7. Overlap of phenomenon
• Kinetics
– Depend upon solid content in bed
• Mass Transfer
– Depends upon particle Re number
• Heat Transfer
– Depends upon solid content in bed and gas Re
• Fluid Dynamics
– Fluidization – function of particle Re
– Particle elution rate – terminal settling rate vs gas
velocity
– Distribution Plate Design to prevent channeling
8. Packed Bed
• Pressure Drop
P vo
LR
vo
Dp
1
3
150 1
( )
Dp
1.75
vo
Void Fraction, ε=0.2-0.4, Fixed
0 0.2 0.4 0.6 0.8
10
100
1 10
3
1 10
4
1 10
5
P v
ft
s
psi
v
9. Now if particles are free to move?
• Void Fraction
0 0.2 0.4
0
0.2
0.4
0.6
0.8
Superficial Gas Velocity (ft/s)
Bed
Void
Fraction
f vo
ft
s
mf
f vR
vo
Gmf
ft
s
vR
ft
s
50 1 f vo
vo 1
150 1
( )
Void Fraction, ε=0.2-0.4 packed
Becomes
εMF=0.19 to εF=0.8.
MF Pressure drop equals the weight of Bed
0
15
2
1
( )
3
vo
Dp
1.75
3
vo
Dp
2
Dp
3
S
g
2
10. Fluid Bed Pressure Drop
• Lower Pressure Drop
@ higher gas velocity
• Highest Pressure
Drop at onset of
fluidization
0 0.2 0.4
0
20
40
60
Superficial Gas Velocity (ft/s)
Pressure
Drop
(psi)
P f vo
ft
s
psi
P mf
psi
P f vR
psi
vo
Gmf
ft
s
vR
ft
s
11. Bed at Fluidization Conditions
• Void Fraction is High
• Solids Content is Low
• Surface Area for Reaction is Low
• Pressure Drop is Low
• Good Heat Transfer
• Good Mass Transfer
12. Distributor Plate Design
• Pressure Drop over the Distributor Plate
should be 30% of Total Pressure Drop (
bed and distributor)
– Pressure drop at distributor is ½ bed pressure
drop.
• Bubble Cap Design is often used
13. Bubble Caps
• Advantages
– Weeping is reduced or totally avoided
• Sbc controls weeping
– Good turndown ratio
– Caps stiffen distributor plate
– Number easily modified
• Disadvantages
– Expensive
– Difficult to avoid stagnant regions
– More subject to bubble coalescence
– Difficult to clean
– Difficult to modify
From Handbook of Fluidization and Fluid-Particle Systems By Wen-Ching Yang
14. Bubble Cap Design
• Pressure drop controlled by
– number of caps
– stand pipe diameter
– number of holes
• Large number of caps
– Good Gas/Solid Contact
• Minimize dead zones
• Less bubble coalescence
– Low Pressure Drop
15. Pressure Drop in Bubble Caps
• Pressure Drop Calculation Method
• Compressible Fluid
• Turbulent Flow
– Sudden Contraction from Plenum to
Bottom of Distributor Plate
– Flow through Pipe
– Sudden Contraction from Pipe to hole
– Flow through hole
– Sudden Expansion into Cap
16. Elution of Particles from Bed
• Particle Terminal
Setting Velocity
• When particles are
small they leave bed
Terminal Settling Velocity
0 50 100 150 200
0
1
2
3
4
ParticleDiameter (microns)
Terminal
Settling
Velocity
(ft/s)
Gas Velocity
vt
4
3
g Dp
f
S
2
Dp
2
2
S
g
9
17. Cyclone
• Used to capture
eluted particles and
return to fluid bed
• Design to capture
most of eluted
particles
• Pressure Drop
Big particles
P i V
( ) 0.24
V
2
18. Cyclone Design
• Inlet Velocity as a function of
Cyclone Size
• Cut Size (D50%)
Cyclone Equations
Perry's HB 5th ed,
+7th ed, 17-28
Vin Dc
QR
Dc
2
4 2
D50 Dc
N Vi
D50 Dc
9
Dc
4
N Vin Dc
Vin Dc
Si
1
2
Dc = Cyclone diameter
20. Size Selectivity Curve
20 40 60
0
0.2
0.4
0.6
0.8
24 in cyclone
14 in cyclone
D50 for 24 in Cyclone
20 in cyclone
Diameter of Eluted Particles
Particle Diameter (microns)
Size
Selectivity SS D
( ) 1 exp 0.693
D
D50
3.12
21. Mass Transfer
• Particle Mass Transfer
– Sh= KMTD/DAB = 2.0 + 0.6 Re1/2 Sc1/3
• Bed Mass Transfer
– Complicated function of
• Gas flow
• Particles influence turbulence
• Particles may shorten BL
• Particles may be inert to MT
22. Fluid Bed Reactor Conclusions
• The hard part is to get the fluid dynamics
correct
• Kinetics, MT and HT are done within the
context of the fluid dynamics
23. Heat Transfer
• Particle Heat Transfer
– Nu= hD/k = 2.0 + 0.6 Re1/2 Pr1/3
• Bed Heat Transfer
– Complicated function of
• Gas flow
• Particle contacts
