eli5 why cant you keep liquid nitrogen in a high pressure tank? even though it wants to turn to gas, at a certain pressure it just has to stay in liquid state, when you open the tank again it would just become cold again right?
In: Chemistry
Liquid nitrogen at room temperature would exert a pressure of ~43000 psi.
For comparison to other substances commonly held as a liquid under pressure; liquid CO2 (e.g. for soft drinks) exerts ~900 psi and propane (e.g. fuel) exerts ~120 psi.
You could keep it in a high pressure tank, but why go through the expense and risk when a low pressure tank and insulation works? The point of having N2 around is typically for the cold, so it’s easiest to keep it cold, and that doesn’t require a high pressure tank.
If you kept it in a high pressure tank when you opened that valve on a high pressure tank the liquid would boil, to the point it may appear to explosively boil. The amount that boiled off would be huge and all wasted.
It exists as a supercritical fluid at room temperature and high pressure. What are the uses of this supercritical liquid nitrogen? Who knows, probably nothing industrial.
It won’t have the same utility as a -320 F at 1 atmosphere liquid though. Hence why you see so much liquid nitrogen transported at 1 atmosphere (or there abouts).
A sufficiently robust pressure vessel (much stronger than the gas cylinders you are thinking of) could theoretically be used in the system you describe, but would have several notable safety hazards.
1. After the tank reaches room temperature, in order to cool it off again you would need to vent some gas out to cool it off. Venting nitrogen gas into a room is an especially bad idea if you are standing in it, as it is an odorless asphyxiant. So your “simple” tank would need a dedicated ventilation system to the outside to not kill you.
2. If the tank does not have a vacuum wall to insulate the exterior surface, then when it is cold enough to get liquid nitrogen out, it is cold enough to give you frostbite from touching it.
3. Cooling the tank through venting will build up ice around the vent, potentially making it impossible to close. You also wouldn’t want a safety release valve buried in ice.
4. Pressure vessels always have a small risk of rupture or exploding. If the nitrogen is room temperature under a high pressure, then such an event will instantly fill the room choking everyone inside. If the tank is full of cold liquid nitrogen under a reasonable pressure, a rupture will not lead to an explosion, and people will likely have time to evacuate.
5. Rapid heating and cooling of a pressure vessel to cryogenic temperatures and back sounds absolutely horrible from a materials standpoint. I would absolutely not trust that vessel unless it was super-thick. And thus super heavy.
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