Sciencemadness Discussion Board

misuse of OB% in some cases?

DubaiAmateurRocketry - 2-1-2014 at 10:17

OB indicate how much oxygen can the compound release, or need extra to burn the whole compound. OB is calculated by amount of oxygen that the compound need/release divided by the molecular weight of the compound.

It sounds efficient, but for organic compounds, its often misleading. Spme good example of my point(the purpose of the topic) is when you try to compare compounds like these.......

Nitroglycerin (NG) VS Ammonium Nitrate (AN) .

Nitroglycerin have OB = +3.5%
Ammonium nitrate OB = +20.0%

Wow, so AN can provide more oxygen to burn other fuels than NG ? The answer is no, NG can burn more other fuel than AN. NG can release oxygen with OB +24.7% if the carbon were turned into CO. however AN still release only +20% for minimum use of oxygen. Therefore, NG can burn more fuel than AN do!

Another example is comparing Tetrazolium Nitrate (5ATZN) and 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX)

Tetrazolium Nitrate (5ATZN) VS RDX
[CN4H4]+ [NO3]- and C3H6N6O6
OB: -11.9 and -21.6

Alright, then in a paper on 5ATZN, author says it is less oxygen negative than RDX. It is true to fully burn the compound, RDX need more oxygen, however both compounds can not provide any extra oxygen. while 5ATZN have almost twice higher OB than RDX.

This problem is caused by the flexible use of oxygen by the carbon, so although organic compounds have a negative OB, sometimes it can provide more oxygen for others than what the number shows ! Yes I know many have seen this problem and chemists made another symbol for 100% CO production OB, however its much much less often used then the traditional calculation for OB


[Edited on 2-1-2014 by DubaiAmateurRocketry]

Praxichys - 2-1-2014 at 10:45

Quote: Originally posted by DubaiAmateurRocketry  
Nitroglycerin have OB = +3.5%
Ammonium nitrate OB = +20%

Wow, so AN can provide more oxygen to burn other fuels than NG ? The answer is no, NG can burn more other fuel than AN. NG can release oxygen with OB +24.7% if the carbon were turned into CO. however AN still release only +20% for minimum use of oxygen. Therefore, NG can burn more fuel than AN do!


But isn't the answer yes? Consider ANFO: By that logic, if AN is +20% but the carbon from the fuel oil goes to CO, then it is really +40%.

All you have done is calculated OB with respect to CO.

For CO, NG = 24.7%, AN = 40%
For CO2, NG = 3.5%, AN = 20%

DubaiAmateurRocketry - 2-1-2014 at 11:09

Quote: Originally posted by Praxichys  
Quote: Originally posted by DubaiAmateurRocketry  
Nitroglycerin have OB = +3.5%
Ammonium nitrate OB = +20%

Wow, so AN can provide more oxygen to burn other fuels than NG ? The answer is no, NG can burn more other fuel than AN. NG can release oxygen with OB +24.7% if the carbon were turned into CO. however AN still release only +20% for minimum use of oxygen. Therefore, NG can burn more fuel than AN do!


But isn't the answer yes? Consider ANFO: By that logic, if AN is +20% but the carbon from the fuel oil goes to CO, then it is really +40%.

All you have done is calculated OB with respect to CO.

For CO, NG = 24.7%, AN = 40%
For CO2, NG = 3.5%, AN = 20%


Umm you didnt understand my point.

Does ammonium nitrate contain carbon atoms ? No. Does NG ? Yes, so if all the carbon atoms in those compounds are only burned to CO, NG can release more oxygen than ammonium nitrate. It does not depend on the fuel.

Lets do some stoichiometry

As I said, assume NG becomes totally CO. This will be the equation.

2 NG = C6H10N6O18 > 6CO + 5H2O + 3N2 + 7O(3.5O2)
AN = NH4 NO3 > N2 + 2H2O + O(0.5O2)

1AN have molecular weight of 80.04352
2NG have molecuar weight of 454.1742

Since so 2 NG compound can burn 7 atoms of carbon. 1 AN can burn 1 carbon. The mixture to burn all carbon to CO is...

Nitroglycerin = 100-[454.1742/(454.1742+84.07546)*100] = 15.62016% Fuel

Ammonium Nitrate = 100-80.04352/(80.04352+12.01078)*100] = 13.0474948 % Fuel.

So as you can see, NG can burn more fuel than AN.


[Edited on 2-1-2014 by DubaiAmateurRocketry]

Turner - 2-1-2014 at 11:19

CO2, H2O and N2 are considered ideal products of detonation. Would NG ever decompose to CO? Or even C for that matter in order to oxidize a metal like aluminum?

