1. DRYING

2. It is a mass transfer process resulting in the removal of moisture (water) or another solvent, by evaporation from a solid, semi-solid or liquid to end in a solid state. To achieve this, there must be a source of heat.
Drying is most commonly used in manufacturing as a unit process in the preparation of granules, which can be dispensed in bulk or converted into tablets or capsules.

3. Purposes of drying
In pharmaceutical technology, drying is carried out for one or more of the following reasons:
To avoid or eliminate moisture which may lead to corrosion and decrease the product or drug stability.
To improve or keep good properties of a material, e.g. flowability, compressibility.
To reduce the cost of transportation of large volume materials ( liquids).
To make the material easy or more suitable for handling.
The final step in: Evaporation- Filtration- Crystallization.

4. Moisture content of wet solids
The moisture content of a wet solid is expressed as kilograms of moisture associated with 1 kg of the moisture. Thus a moisture content of 0.4 means that 0.4 kg of removable water is present per kg of the solid. It is sometimes calculated as percentage moisture content.
Total moisture content: This is the total amount of liquid associated with a wet solid. The easily removable water is known as the free moisture content, and the moisture which is more difficult to remove is the equilibrium moisture content. The easily removable water is known as unbound water.
Unbound water: This water exists as a liquid and exerts its full vapour pressure, it can be removed readily by evaporation. During a drying process this water is easily lost but the resulting solid is not completely free from water molecules.

5. Equilibrium moisture content:
The moisture content present in a solid under steady-state ambient conditions is termed the eq. moisture content. Its value changes with temperature, humidity and the nature of the solid.
Bound water :
Part of the moisture present in a wet solid may be adsorbed on surfaces of the solid or be adsorbed within its structure to such an extent to prevent it from developing its full vapour pressure and from being easily removed by evaporation. Such moisture is described as “bound” and is more difficult to remove than unbound water.

6. DRYING CURVE

7. Constant-rate period: It is found that the evaporation rate from the drying bed is similar to that of the solvent alone from a free liquid surface under the same conditions, indicating that the evaporation takes place from the wet surface of the solid, and that the surface remains wet in this period as a result of the liquid being replaced from below as fast as it is vaporized Controlling factors in this period are the rate at which heat can be transferred and the rate of removal of the vapour.

8. First falling-rate period: As moisture is removed from the surface, a point will be reached when the rate of vaporization is insufficient to saturate the air in contact with the surface. Under these conditions, the rate of drying will be limited by the rate of capillary transfer of the liquid
to the surface of the wet bed, and this becomes increasingly difficult as the bed dries, the solvent level decreases and thus has further to travel to the point of evaporation.
Consequently, the rate of drying
decreases continuously.

9. Second falling-rate period: Any moisture that remains within the drying bed at the end of the first falling –rate period is unable to move, so that drying cannot take place on the surface. i.e. the drying rate depends on the
movement of the vapour through
the pores of the bed to the surface,
in general by molecular diffusion

10. Tray Dryer
A tray Dryer is an enclosed insulated chamber in which trays are placed on top of each other in trolleys. Tray Dryer are used where heating and drying are essential parts of manufacturing process in industries such as Chemicals, Dye stuff, Pharmaceutical, Food Products, Colors etc. The material to be dried either wet or solids are placed in the trays.
The trays or either square or rectangular and about 4-8 square feet in area. Trays are usually loaded inches deep. Heat transfer is by circulation of hot air by electric heaters or steam in radiator coils. Blower fans are installed inside to ensure proper circulation and transfer of heat. A control panel to control the temperature and other parameters is fixed outside the dryer. These dryers are available in Mild Steel, Stainless Steel.

11. AIR CIRCULATION
A highly effective recirculation air system is provided. The heated air, is recirculated with fresh air in selected proportions for optimum drying. The system is designed so that the materials at the top & the bottom dry simultaneously.
Uniform air circulation, controlled temperature, sturdy construction and large working space are the valuables of the oven which is suitably designed to cover wide temperature range, loading and unloading is faster and simple.
CONTROLLED TEMPERATURE & TIME
Digital temperature controller with digital timer are supplied to facilitate working day and night.

13. Air outlet
Trays
Heaters
Air Inlet
Directed circulation tray dryer

14. Fluidized Bed Dryer (FBD)
Principle Of Fluidization
The principle of operation of fluidized systems are based on the fact that if a gas is allowed to flow through a bed of particulate solids at velocity greater than the settling velocity of the particles, the solids get partially suspended in the stream of upward moving gas. The gas stream negates the gravitational pull due to weight of particles to enable the suspended state of the solid.
The resultant mixture of solids and gas behave like a liquid and thus solids are called Fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying. In the process of fluidization there occurs an intense mixing between the solids or gas resulting in uniform condition of temperature, composition and particle size distribution throughout the bed.

15. If the air velocity through the bed is increased gradually and the pressure drop through the bed is measured, a graph of the operation shows several distinct regions.
At first, when the air velocity is low, (point A) , flow takes place between the particles without causing disturbance, but as the velocity is increased point (B), is reached (gravity = force of fluidization on that particle). Rearrangement of the particles occurs to offer least resistance (C), and they are suspended in the air and can move.

