Spray drying is a method for producing a dry powder directly from a solution or slurry. In a continuous process, the liquid feedstock is dispersed as a mist of fine droplets into a stream of hot drying gas, all within a cylindrical drying chamber. In aqueous processes, the drying gas is typically air. In addition, an inert closed cycle is also useful for drying chemical and biologic materials that are sensitive to oxygen. Both air and material either travel in parallel or counterflow. Because the kinetics of drying are so rapid, materials are dried in a fraction of a second which helps to make the process viable for thermal sensitive materials such as foods, extracts and other organic substances.
It atomizes the liquid into small droplets, providing a large surface for mass and heat transfer. Using hot air, the drops are sprayed so that they dry into solid particles.
When moved through a hot gaseous drying medium, spray drying produces dry droplets from liquid atomization. Atomization, mixing spray air, evaporation of moisture, and separation of the dry product from the exit air are the steps before drying occurs in a spray dryer. Atomization is the first step in spray drying. Atomization is the act of turning the liquid feedstock into small droplets of liquid by using a nozzle or rotary atomizer. Following, the solute or suspension is separated from the solvent as a solid and then into a gas. As part of this stage, many desirable properties are developed, such as particle size and viscosity. In the case of atomizers or nozzles, the droplets dried to form a powder that is easily packaged and transported. After the droplets have dried, they form solids. Drums or cyclones are usually used to collect solids. The spray dryer’s design and operation, coupled with the physicochemical properties of the feed, determine the final product. Hot air is commonly used to dry powder. By adjusting the hot air temperature, the powder’s final moisture content can be controlled. After the exhaust gas has been re-circulated, the powder can be recovered from it within a few seconds during the recovery process.