Quote:
Originally Posted by Suzy_A
A few more comments about decanting high-pressure gas into a low pressure cylinder - as Monte points out, if you get it wrong, you may not be around any longer. Or rather, you may be all over the place. A 20 L cylinder at 200 psi will have 260 L of gas in it, or about 10 moles at 20 C. The (pressure) energy is then about 36 MJ, or about the same as in a litre of petrol or a few kg of high explosives. It goes off even better with some O2 added, so everything has to be flushed and O2 free.
However it is possible.
A LPG cylinder is tested to 3.3 MPa (32 atm - that's atmosphere's, not amateur telescope makers!) and has a burst pressure of about double that. The max safe working pressure is about 1/2 that - about 13 atm or 200 psi. Obviously you also need to take into account the temperature effects - as the temp rises, the pressure will too.
So if you limit the gas pressure to something safe, then there should be no problems. Except maybe due to hydrogen embrittlement of the steel cylinder - which is why H2 tanks are lined to protect the steel (and stop leakage) or N2 or something else is added. Transporting H2 by pipeline is a problem due to embrittlement and leakage, but adding 10 - 20% methane stops the embrittlement and leakage - the larger molecules plug up the pores in the metal and also seal the metal crystal lattice. Maybe as a chemist you know all this?
Of course being a chemist you should also know how to make H2 - zinc and HCl - use old die-cast metal bits and pool grade HCl. Or Al and NaOH, but be careful as the Al + NaOH starts off slow due to the Al2O3 coating on the Al but as it dissolves aways and also heats up, the reactions can get much faster.
If you want pure H2, pass the gas you make through water to wash it, and then dry the gas by passing it through a cold trap - pass the gas through a vessel under ice or dry ice. Or pass it through silica gel. Even better, through ice then gel. If you want to store home-made H2, use a fridge compressor and pump it in a LPG cylinder. But you need to pas the gas through another cold trap - dry ice and ether or liquid N2 - to catch the oil vapours from the pump.
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I am aware of hydrogen embrittment of steel , there is another mechanism that you perhaps have not heard of , where hydrogen enters the steel and diffuses to voids and fills them with very high pressure hydrogen which causes the steel to rupture along grain boundaries and can lead to catastrophic failure.