Come on - be a man and shake the canister while you're using it to keep the fuel blended.

Sorry - couldn't resist...

"Now, when you have a mixture of propane and n-butane, you don't have two boiling points, you have only one and it will lie between the two temperature given above, depending on the ratio of the two components of the liquid. Specifically, it depends on the molar weighted fraction."

I don't mean to be too blunt, but this is completely wrong. You do indeed have two very different boiling points. That is the very basic premise of refining oil. If a variety of combined hydrocarbons had a singular boiling point, the entire distillation process would not function and you would never have the two separate products in the first place.. No amount of mixing will ever change the boiling point of either component. They will always work off of their own unique curve.

So Jesse, you are saying that crude oil can be heated to the boiling point of some particular chemical, octane say, and then that chemical and no others will boil off?

How do you think mixtures of chemicals such as naptha are produced?
By mixing together all the constituent chemicals after they have been separated?

If you use a propane/butane mixture at high temperatures like 50 F or whatever, will it maintain the propane/butane ratio more than if you use it close to 30 F?

The "accepted wisdom" is the opposite - if you use it close to minimum temp then most of the propane will be used up first so you're stuck with just butane which then doesn't work so good.

If the percent propane drops as you use it regardless of temperature, then the only thing that will work is to use a full canister if you're close to the minimum temperature.

"...Now, when you have a mixture of propane and n-butane, you don't have two boiling points, you have only one and it will lie between the two temperature given above, depending on the ratio of the two components of the liquid. Specifically, it depends on the molar weighted fraction."

Sorry, I still don't see it except as a special case, ie eutectic. This defines the composition of the gas ratio's in a two part mixture at any given temperture/pressure and can be calculated. In a closed system, NONE of the components actually boils, soo, I do understand what you are trying to say.

But, this contradicts what you stated earlier:
"The vapour pressure vs temperature of butane is a smooth curve, 0C just happens to be 1013mB. Similar smooth curve for propane, just higher pressure at any given temperature. For any blend of propane/butane, the partial pressure of each component is the same smooth curve of the pure companent just reduced by its mole weighted fraction. The net result is that the ratio of the two partial pressures is almost independent of temperature, meaning that the ratio of propane/butane in the gas mixture is almost independent of temperature.

The ratio of propane/butane in the gas state depends on the ratio of propane/butane in the liquid state (vapour pressure), but NOT on temperature.

So, if the GAS is say 50% propane at 10C, it will still be 50% propane at -10C"

Temperature and pressure are HIGHLY and DIRECTLY related, not independent of eachother. To say that the composition of the gas in the canister is the same at 10C and -10C is incorrect. The pressure exerted on the can, from each component, can be calculated for each temp.

However you are correct in that the propane will burn first, but not exclusivlely, in most canisters. Above the boiling point of the n-butane, it will add a considerable portion to any flame. But they will still form a balance at any given pressure (or temperature if you prefer, nearly indistinguishable.) However, below the boiling point of n-butane, it will add only a small amount to the flame. This assumes a wide open jet, of course, as in Bob's case...not enough to burn.

" A new canister with 30% propane and 70% n-butane as liquid will have around 65% propane in the gas mixture, regardless of the temperature."

This is incorrect. At temps below the boiling point of n-butane, we might see a lot more propane. At temps above that we might see a lot less, since there WILL be a difference between raw vapour pressure at -10C and raw vapour pressure at 10C for n-butane. (Since propane is always assumed to be at or above the boiling point, I am ignoring that and calling it a one. We just add a bit of pressure to a highly compressible gas/or vapour pressure if you prefer to look at it that way.)
Again, temperature and pressure are related. The molar fraction of vapour pressure will only change slightly for propane. Burn it at those two temps, I will garantee you that in the lower temp case the propane will be the primary component and the canister will burn normally at the higher temp. The open valve, of course, reverts the system to a more STP like system, I sort of doubt there is ever any actual boiling inside the cannister, though.

Ok, here are some figures.

1) At 10C, the vapour pressure of pure propane is 6350mB, n-butane is 1480mB.
Assume a new canister with 30% propane, 70% n-butane by weight ie. in liquid form.
The mole fraction of the liquid propane is 36% (propane molar weight is 44, butane is 58).
With this mixture, the vapour pressure of the propane is 0.36 x 6350 = 2290mB, the butane is 950mB, add then together to get a total pressure of 3240mB.
The mole fraction of propane in the gas phase is 2290/3240 = 71% and the fraction by weight is 65%.

