Abstract:
An LNG facility employing a heavies enriching stream to increase the flexibility of the LNG facility by allowing feed gas streams of widely varying compositions to be processed while producing on-spec LNG.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas (LNG) facility employing a heavies enriching stream. 
         [0003]    2. Description of the Prior Art 
         [0004]    Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form. 
         [0005]    Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers. 
         [0006]    Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires. 
         [0007]    Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process. 
         [0008]    In general, LNG facilities are designed and operated to provide LNG to a single market in a certain region of the world. Because natural gas specifications, such as, for example, higher heating value (HHV), Wobbe index, methane content, ethane content, C 3+  content, and inerts content, vary widely throughout the world, LNG facilities are typically optimized to meet a certain set of specifications for a single market. Thus, most existing facilities are equipped only to process natural gas feed streams within a relatively narrow composition range. For example, when an LNG facility designed and operated to effectively process a lean (i.e., heavies-lean) natural gas feed stream is forced to process a rich natural gas stream due to, for example, change in feed gas source or upstream process upset, the plant&#39;s LNG production rate and product quality are adversely affected. 
         [0009]    One proposed solution to managing natural gas feed streams having widely varying compositions is to constantly adjust the operating conditions of the distillation column(s) in the heavies removal zone based on the compositional changes in the feed gas. The flexibility of this proposed solution is typically limited by equipment constraints. In addition, frequently altering plant process conditions introduces operational instability and can result in large volumes of off-spec product and/or product loss. Another proposed solution is to equip LNG facilities with auxiliary process equipment (e.g. distillation columns, turboexpanders, and/or compressors) to be used when the facility processes feed gas outside its design composition range. The main drawbacks associated with this proposed solution are the increased capital cost and operational complexity associated with adding process equipment to a new or existing plant configuration. 
         [0010]    Thus, a need exists for an LNG facility capable of managing natural gas feed streams having widely varying compositions in a way that maximizes the production of on-spec LNG product while minimizing capital and operating costs for the entire facility. 
       SUMMARY OF THE INVENTION 
       [0011]    In one embodiment of the present invention, there is provided a process for liquefying a natural gas stream in an LNG facility, the process comprising: (a) combining at least a portion of the natural gas stream with a heavies enriching stream to thereby form a heavies enriched natural gas stream; (b) separating at least a portion of the heavies enriched natural gas stream in a first distillation column to thereby provide a first overhead stream and a first bottoms stream; and (c) separating at least a portion of the first bottoms stream in a second distillation column to thereby provide a second bottoms stream, wherein the heavies enriching stream comprises at least a portion of the second bottoms stream. 
         [0012]    In another embodiment of the present invention, there is provided a process for liquefying a natural gas stream in an LNG facility, the process comprising: (a) introducing a heavies enriching stream into the natural gas stream to thereby produce a heavies enriched natural gas stream; (b) separating at least a portion of the heavies enriched natural gas stream in a distillation column to thereby provide an overhead stream and a bottoms stream, wherein the heavies enriching stream comprises at least a portion of the bottoms stream; and (c) adjusting the flow rate of the heavies enriching stream introduced into the natural gas stream to maintain a C 3 +/C 2  molar ratio in the heavies enriched natural gas stream of at least about 0.3:1. 
         [0013]    In yet another embodiment of the present invention, there is provided an LNG facility comprising a natural gas feed conduit, a first distillation column, and a second distillation column. The first distillation column defines a first fluid inlet, upper outlet, and lower outlet. The first fluid inlet is coupled in fluid flow communication with the natural gas feed conduit. The second distillation column defines a second fluid inlet, upper outlet, and lower outlet. The second fluid inlet is coupled in fluid flow communication with the first lower outlet. The second lower outlet is coupled in fluid flow communication with the natural gas feed conduit at an enrichment location upstream of the first distillation column. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    Certain embodiments of the present invention are described in detail below with reference to the enclosed figures, wherein: 
           [0015]      FIG. 1   a  is a simplified overview of a cascade-type LNG facility configured in accordance with one embodiment of the present invention; 
           [0016]      FIG. 1   b  is a flow chart of the major steps involved in executing one embodiment of the present invention; and 
           [0017]      FIG. 2  is a schematic diagram a cascade-type LNG facility configured in accordance with one embodiment of present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention can be implemented in a facility used to cool natural gas to its liquefaction temperature to thereby produce liquefied natural gas (LNG). The LNG facility generally employs one or more refrigerants to extract heat from the natural gas and then reject the heat to the environment. Numerous configurations of LNG systems exist, and the present invention may be implemented many different types of LNG systems. 
         [0019]    In one embodiment, the present invention can be implemented in a mixed refrigerant LNG system. Examples of mixed refrigerant processes can include, but are not limited to, a single refrigeration system using a mixed refrigerant, a propane pre-cooled mixed refrigerant system, and a dual mixed refrigerant system. 
         [0020]    In another embodiment, the present invention is implemented in a cascade LNG system employing a cascade-type refrigeration process using one or more pure component refrigerants. The refrigerants utilized in cascade-type refrigeration processes can have successively lower boiling points in order to maximize heat removal from the natural gas stream being liquefied. Additionally, cascade-type refrigeration processes can include some level of heat integration. For example, a cascade-type refrigeration process can cool one or more refrigerants having a higher volatility via indirect heat exchange with one or more refrigerants having a lower volatility. In addition to cooling the natural gas stream via indirect heat exchange with one or more refrigerants, cascade and mixed-refrigerant LNG systems can employ one or more expansion cooling stages to simultaneously cool the LNG while reducing its pressure to near atmospheric pressure. 
