k 5b scrubber teknologi

7/30/2019 K 5b Scrubber Teknologi

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Control of Particulate Matter

(Scrubber)


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What is Particulate Matter? Particulate matter (PM) describes a wide variety of airborne

material. PM pollution consists of materials (including dust,smoke, and soot), that are directly emitted into the air or resultfrom the transformation of gaseous pollutants.

Particles come from natural sources (e.g., volcanic eruptions) and

human activities such as burning fossil fuels, incinerating wastes,and smelting metals.


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aracter st cs o art c es

The most important characteristic of particulatematter (PM) is the particle size.

This property has the greatest impact on thebehavior of particulate matter in control

equipment, the atmosphere, and the respiratorytract.

Particles of importance in air pollution controlspan a broad size range from extremely small(0.01 micrometer) to more than 1,000micrometers.

As a frame of reference, a human hair has adiameter of approximately 50 micrometers.


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The chemical composition of the particulatematter is also important.

Absorption and heterogeneous nucleation ofvapor phase pollutants onto existing particles can

create toxic particulate matter.

Other characteristics besides size and chemicalcomposition should be considered when selectingan appropriate particulate control device for a

gas stream.

Other important characteristics of particulatematter in gas streams include stickiness andexplosiveness.


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Formation Mechanisms

Physical attrition occurs when two surfaces rubtogether.

The composition and density of the particles formedare identical to the parent material and range in size

from less than 10 micrometers to almost 1,000micrometers.

Combustion particle burnout occurs when fuelparticles are injected into the hot furnace area of a

combustion process.

as the combustion progresses, are reduced to ashand char particles that are primarily in the 1- to 100-micrometer size range.


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Homogeneous nucleation and heterogeneous

nucleation involve the conversion of vapor

phase materials to a particulate form.

Homogeneous and heterogeneous nucleation

generally create particles that are very small,

often between 0.1 and 1.0 micrometer.


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Droplet evaporation is caused by air pollution control

systems that use solids-containing water recycled

from wet scrubbers to cool the gas streams.

The water streams are atomized during injection into

the hot gas streams.

as these small droplets evaporate to dryness, the

suspended and dissolved solids are released as small

particles.


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Control Techniques

Gravity settling chamber

Mechanical collectors

Particulate wet scrubbers

Electrostatic precipitators

Fabric filters


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Gravity Settling Chambers

This category of control devices relies upon gravitysettling to remove particles from the gas stream.

Gravity settling chambers are used only for verylarge particles in the upper end of the supercoarse

size range (approximately 75 micrometers andlarger).

The very low terminal settling velocities of mostparticles encountered in the field of air pollution

limit the usefulness of gravity settling chambers.

The stringent control requirements adopted in thelate 1960s through early 1970s have resulted in asharp decline in the use of this type of collector.


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Mechanical Collectors

The particulate-laden gas stream is forced to

spin in a cyclonic manner.

The mass of the particles causes them to move

toward the outside of the vortex.

Most of the large-diameter particles enter a

hopper below the cyclonic tubes while the gas

stream turns and exits the tube.


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There are two main types of mechanical collectors:(1) large-diameter cyclones, and (2) small-diametermulti-cyclones.

Large-diameter cyclones are usually one to six feet indiameter; while small-diameter multi-cyclones usuallyhave diameters between 3 and 12 inches.

A typical large-diameter cyclone system is shown in

Figure 1.

The gas stream enters the cyclone tangentially andcreates a weak vortex of spinning gas in the cyclonebody.


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Large-diameter particles move toward the cyclone

body wall and then settle into the hopper of the

cyclone.

The cleaned gas turns and exits the cyclone.

Large-diameter cyclones are used to collect particles

down to 1/16 inch (1.5 mm) diameter and above.


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In systems where the large-diameter cyclone islocated after the fan (positive pressure), thetreated gas is usually discharged directly fromthe cyclone.

In systems where the cyclone is located beforethe fan (negative pressure), the gas stream iseither exhausted from a separate stack or fromthe discharge of the fan itself.

In negative pressure systems, a solids dischargevalve is used to prevent air infiltration up through

the hopper area.


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A small-diameter cyclone tube is shown in Figure2.

Vanes located on the inlet of each of the tubes

create the spinning movement of the gasstream.

Most of the commercial tubes are six, nine, ortwelve inches in diameter.

Due to the limited gas handling capacity of eachtube, large numbers of tubes are mounted inparallel in a single collector.


