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AN INTRODUCTION TO
FLUIDIZATION
BY
MILAN CARSKY
UNIVERSITY OF KWAZULU-NATAL
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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AN INTRODUCTION TO FLUIDIZATION
SUMMARY
• Principle of fluidization (gas-solid fluidization, liquid-solid fluidization, properties of
fluidized beds)
• Components of a fluidized bed unit
• Fluidized bed materials (size, density, shape, strength; definition of particle size; bulk
and material density); classification of materials according to their fluidization
behaviour (Geldart’s diagram)
• Minimum fluidization velocity (experimental determination and literature predictions)
and fluidized bed hydrodynamics
• Special fluidized beds (spouted bed, vibro-fluidized beds)
• Fluidization regimes and transitions
• Use of fluidized bed technology (details later in the course):
� Fluidized bed (catalytic) reactors
� Ore roasting
� Fluidized bed combustion (for power generation)
� Drying of materials which are granular, free flowing, not agglomerating, not
fragile, not vulnerable to oxidation, neither the product nor the vapour released
is toxic or flammable
1. Principle of fluidization
By using gas (or liquid) flowing upwards through a layer of a particulate material supported
on a distributor, at certain fluid upwards velocity the particles start to move. Once that occurs
we have reached onset of fluidization and the fluid velocity at this point is called minimum
fluidization velocity (umf).
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Increasing fluid velocity
Properties of fluidized beds:
• Liquid-like behaviour
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Hydrostatic pressure Particles flow on an Viscosity of a fluidized bed
inclined grid
• Rapid mixing of solids
Time=0 s Time = 3 s (top) and 5 s (bottom)
Tracer at the top
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Average tracer concentration = 20%
• Uniform conditions and slow response to changing operating conditions
Fluidized bed combustion of coal. Temperature profile taken in different times. T1-T5
temperatures above the bed level, T6-T8 temperatures in the fluidized bed, T9 temperature of
the stagnant layer on the grid.
• High mass and heat transfer rates
• Pressure drop is independent of the fluid velocity
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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• Erosion of internal parts because of moving particles
• Entrainment and attrition of the bed material
Liquid-solid fluidization
“Smooth” (particulate) fluidization. Uniform and large
expansion of the bed without presence of bubbles. Mixing by
diffusion of particles.
Gas-solid fluidization
“Aggregative” fluidization. Not
a uniform concentration of
particles in the bed. Presence of
gas bubbles (voids) in the bed
is the main factor of particle
mixing. The regimes may vary
from bubbling to slugging and
turbulent fluidized bed.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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2. Components of a fluidized bed unit
Typical industrial fluidized bed unit
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Typical multi-purpose laboratory fluidize bed unit. 1- blower, 2- gas tank, 3- rupture disc, 4-
filter, 5-dryer, 6-humidification column, 7- mist eliminator, 8-water circulation, 9- humidity
indicator, 10- pressure regulator, 11- rotameters, 12- control valves, 13- windbox, 14-
fluidized bed.
Gas distributors
Function of a distributor:
• To support bed material (and to prevent leaking of a bed material to the plenum
chamber below)
• To distribute gas uniformly into the fluidized bed
• Minimum pressure drop 10-30% of the bed pressure drop
Perforated (single, double) plates
Simple and cheap distributors, but difficult to achieve uniform gas distribution, prone to
weepage of solids, require mechanical support for larger diameters.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Porous plate and “Sandwich” type distributors
Restricted to small-scale (laboratory) units. They prevent bed material from leaking through
the distributor and usually ensure uniform gas distribution. In case of sandwich type
distributors the operation is restricted to ambient temperature (filter cloth between two
perforated plates).
Cap type distributors
Limited weeping, good gas distribution but creating a stagnant region underneath, more
expensive, difficult to clean.
Spargers
Limited weeping, good gas distribution but creating a stagnant region underneath.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Slot-type distributors
May achieve uniform gas distribution and promote particle mixing, no stagnant regions,
difficult to construct, require mechanical support for larger diameters.
Conical distributors
Promotes particle mixing, no stagnant regions, difficult to construct.
Cyclones, filters
Function: To separate solids from the gas at the exit (and to return them into the bed).
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Bag filter
Feeders (variety of constructions)
Laboratory screw feeder
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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3. Fluidized bed materials
Particle size-characterisation
a) Sieve size (dp)
The width of the square aperture in a sieve.
b) Surface diameter (ds)
The diameter of a sphere having the same surface area as the particle.
c) Volume diameter (dv)
The diameter of a sphere having the same volume as the particle.
d) Surface/volume diameter (dsv)
The diameter of a sphere having the same surface to volume ratio as the particle.
e) Median particle diameter (dp50)
Particle size corresponding to the 50% value on particle size vs. wt% cumulative
undersize plot.
Particle shape
Sphericity:
� ������� ��� �� � � �� ������ �� ��� ����� �� �� ������ ����
������ ��� �� �� ������ ����
For spherical particles: � � 1
For non-spherical particles: 0 � � � 1
dp = � dp, meas
dp = mean diameter of non-spherical particles
dp, meas = measured mean particle size
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Density
Particle density Bulk density
If a non-porous particle, particle density = skeletal density
Bed voidage
� ������ �� � ���� � �! ���
����� �� �
Particle strength: Impacts between particles and vessel internals lead to particle attrition
(and entrainment) and/or abrasion of the equipment
Classification of materials according to their fluidization behaviour (Geldart’s
diagram)
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Materials A are easy to fluidize. A typical example is
fluidized bed cracking catalyst. These particles can be
fluidized in a limited range of velocities in a regime similar
to liquid fluidization (i.e. “smooth” fluidization) without the
presence of gas bubbles in the bed.
