Abstract:
A natural gas liquefaction system, the system comprising a first precooling refrigeration system that accepts at least a natural gas feed stream, a second precooling refrigeration system that accepts at least a first refrigerant stream; and a cryogenic heat exchanger fluidly connected to the first precooling refrigeration system and the second precooling refrigeration system that accepts the natural gas feed stream from the first precooling refrigeration system and the first refrigerant stream from the second precooling refrigeration system to liquefy the natural gas feed stream, where the second precooling refrigeration system accepts only stream(s) having a composition different from the stream(s) accepted by the first precooling refrigeration system.

Description:
BACKGROUND 
       [0001]    The present invention relates to a system and method for liquefaction of a gas stream, and more specifically, to a system and method for liquefaction of a natural gas stream in large capacity liquefaction plants. 
         [0002]    Over the past few years, the liquid natural gas (LNG) industry has moved towards using large capacity liquefaction plants to achieve favorable economics associated with the large plants. Scale-up problems arise, however, when refrigerant mass and volume flow rates are increased. For example, the design of compression equipment, particularly the compression equipment associated with precooling, becomes problematic because the increased flow rates require larger compressor impellers with higher tip speeds, thicker and heavier casings, and higher inlet velocities to the impellers. As the equipment is scaled up, the design of the compressor becomes more problematic as fundamental aerodynamic limits are approached and, thus, the scale up may be limited by these considerations. In addition these precooling compressors are large and often contain multiple stages. Moreover, scale-up in many instances requires large, heavy equipment that can be difficult and costly to manufacture and/or install. 
         [0003]    U.S. Pat. No. 6,962,060 (Petrowski et al.) assigned to the assignee of the present invention, discloses one alternative system designed for liquefaction at large plants that includes a compressor system comprising a first compressor having a first stage and a second stage wherein the first stage of the first compressor is adapted to compress a first gas and the second stage of the first compressor is adapted to compress a combination of a fourth gas and an intermediate compressed gas from the first stage of the first compressor; and a second compressor having a first stage and a second stage wherein the first stage of the second compressor is adapted to compress a second gas and the second stage of the second compressor is adapted to compress a combination of a third gas and an intermediate compressed gas from the first stage of the second compressor. 
         [0004]    There is a need for a method and system that provides stable operation at full rates and during turndown for larger capacity liquefaction plants. 
       BRIEF SUMMARY 
       [0005]    Embodiments of the present invention satisfy this need in the art by providing a liquid natural gas liquefaction system and process that is stable and operational at full rates and during turndown for larger capacity liquefaction plants. 
         [0006]    In one exemplary embodiment a natural gas liquefaction system is disclosed, the system comprises: a first precooling refrigeration system that accepts at least a natural gas feed stream; a second precooling refrigeration system that accepts at least a first refrigerant stream; and a cryogenic heat exchanger fluidly connected to the first precooling refrigeration system and the second precooling refrigeration system that accepts the natural gas feed stream from the first precooling refrigeration system and the first refrigerant stream from the second precooling refrigeration system to liquefy the natural gas feed stream, wherein the second precooling refrigeration system accepts only stream(s) having a composition different from the stream(s) accepted by the first precooling refrigeration system. 
         [0007]    In another exemplary embodiment, a method for liquefying natural gas is disclosed, the method comprising the steps of: providing a natural gas feed stream; providing a first refrigerant stream; precooling in a first precooling refrigeration system at least the natural gas feed stream; precooling in a second precooling refrigeration system at least the first refrigerant stream; and vaporizing the precooled first refrigerant stream in a cryogenic heat exchanger to cool the precooled natural gas feed stream through indirect heat exchange, wherein the second precooling refrigeration system precools only stream(s) having a composition different from the stream(s) precooled by the first precooling refrigeration system. 
         [0008]    In yet another exemplary embodiment, a natural gas liquefaction system for large capacity liquefaction plants is disclosed, the system comprising: a first precooling refrigeration system that accepts one stream selected from the group consisting of: 
         [0009]    a natural gas feed stream, and an at least one refrigerant stream; a second precooling refrigeration system that accepts any remaining stream(s) not accepted by the first precooling refrigeration system and from the group consisting of: the natural gas feed stream, and the at least one refrigerant stream; and a cryogenic heat exchanger fluidly connected to the first precooling refrigeration system and the second precooling refrigeration system and adapted to accept the natural gas feed stream and the at least one refrigerant stream from the first precooling refrigeration system and the second precooling refrigeration system, wherein the at least one refrigerant stream is used to liquefy the natural gas feed stream, wherein the second precooling refrigeration system accepts only stream(s) having a composition different from the stream(s) accepted by the first precooling refrigeration system. 
     
