Patent Publication Number: US-11378333-B2

Title: System and method for separating methane and nitrogen with reduced horsepower demands

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a system and method for separating nitrogen from methane and other components from natural gas streams of around 20 MMSCFD or more with reduced energy/horsepower requirements compared to prior art systems and methods. 
     2. Description of Related Art 
     Nitrogen contamination is a frequently encountered problem in the production of natural gas from underground reservoirs. The nitrogen may be naturally occurring or may have been injected into the reservoir as part of an enhanced recovery operation. Transporting pipelines typically do not accept natural gas containing more than 4 mole percent inerts, such as nitrogen. As a result, the natural gas feed stream is generally processed to remove such inerts for sale and transportation of the processed natural gas. 
     One method for removing nitrogen from natural gas is to process the nitrogen and methane containing stream through a Nitrogen Rejection Unit or NRU. The NRU may be comprised of two cryogenic fractionating columns, such as that described in U.S. Pat. Nos. 4,451,275 and 4,609,390. These two column systems have the advantage of achieving high nitrogen purity in the nitrogen vent stream, but require higher capital expenditures for additional plant equipment, including the second column, and may require higher operating expenditures for refrigeration horsepower and for compression horsepower for the resulting methane stream. 
     The NRU may also be comprised of a single fractionating column, such as that described in U.S. Pat. Nos. 5,141,544, 5,257,505, and 5,375,422. Many single column systems have a single sales gas stream exiting the NRU fractionating column, usually at a lower pressure requiring compression to meet pipeline requirements. For example, in U.S. Pat. No. 5,141,544, an NRU feed stream is first processed to remove water and carbon dioxide (to avoid freezing problems associated in carbon dioxide) and is then split into three portions prior to feeding the single column NRU. A first portion is cooled through heat exchange with an overhead stream from the NRU column, a second portion is cooled through heat exchange with the NRU column bottoms stream, and a third portion is cooled through heat exchange with a side stream withdrawn from and returned to the NRU column in a reboiler for the NRU column. The first, second and third portions of the feed stream are recombined, the recombined stream is further cooled through heat exchange with the NRU column bottoms stream, and then passes through a JT valve prior to feeding into the NRU column as a liquid and vapor mixed phase stream around −215° F. and around 170 psia. The overhead stream from the single column NRU is the nitrogen vent stream. The single NRU bottoms stream is a sales gas stream at a pressure around 60 psia in the example in the &#39;544 patent, requiring further compression. 
     Some single column systems also split the NRU column bottoms stream into two streams to allow for additional heat exchange with other process streams and resulting in two sales gas streams at different pressures. For example, in U.S. Pat. No. 5,375,422, an NRU feed stream is first processed to remove water and carbon dioxide and is then split into four portions prior to feeding the single column NRU. A first portion is cooled through heat exchange with an overhead stream from the NRU column; a second portion is cooled through heat exchange with a first portion of the NRU column bottoms stream after passing through the NRU column reboiler, then an internal reflux condenser in the NRU column, and then back through the reboiler; and a third portion is cooled through heat exchange with a second portion of a bottoms stream from the NRU column. The first, second and third portions of the feed stream are recombined and the recombined stream passes through a JT valve prior to feeding into the NRU column as a liquid and vapor mixed phase stream between −60 and −150° F. and around 315 psia. The fourth portion of the feed stream is cooled through two separate heat exchanges, each with a side stream withdrawn from and returned to the NRU column, before passing through a JT valve and feeding into the NRU column as a liquid and vapor mixed stream between −200 and −250° F. and around 315 psia. The fourth portion of the feed stream feeds into the NRU column at a location that is several trays above the recombined first, second, and third portions. The overhead stream from the single column NRU is the nitrogen vent stream. The NRU bottoms stream is split into the first and second portions, each of which is processed differently to achieve the desired heat exchange with other process streams. The different processing of the two portions of the NRU bottoms stream results in two sales gas streams, one at a pressure of around 20 psia and the other at a pressure around 300 psia. Such a single tower system producing only two sales gas streams, the horsepower per inlet MMSCF generally runs around 100 to 110 HP/MMSCF. 
     Compared to two column systems, these single column systems have the advantage of reduced capital expenditures on equipment, including elimination of the second column, and reduced operating expenditures because no external refrigeration equipment is necessary. However, they can also have higher operating expenditures related to energy/horsepower requirements. Many single column systems have horsepower requirements of around 110 HP/MMSCF of inlet feed, particularly for such systems with a single sales gas stream from the NRU column. The HP/MMSCF is improved with prior art single column systems that produce three sales gas streams at differing pressures, typically requiring between 80 and 90 HP/MMSCF. Similarly, prior art conventional two column systems producing a single sales gas stream (such as the &#39;544 patent), the horsepower requirements generally run around 80 to 90 HP/MMSCF of inlet feed. In addition to capital and operating expenditures, many prior NRU systems have limitations associated with processing NRU feed streams containing high concentrations of carbon dioxide. Nitrogen rejection processes involve cryogenic temperatures, which may result in carbon dioxide freezing in certain stages of the process causing blockage of process flow and process disruption. Carbon dioxide is typically removed by conventional methods from the NRU feed stream, to a maximum of approximately 35 parts per million (ppm) carbon dioxide, to avoid these issues. There is a need for a system and method to efficiently separate nitrogen from methane and other components in natural gas streams with reduced energy/horsepower requirements and preferably with the capability to process feed streams with higher concentrations of carbon dioxide. 
     SUMMARY OF THE INVENTION 
     The system and method disclosed herein facilitate the economically efficient removal of nitrogen from methane with substantially reduced energy/horsepower requirements. The system and method are particularly suitable for feed gas flow rates of around 20 MMSCFD or more and having nitrogen contents ranging from 5 mol % to 50 mol %. The system and method are also capable of processing feed gas containing concentrations of carbon dioxide up to approximately 100 ppm for typical nitrogen levels between 5-50%. The system and method have horsepower requirements that are around 50-60% of the horsepower requirements for most prior art single column NRU systems with a single sales gas stream. 
     According to one embodiment of the invention, a system and method are disclosed for processing an NRU feed gas stream containing primarily nitrogen and methane through two fractionating columns to produce three processed sales gas streams, each at a different pressure, which may be further compressed as needed to be meet transporting pipeline requirements (typically around 615 psia). Most preferably, one sales gas stream is a high pressure stream having a pressure between 315-415 psia, a second sales gas stream is an intermediate pressure stream having a pressure between 75-215 psia (more preferably between 115-215 psia), and a third sales gas stream is a low pressure stream having a pressure between 45-115 psia (more preferably between 50-90 psia). An inlet feed stream is preferably separated in a first separator into an overhead vapor stream that feeds into a first stage column and a bottoms liquid stream that may be sent for further processing to recover remaining methane and NGL components. The first stage column is designed as a high pressure NRU column to remove the bulk of the incoming nitrogen from the methane and heavier hydrocarbon components, while the second stage column is operated at a lower pressure. The feed streams to the first stage NRU column and the first stage overhead stream are not cooled to traditional targeted temperatures of −200 to −245 degrees F. This allows the system and method of the invention to feed the first column at a warmer temperature than prior art systems, which increases CO 2  tolerance in the feed stream. The first column also operates at a higher pressure (preferably around 315-415 psia) compared to prior art systems. The second column operates at a lower pressure (preferably around 65-115 psia). The pressure differential between the two columns allows for efficient energy sharing between the columns, including through heat exchange between first and second column streams to provide reflux to the first column and reboil heat to the second column. 
