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Environmental Pollution Control(ETZC362)

Dr. Jegatha Nambi Krishnan

Department of Chemical Engineering

28thJan. 2014

etzc362

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BITS Pilani, K K Birla Goa Campus28thJan. 2014 (Course Code: etzc 362)

Principle

Wet scrubbers are used for removal of particles which have a diameter ofthe order of 0.2 mm or higher.

Wet scrubbers work by spraying a stream of fine liquid droplets on theincoming stream.

The droplets capture the particles

The liquid is subsequently removed for treatment.

Construction and Operation

A wet scrubber consists of a rectangular or circular chamber in whichnozzles are mounted.

The nozzles spray a stream of droplets on the incoming gas stream

The droplets contact the particulate matter, and the particles get sorbed.

The droplet size has to be optimized.

Wet Scrubbers

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o Construction and Operation

Smaller droplets provide better cleaning, but are more difficult to

remove from the cleaned stream.

The polluted spray is collected. Particles are settled out or otherwise removed from the liquid.

The liquid is recycled.

Wet scrubbers are also used for the removal of gases from the air

streams.

Wet Scrubbers

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Advantages of Wet Scrubbers Wet Scrubbers can handle incoming streams at high temperature, thus

removing the need for temperature control equipment.

Wet scrubbers can handle high particle loading.

Loading fluctuations do not affect the removal efficiency. They can handle explosive gases with little risk.

Gas adsorption and dust collection are handled in one unit.

Corrosive gases and dusts are neutralized.

Disadvantages of Wet Scrubbers High potential for corrosive problems

Effluent scrubbing liquid poses a water pollution problem.

Wet Scrubbers

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The major types of scrubbers are

Spray Towers

Centrifugal scrubbers

Packed beds and plate columns

Venturi Scrubbers

low energy scrubber like Spray Towers used forhandling 5-10 micron particle whereas high energyscrubbers like venturi scrubber used for fine particle.

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Spray Tower Centrifugal Scrubber

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7/24BITS Pilani, K K Birla Goa Campus28thJan. 2014 (Course Code: etzc 362)

Spray Tower

Polluted gas flow upward

Particle collection results due to inertial impaction

and interception on droplets

Maximum efficiency by inertial impaction is

independent of particle size

When droplets fall under gravity through still air and

this maximum occurs for droplets of the diameter of

about 800 m

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Centrifugal Scrubbers

The collection efficiency for particles smaller than those

recovered in spray towers can be increased through centrifugal

scrubbers

A simple type of centrifugal scrubber can be constructed by

inserting banks of nozzles inside a conventional dry cyclone.

The spray acts on the particles in outer vortex, and the droplets

loaded with particles are thrown outward against the wet inner

wall of the cyclone.

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28thJan. 2014 (Course Code: etzc 362)

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Centrifugal Scrubbers

The polluted gas introduced tangentially into the lower

portion of the vertical cylinder.

The particle cut diameter (dpc) for centrifugal spray scrubbers is

between 2 and 3 m.

Commercial scrubbers have operating efficiencies of 97 percent

or better for particles larger than 1 m.

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Packed Beds and Plate Columns

In a typical countercurrent-flow packed scrubber the polluted

gas stream moves upward and comes in contact with the

scrubbing liquid stream which is moving downward over the

packing in a film.

The packing provides a target which allows the gas stream to

take a curved path through the pore spaces while the particles

carried by the gas stream are captured by inertial impaction.

GOOD MASS TRANSFER results in efficient collection of fine

particles by diffusion.10


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Packed Beds and Plate Columns

Variety of materials used are Raschig rings, coke or broken stone.

Smaller packing increases the efficiency of collection.

Plugging problems can be reduced by employing sprays to wash

the packing or by using low density spheres agitated by gas flow.

Impingement Baffle Plate Scrubber

The gas stream is passed through a flooded, perforated plate for

Scrubbing.

Impingement baffles are placed right above each perforation.

Particles are collected by inertial impaction11


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Impingement type scrubbers

The superficial gas velocities are 5 times the velocities generally

used for mass transfer operations Ex. Distillation

Main disadvantage of impingement plate systems is their

Tendency towards scaling, resulting in plugging of the perforations

Venturi Scrubbers

High performance collection of fine particles, usually smaller than2 to 3 m in diameter.

Suitable for particulate matters Sticky, Flammable or Corrosive.

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Venturi Scrubbers

High performance of venturi scrubbers is achieved by acceleratin

the gas stream to very high velocities of 60-120 m/s.

High speed action atomizes the feed liquid .

