Integrated Nitrogen Removal in the Production of Liquefied Natural Gas Using Intermediate Feed Gas Separation

Abstract

A method and apparatus for liquefying a natural gas feed stream and removing nitrogen therefrom to produce a nitrogen-depleted LNG product, in which a natural gas feed stream is fed into the warm end of a main heat exchanger, cooled and at least partially liquefied, withdrawn from an intermediate location of the main heat exchanger and separated to form a nitrogen-enriched natural gas vapor stream and a nitrogen-depleted natural gas liquid stream, the liquid and vapor streams being reintroduced into an intermediate location of the main heat exchanger and further cooled in parallel to form a first LNG stream and a first at least partially liquefied nitrogen-enriched natural gas stream, respectively.

1 . A method for producing a nitrogen-depleted LNG product, the method comprising:
 (a) introducing a natural gas feed stream into the warm end of a main heat exchanger, cooling and at least partially liquefying the natural gas feed stream, and withdrawing the cooled and at least partially liquefied stream from an intermediate location of the main heat exchanger;   (b) expanding, partially vaporizing and separating the cooled and at least partially liquefied stream to form a nitrogen-enriched natural gas vapor stream and a nitrogen-depleted natural gas liquid stream;   (c) separately re-introducing said vapor and liquid streams into an intermediate location of the main heat exchanger, further cooling the vapor and liquid streams in parallel, the liquid stream being further cooled to form a first LNG stream and the vapor stream being further cooled and at least partially liquefied to form a first at least partially liquefied nitrogen-enriched natural gas stream, and withdrawing the first LNG stream and the first at least partially liquefied nitrogen-enriched natural gas stream from the cold end of the main heat exchanger;   (d) expanding, partially vaporizing and separating the first at least partially liquefied nitrogen-enriched natural gas stream to form a nitrogen-rich vapor product and a second LNG stream; and   (e) expanding, partially vaporizing and separating the second LNG stream to form a nitrogen-depleted LNG product and a nitrogen-enriched natural gas vapor.   
     
     
         2 . The method of  claim 1 , wherein step (e) further comprises forming a recycle stream from the nitrogen-enriched natural gas vapor or a portion thereof; and wherein the method further comprises;
 (f) compressing the recycle stream to form a compressed recycle stream; and   (g) returning the compressed recycle stream to the main heat exchanger to be cooled and at least partially liquefied in combination with or separately from the natural gas feed stream.   
     
     
         3 . The method of  claim 2 , wherein step (g) comprises adding the compressed recycle stream to the natural gas feed stream such that the recycle stream is cooled and at least partially liquefied in the main heat exchanger in combination with and as part of the natural gas feed stream. 
     
     
         4 . The method of  claim 2 , wherein step (g) comprises introducing the compressed recycle stream into the warm end or an intermediate location of the main heat exchanger, cooling the compressed recycle stream and at least partially liquefying all or a portion thereof, separately from and in parallel with the natural gas feed stream, to form a second at least partially liquefied nitrogen-enriched natural gas stream, and withdrawing the second at least partially liquefied nitrogen-enriched natural gas stream from the cold end of the main heat exchanger. 
     
     
         5 . The method of  claim 1 , wherein step (b) comprises expanding and partially vaporizing the cooled and at least partially liquefied stream and separating said stream in a phase separator into vapor and liquid phases to form the nitrogen-enriched natural gas vapor stream and the nitrogen-depleted natural gas liquid stream 
     
     
         6 . The method of  claim 1 , wherein step (e) comprises expanding the second LNG stream, transferring the expanded stream into an LNG storage tank in which a portion of the LNG vaporizes, thereby forming the nitrogen-enriched natural gas vapor and the nitrogen-depleted LNG product. 
     
     
         7 . The method of  claim 1 , wherein step (d) comprises expanding and partially vaporizing the first at least partially liquefied nitrogen-enriched natural gas stream and separating said stream in a phase separator into vapor and liquid phases to form the nitrogen-rich vapor product and the second LNG stream. 
     
     
         8 . The method of  claim 7 , wherein step (e) further comprises expanding, partially vaporizing and separating the first LNG stream to produce additional nitrogen-depleted LNG product and additional nitrogen-enriched natural gas vapor. 
     
     
         9 . The method of one of  claim 1 , wherein step (d) comprises expanding and partially vaporizing the first at least partially liquefied nitrogen-enriched natural gas stream, introducing said stream into a distillation column to separate the stream into vapor and liquid phases, forming the nitrogen-rich vapor product from overhead vapor withdrawn from the distillation column, and forming the second LNG stream from bottoms liquid withdrawn from the distillation column. 
     
     
         10 . The method of  claim 9 , wherein step (e) further comprises expanding, partially vaporizing and separating the first LNG stream to produce additional nitrogen-depleted LNG product and additional nitrogen-enriched natural gas vapor. 
     
     
         11 . The method of  claim 9 , wherein step (d) further comprises expanding and partially vaporizing the first LNG stream and introducing said stream into the distillation column to separate the stream into vapor and liquid phases, the first LNG stream being introduced into the distillation column at a location below the location at which the first at least partially liquefied nitrogen-enriched natural gas stream is introduced into the distillation column. 
     
