Process for Separating Off Nitrogen from Natural Gas

Abstract

The invention relates to a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas, wherein: a) the feed fraction ( 1 ) is liquefied (E 1 , E 2 ), b) is separated by rectification (T 1 ) into a nitrogen-enriched fraction ( 9 ), the methane content of which is a max. of 1% by volume, and a hydrocarbon-rich, nitrogen-depleted fraction ( 4 ), c) the hydrocarbon-rich, nitrogen-depleted fraction ( 4 ) is subcooled (E 3 ) and expanded (b), d) the expanded hydrocarbon-rich, nitrogen-depleted fraction ( 5 ) is separated (D 1 ) into a hydrocarbon-rich fraction ( 6 ), the nitrogen content of which is a max. of 1% by volume, and a nitrogen-rich fraction ( 7 ), and e) the nitrogen-rich fraction ( 7 ) is added to the feed fraction ( 1 ).

1 . A process for liquefying a hydrocarbon-rich, nitrogen-containing, and methane-containing feed fraction, said process comprising:
 a) liquefying (E 1 , E 2 ) said feed fraction ( 1 ),   b) separating the liquefied feed fraction by rectification (T 1 ) into a nitrogen-enriched fraction ( 9 ) having a methane content of at most 1% by volume, and a hydrocarbon-rich, nitrogen-depleted fraction ( 4 ),   c) subcooling (E 3 ) and expanding (b) said hydrocarbon-rich, nitrogen-depleted fraction ( 4 ),   d) separating (D 1 ) the expanded hydrocarbon-rich, nitrogen-depleted fraction ( 5 ) into a liquid hydrocarbon-rich fraction ( 6 ) having a nitrogen content of at most 1% by volume, and a nitrogen-rich fraction ( 7 ), and   e) adding said nitrogen-rich fraction ( 7 ) to said feed fraction ( 1 ).   
     
     
         2 . The process according to  claim 1 , wherein said hydrocarbon-rich, nitrogen-containing feed fraction is natural gas. 
     
     
         3 . The process according to  claim 1 , wherein
 said nitrogen-rich fraction ( 7 ,  8 ), before being added to said feed fraction ( 1 ), is compressed (C 1 ) in a single-stage or multistage manner and/or cooled (E 5 ), and/or   said nitrogen-rich fraction, after being added to said feed fraction ( 1 ), is compressed in a single-stage or multistage manner.   
     
     
         4 . The process according to  claim 1 , wherein said feed fraction ( 1 ), before being cooled (E 1 ) for liquefaction, is subjected to an adsorptive drying process (A), and said nitrogen-rich fraction ( 7 ), before being added to the feed fraction ( 1 ), is used as regeneration gas for said adsorptive drying process (A). 
     
     
         5 . The process according to  claim 1 , wherein a substream ( 2 ′) of the cooled feed fraction ( 2 ) is introduced as a stripping gas into the separation by rectification (T 1 ) of the liquefied feed fraction ( 3 ). 
     
     
         6 . The process according to  claim 1 , wherein the liquefaction and/or the subcooling of the hydrocarbon-rich, nitrogen-containing feed fraction proceeds using any arbitrary liquefaction process. 
     
     
         7 . The process according to  claim 1 , wherein said liquid hydrocarbon-rich fraction ( 6 ) having a nitrogen content of at most 1% by volume is stored in a tank, and boil-off gas occurring in said tank is added to said nitrogen-rich fraction ( 7 ) and/or said feed fraction ( 1 ). 
     
     
         8 . The process according to  claim 1 , wherein at least a substream of said nitrogen-enriched fraction ( 9 ) is used for precooling said feed fraction ( 1 ). 
     
     
         9 . The process according to  claim 1 , wherein said feed fraction ( 1 ) is liquefied by being cooled in at least a first heat exchanger and a second heat exchanger. 
     
     
         10 . The process according to  claim 9 , wherein after said feed fraction ( 1 ) is cooled in said first heat exchanger, a substream ( 2 ′) of the cooled feed fraction ( 2 ) is introduced as a stripping gas into the separation by rectification (T 1 ) of the liquefied feed fraction ( 3 ). 
     
     
         11 . The process according to  claim 10 , wherein said substream ( 2 ′) is expanded before being introduced as a stripping gas into the separation by rectification (T 1 ) of the liquefied feed fraction ( 3 ). 
     
     
         12 . The process according to  claim 3 , wherein said nitrogen-rich fraction ( 7 ,  8 ), before being added to said feed fraction ( 1 ), is compressed (C 1 ) in a single-stage or multistage manner and/or cooled (E 5 ). 
     
     
         13 . The process according to  claim 3 , wherein said nitrogen-rich fraction, after being added to said feed fraction ( 1 ), is compressed in a single-stage or multistage manner. 
     
     
         14 . The process according to  claim 1 , wherein, before being added to said feed fraction ( 1 ), said nitrogen-rich fraction ( 7 ) is compressed and cooled. 
     
     
         15 . The process according to  claim 1 , wherein at least part of said nitrogen-enriched fraction ( 9 ) from said rectification (T 1 ) is cooled in a reflux condenser (E 4 ), arranged within a separator (D 1 ), by heat exchange with said expanded hydrocarbon-rich, nitrogen-depleted fraction ( 5 ), and delivered to said rectification (T 1 ) as reflux ( 11 ). 
     
     
         16 . The process according to  claim 1 , wherein at least one substream of said nitrogen-enriched fraction ( 10 ) is used to precool said feed fraction ( 1 ). 
     
     
         17 . The process according to  claim 16 , wherein said feed fraction ( 1 ), before being cooled (E 1 ) for liquefaction, is subjected to an adsorptive drying process (A), and precooling of said feed fraction ( 1 ) by said at least one substream of said nitrogen-enriched fraction ( 10 ) is performed upstream of said adsorptive drying process. 
     
     
         18 . The process according to  claim 7 , wherein said boil-off gas occurring in said tank is added to said nitrogen-rich fraction ( 7 ), and is compressed to the pressure of said nitrogen-rich fraction ( 7 ) before being added thereto. 
     
     
         19 . The process according to  claim 7 , wherein said boil-off gas occurring in said tank is added to said feed fraction ( 1 ), and is compressed to the pressure of said feed fraction ( 1 ) before being added thereto.