         [0021]      FIG. 1   a  illustrates one embodiment of a simplified LNG facility employing a heavies recycle stream. The cascade LNG facility of  FIG. 1   a  generally comprises a cascade cooling section  10 , a heavies removal zone  11 , and an expansion cooling section  12 . Cascade cooling section  10  is depicted as comprising a first mechanical refrigeration cycle  13 , a second mechanical refrigeration cycle  14 , and a third mechanical refrigeration cycle  15 . In general, first, second, and third refrigeration cycles  13 ,  14 ,  15  can be closed-loop refrigeration cycles, open-loop refrigeration cycles, or any combination thereof. In one embodiment of the present invention, first and second refrigeration cycles  13  and  14  can be closed-loop cycles, and third refrigeration cycle  15  can be an open-loop cycle that utilizes a refrigerant comprising at least a portion of the natural gas feed stream undergoing liquefaction. 
         [0022]    In accordance with one embodiment of the present invention, first, second, and third refrigeration cycles  13 ,  14 ,  15  can employ respective first, second, and third refrigerants having successively lower boiling points. For example, the first, second, and third refrigerants can have mid-range boiling points at standard pressure (i.e., mid-range standard boiling points) within about 20° F., within about 110° F., or within 5° F. of the standard boiling points of propane, ethylene, and methane, respectively. In one embodiment, the first refrigerant can comprise at least about 75 mole percent, at least about 90 mole percent, at least 95 mole percent, or can consist essentially of propane, propylene, or mixtures thereof. The second refrigerant can comprise at least about 75 mole percent, at least about 90 mole percent, at least 95 mole percent, or can consist essentially of ethane, ethylene, or mixtures thereof. The third refrigerant can comprise at least about 75 mole percent, at least about 90 mole percent, at least 95 mole percent, or can consist essentially of methane. 
         [0023]    As shown in  FIG. 1   a , first refrigeration cycle  13  can comprise a first refrigerant compressor  16 , a first cooler  17 , and a first refrigerant chiller  18 . First refrigerant compressor  16  can discharge a stream of compressed first refrigerant, which can subsequently be cooled and at least partially liquefied in cooler  17 . The resulting refrigerant stream can then enter first refrigerant chiller  18 , wherein at least a portion of the refrigerant stream can cool the incoming natural gas stream in conduit  100  via indirect heat exchange with the vaporizing first refrigerant. The gaseous refrigerant can exit first refrigerant chiller  18  and can then be routed to an inlet port of first refrigerant compressor  16  to be recirculated as previously described. 
         [0024]    First refrigerant chiller  18  can comprise one or more cooling stages operable to reduce the temperature of the incoming natural gas stream in conduit  100  by about 40 to about 210° F., about 50 to about 190° F., or 75 to 150° F. Typically, the natural gas entering first refrigerant chiller  24  via conduit  100  can have a temperature in the range of from about 0 to about 200° F., about 20 to about 180° F., or 50 to 165° F., while the temperature of the cooled natural gas stream exiting first refrigerant chiller  18  can be in the range of from about −65 to about 0° F., about −50 to about −10° F., or −35 to −15° F. In general, the pressure of the natural gas stream in conduit  100  can be in the range of from about 100 to about 3,000 pounds per square inch absolute (psia), about 250 to about 1,000 psia, or 400 to 800 psia. Because the pressure drop across first refrigerant chiller  18  can be less than about 100 psi, less than about 50 psi, or less than 25 psi, the cooled natural gas stream in conduit  101  can have substantially the same pressure as the natural gas stream in conduit  100 . 
         [0025]    The cooled natural gas stream (also referred to herein as the “cooled predominantly methane stream”) exiting first refrigeration cycle  13  can then enter second refrigeration cycle  14 , which can comprise a second refrigerant compressor  19 , a second cooler  20 , and a second refrigerant chiller  21 . Compressed refrigerant can be discharged from second refrigerant compressor  19  and can subsequently be cooled and at least partially liquefied in cooler  20  prior to entering second refrigerant chiller  21 . Second refrigerant chiller  21  can employ a plurality of cooling stages to progressively reduce the temperature of the predominantly methane stream in conduit  101  by about 50 to about 180° F., about 65 to about 150° F., or 95 to 125° F. via indirect heat exchange with the vaporizing second refrigerant. As shown in  FIG. 1   a , the vaporized second refrigerant can then be returned to an inlet port of second refrigerant compressor  19  prior to being recirculated in second refrigeration cycle  14 , as previously described. 
         [0026]    In one embodiment, the natural gas feed stream in conduit  100  can comprise at least about 5 mole percent, at least about 10 mole percent, or at least 15 mole percent C 2 +. The presence of these ethane and heavier components generally results in the formation of a C 2 +-rich liquid phase in one or more of the cooling stages of second refrigeration cycle  14 . In order to remove the desired heavies, at least a portion of the cooled predominantly methane feed stream passing through second refrigerant chiller  21  can be withdrawn via conduit  102  and processed in heavies removal zone  11 . The feed stream entering heavies removal zone  11  in conduit  102  can have a temperature in the range of from about −160 to about −50° F., about −140 to about −65° F., or −115 to −85° F. and a pressure that is within about 5 percent, about 10 percent, or 15 percent of the pressure of the natural gas feed stream in conduit  100 . 
         [0027]    Heavies removal zone  11  can comprise one or more gas-liquid separators operable to remove at least a portion of the heavy hydrocarbon material from the predominantly methane natural gas stream. In one embodiment, as depicted in  FIG. 1   a , heavies removal zone  11  comprises a first distillation column  25  and a second distillation column  26 . First distillation column  25 , also referred to herein as the “heavies removal column,” functions primarily to remove the bulk of the heavy hydrocarbon material, especially components with molecular weights greater than hexane (i.e., C 6 + material) and aromatics such as benzene, toluene, and xylene, which can freeze in downstream processing equipment. The overhead stream exiting heavies removal column  25  via conduit  103  can comprise at least about 75 mole percent, at least about 85 mole percent, at least about 95 mole percent, or at least 99 mole percent methane. Typically, the concentration of C 6 + material in the overhead stream exiting heavies removal column  25  via in conduit  103  can be less than about 0.1 weight percent, less than about 0.05 weight percent, less than about 0.01 weight percent, or less than 0.005 weight percent, based on the total weight of the stream. Generally, heavies removal column  25  can operate with an overhead temperature in the range of from about −200 to about −25° F., about −175 to about −50° F., or about −125 to about −75° F. and an overhead pressure in the range of from about 100 to about 1,000 pounds per square inch absolute (psia), about 250 to about 750 psia, or 400 to 600 psia. 
