Explosion in a sodium chlorate electrolyser

An explosion affected a chemical plant producing sodium chlorate for bleaching pulp and paper. The chemical process utilized electrolytic cells and produced hydrogen a byproduct. Hydrogen was then utilised as a fuel. The explosion occurred in the hydrogen processing system.

At the time of the incident, the plant was at an abnormally low operating level of 25% capacity. A non-routine maintenance operation to repair high-pH liquid piping was in progress. To allow for this intervention, personnel rerouted the high-pH liquid stream to the plant sump. However, the liquid eventually made its way back into the electrolytic process. This caused the pH value of the electrolytic process increasing faster than the computer-controlled acid-injection system could compensate for. (The acid system is designed to maintain proper process pH by lowering pH in the electrolytic process.)
Due to this increase of high-pH electrolyte circulating through the system, oxygen generated by the electrolytic process increased as well. Eventually, the hydrogen concentration fell below the upper flammability limit (UFL).
This situation continued for hours, until the hydrogen-oxygen flammable mixture found an ignition source, resulting in an explosion.

The INITIATING CAUSE was the enrichment of oxygen in the hydrogen stream, triggered by an increase of pH values of the electrolyte.
The reason for the ignition is not provided.

It is unclear how and why the high pH liquid discharged from the pipe section could flow back into the electrolytic cell. H2TOOLS mention that this occurred “by design”. This statement suggests design shortcomings as a ROOT CAUSE component.
H2TOOLS reports further that operations personnel on duty failed to realise the hazards created by this abnormal situation and to act accordingly, despite their training and the existence of standard operating procedures (SOPs). They also questioned instrument readings. This could hint at a human error. However, considering that the operating condition were highly abnormal, with the facility working at very low power level and repairs work on going on a pipe section, another ROOT CAUSE component could be fairly attributed to a lack of supervision.

pipe,
electrolytic cell

DESCRIPTION OF THE PROCESS
Sodium chlorate is an inorganic compound with the chemical formula NaClO3.
Sodium chlorate is produced industrially by electrolysis of concentrated sodium chloride solutions:
NaCl + 3 H2O → NaClO3 + 3 H2
Sodium chlorate was used in the plant of this event for bleaching paper and pulp, after its transformation into ClO2.
Important to understand the background of this incident, is one of the reactions occurring at the anode to form chlorate and oxygen:
12 ClO− + 6 H2O → 4 ClO3− + 12 H+ + 8 Cl− + 3 O2
Further reactions occur away from th eelectrodes, and depends on the pH value of the electrolyte solution.
[Extract from Wikipedia: https://en.wikipedia.org/wiki/Sodium_chlorate]

No injury or secondary fires. The explosion caused extensive damage to piping, process vessels and the containment building

1. Operators received additional training to understand how to assess the hazards of and effectively respond to an increase of oxygen in the hydrogen gas stream. Their knowledge shall be assessed as well.
2. The plant control system was redesigned, with additional interlocks able to automatically shut down the process and safely secure the hydrogen systems on pre-determined pH and oxygen levels thresholds.