Common air pollution Control equipment :
Method of removal Particulate Matter
1. Gravitational Settling Chamber
2. Cyclone
3. Inertial Dust Separator
4. Wet Scrubber
5. Bag Filter
6. Electrostatic Precipitator
Removal of Gaseous Impurities
1. Absorption
2. Adsorption
3. Condensation
4. Combustion
Gravitational Settling Chamber:
- The settling chamber is the simplest type of equipment used
for the collection of solid particles.
- These are mainly used for the removal of larger particles >
40 micron.
- It consist of chamber in which carrier gas velocity is reduced
so as to allow particulate to settle out of the moving stream
under the action of gravity..
- Polluted gas enter from the inlet where velocity is reduced
by gravitational force and particulate matter start settling down
and clean gas emit from outlet.
2. Cyclone :
Cyclone employ centrifugal force to separate smaller particles those can’t be
removed by gravitational force. Rotaing motion of the gas stream generates
a centrifugal force.
- Cyclone separator may be used at high temperature or at high pressure for
wide range of particles size.
- Cyclone have an efficiency range from 60 to 90 % but may not effectively
remove particles of less then 20 micron size.
Cyclone Collector:
- When the dust ladden air enter tangentially at the inlet point under the
influence of centrifugal force by spinning gas, the solid particles thrown to the
wall of cyclone as gas spiral upward at the inside of cyclone.
- The particles are slide down to the wall of cyclone.
- The outlet pipe for the purified gas is a cylindrical opening at top.
cyclone collector
Installation of Cyclone
Inertial Separator
This type of control equipment includes all collectors which utilize relatively
greater inertia of the dispersoid to effect the particulate – gas separation e.g
Dust Trap
Dust Trap
- It is a common type of inertial separator.
- In this device the dust laden gas is introduced into a central pipe (cylindrical
) and is made to undergo a change in direction by 180 0 .
- Dust because of inertia , settle in conical chamber.
Wet Scrubber:
Wet collector remove particulate matter from gas stream by intercepting the
particles into liquid droplet directly on contact .- Gaseous particles can be
easily removed.
- 0.1 to 20 micron size particles are removed in these units.
- Generally water is used as scrubbing liquid.
Example are:
1. Spray Tower
2. Wet Cyclone Scrubber
3. Ventury Scrubber
4. Multiple Cyclone
Spray Chamber
1.
Spray Tower
- It can be used to remove both gaseous and particulate matter
contamination.
- As the gas flow upward the particles of the gas collide with spray
water and these particles are settled by gravity to the chamber.
2. Cyclone Scrubber:
- In a cyclone scrubber , polluted air is tangentially swirled around
whereas water sprinkles into the chamber continuously.
- In a simple wet cyclone scrubber high pressure spray nozzle located
in various place within a cyclone chamber generate a fine spray that
intercept small particles entrained in swirled gas
- Particles are thrown to the wall by centrifugal force.
Venturi Scrubber:
In a ventury scrubber high velocity (60 – 100 m/s ) at throat section atomise
the liquid.
- Particles strike against moving droplets because of the velocity difference
between the droplets and particles, droplet accelerate in throat section.
- The action is not complete in throat section but continue even afterward till
settling.
- Hence throat length is important for travel and collection of particles.
- This type of scrubbing device is very efficient for the removal of particulates
greater than 0.5 microns, due to better capture between high velocity collision
particles and atomised water droplet.
Ventury Scrubber
Advantages of Wet Collector:
- Reliable with high collection efficiency.
- Highly soluble gases like NH3 , HCl, HF are also removed.
- Remove corrosive chemical dusts that may be difficult for removal in filters
or electrostatic precipitators.
Bag Filter or Fabric Filter :
- Filtration is one of the most reliable, efficient and economic method by which particulate
matter can be removed from gases.
- These filter consist of porous structure material , fabric or fibrous matter which has the
property to retain the particulate matter as the dirty gas passes through the void or
interstitial spaces or layers of filter.
- The gas entering through the inlet pipe strikes a baffle plate which causes the larger
particles to fall into hopper due to gravity.
- The carrier gas then flow upwards through fabric leaving the fabric leaving the
particulate matter on the inside of the bag.
