particle technology gas cleaning
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The ninth lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. The different mechanisms for the removal of dust from gases are covered and the design equations used for control, modelling and understanding of the equipment are presented and derived. Examples of industrial equipment for gas cleaning are included.TRANSCRIPT
Gas CleaningChapter 13 in Fundamentals
Professor Richard Holdich
[email protected] Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study.
Watch this lecture at www.vimeo.com
Also visit; http://www.midlandit.co.uk/particletechnology.htm
for further resources.
Gas Cleaning
Inertia Diffusional collection Target efficiency Material balance - e.g. fibrous
filter Types of equipment
Collection mechanisms
DiffusionInertia
Bounce
Sieving
Targ
et c
olle
ctio
n ef
ficie
ncy
Particle diameter, microns.
~0.1 to 1
Dyson vacuum cleaner
The animated images shown above are reproduced by permission of Dyson Limited.
Inertia - rate of change of momentum
How long does it take to reach the terminal settling velocity (gas or liquid)?
Inertial collecting devices Stokes’ law and STOKES NUMBER -
note the difference!
Inertia - rate of change of momentum
t
Um
t
Um
d
d
d
)d(INERTIA
Section 5.3
Force Balance
Apparent (buoyed) mass, drag & inertia:
0d
d3'
t
UmxUgm
Apparent mass is density x volume:
)(66
'33
s
xxmm
Force Balance
)/1ln(3 tUUx
mt
Therefore:
Where m is actual mass of particle - not buoyed mass.
Validity depends on Stokes’ law being applicable.
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 20 40 60 80
Particle diameter, microns.
Tim
e ta
ken
to
rea
ch 9
9% o
f te
rmin
al s
ettl
ing
vel
oci
ty, s
.Force Balance
)/1ln(3 tUUx
mt
Acceleration & Inertia
Particles reach 99% of their terminal settling velocity very quickly.
Can use similar approach to characterise the inertia within a system.
Inertia can be used in gas cleaning systems.
Inertial collection
Gas streamlines/flow bend easily round target.
Inertia carries heavier particles onto target - if they stick this is inertial collection.
Flow
TargetDust
Force Balance - Inertial Collection of Particles
Consider only drag & inertia:
0d
d)(3
t
UmUUx p
pg
mass is density x volume & rearranging:
0d
d
d
d
18 2
22
gs U
t
z
t
zx
t
zU p d
d
where:
Force Balance - Inertial Collection of Particles
Make dimensionless as follows:
0d
d
d
d
18 2
22
gs U
t
z
t
zx
tr
zz *
0
*U
UU g t
r
Ut
t
0*
0*d
*d
*d
*d
18*
2
22
Ut
z
t
z
r
Ux
t
os
Force Balance - Inertial Collection of Particles
The solution to the above equation is the same under all conditions so long as the parameters making up the term on the left may be allowed to vary individually but in such a way as to keep the overall value the same.
0*d
*d
*d
*d
18*
2
22
Ut
z
t
z
r
Ux
t
os
Force Balance - Inertial Collection of Particles
Based on radius or diameter:
0*d
*d
*d
*d
18*
2
22
Ut
z
t
z
r
Ux
t
os
t
os
r
Ux
18
2
t
os
D
Ux
9
2
Stop Distance
Integrating using Ug= 0 and Up=U0, at the start, provides the distance taken for a particle injected into still air to come to a halt - The Stop Distance.
0*d
*d
*d
*d
18*
2
22
Ut
z
t
z
r
Ux
t
os
18
2osUx
z
The Stokes Number
N.B. a dimensionless number and a measure of the SYSTEM inertia.
It has both particle and collection device properties in its definition.
Hint - high inertia given by terms on the top & vice versa for those underneath.
t
os
r
Ux
18
2
t
os
D
Ux
9
2
The Stokes Number
t
os
r
Ux
18
2
t
os
D
Ux
9
2
= Stk =
Particle collectionefficiency
Stokes number
Collection mechanisms
DiffusionInertia
Bounce
Sieving
Targ
et c
olle
ctio
n ef
ficie
ncy
Particle diameter, microns.
~0.1 to 1
Diffusional collection
Small particles move randomly across flow.
Diffusion means that particles can be captured even behind the target.
Flow
Target
Dust
Material Balance
Applicable to any device with a concentration gradient within the collection device. Example quoted is for a fibrous filter of the HEPA (high efficiency particulate air) type - this has a packing density of 2% (ish) fibres, 98% porosity.
