Theories of drying, Drying of solids, Classification of dryers, General methods, Fluidized bed systems, Pneumatic Dryers, Spray dryers, Freeze drying

1. SAMIA GHANI
M. PHIL PHARMACEUTICS

2.  Definition
 Introduction
 Non-thermal processes of drying

3.  Can be defined as
 Removal of liquid from a material by the application of heat, and is accomplished by
transfer of liquid from a surface into an unsaturated vapour phase.
 Drying and evaporation are distinguishable merely by the relative quantities of liquid
removed from the solid.

4.  Drying can be described by simultanoeus operation of three processes:
 Energy transfer from an external source to the water or organic solvent.
(Direct or indirect heat transfer)
 Phase transformation of water/solvent from liquid to vapour state
(Mass transfer (solid charactersitics))
 Transfer vapour generated away from the API and out of drying equipment
 After the moisture is removed, product is maintained at low water levels by the use of
dessicants and/or low moisture transmission packaging material.

5.  There are many nonthermal processes of drying such as:
 Expression (squeezing of a wetted solid or sponge to remove liquid)
 Extraction (of a liquid from solid by using a solvent)
 Adsorption (of water from a solvent by using desiccants such as anhydrous calcium
chloride)
 Absorption (of moisture from gases by passage through a sulphuric acid column)
 Desiccation (of misture from a solid by placing it in a sealed container with a moisture
removing material (silica gel))

6. Difference between Drying and Evaporation
Sr. No. Drying Evaporation
1 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
2 Drying in most of the cases means the
removal of relatively small amounts of
water from solids.
Evaporation include the removal of large
amounts of water from solutions.
3 Drying involves the removal of water
at temperatures below its boiling
point.
Evaporation involves the removal of water
by boiling a solution.
4 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.

7.  To prepare granules (which will be dispensed in bulk or compressed in tablets).
 To process materials (prep of dried aluminium hydroxide, spray drying of lactose, prep
of powderd extract).
 To lower the cost of transportation and storage.
 To prevent microbial or other types of contamination that will aid in preservation.
 To facilitate comminution by making dried substance more friable.
 To improve or keep the good properties of a material, e.g. flowability, compressibility.

8.  Preservation of drug products
 Preparation of bulk drugs
 Improved handling
 Improved characteristics
 Reduction in transport cost
 Purification of crystalline products
 Prevention of corrosion

9. Drying is necessary in order to avoid
deterioration.
A few examples are… Blood products,
tissues undergo microbial growth,
effervescent tablets, synthetic &
semisynthetic
drugs undergo chemical decomposition.

10.  Drying is the final stage of processing.Eg:
dried aluminium hydroxide, spray dried
lactose, powdered extracts

11.  Drying produces materials of spherical shape, uniform size, free flowing &
enhanced solubility.
1. Granules are dried to improve the fluidity & compression characteristics.
These are essential for production of tablets and capsules.
2. Viscous & sticky materials are not free flowing, Drying modifies these
characteristics.

12.  Removal of moisture makes the material light in weight and reduces bulk.
 Thus cost of transportation will be less & storage will be efficient.
 If moisture is present, size reduction of drugs is difficult.
 Drying reduces the moisture content.

13.  Particle size
 Nature of material
 Nature of moisture (bound/unbound)
 Surface area
 Initial and final moisture content
 Thickness of material bed
 Temperature
 Amount of moisture
 Nature of product

14.  Drying involves both heat and mass transfer operations
 Heat must be trasferred to the material to be dried in order to supply the latent heat
required for vaporization of moisture. Drying can be understood easily by considering
a film of liquid at the surface of material to be dried. Rate of evaporation is related to
rate of hea transfer to this film.
 dW/d𝛳= q/λ
 Where dW/d𝛳 is rate of evaporation
 q is the overall rate of heat transfer
 λ is latent heat of vaporization of water

15.  Secondly, mass transfer is involved in diffusion of water through the material to the
evaporating surface, in the subsequent evaporation of water from the surface and in
diffusion of resultant vapor into passing air stream.
 dW/d𝛳=k’ A(HS-Hg)
 Where: dW/d𝛳 is the rate of diffusion
 k’ is the coefficient of mass transfer
 A is the area of evaporating surface
 HS is the absolute humidity at he evaporating surface
 Hg is the absolute humudity of the passing air stream

16.  In a wet solid mass water may be present in two forms;
 1) 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.
 2) 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 exists largely in the voids of solid thus in non-
hygroscopic material, all the liquid is unbound water.

