Page 223 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 201

settle down (sedimentation) or allowing the fluid to . On what factors minimum fluidization velocity
carry away the particles (pneumatic conveyance if depends?
the fluid is air, hydraulic transport if it is water, or & The factors include particle diameter, its density,
carryover in the general sense of the term if the fluid void fraction of the bed at the onset of fluidization,
is a gas or liquid other than water). Initially, particles and viscosity of the fluid. Void fraction of the fluid-
are at rest over a porous support such as a screen or a ized bed is not accurately known but is normally
distributor plate with openings whose size is less than taken as about 0.4 at the onset of fluidization.
the size of the particles. & Commercial gaseous fluidized beds are usually op-
& As the velocity of the fluid from the bottom of the
erated at flow rates many times those at minimum
distributor is increased, the particles start getting fluidization requirements. Typically, 5–20 times the
disturbed and begin to rise, first as aggregates and minimum values, taking care that carryover will not
then as individual particles. At this point, the force of occur.
the pressure drop times the cross-sectional area & Liquid fluidized beds operate at values closer to
equals thegravitational force on the mass of particles.
minimum values.
This is the onset of fluidization.
. What is the relationship between bed height and
& The equation for the onset of fluidization is
porosity?
L 1 =L 2 ¼ð1 « 1 Þ=ð1 « 2 Þ; ð7:10Þ
3
2
V f ¼½ðr r ÞgD =150mŠ½« =ð1 «ÞŠ: ð7:9Þ
p
f
p
where L is the bed height and « is the bed porosity.
. What are the different regimes of fluidization? Illustrate.
& When the superficial velocity V s ¼ V f , the condition
of the bed is referred as one of incipient fluidization. & With increased fluid velocities, the following trans-
formations, ranging from fixed bed to pneumatic
& At a certain velocity of the fluid, called minimum
conveyance, take place: fixed bed ! particulate
fluidization velocity, the particles get fluidized.
regime ! bubbling regime ! slug flow ! turbu-
Minimum fluidization velocity is based on empty
lent regime (bubbling bed, slug flow, and turbulent
cross section of the container (superficial velocity).
regimes constitute aggregative fluidization) ! fast
Liquid fluidized beds are generally characterized by
fluidization ! pneumatic conveyance.
the regular expansion of the bed, which takes place
& Figure 7.8 illustrates the regimes of fluidization.
as the velocity increases from the minimum fluidi-
zation velocity to terminal falling/settling velocity & Six different regimes of fluidization are identified in
of the particles. This phenomenon is termed bed Figure 7.8.
expansion. & Particulate fluidization, class (b) of the figure, is
& The pressure drop increases as the fluid velocity is desirable for most processing since it affords intimate
increased until the onset of minimum fluidization. contacting of phases.
Then, as thevelocity is further increased, the pressure & Fluidization depends primarily on the sizes and
drop decreases very slightly and then it remains densities of the particles, but also on their roughness
practically unchanged as the bed continues to expand and the temperature, pressure, and humidity of the
or increase in porosity with increase in velocity. gas.

FIGURE 7.8 Six regimes of fluidization identified with increasing gas superficial velocities: (a) fixed bed; (b) particulate fluidization;
(c) bubbling fluidization; (d) slugging fluidization; (e) turbulent regime.