1. CONTROL OF GASEOUS POLLUTANTS

2. There are six main processes by which a gaseous pollutant may be removed from an air stream. Table 10.1, taken from the EPA handbook, lists those processes with the advantages and disadvantages of using each one.

3. Thermal Oxidation for VOC Control
Volatile organic compounds (VOCs) generally are fuels that are easily combustible.
Through combustion, which is synomonous with thermal oxidation and incineration, the organic compounds are oxidized to CO2 and water, while trace elements such as sulfur and chlorine are oxidized to species such as SO2 and HCl.

4. Three combustion processes that control vapor emissions by destroying collected vapors to prevent release to the environment are
thermal oxidation — flares,
(b) thermal oxidation and incineration, and
(c) catalytic oxidation
Each of these processes has unique advantages and disadvantages that require consideration for proper application.

5. COMBUSTION BASICS
Combustion requires the three legs of the fire triangle

6. COMPLETE AND INCOMPLETE COMBUSTION

7. COMPLETE COMBUSTION
In a combustion reaction, oxygen combines with another substance and releases energy in the form of heat and light.
When oxygen is available in sufficient amounts complete combustion occurs

8. COMPLETE COMBUSTION
This means that all of the carbon atoms and hydrogen atoms from the hydrocarbon molecules combine with oxygen atoms to form carbon dioxide and water.

9. The general equation of the complete combustion of a hydrocarbon is:
CxHy + O2 –> CO2 + H2O
CxHy + (4x+y)/4 O2 –> x CO2 + y/2 H2O

10. INCOMPLETE COMBUSTION
When a reaction has too little oxygen incomplete combustion is the result.
A bright yellow flame is produced during incomplete combustion. In addition to this, soot and toxic carbon monoxide can also be formed through incomplete combustion

11. INCOMPLETE COMBUSTION
An example equation of the incomplete combustion of propane is:
2C3H 8 (g) + 7O2 (g)  2C(s) + 2CO(g) + 2CO2(g) + 8H2O (g)

12. INCOMPLETE COMBUSTION
The products of incomplete combustion include carbon dioxide and water vapour as well as carbon, carbon monoxide or both
Incomplete combustion is a more inefficient process for generating heat since it has less oxygen and therefore more light is produced rather than heat. This will be seen with a more yellow flame.

13. Carbon Monoxide
Why is the formation of carbon monoxide a serious concern?
Carbon monoxide is a toxic gas that is both colourless and odourless. Carbon monoxide can bind to oxygen in the blood which will decrease the number of available oxygen binding sites in a person.
Symptoms of carbon monoxide poisoning include headache, dizziness, and nausea. Eventually suffocation can be a result of carbon monoxide poisoning.

14. Complete and Incomplete Combustion using Propane
Propane + oxygen  carbon dioxide + water
C3H8 (g) + 5 O2  3 CO2 + 4 H2O
Propane + oxygen  carbon + carbon dioxide + water
C3H8 (g) + 3 ½ O2  1 ½ C + 1 ½ CO2 + 4 H2O
Oxygen is limited and therefore carbon is produced as a result.

15. Stoichiometric Calculations

16. Applications of the Combustion Equation
Stoichiometric proportions for finding the correct air supply rate for a fuel
Composition of the combustion products is useful during the design, commissioning and routine maintenance of a combustion system

17. Combustion Air Requirements: Gaseous Fuels
Calculating the air required for gaseous fuels combustion is most convenient to work on a volumetric basis.
The stoichiometric combustion reaction of methane is : CH4 + 2O2 → CO2 + 2H2O which shows that each volume (normally 1 m3) of methane requires 2 volumes of oxygen to complete its combustion.

18. For the purposes of combustion calculations the composition of air is approximated as a simple mixture of oxygen and nitrogen: oxygen 21% nitrogen 79%

19. The complete relationship for stoichiometric combustion: CH4 + 2O2 + 7
The complete relationship for stoichiometric combustion: CH4 + 2O N2 → CO2 + 2H2O +7.52N2 as the volume of nitrogen will be 2×79÷21=7.52.
A very small amount of nitrogen is oxidized but the resulting oxides of nitrogen (NOX) are not formed in sufficient quantities to concern us here. However, they are highly significant in terms of air pollution.

