Patent Publication Number: US-2023136307-A1

Title: Managing Make-Up Gas Composition Variation for a High Pressure Expander Process

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Application No. 62/721,367, “Managing Make-Up Gas Composition Variation for a High Pressure Expander Process,” filed Aug. 22, 2018; U.S. Provisional Application No. 62/565,725, “Natural Gas Liquefaction by a High Pressure Expansion Process”, filed Sep. 29, 2017; U.S. Provisional Application No. 62/565,733, “Natural Gas Liquefaction by a High Pressure Expansion Process,” filed Sep. 29, 2017; and U.S. Provisional Application No. 62/576,989, “Natural Gas Liquefaction by a High Pressure Expansion Process Using Multiple Turboexpander Compressors”, filed Oct. 25, 2017, the disclosures of which are incorporated by reference herein in their entireties for all purposes. 
     This application is related to U.S. Provisional Application No. 62/721,375, “Primary Loop Start-up Method for a High Pressure Expander Process”; and U.S. Provisional Application No. 62/721,374, “Heat Exchanger Configuration for a High Pressure Expander Process and a Method of Natural Gas Liquefaction Using the Same,” having common ownership and filed on an even date, the disclosures of which are incorporated by reference herein in their entireties for all purposes. 
    
    
     BACKGROUND 
     Field of Disclosure 
     The disclosure relates generally to liquefied natural gas (LNG) production. More specifically, the disclosure relates to LNG production at high pressures. 
     Description of Related Art 
     This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as an admission of prior art. 
     Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, which are great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (LNG) for transport to market. 
     In the design of an LNG plant, one of the most important considerations is the process for converting the natural gas feed stream into LNG. Currently, the most common liquefaction processes use some form of refrigeration system. Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) the “cascade cycle,” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature; (2) the “multi-component refrigeration cycle,” which uses a multi-component refrigerant in specially designed exchangers; and (3) the “expander cycle,” which expands gas from feed gas pressure to a low pressure with a corresponding reduction in temperature. Most natural gas liquefaction cycles use variations or combinations of these three basic types. 
     The refrigerants used in liquefaction processes may comprise a mixture of components such as methane, ethane, propane, butane, and nitrogen in multi-component refrigeration cycles. The refrigerants may also be pure substances such as propane, ethylene, or nitrogen in “cascade cycles.” Substantial volumes of these refrigerants with close control of composition are required. Further, such refrigerants may have to be imported and stored, which impose logistics requirements, especially for LNG production in remote locations. Alternatively, some of the components of the refrigerant may be prepared, typically by a distillation process integrated with the liquefaction process. 
     The use of gas expanders to provide the feed gas cooling, thereby eliminating or reducing the logistical problems of refrigerant handling, is seen in some instances as having advantages over refrigerant-based cooling. The expander system operates on the principle that the refrigerant gas can be allowed to expand through an expansion turbine, thereby performing work and reducing the temperature of the gas. The low temperature gas is then heat exchanged with the feed gas to provide the refrigeration needed. The power obtained from cooling expansions in gas expanders can be used to supply part of the main compression power used in the refrigeration cycle. The typical expander cycle for making LNG operates at the feed gas pressure, typically under about 6,895 kPa (1,000 psia). Supplemental cooling is typically needed to fully liquefy the feed gas and this may be provided by additional refrigerant systems, such as secondary cooling and/or sub-cooling loops. For example, U.S. Pat. Nos. 6,412,302 and 5,916,260 present expander cycles which describe the use of nitrogen as refrigerant in the sub-cooling loop. 
     Previously proposed expander cycles have all been less efficient thermodynamically, however, than the current natural gas liquefaction cycles based on refrigerant systems. Expander cycles have therefore not offered any installed cost advantage to date, and liquefaction cycles involving refrigerants are still the preferred option for natural gas liquefaction. 
     Because expander cycles result in a high recycle gas stream flow rate and high inefficiency for the primary cooling (warm) stage, gas expanders have typically been used to further cool feed gas after it has been pre-cooled to temperatures well below −20° C. using an external refrigerant in a closed cycle, for example. Thus, a common factor in most proposed expander cycles is the requirement for a second, external refrigeration cycle to pre-cool the gas before the gas enters the expander. Such a combined external refrigeration cycle and expander cycle is sometimes referred to as a “hybrid cycle.” While such refrigerant-based pre-cooling eliminates a major source of inefficiency in the use of expanders, it significantly reduces the benefits of the expander cycle, namely the elimination of external refrigerants. 
