module 6-aqm
TRANSCRIPT
Module 6-aqm
Syllabus
Control of Air Pollutants- Particulate emission control-methods, Scrubbing-Cyclones- Filtration- Electrostatic Precipitation-Gaseous emission control- adsorption
NATURAL CONTROL(Natural self cleansing capacity)
The nature itself has a capacity to clean out the pollutants coming into the atmosphere with
natural forces of climate and ecology. The following natural phenomena tries to reduce the
pollutants and remove the pollutants from the air .
Dispersion
Dispersion of the pollutants from the source to larger area by wind so that the concentrations are diluted.
The pollutants emitted through tall chimneys is only a means of spreading our own pollutant around our neighbors
Gravitational settling with or without flocculation
Large heavy SPM from ambient air settle down on building, trees & other surfaces. This process also helps in removing flocculated particles formed in the atmosphere
Gravitational settling with or without flocculation
Large heavy SPM from ambient air settle down on building, trees & other surfaces. This process also helps in removing flocculated particles formed in the atmosphere
When the pollutants emitted to the atmosphere are more than the natural self cleansing capacity
of the atmosphere, the pollutants remain in the air for long periods in a sustainable manner
which result into various ill effects to all the living & non living things on the earth.
Under these circumstances we have to impose control measures to limit the emission into the
atmosphere.
CONTROL MEASURES.
PREVENTION The best control strategy is the prevention of emissions Investigate different approaches in process design Select the process which do not contribute pollution or have minimum
pollution potential Source Correction methods
Raw material change, operational change, modification of process equipment, replacement of equipment etc
Raw material Change
Use purer grade of raw materials & reduce formation of undesirable pollutants as by-products
Ex: use of low Sulphur fuel instead of high sulpur fuel Burning of natural gas instead of Coal
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Fuel desulphurization
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Coal gasification- sulpur & other unwanted materials can be removed from gas coal rathen than ferom solid coal
This can be done by destructive distillationof coal coke is left as bye product
In all the above process sulpur is recovered by passing the gas coal through an absorbing medium.
Process change
Use of new modified techniques Pertoleum refining Volatile susbstances are recovered by condensation Noncondensable gases are recycled Hydrogen sulphide is recycled and used to derive elemental Sulphur
Dust control in Rotary kilns By adjusting operating conditions in Rotary kilns of cement factories
the dust control can be achieved Reduction of gas velocities in Kilns Modification in rate & location of feed introduction Employment of dense curtain of light weight chain at discharge end
Smelting in Paper industries High pollution potential This is replaced by hydro metallurgical separation of ores Use of sulphides in paper making
Steel Industry Sulphur bearing fuel, lime stone & air are injected into a molten iron
bath the combustible matter are oxidized to carbon dioxide The CO coming out is burnt efficiently
Combustion Chambers Reduction of formation of Nitric Oxide in Combustion Chambers by low
excess air combustion in two stages Flue gas recirculation &water injection
Equipment Modification or replacement
Automobiles
Unburnt hydrocarbons & CO can be burnt by injecting air into hot exhaust manifolds
Modification of Carburetors Petroleum Refineries
Direct evaporation of HCs from storage tanks and filling can be minimized by floating roof tanks or by pressurizing the tanks
Use of Electric engines instead of combustion engines completely avoid automobile pollution
Steel industry
Replacement of open hearth furnace by oxygen furnace
EMISSION CONTROL
Control measures- Stationary Sources
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!)what are the advantageous and disadvantageous of electrostatic precipitator
Removal of particulate matter
Various equipment’s used are
1)Scrubbing
2)Cyclones-
3)Electrostatic Precipitation
4)Filtration
Scrubber
Scrubber systems are a diverse group of air pollution control devices that can be used to
remove some particulates and/or gases from industrial exhaust streams. "Scrubber" is a
pollution control devices that use liquid to wash unwanted pollutants from a gas stream.
Scrubbers are one of the primary devices that control gaseous emissions, especially acid gases,
Fumes. Industrial Scrubbers can also be used for heat recovery from hot gases by flue gas
condensation. Scrubber are used for scrubbing the abnoxious fumes such as silicon tetrafluride,
HNO3, HCl, NH3, Phosphoric acid, super phosphate & Fluorine. Manufacturing using quality R.M
these are available in various dimensions & specifications. As per the client requirement this
systems can also be used for recovery of the products.
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Different type scrubbers
(Wet scrubbers and dry scrubbers )
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Liquids are used for scrubbing activities
This is used by industries
Wet scrubbers have lot of merits compared to wet scrubbers
Simultaneous removal of particles and gaseous pollutants
But corrosion is a disadvantage
Different type of scrubbers
1)spray tower
2)Centrifugal scrubbers
3)packed beds and plate columns
4)venturi scrubbers
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Centrifugal scrubber
Centrifugal scrubbers
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Packed beds and Plate columns
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Venturi scrubbers
main purpose of the venturi scrubber is to decrease the particle concentration in the flue gas.