    
     
       BRIEF DESCRIPTION OF THE EXEMPLARY DRAWINGS 
         [0010]    The foregoing brief summary, as well as the following detailed description of exemplary embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating embodiments of the invention, there is shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings: 
           [0011]      FIG. 1  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0012]      FIG. 2A  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0013]      FIG. 2B  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0014]      FIG. 3  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0015]      FIG. 4  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0016]      FIG. 5  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0017]      FIG. 6  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0018]      FIG. 7A  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0019]      FIG. 7B  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; 
           [0020]      FIG. 8A  is a flow chart illustrating an exemplary system and method involving aspects of the present invention; and 
           [0021]      FIG. 8B  is a flow chart illustrating an exemplary system and method involving aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates an exemplary embodiment of the invention as applied to a pre-cooled refrigerant system and process. In this exemplary system  100 , propane is used to precool both a natural gas feed stream  102  and a liquefaction refrigerant stream  104 . The natural gas feed stream  102  may be pretreated, for example. The liquefaction refrigerant stream  104  may be a pure or a mixed refrigerant, for example. It should be noted that while the exemplary embodiments described below may refer to the liquefaction refrigerant stream as a mixed refrigerant stream, the liquefaction refrigerant stream described below may also be a pure refrigerant stream, for example. Depending on refrigerant availability in the local area and system requirements (e.g., adjusting the composition of the mixed refrigerant to match the cooling curve for optimal cooling performance), the liquefaction refrigerant stream  104  may comprise one or more of the following: nitrogen, methane, ethylene, ethane, propylene, propane, iso-butane, n-butane, and iso-pentane, for example. 
         [0023]    The compression of the vapor resulting from the cooling of the natural gas feed stream  102  may occur in one compressor  118  while the compression of the propane vapor generated from cooling of liquefaction refrigerant stream  104  may occur in a separate compressor  126 . 
         [0024]    Precooling of the natural gas feed stream  102  and the mixed refrigerant stream  104  may be accomplished by vaporizing a precooling refrigerant such as propane at four different pressure levels in closed-loop precooling refrigeration system(s). The natural gas feed stream  102  may be precooled because of equipment limitations and for efficiency purposes. It should be noted that while propane may be used as the precooling refrigerant for vaporizing at four different pressure levels (as illustrated in exemplary  FIGS. 1-7A ), carbon dioxide, methane, propane, butane, iso-butane, propylene, ethane, ethylene, R22, HFC refrigerants, including, but not limited to, R410A, R134A, R507, R23, or combinations thereof, may also be used, for example. 
         [0025]    Cooling of the natural gas feed stream  102  is performed in unit  106 . Unit  106  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  102  is cooled by indirect heat exchange against a precooling refrigerant in a series of propane evaporators  202 ,  204 ,  206 ,  208  that may operate at successively lower pressures ( 202  being the highest and  208  being the lowest, for example) producing cooled successive streams  203 ,  205 ,  207 , and  150 . The evaporation of propane at the four pressures results in four propane vapor streams  110 ,  112 ,  114 ,  116  that are then compressed in compressor  118 . The resulting compressed stream  120  is then condensed in propane condenser  122 , producing liquid stream  124  for reintroduction into the series of propane evaporators  202 ,  204 ,  206 ,  208 . Propane condensers used in these types of methods and systems may include, for example, a propane de-superheater, a condenser, an accumulator, and a propane subcooler. It should be noted that while this exemplary embodiment illustrated in  FIGS. 1 ,  2 A,  2 B,  3 ,  4 ,  5 ,  6 , and  7 A uses a four stage pre-cooling system, the pre-cooling system may comprise a single-stage, a two-stage, a three-stage, or systems with greater than four stages, for example, where the series of propane evaporators may operate at successively lower pressures. 
         [0026]    Cooling of the mixed refrigerant stream  104  is performed in unit  108 . Unit  108  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant stream  104  may also be cooled by indirect heat exchange against the precooling refrigerant in a series of propane evaporators  222 ,  224 ,  226 ,  228  that may operate at successively lower pressures ( 222  being the highest and  228  being the lowest, for example) producing cooled successive streams  223 ,  225 ,  227 , and  138 . The evaporation of propane at the four pressures results in four propane vapor streams  130 ,  132 ,  134 , 136  that are then compressed in compressor  126 . The resulting compressed stream  127  is then condensed in propane condenser  128 , producing liquid stream  129  for reintroduction into the series of propane evaporators  222 ,  224 ,  226 ,  228 . 
         [0027]    Cooled mixed refrigerant stream  138  is separated in phase separator  140  into a liquid mixed refrigerant stream  142  and a vapor mixed refrigerant stream  144 . Liquid mixed refrigerant stream  142  is sub-cooled in the cryogenic heat exchanger (MCHE)  146  producing stream  147 . Stream  147  may then be reduced in pressure through isenthalpic valve  148  producing stream  149 . Stream  149  may then be vaporized in the shell side of the MCHE  146  to provide cooling to tubeside streams  142 ,  144 , 150 . 
