CONCEPT OF FLUIDIZATION:
Principle of Fluidization:
The principle of operation of fluidized systems are based on
the fact that if a gas is allowed to flow through a bed of
particulate solids at velocity greater than the settling velocity
of the particles and less than the terminal velocity for
pneumatic conveying and equal to the minimum velocity of
fluidization (V mf ), the solids get partially suspended in the
stream of upward moving gas.
The gas stream negates the gravitational pull due to weight of
particles to enable the suspended state of the solid.
The resultant mixture of solids and gas behave like a liquid
and thus rightly solids are called Fluidized.
Theory Of Fluidization :
i)Phenomenon of Fluidization
Stages of fluidization:- The stages of fluidization is
mostly based on the fluid velocity passing through the
particle bed. According to Ridgeway and Quinn the
stages of fluidization can be summarized as follows.
1) Static bed
2) Expanded bed
3) Mobile bed
4) Bubble formation
5) Pneumatic transport
ii) Role of Fluidization velocity:
A mass of finely divided solids is transformed into a fluidized bed by
lifting action of gas passing through it.
Thus three stages can be identified in the process of fluidizing a bed
of solids basing on the velocity of gas flow through it. They include
1) Fixed bed or Static Bed
2) Expanded bed or particulate fluidization.
3) Mobilized bed
After mobile bed formation if velocity is further increased the bed
expands considerably with increase in voidage and bubble formation
If further increase in velocity of air occurs, eventually the lifting
force of passing air blows particle out of the bed altogether leading to
Pneumatic transport .
As shown in Figure below, the velocity of the fluid through
the bed opposite to the direction of gravity determines whether
the bed is fixed, expanded, or is swept out.
This led to the development of the concept of minimum
fluidization velocity (V mf ) at which the bed just begins to
(a) Slow flow rate (b) Intermediate flow rate (c) High flow rate
Status of approach fluid velocity
(V 0 )
Type of bed formed
V 0 < V mf
V mf ≥ Vo < U t
V 0≥ U t
Where Vo is approach velocity or superficial
velocity, Vmf is the minimized fluidized
velocity, Ut is particle terminal velocity.
iii) Role of pressure drop in Fluidization:
When a fluid flows through a bed of particles in a tube, it will
exert a drag force upon the particles resulting in a pressure
drop across the bed.
As the fluid’s approach velocity is increased, pressure drop is
The pressure drop across the bed, ∆p, then remains constant
(even with further increase in the fluid velocity) and equal to
the effective weight of the .bed per unit area
Different types of Fluidized Bed:
Slugging bed – A slugging bed is a fluid bed in which
air bubbles occupy entire cross sections of the vessel
and divide the bed into layers
Boiling bed – A boiling bed is a fluid bed in which the
air or gas bubbles are approximately the same size as
the solid particles
Channeling bed – A channeling bed is a fluid bed in
which the air (or gas) forms channels in the bed
through which most of the air passes
Spouting bed – A spouting bed is a fluid bed in which
the air forms a single opening through which some
particles flow and fall to the outside
FLUIDIZED BED DRYERS
Fluid bed drying is most widely used technique for drying pharmaceutical powders
The direct contact between particles and air/gas is possible in fluid bed system.
Here any type of inert gas or air is used.
They can be designed in either batch or continuous type fluid bed dryer.
In fluidized bed dryer, hot air is passed at high pressure through a perforated bottom of
the container containing granules to be dried.
The granules are lifted from the bottom and suspended in the stream of air.
This condition is called fluidized state.
The hot gas surrounding every granule to completely
Thus, material or granules are uniformly dried.
The hot air/gas used for drying can be generated by
either steam coils or a combustion furnace.
The angle of repose of gas-solid mixture is zero and
it assumes the shape of the vessel that contains it.
In fluid bed drying uniform conditions of
temperature, composition and particle size distribution
is achieved throughout the bed because of complete
mixing between the solids and gas is obtained.
PARAMETERS TO BE CONTROLLED IN FLUID BED
i) Apparatus Parameters
1) Air distribution plate Position of the air distribution
plate influences the airflow pattern inside the body.
