Abstract:
A system and method for removing nitrogen and producing liquefied natural gas (“LNG”) from methane without the need for external refrigeration. The invention also relates to a system and method for removing nitrogen from methane and for producing liquefied nitrogen in addition to LNG. The system and method of the invention are particularly suitable for use in recovering and processing comparatively small volumes of methane from coal mines or from flash gas captured at an LNG loading site.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/256,053, filed Oct. 29, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a system and method for removing nitrogen and producing liquefied natural gas (“LNG”) from gaseous streams containing methane and other impurities without the need for an external refrigeration system. The invention also relates to a system and method for removing nitrogen from methane and for producing liquefied nitrogen in addition to LNG. The system and method of the invention are particularly suitable for use in recovering and processing comparatively small volumes of methane from coal mine vent streams or streams containing methane and nitrogen captured as flash gas at an LNG loading site. 
     2. Description of Related Art 
     Because many sources of methane produced during mining, energy transport or other industrial applications are not located near a natural gas transmission pipeline or other facility having gas-processing or liquefaction capabilities, a significant amount of methane gas, often combined with other gaseous or vaporous components, is either flared or vented to the atmosphere. This is particularly true in remote or otherwise underdeveloped areas where environmental impact is less of a concern than in the United States and other developed countries. 
     Naturally occurring methane is often encountered in coal mines, where it poses a significant risk to miners and to the mine subsurface equipment and inventory. This risk arises from miners being unable to breathe methane gas and also because air containing more than about 5 percent methane (preferably not more than about 2 percent) poses a significant risk of explosion. For these reasons, vertical shafts are frequently drilled into coal-containing formations ahead of the mining equipment so that any pockets of methane encountered during the drilling can be brought to the surface. Air is also forced down into subterranean mines and circulated through the mine shafts to dilute any residual methane that may be present and force it to the surface as well. Once the mining equipment reaches the vertical shafts drilled to recover methane from the formation, collapses can occur that produce another kind of methane-containing gas referred to as “gob gas,” which is also extremely hazardous. 
     Also, at LNG loading facilities, some LNG is typically vaporized as flash gas when the product first enters the tank, which is typically in an LNG tanker or other transport vessel. Because LNG normally comprises a minor amount of residual nitrogen, and because the nitrogen vaporizes at a lower temperature than LNG, the flash gas thus produced will contain a higher percentage of nitrogen than is contained in the LNG. For this reason, even where the flash gas is captured without exposing it to air, the methane in the flash gas cannot readily be re-liquefied without first removing the nitrogen. Although the amount of methane in the flash gas is relatively minor compared to the total amount being loaded, it may not enough to justify economically the investment and expense required to remove the nitrogen and then re-liquefy the methane using conventional technology. Unfortunately, this can cause operators to resort to the more expedient but less environmentally responsible alternatives of venting or flaring the flash gas. 
     Advantages of recovering coal mine methane for producing LNG, the existing technologies and the importance of accommodating smaller gas flows than conventional natural gas to LNG applications are all discussed in “Coal Mine Methane and LNG,” a paper published in November 2008 by the U.S. Environmental Protection Agency Coalbed Methane Outreach Program Technical Options Series. 
     Prior patents disclosing other gas processing technology invented by Rayburn C. Butts of BCCK Engineering include U.S. Pat. Nos. 5,141,544; 5,257,505; and 5,375,422. 
     Compander technology comprising an integrally geared design with one or more expansion stages and one or more compressor stages has previously been disclosed, for example, by Cryostar Industries. The expansion of gas allows for energy to be extracted or harnessed by the use of an expander device. The expander is coupled with a matching compressor, thereby creating a stage compression as is useful in the process. Auxiliary compression is often required to produce the total amount of compression requirements. 
     SUMMARY OF THE INVENTION 
