Explosion in a pharmaceutical plant during a hydrogenation reaction

In a pharmaceutical company classified Seveso ‘upper-tier establishment’, a deflagration occurs at the end of a hydrogenation reaction involving a chemical raw material, acetic acid, water and a catalyst: palladium.
The operator was performing hydrogen purges, consisting in sending hydrogen under pressure into the reactor via the hydrogen vent. he heard a thud and observed a light above the hydrogen vent, at the bell of the flame breaker. He triggered the emergency stop and started the emergency plan. The staff was evacuated and the other facilities on site secured.
However, the Intervention team did not observe anything anomalous during the reconnaissance and did not have to intervene. The emergency status is called of f30 minutes later.
Before starting the analysis, the reactor involved in the event was secured by placing it under inert atmosphere (nitrogen) and by keeping it at 10 ° C .
A similar incident in all respects had already occurred on 04/06.

By disassembling of the system, palladium trace of palladium was 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.
In the case of the two events, a long pause of the production campaign had took 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, since 4 months a new procedure was put in place, 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.

The INITIATING CAUSE was the presence of dry catalyst in the hydrogen vent, which self-ignited in presence of oxygen.
It is not clear from the referenced source which has been the role of hydrogen, but apparently the hydrogen concentration at the vent were above the LEL, triggering the explosion.

The ROOT CAUSE was related to mishaps during operation (new procedure and failure to take correct action from a previous event).

hydrogen vent, at the bell of the flame breaker

A similar incident in all respects had already occurred on 04/06.

The plan is classified as Seveso upper tiers, but this incident does not belongs to the major events covered by the Seveso directive.

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.