17.  Theory of drying can be discussed under two headings
 A) Equilibrium relationships
 B)Raterelationships
 A) Equilibrium relationships:
 Air of constant humidity and temperature is passed over wet material
after long exposure equilibrium is reached.
 Equilibrium moisture content (EMC):
It is amount of water which exerts vapour pressure equal to the vapour
pressure of atmosphere surrounding it.

18.  Based on the conditions of temperature and humidity solid will either
lose or absorb the moisture;
1) When air is continuously passed over the solid containing moisture
more than EMC then solid lose water till the EMC is reached. This
phenomenon is known as Desorption.
2) When air is continuously passed over the solid containing moisture less
than EMC then solid absorb water till EMC is reached. This phenomenon is
known as Sorption.
 Moisture in solid>EMC=desorption (lose water)
 Moisture in solid<EMC=sorption (gain water)

20.  Free moisture content (FMC): It is the amount of water that is free to
evaporate from solid.
 FMC =Total water content – EMC
 B)Raterelationships:
 Rate relationship is observed by considering a simple model which mimic
the conditions of a dryer. In this model wet slab of solid is considered and
hot humid air is passed over it. The change in weight is determined by
weighing the slab at different time interval and following calculations are
made;

22.  Rate of drying of a sample is determined by suspending wet material on a scale or
balance in a drying cabinet and measuring weight of sample as it dries as a functio, of
time.
 To determine accurate drying rate curve of a material in a oven, than drying condition
should approximate the conditions in a full sized dryer as closely as possible.
 From the data obtained by above experiment, graph is plotted by taking FMC on x-
axis and drying rate on y-axis.
 The curve obtained is known as drying rate curve.

23. Initial
adjustment
First
falling
rate
period
Moisture
content
Constant
rate
period
Second
falling rate
period
Critical
moisture
content
Secon critical
point
Equilibrium
moisture
content
B
A
D
E
C
Drying time

24.  Segment AB-initial adjustment
 Segment BC-constant rate period
 Point C- critical moisture content
 Segment CD-first falling rate period
 Segment DE- second falling rate period
 Point E- equilibrium moisture content (EMC)
 The conditions in which material is in equilibrium with its surrondings neither gainig, nor
losing moisture may be expressed in terms of;
 Equilibrium moisture content (EMC)
 Equilibrium relative humidity (ERH)
 Water activity (aw)

25.  Equilibrium moisture content (EMC):
The moisture content of a material that is in equilibrium with an atmosphere of given
relative humidity is called EMC.
No driving force for mass transfer
 Equilibrium relative humidity (ERH):
The relative humidity surrondig a material at which the material neither gains nor loses
moisture is called ERH.
 Water activity (aw):
The ratio of water vapour pressure exerted by the material to the vapour pressure of
pure water at the same temperature.
The aw of pure water is one when ERH is 100%

30.  Depending upon their drying behavious, Solids are classified into two major
categories:
 Granular/ crystalline type solids:
 Amorphous solids:

31.  Water in crystalline solids is held in shallow and open surface pores as well as
interstitial spaces between particles that are easily accessable to surface .
 Typical pharmaceuticals of this category are calcium sulphate, zinc oxide, magnesium
oxide etc
 Moisture is lost with little hinderance by either gravittioanl of capillary forces.
 Constant rate period is major portion of drying curve
 Materials of this category are usually inorganic substances.
 Not affected by heat (uless temp is to high to change hydrated forms of material)
 Equilibrium moisture contents for these materials are close to zero.

32.  Moisture is an integral part of molecular structure as well as physically entrapped in
fine capilary and small interior pores.
 E.g. materials with fibrous, amorphous or gelatinous structures such as starch, casein,
yeast, insulin and aluminium hydroxide.
 Constant rate period is short.
 Difficult to dry than crystalline solids.
 Drying requires lower temperatures, reduced pressure and increased air flow.

33.  1) Properties of material being handled
 Physical characteristics when dry
 Physical characteristics when wet
 Corrosiveness
 Toxicity
 Flammability
 Particle size
 Abrasiveness

34.  2) Drying characteristics of material:
 Type of moisture (bound/unbound/both)
 Initial moisture content
 Final moisture content
 Permissible drying temperature
 Probable drying time for different dryer
 3) Flow of material to and from dryer:
 Quantity to be handled per hour
 Type of operation (batch/continuous)
 Process prior to drying
 Process subsequent to drying

35.  4) Product qualities:
 Shrinkage
 Contamination
 Uniformity of final moisture content
 Decomposition of product
 Rate of subdivision
 Product temperature
 Bulk density
 5) Recovery problems:
 Dust recovery
 Solvent recovery

