Assistant lecturer: Bareq S.

Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Industrial Pharmacy I
Lec. 4 4th stage

“Drying”
Drying
Is the removal of small amount of a liquid from a material by the application of heat
which is transferred from a surface into an unsaturated vapor phase.
Drying and evaporation are distinguishable merely ‫( ببساطة‬simply) by the relative
quantities of liquid removed from the solid.

Non-thermal methods of drying

1. The expression (Squeezing)
Removing liquid from solid by (the squeezing of a wetted sponge) ‫اسفنجة‬

2. The extraction
The extraction of liquid from a solid by use of a solvent.

3. The adsorption
By the use of desiccants (such as anhydrous calcium chloride).

4. The absorption
By passage through a sulfuric acid column.

5. The desiccation ‫تجفيف‬

By placing the solid in a sealed container with a moisture-removing material
(silica gel).

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Purpose of drying
1. In the preparation of granules in tablets and capsules.

2. In the preparation of dried aluminum hydroxide, the spray drying of

lactose, and powdered extracts.

3. Reduces the bulk and the weight of drying materials (lowering the cost of
transportation and storage).

4. Aids in the preservation of animal and vegetable drugs by minimizing mold

and bacterial growth.

5. Facilitates comminution (miling) and becomes more friable (powdery).

6. Dried products are more stable such as effervescent salts, aspirin,

hygroscopic powders, ascorbic acid, and penicillin; through reducing the
chemical reactivity.
Psychrometry
Psychrometry determines the vapor concentration and vapor carrying capacity of
the gas (air).

Vapor carrying capacity Vapor concentration

Nitrogen, or other gas stream passing The concentration of water vapor in a
over the drying material. gas (air) is called the humidity of the
gas (air).

Note: ‫مالحظة‬
The carrying capacity determines the rate and the extent of drying material (the
lowest moisture content).

Industrial Pharmacy I
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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Psychrometry (humidity) chart

Psychrometric or humidity chart represents the relationship between the
temperature and humidity of the air-water vapor system at constant pressure.

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

1. Dry bulb temperature

Represents the horizontal axis (x-axis) and it is measured by ordinary
thermometer.

2. Absolute humidity (Specific humidity)

Represents the vertical axis (y-axis) (the weight of water vapor per unit weight
of dry air) usually expressed in grams of water vapor per kilogram of dry air.

3. Saturation humidity (CDE curve)

The curve that represents the conditions at which air becomes saturated with
moisture at various temperatures that means 100% relative humidity (RH)
PP of water vapor = VP of free water
PP = Partial pressure VP=Vapor pressure

4. Dew point C (60°F)

The temperature at which air becomes saturated with water vapor and starts
to condense.
It is an indicator of the moisture content in the air (maximum amount of
moisture without condensation).

• When the mixture is cooled below the dew point; the water vapor
condenses into a two-phase system of saturated air (point C) and
droplets of free water.

5. The relative humidity (% RH)

Is the partial pressure of water vapor in the air to the vapor pressure of free
water at the same temperature.

Or
The ratio of the actual amount of water vapor present in the air to the
maximum amount of water vapor the air can hold at that temperature

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

GK with ≈ 50% relative humidity

The relative saturation (RH) represents
Relative humidity = (specific or absolute humidity) / (saturation point) x 100

Relative humidity = (78) / (161) x 100

Relative humidity = 48 % ≈ 50 %

6. Wet Bulb Temperature

The temperature indicated by a thermometer whose bulb is kept wet by a
wick ‫ قطعة قماش مبللة‬saturated with water.
It represents the lowest temperature that can be reached by evaporative
cooling

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Theory of Drying
Drying involves heat and mass transfer operations.
• Heat transfer into the material to supply the latent heat required for
vaporization of the moisture.

• Mass transfer is the diffusion of water through the material to the

evaporating surface, then from the surface to the passing air stream.

The rate of evaporation (mass transfer) is related to the rate of heat transfer by
the equation:
dW/dϴ= q/ λ
• dW/dϴ is the rate of evaporation
• q is the overall rate of heat transfer
• λ is the latent heat of vaporization
The driving force for heat transfer is temperature differential.

The rate of diffusion of moisture (mass transfer) into the air stream is expressed by
the equation:
dW/dϴ=k A(Hs-Hg)
• dW/dϴ is the rate of diffusion expressed as pounds of water/hour
• k is coefficient of mass transfer
• A is the area of evaporating surface
• (Hs Hg) is humidity differential.

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

The coefficient of mass transfer is not constant but varies with the velocity of the air
stream passing over the evaporating surface.
After initial period of adjustment, the rate of heat transfer will equal to the rate of
mass transfer.
dW/dϴ=q/ λ =k A(Hs-Hg)
The rate of heat transfer includes all method of heat transfer which is
• Convection ‫الحمل‬
• Radiation
• Conduction ‫التوصيل‬

(The general principle for efficient or good drying)

The rate of drying may be accelerated ‫ يسرع‬by increasing:
1. The rate of convection of heat transfer which can be achieved by increasing
the air flow rate and by raising the inlet air temperature.

