Abstract:
This invention relates to a system and method for liquefying natural gas. In another aspect, the invention concerns an improved liquefied natural gas facility employing a closed loop methane refrigeration cycle. In another aspect, the invention concerns the utilization of lean boil-off gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority benefit under 35 U.S.C. Section 119(e) to U.S. Provisional Patent Ser. No. 61/146,209 filed on Jan. 21, 2009, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and a system for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas facility employing a closed loop methane refrigeration cycle. In another aspect, the invention concerns the utilization of lean boil-off gas as a refrigerant source. 
       BACKGROUND OF THE INVENTION 
       [0003]    The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure. 
         [0004]    Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires. 
         [0005]    The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers. 
         [0006]    In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). 
         [0007]    A common closed loop LNG design utilizes a recirculating methane refrigerant derived from the feed gas. Utilizing recirculated methane refrigerant derived from the feed gas is acceptable when the feed gas is comprised primarily of 99 mole percent methane with insignificant amounts of heavier components. However, today plants around the world are designed for feed gas with less than 99 mole percent methane, rather containing significant amounts of ethane, propane and heavier components. A feed stream containing heavy components as well as methane is problematic because the heavies will tend to accumulate in the methane flash drums and eventually degrade performance of the feed gas chillers. 
         [0008]    Therefore, a need exists for reducing accumulation of heavies and increasing refrigerant efficiency in closed loop LNG systems which utilize a feed gas stream containing heavy components. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention overcomes the above-mentioned and other shortcomings by providing a novel and improved method, system, and device that meet the aforementioned needs. 
         [0010]    In an embodiment of the present invention, a method for liquefying natural gas, the method includes the steps of: (a) cooling a natural gas stream in a first refrigeration cycle employing a first refrigerant, wherein the first refrigeration cycle includes a plurality of cooling stages; (b) downstream of the first refrigeration cycle, further cooling the natural gas stream in a final refrigeration cycle employing a final refrigerant, wherein the final refrigerant is predominately comprises less than 10 mole percent nitrogen, wherein the final refrigeration cycle includes a plurality of cooling stages, wherein the plurality of cooling stages includes a first cooling stage and a final cooling stage, wherein the temperature of the final refrigeration cycle operates at a lower temperature than the temperature of the first refrigeration cycle; (c) delivering the natural gas stream to a storage tank, wherein evaporation of a portion of the natural gas stream occurs within the storage tank resulting in a boil-off gas; (d) compressing at least a portion of the boil-off gas; and (e) delivering at least a portion of the compressed boil-off gas to the final cooling stage of the final refrigeration cycle, wherein at least a portion of the compressed boil-off gas serves as the final refrigerant in the final cooling stage of the final refrigeration cycle. 
         [0011]    In another embodiment of the present invention, a method for liquefying natural gas, the method includes the steps of: (a) cooling a natural gas stream in a first refrigeration cycle employing a first refrigerant, wherein the first refrigeration cycle includes a plurality of cooling stages; (b) downstream of the first refrigeration cycle, further cooling the natural gas stream in a final refrigeration cycle employing a final refrigerant, wherein the final refrigeration cycle includes a plurality of cooling stages; (c) delivering the natural gas stream to a storage tank, wherein evaporation of a portion of the natural gas stream occurs within the storage tank resulting in a boil-off gas; (d) compressing at least a portion of the boil-off gas; and (e) delivering at least a portion of the compressed boil-off gas to the final cooling stage of the final refrigeration cycle, wherein at least a portion of the compressed boil-off gas serves as the final refrigerant in the final cooling stage of the final refrigeration cycle. 
