Industrial Air Pollution Control

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Industrial Air Pollution Control

• Sell, N.J. (1992) Industrial Pollution Control.
  2nd Edition. Van Nostrand Reinhold, New York

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Methods used to control air pollutants

• Destroys
• Masks
• Counteracts
• Traps

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Air pollution control equipment used depends upon

• Size
• Shape
• Density
• Stickiness
• Electrical properties
• Temp., Moisture content, quantity, economic
  factors

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Physical Principles for particulate collection

• Gravity settling
• Inertial forces
• Filtration
• Electrostatic attraction
• Particle enlarging

In general, the simpler, less energy-intensive
devices are less efficient, making them suitable
mainly for large, dense particulates. Finer
particulate and other types of air pollutants
require the more sophisticated collection
devices.
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Dry Mechanical Collectors

• Gravity Settling Chambers particles between
  40-100 µm

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Dry Mechanical Collectors (cont.)

• Recirculating Baffle Collectors particles
  between 10-50 µm

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Dry Mechanical Collectors (cont.)

• Cyclones Best for 15-50 µm particles. More
  efficient with heavier loads due to the increased
  interparticle interactions.

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• Cyclone efficiency can be calculated using cut
  diameter, dc particle diameter at which 50
  are removed

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Dry Mechanical Collectors (cont.)

• Multiple Cyclones

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Dry Mechanical Collectors (cont.)

• Dynamic Precipitators operated on the
  centrifugal force. Unsuitable for sticky or
  fibrous materials. Remove 5-20 µm

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Dry Mechanical Collectors (cont.)

• Fabric Collectors lt0.5 µm particles can be
  removed thanks to the filter cake

• The fabric must be cleaned Intermittently,
  Periodically, or Continuously
• Various materials can be used to construct the
  fabric, but polyesters are the most widely used
  as they can tolerate high temperature and
  abrasion resistance

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Manual or powered shaking
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Air jet
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Blow ring
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Wet Scrubbers
Pros
Cons

• Use a stream of water to increase the collection
  efficiency
• No limit for inlet air stream temperature and
  moisture content
• Other liquid than water can be used, which could
  neutralise the corrosive air pollutants, e.g. SO2
  

• High operating cost, especially for the
  high-efficiency systems
• Required treatment of the scrubbing liquids and
  disposal of the wet sludge
• Corrosion of structure
• The presence of a highly visible white plume,
  cause by steam discharged to the atmosphere

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Low-energy Types Wet Scrubbers

• Spray Chambers

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Low-energy Types Wet Scrubbers (cont.)

• Spray Towers

• Open Spray Tower
• Best for coarse particulates (gt10 ?m), e.g. in
  the iron pyrite roasting step of steel production
• Good for heavy particulate loads

• Packed Spray Tower
• Suitable for gas cooling and absorption, e.g. for
  fluorine compounds
• Has little solids-handling capacity, and hence is
  not suitable for heavy dust loads

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High-energy Types Wet Scrubbers

• Venturi Scrubbers

• Very efficient (gt99 removal of 0.5 ?m particles)
• Typically used in the steel industry for fine
  particulates, for acid mists, and for lime and
  coal dust
• High operating cost

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Electrostatic Precipitators

• Can operate at over 1000?F
• Can be used for dry particulates or fumes, and
  well as mists
• The dirty gas is channelled between a
  high-voltage electrode (-40,000 V and -50,000 V)
  and a grounded collecting electrode
• Efficiency (?) is estimated by
• 99.9 efficiency, good for hot, corrosive
  substances

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Electrostatic Precipitators (cont.)

• Dust resistivity is also a limiting factor
• The resistivity of dust such as that generated by
  low-sulfur coal can be reduced by adding
  substances, e.g. moisture, ammonia, acid mists,
  and SO3

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Fume Incineration

• HCN is an extremely poisonous, colourless gas,
  used in the production of, e.g. nylon
• Incineration will destroy the toxicity, producing
  a non-toxic product, i.e. CO2, N2, and H2O
• Moreover, incineration is also suitable for
  destroying of paint solvents and odours

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Sorption Techniques
Adsorption
Absorption

• Employs materials, e.g. activated carbon, silica
  gel, or alumina
• Used for VOC removal, sulfur-bearing materials
  and mercury
• Efficiency increases with molecular weight of the
  material adsorbed, degree of cyclisation, polarity

• Utilises water or an organic liquid, e.g.
  low-volatile hydrocarbon or mineral oil
• Occurs whenever the conc. of the organic species
  in the liquid is less than the equilibrium conc.
• Can be used in conjunction with spray chambers,
  packed towers, and venturi scrubbers
• Maximum absorption occurs at low temp., large
  contacting surface area, high liquid-to-gas
  ratio, and conc. Streams
• Inlet conc. 5000 mg/l, gt98 removal can be
  achieved

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Tall Stacks

• To diffuse the pollutants, e.g. SO2, CO2, NOx,
  not to reduce the quantity
• Tendency in causing the acid rain problems
• A rule of thumb the stacks should be 2.5 times
  the height of the tallest nearby building
• The air masses at 100 m have been reported to be
  more stable than had been assumed!!!!

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