Polymerisation runaway reaction, May 1992

Accident summary

In May 1992 an uncontrolled runaway polymerisation occurred on a plant which makes a maleic co-polymer product.

On the day before the incident, the monomer and solvent had been charged to the maleic in the reactor for the polymerisation. A charge of styrene was then made to the reactor to react with the residual maleic followed by a line flush with solvent. During this line flush it was discovered that the monomer line valve was open, and it was subsequently closed. The batch was completed without incident. The following day the batch was charged as normal and heated to 114°C before the first catalyst charge was made. The expected exotherm was exceeded and the batch was put on full cooling. The reaction began to runaway and the pressure in the reactor rose to an estimated 40-50 psig. A 35 psig relief valve lifted, but the 50 psig bursting disc did not burst. However the agitator seal o-ring blew out and a heavy concentration of vapours were released into the plant area.

It was discovered that on the previous day, styrene had back-flowed into the monomer line via the open block valve. A non-return valve in the monomer line failed to prevent this. This styrene was contained in the initial charge of the following batch resulting in a significantly more vigorous reaction than normal, which exceeded the reactor control.

Failings in technical measures

The reactor venting and relief system could not cope with the maleic / styrene co-polymerisation.
Reaction / Product Testing: effect of impurities
Relief Systems / Vent Systems: sizing of vents, especially exothermic reactions
The rating of the agitator seal pressure lower than the bursting disc set pressure resulted in a release of process vapours into the plant area rather than to a safe area.
Design Codes - Plant: rotating equipment seals
There was no fast and effective way to add quench water to absorb the heat of reaction in an emergency.
Quench Systems: sizing
The plant evacuation alarm only sounded when the actuator was held in and was located within the vapour cloud.
Emergency Response / Spill Control: area evacuation
Maloperation of the transfer route block valve allowing the back-flow of material through a faulty non-return valve.
Inspection / Non-Destructive Testing (NDT): proof testing

References

'Polymerisation Runaway Reaction', Loss Prevention Bulletin, Issue No. 110, April 1993, pp 25-26, Institution of Chemical Engineers, ISSN 0260-9576.