         [0012]    In yet another embodiment of the present invention, a system for liquefying natural gas, the system includes: (a) a first refrigeration cycle for cooling a natural gas stream employing a first refrigerant, wherein the first refrigeration cycle includes a plurality of cooling stages; (b) a final refrigeration cycle for cooling the natural gas stream employing a final refrigerant, wherein the final refrigeration cycle includes a plurality of cooling stages; (c) a storage tank for storing the natural gas, wherein evaporation of a portion of the natural gas stream within the storage tank results in a boil-off stream; and (d) a compressor for compressing the boil-off stream, whereby the boil-off stream is delivered to the final refrigeration cycle, and utilizing it as the final refrigerant in the final cooling stage of the final refrigeration cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention is shown by way of example and not by limitation in the accompanying figures, in which: 
           [0014]      FIG. 1  is a simplified flow diagram of a cascade LNG refrigeration process in accord with an embodiment of the present invention. 
           [0015]      FIG. 2  is a flow diagram detailing the methane refrigeration system of the cascade LNG refrigeration process utilizing boil-off gas in accord with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents. 
         [0017]    A cascade refrigeration process uses one or more refrigerants for transferring heat energy from the natural gas stream to the refrigerant and ultimately transferring said heat energy to the environment. In essence, the overall refrigeration system functions as a heat pump by removing heat energy from the natural gas stream as the stream is progressively cooled to lower and lower temperatures. The design of a cascade refrigeration process involves the balancing of thermodynamic efficiencies and capital costs. In heat transfer processes, thermodynamic irreversibilities are reduced as the temperature gradients between heating and cooling fluids become smaller. However, obtaining such small temperature gradients generally requires (1) significant increases in the amount of heat transfer area; (2) major modifications to various process equipment; and (3) the proper selection of flowrates through such equipment so as to ensure both flowrates, approach temperatures and outlet temperatures are compatible with the required heating/cooling duty. 
         [0018]    One of the most efficient and effective means of liquefying natural gas is via an optimized cascade-type operation in combination with expansion-type cooling. Such a liquefaction process involves the cascade-type cooling of a natural gas stream at an elevated pressure, (e.g., about 650 psia) by sequentially cooling the gas stream via passage through a multistage propane or propylene cycle, a multistage ethane or ethylene cycle, and an open-loop methane cycle which utilizes a portion of the feed gas as a source of methane and which includes therein a multistage expansion cycle to further cool the same and reduce the pressure to near-atmospheric pressure. In another embodiment, the methane cycle can be a closed loop system. In the sequence of cooling cycles, the refrigerant having the highest boiling point is utilized first followed by a refrigerant having an intermediate boiling point and finally by a refrigerant having the lowest boiling point. As used herein, the terms “upstream” and “downstream” shall be used to describe the relative positions of various components of a natural gas liquefaction plant along the flow path of natural gas through the plant. 
         [0019]    In cryogenic processing of a natural gas stream an important consideration is contamination. The raw natural gas feed stream suitable for the process of the invention may comprise natural gas obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). The composition of natural gas can vary significantly. While methane is the major desired component of natural gas streams, the typical raw natural gas stream also contains ethane (C 2 ), higher hydrocarbons (C 3 +), and minor amounts of contaminants such as water, carbon dioxide, hydrogen sulfide, nitrogen, butane, hydrocarbons of six or more carbon atoms, dirt, iron sulfide, wax, and crude oil. The solubilities of these contaminants vary with temperature, pressure, and composition. At cryogenic temperatures, CO 2 , water, and other contaminants can form solids, which can plug flow passages in cryogenic heat exchangers. 
         [0020]    Various pretreatment steps provide a means for removing undesirable components, such as acid-gases, mercaptan, mercury, and moisture from the natural gas feed stream delivered to the LNG facility. The composition of this natural gas stream may vary significantly. As used herein, a natural gas stream is any stream principally comprised of methane which originates in major portion from a natural gas feed stream, such feed stream for example containing at least 85 percent methane by volume, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide and minor amounts of other contaminants such as mercury, hydrogen sulfide, and mercaptan. The pretreatment steps may be separate steps located either upstream of the cooling cycles or located downstream of one of the early stages of cooling in the initial cycle. The following is a non-exclusive listing of some of the available means which are readily available to one skilled in the art: (a) acid gases and to a lesser extent mercaptan are routinely removed via a sorption process employing an aqueous amine-bearing solution; (b) a major portion of the water is routinely removed as a liquid via two-phase gas-liquid separation following gas compression and cooling upstream of the initial cooling cycle and also downstream of the first cooling stage in the initial cooling cycle; (c) mercury is routinely removed via mercury sorbent beds; and (d) residual amounts of water and acid gases are routinely removed via the use of properly selected sorbent beds such as regenerable molecular sieves. 
