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hankw

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Reaction Equations for Explosives II
« on: July 13, 2005, 03:34:05 AM »
I would have liked to replied to hmx9123 in my last post but unfortunatly the reply button has been deactivated. I get the impression that my post is ok so i repost so i can reply.

I realize my question might raise a few eyebrows. One can get the reaction equations for Nitroglycerin and TNT all over the internet. RDX and HMX were discovered in the 1940s so they should be available too. I am trying to teach myself science. The only way for me is to make it interesting! I saw a demolition on the Discovery channel and became curious as to how much explosives might have been used. Nothing major. By using video, i am estimating the demolition cloud volume and working backwards- reverse engineering. Since RDX is a common high explosive used in demolition I wanted to convert my TNT estimate into what was used. I know RDX or HMX was likely used because of the signature white-gray smoke in the video.

Once i have the reaction equations i can figure our how many moles of gas was produced. From this i can translate my TNT estimate in an RDX or HMX estimate to find out how many kg of explosives was likely used to demolish the building.

I thought this was a cool problem. Hopefully i don't become an enemy of the state for posting this. I appreciate the help.
Hank  


Below is the original post.

I would like to translate TNT quantity estimates to RDX and HMX. I have been unable to find the reaction equations for RDX and HMX. Anybody know off the top of their heads? Or know any links?

« Last Edit: July 13, 2005, 03:37:07 AM by hankw »

Offline Borek

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Re:Reaction Equations for Explosives II
« Reply #1 on: July 13, 2005, 04:54:15 AM »
I know RDX or HMX was likely used because of the signature white-gray smoke in the video.

IMHO white-grey cloud is not from explosives, but it is rather dust from crushed concrete. Do you remember 9/11 dust cloud? (although this one was gypsium mostly).

Quote
Once i have the reaction equations i can figure our how many moles of gas was produced. From this i can translate my TNT estimate in an RDX or HMX estimate to find out how many kg of explosives was likely used to demolish the building.

I think in your search for estimate you are underestimating the complexity of the problem :)

Common reasoning mode on:

As I understand your question you want to estimate amount of energy stored is explosives. However, amount of energy is not all, as the same energy can be freed slowly or pretty fast (that's were detonation speed comes into account). The higher the detonation speed the higher the power. And for me the real question about explosives is - how this power translates into destruction? Probably it depends on what you are trying to destroy?

Hopefully hmx will shed some light (or posts some links)...
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hankw

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Re:Reaction Equations for Explosives II
« Reply #2 on: July 13, 2005, 01:43:53 PM »
Borek,

Indeed, i could be underestimating the problem. I probably would not be the first newbie to bite off more than he can chew! But i learn best when the challenge is above my head. Throw me in the water and i will swim! My method for solving the problem is this: 1) get a ballpark figure; 2) go back and add detail. I have already learned that i will need to take in consideration air turbulence in the clouds. I don't expect to solve this problem overnight. I am doing it for the learning experience.

Here is a little more about the problem. On the physics side, i can infer the volume of clouds that was inside a floor of the demolished building. This volume is way larger than the volume of air the floor contained at normal atmosphere pressure. Simple subtraction and one is left with a volume of air i call overpressure. This overpressure came from the explosives, directly and indirectly.

http://www.absoluteastronomy.com/encyclopedia/e/ex/explosive_material.htm
From the above link (start with Volume of products of explosion) i have learned that from the chemical reaction equation i can find out the moles of gas produced. From this i can get a volume measurement in liters. Then i can infer from the overpressure volume how many moles of compound produced the overpressure. From this i can infer the amount of compound used in kg. Am i on the right track? I don't want to be doing any pseudo quack science here! :-P

Some thoughts on your "common reasoning mode" comments. Regarding RDX i have read that "gases produced by a high explosive can dilate to 10,000 times the original material volumeā€¦almost instantaneously." "They undergo detonation at rates of 1,000 to 8,500 meters per second." This sounds pretty fast. Would the explosion speed affect the volume of gas produced?

Regarding "power translates into destruction". Does an explosive compound create more than just heat and gas? Am i over-simplifying? One thing i have to consider in the "overpressure" is that it contains air volume that was heated up by the heat released in the explosion reaction. Hmmm... that is probably why my TNT estimates seemed rather large! Is there a simple equation of rule-of-thumb to account for this?

