208
8 Thermal Process Safety
• MTSR: maximum temperature of the synthesis reaction [K]
• T process : temperature of the desired reaction process [K]
• T ad : adiabatic temperature rise [K]
The MTSR is used during the development of the process design as a basis to
ensure reactor stability even in case of malfunctioning (e.g., cooling failure).
8.3.1.3 Time to Maximum Rate Under Adiabatic Conditions
The time to maximum rate under adiabatic conditions (TMR ad ) is the amount of
time needed before a thermal explosion takes place under adiabatic conditions. It
can be calculated solving the heat balance for a zero-order 7 reaction under adiabatic
conditions. This can be done since for large values of T ad , the Arrhenius term of
the reaction rate becomes much more important than the concentration-dependent
term. The TMR ad [s] is then defined as:
TMR ad ∼ =
c p
˙
q(T 0 )
RT 2
0
E A
(8.12)
• c p : specific heat capacity of the reaction mixture [J kg −1 K −1 ]
• T 0 : temperature of the reaction at the initial conditions [K]
• E A : activation energy [J/mol]
• R: universal gas constant [J mol −1 K −1 ]
• ˙
q(T 0 ): rate of heat produced by the chemical reaction at temperature T 0 [W/kg]
The TMR ad can be calculated to represent both (1) how quickly the runaway of
the desired reaction occurs and (2) how fast the runaway of a decomposition reaction
occurs starting at the MTSR. In the first case, the T 0 will be equal to the operating
temperature of the process, and in the second case, the T 0 will be equal to the MTSR.
To calculate TMR ad according to Eq. 8.12, the rate of heat produced ( ˙
q at T =
T 0 ), and the activation energy (E A ) of the reaction must be known. Methods to
experimentally determine these parameters are briefly introduced in Appendix F.
8.3.2 Cooling Failure Scenario
The scenario of a cooling system failure during an exothermic batch reaction has
proven to be a useful case for learning about the thermal risks of chemical processes.
In order to conservatively identify its risks, the following assumptions can be made:
7 Even though the concept of TMR ad was developed for zero-order kinetics, it still provides a
conservative approximation for strongly exothermic reactions with higher kinetic orders (Stoessel,
2008).
8 Thermal Process Safety
• MTSR: maximum temperature of the synthesis reaction [K]
• T process : temperature of the desired reaction process [K]
• T ad : adiabatic temperature rise [K]
The MTSR is used during the development of the process design as a basis to
ensure reactor stability even in case of malfunctioning (e.g., cooling failure).
8.3.1.3 Time to Maximum Rate Under Adiabatic Conditions
The time to maximum rate under adiabatic conditions (TMR ad ) is the amount of
time needed before a thermal explosion takes place under adiabatic conditions. It
can be calculated solving the heat balance for a zero-order 7 reaction under adiabatic
conditions. This can be done since for large values of T ad , the Arrhenius term of
the reaction rate becomes much more important than the concentration-dependent
term. The TMR ad [s] is then defined as:
TMR ad ∼ =
c p
˙
q(T 0 )
RT 2
0
E A
(8.12)
• c p : specific heat capacity of the reaction mixture [J kg −1 K −1 ]
• T 0 : temperature of the reaction at the initial conditions [K]
• E A : activation energy [J/mol]
• R: universal gas constant [J mol −1 K −1 ]
• ˙
q(T 0 ): rate of heat produced by the chemical reaction at temperature T 0 [W/kg]
The TMR ad can be calculated to represent both (1) how quickly the runaway of
the desired reaction occurs and (2) how fast the runaway of a decomposition reaction
occurs starting at the MTSR. In the first case, the T 0 will be equal to the operating
temperature of the process, and in the second case, the T 0 will be equal to the MTSR.
To calculate TMR ad according to Eq. 8.12, the rate of heat produced ( ˙
q at T =
T 0 ), and the activation energy (E A ) of the reaction must be known. Methods to
experimentally determine these parameters are briefly introduced in Appendix F.
8.3.2 Cooling Failure Scenario
The scenario of a cooling system failure during an exothermic batch reaction has
proven to be a useful case for learning about the thermal risks of chemical processes.
In order to conservatively identify its risks, the following assumptions can be made:
7 Even though the concept of TMR ad was developed for zero-order kinetics, it still provides a
conservative approximation for strongly exothermic reactions with higher kinetic orders (Stoessel,
2008).