Method for removing nitrogen

Abstract

A process is described for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction and wherein the methane-rich fraction for the purpose of cold generation is vaporized and superheated at a pressure as high as possible against the feed fraction which is to be cooled. According to the invention the still liquid or partially vaporized methane-rich fraction ( 5 ′) is fed to a circulation vessel (D), only the liquid fraction of the methane-rich fraction ( 5 ′) occurring in the circulation vessel (D) is completely vaporized, preferably in natural circulation, and the top product ( 7 ) of the circulation vessel (D) is superheated (E 1 ).

1 . Process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction and wherein the methane-rich fraction for the purpose of cold generation is vaporized and superheated at a pressure as high as possible against the feed fraction which is to be cooled, characterized in that the still liquid or partially vaporized methane-rich fraction ( 5 ′) is fed to a circulation vessel (D), only the liquid fraction of the methane-rich fraction ( 6 ) occurring in the circulation vessel (D) is at least partially vaporized (E 1 ), fed again to the circulation vessel (D) and the completely vaporized top product ( 7 ) of the circulation vessel (D) is superheated (E 1 ). 
     
     
         2 . Process according to  claim 1 , characterized in that the vaporization of the liquid fraction of the methane-rich fraction ( 6 ) which occurs in the circulation vessel (D) proceeds in natural circulation. 
     
     
         3 . Process according to  claim 1 , wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that, in the upper region of the preseparation column (T 1 ), preferably below the top tray of the preseparation column (T 1 ), a helium-rich fraction ( 8 ) is taken off and expanded (c) into the low-pressure column (T 2 ), preferably into the top region of the low-pressure column (T 2 ). 
     
     
         4 . Process according to  claim 1 , wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that at least at times a substream of the nitrogen-rich fraction ( 9 ) is compressed (C) at least to the pressure of the preseparation column (T 1 ) and/or the low-pressure column (T 2 ) and is fed as reflux stream ( 10 ,  11 ,  12 ) to the preseparation column (T 1 ) and/or the low-pressure column (T 2 ). 
     
     
         5 . Process according to  claim 1 , wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that a substream ( 14 ,  14 ′) of the nitrogen-rich fraction ( 3 ) which is taken off from the top region of the preseparation column (T 1 ) and is to be fed to the low-pressure column (T 2 ) is vaporized under pressure (E 1 ). 
     
     
         6 . Process according to  claim 1 , characterized in that the methane content of the nitrogen-rich fraction ( 4 - 4 ″) obtained by rectification (T 1 /T 2 ) is less than 1% by volume. 
     
     
         7 . Process according to  claim 1 , characterized in that the nitrogen content of the methane-rich fraction ( 5 ) obtained by rectification (T 1 /T 2 ) is less than 5% by volume.