Chlorine-Hydrogen Explosion Disrupts Electrolysis Process
During chlorine-alkali electrolysis an accident occurred in which chlorine was released due to a chlorine-hydrogen explosion. The person charged with the control of the measurements was informed by an acoustic signal that a short circuit had occurred in one of the cells. This occurrence is not exceptional and is generally caused by a slight pressure fluctuation in the hydrogen system, which is passed back through the sloping connection between the hydrogen and the chlorine sections. Searching the cause of the increased hydrogen pressure, the controller found immediately out that the hydrogen compressor, which removes all hydrogen from the plant, had stopped. He tried to restart the compressor, which succeeded in the first instance. But short time afterwards, the aggregate shut down again, due probably to the low-pressure blocking system caused the break down of the aggregate. The crew observed at that time that a considerable release of hydrogen occurred from the small pipes of the amalgam decomposers. The reason of it was the insufficient purge of hydrogen over roof passing through a hydraulic guard foreseen for such cases. This hydraulic guard has the function to transport hydrogen without danger over the roof. On the day of the accident, however, the function of the safety devices was limited by a non-negligible dirt deposition had formed inside, which increased the counter pressure. This was situation was not recognized. Hydrogen reached the cell-space through the mercury pump, which transfers the regenerated mercury again in the electrolytic cell. This fast and massive intrusion of hydrogen caused a local formation of an explosive mixture. The first heavy explosion destroyed a part of the cells as well as the chlorine collection pipe passing under the explosion point. The plant was shut down by emergency shut down by the crew immediately after the explosion, but the ongoing decomposition of the amalgam present did not stopped the production of hydrogen. Following that, another heavy explosion occurred, which destroyed the externally passing chlorine collection pipe.
Event Date
March 4, 1995
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Causes
Cause Comments
Immediate cause was the break down of the hydrogen compressor. Neither electrical causes nor human causes could be determined. It has to be assumed that the break down has been caused by the blocking of the safe hydrogen release system.Therefore at least a contributing cause can be found in lack of safeguards control.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
hydraulic guards, mercury pump, electrolytic cell
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
The electrolysis of chloro-alkali was performed, as it had been for the previous days, near to the technical capacity. The plant produced 15.6 tones chlorine/h and was in normal conditions. There were no evident deviations from the safe operating mode. At the time of the disturbance (accident, failure, breakdown) the shift was manned with a regular crew of 9 persons.
Currency
Lessons Learned
Lessons Learned
The aggregate affected was re-designed, avoiding the deposition of dirt (design change). Further, periodic checks are to be performed on it in the future (change in procedure, change in supervision).
Event Nature
Emergency Action
The plant was immediately shut down after the explosion by the emergency shut down system.
Release Type
Release Substance
Release Amount (kg)
100.00
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
Original description of the event in Database eMARS
ARIA database, event 10316
https://www.aria.developpement-durable.gouv.fr/accident/10316/