Definition of drying
Importance of drying
Difference between drying and evaporation
Drying is defined as removal of the liquid from a material by
application of heat & is accomplished by transfer of a liquid
from the surface into an unsaturated vapor phase .
Drying is the final removal of water from material (usually by
Drying is commonly the last stage in a manufacture process
1- As Squeezing wetted sponge
2- Adsorption by desiccant (desiccation)
Difference between drying and
Sl. No. Drying Evaporation
1. In drying processes, the main operation
usually carried out on solid materials, e.g.
powders, or products
In evaporation processes, the main
operation usually carried out on
liquid materials, e.g. solution, or
2. Drying in most of the cases means the
removal of relatively small amounts of
water from solids
Evaporation include the
removal of large amounts of
water from solutions.
3. In most cases, drying involves the
removal of water at temperatures
below its boiling point
Whereas evaporation means the
removal of water by boiling a
4. In drying , water is usually removed
by circulating air over the material in
order to carry away the water vapour
While in evaporation , water is
removed from the material as
pure water vapour mixed with
other gases. 4
Preservation of drug products
Preparation of bulk drugs
• Drying is necessary in order
to avoid deterioration. A
few examples are…
• --blood products, tissues…
undergo microbial growth
• --effervescent tablets,
synthetic & semi synthetic
drugs undergo…. chemical
• Drying is the final stage
of processing . Eg:
• *dried aluminium
• *spray dried lactose
• *powdered extracts (in
Drying produces materials of spherical shape,
uniform size, free flowing & enhanced
solubility. Some specific areas of imp are…
1. Granules are dried to improve the fluidity
& compression characteristics. These are
essential for production of tablets and
2. Viscous & sticky materials are not free
flowing. Drying modifies these characteristics
Removal of moisture makes the material light in weight and reduces bulk.
Thus cost of transportation will be less & storage will be efficient.
If moisture is present, size reduction of drugs is difficult.
Drying reduces the moisture content.
It is the amount of water (moisture) held by the material that
exerts an equilibrium vapour pressure equal to that of pure
water at the same pressure.
Bound water (moisture)
It is the minimum water(moisture) held by the material that
exerts an equilibrium vapour pressure less than the pure water
at the same temperature.
-Cell and fibre walls
Vapour pressure of
wet solids is less
than the v.p of pure
Water in void spaces
Vapour pressure of
wet solids is equal to
v.p of pure water
Drum Dryer (Film Drying)
It consists of a drum of about 0.75-1.5
m in diameter and 2-4 m in length,
heated internally, usually by steam and
rotated on its longitudinal axis.
Working: The liquid is applied to the
surface and spread to a film, this may be
done in various ways, but the simplest
method is that shown in the diagram,
where the drum dips into a feed pan.
Drying rate is controlled by using a
suitable speed of rotation and the drum
temperature. The product is scraped
from the surface of the drum by means
of a doctor knife.
It is used for drying solution, slurries, suspensions etc and also the dry
products like this
• Milk products
• Starch products
• Ferrous salts
• Suspension of zinc oxide & kaolin
Drying time is less, only in few seconds.
Drum dryer occupies less space
Product are obtained is completely dried and it’s final form
Rate of heat & mass transfer is high
• Maintenance cost is higher
• Skilled operators are essential to control feed rate
• Not suitable for solution of salts with less solubility
Working : Drying of the material in spray dryer involves 3 steps
(1) Atomization of the liquid
(2) Drying of the liquid droplets
(3) Recovery of the dried product
The spray dryer 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
The drying chamber resembles the cyclone ensuring good circulation of air, to
facilitate heat and mass transfer, and that dried particles are separated by the
Spray dryer can be operated efficiently at various feed rates.
Product obtained are better than other dryer
Product quantity to be dried is large
It is continuous process & dried completely 3 to 30 sec.
Labor cost is low
Product of uniform & controllable size is obtained
Products shows excellent solubility
Suitable for drying of sterile products
It is bulky and expensive
Huge equipment not always easy to operate
Thermal efficiency is low
Air flows in direction of the arrows over each shelf in turn.
The wet material is spread on shallow trays resting on the shelves.
Electrical elements or steam-heated pipes are positioned as shown, so that the air is
periodically reheated after it has cooled by passage over the wet material on one
shelf before it passes on the next.
Handling of the material can be done without looses
Tray dryer is operated batch-wise
(a) Each batch of material are separate entity
(b) Equipment is readily adjusted for use in drying a wide
variety of material
It require more labour to load and unload
Fluidized Bed Dryer
‘Fluidized’ means something that behaves like liquid.
In the fluidized bed dryer, the mixture of solids and
gas behave like a liquid and solid are called fluidized.
It provides good contact between hot air and particles
to obtain efficient drying.
The hot air is passed through a mesh, which supports
the conical vessel with a porous base.
This vessel is filled with powder to be dried.
Uses: It Is Popularly Used For The Drying of Granules
in the Production of Tablets.
Require less time for drying i.e., 20 to 40 min.
It is available in different sizes with the drying capacity 5 to 200 kg
Labour cost are low
Also used for mixing the ingredients
Hot spot are not observed in the dryer
Higher drying temp. can be used
Some powder are organic which develop electrostatic
charges to avoid this, efficient electrical earthing of the
dryer is essential.
This equipment is a good example of conduction
drier. The vacuum oven consists of a jacketed vessel
to withstand vacuum within the oven.
There are supports for the shelves giving a larger area
for conduction heat transfer. The oven can be closed
by a door.
The oven is connected through a condenser and liquid
receiver to a vacuum pump.
Operating pressure can be as low as 0.03-0.03 bar, at
which pressures water boils at 25-35 0C.
It provides large surface area for heat transfer
Handling of the material, trays and equipment is easy
It is easy for switching over to the next material
Hot water of desired temp. can be supplied
It has a limited capacity
It is more expensive than tray dryer
Freeze drying is a process used to dry extremely heat-
sensitive materials. It allows the drying , without
excessive damage, of proteins, blood products and
even microorganisms, which retain a small but
In this process the initial liquid solution or suspension
is frozen, the pressure above the frozen state is
reduced and the water removed by sublimation.
Thus a liquid-to-vapour transition takes place, but
here three states of matter involved: liquid to solid,
then solid to vapour.