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# Physics/Pressure of Liquid Nitrogen

Ex-Nuke wrote at 2009-10-16 02:55:55
You can't derive the saturation pressure of a gas using PV/T.  PV/T only applies to gases, but during a situation where you have a liquid/gas saturation mixture it no longer works.  When the volume of the gas in a given space raises so much as to increase pressure greater than saturation pressure for a given temperature, an amount of gas will condense to form liquid and increase the gas/liquid mixture temperature by an amount equal to the latent heat of vaporization for the given substance and therefore, PV/T no longer works.

Zeta Theta wrote at 2011-09-09 13:58:13
I happened to have a similar question, and decided to research the answer. If you can get a copy of http://pubs.acs.org/doi/abs/10.1021/j150656a018 that article, it contains a phase diagram for the super-critical nitrogen fluid.

At room temperature a pressure vessel would need to maintain something close to 1.7Gpa. That's Gigapascal, equating to about 250,000 psi. Yes, two hundred fifty thousand psi.

There's good reason pressure vessels containing LN2 are vented.

Fabian wrote at 2012-04-23 13:32:18
I remember someone once said that the final pressure in a LN2 container that is not vented, and at room temperature will be 1600 psi. Most containers can't withstand that pressure so they vent the gas.

Mark wrote at 2016-02-29 09:19:31
I am sorry but this is a dangerously off answer.

I am not aware of the exact pressure / temp curve but when you bear in mind that many gasses (argon etc) are stored around 2000 psi and even bog standard CO2 and nitrous oxide can reach WELL over 1000 psi on a warm day - the idea that nitrogen is only a little above this is wrong. Dangerously wrong.

Remember, simple paintball air tanks and diving air tanks are often rated at 4000 psi - and if a tank as cheap and cheerful as that was enough to store nitrogen in a compressed liquid state (as we do with CO2 for example), then it would be done.

When I looked last, there was SOME disagreement over things but the lowest esitmate was around 42,000 psi and the highest was just under 50,000 psi. (Only they expressed in in Bars I think).

It is only because of this EXTREME pressure that a lot of industries (eg: large pubs) have to have cryogenic storage for liquid nitrogen (some drinks require nitrogen as well as CO2 - Guinness being one) instead of simply using a cylinder of "compressed to a liquid state" nitrogen.

either you get nitrogen in a gaseous state at around 3000 psi OR you get liquid nitrogen.

I guess if you had the engineering skills and the determination, you could produce a tank perhaps with a metal that wouldd safely handle 150,000 psi (to give some error of safety) and then fit a guage (I guess they must make one that can read that high for some hydraulic systems) and then "try it an see". Ideally using a VERY remote camera to keep an eye on things as the cylinder warms up under a few tonnes of earth in the middle of a deserted field.

Seriously, PLEASE do not think it can be stored in a cylinder rated at just 1600 psi, you are out by an order of magnitude and then by a fact of about three.

I REALLY dislike disputing other people without accurate figures to hand and references but the suggestion that the gas pressure would be under 2000 psi is potentially lethal if anyone tries it. The cylinder WILL explode.

Truly, if you could store the stuff under 10,000 psi, people would do it rather than faff around with LN2 tanks which lose their content whether it is used or not.

i apologise for "lack of politeness" but any suggestion that contained liquid nitrogen will not rupture a cylinder that is not rated at tens of thousands of psi is plain wrong and crazily dangerous. :(

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