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Control Methods
• The most effective means of dealing with the problem of air
pollution is to prevent the formation of the pollutants or
minimize their emission at the source itself.
• In the case of industrial pollutants, this can often be
achieved by investigating various approaches at an early
stage of process design and development, and selecting
those methods which do not contribute to air pollution or
have the minimum air pollution potential. These are known
as “Source Correction Methods”.
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• Application of these methods to existing
plants is difficult, but still some of these
correction methods could be applied
without severely upsetting the economy of
the operation.
• Control of the pollutant at the source can be
accomplished in several ways through raw
material changes, operational changes,
modification or replacement of processes
equipment, and by more effective operation
of existing euipment.
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Methods of air pollution control can be divided into
two categories: the control of particulate emissions
and the control of gaseous emissions.
The term particulate refers to tiny particles of matter
such as smoke, soot, and dust that are released
during industrial, agricultural, or other activities.
Gaseous emissions are industrial products such as
sulfur dioxide, carbon monoxide, and oxides of
nitrogen also released during various manufacturing
operations.
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Source Control Technology
• Air quality management sets the tools to control air
pollutant emissions.
• Control measurements describes the equipment,
processes or actions used to reduce air pollution.
• The extent of pollution reduction varies among
technologies and measures.
• The selection of control technologies depends on
environmental, engineering, economic factors and
pollutant type.
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Gravitational Settling Chamber
• Like settling basins in water and
waste water treatment, settling
chambers provide enlarged areas
to minimize horizontal velocities
of air flow thus allow time for
gravity to carry the particle to the
floor.
• These are simple in design and
operation but they require large
spaces for installation and have
relatively low efficiency
especially for removal of small
particles.
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• Settling chambers use the force of gravity to remove
solid particles.
• The gas stream enters a chamber where the velocity
of the gas is reduced.
• Large particles drop out of the gas and are
recollected in hoppers.
• Because settling chambers are effective in removing
only larger particles, they are used in conjunction
with a more efficient control device.
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Howard Settling Chamber
• In the settling chamber
the gas stream, with its
entrained particles, is
allowed to flow at a
low velocity, allowing
sufficient time for the
particles to settle
down.
• Howard type settling
chamber is a more
elaborate .
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• By inserting several trays, the collection efficiency
of the device is improved since the gas flow velocity
in the chamber remains substantially the same and
yet each particle has a much shorter distance to fall
before reaching the bottom of the passage between
trays.
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Centrifugal Collectors
• Centrifugal collectors employ a centrifugal
force instead of gravity to separate particles
from the gas stream.
• Because the centrifugal forces than can be
generated are several times greater than
gravitational forces, particles can be removed
in centrifugal collectors that are much
smaller than those that can be removed in
gravity settling chambers. i.e particles of 10
µm size.
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Cyclone Separator
• Cyclone separators utilise a
centrifugal force generated by a
spinning gas stream to separate
the particulate matter from the
carrier gas.
• The centrifugal force on the
particles in a spinning gas
stream is much greater than
gravity, therefore, cyclones are
effective in the removal of much
smaller particles than
gravitational settling chambers,
and require much less space to
handle the same gas volumes.
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• The general principle of inertia separation is
that the particulate-laden gas is forced to
change direction. As gas changes direction,
the inertia of the particles causes them to
continue in the original direction and be
separated from the gas stream.
• The walls of the cyclone narrow toward the
bottom of the unit, allowing the particles to
be collected in a hopper.
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• The cleaner air leaves the cyclone through
the top of the chamber, flowing upward in a
spiral vortex, formed within a downward
moving spiral.
Cyclones are efficient in removing large
particles but are not as efficient with
smaller particles. For this reason, they are
used with other particulate control devices.
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Venturi Scrubbers
• Venturi scrubbers use a liquid stream to remove
solid particles.
• In the venturi scrubber, gas laden with particulate
matter passes through a short tube with flared ends
and a constricted middle.
• This constriction causes the gas stream to speed up
when the pressure is increased.
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• The difference in velocity and pressure resulting
from the constriction causes the particles and water
to mix and combine.
