APV Dryer Handbook - Umbc

32
DRYER FEED MOISTURE CONTENT (% W.W.B.)
35
30
25
20
15
10
5
0
B
A
material can be increased from 50% to
60% by evaporation, the amount of
water to be evaporated in the spray
dryer is reduced by 33%. A similar
example is shown in Table 2. In this
case the surplus water is being removed
by filtration, and the resulting cake flash
dried. This results in a smaller and less
expensive dryer because evaporation is
far more thermally efficient than drying.
For example, a double effect
evaporator for kaolin will require only
1/3 the energy per unit of evaporation
than the dryer. The pre-concentration
will save approximately 15 million
BTU/h (3.8 million Kcal/h).
A further example is illustrated in
DRYER HEAT LOAD (BTU/H) x 10
Figure 13. At 35% moisture content as
shown at point “A” , a spray dryer
could be used to dry the material to
0.1%. However, if a rotary vacuum filter can be used to reduce the water content
to 14%, the energy requirements are reduced to less than 1/3. The final drying
can then be performed by a spin flash or pneumatic dryer.
6
5 10 15 20 25 30 35 40
Figure 13. Thermal energy required for drying
vs. feed moisture content (duty as in Table 2)
It is important to re-emphasize that, in almost all drying applications, the best
energy saving technique is to pre-concentrate the product as far as possible prior
to drying. In addition to energy, there are also large savings on the capital cost
of the dryer – which in most cases, far exceed the cost of the preliminary
dewatering equipment.
MAXIMIZE THE TEMPERATURE DROP OF THE DRYING GAS
Unlike evaporators where the latent heat of the evaporated water can usually be
reused in effects operating at lower pressure, the steam from a dryer is not easily
reused apart from some preheating applications. The steam is usually carried in