     The overhead stream from the first stage column preferably feeds the second stage column, as does an overhead stream from a second separator that separates a portion of the first column bottoms stream and the second column bottoms stream. The second column overhead stream is a nitrogen vent stream and the second column bottom stream feeds into the second separator. The bottoms stream from the first column is split into four portions, each of which is expanded and cooled to varying degrees. One portion is combined with a bottoms stream from the second separator. That combined stream and two other portions of the first column bottoms stream are three separate sales gas streams, each at a different pressure. The fourth portion of the first column bottoms stream feeds into the second separator. The second separator is preferably located near grade elevation level to allow for instrumentation critical for optimal operation and for maintenance to be easily accessible. 
     According to another preferred embodiment, the feed stream is cooled in a first heat exchanger prior to feeding the first separator through heat exchange with the first separator bottoms stream, the first column bottoms stream, the second separator bottoms stream, and the second column overhead stream. According to another preferred embodiment, the first separator overhead stream is split into two portions, a first portion of which is recycled back through the first heat exchanger to be further cooled prior to feeding the first column. A second portion is cooled and provides reboil heat to a reboiler for the first column prior to feeding the first column. 
     According to another preferred embodiment, there is heat exchange between streams from the first and second columns. Most preferably a shell and tube style heat exchanger is used, which provides the same function as an internal knockback condenser, but with the flexibility of two independent pieces of equipment, to provide reflux to the top of the first stage column and reboil heat to the bottom of the second stage column. A stream from a top of the first column feeds into a tube side of the heat exchanger, with a liquid portion returning to the column and a vapor portion exiting the column as the first column overhead stream. A portion of the second column liquid bottoms stream enters the shell side of the heat exchanger, where it is warmed to a vapor stream that is combined with a second portion of the second column liquid bottoms stream prior to feeding into the second separator. The second separator overhead stream feeds back into the second column as an ascending vapor stream. According to one preferred embodiment, the two columns are erected independently, most preferably with at least part of the second column being located at an elevation higher than the first column and the heat exchanger being at least partially elevated relative to the first column so that the portion of the second column bottoms stream may feed into the shell side of the heat exchanger by gravity feed. According to another preferred embodiment, the first and second stage columns may be stacked with the second column above the first column, effectively into a single column, as will be understood by those of ordinary skill in the art. According to another preferred embodiment, the two columns may be erected inside a cold box, but a cold box is not required. 
     According to another preferred embodiment, the first column overhead stream is cooled upstream of feeding the second column in a second heat exchanger through heat exchange with the second separator bottoms stream and the second column overhead stream. According to yet another preferred embodiment, the cooled first column overhead stream passes through a third separator or flash drum downstream of the second heat exchanger to allow a desired amount of vapor from the cooled first column overhead stream to pass through a third heat exchanger to further cool the stream and condense it prior to feeding a top of the second column. This additional cooling results from heat exchange with the second column overhead stream in the third heat exchanger. Preferably, the amount of vapor withdrawn from the third separator is controlled to achieve the desired heat balance in the third heat exchanger. Most preferably, the remaining vapor from the cooled first column overhead stream exits the third separator and is combined with the liquid portion of the stream exiting the third separator to feed into a middle section of the second column. 
     The primary advantage of the preferred embodiments of the system and method disclosed herein is substantially reduced energy/horsepower requirements compared to prior art single column systems. By splitting a bottoms stream from the first column into three separate sales gas streams, each at a different pressure, with the low pressure stream preferably between 45 to 115 psia, preferred embodiments of the system and method can achieve a substantial reduction in energy/horsepower requirements to around 65 to 75 HP/MMSCF of inlet feed. Many single column prior art systems having a single sales gas stream exiting the NRU column or even two sales gas streams have horsepower requirements of around 110 HP/MMSCF of inlet feed. The horsepower requirements are reduced in many prior art conventional two column systems producing a single gas stream to around 80 to 90 HP/MMSCF of inlet feed. The horsepower requirements are similarly reduced in many prior art single column systems that produce three sales gas streams at differing pressures to around 80 to 90 HP/MMSCF of inlet feed. However, a further reduction to around 65 to 75 HP/MMSCF of inlet feed is achievable according to preferred embodiments of the system and method of the invention. 
     For inlet feed conditions like those in the computer simulation Example 1 described below, a prior art single column design with the NRU bottoms stream split into two streams at different pressures (like in the &#39;422 patent) would require around 11,000 hp (or around 110 hp per inlet feed MMSCF of gas); however, a preferred embodiment of the invention can process that inlet gas feed stream using only 6,650 hp—a difference of more than 4,350 hp. That difference equates to around $4,300,000 in installed cost plus the added fuel demand and lower associated emissions that are saved using a preferred embodiment of the invention over prior art single column designs. The operating cost savings over the capital cost differential between a prior art single column and two column system according to a preferred embodiment of the invention would be around 25% of the total installed costs. One of the aspects that results in the lower energy/horsepower requirements is the availability of three sales gas streams, each at a different pressure level, exiting the NRU first column. The pressure levels of the three streams is higher than prior art systems that split the NRU column bottoms stream into two or three sales streams. For example, in U.S. Pat. No. 9,816,752 the NRU column bottoms stream is split into three streams—a low pressure sales stream at around 15 psia, an intermediate pressure sales stream at around 111-132 psia, and a high pressure sales stream at around 248-271 psia and requires more HP/MMSCF of inlet feed than preferred embodiments of the system and method herein where the pressures of the three sales streams (particularly the low pressure sale stream) are higher. For example, a low pressure sales stream according to the invention may have a pressure of around 55 psia compared to around 15 psia in the &#39;752 patent. Although this does not seem like a large pressure difference, there is a significant difference in HP required to compress any given volume with this higher pressure. When multiple sales gas streams are produced at different pressures, they typically undergo multiple stages of compression where a lower pressure stream is compressed in a first stage and then combined with a higher pressure stream, the combined stream is then compressed in a second stage, etc. until all of the sales gas streams are recombined into a single, final sales gas stream at the desired pressure (typically around 800 psig for pipeline requirements). Most preferably, systems and method according to the invention will allow the use of at least one less stage of compression to achieve the desired end pressure for the final sales gas stream, resulting in a substantial energy/horsepower reduction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The system and method of the invention are further described and explained in relation to the following drawing wherein: 
