The techniques and devices used to control a gaseous pollutant depend on the properties of the specific gas to be controlled. The techniques or control methods are generally classified as absorption, adsorption, combustion, closed collection and recovery system as well as masking and contraction.
The efficiency of gas absorption depends upon the chemical reactivity of the pollutant gass in the liquid phase. In the gas absorption technique, effluent gases are passed through absorbers (scrubbers), containing liquid absorbents that remove, treat, or modify one or more of the offending constituents in the gas stream.
Gas Adsorption:
In this process, gases react with a solid adsorbent, which takes place when the molecules or atoms adsorbed are concentrated only at the interface of the solid. Gas adsorption is based on the principle of passing effluent gas through solid adsorbers, contained in an adsorption collecting device.
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Adsorption may be physical or chemical in nature. Solid adsorbents of the industrial type are generally capable of adsorbing both organic and inorganic gases. However, their preferential adsorption characteristics and other physical properties make each one more or less specific for a particular application.
As an example, activated alumina, silica gel-6, and their molecular sieves (synthetic, silicate or zeolite) will absorb water preferentially from a gas phase mixture of water vapour and an organic contaminant. Alumina and silica gel are used industrially to dry gases.
Bauxite is used in the treatment of petroleum fractions and drying of gases. Activated carbon is most often used for removal of organic solvent vapours.
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Combustion:
Many organic compounds released from manufacturing operations can be converted to innocuous carbon dioxide and water by combustion. To obtain complete combustion, the proper proportion of oxygen, the right temperature and turbulence must be provided at the right time. Combustion may be categorized as furnace combustion, flare combustion, or catalytic combustion.
Other Controls of Gaseous Pollutants:
In case of petroleum storage tanks, there are condense hydrocarbon vapours in a floating roof tank to prevent loss to the atmosphere. This is referred to as a closed collection and recovery system.