Effect of droplet collisions on evaporation in spray-drying

Excess vapor content in the spray-drying system leads to various issues such as undesired agglomeration or caking of products. The majority of the drying in spray-dryers occurs during the initial phase, where the feed is still liquid. Drying kinetics is very sensitive to the droplet size, and the droplet size is heavily influenced by collisions that can coalesce or split droplets. An analytical model is developed to examine the role of droplet collisions on drying kinetics, and the evaporation rate during the first phase of drying. In addition, droplet collision dynamics are implemented into an Eulerian-Lagrangian framework, where droplets are tracked in the Lagrangian frame, and the background gas is modeled as a continuum. The collision dynamics is capable of predicting the outcome of collisions based on a regime map constructed from the experimental data from the literature. The relative velocity and the Weber number between colliding droplets affect whether droplets coalesce or split into satellite droplets. The modeling framework includes fully coupled interphase heat transfer between the droplet and gas phases. The analytical and numerical models are then used to analyze a simplified pseudo-1D spray system. Both models show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. Further, comparing both methods' results reveals droplet number density as another critical factor influencing the evaporation rate. It is shown that at high droplet number densities, the relationship between the evaporation rate and the Weber number becomes non-linear, and at extremely high droplet number densities, the evaporation rate decreases even at high Weber numbers.

Automated summary

It has been found that excessive vapor content in the spray-drying system can lead to issues such as product agglomeration or caking. The drying process in spray-dryers mainly occurs during the initial phase, where the feed is still in a liquid state. The size of the droplets plays a crucial role in the drying kinetics, and it is heavily influenced by collisions that can cause droplets to coalesce or split. Analytical and numerical models are used to investigate the impact of droplet collisions on drying kinetics and the evaporation rate.