DRYING
Syllabus
Definition, purpose of drying, theory
of drying, loss on drying, moisture content and equilibrium moisture content.
Classification of dryers. Study of tray, rotary, vacuum, fluidized bed, freeze,
spray and flash dryers.
Questions
1. Classify dryers with
examples. (2000) [3]
2. Describe the construction,
operation and applications of Fluidized bed dryer. (2000) [10]
3. Mention the applications of
freeze drying in pharmacy. (2000) [3]
4. Explain working and
application of spray dryer. How can spray-dryer be used for encapsulation in
pharmaceutical industries? (1999) [5]
5. How much moisture is to be
removed from 20 kgwt material form 15% to 5% moisture content. The moisture
contents are expressed in dry weight basis. (1999) [6]
6. Fluidized bed dryer(1999) [4]
7. Sketch a typical drying rate
curve for a porous solid bed. Explain the nature of the curve. What is the
utility of this curve? (1998) [6]
8. What is the principle of
freeze-drying? Explain the different methods of accomplishing freeze drying of
pharmaceuticals. Mention specific application. (1998) [6]
9. How much moisture is to be
removed from 10 kg of wet material from 18% to 5% moisture content, the
moisture contents are expressed in dry basis. (1998) [4]
10. With a neat sketch describe
the operational principle and construction details of a fluidized bed dryer.
[1997] [12]
11. Successive granulation and
drying bed granulation. [1997] [4]
12. How do rotary dryers work?
Explain the term equilibrium moisture content. [1996] [16]
13. Merits of fluidized bed
drying [1995] [4]
14. Falling rate periods of
drying [1995] [4]
15. Define drying. Outline its
application in the field of Pharmacy. Classify the dryers on the basis of heat
transfer. Discuss in detail ‘spray drying’. [1994] [16]
DEFINITION
Drying is defined
as the removal of liquid from a solid by thermal method.
When
large amount of liquid is evaporated from a solution /suspension / slurry the
process is called ‘evaporation’. The final product is a concentrated liquid /
slurry.
When
very small amount of liquid is evaporated from solids the process is called
‘drying’. The final product is a ‘dried solid’.
PURPOSE OF DRYING
1.
Drying is most commonly used in pharmaceutical industries
in the preparation of granules, which can be packed in bulk or compressed into
tablets or filled in capsules.
2.
Drying is required for processing of materials like,
drying of aluminium hydroxide, spray drying of lactose and preparation of
powdered extracts.
3.
Drying is used to reduce the bulk weight that lowers
the transportation and storage costs of that material.
4.
Drugs obtained from plant and animal sources, when
dried, becomes more friable. Thus drying helps in size reduction of natural
drugs.
5.
Animal and vegetable drugs are preserved against mold
growth in dried condition.
6.
Dried products often are less more stable than moist
ones as in the case of effervescent
salts, aspirin, hygroscopic powders, ascorbic acids and penicillin.
SOME DEFINITIONS
Moisture
content (MC)
Loss
on drying (LOD)
THEORY OF DRYING
A typical drying cycle shows the
following phases:
Phase-I: Initial adjustment period
Phase-II: Constant rate period
Phase-III: First falling rate period
Phase-IV: Second falling rate period
The rate of drying
of a sample can be determined by suspending the wet material on a scale or
balance in a drying cabinet and measuring the weight of the sample as it dries
as a function of time.
Phase-I:
Initial adjustment period
A wet material
when kept for drying it begins to absorb the heat and the temperature of the
material increases. At the same time, the moisture begins to evaporate and thus
cools the drying solid. After a period of initial adjustment, the rates of
heating and cooling become equal and the temperature of the drying material
reaches the wet-bulb temperature of
the drying air.
Phase-II: Constant rate period
During this period
there is continuous liquid film over the surface of the solid. Evaporation from
the film (at wet bulb temperature) proceeds at a constant rate and the film is
continuously replaced by the underlying moisture. As long as the delivery of
water from the interior of the material is sufficient to keep the surface
completely wet, the drying rate remains constant.
