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Vacuum drying can be a useful tool for solid products that are heat-sensitive. Here are
some guidelines for the selection and use of various types of vacuum dryers
Drying is an essential unit operation
in a variety of chemical process in-
dustries (CPI) sectors. Food, phar-
maceuticals, chemicals, plastics,
timber, paper and other industries use dry-
ing equipment to eliminate moisture during
product processing. Most dryers are classi-
fied as direct dryers, where hot air (at near
atmospheric pressure) is used to supply the
heat to evaporate water or other solvents
from the product. Another important dryer
category, vacuum dryers, involves the use of
a reduced-pressure atmosphere to surround
the product.
Drying is among the most energy-intensive
unit operations, due to the high latent heat of
vaporization of water and the inherent ineffi-
ciency of using hot air as the (most common)
drying medium. Depending on the specific
product attributes required, different indus-
try sectors require different types of drying
technology. Drying high-value products that
are likely to be heat-sensitive, such as food,
pharmaceuticals and biological products,
demands special attention. When dried by
convection at higher temperatures, these
heat-sensitive products degrade, change
color and appearance and have lower vita-
min or nutrient content. Vacuum dryers offer
an alternate path. This article discusses the
operation and selection of vacuum dryers,
and provides examples of applications in
which vacuum drying is used.
Drying principles
Drying involves two distinct drying periods,
known as the constant drying period and the
falling drying period (Figure 1). Drying occurs
when liquid is vaporized by supplying heat to
the wet feedstock. The liquid removed by the
drying process could be either free moisture
(unbound) or bound within the structure of
the solid. The unbound moisture, normally
present as a liquid film on the surface of a
solid particle, is easily evaporated, while the
bound moisture could be found within the
solid material, trapped in the microstruc-
ture of the solid. In this case, the moisture
must travel to the surface to be evaporated.
When a solid product is subjected to drying,
removal of unbound and bound moisture de-
pend on the rates at which these two pro-
cesses proceed. Removal of unbound mois-
ture depends on external conditions of air or
gas temperature, flow, humidity, area of ex-
posed surface and pressure. The movement
of bound moisture depends on the nature
of the product being dried and the extent of
moisture within the product.
Unbound moisture normally is removed
by evaporation and vaporization. Raising the
temperature facilitates the evaporation and air
draws the moisture away. If the product being
dried is heat-sensitive, then the temperature at
which evaporation occurs (at the boiling point
of water or other solvent) can be reduced by
lowering the pressure with a vacuum.
Vacuum-drying advantages
Vacuum drying is a viable technology that
has been used successfully for many years
in the pharmaceutical, food, plastics and
textile industries, among others in the CPI.
Vacuum Drying:
Basics and Application
Dilip M. Parikh
DPharma Group
Part 1
FIGURE 1: Drying processes involve two distinct phases
Drying cycle time (t)
drying period
drying period
Moisture content (
ChemiCal engineering www.Chemengonline.Com april 2015 49
A major advantage to vacuum dry-
ing is its energy conservation — less
energy is needed for drying, cutting
down on the economic and environ-
mental costs associated with drying
a product for storage, sale or other
purposes. Vacuum-drying pro-
cesses also tend to work faster than
other drying methods, cutting down
on processing times, which can be
important in some facilities where
products are being moved through
quickly. Another advantage of drying
materials in this way is a less dam-
aging drying process. Some materi-
als can experience problems at high
temperatures, such as developing
hard, leathery crusts from heat ex-
posure during the drying process.
Vacuum drying tends to retain the
integrity of the original item without
damaging it with heat. For foods and
pharmaceuticals, this can be valu-
able, as other drying processes can
degrade quality and make the food
less appealing or affect potency of
heat-sensitive drug product.
Using vacuum-drying equipment
also reduces risks to workers. With
other types of drying equipment,
there are vented fumes and particles
that can make people sick or that
force people to wear protective gar-
ments. With a vacuum dryer, ventila-
tion does not occur, and personnel
working near the dryer are safer. It is
also possible to recover the precipi-
tated moisture collected during the
drying for further use.
Vacuum-drying operation
The majority of dryers are of the di-
rect (or convective) type, where hot
air is used both to supply the heat for
evaporation and to carry away the
evaporated moisture from the prod-
uct. Notable exceptions are freeze
and vacuum dryers, which are used
almost exclusively for drying heat-
sensitive products because vacuum
dryers tend to be significantly more
expensive than dryers that operate
near atmospheric pressure.
Vacuum drying is a process in
which materials are dried in a re-
duced pressure environment, which
lowers the heat needed for rapid
drying. Vacuum dryers offer low-
temperature drying of thermolabile
materials and are suitable for solvent
recovery from solid products con-
taining solvents. Heat is usually sup-
plied by passing steam or hot water
through hollow shelves (Figure 2).
Drying temperatures can be care-
fully controlled and, for the major
part of the drying cycle, the mate-
rial remains at the boiling point of
the wetting agent. Drying times are
long, usually about 12 to 48 h. Un-
like a direct-heat dryer in which
the material is immersed directly into
the heating media (usually a hot gas
FIGURE 2: Vacuum tray dryers are common for laboratory and pilot-scale work FIGURE 3: Traditional blenders can be modified to be used as vacuum dryers
(Source Patterson Kelley)
Table 1. SelecTion criTeria for The indirecT dryerS [1]
Dryer type Plate Drum Tumbling Vibrating Conical Thin
Paddle Mixer-
Continuous x x x yy x x x
Discontinuous yy yy x yy x x x
Vacuum x * x x x x
Large surface
and volume
* o o o * x * x
High specific
* x o x * x * x
Friable x yy x x x * x x
Fluid yy X yy yy yy x o x
Viscous/pasty yy x yy yy yy yy o x
Crusty yy yy yy yy yy o * x
Mechanical x x x o x * * *
Thermal * o o * * o o o
yy = not suitable, o = sometimes suitable, * = good, x = ideal
ChemiCal engineering www.Chemengonline.Com april 2015
stream) and is dried by convection
— a vacuum dryer is an indirect-heat
dryer (Table 1). That is, the heat is
transferred to the material as it con-
tacts the dryer’s heated surface, dry-
ing the material by conduction.
