Understanding the Software Options

Solids Processing
FIGURE 4. (Top left) In the test for “aerated” conditions, air
�ows through the sample from top to bottom, which are both
closed by sintered glass discs. The cylindrical section has a 50
mm dia. and a height of 80 mm. (Bottom) For "air over layer"
testing, warm air �ows over the powder layer in the sample
tray. (Top right) The wire basket for “basket testing” is illustrated
more clearly in Figure 5
heat generation rate, increases exponentially
with temperature. Consequently,
the heat generation rate will
exceed the rate of heat loss and the
temperature of the material will rise
higher. This process is referred to as
self-heating. Self-heating begins at a
temperature at which the rate of heat
generation is greater than the rate of
heat loss and this temperature is called
the exothermic onset temperature.
Subsequent effects
Potential smoldering. Self-heating
of solid materials usually results in
smoldering, which can set the material
on fire or cause dust explosions,
particularly when the smoldering
material is disturbed and exposed to
air (Figure 1). Many plants that experience
self-heating incidents have
a history of “near misses” where some
self-heating occurs but does not progress
to full-blown ignition. In such
cases there may be “black spots” in an
otherwise light-colored product, or a
lump of charred product may be found,
a so-called “smoldering nest”. It is important
to recognize such occurrences
as indications of a potentially serious
problem, rather than to learn to live
with it.
Potential release of flammable
46 CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011
gases. Self-heating reactions may
also produce flammable gases, which
may lead to gas explosions in process
vessels or compromise product quality
(Figure 2).
Testing self-heating behavior
The exothermic onset temperature
is influenced not only by the chemical
and physical properties, such as
chemical reaction kinetics and heat
of reaction, but also by other factors,
including the following:
• Dimension and geometry of the solid
bulk
• Ambient airflow
• Availability of oxygen in the bulk
• Additives and contaminants
Usually, the material has to be exposed
to an elevated temperature for a period
of time before it self-heats. This time is
referred to as the induction time, which
is dependent on temperature; and usually
the higher the temperature, the
shorter the induction time will be.
Because of the influencing factors
mentioned, a single test is usually unable
to predict self-heating behavior
for all different drying and storage
conditions. Instead, separate tests
have been developed to simulate the
conditions where the powder is in bulk
form (Figure 3), layer form (with air
FIGURE 5. “Basket test” sample holders, for testing at different
scales, allow extrapolation to large-scale storage conditions.
The baskets typically have sides of 25, 50 and 100 mm
FIGURE 6. This “basket test” sample holder is prepared for
testing inside a laboratory oven
flowing over the powder; Figure 4)
and aerated form (Figure 4), where
air is passing through the bulk of the
product, increasing the oxygen availability
for the reaction and also helping
to remove heat from the reacting
material. For large-scale storage situations
tests are carried out at different
scales so that the effect of the size
of the bulk material can be assessed
(Figure 5). All tests are carried out in
temperature-controlled ovens (Figure
6) that allow screening tests (with the
temperature ramped up at a defined
rate) and isothermal testing (with
a constant temperature controlled
within narrow margins). Because of
the potential for violent reactions during
the self-heating process, all testing
equipment needs to be fitted with explosion
protection.
Learning from a real incident
In one incident, the powder in a fluidized
bed dryer caught fire when the
powder conveyer in the dryer was
turned off in order to fix clogging in
an upstream wet-product conveyer.
During this period, the hot air supply
was continued.
A screening test was conducted to
determine whether this powder could
self-heat under the conditions that ex-