Abstract:
Processes and systems for producing liquefied natural gas (LNG) with a single mixed refrigerant, closed-loop refrigeration cycle are provided. Liquefied natural gas facilities configured according to embodiments of the present invention include refrigeration cycles optimized to provide increased efficiency and enhanced operability, with minimal additional equipment or expense.

Description:
BACKGROUND 
     1. Technical Field 
     One or more embodiments of the present invention generally relate to systems and processes for cooling a feed gas stream with a single closed-loop mixed refrigerant cycle. 
     2. Description of Related Art 
     In recent years, natural gas has become a widely used source of fuel. In addition to its clean burning qualities and convenience, advances in exploration and production technology have permitted previously unreachable gas reserves to become accessible. Because many of these previously unreachable sources of natural gas are remote and are not connected to commercial markets or infrastructure by pipeline, cryogenic liquefaction of natural gas for transportation and storage has become increasingly important. In addition, liquefaction permits long term storage of natural gas, which can help balance out periodic fluctuations in supply and demand. 
     Several methods for liquefying natural gas are currently in practice. Although the specific configuration and/or operation of each facility may vary depending on, for example, the type of refrigeration system used, the rate and composition of feed gas, and other factors, most commercial facilities generally include similar basic components. For example, most facilities typically include a pretreatment area for removing one or more impurities from the incoming gas stream, a liquefaction zone for liquefying the gas stream, a refrigeration system for providing refrigeration to the liquefaction zone, and a storage and/or loading area for receiving, storing, and transporting the final liquefied product. Overall, the cost to construct and operate these facilities can vary widely, but in general, the cost of the refrigeration portion of the plant can account for up to 30 percent or more of the overall cost of the facility. 
     Thus, a need exists for an optimized refrigeration system capable of efficiently producing a liquefied gas product at a desired capacity, but with minimum amount of equipment. Ideally, the refrigeration system would be both robust and operationally flexible in order to handle variations in feed gas composition and flow rate, while still requiring minimal capital outlay and operating at the lowest possible cost. 
     SUMMARY 
     One embodiment of the present invention concerns a process for producing liquefied natural gas (LNG). The process comprises the following steps: (a) cooling a natural gas stream in a first heat exchanger to provide a cooled natural gas stream; (b) compressing a mixed refrigerant stream to provide a compressed refrigerant stream; (c) cooling and at least partially condensing the compressed refrigerant stream to provide a two-phase refrigerant stream; (d) separating the two-phase refrigerant stream into a first refrigerant vapor stream and a first refrigerant liquid stream in a first vapor-liquid separator; (e) combining at least a portion of the first refrigerant vapor stream withdrawn from the first vapor-liquid separator with at least a portion of the first refrigerant liquid stream to provide a combined refrigerant stream; (f) cooling at least a portion of the combined refrigerant stream to provide a cooled combined refrigerant stream; (g) separating the cooled combined refrigerant stream into a second refrigerant vapor stream and a second refrigerant liquid stream in a second vapor-liquid separator; (h) dividing the second refrigerant liquid stream into a first refrigerant liquid fraction and a second refrigerant liquid fraction; (i) cooling at least a portion of the first and second refrigerant liquid fractions to provide respective first and second cooled liquid refrigerant fractions; and (j) introducing the first and second cooled liquid refrigerant fractions into separate inlets of the first heat exchanger, wherein the first and second cooled liquid refrigerant fractions are used to carry out at least a portion of the cooling of step (a). 
     Another embodiment of the present invention concerns a process for producing a liquefied gas stream. The process comprises the following steps: (a) compressing a stream of mixed refrigerant in a first compression stage of a compressor to provide a first compressed refrigerant stream; (b) cooling and at least partially condensing the first compressed refrigerant stream to provide a cooled, compressed refrigerant stream; (c) separating the cooled, compressed refrigerant stream into a first refrigerant vapor stream and a first refrigerant liquid stream; (d) compressing the first refrigerant vapor stream in a second compression stage of the compressor to provide a second compressed refrigerant stream; (e) cooling and at least partially condensing at least a portion of the second compressed refrigerant stream to provide a partially condensed refrigerant stream; (f) separating the partially condensed refrigerant into a second refrigerant vapor stream, a second refrigerant liquid stream, and a third refrigerant liquid stream; (g) cooling the second and third refrigerant liquid streams to provide respective cooled second and third refrigerant liquid streams; (h) expanding at least one of the cooled second and cooled third refrigerant liquid streams to provide at least one cooled, expanded refrigerant stream; (i) cooling a feed gas stream via indirect heat exchange with the at least one cooled, expanded refrigerant stream to provide a cooled feed gas stream and at least one warmed refrigerant stream. 
