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MODULE- IV



Air POLLUTION
   CONTROL
                     Prepared by
            Bibhabasu Mohanty
      Dept. of Civil Engineering
        SALITER, Ahmedabad
Contents…
Air Pollution control- at source-equipments for
control of air pollution-For particulate matter-
Settling chambers-Fabric filters-Scrubbers-
Cyclones-Electrostatic precipitators, For
Gaseous pollutants-control by absorption-
adsorption-scrubbers-secondary combustion
after burners, Working principles advantages
and disadvantages, design criteria and examples
Source correction methods
o Raw material changes

o Process changes

o Equipment modification & replacement
Objectives of control equipment
o Prevention of nuisance
o Prevention of physical damage to property
o Elimination of health hazards to plant personnel
o Recovery of valuable waste product
o Minimization of economic losses
o Improvement of product quality
Particulate control equipment
o Gravitational settling chambers
o Fabric filters
o Scrubbers
o Cyclone separator
o Electrostatic precipitators
o Efficiency of a separating device

η= quantity of particulates collected from gas/
  quantity of particulates present initially
Gravitational settling chamber




o Used to remove particles with size greater
  than 50 μm.
o Velocity of flue gas reduced in large
  chamber.
o Particles settle under gravitational force.
Vs= hV/ L ----------- (i)
L= length of chamber
V= horizontal velocity of carrier gas
Vs= settling velocity of particulates
h= height through which particulates travel before
 settling down
By stokes law
            Vs= g(ρp- ρ)D2/18μ --------- (ii)
D= dia of particle
g= acceleration due to gravity
ρp= density of particle
ρ = density of gas
μ= viscosity of gas
From eq- i and ii

D= [18Vhμ/ Lg (ρp- ρ)]1/2

 D = is minimum size of particle that can be
 removed in a settling chamber
Advantages
o Low initial cost.
o Easy to design.
o Low pressure drop.
o Low maintenance cost.
o Dry and continuous disposal of solid particulates.

Disadvantages
o Require large space.
o Less collection efficiency.
o Only larger size particles can be collected.
Application
o Industrial application is limited.
o Used widely for removal of large solid
  particulates from draft furnace, kilns.
o Sometimes used in process industry, food and
  metallurgical industry.
o Used as pre-cleaners for high efficiency
  collectors.
Cyclone separator
o Centrifugal force is utilized to separate the
  particulate matter.
o It can remove 10 to 50 μm particle size.
o Used mostly in industries.
Dp, min= [9 μ B/ π V Nt(ρp- ρ)] ½
Dp, min= dia of smallest particles that can be
  removed cm
μ = viscosity of the fluid
B= width of cyclone inlet duct
V= avg. inlet velocity
Nt= no of turns made by gas stream in cyclone
ρp = density of particles
ρ = density of fluid
o Design factor having greatest effect on
  collection efficiency is cyclone diameter.
o Smaller dia, higher is efficiency, because
  centrifugal action increase with decreasing radius
  of rotation.
o Cyclone efficiencies > 90 % with particle dia of
  10 μ
o > 95 % with particle dia 20 μ.
Efficiency
o Conventional efficiency
o High efficiency- smaller body dia to create
  greater separating force.
o Increase collection efficiency, if increase in dust
  particle size, dust particle density, gas inlet
  velocity, inlet dust loading, cyclone body length
  (no of gas revolutions)
o Decrease collection efficiency due to increase in
  gas viscosity, cyclone dia, gas outlet dia, inlet
  width, and inlet area
Operating problems

o Erosion

o Corrosion

o Material build up
Advantages
o Low initial cost.
o Require less floor area.
o Simple construction and maintenance.
o Can handle large volume of gas at high temp.
o No moving parts

