Basic Concepts in
Fluidization
Britt Halvorsen
Telemark University College
The phenomenon of fluidization
Fluidization is a process
whereby a granular
material is converted from
a static solid-like state to
a dynamic fluid-like state.
The process occurs when
a fluid (liquid or gas) is
passed up through the
granular material
(powder).
1
Fluidized beds
In a fluidized bed gas is passing upwards through a bed
of particles. The earliest applications of fluidization were
for the purpose of carrying out chemical reactions. Since
that time there have been a number of successful
chemical processes involving fluidized bed reactors, but
also other applications.
Fluidized beds are applied in industry due to their large
contact area between phases, which enhances chemical
reactions, heat transfer and mass transfer. The efficiency
of fluidized beds is highly dependent of flow behaviour
Particle behaviour
In a fluidized bed the frictional forces
between particles are small
Gas/particle assembly behaves like a liquid
with a density equal to the bulk density.
Behaviour of particles in fluidized beds
depends on a combination of their mean
particle size and density.
2
Various stages of fluidization
Classification of particles
Geldart fluidization
diagram:
Identify characteristics
associated with fluidization
of particular powders at
ambient conditions.
Geldart (1973) classification of particles according to their fluidization behaviour.
3
Group A powders
Most commercial fluidized bed catalytic reactors use Geldart
group A powders.
Group A powders:
Easily fluidized
Bed expands considerably before bubbles appear due to:
Inter-particle forces that are present in this group of powders
Inter-particle forces are due to particle wetness, electrostatic charges and
van der Waals forces.
Bubble formation will occur when the gas velocity exceeds the minimum
bubble velocity.
The bubbles rice faster than the gas percolating through the emulsion.
Maximum bubble size usually less than 10 cm and independent of the bed
size. Kunii and Levenspiel (1991)
Group B particles
For group B particles
Inter-particle forces are negligible
Bubbles are formed as the gas velocity reaches the minimum
fluidization velocity.
Bed expansion is small compared to group A particles. Small
bubbles are formed close to the air distributor and the bubble
size increase with distance above the distributor.
Bubble size also increases with the excess gas velocity
Excess gas velocity: difference between the gas velocity and the
minimum fluidization velocity, Geldart (1986).
Coalescence is the dominating phenomena for group B powders
and bubble size is roughly independent of mean particle size. Most
bubbles rise faster than the interstitial gas velocity.
4
Group C and D powders
Group C powders:
Cohesive.
Fluidization
is
extremely difficult
Bubble formation will
not occur.
Geldart group D
powders:
Large
and/or dense
particle powders
Undesirable for
fluidization operations.
Large gas flows are
needed to get these
particles fluidized
Geldart C and D powders give a low degree of solid mixing and gas
back-mixing.
Phenomenon of fluidization
Gas or liquid
Fixed and expanded bed (low fluid velocity)
Fluid percolates through the void between the
particles.
Expanded bed (higher fluid velocity)
Particles move apart
Bed at minimum fluidization
Further increased velocity→particles are
suspended by the upward-flowing fluid
Frictional force between particle and fluid counter
balances.
Pressure drop equals weight of particles+fluid
Minimum fluidization velocity
5
Liquid:
Smoothly
Velocity above minimum
fluidization velocity
Gas and liquid
Lean
fluidized bed
phase fluidization
High velocity, fine particles
Particles carried out of the
bed with the fluid
Pneumatic transport
Gas
Bubbling
fluidized bed
Gas velocity increased above
minimum fluidization velocity
Bed height about minimum
fluidization
Bubbles growing with hight
Slugging
Deep bed with small diameter
Bubble diameter about bed
diameter
Fine particles, axial
sluggs
Coarse particles, flat
slugs
6
Gas
Turbulent
fluidization
Very high gas velocity
Terminal velocity of solids
exceeded
Upper surface of bed
disappears
Entrainment of particles
significant
Turbulent motion of solid
clusters and voids of gas with
various size and shape
When will the bed start to fluidize?