Understanding this distinction is
essential for grasping the advan-
tages and limitations of vacuum dry-
ing, as well as for selecting a vacuum
dryer that efficiently and economi-
cally achieves process goals.
To understand how vacuum op-
eration can aid drying, consider the
following equation, which represents
a simplified drying theory:
Q = U A ΔT (1)
Where Q is the total heat (in British
thermal units [Btu]), U is the over-
all heat-transfer coefficient (in Btu/
[ft2/°F]), A is the effective heat trans-
fer surface area (in ft2), and ΔT is the
temperature difference between the
liquid’s boiling point (that is, its vapor-
ization temperature) and the heating
media’s temperature (in °F). The pro-
cess goal is to achieve an effective
heat transfer (Q) to the material so
that its liquid content is vaporized.
Most often, the material’s prop-
erties and the dryer type effectively
establish the U and A values for the
process. So the process-efficiency
objective should be to maximize
the ΔT value, in order to increase
the Q value. By controlling atmo-
spheric pressure, the vacuum dryer
increases the effective ΔT for a given
process. It reduces the boiling point
(vaporization temperature) required
for removing the liquid.
Effective ΔT can be significantly in-
creased by controlling pressure and
heat to the dryer, facilitating faster dry-
ing than at normal atmospheric pres-
sure. Hence, heat-sensitive materials
such as foods, pharmaceuticals and
antibiotics can be dried with vacuum
drying with shorter drying times and
at lower temperatures. The closed
system also offers the advantage of
handling reactive compounds or haz-
ardous solvents in the product being
dried. The vacuum dryer safely con-
tains and condenses the hazardous
vapors from such substances without
any threat to the workplace environ-
ment or to the outside atmosphere.
Vacuum drying is predominately
operated as a batch unit operation.
However, a vacuum-drying unit can
also be integrated as part of a con-
tinuous process. In those cases,
proper control of the infeed and dis-
charge materials is critical, along with
proper process-control parameters.
Limitations of vacuum dryers are
generally related to the heat-transfer
mode of the equipment. A vacuum
dryer’s upper temperature limit (typi-
cally about 600°F) is lower than that
of a direct-heat dryer. The rate at
which material temperature can
be raised in a vacuum dryer is also
limited. This is because the indirect-
heat vacuum dryer is limited by the
surface area available for heat trans-
fer, unlike a direct-heat dryer, which
is limited only by the hot-gas volume
in the drying chamber. The vacuum
pump is primarily responsible for the
vacuum level inside the dryer.
Vacuum drying options
Most vacuum dryers are adapted
from solids blenders. The two prin-
cipal types of vacuum dryers are
tumble and agitated. A number of
traditional blenders can be modified
for use as vacuum processors (Fig-
ures 3, 4 and 5). Selection of dryers
in general and vacuum dryers in par-
ticular are shown in Tables 1–3.
Tray dryers. The most common
dryer for laboratory and small-scale
FIGURE 4: Vacuum-drying systems for solvent-based products employ condensers to collect solvents (Source: Patterson Kelley)
FIGURE 5: This double-cone blender is another example of modified vacuum dryer (Source: Paul O. Abbé)
Circulating pump
Plant in
Coolant out
vacuum equipmenT
Vacuum pump
Liquid feed pump
Vacuum filter Intensifier bar
Expansion tank
Heat exchanger Steam in
Condensate out
Coolant in
Coolant out
Cooling exchnager
Rotating connector
Condensate water
Heat source
Rotating connector
Frame Tank Filter Seal seat Vacuum valve
ChemiCal engineering www.Chemengonline.Com april 2015 51
pilot work is the vacuum tray dryer
(Figure 2). Heat transfer in this type
is largely by conduction or by ra-
diation. The trays are enclosed in a
large cabinet, which is evacuated.
The water vapor produced is gener-
ally condensed, so that the vacuum
pumps have only to deal with non-
condensable gases. Tray dryers are
reliable and have no moving parts,
but operation and cleaning are la-
bor-intensive. Also, because solvent
wicking can cause a crust to form on
the cake, the product often requires
milling, screening, blending or other
post-drying treatment to ensure ho-
mogeneity. These problems can be
avoided by keeping the cake moving
during drying, which is the major ad-
vantage of cone, paddle and tumble
dryers. Vacuum tray dryers consists
of a main body completely heated by
means of a liquid circulation circuit,
so as to avoid any condensation phe-
nomena, and by a series of shelves
heated by means of a fluid distribu-
tion collector which guarantees heat
homogeneity on all radiating plates.
The inner walls and the shelves are
perfectly polished.
The shelves are heated inside the
vacuum chamber. This technique
can apply heat indirectly to the
product by forcing physical contact
with the shelf. A hot medium flows
through the shelves, thus enabling it
to conduct heat to the tray, which is
positioned on the shelves.
