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Fluidization has been a key technology in Fluid Catalytic
Cracking (FCC) to make gasoline in petroleum
industry in catalytic processes such as partial oxidation of
ammonia to acrylonitrile to prepare acrylic resin
in gas phase polymerization processes ofpolyethylene and polypropylene
in chlorination process of metals such as siliconfor purification in the semiconductor industry
in granulation process for the pharmaceuticalindustry
in fluidized bed combustion (FBC) of solid fuels(coal,waste and biomass) to generate steam forboilers
in waste incineration of solid and sludge
drying, dip powder coating , thermal treatment of
metals by hot or cold sand
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Fluidized bed dryer
Temperature Sensor
Pressure Inlet Sensor
Pressure Outlet Sensor
Air Inlet
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FLUIDIZATION A fluidized bed is formed
by passing a fluid usually a
gas upwards through abed of particles supportedon a distributor.
As a fluid is passedupward through a bed ofparticles, pressure lossdue to frictional resistanceincreases as fluid flowincreases.
At a point, upward dragforce exerted by the fluidon the particle equal toapparent weight ofparticles in the bed.
W
F F F = drag force
W = apparent
weight
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FLUIDIZED BED
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=q2Kyb4gX8f9h5M&tbnid=8GHItsXiYM6c-M:&ved=0CAUQjRw&url=http://www.umich.edu/~elements/12chap/html/12prof2b.htm&ei=Lj93UraWF8aOrgfUpYC4Cw&psig=AFQjCNF6B3oytTaeO1AYKDXNHNJaJeViyg&ust=1383632645400577http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=xa8SXdm_paSq6M&tbnid=4kiIHabcD0PCyM:&ved=0CAUQjRw&url=http://commons.wikimedia.org/wiki/File:Fluidized_Bed_Reactor_Graphic.JPG&ei=5z53UuWAGIGFrge0oIDwAg&psig=AFQjCNF6B3oytTaeO1AYKDXNHNJaJeViyg&ust=13836326454005778/12/2019 FLUIDIZATION 1
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Characteristics of Gas Fluidized Bed
Bed behaves like liquid of the same bulk
density can add or remove particles,
pressure-depth relationship, wave motion,
heavy objects sink, and light ones float.
Rapid particle motion good solid mixing
Very large surface area available 1m3of
100 m particles has a surface area ofabout 30,000 m2, and 1 m3of 50 m
particles 60,000 m2.
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Characteristics of Gas Fluidized Bed
Good heat transfer from surface to bed,
and gas to particles.
Isothermal conditions radially and axially.
Pressure drop through bed depends only
on bed depth and particle density does
not increase with gas velocity.
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Advantages of Fluidized Bed
High mobility Gives superb heat transfer, which usually always a problem
to powders.
Heavily used for drying eg: pharmaceutical industry.
Excellent reactors
Good temperature control A perfect gas/liquid mixing equipment.
Very flexible Can carry out many processes in a single vessel.
Mix, dry, granule, separate etc. in one vessel.
Less number of moving parts Easy to handle
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DISADVANTAGES
Costly Blowing air into the system.
Trap air to make it fluidized.
Cleaning process
Some powders
costly in operation than others.
Not all particles fluidized Cohesive and large particles are difficult to
fluidize.
Difficult distributor design
Maldistribution of fluidizing gas
P across distributor = 30% of bed P.
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PRESSURE DROP
The force balance;
Pressure drop=
Weight of particles - up thrust on particle
Bed cross - sectional area
For a bed of particle density, p, fluidized by a fluid
with fto form a bed of depth, Hand voidage, in a
vessel of cross sectional area,A;
A
gHAP
fp
1 gHP fp 1
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PRESSURE DROP
For a flow of fluid through a packedbed, two distinct types of flow
involved. They are laminar and
turbulent flow. The pressure drop across a fluidized
bed is the only parameter which can
be accurately predicted:PFcm w.g.A
MgPF
AM1.0
where M in kg and A in m2.
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mfis the bed voidage at Umfand a
close approximation to it can beobtained by measuring the aerated ormost loosely packed bulk density, bLP.
