General methods to control gaseous pollutants absorption adsorption combustion

1. Methods to Control Gaseous
Pollutants
Ravi Kumar GARRE

2. Contents covered
 Introduction
 Absorption
• Plate tower
• Packed tower
• Spray tower
 Adsorption
 Combustion
• Flare combustion
• Thermal incineration
• Catalytic combustion

3. Introduction
• A substance in the air which causes harm to humans
and environment is known as pollutant. The pollutants
which are in gaseous form are called as gaseous
pollutants.
• The most common methods for controlling gaseous
pollutants is the addition of add-on control devices to
recover or destroy a pollutant.
• There are three commonly used control technologies
for gaseous pollutants:
 Absorption
 Adsorption
 Combustion (incineration)

4. Absorption
• Absorption is a process in which one state of
matter completely dissolved in another state of
matter.
• absorption is two types: physical absorption and
chemical absorption.
• Generally liquids act as absorbents and pollutant
gases act as absorbates.
• In this process, pollutant gases are passed
through absorbers which contain liquid
absorbents that remove pollutants in gas stream.

5. Absorption
The efficiency of this process depends on
 Amount of surface contact between gas
and liquid
 Time of contact between gas and liquid
 Concentration of absorbing liquid
 Speed of reaction between absorbent and
pollutant gases

6. Absorption
• The absorbents may be either reactive (or) non-
reactive with the pollutants removed by them.
Some of the absorbents may be of regenerative.
• The equipments using the principle of absorption
for the removal of gaseous pollutants includes:
 Packed tower
 Plate tower
 Spray tower

7. Packed tower

8. Packed tower
• Packed tower consists of a
long tower packed with a
suitable inert packing
material such as
polyethylene.
• The absorbent trickles down
from top to downward,
while the gases pass in the
opposite direction from
downward to the top, thus
allowing the maximum
reaction time.

9. Packed tower
• The presence of packing
material makes the
absorbent to trickle down in
thin films to provide
maximum surface area for
contact.
• The packed tower is usually
more economic for corrosive
gases and vapours in view of
the lesser quantities of
corrosion resistant materials
requires for its construction.

10. Plate tower

11. Plate tower
• Plate tower consists of a long
vertical chamber fitted with
perforated circular plates at
equal spacing.
• The gas pass from down to
top of the tower making a
contact with the liquid
present on the each
perforated plate.

12. Plate tower
• The liquid does not fall
through the pores on the
plates as it is held by the
pressure created by the
velocity of the gases.
• The plate towers are the
most suitable when a
frequent cleaning is
required particularly in
case of the liquid which
after absorption contains
high quantities of
particulates and relatively
insoluble and offensive
gases.

13. Spray tower
• Also called as scrubbers.
• It consists of a tower with
spray nozzles, which
generate the droplets for
surface contact.
• Here the liquid droplets
and gaseous stream move
counter current to each
other.
• The contact between gas
and droplets make
absorption possible.

14. Spray tower
• In general smaller the
droplet size, greater the
chance for absorption of
gas.
• Spray tower has less gas
liquid interfacial area so
they are less effective in
removal of gaseous
contaminant.

15. Adsorption
• The phenomenon of concentration or
assimilation of one state of matter (i.e. liquid/
gas) on surface of another state of matter (i.e.
Solid) is called as adsorption.
• In the control of gaseous pollutant emissions, the
pollutant gases act as adsorbates and the solids
act as adsorbents.
• When a gas is brought into contact with a solid it
either enter inside of the solid (or) remain on the
outside surface.

16. Adsorption
• Adsorption may be physical (or) chemical.
• The commonly used solids are: Activated
carbon, Silica gel, Zeolite, Alumina, fuller’s
earth, etc.
• Some times calcium phosphate and calcium
carbonate are also used as adsorbents.

17. Adsorption
• The technique of gas adsorption is based on
the reaction of gases on the solid adsorbent.
• Adsorption is prompted be the increase in
pressure and decrease in temperature.
• Adsorption depend on the available surface
area of the adsorbent.
• The chemical adsorption depends on the
reactivity of the gases and their bond forming
capacity with the surface of the adsorbent.

18. Adsorption Units
• The treatment units used for adsorption of gaseous pollutants on
the solids are called as adsorbers.
• The common design of adsorber is:
 It consists cylindrical/ rectangular chamber
 It should be air tightened
 A bed of adsorbent was placed at centre of the chamber
 The gases are passed from below the adsorbent bed in
vertical chamber
 The gases should sucked/ attracted and adsorbed by
surface of adsorbents
• Generally adsorption units are two types:
 Regenerative type
 Non-regenerative type

19. Adsorption Units
Regenerative type:
• It is a continuous
operation
• It contains two beds of
carbon/ any other
adsorbents
• When one bed is in use,
another bed is being
regenerated for next
operation

20. Adsorption Units
• They have only one bed
of carbon/ any other
adsorbent.
• The spent carbon/
adsorbent bed is
disposed off when it
becomes saturated with
the pollutant.

21. Combustion
• Combustion is an exothermic chemical reaction,
which develops the heat and light at a rapid rate.
So temperature rises considerably.
• This is used when the pollutants in the gas stream
are oxidizable to an inert gas.
• The pollutants like the pollutants like Co,
hydrocarbons can be easily burned, oxidized and
removed from the combustion equipment.

22. Combustion
• If the waste gas contains large amount of combustible
materials then incineration is the simple route to
prevent air pollution.
• The combustion process comprises of destruction of
odours, toxic substances, reactive materials,
prevention of hazards and finally reduction of
atmospheric pollutants.
• The combustion process may be of three types:
1. Direct combustion
2. Thermal incineration
3. Catalytic combusion

23. Combustion
• When a pollutant is to be destroyed, the
combustion must be complete or else
intermediate products shall be formed.
• In order to have complete combustion there
must be sufficient amount of oxygen to come
in contact with the combustible materials at
high temperatures, long residence time, and
adequate turbulence (3Ts).

24. Direct combustion
• Direct combustor is a device in which air and all
the combustible waste gases react at the burner.
• Complete combustion must occur
instantaneously since there is no residence
chamber.
• A flare can be used to control almost any
emission stream containing volatile organic
compounds.
• Studies conducted by EPA have shown that the
destruction efficiency of a flare is about 98%.

25. Thermal incineration
• In thermal incinerators
the combustible waste
gases pass over or around
a burner flame, into a
residence chamber where
oxidation of the waste
gases is completed.
• Thermal incinerators can
destroy gaseous
pollutants at efficiencies
of greater than 99%.

26. Catalytic combustion
• In the catalytic
combustion, a mixture of
diluted organic gases and
oxygen is exposed to a
catalytic surface.
• Commonly used catalysts
are: Platinum, Vanadium,
Palladium and their
oxides.
• Used a temperature and
time of contact less than
that of direct flame
combustion.

27. Catalytic combustion
• Catalysts are usually solids. That are neither
reactants nor products of a reaction but yet alter
the rate of chemical reaction.
• The effect of catalyst is to reduce the
temperature required to oxidize the organic
compounds, and hence the inlet gases need not
be heated to ignition temperature.
• It requires less fuel, less time of contact and low
heat than they required for direct flame
combustion.

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