Granulators. Traditional high-shear
granulators in the pharmaceutical
industry are modified to be vacuum
processors by using vacuum to pass
heated air through the product and
vapors of solvents being transported
out of the processor by vacuum.
The flow-through processors (Fig-
ure 6) are where the drying is ac-
complished, by passing the inert gas
through the product and use vacuum
to remove solvents. By incorporating
condenser systems and selecting
the right pumps, excellent solvent
recovery and a competitive drying
rate can be achieved. The bowl can
be swung to provide gentle turning
of product while undergoing drying
under vacuum.
Microwave vacuum drying
Microwaves are a form of electro-
magnetic radiation with frequencies
ranging from 300 GHz to 300 MHz
(Figure 7). Microwave drying refers to
the use of the dielectric heating prin-
ciple, which relies on high-frequency
electromagnetic oscillations caused
by molecular motion. The mecha-
nism for energy transfer during mi-
crowave heating is delivered directly
to materials by molecular interactions
with an electromagnetic field and the
conversion of electrical field energy
into thermal energy.
Microwave drying (Figures 9 and
10) has significant advantages com-
pared with conventional drying, be-
cause in microwave drying, heat is
generated by directly transforming
the electromagnetic energy into ki-
netic molecular energy. Thus, the
heat is generated deep within the
material to be dried. Especially in
microwave vacuum drying, this ap-
proach has significant advantages
for bulk products with poor ther-
mal conductivity. Microwave drying
is comparatively the fastest drying
method available in single-pot sys-
tems (Figure 8).
Because microwave heating is a
form of dielectric heating, the materi-
als’ dielectric properties are thus the
most important factors. The mole-
cules of such substances form elec-
tric dipoles which, when exposed to
an electric field, assume an orientation
relative to the direction of the field. It
is this orientation polarization that is
responsible for generating energy.
In the rapidly alternating electric
field generated by microwaves, polar
materials orient and reorient them-
selves according to the direction of
the field. With the rapid change in the
field at 2,450 MHz, the orientation of
the field changes 2,450 million times
per second and causes rapid re-ori-
entation of the molecules, resulting
in increased kinetic energy, friction
and heat creation.
Freeze drying
Freeze-drying, also known as lyo-
philization, is a dehydration process
typically used to preserve a perishable
material or make the material more
convenient for transport. Freeze-dry-
ing works by freezing the material and
then reducing the surrounding pres-
sure to allow the frozen water in the
material to sublimate directly from the
solid phase to the gas phase.
Freeze drying is divided into three
An initial freezing process, carried 1.
out such that the product exhibits
the desired crystalline structure.
The product is frozen below its eu-
tectic temperature (the highest al-
lowable product temperature dur-
ing the conditions of sublimation)
FIGURE 6: Inert gas can be passed through prod-
ucts to assist solvent removal in vacuum dryers
(Source: GEA Pharma systems)
FIGURE 7: Microwave radiation frequencies range from 300 GHz to 300 MHz
Gamma ray
1 km 1 cm 10-8 cm 10-4 cm 10-2 cm 10-8 cm 10-8 cm
12.2 cm/2.450 MHz
c = f* E = h * f
Radio Microwave
ChemiCal engineering www.Chemengonline.Com april 2015
A primary drying (sublimation) 2.
phase during which the partial
pressure of the vapor surrounding
the product must be lower than
the pressure of the vapor from
the ice, at the same temperature.
The energy supplied in the form of
heat must remain lower than the
product’s eutectic temperature
A secondary drying phase aimed 3.
at eliminating the final traces of
water, where the partial pressure
of the vapor rising from the prod-
uct will be at its lowest levels
Hybrid technologies
Hybrid, or combined, drying tech-
nologies involve implementation of
different modes of heat transfer and
two or more stages of the same or
different type of dryer [2]. The effi-
ciency of drying, in terms of both en-
ergy and process duration, is an area
of intense research, and some of the
most promising drying methods in-
clude the use of electromagnetic
waves and sonic-assisted drying.
The following describes examples of
areas in which vacuum-drying tech-
nologies are employed.
Vacuum drying pharmaceuticals.
In the preparation of mouth-dis-
solving tablets, granules containing
Nimesulide, camphor, crospovidone
and lactose are prepared by a wet-
granulation technique. Camphor was
sublimed from the dried granules by
exposure to vacuum. The porous
granules were then compressed.
Sublimation of camphor from tab-
lets resulted in superior tablets as
compared with the tablets prepared
from granules that were exposed to
vacuum [3].
Drying proteins. Recently, there
has been an increased level of inter-
est in developing drying technolo-
gies as alternatives to lyophilization
in the formulation of proteins as dry
powders. Mannitol is often added
to dried protein formulations as the
bulking agent because it has the
tendency to crystallize rapidly from
aqueous solutions.
Vacuum-drying hybrid technolo-
gies are being investigated to over-
come some of the issues associated
with the process of lyophilization.
These limitations include long pro-
cessing times (typically 3–5 d), ex-
pensive setup and maintenance of
the lyophilization plants, and, most
of all, the steps inherent in freeze-
drying can lead to instabilities in the
protein structure. Due to the complex
structural properties, proteins have a
tendency to denature and undergo
irreversible aggregation during vari-
ous processing steps of drying [4].
Drying bacteria. Normally, probiotic
bacterial strains and starter cultures
are dry-frozen to preserve them until
use. That means, they are first deep-
frozen and afterward, dehydrated in
a vacuum. This procedure has two
major disadvantages in practice: First,
it consumes a very high amount of
energy; and second, some bacterial
strains do not survive temperatures
below 0°C. The Technical University
of Munich ( has devel-
oped low-temperature vacuum dry-
ing (LTVD) for industrial processes.