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The small-diameter of the cyclone tube creates more rapid

spinning of the gas stream than in large-diameter cyclones.

The particles moving outward in the spinning gas stream have a

relatively shorter distance to travel in a small-diameter multi-

cyclone tube before they reach the cyclone body wall.

These features allow small-diameter multi-cyclones to collect

considerably smaller particles than large-diameter cyclones can.


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Small-diameter multi-cyclones, such as the one shown in Figure2 are capable of removing particles having diameters down to 5

micrometers.

Small-diameter multi-cyclones are not generally used for very

large diameter material, such as 3 mm and above, because

large particles may plug the spinner vanes in the multi-cyclone

tubes.

Some mechanical collectors are specially designed to provide

high-efficiency PM collection down to a particle size of one

micrometer.


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These have higher gas velocities within the cyclone tubes and

different cyclone geometries than those shown in Figure 2.

A typical application of a conventional multi-cyclone collector is

shown in Figure 3.

In this example, the multi-cyclone is located after a small,

wood-fired boiler and is used as a pre-collector for the fabric

filter.


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Mechanical collectors are used whenever the particle


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Mechanical collectors are used whenever the particlesize relatively large (> 5 micrometers) and/or thecontrol efficiency requirements are in the low-to-moderate range of 50 to 90%.

They are also used as the pre-collector of large-diameter embers generated in some combustionsystems.

Removal of the embers is necessary to protect high-

efficiency particulate control systems downstreamfrom the mechanical collectors.

Most mechanical collectors are not applicable toindustrial sources that generate sticky and/or wet

particulate matter.

These materials can accumulate on the cyclone bodywall or the inlet spinner vanes of conventional multi-cyclone collectors.


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Particulate Wet Scrubbers

There are a number of major categories ofparticulate wet scrubbers:

- Venturis -Impingement and SievePlates

- Spray Towers -Mechanically Aided- Condensation Growth -Packed Beds- Ejector -Mobile Bed- Caternary Grid -Froth Tower

-Oriented Fiber Pad -Wetted MistEliminators

We will discuss three of the above types ofscrubbers: venturis, impingement plate

scrubbers, and spray towers.


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Venturi Scrubbers

A typical venturi throat is shown in Figure 4.

Particulate matter, which accelerates as it entersthe throat, is driven into the slow moving, largewater droplets that are introduced near the highvelocity point at the inlet of the venturi throat.

The adjustable dampers in the unit illustratedare used to adjust the open cross-sectional areaand thereby affect the speed of the particlesentrained in the inlet gas stream.


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Impingement Plate Scrubbers

An impingement plate scrubber is shown in Figure5.

These scrubbers usually have one to three horizontal

plates, each of which has a large number of smallholes.

The gas stream accelerating through the holesatomizes some water droplets in the water layer

above the plate.

Particles impact into these water droplets.


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Spray Tower Scrubbers

A typical spray tower scrubber is shown in Figure 6.

This is the simplest type of particulate wet scrubber

in commercial service.

Sets of spray nozzles located near the top of the

scrubber vessel generate water droplets that impactwith particles in the gas stream as the gas stream

moves upwards.


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E h f th t i f ti l t t


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Each of the categories of particulate wetscrubbers listed earlier has a large number ofdifferent design types.

For example, venturi scrubbers include thefollowing different design types: (1) fixed throat,(2) adjustable throat.

Spray tower scrubbers include these designtypes: (1) open, (2) cyclonic.

The scrubber categories listed above comprisemore than fifty different types of scrubbers incommon commercial use.

Scrubbers are by far the most diverse group ofair pollution control devices used for particulatecontrol.


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Wet Scrubbing Systems

Each particulate wet scrubber vessel is part ofa large, and sometimes complex, wet scrubbingsystem.

For example, Figure 7 illustrates a venturiscrubber in a scrubbing system.

The evaporative cooler, located before theventuri scrubber in the system, cools the gasstream, which serves the following purpose:

1- It protects the construction materials of theventuri throat.

2 It helps to homogeneously and heterogeneously


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2- It helps to homogeneously and heterogeneouslynucleate vapor phase material emitted from theprocess before it reaches the scrubbing system.

3-It prevents the water droplets from evaporatingand inhibiting inertial impaction.

L t d ft th t i bb th l i


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Located after the venturi scrubber, the cyclonicseparator removes entrained water droplets fromthe gas stream leaving the venturi.

The cyclonic separator consists of a cyclonicvessel and a horizontal mist eliminator.

