Control of Gaseous Pollutants

CONTROL OF GASEOUS AIR POLLUTANTS. Page 1
VEERMATA JIJAMATA TECHNOLOGICAL INSTITUTE,
MATUNGA, MUMBAI.
DEPARTMENT OF CIVIL ENGINEERING
SEMINAR ON
A CONTROL DEVICES FOR GASEOUS AIR POLLUTANTS
BY-
SOURABH M. KULKARNI
M. Tech (ENVIRONMENTAL ENGG.)
ROLL NO. - 112020016
UNDER THE GUIDEANCE OF -
DR. P.P BHAVE

ACKNOWLEDGEMENT
I am very thankful to Dr. P.P.Bhave for his constant support and valuable guidance. I am also
thankful to my friends for valuable suggestions for the preparation of my seminar.
Mr.Sourabh M. Kulkarni.
M.Tech (Environmental Engg.)
Roll No. 112020016.

INDEX -
Sr no Description Page no Remark
1 Introduction
2 Sources
3 Methods of
control of
gaseous
Pollutants
4 Absorption
5 Adsorption
6 Combustion
7 Condensation
8 Control devices
9 References

 ABSTRACT–
The pollution is the one of the greater issue in the world today. Now days the pollution problems
are go on increasing in the world. The pollution due to air is also global serious issue today. There
are various sources of the air pollution such as natural, manmade sources etc.Their also adverse
effect of this pollution on environment & man. So there is need to control the pollution. Due to
heavy industrialisation in the world the air pollution problems are also becoming a serious.
Harmful air pollutants are emitted from the industries during the processes. So there is need to
control the pollutants which are emitted from the industries. There are some control methods to
control the pollution of air from the industries.
The current topic is related with the control devices which are used for the control of gaseous air
pollutants from the industries. It also includes the introduction and sources of gaseous pollutants
& of techniques involved in the control of gaseous air pollutants such as absorption, adsorption,
combustion, closed collection & recovery system etc.
The control devices which are used for the gaseous air pollution control such as packed tower,
wet scrubber, plate tower, spray tower, catalytic convertor etc.

 INTRODUCTION-
A substance in the air that can cause harm to humans and the environment is known as an air
pollutant. Pollutants can be in the form of solid particles, liquid droplets, or gases. In addition,
they may be natural or man-made.
Pollutants can be classified as primary or secondary. Usually, primary pollutants are directly
emitted from a process, such as ash from a volcanic eruption, the carbon monoxide gas from a
motor vehicle exhaust or sulphur dioxide released from factories. Secondary pollutants are not
emitted directly. Rather, they form in the air when primary pollutants react or interact. An
important example of a secondary pollutant is ground level ozone — one of the many secondary
pollutants that make up photochemical smog. Some pollutants may be both primary and
secondary: that is, they are both emitted directly and formed from other primary pollutants.
 Pollutants -
Major primary pollutants produced by human activity include:
 Sulfur oxides (SOx) - especially sulphur dioxide, a chemical compound with the formula
SO2. SO2 is produced by volcanoes and in various industrial processes. Since coal and
petroleum often contain sulphur compounds, their combustion generates sulfur dioxide.
Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4,
and thus acid rain. This is one of the causes for concern over the environmental impact of the
use of these fuels as power sources.
 Nitrogen oxides (NOx) - especially nitrogen dioxide are emitted from high temperature
combustion. Can be seen as the brown haze dome above or plume downwind of cities.
Nitrogen dioxide is the chemical compound with the formula NO2. It is one of the several
nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO2 is
one of the most prominent air pollutants.
 Carbon monoxide - is a colourless, odorless, non-irritating but very poisonous gas. It is a
product by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular
exhaust is a major source of carbon monoxide.
 Carbon dioxide (CO2) - a colourless, odorless, non-toxic greenhouse gas associated with
ocean acidification, emitted from sources such as combustion, cement production, and
respiration.
 Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they
are often divided into the separate categories of methane (CH4) and non-methane
(NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhance
global warming. Other hydrocarbon VOCs are also significant greenhouse gases via their
role in creating ozone and in prolonging the life of methane in the atmosphere, although the
effect varies depending on local air quality. Within the NMVOCs, the aromatic compounds
benzene, toluene and xylene are suspected carcinogens and may lead to leukemia through
prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated
with industrial uses.

