1. Technology for Air Pollution Control – Part2

2. Cyclones
Principle
The particles are removed by the application of a centrifugal force. The polluted gas stream is forced into a vortex. the motion of the gas exerts a centrifugal force on the particles, and they get deposited on the inner surface of the cyclones

3. Cyclones

4. Cyclones (contd.) Construction and Operation
The gas enters through the inlet, and is forced into a spiral.
At the bottom, the gas reverses direction and flows upwards.
To prevent particles in the incoming stream from contaminating the clean gas, a vortex finder is provided to separate them. the cleaned gas flows out through the vortex finder.

5. Cyclones (contd.) Advantages of Cyclones Disadvantages of Cyclones
Cyclones have a lost capital cost
Reasonable high efficiency for specially designed cyclones.
They can be used under almost any operating condition.
Cyclones can be constructed of a wide variety of materials.
There are no moving parts, so there are no maintenance requirements.
Disadvantages of Cyclones
They can be used for small particles
High pressure drops contribute to increased costs of operation.

7. Problem
A cyclone with a flow rate of 150 m3/min has an efficiency of 80%. Estimate the efficiency if the flow rate is doubled.

8. Solution Step 2 Final Efficiency = 1- Pt2 = 86% Step 1 Q1 = 150 m3/min
Pt2/Pt1 = (Q1/Q2)0.5
Step 2
Final Efficiency = 1- Pt2
= 86%

9. Dimensions of a Standard Cyclone

11. Problem Design of Cyclone Body diameter = 0.75 m Flow rate = 2.75 m3/s
ρp = 1600 kg/m3
ρg = 1.1 kg/m3
µ = 2.5*10-5 kg/m-s
High throughput
H = 0.8 * body diameter
W = 0.35 * body diameter
Lb = 1.7 * body diameter
Lc = 2.0 * body diameter
Calculate the diameter of particle with 50% efficiency

12. Solution Step 1: Inlet Velocity = Q/A = 17.46 m/s Step 2:
Number of effective turns
Ne = (Lb + Lc/2)/H
Ne = 3.375
Step 3: Diameter of particle
dpc = 10 µm

13. Filter Cloth

14. Fabric Filters
Fabric Filter

15. Fabric Filters Principle
The filters retain particles larger than the mesh size
Air and most of the smaller particles flow through. Some of the smaller particles are retained due to interception and diffusion.
The retained particles cause a reduction in the mesh size.
The primary collection is on the layer of previously deposited particles.

16. Design of Fabric Filters
The equation for fabric filters is based on Darcy’s law for flow through porous media.
Fabric filtration can be represented by the following equation:
S = Ke + Ksw
Where,
S = filter drag, N-min/m3 S = ∆P/V
Ke = extrapolated clean filter drag, N-min/m3
Ks = slope constant. Varies with the dust, gas and fabric, N-min/kg-m
W= Areal dust density = L V t
L = dust loading (g/m3), V = velocity (m/s)
Both Ke and Ks are determined empirically from pilot tests.

17. Empirical Determination of Ke and Ks

18. Problem Estimate the values of Ke and Ks for the filter drag model:
Limestone dust loading L = 1.00 g/m3 Fabric Area A = 1.00 m2 Air flow rate Q = 0.80 m3/min
Time (min) 5 10 15 20 25 30
Filter ∆P (Pa)
330
490
550
600
640
700

19. Solution Step 1: Calculate the air velocity
Air velocity =  0.80 (m3/min)/1.00 m2
= 0.80 m/min
Step 2:
Step 3:
Determine Ke and Ks graphically
Ke = 470 N-min/m3 Ks = N-min/g-m
S = ∆P/V
412.5
612.5
687.5
750
800
875
W = LVt 4 8 12 16 20 24

20. Fabric Filters Δ P Total pressure drop
Δ Pf Pressure drop due to the fabric
Δ Pp Pressure drop due to the particulate layer
Δ Ps Pressure drop due to the bag house structure

21. Advantages of Fabric Filters
Very high collection efficiency
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

22. Fabric Filters (contd.)
Disadvantages of Fabric Filters
Fabric Filters require a large floor area.
The fabric is damaged at high temperature.
Ordinary fabrics cannot handle corrosive gases.
Fabric Filters cannot handle moist gas streams
A fabric filtration unit is a potential fire hazard

23. Darcy’s equation ΔPf Pressure drop N/m2 ΔPp Pressure drop N/m2
Df Depth of filter in the direction of flow (m)
Dp Depth of particulate layer in the direction of flow (m)
μ Gas viscosity kg/m-s
V superficial filtering velocity m/min
Kf, Kp Permeability (filter & particulate layer m2)
60 Conversion factor δ/min
V = Q/A
Q volumetric gas flow rate m3/min
A cloth area m2

24. Dust Layer ΔP = ΔPf + ΔPp Filter Drag S = ΔP/V
L Dust loading kg/m3
t time of operation min
ρL Bulk density of the particulate layer kg/m3
ΔP = ΔPf + ΔPp
Filter Drag S = ΔP/V
Areal dust density W = LVt
S= k1+k2W

25. Problem
Estimate the net cloth area for a shaker bag house that must filter 40,000 cfm of air with 10 grams of flour dust per cubic foot of air. Also specify the number of components to be used and calculate the total number of bags required if each bag is 8 feet long and 0.5 feet in diameter. The maximum filtering velocity for flour dust is 2.5 ft/min.