Materials B are also very common for
fluidization but unlike group A materials
bubbles are always present in the bed
(aggregative fluidization). Typical example of
materials B is sand.
Materials C are very fine particles (eg. flour) which are difficult and often impossible to
fluidize because of large surface (cohesive) forces holding particles together.
Materials D consist of large particles. Their
fluidization is uneven, vigorous, fountain-
like, results in equipment shaking
vigorously.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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4. Minimum fluidization velocity and fluidized bed hydrodynamics
Experimental determination of minimum fluidization velocity
Ergun equation for a pressure drop in a static bed:
where ∆P is pressure drop (Pa), h height of the bed (m), ε bed voidage, U gas velocity (m/s),
and G gas mass velocity (kg/(m2.s).
( ) ( )
ppd
GU
d
U
h
P
−+
−=
∆
ε
εµ
ε
ε 175.1
1150
2
2
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Pressure drop for a fluidized bed:
Correlations for minimum fluidization velocity (examples)
Broadhurst and Becker:
Geldart:
Kunii and Levenspiel:
A correlation given below (see table below for values for C1 and C2) was developed by
multiple researchers:
where:
Remf Reynolds number at the onset of fluidization
Ar Archimedes number
ρp Particle density (kg/m3)
ρf or ρg Fluid density (kg/m3)
Dp or dp Particle size (diameter) (m)
g Acceleration due to gravity (9.81 m/s2)
µ Fluid viscosity (Pa.s)
umf Minimum fluidization velocity (m/s)
hgP fp ))(1( ρρε −−=∆
1 . 8 52
0 . 8 5 0 . 1 3
3
2
R e
3 7 . 7 2 4 2 0 0 0 ( )
( )
R e
m fp
f
p f p f
m f
m f
p f
A r
A r
D gA r
UD
ρ
ρ
ρ ρ ρ
µ
µ
ρ
=
+
−=
=
( ) 85.013.05
85.1
7.371042.2 Ar
ArU
fp
mf
+−×=
ρρ
µ
ρgmfp
mf
mf
Ud
CArCC
=
−+=
Re
Re 12
2
1
( )[ ]7.330408.07.11355.0
−+= Ard
Upf
mfρ
µ
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Reference C1 C2
Wen&Yu 33.7 0.0408
Richardson 25.7 0.0365
Saxena&Vogel 25.3 0.0571
Babu et al. 25.25 0.0651
Grace 27.2 0.0408
Chitester et al. 28.7 0.0494
Fluidized bed hydrodynamics
A = static bed, B = bed at minimum
fluidization conditions, C = fluidized
bed
uA < uB < uc
h0(A) < h(B) < h(C)
∆p(A) < ∆p(B) = ∆p(C)
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Pressure drop increases with gas velocity in
the static bed; is constant in the fluidized bed
and decreases as the particles are entrained
from the bed.
Bed height is constant for the static bed and
increases in the fluidized bed to the column
height h’.
Bed voidage is constant for the static bed and
increases to 1 in the fluidized bed.
Particle concentration is constant for the
static bed and decreases to zero in the
fluidized bed.
Packed (static) bed Fluidized bed
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Onset of fluidization for a mixture
A = Fine (light) particles start to fluidize
B = Minimum fluidization velocity
determined experimentally for a mixture
C = All bed material fluidizes
Mixing vs. segregation for a binary mixture of particles in a fluidized bed
5. Special fluidized beds
Vibro-fluidized beds
Used for materials which are difficult to fluidize otherwise (namely materials “C”).
Mechanical vibrations imposed on fluidized bed units.
Vibrations either vertical (mostly) or horizontal.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Support for the vibrated fluidized bed unit with motors.
Vibro-fluidized bed unit
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Spouted beds
A distributor with a single aperture. The material starts moving even
before reaching its minimum fluidization velocity. Suitable for
operations with materials which are difficult to fluidize otherwise.
Intensive particle circulation.
6. Fluidization regimes and transitions
A ... Static bed. Gas velocity is below minimum fluidization velocity.
B ... Particulate regime. Occurs between minimum fluidization and minimum bubbling
velocities for Geldart group A materials. Fluidized bed without bubbles.
C ... Bubbling fluidized bed. Distinct bubbles in the bed. Occurs at velocities higher than
minimum bubbling velocity for Geldart group A materials; and higher than minimum
fluidization velocity for Geldart group B and D materials.
D ... Turbulent bed. No distinct bubbles. Highly turbulent mixing of solids and gas. Large
particle entrainment.
E ... Slugging bed. Occurs in small diameter column where a bubble diameter reaches the
column diameter. High pressure fluctuations, poor particle-solids contact. Unwanted regime.
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Development of slugs and their different types. a) Axisymmetric slug, b) asymmetric slug, c)
“plug” slug.
Correlations for bubble and slug velocity:
F ... Channelling in the bed. Typical for Geldart group C materials. Poor particle-solids
contact. Unwanted regime
IFSA 2011 short-course on fluidization: Concepts in industrial fluidization
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Transitions between regimes:
7. Use of fluidized bed technology (details later in the course):
a. Fluidized bed (catalytic) reactors
b. Ore roasting
c. Fluidized bed combustion (for power generation)
d. Drying of materials which are granular, free flowing, not agglomerating, not
fragile, not vulnerable to oxidation, neither the product nor the vapour released
is toxic or flammable
Fluidized bed dryer
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