objectives of control equipment · 2017-01-25 · • air quality management sets the tools to...
TRANSCRIPT
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ENVIRONMENTAL
Philadelphia UniversityFaculty of Engineering
Department of Civil EngineeringFirst Semester, 2013/2014
ENVIRONMENTALENGINEERING
343Lecture 12:
Air pollution 3
SOURCE CONTROL TECHNOLOGY
• Air quality management sets the tools to control air pollutant emissionspollutant emissions.
• Control measurements describes the equipment, processes or actions used to reduce air pollution.
• The extent of pollution reduction varies among technologies and measures.
• The selection of control technologies depends on environmental, engineering, economic factors and pollutant type.
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OBJECTIVES OF CONTROL EQUIPMENT
o Prevention of nuisanceo Prevention of physical damage to Environmento Prevention of physical damage to Environmento Elimination of health hazardso Recovery of valuable waste producto Minimization of economic losseso Improvement of product quality
CONTROL OF POLLUTANTS FROM STATIONARYSOURCESThe most common method for controlling pollutants is the addition
of add-on control devices to recover or destroy a pollutantAir Pollution control- at source-equipments for control of air
pollution-p
I- For Gaseous pollutants1.1 Absorption, 1.2 Adsorption, 1.3 Condensation, and 1.3 Incineration (combustion)
II- For particulate matter
2.1 Settling chambers-
2.2 Fabric filters
2.3 Scrubbers
2.4 Cyclones
2.5 Electrostatic precipitators
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1.1 ABSORPTION• The removal of one or more selected components from a
gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions.
• Absorption is a process in which a gaseous pollutant is dissolved in a liquid. q
• As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred.
• Absorbers are often referred to as scrubbers, and there are various types of absorption equipment.
• The principal types of gas absorption equipment include p p yp g p q pspray towers, packed columns, spray chambers, and venture scrubbers.
In general, absorbers can achieve removal efficiencies grater than 95 percent. One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.
1.1 ABSORPTION
o Effluent gas passed through absorbers (scrubbers), which contain liquid absorbent.
o Efficiency depends ony p1. Amount of surface contact between gas and liquid2. Contact time3. Conc. of absorbing medium4. Speed of reaction between the absorbent and gases
o Absorbents used to remove SO2, H2S, SO3, F and oxides of nitrogen.
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Equipments using principles of absorption for removal of gaseous pollutants
S t / t bbo Spray tower/ wet scrubber
Gaseous pollutants
Common absorbents used in solution form
SO2 Dimethylaniline, ammonium sulphite, ammonium sulphate, sodium sulphide, calcium
l hit lk li tsulphite, alkaline water, H2S NaOH and phenol mix (3:2), tripottasium
phosphate, sodium alamine, sodium thioarsenate, soda ash
HF Water, NaOHNOX Water, aqueous nitric acid, q
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1.2 ADSORPTION• When a gas or vapor is brought into contact with a solid, part of
it is taken up by the solid. The molecules that disappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption.
• The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight.
• Activated carbon is the universal standard for purification and removal of trace organic contaminants from liquid and vaporstreams. Carbon adsorption systems are either regenerative or non-regenerative. g- Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use. - Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant.
Equipments using principles of adsorption for removal of gaseous pollutants
o Packed tower
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1.2 ADSORPTION
o Surface phenomenon, require large solid surfaceo Adsorption towers use adsorbents to remove the
i i i f himpurities 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.
Advantages of Adsorption TowersV l i f ll bo 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.
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Gaseous pollutants Adsorbents used in solid form
SO Pulverized limestone orSO2 Pulverized limestone or dolomite, alkalized alumina
H2S Iron oxideHF Lump limestone, porous sodium
fluoride pelletsNOX Silica gelOrganic solvent vapours Activated carbon
1.3 INCINERATION
• Incineration, also known as combustion, is most used to control the emissions of organic compounds from process industries. This control technique refers to the rapid• This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor. E i t d t t l t b• Equipment used to control waste gases by combustion can be divided in three categories: - Direct combustion or flaring, - Thermal incineration and - Catalytic incineration.
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Fume incinerators
o Used to control effluent gases, fumes and odors from refineries, burning waste, cracking gases and chemical plants
Method is expensive when 1. Fuel values of gaseous discharge low2. Moisture content of discharge is high3. Exhaust volume is extremely large
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Factors considered in design of incinerators are1. Sufficient air for combustion reaction2. 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 higherhigher.
o When conc. of combustible portion of gas stream below flammable range, catalytic combustion process used.
1.3.2 CATALYTIC INCINERATORS
Catalytic incinerators are very similar to thermal incinerators. The main difference is that after passingincinerators. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95 percent are possible using a catalytic incinerator.
Catalytic incinerator
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1.4 CONDENSATION
• Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a liquid by lowering its temperature and/or increasing its pressure.
• Condensers are typically used as pretreatment devices. Th b d h d f b b d b dThey can be used ahead of absorbers, adsorbers, and incinerators to reduce the total gas volume to be treated by more expensive control equipment. Condensers used for pollution control are contact condensers and surface condensers.
• In a contact condenser, the gas comes into contact with cold liquid.
• In a surface condenser, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the outside of the tube.
Removal efficiencies of condensers typically range from 50 percent to more than 95 percent, depending on design and applications.