2) Shape of instrument body Annular base gives better
product and fluidization
3) Nozzle height in case of coater and granulator. It
plays vital role as in coating, the atomized coating
solution should not get dried before reaching the tablet
4) Positive and negative pressure operation.
ii) Process Parameters:
The following inlet air parameters are critical
As the inlet air temperature increases the rate of drying
increases and vice versa.
This approach to increase the rate of drying can not be used
always because some materials are harmed by high
temperature, e.g. Ibuprofen liquefies above 60°C temperature
of inlet air should be optimized without any impact on product
If temperature is high, it leads to blistering. If temperature is
low, soft spot can be formed.
Humidity in the inlet air should be as low as possible and ideally
dehumidified air should be used for faster drying rate because as the
humidity of inlet air decreases the rate of drying
3) Air flow rate Air flow rate should be controlled
properly in order to get efficient use of drying air.
Air flow rate should not be too fast or too slow but
optimized to have efficient drying.
iii) Product Parameters:
1) Initial moisture content of material.
It should not be high otherwise it increases
2) Batch size.
It should be small and optimized based on
TYPES OF FLUID BED DRYER
A)Batch Type Vertical Fluid Bed Dryer With
I. Reverse Turning Bed Type
In this equipment, by turning the gas dispersion plate
(the reverse turning bed) in 90° direction with the
control motor, all the dried material can be
discharged at once.
II. Rotating Discharge Type
Dried material is discharged by opening the discharge
gate equipped at the side of the Dryer. As the
perforated plate is used as the gas dispersion plate,
the gas inside the equipment whirls and pushes the
dried material out from the discharge gate.
Characteristics Of Batch Type
Fluidized Bed Dryer
The residence period of the dried
material can be controlled which
results in uniform drying.
It is most suitable in case where
an accurate control of the
residence period is required at the
decreasing rate drying zone.
Small destruction of particle occurs
therefore suitable for granular or
Easy operation can be achieved by
an automatic control of material
feeding, drying discharging etc.
When multiple stage system us
adopted, the exhaust gas heat can be
B) Continuous Type Horizontal Vibrating Conveyor
Fluid Bed Dryer
Residence time in any
drying zone is dependent
(1) Length of the zone
(2) The frequency and the
amplitude of the vibration
(3) Use of dams
Heat transfer units such as
tube or plate, are built
inside the equipment.
These unit supplies 60-80
% heat necessary for
Characteristics Of Continuous Fluidized Bed Dryer
(1) The materials with relative high moisture content can also
(2) At and after a second drying chamber, piston flow ability
can be achieved by arranging numbers of the partition plates
as per the required residence period.
The perforated plate at the fixed direction ensures easy
(3) Small destruction of particles, so suitable for granules or
In multiple zones fluid bed dryers, heating and cooling
occurs in same unit.
Each zone has independent control for temperature, dewpoint
and velocity of air/gas.
By adjusting the weir height for each zone, residence time
can vary up to four fold in the unit.
The construction of a vertical fluidised bed dryer
The dryer is made of stainless steel or plastic.
A detachable bowl is placed at the bottom of the dryer, which
is used for charging and discharging.
The bowl 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 temperatures.
The temperature of hot air and exit are monitored.
Bag filters are placed above the drying bowl for recovery of
When the velocity of the air is greater than the settling
velocity of granules, the granules remain partially suspended
in the gas stream.
After some time, a point of pressure is reached at which
frictional drag in the particles is equal to the force of gravity.
The granules rise in the container because of high velocity
gas and fell back in a random boiling motion.
This condition is said to be fluidized state.
Drying is achieved at constant rate and falling rate period
is very short.
1) It requires less time to complete drying i.e.,20 to 40 mins
2)To 24 hrs of tray dryer. Handling time is also short. It is 15
times faster than tray dryer.
3)Hot spots are not observed in the dryer, because of its
excellent mixing and drying capacities.
4)It facilitates the drying of thermo labile substances, since the
contact time is short.
5)It can be used either as batch type or continuous type.
6)The free movement of individual particles eliminates the
risk of soluble material migrating as may occur in static bed.
1)Many organic powders develop
electrostatic charges during drying which
can be avoided by efficient electrical
earthing of the dryer.
2)The turbulence of the fluidized state of
granules may cause attrition of some
materials resulting in production of fines
which can be avoided by using suitable
FLUIDIZED BED COATING
Principle of operation:
Particles are fluidized and coating fluid is sprayed on and
dried.Small droplets and low viscosity of the spray
medium ensure an even product coating.