2) Now repeat at -10C.
The vapour pressure of pure propane is now 3450mB, n-butane is 700mB.
Assuming the same 30% propane, 70% n-butane by weight so the mole fraction of the liquid propane is still 36%.
The vapour pressure of the propane is now 0.36 x 3450 = 1240mB, the butane is 440mB, add then together to get a total pressure of 1680mB.
Note that the individual vapour pressures and the total pressure are all around half what they were at 10C.
The mole fraction of propane in the gas phase is now 1240/1680 = 74% and the fraction by weight is 68%.

At -10C the total pressure in the canister is 52% of what it was at 10C, but the fraction of propane in the gas phase has only changed from 65% to 68%

I rest my case.

Stuart is right. All un-named persons arguing with him are less right (they are getting some things correct, but not everything).

I'll try in a separate post to explain the concepts. I thought Stuart did a good job, but then I've had rather a lot of classes and work experience in this area and he's speaking my language, (give or take the British spellings).

Not to be annoyingly pedantic, but like when my MD wife's medical opinion differs from some poster's, maybe it's helpful to state quals: BS Chem Eng, UC Berkeley; 23 years of professional engineering on scores of Chevron, Shell and Tesoro facilities including a dozen refineries.

As Roger listed:

Options (which may be all used together):
* use a canister which is labelled 'isobutane/propane'
* use an inverted canister stove, with the canister inverted
* put the canister in a dish of luke-warm water
* put a windshield around the stove so the canister gets warm to the touch (but NOT 'hot')
and of course:
* subscribe to BPL and read our many comprehensive technical articles on all aspects of this subject.

which I would modify:

- Pretty darn hot water is also fine. In my BPing hot tub, I have to immerse the propane tank in the hot tub, 40C/104F which isn't even a hot day in Phoenix and their propane tanks don't go *boom*. All the factory-filled canisters are good to at least 140F and so are mine, unless I've consciously opted to overfill them for some reason (and then I mark them very clearly).

and to which I would add:

- a metal heat conductor to conduct heat from the flame to the canister. Someone suggested copper wire and while that is compact, easily formed, and absorbs heat nicely from the flame, its round cross-section doesn't contact the canister very well. I'd suggest a strip of aluminum (Ohhh!, we could argue about how to pronounce that!), about stop-sign gauge, 10 cm long, 1-2 cm wide, hammered to curvative of the canister. Secure it with nylon thread or a hose clamp. If you want to deal with sticky white goo, get a little heat-sink compound and put it between the canister and your aluminum bar.

- a wider pot (better yet, painted black) will radiant more heat downwards and help keep the canister at temperature. Alas, like many of these hints, it mostly heats the TOP of the canister and therefore inefficiently heats the liquid fuel itself.

I've done the strip of aluminum which works. Just use 18 gauge galvanized wire, through holes in aluminum strip, bent to the right length so it holds the strip tight to the canister.

But someone had a better idea, which they got from some book, which I tried - 1.5 foot length of copper or aluminum wire - solid core - maybe #16 or #18. Wrap it around canister once and then up into flame.

OK. I give up. Believe what you will...I believe in distilation, myself.

+1 to another discussion of Boyle's Law on BPL.

First off, why does it SEEM like propane boils off first ONLY at cold temps? Because at >50F, you don't miss the propane. More of its mole fraction (and weight fraction) boil off regardless of ambient temperature, but you don't have a problem maintaining a vapor pressure of the remaining mix of fuel if the liquid fuel stays above about 30F.

Therefore, if your mental model is of discrete boiling points (all propane boils and then butane starts or no butane boiling below 0C), that model seems to be confirmed by lousy performance below 0C and okay performance above 10C.

Now, onto a parallel example that hopefully you have experience with: flambe'ing brandied pears or plantains in a fry pan. You've got a hot pan. You add some brandy. Enough alcohol vaporizes that you can light the vapors on fire and bring a flaming pan to the table and impress your dinner guests. It isn't ONLY ethanol boiling off, but water was too. You can see this in how little liquid remains (brandy is only 40% alcohol, most of it is water). And, even though a lot of alcohol and water have boiled off, some of each still remain. You can still taste alcohol in the dessert, despite what we say to rationalize serving it to children.

