Why can't liquid nitrogen be sealed in gas cylinders? By observing the phase diagram of nitrogen, hydrogen and carbon dioxide, I discovered the supercritical region.
I can easily buy a gas tank of carbon dioxide at 250 Bars. However I never saw any cylinder of water or liquid nitrogen at 250 Bar. Why not? At my university, we usually use an thermally isolated, but opened, vessel. In my opinion, a cylinder of liquid nitrogen would be more convenient to manipulate. Is that not true? 
According to this diagram, at the temperature of 300K and pressure of 25 MPa (250 Bars), I am just in the supercritical region. Isn't that possible?
 A: I looked up the vapour pressuer of carbon dioxide at room temp (20 deg. C) and it is 57 bar, so I don't think you can have seen a cylinder of CO2 at 250 bar. If it is advertised as containing '250 bar' then the reality will be a lower pressure plus some solid CO2 in the bottle as well.
For nitrogen, commercial cylinders can be supplied up to about 200 or 250 bars or more. 
For water, supercritical water is highly corrosive - it is both strong acid and strong alkali, dissolves organic material and is not easy to handle. People who do research with supercritical water tend to use small volumes (for safety) and special pumps to pressurize and depressurize the fluid. A container or water at 250 bar (and at temperature where the vapour pressure is 250 bar) is potentially very explosive and dangerous.
A: At research laboratories where liquid nitrogen (and oxygen) are commonly used every day, closed containers of LN and LOx are routinely transferred and stored. These pressurized tanks typically operate at around 20 atmospheres and hold between 50 and 250 liters.
For short-term needs and operations, the LN will be transferred to a vented vacuum-jacketed dewar. These come in various sizes, but are not pressurized (as you have seen).
