Transcript Slide 1

‫بنام خدا‬
‫با سالم‬
‫انتظار میرود دانشجو در پایان جلسه بتواند‪:‬‬
‫‪ -1‬اساس كار رسوب دهنده هاي ثقلي را شرح دهد‪.‬‬
‫‪ -2‬اساس كار سیكلونها را توضیح دهد‪.‬‬
‫‪ -3‬اساس كار الكترو فیلترها را توضیح دهد‪.‬‬
‫‪ -4‬اساس كار فیلترهاي پارچه اي را شرح دهد‪.‬‬
‫‪ -5‬مزایا و معایب دستگاههاي فوق را بیان كند‪.‬‬
AIR POLLUTION CONTROL
Environment technologies
2. AIR POLLUTION
CONTROL
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AIR POLLUTION CONTROL
Environment technologies
CONTENTS
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•
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Background
Equipment
Techniques
Factors
Selecting technology
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AIR POLLUTION CONTROL
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BACKGROUND
• Controlling the emission of pollutants from industrial and
domestic sources is important in protecting the quality of
air. Air pollutants can exist in the form of particulate
matter or as gases.
• Air cleaning devices have been reducing pollutant
emissions from various sources for many years.
• Originally, air cleaning equipment was used only if the
contaminant was highly toxic or had some recovery
value.
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EQUIPMENT
Equipment used to control particulate emissions are:
• Gravity settlers (often referred to as settling
chambers)
• Mechanical collectors (cyclones)
• Electrostatic precipitators (ESPs)
• Scrubbers
• Fabric filters
• Hybrid systems
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EQUIPMENT
Gravity settlers (often referred to as settling chambers)
• Gravity settlers, or gravity settling chambers, are used
industrially for the removal of solid and liquid waste
materials from gaseous streams.
• Advantages accounting for their use are simple
construction, low initial cost and maintenance, low
pressure losses, and simple disposal of waste materials.
Mechanical collectors (cyclones)
• Centrifugal separators, commonly referred to as
cyclones, are widely used in industry for the removal of
solid and liquid particles (or particulates) from gas
streams.
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AIR POLLUTION CONTROL
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EQUIPMENT
Electrostatic precipitators (ESPs)
• They are satisfactory devices for removing small
particles from moving gas streams at high
collection efficiencies. They have been used
almost universally in power plants for removing
fly ash from the gases prior to discharge.
• Two major types of high-voltage ESP
configurations currently used are tubular and
plate. Tubular precipitators consist of cylindrical
collection tubes with discharge electrodes
located on the axis of the cylinder. Vast majority
of ESPs installed are of the plate type.
• Collected particles are usually removed by
rapping.
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AIR POLLUTION CONTROL
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EQUIPMENT
Scrubbers (venturi scrubbers)
• Wet scrubbing involves the technique of bringing a
contaminated gas stream into intimate contact with a
liquid.
• Wet scrubbers include all the various types of gas
absorption equipment.
• The term "scrubber" will be restricted to those systems
which utilize a liquid, usually water, to achieve or assist
in the removal of particulate matter from a carrier gas
stream.
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EQUIPMENT
Fabric filters (bag houses)
• Filtration process may be conducted in many
different types of fabric filters.Differences may
be related to:
–
–
–
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Type of fabric
Cleaning mechanism
Equipment
Mode of operation
• Gases to be cleaned can be either "pushed" or
"pulled" through the bag house.
• In the pressure system (push through) the gases
may enter through the cleanout, hopper in the
bottom or through the top of the bags.
• In the suction type (pull through) the dirty gases
are usually forced through the inside of the bag
and exit through the outside.
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AIR POLLUTION CONTROL
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EQUIPMENT
• Hybrid systems are defined as those types of control
devices
that
involve
combinations
of
control
mechanisms-for example, fabric filtration combined with
electrostatic precipitation.
• Four of the major hybrid systems found in practice today
include:
– Wet electrostatic precipitators,
– Ionizing wet scrubbers,
– Dry scrubbers, and
– Electrostatically augmented fabric filtration.