     U. S. Patent Application US2009/0217701 introduced the concept of using high pressure within the primary cooling loop to eliminate the need for external refrigerant and improve efficiency, at least comparable to that of refrigerant-based cycles currently in use. The high pressure expander process (HPXP), disclosed in U. S. Patent Application US2009/0217701, is an expander cycle which uses high pressure expanders in a manner distinguishing from other expander cycles. A portion of the feed gas stream may be extracted and used as the refrigerant in either an open loop or closed loop refrigeration cycle to cool the feed gas stream below its critical temperature. Alternatively, a portion of LNG boil-off gas may be extracted and used as the refrigerant in a closed loop refrigeration cycle to cool the feed gas stream below its critical temperature. This refrigeration cycle is referred to as the primary cooling loop. The primary cooling loop is followed by a sub-cooling loop which acts to further cool the feed gas. Within the primary cooling loop, the refrigerant is compressed to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia. The refrigerant is then cooled against an ambient cooling medium (air or water) prior to being near isentropically expanded to provide the cold refrigerant needed to liquefy the feed gas. 
       FIG.  1    depicts an example of a known HPXP liquefaction process  100 , and is similar to one or more processes disclosed in U. S. Patent Application US2009/0217701. In  FIG.  1   , an expander loop  102  (i.e., an expander cycle) and a sub-cooling loop  104  are used. Feed gas stream  106  enters the HPXP liquefaction process at a pressure less than about 1,200 psia, or less than about 1,100 psia, or less than about 1,000 psia, or less than about 900 psia, or less than about 800 psia, or less than about 700 psia, or less than about 600 psia. Typically, the pressure of feed gas stream  106  will be about 800 psia. Feed gas stream  106  generally comprises natural gas that has been treated to remove contaminants using processes and equipment that are well known in the art. 
     In the expander loop  102 , a compression unit  108  compresses a refrigerant stream  109  (which may be a treated gas stream) to a pressure greater than or equal to about 1,500 psia, thus providing a compressed refrigerant stream  110 . Alternatively, the refrigerant stream  109  may be compressed to a pressure greater than or equal to about 1,600 psia, or greater than or equal to about 1,700 psia, or greater than or equal to about 1,800 psia, or greater than or equal to about 1,900 psia, or greater than or equal to about 2,000 psia, or greater than or equal to about 2,500 psia, or greater than or equal to about 3,000 psia, thus providing compressed refrigerant stream  110 . After exiting compression unit  108 , compressed refrigerant stream  110  is passed to a cooler  112  where it is cooled by indirect heat exchange with a suitable cooling fluid to provide a compressed, cooled refrigerant stream  114 . Cooler  112  may be of the type that provides water or air as the cooling fluid, although any type of cooler can be used. The temperature of the compressed, cooled refrigerant stream  114  depends on the ambient conditions and the cooling medium used, and is typically from about 35° F. to about 105° F. Compressed, cooled refrigerant stream  114  is then passed to an expander  116  where it is expanded and consequently cooled to form an expanded refrigerant stream  118 . Expander  116  is a work-expansion device, such as a gas expander, which produces work that may be extracted and used for compression. Expanded refrigerant stream  118  is passed to a first heat exchanger  120 , and provides at least part of the refrigeration duty for first heat exchanger  120 . Upon exiting first heat exchanger  120 , expanded refrigerant stream  118  is fed to a compression unit  122  for pressurization to form refrigerant stream  109 . 
     Feed gas stream  106  flows through first heat exchanger  120  where it is cooled, at least in part, by indirect heat exchange with expanded refrigerant stream  118 . After exiting first heat exchanger  120 , the feed gas stream  106  is passed to a second heat exchanger  124 . The principal function of second heat exchanger  124  is to sub-cool the feed gas stream. Thus, in second heat exchanger  124  the feed gas stream  106  is sub-cooled by sub-cooling loop  104  (described below) to produce sub-cooled stream  126 . Sub-cooled stream  126  is then expanded to a lower pressure in expander  128  to form a liquid fraction and a remaining vapor fraction. Expander  128  may be any pressure reducing device, including, but not limited to a valve, control valve, Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and the like. The sub-cooled stream  126 , which is now at a lower pressure and partially liquefied, is passed to a surge tank  130  where the liquefied fraction  132  is withdrawn from the process as an LNG stream  134 , which has a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream  136  may be used as fuel to power the compressor units. 
