B.D.E
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does autoignition temperature even matter when refluxing a liquid?,
I mean, the max temprature probably going to be very close to the b.p, isn't it?
I'm asking because I now refluxing n-heptane(b.p=98C, a.i=223C) in a soxhelt apperatus, and it just won't reflux efficiently if my hotplate set to
below at least 300C.
Thanks people 
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DavidJR
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Usually not a concern. The temperature of the flask contents is not going to be anywhere near as hot as the hotplate surface, because you're
continually removing heat using the condenser. Also, I would think that after refluxing for a while there would be little oxygen left in the
apparatus.
It might be an issue if your cooling were to fail, causing vapours to leave the flask and potentially be ignited by the hot surface. Good ventilation
to prevent accumulation of flammable levels of vapours should help mitigate that risk.
[Edited on 6-7-2020 by DavidJR]
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Metacelsus
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If you have a system above the autoignition temperature, any leak will result in a fire. But in your case the heptane will not ignite unless it comes
in contact with the hotplate.
In one experiment I was doing while an undergrad, I had to heat my compound to 285 °C in decalin (boiling point 190 °C, autoignition temp 250 °C)
in a sealed pressure vessel. At those temperatures, decalin will transfer hydrogen onto unsaturated hydrocarbons, forming tetrahydronaphthalene as a
byproduct.
I made sure to always do this behind a blast shield! It was called a "bomb reactor" for a reason . . .
[Edited on 2020-7-6 by Metacelsus]
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Refinery
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The concept of vapor leaks and autoignition remains the same, but you added a fear factor of 10bar pressure vessel in the equation. 
If no reacting gas (in this instance, oxygen) is no present due to vapors displacing it or a non-reactive gas being used, autoignition temperature is
null. Only the decomposition temperature effects, and pressure has an effect on boiling point. Conceptually you could inertize reaction vessels with
propane, although this can lead to major issues if parts are not gas tight. I understand that someone here actually did use common propane gas to
protect benzaldehyde in need of actual inert gas.
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B.D.E
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Ace 
Thanks for the useful answers!
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chornedsnorkack
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Quote: Originally posted by DavidJR  | Usually not a concern. The temperature of the flask contents is not going to be anywhere near as hot as the hotplate surface, because you're
continually removing heat using the condenser. Also, I would think that after refluxing for a while there would be little oxygen left in the
apparatus.
It might be an issue if your cooling were to fail, causing vapours to leave the flask and potentially be ignited by the hot surface. Good ventilation
to prevent accumulation of flammable levels of vapours should help mitigate that risk.
[Edited on 6-7-2020 by DavidJR] |
The flashpoint of heptane is -4. Even if your condenser is cooled with, say, +5 degree water, the vapours at the exit of condenser will be above the
lower explosive limit.
Now the issue is whether the vapours can travel from the condenser exit to the hotplate while remaining above the explosive limit.
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Tsjerk
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As long as the condensor is cooled there are no vapors at the exit of the condensor.
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chornedsnorkack
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There would be no vapours as long as the condenser were cooled to absolute zero.
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Tsjerk
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Practically speaking: have you ever tried getting a gas past a condensor which is cooled to 80 degrees below the boiling point of that gas? Assuming
the rate of gas production is normal, like during a reflux.. Let's say the coolant is 20 degrees water and the solvent is heptane, bp 98 degrees.
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