NOVEMBER 2016
| WORLD FERTILIZER |
93
takes place. Temperatures will drop slightly as water volume
increases. This will cause the salt particles to agglomerate
and hence grow in size. The result is that the salt particles
grow sufficiently large enough to allow their capture in the
standard B-GON coalescing mist eliminator.
The same mechanism occurs with acid aerosols of
small droplet size, typically
<
1
µ
m. When an acid, such as
HCl or H
2
SO
4
, forms an aerosol, the droplets formed are by
definition composed of pure acid. The concentrated acid
will have a lower vapor pressure than pure water. Thus,
even if the acid aerosol is initially very fine (note, the CRC
Handbook defines sulfuric acid aerosols as being
0.3 – 4
µ
m in size) and as the acid droplets pass through
the 100% humidity environment in the AEROSEP stage, the
small droplets will tend to attract water and grow in size.
The Kimre SXF scrubber can easily be modified to
include an AEROSEP particle growth stage. Typically, the
strategy of particle removal is to first capture everything
>
3
µ
m in size in one or two stages of KON-TANE packing.
In the AEROSEP stage, submicron sized particles are grown
to a 1
µ
m size (and larger), allowing for capture and
scrubbing in B-GON coalescing mist eliminator. Final
B-GON mist capture is accomplished before removal from
the scrubber.
Wet scrubber selection
Choosing a scrubber for a particular air pollution problem
largely depends on the following criteria:
n
n
Process.
n
n
Type of contaminants/characteristics.
n
n
Separation efficiency.
n
n
Design considerations for:
§
§
Single or stagewise absorption.
§
§
Packing type.
§
§
Pressure drop.
§
§
Liquid (L) to gas (G) ratio.
§
§
Solution recovery.
§
§
Temperature change.
n
n
Flexibility of operation/maintenance.
n
n
Costs (installation and operation).
n
n
Performance history.
Wet scrubber applications
The major contaminants for air pollution in the fertilizer
industry are NH
3
, NO
X
, SO
X
, fluorides, ammonium nitrate,
ammonium phosphate, urea, superphosphates
(normal/triple) and NPK dusts.
Ammonia processes use high- and low-pressure spray
type water scrubbers with a mist eliminator to recover
95 – 96% ammonia. This can further be improved with a
filament type B-GON mist eliminator.
Nitric acid processes can use similar droplet separators
in order to lower NO
X
emission levels below 100 ppm.
Ammonium nitrate/CAN/NPK processes have been
using packed column or venturi cyclonic scrubbers and a
demister for removal of ammonia less than 15 mg/Nm
3
.
However, removal of submicron ammonium nitrate
particles can be difficult using regular scrubbing methods
as has been experienced in urea and mono/diammonium
phosphate (MAP/DAP) processes.
To reduce emissions of ‘aerosols’ and submicron
particles in air, cross/semi-cross flow scrubbers using
KON-TANE, B-GON or AEROSEP filament type media have
been found most suitable for dealing with the following:
n
n
High dust loads.
n
n
‘Aerosol’/submicron size (NH
4
, NO
3
, NH
4
F, NH
4
, HF and
other reaction products in gaseous phase).
n
n
High air volumes from prilling/granulation.
n
n
Fugitive ammonia/fluoride gases.
n
n
Unstable operating conditions.
n
n
Increased maintenance problems.
n
n
Low operating cost.
n
n
Most stringent air pollution limits under the current
and proposed US Clean Air Quality Act.
Industrial experiences
Proven applications related to phosphate fertilizers/acids
plants include the following:
n
n
Rock dryer
: B-GON mist eliminator for maximum rock
particle recovery.
n
n
Fume scrubbers
: KON-TANE packing followed by
B-GON mist eliminator for maximum fluorides and
particulate recovery worldwide.
n
n
Fluosilicic acid scrubbers
: B-GON mist eliminator for
fluorides recovery.
n
n
Evaporators
: B-GON mist eliminator for P
2
O
5
recovery.
n
n
Ammonium phosphate/single/triple super phosphate,
urea or CAN scrubbers
: widely used with KON-TANE
packing media followed by B-GON mist eliminator in
cross/semi-cross flow (SXF) scrubbers for chemical
recovery and minimum fluorides/particulates in exit
air in Australia, Brazil, Canada, China, France, India, the
Philippines, South Korea, the US, etc.
n
n
Sulfuric acid absorption/drying tower
: B-GON mist
eliminators offer superior efficiency in the US and
abroad.
Retrofit applications related to the nitrogenous
fertilizer industry have proved B-GON media to be less
susceptible to fouling, easy to clean and able to reduce
droplet and particulate entrainment from ammonium
nitrate and urea, granulation and prilling towers in the
US.
Conclusion
The selection of a scrubber is critical to tackling an
increased dust load and sub-micron particles or aerosols,
in order to avoid ‘blue haze’ and other visible plumes in
the stack and meet USEPA and European Community (EC)
environmental legislations. Cross/semi-cross flow
scrubbers with a filament media (such as
KON-TANE/B-GON/AEROSEP) have been used
successfully in phosphate and nitrogenous fertilizer
plants to meet these air pollution standards, either as a
primary or secondary unit. Present pollution prevention
by implementing advanced gas cleaning technology has
already led to fluosilicic acid solution recovery in super
phosphate plants and therefore minimisation of wastes
and hazards in the fertilizer industry.
