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 choosed a rather high inlet temperature (973 K): For the simulation, drop size spectrum is represented by 6 discrete droplets 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. Using superheated steam or air as a drying medium shows only slight differences in flow patterns, except for the recirculation which is tighter in steam. The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a "cool" zone which is narrower in steam than in air. Finally, the particle deposit problem seems to be more pronounced in air than in steam. Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular, under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.