AIR QUALITY AND POLLUTION (TKA 3301) LECTURE NOTES 11- Control Procedure PM
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Transcript of AIR QUALITY AND POLLUTION (TKA 3301) LECTURE NOTES 11- Control Procedure PM
Control ProcedureControl ProcedureControl ProcedureControl Procedure
Dr. Marzuki Hj. IsmailDr. Marzuki Hj. Ismail
Semester I 2009/10 Semester I 2009/10
ENTECHENTECH
IntroductionIntroductionIntroductionIntroduction
Stationary sources produce significant amount of criteria pollutants.
Control of air pollutants form stationary sources is sensitive issue
1. generate thousands of useful consumer products;
2. create millions of jobs and 3. provide essential goods and services.
4 Steps Process4 Steps Process
1. Elimination of the problem source or operation
2. Modification of the source operation3. Relocation of the source4. Selection and application of the
appropriate control technology
Controlling Particulates
Can use several types of treatment Settling chambers Cyclones Fiber filter/Bag house Scrubber Electrostatic precipitator (ESP)
Comparison of Air Pollution Comparison of Air Pollution Control DevicesControl Devices
A Setling ChamberB Simple CycloneC High efficiency cycloneD Electrostatic PrecipitatorE Spray Tower Wet ScrubberF Venturi ScrubberG Bag Filter
Settling chambers Settling chambers use the force of gravity to
remove solid particles. The gas stream enters a expansion chamber in
which gas velocity is reduced, allowing time for particles to settle out under the action of gravity.
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.
Types and ComponentsTypes and Components
Simple expansion chamber
Multiple-tray settling chamberMultiple-tray settling chamber
CyclonesCyclones
Use inertial force to separate particles from a rotating gas stream.
Two main types:1. large diameter cyclones and 2. small diameter multi-cyclones
Large diameter cyclonesLarge diameter cyclones
Multi-cyclone collectorMulti-cyclone collector
Fabric filtersFabric filters
Also referred to as baghouses, Capable of high-efficiency PM (0.1 μm to
1,000 μm) removal in a wide variety of industrial applications.
4 major types of fabric filters: shaker, reverse air, pulse jet, and cartridge.
ImpactionImpaction
Impaction is most effective on particles larger than about 1 μm.
Brownian diffusionBrownian diffusion
Brownian diffusion is quite effective for collecting sub-m particles because of the close contact between the gas stream and the dust cake.
Electrostatic attractionElectrostatic attraction
Particles can be attracted to the dust layer and fabric due to the moderate electrical charges that accumulate on the fabrics, the dust layers, and the particles.
Both +ve and -ve charges can be generated, depending on the chemical make-up of the materials.
Fabric Filter Systems
Shaker Reverse air Pulse jet Cartridge Filters
ShakerShaker The dust laden gases
enter through the hopper, where some of the larger particles in the gas stream settle out.
Most of the dust will be carried by the gas stream as it passes up through the filter bag and will be deposited on the inside of the bag.
Shaker collectors use woven fabrics
Shaker Collectors
Cleans the bags by gently shaking. SC has a tube sheet between the vertical
casing and the hopper. The open bottom of cylindrical bag are
attach to holes, the closed top of the bag attach to shaking mechanism in the top of casing.
Contaminated gas stream enter the hopper, flow thro holes in the tube sheet and into the inside of vertical bags.
Since top of bags are closed, gas stream flow thro the bags leaving dust cake on the inside.
Periodically, gas flow is stop, bag shaken to clear it.
Dustcake fall into hopper, removed from the collector.
Reverse airReverse air
The construction and operation of reverse air fabric filters is very similar to shaker collectors.
The difference is in the cleaning mechanism – reverse air is passed from the outside of the bags to the inside in order to remove some of the dust cake.
Reverse Air Collector
Very similar to SC. The collector has tube sheet between casing and hopper.
Bottom of bags are attach to holes in tube sheet.
Contaminate gas stream enter the hopper, flow into and thro’ the bag, leaving dust cake on the inside.
When time to clean bags, gas flow is stop; another gas flow is introduce in reverse direction – use the cleansed gas stream discharge.
Dust cake falls into hopper, and removed.
Pulse jetPulse jet
View of the bottoms of pulse jet bags
Pulse Jet Collectors Clean bags using short duration
pulse of compress air. PJC has a tube sheet located
near top of vertical casing; bags hang from the hole in tube sheet.