II-PARTICULATE CONTROL EQUIPMENT
1. Gravitational settling chambers
F b i fil2. Fabric filters
3. Scrubbers
4. Cyclone separator
5. Electrostatic precipitators
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2.1 SETTLING CHAMBERS/ GRAVITATIONALSETTLING CHAMBER
• Settling chambers use the force of gravity to remove solid particles.
• The gas stream enters a chamber where the velocity of the gasThe gas stream enters a chamber where the velocity of the gas is reduced. Large particles drop out of the gas and are recollected in hoppers. Because settling chambers are effective in removing only larger particles, they are used in conjunction with a more efficient control device.
o Used to remove particles with size greater than 50 μm.o Velocity of flue gas reduced in large chamber.
2.1 SETTLING CHAMBERS/ GRAVITATIONAL
SETTLING CHAMBER
o Particles settle under gravitational force. Application
o Industrial application is limited.o Used widely for removal of large solid particulateso Sometimes used in process industry, food and metallurgical
industryindustry.o Used as pre-cleaners for high efficiency collectors.
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Advantages
o Low initial cost.
2.1 Settling Chambers/ Gravitational settling chamber
o Easy to design.o Low pressure drop.o Low maintenance cost.o Dry and continuous disposal of solid particulates.DisadvantagesDisadvantages
o Require large space.o Less collection efficiency.o Only larger size particles can be collected.
2.2 CYCLONES
• The general principle of inertia separation is that the particulate-laden gas is forced to change direction. As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream.
• The walls of the cyclone narrow toward the bottom of the unit, allowing the particles to be collected in a hopper.
The cleaner air leaves the cyclone through the top of the chamber, flowing upward in a spiral vortex, formed within a downward moving spiral.
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2.2 Cyclone separatoro Cyclones are efficient in removing large particles but are not
as efficient with smaller particles. For this reason, they are used with other particulate control devices.
o Centrifugal force is utilized to separate the particulate matter.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 Used mostly in industrieso Used mostly in industries.o It can remove 10 to 50 μm particle size.o Cyclone efficiencies > 90 % with particle dia of 10 μo > 95 % with particle dia 20 μ.
Efficiency
o Conventional efficiencyo 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 Corrosiono Material build up
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Advantages
o Low initial cost.o Require less floor area.o Simple construction and maintenanceo 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.
2.3 ELECTROSTATIC PRECIPITATORS (ESPS)
• An ESP is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates.
The ESP places electrical charges on the particles causing them to be• The ESP places electrical charges on the particles, causing them to be attracted to oppositely charged metal plates located in the precipitator.
• The particles are removed from the plates by "rapping" and collected in a hopper located below the unit.
• The removal efficiencies for ESPs are highly variable; however, for very small particles alone the removal efficiency is about 99 percentvery small particles alone, the removal efficiency is about 99 percent.
Electrostatic precipitators are not only used in utility applications but also other industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake & lime dust), petrochemicals (sulfuricacid mist), and steel (dust & fumes).
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ELECTROSTATIC PRECIPITATORS (ESPS)
Figure: Electrostatic precipitator components
Electrostatic precipitators
o Works on the principle of electrical charging of particulate Matter (-ve) and collecting it in a (+ve) charged surface.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.
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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.q g go Collection efficiency reduce with time.o Space requirement is more.o Possible of explosion during collection of
combustible gases or particulates.
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2.4 Fabric filters or cloth filters
o Flue gas is allowed to pass through a woven g p gfabric, 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%.
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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
Filter cleaning
o Rapping Sh kio Shaking
o Back washo Pulse jet .
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Fabric Max. operating temp. (˚ C)
Acid resistance
Alkali resistance
Abrasion resistance
Tensile strength Kg/cm2
Cotton 82 Poor Good Very good 4920Wool 93 Very good Poor Fair to
good1755
Nylon 93 Poor to fair
Excellent Excellent 5625
Dacron 135 Good Good Very good 5625Polypropyl 93 Excellent Excellent Excellent 7730yp pyeneFiber glass 290 Fair to
goodFair to good
Fair 14,060
Physical properties of bag filters
Advantages
o Higher collection efficiency for smaller than 10 μm particle size.
o Performance decrease becomes visible, giving o Pe o a ce dec ease beco es v s b e, g v gprewarning.
o Normal power consumption.Disadvantages
o High temp. gases need to be cooled.Hi h i t d f bri r pl t to High maintenance and fabric replacement cost.
o Large size equipment.o Fabric is liable to chemical attack.
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2.5 Venturi
scrubber/
sprayer
tower
Packed wet scrubbers
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Cyclone scrubber
Mechanical scrubbers
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CHOICE OF EQUIPMENT
1. Particulate size2. Particulate loading 3. Efficiency required4. Properties of carrier gas
1. Composition 6. Humidity 2. Temp. 7. Combustibility 3. Pressure 8. Reactivity4 Viscosity 9 Toxicity4. Viscosity 9. Toxicity5. Density 10. Electrical property
5. Flow characteristics of carrier gas1. Flow rate2. Variation in flow rate
6. Specific property of contaminant C i i 6 T i i1. Composition 6.Toxicity
2. Contaminant phase 7. Hygroscopicity3. Solubility 8. Agglomerating characteristics 4. Combustibility 9. Electrical and sonic
properties5. Reactivity 10. Catalyst poisoning
7. Allowable pressure drop8. Contaminate disposal9. Capital and operating cost of equipment 10. Ease of maintenance and reliability
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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 moderatep p
4. Scrubbers :- installation cost moderate, maintenance cost not high, high rate of power consumption.