Jangam, Mujumdar - Classification and Selection3.7. CASE STUDIES
3.7.1. Production of milk powder
It should be noted that the selection of dryer or drying system not only depends on
the physical properties of the feedstock but also on the mode of operation, capacity re-
quired, heat sensitivity. Apart from this, the initial and required final moisture content,
drying kinetics also affects the dryer selection. In case of the production of milk powder,
the feedstock is in a liquid form hence the basic choice of dryers is limited referring to
the Table 3 (spray dryer, drum dryer and fluidized bed dryer with some improvements
which is difficult to use for this application). However, if the final material is required to
be in a sufficiently free flowing form, it is best choice to use spray dryer. Nevertheless,
spray dryer alone does not serve the purpose as the required final moisture content
cannot be reached. It is recommended to go for two-stage drying option with fluidized
bed as a second dryer which takes care of final drying of relatively wet powder from
spray dryer. The choice of fluidized bed depends on the physical properties of material
to be dried. The milk powder from spray dryer is of Geldart type 'A/B' with porous
structure and good fluidization characteristics. In addition milk powder is heat sensitive
hence it can be seen from Figure 3. 3 that, one can have three options of fluidized bed
(multi-stage fluidized bed, vibrated fluidized bed and spouted bed). However because of
the colloidal nature of the milk powder it is recommended to use vibrated fluid bed
dryer. Although these are just suggestions the detailed experimental validation should
be done on pilot scale dryers.
3.7.2. Drying of grains
Most of the grains fall in the Geldart type 'D' of particles and have both surface as
well as the internal moisture. For particulate materials fluidized beds are the best choice
because of high heat and mass transfer. However, this class of particles have poor fluidi-
zation quality (because of the funny shapes of particles) hence the use of well mixed or
plug flow fluidized bed dryer can be ruled out. Referring to Figure 3. 3 suggest that for
class D of particles with surface and internal moisture, the best choice is to use multi-
stage fluidized bed dryers or spouted bed dryer. In multi-stage drying of grains, the first
stage can be flash dryer or cyclone dryer to remove the surface moisture and the second
stage can be a plug flow fluid bed dryer or vibrated fluid bed dryer depending on the
type of grain to be dried. If the spouted bed dryer is used alone, higher drying gas flow
rates can be used in initial staged to remove surface moisture and low gas flow in later
stage.