It has 3 main sections; namely, Convergent section, Venturi throat, Divergent section
Gas enters from the top of the unit and moves in the same direction with water
Main reduction is achieved in the throat section of the unit.
For efficient removal gas is passing through the stack with very high speed.
There is a high pressure drop in this unit. The main reason is high speed difference through the venturi.
Even small particles can be removed by venturi scrubber.
In addition to the particle reduction, temperature decrease is achieved in the venturi scrubber.
It has very high removal rates when it is compared with the other treatment methods.
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Suitable for high performance removal of fine particles
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Advantage of wet scrubber over dry Scrubber
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How to calculate the efficiency of a venture scrubber –see
how to do the problem
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ρg is taken as zero in the above equation
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Advantages
Spray Tower & Ventury Scrubber can be used to remove gas pollutants also
Hot gases can be cooled down
Corrosive gases can be recovered & neutralized
Useful by-products can be generated when aqueous chemical solutions are used
Disadvantages
Water requirement is high
Slurry of SPM & water coming out need to be treated & disposed
Maintenance cost is high
Wet outlet gases cannot rise high from the stacks
Poses freezing problem in cold countries
Plume ( Gases coming out from Chimneys) contain water vapour & may be viscible
Selection criteria for equipment for particulate
removal
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Electrostatic precipitator An electrostatic precipitator (ESP) removes particles from a gas stream by using electrical energy to charge particles either positively or negatively. The charged particles are then attracted to collector plates carrying the opposite charge. The collected particles may be removed from the collector plates as dry material (dry ESPs), or they may be washed from the plates with water (wet ESPs). ESPs are capable of collection efficiencies greater than 99 percent. An ESP is primarily made up of the following four components: gas distribution plates, discharge electrodes, collection surfaces (either plates or pipes) and rappers. The gas distribution plates consist of several perforated plates which help maintain proper flow distribution of the entering gas stream. The discharge electrodes are divided into fields. Most ESPs have three or four fields in series; however, very large units may have as many as fourteen fields in series. Discharge electrodes are energized by a single transformer-rectifier (T-R) set power supply. The energized electrodes create ions that collide with the particles and apply the electrical charge to the particles contained in the incoming gas stream. The collection plates or pipes provide the collection surfaces for the charged particulate matter. The rapping system is responsible for removing the collected particulate matter from the collection surfaces. ESPs are generally classified as dry ESPs (the most commonly used) and wet ESPs. The primary difference between the two classifications is the method by which the collector plates are cleaned. In dry ESPs, the collector plates are cleaned by applying mechanical impulses or vibration to the plates, which knocks loose the collected particulate matter (referred to as rapping). In wet ESPs, the collector plates are cleaned by rinsing with water. Wet ESPs are typically employed when gas streams contain sticky particles with low resistivity. ESP performance can be affected by particle resistivity. Particle resistivity is the property that influences the deposition and removal of particles from the collection plates. The desirable situation is to have particles that conduct away some of their charge once they reach the plate, so that the deposition of other particles is not inhibited, but retain enough of their charge to lightly hold them to the plate. The characteristic is termed moderate resistivity. If the particles have very high resistivity, they are slow to conduct away their charge, causing a negative charge to build up on the plates that inhibit other particles from depositing. If the particles have very low resistivity, they rapidly lose their charge when reaching the plate and pick up the charge of the plate, causing them to be repelled back into the gas stream where they are recharged negatively.
