Nitrogen Rejection Column Reboiler Configuration

Abstract

A process is provided for denitrogenation of a crude LNG stream. A crude LNG stream comprising between about 1% and 10% nitrogen, and the remainder methane and heavier hydrocarbons, is expanded in a means for expansion, and cooled. Resultant crude LNG stream is introduced into nitrogen rejection column, wherein nitrogen content of LNG is reduced. A nitrogen-enriched vapor stream is withdrawn from top of the column, and a nitrogen-diminished liquid stream is withdrawn from bottom of the column. The nitrogen-diminished bottoms LNG stream is pumped to higher pressure and then divided into two streams. The second stream is reduced in pressure and then passed through reboiler heat exchanger, thus cooling the crude LNG stream. Partially vaporized second stream is reinjected into column at a level above the level of withdrawal of nitrogen-diminished bottoms LNG stream and below the level of introduction of crude LNG feed stream to provide column boilup.

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 reboiler heat exchanger to form a cooled, expanded LNG stream, wherein the expansion 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;   (c) withdrawing a nitrogen-enriched overhead vapor stream from the column;   (d) withdrawing a nitrogen-diminished bottoms liquid stream from the column;   (e) passing the bottoms stream from step (d) through a pump to increase the pressure of the nitrogen-diminished bottoms liquid stream of step (d);   (f) dividing the bottoms stream into a first stream and a second stream;   (g) at least partially vaporizing the second stream by reducing said second stream in pressure and then passing through the reboiler heat exchanger of step (a);   (h) injecting the partially vaporized second stream of step (g) into the column at a position of the column above that from which the bottoms stream of step (d) is withdrawn and below that which received the LNG feed stream of step (b), to provide boilup for the column.   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 means provided to expand the LNG feed stream is a dense fluid expander. 
   
   
       3 . The process of  claim 1 , wherein the reduction in pressure of step (g) is achieved by passing through a Joule-Thomson valve. 
   
   
       4 . The process of  claim 1 , wherein the first stream of step (f) is collected as an LNG product. 
   
   
       5 . 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. 
   
   
       6 . The process of  claim 1 , wherein the expanding of step (a) is performed before the cooling of step (a). 
   
   
       7 . The process of  claim 1 , wherein the cooling of step (a) is performed before the expanding of step (a). 
   
   
       8 . The process of  claim 1 , wherein the reboiler heat exchanger of step (a) is a plate-fin heat exchanger. 
   
   
       9 . The process of  claim 1 , wherein the flow rate of the second stream of step (f) is less than about 20% of the flow rate of the first stream of step (f). 
   
   
       10 . The process of  claim 1 , wherein the cooled, expanded LNG stream of step (b) is introduced into the top of the nitrogen rejection column.