Gadget to stop wasting camping gas near empty cans

I was quoting someone else but to be clear what I (and others) have been asking is whether a cannister of 30/70 propane butane that’s had a lot of contents used on a stove/whatever is still at the 30/70 ratio.

If the 70/30 mixture is an azeotrope then yes, otherwise no.

I always empty the receiving can completely.

You don’t. If you did I suspect the cannister would be crushed to some degree. I expect what you do is take it down to 1atm.

…but that doesn’t matter because the cans must be designed to handle a pressure of 1atm multiple times because the gas itself can reach 1atm. So being completely de pressurized is a normal situation, even in an unused cannister straight out of the factory.

I was quoting someone else but to be clear what I (and others) have been asking is whether a cannister of 30/70 propane butane that’s had a lot of contents used on a stove/whatever is still at the 30/70 ratio.

If the 70/30 mixture is an azeotrope then yes, otherwise no.

It’s the 70/30 butane/propane mix. Is that azeotropic?

Copy and paste of my post from page 3

It will vary depending on the temperature when it’s used.

At 30C (i.e. summer) a can containing a 70/30 mix would have a vapour consisting of about 3 parts butane 10.75 parts propane (those being the vapour pressures in bara at 30C).

At -10C the only (to within a few decimal places, there will be butane but only a very small amount) component in the vapour space is Propane (at about 3.75bara).

Unless a mixture has an azeotrope (a point in the distillation curve where the vapour composition matches the liquid composition so neither can change) then one will always boil off before the other. A practical example is ethanol, you can’t distil past 95%, to get purer alcohol than that you have to distil it to 95% then strip the water some other way e.g. by adsorbing it onto a dessicant. Or you absorb the alcohol into something else (a high molecular weight hydrocarbon like diesel might work for example), then distil the diesel/ethanol mixture.

(there is no azeotropic point for propane/butane)

[edit] I typed that quickly and ommitted Rault’s law, the proportions should read:

0.7*3 = proportion butane = 2.1bara
0.3*10.75 = proportion butane = 3.225bara

About 40% butane and 60% propane (i.e. significantly higher proportion propane than in the liquid)

and pressure at -10 should read
3.75*0.3 = pressure of propane.

You don’t. If you did I suspect the cannister would be crushed to some degree. I expect what you do is take it down to 1atm.

It shouldn’t crush. As I showed on the last page you can produce an almost complete vacuum in one of those cylinders under relatively benign conditions. Without the pressure inside it’s probably easier to crush the canister (I’ve never tried, and TBH I wouldn’t advise trying it) but they shouldn’t crush under atmospheric pressure otherwise they’d be unsafe. As I said on the last page, you can have a brand new, “full” cannister, and still have a vacuum inside if the temperature drops below -1.

Back in the game 😃.

Ok yes I was not focussed on the issue the vapour pressure is important relative to temp. But as this increases then the liquification has a relieving effect. I was stuck thinking of only the gaseous part.

Is this not partly the reason the cans have a concave base? So if temp gets silly and pressure dangerous then there is material to allow the volume to expand.

Can I covert a 500ml fanta/coke bottle into a portable gas reservoir? Thinking if I cut out a valve from an empty gas canister and epoxy it onto the screw lid from the fanta bottle I can fill this with gas and have it as a back up to my lightweight bikepacking gas canister. If that runs out then I can attach the fanta bottle to the canister and squeeze the gas from it into the canister?

Why not just ditch the gas canister and just attach your stove to the Fanta bottle? Think of the weight savings!

just ditch the gas canister and just attach your stove to the Fanta bottle?

I didn’t realise Fanta had propane/butane in it. That must be why it give me such bad farts.

:p

Is this not partly the reason the cans have a concave base? So if temp gets silly and pressure dangerous then there is material to allow the volume to expand.

Don’t think so, the dome shape is incredibly strong. I think it’s just to give it a flat base!

Failures in cylinders are almost always longitudinal as the hoop stress is greater than the longitudinal. Practical example, sausages always split end to end as they cook, they don’t split around the circumference. The only time you get a failure the other way is due to a weak seam or weld.

If you were designing a gas canister you would want it to fail in a more controlled way when it reached it’s design pressure. I suspect either the valve is frangible and would pop off allowing the gas to vent in an uncontrolled manner but not explosively* or the valve itself might be spring in such a way that it opens to relieve some gas but closes again as the pressure falls. What you wouldn’t want is the canister itself to fail as that would imply very high temperatures and pressures, the auto ignition temp of butane for example is 287C so if it was released at that temperature it would spontaneously combust. You would also get what’s called a BLEVE (boiling liquid expanding vapour explosion) which is where the drop in pressure causes the instant boiling of the liquid (which if it’s above it’s auto ignition temp then mixes with the air causing an even bigger explosion).

*the resulting vapour cloud might explode though.

Don’t drink cans pop their bottom on a plane? But yes I can see the adaptor (area) failing first under pressure.

Can I covert a 500ml fanta/coke bottle into a portable gas reservoir?

If it’s cold enough then yes, and you won’t even need to put a lid on it.

According a google a coke bottle is good for 100psi which will hold the pressure of 30/70 Propane/Butane well over room temperature, somewhere around 40c.

😀

I just had to google to see if anyone had tried it:

Thought I should update this with an experience over the weekend. The 2 cans I had with me on an overnight trip failed. Both full of gas, neither would let gas thru into the burner. Checked once home – burner works with other gas cans, those gas cans will let gas out when the refill valve is attached.

These are old cans refilled many times. I can only assume wear on the valve in the can

I will continue to refll cans but obviously need to check they are working with the burner after refilling