Abstract:
The current invention provides a methodology and apparatus for the liquefaction of normally gaseous material, most notably natural gas, which reduces the number of process vessels required and/or reduces space requirements over convention apparatus.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The inventive methodology and associated apparatus disclosed herein relates to the liquefaction of a normally gaseous material, most notably natural gas. In one aspect, the invention concerns a liquified natural gas (LNG) production system that operates with a reduced number of process vessels and in a smaller space than conventional LNG production systems. 
         [0003]    2. Description of the Prior Art 
         [0004]    It is common practice to cryogenically treat natural gas to liquefy the same for transport and storage. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from a supply source to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. 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, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand 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]    Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. 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 which in turn 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 it 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 heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, or a combination of one or more of the preceding. The refrigerants are sometimes arranged in a cascaded manner. Further cooling of the liquid is possible by expanding the liquefied natural gas to atmospheric pressure in one or more expansion stages. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase, gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a temperature suitable for liquefied gas storage at near-atmospheric pressure. 
         [0007]    As previously noted, the present invention concerns the arrangement/selection of apparatus and associated process methodologies whereby the number of process vessels in the overall system is reduced. This reduction in the number of process vessels also reduces space requirements. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0008]    It is an object of this invention to reduce the number of process vessels required for liquefying natural gas. 
         [0009]    It is another object of this invention to reduce the space requirements of a process for liquefying natural gas. 
         [0010]    It is still yet another object of this invention to develop a process methodology and associated apparatus for liquefying natural gas which is less capital intensive than alternative liquefaction methodologies. 
         [0011]    One embodiment of the present invention concerns a process for liquefying natural gas that includes the following steps: (a) cooling a natural gas stream in a first refrigeration cycle via indirect heat exchange with a first refrigerant; and (b) downstream of the first refrigeration cycle, cooling the natural gas stream in a second refrigeration cycle via indirect heat exchange with a second refrigerant. At least one of the first and second refrigerants is a pure component refrigerant, and less than about 10 percent of the natural gas mechanical cooling duty of at least one of the first and second refrigeration cycles is provided by core-in-kettle heat exchangers. 
         [0012]    Another embodiment of the present invention concerns a process for liquefying natural gas that includes the following steps: (a) cooling a natural gas stream in a first refrigeration cycle employing a first refrigerant; (b) downstream of the first refrigeration cycle, cooling the natural gas stream in a second refrigeration cycle employing a second refrigerant; (c) downstream of the second refrigeration cycle, cooling the natural gas stream in a third refrigeration cycle employing a third refrigerant. The third refrigeration cycle is an open refrigeration cycle that uses a portion of the natural gas stream as the third refrigerant, and at least about 90 percent of the combined natural gas mechanical cooling duty of the first, second, and third refrigeration cycles is provided by plate-fin heat exchangers. 
         [0013]    Still another embodiment of the present invention concerns a process for liquefying natural gas comprising the following steps: (a) cooling a natural gas stream in a first methane heat exchanger via indirect heat exchange with at least one predominately-methane first refrigerant stream to thereby produce a first cooled natural gas stream; (b) dividing the first cooled natural gas stream into a first refrigerant portion and a first product portion; (c) expanding the first refrigerant portion to thereby produce a first expanded refrigerant portion; and (d) using the first expanded refrigerant portion as at least a portion of the first refrigerant stream in the first methane heat exchanger. 
         [0014]    Yet another embodiment of the present invention concerns a facility for producing LNG. The facility includes the following components: (a) a first refrigeration cycle for cooling natural gas with a first refrigerant; and (b) a second refrigeration cycle for cooling the natural gas with a second refrigerant. At least one of the first and second refrigerants is a pure component refrigerant, and at least one of the first and second refrigeration cycles does not include any core-in-kettle heat exchangers that are operable to significantly cool the natural gas. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0015]      FIG. 1  is a simplified flow diagram of a cryogenic LNG production process which illustrates one embodiment of the methodology and apparatus of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    As used herein, the term “natural gas” or “natural gas stream” shall denote any stream principally comprised of methane, which originates in major portion from a natural gas feed stream. A natural gas stream typically contains at least 85 mole percent methane, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide, and minor amounts of other contaminates such as, for example, mercury, hydrogen sulfide, and mercaptans. As used herein, the terms “principally,” “predominately,” “primarily,” and “in major portion,” when used to describe the presence of a particular component of a fluid stream, shall mean that the fluid stream contains at least 50 mole percent of the stated component. For example, a “predominately” methane stream, a “primarily” methane stream, a stream “principally” comprised of methane, or a stream comprised “in major portion” of methane, each denote a stream containing at least 50 mole percent methane. 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 main flow path of natural gas through the plant. 