Accumulation
Accumulation is (SI units of kg s-1):
Interstitial . Projected . Mass . concentration
Collection
velocity target areaof the dust
efficiency
Projected target area
mass input - mass output = accumulation
volume of fibres in height dL is
aAdL
The length of fibres in dL is fibre volume over fibre area, i.e.
ap
AdL
df( / )4 2
Projected target area
Projected area to the gas flow is the product ofthe length and diameter of the fibre
4apAdLdf
ap
AdL
df( / )4 2df =
Accumulation
Accumulation is (SI units of kg s-1):
Interstitial . Projected . Mass . concentration
Collection
velocity target areaof the dust
efficiency
U AdLd
Cg
fs s1
4- a
ap
r h. . .
Mass Balance
rate of dust input into layer is CU Ag sr
rate of dust output from layer is CU UCL
dL Ag g s+éëê
ùûú
¶¶
r
hence accumulation is -U dC Ag sr
Mass Balance
accumulation
U dC Ag sr
U AdLd Cg
fs s1
4- a
ap r h. . .=
Mass Balance & Accumulation
Hence,
- =-
dCC
dLd
s
f
41
h ap a( )
C=Co at L=0 to C=C at L=L to give OVERALLefficiency of
hh a
p a= - = - -
-é
ëê
ù
ûú1 1
41
CC
Ldo
s
fexp
( )
Single target efficiency minimum at approx 0.4mm.
In turbulent flow:
Critical trajectory within a boundary layer
Particle Collection Efficiency
Hence,
t
yU
pt
zu
g
zu
Uy
g
p
and
Thus, equating the times
Particle Collection Efficiency
Model based on fraction particles removed = fraction volume particles are being removed from:
H
z
u
U
zWH
zyW
N
N
g
pd
Particle Collection Efficiency
Negative sign as removal
Particle Collection Efficiency
Integrate over full length, and we want fractioncollected – not fraction remaining, hence:
Deutch Equation – forelectrostatic precipitators, whereUp is function of electric fieldstrength
Hu
LU
N
N
g
p
o
exp11
Scrubber and Venturi Scrubber
Image located at http://en.wikipedia.org/wiki/File:Adjthroatplunger.jpg
Spray Tower Efficiency
Equipment Collection efficiency (%) at following sizes:50 m 5 m 1 m High
temperatureRelative
cost*Inertial collector 95 16 3 yes 1Medium efficiency cyclone 94 27 8 yes 3Low resistance cellular cyclone 98 42 13 yes 2High-efficiency cyclone 96 73 27 yes 4Impingement scrubber 98 83 38 no 7Self-induced spray deduster 100 93 40 no 5Void spray tower 99 94 55 no 11Fluidised bed scrubber >99 99 60 no 8Irrigated target scrubber 100 97 80 no 6Electrostatic precipitator >99 99 86 yes 9Irrigated electrostatic precipitator >99 98 92 no 13Flooded-disc scrubber - low energy 100 99 96 no 10Flooded-disc scrubber - medium energy 100 >99 97 no 15Venturi scrubber - medium energy 100 >99 97 no 14High efficiency electrostatic precipitator 100 >99 98 yes 16Venturi scrubber - high energy 100 >99 98 no 18Shaker type fabric filter >99 >99 99 no 12Reverse jet fabric filter 100 >99 99 no 17Ceramic filter elements 100 >99 >99 yes ???*relative cost per 1000 m3 of gas treated - the lower value the better
Electrostatic Precipitator
Electrostatic Precipitator
Image located at http://www.arb.ca.gov/training/images/281.jpg
Electrostatic Precipitator
Image removed for copyright reasons.
For a suitable example see
http://www.alentecinc.com/company_profile.htm#Electrostatic%20precipitation
.
Equipment Combined - Flowsheet
Image located at http://www.tfhrc.gov/hnr20/recycle/waste/images/cfa.gif
Industrial SME
NotesThe gas cyclone uses INERTIAL collection of dust whereas the hydrocyclone uses a centrifugal field force - it operates in a much higher viscosity medium. The two have very different operating principles.
This resource was created by Loughborough University and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.
The animated images shown on slide 4 are reproduced by permission of Dyson Limited.
Slide 33. Image of an adjustable throat venturi scrubber located on http://en.wikipedia.org/wiki/File:Adjthroatplunger.jpg.
Slide 37. Image of an electrostatic precipitator reproduced with permission from http://www.arb.ca.gov/training/images/281.jpg.
Slide 39. Public domain image located at http://www.tfhrc.gov/hnr20/recycle/waste/images/cfa.gif
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