Comparison of superheated steam and air operated spray dryers using computational fluid dynamics

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase. The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air. Considering that there is no risk of powder ignition in superheated steam, we chose a rather high inlet temperature (973K). For the simulation, drop size spectrum is represented by 6 discrete diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice. It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam.