Abstract:
A method for recovering liquefied natural gas from a gas mixture containing natural gas and impurities by subjecting the natural gas to a series of steps beginning with feeding a natural gas stream containing impurities to a nitrogen rejection unit; feeding the purified natural gas stream to a liquefier heat exchanger; expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel; flashing the liquid natural gas and separating the liquefied natural gas from the flash gas; and feeding the liquefied natural gas to storage and the flash gas to the nitrogen rejection unit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional patent application Ser. No. 61/317,466, filed Mar. 25, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to the integration of a liquefied natural gas (LNG) liquefier system with a nitrogen rejection unit (NRU) so as to minimize the capital and operating costs while maintaining liquefied natural gas product purity requirements. 
         [0003]    Renewable methane can be recovered from a number of sources, such as anaerobic digestion of municipal or industrial waste streams, the degradation of biomass in landfills, the gasification of waste and biomass streams, amongst others. In many instances, this renewable methane require purification before it can be used and/or sold into higher valued markets, such as injection into the pipeline grid, as a feedstock for liquefied natural gas, as a vehicle fuel, or as a feedstock for the production of hydrogen. Further, the energy that is required to purify the renewable methane is significant. 
         [0004]    The cleanup of biogas/landfill gas is both capital and power intensive because it contains a large number of trace and bulk contaminants in fairly large concentrations. Various methods are employed to remove these including chilling, cryogenic methods and various adsorption and scrubbing processes. However, these processes can be expensive in both capital and operating costs and it is important to minimize these costs to achieve an economically viable process. 
         [0005]    A typical process for the purification of the methane from biogas/landfill gas requires several steps. Sulfur removal is generally followed by drying. The dried gas stream is then treated for contaminants such a volatile organic compounds by process such as adsorption, CO 2  washing or by cryogenic methods. The stream is then treated for bulk carbon dioxide removal by a membrane or adsorption process and then is treated for removal of nitrogen. All these purification steps are necessary before the biogas/landfill gas can be liquefied and stored in anticipation of being dispensed, or directed towards other uses, such as pipeline injection, energy production with fuel cells or small-scale hydrogen production. LNG production is particularly challenging since all condensable contaminants including carbon dioxide must be removed to low ppm levels. 
         [0006]    The invention will allow for maximizing the methane recovery while maintaining high liquefied natural gas product purity. The operator can utilize a smaller nitrogen rejection unit and can optimize power consumption. The process of using the nitrogen rejection unit integrated with the liquefied natural gas liquefier system achieves greater product purity (&gt;96 mol % methane) and greater than 89% methane recovery than conventional non-integrated combinations. 
         [0007]    By integration of the liquefied natural gas liquefier system with a nitrogen rejection unit, the overall system becomes more compact and efficient. This further enables the operator to maximize methane recovery while maintaining high liquefied natural gas product purity while enable a smaller nitrogen rejection unit. The invention further allows the operator to optimize power consumption while allowing for significantly higher product purity and methane recovery than conventional or unitegrated NRU and liquefier combinations which are limited to 96 mol % methane and 80% methane recovery. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is a method for recovering liquefied natural gas comprising the steps: 
         [0000]    Feeding a natural gas stream containing impurities to a nitrogen rejection unit;
 
Feeding the purified natural gas stream to a liquefier heat exchanger;
 
Expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;
 
Flashing the liquid natural gas and separating the liquefied natural gas from the flash gas;
 
Feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.
 
         [0009]    Alternatively, the invention is a method for recovering liquefied natural gas comprising the steps: 
         [0000]    Feeding a natural gas stream containing impurities to a nitrogen rejection unit;
 
Feeding the purified natural gas stream to a liquefier heat exchanger;
 
Expanding the liquefied natural gas and feeding the expanded liquefied natural gas to a flash vessel;
 
Flashing the liquid natural gas and separating the liquefied natural gas from the flash gas;
 
Recovering refrigeration from said flash gas; and
 
Feeding the liquefied natural gas to storage and the flash gas to said nitrogen rejection unit.
 