 Persistent free radicals connected to airborne fine particles could cause cardiopulmonary
disease.
 Toxic metals, such as lead, cadmium and copper.
 Chlorofluorocarbons (CFCs) - harmful to the ozone layer emitted from products currently
banned from use.
 Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the
formula NH3. It is normally encountered as a gas with a characteristic pungent odor.
Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving
as a precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, is also a
building block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is
both caustic and hazardous.
 Odors — such as from garbage, sewage, and industrial processes
 Radioactive pollutants - produced by nuclear explosions, war explosives, and natural
processes such as the radioactive decay of radon.
 Particulate matter - Particulates, alternatively referred to as particulate matter (PM) or
fine particles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol
refers to particles and the gas together. Sources of particulate matter can be man made or
natural. Some particulates occur naturally, originating from volcanoes, dust storms, forest
and grassland fires, living vegetation, and sea spray. Human activities, such as the
burning of fossil fuels in vehicles, power plants and various industrial processes also
generate significant amounts of aerosols. Averaged over the globe, anthropogenic
aerosols—those made by human activities—currently account for about 10 percent of the
total amount of aerosols in our atmosphere. Increased levels of fine particles in the air are
linked to health hazards such as heart disease, altered lung function and lung cancer.
Secondary pollutants include:-
 Particulate matter formed from gaseous primary pollutants and compounds in photochemical
smog. Smog is a kind of air pollution; the word "smog" is a portmanteau of smoke and fog.
Classic smog results from large amounts of coal burning in an area caused by a mixture of
smoke and sulfur dioxide. Modern smog does not usually come from coal but from vehicular
and industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun
to form secondary pollutants that also combine with the primary emissions to form
photochemical smog.
 Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of the
troposphere. It is also an important constituent of certain regions of the stratosphere
commonly known as the Ozone layer. Photochemical and chemical reactions involving it
drive many of the chemical processes that occur in the atmosphere by day and by night. At
abnormally high concentrations brought about by human activities (largely the combustion
of fossil fuel), it is a pollutant, and a constituent of smog.
 Peroxyacetyl nitrate (PAN) - similarly formed from NOx and VOCs.

Minor air pollutants include:
 A large number of minor hazardous air pollutants. Some of these are regulated in USA under
the Clean Air Act and in Europe under the Air Framework Directive.
 A variety of persistent organic pollutants, which can attach to particulate matter.
Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental
degradation through chemical, biological, and photolytic processes. Because of this, they have
been observed to persist in the environment, to be capable of long-range transport,
bioaccumulate in human and animal tissue, biomagnify in food chains, and to have potential
significant impacts on human health and the environment.
 Sources of air pollutants –
Sources of air pollution refer to the various locations, activities or factors which are responsible
for the releasing of pollutants into the atmosphere. These sources can be classified into two
major categories which are:
Anthropogenic sources (human activity) mostly related to burning different kinds of fuel-
 "Stationary Sources" include smoke stacks of power plants, manufacturing facilities
(factories) and waste incinerators, as well as furnaces and other types of fuel-burning heating
devices.
 "Mobile Sources" include motor vehicles, marine vessels, aircraft and the effect of sound etc.
 Chemicals, dust and controlled burn practices in agriculture and forestry management.
Controlled or prescribed burning is a technique sometimes used in forest management,
farming, prairie restoration or greenhouse gas abatement. Fire is a natural part of both forest
and grassland ecology and controlled fire can be a tool for foresters. Controlled burning
stimulates the germination of some desirable forest trees, thus renewing the forest.
 Fumes from paint, hair spray, varnish, aerosol sprays and other solvents.