LTVD can dry unstable substances
at moderate temperatures above
zero without causing too much dam-
age to the cell structure.
Vaccines and other injectables.
Freeze drying is routinely employed
to produce pharmaceutical products
[5]. Pharmaceutical companies often
use freeze-drying to increase the
shelf life of products, such as vac-
cines and other injectables. By re-
moving water from the material and
sealing the material in a vial, the ma-
terial can be easily stored, shipped,
and later reconstituted to its original
form for injection. Another example
from the pharmaceutical industry is
the use of freeze-drying to produce
FIGURE 8: Microwave-assisted vacuum dryers can achieve faster drying times compared to vacuum
alone and inert gas with vacuum (Source: GEA Pharma Systems)
FIGURE 9: Microwave vacuum drying at production scale depends a great deal on a materials’ dielectric
Minutes drying
Basic granulator and cavity unit
Moisture content %
0 15 30 45 60 75 90 105
unit No
Contact thermometer
Video system
(chopper) (mixer)
MW system
Vacuum and condenser system
ChemiCal engineering www.Chemengonline.Com april 2015 53
tablets or wafers, the advantage of
which is that less excipient material
is required. In addition, freeze-drying
allows a rapidly absorbed and easily
administered dosage form.
Food industry. As mentioned
above, reasons for the growing in-
terest on microwave heating can be
found in its peculiar mechanism for
energy transfer: during microwave
heating, energy is delivered directly
to materials through molecular in-
teractions with an electromagnetic
field via conversion of electrical field
energy into thermal energy [6, 7, 8].
This can allow unique benefits, such
as high efficiency of energy conver-
sion and shorter processing times,
thus leading to reductions in manu-
facturing costs due to energy saving
(microwave heating can be a tool of
process intensification). By combin-
ing vacuum with microwave as the
source of thermal energy, the prod-
uct is dried faster and at a lower
temperature, thus avoiding the dam-
age to the product.
By using pulsed-microwave vac-
uum drying, researchers have been
able to dry cranberries [9]. Research-
ers have microwave-vacuum-dried
mint leaves and compared the pro-
cess with air drying [10]. The effective
moisture diffusivity was significantly
increased when microwave drying
was applied under vacuum condi-
tions, compared with hot-air drying.
For color, the microwave-vacuum-
dried mint leaves were light green/
yellow, whereas the hot-air-dried
mint leaves were dark brown.
Plastics production. The plastics
industry uses vacuum to remove
moisture from engineered plas-
tics. If the moisture is not extracted
from pellets before melt processing,
streaks, bubbles, burning, brittle-
ness and other critical defects in the
molded or extruded part can occur.
The textile and lumber industries also
use vacuum drying.
Optimizing drying processes
To optimize dryer performance, it is
important to adjust the peripheral
equipment to match the specific
needs of the drying operation. This
equipment includes the dryers’ heat-
ing and cooling system, dust filter,
condenser and the vacuum-pump.
Condensers are used mostly to re-
cover process solvents, which are
evaporated during drying. They are
typically shell-and-tube type surface
condensers, arranged either verti-
cally or horizontally. Condensate
collection can be measured and
combined with a mass balance to
allow indirect, realtime monitoring of
product moisture during the drying
Drying efficiency. To increase the
efficiency of drying processes, em-
phasis has recently been placed on
the development of new technolo-
gies that use alternative sources of
energy to enhance the heat trans-
fer between the product and heat
source (for example, microwave, ra-
diofrequency and infrared radiation).
New technologies can also intensify
the dehydration rate without increas-
ing the amount of heat supplied to
the product (for example, organic
solvents, ultrasonic waves). Some
of these technologies have already
FIGURE 10: Microwave drying can help avoid
damaging food products (Source: Bohle)
Table 3. vacuum drying applicaTion in food induSTry
Products Dryer type Comment Reference
Strawberries and
Microwave Preliminary drying studies 11
Pistachios Vacuum-infrared modeling 12
Eggplants Vacuum Drying characteristics studies 13
Probiotics Ultrasonic vacuum spray dryer Making highly viable probiotics 14
Chilis Combined microwave-vacuum
rotary drum dryer
Experimental studies 15
Cranberries Microwave-vacuum dryer Energy use and efficiency studies 9
Cranberries Microwave-vacuum dryer Evaluation of quality 9A
Fruits and veg-
Microwave vacuum dryer Drying systems in agricultural
Food (general) Microwave Microwave applications in ther-
mal food processing
Table 2. quanTiTaTive informaTion needed To arrive aT a SuiTable dryer [20]
Dryer throughput; mode of feedstock production (batch versus continuous)•
Physical, chemical and biochemical properties of the wet feed, as well as desired product specifications; •
expected variability in feed characteristics
Upstream and downstream processing operations•
Moisture content of the feed and product•
Drying kinetics; moist solid sorption isotherms•
Quality parameters (physical, chemical, biochemical)•
Safety aspects (fire hazard and explosion hazards, toxicity)•
Value of the product•
Need for automatic control•
Toxicological properties of the product•
Turndown ratio, flexibility in capacity requirements•
Type and cost of fuel, cost of electricity•
Environmental regulations•
Space in plant•
ChemiCal engineering www.Chemengonline.Com april 2015
been developed and tested for use
in freeze-drying foods [18].
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
of dryers.
