Process and apparatus for separation of nitrogen from lng

Abstract

The present application is concerned with processes and apparatuses for the separation of nitrogen from liquefied natural gas feeds. The processes comprise the steps of: (i) cooling the feed and passing the feed to a fractionation column; (ii) withdrawing from the fractionation column an overhead vapour stream having an enriched nitrogen content, and a liquid stream having a reduced nitrogen content; (iii) dividing the overhead vapour stream from step (ii) into at least first and second overhead streams; (iv) compressing, cooling and at least partially condensing at least the first overhead stream from step (iii); and (v) expanding the stream from step (iv) and passing the expanded stream to the fractionation column as reflux, and wherein cooling in step (iv) is provided, at least in part, by heat exchange with one or more streams from the fractionation column.

1 . A process for the separation of nitrogen from a liquefied natural gas feed, the process comprising the steps of:
 (i) cooling the feed and passing the feed to a fractionation column;   (ii) withdrawing from the fractionation column an overhead vapour stream having an enriched nitrogen content, and a liquid stream having a reduced nitrogen content;   (iii) dividing the overhead vapour stream from step (ii) into at least first and second overhead streams;   (iv) compressing, cooling and at least partially condensing at least the first overhead stream from step (iii); and   (v) expanding the stream from step (iv) and passing the expanded stream to the fractionation column as reflux,   
       wherein cooling in step (iv) is provided, at least in part, by heat exchange with one or more streams from the fractionation column. 
     
     
         2 . A process according to  claim 1 , wherein the liquefied natural gas feed comprises from 1 mol % to 40 mol % nitrogen. 
     
     
         3 . A process according to  claim 1 , wherein cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the overhead vapour stream withdrawn from the fractionation column. 
     
     
         4 . A process according to  claim 1 , wherein cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the liquid stream withdrawn from the fractionation column. 
     
     
         5 . A process according to  claim 4 , wherein at least a portion of the condensed stream withdrawn from the fractionation column is expanded before being passed in heat exchange with the compressed first overhead vapour stream. 
     
     
         6 . A process according to  claim 1 , wherein heat to the fractionation column is provided, at least in part, by a reboiler. 
     
     
         7 . A process according to  claim 6 , wherein cooling in step (iv) is provided, at least in part, by heat exchange with a liquid stream from the fractionation column in the reboiler. 
     
     
         8 . A process according to  claim 6 , wherein the reboiler is an internal reboiler located within the fractionation column. 
     
     
         9 . A process according to  claim 6 , wherein the reboiler is external to the column. 
     
     
         10 . A process according to  claim 9 , wherein the feed to the reboiler is a stream withdrawn from the bottom of the fractionation column. 
     
     
         11 . A process according to  claim 1 , wherein further cooling of the first overhead stream from step (iii) is provided by heat exchange with the liquefied natural gas feed. 
     
     
         12 . A process according to  claim 6 , wherein cooling in step (i) is provided, at least in part, by heat exchange with a liquid stream from the fractionation column in the reboiler. 
     
     
         13 . The process according to  claim 1 , wherein the cooled feed in step (i) is expanded to form a two-phase feed to the fractionation column. 
     
     
         14 . A process according to  claim 1 , wherein the cooled and at least partially condensed first overhead stream from step (iv) is expanded before being fed to the fractionation as reflux in step (v). 
     
     
         15 . A process according to  claim 1 , wherein the reflux stream of step (v) is at a temperature of from 5 to 50° C. below the temperature of the feed to the column in step (i). 
     
     
         16 . An apparatus for the separation of nitrogen from a feed comprising liquefied natural gas and nitrogen, the apparatus comprising:
 (i) means for cooling and expanding the feed;   (ii) a fractionation column for producing an overhead vapour stream and a bottom liquid stream;   (iii) means for conveying the cooled and expanded feed from step (i) to the fractionation column;   (iv) means for dividing the overhead vapour stream from the fractionation column into at least first and second overhead streams;   (v) means for compressing at least the first overhead stream;   (vi) one or more heat exchangers for cooling and at least partially condensing the compressed stream from step (v) in heat exchange with one or more streams from the fractionation column;   (vii) means for conveying at least one stream from step (v) to the one or more heat exchangers; and   (viii) means for conveying the compressed, cooled and at least partially condensed first overhead stream from step (vi) to an expanding means and from the expanding means to the fractionation column as reflux.