     Yet another embodiment of the present invention concerns a system for cooling a natural gas stream. The system comprises a first heat exchanger for cooling a natural gas feed stream. The first heat exchanger comprises a first cooling pass having a feed gas inlet and a cool natural gas outlet, a second cooling pass for receiving and cooling a first stream of refrigerant liquid, wherein the second cooling pass has a first warm refrigerant inlet and a first cool refrigerant outlet; a third cooling pass for receiving and cooling a second stream of refrigerant liquid, wherein the third cooling pass has a second warm refrigerant inlet and a second cool refrigerant outlet; a first warming pass for receiving and warming a first stream of cooled refrigerant, wherein the first warming pass has a first cool refrigerant inlet and a first warm refrigerant outlet; and a second warming pass for receiving and warming a second stream of cooled refrigerant liquid, wherein the second warming pass has a second cool refrigerant inlet and a second warm refrigerant outlet. The first cool refrigerant outlet of the second cooling pass is in fluid flow communication with the first cool refrigerant inlet of the first warming pass, and the second cool refrigerant outlet of the third cooling pass is in fluid flow communication with the second cool refrigerant inlet of the second warming pass. The system also comprises at least one compressor for receiving and pressurizing a stream of mixed refrigerant. The compressor has a low pressure inlet and a high pressure outlet and the low pressure inlet is in fluid flow communication with at least one of the first warm refrigerant outlet of the first warming pass and the second warm refrigerant outlet of the second warming pass. The system also comprises a first cooler for cooling the pressurized stream of mixed refrigerant. The first cooler has a first warm fluid inlet and a first cool fluid outlet and the first warm fluid inlet is in fluid flow communication with the high pressure outlet of the compressor. The system also comprises a first vapor-liquid separator for separating a portion of the cooled refrigerant stream. The vapor-liquid separator comprises a first fluid inlet, a first vapor outlet, and a first liquid outlet and the first fluid inlet of the first vapor-liquid separator is in fluid flow communication with the first cool fluid outlet of the first cooler. The system also comprises a first liquid conduit for transporting at least a portion of the liquid exiting the first vapor-liquid separator. The first liquid conduit has a refrigerant liquid inlet and a pair of refrigerant liquid outlets. The refrigerant liquid inlet is in fluid flow communication with the first liquid outlet of the first vapor-liquid separator. One of the pair of refrigerant liquid outlets is in fluid flow communication with the first warm refrigerant inlet of the second cooling pass and the other of the pair of refrigerant liquid outlets is in fluid flow communication with the second warm refrigerant inlet of the third cooling pass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention are described in detail below with reference to the attached Figures, wherein: 
         FIG. 1  provides a schematic depiction of a liquefied natural gas (LNG) facility configured according to one embodiment of the present invention, particularly illustrating an optimized mixed refrigerant system; 
         FIG. 2  provides a schematic depiction of a liquefied natural gas (LNG) facility configured according to another embodiment of the present invention, similar to the embodiment depicted in  FIG. 1 , but including a method for recycling refrigerant liquids; and 
         FIG. 3  provides a schematic depiction of a liquefied natural gas (LNG) facility configured according to another embodiment of the present invention, similar to the embodiment depicted in  FIG. 1 , but including another method for recycling refrigerant liquids. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of embodiments of the invention references the accompanying drawings. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the claims. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     The present invention generally relates to processes and systems for liquefying a natural gas feed stream to thereby provide a liquefied natural gas (LNG) product. In particular, the present invention relates to optimized refrigeration processes and systems for cooling the incoming gas. As described in further detail below, the incoming feed gas stream can be cooled and at least partially condensed with a closed-loop refrigeration system employing a single mixed refrigerant. According to various embodiments of the present invention, the refrigeration system may be optimized to provide efficient cooling for the feed gas stream, while minimizing the expenses associated with the equipment and operating costs of the facility. 