Disadvantages
o Requires large head room.
o Less efficiency for smaller particles (<10μm).
o Sensitive to variable dust load and flow rate.
Applications
o Control gas borne particulates from industries
  like cement, feed and grain processing, food and
  beverage processing, mineral processing, paper
  and textile industries and wood working
  industries.
o Used in recovery of catalyst ducts in petroleum
  industry and reduction of fly ash emission.
Electrostatic precipitators

o Works on the principle of electrical charging
  of particulate Matter (-ve) and collecting it in a
  (+ve) charged surface.
o 99% efficiency.
o Can remove particle size range of 0.1 μm to 1
  μm.
Six major components
o A source of high voltage
o Discharge electrodes and collecting electrodes
o Inlet and outlet for gas
o A hopper for disposal of collected material
o An electronic cleaning system
o An outer casing to form an enclosure around
  electrodes
Principles
o Gas stream passed two electrodes.
o High potential difference is maintained.
o Out of two electrodes, one is discharging other
  collecting.
o Potentials of 100 kv are used.
o Ionization creates active glow zone called
  “corona”.
o Gas ionization is dissociation of gas molecules
  into free ions.
o As particulates pass through field, they get
  charged and migrate to oppositely charged
  electrode.

o Particles deposited on collecting electrodes, lose
  charge and removed mechanically by
  rapping., vibration or washing to a hopper.
Single stage and two stage precipitators

o Single stage gas ionization and particulate
  collection in a single stage.
o Two stage, particle ionized in first chamber and
  collected in second chamber.
o Industrial precipitators single stage design.
o Two stage used for lightly loaded gases.
o Single stage for more heavily loaded gas streams.
Efficiency
o General collection efficiency is high, nearly
  100%
o Installations operate 98 and 99% efficiency.
o Acid mist and catalyst recovery efficiencies in
  excess of 99%.
o Carbon black, because of agglomeration
  tendency collection efficiency less than 35%.
E= 1- e -Utf

E= collection efficiency
f= specific collecting area of precipitator,
  expressed as sq m of collecting electrode area
  per cubic m of gas handled per s.
Ut= migration velocity of particle towards
  collection electrode
e= napierian log base
Design parameter
o Volumetric flow rate
o Composition
o Temperature
o Dew point
o Dust particle conc.
o Size of particle
o Bulk density
o Tendency of allgomorate
Advantages
o High collection efficiency.
o Particles may be collected dry or wet.
o Can be operated at high temp. (300-450˚c).
o Maintenance is normal.
o Few moving parts.
Disadvantages
o High initial cost.
o Require high voltage.
o Collection efficiency reduce with time.
o Space requirement is more.
o Possible of explosion during collection of
  combustible gases or particulates.
Application
o Cement factories
o Pulp and paper mills
o Steel plants
o Non- ferrous metal industry
o Chemical industry
o Petroleum industry
o Carbon black industry
o Electric power industry
Fabric filters or cloth filters
o Flue gas is allowed to pass through a woven
  fabric, which filters out particulate matter.
o Small particles are retained on the fabric.
o Consists of numerous vertical bags 120-400
  mm dia and 2-10 m long.
o Remove particles up to 1 μm.
o Its efficiency up to 99%.
Factors affecting efficiency
Efficiency decrease due to
o Excessive filter ratio:- ratio of carrier gas vs
  gross filter area

o Improper selection of filter media:- temp.
  resistance, resistance to chemical attack and
  abrasion resistance taken into consideration.
Operating problems

o Cleaning
o Rupture of cloth
o Temperature
o Bleeding
o Humidity
o Chemical attack
Filter cleaning
o Rapping
o Shaking
o Back wash
o Pulse jet
Filter medium
o Carrier gas temp.
o Carrier gas composition
o Gas flow rate
o Size and shape of dust particles
Fabric        Max.       Acid       Alkali     Abrasion Tensile
              operatin   resistance resistance resistance strength
              g temp.                                     Kg/cm2
              (˚ C)
Cotton        82         Poor        Good        Very good 4920
Wool          93         Very good Poor          Fair to     1755
                                                 good
Nylon         93         Poor to     Excellent   Excellent   5625
                         fair
Dacron        135        Good        Good        Very good 5625
Polypropyl    93         Excellent   Excellent   Excellent   7730
ene
Fiber glass   290        Fair to     Fair to     Fair        14,060
                         good        good

