Causes of Runaway Incidents
A review of incidents involving thermal runaway reactions in
batch reactors shows that the main causes
are:
- Inadequate understanding of the process chemistry and thermochemistry
- Inadequate engineering design: for example heat transfer
system, agitation system, control systems and safety back-up systems
- Inadequate operational procedures, including training
- Human factors
Examples of inadequate understanding
of the process chemistry and thermochemistry
include the following:
- Decomposition of product mixture
- Unstable and shock-sensitive by-products were produced
- All reagents added simultaneously at the start of the reaction
- Unintended oxidation occurred instead of nitration
- Reaction was carried out with reactants at too high a concentration
- Reaction was carried out with too low a temperature resulting
in accumulation of reactants and subsequent en masse reaction
- Reaction was accelerated due to catalysis by materials of
construction of the reactor
- Reaction was accelerated by a phase change of the product
(to the vapor state)
- Contamination or presence of other impurities in raw materials
Examples of inadequate engineering
design include the following:
- Failure to control
steam pressure or duration of steam heating
- Probe wrongly
positioned to monitor reaction temperature
- Failure of temperature
control system
- Failure to monitor
loss of cooling water
- Failure to provide
sufficient separation distance between reactor and adjacent hot plant
- Too rapid heating
at reaction initiation
- Thermocouple
coated with polymer giving slow response
- Inadequate stirrer
specification
- Stirrer switched
on agitator too late, causing accumulation of reactants followed by en masse
reaction
- Loss of power
with no back-up systems
Examples of inadequate operational
procedures include the following:
- Over-charging, e.g. wrong volume of
reaction mixture, or reaction mixture contained wrong proportions of reactants
- Too rapid or too slow addition of
reactants
- Wrong sequence of addition of reactants
- Wrong materials charged
- Agitation stopped by operator to make
an addition
- Residues from previous batch
- Unauthorized modifications
- Water in transfer lines
The effects of a thermal runaway is briefly discussed in the
next Section.
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