Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 65 users online :: 3 registered, 0 hidden and 62 guests


Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

Registered users: Bing [Bot], Google [Bot], Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

propane newbie

A place to ask general spud cannon related questions.
Sponsored 
  • Author
    Message

Re: propane newbie

Unread postAuthor: DYI » Sun Sep 28, 2014 4:55 pm

Regarding the use of a heavier / higher friction potato slug: using a propane/air mixture starting at atmospheric pressure, the maximum theoretical pressure is about 120psi. This would be, roughly, the pressure generated in a sealed, perfectly insulated chamber with no significant contribution from shock waves. The question becomes, is the pipe you're using rated for the maximum combustion pressure? If it isn't, I'd be wary about using it at all, given the shot-to-shot variability. I'm aware that most, if not all firearms will not withstand the load's maximum potential (plugged barrel) pressure, but they are much more carefully designed and manufactured. Counting on a potato gun to not generate its maximum pressure when people are nearby is a dangerous practice.

Regarding the acetylene question: the bond is not "stronger" with acetylene, it is higher potential energy. "Stronger" would imply harder to break or more energetically favorable than other structurally similar configurations, and acetylene is absolutely not either of those things. Because the bond is high energy compared to other reachable configurations, and the energy required to initiate such a configuration change is comparatively low, acetylene is fragile. If anything, it would be more appropriate to call the bond "weak".
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Re: propane newbie

Unread postAuthor: Chris spud » Mon Sep 29, 2014 1:26 am

120psi, that's more than 8 bar. The combustion chamber PVC is rated to 2 bar...not good. I did wrap a lot of duck tape around it, should it explode, hopefully that would mitigate the effects of flying shrapnel. Maybe then I'd rather stick with lower friction. How any of you experimented with lubing the front of the barrel with a light lubricating oil, or something like WD40? Does it work? I'm thinking that as the spud travels along the barrel the pressure might drop, and acceleration decrease. If you decrease friction for that last bit in the barrel, it might just help a bit

RE acetylene. single carbon carbon bonds: 348kJ/mol - that is 348 kJ of energy (typically in the form of thermal energy) required to break one mole of Carbon-Carbon single bonds. Carbon-carbon double bonds: 614kJ/mol. Carbon-carbon triple bonds: 839kJ/mol. Bond energy is defined as the amount of energy required to break (a mol) of specific bonds between atoms.

This is where your investing energy, it's called the activation energy. If you look at the energy profile for combustion reactions, you'll the see the graph curves up - that's the amount of energy you need to get the reaction going - to break the reactant bonds, so the atoms are free (activated complex forms) to rearrange and form product bonds.

Acetylene has a very strong carbon-carbon triple bond, it should actually be quite stable, low or low energy due to this. When it reacts with oxygen, with complete combustion, you get water and carbon dioxide gases. If you look at the heat of combustion for acetylene in kJ/mol it's 1300, which is less than propane or butane. This is because a mol of acetylene burns (complete combustion) to provide 2 mol of carbon dioxide, and 1 mol of water. Compare that to butane, where one mol of butane reacts with oxygen to given 4 mol CO2 and 5 mol water. In the reaction, butane also requires more oxygen than acetylene does.

So...I'm a bit confused as to why acetylene seems fragile, or very reactive as the carbon-carbon triple bond is quite strong. My thinking as to why it's dangerous, is because of the burn rate which is high and can even lead to detonation where all the energy is released at once with shock waves travelling supersonic speeds (compared to a few meter per second). I'll try and research it a bit more. thanks
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: Chris spud » Mon Sep 29, 2014 2:42 am

Ok...this makes sense: double and triple bonds between carbon atoms are pi bonds, as opposed to sigma bonds (single bonds). There's stronger overlap with sigma bonds, compared to the weak overlap between p orbitals for additional bonds. So, alkene and alkynes are more reactive compared to alkanes... of course. It's just funny to me that despite the fact that pi bonds are weaker, the bond energies are higher for double and triple covalent bonds... weird..

so, it seems that acetylene then would be unstable, reactive, despite having a strong triple bond between it's two carbon atoms - seems almost contradictory

Molecular nitrogen has a triple bond (N2), and it's bond energy is 941kJ/mol - a bit stronger than the carbon carbon triple bond, however, molecular nitrogen is very stable, low energy. I guess you have to go into molecular orbital theory to really see why acetylene is reactive, and nitrogen inert.

