The removal of one or more selected components from a gas
mixture by absorption is probably the most important operation in
the control of gaseous pollutant emissions.
Absorption is a process in which a gaseous pollutant is dissolved in a
Water is the most commonly used absorbent liquid. As the gas
stream passes through the liquid, the liquid absorbs the gas, in
much the same way that sugar is absorbed in a glass of water when
Absorption is commonly used to recover products or to purify gas
streams that have high concentrations of organic compounds.
Absorption equipment is designed to get as much mixing between
the gas and liquid as possible.
Absorbers are often referred to as scrubbers, and there
are various types of absorption equipment.
The principal types of gas absorption
1. spray towers,
2. packed columns,
3. spray chambers, and
4. venture scrubbers.
The packed column is by far the most commonly used for
the absorption of gaseous pollutants.
The packed column absorber has a column filled with an inert
(nonreactive) substance, such as plastic or ceramic, which
increases the liquid surface area for the liquid/gas interface.
The inert material helps to maximize the absorption capability
of the column. In addition, the introduction of the gas and
liquid at opposite ends of the column causes mixing to be
more efficient because of the counter-current flow through
the column. In general, absorbers can achieve removal
efficiencies grater than 95 percent.
One potential problem with absorption is the generation of
waste-water, which converts an air pollution problem to
a water pollution problem.
1. Absorbent: the liquid, usually water mixed with neutralizing
agents, into which the contaminant is absorbed
2. Solute: the gaseous contaminant being absorbed, such as SO2,
H2S, and so forth
3. Carrier gas : the inert portion of the gas stream, usually flue
gas, from which the contaminant is to be removed
4. Interface : the area where the gas phase and the absorbent
contact each other
5. Solubility : the capability of a gas to be dissolved in a liquid
Adsorption is used when
1. The pollutant gas is incombustible or difficult to burn
2. The pollutant is sufficiently valuable to warrant recovery
3. The pollutant is in very dilute concentration in the exhaust
The technique is based on the reaction of gases on the solid
The adsorption may be physical or chemical.
In this method gas is passed through a bed of adsorbents
packed in the specially designed towers to allow the
maximum contact between the two
Physical adsorption depends on the temperature and pressure
conditions. Adsorption is promoted by increase in
pressure and decrease in temperature
Chemical adsorption depends on the reactivity of the gases
and their bond forming capacity with the surface of the
adsorbent, which provides surface for the reaction.
Adsorbent can be regenerated for continuous
In some cases if is not economical to regenerate, it better to
dispose the pollutant together with the adsorbent
Adsorption is mostly pollutant specific.
e.g – Activated carbon, silica gel and diatomaceous earth
are suitable for adsorption of water vapours from a gas
phase. It can also adsorb SO2 and NH3.
Activated carbon is most suited for removal of organic
gases from gas stream.
Carbon adsorption systems are either regenerative or
Regenerative system usually contains more than one
carbon bed. As one bed actively removes pollutants,
another bed is being regenerated for future use.
Non-regenerative systems have thinner beds of activated
carbon. In a non-regenerative adsorber, the spent carbon
is disposed of when it becomes saturated with the
In many cases it is not possible to remove the required
amount of specific pollutant from an exhaust stream by
techniques such as absorption or adsorption.
The other technique available is Combustion
Combustion refers to rapid oxidation of substances (usually
referred as fuels) with evolution of heat. To summarize,
Combustion is defined as rapid, hightemperature gas-
Simply, the contaminant (a carbon-hydrogen substance) is
burned with air and converted to carbon dioxide and water
Combustion process involves three distinct components.
1. Fuel : -A solid, liquid or gaseous substance with energy rich
C-C or C-H bonds among others, which are broken up during
2. Oxidant:- A substance which aids in combustion process by
breaking the chemical bonds allowing the release of heat
3. Diluent:- A substance that does not take part in the
combustion process but acts as carrier of the fuel or the
oxidants. Most common diluents is Nitrogen present in the
Combustible gases are burned in open air, which produces
The flare is usually employed to remove hydrocarbons and
organic vapours, odorous compounds in refineries and
It can also burn gases such as NH3, HCN or other toxic or
If aromatic hydrocarbons are present, they burn with Smokey
flame. This can be avoided by injecting a steam into the flame,
which reacts and forms hydrogen and CO both burn
However such steam-injected flare are little noisy