ChemiCal engineering www.Chemengonline.Com april 2015
been developed and tested for use
in freeze-drying foods .
Selection of dryers. Determining
which vacuum dryer is best for a par-
ticular application (Table 4) depends
in part on knowing a solid material’s
moisture content. In addition, it is im-
portant to understand the material’s
particle characteristics, because
these characteristics can vary in un-
expected ways at different moisture
levels. For instance, a filter cake con-
taining 40% moisture can flow better
than one with 15% moisture. For this
reason, end-users of dryer technol-
ogy should expect dryer manufac-
turers to test solid materials before
determining which dryer type is best
capable of handling it.
Several dryer types (or drying sys-
tems) may be equally suited (techni-
cally and economically) for a given
application. A careful evaluation of
as many of the possible factors af-
fecting the selection will help reduce
the number of options. For a new ap-
plication (a new product or new pro-
cess), it is important to follow a care-
ful procedure leading to the choice
Vacuum operation also eases the
recovery of solvents by direct con-
densation, thus alleviating possible
serious environmental problems.
Dust recovery is simpler, so that vac-
uum dryers are especially suited for
drying toxic, dusty products, which
must not be entrained in gases. Fur-
thermore, vacuum operation lowers
the boiling point of the liquid being
removed, thus allowing the drying
of heat-sensitive solids at relatively
fast rates. Dryer selection can have a
major impact on product quality, par-
ticularly in the case of thermally sen-
sitive materials, such as foodstuffs.
Fuzzy logic. The use of a fuzzy ex-
pert system as an aid in the prelimi-
nary selection of a batch dryer can
be a helpful tool. This incorporates
a novel “multiple goal” approach in
which different facets, such as dryer
type, single versus multiple dryers,
and atmospheric versus vacuum
operation, are determined indepen-
dently . n
Edited by Scott Jenkins
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and Valérie Orsat, Microwave vacuum dryer setup and
preliminary drying studies on strawberries and carrots,
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of vacuum-infrared drying of pistachios; Agric. Eng. Int:
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Faculty of Horticulture, Chiba University, 648, Matsudo,
Matsudo, Chiba, Japan.
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Wongwises; Experimental study on drying of chili in a
combined microwave-ratary drum dryer. King Mongkut’s
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Dilip M. Parikh is president of the
pharmaceutical technology devel-
opment and consulting group DP-
harma Group Inc. (Ellicott City, MD
21042; Email: dpharma@gmail.
com). As an industrial pharmacist,
Parikh has more than 35 years of
experience in product develop-
ment, manufacturing, plant opera-
tions and process engineering at
various major pharmaceutical companies in Canada and
the U.S. Prior to starting DPharma Group, he held the
position of vice president of operations and technology
at Synthon Pharmaceuticals in North Carolina and vice
president and general manager at Atlantic Pharmaceuti-
cals Services in Maryland. He is the editor of “Handbook
of Pharmaceutical Granulation” 3rd ed. He has authored
several book chapters and articles on various pharma-
ceutical technologies, including quality by design, pro-
cess assessment and contract manufacturing. He has
been an invited speaker at scientific conferences world-
wide on solid-dosage technologies development and
Table 4. SuggeSTed vacuum dryer SelecTion approach 
1 What is the batch size you want to process?
2 Consider which dryer type will drive up the temperature difference between your material and the
heating media (ΔT)
3 Determine the moisture (liquid) content of your product to be dried, as well as the nature of that
liquid (water or solvents)
4 Determine bound liquid and un-bound liquid is in the product
5 Based on the liquid, determine the vapor pressure profile with the temperature to be used
6 What is final product specification regarding percentage of liquid remaining in the product?
7 What utilities are available in the plant?
8 Consult a reputable dryer manufacture, discuss the product and determine the budget
9 Perform testing of your product in the dryer manufacturer’s facility
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