Explosion in a pharmaceutical plant during a hydrogenation reaction

The event occurred at a pharmaceutical company classified Seveso ‘upper-tier establishment’. An explosion occurred at a hydrogenation reactor involving a chemical raw material, acetic acid, water and palladium as catalyst.

The explosion took place at the end of a reaction, when the operator was performing hydrogen purges, consisting in sending hydrogen under pressure into the reactor via the hydrogen vent. The operator heard a thud and observed a light above the hydrogen vent, at the bell of the flame breaker.

The emergency stop was actuated, the emergency plan started, the staff was evacuated, and the other facilities on site secured. Not having observed anything anomalous during the inspection tour, the emergency status was terminated called 30 minutes later.

The reactor involved in the event was secured by placing it under an inert atmosphere of nitrogen and by keeping it at 10 ° C. By disassembling the unit, traces of palladium were found in the vent line, possibly as consequence of foaming of the reaction medium which dragged the palladium up to the flame breaker. The palladium, dried out during the purge operations may have self-ignited at the outlet of the vent, in presence of oxygen.

The INITIATING CAUSE was the ignition of hydrogen due to the accidental presence of dry catalyst in the hydrogen vent, which self-ignited in presence of oxygen.

A very similar incident had already occurred two months before this one. In the case of the two events, a long pause of the production campaign had taken place beforehand. This has probably favoured the drying of the palladium in the vent line. Following the first event, the flame breaker was cleaned, but apparently the cleaning had been insufficient.

Another contribution to the incident was the fact that a new procedure was put in place 4 months before, requiring operators to empty the system at each batch. Before the introduction of this new practice, the presence of liquid in the system avoided the drying of the catalyst and therefore its auto-ignition.

Despite some lack of details of the ARIA report, the ROOT CAUSE could be attributed to mishaps of the management of the operations (new procedure and failure to take correct action from a previous event).

hydrogen vent, at the bell of the flame breaker

The plan was classified as Seveso upper tiers, but this incident was not classified as ' major event' covered by the Seveso directive.

The operator triggered the emergency stop and the emergency plan.
The staff was evacuated. The other facilities were secured.
Intervention team members did not observe anything abnormal during the reconnaissance and did not have to intervene. The emergency plan was called off in 1/2 hour.

Following the accidents, the plant operator has taken the following actions:
1. To reduce the risk of foaming, which implied the use of a non-foaming raw material, avoid the use of vacuum to create inert conditions (vacuum produces foams), wall –cleaning system avoiding foam

2. To inject nitrogen at the hydrogen vent to maintain the hydrogen concentration below its LEL;

3. To lower the decompression rate of hydrogen degassing to avoid the engulfment of particles;

4. To study the establishment of a hydrogenation reactor of greater capacity or the reduction of the charged quantities to limit the level of filling to 50-75%;

5. To modify the instructions for cleaning the vent lines: cleaning at the end of the campaign + cleaning the flame breaker in case of prolonged campaign stoppage; annual cleaning after complete disassembly of the line.