Patent Publication Number: US-7591149-B2

Title: LNG system with enhanced refrigeration efficiency

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for liquefying natural gas. In particular, the invention concerns an improved cascade-type liquefied natural gas (LNG) facility employing additional refrigeration levels in the heat exchanging economizers of one or both of the ethylene and methane refrigeration cycles, thereby enhancing thermodynamic efficiencies without significant additional capital cost. 
     2. Description of the Prior Art 
     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. 
     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. 
     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. 
     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). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream. 
     Currently, the methane refrigeration systems of cascade-type LNG processes are designed so that the natural gas feed stream leaves the ethylene cooling system as a subcooled liquid and enters the methane system for further subcooling. The feed stream is subcooled by the vapors generated from lower-stage flashes and is then expanded into a high-pressure flash drum. The stream changes from a subcooled liquid stream to a liquid/vapor mixture at lower pressure. The vapor phase is returned through the methane economizer where it extracts heat from the predominantly methane feed and is ultimately directed to the methane compressor for recompression. The liquid fraction leaves the high-stage flash drum and enters another economizer stage where it transfers heat to lower-stage flash vapors. The stream is then expanded via an expansion valve into an intermediate-stage flash drum where the stream changes from a subcooled liquid to a vapor/liquid mixture which is separated in the intermediate-stage flash drum. The vapor leaves the intermediate-stage flash drum and is directed through the economizers wherein it extracts heat from the processed natural gas feed and is ultimately recompressed in the methane compressor. The liquid leaving the intermediate-stage flash drum is then expanded and introduced into a low-stage flash drum. The vapor leaves the low-stage flash drum and passes through the economizers, where it extracts heat from the processed natural feed and is then recompressed. The liquid leaves the low-stage flash drum to LNG storage. 
     In the ethylene refrigeration systems of conventional LNG processes, the ethylene refrigerant is condensed in the propane refrigeration system. Thereafter, the ethylene is subcooled in the ethylene economizer and is then expanded into the high-stage ethylene chiller where it is used to cool the natural gas feed. 
     Designers of LNG processes are constantly seeking ways to improve the thermodynamic efficiencies of the systems. While in theory this can be accomplished by providing additional refrigeration capacity, there is a point of diminishing returns where the capital costs associated with the added capacity are greater than the return. Therefore, the goal is to have maximum thermodynamic efficiencies coupled with the lowest possible costs. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a novel natural gas liquefaction system that is characterized by high thermodynamic efficiency with only minimal additional capital cost. 
     A further object of the invention is to provide an LNG process which derives increased thermodynamic efficiency in both the ethylene and methane cooling systems thereof, by the addition of extra cooling stages within the system economizers. 
     Another object of the invention is to provide such cooling system efficiencies with only a minimum of additional equipment, as compared with present-day ethylene and methane cooling systems. 
     It should be understood that the above objects are exemplary and need not all be accomplished by the invention claimed herein. Other objects and advantages of the invention will be apparent from the written description and drawings. 
     Accordingly, one aspect of the present invention concerns a process for liquefying a predominantly methane stream comprising the following steps: (a) generating at least four distinct refrigerant streams from a common first refrigerant stream; and (b) cooling at least a portion of the predominantly methane stream via indirect heat exchange with at least a portion of each of the distinct refrigerant streams. 
     Another aspect of the present invention concerns a process of liquefying a natural gas stream comprising the following steps: (a) cooling at least a portion of the natural gas stream via indirect heat exchange with a refrigerant in a first heat exchanging chiller; (b) generating at least three distinct refrigerant streams from the refrigerant employed in the first heat exchanging chiller; and (c) cooling at least a portion of the natural gas via indirect heat exchange with at least a portion of each of the distinct refrigerant streams in a heat exchanging economizer different than the first heat exchanging chiller. 
