[derekmjenkins]

I could use some verification on my calculations. 

We have a closed device with limited to no air exchange that stores vials of compounds in Tetrahydrofuran (THF) for presentation to an autosampler for HPLC analysis.  THF is a common solvent.  It came to our attention that we could not guarantee that the vials were crimped correctly and thus the septums might be open on some or all vials (approximately 400 2ml vials for example).  Therefore we needed to look into whether an explosive vapor might build up in this unit.

I took the approach of working backward from the hazardous limit of THF.   I discovered the Lower Explosion Limit (LEL) and Upper Explosion Limit (UEL) for Tetrahydrofuran (THF) are 2% and 12%.  Please see http://en.wikipedia.org/wiki/Flammability_limit

On that page it also states that  “Control of the explosion hazard is usually achieved by sufficient natural or mechanical ventilation, to limit the concentration of flammable gases or vapors to a maximum level of 25% of their lower explosive or flammable limit”

The total air volume of the device is 49 Liters.  We therefore infer from the information that a build-up of 0.98 Liters of THF will mean we have reached the LEL.  If we take 25% of this as indicated above, we would be designing to maintain for 0.245 L.

Here is where the calculations begin.

For an ideal gas PV=nRT, and therefore to calculate the number of moles of THF:
P = 1 atm
V = 0.245 L
T = 20 deg. C or 293.15 deg. K
R = 0.0821 L atm mol-1 K-1  (See http://en.wikipedia.org/wiki/Gas_constant )
n = PV/RT = (1 x 0.245)/(0.0821 x 293.15) = 0.01018 moles

Here is where things get vague.  Is the following calculation valid to see how many ml of liquid would need to evaporate into the closed chamber volume to get to the volume limit stated above?

THF density (ρ) = 889.20 kg/m3 = 0.8892 g/ml (@ 20 deg. C liquid)
THF molar mass = 72.11 g/mol

(0.01018 mol x 72.11 g/mol) / 0.8892 g/ml = 0.826 ml

In other words, if 0.826 ml of THF would evaporate into a fixed volume of 49 Liters, then this would mean that the volume would contain 0.5% THF vapor/gas.

If so, then my next steps will be to look at the rate at which THF evaporates and therefore direct the designers on the amount of air exchange necessary to prevent this dangerous build-up.

Any thoughts would be appreciated.