34 gaseous control technologies.pdf

8/10/2019 34 Gaseous Control Technologies.pdf

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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)

34 gaseous control technologies.pdf

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