     A further aspect of the present invention concerns an apparatus for liquefying a predominantly methane stream. The apparatus includes first and second mechanical refrigeration cycles. The first mechanical refrigeration cycle employs a first refrigerant to cool at least a portion of the predominantly methane stream. The second mechanical refrigeration cycle employs a second refrigerant to cool at least a portion of the predominantly methane stream downstream of the first refrigeration cycle. At least one of the first and second mechanical refrigeration cycles includes a heat exchanging economizer defining at least one cooling pass through which at least a portion of the predominately methane stream flows and at least four warming passes though which at least four distinct refrigerant streams flow. The heat exchanging economizer facilitates indirect heat exchange between the predominately methane stream and each of said four distinct refrigerant streams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURE 
       A preferred embodiment of the present invention is described below with reference to the attached drawing FIGURE wherein: 
         FIG. 1  is a simplified flow diagram of a cascaded refrigeration process for LNG production which employs enhanced refrigeration apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A cascaded 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 cascaded refrigeration process involves a 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, but obtaining such small temperature gradients generally requires significant increases in the amount of heat transfer area, major modifications to various process equipment, and the proper selection of flow rates through such equipment so as to ensure that both flow rates and approach and outlet temperatures are compatible with the required heating/cooling duty. 
     As used herein, the term “open-cycle cascaded refrigeration process” refers to a cascaded refrigeration process comprising at least one closed mechanical refrigeration cycle and one open mechanical refrigeration cycle where the boiling point of the refrigerant/cooling agent employed in the open cycle is less than the boiling point of the refrigerating agent or agents employed in the closed cycle(s) and a portion of the cooling duty to condense the compressed open-cycle refrigerant/cooling agent is provided by one or more of the closed cycles. In the current invention, a predominately methane stream is employed as the refrigerant/cooling agent in the open cycle. This predominantly methane stream originates from the processed natural gas feed stream and can include the compressed open methane cycle gas streams. As used herein, the terms “predominantly,” “primarily,” “principally,” and “in major portion,” when used to describe the presence of a particular component of a fluid stream, shall mean that the fluid stream comprises at least 50 mole percent of the stated component. For example, a “predominantly” 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 comprising at least 50 mole percent methane. 
     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 multi-stage propane cycle, a multi-stage ethane or ethylene cycle, and an open-end methane cycle which utilizes a portion of the feed gas as a source of methane and which includes therein a multi-stage expansion cycle to further cool the same and reduce the pressure to near-atmospheric pressure. 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. 
     Various pretreatment steps provide a means for removing certain undesirable components (e.g., acid gases, mercaptan, mercury, and moisture) from the natural gas feed stream delivered to the LNG facility. The composition of this 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 mole percent methane, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide, and a minor amount 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-inclusive listing of some of the available means which are readily known to one skilled in the art. Acid gases and to a lesser extent mercaptan are routinely removed via a chemical reaction process employing an aqueous amine-bearing solution. This treatment step is generally performed upstream of the cooling stages in the initial cycle. 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. Mercury is routinely removed via mercury sorbent beds. Residual amounts of water and acid gases are routinely removed via the use of properly selected sorbent beds such as regenerable molecular sieves. 
     The pretreated natural gas feed stream is generally delivered to the liquefaction process at an elevated pressure or is compressed to an elevated pressure generally greater than 500 psia, preferably about 500 psia to about 3000 psia, still more preferably about 500 psia to about 1000 psia, still yet more preferably about 600 psia to about 800 psia. The feed stream temperature is typically near ambient to slightly above ambient. A representative temperature range being 60° F. to 150° F. 
     As previously noted, the natural gas feed stream is cooled in a plurality of multistage refrigeration cycles or steps (preferably three) by indirect heat exchange with a plurality of different refrigerants (preferably three). Preferably, each of the refrigerants associated with each refrigeration cycle is a single component refrigerant (i.e., not a mixed refrigerant). 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 feed gas is preferably passed through an effective number of refrigeration stages (nominally two, preferably two to four, and more preferably three stages) in the first closed mechanical refrigeration cycle utilizing a relatively high boiling refrigerant. Such relatively high boiling point 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. Thereafter, the processed feed gas flows through an effective number of stages (nominally three, preferably three to six, and more preferably three to five) in a second closed mechanical refrigeration cycle in heat exchange with a refrigerant having a lower boiling point. Such lower boiling point 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. Each cooling stage comprises a separate cooling zone. As previously noted, the processed natural gas feed stream is preferably combined with one or more recycle streams (i.e., compressed open methane cycle gas streams) at various locations in the second refrigeration 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 feed gas to the first stage of the first mechanical refrigeration cycle. 
