Drying is the removal of a liquid from a material through the application of heat, and is accomplished by the transfer of a liquid from a surface into an unsaturated vapor phase.
Uses of drying in Pharmaceutical industry
- Drying is commonly used in granulation as the final stage of Pharmaceutical processing for reduction of bulk weight, and to yield stable homogenous product for preservation,
- It’s used in improving flow and for packaging and storage.
- Drying is also used to convert wet filter cake to free flowing powder,
- It’s used to evaporate and dry (drum-drying, freeze-drying or spray-drying of solutions (especially vegetable solutions) in one swoop
- It’s used to partly facilitate grinding.
The Rate of drying and distribution of moisture
There are three stages of moisture distribution in a drying particle bed
When wet material is heated, the wetting liquid acquire more kinetic energy, begin to agitate and detach from material bulk and evaporates. At constant heat, this evaporation continues in stages until dryness.
The stages are:
- Constant rate period; water is evaporating from the inner material layer in a continuous film up to the surface and into the atmosphere on major spots. Rate of drying here is determined by the temperature and humidity of the surface surrounding air when air is moving at constant velocity.
- First falling rate period: After the critical moisture temperature is reached, the rate of evaporation starts declining and the number of continuous film of evaporating water reduces. Dry spot begins to appear on some spots. Rate of drying here is controlled by the rate at which vapour can diffuse through the surface air film. The thickness of the surface air film will influence the rate of diffusion of vapour through it. High air velocity will reduce this film thickness and increase rate of diffusion and drying.
- Second falling rate period; after the second critical point, the rate of drying declines even further and all the surfaces are dried and there in no more chain film evaluation. Here the drying is achieved through diffusion of fluid from material inner core to material surface and evaporation. It is slow. Air velocity does not affect this falling rate.Thus, in other to dry a material, two processes are necessary:
- Heat supply to provide latent heat of vaporization
- Liberated vapour must be removed, generally, by moving air stream.
The rate of movement (mass transfer) of water from the wet solid to moving air is directly proportional to the difference between the humidity of the air at the liquid surface of the solid (H=saturated air) and the bulk of the air (h=air stream), and the area exposed to the air (K);
Rate of mass transfer = A (H – h) K.
But rate of heat transfer from air to wet solid is = U.A∇.t
U is overall coefficient of heat transfer, ∇t is temperature difference between air and surface of liquid.
Latent heat of vaporization L = Rate of heat transfer / rate of mass transfer.
U.A.∇t = A (H – h)K .L
H – h = U.A.∇t / K.L
Factors involved in the selection of methods and choice of drying equipment
- Heat sensitivity of the material to be dried
- Physical characteristics of the material
- The necessity for asepsis
- Nature of liquid to be removed
- The scale of the operation
- Achievable source of heat
Ideals of drying equipment
- Large surface area for drying
- Efficient heat transfer per unit area to supply sufficient heat for latent vaporization
- Efficient mass transfer of evaporated water through any boundary, turbulence and to minimize boundary layer thickness
- Efficient vapor removal at low relative humidity
Types of drying methods
CONVECTIVE: Heat is transferred through air that is forced through the wet material. Example is fixed bed convective drying with tray drier, dynamic bed convective drying with fluidized bed drier.
CONDUCTIVE: Heat transferred through contact with heated surface. Example is conductive drying with vacuum oven or with vacuum tumbling drier.
RADIATION: Radiation is used to create internal agitation and heat. Example is radiant heat transfer using infrared radiation or microwave.
DRIERS FOR DILUTE SOLUTIONS AND SUSPENSIONS; The material to be dried (usually in solution/suspension is transferred/sprayed into a chamber of reduced pressure to vaporize at reduced temperature.
FREEZE DRYING: Lower pressure and temperature is introduced to freeze and sublime fluid from wet material.
Classification of dryers
- MOVING-BED (drum, turbo-tray and pan dryers).
- STATIC-BED (tray dryers and truck dryers {with tray loaded in truck})
- STATIC FLUIDIZED-BED (equipment for both mixing and drying; vertical FBD and horizontal FBD):
- PNEUMATIC (Spray dryers and flash dryers)
- SPECIALIZED DRYING (microwave dryers) systems.
- FREEZE DRYING
MOVING-BED (drum, turbo-tray and pan dryers):
The wet mass or suspension/liquid is placed in a bowl, and heated drum is allowed to roll on top of the material at just enough distance to make calculated contact. The wet mass picked is dried by the hot revolving drum and scraped off with a knife.
Advantages
The advantages of moving bed dryers are
- rapid drying,
- thin film spreads over large surface,
- short heating time,
- compact equipment that can be enclosed, and
- formation of compressible flakes.
Disadvantages
These include
- the care in ensuring adequate temperature, drum speed, material uptake and film thickness.
STATIC-BED (tray dryers and truck dryers {with tray loaded in truck})
Tray dryer is an enclosed insulation chamber with single or double gasket doors, stainless steel trays stacked on trolley, and a control panel. The walls of the chamber are filled with high density fiber glass wool insulation material to avoid heat transfer. Materials to be dried are placed on the trays, and heat by convection of a blow fan is transferred from electric heaters or steam radiators coils.
STATIC FLUIDIZED BED DRYERS
Fluidized bed dryers works on the principle of fluidization of the wet material. This is achieved by introducing high pressures hot air through the perforated bottom of a bowl containing the wet mass. The wet mass is lifted from the bottom and suspended in the stream of air (fluidized state). The suspended wet mass is kept in space by fingered musilin-like cloth. The direct contact of wet mass and hot air ensures heat transfer and vaporization of liquid in wet mass. The vaporized liquid is carried away by the hot gas. The cloth allows the vapoutized liquid and air to pass through, but retains the drying mass.
Advantages
- They have efficient heat and mass transfer,
- They ensure that drying is not only at the surface of mass but at surface of each particle
- Uniform temperature throughout,
- turbulence causes attrition and free flow.
Disadvantages
- The disadvantages is that turbulence can cause excessive attrition, fines and charges.
The two types of Fluidized bed dryers are
- Vertical fluidized bed dryer
- Horizontal Fluidized bed dryer
PNEUMATIC (spray and flash dryers);
The solution or suspension to be dried (together with a vaporizer solvent and others) is sprayed from the top of a chamber at low pressure/vacuum with streaming hot air. The liquid escapes and dry mass falls off.
Advantage
- low labour cost,
- rapid drying,
- free flowing particles with high density and surface area,
- uniform and controllable particle size.
Disadvantages
- Its equipments are bulky and
- They have low thermal efficiency.
SPECIALIZED DRYING (microwave dryers) systems.
The magnetron device produces microwave radiation into the drying chamber. These waves penetrate wet material and small polar molecules resonates in sympathy with the waves. The resonating molecules creates friction on contact with each other, and generates heat uniformly all through the material and evaporates off. Dry substances do not resonate as much as well as water, thereby avoiding overheating after drying.
Advantages
- Thermal efficiency here is high,
- bed and material are static,
- no charge from attrition,
- and solute migration is reduced.
Disadvantage
- Size of material and
- fear of radiation.
FREEZE DRYING (For freezing and sublimation drying).
Material to be dried (often heat sensitive) is place in the drying equipment. Pressure and temperature is reduced to freeze the material. At slightly elevated temperature, the liquid in the frozen mass sublimes off.
Advantages
- low temperature,
- light and porous material,
- no concentration or migration of solute, static powder,
Disadvantage
- the process is slow
- material can be overly porous and soluble