• The reduced velocity at the expanded section of the
throat allows the droplets of water containing the
particles to drop out of the gas stream.
• Venturi scrubbers are effective in removing small
particles, with removal efficiencies of up to 99
percent.
• One drawback of this device, however, is the
production of wastewater.
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Rectangular throat venturi scrubber
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Table 1. Operating characteristics of venturi scrubbers
Pollutant
Pressure
drop (Δp)
Liquid-to-
gas ratio
(L/G)
Liquid-inlet
pressure
(pL
)
Removal
efficiency
Gases
13-250 cm
of water (5-
100 in of
water)
2.7-5.3 l/m3
(20-40
gal/1,000 ft3
)
< 7-100 kPa
(< 1-15 psig)
30-60% per
venturi,
depending
on pollutant
solubility
Particles
50-250 cm
of water (50-
150 cm of
water is
common)
20-100 in of
water (20-60
in. of water
is common)
0.67-1.34
l/m3
(5-10
gal/1,000 ft3
)
90-99% is
typical
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Filters
• Filters consist of porous structure composed of
granular or fibrous material which tends to
retain the particulate matter as the carrier gas
passes through the voids of the filter.
• Filtration may be by the fabrics or by mats i.e.
fibrous media, felted media, porous media.
• They stop the particles due to impaction,
interception, diffusion, settling, sieving action
and electrostatic attraction forces.
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• These filters may have single layer where dust
particles are collected in the interstitial spaces.
• They may be packed beds where air passes through
the length of a filtering medium.
• Fabric filters require frequent cleaning. Filters have
high collection efficiency, flexibility of operation
and continuous removal is possible.
• These filters may be Bag house filters or
Electrostatic filters.
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Fabric filters, or baghouses
• Fabric filters, or baghouses, remove dust from a gas
stream by passing the stream through a porous
fabric. The fabric filter is efficient at removing fine
particles and can exceed efficiencies of 99 percent
in most applications.
• The selection of the fiber material and fabric
construction is important to baghouse performance.
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• The fiber material from which the fabric is made
must have adequate strength characteristics at the
maximum gas temperature expected and
adequate chemical compatibility with both the
gas and the collected dust.
• One disadvantage of the fabric filter is that high-
temperature gases often have to be cooled before
contacting the filter medium.
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• Fig. shows a simple bag
house filter. Dirty air passes
through the felt pad and the
solid particles are collected.
• A shaker mechanism helps
in collecting the dust
through the hopper outlet at
the bottom of the filter.
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Fabric filter (baghouse) components
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Electrostatic Precipitators
• Electrostatic precipitators provide mutual
attraction between particles of one electric charge
and a collecting electrode of opposite polarity.
• They may be used to handle large gas volume or
high temperature gases effectively.
• Deposition of material takes place on a grounded
collection surface.
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• Adhesive, cohesive and electrical forces should
prevent reentering of particles into the gas stream.
• Efficiency of collection depends on dust
concentration, collector area, gas flow rate,
electrical field intensity and other reasons.
• 0.05 µm to 200 µm particles sizes also can be
removed.
• These esp devises have the advantage of range of
size of particles collected, complete collection of
both solid and liquid particles and high efficiency
even with small size particle collection.
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• First an electrostatic charge is
imparted to the particles and
an electric field is generated
in the flow region.
• An attractive force is exerted
on the particles and causes
them to migrate towards the
opposite charged electrodes.
• Particles are collected by
gravity, shaking the
electrodes (rapping) helps in
the collection of particles at
the bottom. Dust or slurry is
removed by flushing.
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• For industrial applications, vertical plates exposed
to horizontal gas flow are normally used.
• In this type of collector the gas flows between two
vertical parallel plates between which are suspended
a number of vertical wires held in place by weights
attached at the bottom.
• These wires form the discharge electrode, while the
vertical plates form the collection electrode.
• A unique feature of electrostatic precipitators is that
the separating force is applied directly to the
particles without necessity of accelerating the gas as
is done for all other particulate collection devices.
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Electrostatic precipitator components
