Chemical Engineering April 2021 - 50

Solids Processing
Operational Best Practices for Rotary Dryers
To maximize the service life and reliability of rotary dryers for solid materials, adhere to
this set of practices
Shane Le Capitaine and
Carrie Carlson
FEECO International
R
otary dryers have a welldeserved
reputation for
reliability, often providing
decades of trouble-free
operation in solids-handling processes.
As with any type of process
equipment, however, the reliability of
a rotary dryer and its ability to consistently
dry bulk solids hinges on
proper operation and maintenance
of the unit (Figure 1). This article covers
best practices for rotary-dryer
operation that will help to maximize
the service life and reliability of the
rotary dryer, while maintaining product
integrity in industrial settings.
Maintain a consistent feed
Rotary dryers are known for their
tolerance to variation in feedstock
conditions, such as particle size and
moisture content. And while the tolerance
for feedstock variation is real,
the best results are still achieved by
maintaining a feedstock that is as
uniform as possible in all aspects.
The more uniform the material going
into the dryer, the more uniform the
material will be coming out.
Operators struggling with product
consistency issues often find that a
large variance in feedstock conditions
is to blame. In some settings,
maintaining a uniform feedstock for
a dryer may not be possible, but options
do exist for minimizing excessive
variation. One way to improve
the uniformity of feedstock going to
the dryer is to use a technique called
" backmixing, " which is achieved by
mixing finished (dried) product with
the raw feedstock before it enters
the dryer (Figure 2). This evens out
the overall moisture content of the
material entering the dryer.
A more efficient way to manage
variation is to eliminate it from the
source material. This may involve a
change in vendor specifications (or
50
FIGURE 1. Rotary dryers like the one shown here, can be highly reliable
when properly operated and maintained
even a change in vendors), adding
a pre-conditioning system, or making
improvements in the process upstream
of the dryer. In many cases, a
process audit will reveal that by optimizing
the performance of equipment
upstream of the dryer, variation
can largely be mitigated.
Never run empty with heat
One rule all operators and maintenance
personnel should understand
is to never run the dryer empty with
heat. When the drum runs empty,
there is no material to absorb the
heat and protect the dryer. Similarly,
the burner should never be left running
while the dryer is not rotating.
Running the dryer empty or without
rotation in the presence of heat
has the potential to cause warping
or deflection of the drum shell as a
result of uneven heat distribution. A
drum that is heated without rotation
will result in localized heating, which
causes the drum to sag.
If the temperature of the shell is allowed
to exceed the yield strength
of the material of construction, deformation
of the shell will be permanent.
In most cases where deflection
has been allowed to occur, total shell
replacement has been necessary.
Deflection causes additional problems
where refractory material is employed,
because the refractory will
experience heat stress at a different
rate than the shell, causing cracks or
breaks in the refractory.
Minimize buildup
Depending on the material being
dried and the design of the rotary
dryer, buildup of material inside the
drum can become an issue. Minimizing
the amount of buildup that
is allowed to occur is critical to the
drum's longevity and efficiency.
Buildup in the dryer is problematic
for a number of reasons. First, it reduces
the overall efficiency of the drying
process and often results in offspecification
material. Buildup also
has the potential to cause clumps of
material that eventually break away
from the dryer wall. When this happens,
it can damage both the interior
of the dryer and the flights (material
lifters; described below), as well as
the discharge breeching (where the
product exits the dryer). Buildup that
does not break away can pose a risk
of fire for some materials. If the material
is corrosive, buildup promotes
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