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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16. 16
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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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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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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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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