A flask containing a toluene / water reaction mixture was being refluxed to remove the water as it was formed. The mixture had been running for some 6 hrs without problem, with the oil bath set to 130°C. Without warning the reflux became very vigorous and it was found that the oil was now at 190°C. The bath was turned off, lowered and allowed to cool.
The bath in question had been used with an auxiliary controller with probe, set to the desired temperature, and the original temperature control had been used as a "backup" to prevent over heating. However, it was not realized that both the internal controller and the auxiliary controller both worked the same internal relay. Thus when the relay failed neither controller had any effect.
- The relay failed in the "on" position, probably by the contacts welding shut, therefore the bath over heated.
- There was no safety cutout, the use of the internal controller as a backup was logically flawed.
When electrical items have to be left on for some time (i.e. they are not under 100% surveillance), then there needs to be a separate safety backup device to prevent over heating in the event of a failure.
This is not the first time this has been seen. A similar incident was reported here