     Referring initially to  FIG. 1 , one embodiment of an LNG production facility  10  is illustrated as comprising a closed-loop, mixed refrigerant refrigeration system  12  and a gas separation zone  14 . As shown in  FIG. 1 , the incoming feed gas stream in conduit  110  can be cooled and at least partially condensed in a primary heat exchanger  16  of refrigeration cycle  12  before being separated and further cooled in gas separation zone  14  to provide the LNG product. Additional details regarding the configuration and operation of LNG facility  10 , according to various embodiments of the present invention, are described below with reference to  FIG. 1 . 
     As shown in  FIG. 1 , a feed gas stream can be introduced into LNG facility  10  via conduit  110 . The incoming gas stream in conduit  110  can be any gas stream requiring cooling and, in some embodiments, can be a natural gas feed stream originating from one or more gas sources (not shown). Examples of suitable gas sources can include, but are not limited to, natural sources such as, subterranean formations and petroleum production wells, and/or refining units such as fluidized catalytic crackers, petroleum cokers, or heavy oil processing units, such as oil sands upgraders. Depending on the origin and composition of the feed gas stream, LNG facility  10  can include one or more additional processing units or zones (not shown) upstream of primary heat exchanger  16  for removing unwanted components such as water, sulfur, mercury, nitrogen, and heavy (C 6   + ) hydrocarbon materials from the feed gas stream prior to its liquefaction. 
     According to one embodiment, the feed gas stream in conduit  110  can comprise at least about 65, at least about 75, at least about 85, at least about 95, at least 99 weight percent methane, based on the total weight of the stream. Typically, heavier components such as C 2 , C 3 , and heavier hydrocarbons, and trace amounts of components such as hydrogen and nitrogen, can make up the balance of the composition fo the feed gas stream. As discussed previously, the stream in conduit  110  may have undergone one or more pretreatment steps to reduce the amount of or remove one or more components other than methane from the feed gas stream. In one embodiment, the feed gas stream in conduit  110  comprises less than about 25, less than about 20, less than about 15, less than about 10, or less than about 5 percent of components other than methane. Depending on the source and composition of the feed gas stream, the undesired components removed in the pretreatment steps can include, but are not limited to, water, mercury, sulfur compounds, and other materials. 
     As shown in  FIG. 1 , the feed gas stream in conduit  110  can be introduced into a first cooling pass  18  of a primary heat exchanger  16 , wherein the stream may be cooled and at least partially condensed via indirect heat exchange with at least one yet-to-be-discussed stream of mixed refrigerant. Terms such as “first,” “second,” and “third,” are used herein and in the appended claims to describe various elements of systems and processes of the present invention, and such elements should not be limited to by these terms. These terms are only used to distinguish one element from another and do not necessarily imply a specific order or even a specific element. For example, an element may be regarded as a “first” element in the description and a “second element” in the claims without departing from the scope of the present invention. Consistency is maintained within the description and each independent claim, but such nomenclature is not necessarily intended to be consistent therebetween. 
     Primary heat exchanger  16  shown in  FIG. 1  can be any type of heat exchanger, or a series of heat exchangers, operable to cool and at least partially condense the feed gas stream in conduit  110 . For example, in some embodiments, primary heat exchanger  16  can be a brazed aluminum heat exchanger comprising a plurality of warming and cooling passes (e.g., cores) disposed within the exchanger configured to facilitate indirect heat exchange between one or more process streams and one or more refrigerant streams. In some embodiments, one or more of the warming and/or cooling passes may be alternately defined between a plurality of plates disposed within the external “shell” of exchanger  16 . It should be understood that, although generally illustrated in  FIG. 1  as comprising a single shell, primary heat exchanger  16  may, in some embodiments, comprise two or more separate shells optionally encompassed by a “cold box” to minimize heat loss to the surrounding environment. Other types or configurations of primary heat exchanger  16  may also be suitable and are contemplated to be within the scope of the present invention. 
