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, ε=0.2-0.4, Fixed

9. Now if particles are free to move? • Void Fraction Void Fraction, ε=0.2-0.4 packed Becomes εMF=0.19 to εF=0.8. MF Pressure drop equals the weight of Bed

10. Fluid Bed Pressure Drop • Lower Pressure Drop @ 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 SystemsBy 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