Chemical reaction producing hydrogen, followed by an explosion.

The incident occurred in a pharmaceutical plant. A disc ruptured on the vent of a reactor and the reaction mixture containing hydrogen was projected onto a frontage of the synthesis workshop. It caused a spill of 700 l of a mixture of mono-chlor-benzene (which is flammable liquid) and sodium borohydride (which is toxic).

The plant operator manually activated the emergency response plan, evacuating the personnel. The internal emergency services set up a foam blanket on the spill, then carry out a water washing. The discharge valves were closed and the product was recovered in the fire retention basins. The site's response teams marked out the impacted area and took VOC measurements. Two hours later, the plant operator lifted the emergency response plan and set up a restriction zone around the building.
Nine employees were slightly affected by odours.

The INITIATING CAUSE cause was the formation of amount of hydrogen which could not be vented. The disc rupture was caused by an excessively rapid flow of propionic acid. The reaction between this acid and sodium borohydride releases hydrogen and causes the solution to foam. The flow should last between 3 and 5 hours with a target flow rate of 2 L/min. Investigations revealed that the flow occurred at a rate of 7 L/min. This rate led to the rapid formation of foam and hydrogen, which could not escape through the vent designed for this purpose, as it was blocked by the foam. The pressure in the reactor rose until it reached the pressure at which the disc ruptured.

According to the ARIA source, the root causes of this event were identified as:
(1) a lack of training of the operator carrying out the process,
(2) an inaccuracy on the manufacturing sheet as to the flow rate of pouring and
(3) safety barriers rendered ineffective by the foaming phenomenon.

chemical reactor, monochlorobenzene and sodium borohydride

Nine employees were slightly affected by odours. They were taken care of by the site's medical service. A gaseous emission to the atmosphere occurred via the bursting disc outlet. Aqueous effluents are treated by the site's wastewater treatment plant. Solid waste (material) is sent to a specific treatment facility.

Before restarting, the following actions were taken:
(1) improvement of procedures: modification of the manufacturing sheet to specify the pouring rate;
(2) Selection of experienced operators to perform this operation;
(3) passive safety barrier: resizing the orifice restrictor on the propionic acid supply pipe to limit the pouring rate.