         [0010]    The invention further comprises an apparatus comprising a nitrogen rejection unit, a liquefier heat exchanger and a flash vessel. 
         [0011]    The raw feed gas is first compressed and pre-conditioned which entails the removal of water, carbon dioxide, non-methane organic compounds (NMOCs) and sulfur compounds by known methods. The partially purified gas is fed to the nitrogen rejection unit where much of the nitrogen is rejected. Since product purity and methane recovery are inversely related, nitrogen rejection is limited to maximize the methane recovery for the smallest equipment cost. The resulting gas which contains significantly lower amounts of inerts is fed to the liquefier heat exchanger where it is liquefied at pressure to a subcooled state. Typical pressures range from 30 bar to 6 bar with a tradeoff between mixed refrigeration compressor power and compression power for the purification system. This liquid is expanded through a valve whereby further cooling is effected to about 2 bar (range is 1 to 5 bar). 
         [0012]    The two-phase mixture is separated in a flash vessel and the resulting liquid is directed to the storage tanks, while the flash gas which is richer in nitrogen is recycled back to the nitrogen rejection unit. The flash gas can also be combined with the raw natural gas/biogas at the front end of the overall process if the nitrogen rejection unit does not have a recycle compressor. Clearly additional flash gas from the storage tank will be produced. This too is recycled back to the nitrogen rejection unit or to the front end of the cleanup process. The only methane that will be lost is the nitrogen rejection unit waste stream which is nitrogen-rich but otherwise very pure and can be flared or converted into power using a gas engine or a fuel cell. 
         [0013]    The end flash from the flash vessel has an additional advantage in that the liquid outlet of a flash is at equilibrium which implies that it will produce some more gas inside the line between the end flash and storage tank because of product line pressure drop. Therefore, it is best practice that the flash pressure be lower than the storage pressure. To maximize liquefied natural gas production and minimize product flash losses, the first flash within the flash vessel is effected at a pressure lower than the storage tank pressure, whence, the liquid coming down from the end-flash tank will be sub-cooled at storage pressure. A cryogenic pump can be utilized to overcome this concern, but involves additional cost, maintenance and potential reliability issues. Therefore it is advisable to have horizontal storage tanks, and a cold box layout so that the liquid level inside the end flash will be higher by a few hundreds of mbar of equivalent liquefied natural gas head that the top of the storage and to use that additional head to pressurize  FIG. 2   c.    
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a typical mixed refrigerant liquefied natural gas liquefier. 
           [0015]      FIG. 2   a  is an integrated mixed refrigerant liquefied natural gas liquefier. 
           [0016]      FIG. 2   b  depicts a different embodiment of an integrated mixed refrigerant liquefied natural gas liquefier. 
           [0017]      FIG. 2   c  depicts another embodiment of an integrated mixed refrigerant liquefied natural gas liquefier. 
           [0018]      FIG. 3  shows methane recovery versus nitrogen rejection unit product methane content. 
           [0019]      FIG. 4  shows a liquefier embodied in the invention. 
           [0020]      FIG. 5   a  shows a liquefier having a lower cost to operate. 
           [0021]      FIG. 5   b  shows a different embodiment of a lower cost to operate liquefier. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Landfill gas is purified and all the water, sulfur compounds, NMOCs and carbon dioxide are removed in a pre-purification process. The purified gas contains methane, nitrogen and oxygen and has the following composition: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 NRU Feed Gas Composition 
               
             
          
           
               
                   
                 Species 
                 Mole Fraction 
               
               
                   
                   
               
               
                   
                 Carbon Dioxide 
                 0.0100 
               
               
                   
                 Nitrogen 
                 0.2160 
               
               
                   
                 Methane 
                 0.7720 
               
               
                   
                 Oxygen 
                 0.0020 
               
               
                   
                   
               
             
          
         
       
