Explosion in a hydrogenation reactor

A explosion occurred at a hydrogenation reactorin of a chemical manufacturing plant for the pharmaceutical and cosmetics industries (classified as high-level Seveso). 
At end of the hydrogenation cycle, the catalyst needed to be reactivated by exposure to hydrogen at high temperature.  After that, two buffer tanks belonging to the hydrogenation unit were opened for cleaning, when the explosion occurred.

The operator initiated the emergency plan, evacuated personnel and secured the area. Two operators were injured.

INITIATING CAUSE of the explosion was the presence of hydrogen when opening to air the buffer tanks. According to the ARIA report, the explosion was probably be due to the presence of hydrogen in the reactor, because the nitrogen purging was not performed despite the procedures in place. The flammable mixture created by the entrance of air at the opening of the tanks was then ignited by a hot spot formed by a catalyst deposit (ruthenium on coal), which probably had become pyrophoric after activation. Contributing to the ROOT CAUSE was the failing to apply the procedure to purge the tank with nitrogen. This failure could be attributed to a human error, possibly triggered by lack of training or lack of clarity of the procedures. ROOT CAUSE is unknown, because it is not known why the procedure was no followed.

hydrogenation reactor

Periodic catalyst regeneration or reactivation is necessary, because over time, the catalysts lose activity due to coking (buildup of carbonaceous deposits/heavy polymers on the surface) or poisoning (irreversible binding of impuri-ties like sulfur). Regeneration strips away these contaminants to restore the active surface area. It is achieved through two main pathways depending on the deactivation phenomena: 

• Oxidative Regeneration (Controlled Burning): This is the most common method for removing carbon (coke) deposits. The catalyst is exposed to an oxygen-containing gas stream at elevated temperatures to burn the carbon off as carbon dioxide and water. During coke burn-off, hydrogen must be strictly avoided. Mixing hydrogen with oxygen at high temperatures presents a severe explosion hazard.
• Reductive Regeneration (Hydrogen Stripping): Used when the catalyst surface has become oxidized or lightly fouled by volatile hydrocarbons. It is usually the second step of the regeneration process, because the burning tends to ocidise the catalysts. 

Two operators in the vicinity of the reactor at the time of the explosion were injured. The system was not damaged.

Following the explosion, the operator initiated the emergency plan, evacuated personnel and secured the area.

An additional measure, not mentioned in the ARIA report, would come from answering the question on why the procedure was not followed, and what can be put in place to guarantee/monitor that they are followed in the future.

The operator introduced the following two improvements 1. Review and re-assessment of the operative procedures 2. new training of the workers.