         [0028]    Vapor mixed refrigerant steam  144  is condensed and sub-cooled in the MCHE  146  to produce stream  151 . Stream  151  may then be reduced in pressure through isenthalpic valve  152  to produce stream  153 . Stream  153  may then be vaporized in the shell side of the MCHE  146  to provide cooling to tubeside streams  142 ,  144 ,  150 . 
         [0029]    The cooled natural gas feed stream  150  may enter the MCHE  146  where it is further cooled producing product stream  166  that may be, for example, liquid natural gas (LNG). 
         [0030]    Low pressure mixed refrigerant stream  145  exiting the MCHE  146  is compressed in the low pressure mixed refrigerant compressor  154  to produce stream  155 . It should be noted that the refrigerant compressors of all of the exemplary embodiments may include one or more intercoolers and compressor casings. For example, mixed refrigerant compressor  154  may include one or more intercoolers and at least one compressor casing. Intercoolers and aftercoolers use an ambient heat sink (air or water) to reject compression heat to the environment. 
         [0031]    Stream  155  is cooled in intercooler  156  to produce stream  157 . Stream  157  is further compressed in the medium pressure mixed refrigerant compressor  158  to produce stream  159 . Stream  159  is cooled in intercooler  160  to produce stream  161 . Stream  161  is further compressed in high pressure mixed refrigerant compressor  162  to produce stream  163 . Stream  163  is cooled in aftercooler  164  to be recycled back as original mixed refrigerant stream  104 . 
         [0032]    The exemplary embodiment illustrated in  FIG. 1  shows how the power supplied to the refrigeration compressors  118 ,  126 ,  154 ,  158 ,  162  are provided by two equal sized directly connected gas turbines  180 ,  182 . For example, mixed refrigerant compressors  154 ,  158  are driven by gas turbine driver  180  while mixed refrigerant compressor  160  and the propane compressors  118 ,  126  are driven by gas turbine driver  182 . In this exemplary embodiment, the design pressure level between the mixed refrigerant compressors  158  and  162  may be chosen such that the work required by the two gas turbine drivers  180 ,  182  is essentially equal. The gas turbine drivers in all exemplary embodiments may be single-shaft gas turbines or multi-shaft gas turbines, for example. 
         [0033]    This exemplary embodiment is independent of the method used to power the refrigeration compressors  118 ,  126 ,  154 ,  158  and  162 . The refrigeration compressors  118 ,  126 ,  154 ,  158  and  162 , and the refrigeration compressors of the other exemplary embodiments may be driven by one or more gas turbines, electric motors, steam turbines, or a combination of different drivers. As illustrated in  FIG. 1 , the gas turbines  180 ,  182  may include starter/helper electric motors  184 ,  186  respectively to assist in starting the gas turbines  180 ,  182  and optimally, to provide additional power to assist the gas turbines  180 ,  182 , or to generate power for exportation into the power grid when excess power is available from the gas turbines. Moreover, for the exemplary embodiment illustrated in  FIG. 1 , and all other exemplary embodiments disclosed, the order of the compressor bodies and the starter/helper electric motors coupled to each driver is not fixed and may be manipulated/altered pursuant to any system requirements, maintenance requirements, and/or plant design requirements. For example, starter/helper electric motor  186  in  FIG. 1  may be positioned away from and not adjacent to driver  182  (i.e., at the opposite end of the driver string). The positions of the compressor bodies  118 ,  126 ,  162  may also be exchanged. 
         [0034]      FIG. 3  illustrates another exemplary embodiment  300  where the propane compressors  318 ,  326  are powered by different drivers  380 ,  382  respectively. In this exemplary embodiment, the power demand from the equivalent gas turbine drivers  380 ,  382  may be balanced by adjustment of the discharge pressure of low pressure mixed refrigerant compressor  354 . 
         [0035]    As illustrated in the exemplary embodiment  300  in  FIG. 3 , cooling of the natural gas feed stream  302  is performed in unit  306 . Like unit  106  of  FIG. 1 , unit  306  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  302  is cooled by indirect heat exchange to ultimately produce cooled stream  350 . The evaporation of propane at the four pressures results in four propane vapor streams  310 ,  312 ,  314 ,  316  that may then be compressed in compressor  318 . The resulting compressed stream  320  may then be condensed in propane condenser  322 , producing liquid stream  324  for reintroduction into the series of propane evaporators as shown in  FIG. 2A . 
         [0036]    Cooling of the mixed refrigerant stream  304  is performed in unit  308 . Unit  308  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant stream  304  may also be cooled by indirect heat exchange to ultimately produce cooled stream  338 . The evaporation of propane at the four pressures results in four propane vapor streams  330 ,  332 ,  334 ,  336  that may then be compressed in compressor  326 . The resulting compressed stream  327  may then be condensed in propane condenser  328 , producing liquid stream  329  for reintroduction into the series of propane evaporators as shown in  FIG. 2B . 
         [0037]    Again cooled mixed refrigerant stream  338  is separated in phase separator  340  into a liquid mixed refrigerant stream  342  and a vapor mixed refrigerant stream  344 . Liquid mixed refrigerant stream  342  is sub-cooled in the cryogenic heat exchanger (MCHE)  346  producing stream  347 . Stream  347  may then be reduced in pressure through isenthalpic valve  348  producing stream  349 . Stream  349  may then be vaporized in the shell side of the MCHE  346  to provide cooling to tubeside streams  342 ,  344 ,  350 . 