     After being processed in the second mechanical refrigeration cycle, the pressurized LNG-bearing stream is then further cooled in a third refrigeration cycle referred to as the open methane cycle via contact in a methane economizer with flash gases generated in this third cycle in a manner to be described later and via sequential expansion of the pressurized LNG-bearing stream to near atmospheric pressure. The flash gasses used as a refrigerant in the third refrigeration cycle are preferably comprised in major portion of methane, more preferably the flash gas refrigerant comprises at least 75 mole percent methane, still more preferably at least 90 mole percent methane, and most preferably the refrigerant consists essentially of methane. During expansion of the pressurized LNG-bearing stream to near atmospheric pressure, the pressurized LNG-bearing stream is preferably cooled via at least three sequential expansions where each expansion employs an expansion device as a pressure reduction means. Suitable expansion devices include, for example, either Joule-Thomson expansion valves or hydraulic expanders. The expansion is followed by a separation of the gas-liquid product with a separator. 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 flash expansion step will frequently more than off-set the higher capital and operating costs associated with the expander. In one embodiment, additional cooling of the pressurized LNG-bearing stream prior to flashing is made possible by first flashing a portion of this stream via one or more hydraulic expanders and then via indirect heat exchange means employing the flash gas stream to cool the remaining portion of the pressurized LNG-bearing stream prior to flashing. The warmed flash gas stream is then recycled via return to an appropriate location, based on temperature and pressure considerations, in the open methane cycle and will be recompressed. 
     The liquefaction process described herein may use one of several types of cooling which include but are 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 undergone 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. Certain portions of the present disclosure describe indirect heat exchange that is carried out in heat exchanging “chillers” and heat exchanging “economizers.” Such chillers and economizers can have any configuration that facilitates indirect heat exchange between fluids passed therethrough. In a preferred embodiment of the present invention, the chillers have a core-in-kettle configuration while the economizers have a plate-fin configuration. 
     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. 
     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-reducing expansion device. In one embodiment, this expansion device is a Joule-Thomson expansion valve. In another embodiment, the expansion device is either a hydraulic or gas expander. Because expanders recover work energy from the expansion process, lower process stream temperatures are possible upon expansion. 
     The flow schematic and apparatus set forth in  FIG. 1  represents a preferred embodiment of the inventive LNG facility employing enhanced ethylene and methane refrigeration cycles. Those skilled in the art will recognized that  FIG. 1  is schematic only 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, and valves, etc. These items would be provided in accordance with standard engineering practice. 
     To facilitate an understanding of  FIG. 1 , the following numbering nomenclature was employed. Items numbered  1  through  99  are process vessels and equipment which are directly associated with the liquefaction process. Items numbered  100  through  199  correspond to flow lines or conduits which contain predominantly methane streams. Items numbered  200  through  299  correspond to flow lines or conduits which contain predominantly ethylene streams. Items numbered  300  through  399  correspond to flow lines or conduits which contain predominantly propane streams. 
     Referring to  FIG. 1 , gaseous propane is compressed in a multistage (preferably three-stage) compressor  18  driven by a gas turbine driver (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 a cooler  20  where it is cooled and liquefied. A representative pressure and temperature of the liquefied propane refrigerant prior to flashing is about 100° F. and about 190 psia. The stream from cooler  20  is passed through conduit  302  to a pressure reduction means, illustrated as 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 a high-stage propane chiller  2  wherein 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 . The gas in conduit  154  is fed to a main methane economizer  74  which will be discussed in greater detail in a subsequent section and wherein the stream is cooled via indirect heat exchange means/pass  98 . 
