Fluidization
Safat Anam
Lecturer
Dept of ChE, RUET
1
Understand the basic principles of fluidization
Relate the equation with packed bed pressure drop equation,
already derived
Understand the conditions for fluidization (umf, Lmf etc)
Applications of fluidization operation
Ref: Foust, McCabe-Smith
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Fig: Bed Fluidization
occurs as the gas velocity
increases and
counterbalance the
gravitational forces.
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Fluidized beds in chemical processing
industries used for separations, rapid mass
and heat transfer operations, and catalytic
reactions.
A cylindrical column that contains
particles and through which fluid, either
gaseous or liquid, flows.
Fluidized bed reactors: particles
contain catalyst
Separations: particles might be an
absorbent or adsorbate.
Velocity of the fluid is sufficiently high to
suspend, or fluidized, the particles within
the column, providing a large surface area
for the fluid to contact, which is the chief
advantage of fluidized beds.
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Fundamentals/Description
When a fluid is passed upwards through a bed of
particles the pressure loss in the fluid due to
frictional resistance increases with increasing fluid
flow
A point is reached when the upward drag force
exerted by the fluid on the particles is equal to the
apparent weight of particles in the bed
Pressure drop = (weight of particles – upthrust on
particle)/Bed cross-sectional area
At this point the particles are lifted by the fluid,
the separation of the particles increases and the bed
becomes fluidized
The force balance across the fluidized bed
dictates that the fluid pressure loss across the bed
of particles is equal to the apparent weight of the
particles per unit area of the bed
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At Umf, the hydrodynamic drag force on the
particles Fd, due to the flow of gas through the
packed bed of particles, matches (or just
exceeds), the net gravitational forces Fg,
0 = Fg + Fd,
0 = (W − Fb )+ Fd,
(ρp− ρf ) gVp= − Fd
where the balance had been made in the direction of gravity.
Also The drag force on the bed of particles can then be calculated by
multiplying by the cross-sectional area A of the column,
Fd= ΔPA ΔP = Fd /A
(The net gravitational forces on the bed of particles must consider the weight W
such that
Fg = W − Fb= (ρp − ρf ) g Vp
where ρp is the density of the particles, ρf is the density of the fluid, g is the
gravitational acceleration constant, and Vp is the total volume of particles within
the fluidized bed.) 6
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Plot of fluid pressure loss across the bed vs superficial fluid velocity
through the bed:
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Fluidized Bed Setup (BUET) 10
Response to Superficial velocity
Understand the graph.
Why there is a
Hysteresis?
Fig – 7.9 (McCabe-Smith)
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Expression of minimum fluidization velocity.
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Flow Regimes in Fluidized Beds
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Flow Regimes : Bubbling Fluidization
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Application of Fluidization
(a) Fluid-Solid conveying(pneumatic conveying):
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(b) Drying of solids
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(c) ) Fluid catalytic cracking process
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(d) Gas fluidized bed/Fluidized bed reactor
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Problems
1. A packed bed of solid particles of density 2500 kg/m3, occupies a depth of 1
m in a vessel of cross-sectional area 0.04 m2 The mass of solids in the bed is
50 kg and the surface-volume mean diameter of the particles is 1 mm. A
liquid of density 800 kg/m3 and viscosity 0.002 Pa.s flows upwards through
the bed.
(i) Calculate the voidage (volume fraction occupied by voids) of the bed
(ii) Calculate the pressure drop across the bed when the volume flow rate of
liquid is 1.44 m3/h.
(iii) Calculate the pressure drop across the bed when it becomes fluidized.
2. 50 kg of 500 μm Copper particles (ρp = 8.8 gm/cc) is fluidized using water
(viscosity = 0.01 g/cm.s) at ordinary conditions in a column of diameter 20
cm. What is the bed height when the velocity is 4.2 cm/s?
3. Copper particle (ρp = 8.8 gm/cc) of size 1 mm diameter is fluidized using
water at ordinary condition. What is the minimum fluidizing velocity?
(Assume bed porosity is 0.42). 24
4. Boiler feed water is deionized by pumping it downward through a bed of
ionexchange resin. The resin has spherical particles sized to -8 + 10 mesh.
The deionizer is 4 ft. in diameter by 6 ft. long and operating flow rate of 20
gal/min/ft3 of bed in used. What pressure drop exists through the bed?
If the ion-exchange resin has a particle density of 80 Ib/ft3 and it is
regenerated by back-flushing with 10 percent brine solution (p = 1.07
gm/cm3 ) at a rate such that the bed is expanded 100 percent, what rate of
brine flow is required?
5. A sand filter consists of uniform spherical particles of -20+28 mesh size.
After back flushing with water, the sand bed has settled to a stable depth of
6 ft and is flooded with water to a depth of 3 ft above the top of the sand. If
the drained valves are opened, how long will it take the bed to drain until
the water level is even with the top of the bed?
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