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 [19]. n
Edited by Scott Jenkins
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17. Mohamed Shaheen et al., Microwave Applications, In
“Thermal Food Processing” Chapter 1, accessed at
18. R. Pisano; D. Fissore; and A.A. Barresi. Sustainable
freeze drying in the pharmaceutical and food industry.
In E. Tsotsas and A.S. Mujumdar (eds.); “Modern Drying
Technology, Volume 5: Process Intensification,” Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim, Germany,
pp. 131–161.
19. Haitham M.S., Lababidi, C., G.J. Baker, An expert sys-
tem for dryer selection using fuzzy logic; Computers
& Chemical Engineering, 23, Supplement, pp. S691–
S694, June 1999.
20. Mujumdar, A. (ed.), “Handbook of Industrial Drying,” 2nd
ed., Marcel Dekker, 1995.
21. Cox, D., Using a batch vacuum dryer to protect workers,
the environment, and heat-sensitive materials. Powder
and Bulk Engineering, pp. 40–46. 1991.
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 [21]
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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For more than 40 years, chemical process industries professionals- engineers, manager
and technicians, have used Chemical Engineering’s Plant Cost Index to adjust process
plant construction costs from one period to another.
This database includes all annual archives (1947 to present) and monthly data archives (1970 to present).
Instead of waiting more than two weeks for the print or online version of Chemical Engineering to arrive,
subscribers can access new data as soon as it’s calculated.
Subscribe today at
Jan Feb Mar Apr May Jun Jul Au g Sep Oct Nov Dec
Sep ‘06
Aug ‘06
Sep ‘05
CE Index 513.1 510.0 467.2
Equipment 606.5 602.3 541.2
Heat Exchanges and Tanks 565.1 560.9 509.2
Process Machinery 559.6 556.2 521.7
Pipe, valves and fi ttings 734.7 731.7 620.8
Process Instruments 441.4 437.2 379.5
Pumps and Compressions 788.9 788.3 756.3
Electrical equipment 418.9 414.2 374.6
Structural supports 643.7 637.7 579.3
Construction Labor 314.7 312.9 309.1
Buildings 476.9 475.2 444.7
Engineering Supervision 350.7 351.9 346.9
Resources included with Chemical
Engineering’s Plant Cost Index:
Electronic notifi cation of monthly updates
as soon as they are available
• All annual data archives (1947 to present)
• Monthly data archives (1970 to present)
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... The other advantages of vacuum drying are no oxidative reactions as well as the preservation of appearance, color, quality, texture, vitamins, nutrients, etc. Therefore, vacuum dryers are considered to be suitable for drying products with bioactive components such as food and medicine [81]. ...
... The total energy demand of the vacuum dryer is the sum of electrical energy consumption of the vacuum pump and thermal energy needed to heat the drying tray or shelf. The operating temperature, therefore, the thermal energy requirement, has a higher effect on the energy consumption of a vacuum dryer [81]. ...
... In cases where such data are not found, the corresponding method is not taken into account for those performance criteria. Figure 8 shows the final heat use comparison of agitated thin film (data from [23][24][25][26]), refractance window (data from [66][67][68][69][70][71]), superheated steam (data from [76][77][78][79]), and vacuum drying (data from [80][81][82][83][84]). Here, it can be seen that agitated thin film and vacuum drying have the lowest final heat use of 0.73 kWh/kg water removed because of their low direct heat requirement. ...
Conventional drying technologies generally rely on drying processes driven by using hot air, where these processes have significantly high energy consumption and greenhouse gas emissions. With our knowledge based on the limited nature of fossil fuels and the impact of climate change, it is clear that every technology has to be cleaner, cheaper, and more efficient to limit our carbon footprint while maintaining sustainable development. In this regard, the present study comprehensively reviews the recent advances in drying technologies and energy consumption and the efficiency and economic performances of several alternative drying technologies. The reviewed drying technologies include adsorption mediated, agitated thin film, electrotechnologies, hybrid systems, impingement, heat pumps, microwave, ohmic-heating, refractance window, rotating drum, superheated steam, and vacuum. The selected performance criteria are final heat use, final electricity use, primary energy use, thermal and energy efficiencies, and operating cost. With these criteria taken into account, the results show that the refractance window has the highest sustainability ranking (8.28/10), and superheated steam has the lowest sustainability ranking (2.90/10). Besides, the sustainability of the different drying fuels and technologies is conducted, and the results show that renewables have the highest sustainability ranking. Besides, it is seen that some novel approaches in drying, such as solar-based hybrid drying systems and refractance window drying, are amongst the most promising sustainable technologies. They could be the key to clean, affordable, reliable, and efficient drying of different kinds of products in industrial processes.
... The food is placed on trays inside the shelves. The product is indirectly heated by conduction and radiation through the shelves (Parikh, 2015). A vacuum pump is used to build up the vacuum and reduce the chamber pressure. ...
... Depending on product properties and drying parameters, drying times differ between 4 and 20 h (Brennan, 2011). On the one hand, with vacuum-drying of fruits or vegetables better product quality is reached compared to air-dried products, due to lower heat exposure and less heat damage risks (Parikh, 2015). On the other hand, costs of vacuum dryers are higher, and throughput is lower, compared to most types of conventional hot air dryers. ...
