air pollution and its abatement and control in chemical industry

8/12/2019 Air Pollution and Its Abatement and Control in Chemical Industry

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Air pollution and itsabatement and control in

chemical industry

Group Members

Name Reg.no

Uzair Wahid UW-10-BSc-Ch.e-041

Saif ur Rehman UW-10-BSc-Ch.e-009

Khayaam Manzoor UW-10-BSc-Ch.e-004


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Table of Contents

Ch#1 Understanding the basics .......................................................................... 5

1.1. What is air pollution? ................................................................................. 51.1.1. What are pollutants? .............................................................................. 5

1.1.2. Main sources and effects of pollutants? ..................................................... 7

1.2. What is industrial air pollution? ..................................................................... 7

1.2.1. Chemical composition of normal air .......................................................... 7

1.2.2. Comparison of pure air and polluted atmosphere ....................................... 8

1.3. A brief history of air pollution ........................................................................ 8

1.3.1. Ranges of Concentrations of Gaseous Pollutants A Historical Record from

1956 ............................................................................................................. 9

1.3.2. Annual production of air pollution in 1998 ................................................. 9

Ch#2 Effects of air pollutants on human health ............................................... 10

2.1. Effects on human....................................................................................... 10

2.2. Specific pollutants...................................................................................... 11

2.3. Safety and protection factors that must be used ............................................ 12

2.3. Persons Needing Special Protection .............................................................. 12

2.4. Carcinogenicity, Mutagenicity, and Teratogenicity .......................................... 13

Ch#3 Environmentally Relevant Air Pollutants ................................................. 13

3.1. Sulfur Dioxide ........................................................................................... 13

3.1.1. Nature ................................................................................................ 14

3.1.2. Quantity ............................................................................................. 14

3.1.3. Sources .............................................................................................. 14

3.1.4. Methods for controlling sulphur dioxide emissions .................................... 15

3.1.5. Conversion of SO2to SO3and then to H2SO4........................................... 17

3.1.6. The sulphuric acid mist ......................................................................... 17

3.1.7. Effects of SO2on Human Health and the Environment: ............................. 18

3.2. Nitrogen Oxides ......................................................................................... 18

3.2.1. Nature, Effects on Human Health and the Environment ............................. 19

3.2.2. Quantity ............................................................................................. 19

3.2.3. Sources .............................................................................................. 19

3.2.4. The Fate of Atmospheric NOx: ............................................................... 20

3.3. Ozone ...................................................................................................... 20

3.4. Carbon Monoxide ....................................................................................... 21

3.5. Dusts ....................................................................................................... 21


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3.6. Lead ........................................................................................................ 22

3.7. Cadmium .................................................................................................. 23

3.8. Arsenic ..................................................................................................... 23

Ch#4 Industrial air pollution sources and its prevention ................................. 24

4.1. Potential sources of air pollution in chemical industry .........Error! Bookmark not

defined.

4.2. Air pollution from chlor-alkali plants ............................................................. 27

4.2.1. Its prevention ..................................................................................... 27

4.3. Air pollution from agro-industry chemicals .................................................... 28

4.3.1 Mixed Fertilizer Plants............................................................................ 28

4.3.2. Its prevention ..................................................................................... 28

Ch#5 Air pollution control equipment .............................................................. 31

5.1 Wet scrubbers ............................................................................................ 33

5.1.1. Diagram ............................................................................................. 33

5.1.2. Advantages ......................................................................................... 34

5.1.3. Disadvantages ..................................................................................... 35

5.1.4. Collection mechanisms and efficiency ..................................................... 35

5.2. Dry cyclone collectors ................................................................................ 37

5.2.1. Diagram ............................................................................................. 38

5.2.3. Collection mechanism ........................................................................... 38

5.2.3. Advantages ......................................................................................... 39

5.2.4. Disadvantages ..................................................................................... 39

5.2.5. Typical cyclone dimensions ................................................................... 39

5.3. Electrostatic precipitators ........................................................................... 40

5.3.1. Diagram ............................................................................................. 41


5.3.2. Advantages ......................................................................................... 42

5.3.3. Disadvantages ..................................................................................... 42

5.4. Fabric filter collectors ................................................................................. 43

5.4.1. Diagram ............................................................................................. 43


5.4.3. Types ................................................................................................. 44

5.4.4. Advantages ......................................................................................... 44

5.4.5. Disadvantages ..................................................................................... 45

5.5. Settling Chambers ..................................................................................... 45

5.5.1. Diagram ............................................................................................. 465.5.2. Collection Mechanism ........................................................................... 46


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5.5.3. Advantages ......................................................................................... 47

5.5.4. Disadvantages ..................................................................................... 47


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Ch#1 Understanding the basics

.1. What is air pollution?Air pollution occurs when the air contains gases, dust, fumes or odour in harmful

amounts. That is, amounts which could be harmful to the health or comfort of

humans and animals or which could cause damage to plants and materials.

1.1.1. What are pollutants?

The substances that cause air pollution are called pollutants. Pollutants that are

pumped into our atmosphere and directly pollute the air are called primary

pollutants. Primary pollutant examples include carbon monoxide from car exhausts

and sulfur dioxide from the combustion of coal.

The main primary pollutants known to cause harm in high enough concentrations

are the following:

Carbon compounds, such as CO, CO2, CH4, and VOCs

Nitrogen compounds, such as NO, N2O, and NH3

Sulfur compounds, such as H2S and SO2

Halogen compounds, such as chlorides, fluorides, and bromides

Particulate Matter (PM or aerosols), either in solid or liquid form, whichis usually

categorized into these groups based on the aerodynamic diameter of the particles

1. Particles less than 100 microns, which are also called inhalable10since they can

easily enter the nose and mouth.

2. Particles less than 10 microns (PM10, often labeled fine in Europe). These

particles are also called thoracicsince they can penetrate deep in the respiratory

system.

3. Particles less than 4 microns. These particles are often called respirable 12

because they are small enough to pass completely through the respiratory system

and enter the bloodstream.


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4. Particles less than 2.5 microns (PM2.5, labeled fine in the US).

5. Particles less than 0.1 microns (PM0.1, ultrafine).

Further pollution can arise if primary pollutants in the atmosphere undergo chemical

reactions. The resulting compounds are called secondary pollutants. Photochemical

smog is an example of this.


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1.1.2. Main sources and effects of pollutants?

Air pollutants mainly occur as a result of gaseous discharges from industry and

motor vehicles. There are also natural sources such as wind-blown dust and smoke

from fires.

Some forms of air pollution create global problems, such as upper atmosphere

ozone depletion and global warming. These problems are very complex, and require

international cooperative efforts to find solutions. The table listed below gives some

information about some main pollutants and their effect on human health.

1.2. What is industrial air pollution?

Pollution resulting from an industrial plant discharging pollutants into the atmosphere

that are injurious to health.