Since it appears you have considerable science knowledge, maybe you can suggest what things i need to address in this problem to get a credible estimate? I take my science studies seriously!

Thank for the guidance.
Hank

Offline hmx9123

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Re:Reaction Equations for Explosives II
« Reply #3 on: July 13, 2005, 03:03:27 PM »
OK, there's a lot of misnomers here.  

1. RDX is not the most common demolition agent.  It is used primarily by the military.  About 95% of all explosives now are ANFO (ammonium nitrate fuel oil) explosives.  When doing controlled demolitions of buildings, usually a gel is used: nitroglycerin dissolved in nitrocellulose.

2. A very small amound of explosive is actually used in the demolition of a building.  They are placed at strategic points and gravity is used to collapse the building.

3. The dust cloud you see has little, if anything, to do with the energy left behind by the explosive.  It is, as Borek said, just parts of the building.  By the time you see the dust from the explosion, the shockwave has already long since passed that way.

4. Explosives do not necessairly form gas when they detonate.  All explosives release a tremendous amount of energy, this is true, and most release gas, but not all.  The expansion is from the relase of energy instantaneously.  Take cuprous acetylide, for instance: you only get copper and carbon out as detonation products, carbon as solid, copper as solid or liquid.  No gas is produced, but it is a brisant explosive.  The power comes from the fact that most high explosives release enough energy to locally heat the area to 4500K or higher and generate an overpressure (above atmospheric pressure) of around 210,000 atm.  This forms the shockwave from the compressed medium around it and on average is about 0.2mm thick and travels between 4000-10,000 m/s.

The website you have listed is interesting and has some good information to get you started.  It neglects a lot of very important things if you really want to calculate volume of gas, etc., though.  You'll need more than a video to do it, too.

It sounds like you are into the physics of explosives.  I would strongly recommend one title to you for this: Explosives Engineering by Paul W. Cooper.  You can get it at Amazon.  As long as you have a little calculus, the book will be perfect.  You don't really need the calc, but it helps to have it so you're not scared off by the math.  It goes over the physics, calculations you would want to make, the thermodynamics, the decomposition products, etc.  I think you'll really get into it.

There is no simple answer to what you're trying to do.  The fact is that if you try and calculate based on the dust cloud, you might as well guess.  I suppose if you wanted a very rough estimate of volume, you could use the ideal gas law, but you're so far from STP at explosive conditions, you'd really be grasping at straws.  The relation:

PV=nRT

Doesn't hold up so well under extremely non-ideal conditions, but perhaps you could get a vague ballpark of volume by inserting the average numbers I quoted above.

Offline Borek

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hankw

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Re:Reaction Equations for Explosives II
« Reply #5 on: July 14, 2005, 03:31:57 AM »
hmx9123,

I somehow thought RDX was used. I read about a bridge taken down using RDX. I read elsewhere that civilian use of RDX amd HMX is demolition. So i thought RDX would be used on large buldings. Also, since this building caught on fire before collapse, i inferred that a high temperature explosive was used. I could be wrong, i am inferring based on what i can observe.

Keep in mind that because of fire, smoke was produced. This marks the otherwise invisible air. When the collapse occurred, well-defined clouds were ejected from the sides of the building. Would this not faithfully represent the air volume inside the building? I can measure how much clouds were ejected when the first floor collapsed. This likely represented what other floors ejected when collapsing one floor. Using this measurement i can then measure the cloud volume at each floor and infer how much cloud volume the first floor ejected. I then account for air turbulence. I have read that a rule of thumb, as long as clouds are expanding and are well defined (no noticeable diffusion with the air), that one subtracts 1/3 of the volume. So i am left with a volume of air that was inside one floor when the explosives went off. I then account for the floor's original air volume. I calculate its size when heated by explosives and then subtract. I am left with a volume of air representing the overpressure caused by explosives. Am i oversimplifying this?

From my understanding of TNT, in the reaction it produces three things: 1) heat; 2) gas; 3) carbon. If TNT was used then the overpressure volume was made up directly by the produced gas and indirectly by the heat. Is this right? Is there a rule of thumb that establishes "total" air volume that a certain amount of explosive produces? Could i then work backwards and infer how much explosives was used?

With the measurements i have outlined, what is missing that would NOT make a credible estimate? I am confused. I wish i could post the video but i am unable to video capture my VCR.

