Air Treatment

Industries make a major contribution towards causing air pollution. Formation of pollutants can be prevented and their emission can be minimized at the source itself. By carefully investigating the early stages of design and development in industrial processes e.g., those methods which have minimum air pollution po¬tential can be selected to accomplish air-pollution control at source itself like Substitution of raw materials, Process Modification, Modification of Existing Equipment, Maintenance of Equipment. Sometimes pollution control at source is not possible by preventing the emis¬sion of pollutants. Then it becomes necessary to install pollution control equip¬ment to remove the gaseous pollutants from the main gas stream. The pollutants are present in high concentration at the source and as their distance from the source increases they become diluted by diffusing with envi¬ronmental air.

Air Treatment Process:

(1) Gravitational Settling Chamber:

For removal of particles exceeding 50 µm in size from polluted gas streams, gravitational settling chambers are put to use. Settling chambers use the force of gravity to remove solid particles. The gas stream enter the chamber where the velocity of the gas is reduced. Large particles drop out of the gas and are recollected in hoppers. Because settling chambers are effective in removing only large particles, they are used in conjunction with a more efficient control device.

The particulates having higher density obey Stoke’s law and settle at the bot¬tom of the chamber from where they are removed ultimately.

(2) Cyclone Separators (Reverse flow Cyclone):

Instead of gravitational force, centrifugal force is utilized by cyclone separators, to separate the particulate matter from the polluted gas. Centrifugal force, several times greater than gravitational

force, can be generated by a spinning gas stream and this qual¬ity makes cyclone separators more effec¬tive in removing much smaller parti¬culates than can possibly be removed by gravitational settling chambers.

(3) Fabric Filters (Bag house Filters):

In a fabric filter system, a stream of the polluted gas is made to pass through a fabric that filters out the particulate pollutant and allows the clear gas to pass through. The particulate matter is left in the form of a thin dust mat on the insides of the bag. This dust mat acts as a filtering medium for further removal of particulates increasing the efficiency of the filter bag to sieve more sub-mi-cron particles (0.5 µm).

force, can be generated by a spinning gas stream and this qual¬ity makes cyclone separators more effec¬tive in removing much smaller parti¬culates than can possibly be removed by gravitational settling chambers.

(4) Electrostatic Precipitators:

An ESP is a particle control device that use electrical forces to move the particles out of the flowing gas stream and onto collector plates. The ESP places electrical charges on the particles, causing them to be attracted to be oppositely charged metal plates located in the precipitator.

The particles are removed from the plates by “trapping” and collected in a hopper located below the unit. The removal efficiency for ESPs are highly variable however for very small particles alone, the removal efficiency is about 99%. This type of equipment is mainly used in industries like cement (dust), pulp and paper (salt cake & lime dust), petrochemicals (sulfuric acid mist) and steel (dust & fumes).

(5) Wet Collectors(Scrubbers):

In wet collectors or scrubbers, the particulate contaminants are removed from the polluted gas stream by incorporating the particulates into liquid droplets.

							(a) Spray Tower
							(b) Venturi Scrubber
							(c) Cyclone Scrubber
							

(I)Spray Tower:

Water is introduced into a spray tower by means of a spray nozzle (i.e. there is downward flow of water). As the polluted gas flows upwards, the particulates (size exceeding 10 µm) present collide with the water droplets be¬ing sprayed downward from the spray nozzles. Under the influence of gravita¬tional force, the liquid droplets containing the particulates settle to the bottom of the spray tower.

(II) Venturi Scrubber:

Submicron particulates (size 0.5 to 5 µn) associated with smoke and fumes are very effectively removed by the highly efficient Venturi Scrubbers. Venturi Scrubber has a Venturi shaped throat section. A coarse water stream is injected upwards into the throat where it gets atomized (i.e. breaks the water into droplets) due to the impact of high velocity of the gas.

(III)Cyclone Scrubber:

The dry cyclone chamber can be converted into a wet cyclone scrubber by in¬serting high pressure spray nozzles at various places within the dry chamber.

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