         [0038]    Vapor mixed refrigerant steam  344  is condensed and sub-cooled in the MCHE  346  to produce stream  351 . Stream  351  may then be reduced in pressure through isenthalpic valve  352  to produce stream  353 . Stream  353  may then be vaporized in the shell side of the MCHE  346  to provide cooling to tubeside streams  342 ,  344 ,  350 . 
         [0039]    The cooled natural gas feed stream  350  may enter the MCHE  346  where it is further cooled producing product stream  366  that may be, for example, liquid natural gas (LNG). 
         [0040]    Low pressure mixed refrigerant stream  345  exiting the MCHE  346  is compressed in the low pressure refrigerant compressor  354  to produce stream  355 . Stream  355  is cooled in intercooler  356  to produce stream  357 . Stream  357  is further compressed in the high pressure refrigerant compressor  362  to produce stream  363 . Stream  363  is cooled in aftercooler  364  to be recycled back as original mixed refrigerant stream  304 . 
         [0041]    Power is supplied to the refrigeration compressors  318 ,  326 ,  354 ,  362  by two equal sized directly connected gas turbines  380 ,  382 . As illustrated in  FIG. 3 , the gas turbines  380 ,  382  may include starter/helper electric motors  384 ,  386  respectively to assist in starting the gas turbines  380 ,  382  and optimally, to provide additional power to assist the gas turbines  380 ,  382 , or for exportation into the power grid when excess power is available from the gas turbines. 
         [0042]      FIG. 4  illustrates another exemplary embodiment  400  where the position of compressors  418 ,  426  of  FIG. 3  may be swapped such that one of the drivers provides power to the propane compressor  418  and the high pressure refrigerant compressor  462 , while the other driver provides power to the propane compressor  426  and the low pressure refrigerant compressor  454 . 
         [0043]    As illustrated in the exemplary embodiment  400  in  FIG. 4 , cooling of the natural gas feed stream  402  is performed in unit  406 . Like unit  106  of  FIG. 1 , unit  406  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  402  is cooled by indirect heat exchange to ultimately produce cooled stream  450 . The evaporation of propane at the four pressures results in four propane vapor streams  410 ,  412 ,  414 ,  416  that may then be compressed in compressor  418 . The resulting compressed stream  420  may then be condensed in propane condenser  422 , producing liquid stream  424  for reintroduction into the series of propane evaporators as shown in  FIG. 2A . 
         [0044]    Cooling of the mixed refrigerant stream  404  is performed in unit  408 . Unit  408  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant stream  404  may also be cooled by indirect heat exchange to ultimately produce cooled stream  438 . The evaporation of propane at the four pressures results in four propane vapor streams  430 ,  432 ,  434 ,  436  that may then be compressed in compressor  426 . The resulting compressed stream  427  may then be condensed in propane condenser  428 , producing liquid stream  429  for reintroduction into the series of propane evaporators as shown in  FIG. 2B . 
         [0045]    Again cooled mixed refrigerant stream  438  is separated in phase separator  440  into a liquid mixed refrigerant stream  442  and a vapor mixed refrigerant stream  444 . Liquid mixed refrigerant stream  442  is sub-cooled in the cryogenic heat exchanger (MCHE)  446  producing stream  447 . Stream  447  may then be reduced in pressure through isenthalpic valve  448  producing stream  449 . Stream  449  may then be vaporized in the shell side of the MCHE  446  to provide cooling to tubeside streams  442 ,  444 ,  450 . 
         [0046]    Vapor mixed refrigerant steam  444  is condensed and sub-cooled in the MCHE  446  to produce stream  451 . Stream  451  may then be reduced in pressure through isenthalpic valve  452  to produce stream  453 . Stream  453  may then be vaporized in the shell side of the MCHE  446  to provide cooling to tubeside streams  442 ,  444 ,  450 . 
         [0047]    The cooled natural gas feed stream  450  may enter the MCHE  446  where it is further cooled producing product stream  466  that may be, for example, liquid natural gas (LNG). 
         [0048]    Low pressure mixed refrigerant stream  445  exiting the MCHE  446  is compressed in the low pressure refrigerant compressor  454  to produce stream  455 . Stream  455  is cooled in intercooler  456  to produce stream  457 . Stream  457  is further compressed in high pressure refrigerant compressor  462  to produce stream  463 . Stream  463  is cooled in aftercooler  464  to be recycled back as original mixed refrigerant stream  404 . 
         [0049]    Power is supplied to the refrigeration compressors  418 ,  426 ,  454 ,  462  by two equal sized directly connected gas turbines  480 ,  482 . As illustrated in  FIG. 4 , the gas turbines  480 ,  482  may include starter/helper electric motors  484 ,  486  respectively to assist in starting the gas turbines  480 ,  482  and optimally, to provide additional power to assist the gas turbines  480 ,  482 , or for exportation into the power grid when excess power is available from the gas turbines. 
         [0050]      FIG. 5  illustrates yet another exemplary embodiment  500  as applied to a three loop refrigeration system. In this exemplary embodiment  500 , unit  506  precools a third refrigerant stream  503  in addition to the natural gas feed stream  502 . Like unit  106  of  FIG. 1 , unit  506  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  502  is cooled by indirect heat exchange to ultimately produce cooled stream  550 . The evaporation of propane at the four pressures results in four propane vapor streams  510 ,  512 ,  514 ,  516  that may then be compressed in compressor  518 . The resulting compressed stream  520  may then be condensed in propane condenser  522 , producing liquid stream  524  for reintroduction into the series of propane evaporators as shown in  FIG. 2A . 