A more controlled, instrumented, parallel example: My uncle owned a distillery, making European-style brandies from fruit wines (plum, peach, apricot and, once, an ill-advised experiment with kiwi fruit). The wine is put into a "distillation column" in ChemE speak also called a "retort" by chemists and a "reflux column" by winemakers. As heat is applied, an ever changing mix of water vapor and alcohol vapors (with various flavor elements and oddball aldehydes and ketones) boils off. At first, the vapors coming off are relatively high in alcohol because (1) alcohol is more volatile and (2) there is a decent mole fraction of alcohol in the liquid (10-15%). As the distillation progresses, the temperature of the boiling liquid increases as the mole fraction of alcohol decreases. Just by looking at a thermometer, the distiller can know precisely what "proof" is coming off at the moment.

There's water coming off at all times - you never get 100% alcohol. There's alcohol coming off at all times, too, but in ever-decreasing amounts. He'd cut off the distillation while there was still more alcohol in the vapors than in the original wine because he wanted to avoid some higher-boiling, bitter flavor compounds, but that doesn't effect us with our stoves.

The bit to focus on in this example is that as a larger fraction of the more volatile component (alcohol/propane) boils off, less remains in the liquid phase. When the liquid phase contains less of the volatile component, the temperature needed to boil the liquid mixture rises. This is bad in cold weather operation.

"another discussion of Boyle's Law on BPL"

Bruce, what did Boyle have for a base weight?

--B.G.--

Rather than try to explain the chemistry (again) in a single paragraph, can I refer everyone to our full-length technical article (by Stuart and myself) on the subject?
Temperature Effects on Gas Canister

I believe that what I wrote in this thread was completely consistent with what the article says.

Cheers
PS: and what's the matter with distilling Kiwi fruit? Enquiring minds want to know.

Roger, the explanation by you and Stuart make it sound like a university physics lecture. David's explanation makes it sound like something he could not admit to until after the Statute of Limitations had run out.

--B.G.--

Roger: I find no fault or errors in your post or the full-length article and I, personally, appreciated all the details and data in the article.

I was trying to give a real-world example that people might be able to relate to because it seems some folks got the overview of distillation without spending two semesters modelling it in detail. The overview being close to this: the most volatile component boils off before the next most volatile component boils off. That overview explains many distillation processes adequately (fractionating air, for instance) or even refining petroleum **as long as you don't get deeply into the details or purity of products**.

In practice, those tall columns in oil refineries are used to give a better separation between fractions (remember our canisters are but one stage compared to scores in a distillation column) and even those tall columns don't achieve perfect separations or "cuts" between fractions. The heavier fractions within gasoline are also the lighter fractions of diesels (hydrocarbons with 11 to 15 carbons, for instance). And so on, through various fuel oils and on through the asphalt fraction. Think of a bell-shaped distribution that has a certain midpoint, but a wide range of individual chemicals of wide-ranging boiling points. But an overview of "first fuel gases boil off, and then gasoline, then diesel, etc" is accurate so far as it goes. All of which, I know you know, Roger but I include it here for others.

He never let anyone try the Kiwi fruit brandy, so it must have been pretty bad. Are orange-flavored liquors fermented from orange juice, or just flavored with oranges? I can imagine the highly acidic juice and/or some of the volatile elements causing a disagreeable taste in the wine/brandy. Are there wines made from kiwi juice? I'll ask my winemaker friend up here next time I see him.

"David's explanation makes it sound like something he could not admit to until after the Statute of Limitations had run out."

Must be a "Breaking Bad" reference : )

"Are there wines made from kiwi juice? I'll ask my winemaker friend up here next time I see him."

Yes, kiwifruit wine has been made in New Zealand for quite some time, for at least 30 years, but it's not that common. I tried some a long time ago and didn't like it. Life's too short to drink bad wine.

Thank you, David. This was exactly what I was driving at.

BTW, I worked in Chemical Engineering for over ten years at a large university, though we changed the department name several years ago.

So, uh, like what brand of like fuel should I totally use without issues with boiling off or loosing elasiticity or whatever you guys were like talking about? I guess the Coleman canisters are like, "not amazing"?