         FIG. 1  is a process flow diagram illustrating a preferred embodiment of a methane and nitrogen separation system and method according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , system  10  for separating nitrogen from methane from an NRU feed stream  12  according to one preferred embodiment of the invention is depicted. Where present, it is generally preferable for purposes of the present invention to remove as much of the water vapor and other contaminants from the NRU feed gas  12  as is reasonably possible prior to processing stream  12  through system  10 . It may also be desirable to remove excess amounts of carbon dioxide prior to separating the nitrogen and methane; however, the method and system are capable of processing NRU feed streams containing up to approximately 100 ppm carbon dioxide without encountering the freeze-out problems associated with prior systems and methods. Methods for removing water vapor, carbon dioxide, and other contaminants are generally known to those of ordinary skill in the art and are not described herein. 
     NRU feed stream  12  preferably comprises around 5-50% nitrogen, more preferably around 10-40% nitrogen and is at a temperature between 50-120 F, more preferably between 80-100 F, and a pressure of 550-1015 psia. Feed stream  12  is preferably cooled in a first heat exchanger  14  to a temperature between 0 to −75° F. before feeding into a first separator  18  as stream  16 . A bottoms liquid stream  158  from first separator  18  is warmed in first heat exchanger  14  and is then sent for further processing as stream  164  to refine contained NGL components. An overhead vapor stream  20  from first separator  18  is split into streams  24  and  34 . Stream  24  is recycled back through first heat exchanger  14  where it is cooled and condensed prior to passing through a JT valve  28  and then feeding into an upper level of first fractionating column  32  as liquid stream  30 . Stream  34  passes through a tube side of a reboiler  36  for the first column  32  where it is cooled and partially condensed before passing through valve  40  (most preferably a throttle valve) and then feeding into a mid-to-lower level of first fractionating column  32  as mixed liquid-vapor stream  42 . First column  32  is preferably operated at pressures ranging from 315-415 psia, more preferably from 325-385 psia with feed stream (streams  30  and  42 ) temperatures ranging from −210 to −170 F, more preferably −205 to −175 F. 
     A liquid stream  46  from a bottom of first column  32  passes through a shell side of reboiler  36  with a vapor portion  44  returning to the bottom of column  32  and a liquid portion  48  exiting as a first column bottoms stream. Bottoms stream  48  preferably comprises around 1-4% nitrogen, more preferably 2-3% nitrogen. Bottoms stream  48  is preferably split into four portions  52  (first portion),  60  (second portion),  68  (third portion), and  152  (fourth portion) in splitter  50 . Each portion passes through a valve  54 ,  62 ,  70 ,  154  where it is partially vaporized, reducing the temperature and pressure of the exiting streams  56  (first portion),  64  (second portion),  72  (third portion), and  156  (fourth portion) to varying degrees. Stream  56  preferably has a pressure of 325-385 psia and a temperature of −145 to −165° F. before being warmed in first heat exchanger  14  to become a high pressure sales gas stream  58 . Stream  64  preferably has a pressure of 150-175 psia and a temperature of −175 to −200° F. before being warmed in first heat exchanger  14  to become an intermediate pressure sales gas stream  66 . Stream  72  preferably has a pressure of 45-105 psia and a temperature of −200 to −235° F. before being mixed in mixer  74  with a bottoms stream from second separator  132  to form stream  76 . Stream  76  preferably has a pressure of 45-105 psia and a temperature of −200 to −235° F. before being warmed in first heat exchanger  14  to become a low pressure sales gas stream  78 . 
     Most preferably, high pressure sales gas stream  58  is at a pressure between 315-415 psia, and is at a pressure higher than intermediate sales gas stream  66  and higher than low pressure sales gas stream  78 . Most preferably, intermediate pressure sales gas stream  66  is at a pressure between 145-215 psia, and is at a pressure lower than high sales gas stream  58  and higher than low pressure sales gas stream  78 . Most preferably, low pressure sales gas stream  78  is at a pressure between 45-105 psia, and is at a pressure lower than intermediate sales gas stream  66  and lower than high pressure sales gas stream  58 . The pressures of high pressure sales gas stream  58  and lower pressure sales gas stream  78  are substantially higher than prior art systems, such as U.S. Pat. No. 9,816,752, where the bottoms stream from the NRU column is separated into multiple streams at different pressures. The pressures of the high pressure sales gas stream  58  and intermediate sales gas stream  66  are also substantially higher than other prior art systems having only a single sales gas stream from the bottoms of the NRU column, such as U.S. Pat. No. 5,141,544. Each sales gas stream preferably comprises at no more than 4% nitrogen. 
     First fractionating column overhead stream  86  preferably comprises around 20-40% methane and 60-80% nitrogen. First column overhead stream  86  is cooled and partially condensed in a second heat exchanger  88 , before entering a third separator or flash drum  92  as stream  90 . Cooled first column overhead stream  90  is separated in third separator  92  into a primarily liquid bottoms portion  98  and a vapor overhead portion  144 . The amount of vapor exiting the third separator  92  is controlled by the amount of vapor needed to achieve certain thermal conditions as dictated by the requirements of the heat exchanger  112 . Specifically, the amount of vapor entering the third exchanger  112  is determined by the difference in temperature between streams  144  and  114  so that stream  114  preferably exits the third heat exchanger  112  at temperature approximately 2 to 5° F. colder than stream  144 . The excess vapor, not required by the heat exchanger  112 , exits the third separator  92  from the bottom of the separator with the exiting liquid as stream  98 . Vapor stream  144  is then cooled and condensed in the third heat exchanger  112  prior to feeding into a top of the second column  104  as a liquid reflux stream  150 . Third separator  92  is designed to allow a measured amount of vapor flow from the cooled first column overhead stream  90 , to pass through third heat exchanger  112  to control subcooling stream  144  prior to feeding into the top of the second column  104  as stream  150 . The amount of subcooling achieved in the third exchanger  112  is preferably approximately 40 to 80° F. This subcooling is required to cool the overhead of the second tower, stage 1, to an adequately low temperature to create reflux inside of the second tower  104 . This reflux is required to achieve a high degree of methane/nitrogen separation within the second tower  104  and to achieve a preferred purity of nitrogen exiting the second tower  104  of approximately 96-99%, most preferably at least approximately 98%. The balance of the vapor present in stream  90  and not utilized by the exchanger  112  exits the third separator along with the liquid present in stream  90  as stream  98 . The two phase stream  98  then enters the expansion valve  100  where the pressure and temperature are preferably reduced 55-75 psia, more preferably around 70 psia, and a temperature of −265 to −285° F., more preferably around −275° F. respectively. 