         [0017]    One of the most efficient and effective methodologies for natural gas liquefaction is a cascade-type operation in combination with expansion-type cooling. Cascaded processes utilize one or more refrigerants to transfer heat energy from the natural gas stream to the refrigerant and ultimately to the environment. In essence, the refrigeration system functions as a heat pump by removing thermal energy from the natural gas stream as the stream is progressively cooled to lower and lower temperatures. In so doing, the thermal energy removed from the natural gas stream is ultimately rejected (pumped) to the environment via energy exchange with one or more refrigerants. 
         [0018]    In a preferred embodiment, the present invention employs a cascaded refrigerant system that cools the natural gas stream at an elevated pressure (e.g., about 650 psia), by sequentially passing the natural gas stream through an initial refrigeration cycle, an intermediate refrigeration cycle, and a final refrigeration cycle. In a preferred embodiment of the invention, the initial and intermediate refrigeration cycles are closed refrigeration cycles, while the final refrigeration cycle is an open refrigeration cycle that utilizes a portion of the feed gas as a source of refrigerant and which includes therein a multi-stage expansion cycle to further cool the feed gas and reduce its pressure to near-atmospheric pressure. 
         [0019]    The refrigerants employed in the initial, intermediate, and final refrigeration cycles preferably have their own distinct compositions. In other words, it is preferred for pure component refrigerants, rather than mixed refrigerants, to be employed in the initial, intermediate, and final refrigeration cycles of the present invention. As used herein, the term “mixed refrigerant” denotes a refrigerant that does not contain more than 80 mole percent of any single refrigerant component. As used herein, the term “pure component refrigerant” denotes a refrigerant that is not a mixed refrigerant. Preferably, a pure component refrigerant comprises at least about 80 mole percent of a single refrigerant component, more preferably at least about 90 mole percent of a single hydrocarbon refrigerant component, and most preferably at least 95 mole percent of a single hydrocarbon refrigerant component. In the system of the present invention, it is preferred for the refrigerant having the highest boiling point to be utilized in the initial refrigeration cycle, followed by a refrigerant having an intermediate boiling point employed in the intermediate refrigeration cycle, and finally a refrigerant having the lowest boiling point is employed in the final refrigeration cycle. 
         [0020]    In a preferred embodiment of the present invention, the initial refrigerant employed in the initial refrigeration cycle contains primarily propane, propylene, and/or carbon dioxide. More preferably, the initial refrigerant comprises predominately propane, most preferably, the initial refrigerant consists essentially of propane. The intermediate refrigerant preferably comprises predominately ethane and/or ethylene. More preferably, the intermediate refrigerant comprises predominately ethylene. Most preferably, the intermediate refrigerant consists essentially of ethylene. The final refrigerant preferably comprises predominately methane. Most preferably, the final refrigerant consists essentially of methane. 
         [0021]    Preferably, each of the initial, intermediate, and final refrigeration cycles employs a plurality of distinct cooling steps carried out in one or more heat exchangers. In a preferred embodiment of the present invention, less than about 10 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by core-in-kettle and/or spiral-wound heat exchangers, more preferably less than about 5 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by core-in-kettle and/or spiral-wound heat exchangers, still more preferably less than 2 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by core-in-kettle and/or spiral-wound heat exchangers. Most preferably, none of the natural gas-cooling heat exchangers employed in the initial, intermediate, and final refrigeration cycles are core-in-kettle heat exchangers and/or spiral-wound heat exchangers. Rather, it is preferred that at least about 90 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by plate-fin heat exchangers, more preferably at least about 95 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by plate-fin heat exchangers, still more preferably at least 98 percent of the natural gas mechanical cooling duty of the initial, intermediate, and/or final refrigeration cycles is provided by plate-fin heat exchangers. Most preferably, all of the natural gas-cooling heat exchangers employed in the initial, intermediate, and final refrigeration cycles are plate-fin heat exchangers. It is particularly preferred for the plate-fin heat exchangers to be brazed aluminum plate-fin heat exchangers. 
