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B refilling butane lighter the physics way

  1. Jan 11, 2017 #1

    DaveC426913

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    Please move if not appropriate.

    I'd like to understand how filling my butane lighter works from a more rigorous point of view than just guessing.

    How I can tell when my refill cannister is getting inefficient? (i.e. more rigorously than just shaking it or weighing it) I hope to detemine how full a random refill is by observing its buoyancy compared to a full and an empty cannister.

    So - the ideal goal is to fill the lighter as full as possible with liquid butane.

    When I put a butane refill, it is liquid butane under pressure.
    When I fill the lighter, it will equalize pressure. Is liquid butane compressible?
    As the refill loses liquid, the pressure will drop. Does the butane start to evaporate?

    Ultimately, I hope to establish a function relating amount of refill left (in grams) to effficiency of lighter filling.

    As is apparent by the scattered nature of my comments and questions, I'm discovering it to be a lot more complex than I thought at first.
     
    Last edited: Jan 11, 2017
  2. jcsd
  3. Jan 11, 2017 #2
    The following is a guess ....

    So ... we have the lighter in need of refilling , upside down with the valve uppermost. We have the canister pointing downwards , about to engage the valve of the lighter .... Both will contain some liquid and some gas ....the pressure in both is the same .

    On pressing the two together the valve must open to allow liquid to to move from the canister into the lighter , but this will only happen if the lighter valve also allows venting to the atmosphere. The valve is not a simple thing , it must allow in the fluid at the same time letting gas out .

    When the lighter is full gas is no longer vented , but the liquid is .

    The canister will fill with the same efficiency, whether full or nearly empty .
     
  4. Jan 12, 2017 #3
    Butane has this amazingly useful property: it makes its own pressure. You see, the vapor pressure of Butane at room temperature is about 2 atmospheres. In a closed volume liquid butane will evaporate until the pressure of Butane gas above the liquid equals 2 atmospheres. As you dispense the liquid a little liquid evaporates to fill the new extra volume. The Butane gas occupies 11 liters/mole, so it takes very little of the liquid to fill the new available volume. What's more, the amount of evaporation required depends only on the amount dispensed, not on the total volume, so it doesn't make any difference how full the can is.

    So there you have it. The liquid is being pushed out by 2 atmospheres of gas no matter how full the can is so long as there is some liquid left. The efficiency as you put it which I will define as liquid dispensed over liquid used doesn't depend on how full the can is either.

    An interesting side effect: the evaporation uses heat from the surroundings. That's why the can gets cold. The amount of evaporation and so the amount of cooling depends on the amount dispensed. As the system cools the vapor pressure falls. So the pressure doesn't depend on how full the can is, but it does depend on how much you dispense before the can can warm back up.
     
  5. Jan 12, 2017 #4

    NascentOxygen

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    With pressures equal, why does liquid move from one container into the other?
     
  6. Jan 12, 2017 #5
    I think Oz misspoke, but he immediately went on to point out the answer. At the same time the valve opens to allow liquid to enter from the refill it must also open a path to allow the gas to flow from the lighter. This lowers the pressure in the lighter. With the gas flowing this is no longer an equilibrium condition and it becomes a rate problem. How fast the gas is let out of the lighter compared to how fast the liquid evaporates determines the drop in pressure. The conductance of the outlet hole must be engineered to some particular rate that best balances the need for a pressure differential to allow flow and the desire not to waste too much butane.
     
  7. Jan 12, 2017 #6

    DaveC426913

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    Thank you!
    This is very informative. I was grappling with this idea myself and realizing it made no sense, or at least I didn't understand how pressurized fluids behave.

    Is this a unique property of butane? Or a general property of similar compounds?
    Do other compounds have such a sweet spot such as 2 atmospheres? Does water?
     
  8. Jan 12, 2017 #7
    Every liquid in thermodynamic equilibrium with its own vapour has a (saturated vapour) pressure which is function of temperature only and not of the amount/volume of the liquid present (as long as some liquid is present).
    Or I haven't understood your concern.
    So the OP question has not a simple answer to me, apart the fact that canister fills the lighter because of gravity and because of little pressure differences due to little temperature differences in the two containers.

    --
    lightarrow
     
  9. Jan 12, 2017 #8
    As light arrow indicated all liquids have a vapor pressure, but not all vapor pressures are 2 atmospheres. That is a particularly nice value in that pressure vessels don't have to be particularly strong. You can transport a large quantity as a liquid in lightweight containers like your lighter or your refill can and yet use it as a gas. It has enough pressure to allow you to easily refill from another container, but not so much that the pressure breaks anything. Other gasses have a similar but not quite as convenient vapor pressure, so you wont see any pocket propane lighters in the checkout aisle. However many gasses do have a similar convenient value like some of the more useful freons or aerosol propellants.

    Things which remain liquid in the open and don't rapidly evaporate or boil have low vapor pressure. Alcohol and lighter fluid have fairly high vapor pressures so they evaporate quickly but that is still well below 1 atm.

    Many gasses have high vapor pressures and don't liquefy when you put them in high pressure gas cylinders.

    However Butane is not unique. The propane you use in your home or your barbecue is held at room temperature as a liquid under its own vapor pressure. Here the pressure is more like 20 atmospheres, so the vessels are more substantial and you aren't permitted to refill them yourself, but it's the same thing. Liquified Petroleum Gas is another example. The Freon we used to put in our car air conditioners. In fact all those flourocarbon powered aerosol cans. The "air duster" can you have by your computer is actually some fluorocarbon. Carbon dioxide is distributed in high presssure gas bottles, but the pressure isn't all that high and the gas has compressed to liquid. There are many others.