     
         [0023]    The gas is further purified in an adsorption system so that the carbon dioxide level is reduced below 50 ppmv and a large portion of the nitrogen is removed. Oxygen usually does not adsorb appreciably and about 50% of the oxygen is removed in each case.  FIG. 1  illustrates a typical MR liquefier stream without process integration. In this case both storage tank losses and nitrogen rejection unit waste streams are not recovered. 
         [0024]    Turning to the figures,  FIG. 1 , represents a base case mixed refrigerant liquefied natural gas liquefier. Purified natural gas is fed through line  1  through main heat exchanger A where it will be warmed and fed through valve V 1  and line  2  as liquefied natural gas to a storage container, not shown. 
         [0025]    A cold water stream (CWS) is fed through line  11  to heat exchanger E as well as through valve V 6  and line  13  to line  12  as cold water return (CWR). The cold mixed refrigerant is fed through line  9  to knockout drum B where it will proceed through line  7 A to refrigerant pump C and through open valve V 4  to contact line  3  in heat exchanger A. When valve V 4  is closed and valve V 5  is open the mixed refrigerant will re-enter knock out drum B through line  7 . The overhead from knockout drum B will travel through line  3  to heat exchanger A and enter valve V 2  to column unit D where it will exit unit D through overhead line  4  as well as through the bottom of unit D through line  5 A. This will exit heat exchanger A through line  5  and connect to inlet separator G where the bottoms will travel through line  8  and transfer pump H to line  6  which will enter the knockout drum B. The refrigerant will leave the inlet separator G through line  8 A and connect to mixed refrigerant column unit F where the mixed refrigerant will travel through line  10  back to heat exchanger E. 
         [0026]    In  FIG. 2   a , an integrated mixed refrigerant liquefied natural gas liquefier system is shown per the operation of the invention. Nitrogen containing biogas or other source of natural gas such as landfill gas feed is fed through line  23  to nitrogen rejection unit R which is typically a vacuum swing adsorption (VSA) system. Waste gas is fed through line  25  to blower S and released into the atmosphere. Depressurization gas is released through line  25 A into line  22  where it will travel through recycle compressor Q and reenter the nitrogen containing landfill or biogas feed line  23 . 
         [0027]    The nitrogen recovery unit product/liquefier feed natural gas is fed through line  24  into heat exchanger I where it will pass through valve V 12  and enter flash tank J. The now liquefied natural gas will exit through valve V 11  and line  25  to line  26  where it will enter storage tank K and can be accessed through line  21  and valve V 10  for later use. Vent gas from the storage tank K will exit through line  20  where it will join line  22  and be fed back through the recycle compressor Q to the nitrogen containing biogas feed line  23 . 
         [0028]    Heat exchanger T is fed cold water through line  35 A to provide a cooling medium which will also feed to the cold water return line  35  through valve V 16 . Line  24 A directs warm water leaving heat exchanger T. The cold refrigerant is fed through line  36  to knock out drum P which feeds the cold refrigerant to the heat exchanger I through line  28  and which passes through valve V 13  to the column unit L where refrigerant from the top exits through line  31  and through the bottom through  31 A which joins line  31  and passes through heat exchanger I and line  31  will be fed to inlet separator M where refrigerant exits through line  33  and is fed to mixed refrigerant column unit U which feeds mixed refrigerant, now warmer to the heat exchanger T through line  24 . The bottoms from the inlet separator M is fed through line  32  and transfer pump N back to knock out drum P. The bottoms from the knockout drum P are fed through line  30  and refrigerant pump O to valve V 15  for reentry back into the knockout drum P. BZ designates the cold box boundary. 
         [0029]    The refrigerant from the knockout drum P may also enter line  29  and open valve V 14  where it will feed into line  31  and entry into the inlet separator M. 
         [0030]      FIG. 2   b  is a similar version of the integrated mixed refrigerant liquefied natural gas liquefier system of  FIG. 2A  with the numbering being the same as  FIG. 2A . This embodiment has compressor Q on line  23  rather than line  22  and no return line  25 A from the nitrogen rejection unit R to line  22 . Also, line  30 A connects with valve V 15 A to heat exchanger I such that refrigerant from knock out drum P is directed to the heat exchanger I. 
         [0031]      FIG. 2   c  is another embodiment of the invention showing an integrated mixed refrigerant liquefied natural gas liquefier. Natural gas such as that from landfill gas or biogas from a nitrogen recovery unit, not shown, is fed through line  40  into heat exchanger V. The natural gas is liquefied and its pressure is higher as it exits through line  40 A through temperature control valve V 17 . The liquefied natural gas enters end flash unit W where the flashed liquefied natural gas is fed through line  41  and open pressure control valve V 18  and recycled back to heat exchanger V where it will exit and be fed through line  42  to a nitrogen recovery unit, not shown. 
         [0032]    The bottoms from the flash unit W exit through line  43  and open valve V 19  where it will enter horizontal cryogenic storage tank Y. Additional static head is maintained between the liquid level in the end flash unit W and the horizontal cryogenic storage tank Y such that it is equivalent to subcooling at storage level and pressure. Line BZ represents the cold box boundary. 
         [0033]      FIG. 3  shows the effect of methane product purity on methane recovery. Methane recovery decreases as the nitrogen recovery unit product methane content in mole % increases. 