26. Solution
Step 1:   
Calculate total area and number of components required. A = Q/V
Step 2:
Calculate the area of each bag.
A = Π (d) l
Step 3:
Calculate the total number of bags required.
Number of bags required = Total area / Area per bag
= 1270 bags
Number of compartments sq. ft. / compartment

27. Electrostatic Precipitator
Electrostatic Charging of Dust Particles
Cutaway of Electrostatic Precipitator

28. Electrostatic Precipitator
Principle
The particles in a polluted gas stream are charged by passing them through an electric field.
The charged particles are led through collector plates
The collector plates carry charges opposite to that on the particles
The particles are attracted to these collector plates and are thus removed from the gas steam
Construction and Operation of Electrostatic Precipitator
Charging Electrodes in the form of thin wires are placed in the path of the influent gas.
The charging electrodes generate a strong electric field, which charges the particles as they flow through it.
The collector plates get deposited with the particles. the particles are occasionally removed either by rapping or by washing the collector plates.

29. Design of Electrostatic Precipitators
The efficiency of removal of particles by an Electrostatic Precipitator is given by
η = fractional collection efficiency
w = drift velocity, m/min.
A = available collection area, m2
Q = volumetric flow rate m3/min

30. Migration velocity Where, q = charge (Columbus)
Ep = collection field intensity (volts/m)
r = particle radius (m)
μ = dynamic viscosity of gas (Pa-S)
c = Cunningham correction factor

31. Cunningham correction factor
where,
T = absolute temperature (°k)
dp = diameter of particle (μm)

32. Problem
An ESP is designed to treat 50,000 m3/min with 97 % efficiency. Assuming an effective drift velocity of 2.5 m/min, calculate the required plate area and the number of plates. The plate size is 10 m by 5 m (height by length).

33. Solution
Step 1:
Efficiency of an Electrostatic Precipitator is given by
A =-[ (Q/w)*ln(1- η)]
A = 70,000 m2
Step 2:
Number of plates = total area/plate area
= 1400

34. ELECTROSTATIC PRECIPITATOR (contd.)
Advantages of Electrostatic Precipitators
Electrostatic precipitators are capable very high efficiency, generally of the order of %.
Since the electrostatic precipitators act on the particles and not on the air, they can handle higher loads with lower pressure drops.
They can operate at higher temperatures.
The operating costs are generally low.
Disadvantages of Electrostatic Precipitators
The initial capital costs are high.
Although they can be designed for a variety of operating conditions, they are not very flexible to changes in the operating conditions, once installed.
Particulate with high resistivity may go uncollected.

35. Wet Scrubbers
Aeromix Wet Scrubber

36. Wet Scrubbers
Floating Bed Wet Scrubber

37. Wet Scrubbers Principle Construction and Operation
Wet scrubbers are used for removal of particles which have a diameter of the order of 0.2 mm or higher.
Wet scrubbers work by spraying a stream of fine liquid droplets on the incoming stream.
The droplets capture the particles
The liquid is subsequently removed for treatment.
Construction and Operation
A wet scrubber consists of a rectangular or circular chamber in which nozzles are mounted.
The nozzles spray a stream of droplets on the incoming gas stream
The droplets contact the particulate matter, and the particles get sorbed.
The droplet size has to be optimized.

38. Wet Scrubbers (contd.) Construction and Operation (contd.)
Smaller droplets provide better cleaning, but are more difficult to remove from the cleaned stream.
The polluted spray is collected.
Particles are settled out or otherwise removed from the liquid.
The liquid is recycled.
Wet scrubbers are also used for the removal of gases from the air streams.

39. Scrubber Efficiency where,
k = Scrubber coefficient (m3 of gas/ m3 of liquid)
R = Liquid-to-gas flow rate (QL/QG)
ψ = internal impaction parameter
Internal impaction parameter
c = Cunningham correction factor
ρp = particle density (kg/m3)
Vg = speed of gas at throat (m/sec)
dp = diameter of particle (m)
dd = diameter of droplet (m)
μ = dynamic viscosity of gas, (Pa-S)

40. Wet Scrubbers (contd.) Advantages of Wet Scrubbers
Wet Scrubbers can handle incoming streams at high temperature, thus removing the need for temperature control equipment.
Wet scrubbers can handle high particle loading.
Loading fluctuations do not affect the removal efficiency.
They can handle explosive gases with little risk.
Gas adsorption and dust collection are handled in one unit.
Corrosive gases and dusts are neutralized.
Disadvantages of Wet Scrubbers
High potential for corrosive problems
Effluent scrubbing liquid poses a water pollution problem.