 Waste deposition in landfills, which generate methane. Methane is not toxic; however, it is
highly flammable and may form explosive mixtures with air. Methane is also an asphyxiant
and may displace oxygen in an enclosed space. Asphyxia or suffocation may result if the
oxygen concentration is reduced to below 19.5% by displacement
 Military, such as nuclear weapons, toxic gases, germ warfare and rocketry
Natural sources-
 Dust from natural sources, usually large areas of land with little or no vegetation.
 Methane, emitted by the digestion of food by animals, for example cattle.
 Radon gas from radioactive decay within the Earth's crust. Radon is a colourless, odourless,
naturally occurring, radioactive noble gas that is formed from the decay of radium. It is
considered to be a health hazard. Radon gas from natural sources can accumulate in
buildings, especially in confined areas such as the basement and it is the second most
frequent cause of lung cancer, after cigarette smoking.
 Smoke and carbon monoxide from wildfires.
 Vegetation, in some regions, emits environmentally significant amounts of VOCs on warmer
days. These VOCs react with primary anthropogenic pollutants—specifically, NOx, SO2, and
anthropogenic organic carbon compounds—to produce a seasonal haze of secondary
pollutants.
 Volcanic activity, which produce sulphur, chlorine, and ash particulates.
 METHODS OF CONTROL OF GASEOUS POLLUTANTS –
ABSORPTION –
In this process effluent gases are passed through the absorbers which contain liquid absorbents
that remove the one or more of the gaseous air pollutants in the gas stream.
The efficiency of this process depends on
1. Amount of surface contact between gas & liquid.
2. Contact time.
3. Concentration of absorbing medium.
4. Speed of the reaction between the absorbent & gases.
The following equipments are used to remove the gaseous pollutants using principle of
absorption-
1. Packed bed tower.
2. Plate tower.
3. Bubble- cap plate tower.
4. Spray tower.
5. Liquid jet scrubber absorber.

Absorption devices-
1. Gas scrubber.
2. Venture scrubber.
3. Cyclone scrubber.
Some of the absorbing solutions that are used in the removing different gaseous pollutants from
gas streams are-
SO2 - Ammonium sulphate, sodium sulphite, alkaline water, calcium sulphite, calcium sulphate.
H2S - NaOH & phenol mix, sodium alamine & potassium dimethyl glycin, ethanolamines etc
HF - water, sodium hydroxide
Oxides of nitrogen- water, aquieous nitric acid.
 ADSORPTION-
In this process the effluent gases are passed through adsorber which contains the solids of porous
structure. The commonly used adsorbers are activated carbons, silica gel, activated alumina,
activated bauxite etc.
Activated carbon-
also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been
processed to make it extremely porous and thus to have a very large surface area available for
adsorption or chemical reactions.
Silica gel-
Is a chemically inert, nontoxic, polar and dimensionally stable (< 400 °C or 750 °F) amorphous
form of SiO2. It is prepared by the reaction between sodium silicate and acetic acid, which is
followed by a series of after-treatment processes such as aging, pickling, etc. These after
treatment methods results in various pore size distributions.
Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy (polar)
hydrocarbons from natural gas.

Zeolites –
Are natural or synthetic crystalline aluminosilicates which have a repeating pore network and
release water at high temperature. Zeolites are polar in nature.
They are manufactured by hydrothermal synthesis of sodium aluminosilicate or another silica
source in an autoclave followed by ion exchange with certain cations (Na+
, Li+
, Ca2+
, K+
, NH4
+
).
The channel diameter of zeolite cages usually ranges from 2 to 9 Å (200 to 900 pm). The ion
exchange process is followed by drying of the crystals, which can be pelletized with a binder to
form macroporous pellets.
Zeolites are applied in drying of process air, CO2 removal from natural gas, CO removal from
reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming.
Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by
dealumination of aluminum-containing zeolites. The dealumination process is done by treating
the zeolite with steam at elevated temperatures, typically greater than 500 °C (930 °F). This high
temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is
expelled from the zeolite framework.
The following points are important in the effective removal of gaseous pollutants adsorbents are
1. Contact of gaseous pollutants with solid adsorbent.
2. Separation of adsorbed gaseous pollutants gaseous pollutants from the solid adsorbent by
replacement of adsorbent.

