Sickman
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Liquid N2 As Moderator In Nitrations
I've been thinking lately that liquid nitrogen may be useful
in moderating exotherm in nitrations, espeacially when
safety is a factor, and it usually is.
I've been thinking that liquid nitrogen can be fed directly into
the reaction mixture, with stirring, at a specific rate to maintain the desired temperature, which would eliminate the need for external cooling with
ice and dry ice in reactions which require a cooler temperatature such as the safe production of nitroglycerin ect.
Such a setup may present several advantages including a more direct control of temperature while being inert to the reaction and also the nitrogen
bubbles will likely help in the uniform mixing of reactants when combined with stirring.
I believe that if a reaction got out of hand and began to runaway feeding liquid nitrogen into the reaction mixture, with stirring would very
effectivly 'kill" the threat dead in it's tracks, and would also allow the reaction to continue without interuption. It has been a precaution in
nitrating glycerin to dump reactants in ice water if things get scary. Such an event would dilute the acids significantly and lower yields if not
completly ruin the entire synthesis. While liquid nitrogen would just bubble out leaving the acids at thier original concentrations.
Does anyone have any thoughts about using liquid nitrogen in moderating nitration reactions? See any possible pitfalls ect.?
Thank you for your replies in advance!
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12AX7
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Hum, stirring it in? I don't know if you'd get it to actually combine, but I also have bad images of what happens when you stir water into molten
lead for instance.
Tim
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The_Davster
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I think LN2 would be best used to be poured directly in if the reaction begins to run away...
It might work, but kinda more difficult than an ice bath eh?
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Nerro
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Wouldn't all dissolved product immedeately crystallize? Also, the cooling could be a very local process this way. LN2 doesnt take up a whole lot of
heat before it vapourizes.
It's also pretty expensive, ice baths cost nothing and when the stirring is vigourous enough a carefull dripfeed of eg. toluene into the nitration mix
should do the trick. Why change a winning team right?
[Edited on Sun/Jun/2006 by Nerro]
#261501 +(11351)- [X]
the \"bishop\" came to our church today
he was a fucken impostor
never once moved diagonally
courtesy of bash
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nitro-genes
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| Quote: | Originally posted by Nerro
LN2 doesnt take up a whole lot of heat before it vapourizes.
[Edited on Sun/Jun/2006 by Nerro] |
Thats right, when i'm in a hurry on the lab, I just grab the eps with bare hands (two fingers that is) out of the LN2. If you do it quick enough you
won't get any frost bite at all because of LN2's low specific heat capacity...
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neutrino
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What exactly are esp’s? In any case, the reason that works is because the nitrogen evaporating from the heat of your hand forms a gaseous layer
between your finger and the liquid nitrogen. This is the same reason you can hold the stuff on your tongue.
> liquid nitrogen would just bubble out leaving the acids at thier original concentrations.
Sorry, the nitric acid will evaporate away with the nitrogen. Nitric acid has a significant vapor pressure at most temperatures, so a gas blown
through it will pick up some of the acid.
What temperature are you planning on conducting the nitration at? Remember the old rule of thumb: decreasing the temperature by 10*C decreases the
reaction rate by half. Too cold and nothing happens, or the reactants freeze solid…
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Sickman
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My main interest in using liquid nitrogen is just having it as a stand by in the event of a runaway. The use of liquid nitrogen as a sustitute for ice
or refrigeration was just a secondary thought, the main thought is stopping a runaway and preventing explosive mishaps.
Although, I think I have reason to believe that the gaseous nitrogen being released from liquid nitrogen, being bubbled though a reaction mixture
could be a substitute for ice in controlling temperature and it's widely available to me.
I believe what triggered my thinking towards this area is when I was reading in The Chemistry and Technology of Explosives by Urbanski. As I studied
the many methods developed for nitrating materials such as phenol and toluene I noticed that most of the nitration vessels are equipt with mechanical
stirrers, which makes sense to me, but what was also used and what is kind of confusing to me in light of some of the above statements is the use of
compressed air, which is continuosly bubbled through the nitration mixture.
I thought that maybe the compressed air being bubbled was being used perhaps to help regulate temperature and assist in the uniform mixing of
reactants, but if what is said above is true it would seem that this compressed air would carry away some of the acids with it which would be
undesirable.
I reasoned that liquid nitrogen could be allowed to evaporate/boil and the resulting "cool" nitrogen gas could be bubbled through the nitration
mixture the same way compressed air may be while eliminating the prescence of unnessesary oxygen that could contribute to the oxidation of reactants.
It should be noted that in Urbanski, when it is describing these nitration vessels, the scale is industrial, in the tens of kilograms of reactants and
that nitrogen vapors rising from the vessels are ultimatly recovered as nitric acid in kilogram amounts from a single batch.
Furthermore, I would very much like to know why compressed air was used in these nitration vessels described in the literature.
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Marvin
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Compressed air is for stirring and also helps remove oxides of nitrogen. I don't recall it being used in phenol/toluene nitrators but I do recall it
being used in glycerine nitrators where mechanical stirring is forbidden. Nitrogen gas would not improve matters, its the nitrating mixture that
oxidises.
Liquid nitrogen would invite non detonating explosions as hinted at and is not useful for reasons of heat capacity allready mentioned.
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