     Referring back to  FIG. 1 , the cooled, two-phase stream withdrawn from cooling pass  18  of primary heat exchanger  16  via conduit  112  may subsequently be introduced into a vapor-liquid separator  20 . Separator  20  can be any suitable type of vapor-liquid separation vessel and may include any number of actual or theoretical separation stages. In one embodiment, vapor-liquid separation vessel may comprise a single separation stage, while, in other embodiments, separation vessel  20  can include at least about 2, at least about 5, at least about 10 and/or not more than about 50, not more than about 40, not more than about 25 actual or theoretical separation stages. Separator  20  may include any suitable type of column internals, including, for example, mist eliminators, mesh pads, vapor-liquid contacting trays, random packing, and/or structured packing in order to facilitate heat and/or mass transfer between the vapor and liquid streams. In some embodiments, when separator  20  comprises a single-stage separation vessel, few or no column internals may be employed. Additionally, gas separation zone  14  may include one or more other separation vessels (not shown) arranged in parallel or in series with separator  20 . When gas separation zone  14  includes one or more additional vapor-liquid separators, each of the additional separators may configured similarly to or different than separator  20 . 
     As shown in  FIG. 1 , separator  20  can separate the two-phase fluid stream in conduit  112  into an overhead vapor stream in conduit  114  and a bottoms liquid stream in conduit  116 . Typically, the overhead vapor stream withdrawn from separator  20  via conduit  114  may be enriched in methane and lighter components, while the bottoms liquid stream in conduit  116  may be a methane-depleted stream enriched one or more heavier components, such as ethane, propane, and others. In some embodiments, the bottoms liquid stream in conduit  116  may be recovered as a separate natural gas liquids (NGL) product stream and may be subjected to further downstream processing and/or separation (not shown). 
     As shown in one embodiment depicted in  FIG. 1 , the overhead vapor stream withdrawn from separator  20  via conduit  114  may be routed into a second natural gas cooling pass  22  of primary heat exchanger  16 . In cooling pass  22 , the cooled gas stream may be further cooled, condensed, and optionally sub-cooled, via indirect heat exchange with one or more yet-to-be-discussed refrigerant streams. As shown in  FIG. 1 , the resulting sub-cooled LNG product stream may be withdrawn from primary heat exchanger  16  via conduit  118 . In some embodiments, the LNG product stream in conduit  118  may have a temperature in the range of from about −200° F. to about −290° F., about −220° F. to about −280° F., or about −240° F. to about −275° F. and/or a pressure of less than about 50 psia, less than about 40 psia, less than about 30 psia, or less than about 20 psia. Although not illustrated in  FIG. 1 , LNG facility  10  may also include additional processing units and/or storage facilities downstream of primary heat exchanger  16  to further process, separate, and/or store the LNG product stream in conduit  118 . In some embodiments, at least a portion of the LNG product may be transported from LNG facility  10  to one or more separate facilities (not shown) for subsequent storage, processing, and/or use. 
     Turning now the embodiment of refrigeration system  12  of LNG facility  10  depicted in  FIG. 1 , refrigeration cycle  12  illustrated as generally including a refrigerant suction drum  28 , a multi-stage refrigerant compressor  30 , an interstage cooler  32 , an interstage accumulator  34 , an interstage refrigerant pump  36 , a refrigerant condenser  38 , a refrigerant accumulator  40 , and a refrigerant pump  42 . Additionally, refrigeration system  12  includes a pair of refrigerant cooling passes  52  and  58  and a pair of refrigerant warming passes  56  and  62 , each having an expansion device  54  and  60 , respectively disposed between cooling pass  52  and warming pass  56  and cooling pass  58  and warming pass  62 . 
     According to one embodiment of the present invention, the refrigerant utilized in closed-loop refrigeration cycle  12  may be a mixed refrigerant. As used herein, the term “mixed refrigerant” refers to a refrigerant composition comprising two or more constituents. In one embodiment, the mixed refrigerant utilized by refrigeration cycle  12  may be a single mixed refrigerant and can comprise two or more components selected from the group consisting of methane, ethylene, ethane, propylene, propane, isobutane, n-butane, isopentane, n-pentane, and combinations thereof. In some embodiments, the refrigerant composition can comprise methane, ethane, propane, normal butane, and isopentane and can exclude certain components, including, for example, nitrogen or halogenated hydrocarbons. Various specific refrigerant compositions are contemplated according to embodiments of the present invention. Table 1, below, summarizes broad, intermediate, and narrow ranges for several exemplary components that may be employed in refrigerant mixtures suitable for use in refrigerant cycle  12 , according to various embodiments of the present invention. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary Mixed Refrigerant Compositions 
               