2. The rate of radiation heat transfer.

3. The rate of conduction can be increased by reducing the thickness of the

material to be dried (large surface area) by allowing it to become in close
contact with raised temperature surface.

4. Increasing the air velocity also speed up the rate of drying by increasing the
coefficient of mass transfer (sufficient turbulence to minimize boundary layer
thickness)

5. Dehumidifying the inlet air increase the humidity differential also speed up
the rate of drying (low relative humidity)

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Drying of Solids
The moisture in a solid is represented as a wet weight or dry-weight.
• Wet-weight, the water content is a percentage of the weight of the wet solid.

𝑤𝑡. 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒

𝑤𝑡. 𝑜𝑓 𝒘𝒆𝒕 𝑠𝑎𝑚𝑝𝑙𝑒

• Dry-weight, the water content is a percentage of the weight of the dry solid.

𝑤𝑡. 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒

𝑤𝑡. 𝑜𝑓 𝒅𝒓𝒚 𝑠𝑎𝑚𝑝𝑙𝑒

Loss on drying (LOD)

𝑤𝑡. 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒
Is calculated by using % LOD= ×100
𝑤𝑡. 𝑜𝑓 𝒘𝒆𝒕 𝑠𝑎𝑚𝑝𝑙𝑒

• Moisture balance is having heat source, the weighted wet sample allowed to
dry and obtain %LOD.
Moisture Content
The moisture content in the sample is determined on a dry-weight basis:
𝑤𝑡. 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒
% MC= ×100
𝑤𝑡. 𝑜𝑓 𝒅𝒓𝒚 𝑠𝑎𝑚𝑝𝑙𝑒

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Behavior of Solids during Drying/Rate of Drying

The study of drying rate is crucial to understand the solid behavior during drying.
Rate of drying could be determined by suspending the wet sample on a scale in
a drying cabinet and measuring the weight of dry sample as a function of time.
The data from drying rate is plotted as moisture content versus time.
1. Moisture content vs drying time (Figure A)
2. Drying rate vs moisture content (Figure B)
After a period of initial adjustment;
Heating rate=Cooling rate till drying temperature stabilizes.

• This period of initial adjustment is shown as segment AB

• B (the temperature stabilized) as long as there is a moisture film at the surface
of the drying solid.
• BC (constant rate period), the moisture evaporating from the surface is
replaced by water diffusing from the interior of the solid
(Evaporation rate= Diffusion rate) of evaporation.
• C (critical moisture content), no more moisture is replaced and dry spot
appears; (Drying rate falls off).
• C-D (first falling rate period or the period of unsaturated surface drying), the
spots number increases (Drying falls steadily).

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

• D (second critical point), complete evaporation of surface film (Drying rate

depends on diffusion rate).
• D-E (second falling rate period), drying rate falls rapidly.
• E (equilibrium moisture period begins), drying rate=Zero; the temperature and
moisture content remain constant.

Continued drying after this point is a waste of time and energy

Classification of Solid Based on Drying Behavior

Solids to be dried may be classified based on their drying behavior into:
1. Crystalline solids in which the water is held in open surface pores and
interstitial spaces between particles which are easily accessible to the surface.

2. Amorphous solids, the moisture here is integral part of the molecular

structure and entrapped in fine capillaries and pores, accordingly it is difficult
to dry than crystalline solids.

Types of Dryers
When considering how to dry a material, certain points should be considered:
1. Heat sensitivity of the material to be dried
2. Physical nature of the material
3. Nature of liquid to be removed
4. The scale of operation
Dryers can be classified based on
1. Method of heat transfer (dryer design and energy requirement are important)
2. Method of sample handling (attention given to the type of the substance to
be dried)

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

In the method of sample handling the presence or absence of agitation is the major
criteria:
1. Static-bed dryers
2. Moving-bed dryers
3. Fluidized-bed dryers
4. Pneumatic dryers
Static Bed System (Tray dryer)
• Tray dryer consist of cabinet ‫ كابينة‬shelf or compartment in which the material
to be dried is spread on trays.
• There is no particles movement; only the bulk motion, the exposed surface
can be increased by decreasing the thickness of the bed.
Moving-Bed Systems (Turbo-Tray Dryers)
• This type is a continuous shelf, moving-bed dryer; consists of series of
rotating trays arranged in a vertical stack-Rotating slowly at 0.1 to 1.0 rpm.

• Heated air is circulated by turbo-type

fans mounted in the stack center.