              Physical properties of bag filters
Advantages
o Higher collection efficiency for smaller than 10 μm
  particle size.
o Performance decrease becomes visible, giving
  prewarning.
o Normal power consumption.
Disadvantages
o High temp. gases need to be cooled.
o High maintenance and fabric replacement cost.
o Large size equipment.
o Fabric is liable to chemical attack.
Application

o Metallurgical industry
o Foundries
o Cement industry
o Chalk and lime
o Brick works
o Ceramic industry
o Flour mills
Scrubbers or wet collectors

o Particulate matters are incorporated into liquid droplets
  and removed from the gas stream.
o Flue gas made to push up against a down falling water
   current.
o Particulate matter mix up with water thus falls down
   and gets removed.
Collection mechanism
o Impingement
o Interception
o Diffusion
o Condensation
Types of scrubbers

o Spray towers
o Venturi scrubbers
o Cyclone scrubbers
o Packed scrubbers
o Mechanical scrubbers
Spray towers
Venturi scrubber
Cyclone scrubber
Packed scrubbers
Mechanical scrubbers
Advantages
o Simultaneously remove particulates and gaseous
  pollutants.
o Hot gases can be cooled down.
o Corrosive gases can be recovered and neutralize.

Disadvantages
o Lot of waste waters produced.
o Poses freezing problem in cold countries.
o Maintenance cost is high when corrosive materials are
  collected.
Choice of equipment
1.   Particulate size
2.   Particulate loading
3.   Efficiency required
4.   Properties of carrier gas
     1.   Composition    6. Humidity
     2.   Temp.          7. Combustibility
     3.   Pressure       8. Reactivity
     4.   Viscosity      9. Toxicity
     5.   Density        10. Electrical property
5. Flow characteristics of carrier gas
     1.   Flow rate
     2.   Variation in flow rate
6. Specific property of contaminant
     1.   Composition              6.Toxicity
     2.   Contaminant phase        7. Hygroscopicity
     3.   Solubility               8. Agglomerating
          characteristics
     4.   Combustibility           9. Electrical and sonic
                                   properties
     5.   Reactivity               10. Catalyst poisoning
7. Allowable pressure drop
8. Contaminate disposal
9. Capital and operating cost of equipment
10. Ease of maintenance and reliability
Economical aspects
1. Cyclones:- cheap to install, power consumption
   moderate, maintenance cost normal.
2. Filters:- expensive to install, power consumption
   moderate. Maintenance cost high.
3. Electrostatic precipitators:- most expensive regarding
   installation, power consumption moderate to low as
   pressure drops. Maintenance cost moderate
4. Scrubbers :- installation cost moderate, maintenance
   cost not high, high rate of power consumption.
Control of gaseous contaminants
Methods of control include:
o Absorption
o Adsorption
o Secondary combustion
Absorption
o Effluent gas passed through absorbers
  (scrubbers), which contain liquid absorbent.
o Efficiency depends on
     1.   Amount of surface contact between gas and liquid
     2.   Contact time
     3.   Conc. of absorbing medium
     4.   Speed of reaction between the absorbent and gases
o Absorbents used to remove SO2, H2S, SO3, F
  and oxides of nitrogen.
Equipments using principles of absorption for
 removal of gaseous pollutants