     The system and method disclosed herein facilitate the economically efficient and environmentally friendly removal of nitrogen from methane and the production of LNG without the use of an external refrigeration system. As used throughout this specification and claims, the terms “external refrigeration” and “recirculated refrigerant” refer to cooling by means of a recirculated coolant that is external to the process streams emanating directly or indirectly from the inlet gas, and also include cascade refrigeration or mixed refrigerant processes as those conventional cascade and mixed refrigerant processes are known to and understood by those of ordinary skill in the art. According to one embodiment of the invention, nitrogen removed from the methane stream is also liquefied and produced in addition to LNG. The system and method of the invention are suitable for use in processing relatively small volumes of methane in comparison to conventional natural gas processing plants, and are particularly suitable for use in processing methane recovered from coal mines and from LNG loading facilities. 
     It has now been discovered that integration of some of the nitrogen removal technology previously disclosed, for example, in U.S. Pat. Nos. 5,375,422, 5,257,505 and 5,141,544 with additional technology as disclosed herein, offers significant advantages not previously achievable by those of ordinary skill in the art using existing technologies. These advantages include, for example, an ability to process and liquefy methane at relatively low temperatures through the use of strategically placed turbo expander or compander units without the need for an external refrigeration system, thereby substantially reducing horsepower and compressor requirements, with attendant reductions in capital investment and operating costs. Moreover, because the economic and operational advantages of the subject system and method can be realized in facilities processing comparatively small volumes of methane, the technology can be provided and practiced at locations where methane would otherwise be flared or vented to the atmosphere, thereby eliminating or significantly reducing any adverse environmental impact. 
     According to one embodiment of the invention, a system is disclosed for removing nitrogen and for producing LNG from methane gas comprising other gaseous components, the system comprising a source of methane gas disposed proximally to a processing site, the source being accessible to the site without transport through an external pipeline; and a nitrogen removal section configured to remove nitrogen gas from the methane gas and to liquefy a substantial portion of the methane gas to produce LNG without use of a recirculated refrigerant. 
     According to another embodiment of the invention, a system is disclosed for producing LNG from methane gas comprising other gaseous components, the system comprising a source of methane gas disposed proximally to a processing site, the source being accessible to the site without transport through an external pipeline; a first processing section configured to remove oxygen gas from the methane gas; a second processing section configured to remove carbon dioxide from the methane gas; a third processing section configured to dehydrate the methane gas; a fourth processing section configured to remove nitrogen gas from the methane gas and to liquefy a substantial portion of the methane gas to produce LNG without use of a recirculated refrigerant; an LNG subcooler section disposed downstream of the fourth processing section; wherein the LNG subcooler section is configured to further cool LNG received from the fourth processing section without use of a recirculated refrigerant; conduits through which the methane gas received from the source can flow into and out of the first, second, third and fourth processing sections and the LNG subcooler section; and a receptacle for LNG received from the LNG subcooler section. 
     According to another embodiment of the invention, a method is disclosed for removing nitrogen and for producing LNG from methane gas comprising other gaseous components, the method comprising: providing a source of methane gas disposed proximally to a processing site, the source being accessible to the site without transport through an external pipeline; and removing nitrogen from the methane gas and liquefying a substantial portion of the methane gas to produce LNG without use of a recirculated refrigerant. 
     According to another embodiment of the invention, a method is disclosed for producing LNG from methane gas containing other gaseous components, the method comprising: providing a source of methane gas disposed proximally to a processing site, the source being accessible to the site without transport through an external pipeline; introducing the methane gas into a first processing section configured to remove oxygen gas from the methane gas; introducing the methane gas into a second processing section configured to remove carbon dioxide from the methane gas; introducing the methane gas into a third processing section configured to dehydrate the methane gas; introducing the methane gas into a fourth processing section configured to remove nitrogen gas from the methane gas and to liquefy a substantial portion of the methane gas to produce LNG without use of a recirculated refrigerant; introducing the LNG received from the fourth processing station into an LNG subcooling section to further cool LNG received from the fourth processing section without use of a recirculated refrigerant; and introducing LNG received from the LNG subcooler section into a receptacle. 
     According to another embodiment of the invention, a system and method are disclosed for producing liquid nitrogen and LNG from methane as separate product streams without use of a recirculated refrigerant. 
     According to another embodiment of the invention, a system and method are disclosed for producing LNG from methane recovered from a coal mine or from an LNG loading station or facility. 
