Drying

Drying

INTRODUCTION

Drying is a ubiquitous process in the handling and preparation of pharma-ceuticals, and it may be 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. The change of phase from liquid to vapor distinguishes drying from mechanical methods of separating solids from liquids such as filtration. The latter often precede drying since, where applicable, they offer a cost-effective method for removing a large part of the liquid.

Drying, as defined above, may still be confused with evaporation. Greater precision is not possible because the division of the two operations is to some extent arbitrary. Drying is normally associated with the removal of relatively small quantities of liquid to give a dry product. Evaporation is more often applied to the concentration of solutions. However, exceptions to these generalizations occur.

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. improve handling characteristics, as in bulk powder filling and other operations involving powder flow and

2. stabilize moisture sensitive materials, such as aspirin and ascorbic acid.

A wide range of drying equipments 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. In the pharmaceutical industry, batch sizes are frequently small and of high value and the same dryer may be used to dry different materials. These factors limit the application of continuous dryers and promote the use of batch dryers that give low product retention and are easily cleaned. Recovery of solvents, where economically justified, may be another factor affecting the choice of equipment.