What is Spray Drying?
Spray drying is a method frequently used for microencapsulation in which an active ingredient is dissolved into a polymer solution, sprayed through a nozzle, and trapped in the resulting dried particles. Here is an introduction to the process and applications of spray drying:
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some industrial products such as catalysts. Air is the heated drying media; however, if the liquid is a flammable solvent, such as ethanol, or the product is oxygen sensitive nitrogen is used.
All spray dryers use some type of atomizer or spray nozzle to disperse the liquid or slurry into a controlled drop size spray. The most common of these are rotary nozzles and single-fluid pressure swirl nozzles. Alternatively, for some applications two-fluid or ultrasonic nozzle are used. Depending on the process needs drop sizes from 10 to 500 micron can be achieved with the appropriate choices. The most common applications are in the 100 to 200 micron diameter range.
The hot drying gas can be passed as a co-current or counter-current flow to the atomiser direction. The co-current flow enables the particles to have a lower residence time within the system and the particle separator (typically a cyclone device) operates more efficiently. The counter-current flow method enables a greater residence time of the particles in the chamber and usually is paired with a fluidised bed system.
Spray drying often is used as an encapsulation technique by the food and pharmaceutical industries. A substance to be encapsulated (the load) and an amphipathic carrier (usually some sort of modified starch) are homogenized as a suspension in water (the slurry). The slurry is then fed into a spray drier, usually a tower heated to temperatures well over the boiling point of water.
As the slurry enters the tower, it is atomized. Partly because of the high surface tension of water and partly because of the hydrophobic/hydrophilic interactions between the amphipathic carrier, the water, and the load, the atomized slurry forms micelles. The small size of the drops (averaging 100 micrometers in diameter) results in a relatively large surface area which dries quickly. As the water dries, the carrier forms a hardened shell around the load. (Text from http://en.wikipedia.org/wiki/Spray_drying, released under a CC-ASL 3.0 license)