Spray drying is a process where solid material is made by ejecting liquids or slurries, mixtures of solids and liquids, from nozzles into heated air streams. This is used to make a wide range of powdered and solid chemicals, ranging from fertilizers to powdered pharmaceuticals. Creating solids from liquids or mixtures reduces shipping costs, improves storage life, and provides the option of making different shaped particles for specific applications. Spray drying technology was first developed in the late 19th century, but was not seen commercially until the 1920s when improvements made continuous operation possible.
The process of spray drying can occur in open or closed tanks or storage buildings. Products can be sprayed at the top of open storage tanks or bins, and the liquid evaporates before the product reaches the bottom. Heated air may be introduced at the top along with the product, which is called co-current flow because the air and product are moving in the same direction. Counter-current flow occurs when the product is flows down and the air moves from the bottom to the top of the tank.
A mixed-flow system can be used for different materials and drying patterns. The nozzle normally points up, creating a pattern in an upward direction. Air is introduced at the bottom of the tank. The particles first move upward in the air stream, then fall down against the air stream, causing a mixed flow. These spray drying systems can be useful for larger particle sizes, or materials that require additional drying time.
Products produced from spray drying systems are usually round spheres, but other shapes can be made. The aspect ratio, or the maximum length to width ratio of the product shape, is normally used to measure non-spherical shapes. Material can be produced that is spherical, but hollow in the middle to reduce weight. A two-stage system can be used, where the first step creates product that is still slightly wet, and the material sticks together in a second step to form larger particles called aggregates.
The nozzle can be changed for different products or particle sizes. Atomizers, both stationary and rotary, are used to create spheres. Rotary atomizers use a spinning nozzle to eject the material in fine droplets. Two-fluid nozzles can use a mixture of product and compressed air or steam to create a spray pattern. Using compressed air or steam adds cost to the product, but may be needed for materials that are very thick or require steam heating to be liquids.
Freeze drying can be used to dry products by spraying liquids into refrigerated storage tanks. The liquids are removed by freezing and sublimation, or changing directly from a solid to a gas without melting. This technique may be useful for creating powered drugs or other chemicals that are heat sensitive.
