Explosion in a chemical plant

The incident occurred at a silane production plant, where a ‘neutralization container’, part of the "Contact Mass Preparation" process unit exploded. The bursting was caused by the accidental production of hydrogen, causing (1) an over-pressurisation and failure of the tank, (2) the release of hydrogen and (3) a second, combustion explosion following ignition. This second explosion caused significant property damage to plant equipment and buildings, but no serious personal injuries. 

The INITIATING CAUSE was the accidental production of hydrogen from a run-away reaction. The production of hydrogen was due to an overdose of caustic soda in a silicon dust suspension tank, which caused its pH value to increase beyond a value of 11. This allowed a large amount of hydrogen to evolve in a very short time. 

There was no interlock which could have prevented the run-away reaction. The plant had been designed to operate on a manual basis. The pH probe was not used as a risk-monitoring tool. On top of that, the personnel forgot to activate the probe, probably due to malfunctioning of process components, catching their whole attention. 

The ROOT CAUSE was a lack of proper risk assessment design and lack of proper personnel training during emergencies.

chemical reactor

The process conditions were: alkaline solution with Si sludge, pressure <= 300 mb, temp. approx. 50° C

Silane (SiH4) could be used as precursor of semiconductors grade silicon, or as surface treatment (coating, adhesion) as precursor of organosilanes. The sources of this event do not disclose the purpose of the silane production. Therefore, also the production route could not be identified with certainty. 
It is probable that silane in this plant was produced via the reaction of metallurgical silicon dust with hydrogen and chlorine compounds. The “contact mass preparation“ mentioned in the description of the event consists in the   physical blending of silicon powder, the copper catalyst and chemical promoters. This solid blend is fed into a fluidized-bed reactor where it comes into contact with reacting gases (like hydrogen chloride or methyl chloride).
The reaction has acidic byproducts such as hydrochloric acid (HCl) and various chlorosilanes, which can contaminate the silicon dust slurry. Therefore, natron lye (NaOH) is introduced to neutralize these highly corrosive acidic products. If an overdose of natron lye occurs and pushes the pH value beyond 11, the caustic soda begins to chemically attack and dissolve the fine silicon dust itself, with unwanted release of hydrogen.

Two people with minor injuries were taken to hospital (ZEMA mentions only 1). Damage to adjacent buildings, to cars in surrounding area. Damage outside of plant: Damage to pergola

The onsite fire brigade arrived on the scene immediately after the explosion. They first searched for injured persons and did a headcount to check that everyone from in and around the plant was accounted for. The emergency response control centre also requested the intervention of an emergency doctor and ambulances. Smaller cable fires in the plant were extinguished from turntable ladders. Measuring vehicles were unable to determine the concentration of toxic substances. The police provided assistance and checked the application of pollution control measures. The extinguishing water was channelled to the internal clarification plant for treatment.

This incident example of lack of proper process variable monitoring and of interlocks system able to bring automatically the system to a safe state. In the absences of these, the whole responsibility is shifted to personnel. In case of unexpected (thus not tested, exercised) series of events, the personnel has great difficulty in (i) understanding what is happening, (ii) focussing on the most critical emergency actions required to manage the emergency.

A ‘plausibility check’ was introduced as operative risk assessment tool. for those parts of the system which were not the object of a systematic safety-related testing beyond design and needed to be assessed regarding possible generation of unacceptable energy releases. This check is based on the logical concept that inadmissible energy conditions can occur only if: (i) energy generated as intended cannot be dissipated, (ii) energy potentials deactivated as intended are suddenly released, (iii) or energy is introduced from outside. Moreover, the company modified the process of ‘contact mass’ processing, particularly the neutralization stage relevant to the event.