Dehydration is a technique that has been used since ancient times. The need to develop more efficient processes to obtain dehydrated foods of higher quality from the organoleptic and nutritional point of view has led to the study of different techniques. For instance, convection drying, freeze-drying, spray drying, vacuum drying, microwave vacuum drying, infrared radiation drying, osmotic dehydration, among others have been investigated. Over the last years, pulsed electric fields (PEF)-assisted drying has attracted the interest of several researchers due to its ability for reducing drying time, preserving at the same time some thermolabile compounds which are responsible for the aroma, nutritional and bioactive properties of food products. Therefore, in this article, some of the most important studies regarding the application of PEF-assisted drying in food processing will be discussed.
... Vacuum drying is one of the most preferred methods to overcome the disadvantages observed in HAD. During vacuum drying, the decrease in the vapor pressure shortens the drying time and provides the opportunity to work at low temperatures (Bhandari & Adhikari, 2008;Parikh, 2015). Besides, the absence of oxygen during vacuum drying reduces the degradation of bioactive components such as phenolic compounds and vitamins (Karaaslan et al., 2014;Karasu et al., 2015). ...
Full-text available
This study aimed to investigate the effect of different drying methods on the drying kinetic, total bioactive content, in-vitro bioaccessibility of bioactive compounds, and color and microstructural properties of pomegranate arils. Drying methods significantly affected all selected parameters of dried pomegranate arils (P<0.05). Freeze-drying (FD) showed higher bioactive compounds, lower shrinkage, and better color quality than those of other samples. Ultrasound-assisted vacuum drying (UAVD) showed lower drying times, lower shrinkage, and higher bioactive compounds than vacuum drying (VD) and hot air drying (HAD). The ABTS, CUPRAC, and DPPH results were 34.53-63.71 μmol TE/g, 29.70-61.60 μmol TE/g and 64.82-93.69% DPPH radical scavenging activity, respectively. Also, the highest values of antioxidant capacity for all methods were obtained from the samples dried with FD followed by UAVD, VD, and HAD. The recovery of TPC in dried and fresh samples changed from 2.58% (fresh pomegranate) to 10.32% (vacuum-dried pomegranate). The recovery of TPC for freeze-dried samples (2.62%) was closest to the fresh ones and VD showed the highest TPC recovery. This study suggested that UAVD and VD should be used as alternative methods to HAD due to higher bioactive compounds retention, better color and surface quality, and higher recovery of bioactive compounds.
... The drying process, which removes the air surrounding the product inside a closed chamber by a vacuum pump, and results in lower pressure for artificially increasing the water vapor pressure difference between the product and surrounding, is called vacuum drying. The unbound moisture can be easily removed from the heat-sensitive food product using vacuum drying (Parikh, 2015). The same technology has been successfully used in raisin processing where the grapes are dried under subatmospheric pressure. ...
Full-text available
Raisins are highly produced value‐added products that are extracted from grape processing. The most important unit operations in raisin production are pretreatment, followed by drying. Due to the presence of waxy layer, the drying cannot be used alone; however, the layer can be removed by pretreating the grapes with chemicals or physical abrasion. The food safety and environmental issue with the chemicals have forced to find alternative methods. The physical method, such as surface abrasion, is explored but showed negative effect of color. The nonthermal methods such as microwave (MW) heating, pulsed electric field, ultrasound, and ohmic heating are observed, and their outcomes are discussed in this review. Carbonic maceration, high‐humidity hot air impingement blanching, and MW hydrodiffusion and gravity are the novel methods that showed promising results. In the latter part, the traditional drying process of grapes is discussed, where shade drying is found to be used remarkably in raisin production. However, the time of drying was long, and the process required large land area. Alternatively, novel methods such as hot air, MW, vacuum, and infrared drying provide higher drying rate and good‐quality raisins. However, the combined drying methods such as MW‐assisted air drying, pulsed vacuum drying, and MW vacuum drying have shown better results than single drying. Pretreatment and drying cause changes in physical and chemical properties of grapes, such as changes in color, size, nutrient content, and polyphenol content. Overall, the review covered the crux output with the advancement in raisin development, which could be useful for novel research on grape processing.
... Vacuum drying has been utilized in the pharmaceutical and food industry as a viable drying process. It offers many advantages such as low energy consumption and prevents uneven drying and is less damaging to the sample being dried (Parikh 2015). The vacuum dryer apparatus included a VDO-23i commercial-grade vacuum oven and RS-2 vacuum pump; in addition, temperature, pressure, power, and weight sensors were used to monitor the experiment conditions including the change of weight of the microalgae (Fig. 1). ...
Full-text available
Algal-based biofuels offer distinct advantages over other types of biofuels currently available within the fuel industry. However, one important disadvantage is that over their entire life cycle, they consume significant amounts of energy through cultivation, pretreatment, and production. Under pretreatment, drying is an energy-intensive yet highly critical process in standardizing the production of algal biofuel products. The current study proposes the use of artificial intelligence (AI) methods in optimizing the efficiency of a vacuum drying process. Previously, vacuum drying was modeled using least-squares regression methods, which captured the general linear or non-linear trend of the samples, but secured poor accuracy for individual sample points. In addition, these methods are unsuitable for online parameter optimization. Three AI-based models were developed to model the vacuum drying process, specifically an artificial neural network (ANN), a support vector machine (SVM), and an extreme gradient boosting machine (XGB). Based on error values, the ANN (RMSE = 0.0437) and XGB (RMSE = 0.0308) outperformed polynomial regression, and all models obtained meaningful lower values than multivariate linear regression (MLR). There is a notable difference in the ability of XGB to approximate individual sample points, particularly at high and low tail-ends of the dataset. Overall, the AI methods exhibited higher accuracy in estimating the drying characteristics for the chosen strain of algae. The current study may be extended to optimization by relating the control parameters to energy consumption, and automation based on the mathematical model. Graphical abstract
... Since the physical and chemical properties of dried foods depend on the drying methods and drying conditions, it is critical to determine the appropriate drying method and drying conditions. Vacuum drying is a process in which materials are dried in a reduced pressure environment, which lowers the heat needed for rapid drying (Parikh, 2015). ...