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AIR POLLUTION CONTROL
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TECHNIQUES
• Applicability of a given technique depends on the
physical and chemical properties of the pollutant and the
exhaust stream.
• More than one technique may be capable of controlling
emissions from a given source
• Techniques used to control gaseous emissions are:
Absorption
Adsorption
Combustion
Condensation
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TECHNIQUES
Absorption
• Mass transfer operation in which a gas is dissolved in a
liquid.
• A contaminant (pollutant exhaust stream) contacts a liquid,
and the contaminant diffuses from the gas phase into the
liquid phase.
• The liquid most often used for absorption is water.
• Reagents can be added to the absorbing water to increase
the removal efficiency of the system. Gas absorbers or wet
scrubbers are designed to provide good mixing of the gas
and liquid phases.
• The devices used for gas absorption are often the same as
those used in particulate emission scrubbing.
• These include packed towers, plate towers, spray columns,
and venturi scrubbers.
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TECHNIQUES
Adsorption
• Mass transfer process that involves removing a gaseous
contaminant by adhering it to the surface of a solid.
• It can be classified as physical or chemical. In physical
adsorption, a gas molecule adheres to the surface of the
solid due to an imbalance of natural forces (electron
distribution).
• In chemisorption, once the gas molecule adheres to the
surface, it reacts chemically with it.
• The major distinction is that physical adsorption is readily
reversible whereas chemisorption is not.
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TECHNIQUES
Combustion
• Combustion is defined as rapid, high-temperature gas-phase
oxidation.
• Simply, the contaminant (a carbon-hydrogen substance) is
burned with air and converted to carbon dioxide and water
vapor.
• The operation of any combustion source is governed by the
three T's of combustion; temperature, turbulence, and time.
• Combustion devices can be categorized as flares, thermal
incinerators, or catalytic incinerators
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TECHNIQUES
Condensation
• Process in which the volatile gases are removed from the
contaminant stream and changed into a liquid.
• It is usually achieved by reducing the temperature of a
vapor mixture until the partial pressure of the
condensable component equals its vapor pressure.
• Requires low temperatures to liquefy most pure
contaminant vapors.
• It is affected by the composition of the contaminant gas
stream.
• Condensers are normally used in combination with
primary control devices.
• Condensers can be located upstream of (before) an
incinerator, adsorber, or absorber.
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AIR POLLUTION CONTROL
Environment technologies
FACTORS
• There are a number of factors to be considered prior
to selecting a particular piece of air pollution control
hardware.
Economic
Environ
mental
Engineering
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FACTORS
Environmental factors
• Equipment location
Environ
• Available space
mental
• Ambient conditions
• Availability of adequate utilities (i.e., power, water, etc.) and
ancillary system facilities (i.e., waste treatment and disposal,
etc.)
• Maximum allowable emissions (air regulations)
• Aesthetic considerations
• Contribution of air pollution control system to wastewater
and solid waste
• Contribution of air pollution control system to plant noise
levels
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FACTORS
Economic
• Capital cost (equipment, installation, engineering, etc.)
• Operating cost (utilities, maintenance, etc.)
• Expected equipment lifetime and salvage value
Economic
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FACTORS
Engineering
• Contaminant characteristics (i.e., physical and chemical
properties, concentration, particulate shape and size)
• Gas stream characteristics (i.e., volume flow rate,
temperature, pressure, humidity, composition, viscosity,
density, reactivity, combustibility, corrosivity, toxicity, etc.)
• Design and performance characteristics of the particular
control system(i.e., size and weight, fractional efficiency
curves, etc)
Engineering
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SELECTING TECHNOLOGY
• Final choice in equipment selection is usually dictated by
that equipment capable of achieving compliance with
regulatory codes at the lowest uniform annual cost
(amortized capital investment plus operation and
maintenance costs).
• In order to compare specific control equipment alternatives,
knowledge of the particular application and site is essentials.
• A preliminary screening, however, may be performed by
reviewing the advantages and disadvantages of each type of
air pollution control equipment.
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