Saturday, August 6, 2011

Pollution Control

Pollution control is the process of reducing or eliminating the release of pollutants into the enviroment. It is regulated by various environmental agencies which establish pollutant discharge limits for air, water, and land.
Air pollution control strategies can be divided into two categories, the control of particulate emission and the control of gaseous emissions. There are many kinds of equipment which can be used to reduce particulate emissions. Physical separation of the particulate from the air using settling chambers, cyclone collectors, impingers, wet scrubbers, electrostatic precipitators, and filtration devices, are all processes that are typically employed.
Settling chambers use gravity separation to reduce particulate emissions. The air stream is directed through a settling chamber, which is relatively long and has a large cross section, causing the velocity of the air stream to be greatly decreased and allowing sufficient time for the settling of solid particles.
A cyclone collector is a cylindrical device with a conical bottom which is used to create a tornado-like air stream. A centrifugal force is thus imparted to the particles, causing them to cling to the wall and roll downward, while the cleaner air stream exits through the top of the device.
An impinger is a device which uses the inertia of the air stream to impinge mists and dry particles on a solid surface. Mists are collected on the impinger plate as liquid forms and then drips off, while dry particles tend to build up or reenter the air stream. It is for this reason that liquid sprays are used to wash the impinger surface as well, to improve the collection efficiency.
Wet scrubbers control particulate emissions by wetting the particles in order to enhance their removal from the air stream. Wet scrubbers typically operate against the current by a water spray contacting with the gas flow. The particulate matter becomes entrained in the water droplets, and it is then separated from the gas stream. Wet scrubbers such as packed bed, venturi, or plate scrubbers utilize initial impaction, and cyclone scrubbers use a centrifugal force.
Electrostatic precipitators are devices which use an electrostatic field to induce a charge on dust particles and collect them on grounded electrodes. Electrostatic precipitators are usually operated dry, but wet systems are also used, mainly by providing a water mist to aid in the process of cleaning the particles off the collection plate.
One of the oldest and most efficient methods of particulate control, however, is filtration. The most commonly-used filtration device is known as a baghouse and consists of fabric bags through which the air stream is directed. Particles become trapped in the fiber mesh on the fabric bags, as well as the filter cake which is subsequently formed.
Gaseous emissions are controlled by similar devices and typically can be used in conjunction with particulate control options. Such devices include scrubbers, absorption systems, condensers, flares, and incinerators.
Scrubbers utilize the phenomena of adsorption to remove gaseous pollutants from the air stream. There is a wide variety of scrubbers available for use, including spray towers, packed towers, and venturi scrubbers. A wide variety of solutions can be used in this process as absorbing agents. Lime, magnesium oxide, and sodium hydroxide are typically used.

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