         [0022]    As used herein, the term “natural gas mechanical cooling duty” denotes a responsibility for extracting heat from natural gas via indirect heat exchange, expressed in terms of energy per units of time (e.g., BTU/hr). As used herein, the term “core-in-kettle heat exchanger” denotes a heat exchange device comprising an outer vessel shell and an inner core disposed in the vessel shell. A core-in-kettle heat exchanger facilitates indirect heat transfer between a first fluid contained in the vessel shell and a second fluid flowing through the core while the core is at least partly submerged in the first fluid. As used here, the term “spiral-wound heat exchanger” denotes a heat exchange device comprising an outer vessel shell and an inner core of wound tubes disposed in the shell. As used herein, the term “plate-fin heat exchanger” denotes a device that defines a plurality of distinct fluid passageways separated by plates. A plate-fin heat exchanger facilitates indirect heat transfer between a first fluid flowing through a first group of fluid passageways and a second fluid flowing through a second group of fluid passageways. Heat is transferred between the first and second fluids via heat flux through the plates. Thus, plate-fin heat exchangers do not require the use of large containment vessels because the first and second fluids are contained in the fluid passageways during heat transfer. As used herein, the term “brazed aluminum plate-fin heat exchanger” denotes a plate-fin heat exchanger constructed of multiple aluminum plates brazed to one another. 
         [0023]    In a preferred embodiment of the present invention, the natural gas stream is delivered to the initial refrigeration cycle at an elevated pressure or is compressed to an elevated pressure, that being a pressure greater than about 500 psia, preferably about 500 to about 900 psia, still more preferably about 550 to about 675 psia, still yet more preferably about 575 to about 650 psia, and most preferably about 600 psia. The stream temperature is typically near ambient to slightly above ambient. A representative temperature range being about 60° F. to about 120° F. 
         [0024]    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 stages of the initial and/or intermediate refrigeration 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 in each stage of the initial and/or intermediate refrigeration cycles 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 first gas stream predominating in methane and a second 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 stages for the removal of liquids streams rich in C 2+  components. The exact locations and number of gas/liquid separation means will be dependent on a number of operating parameters, such as the C 2+  composition of the natural gas feed stream, the desired BTU content of the final 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 operation. The C 2  hydrocarbon stream or streams may be demethanized via a single stage flash or a fractionation column. In the former case, the methane-rich stream can be repressurized and recycled or can be used as fuel gas. In the latter case, the methane-rich stream can be directly returned at pressure to the liquefaction process. 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+ ) 
         [0025]    In the last cooling stage of the intermediate refrigeration cycle, the processed natural gas stream, which is predominantly methane (typically greater than 95 mole percent methane and more typically greater than 97 mole percent methane), is condensed (i.e., liquefied) in major portion, preferably in its entirety. The cooled and condensed natural gas stream exiting the intermediate refrigeration cycle is then further cooled in the final refrigeration cycle via indirect heat exchange with the final refrigerant. In a preferred embodiment of the present invention, the final refrigeration cycle is an open methane refrigeration cycle employing a predominantly-methane refrigerant that originates from the natural gas feed stream. 