     According to another embodiment of the invention, a system and method are disclosed for producing LNG and liquid nitrogen from methane recovered from a coal mine or from an LNG loading station or facility, 
     It will be appreciated by those of ordinary skill in the art upon reading this disclosure that additional processing sections for removing oxygen, carbon dioxide, water vapor, and possibly other components or contaminants that are present with methane in the inlet gas 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 or impurities or contaminants as are present in the inlet gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The system and method of the invention are further described and explained in relation to the following drawings wherein: 
         FIG. 1  is a simplified process flow diagram illustrating principal processing stages of one embodiment of a system and method for producing LNG from an inlet gas containing methane and other contaminants; 
         FIG. 2  is a simplified process flow diagram illustrating principal processing stages of another embodiment of a system and method for producing LNG and liquid nitrogen (“LIN”) from an inlet gas containing methane and nitrogen; 
         FIG. 3  is a more detailed process flow diagram illustrating one embodiment of the nitrogen removal section of the simplified process flow diagrams of  FIGS. 1 and 2 ; 
         FIG. 4  is a more detailed process flow diagram illustrating one embodiment of the LNG production section of the simplified process flow diagrams of  FIGS. 1 and 2 ; and 
         FIG. 5  is a modified version of the detailed process flow diagram of  FIG. 4  illustrating an alternate embodiment in which a liquid nitrogen stream is produced as another byproduct of the system and method of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , one satisfactory system  10  of the invention comprises processing equipment useful for receiving methane gas and cooling it to form LNG without the use of external refrigeration or a recirculated refrigerant. Although the source of the methane gas is not critical to the system and method of the invention, some suitable sources of methane gas for use in the invention are coal mines, LNG loading facilities, and other industrial or geologic sources. The methane used as inlet gas stream  12  will typically contain other gases as well, with nitrogen, oxygen, carbon dioxide and water vapor being the most notable examples. Where present, it is generally preferable for purposes of the present invention to remove as much of the oxygen, carbon dioxide and water vapor as is reasonably possible prior to implementing the nitrogen removal and methane liquefaction portions of the invention. For this reason, system  10  of the invention as depicted in  FIG. 1  includes first, second, third processing sections  14 ,  16 ,  18  for the removal of oxygen, carbon dioxide and water vapor, respectively, upstream of the nitrogen removal section  20  and the LNG subcooler section  22  and LNG storage  24  for the LNG product  26 . Conventional technologies for removing oxygen and carbon dioxide from methane, and for dehydrating the methane stream to remove water vapor are generally well known and are already commercially available from various sources. For this reason, this disclosure is primarily directed to enabling those of ordinary skill in the art to produce LNG and, optionally, liquid nitrogen from a methane inlet stream without the need for external refrigeration (including cascade refrigeration or mixed refrigerant processes). 
     Referring to  FIG. 2 , system  30  is disclosed as another suitable alternative embodiment of the invention. In this embodiment, the inlet gas  12 , oxygen removal section  14 , carbon dioxide removal section  16  and dehydration section  18  of the invention are provided as discussed above in relation to  FIG. 1 . In nitrogen removal section  20 , however, a stream of nitrogen gas recovered from the methane is diverted to a nitrogen expander  175  to liquefy at least a portion of the stream, and then to a liquid nitrogen separator  196  to produce a liquid nitrogen product  204  in addition to LNG product  26  produced substantially as disclosed in relation to system  10  of  FIG. 1 . 
     Nitrogen removal section  20  of the invention as seen in  FIGS. 1 and 2  is further described and explained in relation to  FIG. 3 . Referring to  FIG. 3 , a nitrogen-containing methane feed stream  56  is combined in manifold  60  with recycled methane stream  62  from an expander-compressor section that is part of LNG subcooler section  22  and is further described below in relation to  FIG. 4 . Combined inlet stream  57  is directed to plate fin cooler  64  or another similarly suitable heat exchange device and emerges as stream  66 . Stream  66  is controlled by valve  68  to produce stream  70  having substantially the same temperature but approximately half the pressure of stream  66  before entering nitrogen fractionation tower  71 . Tower  71  operates at approximately −230° F. and 300 psia, and causes the nitrogen gas to separate from the methane and flow upwardly through the tower as a vapor. 
     Acceptable inlet compositions in which this invention may operate satisfactorily are listed in the following Table 1: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 INLET STREAM COMPOSITIONS 
               