     The 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 an intermediate-stage propane chiller  22  through conduit  310 , thereby providing a coolant for chiller  22 . The cooled feed gas stream from chiller  2  flows via conduit  102  to separation equipment  10  wherein gas and liquid phases are separated. The liquid phase, which can be rich in C 3 +components, is removed via conduit  103 . The gas phase is removed via conduit  104  and then split into two separate streams which are conveyed via conduits  106  and  108 . The stream in conduit  106  is fed to propane chiller  22 , while the stream in conduit  108  becomes the feed to heat exchanger  62  and ultimately becomes the stripping gas to heavies removal column  60 , discussed in more detail below. Ethylene refrigerant from chiller  2  is introduced to chiller  22  via conduit  204 . In chiller  22 , the feed gas stream, also referred to herein as a methane-rich stream, and the ethylene refrigerant streams are respectively cooled via indirect heat transfer means  24  and  26 , thereby producing cooled methane-rich and ethylene refrigerant streams via conduits  110  and  206 . The thus evaporated portion of the propane refrigerant in chiller  22  is separated and passed through conduit  311  to the intermediate-stage inlet of compressor  18 . Liquid propane refrigerant from chiller  22  is removed via conduit  314 , flashed across a pressure reduction means, illustrated as expansion valve  16 , and then fed to a low-stage propane chiller/condenser  28  via conduit  316 . 
     As illustrated in  FIG. 1 , the methane-rich stream flows from intermediate-stage propane chiller  22  to the low-stage propane chiller  28  via conduit  110 . In chiller  28 , 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 low-stage propane chiller  28  via conduit  206 . In the latter, the ethylene refrigerant is totally condensed or condensed in nearly its entirety via indirect heat exchange means  32 . The vaporized propane is removed from low-stage propane chiller  28  and returned to the low-stage inlet of compressor  18  via conduit  320 . 
     The methane-rich stream exiting low-stage propane chiller  28  in conduit  112  enters the ethylene refrigeration cycle where it first passes through a feed cooling pass/exchanger  41  of an ethylene economizer  34  for cooling via indirect heat exchange with four distinct ethylene refrigerant vapor streams flowing through warming passes  39 ,  46 ,  57 , and  58  of ethylene economizer  34 . As discussed in greater detail below, the ethylene refrigeration cycle employs a unique system for generating the streams processed in warming passes  39 ,  46 ,  57 , and  58  from the ethylene refrigerant originating from conduit  210 . The initially cooled methane-rich stream then exits ethylene economizer  34  via line  113  and is introduced into a high-stage ethylene chiller  42 . 
     Ethylene refrigerant exits low-stage propane chiller  28  via conduit  208  and is preferably fed to a separation vessel  37  wherein light components are removed via conduit  209  and condensed ethylene is removed via conduit  210 . 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 then flows through line  210  to ethylene economizer  34  wherein it is cooled in a refrigerant cooling pass/exchanger  38  via indirect heat exchange with the distinct refrigerant streams in warming passes  39 ,  46 ,  57 , and  58 . The resulting cooled ethylene refrigerant stream exits ethylene economizer  34  via conduit  211 . The refrigerant stream in conduit  211  is split into two fractions, respectively passing through lines  211   a  and  211   b . The fraction passing through line  211   a  is directed to a pressure reduction means, illustrated as an expansion valve  40 , whereupon the refrigerant is flashed to a preselected temperature and pressure and fed to high-stage ethylene chiller  42  via conduit  212 . The remaining fraction is routed via line  211   b  to a pressure reduction means, here illustrated as expansion valve  40   a , where it is flashed to generate a two-phase stream that is thereafter conducted to warming pass  39  of ethylene economizer  34 , where it is employed as a coolant. Preferably, the two-phase stream in conduit  211   b  contains at least about 10 mole percent liquid. In warming pass  39 , such liquid boils so that the stream exiting pass  39  is substantially all vapor. The output from warming pass  39  is conveyed via conduit  211   c  to a first stage of ethylene compressor  48 . 
