What happens to a liquid nitrogen canister under pressure if left at room temperature? Will it eventually explode? Or forever stay cold? If I put a strong, sealed container of liquid nitrogen on a table at room temperature, will it always stay cold? Or will it slowly heat up until the canister bursts? Assume it is an extremely strong container made of reinforced materials? I suppose no matter how strong, it will still explode one day. Any idea how long a standard reinforced LNG bottle will last before it explodes?
 A: The liquid nitrogen will certainly warm up to room temperture, and a high pressure will build up inside the container (I assume, your container provides no thermal isolation). If the container is firmly sealed, it will eventually explode, if the pressure becomes higher than what the container can stand. For example, this happens when you fill liquid nitrogen into a plastic bottle (don't try it out!!!, there is serious risk for your health doing it).
However, special tailored containers may stand these high pressures, e.g., special steel containers.
As a rough order of magnitude estimate: at normal pressure, the gas takes roughly 1000 times the volume that the liquid needed. If the volume is kept constant, the pressure will rise roughly by a factor of 1000.
Note that this is a bit handwaving, but suffices for most practical cases. For a more precise description of what is going to happen, you have to consult the phase diagram for nitrogen.
A: There is no such thing as perfectly isolated cryogenic tank. Some heat will always transfer to the contents. However, cryogenic liquid tanks are not completely sealed. They instead have pressure relief valves. The valve opens when the internal pressure exceeds some upper limit, then closes when the pressure drops below some lower limit. This is the mechanism by which cryogenic liquid tanks (including liquid nitrogen tanks) stay cold.
Suppose a full liquid nitrogen tank is abandoned for decades. What happens over time won't be very exciting. A decade later, the tank pressure will be somewhere between the relief valve's upper and lower pressure limits, the temperature will be very close to ambient, and the mass of nitrogen in the tank will be very small. Almost all of the nitrogen will have boiled off and been vented to the atmosphere.
Now suppose the tank is abandoned and the relief valve fails in the closed position. What happens in the ensuing decades will briefly be exciting. Something will fail, catastrophically. It might be the relief valve, it might be some other valve, or it might be the tank itself. The end result will be an empty tank (or perhaps an empty remnant of a tank).
Finally, let's suppose the tank has no valves and is incredibly strong. For lack of better words, it's made of unobtainium. (But not Thermodynamically Isolated Unobtainium©; that stuff is ridiculously expensive.) After decades of abandonment, the contents of the tank will be at ambient temperature and at a ridiculously high pressure. The contents won't be a liquid (nitrogen's critical temperature is -146.9 °C), but it won't quite be a gas, either. The reactions of a highly pressurized supercritical fluid are somewhere in between those of things we call liquids and things we call gases.