             
          
           
               
                   
                   
                 Acceptable Inlet Composition Ranges 
               
               
                   
                 Inlet Component 
                 (Inlet Percent) 
               
               
                   
                   
               
               
                   
                 Methane 
                 20 to 100 
               
               
                   
                 Oxygen 
                 0 to 15 
               
               
                   
                 Carbon Dioxide 
                 0 to 5  
               
               
                   
                 Nitrogen 
                 0 to 80 
               
               
                   
                   
               
             
          
         
       
     
     The flow rates, temperatures and pressures of various flow streams referred to in connection with the discussion of the system and method of the invention in relation to  FIGS. 3 and 4  for a nominal inlet flow rate in this example of 19 MMSCFD appear in Table 2 below: 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 FLOW STREAM PROPERTIES 
               
             
          
           
               
                   
                 Stream 
                   
                   
                   
               
               
                   
                 Reference 
                 Flow Rate 
                 Temperature 
                 Pressure 
               
               
                   
                 Numeral 
                 (lbmol/h) 
                 (deg. F.) 
                 (psia) 
               
               
                   
                   
               
             
          
           
               
                   
                  26 
                 575 
                 −263 
                 17 
               
               
                   
                  56 
                 1617 
                 121 
                 640 
               
               
                   
                  57 
                 1706 
                 119 
                 640 
               
               
                   
                  62 
                 89 
                 120 
                 1033 
               
               
                   
                  66 
                 1706 
                 −230 
                 639 
               
               
                   
                  70 
                 1706 
                 −231 
                 315 
               
               
                   
                  72 
                 1811 
                 −252 
                 280 
               
               
                   
                  75 
                 769 
                 −252 
                 280 
               
               
                   
                  76 
                 1043 
                 −252 
                 280 
               
               
                   
                      76′ 
                 1043 
                 −230 
                 278 
               
               
                   
                  77 
                 1769 
                 −224 
                 280 
               
               
                   
                  78 
                 1078 
                 −173 
                 280 
               
               
                   
                  79 
                 1769 
                 −206 
                 280 
               
               
                   
                  82 
                 415 
                 −168 
                 280 
               
               
                   
                  84 
                 663 
                 −168 
                 280 
               
               
                   
                  88 
                 313 
                 −168 
                 280 
               
               
                   
                  89 
                 1043 
                 −308 
                 30 
               
               
                   
                  90 
                 350 
                 −168 
                 280 
               
               
                   
                  94 
                 350 
                 −250 
                 278 
               
               
                   
                  98 
                 350 
                 −250 
                 30 
               
               
                   
                 100 
                 350 
                 −257 
                 16 
               
               
                   
                 102 
                 350 
                 −235 
                 14 
               
               
                   
                 104 
                 350 
                 110 
                 14 
               
               
                   
                 106 
                 1421 
                 −258 
                 17 
               
               
                   
                 108 
                 1421 
                 105 
                 16 
               
               
                   
                 112 
                 1771 
                 106 
                 14 
               
               
                   
                 115 
                 1771 
                 363 
                 55 
               
               
                   
                 117 
                 1771 
                 120 
                 50 
               
               
                   
                 119 
                 1771 
                 313 
                 140 
               
               
                   
                 121 
                 1771 
                 120 
                 135 
               
               
                   
                 123 
                 1774 
                 325 
                 400 
               
               
                   
                 126 
                 1771 
                 120 
                 395 
               
               
                   
                 128 
                 1771 
                 120 
                 1034 
               
               
                   
                 134 
                 1683 
                 120 
                 1033 
               
               
                   
                 136 
                 1683 
                 −12.5 
                 1028 
               
               
                   
                 138 
                 1043 
                 −225 
                 28 
               
               
                   
                 140 
                 1043 
                 110 
                 27 
               
               
                   
                 146 
                 1771 
                 120 
                 635 
               
               
                   
                 152 
                 1771 
                 120 
                 864 
               
               
                   
                 156 
                 1771 
                 158 
                 1039 
               
               
                   
                 160 
                 313 
                 −270 
                 278 
               
               
                   
                 166 
                 1683 
                 −200 
                 125 
               
               
                   
                 169 
                 8 
                 −200 
                 124 
               
               
                   
                 170 
                 1675 
                 −200 
                 124 
               
               
                   