Contaminate gas stream enter thro the side or hopper and flow into the bag and up thro’ the tube sheet, leaving dust cake on outside of the bags.
Wire-mesh cage is locate inside the bag to keep from collapse.
Cleaning – directing compressed air pulse to the bags.
Dust cake fall into hopper, remove from collector.
Cartridge Filters Similar to pulse jet
fabric filter systems Gas stream to be
filtered passes from the outside of the filter element to the inside.
Filtering is performed by the filter media and the dust cake supported on the exterior of the filter media
Pleated cartridge filter elementPleated cartridge filter element
Flat cartridge filter elementFlat cartridge filter element
Types of Types of FabricsFabrics
Wet ScrubbersWet Scrubbers
Wet scrubbers use a three-step process for the treatment of particulate-laden gas streams:
1. Particle capture in either droplets, liquid sheets, or liquid jets
2. Capture of the liquid droplets entrained in the gas stream
3. Treatment of the contaminated liquid prior to reuse or discharge
The primary mechanism by which particles are collected in wet scrubbers is impaction.
Wet Scrubber Systems
3 types: Low energy wet scrubber Medium energy wet scrubber High energy wet scrubber
Wet Scrubber Systems – Spray Tower
A low energy wet scrubber
Spray tower scrubberSpray tower scrubber
Packed Bed ScrubbersPacked Bed Scrubbers
A medium energy wet scrubber
Venturi Scrubber (VS)Venturi Scrubber (VS)
Most common high energy wet scrubber.
In fixed-throat VS, gas stream enters a converging section where it is accelerated towards the throat section.
Here, the high velocity gas stream strikes liquid streams that are injected at right angle, shatter the liquid into small drops.
PM are collected when they impact the slower moving drops.
After throat section, gas stream pass thro diverging section that reduce the velocity (some PM collection also occurs here).
Con’t
Other variation - variable throat venturi. Since scrubbing is a function of the gas velocity in the
throat, venturi that can change throat dimensions are used when gas flow rate from the process varies.
This mechanism is set to maintain fixed pressure drop across the collector.
Mist Eliminators In all WS, process of contact between gas
and liquid streams will produce entrained droplets.
Because these droplets contain contaminants, it must be removed before its exits the unit.
This is called – mist elimination or entrainment separation.
3 common mist elimination – chevrons, mesh pad, cyclones.
Chevrons – zig-zag baffles that cause gas stream to turn several times when passing mist elimination. Liquid droplets are collected on the blade and drain back into scrubber.
Mesh pad – made from interfaced fiber to collect droplets.
Cyclone – removed by centrifugal force.
Electrostatic PrecipitatorsElectrostatic Precipitators
Operating PrinciplesOperating Principles In all types of electrostatic precipitators,
there are 3 basic steps to PM collection: Step 1 - electrical charging and migration of
particles toward a vertical collection surface. Step 2 - involves the gravity settling (or
draining in the case of liquids) of the collected material from the vertical collection surfaces.
Step 3 - removal of the accumulated solids or liquids from the hopper or sump below the electrically energized zone.
Particle Charging and MigrationParticle Charging and Migration
Negative corona particle chargingNegative corona particle charging
Precipitator SystemsPrecipitator Systems
ESP’s remove PM from gas stream by charging them either tve/ -ve and cause them to be deposit on grounded collection plate.
There are three categories of electrostatic precipitators (ESPs). These units serve entirely different industrial applications. Dry, negative corona; Wet, negative corona; Wet, positive corona.
Dry, Negative Corona ESP’s The most common type of ESP. As gas stream enters, the velocity is
reduce as it pass thro’ expanding transition section.
Several perforate plates are mounted in this section to maintain proper flow distribution,
Gas stream exiting transition section flows horizontally thro’ a large no. of parallel gas passage with vertical discharge electrode mounted on the centre and vertical grounded collection plates on either side.
Wet, Positive Corona Designs Used to collect organic mists
from small, medium industry. Here, discharge electrodes
are separated from grounded collection plate (sometime call 2-stage precipitator).
First, gas stream enter charging section, where +ve discharge electrodes charge the mist particle +ve.
The charge particle then move thro’ collector section; they are attracted to grounded plate and then drain to sump.
Plates are clean manually.