Release and Fire due to Inadvertent Polymerization of Acrylic Acid
At 10:00 am, the sight glass of a 2.4 m3 dosing vessel charged with acrylic acid burst after pressure increase due to an inadvertent polymerization inside the vessel. Vapourized and liquid acrylic acid as well as polymerized material spilled out and ignited at an ignition source. It is a normal procedure for this product line, to load the raw materials and catalyst into the dosing vessels during the night shift and to start the reaction in the main 6.4 m3 reactor in the morning when the technical personnel is present. The vessel had been inerted with nitrogen and then loaded at 4:15 am.
The major part of the acrylic acid used was delivered in drums in the afternoon, the day before the incident, from the site warehouse; it was kept in the storage area of the plant until night when it was charged into the vessel with a pneumatic pump. In the same area there was also stored a drum containing some 70 kg left over from a campaign that took place two weeks earlier. This drum had been exposed to temperatures, as low as 5°C during the cold period three days before the incident. During the cold period the material in this drum crystallized and melted one or several times, and caused loss of stabilization in the acrylic acid phase. Not sufficiently stabilized acrylic acid was then transferred into the dosage vessel and there the polymerization process started, which was accelerated by the fact, that the vessel was inerted with nitrogen. The chemical stabilization mechanism of acrylic acid is less effective under inert conditions. The exothermic self-polymerization ended in a run-away causing increase of temperature and pressure. The safety valve was undersized for this type of pressure increase. The sight glass was broken and the release of the reaction mixture probably damaged a lighting installation which most likely became the ignition source for the subsequent fire.
The whole process was re-analyzed from the delivery of the raw materials to the finished product and based on this analysis, the following measures were taken:
- Installation of the following instrumentation / interlocks / safety devices:
- On the dosage vessel
- High temperature alarm
- A rupture disk (for pressure release in case of inadverted polymerization)
- A shortstop inhibition system (phenothiazine) was introduced, for interrupting an incipient unintended polymerization in case of high temperature or fire.
- These alarms and interlocks were classified as safety related process control systems.
- Refresher training was provided to the plant staff.
- Acrylic acid may polymerize inadvertently if it is not stored between 18 and 30°C. Furthermore it is known that contamination, for example with heavy metals (e.g. rust) and reducing agents, can trigger polymerization.
- Provide appropriate training and instruction to production staff.
- Consider that nitrogen blanketing to Acrylic Acid, affects the stabilization mechanism.
- Keep the Process Risk Analysis updated.
- Design relief systems according to possible scenarios