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Air Pollution Control Devices

Air Pollution Control Methods • • •

Prevent the formation of pollutants. Minimize their emission at the source itself. Control of the pollutant at the source can be accomplished in several ways: – – – –

Raw material changes. Operational changes. Modification or replacement of process equipment. More effective operation of existing equipment.

Raw material changes • Use of a purer grade of raw material. • Use of good quality of fuel.

Operational changes • New or modified techniques.

Modification or replacement of process equipment.

Air pollution • • • • •

Particulate Emission Control of Gaseous Emissions: Absorption by Liquid Adsorption by Solid Combustion.

Air pollution control equipments • • • • • •

Particulate Emission Control: Gravitational settling Chambers CYCLONE SEPARATORS Fabric Filters Electrostatic precipitators. Wet scrubbers.

Control of Gaseous Emissions: • Absorption by Liquid • Adsorption by Solid • Combustion.

Selection of control equipment • Quantity of gas to be treated. • Concentration of particulate matter to be removed. • Temperature and pressure of the gas stream • Efficiency of removal of particulates required.

Efficiency of a separating device

Qty of particulates collected from the gas ď ¨ď€˝ Qty of particulates initially present in the gas

Gravitational settling chamber • Generally used to remove large, abrasive particles (usually >50 μm) from gas streams. • They offer low pressure drop and require simple maintenance. • Efficiencies are quite low for particles smaller than 50 μm. • In the settling chamber the gas stream, with solid particles, is allowed to flow at a low velocity (< 2 m/s), allowing sufficient time for the particles to settle down.

Advantages • • • •

Low initial cost Simple construction Low maintenance cost Low pressure drop

Disadvantages â&#x20AC;˘ Large space requirements â&#x20AC;˘ Only comparatively large particles can be collected (used to remove particulates above 40 Âľm in diameter)

CYCLONE SEPARATORS â&#x20AC;¢ A cyclone separator is a very useful piece of equipment for the removal from air streams of particles above 10 micrometer in diameter.

Operating principles of cyclone separators • the gas enters a cylinder tangentially. • it spins in a vortex as it proceeds down the cylinder. • A cone section causes the vortex diameter to decrease until the gas reverses on itself and spins up the center to the outlet pipe or vortex finder. • Dust particles are centrifuged toward the wall. • The collected dust flows down.

Electrostatic precipitators â&#x20AC;˘ The electrostatic precipitator is one of the most widely used devices for controlling particulate emissions at industrial installations ranging from power plants, cement and paper mills to oil refineries.

Features â&#x20AC;˘ Utilize electric energy â&#x20AC;˘ Used for removing fine dust.

â&#x20AC;˘ Electrostatic precipitation is a physical process by which particles suspended in gas stream are charged electrically and, under the influence of the electrical field, separated from the gas stream. â&#x20AC;˘ The precipitation system consists of a positively charged (grounded) collecting surface and a high-voltage discharge electrode wire suspended from an insulator at the top and held in position by a weight at the bottom. At a very high DC voltage of the order of 50 kV, a corona discharge occurs close to the negative electrode, setting up an electric field between the emitter and the ground surface.

Collection Efficiency â&#x20AC;˘ The collection efficiency of an electrostatic precipitator as a function of gas flow rate and precipitator size. â&#x20AC;˘ A particle that has entered the precipitator and received an electric charge moves towards the collection electrode.

Four steps • Place the charge on the particle to be collected. • Migrate the particle to the collector. • Neutralize the charge at the collector. • Remove the collected particle.

v z y



vpm dX L

Advantages • High collection efficiency. • Particles as small as 0.1µ may be removed. • Low maintenance and operating costs • Less pressure drop. • Treatment time is negligible.

Disadvantages • High initial cost • Space requirement is more because of large size of the equipment. • Possible explosion hazards during collection of combustible gases or particulates.

