By definition, spray drying is the transformation of feed from a fluid state into a dried form by spraying the feed into a hot drying medium. The process is a one step continuous operation. The feed can be either a solution, suspension or a paste. The spray dried product conforms to powder consisting of single particles or agglomerates, depending upon the physical and chemical properties of the feed and the dryer design and operation. During the last three decades spray drying has undergone an intensive research and development, so that modern spray drying equipment can meet the requirements to produce a powder with tailor-made specifications required by the end-user.
Spray drying and the dairy industry
Spray drying in the dairy industry dates back to around 1800, but it was not until 1850 that it became possible in major scale to dry milk. All processes, however, required addition of sugar, sulphuric acid or alkali, so the end product could not be considered pure.
Commercial use of spray drying
One of the first spray drying patents was applied for in 1901 by the German Mr. Stauf, who sprayed the milk by nozzles into a chamber with warm air. The first real break-through, however, was in USA in 1913, when the American Mr. Grey and the Dane Mr. Jensen developed a nozzle spray dryer and started to produce and sell drying installations on a commercial scale.
The first rotary atomizer was developed by the German Mr. Kraus in 1912, but not until 1933, when the Danish engineer Mr. Nyrop filed his world patent, which was the real break-through of atomization.
The basis for the mordern dry milk industry, formed by these pioneers lead to new developments and the spray drying equipment installed nowadays is in most cases very sophisticated and involves a highly technical and technological design.
A conventional spray dryer consists of the following main components:
Fig. 38. Spray drying plant
Operation of a conventional spray dryer
(Fig. 38) The feed is pumped from the product feed tank to the atomizing device, located in the air disperser at the top of the drying chamber. The drying air is drawn from the atmosphere via a filter by a supply fan and is passed through the air heater to the air disperser. The atomized droplets meet the hot air and the evaporation takes place, while cooling of the air happen simultaneously. After the spray is dried in the drying chamber, the majority of the dried product falls to the bottom of the chamber and enters a pneumatic conveying and cooling system.
The fines, which are the particles with a small diameter, will remain in the air, and it is therefore necessary to pass the air through cyclones to separate the fines. The fines leave the cyclone at the bottom via a locking device and enter the pneumatic system, too. The air passes from the cyclone to the atmosphere via the exhaust fan. The two fractions of powder are collected in the pneumatic system for conveying and cooling and are passed through a cyclone for separation, after which they are bagged off. The instrumentation comprises indication of the temperature of the inlet and outlet air, as well as automatic control of the inlet temperature by altering the steam pressure, amount of oil or gas to the air heater, and automatic control of the outlet temperature by altering the amount of feed pumped to the atomizing device.