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EXPLOSION IN A RESEARCH LABORATORY

The following is a summary of an incident which occurred in a research laboratory.

The Incident

A chemist was attempting to dispose of ca 1 litre of a solution containing a solvent/reactant (A) and a second reactant (B). The solution was in a 2 litre resin flask fitted with a water cooled reflux condenser. Reactant B can be reacted with sulphuric acid to produce a volatile intermediate material (C). It was intended to remove all of reactant B, by treating it with sulphuric acid to convert it to C, and recover the solvent reactant A. The chemist added a small amount of sulphuric acid to the reaction mixture at ambient temperature. No reaction was observed. The flask contents were then gently heated in a water bath. An exothermic reaction was first observed at 42°C. Cooling was applied in the form of a bucket of cold water. The temperature of the flask contents continued to rise and at ca 60°C a rapid and violent reaction occurred which resulted in the flask exploding. The force of the explosion also managed to smash the fume-cupboard window. Fortunately no one was injured as a result of this incident.

Chemistry

In previous laboratory work the solution under disposal had been used in a continuous laboratory reactor. The reaction proceeds via two keystages to make the product (D).

(i) B    -------(H2SO4)---------->   3C
(ii) 3A + 3C   -----(H2SO4)----->   D

Both of these stages rely on acid catalysis. The reaction is normally carried out under continuous or semi-batch conditions at ca 80°C. The rate of heat output from the reaction is controlled by the feed rates of the materials.

The Investigation

Carius tube thermal stability screening tests had shown that D was thermally unstable from temperatures of ca 140°C. The estimated heat of decomposition of D using Benson’s group contribution theory, has been calculated to be >-3500J.g-1.

Isothermal calorimetry had shown that the overall heat of reaction for the preparation of D was ca -400kJ mol -1 of B. Reaction hazard studies of the mixture of A and B revealed no adverse chemical reaction hazards in the absence of acid. However, the presence of a small quantity of sulphuric acid resulted in exothermic activity being observed from ca 40°C in screening tests, becoming much more marked at temperatures above 60°C.

Adiabatic stability tests showed that the addition of acid to the mixture at ambient temperature did result in a small exotherm of ca 10K. This was sufficient to access a second accelerating exotherm from ca 30°C to 60°C. During this exotherm no significant rise in pressure was observed. At 60°C a sudden, violent reaction occurred with rates of temperature rise in excess of 3300 K min-1 being recorded. This exotherm was accompanied by a sudden rise in vapour pressure of at least 1000 psi min-1.

It was the finding of the investigation that the sudden rise in pressure accompanying the rapid exotherm was probably the cause of the explosion. It is unclear whether the decomposition reaction of D was accessed during this incident.

Lessons Learnt

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