Drying of multiphase single droplets in ultrasonic levitator

Spray drying is widely used in many industries, e. g. chemical, food and pharmaceutical processing, in which the spray generation is followed by solvent/liquid evaporation leading to a final powder product. As the liquid component evaporates, the droplet weight decreases. This evaporation process is accompanied by phase change of the volatile component. For a multiphase droplet, solid material is suspended or dissolved in the liquid phase. In such cases, it is important to have knowledge of transport phenomena for single droplets as it pertains to their evaporation and component distribution. The investigation of the evaporation of single droplets is particularly applicable to the study of spray drying and atomization. There are several different measurement techniques that can be used to analyze droplets of various components such as pure liquids, mixtures, solutions, and colloidal suspensions. The principle of ultrasonic levitation is unique in that it allows the isolation of a single droplet in space rather than a myriad of droplets. Having a multiphase droplet suspended in the acoustic field facilitates the study of the drying rate at different operational parameters, and tracking the morphological changes during the course of the drying process. The experiments conducted in this work were performed for a single droplet or separation of two single droplets. Single droplet analysis allows for characterization of the droplets drying rate. The multi-droplet analysis is pertinent to sound pressure level calibration. Understanding the sub-models for these processes is largely beneficial to the advance of modeling the overall spray drying process. In the present work, the drying of further example systems has been studied, and a characterization of the outer crust formed at the end of the first drying stage is presented. These are accompanied by „quasi“ 2-D model formulations for the simulation of individual drop drying to describe the solid concentration profiles and the creation of the crust at the end of the constant rate period, when the solids concentration at the surface exceed the saturation concentration.