3. Recovery of gases for final disposal.
The efficiency of removal of gases from adsorbents depends on-
4. Physical & chemical characteristics of the adsorbent.
5. The concentration & nature of gas to be adsorbed.
The commonly adsorbents used for the removal of gases are given below-
SO2- limestone.
H2S- iron oxide.
HF- lump limestone.
Oxides of nitrogen- silica gel.
Organic solvent vapours- Activated carbon.
 CONTROL DEVICES -
1. Fixed bed absorber.
2. Moving bed absorber.
Fixed bed absorber -
It consists of an absorbent bed (granulated activated carbon) through which the polluted gas is
passed from the top, which then travels downwards and leaves through the bottom. The single
bed absorber may be operated to the breakthrough point and then must be regenerated.
Moving bed absorber-
• It consists of a cylindrical bed which slowly rotates about its axis.
• The absorber bed moves from polluted fluid to the regeneration fluid to the drying &
cooling fluid.

• There are three sections: adsorption section, a regeneration section and a drying &
cooling section.
 COMBUSTION -
Incineration, also known as combustion, is most used to control the emissions of organic
compounds from process industries. This control technique refers to the rapid oxidation of a
substance through the combination of oxygen with a combustible material in the presence of
heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and
water vapour.
Equipment used to control waste gases by combustion can be divided in three categories:
- Direct combustion or flaring,
- Thermal incineration.
- Catalytic incineration.
 Direct combustor –
Direct combustor is a device in which air and all the combustible waste gases react at the
burner. Complete combustion must occur instantaneously since there is no residence
chamber.

A flare can be used to control almost any emission stream containing volatile organic
compounds. Studies conducted by EPA have shown that the destruction efficiency of a flare
is about 98 percent.
 Thermal incenaration-
In thermal incinerators the combustible waste gases pass over or around a burner flame into a
residence chamber where oxidation of the waste gases is completed. Thermal incinerators can
destroy gaseous pollutants at efficiencies of greater than 99 percent when operated correctly.
 Catalytic incinerators -
Catalytic incinerators are very similar to thermal incinerators. The main difference is that after
passing through the flame area, the gases pass over a catalyst bed. A catalyst promotes oxidation
at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95
percent are possible using a catalytic incinerator.
Catalytic incinerator

 CONDENSATION-
Condensation is the process of converting a gas or vapour to liquid. Any gas can be reduced to
a liquid by lowering its temperature and/or increasing its pressure.
Condensers are typically used as pretreatment devices. They can be used ahead of absorbers,
absorbers, and incinerators to reduce the total gas volume to be treated by more expensive
control equipment. Condensers used for pollution control are contact condensers and surface
condensers.
In a contact condenser, the gas comes into contact with cold liquid
In a surface condenser, the gas contacts a cooled surface in which cooled liquid or gas is
circulated, such as the outside of the tube. Removal efficiencies of condensers typically
range from 50 percent to more than 95 percent, depending on design and applications.

CONTROL DEVICES FOR GASEOUS POLLUTANTS –
 Spray Tower –
Spray towers or spray chambers are a form of pollution control technology. They consist of
empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels.
The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is
sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite
direction is called countercurrent flow. Fig. shows a typical countercurrent-flow spray tower.
This type of technology is a part of the group of air pollution controls collectively referred to as
wet scrubbers.
Countercurrent flow exposes the outlet gas with the lowest pollutant concentration to the freshest
scrubbing liquid. Many nozzles are placed across the tower at different heights to spray all of the
gas as it moves up through the tower. The reasons for using many nozzles is to maximize the
number of fine droplets impacting the pollutant particles and to provide a large surface area for
absorbing gas.
Theoretically, the smaller the droplets formed, the higher the collection efficiency achieved for
both gaseous and particulate pollutants. However, the liquid droplets must be large enough to not
be carried out of the scrubber by the scrubbed outlet gas stream. Therefore, spray towers use
nozzles to produce droplets that are usually 500 to 1,000 µm in diameter. Although small in size,
these droplets are large compared to those created in the venturi scrubbers that are 10 to 50 µm