     Second column  104  is preferably operated at pressures ranging from 50-115 psia, more preferably from 55-75 psia with feed stream (streams  150 ,  102 ,  134 ). The approximate feed temperature of stream  150  feeding the top of the second tower is approximately −295° F. The temperature feeding the intermediate feed, mid column is approximately −275° F. and the temperature feeding the column bottom is approximately −225° F. The subcooled liquid stream  150  entering the column top into tray  1  provides the required reflux for the column and the vapor entering as stream  134  provides the reflux vapor. An overhead stream  106  from the second column  104  is routed to an expansion valve  108  where the temperature and pressure are further reduced. The approximate temperature at this point is preferably −290 to −310° F., most preferably approximately −300° F. The vapor exiting the expansion valve  108  is then warmed in third heat exchanger  112 , then warmed again in second heat exchanger  88 , then warmed again in the first heat exchanger  14  before exiting system  10  as nitrogen vent stream  118 . 
     Nitrogen vent stream  118  preferably comprises less than 2% methane and more than 98% nitrogen. A liquid bottoms stream  120  from second column  104  is split in splitter  122  into two portions  124  and  180  that are later recombined, along with a fourth portion of the bottoms stream from first column  32 , in mixer  128  to form stream  130 , which feeds into second separator  132 . A first portion of the bottoms stream from column  104 , stream  124 , is warmed in a shell side of heat exchanger  82  upstream of mixer  128 . A second portion of the bottoms stream from column  104 , stream  180 , enters temperature control valve  182  upstream of mixer  128 . The placement of this control valve  182 , and the piping configuration involving streams  124 ,  180 ,  184 , and  126 , are important aspects to operation of system  10  in that it provides the pressure drop necessary to offset the pressure loss through the shell side of heat exchanger  82 . 
     Stream  130  preferably feeds into second separator  132  at a temperature −220 to −235° F. and a pressure between 50-75 psia. An additional two phase stream  156  (a partially vaporized fourth portion of the first column bottoms stream, preferably at a temperature of −220 to −210° F. and a pressure between 50-115 psia) is added to separator  132  to provide additional refrigeration as required to allow exchanger  88  to function properly. Stream  156  is preferably mixed with two portions of the bottoms stream from second column  104  in mixer  128  to form stream  130  prior to feeding into second separator  132 . A vapor stream  134  exits the separator  132  and is then routed to the second column  104 . Likewise, a liquid stream  166 , preferably comprising less than 4% nitrogen and more preferably less than 2% nitrogen, exits the separator  132 . Second column  104  preferably does not comprise a reboiler, but uses heat exchanger  82  and second separator  132  to effectively act as a reboiler with stream  134  being returned to a bottom of column  104  as an ascending vapor stream. Bottoms stream  166  from second separator  132  is then routed to level valve  168  as required to hold a desired liquid level in the separator  132 . Stream  166  exits the level valve  168  as stream  170  where it then enters heat exchanger  88 . Stream  170  is warmed in second heat exchanger  88  before mixing in mixer  74  with a third portion  72  of the bottoms stream from first column  32  to form low pressure sales gas stream  78 . 
     System  10  utilizes efficient heat exchange between various process streams to improve process performance. In first heat exchanger  14 , feed stream  12  and a portion  24  of an overhead stream from first separator  18  are cooled through heat exchange with first portion  66  of the first column bottoms stream, second portion  64  of the first column bottoms stream, mixed stream  76 , overhead stream  116  from the second column  104  (downstream of heat exchange in second heat exchanger  88  and third heat exchanger  112 ) and a bottoms stream  162  from the first separator  18 . The feed stream  12  is cooled in first heat exchanger  14  upstream of feeding first separator  18 . The purpose of separator  18  is to provide separation of heavier hydrocarbon components such as propane, butanes and gasolines from the inlet feed stream  12  before entering the colder part of the system  10 . Portion  24  is cooled in first heat exchanger  14  upstream of routing the stream to the first column  32 . In second heat exchanger  88 , overhead stream  86  from first column  32  is cooled through heat exchange with overhead stream  114  from second column  104  (downstream of heat exchanger in third heat exchanger  112 ) and bottoms stream  170  from second separator  132 . Overhead stream  86  is cooled in second heat exchanger  88  prior to feeding third separator  92 . In third heat exchanger  112 , stream  144  from third separator  92  is subcooled through heat exchange with overhead stream  110  from second column  104 . System  10  also preferably allows for heat exchange between a second portion  34  of the overhead stream from the first separator  18  and a liquid stream  46  from a bottom of column  32  in a reboiler  36 . The exchanger  36  (tube) is the tube side of a shell and tube style heat exchanger used to provide the necessary heat source for the bottom of the first column  32 . The exchanger depicted as  36  (shell) is the shell side of the exchanger  36 . 
     System  10  preferably also comprises a fourth heat exchanger comprising a tube side  82  (tube) and a shell side  82  (shell), that are independent pieces of equipment configured as a vertical tube, falling film condenser. Heat exchanger  82  (tube) and  82  (shell) provide the similar function as an internal knockback condenser (like that described in U.S. Patent Application Publication 2007/0180855, incorporated herein by reference). A vapor stream  80  from a top of first column  32  passes through a tube side  82  (tube) of a heat exchanger  82  (tube), where it is partially condensed, with a vapor portion exiting as first fractionating column overhead stream  86  and a liquid portion  84  returning to column  32 . The refrigerant source for heat exchanger  82  is a first portion of the bottom fluid from the second column  104 , which is routed to the shell side of the exchanger  82 , and the condensed liquid from first column overhead stream is designed to operate on the tube side of exchanger  82 . The first portion  124  of the bottoms stream from second column  104  passes through the shell side  82  (shell), preferably by gravity feed, where heat is added resulting in a partial or total vaporization of stream  124  and exiting the exchanger  82  (shell) as stream  126 . Stream  126  is then mixed with the liquid second portion of the bottoms stream from the second column  104  to form stream  130 , which feeds into second separator  132 . Column  104  is preferably located in an elevated position relative to column  32 , and the two may be stacked together to effectively form a single column, with elevated heat exchanger  82  preferably mounted between column  104  and column  32  and at least partially elevated relative to column  32 . This allows gravity feed of the liquid from stream  124  through the shell side  82  (shell) of the fourth heat exchanger, like in a knockback condenser, so that it is not necessary to use a conventional reflux condenser that requires a pump to circulate the refrigerant liquid, which can add undesirable heat to the liquid. Utilizing fourth heat exchanger  82  allows system  10  to operate with less refrigerant (horsepower) resulting in lower cost and greater flexibility. This fourth heat exchanger provides reflux to column  32  and, coupled with second separator  132 , reboil heat to column  104 . Although it is known in the prior art to use a knockback condenser, the configuration of heat exchanger  82  (shell) and  82  (tube) and the pressures and temperatures used in system  10  are different from the prior art. In the prior art, the knock back condenser had a single purpose, which is to remove heat from the column  32  overhead. In the configuration of exchanger  82  in system  10 , the purpose is twofold. As with the prior art, the exchanger  82  is still utilized to provide the removal of heat from the overhead of column  32 , but the primary purpose of exchanger  82  is to provide a heat source to reboil the second column  104 . In operation, the controls are adjusted to provide for the second column heat and are not designed to remove heat from the first column  32  against a specific target. The pressure difference between the two columns allows for this interchange of heat. The piping configuration to allow satisfactory operation of this exchanger  82  is an important aspect of system  10  must be designed so as to allow for the correct amount of heat input into stream  124 . 
     Acceptable inlet compositions in which this invention may operate satisfactorily are listed in the following Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 INLET STREAM COMPOSITIONS 
               
