drying Flashcards
what is drying
defined as the removal of all or most of
the liquid by supplying latent heat to cause
thermal vaporization
what is the importance of drying
drying is usually
the last step of manufacturing before
packaging.
1-It is important that the residual
moisture is rendered low enough to prevent
product deterioration during storage.
2-drying is
important for the drying of wet granulation prior
to tablet compaction. Stability, flow properties
and compactibility are all influenced by the
residual moisture.
what is relative humidity
the percent of saturation
humidity, generally calculated in relation to
saturated vapor density (or corresponding vapor
pressure). The most common unit for vapor
density is gm/m3.
RH%= vapor pressure of water in the air\vapor pressure in water in air saturated at the same temp x 100
what is the Equilibrium moisture content
the amount of moist a dry solid would gain when exposed to air
what is the Relationship between equilibrium
moisture content and relative humidity
Ordinary atmospheric conditions are of the
order of 20 oC and 70-75% relative humidity
depending on the substance some would absorb as low as 1% such as kaolin while others would absorb as much as 30+
should we dry for lower than the atmosphere humidity
there is no advantage in drying to a moisture
content lower than that which the material will
have under the conditions of use.
what are static bed driers? (directed circulation dryer)
-no relative movement among the solid particles being dried.
-Only a fraction of the total number of particles is
directly exposed to the heat source.
-The dryers are made of trays held in a cabinet
which is connected to a source of air heated. The
air temperature is usually controlled by a
thermostat.
The air enters the bottom of the chamber below
the trays and then rises, through the trays of
material being dried, and exits from an opening in
the top of the chamber.
-One type of static bed drier is the directed
circulation dryer.
what is Intergranular migration
happens in static bed driers
During granulation, ingredients such as drug, binder
or colorant could dissolve in the granulating solvent.
During drying, the dissolved ingredients may
migrate with the evaporating solvent to the drying
surface.
The type of migration in tray dryers is intergranular
from granule to granule.
Consequently, the upper layer of the static bed will
be enriched with the migrated substance, while the
bottom layers will be relatively depleted
how do we solve Intergranular migration
- Switching to a solvent in which the migrated
substance is minimally soluble. - Choosing less soluble form of the drug or
color. - Switching the drying method to Fluidizedbed drying
what are fluidized bed driers
Good contact between the
warm drying air and wet
particles is found in the
fluidized – bed drier.
Principles of fluidization: the
particulate matter is
contained in a vessel, which
is perforated, enabling a
fluid to pass through the bed
of solids from below.
what are the advantages of fluidised bed driers
- Multifunctional equipment:
a) it could be used to dry wet granules made with
another equipment
b) or granules could be formed and dried in situ. - High drying rates: each particle is completely
surrounded by the drying air, leading to huge surface
area for heat transfer → drying times are shorter than
with static-bed convection driers. This makes the
process economic and more suitable to heat-labile
materials than static driers. - The free movement of individual particles eliminates the
risk of intergranular migration
what are the disvantages of fluidised bed driers
v1. The turbulence of the fluidized state may cause
excessive attrition of some materials (size
reduction), with damage to some granules and the
production of too much dust.
2. Fine particles may become carried along the
fluidizing air and must be collected by bag filters,
possibly leading to segregation and loss of fines.
3. Intragranular migration: migration of material from
the interior of each granule to its surface. This leads
to enriched periphery and depleted interior of the
granules.
what is Intragranular color migration disadvantage
May lead to mottling upon compression, if fragmentation of the
granules happened during compression. This is because the
interior (colorless) would be exposed on the surface
what are spray dryers
Differs from most other dryers. They can handle
only liquid-spray-able materials such as liquids,
slurries and thin pastes.
However, it cannot be used for drying wet
granulation.
One example of spray drying is the conversion
of liquid-milk to milk powder for reconstitution
Liquid droplets are sprayed into a stream of hot air, so that
each droplet dries to a solid particle.
The air enters the chamber tangentially and rotates the
drying droplets around the chamb.
what are the advantages of spray dryers
- Very efficient and rapid: As It gives huge surface
area for drying, the evaporation is rapid. The
actual drying time for a droplet is a fraction of a
second and over all time in the drier is only few
seconds. - Because the evaporation is very rapid the droplets
do not reach a high temperature. Most of the
heat is used as latent heat of vaporization (heat
absorbed by a liquid as it evaporates).
→ can be suitable for heat-labile materials - The powder will have a uniform and
controllable particle size. - The product is free-flowing, with almost
spherical particle
but Expensive equipment and expensive to run
