INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

1. Introduction
Drying has found applications in many areas of industry such as chemical, food, pharmaceutical,
polymer, ceramics and mineral processing. The basic idea of drying is to
remove liquid by evaporation from the material that has either dissolved or suspended
solids to a dried powder. The objectives can be, to reduce the transportation costs,
to increase the shelf life of a material, which is done for instance in the preservation
of milk, tomato, etc. or because the material has better properties in dry form than
when dissolved.
Spray drying is one of the most widely used drying techniques and it is a process
for converting a liquid feed into a powder by evaporating the solvent. The other drying
techniques to produce powders mainly include freeze drying, supercritical drying and
vacuum drying. Compared to other evaporation processes, spray drying has the great
advantage that products can be dried without much loss of volatile or thermally unsteady
or degradable compounds. These advantages are especially important in the production
of pharmaceutical bulk materials such as polymers (e.g. polyvinylpyrrolidone),
carbohydrates (e.g. mannitol) and food powders (e.g. milk and coffee powders) [1, 2].
Spray dryers are extensively used in industry, usually placed at the end-point of a
process plant, which play an important role in the whole process, not because of their
capital investment, size and operating costs but mainly because of their high energy
efficiency and throughput production.
Spray drying can be structured into several steps primarily consisting of the atomization
of liquid feed into a population of poly-disperse droplets followed by the
convection of droplets and gas, the evaporation of the liquid solvent from the droplets,
droplet-droplet collisions, which might lead to coalescence, aggregation and eventually
breakup. In some cases, chemical reactions are also involved, e.g. in the production
of polymer via monomer polymerization using spray drying. These sub-processes are
inter-dependent. The atomization leads to droplets with specific size distribution and
kinetic energy, thus influencing the droplets convection and probability of collision.
The drying gas in most of the cases is the ambient air heated to a desired temperature
with controlled relative humidity and this hot air is either co-flow or counter-flow
depending on the system requirement and the thermal sensibility of the material. A
schematic diagram of the typical spray drying process in shown in Fig. 1.1 [3]. A review
of available spray drying designs, process types and applications is given by Masters [1].