         [0028]    As illustrated in  FIG. 1   a , a heavies-rich stream having a temperature in the range of from about −20 to about −100° F., about −35 to about −85° F., or −45 to −65° F. can exit first distillation column  25  via conduit  102   a , whereafter the stream can enter second distillation column  26 . Second distillation column  26 , also referred to herein as the “NGL recovery column,” concentrates residual heavy hydrocarbon components into an NGL product stream. Examples of typical hydrocarbon components included in NGL streams can include ethane, propane, butane isomers, pentane isomers, and C 6 + material. The operating conditions (e.g., overhead temperature and pressure) of second distillation column  26  can vary according to the degree of NGL recovery desired. In one embodiment, NGL recovery column  26  can have an overhead temperature in the range of from about 10 to about 80° F., about 20 to about 70° F., or 30 to 60° F. and an overhead pressure in the range of from about 150 to about 900 psia, about 275 to about 725 psia, or about 350 to about 500 psia. The NGL product stream exiting heavies removal zone  11 , which can have a temperature in the range of from about 150 to about 350° F., about 200 to about 305° F., or 220 to 280° F., can be subjected to further fractionation (not shown) in order to obtain one or more substantially pure component streams. Often, NGL and/or the substantially pure product streams derived therefrom can be desirable blendstocks for gasoline and other fuels. 
         [0029]    According to one embodiment, the natural gas feed stream in conduit  100  can fluctuate between comprising a lean natural gas feed stream and a rich natural gas feed stream. In general, a lean natural gas feed stream can comprise less than about 1 mole percent, less than about 0.5 mole percent, or less than 0.25 mole percent C 3 + components. A rich natural gas stream typically comprises greater than about 1.1 mole percent, greater than about 2 mole percent, or greater than 5 mole percent C 3 + components. In order to produce an on-spec LNG and/or NGL product despite fluctuations in the natural gas feed composition to the plant, the LNG facility depicted in  FIG. 1   a  can employ a heavies enriching stream. A heavies enriching stream can be any stream operable to enrich (i.e., increase the heavies content of) the stream with which it is combined. Typically, the heavies enriching stream can comprise at least about 1 percent, at least about 5 percent, at least about 10 percent, or at least 20 percent more heavy hydrocarbon material than the stream being enriched. In one embodiment, the heavies enriching stream can comprise at least about 50 mole percent, at least about 75 mole percent, or at least about 90 mole percent C 3 + components. Typically, the ratio of the volumetric flow rate of the heavies enriching stream to the volumetric flow rate of the stream being enriched can be in the range of from about 0.0001 to about 0.75, about 0.0005 to about 0.60, or 0.001 to 0.50. 
         [0030]    The heavies enriching stream can be withdrawn from one or more of several locations within the LNG facility or can originate from an external source, such as, for example, a gas plant or other location. In one embodiment of the present invention depicted in  FIG. 1   a , the heavies enriching stream in conduit  330  can originate from the bottom product stream of first and/or second distillation columns  25 ,  26  in heavies removal zone  11 . If desired, a cooler  28  can be employed to cool the heavies enriching stream to a temperature within about 2 to about 50° F., about 5 to about 25° F., or 10 to 15° F. of ambient air or water temperature via indirect heat exchange with an external fluid (e.g., air or water) or an intermediate process stream (not shown). The heavies enriching stream can then be combined with the natural gas feed stream in conduit  100  to produce a heavies enriched natural gas stream in conduit  100   a , as shown in  FIG. 1   a.    
         [0031]    Employing a heavies enriching stream can increase overall production of on-spec LNG by helping stabilize plant operations and by increasing the separation efficiency of difficult-to-remove components (e.g., ethane) from the predominantly methane stream processed in heavies removal zone  11 . For example, in one embodiment, the molar ratio of the ethane content of the overhead product stream exiting heavies removal column  25  to the ethane content of the bottoms product stream exiting heavies removal column  25  can be less than about 0.25:1, less than about 0.10:1, or less than 0.05:1. Typically, when a heavies enriching stream is employed, an overhead product exiting heavies removal column  25  via conduit  103  can have an ethane content of less than about 10 mole percent, less than about 8 mole percent, less than about 6 mole percent, or less than 5 mole percent. As a result, the LNG produced in the LNG facility can comprise less than about 10 mole percent, less than about 8 mole percent, or less than 6 mole percent C 2 + components. This allows the LNG produced to meet strict market requirements, such as, for example, the North American West Coast specification (NAWC spec), which requires LNG having an ethane content less than 6 mole percent at the product terminal. 
         [0032]    Referring now to  FIG. 1   b , the major steps of one embodiment of a method for utilizing a heavies enriching stream in an LNG facility are presented. First, as depicted in block  500 , at least one compositional property of one process stream in the LNG facility can be determined. Suitable process streams can include, for example, the natural gas feed stream ( 100 ), the heavies enriched natural gas feed stream ( 100   a ), the feed stream to heavies removal zone ( 102 ), the heavies enriching stream ( 330 ), the overhead and/or bottoms streams from first and/or second distillation columns  25 ,  26 . Examples of determined compositional properties can include, but are not limited to, C 2  content, C 2 + content, C 3  content, C 3 + content, C 3 +/C 2  molar ratio, C 3 /C 2  molar ratio, molecular weight, and specific gravity, and any combination thereof. The value of the property selected can be determined using any property measurement device, such as, for example, a gas chromatograph (GC), a mass spectrometer, an online analyzer, or any other suitable device for determining the selected compositional property. According to one embodiment depicted in  FIG. 1   a , a property measurement device  27  can be used to determine the C 3 +/C 2  molar ratio in the enriched natural gas feed stream. 
         [0033]    As shown by block  502  in  FIG. 1   b , the next step comprises setting a target value for the stream-specific compositional property determined in the previous step. For example, when the determined compositional property is C 3 +/C 2  molar ratio of the heavies enriched natural gas stream, the target value can be at least about 0.3:1, or in the range of from about 0.45:1 to about 10:1, or 0.5:1 to 5.0:1. In addition, as indicated in  FIG. 1   b , the comparison threshold, or maximum acceptable difference between the target value and the determined value of the compositional property selected, can also be established. In one embodiment, the comparison threshold can be less than about 50 percent, less than about 25 percent, less than about 10 percent, or less than 5 percent. 
