Electrostatic precipitator ESP are particulate collection devices that utilize electrical energy directly to assist in the removal of the particulate matter. They have been used for the removal of fine dust from all kinds of waste gases with very high efficiency.
The principle on which this equipment operates is that when a gas containing aerosols is passed between two electrodes that are electrically insulated from each other and between which there is a considerable difference in electrical potential.
Aerosol particles precipitates on the low potential electrode. Out of the two electrodes one is a discharging electrode and the other a collecting electrode.
The four steps in the process are
1. Place the charge on the partice to be collected
2. Migrate the particle to the collector
3. Neutralize the charge at the collector
4. Remove the collected particles
An ESP consists of six major components
1. Source of high voltage
2. Discharge and collecting electrodes
3. Inlet and outlet for the gas
4. Hopper for the disposal of the collected material
5. An electronic cleaning system
6. An outer casing to form an enclosure around the electrodes.
The entire ESP is enclosed in a casing
The collection efficiency of the ESP is high approaching 100%.
What are advantages disadvantages and applications of electrostatic precipitator?
1. High collection efficiency
2. Particles as small as 0.1 micro meter can be removed
3. Low maintenance and operating costs
4. Low pressure drop
5. Satisfactory handling of a large volume of high temperature gas
6. Treatment time is negligible (0.1-10 sec)
7. Cleaning is easy by removing units of the precipitator for maintenance and operation
8. There is no limit to solid, liquid or corrosive chemical usage
1. High initial cost
2.Space requirement is more because of the large size of the equipment
3. Possible explosion hazards during collection of combustible gases or particulates
4. Precautions are necessary to maintain safety during operation Proper gas flow distribution, gas resistivity, particulate conductivity, and corona spark over rate must be maintained carefully
5. The poisonous gas ozone is produced by the negatively charged discharge electrodes during gas ionization.
ELECTROSTATIC PRECIPITATOR [ESP] || FUNCTION OF ESP & IT’S COMPONENTS || [Hindi]
Application of ESP in industries are as follows
a. Cleaning the flue gas from cement kilns
b. Recovery of cement dust from kilns
2. Paper and pulp mills
a. Soda fume recovery in kraft pulp mills
3. Steel plants
a. Cleaning blast furnace gas to use it as a fuel
b. Removing tars from coke oven gases
C. Cleaning open hearth and electric furnace gases
4. Non ferrous metal industry
a. Recovering valuable material from flue gases
b. Collecting acid mist
5. Chemical industry
a. Collecting sulphuric and phosphoric acid mist
b. Cleaning various types of gas such as hydrogen, CO2 and SO2
c. Removing the dust from elemental phosphorus in the vapour state
6. Petroleum industry,
a. Recovery of catalyst dust
7. Carbon black industry.
a. Agglomeration and collection of carbon black
8. Electric power industry.
a. Collecting fly ash from coal fired boilers
ESP are used for cleaning air in public buildings, theaters, ships, railway cars and some private homes and club houses.
Explain the mechanism or principle of working of scrubbers for particulate removal
Scrubbers or wet collectors utilize a liquid (generally water) to assist in the removal of particles from the carrier gas stream. The dust is agglomerated with the water and then separated from the gas together with the water.
These can remove materials less than 0.2 microns in diameter to a large particles which can be suspended in the gas phase.
Four major steps are involved in collecting particles. The first of these is transport The particles must be moved to the vicinity of the water droplets which are usually 10 to 1000 times larger.
The second step is collision. The particles must collide with the droplets. The third step is adhesion. Adherence is promoted by surface tension. The fourth step is precipitation, or removal of the droplets containing the dust particles from the gas phase.
Particles are removed from the gas stream by one or a combination of the following mechanisms.
a. Impingement-When gas containing dust is swept through an area containing liquid droplets, dust particles will impinge upon the droplets and if they adhere they will be collected by them. The efficiency is more if the liquid droplet size is 100 to 300 times the size of dust particles.
b. Interception-Particles that move with the gas stream may not impinge on the droplets but can be captured because they brush against the droplets and adhere there. This is known as interception.
c. Diffusion-Diffusion of the dispersoid on to the liquid medium helps in removal of the particulate matter.
d. Condensation-Condensation of the liquid medium vapours on the particulates increases the size and weight of particles. It helps in the easy removal of particulates.
Enumerate the factors on which the collection efficiency of a cyclone depends. Give the merits and demerits of cyclone and its applications.
Increase in collection efficiency will result with an increase in any of the following:
dust particle size, dust particle density, gas inlet velocity, inlet dust loading, cyclone body length (no. of gas revolutions) and ratio of body diameter to gas outlet tube diameter.
On the contrary, collector efficiency will decrease if there is an increase in gas viscosity or density, cyclone diameter, gas outlet diameter, inlet width and inlet area.
Merits of cyclone separator
1. Low initial cost
2. Simple construction and operation
3. Low pressure drop
4. Low maintenance requirements
5. It has no moving parts
6. Continuous disposal of solid particulates
7. They can be constructed of any material which will meet the temperature and pressure requirements and the corrosion potential of the carrier gas stream
1. Low collection efficiencies for the particles below 5-10 microns in diameters
2 Equipment is subject to severe abrasive deterioration
3. Decreasing collection efficiency for decreasing dispersoid concentrations in the gas stream.
Cyclones are used widely for the control of gas borne particulates in such industrial operations as cement manufacture, feed and grain processing, food and beverage processing, mineral processing, paper and textile industries and wood working industries.
In disintegration operations such as rock crushing, ore handling and sand conditioning in industries. They are also used in the recovery of catalyst dusts in the petroleum industry and in the reduction of fly ash emissions.
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