0.72 T/ton of Urea
28 th
Sept, 2022
Stripper Vs decomposer water carryover
Stripper
Ammonia stripping Process CO2 Stripping Process
More in same plant Load because the double
load of heating(CO2 as well as Reactor
effluents)
About 150 % tubes more
(Except Mega bond) Very Costly at same plant load
In case of Leakage the pH of
Steam condensate is more
than 9.0-10.0 because the
ammonia content is higher,
Hence no problem of
corrosion, you can run the
plant at minor leakage by
separate draining of
condensate,
In case of minor leakage , take shut down
immediately because the pH of steam
condensate come down below 7.0 due to
CO2 contents is higher and hence corrosion
start in CS shell, otherwise equipment will
damage and cause major incident.
T/ton of Urea 0.84 T / ton of urea
5
because the double
as well as Reactor
Very Costly at same plant load
In case of minor leakage , take shut down
mmediately because the pH of steam
condensate come down below 7.0 due to
and hence corrosion
start in CS shell, otherwise equipment will
damage and cause major incident.

DOI: 10.13140/RG.2.2.16894.66880
6 Stripper inlet
Temp, 0
C
7 Stripper outlet
Temp, 0
C
204(211 Mega
8 Stripper vapour
Temp, 0
C
9 CO2 to stripper
Temp 0
C
10 O2 in CO2 %
Fig-mega bond tube
28 th
Sept, 2022
188 185
(211 Mega bond) 176
190 189
NA 120
0.25 0.6
The Omega bond stripper is so costly,
everyone can dare. Replacing the old stripper in
old unit is one choice. In the Titanium stripper
bottom temperature easily maintain 210
without any problem ig ignore some problem
like tube end corrosion. This 210
temperature at which challenging all the process
we can run the plant at any load. The ideal
temperature for urea stripping is between 210
– 212˚C However, due to the upper temperature
limits of stainless steel, plants cannot operate at
their optimum capabilities, sacrificing
significant plant capacity. Passivation air is the
main factor in the urea process on which the
plant safety explosion etc and losses also
depends. Passivation air must also be removed
after stripping. This adds process costs and
hazards resulting from the co
oxygen and hydrogen—an explosive gas mixture
the mega bond is the solution of all problems
this is the mother/father of urea plants.
Passivation air is not necessary with titanium
and zirconium. Eliminating the need to remove
passivation air downstream improves yield,
6
The Omega bond stripper is so costly, not
everyone can dare. Replacing the old stripper in
old unit is one choice. In the Titanium stripper
bottom temperature easily maintain 2100
C
without any problem ig ignore some problem
like tube end corrosion. This 2100
C is the
enging all the process
we can run the plant at any load. The ideal
temperature for urea stripping is between 210˚C
˚C However, due to the upper temperature
limits of stainless steel, plants cannot operate at
their optimum capabilities, sacrificing
nificant plant capacity. Passivation air is the
main factor in the urea process on which the
plant safety explosion etc and losses also
depends. Passivation air must also be removed
after stripping. This adds process costs and
hazards resulting from the combination of
an explosive gas mixture
the mega bond is the solution of all problems
this is the mother/father of urea plants.
Passivation air is not necessary with titanium
and zirconium. Eliminating the need to remove
ownstream improves yield,

Sept, 2022
7
reduces environmental concerns and improves
plant safety. In addition, next-generation urea
tubing eliminates temperature limitations
inherent with steel alternatives, allowing the
strippers to run at 212°C, thus improving
capacity by up to 15 percent. In addition,
corrosion is eliminated in the stripper, ensuring
impurities such as iron, nickel and chrome—the
core components of stainless steel—do not exist
in the final product. Erosion inside the tubes is
also eliminated, ensuring an extensive stripper
life without the need to flip the stripper. Next-
generation urea tubing works with standard
titanium stripper designs, replacing pure
titanium tubes. Omega Bond tubing is the next
generation solution; it is the only technology that
combines the benefits of Zirconium and
Titanium, ideally optimizing urea processing.
ATI is a world leader in specialty metal
manufacturing, with over 50 years of experience
from raw material to final product ensuring a
reliable and high-quality solution. Omega Bond
Tubing combines titanium grade 3 and
Zirconium 702 using a unique metallurgical
process that creates high-quality corrosion- and
erosion-resistant tubing designed to leverage the
strengths of each metal. The result:
maintenance-free operations, less downtime and
higher quality output.

