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Friday, April 13, 2012

Gas in Cold Weather: The Myth of "Fractioning"

I'd like to dispel a myth that's out there: the myth of "fractioning."

Supposedly, if you've got a typical canister of gas (i.e. containing propane and some form of butane), in cold weather (temps below the vaporization point of all but the propane), only that fraction of the fuel that is propane will burn. Supposedly.

Here's an example of the myth: You have a canister that is 70/30 butane/propane. Propane vaporizes at -44F/-42C; butane at +31F/-0.5C. Now, say it's 20F/-7C out. According to the myth, only 30% (that fraction of the mix that is propane) will burn off.

That's the myth.  What really happens?  Well, take a 70/30 canister out in 20F/-7C weather and burn it until it's empty. Now weight it. You'll find the weight is lower than if only 30% of the fuel had burned off.

Why? Well, when you mix liquid hydrocarbons, they combine to form a blend. The blend has properties that lie "between" the properties of the constituent fuels. For example, the boiling point of a 70/30 blend of propane and butane will have a boiling point between -44F/-42C and +31F/-0.5C. The math is non-trivial, but if you know the formulae, you can calculate the resultant boiling point. In this case, a 70/30 blend has a boiling point of about 0F at standard pressure (1 atmosphere/1013mBar).

Now there is a little bit of a catch to all this. This "blend" of liquid hydrocarbons in a canister of gas (recall that there's so much pressure inside a cansiter that our gasses liquefy) still has two distinct molecules in it: Propane molecules (C3H8) and butane molecules (C4H10). The propane molecules have a higher vapor pressure than the butane molecules. In other words, the propane molecules tend to vaporize more than the butane molecules in the blend. How does a canister typically work? Well, you draw vapor off the top, and then more of the liquid portion of the fuel vaporizes to replace the drawn off vapor. If the propane is what most readily vaporizes, then the propane will be removed from the blend more rapidly than the butane. This is called "preferential" vaporization. Because the propane is vaporizing faster than the butane, your blend will change over time as the canister empties.  As the blend changes, so will the vaporization point.  Toward the end of the life of the cansiter, all the propane will be gone and only your secondary fuel will remain.

Uh, isn't this "preferential" vaporization the same as "fractioning?"

No, not at all. Remember that the myth of fractioning calls for only the propane to burn leaving behind all of the butane. What happens in reality is that the blend vaporizes some butane and some propane throughout the life of the canister until the propane is gone. Yes, you will reach a point where all of the propane is gone and only butane remains, but the fact of the matter is that you will be burning off far more fuel than if only propane alone were burning.

With fractioning (if it were true), you could only burn of 30% of the fuel in a 70/30 canister. With preferential burning, you burn more like 75% of your fuel before you wind up with nothing but butane left (the actual amount burned varies with altitude and temperature of course).

This brings up two important points:
1. Don't use canisters with regular butane in cold weather. Eventually all your propane will burn off, leaving behind whatever else is in your canister. If you've got isobutane (vaporizes at +11F/-12C) as your secondary fuel you've got a 20F (10C) degree advantage over a canister that has regular butane (vaporizes at +31F/-0.5C) as its secondary fuel.

2. Stoves that can handle running with the canister upside down (inverted canister stoves) draw fuel off the bottom of the canister. If you're drawing fuel off the bottom, you're only drawing liquid. If you're not drawing vapor, then it doesn't matter which fuel vaporizes at what rate. Inverted canister stoves start and end with the same fuel blend. If you start with 70/30, you end with 70/30, and your vaporization point stays low, allowing you to run your stove in much colder weather than a conventional upright (screws onto the top of the canister) gas stove. With an inverted canister stove, you can burn all of your fuel in cold weather (assuming that your blend's vaporization point is sufficiently high for the weather).

So, there you have it: What's really going on in that canister of yours in cold weather. I hope this puts the myth of "fractioning" to bed.

I thank you all for joining me on another Adventure in Stoving,

HJ

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4 comments:

  1. As always very informative and interesting. :) But I really want to see that math! Can you do a problem for us? :D

    I do like the concept of the invertible stoves. Can this be done with any stove that has a remote canister or only certain stoves?

    Wolfman

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    Replies
    1. Wolfman,

      No, you can't invert all remote canister stoves. Some will be very dangerous if you try to invert them. See my article on Seattle Backpackers Magazine.

      HJ

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