20. It can be seen that the complete combustion of one volume of methane will require (2+7.52=9.52) volumes of air, so the stoichiometric air-to-fuel (A/F) ratio for methane is 9.52.

21. 3. Flue Gas Composition-Gaseous Fuels
The composition of the stoichiometric combustion products of methane is: 1 volume CO volumes N2 2 volumes H2O
Given a total product volume, per volume of fuel burned, of if water is in the vapor phase, or 8.52 if the water is condensed to a liquid. The two cases are usually abbreviated to “wet” and “dry”.

22. The proportion of carbon dioxide in this mixture is therefore

23. Considering the combustion of methane with 20% excess air, the excess air (0.2×9.52) of 1.9 volumes will appear in the flue gases as (0.21×1.9)=0.4 volumes of oxygen and ( )=1.5 volumes of nitrogen.
The complete composition will be: constituent vol/vol methane CO O N H2O 2 giving a total product volume of (wet) or (dry).

24. Example 1: A gas consists of 70% propane (C3H8) and 30% butane (C4H10) by volume. Find: (a) The stoichiometric air-to-fuel ratio and (b) The percentage excess air present if a dry analysis of the combustion products shows 9% CO2 (assume complete combustion).
Solution: The combustion reactions for propane and butane are:

25. (a) Stoichiometric Air Requirement On the basis of 1 volume of the fuel gas, the propane content requires 0.7 × ( ) = 16.7 vols air and the butane requires 0.3 × ( ) = 6.3 vols air Hence the stoichiometric air-to-fuel ratio is 23:1.

26. (b) Excess Air. The combustion products (dry) will contain. (0
(b) Excess Air The combustion products (dry) will contain (0.7 × 3) + (0.3 × 4) = 3.3 vols CO2 (0.7 × 18.8) + (0.3 × 24.5) = 20.5 vols N2 plus υ volumes excess air, giving a total volume of products of ( υ ).
Given that the measured CO2 in the products is 9%, we can write: hence υ = vols
The stoichiometric air requirement is 23 vols so the percentage excess air is:

27. 4. Combustion Air Requirements-Solid and Liquid Fuels
The way in which the combustion equation is used reflects the available information on the analysis of the solid or liquid fuels. This takes the form of an element-by-element analysis (referred to as an ultimate analysis) which gives the percentage by mass of each element present in the fuel.
An example of an ultimate analysis of a liquid fuel (oil) might be : Component % by mass Carbon (C) 86 Hydrogen(H2) 14

28. Each constituent is considered separately via its own combustion equation. For the carbon: C + O2 → CO2 12kg 32kg 44kg or for 1 kg of fuel
So each kg of oil requires 2.29 kg oxygen for combustion of its carbon and produces 3.15 kg CO2 as product.

29. Similarly H2 + ½ O2 → H2O 2kg 16kg 18kg or per kg of fuel
In order to burn the hydrogen content of the oil 1.12 kg oxygen are needed and 1.26 kg water is formed.

30. We can now establish that 3
We can now establish that 3.41 kg oxygen, which is the stoichiometric requirement, will be associated with:
The stoichiometric air-to-fuel ratio is thus = 14.6 : 1

31. 5. Combustion Products-Solid and Liquid Fuels
The stoichiometric combustion products from combustion of the oil are: CO kg H2O kg N kg
The combustion products would normally be needed as a volume percentage, so the reverse operation to that which was performed for air above is required.

32. CONTROL OF GASEOUS POLLUTANTS BY COMBUSTION
Though it is a major source of air pollution, combustion, or incineration, is also the basis for an important air-pollution-control process in which the objective is to convert the air contaminants (usually hydrocarbons or carbon monoxide) to innocuous carbon dioxide and water.
The combustion equipment used to control air pollution emissions is designed to push oxidation reactions as close as possible to completion, leaving a minimum of unburned compounds. For efficient combustion to occur, it is necessary to have the proper combination of four basic elements: oxygen, temperature, turbulence, and time.