1.2.1. Chemical composition of normal air

Table 1.11

Table 1.1 records the chemical composition of air. Air normally contains water vapor

which would be somewhere around 1% by volume of the total mixture. The

concentrations in Table 1.1 remain nearly constant or vary slowly. The following are

variable in their concentration:2

Water > variable 1.0% by volume

Meteoric dust

Pollen

Substance % by volume in dry air

N2 78.09

O2 20.94

Ar 0.93

CO2 0.03

Ne 0.0018

He 0.00052

CH4 0.00022

Kr 0.00010

N2O 0.00010

H2 0.00005

Xe 0.00008


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Bacteria

Sodium chloride

Spores

Soil

Condensation nuclei

NO2 formed by electric discharge

SO2 volcanic oxygen

O3formed by electric discharge

HCl volcanic origin

HF of volcanic origin

1.2.2. Comparison of pure air and polluted atmosphere

Table 1.2

1.3. A brief history of air pollution

Media reports about air pollution might lead us to think of air pollution as being

something that developed in the second half of the 20th century. But this is not so. The

kind of air pollution to which human beings have been exposed has changed with time,

but air pollution has been known in larger cities at least from the 14th century when

people first started using coal for heating their homes.


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During World War II a new type of air pollution had been discovered in the Los Angeles

atmosphere. New effects were manifest in the form of eye and skin irritation and plant

damage not evident from simple smoke pollution. It was the result of a photochemical

smog that was at first attributed to the oil refineries and storage facilities. When controls

of these facilities did not result in a significant reduction of the problem, it was then

discovered that the internal combustion engine was a major cause of this new type of

pollution. The result of photochemical oxidation is seen in the brown haze apparent in

the upper layer of the atmosphere.

1.3.1. Ranges of Concentrations of Gaseous PollutantsA Historical Record from 1956

Table 1.3

1.3.2. Annual production of air pollution in 1998


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Fig.1.13

Ch#2 Effects of air pollutants on human health

2.1. Effects on human


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Air pollution has always been an undesirable byproduct of human activities, and has

presumably had harmful effects on human health ever since the cave dwellers lit their

first fire. Continuing structural change in post-industrial society, including a decrease in

classical industrial operations (e.g., the coal and steel industries) and a simultaneous

increase in the importance of the service industries, has led to considerable changes in

the release of emissions, with consequent reductions in the extent to which the

population is exposed to air pollutants.

The effects of air pollutants on humans can range from simple nuisance (e.g., an odor)

to serious health damage A large number of substances cause air pollution, and their

concentration in the air varies greatly with time and location, depending on weather

conditions (transmission effect) and the various types of emitters involved. Today, their

concentrations are in general very low (e.g., compared with those at the workplace), so

that an assessment of health damage can only be carried out by considering combination

effects in the low ose range. Competing effects from the private actions of individuals

(e.g., smoking, medication) and exposure by other routes (e.g., water and food) must

also be considered. Especially when long-term (chronic) effects are being investigated,

epidemiological studies on representative population groups are carried out.

The respiratory tract, with its large inner surface area, is the main site of damage by air

pollutants. However, there can also be effects on the heart and circulatory system, bloodformation, the kidneys, the immune and nervous systems, and the skin. Air pollutants

relevant to environmental medicine include sulfur dioxide, nitrogen dioxide, ozone,

carbon monoxide, dusts, heavy metals, and a wide range of hydrocarbons and

chlorinated hydrocarbons. Of special significance are air pollutants that are regarded as

potential carcinogens. Important substances of this type include arsenic, benzene,

cadmium, diesel motor emissions (DME), polycyclic aromatic hydrocarbons (PAH), and

2,3,7,8-TCDD (dioxin).4

2.2. Specific pollutants

It is considered that an effect on humans by an environmental pollutant has been caused

if reversible or irreversible changes to normal physiological processes are caused in the

human organism. In the case of substances that have an irreversible, carcinogenic,

mutagenic, or allergenic effect, it is assumed that an effect threshold does not exist.


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Since a risk is present even at low concentrations, special efforts must be made to

reduce or limit such pollutants.

The specific characteristic effects of a substance are mainly a function of a combination

of dose, exposure time, the nature of the effects, and the fundamental mechanism of

these effects. In the context of environmental pollutants, effect means any change

which is brought about by a substance after acute or chronic exposure. Damage to

health means reversible or irreversible undesired changes caused by a substance or

factor. Toxicity is the ability of a substance or factor to cause such damage, depending

on the applied dose and exposure time. Risk is defined as the probability that a given

damage occurs in that part of a population exposed to a harmful agent.

2.3. Safety and protection factors that must be used

A safety margin in the estimation of a no effect level

Interindividual differences (toxicokinetic and toxicodynamic differences)

Interspecies differences (toxicokinetic and toxicodynamic differences)

simultaneous effect of several substances, or their combined effects

Variation in the sensitivity of persons (special risk groups)

Severity of the effects

Exposure by other routes (e.g., food contamination)5

2.3. Persons Needing Special Protection

Because of their general sensitivity to the pollutants, children, old and ill persons, and

persons with a genetic predisposition must receive special consideration when assessing

the effects of (air) pollutants. For example, persons with chronic bronchitis or bronchial

hypersensitivity are especially susceptible to substances which attack the respiratory

tract or lungs.

This is exacerbated in children, because they have higher volume per minute respiration

rates per unit body weight than adults, and are more sensitive to infections.

Persons sensitive to (air) pollutants also include allergic persons for whom the frequency

and severity of allergic reactions can be intensified by certain nonallergenic foreign

substances in the air. Thus, the percentage of children with hay fever is significantly


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higher in areas of high traffic density than in traffic-free areas, even though the

pollutants themselves do not have an allergenic effect.

2.4. Carcinogenicity, Mutagenicity, and Teratogenicity

Carcinogenicity is the ability of a substance or physical factor to cause cancer in humans

or animals (CarcinogenicAgents). Cancer-risk factors can lead to the uncontrolled and

unorganized formation of neoplasms, which include bothbenign and malignant tumors.

Carcinogenic factors can include ionizing and UV radiation, numerous chemical

substances,and oncogenic viruses. Also, immune deficiencies, hormonal influences, and

inherited predispositions affect the formationof cancer.

Humans are exposed to a large number of carcinogenic substances via various

environmental media (e.g., water, soil, air, food), and physical and biological factors can

also play an important role. The risk of cancer can be obscured by general environmental

factors, individual behavior patterns (e.g., smoking, diet, and hobbies), and/or by stress

at the workplace. The carcinogenicity of a substance is usually expressed as the unit risk

(risk coefficient), which indicates the risk of cancer from lifelong exposure, e.g., to 1 g

of a carcinogenic air pollutant per cubic meter of respired air.

Mutagenicity is the ability of a substance or physical agent to cause permanent structural

changes in the genetic information of cells in vivo and in vitro (Mutagenic Agents). The

changes can involve one or more genes, one or more chromosomes, or sections of

chromosomes. Mutations in gametes are transmitted to their descendants. Mutations in

body cells remain in the organism in which they were induced.

Teratogenicity is the ability of certain chemical substances, microorganisms, or ionizing

radiation to cause nontransmissible malformations, mainly in mammalian embryos

(Teratogenic Agents).

Ch#3 Environmentally Relevant Air Pollutants

3.1. Sulfur Dioxide


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In children, an increase in the long-term chronic exposure to 181 275 g SO2/m3 and

230 310 g airborne dust/m3 led to increased incidence of diseases of the lower

respiratory tract compared with children from a control region with lower air pollution.

