UNIT 3 DRYING.pptx

PHARMACEUTICAL ENGINEERING
(BP304T)
Unit-IiI
Drying
Name: Mrs. Pooja Eknathji Waware
Assistant Professor
G H RAISONI UNIVERSITY SCHOOL OF PHARMACY

CONTENTS:
• OBJECTIVES OF DRYING
• APPLICATIONS OF DRYING
• MECHANISM OF DRYING PROCESS
• RATE OF DRYING CURVE
• CLASSIFICATION OF DRYERS
• TRAY DRYER
• DRUM DRYER OR ROLL DRYER
• SPRAY DRYER
• FLUIDIZED BED DRYER (FBD)
• VACUUM DRYER
• FREEZE DRYER

Drying
•Drying is define as the removal of small amounts of
water or other liquid from a material by application
of heat.
• In general drying process involves two steps.
• 1) Heat transfer 2) Mass transfer
•Drying and Evaporation are relatively same term
are distinguish in terms of removal of water. Hear
are some differences between drying and
evaporation

 Difference between Drying and Evaporation
Drying Evaporation
In drying processes the main operation
usually carried out on solid materials, e.g.
powders or products
In Evaporation processes the main operation
usually carried out on liquid materials, e.g.
solution or products
Drying in the most of the cases means the
removal of relatively small amounts of water
from solids.
Evaporation includes the removal of large
amounts of water from solutions.
Drying involves the removal of water at
temperatures below its boiling point.
Evaporation involves the removal of water by
boiling solutions.
In drying, water is usually removed by
circulating air over the material in order to
carry away the water vapour.
While in evaporation, water is removed from
the material as pure water vapour mixed with
other gases.

OBJECTIVES OF DRYING
 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
following 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.
Flow ability, compressibility

APPLICATIONS OF DRYING
Preservation of drug product: Drying is necessary in order to
avoid deterioration.
• For examples protection of blood products, skin, tissues and
crude drugs from microbial growth.
• Effervesant tablets, synthetic and semisynthetic drugs undergo
chemical decomposition.
 Preparation of bulk drugs: In the preparation of bulk drugs,
drying is the final stage of processing.
• examples – dried aluminium hydroxide,
• spray dried lactose
• and powdered extracts.

Improved characteristics: Drying produces materials
of spherical shape, uniform size, free flowing and
enhanced solubility.
Granules are dried to improve the fluidity & compression
characteristics these are essential for production of
tablets and capsules.

Improved handling
• Removal of moisture makes the material light in weight and
reduces bulk.
• Thus cost of transportation will be less & storage will be
efficient.
• To make the materials easy or more stable for handling.
• Drying reduces moisture content

Theory of drying
• In a wet solid mass, water may be present as bound water and unbound water.
• Bound Water: bound water is the minimum water held by the material that
exerts an equilibrium vapour pressure less than the pure water at the same
temperature.
• Substances containing bound water are often called Hygroscopic substances.
• Unbound water : it is the amount of water held by the material that exerts an
equilibrium vapour pressure equal to that of pure water at the same
temperature.
• Unbound water exist mostly in the voids of solids. Thus, in a non hygroscopic
material, all the liquid is unbound water.

In hygroscopic material, the unbound moisture is the liquid in excess of the
equilibrium moisture content.
Measurement of the moisture in a wet solid is referred as moisture content,
or MC.
% 𝑴𝒐𝒊𝒔𝒕𝒖𝒓𝒆 𝒄𝒐𝒏𝒕𝒆𝒏𝒕 = 𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒘𝒂𝒕𝒆𝒓 𝒊𝒏 𝒔𝒂𝒎𝒑𝒍𝒆
𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒅𝒓𝒚 𝒔𝒂𝒎𝒑𝒍𝒆 × 𝟏𝟎𝟎

Hygroscopic
materials
Water in:
- fine capillaries
- Cell and fiber walls
- Physical interaction
Vapour pressure of
wet solids is less
than the v.p of pure
water
Non-Hygroscopic
materials
Water in void spaces
Vapour pressure of wet
solids is equal to v.p of
pure water
Bound water
UnBound water

Mechanism of Drying Process
•Drying involve two steps-
•Heat Transfer
It takes place from the heating medium to the
solid material.
•Mass Transfer
It involves the transfer of moisture to the
surface of solids and subsequently vapourisation from the
surface into surrounding.

