Drying involves the removal of solvents like water or other liquids from a formulation using heat. It is done to stabilize moisture sensitive materials, aid preservation, and prepare granules for tablets and capsules. Common drying methods include hot air oven dryers, vacuum oven dryers, fluidized bed dryers, freeze dryers, drum dryers, and spray dryers. Hot air oven dryers operate by circulating hot air over solid materials spread on trays while vacuum oven dryers dry materials that are heat sensitive under low pressure. Spray dryers dry dilute solutions by spraying the liquid into a hot air stream where each droplet dries quickly.

1. DRYING
By: Chernet Tafere
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Love each other

2. Objective
 At the end of this sub-topic, the student will be able to:
 Define drying
 Describe the applications of drying
 Discuss the drying of different dosage forms
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3. Drying
• Drying can be defined as the process of removal of solvents (i.e. water
or other liquid) from a formulation with the help of heat
• It involves evaporation or sublimation of the liquid phase and results in
dry solid or powder
• What is the difference between drying and filtration??
• The change of phase from liquid to vapor distinguishes drying
from mechanical methods of separating solids from liquids such as
filtration
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4.  Why drying?
 To avoid moisture which may decrease product stability.
 aid preservation by minimizing mold or bacterial growth
 To stabilize moisture sensitive materials
 prevent deterioration of the drugs by moisture
 To prepare granules for tablets & capsules.
 To reduce the cost of transportation and storage
 By reducing bulk density and weight.
 To facilitate comminution
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5. Choice of drying method
 When considering how to dry a material, the following points
should be considered:
 Heat sensitivity of the material being dried
 Physical characteristics of the material
 Nature of the liquid to be removed
 The scale of the operation
 The necessity for asepsis
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6. Dryers
 Dryers can be categorized based on mode of operation
into
 Dryers for solid materials
 Compartment/tray/shelf dryer (Hot air ovens)
 Vacuum oven dryer
 Fluidized bed dryer
 Freeze dyer/sublimation dryer (Special dryer)
 Dryers for dilute solutions and suspensions
 Drum dryer (film dryer)
 Spray dryer
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7. Dryers for solid
 Hot Air Ovens
• Operates by passing hot air over the
surface of a wet solid that is spread over
trays arranged in racks
• Fan: Circulate the air
• Trays containing the load remain in the
dryer until drying is complete
• After w/h they will be emptied &
recharge for the next batch
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8.  Advantages
 Low initial cost
 Different products can be dried
 Versatility
 Materials of almost any physical form may be dried (sticky materials,
granular mass, crude drugs, paste..
 Limitations
 A large floor space is required for the oven and tray-loading facilities
Labor cost for loading and unloading the trays is high
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9. Drying…
Limitation…
drying process is time consuming
Not good for thermolabile products
Usually used for small sized batch
Solvents recovery from the air is difficult
 Not connected to condenser
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10.  Vacuum oven dryer
 The oven is connected to a vacuum pump through a condenser
Pressure decreased by vacuum pump
 The operating pressure is usually about 0.03 to 0.06 bar
• At this pressure water boils at 25 to 35oC
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11.  Advantages
 Suitable for materials that are unstable at highT⸰
 low oxygen content when drying under low pressure prevents oxidation
Easy for batch process of different compounds.
The use of condenser enables solvents to be recovered, which is useful
with expensive or inflammable solvents
• Disadvantage
• Higher investment cost
• Labor and running costs are high
• Low production efficiency-low output
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12.  Fluidized Bed Dryers
• Fluidization
Principal: hot air through a bed of granules
Materials will be loaded – detachable perforated basket (bowl).
• Fresh air enter – inlet- heater- hot air- passes through bed of
granules with optimum velocity.
• Granules lifted from bottom & suspend in air (Fluidized state).
Then drying from individual particles takes place.
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14.  Advantage
 Efficient heat transfer gives fast drying rate than static bed dryers
The fluidized state of the particles enables drying from individual
particles
 Easy handling reduce labor cost
High output from a small floor space
Heating time of thermo labile substance is minimized-
Disadvantage
• The turbulence may cause attrition of some material, with the production
of fines
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15.  Freeze drying (Sublimation drying)
(lyophilization)
Lyophilization or freeze drying is a process in which water is frozen
followed by its removal from the sample.
• Drying takes place by subliming the frozen solid.
Dehydration process is performed under vacuum while the
substance is in a frozen state
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The phase diagram for water with freeze drying process
super imposed.
• The drying is initiated by reducing the surrounding
pressure (below the triple point).

17. • Stages of the freeze-drying process
 Freezing stage
• The product is placed into the lyophilizer chamber and
frozen
Results is an amorphous solid product and water crystals
Temperature necessary to achieve complete freezing of
formulation will depend on the nature of solution
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18.  Primary drying
• Drying is initiated by introducing vacuum in the chamber (4mm Hg)
• Pressure inside the chamber is reduced and heat is applied to initiate
the process of sublimation of ice crystals formed in freezing stage.
(20⸰ c or higher)
• When all frozen water is removed via sublimation, primary drying is
complete
Secondary drying
• Secondary drying is to reduce the bound unfrozen water to a level that is
optimal for stability (less than 1%) of final product
 at higher temperature than primary drying.
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19.  Advantages
• Drying takes place at very low temperature,
• Suitable for thermolabile products
• This reduces the extent of decomposition or loss of activity in biological
products,…..
• The product is light and porous
• Easy for reconstitution and transportation
• Oxidation is minimized
Under high vacuum there is no contact with oxygen
• less chances of contamination as its operated in a controlled
environment
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20. Disadvantages
• Should be dried in the final container
• The process is very slow and very expensive.
• Limited to certain types of valuable products that cannot be dried by any
other means
 Biological products (antibiotics, vaccine, blood products etc.)
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21. Dryers for dilute solutions and
suspensions
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22.  Spray Dryers
 The solution or suspension to be dried is sprayed into a hot airstream.
Each droplet dries to an individual solid particle
The process can be described in three major phases:
(Atomization- droplet to particle conversion-collection)
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23.  Advantage
• drying takes only few seconds
 The droplets are small, giving a large surface area for heat
and mass transfer
• Uniform and controllable particle size
• The product is free-flowing
 Spherical particle
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Disadvantage
• Expensive

24. Drum dryers
• Working principle:
• heated drum is slowly rotated – through feed pan.
• Material adhere to pan and a thin layer form.
• Material dry during rotation.
• knife- Dry material is scrap –fall in storage bin.
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25. Drum Dryers
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26.  Advantage
 Rapid drying
The thin film spread over a large area resulting in rapid heat and
mass transfer
 The equipment is compact
Occupy less space
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27.  Disadvantage
 High maintenance cost.
 Skilled operator required
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28. summary
 Drying- vaporization and removal of water or other liquid from a
product to form a dry solid
 Why drying? To stabilize, improve flow and compression, ease
handling and transportation, facilitate size reduction
 Dryers for solid materials: hot air oven dryers, vacuum oven dryers
Fluidized bed dryer, Freeze dyer/sublimation dryer
 Dryers for dilute solutions and suspensions: Drum dryer (film
dryer), Spray dryer
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