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A spray dryer uses a hot gas to produce dry powdered substances from fluid slurries. It has fast drying times and uses various nozzles and gases depending on the product. The process is suitable for the food and pharmaceutical industries due to short heating times and can increase productivity and reduce costs.
A spray dryer is a device used to produce dry powdered substances from fluid slurries or slurries by exposing a spray stream of the slurry to a hot gas. It’s a simple one-step evaporation system that has minimal moving parts and fast drying times. Spray drying processes use various nozzles, gases, sprays and gas streams depending on the type of solids involved and the desired consistencies of the finished product. This method of defluiding exposes the slurry to minimal heat and is the preferred method of drying for many food and pharmaceutical products.
The substances to be dried are mixed with water then fed under pressure into the nozzle system which nebulizes the slurry into droplets of controlled size and passes them through a heated gas stream. This fine nebulization allows the liquid part of the suspension to evaporate very quickly, allowing the dried solid granules to collect. The spray dryer generally uses heated air as the drying medium but, in the case of flammable solutes such as ethanol and solids sensitive to oxygen exposure, nitrogen gas may be used. This heated gas stream can 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 and consistent droplet size producing dry solids that are uniform and often free flowing. This high level of particle size consistency is essential for the successful manufacture of many products such as chemical catalysts. Droplet sizes generally range from 100 to 200 microns, but the use of specialty nozzles such as ultrasonic variants can produce finer sprays with droplet sizes of 10 microns. These specialty nozzles are typically used when a two fluid suspension system is being applied and where an extremely fine finished product is required.
The spray dryer is particularly suitable for the food and pharmaceutical industries due to the short heating times involved. Food products such as powdered milk, tea, spices and medicines such as antibiotics degrade when exposed to excessive heat, making spray drying an attractive option. The products can also be mixed with encapsulating agents in the suspension phase, thus adding further value to the system. The process increases productivity and reduces operating costs, which makes spray-dried products more profitable to produce.
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