Lecture 19 Engineered Systems for Air Pollution Control

ChE 150 Environmental Process Engineering 2nd Semester AY 2014-2015Engr. Kristian July R. Yap University of the Philippines Diliman

ENGINEERED SYSTEMS FORENGINEERED SYSTEMS FORENGINEERED SYSTEMS FORENGINEERED SYSTEMS FOR

AIR POLLUTION CONTROLAIR POLLUTION CONTROLAIR POLLUTION CONTROLAIR POLLUTION CONTROLChE 150 Environmental Process Engineering

2nd Semester AY 2014-2015

ChE 150 Environmental Process Engineering 2nd Semester AY 2014-2015

OUTLINE

Settling Chambers

Cyclones

Baghouse Filters

Electrostatic Precipitators (ESP)

Absorption


SETTLING CHAMBERS


SETTLING CHAMBERS

Usual velocity through settling chambers

-- Between 0.5 and 2.5 m/s

-- For best results, gas flow should bemaintained at less than 0.3 m/s

Assuming that Stokes law applies, anexpression can be derived for calculating the minimum diameter of a particle collected at100 percent theoretical efficiency in a chamberof length L.


SETTLING CHAMBERS

(1)

where vt = terminal settling velocity, m/s

H = height of settling chamber, m

vh = horizontal flow-through velocity, m/s

L = length of settling chamber, m

t hv v

H L=


SETTLING CHAMBERS

From Stokes Law:

(2)

Equating the vt expressions from (1) and (2):

(3)

( ) 218

p a p

t

g dv

=

( ) 218

p a phg dv H

L

=


SETTLING CHAMBERS

can be dropped from equation (3)since (


SETTLING CHAMBERS

EXAMPLE

Calculate the minimum size of the particle that will be removed with 100% efficiency from a settling chamber under the ff. conditions:

Air: Horizontal velocity is 0.3 m/s.

Temperature is 77C.

-- viscosity of air is 2.1 x 10-5 kg/ms

Particle: Specific gravity is 2.0.

Chamber: Length is 7.5 m.

Height is 1.5 m. *Use .2 =


CYCLONES

Standard-dimension cyclone collector

(Lapplestandard cyclone)


CYCLONES

The operating or separating efficiency of a cyclone depends on the magnitude of the centrifugal force exerted on the particles.

where Fc = centrifugal force, N

Mp = particulate mass, kg

vi = particle velocity, m/s

R = radius of cyclone, m

2

ic p

vF M

R=


CYCLONES

Performance of Lapple standard cyclonescan be described by a single grade fractional efficiency curve:


CYCLONES

The size of the particle is determined by the following equation based from Lapples work:

where d50 = diameter of particle that is collected with 50% efficiency, m

= gas viscosity, kg/ms

b = width of cyclone inlet, m

Ne = no. of effective turns within the cyclone

vi = inlet gas velocity, m/s

p = density of particulate matter, kg/m3

50

9

2 e i p

bd

N v

= 21

1

2e

LN L

h

= +


CYCLONES

Calvert and Englund (1984) suggest an equation for the pressure drop of the Lapple standard cyclone:

where = actual gas density, g/cm3

Q = actual gas volumetric flow rate, m3/s

b,h = inlet dimensions, m

*Acceptable pressure drop is generally less than 20 cm H2O.

( )2

2cm H O 40.96Q

Pbh

=


CYCLONES

EXAMPLE

An air stream with a flow rate of 7 m3/s is passed through a cyclone of standard proportions. The diameter of the cyclone is 2.0 m, and the air temperature is 77C.

a) Determine the removal efficiency for a particle with density of 1.5 g/cm3 and diameter 10 m.

b) Determine the collection efficiency based on the above if a bank of 64 cyclones with diameters of 24 cm are used instead of the single large unit.


CYCLONES

EXAMPLE


BAGHOUSE FILTERS


BAGHOUSE FILTERS

EXAMPLE

A fabric filter is to be constructed using bagsthat are 0.3 m in diameter and 6.0 m long.The baghouse is to receive 10 m3/s of air, and the appropriate filtering velocity hasbeen determined to be 2.0 m/min. Determinethe number of bags required for a continuously cleaned operation.


ELECTROSTATIC PRECIPITATORS



The size-efficiency relationship for an ESP is a curvilinear function similar to that for a cyclone. [Deutsch equation]

where = efficiency

A = area of the collection plates, m2

w = drift velocity of charged particles, m/s

Q = flow rate of the gas stream, m3/s

1 expAw

Q =



The drift velocity w is the velocity at which the particle approaches the collection plate.

-- analogous to terminal settling velocityin gravity settling

where a = [s-1] function of:> charging field

> carrier gas properties

> ability of particles to accept an electrical charge

pw ad=



EXAMPLE

An ESP is to be constructed to remove fly ash particles from stack gases flowing at 10 m3/s. Analysis of a similar system shows that the drift velocity can be taken as

Determine the plate area required to collect a 0.5 m particle with (a) 90% efficiency, and (b) 99% efficiency.

53.0 10 m/spw d=


ABSORPTION

Review design equations in ChE 133.

>> Equilibrium Curve

>> Operating Line

>> Limiting flow rates

>> Film and overall mass transfer coefficients

>> For packed towers:

Z = HOGNOG = HOLNOLZ = HGNG = HLNL

ChE 150 Environmental Process Engineering 2nd Semester AY 2014-2015Engr. Kristian July R. Yap University of the Philippines Diliman

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Lecture 19 Engineered Systems for Air Pollution Control