         [0021]    The pretreated natural gas feed stream is generally delivered to the liquefaction process at an elevated pressure or is compressed to an elevated pressure, that being a pressure greater than 500 psia, preferably about 500 psia to about 900 psia, still more preferably about 500 psia to about 675 psia, still yet more preferably about 600 psia to about 675 psia, and most preferably about 625 psia. The natural gas feed stream temperature is typically near ambient to slightly above ambient. A representative temperature range being 60° F. to 138° F. 
         [0022]    As previously noted, the natural gas feed stream is cooled in a plurality of multistage (for example, three) cycles or steps by indirect heat exchange with a plurality of refrigerants, preferably three. The overall cooling efficiency for a given cycle improves as the number of stages increases but this increase in efficiency is accompanied by corresponding increases in net capital cost and process complexity. The natural gas feed stream is preferably passed through an effective number of refrigeration stages, nominally two, preferably two to four, and more preferably three stages, in the first refrigeration cycle, also referred herein as the first cooling cycle, utilizing a first refrigerant having relatively high boiling refrigerant. Such refrigerant is preferably comprised in major portion of propane, propylene or mixtures thereof, more preferably the refrigerant comprises at least about 75 mole percent propane, even more preferably at least 90 mole percent propane, and most preferably the refrigerant consists essentially of propane. 
         [0023]    Thereafter, the processed natural gas feed stream flows through an effective number of stages, nominally two, preferably two to four, and more preferably two or three, in a second refrigeration cycle, also referred herein as the second cooling cycle, in heat exchange with a second refrigerant having a lower boiling point. Such refrigerant is preferably comprised in major portion of ethane, ethylene or mixtures thereof, more preferably the refrigerant comprises at least about 75 mole percent ethylene, even more preferably at least 90 mole percent ethylene, and most preferably the refrigerant consists essentially of ethylene. As previously noted, the processed natural gas feed stream is combined with one or more recycle streams at various locations in the second cycle thereby producing a liquefaction stream. In the last stage of the second cooling cycle, the liquefaction stream is condensed (i.e., liquefied) in major portion, preferably in its entirety thereby producing a pressurized LNG-bearing stream. Generally, the process pressure at this location is only slightly lower than the pressure of the pretreated natural gas feed stream of the first stage of the first cycle. 
         [0024]    Thereafter, the processed natural gas feed stream flows through an effective number of stages, nominally two, preferably two to four, and more preferably three, in a final refrigeration cycle in indirect heat exchange with a final refrigerant. The final refrigerant consists essentially of methane. In a particularly preferred embodiment, the predominately methane refrigerant comprises less than 10 mole percent nitrogen, most preferably less than 5 mole percent nitrogen. Each cooling stage comprises a separate cooling zone. 
