How to control prilling tower dust emission

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HOW TO CONTROL PRILLING TOWER DUST EMISSION
Presentation · February 2022
DOI: 10.13140/RG.2.2.22899.20004
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HOW TO CONTROL PRILLING TOWER DUST EMISSION
By
Prem Baboo
Retired from National Fertilizers Ltd, India
&
Dangote Fertilizers Ltd, Lagos, Nigeria
Abstract
Urea dust emission is the major problem for environment. In India fertilizers Produced by Prilling
routs for Urea and Ammonium Nitrate. Prilling is the common process. The revamp of emission
control system in prilling tower is a considerable burden. It not only presents a substantial investment
but also raises the running costs and energy may increase up to 0.01 Gcal/ton of urea.. In the face of
strong demand for environment friendliness and effective use of power it is then an issue of utmost
importance to pick the legally emission control solution, the one that can guarantee, if not a full
return on the investment, then at least cutting the cost to absolute minimum. In order to remove urea
dust and ammonia, wet processes are generally applied. The available technologies vary with regard to
the scrubber design, type of demisters and the gas moisturizing/spraying system. Dust emission is
directly proportional to temperature. Dust emission can control by internal and external process. In
India generally followed internal routs. The pollution control Board sample should be ok just thinking
so. Either reduction of plant load or bypassing the recovery system at the time of sampling or
manipulating data. It is the bitter truth. In reality dust emission control system should be installed in
prilling tower. It is not costly; slightly per ton of urea energy will increase but it is necessary for all
urea plants. Number of Revamp Companies are available in market.
Key words
Dust emission, Prilling tower, Pollution,
environment.
Introduction
Urea is produced by synthesis from liquid
ammonia and gaseous carbon dioxide. Ammonia
and carbon dioxide react to form ammonium
carbamate, a portion of which dehydrates to
form urea and water. The reaction of ammonium
carbamate dehydration is influenced by the ratio
of various reactants, operating pressure,
temperature and residence time in reactor. The
reaction of ammonia and carbon dioxide takes
place in two stages to produce urea.
Formation of ammonium Carbamate, at 25 0
C
2 NH3 (g) + CO2 (g) → NH4COONH2 (s)
-38 086 cal/gmol
Dehydration of ammonium Carbamate to urea,
at 25 0
C
NH4COONH2 (s) →NH2CONH2 (l) + H2O (l)
+10 330 cal/gmol
Overall reaction, at 25 0
C
2 NH3 (g) + CO2 (g) → NH2CONH2 (l) +H2O
(l) -27 756 cal/gmol
The first reaction is highly exothermic and heat
is liberated as the reaction occurs. With excess
NH3, the CO2 conversion to carbamate is almost
100%, provided solution pressure is greater than
decomposition pressure. The decomposition
pressure is the pressure at which carbamate will
decompose back into CO2 and NH3 i.e
NH2 COO NH4 (s) →2 NH3 (g) + CO2 (g)
Decomposition pressure is a function of NH3
concentration in the feed and the solution