Hank



« Last Edit: July 14, 2005, 03:38:37 AM by hankw »

Offline jdurg

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Re:Reaction Equations for Explosives II
« Reply #6 on: July 14, 2005, 12:47:53 PM »
The whole reason why ANFO is used and not RDX/HMX is price.  RDX/HMX, if I'm not mistaken, cost a fair amount of money more than ANFO.  As a result, it's much more cost effective over the long haul to use large quantities of ANFO than it is to use the same quantities of RDX/HMX.
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Offline hmx9123

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Re:Reaction Equations for Explosives II
« Reply #7 on: July 14, 2005, 03:35:05 PM »
AFAIK, RDX and HMX are rarely used in building demolition because of cost.  Blasting gelatin is cheaper and easier to obtain.  RDX and HMX are also fairly poor at destroying concrete because of their high brisance.  They are used for blowing up bridges by the military, and maybe by civilians, but that's a little different game than controlled demolition of a building.  You'd need to talk to an explosives engineer to find out exactly what is used.

Jdurg is correct, ANFO is used because it's cheaper than dirt.  Obviously it's not used for controlled demolition of buildings because it would require a lot and would blow bits of the building everywhere.

When you mention 'high temperature' explosive, I cannot think of one that is not.  All explosives produce very high temperatures and thus could create fires.  If there is a big fire, most likely something in the building is burning.  If it's just a flash, it could be an afterburn for a poorly oxygen balanced explosive.  It's hard to tell without seeing the video.

Perhaps you could estimate air volume by the cloud, but you would be better off estimating it by the size of the building.  Just knowing the dimensions of the building gives you a good idea of air volume inside, and that is a lot more accurate than trying to screw around with smoke clouds.

When TNT explodes (which is irrelevant here since the building almost assuredly didn't come down by TNT) you get:

C7H5N3O6 --> 1.5 N2 + 2.5 H2O + 3.5 CO + 3.5 C

But, in aerobic conditions, you usually get an afterburn, you you're left with 7 CO2 in place of the CO and the C.

If TNT was used, the overpressure came from the energy released as heat, which served to heat the local air but also heat the gasses produced.  The volume of the gasses produced also contribute to this, but really it's their high temperature that causes them to expand so violently.

There is no rule of thumb that establishes how much volume a certain amount of explosive produces.  You are not dealing with a closed system.  You might be able to guess at it with some engineering equations, but again, I refer you to the Cooper book.  Maybe alt.engr.explosives may be a better place to ask, too.

The problem with the video is that you're assuming that the volume of the smoke cloud somehow is related to the volume of air of the explosive after detonation.  That simply isn't the case, so everything else from that point on is flawed.

hankw

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Re:Reaction Equations for Explosives II
« Reply #8 on: July 15, 2005, 05:11:22 AM »
I checked out that book at amazon.com. Through the amazon search engine i got the reaction equation for RDX. C3H6N606 -> 3N2 + 3H20+ 3CO.

When i refer to high temperature, i really mean the explosive had a high melting point. RDX melts at 205C and HMX melts at 285C. They can withstand the temperature of fire without premature detonation or failing. Add fire proofing and they can withstand hotter temperatures. This is why i don't think any other explosive was used in the building collapse.

Thanks for the forum suggestion. The types of questions i am getting into seem to be more physics based or explosive-specific.

Thanks
Hank



Offline Borek

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Re:Reaction Equations for Explosives II
« Reply #9 on: July 15, 2005, 06:13:04 AM »
They can withstand the temperature of fire without premature detonation or failing. Add fire proofing and they can withstand hotter temperatures. This is why i don't think any other explosive was used in the building collapse.

Where do you see need to whithstand fire and high temperatures in building destruction?

Before the destruction building is ripped of everything that can catch fire. What is left is just a skeleton, already weakened by removing some elements. There will be no fire before the explosion, unless the concrete can burn. Flames after the explosion can show if the explosion products are flammable ("poorly oxygen balanced explosive", to quote hmx).
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Re:Reaction Equations for Explosives II
« Reply #10 on: July 15, 2005, 06:21:14 AM »
RDX and HMX are also fairly poor at destroying concrete because of their high brisance.

