DRYING

·      Drying involves removal of water or another
solvent by evaporation from a solid, semi–solid or liquid by application of heat and finally a liquid free solid product is obtained.

·        
Drying is accomplished by thermal techniques but non–thermal drying processes such as
squeezing wetted sponge, adsorption by desiccant (desiccation) and extraction are also used.

·      In bio products like food, grains, and
pharmaceuticals like vaccines, the
solvent
to be removed is almost invariably water.

Difference between Drying
and
Evaporation

·        
The main objectives of drying include to preserve foods and increase their shelf life by reducing
the water content and water activity; avoid the need for use of refrigeration systems
for transport and storage (expensive);
reduce space requirements
for storage and transport.

·        
In pharmaceutical technology, drying is
carried out for one or more of the fallowing reasons:

1.
To avoid or eliminate moisture which may
lead to corrosion and decrease the
product or drug stability.

2.      
To improve or keep the good properties of a material like
granules, e.g. Flowability,
compressibility.

APPLICATIONS OF DRYING

·        
Preparation of bulk drugs: In the preparation of bulk drugs, drying is the final stage of processing. A few examples are – dried aluminum hydroxide, spray dried lactose and powdered extracts.

·        
Preservation of drug products: Drying is necessary in order to avoid deterioration. For examples protection of blood products, skin, tissues and crude drugs from microbial growth.

·        
Improved characteristics: Drying produces materials of spherical shape, uniform size, free flowing
and enhanced solubility.

·        
Improved
handling:
To reduce the cost
of transportation of large volume materials.

·        
To make the materials easy or more stable for handling. Drying reduces moisture
content.

·        
Drying
as final step: Drying is the final step in evaporation, filtration, and
crystallization

Mechanism
of drying

• Involves both heat and mass transfer

• Rate of evaporation is related to rate of heat transfer as

             dW/d∅=q/ƛ

• The rate equation for mass transfer

            dW/d∅=KA (Hs-H
g )

Moisture
content

• Bound moisture and free moisture

• % moisture content=  
weight of water in samplex100/ weight of dry sample

• If exactly 5 g of moist sample is brought to a constant dry
weight of 4g, calculate loss on drying and moisture content

• Percent MC is more realistic value in determination of dryer
load capacity

• Equilibrium moisture content depends on

– Temperature and humidity of air

– Properties of the material

Rate of
drying & Time of drying

Classification
and types of dryers

·        
Static Bed
Dryers (Ex. Tray Dryers, Freeze Dryer)

·        
Moving Bed Dryer (Ex. Drum Dryer)

·        
Fluidised Bed Dryer

·        
Vacuum Dryer

·        
Pneumatic Dryer (Ex. Spray Dryer)

TRAY DRYER

Principle

·        
The basic working principle of this incredible machine is the continuous circulation
of hot air.

·  
In the tray dryer,
moisture is removed from the solids that are placed in the tray by
a forced convectional heating.

·        
The moist air is removed is partially but in a simultaneous
fashion.

Construction

·   
Tray Dryer is used for the best drying results in conventional process.

·        
It is a double
walled cabinet with Single or Two doors.

·        
The gap between two walls is filled with high density
fibre glass wool insulation material to avoid heat transfer.

·        
Doors are provided
with gaskets.

·        
Stainless steel trays are placed on the movable trolleys.

·        
Tray Dryer is provided with control
panel board, process timer, Digital temperature controller cum indicator etc.

·        
Tray Dryer is
available
in capacities ranging from
6, 12, 24, 48, 96, 192 trays.

Working

·        
In tray dryer hot air is continuously circulated. Forced convection heating takes place to remove
moister from the solids
placed in trays.

·        
Simultaneously the
moist air is removed partially.

·        
Wet solid
is loaded in to the trays.
Trays are placed in the chamber.

·        
Fresh air is introduced through in let, which passes through the heaters and gets
heated up.

·        
The hot air
is circulated by means
of fans at 2 to 5 meter per second.

·        
Turbulent flow lowers the partial vapour pressure in the atmosphere and also reduces the thickness of
the air boundary layer.

·        
The water is picked up by the air. As the water evaporates from the surface, the water
diffuses from the interior of the solids
by
the capillary action.

·        
These events occur in a single pass of air. The time of contact is short and amount of water picked up in
a single pass is small.