41. Cyclone Spray Chambers
These scrubbers combine a cyclone with a spray nozzle.
The added centrifugal force permits good separation of the droplets, hence a smaller droplet size can be used.
Cyclone spray chambers provide up to 95% removal of particles > 5 micron.

42. Orifice Scrubbers
The gas is impacted onto a layer of the scrubbing liquid.
The gas passes through the liquid, thus removing almost all the particulate matter, and a large portion of the probable gases.
After coming out of the liquid, the gas is passed through baffles to remove the liquid droplets.

43. Impingement Scrubbers
In Impingement scrubbers, the gas impacts a layer of liquid/froth through a perforated tray.
Passing through this layer removes the particulate matter.
The wet gas stream is then passed through a mist collector.

44. Venturi Scrubbers

45. Venturi Scrubbers
The dirty gas is led in to the chamber at high inlet velocities.
At the inlet throat, liquid at low pressure is added to the gas stream
This increases the relative velocity between the gas and the droplets, thus increasing the efficiency of removal.
Efficiencies of the range of 95% for particles larger than mm have been obtained.

46. Venturi Scrubber Absolute Pressure Drop
Δp = pressure drop ( cm of water)
ug = gas velocity (cm/s)
Qt = liquid volume flow rate
Qg = gas volume flow rate

47. Problem
Water is introduced into the throat of a venturi scrubber. The air velocity through the scrubber is 550 fps and the liquid to gas ratio is 8.5 gal/1000 actual ft3. Determine the pressure drop?

48. Solution
Step 1:
Absolute pressure drop is given by
Δp = 4.8

49. HYDROCARBON CONTROL

50. General Methods For Control Of Hydrocarbon Emissions
Incineration or after burning
Direct flame incineration
Thermal incineration
Catalytic incineration

51. Direct Flame Incineration

52. Catalytic incineration

53. Thermal Incinerator

54. Catalytic Incinerator with Heat Recovery

55. Problem
Calculate the removal efficiency of a burner if the concentration of HC was reduced from 1300 ppm to 100 ppm.

56. Solution Step 1: Removal Efficiency of the burner
Efficiency = (initial concentration – final concentration)/ initial concentration
Efficiency of the burner = 92.3%

57. VOC Incinerators

58. CxHy + ( x + y/4 ) O2 x CO2 + (y/2) H2O
VOC Incinerators
Principle
VOC incinerators thermally oxidize the effluent stream, in the presence of excess air.
The complete oxidation of the VOC results in the formation of carbon monoxide and water. The reaction proceeds as follows:
CxHy + ( x + y/4 ) O2 x CO2 + (y/2) H2O
Operation
The most important parameters in the design and operation of an incineration system are what are called the
' three T's ' Temperature, Turbulence, and residence Time.

59. VOC Incinerators (contd.)
Temperature
The reaction kinetics are very sensitive to temperature
The higher the temperature, the faster the reaction
Timing
A certain time has to be provided for the reaction to proceed
Turbulence
Turbulence promotes mixing between the VOC's and oxygen
Proper mixing helps the reaction to proceed to completion in the given time.

63. VOC Incinerators (contd.)
The various methods for incineration are:
Elevated fires, for concentrated streams
Direct thermal oxidation, for dilute streams
Catalytic oxidation, for dilute streams.

64. Problem In a workshop a mixture of chemicals were released,
Benzene – 3000 ppm
Toulene – 1000 ppm
Methane – 2000 ppm
Calculate the lower exposure limit (LEL) of the mixture
LEL by volume for each chemical
Benzene – 1.4 %
Toulene – 1.27 %
Methane – 5.00%

65. Solution LEL of a mixture:
Xi - Volume of i component in the mixture
Xm - Volume of mixture
LELi - LEL of i component
LELmix = [3000/(6000*1.4) /(6000*1.27) /(6000*5)]-1
= 1.79%

66. GASES

67. Air Pollution Control For Gases
Adsorption Towers
Thermal Incernation
Catalytic Combustion
Air Correction Equipment for Gases and Vapors

68. Adsorption Towers Principle
Adsorption towers use adsorbents to remove the impurities from the gas stream.
The impurities bind either physically or chemically to the adsorbing material.
The impurities can be recovered by regenerating the adsorbent.
Adsorption towers can remove low concentrations of impurities from the flue gas stream.

69. Adsorption Towers (contd.)
Construction and Operation
Adsorption towers consist of cylinders packed with the adsorbent.
The adsorbent is supported on a heavy screen
Since adsorption is temperature dependent, the flue gas is temperature conditioned.
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
Very low concentrations of pollutants can be removed.
Energy consumption is low.
Do not need much maintenance.
Economically valuable material can be recovered during regeneration.