Patent ID: 6837071

Claim:
A method of rejecting nitrogen from a 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 first condenser-reboiler, the first condenser-reboiler placing the top region of the higher pressure rectification column in heat exchange relationship with a bottom region of the lower pressure rectification column, recycling a flow of gas from the lower pressure rectification column to the higher pressure rectification column, the recycle gas flow being warmed, a first part of the warmed recycle gas being compressed to a first pressure, cooled, and introduced at least in part into the higher 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, wherein a second part of the warmed recycle gas flow is compressed to a second pressure lower than the first pressure, is cooled, and is condensed in a second condenser-reboiler associated with an intermediate mass exchange region of the lower pressure rectification column.