         [0026]    The liquefied gas entering the final refrigeration cycle preferably has a pressure of at least about 250 psia, more preferably at least about 400 psia, and most preferably in the range of from 500 to 800 psia. It is preferred that the expansion section of the final refrigeration cycle is operable to reduce the pressure of the liquefied gas stream by at least about 100 psi, more preferably at least about 250 psi, and most preferably at least 400 psi. The pressure reduction in the expansion section of the final refrigeration cycle is preferably accomplished via a plurality of sequential expansion steps carried out in a plurality of expansion devices. Each expansion device can be a Joule-Thomson expansion valve or a hydraulic expander. As used herein, the term “hydraulic expander” is not limited to an expander which receives and produces liquid streams but is inclusive of expanders which receive a predominantly liquid-phase stream and produce a two-phase (gas/liquid) stream. When a hydraulic expander is employed and properly operated, the greater efficiencies associated with the recovery of power, a greater reduction in stream temperature, and the production of less vapor during the expansion step will frequently be cost-effective even in light of increased capital and operating costs associated with the expander. The pressure of the liquid product entering the final refrigeration cycle is preferably reduced to near atmospheric pressure so that the final LNG product has a near-atmospheric pressure and a temperature of −240° F. to −260° F. 
         [0027]    One embodiment of the present invention provides a final refrigeration cycle having a reduced number of process vessels compare to similar refrigeration cycles employing multi-step expansion cooling of the liquefied gas stream. In particular, in one embodiment of the present invention, the expansion section of the final refrigeration cycle employs less than three vapor/liquid separation vessels (e.g., flash drums), most preferably less than two vapor/liquid separation vessels.  FIG. 1  illustrates one configuration of a final refrigeration cycle that reduces the number of process vessels (e.g., flash drums) relative to similar conventional expansion-type refrigeration cycles. 
         [0028]    The flow schematic and apparatus set forth in  FIG. 1  represents a preferred embodiment of the invention employed in an open-cycle cascaded liquefaction process. Those skilled in the art will also recognized that  FIG. 1  is a schematic representation and, therefore, many items of equipment that would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items might include, for example, compressor controls, flow and level measurements and corresponding controllers, temperature and pressure controls, pumps, motors, filters, additional heat exchangers, valves, etc. These items would be provided in accordance with standard engineering practice. 
         [0029]    To facilitate an understanding of  FIG. 1 , items numbered  1  through  99  generally correspond to process vessels and equipment directly associated with the liquefaction process. Items numbered  100  through  199  correspond to flow lines or conduits which contain methane in major portion. Items numbered  200  through  299  correspond to flow lines or conduits which contain the refrigerant ethylene or optionally, ethane. Items numbered  300  through  399  correspond to flow lines or conduits which contain the refrigerant propane. 
         [0030]    Referring to  FIG. 1 , gaseous propane is compressed in multistage compressor  18  driven by a gas turbine driver which is not illustrated. The three stages of compression preferably exist in a single unit although each stage of compression may be a separate unit and the units mechanically coupled to be driven by a single driver. Upon compression, the compressed propane is passed through conduit  300  to cooler  16  where it is liquefied. A representative pressure and temperature of this liquefied propane stream exiting cooler  16  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  16  and upstream of the high-stage propane brazed aluminum plate-fin heat exchanger  2 , for the removal of residual light components from the liquefied propane and to provide surge control for the system. The refrigerant stream from this vessel or the stream from cooler  16 , as the case may be, is passed through conduit  302  to a high-stage propane brazed aluminum plate-fin heat exchanger  2 , wherein the stream flows through core passages  10  and is cooled by indirect heat exchange. The cooled or second propane refrigerant stream is produced via conduit  303 . This stream is then split via a splitting or separation means (illustrated but not numbered) into two portions, third and fourth propane refrigerant streams, and produced via conduits  304  and  307 . The third propane refrigerant stream flows via conduit  304  to a pressure reduction means, illustrated as expansion valve  14 , wherein the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion thereof and producing a high-stage refrigeration stream. This stream then flows through conduit  305  and through core passages  12 , wherein the stream flows countercurrent to the stream in passage  10  and the yet to be described streams in passages  4 ,  6 , and  8  and wherein indirect heat exchange occurs. The resulting stream, the high-stage propane recycle stream, is routed via conduit  306  to the high-stage inlet port of propane compressor  18 . In the course of such routing, the stream will generally pass through a suction scrubber. 