                 174 
                 1675 
                 −253 
                 20 
               
               
                   
                 178 
                 1526 
                 −255 
                 19 
               
               
                   
                 180 
                 1526 
                 −258 
                 17 
               
               
                   
                 184 
                 105 
                 −258 
                 17 
               
               
                   
                 186 
                 156 
                 −255 
                 19 
               
               
                   
                 188 
                 575 
                 −263 
                 17 
               
               
                   
                 192 
                 0 
                 −263 
                 17 
               
               
                   
                   
               
             
          
         
       
     
     Overhead nitrogen gas stream  72 , shown as being external to tower  71  for purposes of illustration, is directed to condenser  74 , but in practice condenser  74  is preferably a knockback condenser section that is internal to the tower, and is previously known. Condensate  75  is returned to the fractionation section of tower  71 , and stream  76  of nitrogen gas is preferably directed to an N 2  expander that is further discussed below in relation to  FIGS. 4 and 5 . Q-1 represents the energy transferred to heat exchanger  99  from knockback condenser  74 . Representative energy values for Q-1 and other energy streams that are identified in  FIGS. 3 and 4  appear in Table 3 below: 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 ENERGY STREAM REPORT 
               
             
          
           
               
                 Energy Stream 
                 Energy Rate 
                   
                   
               
               
                 Reference Numeral 
                 (Btu/h) 
                 From 
                 To 
               
               
                   
               
               
                 Q-1 
                 1.08E+06 
                 Virtual 
                 KB 
               
               
                   
                   
                 KB 
                 Condenser 
               
               
                 Q-2 
                 1.05E+06 
                 Plate Fin 
                 Virtual 
               
               
                   
                   
                   
                 Reboiler 
               
               
                 Q-4 
                 4.26E+06 
                   
                 Sales 
               
               
                   
                   
                   
                 1 st  Stage 
               
               
                 Q-5 
                 4.07E+06 
                 1 st  Sales 
               
               
                   
                   
                 Cooler 
               
               
                 Q-6 
                 3.13E+06 
                   
                 Sales 
               
               
                   
                   
                   
                 2 nd  Stage 
               
               
                 Q-7 
                 3.20E+06 
                 2 nd  Sales 
               
               
                   
                   
                 Cooler 
               
               
                 Q-8 
                 3.31E+06 
                   
                 Sales 
               
               
                   
                   
                   
                 3 rd  Stage 
               
               
                 Q-9 
                 3.52E+06 
                 3 rd  Sales 
               
               
                   
                   
                 Cooler 
               
               
                  Q-10 
                 1.55E+06 
                 Warm 
                 Warm 
               
               
                   
                   
                 Expander 
                 Comp 
               
               
                  Q-11 
                 965593 
                 Low Temp 
                 Low Temp 
               
               
                   
                   
                 Exp 
                 Comp 
               
               
                  Q-12 
                 552059 
                 N2 
                 Nitrogen 
               
               
                   
                   
                 Exp 
                 Comp 
               
               
                  Q-13 
                 1.74E+06 
                 Warm 
               
               
                   
                   
                 Comp 
               
               
                   
                   
                 Cooler 
               
               
                  Q-14 
                 1.14E+06 
                 LT Comp 
               
               
                   
                   
                 Cooler 
               
               
                  Q-15 
                 679952 
                 N2 Comp 
               
               
                   
                   
                 Cooler 
               
               
                   
               
             
          
         
       
     