3.1.1. Nature

Sulphur dioxide is a colorless gas with sharp, choking odor. It is primary pollutant

because it is directly emitted in the form of SO2 Sulfur dioxide (SO2) is an irritating gas

which can be retained to 90 % in the nasal passages on inhalation owing to its high

solubility in water.

It can reach the lower respiratory passages if its concentration is very high, if it is

adsorbed on dust particles, or if breathing is mainly through the mouth. Dust particles

can then catalyze the production of sulfur trioxide (SO3).

SO2 is absorbed from the respiratory tract into the blood. After biological conversion to

sulfate, it is mainly excreted in the urine. Only a few minutes after exposure, the

distribution of sulfates by the blood into the tissues and organs and their excretion in the

urine can be detected. Radioactively labeled sulfur compounds can be found in the lungs

up to one week after exposure.

The effects of sulfur dioxide and of the acids formed from it vary in severity, depending

on the state of health of the exposed person. Numerous studies have shown that

changes in lung function only occur in healthy subjects at very high SO2 concentrations.

3.1.2. Quantity

Roughly 25 million tons of SO2 are discharged into the atmosphere in US alone annually.

3.1.3. Sources

1. Coal, oil and all other fossil fuels naturally contain some sulphur, because the

plant material, from which they are formed, included sulphur containing

compounds.


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2. Coal frequently contains additional sulphur compounds in the form of mineral

pyrite (FeS).

3. Coal mined from different locations, contains typically between 1% and 7% (by

weight).

4.

More than 80% of the emissions result from fossil fuel combustion, and most of

them is from electric utility power plants.

5. Only a very small amount comes from mobile sources.

6. Other sources of SO2 emissions are petroleum refining, copper smelting and the

making of cement.

Fig no.3.1

3.1.4. Methods for controlling sulphur dioxide emissions

As a result of clean air act 1970 and amendments made to it in 1990, coal fired electric

power plants were required to make significant reductions in their emissions of SO 2.

Reductions can be achieved in two ways.


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1. Sulphur can be removed before combustion, or

2. Sulphur dioxide can be removed from the smokestack after combustion but

before it reaches the atmosphere. The second cheaper source is generally chosen.

The most commonly used method is flue gas desulfurization (FGD) in which

sulphur containing compounds are washed out (or scrubbed) by passing the flue

gases through a slurry of water mixed with finely ground limestone (CaCO3) or

dolomite Ca.Mg(CO3)2 or both. On heating, the basic calcium carbonate reacts

with acidic SO2 and oxygen, to form calcium sulfate. Scrubbers which remove up

to 99% of the SO2in the flue gas are easily available.

2SO2+2CaCO3+O22CaSO4+ 2CO2

The figure shown below illustrates flue gas desulfurization

Fig no.3.2

3. A promising newer method is fluidized bed combustion (FBC) a process in

which a mixture of pulverized coal and powdered limestone is burned, with air

being introduced to keep the mixture in semi fluid state. The limestone is

converted to CaSO4 according to the previous equation. In this process because

the coal is so finely divided the reaction occurs at a lower temperature, and as a


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result, the quantity of NOx emitted is much low (because NOx formation is

reduced at lower temperature).

The figure shown below illustrates fluidized bed combustion (FBC)

Fig no.3.3

3.1.5. Conversion of SO2to SO3and then to H2SO4

(In additional chemical reactions, thus forming secondary pollutants i.e. SO 3and H2SO4)

SO2reacts with oxygen to form SO3, which then reacts with water vapour to form a mist

of sulfuric acid.

2SO2+ O2 2SO3

SO3+ H2O H2SO4

3.1.6. The sulphuric acid mist

The sulphuric acid mist is a secondary pollutant because it is not emitted directly, but is

formed subsequently in the atmosphere. It is a constituent of acid rain, an importantair pollution problem.


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Sulfuric acid molecules may also condense on existing particles in the air, and sulfate

aerosols (a suspension of fine particles in a gas, e.g. fog) often compose a significant

fraction af particulate pollution in the atmosphere. The sulfur pollution eventually

reaches the ground by wet deposition (i.e. as acid rain) or dry deposition (without

precipitation).

3.1.7. Effects of SO2on Human Health and the Environment:

1. SO2 is a colorless, toxic gas with sharp pungent odor.

2. Exposure to it cause irritation to the eyes and respiratory passages and

aggravates symptoms of respiratory diseases.

3.

Children and elderly are especially susceptible to its effects.

4. SO2 is also harmful to plants. Crop such as barley, cotton and wheat are

particularly affected adversely.

5. Tall smokestacks (1200-feet high), used to protect local agriculture form the

release of SO2, produces a problem elsewhere; SO2 emissions from North

America have been detected as far away as Greenland.

3.2. Nitrogen Oxides

The nitrogen oxides NO and NO2are important air pollutants. Health hazards are almost

exclusively associated with NO2and its reaction products, while NO acts as a reservoir

from which nitrogen dioxide is produced. Nitrogen dioxide is a strongly irritant gas. On

inhalation, the mucous membranes of the respiratory tract are attacked, the gas reacting

immediately with the moisture present there and on alveolar surfaces. It probably does

not reach the lung capillaries, although lung function is affected. On inhalation, 80 90

% of NO2is absorbed in the respiratory tract. Absorption possibly occurs by formation of

nitrous and nitric acids or their salts, as indicated by the detection of nitrite and nitrate

in the urine and blood after NO2inhalation.

The odor threshold lies in the range 200 440 g/m3. Under controlled conditions,

effects on the lung function of healthy experimental subjects are detectable above a

concentration of 3800 g NO2/m3.


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3.2.1. Nature, Effects on Human Health and the Environment

1) There are many forms of nitrogen oxides, but the one of greatest importance is

nitrogen dioxide (NO2).

2)

Most emissions are initially in the form of Nitric Oxide (NO) , which by itself is not

harmful at concentrations usually found in the atmosphere, NO is colourless. But

NO is readily oxidized to NO2, which in the presence of sunlight can further react

with hydrocarbons to form photochemical smog. Smog is of course harmful.

3) NO2 is a pungent, irritating, red-brown gas that tends to give smog a reddish

brown colour.

4) NO2also reacts with the hydroxyl radical (OH) to form nitric acid (HNO3), which

contributes to the problem of acid rain.

5)

The harmful effect of nitrous oxide (NO2) on the ozone layer in the upper

atmosphere has already being recognized, but inadequate information is available

at the moment to make clear, its mechanism of formation, and the exact

magnitude of its effect.

3.2.2. Quantity

More than 20 million tons of nitrogen oxides are discharged each year in the UnitedSates.

3.2.3. Sources

1) The largest source is from the oxidation of nitrogen compounds, during the

combustion of certain fossil fuels such as coal or gasoline.

2)

Nitrogen oxides are also formed when temperatures are high enough to oxidizenitrogen in the combustion air.

3) At normal atmospheric temperature nitrogen and oxygen, the two main

components of air, do not react with each other, however at the very high

temperatures that exist in the internal combustion engine and in industrial

furnaces, normally unreactive atmospheric nitrogen reacts with oxygen to form

NO2:

N2+ O2 2NO

When released to atmosphere, NO combines rapidly with atmospheric oxygen to

form NO2:


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2NO + O2 2NO2

4) Stationary sources are the major contributions of nitrogen oxides (power Plants),

although mobile sources (automobiles air crafts) are also important.

