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 comput ational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differenti al equations of momentum, heat and mass conservation for both gas and dispersed phase. The model is used to simulate the behaviour of a pilo t scale spray dryer operated with two drying media : superheated steam and air. Considering that there is no risk of powder ignition in supe rheated steam, we choosed a rather high inlet temperature (973 K): For the simulation, drop size spectrum is represented by 6 discrete dropl ets diameters, fitting to an experimental droplets size distribution a nd all droplets are injected at the same velocity, equal to the calcul ated velocity of the liquid sheet at the nozzle orifice. It is showed that the model can evaluate the most important features of a spray dry er : temperature distribution inside the chamber, velocity of gas, dro plets trajectories as well as deposits on the walls. The model predict s a fast down flowing core jet surrounded by a large recirculation zon e. Using superheated steam or air as a drying medium shows only slight differences in flow patterns, except for the recirculation which is t ighter 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 recirculatio n zone. Also, they evaporate faster. The contours of gas temperature r eflect these differences as these two aspects are strongly coupled. In both air and steam there is a ''cool'' zone which is narrower in stea m than in air. Finally, the particle deposit problem seems to be more pronounced in air than in steam. Adding to the inherent interest in us ing superheated steam as a drying medium, the model predicts attractiv e behaviour for spray drying with superheated steam. In particular, un der the conditions tested with the model, a higher volumetric drying r ate is obtained in superheated steam.