Exploring agglomeration: The role of particle size and shape in binary collisions

In spray drying operations, fines are recovered and recirculated into the dryer to enhance agglomeration. Since the properties of fines may affect collision outcomes, this study aims to specifically investigate the impact of varying particle size and shape on collisions. Given the challenging conditions of a spray dryer, a custom-built single droplet dryer was used to study collision behavior. This allowed for dispensing and drying of a sessile droplet and controlled collisions of small fines particles using the air flow. First, glass beads with different size ranges were employed to collide with drying maltodextrin droplets. Sequential merging, sticking and bouncing regimes were observed, where the regimes were affected by fine particle size. A larger particle size led to more sticking and fewer bouncing collisions. Additional experiments were carried out with maltodextrin particles which have a lower density and a non-spherical shape. Minor differences were found if maltodextrin particles and glass beads were compared. Mostly, it was observed that drying time and thus the state of the droplet skin is dominant in determining the collision outcome. Obtained insights are useful to establish future guidelines for nozzle zone agglomeration in spray drying.