PARAMETERS TO BE CONTROLLED:
IN COATING PROCESS:
1) Related To Spray Nozzle
Distance of spray nozzle.
Efficiency of coating depends on the quality of the coating solution.
The coating solution should not get dried before reaching the
fluidized substances viz. tablet, particles, and granule surface.
2) Droplet size.
Quality of the coat depends on the droplet size. So it should neither
be too big nor be too small.
3) Spray rate.
Flow rate should not be too fast or too slow, but should be of
optimized rate for efficient coating.
4) Spray pressure.
Atomization of coating solution depends on the spray pressure,
thus for proper atomization droplet size should be optimum.
1) Moisture content in processing compartment. Moisture should not
be present in case of hygroscopic materials.
2) Method used for coating should be chosen on basis of the purpose
for which it is used. e.g. SR, ER, etc.
3) Time of drying should be determined on bases of the product and
quality of the coat desired.
1) Coating agent.
Selection of coating agent should be done according to type
of coating required e.g. Enteric coating, Sugar coating, etc.
Solvent should be selected according to the properties of the
coating agent. If solvent is volatile, it should be checked for
If solvent is volatile, it should be checked for
inflammability. Concentration of granulating
agent should be optimized for uniform
spreading and droplet formation. Temperature
of the coating agent should not be so high that
coating solution get dried before reaching to
the tablet surface.
2) Starting material.
Shape of tablets greatly affects the coating
process. In case of powder coating the particles
shape and density affects the coating process.
TOP SPRAY COATING
•This process is used for general coatings
right up to enteric coating. With top spray
coating in the fluid bed (batch and
continuous), particles are fluidized in the
flow of heated air, which is introduced
into the product container via a base
•The coating liquid is sprayed into the
fluid bed from above against the air flow
(countercurrent) by means of a nozzle.
Drying takes place as the particles
continue to move upwards in the air flow.
• Small droplets and a low viscosity of
the spray medium ensure that the
distribution is uniform.
•Coating in the continuous fluid bed is
particularly suitable for protective coatings/color
coatings where the product throughput rates are
•The product is continuously fed into one side of
the machine and is transported onwards via the
sieve bottom by means of the air flow.
•Depending on the application, the system is
sub-divided into pre-heating zones, spray zones
and drying zones. The dry, coated particles are
BOTTOM SPRAY COATING (WURSTER COATING)
•This process is particularly
suitable for a controlled release/
Extended Release and delayed/
enteric coating of active
ingredients layered in the form
•In the Wurster process, a
complete sealing of the surface
can be achieved with a low
usage of coating substance.
•The spray nozzle is fitted in the base plate resulting in
a spray pattern that is concurrent with the air feed. By
using a Wurster cylinder and a base plate with
different perforations, the particles to be coated are
accelerated inside the Wurster tube and fed through
the spray cone concurrently.
•As the particles continue traveling upwards, they dry
and fall outside the Wurster tube back towards the
base plate. They are guided from the outside back to
the inside of the tube where they are once again
accelerated by the spray. This produces an extremely
even film. Particles of different sizes are evenly
TANGENTIAL SPRAY COATING (ROTOR PELLET COATING)
•Ideal for coatings with high
solid content. The product is set
into a spiral motion by means of
a rotating base plate, which has
air fed into the powder bed at its
•The spray nozzle is arranged
tangentially to the rotor disc and
also sprays concurrently into the
powder bed. Very thick film
layers can be applied
HUTTLIN KUGEL COATER
•The product container of kugel
coater is spherical.
•Fluidized air is introduced by tube
that passes down the center of the
•A series of spray nozzles are also
located at the bottom of the product
container in such a way that
fluidizing air create a ballon effect to
keep the product been coated away
from the spray nozzle
•Addition, of multiple spray nozzles
intended to maximize uniformity of
distribution of coating.
FLUIDIZED BED GRANULATORS
Principle of granulation:
•The powder is fluidized by the hot air in fluid bed granulator.