         [0034]      FIG. 4  shows a preferred liquefier embodiment. Natural gas such as that found in landfill gas or biogas is fed through line  64  to heat exchanger AA where it will exit as liquefied natural gas through open valve V 20  and be fed to flash tank AB. The liquefied natural gas from the bottoms of the flash tank AB will exit through line  66  and open valve V 22  where it will be fed to storage, not shown. The gaseous natural gas tops of the flash tank will exit through line  65  and re-enter heat exchanger AA where it will be fed to a mixed gas nitrogen recovery unit, not shown. 
         [0035]    Cold water is fed through line  60  into heat exchanger AI to provide a cooling medium and also fed through line  61  and open valve V 25  to the cold water return line  62 . Refrigerant will exit through line  64  and be fed through to a knockout drum AD where refrigerant is fed through line  51  and refrigerant pump AE through open valve V 24  to line  52  passing through heat exchanger AA. When valve V 24  is closed and valve V 24 A is open, the refrigerant is fed through line  55  back to knockout drum AD. Refrigerant is also fed through line  56  from the top of the knockout drum AD to line  52  passing through heat exchanger AA. Line  52  will deliver the refrigerant through open valve V 21  to a column unit AC where the bottoms from said unit are fed through line  53  to rejoin with the tops which exit unit AC through line  54 . Line  54  passes through heat exchanger AA where it will be fed to inlet separator AF. 
         [0036]    The refrigerant in line  54  is occasionally supplemented from the knockout drum AD through open valve V 23  and line  57  which connects with the tops from the knockout drum AD through line  56 . Line  54  will enter the inlet separator AF where its bottoms are transferred through line  58 A and transfer pump AG to line  50  which returns to the knockout drum AD. The tops from the inlet separator AF exit through line  58  and enter mixed refrigerant column unit AH where mixed refrigerant will enter the heat exchanger AI for cooling and reentry into the knockout drum AD for entry into heat exchanger AA. 
         [0037]      FIG. 5   a  shows a lower cost embodiment liquefier. Natural gas such as that found in landfill gas or biogas is fed through line  79  and open valve V 30  where it will enter flash tank BA. Liquefied natural gas exits through line  77  and open valve V 33  to storage, not shown. Natural gas will exit the flash tank BA through line  78  where it will pass through economizer BC and exit to a nitrogen recovery unit, not shown. Valve V 32  can be opened and excess nitrogen can be recovered through line  78 A, unit TIC back into flash tank BA. 
         [0038]    Part of the natural gas feed from line  78  is fed through open valve V 31  to line  76  which passes through heat exchanger BD and open valve V 34  back to the flash tank BA as liquefied natural gas. 
         [0039]    Cold water is fed through line  83  to heat exchanger BJ and through line  85  and open valve V 39  to cold water return line  84 . Refrigerant will exit through line  85 A and be fed to knockout drum BH where it will exit through the bottom of the knockout drum through open valve V 36  and refrigerant pump BI to be fed to line  74  passing through heat exchanger BD. Valve V 36  can be closed and valve V 38  open such that refrigerant will pass through line  75  back to knockout drum BR 
         [0040]    The tops from the knockout drum BH will be fed through line  70  to line  74  passing through heat exchanger BD. The refrigerant will pass through open valve V 35  and be fed to column unit BE where the bottoms from the unit exit through line  71  and join with the tops from the unit BE line  72  which passes refrigerant through heat exchanger BD. This refrigerant will enter inlet separator BG through line  72  where the bottoms from the inlet separator BG are fed through line  80  and transfer pump BF back to the knockout drum BH. 
         [0041]    The tops from the inlet separator will exit through line  81  to mixed refrigerant unit BK. The mixed refrigerant from unit BK is fed back to heat exchanger BJ as a warm fluid through line  82  where it will be cooled down and ultimately fed back into heat exchanger BD after passing through knockout drum BH. Line BZ designates the cold box boundary. 
         [0042]      FIG. 5   b  is virtually identical to  FIG. 5   a  designating a lower cost liquefier embodiment. In this embodiment, the numbering is the same and there is no return embodiment on top of the flash tank BA, thus line  78 A, valve V 32  and TIC control mechanism are not present. In  FIG. 5   b , the cold box boundary BZ is also broader and covers the flash tank BA which is not seen in  FIG. 5   a.    
         [0043]    Typical nitrogen rejection performance is shown in  FIG. 3  for a vacuum swing adsorption (VSA) nitrogen rejection unit. The invention is shown in  FIG. 2   a . The nitrogen rejection amount was varied while ensuring that the final LNG product contained 98%+methane. Three cases were considered for illustrative purposes where the NRU product/liquefier feed gas contained 90.6, 98.2 and 98% methane (C1). The relative equipment size, which determines capital cost and the power were calculated and compared. The results are as indicated in Tables 2 and 3 below. In Table 2, both the pre-cleanup system, which is used to remove all contaminants other than nitrogen and oxygen, and the four bed VSA system are compared in terms of size which is directly proportional to the kg-moles/hr of NRU feed to be processed or the nitrogen to be rejected. Case 3 clearly shows significant benefits when a less pure NRU product is fed to the liquefier with a pre-cleanup system that is 17% smaller and a NRU that is 23% smaller than the first case. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Effect of Liquefier Feed Composition on Overall Methane 
               
