Process for separating off nitrogen

Abstract

The invention relates to a process for separating off C 2+ -hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, wherein a) the feed fraction ( 1, 20 ) is partially condensed (E 1, E 1′, E 3 ) and separated by rectification (T) into a C 2+ -hydrocarbon-rich fraction ( 11 ) and a C 2+ -hydrocarbon-depleted fraction ( 2 ), b) the C 2+ -hydrocarbon-depleted fraction ( 2 ) is partially condensed (E 2 ) and separated into a liquid fraction which forms at least in part the reflux ( 3 ) for the separation by rectification (T), and a C 2+ -hydrocarbon-depleted gas fraction ( 4 ), and c) the C 2+ -hydrocarbon-depleted gas fraction ( 4 ) is separated in a double-column process (N) into a nitrogen-rich fraction ( 8′ ) and a methane-rich fraction ( 7″ ). According to the invention, the liquid fraction obtained in process step b) is fed ( 10 ) at least in part likewise to the double-column process (N) and separated therein into a nitrogen-rich fraction ( 8′ ) and a methane-rich fraction ( 7″ ).

1 . A process for separating off C 2+ -hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, said process comprising:
 a) partially condensing (E 1 , E 1 ′, E 3 ) said feed fraction ( 1 ,  20 ) and separating said feed fraction by rectification in rectification column (T) into a C 2+ -hydrocarbon-rich fraction ( 11 ) and a C 2+ -hydrocarbon-depleted fraction ( 2 ),   b) partially condensing (E 2 ) said C 2+ -hydrocarbon-depleted fraction ( 2 ) and separating said C 2+ -hydrocarbon-depleted fraction into a liquid fraction which forms at least in part reflux ( 3 ) for the separation by rectification (T), and a C 2+ -hydrocarbon-depleted gas fraction ( 4 ),   c) separating said C 2+ -hydrocarbon-depleted gas fraction ( 4 ) in a double-column separator (N) into a nitrogen-rich fraction ( 8 ) and a methane-rich fraction ( 7 ), and   wherein at least part of said liquid fraction obtained in process step b) is fed ( 10 ) to said double-column process (N) and is separated in said double-column separator (N) into said nitrogen-rich fraction ( 8 ) and said methane-rich fraction ( 7 ).   
     
     
         2 . The process according to  claim 1 , wherein said feed fraction is divided into a plurality of substreams ( 1 ,  20 ), and said plurality of substreams are partially condensed (E 1 , E 1 ′, E 3 ), separately from one another, and then separated by rectification in said rectification column (T). 
     
     
         3 . The process according to  claim 2 , wherein cooling of the substreams ( 1 ,  20 ) of said feed fraction is performed in double-pipe heat exchangers (E 1 , E 1 ′, E 3 ). 
     
     
         4 . The process according to  claim 3 , wherein the cooling or partial condensation of the substreams ( 1 ,  20 ) proceeds in the tubes of said double-pipe heat exchangers and the vaporization or warming of cold fractionation products ( 7 ′,  8 ) proceeds on the shell side of said double-pipe heat exchangers. 
     
     
         5 . The process according to  claim 3 , wherein the cooling or partial condensation of the substreams ( 1 ,  20 ) proceeds in an ascending manner on the tube side of said double-pipe heat and the warming or vaporization of fractionation products ( 7 ′,  8 ) proceeds in a falling manner on the shell side of said double-pipe heat. 
     
     
         6 . The process according to  claim 4 , wherein the cooling or partial condensation of the substreams ( 1 ,  20 ) proceeds in an ascending manner on the tube side of said double-pipe heat and the warming or vaporization of fractionation products ( 7 ′,  8 ) proceeds in a falling manner on the shell side of said double-pipe heat. 
     
     
         7 . The process according to  claim 3 , wherein said double-pipe heat exchangers are helically coiled heat exchangers (E 1 , E 1 ′). 
     
     
         8 . The process according to  claim 4 , wherein said double-pipe heat exchangers are helically coiled heat exchangers (E 1 , E 1 ′). 
     
     
         9 . The process according to  claim 5 , wherein said double-pipe heat exchangers are helically coiled heat exchangers (E 1 , E 1 ′). 
     
     
         10 . The process according to  claim 6 , wherein said double-pipe heat exchangers are helically coiled heat exchangers (E 1 , E 1 ′). 
     
     
         11 . The process according to  claim 2 , wherein at least one of the substreams ( 1 ,  20 ) of said feed fraction is separated (D′) into a gas fraction ( 22 ) and a liquid fraction ( 21 ), and said gas fraction ( 22 ) and liquid fraction ( 21 ) are fed separately from one another to the separation by rectification in said rectification column (T). 
     
     
         12 . The process according to  claim 1 , wherein liquid from the bottom of said rectification column T is not introduced into said double-column separator N. 
     
     
         13 . The process according to  claim 1 , wherein said methane-rich fraction ( 7 ,  7 ′) removed from said double-column separator (N) is heated in a top condenser of said rectification column T and further heated in a heat exchanger against said feed fraction ( 1 ,  20 ). 
     
     
         14 . The process according to  claim 13 , wherein a liquid fraction ( 11 ) is removed from the bottom of said rectification column T, expanded, and added to said methane-rich fraction at a point between said top condenser (E 2 ) and said heat exchanger (E 1 ). 
     
     
         15 . The process according to  claim 1 , wherein the content of C 2+ -hydrocarbons in said C 2+ -hydrocarbon-depleted fraction ( 2 ) is at most 0.1% by volume (1000 vppm). 
     
     
         16 . The process according to  claim 15 , wherein the content of C 2+ -hydrocarbons in said C 2+ -hydrocarbon-depleted fraction ( 2 ) is at most 0.01% by volume (100 vppm).