•The binding liquid such as solution ,suspension is sprayed on
the fluidized powder to build liquid bridges among them to
•The liquid bridge that serve to hold the particle together in two
1) By surface tension at the air liquid interface
2) By hydrostatic suction
• The liquid bridges are dried by the hot fluid air to stick the
• While the liquid sprayed continuously , the particles grow
bigger to a desire granule size.
• The process is carried out continuously.
• Finally it forms uniform porous granules.
FLUID BED GRANULATOR
1. Inlet air filter
4. Inlet air Heater
5. HEPA filter
6. Inlet air
7. Inlet air plenum
8. Gas distributor plate
9. Product container
10. Conical expansion zone
11. Filter housing
12. Product filter
13. Outlet air
14. HEPA filter
16. Spray gun
•A suction fan mounted at the top portion generates
the airflow necessary for fluidization of powders. The
air used for fluidization is heated to the desired
temperature by an air heater.
•The liquid granulating agent is pumped from its
container & sprayed as a fine mist through a spray
head onto the fluidized powder.
•The wetted particles undergo agglomeration
through particle contacts. After appropriate
agglomeration is achieved, the spray operation is
discontinued and the material is dried and discharged
Types Of Fluidized Bed Granulator:
1) Top Spray Fluid Bed Granulator
•The recrystallization and hardening binder technology are
generally carried out in the top spray granulator. In this
equipment spray nozzle located at the top the base of the
product container is equipped with a fine – mesh retention
screen to allow small particle size.
•Spray nozzle to permit positioning above the static bed in
the lengthened expansion chamber. The granulator is
operated by fluidizing the bed of powder & spraying the
granulating solution at the controlled rate.
•Proper agglomeration achieved, the liquid spray is cut off
and the material allows drying to the desired moisture
2) Rotating Disk Fluid Bed Granulator
With Dryer Option:
•Layering technology carried out by rotating
disk granulator and coater. The technique have
been extended to coating operation and
combined with an expansion chamber to form
the rotating disk granulator & coater fluid bed
•The rotating disk can be moved up or down to
create a variable slit opening between the outer
perimeter of the disk and the side wall of the
•This allows independent control of air velocity over
air volume, air is drawn into the product container
through the slit under negative pressure. At the same
time the disk rotate at varying speed & product move
under centrifugal force to the outer positions where it
is lifted by the fluidizing air stream into the expansion
•As the material fall to the center of the rotating disk
and repeat the processes. This fluidization pattern
also described as a spiraling helix or rope like pattern
around the inside the rotor chamber.
•The motion of fluidization of the particle controlled
by the forces like fluidization, centrifugal force and
•In the layering technique dry powder can be fed into the wet
bed resulting in the build up the layers of the powder on to
the particle substrate. At the end of the coating process the
liquid spray is cut off and the material in the product
chamber is dried by increasing the fluidizing air volume and
•Liquid like behavior, easy to control
Rapid mixing, uniform temperature and
•Resists rapid temperature changes, hence responds
slowly to changes in operating conditions and avoids
temperature runaway with exothermic reactions.
•Applicable for large or small scale operations.
Heat and mass transfer rates are high, requiring
•Ease of process control due to stable conditions.
•Bubbling beds of fine particles are difficult to predict
and are less efficient.
•Particle comminution (breakup) is common.
•Pipe and vessel walls erode due to collisions by
•Non-uniform flow patterns (difficult to predict).
Size and type of particles, which can be handled by
this technique, are limited.
•Due to the complexity of fluidized bed behavior,
there are often difficulties in attempting to scale-up
from smaller scale to industrial units.
•Fluidized bed dryers are used in drying of various materials such as
powders, tablets, granules, coals, fertilizers, plastic materials.
•This process is being used in granulation of pharmaceutical powders.
•Fluidized bed coaters are used widely for coating of powders,
granules, tablets, pellets, beads held in suspension by column of air.
•The three types (Top spray, Bottom spray, Tangential spray) are mainly
used for aqueous or organic solvent-based polymer film coatings.
•Top-spray fluidized bed coating is used for taste masking, enteric
release and barrier films on particles/tablets. Bottom spray coating is
used for sustained release and enteric release and Tangential spray
coating is used for SR and enteric coating products.
•REVIEW ARTICLE :FLUIDIZED BED
•HANDBOOK OF PHARMACEUTICAL
GRANULATION TECHNOLOGY 2ND EDITION