o Packed tower
o Plate tower
o Bubble cap plate tower
o Spray tower
o Liquid jet scrubber absorbers
Gaseous      Common absorbents used in solution form
pollutants
SO2          Dimethylaniline, ammonium sulphite,
             ammonium sulphate, sodium sulphide, calcium
             sulphite, alkaline water,
H2S          NaOH and phenol mix (3:2), tripottasium
             phosphate, sodium alamine, sodium
             thioarsenate, soda ash
HF           Water, NaOH
NOX          Water, aqueous nitric acid
Adsorption
o Surface phenomenon, require large solid surface
o Adsorption towers use adsorbents to remove the
  impurities from the gas stream.
o The impurities bind either physically or chemically to
  the adsorbing material.
o The impurities can be recovered by regenerating the
  adsorbent.
o Adsorption towers can remove low concentrations of
  impurities from the flue gas stream.
Construction and Operation
o Adsorption towers consist of cylinders packed with the
  adsorbent.
o The adsorbent is supported on a heavy screen
o Since adsorption is temperature dependent, the flue gas
  is temperature conditioned.
o Vapor monitors are provided to detect for large
  concentrations in the effluent. Large concentrations of
  the pollutant in the effluent indicate that the adsorbent
  needs to be regenerated.
Advantages of Adsorption Towers
o   Very low concentrations of pollutants can be removed.
o   Energy consumption is low.
o   Do not need much maintenance.
o   Economically valuable material can be recovered during
    regeneration.
Gaseous pollutants        Adsorbents used in solid form
SO2                       Pulverized limestone or
                          dolomite, alkalized alumina
H2S                       Iron oxide
HF                        Lump limestone, porous sodium
                          fluoride pellets
NOX                       Silica gel
Organic solvent vapours   Activated carbon
Secondary combustion
o Flame or catalytic combustion can be utilized
  when gases or vapors to be controlled.

o Fume and vapor incinerators
o After burners
o Flares, either with steam injection or venturi
  flare
o After burners on incinerators met with varying
  success depending on kind of after burner used
  and type of incinerator.
o Flare design for smokeless combustion gases of
  variable composition and wide range of flow
  rates.
o Venturi flares, mix air with gases in proper ratio
  prior to ignition to achieve smokeless burning.
Fume incinerators
o When conc. of combustible portion of gas
  stream below flammable range, catalytic
  combustion process used.

o Used to control effluent gases, fumes and odors
  from refineries, burning waste, cracking gases,
  chemical plants, paint and enamel ovens.
Method is expensive when

1. Fuel values of gaseous discharge low

2. Moisture content of discharge is high

3. Exhaust volume is extremely large
Factors considered in design of incinerators are

1. Sufficient air for combustion reaction

2. Adequate temp.

3. Adequate retention time
o Incineration equipment, single combustion
  chamber.
o Combustion chamber proportioned that gas
  velocity, gas flow patterns, established produce
  adequate retention time.
o Avg. retention time 0.2-0.3 s at temp. 650 ˚C
  and higher.
Air Pollution Control Equipment

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Air Pollution Control Equipment

