L 29 final

L-29
Wet scrubbers
Air Pollution and Control
(Elective-I)

Scrubbers
• Scrubbers are devices that remove
particulate matter by contacting the dirty
gas stream with liquid drops. Generally
water is used as the scrubbing fluid. In a
wet collector, the dust is agglomerated
with water and then separated from the
gas together with the water.

• The mechanism of particulate
collection and removal by a scrubber
can be described as a four-step
process.
i) Transport : The particle must be
transported to the vicinity of the
water droplets which are usually 10
to 1000 times larger.

• ii) Collision : The particle must
collide with the droplet.
iii) Adhesion : This is promoted by
the surface tension property.
iv) Precipitation: This involves the
removal of the droplets, containing
the dust particles from the gas
phase.
• Collection mechanism consists of
combination of many mechanisms

Types of scrubbers
i) Spray towers.
ii) Venturi scrubbers.
iii) Cyclone scrubbers.
iv) Packed scrubbers.

• Can be used for removing both
particulates and gaseous contaminants.
• Less pressure loss.
• Can handle large volume of gases
• Gas flows upwards, entrained particles
colloid with droplets sprayed across the
flow passage and liquid droplets
containing the particles settle by gravity
to bottom of the chamber.

• Most efficient in removing particles in the range
of 0.5 µ to 5 µ, that makes them specially
effective in removing submicron particles
associated with smoke and fumes.
• A coarse water spray is injected into throat,
which gets atomized by high gas velocities.
• Liquid droplets colloid with particles in the gas
stream and water particles fall down for later
removal.
• Highly efficient for submicron removal i.e. 90%
efficient.

• High-pressure nozzle sprays are located in
various places within the cyclone chamber
generate a fine spray that intercepts particles
entrained in the swirling gases.
• The particulate matter is thrown on to the
walls by centrifugal action and then drained to
the collection sump.
• For droplets of 100 µ size efficiency
approaches 100%.

• The exhaust stream being treated enters the
bottom of the tower and flows upward over
the packing material.
• Liquid is introduced at the top of the packing
by sprays or weirs, and it flows downward
over the packing material.
• As the exhaust stream moves up through the
packing, it is forced to make many winding
changes in direction, resulting in intimate
mixing of both the exhaust gas and liquid
streams.

Applications of Scrubbers
i) They’re particularly useful in the case
of a hot gas that must be cooled for
some reason.
ii) If the particulate matter is
combustible or if any flammable gas is
present, even in trace amounts, in the
bulk gas phase, a scrubber is preferred
to
an
electrostatic
precipitator.

• iii) Scrubbers can be used when
there are waste water treatment
systems available on the site,
with adequate reserve capacity to
handle the liquid effluent.
iv) Scrubbers are also used when
gas reaction and absorption are
required simultaneously with
particulate control.

Theory Questions
Q1. Write short notes on
 Advantages and disadvantages of
scrubbers
Q2. Explain working principle of scrubbers
and explain spray tower.
Q3. Draw sketches for
1. Venturi scrubber.
2. Cyclonic scrubbers.

L 29 final

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L 29 final