·        
Therefore, the discharged air to the tune of 80 to 90 % is circulated back through the fans.
Only
10 to 20% of fresh
air is introduced.

·        
Moist air is discharged through outlet. Thus, constant temperature and uniform air
flow over the materials can
be maintained for achieving uniform
drying.

·        
In case of the wet granules as in tablets and capsules drying is continued until the
desired moister content is
obtained.

·        
At the end of the drying trays or trucks are pulled out of the chamber and taken to a
tray
dumping station.

Advantages

·        
Each batch
is handled
as
a separate
entity.

·        
It
is more efficient in fuel
consumption.

·        
It
is operated batch–wise.

·        
It
is simple to use.

·        
It provides
tendency to over–dry the lower trays.

·        
It
requires little
labor costs –
merely load and
then unload.

Disadvantages

·        
The process is time-consuming.

·        
It
requires extra
cost.

·        
Not
suitable
for oxidizable
and thermolabile
substances.

Uses

·        
Tray dryer
is used in the drying of
the
sticky materials.

·        
It is used in the drying of the granular mass or crystalline materials.

·        
Plastic
substances can be dried by the tray dryers.

·        
Wet
mass
preparations and
pastes can be dried
in a
tray dryer.

·        
In the tray dryers
the crude drugs, chemicals, powders and
tablet granules
are
also dried to obtain free flowing materials.

·        
Some
types of equipments can
be dried in the tray dryers.

Variants:

·     
Tray dryer may be operated under vacuum called vacuum tray dryers.

·   
Another is Tunnel dryer. In this type, trucks are loaded with wet materials at one end of the tunnel.

·        
The
tunnel comprised of a number of units, each of which is electro–statically
controlled.

·        
The solids
get
dried and the product is
discharged at
the
other end of the
tunnel.

DRUM DRYER
OR ROLL DRYER

Principle

·        
In drum drying,
the heated surface is
the
envelope of a rotating horizontal metal cylinder.

·        
The cylinder
is heated by steam
condensing inside,
at a pressure in the range of
200 to 500 kPa bringing the temperature of
the cylinder
wall to 120–155°C.

Construction

·        
A drum dryer consists of one or two horizontally mounted hollow cylinder(s) or drums
of about 0.75–1.5 m in
diameter and 2–4 m in length, made of high-grade cast iron or stainless steel, a supporting
frame, a product feeding system, a scraper, and auxiliaries.

·        
The drum is
heated internally by
steam, and rotated on its longitudinal axis.

·        
The external surface of the
drum is polished.

·        
Liquid or slurry
is placed as feed in a pan. The drum is partially
dipped in pan.

·        
The spreader is used to spread liquid film evenly
on roller.

·    The rotation of the drum adjusted so that all of the liquid is fully vaporized.

·    The drum is rotated continuously.

·        
The
dried deposits can be scrapped
off with the help of
doctor knife.

Working

·        
As the drum rotates, the liquid material gets adhere to external surface of drum.

·      The liquid is spread as film on to the surface. The drying of the material is done by process of steam when
passed in to the drum.

·        
By the mechanism of conduction, the heat gets transferred in to drum
and
drying process takes place.

·        
The material is completely
dried during whole process during its
revolution.

·        
The dried material
is scrapped by the knife
and that fall in
to the bin.

Advantages

·        
Drying takes place in
less time.

·        
It
is suitable for thermo sensitive drugs.

·        
It
occupies
less space.

·        
In order to reduce the temperature of drying the drum can be enclosed in a vacuum
chamber.

·        
Rapid drying takes place due to
rapid heat and mass
transfer.

Disadvantages

·        
Maintenance cost is
high.

·        
Skilled operations
are
essential
to control
thickness of film.

·        
It
is not suitable for less
solubility products.

·        
The operating conditions are critical.

·   
It is necessary to introduce careful control
on feed rate, film thickness, and
speed of drum rotation
and drum temperature.

Pharmaceutical
Uses
of Drum Dryer or Roll Dryer:

·        
Drum dryer takes viscous liquids, slurries, suspensions, and pastes as input material to be dried and
produces the output
as powders or flakes.

·        
Drum  dryers
 find
 application
 majorly
 in
 Food  &  Dairy
 industry  and
 Chemical & Pharmaceutical Industries for
drying various type of pasts and
slurries.

