Thermosyphon reboiler for the denitrogenation of liquid natural gas

Abstract

Processes are provided for the denitrogenation of a crude LNG stream. A crude LNG stream is expanded and then cooled in a thermosyphon reboiler. The resultant crude LNG stream is introduced into a nitrogen rejection column, wherein the nitrogen content of the LNG is reduced as the liquid flows down the column. A nitrogen-enriched vapor stream is withdrawn from the top of the column, and a first, and optionally a second, nitrogen-diminished liquid stream is withdrawn from the bottom of the column. The first stream may be recovered as a LNG product, and the second stream is passed through the thermosyphon reboiler, cooling the crude LNG stream, while partially vaporizing the second stream. The vaporized second stream is reinjected into the column at a level above the level of withdrawal of the second nitrogen-diminished bottoms LNG stream to provide column boilup. Alternatively, the thermosyphon reboiler is placed within the column.

1 . A process for the denitrogenation of a liquid natural gas (LNG) feed stream, wherein said LNG stream comprises 1-10 mol % nitrogen, comprising:
 (a) expanding the LNG feed stream via means provided to expand the LNG feed stream and cooling the LNG feed stream in a thermosyphon reboiler to form a cooled, expanded LNG stream, wherein the expansion is performed before the cooling or the cooling is performed before the expansion;   (b) introducing the cooled, expanded LNG stream into a nitrogen rejection column, comprising a distillation section and a sump;   (c) withdrawing a nitrogen-enriched overhead vapor stream from the column;   (d) withdrawing a first nitrogen-diminished bottoms liquid stream from the column;   (e) withdrawing a second nitrogen-diminished bottoms liquid stream from the column;   (f) at least partially vaporizing the second bottoms liquid stream by passing said second bottoms liquid stream through the thermosyphon reboiler of step (a); and   (g) injecting the partially vaporized second stream of step (f) into the column at a position of the column above that from which the second bottoms stream of step (e) is withdrawn and below that which received the LNG feed stream of step (b), to provide boilup for the column.   
   
   
       2 . The process of  claim 1 , wherein the first nitrogen-diminished bottoms liquid stream of step (d) and the second nitrogen-diminished bottoms liquid stream of step (e) are withdrawn together from the column through the same conduit, and are divided following their withdrawal. 
   
   
       3 . A process for the denitrogenation of a liquid natural gas (LNG) feed stream, wherein said LNG stream comprises 1-10 mol % nitrogen, comprising:
 (a) expanding the LNG feed stream via means provided to expand the LNG feed stream and cooling the LNG feed stream in a thermosyphon reboiler to form a cooled, expanded LNG stream, wherein the expanding is performed before the cooling or the cooling is performed before the expanding;   (b) introducing the cooled, expanded LNG stream into a nitrogen rejection column, comprising a distillation section and a sump;   (c) withdrawing a nitrogen-enriched overhead vapor stream from the column;   (d) withdrawing a first nitrogen-diminished bottoms liquid stream from the column;   (e) passing a portion of the liquid in the sump of the column through the thermosyphon reboiler of step (a) to provide boilup for the column;   
     wherein the thermosyphon reboiler is positioned within the sump of the column. 
   
   
       4 . The process of  claim 1 , wherein the means provided to expand the LNG feed stream is a dense fluid expander. 
   
   
       5 . The process of  claim 1 , wherein the first nitrogen-diminished bottoms stream of step (d) is collected as a LNG product. 
   
   
       6 . The process of  claim 1 , further comprising passing the cooled, expanded LNG stream of step (a) through a valve before introducing the cooled, expanded LNG stream into the nitrogen rejection column. 
   
   
       7 . The process of  claim 1 , wherein the expanding of step (a) is performed before the cooling of step (a). 
   
   
       8 . The process of  claim 1 , wherein the cooling of step (a) is performed before the expanding of step (a). 
   
   
       9 . The process of  claim 3 , wherein the means provided to expand the LNG feed stream is a dense fluid expander. 
   
   
       10 . The process of  claim 3 , wherein the first nitrogen-diminished bottoms stream of step (d) is collected as a LNG product. 
   
   
       11 . The process of  claim 3 , further comprising passing the cooled, expanded LNG stream of step (a) through a valve before introducing the cooled, expanded LNG stream into the nitrogen rejection column. 
   
   
       12 . The process of  claim 3 , wherein the expanding of step (a) is performed before the cooling of step (a). 
   
   
       13 . The process of  claim 3 , wherein the cooling of step (a) is performed before the expanding of step (a).