Full-text available
Thanks to their high fiber, antioxidant activity, vitamin and mineral content, dried fruits are much healthier and more nutritious snacks than other processed snack foods. However, many research shows that certain compounds may degrade in the drying process, notably antioxidants, so drying method is very important to obtain desirable and healthy dried products. In this study, the comparison of quality characteristics of fluidized bed, microwave and vacuum dried pineapples was investigated. Pineapple slices were dried using fluidized bed dryer at 80 °C for 3 h. Vacuum drying treatments were performed using vacuum oven at 21.5 kPa and 60 °C for 12 h. Drying treatments in a microwave oven were performed at 360 W for 24 min. Color, rehydration capacity, shrinking ratio, antioxidant activity, total phenolic content measurements and sensory analysis were conducted in order to compare the physical, chemical and sensory properties of dried pineapples. Microwave dried samples had the highest L* value, total phenolic content and antioxidant activity compared to other samples (p < 0.05). On the other hand, vacuum dried samples took the highest flavor and overall impression values by the panelists (p < 0.05). When it was compared to other drying methods used in this study, fluidized bed drying caused the highest decrease in physicochemical, antioxidant activity and of pineapple samples. In the result of this study, it was revealed that being quick, cheap, and practical method, microwave drying has a promising potential for high-quality dried products.
... Vacuum drying is one of the major operational processes in numerous industries such as chemical, pharmaceutical, food, plastic, textile etc. [1]. Modeling and simulation of vacuum drying process with a view to predict system properties (i.e., temperature, pressure, water content) has been therefore a topic of interest in the respective research community. ...
... Vacuum drying biasanya digunakan untuk mengeringkan zat yang hidroskopis dan sensitif terhadap panas sehingga ditambahkan vakum untuk menurunkan tekanan di bawah tekanan uap air dan menurunkan titik didih air untuk menigkatkan kecepatan penguapan [10]. Pada oven vacuum suhu pengeringan dapat diturunkan dengan adanya vakum sehingga tidak merusak bahan. ...
Full-text available
Serbuk lidah buaya merupakan salah satu produk olahan lidah buaya yang banyak digunakan industri kosmetik sebagai bahan tambahan. Tujuan dari penelitian ini adalah untuk menentukan kondisi operasi dan konsentrasi zat aditif dengan variasi temperatur operasi (50oC, 60oC, dan 70oC) dan variasi konsentrasi maltodekstrin (6%, 7%, 8%, dan 9%) dalam pembuatan serbuk lidah buaya untuk mencapai kandungan antrakuinon terendah dan kandungan glukomanan tertinggi. Bahan yang digunakan adalah lidah buaya (Aloe Vera barbadensis.) yang dilakukan ­perlakuan awal sebelum pengeringan. Serbuk lidah buaya diukur kadar air, rendemen, kandungan antrakuinon dan kandungan glukomanan. Hasil menunjukan bahwa pada temperatur 60oC dan konsentrasi maltodekstrin 8%, serbuk lidah buaya memiliki kadar air 6,92%; rendemen 9% kandungan antrakuinon 20,8%; dan kandungan glukomanan 11,5% merupakan hasil terbaik.Aloe vera powder is one of processed aloe products are widely used cosmetic industry as an additive. The purpose of this study was to determine the optimum operating conditions and concentrations of additives by variations in the operating temperature (50, 60, and 70°C) and the variation of the concentration of maltodextrin (6, 7, 8, and 9%) in the manufacture of aloe vera powder to reach the lowest anthraquinone content and highest glucomannan content. Materials used are pre treated aloe vera (Aloe Vera barbadensis.) before drying process. Aloe vera powder measured water content, yield, anthraquinone content, and glucomannan content. Results showed that at temperature of 60° C and a concentration of 8%. Aloe vera powder has a water content of 6,92%; 9,00% yield; glucomannan content of 11,50%; and 27.30% anthraquinone content is the best results.
Lingzhi (Ganoderma lucidum) is a high‐potential mushroom which is used as a dietary supplement or a source of nutrients and antioxidant agents. The aims were to investigate the effect of drying conditions (hot air and vacuum dry) and to compare the extraction methods of microwave‐assisted extraction (MAE) and pulsed electric field extraction (PEF) for obtaining triterpenes, flavonoid and polysaccharides. The favourable drying condition was 80 °C for 1 h and 37 min, which achieved 1.17 ± 0.05 mg quercetin/g of flavonoids and 11.49 ± 0.87 mg ursolic acid/g of triterpenes. The MAE condition was 800 W, 1 min 30 s extraction time and 65.35% ethanol, achieving polysaccharides of 13.08 mg glucose/g and triterpenes of 9.15 mg ursolic acid/g. Additionally, MAE was significantly more efficient than reflux and PEF methods. The application of microwave radiation is beneficial in saving time and solvent and also provides high extraction efficiency for dried Lingzhi.