     Stream  78  from the bottom of tower  71  is desirably directed to virtual reboiler  80  that receives heat (designated by energy stream Q-2) from plate fin cooler  64 . Vapor stream  82  is returned to tower  71  and liquid methane stream  84  is directed through splitter manifold  86  to form two streams  88 ,  90  having comparable flow rates, temperatures and pressures. LNG stream  88  is directed to the LNG subcooling section  22  described below in relation to  FIG. 4 , and stream  90  is circulated through subcooler  92 , valve  96  and heat exchanger  99 , then back through subcooler  92  to plate fin cooler  64  as stream  102 , through which it passes countercurrent to combined inlet stream  57 . In this loop, the pressure of stream  90  is dropped more than about 260 psi and the stream is cooled more than 65 degrees before returning to plate fin cooler  64 . In this manner, a portion of the LNG stream  84  produced in tower  71  can be recirculated for use an “internal” refrigerant for inlet stream  56 . Sections of stream  90  are also designated by reference numerals  94 ,  98  and  100  at intermediate points between its passes through subcooler  92  to facilitate illustrating the temperature and pressure changes at various points in the loop. 
     Referring again to nitrogen fractionation tower  71 , a sidestream  77  drawn, for example, from tray  13  of tower  71  is also directed back to and through plate fin cooler  64 , again countercurrent to combined inlet stream  57 , before returning as stream  79  to a lower position in tower  71 , in this case tray  14 . By reference to Table 2, it is seen that the temperature of the sidestream is increased by about 18° F. with virtually no change in pressure before reentering tower  71 , thereby again serving as an “internal” refrigerant for inlet gas stream  56 . 
     Stream  104  exits plate fin cooler  64  and is directed to mixing manifold  110  where it is desirably combined with stream  108  that emerges from plate fin cooler  64  after being returned as stream  106  from final LNG separator  182  of LNG subcooler section  22  as discussed below in relation to  FIG. 4 . Combined stream  112  is thereafter directed through an alternating series of compression stages  114 ,  116 ,  118  and sales coolers  120 ,  122 ,  124  in which the stream undergoes a net temperature increase of about 15 degrees and a net pressure increase of about 380 psi before flowing as stream  126  to a series of compression stages that are connected to and are driven by expanders, which extract mechanical energy from the expansion of gas streams that are further discussed below in relation to  FIG. 4 . Reference numerals  115 ,  117 ,  119 ,  121  and  123  are used to better illustrate the changes in temperature and pressure that the recycled material in stream  112  undergoes at intermediate points as it passes through the sales coolers before emerging as stream  126  in  FIG. 3 . 
     In summary, it is apparent from the foregoing discussion of nitrogen removal section  20  in relation to  FIG. 3  and to the illustrative stream properties presented in Table 2 that substantial cooling of the inlet stream of mixed methane and nitrogen is achieved before reaching nitrogen fractionation tower  71  by strategically controlling the flows, temperatures and pressures of internal process streams and not through the use of external refrigeration. 
     Referring back to  FIG. 1 , the portion of system  10  that is inside dashed outline  200  is further described and explained in relation to  FIG. 4 . Referring to  FIG. 4 , stream  88  of LNG received from nitrogen removal section  20  of  FIG. 3  is directed to subcooler  142 , which is preferably a plate fin cooler or other similarly effective exchanger apparatus. The temperature of stream  88  is reduced approximately 100° F. with minimal pressure drop as it passes through subcooler  142 , from which it emerges as stream  160  and is directed through manifold  162  into LNG storage section  24 , from which LNG product  26  is produced. Referring to Table 2, LNG product can be produced according to the system and method of the invention at temperatures below 250° F. and pressures only slightly above atmospheric. LNG storage section  24  is desirably configured and adapted to recover any vapor that is flashed as stream  192 . The substantial cooling provided by subcooler  142  to further lower the temperature of LNG received from nitrogen removal section  20  is again achieved through the use and control of internal process streams and not through use of external refrigeration. 
     One source of cooling within subcooler  142  is provided by expanding the gaseous nitrogen received from nitrogen removal section  20  in stream  76 . Stream  76  is desirably directed to N 2  expander  175 , from which it exits as stream  89 , which is then directed to subcooler  142  countercurrent to the incoming flow of LNG in stream  88 . Inside N 2  expander  175 , the stream pressure is reduced by about 250 psi, with an attendant temperature reduction of about 55° F., to below −300° F. After emerging from subcooler  142 , nitrogen stream  138  is returned to plate fin cooler  64  countercurrent to combined inlet stream  57  as described above, after which it exits as vent stream  140 . 