  • 1. MODULE- IV Air POLLUTION CONTROL Prepared by Bibhabasu Mohanty Dept. of Civil Engineering SALITER, Ahmedabad
  • 2. Contents… Air Pollution control- at source-equipments for control of air pollution-For particulate matter- Settling chambers-Fabric filters-Scrubbers- Cyclones-Electrostatic precipitators, For Gaseous pollutants-control by absorption- adsorption-scrubbers-secondary combustion after burners, Working principles advantages and disadvantages, design criteria and examples
  • 3. Source correction methods o Raw material changes o Process changes o Equipment modification & replacement
  • 4. Objectives of control equipment o Prevention of nuisance o Prevention of physical damage to property o Elimination of health hazards to plant personnel o Recovery of valuable waste product o Minimization of economic losses o Improvement of product quality
  • 5. Particulate control equipment o Gravitational settling chambers o Fabric filters o Scrubbers o Cyclone separator o Electrostatic precipitators
  • 6. o Efficiency of a separating device η= quantity of particulates collected from gas/ quantity of particulates present initially
  • 7. Gravitational settling chamber o Used to remove particles with size greater than 50 μm. o Velocity of flue gas reduced in large chamber. o Particles settle under gravitational force.
  • 8. Vs= hV/ L ----------- (i) L= length of chamber V= horizontal velocity of carrier gas Vs= settling velocity of particulates h= height through which particulates travel before settling down
  • 9. By stokes law Vs= g(ρp- ρ)D2/18μ --------- (ii) D= dia of particle g= acceleration due to gravity ρp= density of particle ρ = density of gas μ= viscosity of gas
  • 10. From eq- i and ii D= [18Vhμ/ Lg (ρp- ρ)]1/2 D = is minimum size of particle that can be removed in a settling chamber
  • 11. Advantages o Low initial cost. o Easy to design. o Low pressure drop. o Low maintenance cost. o Dry and continuous disposal of solid particulates. Disadvantages o Require large space. o Less collection efficiency. o Only larger size particles can be collected.
  • 12. Application o Industrial application is limited. o Used widely for removal of large solid particulates from draft furnace, kilns. o Sometimes used in process industry, food and metallurgical industry. o Used as pre-cleaners for high efficiency collectors.
  • 13. Cyclone separator o Centrifugal force is utilized to separate the particulate matter. o It can remove 10 to 50 μm particle size. o Used mostly in industries.
  • 14.
  • 15. Dp, min= [9 μ B/ π V Nt(ρp- ρ)] ½ Dp, min= dia of smallest particles that can be removed cm μ = viscosity of the fluid B= width of cyclone inlet duct V= avg. inlet velocity Nt= no of turns made by gas stream in cyclone ρp = density of particles ρ = density of fluid
  • 16. o Design factor having greatest effect on collection efficiency is cyclone diameter. o Smaller dia, higher is efficiency, because centrifugal action increase with decreasing radius of rotation. o Cyclone efficiencies > 90 % with particle dia of 10 μ o > 95 % with particle dia 20 μ.
  • 17. Efficiency o Conventional efficiency o High efficiency- smaller body dia to create greater separating force. o Increase collection efficiency, if increase in dust particle size, dust particle density, gas inlet velocity, inlet dust loading, cyclone body length (no of gas revolutions) o Decrease collection efficiency due to increase in gas viscosity, cyclone dia, gas outlet dia, inlet width, and inlet area
  • 18. Operating problems o Erosion o Corrosion o Material build up
  • 19. Advantages o Low initial cost. o Require less floor area. o Simple construction and maintenance. o Can handle large volume of gas at high temp. o No moving parts Disadvantages o Requires large head room. o Less efficiency for smaller particles (<10μm). o Sensitive to variable dust load and flow rate.
  • 20. Applications o Control gas borne particulates from industries like cement, feed and grain processing, food and beverage processing, mineral processing, paper and textile industries and wood working industries. o Used in recovery of catalyst ducts in petroleum industry and reduction of fly ash emission.
  • 21. Electrostatic precipitators o Works on the principle of electrical charging of particulate Matter (-ve) and collecting it in a (+ve) charged surface. o 99% efficiency. o Can remove particle size range of 0.1 μm to 1 μm.
  • 22. Six major components o A source of high voltage o Discharge electrodes and collecting electrodes o Inlet and outlet for gas o A hopper for disposal of collected material o An electronic cleaning system o An outer casing to form an enclosure around electrodes
  • 23.
  • 24.