Drying rate at
this phase is given by:
where, 
= Rate of drying in 
= Rate of drying in
A =
Area exposed to drying
td, tw = Dry bulb and wet bulb temperature
of the drying air respectively
r
= Bulk density of solid
L = Thickness of solid bed
l
= Latent heat of vaporization of water
hc = Convection heat transfer
coefficient
Phase-III: First falling rate period
Due to dry spots
on the surface, the area of constant mass transfer decreases and the heat
transferred to the dry spots will be utilized to raise the temperature of the
material to the dry bulb temperature. Thus as the number of dry spots increase,
heat transfer and mass transfer rates fall which is called the first falling
rate period. The point from which Phase-II starts is called critical point (B). Moisture content at
which this point appears is called critical
moisture content (CMC).
Phase-IV: Second falling rate period
In this period the
capillaries are empty, no film is present on the surface. The moisture movement
takes place only by diffusion. In this phase the rate of drying falls and it is
lesser than first falling rate. The starting of second falling rate period is
called second critical point (C).
At the end the
drying rate becomes zero. The moisture content at this point is called Equilibrium Moisture Content (EMC). EMC
may be defined as the moisture content of
the solid when drying limit has attained by use of air at a given temperature
(dry bulb) and humidity of air. EMC depends on the nature of the material, temperature and humidity.
Utility of
drying curve
1.
From the drying curve the time for drying a batch of material in a certain dryer can be
estimated.
2.
The size of a particular dryer can be determined for
drying a substance from one moisture level to the desired moisture content.
CLASSIFICATION OF
DRYERS
Classification based on solid handling
Classification based on heat transfer mechanism
A.
Convection dryers
(a)
Tray or shelf dryers (b) Tunnel
dryers (c)
Rotary dryers (d)
Fluidized bed dryers
B.
Conduction dryers
(a)
Vacuum oven (b)
Freeze dryers
C.
Radiant heat dryers
(a)
Infra-red dryers
TRAY DRYER / TRUCK
DRYER / SHELF DRYER / CABINET DRYER / COMPARTMENT DRYER
Construction
It consists of a small
cabinet or a large compartment in which trays containing wet materials are
placed. The compartment wall is insulated to reduce heat loss.
§
In tray dryers the trays are directly
placed inside the cabinet.
§
The truck dryer the trays are loaded on
to the trucks (shelves on wheel) and then the trucks are introduced inside the
heating cabinet.
The bottom of the
trays are either perforated or having wire-mesh bottom.
§
The material is heated by hot air circulated by
means of fans that removes the humid air from the cabinet.
The trays
containing the load remain in the dryer until drying is complete, after which
they are withdrawn, emptied and recharged for drying the next batch.
Energy sources:
Dry air can be heated either by electricity or steam.
Applications
§
Drying of crude drugs, chemicals, powders,
tablet granules etc.
§
It is a batch process and materials can be
handled separately.
ROTARY DRYERS
Construction
 |
It is a cylindrical shell (10 m length) mounted with a slight slope so that the material will move through the shell as it is slowly rotated at about 10 rpm. To improve contact the shell contains baffles or flights, which lift the solids and spill the particles through the air stream.
The hot air flows
counter current to the flow off material.
Application
It is used for
continuous drying on a large scale of any powdered or granular solid.
FLUIDIZED BED DRYER
Principle
Let us consider a
situation where a bed of granules is placed over a perforated bottom container
and hot air is flown from bottom through the bed. The pressure drop (DP) across the bed and the
air velocity (V) are measured. If the air velocity is gradually
increased and DP
is plotted against V then the following curve is observed.
§
Point A:
When the air velocity is very low flow takes place between the particles
without causing any disturbance.
§
Point B:
When the velocity is increased to a certain value the frictional drag on the
particles become equal the force of gravity of the particle.
§
Point C:
Rearrangement of the particles occurs to offer least resistance.
§
Point D:
Eventually the particles are suspended in the air and can move, DP
decreases slightly because of greater porosity.
§
Further increase in the air velocity causes the
particles to separate and move freely, and the bed is fully fluidized. Any additional increase in
velocity separate the particles further, i.e. the bed expands, without
appreciable change in DP until E.
§
In the D-E region the
air flows through the bed in the form of bubbles – the term boiling bed is generally used for this stage.
