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Air Washers & Wet Gas Scrubbers Overview
A Safe Way to minimize dust fire and explosion hazards
Inertial separators had the drawback of
dust
particles not accumulating properly on the collecting surface, resulting in
finer particles being swept back into the air stream. Air washers
or wet scrubbers, use liquid (usually water) to absorb the
finer dust particles (scrubbed). Water comes into contact with the dust filled
air stream and captures dust in water droplets. The dirty water droplets
accumulate, and are collected and removed. The cleaned air in air washers
contains fine water droplets forming a mist. These water droplets can be removed
from the cleaned air by a mist eliminator. Mist eliminators are similar in
design to inertial separators where mist and air are separated either by change
in air flow or by the spin in air stream which creates the centrifugal forces.
The dust-water mixture is either cleaned or recycled into the scrubber. The
accumulated dust is removed by using a clarification tank or a drag chain tank,
where dust particles can be cleaned after they settle to the bottom.
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 The diagram to the right shows a
standard
design of a packed bed wet scrubber or spray tower wet scrubber.
The three important operations of air washers or wet gas scrubbers:
- Humidification (of air): This process increases size of fine dust particles
and makes collection easier
- Contact of liquid and dust filled air: This is the key operation that
governs collection efficiency of the scrubber. Water droplets in the path of the
air stream collect dust particles while the air stream flows around it
- Separation of dirty liquid and clean air: During this operation, the
dust-water mixture accumulates, and as they grow larger, they collect into the
hopper.
How do Wet Scrubbers Work?
In the simplest form of air scrubbers, water is sprayed on the interior walls of
the cyclone to improve collection efficiency. Water thus mops up fine dust
particles that otherwise do not stay on the collection surfaces and tend to be
swept back into the air stream. However air scrubbers suffered operational
problems of
(i) keeping the interior walls coated,
(ii) maintaining water distribution on interior walls and
(iii) cleaning mud and sludge from collection interior.
An improvement of this design sprayed water into the polluted air inlet. In
these collectors, the dust-water slurry had to travel a long distance inside the
collector. Another drawback was that slurry droplets were swept upward to the
outlet with the inner vortex that disturbed water dropping on the expansion
chamber. The collection efficiencies of these units was however, much higher
than the dry units.
High collection efficiencies can be achieved in scrubbers by maintaining
velocities of air that are high enough to drive dust through the surface tension
of water droplets. A good design scrubber produced a secondary generation of
water droplets and induced a mist collection section.
Wet scrubbers are normally categorized on the basis of pressure drop (the
difference in pressure between two points in a system, caused by resistance to
flow, and measured in inches water gauge) as the following:
i) Low energy scrubbers (between 0.5 to 2.5 inches wg)
ii) Low medium energy scrubbers (between 2.5 to 6 inches wg)
iii) Medium to high energy scrubbers (between 6 to 15 inches wg)
iv) High energy scrubbers (greater than 15 inches wg).
Low Energy Wet Scrubbers
Dynamic wet precipitators are similar to centrifugal type
dry collectors in design. The difference is that they are equipped with water
sprays and accordingly the blade is designed to handle dust and water. Centered
in the inlet the water sprays spray the blades with water. When dust filled air
stream enters the inlet, water and dust come into contact, mix, and form a
slurry. The slurry is thrown against the walls of the collector due to the
centrifugal force and is collected in the drain. Water and mist (clean air) have
separate outlets, the water going into the centrifugal separator and the mist
into the drain. This type of collectors have limited use due to high wear on the
blades caused by high solid content in dust particles.
Orifice scrubbers are similar to inertial separators. The
difference being that instead of baffles, orifice scrubbers use a water surface.
The air stream changes its direction abruptly after colliding against the water
surface, causing a pressure drop of almost 3-6 inch w.c. Greater dispersion is
achieved when equipped with spray nozzles. Dust particles are captured by water
to from coarse droplets and clean air is separated. Orifice scrubbers can be
installed inside the plant being shorter than other scrubbers.
Design considerations for higher efficiencies:
Some design considerations for greater efficiencies of low pressure scrubbers
are
- By increasing speed of blades, dust particles hit the water surface at a
faster rate
- By separating dust into individual streams, dust stays in contact with water
for longer time (collectors can be designed with orifice plates, orifices of
1/10 to ¼ inch in diameter).
- Orifices designed with smooth spheres on a coarse grid have an effective
scrubbing action
- By increasing water velocity, finer particles can be collected
Limitations of low pressure scrubbers:
One of the main limitations of low pressure scrubbers is that fine particles are
deflected away from water surface due to water surface tension.
High Pressure Scrubbers
 Venturi Scrubbers: A venturi is a tube with a narrow throat
that increases the velocity and lowers the pressure of the liquid passing
through it. In venturi scrubbers the inlet for the dust filled air stream is
shaped like a venturi. When dusty air enters it, the speed of the air stream is
increased to 15000-20000 feet / minute. By adding 4-6 gallons of water pr 1000
cfm of cleaned air at the venturi throat, dust is trapped in slurry to form very
fine droplets. The high air velocity helps fine dust particles to be collected
efficiently. After dust is captured by the water and formed into slurry, mist is
eliminated from clean air stream by mist eliminator. Slurry is directed to the
scrubber outlet.
It is necessary to keep the air stream close to saturation point or in a
‘humidified’ state to prevent dust from escaping as slurry evaporates. This
could be a problem since liberated air normally does not return to the work
environment. Another phenomenon that could occur is that of plumes that form due
to mixing of hot air and cold air when condensed water vapor descends ground
ward due to heavy water droplets, sometimes miles away.
Uses of Wet Scrubbers:
Wet Scrubbers are most often used
- To separate explosive solids from air streams
- Where slurry could be reused (either in other parts of process or sold)
- Where chemical reactions could be generated with scrubbing action
- To absorb air
Wet scrubbers have low capital costs and require small spaces to install. They
can be used to treat high temperature and high humidity air streams and have the
capability to capture both air and sticky particulates. However they have high
operating costs, require a pre-cleaner for heavier dust loadings, cause water
pollution and are likely to erode at high velocities.
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