            
           
           
               
               
            
               
                   
                 Acceptable Inlet  
               
               
                 Inlet Component 
                 Composition Ranges 
               
               
                   
               
               
                 Methane 
                 50-95% 
               
               
                 Ethane and Heavier Components 
                  0-20% 
               
               
                 Carbon Dioxide 
                  0-100 ppm 
               
               
                 Nitrogen 
                  5-50% 
               
               
                   
               
            
           
         
       
     
     Example 1—Computer Simulation for 100 MMSCFD Feed with 20% Nitrogen 
     Still referring to  FIG. 1 , a system and method for processing a 100 MMSCFD NRU feed stream  12 , comprising approximately 20 mol % nitrogen and 72 mol % methane at 120° F. and 664.5 psia based on a computer simulation is shown and described below. Feed stream  12  passes through first heat exchanger  14 , which preferably comprises a plate-fin heat exchanger. The feed stream emerges from the heat exchanger and enters separator  18  having been cooled to −17.4° F. as stream  16 . This cooling is the result of heat exchange with other process streams  56 ,  64 ,  76 ,  116 , and  162 . The cooled stream  16  is then separated into an overhead vapor stream  20  and a bottoms liquid stream  158 . Bottoms liquid stream  158  comprises around 1.8% nitrogen, 26% methane, 10% ethane, and 14% propane. The pressure of stream  158  is reduced in valve  160  to around 165 psia in mixed liquid-vapor stream  162 . Stream  162  is then warmed in heat exchanger  14 , exiting as stream  164  at 101.7° F. and 160 psia. Stream  164  may be sent to a stabilizer column (not shown) for further processing. 
     Overhead vapor stream  20 , comprising around 20% nitrogen and around 73% methane is split in splitter  22  into streams  24  and  34 . Stream  24  is then routed for another pass through heat exchanger  14 , exiting as a subcooled liquid stream  26  having been cooled to −195° F. Stream  26  passes through a pressure reducing valve  28 , exiting as stream  30  with a pressure around 380 psia. Stream  30  feeds into an upper tray level on first fractionating column  32 . First fractionating column  32  is preferably a high pressure column upstream of a low pressure second fractionating column  104 . Vapor stream  34 , the other portion of the first separator overhead stream, passes through the tube side of exchanger  36  in order to provide heat for the reboiler  36  for first fractionating column  32 , exiting as mixed liquid-vapor stream  38  having been cooled to around −138° F. Around 8.04 million Btu/Hr of heat energy (Q-4) passes from tube side of reboiler  36  (tube) (from stream  34 ) to shell side of reboiler  36  (shell) (to stream  46 ). Stream  38  passes through temperature control valve  40  (preferably a throttling valve), exiting as stream  42  with a reduced pressure of around 391 psia. Mixed liquid-vapor stream  42  feeds into first fractionating column  32  near a mid-level tray location. Stream  80  comprising around 59% nitrogen and 40.5% methane at −189° F. from the top of column  32  feeds into a tube side  82  (shell) of a shell and tube heat exchanger that acts as a condenser for column  32 . A liquid portion of stream  80  returns to column  32  as stream  84  and a vapor portion exits tube side  82  (tube) as overhead stream  86  comprising around 66% nitrogen and 34% methane at −199° F. and 385 psia. Around 1.86 million Btu/hr of heat energy (Q-1) passes from tube side  82  (tube) to shell side  82  (shell). 
     First column overhead stream  86  passes through second heat exchanger  88 , which preferably comprises a plate-fin heat exchanger, exiting as cooled, mixed liquid-vapor stream  90  at −224° F. Stream  90  then enters a third separator or flash drum  92  where it is separated into liquid stream  98  and vapor stream  144 . Stream  98  comprises 63% nitrogen and 37% methane at −224° F. and 379 psia. Stream  98  passes through valve  100 , existing as stream  102  at −276° F. with a pressure of around 70 psia. Stream  102  feeds into a mid-level of second fractionating column  104 . Vapor stream  144  passes through third heat exchanger  112 , which preferably comprises a plate-fin heat exchanger, exiting as stream  146  having been subcooled to around −296° F. Stream  146  then passes through valve  148  to reduce the pressure of exiting stream  150  to around 70 psia. Stream  150  comprising around 86% nitrogen and 14% methane at −295° F. and 70 psia then feeds into an upper level of column  104 . A third stream, stream  134  comprising around 20% nitrogen and 80% methane at −226° F. and 65 psia, also feeds into a lower level of column  104  as an ascending vapor stream. 
     Components of feed streams  150 ,  102 , and  134  are separated in second fractionating column  104  into an overhead stream  106  and a bottoms stream  120 . Overhead stream  106  comprises around 98% nitrogen and less than 2% methane at −290° F. and 62.5 psia before passing through valve  108 , existing at stream  110  at −300° F. and 20 psia. Stream  110  passes through third heat exchanger  112 , exiting as stream  114  warmed to −229° F. Stream  114  then passes through second heat exchanger  88 , exiting as stream  116  warmed to −204° F. Stream  116  then passes through first heat exchanger  14 , exiting as stream  118  warmed to 101.7° F. Stream  118  is the nitrogen vent stream for system  10 . 
     Bottoms stream  120  comprising around 9% nitrogen and 91% methane at −246° F. and 65 psia is split in splitter  122  into streams  124  and  180 . Liquid stream  124  passes through the shell side  82  (shell) of a shell and tube heat exchanger that acts as a condenser for column  32 , exiting as vapor stream  126  at around −221° F. Stream  180  passes through valve  182 , exiting as stream  184 . Streams  184  and  126  are mixed in mixer  128  to form stream  130  that feeds into a low pressure second separator  132 . Valve  182  is used to control the temperature of mixed stream  130  feeding into separator  132 , by controlling a flow rate of stream  180  inversely relative to stream  124 . Stream  156  is also preferably mixed in mixer  128  to form stream  130 , but may also be separately fed into separator  132 . Stream  130  (and  156  if separate from  130 ) are separated in separator  132  into overhead vapor stream  134  and bottoms liquid stream  166 . Stream  134  is returned to second fractionating column  104  as an ascending vapor stream providing heat to the second column as is similar to having a reboiler in second column  104 . Bottoms stream  166  comprises less than 2% nitrogen and around 96% methane at −226° F. and 65 psia. Stream  166  passes through level valve  168 , exiting as stream  170  with a slight pressure reduction to 60 psia. Stream  170  passes through heat exchanger  88 , exiting as stream  172  having been warmed to −204° F. Stream  172  is mixed with a partially vaporized third portion  72  of a bottoms stream from fractionating column  32  in mixer  74  to form mixed stream  76 . 
     Liquid stream  46  from a bottom of column  32  passes through reboiler  36  (shell) where there is heat exchange with stream  34  (which is a portion of first separator overhead stream for system  10 ). A vapor portion  44  of stream  46  returns to the bottom of column  32  and a liquid portion exits as bottoms stream  48  comprising less than 2% nitrogen and around 89% methane at −145° F. and 388.5 psia. Bottoms stream  48  is then split in splitter  50  into streams  52 ,  60 ,  68  and  152 . Stream  52  passes through valve  54 , exiting as stream  56  at 345 psia. Stream  56  then passes through heat exchanger  14 , exiting as stream  58  having been warmed to around 101.5° F. and at a pressure of 340 psia. Stream  58  is one of the three sales gas streams. Stream  60  passes through valve  62 , exiting as stream  64  at −183° F. and a pressure of 165 psia. Stream  64  then passes through heat exchanger  14 , exiting as stream  66  having been warmed to around 101.7° F. and a pressure of 160 psia. Stream  66  is a second of the sales gas streams. Stream  68  passes through valve  70 , exiting as stream  72  having been cooled to −216° F. at a pressure of 65 psia. Stream  72  is mixed with stream  172  in mixer  74  to form stream  76  at −217.8° F. and 57.5 psia, which passes through heat exchanger  14  exiting as stream  78  at 101.7° F. and 55 psia. Stream  78  is a third sales gas stream. Of the sales gas streams, stream  58  is a high pressure stream (higher than streams  66  and  78 ) and depending on the requirements of the installation, this stream may not need further compression to enter existing facility equipment or the compression requirements would be significantly reduced when compared with existing nitrogen rejection technologies. Stream  66  is an intermediate pressure stream (lower pressure than stream  58  but higher pressure than stream  78 ), and stream  78  is a low pressure stream (lower pressure than streams  58  and  66 ). These streams  66  and  78  may be further compressed as needed to meet pipeline requirements. 
     Stream  152 , the fourth portion split from bottoms stream  48 , passes through valve  154 , exiting as partially vaporized stream  156  having been cooled to −214° F. at a pressure of 70 psia. Stream  156  is the third stream to enter mixer  128 . The mixed stream from  128  exits as stream  130  and feeds into second separator  132 . 
     For inlet feed conditions in Example 1, a prior art single column design would require around 11,000 hp (or around 110 hp per inlet feed MMSCF of gas); however, a preferred embodiment of the invention according to  FIG. 1  can process that inlet gas feed stream using only 6,650 hp, which is around 60% of the horsepower required in the prior art system. That difference equates to around $4,300,000 in installed cost plus the added fuel demand that are saved using a preferred embodiment of the invention as depicted in  FIG. 1  over prior art single column designs. The operating cost savings over the capital cost differential between a prior art single column and two column system according to the preferred embodiment in  FIG. 1  would be around 25% of the total installed costs. 
     The specific flow rates, temperatures, pressures, and compositions of various flow streams referred to in connection with the above discussion of a computer simulation for a system  10  appear in Table 2 below. These values are based on a feed gas stream  12  comprising 20% nitrogen, around 73% methane, and 50 ppm of carbon dioxide with a flow rate of 100 MMSCFD. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 FLOW STREAM PROPERTIES 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Property 
                 12 
                 16 
                 20 
                 24 
                 26 
                 30 
                 34 
               
