This document discusses various drying processes used in pharmaceutical manufacturing. It begins by describing the objectives of drying and then discusses different evaporation methods like hot air drying, infrared drying, and vacuum drying. The main types of dryers covered are drum dryers, spray dryers, tray dryers, tunnel dryers, rotary dryers, fluidized bed dryers, vacuum dryers, and freeze dryers. Key factors that influence the selection of drying methods and equipment are also outlined.

1. Drying processes
3rd year of Pharmacy
Dr. Kuyhuong NAY, PharmD
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2. Objectives
At the end of this course, the student will be able to :
- Describe the different drying process in
manufacture of drug preparations.
- Identify the type and mechanism of dryer used for
drying process.
- Choose the method and equipment used for
drying process.
- Explain the advantage, disadvantage and used of
the process in pharmaceutical application.
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3. Table content
• Introduction
• Evaporation method
• Types of dryer
• Factors affecting method and equipment
• Dryer for dilute solutions and suspensions
• Dryers for solid materials
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4. - Drying is defined as a process in which the
liquid is removed from a material by the
application of heat and is accomplished by the
transfer of a liquid from a surface into an
unsaturated vapor phase.
- Objectives of drying :
• Eliminate all volatile liquid contained in non
volatile materials
• Get rid of water contained in solid materials
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Introduction

5. - Drying is needed for the following purposes :
For the preparation of granules which can be
dispensed in bulk, compressed in the form of tablets
or filled in capsules.
For the preparation of certain products like dried
aluminium hydroxide, spray dried lactose and
powdered extracts.
For reducing the bulk and weight, thereby lowering
the cost of transportation and storage.
Vegetable drugs are dried before extraction to
facilitate grinding and to avoid deterioration on
storage.
As dried products are more stable than moist ones
therefore heat is required for drying. 5

6. Evaporation methods
• Hot air : discontinuous dryer, continuous
dryer.
• Infrared
• Under vacuum
• Drying in the presence of desiccants
• Cylinder drying
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7. - Dryers for dilute solutions, suspensions :
Drum dryer
Spray dryer
- Dryers for damp solid materials :
Tray or shelf dryer
Tunnel dryer
Rotary dryer
Fluidised bed dryer
Vacuum dryer
Freeze dryer
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Types of dryer

8. - Various methods and equipments for drying are available
the choice of which depends on the following factors :
Product is sensitive to heat or not.
Physical characteristics of the product before drying.
Nature of the solvent to be removed.
Process is to be carried out under aseptic conditions
or not.
Quantity of the product to be dried.
Available source of heat.
Cost involved.
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Factors affecting method and
equipment

9. Dryer for dilute solutions and
suspensions
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10. Dryer for dilute solutions and
suspensions
• The objective of these dryers is to spread the
liquid to a large surface area for heat and mass
transfer and to provide an effective means of
collecting the dry solid.
• Two main types are used :
- Spreading the liquid to a thin film
- Dispersing the liquid to a spray of small
droplets.
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11. - Consists of a drum 0.75 to 1.5 m in diameter and 2-4 m in length,
heated internally, usually by steam, and rotated on its longitudinal
axis.
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- The liquid to be dried is kept in a
feeding pan in which the heated
drum is applied to the surface and
spreads to a film which dries
rapidly.
- The dried product is scrapped from
the surface of the drum by means
of a doctor knife, in the form of
flakes.
- The drying rate can be controlled by adjusting rotation speed
and temperature of the drum (140-150oC, 6-30t/mn)
Drum dryer (Film dryer)

12. - Avantages :
 The method gives rapid drying, the thin film spread over a large area
resulting in rapid heat and mass transfer.
 The equipment is compact, occupying much less space than the spray dryer,
for example.
 Heating time is short, being only a few seconds.
 The drum can be enclosed in a vacuum jacket, enabling the temperature of
drying to be reduced.
 The product is obtained in flake form, which is convenient for many
purposes.
- Disadvantage is that operating conditions are critical and it is necessary to
impose careful control on :
 feed rate,
 film thickness,
 speed of rotation and
 temperature of drum. 12
Drum dryer (Film dryer)

13. • Uses :
It can handle a variety of materials, either
as solutions or suspensions, substances
that are dried by this method include :
- milk products,
- starch products,
- ferrous salts, and
- suspensions of kaolin or zinc oxide.
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Drum dryer (Film dryer)

