Single-droplet drying processes at varying environmental conditions

Abstract

Modeling and numerical simulations of the single-droplet drying process are commonly executed based on the assumption of constant boundary conditions. In this study, the validity of this assumption and its influence on the model prediction of the final particle morphology is investigated. Accordingly, numerical simulations of the single-droplet drying process were conducted at different drying conditions. Five boundary conditions were examined: (a) constant inlet drying air condition based on the most commonly used assumption, (b) varying drying conditions, (c) constant average drying conditions, (d) partially varying drying conditions whereby all drying conditions were changed apart from the droplet’s relative velocity, and (e) velocity varying condition whereby all the drying conditions were averaged apart from the relative velocity of the droplet. The boundary conditions for the last four cases were calculated using three-dimensional simulations of the spray drying process of the particles that possessed pure solvent properties. The final product morphologies have been examined, i.e., the morphologies of hollow or full porous particles as predicted by the simulations of the single-droplet drying process. It has been found that the variation of the drying boundary conditions had considerable influence on the droplet’s shrinking rate as well as on the efficiency of the process, while the dried particle morphology depended on its shell strength. The influence was more pronounced when the initial droplet diameter and its shell strength increased.