introduction to environmental engineering code no. (pe389) lec. 10

WELCOME

DR.KHALED ALI

Introduction to Environmental EngineeringCode No. (PE389)

Lec. 10

Introduction

• Limiting emissions into the air is both technically difficult and expensive. However,

• since rain is the only air-cleaning mechanism available but not very efficient, air quality depends on pollution prevention and on limiting what is emitted.

• The control of air emissions may be realized in a number of ways.

• Figure 1 shows five separate possibilities for control.

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FIGURE 1. Points of possible air pollution control

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SOURCE CORRECTION1. Changing or eliminating a process that produces a polluting air

emission is often easier than trying to trap the pollutant.

For example, automobile exhaust once caused high lead levels in urban air. Elimination of lead from gasoline, which was needed for proper catalytic converter operation, also reduced lead. Similarly, removal of sulfur from coal and oil before the fuel is burned has reduced the amount of SO2 emitted. In these cases, the source of air pollution was corrected.

2. Processes may also be modified to reduce air pollution.(Odors from municipal incinerators may be controlled by operating the

incinerator at a high enough temperature to effect more complete oxidation of odor-producing organic compounds.

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COLLECTION OF POLLUTANTS

• Collection of pollutants for treatment can be the most serious problem in air pollution control. Automobile exhaust is a notorious polluter mainly because it is so difficult to trap and treat.

• If automobile exhaust could be channeled to a central treatment facility, treatment could be more efficient in controlling individual cars.

• Many stationary industries recycle exhaust gases usually CO and volatile organic compounds as fuel for the process, since even CO releases heat when burned to CO2.

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COOLINGThe exhaust gases to be treated are sometimes too hot for the

control equipment and must be cooled first. Cooling may also drop the temperature below the condensation

point of some pollutants so that they can be collected as liquids. Dilution, quenching, and heat exchange, shown in Figure 2, are all

acceptable cooling methods. Quenching has the added advantage of scrubbing (clean) out

some gases and particulate matter, but may yield a dirty, hot liquid that itself requires disposal.

Cooling coils (heat exchange) are probably the most widely used cooling method and are especially appropriate where heat can be conserved.

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FIGURE 2. Cooling hot waste gases

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TREATMENTSelection of the correct treatment device requires matching its

features with the characteristics of the pollutant. Pollutant particles vary in size over many orders of magnitude, from

ideal gas molecules to macroscopic particles several mm in diameter. One device will not be effective and efficient for all pollutants, or

even for all pollutants coming from the same stack. The chemical behavior of pollutants may also dictate selection of a

control process. The various air pollution control devices are conveniently divided

into 1. those that control particulate matter and,2. those that control gaseous pollutants.

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CyclonesThe cyclone is a popular, economical, and effective means of

controlling particulates.As illustrated in Figure 3, dirty air enters the cyclone off-

center; a violent swirl of air is thus created in the cone and particles are accelerated centrifugally toward the wall.

Friction at the wall slows the particles and they slide to the bottom, where they can be collected, and clean air exits at the center of the top of the cone.

Cyclones are reasonably efficient for large particle collection and are widely used as the first stage of dust removal.

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Figure 3. Cyclone

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Fabric FiltersFabric filters used for controlling particulate matter (Figure 4) operate like a

vacuum cleaner. Dirty gas is blown or sucked through a fabric filter bag, which collects the dust. The dust is removed periodically when the bag is shaken. Fabric filters can be very efficient collectors for even sub-micron-sized particles

and are widely used in industrial applications.Very hazardous or toxic particulate matter of a diameter less than 1 gm

sometimes must be controlled to better than 99.9%. A single stage of High Efficiency Particle Attenuation (HEPA) micropore or

glass filters, through which the precleaned gas is forced or sucked by vacuum, can achieve this level of control, and four to six HEPA filter stages in series can achieve 99.9999% control.

HEPA filters are commonly used to control emission of radioactive particles, for example.


FIGURE 4. Industrial fabric filter apparatus


Wet Collectors

• The spray tower, or scrubber, pictured in Figure 21-5 can effectively remove larger particles.

• More efficient scrubbers promote the contact between air and water by violent action in a narrow throat section into which the water is introduced.

• Generally, the more violent the encounter, the smaller the gas bubbles or water droplets, hence the more effective the scrubbing.


Wet CollectorsAlthough wet scrubbers are very efficient and can trap gaseous

pollutants as well as very fine particulate matter, they have their disadvantages.

Scrubbers use a great deal of water that itself either requires further treatment or has limited use after being used to scrub dirty gas.

In places where water supplies are limited, a scrubber may have a very low priority among other uses for available water.

Moreover, scrubbers use energy and are expensive to construct as well as operate.

Finally, scrubbers usually produce a visible plume (spiral- cloud) of water vapor.


Spray towers and Scrubbers


CONTROL OF GASEOUS POLLUTANTS

Gaseous pollutants may be removed from the effluent stream by 1. trapping, by2. chemical change, or by 3. a change in the process that produces them.The wet scrubbers discussed can remove pollutants by dissolving

them in the scrubber solution. SO2 and NO2 in power plant off-gases are often controlled in this way.

Packed scrubbers spray towers packed with glass platelets or glass Adsorption, or chemisorption, is the removal of organic compounds

with an adsorbent like activated charcoal.Incineration is used when an organic pollutant can be oxidized to CO2

and water, or in oxidizing H2S to SO2.


Assignment # 9

Dr. Ola Abdelwahab 17

• Mention five separate possibilities for air pollution control

• Write short notes on each of the following methods of air pollution control

source correctioncollection of pollutantscooling of exhausted gas• Mention various air pollution control devices

and compare between them

Self Study Topics

• Comparison of Particulate Control Devices• Disposal of unprocessed refuse in sanitary

landfills• Siting Landfills


Lec 10

Air Pollution Control

Dr. Ola Abdelwahab