Process for separating off 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, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream. According to the invention at least occasionally at least a substream ( 16 ) of the compressed (C) nitrogen-rich fraction ( 9 ′) is expanded (f) after condensation (E 1 ) thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized (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, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream, characterized in that, at least occasionally at least a substream ( 16 ) of the compressed (C) nitrogen-rich fraction ( 9 ′) is expanded (f) after condensation (E 1 ) thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized (E 1 ). 
     
     
         2 . Process according to  claim 1 , characterized in that the nitrogen-rich fraction ( 9 ) is compressed (C) to a pressure between 20 and 80 bar and after condensation (E 1 ) thereof is expanded (f) to a pressure between 1 and 5 bar. 
     
     
         3 . Process according to  claim 1 , characterized in that at least occasionally at least a substream ( 15 ) of the compressed (C) nitrogen-rich fraction ( 9 ′) after cooling (E 1 ) thereof is cold-producingly expanded (X), preferably expanded (X) to a pressure between 10 and 20 bar, and optionally vaporized and warmed (E 1 ) for the purpose of cold generation. 
     
     
         4 . Process according to  claim 1 , characterized in that the still incompletely vaporized methane-rich fraction ( 5 ′) is fed to a circulation vessel (D), only the liquid fraction occurring in the circulation vessel (D) of the methane-rich fraction ( 5 ′) is partially vaporized (E 1 ) and fed back to the circulation vessel (D) and the completely vaporized top product ( 7 ) of the circulation vessel (D) is superheated (E 1 ). 
     
     
         5 . 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. 
     
     
         6 . 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. 
     
     
         7 . Process according to  claim 1 , wherein the feed fraction is separated by rectification 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 above 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 ).