Full-text available
The vacuum-infrared drying of two varieties of Iranian pistachios (Khany and Abasali) was performed in a laboratory scale vacuum dryer, which was developed for this purpose. A non-linear regression Logarithmic model represents good agreement with experimental data with coefficient of determination and mean square of deviation as 0.9942 and 2.035×10 -5 for Khany variety and 0.9951 and 2.365×10 -5 for Abasali pistachios respectively. Using combined vacuum infrared radiation to drying pistachios has a number of advantages. Pistachios can be dried approximately eight to ten times faster than dryers that rely on conventional convection and conduction processes only; drying at near room temperatures is the most economical way and it seems the quality in the drying process is enhanced. Citation: Ahmad Kouchakzadeh, Kourosh Haghighi. 2011. Modeling of vacuum-infrared drying of pistachios. Agric Eng Int: CIGR Journal, 13(3): -.
Dryer selection can have a major impact on product quality, particularly in the case of thermally sensitive materials such as foodstuffs. This paper discusses some of the principal factors that influence this process. The use of a fuzzy expert system as an aid in the preliminary selection of a batch dryer is described. This incorporates a novel ‘multiple goal’ approach in which different facets, such as dryer type, single or multiple dryers, and atmospheric or vacuum operation, are determined independently.
The vacuum drying characteristics of eggplant were investigated. Drying experiments were carried out at vacuum chamber pressures of 2.5, 5 and 10 kPa, and drying temperature ranging from 30 to 50 °C. The effects of drying pressure and temperature on the drying rate and drying shrinkage of the eggplant samples were evaluated. The suitable model for describing the vacuum drying process was chosen by fitting four commonly used drying models and a suggested polynomial model to the experimental data; the effective moisture diffusivity and activation energy were calculated using an infinite series solution of Fick’s diffusion equation. The results showed that increasing drying temperature accelerated the vacuum drying process, while drying chamber pressure did not show significant effect on the drying process within the temperature range investigated. Drying shrinkage of the samples was observed to be independent of drying temperature, but increased notably with an increase in drying chamber pressure. A linear relationship between drying shrinkage ratio and dry basis moisture content was observed. The goodness of fit tests indicated that the proposed polynomial model gave the best fit to experimental results among the five tested drying models. The temperature dependence of the effective moisture diffusivity for the vacuum drying of the eggplant samples was satisfactorily described by an Arrhenius-type relationship.
Color, texture and water activity of microwave-vacuum dried cranberries were evaluated and compared with the corresponding properties of hot-air dried and store-bought cranberries. The microwave drying was done both in the pulsed and continuous modes. The microwave-vacuum dried cranberries were redder and had a softer texture than those dried by the conventional hot-air method. The storage stability of the product dried by microwave-vacuum method was comparable to that of conventionally-dried cranberries. The microwave operating conditions have an effect on the quality of the dried cranberries.
Mint (Mentha cordifolia Opiz ex Fresen) was subjected to microwave vacuum drying and hot air drying, respectively. For microwave vacuum drying, three microwave intensities i.e. 8.0 W g−1, 9.6 W g−1 and 11.2 W g−1 were applied with pressure controlled at 13.33 kPa. For hot air drying, two drying temperatures of 60 °C and 70 °C were examined. Lewis’s, Page’s and Fick’s models were used to describe drying kinetics under various drying conditions. Effective moisture diffusivities were determined to be 4.6999 × 10−11, 7.2620 × 10−11, 9.7838 × 10−11, 0.9648 × 10−11 and 1.1900 × 10−11 m2 s−1 for microwave vacuum drying at 8.0 W g−1, 9.6 W g−1 and 11.2 W g−1, hot air drying at 60 °C and 70 °C, respectively. The microwave vacuum drying could reduce drying time of mint leaves by 85–90%, compared with the hot air drying. In addition, color change during drying was investigated. Lightness, greenness and yellowness of the microwave vacuum dried mint leaves were higher than those of the hot air dried mint leaves. From scanning electron micrographs, the microwave vacuum dried mint leaves had a more porous and uniform structure than the hot air dried ones. From rehydration test at 30 °C, rehydration rate constants of the dried mint leaves by the microwave vacuum drying at 9.6 W g−1 and 11.2 W g−1 microwave intensity were significantly higher than those by the hot air drying at 60 °C and 70 °C (p ⩽ 0.05).
Microwave-vacuum drying was investigated as a potential method for drying cranberries. Cranberries were pretreated with either 30°B or 60°B high fructose corn syrup solution for 24 h. They were dried using a laboratory-scale microwave-vacuum oven operating either in continuous or pulsed mode until the final moisture content reached 15% (wet basis). In the continuous mode, two levels of microwave power (250, 500 W) and absolute pressure (5.33, 10.67 kPa) were applied. In the pulsed mode, microwave power of 250 W and two levels of pressure (5.33, 10.67 kPa) were used with two levels of power-on time (30, 60s) and three levels of power-off time (60, 90, 150s).Pulsed application of microwave energy was more efficient than continuous application. In both cases, drying efficiency improved when lower pressure (5.33 kPa) was applied. Shorter power-on time and longer power-off time provided more favorable drying efficiency in pulsed mode. Power-on time of 30 s and power-off time of 150 s was the most suitable setting for maximum drying efficiency.
This paper analyses transport phenomena that occur during microwave heating. In particular, a transient one-dimensional energy balance equation has been adopted to describe the heating of a body in a microwave cavity (single-mode applicator). In the energy balance, a kinetic term has been introduced to take into account released or absorbed heat due to chemical reaction. Thus, the energy equation has been coupled with a mass balance and the relevant initial and boundary conditions. Modeled heating profiles are shown to describe the energy transfer in different materials and, the influence, on the heating process, of the fluid-dynamics outside the microwave cavity is studied and discussed.