DRYING
The process in which water and other solvents are removed from a substance by applying heat is called drying. Removal of liquid from solids can be done either mechanically (using filter press or centrifuge) or thermally (by vaporisation).
OBJECTIVES OF DRYING
- To decrease food decomposition caused by moisture present.
- To prevent decomposition by microbes so product can be used for longer duration.
- To reduce transportation and storage cost by reducing bulk and weight.
- To ensure additional benefit as dried goods of superior quality have more market value.
- To aid in granule formation.
- To facilitate material processing (spray-dried lactose, dried aluminium hydroxide, ferric ammonium citrate).
- To enable size reduction.
- To prevent bacterial and mould growth, thus increases product stability.
- To produce stability to some substances like aspirin, penicillin, ascorbic acid in their dry form.
MECHANISM OF DRYING
Drying is a diffusion process in which moisture is transferred from the inside of the solid to its surface as soon as some of the surface moisture evaporates. This process releases moisture into the surrounding medium. An external drying medium (typically air or hot gas) supplies the heat needed to evaporate the moisture from the drying product.
APPLICATIONS OF DRYING
1. Preparation of Bulk Drugs
- Dried aluminium hydroxide
- Spray-dried lactose
- Powdered extracts
2. Preservation of Drug Products
| Drugs | Type of Decomposition Prevented |
|---|---|
| Crude drugs (animal and vegetable origin) | Chemical decomposition |
| Blood products, skin, and tissue | Microbial growth |
| Synthetic and semi-synthetic drugs | Chemical decomposition |
| Effervescent tablets (aspirin, penicillin) | Chemical decomposition |
3. Improved Characteristics
Drying of granules improves fluidity and compression for tablet and capsule production. Drying enhances free-flowing nature of viscous and sticky materials (male fern extract, malt extract, oleoresin).
4. Improved Handling
Material becomes light-weight and bulk reduces if moisture content is reduced, reducing transportation cost and making storage efficient.
5. Purification of Crystalline Products
Drying purifies a crystalline product by removing the solvent adhering to the crystals.
6. Prevention of Corrosion
Drying prevents corrosion occurring due to presence of moisture (e.g., moisture in dry chlorine gas makes it very corrosive).
FACTORS AFFECTING DRYING
- Particulate Diameter: Affects drying rate as diameter changes surface area exposed for drying.
- Size Uniformity: Non-uniform size affects path length for diffusion and capillary action.
- Physical Properties of Wet and Dry Solid Particles: Particle size, surface area, pore size, and powder rheology affect drying rate.
- Product Mass Flow Rate: Bed surface constantly changing at optimised speed increases drying rate.
- Hot Air Mass Flow Rate: Continuous change in hot air movement avoids moisture saturation.
- Diameter of the Dryer Section: Altering mass flow rate of hot air increases drying rate.
- Critical Moisture Content of the Material: Moisture content at which constant rate period ends and falling rate period starts.
CLASSIFICATION OF DRYERS
- Direct Dryers: Employ convective drying – heat substance by direct contact with heated air, gas, or combusted gas product.
- Indirect Dryers: Employ convective drying – heat substance by direct contact with a heated wall.
- Common Dryers: Tray dryer, Fluidised bed dryer, Vacuum dryer, Freeze dryer.
FLUIDISED BED DRYER (FBD)
Principle
Hot air or gas at high pressure is introduced through perforated bottom of container filled with granules. The gas lifts granules and suspends them in air stream, resulting in a fluidised state. This ensures uniform drying as hot gas surrounds all granules completely.
Construction
- Made of stainless steel or plastic
- Detachable bowl at bottom for charging/discharging with perforated bottom and wire mesh
- Upper part has fan for circulating hot air
- Air heated via fresh air inlet, pre-filter, and heat exchanger
- Filter bags above drying bowl to recover fines
Working
- Wet granules placed in detachable bowl and placed into dryer
- Fresh air passes through pre-filter then heat exchanger (heats up)
- Hot air flows through bottom of bowl; fan rotates
- When air velocity exceeds settling velocity (1.5 to 7.5 m/min) fluidised state achieved
- Hot air surrounds each granule, completing drying
- Air passes through filter bags and exits
- Residence time is 40 minutes
Applications
- Drying granules for tablet compression
- Mixing, granulation, and drying
- Coating granules
- Mixing ingredients
FREEZE DRYER (LYOPHILISATION)
Principle
Freeze drying involves removal of water from frozen material by sublimation. Exposing the material to temperature and pressures below the triple point of water (0.0098°C and 0.533 kPa) results in drying.
Construction
- Drying chamber for loading trays
- Radiation source or heating coils for supplying heat
- Vapour condensing or adsorption system
- Vacuum pump or steam ejector (or both)
Process of Freeze Drying
- Preparation and Pre-Treatment: Solution pre-concentrated using tray drying under vacuum, reducing actual drying rate by 8-10 times.
- Pre-Freezing to Solidify Water: Aqueous solutions frozen at -50°C; normal cooling rate 1-3 K/min for large ice crystals.
- Primary Drying (Sublimation of Ice under Vacuum): Temperature and pressure below triple point (0.0098°C, 0.533 kPa); vacuum pressure 3 mm Hg; removes 98-99 percent water.
- Secondary Drying (Removal of Residual Moisture): Traces of moisture eliminated by heating at 50-60°C under vacuum less than 50 mm Hg; takes 10-20 hours.
- Packing: After replacing vacuum with inert gas, bottles and vials are closed.
Applications
- Manufacturing injections, solutions, and suspensions
- Drying blood plasma and fractionated products
- Drying bacterial and viral cultures
- Drying human tissue (arteries and corneal tissue)
- Drying antibiotics and plant extracts
- Drying steroids, vitamins, and enzymes
- Food items: prawns, mushrooms, meat, poultry, coffee, tea concentrates, citrus fruit juices
FREEZE DRYING STAGES
| Stage | Process | Parameters / Duration |
|---|---|---|
| Pre-treatment | Pre-concentration using tray drying | Reduces drying rate by 8-10 times |
| Pre-freezing | Freezing at -50°C | Cooling rate 1-3 K/min |
| Primary Drying | Sublimation of ice under vacuum | Below triple point (0.0098°C, 0.533 kPa); removes 98-99% water |
| Secondary Drying | Removal of residual moisture | 50-60°C, vacuum less than 50 mm Hg, 10-20 hours |
