UNIT III PARTICULATE SCRUBBES

PARTICULATE SCRUBBERS or WET COLLECTORS Scrubbers are air pollution control devices that use liquid(often water) to remove particulate matter or gases from an industrial exhaust Through a nozzle a scrubbing liquid is atomized and dispersed into the gas stream The droplets capture dust particles and collects the particles in a filter or separation process Many air scrubbers, wet scrubbers and gas scrubbers are available with pre filter and final filter

Pre filter- installed upstream(removes large particles) Scrubber- smaller particles Final filter- installed down stream (catch fine particles)

Dry Scrubber Dry scrubber is used for the removal of odorous and corrosive gases (acid gases- SO2,HCl) – Dry sorbent injector(alkaline material react with acid gas and produces solid salt which are removed in control device) – Dry spray absorbers(packed absorbing tower where acid gases are contacted with alkaline slurry)

Wet Scrubber It removes particulate matter from gas streams by incorporating the particles into liquid droplets directly on contact CaCo3(solid)+So2(gas)  CaSo3(solid)+Co2(gas) Types: – Plate scrubber – Packed-bed scrubber – Spray scrubber – Venturi scrubber – Cyclone scrubber – Baffle scrubber – Impingement scrubber – Fluidized bed scrubber

Plate scrubber(efficiency 50%) – It contains vertical tower containing one or more horizontal plates (trays) – Gas enters the bottom of the tower and must pass through perforations(hole) in each plate as it flows vertically to the water steam

Packed bed scrubber – Operates similarly to packed bed gas absorber – Collection efficiency increases as packing size

Spray scrubber – Particles are collected by liquid drops that have been atomized by spray nozzles – Water is introduced into a spray tower by means of spray nozzle – Under the gravitational force the liquid droplet containing the particles settle to the bottom of the spray tower

Venturi scrubber – A moving gas stream is used to atomize liquids into droplets. – The polluted gas stream passes downwards through the throat at velocity 60 to 180 m/s – The water stream is injected upwards into the throat where it gets atomised (i.e breaks the water into droplets) due to the impact of high velocity of the gas

Orifice or baffle scrubber – It is similar to veturi scrubber in that gas stream flows at high velocity through a narrow section (orifice) to atomize the liquid into droplets – As it exits droplets and pollutants are removed through impingment(collision)on a series of baffles

Cyclone or dynamic scrubber – Drops can be introduced into the gas stream of a cyclone to collect particles

Orientation: – Cross-current or horizontal flow(perpendicular) – Co-current flow (same direction) – Counter current or vertical ( oppposite)

Scrubber model – Stationary – Portable – Skid – Truck Performance specification: Gas flow rate Water flow rate Minimum particle size Collection efficiency

Electrostatic Precipitators The electrostatic precipitator is one of the most widely used collection devices for particulates It is a Paniculate collection device that removes particles from a flowing gaseous stream using the force of an induced electrostatic charge It has electrodes with supply of several thousand volts of electric current, which produce a corona that releases electrons. These electrons attach to particles giving them net negative charge. The collecting plates are positively charged and attract the negatively charged particles.

Electrostatic precipitator is used, which can remove 99% particulate matter(0.1 micrometer to 1micrometer) present in the exhaust of thermal power plants.

Principle of ESP: (four distinct phases) I)Ionization or corona generation – when the potential difference between the wire and electrode increases, a voltage is reached where an electrical breakdown of the gas occurs near the wire. This break down is called corona formation – Negative corona(discharge electrode is negative polarity) – Positive corona (discharge electrode when positive voltage is applied) II)Charging of particles Field charge is dominant for particles with diameters greater than about 1 micron, while diffusion charge is dominant for smaller particles III)Migration and Precipitation IV)Removal of deposited dust(coalescing and draining)

Types of Electrostatic precipitators: Single stage precipitators(charging and collection occurs in same stage) – Plate-wire precipitators(gas flow between parallel plates of sheet metals and high voltage electrode) – Flat plate precipitators(use flat plates instead of wires for the high voltage electrode) – Tubular precipitators(tubular like smokestacks) – Wet precipitators(water is applied to wash particles) Two stage precipitators – Two stage precipitators has separate particle charging and collection stages

Advantages: High collection efficiency Easy cleaning Handles large volume of high temperature gas Disadvantages: High initial cost More space requirements Applications: Pulp and paper mill Cement recovery furnace Petroleum industry