RogueRose
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Potential energy of anhydrous ammonia in explosion
I'm doing research into some accidents in history where anhydrous ammonia tanks have exploded, often in conjunction with other substances and I'm
trying to figure out how to determine what percent of the blast may be attributed to the ammonia.
The burning of ammonia looks like it produces the following: 4NH3 + 3O2 + heat → 2N2 + 6H2O and I would suspect that a large amount of the H2O would
be in gaseous form due to the heat.
I'd like to be able to determine this based on the lbs of ammonia but I'm wondering if there are other factors which may contribute to the
explosiveness such as the containment tank being in the middle of a fire, causing the temps & pressure to rise and if this would increase the
potential energy of the ammonia. Example - if a 7,000lb tank was in a burning building of 1,600F, would the effect of the flame on the tank
(transferring heat to the ammonia & weakening the tank) increase the potential effect of the explosion?
Also, since there is a need for a good bit of O2, might this explosion act like a fuel/air bomb?
The one incident I'm looking at is the recent Texas fert company and am wondering if the AN burning may have liberated lots of O2 as well as when it
went critical liberated the remaining O2 (AN has a positive O2 balance, correct? Isn't this why fuel is usually added to it to make use of the extra
O2 available?)
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Vomaturge
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That's a good question, and one I've wondered about too. Apparently, ammonia has an enthalpy of formation of around -45.9 kj/mol.
http://webbook.nist.gov/cgi/cbook.cgi?ID=C7664417&Mask=1
Its decomposition into nitrogen and hydrogen would actually absorb energy. With some oxygen present, on the other hand, it could burn. The enthalpy of
formation of water is about -241.8 kj/mol.
http://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=1
The reaction could go like this: 4NH3+3O2>6H2O+2N2. This would emit about 1267 kj, after subtracting the energy to break up the NH3 in the first
place. It takes 4mols of ammonia (around 68 grams) being burned completely to produce this energy, so the heat of combustion should be about 18.6
kj/gram. Apparently, ammonia is not especially flammable, and has narrow flammable limits in air https://en.m.wikipedia.org/wiki/Ammonia#Combustion
But, if it's already hot from another fire or explosion, it should add some heat.
Okay, so with the combustible properties of ammonia out of the way, let's look at gas explosions in general. When a cylinder of liquefied gas is
heated, it's pressure will rise along with the temperature. It will add energy, but it should be a small amount compared to the 18.6 mj/kg produced by
burning it. Explosions from overheated gas cylinders usually happen in two stages. First, the tank bursts due to internal pressure increases, and
possibly walls being weakened by heat. The thermodynamics needed to calculate the energy released here are beyond me. Really, this stage can also
happen with inert materials like water or liquid CO2. This is a rapid, mechanically forceful blast, but the total energy is small compared with say,
an equal weight of a chemical explosive. Next, if the hot gas is flammable, it starts to mix with the air and burn. This stage releases much more
energy, in the form of heat. This isn't a detonation, and it usually is not even a fast deflagration, because the fuel and air are not well mixed.
Still, it fills a wide area with flame, and would cause an increase in pressure if it were confined in a building, for instance. If you chose to count
all of this heat and flame as part of the explosion, energy, then a pound of burning ammonia is far more energetic than a pound of TNT! Of course,
that's also true of a pound of charcoal briquettes or candles.
Here is a video of a tank of propane (a much more powerful fuel than ammonia) exploding due to excessive heat.
https://m.youtube.com/watch?v=UM0jtD_OWLU
Note that the initial rupture creates a blast wave strong enough to force condensation in the air, and throw dirt up, before the fuel ignites fully.
The burning fuel creates no such blast wave, but it does produce intense heat for a relatively long time.
As for ammonium nitrate reacting with anhydrous ammonia, I can imagine that happening too. The ammonium nitrate could explode, creating an oxygen rich
fireball (you were right about ammonium nitrate's oxygen balance), and the ammonia could combine with it to create extra heat and pressure.
Incidentally, I think the most powerful ratio would be 2 mols of ammonia to 3 mols of ammonium nitrate, or 87.6% NH4NO3 and 12.4% NH3 by weight. I
would expect the two to combine in the gas phase, after the initial ammonium nitrate detonation. I hope that helps.
[Edited on 3-2-2018 by Vomaturge]
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joseph6355
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Quote: Originally posted by RogueRose  |
I'd like to be able to determine this based on the lbs of ammonia but I'm wondering if there are other factors which may contribute to the
explosiveness such as the containment tank being in the middle of a fire, causing the temps & pressure to rise and if this would increase the
potential energy of the ammonia. Example - if a 7,000lb tank was in a burning building of 1,600F, would the effect of the flame on the tank
(transferring heat to the ammonia & weakening the tank) increase the potential effect of the explosion?
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I believe so. I could be wrong because I'm an amateur, but I think that the expansion of the ammonia inside the tank would add up to the gas expansion
resulting from the deflagration.
First the tank would rupture and eject high pressure ammonia, which then would deflagrate from the heat and flame, adding up to the final velocity of
deflagration.
[Edited on 3/2/18 by joseph6355]
Oh, hello!  |
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RogueRose
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Thanks for the replies. Looking at the "recent" tragedy at the Texas fertilizer plant, it seems that there was close to stoichiometric amounts of NH3
and NH4NO3 to create a near ideal explosion of 540,000/110,000 lbs of AN to ammonia.