         [0051]    Cooling of the mixed refrigerant stream  504  is performed in unit  508 . Unit  508  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant stream  504  may also be cooled by indirect heat exchange to ultimately produce cooled stream  538 . The evaporation of propane at the four pressures results in four propane vapor streams  530 ,  532 ,  534 ,  536  that may then be compressed in compressor  526 . The resulting compressed stream  527  may then be condensed in propane condenser  528 , producing liquid stream  529  for reintroduction into the series of propane evaporators as shown in  FIG. 2B . 
         [0052]    Cooled mixed refrigerant stream  538  is subcooled in the cryogenic heat exchanger (MCHE)  546  producing stream  547 . Stream  547  may then be reduced in pressure through isenthalpic valve  548  producing stream  549 . Stream  549  may then be vaporized in the shell side of the MCHE  546  to provide cooling to tubeside streams  505 ,  538 , and  550 . 
         [0053]    Cooled mixed refrigerant stream  505  may also be subcooled and liquefied in MCHE  546  producing stream  569  then subcooled in exchanger  568  producing stream  551 . Exchanger  568  may be a wound coil type exchanger, for example. The resulting stream  551  may then be reduced in pressure through isenthalpic valve  552  to produce stream  553 . Stream  553  may then be vaporized in exchanger  568  to provide refrigeration for subcooling both the feed gas stream (entering as stream  567  and exiting as  566 ) and the third refrigerant stream  569 . After vaporization and warming, third refrigerant stream  553  exits exchanger  568  as stream  593  and is then compressed by compressor  594  to produce stream  595 . Stream  595  is then cooled in the mixed refrigerant intercooler  596  to produce stream  597 . Stream  597  is compressed in compressor  598  to produce stream  599 . Stream  599  is then cooled in mixed refrigerant aftercooler  501  to be recycled back as original stream  503 . 
         [0054]    The cooled natural gas feed stream  550  may enter the MCHE  546  where it is further cooled producing stream  567 . Stream  567  may then be subcooled in exchanger  568  to produce product stream  566  that may be, for example, liquid natural gas (LNG). 
         [0055]    Low pressure mixed refrigerant stream  545  exiting the MCHE  546  is compressed in the low pressure refrigerant compressor  554  to produce stream  555 . Stream  555  is cooled in intercooler  556  to produce stream  557 . Stream  557  is further compressed in high pressure refrigerant compressor  558  to produce stream  559 . Stream  559  is cooled in aftercooler  564  to be recycled back as original mixed refrigerant stream  504 . 
         [0056]    Power is supplied to the refrigeration compressors  518 ,  526 ,  554 ,  558 ,  594 ,  598  by three equal sized directly connected gas turbines  580 ,  582 ,  592 . As illustrated in  FIGS. 1 ,  3 , and  4 , the gas turbines may include starter/helper electric motors (not shown in this embodiment) to assist in starting the gas turbines and optimally, to provide additional power to assist the gas turbines, or for exportation into the power grid when excess power is available from the gas turbines. 
         [0057]      FIG. 6  illustrates yet another exemplary embodiment  600  as applied to another three loop refrigeration system. In this exemplary embodiment  600 , unit  606  precools the natural gas feed stream  602  only. Like unit  106  of  FIG. 1 , unit  606  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  602  is cooled by indirect heat exchange to ultimately produce cooled stream  650 . The evaporation of propane at the four pressures results in four propane vapor streams  610 ,  612 ,  614 ,  616  that may then be compressed in compressor  618 . The resulting compressed stream  620  may then be condensed in propane condenser  622 , producing liquid stream  624  for reintroduction into the series of propane evaporators as shown in  FIG. 2A . 
         [0058]    In this exemplary embodiment, both mixed refrigerant streams  603 ,  604  are cooled in unit  608 . Unit  608  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant streams  603 ,  604  may also be cooled by indirect heat exchange to ultimately produce cooled streams  605 ,  638 . The evaporation of propane at the four pressures results in four propane vapor streams  630 ,  632 ,  634 ,  636  that may then be compressed in compressor  626 . The resulting compressed stream  627  may then be condensed in propane condenser  628 , producing liquid stream  629  for reintroduction into the series of propane evaporators as shown in  FIG. 2B . 
         [0059]    Cooled mixed refrigerant stream  638  is subcooled in the cryogenic heat exchanger (MCHE)  646  producing stream  647 . Stream  647  may then be reduced in pressure through isenthalpic valve  648  producing stream  649 . Stream  649  may then be vaporized in the shell side of the MCHE  646  to provide cooling to tubeside streams  605 ,  638 , and  650 . 
         [0060]    Cooled mixed refrigerant stream  605  may also be subcooled and liquefied in MCHE  646  producing stream  669  then subcooled in exchanger  668  producing stream  651 . Exchanger  668  may be a wound coil type exchanger, for example. The resulting stream  651  may then be reduced in pressure through isenthalpic valve  652  to produce stream  653 . Stream  653  may then be vaporized in exchanger  668  to provide refrigeration for subcooling both the feed gas stream (entering as stream  667  and exiting as  666 ) and the third refrigerant stream  669 . After vaporization and warming, third refrigerant stream  653  exits exchanger  668  as stream  693  and is then compressed by compressor  694  to produce stream  695 . Stream  695  is then cooled in the mixed refrigerant intercooler  696  to produce stream  697 . Stream  697  is compressed in compressor  698  to produce stream  699 . Stream  699  is then cooled in mixed refrigerant aftercooler  601  to be recycled back as original stream  603 . 