Feb. 2022
temperature and increases if either temperature
or NH3 recycle is increased. It is desirable to
operate at higher pressure and high mole ratio of
NH3 to CO2 provided reactor operating pressure
is quite high enough to prevent
carbamate from decomposition into NH3 and
CO2. This will maximise CO2 conversion to urea
towards the reaction (ii). The second reaction is
endothermic, therefore heat is required for this
reaction to start. The heat for this reaction comes
from the heat of formation of carbamate. This
reaction is a function of temperature and
ammonia concentration in the feed. The solution
effluent from the reactor being mixture of urea
solution, ammonium carbamate, unreacted
ammonia, water and CO2 is extremely corrosive
in nature. The subsequent stages of process
consist of decomposition of unconverted
carbamate, recovery of resulting ammonia and
carbon dioxide for recycle, concentration and
prilling of urea solution.
Prilling
Prilling is defined as distribution of molten
droplets into a column of rising air which
removes the heat of fusion and yield a solid
product. Two most important aspect of prilling
are droplet formation and distribution of droplets
over maximum cross-section area of prilling
tower. The jet coming out from the hole on the
prilling bucket becomes unstable and becomes
ready to disrupt when its length becomes about
4.5 times as that of hole diameter and dia of the
prills becomes 1.89 times as that of hole
diameter. The tendency of jet to disrupt can be
expressed in terms of viscosity, density, surface
tension and jet size. The disturbance that leads to
break up of liquid jets into small droplets must
be as similar as possible to produce urea prills of
maximum uniformity of size and shape.
• As increase in the static pressure will result in
small increase in average prills diameter. Prills
size and distribution are function of three
parameters namely vibration frequency, orifice
diameter and static fluid pressure.
• The shape of the size distribution curve of a
product is not affected by the change in bucket
design. Size distribution is a natural
phenomenon and cannot be changed. Only mean
prills diameter can be changed.
Aside from technique of dividing the jets into
droplets other variables which control the urea
prills are feed temperature, pressure and
composition, tower diameter, forced or natural
draft, air velocity in tower, height of free fall,
ambient conditions and pollution control. At
critical disturbance frequency, the jet is
disrupting to form a prills. This frequency is a
function of velocity of jet and distance between
drops. Frequency of distribution can be kept
constant by applying forced vibrations, which
cooled lead to more uniform prills. Some
research work is going on abroad wherein
bucket will be getting forced vertical vibrations
as it rotation vibrations of undesired frequency
are to be masked.
In prilling processes, urea solution is
concentrated to 99.7% in two steps under
vacuum and resultant molten urea is distributed
in the form of droplets in a prilling tower. This
distribution is performed either by showerheads
or by using a rotating prilling bucket equipped
with holes. Urea droplets solidify as they fall
down the tower, being cooled counter currently
with upward flowing air.
Zones of free fall Height
Three zones of states take place for the prills
falling from top to bottom. In first zone droplet
loses its sensible heat and cools to the
temperature above crystallization temp. In the
second zone, the most outer layer of drop starts
to crystallize and latent heat of crystallization
transfers to the cooling air. In this zone most
innermost layer of prills gets solidified and prills
becomes completely solid.In third layer prills

Feb. 2022
further cools down to lower temperature.
Smaller particles would crystallize quickly and
reach zone three whereas large ones may never
approach zone three during their residency in the
tower.Air velocity increases along the height of
the tower due to the decrease in density of air
because of temperature rise. Humidity of air
along the height of tower increases due to
evaporation of moisture from prills. Rate of
change of humidity at the top is more than that at
the bottom indicates most of the moisture is
removed at the top when the prills is in the liquid
stage. Prills size varies inversely proportional to
RPM of bucket and feed liquid density but varies
directly proportional to feed rate, feed viscosity,
feed surface tension The Urea Melt inside the
bucket takes shape of a vortex, practically
parallel to the bucket wall. The Prills drum has
an angle of about 5 deg to the vertical plane.
This wall thickness of liquid has to be consistent
all over to ensure air is not entrapped leading to
hollow prills
Fig-1
Types of Prilling Tower
There are two types of prilling tower
1. Natural prilling tower
2. Force draft prilling tower.
(i) Cross flow Prilling Tower
(ii) Unidirectional Prilling Tower
Natural Prilling Tower
The natural prilling towers are simple in
construction. No ID/FD Fan and no blower.
Only natural cooling takes place by prilling

Feb. 2022
tower height natural draft. Power consumption is
minimum. Sufficient prilling height for cool
down prills. The prilling bucket is installed
central part of the cooling tower. A conical or
flat scrapper is used to collect urea on belts
conveyors.
Fig-2(Images of Natural Prilling Towers)
Natural prilling tower height depends upon plant
load. Generally 90meters to 150 meters for 2000
TPD to 5000TPD plants respectively; Prilling
height also varies according to plant load and
ambient conditions, e.g 70 meters to 120 meters.
Dia of the prilling tower according to plant
loads.It may be
Design of Prilling Tower
Transformation of urea from melt to solid prills
takes place in the urea prilling tower. In the
prilling process, urea melt is pumped to the top
of 70 to 150 meter (above ground) cylindrical
concrete tower where it is fed to the prilling
device that called rotating bucket. The rotating
bucket is a sieve-like cylindrical or conical drum
that rotates about its axis. Liquid jets emerge
from the various holes on the curved surface of
the drum, and break up due to centrifugal and
capillary instability. The liquid urea droplets
formed fall downward the prilling tower. A
counter current cooling air stream enters from
intake openings located around the
circumference of the tower at a height
approximately 5-7 meters from the ground level
of the tower. The design of the prilling tower
according to plant load.
Sr.
No.
Urea
Prod,
TPD
Prilling
Tower
Total
height,
meter
Free
Fall
Height,
meter
Prilling
Tower
Diameter,
meter
1 1200-
1600
96 76 27
2 1600-
2000
102 80 27
3 2100-
2500
110 86 27
4 2600-
3000
120 90 27
5 3100-
3500
130 98 27
6 3600-
4000
140 100 28
7 4100-
4500
142 102 28
8 4600-
5000
144 104 29
Table-1

