Pressure Vessel

[BJT]

I'm not sure where to ask this. This is just something I've thought about and can't resolve.

 

Let's say you had a bunch of liquid hydrogen, helium, or air. It doesn't matter. Something that condenses only at extremely low temperatures. Now let's say you put that liquid in a thick vessel and filled it up. What would happen when you let the temperature inside increase? Let's say the temperature increases to room temperature. Or maybe you're even heating it. What happens to the liquid inside assuming your pressure vessel is so think it cannot be breached. Or would the gas find a way out no matter how thick the vessel was?

 

I'm a degreed engineer but I can't make any sense of this problem. The very intelligent engineer who sits next to me says the vessel would break because the liquid would have to expand once it reaches it's critical point, but he never explained clearly how it would do so if the vessel was thick enough that it could theoretically take the pressure.

[J.C.MacSwell]

I'm not sure where to ask this. This is just something I've thought about and can't resolve.

 

Let's say you had a bunch of liquid hydrogen, helium, or air. It doesn't matter. Something that condenses only at extremely low temperatures. Now let's say you put that liquid in a thick vessel and filled it up. What would happen when you let the temperature inside increase? Let's say the temperature increases to room temperature. Or maybe you're even heating it. What happens to the liquid inside assuming your pressure vessel is so think it cannot be breached. Or would the gas find a way out no matter how thick the vessel was?

 

I'm a degreed engineer but I can't make any sense of this problem. The very intelligent engineer who sits next to me says the vessel would break because the liquid would have to expand once it reaches it's critical point, but he never explained clearly how it would do so if the vessel was thick enough that it could theoretically take the pressure.

 

They also condense at ambient temperatures...at higher pressures.

 

edit: this is wrong see CP below

Edited May 17, 2011 by J.C.MacSwell

[CaptainPanic]

Let's assume a few things before answering this question:

 

The container is a theoretical one - which cannot break under any circumstances.

The container is filled with any of the gases mentioned (hydrogen, helium or air).

The container is not completely filled to the brim with liquid. There is a little space on top for gases.

 

So, we're starting off at extremely low temperatures, where the hydrogen/helium/air is in fact a liquid at atmospheric pressure.

If we start to increase the temperature, the liquid itself expands - and that's why we need a little space on top. Otherwise, the pressure would increase extremely rapidly, and we'd be discussing something very different.

Now, as we pass the critical point of the gases (helium is already at 5.19 Kelvin!) we no longer speak of a liquid or a gas. We have a supercritical fluid.

 

The thing with a supercritical fluid is that there is NO transition from gas to supercritical to liquid. You don't see a sudden change. So, it's also difficult to say where the transition is exactly (hence the dotted lines in the pictures in the wikipedia links). But I think it's safe to assume that hydrogen, helium and air alike all would remain in their supercritical state up until room temperature.

 

Note: this also means I strongly disagree with J.C.MacSwell's response that there is a 'condensation' at room temperature. That is NOT true. You cannot get any of those gases in a liquid state at room temperature by merely increasing the pressure. You will just get them supercritical, not liquid. The liquid state (and any boiling / condensation effects) can only be achieved at temperatures below the supercritical point. If we assume that air would be pure nitrogen, then the highest of the critical points is 126 K. Above that, and there is NO gas/liquid, only supercritical fluids.

 

Slightly confusing: when the gas is thin enough (although in temperature well above its critical point) we still call it a gas. Therefore, air is a gas, not a supercritical fluid.

 

To make it slightly more complicated: there is still a pressure so high that your gas can turn into a solid. (Transition from either liquid to solid is possible, but also from supercritical to solid).

 

Of course, in a real life test, you're always risk breaking your vessel... and these kinds of tests are typically done in special equipment.

[InigoMontoya]

Note: For a "normal life" example of the situation being asked about... Go get one of those 12 gram CO2 cylinders that run BB guns and the like. Heat it up to 100 F. Congrats, you now have a real life pressure vessel containing a supercritical fluid.

[Spyman]

"Do not expose to sun and heat. Explosion danger - 50 C max temperature."

(From a CO² BB gun cartridge.)

[CaptainPanic]

Note: For a "normal life" example of the situation being asked about... Go get one of those 12 gram CO2 cylinders that run BB guns and the like. Heat it up to 100 F. Congrats, you now have a real life pressure vessel containing a supercritical fluid.

 

"Do not expose to sun and heat. Explosion danger - 50 C max temperature."

(From a CO₂ BB gun cartridge.)

 

Note that the critical point of CO2 is at 31.1°C and 7.38 MPa (73.8 bar). That's within the safety limits of that cartridge.

 

The 100 F mentioned by InigoMontoya equals 37.8°C.

 

Anyway, please don't see this as criticism... it is never a bad thing to mention safety.

[Spyman]

Note that the critical point of CO2 is at 31.1°C and 7.38 MPa (73.8 bar). That's within the safety limits of that cartridge.

 

The 100 F mentioned by InigoMontoya equals 37.8°C.

Yes and ~12° Celcius is probably a good enough safety limit for someone who knows what they are doing, but for a less experienced youngster eager to experiment, those low and few 12 degrees are very easy to overcome.

 

 

Anyway, please don't see this as criticism... it is never a bad thing to mention safety.

Sure, I didn't intend to critique InigoMontoya either - I only wanted to point out the hazard involved.