Variants:

·        
A vacuum drum dryer encloses
both drum and feed line in a vacuum chamber to facilitate
drying of
heat sensitive materials.

·        
In large scale, instead of one drum, two drums are set in parallel, rotating in opposite direction with a common feed inlet.

SPRAY
DRYER

Principle

Spray drying is an industrial process for dehydration of a liquid
feed containing dissolved and/or  dispersed 
solids,  by transforming  that 
liquid  into  a 
spray  of  small 
droplets  and exposing these
droplets to a flow of hot air.

Construction

·        
A spray dryer is composed of a feed pump,
atomizer, air heating unit, air dispenser, drying chamber (diameter of the
drying chamber ranges between 2.5 to 9.0 m and height is 25 m or more) and also
systems for exhaust air cleansing and also powder recovery/separator. 

·        
The spray disk atomizer is about 300
millimeters in diameter and rotates at a speed of 3,000 to 50,000 revolutions
per minutes.

·        
In the spray dryer the liquid to be dried is
atomized into the good droplets that are tossed radially into a relocating
stream of warm gas.

Working

·        
A spray dryer takes a liquid stream and
separates the solute or suspension as a solid and the solvent into a vapor. The
solid is usually collected in a drum or cyclone. The liquid input stream is
sprayed through a nozzle into a hot vapor stream and vaporized. Solids form as
moisture quickly leaves the droplets.

·        
The three stages that occur in a spray dryer
before drying is accomplished include:

§ 
Atomization

§ 
Spray-air mixing and moisture evaporation.

§ 
Dry product separation from the exit air.

·        
The nature of the final product obtained after
drying in a spray dryer depends on;

§ 
The design and operation of the spray dryer.

§ 
The physicochemical properties of the feed.

Advantages       

·        
Product quality and properties can be
effectively controlled and maintained through the entire drying operation.

·        
Thermolabile products/ pharmaceuticals can be
dried at atmospheric pressure and low temperature.

·        
Spray dryer permits high- tonnage production
in continuous operation adaptable to conventional PLC control (Programmable
Logic Controller) and it is relatively simple to operate.

·        
Feedstock 
in  solution,  slurry, 
emulsion,  paste,  and 
melt  form  can 
be  dried  if pumpable.

·        
Corrosion problem is minimal and the selection
of materials of construction of spray dryer is simplified since the dried
material comes in contact with the equipment surfaces in an anhydrous
condition.

·        
Spray dryer produces dry powder particles of
controllable particle size, shape, form, moisture content, and other specific
properties irrespective of dryer capacity and heat sensitivity.

·        
Spray 
dryer  handles  a 
wide  range  of 
production  rates  and 
provides  extensive flexibility
in  its 
design  that  is 
product  specification  are 
readily met  through  the selection of appropriate spray dryer
design and its operation from a wide range of available design.

·        
It is energy-intensive equipment because
specific heat of evaporation can be supplied in a short time. The temperature
difference across the drying chamber is relatively small and an appreciable
amount of heat is lost with exhaust air.

Disadvantages

·        
Spray dryer is bulky and also expensive to
install.

·        
It is difficult to clean after use.

·        
It has a low thermal efficiency that is a lot
of heat is wasted during operation.

·        
Solid materials cannot be dried using spray
dryers.

·        
Product degradation or fire hazard may result
from product deposit on the drying chamber.

Uses

·        
Spray 
dryer  is  used 
in  drying  pharmaceuticals  like 
penicillin,  blood  products, enzymes, vaccines, etc.

·        
It is used in the production of excipients and
co-processed excipients with increased flowability, compatibility, and tablet
disintegration.

·        
To improve drug compressibility and reduce
capping tendencies in crystals.

·        
It 
is  equally  used 
in  the  preparation 
of  matrix  microcapsule 
containing  drug substances and a
biodegradable polymer in order to obtain controlled drug release formulation.

·        
It is employed in enhancing solubility and
dissolution rates of poorly soluble drugs by formation of pharmaceutical
complexes or via the development of solid dispersion thus increasing bioavailability.

·        
It  is  used  in  the  production
 of
 dry  powder  formulation/dry  powder
 aerosol
 and thermolabile materials.

·        
Apart from its applications in the pharmaceutical industries, spray dryers also find use
in; Chemical industries, Ceramic industries, Food industries,
etc.

