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partially dried material is held in circulation. The split
is varied by adjusting the positions of suitable deflec-
tors, introduced in the flow loop. This type of FD is
available in both multistage and closed-circuit designs
with both direct and indirect heating options for re-
moving both surface and bound moistures, as well as
solvent removal and recovery.
41.3.4.3 Spray Dryers
In spray dryers, the feed material, in the form of a
solution, suspension, slurry, or paste, is sprayed in a
high-temperature gas zone by centrifugal disks or
pressure nozzles. Such dryers are used in polymer
industries in which the polymers cannot be separated
mechanically from the carrier liquid, e.g., emulsion-
polymerized PVC.
In polymer industries, wherever spray dryers are
used they are primarily used as predryers of a multi-
stage system. Final drying is normally done in a fluid
bed, which is either stationary or vibrated type. Sta-
tionary fluid beds are used when spray-dried powder
leaving the drying chamber is directly fluidizable. The
vibrated type of fluid bed is used for products that, on
leaving the spray dryer, are not readily in a fluidizable
state owing to their particle form, size distribution, or
wetness.
In such a multistage system, the higher moisture
content powder leaving the spray-drying chamber is
transferred to the second stage, which is a fluid bed
for completion of drying. The higher inlet temp-
erature and lower outlet temperature operation in
such a system give improved dryer thermal efficiency
and increased dryer capacity without product quality
degradation.
41.3.4.4 Fluidized Bed Dryers
Fluidized bed dryers (FBDs) involve the suspension
of solid particles in an upwardly moving stream of
gas, which is introduced through a distribution plate
that may be cooled for heat-sensitive polymers. Such
a dryer may operate batchwise.
The advantages offered by FBDs are: (1) the
even flow of fluidized particles permits continuous,
automatically controlled, large-scale operation with
easy handling of feed and product; (2) no mechanical
moving parts, i.e., low maintenance; (3) high heat
and mass transfer rates between gas and particles—
this is well mixed, which also avoids overheating of
the particles; (4) heat transfer rates between fluidized
bed and immersed objects, e.g., heating panels, are
high; and (5) mixing of solids is rapid and causes
nearly isothermal conditions throughout the bed,
thereby facilitating easy and reliable control of the
drying process.
Using the solvent being removed as the heat car-
rier and fluidizing medium (i.e., a superheated vapor)
has proved a feasible and beneficial design. Its advan-
tages include: (1) reduction in size of condensing and
recovery equipment; (2) increase in drying rate due to
the elimination of the gas-film resistance of the for-
eign vapor; (3) volumetric heat capacity of various
vapors is usually greater than that of air; and (4)
space velocity for fluidization is lower than with air,
which reduces the volumetric vapor flow and conse-
quently the size of the dust collector, air moving
equipment, and other parts.
Drying of polystyrene beads is a typical example
for industrial use of these dryers because of the close
range of bead particle size. Also, the size of the beads
permits high fluidizing velocities and therefore eco-
nomic dryer sizes.
In recent years, indirect-heated fluidized beds have
made inroads in almost all industries. Some of their
advantages over the direct-heated FBD are: (1) the
indirect heat transfer rate significantly reduces gas
flow requirements; (2) there is tremendous leverage
gained by the multiple of the heat transfer coefficient,
LMTD, and heat transfer surface density permits
very high heat inputs into low-temperature, heat-
sensitive applications; (3) when a plug-flow, rectan-
gular indirect fluid bed or low bed height is used, the
solids flow counter to the thermal fluid, behaving like
a countercurrent heat exchanger with all its attendant
benefits; and (4) since the heat source is decoupled
from the fluidizing gas source, vessel diameters and
pollution-control equipment are much smaller.
Indirect fluid beds have already proved efficient
in drying very heat-sensitive polymers with large
constant-rate drying periods, as in drying PVC,
polyethylene, acrylonitrile–butadiene–styrene (ABS)
copolymers, and polycarbonates (PC).
41.3.4.5 Vibrated Fluidized Beds
A vibrated fluidized bed (VFB) is basically a long
rectangular trough vibrated at a frequency of 5 to
25 Hz with a half amplitude of a few millimeters
(2 to 5 mm). This kind of dryer can be used for drying
ß
2006 by Taylor & Francis Group, LLC.
wet, sticky , an d granula r media an d has be en used
success fully for drying pol ymers. It is often use d as a
second-s tage dryer afte r a flash or spray dryer in
many polyme r-drying applic ations .
Bene fits achieve d from such dryers are: (1)
unifor m resid ence tim e distribut ion regardless of
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