36.  6) Facilities available at site of installation:
 Space
 Temperature
 Humidity
 Cleanliness of air
 Available fuels
 Available electric power
 Source of wet feed
 Permissible noise, vibration, dust or heat losses
 Exhaust-gas outlets

37. Classification of dryers
Criteria Types
1. Mode of action • Batch
• Continuous*
2. Heat Input type • Conduction
• Convection*
• Radiation
3. State of Material in dryer • Stationary
• Moving, agitated, dispersed
4. Drying Temperature • Below boiling temperature*
• Above boiling temperature
• Below freezing point
5. Operating Pressure • Vacuum*
• Atmospheric
6. Relative Motion between drying medium
and drying solids
• Co-current
• Countercurrent
• Mixed flow
6. Number of Stages • Single*
• Multi-Stage

40.  In its simplest form, there was a cabinet with heater at the bottom to assist convection.
Disadvantage:
 No control over heat and humidity
This situation can be controlled by including a fan, so that forced convection can take place.

43.  Air is heated and directed across the material in a controlled form.
 Heaters are positioned in a way that air is reheated before passing over each shelf.
 Further, economy of heat can be affected by affected by re- circulating a portion of air
through the duct connecting the outlet back to the inlet.
 Advantage: Controlled heating and Humidity
 Disadvantage: Limited output due to size (usually 6 shelves) and batch operation.

45.  Uses
 Drying of crude drugs
 Drying of Powders
 Drying of Tablet granules
 Drying of parts of Equipment

47.  Conduction is used as principal method of heat transfer in dryers which are operated
under vacuum; when air is absent
 For example;
 Vacuum Oven

49.  It is a jacketed vessel constructed in a way to withstand pressure in oven.
 It contains shelves, giving large surface area for conduction heat transfer.
 Usually 20 shelves – 45 – 50 m² area
 The oven can be closed by a door to give air tight seal.
 The oven is connected to a condenser and receiver to a vacuum pump.
 Operating pressure is usually; 0.03 to 0.06 bar.
 At this pressure water boils at 25 to 35°C

50.  Vacuum oven are suitable for unstable materials as drying takes place at low
temperatures.
 Vacuum operation reduces the risk of oxidation.
 Use of condenser enables solvent recovery.

51.  Low heat transfer coefficients
 Vacuum oven have limited capacity
 Labor and running costs are high
 Trained labors are required
 Excessive temperature gradient may lead to product decomposition or damage to
vessel.

53. FREEZE DRYING
OR
LYOPHILIZATION
 Freeze drying is a process used to dry extremely heat – sensitive materials. It
allows the drying , without excessive damage, of proteins, blood products and even
microorganisms, which retain a small but with significant viability.
 Definition: “Lyophilization or freeze drying is a process in which water is frozen,
followed by its removal from the sample, initially by sublimation (primary drying) and
then by desorption (secondary drying)”.

54. FREEZE DRYER

55.  Vapour pressure of water on the surface of material being dried must be higher than
the partial pressure of enveloping atmosphere i.e. there must be a positive vapour
pressure driving force.
 The latent heat of vaporization must be introduced to the drying solid at such a rate as
to maintain desirable temperature levels at both the surface and interior.
 Provision must be made for the removal of evaporated moisture .

56.  Chamber for vaccum drying: generally designed for batch operation and thus can be
compared with vaccum shelf dryer.
 A vaccum source: vaccum is achieved by pumps, steam ejectors, or combination of
both.
 Heat source: heat is provided by conduction or radiation or by both ways.
 A vapor-removal system: removal of water vapors are employed by condensers,
desiccants and pumps.

57. STAGES OF THE FREEZE DRYING PROCESS
 1- Freezing stage:
 The liquid material is frozen before the application of vacuum to avoid frothing, and several
methods are used to produce a large frozen surface.
 a- Shell freezing : This is employed for large volumes such as blood products. The bottles
are rotated slowly and almost horizontally in a refrigerated bath. The liquid freezes in a
thin shell around the inner surface of the bottle.
 Freezing is slow and large ice crystals form, which is a drawback of this method.
 In vertical spin freezing the bottles are spun individually in a vertical position , centrifuged
and cooled by a blast of cold air. The solution super cools and freezes rapidly, with the
formation of small ice crystals.
 b- Centrifugal evaporative freezing: The solution is spun in small containers within a
centrifuge. This prevents the foaming when a vacuum is applied.