    Other gases are held in equilibrium with their vapor at their boiling point. Liquid nitrogen dewars for example.

    So the property is universal, but the convenient value is realized by only a few gasses and those gasses get a lot of use in daily life.
     
  10. Jan 12, 2017 #9

    DaveC426913

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    This is fascinating, thank you. I knew bits fo the peices, but never connected the dots like this.

    I guess the crux of my question has been answered: that the efficiency of the refill is essentially constant as long as there is still some liquid in the cannister.
    Now I know why that's true.
     
  11. Jan 12, 2017 #10
    Gravity has virtually no effect (less than 0.1%) neither does temperature difference ...compared to the two atms. pushing the liquid into the lighter ... the key is , pushing the canister nozzle into the lighter valve opens the lighter chamber to atmospheric pressure.
     
    Last edited: Jan 12, 2017
  12. Jan 12, 2017 #11
    Not even did I misspeak .....Quote .... "We have the canister pointing downwards , about to engage the valve of the lighter .... Both will contain some liquid and some gas ....the pressure in both is the same ..."
     
  13. Jan 13, 2017 #12
    Sorry but I don't understand: when the canister nozzle is pushed on the lighter valve, both the canister and the lighter opens partially to the atmosphere (because the canister nozzle and the lighter valve don't connect perfectly each other). The two containers also become in communication each other and from this moment on, what pushes the liquid from the canister to the lighter? If the lighter tank is not totally empty, why there should be a difference of pressure between the two containers?

    --
    lightarrow
     
  14. Jan 13, 2017 #13
    I stand by my original post(#2) .... the two are not meant to 'connect perfectly' if they did , as you correctly say only gravity would cause the transfer ... which would take forever ... in fact it probably wouldn't happen , because the lighter is smaller and will heat up from the hand holding it , more quickly than the large refill canister...
    The act of connection must surely be to give a path for the liquid transfer and also connect the lighter chamber to the atmosphere to allow gas release and the pressure to drastically drop.
     
  15. Jan 13, 2017 #14
    Now I see what you mean, I find it reasonable.
    Thank you.

    --
    lightarrow
     
  16. Jan 14, 2017 #15

    sophiecentaur

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    It makes its contribution because the upper (refill) is pointing downwards so the difference in pressure causes the liquid in the bottom of the refit to flow out. If you held them the other way up, how would liquid flow the way you want it to?
    I'm still struggling with precisely what happens about the pressures in the two containers. Doesn't it have something to do with the metal refill which conducts heat from your hand (the pump energy source) and the plastic lighter reservoir (or internal metal tank) which ends up at a lower temperature as soon as the initial 'squirt' takes place at connection time? (If someone reckons they have already said that then I apologise)
     
  17. Jan 14, 2017 #16
    Sure it makes a contribution , but an insignificant one .... pressure in canister and lighter is over 2 atmospheres... equivalent to 20 meters of water or 30 meters of liquid butane .... what would be the height of the butane when plugged into the lighter ? ....between 3cm to 12 cm ... that's only 0.1% to 0.4% of the pressure from the butane ... of course the canister must point down otherwise only gas would be transferred.
    A clue is the speed at which the transfer takes place the transfer nozzle has a very small diameter , the liquid is being pushed through at great pressure ... also the liquid in the lighter gets very cold , I would think close to 0C ... Bpt butane is -0.4C ... this shows the pressure inside of the lighter MUST be close to atmospheric pressure during the transfer ..

    It would be interesting to know the % of wastage during a typical re-fill.
     
    Last edited: Jan 14, 2017
  18. Jan 15, 2017 #17

    sophiecentaur

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    Isn't that a gravitational effect - and very significant to the operation? It's what I was referring to explicitly in my post.
    But what is the energy source to make the transfer operate? It surely has to be heat from the hand around the refill. There is work done on the fluids in the (receiving) lighter and that can only produce a rise in temperature, if there is any change. Is it Joule Kelvin cooling, whilst the butane enters the lighter? Can the vapour pressure in the lighter change as more liquid is admitted?
    So does to drop as soon as the connection is made? They seem like contradictory statements and I don't understand the mechanism.

    I remember this toy, which seems to operate on the same principle.
     
  19. Jan 15, 2017 #18

    NascentOxygen

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    I'm not so sure that there is need for gas release from the lighter as it's being refilled. My evidence: sometimes I'm successful in immediately perfectly mating the canister with the lighter valve and the filling operation proceeds from start to finish almost silently—and it's only as I separate the two at the end of the filling that the first butane is lost.

    Perhaps the chilled liquid spraying into the lighter's tank immediately condenses sufficient gaseous butane that no venting of the tank is actually needed?
     
  20. Jan 15, 2017 #19

    sophiecentaur

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    The Bird toy works without any venting. But that toy is not the best example that I have seen. In a school lab, I have seen a blown glass demo with two bulbs and a joining tube with fluid pumped from one to the other just by the application of a warm hand to the full bulb.
    Any explanation about the Butane filling has to include a description of Energy transfer, surely. Pressure, on its own, does no work and makes no difference.
     
  21. Jan 15, 2017 #20
    It the lighter is completely empty the pressure inside it will reduce to 1 atmosphere. When you start refilling the lighter the gas will initially flow due to the higher pressure in the canister. But very soon after that the pressures will equalise it will be gravity that causes the fluid to flow from the cannister into the lighter.
     
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