         [0034]    According to decision block  504 , the next step comprises comparing the determined and target property values of the selected stream. If the difference between the target and the determined values are within the comparison threshold established in the previous step, the flow rate of the heavies enriching stream can be maintained at its current rate, as indicated by block  506   a . Alternatively, if the difference between the determined value and the target value of the selected compositional property exceeds the threshold limit established in the previous step, the flow rate of the heavies enriching stream can be adjusted accordingly, as shown by block  506   b.    
         [0035]    Typically, a flow control system can be employed to perform the steps depicted in blocks  504  and  506   a,b . One embodiment illustrated in  FIG. 1   a , a flow control system  28  is illustrated as generally comprising a processor  29  and a flow control device  30 . Processor  29  compares the determined value of the property communicated from property measurement device  27  (via an electronic, pneumatic, or other type of signal) to a target value and can manipulate the position of flow control device  30  in order to affect the flow rate of the heavies enriching stream in conduit  330 . Flow control device  30  can be a manual flow control valve operated by, for example, a human operator or an automatic flow control valve operated by, for example, a computerized operator. Once the flow rate of the heavies enriching stream has been adjusted, the above-described process should be repeated until an acceptable difference between the target and determined values has been achieved. 
         [0036]    Referring back to heavies removal zone  11  illustrated in  FIG. 1   a , a heavies-depleted, predominantly methane overhead stream can be withdrawn from heavies removal column  25  via conduit  103  prior to being routed back to second refrigeration cycle  14 . The stream in conduit  103  can have a temperature in the range of from about −140 to about −50° F., about −125 to about −60° F., or −110 to −75° F. and a pressure in the range of from about 200 to about 1,200 psia, about 350 to about 850 psia, or 500 to 700 psia. As shown in  FIG. 1   a , the predominantly methane stream in conduit  103  can subsequently be further cooled via second refrigerant chiller  21 . In one embodiment, the stream exiting second refrigerant chiller  21  via conduit  104  can be completely liquefied and can have a temperature in the range of from about −205 to about −70° F., about −175 to about −95° F., or −140 to −125° F. Generally, the stream in conduit  104  can be at approximately the same pressure the natural gas stream entering the LNG facility in conduit  100 . 
         [0037]    As illustrated in  FIG. 1   a , the pressurized LNG-bearing stream in conduit  104  can combine with a yet-to-be-discussed stream in conduit  109  prior to entering third refrigeration cycle  15 , which is depicted as generally comprising a third refrigerant compressor  22 , a cooler  23 , and a third refrigerant chiller  24 . Compressed refrigerant discharged from third refrigerant compressor  22  enters cooler  23 , wherein the refrigerant stream is cooled and at least partially liquefied prior to entering third refrigerant chiller  24 . Third refrigerant chiller  24  can comprise one or more cooling stages operable to subcool the pressurized predominantly methane stream via indirect heat exchange with the vaporizing refrigerant. In one embodiment, the temperature of the pressurized LNG-bearing stream can be reduced by about 2 to about 60° F., about 5 to about 50° F., or 10 to 40° F. in third refrigerant chiller  24 . In general, the temperature of the pressurized LNG-bearing stream exiting third refrigerant chiller  24  via conduit  105  can be in the range of from about −275 to about −75° F., about −225 to about −100° F., or −200 to −125° F. 
         [0038]    As shown in  FIG. 1   a , the pressurized LNG-bearing stream in conduit  105  can be then routed to expansion cooling section  12 , wherein the stream is subcooled via sequential pressure reduction to near atmospheric pressure by passage through one or more expansion stages. In one embodiment, each expansion stage can reduce the temperature of the LNG-bearing stream by about 10 to about 60° F., about 15 to about 50° F., or 20 to 40° F. Each expansion stage comprises one or more expansion devices, which reduce the pressure of the liquefied stream to thereby evaporate or flash a portion thereof. Examples of suitable expansion devices can include, but are not limited to, Joule-Thompson valves, venturi nozzles, and turboexpanders. Expansion section  12  can employ any number of expansion stages and one or more expansion stages may be integrated with one or more cooling stages of third refrigerant chiller  24 . In one embodiment of the present invention, expansion section  12  can reduce the pressure of the LNG-bearing stream in conduit  105  by about 75 to about 450 psi, about 125 to about 300 psi, or 150 to 225 psi. 
         [0039]    Each expansion stage may additionally employ one or more vapor-liquid separators operable to separate the vapor phase (i.e., the flash gas stream) from the cooled liquid stream. As previously discussed, third refrigeration cycle  15  can comprise an open-loop refrigeration cycle, closed-loop refrigeration cycle, or any combination thereof. When third refrigeration cycle  15  comprises a closed-loop refrigeration cycle, the flash gas stream can be used as fuel within the facility or routed downstream for storage, further processing, and/or disposal. When third refrigeration cycle  15  comprises an open-loop refrigeration cycle, at least a portion of the flash gas stream exiting expansion section  12  be used as a refrigerant to cool at least a portion of the natural gas stream in conduit  104 . Generally, when third refrigerant cycle  15  comprises an open-loop cycle, the third refrigerant can comprise at least 50 weight percent, at least about 75 weight percent, or at least 90 weight percent of flash gas from expansion section  12 , based on the total weight of the stream. As illustrated in  FIG. 1   a , the flash gas exiting expansion section  12  via conduit  106  can enter third refrigerant chiller  24 , wherein the stream can cool at least a portion of the natural gas stream entering third refrigerant chiller  24  via conduit  104 . The resulting warmed refrigerant stream can then exit third refrigerant chiller  24  via conduit  108  and can thereafter be routed to an inlet port of third refrigerant compressor  22 . As shown in  FIG. 1   a , third refrigerant compressor  22  discharges a stream of compressed third refrigerant, which is thereafter cooled in cooler  23 . The resulting cooled methane stream in conduit  109  can then combine with the natural gas stream in conduit  104  prior to entering third refrigerant chiller  24 , as previously discussed. 
         [0040]    As shown in  FIG. 1   a , the liquid stream exiting expansion section  12  via conduit  107  comprises LNG. In one embodiment, the LNG in conduit  107  can have a temperature in the range of from about −200 to about −300° F., about −225 to about −275° F., or −240 to −260° F. and a pressure in the range of from about 0 to about 40 psia, about 5 to about 25 psia, 10 to 20 psia, or about atmospheric. The LNG in conduit  107  can subsequently be routed to storage and/or shipped to another location via pipeline, ocean-going vessel, truck, or any other suitable transportation means. In one embodiment, at least a portion of the LNG can be subsequently vaporized for pipeline transportation or use in applications requiring vapor-phase natural gas. 
