[sirluke90]

Hello to everyone

I have one question regarding the connection between flash point, vapor pressure and LFL that is really bugging me. In preparation for my exam i run in this exercise: two substances, A and B, have different vapor pressures: P_vA= 0.1 bar and P_vB=0.3 bar at 20°;moreover the LFL of A is 5% and the B one is 3%. Which substance has the lowest Flash Point?

Intuitively I know that B is more volatile since it has the highest vapor pressure, so it makes sense that it will have also the lowest flash point, but i can't really demonstrate mathematically; could you help me? Thanks

[Enthalpy]

Hi sirluke90,

The flash point depends not only on the vapour pressure, but also on the lowest vapour concentration that makes the air mix flammable. Logically enough, since a non-flammable compound like trichlorofluoromethane may be very volatile, but the mix won't catch fire.

Admitting that 5% and 3% are by volume, and that atmospheric pressure is nearly 1 bar (it's not exactly) for simpler figures, at +20°C equilibrium compound A has 10% volume concentration in air, or 2× its LFL, while B has 30% or 10× its LFL. You might expect that at some lower temperature hence vapour pressure, A is below its closed-cup flash point and B above.

Though, the question is over-simplified and formally unanswerable, because we ignore how the vapour pressures of A and B vary with the temperature. This differs among the compounds. The vapour pressure of B could drop more steeply at cold, so that some temperature exists where A is above its flash point and B is below its flash point.

If your examiner wrote the question, find some delicate way to formulate the objection, like "we suppose that the vapour pressures of A and B vary similarly with the temperature".