58.  2 - Vacuum application stage: The containers and the frozen material must be connected to a vacuum
source sufficient to drop the pressure below the triple point and remove the larger volumes of low –
pressure vapour formed during drying.
 3 - Sublimation stage:
 Heat of sublimation must be supplied. Under these conditions the ice slowly sublimes, leaving a porous
solid which still contains about 0.5% moisture after primary drying .
 Primary drying: It can reduce the moisture content of a freeze-dried solid to around 0.5%. Further
reduction can be affected by secondary drying .
 Heat transfer: Insufficient heat input prolongs the process, which is already slow, and excess heat will
cause melting.
 Vapour removal: The vapour formed must be continually removed to avoid a pressure rise that would
stop sublimation.
 Rate of drying: The rate of drying in freeze drying is very slow, the ice being removed at a rate of about
only 1mm depth per hour.

59.  4- Secondary drying:
 The removal of residual moisture at the end of primary drying is performed by raising the temperature of the solid
to as high as 50 or 60 C or by using desiccants.. This can cause removal of bound water or traces of water left
after primary drying.
 5- Packaging:
 Attention must be paid to packaging freeze-dried products to ensure protection from moisture. Containers
should be closed without contacting the atmosphere.
 Advantages of freeze drying
 1- Drying takes place at very low temperatures, so the chemical decomposition, particularly hydrolysis is
minimized.
 2- The product is light and porous.
 3- The porous form of the product gives ready solubility.
 4- As the process takes place under high vacuum there is little contact with air, and oxidation is minimized.

60. DISADVANTAGES & USES OF FREEZE DRYING
 Disadvantages:
There are two main disadvantages:
1-The porosity, ready solubility and complete dryness yield a very hygroscopic product.
Packing require special conditions.
2-The process is very slow and uses complicated plant, which is very expensive .
It is not a general method of drying but limited to certain types of valuable products.
Uses of freeze drying
 The method is used for products that can not be dried by any other heat method.
These include biological products, e.g. antibiotics, blood products, vaccines (BCG,
yellow fever, small pox), enzyme preparations (hyaluranidase) and microbiological
cultures.

62.  In tunnel dryers, material is moved from one end of tunnel to the other in a way that outgoing
material meets the incoming hot air to ensure maximum drying.
 Such counter current movement of air and drying solid is arranged by following ways;
 Placement of solid materials on rails from inlet and collecting product from other end.
 Mechanical conveyers having automatic speed control, temperature and humidity control, including
pre heating if necessary.

63.  Advantages:
 Continuous operation compared to Trays
 Large Scale Production
 Uses
 Drying of crude drugs
 Drying of Powders
 Drying of Tablet granules
 Drying of parts of Equipment

67. DRUM DRYER (FILM DRYING)
It consists of a drum of about 0.75-1.5 m in diameter and 2-4 m in length, heated internally,
usually by steam, and rotated on its longitudinal axis.
 Operation: The liquid is applied to the surface and spread to a film, this may be done in
various ways, but the simplest method is that shown in the diagram, where the drum dips into
a feed pan. Drying rate is controlled by using a suitable speed of rotation and the drum
temperature. The product is scraped from the surface of the drum by means of a doctor
knife (two bladded knife).
 Fig. Drum dryer

70. ADVANTAGES OF THE DRUM DRYER
1- The method gives rapid drying, the thin film spread over a large area resulting in rapid heat and mass
transfer.
2- The equipment is compact, occupying much less space than other dryers.
3- Heating time is short, being only a few seconds.
4- The drum can be enclosed in a vacuum jacket, enabling the temperature of drying to be reduced.
5- The product is obtained in flake form, which is convenient for many purposes.
 DISADVANTAGES
 The only disadvantage : is that operating conditions are critical and it is necessary to introduce careful
control on feed rate, film thickness, speed of drum rotation and drum temperature.
 Uses:-
It can handle a variety of materials, either as solutions or as suspensions e.g. starch products, ferrous
salts and suspensions of kaolin.

72.  Prolonged drying times as in case of static dryers can be overcomed by use of vacuum
tumble dryers.
 Working under vacuum condition, this dryer provides controlled low-temperature drying,
possibility of solvent recovery, increase rates of drying.
 Most common shape is double cone vacuum dryer.
 Results in the achievement of uniform powders distinct fro cakes as in static-bed drying.
 Typical rotational speed of vacuum dryer is 6-8rpm.
 Vacuum is supplied by conventional pumps, blowers or steam jets.
 Heat is supplied to tumbling charge by contact with heated shell and by heat transfer
through vapor.
 Heated fluid enters through a jacket & enters & exits through dynamic seals along the axis
of rotation.
 Waxy solids can not be dried by this method.