14. - Provides a large surface area for
heat and mass transfer by
atomizing the liquid to small
droplets.
- These are sprayed into a stream of
hot air, so that each droplet dries
to a solid particle.
- The disc of the atomizer is driven
by an air turbine and spins at
35000 rpm.
- Air is introduced with the help of
fan, which is heated by means of
electric heater to a maximum
temperature of 350oC. 14
Spray dryer

15. • Advantages :
Droplets are small, giving a large surface area for heat
and mass transfer, so that evaporation is very rapid.
Because evaporation is very rapid, the droplets do not
attain a high temperature.
Characteristic particle form gives the product a high
bulk density and, in turn, ready solubility.
Powder will have a uniform and controllable particle
size.
Product is free-flowing, with almost spherical
particles, and is especially convenient for tablet
manufactures.
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Spray dryer

16. • Disadvantages :
The equipment is very bulky (15 m in height and 6 m
in diameter) and with the ancillary equipment (fans,
heaters, separations, etc.) is expensive.
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Spray dryer

17. • Uses :
The substances that are spray dried
include :
- borax, citric acid, hexamine, sodium
phosphate, gelatin, acacia and
extracts,
- starch barium sulphate and calcium
phosphate, milk, soap and detergents.
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Spray dryer

18. Dryers for solid materials
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19. Tray or shelf or compartment dryers
- Is a cabinet with a heater, usually at
the bottom to assist convection, but it
is of limited value, giving virtually no
control of heat transfer or humidity.
The situation can be improved by
including a fan, so that forced
convection takes place, with increased
heat transfer and reduced local vapor
concentrations, but control is still
inadequate.
- Uses of the compartment dryer are
very varied because of the versatility
of the method, and include the drying
of crude drugs, chemicals, powders,
tablet granules, or items of
equipment. 19

20. - The drying method resembles the compartment dryer,
but takes the form of a long tunnel, with heated air
entering at one end and some means of moving the
material to be dried at the opposite end.
- As compared to the tray dryers, the tunnel dryer has
the advantage that it is semi-continuous or continuous
in operation, applications are similar, but more suitable
for large-scale production.
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Tunnel dryers

21. - Is a modified form of the tunnel dryer in which the
particles are passed through a rotating cylinder,
counter-current to a stream of heated air. Due to the
rotation of the cylinder, the material is turned over
and drying takes place from individual particles and
not from a static bed.
- The rotary dryer is used for continuous drying on a
large-scale of any powdered or granular solid.
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Rotary dryer

22. - This technique is very efficient and is used for drying granular
solids. With these types of dryers materials ranging from 5kg
to 200kg can be dried in about 20-40min.
- It consists of stainless steel chamber with a perforated
bottom, into which the wet material to be dried is placed.
For loading and unloading the drying chamber is removed
from the unit. The air is introduced from below which is
heated by means of heaters fitted therein, filtered and then
passes through the powder bed by means of induction fan
fitted in the upper part of the apparatus. The air is heated to
the required temperature and its flow rate is adjusted.
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Fluidised Bed Dryer

23. 23
Fluidised Bed Dryer

24. - Advantages :
The material can be dried within 20-40 min
compared with several hour in tray dryers.
Drying takes place from individual particles and not
from whole bed.
Temperature of a fluidised bed is uniform and can
be controlled to the desired temperature.
A free flowing product is produced.
Since the containers are mobile which make
handling easy thereby labor costs are reduced.
Due to short drying time the unit has a high output.
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Fluidised Bed Dryer

25. • Disadvantages :
Due to turbulence of the fluidised state, large
quantities of fines are produced but the use of
suitable binders during granulation can
overcome this defect.
Too wet granules stick together on drying
therefore too wet granules must not be used.
Many organic powders develop electrostatic
charges during fluidisation so efficient
electrical earthing of the drying chamber and
the cloth filters is necessary.
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Fluidised Bed Dryer

26. - Is a modified form of vacuum still and consists of a jacketed
vessel sufficiently hard in construction to withstand vacuum
within the oven and steam pressure in the jacket. The oven is
connected through a condenser and receiver to a vacuum pump,
the vacuum pump can be connected directly to the oven. For
drying the material is spread on trays which are placed on steam
heated shelves in the jacket of vacuum oven.
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Steam or
Water Jacket
Condenser
To Vacuum Pump
Receiver
Vacuum dryer (Vacuum oven)

27. - Advantages :
Thermolabile substances can be dried at a low
temperature and chances of oxidation are
reduced.
During the manufacture of a dry extract, a
porous and friable product is obtained which
is especially useful in tablet manufacturing.
By the use of condenser the solvent can be
recovered from the product.
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Vacuum dryer (Vacuum oven)