·        
Biochemical industries e.g.
algae, fodder antibiotics, yeast
extracts,
enzymes,
etc.

·        
Environmental pollution control e.g. flue gas desulfurization, black liquor from paper– making
etc.

Variants

·        
Spray dryer can be constructed in such a way as
to suit sterile products.

·        
It
can
be operated under closed
conditions to recover
solvents.

·        
It
can
be operated under oxygen free environment.

·        
The same
equipment can be used for
spray congealing.

·        
It
is useful for encapsulation (coating)
of solid and liquid particles.

FLUIDIZED BED
DRYER

Principle 

·        
The equipment works on a principle of
fluidization of the
feed materials.

·        
In fluidization process, hot
air is introduced
at high pressure
through a perforated bed of moist solid particulate.

·        
The wet solids are lifted from the bottom and
suspended in a stream of air
(fluidized state).

Construction

·        





The dryer
is made up of stainless steel or plastic.

·        
A detachable bowel is placed at the bottom of the dryer, which is used for charging
and discharging.

·        
The bowel has a perforated bottom with a wire mesh support for placing materials to
be dried.

·        
A
fan is mounted in the upper
part for circulating hot air.

·        
Fresh air inlet, prefilter and heat exchanger are connected serially to heat the air to the required temperature.

·        
The temperature of hot air
and exit air
are
monitored.

·        
Bag filters are placed above the drying bowl for the recovery of fines.

Working

·        
The wet granules to be dried are placed in a detachable bowl. The bowl is inserted in
the drier.

·        
Fresh air can pass through a pre-filter, which is then heated when passing through a heat
exchanger.

·        
Hot
air flows through the bottom of the
bowl.

·        
At
the same time, fan start
to rotate.

·        
The air speed increases gradually.

·        
When the velocity of air is greater than the sedimentation
rate of the granules,
the granules remain suspended in the gas
stream.

·        
After specific time, a pressure point is reached in which the friction drag on a particle
is equal to the force of gravity.

·        
The granules rise in the container due to high gas velocity of 1.5 to 7.5 meter per minute
and then fall back.
This state is known as fluidized
state.

·        
The gas
surround to each granule
do dry them completely.

·        
The air comes
out of the dryer passing through the filters in the bag.

·        
The entrained
particles remain adhered to the interior
surface of bags.

·        
Periodically bags
are shaken to remove
entrained particles.

Advantages

·        
It
takes
less time to complete drying as
compared to other
dryer.

·        
Drying is achieved at
constant rate.

·        
Handling time
is also short.

·        
It
is available
at different sizes with different
drying capacity.

·        
The equipment is
simple and less labor cost required.

·        
More thermal
efficiency.

·        
Drying capacity is
more than other
dryer.

·        
It facilitates the drying of thermo labile substances since the contact time of drying is short.

·        
It
is batch
type or
continuous type process.

Disadvantages

·        
Many organic powders develop electrostatic charge during drying. 
To avoid this efficient electrical grounding of
the dryer is essential.

·        
Chances
of attrition of some materials resulting in production of fines.

Pharmaceutical

·        
It
is used for drying of granules
in the production of
tablets.

·        
It
is used for coating of granules.

·        
It can
be used for three operations
such as mixing, granulation
and drying.

Variants:

·        
Plug flow dryer: It is a rectangular fluid bed dryer having different compartments for fluidisation.

·        
The material is made to move
from inlet through different compartments to outlet.

·        
Different drying condition can be maintained in the compartments.

·        
Often
the last compartment is
fluidised with cold gas to cool the solids before discharge.

VACUUM DRYER

Principle

Vacuum drying is generally used for the drying
of substances which are hygroscopic and heat
sensitive, and is based on the principle of creating a vacuum to decrease the chamber
pressure
below the vapour pressure of
the water, causing it to
boil. Hence, water evaporates faster.
The heat
transfer
becomes, i.e., rate of drying
enhances substantially.

Construction

·        
The oven is divided into hollow trays which increases the surface area for heat conduction.
The oven door is locked air tight and is connected to vacuum
pump to reduce the pressure. The materials to be dried are kept on the trays inside the vacuum dryer and pressure is reduced by means of vacuum pump.

