What is Boil off in Liquid Hydrogen?

[Booter]

Liquid Hydrogen Boil off ?

I am a little bit confused as to what boil off is, I can only assume it is the loss of gas but it is still unclear to me as to how a gas can escape from a steel container. Can anyone try to explain this to me better. Thank you very much

 

[Danger]

Welcome to PF, Booter.
I've always thought of 'boil-off' as simply the part that gets away before you can get the lid on, or as something vented to prevent an overpressure mishap.
Your question appears to be about what I call 'outgassing'. The molecules of hydrogen are small enough to fit through the intermolecular spaces in most materials. It's the same reason that you have to use special material for helium balloons to keep them from deflating right away. You can sort of think of it on a larger scale by figuring what would happen if you had a bunch of sand on a vibrating screen, the way archaeologists sift for fossils.

 

[mgb_phys]

but it is still unclear to me as to how a gas can escape from a steel container.

Assuming the container and you are still in one piece it will have an overpressure (boil-off) valve!
Depending on the size of the container and the quality of the insulation you might not be able to hear/feel anything coming out.

 

[Booter]

so once the container is sealed the only loss will be due to outgassing, and if I understand correctly this will be a very minute amount

 

[Bystander]

Nerp --- "seal" a container of liquid hydrogen and be prepared for a very abrupt failure of the container. The "boil-off" is to carry off heat leaking into the container through the insulation.

 

[mgb_phys]

Careful with the terminology.
Outgassing usually means (in a vacuum) any small amounts of air trapped in corners or absorbed on the surfaces being slowly released.
Offgassing means the boil off of a cryogenic liquid as heat leaks into it.

As bystander said - liquid hydrogen is the worst possible substance to trap in a sealed container. 1 litre of H2(l) will boil to fill a cube about 6" on a side!
Your lab will have rules about not traveling in elevators with liquid gas containers for this reason.

 

[Danger]

Careful with the terminology.
Outgassing usually means (in a vacuum) any small amounts of air trapped in corners or absorbed on the surfaces being slowly released.

Sorry. So is there a name for that leakage that I was referring to, or is it just called 'leakage'?

 

[mgb_phys]

I would probably call it "diffusion" but leakage is as good.

 

[Bystander]

Sorry. So is there a name for that leakage that I was referring to, or is it just called 'leakage'?

Permeation, permeability --- as H₂ through Pd (and every other metal), Hg through Au, O₂ through Cu, He through quartz --- and, given enough time, just about anything from a condition of high chemical potential through just about anything else to a state of lower chemical potential.

 

[Q_Goest]

Hi Booter,
The term "boil off" is a term used in the industrial gas industry for the amount of cryogenic liquid which converts to a gas by boiling inside a cryogenic tank. It has a very specific meaning, but it also has a very general meaning.

Every vacuum insulated cryogenic container has some amount of heat leak, and that heat will boil away the cryogenic liquid. If you maintain a constant pressure in the tank over time then you'll obviously need to vent gas off the tank. That rate of venting is the very general use of the term.

Boil off can also have a very specific meaning though which regards the "normal (or nominal) evaporation rate" (NER). Cryogas International is an industry journal, and they have a pretty good write up on it here:
http://www.cryogas.com/news/basiccryogenics.php

Boil off in the sense of NER boil off rates can mean one of two things. It can be the actual mass of cryogenic liquid that boils off per unit time, or it can be the actual mass of vapor vented from the tank. The amount of liquid that changes state is actually slightly higher than the amount of vapor vented since some vapor will stay inside the vessel to displace the liquid that just boiled. Generally, the term "boil off" is the amount which boils and NER is the amount vented, which is slightly less.

Note that boil off has nothing to do with outgassing. Outgassing is a phenomena seen in vacuums whereby gas comes off of the surface of a solid material as indicated by mgb_phys.

 

[Danger]

Thanks for the clarification about 'outgassing', guys. I'm not sure where I got the impression that it meant what I thought it did, but it's been with me a long time.

 

[xez]

Liquid carbohydrates can lead to hydrogen (sulfide)
outgassing, too. :)

 

[Booter]

so although the liquid hydrogen would become gassious hydrogen I assume in a closed container that it would increase in pressure until an equilibrium was met between temperature and pressure. on that point even though boil off would occur would the temperature also increase in the pressure vessel? Thank you for all the great replies

 

[Q_Goest]

so although the liquid hydrogen would become gassious hydrogen I assume in a closed container that it would increase in pressure until an equilibrium was met between temperature and pressure. on that point even though boil off would occur would the temperature also increase in the pressure vessel?

How is an equilibrium going to exist when liquid hydrogen is roughly -420 F? The liquid will all boil off, convert to gas, and as the gas warms up, pressure will continue to rise. The only way to come to some equilibrium temperature and pressure is for the temperature to rise to ambient such that there's no more heat transfer.