In the D-E region the
air flows through the bed in the form of bubbles – the term boiling bed is generally used for this stage.
§
Above point E the solid particles entrain into
the gaseous phase and the particles float in the gas.
Construction
Two types of
fluidized bed dryers are there
(i)
vertical fluidized bed dryer – for batch process
(ii)
horizontal fluidized dryer – for continuous process
The dryer consists
of:
(a)
Air handler: This is a source of dry and hot
air. It is also attached by means of heating and dehumidifying air, if
necessary.
(b)
Plenum: It consists of a screen or plate to
distribute the incoming air as it enters the dryer.
(c)
Product container: This container holds the
product that is to be dried.
(d)
Expansion chamber: This chamber is situated
above the product container and holds the suspended material.
(e)
Filter: The upper part of the expansion chamber
has bag filters. It prevents fines from escaping into the atmosphere or
collecting on the blades that pulls the air through the dryer.
Applications
1. Wet
granulation:
(a)
Fluidized bed dryers are used to dry the previously
prepared wet granules.
(b)
Powders are agglomerated in the drying chamber by
spraying liquid binder over it, while the hot air dries the agglomerates to
form dry granules.
2. Coating of
tablets
The fluidized bed
dryer can be used for coating granules also. This technique is called Wurster
technique. Three basic designs are employed
(a) top spray (b) bottom spray (c) tangential spray
In fluidized
condition the powder is coated by coating solution sprayed from the nozzles. As
the particles are coated they become heavier. When the mass developed becomes
higher than the drag force given by the fixed air velocity the particles no
longer floats. They fall back, which is then collected as product.
Advantages
1.
Efficient heat and mass transfer facilitate high drying
rates. Heating time of thermolabile materials is minimized.
2.
Individual particles of the bed get dried in the
fluidized state. So, most of the drying will be at constant rate and the
falling rate period is very short.
3.
Temperature can be controlled uniformly.
4.
A free-flowing product is obtained.
5.
Since the bed is not static, free movement of
individual particles eliminates the risk of soluble materials migrating.
6.
Short time yields a high output from a small floor
space.
Disadvantages
1.
Turbulence of fluidized state may produce fine
particles due to attrition.
2.
Fine particles lead to segregation, so they must be
collected by bag filters.
3.
Static charges may be produced due to vigorous movement
of particles in hot dry air.
VACUUM DRYER
Conduction is used as
the principle method of heat transfer in dryers that are operated under vacuum.
Convection cannot take place when air is nearly absent.
Construction
It is a jacketed
vessel through which steam or hot water is passed. The vessel can be closed
airtight. The oven is connected through a condenser and receiver to a vacuum
pump. The supports of the shelves form part of the jacket, giving a larger area
for heat conduction. Materials to be dried are kept in a tray and placed on the
shelves. Hot water or steam is passed through the jacket, a vacuum pump is
connected to the chamber.
Advantages:
1.
Drying takes place at low temperature, so thermolabile
materials can be dried.
2.
It reduces the risk of oxidation during drying.
3.
It produces porous and friable granules. [N.B. Because under vacuum the
vapor forms bubbles and in this condition the material is dried.]
4.
The solvent can be recovered from the condenser.
Disadvantages
1.
Heat coefficients are low. Most of the heating takes
place by conduction, some is from radiation
from the wall of the jacket around. So the drying rate is slow.
2.
Labor and running costs are high.
Applications:
1.
To dry a thermolabile material like Penicillin.
2.
To produce porous form such as dry extract.
3.
To recover the solvent, for example to recover ethanol
from ethanol extractives.
FREEZE DRYER
Principle
The temperature and
pressure of the material is reduced below the triple point of solvent to be
dried. Under these conditions, any heat transferred is used as latent heat and
the ice sublimes directly to vapor state (without formation of liquid state).
Triple point of
pure water is 4579 mm
of Hg and 0.00990C. Pharmaceutical products remain in solution. In
this case the pressure and temperature below which water evaporates directly
from ice to vapor state is called eutectic
point. In freeze dryer the pressure and temperature is maintained well
below the eutectic point. Generally
it is carried out at –100C to –400C, and at pressure of
2000 to 100 mm
Hg.