  • 25. Principles o Gas stream passed two electrodes. o High potential difference is maintained. o Out of two electrodes, one is discharging other collecting. o Potentials of 100 kv are used. o Ionization creates active glow zone called “corona”. o Gas ionization is dissociation of gas molecules into free ions.
  • 26. o As particulates pass through field, they get charged and migrate to oppositely charged electrode. o Particles deposited on collecting electrodes, lose charge and removed mechanically by rapping., vibration or washing to a hopper.
  • 27. Single stage and two stage precipitators o Single stage gas ionization and particulate collection in a single stage. o Two stage, particle ionized in first chamber and collected in second chamber. o Industrial precipitators single stage design. o Two stage used for lightly loaded gases. o Single stage for more heavily loaded gas streams.
  • 28. Efficiency o General collection efficiency is high, nearly 100% o Installations operate 98 and 99% efficiency. o Acid mist and catalyst recovery efficiencies in excess of 99%. o Carbon black, because of agglomeration tendency collection efficiency less than 35%.
  • 29. E= 1- e -Utf E= collection efficiency f= specific collecting area of precipitator, expressed as sq m of collecting electrode area per cubic m of gas handled per s. Ut= migration velocity of particle towards collection electrode e= napierian log base
  • 30. Design parameter o Volumetric flow rate o Composition o Temperature o Dew point o Dust particle conc. o Size of particle o Bulk density o Tendency of allgomorate
  • 31. Advantages o High collection efficiency. o Particles may be collected dry or wet. o Can be operated at high temp. (300-450˚c). o Maintenance is normal. o Few moving parts. Disadvantages o High initial cost. o Require high voltage. o Collection efficiency reduce with time. o Space requirement is more. o Possible of explosion during collection of combustible gases or particulates.
  • 32. Application o Cement factories o Pulp and paper mills o Steel plants o Non- ferrous metal industry o Chemical industry o Petroleum industry o Carbon black industry o Electric power industry
  • 33. Fabric filters or cloth filters o Flue gas is allowed to pass through a woven fabric, which filters out particulate matter. o Small particles are retained on the fabric. o Consists of numerous vertical bags 120-400 mm dia and 2-10 m long. o Remove particles up to 1 μm. o Its efficiency up to 99%.
  • 34.
  • 35.
  • 36. Factors affecting efficiency Efficiency decrease due to o Excessive filter ratio:- ratio of carrier gas vs gross filter area o Improper selection of filter media:- temp. resistance, resistance to chemical attack and abrasion resistance taken into consideration.
  • 37. Operating problems o Cleaning o Rupture of cloth o Temperature o Bleeding o Humidity o Chemical attack
  • 38. Filter cleaning o Rapping o Shaking o Back wash o Pulse jet
  • 39. Filter medium o Carrier gas temp. o Carrier gas composition o Gas flow rate o Size and shape of dust particles
  • 40. Fabric Max. Acid Alkali Abrasion Tensile operatin resistance resistance resistance strength g temp. Kg/cm2 (˚ C) Cotton 82 Poor Good Very good 4920 Wool 93 Very good Poor Fair to 1755 good Nylon 93 Poor to Excellent Excellent 5625 fair Dacron 135 Good Good Very good 5625 Polypropyl 93 Excellent Excellent Excellent 7730 ene Fiber glass 290 Fair to Fair to Fair 14,060 good good Physical properties of bag filters
  • 41. Advantages o Higher collection efficiency for smaller than 10 μm particle size. o Performance decrease becomes visible, giving prewarning. o Normal power consumption. Disadvantages o High temp. gases need to be cooled. o High maintenance and fabric replacement cost. o Large size equipment. o Fabric is liable to chemical attack.
  • 42. Application o Metallurgical industry o Foundries o Cement industry o Chalk and lime o Brick works o Ceramic industry o Flour mills
  • 43. Scrubbers or wet collectors o Particulate matters are incorporated into liquid droplets and removed from the gas stream. o Flue gas made to push up against a down falling water current. o Particulate matter mix up with water thus falls down and gets removed.
  • 44. Collection mechanism o Impingement o Interception o Diffusion o Condensation
  • 45. Types of scrubbers o Spray towers o Venturi scrubbers o Cyclone scrubbers o Packed scrubbers o Mechanical scrubbers
  • 51. Advantages o Simultaneously remove particulates and gaseous pollutants. o Hot gases can be cooled down. o Corrosive gases can be recovered and neutralize. Disadvantages o Lot of waste waters produced. o Poses freezing problem in cold countries. o Maintenance cost is high when corrosive materials are collected.
