Chemical Engineering September 2022 - 45

FEECO International
FEECO International
the end product; excess moisture
would not only inflate shipping costs,
but could also promote mold or bacterial
growth, making the product
unfit for its intended use. Similarly, an
over-dried product may be too dusty
for acceptance on the market.
The producer may also want to test
FIGURE 2. The flights used in a rotary dryer can
be customized to match the requirements of a
given application
* Exhaust gas flow
* Burner fuel usage
* System pressures
* Gas sampling and analysis
* Baghouse, cyclone, scrubber
efficiency
Establishing optimal flight design.
The flights, or material lifters employed
in a rotary drum (Figure 2),
create a cascading effect, or " curtain "
of material across the drum's
cross section. This maximizes the
amount of contact, and ultimately
heat transfer, between the material
and products of combustion.
Flights are available in a range
of standard and custom designs.
Standard designs include straight,
single-bend, and double-bend,
among others.
In addition to flight design, the
placement of flights throughout the
drum, or pattern, is also highly customizable.
It is not uncommon for
flight design or pattern (or both) to
change along the length of the drum,
as process and material conditions
change during the drying process.
Flight design is based around
specific material properties, such as
moisture content, angle of repose,
potential for sticking and more,
making this a very custom aspect
of the design process. When flight
selection is not readily apparent, a
flight simulator can be enlisted in
the testing process to identify the
most suitable design for the material
at given stages throughout the
drying process.
A flight simulator is an offline rotating
drum with flights that are easily
changed out for testing. A clear end
cap allows for visual observance of
how the selected flights work with
the material.
Targeting particle characteristics.
Whether drying a finished product or
an intermediary material, producers
are often looking to target specific
parameters in the material exiting the
FIGURE 3. A look inside a testing center, with a
pilot dryer visible in the background
dryer. Testing provides the environment
necessary to refine the process
variables necessary to achieve those
qualities. The most commonly targeted
parameters during drying include
the following:
* Outlet moisture content
* Level of attrition
(product
degradation)
* Bulk density
* Crush strength
* Particle size distribution (PSD)
By manipulating the design of the
dryer, in combination with process
parameters during testing, producers
can reveal the conditions necessary
to reach their target specifications.
The testing process varies depending
on the user's goals, but it may
also identify inefficiencies and opportunities
for improvement, both in the
process and the product.
A case study: paper sludge
The paper industry has been under
increasing pressure to find beneficial
reuse applications for the
sludge left over from the recycling
process. In this effort, producers
have begun drying their sludge for
reuse in products, such as absorbents,
cat litter, animal bedding,
lawn products and more.
Depending on how the paper pulp
was processed and dewatered at
the
plant,
its
characteristics, and
particularly its moisture and clay
content, can vary, making testing
a critical step in developing a commercial-scale
system. Optimizing
for efficiency is especially important
when working with wastes and byproducts,
as the margin for profit is
often slim.
Whatever the intended product,
testing helps to reveal the process
conditions necessary to consistently
produce product to specification.
Under-dried material could present
a host of issues depending on
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SEPTEMBER 2022
in order to determine the highest level
of moisture they can run through their
dryer, since paper waste streams are
not always consistent.
In this example, producers are
likely to focus on finding the right
combination of volumetric fill, retention
time, dryer temperature and
air volume to
consistently
reach
the target moisture content. Paper
producers often find that they must
employ a size-reduction step prior
to drying in order for the process to
be effective.
Concluding remarks
Rotary dryers are essential in processing
bulk solids, but maximizing
the efficiency of a dryer can be challenging
without a thorough testing
program. Whether developing a new
process, or optimizing an existing
one, testing facilities such as the one
shown in Figure 3, offer an invaluable
opportunity to establish a highly efficient
operation.
Testing not only illustrates proof
of process and provides other critical
assurances, but it also reveals
the parameters necessary to reliably
yield consistent results, giving producers
a leg up on commercializing
and refining their operations.
n
Edited by Gerald Ondrey
Authors
Shane Le Capitaine is a process
sales engineer at FEECO International
(3913 Algoma Rd., Green
Bay, WI 54311; Phone: 1-800373-9347;
Email: slecapitaine@
feeco.com), where he specializes
in bulk solids drying,
fertilizer
granulation, and rotary drum applications.
Le Capitaine received a
B.S.Ch.E. from the University of
Wisconsin-Madison and has over 20 years of experience
in his field.
Carrie Carlson is a technical
writer at FEECO International
(same address as above; Email:
ccarlson@feeco.com) with more
than a decade of experience. She
works closely with engineers and
process experts to turn complex
ideas into easy-to-understand
literature.
45
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