             
          
           
               
                   
                   
                 Broad 
                 Intermediate 
                 Narrow 
               
               
                   
                   
                 Range, 
                 Range, 
                 Range, 
               
               
                   
                 Component 
                 mole % 
                 mole % 
                 mole % 
               
               
                   
                   
               
               
                   
                 methane 
                 0 to 50 
                 5 to 40 
                 10 to 30 
               
               
                   
                 ethylene 
                 0 to 50 
                 5 to 40 
                 10 to 30 
               
               
                   
                 ethane 
                 0 to 50 
                 5 to 40 
                 10 to 30 
               
               
                   
                 propylene 
                 0 to 50 
                 5 to 40 
                  5 to 30 
               
               
                   
                 propane 
                 0 to 50 
                 5 to 40 
                  5 to 30 
               
               
                   
                 i-butane 
                 0 to 10 
                 0 to 5  
                 0 to 2 
               
               
                   
                 n-butane 
                 0 to 25 
                 1 to 20 
                  5 to 15 
               
               
                   
                 i-pentane 
                 0 to 30 
                 1 to 20 
                  2 to 15 
               
               
                   
                 n-pentane 
                 0 to 10 
                 0 to 5  
                 0 to 2 
               
               
                   
                 nitrogen 
                 0 to 30 
                 0 to 25 
                  0 to 20 
               
               
                   
                   
               
             
          
         
       