               
                 Recovery and Equipment Size 
               
             
          
           
               
                   
                 C1 in Liquefier 
                 Pre-Cleanup 
                 NRU 
                 Wobbe Index 
               
               
                   
                 Feed (mol %) 
                 Relative Size 
                 Relative Size 
                 (MJ/m 3 ) 
               
               
                   
                   
               
             
          
           
               
                 Case 1 
                 98.0 
                 1.17 
                 1.23 
                 50.37 
               
               
                 Case 2 
                 96.2 
                 1.06 
                 1.10 
                 50.11 
               
               
                 Case 3 
                 90.6 
                 1.00 
                 1.00 
                 49.35 
               
               
                   
               
             
          
         
       
     
         [0044]    In addition, the relative power for all 3 cases is compared in Table 3 which shows that the extra power needed for liquefaction and recycle with higher inerts (case 1) is compensated by the vacuum pump power needed for higher NRU purity (case 3). Hence, there is no appreciable net power penalty. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Effect of Liquefier Feed Composition on Net Power 
               
             
          
           
               
                   
                 C1 in Liquefier Feed 
                 Relative Power 
               
               
                   
                 (mol %) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 Case 1 
                 98.0 
                 100.5 
               
               
                   
                 Case 2 
                 96.2 
                 99.6 
               
               
                   
                 Case 3 
                 90.6 
                 100.0 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    Other embodiments of the invention are illustrated in  FIGS. 5   a  and  5   b , both of which are lower capital cost options and do not require a separate pass in the main heat exchanger, or a larger coldbox. Nevertheless, both embodiments do not allow for full cold recovery and are less efficient. Additionally, if all the purified natural gas from the NRU is fed to the economizer, a very large temperature gradient will result at the cold end of this exchanger. Therefore, it is desired that only a portion of the NRU product is fed to the economizer so that it can be liquefied, or cooled close to the liquefaction temperature. The portion of the NRU product gas cooled in the economizer can be sent to flash tank labeled BA in  FIGS. 5   a  and  5   b  as sub-cooled liquid or to the main heat exchanger. 
         [0046]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.