Method of Separating Off Nitrogen from Liquefied Natural Gas

Abstract

A method of separating off an N 2 -rich fraction from a liquefied hydrocarbon-rich fraction is described, in particular from liquefied natural gas, wherein the liquefied hydrocarbon-rich fraction, after its liquefaction and subcooling, is fed to a stripping column which serves for separating off the N 2 -rich fraction. According to the invention, a first substream of the liquefied hydrocarbon-rich fraction ( 2, 2′ ) is applied to the stripping column (S) as reflux, whereas a second substream of the liquefied hydrocarbon-rich fraction ( 3, 3′ ) is fed to the bottom of the stripping column (S), wherein the second substream of the liquefied hydrocarbon-rich fraction ( 3, 3′ ) has a higher temperature than the first substream of the liquefied hydrocarbon-rich fraction ( 2, 2′ ).

1 . A method of separating off an N 2 -rich fraction from a feed of a nitrogen-containing liquefied hydrocarbon-rich fraction, wherein the liquefied hydrocarbon-rich fraction, after its liquefaction and subcooling, is fed to a stripping column having a top section and a bottom section, which serves to separate off the N 2 -rich fraction, characterized in that the feed nitrogen-containing liquefied hydrocarbon net fraction is split into at least 2 substreams, a first substream ( 2 ,  2 ′) is passed to a top section of a stripping column (S) as reflux, and a second substream ( 3 ,  3 ′) is fed to a bottom section of the stripping column (S), wherein the second substream of the liquefied hydrocarbon-rich fraction ( 3 ,  3 ′) has a higher temperature than the first substream of the liquefied hydrocarbon-rich fraction ( 2 ,  2 ′). 
     
     
         2 . A method according to  claim 1 , characterized in that the first ( 2 ,  2 ′) substream and/or the second substream of the liquefied hydrocarbon-rich fraction ( 3 ,  3 ′), before being fed into the stripping column (S) is or are subjected to expansion (a, b, X 1 , X 2 ). 
     
     
         3 . A method according to  claim 1 , characterized in that prior to expansion the temperature difference between the first ( 2 ,  2 ′) substream and the second substream of the liquefied hydrocarbon-rich fraction ( 3 ,  3 ′) is between 40 and 100° C. 
     
     
         4 . A method according to  claim 2 , characterized in that the first substream of the liquefied hydrocarbon-rich fraction ( 2 ,  2 ′) which is passed to the stripping column (S) as reflux has a gas fraction of at most 80% by volume. 
     
     
         5 . A method according to  claim 1 , characterized in that the quantitative ratio between the first substream and second substream of the hydrocarbon-rich fraction ( 2 ,  2 ′,  3 ,  3 ′) which is fed to the stripping column (S) is variable. 
     
     
         6 . A method according to  claim 1 , characterized in that, in addition to the second substream of the liquefied hydrocarbon-rich fraction ( 3 ,  3 ′), at least one further methane-rich or nitrogen-rich stream ( 8 ,  11 ,  12 ) is fed to the bottom section of the stripping column (S). 
     
     
         7 . A method according to  claim 3 , wherein the temperature difference is between 60° and 80° C. 
     
     
         8 . A method according to  claim 1 , wherein the feed is natural gas. 
     
     
         9 . A method according to  claim 4 , wherein the reflux after expansion has a vapor fraction of 10-20% by volume.