74.  Indirect type moving bed dryers that may operate under atmospheric pressure or
vacuum & are generally used to dry small batches of pastes or slurries.
 Structural design: consists of shallow, circular, jacketed pan having 3-6 feet diameter
& 1-2 feet depth with flat bottom & vertical sides.
 There is a set of rotating plows in the pan that revolve slowly, scrapping the moisture-
laden mass from the walls and exposing new surfaces to contact with heated sides
and bottom.
 Heat is supplied by steam or hot water.
 Atmospheric pan drying allows the moisture to escape whereas in vaccum dryers
where pan is completely enclosed, solvents are recoverabe if evacuated vapors pass
through condenser.
 Dried material is discharged through a door on the bottom of pan.

76.  Rotary dryers are modified Tunnel dryers in which material passes through a rotating
cylinder, counter current to a stream of heated air.
 Due to rotation of material in cylinder, the material is turned over and drying takes
place from individual particle and not from a static bed.

77.  Advantages:
 Large surface area; Cylinder length upto10 m.
 Output of 4 Mg/h. = 4.4 Ton
 Continuous drying operation

83.  Can be used for;
 Drying
 Coating and
 Granulation
 Principle: Particle materials is fluidized by a fluid (gas or liquid) and this fluid is
enabled to pass through the bed of solids from below (perforated base).

84.  Advantages:
 Short Drying Time compared to static bed
 In fluidized state, individual particle drying
 Uniform temperature control
 Free flowing product
 High out put (up to 100 Kg)
 Disadvantages:
 Turbulence may cause attrition of some materials and leading to fine production.

90. SPRAY DRYER
The spray dryer provides a large surface area for heat and mass transfer by atomizing the
liquid to small droplets. These are sprayed into a stream of hot air, so that each droplet dries
to a solid particle.
 The drying chamber resembles the cyclone ensuring good circulation of air, to facilitate heat
and mass transfer, and that dried particles are separated by the centrifugal action.....

91.  The character of the particles is controlled by the droplet form; hence the type of atomizer is
important.
 Rotary atomizer is preferable than jet which is easily blocked. Liquid is fed to the disc of the
atomizer which is rotated at high speed (up to 20,000 rpm), a film is formed and spread as
uniform spray. In addition, the rotary atomizer is effective with suspensions. It can be operated
efficiently at various feed rates.
 Fig. Rotary atomizer

94. CHARACTERIZATION OF SPRAY DRIED PRODUCTS ;
The products are uniform in appearance and have characteristic shape, in the form of
hollow spheres with a small hole. This arises from the drying process, since the
droplet enters the hot air stream, and dries on the outside to form an outer crust with
liquid still in the center. This liquid then vaporizes, the vapour escaping by leaving a
hole in the sphere.
 This method of drying allows a dry product to retain some properties of feed , e.g., a
drop from an emulsion dries with continuous phase on the outside. When
reconstituted, the emulsion is easily re- formed.

96. ADVANTAGES OF THE SPRAY DRYING PROCESS
 1-The droplets are small, giving a large surface area for heat transfer, so that evaporation
is very rapid. The actual drying time of a droplet is only a fraction of a second, and the
overall time in the dryer is only a few seconds.
 2- Because evaporation is very rapid, the droplets do not attain a high temperature, most
of the heat being used as latent heat of vaporization.
 3- The characteristic particle form gives the product a high bulk density and, in turn,
ready solubility.
 4- The powder will have a uniform and controllable particle size.

97.  5- The product is free-flowing, with almost spherical particles, and is especially convenient for
tablet manufacture.
 6- Labour costs are low, the process yielding a dry, free-flowing powder from a dilute solution,
in a single operation with no handling.
 7- It is possible to operate spray driers aseptically using heated filtered air to dry products
such as serum hydrolysate.

98. DISADVANTAGES & USES
 The equipment is very bulky, connected to accessories, fans, heaters,)
 That is make it expensive.
 Uses:
 1- Drying of any substance in solution or in suspension form.
 2- It is most useful for drying of thermolabile materials e.g. antibiotics.
 3- Suitable for large quantities solution.
 4- Suitable for both soluble and insoluble substances e.g. citric acid, gelatin, starch.
 5- It can produce spherical particles in the respiratory range e.g. dry powder inhalers.
 6- Drying of milk, soap and detergents which is pharmaceutically related compounds.

103.  Microwave drying: Energy in the form of microwaves is converted into internal heat by
interaction with the material itself.
Heating effect is produced by interaction of rapidly oscillating electric field (915 or 2450)
with polaized molecules and ions inthe material.
Can be used for drying of pharmaceutical materials at low ambient temperatures,
avoiding high surface temperatures, case hardening and solute migration
Microwave vacuum drying at low pressure (1 to 20 mm Hg) and moderate temperature
(30 to 40°C) can be used for drying thermolabile materials such as vitamins, enzymes,
proteins and flavours.