     
     In some embodiments of the present invention, it may be desirable to adjust the composition of the mixed refrigerant to thereby alter its cooling curve and, therefore, its refrigeration potential. Such a modification may be utilized to accommodate, for example, changes in composition and/or flow rate of the feed gas stream introduced into LNG facility  10 . In one embodiment, the composition of the mixed refrigerant can be adjusted such that the heating curve of the vaporizing refrigerant more closely matches the cooling curve of the feed gas stream. One method for such curve matching is described in detail in U.S. Pat. No. 4,033,735, the disclosure of which is incorporated herein by reference in its entirety and to the extent not inconsistent with the present disclosure. In some embodiments, ability to alter the composition and, consequently, the heating curve of the refrigerant provides increased flexibility and operability to the facility, enabling it to receive and efficiently process feed streams having a wider variety of gas compositions. 
     Referring again to refrigeration cycle  12  shown in the embodiment of facility  10  in  FIG. 1 , a stream of mixed refrigerant in conduit  120  may be introduced into a fluid inlet of refrigerant suction drum  28 , wherein any liquid present may be separated from the vapor phase. When present, the liquids may then be withdrawn from a lower liquid outlet of suction drum  28  and can be returned to the circulating system (not shown). As shown in  FIG. 1 , a vapor phase stream of mixed refrigerant can be withdrawn from an upper vapor outlet of suction drum  28  and routed to a low pressure inlet of a low pressure compression stage  44  of multi-stage compressor  30 . Multi-stage compressor  30  may be any type of compressor suitable to increase the pressure of the mixed refrigerant in closed-loop mixed refrigeration cycle  12 . Although illustrated in  FIG. 1  as generally comprising two compression stages, multi-stage compressor  30  may include three or more stages, in accordance with other embodiments of the present invention. 
     As shown in  FIG. 1 , the compressed refrigerant stream withdrawn from the intermediate pressure outlet of low pressure compression stage  44  of refrigerant compressor  30  via conduit  126  can be routed to the warm fluid inlet of interstage cooler  32 , wherein the stream can be cooled and at least partially condensed via indirect heat exchange with at least one coolant stream (e.g., air or cooling water). The resulting two-phase refrigerant stream in conduit  128  can then be routed to an interstage accumulator  34 , wherein the vapor and liquid phases may be separated. As shown in  FIG. 1 , the vapor stream withdrawn from interstage accumulator  34  via conduit  132  can be introduced into an intermediate pressure inlet of a high pressure compression stage  46  of multi-stage compressor, which can be connected to low pressure compression stage  44  via shaft  48 . In high pressure compression stage  46 , the mixed refrigerant stream may be further compressed before being discharged from a high-pressure outlet of high pressure compression stage  46  into conduit  134 . Additionally, as depicted in the embodiment shown in  FIG. 1 , the liquid portion of the refrigerant stream withdrawn from interstage accumulator  34  via conduit  130  may be pumped to a higher pressure via refrigerant pump  36 , before being combined with the compressed refrigerant stream in conduit  134 . In one embodiment, the pressure of the liquid stream discharged from refrigerant pump  36  in conduit  136  can be within about 100, within about 50, within about 20, within about 10, or within about 5 psi of the pressure of the vapor stream in conduit  134  prior to combination of the two streams. 
     The combined refrigerant stream in conduit  138  can then be introduced into a refrigerant condenser  38 , wherein the stream may be cooled and at least partially condensed via indirect heat exchange with a coolant stream (e.g., cooling water). The resulting cooled, at least partially condensed refrigerant stream in conduit  140  may then be introduced into a refrigerant accumulator  40 , wherein the vapor and liquid phases may be separated. As shown in  FIG. 1 , the vapor phase refrigerant stream in conduit  142  may be withdrawn and combined with a yet-to-be-discussed liquid refrigerant stream before being introduced into primary heat exchanger  16 . 
     According to one embodiment of the present invention, the liquid refrigerant stream withdrawn from refrigerant accumulator  40  via conduit  144  can be pressurized via refrigerant pump  40  and the resulting stream discharged into conduit  146  may be passed through a dividing device  50 , which can be configured to divide the pressurized liquid refrigerant into two separate portions in conduits  148  and  150 . As shown in  FIG. 1 , dividing device  50  is not a vapor-liquid separator, but, instead, can be any device configured to divide the liquid stream in conduit  146  into two streams of similar composition and state. The flow rates of the individual streams in conduits  148  and  150  may be similar or different. For example, in some embodiments, the ratio of the mass flow rate of the stream in conduit  148  to the mass flow rate of the stream in conduit  150  can be at least about 0.5:1, at least about 0.75:1, at least about 0.95:1 and/or not more than about 2:1, not more than about 1.75:1, not more than about 1.5:1, not more than about 1.25:1. In the same or other embodiments, the ratio of the mass flow rate of the stream in conduit  148  to the mass flow rate of the stream in conduit  150  can be approximately 1:1. 
