Nitrogen rejection method and apparatus

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, and vaporising the liquid product methane stream, at least part of the vaporisation being performed in the main heat exchanger, wherein the operating pressure of the lower pressure rectification column is periodically increased in response to increases in the mole fraction of nitrogen in the pressurised feed gas stream, a stream of gas withdrawn from the lower pressure rectification column is expanded with the performance of external work, and over a range of feed gas stream pressures the pressurised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.

An apparatus for rejecting nitrogen from a pressurised feed gas stream comprising methane and nitrogen so as to form a methane product, comprising a main heat exchanger for cooling the feed gas stream, a double rectification column for rectifying the feed gas stream 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, and a liquid pump for withdrawing and pressurising a stream of product methane in liquid state from the lower pressure rectification column, the liquid pump having an outlet communicating with vaporising passages in the main heat exchanger, wherein the apparatus additionally includes upstream of the main heat exchanger an expansion turbine operable over a range of feed gas pressures to expand the feed gas with the performance of external work.