16. At point (D), pressure drop through the bed decreases slightly because of the greater porosity. Further increase in the air velocity causes the particles to separate and move freely and the bed is fully fluidized. Any additional increase in velocity separates the particles further, that is, they expands without appreciable change in the pressure drop, until (E), when the air velocity is sufficient to transport the particles to the top of the bed.

17. Fluid bed dryer is designed to introduce the hot air stream at the base of the product container which is filled with the material.
Induced draught is created by means of blower and fresh air is sucked in to the unit. This hot air system expands the bed of material at certain velocity & creating turbulence in the product. The phenomenon is known as fluidization and offer conditions which are almost ideal for drying.
Fluidization produces full agitation of solid particles and since each particle gets surrounded by hot air, heat transfer is extremely high and uniform.
The product is dried fast without appreciable loss of heat. Filter bags prevent particles escaping from the dryer.
Fluid Bed Dryer occupy lesser floor space compare to conventional dryer and are very easy to operate and can dry material in least time as compared to tray dryer.

18. In fluid Bed Dryer, temperature distribution through out the product is uniform and the heat transfer rate is very high. High production rates are achieved due to reduce drying time.
As the product is in close contact with drying air at low temperature and also for short duration, the physical and chemical properties of the products are generally not effected and therefore the dryer can effectively be used for heat sensitive products.
Due to the continuous movements of product during drying lump formation, case hardening etc. are minimized.
Can be used for granulation, coating, blending and microencapsulation

19. Types of FBD
1. Vertical (Batch type) FBD 2. Horizontal (Continuous) FBD

21. Spray dryer
It provides a large surface area for heat and mass transfer by atomizing the liquid to small droplets. These are sprayed into a stream of hot air, so that each droplet dries to a solid particle. The drying chamber resembles the cyclone ensuring good circulation of air, to facilitate heat and mass transfer, and that dried particles are separated by the centrifugal action.

22. The character of the particles is controlled by the droplet form; hence the type of atomizer is important.
Rotary atomizer is preferable than jet which is easily blocked. Liquid is fed to the disc of the atomizer which is rotated at high speed (up to 20,000 rpm), a film is formed and spread as uniform spray. In addition, the rotary atomizer is effective with suspensions. It can be operated efficiently at various feed rates.

23. The products are uniform in appearance and have characteristic shape, in the form of hollow spheres with a small hole. This arises from the drying process, since the droplet enters the hot air stream, and dries on the outside to form an outer crust with liquid still in the center. This liquid then vaporizes, the vapour escaping by blowing a hole in the sphere.
Advantages:
The droplets are small, giving a large surface area for heat transfer, so that evaporation is very rapid. The actual drying time of a droplet is only a fraction of a second, and the overall time in the dryer is only a few seconds.

24. Because evaporation is very rapid, the droplets do not attain a high temperature, most of the heat being used as latent heat of vaporization.
The characteristic particle form gives the product a high bulk density and, in turn, ready solubility.
The powder will have a uniform and controllable particle size.
Being spherical in nature the particles have good flowability.

25. Disadvantages:
The equipment is very bulky, connected to accessories, fans, heaters,), that make it expensive.
Uses:
Drying of any substance in solution or in suspension form.
It is most useful for drying of thermolabile materials e.g. antibiotics.
Suitable for large quantities solution.
Suitable for both soluble and insoluble substances e.g. citric acid, gelatin, starch.
It can produce spherical particles in the respiratory range e.g. dry powder inhalers.
Drying of milk, soap and detergents.

26. Lyophilization
This is also known as freeze drying, gelsicsation or drying by sublimation.
The product to be dried is first frozen and then subjected to drying under high vacuum through sublimation.

27. Tripple point for water as shown in figure is
Temperature = oC
Pressure = mm (4579 microns)
The water in pharmaceutical products intended for freeze drying contains dissolved solids, resulting in different temperature- pressure relationship for each solute. In such cases the point at which solid water is sublimed is known as eutectic point
Freeze drying condition
Temperature = -10 to -40 oC
Pressure = 2000 to 100 microns
Composition of Freeze dryer
Chamber for vacuum drying
Vacuum source (Vacuum pump, steam ejectors)
Heat source
Vapour removal system (Condensors, Dessicants, pumps)

30. Advantages:
Minimum damage and loss of activity in delicate heat-liable materials
Speed and completeness of rehydration
Possibility of accurate, clean dosing into final product containers
Porous, friable structure
Disadvantages:
High capital cost of equipment (about three times more than other methods)
High energy costs (2-3 times more than other methods)
Long process time (typically 24 hour drying cycle)
Uses
Lyophilization should be used when the product meets one or more of the following criteria: unstable; heat liable; minimum particulates required; accurate dosing needed; quick; complete rehydration needed; high value.
Hormones
Blood serum
Plasma
Antibiotics
Bacterial culture
foodstuff

31. TLC= Thermodynamic Lyophilization Control