         [0025]    Generally, the natural gas feed stream will contain such quantities of C 2 + components so as to result in the formation of a C 2 + rich liquid in one or more of the cooling cycles. This liquid is removed via gas-liquid separation means, preferably one or more conventional gas-liquid separators. Generally, the sequential cooling of the natural gas stream in each stage is controlled so as to remove as much as possible of the C 2  and higher molecular weight hydrocarbons from the gas to produce a gas stream predominating in methane and a liquid stream containing significant amounts of ethane and heavier components. An effective number of gas/liquid separation means are located at strategic locations downstream of the cooling zones for the removal of liquids streams rich in C 2 + components. The exact locations and number of gas/liquid separation means, preferably conventional gas/liquid separators, will be dependant on a number of operating parameters, such as the C 2 + composition of the natural gas feed stream, the desired BTU content of the LNG product, the value of the C 2 + components for other applications and other factors routinely considered by those skilled in the art of LNG plant and gas plant operations. The C 2 + hydrocarbon stream or streams may be demethanized via a single stage flash or a fractionation column. In the latter case, the resulting natural gas stream can be directly returned at pressure to the liquefaction process. In the former case, this natural gas stream can be repressurized and recycled or can be used as fuel gas. The C 2 + hydrocarbon stream or streams or the demethanized C 2 + hydrocarbon stream may be used as fuel or may be further processed such as by fractionation in one or more fractionation zones to produce individual streams rich in specific chemical constituents (e.g., C 2 , C 3 , C 4  and C 5 +). 
         [0026]    The liquefaction process may use one of several types of cooling which include but is not limited to (a) indirect heat exchange, (b) vaporization, and (c) expansion or pressure reduction. Indirect heat exchange, as used herein, refers to a process wherein the refrigerant cools the substance to be cooled without actual physical contact between the refrigerating agent and the substance to be cooled. Specific examples of indirect heat exchange means include heat exchange in a shell-and-tube heat exchanger, a core-in-kettle heat exchanger, and a brazed aluminum plate-fin heat exchanger. The physical state of the refrigerant and substance to be cooled can vary depending on the demands of the system and the type of heat exchanger chosen. Thus, a shell-and-tube heat exchanger will typically be utilized where the refrigerating agent is in a liquid state and the substance to be cooled is in a liquid or gaseous state or when one of the substances undergoes a phase change and process conditions do not favor the use of a core-in-kettle heat exchanger. As an example, aluminum and aluminum alloys are preferred materials of construction for the core but such materials may not be suitable for use at the designated process conditions. A plate-fin heat exchanger will typically be utilized where the refrigerant is in a gaseous state and the substance to be cooled is in a liquid or gaseous state. Finally, the core-in-kettle heat exchanger will typically be utilized where the substance to be cooled is liquid or gas and the refrigerant undergoes a phase change from a liquid state to a gaseous state during the heat exchange. 
         [0027]    Vaporization cooling refers to the cooling of a substance by the evaporation or vaporization of a portion of the substance with the system maintained at a constant pressure. Thus, during the vaporization, the portion of the substance which evaporates absorbs heat from the portion of the substance which remains in a liquid state and hence, cools the liquid portion. 
         [0028]    Finally, expansion or pressure reduction cooling refers to cooling which occurs when the pressure of a gas, liquid or a two-phase system is decreased by passing through a pressure reduction means. In one embodiment, this expansion means is a Joule-Thomson expansion valve. In another embodiment, the expansion means is either a hydraulic or gas expander. Because expanders recover work energy from the expansion process, lower process stream temperatures are possible upon expansion. 
         [0029]    Referring to  FIG. 1 , a feed gas stream, as previously described, is introduced to the system through conduit  100 . Gaseous propane is compressed in multistage compressor  18  driven by a gas turbine driver which is not illustrated. The three stages preferably form a single unit although they may be separate units mechanically coupled together to be driven by a single driver. Upon compression, the compressed propane is passed through conduit  300  to cooler  20  where it is liquefied. A representative temperature and pressure of the liquefied propane refrigerant prior to flashing is about 100° F. and about 190 psia. Although not illustrated in  FIG. 1 , it is preferable that a separation vessel be located downstream of cooler  20  and upstream of expansion valve  12  for the removal of residual light components from the liquefied propane. Such vessels may be comprised of a single-stage gas liquid separator or may be more sophisticated and comprised of an accumulator section, a condenser section and an absorber section, the latter two of which may be continuously operated or periodically brought on-line for removing residual light components from the propane. The stream from this vessel or the stream from cooler  20 , as the case may be, is passed through conduit  302  to a pressure reduction means such as an expansion valve  12  wherein the pressure of the liquefied propane is reduced thereby evaporating or flashing a portion thereof. The resulting two-phase product then flows through conduit  304  into high-stage propane chiller  2  wherein indirect heat exchange with gaseous methane refrigerant introduced via conduit  152 , natural gas feed introduced via conduit  100  and gaseous ethylene refrigerant introduced via conduit  202  are respectively cooled via indirect heat exchange means  4 ,  6  and  8  thereby producing cooled gas streams respectively produced via conduits  154 ,  102  and  204 . 