5) Ironically, modifications in the operation of internal combustion engines, meant to

control carbon monoxide emissions, such as increasing the air supply and raising

the combustion temperature, tend to make the NOx problem worse.

6) A: Far more NOx are released to the atmosphere by natural processes than by

human activities, During electrical storms, atmospheric nitrogen and oxygen react

to form NO, which then rapidly combines with more atmospheric oxygen to form

NO2, as shown in the previous equations.

B: Bacteria decomposition of nitrogen-containing organic matter in soil is another

natural source of NOx. Because emissions from natural processes are widely

dispersed, they do not have an adverse effect on the environment.

3.2.4. The Fate of Atmospheric NOx:

1) NO2 regardless of its source is ultimately removed from the atmosphere as nitric

acid in rainfall and dust (fine particles of solid matter, whether lying as a deposit

or carried in the air).

2) NO2 combines with water vapour to form nitric acid.

4NO2+ 2H2O + O2 4HNO3

3) Much of the nitric acid in the atmosphere is formed within aqueous aerosols. If

weather conditions are right, the aerosols form larger droplets in clouds and the

result is acid rain.

4) Some of the nitric acid formed reacts with ammonia and metallic particles on the

atmosphere to form nitrates:

HNO3+ NH3 NH4NO3

5) Nitrates dissolve in rain and snow, or settle as particles. The combined fallout

contributes to acid deposition.

3.3. Ozone

Ozone (O3) is a highly reactive gas that can change or decompose enzymes, coenzymes,

and proteins by oxidizing thiol groups, and can oxidize fatty acids to toxic fatty acid

peroxides. Protein and lipid membranes are the main sites of attack by ozone. Ozone is


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mainly absorbed via the respiratory system, where rapid reaction with the tissues

occurs. In many animals, acute exposure to high concentrations of the gas (440 1000

g/m3) causes rapid, shallow breathing, increased resistance of the respiratory system,

reduced vital capacity, and bronchial hyper reactivity. Younger animals show higher

sensitivity to these concentrations of ozone. An 8 C increase in the air temperature

doubles the toxic effects.

3.4. Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas, with low solubility in

water. It is formed by then incomplete combustion of carbon or carbon-containing

compounds. Carbon monoxide affects humans by combining with hemoglobin (Hb) to

form carboxyhemoglobin (COHb). Carbon monoxide blocks the divalent iron center of Hb

by displacing oxygen without change of oxidation state. Carbon monoxide forms a bond

with hemoglobin that is ca. 240 times as strong as that formed by oxygen. Therefore,

even at comparatively very low concentrations it blocks the oxygen-binding site, and

prevents oxygen transport by the blood. All the symptoms of carbon monoxide

intoxication are due to oxygen deficiency in the tissues and the accumulation of carbon

dioxide during normal metabolism.

The symptoms of carbon monoxide intoxication with increasing COHb content of the

blood are as follows

5 10 % Slight, measurable visual impairment (threshold of fusion frequency)

10 20 % Slight headache, lassitude, malaise, shortness of breath with exertion,

palpitations

20 30 % Vertigo, partial loss of consciousness, limpness and paralysis of limbs

30 40 % Pink skin color, loss of consciousness, shallow breathing, circulatory

collapse

40 60 % Deep unconsciousness, paralysis, Cheyne-Stokes respiration, decrease

in body temperature

60 70 % Death within 10 min to 1 h

>70 % Death within a few minutes6

3.5. Dusts


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Dusts are defined as dispersions of solid materials in gases. The particles can be of any

shape, structure, and density, and can be up to ca. 100 m in size. They are formed

either by entrainment or mechanical processes. Like smoke and fog, dusts are types of

aerosol. The health hazard due to dusts depends on chemical composition,

concentration, exposure time, and especially particle size. This last parameter

distinguishes dusts from gases and vapors. Dusts are principally absorbed by respiration.

The transport and deposition of a dust in the respiratory tract are mainly determined by

the behavior of particles in flowing gases.

The characteristic parameter is the aerodynamic diameter dae of a particle. The

aerodynamic diameter of a particle of a given shape and density is given by the diameter

of a sphere of unit density (1 g/cm3) which has the same sedimentation rate as that of

the particle in still air or air moving with laminar flow. Fine particles of size 50 m, medium-size dust: 10 50 m, fine dust: 0.5

10 m, and very fine dust:


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various environmental media (air, water, soil, and food) and into the toxicity of lead to

humans and animals. These are documented in the literature in a large number of

comprehensive reports. Lead absorption by the human organism depends on the

solubility of the lead compounds and the composition of the food. Deficiencies of calcium

and vitamin D increase the absorption. The amount of inhaled lead-containing dust taken

up by the organism is determined by the deposition, elimination, and absorption of the

inhaled dust particles. It is assumed that 90 % of inhaled lead is absorbed.

The absorbed lead first enters the blood, and is distributed among the various organs

and tissues. Approximately 90 % of the lead in the blood is bound to components of the

erythrocytes. Lead inhibits various enzymes and thus affects several stages of

hemoglobin synthesis.

3.7. Cadmium

The heavy metal cadmium is a nonessential trace element. It is absorbed orally and by

inhalation, skin absorption being practically insignificant. The effects of cadmium depend

on the quantity of cadmium absorbed into the blood. Cadmium has a marked tendency

to accumulate in the human organism, mainly in the kidneys and liver. Thus, cadmium

concentrations in the liver and kidneys increase with increasing age, and also in the

blood and the urine. The most important factor is cigarette smoking. Cadmium

concentrations in the blood of smokers are 3 5 times as high as those for nonsmokers,

and cadmium concentrations in the renal cortex of smokers are ca. twice those for

nonsmokers.

3.8. Arsenic

Arsenic is a metalloid which is ubiquitous in the earth's crust in various forms.

Toxicologically, inorganic arsenic compounds, organic arsenic compounds, and arsenic

hydride (arsine) must be considered separately. Arsenic is mainly ingested orally by

humans in food and drinking water, but is also inhaled. The extent of absorption depends

on the type of arsenic compound. Arsenic can also be absorbed through the skin.

Organic arsenic compounds are ingested mainly during the consumption of fish, mussels,

and crustaceans, the main compounds concerned being arsenobetaine, arsenocholine,

and trimethylarsonium lactate and its derivatives.


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Arsenic is rapidly transported within the body by the blood. It becomes distributed in

human tissue in varying concentrations, being more concentrated in the skin, hair and

nails, the lungs, the bones, and the brain.7

Ch#4 Air pollution sources and its prevention

The sources of air pollution are nearly as numerous as the grains of sand. In fact, the

grains of sand themselves are air pollutants when the wind entrains them and they

become airborne. We would class them as a natural. There are mainly two types of air

pollution sources

Natural sources

Anthropogenic sources

4.1. Natural sources

An erupting volcano emits particulate matter. Pollutant gases such as SO2/ H2S, and

methane are also emitted. The emission from an eruption may be of such magnitude

as to harm the environment for a considerable distance from the volcanic source.

