APV Dryer Handbook - Umbc

48
S P I N F L A S H
D R Y E R S
for continuous powder production from pastes and filter cakes
While mechanical dewatering of a feed slurry is significantly less expensive than
thermal drying, this process results in a paste or filter cake that cannot be spray
dried and can be difficult to handle in other types of dryers. This article describes
the Spin Flash dryer, shown in Figure 21, as one option available for continuous
powder production from pastes and filter cakes without the need for grinding.
Powder production generally involves some form of drying operation. There are
several generic types of dryers but all must involve the evaporation of water, which
can take anywhere from 1000 to 2500 BTU/lb (555 to 1390 Kcals/kg),
depending on dryer type. Due to its ability to produce a uniform powder at
relatively low temperatures, the spray dryer is probably the most common of these
dryers. However, by its definition, a spray dryer requires a fluid feed material
to allow its atomization device to be employed. Generally, there is a maximum
viscosity limitation in the range of 250 to 300 Centipoise.
Figure 22 (on page 49) illustrates the amount of water which must be evaporated
to produce one pound of bone dry powder from a range of different feed solids.
It can be clearly seen that even a 5%
increase in total solids will reduce the water
evaporation and hence, the dryer operating
costs by about 20%. If this water removal
can be done mechanically by, for example,
filtration or centrifuging, the cost will be
significantly lower than that required to heat
and evaporate the same water. The direct
energy cost can be calculated as equivalent
to 3-8 BTU/lb (1.7-4.5 Kcals/kg) compared
to an average 1500 BTU/lb (830
Kcals/kg) for evaporation. This increase in
Figure 21. Spin Flash dryer
solids, however, inevitably will result in an