         [0061]    The cooled natural gas feed stream  650  may enter the MCHE  646  where it is further cooled producing stream  667 . Stream  667  may then be subcooled in exchanger  668  to produce product stream  666  that may be, for example, liquid natural gas (LNG). 
         [0062]    Low pressure mixed refrigerant stream  645  exiting the MCHE  646  is compressed in the low pressure refrigerant compressor  654  to produce stream  655 . Stream  655  is cooled in intercooler  656  to produce stream  657 . Stream  657  is further compressed in the high pressure refrigerant compressor  658  to produce stream  659 . Stream  659  is cooled in aftercooler  664  to be recycled back as original mixed refrigerant stream  604 . 
         [0063]    Power is supplied to the refrigeration compressors  618 ,  626 ,  654 ,  658 ,  694 ,  698  by three equal sized directly connected gas turbines  680 ,  682 ,  692 . As illustrated in  FIGS. 1 ,  3 , and  4 , the gas turbines may include starter/helper electric motors (not shown in this embodiment) to assist in starting the gas turbines and optimally, to provide additional power to assist the gas turbines, or for exportation into the power grid when excess power is available from the gas turbines. 
         [0064]      FIG. 7A  illustrates another exemplary embodiment  700 A as applied to yet another three loop refrigeration system. In this exemplary embodiment  700 A, unit  706  precools the natural gas feed stream  702  and the mixed refrigerant stream  704 . Like unit  106  of  FIG. 1 , unit  706  may comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2A . Natural gas feed stream  702  and mixed refrigerant stream  704  is cooled by indirect heat exchange to ultimately produce cooled streams  750 ,  738 . The evaporation of propane at the four pressures results in four propane vapor streams  710 ,  712 ,  714 ,  716  that may then be compressed in compressor  718 . The resulting compressed stream  720  may then be condensed in propane condenser  722 , producing liquid stream  724  for reintroduction into the series of propane evaporators as shown in  FIG. 2A . 
         [0065]    In this exemplary embodiment, only mixed refrigerant stream  703  is cooled in unit  708 . Unit  708  may also comprise a series of heat exchangers, valves, and separators as illustrated in  FIG. 2B . The mixed refrigerant stream  703  is cooled by indirect heat exchange to ultimately produce cooled streams  705 . The evaporation of propane at the four pressures results in four propane vapor streams  730 ,  732 ,  734 ,  736  that may then be compressed in compressor  726 . The resulting compressed stream  727  may then be condensed in propane condenser  728 , producing liquid stream  729  for reintroduction into the series of propane evaporators as shown in  FIG. 2B . 
         [0066]    Cooled mixed refrigerant stream  738  is subcooled in the cryogenic heat exchanger (MCHE)  746  producing stream  747 . Stream  747  may then be reduced in pressure through isenthalpic valve  748  producing stream  749 . Stream  749  may then be vaporized in the shell side of the MCHE  746  to provide cooling to tubeside streams  705 ,  738 , and  750 . 
         [0067]    Cooled mixed refrigerant stream  705  may also be subcooled and liquefied in MCHE  746  producing stream  769  then subcooled in exchanger  768  producing stream  751 . Exchanger  768  may be a wound coil type exchanger, for example. The resulting stream  751  may then be reduced in pressure through isenthalpic valve  752  to produce stream  753 . Stream  753  may then be vaporized in exchanger  768  to provide refrigeration for subcooling both the feed gas stream (entering as stream  767  and exiting as  766 ) and the third refrigerant stream  769 . After vaporization and warming, third refrigerant stream  753  exits exchanger  768  as stream  793  and is then compressed by compressor  794  to produce stream  795 . Stream  795  is then cooled in the mixed refrigerant intercooler  796  to produce stream  797 . Stream  797  is compressed in compressor  798  to produce stream  799 . Stream  799  is then cooled in mixed refrigerant aftercooler  701  to be recycled back as original stream  703 . 
         [0068]    The cooled natural gas feed stream  750  may enter the MCHE  746  where it is further cooled producing stream  767 . Stream  767  may then be subcooled in exchanger  768  to produce product stream  766  that may be, for example, liquid natural gas (LNG). 
         [0069]    Low pressure mixed refrigerant stream  745  exiting the MCHE  746  is compressed in the low pressure refrigerant compressor  754  to produce stream  755 . Stream  755  is cooled in intercooler  756  to produce stream  757 . Stream  757  is further compressed in the high pressure refrigerant compressor  758  to produce stream  759 . Stream  759  is cooled in aftercooler  764  to be recycled back as original mixed refrigerant stream  704 . 
         [0070]    Power is supplied to the refrigeration compressors  718 ,  726 ,  754 ,  758 ,  794 ,  798  by three equal sized directly connected gas turbines  780 ,  782 ,  792 . As illustrated in  FIGS. 1 ,  3 , and  4 , the gas turbines may include starter/helper electric motors (not shown in this embodiment) to assist in starting the gas turbines and optimally, to provide additional power to assist the gas turbines, or for exportation into the power grid when excess power is available from the gas turbines. 