At the velocity where the buoyant forces equals the drag
forces. (wall friction and solids stress are neglected)
Minimum fluidization velocity developed from the :
(1 − ε )(ρ
g
s
− ρ g )⋅ g =
Φ sg
εg
(u g − u s )
ε g = void (gas) fraction
[
ρs , ρg = solid and gas density kg/m3
[
g = acceleration of gravity m/s 2
]
]
[
Φ sg = Gas − particle drag coefficient m/s 2
]
7
Minimum fluidization velocity
At minimum fluidization:
Solid
velocity is zero:
U mf =
ε mf 2 (1 − ε mf )(ρ s − ρ g )g
Φ sg
Calculated
minimum fluidization velocity:
Deviation from experimental;
Void fraction
Particle size distribution
Mean particle diameter
Drag models
Different drag models are developed
Ergun
equation :
(1 − ε ) µ
2
Φ sg = 150
g
ε g (d s ψs )
g
2
+ 1.75
ρ g v g − vs ε s
d s ψs
, for ε g ≤ 0.8
ε s = Solid volume fraction
d s = Particle diameter
ψ s = Form factor
µ g = Gas viscosity
v g , vs = Gas and solid velocity
8
Syamlal & O’Brien drag model
Empirical model
Φ sg =
3
1
C Dρ g 2 ε g (1 − ε g ) v g − v s
4 ds
Rt
(
R t = 0.5 A − 0.06Re s + 0.0036Re s2 + 0.12Re s (2B − A) + A 2
A=ε
)
4.14
g
B = 0.8ε 1.28
g , for ε g < 0.85
B = ε 2.65
g
, for ε g > 0.85

R t 
C D =  0.63 + 4.8
Re s 

ε g ⋅ ρ g ⋅ (v g − v s )⋅ d p
Re s =
µg
Calculated and experimental minimum fluidization
velocities
Minimum fluidization velocity [m/s]
0.6
0.5
Ex. Small+mix small
0.4
Ex. Medium + mix medium
Ex. Large
Calc., void=0.37
0.3
Calc., void=0.38
Calc., void=0.40
0.2
Calc., void=0.42
Calc., void=0.44
0.1
0
0
200
400
600
800
1000
1200
Mean particle diameter [µ
µ m]
•Particle size distribution influences on:
Minimum fluidization velocity, Bubble
formation,
Segregation, Pressure drop
9
Pressure drop
a. Uniform distributed sand
( Figure from: Kuuni and Levenspiel,
Fluidization Engineering)
Wide distribution of particle sizes
(180-1400 µm)
( Figure from: Kuuni and Levenspiel,
Fluidization Engineering)
Pressure drop
(Experiments performed at TUC)
10
Efficiency
dx
Bubble
size
dy
Bubble
Distribution
Efficiency
Bubble
velocity
Bubble
frequency
V=dy/dt
Particle segregation
Experiment: Mixture of two powders.
Particle range: 100-200 µm (153) and 750-1000 µm (960)
Simulation: Two particle phases: Mean diam. 153 µm and 960 µm
11
Experimental set-up, 3-D bed
2.0 m
0.75 m
0.35 m
0.25 m
Fibre optical probe
Probe head
Light reflection
Signal [V]
distance between lights: 2.7 mm
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
Signal 1
Signal 2
0
0.1
0.2
0.3
0.4
0.5
Time [s]
12
Labview program
Pneumatic transfer
Test facility [Akilli H et al., 2001]
Lab scale set-up for experimental study of pneumatic transport.
Characteristic for industrial transport systems:
•Long pipes with rather small diameters.
•Pipe walls influence significantly on the flow behaviour.
