Fluid Bed Reactors
Chapter (Not in book)
CH EN 4393
Terry A. Ring
Fluidization
• Minimum Fluidization
– Void Fraction
– Superficial Velocity
• Bubbling Bed Expansion
• Prevent Slugging
– Poor gas/solid contact
Fluidization
• Fluid Bed
– Particles
– mean particle size, Angular
• Shape Factor
• Void fraction = 0.4 (bulk density)
Geldart, D. Powder Technology
7,285(1973), 19,133(1978)
Fluidization
Regimes
Fluidization Regimes
•
•
•
•
•
Packed Bed
Minimum Fluidization
Bubbling Fluidization
Slugging (in some cases)
Turbulent Fluidization
Minimum Fluidization
• Bed Void Fraction at Minimum Fluidization
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
Packed Bed
• Pressure Drop
 v o  1     1 50 ( 1   )  

P  v o   LR 


 1 .75
  v o
Dp  3  
Dp





5
110
Void Fraction, ε=0.2-0.4, Fixed
4


 P  v 
ft 
s


110
3
110
psi
100
10
0
0.2
0.4
0.6
v
0.8
Now if particles are free to move?
• Void Fraction
2
Dp   S     g
15
  ( 1   )  vo Dp
1.75
   vo Dp 



 
3

3 

2




0
3
Void Fraction, ε=0.2-0.4 packed
Becomes
εMF=0.19 to εF=0.8.
Bed Void Fraction
2


 f vo 
ft 0.8
s


0.6
 mf
0.4
 
 f vR
0.2
0
0
0.2
MF Pressure drop equals the weight of Bed
50  1    v
0.4
Gmf
vo 

ft
s
 

vR
ft
s
Superficial Ga s Velocity (ft/s)
v
Fluid Bed Pressure Drop


P f vo 
Pressure Drop (psi)
• Lower Pressure Drop
@ higher gas velocity
• Highest Pressure
Drop at onset of
fluidization
ft
s

 60
 
psi
P mf
40
psi

P f vR 

20
psi
0
0
0.2
vo 
0.4
Gmf

ft
s

vR
ft
s
Superficial Gas Velocity (ft/s)
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
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
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
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
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
Elution of Particles from Bed
Terminal Settling Velocity
• Particle Terminal
Setting Velocity
2

 
2
 
2 
Dp 
S   g
9 
• When particles are
small they leave bed
Terminal Settling Velocity ( ft/s)
vt
4 g  Dp  S   



3 f   
4
3
2
1
0
0
50
100
150
Gas Velocity
Particle Diameter (microns)
200
Cyclone
• Used to capture
eluted particles and
return to fluid bed
• Design to capture
most of eluted
particles
• Pressure Drop
2
P i( V)   0 .24
 V
Big particles
C y clone
Cyclone Design
Equatio
Perry 's H B 5th e
+7th ed,of17-28
• Inlet Velocity as a function
Cyclone Size
QR
Vin Dc 
2
Dc
Dc = Cyclone diameter
• Cut Size (D50%)
4 2
1

Dc


9  



4
D50 Dc   
 D  
D
 c
   N  Vin Dc   Vin Dc   Si  50
   N Vi
2
 

• Diameter where
50% leave, 50%
captured
1
D50
Dc


9 


4


   N Vin  S  
2
Cut Size Particle Diameter (mic rons)
Cyclone Cut Size
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
Cyclone Diam eter(ft)
4
Size Selectivity Curve
3.12

D 

SS( D) 1  exp0 .69 3 
 
D

 50  
Size Selec tivity
0.8
0.6
0.4
0.2
0
20
40
P article Dia meter (m icrons)
24 in cy clone
14 in cy clone
D50 for 24 in Cy clone
20 in cy clone
Diam eter of Eluted Particles
60
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
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
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