If a feedstock can be pumped
and atomised, a spray dryer can be used to create powders
with desirable properties including size, size distribution,
particle shape, form, density, colour, flowability and
dispersibility. The development of scientifically based
design procedures has proved to be a daunting task due
to the complex nature of the various stages in the spray
drying process. These include atomisation of the feed,
intimate contact between the spray and hot gas, moisture
evaporation from the spray, and separation and collection
of the dried product from the exhaust gas. Current spray
dryer designs are therefore invariably based on the
results of pilot plant trials and the manufacturer's
accumulated experience. It is against this background
that this volume is presented. It is divided into five
major self-contained parts.
Volume
DRY V Part 1 A practical guide to selection and design.
This part gives
a manufacturer's guide to selection and design. It includes
a simple preliminary chamber sizing calculation procedure
and goes on to discuss the various questions that must be
addressed and answered to ensure a proper basis for consultation
with vendors.
Volume
DRY V Part 2 Atomisation of spray
dryer feedstocks.
Part 2 is a critical review of
atomisation, which is the vital process in a spray dryer,
as it converts the feedstock into a spray of fine droplets.
The droplet size and initial velocity and the spray angle
play a major role in determining the dimensions of the drying
chamber. Rotary atomisers, pressure and pneumatic nozzle options
are described. The criteria for selecting and applying a particular
type of atomiser are considered together with their operation
and economics. The importance of feedstock rheology to atomisation
is emphasised for realistic feedstocks such as thick slurries,
emulsions and thixotropic pastes. Current scientific understanding
of the mechanisms of atomisation is reviewed. An assessment
is made of published correlations for the prediction of mean
drop size. The important effect of gas entrapment on droplet
formation is considered. Finally, new developments in atomisation
techniques, which may find future applications in industry,
are described.
Volume
DRY V Part 3 Description of programs SPRY1 and SPRY2 and generation
of experimental data.
In SPRY1 the spray is described
by the Sauter mean diameter, whilst in SPRY2 the droplet size
distribution is taken into account. These programs contain
detailed flowsheeting calculations and an algorithm based
on a simplified model of spray drying in which the dryer is
characterised by a so-called "Chamber coefficient".
Volume
DRY V Part 4 User's guide to computer programs SPRY1 and SPRY2.
This part is the comprehensive
"User guide" to the computer programs.
Volume
DRY V Part 5 Scientific
background.
This part covers such topics
as atomisation, particle drying, product quality, control,
aerodynamics, particle motion and overall simulation models.