Droplets accelerate in the throat section and due to the velocity

difference between

The particles and the droplets the particles are impacted against

the slow-moving droplets. This acceleration of droplets is not likel

to be complete at the end of the throat, so that particle collection

continues to some extent into the diverging section of the venturi.13


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The gas-liquid mixture is then directed to a separation device

such as cyclone separator where the droplets carrying the

particulate matter are separated from the gas stream.

Mechanisms which affect the collection of particulates in the

venturi scrubber are inertial impaction, diffusion, electrostatic-

phenomenon, and condensation and agglomeration.

Eventhough condensation plays a part in the effectiveness of

scrubber by helping the particles to agglomerate, the predominan

mechanism is inertial impaction.

Venturi Scrubbers

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Packed bed Scrubber Venturi Scrubber

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Venturi Scrubber

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The most popular collection efficiency equation is proposed by Johnstoneand field and Tassler:

Design Aspects for VenturiScrubber

m.inparticleofd

Kre,temperatuAbsolute-T;)(10*21.6

1C

dropletofd

at throatgasof

factorcorrection,

18

)(parameterimpactioninertial-

gas/mmrate,flowliquid-R

liquidgas/mmt,coefficienncorrelatio

)exp(1

4

(Hesketh)

d

g

2

33

33

diameter

d

T

diameter

speedv

CunninghamCWhere

d

dvC

of

ofofK

R

p

p

gd

pgp

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Design Aspects for VenturiScrubber

smroattventurithstoliquidalocityofgarelativevev

termetresofwaessuredropP

/,

,Pr

r

P = 5 x 10-5 vrL

The pressure drop through the venturi scrubber can be predicted

on the basis of the equation suggested by Calvert

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Scrubber Performance and Efficiency

Above ideas have led Semrau to correlate the scrubber

efficiency in terms of contacting power. It is a concept used to

describe the power expended in a wet scrubber to force contact

between the liquid and solid phases and is usually in terms of power

per unit of volumetric gas flow rate that is dissipated in contacting

and is ultimately converted into heat.

Total contacting power PT

Sum of the gas-phase contacting power, PGLiquid phase contacting power, PLand

mechanical contacting power, PM

PT= PG+ PL + PM19


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The most effective way to deal with air pollution problem is to prevent theformation of the pollutants or minimize their emission at the source itself.

In the case of industrial pollutants, source correction methods ( selection of

those methods which do not contribute to air pollution or have the

minimum air pollution potential) can be used at an early stage of processdesign and development to prevent or minimize the pollutant level.

Control of pollutant at the source can be achieved in different ways

through raw material changes, operational changes, modification or

replacement of process equipment etc.

If the source correction method is not useful to control air pollution, we

can try effluent gas cleaning techniques (involve many of the chemical

Eng. Unit operations). 20


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1. Raw material changes : If a particular raw material is responsiblefor causing air pollution, purer grade of that raw material can be

used to reduce the formation of undesirable impurities (Ex. Use of

low sulfur fuel in place of high sulfur ones).

2. Process changes: Involve new or modified techniques to lower

atmospheric pollutant emissions. H2S, which was once flared inrefineries, can be recycled and used in Claus process to recover

elemental S.

3. Equipment modification or Replacement: Air pollution emissions

can be reduced by suitable modification or replacement of process

equipment. Ex; The unburnt CO and hydrocarbons in the cylinders

of an automobile engine can be burnt by injecting air into the hot

exhaust manifold of the engine.

4. Other methods: Proper equipment maintenance, housekeeping and

cleanliness in the facilities and premises. 21


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Cleaning of gaseous effluents

The cleaning techniques are applied to those caseswhere the emission of pollutants can not be prevented

and pollution control equipment is necessary to remove

them from the main gas stream.

Emission control equipment is divided into two types:

1. Particulate control type

2. Gases and odours control type Wet absorption and dry adsorptionChemical alteration of the pollutant

(mechanism: diffusion of the

Particular gas) 22


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BITS Pilani, K K Birla Goa Campus

Particulate Emission Control

Discrete and minute suspended particles emitted (size range from

100 m down to 0.1 m )are called particulates.

The choice of collection devices depend up on : The physical and

chemical characteristics of particulates, size and concentration inthe gas, volume to be handled and the temp. and humidity of

gaseous medium.

Collection efficiency,

T= 100 X Weight of material collected/ Total

amount entering collector

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Particulate Control Equipment

1. Gravitational settling chambers: Used to remove large abrasive

particles

2. Cyclone separators: Effective in removal of smaller particles than

gravitational settling chambers

3. Filters :Most versatile and efficient methods for removing particulate

matter from industrial gases.

4. Electrostatic precipitators: Used for controlling particulate emissionsat industrial installations ranging from power plants, cement and paper

mills to refineries.

5. Wet collectors: Simultaneous removal of particles and gaseous

pollutants.

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28th J 2014 (C C d t 362)

Documents

ET ZC362-L6