Page 225 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 203

The bubbles may be carrying some fines within them. & Generally liquid fluidized beds exhibit particulate
Thewhole bed resembles that of a boiling liquid. This behavior while gas fluidized beds tend to be aggre-
bubbling bed results at low velocities above mini- gative in character. Very dense, coarse particles can
mum fluidization velocity. fluidize aggregatively while fine particles in dense
. What are the differences between particulate and ag- gas may exhibit particulate fluidization tendencies.
gregative fluidization? . What is slugging in a fluidized bed?
& Above minimum fluidization velocity, the bed can & Particulates will be distributed nonuniformly form-
expand in two ways: ing aggregates of slugs in the gas–solid systems.
i. Particulate Fluidization: As the fluid velocity is & Slugging can occur in bubbling fluidization since the
increased, the bed continues to expand and re- bubbles tend to coalesce and grow as they rise in the
mains homogeneous, with the particles moving bed. If the column is small in diameter with a deep
farther apart more rapidly. The average bed den- bed, bubbles can become largeand fill the entire cross
sity at a givenvelocity is the same in all regions of section and travel up the column separated by slugs
the bed. The bed voidage increases with fluid of solids.
velocity, but remains uniform. This type of flu- . What is a tapered bed? What are its advantages?
idization is very desirable in promoting intimate
& In a tapered bed, cross-sectional area of the bed
contact between the gas and solids. Liquids often
increases from a minimum at the bottom to a uniform
give particulate fluidization. Particulates in this
cross section (that of the vessel) at some height.
type of fluidization are classified, based on their
& When there is a wide range of particle sizes in a
density and size range, as follows:
powder, fluidization will be more even in a bed that
3
Dr ¼ðr rÞ¼ 2000 kg=m ; D p ¼ 20 125 mm; is tapered so as to provide minimum cross-sectional
p
area at the bottom. If pressure gradient is low and
the gas does not significantly expand, velocity will
3
Dr ¼ 1000 kg=m ; D p ¼ 25 250 mm;
decrease in the direction of flow.
& Coarse particles that tend to fluidize at the bottom
3
Dr ¼ 500 kg=m ; D p ¼ 40 450 mm;
(high velocity) assist in the dispersion of the fluidized
gas. Carryover of fines from top will be reduced
3
Dr ¼ 200 kg=m ; D p ¼ 100 1000 mm:
because of the lower velocity at the exit.
. What is a centrifugal fluidized bed?
ii. Aggregative Fluidization: The bed is very non-
& In the centrifugal fluidized bed, the solids are rotated
uniform. Most of the fluid (usually gas or vapor)
in a basket and the gravitational field is replaced by a
passes through the bed in fast moving bubbles that
centrifugal field that is usually sufficiently strong for
are nearly free from particles, while the rest of the
gravitational effects to be neglected. Gas is fed in at
bed, the emulsion phase, remains close to mini-
the periphery and travels inward through the bed.
mum fluidization conditions. It may become quite
& The equipment required for a centrifugal fluidized
rough/violent at high fluid velocities and bed
bed is much more complex than that needed for con-
structure and flow patterns are much more com-
ventional fluidized bed, and therefore, it has found
plex than those in particulate fluidization. The
use only in highly specialized situations. One such
expansion of the bed is small as gas velocity is
application is in zero gravity situations, such as
increased. Sand and glass beads provideexamples
spacecraft, for the absorption of carbon dioxide from
of this behavior. This behavior is sometimes
the atmosphere inside.
described as bubbling fluidization. Particulates
in this type of fluidization are classified as . What is a recirculation fluidized bed? Explain with an
follows: example.
& The recirculating fluidized bed with a draft tube
3
Dr ¼ 2000 kg=m ; D p ¼ 125 700 mm: concept is briefly illustrated in Figure 7.10.
& In application as a coal devolatilizer, dry coal is
3
Dr ¼ 1000 kg=m ; D p ¼ 250 1000 mm: introduced into the devolatilizer below the bottom
of the draft tube through a coal feeding tube con-
3
Dr ¼ 500 kg=m ; D p ¼ 450 1500 mm: centric with the draft tube gas supply.
& The coal feed and recycled char at up to 100 times the
3
Dr ¼ 200 kg=m ; D p ¼ 1000 2000 mm: coal feed rate are mixed inside the draft tube and