               
                   
               
               
                 Nitrogen 
                 20.0000*   
                 20.0000   
                 20.1842 
                 20.1842 
                 20.1842   
                 20.1842   
                 20.1842 
               
               
                 CO2 
                 0.005*   
                 0.005   
                 0.00499903 
                 0.00499903 
                  0.00499903 
                  0.00499903 
                 0.00499903 
               
               
                 Methane 
                 72.7672*   
                 72.7672   
                 73.2420 
                 73.2420 
                 73.2420   
                 73.2420   
                 73.2420 
               
               
                 Ethane 
                 4.28875*  
                 4.28875  
                 4.22698 
                 4.22698 
                 4.22698  
                 4.22698  
                 4.22698 
               
               
                 Propane 
                 1.64580*  
                 1.64580  
                 1.51655 
                 1.51655 
                 1.51655  
                 1.51655  
                 1.51655 
               
               
                 i-Butane 
                 0.313443* 
                 0.313443 
                 0.251551 
                 0.251551 
                 0.251551  
                 0.251551  
                 0.251551 
               
               
                 n-Butane 
                 0.616397* 
                 0.616397 
                 0.445057 
                 0.445057 
                 0.445057  
                 0.445057  
                 0.445057 
               
               
                 i-Pentane 
                 0.126174* 
                 0.126174 
                 0.0640669 
                 0.0640669 
                 0.0640669 
                 0.0640669 
                 0.0640669 
               
               
                 n-Pentane 
                 0.103348* 
                 0.103348 
                 0.0447387 
                 0.0447387 
                 0.0447387 
                 0.0447387 
                 0.0447387 
               
               
                 Hexane 
                 0.133944* 
                 0.133944 
                 0.0198272 
                 0.0198272 
                 0.0198272 
                 0.0198272 
                 0.0198272 
               
               
                 Temperature 
                 120*      
                 −17.4194   
                 −17.4875 
                 −17.4875 
                 −195*       
                 −195.038     
                 −17.4875 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 664.5*     
                 659.5     
                 658.5 
                 658.5 
                 653.5     
                 380*      
                 658.5 
               
               
                 Mole Fraction 
                 100       
                 99*     
                 100 
                 100 
                 0      
                 0      
                 100 
               
               
                 Vapor % 
               
               
                 Std Vapor 
                 100*      
                 100      
                 98.9982 
                 70.5388 
                 70.5388   
                 70.5388   
                 28.4594 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Property 
                 38 
                 42 
                 44 
                 46 
                 48 
                 52 
                 56 
               
               
                   
               
               
                 Nitrogen 
                 20.1842 
                 20.1842 
                 7.76154 
                 3.73594 
                 1.93914 
                 1.93914 
                 1.93914  
               
               
                 CO2 
                 0.00499903 
                 0.00499903 
                 0.00166185 
                 0.00531146 
                 0.00694044 
                 0.00694044 
                  0.00694044 
               
               
                 Methane 
                 73.2420 
                 73.2420 
                 91.6747 
                 89.7532 
                 88.8955 
                 88.8955 
                 88.8955   
               
               
                 Ethane 
                 4.22698 
                 4.22698 
                 0.527887 
                 4.23647 
                 5.89178 
                 5.89178 
                 5.89178  
               
               
                 Propane 
                 1.51655 
                 1.51655 
                 0.0315056 
                 1.47234 
                 2.11545 
                 2.11545 
                 2.11545  
               
               
                 i-Butane 
                 0.251551 
                 0.251551 
                 0.00111929 
                 0.242955 
                 0.350896 
                 0.350896 
                 0.350896  
               
               
                 n-Butane 
                 0.445057 
                 0.445057 
                 0.00154193 
                 0.429712 
                 0.620824 
                 0.620824 
                 0.620824  
               
               
                 i-Pentane 
                 0.0640669 
                 0.0640669 
                 2.12102E−05 
                 0.0617961 
                 0.0893689 
                 0.0893689 
                 0.0893689 
               
               
                 n-Pentane 
                 0.0447387 
                 0.0447387 
                 2.53333E−05 
                 0.0431562 
                 0.0624074 
                 0.0624074 
                 0.0624074 
               
               
                 Hexane 
                 0.0198272 
                 0.0198272 
                 1.62426E−06 
                 0.0191229 
                 0.0276576 
                 0.0276576 
                 0.0276576 
               
               
                 Temperature 
                 −137.715* 
                 −160.830 
                 −145.335 
                 −151.495 
                 −145.335 
                 −145.335 
                 −151.019     
               
               
                 ° F. 
               
               
                 Pressure psia 
                 653.5 
                 391.273* 
                 388.5 
                 388.5 
                 388.5 
                 388.5 
                 345*      
               
               
                 Mole Fraction 
                 40.1571 
                 50.8018 
                 100 
                 0 
                 0 
                 0 
                 4.97369  
               
               
                 Vapor % 
               
               
                 Std Vapor 
                 28.4594 
                 28.4594 
                 31.6770 
                 102.647 
                 70.9699 
                 42.2528 
                 42.2528   
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Property 
                 58 
                 60 
                 64 
                 66 
                 68 
                 72 
                 76 
               
               
                   
               
               
                 Nitrogen 
                 1.93914 
                 1.93914 
                 1.93914  
                 1.93914 
                 1.93914 
                 1.93914  
                 1.91624 
               
               
                 CO2 
                 0.00694044 
                 0.00694044 
                  0.00694044 
                 0.00694044 
                 0.00694044 
                  0.00694044 
                 0.00390743 
               
               
                 Methane 
                 88.8955 
                 88.8955 
                 88.8955   
                 88.8955 
                 88.8955 
                 88.8955   
                 93.0578 
               
               
                 Ethane 
                 5.89178 
                 5.89178 
                 5.89178  
                 5.89178 
                 5.89178 
                 5.89178  
                 3.23637 
               
               
                 Propane 
                 2.11545 
                 2.11545 
                 2.11545  
                 2.11545 
                 2.11545 
                 2.11545  
                 1.15643 
               
               
                 i-Butane 
                 0.350896 
                 0.350896 
                 0.350896  
                 0.350896 
                 0.350896 
                 0.350896  
                 0.191808 
               