         [0041]      FIG. 2  presents one embodiment of a specific configuration of the LNG facility described previously with respect to  FIG. 1   a . To facilitate an understanding of  FIG. 2 , the following numeric nomenclature was employed. Items numbered  31  through  49  are process vessels and equipment directly associated with first propane refrigeration cycle  30 , and items numbered  51  through  69  are process vessels and equipment related to second ethylene refrigeration cycle  50 . Items numbered  71  through  94  correspond to process vessels and equipment associated with third methane refrigeration cycle  70  and/or expansion section  80 . Items numbered  96  through  99  are process vessels and equipment associated with heavies removal zone  95 . Items numbered  100  through  199  correspond to flow lines or conduits that 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 that contain predominantly propane streams. 
         [0042]    Referring now to  FIG. 2 , a cascade-type LNG facility in accordance with one embodiment of the present invention is illustrated. The LNG facility depicted in  FIG. 2  generally comprises a propane refrigeration cycle  30 , a ethylene refrigeration cycle  50 , a methane refrigeration cycle  70  with an expansion section  80 , and a heavies removal zone  95 . While “propane,” “ethylene,” and “methane” are used to refer to respective first, second, and third refrigerants, it should be understood that the embodiment illustrated in  FIG. 2  and described herein can apply to any combination of suitable refrigerants. The main components of propane refrigeration cycle  30  include a propane compressor  31 , a propane cooler  32 , a high-stage propane chiller  33 , an intermediate-stage propane chiller  34 , and a low-stage propane chiller  35 . The main components of ethylene refrigeration cycle  50  include an ethylene compressor  51 , an ethylene cooler  52 , a high-stage ethylene chiller  53 , an intermediate-stage ethylene chiller  54 , a low-stage ethylene chiller/condenser  55 , and an ethylene economizer  56 . The main components of methane refrigeration cycle  70  include a methane compressor  71 , a methane cooler  72 , a main methane economizer  73 , and a secondary methane economizer  74 . The main components of expansion section  80  include a high-stage methane expansion device  81 , a high-stage methane flash drum  82 , an intermediate-stage methane expansion device  83 , an intermediate-stage methane flash drum  84 , a low-stage methane expansion device  85 , and a low-stage methane flash drum  86 . The LNG facility of  FIG. 2  also includes heavies removal zone located downstream of intermediate-stage ethylene chiller  54  for removing heavy hydrocarbon components from the processed natural gas and recovering the resulting natural gas liquids. The heavies removal zone  95  of  FIG. 2  is shown as generally comprising a first distillation column  96  and a second distillation column  97 . 
         [0043]    The operation of the LNG facility illustrated in  FIG. 2  will now be described in more detail, beginning with propane refrigeration cycle  30 . Propane is compressed in multi-stage (e.g., three-stage) propane compressor  31  driven by, for example, 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 propane is passed through conduit  300  to propane cooler  32 , wherein it is cooled and liquefied via indirect heat exchange with an external fluid (e.g., air or water). A representative temperature and pressure of the liquefied propane refrigerant exiting cooler  32  is about 100° F. and about 190 psia. The stream from propane cooler  32  can then be passed through conduit  302  to a pressure reduction means, illustrated as expansion valve  36 , wherein the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion thereof. The resulting two-phase stream then flows via conduit  304  into high-stage propane chiller  33 . High stage propane chiller  33  uses indirect heat exchange means  37 ,  38 , and  39  to cool respectively, the incoming gas streams, including a yet-to-be-discussed methane refrigerant stream in conduit  112 , a yet-to-be-discussed heavies enriched natural gas feed stream in conduit  110   a , and a yet-to-be-discussed ethylene refrigerant stream in conduit  202  via indirect heat exchange with the vaporizing refrigerant. The cooled methane refrigerant stream exits high-stage propane chiller  33  via conduit  130  and can subsequently be routed to the inlet of main methane economizer  73 , which will be discussed in greater detail in a subsequent section. 
         [0044]    The cooled natural gas stream from high-stage propane chiller  33  (also referred to herein as the “methane-rich stream”) flows via conduit  114  to a separation vessel  40 , wherein the gaseous and liquid phases are separated. The liquid phase, which can be rich in propane and heavier components (C 3 +), is removed via conduit  303 . The predominately vapor phase exits separator  40  via conduit  116  and can then enter intermediate-stage propane chiller  34 , wherein the stream is cooled in indirect heat exchange means  41  via indirect heat exchange with a yet-to-be-discussed propane refrigerant stream. The resulting two-phase methane-rich stream in conduit  118  can then be routed to low-stage propane chiller  35 , wherein the stream can be further cooled via indirect heat exchange means  42 . The resultant predominantly methane stream can then exit low-stage propane chiller  35  via conduit  120 . Subsequently, the cooled methane-rich stream in conduit  120  can be routed to high-stage ethylene chiller  53 , which will be discussed in more detail shortly. 
         [0045]    The vaporized propane refrigerant exiting high-stage propane chiller  33  is returned to the high-stage inlet port of propane compressor  31  via conduit  306 . The residual liquid propane refrigerant in high-stage propane chiller  33  can be passed via conduit  308  through a pressure reduction means, illustrated here as expansion valve  43 , whereupon a portion of the liquefied refrigerant is flashed or vaporized. The resulting cooled, two-phase refrigerant stream can then enter intermediate-stage propane chiller  34  via conduit  310 , thereby providing coolant for the natural gas stream and yet-to-be-discussed ethylene refrigerant stream entering intermediate-stage propane chiller  34 . The vaporized propane refrigerant exits intermediate-stage propane chiller  34  via conduit  312  and can then enter the intermediate-stage inlet port of propane compressor  31 . The remaining liquefied propane refrigerant exits intermediate-stage propane chiller  34  via conduit  314  and is passed through a pressure-reduction means, illustrated here as expansion valve  44 , whereupon the pressure of the stream is reduced to thereby flash or vaporize a portion thereof. The resulting vapor-liquid refrigerant stream then enters low-stage propane chiller  35  via conduit  316  and cools the methane-rich and yet-to-be-discussed ethylene refrigerant streams entering low-stage propane chiller  35  via conduits  118  and  206 , respectively. The vaporized propane refrigerant stream then exits low-stage propane chiller  35  and is routed to the low-stage inlet port of propane compressor  31  via conduit  318  wherein it is compressed and recycled as previously described. 
