OK, I know Isopropanol is smoky and generally considered a non-starter alcohol stove fuel. But I've read that isopropanol has more stored energy than ethanol or methanol, and Zelph among others has clean-burning isopro stoves so "isopropanol = sooty" is not true all the time.

Isopropanol is generally available in 91% (9% water) max. at drug stores, and 99% (1% ??? as Iso-HEET) in auto parts shops and gas stations. When the 91% burns, the 9% water is left behind as a puddle.

My question is three-parts: putting the soot issue aside, does the 91% isopro deliver an energy advantage over methanol, or is that only with the Iso-HEET? Is the energy advantage of isopro lost because of the sooty flame? (i.e., the incomplete combustion negates the advantage). And do the stoves that do burn clean lose the isopro energy advantage because the flame is modified to burn clean?

You'll have to carry 9% extra fuel to offset the water weight.

Well, a modified burner *is* possible

"Isopropanol is generally available in 91% (9% water) max. at drug stores, and 99% (1% ??? as Iso-HEET) in auto parts shops and gas stations. When the 91% burns, the 9% water is left behind as a puddle."

Ha ha, possible, but really doubtfull. At the heat required for vaporization of the fuel, needed to support any real burning, the water will simply vaporize off too. The only way to get a puddle of water is really dilute the fuel with water down to like 75%, or, like 150 proof for ethanol. Generally, there is a rather complicated formula for figuring out the vapour pressures and temperatures, etc. With any fuel grade alcohols, you won't see a puddle in a well designed stove. But, this does mean a lot of lost heat.
1) The water phase change from liquid to gas takes a lot of heat away from the burner. It will "cool" a stove's output.
2) A eutectic mixture forms, ie, where vapor pressures are equal between alcohol and water, at some temperature. Pure distilled ethanol is not possible, for an example...it only ever gets to be about 95% ethanol/5% water. All miscible liquids do this. Methanol does this, too...sort of an aside.


Soo, if you are using anything with more than 5% water as fuel, chances are you are simply wasting a lot of heat. It just produces a lot of steam that means lost heat. I found that using a general denatured alcohol, like SLX, was much better because this is industrial grade, ie, produced from chemical factories/petrolium factories *without* water. It is as good as everclear and isopronol, due to the larger water content the others have. It lights easier and burns hotter, though slightly less time...about 20secs in 6 minutes. Soo, it is a wash. Send me a link to the exact stove that burns isopropynol cleanly. I have never really seen one...

I haven't done any timed testing, but 75% isopropanol seems a lot more efficient than 99%, at least for my pot.

OK good info here to digest, thanks...

James: are you saying that the 9% water content of isopro that burns off as steam negates entirely its higher energy content over methanol? (I'm comparing strictly isopro to methanol--Heet). 100% isopro has 25% more energy than methanol, I assume 91% is 9% less, so around 22% net. Still sounds high to me, offsetting the extra weight and maybe longer boil times.

Eugene: are you using an isopro stove like Zelph's Ring of Fire? Why do you think the 75% isopro is more efficient?

Hi Simon,

There is a comprehensive article on alcohol water mixes here, http://www.backpackinglight.com/cgi-bin/backpackinglight/alcohol_fuels_part_two.html

Tony

"James: are you saying that the 9% water content of isopro that burns off as steam negates entirely its higher energy content over methanol? (I'm comparing strictly isopro to methanol--Heet). 100% isopro has 25% more energy than methanol, I assume 91% is 9% less, so around 22% net. Still sounds high to me, offsetting the extra weight and maybe longer boil times."

I don't really care for HEET. It burns way to quickly...probably due to the close to 100% methanol content.
Methanaol, 9800BTU/lb ethanol, 12800BTU/lb, Propanol 14500BTU/lb
Note that Isopropanol is slightly less. I would guess around 14100 or so.

SLX is mostly ethanol with some methanol as denaturizer. But it varies from about 55% to about 75% ethanol. Just to make crunching some numbers easy, let's say it is a 50/50 blend. This gives us about 11300BTU/lb of industrial alcohol.

So, now we take a 75% isopropanol blend. First off water doesn't burn, soo, from our 14500 we deduct 25%. This gives us 10875BTU/lb. Not counting the extra weight in water we are carrying. So, this is clearly out.

At 90% (I hate fiddling with fractions a lot) we get 13050. Ahh, much better.
But we still have to figure the additional weight for the 10% water. It doesn't burn, so it is just weight. The specific gravity of water is 1. The specific gravity for alcohols is about .79 or .80 (I forget whitch and it varies with type.) So in 1 12floz bottle only 10.8floz is fuel, the other 1.2floz is water. But, this is not a big deal...I carry around more than that in my bladder. What IS a big deal is boiling that 10% water off. Generally we don't see it but this will absorb about half again as much heat as just getting it to vaporize off. It will NOT collect on the pot (except at first) because the pot is warmer than vapor presure. Soo that heat is simply carried away. About 1000BTU/lb is latent heat (well actually 970BTU/lb.) We will ignore just warming it up to the boiling point, since some of that will be recovered. Deducting the 1000 we get 12,050 BTU/lb as an actual estimate of the heat value. But it is heavier to carry, soo, I am saying it is so close to being even, that unless you can swipe lab grade stuff from a lab, it isn't worth it. They are about the same.

Heat value per pounds carried is about the same, but, the ISO will still need a heavier stove than a 3gram alcohol burner...hence my question about the stove at the end. None of the stoves you mention recomend Iso as a fuel. Always willing to learn, though.

I crunched the numbers and did a lot of checking on stoves and lab grade isopropanol about 10 years ago or so. Unfortunaly, physics don't change. Backpacking is even slower.

@Simon

I have an E2E Pro stove. The 75% burns longer and definitely gets the water to boil, sometimes with lots of burn time leftover...I almost got two pots to boil before, and probably would have if I could refill or switch pots more quickly. 99% doesn't always get a rolling boil even when I fill the stove completely.

I think this is because 99% burns too intensely. It shoots the flames much further, which means less of the flame touches the pot. It also burns much more quickly. Burning ethanol gets similar results, except without the soot.

I don't use isopropanol anymore because of the soot, but I do add a little water when using ethanol to make it burn more efficiently. The other nice thing about adding water is that I don't have to carry it's weight the entire trip, unlike the alcohol.

Edited by leaftye on 02/12/2012 21:54:59 MST.

Does anyone remember the MBD iso stove designed to run on isopropanol? It used a wick arrangement. I don't know if he's still making them anymore or not... it was several years ago...

BM

(just did some googling and there's apparently little information on that stove out there)

Does anyone remember the MBD iso stove designed to run on isopropanol? It used a wick arrangement. I don't know if he's still making them anymore or not... it was several years ago...

BM

Ha ha. Yeah, my first chemistry set had a little alcohol burner that took a little blow pipe to get it hot enough to melt glass. I just used cheap Isopropanol in it.
It sorta burned like a candle except when blowing through the blow-pipe. Then it jetted a nice blue flame out. That said, it is certanly possible. But it *did* require more air than could be gotten through normal combustion. Note that I was out checking at the drug store this morning and there was some 99% isopropanol there. I was a bit shocked. After some rather intensive searching about 10 years ago, the best I could find was 91%. Soo, I might revisit this again at some future date with the 99% stuff.

If you buy denatured alcohol at the local hardware store in large containers, it is much cheaper than iso and it's 99% alcohol.

If you don't like the short burn time, use a stove with a slower burn time.

Zelph and others have designs that burn at various speeds efficiently. Some are adjustable.

I know my Zelph Super burns slower than most others, but also seems to work more efficiently. There are also options with adjuster rings.