Drying
Prepared by:
Mohammad Mahareeq
Definitions
Definitions
• Equilibrium moisture X*. This is the moisture content of a
substance when at equilibrium with a given partial pressure of
the vapor.
• Bound moisture. This refers to the moisture contained by a
substance which exerts an equilibrium vapor pressure less than
that of the pure liquid at the same temperature.
• Unbound moisture. This refers to the moisture contained by a
substance which exerts an equilibrium vapor pressure equal to
that of the pure liquid at the same temperature.
• Free moisture. Free moisture is that moisture contained by a
substance in excess of the equilibrium moisture: X – X*. Only
free moisture can be evaporated, and the free-moisture content
of a solid depends upon the vapor concentration in the gas.
• These relations are shown graphically in the following figure for
a solid of moisture content X exposed to a gas of relative
humidity A.
Types of Moisture
Definitions
Example
Drying
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Definition:
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The removal of a liquid from a material by
the application of heat.
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It is accomplished by the transfer of the liquid
from the surface of the material into the vapor
phase.
There are other methods to achieve drying
than with the application of heat such as:
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Desiccation in a sealed container with silica gel.
Freeze drying by application of vacuum.
Drying
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Purpose:
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It is most commonly used in preparation of
granules.
Other uses include:
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Spray drying of lactose.
Increase stability of moisture sensitive drugs
(aspirin)
Preservation of animal and vegetable drugs.
Psychrometry
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Psychrometry: is the determination of the
vapor concentration and carrying capacity of
the gas.
The most common drying in pharmaceutics
involves removal of water from the material
by air (air-water system).
The carrying capacity determines:
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Rate of drying.
Extent of drying (equilibrium content).
Psychrometry
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Psychrometric charts (humidity charts)
are helpful in understanding the
relationship between temperature and
humidity of the air-water vapor system
at constant pressure.
Psychrometry
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Humidity: is the concentration of water vapor in a
gas.
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Absolute humidity: weight of water vapor per unit weight of
dry air.
Saturation humidity: The absolute humidity at which the
partial pressure of water vapor in the air is equal to the
vapor pressure of free water at the same temperature
(100% relative humidity), curve CDE.
Relative humidity: the ratio of the partial pressure of water
vapor in the air to the vapor pressure of free water at the
same temperature.
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Which is also the ratio of the absolute humidity to the saturated
humidity at that temperature.
Psychrometry
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Dew point: is the temperature to which
a water vapor-air system must be
cooled to become saturated.
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The air holds the maximum amount of
water without condensation taking place.
When the vapor-air mixture is cooled below
its dew point water droplet form (2-phase
system).
Psychrometry
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Raising the temperature of a given air
leads to reducing its relative humidity
and thus can accept more water during
the drying process.
Psychrometry
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Wet bulb temperature: the equilibrium
temperature reached by an evaporating
surface when the rate of heat transferred to
the surface by convection is equal to the rate
of heat lost by evaporation.
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it can be measured by a thermometer whose bulb
is covered by a wick saturated with water.
The actual temperature of the air measured by an
ordinary thermometer is the dry bulb
temperatures.
Psychrometry
Psychrometry
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The wet bulb temperature is a function of the
temperature and the humidity of the air thus it can
be used to determine the humidity.
The constant wet bulb temperature line (AD) can be
used to determine the relative humidity:
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If the dry bulb temperature is 60 F and the wet bulb
temperature is 54 F.
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The intersect of 60 F and the 54 F constant wet bulb
temperature corresponds to 53 G/p absolute humidity.
The same vapor-air mixture at 60 F is saturated at point C
corresponding to 78 G/p.
So the relative humidity is equal to (53/78)*100% ~70%
Humidity measurement
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Gravimetric methods: A known amount of air is passed on a
pre-weighed amount of phosphorous pentoxide. The increase of
weight of phosphorous pentoxide represents the amount of
water in that sample.
Wet-bulb and dry bulb thermometers.
Hygrometer: the instrument utilizes a material that has different
properties at different humidities.
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Example hair or wood fiber that expands or shrinks with changes in
humidity.
Digital hygrometer.
Dew point hygrometer: by observing the temperature at which
moisture begins to form on a polished surface in contact with
air.
Theory of drying
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Drying involves both heat and mass transfer.
Heat must be transferred to the surface of
the material in order to provide the latent
heat of evaporation for the moisture.
Mass transfer involves the diffusion of water
through the material to the evaporating
surface, diffusion of the resultant vapor into
the passing air stream.
Drying Tests
• Rate of drying can be determined for a
sample of a substance by suspending it in a
cabinet or duct in a stream of air.
• The weight of the sample can be measured
as a function of time.
• The following conditions should be as
closely as possible to the large-scale
operation:
Drying Tests
 The sample should not be too small.
 The sample should be similarly supported in a tray or
frame.
 It should have the same ratio of drying to nondrying
surface.
 It should be subject to similar conditions of radiantheat transfer.
