Drying (principles)

DRYING 1
Md. Saiful Islam
BPharm, MSc
North South University
Fb Group: Pharmacy Universe

DEFINITION
Drying is defined as the removal of a liquid (moisture
content) from a material by the application of heat
and accomplished by the transfer of a liquid from a
surface into an unsaturated vapour phase.
In drying, dry solid is the product.
Drying is an important operation in pharmaceutical
practice, since it is commonly the last stage of the
process before packaging.

IMPORTANCE
1. Preparation of bulk drugs: In the preparation of
bulk drugs, drying is the final stage of processing. A
few examples are:
- dried aluminium hydroxide
- spray dried lactose
Drying step is essential after certain operations such
as crystallization and filtration.

2. Preservation of drug products.: Drying is necessary
in order to avoid deterioration.
- Crude drugs of animal and vegetable origin-
chemical decomposition
- Blood products, skin, tissue - microbial growth
3. Improved characteristics : Drying produces
materials of spherical shape, uniform size, free
flowing and enhanced solubility. Some specific of
importance are:

- Granules are dried to improve the fluidity and
compression characteristics. These are essential for
the production of tablets and capsules.
- Viscous and sticky materials are not free flowing,
can be modified by drying. Examples are: male
fern extract, malt extract and oleoresin.

4. Improved handling: Removal of moisture
makes the material light in weight and reduces
the bulk,
- thus cost of transportation will be less and
storage will be efficient.
- If moisture is present, size reduction of drugs
become difficult.

THEORY OF DRYING:
Drying involves both heat and mass transfer operations. This process can be
expressed as follows-
Wet solid mass
Heat transfer operation Mass transfer operation
Heat transferred into materials Diffusion of water through the material
and supply the latent heat to the evaporating surface
to vaporize the moisture
Subsequently evaporation into the air
stream
Drying

Moisture content of wet solids
The moisture content of a wet solid is expressed as kilograms of
moisture associated with 1 kg of the moisture – free solid. Thus a
moisture content of 0.4 means that 0.4 kg of removable water is
present per kg of the solid. It is sometimes calculated as percentage
moisture content.
Total moisture content: This is the total amount of liquid associated with
a wet solid. The total removable water is known as the free moisture
content, and the moisture which is more difficult to remove is the
equilibrium moisture content. The easily removable water is known
as unbound water.
Unbound water: This water exists as a liquid and exerts its full vapour
pressure, it can be removed readily by evaporation. During a drying
process this water is easily lost but the resulting solid is not
completely free from water molecules.

Equilibrium moisture content:
The moisture content present in a solid under steady-state ambient
conditions is termed the eq. moisture content. Its value changes with
temperature, humidity and the nature of the solid. It is the state when
an object is neither gaining nor losing moisture
Bound water :
Part of the moisture present in a wet solid may be adsorbed on surfaces
of the solid or be adsorbed within its structure to such an extent to
prevent it from developing its full vapour pressure and from being
easily removed by evaporation. Such moisture is described as “bound”
and is more difficult to remove than unbound water.

Theory can be discussed in two ways namely-
1. Equilibrium relationships
2. Rate relationships
1. Equilibrium Relationships:
Air of constant temperature and humidity is passed over the wet solid.
After a long exposure, an equilibrium is reached i.e. vapour pressure
of the wet solids is equal to that of the surrounding atmosphere.
*At this stage, the amount of water present in the solid is called
Equilibrium moisture content (EMC).
*Hence there is no driving force for mass transfer.

2. Rate Relationships
When a solid dries, two fundamental and simultaneous
processes occur:
i) Heat is transferred to evaporate liquid
ii) Mass is transferred as a liquid or vapor within
the solid and as a vapor from the surface.
These factors governing the drying processes and help to
determine the drying rate.
The rate at which drying occurs can be shown by certain phases, where the change in moisture content is
plotted against time.

FIG. THE DRYING CURVE
FIG. REPRESENTS CERTAIN PHASES:
1. A TO B THE RELATIONSHIP IS LINEAR & KNOWN AS CONSTANT RATE PERIOD.
2. B TO C THE RATE OF LOSS OF MOISTURE DECREASES & IS KNOWN AS
FALLING RATE PERIOD
3. AT THE END OF THE CONSTANT RATE PERIOD, B IS REFEREED TO AS THE
CRITICAL MOISTURE CONTENT (CMC)

RELATIVE HUMIDITY (RH) OF AIR
Air at a given temperature is capable of taking up water vapour
until it is saturated (at 100% RH ). If the temperature is raised
then the air will be able to take up more moisture and the
relative humidity falls.
The RH of air is dependent not only on the amount of moisture in
the air , but also on its temperature, as the amount of water
required to saturate air is itself dependent on temperature.
It should be noted that in convective drying, where warm air is
passed over the surface of a wet solid, the relative humidity
may rise during the drying process as a result of two separate
factors:-
1- Uptake of evaporated water vapour from the wet solid,
2- The cooling of the supply air as it transfers heat to the wet
solid (evaporative cooling).
If the cooling is excessive the temperature of the air may fall to a
value known as the dew point, when liquid water will
condense and be deposited.

RELATIONSHIP BETWEEN EQUILIBRIUM MOISTURE CONTENT
(EMC) AND RELATIVE HUMIDITY
The EMC of a solid exposed to moist air varies with
the relative humidity. Ordinary atmospheric
conditions are of the order of 20 C and 70-75 RH
so that if exposed to atmosphere a material such as
kaolin will contain about 1% moisture, whereas a
starch –based product may have as much as 30%
or more. Materials exposed to humid conditions will
regain moisture, and so there is no advantage in
drying to moisture content lower than that which the
material will have under the conditions of use

LOSS OF WATER FROM WET SOLIDS
Unbound water is easily lost by evaporation until the
equilibrium moisture content of the solid is reached.
Once the solid reaches its EMC , extending the time of
drying will not change the moisture content as an
equilibrium situation has been reached. The only way to
reduce the moisture content is to reduce the RH of the
ambient air. This can be done mechanically with an air-
conditioning system.
On small scale, desiccators are used.

DIFFERENT TECHNIQUES OF DRYING OF SOLIDS (
DRYING METHODS)
The following points should be considered before the
selection of the suitable drying method:
1- Heat sensitivity the material being dried.
2- Physical characteristics of the material.
3- Nature of the liquid to be removed.
4- The scale of the operation.
5- Available sources of heat (steam, electrical).

THE GENERAL PRINCIPLES FOR EFFICIENT DRYING CAN
BE SUMMARIZED AS FOLLOWS
1- Large surface area for heat transfer.
2- Efficient heat transfer per unit area (to supply sufficient
latent heat of vaporization or heat of sublimation in case
of freeze-drying)
3- Efficient mass transfer of evaporated water through any
surrounding boundary layers, i.e. sufficient turbulence to
minimize boundary layer thickness.
4- Efficient vapour removal , i.e. low relative humidity air at
adequate velocity.
It is convenient to categorize pharmaceutical driers
according to the heat transfer method they use, i.e.
convective, conductive or radiant.

Drying (principles)

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