THERMAL SAFETY
Molar enthalpy of a reaction – is the heat released (or absorbed) in
a chemical reaction at constant pressure when simple substances
combine into complex product;
Δ HR [kJ mol-1];
Specific heat of a reaction – the amount of heat energy required to
raise the temperature of a body per unit of mass (standard – in J by 1
K for 1 g; e.g. water has 4.18 J);
QR
’ [kJ kg-1];
Heat capacity – the amount of energy required to raise the energy of
the system by 1 K;
Cp [J K-1];
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THERMAL SAFETY
Time to Maximum Rate (under Adiabatic Conditions) (TMRad) –
the higher the temperature the faster the reaction and the shorter
TMRad, can be determined by the DSC measurement;
Time of No Return (TNR)
MTSR … Maximum Temperature of the Synthesis Reaction
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COOLING FAILURE SCENARIO
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
1.Can the proces temperature be controlled by the cooling
system?
• Sufficient cooling of the system depends on e.g. viscosity of the
mixture, power of a cooling system, possible fouling of the
reactor walls, an area for heat exchange, efficient stirring;
• Heat release rate of the reaction is relatively easily obtained from
reaction calorimetry measurement.
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
2. What temperature can be attained after runaway of the
desired reaction?
• After the cooling failure unconverted reactants will react in an
uncontrolled way and it leads to an adiabatic temperature
increase;
• The available energy is proportional to the accumulated fraction;
• At higher temperature even (desired) products can further react
providing additional heat increase.
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
2. What temperature can be attained after runaway of the
desired reaction?
• The Concept of Maximum Temperature of the Synthesis
Reaction (MTSR)
MTSR = Tp + Xac x ΔTad
Tp … desired reaction temperature
Xac … degree of accumulation of unconverted reactants
ΔTad … the adiabatic temperature raise
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
3. What temperature can be attained after runaway of the
secondary reaction?
• At higher temperature the secondary reactions might be triggered
– it leads to further runaway;
• At higher temperature even (desired) products can further react
providing additional heat increase;
• Data can be obtained from the DSC and adiabatic calorimetry
measurement.
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
4. At which moment does the cooling failure have the worst
consequences?
• The time where the accumulation is at a maximum and/or the
thermal stability of the reaction mixture is critical;
• In order to answer this question both the synthesis reaction
and secondary reactions must be known;
• Data obtained from the reaction and adiabatic calorimetry, and
the DSC measurements can help to answer this question.
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
5. How fast is the runaway of the desired reaction?
• Usually, the industrial reactors are operated at temperature
where the desired reaction is relatively fast;
• A temperature increase above the normal proces temperature
thus will cause a significant acceleration of the reaction rate
(the van’t Hoff criterion);
• Duration of the main reaction runaway may be estimated using
the initial heat release rate of the reaction and the concept of
the Time to Maximum Rate (TMR).
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COOLING FAILURE SCENARIO
SIX PRINCIPAL QUESTIONS
6.How fast is the runaway of the decomposition reaction
starting at MTSR?
• The dynamics of the secondary reactions plays an important role in
the determination of the probability of an incident;
• Again, the concept of the Time to Maximum Rate (TMR) is useful.
The answers to all six questions represent a systematic way of
analysing the thermal safety of a proces and building the cooling
failure scenario.
Thermal risk assessment based on severity and probability of the
event.
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SEVERITY OF COOLING FAILURE SCENARIO
Δ Tad=
𝑄′
𝑐 𝑝
′
Δ Tad … increase of temperature under adiabatic conditions
Q’ … specific energy of the reaction
cp’ … specific heat capacity (water 4.2 kJ kg-1 K-1; organic solvents
around 1.8 kJ kg-1 K-1; inorganic acids around 1.3 kJ kg-1 K-1)
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PROBABILITY OF COOLING FAILURE SCENARIO
The probability can be evaluated using the time scale.
If, after the cooling failure, there is enough time left to take
emergency measures before the runaway becomes too fast, the
probability of the runaway will remain low.
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Tp … proces temperature; MTSR … Maximum Temperature of the Synthesis Reaction; TD24 … temperature at
which the Time to Maximum Rate is 24 h; MTT … Maximum Technical Temperature (e.g. boiling point)
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Mettler Toledo 14
COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Criticality Class 1
• MTSR < MTT
• Very safe
• Evaporative cooling serves as an additional
safety barrier
• Reaction mass should not be held for a very
long time under heat accumulation
conditions
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Criticality Class 2
• MTSR < MTT;
• but MTT > TD24 , the decomposition reaction
can be triggered if the reaction mass is
maintained for a long time under heat
accumulation conditions;
• Still low risk scenario
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Criticality Class 3
• MTSR > MTT;
• Safety of the process depends on the heat release
rate of the synthesis reaction at MTT;
• Get ready to do pressure release;
• Decomposition reaction should not be triggered in 24
hours
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Criticality Class 4
• MTSR > MTT;
• Moreover, MTSR > TD24;
• Safety of the proces depends on the heat release
rate of both the synthesis reaction and the
decomposition reaction;
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COOLING FAILURE SCENARIO
CRITICALITY CLASSES
Criticality Class 5
• MTSR > TD24; after loss of control of the reaction the
decomposition will be triggered;
• It is very unlikely that evaporative cooling or the
pressure relief can serve as a safety barrier;
• This is very dangerous and unacceptable scenario;
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