How to calculate N2 volume or mass when there is some pressure differences.
e.q :

50 kg N2 liquid at 16 bar -->... m3 N2 gas at 7 bar

well you know how many moles there are from the liquid, and you know that a mole of ideal gas takes up 22.4L at STP, so you should be able to use PV=nRT to work out how much volume you have at 7 bar.

I'm not sure if you should assume the temperature is room or standard, though.