Dust storms that entrain large amounts of particulate matter are a common natural

source of air pollution in many parts of the world. Even a relatively small dust stormcan result in suspended particulate matter readings one or two orders of magnitude

above ambient air quality standards. Visibility reduction during major dust storms is

frequently the cause of severe highway accidents and can even affect air travel. The

particulate matter transferred by dust storms from the desert to urban areas causes

problems to householders, industry, and automobiles. The materials removed by the

air cleaner of an automobile are primarily may become airborne.

An extensive source of natural pollutants is the plants and trees of the earth. Even

though these green plants play a large part in the conversion of carbon dioxide to

oxygen through photosynthesis, they are still the major source of hydrocarbons on


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the planet. The familiar blue haze over forested areas is nearly all from the

atmospheric reactions of the volatile organics given off by the trees of the forest (1).

Another air pollutant problem, which can be attributed to plant life, is the pollens

which cause respiratory distress and allergic reactions in humans.

Other natural sources, such as alkaline and saltwater lakes, are usually quite local in

their effect on the environment. Sulfurous gases from hot springs also fall into this

category in that the odor is extremely strong when close to source.

4.2. Anthropogenic sources

4.2.2. Industrial sources

A great deal of industrial pollution comes from manufacturing products from raw

materials(1) iron from ore, (2) lumber from trees, (3) gasoline from crude oil, and (4)

stone from quarries. Each of these manufacturing processes produces a product, along

with several waste products which we term pollutants. Occasionally, part or all of the

polluting material can be recovered and converted into a usable product.

Industrial pollution is also emitted by industries that convert products to other

products(1) automobile bodies from steel, (2) furniture from lumber, (3) paint from

solids and solvents, and (4) asphaltic paving from rock and oil.

Industrial sources are stationary, and each emits relatively consistent qualities and

quantities of pollutants. A paper mill, for example, will be in the same place tomorrow

that it is today, emitting the same quantity of the same kinds of pollutants unless a

major process change is made. Control of industrial sources can usually be accomplished

by applying known technology.

The most effective regulatory control is that which is applied uniformly within all

segments of industries in a given region, e.g., "Emission from all asphalt plant dryers in

this region shall not exceed 230 mg of particulate matter.


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Table 4.2.2.18


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4.2.2.1. Air pollution from chlor-alkali plants

There are three basic processes for the manufacture of chlorine and caustic soda from

brine: the mercury cell, the diaphragm cell, and the membrane cell. Among these

technologies, the membrane cell is the most modem and has both economic and

environmental advantages. The other two processes generate hazardous wastes

containing mercury or asbestos. Mercury cell technology is being phased out in

worldwide production.

Chlorine is a highly toxic gas, and strict precautions are necessary to minimize risk to

workers and possible releases during its handling. Major sources of fugitive air emissions

of chlorine and hydrogen are vents, seals, and transfer operations.

For the chlor-alkali industry, an emergency preparedness and response plan is

mandatory for potential uncontrolled chlorine and other releases. Carbon tetrachloride is

sometimes used to scrub nitrogen trichloride (formed in the process) and to maintain its

levels below 4% to avoid fire and explosion.

Substitutes for carbon tetrachloride may have to be used, as the use of carbon

tetrachloride may be banned in the near future due to its carcinogenicity.

4.2.2.1.1. Its prevention

Implementation of cleaner production processes and pollution prevention measures can

yield both economic and environmental benefits. The primary treatment technologies

afforded to this manufacturing include the following:

Caustic scrubber systems should be installed to control chlorine emissions from

condensers and at storage and transfer points for liquid chlorine.

Sulfuric acid used for drying chlorine should be neutralized before discharge.

Chlorine monitors should be strategically located within the plant to detect

chlorine releases or leaks on a continuous basis.

Monitoring data should be analyzed and reviewed at regular intervals and

compared with the operating standards so that any necessary corrective actions

can be taken.

Records of monitoring results should be kept in an acceptable format.

The results should be reported to the responsible authorities and relevant parties,

as required.


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Preference should be given to the membrane process due to its less polluting

characteristics over other technologies.

In addition, the scrubbing of chlorine from tail gases to produce hypochlorite is

highly recommended.

4.2.2.2. Air pollution from agro-industry chemicals

4.2.2.2.1 Mixed Fertilizer Plants

Mixed fertilizers contain two or more of the elements nitrogen, phosphorus, and

potassium (NPK), which are essential for good plant growth and high crop yields. This

subsection briefly addresses the production of ammonium phosphates (monoammonium

phosphate, or MAP, and diammonium phosphate, or DAP), nitrophosphates, potash, and

compound fertilizers.

Compound fertilizers can be made by blending basic fertilizers such as ammonium

nitrate, MAP, DAP, and granular potash; this route may involve a granulation process.

The principal pollutants from the production of MAP and DAP are ammonia and fluorides,

which are given off in the steam from the reaction. Fluorides and dust are released from

materials-handling operations. Ammonia in uncontrolled air emissions has been reported

to range from 0.1 to 7.8 kilograms of nitrogen per metric ton (kg/t) of product, with

phosphorus ranging from 0.02 to 2.5 kg/t product (as phosphorous pentoxide, P2O5).

In nitrophosphate production, dust will also contain fluorides. Nitrogen oxides NO, are

given off at the digester. In the evaporation stage, fluorine compounds and ammonia are

released. Unabated emissions for nitrogen oxides from selected processes are less than

1,OOOmilligrams per cubic meter (mg/m3) from digestion of phosphate rock with nitric

acid, 50 to 200 (mg/m3

) from neutralization with ammonia, and 30 to 200 mg/m3

fromgranulation and drying. Dust is the primary air pollutant from potash manufacturing.

4.2.2.2.1.1. Its prevention

Materials handling and milling of phosphate should be carried out in closed

buildings. Fugitive emissions can be controlled by, for example, hoods on

conveying equipment, with capture of the dust in fabric filters.

In the ammonium phosphate plant, the gas streams from the reactor, granulator,

dryer, and cooler should be passed through cyclones and scrubbers, using


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phosphoric acid as the scrubbing liquid, to recover particulates, ammonia, and

other materials for recycling.

In the nitrophosphate plant, nitrogen oxide (NO,) emissions should be avoided by

adding urea to the digestion stage.

Fluoride emissions should be prevented by scrubbing the gases with water.

Ammonia should be removed by scrubbing.

Phosphoric acid may be used for scrubbing where the ammonia load is high.

The process water system should be balanced, if necessary, by the use of holding

tanks to avoid the discharge of an effluent.

Air emissions at point of discharge should be monitored continuously for fluorides

and particulates and annually for ammonia and nitrogen oxides.

Monitoring data should be analyzed and reviewed at regular intervals and

compared with the operating standards so that any necessary corrective actions

can be taken. Records of monitoring results should be kept in an acceptable

format. The results should be reported to the responsible authorities and relevant

parties, as required.

Maximize product recovery and minimize air emissions by appropriate

maintenance and operation of scrubbers and bag houses.

Prepare and implement an emergency preparedness and response plan. Such a

plan is required because of the large quantities of ammonia and other hazardous

materials stored and handled on site. Where possible, use natural gas as the feedstock for the ammonia plant, to

minimize air emissions.

Use hot process gas from the secondary reformer to heat the primary reformer

tubes (the exchanger-reformer concept), thus reducing the need for natural gas.