         [0030]    The flashed propane gas from chiller  2  is returned to compressor  18  through conduit  306 . This gas is fed to the high stage inlet port of compressor  18 . The remaining liquid propane is passed through conduit  308 , the pressure further reduced by passage through a pressure reduction means, illustrated as expansion valve  14 , whereupon an additional portion of the liquefied propane is flashed. The resulting two-phase stream is then fed to chiller  22  through conduit  310  thereby providing a coolant for chiller  22 . 
         [0031]    The cooled feed gas stream from chiller  2  flows via conduit  102  to a knock-out vessel  10  wherein gas and liquid phases are separated. The liquid phase which is rich in C 3 + components is removed via conduit  103 . The gaseous phase is removed via conduit  104  and conveyed to propane chiller  22 . Ethylene refrigerant is introduced to chiller  22  via conduit  204 . In the chiller, the natural gas and ethylene refrigerant streams are respectively cooled via indirect heat transfer means  24  and  26  thereby producing cooled natural gas and ethylene refrigerant streams via conduits  110  and  206 . The evaporated portion of the propane refrigerant is separated and passed through conduit  311  to the intermediate-stage inlet of compressor  18 . 
         [0032]    As illustrated in  FIG. 1 , the natural gas stream flows from the intermediate-stage propane chiller  22  to the low-stage propane chiller/condenser  28  via conduit  110 . In this chiller, the stream is cooled via indirect heat exchange means  30 . In a like manner, the ethylene refrigerant stream flows from the intermediate stage propane chiller  22  to the low-stage propane chiller/condenser  28  via conduit  206 . In the latter, the ethylene-refrigerant is condensed via an indirect heat exchange means  32  in nearly its entirety. The vaporized propane is removed from the low-stage propane chiller/condenser  28  and returned to the low-stage inlet at the compressor  18  via conduit  320 . Although  FIG. 1  illustrates cooling of streams provided by conduits  110  and  206  to occur in the same vessel, the chilling of stream  110  and the cooling and condensing of stream  206  may respectively take place in separate process vessels (ex., a separate chiller and a separate condenser, respectively). 
         [0033]    As illustrated in  FIG. 1 , the natural gas stream exiting the low-stage propane chiller is introduced to the high-stage ethylene chiller  42  via conduit  112 . Ethylene refrigerant exits the low-stage propane chiller  28  via conduit  208  and is fed to a separation vessel  37  wherein light components are removed via conduit  209  and condensed ethylene is removed via conduit  210 . The separation vessel is analogous to the earlier discussed for the removal of light components from liquefied propane refrigerant and may be a single-stage gas/liquid separator or may be a multiple stage operation resulting in a greater selectivity of the light components removed from the system. The ethylene refrigerant at this location in the process is generally at a temperature of about −24° F. and a pressure of about 285 psia. The ethylene refrigerant via conduit  210  then flows to the ethylene economizer  34  wherein it is cooled via indirect heat exchange means  38  and removed via conduit  211  and passed to a pressure reduction means such as an expansion valve  40  whereupon the refrigerant is flashed to a preselected temperature and pressure and fed to the high-stage ethylene chiller  42  via conduit  212 . Vapor is removed from this chiller via conduit  214  and routed to the ethane economizer  34  wherein the vapor functions as a coolant via indirect heat exchange means  46 . The ethylene vapor is then removed from the ethylene economizer via conduit  216  and feed to the high-stage inlet on the ethylene compressor  48 . The ethylene refrigerant which is not vaporized in the high-stage stage ethylene chiller  42  is removed via conduit  218  and returned to the ethylene economizer  34  for further cooling via indirect heat exchange means  50 , removed from the ethylene economizer via conduit  220  and flashed in a pressure reduction means illustrated as expansion valve  52  whereupon the resulting two-phase product is introduced into the low-stage ethylene chiller  54  via conduit  222 . The natural gas stream is removed from the high-stage ethylene chiller  42  via conduit  116  and directly fed to the