The overall scrubbing system includes pumps for

liquid recirculation, a tank to treat the liquidbeing recirculated, an alkali addition unit tocontrol the liquid pH, a purged liquid treatmentunit, a fan for gas movement, and a stack.

There are a wide variety of wet scrubber systemdesigns; however, these components are presentin many systems, regardless of which type ofparticulate matter scrubber is used.


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Scrubber Operating Principles

The ability of a particulate wet scrubber to remove

particles depends on two or more of the following

variables:

The size (aerodynamic diameter) of the particle

The velocity of the particle

The velocity of the droplet.


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Collection Efficiency of Wet Scrubbers

The velocities of the particle-laden gas stream

and the liquid targets vary substantially.

There are substantial differences in the ability of

particulate wet scrubbers to collect particles less

than approximately 5 micrometers.

This is illustrated in Figure 8.


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If a significant portion of the particulate mattermass is composed of particles < 5 micrometers,care is needed to select the type of scrubber thatis effective in this size range.

It should be noted that some types of wetscrubbers have limited capability to removeparticles in the less than 0.3-micrometer range.

These particles are so small that their movementis influenced by collisions with individualmolecules in the gas stream.


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Advantages and Disadvantages ofScrubbers

Many types of particulate wet scrubbers canprovide high efficiency control of particulatematter.

One of the main advantages of particulate wet

scrubbers is that they are often able tosimultaneously collect particulate matter andgaseous pollutants.

Also, wet scrubbers can often be used on sources

that have potentially explosive gases orparticulate matter.

They are compact and can often be retrofittedinto existing plants with very limited space.


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One of the main disadvantages of particulate

wet scrubbers is that they require make-up

water to replace the water vaporized into the

gas stream and lost to purge liquid and sludge

removed from the scrubber system.

Wet scrubbers generate a waste stream that

must be treated properly.


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Electrostatic Precipitators

An electrostatic precipitator (ESP) uses non-uniform,high-voltage fields to apply large electrical charges toparticles moving through the field.

The charged particles move toward an oppositely

charged collection surface, where they accumulate.

There are three main styles of electrostatic

precipitators: (1) negatively charged dryprecipitators, (2) negatively charged wetted-wall

precipitators, and (3) positively charged two-stage

precipitators.


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The negatively charged dry precipitators are thetype most frequently used on large applicationssuch as coal-fired boilers, cement kilns, and kraftpulp mills.

Wetted-wall precipitators (wet precipitators) areoften used to collect mist and/or solid materialthat is moderately sticky.

The positively charged two-stage precipitators are

used only for the removal of mists.

The discussions will focus only on negativelycharged dry precipitators because these are themost common types of precipitators.


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Figure 9 shows the scale of a typical electrostaticprecipitator used at a coal-fired boiler.


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Essentially all of these units are divided into anumber of separately energized areas that aretermed fields (see Figure 10).

Most precipitators have between three and ten

fields in series along the gas flow path.

On large units, the precipitators are divided into anumber of separate, parallel chambers, each ofwhich has an equal number of fields in series.

There is a solid partition or physical separationbetween the 2 to 8 chambers that are present onthe large systems.


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Advantages and Disadvantages of ESPs

Electrostatic precipitators can have very highefficiencies due to the strong electrical forcesapplied to the small particles.

These types of collectors can be used when thegas stream is not explosive and does not containentrained droplets or other sticky material.

The composition of the particulate matter is veryimportant because it influences the electricalconductivity within the dust layers on thecollection plate.


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Resistivity, an important concept associated withelectrostatic precipitators, is a measure of theability of the particulate matter to conductelectricity and is expressed in units of ohm-cm.

As the resistivity increases, the ability of theparticulate matter to conduct electricitydecreases.

Precipitators can be designed to work in anyresistivity range; however, they usually workbest when the resistivity is in the moderaterange (108 to 1010 ohms-cm).


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Collection Efficiency of ESPs

The efficiency is usually at a minimum in therange of 0.1 to 0.5 micrometers.

The shape of the efficiency curve is the combinedeffect of two particle electrical charging

mechanisms, neither of which is highly effectivein this particle size range.

It should be noted that this decrease in efficiencyoccurs in the same particle size range as for

particulate wet scrubbers.

However, the reason for this decreased efficiencyzone is entirely different than that for particulatewet scrubbers.


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Fabric Filters

Operating Principles Fabric filters collect particulate matter on the

surfaces of filter bags.

