Drying is one of the most essential unit operations in pharmaceutics. It involves the removal of moisture or solvent from a wet substance using heat energy. The process helps in improving product stability, ease of handling, and shelf life. In the pharmaceutical industry, drying is crucial for the preparation of powders, granules, and biological products.
What is Drying?
Definition:
Drying is the process of removing moisture from a substance by vaporization or sublimation using heat energy.
The main purpose of drying is to:
- Prevent microbial growth
- Improve stability of drugs
- Reduce weight and volume
- Prepare materials for further processing like granulation or coating
ORIGINAL NOTES:
Principle of Drying
Drying works on the principle of evaporation.
When heat is supplied to a wet material, the liquid (usually water) present in it is vaporized and then removed by a stream of air or vacuum.
The rate of drying depends on:
- Temperature of drying air
- Air velocity and humidity
- Surface area of the material
- Nature and moisture content of the substance
Types of Drying
Pharmaceutical drying can be performed by different techniques such as:
- Conduction drying: e.g., tray dryer
- Convection drying: e.g., fluidized bed dryer
- Radiation drying: using infrared radiation
- Dielectric drying: using microwave or radio frequency
- Sublimation drying: e.g., freeze drying
Fluidized Bed Dryer (FBD)
Principle
The fluidized bed dryer works on the principle of fluidization.
When hot air is passed at high velocity through a bed of wet granules, the solid particles are lifted and suspended in the air stream — behaving like a boiling liquid. This ensures uniform heat transfer and efficient drying.
Construction
A typical FBD consists of:
- Air Inlet and Filter: supplies clean, filtered air
- Heater: warms the air to the required temperature
- Product Container: holds the wet material
- Perforated Plate: supports the granules and allows air passage
- Blower/Fan: circulates hot air through the product
- Exhaust System: removes moist air through filters
- Control Panel: regulates temperature and air flow
Working
- Wet granules are loaded into the product container.
- Heated air passes through the perforated bottom plate.
- The air velocity is adjusted until the granules become fluidized.
- The hot air evaporates the moisture rapidly.
- The moist air is exhausted, leaving dry granules behind.
Advantages
- Fast and uniform drying
- High thermal efficiency
- Short drying time
- Easy to clean and operate
- Suitable for heat-sensitive materials
Disadvantages
- Not suitable for sticky or dense materials
- Requires skilled operation
- Higher equipment cost
Applications
- Drying of tablet granules before compression
- Drying of powders, pellets, and crystals
- Used in food and chemical industries
Freeze Drying (Lyophilization)
Principle
Freeze drying works on the principle of sublimation, where ice changes directly into vapor without becoming liquid.
The process occurs under low temperature and vacuum, preserving the chemical and biological stability of materials.
Process of Freeze Drying
- Freezing:
- The liquid product is frozen below –40°C, converting water to ice.
- Primary Drying (Sublimation):
- Pressure is reduced under vacuum.
- Gentle heat is applied, causing ice to vaporize directly.
- Around 95% moisture is removed.
- Secondary Drying (Desorption):
- Temperature is slightly increased to remove bound water.
- Final moisture content remains around 1–4%.
Advantages
- Ideal for heat-sensitive substances (enzymes, vaccines, proteins).
- Produces porous, stable, and easily reconstitutable products.
- Minimal loss of volatile components.
Disadvantages
- Very expensive and time-consuming.
- Requires vacuum equipment and skilled operation.
Applications
- Drying of biological products (antibiotics, vaccines).
- Preservation of heat-labile drugs.
- Used in food, biotechnology, and cosmetic industries.
