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Runaway Polymerisation of Glacial Acrylic Acid

The Incident

A 1.5 te batch of glacial Acrylic Acid was charged to a feed vessel from drums. To this was added ca.200 kg of n-Dodecyl Mercaptan (nDM). The vessel contents should have been immediately transferred to a reactor containing a heel of water and methanol but, due to delays in obtaining raw materials, this transfer did not take place. 4.5 hours after charging the nDM a violent runaway reaction occurred. The bursting disc on the feed vessel ruptured and polymer was sprayed out of the works and into a nearby, on-site car park. Several cars were damaged and one person was taken to hospital suffering from the effects of vapour inhalation.

The Investigation

Carius tube screening tests revealed that nDM lowered the onset temperature of thermal polymerisation of acrylic acid by ca.70 K.

The acrylic acid used at the time of the incident was adequately inhibited with 210 ppm MEHQ. However, the normal plant procedure for charging glacial acrylic acid is by placing the feed vessel under vacuum and sucking the monomer in. It was shown experimentally that de-aeration of the acrylic acid (which may arise as a result of the charging method) destabilised the monomer - this is unsurprising since MEHQ requires the presence of dissolved oxygen to remain effective.

In adiabatic Dewar studies the feed vessel contents at the time of the incident showed accelerating exothermic behaviour from ambient temperature. After 1.5 hours the temperature had increased to ca. 80°C, at which point the temperature increased rapidly to 160°C in under 5 minutes. Polymeric material was violently ejected from the Dewar flask.

Conclusions

The incident arose as a result of glacial acrylic acid being stored in the presence of a highly reductive mercaptan for extended periods of time. The monomer had been further destabilised as a result of the charging process.

Lessons to be Learnt:

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