systemloc
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Hydrogen gas generator?
Hi, I'm looking into different ways to produce hydrogen gas in order to fill a large balloon (20-60 cu.ft.) within 5-10 minutes or so (5-10
cu.ft./min).
Obviously, the first thing I tried was aluminum in sodium hydroxide. This would take a massive amount of foil, so I tried some 1/8"x1" aluminum strip.
That took over an hour to make one liter or so of gas.
An alternative would be a large zinc anode for boat hulls in a strong acid. This would probably work, but would also be susceptible to surface area
limitations, and a bit expensive to buy the zinc. It appears to be easy to plate back zinc chloride, so this would be recyclable. Comments?
Electrochemically, I can just use water and current, however this generates oxygen as well, and keeping this separate, and pressure equalized with a
balloon seems a bit painful. I've been trying to find some side reaction for the anode that wouldn't produce a gas and would be easily regenerated.
I'd appreciate any advice on how to reclaim the zinc for the zinc/acid solution, or a side reaction for water electrolysis that would allow for only
hydrogen production, and would be easily reversible.
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ScienceSquirrel
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I think you are going to need a cylinder for that.
The only way to make that sort of amount of hydrogen on a home chemist basis in that time would be fine aluminium powder with say hydrochloric acid
and copper salts.
The reaction is going to have to run very fast and it is going to be highly exothermic plus you will have foaming as well.
If I were doing it on a professional basis I would use sodium borohydride and water. Very high hydrogen content and a very cool and controllable
reaction.
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zed
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Nope. Easiest way to do it, is the regular way. Either a tank of hydrogen gas, or Calcium hydride and water.. You can make Calcium Hydride, or you
can buy it surplus.
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Random
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How could I synthesize calcium hydride at home (the easiest way)?
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peach
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20cu.ft of hydrogen
First google;
"hydrogen gas volume per mole"
22.4 Liters <--- Answers.com
Second google;
"al naoh h"
2 Al(s) + 6 NaOH(aq) → 3 H2(g) + 2 Na3AlO3(aq) <--- Wiki
Third google;
"20 cubic feet to liters"
20 (cubic feet) = 566.336932 liters <--- Google calculator
Fourth google;
"566.336932 / 22.4"
25.2828988 <--- Google calculator
Working it out;
25 moles of hydrogen, reaction above gives three so...
Fifth google;
"25/3"
8.33333333 <--- Google calculator, multiply by 8
Reaction becomes;
2 Al(s) + 6 NaOH(aq) → 3 H2(g) + 2 Na3AlO3(aq)
x8
16 moles aluminium, 48 moles NaOH
Sixth google;
"Aluminium"
26.98 standard atomic weight
Seventh google;
"NaOH"
33.99 g molar mass
Eighth google;
"26.98 x 16"
431.68g aluminium
Ninth google;
"33.99 x 48"
1.6kg
So.... you're going to need around half a kilo of aluminium and one and half of drain cleaner.
I can tell you from past experience filling bin bags with hydrogen this way, the heat of the reaction will massively contaminate the hydrogen with
moisture, which will negate the lifting effect and easily make it heavier than it would be if it was filled with air. You'd want to blow the hydrogen
through some silica and probably some solid NaOH as well to strip all the moisture back out.
If you're trying to make a mini Hindenberg, a cylinder or two is likely a better idea, if you're not interested in the reaction.
That quantity of drain cleaner is not too hard to buy or too expensive. At a very rough guess, it'd be about £10-15 pounds worth off the shelf in the
DIY store.
Finding the half kilo of aluminium may be more tricky. Scrap dealers and machine shops have bins full of it as chips.
If you don't have a gas supplier account yet and haven't already rented a cylinder (and don't have some other use for it), it may still be cheaper to
generate it yourself, as cylinder rental can be painful. BOC do them by the month now, which is more suited to this kind of thing. It's usually about
£40 - 60 to rent a cylinder for a year, lots more if it's a reactive one.
This reaction is fond of extreme thermal runaways when all the ingredients are put in at once, as they likely will be in an improvised generator. It
will get so out of control hoses will be popping off, bungs popping out, the glass may burst or the plastic may melt and there will likely be a plume
of steam rocketing out of the container, meaning you will have to immediately bail out and turn the power off (might be an idea to do that first).
Care must be taken! 
Which is of coarse to say nothing of what will happen if there are any ignition sources around.
Google some more;
"Hydrogen"
Minimum ignition limit 4% with air <--- Wiki
Google;
"100 / 4"
Multiply volumes by 25
Google:
"20 x 25"
The generator will fill a 500 cubic foot room with an ignitable mixture.
I remember them testing that on myth busters, the guy who strapped helium balloons to his deck chair and went for a flight one afternoon. They ended
up spending thousands sucking all the cylinders out of the local suppliers.
Along with that, the lead balloon episode is a highly recommended watch.
I would highly not recommend this reaction at this scale if you struggle with any of the above maths & working out.
[Edited on 1-11-2010 by peach]
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systemloc
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Helium tanks are significantly more expensive, and don't lend themselves to long-term storage.
Wow, I wasn't aware of the calcium hydride. That looks to be a very nice solution, except for obtaining it. Fair radio has some cheap, hopefully in
quantity. This may turn out to be my solution.
Also, as for the Al + NaOH reaction, I haven't been able to find why copper chloride seems to be added to speed the reaction. I did another little
test run adding Al + NaOH + CuSO4 + NaCl and it did appear to be significantly more vigorous (Didn't have CuCl2 handy). Any comments on what copper
chloride does?
