Drying is defined as the removal of small amount of water or other liquids from a material by the application of heat.

1. Drying
By
S. I Makrani

2. Content
 Fluidized Bed Dryer (FBD) & Freeze Dryer
 Principle
 Construction
 Working
 Uses
Advantages
 Disadvantages
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3. Fluidized Bed Dryer (FBD)
 Principle:
• In fluidized bed dryer, hot air (gas) is passed at high pressure
through a perforated bottom of the container containing
granules to be dried.
• The granules are lifted from the bottom and suspended in the
stream of air this condition is called fluidised state.
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4.  Construction :
Figure. Construction of Fluidized Bed Dryer
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5.  2 types of bed dryer are available, Vertical fluid bed dryer and
horizontal fluid bed dryer.
 The construction of a vertical FBD is shown in figure.
 The dryer is made up of stainless steel or plastic.
 A detachable bowl is placed at the bottom of the dryer, which
is used for charging and discharging.
 The bowl 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 temperatures.
 Bag filters are placed above the drying bowl for the recovery 4

6.  Working:
 The wet granules to be dried are placed in the detachable
bowl.
 The bowl is pushed into the dryer. Fresh air is allowed to pass
through a prefilter, which subsequently gets heated by passing
through a heat exchanger.
 The hot air flows through the bottom of the bowl.
 Simultaneously fan is allowed to rotate. The air velocity is
gradually increased.
 When the velocity of the air is greater than settling velocity of
granules. The granules rise in the container because of high
velocity gas and later fall back in a random boiling motion.
This condition is said to be fludised state. 5

7.  Uses:
 Fluidised bed dryer is popularly used for drying of granules in
the production of tablets.
 Fluidised bed dryer can be used for three operations such as
mixing, granulation and drying.
 Advantages:
 Fluidised bed dryer requires less time to complete drying. i.e.,
20 to 40 minutes compared to 24 hours of tray dryer.
 It is available in different sizes with the drying capacity
ranging from 5 to 200 kg per hour.
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8.  The drying containers are mobile, making handling simple
and reducing labour costs.
 It can be used either batch type or continues type
 Disadvantages:
 Poor fluidization and low flexibility especially if the feed is
too wet.
 Not the best choice of equipment when organic solvents need
to be removed during drying.
 Non-uniform product quality for certain types of fluidized bed
dryers.
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9. Freeze Dryer
• Freeze drying is also known as Lyophillization. This is a
techniques used for preservation of foods etc.
 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 a liquid phase.
• The process of Lyophillization or freezing drying include the
conversion of the material directly from the solid phase to the
gaseous phase, without going through the liquid phase
(sublimation)
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10.  Construction:
Figure. Schematic diagram of freeze dryer 9

11. • The construction of a freeze dryer is shown in Figure .
• It consists of:
 1. Drying chamber in which trays are loaded.
 2. Heat supply in the form of radiation source, heating coils
 3. Vapour condensing or adsorption system.
 4. Vacuum pump or steam ejector or both.
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12.  Working:
A freeze dryer works in three phases
 1. Freezing Phase
 2. Primary Drying (Sublimation) Phase
 3. Secondary Drying (Adsorption) Phase
 1. Freezing Phase
• Freeze dryers use various methods to freeze the product.
Freezing can be done in a freezer, a chilled bath (shell
freezer), or on a shelf in the freeze dryer. The freeze dryer
cools the material below its triple point to ensure that
sublimation, rather than melting, will occur. This preserves the
material’s physical form. 11

13.  2. Primary Drying (Sublimation) Phase
• A freeze dryer’s second phase is primary drying (sublimation),
in which the pressure is lowered and heat is added to the
material in order for the water to sublimate.
• Direct change of water from solid into vapour without
conversion to a liquid phase.
• About 95% of the water in the material is removed in this
phase.
• Primary drying can be a slow process.
• Too much heat can alter the structure of the material.
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14.  3. Secondary Drying (Adsorption) Phase
• A freeze dryer’s final phase is secondary drying (adsorption),
during which the ionically bound water molecules are
removed.
• By raising the temperature higher than in the primary drying
phase, the bonds are broken between the material and the
water molecules.
• Freeze dried materials retain a porous structure. After the
freeze dryer completes its process, the vacuum can be broken
with an inert gas before the material is sealed.
• Most materials can be dried to 1-5% residual moisture.
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15.  Uses :
Freeze dryer is most commonly used in the production of
dosage forms, such as injections, solutions and suspensions.
 It is used for drying of a number of products.
 Blood plasma and its fractionated products.
 Bacterial and viral cultures.
 Antibiotics and plant extracts.
 Steroids, vitamins and enzymes.
 Several other products such as food items (mushrooms, meat
and poultry products), coffee and tea concentrates and citrus
fruit juices are dried. 14

16.  Advantages:
 The entire operation is carried out well below the freezing
point
 Thermolabile materials (heat sensitive materials) can be
dried
 loss of volatile material is less
 Sterility can be maintained
 Material can be dried in it final container such as single dose
and multiple dose vials
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17.  Disadvantages:
 Equipment's and running cost are high
 The period of drying is high
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