33. Depending upon the contaminant being oxidized following methods can be used to control air pollution.
(1)Direct-flame combustion,
(2)Thermal-combustion (afterburners)
(3)Catalytic combustion

40. Flare Height
The height of a flare is determined based on the ground level limitations of thermal radiation intensity, luminosity, noise, height of surrounding structures, and the dispersion of the exhaust gases.
In addition, consideration must also be given for plume dispersion in case of possible emission ignition failure.
L, required from the center of the flare flame and a point of exposure where thermal radiation must be limited

46. How it works?
The converter uses simple oxidation and reduction reactions to convert the unwanted fumes. The precious metals such as platinum, rhodium, and palladium promote the transfer of electrons and, in turn, the conversion of toxic fumes.

48. CONTROL OF VOLATILE ORGANIC
COMPOUNDS (VOCs)

49. Volatile organic compounds (VOCs) are liquids or solids that contain organic carbon (carbon bonded to carbon, hydrogen, nitrogen, or sulfur, but not carbonate carbon as in CaC03 nor carbide carbon as in CaC2 or CO or C02), which vaporize at significant rates. VOCs are probably the second-most widespread and diverse class of emissions after particulates.

50. Most VOCs are believed not to be toxic (or not very toxic) to humans.
Our principal concern with VOCs is that they participate in the "smog" reaction and also in the formation of secondary particles in the atmosphere.
Some VOCs are powerful infrared absorbers and thus contribute to the problem of global warning.

53. VAPOR PRESSURE, EQUILIBRIUM VAPOR CONTENT, EVAPORATION
To understand which chemicals are volatile (evaporate at significant rates) we must consider the idea of vapor pressure.
At 212°F, its normal boiling point (NBP), water has a pressure of psia (= 760 torr= 1 atmosphere= kPa ~ 14.7 psia).
The normal (atmospheric) boiling point is the temperature at which the vapor pressure equals the atmospheric pressure and the liquid converts to a vapor by the vigorous bubble formation we call boiling.
At room temperature (68°F = 20°C) the vapor pressure of water is psia = 17.5 torr= atm. At this temperature water does not boil. But it does evaporate if the surrounding air is not saturated. (Wet clothes and swimsuits dry slowly at this temperature, faster in a dry climate than in a wet one.

55. Substances like ethane, propane, and n-butane (C2H6, C3H8, and C4H10) have vapor pressures above atmospheric pressure at room temperature.
These must be kept in closed, pressurized containers or they will immediately boil away at room temperature.

56. CONTROL BY PREVENTION
If possible, we prevent the formation of a VOC-containing air or gas stream, which we must -treat by some kind of tailpipe control device.
The ways of doing this for VOCs are substitution, process modification, and leakage control.

57. Substitution
Oil-based paints, coatings, and inks harden by the evaporation of VOC solvents such as paint thinner into the atmosphere. Water-based paints are concentrated oil based paints, emulsified in water. After the water evaporates, the small amount of organic solvent in the remaining paint must also evaporate for the paint to harden.
Switching from oil- to water-based paints, coatings, and inks greatly reduces but does not totally eliminate the emissions of VOCs from painting, coating, or printing.

58. Replacing gasoline as a motor fuel with compressed natural gas or propane is also a form of substitution that reduces the emissions of VOCs, because those fuels can be handled, metered, and burned with fewer VOC emissions than can gasoline.

59. Process Modification
Process modification to prevent or reduce the formation of the VOC stream may be more economical than applying the control options.
EXAMPLE:
Replacing gasoline-powered vehicles with electric-powered vehicles is a form of process modification that reduces the emissions of VOCs, as well as emission of carbon monoxide and nitrogen oxides.
Many coating, finishing, and decoration processes that at one time depended on evaporating solvents have been replaced by others that do not, e.g., fluidized-bed powder coating and ultraviolet lithography.

60. Leakage Control
Tanks containing liquid VOCs can emit VOC vapors because of filling and emptying activities as well as changes in temperature and atmospheric pressure. These emissions are called filling or displacement losses, emptying losses, and breathing losses, or, collectively, working Josses.

63. CONTROL BY CONCENTRATION AND RECOVERY
Most VOCs are valuable fuels or solvents; if we can recover them in pure or nearly pure form we can reuse or sell them for a profit. For large VOC-containing gas streams this is often economical, but not often for small streams.
We can concentrate and recover VOC by condensation, adsorption, and absorption.

