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Fundamentals of Fluid Bed Reactors: Dynamics, Design, and Fluidization Regimes

This chapter explores the essential concepts of fluid bed reactors, focusing on fluidization dynamics, minimum fluidization conditions, and the significance of void fraction. It discusses the various fluidization regimes, including bubbling and turbulent fluidization, and examines the implications of particle size, shape, and distribution on heat and mass transfer. Emphasis is placed on distributor plate design, pressure drop management, and strategies to enhance gas/solid contact. The intricacies of cyclone design for particle capture and elution are also detailed, providing a comprehensive understanding of fluid bed reactor operation.

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Fundamentals of Fluid Bed Reactors: Dynamics, Design, and Fluidization Regimes

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  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

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