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SAFETY
THERMAL RISK ASSESSMENT
Petr Beňovský

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SAFETY
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SAFETY
Francis Stoessel Thermal Safety of Chemical Processes: Risk Assessment and Process Design, Wiley
VCH 2008
Seveso, Bhopal terrible accidents;
Risk analysis to advocate benefits of chemical industry;
Toxicity (in silico studies), environmental aspects (including waste disposal), process safety,
contamination, elimination of human errors, property protection;
Material and Safety Data Sheets (MSDS) accompaning products
Risk Perception – comparing fatalities in different activities – the Fatal Accident Rate Index
(FAR) that gives the number of fatalities per 108 hours of exposure to the hazard.
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SAFETY
Francis Stoessel Thermal Safety of Chemical Processes: Risk Assessment and Process Design, Wiley
VCH 2008
Risk – according to a definition of the European Federation of Chemical Engineering (EFCE) it is a
measure of loss potential, or damage to the environment or persons in terms of probability and
severity
RISK = probability x severity ( x occurence)
Risk tools – e.g. Failure Mode and Effect Analysis (FMEA)
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SAFETY
Francis Stoessel Thermal Safety of Chemical Processes: Risk Assessment and Process Design, Wiley
VCH 2008
The Hazard and Operatibility Study (HAZOP) developed in early 1970s in ICI after the Flixborough
incident. It is derived from FMEA concept, but specially adopted for the process industry;
Essentially oriented towards the identification of risks stemming from the process equipment using
the proces and instruments design and the process flow diagram.
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SAFETY
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SAFETY
Fatal Accident Rate Index (FAR) - the number of fatalities per 108 hours of exposure to the hazard.
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SAFETY
A quiet situation resulting from the real absence of any hazard
But, is it attainable??
The risk is always there, a hazard should be minimized or eliminated as much as possible
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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 given mass of the system
by 1 K;
Cp [J K-1];
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TYPICAL VALUES OF SPECIFIC HEAT CAPACITIES
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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?
2.
•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?
1.
•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?
1.
•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?
1.
•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?
1.
•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?
1.
•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?
7.
•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.
1.
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SEVERITY OF COOLING FAILURE SCENARIO
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
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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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CALORIMETRY
Antoine Lavoisier
Pierre-Simon Laplace
Mettler RC-1
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PRACTICAL EXAMPLE
Grimm, J.S. et al Org.Process Res. Dev. 6, 938 (2002)
Original conditions:
• 3 mol eq. of SOCl2 as reaction solvent
• upon completion rxn becomes thick, unstirrable
• added MTBE, filtered
Problems:
• large amounts of unused SOCl2 to be disposed
• although MTBE meets all requirements at higher temp it could decompose
  should be OK at RT
Decision:
Run rxn with 1.6 eq of SOCl2 in MTBE as rxn solvent at RT
This was OK up to 2.0 mol scale, then sluggish
Concern – prolonged exposure of MTBE to acidic conditions
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PRACTICAL EXAMPLE
Adiabatic temperature rise – temp.rise when all the heat generated stays in the RM
    (cooling, stirring failure)
specific heat capacity (kJ/kg.K)
specific weight (kg/m3)
conversion
MTSR – maximum temperature of synthesis reaction
MTSR = Tp + DTad
process temperature
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Calorimetric measurement at 40-45°C (RC-1)
Problems:
• rxn not „feed controlled“
  (sudden delayed heat release)
• sudden uncontrolled outgassing
• gas contained isobutylene as well
  (separate exp. showed decomp. at 40ºC)
• DTad is 25ºC, MTSR=65ºC,
   b.p. of MTBE 55ºC
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Calorimetric measurement at RT
Again:
• rxn not „feed controlled“
  (sudden delayed heat release)
• 2 exotherms observed
• DTad is 53ºC, MTSR=78ºC,
   b.p. of MTBE 55ºC
•
Still hazardous !!!
Decision:
abandon MTBE in favor of toluene
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Calorimetric measurement in toluene at RT
Problems:
• delay even worse (entire charge)
• uncontrolled outgassing
Still hazardous !!!
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Calorimetric measurement in toluene at 45 oC
Problems:
• Still the same problem – delay of massive outgassing
•  DTad is 23ºC, MTSR=68ºC,
   b.p. of toluene 111ºC
Still somewhat hazardous due to uncontrolled outgassing
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Measurement in toluene/DMF at 25ºC
Finally:
• Rxn is more or less feed controlled
• Outgassing is also under control
•  DTad is 20ºC, MTSR=45ºC,
   b.p. of toluene 111ºC
•
Not ideal but safe within reason
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