         [0031]    Also fed to plate-fin heat exchanger  2  are the natural gas stream via conduit  100 , a gaseous ethylene stream via conduit  202 , and a methane-rich stream via conduit  152 . These streams in flow passages  6 ,  8 , and  4  and the propane refrigerant stream in passage  10  flow countercurrent, more preferably counterflow, to the propane stream in passage  12 . Indirect heat exchange occurs between such streams. The streams respectively flowing in passages  4 ,  6 , and  8  are produced via conduits  154 ,  102 , and  204 . The stream in conduit  204  will be referred to as a first cooled ethylene stream. 
         [0032]    The cooled natural gas stream in conduit  102 , the first cooled ethylene stream in conduit  204 , and the fourth propane refrigerant stream in conduit  307  respectively flow through passages  22 ,  24 , and  25  in brazed aluminum plate-fin heat exchanger  20  countercurrent, more preferably counterflow, to a yet to be identified refrigeration stream thereby producing a further cooled natural gas stream, a second cooled ethylene stream, and a fifth propane refrigerant stream which are produced via conduits  110 ,  206 , and  308 . The fifth propane refrigerant stream is then split via a splitting or separation means (illustrated but not numbered) into two portions, the sixth and seventh propane refrigerant streams, and respectively produced via conduits  309  and  312 . The sixth propane refrigerant stream flows via conduit  309  to a pressure reduction means, illustrated as expansion valve  27 . In expansion valve  27 , the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion thereof and producing a intermediate-stage propane refrigeration stream. This stream then flows through conduit  310  and through core passage  26  wherein said stream flows countercurrent to the streams in passages  22 ,  24 , and  25  and wherein indirect heat exchange occurs. The resulting stream is produced as an intermediate-stage propane recycle stream via conduit  311 . This stream is returned to the intermediate-stage inlet port of propane compressor  18 , again preferably after passing through a suction scrubber. 
         [0033]    The further cooled natural gas stream and the second cooled ethylene stream are respectively routed via conduits  110  and  206  to respective passages  36  and  38  in brazed aluminum plate-fin heat exchanger  34  wherein the natural gas stream is yet further cooled. The natural gas and ethylene streams are produced from plate-fin heat exchanger  34  via conduits  112  and  208 , respectively. 
         [0034]    The seventh propane refrigerant stream in conduit  312  is connected to brazed aluminum plate-fin heat exchanger  28  wherein the stream flows via passage  29  countercurrent, more preferably counterflow, to and in indirect heat exchange with a low-stage propane refrigerant flowing via passage  30  thereby producing an eighth propane refrigerant stream via conduit  314 . The eighth propane refrigerant flows via conduit  314  to a pressure reduction means, illustrated as expansion valve  32 , wherein the pressure of the liquefied propane is reduced thereby evaporating or flashing a portion thereof and producing a two-phase refrigerant stream. The expanded refrigerant stream is carried to brazed aluminum plate-fin heat exchanger  34  where it is employed as a cooling agent in passage  37 . A low-stage propane refrigeration stream is removed from heat exchanger  34  via conduit  318 . This conduit is connected to passage  30  in heat exchanger  28  wherein said stream flows countercurrent and is in indirect heat exchange with the seventh propane refrigerant stream in passage  29  thereby producing a low-stage propane recycle stream. The low-stage propane recycle stream is then returned to the low-stage inlet port of compressor  18 , preferably after flow through a suction scrubber, via conduit  320 . In compressor  18 , the low-stage propane recycle stream is compressed, combined with the intermediate-stage propane recycle stream, and compressed to form a compressed intermediate-stage recycle stream. This stream is then combined with the high-stage propane recycle stream to form a combined high-stage propane recycle stream which is compressed to form the compressed propane refrigerant stream produced via conduit  300 . 
         [0035]    In one embodiment of the invention, the brazed aluminum plate-fin heat exchangers  2 ,  20 ,  28 , and  34  of the initial (propane) refrigeration cycle are separate heat exchangers. In another embodiment, the heat exchangers are combined into one or more exchangers. Although resulting in a more complex heat exchanger which possesses intermediate headers, combined approach can offer advantages from a lay-out and cost perspective. 
