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Environmental Pollution Control(ETZC362)

Dr. Jegatha Nambi Krishnan

Department of Chemical Engineering

21stJan. 2014

etzc362

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BITS Pilani, K K Birla Goa Campus25thJan. 2014 (Course Code: etzc 362)

Chapter 5: Air Pollution ControlMethods and Equipment

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Air pollution can be controlled by removing/cleaning/

Chemically transforming the pollutant from main stream

before release to atmosphere.

Emission control equipment can be classified into two groups

(i) for particulate matter(ii) for gaseous pollutant

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Common Methods & Equipmentsfor SPM Control

Methods

Gravitational settling

Centrifugal impaction

Inertial impaction

Direct interception

Diffusion

Electrostatic precipitation

Equipments

Gravitational settler

Cyclone separators

Fabric filters

Electrostatic precipitators

Wet Scrubbers

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Gravitational Settling Chambers

Mainly used to remove large (d>50 m) particles

Advantages

- Simple design

- low pressure drop- Simple maintenance

Disadvantages

- Low efficient for particles d < 50 m.

Normally used as pre-cleaners prior to passing the gas

stream through high efficiency collection devices

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In settling chamber SPM containing gas streamallowed to flow at low velocity, so that particles getsufficient time for its settling

Howard type settling chamber is one of the widelyused settler

By inserting several trays, collection efficiency of thedevice is improved since each particle has a muchshorter distance to fall before reaching the bottom of thepassage

Gravitational Settling Chambers

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Design concepts

Let a settling chamber ofdimensions L x W x H, &with n numbers of tray

equivalent dia. for flowpassage between thetrays is given by

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7/37BITS Pilani, K K Birla Goa Campus25thJan. 2014 (Course Code: etzc 362)

If particles distribution are uniform, then efficiency

() of collection can be written as

Vtfor Stokes region, (NRe,p< 1) can be given as

Therefore,7

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The minimum particle size that can be removedwith 100% efficiency can be found from the

following Eq.

For turbulent flow, turbcan be written as

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Centrifugal Separators

Centrifugal separators (like cyclones) utilize a centrifugalforce to separate the particles from carrier gas

Works on the principle of spinning the gas so that particlesof higher mass fall out in proportion to the velocity

Cyclones are more efficient for the removal of smaller

particles than a gravity settler

Require less space for same volume of gas

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The polluted gas stream is

forced into a vortex.

The motion of the gas exertsa centrifugal force on the

particles, and they get

deposited on the innersurface of the cyclones.

Cyclones

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Construction and Operation

The gas enters through the inlet,and is forced into a spiral.

At the bottom, the gas reversesdirection and flows upwards.

To prevent particles in the incomingstream from contaminating the clean

gas, a vortex finder is provided toseparate them. The cleaned gas flowsout through the vortex finder.

Cyclones (contd.)

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Advantages of Cyclones

Reasonable high efficiency for specially designed cyclones.

They can be used under almost any operating condition.

Cyclones can be constructed of a wide variety of materials.

There are no moving parts, so there are no maintenancerequirements.

Disadvantages of Cyclones

High pressure drop (0.5 - 2.5 kPa), depending on of the

construction version Low output for low particle diameter

Emission of effluent at wet cyclone

Erosion sensitive and constipation danger to the entrance

Cyclones

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Design of cyclone Separator

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Centrifugal force Fcacting on particle is given byFc= m.rw

2

r = radial distance, w = angular velocity, m = mass of particle

Angular velocity w = V/r

V - tangential velocity along the circular path.

Higher separation factor means, better cyclone performance 15

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since the tangential velocity between the gas and particle is equal to

the radial velocity of gas therefore equating the gravitational minus

buoyant force to the stokes form of the drag force for stocks law

regime, we can write

Only the outer helix contributes to collection, inner helix particle

move upward & escape.

Thus the outer helix is equivalent to the gravity settler & let the

max. distance (radial direction) for any particle to reach the wall is w1.

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Then H in gravity settler is = Wi. The length of the flow

path is ND, where N is the number of turns, & D is

the Diameter of the cyclone. This length of the flow

path corresponds to L in the gravity settler. SubstitutingH = Wi & L = ND, efficiency equation for gravity settler.

