Abstract
The use of population balances to describe the quality of solid particles received from spray drying processes is outlined. To describe the process, a single droplet model is developed. At the beginning of the process the droplet consists of at least two components, the solvent and the substance to be crystallized. During the process mass and heat transfer occur, because the droplet is located in a hot gaseous atmosphere. So the solvent evaporizes and diffuses into the continuous phase which leads to a decrease of the liquid droplet volume. Simultaneously, solid hollow structured particles are created, because the combined mass and energy transport generates a supersaturation inside the droplet.