Abstract:
An LNG facility capable of producing a domestic gas product from an intermediate stream in the LNG facility. Withdrawing the domestic gas product from a location within the LNG facility can minimize operational disturbances typically caused by extracting a domestic gas product stream upstream of the liquefaction portion of the LNG facility. In addition, withdrawing the domestic gas product from this location can provide increased control of light contaminants (e.g., nitrogen) in open-loop refrigeration cycles and can ultimately result in incremental LNG and/or NGL production.

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 an LNG facility capable of simultaneously producing liquefied natural gas (LNG) and a domestic gas product. 
         [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, 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 addition to LNG, some LNG facilities also produce a domestic gas product. As used herein, the term “domestic gas product” refers to any gaseous, predominantly methane stream originating from an LNG facility that is routed to a location external to the LNG facility for sale and/or use. Typically, domestic gas products from LNG facilities are transported via pipeline to the local natural gas market for subsequent sale. The domestic gas product from most LNG facilities originates as a slip stream of the natural gas feed entering the liquefaction portion of the LNG facility. In order to ensure the domestic gas product meets certain pipeline specifications (e.g., hydrocarbon dew point), the withdrawn natural gas stream is often subjected to further processing (e.g., distillation) in order to produce a compliant domestic gas product. Often, the remaining portion of the domestic gas stream is recombined with the natural gas feed stream entering the LNG facility, a practice which can cause in drastic changes in the composition of the natural gas feed. These drastic changes can adversely affect the operation of the LNG facility and can ultimately result in off-spec LNG product and/or reduced LNG production. 
         [0009]    Thus, a need exists for an LNG facility that is capable of efficiently and consistently producing on-spec LNG and a pipeline-compliant domestic gas product without requiring additional process equipment in order to maximize facility production while minimizing capital and operating costs. 
       SUMMARY OF THE INVENTION 
       [0010]    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) cooling at least a portion of the natural gas stream in a first refrigeration cycle via indirect heat exchange with a predominantly methane refrigerant; (b) flashing at least a portion of the cooled natural gas stream to thereby provide a predominantly liquid product stream and a predominantly vapor refrigerant stream; (c) compressing at least a portion of the predominantly vapor refrigerant stream to thereby provide a compressed refrigerant stream; and (d) separating at least a portion of the compressed refrigerant stream into a domestic gas fraction and a compressed refrigerant fraction. 
         [0011]    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) cooling at least a portion of the natural gas stream in an upstream refrigeration cycle of the LNG facility via indirect heat exchange with an upstream refrigerant to thereby provide a cooled natural gas stream; (b) further cooling at least a portion of the cooled natural gas stream via indirect heat exchange with a predominantly methane refrigerant stream in a methane refrigeration cycle to thereby produce a further cooled natural gas stream and a warmed refrigerant stream; (c) separating at least a portion of the warmed refrigerant stream into a domestic gas fraction and a refrigerant fraction; and (d) cooling at least a portion of the refrigerant fraction in the upstream refrigeration cycle via indirect heat exchange with the upstream refrigerant. 
         [0012]    In yet 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) cooling the natural gas stream in a first refrigeration cycle via indirect heat exchange with a first refrigerant to thereby produce a cooled predominantly methane stream; (b) separating at least a portion of the cooled predominantly methane stream in a distillation column to thereby produce a heavies-rich stream and a heavies-depleted stream; (c) subjecting at least a portion of the heavies-depleted stream to expansion cooling to thereby produce LNG having a pressure in the range of from about 0 to about 40 psia; and (d) prior to at least a portion of the expansion cooling of step (c), withdrawing a domestic gas fraction from the heavies-depleted stream. 
