.3.3 SAFETY ASPECTSIn spray drying

.3.3 SAFETY ASPECTS
In spray drying operations a potential danger of explosion and fire can exist under certain conditions [7].
Fire hazards exist if any of the following conditions occur:
. The temperature of the air–product mixture
reaches a flammability limit (see Table 10.8).
. The oxygen content in the drying medium is
high.
(See relevant chapter of this handbook for details on
fire and explosion hazards.)
Dry skim milk has the lowest inflammation temperature in a layer; sugar and some detergents have
the lowest temperature when in a cloud in a spray
dryer. Temperature data for the drying medium in a
spray dryer for three typical materials are shown in
Figure 10.43. The temperatures are given at the following locations: T1, heater outlet; T2, drying chamber inlet; T3, drying chamber outlet; T4, exhaust air;
T5, pneumatic transport location. Useful data for minimum ignition temperatures of milk products can
be found in Ref. [34].
It is evident that the drying air temperature nearly
always exceeds the inflammation temperature in a
layer. This means that ignition is possible whenever
dry powder deposits are formed. In the case of detergents, ignition is also possible in the cloud.
Ignition may be initiated for any one of the following reasons:
. Spontaneous combustion in product deposits
. Hot solid particles entering the dryer with
drying gas
. Spark generation through friction
. Electrical failure
. Static electricity discharge
Fire prevention methods are based on proper operation and cleaning of the dryer. An important safety
device is an automatic control system (ACS), as it can
maintain the temperature within +18C range whereas
the manually controlled temperature may vary +108C.
The control system should include a detection system
of parameter deviation, deposit detection, drying gas
analysis, and continuous monitoring of the humidity
and temperature at selected locations.
The explosion hazard depends on the value of the
critical particle concentration, oxidation velocity, and
the explosion pressure. If any one of these conditions
is reached, there is danger of ignition. A comparison
of both explosive and process concentration for
three products is shown in Figure 10.44. The powder
concentrations are given in the following locations:
C1, drying chamber; C2, cyclones; C3, exhaust fan;
C4, wet scrubber; and C5, pneumatic transport. Figure 10.44 shows that even under normal operating conditions the danger of explosion does exist in a
normal spray dryer. The explosion hazard is high
above the solid line. The zone between the thick and
dotted lines is potentially dangerous as the powder
concentration there exceeds 50%. All process
equipment in which the operating dust concentration
exceeds 50% of the lower explosive concentration
must be provided with a safety device. The spray
drying chamber, cyclones, and pipes should be
equipped with safety doors and explosion vents [19].
When highly inflammable materials are dried it is
necessary to use inert atmosphere. A gastight closedcycle spray dryer with nitrogen as the drying medium
may be used in such cases. When explosion hazard
exists, a self-inertizing system is recommended. Good
maintenance practice cannot be overemphasized.
For additional information and further references
on fire and explosion hazards in drying of solids, the
reader is referred to the appropriate chapter in this
handbook.