low-stage ethylene chiller  54  wherein it undergoes additional cooling and partial condensation via indirect heat exchange means  56 . The resulting two-phase stream then flows via conduit  118  to a two phase separator  60  from which is produced a natural gas vapor stream via conduit  120  and via conduit  117 , a liquid stream rich in C 2 + components which is subsequently flashed or fractionated in vessel  67  thereby producing via conduit  125   a  heavies stream and a second natural gas stream which is transferred via conduit  121  and after combination with a second stream via conduit  128  is fed to the high pressure inlet port on the methane compressor  83 . The stream in conduit  120  and the stream in conduit  158  which contains a cooled compressed methane recycle stream are combined and fed to the low-stage ethylene condenser  68  wherein this exchanger heats via indirect heat exchange means  70  with the liquid effluent from the low-stage ethylene chiller  54  which is routed to the low-stage ethylene condenser  68  via conduit  226 . In condenser  68 , combined streams respectively provided via conduits  120  and  158  are condensed and produced from condenser  68  via conduit  122 . The vapor from the low-stage ethylene chiller  54  via conduit  224  and low-stage ethylene condenser  68  via conduit  228  are combined and routed via conduit  230  to the ethylene economizer  34  wherein the vapors function as a coolant via indirect heat exchange means  58 . The stream is then routed via conduit  232  from the ethylene economizer  34  to the low-stage side of the ethylene compressor  48 . As noted in  FIG. 1 , the compressor effluent from vapor introduced via the low-stage side is removed via conduit  234 , cooled via inter-stage cooler  71  and returned to compressor  48  via conduit  236  for injection with the high-stage stream present in conduit  216 . Preferably, the two-stages are a single module although they may each be a separate module and the modules mechanically coupled to a common driver. The compressed ethylene product from the compressor is routed to a downstream cooler  72  via conduit  200 . The product from the cooler flows via conduit  202  and is introduced, as previously discussed, to the high-stage propane chiller  2 . 
         [0034]    Referring to  FIG. 2 , the natural gas stream in conduit  122  is generally at a representative temperature and pressure of about −125° F. and about 600 psi. This stream passes via conduit  122  through the main methane economizer  74  wherein the stream is further cooled by indirect heat exchange means  88   a . The stream exits main methane economizer  74  via conduit  131  and is then introduced to first methane heat exchanger  63  wherein the stream undergoes indirect heat exchange with a methane refrigerant stream in conduit  123 . Prior to entry into the first methane heat exchanger  63  the predominately methane refrigerant stream in conduit  123  is introduced to a pressure reduction means such as expansion valve  78  wherein the pressure of the predominately methane refrigerant is reduced thereby evaporating or flashing a portion thereof resulting in a two-phase stream. The two-phase predominately methane refrigerant is then introduced to the first methane heat exchanger  63 . 
         [0035]    Upon entering the first methane heat exchanger  63  and undergoing indirect heat exchange, the two-phase predominately methane refrigerant exits the heat exchanger as a gas-phase methane predominately methane stream via conduit  126  and a liquid phase predominately methane refrigerant via conduit  130 . The gas phase predominately methane refrigerant exits first methane heat exchanger  63  via conduit  126  and is introduced to main methane economizer  74  wherein the stream is further cooled by indirect heat exchange means  82 . The predominately methane refrigerant stream exits main methane economizer  74  via conduit  128  and is introduced to high stage methane compressor  83 . The liquid phase predominately methane refrigerant exits first methane heat exchanger  63  via conduit  130  and is subsequently introduced to pressure reduction means such as expansion valve  91  wherein the pressure of the predominately methane refrigerant is reduced thereby evaporating or flashing a portion thereof resulting in a two-phase predominately methane refrigerant. The two-phase predominately methane refrigerant is then introduced to the second methane heat exchanger  71 . 