         [0036]    In the intermediate refrigeration cycle depicted in  FIG. 1 , the natural gas stream is cooled via indirect heat exchange with an ethylene refrigeration stream until it is substantially condensed. As illustrated in  FIG. 1 , a low-stage ethylene recycle stream delivered via conduit  232  is compressed in compressor  40  and the resulting compressed low-stage ethylene recycle stream is preferably removed from compressor  40  via conduit  234 , cooled via inter-stage cooler  71 , returned to the compressor via conduit  236  and combined with a high-stage ethylene recycle stream delivered via conduit  216  whereupon the combined stream is compressed to thereby producing a compressed ethylene refrigerant stream via conduit  200 . A preferred pressure for this compressed ethylene refrigerant stream is approximately  300  psia. Preferably, the two compressor stages of compressor  40  are a single module although they may each be a separate module and the modules mechanically coupled to a common driver. The compressed ethylene refrigerant stream is routed from the compressor  40  to the downstream cooler  72  via conduit  200 . The product from cooler  72  flows via conduit  202  and is introduced, as previously discussed, to the initial refrigeration cycle wherein the stream is further cooled and liquefied via heat exchange passages  8 ,  24 , and  38  and then returned to the intermediate refrigeration cycle via conduit  208 . This stream in conduit  208  preferably flows to a separation vessel  41  which provides for the removal of residual light components from the liquefied stream and which also provides surge volume for the refrigeration system. A refrigerant stream, referred to herein with regard to the intermediate refrigeration cycle as a first ethylene refrigerant stream, is produced from vessel  41  via conduit  209 . 
         [0037]    The cooled natural gas stream produced from the initial refrigeration cycle via conduit  112  is combined with a yet to be described methane-rich stream provided via conduit  156 . This combined stream in conduit  114  and the first refrigerant ethylene stream in conduit  209  are routed to a brazed aluminum plate fin-heat exchanger  42  wherein these streams flow through core passages  44  and  46  countercurrent, more preferably counterflow, to and in indirect heat exchange with a yet to be described high-stage ethylene refrigerant stream and optionally, a low-stage ethylene refrigerant stream respectively flowing in passages  48  and  50 . A cooled stream referred to herein as a second ethylene refrigerant stream is produced from passage  46  via conduit  210 . This stream is then split via a splitting or separation means (illustrated but not numbered) into two portions, third and fourth ethylene refrigerant streams, and produced via conduits  212  and  218 . The third ethylene refrigerant stream flows via conduit  212  to a pressure reduction means, illustrated as expansion valve  52 , wherein the pressure of the liquefied ethylene is reduced thereby evaporating or flashing a portion thereof and producing a high-stage ethylene refrigeration stream. This stream then flows through conduit  214  and through core passage  48  thereby producing a high-stage ethylene recycle stream which is transported via conduit  216  to the high-stage inlet port of compressor  40 . 
         [0038]    A further cooled natural gas stream is produced from passage  44  via conduit  116  and is optionally combined with a methane-rich recycle stream delivered via conduit  158 . The resulting stream is routed via conduit  120  to a passage  59  of a brazed aluminum plate-fin heat exchanger  58  wherein the stream is cooled and liquefied in major portion and the resulting stream is produced via conduit  122 . 
         [0039]    The fourth ethylene refrigerant stream is transported via conduit  218  to a passage  54  in a brazed aluminum plate-fin heat exchanger  53 . The fourth ethylene refrigerant stream flows countercurrent, more preferably counterflow, to and is in indirect heat exchange with a low-stage ethylene refrigerant flowing via passage  55  in heat exchanger  53 , thereby producing a fifth ethylene refrigerant stream via conduit  220 . The fifth ethylene refrigerant stream flows via conduit  220  to a pressure reduction means, illustrated as expansion valve  56 , wherein the pressure of the liquefied ethylene is reduced, thereby evaporating or flashing a portion thereof and producing a two-phase ethylene refrigerant stream. The resulting two-phase ethylene refrigerant stream is carried via conduit  226  to heat exchanger  58  wherein the stream is employed as a cooling agent in passage  57 . A low-stage ethylene refrigeration stream is removed from heat exchanger  58  via conduit  228 . Conduit  228  is connected to passage  55  in heat exchanger  53  wherein said stream flows countercurrent and is in indirect heat exchange with the fluid in passage  54  thereby producing a low-stage ethylene recycle stream. This stream is returned to the low-stage inlet port of compressor  40  via conduit  232 . Optionally, and as depicted in  FIG. 1 , this stream may also flow to plate-fin heat exchanger  42  via conduit  230  and through passage  50  wherein said stream flows countercurrent, more preferably counterflow, to the fluids in passages  44  and  46  and is further warmed prior to flow to the compressor  40  via conduit  232 . 
