3. Basic concepts of solid drying

The central stage of the spray-drying operation is the drying of the particles, more or less moist, to the desired degree of dehydration. It is therefore necessary to recall a few general principles of solids drying. This reminder is limited to water evaporation (representing the majority of industrial operations), while maintaining the general character of the approach. The case of other solvents can be treated by analogy.

In the spray-drying process, water is removed by entrainment evaporation using a hot, dry gas stream (often air). More precisely, the operating conditions (temperature and relative humidity) must be such that the water activity in the gas stream remains below its equilibrium activity at the same temperature. By exposing a moist solid body to this gas stream, a simultaneous exchange of heat and matter takes place, leading, on the one hand, to...