Hydrogen weirdness

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Grumble
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Hydrogen weirdness

Post by Grumble » Wed Nov 30, 2022 11:27 am

Did you know that hydrogen and to a lesser extent helium heat up as they expand? This is counter to the typical behaviour for gases which cool as they expand.

I’ve just had to buy relief valves for helium and argon and they require different seal materials from each other due to the temperature difference on relief. Argon goes really cold.

I had always thought there was a straightforward relationship between expansion/compression and temperature but it seems IABMCTT.
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dyqik
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Re: Hydrogen weirdness

Post by dyqik » Wed Nov 30, 2022 11:34 am

Grumble wrote:
Wed Nov 30, 2022 11:27 am
Did you know that hydrogen and to a lesser extent helium heat up as they expand? This is counter to the typical behaviour for gases which cool as they expand.

I’ve just had to buy relief valves for helium and argon and they require different seal materials from each other due to the temperature difference on relief. Argon goes really cold.

I had always thought there was a straightforward relationship between expansion/compression and temperature but it seems IABMCTT.
Not sure about hydrogen, but helium is an ideal gas at almost all lab pressures and any temperature above 4.2K. it definitely cools as it expands, and cryocoolers in e.g. MRI machines or my receivers wouldn't work if it didn't.

Helium needs particular seals because it can leak through a lot of common seal materials. It can also convert from liquid to gas while at very low temperatures, which means you might need pressure relief at very low temperatures, and gas expansion can work to very low temperatures. OTOH, it's got very low heat capacity, so it doesn't carry much enthalpy away when it does expand.

Googling, hydrogen is also an ideal gas above 20K at standard pressure, and at least up to 13 bar. It has a supercritical point at 13.3 bar and -239C, but I doubt that makes much difference in non-cryogenic work.

It is fairly reactive though, so that might affect the seal material. Hydrogen embrittlement of valve parts might also be an issue.

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Grumble
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Re: Hydrogen weirdness

Post by Grumble » Wed Nov 30, 2022 11:45 am

It’s a real gas, not an ideal gas. You have to consider the Joule-Thomson expansion. In an ideal gas pV remains constant during expansion, but in a real gas it doesn’t. The difference is whether or not there are inter-molecular forces. For most circumstances assuming ideal gas is fine, but during expansion it doesn’t quite work.
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Re: Hydrogen weirdness

Post by dyqik » Wed Nov 30, 2022 11:54 am

Grumble wrote:
Wed Nov 30, 2022 11:45 am
It’s a real gas, not an ideal gas. You have to consider the Joule-Thomson expansion. In an ideal gas pV remains constant during expansion, but in a real gas it doesn’t. The difference is whether or not there are inter-molecular forces. For most circumstances assuming ideal gas is fine, but during expansion it doesn’t quite work.
Hydrogen, helium and neon don't cool in Joule-Thomson expansion at room temperature. They're ideal gases in this context.

https://en.m.wikipedia.org/wiki/Joule%E ... son_effect

Helium J-T expansion does produce cooling at low temperatures though. Some of our cryocoolers depend on it. (I guess my 4.2K above should be a bit higher)

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Re: Hydrogen weirdness

Post by shpalman » Wed Nov 30, 2022 12:06 pm

It's not the expansion itself which causes (ideal) gases to cool as they expand, but they work they do pushing the atmosphere out of the way. If you let a gas expand into a vacuum there's no temperature change (Joule expansion).
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Re: Hydrogen weirdness

Post by Martin_B » Wed Nov 30, 2022 10:35 pm

Talking about PV isn't quite the whole story. The equation is PV = nZRT, with the terms being pressure (P), volume (V), number of molecules (n), compressibility factor (Z), gas constant (R) and temperature (T). Rearranging, T = PV/nRZ

When a gas is expanding (P dropping, V increasing), both n and R are constant, so T is proportional to PV/Z. Ideal gases have a Z factor of 1, but in practice no gases are ideal over their entire pressure range and most vary with both pressure and temperature.

Generally, increased temperature reduces the value of Z while increased pressure increases the value of Z but where the pressure is below the critical pressure you will generally find that the temperature component dominates and Z is below 1 and above the critical pressure Z can be above 1.

The practical outcome of this is that for most gases in most applications where pressure relief is taking place, Z is below 1 but moving towards 1 as the pressure is decreasing (the temperature component dominating) and so as Z is increasing in PV/Z this results in Joule-Thomson cooling.

However, if pressure relief is taking place from above the critical pressure, where Z is above 1, as the pressure reduces the value of Z drops towards 1 and in this case PV/Z results in T increasing and Joule-Thomson heating.

Hydrogen's critical pressure is ~13 atmospheres, so it's quite easy to compress it to above the critical pressure. Helium's critical pressure is only 2.2 atmospheres. In contrast, Methane's critical pressure is 46 atmospheres and Oxygen's is 50 atmospheres.
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