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.