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

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 Void Fraction, ε= , Fixed

Now if particles are free to move? Void Fraction Void Fraction, ε= packed Becomes ε MF =0.19 to ε F =0.8. MF Pressure drop equals the weight of Bed

Fluid Bed Pressure Drop Lower Pressure higher gas velocity Highest Pressure Drop at onset of fluidization

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 S bc 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 Particle Terminal Setting Velocity When particles are small they leave bed Gas Velocity

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

Cyclone Design Inlet Velocity as a function of Cyclone Size Cut Size (D 50% ) D c = Cyclone diameter

Cyclone Cut Size Diameter where 50% leave, 50% captured

Size Selectivity Curve

Mass Transfer Particle Mass Transfer –Sh= K MT D/D AB = Re 1/2 Sc 1/3 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 = Re 1/2 Pr 1/3 Bed Heat Transfer –Complicated function of Gas flow Particle contacts