in size. The gas velocity is kept low, from 0.3 to 1.2 m/s (1 to 4 ft/s) to prevent excess droplets
from being carried out of the tower.
In order to maintain low gas velocities, spray towers must be larger than other scrubbers that
handle they tend to agglomerate or hit the walls of the tower. Consequently, the total liquid
surface area for contact is reduced, reducing the collection efficiency of the scrubber.
 Gas collection -
Spray towers can be used for gas absorption, but they are not as effective as packed or plate
towers. Spray towers can be very effective in removing pollutants if the pollutants are highly
soluble or if a chemical reagent is added to the liquid.
For example, spray towers are used to remove HCl gas from the tail-gas exhaust in
manufacturing hydrochloric acid. In the production of superphosphate used in manufacturing
fertilizer, SiF4 and HF gases are vented from various points in the processes. Spray towers have
been used to remove these highly soluble compounds. Spray towers are also used for odor
removal in bone meal and tallow manufacturing industries by scrubbing the exhaust gases with a
solution of KMnO4.
Because of their ability to handle large gas volumes in corrosive atmospheres, spray towers are
also used in a number of flue gas desulfurization systems as the first or second stage in the
pollutant removal process.
In a spray tower, absorption can be increased by decreasing the size of the liquid droplets and/or
increasing the liquid-to-gas ratio (L/G). However, to accomplish either of these, an increase in
both power consumed and operating cost is required. In addition, the physical size of the spray
tower will limit the amount of liquid and the size of droplets that can be used.

 Packed bed tower -
The Packed Bed Scrubber, or Packed Tower, is designed to remove liquor that absorbs or
chemically reacts with the pollutants. Some vapors can be simply removed by condensation
through the cooling effect of the circulating liquid. The cleaned air is then discharged to the
atmosphere and the contaminated scrubbing liquor is either disposed of in an approved manner
or chemically treated and recycled. In some cases, the collected contaminants can be recovered
and reused in the original or other processesgaseous or vaporous pollutants from similar gas
stream flow rates. Another problem occurring in spray towers is that after the droplets fall short
distances, an air stream. The process is accomplished by contacting the contaminated air stream
with a scrubbing.
Typical Applications-
 Chemical production
 Fertilizer production and processing
 Pulp and paper
 Petrochemical
 Pharmaceutical.
Packed bed material-
In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a
packing material. The packing can be randomly filled with small objects like Raschig rings or
else it can be a specifically designed structured packing. Packed beds may also contain catalyst
particles or adsorbents such as zeolite pellets, granular activated carbon, etc.

 The purpose of a packed bed is typically to improve contact between two phases in a
chemical or similar process. Packed beds can be used in a chemical reactor, a distillation
process, or a scrubber, but packed beds have also been used to store heat in chemical
plants. In this case, hot gases are allowed to escape through a vessel that is packed with a
refractory material until the packing is hot. Air or other cool gas is then fed back to the
plant through the hot bed, thereby pre-heating the air or gas feed.

 Bubble cap plate tower-
Plate scrubbers are counter-flow devices in which liquid moves downward and gas moves
upward. Liquid-gas contact is obtained in a mixing zone consisting of a plate with some type of
openings on it.
Openings can be perforations, valves, or slots. Often multiple plates are used. Liquid flows
downward from plate to plate. The simplest plate is the perforated plate or slotted plate. Liquid
flows over the perforations. The velocity of the air stream is sufficient to prevent weeping
(liquid flow) through the openings. The action creates a frothing column above the plate. Liquid
flows across the plate to a downcomer and then downward to the next plate.
Bubble cap towers utilize the principle just described, except that the perforations are covered
with caps. Air bubbles out of slots or notches in the cap. This arrangement causes effective gas
dispersion and prevents liquid from weeping if the flow is momentarily reduced.
Some plate towers do not utilize downcomers and are therefore truly counter-flow. Rod decks
are examples of this type. Plate towers have been widely used for mass transfer but also have
been used for particulate collection.