• Wet mass fed from the roof (top)

of dryer; which leveled by
a stationary wiper, then the dried
material is pushed through radial
slots ‫ فتحات‬onto the tray below.

• After each cycle the mass transfers

to the next shelf until discharge at
the bottom.
• Drying rate is faster than tunnel-dryer due to the continuous exposes to the
air.

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Fluidized-Bed Systems
• The solids are partially suspended in the gas stream (the mixture behaves like
a liquid) and the solid is fluidized.

• The fluidization technique is used for drying granular solids, as each particle
is surrounded by the drying gas.

• The intense mixing results in uniform conditions of temperature,

composition, and particle size distribution throughout the bed.

• The resultant granules are not wet nor completely dried to avoid cracking.

Advantages of Fluidized Bed Dryer

1. High drying rate due to efficient heat and mass transfer
2. The drying occurs at constant rate because all the surface of particles
subjected to heat uniformly
3. More spherical free- flowing particles can be obtained reduce the problems
of aggregation and migration of color.

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Pneumatic Systems
A. Spray Dryers
• It can handle only fluid materials such as solutions, slurries, and thin pastes.

• Feeding of fine droplets of fluid is into the hot gas stream.

• When the liquid droplets contact the hot gas, the surface liquid is quickly
evaporated, and form a tough shell ‫ غالف خشن‬of solids.

• The diffusion rate of the liquid is slower than heat transfer.

• If the shell is nonelastic or impermeable the internal pressure causes the

droplet to swell and the shell becomes thinner, allowing faster diffusion, it
ruptures, producing either fragments or budlike forms.

• Thus, spray-dried material consists of intact spheres ‫ كاملة‬, spheres with

buds ‫انتفاخات‬, ruptured hollow spheres ‫ تحتوي فجوات‬, or sphere fragments
‫محطمة‬
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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

B. Spray drying and spray congealing of pharmaceuticals.

Spray drying is rapid drying and the unique form product.
There are three major uses:
1. Drying heat-sensitive materials,
2. Changing the physical form of materials (in tablet and capsule manufacture)
3. Encapsulating solid and liquid particles

• Spray drying is use in tablet and capsule formulations.

• Drying process changes the shape, size, and bulk density of the product.
• The spherical particles flow better than the same product dried by
conventional method due to size and shape uniformity.

• Spray drying is used in the coating and encapsulation of both solids and
liquids.
• Chilling spray (congealing) consists of suspending the particles in a molten
coating material and pumping the slurry into a spray dryer in which cold air
is circulated.

• Spray congealed coatings are used mainly for taste masking and for
sustained-release formulations.

C. Flash Dryers
• The moistened solid is suspended in a finely divided state [velocity (3000-
6000 feet/min)] at [temperature (300-1300°F) air stream]
• The flash drying is a short-time process.

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Specialized Drying Methods

Freeze Dryers
• Freeze drying is also referred to lyophilization, or sublimation.‫التسامي‬
• The drying of heat-sensitive materials must be dehydrated to a solid state to
maintain the stability through frozen then under a high vacuum to heat (by
conduction or radiation) to sublime the frozen liquid leaving only the solid.

• Example; blood serum, plasma, antibiotics, hormones, bacterial cultures,

vaccines, and food stuffs.

• The dried product is reconstitution (re-dissolved or re-suspended) by the

addition of water before use.

• In freeze drying the water passes directly from solid state (ice) to vapor
state without passing through the liquid state. As shown in the schematic
pressure-temperature diagram for water.

• Sublimation occurs at P and T below the triple point, 4.579 mmHg and 0.0099°C. ‫ة‬ ‫م‬

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 Assistant lecturer: Bareq S. Mohammed
Al-Yarmouk University College / Department of Pharmacy 2023-2024

Stages of Freeze Drying

1. Freezing (Pre-freezing)
Material is frozen below or at −20°C.
• Pre-freezing the material before application of vacuum avoids foaming.

2. Vacuum
Rotary pumps on small scale, and ejector pumps on large-scale, are used to
reduce the pressure sufficiently.

3. Primary drying
During primary drying the latent heat of sublimation must be provided and
the vapour removed.
• Primary drying stage by sublimation can remove the unbound water.

4. Secondary drying
It is used to remove bound water or traces of water left after primary drying.
The temperature is raised (up to 50°C) or desiccant is used to carry secondary
drying.

Microwave Drying
The application of microwave energy to the drying of solids represents a
radical departure from conventional means of drying.

Instead of applying heat externally to a material, energy in the form of

microwaves is converted into internal heat by interaction with the
material itself.
This permits extremely rapid heat transfer throughout the material lead to
rapid drying.

The moisture is mobilized as a vapor rather than a liquid, and its

movement to the surface can be extremely rapid because it does not depend
on mass concentration gradients or on slow liquid diffusion rate.

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