Simulation of spray drying in superheated steam using computational fluid dynamics.

This paper presents a numerical simulation and experimental validation of a spray dryer using superheated steam instead of air as drying medium, model ized with a computational fluid dynamics (CFD) code. The model describes mo mentum, heat and mass transfer between two phases - a discrete phase of dro plets and a continuous gas phase - through a finite volume method. For the simulation, droplet size distribution is represented by 6 discrete classes of diameter, fitting to the experimental distribution injected from the noz zle orifice, taking into account their peculiar shrinkage during drying. This model is able to predict the most important features of the dryer : fi elds of gas temperature and gas velocity inside the chamber, droplets traje ctories and eventual deposits on to the wall. The results of simulation are compared to a pilot scale dryer, using water. In the absence of risk of po wder ignition in steam, we have tested rather high steam inlet temperature (973K), thus obtaining a high volumic efficiency. The model is validated by comparison between experimental and predicted values of temperature inside the chamber, verifying the coupling between the 3 different types of trans fer without adjustment. This type of model can be used for chamber design, or scale up. Using superheated steam instead of air in a spray dryer can al low a high volumic evaporation rate (20 kg.h(-1).m(-3)), high energy recove ry and better environment control.