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L-34
GASEOUS CONTROL
TECHNOLOGIES
Air Pollution and Control
Elective- I
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CONTENTS OF UNIT- VIII
L-32 Principles of removal of gaseous
pollutants, details of incineration, absorptionadsorption systems.
L-33 Vehicular pollution, composition,
quantity and control.Status of air pollution in India, Air pollution
control act and strategy for effective control of
air pollution.
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1) ABSORPTION
The removal of one or more selectedcomponents from a gas mixture by
absorption is probably the most
important operation in the control ofgaseous pollutant emissions.
Absorption is a process in which a
gaseous pollutant is dissolved in aliquid.
Water is the most commonly used
absorbent liquid.
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As the gas stream passes through the
liquid, the liquid absorbs the gas, in
much the same way that sugar isabsorbed in a glass of water when
stirred.
Absorption is commonly used to recoverproducts or to purify gas streams that
have high concentrations of organic
compounds.Absorption equipment is designed to get
as much mixing between the gas and
liquid as possible.
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Absorbers are often referred to asscrubbers, and there are various typesof absorption equipment.
The principal types of gas absorptionequipment include
1. spray towers,
2. packed columns,
3. spray chambers, and
4. venture scrubbers. The packed column is by far the most
commonly used for the absorption ofgaseous pollutants.
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The packed column absorber has acolumn filled with an inert (non-
reactive) substance, such as plastic orceramic, which increases the liquidsurface area for the liquid/gas interface.
The inert material helps to maximizethe absorption capability of the column.In addition, the introduction of thegas and liquid at opposite ends ofthe column causes mixing to be moreefficient because of the counter-currentflow through the column.
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In general, absorbers can achieve
removal efficiencies grater than 95
percent. One potential problem with absorption
is the generation of waste-water, which
converts an air pollution problem toa water pollution problem.
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KEY TERMS
1. Absorbent: the liquid, usually water mixed
with neutralizing agents, into which the
contaminant is absorbed
2. Solute: the gaseous contaminant being
absorbed, such as SO2, H2S, and so forth3. Carrier gas : the inert portion of the gas
stream, usually flue gas, from which the
contaminant is to be removed
4. Interface : the area where the gas phase and
the absorbent contact each other
5. Solubility : the capability of a gas to be
dissolved in a liquid
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SPRAYTOWER
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PlateTower
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PACKED
TOWER
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PACKING MATERIAL USED IN PACKED TOWER
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BUBBLE
CAPPED TRAY
TOWER
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2) ADSORPTIONAdsorption is used when
1. The pollutant gas is incombustible or
difficult to burn
2. The pollutant is sufficiently valuable towarrant recovery
3. The pollutant is in very dilute
concentration in the exhaust system
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The technique is based on the
reaction of gases on the solid
adsorbents.The adsorption may be physical or
chemical.
In this method gas is passed through
a bed of adsorbents packed in the
specially designed towers to allow themaximum contact between the two
Ph i l d i d d h
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Physical adsorption depends on the
temperature and pressure conditions.
Adsorption is promoted by increase in
pressure and decrease in temperature
Chemical adsorption depends on the
reactivity of the gases and their bond forming
capacity with the surface of the adsorbent,which provides surface for the reaction.
Adsorbent can be regenerated for continuous
reuse.In some cases if is not economical to
regenerate, it better to dispose the pollutant
together with the adsorbent
Ad i i l ll ifi
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Adsorption is mostly pollutant specific.
e.g Activated carbon, silica gel and
diatomaceous earth are suitable foradsorption of water vapours from a gas
phase. It can also adsorb SO2 and NH3.
Activated carbon is most suited forremoval of organic gases from gas
stream.
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Carbon adsorption systems are either
regenerative or non-regenerative.
Regenerative system usually containsmore than one carbon bed. As one bed
actively removes pollutants, another bed
is being regenerated for future use. Non-regenerative systems have
thinner beds of activated carbon. In a
non-regenerative adsorber, the spentcarbon is disposed of when it becomes
saturated with the pollutant.
R ti C b Ad ti S t
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Regenerative Carbon Adsorption Syste
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Non-Regenerative
Carbon
Adsorption
System
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PART-II
GASEOUS POLLUTION CONTROL
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3) COMBUSTIONIn many cases it is not possible to
remove the required amount of specificpollutant from an exhaust stream by
techniques such as absorption or
adsorption.The other technique available is
Combustion
Combustion refers to rapid oxidation of
substances (usually referred as fuels)
with evolution of heat.
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To summarize, Combustion is
defined as rapid, high-temperature gas-phaseoxidation.
Simply, the contaminant (acarbon-hydrogen substance) isburned with air and convertedto carbon dioxide and watervapor.
Combustion process involves three distinct
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Combustion process involves three distinctcomponents
1. Fuel : -A solid, liquid or gaseous substance
with energy rich C-C or C-H bonds amongothers, which are broken up duringcombustion
2. Oxidant:- A substance which aids incombustion process by breaking thechemical bonds allowing the release of heat.
3. Diluent:- A substance that does not take
part in the combustion process but acts ascarrier of the fuel or the oxidants. Mostcommon diluents is Nitrogen present inthe air
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I) FLARE OR
DIRECT FLAME COMBUSTION
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Flare
Combustible gases are burned in open air which
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Combustible gases are burned in open air, which
produces flare
The flare is usually employed to remove
hydrocarbons and organic vapours, odorouscompounds in refineries and chemical works.
It can also burn gases such as NH3, HCN or other
toxic or dangerous gases.
If aromatic hydrocarbons are present, they
burn with Smokey flame. This can be avoided
by injecting a steam into the flame, which
reacts and forms hydrogen and CO both burnsmokelessly.
However such steam-injected flare are little noisy
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ADVANTAGES OF FLARE
1. Can be an economical way to dispose ofsudden releases of large amounts of
gas;
2. In many cases do not require
auxiliary fuel to support combustion;
and
3. Can be used to control intermittent or
fluctuating waste streams.
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DISADVANTAGES OF FLARE
1. Can produce undesirable noise, smoke,heat radiation, and light.
2. Can be a source of SOx, NOx, and CO;
3. Cannot be used to treat waste streams
with halogenated compounds; and.
4. Released heat from combustion is lost
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4) INCINERATION ORAFTERBURNING
Incineration is method of reducinggases, liquid and solid waste streams by
chemically altering the pollutant species
once they are formed.It is used to remove combustible air
pollutants (gases, vapours or odours)
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I) THERMAL INCINERATION
+
Air
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In this method combustibles in the gas
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In this method combustibles in the gasstream are brought above autoignition temperatures and burn
with oxygen usually present in thegas stream.
If sufficient oxygen is not
available, air is added by means ofblower fan.
Thermal incineration is carried out in
the temperature range of 10000 F to15000F
Because of this cost is less and NOx
formation is also less.
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Important considerations in Design of
thermal incinerators are the Three Ts
Time- residence time should be 0.2 to
0.8 sec with 0.5 sec as a reasonable
guidelineTemperature (refer next slide)
Turbulence- complete mixing is very
important in case of odour control thanhydrocarbons, Less residence time is
required if proper mixing occurs
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APPROXIMATE AVERAGE TEMPERATURE
REQUIREMENTS
Average temperature
range (0K)
Hydrocarbon oxidation 780 1030
Carbon monoxide
oxidation
950- 1060
Odour control viaoxidation
750 - 980
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Thermal
Incinerator
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ADVANTAGES
Incinerators are one of the most
positive and proven methods for
destroying VOC, with efficiencies up
to 99.9999% possible.
Thermal incinerators are often the
best choice when high efficiencies
are needed and the waste gas isabove 20% of the LEL (Low explosive
limit).
II) RECUPERATIVE
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II) RECUPERATIVE
INCINERATION
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Whichever may be the method,
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W c eve ay e e e o ,Temperature of gases leaving thesystem vary from 700 to 2000 0F
Thus considerable energy at hightemperatures is associated with the gasstream.
So that heat can be used to preheat thecontaminated gas entering into thereactor.
Heat exchanger used for this purpose
is called as recuperator or regeneratorUse of recuperator reduce use of fuel and
makes it economical.
Initial cost is high.
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ADVANTAGES
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ADVANTAGES
1. 99.9999% efficiency can be achieved
2. Recuperative incinerators usually are moreeconomical than straight thermal
incinerators because they recover about
70% of the waste heat from the exhaust
gases.
3. This heat can be used to preheat incoming
air, and of ten times, sufficient waste heat
will be available for process heating, or togenerate steam or hot water
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DISADVANTAGES
Thermal incinerators, including recuperative types,
are not well suited to streams with highlyvariable flow because of the reduced residence
time and poor mixing during increased flow
conditions which decreases the completeness
of combustion.
Incinerators, in general, are not recommended for
controlling gases containing halogen- or sulfur-
containing compounds because of theformation of highly corrosive acid gases.
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III) CATALYTIC INCINERATION
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A catalyst accelerates rate of chemical
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yreaction without undergoing a chemicalchange itself.
Residence time is in the range of 0.3 to 0.9sec.
Combustion reaction occurs on the surfaceof the catalyst
Most gases containing combustiblepollutants from industrial processesare at a fairly low temperature.
Therefore some type of preheatingburner is used to bring waste gas up totemperature , at which catalyst will beeffective.
Temperature range is 590 to 810 0K
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TEMPERATURE RANGES FOR DIFFERENT
POLLUTANT GASES
Temperature range is 590 to 810 K
Efficiency is 95% to 98%
Effluent gases are CO2, vapours andnitrogen
Industrial pollutant Average temperature
range (0K)
Solvents 530 730
Vegetable and animal fats 530 - 640
Chemical process exhausts 480 - 670
Catalyst should be
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y
- Cheap
- Long lasting
- Should be able to function at required
temperatures.
- Capable of formed into variety of
shapes
Examples of catalyst are
Platinum
Palladium
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Catalytic
Incinerator
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ADVANTAGES
1. Lower fuel requirements;2. Lower operating temperatures;
3. Little or no insulation requirements;
4. Reduced fire hazards; and5. Less volume/size required
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DISADVANTAGES
1. High initial cost;2. Catalyst poisoning is possible;
3. Particulate often must first be removed;
and4. Spent catalyst that cannot be regenerated
may need to be disposed
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THEORY QUESTIONS
Write short notes on1. Absorption 2. Adsorption 3. Catalytic
incineration 4. Recuperation 5. Flare
6. Thermal incinerationA. Gaseous control of pollutants (note:- List all and
explain any one or two in detail)
B. Combustion method of gaseous pollution
control (note:-list all methods under combustion andincineration and explain any one with figure)