         [0046]    As shown in  FIG. 2 , a stream of ethylene refrigerant in conduit  202  enters high-stage propane chiller, wherein the ethylene stream is cooled via indirect heat exchange means  39 . The resulting cooled stream in conduit  204  then exits high-stage propane chiller  33 , whereafter the at least partially condensed stream enters intermediate-stage propane chiller  34 . Upon entering intermediate-stage propane chiller  34 , the ethylene refrigerant stream can be further cooled via indirect heat exchange means  45 . The resulting two-phase ethylene stream can then exit intermediate-stage propane chiller  34  prior to entering low-stage propane chiller  35  via conduit  206 . In low-stage propane chiller  35 , the ethylene refrigerant stream can be at least partially condensed, or condensed in its entirety, via indirect heat exchange means  46 . The resulting stream exits low-stage propane chiller  35  via conduit  208  and can subsequently be routed to a separation vessel  47 , wherein the vapor portion of the stream, if present, can be removed via conduit  210 . The liquefied ethylene refrigerant stream exiting separator  47  via conduit  212  can have a representative temperature and pressure of about −24° F. and about 285 psia. 
         [0047]    Turning now to ethylene refrigeration cycle  50  in  FIG. 2 , the liquefied ethylene refrigerant stream in conduit  212  can enter ethylene economizer  56 , wherein the stream can be further cooled by an indirect heat exchange means  57 . The sub-cooled liquid ethylene stream in conduit  214  can then be routed through a pressure reduction means, illustrated here as expansion valve  58 , whereupon the pressure of the stream is reduced to thereby flash or vaporize a portion thereof. The cooled, two-phase stream in conduit  215  can then enter high-stage ethylene chiller  53 , wherein at least a portion of the ethylene refrigerant stream can vaporize to thereby cool the methane-rich stream entering an indirect heat exchange means  59  of high-stage ethylene chiller  53  via conduit  120 . The vaporized and remaining liquefied refrigerant exit high-stage ethylene chiller  53  via respective conduits  216  and  220 . The vaporized ethylene refrigerant in conduit  216  can re-enter ethylene economizer  56 , wherein the stream can be warmed via an indirect heat exchange means  60  prior to entering the high-stage inlet port of ethylene compressor  51  via conduit  218 , as shown in  FIG. 2 . 
         [0048]    The remaining liquefied refrigerant in conduit  220  can re-enter ethylene economizer  56 , wherein the stream can be further sub-cooled by an indirect heat exchange means  61 . The resulting cooled refrigerant stream exits ethylene economizer  56  via conduit  222  and can subsequently be routed to a pressure reduction means, illustrated here as expansion valve  62 , whereupon the pressure of the stream is reduced to thereby vaporize or flash a portion thereof. The resulting, cooled two-phase stream in conduit  224  enters intermediate-stage ethylene chiller  54 , wherein the refrigerant stream can cool the natural gas stream in conduit  122  entering intermediate-stage ethylene chiller  54  via an indirect heat exchange means  63 . As shown in  FIG. 2 , the resulting cooled methane-rich stream exiting intermediate stage ethylene chiller  54  can then be routed to heavies removal zone  95 . Heavies removal zone  95  will be discussed in detail in a subsequent section. 
         [0049]    The vaporized ethylene refrigerant exits intermediate-stage ethylene chiller  54  via conduit  226 , whereafter the stream can combine with a yet-to-be-discussed ethylene vapor stream in conduit  238 . The combined stream in conduit  239  can enter ethylene economizer  56 , wherein the stream is warmed in an indirect heat exchange means  64  prior to being fed into the low-stage inlet port of ethylene compressor  51  via conduit  230 . As shown in  FIG. 2 , a stream of compressed ethylene refrigerant in conduit  236  can subsequently be routed to ethylene cooler  52 , wherein the ethylene stream can be cooled via indirect heat exchange with an external fluid (e.g., water or air). The resulting, at least partially condensed ethylene stream can then be introduced via conduit  202  into high-stage propylene chiller  33  for additional cooling as previously described. 
         [0050]    The remaining liquefied ethylene refrigerant exits intermediate-stage ethylene chiller  54  via conduit  228  prior to entering low-stage ethylene chiller/condenser  55 , wherein the refrigerant can cool the methane-rich stream entering low-stage ethylene chiller/condenser via conduit  128  in an indirect heat exchange means  65 . In one embodiment shown in  FIG. 2 , the stream in conduit  128  results from the combination of a heavies-depleted (i.e., light hydrocarbon rich) stream exiting heavies removal zone  95  via conduit  126  and a yet-to-be-discussed methane refrigerant stream in conduit  168 . As shown in  FIG. 2 , the vaporized ethylene refrigerant can then exit low-stage ethylene chiller/condenser  55  via conduit  238  prior to combining with the vaporized ethylene exiting intermediate-stage ethylene chiller  54  and entering the low-stage inlet port of ethylene compressor  51 , as previously discussed. 
         [0051]    The cooled natural gas stream exiting low-stage ethylene chiller/condenser can also be referred to as the “pressurized LNG-bearing stream.” As shown in  FIG. 2 , the pressurized LNG-bearing stream exits low-stage ethylene chiller/condenser  55  via conduit  132  prior to entering main methane economizer  73 . In main methane economizer  73 , the methane-rich stream can be cooled in an indirect heat exchange means  75  via indirect heat exchange with one or more yet-to-be discussed methane refrigerant streams. The cooled, pressurized LNG-bearing stream exits main methane economizer  73  and can then be routed via conduit  134  into expansion section  80  of methane refrigeration cycle  70 . In expansion section  80 , the cooled predominantly methane stream passes through high-stage methane expansion device  81 , whereupon the pressure of the stream is reduced to thereby vaporize or flash a portion thereof. The resulting two-phase methane-rich stream in conduit  136  can then enter high-stage methane flash drum  82 , whereupon the vapor and liquid portions can be separated. The vapor portion exiting high-stage methane flash drum  82  (i.e., the high-stage flash gas) via conduit  143  can then enter main methane economizer  73 , wherein the stream is heated via indirect heat exchange means  76 . The resulting warmed vapor stream exits main methane economizer  73  and subsequently combines with a yet-to-be-discussed vapor stream exiting heavies removal zone  95  in conduit  140 . The combined stream in conduit  141  can then be routed to the high-stage inlet port of methane compressor  71 , as shown in  FIG. 2 . 
         [0052]    The liquid phase exiting high-stage methane flash drum  82  via conduit  142  can enter secondary methane economizer  74 , wherein the methane stream can be cooled via indirect heat exchange means  92 . The resulting cooled stream in conduit  144  can then be routed to a second expansion stage, illustrated here as intermediate-stage expansion device  83 . Intermediate-stage expansion device  83  reduces the pressure of the methane stream passing therethrough to thereby reduce the stream&#39;s temperature by vaporizing or flashing a portion thereof. The resulting two-phase methane-rich stream in conduit  146  can then enter intermediate-stage methane flash drum  84 , wherein the liquid and vapor portions of the stream can be separated and can exit the intermediate-stage flash drum via respective conduits  148  and  150 . The vapor portion (i.e., the intermediate-stage flash gas) in conduit  150  can re-enter secondary methane economizer  74 , wherein the stream can be heated via an indirect heat exchange means  87 . The warmed stream can then be routed via conduit  152  to main methane economizer  73 , wherein the stream can be further warmed via an indirect heat exchange means  77  prior to entering the intermediate-stage inlet port of methane compressor  71  via conduit  154 . 