         [0013]    In a still further embodiment of the present invention, there is provided an LNG facility for liquefying a natural gas stream. The LNG facility comprises an open-loop refrigeration cycle operable to cool at least a portion of the natural gas stream via indirect heat exchange with a first refrigerant. The open-loop refrigeration cycle comprises a first heat exchanger defining a first cooling pass and a first refrigerant pass. The first heat exchanger is operable to cool at least a portion of the natural gas stream in the first cooling pass via indirect heat exchange with the first refrigerant in the first refrigerant pass. The open-loop refrigeration cycle also comprises an expander defining an expander inlet and an expander outlet. The expander inlet is in fluid communication with the first cooling pass. The open-loop refrigeration cycle further comprises a vapor-liquid separator defining a separator inlet, a lower liquid outlet, and an upper vapor outlet. The separator inlet is coupled in fluid flow communication with the expander outlet and the upper vapor outlet is coupled in fluid flow communication with the first refrigerant pass. The open-loop refrigeration cycle also comprises a first refrigerant compressor defining an inlet port and an outlet port. The inlet port is coupled in fluid flow communication with the first refrigerant pass. The open-loop refrigeration cycle additionally comprises a compressed refrigerant conduit for routing fluid flow from the outlet port of the compressor to a location within the LNG facility and a domestic gas conduit for routing fluid flow from the outlet port of the compressor and/or the compressed refrigerant conduit to a location outside the LNG facility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Certain embodiments of the present invention are described in detail below with reference to the enclosed figures, wherein: 
           [0015]      FIG. 1  is a simplified overview of a cascade-type LNG facility in configured in accordance with one embodiment of the present invention; and 
           [0016]      FIG. 2  is a schematic diagram a cascade-type LNG facility configured in accordance with one embodiment of present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]      FIG. 1  illustrates one embodiment of a simplified LNG facility capable of simultaneously producing LNG and a domestic gas product. The cascade-type LNG facility of  FIG. 1  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. 
         [0021]    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 10° 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. 
         [0022]    As shown in  FIG. 1 , 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. 
         [0023]    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 . 
         [0024]    As illustrated in  FIG. 1 , 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 , 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. 
         [0025]    The natural gas feed stream in conduit  100  will usually contain ethane and heavier components (C 2 +), which can result 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 undesired heavies material from the predominantly methane stream prior to complete liquefaction, at least a portion of the natural gas stream passing through second refrigerant chiller  21  can be withdrawn via conduit  102  and processed in heavies removal zone  11 , as shown in  FIG. 1 . The natural gas stream 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 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 . 
         [0026]    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 stream. Typically, heavies removal zone  11  can be operated to remove benzene and other high molecular weight aromatic components, which will freeze in subsequent liquefaction steps and plug downstream process equipment. In addition, heavies removal zone  11  can be operated to recover the heavy hydrocarbons as a natural gas liquids (NGL) product stream. Examples of typical hydrocarbon components comprising NGL streams can include ethane, propane, butane isomers, pentane isomers, and hexane and heavier components (i.e., C 6 +). The extent of NGL recovery from the predominantly methane stream can ultimately impact one or more final characteristics of the LNG product, such as, for example, Wobbe index, BTU content, higher heating value (HHV), ethane content, and the like. In one embodiment, the NGL product stream exiting heavies removal zone  11  can be subjected to further fractionation in order to produce one or more pure component streams. Often, NGL product streams and/or their constituents can be used as gasoline blendstock. 
         [0027]    The predominantly methane stream exiting heavies removal zone  11  via conduit  103  can comprise less than about 1 weight percent, less than about 0.5 weight percent, less than about 0.1 weight percent, or less than 0.01 weight percent of C 6 + material, based on the total weight of the stream. Typically, the predominantly methane 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 , the stream exiting heavies removal zone  12  via conduit  103  can subsequently be routed back to second refrigeration cycle  14 , wherein the stream can 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 . 
         [0028]    As illustrated in  FIG. 1 , the pressurized LNG-bearing stream in conduit  104  enters 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. 
         [0029]    As shown in  FIG. 1 , the pressurized LNG-bearing stream in conduit  105  can be then routed to expansion cooling section  12 , wherein the stream is sub-cooled 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 expanders, which reduce the pressure of the liquefied stream to thereby evaporate or flash a portion thereof. Examples of suitable expanders 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. 
         [0030]    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  can 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 , 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 . 
         [0031]    As shown in  FIG. 1 , third refrigerant compressor  22  discharges a stream of compressed third refrigerant, which is thereafter cooled in cooler  23 . The cooled refrigerant stream can then be split into two portions. The first portion in conduit  109   a  can comprise the domestic gas product stream and can subsequently be routed to a location external to the LNG facility depicted in  FIG. 1 . The second portion of cooled refrigerant in conduit  109   b  can combine with the natural gas stream in conduit  104  prior to re-entering third refrigerant chiller  24 , as previously discussed. 
         [0032]    As shown in  FIG. 1 , 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, or 10 to 20 psia. 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 uses in applications requiring vapor-phase natural gas. 
         [0033]    In addition to producing LNG in conduit  107 , the LNG facility depicted in  FIG. 1  can also produce a domestic gas product in conduit  109   a . As shown in  FIG. 1 , the domestic gas product can be withdrawn from an intermediate stream within the LNG facility, typically at a location downstream of heavies removal zone  95 . Because the domestic gas stream can be withdrawn downstream of heavies removal zone  95 , the domestic gas product can have a concentration of C 6 + material that is less than about 1 weight percent, less than about 0.5 weight percent, less than about 0.1 weight percent, or less than 0.01 weight percent, based on the total weight of the domestic gas stream. As a result, the domestic gas product withdrawn from the LNG facility of  FIG. 1  via conduit  109   a  can comply with most or all of the local natural gas pipeline product specifications, including, for example, hydrocarbon dew point, with little or no additional processing. 
