Secondary Reaction Cause Explosion

The investigation team found that the reaction accelerated beyond the heat-removal capacity of the reactor. The resulting high temperature led to a secondary runaway decomposition reaction, causing an explosion that blew the hatch off the reactor and allowed the release of the contents from the vessel. 

This company s initial research for the process identified and described two exothermic chemical reactions (1) The desired exothermic reaction is initiated at an onset temperature of 38°C, and it proceeds rapidly at 75°C (2) an undesirable decomposition (the dye) reaction has an onset temperature of 195°C. 

The operating plant was not aware of the decomposition reaction. The plant s operating and process information described the desired exothermic reaction, but they did not include information on the undesirable decomposition reaction. Information on their MSDS was also misleading (mentioning a lower reactivity and a much lower boiling point than the actual values). 

The root cause of this accident was poor operating procedures and poor process infoiv mation. The operating procedure, for example, did not cover the safety consequences of deviations from the normal operating conditions, such as the possibility of a runaway reaction and the specific steps to be taken to avoid or recover from such deviations. 

Chemical Manufacturing Incident, Report 1998-06-I-NY. Available at http //www.chemsafety.gov/ reports/2000/morton/index.htm. 

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Decomposition reactions are often involved in thermal explosions or runaway reactions, in certain cases as a direct cause, in others indirectly as they are triggered by a desired synthesis reaction that goes out of control. A statistical survey from Great Britain [1, 2] revealed that out of 48 runaway reactions, 32 were directly caused by secondary reactions, whereas in the other cases, secondary reactions were probably involved too, but are not explicitly mentioned (Figure 11.1). Therefore, characterizing secondary decomposition reactions is of primary importance when assessing the thermal hazards of a process. 

For reactions characterized by high values of Q, the onset of a thermal explosion can be controlled by adjusting the batch time, tt, the explosion occurs if t, > and does not in the opposite case. Hence, the reactor performance shows a typical on-off behavior, being characterized by either complete or negligible reactant conversion. It follows that reactions of interest must be carried out under explosion conditions, provided that the reactor vessel withstands the final internal pressure and the thermal shock caused by the sudden temperature increase. A similar map can be drawn with reference to undesired secondary reactions and, in this respect, operative parameters must be adjusted in order to avoid the ignition stage within the batch length. 

The released energy might result from the wanted reaction or from the reaction mass if the materials involved are thermodynamically unstable. The accumulation of the starting materials or intermediate products is an initial stage of a runaway reaction. Figure 12-6 illustrates the common causes of reactant accumulation. The energy release with the reactant accumulation can cause the batch temperature to rise to a critical level thereby triggering the secondary (unwanted) reactions. Thermal runaway starts slowly and then accelerates until finally it may lead to an explosion. 

In a closed system in which the volume is greater than the initial volume of the reactant, the extra or free volume must be taken into account. If this free volume is filled with air, then the oxygen in the air can react with excess carbon monoxide and free carbon from underoxidized explosives or propellants. A large air-filled free volume causes the explosive to undergo a complete afterbum or secondary fireball reaction. Both the products of this reaction and the additional heat developed by it must be taken into account. 

It appears that insofar as growth to stable detonation is concerned lead azide displays characteristics similar to those of heterogeneous secondary explosives. The delayed stress excursions evident in measured stress profiles were interpreted as reactions behind the shock front. Reactions produce pressure waves which travel through the explosive at a velocity at least equal to its velocity of sound and interact with the undecomposed explosive ahead of the reaction front, causing a nonuniform rate of growth. 

Properties and reactions of nitramines Secondary nitramines are neutral, primary nitramines form salts with bases, but an excess of alkali often causes decomposition to the carbonyl compound, nitrogen, and water. Secondary nitramines and aqueous alkali afford nitrous acid, aldehyde, and primary amine. Acids decompose primary aliphatic nitramines with formation of nitrous oxide in a reaction that has not yet been clarified thus these compounds cannot be hydrolysed by acid to amines in the same way as nitrosamines, although, like the latter, they can be reduced to hydrazines. Primary and secondary aromatic nitramines readily rearrange to C-nitroarylamines in acid solution. Most nitramines decompose explosively when heated, but the lower aliphatic secondary nitramines can be distilled in a vacuum. 

The results of the runaway incidents ranged from a simple foam-over of the reaction mass, to large increases in temperature and pressure leading to violent loss of containment. In some instances this caused the release to the environment of quantities of flammable or toxic materials up to several tonnes. In a few cases where flammable materials were released, a fire or a secondary explosion followed. Thermal runaways caused four fatalities and 82 injuries (as defined in relevant health and safety legislation ) in the period 1962-1987. 

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