- Bag are usually of 15 cm diameter and 3 m long and maintain 0.5 to 1 m /minutes flow
velocities.
- Common material used are cotton (natural, fiber, cellulose), wool, nylon, telfon, fiber
glass and stainless steel.
- The efficeincy is estimated to be 95 – 99 % for removing fine particles (I micron size) .
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Bag Filter
Electrostatic Precipitator:
- These are widely used in cement, power and metallurgical industries because of
their capabilities to treat large gas flows, under high temperature and corrosive
atmosphere and remove submicron particles.
- In electrostatic precipitator the gas stream is passed between two electrode,
across which a high potential difference is maintained.
- Out of two electrode one is discharging electrode and other one is collecting
electrode.
- Because of high potential difference and the discharge system a powerful
ionisinsg field is formed. Potential as high as 100 kilovolts are used.
- Consequnetly, ionization creates an active glow zone (blue electic discharge)
called the corona glow.
Electro- static Precipitator
Electro static precipitator
The following steps are involved in the functioning of an electro static precipitator:
1. Impart electrostatic charge to particles.
2. Create electric field in the flow region.
3. Particles develop force of attraction.
4. Particulate migrate towards oppositely charged electrode.
5. Electrode collect the impurities.
6. Particulate dust is removed by shaking or raping electrode.
7. The dry particulate matter is collected in the hopper below.
Basically two type of precipitator are available
1. Pipe Type electrostatic precipitator
2. Plate Type electrostatic precipitator
Corona forms between an active high voltage electrode . It is a mechanism of formation
of ions in the gas.
Dust particles are carried aut through this space, instantaneously become highly charged
and immediately migrate and attach to a collecting electrode
The efficiency of ESP is almost 100 %
,ESP have a number of mechanical part and electrical component and so have a number
of operational and maintenance problem.
Control of Gaseous Impurities
1. Absorption
2. Adsorption
3. Condensation
4. Combustion
1. Absorption:
By selecting suitable absorbent solution it is possible to absorb even minute traces of
many gases and vapour of oxide of N2 , S , H2S, HCl, and ammonia.
- It is suitable method for collection of gaseous and vapour pollutant.
- Pure water : collecting some gasoues pollutant like HF
- Alkaline solution : absorbing acidic gases
- Hydrocarbon
: absorbed in oil
Effluent gas pass through packed beds or liquid absorbent so
that pollutant are removed. Absorption or scrubbing is a
diffusion process in which gas molecule are transferred into
a liquid phase.
When soluble gas contacts solvent liquid, mass transfer take
place due to difference in pressure of soluble gas in gas
mixture and vapour pressure of solute gas in liquid film within
gas.
Water is good solvent for NH3 , HCl, HF, Cl2, NOx etc.
SO2 may be absorbed using lime or limestone
H2S is absorbed by iron oxide granules bed.
Adsorption:
- Air or gaseous pollutant may be removed by adsorption colum which
may contain absorbing material such as Activated Carbon or Silica.
- Activated carbon (manufactured using coconut shell , bituminous coal or
petroleum residues) , siliceous compound ( silica gels, fullers, and
diatomaceous earth, synthetic zeolites) alumina anhydrous CaSO4 , Ca or
Mg silicate are used too. It is best dealing for organic vapour.
- Iron oxide bed removes benezene and H2S.
- Silica gel absobs oxide of N2 and Bromine.
Condensation :
- Condensation of a vapour from dirty air is possible either by
increasing pressure or controlling the temperature. It is
suitable material with low pressures at moderately high
temperature, such as hydrocarbons.
- In a condenser, coolant is circulated in the tubes.
- Air vapor stream circulates outside the tubes and vapor
condensed on the cool surface and drop as liquid.
- This is collected at the bottom of a condenser.
Combustion
- Incinerator are simple, safe and reliable. They are widely
used for combustion of a variety of pollutant gases.
- In this process pollutant are exposed to high temperature
> 650 0C.
- CO and hydrocarbons are oxidized into carbon dioxide at
higher temperature (>650 0C).
- Combustion of organics and gases, with the aid of auxillary
fuel and a catalyst enhances kinetics of combustion.