Solvent Still Fire in a Fume Hood

July 2026

What Happened?

On the day of the incident, a student was working with four solvent stills in one of their lab’s fume hoods. The student planned to use one still at a time, starting with the still for purifying toluene. This toluene still was prepped with about 1 L of toluene in a large round bottom flask which was then connected to a solvent distillation column. The system was purged with N2 and the coolant system activated and chilled to 6.5°C. Once the still was finished being prepped, the distillation was initiated by heating the round bottom flask with a heating mantle connected to a Variac power supply. The other three stills contained about 1 L of various solvents but were not activated and remained at room temperature.  

When heating was first initiated, the student verified that the apparatus was set up properly then they left the hood area to allow the solution to reach temperature. After approximately 10-15 minutes of heating, they returned to check on the system and saw no apparent issues and so they lowered the fume hood sash and resumed working on another task on a nearby bench. A few minutes later, they heard and witnessed the round bottom flask stopper becoming dislodged which allowed the hot toluene to spray across half of the fume hood area. Immediately after, the fumes were ignited by an unknown ignition source causing an intense fire and deflagration (detonation with a slow-moving shock wave). The fire was contained behind the fume hood sash and was quickly extinguished by another lab member using a building fire extinguisher. No injuries were reported besides slight ringing in the ears and dizziness of the student closest to the fire. The fume hood and ductwork were damaged in the incident and extensive repairs were required for the equipment to return to full function and to correct the resulting negative air pressure differential in the room. 

What Caused the Accident? 

• The initiating event to this incident was the loss of containment of toluene from the solvent still due to over-pressurization.  
  • The specific cause to the over-pressurization cannot be determined from the aftermath of the incident or the witness accounts but is most likely due to clogging of the glassware, failure to fully open the nitrogen valve to allow proper air exchange within the column, or clogging/failure of the nitrogen inlet/outlet valve.
•  The lab group had an SOP (standard operating procedure) posted in the immediate area for using solvent stills to purify solvents but this copy was burned up in the fire and an additional copy could not be provided to review and verify the established procedure. 

How Can We Prevent Future Accidents and Minimize Injuries? 

Actions taken to address this incident included the following: 

• Solvent stills are notoriously dangerous apparatuses that can easily fail if proper precautions are not taken but safer alternatives are readily available – specifically solvent purification systems which remove oxygen and water from solvents at room temperature. Many of the solvents used by this group and others in the department were switched to this type of purification method to eliminate the need to utilize solvent stills on a regular basis. 
• For solvents that are unable to be switched to the purification system, a new SOP for using solvent stills was established and reviewed by EHS to ensure that proper precautions are taken to avoid such incidents in the future, including regular dismantling and cleaning of the apparatus, proper PPE selection, and two-person minimum safety checks. 

Final Thoughts

Solvent stills must be always treated and handled with the proper precautions as they are proven to have the potential to cause major fires and incidents in labs.  

Benzene Solvent Still Fire 2001 

This incident very easily could have been much more severe if specific aspects had been slightly different. If the student was in closer proximity to the fire, they would have sustained burns and other serious injuries beyond what was experienced from the deflagration shock wave. The fume hood sash was properly lowered but not fully closed which allowed the flames to be mostly confined to the fume hood area while also giving the other student access to extinguish the fire. Only one still was involved in the fire and the remaining 3 L of flammable solvents were seemingly unaffected but could have also ignited and enhanced the severity of the fire. If the student who used the fire extinguisher was not properly trained or able to access an extinguisher, then the damage could have easily been intensified or extended beyond the fume hood. 

Although the handling of this incident was proper, and the impact was limited to property damage rather than serious personnel injuries, the lesson to be learned is to always respect the dangers presented in the lab by the chemicals and procedures conducted. This incident identified the need to review fire safety and establish a standardized solvent still operating procedure with robust safety precautions so that more serious incidents can be prevented before they start.