Equations are used to predict thetheoretical pressure drop comparing toexperimental one.
gH
Pgpmf
F 1
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Laminar flowThrough the work of
Darcy and Poiseuille, ithas been known formore than 120 yearsthat the averagevelocity through a
packed bed, orthrough a pipe, isproportional to thepressure gradient.
Pressure gradient fluid velocity
Based on Carmen-Kozeny (1927, 1933and 1937),
U
H
P
32
21180
pd
U
H
P
Carmen-
Kozeny equationfor laminar flow.
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TURBULENT FLOW
Burke Plumme equation for turbulentflow through a randomly packed bed
of monosized spheres of diameter, dp.
3
2 175.1
p
g
d
U
H
P
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Laminar and turbulent flow
Based on experimental data covering awide range of size and shape of
particles, Ergun (1952) suggested the
following general equation for any flowconditions;
Ergun equation
3
2
32
2 175.11150
p
g
p d
U
d
U
H
P
Laminar
component
Turbulent
component
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Reynold number,
For Re* < 10, laminar flowFor Re* > 2000, turbulent flow
Ergun also expressed flow through a
packed bed in terms of friction factor;
Friction factor,
Compare this friction factor with
Fanning friction factor.Then it
becomes
1Re
* Ud gp
1*
3
2U
d
H
Pf
g
p
75.1Re*
150* f
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with for Re* < 10
and for Re* > 2000
For non-spherical particles; dpis
replaced by dsv,then,
Re*
150* f
75.1* f
3
2
32
2 175.11150
sv
g
sv d
U
d
U
H
P
The surface/volume size, dsv is used: if only sieve sizes are
available, depending on the particle shape, an approximation
can be used for non-spherical particles;
psv dd 87.0 pv dd 13.1Recalling,
where dpis the mean sieve size.
Minim m Fl idi ation Velocit U
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Minimum Fluidization Velocity, Umf
.
A plot of pressure drop across the bed vs. fluid velocity
Line OA packed bed region. Solid
particles do not move relative to one anotherand their separation is constant.
Region BC: fluidized bed region
Point A: Phigher than predicted value n.
ABed pressure
drop, p
Gas velocity, U
B
O
C
Umf
ppp
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This is due to powders, which have beencompacted to some extent before thefluidization process takes place.
Higher Pis associated with the extraforce required to overcome inter particleattractive forces.
Minimum fluidization velocity, Umf:superficial fluid velocity at packed bedbecomes a fluidized bed (as marked ongraph).
Also known as incipient fluidizationvelocity.
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Umfincreases with particle size and particle
density and affected by fluid properties.
Recalling Ergun (1952) for any flowcondition;
(1)
and (2)
substituting (1) into (2),
Rearrange:
3
2
32
2 175.11150
sv
g
sv d
U
d
U
H
P
gHP fp 1
3
2
32
2175.11150
1
sv
mfg
sv
mf
fpd
U
d
Ug
2
222
3
2
3
3
2
3
2
..175.1
..1150
1
fsvmf
svf
fsvmf
svf
fp
dU
d
dU
dg
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2,3
,3
2
2
3
.175.1
.1150
1
mfe
mfe
svf
fp
R
Rd
g
2,3,3
2
.175.1
.1150
mfemfe RRAr
2
3
svfpf gdAr
svmff dURe
or
where,
Archimedes number
- Reynolds number
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Wen and Yu (1966) correlation
for Umf.
for spheres ranging 0.01 < Re,mf< 1000
used for particles larger than 100 m
use dvinstead of dsvfor Wen and Yu
Please check the Wen & Yu correlation in determining Umffrom Data Booklet.
687.1
,, 1591060 mfemfe RRAr
11059.317.33 5.05, ArR mfe
or
B d G ld t
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Baeyens and Geldart
for particles, dp< 100 m;
066.0
f
87.0
f
8.1
p
934.0934.0
fp
mf1110
dg
U
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Assignment
A bed of angular sand of mean sieve size778 m is fluidized by air. The particledensity is 2540 kg/m3, g(air) = 18.4 10-6kg/ms, g= 1.2 kg/m
3and 24.75 kg
of the sand are charged to the bed 0.216m in diameter. The bed height atincipient fluidization is 0.447 m. Find;
mf
The pressure drop across the bubblingbed in cm water gauge. The incipient fluidization velocity, Umf.