Drying Process in Pharmaceutical Industry

Drying is a critical process in the handling and preparation of pharmaceuticals. It is defined as the vaporization and removal of water or other liquid from a solution, suspension, or other solid-liquid mixture to form a dry solid.


Purpose of Drying

Adjustment and control of moisture levels by drying is important in the manufacture and development of pharmaceutical products. Apart from the obvious requirement of dry solids for many operations, drying may be carried out to -

1. To improve handling characteristics,

2. To improve flow properties,

3. To prepare desired granules for tablet or capsule preparation,

4. To stabilize moisture sensitive materials, such as aspirin and ascorbic acid etc.


A wide range of drying equipment are available to meet these ends, but in practice, the choice is limited by the scale of the operation and may be determined partly or completely by the thermal stability of the material and the physical form in which it is required.


Types of Dryers

  • Tray Dryer
  • Fluidized Bed Dryer (FBD)
  • Spray Dryer
  • Tunnel Dryer
  • Rotary Dryer
  • Vacuum Dryer
  • Freeze Dryer

The following terms are employed in discussing drying:

  • Humidity,
  • Humidity of saturated air,
  • Relative humidity,
  • Moisture content,
  • Equilibrium moisture content,
  • Wet bulb temperature, 
  • Adiabatic cooling line etc.

The relation between equilibrium moisture content and relative humidity for a hygroscopic solid.


If a material is exposed to air at a given temperature and humidity, it will gain or lose moisture until equilibrium is reached. The moisture present at this point is defined as the equilibrium moisture content for the given exposure conditions.

Any moisture present in excess of the equilibrium moisture content is called “free water.”


The rate at which water vapor is transferred from the saturated layer at the surface to the drying stream is described by equation as:

Where, Pwi is the partial pressure of the water vapor at the surface and Pwa is the partial pressure of water vapor in the air. kg is a mass transfer coefficient, and N is the number of moles of vapor transferred from unit area in unit time. Rewriting this in terms of the total mass, W, transferred in unit time from the entire drying surface, A,

Where, Mw is the molecular weight of water vapor, R is the gas constant, and T is the absolute temperature.

The mass transfer coefficient, kg, will itself be a function of the temperature, the air velocity, and its angle of incidence. A high velocity or angle of incidence diminishes the thickness of the stationary air layer in contact with the liquid surface and, therefore, lowers the diffusional resistance.

The rate of evaporation may also be expressed in terms of the heat transferred across the laminar film from the drying gases to the surface. This is described by equation as:

Where, Q is the rate of heat transfer, A is the area of the surface, Ta and Ts are the temperatures of the drying air and the surface, respectively, and h is the heat transfer coefficient.


Extensive studies have been made to determine the nature of the forces that initially convey moisture to the surface at a rate sufficient to maintain saturation and their subsequent failure. Movement of liquid may occur by diffusion under the concentration gradient created by depletion of water at the surface by evaporation, as the result of capillary forces, through a cycle of vaporization and condensation, or by osmotic effects. Of these, capillary forces offer a coherent explanation for the drying periods of many materials.

Read also: Mixing Process in Pharmaceutical Industry

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