64. Condensation
One can remove most of the VOCs from an air or gas stream by cooling the stream to a low enough temperature that most of the VOCs are condensed as a liquid and then separated from the gas stream by gravity.

65. 1 pound per square inch = 6 894.75729 pascals

69. Adsorption Lecture Notes
Difference b/w ADSORPTION and ABSORPTION

70. Useful When The pollutant gas is noncombustible or difficult to burn
The pollutant is sufficiently valuable to warrant recovery
The pollutant is in very dilute concentration
It is also used for purification of gases containing only small amounts of pollutants that are difficult to clean by other means

71. Adsorption Equilibrium
Adsorption vs. Absorption
Adsorption is accumulation of molecules on a surface (a surface layer of molecules) in contact with an air or water phase
Absorption is dissolution of molecules within a phase, e.g., within an organic phase in contact with an air or water phase

72. Adsorption
PHASE I
‘PHASE’ 2
Absorption (“partitioning”)
PHASE I
PHASE 2

73. Adsorption Process Classified as Physical and Chemical Ads.
1) Physical adsorption
The gas molecules adhere to the surface of the solid adsorbent as a result of intermolecular attractive forces (van der Waals forces) between them
The process is exothermic: the heat liberated is in the order of the the enthalpy of condensation of vapor (2-20 kJ/gmole)
The process is reversible (recovery of adsorbent material or adsorbed gas is possible) by increasing the temperature or lowering the adsorbate conc.
Physical adsorption usually directly proportional to the amount of solid surface area
Adsorbate can be adsorbed on a monolayer or a number of layers
The adsorption rate is generally quite rapid

74. Adsorption Process 2) Chemical adsorption
Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption
Heat liberated during chemisorption is in the range of kj/g mole
It is frequently irreversible. On desorption the chemical nature of the original adsorbate will have undergone a change.
Only a monomolecular layer of adsorbate appears on the adsorbing medium
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76. Adsorption Mechanism 2) Chemical adsorption
Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption
Heat liberated during chemisorption is in the range of kj/g mole
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Aerosol & Particulate Research Lab 76

77. Properties of Activated Carbon
Adsorbent Material
Silica gel
Activated alumina
Activated carbon
Synthetic zeolite
Molecular sieve
Dehyrdating purposes
Properties of Activated Carbon
Bulk Density
22-34 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
15-25 Å
Regeneration Temperature
(Steaming) oC
Maximum Allowable
Temperature
150 oC
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78. Properties of Silica Gel Properties of Activated Alumina
Adsorbent Material
Properties of Silica Gel
Bulk Density
44-56 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
0.37 cm3/g
Surface Area
750 m2/g
Average Pore Diameter
22 Å
Regeneration Temperature oC
Maximum Allowable Temperature
400 oC
Properties of Activated Alumina
Bulk Density
Granules
38-42 lb/ft3
Pellets
54-58 lb/ft3
Specific Heat
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
18-48 Å
Regeneration Temperature (Steaming) oC
Maximum Allowable Temperature
500 oC
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79. Properties of Molecular Sieves
Adsorbent Materials
Properties of Molecular Sieves
Anhydrous Sodium Aluminosilicate
Anhydrous Calcium Aluminosilicate
Anhydrous Aluminosilicate
Type 4A 5A
13X
Density in bulk (lb/ft3) 44 38
Specific Heat (BTU/lboF)
0.19 -
Effective diameter of pores (Å) 4 5 13
Regeneration Temperature (oC)
Maximum Allowable Temperature (oC)
600
Crystalline zeolite
Uniform pores to selectively separate compounds by size & shape
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80. Adsorption Isotherm
The amount of gas adsorbed per unit of adsorbent at equilibrium is measured against the partial pressure of the adsorbate in the gas phase gives equilibrium adsorption isotherm
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81. Adsorption Isotherm
In general, an adsorption isotherm relates the volume or mass adsorbed to the partial pressure or concentration of the adsorbate in the main gas stream at a given temperature
The equilibrium concentration adsorbed is very sensitive to T
There are many equations proposed to fit analyticaly the various experimental istoherms
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85. For large-scale air pollution applications, like collecting the solvent vapors coming off a large paint-drying oven or a large printing press, the normal procedure is to use several adsorption beds.

97. Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream.