concentration
The feedstock is usually concentrated before being introduced into the spray dryer. The concentration stage increases the solids content and thereby reduces the amount of liquid that needs to be evaporated in the spray dryer.
atomization
The characteristics of the spray dryer are the atomization of the raw material and the contact between the spray and hot air. The atomization stage is designed to create excellent conditions for transpiration and produce a dry product with the desired properties. Nozzles and rotary atomizers are used to form the spray. Dryers range from one nozzle to over fifty.
Droplet - air touch
The central component of the spray dryer is the spray drying chamber. In a chamber, the atomized liquid is brought into contact with a hot gas (usually air under a vacuum), causing more than 95% of the water droplets to evaporate within a few seconds. The way the spray comes into contact with the air in the dryer affects the behavior of the droplets during the drying phase and directly affects the properties of the dried product. The type of contact between spray and air is determined by the position of the atomizer relative to the air inlet. The nozzle head is usually located at the top of the dryer and sprays downward.
droplet drying
Water transpiration occurs in two stages. During the preliminary phase, the temperature in the saturated air at the droplet's surface is approximately equal to the wet-bulb temperature of the dry air. There is enough water in the droplets to replace the liquid evaporating on the surface, and the evaporation occurs at a relatively stable rate. At the beginning of the second stage, there is no longer enough moisture to maintain a saturated state on the surface of the droplet, resulting in the formation of a dry shell on the surface. Transpiration then depends on the diffusion of water through the shell, which increases in thickness. In the second stage, the transpiration rate decreases rapidly. Different products have different transpiration and particle composition characteristics. Some swell, some shrink, crack or disintegrate.
The resulting particles can be relatively uniform hollow spheres, or porous and irregularly shaped.
separation
After drying is completed, the product particles need to be separated from the dry air. Primary separation is accomplished by the particles simply falling to the bottom of the chamber. A small portion of the particles remains entrained in the air and needs to be recovered in a separation device. Cyclone separators, bag filters and electrostatic precipitators can be used in the final separation stage. Wet scrubbers are then often used to clean and cool the air so it can be released into the atmosphere.
