Nitrogen rejection methods and systems

Abstract

Methods and systems for removing nitrogen from a natural gas feed stream. The systems and methods generally include a heat exchange unit, a separation unit, and a liquid methane pump unit, where the separation unit produces a liquid methane bottoms stream and a gaseous overhead stream enriched in nitrogen and the liquid methane pump unit compresses the liquid methane bottoms stream and then pumps the stream through the heat exchange unit to cool a natural gas feed stream. In some embodiments the liquid methane pump unit is a sleeve bearing type unit. Beneficially, the disclosed systems and methods incorporate high head pumps for liquid methane compression instead of vaporizing the liquid methane and compressing it in a gaseous compression units that are typically used for this purpose, saving space, materials, and power.

What is claimed is: 
     
         1 . A nitrogen rejection system, comprising:
 a natural gas feed stream comprising nitrogen and methane and having a temperature above cryogenic conditions;   a feed stream heat exchanger configured to reduce the temperature of the natural gas feed stream to form a majority liquefied natural gas feed stream;   a separation unit configured to receive the cooled natural gas feed stream and produce an overhead stream enriched in nitrogen and a bottoms stream enriched in methane (“liquefied methane stream”); and   a liquid methane pump configured to pump the liquefied methane stream to a sales compression pressure to form a pressurized liquefied methane stream, wherein the pressurized liquefied methane stream is substantially vaporized in the feed stream heat exchanger to form a methane product stream.   
     
     
         2 . The system of  claim 1 , wherein the liquid methane pump is a sleeve bearing type pump. 
     
     
         3 . The system of  claim 2 , wherein the liquid methane pump comprises a magnetic thrust bearing configured to reduce a gravity thrust load on an axial bearing of the liquid methane pump. 
     
     
         4 . The system of  claim 3 , wherein the configuration of the sleeve bearing type pump is selected from the group consisting of: a single pump, a series of at least two pumps, a parallel configuration of at least two pumps, a multistage pump, and any combination thereof. 
     
     
         5 . The system of  claim 3 , wherein the separation unit is configured to operate at a pressure of at least about 200 pounds per square inch (psi) to about 500 psi and a temperature of at least about −220 degrees Fahrenheit (° F.) to about −120° F. 
     
     
         6 . The system of  claim 5 , wherein the separation unit is a tower having a top feed stripper portion and a lower cryogenic reboiler portion configured to separate gaseous nitrogen from the liquefied methane stream. 
     
     
         7 . The system of  claim 5 , wherein at least a portion of the overhead stream enriched in nitrogen is fed to the feed stream heat exchanger to form a warmed nitrogen enriched stream. 
     
     
         8 . The system of  claim 7 , further comprising:
 a compressor configured to compress the warmed nitrogen enriched stream to form a compressed nitrogen enriched stream; and   a nitrogen rejection unit (NRU) configured to receive the compressed nitrogen enriched stream to form a methane enriched stream.   
     
     
         9 . The system of  claim 8 , wherein the warmed nitrogen stream is less than about 50 volume percent (vol %) of the natural gas feed stream. 
     
     
         10 . The system of  claim 5 , wherein at least a portion of the overhead stream enriched in nitrogen is fed to a power generation unit configured to generate power using the at least a portion of the overhead stream enriched in nitrogen. 
     
     
         11 . The system of  claim 5 , further comprising:
 a reboiler feed stream from the separation unit;   a slip stream from the substantially liquefied natural gas feed stream; and   a reboiler heat exchanger configured to exchange heat energy from the slip stream to the reboiler feed stream to generate a nitrogen containing vapor from the reboiler feed stream, wherein the slip stream is then re-mixed with the substantially liquefied natural gas feed stream.   
     
     
         12 . The system of  claim 11 , further comprising an expansion device configured to receive the substantially liquefied natural gas feed stream and hold a back-pressure on a feed condensing pass of the feed stream heat exchanger, wherein the expansion device is selected from the group consisting of a flow control device, a level control device, a back-pressure control valve, and any combination thereof. 
     
     
         13 . The system of  claim 11 , further comprising:
 a feed separator configured to produce a nitrogen enriched gas stream and a bottoms stream enriched in methane; and   at least one level control valve configured to maintain a liquid level in the feed separator.   
     
