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REPORT ON INVESTIGATION OF AN INCIDENT
The site at Margate (Kent) Investigation 20/2/2008
Report attached below (4 pages)
The main Thor site in the UK appears to be at Wincham (Cheshire) although they have facilities in Germany. Business at Wincham includes flame retardant textiles, biocides for paints and ingredients for personal care products.
The site at Margate (Kent) was stated to be undergoing decommissioning, the intention being to level the site for redevelopment. Groundwater remediation was stated to be underway.
The incident occurred on 19th December 2007. ************were carrying out scale-up development for a flame retardant based on a laboratory recipe provided by chemists in Germany. The aim was to develop a scale-up route that could be transferred to a plant at Wincham (or elsewhere) to produce one tonne of development product for customer trials.
The Margate site was chosen, as it had a pilot scale reactor with adequate heating. The company stated that the scale-up on 18/19th December was the second attempt at developing the scale-up methodology, the first attempt occurring earlier in the year (when the German chemists were present). The synthetic route can be separated into two stages. Stage one ends in an intermediate bicyclic phosphate (CAS: 873-93-8). The second stage ends in a diphosphasprio compound (CAS: 3001-98-7) known as development TL 1171. The company completed stage one on 18th December and had removed toluene solvent to a glass vessel near the plant. Stage 2 was started on 19th December and resulted in uncontrolled events, reactor explosion and fire from the nearby stored toluene.
Information about the reactive chemistry, the risk assessment and consequences were obtained by examining information provided by the company and via discussion on 20/2/08.
The Information in Context
The Pilot Scale Reactor
The P&ID diagram supplied by the company was simplistic and this was borne out by examining the plant after the incident.
The plant stirrer was glass coated metal tripod bar design. This would only have been effective for very mobile fluids that covered the stirrer. Stirrer operation was via an overhead electric motor.
Heating was via circulating hot oil. The company measured inlet temperature and temperature within the pot. Pot temperature measurement was stated to be by platinum resistance thermometer, but this was within a large metal sheath approximately 2 -2.5 inches in diameter. There was no provision for crash cooling the reactor.
Temperature logging and plant control was via a control panel no more than 1-2 metres from the reactor and there were the remains of a chair next to the reactor and control panel.
The reactor bursting disc was sized for the largest port on the reactor. No formal calculations had been carried out to size the bursting disc based on an analysis of the chemical reaction hazards.
2.2 The Company Risk Assessment Ref No: TL1171 May 2007
The risk assessment appears to focus on the flammable and toxic hazards of the reactive ingredients, commenting on how they should be handled and what PPE should be required. Exothermic reaction hazard for stage one was considered, as medium likelihood and serious severity. Measures were stated to be toluene reflux and quench agent. The quench agent was not specified.
No consideration of thermal runaway appears for stage 2 reaction.
Discussions with ********** uncovered that for the stage two reaction, a quench medium was considered to be uncontrolled pouring in of dimethylmethyl phosphonate (DMMP) one of the reactants. This is inappropriate for a runaway reaction (adding more fuel to the fire).
2.3 What the Company Knew
It appears that little information had been provided to the Margate development team apart from two pages of A4. The first page highlights in two small paragraphs what the differences were in the manufacturing process. The second page provides summary information in very few words. ********** commented that the chlorination section related to Stage 1 of the synthesis. The rearrangement section related to Stage 2 of the synthesis.
Information for Stage 2
Information from Germany stated that when the reaction reached 240oC it became extremely exothermic with no possibility to stop reaction. At lab scale there was no exothermic reaction to 225oC, but the scale-up situation was unclear.
Information for Stage 1
A compound similar to the product from stage one of the synthesis (made
from trimethylol propane) was subjected to accelerated rate calorimetry
and found to runaway after 235oC. The company did not carry out
calorimetry on the actual product of stage 1 synthesis.
2.4 The Synthetic Route
An overview of the synthetic route carried out on 18/19th December 2007
is shown in Document “TL1171 Revised Process ref H&S 12/12/2007.”
For stage one the final stages of the process describe formation of a “thick
product but stirrable”, a “lumpy gel” and finally removal of the toluene
solvent prior to stage 2. ************** stated that the final product was
a stirrable liquid. It seems unlikely that the product from Stage 1 would be
Early steps in Stage 2 involved adding iodine/ dimethly methyl phosphonate (DMMP) to stage 1 product. No toluene was available to act as a heat transfer agent or temper any exothermic reaction. Evidence suggested that both Stage 1 product and Stage 2 product might produce an unstoppable exothermic reaction if temperature reached around 240oC and the reaction for Stage 2 was to be initiated at 175oC. This allowed a window of around 65oC to safely carry out the reaction without thermal runaway.
The company knew that danger existed from both Stage 1 and Stage 2 products, but failed to carry out an adequate programme to determine the chemical reaction hazards and determine an adequate basis of safety that could be engineered into the pilot plant at Margate.
There was a window of about 65oC between the reaction temperature for Stage 2 and the point at which thermal runaway was known to occur. It is questionable whether the synthetic route chosen would have allowed adequate temperature control.
The reactor whistle from the bursting disc is consistent with a runaway reaction. It was clear that the vent size was inadequate and this is likely to be the cause of the vessel overpressure event.
The explosion fractured the nearby toluene solvent (50Kg) and this resulted in fire. Should a person have been taking temperature readings or carrying out any other supervision of the plant at the time of the incident, they would have most likely suffered fatality.
Danger from chemical reaction hazards is well known and has been the subject of several publications (1,2). The company failed in its duty of care to ensure that harm was adequately risk assessed and controlled.
All pilot scale operations should cease at the Margate site, until the company has adequate processes to carry out assessment of Chemical Reaction hazards and implement a suitable basis of safety for plant operations.
Issues raised should be highlighted to Inspectors that deal with the Wincham site (COMAH top tier).
Relevant literature setting out the principles behind the study of chemical reaction hazards should be made known to the company (ref 1, 2).
Designing and operating safe chemical reaction processes, HSG 143, ISBN 0-7176-1051-9.
2. Chemical Reaction Hazards, John barton and Richard Rogers, IChemE, ISBN 0-85295-284-8.