Phosphorus oxychloride and acetone is a dangerous mixture
An unexpected and severe chemical reaction between the solvent and cleaning agent acetone and the chemical phosphorus oxychloride led to an accident in a member company that luckily did not cause any personal injury.
In a pharmaceutical company, phosphorus oxychloride is used as a solvent and reagent within the scope of a chemical process. On the day of the accident, after the end of the synthesis reaction the surplus phosphorus oxychloride was removed from the reaction mixture by means of distillation, drained into a non-removable head steel drum with PE inlay, sealed and stored. Around ten hours later, the drum burst and was torn open at the top edge. Hydrochloric acid vapours escaped and some of the phosphorus oxychloride was expelled.
Through comprehensive laboratory experiments, the incident could be attributed to a reaction of the phosphorus oxychloride with the acetone. The acetone had previously been used in the production facility for removing water residue. When filling the phosphorus oxychloride, acetone residue unexpectedly entered the drum from "dead spaces" in the filling hose and the pump.
Analysis of the reaction
In the company's safety laboratory, experiments regarding the thermal stability of the chemical mixture were carried out in an adiabatic reaction calorimeter. To replicate the conditions under which the accident occurred as precisely as possible, phosphorus oxychloride was overlaid with acetone in a so-called closed-cell test. At a starting temperature of 30°C, the mixture was left to stand without stirring while heat was allowed to accumulate. The temperature initially increased continuously for around two hours up to 55°C. After this, a self-accelerating, exponential reaction process was measured: the temperature climbed to more than 180°C within just a few minutes. At the same time, the pressure increased suddenly to almost 50 bar. After cooling down, the pressure in the measuring vessel was still 4.8 bar and a black liquid remained. The conclusion was therefore drawn that decomposition with gas separation had occurred.
No reference could initially be found in literature regarding the observed reaction. A thorough search eventually uncovered a similar accident report. However, this accident took place in the reverse sequence. Phosphorus oxychloride was distilled in an American laboratory and the vacuum pump and distillation vessels used were subsequently cleaned using ethyl acetate and acetone.
Phosphorus oxychloride, acetone and ethyl acetate were poured into a waste container together. This exploded around two hours later. Safety checks revealed that the phosphorus oxychloride had reacted with acetone in a delayed, highly exothermic reaction with a rapid pressure increase.
See: Sichere Chemiearbeit 2-2005, page 19