At the higher temperature of combustion, I wonder if nitrogen in air does not react with oxygen to form nitrogen oxides. Anyone know? This would not be good, as it is an endothermic reaction which will take up some of the thermal energy produced through combustion

EDIT: this adds to the confusion, the bond order for both nitrogen and acetylene is 3... therefore, the triple bonds in both should result in high stability (as bond energy for these triple bonds indicate)..yet...confused. Still researching
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: DYI » Mon Sep 29, 2014 9:30 am

This is actually a pretty interesting question, now that I'm thinking about it. I'll bet that Jimmy will have a more satisfactory answer. The listed heat of formation for acetylene (+225 kJ/mol) suggests it to be energetically favourable for acetylene to decompose into carbon and dihydrogen, which is what we observe in practice as well.

Also: nitrogen oxides are produced in combustion processes in air, and they do drain energy from the reaction into bond formation. There are various other important processes acting as temporary or permanent sinks of the heat of combustion as well.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Re: propane newbie

Unread postAuthor: mobile chernobyl » Mon Sep 29, 2014 11:05 am

I don't really have anything extra to add to this conversation... but - I have, as a spudder, going through chemistry classes in college wondered about the direct effect of activation energies in spudding regarding different chemistries and ratios and I'm glad a conversation has started on this topic!
  • 0

User avatar
mobile chernobyl
Colonel
Colonel
 
Posts: 732
Joined: Sun Dec 03, 2006 11:53 am
Reputation: 9

Re: propane newbie

Unread postAuthor: jimmy101 » Tue Sep 30, 2014 2:25 am

Activation energy has no effect on the amount of heat (energy) released in a chemical reaction. All the energy used to raise the reactants to the energy peak is recovered as the products fall down the energy profile after the rate limiting step. The heat of combustion is calculated as the difference in energy between the reactants and products so the activation energy doesn't contribute. In spud guns (and combustion in general) the only thing the activation energy effects is how much energy is required to get the reaction going. Generally there is a minimum energy density (energy per unit volume) required and that volume can be a tiny tiny fraction of the total reaction volume. In practice the ignition energy is tiny compared to the chemical energy in a spudgun. It has been estimated that it takes about 0.5 millijoules in a volume less than 1mm^3 to get most gas mixes going, regardless of how big the combustion chamber is. If the ignition system supplies much more energy than that the extra energy is basically wasted since it is more than is needed for ignition but much less than the energy that combustion will release (so the extra ignition energy really doesn't boost the gun's performance).

So, an ignition system that works reliably with a mini-spudgun will work just as reliably in a giant cannon. Though there may be better ignition systems for a large gun, they won't be any more reliable in starting combustion than a simpler system.

A catalyst lowers the activation energy of a reaction but doesn't change the amount of energy released. So a catalyst makes it easier to start a reaction but doesn't in anyway change the progress of the reaction. And a catalyst is unchanged by the reaction.

Edit: The important thing about the activation energy is it is what keeps the fuel air mixture (or gasoline in your gas tank, or a hunk of firewood laying on the ground) from spontaneously combusting. That of course would be a bad thing (in spud guns, cars and firewood piles).
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Re: propane newbie

Unread postAuthor: Chris spud » Thu Oct 02, 2014 3:52 am

Activation energy is equal to the bond energy of the products. Two ways to calculate heat of combustion of change in enthalpy. One is like you described, the other is by subtracting the energy released from the energy absorbed (where the energy released on the energy profile is from the highest peak to the bottom (products), and the energy absorbed is from the reactants (where the reaction starts) to the highest peak.