         [0034]    In one embodiment shown in  FIG. 1 , the domestic gas product stream can be withdrawn from the compressed third refrigerant stream exiting third refrigerant compressor  22  via conduit  109   a . Typically, the pressure of the domestic gas stream can be in the range of from about 15 to about 100 bar gauge (barg), about 25 to about 90 barg, or 35 to 75 barg. In order to produce a domestic gas product having a mass flow rate that is at least about 2 percent, at least about 5 percent, at least about 10 percent, or at least 25 percent of the mass flow rate of the total compressed third refrigerant stream exiting third refrigerant compressor  22 , the LNG facility of  FIG. 1  can process additional natural gas feed. By processing additional feed gas, additional refrigeration duty can be recovered in the third refrigeration cycle, which can ultimately result in incremental LNG and/or NGL production. In addition, when the domestic gas product is withdrawn from an open-loop cycle, as illustrated in  FIG. 1 , producing a domestic gas stream can help control the concentration of light contaminants (e.g., nitrogen) in the refrigeration loop, thereby allowing the LNG facility increased processing flexibility. Further, because of the relatively low concentration of heavies and other contaminants in the domestic gas product in conduit  109   a , at least a portion of the domestic gas product can subsequently be blended with an unprocessed or off-spec domestic gas stream from another source (not shown) in order to produce a saleable domestic gas product. Optionally, one or more fuel gas streams (not shown) for use within the LNG facility can be withdrawn from the domestic gas stream and/or the compressed refrigerant stream in conduits  109   a ,  109   b . Typically, at least a portion of the fuel gas stream can be used to power one or more gas turbine used to drive at least one refrigerant compressor. 
         [0035]      FIG. 2  presents one embodiment of a specific configuration of the LNG facility shown in  FIG. 1 . 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 LNG facility depicted in  FIG. 2  generally comprises a propane refrigeration cycle  30 , an ethylene refrigeration cycle  50 , a methane refrigeration cycle  70  with an expansion section  80 , and a heavies removal zone  95 . 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 propane refrigeration cycle  30 , and items numbered  51  through  69  are process vessels and equipment related to ethylene refrigeration cycle  50 . Items numbered  71  through  94  correspond to process vessels and equipment associated with 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. 
         [0036]    Referring to  FIG. 2 , 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 expander  81 , a high-stage methane flash drum  82 , an intermediate-stage methane expander  83 , an intermediate-stage methane flash drum  84 , a low-stage methane expander  85 , and a low-stage methane flash drum  86 . The LNG facility of  FIG. 2  also includes heavies removal zone  95  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 . 
         [0037]    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  31   a . 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 natural gas feed stream in conduit  110 , 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. 
         [0038]    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  34  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. 
         [0039]    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 via conduit  318  to the low-stage inlet port of propane compressor  31 , wherein the stream is compressed and recycled as previously described. 
         [0040]    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. 
         [0041]    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 . 
         [0042]    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  via conduit  124 . Heavies removal zone  95  will be discussed in detail in a subsequent section. 
         [0043]    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 then 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 . Ethylene compressor  51  can be driven by, for example, a gas turbine driver  51   a . Ethylene compressor  51  comprises at least one stage of compression, and, when multiple stages are employed, the stages can exist in a single unit or can be separate units mechanically coupled to a common driver. Generally, when ethylene 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 , 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. 
         [0044]    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  via conduit  226  and entering the low-stage inlet port of ethylene compressor  51 , as previously discussed. 
         [0045]    The cooled natural gas stream exiting low-stage ethylene chiller/condenser in conduit  132  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 expander  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  via conduit  138  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 . 
         [0046]    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 expander  83 , wherein the pressure of the stream can be reduced to thereby evaporate or flash 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 . 
         [0047]    The liquid stream exiting intermediate-stage methane flash drum  84  via conduit  148  can then pass through a low-stage expander  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. 