Feb. 2022
Fig-3
Urea finishing Technology
Following technologies are used for size and
shape of product and to improve various
physical properties of urea.
Granulation
A urea melt stream with a urea concentration of
98.5 wt% is introduced into the fluidbed
granulator through the injection headers, which
are connected to the urea melt line and the
secondary air system. Each injection header
comprises vertically placed risers fitted with
spray nozzles that spray the urea melt onto the
seed particles. The secondary air, required to
transport the granules through the urea melt film,
is provided by a secondary air blower. In
granulation, urea formaldehyde is added to the
urea melt as an additive and anti-caking agent.
This also improves the granule crushing
strength. Various past and present technologies
for granulation are given below.
Various past and present technologies for
granulation are given below
Sr.No. Company Granulation Technology
1 C & I Gridler (Now Bechtel) Drum Granulation Technology
2 Kaltenbach-Thuring Drum Granulation Technology
3 Montedison Drum Granulation Technology
4 Tennessee Valley Authority (TVA) Pan Granulation Technology
5 Norsk Hydro Pan Granulation Technology
6 Hydro Agri Fluid bed Granulation Technology
7 Uhde Fertiliser Technology (UFT) Fluidized Bed Granulation Technology
8 Stamicarbon Fluidized Bed Granulation Technology
9 Toyo Engineering Corporation Spouted Bed Granulation Technology
10 Saipem Rotary Drum Granulation Technology
Table-2

Feb. 2022
Other finishing technologies
In the 2000s, some other new finishing
technologies have their entry into the urea
market such as Pastilization & Compression
Prilling Tower Scraper
Two Type of Scrapper
1. Flat Scrapper
2. Conical scrapper
A provision is made to raise/lower and to rotate
the bucket manually for the purpose of bucket
change over. The hot urea prills of 80° C - 95° C
fall on the scraper floor at the bottom of the prill
tower . Prills are scraped by a rotating double
arm straight scraper and fed to the prill tower
conveyor. The conveyor is running across the
diameter of the prilling tower, through opening
in the rake floor. The scraper is driven by two
motors .The motor coupled scrapper with
hydraulic coupling. Placed below the rake floor.
These motors are connected to the gear of the
scraper control shaft through gearboxes of
drivers. The scraper central shaft extends up
through the rake floor and the scraper arms are
attached to it. The scraper arms are fitted with
adjustable blades. Effective sealing for the
central shaft is provided to prevent urea dust
entering the bearings.
Fig-4
The cleaning of scraper arms inside a prill tower
is a great safety concern in Urea Plant as it is a
high risk activity with potential severe
consequences. Several accidents have occurred
during the manual chipping of Urea deposition
on scrapper arms all over the World. Entering the
scraper floor is dangerous even with all safety
protections like steel or wooden movable
structures as well as personnel protective
equipment (PPE). Urea lumps can fall with high
velocity due to the height and even small lumps
can cause severe injuries to the personnel
working inside. Some industries are using
movable scraper floor is an issue. steel structures
under which workers carry out cleaning. The
Scrapper cleaning activities generally done every
month. The time taken about 1=2 hrs depends
upon the condition. At that time the plant load
can be reduced.