     In sub-cooling loop  104 , an expanded sub-cooling refrigerant stream  138  (preferably comprising nitrogen) is discharged from an expander  140  and drawn through second and first heat exchangers  124 ,  120 . Expanded sub-cooling refrigerant stream  138  is then sent to a compression unit  142  where it is re-compressed to a higher pressure and warmed. After exiting compression unit  142 , the re-compressed sub-cooling refrigerant stream  144  is cooled in a cooler  146 , which can be of the same type as cooler  112 , although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed to first heat exchanger  120  where it is further cooled by indirect heat exchange with expanded refrigerant stream  118  and expanded sub-cooling refrigerant stream  138 . After exiting first heat exchanger  120 , the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander  140  to provide a cooled stream which is then passed through second heat exchanger  124  to sub-cool the portion of the feed gas stream to be finally expanded to produce LNG. 
     U. S. Patent Application US2010/0107684 disclosed an improvement to the performance of the HPXP through the discovery that adding external cooling to further cool the compressed refrigerant to temperatures below ambient conditions provides significant advantages which in certain situations justifies the added equipment associated with external cooling. The HPXP embodiments described in the aforementioned patent applications perform comparably to alternative mixed external refrigerant LNG production processes such as single mixed refrigerant processes. However, there remains a need to further improve the efficiency of the HPXP as well as overall train capacity. There remains a particular need to improve the efficiency of the HPXP in cases where the feed gas pressure is less than 1,200 psia. 
     U. S. Patent Application 2010/0186445 disclosed the incorporation of feed compression up to 4,500 psia to the HPXP. Compressing the feed gas prior to liquefying the gas in the HPXP&#39;s primary cooling loop has the advantage of increasing the overall process efficiency. For a given production rate, this also has the advantage of significantly reducing the required flow rate of the refrigerant within the primary cooling loop which enables the use of compact equipment, which is particularly attractive for floating LNG applications. Furthermore, feed compression provides a means of increasing the LNG production of an HPXP train by more than 30% for a fixed amount of power going to the primary cooling and sub-cooling loops. This flexibility in production rate is again particularly attractive for floating LNG applications where there are more restrictions than land based applications in matching the choice of refrigerant loop drivers with desired production rates. 
     For LNG production via an HPXP process, the refrigerant used in primary cooling loop needs to be built up during start-up procedures, and must also be made up during normal operation. In known processes, the primary cooling loop refrigerant make-up source may be feed gas or boil-off gas (BOG) from an LNG storage tank. However, the compositions of feed gas and/or BOG gas compositions could change with reservoir conditions and/or gas plant operation conditions. The changes in gaseous refrigerant composition could affect liquefaction performance, causing the process to deviate from optimum operating conditions. If using feed gas for start-up or make-up processes, the primary cooling loop refrigerant should have sufficiently low C 2+  content to stay at one phase before entering the suction sides of compressors and turboexpander compressors. Furthermore, liquid pooling in the primary loop passages of the main cryogenic heat exchanger could also cause gas mal-distribution, which is undesirable for efficient operation of the main cryogenic heat exchanger. Using BOG as for start-up and make-up processes, on the other hand, could avoid the issues related to heavy components breakthrough. However, BOG is generally has much higher N 2  content than feed gas. Generally, too high of a nitrogen concentration negatively impacts the effectiveness of the primary loop refrigerant. In addition, the BOG composition is very sensitive to variations in composition of light ends such as nitrogen, hydrogen, helium in the feed gas. As shown in Table 1, an increase in the nitrogen concentration by 0.2% in the feed gas would result in an increase in BOG nitrogen concentration by 2%. For these reasons, there remains a need to manage variations in the feed gas composition during normal operation—both for the light contents (i.e., nitrogen, hydrogen, helium, etc.) and the heavy contents (i.e., C 2+ ). There is also a need to provide for efficient start-up operations of a high-pressure LNG liquefaction process. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 BOG Gas N 2  content sensitivity to 
               