 The air should have the same temperature, humidity
and velocity (both speed and direction).
• If possible several tests should be made on
samples of different thicknesses.
Rate-of-drying Curve
• From the data obtained during the test, a curve of moisture
content as a function of time can be plotted (fig a).
• This is useful in determining the time required for drying
larger batches under the same drying conditions.
• The data is then converted into rates of drying, expressed as
N = mass/(area) (time) and plotted against moisture content
(fig. b)
a
b
Rate-of-drying Curve
• This can be done by:
 Measuring the slopes of tangents drawn to the curve of
Fig a) or by,
 Determining from the curve; small changes in moisture
content (∆X) for corresponding small changes in time
(∆θ).
• Calculating the rate as: (N=Ss ∆X/A ∆θ) where;
Ss: the mass of dry solid.
A: the wet surface over which the gas blows and through which
evaporation takes place.
• There are two major parts to the rate curve of Fig(b)
 A period of constant rate (B-C).
 A period of falling rate (C-E).
Rate-of-drying Curve
• If a solid is initially very wet, the surface will be covered with a
thin film of liquid. The rate at which moisture evaporates can be
described in terms of a gas mass-transfer coefficient ky:
Nc = Ky (Ys – Y)
Ys : humidity of the gas at liquid surface which is the saturated
humidity at temperature ts.
Y: humidity of the main gas stream.
• Evaporation of moisture absorbs latent heat of evaporation, the
liquid surface will come to, and remain at, an equilibrium temp.
such that the rate of heat flow from the surroundings to the
surface exactly equals the rate of heat absorption.
• When the average moisture content of the solid has reached a
value Xc (the critical moisture content), so the surface film of
moisture has been reduced by evaporation that further drying
causes dry spots to appear.
Rate-of-drying Curve
• The value of N must fall and this gives rise to the
first part of the falling—rate (the period of
unsaturated surface drying). From points C to D.
• On further drying (points D to E) the rate of
drying falls more rapidly than before. At point E
the moisture content of the equilibrium value X*
for the prevailing air temp. and humidity, and
drying stops.
Time of Drying
Time of drying
The Mechanisms of Batch Drying
•
Consider the section of a
material drying in a stream of
gas as shown in the (Fig.
12.11)
Zs: solid thickness.
Zm: tray thickness.
TG: temp. of drying gas.
Y: humidity of the gas (mass
moisture/mass dry gas).
G: mass velocity of the gas
(mass/(time)(area)).
A: evaporation surface area
(upper).
Ts: temp. of the surface.
The Mechanisms of Batch Drying
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The drying surface receives heat from several sources:
- qc by convection from the gas stream.
- qk by conduction through the solid.
- qr by direct radiation from a hot surface at temp. TR.
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q = qc + qk + qr
The flux of evaporation is equal to Ns.
N: the flux of drying (mass of moisture
evaporated/(area) (time)
s: the latent heat of evaporation.
N s = q
The Mechanisms of Batch Drying
Acceleration of Drying Rate
Classification of Dryers
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Two useful classifications:
 Based on the method of heat transfer.
 On the method of solid handling.
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Based on the method of solids handling, dryers
are divided into:
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Static-bed dryers.
Moving bed dryers.
Fluidized-bed dryers.
Pneumatic dryers.
Static-Bed Systems
Tray and Truck Dryers:
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Most commonly used in pharm. operations.
Tray dryers are called shelf, cabinet or
compartment dryers.
Truck dryer is one in which the trays are loaded
on trucks (racks equipped with wheels).
Trays are loaded from 0.5-4.0 inches deep with
at least 1.5 inches clearance between the surface
and the bottom of the tray above.
Batch type dryers.
Static-Bed Systems
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Widely used in pharm. Industry for:
 Each batch can be handled as separate entity.
 Batch size of pharm. Industry is relatively small
compared to chem. Ind.
 The same equipment can be used for drying a wide
variety of materials.
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Classified as direct or indirect:
 Most are direct, in which heating is accomplished by
the forced circulation of heated air.
 Indirect type utilize heated shelves or radiant heat
sources inside the drying chamber.
Static-Bed Systems
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Trays used have solid, perforated, or wire mesh
bottoms.
To achieve uniform drying, there must be a
constant temp. and uniform airflow over the
material being dried.
Energy sources for heating the drying air are
electricity for small units and steam for larger
units.
Controlled air temp. between 40 – 120oC permit
heat sensitive materials to be dried.
Tunnel and Conveyor Dryers
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Truck dryers for continuous drying.
Semi continuous operation for tunnel dryers type.
Conveyor dryers are truly continuous.
Turbo-Tray Dryers
Moving-Bed Systems
Turbo-Tray Dryers:
• A continuous shelf moving-bed dryer.
• Consists of a series of rotating annular trays arranged in a
vertical stack, rotating slowly at about 0.1 -1.9rpm.
• Does low temperature drying as low as 60°F without vacuum or
up to 1200°F
• Heated air is circulated over the trays by turbo-type fans.