Of course, heat production is going to be a problem. I was considering water bath vs a condenser of some sort to chill the gas before it goes into the
balloon.
I'm also interested in an electrolytic solution if anyone has some ideas.
peach: The math is trivial. Now that you mention it, I should calculate the heat production as well.. Also, scrap aluminum is a good idea, thanks.
Ironically, my purpose was exactly what the surplus calcium hydride at Fair Radio was for. I'm developing an emergency antenna deployment system for
ham radio. A light weight wire could easily be suspended vertically to over 100' using a balloon or a kite (depending on weather). I was hoping to
have a working solution by Field Day in June.
Yeah, I'm aware of the irony of the Hindenburg, as well as the flammability and spark risk of a hydrogen balloon. It will be well insulated from any
ignition source and kept at a safe distance from bystanders.
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The WiZard is In
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Quote: Originally posted by systemloc  | | Hi, I'm looking into different ways to produce hydrogen gas in order to fill a large balloon (20-60 cu.ft.) within 5-10 minutes or so (5-10
cu.ft./min). |
Being an analogue person I own an original copy of —
The Chemistry and Manufacture of Hydrogen
P. Litherland Teed
Longmans, Green and Co.
1919
They filled a SL of balloons w/ hydrogen during the great war.
The methods they used to generate hydrogen are therein well described in Teed's book.
La book is only 152 small pages and a good read.
DL'd it from Google.com/books
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hissingnoise
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Ham radio? In the twentyfirst century? WTF! 
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watson.fawkes
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CaH2 at Fair Radio. It's $13 for 1200g.
[Edited on 1-11-2010 by watson.fawkes]
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peach
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Hydrogen generator + antenna + emergency + potential lightning strikes / static discharge does not sound good at all.
Neither is trying to cool the reaction such a great idea, since the cooling for something that large and generating that much heat is going to need
testing and experience. I can assure you, the reaction will rocket out of control faster than the balloon will go up if given the chance.
If you're going to cool it, that means the power has to be on to shift the vast quantities of heat it'll produce, batteries will need to be huge,
maintained and probably not all that effective. You need to control the rate from the start, as you would with an addition funnel. Which, in this
case, would be probably be a plastic container of some sorts dripping the hydroxide onto the aluminium.
If the maths and working this all out were trivial, you could have done it yourself prior to checking it with us right? Since it took me about five
minutes and I'm not all that interested in making gigantic hydrogen balloons. 
The MIR space station had an emergency oxygen generator on board, an oxygen candle that burns and releases O2. Despite the vast quantities of research
and money put into it, it caught fire. In orbit.
For this application, you need to BUY a cylinder. I am being what will likely appear rudely blunt, but if you want the balloon to work in an
emergency, DIY'ing such a large scale generator and expecting it to work under those circumstances, quickly and without failing, that is going to take
a fair bit of work and checking. 5-10 minutes is fast. More so than checking the heat generated, you want to compare that to the thermal
conductivities of what it's contained in, to see how many minutes it'll be before it's boiling, literally.
MIR used an oxygen candle because it'd store more oxygen in the same volume, and volume / weight costs a phenomenal amount when firing things into
space.
[Edited on 1-11-2010 by peach]
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ScienceSquirrel
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I would go for a small cylinder. It's sheer convenience etc would make it highly convenient.
Any kind of metal plus acid or base plant is going to be a nightmare in the field.
Calcium hydride is cheap and pretty convenient but I would go for sodium borohydride if you can get it.
It reacts with tap water in the presence of a catalyst to produce very pure cold hydrogen. It produces four moles of hydrogen ( 88 litres ) from 38
grams of compound and the chemistry is well described as it is used for fuel cells etc.
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The WiZard is In
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| Quote: Originally posted by peach |
The MIR space station had an emergency oxygen generator on board, an oxygen candle that burns and releases O2. Despite the vast quantities of research
and money put into it, it caught fire. In orbit. |
ValuJet Flight 592 1996 110 killed.
http://en.wikipedia.org/wiki/ValuJet_Flight_592
Byda VJ and MIR oxygen generators are a lot more sophisticated than an "oxygen candle". Though chemically ....
Found at Wiki-P NTSB oxygen generator test - fire!
http://www.youtube.com/watch?v=6i_l_ux3R-4
At no extra charge - An O2 cylinder explosion.
http://www.youtube.com/watch?v=9lw_fhNAIQc&feature=relat...
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peach
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What the science was he doing touching a cylinder in the first place, let alone one to do with oxygen and medical care?
"Might be empty, might be fully pressurized, I'll just unscrew the valve"
As silly as that sounds, I have seen 'professional HAZMAT' teams doing exactly the same thing with cylinders of ammonia and hydrogen chloride just to
get it done as fast as possible. They didn't even bother bracing the cylinder, they went straight to the valve with a wrench; charged cylinders.
Ridiculous.
Indeed, a meth lab is dangerous, particularly so when you start doing that.
Given the markings on the cylinders and the type of work involved, there is a good chance things like that may have been back filled with anything.
Anything which may spontaneously ignite or explode when opened that way.
This is where I begin figuring the BOC rental charge is worth it, so I can be reasonably sure someone who knows what they're doing has played with it
and checked it. Not someone DIY'ing the valves in a makeshift workshop.
{edit}I have seen that video before, and many others like it. But they're always worth a rewatch.
[Edited on 1-11-2010 by peach]
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