Chemical Engineering September 2022 - 42

trolling the product moisture. Some
dryer operators control the wet-bulb
or the dewpoint temperature to
control product moisture. The temperature
and water-vapor loading of
the outlet air stream will relate to the
final product moisture.
Dryer types
Brief descriptions of various types of
dryers used in industrial drying applications
follow.
Rotary dryers. Rotary dryers are designed
with rotating, tumbling drums
and internal lifters that cause the
moist solid to " rain " down through
the passing and heated drying air.
The challenge related to this dryer is
maximizing the time the wet feed is
exposed to the heated drying air: the
better the contact, the shorter the
rotary dryer's length and the lower
the cost. Final design will depend on
the retention time required.
Conveyor belt dryers. Conveyor
dryers (also called belt, tunnel or
apron dryers) are rotating belt conveyors
typically equipped with a
perforated belt to allow heated air
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to circulate through the belt and the
wet feed. The challenge for this dryer
is getting the heated air into contact
with the inner layer of the feed on the
belt. Some use multiple belts to reorient
the feed and expose internal
surfaces to the heated air.
Spray dryers. A spray dryer begins
with a liquid feed or slurry, progresses
to atomization of the liquid
or converts it into smaller droplets.
The goal is rapid drying of liquid material
into a powdered material using
heated gas. Many different types of
spray dryers are available and commonly
used within the food, chemical
and pharmaceutical industries.
Spray dryers often will be used in
conjunction with a fluidized-bed
dryer, as complementary systems to
achieve the target moisture level.
Flash dryers. A flash dryer is a
hot pneumatic conveyor. The challenge
is to increase the exposure
time from a few seconds. There are
variations, such as recycling large
particles or inline particle size reduction
to aid in drying. Flash dryers are
used in processing food or minerals
that may be sensitive to prolonged
heat exposure.
Fluidized-bed dryers
Fluidized-bed dryers use heated air
to fluidize and dry feed material and
are either static or vibrating in design.
Static fluidized beds can have
bed depths that minimize the size
of the dryer and the amount of drying
air required. The inefficiency of
a direct dryer is the exhausted hot
air; minimizing the amount of air increases
the dryer's efficiency.
Fluidized-bed dryers come in two
different shapes: rectangular or circular.
Fluidized-bed dryers (and
coolers) offer batch and continuous
processing capabilities.
Rectangular versus circular fluidbed
dryers. A rectangular fluidizedbed
dryer operates on a conveyorstyle
movement for material transport
through the dryer. These can be built
to higher capacities and with multiple
stages. Rectangular fluidized-bed
dryers offer a viable option for a highvolume,
continuous application that
does not require stringent adherence
to rigid sanitation standards. However,
the larger size requires a substantial
plant footprint and can be a
challenge to install, clean and maintain,
with the largest sizes requiring a
crane or forklift to maneuver.
One of the challenges for a rectangular
fluidized-bed dryer is the
" dead " areas in the dryer's corners
that can trap or retain previously
processed materials or harbor bacterial
growth. In case of product or
material retention, the material with
a longer dwell time can overdry
some materials. In food, pharmaceutical
and similar applications,
fast and thorough cleaning capabilities
are critical to comply with 3-A,
USDA, FDA, BISCC and other stringent
sanitary and safety standards.
Circular fluidized-bed dryers offer
batch and continuous dryboth
ing
capabilities, with batch offering
greater operator control of the final
moisture level. In applications where
the specifications are not as tight
for final moisture levels or if they fall
within a range, the operator can opt
for a continuous drying process.
The circular design is free from
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internal corners or edges, offering
easier maintenance and cleaning
options than a rectangular version. A
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Chemical Engineering September 2022

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