Patent ID: 7373790

Claim:
A method of rejecting nitrogen from a pressurised feed gas stream comprising methane and nitrogen so as to form a methane product, wherein the mole fraction of methane in said feed gas stream decreases during operation of said method comprising cooling the feed gas stream in a main heat exchanger, rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser/reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, withdrawing a product methane stream in liquid state from the lower pressure rectification column, raising the pressure of the liquid product methane stream, and vaporising the liquid product methane stream, at least part of the vaporisation being performed in the main heat exchanger, wherein over a range of feed gas stream pressures the pressurised feed gas stream is expanded to a chosen pressure with the performance of external work upstream of the main heat exchanger and the chosen pressure is increased with decreasing mole fraction of methane in the feed gas stream.