The heat of combustion for acetylene is nothing special - for a similar size molecule like ethanol or ethane, it's about the same, yet..we know that burning ethanol is not nearly as dangerous as acetylene. I think the danger with acetylene is because of the burn rate which is higher. How can nitrogen be stable and acetylene unstable (related to the burn rate I believe)? We always teach the kids that nitrogen is stable because of the strong triple bond.. triple bonds are stronger than double bonds is stronger than single bonds (it takes more energy to break a triple bond, than a double, than a single)...the contradicting thing is the stability of nitrogen, and 'unstability' of acetylene (both should be as stable as the other because both has a bond order of 3)..so what's going on?
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Sponsored

Sponsor
 


Re: propane newbie

Unread postAuthor: Chris spud » Thu Oct 02, 2014 1:13 pm

OK...the back screw-on end blew into bits today.. using LPG (70% propane, 30% butane), calculated that we needed 245cm3 of fuel for a 69cm long combustion chamber (11cm diameter)...yes, it's huge. Barrel = 2m. It was my fault..loaded the spud parallel to the barrel..note to self - don't do that again. I slid the spud down the barrel, and thought that the friction was not that much, should be fine. What seemed to happen was detonation, instead of deflagration - I know this happened bacause the spud still flew out, but the back was blown off - so the shock wave from detonation moved so fast that the spud could not acceleration fast enough to relieve the pressure. The increased pressure increased the rate of burning to the point where the shock wave was compressing the gases in front of it sufficiently to it's auto ignition temp, so where normally the shock wave travels in front of the reacting gases, now the shock wave and reaction between gases coincide. That's what I read on wikipedia. Man... it was an explosion. When you get the exact amount of fuel you need, your efficiency increases. We've got twin spark, a good fan to mix the gases, and one HUGE combustion chamber with the exact amount of LPG we need (even the air pushed out of the chamber as the fuel is loaded is taken into consideration) = fun (as long as the screw on back remains in tact). So, need to do the fan again. Any suggestions as to how to ensure that this does not happen again?

I thought that since were not using pure oxygen, and/or acetylene, we could optimize the gun safely...seems there's limits even with propane
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: jimmy101 » Thu Oct 02, 2014 5:00 pm

Activation energy is equal to the bond energy of the products.
Nope. Activation energy has nothing to do with the bond energy of the products (or the reactants). Activation energy is a kinetic parameter that controls how fast something happens and whether it happens spontaneously or not. Activation energy has no correlation with the energy of the reactants, products or the difference in energy of the reactants and products. There are high energy (highly exothermic) reactions with low activation energies and high energy reactions with high activation energies. For example, nitroglycerin has a very low activation energy whereas C4 has a very high activation energy. The activation energy has no effect on the energy released it simply controls how easy is is to get the reaction going. The difference in energy in a pound of nitroglycerin versus a pound of C4 cannot in any way be rationalized by their activation energies. The ease with which they can be ignited has everything to do with their activation energies. (Random factoid: there is more chemical potential energy in a pound of propane than there is in a pound of C4.)

Two ways to calculate heat of combustion of change in enthalpy. One is like you described, the other is by subtracting the energy released from the energy absorbed (where the energy released on the energy profile is from the highest peak to the bottom (products), and the energy absorbed is from the reactants (where the reaction starts) to the highest peak.
Nobody ever does the calculation that second way, mostly because the activation energy is hard to measure and is a kinetic parameter. The change in energy (or heat released) in a reaction is much easier to measure using just the starting material and the products. That way you are measuring a steady state (or equilibrium), which is time-independent and much easier to do.