     In high-stage ethylene chiller  42 , the methane-rich stream flowing through indirect heat exchange means  44  is cooled by the ethylene refrigerant entering via conduit  212 . Vaporized ethylene is removed from chiller  42  via conduit  214  and routed to ethylene economizer  34  wherein the vapor functions as a coolant in warming pass  46 . The ethylene vapor is then removed from ethylene economizer  34  via conduit  216  and fed to a second stage of ethylene compressor  48 . The ethylene refrigerant which is not vaporized in high-stage ethylene chiller  42  is removed via conduit  218  and returned to ethylene economizer  34  for further cooling in refrigerant cooling pass  50 , and is removed from ethylene economizer  34  via conduit  220 . This stream is then split into two fractions. One fraction is conveyed via line  222   a  and is flashed in a pressure reduction means, illustrated as expansion valve  52 , whereupon the resulting two-phase product is introduced into a low-stage ethylene chiller  54 . The other fraction is conveyed in conduit  222   b , and is flashed in a pressure reduction means, illustrated as expansion valve  52   a , whereupon the resulting two-phase product is introduced into warming pass  57  of ethylene economizer  34  for use as a coolant. Preferably, the stream in conduit  222   b  contains at least about 10 mole percent liquid. In warming pass  57 , such liquid boils so that the stream exiting pass  57  is substantially all vapor. The output from warming pass  57  is directed via conduit  222   c  to a third stage of ethylene compressor  48 . 
     After cooling in indirect heat exchange means  44 , the methane-rich stream is removed from high-stage ethylene chiller  42  via conduit  116 , and passes through feed cooling pass  41   a  of ethylene economizer  34 . The cooled stream then exits economizer  34  via line  116   a  and is introduced into heat exchange means  56  of low-stage ethylene chiller  54  for cooling via indirect heat exchange with the ethylene refrigerant entering via conduit  222   a . A two-phase stream is produced from heat exchange means  56  of chiller  54  and flows via conduit  118  to a heavies removal column  60 . As previously noted, the methane-rich stream in line  104  was split so as to flow via conduits  106  and  108 . The contents of conduit  108 , which is referred to herein as the stripping gas, is first fed to heat exchanger  62  wherein this stream is cooled via indirect heat exchange means  66  thereby becoming a cooled stripping gas stream which then flows via conduit  109  to heavies removal column  60 . A heavies-rich liquid stream containing a significant concentration of C 4 +hydrocarbons, such as benzene, cyclohexane, other aromatics, and/or heavier hydrocarbon components, is removed from heavies removal column  60  via conduit  114 , preferably flashed via a flow control means  97 , preferably a control valve which can also function as a pressure reduction, and transported to heat exchanger  62  via conduit  117 . Preferably, the stream flashed via flow control means  97  is flashed to a pressure about or greater than the pressure at the high stage inlet port to methane compressor  83 . Flashing also imparts greater cooling capacity to the stream. In heat exchanger  62 , the stream delivered by conduit  117  provides cooling capabilities via indirect heat exchange means  64  and exits heat exchanger  62  via conduit  119 . In heavies removal column  60 , the two-phase stream introduced via conduit  118  is contacted with the cooled stripping gas stream introduced via conduit  109  in a countercurrent manner thereby producing a heavies-depleted vapor stream via conduit  120  and a heavies-rich liquid stream via conduit  114 . 
     The heavies-rich stream in conduit  119  is subsequently separated into liquid and vapor portions or preferably is flashed or fractionated in vessel  67 . In either case, a heavies-rich liquid stream is produced via conduit  123  and a second methane-rich vapor stream is produced via conduit  121 . In the preferred embodiment, the stream in conduit  121  is fed to the high-stage inlet port of the methane compressor  83 . 