  • 52. Choice of equipment 1. Particulate size 2. Particulate loading 3. Efficiency required 4. Properties of carrier gas 1. Composition 6. Humidity 2. Temp. 7. Combustibility 3. Pressure 8. Reactivity 4. Viscosity 9. Toxicity 5. Density 10. Electrical property
  • 53. 5. Flow characteristics of carrier gas 1. Flow rate 2. Variation in flow rate 6. Specific property of contaminant 1. Composition 6.Toxicity 2. Contaminant phase 7. Hygroscopicity 3. Solubility 8. Agglomerating characteristics 4. Combustibility 9. Electrical and sonic properties 5. Reactivity 10. Catalyst poisoning
  • 54. 7. Allowable pressure drop 8. Contaminate disposal 9. Capital and operating cost of equipment 10. Ease of maintenance and reliability
  • 55. Economical aspects 1. Cyclones:- cheap to install, power consumption moderate, maintenance cost normal. 2. Filters:- expensive to install, power consumption moderate. Maintenance cost high. 3. Electrostatic precipitators:- most expensive regarding installation, power consumption moderate to low as pressure drops. Maintenance cost moderate 4. Scrubbers :- installation cost moderate, maintenance cost not high, high rate of power consumption.
  • 56. Control of gaseous contaminants Methods of control include: o Absorption o Adsorption o Secondary combustion
  • 57. Absorption o Effluent gas passed through absorbers (scrubbers), which contain liquid absorbent. o Efficiency depends on 1. Amount of surface contact between gas and liquid 2. Contact time 3. Conc. of absorbing medium 4. Speed of reaction between the absorbent and gases o Absorbents used to remove SO2, H2S, SO3, F and oxides of nitrogen.
  • 58. Equipments using principles of absorption for removal of gaseous pollutants o Packed tower o Plate tower o Bubble cap plate tower o Spray tower o Liquid jet scrubber absorbers
  • 59. Gaseous Common absorbents used in solution form pollutants SO2 Dimethylaniline, ammonium sulphite, ammonium sulphate, sodium sulphide, calcium sulphite, alkaline water, H2S NaOH and phenol mix (3:2), tripottasium phosphate, sodium alamine, sodium thioarsenate, soda ash HF Water, NaOH NOX Water, aqueous nitric acid
  • 60. Adsorption o Surface phenomenon, require large solid surface o Adsorption towers use adsorbents to remove the impurities from the gas stream. o The impurities bind either physically or chemically to the adsorbing material. o The impurities can be recovered by regenerating the adsorbent. o Adsorption towers can remove low concentrations of impurities from the flue gas stream.
  • 61. Construction and Operation o Adsorption towers consist of cylinders packed with the adsorbent. o The adsorbent is supported on a heavy screen o Since adsorption is temperature dependent, the flue gas is temperature conditioned. o Vapor monitors are provided to detect for large concentrations in the effluent. Large concentrations of the pollutant in the effluent indicate that the adsorbent needs to be regenerated.
  • 62. Advantages of Adsorption Towers o Very low concentrations of pollutants can be removed. o Energy consumption is low. o Do not need much maintenance. o Economically valuable material can be recovered during regeneration.
  • 63. Gaseous pollutants Adsorbents used in solid form SO2 Pulverized limestone or dolomite, alkalized alumina H2S Iron oxide HF Lump limestone, porous sodium fluoride pellets NOX Silica gel Organic solvent vapours Activated carbon
  • 64. Secondary combustion o Flame or catalytic combustion can be utilized when gases or vapors to be controlled. o Fume and vapor incinerators o After burners o Flares, either with steam injection or venturi flare
  • 65. o After burners on incinerators met with varying success depending on kind of after burner used and type of incinerator. o Flare design for smokeless combustion gases of variable composition and wide range of flow rates. o Venturi flares, mix air with gases in proper ratio prior to ignition to achieve smokeless burning.
  • 67. o When conc. of combustible portion of gas stream below flammable range, catalytic combustion process used. o Used to control effluent gases, fumes and odors from refineries, burning waste, cracking gases, chemical plants, paint and enamel ovens.
  • 68. Method is expensive when 1. Fuel values of gaseous discharge low 2. Moisture content of discharge is high 3. Exhaust volume is extremely large
  • 69. Factors considered in design of incinerators are 1. Sufficient air for combustion reaction 2. Adequate temp. 3. Adequate retention time
  • 70. o Incineration equipment, single combustion chamber. o Combustion chamber proportioned that gas velocity, gas flow patterns, established produce adequate retention time. o Avg. retention time 0.2-0.3 s at temp. 650 ˚C and higher.