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Another form is

where Q is the volumetric flow rate, 1= angle = 2 N,

r1& r2is inner & outer shell diameter respectively.

Generally we assume N = 5 or 6

Cut-diameter : It gives a measure of the size of particles

caught & the size passes through

Conventionally cut diameter is defined as the diameter

of a particle for which the efficiency is 0.5

Cut size for 50% efficiency18

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Typical Cyclone Dimensions

Cyclone dia. D2

Length of Cylinder L1= 2D

Length of Cone L2= 2D

Dia of exit De = D

Height of entrance H = D

Width of entrance W = D

Dia of dust exit Dd= D

Length of exit duct L3= D19

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Pressure Drop in Cyclones

As cyclone diameter efficiency

As cyclone diameter pressure drop (P)

Shepherd & Lapples correlation for pressure drop

Where, K = 16 for std. Cyclone (tangential gas entry)

Viis inlet gas velocity 20

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In principle, the dust laden gas passes through the filter in

which particulates are trapped on to the fibers by the

mechanisms of inertial impaction, direct interception and

diffusion.

Overall collection efficiency of a packed filter:

Face area of filter =

Where, Q is the volume flow rate of the gas

Ffis fiber solids fraction, given by

Packed filters

(1 )

f

f

QA

v f

1

1 ff

Af

A

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Where A1is the cross sectional area for as moving inside

the filter and Afis the face area of the filter.

Length of the filter

Where dfis the diameter of the fiber andfis theindividual fiber efficiency

(1 )ln(1 )

4

f f

f f

d fL

f

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Principle The filters retain particles larger than the mesh

size

Air and most of the smaller particles flowthrough. Some of the smaller particles areretained due to interception and diffusion.

The retained particles cause a reduction in themesh size.

The primary collection is on the layer ofpreviously deposited particles.

Fabric Filters

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The equation for fabric filters is based on Darcys law for flowthrough porous media.

Fabric filtration can be represented by the following equation:

S = Ke+ Ksw

Where,S = filter drag, N-min/m3

Ke= extrapolated clean filter drag, N-min/m3

Ks= slope constant. Varies with the dust, gas and fabric, N-min/kg-m

W= Areal dust density = LVt, where

L = dust loading (g/m3), V = velocity (m/s)

Both Keand Ksare determined empirically from pilot tests.

Design of Fabric Filters

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P Total pressure drop

Pf Pressure drop due to the fabric

Pp Pressure drop due to the particulate layer

Ps Pressure drop due to the bag house structure

Fabric Filters

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Very high collection efficiency They can operate over a wide range of volumetric flow rates

The pressure drops are reasonably low.

Fabric Filter houses are modular in design, and can be pre-

assembled at the factory

Advantages of Fabric Filters

Disadvantages of Fabric Filters

Fabric Filters require a large floor area.

The fabric is damaged at high temperature. Ordinary fabrics cannot handle corrosive gases.

Fabric Filters cannot handle moist gas streams

A fabric filtration unit is a potential fire hazard27

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Principle

The particles in a polluted gas stream are charged by passing themthrough an electric field.

The charged particles are led through collector plates The collector plates carry charges opposite to that on the particles

The particles are attracted to these collector plates and are thusremoved from the gas steam

Construction and Operation of Electrostatic Precipitator

Charging Electrodes in the form of thin wires are placed in the path ofthe influent gas.

The charging electrodes generate a strong electric field, which chargesthe particles as they flow through it.

The collector plates get deposited with the particles. the particles areoccasionally removed either by rapping or by washing the collectorplates.

Electrostatic Precipitator

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Electrostatic Precipitator

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factorcorrectionCunninghamC

strengthfieldElectricalE

chargeParticleq

gastheofrateflowVolumetricQ

areaSurface

3

q

)exp(1,

p

p

CollectorA

d

ECvelocitymigrationorDrif tv

Q

Av

Eff iciencyCollection

c

pg

pm

cpm

m.0.066air,for

moleculesgastheofpathfree

)4.0257.1(2

1/55.0

Mean

edC pd

p

a process by which particles suspended ingas stream are charged electrically under theinfluence of electrical field & separated out

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ET ZC362-L5

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