Diffusion theory
• Diffusion theory the rate of flow of water is proportional to moisture
gradient.
• According to this theory, moisture movement may be as follows:
i. Water diffuses through the solid to the surface and subsequently
evaporates in to surroundings.
ii. Evaporation of water occurs at an intermediate zone, much below
the solid surface, then vapours diffuse through the solid into air.

Capillarity Theory
• Capillary theory is applicable to porous granular solids with network
of inter-connected pores and channels.
• As the drying starts, a meniscus is formed in the capillary and exerts a
force.
• This is the driving force is greater in small pores compared to the
large pores. Therefore, small pores pull more water from the larger
pores and thus large pores get emptied first.
• This theory is applicable to hygroscopic material.

Equilibrium Moisture Content (EMC) :
It is the amount of water present in the solid which exerts a vapour pressure equal to the
vapour pressure of the atmosphere surrounding it.
• Depending upon temperature and humidity conditions solids may absorbed or loss
moisture.
• Desorption:
When air is continuously passed over the solid containing moisture more than
EMC, then solid loses water continuously till EMC is reached. This phenomenon is
known as desorption.
• Sorption:
When air is continuously passed over the solid containing moisture less than
EMC, then solid absorbs water continuously till EMC reached. This phenomenon is
known as sorption.

•Measurement of EMC :
The solid samples are placed in a series of closed chamber such
as desiccators.
Each chamber consist of solutions which maintain a fixed
relative humidity in the enclosed air spaces.
Solids samples are exposed to several humidity conditions,
the exposure is continued until the material attains a constant
weight.
The difference in the final & initial weights gives the
moisture content.

Factor affecting EMC
• Nature of material :
Nonporous insoluble solids have an EMC zero ex- Talc
For fibrous or colloidal organic substances, EMC values are
high.
For porous solids, EMC values are much higher & variable.
• Nature of Air:
For air zero humidity, EMC of all materials is zero.
As the temperature of air increases, the EMC of solid
decreases.

Free Moisture Content (FMC):
It is the amount of water that is free to evaporate from the
solid surface.
Free moisture content (FMC) = Total water content – EMC
The moisture present in the solid can be expressed on a wet weight or
dry weight basis.
% loss on Drying(LOD) = Mass of water in sample(kg)/ total mass of
wet sample (kg) ×100
% moisture content(MC) = Mass of water in sample(kg)/ total mass of
dry sample (kg) ×100

Behavior of solids During Drying or Rate of Drying
•The rate of drying of a sample can be determined by
suspending the weight material on a balance in a
drying cabinet and measuring the weight of sample
as it dries as a function of time.
•Drying rate = Weight of water in sample (kg)/
Time(h) ×Wt. of dry solid (kg)

Drying rate curve
•It is obtained by plotting a graph of FMC on x-axis &
drying rate on y-axis.
•Depending on external conditions & internal mechanism
of fluid flow,
solids show different drying patterns.

1. Initial adjustment period
• It is time corresponding to AB curve,
which is also called as “Heating Up
Period”.
• During this period solids absorbs heat &
temperature is increases i.e. an wetted
substance when kept for drying it absorbed
heat from surrounding & vaporization of
moisture takes place which cools the
surface.
• Heat flows to the cooled surface at higher
rates, leads to rise in temperature &
evaporation again.

2. Constant rate period
• It is time corresponding to BC curve in graph. The
temperature remains constant & rate of drying is
constant.
• During this period, there is a continuous liquid
film over the surface of solid.
• Moisture evaporating from the surface is replace
by the water diffusing from the interior of the
solid. Also the drying rate remains constant as
show in curve by region BC.
• Rate of diffusion = Rate of Evaporation

3. First Falling Rate Period
• Also known as period of unsaturated
surface drying. It is time corresponding to
CD Curve.
• During this period, surface water is no
longer replaced at a rate fast enough to
maintain a continuous film on the surface.
• Dry spots begin to appear & rate of drying
begins to fall off. The point D is referred
to as second critical point.

