An explosion occurred in a hydrogen liquefier/purifier commissioned in 1987, after it had previously operated safely for many years. The explosion took place in the nitrogen cold box section of the hydrogen liquefaction process in an activated carbon cold adsorber vessel. Process records showed that the explosion occurred at the beginning of the regeneration phase of the activated carbon adsorber. When the explosion took place, the outlet temperature of the activated carbon bed was still at -190C. The force of the explosion was estimated from a mapping of the debris to be between 10 and 100 kg TNT equivalent.
Activated carbon is a general term that covers carbon material mostly derived from charcoal. It has an exceptionally high surface area and can adsorb large quantities of gases. It is believed that condensation of an oxygen-enriched phase occurred and reacted with the activated carbon adsorbent, resulting in an explosion. The root causes of the explosion were:
- The catalyst used in the oxygen reduction system upstream of the hydrogen purifier was rapidly inhibited during operation, most likely due to an unexpected contaminant in the feed from the unpurified hydrogen source.
- As a result, oxygen passed downstream into the purification section and was adsorbed on the activated carbon of the cold adsorbers.
- The regeneration blower generated a spark, which ignited a hydrogen/oxygen mixture and triggered a flashback to the adsorber bed.
- The explosion in the adsorber vessel was caused by the activated carbon reaction with the adsorbed oxygen.
The safety relief valves were found to be operational after the explosions. This indicates that the reaction leading to the explosion was so rapid that the purifier vessel could not be protected by conventional pressure relief devices (PRDs). In other words, the reaction was much faster than PRD actuation and opening time.
The ignition mechanism of this incident is not fully understood. However, it is known that activated carbon in the presence of condensed oxygen or liquid air is an unstable combination that can result in a powerful explosion. One kg of oxygen adsorbed in activated carbon is said to be 2.6 times more powerful than an equivalent weight of TNT.
Potential ignition sources include:
- Presence of high temperature or hot spot during adsorbent regeneration.
- Impact of particles entrained in the gas stream.
- Rapid flow change through the bed (e.g., during operation of process flow valves).
- Heat generated by adiabatic compression.
- The presence of other flammable impurities (e.g., oil carryover from compressors, hydrocarbon contamination of the gas) is an additional hazard and should be eliminated before cryogenic purification.
- Potential sources of oxygen in the system include:
- Oxygen or air impurity entrainment in the main gas to be purified in conjunction with failure of the upstream processes to remove these impurities.
- Regeneration of previously desorbed oxygen in the activated carbon bed.
- Accidental ingress of oxygen or air in the upstream processes (e.g., during vacuum or low-pressure processes).
- Risk mitigation methods for processes involving cryogenic activated carbon adsorbers include:
- Identify and eliminate by design the following:
- Potential formation of liquid oxygen and its contact with the activated carbon bed. Sources of liquid oxygen could be air ingress or oxygen impurity being entrained in the main gas to be purified/liquefied.
- Potential ignition mechanisms/sources (e.g., electric heaters used for regeneration).
- Use of alternative/substitute adsorbent that is noncombustible with liquid oxygen (e.g., silica gel, molecular sieve).
- Add an oxygen trap (e.g., silica gel, liquid nitrogen trap/knockout pot system) upstream of the activated carbon adsorbent bed.
- Avoid having sudden flow or pressure changes through the activated carbon adsorber, since these would increase the likelihood of ignition.
- Use sensors to identify the presence of oxygen or air ingress upstream and actuate alarms or system emergency stop when concentrations exceed predetermined set points.
- Purge the adsorber bed with an inert gas to reduce the risk of ignition if high oxygen concentrations are present.
- Alternatively, pull a vacuum on the adsorber to remove any adsorbed oxygen before the adsorber is put back online.
- Look for alternatives to or add-on features for pressure-reducing devices.
- Identify and eliminate by design the following:
- Years of satisfactory service without incident should not be taken as proof of safe operation of cryogenic activated carbon adsorbers.