     Another source of cooling within subcooler  142  is provided by sequentially expanding high pressure stream  136 , which passes sequentially through warm expander  164 , low temperature expander scrubber  168 , low temperature expander  172 , and LNG separator  176 . In LNG separator  176 , the material from stream  136  separates into streams  178 ,  186 , respectively, with the flow rate of stream  178  being substantially greater (by a factor of about 10) than the flow rate of stream  186 . During the progression from stream  136  to stream  178 , the temperature drops about 240° as the pressure drops more than 1000 psi. Reference numerals  166 ,  170  and  174  are used to designate stream  136  at intermediate points between warm expander  164  and LNG separator  176  to assist in identifying the temperatures and pressures of the steam at those points. 
     As stream  178  passes through subcooler  142 , it cools slightly more and exits as stream  180  into final LNG separator  182 . In LNG separator  182 , the material from stream  180  separates into streams  106  and  184 , respectively, with the flow rate of stream  106  again being substantially greater than the flow rate of stream  184 . Stream  106  is directed back to nitrogen removal section  20  of  FIG. 3 , where it enters and passes through plate fin cooler  64 , from which it exits as stream  108  that is combined with stream  104  in manifold  110  to produce stream  112  as discussed above in relation to  FIG. 3 . Referring again to  FIG. 4 , stream  186  from LNG separator  176  and stream  184  from final LNG separator  182  are then combined in manifold  162  to form combined stream  188  that flows into LNG storage tank  24 , from which LNG product  26  is produced. 
     Stream  136  as described above is received by warm expander  164  from plate fin cooler  64  in nitrogen removal section  20  of  FIG. 3 , which enters plate fin cooler  64  as stream  134 . Stream  134  is formed when stream  128  as shown in  FIG. 3  is split into streams  62  and  134  in manifold  132 , after which stream  62  is combined with inlet stream  56  in manifold  60 . Stream  128 , in turn, originates from stream  126  of  FIG. 3 , after passing through a loop that is further described and explained in relation to LNG subcooler section  22  in  FIG. 4 . 
     Referring again to  FIG. 4 , stream  126  is received from nitrogen removal section  20  and passes successively through warm compressor  142 , warm compressor cooler  144 , low temperature compressor  148 , low temperature compressor cooler  150 , nitrogen compressor  154  and N 2  compressor cooler  158 , before returning to nitrogen removal section  20  as stream  182 , discussed above. Intermediate stream designations  146 ,  150 ,  152  and  156  are provided for use in tracking relative temperatures and pressures through this portion of system  10  of the invention. As compared to stream  126 , the temperature of stream  128  is increased by less than 50° F. but the pressure is increased by more than 600 psi. Illustrative energy streams corresponding to the movement of the material of stream  126  through the various devices as identified above between stream  126  and stream  128  are reported in Table 2. All devices identified in relation to  FIGS. 3 and 4  are believed to be commercially available from sources known to those of ordinary skill in the art, and particular equipment specifications will depend upon factors that can vary, for example, according to the intended application, use site, inlet gas composition, throughputs and operating conditions. 
     In accordance with another alternative embodiment of the invention in which liquid nitrogen is also produced according to the system and method of the invention, which corresponds to that portion of  FIG. 2  that is identified by dashed box  300  and which is further described and explained in relation to  FIG. 5 , stream  89  can be directed to liquid nitrogen separator  196 , from which overhead stream  197  is returned to subcooler  142 . Stream  197  enters subcooler  142  countercurrent to LNG stream  88  in substantially the same manner that stream  89  did in the embodiment described in relation to  FIG. 4 , and exits as stream  138 . Stream  138  is then returned to plate fin cooler  64  in nitrogen removal section  20 , as previously shown and described in relation to  FIG. 3 . Liquid nitrogen stream  198 , which exits from the bottom of liquid nitrogen separator  196 , is desirably directed to storage vessel  201 , from which liquid nitrogen product stream  204  is produced, with any flashed nitrogen vapor exiting vessel  201  as vent stream  202 . 
     It should be appreciated by those of ordinary skill in the art upon reading this disclosure that the flow rate, temperature and pressure of stream  138  as shown in  FIG. 5  will differ somewhat from the values as reported in Table 2 for the embodiment described in relation to  FIGS. 3 and 4 , which can in turn have a slight effect on the temperatures, pressures and/or energy values for other streams reported in Tables 2 and 3 to the extent that those streams are also referred to in the alternative embodiment of  FIG. 5 . Otherwise, the streams and flow configurations previously described in relation to  FIGS. 3 and 4  are likewise applicable to like-numbered streams in  FIG. 5 . 
     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.