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Author: Subject: Dangers of distilling unstable compounds like nitromethane
cnidocyte
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[*] posted on 22-2-2011 at 11:41
Dangers of distilling unstable compounds like nitromethane


I distilled the majority of the methanol out of nitro fuel containing 25% nitromethane so what I have left is mainly nitromethane. Nitro fuel is mainly methanol so this is a good OTC source of MeOH but my plan was to distill the nitromethane too. I'm having 2nd thoughts about this now, MeNO2 has a NFPA 704 yellow rating of 4, up there with nitroglycerin

I'm even questioning how safe it was doing distilling methanol from a mixture containing this stuff. What would it take to cause nitromethane to ignite or detonate (in the context of things that may happen when distilling it or storing it in the lab)? Would a flash boil (from superheating) be enough to cause an explosion?

[Edited on 22-2-2011 by cnidocyte]
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Melgar
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[*] posted on 22-2-2011 at 12:00


Considering they didn't even realize nitromethane could detonate until a train carrying a tanker of it crashed, I'd say your chances of detonating it are quite remote. It's not easy to set off at all. It's used in engine fuel, you know, without causing any detonation explosions.
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[*] posted on 22-2-2011 at 12:24


But your caution seems at least a bit warranted, given that in a distillation, the flash point of nitromethane is less than the boiling point of methanol. MSDSs tend to overstate things, but it sounds at least feasible that a problem could occur.
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[*] posted on 22-2-2011 at 13:01


Why not lower pressure, have distilled litres of the stuff in batches up to 500 mL on a rotavap, did not take a bp. but had an running pressure of about 20 mbar.

Mine came also from nitrofuel, rotavapped the fuel mix to remove oil, froze majority of the MeNO2 from the MeOH in a -80 freezer (for a week in batches) and filtered it through a glass frit funnel then distilled the majority of the MeOH off with the rotavap, and collected the MeNO2 fraction in the end...

Wanted to do a writeup on it, but as many of my projects it just got forgotten...




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cnidocyte
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[*] posted on 22-2-2011 at 23:24


Quote: Originally posted by bahamuth  
Why not lower pressure, have distilled litres of the stuff in batches up to 500 mL on a rotavap, did not take a bp. but had an running pressure of about 20 mbar.

I'm going to start doing this once I setup a vacuum source.
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peach
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[*] posted on 23-2-2011 at 00:43


This has come up before, nitromethane and fires.

Personally, I much prefer the NFPA system of warnings to the simple flammable / toxic style stickers, at it at least attempts to give some qualification to the hazard in a quick format.

The NFPA is really designed for a fire fighters, so they can quickly get a rough idea by looking at the diamond on the side of the container. They usually arrive once something is already on fire. If you want to stop things ending up in that state, it's a good idea to have a read about the more detailed properties.

The flash point, is merely the point at which the thing will form a flammable mixture with air; base on how much of it will mix with the atmosphere due to the temperature alone. But it needs an ignition source to start it burning, like a spark or open flame. A far worse thing is the autoignition point, as this is the point at which will both form the mixture and ignite without a spark; so an explosion proof lab doesn't help with that. For something like the synthesis of ether, the temperature the process is being run at is not far off the latter, which is why it is so risky compared to simply distilling flammable solvents.

The autoignition temperature of nitromethane is around 400C according to my quick google. For diethyl ether, just 160C!

Distilling under vacuum will lower the temperatures, but you will need to be careful doing things like, disconnecting the vacuum whilst the still is warm; where the mixture of gases and temperatures changes drastically.

The same applies for toxic things. You will often find simple blistering agents being given high NFPA ratings yet, in my opinion, they are more of an annoyance than something that will actually end up inside my bloodstream, messing with internal organs or bits of DNA. One of the first things I do when I see something being described as toxic or dangerous is check whether or not it is also mutagenic or accumulatively toxic, or interferes with metabolism.

Quote:
did not take a bp. but had an running pressure of about 20 mbar.


Probably around 10C I expect.

How I did this for anyone wondering

The main article for Nitromethane lists it's BP at 1ATM as about 100C.

A lot of chemistry articles have a data page associated with them, not immediately visible in the main article. Here's the one for Nitromethane. Under thermodynamic properties it tells me the enthalpy of vapourisation is 38.2 kJ/mol; the energy required to turn a mole of the liquid into a gas. You can often find these by searching google for "[compound] enthalpy of vapourisation kj/mol". For a lot of solvents and easily boiled things, it's around 40.

I use google to change Bahamuth's mBar to Torr (which is the opposite of what I'd prefer), by searching for "20mbar to torr"

I visited this boiling point calculator, enter 760 for the pressure in the top left and 100 for the temperature in the top right. Then 38.3 for the heat of vapourisation and then 15 torr for the new pressure. 10C shows up in the new temperature box

It can also be done, quicker, on a nomograph if the temperature is 100C or above at 1ATM, with a ruler. This one from Sigma has some neat features on it though. Click launch. Now go over to the right and click 'physical properties'. Under the green numbers, on the right, you'll see a scrollable list of compounds. Scroll to nitromethane. Now enter 15 in the box to the left where it says 'mmHg' (which is Torr) and click 'Calc T'

Azeoptropes will mess around with the BP's, and vacuum or pressure distillation will mess around with the azeotropes.

[Edited on 23-2-2011 by peach]




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[*] posted on 23-2-2011 at 03:44


I've distilled rc car fuel which was a mixture of nitromethane, methanol, dye, and a small amount of castor oil. Never had any issues distilling at atmospheric pressure.
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[*] posted on 24-2-2011 at 00:00


I have pure CH3NO2 (99.9%). It is a colorless liquid, looking like water with a faint smell.

I tried igniting some by pouring a little on a glass plate and keeping a flame nearby. I was not able to ignite it this way. It simply doesn't take fire.

I also put a drop of the material in Al-foil and folded this. When this is hit with a hammer, no bang occurs. So, although it may be shock sensitive, it is not highly sensitive.

Finally, I took a drop of the liquid, put this in some Al-foil and wrapped this. Next, I put this in the flame of a propane torch. The liquid quickly boils of and it burns in the flame of the propane torch (it adds a pale orange color to the blue flame).

From these experiments I conclude that nitromethane is not very sensitive and also is is not very flammable. Of course, when there already is a fire, it certainly will contribute to it and probably it can do so violently, but it does not easily ignite. Lamp oil is more flammable.

Probably a very strong mechanical shock is capable of detonating the MeNO2 and then the MeNO2 itself also adds a strong shock, but triggering this is not easy at all.




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