Jangam, Mujumdar - Classification and Selection3.5. SELECTION OF SPRAY DRYERS
Spray dryer is the second most important dryer type used in the food processing
sector. Some of the common applications are drying of fruit juices, herbal extracts, milk
and many more. The detailed chapter on spray drying of food products is covered in this
book. However, we have tried to give some information on selection criteria as the spray
dryers are available in a wide range of designs. The selection choices can be limited to
the following categories of dryers.
Co-current single-stage dryer with pneumatic powder conveying: This basic design is
widely used to process relatively easy to dry, non- and low-fat formulation. Fairly high
outlet temperatures are used to achieve the desired final outlet moisture content before
the powder is discharged into the pneumatic conveying system.
Co-current, two-stage dryer with an external fluidized bed: This versatile layout can
be used to produce standard, semi-agglomerated and fully agglomerated powders. The
externally mounted vibrating fluidized bed acts as a post-dryer, a cooler or a combina-
tion of the two.
Co-current, two-stage dryer with integrated fluidized bed and pneumatic conveying
system: In this variant, an annular fluidized bed is located in the base of the spray-drying
chamber. This enables higher moisture contents to be handled in the drying chamber.
Moreover, the drying process can be completed at relatively low product temperatures,
which can be of advantage in some circumstances. It is, however, restricted to non- or
low-fat formulations. The product is a standard or a semi-agglomerated powder.
Figure 3. 4 elaborates the general classification of spray dryers based on their oper-
ating characteristics. Note that selection of the spray drying system also must consider
product quality and pilot scale test results
Another important part in spray dryer selection is the choice of a suitable atomizer.
A spray dryer is indicated when a pumpable slurry, solution or suspension is to be re-
duced to a free-flowing powder. With proper choice of atomizer, spray chamber design,
gas temperature and flow rate it is possible to “engineer” powders of desired particle
size and size distribution.
Table 5 shows how the choice of the atomizer affects chamber design, size, as well as
energy consumption for atomization and particle size distribution. The newly developed
two-fluid sonic nozzles appear to be especially attractive choices when nearly monodis-
persed powders need to be produced from relatively moderate viscosity feeds (e.g., un-
der 250 cp) at capacities up to 80 t/h by using multiple nozzles. More examples may be
found in Masters (1985) and Mujumdar (2006). Table 3. 5 is a case study not from the
food industry but it is included here to demonstrate the effect of atomizer selection on
overall performance of a spray dryer.