Could you elaborate? Seems to me counterintuitive - I would rather expect high brisance to be highly effective in case of concrete. But then I have no idea how the brisance is defined.
« Last Edit: July 15, 2005, 09:34:51 AM by Borek »
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hankw

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Re:Reaction Equations for Explosives II
« Reply #11 on: July 15, 2005, 07:17:12 AM »
Borek,

From my research i think what hmx9123 means is that the explosive is too good. In quarrying and mining, one does not want to use HMX because then all one gets is dust. It appears that RDX and HMX are used to "cut" steel in demolition- thus its use on steel bridges.

In response to why the building was not gutted, maybe the building was contaminated or something? I don't know. I saw it on TV and kept a clip of it. All i know is that the building looks intact and it had a decent fire going. Since the smoke marked the air- like adding dye to water- one is able to credibly measure the amount of air ejected when the first floor collapsed.

As far as i know, the only thing other than air pressure that could have created the clouds is air turbulance. But someone worked out a rule-of-thumb, take off 1/3 of the volume for this. Since the cloud formation occurs in less than 2 secs, i don't think much could have occurred other than expanding gas. But don't take my word for it, i'm a newbie!

So i have two obstacles i need to overcome for this problem.

1) Estimate total cloud volume based on data obtained from picture analysis. (a physics problem)
2) Estimate the the total air volume created for different types of explosives. (a chemistry problem)

In my searching on the internet i found something that relates to what i am doing.

http://globalresearch.ca.myforums.net/viewtopic.php?t=318

In this link (read the red comments about half way down the page) someone has worked out the rule-of-thumb for air produced by the high explosive Amatol. Can anyone verify the accuracy of this work? Is this real or bogus? This may be just what i am looking for. I have trouble following some of the math. Does the calculations include the surrounding air volume or just deal with the gas produced by the explosive reaction?

I think this problem is doable and maybe is only about a few steps away from being solved.

Thanks for *delete me*
Hank

« Last Edit: July 15, 2005, 10:06:30 AM by hankw »

Offline Borek

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Re:Reaction Equations for Explosives II
« Reply #12 on: July 15, 2005, 09:55:42 AM »
I think this problem is doable and maybe is only about a few steps away from being solved.

Bulding my Mother lives in has 8 small flats on every level. Every flat is about 40 square meters and 2.5 meter high. It gives 8*40*2.5 = 800 cubic meters. Add all the communication areas and you get about 1000 cubic meters per every level.

How much explosives could be used for one level? Let's say it is 1kg of RDX. Using your reaction equation - every 222g of RDX produces 200L of gases at STP (treating water as gas). It gives about 1 cubic meter of gases per 1 kg of RDX. Let the temperature of gases be 3000 K - it makes volume about ten times as large. So 1 kg of RDX produces amount of gases that may account for about 1% of the gases thrown away from the building because of collapse. You will be not able to estimate anything, as the accuracy of your gas volume estimation will be much lower.

Besides, I believe hmx that other explosives are used. Demolition must be cost effective.
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hankw

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Re:Reaction Equations for Explosives II
« Reply #13 on: July 15, 2005, 11:27:55 AM »
Borek,

Thanks for estimating the total air for RDX. Are you saying that only 1% of the air volume produced in a demolition is caused by the explosives? If that is true then i am at a loss to explain the large volume of air i have measured. I adjust for air turbulence and the volume is still very big. If unaccounted air volume is not from explosives, and if it is not from air turbulence, then what other factors would explain it? In my internet searching i have not come across anything else. All other factors would make the volume smaller. Example, if the compressed air pushed out the walls, then energy would be lost due to work. True, it could be my error in measuring, but i don't think i could be of by so much. It is not too hard to measure objects in a picture with reference points.

I am truly puzzled.

Hank


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Re:Reaction Equations for Explosives II
« Reply #14 on: July 15, 2005, 12:58:39 PM »
You also appear to be assuming that the air is evacuated from the space in completely the same manner on all sides of the building.  Due to building setup or from the design of the implosion, the air coming out may be greater on one side than the other, and the side you're seeing is the one which had the greater explusion of air.  Also, in an implosion everything doesn't just fall down on top of each other.  The inside of the building basically crushes itself, so the 'expelled air' may also contain air from various levels of the building.  Basically, what I'm trying to say is that you cannot be even remotely confident that the dust cloud you're seeing is from just a certain level of the building.
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