Method for separating off nitrogen and hydrogen from natural gas

Abstract

The invention relates to a method for separating off nitrogen and lighter components, in particular hydrogen, carbon monoxide, neon and argon, from a feed fraction (e.g., natural gas) that is to be liquefied containing at least methane, nitrogen and hydrogen. The cooling and liquefaction of the feed fraction proceeds against the refrigerant or mixed refrigerant of at least one refrigeration cycle. In the inventive method, the feed fraction ( 1 ) is partially condensed (E 1 ), and separated in at least one rectification column (T) into a methane-rich fraction ( 6 ) and a fraction ( 4 ) containing nitrogen and lighter components. The methane-rich fraction ( 6 ) is subcooled. Additionally, cooling of the top condenser (E 2 ) of the rectification column (T) proceeds via a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of at least one, refrigeration cycle ( 20 - 24 ).

1 . A method for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen, comprising:
 a) partially condensing the feed fraction ( 1 ) (E 1 ),   b) separating the partially condensed feed fraction in at least one rectification column (T), having a top condenser (E 2 ), into a methane-rich fraction ( 6 ) and a fraction ( 4 ) containing nitrogen and lighter components, and   c) subcooling the methane-rich fraction ( 6 ),   
       wherein cooling of said top condenser (E 2 ) of said rectification column (T) is performed using a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of at least one refrigeration cycle ( 20 - 24 ). 
     
     
         2 . The method according to  claim 1 , wherein said feed fraction ( 1 ) is natural gas. 
     
     
         3 . The method according to  claim 1 , wherein the partial condensing of said feed fraction ( 1 ) is performed in a heat exchanger (E 1 ) by heat exchange with said refrigerant or mixed refrigerant of at least one refrigeration cycle. 
     
     
         4 . The method according to  claim 1 , wherein said feed fraction ( 1 ) has a methane content of 50-90% by volume, a nitrogen content of 1-30% by volume, and a hydrogen content of 1-30% by volume. 
     
     
         5 . The method according to  claim 1 , wherein said fraction ( 4 ) containing nitrogen and lighter components is warmed (E 1 ) against the feed fraction ( 1 ,  2 ) that is to be cooled. 
     
     
         6 . The method according to  claim 5 , further comprising drying the feed fraction, wherein cooling of the feed fraction ( 1 ) proceeds before said drying (R) of the feed fraction ( 1 ). 
     
     
         7 . The method according to  claim 5 , further comprising drying the feed fraction, wherein cooling of the feed fraction ( 1 ) proceeds after said drying (R) of the feed fraction ( 1 ). 
     
     
         8 . The method according to  claim 1 , wherein the condenser temperature of said top condenser (E 2 ) of said rectification column (T) can be varied. 
     
     
         9 . The method according to  claim 1 , wherein subcooling of the methane-rich fraction yields a liquefied methane-rich fraction, which is temporarily stored, and tank boil-off gas ( 8 ) occurring during the temporary storage (S) of said liquefied methane-rich fraction is compressed (V) and fed to the feed fraction ( 1 ) that is to be liquefied. 
     
     
         10 . The method according to  claim 1 , wherein a substream of the feed fraction ( 1 ) that is to be liquefied
 (a) is fed to the bottom portion of said rectification column (T) as a stripping gas stream, and/or   (b) is cooled in a reboiler (E 3 ) of said rectification column (T) and fed to the rectification column (T) as a further feed stream.   
     
     
         11 . The method according to  claim 1 , wherein the cooling and liquefaction of said feed fraction proceeds against refrigerant and/or mixed refrigerants of at least two refrigeration cycles, and said top condenser (E 2 ) of said rectification column (T) is cooled by means of a separate refrigeration cycle or by at least a substream of the refrigerant or mixed refrigerant of the lightest refrigeration cycle.