DubaiAmateurRocketry - 2-1-2014 at 11:24

Quote: Originally posted by Turner  
CO2, H2O and N2 are considered ideal products of detonation. Would NG ever decompose to CO? Or even C for that matter in order to oxidize a metal like aluminum?


CO is more ideal for detonation/propellant since it takes up much more volume than CO2. However for some explosives, CO2's heat can expand other gases such as H2O, N2, then CO2 might be prefered if the total volume in the end is higher than the CO one.

Yes, if enough fuel, such as aluminum is added. The Aluminum will steel the oxygen from water, and the oxygenless water(hydrogen) will steel oxygen from CO2, to form CO, and if enough is added, ultimately C is formed.

Zyklon-A - 2-1-2014 at 11:39

Dang, that's an interesting series of reactions to happen in such a short amount of time.

Praxichys - 2-1-2014 at 11:46

Ahh, I was flawed in my calculations having assumed 20% OB for AN was + O2 and not O.

Ral123 - 2-1-2014 at 11:57

I believe the real issue would be the energy density(and gases generated) of the final composition we chose. A significant problem I suspect is when the OB is negative, different products form at different pressures. My TNP tests gave black smoke, even trough TNP contains enough oxygen to fully oxidize all of the carbon to CO. My Tetryl gives black smoke when burned in CO2 and white smoke when it detonates:
https://www.youtube.com/watch?v=gNamPNunsc0

Dornier 335A - 2-1-2014 at 12:41

Quote: Originally posted by Ral123  
I believe the real issue would be the energy density(and gases generated) of the final composition we chose. A significant problem I suspect is when the OB is negative, different products form at different pressures. My TNP tests gave black smoke, even trough TNP contains enough oxygen to fully oxidize all of the carbon to CO. My Tetryl gives black smoke when burned in CO2 and white smoke when it detonates:
https://www.youtube.com/watch?v=gNamPNunsc0


That's correct. High pressures and low temperatures favour CO2 and carbon, while CO is formed at lower pressures and higher temperatures. TNP doesn't have enough oxygen to oxidize all carbon to CO though. At least not if all hydrogen is oxidized to water first. C6H3N3O7 → 1.5 H2O + 5.5 CO + 0.5 C + 1.5 N2

Carbon monoxide is not a better product than carbon dioxide. The production of CO2 will liberate much more energy than the production of CO. And if no water is present, carbon will be an important product and lower the gas volume.

Ral123 - 2-1-2014 at 13:46

I suspected the loading density would have an effect on the output equilibrium and energy...
So in optimum conditions, one can expect similar energy gram per gram from RDX and CL-20.
Were you the one who said that in the high temperatures and low pressures of a rocket engine CO2 can't form and they are balance for CO?(Witch still seems somewhat strange)

DubaiAmateurRocketry - 2-1-2014 at 14:01

Quote: Originally posted by Ral123  
I suspected the loading density would have an effect on the output equilibrium and energy...
So in optimum conditions, one can expect similar energy gram per gram from RDX and CL-20.
Were you the one who said that in the high temperatures and low pressures of a rocket engine CO2 can't form and they are balance for CO?(Witch still seems somewhat strange)


Propellants prefer CO much more than CO2. burning CO to CO2 will give little increase in performace but the temperature is too high for mechanical challange of the nozzle. Adding a cooling system or changing to a new design Adds weight to the rocket. that if the extra weight of nozzle counts in, the efficiency will become lower. Which is why most rocket engine have the large long flame behind the engine. CO burns outside the engine with air 2CO + O2 = 2CO2

Dornier 335A - 2-1-2014 at 14:17

Quote: Originally posted by Ral123  
I suspected the loading density would have an effect on the output equilibrium and energy...
So in optimum conditions, one can expect similar energy gram per gram from RDX and CL-20.
Were you the one who said that in the high temperatures and low pressures of a rocket engine CO2 can't form and they are balance for CO?(Witch still seems somewhat strange)

No, CL-20 is close to oxygen balanced so it has almost the same energy output at all loading densities (about 6.05 MJ/kg). RDX reaches 5.8 MJ/kg at maximum density.

I have never said that CO2 can't form in rocket engines. A quick calculation reveals that even in a detonation of gaseous ozone and dicyanoacetylene, where the temperature reaches 4800 K and the pressure is only 30 bar (I just chose a typical gas detonation pressure), as much as 8% of the carbon is oxidized to CO2.

Ral123 - 2-1-2014 at 14:43

That CO balancing kinda explains why hydrazine is such a good fuel. Higher energy then CO balanced organic, output is nice small molecules and a reasonable temperature. If I'm not mistaking, Russia has used LOX/N2H4 for some rockets. Guess it's too toxic for the west and they use RP1.