8.3 Assessing Thermal Risks
213
MTSR stays below the temperature required to reach T 24 . These reactions can be
considered inherently safe.
• Criticality class 2: This type of reaction is also not critical; however, the MTT
of the system is no longer available to serve as a safety buffer to avoid a
decomposition reaction.
• Criticality class 3: In these reactions, the MTSR exceeds the MTT; however a
decomposition reaction cannot be triggered. Careful attention should be paid to
ensure a sufficient design of the evaporative cooling system or depressurization
control. However, this reaction scenario is otherwise not critical.
• Criticality class 4: Within this class, the thermal potential of the desired reaction
exceeds the T 24 , and a thermal decomposition could occur. The lower MTT value
of the system can at best help serve as a safety buffer to stabilize the temperature,
but careful attention should be paid to a sufficient design of the condensation
system. In any case, this scenario is critical, and a technical safety measure is
required to manage the risk.
• Criticality class 5: In this highest class, the thermal potential of the desired
reaction exceeds the T 24 , and a thermal decomposition could occur without any
safety buffer provided by the MTT. Such reactions are critical and should be
shifted to a lower criticality class through a redesign of the reaction process.
Inherent process safety decreases from criticality class 1 (highest inherent safety)
to criticality class 5 (no inherent safety).
8.3.6 Risk Management
For risks determined to be not acceptable, risk management actions should be taken
to reduce their level of risk. The principles of inherent process safety and functional
process safety (layers of protection) introduced in Chap. 7 are also applicable to the
risk reduction of thermal hazards. In decreasing order of importance, some of the
measures that can be taken include (Kletz and Amyotte, 2010):
Inherently safer design:
• Minimization of material and energy content in the process (e.g., use of a
continuous process instead of a batch process)
• Substitution of the reactants and solvents (e.g., with non-hazardous substances, ensure no unstable intermediates are generated, no highly energetic
compounds used).
• Use of less hazardous process conditions (e.g., lower temperatures, 9 dilution, 10 etc.).
9 Attention: this could result in a high accumulation of unreacted reactants.
10 Note that a side effect of diluting the reaction mixture could be adverse environmental and
economic impacts.
213
MTSR stays below the temperature required to reach T 24 . These reactions can be
considered inherently safe.
• Criticality class 2: This type of reaction is also not critical; however, the MTT
of the system is no longer available to serve as a safety buffer to avoid a
decomposition reaction.
• Criticality class 3: In these reactions, the MTSR exceeds the MTT; however a
decomposition reaction cannot be triggered. Careful attention should be paid to
ensure a sufficient design of the evaporative cooling system or depressurization
control. However, this reaction scenario is otherwise not critical.
• Criticality class 4: Within this class, the thermal potential of the desired reaction
exceeds the T 24 , and a thermal decomposition could occur. The lower MTT value
of the system can at best help serve as a safety buffer to stabilize the temperature,
but careful attention should be paid to a sufficient design of the condensation
system. In any case, this scenario is critical, and a technical safety measure is
required to manage the risk.
• Criticality class 5: In this highest class, the thermal potential of the desired
reaction exceeds the T 24 , and a thermal decomposition could occur without any
safety buffer provided by the MTT. Such reactions are critical and should be
shifted to a lower criticality class through a redesign of the reaction process.
Inherent process safety decreases from criticality class 1 (highest inherent safety)
to criticality class 5 (no inherent safety).
8.3.6 Risk Management
For risks determined to be not acceptable, risk management actions should be taken
to reduce their level of risk. The principles of inherent process safety and functional
process safety (layers of protection) introduced in Chap. 7 are also applicable to the
risk reduction of thermal hazards. In decreasing order of importance, some of the
measures that can be taken include (Kletz and Amyotte, 2010):
Inherently safer design:
• Minimization of material and energy content in the process (e.g., use of a
continuous process instead of a batch process)
• Substitution of the reactants and solvents (e.g., with non-hazardous substances, ensure no unstable intermediates are generated, no highly energetic
compounds used).
• Use of less hazardous process conditions (e.g., lower temperatures, 9 dilution, 10 etc.).
9 Attention: this could result in a high accumulation of unreacted reactants.
10 Note that a side effect of diluting the reaction mixture could be adverse environmental and
economic impacts.