Spray drying is a method of producing dry powders from liquids or slurries by rapid drying with hot gases. This is the preferred method for drying many heat sensitive materials such as food and pharmaceuticals. A consistent particle size distribution is the reason for drying some industrial products such as catalyst sprays. Air is the hot drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is oxygen sensitive, then use nitrogen.
All spray dryers use some type of atomizer or nozzle to disperse a liquid or slurry into a spray of controlled droplet size. The most common are rotating disks and single-fluid high-pressure swirl nozzles.
Carburizer wheels are known to provide a broad particle size distribution, but both methods allow for consistent particle size distributions. Alternatively, for some applications two-fluid or ultrasonic nozzles are used. Droplet sizes from 10 to 500 microns can be achieved by proper selection, depending on the needs of the process. ^Common applications are in the 100 to 200 micron diameter range. Dry powders are generally free-flowing.
The most common spray dryers are called single effect because here there is only one drying air at the top of the drying chamber. In most cases, air is blown into the cocurrent jet of liquid. The powder obtained by this type of dryer has a lot of fine dust and poor fluidity. In order to reduce dust and increase the fluidity of powder, there is a new generation of spray dryer called multi-effect spray dryer since more than 20 years ago. One at the top (according to single effect) and one for the integrated static bed at the bottom of the chamber: instead of drying in one stage?
The liquid, the drying is done in two steps. The integration of this fluidized bed allows, by fluidizing the powder inside a humid atmosphere, to agglomerate fine particles and have a median particle size usually in the range of 100 to 300 microns, to obtain granules. Due to this large particle size, these powders are free flowing.
The fine powder produced by the first stage of drying can be recovered in the continuous flow either at the top of the chamber (around the spray liquid), or at the bottom of the integrated fluidized bed. Drying of the powder can be done in an external vibrating fluidized bed.
The hot drying gas can be delivered in a co-current or counter-current direction to the atomizer. The counterflow method enables a greater residence time of the particles in the chamber and is usually paired with co-flow enabling the particles to have a lower residence time within the system and the particle separator (usually a cyclone device) to work more efficiently Fluidized bed system.
What are the alternatives to replace the spray dryer?
Freeze Dryer: A more expensive batch method for degrading spray-dried products. Dry product is not free flowing.
Tumble Dryer: A less expensive continuous process for low-value products; creates flakes instead of free-flowing powders.
Pulse Burner: A less expensive continuous process that can handle higher viscosities and solids loadings than spray dryers, and sometimes freeze-dried quality powders that are free-flowing.
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