14-L1-Fluid Bed Reactors.ppt
- 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)
- 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 P vo LR vo Dp 1 3 150 1 ( ) Dp 1.75 vo Void Fraction, ε=0.2-0.4, Fixed 0 0.2 0.4 0.6 0.8 10 100 1 10 3 1 10 4 1 10 5 P v ft s psi v
- 9. Now if particles are free to move? • Void Fraction 0 0.2 0.4 0 0.2 0.4 0.6 0.8 Superficial Gas Velocity (ft/s) Bed Void Fraction f vo ft s mf f vR vo Gmf ft s vR ft s 50 1 f vo vo 1 150 1 ( ) Void Fraction, ε=0.2-0.4 packed Becomes εMF=0.19 to εF=0.8. MF Pressure drop equals the weight of Bed 0 15 2 1 ( ) 3 vo Dp 1.75 3 vo Dp 2 Dp 3 S g 2
- 10. Fluid Bed Pressure Drop • Lower Pressure Drop @ higher gas velocity • Highest Pressure Drop at onset of fluidization 0 0.2 0.4 0 20 40 60 Superficial Gas Velocity (ft/s) Pressure Drop (psi) P f vo ft s psi P mf psi P f vR psi vo Gmf ft s vR ft s
- 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 Terminal Settling Velocity 0 50 100 150 200 0 1 2 3 4 ParticleDiameter (microns) Terminal Settling Velocity (ft/s) Gas Velocity vt 4 3 g Dp f S 2 Dp 2 2 S g 9
- 17. Cyclone • Used to capture eluted particles and return to fluid bed • Design to capture most of eluted particles • Pressure Drop Big particles P i V ( ) 0.24 V 2
- 18. Cyclone Design • Inlet Velocity as a function of Cyclone Size • Cut Size (D50%) Cyclone Equations Perry's HB 5th ed, +7th ed, 17-28 Vin Dc QR Dc 2 4 2 D50 Dc N Vi D50 Dc 9 Dc 4 N Vin Dc Vin Dc Si 1 2 Dc = Cyclone diameter
- 19. Cyclone Cut Size • Diameter where 50% leave, 50% captured 0 1 2 3 4 0 10 20 30 40 50 60 70 80 90 100 Cyclone Diameter(ft) Cut Size Particle Diameter (microns) D50 9 Dc 4 N Vin S 1 2
- 20. Size Selectivity Curve 20 40 60 0 0.2 0.4 0.6 0.8 24 in cyclone 14 in cyclone D50 for 24 in Cyclone 20 in cyclone Diameter of Eluted Particles Particle Diameter (microns) Size Selectivity SS D ( ) 1 exp 0.693 D D50 3.12
- 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