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Hydrogen generation

  1. Jun 8, 2010 #1
    Hi All,

    I am currently designing a tethered balloon. The balloon is 36m3 and I was hoping to fill with hydrogen. The balloon will only be fully inflated at 400ft as I am aware of the risk of a hydrogen explosion.

    Unfortunately I live in Australia and 36m3 of hydrogen is USD$1800!!

    I am considering using a reaction of hydrochloric acid and aluminium as both substances are cheap here. (approx USD$120 for 200L 32% Hydrochloric acid technical grade, $20/kilo 200mesh aluminium powder[74micron particle size])

    The system needs to be portable as I will be using it in outback Australia.

    Since I haven't studied chemistry in about 15years I am guessing I will need around 80L of hydrochloric acid to generate around 3.5kg of hydrogen gas. Firstly I need to know if I am correct in my calculations and how long the reaction would take?

    If you have any simpler/cheaper solutions to make hydrogen gas I am all ears!
     
  2. jcsd
  3. Jun 8, 2010 #2

    phyzguy

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    Science Advisor

    A couple of comments:

    (1) I calculated 105 L of HCl needed, so you are close.
    (2) I priced hydrogen gas at ~ $3.00 US/kg. How did you get to $1800?
    (3) I think electrolysis of water would be a vastly easier, cheaper, and safer way to produce the hydrogen than your HCl/Al reaction.
    (4) Be careful!
     
  4. Jun 8, 2010 #3

    Lok

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    No ideea about price, but electrolysis is not an easy way to produce hydrogen. Electricity has it's cost, and the efficiency is not that good with out good equipment.

    Usually the carbon cathode, be it graphite has a short life and will need replacing. I've no ideea how you will store the hydrogen or use it to fill the baloon, as this will complicate stuff. And this is a quite long lasting thing as the Amperage can go just as high as the surface circumference of the cathode.
     
  5. Jun 8, 2010 #4
    I'm in Australia. I have several quotes from our major gas suppliers, and USD$1800 (AUD$2200) is the best I could get, including cylinder rental.

    I need the system to be portable, so that really rules out electrolysis. (Also, seperating the O2 is also an issue, as well as the time it takes)

    May I ask how you got 105L of HCl? How much Alauminium did you conclude?

    Also, I have no idea of the completeness of the last part of the reaction either, ie how long it would take, at say 300DegC 300psi to completely convert? (i'm hoping to use the addition of the HCl as a moderator for the reaction, rather than just dumping it in an hoping for the best)
     
  6. Jun 8, 2010 #5

    Borek

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    Staff: Mentor

    You need around 3 kmoles of HCl, or 110 kg, at 32% that means 341 kg of solution, assuming density of around 1.16 g/mL thats 300 L.

    Check if iron won't be cheaper than Al (just remember it will get oxidized just to Fe2+). And start with a small scale test - check if Al really reacts the way you want it to. While Al is quite reactive, oxides on the surface make it relatively resistant, especially to acids, so the reaction can be much slower than you expect it.

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    methods
     
  7. Jun 8, 2010 #6
    Are you sure none of the water in the HCl solution is converted?

    My understanding is that the aluminium chloride reacts with water to produce HCl and aluminium oxides, is this not correct?

    This is what I'm unsure about, that the aluminium is just a catalyst in the first part of the reaction.
    Something like this (very, very roughly)
    Al + HCl = H + AlCl

    AlCl + H2O = H + HCl + AlO

    I've believe that HCl with react with aluminium oxides, and having oxidised aluminium markedly speeds up the reaction.

    Can anyone explain the effect of copper sulfate on the reaction? I've been informed that it can significantly increase the speed of the reaction.

    If any of this is the case, do I need to react away any additional oxygen that is produced?
     
  8. Jun 8, 2010 #7

    Borek

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    Yes, I am sure.

    It is correct in the case of anhydrous aluminum chloride. Here chloride is produced in the solution, and Al3+ are already hydrated, thats different situation.

    So the only reaction that will take place (and the one that calculations have to be based on) is

    2Al + 6HCl -> 2AlCl3 + 3H2

    From what I know that's not the case, presence of oxides makes the reaction slower than expected. It is much more prominent in other strong acids, and not that bad if you dissolve aluminum in hydrochloric acid, as chlorides are able to enter oxide structure and somehow they help transfer charge between metal and solution. Still, I would try it on a small scale first.

    Sounds reasonably. Copper can get reduced on the Al surface to metallic copper and it will create small galvanic cells, that will make aluminum more prone to H+ attacks. That is not much different from using sacrificial anodes to prevent corrosion of iron, only in this case we make Al a sacrificial anode to speed up its corrosion.

    I don't see any source of oxygen here?

    --
    methods
     
  9. Jun 8, 2010 #8
    I was refering to the oxygen released if H2O was involved.

    I'm going to purchase some aluminium powder and give this a go tomorrow.

    (Stupid Quastion) Is it possible to dehydate AlCl(H20)6 then recombine with stream? At what tempreture/pressure?

    Does anyone know of a more efficient (read: cheaper/denser/easier) method of producing hydrogen?

    btw, Thanks for all the input guys, I've forgotten almost everything I knew about chemistry.
     
    Last edited: Jun 8, 2010
  10. Jun 8, 2010 #9

    Borek

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    Staff: Mentor

    Possible - yes. But difficult. If you try just to heat the salt it will decompose and you will be left with Al2O3 and HCl.
     
  11. Jun 9, 2010 #10

    Lok

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    Mg + H20 + temp' -- although water vapor might be an issue. No idea about pricing.
    Na + H20 -- clearly not cheap, but very easy to do.

    Btw. separating oxygen from hydrogen in electrolysis is very easy, look it up, all you need is a good distance between anode & cathode and dump a fist full of NaOH in the solution.
     
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