Dust and Mist Collection

8/10/2019 Dust and Mist Collection

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CHE 512Introduction to ParticleTechnology


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purpose of dust collection

Dust collection is concerned with the removal or

collection of solid dispersoids in gases for purposes

of:

Air pollution control

Equipment maintenance reduction

Safety or health hazard elimination

Product-quality improvement

Recovery of a valuable product

Powdered-product collection


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mechanism of dust collection

The basic operations in dust collection by any device

are:

Separation of the gas-borne particles from the gas

stream by deposition on a collecting surface

Retention of the deposit on the surface

Removal of the deposit from the surface for recovery

or disposal


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Performance of dust collectors

The performance of a dust collector is most

commonly expressed as the collection efficiency ,

the weight ratio of the dust collected to the dust

entering the apparatus.

However, the collection efficiency is usually related

exponentially to the properties of the dust and gas

and the operating conditions of most types of

collectors and hence is an insensitive function of thecollector operating conditions as its value

approaches 1.0.


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Dust-collection equipment

Gravi ty s ett l ing chambers

Probably the simplest and earliest type of dust-

collection equipment consisting of a chamber in

which the gas velocity is reduced to enable dust to

settle out by the action of gravity.

Its industrial utility is limited to removing particles

larger than 325 mesh.

Gravity collectors are generally built in the form of

long, empty, horizontal, rectangular chambers with

an inlet at one end and an outlet at the side or top of

the other end.


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Gravi ty s ett l ing chambers

Horizontal plates or vertical baffles are used in some

designs to improve the separation. Settling

chambers offer little resistance to the gas flow, and

can be designed for operation at high temperature

and high pressure, and for use in corrosive

atmospheres.

The length of chamber required to settle a given

particle size can be estimated from the settlingvelocity (calculated using Stokes law) and the gas

velocity.


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Gravi ty settl ing chamber


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advantages

Low capital and energy costs

No moving parts, therefore, few maintenance

requirements and low operating costs

Excellent reliabilityLow pressure drop through device;

Device not subject to abrasion due to low gas

velocity;

Provide incidental cooling of gas stream;

Temperature and pressure limitations are only

dependent on the materials of construction; and

Dry collection and disposal


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disadvantages

Relatively low particulate matter collection

efficiencies, particularly for particulate matter less

than 50 m in size

Unable to handle sticky or tacky materials

Large physical size, and

Trays in multiple-tray settling chamber may warp

during high-temperature operations


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Gravity settling chambers

By assuming a low degree of turbulence relative to

the settling velocity of the dust particle in question,

the performance of a gravity settling chamber is given

by

Assuming Stokeslaw, the smallest particle that can

be completely separated out corresponds to = 1.0, is

given by


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Impingement separators

Impingement separators employ baffles to achievethe separation.

The gas stream flows easily round the baffles,

whereas the solid particles, due to their highermomentum, tend to continue in their line of flight,

strike the baffles and are collected.

Impingement separators cause a higher pressuredrop than settling chambers, but are capable of

separating smaller particle sizes, 10-20 m.


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Impingement separators


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Cyclone separators

Most widely used type of dust collection equipment,

in which dust-laden gas enters a cylindrical or

conical chamber tangentially at one or more points

and leaves through a central opening.

Cyclones are suitable for separating particles

above about 5 m diameter; smaller particles,

down to about 0.5 m, can be separated where

agglomeration occurs.

The most commonly used design is the reversedflow cyclone.

Can be designed for both high temperature and

pressure operations.


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Cyclone separators


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Cyclone separators- design

Stairmand developed two standard designs for gas-

solid cyclones: a high-efficiency cyclone (a), and a

high throughput (b) design.

The following scaling equation, can be used to

transform the performance curves of the two designs

to other cyclone sizes and operating conditions.


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Stairmand developed two standard designs for gas-

solid cyclones: a high-efficiency cyclone (a), and a

high throughput (b) design.

The following scaling equation, can be used to

transform the performance curves of the two designs

to other cyclone sizes and operating conditions.


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The pressure drop in a cyclone will be due to the

entry and exit losses, and friction and kinetic energy

losses in the cyclone. The empirical equation given

by Stairmand (1949) can be used to estimate the

pressure drop:


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Mechanical centrifugal separators

Collectors in which the centrifugal field is supplied by

a rotating member, for example the exhauster or fan

and dust collector are combined as a single unit.

The chief advantage of these units lies in their

compactness, which may be a prime considerations

for large installations or plants requiring a large

number of individual collectors.

High maintenance costs may be encountered from

plugging and rotor unbalancing.


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Mechanical centrifugal separators


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particulate scrubbers

Wet collectors, or scrubbers, form a class of devicesin which a liquid is used to assist or accomplish the

collection of dust or mists.

The principal mechanism involved is the impact

(impingement) of the dust particles and the waterdroplets. Particle sizes down to 0.5 m can be

removed in suitably designed scrubbers. In addition

to removing solids, wet scrubbers can be used to

simultaneously cool the gas and neutralize any

corrosive constituents.


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Particulate scrubbers

Spray towers, and plate and packed columns are

used, as well as a variety of proprietary designs.

Venturi scrubbers are the most efficient of the wet

collectors, achieving efficiencies of more than 98

percent for particles larger than 0.5 min diameter..

Venturi scrubbers achieve high relative velocities by

injecting water into the throat of a venturi channela

constriction in the flow paththrough whichparticulate-laden air is passing at high speed.


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Particulate scrubbers


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Fabric filters

The filters used for gas cleaning separate the solid

particles by a combination of impingement and

filtration; the pore sizes in the filter media used are

too large simply to filter out the particles.

The separating action relies on the precoating of the

filter medium by the first particles separated; which

are separated by impingement on the filter medium

fibres.

Fabric filters have been used for collection of finedusts and fumes for over 100 years.

Greatest application limitation- temperature.


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Fabric filters


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Electrostatic precipitators

Electrostatic precipitators are capable of collecting

very fine particles,


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Electrostatic

precipitator


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references

air pollution control. (2012). EncyclopdiaBritannica. Encyclopdia Britannica UltimateReference Suite. Chicago: EncyclopdiaBritannica.

LEE, J.C. (1994) Coulson and RichardsonsChemical Engineering (3rd ed.), Vol. 3, (PergamonPress, London)

PERRY, R. H., GREEN, D. W. and MALONEY, J. O.(eds) (1997) Perrys Chemical EngineersHandbook, 7thedition(McGraw-Hill)

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Dust and Mist Collection