For mass transfer applications, many of the towers have been individually designed. However, a
number of companies are quite active in the sale of plate towers for absorption purposes.
Absorption Efficiency-
Plate towers are effective for absorption. Efficiency can be increased by the addition of plates
and tower height: Contact takes place in bubbles and droplets. Since the leanest gas contacts the
fresh slurry at the top of a tower after passing through the lower stages where absorption takes
place, low outlet emissions can be achieved. The pressure drop across the plate effects the
efficiency. Greater contact is achieved with higher pressure drop per plate. Liquid flows can
also be increased to enhance absorption efficiency.
Plate Scrubbers-
Physical Description.
Plate scrubbers are counter-flow devices in which liquid moves downward and gas moves
upward. Liquid-gas contact is obtained in a mixing zone consisting of a plate with some type of
openings on it.
Openings can be perforations, valves, or slots. Often multiple plates are used. Liquid flows
downward from plate to plate. The simplest plate is the perforated plate or slotted plate. Liquid
flows over the perforations. The velocity of the air stream is sufficient to prevent weeping
(liquid flow) through the openings. The action creates a frothing column above the plate. Liquid
flows across the plate to a downcomer and then downward to the next plate. Figure III-1
illustrates a scrubber of this design.
Bubble cap towers utilize the principle just described, except that the perforations are covered
with caps. Air bubbles out of slots or notches in the cap. This arrangement causes effective gas
dispersion and prevents liquid from weeping if the flow is momentarily reduced.
Some plate towers do not utilize downcomers and are therefore truly counter-flow. Rod decks
are examples of this type. Plate towers have been widely used for mass transfer but also have
been used for particulate collection.
For mass transfer applications, many of the towers have been individually designed. However, a
number of companies are quite active in the sale of plate towers for absorption purposes.

Absorption Efficiency.
Plate towers are effective for absorption. Efficiency can be increased by the addition of plates
and tower height: Contact takes place in bubbles and droplets. Since the leanest gas contacts the
fresh slurry at the top of a tower after passing through the lower stages where absorption takes
place, low outlet emissions can be achieved. The pressure drop across the plate effects the
efficiency. Greater contact is achieved with higher pressure drop per plate. Liquid flows can
also be increased to enhance absorption efficiency.
Particulate Collection Efficiency.
Multiple plate towers are effective in removing particles above approximately one micron in
size. They are not nearly as effective on submicron particulate as are venturi scrubbers.
Increasing the velocity of the gas through the plates will increase particulate removal efficiency.
Adding additional trays is not likely to have much effect. A three-tray unit at 2" pressure drop
per tray or 6" total pressure drop might not be as efficient as one tray at 3" pressure drop. The
axiom is that for particulate collection, the energy should be consumed in one place for
maximum efficiency. Increasing L/G ratios can have some effect on particulate removal but very
minor. Doubling the liquid rate might not be more beneficial than a 10 percent increase in
pressure drop across the plate.
Maintenance Characteristics.
Tray towers are nearly always designed so that there is access to each tray section from outside
the unit. Thus, the units can be cleaned much more easily than packed towers if build-up
occurs. The underside of the bottom tray section can be sprayed with the scrubbing slurry to
eliminate wet-dry interfaces which cause plugging. In some special cases, the trays have been
designed for easy removal from the unit. The purpose was to enable cleaning of the trays outside
the unit. If trays are not level, the liquid-gas contact will be reduced. Care must be taken on
installation to achieve good level conditions. If plates are not sealed to the side wall, gas can
bypass the contact areas reducing efficiency. Flooding will occur in plate towers with either an
excessive liquid rate or gas velocity. This will cause increased pressure drop across the towers
and decreased gas flow. Plate towers do not resist plugging and scaling to the extent of venturis,