         [0071]      FIG. 7B  illustrates yet another exemplary embodiment  700 B similar to  700 A, however, in this exemplary embodiment  700 B, unit  706  precools the natural gas feed stream  702  and the mixed refrigerant stream  704  through indirect heat exchange with a mixed refrigerant stream in a two-stage mixed refrigerant precooling system. While  FIG. 7B  discloses use of a two-stage mixed refrigerant precooling system, the precooling may be performed using a single-stage mixed refrigerant precooling system, or mixed refrigerant precooling systems with greater than two stages, for example. Additionally, a mixed refrigerant precooling system may be interchanged with the propane precooling systems disclosed in any of the exemplary embodiments. 
         [0072]      FIGS. 8A and 8B  illustrate exemplary units  706  and  708  shown in  FIG. 7B . Unit  706  may comprise two heat exchangers  810 ,  812  where streams  702 ,  704 , and at least a portion of stream  724  are cooled through indirect heat exchange against stream  713  in heat exchanger  810 . Stream  724  enters heat exchanger  810  and is cooled producing stream  830 . Stream  830  is split into two streams  831 ,  832  where stream  831  is further cooled in heat exchanger  812  while stream  832  is let down in pressure across isenthalpic valve  814  to produce stream  833 . Stream  833  then enters heat exchanger  810  to provide cooling to streams  702 ,  704 ,  724  and exits the heat exchanger  810  as stream  713 . 
         [0073]    After stream  831  is cooled in heat exchanger  812  to produce stream  834  and let down in pressure across isenthalpic valve  816 , the resulting stream  835  is introduced into heat exchanger  812  to provide further cooling for resultant streams  738 ,  750 ,  834 . 
         [0074]    Unit  708  may comprise two heat exchangers  818 ,  820  where streams  703 ,  729  are cooled through indirect heat exchange against stream  733  in heat exchanger  818 . Stream  729  enters heat exchanger  818  and is cooled producing stream  840 . Stream  840  is split into two streams  841 ,  842  where stream  841  is further cooled in heat exchanger  820  while stream  842  is let down in pressure across isenthalpic valve  822  to produce stream  843 . Stream  843  then enters heat exchanger  818  to provide cooling to streams  703 ,  729  and exits the heat exchanger  818  as stream  733 . 
         [0075]    After stream  841  is cooled in heat exchanger  820  to produce stream  844  and let down in pressure across isenthalpic valve  824 , the resulting stream  845  is introduced into heat exchanger  820  to provide further cooling for resultant streams  705 ,  844 . 
         [0076]    Heat exchangers  810 ,  812 ,  818 ,  820  may be wound-coil heat exchangers, plate-and-fin brazed aluminum (core) type heat exchangers, or shell and tube heat exchangers, for example. Heat exchangers  810 ,  812  may be combined into a single heat exchanger, for example. Heat exchangers  818 ,  820  may also be combined into a single heat exchanger, for example. Finally, heat exchangers  810 ,  812 ,  818 ,  820  may be combined into a single heat exchanger, for example. Heat exchangers  810 ,  812 ,  818 ,  820  may accept two or more load streams, for example. 
         [0077]    Pre-cooling in units  106 ,  108  may provide, for example, enough cooling to feed stream  102  and liquefaction refrigerant stream  104  such that the temperatures of streams  150  and  138  may reach +60° F. to as low as −100° F. before further cooling in the MCHE  146 . The same cooling ranges may be achieved in  FIGS. 3-7B . In one embodiment, for example, propane may be used as the pre-cooling refrigerant to reach the temperature range of +20° F. to −40° F. 
         [0078]    The isenthalpic valves  148 ,  152  (and the corresponding isenthalpic valves in  FIGS. 3-7B ) may optionally be replaced by work extracting liquid turbines, for example, to improve efficiency. Additionally, propane condensers  122 ,  128  (and the corresponding propane condensers in  FIGS. 3-7A ) may be ambient heat sink coolers used to condense, desuperheat, and/or optimally subcool precooling refrigerant, for example. 
       EXAMPLE 
       [0079]    The following example is based on a computer simulation of  FIGS. 1 ,  2 A, and  2 B as applied to a propane precooled mixed refrigerant process. As in  FIG. 1 , the natural gas feed stream  102  entered unit  106  after pretreatment, including the removal of moisture (H 2 O), carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), mercury, and other heavy components, including, but not limited to, benzene, ethylbenzene, and toluene, if they exist in the natural gas feed stream  102  in concentrations that would lead to freezing in the MCHE  146 . The pretreated natural gas feed stream  102  was at 35° C. and 40 bar absolute and had a flow rate of 12,260 kg-mole/hr. Natural gas feed stream  102  was cooled by indirect heat exchange in a series of propane evaporators  202 ,  204 ,  206 ,  208  (illustrated in  FIG. 2A ) that operate at successively lower pressures of 7.16 bar, 4.25 bar, 2.54 bar and 1.47 bar, where propane evaporator  202  is at the highest pressure and propane evaporator  208  is at the lowest pressure. The evaporation of propane at the four pressures resulted in four propane vapor streams  110 ,  112 ,  114 ,  116  that were then compressed in compressor  118 . Resulting stream  120  (at 16.2 bar, and 10,930 kgmole/hr) was then condensed in propane condenser  122  using an ambient heat sink (air or water), producing liquid stream  124 . 
         [0080]    The natural gas feed stream  102  was precooled by the propane to −22.5° C. Resulting cooled stream  150  was then cooled and liquefied in MCHE  146  by vaporizing mixed refrigerant producing liquid natural gas (LNG) stream  166  at −163.3° C. 
         [0081]    The mixed refrigerant stream  104  had a molar composition as follows: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Component 
                 Mole Composition (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Nitrogen 
                 12 
               
