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Process
Spray drying is the most widely used industrial process involving particle
formation and drying. It is highly suited for the continuous production of dry
solids in either powder, granulate or agglomerate form from liquid feedstocks as
solutions, emulsions and pumpable suspensions. Therefore, spray drying is an
ideal process where the end-product must comply to precise quality standards
regarding particle size distribution, residual moisture content, bulk density,
and particle shape.
Spray drying involves the atomization of a liquid feedstock into a spray of
droplets and contacting the droplets with hot air in a drying chamber. The
sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of
moisture from the droplets and formation of dry particles proceed under
controlled temperature and airflow conditions. Powder is discharged continuously
from the drying chamber. Operating conditions and dryer design are selected
according to the drying characteristics of the product and powder specification.
Principles
Every spray dryer consists of feed pump, atomizer, air heater, air disperser,
drying chamber, and systems for exhaust air cleaning and powder recovery.
Widely varying drying characteristics and quality requirements of the
thousands of products spray dried determine the selection of the atomizer, the
most suitable airflow pattern, and the drying chamber design.
Atomization
The formation of sprays having the required droplet size distribution is vital
to any successful spray dryer operation so that powder specifications can be
met. Atomization is a high technology area where Niro has played a central role
in the development and use of nozzles and rotary atomizers in spray drying.
Airflow
The initial contact between spray droplets and drying air controls evaporation
rates and product temperatures in the dryer. There are three modes of contact:
Co-current
Drying air and particles move through the drying chamber in the same direction.
Product temperatures on discharge from the dryer are lower than the exhaust air
temperature, and hence this is an ideal mode for drying heat sensitive products.
When operating with rotary atomizer, the air disperser creates a high degree of
air rotation, giving uniform temperatures throughout the drying chamber.
However, an alternative non-rotating airflow is often used in tower or Filtermat-type
spray dryers using nozzle atomizers with equal success.
Counter-current
Drying air and particles move through the drying chamber in opposite directions.
This mode is suitable for products which require a degree of heat treatment
during drying. The temperature of the powder leaving the dryer is usually higher
than the exhaust air temperature.
Mixed flow
Particle movement through the drying chamber experiences both co-current and
counter-current phases. This mode is suitable for heat stable products where
coarse powder requirements necessitate the use of nozzle atomizers, spraying
upwards into an incoming airflow, or for heat sensitive products where the
atomizer sprays droplets downwards towards an integrated fluid bed and the air
inlet and outlet are located at the top of the drying chamber. |
