Download 2. Pollution Control Equipments: When the pollutant is present in

2. Pollution Control Equipments: When the pollutant is present in high concentrations in
emissions and they have to be removed at the source.
(i) Removal of Particulate Pollutants:
1.Electrostatic Precipitator:
The most basic precipitator contains a row of thin vertical wires, and followed by a stack
of large flat metal plates oriented vertically, with the plates typically spaced about 1 cm to 18 cm
apart, depending on the application. The air or gas stream flows horizontally through the spaces
between the wires, and then passes through the stack of plates.
A negative voltage of several thousand volts is applied between wire and plate. If the
applied voltage is high enough, an electric corona discharge ionizes the gas around the
electrodes. Negative ions flow to the plates and charge the gas-flow particles.
The ionized particles, following the negative electric field created by the power supply,
move to the grounded plates. Particles build up on the collection plates and form a layer. A jerky
motion will detach the dust particles which collect at the conical bottom.
Fig. Electrostatic Precipitator
2.Baghouse Filter
Most baghouses use long, cylindrical bags (or tubes) made of woven or felted fabric
as a filter medium. For applications where there is relatively low dust loading and gas
temperatures are 250 °F or less, pleated, nonwoven cartridges are sometimes used as filtering
media instead of bags. Dust-laden gas or air enters the baghouse through hoppers (large funnelshaped containers used for storing and dispensing particulate) and is directed into the baghouse
compartment. The gas is drawn through the bags, either on the inside or the outside depending on
cleaning method, and a layer of dust accumulates on the filter media surface until air can no
longer move through it. The bags are cleaned before normal filtering resumes. The filter must be
cleaned periodically.
3.Venturi scrubber:
A venturi scrubber consists of three sections: a converging section, a throat section, and a
diverging section. The inlet gas stream enters the converging section and, as the area decreases,
gas velocity increases (in accordance with the Bernoulli equation). Liquid is introduced either at
the throat or at the entrance to the converging section.
The inlet gas, forced to move at extremely high velocities in the small throat section, shears
the liquid from its walls, producing an enormous number of very tiny droplets.
Particle and gas removal occur in the throat section as the inlet gas stream mixes with the
fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is
forced to slow down.
Venturis can be used to collect both particulate and gaseous pollutants, but they are more
effective in removing particles than gaseous pollutants.
4. Cyclonic separation:
Cyclone separator is a method of removing particulates from an air, gas or liquid stream,
without the use of filters, through vortex separation. Rotational effects and gravity are used to
separate mixtures of solids and fluids. The method can also be used to separate fine droplets of
liquid from a gaseous stream.
A high speed rotating (air)flow is established within a cylindrical or conical container
called a cyclone. Air flows in a helical pattern, beginning at the top (wide end) of the cyclone
and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the
center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too
much inertia to follow the tight curve of the stream, and strike the outside wall, then fall to the
bottom of the cyclone where they can be removed. In a conical system, as the rotating flow
moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus
separating smaller and smaller particles. The cyclone geometry, together with flow rate, defines
the cut point of the cyclone. This is the size of particle that will be removed from the stream with
a 50% efficiency. Particles larger than the cut point will be removed with a greater efficiency,
and smaller particles with a lower efficiency.
(ii) Removal of Gaseous pollutants:
(i) Absorption Techniques: The effluent gas stream containing the pollutants is absorbed in
suitable solvents or scrubbed by the liquid in packed towers, plate towers or venturi scrubbers.
Absorbing Liquids for :
Sulphur dioxide : NaOH, Na2 SO3, Mg(OH)2, CaCO3, CaO and Ca(OH)2 solutions
Nitrogen dioxide : NaOH, Mg(OH)2, Ca(OH)2, NH4HCO3, and ag. HNO3
Hydrogen sulphide : NaOH, KOH, sod. Arsenite solutions & NaOH: phenol (3:2)
Hydrogen chloride: H2 O, NH3, Mg(OH)2, Ca(OH)2 solutions