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The plates can be circular or rectangular
qp=charge possessed by the particle
EC=electric field present
C= cunnighams coefficient given by
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Probem
qp is the charge in the particle Ec is the C is the cunnigham
correction factor given as below
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First part of the problem
No of channels 10
Taking area on both sides Ac=2(2x2)=8m2
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Advantageous and disadvantageous of electrostatics precipitator
Sl no advantageous disadvantageous
1 Pressure drop and hence power requirement is small compared to other instruments
relatively high initial cost and large space requirement
2 99%effciency can be obtained, small particles can be collected wet or dry
sensible to variable particulate loading or flow rats
3 can handle both gases and mists for high volume flow safeguard of operating staff from high voltage necessary
4 for moving parts can be operated at high temperature and pressure
collection efficiency can be reduced on usage
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Cyclone Separator
A cyclone separator consists of a cylindrical shell, conical base, dust hopper and an inlet where the dust-
laden gas enters tangentially. Under the influence of the centrifugal force generated by the spinning gas,
the solid particles are thrown to the wall of the cyclone as the gas spirals upward at the inside of the
cone. The particles slide down the walls of the cone and into the hopper. The operating efficiency of a
cyclone depends on the magnitude of the centrifugal force exerted on the particles. The greater the
centrifugal force grater is the efficiency
Cyclone collector is a closed chamber in which a vortex motion is created
The particulate matter is thrown out to the walls of chamber due to centrifugal action of
thvortex flow
These particulates slide down to the bottom hopper and removed
The efficiency depends on the centrifugal force
Relatively cost effective& Simple
Area requirement is less
Continuous dry disposal of dusts
Low- moderate Pressure lossLarge Volumes Can be handled at high temperatures upto 90 0C
Used in cement, petroleum, fertilizer, Asphalt, Grain & Cotton industries
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Formula for efficiency
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The efficiency depends on the centrifugal force
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ϴ1=2πNe where Ne=6 for all practical cases Ne is constant
dpc is the cut size of the particle that are collected with 50 % efficiency
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Types of filters
Filtration is the oldest method for removal of particulate matter from air . two types of filters are
1)packed filter also called fibrous or deep bed filters
2)fabric or cloth filter or bag house filter
In a packed filter fibres are loosely packed inside the enclosure where the dust laden gas takes a long path on its way through the filter .In a fabric filter fibers are woven in to a thin layer of fabric (bag house )usually made from natural or synthetic , metal or glass fibers . fabric filters are mainly used in industry whereas packed filtres are used for air conditioning
Mechanism of filtration Inertial impaction
It is due to departure of the particle from the fluid stream line around the fibre .this allows the particle to be deposited on the curved suface( see figure below)
Direct interception
Less inertia almost follow the streamline of the fluid but its outer peripheries come in contact with the fibre
Diffusion
It is applicable for very small particles ,having less inertia force .usually it does not follow the stream line but zig zag movement cause th particle to adhere to the surface of the fiber
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fabric or cloth filter or bag house filter
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Bag Filter is one of the most common techniques to collect particulate matter from the
gases. Filters are reliable, efficient and economic method to remove particulates from gas
stream. Those filters consist of numerous long vertically hanging bags. Filtration efficiencies
are very high across a wide range of particle sizes. Compatibility of the filter medium with
the characteristics of the flue-gas and the dust, and the process temperature of the filter
are important for effective performance..
In continuous operation, there is gradual loss of pressure across the filtering media due to
the deposit of particles. When dry sorption systems are used, the formation of a cake on the
media helps to provide the acid removal.
. In bag filter unit, the bags are supported on metal wire cages that are suspended from the
top of the unit. Particulate-laden gas flows around the outside of the bags, and a dust cake
accumulates on the exterior surfaces. When cleaning is needed, a very-short-duration pulse
of compressed air is injected at the top inside part of each bag in the row of bags being
cleaned. The compressed air pulse generates a pressure wave that moves down each bag
and, in the process, dislodges some of the dust cake from the bag.
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Methods used for cleaning of filters
1)
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Control of Gaseous Pollutants
Various methods used are
Combustion
Useful for gases containing organic matter
During combustion it is converted as carbon dioxide and water –types
combustion equipment's
Catalytic combustion
Catalytic combustion is a chemical process which uses a catalyst to speed desired oxidation reactions of a fuel and so reduce
the formation of undesired products, especially pollutant nitrogen oxide gases (NOx) far below what can be achieved
without catalysts. The process was discovered in the 1970s at Engelhard Corp.
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Absorption
Packed column
Packed column. In industry, a packed column is a type of packed bed used to perform separation processes, such as
absorption, stripping, and distillation. A packed column is a pressure vessel that has a packed section
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Plate tower
The plate tower scrubber consists of hollow shell which is divided laterally by a number of perforated plates.The operation
is counter-current, with the scrubbing liquid being introduced onto the plate.
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The gas passing through the holes intermixes with the liquid layer, and it is in this highly turbulent zone that dust collection
occurs.
Bubble cap plate tower
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Adsorption
In adsorption process the contaminant removal is done by passing a stream of effluent gas through a
pours solid material (adsorbent) contained in adsorption bed. The surface of porous solid material
attracts and holds the gas (the adsorbate) by either by physical or chemical
adsorption. The basic difference between physical and chemical adsorption is the manner inwhich the
gas molecule is bonded to the adsorbent.
Salient features of adsorption
Adsorption processes are used extensively on large-scale applications having solvent vapor concentrations
in the range of 10 to 10,000 ppm.
(2) Prior to becoming saturated with the solvents, the adsorbent is isolated from the gas stream and treated to
drive the solvent compounds out of the solid adsorbent and into a small volume, high concentration gas
stream.
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(3) The desorbed gas stream is then treated to recover and reuse the solvents.
(4) The adsorbent is cooled (if necessary) and returned to adsorption service.
(5) Because the adsorbent is treated and placed back in service, these adsorption processes are
termed regenerative.
(6) Adsorption processes usually operate at efficiencies of 90% to 98% over long time periods.
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