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Old Apr 20, 2016, 07:20 PM
HarryWA is offline
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Storing Hydrogen

Hello all,

WARNING THIS IS A RATHER LONG AND BORING POSIT ON STORING
HYDROGEN. IF YOU ARE NOT INTO HYDROGEN IT MAY NOT BE GOOD
FOR YOUR BRAIN.

Given that one would like to store 40 ft3 of hydrogen in N
number of 55 gallons drums where the gauge pressure would
be reasonable (say 20 - 30 psi gauge).

Given 55 gallons = 7.35 ft3 and Boyle's law P1*V1 = P2*V2:
Then: Pdrum * Vdrum = Pblimp * Vblimp or:
Pdrum * 7.35 = Pblimp * 40

Pdrum = Pblimp * 40/ 7.35 = Pblimp * 5.44

But what is the pressure in the blimp (Pblimp) ?

I gather one could use a pressure slightly above standard
atmospheric pressure which is (14.7 psi or 1.03 kg/cm2 ) see
note below. Lets say 15 psi or 1.05 kg/cm2.

So Pdrum = 15 * 5.44 = 81.6 psi or 5.7 kg/cm2 that is a lot
for steel drum?

For three drums then = 27.21 psi or 1.9 kg/cm2 that should be
save enough.

These blue plastic shipping containers/drums blow at
around 65 lbs.

Now what would one use as pump? One needs a "hose" on
each end of the pump to move the gas from the blimp to the
drum. Pressure in the drums moves the gas to the blimp I
would think.

Also inflating the blimp directly from the hydrogen generator
means that the generator only needs to work into low
pressure of the blimp rather then trying to pump from the
generator to the drums in small quantities at a time.


Does that seem correct? Don't want to make my own
hydrogen bomb!

Note:
"pressure airships, maintain their shape only because the
pressure inside the envelope, or gas bag, is slightly higher
than the external air pressure.."
http://blog.nasm.si.edu/aviation/blimp/

When I inflated a latex balloon I could not get a reading on the
gauge as the pressure was to low. See photo.
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Old Apr 24, 2016, 10:31 PM
pyronaught is offline
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The pressure inside a blimp is very low, and gets lower the bigger the blimp is. A small blimp would not likely ever be more than 2 psi, and it goes down from there. You need a gauge that measures inches of water instead of psi so that you can read these very low pressures.

As for storing hydrogen, you need to be 100% sure there is no oxygen mixed with it. Compressing mixtures of hydrogen and oxygen will result in an explosion if the oxygen content is high enough. It's not something I would want to screw around with, but if you do you will need some oxygen sensors so that you can monitor the concentration going into the tank. It is safer to make a hydrogen generator that generates gas fast enough to just run the output right to the blimp rather than trying to compress it in a tank. If you need compressed hydrogen for some reason it is better to just buy industrial hydrogen from a local supplier.
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Old Apr 30, 2016, 10:23 AM
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hydrogen

Your points are well made,

There are oxygen absorbers for the long term storage
of dry foods like Oxy-Sorb that are interesting.

They can take the oxygen level down below 0.1%
It's interesting that they make beer bottle
caps lined with such material to remove the oxygen,

I have been looking for calcium hydride for a desiccant.

The reaction of CaH2 with water:
CaH2 + 2 H2O → Ca(OH)2 + 2 H2

from Wikipedia:
"CaH2 has been used for hydrogen production. In the 1940s,
it was available under the trade name "Hydrolith" as a
source of hydrogen:

'The trade name for this compound is "hydrolith"; in cases of emergency,
it can be used as a portable source of hydrogen, for filling airships.
It is rather expensive for this use' "

It would make the ideal drying agent for the hydrogen gas,
But I can not find a inexpensive supplier except in China.
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Last edited by HarryWA; Apr 30, 2016 at 10:25 AM. Reason: corrections
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Old Apr 30, 2016, 11:43 AM
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How long does it take to absorb a given amount of oxygen though, and how much can it absorb before becoming saturated and no longer absorbs any? To scrub hydrogen on-the-fly as it is being generated would take a pretty fast acting absorber, and then you'd have to be making some pretty pure hydrogen to begin with in order not to saturate it too quickly.

Another method I've seen is a catalytic converter, which basically removes the oxygen by recombining it with the hydrogen to produce heat and water. The advantage is it can be done on-the-fly and never saturates, but the down side is you lose some hydrogen in the process and you also have to dissipate the heat. Keeping the initial oxygen contamination low out of the generator makes this method viable and low cost though.
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