210
8 Thermal Process Safety
The key questions to answer here when assessing the risks related to a runaway
scenario can be formulated in terms of temperature and time as follows:
(i) Temperature:
• What process operating temperature (T process ) allows for the safe dissipation
of the heat of reaction during normal operation?
• What is the maximum temperature achieved by the runaway of the synthesis
reaction (MTSR) in the event of a cooling breakdown?
• Which final temperature (T end ) could be reached following a possible
subsequent decomposition reaction?
(ii) Time:
• At what point in time would a failure of the cooling system be most critical?
• How much time does a runaway of the desired chemical reaction need?
(However, this time frame is usually short and therefore does not provide
a safety factor.)
• How much time does the runaway of the decomposition reaction need?
(TMR ad represents the time available to take corrective measures.)
Identifying the T ad , MTSR, and TMR ad helps to answer these questions and
ensures that the thermal risks for the chemical process have been identified.
Subsequently, these values can also be used to characterize the risk of a runaway
reaction by its consequences and probability of occurrence.
8.3.3 Consequences of a Thermal Runaway
The adiabatic temperature rise (T ad ) defined in Eq. 8.10 can be used as an indicator
for the potential consequences of a thermal runaway reaction. Table 8.3 shows
classifications of T ad that can be used as a descriptor of the potential impact of
a thermal runaway.
Table 8.3 Classification of
the impact of a thermal
runaway using the value of
adiabatic temperature rise
(Stoessel, 2008)
T ad [K]
Impact
>200
High
50 < <50 a
Low
a Boiling point not exceeded as
an additional condition