In pharmaceutical factories, two quiet yet powerful operations determine the quality of tablets, capsules, and suspensions: drying and mixing. Excess moisture can spoil drugs, while poor mixing leads to uneven dosage. UNIT 3 focuses on these core engineering processes, showing how controlled moisture removal and uniform blending ensure safety, stability, and therapeutic accuracy.
From tray dryers to fluidized beds and from double-cone blenders to high-speed emulsifiers, these technologies shape modern drug manufacturing.
Drying in Pharmaceutical Engineering
Objectives and Applications of Drying
Drying removes moisture from materials to improve stability and handling.
Main Objectives
- Prevent microbial growth
- Improve product stability
- Reduce weight and volume
- Enhance powder flow
Applications
Drying is used in granules, powders, herbal extracts, and injectables.
Mechanism of Drying Process
Drying involves heat transfer and mass transfer. Heat supplies energy to evaporate water, while moisture diffuses from inside the material to the surface.
The process generally occurs in two stages:
- Constant rate period
- Falling rate period
Equilibrium Moisture Content (EMC)
Definition
EMC is the moisture content at which the material neither gains nor loses water when exposed to air.
Applications
- Predicts storage stability
- Prevents overdrying
- Guides packaging requirements
Rate of Drying Curve
The drying curve shows how moisture removal changes with time. It helps determine optimal drying time and energy efficiency in industrial operations.
Types of Dryers Used in Pharmacy
Tray Dryer
Simple cabinet with heated air circulation.
Merits: Easy, low cost
Demerits: Slow, non-uniform drying
Drum Dryer
Rotating heated drum dries thin films of liquid.
Uses: Pastes and slurries
Limitation: Possible heat degradation
Spray Dryer
Converts liquid into dry powder by spraying into hot air.
Advantages: Rapid drying, fine particles
Application: Milk powder, antibiotics
Fluidized Bed Dryer
Hot air suspends particles for uniform drying.
Benefits: Fast and efficient
Widely used for: Granules and tablets
Vacuum Dryer
Operates under reduced pressure.
Suitable for: Heat-sensitive drugs
Drawback: Expensive
Freeze Dryer (Lyophilizer)
Removes water by sublimation under vacuum.
Advantages: Preserves biological activity
Application: Vaccines, proteins
Mixing: Achieving Uniformity in Formulations
Objectives and Applications of Mixing
Mixing ensures uniform distribution of ingredients.
Goals
- Uniform dosage
- Improved dissolution
- Consistent product quality
- Enhanced stability
Used in tablets, capsules, suspensions, creams, and ointments.
Factors Affecting Mixing
Mixing efficiency depends on:
- Particle size
- Shape
- Density
- Viscosity
- Equipment speed
Proper control prevents segregation.
Difference Between Solid and Liquid Mixing
- Solid mixing → blending of powders
- Liquid mixing → stirring and dissolution
- Semisolid mixing → kneading and folding
Each requires different mechanisms.
Mechanisms of Mixing
Solid Mixing
- Convection
- Shear
- Diffusion
Liquid Mixing
- Turbulence
- Molecular diffusion
Semisolid Mixing
- Kneading
- Folding
Common Mixing Equipment
Double Cone Blender
Gentle tumbling action.
Best for: Dry powders
Advantage: Low shear
Twin Shell Blender
Efficient mixing through tumbling.
Good for: Free-flowing powders
Ribbon Blender
Helical ribbons move materials inside trough.
Used for: Large batches
Sigma Blade Mixer
Heavy-duty kneading action.
Suitable for: Pastes and ointments
Planetary Mixer
Rotating blades and bowl.
Used for: Creams and semisolids
Propellers, Turbines, and Paddles
Common liquid mixers for low to medium viscosity fluids.
Silverson Emulsifier
High-speed rotor–stator system.
Application: Emulsions and suspensions