People often confuse thermodynamics with kinetics. Thermodynamics describe the energy changes in a chemical reaction. Kinetics describe how fast a reaction proceeds. There is no reliable relationship between the thermodynamics of a given process and the kinetics of that process.
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Re: propane newbie

Unread postAuthor: Chris spud » Sat Oct 04, 2014 9:12 am

That's interesting... if reactant bonds need to be broken in order to start the reaction, then one would think that the activation energy is equal to the bond energy of the reactants. OK, I need to go read up and do a bit of research - any good links?

So why is acetylene reactive, while molecular nitrogen is not? The carbon atoms is slightly bigger than nitrogen, also the nitrogen-nitrogen triple bond is slightly shorter than carbon-carbon triple bond (10pm).

Also, maybe the way the molecules collide has something to do with it - kinetics. Molecular oxygen is not polar, nitrogen also, but the carbons on acetylene molecule is slightly negative, and the hydrogen slightly positive.
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: jimmy101 » Sat Oct 04, 2014 2:03 pm

The Google http://en.wikipedia.org/wiki/Activation_energy is OK.

A typical reaction energy diagram for an exothermic reaction:
Image
The energy put into the system to get a small number of molecules high enough in energy to start the reaction is 100% recovered when the intermediate high-energy state falls down to the products. The net heat released (delta-H in the diagram) is used to raise the energy of more reactants above the activation barrier. When they in turn fall in energy to create products the additional energy released can activate more reactants. Repeat until all the reactants are gone. In each cycle the activation energy is (mostly) recovered and each cycle releases addition energy.

It is difficult to directly compare two molecules based on just the bond energies, particularly if the two molecules are radically different. For acetylene vs molecular nitrogen (N2) the bond energy is a somewhat vague concept since usually you don't just brake bonds and end up with atomic fragments. The fragments react with something creating products, this is certainly the case in all combustion processes. The measured energy of a reaction is the difference in energy between the reactants and products. So, to compare acetylene with nitrogen you need to know the energy of both the reactants and the products. The products would be CO2+water versus various nitrogen oxides and the bond energies of the reactants is a very poor predictor since those energies have no "knowledge" of what the products are going to be.

Overall the triple bond in acetylene is extremely strong but it isn't three times stronger than the single C-C bond in ethane. If you convert the triple bond to a double bond and add two hydrogens (to make ethylene) the energy of the molecule decreases and you release energy. The 2 CH bonds are lower in energy than the single triple bond. The 2 C-H bonds are "stronger" than the difference in strength between the carbon-carbon triple bond and the carbon-carbon double bond.

Polarity sometimes does and sometimes does not affect reactions. Collision geometry always affects a reaction rate but generally the collision geometry is random and the only real external control is from temperature, which controls both collision energy and frequency.

"Kinetics" describe just the rate of a reaction and says nothing about the energy released in a reaction. "Thermodynamics" generally describes just the energy released (or absorbed) without saying anything about the rate of the reaction. Real world chemistry (including a combustion spud gun) are affected by both kinetics and thermodynamics. Extensive knowledge of just one domain is rarely able to predict how the real world operates. Few chemical systems on earth exist at their thermodynamic minimum. Life exists at molecular energies well above the lowest possible energy state of the atoms involved in the molecules (in other words, virtually everything that is alive can be burned and the result of burning is always death). Life exists because the molecules are kinetically trapped in their high energy state. They should fall to lower energy (death) states but there isn't an energetically feasible path from the high energy molecules of life to the low energy molecules (or atoms) of death. If you heat the system up you provide kinetically feasible pathways for decomposition and the system progresses to death spontaneously.
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Re: propane newbie

Unread postAuthor: Chris spud » Sun Oct 05, 2014 3:16 am

very interesting, thanks. It's always more complex that you'd first imagined - something I've learnt along the way. How about this explanation for the inertness of molecular nitrogen VS the unstable-ness of acetylene: the activation energy for the combustion of acetylene is very low (kinetically favourable) compared to N2 reacting with O2 (kinetically unfavourable). The combustion reaction for acetylene is exothermic (thermodynamic-ally favourable), while the N2/O2 reaction is endothermic (thermodynamically unfavourable).

