Fluid Bed ReactorsChapter (Not in book)CH EN 4393Terry A. Ring

FluidizationMinimum FluidizationVoid FractionSuperficial VelocityBubbling Bed ExpansionPrevent SluggingPoor gas/solid contact

FluidizationFluid BedParticlesmean particle size, Angular Shape FactorVoid fraction = 0.4 (bulk density)Geldart, D. Powder Technology 7,285(1973), 19,133(1978)

FluidizationRegimes

Fluidization RegimesPacked BedMinimum FluidizationBubbling FluidizationSlugging (in some cases)Turbulent Fluidization

Minimum FluidizationBed Void Fraction at Minimum Fluidization

Overlap of phenomenonKineticsDepend upon solid content in bedMass TransferDepends upon particle Re numberHeat TransferDepends upon solid content in bed and gas ReFluid DynamicsFluidization function of particle ReParticle elution rate terminal settling rate vs gas velocityDistribution Plate Design to prevent channeling

Packed BedPressure DropVoid Fraction, =0.2-0.4, Fixed

Now if particles are free to move?Void FractionVoid Fraction, =0.2-0.4 packed BecomesMF=0.19 to F=0.8.MF Pressure drop equals the weight of Bed

Fluid Bed Pressure DropLower Pressure Drop @ higher gas velocityHighest Pressure Drop at onset of fluidization

Bed at Fluidization ConditionsVoid Fraction is HighSolids Content is LowSurface Area for Reaction is LowPressure Drop is LowGood Heat TransferGood Mass Transfer

Distributor Plate DesignPressure Drop over the Distributor Plate should be 30% of Total Pressure Drop ( bed and distributor) Pressure drop at distributor is bed pressure drop.Bubble Cap Design is often used

Bubble CapsAdvantagesWeeping is reduced or totally avoidedSbc controls weepingGood turndown ratioCaps stiffen distributor plateNumber easily modifiedDisadvantagesExpensiveDifficult to avoid stagnant regionsMore subject to bubble coalescenceDifficult to cleanDifficult to modifyFrom Handbook of Fluidization and Fluid-Particle Systems By Wen-Ching Yang

Bubble Cap DesignPressure drop controlled by number of capsstand pipe diameternumber of holesLarge number of capsGood Gas/Solid ContactMinimize dead zonesLess bubble coalescenceLow Pressure Drop

Pressure Drop in Bubble CapsPressure Drop Calculation MethodCompressible FluidTurbulent FlowSudden Contraction from Plenum to Bottom of Distributor PlateFlow through PipeSudden Contraction from Pipe to holeFlow through holeSudden Expansion into Cap

Elution of Particles from BedParticle Terminal Setting Velocity

When particles are small they leave bed

Gas Velocity

CycloneUsed to capture eluted particles and return to fluid bedDesign to capture most of eluted particlesPressure DropBig particles

Cyclone DesignInlet Velocity as a function of Cyclone Size

Cut Size (D50%)Dc = Cyclone diameter

Cyclone Cut SizeDiameter where 50% leave, 50% captured

Size Selectivity Curve

Mass TransferParticle Mass TransferSh= KMTD/DAB = 2.0 + 0.6 Re1/2 Sc1/3Bed Mass TransferComplicated function ofGas flowParticles influence turbulenceParticles may shorten BLParticles may be inert to MT

Fluid Bed Reactor ConclusionsThe hard part is to get the fluid dynamics correctKinetics, MT and HT are done within the context of the fluid dynamics

Heat TransferParticle Heat TransferNu= hD/k = 2.0 + 0.6 Re1/2 Pr1/3Bed Heat TransferComplicated function ofGas flowParticle contacts