Atomization is the first key element of the Spay Drying process.
The liquid feed is broken down into droplets which are contacted
with the drying air in a highly controlled manner due to the interaction of the
atomizer and air disperser, which controls the flow pattern within the drying
chamber.
The characteristics of the feed material to be spay dryed and the required
properties of the resulting powder influence the choice of atomization
device.
Rotary Atomization
The spray drying process using rotary atomization is characterised by a
co-current drying regime, with an air disperser which imparts both horizontal
and vertical vectors to the incoming drying air.
Chambers are generally of
larger diameter than for nozzle atomization and must be oversized where larger
particles are required.
Atomization occurs at the wheel periphery and
it is the peripheral speed normally in the range 100 to 300 m/s which controls
particle size for any given product.
The rotary atomizer has the
advantage that high feed rates can be accommodated by a single atomizer, it
requires only a low pressure feed system, it is resistant to abrasion or
clogging and that particle properties are not sensitive to feed rate.
Pressure Nozzle
In the hydraulic pressure nozzle atomizer the feed material is supplied to
the nozzle under pressure. The pressure energy is converted to kinetic energy
and the feed issues form the orifice as a high speed film that readily
disintegrates into droplets.
The droplet size produced from a pressure nozzle varies inversely with
pressure and directly with feed rate and feed viscosity.
The pressure
nozzle is suitable for both co-current and for materials which are not heat
sensitive counter co-current operation.
The pressure nozzle tends to
produce a coarser free flowing powder with a tighter particle size distribution
than a rotary atomizer.
Two Fluid Nozzle
The third alternative for atomization is the multi-fluid nozzle of which the
two fluid nozzle is an example.
In this atomization device the energy
for atomization is provided by the rapid expansion of gas which is mixed with
the feed within the body of the nozzle (internally mixing) or at its tip
(externally mixing).
Particle size is controlled by the feed to air
ratio.
Whilst this type of atomization is not generally considered
economic for high capacity applications, it is a suitable alternative for
smaller specialist plant due to its relatively low pressure and resulting
particle velocity and shorter required drying path.
It is particularly
well suited for the production of particles with a sub 5 micron mean particle
size such as particles required for inhalation products within the
pharmaceutical industry.