4. Second Falling Rate Period
• It is time corresponding to DE in graph.
• During this period, rate of drying falls
even more rapidly than the first falling
rate & no film is present on surface.
• At the end, the drying rate becomes zero
& moisture content of solids at this point
( E) referred to as Equilibrium Moisture
Content(EMC) .

Classification of Dryers
Static Bed Dryers (Ex. Tray Dryers, Freeze
Dryer)
Moving Bed Dryer (Ex. Drum
Dryer)
Fluidized Bed Dryer
Vacuum Dryer
Pneumatic Dryer (ex. Spray
Dryer)

TRAY DRIER
Principle of Tray Dryer:
In tray dryer hot air is continuously
passed over wet mass.
Heat transfer takes place by forced
convection to remove moisture from solids
placed in trays.
Construction :
It consists of a rectangular chamber whose
walls are insulated.
 Trays are placed inside the heating
chamber.
The number of trays may vary with the size
of the dryer.

Each tray is rectangular or square and about 1.2 to 2.4
meters square in area .
Trays are usually loaded from 10.0 to 100 millimeters
deep. Dryer is fitted with a fan for circulating air over the
trays.
Electrically heated elements are provided inside (rather
than outside) to heat the air. In the corner of the chamber,
direction vanes are placed to direct air in the expected path.

WORKING
• Wet solid is loaded into trays. Trays are placed in the chamber.
• Fresh air is introduced through inlet which passes through the heaters and
heated up.
• The hot air is circulated by means of fans at 2 to 5 meter per second.
• The water is picked up by air and moist air is removed from outlet.
• During the cycle of drying only 10 to 20 % of fresh air is introduced and
80 to 90% air is circulated back.
• https://www.youtube.com/watch?v=dSTb3e80400

MERITS
• The method is operated batch wise.
• Attrition is not observed.
• Loading and unloading can be done without losses.
• DEMERITS
• Only a fraction of the solid particles is directly exposed.
• The method is costly and time consuming.
• USE
• Sticky materials, plastic substances, granular mass or crystalline
materials, precipitates and paste can be dried in a tray dryer.

DRUM DRYER OR ROLLER DRYER
Principle
• In drum dryer heated hollow cylindrical drum is rotate on longitudinal axis,
which is dipped into the solution to be dried.
• The solution is carried as a film on the surface of the dryer and dried to form
a layer.
• Dried material is removed with the help of the knife.
Construction
• The drum dryer consists of a horizontally mounted hollow steel drum of 0.6
to 3.0 meters diameter and 0.6 to 4.0 meters length, whose external surface is
smoothly polished.
• Below the drum, feed pan is placed in such a way that the drum dips partially
into the feed.

• On one side of the drum a spreader
is placed and on the other side a
doctor’s knife is placed to scrap the
dried material.
• A storage bin (or a conveyor ) is
placed connecting the knife to
collect the material.

WORKING
• Steam is passed inside the drum. Heat transfer coefficient of
the drum metal is high. Heat is transferred by conduction to
the material.
• Simultaneously drum is rotated at the rate of 1-10
revolutions per minute. The liquid material present in the
feed pan adheres as a thin layer to the external surface of the
drum during its rotation.
• The materials are completely dried during its journey in
slightly less than one rotation (from one side to another side
of the drum).
• The dried materials are scrapped by the doctor’s knife, which

• The dried materials are scrapped by the doctor’s
knife, which than falls into a storage bin.
The time of contact of the material with hot metal is
6 to 15 seconds only.
Therefore processing conditions such as film
thickness, steam temperature are closely controlled.