For those who like things that go boom, you have to check this out if you've never seen it (and even if you have!!!)
The fun is after 1:20
https://www.youtube.com/watch?v=jzDC3iKbTzY
The amounts of AN & NH3 are listed above. The blast was felt something like 40-60 miles away. And to think, this was all because some guy was
cooking meth in a trailor He needed that 110,000 lbs of anhydrous!!
[Edited on 2-3-2018 by RogueRose]
[Edited on 2-3-2018 by RogueRose]
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joseph6355
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| Quote: Originally posted by RogueRose | Thanks for the replies. Looking at the "recent" tragedy at the Texas fertilizer plant, it seems that there was close to stoichiometric amounts of NH3
and NH4NO3 to create a near ideal explosion of 540,000/110,000 lbs of AN to ammonia.
For those who like things that go boom, you have to check this out if you've never seen it (and even if you have!!!)
The fun is after 1:20
https://www.youtube.com/watch?v=jzDC3iKbTzY
The amounts of AN & NH3 are listed above. The blast was felt something like 40-60 miles away. And to think, this was all because some guy was
cooking meth in a trailor He needed that 110,000 lbs of anhydrous!!
[Edited on 2-3-2018 by RogueRose]
[Edited on 2-3-2018 by RogueRose] |
WOW. That is a big explosion.
Did they keep both of substances mixed together?
Oh, hello! |
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unionised
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No
The explosion was nothing to do with the ammonia on site; it was just the ammonium nitrate (and probably impurities derived from things like the
burning roof of the building) which exploded.
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RogueRose
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| Quote: Originally posted by joseph6355 | | Quote: Originally posted by RogueRose | Thanks for the replies. Looking at the "recent" tragedy at the Texas fertilizer plant, it seems that there was close to stoichiometric amounts of NH3
and NH4NO3 to create a near ideal explosion of 540,000/110,000 lbs of AN to ammonia.
For those who like things that go boom, you have to check this out if you've never seen it (and even if you have!!!)
The fun is after 1:20
https://www.youtube.com/watch?v=jzDC3iKbTzY
The amounts of AN & NH3 are listed above. The blast was felt something like 40-60 miles away. And to think, this was all because some guy was
cooking meth in a trailor He needed that 110,000 lbs of anhydrous!!
[Edited on 2-3-2018 by RogueRose]
[Edited on 2-3-2018 by RogueRose] |
WOW. That is a big explosion.
Did they keep both of substances mixed together? |
Yeah, I couldn't imagine what it was like to be there. I'm guessing the people in that truck were cleaning the seats (and underwear) after the boom.
I'd bet they thought (at least the kid) that they died when it happened. Talk about traumatic and dumb for the elder to sit there and watch it.
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Bert
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Ammonia vapor burns, but not as easily or over as wide a range of mixtures as most hydrocarbon fuel which are gasses at STP. See below- The major risk
is what it does to unprotected body parts- lungs, eyes & etc. It's a pretty poor fuel.
If you have ever seen of the of the mobile "nurse tanks" being repaired they are THICK. Stick one in a fire until it lets go, the energy could be
considerable without even setting the gas cloud on fire.
Attachment: Ammia vapor explosive limits&initiation energy.pdf (1.4MB)
This file has been downloaded 1028 times
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
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2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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RogueRose
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| Quote: Originally posted by Bert | Ammonia vapor burns, but not as easily or over as wide a range of mixtures as most hydrocarbon fuel which are gasses at STP. See below- The major risk
is what it does to unprotected body parts- lungs, eyes & etc. It's a pretty poor fuel.
If you have ever seen of the of the mobile "nurse tanks" being repaired they are THICK. Stick one in a fire until it lets go, the energy could be
considerable without even setting the gas cloud on fire. |
Little confused here about what a mobile nurse tank is, are we talking about O2 tanks?? I can't see NH3 tanks being used in medicine thought I could
be unaware of a use.. Could you describe it or link to a name/image?
I have also been reading a good bit about the potential use of NH3 as a fuel and I believe it may be in the realm of the "hydrogen economy". Does it
need a catalyst to break the 2NH3 into N2 + 3H2 - which i suspect is the target for using NH3 as a fuel.
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Vomaturge
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As far as using ammonia as a fuel, I think it can be used directly. In an engine, you can get the perfect ratio of fuel and air, and the mix can.be
preheated, insulated, etc. You can set up perfect conditions to burn an otherwise hard-to-ignite gas. The biggest issue probably, would be toxicity.
Do you really want your garage filling up with 100 lbs of caustic gas if a fuel line comes loose? On the bright side, anhydrous ammonia would be
completely OTC, no questions asked
A tank of any liquid can blow up if it is heated and the vapor pressure gets too high. Normal home water heaters have caused serious explosions when
the temperature control fails and they heated up to a few hundred degrees C. In the case of the West Texas explosion, the force of the tank bursting
was probably negligible compared with 270 tons of AN. Whether or not the ammonia ignited rapidly, I'm not sure.
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Bert
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Tbey are called "nurse tanks" because they feed liquid to the rig that goes out into the field... like how the Germans called a submarine fuel tanker
used to keep atrack subs fueled for long mid atlantic patrols a "milch cow"
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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