     
         14 . The system of any one of  claims 12 - 13 , further comprising a flow integrated controller configured to control at least the back-pressure on the feed condensing pass of the feed stream heat exchanger. 
     
     
         15 . A method of nitrogen rejection, comprising:
 cooling a natural gas feed stream comprising nitrogen and methane in a feed stream heat exchanger to form a majority liquefied natural gas feed stream;   separating the substantially liquefied natural gas feed stream in a separator to produce an overhead stream enriched in nitrogen and a liquid bottoms stream enriched in methane (“liquefied methane stream”);   pressurizing the liquefied methane stream in a liquid methane pump to a sales compression pressure to form a pressurized liquefied methane stream; and   exchanging heat from the natural gas feed stream to the pressurized liquefied methane stream in the feed stream heat exchanger to form a methane product stream.   
     
     
         16 . The method of  claim 15 , wherein the liquid methane pump is a sleeve bearing type pump. 
     
     
         17 . The method of  claim 16 , wherein the liquid methane pump comprises a magnetic thrust bearing configured to reduce a gravity thrust load on an axial bearing of the liquid methane pump. 
     
     
         18 . The method of  claim 17 , wherein the configuration of the sleeve bearing type pump is selected from the group consisting of: a single pump, a series of at least two pumps, a parallel configuration of at least two pumps, a multistage pump, and any combination thereof. 
     
     
         19 . The method of  claim 17 , wherein the separation unit is configured to operate at a pressure of at least about 200 pounds per square inch (psi) to about 500 psi and a temperature of at least about −220 degrees Fahrenheit (° F.) to about −120° F. 
     
     
         20 . The system of  claim 19 , wherein the separation unit is a tower having a top feed stripper portion and a lower cryogenic reboiler portion configured to separate gaseous nitrogen from the liquefied methane stream. 
     
     
         21 . The method of  claim 19 , further comprising feeding at least a portion of the overhead stream enriched in nitrogen to the feed stream heat exchanger to form a warmed nitrogen enriched stream. 
     
     
         22 . The method of  claim 21 , further comprising:
 compressing the warmed nitrogen enriched stream in a compressor to form a compressed nitrogen enriched stream; and   feeding the compressed nitrogen enriched stream to a nitrogen rejection unit (NRU) to form a methane enriched stream.   
     
     
         23 . The method of  claim 22 , wherein the warmed nitrogen stream is less than about 50 volume percent (vol %) of the natural gas feed stream. 
     
     
         24 . The method of  claim 19 , further comprising:
 feeding at least a portion of the overhead stream enriched in nitrogen to a power generation unit; and   generating power in the power generation unit.   
     
     
         25 . The method of  claim 19 , further comprising:
 taking a reboiler feed stream from the separation unit;   taking a slip stream from the substantially liquefied natural gas feed stream;   exchanging heat energy from the slip stream to the reboiler feed stream in a reboiler heat exchanger to generate a nitrogen containing vapor from the reboiler feed stream; and   re-mixing the slip stream with the substantially liquefied natural gas feed stream.   
     
     
         26 . The method of  claim 25 , further comprising:
 maintaining a back-pressure on a feed condensing pass of the feed stream heat exchanger using an expansion device configured to receive the substantially liquefied natural gas feed stream, wherein the expansion device is selected from the group consisting of a flow control device, a level control device, a back-pressure control valve, and any combination thereof.   
     
     
         27 . The method of  claim 25 , further comprising:
 producing a nitrogen enriched gas stream and a bottoms stream enriched in methane in a feed separator; and   maintaining a liquid level in the feed separator using a level control valve.   
     
     
         28 . The method of any one of  claims 26 - 27 , further comprising controlling at least the back-pressure on a feed condensing pass of the feed stream heat exchanger and the back-pressure on the separation unit using flow integrated controller. 
     
     
         29 . The nitrogen rejection system of  claim 1 , wherein the liquefied methane stream comprises ethane and heavier hydrocarbons. 
     
     
         30 . The method of nitrogen rejection of  claim 15 , wherein the liquefied methane stream comprises ethane and heavier hydrocarbons.