Spray drying produces solid materials by expelling liquids or slurries from nozzles in heated air currents. It is used to create a range of powdered and solid chemicals, and can take place in open or closed tanks. Products are usually round spheres, but other shapes can be made. The nozzle can be changed for different products or particle sizes, and freeze drying can also be used.
Spray drying is a process in which solid material is produced by expelling liquids or slurries, mixtures of solids and liquids, from nozzles in heated air currents. This is used to produce a wide range of powdered and solid chemicals, ranging from fertilizers to powdered pharmaceuticals. Creating solids from liquids or mixtures reduces shipping costs, improves shelf life, and provides the ability to create differently shaped particles for specific applications. Spray drying technology was first developed in the late 19th century but was not commercialized until the 1919s when improvements made continuous operation possible.
The spray drying process can take place in open or closed tanks or storage buildings. Products can be sprayed to the top of open tanks or containers and the liquid will evaporate before the product reaches the bottom. The heated air can be introduced to the top together with the product, which is called co-current flow because the air and the product move in the same direction. Counterflow occurs when product flows down and air moves from the bottom of the tank to the top.
A mixed flow system can be used for different drying materials and patterns. The nozzle is normally pointing up, creating an upward pattern. The air is introduced into the bottom of the tank. The particles first move up in the air stream, then fall against the air stream, resulting in a mixed stream. These spray drying systems can be useful for larger particle sizes or materials that require additional drying time.
Products produced by spray drying systems are usually round spheres, but other shapes can be made. The aspect ratio, or the maximum ratio of the length to the width of the product shape, is normally used to measure non-spherical shapes. It is possible to produce material that is spherical, but hollow in the center to reduce weight. A two-stage system can be used, where the first pass creates a still slightly moist product and the material sticks together in a second pass to form larger particles called aggregates.
The nozzle can be changed for different products or particle sizes. Atomizers, both stationary and rotating, are used to create spheres. Rotary atomizers use a rotating nozzle to eject 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 increases the cost of the product, but may be necessary for materials that are very thick or require steam heating to be liquid.
Freeze drying can be used to dry products by spraying liquids into refrigerated storage tanks. Liquids are removed by freezing and sublimation, or by going directly from solid to gas without melting. This technique can be useful for creating enhanced drugs or other heat-sensitive chemicals.
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