APV Dryer Handbook - Umbc

58
S P R A Y D R Y E R S
SPRAY DRYER OPTIMIZATION
Spray drying is a technique used to convert either solutions or free-flowing slurries,
into powder particles having specific characteristics. There is a wide variety of
spray dryer configurations, each suited to a particular type of powder or feed
consistency.
The following notes are directed towards APV Anhydro Spray dryers, but will be
generally applicable to many other spray dryers made by different manufacturers.
Spray drying remains – to a great extent – an art, although there is a scientific
basis to all of the many adjustments which must be made before a particular
dryer is optimized to dry a particular feed stream and produce the desired
powder characteristics.
ATOMIZATION
This is the process which turns a liquid feed into small droplets immediately prior to
drying. The size of the droplet formed controls the size of the powder particles
coming out of a single stage spray dryer. The two principle means of atomization
are centrifugal and pressure nozzle – both have their advantages and
disadvantages and the choice is dependent on both feed and powder properties
of the specific application.
VISCOSITY
The ability to atomize a liquid is largely dependent on the viscosity at the
atomizing device. A general rule of thumb is to keep the viscosity below 250
centipoise at the atomizer. When the viscosity is too high, the liquid tends to form
non-spherical “prolates” (potato shaped large particles), and ultimately strings or
threads, like cotton candy. Poor atomization can also be indicated by a narrow
ring around the dryer wall level with the atomizer wheel, or by speckled deposits
on the lower walls and cone of a nozzle spray dryer. The simplest way to reduce
viscosity is to add water, but this dramatically reduces the capacity of the spray