If a 220g style threaded cannister can stand 30% propane with 70% other gas, then presumably it can hold more pure propane than this: i.e. could it stand 40% of 220g, 50% of 220g (=110g) propane?


Would this be worth it for allowing cannister stoves to go lower in temps without need for remote hose?


Would it be worth such things being commercially available?

Edited by ahbradley on 10/09/2011 06:13:20 MDT.

The greater the percentage of propane, the higher the pressure in the canister.
Commercial canisters must meet safety standards, which are different in different countries but for example no leak or deformation of the canister at 131F.
That is why you will not get more than 30% propane in a 220g style canister.
Other, much heavier, canisters can hold 100% propane.

I was hoping if the cartridge was pure propane, (rather than 30% propane 70% butane) then, the can could hold an amount of extra amount of propane that giving pressure equal to the 70% butane it is in leau of:

obviously, will be a lesser amount than the butane, but I was hoping might be significant.

I understand what your saying and it makes sense but I have no idea of how much it could hold

A standard screw thread canister would hold round about 220 g of propane. The internal pressure would be about 4 times higher than for a 30% propane 70% butane mix. There is every chance that on a warm day the canister would explode.

Whether this would be worth while is up to you. Me, I would prefer to be sure I was not going to have it blow up in a fireball in my face.

Coleman sell propane canisters, but they are somewhat heavier. They have to be to withstand the pressure. Some have used them in the winter at base camps.

No, putting a lower weight of 100% propane in the canister would NOT reduce the pressure at all.

We have an article on exploding canisters - read it.

Cheers

If you removed the butane from a 70% butane/30% propane canister, would the pressure go up?

I wasnt asking about home refilling, just about how much pure propane a (commercially backed) I meant a 220g style can, NOT a can holding 220g of gas, ie if said can is partially filled with pure propane, how much could it hold and still pass the same tests as the usual mix, I presumed it could be at least 30% of 220g.

I read the article (in the past, and have just jumped to Pressure-Temperature Relationship for Common Canister Mixes section), and it seems counter intuitive that a canister could stand 220g of 30/70 propane butane mix but not 30% of 220 = 66g of pure propane.

Edited by ahbradley on 10/09/2011 16:15:57 MDT.

You would be able to fit more Propane (measured in mass) than would be in in a 30%propane / 70% butane canister by eliminating butane from the can.

If the percentages are by mass (as opposed to volume) then I would guess you could get maybe 100g propane (0g butane) and have similar canister pressures at any temp.

The vapor pressure of butane at 80F is about 23 psig, pure propane is about 128 psig.

The 30% propane blend has a vapor pressure of around 54 psig at 80F.

Pressure is dependent on the chemical vapor pressure in the cannister, and not on how much is in it. If you increase propane percentage, the pressure will go up, whether the cannister has one gram of liquid in it, or 100 grams of liquid.


This is because when you make a solution of two chemicals that each has their own vapor pressure, they no longer exert it. Their new vapor pressure in this case is approximately equal to their mole fractions in the liquid phase x their pure component vapor pressures. Adding the individual vapor pressures gives the new total vapor pressure. At 99.9 % propane it will be essentially pure propane = 128 psig @ 80F, and at 1% propane it will be essentially pure butane = 23 psig at 80F, and everything in between is ..in between.


At 120 F you are looking at a difference of 110 psig for 30% vs about 240 psig for pure propane.

Not only would cannister safety be a possible issue, but stove operation might change as well due to the higher cannister pressure. The flow of gas could possibly be 40-50% higher at samr settings than for 30% mix. Whether a given stove could adequately compensate for that, is unknown.

Now, unless 30% is an azeotrope (where the vapor has the same conc. as the liquid phase), as liquid fuel is used, the composition in the cannister will change, for better, or for worse, as the more volatile component is preferentially vaporized. Which would create its own set of undesireable issues. If you think about it, would you want a cannister that became pure butane by the time it got to the end? Of course not. Or that the pressure/composition was constantly changing so that the flame wasnt stable for long periods of heating, etc? Nope.

Edited by livingontheroad on 10/09/2011 19:38:38 MDT.

Pressure depends everybit as much on quantity as it does on physical properties. Take a look at the ideal gas law and daltons law of partial pressures.

You are missing the point. He is asking how much pure propane (moles, grams whatever) he could place in a fixed volume canister and have the same safety margin as with 30/70 propane/butane mix.

Take your 80F example: for the mixture you come up with 54psig. If you take all of the butane out, w/out adding anything in its place would the pressure go up to 128psig?? that sounds like what you are saying. How could that be -- you are giving the propane molecules more room to jump around.

What happens with the 100% propane canister at 128psig as you burn of the fuel, will it stay constant? Of course not it will decrease towards 0psig as it is used up.

You will be able to fit a propane in a butaneless canister than the amount of propane you could in a mixture.

Pressure does not depend on the quanity when you are talking about Vapor Liquid Equilibrium between a liquid and the vapor space.

IF you had just pressurized gas, yes, you would be correct. Or if you had some non-condensable also present like nitrogen, it would have an effect too, but that is not there either. That isnt what you have. You have liquid. And as long as there is 1 drop of liquid inside that cartridge, the pressure in vapor space will be the same as when it is 99% full of liquid. It is the vapor pressure of the liquid solution.

That liquid vapor pressure varies with temperature, which is why the stove doesnt work well below a certain temp, the vapor pressure of the fuel drops below the range it was designed to work in. If it has a regulator, it may not work at all without enough pressure to overcome the spring setting. If it works by an orifice, its flow may be reduced until it is too lean to burn, etc.

