Fire at a chlorine electrolyser plant

Chlorine was released in the liquefaction unit of a chlorine electrolysis plant, due to the rupture of a pipe. THis was the consequence of an attempt by the operator to reduce hydrogen concentration in the chlorine flow, by increasing

During the re-start phase of an electrolysis workshop in a chemical plant, chlorine (Cl2) gas leaked from a pipeline in a confined chlorine liquefaction unit, specifically at the pipeline carrying residual chlorine from the gas/liquid separator to a liquid bleach manufacturing plant.
The leak took place when few tens of cm of this pipeline failed, due to the combustion of the pipeline steel by chlorine, which in turn was caused by the presence of hydrogen in the pipeline due to lack of monitoring.
The flame stabilised near a valve and a flow meter and caused the combustion of iron / chlorine when the temperature reached 130 °C.

[Note of HIAD event validator: this is a chlorine incident, initated by a hydrogen abnormal event].

The INITIATING cause of the accident was the ignition of the chlorine in the pipeline due to a hydrogen content above the allowed maximum.

According to the rules, hydrogen concentration in the residual chlorine must remain below the 4.6 %vol value (LEL, H2 low flammability/explosivity limit in Cl2). H2 concentration was monitored and manually controlled by the rate of evaporation of chlorine in the gas/liquid separator . In this incident, the technician failed to reduce the increasing hydrogen concentration in the pipeline, that reached 6.7%, well above the LEL. To bring down the concentration, the evaporation rate of chlorine was increased causing a rise in the residual chlorine flow rate. This probably provided the low energy needed to start the H2-Cl2 combustion.
This ignition resulted in the rupture of the steel pipe : tens centimetres of pipe disappeared resulting in a Cl2 leak.
Nothing is said on the reason of this hydrogen increase, which could tentatively attributed to an operation error.

liquefaction of chlorine, pipeline

DESCRIPTION OF THE PROCESS
Sodium chlorate (NaClO3) is produced via electrolysis of sodium chloride according to the formula:
NaCl + 3 H2O --> NaClO3 + 3 H2
From a stoichiometric viewpoint, for each tonne of sodium chlorate, 56 kg of hydrogen is formed as by-product. However, since the cathodic and the anodic current efficiencies can be different the real figure can be ± 1-3 kg different.
In Finland sodium chlorate is used in the pulp industry.
[https://www.nordichydrogenpartnership.com/files/assets/uploads/available-by-product-hydrogen-in-the-nordic-countries.pdf]

According to the operator, hundreds of kg of Cl2 were released in the building, but only 3 kg were leaked into the atmosphere thanks to the dynamic confinement system that opened into a neutralisation tower. The Cl2 detectors indicated that the 5 ppm threshold was crossed for 20 seconds at the boundaries of the site and for 20 min near the workshop. No environmental or human (including both staff and civil population) consequences were reported

This is the detailed emergency sequence:
(1) After checking that everyone was out of the lab, the supervisor paged all staff in the vicinity to immediately evacuate to the staging area.
(2) Facility management and ES&H management were notified about the situation, and they contacted the local fire department to respond to the site in case the venting gas was ignited.
(3) On their arrival, the first responders agreed with the facility management and the site safety officer to allow the tank to vent until emptying.
(4) The hydrogen gas supplier was called, but a specialist was unavailable at that time. Based on information from the hydrogen supplier's website and some basic estimates of the hydrogen release rate, facility staff determined when the contents of the tank would have been completely vented.
(5) It was decided that first responders would approach the lab in full bunker gear with Self-Contained Breathing Apparatus and a gas detector, to assess any gas present.
(6) Two first responders entered the lab and two were stationed as back-up. No hissing was heard and the detector confirmed that the cylinder had vented and the lab was clear of flammable H2-air mixture.
(7) The hydrogen cylinder was moved outside and secured and later returned to the supplier.

The lesson learnt from this event are contained in the full ARIA investigation report provided under references.

To reduce the chances of reoccurrence of such an accident, the operator decided to improve monitoring of the hydrogen concentration and shutdown the electrolysis workshop when the H2 concentration crosses 3.5 % in residual chlorine. Furthermore this concentration was guaranteed by continuously performing the material balance of pure chlorine.