Particle Technology Gas Cleaning

Gas Cleaning
Chapter 13 in Fundamentals
Watch this lecture at www.vimeo.com
Also visit; http://www.midlandit.co.uk/particletechnology.htm for further resources.
Course details:
Particle Technology, module code: CGB019 and CGB919, 2nd year of study.
Professor Richard Holdich
R.G.Holdich@Lboro.ac.uk

Gas Cleaning
Inertia
Diffusional collection
Target efficiency
Material balance - e.g. fibrous filter
Types of equipment

Collection mechanisms
Diffusion
Inertia
Bounce
Sieving
Target collection efficiency
~0.1 to 1
Particle diameter, microns.

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
Section 5.3

Force Balance
Apparent (buoyed) mass, drag & inertia:
Apparent mass is density x volume:

Force Balance
Therefore:
Where m is actual mass of particle - not buoyed mass.
Validity depends on Stokes’ law being applicable.

Force Balance

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.
Flow
Target
Dust
Inertia carries heavier particles onto target - if they stick this is inertial collection.

Force Balance - Inertial Collection of Particles
Consider only drag & inertia:
mass is density x volume & rearranging:
where:

Make dimensionless as follows:

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.

Based on radius or diameter:

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.

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.

The Stokes Number
= Stk =
Particle collectionefficiency
Stokes number

Collection mechanisms
Diffusion
Inertia
Bounce
Sieving
Target collection efficiency
~0.1 to 1
Particle diameter, microns.

Diffusional collection
Small particles move randomly across flow.
Flow
Target
Dust
Diffusion means that particles can be captured even behind the target.

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
-1
):
Accumulation is (SI units of kg s
Collection
Interstitial . Projected . Mass .
concentration
efficiency
target area
velocity
of the dust

Projected target area
a
AdL
a
AdL
2
p
(
/
)
d
4
f
mass input - mass output = accumulation
volume of fibres in height dL is
The length of fibres in dL is
fibre volume over fibre area, i.e.

Projected target area
a
AdL
4
p
d
f
a
AdL
2
p
(
/
)
d
4
f
Projected area to the gas flow is the product of
the length and diameter of the fibre
d
=
f

Accumulation
-1
):
Accumulation is (SI units of kg s
Collection
Interstitial . Projected . Mass .
concentration
efficiency
target area
velocity
of the dust
U
a
AdL
4
g
r
h
.
.
.
C
s
s
-
a
p
d
1
f

Mass Balance
r
CU
A
g
s
¶
C
é
ù
+
r
CU
U
dL
A
ê
ú
g
g
s
¶
L
ë
û
-
r
U
dC
A
g
s
rate of dust input into layer is
rate of dust output from layer is
hence accumulation is

Mass Balance
U
a
AdL
4
g
r
h
.
.
.
C
s
s
-
a
p
d
1
f
accumulation
U
dC A
r
g
s
=

Mass Balance & Accumulation
é
ù
h
a
L
4
C
s
h
=
-
=
-
-
exp
1
1
ê
ú
p
a
-
C
d
(
)
1
ë
û
o
f
Hence,
dL
h
a
4
dC
s
-
=
-
p
a
C
d
(
)
1
f
at
L=0
to
C=C
at
L=L
to give OVERALL
C=C
o
efficiency of
m
Single target efficiency minimum at approx 0.4
m.

In turbulent flow:
Critical trajectory within a boundary layer
Particle Collection Efficiency

Hence,
and
Thus, equating the times

Model based on fraction particles removed = fraction volume particles are being removed from:
Negative sign as removal

Integrate over full length, and we want fractioncollected – not fraction remaining, hence:
Deutch Equation – forelectrostatic precipitators, where
Upis function of electric field
strength

Scrubber and Venturi Scrubber
Image located at http://en.wikipedia.org/wiki/File:Adjthroatplunger.jpg

Spray Tower Efficiency

Electrostatic Precipitator

Image located at http://www.arb.ca.gov/training/images/281.jpg

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
© 2009 Loughborough University
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Particle Technology Gas Cleaning

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