               
                 n-Butane 
                 0.620824 
                 0.620824 
                 0.620824  
                 0.620824 
                 0.620824 
                 0.620824  
                 0.339356 
               
               
                 i-Pentane 
                 0.0893689 
                 0.0893689 
                 0.0893689 
                 0.0893689 
                 0.0893689 
                 0.0893689 
                 0.0488510 
               
               
                 n-Pentane 
                 0.0624074 
                 0.0624074 
                 0.0624074 
                 0.0624074 
                 0.0624074 
                 0.0624074 
                 0.0341132 
               
               
                 Hexane 
                 0.0276576 
                 0.0276576 
                 0.0276576 
                 0.0276576 
                 0.0276576 
                 0.0276576 
                 0.0151182 
               
               
                 Temperature 
                 101.540 
                 −145.335 
                 −183.260     
                 101.727* 
                 −145.335 
                 −216.425     
                 −217.785 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 340 
                 388.5 
                 165*      
                 160 
                 388.5 
                 65*      
                 57.5 
               
               
                 Mole Fraction 
                 100 
                 0 
                 23.9490   
                 100 
                 0 
                 36.8655   
                 75.7586 
               
               
                 Vapor % 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Property 
                 78 
                 80 
                 84 
                 86 
                 90 
                 98 
               
               
                   
               
               
                 Nitrogen 
                 1.91624 
                 59.4153 
                 31.3690 
                 66.3824 
                 66.3824 
                 63.1382 
               
               
                 CO2 
                 0.00390743 
                 0.000326395 
                 0.00130540 
                 8.31995E−05 
                 8.31995E−05 
                 9.63113E−05 
               
               
                 Methane 
                 93.0578 
                 40.4845 
                 68.1745 
                 33.6059 
                 33.6059 
                 36.8483 
               
               
                 Ethane 
                 3.23637 
                 0.0959952 
                 0.435886 
                 0.0115625 
                   0.0115625 
                 0.0134116 
               
               
                 Propane 
                 1.15643 
                 0.00367169 
                 0.0182156 
                 5.88179E−05 
                 5.88179E−05 
                 6.84285E−05 
               
               
                 i-Butane 
                 0.191808 
                 9.24394E−05 
                 0.000463516 
                 2.59683E−07 
                 2.59683E−07 
                 3.02223E−07 
               
               
                 n-Butane 
                 0.339356 
                 0.000126703 
                 0.000635589 
                 2.90618E−07 
                 2.90618E−07 
                 3.38227E−07 
               
               
                 i-Pentane 
                 0.0488510 
                 8.01840E−07 
                 4.02942E−06 
                 7.25372E−11 
                 7.25372E−11 
                 8.44290E−11 
               
               
                 n-Pentane 
                 0.0341132 
                 1.29730E−06 
                 6.51838E−06 
                 3.23020E−10 
                 3.23020E−10 
                 3.75974E−10 
               
               
                 Hexane 
                 0.0151182 
                 8.00758E−08 
                 4.02408E−07 
                 4.85067E−12 
                 4.85067E−12 
                 5.64582E−12 
               
               
                 Temperature 
                 101.727* 
                 −189.094 
                 −199.103 
                 −199.103 
                 −223.793   
                 −223.896 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 55 
                 385 
                 385 
                 385 
                 380     
                 379 
               
               
                 Mole Fraction 
                 100 
                 100 
                 0 
                 100 
                 15*    
                 1.22020 
               
               
                 Vapor % 
               
               
                 Std Vapor 
                 20.5208 
                 34.9908 
                 6.96253 
                 28.0282 
                 28.0282 
                 24.0804 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Property 
                 102 
                 106 
                 110 
                 114 
                 116 
                 118 
                 120 
               
               
                   
               
               
                 Nitrogen 
                 63.1382 
                 98.4286 
                    98.4286 
                 98.4286 
                 98.4286 
                 98.4286 
                 8.92683 
               
               
                 CO2 
                 9.63113E−05 
                 4.30859E−10 
                 4.30859E−10 
                 4.30859E−10 
                 4.30859E−10 
                 4.30859E−10 
                 0.000178861 
               
               
                 Methane 
                 36.8483 
                 1.57143 
                     1.57143 
                 1.57143 
                 1.57143 
                 1.57143 
                 91.0478 
               
               
                 Ethane 
                   0.0134116 
                 4.62270E−08 
                 4.62270E−08 
                 4.62270E−08 
                 4.62270E−08 
                 4.62270E−08 
                 0.0250017 
               
               
                 Propane 
                 6.84285E−05 
                 5.06148E−13 
                 5.06148E−13 
                 5.06148E−13 
                 5.06148E−13 
                 5.06148E−13 
                 0.000145857 
               
               
                 i-Butane 
                 3.02223E−07 
                 0 
                 0 
                 0 
                 0 
                 0 
                 7.50616E−07 
               
               
                 n-Butane 
                 3.38227E−07 
                 0 
                 0 
                 0 
                 0 
                 0 
                 8.64757E−07 
               
               
                 i-Pentane 
                 8.44290E−11 
                 0 
                 0 
                 0 
                 0 
                 0 
                 4.25543E−10 
               
               
                 n-Pentane 
                 3.75974E−10 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1.57601E−09 
               
               
                 Hexane 
                 5.64582E−12 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1.78131E−11 
               
               
                 Temperature 
                 −275.993   
                 −290.157 
                  −299.700 
                 −228.767 
                 −204.101* 
                 101.727* 
                 −245.576 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 70*    
                 62.5 
                 20* 
                 19 
                 18 
                 17 
                 65 
               
               
                 Std Vapor 
                 24.0804 
                 18.7245 
                    18.7245 
                 18.7245 
                 18.7245 
                 18.7245 
                 15.2885 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
            
               
                 Property 
                 124 
                 126 
                 130 
                 134 
                 144 
               
               
                   
               
               
                 Nitrogen 
                 8.92683 
                 8.92683 
                 7.71205 
                 19.8681 
                 86.1708 
               
               
                 CO2 
                 0.000178861 
                 0.000178861 
                 0.00135433 
                 6.72785E−05 
                 3.22227E−06 
               
               
                 Methane 
                 91.0478 
                 91.0478 
                 90.6737 
                 80.1220 
                 13.8289 
               
               
                 Ethane 
                 0.0250017 
                 0.0250017 
                 1.04492 
                 0.00971969 
                 0.000283701 
               
               
                 Propane 
                 0.000145857 
                 0.000145857 
                 0.367883 
                 9.71549E−05 
                 1.96930E−07 
               
               
                 i-Butane 
                 7.50616E−07 
                 7.50616E−07 
                 0.0610024 
                 7.01444E−07 
                 2.08579E−10 
               
               
                 n-Butane 
                 8.64757E−07 
                 8.64757E−07 
                 0.107928 
                 8.48175E−07 
                 2.15697E−10 
               
               
                 i-Pentane 
                 4.25543E−10 
                 4.25543E−10 
                 0.0155364 
                 7.47517E−10 
                 1.60551E−15 
               
               
                 n-Pentane 
                 1.57601E−09 
                 1.57601E−09 
                 0.0108492 
                 2.51368E−09 
                 2.50525E−14 
               
               
                 Hexane 
                 1.78131E−11 
                 1.78131E−11 
                 0.00480815 
                 2.27920E−11 
                 5.04461E−16 
               