Direct hydrogen cyanide (HCN) gas in a fuel oil gasification plant to a combustion

unit to prevent its release.

Consider using purge gases from the synthesis process to fire the reformer; strip

condensates to reduce ammonia and methanol. Use carbon dioxide removal processes that do not release toxics to the

environment. When monoethanolamine (MEA) or other processes, such as hot

potassium carbonate, are used in carbon dioxide removal, proper operation and

maintenance procedures should be followed to minimize releases to the

environment.


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4.2.2.2.2. Nitrogenous fertilizer plants

Emissions to the atmosphere from ammonia plants include sulfur dioxide (SO2), nitrogen

oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S),

volatile organic compounds (VOCs), particulate matter, methane, hydrogen cyanide, and

ammonia. The two primary sources of pollutants, with typical reported values, in

kilograms per ton (kg/t) for the important pollutants, are as follows:

Flue gas from primary reformer: CO2: 500 kg/t NH3NOx: 0.6 to 1.3 kg/t

NH3as NO2SO2: less than 0.1 kg/t; CO: less than 0.03 kg/t.

Carbon dioxide removal: CO2: 1,200 kg/t.

4.2.2.2.2.1. Prevention

Nitrogen oxides are reduced, for example, when there is low excess oxygen, with

steam injection when post-combustion measures are in place; and when low-NO,

burners are in use. Other measures will also reduce the total amount of nitrogen

oxides emitted.

4.2.2.3. Pollution from petroleum refineries

Natural gas and crude distillates such as naphtha from petroleum refining are used as

feedstock to manufacture a wide range of petrochemicals that are in turn used in the

manufacture of consumer goods. Basic petrochemicals are manufactured by cracking,

reforming, and other processes, and include olefins (such as ethylene, propylene,

butylenes, and butadiene) and aromatics (such as benzene, toluene, and xylenes). The

capacity of naphtha crackers is generally of the order of 250,000 to 750,000 metric tons

per year (tpy) of ethylene production.

Compounds considered carcinogenic that may be present in air emissions include

benzene, butadiene, 1,2-dichloroethane, and vinyl chloride. A typical naphtha cracker at

a petrochemical complex may release annually about 2,500 metric tons of alkenes, such

as propylenes and ethylene, in producing 500,000 metric tons of ethylene. Boilers,

process heaters, flares, and other process equipment (which in some cases may include

catalyst regenerators) are responsible for the emission of PM (particulate matter),

carbon monoxide, nitrogen oxides (200 tpy), based on 500,000 tpy of ethylene capacity,and sulfur oxides (600 tpy).


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The release of volatile organic compounds (VOCs) into the air depends on the products

handled at the plant. VOCs released may include acetaldehyde acetone, benzene,

toluene, trichloroethylene, trichlorotoluene, and xylene. VOC emissions are mostly

fugitive and depend on the production processes, materials handling and effluent-

treatment procedures, equipment maintenance, and climatic conditions. VOC emissions

from a naphtha cracker range from 0.6 to 10 kilograms per metric ton (kg/t) of ethylene

produced. Of these emissions, 75% consists of alkanes, 20% of unsaturated

hydrocarbons, about half of which is ethylene, and 5% of aromatics.

The WHO (World Health Organization) recommended air emissions levels are

summarized in table below although these are not legal standards, they provide us with

a general sense of emission targets to strive for to meet safe health risk exposure levels.

Table 4.2.2.3.1.8a

4.3. Pollution caused by motor vehicles

The principal pollutants emitted from simple gasoline-powered IC engines are carbon

monoxide, hydrocarbons, and nitrogen oxides. All these are formed in all other

combustion processes, e.g., fossil fuel power plants, kitchen stoves, campfires, and

charcoal barbecues. Auto engines produce more of them per unit of fuel burned

principally for the following reasons:

Auto engines are often oxygen deficient, which most other combustion systemsare not.


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Auto engines preheat their air-fuel mixtures, which most combustion systems do

not.

Auto engines have unsteady combustion, in which each flame lasts about

0.0025s.

Almost all other combustion systems have steady flames that stand still while the

materials burned pass through them.

Auto engines have flames that directly contact cooled surfaces, which is not

common in other combustion systems.

Table 4.3.1.

4.3.1. Prevention

The exhaust emissions from gasoline-powered vehicles are the most difficult to control.

These emissions are influenced by such factors as gasoline formulation, air-fuel ratio,

ignition timing, compression ratio, engine speed and load, engine deposits, engine

condition, coolant temperature, and combustion chamber configuration. Consideration of

control methods must be based on elimination or destruction of unburned hydrocarbons,

carbon monoxide, and oxides of nitrogen. Methods used to control one pollutant may

actually increase the emission of another requiring even more extensive controls.

Fuel modification in terms of volatility, hydrocarbon types, or additive content.

Some of the fuels currently being used are liquefied petroleum gas (LPG),

liquefied natural gas (LNG), compressed natural gas (CNG), fuels with alcohol

additives, and unleaded gasoline. The supply of some of these fuels is very

limited. Other fuel problems involving storage, distribution, power requirements

have to be considered.

Minimization of pollutants from the combustion chamber. This approach consists

of designing the engine with improved fuel-air distribution systems, ignition


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timing, fuel-air ratios, coolant and mixture temperatures, and engine speeds for

minimum emissions.

Further oxidation of the pollutants outside the combustion chamber. This

oxidation may be either by normal combustion or by catalytic oxidation. These

systems require the addition of air into the exhaust manifold at a point

downstream from the exhaust valve.

Ch#5 Air pollution control equipment

5.1 Wet scrubbers

In wet scrubbing, an atomized liquid, usually water, is used to capture particulate dust

or to increase the size of aerosols. Increasing size facilitates separation of the particulate

from the carrier gas. Wet scrubbing can effectively remove fine particles in the range

from 0.1m to 20m.

The particles may be caught first by the liquid, or first on the scrubber structure, and

then washed off by the liquid. Because most conventional scrubbers depend upon some

form of inertial collection of particulates as the primary mechanism of capture, scrubbers

when used in a conventional way have a limited capacity for controlling fine particulates.

Unfortunately inertial forces become insignificantly small as particle size decreases, and

collection efficiency decreases rapidly as particle size decreases. As a result, it becomes

necessary to greatly increase the energy input to a wet scrubber to significantly improve

the efficiency of collection of fine particles.

5.1.1. Diagram


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Fig no.5.110

5.1.2. Advantages

Wet scrubbers have some unique characteristics useful for fine particulate control.

Since the captured particles are trapped in a liquid, re-entrainment is avoided,

and the trapped particles can be easily removed from the collection device.

Wet scrubbers can be used with high-temperature gases where cooling of the gas

is acceptable and also with potentially explosive gases.

Scrubbers are relatively inexpensive when removal of fine particulates is not

critical.

Also, scrubbers are operated more easily than other sophisticated types of

particulate removal equipment.

Wet scrubbers can be employed for the dual purpose of absorbing gaseous

pollutants while removing particulates.


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Both horizontal and vertical spray towers have been used extensively to control

gaseous emissions when particulates are present.

Cyclonic spray towers may provide slightly better particulate collection as well as

higher mass transfer coefficients and more transfer units per tower than other

designs.