DOI: 10.13140/RG.2.2.16894.66880
Fig- Comparison of stripping process
Compression of NH
Operating Feutures
Particular
Fluid circulated
Total fluid entering
Fluid vaporised or condensed Kg/Hr
Temperature in
Temperature out
Operating pressure kg/cm
Design pressure kg/cm
Design temperature
Table –Comparison of stripper
28 th
Sept, 2022
Comparison of stripping process
Compression of NH3 & CO2 Stripper at same Load
CO2 Stripper Detail Ammonia Stripper
unit shell side Tube side
shell
side
Steam
Urea-
carbamate
solution and
CO2
Steam
Urea-carbamate solution
294066
Kg/Hr 58562 100915 42000
0
C 215.9 185 222
0
C 215.9 176 222 204,211(Megabond)
kg/cm2
a 22.8 153 24
kg/cm2
a 26 170 27
0
C 300 230 300
8
Ammonia Stripper
Tube side
carbamate solution
and CO2
211885
49589
188
204,211(Megabond)
150
175
235

Sept, 2022
9
Mechanical features
Parameters Ammonia stripping CO2 Stripping
No. of tubes 2600 1677
Tube material
X2 25-22-2 (Cr-Ni-Mo-N)
Stainless steel
Bimetallic, 2RE69+Zr, Mega bond
(Ti+ Zr)
Tube I.D,mm 25 20
Tube O.D.,mm 31 25.4
Tube length,mm 6000 6000
Tube pitch,mm 41 41
Surface area, m2 1519 632
Shell material carbon steel carbon steel
Stripper Material and Heat balance
Stripper Inlet stream
Stripper Inlet
Components Kg/hr K mole/hr mole fraction, x sp heat KJ/K
mol0
C,Cp
Σ x.cp
Ammonia 70350 4138.24 0.51 98.90 50.10
CO2 29200 663.64 0.08 58.96 4.79
Urea 73560 1226.00 0.15 121.32 18.21
Water 38500 2138.89 0.26 74.00 19.38
Biuret 240 2.33 0.0003 183.80 0.05
Air 803 27.69 0.0034 31.61 0.11
Total 212653 8169.09 1.00 568.59 92.63
Heat Input 143776631 KJ/HR
Stripper outlet stream (Liquid)
Stripper Outlet Liquid
Components
Kg/hr Wt % mole % Mol Fraction, x Sp. Heat, Cp Kg Mol/Hr Σ x.cp
Ammonia 38234 23.46 1.38001081 0.382123939 83.6 2249.06 31.95
CO2 12840 7.88 0.17905812 0.049581057 56.8 291.82 2.82
Urea 73532 45.12 0.75198089 0.208222933 186 1225.53 38.73
Water 38100 23.38 1.29877567 0.359629991 79.9 2116.67 28.73
Biuret 268 0.16 0.00159654 0.00044208 202 2.60 0.09
Total 162974 100 3.61142203 1.00 5885.68 102.31
Heat carried out by stream=5885.68*102.31*205=12.34*107
kJ/Hr

Sept, 2022
10
Stripper outlet stream (Vapour)
Stripper Outlet Vapour
Components Kg/hr Wt % mole
%
Mol
Fraction, x
Sp. Heat,
Cp
Kg
Mol/Hr
Σ x. cp M *λ
Ammonia 34116 68.13% 0.04 0.85 106.76 2006.82 90.96 52879800
CO2 14360 28.68% 0.01 0.14 59.96 326.36 8.31 9334000
Water 400 0.80% 0.0004 0.01 79.80 22.22 0.75 744000
Inerts 1200 2.40% 0.0009 0.02 34.00 42.86 0.62
Total 50076 100.00% 0.05 1.00 246.52 2355.41 100.02 62957800
Sensible Heat of vapour 45940162.51 KJ/Hr
Latent Heat of Vaporization 62957800 KJ/Hr
Total 108897962.5 KJ/Hr
Heat (steam) supplied to stripper in shell side
Steam Flow- 50 .0 T/Hr, at 24 ata, 2220
C, Latent heat 1771.4 KJ/Kg
Total Heat= 50000 *1771=8.857 *107
KJ/Hr
Now Heat input=Heat output
14.37 *107
+ 8.857*107
=12.34*107
+10.88*107
23.22*107
KJ/Hr=23.22*107
KJ/Hr.
Calculations for estimation of additional energy to be removed to bring down CO2 temperature
from 50ºC to 40ºC
CO2 flow at the exit of the Separator B-3306
Components Flow (Nm³/h
CO2 49349
N2 50
H2 323
H2O 4446
Total flow (dry) 49722
Total flow (wet) 54168
Water (liquid) at the bottom of the separator=15764 kg/hr
=15764/18*22.414 Nm³/h=19630 Nm3
/hr
Hence total water vapour at the inlet of 1st
regeneration overhead condenser=4446+19630-24076 Nm3
/hr.
CO2 Flow at inlet of regenerator overhead condenser.
Sr.
No.
Components Flow,
Nm3
/hr
Mole Fraction Mole
fraction
value
Molecular
weight with
mole fraction
Molecular
weight,
kg
1 CO2 49349 49349/73798= 0.6687 44*0.6687= 29.42
2 N2 50 50/73798= 0.0007 28*0.0007= 0.0196
3 H2 323 323/73798= 0.0044 2*0.0044= 0.00288
4 H2O 24076 24076/73798= 0.3262 18*0.3262= 5.816