     As previously noted, the gas exiting high-stage propane chiller  2  via conduit  154  is fed to main methane economizer  74  wherein the stream is cooled via indirect heat exchange means  98 . The resulting cooled compressed methane recycle or refrigerant stream in conduit  158  is combined in the preferred embodiment with the heavies-depleted vapor stream from heavies removal column  60 , delivered via conduit  120 , and fed to a low-stage ethylene chiller/condenser  68 . In ethylene condenser  68 , this stream is cooled and condensed via indirect heat exchange means  70  with a fraction of the expanded ethylene refrigerant which is routed to ethylene chiller  68  via conduit  226 . The condensed methane-rich product from condenser  68  is delivered via conduit  122 . The vaporized ethylene from ethylene chiller  54 , withdrawn via conduit  224 , and the vaporized ethylene from ethylene condenser  68 , withdrawn via conduit  228 , are combined and routed, via conduit  230 , to ethylene economizer  34  wherein the vapors function as a coolant in warming pass  58 . The stream produced from pass  58  is then routed via conduit  232  from ethylene economizer  34  to a fourth stage of ethylene compressor  48 . The compressed ethylene product from compressor  48  is routed to a downstream cooler  72  via conduit  200 . The product from cooler  72  flows via conduit  202  and is introduced, as previously discussed, to high-stage propane chiller  2 . 
     The pressurized LNG-bearing stream exiting the ethylene refrigeration cycle via conduit  122 , preferably a subcooled liquid stream in its entirety, is typically at a temperature in the range of from about −180 to about −75° F., more preferably in the range of from about −150 to about −100° F., most preferably in the range of from −135 to −115° F. The pressure of the stream in conduit  122  is preferably in the range of from about 500 to about 700 psia, most preferably in the range of from 550 to 725 psia. 
     The stream in conduit  122  is introduced into the methane refrigeration cycle where it is first directed to a main methane economizer  74 . In methane economizer  74 , the stream passes through feed cooling exchanger/pass  76  wherein it is cooled via indirect exchange with distinct refrigerant streams in warming passes  82 ,  82   a ,  95 ,  96 , and  96   a . As discussed in further detail below, the methane refrigeration cycle includes a unique system for generating the distinct refrigerant streams that flow through passes  82 ,  82   a ,  95 ,  96 , and  96   a  of main methane economizer  74 , as well as passes  89 ,  89   a , and  90  of secondary methane economizer  87 . The cooled stream from pass  76  exits methane economizer  74  via conduit  124 . It is preferred for the temperature of the stream in conduit  124  to be at least about 10° F. less than the temperature of the stream in conduit  122 , more preferably at least about 20° F. less than the temperature of the stream in conduit  122 . Most preferably, the temperature of the stream in conduit  124  is in the range of from about −175 to about −130° F. 
     The stream in conduit  124  is thereafter separated into two fractions. The first fraction is routed via line  124   a  through a pressure reduction means, here shown as an expansion valve  78   a , to produce a two-phase stream that is then routed through a warming pass/exchanger  82   a  of main methane economizer  74  to assist in the cooling of the stream in pass  76 . Preferably, the stream entering warming pass  82   a  contains at least about 10 mole percent liquid. As the stream flows through pass  82   a  such liquid boils so that the stream exiting pass  82   a  is substantially all vapor. The warmed vapor from pass  82   a  is conveyed via conduit  124   b  to a first stage of methane compressor  83 . 
     The second fraction derived from conduit  124  is directed to a pressure reduction means, illustrated as expansion valve  78 , which evaporates or flashes a portion of the liquid stream thereby generating a two-phase stream. The two-phase stream from expansion valve  78  is then passed to high-stage methane flash drum  80  where it is separated into a flash gas stream discharged through conduit  126  and a liquid phase stream discharged through conduit  130 . The flash gas stream from flash drum  80  is then transferred to main methane economizer  74  via conduit  126  wherein the stream functions as a coolant in warming pass  82  and aids in the cooling of the stream in cooling pass  76 . The warmed refrigerant stream exits heat exchanger pass  82  and methane economizer  74  via conduit  128 , and is directed to a second stage of methane compressor  83 . The liquid-phase stream exiting high-stage flash drum  80  via conduit  130  is passed through a secondary methane economizer  87  wherein the liquid is further cooled by downstream flash vapors in cooling pass  88 . The cooled liquid exits secondary methane economizer  87  via conduit  132  and is split into two portions. One portion is passed via line  132   b  through a pressure reduction means, here illustrated as expansion valve  91   a , to produce a two-phase stream (containing &gt;10 mole percent liquid) that is thereafter directed through a warming pass  89   a  of secondary methane economizer  87  where it assists in cooling the predominantly methane stream in cooling pass  88 , thereby causing the liquid phase of the stream in pass  89   a  to boil. The output from pass  89   a  is conveyed through line  133  to warming pass  96   a  of main methane economizer  74  where it is employed to cool the stream in cooling pass  76 . The vapor output from warming pass  96   a  is routed via line  133   a  to a third stage of methane compressor  83 . 
