8.3 Assessing Thermal Risks
207
8.3
Assessing Thermal Risks
Having introduced the fundamental concept of heat balances for ensuring reactor
stability, this section goes on to present some of the key indicators used to assess
thermal risks within a process. It also provides an overview of some of the basic
guidelines used to estimate the impact and probability of a thermal runaway reaction
as well as methods to evaluate and manage identified thermal risks.
8.3.1 Thermokinetic Concepts
Fundamental to understanding the thermal hazards within a system are the concepts
of adiabatic temperature rise (T ad ), the maximum temperature of the synthesis
reaction (MTSR), and the time to maximum rate under adiabatic conditions (TMR ad ).
The calculations behind each of these are briefly introduced here, and they will be
further examined and applied later in the chapter.
8.3.1.1 Adiabatic Temperature Rise
The adiabatic temperature rise (T ad ) is the increase in temperature in the system
without heat exchange with the surroundings (i.e., under adiabatic conditions).
Considering this, it can be derived from the heat balance defined in Eq. 8.8 to be:
T ad =
Q r
c p
=
− R × C 0
ρ × c p
(8.10)
• T ad : adiabatic temperature rise [K]
• Q r : heat produced by chemical reaction [J/kg]
• c p : specific heat capacity of the reaction mass [J kg −1 K −1 ]
• R : reaction enthalpy [J/mol]
• C 0 : initial reactant concentration [mol/m 3 ]
• ρ: density of the reaction mass [kg/m 3 ]
The experimental determination of this key indicator for thermal risk is briefly
introduced in Appendix F.
8.3.1.2 Maximum Temperature of the Synthesis Reaction
The maximum temperature of the synthesis reaction (MTSR) is the maximum
temperature that can be reached by the synthesis reaction under adiabatic conditions.
This is dependent on the process temperature, the accumulation of reactant, and the
adiabatic temperature rise. For batch reactions, the accumulation is greatest at the
beginning of the reaction. In this case, the MTSR can be expressed according to
Eq. 8.11:
MTSR ∼ = T process + T ad
(8.11)
207
8.3
Assessing Thermal Risks
Having introduced the fundamental concept of heat balances for ensuring reactor
stability, this section goes on to present some of the key indicators used to assess
thermal risks within a process. It also provides an overview of some of the basic
guidelines used to estimate the impact and probability of a thermal runaway reaction
as well as methods to evaluate and manage identified thermal risks.
8.3.1 Thermokinetic Concepts
Fundamental to understanding the thermal hazards within a system are the concepts
of adiabatic temperature rise (T ad ), the maximum temperature of the synthesis
reaction (MTSR), and the time to maximum rate under adiabatic conditions (TMR ad ).
The calculations behind each of these are briefly introduced here, and they will be
further examined and applied later in the chapter.
8.3.1.1 Adiabatic Temperature Rise
The adiabatic temperature rise (T ad ) is the increase in temperature in the system
without heat exchange with the surroundings (i.e., under adiabatic conditions).
Considering this, it can be derived from the heat balance defined in Eq. 8.8 to be:
T ad =
Q r
c p
=
− R × C 0
ρ × c p
(8.10)
• T ad : adiabatic temperature rise [K]
• Q r : heat produced by chemical reaction [J/kg]
• c p : specific heat capacity of the reaction mass [J kg −1 K −1 ]
• R : reaction enthalpy [J/mol]
• C 0 : initial reactant concentration [mol/m 3 ]
• ρ: density of the reaction mass [kg/m 3 ]
The experimental determination of this key indicator for thermal risk is briefly
introduced in Appendix F.
8.3.1.2 Maximum Temperature of the Synthesis Reaction
The maximum temperature of the synthesis reaction (MTSR) is the maximum
temperature that can be reached by the synthesis reaction under adiabatic conditions.
This is dependent on the process temperature, the accumulation of reactant, and the
adiabatic temperature rise. For batch reactions, the accumulation is greatest at the
beginning of the reaction. In this case, the MTSR can be expressed according to
Eq. 8.11:
MTSR ∼ = T process + T ad
(8.11)