Fluid Bed Reactors

1
Fluid Bed Reactors

• Chapter (Not in book)
• CH EN 4393
• Terry A. Ring

2
Fluidization

• Minimum Fluidization
• Void Fraction
• Superficial Velocity
• Bubbling Bed Expansion
• Prevent Slugging
• Poor gas/solid contact

3
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)
4
FluidizationRegimes
5
Fluidization Regimes

• Packed Bed
• Minimum Fluidization
• Bubbling Fluidization
• Slugging (in some cases)
• Turbulent Fluidization

6
Minimum Fluidization

• Bed Void Fraction at Minimum Fluidization

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

Void Fraction, e0.2-0.4, Fixed
9
Now if particles are free to move?

• Void Fraction

Void Fraction, e0.2-0.4 packed Becomes eMF0.19
to eF0.8.
MF Pressure drop equals the weight of Bed
10
Fluid Bed Pressure Drop

• Lower Pressure Drop _at_ higher gas velocity
• Highest Pressure Drop at onset of fluidization

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

Gas Velocity
17
Cyclone

• Used to capture eluted particles and return to
  fluid bed
• Design to capture most of eluted particles
• Pressure Drop

Big particles
18
Cyclone Design

• Inlet Velocity as a function of Cyclone Size
• Cut Size (D50)

Dc Cyclone diameter
19
Cyclone Cut Size

• Diameter where 50 leave, 50 captured

20
Size Selectivity Curve
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