blended butane failure

[Stuart R]

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.

[David Thomas]

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.

[David Thomas]

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.

[Jerry Adams]

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.

[James Marco]

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

[Bruce Tolley]

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

[David Thomas]

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.

[Bob Gross]

"another discussion of Boyle's Law on BPL"

Bruce, what did Boyle have for a base weight?

–B.G.–

[Roger Caffin]

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.

[Bob Gross]

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.–

[David Thomas]

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.

[Jerry Adams]

"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 : )

[Barry Cuthbert]

"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.

[James Marco]

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.

[Erik Basil]

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"?

[Jerry Adams]

"So, uh, like what brand of like fuel should I totally use"

Giga Power or MSR at REI are good

It has to say "iso-butane" on the canister

Then it's good down to 20 to 25 F – some people can use it down to 20 F, other people say 25 F

If it says "high performance fuel" or some other non specific, then it's probably butane and only good down to 30 to 35 F. Or even 40 F

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