     The second portion derived from conduit  132  is conveyed via conduit  132   a  and is expanded or flashed via pressure reduction means, illustrated as expansion valve  91 , to further reduce the pressure and, at the same time, vaporize a portion thereof. The resulting two-phase stream is passed to an intermediate-stage methane flash drum  92  where the stream is separated into a gas phase passing through conduit  136  and a liquid phase passing through conduit  134 . The gas phase flows through conduit  136  to warming pass  89  of secondary methane economizer  87  wherein the vapor helps cool the stream in pass  88 . Conduit  138  serves as a flow conduit between warming pass  89  in secondary methane economizer  87  and warming pass  95  in main methane economizer  74 . The refrigerant stream in warming pass  95  helps cool the predominately methane stream in pass  76 . The warmed vapor stream from warming pass  95  exits main methane economizer  74  via conduit  140  and is directed to a fourth stage of methane compressor  83 . 
     The liquid phase exiting intermediate-stage flash drum  92  via conduit  134  is further reduced in pressure by passage through a pressure reduction means, illustrated as a expansion valve  93 . The two-phase stream from expansion valve  93  is passed to a final or low-stage flash drum  94 . In flash drum  94 , a vapor phase is separated and passed through conduit  144  to secondary methane economizer  87  wherein the vapor functions as a coolant via warming pass  90 , exits secondary methane economizer  87  via conduit  146 , which is connected to the first methane economizer  74  wherein the vapor functions as a coolant in warming pass  96 . The warmed vapor stream from pass  96  exits main methane economizer  74  via conduit  148  and is directed to a fifth stage of methane compressor  83 . 
     Compressed methane gas is discharged from methane compressor through conduit  150 , is cooled in cooler  86 , and is routed to the high pressure propane chiller  2  via conduit  152  as previously discussed. The stream is cooled in chiller  2  via indirect heat exchange means  4  and flows to main methane economizer  74  via conduit  154 . The compressed open methane cycle gas stream from chiller  2  which enters the main methane economizer  74  undergoes cooling in its entirety via flow through indirect heat exchange means  98 . This cooled stream is then removed via conduit  158  and combined with the processed natural gas feed stream upstream of the first stage of ethylene cooling. 
     The liquefied natural gas product from low-stage flash drum  94 , which is at approximately atmospheric pressure, is passed through conduit  142  to an LNG storage tank  99 . In accordance with conventional practice, the liquefied natural gas in storage tank  99  can be transported to a desired location (typically via an ocean-going LNG tanker). The LNG can then be vaporized at an onshore LNG terminal for transport in the gaseous state via conventional natural gas pipelines. 
     In one embodiment of the present invention, main methane economizer  74  and secondary methane economizer  87  are combined into a single unit (e.g., a single plate-fin heat exchanger with multiple passes). In such a configuration, the combined methane economizer receives five distinct methane refrigerant streams via conduits  124   a ,  126 ,  132   b,    136 , and  144 . The distinct methane refrigerant streams in conduits  124   a ,  126 ,  132   b ,  136 , and  144  preferably each have different temperatures and pressures. In addition, it is preferred for the four distinct ethylene refrigerant streams introduced into ethylene economizer  34  via conduits  211   b ,  214 ,  222   b , and  230  to each have different temperatures and pressures. Preferably, the minimum temperature difference between any two of the distinct refrigerant streams fed to an economizer (ethylene and/or methane economizer) is about 5° F., more preferably about 10° F., and most preferably 15° F., while the minimum pressure difference is about 25 psi, 50 psi, or 75 psi. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.