         [0053]    The liquid stream exiting intermediate-stage methane flash drum  84  via conduit  148  can then pass through a low-stage expansion device  85 , whereupon the pressure of the liquefied methane-rich stream can be further reduced to thereby vaporize or flash a portion thereof. The resulting cooled, two-phase stream in conduit  156  can then enter low-stage methane flash drum  86 , wherein the vapor and liquid phases can be separated. The liquid stream exiting low-stage methane flash drum  86  can comprise the liquefied natural gas (LNG) product. The LNG product, which is at about atmospheric pressure, can be routed via conduit  158  downstream for subsequent storage, transportation, and/or use. 
         [0054]    The vapor stream exiting low-stage methane flash drum (i.e., the low-stage methane flash gas) in conduit  160  can be routed to secondary methane economizer  74 , wherein the stream can be warmed via an indirect heat exchange means  89 . The resulting stream can exit secondary methane economizer  74  via conduit  162 , whereafter the stream can be routed to main methane economizer  73  to be further heated via indirect heat exchange means  78 . The warmed methane vapor stream can then exit main methane economizer  73  via conduit  164  prior to being routed to the low-stage inlet port of methane compressor  71 . Methane compressor  71  can comprise one or more compression stages. In one embodiment, methane compressor  71  comprises three compression stages in a single module. In another embodiment, the compression modules can be separate, but can be mechanically coupled to a common driver. Generally, when methane compressor  71  comprises two or more compression stages, one or more intercoolers (not shown) can be provided between subsequent compression stages. As shown in  FIG. 2 , the compressed methane refrigerant stream exiting methane compressor  71  can be discharged into conduit  166 , whereafter the stream can be cooled via indirect heat exchange with an external fluid (e.g., air or water) in methane cooler  72 . The cooled methane refrigerant stream exiting methane cooler  72  can then enter conduit  112 , whereafter the methane refrigerant stream can be further cooled in propane refrigeration cycle  30 , as described in detail previously. 
         [0055]    Upon being cooled in propane refrigeration cycle  30 , the methane refrigerant stream can be discharged into conduit  130  and subsequently routed to main methane economizer  73 , wherein the stream can be further cooled via indirect heat exchange means  79 . The resulting sub-cooled stream exits main methane economizer  73  via conduit  168  and can then combined with the heavies-depleted stream exiting heavies removal zone  95  via conduit  126 , as previously discussed. 
         [0056]    Turning now to heavies removal zone  95 , the cooled, at least partially condensed effluent exiting intermediate-stage ethylene chiller  54  via conduit  124  can be routed into the inlet of first distillation column  96 , as shown in  FIG. 2 . A predominantly methane overhead vapor product stream can exit an upper outlet of first distillation column  96  via conduit  126 . As discussed previously, the stream in conduit  126  can subsequently combine with the methane refrigerant stream in conduit  168  prior to entering low-stage ethylene chiller/condenser  55  via conduit  128 . Referring back to heavies removal zone  95 , a heavies-rich bottoms liquid product stream exiting a lower outlet of first distillation column  96  via conduit  170  can then be routed to an inlet of second distillation column  97 . An overhead vapor product stream can exit an upper outlet of second distillation column  97  via conduit  140  prior to being combined with the warmed methane refrigerant stream in conduit  138 , as discussed in detail previously. The bottoms liquid product exiting a lower outlet of second distillation column  97  can comprise the natural gas liquids (NGL) product. The NGL product, which can comprise a significant concentration of butane and heavier hydrocarbons, such as benzene, cyclohexane, and other aromatics, can be routed to further storage, processing, and/or use via conduit  171 . 
         [0057]    As illustrated in  FIG. 2 , at least a portion of the NGL product exiting the lower outlet of second distillation column  97  in conduit  171  can be withdrawn via conduit  324  and subsequently cooled via indirect heat exchange with an external fluid (e.g., air or water) in cooler  98 . Optionally, a heavies stream originating from an external C 3 + source (e.g., a gas plant or other storage location) via conduit  326  can be routed into the LNG facility depicted in  FIG. 1  and can combine with the cooled stream exiting cooler  98 . The resulting stream can then enter the suction of pump  99 , whereafter the pressurized stream can be discharged into conduit  330 . The heavies enriching stream in conduit  330  can then be routed to combine with the natural gas feed stream in conduit  110  to thereby produce the heavies enriched natural gas feed stream in conduit  110   a , as shown in  FIG. 2 . The heavies enriched natural gas stream can then continue through the LNG facility as previously described. 
         [0058]    In one embodiment of the present invention, the LNG production systems illustrated in  FIGS. 1   a  and  2  are simulated on a computer using conventional process simulation software in order to generate process simulation data in a human-readable form. In one embodiment, the process simulation data can be in the form of a computer print out. In another embodiment, the process simulation data can be displayed on a screen, a monitor, or other viewing device. The simulation data can then be used to manipulate the LNG system. In one embodiment, the simulation results can be used to design a new LNG facility and/or revamp or expand an existing facility. In another embodiment, the simulation results can be used to optimize the LNG facility according to one or more operating parameters. Examples of suitable software for producing the simulation results include HYSYS™ or Aspen Plus® from Aspen Technology, Inc., and PRO/II® from Simulation Sciences Inc. 
       EXAMPLE 
       [0059]    The LNG facility depicted in  FIG. 2  was simulated using HYSYS™ simulation software to illustrate the effect of the heavies enriching stream on the composition of the LNG product. The ratio of the volumetric flow rate of the heavies enriching stream in conduit  330  to the volumetric flow rate of the natural gas stream in conduit  110  was varied in order to achieve various C 3 +/C 2  ratios in the heavies enriched natural gas feed stream in conduit  110   a . The composition of the overhead product stream of first distillation column  96  in conduit  126  was determined for each trial run and the results for are presented in Table 1, below. First distillation column  96  was simulated at an overhead temperature of −107° F. and an overhead pressure of 500 psia. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Results of HYSIS ™ Simulation for Various Heavies 
               