         [0040]    In one embodiment of the invention, brazed aluminum plate-fin heat exchangers  42 ,  53 , and  58  of the intermediate refrigeration cycle are separate heat exchangers. In another embodiment, the heat exchangers are combined into a single exchanger. 
         [0041]    The liquefied natural gas stream produced from plate-fin heat exchanger  58  via conduit  122  is generally at a temperature of about −125° F. and a pressure of about 600 psi. The liquefied stream in conduit  122  is introduced into the final refrigeration cycle where it undergoes cooling by indirect heat exchange with a methane refrigerant and by expansion. The stream in conduit  122  is initially cooled in a main methane economizer  74  via indirect heat exchange with methane refrigerant streams in passages  82 ,  95 , and  96 . In a preferred embodiment of the present invention, the methane refrigerant employed in the final refrigeration cycle is derived from the processed natural gas stream, thereby making the final refrigeration cycle an open methane refrigeration cycle. Main methane economizer  74  is preferably a plate-fin heat exchanger, most preferably a brazed aluminum plate-fin heat exchanger. The liquefied natural gas stream introduced into main methane economizer  74  via conduit  122  is cooled in passage  76  and then exits main methane economizer  74  via conduit  124 . The cooled stream in conduit  124  is subsequently divided into a first refrigerant portion carried in conduit  125  and a first product portion carried in conduit  126 . The first refrigerant portion in conduit  125  is transported to an expansion means (illustrated as expansion valve  78 ), wherein the stream is reduced in pressure to thereby produce a first expanded refrigerant portion in conduit  127 . The first expanded refrigerant portion in conduit  127  is then introduced into passage  82  of main methane economizer  74  wherein it is employed as a refrigerant to cool the natural gas stream in passage  76 . The warmed first refrigerant stream exits passage  82  and methane economizer  74  via conduit  128 , and is introduced into the high-stage inlet port of methane compressor  83 . 
         [0042]    The first product portion in conduit  126  is carried to a second methane economizer  87  for further cooling. The second methane economizer  87  is preferably a plate-fin heat exchanger, most preferably a brazed aluminum plate-fin heat exchanger. In second methane economizer  87 , the first product portion is cooled as it passes through passage  88  and indirectly exchanges heat with the refrigerant streams passing through passages  89  and  90 , described in more detail below. A second cooled natural gas stream is produced from second methane economizer  87  via conduit  129 . The second cooled natural gas stream in conduit  129  is subsequently divided into a second refrigerant portion carried in conduit  130  and a second product portion carried in conduit  131 . The second refrigerant portion carried in conduit  130  is subsequently expanded in expansion valve  91  to thereby produce a second expanded refrigerant portion. The second product portion in conduit  131  is subsequently expanded in expansion valve  92  to thereby produce a two-phase second stream that is subsequently carried to a phase separator  93  via conduit  132 . The second expanded refrigerant portion expander  91  is transported via conduit  133  to second methane economizer  87  wherein the second expanded refrigeration portion is employed as a refrigerant in passage  89  to cool the stream flowing in passage  88 . After being employed as a cooling agent in passage  89 , the warmed second refrigerant portion is removed from second methane economizer  87  via conduit  134  and subsequently introduced into a passage  95  of main methane economizer  74  wherein the warmed second refrigerant portion is used to cool the stream in passage  76 . The further warmed second refrigerant portion exits methane economizer  74  via conduit  135  and is subsequently introduced into the intermediate-stage inlet port of methane compressor  83 . 
