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CIRCULAR FLUIDIZED BED DRYERS: LATEST ENTRANT
ON DRYER SCENE CREATES NEW APPLICATION NICHE
By Henry Alamzad
The large and growing list of methods to dry bulk materials has
narrowed the applications in which each category of dryer excels.
The latest example is fluid bed drying, where circular fluid bed
designs have intruded on turf previously dominated by rectangular
units. This article updates specifiers by comparing fluid bed dryers
with other methods in general,and circular fluid bed designs with
rectangular designs in particular.
Comparing fluid bed dryers with spray dryers, rotary, flash,and
tray dryers is highly application dependent. Usually the nature
of the drying problem dictates the type of dryer to use, or limits
the choice to two or three possibilities.
Considerations for selection include the feed's moisture content
and form of the feed—liquid, semi-solid,or free-flowing. What is
its sensitivity to heat, agitation? What is the dryer's ability
to handle the feed? What is the capacity requirement? Can the feed
be preconditioned?
Where fluid bed dryers fit among available choices hinges on how
closely your process parameters and material characteristics dovetail
with the dryers'capabilities. Various dryers occupy niches for high-water-content
or solid feeds. Fluid bed dryers mainly handle solid feeds comprised
of discrete particles,but with exceptions. Fluid bed processors
can also cool or moisturize when the heating unit, located between
the blower and fluid bed processing unit, is substituted with a
remote chiller, or a moisturizer.
Dryers for high-water-content feed Liquid and pumpable semi-solid
feeds, such as slurries,are best suited for spray, rotary,or drum
dryers. A spray dryer dries liquid or slurry feeds at high rates.
The slurry is atomized,and the droplets are dried in contact with
a hot air stream. Moisture is rapidly vaporized from the droplets,
leaving residual particles of dry solid. Dwell time is very short,
permitting drying of heat-sensitive materials. Since the droplets
must not strike solid surfaces before drying is complete,drying
chambers are necessarily large – 8-30 ft (245-915 cm) diameter,
with dryer heights as great as 80 ft. (24 m). Much heat is lost
in the discharged gases, reducing efficiency of spray dryers.
A rotary dryer processes liquid, pumpable and non-pumpable semisolids,
and free-flowing particle feeds at high throughputs. The dryer consists
of a rotating cylinder, slightly inclined from horizontal, in which
a hot air stream dries flowing parallel or counter to the feed.
As the cylinder rotates, internal flights lift and shower down the
solids through the interior of the shell.
The dryer can also operate under vacuum for drying heat-sensitive
materials at a lower temperature. The rotary dryer's horizontal
configuration,however, can consume much floor space. It is energy
intensive, and incurs relatively high initial cost.
Dryers for solid feeds
Flash, tray, rotary tray and fluid bed drying are among the most
popular methods for drying solid and semi-solid feeds in the form
of cakes,powders,and granules. A flash dryer dries preconditioned
particles at high throughput rates. A heated pneumatic conveying
system entrains the particles in a hot air stream. Temperature is
high (up to 1200°F [649°C ]) at the flash dryer inlet, but the feed
temperature rarely rises above 90°F (32°C) because residence time
is short— between 0.5 and 10 seconds. Thus, flash drying can process
heat-sensitive materials that otherwise would require indirect drying
by a lower temperature method. (Indirect drying transfers heat through
a medium such as pipes or retaining wall, as opposed to direct contact
with the hot gas or air.)
A flash dryer is energy intensive, requiring high volumes of hot
air. Configurations range from a single long tube to a loop design,
to a series of cyclones. Its vertical configuration may require
a costly multiple floor support structure.
Tray dryers are useful for low capacity applications, and for material
that cannot be agitated. A tray dryer consists of a housing into
which shelves or trays of material are manually loaded, around which
a hot air stream circulates. Labor required for loading and unloading
can make them costly to operate.
For precise temperature control, a rotary tray dryer houses a stack
of rotating circular trays within a hot air stream creating different
temperature zones. After one tray revolution,a wiper sweeps the
material to the next lower tray,with the same action repeating for
the entire stack of as many as 20 trays. The feed discharges as
dry product at the bottom of the housing.
Fluid bed drying
In general, fluidized bed drying is suited for products containing
40% or less moisture, the point at which most products become free-flowing
enough to fluidize in the fluid bed chamber. The feed needs to be
in the form of discrete particles, i.e., free-flowing powder,granules,
crystals, flakes,or pulverized material that can be fluidized.
Fluidized bed drying,depending on the application,may provide less
expensive drying and a simpler process than spray, rotary, flash
and other methods. It has the fewest moving parts and is therefore
simple to operate and maintain. Energy consumption is low, further
reducing operating costs.
Fluid bed operation can be batch or continuous. Continuous operation
requires a feed control device such as a screw feeder,belt feeder,or
rotary valve. Batch processing normally employs a discharge spout
gate that remains closed while the batch is being dried,and opens
after the drying run is complete.
Fluidized bed drying produces high thermal efficiency, while preventing
overheating of individual particles,making it a good choice for
temperature-sensitive products.
In operation,material vibrates on a screen or perforated surface
within a rising column of heated air.The continuous airflow and
vibration separate and fluidize individual particles,maximizing
the surface area of material and, accordingly, the drying rate.The
hot air surrounding each particle rapidly transfers heat. Particle
fluidization eases material transport for gentle handling of the
feed. Fluidized bed drying produces high thermal efficiency, while
preventing overheating of individual particles,making it a good
choice for temperature-sensitive products.
Residence time of products within a circular fluid bed dryer operating
continuously ranges from 30 seconds to 15 minutes.For heat sensitive
materials, shorter residence times are preferred. Air temperature
can also be reduced to below the material's temperature limit as
a precaution.
Residence times for batch fluid bed drying can extend as long as
necessary to attain the desired drying level, and can be varied
using an adjustable gate at the discharge spout called a “weir."
Exceptions to the rule
Although semi-solid lumpy or caked feeds are normally handled by
rotary, tray, flash,plate,or conical dryers,a fluid bed dryer can
be utilized,providing a granulator,de-agglomerator or centrifugal
sifter is installed upstream to precondition the material into discrete
particles.
Conclusion
Circular fluidized bed dryers make a good choice when the drying
rate matches that of rectangular fluid bed dryers,due to their smaller
size, lower energy use, and easier cleaning.
The circular design has no corners or crevices for material to
lodge and cause contamination or hamper cleaning. Rectangular fluidized
bed dryers of equivalent capacity occupy about twice the space of
circular units, and require higher airflow, consuming more energy.
Compared with rectangular fluid bed dryers,a circular fluid bed
dryer's initial and operating costs are lower.
The circular design is inherently stronger than rectangular designs,
permitting lightweight construction at less cost; materials can
be down-gauged,and motors and associated components can be downsized,
reducing initial and operating costs. A circular fluid bed drying
system can also be self-contained on a skid. Equipped with casters,
it can be mobile for multiple duties.
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Edition. New York,McGraw-Hill, 1993.
2.Van Nostrand's Scientific Encyclopedia, “Dryers."
3.Traub, Darren A., “Fluid Bed Dryers," Process Heating, May 2001
4.Traub, Darren A.,“Flash Drying,"Process Heating,June 2001
5. Science Citation,“Rotary Dryers,"http:/pubsci.osti.gov
6.“Industrial Dryers,"www.singhasini.com |