Most of the particles are captured by inertial

impaction, interception, Brownian diffusion, andsieving on already collected particles that haveformed a dust layer on the bags.

The fabric material itself can capture particles that

have penetrated through the dust layers.

Electrostatic attraction may also contribute toparticle capture in the dust layer and in the fabricitself.

Due to the multiple mechanisms of particle captureibl f b i filt b hi hl ffi i t f th


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possible, fabric filters can be highly efficient for theentire particle size range of interest in air pollutioncontrol.

Types of Fabric Filters

A reverse-air-type fabric filter, shown in Figure 13, isone of the major categories of fabric filters. It is usedmainly for large industrial sources.

In this type, the particle-laden gas stream enters fromthe bottom and passes into the inside of the bags.

The dust cake accumulates on the inside surfaces ofthe bags.

Filtered gas passes through the bags and is exhausted


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When cleaning is necessary, dampers are used toisolate a compartment of bags from the inlet gas

flow.

Then, some of the filtered gas passes in the reverse

direction (from the outside of the bag to the inside) in

order to remove some of the dust cake.

The gas used for reverse air cleaning is re-filtered and

released.


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Another common type of fabric filter is the pulsejet shown in Figure 14.

In this type, the bags are supported on metalwire cages that are suspended from the top of

the unit.

Particulate-laden gas flows around the outside ofthe bags, and a dust cake accumulates on theexterior surfaces.

When cleaning is needed, a very-short-durationpulse of compressed air is injected at the topinside part of each bag in the row of bags beingcleaned.

The compressed air pulse generates a pressure


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p p g pwave that moves down each bag and, in theprocess, dislodges some of the dust cake fromthe bag.


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Advantages and Disadvantages of FabricFilters

Fabric filters are used in a wide variety of applicationswhere high efficiency particulate collection is needed.

The control efficiencies usually range from 99% to greater

than 99.5% depending on the characteristics of the

particulate matter and the fabric filter design.

Fabric filters can be very efficient at collecting particles inthe entire size range of interest in air pollution control.


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The performance of fabric filters is usuallyindependent of the chemical composition of theparticulate matter.

They are not used when the gas stream

generated by the process equipment includescorrosive materials that could chemically attackthe filter media.

Fabric filters are also not used when there are

sticky or wet particles in the gas stream.

These materials accumulate on the filter mediasurface and block gas movement.


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Fabric filters must be designed carefully if there are

potentially combustible or explosive particulate

matter, gases, or vapors in the gas stream beingtreated.

If these conditions are severe, alternative controltechniques, such as wet scrubbers, are often used.


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General Applicability of ParticulateControl Systems

Particulate matter control systems are often selectedbased on the general criteria listed in Figure 15.


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If there is a high concentration of wet and/or stickyparticulate matter, either a particulate wet scrubberor a wet electrostatic precipitator is used.

If wet or sticky materials are present withcombustible materials or explosive gases or vapors,the particulate wet scrubber is most appropriate.

If the particulate matter is primarily dry, mechanicalcollectors, particulate wet scrubbers, conventionalelectrostatic precipitators, and fabric filters can beused.


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The next step in the selection process is todetermine if the particulate matter and/or gasesand vapors in the gas stream are combustible orexplosive.

If so, then mechanical collectors or particulatewet scrubbers can be used because both ofthese categories of systems can be designed tominimize the risks of ignition.

In some cases, a fabric filter can also be used ifit includes the appropriate safety equipment.


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An electrostatic precipitator is not used due tothe risk of ignition caused by electrical sparkingin the precipitator fields.

When selecting between mechanical collectors

and wet scrubbers, mechanical collectors are themore economical choice.

They have a lower purchase cost and a loweroperating cost than wet scrubbers.

If the dry particulate matter is present in a gasstream that is not combustible or explosive, theselection depends on the particle size range andthe control efficiency requirements.


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If a significant portion of the gas stream is in theless than 0.5-micrometer size range, and highefficiency control is needed, a fabric filter is themost common choice.

If a significant portion of the particulate matter isin the 0.5- to 5-micrometer size range, and highefficiency control is needed, fabric filters,electrostatic precipitators, or particulate wetscrubbers (certain types) could be used.

If most of the particulate matter is larger than 5micrometers, any of the four main types ofparticulate control systems could be used.


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There are numerous exceptions to the generalapplicability information presented above due to site-

specific process conditions and unique particulate

matter control systems.

Nevertheless, this chart provides a general indication

of the uses and limitations of many commercially

available particulate matter control systems.

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