•Elbows highly influence the flow behaviour
13
Conveying system
Horizontal pipes
Vertical pipes
Bend
Horizontal-vertical
Vertical-horizontal
Horizontal-horizontal
U-bend
Distance between bends
Ratio diameter/length
Different flow regimes
can arise
Flow regime can change
during the transport
Change in velocity
direction influence the
flow behaviour
Particle collisions with
walls
Particle motion
Influenced by:
Gravitational
settling in horizontal pipes
Inertial behaviour in bends and branches
Turbulent dispersion
Lift forces, friction forces
Particle-wall collisions
Rough walls/smooth walls
Particle-particle
collisions
(Huber and Sommerfeld, 1998)
14
Flow regimes
Flow regimes in horizontal pipes, Crowe et al.(1997)
Homogeneous flow: Dilute flow transport particles are well mixed homogenious
state by turbulent mixing
Dune flow: Particles start to settle
Slug flow: Includes regions with settled particles – and regions with particles in
suspension. Slug flow is used in dense flow transport. Higher pressure drop and solid
loading. Low velocity less material degradation and line erosion.
Packed bed: Lower velocity. No transport
Flow regimes
Pressure drop as a function of gas velocity, Kunii and Levenspiel (1991)
Saltation velocity: Superficial gas velocity at which particles begin to
separate from the gas phase and slide or roll along the bottom of the pipe.
→Changes from suspended to non-suspended transport
15
Wall roughness
Smooth pipes
Particles
settle
Stronger coupling with gas phase
Deformation of stream-wise velocity profile (ref)
Rough walls
Increase
of rebound angle compared to compact
angle
Particles re-suspended
Gravitational settling reduced
Secondary flow
Increasing pressure loss
Huber and Sommerfeldt, 1998
Wall roughness
Figure: Particle mass flux in smooth and rough pipe. (Huber and Sommerfeld, 1998)
16
Rope
Formation and disintegration:
•Centrifugal forces
•Secondary flows
•Gas velocity
•Bend radius
•Solid loading
•Pipe orientation
•Particle size
Figure: Rope formation and dispersion [Akilli et al., 2001]
•Particle density
Mc Clusky et al. (1989)
Levy and Mason (1998)
Yilmaz and Levy (2001)
Rope
Centrifugal forces in the elbow cause gas and particles to segregate
The rope formation and dispersion is dependent of
solid particles impinging on the outer wall of the bend, forming relatively dense
phase structure (rope).
Centrifugal forces
Secondary flows
Gas velocity
Bend radius
Solid loading
Pipe orientation
Particle size
Particle density
The particles are decelerated in the bend due to particle-particle collisions
and particle-wall collisions, and have to be re-accelerated downstream the
bend to the conveying velocity.
Mc Clusky et al. (1989), Levy and Mason (1998), Yilmaz and Levy (2001)
17
Pressure drop
Pressure drop influenced by:
Gas velocity
Dictates flow regime
Great effect on total pressure drop
Increase in gas velocity strongly increases the total pressure drop
Operation at low gas velocity
Increasing slip velocity
Increasing total pressure drop
Particle size
Particle concentration
cause problems in suspending particles.
Particle density
cause high energy losses.
Slight decrease in slip velocity
Increase in total pressure drop
Particle-wall collision
Bend, radius ratio
Pipe dimensions
Hidayat and Rasmuson, (2005)
Turbulence
Particle influence
Displacement
of flow field
Generation of wakes
Dissipation to the motion of dispersed phase
Modification of velocity gradients
Additional length scales
Particle-particle interaction (0.1)
18
Concluding remarks
Fluidization behaviour depends on:
Particle size and particle size distribution
Particle density
Fluid properties
Efficiency of fluidsized bed depends on:
Contact between particles and fluid
Economy
Bubble size
Bubble frequency
Bubble velocity
Bubble distribution
Low pressure drop
Good mixing
High degree of conversion
Low degree of energy loss
Challenges
(A lot of research still remaining)
Develop
good models
(Empirical)
Physical
CFD
simulations
Scaling
Reduce simulation time
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