centrifugals and other open design scrubbers. Their application should be limited to applications
where plugging potential is not severe.
Size.
Plate towers are found more frequently in larger mass transfer applications as opposed to packed
towers. The plates themselves can be designed to any diameter tower. Moisture eliminator
designs offer a more complicated sizing problem.
Materials of Construction.
Plate towers can be designed out of many materials of construction. However, in comparison
with packed towers, the cost of internals rises sharply with need for corrosion resistance. The
plates in particular present a problem. In units with liquid under-spray, the nozzles and piping
are expensive. The problem is accentuated if the inlet temperatures are high. The total weight of
plate towers is generally less than packed towers.
Liquid-to-Gas Ratios.
Rates as low as 2 gal./1000 cfm are used for particulate collection. For absorption, high liquid
rates can be used. Plate towers can usually handle higher liquid rates than packed towers.
Pressure Drop.
The pressure drop depending on design can range from 1/4" to several inches water gauge per
plate.
Advantages-
(a) High mass transfer rates with multiple plates.
(b) Both particulate removal and absorption can be accomplished in one tower.
(c) Can handle high liquid rates.
(d) Can be built in large sizes.
(e) Can handle volume fluctuations.
(f) Can handle temperature fluctuation.
Disadvantages-
(a) Cannot handle applications with high scaling potential such as a limestone SO2
scrubbing
(b) Cannot handle foamy liquids

(c) Corrosion resistant design is expensive
(d) Cannot remove fine particulate.
 Wet scrubber -
The term wet scrubber describes a variety of devices that remove pollutants from a furnace flue
gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact
with the scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or
by some other contact method, so as to remove the pollutants.
The design of wet scrubbers or any air pollution control device depends on the industrial process
conditions and the nature of the air pollutants involved.
Inlet ga s characteristics and dust properties (if particles are present) are of primary importance.
Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers
remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases
by dissolving or absorbing them into the liquid.
Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by
means of another device referred to as a mist eliminator or entrainment separator (these terms are
interchangeable). Also, the resultant scrubbing liquid must be treated prior to any ultimate
discharge or being reused in the plant.
There are numerous configurations of scrubbers and scrubbing systems, all designed to provide
good contact between the liquid and polluted gas stream.

Advantages of wet scrubber -
1.Small space requirements
Scrubbers reduce the temperature and volume of the unsaturated exhaust stream. Therefore,
vessel sizes, including fans and ducts downstream, are smaller than those of other control
devices. Smaller sizes result in lower capital costs and more flexibility in site location of the
scrubber.
2. No secondary dust sources-
Once particulate matter is collected, it cannot escape from hoppers or during transport.
3. Handles high-temperature, high-humidity gas streams-
No temperature limits or condensation problems can occur as in baghouses or ESPs.
4. Minimal fire and explosion hazards.
Various dry dusts are flammable. Using water eliminates the possibility of explosions.
Ability to collect both gases and particulate matter.
Disadvantages of wet scrubber -
1 .Corrosion problems-Water and dissolved pollutants can form highly corrosive acid solutions.
Proper construction materials are very important. Also, wet-dry interface areas can result in
corrosion.
2. High power requirements-High collection efficiencies for particulate matter are attainable only
at high pressure drops, resulting in high operating costs.
3.Water-disposal problems-Settling ponds or sludge clarifiers may be needed to meet waste-
water regulations.
4. Difficult product recovery-Dewatering and drying of scrubber sludge make recovery of any
dust for reuse very expensive and difficult.
 Catalytic converter -
A catalytic converter (colloquially, "cat" or "catcon") is a device used to convert toxic exhaust
emissions from an internal combustion engine into non-toxic substances. Inside a catalytic
converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion

undergo a chemical reaction. The type of chemical reaction varies depending upon the type of
catalyst installed, for example current North American gasoline powered Light Duty Vehicles
are fitted with a Three-way Catalytic Converter which reduces carbon monoxide(CO), unburned
hydrocarbons(HC), and oxides of nitrogen(NO, NO2, & N2O) to produce carbon dioxide(CO2),
nitrogen(N2), and water(H2O).
Catalytic converters were first widely introduced on series-production automobiles in the United
States market for the 1975 model year to comply with tightening U.S. Environmental Protection
Agency regulations on automobile exhaust emissions. Catalytic converters are still most
commonly used on motor vehicle exhaust systems, but are also used on generator sets, forklifts,
mining equipment, trucks, buses, locomotives, airplanes and other engine fitted devices. Usually
this is in response to government regulation.
 Cyclonic spray scrubber -
Irrigated cyclone scrubber. Cyclonic spray scrubber.
Cyclonic spray scrubbers are an air pollution control technology. They use the features of both
the dry cyclone and the spray chamber to remove pollutants from gas streams.
Generally, the inlet gas enters the chamber tangentially, swirls through the chamber in a
corkscrew motion, and exits. At the same time, liquid is sprayed inside the chamber. As the gas
swirls around the chamber, pollutants are removed when they impact on liquid droplets, are
thrown to the walls, and washed back down and out.
Cyclonic scrubbers are generally low- to medium-energy devices, with pressure drops of 4 to 25
cm (1.5 to 10 in) of water. Commercially available designs include the irrigated cyclone scrubber
and the cyclonic spray scrubber.
In the irrigated cyclone the inlet gas enters near the top of the scrubber into the water sprays.
The gas is forced to swirl downward, then change directions, and return upward in a tighter