·        
The enclosed space (approximately
1.5 meter cube) is divided in to a number of portions by means of 20 hollow shelves, which are part of the jacket. These shelves provide
larger
surface area (about 45 to 50 meter square) for conduction of heat. Over the
shelves, metal
trays are placed for keeping the material. The oven door can be locked tightly to give an air
tight
seal. The oven is connected to a vacuum pump
by
placing condenser in between.

Working

·        
The tray that are present in the dryer are used to dry the material that are placed in the
shelves and the pressure
is reduced to 30 to 60 Kps
by vacuum pump.

·        
The door
closes firmly and
steam passes through the
jacket space and the shelves.

·        
So
the heat transfer is carried
out by the conduction
mechanism.

·        
When evaporating under vacuum,
the water is evaporated from the material at 25 – 30⁰C.

·        
The vapour goes to the condenser.

·        
After drying vacuum
line
is disconnected.

·        
Then the materials are
collected
from the tray.

Advantages

·        
Material
handling is easy.

·        
Hollow
shelves which are electrically heated can be used.

·        
It provides large surface area. So the heat can be easily transfer through the body of
the dryer and
last drying action
takes place.

·        
Hot water can be supplied through the dryer, which help in drying process at the desired temperature.

Disadvantages

·        
Dryer is a batch type process.

·        
It has low efficiency.

·        
It is more expensive.

·        
Labour cost is too high.

·        
Needs high maintenance.

·        
There is a danger of overheating due to
vacuum.

Uses

·        
Vacuum dryer can be used for drying of
following:

·        
Heat sensitive materials, which undergo
decomposition.

·        
Dusty and hygroscopic material.

·        
Drugs containing toxic solvents. These can be
separated in to closed containers.

·        
Feed containing valuable solvents. These are
recovered by condensation.

·        
Drugs which are required as porous end
products.

·        
Friable dry extracts.

FREEZE
DRYER

Principle

Freeze drying or lyophilisation is a drying
process used to convert solutions or suspensions of labile
materials into solids of sufficient stability for distribution
and storage. The fundamental
principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like
evaporation, sublimation occurs when a molecule gains enough energy to break free from the
molecules around it. Drying is
achieved by subjecting the materials to temperature and pressures below the triple point.

Construction

·        
It consists of drying chamber in which trays
are loaded.

·        
Heat supply in radiation source, heating
coils.

·        
Vapour condensing or adsorption system.

·        
Vacuum pump or stream ejector or both.

·        
The chamber for vacuum drying is generally
designed for batch operation.

·        
It consists of shelves for keeping the
material.

·        
The distance between subliming and condenser
must be less than the mean path of molecules. This increases the rate of
drying.

·        
The condenser consists of relatively large
surface cooled by solid carbon dioxide slurred with acetone or ethanol.

·        
The temperature of condenser must be much
lower than evaporated surface of frozen substance.

·        
In order to maintain this condition, the
condenser surface is cleaned repeatedly.

Working

The following steps are involved
in
the working of freeze dryer

1.      
Preparation
and
pretreatment

2.      
Prefreezing for solidifying
water

3.      
Primary
Drying

4.      
Secondary Drying

5.      
Packing

·        
Primary drying is a top–down process with a well–defined sublimation front moving through the product as it dries. Above the ice
surface interface is dried product, or “cake”; below the
interface is product
with
ice crystals still
remaining to be sublimed.

·        
After
primary freeze–drying is complete and all ice
has sublimed, in the area where
ice
has
been removed, desorption of
water from the cake
occurs. This process is called secondary
drying and has started in the primary
drying phase.

·        
The product appears to be dry but the
residual moisture content may be as
high
as 7–8%.

·        
Packing is done by replacing vacuum with
inert gas, bottles
and vials are closed.

Advantages

·        
It
is suitable for drying heat
sensitive products

·        
Freeze dried
products is porous and
easy to dehydrated and
instantly dissolved.

·        
Drying takes place at very low temperature, so that enzyme action is inhibited and chemical decomposition, particularly hydrolysis,
is minimized.

·        
Denaturation
of protein does not occur.

·        
Loss
of volatile material is
less.

·        
Sterility can be
maintained.

Disadvantages

·        
The process
is very slow.

·        
Expensive
process.

·        
It is not a general method of drying, but it is limited to certain type of valuable
products that cannot be dried by any other means.

·        
The period
of drying is
high.