Construction
Freeze dryer
consists of
1.
a chamber for
vacuum drying:
Two
types of chambers are there, one for batch type and another for continuous type
operation.
2.
a vacuum source:
Vacuum
is achieved either by vacuum pump or by steam ejector or a combination of two.
3.
a heat source:
Heat
is provided by conduction or radiation.
4.
a vapor removal
system:
For
removal of water vapor condensers, desiccants, pumps or scrapper blades are
employed.
Stages of freeze drying process
(a) Preparation
and pretreatment:
Protein solutions
take 8 to 10 times longer period than pure water. Therefore, in such cases, it
is desirable to concentrate the solution under normal vacuum tray dryer.
(b) Pre-freezing
The
aqueous solutions to be dried are packed in vials, ampoules or bottles. They
are then cooled to solidify the water. Cooling can be done by using
cold-shelves (–500C), alcohol baths (–500C) or liquid
nitrogen bath (–1950C).
§
Thinner the layer of frozen material higher is
the drying rate. The usual thickness is kept at 0.5 to 0.75 inches.
§
Low freezing rates produces larger crystals of
ice. Sublimation of water from this material leaves large pores. So freezing
rate is generally maintained at 3 to 250C/min resulted in a product
having pore size of 1 to 45 mm.
(c) Primary
drying (Sublimation of ice under vacuum)
A
vacuum of 0.5 bar is applied on the frozen materials. The temperature is
increased to 300C within 2 hours. Then the temperature is kept
constant. During this stage around 98 to 99% water is removed from the
materials.
(d) Secondary
drying (Removal of residual moisture
under high vacuum)
Temperature
is maintained at 300C continuously and vacuum is lowered to a
pressure of 0.07 bar. The rate of drying is very low it takes 10 to 20 hours to
dry 1% moisture.
(e) Packing
Inert
gas is introduced inside the dryer to break the vacuum. Then the vials and
ampoules are sealed within the dryer to reduce the contact of atmospheric
gases.
Advantages
1.
Drying takes place at a very low temperature, so that
the enzyme action is inhibited, and decomposition (e.g. hydrolysis) is
minimized.
2.
The solution is frozen, so that the final dry product
is a network of solid occupying the same volume as the original solution. Thus
there is no case-hardening and the product is light and porous.
3.
The dried products are readily re-dissolved or
re-suspended by the addition of water prior to use (this procedure is termed as
reconstitution).
4.
The solutions do not concentrate during drying (like in
other drying methods). Hence salts do not concentrate and denature the proteins
present in the same solution.
5.
Under high vacuum there is no contact with air, and
oxidation is minimized.
Disadvantage
1.
It produces a very hygroscopic product, hence should be
sealed in the final package within the dryer.
2.
The process is very slow.
3.
The instruments are very costly.
Applications:
1.
Maintenance and preservation of microbial culture.
2.
Solution of penicillin can be stored at 0 – 20C
and used within two-three days, but if freeze dried then it is stable for
several months.
3.
To produce fibrin foam [N.B. Fibrinogen is dissolved in sodium chloride
injection and whipped into a foam that is then clotted by addition of human
thrombin. The foam is then freeze dried].
4.
To prepare gelatin sponge [N.B. A solution of gelatin containing traces of
formaldehyde is foamed, freeze dried, sterilized and used as surgical
dressing.]
5.
Used
to dry sera, blood products, certain enzymes, plant extracts, diagnostics,
mammalian tissues useful in skin and bone graft surgery.
SPRAY DRYING
In spray dryer the
liquid is sprayed in small droplets by atomizer in a drying chamber so that
each droplet dries to a solid particle. The feed may either be solution,
suspension or paste.
Construction and working
The spray drying
process involves four basic stages:
(A) Atomization
of feed into spray
When a feed is atomized into
small droplet its specific surface area increases many folds. So drying takes
place at a very fast rate. The instrument that produces small droplets is
called atomizer. Atomizers are
classified as follows on the mechanisms involved in breaking the bulk of the
liquid.
(a) Centrifugal atomizer
e.g.