               
                 Enriched Feed Stream Compositions 
               
             
          
           
               
                 Volumetric Ratio of 
                 C 3 +/C 2  Molar Ratio in 
                 Mole % C 2  in First 
               
               
                 Heavies Enriching Stream 
                 Heavies Enriched Feed 
                 Distillation Column 
               
               
                 (330) to Feed Stream (110) 
                 Stream (110a) 
                 Overhead (126) 
               
               
                   
               
             
          
           
               
                 0 
                 0.03 
                 6.18 
               
               
                 0.005 
                 0.08 
                 5.80 
               
               
                 0.01 
                 0.19 
                 5.46 
               
               
                 0.02 
                 0.22 
                 4.89 
               
               
                 0.03 
                 0.32 
                 4.43 
               
               
                 0.04 
                 0.42 
                 4.06 
               
               
                 0.044 
                 0.45 
                 3.93 
               
               
                   
               
             
          
         
       
     
         [0060]    As illustrated by the results presented in Table 1, increasing the volumetric flow rate of the heavies enriching stream introduced into the natural gas feed stream (to thereby increase the C 3 +/C 2  molar ratio in the enriched heavies removal stream) reduces the ethane content of the overhead stream withdrawn from first distillation column  96 . Because the overhead stream exiting first distillation column  96  ultimately becomes the final LNG product, utilizing a heavies enriching stream can help control the ethane content of the final LNG product. 
       Numerical Ranges 
       [0061]    The present description uses numerical ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claims limitation that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds). 
       DEFINITIONS 
       [0062]    As used herein, the terms “a,” “an,” “the,” and “said” means one or more. 
         [0063]    As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself, or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. 
         [0064]    As used herein, the term “cascade-type refrigeration process” refers to a refrigeration process that employs a plurality of refrigeration cycles, each employing a different refrigerant to successively cool natural gas. 
         [0065]    As used herein, the term “closed-loop refrigeration cycle” refers to a refrigeration cycle wherein substantially no refrigerant enters or exits the cycle during normal operation. 
         [0066]    As used herein, the term “compositional property” refers to a property associated with the composition of a stream. 
         [0067]    As used herein, the terms “comprising,” “comprises,” and “comprise” are open-ended transition terms used to transition from a subject recited before the term to one or elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up of the subject. 
         [0068]    As used herein, the terms “containing,” “contains,” and “contain” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided above. 
         [0069]    As used herein, the terms “economizer” or “economizing heat exchanger” refer to a configuration utilizing a plurality of heat exchangers employing indirect heat exchange means to efficiently transfer heat between process streams. 
         [0070]    As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided above. 
         [0071]    As used herein, the term “heavies enriching stream” refers to any stream operable to enrich (i.e., increase the heavies content of) the stream with which it is combined. 
         [0072]    As used herein, the terms “heavy hydrocarbon” and “heavies” refer to any component that is less volatile (i.e., has a higher boiling point) than methane. 
         [0073]    As used herein, the terms “including,” “includes,” and “include” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided above. 
         [0074]    As used herein, the term “lean natural gas” refers to natural gas comprising less than about 1 mole percent C 3 + material. 
         [0075]    As used herein, the term “mid-range standard boiling point” refers to the temperature at which half of the weight of a mixture of physical components has been vaporized (i.e., boiled off) at standard pressure. 
         [0076]    As used herein, the term “mixed refrigerant” refers to a refrigerant containing a plurality of different components, where no single component makes up more than 75 mole percent of the refrigerant. 
         [0077]    As used herein, the term “natural gas” means a stream containing at least 75 mole percent methane, with the balance being ethane, higher hydrocarbons, nitrogen, carbon dioxide, and/or a minor amount of other contaminants such as mercury, hydrogen sulfide, and mercaptan. 
         [0078]    As used herein, the terms “natural gas liquids” or “NGL” refer to mixtures of hydrocarbons whose components are, for example, typically heavier than ethane. Some examples of hydrocarbon components of NGL streams include propane, butane, and pentane isomers, benzene, toluene, and other aromatic compounds. 
         [0079]    As used herein, the term “open-loop refrigeration cycle” refers to a refrigeration cycle wherein at least a portion of the refrigerant employed during normal operation originates from the fluid being cooled by the refrigeration cycle. 
         [0080]    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, means 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. 
         [0081]    As used herein, the term “pure component refrigerant” means a refrigerant that is not a mixed refrigerant. 
         [0082]    As used herein, the term “rich natural gas” refers to natural gas having greater than about 1.1 mole percent C 3 + material. 
         [0083]    As used herein, the terms “upstream” and “downstream” refer to the relative positions of various components of a natural gas liquefaction facility along the main flow path of natural gas through the facility. 
       CLAIMS NOT LIMITED TO DISCLOSED EMBODIMENTS 
       [0084]    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. 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. 
         [0085]    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.