Filters Filtration is one of the efficient and economic method of removing particulate matter. â&#x20AC;˘ Fabric or cloth filter (in the form of fabric bag arrangement i.e. tubular bags) â&#x20AC;˘ Fibrous or deep bed filter (a fibrous medium like mats of wool act as a separator)

Fabric or Cloth filter • Most common type is tubular type consists of tubular bags • Bag diameter: 120 – 400 mm and 2-4 m long. • Hopper at the bottom

Fabric and Fiber Characteristics â&#x20AC;˘ The choice of fabric is based primarily on operating temperature and the corrosiveness of the particles. â&#x20AC;˘ Cotton is the least expensive, and is used in low temperature dust collection. â&#x20AC;˘ Silicon coated is commonly employed in high temperature applications.

Fabric Filter system â&#x20AC;˘ Fabric filter system typically consist of a tubular bag, suspended or mounted in such a manner that the collected particles fall into a hopper when dislodged from fabric. â&#x20AC;˘ The structure in which the bags hang is know as a baghouse.

â&#x20AC;˘ Dusty gas enters the bag at the bottom and passes through the fabric while the particle are deposited on the inside of the bag. â&#x20AC;˘ Cleaning is accomplished by shaking at fixed intervals of time.

â&#x20AC;˘ The bags should be arranged in each compartment in such a manner that the available space is utilized effectively and proper access to each bag is provided for its replacement. â&#x20AC;˘ The cleaning may be accomplished by shaking the bags or by increasing the air pressure on the bag in a manner that causes the bag to collapse to dislodge the accumulated dust.

The following advantages make fabric filters the best choice in many cases: • • • • • •

Very high efficiencies. Retention of finest particles. Collection of particulates in dry form. Relatively low pressure drop. Simple construction and operation. Nominal power consumption.

Disadvantages • • • •

Their large size. High construction costs Operation temperature below 285 C. High maintenance and fabric replacement costs.

Techniques for controlling gaseous emissions • Recovery technique • Destruction technique

Recovery Technique • Absorption • Adsorption

Destruction Techniques • Thermal Combustion • Catalytic Combustion

Recovery Techniques ABSORPTION • Gas-Liquid operation • It involves the transfer of the pollutant from the gas phase to liquid phase by using an appropriate solvent. • Two types of towers are generally used i.e. tray tower and packed tower. • Effectiveness of an absorption process for air pollution control mostly depends upon proper selection of solvent.

Suitable solvents for various gaseous pollutants 1. Sulphur dioxide 2. Nitrogen Oxides 3. HCl

NaOH, Na2SO3, MgO Ca(OH)2, Mg(OH)2 Water, Ammonia solution

Gas Out Conc. of pollutant in gas phase x1 Solvent in Conc. of pollutant in liquid phase C1 =0

Here x1>x2 & c2>c1

Gas in Conc. of pollutant in gas phase x2

Liquid Out Conc. of pollutant in liquid phase C2

Adsorption by solids • Gas-Solid Operation • Adsorption is the surface phenomenon by which pollutant gas gets captured by and adhere to the surface of a solid adsorbent. • The adsorbed material can be desorbed or removed by increasing the temperature. • This technique is widely used in removing small quantities of pollutants present in a large volume of air.

Adsorption by solids • This technique is used for collecting valuable organic substances. • The rate of adsorption depends on the concentration of material around the adsorbent and the surface area of the adsorbent. • Adsorption of solids may be carried out in a batch wise or continuous manner of operation.

Fixed Bed Adsorption Column D Clean gas out


Pollutant gas in Conc. of pollutant gas in = C 1

Combustion • It is a destruction technique • Many industrial processes produce gas streams that have no recovery value, so adsorption and absorption methods may not be economically feasible. • If waste gas contains sufficient combustible material then incineration/combustion is the best option.

Types of Combustion • Thermal Combustion • Catalytic Combustion

Thermal Combustion • Most efficient technique for destroying diluted gas streams. • Here the waste gas is preheated over an auxiliary fuel-fired burner and passed into a combustion chamber where a temperature of around 700°C is maintained. • At this temperature the complete oxidation/destruction takes place. • The major operating cost is in the form of fuel required to sustain appropriate temperature levels.

Catalytic Oxidation • This technique is used when thermal combustion of very diluted pollutants is not economically feasible due to additional fuel cost. • Here combustion takes place on a catalyst usually at lower temperatures (i.e. at around 400°C)

Other Techniques • Condensation • Membrane Separation • Bio-filtration

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