Fire at a chlorine electrolyser plant

A release of gaseous chlorine occurred in a chlorine liquefaction unit of a plant producing basic inorganic products. The event happened during the restart phase of the electrolysis unit, which had been stopped due to a loss of cooling water.
The release was caused by the destruction of section of a pipe carrying residual gases from a gas/liquid separator. The pipe failure was due to the combustion of the steel pipe by a chlorine.

The purpose of the chlorine liquefaction unit is to separate the residual hydrogen contained in the chlorine from the electrolyser, and to provide liquid chlorine to the other production unit of the plant.
Part of the chlorine remains in the gaseous phase, together with the hydrogen and air. A safeguard was the measurement and control of the hydrogen relative concentration in this phase. When the detector measured a hydrogen concentration (6.7%vol.) higher than the allowed value of 4.6%vol., the operator increased the evaporation of chlorine. This was the mitigating measure foreseen to reduce hydrogen relative concentration.

However, this created the condition for the formation of a chlorine-hydrogen flammable mixture. This initial fire triggered the reaction of chlorine with the iron of the pipe. The self-sustaining combustion of iron in chlorine occurs above a temperature of 130°C.

The INITIATING CAUSE was the ignition of a hydrogen-chlorine gaseous mixture, which allowed the onset of an iron-chlorine reaction.

The IGNITION SOURCE is believed to be due to the energy provided by the increase of the chlorine stream to the pipe.

During the start-up phase, the automatic and continuous monitoring of hydrogen was not possible, due to a too high concentration of nitrogen. Therefore, the hydrogen was measured manually and at interval. Probably because of the manual character of this operation, the operator failed to react on time to a quick rise of hydrogen beyond the maximum allowed value. The mitigating measures
The operative safety barriers were weak, worsened by unclear instructions on the number of hydrogen measurement measurements. Moreover, the relatively long time necessary to isolate the leak revealed inadequate preparedness and weak emergency strategies.

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

The operators shutdown the facilities and evacuated the premises. The internal emergency plan of the site was triggered and lifted after 1 hour and 30 minutes.

The consequences of this event were very limited. The building did not experience any pressure increase, and the containment of the released chlorine functioned as designed.
Nevertheless, the event revealed the following:

(1) A guillotine rupture scenario is possible and should be considered as possible scenario when performing a risk assessment of the facility.
(2) Combustion of iron in chlorine shall be considered as real hazard and covered by a dedicated risk assessment.
(3) The importance of considering specific accidental scenarios related to transient phases and the need to implement dedicated safety measures.
(4) It is not only about hydrogen concentration in the residual gases, also the chlorine concentration is important, and the risk of a chlorine-hydrogen considered.
(5) The responsibility of both the monitoring of the operative parameters and their control shall not be left to only one operator, and certainly not during critical transient, dynamic phases.

The following actions were implemented:
(1) Implementation of a material balance to monitor the hydrogen concentration in the waste gases based on the liquefaction rate and the hydrogen content at the outlet of the electrolysis room.
(2) Installation of two continuous hydrogen analysers usable in the presence of inert materials.
(3) Supervision of the start-up phases by a manager.
(4) Modification of operating procedures.
(5) Review of the emergency actions.
(6) Shutdown of the installations in the event of a hydrogen concentration exceeding 3.5%.