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. It is a one
step continuous processing operation. The feed can be either a solution,
suspension or a paste. The dried product conforms to powder consisting of
single particles or agglomerates, all 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 equipment can meet the requirements to produce a powder with
tailormade specifications required by the end-user.
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 the milk. All
processes, however, required addition of sugar, sulphuric acid or alkali, so
that the end product could not be considered pure.
One of the first spray drying patents was applied for in 1901 by the German
Mr. Stauf who sprayed the milk by means of 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 un-til 1933, when the Danish engineer Mr. Nyrop filed his world patent,
this way of at-omization had a break-through.
After these pioneers had formed the basis for the modern dry milk industry
the development went fast, and the equipment installed nowadays is in most
cases very sophisticated and involves a highly technical and technological
design.
A conventional spray dryer operates in the following way, see Fig. 38:
Fig. 38
Spray drying plant
The feed is pumped from the product feed tank to the atomizing device which
is located in the air disperser in 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 cooling the air at the same time. After the
drying of the spray in the 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 entrained
in the air, and it is therefore necessary to pass the air through cyclones for
separation of fines. The fines leave the cyclone at the bottom via a locking
device and enters 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
com-prises 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.
A conventional spray dryer consists of the following main components:
- Drying chamber (1)
- Hot air system and air distribution (2)
- Feed system (3)
- Atomizing device (4)
- Powder separation system (5)
- Pneumatic conveying and cooling system (6)
- Fluid bed after-drying/cooling (7)
- Instrumentation and automation (8)