28. - Disadvantages :
Heat transfer may be low and non uniform.
Capacity of vacuum oven is limited therefore may
not be suitable for large quantities.
Labor and running costs are high.
After drying a grinding process is necessary to get
a powdered product.
There is danger that the finally powdered
material may be drawn off by the vacuum pump,
and filters which require frequent cleaning have
to be provided.
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Vacuum dryer (Vacuum oven)

29. Infrared drying
• The most effective wavelengths are of 10000
and 12000Ao, is the caloric effects of infrared
radiation that are used.
• The heat energy is directly transferred from the
infrared source to the product to be heated
without the need of an intermediate such as air
or water. All materials will absorb, reflect
and allow a fraction of the infrared spectrum to
pass through.
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30. Infrared drying
• It is important to select the infrared emitters with the suitable
spectra according to the product to have an optimized
absorption of the radiation that will be subsequently
converted into heat and the product should be distribute in
the thin film to be dry. The lamps used are tungsten filament
lamps , are placed approximately 30cm of the product to dry
out. Drying can be done on trays or in a tunnel on an endless
belt .
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31. Drying in the presence of desiccants
• Drying in the presence of desiccants is performed generally
under vacuum at room temperature where the vapor is
captured by a chemical bonding of water molecules : a
desiccant.
• A desiccant is a substance that absorbs water. It is most
commonly used to remove humidity that would normally
degrade or even destroy products sensitive to moisture.
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32. Drying in the presence of desiccants
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• This table shows that P2O5 is the best desiccant, it deprives the
atmosphere almost completely moisture . Most used for drying are P2O5
and H2SO4 pure. The others are mainly used to prevent rehydration
products stored in a closed container or a packaging container . Anyway,
the absorption capacity of these desiccant is very limited. They can only be
used for drying or dehydrating a little substance rich in water.
Residual water in mg per liter of water
Calcium Chloride ……………………………
Potassium hydroxide ……………………..
H2SO4 95% …………………………………….
Silicagel ………………………………………….
Potassium hydroxide ……………………..
Calcium Sulfate ………………………………
H2SO4 pure …………………………………….
Phosphorus pentoxide (P2O5)………….
1,5
0,8
0,3
0,03
0,014
0,005
0,003
0,00002

33. - Freeze drying (Lyophilisation or Cryodessication ) Is the process in which
water is removed from liquid product by sublimation hence this process
is also known as sublimation drying.
- The frozen water in the material is vaporized directly from the solid
phase to the gas phase without intermediate passing through the liquid
state .
- The sublimation is obtained industrially under a vacuum of 0.10 mm Hg
with a corresponding vaporization temperature of -40oC.
- Usually, the normal pressure 760mmHg at temperature below 0OC, the
water is in solid form (ice) but it is in liquid form at 100OC which it
passes as vapor at this temperature. For a pressure 4,58mmHg at
0,0076oC, the three phases are in equilibrium, it is the triple point.
- The diagram show that it is possible to change directly of the substance
from solid to vapor by making a small changes in pressure and
temperature. 33
Freeze drying

34. Freeze drying
In thermodynamics, the triple point of a substance is the temperature and
pressure at which the three phases (gaz, liquid, and solid) of that substance
coexist in thermodynamic equilibrium.
Ex : Triple point of water is at 4.58mmHg-0.0076oC.
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35. Freeze drying
A freeze dryer comprises two chambers connected to
each other, the chamber A (sublimation or evaporator) is
cooled to a temperature which allows the freezing of the
product to dry, for example -40oC. At this temperature,
the sublimation of the ice is achieved in a pressure in the
chamber is less than or equal to 0,1mmHg, which is
achieved through a vacuum pump. In chamber B
(condenser) refrigerant system of the freeze dryer allows
to be at a lower temperature, for example -50 ° C, this
temperature allows the saturation vapor pressure,
obtained in A, to be reduced in B. This imbalance permits
movement of the steam from A to B,.
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36. Freeze drying
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37. • Uses :
- Freeze drying is used for the manufacture of
certain pharmaceutical and biological
products which are thermolabile.
- It is also used for drying :
Blood plasma
Vitamins
Enzymes
Hormones
Antibiotics.
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Freeze drying

38. References
1. Introduction to pharmaceutics-1. Ashok
K.Gupta. 3rd edition. 1991.
2. A. LEHIR -Pharmacie Galénique, bonnes
pratiques de fabrication des médicaments.
Masson. Paris. 8ème édition 2001.
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