SIMULATION OF SPRAY DRYING IN SUPERHEATED STEAM USING COMPUTATIONAL FLUID DYNAMICS

ABSTRACT

This paper presents a numerical simulation and experimental validation of a spray dryer using superheated steam instead of air as drying medium, modelized with a computational fluid dynamics (CFD) code. The model describes momentum, heat and mass transfer between two phases - a discrete phase of droplets 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 nozzle orifice, taking into account their peculiar shrinkage during drying

This model is able to predict the most important features of the dryer : fields of gas temperature and gas velocity inside the chamber, droplets trajectories 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 powder 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 transfer 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 allow a high volumic evaporation rate (20 kg.h⁻¹.m⁻³), high energy recovery and better environment control.

Key words:

• computational fluid dynamics
• heat and mass transfer
• spray drying
• superheated steam