         [0043]    The two-phase in conduit  132  is separated in vapor/liquid separator  93  to thereby produce a gaseous third refrigerant portion via conduit  136  and a liquid third product portion via conduit  142 . The gaseous third refrigerant stream in conduit  136  is combined with the compressed stream in conduit  138 , described in further detail below. The resulting combined stream flows via conduit  139  to second methane economizer  87  wherein the combined stream is employed as a refrigerant in passage  90  to cool the stream in passage  88 . The warmed third portion exits passage  90  of second methane economizer  87  via conduit  140  and is carried to passage  96  of main methane economizer  74  wherein the refrigerant stream is used to cool the stream in passage  76 . The further warmed third refrigerant portion exits passage  96  and main methane economizer  74  via conduit  141  and is passed to the low-stage inlet port of methane compressor  83 . 
         [0044]    The liquid third product portion that exits separator  93  via conduit  142  is expanded in expansion valve  94  to thereby produce a two-phase expanded third product stream which is carried to LNG storage tank  99  via conduit  143 . The vapor portion of the stream introduced in to LNG storage tank  99  and any boil-off vapors generated in tank  99  are removed from tank  99  via conduit  144 . This vapor stream in conduit  144  is compressed in compressor  96  to produce the compressed gas stream in conduit  138  that is subsequently combined with the separated vapor stream in conduit  136  before being employed as a refrigerant in the second methane economizer  87  and the main methane economizer  74 . The LNG in tank  99  can be stored and subsequently transported to a distant market where it is gasified for use as an energy source. 
         [0045]    As shown in  FIG. 1 , the three stages of compression provided by methane compressor  83  are preferably contained in a single unit. However, each compression stage may exist as a separate unit where the units are mechanically coupled together to be driven by a single driver. The compressed gas from the low-stage section of compressor  83  preferably passes through an inter-stage cooler  85  and is combined with the intermediate pressure gas in conduit  140  prior to the second-stage of compression. The compressed gas from the intermediate stage of compressor  83  is preferably passed through an inter-stage cooler  84  and is combined with the high pressure gas in conduit  140  prior to the third stage of compression. The compressed gas is discharged from the high-stage methane compressor through conduit  150 , is cooled in cooler  86  and is routed to the high-stage propane chiller  2  via conduit  152 . The methane-rich stream exiting chiller  2  via conduit  154  is fed to main methane economizer  74  wherein the stream is cooled via indirect heat exchange with one or more of the streams in passages  82 ,  94 , and/or  96 . In one embodiment and as illustrated in  FIG. 1 , the stream delivered via conduit  154  is cooled in the main methane economizer  74  via indirect heat exchange means  97 , a portion removed via conduit  156  and the remaining stream further cooled via indirect heat exchange means  98  and produced via conduit  158 . This is a preferred embodiment. In this split stream embodiment, a portion of the compressed methane recycle stream delivered via conduit  156  is combined with the natural gas stream via conduit  112  immediately upstream of the intermediate refrigeration cycle and the remaining portion delivered via conduit  158  combined with the stream in conduit  116  immediately upstream of brazed aluminum plate-fin heat exchanger  58  wherein the majority of liquefaction of the natural gas stream occurs. In a simpler embodiment (i.e., less preferred from a process efficiency perspective), the methane recycle stream is cooled in its entirety in the main methane economizer  74  and combined via conduit  158  with the natural gas stream in conduit  112  immediately upstream of the second cycle. 
         [0046]    With regard to the compressor/driver units employed in the process,  FIG. 1  depicts individual compressor/driver units (i.e., a single compression train) for the propane, ethylene and open-cycle methane compression stages. However in a preferred embodiment for any cascaded process, process reliability can be improved significantly by employing a multiple compression train comprising two or more compressor/driver combinations in parallel in lieu of the depicted single compressor/driver units. In the event that a compressor/driver unit becomes unavailable, the process can still be operated at a reduced capacity. 
         [0047]    In one embodiment of the present invention, the LNG production system of  FIG. 1  is simulated on a computer using conventional process simulation software. Examples of suitable simulation software include HYSYS™ from Hyprotech, Aspen Plus® from Aspen Technology Inc., and PRO\II® from Simulation Sciences Inc. 
         [0048]    While specific cryogenic methods, materials, items of equipment and control instruments are referred to herein, it is to be understood that such specific recitals are not to be considered limiting but are included by way of illustration and to set forth the best mode in accordance with the present invention.