Nitrogen Rejection Unit System and Method for Low Nitrogen Concentration Feeds

Abstract

A system and methods for removing nitrogen from a natural gas or liquid natural gas stream. More particularly, a system and method for removing nitrogen from a natural gas feed stream that uses a two-column system with a first column operating at a pressure that is lower than the operating pressure of the second column. A first nitrogen-enriched vapor stream from the first column is compressed and cooled and the resulting stream is then directed to the second column, which produces a second nitrogen-enriched vapor stream and a second methane-enriched liquid stream. Cooling in the system is provided in heat exchangers that warm a first methane-enriched liquid stream from the first column and the second methane-enriched liquid stream from the second column.

What is claimed is: 
     
         1 . A system for removing nitrogen from a natural gas fluid stream comprising:
 a. a feed stream heat exchanger configured to receive and cool the natural gas fluid stream;   b. a first expansion device configured to receive and expand fluid from the feed stream heat exchanger;   c. a first feed stream separation device configured to receive expanded fluid from the first expansion device, said first feed stream separation device including a vapor outlet and a liquid outlet;   d. a first column in fluid communication with the liquid and vapor outlets of the first feed stream separation device and configured to operate at a first operating pressure, said first column including a first column vapor outlet and a first column liquid outlet and configured to so that a first nitrogen-enriched vapor stream exits the first column vapor outlet and a first methane-enriched liquid stream exits the first column liquid outlet;   e. a first compressor configured to receive and compress the first nitrogen-enriched vapor stream;   f. a first after-cooler configured to receive the compressed first nitrogen-enriched vapor stream from the first compressor;   g. a second expansion device configured to received fluid from the first after-cooler;   h. a second column in fluid communication with the second expansion device and configured to operate at a second operating pressure, where the second operating pressure is greater than the first operating pressure, said second column including a second column vapor outlet and a second column liquid outlet and configured so that a second nitrogen-enriched vapor stream exits the second column vapor outlet and a second methane-enriched liquid stream exits the second column liquid outlet; and   i. said feed stream heat exchanger in fluid communication with the second column vapor outlet and the first and second column liquid outlets and configured to warm nitrogen-enriched fluid and methane-enriched fluid to provide refrigeration within the feed stream heat exchanger.   
     
     
         2 . The system of  claim 1  further comprising a feed stream compressor configured to receive and cool the natural gas fluid stream and a feed stream after-cooler configured to receive and cool a cooled natural gas fluid stream from the feed stream compressor, said feed stream after-cooler configured to direct fluid to the feed stream heat exchanger. 
     
     
         3 . The system of  claim 1  further comprising a third expansion device and a fourth expansion device, said third expansion device configured to receive a liquid stream from the liquid outlet of the first feed stream separation device and a second feed stream separation device configured to receive fluid from the third expansion device, said second feed stream separation device having a liquid outlet and a vapor outlet, said liquid outlet of the second feed stream separation device configured to direct a fluid stream to the feed stream heat exchanger to provide refrigeration within the feed stream heat exchanger and said vapor outlet of the second feed stream separation device configured to direct vapor to the fourth expansion device wherein said fourth expansion device is configured to direct fluid to the first column. 
     
     
         4 . The system of  claim 1  further comprising a fifth expansion device, a sixth expansion device and a first column reboiler heat exchanger having a first column reboiler warming passage and a first column reboiler cooling passage,
 i) said first column reboiler cooling passage having an inlet configured to receive a portion of the vapor stream from the vapor outlet of the first feed stream separation device and an outlet configured to direct a cooled fluid stream to the fifth expansion device and said fifth expansion device configured to direct a cooled fluid stream to the first column; 
 ii) said sixth expansion device configured to receive and expand a first reboiler stream from the first column; 
 iii) said first column reboiler warming passage configured to receive an expanded first reboiler fluid stream from the sixth expansion device, warm the expanded first reboiler fluid stream so that refrigeration is provided within the first column reboiler heat exchanger and to return a warmed first reboiler fluid stream to the first column. 
 
     
     
         5 . The system of  claim 1  further comprising a first main heat exchanger configured to receive and cool a vapor stream from the vapor outlet of the first feed stream expansion device, direct a cooled fluid stream to the feed stream expansion device and to receive and warm methane-enriched fluid streams from the first column liquid outlet and the second column liquid outlet and a nitrogen-enriched fluid stream from the second column vapor outlet to provide refrigeration within the first main heat exchanger. 
     