     As shown in  FIG. 1 , the first portion of the liquid refrigerant stream in conduit  148  may be combined with the vapor phase refrigerant stream withdrawn from refrigerant accumulator  40  in conduit  142 . The amount of vapor and/or liquid introduced into conduits  142  and/or  148  may be controlled to achieve a desired ratio of vapor to liquid introduced into a refrigerant cooling pass  58  disposed within primary heat exchanger  16 . In one embodiment, the combined stream introduced into cooling pass  58  can have a vapor fraction of at least about 0.45, at least about 0.55, at least about 0.65 and/or not more than about 0.95, not more than about 0.90, not more than about 0.85. Although illustrated as being combined just prior to introduction into cooling pass  58 , it should be understood that the liquid stream in conduit  148  and the vapor phase refrigerant stream in conduit  142  may be alternatively be combined within primary heat exchanger  16  or may be combined at a different location further upstream of heat exchanger  16 , so that the combined stream may be introduced into cooling pass  58  via a common conduit external to primary heat exchanger  16  (embodiment not shown in  FIG. 1 ). 
     As shown in  FIG. 1 , the combined refrigerant stream introduced into primary heat exchanger  16  descends vertically downward through cooling pass  58 , wherein it can be cooled and condensed via indirect heat exchange with one or more refrigerant streams. The resulting condensed and subcooled liquid stream can be withdrawn from the lower portion of primary heat exchanger  16  via conduit  158 . As shown in  FIG. 1 , the liquid refrigerant stream in conduit  158  may then be passed through an expansion device  60 , wherein the pressure of the stream can be reduced to thereby flash a portion thereof. The resulting cooled, two-phase stream in conduit  160  can then be introduced into refrigerant warming pass  62 , wherein the stream may be warmed as it ascends vertically upwardly through primary heat exchanger  16 . As the ascending refrigerant stream is warmed, it can provide refrigeration to one or more of the streams being cooled, as described previously. 
     According to one embodiment of the present invention, the second portion of the liquid refrigerant stream withdrawn from refrigerant accumulator  40  via conduit  150  can be separately introduced into a second refrigerant cooling pass  52  disposed within primary heat exchanger  16 . As the liquid stream travels vertically downward through cooling pass  52 , it is cooled and condensed via indirect heat exchange with one or more refrigerant streams. The resulting liquid refrigerant stream exiting cooling pass  52  in conduit  152  can then be passed through expansion device  54 , wherein the pressure of the stream can be reduced to thereby flash a portion of the stream. Although generally depicted as being an expansion valve or Joule-Thompson (JT) valve in  FIG. 1 , it should also be understood that expansion device  54  may comprise any suitable type of expander, including, for example, a JT orifice or a turboexpander (not shown). Similarly, expansion device  54  may include, in some embodiments, two or more expansion devices, arranged in parallel or in series, configured to reduce the pressure of the liquid refrigerant stream in conduit  152 . 
     The resulting cooled, two-phase refrigerant stream in conduit  154  may then be reintroduced into another refrigerant warming pass  56  of primary heat exchanger  16 , wherein the stream can be warmed to thereby providing refrigeration to one or more other fluid streams being cooled in primary heat exchanger  16 , including the refrigerant streams in conduits  150  and  158  in respective cooling passes  52  and  58 , the natural gas feed stream in conduit  110  in cooling pass  18 , and/or the overhead vapor stream in conduit  114  in cooling pass  22 . 
     According to one embodiment depicted in  FIG. 1 , the overall length of refrigerant cooling pass  52  can be less than the overall length of refrigerant cooling pass  58 . Consequently, the cooled refrigerant stream exiting refrigerant cooling pass  52  via conduit  152  may be withdrawn from a higher vertical elevation along the height of primary heat exchanger  16  than the cooled refrigerant stream withdrawn from refrigerant cooling pass  58 . For example, in one embodiment depicted in  FIG. 1 , the cooled refrigerant stream exiting refrigerant cooling pass  52  may be withdrawn from a vertical mid-point of primary exchanger  16 , while the cooled refrigerant stream exiting refrigerant cooling pass  58  may be withdrawn from an outlet positioned near the lower vertical end of primary exchanger  16 . According to one embodiment, the ratio of the total length of refrigerant cooling pass  52  to the total length of refrigerant cooling pass  58  can be at least about 0.15:1, at least about 0.25:1, at least about 0.35:1 and/or not more than about 0.75:1, not more than about 0.65:1, not more than about 0.50:1, or in the range of from about 0.15:1 to about 0.75:1, about 0.25:1 to about 0.65:1, or about 0.25:1 to about 0.50:1. In the same or other embodiments, the ratio of the total length of refrigerant cooling pass  52  to the overall height (i.e., vertical dimension) of primary heat exchanger  16  can be at least about 0.15:1, at least about 0.25:1, at least about 0.35:1 and/or not more than about 0.75:1, not more than about 0.65:1, not more than about 0.55:1, while the ratio of the total length of cooling pass  58  to the overall height of primary heat exchanger  16  can be about 1:1. 
     As shown in  FIG. 1 , a first warmed mixed refrigerant stream, which may have a vapor fraction of at least about 0.85, at least about 0.90, at least about 0.95, can be withdrawn from warming pass  62  via conduit  162  and a second warmed refrigerant stream having a similar vapor fraction may be withdrawn from warming pass  58  via conduit  156 . According to one embodiment depicted in  FIG. 1 , the two streams of warmed refrigerant stream may then be combined and the resulting stream in conduit  120  may thereafter be recirculated to the inlet of refrigerant suction drum  28 , as described in detail previously. 
     Turning now to  FIG. 2 , another embodiment of LNG facility  10  is illustrated. The embodiment of LNG facility  10  shown in  FIG. 2  is similar to the embodiment depicted in  FIG. 1 , but includes a different configuration of various components of refrigeration system  12 . The main components of LNG facility  10  shown in  FIG. 2  are numbered the same as those depicted in  FIG. 1 . The operation of LNG facility  10  illustrated in  FIG. 2 , as it differs from that previously discussed with respect to  FIG. 1 , will now be described in detail below. 
