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A spray dryer is a device used to produce dry, powdered substances from fluid suspensions or slurries by exposing a sprayed stream of the suspension to a hot gas. It is a simple, one step evaporation system which features minimal moving parts and rapid drying times. Spray drying processes uses various nozzles, gasses, spray, and gas flow patterns depending on the type of solids involved and the desired consistencies of the finished product. This method of removing fluids exposes the suspension to a minimum of heat and is the preferred method of drying for many food and pharmaceutical products.
Substances to be dried are mixed with water then fed under pressure into the nozzle system which atomizes the slurry into droplets of controlled size and passes them through a flow of heated gas. This fine spray allows the liquid part of the suspension to evaporate very rapidly, leaving granules of dried solid to be collected. The spray dryer generally uses heated air as a drying medium but, in cases of flammable solutes such as ethanol and solids that are sensitive to oxygen exposure, nitrogen may be used. This flow of heated gas may be applied with or against the direction of the slurry spray depending on how long the solids are to be exposed to the drying process.
Spray dryer nozzles are designed to produce a controlled, constant droplet size producing dried solids that are uniform and often free flowing. This high level of consistency in particle size is essential to the successful production of many products such as chemical catalysts. Droplet sizes generally range from 100 to 200 microns but the use of specialist nozzles such as ultrasonic variants can produce finer sprays with droplet sizes of 10 microns. These specialist nozzles are generally used when a two-fluid suspension system is applied and where an extremely fine finished product is called for.
The spray dryer is particularly well suited to the food and pharmaceutical industry due to the short heating times involved. Food products such as milk powder, tea, spices, and drugs such as antibiotics degrade if exposed to excessive heat making spray drying an attractive option. Products may also be mixed with encapsulating agents in the slurry stage, thereby adding further value to the system. The process increases productivity and reduces running costs which makes spray dried products more profitable to produce.
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