• MERITS:
• The method gives rapid heat drying and mass transfer are higher The entire material is
continuously exposed to heat source.
• The equipment is compact.
• Heating time is short being only a few seconds.
• The product obtained is completely dried and is in the final form.
• DEMERITS:
• Operating conditions are critical.
• Attrition is not possible.
• Skilled operators are essential to control feed rate ,film thickness, speed of rotation
and temperature.
• Maintenance cost of a drum dryer is higher than spray dryer.
• It is not suitable for solutions of salts with less solubility.

Use
• Drum dryer is used for drying solution, slurries, suspensions etc.
• The products dried are milk products, starch products, ferrous salts,
suspensions of zinc oxide, suspension of kaolin, yeasts, pigments, malt
extracts, antibiotics, DDT, calcium, insecticides and barium carbonates

SPRAY DRYING
• Principle:
• In spray drier the fluid to be dried is atomized in to fine droplets,
which are thrown radially into a moving stream of hot gas.
• Temperature of droplets is immediately increased & fine droplets
get dried in the form of spherical particles.
• This process completes in few seconds before the droplets reach
the wall of dryer.

Construction
• It consist of large cylindrical drying
chamber with a short conical bottom made
up of stainless steel.(diameter 2.5 to 9.0m
and height 25m or more)
• An inlet for hot air placed in the roof and
another inlet carrying a spray disk
atomizer is also set in the roof.
• The spray disk atomizer is about 300 mm
in diameter and rotates at a speed of 3000
to 50,000 rpm.
• Bottom of the dryer is connected to cyclone
separator.

Working of Spray Dryer
Drying of material in spray dryer involves 3 stages:
Atomization of liquids:
The feed is introduced through the atomizer either by gravity or using suitable
pump.
Drying of the liquid droplets:
Fine droplets are dried in the drying chamber by supplying hot air through the inlet.
Recovery of the dried product:
Centrifugal force of atomizer drives the droplets to follow helical path. Particles are
dried and collected at the conical bottom.
 All these processes are completed in few seconds.
Particle size obtained is ranging from 2 to 500 mm. maximum size of spray dryer has capacity
of 2000 kg/hr.
https://www.youtube.com/watch?v=0o4ZCjHnaRw

Uses
• It is used in the production of dry powder formulation/dry
powder aerosol and thermolabile materials.
• Product quantity to be dried is large.
• Spray dryer is used in drying pharmaceuticals like
penicillin, blood products, enzymes, vaccines, etc
• Apart from its applications in the pharmaceutical
industries, spray dryers also find use in; chemical
industries, ceramic industries, food industries, etc.

Advantages
• It is continuous process & dried completely 3 to 30 sec.
• Labor cost is low
• Product of uniform & controllable size is obtained
• Products shows excellent solubility.
• Disadvantages
• It is bulky and expensive
• Huge equipment not always easy to operate
• Thermal efficiency is low
• Suitable for drying of sterile products

Fluidized Bed Dryer
• ‘Fluidized’ means something that behaves like liquid. In the
fluidized bed dryer, the mixture of solids and gas behave like a
liquid and solid are called fluidized.
• It provides good contact between hot air and particles to obtain
efficient drying.
• The hot air is passed through a mesh, which supports the
conical vessel with a porous base.
• This vessel is filled with powder to be dried.

Principle :
• In fluidized bed dryer hot air is passed at high pressure through
a perforated bottom of the container.
• The granules are lifted from the bottom and suspended in the
stream of air, this condition is called as fluidized state.
• The hot gas surrounding every granules to completely dry
them.

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 trough a prefilter, which is then heated when passing
trough 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.
Uses:
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

Advantages
• Require less time for drying i.e., 20 to 40 min.
• It is available in different sizes with the drying capacity 5 to 200 kg
• Labour cost are low
• Also used for mixing the ingredients
• Hot spot are not observed in the dryer
• Higher drying temp. can be used

Disadvantages
• Some powder are organic which develop electrostatic charges to
avoid this, efficient electrical earthing of the dryer is essential
• Chances of attrition of some materials resulting in production of
fines.