               
                 Temperature 
                 −245.576 
                 −221.201 
                 −225.657 
                 −225.657 
                 −223.896 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 65 
                 65 
                 65 
                 65 
                 379 
               
               
                 Std Vapor 
                 5.12485 
                 5.12485 
                 18.5056 
                 5.98481 
                 3.94784* 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Property 
                 146 
                 150 
                 152 
                 156 
                 158 
                 162 
                 164 
               
               
                   
               
               
                 Nitrogen 
                 86.1708 
                 86.1708 
                 1.93914 
                 1.93914  
                 1.79515 
                  1.79515 
                 1.79515 
               
               
                 CO2 
                 3.22227E−06 
                 3.22227E−06 
                 0.00694044 
                  0.00694044 
                 0.00509588 
                    0.00509588 
                 0.00509588 
               
               
                 Methane 
                 13.8289 
                 13.8289 
                 88.8955 
                 88.8955   
                 25.8431 
                 25.8431 
                 25.8431 
               
               
                 Ethane 
                 0.000283701 
                    0.000283701 
                 5.89178 
                 5.89178  
                 10.3922 
                 10.3922 
                 10.3922 
               
               
                 Propane 
                 1.96930E−07 
                 1.96930E−07 
                 2.11545 
                 2.11545  
                 14.4181 
                 14.4181 
                 14.4181 
               
               
                 i-Butane 
                 2.08579E−10 
                 2.08579E−10 
                 0.350896 
                 0.350896  
                 6.42948 
                  6.42948 
                 6.42948 
               
               
                 n-Butane 
                 2.15697E−10 
                 2.15697E−10 
                 0.620824 
                 0.620824  
                 17.5478 
                 17.5478 
                 17.5478 
               
               
                 i-Pentane 
                 1.60551E−15 
                 1.60551E−15 
                 0.0893689 
                 0.0893689 
                 6.26342 
                  6.26342 
                 6.26342 
               
               
                 n-Pentane 
                 2.50525E−14 
                 2.50525E−14 
                 0.0624074 
                 0.0624074 
                 5.89497 
                  5.89497 
                 5.89497 
               
               
                 Hexane 
                 5.04461E−16 
                 5.04461E−16 
                 0.0276576 
                 0.0276576 
                 11.4107 
                 11.4107 
                 11.4107 
               
               
                 Temperature 
                 −295.724* 
                 −294.945   
                 −145.335 
                 −214.065     
                 −17.4875 
                 −38.8154  
                 101.727* 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 374 
                 70*    
                 388.5 
                 70*      
                 658.5 
                 165*    
                 160 
               
               
                 Mole Fraction 
                 0 
                 0    
                 0 
                 36.0482   
                 0 
                 23.0297 
                 53.0054 
               
               
                 Vapor % 
               
               
                 Std Vapor 
                 3.94784 
                  3.94784 
                 3.21712 
                 3.21712  
                 1.00183 
                  1.00183 
                 1.00183 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
           
               
               
            
               
                 Mole Fraction/ 
                 Stream No. 
               
            
           
           
               
               
               
               
               
               
            
               
                 Property 
                 166 
                 170 
                 172 
                 180 
                 184 
               
               
                   
               
               
                 Nitrogen 
                 1.90160 
                 1.90160  
                 1.90160 
                 8.92683 
                 8.92683 
               
               
                 CO2 
                 0.00196953 
                  0.00196953 
                 0.00196953 
                 0.000178861 
                 0.000178861 
               
               
                 Methane 
                 95.7172 
                 95.7172   
                 95.7172 
                 91.0478 
                 91.0478 
               
               
                 Ethane 
                 1.53973 
                 1.53973  
                 1.53973 
                 0.0250017 
                 0.0250017 
               
               
                 Propane 
                 0.543680 
                 0.543680  
                 0.543680 
                 0.000145857 
                 0.000145857 
               
               
                 i-Butane 
                 0.0901606 
                 0.0901606 
                 0.0901606 
                 7.50616E−07 
                 7.50616E−07 
               
               
                 n-Butane 
                 0.159516 
                 0.159516  
                 0.159516 
                 8.64757E−07 
                 8.64757E−07 
               
               
                 i-Pentane 
                 0.0229626 
                 0.0229626 
                 0.0229626 
                 4.25543E−10 
                 4.25543E−10 
               
               
                 n-Pentane 
                 0.0160351 
                 0.0160351 
                 0.0160351 
                 1.57601E−09 
                 1.57601E−09 
               
               
                 Hexane 
                 0.00710639 
                  0.00710639 
                 0.00710639 
                 1.78131E−11 
                 1.78131E−11 
               
               
                 Temperature 
                 −225.657 
                 −227.698     
                 −204.007 
                 −245.576 
                 −245.576 
               
               
                 ° F. 
               
               
                 Pressure psia 
                 65 
                 60*      
                 57.5 
                 65 
                 65 
               
               
                 Mole Fraction 
                 0 
                 0.990159  
                 96.2238 
                 0 
                 0 
               
               
                 Vapor % 
               
               
                 Std Vapor 
                 12.5208 
                 12.5208   
                 12.5208 
                 10.1637 
                 10.1637 
               
               
                 Volumetric 
               
               
                 Flow MMSCFD 
               
               
                   
               
            
           
         
       
     
     It will be appreciated by those of ordinary skill in the art that these values are based on the particular parameters and composition of the feed stream in the above computer simulation example. The temperature, pressure, and compositional values will differ depending on the parameters and composition of the NRU Feed stream  12  and specific operating parameters for various pieces of equipment in system  10 . 
     According to another preferred embodiment, a natural gas expander may be used in place of valve  108 , which would provide a higher degree of cooling of the second column overhead stream than with the valve alone. For example, where the differential across the valve (stream  106  to stream  110 ) is calculated to be approximately 10° F., the differential across an expander is approximately 37° F. This higher degree of cooling results in a slightly higher purity of nitrogen to be vented in stream  118  of approximately 0.5 to 1 percent higher than when a valve  108  is used, but also significantly reduces the residue compression required. With a standard control valve in the position of valve  108  the amount of compression is calculated to be approximately 66.5 BHP/MMSCF of inlet gas. The calculated residue HP required with the expander in place instead of the valve  108  is approximately 56.4 BHP/MMSCF. This represents a near 18% reduction in compression HP along with the associated reduction in fuel or power and the associated reduction in environmental impact. 
     It will also be appreciated by those of ordinary skill in the art upon reading this disclosure that references to separation of nitrogen and methane used herein refer to processing an NRU feed gas to produce various multi-component product streams containing large amounts of the particular desired component, but not pure streams of any particular component. One of those product streams is a nitrogen vent stream, which is primarily comprised of nitrogen but may have small amounts of other components, such as methane and ethane. Another product stream is a processed gas stream, or sales gas stream, which is primarily comprised of methane but may have small amounts of other components, such as nitrogen, ethane, and propane. Amounts of components in the various streams described herein as a percentage are mole fraction percentage. 
     It will also be appreciated by those of ordinary skill in the art upon reading this disclosure that additional processing sections for removing carbon dioxide, water vapor, and possibly other components or contaminants that are present in the NRU feed stream, can also be included in the system and method of the invention, depending upon factors such as, for example, the origin and intended disposition of the product streams and the amounts of such other gases, impurities or contaminants as are present in the NRU feed stream. Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading this specification in view of the accompanying drawings, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventor is legally entitled.