Rotary atomizer. In rotary atomizer the liquid is distributed centrally on a
wheel disc or cup that is rotated at high speed. Due to centrifugal force the
liquid gets high velocity in the radial direction. The liquid is extended as a
thin film over the disc. Then the liquid films disintegrate to produce fine
droplets.
Use: Used for slurries or pastes having
thermolabile, abrasive, corrosive or high viscous properties.
(b) Pressure atomizer
e.g.
Centrifugal pressure nozzle. The liquid is fed through a nozzle under pressure.
The feed is issued from the nozzle as a thin film that readily disintegrates
into a spray. The feed rotates inside the nozzle so that a cone shaped spray
pattern is obtained.
Use: Used to obtain coarse particles.
(c) Pneumatic atomizer
e.g.
Two fluid Nozzle atomizer. It involves impact of liquid with a high velocity
gas or air. The high velocity gas creates a high frictional force over the
liquid surfaces leading to disintegration of liquid into droplets. The stream
of gas is rotated inside the nozzle and may come in contact with the liquid
wither within the nozzle or after emerging of liquid from the nozzle.
Use: Feed of high viscosity feed produce
coarse and less uniform product. Feed of low viscosity produces smaller size
particles and more uniform particles.
(B) Spray-Air
contact
The
spray and hot air can flow in two ways: (i) co-current flow, (ii) in
counter-current flow.
In co-current flow the spray and the hot
air is passed in the same direction. Here fresh droplets come in contact with
the hottest air. Solvent evaporates and the droplet temperature falls to wet
bulb temperature. So heat labile materials can be dried.
In counter-current flow the spray and hot
air flows in opposite directions. Here the dried product comes in contact with
the hottest air hence this type of flow is suitable for thermostable materials.
(C) Droplet
drying
Heat is transferred to droplets
by convection from air. The moisture from the interior migrates to the surface
and evaporation takes place from the surface. The air temperature is so high that
migration of moisture from the interior of the droplet cannot provide the
moisture necessary to keep the surface wet. So the surface becomes dried
quickly and the moisture in the interior produces vapor that expands the
droplets. The final product depends on the nature of the surface layer:
Layer
is porous and rigid ® No change
Layer
is less porous and rigid ® Broken particles
Layer
is non-porous and plastic ® Expanded hollow particles
(D) Separation
and recovery of dried products
Majority of dried product
falls to the base of the chamber where primary separation takes place. The
particles entrained in the air is recovered by any one of the separation
equipment like cyclone separator, bag filters, electrostatic precipitators or
wet scrubbers.
Applications
1. For drying pharmaceutical products three
types of spray dryers are used:
(i)
Standard open
cycle type: Used from drying infusions, extracts, adrenaline etc. Air is
used for drying.
(ii)
Aseptic open
cycle type: Used to dry antibiotics, vitamins, destrans, hormones, enzymes
etc. in aseptic condition. The air is made sterile by passing through HEPA
filter.
(iii) Closed cycle type: This type is commonly
used to remove organic solvent by using nitrogen instead of air. This type is
used for inflammable solvents, for drying toxic substances and for complete
recovery of solvent.
2. For spray
congealing: In this process solid drug particles are suspended in molten
waxes (or congealing polymers) and sprayed in a chamber containing cool air.
The droplets of wax congeal around the drug particles. Thus the taste of
unpalatable drugs can be masked and sustained release products can be produced.
3. Spray coating:
Solid drug particles are suspended in a coating solution (containing polymer)
and the slurry is sprayed in a drying chamber. The solvent of the coating
solution evaporates leaving a coating on the solid drug particles. Liquids can
also be coated by this method. An emulsion is prepared with the liquid drug as
internal phase and coating solution as the external phase. The emulsion is
sprayed in the drying chamber. Solvent is evaporated from the external phase
leaving a coating over the dispersed liquid drug.
FLASH DRYERS
Principle
The wet solid mass
is suspended in a finely divided state in a high-temperature (3000 to 6000
ft/min), high temperature (3000F to 13000F) air stream.
The resultant attrition exposes new surface for rapid drying. This process is
called flash drying because the
drying time is extremely short.
The dried product
is collected by a cyclone separator, followed by a bag filter or wet scrubber.
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