Working of Spray drying system is a one-step continuous unit operation that employs liquid atomization to produce droplets that are dried to individual particles when moved in a hot gaseous drying medium. The three stages that occur in a spray dryer before drying is accomplished include atomization, spray-air mixing, and moisture evaporation, and dry product separation from the exit air. The spray drying process begins with atomization. During atomization, a nozzle or rotary atomizer turns the liquid feed stock into small liquid droplets. This is followed by the separation of the solute or suspension as a solid and the solvent into a vapour. It is during this stage that many of the desired product qualities such as particle size and viscosity are developed. When droplets exit the nozzles or atomizer, they are dried to form a powder that is easily packed and transported. Solids form as mois ture quickly leaves the droplets. The solid is usually collected in a drum or cyclone. The nature of the final product depends on the design and operation of the spray dryer and the physicochemical properties of the feed. Drying of the powder is commonly completed using hot air. The final moisture content in the powder is controlled by adjusting the hot air temperature. The recov ery process is the last step that takes a few seconds to recover the powder from the exhaust gas within the cyclone.
A high speed rotating disc is used for atomizing the liquid feed. The disc throws the liquid outward or tangentially at high veloci ties forming fine droplets / mist. Instant drying takes place in contact with hot air forming free flowing dry powder. Discs designs include straight / curved vanes, pin wheels, multilayer discs in diameters of 60mm, 100mm, 160mm, 250mm & 350mm for different capacities. For high temperature applications special bearing cooling arrangements are provided.
The main advantage of spray drying is its versatility of the technology. Spray drying offers multiple opportunities that no other single drying technology provides. The flexibility and reproducibility of spray dryer makes spray drying the process of choice for many industrial drying operations. It has the following other advantages :
1. This technique has the ability to operate in aseptic pharmaceutical processing.
2. Feed rates in spray drying can range from a few pounds per hour to over 100 tons per hour and thus can be designed to any capacity required.
3. The spray drying process is very rapid, with the major portion of evaporation taking place in less than a few seconds