• Each tray is provided with a slot cut into the tray, as well as a
leaving rate for spreading the solid.
• Solid fed in the top is spread upon the top tray to a uniform
thickness.
• As the tray revolves, the solid is pushed through the slot by a
separate wiper rake, to fall upon the tray beneath.
• This type is considered faster than tunnel dryers for the turbotray dryer continuously exposes new surfaces to the air.
Tumbling Dryers
Tumbling Dryers
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Batch type dryer.
Double cone shape is the most common.
Operates under vacuum.
Provides controlled low-temp. drying.
Increased rate of drying.
Heat is supplied to the tumbling charge by
contact with the heated shell and by heat
transfer through the vapor.
Optimum conditions are established by varying
the vacuum, temp. and rotation speed.
Tumbling Dryers
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The normal charge would be about 60% of the
total volume.
Drying times of 2-12 hrs are expected.
For drying tablet granules, 2-4 hrs periods are
expected instead of the 18-24hrs obtained with
hot air ovens.
Mixing and granulation can be done in the same
equipment and could precede drying.
Fluidized-Bed Systems
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Fluidization is applied to processes in which a
loose, porous bed of solids is converted to a
fluid system having the properties of surface
leveling flow and pressure-depth relationships,
by passing the fluid up through the bed.
Gas velocity is greater than the setting velocity
of the particles and less than the velocity for
pneumatic conveying.
Fluidization technique is efficient for the drying
of granular solids.
Fluidized-Bed Systems
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Advantages over conventional tray drying:
 Granules have the proper particle size, for good
fluidization. (i.e no need for further grinding).
 It has a two fold to six fold advantage in the thermal
efficiency.
 Faster in both drying and handling time than the tray
dryer, because of the high interfacial contact
between drying air and solids.
 No hot spots are produced and higher drying temp.
can be employed.
Type of Fluidized-Bed Systems
1. Vertical.
2. Horizental.
Type of Fluidized-Bed Systems
1. Vertical FBD:
• The unit is of batch type.
• The fluidizing air stream is induced by a fan
in the upper part of the equipment.
• Air is heated in an air heater and flows
upward through the wet material.
• Air flow rate is adjusted by means of damper.
• The bag filter at the top of the drying
chamber is to prevent carrying over of fine
particles.
Type of Fluidized-Bed Systems
• Capacity varies from 5kg to 200kg.
• Average drying time from 20-40 min.
• Designed for the direct preparation of tablet
granulations as well as for the drying of
conventionally produced wet granulations.
• The unit can be used as granulator, the dry
ingredients are placed in the chamber and
fluidized while the granulating liquid is
sprayed into the bed. Granules are dried by
heating the fluidizing air.
Freeze Drying
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Used for pharm. products:
 Which lose their viability in the liquid state.
 Which deteriorate if dried in air at normal
atmospheric pressures.
 Which are heat-sensitive.
 Which may react with oxygen.
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Materials or products need such type of drying
as blood serum, plasma, antibiotics, hormones,
bacterial cultures, vaccines and many food
stuffs are dehydrate by freeze drying.
Freeze Drying
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The process is also refered as lyophilization,
gelsication or drying by sublimation.
The material to be dried is:
 First cooled and frozen.
 Then subjected under a high vacuum to heat
(supplied by conduction or radiation or both).
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The frozen liquid sublimes leaving only the
solid.
The process depends on the phenomenon of
sublimation, whereby water passes directly
from the solid state (ice) to the vapor state.
Freeze Drying
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Sublimation can take place at pressures and
temperatures below the triple point. (4.579) mm
Hg absolute (4579 microns) and 0.0099oC)
The pressure and temperature at which the
frozen solid vaporizes without conversion to a
liquid is referred to as the eutectic point.
Freeze drying is carried out at temperatures and
pressures well below this point, to prevent the
frozen water from melting.
Freeze Drying
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In practice freeze drying of pharmaceuticals is
carried out at temperatures of 10oC to 40oC and
pressures of 2000 to 100 microns.
Basic requirements for freeze drying:
 The vapor pressure of the water on the surface of the
material must be higher than the partial pressure of the
enveloping atmosphere.
 Latent heat of vaporization must be introduced to the
drying solid at such a rate as to maintain desirable
temp. levels at both the surface and interior.
 Evaporated moisture must be removed.
Freeze Drying
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Freeze dryers are composed of four basic
components:
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Chamber for vacuum drying.
Vacuum source.
Heat source.
Vapor-removal system.
Vacuum is achieved by pumps, steam ejectors, or
a combination of the two.
Heat is provided by conduction or radiation or by
both.
Freeze Drying
• Water vapor is removed by:
 Condensers: water vapor removed from the
chamber condenses in a from of thin layer of
ice, the ice is removed by melting it with a
heated fluid or by means of scraper blades in
the continous operation.
 Desiccants: liquid or solid desiccants are
employed in the initial vapor removal to
enhance the efficiency of the pumps removing
the water vapor.
 Pumps.