     
         6 . The system of  claim 5  further comprising a second main heat exchanger configured to receive and cool fluid from the first after-cooler, direct fluid to the first expansion valve and receive and warm fluid streams from the first column liquid outlet and the second column liquid outlet to provide refrigeration within the first main heat exchanger. 
     
     
         7 . The system of  claim 6  further comprising a nineth expansion device and a third main heat exchanger configured to receive and cool a reflux stream from the second column and to return the cooled reflux stream to the second column, receive and cool a liquid stream from the second column liquid outlet, direct the cooled liquid stream from the second column to the nineth expansion device, receive and warm a nitrogen-enriched fluid stream from the second column vapor outlet and receive and warm an expanded methane-enriched fluid stream from the nineth expansion device to provide refrigeration within the third main heat exchanger. 
     
     
         8 . The system of  claim 7  further comprising a product gas compressor configured to receive warmed methane-enriched fluid from the feed stream head exchanger and a product gas after-cooler configured to receive and cool compressed methane-enriched fluid from the venting compressor. 
     
     
         9 . The system of  claim 8  wherein the outlet of the product gas after-cooler is configured to direct fluid to a junction with the first methane-enriched stream exiting the feed stream heat exchanger. 
     
     
         10 . The system of  claim 1  further comprising a seventh expansion device, an eighth expansion device and a second column reboiler heat exchanger having a second column reboiler warming passage and a second column reboiler cooling passage,
 i) said second column reboiler cooling passage having an inlet configured to receive a portion of a fluid stream from the first after-cooler and an outlet configured to direct a cooled fluid stream to the seventh expansion device, said seventh expansion device configured to direct a cooled fluid stream to the second column; 
 ii) said eighth expansion device configured to receive and expand a second reboiler stream from the second column; 
 iii) said second column reboiler warming passage configured to receive an expanded reboiler fluid stream from the eighth expansion device, warm the expanded reboiler fluid stream so that refrigeration is provided within the second column reboiler heat exchanger and to return a warmed second reboiler fluid stream to the first column. 
 
     
     
         11 . The system of  claim 1  wherein the first column is positioned within a first cold box and the second column is positioned within a second cold box. 
     
     
         12 . The system of  claim 1  wherein the first operating pressure of the first column is about 230 to 320 psig. 
     
     
         13 . The system of  claim 12  wherein the second operating pressure of the second column is about 300 to 350 psig. 
     
     
         14 . The system of  claim 1  wherein the second operating pressure of the second column is about 300 to 350 psig. 
     
     
         15 . The system of  claim 1  wherein the first nitrogen-enriched stream exiting the feed stream heat exchanger is directed to a vent. 
     
     
         16 . A method for removing nitrogen from a natural gas fluid feed stream comprising the steps of:
 directing a feed stream to a first column, wherein the first column operates at a first operating pressure;   separating the feed stream into a first nitrogen-enriched vapor and a first methane-enriched liquid in the first column;   compressing and cooling the first nitrogen-enriched vapor;   directing the compressed and cooled first nitrogen-enriched vapor to a second column, wherein the second column operates at a second operating pressure that is greater than the first operating pressure;   separating the first nitrogen-enriched vapor into a second nitrogen-enriched vapor and a second methane-enriched liquid in the second column;   venting the second nitrogen-enriched vapor from the second column; and   combining the first methane-enriched liquid from the first column and the second methane-enriched liquid from the second column so as to form a product methane-enriched liquid stream.   
     
     
         17 . The method of  claim 16  wherein the first operating pressure is about 230-320 psig. 
     
     
         18 . The method of  claim 17  wherein the second operating pressure is about 300-350 psig. 
     
     
         19 . The method of  claim 16  wherein the second operating pressure is about 300-350 psig. 
     
     
         20 . The method of  claim 16 , comprising compressing the feed stream to about 400-450 psig and cooling the feed stream to about 100-120° F. before entering the first column. 
     
     
         21 . The method of  claim 16 , wherein the feed stream comprises about 4-7 mole % nitrogen and is about 200-290 psig. 
     
     
         22 . The method of  claim 16 , wherein the first nitrogen-enriched vapor is about 12 to 24 mole % nitrogen and the first methane-enriched stream is above 0.5 to 1.5 mole % nitrogen. 
     
     
         23 . The method of  claim 16  wherein the first nitrogen-enriched vapor is compressed to above 450 to 550 psig and cooled to about 100 to 120° F. prior to being directed to the second column. 
     
     
         24 . The method of  claim 16 , wherein the second nitrogen-enriched stream comprises about 0.5-2.5 mole % methane and the second methane-enriched liquid comprises about 0.5-1.5 mole % nitrogen.