LN2 Critical Point

[ktdough]

I have a “science-guy” question for you. What would happen if an LN2 tank filled with liquid nitrogen was sealed off and could take an infinite amount of pressure? In other words what pressure would it go up to if none of the gas could escape and the tank was made to take it?

 

[iainuts]

This depends on the initial pressure, and how much liquid vs. gas there is. The following assumes the LN2 is saturated at 14.7 psia.

If the vessel were completely filled with LN2 (no vapor) with density of 52.3 lbm/ft3 (aprox) then the final pressure at 70 F (ambient) would be about 49,000 psig per my database.

If the vessel was filled to 90% liquid by volume, 10% gas by volume, the final pressure at ambient is around 39,000 psig.

Note: Different databases have different values for density vs. temp and this is only an approximation.

 

[ktdough]

Thanks for the information! Is there any chance you could point me to a database like the ones you mentioned or tell me how you compute such a thing?

 

[iainuts]

The database I use is company proprietary. Most databases are owned by folks that want to be paid them, such as the ones at NIST

http://cryogenics.nist.gov/

Check out the "Fluid Properties" link on this page.

The way this is calculated is very simple. You have a known mass of nitrogen in the vessel as a cryogenic fluid which does not change since the vessel is assumed to be sealed. Therefore, you know the density of the gas as it warms up. Knowing the final temperature and density gives you a pressure which can be obtained from the database. This is far out of the ideal gas range, though there are also correlations in many handbooks that improve accuracy as the fluid deviates from ideal gas if you found those equations. I'm sure there are things on the internet that might help.

Hope that makes sense.

Dave.