Sept, 2022
11
5 Total (dry) 49722
6 Total (wet) 73798
7 Molecular weight of the Gas 35.32
Quantity of water condensation at 400
C
Vapour of water at 400
C=0.073 kg/cm2
a
Total Pressure=0.5 kg/cm2
g
Hence Absolute pressure=0.5 +0.9928 kg/cm2
a=1.4928 kg/cm2
a
Hence water vapour at the exit of the separator at 400
C=
𝟒𝟗𝟕𝟐𝟐∗𝟎.𝟎𝟕𝟑
𝟏.𝟒𝟗𝟐𝟖 𝟎.𝟎𝟕𝟑
=2556 Nm3/hr
Hence Total water vapour condensed =24056-2556=21520 Nm3
/hr
𝐇𝐞𝐧𝐜𝐞 𝐓𝐨𝐭𝐚𝐥 𝐰𝐚𝐭𝐞𝐫 𝐯𝐚𝐩𝐨𝐮𝐫 𝐜𝐨𝐧𝐝𝐞𝐧𝐬𝐞𝐝 =
𝟐𝟏𝟓𝟐𝟎∗𝟏𝟖
𝟐𝟐.𝟒
Kg/Hr=17282 kg/hr.
Cp of the gas=0.24 kcal
Gas inlet temperature to separator=93.90
C
Gas outlet temperature to separator=400
C
Latent Heat of water at 400
C=577 k.cal/kg
Hence the Total Heat load= =73798*0.24*53.9*35/22.414+17282*577
=11462424 k.cal/hr
11.46 G.Cal/hr
Heat load of the existing condenser in revamp case=10.24 G.Cal/Hr
Hence, requirement of additional heat load=11.46-10.24=1.22 G.Cal/hr
Pool Reactor calculation
Sr. No Components % Moles
1 Ammonia 29.60 1.741
2 CO2 18.90 0.430
3 Urea(include biuret) 33.00 0.550
4 Water 18.50 1.028
5 Total 100.00 3.748
Sr. No. CO2 required for 0.55 moles of urea 0.550 moles
1 Reactor outlet CO2 is 0.430 moles
Therefore Reactor inlet CO2 would be 0.550+0.430=0.980 moles
2 NH3 required for 0.550 moles of Urea 0.550*2=1.10 moles
Reactor outlet NH3 is 1.741 moles
Therefore Reactor inlet NH3 would be 1.10+1.741=2.841 moles
3 H2O at Reactor inlet 1.028-0.550=0.478 moles
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 =
Inlet CO2 − Outlet CO2
Inlet CO2
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 =
. .
.
==56.1%
N/C Ratio=2.841/0.980=2.9.H/C ratio=0.478/0.980=0.49

Sept, 2022
12
Fig- CO2 stripping (small plant 188 TPD)
Conclusion
The era of small plants has gone, now is the era of big mega urea plants, the stripper has a huge role in
that. The plant’s capacity depends upon stripper. Now the era is one ammonia and one urea single stream
plants. Earlier there were one ammonia and two urea plants. Now the era is of 4000-5000 TPD urea
plants.Big plants mean everything is bigger too much. explosive hazards and more stress on equipments
lines. There are many problems in start up and shut down in mega plants. The more passivation air also
creates safety issues. Fixing corroded stripper tubes is exceptionally costly. Even a few days of downtime
can cost businesses millions in lost revenue. Replacing strippers every 10 years is also a time-consuming
and costly procedure.
************************************************************************************

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