The composition of the vapor space is fixed by chemical equilibria with the liquid. As the liquid level drops, more is vaporized to make the partial pressure of the component stay in equilibrium with the liquid.

Edited by livingontheroad on 10/09/2011 20:11:58 MDT.

He could add no more propane if he values his life. A butane canister isn't made to add any more propane than what was present, with or without butane content.

Edited by jshann on 10/09/2011 20:02:40 MDT.

Ahh, you called it...

I was forgetting the important fact that we are talking about a saturated liquid not gases.

After a little reminding (thank you, it was me who was missing the point) I do agree with the comments about not being able to increase the amount without decreasing the safety margin.

Martin has it absolutely right.

I however, have run out of patience to re-explain these basics over and over, given that there are BPL articles on this subject.

OK, thanks, but it did seem rather non-intuitive (but some science is): however, thinking about it, if the gas is in liquid form at above its bp, then it is compressed and will exert pressure because it "wants" to be a gas.


I have reread the article and found the line "Note that the pressure is the same no matter whether the canister is full or not, provided that the LPG mixture is the same" buried in the "Test Requirements to meet Standards". And also mentioned more in attached thread.

Edit: the article http://www.backpackinglight.com/cgi-bin/backpackinglight/effect_of_cold_on_gas_canisters.html gives the mix vapour pressure equation.

So the idea is a non starter.

Thanks anyway. :)

Edited by ahbradley on 10/11/2011 12:33:06 MDT.

yeah, it seems non-intuitive that if you removed the butane from a propane/butane mixture, the pressure would go up.

"...have run out of patience..."
Oh, you poor soul. How hard shouldering that burden must be :)

I had read the article as it does appear most people in this thread have. I greatly appreciate the content you and roger put together there -- what I really took away from it were the graphs showing cold weather performance of various mixtures of fuel as the mix is consumed.

Regarding the basics you mention here...I vividly remember these concepts tripping up many in my Mech Engr Thermodynamics class (I am sure my prof ran outta patience quite often). Even with direct instruction, when attempting to generalize, the mind has a way a reverting back to a given (typically inexact) pt of reference until a solid level of mastery is attained. Unfortunalty once mastery is attained, the trickier parts of getting there are sometimes forgotten. I think of this constantly as I try to teach my daughter anything that feels "basic" to me.

I think the crux of understanding "why not" to the original question is understanding the behavior of a substance in some form of phase equilibrium (eg gas and liquid) and how it differs from its behavior in a pure phase (eg gas or liguid). And a related tripping point I see here is the quick assumption that substance's phase is its most commonly excountered phase, regardless of physical properties (ie propane is gas phase or water is liquid phase).

HERE IS MY ATTEMPT TO CONTRIBUTE TO THIS THREAD(for anyone still slightly confused).

1)Take a nearly empty (say it has 1 molecule of propane), very strong canister (fixed volume) and hold it at 80F. Its pressure will be a near vacuum: ~0psia.
2)Let's change the canister by slowly adding some propane molecules (and changing nothing else). With these early additions, pressure increases proportionaly with molecule additions*.
3)Continuing to add molecules...the pressure continues increasing. As the pressure in the canister approaches ~145psia the changes in pressure become less and less proportional** - whereby it takes greater and greater additions of molecules to get the same change in pressure but the general relationship still holds. Also, very important...up to this point the propane exists entirely in the gas phase.
4)At ~145psia the propane gas will begin to condense and form some liquid***. Again, just before this pressure, additional molecules resulted in increase the pressure of propane gas. Just after this pressure is attained, increasing molecules no longer changes pressure. What changes is the amount of propane (in percentage) existing as a gas. With each additional molecule the percentage of liquid propane increase.
5)This continues until you have 100% liquid propane @~145psia, after which additional molecules begin to increase pressure of the liquid propane.

*think of pressure as the net force of all molecules striking the wall of the canister at any time, if you increase the # of molecules than you increase this effect and thus the pressure.
**as you continue to add molecules they contact each other more frequently before hitting the canister, losing a little energy in the process -- hence the breakdown in proportionality.
***around this pt the molecules are so close (on average) and the number of molecule-molecule collisions so frequent that some of the collisions are between molecules which have slown enough (or lost enough energy) to stick to each other on contact -- this sticking is liquid formation or condensing.

One last point, to answer the OP's question. There is a mass of 100% propane that you could fit in the canister w/out increasing the safetly factor. Knowing the volume of the canister, molecular weight of propane you could use the ideal gas equation to estimate that number. The mass would be very low relative to 220g. Unfortunatly, its late and I am outta steam.

good luck to anyone who tries to interpret this long mess (I am no good at succinct),
James

Pressure does not depend on the quanity when you are talking about Vapor Liquid Equilibrium between a liquid and the vapor space.

IF you had just pressurized gas, yes, you would be correct. Or if you had some non-condensable also present like nitrogen, it would have an effect too, but that is not there either. That isnt what you have. You have liquid. And as long as there is 1 drop of liquid inside that cartridge, the pressure in vapor space will be the same as when it is 99% full of liquid. It is the vapor pressure of the liquid solution.

That liquid vapor pressure varies with temperature, which is why the stove doesnt work well below a certain temp, the vapor pressure of the fuel drops below the range it was designed to work in. If it has a regulator, it may not work at all without enough pressure to overcome the spring setting. If it works by an orifice, its flow may be reduced until it is too lean to burn, etc.

The composition of the vapor space is fixed by chemical equilibria with the liquid. As the liquid level drops, more is vaporized to make the partial pressure of the component stay in equilibrium with the liquid.