         [0036]    The cooled natural gas stream exits first methane heat exchanger  63  and is introduced to main methane economizer  74  via conduit  125 . The cooled natural gas stream is cooled via indirect heat exchange means  88   b . The cooled natural gas stream is then passed to second methane heat exchange  71  via conduit  132 . 
         [0037]    The natural gas stream exiting the main methane economizer  74  in conduit  132  then flows to the second methane heat exchanger  71  wherein it is cooled via indirect heat exchange with the two-phase predominately methane refrigerant originating from conduit  130 . The two-phase predominately methane refrigerant includes a gas phase and a liquid phase. The gas phase predominately methane refrigerant is discharged from the second methane heat exchanger  71  via conduit  136 , while the liquid phase predominately methane refrigerant is discharged from the second methane heat exchanger  71  via conduit  129 . The gas phase predominately methane refrigerant in conduit  136  is introduced into main economizer  74  wherein it is cooled via indirect heat exchange means  89 . The resulting predominately methane refrigerant exits main economizer  74  via conduit  138  and is introduced to high stage methane compressor  83 . The liquid phase predominately methane refrigerant undergoes pressure reduction means such as expansion valve  92  wherein the pressure of the predominately methane refrigerant is reduced thereby evaporating or flashing a portion thereof resulting in a two-phase predominately methane refrigerant. The two-phase predominately methane refrigerant is then introduced to the third methane heat exchanger  73 . 
         [0038]    The natural gas stream discharged from second methane heat exchanger  71  via conduit  137  is introduced to main methane economizer  74  wherein it is further cooled via indirect heat exchange means  88   c  with predominately methane refrigerant. The natural gas stream cooled via indirect heat exchange means  88   c  in main economizer  74  is then passed to the third methane heat exchanger  73  via conduit  134 . 
         [0039]    The two-phase predominantly methane refrigerant is introduced into third methane heat exchanger  73  via conduit  129 . The natural gas stream is further cooled by indirect heat exchange with the predominately methane refrigerant. The two-phase predominately methane refrigerant exits the heat exchanger as a gas phase predominately methane refrigerant in conduit  146  and a liquid phase predominately methane refrigerant in conduit  135 . The gaseous predominately methane refrigerant in conduit  146  is introduced into main methane economizer  74  wherein it is employed in indirect heat exchange means  90  and subsequently exits main methane economizer  74  via conduit  148  and is carried to high stage methane compressor  83 . 
         [0040]    The natural gas stream cooled in indirect heat exchange is discharged from third methane heat exchanger  73  via conduit  135  and is flashed in pressure reducing means  94 . The flashed stream is then introduced into separator vessel  75  via conduit  139 . Separator vessel  75  is operable to separate the predominately liquid and predominately gas phases of the stream introduced via conduit  139 . Liquefied natural gas exits separator  75  via conduit  150 . The liquefied natural gas product from separator vessel  75 , which is at approximately atmospheric pressure, is passed through conduit  150  to a liquefied natural gas storage tank  27 . In liquefied natural gas storage tank  27 , “boil-off” vapors from the liquefied natural gas are then removed from liquefied natural gas storage tank  27  via conduit  178 . The boil off stream in conduit  178  is flashed in a compressor  95 , which is then delivered to the third methane heat exchange  73  via the flashed methane refrigerant  129 . 
         [0041]    The preferred embodiment of the present invention has been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described in the present invention. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims below and the description, abstract and drawings not to be used to limit the scope of the invention.