Feb. 2022
Fig-4
Advantages:
1. Emission Control with Product recovery
2. Trouble free operation.
3. Flexibility to install before or after ID
Fan.
4. Inherent quality to reduce Low water or
aqueous solution requirement.
5. Low pressure drop ( 60 mmwc ).
6. Capable to reduce 0.7μm particulate up
to 90% with reduction in dust and
ammonia up to approx. 25 mg/Nm3 .
7. Wet Scrubber for Prill Tower
8. Process Description
9. Multiple Wet Scrubber units are
designed to handle large quantity of
dusty air from the prill tower. Each
scrubber has two circulating/ spraying
loops.
10. In one of them there is a circulation of
urea solution (U) or urea/ammonium
sulphate aqueous solution (UAS) or
urea/ammonium nitrate aqueous solution
(UAN) depending on the type of acid
used to react with ammonia.
11. The second loop contains process water
used to constant flushing of demisters
and dilution of obtained product up to
the required concentration.
12. To remove ammonia, in most cases,
sulphuric or nitric acid is used
depending on their availability as well as
possibility of product
distribution/utilization.
How to control Dust and Ammonia emission
on Prilling Tower
Dust and ammonia can be controlled by two
ways
1. Internal Process
2. External Process
(i) Dust control system from Top
of the prilling tower
(ii) Dust control system from
bottom of the prilling tower.
Internal Process
Temperature of prills is directly proportional to
dust emission. Following parameters of the
process you can control dust emission
1. Biuret Control
If you reduced the Biuret then save one molecule
of ammonia
Formation of biuret takes place when urea is
heated to its melting point it starts
decomposition with evolution of ammonia, urea
first isomerizes which dissociates into isocynic
acid and ammonia.
CO(NH2)2 = NH4CNO +NH3.
(UREA) (AMM.CYNATE)

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The isocynic acid reacts with urea to form
biuret.
NHCO + CO(NH2)2 = NH2CONHCONH
In the presence of excess ammonia biuret is
formed at substantially lower rate by direct
reaction between urea molecules.
2CO (NH2)2 = NH2CONHCONH2 + NH
(UREA) BIURET AMMONIA
Biuret Favorable Condition-
1. High temperature, low pressure
2. High residence time.
3. High concentration
Fig-5
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The isocynic acid reacts with urea to form
CONHCONH2
In the presence of excess ammonia biuret is
formed at substantially lower rate by direct
+ NH3.
AMMONIA
1. High temperature, low pressure
4. Low Ammonia.
Dust emission Control by Vibro priller
The dust emission of prilling tower can be
controlled by vibro priller. You can install Vibro
priller with replacement of existing prilling
Bucket or Prilling distributers. Prilles quality
improved with Vibro priller because the dust
emission will be minimum. it has wide capacity
range and simple in design.Environment
friendship.95% normal size and minimum dust
emission. Average prills size 2
cooling of prilles in summer season.
shown in the figure- 5 The Vibro priller can be
installed from Russian Company NIIK.
22nd
Feb. 2022
Dust emission Control by Vibro priller
The dust emission of prilling tower can be
controlled by vibro priller. You can install Vibro
priller with replacement of existing prilling
Bucket or Prilling distributers. Prilles quality
improved with Vibro priller because the dust
it has wide capacity
range and simple in design.Environment
friendship.95% normal size and minimum dust
emission. Average prills size 2-4 mm and
cooling of prilles in summer season. Detail as
The Vibro priller can be
rom Russian Company NIIK.

Feb. 2022
Fig-6
Urea Dust Recovery system from bottom of
the prilling Tower.
Urea dust recovery system in prilling comprising
conducting the prilling operation in a co-current
stream of air (moving downward with the
solidifying prills through the prilling tower) and
then collecting the prills and substantially all by-
product urea dust (fines) at the bottom of the
tower.some of the smaller particles are entrained
in the cooling air stream and are carried out the
top of the tower, where they are vented to the
atmosphere and lost. this 1oss to be a minimum
of 2-3 ton/day for fertilizer prills(as per plant
capacity) Because of the smaller average product
particle size, countercurrent air flow in the
prilling tower during the production of prills
must be limited to natural draft. The use of
forced draft as used in the production of
fertilizer prills would result in excessive dust
losses about 150-250 mg/Nm3,while in natural
prilling tower this value is 60-90 mg/Nm3 after
implementation of this dust recovery from
bottom of the prilling tower it is reduced to 30-
50 mg/Nm3. Reduced air flow (natural draft),
however, introduces a problem of its own.
Because of insufficient prill cooling during
periods of high production or of high ambient
temperature in summer season , formations of
urea are deposited on the walls and on the
bottom collecting cone of the prilling tower.This
formation is very dangerous can be removed
shutdown to shut down.