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 :
• It is made up of cast and iron heavy jacketed vessel. It is so strong
that it can withstand high vacuum within the oven & steam pressure
in the jacket.
• The enclosed space is divided into a number of portions by means
of 20 hollow shelves, which are part of jacket.
• These shelves provide large area 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.
• 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 30℃.
• The vapour goes to the condenser.
• After drying vacuum line is disconnected.
• Then the materials are collected from the tray.
• https://www.youtube.com/watch?v=HqP45slStpw

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
• 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
It is also known as lyophilization i.e. system is
made solvent loving for removing the same.
• Principle:
• In freeze drying , water is removed from the frozen state
by sublimation i.e. direct change of water from solid into
vapour without conversion to liquid phase.
• Sold-liquid-vapour equilibrium phase diagram of water Is
usefull to decide the experimental conditions.

• The drying is achieved by subjecting material to temperature & pressure
below the triple point.
• under this conditions, any heat transferred is used as a latent heat & ice
sublimes directly into vapour state
• The water vapour is removed from the system by condensation in a cold
trap maintained at a temperature lower than frozen material.

construction
• It consists of
• Drying chamber in which trays are loaded.
• Heat supply in radiation Vacuum pump or
stream ejector or both. source, heating
coils.
• Vapour condensing or adsorption system.
• Vacuum pump or ejector or both.

Working
• The following Preparation and pretreatment
Preparation and pretreatment
Prefreezing for solidifying water
Secondary Drying
Primary Drying
Packing

1.Preparation & Pretreatment
• The volume of solution introduced into the container is limited
by its capacity. Therefore pretreatment is essential. The
solutions are preconcentrated under the normal vacuum tray
drying. This reduces the actual drying by 8 to 10 times.
• 2. Prefreezing for solidifying water: Vials, ampoules or bottles
in which the aq. Solution is packed are frozen in cold shelves (-
50ºc)
• The normal cooling rate is about 1 to 3 Kevin/minute so tghat
large ice crystals with relatively large holes are formed on
sublimation of ice. This is also responsible for giving a porous
product.

3. Primary Drying:
• It means sublimation of ice under vacuum. The temp. & pressure
should be below the triple point of water i.e 0.0098ºc & 4.58 for
mmHg, sublimation, when water is alone present.
• when a solution a solid is dried, the depression of freezing pont of
water occurs.
• Hence, it is essential that the temperature be brought below the
eutectic point.
• The pressure & temp. at which the frozen solid vaporizes without
conversion to liquid is referred to as the eutectic point.
• Depending on the drug substances dissolved in water, the eutectic
point is determined. This usual range is from -10ºc to 30ºc.
• The condition of 1 to 8 K below eutectic point is sufficient.

• Vacuum is applied to the tune of about 3 mmHg on the
frozen sample and the temp. is linearly increased about 30ºc
in a span of 2hrs.
• Heat (About 2900 kilojoules /kg) is supplied which transfer
as latent heat & ice sublimes directly in to vapour state.
• As the drying proceeds, thickness of dried solids increases.
• Primary drying stage removes easily removable water, about
98% to 99%.

Secondary Drying
• It is removable of residual moisture under high vacuum.
• The temp. of solid is raised to as high as 50 to 60ºc but
vacuum is lowered below that is used in primary drying.
• The rate of drying is very low & it takes about 10 to 20 hrs.
•Packing:
After vacuum is replaced by inert gas, the bottles & vials
are closed.

Uses
•It is used in production of injection, solutions, and
suspension
•used for production of blood plasma and its
fractionated products
•bacterial and viral cultures
• antibiotics and plant extracts
• steroids, vitamins and enzymes.

Advantages
•Denaturation of protein does not occur.
•Loss of volatile material is less.
•Sterility can be maintained
•Thermo labile substances can be dried.
• migration level can be kept as low as possible.
•Material can be dried in its final container such as
single done & multiple dose.

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.
• The product is prone to oxidation, due to the high
porosity and large surface area.
• Therefore, the product must be vacuum packed or with
an inert gas or in container.

UNIT 3 DRYING.pptx

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