Explosion at a chlorine production plant

An explosion caused by an increased hydrogen concentration in the chlorine gas stream. Hydrogen and chlorine concentrations at this plant were measured once each shift. On the morning of the explosion, the hydrogen concentration in the chlorine header leaving the cell bank was 0.47 % vol. After passing through the chlorine coolers and liquid/gas separators, the hydrogen concentration of the gas streams increased to 2.5% - 3.2% vol.

About 5 hr after those measurements were made, the d.c. power to the electrolysis cell bank was shut down because of intermittent power supply problems. At that time a low order explosion was heard from the chlorine drier area of the plant. Thirty seconds later, chlorine gas began escaping from the chlorine header pumps, and another explosion occurred in the electrolysis cell room. There was considerable damage to the dryer, the electrolyser cells and other chlorine processing equipment.

[Zalosh and Short, 1978]

The post-incident investigation indicated that the hydrogen concentration in the chlorine header probably increased when the power to the cells was cut off without completely shutting off the chlorine vacuum pumps. Thus, the pumps probably drew additional hydrogen into the chlorine header and increased the hydrogen concentration further downstream where ignition occurred.

The ROOT CAUSE can be attributed to inadequate operation procedures when dealing with transients and emergencies such as the one occurred in this event (partial loss of power, shutdown). Moreover, it is also related to risk management ( too low frequency of gases concentrations, no interlocks).

electrolityc cell, dryer, electrolysis equipment , chlorine dryer

DESCRIPTION OF THE CHLORINE PRODUCTION PROCESS BY ELECTROLYSIS
The chlorine is generated by the electrolysis of brine, (sodium chloride water solution) around 80 C. A low-voltage, high electrical current passing through the electrolysis cell produces chlorine gas at the anode and sodium at the cathode.
(1) In a diaphragm type cell, the sodium immediately reacts with water to produce sodium hydroxide and hydrogen at the cathode, which is coated with a porous asbestos diaphragm.
(2) In a mercury amalgam type cell, most of the sodium reacts with mercury at the cathode to form an amalgam, which is then decomposed under the influence of water to form sodium hydroxide, mercury, and hydrogen.
The chlorine outlet of each cell is connected to a chlorine header pipe which leads to coolers and driers or to further chemical process streams.
The hydrogen output from the electrolysis cells goes to a hydrogen header which may lead to a compressor, a drier, or a vent stack depending upon the end use of the hydrogen.
[https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2019-11/CAK_BREF_102014.pdf]

FLAMMABILITY OF THE HYDROGEN-CHLORINE SYTSTEM
At room temperature and pressure, the lower flammability for H2 in Cl2 is 4.1% vol. This values reduce to 3.6% vol. around 3 bar, and reduces further at higher pressures.
[ A. W. Umland, Explosive Limits of Hydrogen‐Chlorine Mixtures, 1954 J. Electrochem. Soc. 101 626]

The first explosion blew off the steel plate cover on the drier by shearing off the 3/8-in. (0.95-cm) bolts on the cover. The second explosion blew off the heads of two electrolysis cells and broke 108 glass connections between the chlorine header and the cell bank.
Zalosh & Short reported a range of property damage 20,000 to 100,000

According to the summary of Zalosh’ and Short, this event is a classic example for most of the hydrogen-chlorine incidents, typically due to hydrogen inadvertently entering the chlorine header for various reasons. As the chlorine is gradually cooled and liquefied, the concentration of hydrogen increases in the vapor mixture. Since the lower flammability limit of hydrogen in chlorine is approximately 3% at room temperature, most of the ignitions have occurred in the chlorine processing equipment downstream of the electrolysis cells.

Specific to this case is the fact that the hydrogen and chlorine concentrations in the electrolyser were measured once each shift. This frequency may have been sufficient for steady-state operation, but was not sufficient during transients, especially during unplanned events such as a loss of power, which may trigger unexpected operative conditions. The need for accurate and frequent monitoring of hydrogen concentrations was also required by the fact that hydrogen concentration of hydrogen in the chlorine stream had been measures not far away from the lower flammability limit of hydrogen in chlorine: the value 3.2% vol is approximately 78% of the flammability limit at 1 bar, a value whucg should be of high concern for operators).