Understanding the Software Options

Solids Processing
Preventing Self-Heating and
Ignition in Drying Operations
Incident investigation reveals that the
most common root cause is lack of understanding
Pieter Zeeuwen and Vahid Ebadat
Chilworth Global
Many solid materials can
exhibit self-heating, which
— if unchecked — can
progress to a fire or even
explosion. And even if the situation
does not get that far, it is likely to affect
the output of the process, in terms
of product quality degradation, for example.
Recognizing that your product
in powder or granular form can selfheat
is the first step in controlling the
risks associated with self-heating.
Whenever self-heating incidents are
investigated, we find that a common
root cause is a lack of understanding
of self-heating phenomena. This
article provides an introduction to
self-heating phenomena and suggests
measures to control this type of ignition
source.
What is self-heating?
Not all particulate solids that are classified
as combustible dust (in other
words, pose a dust explosion hazard)
will self-heat at normal processing
temperatures, and conversely, some of
the materials that do self-heat react
too slowly to pose a dust explosion
hazard. Some materials can self-heat
at ambient temperatures, especially
in large-scale storage, but for most
materials the hazards arise when they
are heated.
Self-heating can arise by one of two
different mechanisms: by exothermic
(heat releasing) chemical reactions
and by exothermic decomposition.
The chemical reactions are often the
same as what occurs during a fire or
explosion: an oxidation reaction with
the oxygen in the air. At the start of
the self-heating process, the reaction
is very slow, like steel that oxidizes
FIGURE 1. After completion of a test,
in which self-heating of the product took
place, the product was completely burnt.
The charred and partly molten remains no
longer �t inside the sample holder
with atmospheric oxygen to form rust.
Decomposition happens in a material
that is unstable, and the material will
fall apart while releasing heat. A significant
difference between the two
mechanisms is that decomposition
does not require additional reactants
and is therefore largely independent
of the environment, while an oxidation
reaction only happens if certain conditions
are present, making it more difficult
to predict its occurrence without
detailed experimental studies.
What happens in self-heating?
Step 1. Rate of heat generation exceeds
rate of heat loss. If a material
undergoes an exothermic chemical
reaction (or multiple reactions) or decomposes
exothermically, the temperature
of the material will rise due to the
heat released from the exothermic reaction
or decomposition. In the meantime,
some of the heat is lost to the
environment. If the rate of heat loss
exceeds the rate of heat generation,
the temperature of the material will
be the same as the ambient temperature,
otherwise, it will increase. Due
to the poor thermal conductivities of
many solids, a large portion of the re-
FIGURE 2. Product in the test cell
(right) is discolored signi�cantly after
the test compared to the original sample
(left), even though the self-heating
has not led to smoldering or burning of
the material
FIGURE 3. In this typical test cell for
“bulk” conditions (50-mm dia., 80-mm
height), air can diffuse into the sample
through the open top of the cell and
through the bottom of the cell, which is
closed with a sintered glass disc. The
sample temperature is measured continuously
at various locations along the
height of the cell
action heat is retained in the powder.
Step 2. Resulting temperature
rise further increases chemical
reaction rate exponentially. The
temperature rise of the material due
to the exothermic reaction will further
increase the chemical reaction rate,
which in turn will cause the temperature
to increase further. The increase
of material temperature also results
in an increase in the rate of heat loss.
However, the rate of heat loss increases
linearly with temperature, while the
chemical reaction rate, and thus the
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 45