So, when an acetylene molecule reacts with oxygen, a lot of thermal energy is released which is sufficient to cause the next molecule to react with oxygen (collision theory - chemical reactions for gases is all about reactant molecules colliding with sufficient force to break bonds and get to activation complex), and this continues until all fuel is spent.

With N2/O2, if you provide the activation energy, the amount of energy released is not enough to cause a chain reaction.

So, acetylene is unstable because it's reaction with O2 in air is both kinetically and thermodynamically favourable - causing it to react with O2 easily, and when it does, a chain reaction occur.

I've also read that if a shock wave goes through acetylene, it can decompose into hydrogen and carbon http://en.wikipedia.org/wiki/Acetylene which might be an exothermic reaction itself, but the hydrogen can now also react with molecular oxygen - the rate of this reaction is even higher than acetylene and oxygen (what I've read) - higher chance of detonation occurring, instead of burning

Is this kind of reasoning OK? The energy profile you've given in your last post would not refer to an unstable chemical - if it reacts with oxygen, because it's kinetically unfavourable to the point that the energy released will not be enough to cause a chain reaction. Acetylene reacting with O2's energy profile will also be exothermic, but the activation energy will be little compared to the diagram you posted. Any take on what happened with our one spudgun? - using LPG, the potato shot out at low speed, but the screw-on back was blown to bits. That sounds like a detonation to me, instead of deflagration. Could have occurred because the pressure built too much with the big potato slug, increasing the rate of reaction, until the unburnt gases were compressed by a shock wave to it's auto-ignition temp - all the energy released at once. The potato did not have a lot of friction with the barrel, I could easily slide it back, that's why I thought we are safe - however, the heavier slug had more inertia - it could not accelerate quick enough to release pressure inside of chamber..fortunately, nobody was hurt (except our ears!)
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: jimmy101 » Sun Oct 05, 2014 9:01 am

I doubt you got detonation with LPG. Even with acetylene or hydrogen getting a detonation is pretty unlikely. The more likely explanation is that the "screw on back" is a non-pressure rated component and it just couldn't handle the pressure. (PVC clean out plugs are always non-pressure rated components). If you aren't metering your fuel then most of your shots are low power because of the incorrect fuel mixture. Occasionally you'll get a proper mix and the pressure (and performance) will go up significantly.

A more massive projectile and/or tighter fitting projectile will increase chamber pressure but the peak pressure at 1x fueling is only about 135 PSI. A properly constructed PVC gun should be able to handle that without any problems.

It is pretty widely recognized that increasing ammo to barrel friction in a typical combustion spud gun improves performance, which is somewhat counter intuitive. For the best performance with spud ammo you should double bevel the muzzle; both the inner and outer edge of the barrel is sharpened, which cuts the spud a bit bigger than the barrel giving more static friction.
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3130
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Re: propane newbie

Unread postAuthor: Chris spud » Mon Oct 06, 2014 4:16 am

now that is smart..i will definitely double bezel the barrel. We are metering the fuel, using syringes. My chamber is using 195ml of Propane, so that's basically 2 X 100ml syringe. I think the screw-on back was weakened by the holes we drilled for the fan. I hope this does not happen again, otherwise it might be best to go back to hairspray...I wonder if you could strengthen the screw-on back with a metal disk or something, but I guess it can also fail on the thread...something to consider for the next spud gun. cheers
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

Re: propane newbie

Unread postAuthor: Chris spud » Tue Oct 07, 2014 7:27 am

Question: does lubing the barrel help at all? It seems logical, but it might decrease force on potato in the end
  • 0


Chris spud
Private
Private
 
Posts: 18
Joined: Sun Sep 14, 2014 2:36 pm
Country: South Africa (za)
Reputation: 0

PreviousNext

Return to General Spud Cannon Related

Who is online

Registered users: Bing [Bot], Google [Bot], Yahoo [Bot]

Reputation System ©'