Natural gas liquefaction with nitrogen rejection stabilization

In a cascade refrigeration system for liquid natural gas containing at least 1.5 percent nitrogen in which the feed is cooled by refrigeration and heat exchange with intial flashing of liquid to a pressure sufficient to produce a flashed vapor stream containing substantially all of the contained nitrogen and a flashed liquid stream and with recycled vapors used for heat exchange, recompressed, and combined with the incoming feed, a method for stabilizing the nitrogen content of the recycled vapors comprising:

• (a) passing the natural gas containing at least 1.5 percent nitrogen through an indirect heat exchanger for reboiling the kettle of a nitrogen stripping column;
• (b) initially flashing the effluent natural gas from (a) to a pressure sufficient to produce a flashed vapor stream containing substantially all of the contained nitrogen and a flashed liquid stream;
• (c) subjecting the flashed vapors from the initial flashing of liquid to a stripping operation in a nitrogen stripping column to remove at least part of the nitrogen content;
• (d) recovering refrigeration from the vapor stripped from said flashed vapors by heat exchange within the system;
• (e) yielding the stripped vapors from the system;
• (f) returning as a liquid the stream from which nitrogen has been stripped to the liquid stream produced from the initial flashing of liquid;
• and
• (g) further flashing flashed liquid from the initial flashing of natural gas with liquid produced from this flashing operation used in part as cooling liquid for indirect heat exchange with the vapors from the initial flashing and in part for further cooling and flashing to produce a liquified natural gas.