5.1.3. Disadvantages

The disadvantages of wet scrubbers include the necessity of reheating cooled

scrubber effluents for discharge up a stack.

Furthermore, the water solutions may freeze in winter and become corrosive at

other times.

In some cases, the resultant liquid sludge discharge may have to be treated for

disposal.

It should be noted also that operating costs can become excessive due to the

high energy requirements to achieve high collection efficiencies for removal of

fine particulates.

Even with great energy inputs, wet scrubber collection efficiencies are not high

with particles less than 1.0m in size.11

5.1.4. Collection mechanisms and efficiency

In wet scrubbers, collection mechanisms such as inertial impaction, direct interception,

Brownian diffusion, and gravity settling apply in the collection process. Most wet

scrubbers will use a combination of these mechanisms; therefore, it is difficult to classify

a scrubber as predominately using one particular type of collection mechanism.

Nearly all wet separation devices use the same three capture mechanisms.

These are:

Impaction

Interception

Diffusion

As discussed above.

However, inertial impaction and direct interception play major roles in most wet

scrubbers. Thus, in order to capture finer particles efficiently, greater energy must be

expended on the gas. This energy may be expended primarily in the gas pressure drop

or in atomization of large quantities of water. Efficiency of collection may be

unexpectedly enhanced in a wet scrubber through methods that cause particle growth.

Particle growth can be brought about by vapor condensation, high turbulence, or thermal

forces in the confines of the narrow passages in the scrubber structure.


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Condensation, the most common growth mechanism, occurs when a hot gas is cooled or

compressed. The condensation will occur preferentially on existing particles rather than

producing new nuclei.

Thus, the dust particles will grow larger and will be more easily collected. When

hydrophobic dust particles must be collected, there is evidence that the addition of small

quantities of non foaming surfactants may enhance collection. The table given below

shows various collection efficiencies of different wet scrubbers.12

Table no.5.1

Figure 5.2shows a target droplet being impacted by a particle. The particle has sufficient

inertia to follow a predicted course into the droplet. Once inside the droplet, the

combined particle/droplet size is aerodynamically much larger, therefore the separation

task becomes easier. Simply separate the droplet from the gas stream (more on that

later) and one removes the particle(s).

Fig no.5.2


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Figure 5.3 shows a particle, perhaps a bit smaller, moving along the gas stream lines

and being intercepted at the droplet surface. The particle in this case comes close

enough to the droplet surface that it is attracted to that surface and is combined with the

droplet. Again, once the particles are intercepted, the bigger droplet is easier to remove.

Fig no.5.313

5.2. Dry cyclone collectors

Cyclone collectors are used for product recovery of dry dusts and powders and as

primary collectors on high dust loading (more than 2 to 5 grs/dscf) air pollution control

applications. Cyclones are very common particulate control devices used in many

applications, especially those where relatively large particles need to be collected.

Cyclones are very simple devices that use centrifugal force to separate particles from a

gas stream. They commonly are constructed of sheet metal, although other materials

can be used.

A common application is the rotary dryer. Used to dehydrate various products from grain

to manure, direct or indirect fired rotary dryers often use cyclone collectors to capture

the entrained dust prior to a secondary collector (such as a Venturi scrubber). The

rotating action of the dryer entrains a portion of the product as the product tumbles

through the hot, drying air. This product is often valuable in dry form so the cyclone is

used to disengage the dust from the gas stream and be recovered. The residual dust is

air-conveyed to the downstream device.

Another application is on woodwaste or bagasse (sugar cane) boilers where light

entrained ash can be collected in suitably designed cyclones. On woodwaste applications,

smaller diameter cyclones are often used in banks where each cyclone handles less

than 1000 acfm of flue gas. These are called multiple cyclone collectors.

14


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5.2.1. Diagram

Fig no. 5.415

5.2.3. Collection mechanism

The operating principle of a cyclone is based on using centrifugal force to move particles

to the cyclone wall. The gas stream is typically directed into a cylindrical portion of the

device so that a spinning motion is created and sustained for a required number of turns

or revolutions to achieve the desired separation.

In general, the more spin cycles or turns imparted to the gas stream, the greater the

separation efficiency. Cyclone collector housings are therefore designed to provide

varying number of spins or turns, depending on the application. Centrifugal force and, to

a lesser extent, settling are the forces used in cyclone collectors. Contradicting forces

and effects are same-charge electrostatic forces that could inhibit separation as well as

the friability of the particles themselves.


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Some particulate acquires a charge as it passes through ductwork or a cyclone

(piezoelectric effect) thereby making separation more difficult. If the particulate or dust

becomes reduced in size, it makes it more difficult to collect because the effective

centrifugal force applied to the particle is a function of its mass.

5.2.3. Advantages

Cyclones are able to handle very heavy dust loading.

They can be used in high temperature gas streams.

Cyclones are cost-effective devices.

Require low maintenance.

They also reduce loading on the primary collector and allow for the dry recovery

of product.


High operating costs (owing to power required to overcome pressure drop).

Cyclones have low efficiencies in removing fine particulate. Because small

particles have little mass that can generate a centrifugal force.16

5.2.5. Typical cyclone dimensions

Table no. 5.217


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5.3. Electrostatic precipitators

Electrostatic precipitators are used for the purpose of removing dry particulate matter

from gas streams. They basically apply an electrostatic charge to the particulate and

provide sufficient surface area for that particulate to migrate to the collecting plate and

be captured. The collecting plates are rapped periodically to disengage the collected

particulate into a receiving hopper.

An ESP controls particulate emissions by: (1) charging the particles, (2) applying an

electric field to move the particles out of the gas stream, and then (3) removing the

collected dust. Particles are charged by gas ions that are formed by corona discharge

from the electrodes. The ions become attached to the particles, thus providing the

charge.

In a typical ESP, vertical wires are used as the negative discharge electrode between

vertical, flat, grounded plates. The dirty gas stream passes horizontally between the

plates and a dust layer of particulate collects on the plates. The typical spacing between

the discharge electrode and the collector plate is 4 to 6 in. The dust layer is removed

from the plates by rapping, or in the case of a wet ESP, by washing with water.

An alternative to the alternative to the plate and wire design is the tube and wire design,

in which the discharge electrode wire is fixed in the center of a vertical tubular collection

electrode. In this configuration, the gas flow is parallel to the discharge electrode.

Dry electrostatic precipitators are used to remove particulate matter from flue gas

streams exiting cement kilns, utility and industrial power boilers, catalytic crackers,

paper mills, metals processing, glass furnaces, and a wide variety of industrial

applications. An electrostatic precipitator is a constant pressure drop, variable emission

particulate removal device offering exceptionally high particulate removal efficiency.18


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5.3.1. Diagram

Fig no.5.519


The basic principle of an electrostatic precipitator is to attract charged dust particles to

the collecting plates where they can be removed from the gas stream. Dust entering the

precipitator is charged by a corona discharge leaving the electrodes. Corona is plasma

containing electrons and negatively charged ions. Most industrial electrostatic

precipitators use negative discharge corona for charging dust.

When charged, the dust particles are driven toward the collecting plates by the

electromagnetic force created by the voltage potential applied to the dischargeelectrodes. An electrostatic precipitator contains multiple mechanical fields located in

series and parallel to the direction of gas flow.