spiral. The liquid droplets produced capture the pollutants, are eventually thrown to the side
walls, and carried out of the collector. The "cleaned" gas leaves through the top of the chamber.
Gas collection -
High gas velocities through these devices reduce the gas-liquid contact time, thus reducing
absorption efficiency. Cyclonic spray scrubbers are capable of effectively removing some gases;
however, they are rarely chosen when gaseous pollutant removal is the only concern.
 Venturi scrubber-
This type of technology is a part of the group of air pollution controls collectively referred to as
wet scrubbers.
An ejector or venturi scrubber is an industrial pollution control device, usually installed on the
exhaust flue gas stacks of large furnaces, but may also be used on any number of other air
exhaust systems. To this end, an ejector venturi scrubber (as well as the spray tower) uses a
preformed spray, the difference is that only a single nozzle is used instead of many nozzles. This
nozzle operates at higher pressures and higher injection rates than those in most spray chambers.
The high-pressure spray nozzle (up to 689 kPa or 100 psig) is aimed at the throat section of a
venturi constriction.
The ejector venturi is unique among available scrubbing systems since it can move the process
gas without the aid of a blower or fan. The liquid spray coming from the nozzle creates a partial
vacuum in the side duct of the scrubber. This has the same effect as the water aspirator used in
high school chemistry labs to pull a small vacuum for filtering precipitated materials (due to the
Bernoulli effect). This partial vacuum can be used to move the process gas through the venturi as
well as through the facility's process system. In the case of explosive or extremely corrosive
atmospheres, the elimination of a fan in the system can avoid many potential problems.

The energy for the formation of scrubbing droplets comes from the injected liquid. The high
pressure sprays passing through the venturi throat form numerous fine liquid droplets that
provide turbulent mixing between the gas and liquid phases. Very high liquid-injection rates are
used to provide the gas-moving capability and higher collection efficiencies. As with other types
of venturis, a means of separating entrained liquid from the gas stream must be installed.
Entrainment separators are commonly used to remove remaining small droplets.
Gas collection -
Ejector venturis have a short gas-liquid contact time because the exhaust gas velocities through
the vessel are very high. This short contact time limits the absorption efficiency of the system.
Although ejector venturis are not used primarily for gas removal, they can be effective if the gas
is very soluble or if a very reactive scrubbing reagent is used. In these instances, removal
efficiencies of as high as 95% can be achieved.

REFERENCES-
 http://www.springerlink.com/content/lw47r8560x143282/
 http://www.springerlink.com/content/973k50v33311543n/
 http://www.sciencedirect.com/science/article/pii/0307904X89901947
 http://ascelibrary.org/eeo/resource/1/joeedu/v132/i5/p463_s1?isAuthorized=
no
 http://www.sciencedirect.com/science/article/pii/S0920586106000757
 http://www.sciencedirect.com/science/article/pii/S0956053X10005714
 http://www.springerlink.com/content/h46155h71p60066g/
 http://www.springerlink.com/content/563g7t41n641116m/
 http://www.springerlink.com/content/3x6w532181022286/
 http://www.springerlink.com/content/j68k4457u2851837/
 http://www.springerlink.com/content/g07624551t212736/
 http://www.springerlink.com/content/w5671v506110n53g/

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