Introduction to Drying.

Definition

• Drying is defined as the removal of liquid from a product usually with application of heat.

• When a large amount of liquid is evaporated from a product with liquid consistency the process is called ‘evaporation’. The final product is a concentrated liquid / slurry.

• When a very small amount of liquid is evaporated from solids the process is called ‘drying’. The final product is a ‘dried solid’.

• It's a process of heat and mass transfer.

• Drying involves:

• Energy transfer from an external source to water or other liquid (Heat transfer). 

• Phase transformation of water/ other liquid from a liquid state to a vapour state and,

• Carry vapour generated away from the material under drying process and out of the dryer.

DIFFERENCE BETWEEN EVAPORATION AND DRYING

Drying	Evaporation
Drying is mainly carried out on solid materials.	Evaporation is mainly carried out on liquid materials.
It involves removal of small amounts of water from solids.	It involves removal of large amounts of water from solutions.
Water is removed at a temperature below its boiling point.	Water is removed at its boiling point.
Usually circulating air over the material is used to carry away the water vapour.	Water is removed from the material as pure water vapour mixed with other gases.
Product obtained is dry solid.	The product obtained is slurry or suspension.

OBJECTIVES OF DRYING PROCESS.

1. One of the primary objectives of drying operation is the removal of solvent to meet specifications in the product.

2. Another important objective of drying is to either achieve or maintain a specified crystallographic form.

3. To improve handling characteristics, as in bulk powder filling and other operations involving powder flow, ensuring free flowing properties. 

4. To stabilize moisture-sensitive materials.

APPLICATIONS OF DRYING PROCESS.

1. Preservation of the crude drugs.

1. Moisture content in the drugs accelerate many reactions leading to their breakdown e.g. Digitalis, Aspirin etc.

2. The quality of many pharmaceuticals is decided on their dried form,

1. E.g. Titanium dioxide should contain NLT 98% of TiO2 calculated with reference to the dried substance.

3. Drying is a final process of preparation of certain drugs and excipients. E.g. Dried aluminium hydroxide, Spray dried lactose, herbal extracts, etc.

4. Final production step before selling out products. 

5. Drying is an essential step after crystallization.

6. Drying improves physical characteristics such as spherical shape, free flowing, uniform size and solubility of granules.

7. Drying reduces the weight of the materials, hence reduces transportation cost .

8. Drying makes materials more efficient for storage.

9. Makes size reduction of materials easier.

10. Reduces the chemical reactivity of the remaining water and increases the stability of the product.

Commonly Asked Questions.

1. Define Drying and give its objectives and applications in pharmacy.