Patent ID: 7127915

Claim:
A method of rejecting nitrogen from a pressurised feed gas stream comprising methane and nitrogen so as to form a methane product, 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 to a pressure which does not vary, 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 with the performance of external work upstream of the main heat exchanger and the operating pressure of said lower pressure rectification column is periodically increased in response to increases in the mole fraction of nitrogen in said pressurised feed gas stream.