Each mechanical field is comprised of a group of collecting plates that define a series of

parallel gas passages. These passages run in the direction of gas flow. Bisecting the gas

passage is a series of discharge electrodes, also running in the direction of gas flow. A

mechanical field contains one or more electrical fields. A single transformer rectifier

serves each electrical field. There can be multiple electrical sections contained in a singleelectrical field.20


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5.3.2. Advantages

An electrostatic precipitator is a constant pressure drop, variable emission

particulate removal device offering exceptionally high particulate removal

efficiency.

This is more effective to remove very small particles like smoke, mist and fly ash.

Its range of dust removal is sufficiently large (0.01 micron to 1.00 micron).

The small dust particles below 10 microns cannot be removed with the help of

mechanical separators and wet scrubbers cannot be used if sufficient water is not

available. Under these circumstances, this type is very effective.

This is also most effective for high dust loaded gas (as high as 100 grams per

cu.meter)

The draught loss of this system is the least of all forms(1 cm of water)

It provides ease of operation.

The dust is collected in dry form and can be removed either dry or wet.


The direct current is not available with the modern plants, therefore considerable

electrical equipment is necessary to convert low voltage (400 V) A.C to high

voltage (60000 V) D.C. This increases the capital cost of the equipment as highas 40 to 60 cents per 1000 kg of rated installed steam generating capacity.

The running charges are also considerably high as the amount of power required

for charging is considerably large.

The space required is larger than the wet system.

The efficiency of the collector is not maintained if the gas velocity exceeds that

for which the plant is designed. The dust carried with the gases increases with an

increase of gas velocity.

Because of closeness of the charged plates and high potential used, it is

necessary to protect the entire collector from sparking by providing a fine mesh

before the ionizing chamber. This is necessary because even a smallest piece of

paper might cause sparking when it would be carried across adjacent plates or

wires.21


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5.4. Fabric filter collectors

Fabric filter collectors, or baghouses, separate particulate from gas stream by causing

the particulate to pass through a filtering media, a layer of previously collected (or

purposely deposited) particulate, or both. The gasborne particulate is intercepted by the

fibers of the filtering media, by the particulate already present on the media surface, or

both.

To prevent excessive pressure drop as the particulate accumulates, these devices use

various mechanisms to disengage the particulate from the media. The filter bags are

mounted on a tube sheet and enclosed in a sheet-metal housing. It must be understood

that the mechanism that achieves filtration of small particles from a gas stream is not

simple sieving. The spacing between fabric threads may be on the order of 50 to 75

microns, yet particles of 1 micron diameter and less are collected efficiently.22

5.4.1. Diagram

Figure no.5.623


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Fabric filter collectors primarily use sieving (a combination of impaction and interception)

as the collecting mechanism. The combined porosity of the media and any previously

accumulated particulate serve to produce small pores through which the new particulate

must attempt to pass. This filtering or sieving action relies on the fact that the net

opening at any given time is smaller than the particulate. Because the particle is bigger

than the opening, it cannot pass through. After collection on the media surface or in the

dust cake, various mechanisms are used to remove the particulate from the media. After

that, the particulate settles by gravity in the devices housing.24

5.4.3. Types

The three common types of baghouses based on cleaning methods

Reverse-air

Shaker

Pulse-jet

5.4.4. Advantages

Very high collection efficiencies possible (99.9 + per cent) with a wide range of

inlet grain loadings and particle size variations. Within certain limits fabric

collectors have a constancy of static pressure and efficiency, for a wider range of

particle sizes and concentrations than any other type of single dust collector.

Collection efficiency not affected by sulfur content of the combustion fuel as in

ESPs.

Reduced sensitivity to particle size distribution.

No high voltage requirements. Flammable dust may be collected.

Use of special fibers or filter aids enables sub-micron removal of smoke and

fumes.

Collectors available in a wide range of configurations, sizes, and inlet and outlet

locations.

They can operate over a wide range of volumetric flow rates

The pressure drops are reasonably low.

Fabric Filter houses are modular in design, and can be pre-assembled at the

factory.


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Fabric life may be substantially shortened in the presence of high acid or alkaline

atmospheres, especially at elevated temperatures.

Maximum operating temperature is limited to 550 degrees Fahrenheit, unless

special fabrics are used.

Collection of hygroscopic materials or condensation of moisture can lead to fabric

plugging, loss of cleaning efficiency, large pressure losses.

Certain dusts may require special fabric treatments to aid in reducing leakage or

to assist in cake removal.

Fabric bags tend to burn or melt readily at temperature extremes.

Fabric Filters require a large floor area.

The fabric is damaged at high temperature.

Ordinary fabrics cannot handle corrosive gases.25

5.5. Settling Chambers

One of the simplest (and oldest) air pollution control devices is the Settling chamber

These are also sometimes called knock out boxes or drop out boxes .The equipment is in

the form of a large chamber, which allows reduction of the gas velocity to a point where

the particulate it carries simply drops out. Today, settling chambers are used for coarse

removal of large particulate in advance of higher efficiency particulate control

equipment. They are rarely, if ever, used as the final gas cleaning device.

Settling chambers are primarily used to reduce the loading of particulate from sources

such as kilns, calciners, and mills or grinders that inherently produce high particulate

concentrations. If the particulate is valuable in a dry form, the settling chamber usually

is designed to settle out the smallest size particle that can economically be separated. If

the product is not valuable or further downstream particulate separation is to be used

(such as a cyclone, scrubber, or fabric filter collector), the chamber is usually sized to

afford some basic separation at low cost. They are often followed by product recovery

cyclones which are, in turn, followed by collectors designed for high efficiency collection

of the fine particulate that pass through the upstream devices.26


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5.5.1. Diagram

Figure no.5.727

5.5.2. Collection Mechanism

A settling chamber operates on the principle that if you slow a gas stream down

sufficiently, the solid particulate contained within that gas stream will settle out by

gravity. In general the larger the particle the faster the settling rate. In addition, larger

particles will settle out faster in a given moving gas stream than smaller particles.

Settling chambers are therefore designed to allow the mean gas stream velocity to slow

down to a point at or below the target particles settling velocity so that the particle

drops out within the confines of the chamber.

Because the particle settles at a given rate (i.e., distance per unit time) as predicted by

Stokes Law, the chamber must be sufficiently long to allow this settling to be completed

before the gas reaches the devices gas outlet. Settling chambers are therefore large in

cross-sectional area (to slow the gas stream down), and long, to allow sufficient time for

settling. The primary mechanism used is the drag force applied on the particle by the

viscosity of the carrier gas. As the gas stream slows down, the influence of the viscous

force of the gas on the particle is reduced and the particle begins to settle by primarily

gravitational forces.


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5.5.3. Advantages

Low capital cost.

Very low energy cost.

No moving parts therefore low maintenance requirements and low operating cost.

Excellent reliability.

Low pressure drop through device.

Device not subjected to abrasion due to low gas velocity.

Provide incidental cooling of gas stream.

Temperature and pressure limitations only dependant on the material of

construction.


Relatively low PM collection efficiencies, particularly for PM less than 50m in

size.

Unable to handle sticky materials.

Large physical size.

Trays may warp during high temperature operations.28

air pollution and its abatement and control in chemical industry

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