What gas velocities are required For particles larger

What gas velocities are required? • For particles larger than 100 m – Wen&Yu correlation • Remf=33. 7[(1+3. 59*10 -5 Ar)0. 5 -1] – Valid for spheres in the range 0. 01< Remf 1000 • For particles less than 100 m(x. P=particle diameter) • For fluidized beds-harmonic mean of mass distribution used as mean

Bubbles vs. No Bubbles • umb=superficial velocity at which bubbles first appear • umb(Abrahamsen &Fieldart, 1980) for • For groups B&D powders, they only bubble, umf= umb • For group C, bubbles never form (cohesive force too high) & channeling occurs

Slugging • When size of bubbles is greater than 1/3 of diam. of bed, rise velocity is controlled by equipment • Slugging leads to large pressure fluctuations & vibrations • Don’t want slugging! • Yagi&Muchi(1952) criteria to avoid slugging (Hmf: bed height at onset of fluidization, D: diameter of bed)

Expansion of a fluidized bed • For non bubbling, there’s a region where u increases, particle separation increases but P/H remains constant • u is related to u. T –single particle terminal velocity in general u= u. T n, =voidage of the bed u= u. T 4. 65 Re. P > 500 • Between - Khan & Richardson, 1989 u= u. T 2. 4

More Bed Stuff Expansion for bubbling beds • Simple theory-any gas excess of that needed for fluidization could form bubbles (not perfect since for low cohesive powders, much increase in gas velocity can occur before bubbling & increase leads to lower density, bigger bed volume) • Relationship between gas as bubbles & gas doing fluidization depends on type of powder Entrainment • Removal of particles from bed by fluidizing gas • Rate of entrainment & size distribution of entrained particles will depend on particle size & density, gas density & viscosity, gas velocity & fluctuations, gas flow regime, radial position, vessel diameter

Entrainment All particles are carried up & particle flux+suspension concentration are constant with height Disengagement zone-upward flux and suspension concentration of fine particles decreases with increasing height Coarse particles fall back down

Applications for fluidized beds • Drying – minerals, sand, polymers, pharmaceuticals, fertilizers • Mixing – all kinds of materials • Granulation – process of making particles cluster by adding a binder • Coating • Heating/cooling – provides uniform temperature and good heat transport

Issues to consider • Gas distribution screen • Erosion – solid, hard particles may cause wear in bed • Loss of fines- reduces quality of fluidization lowers gas-solid contact area, reduces catalytic activity • Cyclones – can be used to separate entrained fines for recycle

Feeding the bed • May need to feed fluidized bed • Important for drying, granulation, recycle of fines • Methods of solids feeding – Screw conveyors – Pneumatic conveying