     As shown in  FIG. 2 , the stream of mixed refrigerant in conduit  120  introduced into refrigerant suction drum  28  can be separated into an overhead vapor stream in conduit  124  and a bottoms liquid stream in conduit  122 . According to the embodiment depicted in  FIG. 2 , the bottoms liquid stream in conduit  122  withdrawn from refrigerant suction drum  28  can be pressurized via a refrigerant pump  64  and the resulting stream in conduit  123  may then be combined with the two-phase refrigerant stream in conduit  138 . Thereafter, the combined refrigerant stream in conduit  138  can be introduced into refrigerant condenser  38  and the resulting cooled stream can then pass through the remainder of refrigeration cycle  12 , as discussed in detail previously with respect to  FIG. 1 . In one embodiment (not shown in  FIG. 2 ), it may be possible to combine the pressurized liquid bottoms stream in conduit  123  with the compressed vapor refrigerant stream exiting the high pressure compression stage  46  in conduit  134  to produce a combined stream, which can subsequently be combined with the pressurized liquid phase refrigerant stream discharged from interstage pump  36  in conduit  136 . 
     According to one embodiment, the addition of refrigerant pump  64  to the lower liquid conduit  122  of refrigeration suction drum  28  may permit refrigeration cycle  12  to utilize refrigerants having different compositions than those suitable for use in the embodiment of LNG facility  10  shown in  FIG. 1 . In particular, the employment of a refrigeration liquid recycle conduit  123  as shown in the embodiment of LNG facility  10  depicted in  FIG. 2 , may allow refrigeration cycle  12  to employ a mixed refrigerant that includes a higher concentration of heavy hydrocarbons than the mixed refrigerant utilized in LNG facility  10  shown in  FIG. 1 . As described previously, it may be desirable to alter the composition of the mixed refrigerant employed in refrigeration cycle  12  to, for example, accommodate changes in composition of the feed gas stream and more closely match the heating curve of the mixed refrigerant with the cooling curve of the natural gas stream. In some embodiments, the option to utilize mixed refrigerants of varying composition, including those refrigerant compositions including a higher amount of heavier components, may impart even more operating flexibility to LNG facilities configured according to embodiments of the present invention. 
     Turning now to  FIG. 3 , yet another embodiment of LNG facility  10  is illustrated. The embodiment of LNG facility  10  shown in  FIG. 3  is similar to the embodiment depicted in  FIG. 1 , but includes a different configuration of various components of refrigeration system  12 . The main components of LNG facility  10  shown in  FIG. 3  are numbered the same as those depicted in  FIG. 1 . The operation of LNG facility  10  illustrated in  FIG. 3 , as it differs from that previously discussed with respect to  FIG. 1 , will now be described. 
     As shown in  FIG. 3 , two streams of warmed mixed refrigerant can be withdrawn from refrigerant warming pass  56  and refrigerant warming pass  62  via respective conduits  156  and  162 . Rather than being combined, as shown in the embodiment depicted in  FIG. 1 , the warmed refrigerant streams in conduits  156  and  162  remain separate as shown in the embodiment of LNG facility  10  shown in  FIG. 3 . As shown in  FIG. 3 , the warmed refrigerant vapor stream in conduit  156 , which may have a temperature that is at least about 25° F., at least about 50° F., at least about 75° F. and/or not more than about 150° F., not more than about 125° F., not more than about 100° F. warmer than the warmed refrigerant vapor stream in conduit  162 , may be routed to a fluid inlet of a refrigerant separator  68 , wherein the vapor and liquid portions may be separated from each other. Refrigerant separator  68  may be any suitable type of vapor-liquid separator and can optionally include one or more tower internals described in detail previously with respect to separator  20 . 
     As shown in  FIG. 3 , the liquid portion of the warmed refrigerant stream introduced into refrigerant separator  68  may be withdrawn from separator  68  via conduit  166  and pumped to a higher pressure via a refrigerant pump  70 . The resulting, pressurized stream of liquid refrigerant in conduit  168  may then be combined with the previously-discussed two-phase pressurized refrigerant stream in conduit  138 . The resulting combined refrigerant stream in conduit  139  may then be introduced into refrigerant condenser  38 , wherein the stream can be cooled and at least partially condensed before continuing through the remaining portions of refrigeration cycle  12  as described previously with respect to  FIG. 1 . 
     Referring again to  FIG. 3 , the vapor portion of the warmed refrigerant stream introduced into refrigerant separator  68  may be withdrawn from the upper portion of separator  68  via conduit  164  and combined with the second warmed refrigerant stream withdrawn from refrigerant warming pass  62  in conduit  162 . The resulting combined vapor-phase refrigerant stream in conduit  120  can then be routed to the inlet of refrigerant suction drum  28 , wherein the stream may be separated into vapor and liquid portions withdrawn from drum  28  via respective conduits  124  and  122 , as shown in  FIG. 3 . Thereafter, each of the vapor and liquid portions may continue through the remainder of refrigeration cycle  12  as discussed in detail previously with respect to  FIG. 1 . 
     Although described herein with respect to liquefying a natural gas stream, it should it should also be understood that processes and systems of the present invention may also be suitable for use in other gas processing and separation applications, including, but not limited to, ethane recovery and liquefaction, recovery of natural gas liquids (NGL), syngas separation and methane recovery, and cooling and separation of nitrogen and/or oxygen from various hydrocarbon-containing gas streams. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary one embodiment, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.