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