         [0048]    The vapor stream exiting low-stage methane flash drum  86  (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 , whereafter the stream can be split into two portions. The first portion in conduit  164  can enter the low-stage inlet port of methane compressor  71 , which will be discussed in detail shortly. The second portion in conduit  164   a  can be routed to an inlet port of a sales gas compressor  91 . The compressed gas product exiting sales gas compressor  91  via conduit  172   e  can then cooled (not shown) and routed to a location external to the LNG facility for use as a domestic gas product. Optionally, as shown in  FIG. 2 , at least a portion of the compressed gas stream in conduit  172   e  can be routed via conduit  160   b  to recombine with the warmed refrigerant stream in conduit  164 . 
         [0049]    As previously discussed, the warmed methane refrigerant stream in conduit  164  can enter the low-stage inlet port of methane compressor  71 . Methane compressor  71  can be driven by, for example, a gas turbine driver  71   a . Methane compressor  71  comprises at least one stage of compression, and, when multiple stages are employed, the stages can exist in a single unit or can be separate units 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. 
         [0050]    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 . In one embodiment, the cooled compressed refrigerant stream can then be split into a compressed refrigerant fraction in conduit  112  and a domestic gas fraction in conduit  172   a . Optionally, a fuel gas stream can be withdrawn from the domestic gas fraction via conduit  174   a  and/or from the compressed refrigerant fraction via conduit  176   a . The domestic gas fraction in conduit  172   a  can subsequently be routed to a location outside the LNG facility, whereafter the domestic gas stream can optionally be combined with another gas stream (e.g., a portion of the feed natural gas) prior to being transported and sold to subsequent users. The fuel gas stream, if present, can be routed to one or more fuel gas consumers (e.g., gas turbine drivers  31   a ,  51   a , and  71   a  of respective propane, ethylene, and methane compressors  31 ,  51 ,  71 ) within the LNG facility. In another embodiment, a domestic gas fraction can be withdrawn from the streams exiting the discharge of the low-stage, intermediate-stage, and/or high-stage of methane compressor  71 , as indicated in  FIG. 1  by respective lines  172   b ,  172   c ,  172   d . In addition, optional fuel gas streams  174   b - d  can be withdrawn from the domestic gas fractions in corresponding conduits  172   b - d  or from the remaining compressed refrigerant fractions exiting the low, intermediate, and high stages of methane compressor  71  (not shown). As illustrated in  FIG. 2 , the compressed refrigerant fraction in conduit  112  can be further cooled in propane refrigeration cycle  30 , as described in detail previously. 
         [0051]    Upon being cooled in propane refrigeration cycle  30 , the compressed methane refrigerant fraction 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. 
         [0052]    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 vapor overhead 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 . 
         [0053]    In one embodiment of the present invention, the LNG production systems illustrated in  FIGS. 1 and 2  are simulated on a computer using conventional process simulation software in order to produce simulation results. In one embodiment, the simulation results can be in the form of a computer print out. In another embodiment, the simulation results can be displayed on a screen, monitor, or other viewing device. In yet another embodiment, the simulation results may be electronic signals directly communicated into the LNG system for direct control and/or optimization of the system. 
         [0054]    The simulation results 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 LNG facility. In another embodiment, the simulation results can be used to optimize the LNG facility according to one or more operating parameters. In a further embodiment, the computer simulation can directly control the operation of the LNG facility by, for example, manipulating control valve output. 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. 
       Numerical Ranges 
       [0055]    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 
       [0056]    As used herein, the terms “a,” “an,” “the,” and “said” means one or more. 
         [0057]    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. 
         [0058]    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 pure component refrigerant to successively cool natural gas. 
         [0059]    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. 
         [0060]    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. 
         [0061]    As used herein, the terms “containing,” “contains,” and “contain” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided below. 
         [0062]    As used herein, the term “domestic gas product” refers to any gaseous, predominantly methane stream originating from within an LNG facility that is routed to a location external to the LNG facility prior to sale and/or use. 
         [0063]    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. 
         [0064]    As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided above. 
         [0065]    As used herein, the terms “heavy hydrocarbon” and “heavies” refer to any hydrocarbon component having a molecular weight greater than methane. 
         [0066]    As used herein, the terms “including,” “includes,” and “include” have the same open-ended meaning as “comprising,” “comprises,” and “comprise,” provided above. 
         [0067]    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. 
         [0068]    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. 
         [0069]    As used herein, the term “natural gas” means a stream containing at least 85 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. 
         [0070]    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. 
         [0071]    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 an external source. 
         [0072]    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. 
         [0073]    As used herein, the term “pure component refrigerant” means a refrigerant that is not a mixed refrigerant. 
         [0074]    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 plant. 
       Claims not Limited to Disclosed Embodiments 
       [0075]    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. 
         [0076]    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 or process not materially departing from but outside the literal scope of the invention as set forth in the following claims.