Spray dryers convert solutions, suspensions or pastes into dry products by spraying a fluid into a warm drying medium, usually air. They can be countercurrent, cocurrent or mixed flow processes.
Equipment design:
In a typical spray dryer, the cylindrical chamber has a short conical bottom. The liquid feed is pumped into a spray pan atomizer located in the top chamber. The liquid is atomized into small droplets that are projected radially into the hot gas stream entering near the top of the chamber. Manipulating the temperature of this hot gas stream can control the porosity of the dried particles. Once drying is complete, the gas and solids are cooled and separated in a cyclone where any entrained solid particles are removed.
The atomizer is the most important part of any spray dryer. It determines the droplet size, size distribution, trajectory and velocity. The droplet size determines the drying rate by adjusting the available heat transfer surface and also controls the size of the drying particles. Additionally, larger droplet sizes require larger drying chambers. Drying chambers are usually found to be 20 meters or higher. Once the material has passed, if it is a coarse powder, it can be easily collected from the bottom of the chamber, but fine powders must be collected from the bag filter.
Since spray drying pushes large quantities of fine dust into the air, there is a risk of fire or dust explosion. However, with proper safety measures, these explosions are very rare. Spray dryers are equipped with a pressurized exhaust system and typically operate under an inert atmosphere to prevent ignition.
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