     
         12 . The method of  claim 11 , wherein the first LNG stream is introduced into the distillation column at an intermediate location of the column, and boil-up for the distillation column is provided by heating and vaporizing a portion of the bottoms liquid in a reboiler heat exchanger via indirect heat exchange with the first LNG stream prior to introduction of the first LNG stream into the distillation column. 
     
     
         13 . The method of  claim 11 , wherein the first LNG stream is introduced into the bottom of the distillation column. 
     
     
         14 . The method of one of  claim 9 , wherein boil-up for the distillation column is provided by heating and vaporizing a portion of the bottoms liquid in a reboiler heat exchanger via indirect heat exchange with all or a portion of the first at least partially liquefied nitrogen-enriched natural gas stream prior to the introduction of said stream into the distillation column. 
     
     
         15 . The method of  claim 9 , wherein:
 step (b) comprises expanding, partially vaporizing and separating the cooled and at least partially liquefied stream to form the nitrogen-enriched natural gas vapor stream, a stripping gas stream composed of nitrogen-enriched natural gas vapor, and the nitrogen-depleted natural gas liquid stream; and   step (d) further comprises introducing the stripping gas stream into the bottom of the distillation column.   
     
     
         16 . The method of  claim 4 , wherein step (d) comprises expanding and partially vaporizing the first at least partially liquefied nitrogen-enriched natural gas stream and introducing said stream into a distillation column to separate the stream into vapor and liquid phases, expanding and partially vaporizing the second at least partially liquefied nitrogen-enriched natural gas stream and introducing said stream into the distillation column to separate the stream into vapor and liquid phases, forming the nitrogen-rich vapor product from overhead vapor withdrawn from the distillation column, and forming the second LNG stream from bottoms liquid withdrawn from the distillation column. 
     
     
         17 . The method of  claim 16 , wherein the second at least partially liquefied nitrogen-enriched natural gas stream is introduced into the top of the distillation column. 
     
     
         18 . The method of  claim 9 , wherein the first at least partially liquefied nitrogen-enriched natural gas stream is introduced into the top of the distillation column. 
     
     
         19 . The method of  claim 9 , wherein reflux for the distillation column is provided by condensing a portion of the overhead vapor from the distillation column in a condenser heat exchanger. 
     
     
         20 . The method of  claim 19 , wherein refrigeration for the condenser heat exchanger is provided by warming overhead vapor withdrawn from the distillation column. 
     
     
         21 . The method of  claim 19 , wherein refrigeration for the condenser heat exchanger is provided by a closed loop refrigeration system that likewise provides refrigeration for the main heat exchanger, refrigerant circulated by the closed loop refrigeration system passing through and being warmed in the condenser heat exchanger. 
     
     
         22 . The method of  claim 1 , wherein refrigeration for the main heat exchanger is provided by a closed loop refrigeration system, refrigerant circulated by the closed loop refrigeration system passing through and being warmed in the main heat exchanger. 
     
     
         23 . An apparatus for producing a nitrogen-depleted LNG product, the apparatus comprising:
 a main heat exchanger having (i) a first cooling passage, extending from a warm end of the heat exchanger to an intermediate location of the heat exchanger, for receiving a natural gas feed stream and cooling and at least partially liquefying said stream so as to produce a cooled and at least partially liquefied stream, (ii) a second cooling passage extending from an intermediate location of the heat exchanger to a cold end of the heat exchanger, for receiving and further cooling a nitrogen-depleted natural gas liquid stream to form a first LNG stream, and (iii) a third cooling passage extending from an intermediate location of the heat exchanger to the cold end of the heat exchanger, for receiving and further cooling a nitrogen-enriched natural gas vapor stream, separately from and in parallel with the nitrogen-depleted natural gas liquid stream, to form a first at least partially liquefied nitrogen-enriched natural gas stream;   a refrigeration system for supplying refrigerant to the main heat exchanger for cooling the cooling passages;   a first separation system, in fluid flow communication with the main heat exchanger, for (i) receiving the cooled and at least partially liquefied stream from the first cooling passage of the main heat exchanger, (ii) expanding, partially vaporizing and separating said stream to form the nitrogen-enriched natural gas vapor stream and the nitrogen-depleted natural gas liquid stream, and (iii) returning said liquid and vapor streams to, respectively, the second and third cooling passages of the main heat exchanger;   a second separation system, in fluid flow communication with the main heat exchanger, for receiving, expanding, partially vaporizing and separating the first at least partially liquefied nitrogen-enriched natural gas stream to form a nitrogen-rich vapor product and a second LNG stream; and   a third separation system, in fluid flow communication with the second separation system, for receiving, expanding, partially vaporizing and separating the second LNG stream to form a nitrogen-depleted LNG product and a nitrogen-enriched natural gas vapor.   
     
     
         24 . An apparatus according to  claim 23 , wherein the apparatus further comprises a compressor system, in fluid flow communication with the third separation system and main heat exchanger, for receiving a recycle stream from the third separation system, formed from the nitrogen-enriched natural gas vapor or a portion thereof, compressing the recycle stream to form a compressed recycle stream, and returning the compressed recycle stream to the main heat exchanger to be cooled and at least partially liquefied in combination with or separately from the natural gas feed stream. 
     
     
         25 . An apparatus according to  claim 23 , wherein the refrigeration system is a closed loop refrigeration system, the first separation system comprises an expansion device and a phase separator, the second separation system comprises an expansion device and a phase separator or a distillation column, and the third separation system comprises an expansion device and an LNG tank.