Following advantages
1. Dust losses at the top of the pril tower
can be reduced
2. Product build-up on the walls of the prill
tower during the production of sm
size can be reduced
Fig-7
Dust Recovery from top of the Prilling Tower
For recovery of the dust from prilling tower top
two number of steel duct are provided.
possibility to place the equipment on the ground
level but it is less efficient with regard to both
ust losses at the top of the pril tower
up on the walls of the prill
tower during the production of smaller
3. Heat and fumes at the top of the priliing
tower can be controlled for
environments
Dust Recovery from top of the Prilling Tower
For recovery of the dust from prilling tower top
steel duct are provided. There is a
possibility to place the equipment on the ground
level but it is less efficient with regard to both,
investment and operation. As shown in the
Figure-8
Figure -8 represents the proposed configuration
of the Scrubber, which comprises the following
main equipment. Note the final dimensions and
details are subject to completion of detailed
22nd
Feb. 2022
Heat and fumes at the top of the priliing
tower can be controlled for
investment and operation. As shown in the
represents the proposed configuration
of the Scrubber, which comprises the following
main equipment. Note the final dimensions and
details are subject to completion of detailed

design: Air collection duct and down comer
duct.
2. ting prill tower fan room and supported
from the tower while a stainless steel
down comer duct with free standing
support structure installed to bring the
un-scrubbed gas into the scrubber;
3. A stainless steel scrubber vessel which
contains filtration medium;
4. Scrubber fan with variable speed drive.
and , stainless steel self-supporting
exhaust stack
5. Vibro priller provided instead of
distributers or prilling bucket to control
product quality.
The urea solution is used for scrubbing the urea
dust. Dil H2SO4 and Dil HNO3 are also used to
recover dust and ammonia.
For installing dedusting system following
aspects should be considered:
1. Gas flow through prilling towers
Fig-8
design: Air collection duct and down comer 1. An air collection manifold will be fitted
beneath the exis
ting prill tower fan room and supported
from the tower while a stainless steel
down comer duct with free standing
support structure installed to bring the
scrubbed gas into the scrubber;
l scrubber vessel which
contains filtration medium;
Scrubber fan with variable speed drive.
supporting
Vibro priller provided instead of
distributers or prilling bucket to control
on is used for scrubbing the urea
are also used to
For installing dedusting system following
Gas flow through prilling towers
2. Dust and ammonia content in mg/Nm3
3. The temperature of gas. Amvient
temperature maximum and minimum
4. Whether only urea dust or urea dust and
ammonia is to be removed, and in case
the emission of both pollutants should
be controlled, whether they are to be
recovered as one or two separate
streams.
Following equipments are required for
dedusting system
1. Scrubber SS 304 L
2. Blower with motor 3.3 KV
3. Urea solution recycle pump
4. Filter
5. Condensate tank
6. Additives H2SO4 or HNO
7. Condensate/DM water
22nd
Feb. 2022
ction manifold will be fitted
Dust and ammonia content in mg/Nm3
rature of gas. Amvient
temperature maximum and minimum
Whether only urea dust or urea dust and
ammonia is to be removed, and in case
the emission of both pollutants should
be controlled, whether they are to be
recovered as one or two separate
lowing equipments are required for
Scrubber SS 304 L
Blower with motor 3.3 KV
Urea solution recycle pump
or HNO3
Condensate/DM water

Feb. 2022
Conclusion
The existing prilling tower revamp task is
difficult but not impossible. It is necessary for
legally environment law and regulation
according to world standard. An important
consideration in prilling tower design is the
selection of suitable criteria against which the
adequacy of the design results can be tested. A
number of such criteria suggest themselves. The
criterion selected in the present design method
requires that a prilling device obtains
substantially mono-dispersed droplets under
creation a relative quiescent zone near the
showerheads and selected uses the solidifying
time as the key parameter in determining the size
of prilling unit needed. About 0.01 G cal /ton of
energy will be increased for prilling tower
revamp because the 3.3 KV motors are required
for large capacity blowers.
References
1. An advance book on Fertilizers
technology (Pure knowledge) by Prem
Baboo. Published in Notion press.
2. NIIK Russia References.
3. PROZAP Engineering Ltd Puławy,
Poland.
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