Thermal Explosion after Mischarge of concentrated Nitric Acid
In a production plant, a strongly exothermic reaction was carried out in an inerted 50 m3 vessel(pressure proof up to 16 bar). The reaction got out of control and lead to a fast increase in pressure, which was no compensated by pressure relief via the safety valve. As a result, a short tube between the vessel and the safety valve was ruptured and a flange towards the cellulose dosage system broke apart and was pressed upwards where further parts of the plant were damaged. Hot combustion gases escaped from the vessel and damaged cable isolations and flexible tubes of the pneumatic system. Furthermore the pressure wave blew away parts of the wall cladding. A cloud of nitrous gas escaped from the building and was dispersed beyond the site fence. Other gases were absorbed by water curtains set by the fire brigade. No persons were injured.
The manufacturing process includes as one of the last process steps in the reactor the neutralization of the reaction mixture with nitric acid. This process step was not fully completed by the automatic process control system (PCS). Therefore a manual operation was necessary to complete the process phase. This was an admitted and necessary action, very similar to manual operations that allow for re-start of automatic control after some checks by the operator. During this manual operation the dosage program for nitric acid was unintentionally activated for a second time. The activation was not visible on the screen. However no acid was dosed, as the respective meter had already reached the full set amount. But the dosage program remained activated anyway - unnoticed by the operators, as the next charge was started, all counters were re-set to zero, and as a result, the still active program initiated the dosage of 2098 kg nitric acid immediately (i.e. in the wrong process phase), as the condition for start ("metering =0") was fulfilled and there was no interlock for acid dosage in the initial process phase. The incident occurred, when approx. 110 kg t-Butanol was added to the erroneously dosed nitric acid. Concentrated nitric acid is a strongly oxidizing agent. Contact with organic substance leads to strongly exothermic oxidation, which can result in the formation of toxic nitrous gases (NOx). Mixtures of nitric acid with alcohols are particularly instable. The decomposition is unpredictable and may occur very violent, i.e. like an explosion. (see: L. Bretherik, "Handbook of Chemical Reaction Hazards", 5.Ed., Butterworth, 1995)
- The supply of Nitric Acid is now interlocked by the PLC (Programmable Logic Controller) in such a way, that the dosage can be initiated only during a defined process phase and if the specified conditions for the process parameters are fulfilled. In addition, a matrix is established to assess the risk of credible unintended interactions between reactants involved in this process. Depending on the respective findings, further interlocks will be programmed and/or additional protective installations will be set up.
- A protective scheme will be integrated in the PSC that will avoid the unintended initiation of subroutines during manual operations.
- All subroutines running will be displayed on the control screen, such that the operators notice them.
- PCS-specialists will provide additional and more detailed training on relevant aspects of PCS will be carried out for the operators.
- Flexibility is a prerequisite for modern process control systems. However, it is just this high flexibility that increases the risk of unintended activation of subroutines. Control systems should therefore be designed such that manual interventions are possible only as far as necessary for the correct completion of the process. (PLC-)Interlocks are to be foreseen to avoid dangerous deviations.
- Active programs must be displayed such that the operating personnel will notice any incorrect activation of subroutines and can take the necessary corrective actions in time.