Thermal Explosion of Zoalene

see also: Thermal Simulations, Algorithm of Thermal Simulations, Application Epoxy Resin

Materials with highly exothermal decomposition potential are liable to explode under certain conditions. In this category, solid materials are particularly dangerous when decomposition starts below the melting temperature. Due to insufficient convection and limited thermal conductivity, a progressive temperature increase can easily take place, resulting in a thermal explosion.

Frequent observations show that the kinetic decomposition mechanism often changes with temperature. This happens when the decomposition process is multi-staged and, as a function of temperature and/or time, the step which determines the speed shifts to another stage.

All known theories and also more recent papers take into account only one-step reactions for the heat generating process. The correct description of the decomposition reaction process is the essential basis of an accurate prediction. It is therefore of great importance from the point of view of safety regulations, when the known theory can be extended to include a description of the decomposition process by more complex reactions.

On the other hand, the simulation can be very closely fitted to the surrounding conditions by free selection of heat capacity, heat conductivity and heat dissipation over the surface. In this way, even borderline cases are made accessible to adiabatic behavior.

Zoalene is a model substance which has proven ideal for testing the thermal simulation program because:

  • the decomposition potential is very high, 3000 J/g
  • the decomposition kinetics are relatively complex,
  • an exothermal decomposition can be determined with highly sensitive micro
    calorimeters, even at more than 100 ¡C below the melting point,
  • Zoalene caused a bad explosion in England during 1970s. By comparing the
    simulation results with the facts that were gathered after the explosion,
    the algorithm¥s efficiency can be checked.
Thermal Simulation needs that information about physical properties of material: size and Geometry; temperature dependent heat capacity, density, heat conductivity.
Kinetic analysis of Zoalene contains endothermal and two following exothermal decomposition steps

DSC measurements of Zoalene in an autoclave crucible with results of the fitting

The melting process is approximated by a n-th order reaction of high activation energy and a small reaction order. The decomposition itself is a double-step reaction, described by two first order reactions with autocatalysis. 

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Kinetic Parameters for Melting and for Decomposition Reaction
above the Melting Temperature
#ParameterValue
0lg (A1/s^-1) 150.00
1E1/(kJ/mol) 1310.00
2React.order10.0015
3lg (A2 /s^-1) 8.78
4E2 /(kJ/mol) 117.42
5lg K-cat 2 0.2007
6lg (A2 /s^-1) 2.76
7E2 /(kJ/mol) 64.94
8lg K-cat 2 0.3737
9FollReact. 1- 0.044
10FollReact. 20.606
11Area /(J/g) 2400
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Kinetic Parameters for Decomposition Reaction
below the Melting Temperature (first order reaction)
 
#ParameterValue
0lg (A1/s^-1) 2.646
1E1/(kJ/mol)75.13
2Area /(J/g) 2400
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Conditions of Simulation 
ConditionValue
Density/(g/cm^3) 0.34
Cp/(J/gK) 1.34
Thermal conductivity/(W/cmK) 0.001
Heat transfer /(W/cm^2K) 0.0002
Temperature profiles on the different distances from the center
Temperature profile as the function of time and distance from center
Temperature profiles for zoalene: start at 40°C, ambiente 135°C.
Adiabatic heating for different initial temperatures
TMR: time to maximum rate for zoalene
Critical radius for zoalene calculated according to Frank-Kamenetzkiy
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