NITROGEN REJECTION METHOD AND APPARATUS

摘要

Nitrogen is rejected from a feed natural gas stream comprising methane a nd nitrogen so as to form a primary methane product. The mole fraction of nitrogen in the feed natural gas increases over a period of time. The feed natural gas stream is cooled in a main heat exchanger 10 and is rectified in a double rectificatio n column 20. A primary product methane stream and a secondary nitrogen-enriched produ ct stream are withdrawn from the rectification column 20. The secondary nitroge n- enriched product stream has a mole fraction of methane at or above a chosen minimum value when the said mole fraction of nitrogen is at a minimum. When the said mole fraction of nitrogen rises to a value at which the mole fraction o f methane in the secondary nitrogen-enriched product stream falls below the chosen minimum, a part of the feed gas is introduced through conduit 90 into the secondary nitrogen- enriched product stream so as to restore its mole fraction of methane to the chosen minimum value or a value thereabove.