UNIT OPERATIONS
Unit operation is a simple step in a process which includes a physical change during the processes, such as polymerisation, separation, evaporation, crystallisation, filtration, isomerisation, and several other reactions. Unit process is a process in which chemical reactions takes place, such as bromination, halogenation, sulphonation, etc.
CLASSIFICATION OF UNIT OPERATIONS
1. Material Handling and Transportation / Fluid Flow Process
- Pumping
- Compression
- Fluidisation
2. Mechanical Unit Operations
- Size reduction
- Size enlargement
- Mixing
- Agitation
- Blending
3. Mass Transfer Operations
- Evaporation
- Distillation
- Absorption
- Extraction
- Leaching
4. Heat Transfer Operations
- Conduction
- Convection
- Radiation
SIZE REDUCTION
Size Reduction is a process in which the particle size of a substance is reduced from its smaller size to a finer state of sub-division (coarse or powdered state). In some cases, the process of size reduction is also known as comminution and grinding. Milling is the process in which the particle size of solid substance is reduced into coarse or powdered state by employing mechanical strength.
Objectives of Size Reduction
- To increase the surface area of the drug.
- To enhance solvent penetration capacity.
- To increase therapeutic efficacy.
- To facilitate the mixing process and yield a uniform product.
- To increase the stability of emulsions.
- To enhance the physical appearance of ointments, pastes, and creams.
- To ensure stability of suspension.
- To minimise the chance of irritation causing coarse particle present in ophthalmic preparation.
- To increase the rate of drying of the wet masses by milling.
- To reduce the bulkiness of drugs.
Factors Affecting Size Reduction
- Toughness: Crude drugs with high water content or soft/fibrous nature are tough; soft but tough substances are harder to reduce than hard and fragile substances.
- Hardness: Soft materials are easier to reduce than hard substances.
- Stickiness: Resinous or gummy substances stick to grinding or sieve surfaces, making size reduction difficult.
- Moisture Content: Affects physical properties like stickiness, toughness, or hardness.
- Softening Temperature: Wax-like materials become soft due to temperature rise in the mill.
- Purity Required: Grinding surface wear and tear may mix impurities with the powder.
- Material Structure: Vegetable drugs with cellular structure produce long fibrous particles.
- Physiological Effect: Highly potent drugs should be processed in closed mills to avoid operator exposure to dust.
- Ratio of Feed Size to Product Size: Feed size should be small for fine powder output.
- Bulk Density: Determines the output of material through the size reduction process.
Methods / Mechanism of Size Reduction
- Cutting: Material is cut with sharp blades (e.g., cutter mill).
- Compression: Pressure is applied to crush material into smaller particles (e.g., roller mill).
- Impact: High-speed object strikes stationary material; or moving particles strike non-moving surface.
- Attrition: Both surfaces (machine and material) move relative to each other, producing shear forces (e.g., fluid energy mill).
EQUIPMENT FOR SIZE REDUCTION
The common size reduction mills used in pharmaceutical industries are: Hammer mill, Ball mill, Fluid energy mill, and Disintegrator. The two most commonly used equipment – hammer mill and ball mill – are discussed below.
Hammer Mill
Principle
Hammer mill works on the principle of impact, in which an object moving with high speed is made to hit a stationary substance. Thus, particle size is reduced by pulverisation or grinding.
Construction
- Either a horizontal or vertical shaft type
- Hammers made of hardened or stainless steel
- Impact surface made of haystellite and carboloy (abrasion resistant materials)
- Hammer shapes: stirrup and bar (bar-shaped preferred for granulating tablets)
- Hammer edges: flat or sharp (or both)
- Hammers are either rigid or swing-type
- Chamber has a removable screen or grid made of perforated metal sheets
Working
The material to be crushed is fed into the hopper connected to a drum. Fast rotating hammers powder the material into required size. The resultant powder (coarse to moderately fine) is collected beneath the screen. Hammer mill operates continuously because the hammers are not fixed, reducing the chance of choking.
Applications
- Used in wet or dry granulations
- Grinding pharmaceutical raw materials, herbal medicines, and sugar
- Making powder of barks, leaves, and roots having medicinal properties
- Milling Active Pharmaceutical Ingredients (APIs), excipients, etc.
Ball Mill
Principle
Ball mill works on the principle of impact and attrition, in which size reduction takes place as the balls drop from the near top of a rotating hollow metal cylinder.
Construction
- Hollow Cylinder: Made of metal with chromium lining; attached to a metallic frame for rotation at its longitudinal axis; about 30-50% volume occupied by steel balls.
- Balls: Made of metal coated with chromium (sometimes lined with rubber or porcelain); weight kept constant; size depends on diameter of mill and feed.
Working
- Cylinder is filled with drug substance and rotated.
- Speed of rotation plays key role:
- At Low Speed (Sliding): Balls roll and slide over each other; very small quantity reduced.
- At High Speed (Centrifugation): Centrifugal force moves balls toward walls; no grinding occurs.
- At Correct Speed (Cascading): Centrifugal force moves balls to roof then fall off; maximum size reduction due to attrition and impact.
- Material is withdrawn after specific time and passed through suitable sieve for desired particle size.
Applications
- Key equipment for regrinding
- Widely used for cement, silicate products, building materials, fire-proof materials, chemical fertilisers, black and non-ferrous metals, glass, ceramics
- Can grind ores or other materials by wet process or dry process
SUMMARY TABLE: SIZE REDUCTION METHODS
| Method | Description | Example Equipment |
|---|---|---|
| Cutting | Material cut with sharp blades | Cutter mill |
| Compression | Pressure applied to crush material | Roller mill |
| Impact | High-speed object strikes stationary material | Hammer mill |
| Attrition | Shear forces from relative movement of surfaces | Fluid energy mill |
COMPARISON: HAMMER MILL vs BALL MILL
| Feature | Hammer Mill | Ball Mill |
|---|---|---|
| Principle | Impact | Impact and Attrition |
| Speed Effect | High speed rotating hammers | Cascading speed required for maximum grinding |
| Product Particle Size | Coarse to moderately fine | Superfine |
| Continuous Operation | Yes (hammers not fixed, less choking) | Batch or continuous |