               
                   
                 Methane 
                 38 
               
               
                   
                 Ethane 
                 42 
               
               
                   
                 Propane 
                 8 
               
               
                   
                   
               
             
          
         
       
     
         [0082]    The mixed refrigerant stream  104  was at 35° C. and 62 bar absolute and had a flow rate of 50,250 kg-mole/hr. The mixed refrigerant stream  104  was cooled by indirect heat exchange in a series of propane evaporators  222 ,  224 ,  226 ,  228  (illustrated in  FIG. 2B ) that operate at successively lower pressures of 7.16 bar absolute, 4.25 bar, 2.54 bar and 1.47 bar where propane evaporator  203  is the highest and propane evaporator  209  is the lowest. The evaporation of propane at the four pressures results in four propane vapor streams  130 ,  132 ,  134 ,  138  which are then compressed in compressor  126 . Resulting stream  127  (at 16.2 bar absolute and 31,600 kgmole/hr) is condensed in propane condenser  128  using an ambient heat sink (air or water), producing liquid stream  129 . 
         [0083]    The precooled mixed refrigerant stream  138  is then separated into liquid stream  142  and vapor stream  144  in phase separator  140 . Liquid stream  142  is then subcooled to −125° C., flashed isenthalpically through valve  148 , and then vaporized in the shell side of exchanger  146  to provide cooling to the tubeside streams  142 ,  144 ,  150 . Vapor stream  144  is liquefied, subcooled to a temperature of −163° C., flashed isenthalpically through valve  152 , and then vaporized and warmed in the shell side of exchanger  146  to provide cooling to the tubeside streams  142 ,  144 ,  150 . After vaporization and warming, the combined mixed refrigerant stream  145  exits the MCHE  146  at a temperature of −32.7° C. and a pressure of 4.14 bar absolute. The combined mixed refrigerant stream  154  is then compressed in three stages of compressors  156 ,  158 ,  160  back to a pressure of 62 bar absolute, completing the loop. 
       Comparison with U.S. Pat. No. 6,962,060  
       [0084]    Computer simulations of the exemplary embodiment illustrated in  FIG. 1  were performed on the same basis as the simulation of a propane precooled mixed refrigerant process utilizing the precooling arrangement of U.S. Pat. No. 6,962,060. 
         [0085]    Results for the simulations are listed in Table II below. For both simulations, the same propane low pressure suction pressure was assumed and two compressor casings were required. For both simulations, preliminary sizing calculations for the compressors were performed. In the case of the exemplary embodiment illustrated in  FIG. 1 , the compressor casings  118  and  126  were smaller in diameter and had lower volumetric flow rates translating into lower cost. In addition, depending on the vendor and the scale of the plant, construction of large diameter impellers and casings may not have been feasible, thus, the solution utilizing the prior art may have been more limited in scale-up potential. 
         [0086]    As illustrated in Table II, the exemplary embodiment of  FIG. 1  allows more optimal and feasible compressor designs than the system disclosed in U.S. Pat. No. 6,962,061 using the same number of compressor casings and providing the same pre-cooling service. This is achieved by segregating the heat loads requiring pre-cooling refrigeration into two independent systems. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 U.S. Pat. 
                 Exemplary 
               
               
                   
                 No. 
                 Embodiment in 
               
               
                   
                 6,962,060 
                 FIG. 1 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Precooling 
                 −30.2 
                 −30.2 
               
               
                   
                 Temperature (° C.) 
               
               
                   
                 Liquid Natural Gas 
                 490,000 
                 490,000 
               
               
                   
                 Production (kg/h) 
               
               
                 Compressor 1 
                 Identifier 
                 Compressor 43 
                 Compressor 126 
               
               
                   
                 Maximum Impeller 
                 55 
                 50 
               
               
                   
                 Diameter (inches) 
               
               
                   
                 Maximum Volume 
                 149,000 
                 119,000 
               
               
                   
                 Flow Rate (m 3 /hr) 
               
               
                 Compressor 2 
                 Identifier 
                 Compressor 49 
                 Compressor 118 
               
               
                   
                 Maximum Impeller 
                 52 
                 51 
               
               
                   
                 Diameter (inches) 
               
               
                   
                 Maximum Volume 
                 78,000 
                 57,000 
               
               
                   
                 Flow Rate (m 3 /hr) 
               
               
                   
               
             
          
         
       
     
         [0087]    While aspects of the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the claimed invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims.