Air Pollution and Control : Bag Filter

Sources and types of air pollution in chemical industry.

Air pollution in the chemical industry can come from a variety of sources, including: 

Reaction or distillation facilities

Combustion plants

Oil refineries

                                   

Industry	Major Air Pollutants
Thermal power Plant	NO2 N2O SO2
Steel Industry	CO2 Fluoride
Petroleum Refineries	Smoke, SO2
Metal Smelters	Smoke, NO2 N2O SO2
Fertilizer Plant	NO2, N2O, SO2
Acid Plant	NO2, N2O, SO2
Cement Plant	Smoke, SO2
Soap and Detergent  Plant	Odour
Paper Mills	Odour SO2

                                                                                      

 

 Types of Air Pollution in Chemical Industry:

    When pollutants come directly from a source, they are called primary pollutants. These include particles such as dust and industrial emissions. Secondary pollutants are created when a primary gas-phase pollutant is oxidised by a chemical reaction. For example, when volatile organic compounds (VOCs) are combined with nitrogen oxides (NO_x), ozone and fine particles such as PM_2.5 may be produced. Chemical reactions can also occur on particle surfaces, and substances can condense onto or dissolve into surface layers.

Types of Pollutant	Pollutant	Major sources
Primary pollutants	Carbon monoxide	·         Combustion, including biomass (vegetation) burning in domestic wood heaters, prescribed burns and bushfires, motor vehicles and metal manufacturing
	Nitrogen dioxide (NO2) and nitric oxide (NO), together termed NOx	·         Combination of nitrogen and oxygen during high-temperature combustion of fossil fuels ·         Motor vehicle exhaust (responsible for about 80% of urban NO2) ·         Electricity generation in fossil-fuelled power stations, petrol and metal refining, food processing and other manufacturing industries ·         NOx naturally emitted from lightning activity and from microbial action in soils
	Sulfur dioxide	·         Electricity generation in coal-fired power stations; metal smelting of sulfurous ores, including lead, copper, zinc, aluminium and iron
	Coarse particulate matter (PM10)	·         Burning in domestic wood heaters, prescribed burns and bushfires; mining; other land uses; and road dust ·         Sea salt and windblown dust
	Fine particulate matter (PM2.5)	·         Combustion ·         Chemical processes – can form in the atmosphere during chemical reactions, such as those involving secondary nitrates and sulfates, and secondary organic aerosols
	Volatile organic compounds (VOCs)	·         ndustrial applications involving paints, solvents and evaporative fuels ·         Naturally occurring VOCs emitted from vegetation ·         Cleaning and personal hygiene products
Secondary pollutants	Ozone	·         Atmospheric photochemical reactions of primary pollutants, NOx and VOCs from motor vehicles, industry and natural sources ·         Naturally occurring background ozone and stratospheric intrusions
Hazardous substances	Mercury	·         Mining, metal processing industries and power stations ·         Soil and vegetation, biomass burning and geological sources
	Persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAHs)	·         POPs in pesticides and industrial chemicals, and produced as a by-product of industrial processes ·         PAHs generated during the incomplete combustion of organic materials
Allergens	Pollen, fungal spores	·         Plants and fungi

       

                      Particulate Pollution and Control: 

                      Bag Filter

A bag filter is a type of air pollution control device that uses a series of bags to remove particulate matter from gas streams. The bags are made of a filter media, such as polyester, nylon, or glass fiber, that has small pores that trap the particles. The gas stream flows through the bags, and the particles are captured on the surface of the bags. The bags are then cleaned periodically to remove the captured particles.

A bag filter is an air pollution control device that is commonly used in industrial settings, including boiler houses, to remove particulate matter (PM) from flue gases. PM, also known as soot or dust, is a type of air pollution that consists of tiny particles that can be harmful to human health and the environment.

Bag filters are highly efficient at removing PM from flue gases, and they can achieve removal rates of up to 99.9%. This makes them an effective way to control air pollution from boiler and other industrial processes.

 

Working Principle of Bag Filter:

    The working principle of a bag filter is based on the filtration of particulate matter from a gas stream. The gas stream enters the filter and flows through a series of bags. The bags are made of a filter media, such as polyester, nylon, or glass fiber, that has small pores that trap the particles. The particles are captured on the surface of the bags, and the clean gas flows out of the filter.

                    

Construction of Bag Filter:

This equipment consists of a big metal vessel (baghouse) with a series of fabric bags in compartments, Fig.1.2 the bags are made up of woven cotton, wool, membranes, sintered metal fibers, or ceramic cartridges. The selection is based upon the operating temperature and pressure and stability of the filter medium to these conditions. Filter bags are suspended in an invert position in the vessel. The length of bags varies from 2 to 10 m with a diameter up to 40 cm. The open ends of the bags are attached to the manifold. The number of bags in a vessel varies from 100 to 1000 or more depending on the baghouse. In the bottom, a portion hopper is provided to collect dust held by the filter.

Working of Bag Filter:

The processing gas enters through the inlet pipe that strikes the baffle plate. This striking causes large particles to fall due to gravity in the hopper at the bottom. Carrier gas then flows in an upward direction and leaves through the bags leaving behind fine particulate matter on its interior surface. Normally the filtration velocity of the process gas passing through the filter cloth is about 0.3-2 m/min, and the pressure loss is 1-2 kPa. As the dust layer collected on the surface of the filter cloth becomes thicker, the pressure loss of the filter cloth increases, so the collected dust is intermittently removed. Removing the dust layer is carried out by mechanical shaking, reverse pressure, or pulse jet. In most cases, the dust collection efficiency of fabric filters is 99% or higher, and the dust concentration at the outlet is less than 10 mg/m³. To achieve better efficiency filter bags are cleaned, maintained, and changed intermittently.

Construction of Bag Filter

Advantages of Bag Filter:

1.      A filter bag is the best method amongst all for removing fines from the air.

2.      Electricity consumption is low.

3.      It helps to maintain and protect a healthy environment.

4.      They are simple in construction and operation.

5.      It has versatility and an effective design.

1.      It helps to reduce housekeeping and better product quality.

2.      Filter bags have effective design according to American Ventilation System Standards

3.      High-quality filter  bags have a trouble-free operation.

4.      It has a robust construction.

 

Disadvantages of Bag Filter:

1.      It has limitations for its operation due to high gas temperature and high humidity.

2.      The maintenance cost is high as the fabric used is costly.

3.      The characteristics of fabric change with operating parameters.

4.      Comparatively, it is large.

5.      Condensation of vapors and the presence of hygroscopic material reduce its efficiency.

 

Applications of Bag Filter:

1.      Bag filters are used in industries to separate dust particles from the air.

2.      These are used to clean the air in working areas.

3.      They are extensively used in large industries that produce different kinds of dust such as metals, cement, chalk and lime, ceramics, flour, and foundries.

4.      It is most commonly used in fluidized bed dryers.

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