               
                 the feed gas N 2  content variation 
               
            
           
           
               
               
               
            
               
                   
                 N2/(N2 + C1) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Scrubber 
                 Scrubber 
                   
                   
               
               
                 Case 
                 Feed 
                 OVHD 
                 LNG 
                 BOG 
               
               
                   
               
               
                 Base 
                 0.56% 
                 0.56% 
                 0.23% 
                 5.8% 
               
               
                 1 
                 0.61% 
                 0.62% 
                 0.25% 
                 6.3% 
               
               
                 2 
                  067% 
                 0.67% 
                 0.27% 
                 6.9% 
               
               
                 3 
                 0.72% 
                 0.73% 
                 0.29% 
                 7.4% 
               
               
                 4 
                 0.78% 
                 0.78% 
                 0.31% 
                 7.9% 
               
               
                   
               
            
           
         
       
     
     SUMMARY 
     According to disclosed aspects, a method is provided for liquefying a feed gas stream rich in methane. According to the method, The feed gas stream is provided at a pressure less than 1,200 psia. A compressed refrigerant stream with a pressure greater than or equal to 1,500 psia is provided. The compressed refrigerant stream is cooled by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream. The compressed, cooled refrigerant stream is expanded in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream. Part or all of the expanded, cooled refrigerant stream is mixed with a make-up refrigerant stream in a separator, thereby condensing heavy hydrocarbon components from the make-up refrigerant stream and forming a gaseous expanded, cooled refrigerant stream. The gaseous expanded, cooled refrigerant stream is passed through a heat exchanger zone to form a warm refrigerant stream. The feed gas stream is passed through the heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream. The warm refrigerant stream is compressed to produce the compressed refrigerant stream. 
     According to another aspect of the disclosure, a method is provided for liquefying a feed gas stream rich in methane in a system having a first heat exchanger zone and a second heat exchanger zone. A compressed refrigerant stream with a pressure greater than or equal to 1,500 psia is provided. The compressed refrigerant stream is cooled by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream. The compressed, cooled refrigerant stream is directed to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream. The compressed, additionally cooled refrigerant stream is expanded in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream. Part or all of the expanded, cooled refrigerant stream is routed to at least one separator, such as a separation vessel. The expanded, cooled refrigerant stream is mixed with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream. The gaseous overhead refrigerant stream is combined with the remaining expanded, cooled refrigerant stream to form a cold primary refrigerant mixture. The cold primary refrigerant mixture is passed through the first heat exchanger zone to form a warm refrigerant stream. The warm refrigerant stream may have a temperature that is cooler by at least 5° F. of the highest fluid temperature within the first heat exchanger zone. The heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone. The feed gas stream is passed through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream. The warm refrigerant stream is compressed to produce the compressed refrigerant stream. 
     According to still other aspects of the disclosure, a method is disclosed for liquefying a feed gas stream rich in methane. According to the method, the feed gas stream is provided at a pressure less than 1,200 psia. The feed gas stream is compressed to a pressure of at least 1,500 psia to form a compressed gas stream. The compressed gas stream is cooled by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream. The compressed, cooled gas stream is expanded in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream. A compressed refrigerant stream with a pressure greater than or equal to 1,500 psia is provided. The compressed refrigerant stream is cooled by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream. The compressed, cooled refrigerant stream is expanded in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream. Part or all of the expanded, cooled refrigerant stream is routed to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant stream therein with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream. The gaseous overhead refrigerant stream is combined with the remaining expanded, cooled refrigerant to form a cold primary refrigerant mixture. The cold primary refrigerant mixture is passed through a heat exchanger zone to form a warm refrigerant stream. The chilled gas stream is passed through the heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream. The warm refrigerant stream is compressed to produce the compressed refrigerant stream. 
     The foregoing has broadly outlined the features of the present disclosure so that the detailed description that follows may be better understood. Additional features will also be described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below. 
         FIG.  1    is a schematic diagram of a system for LNG production according to known principles. 
         FIG.  2    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  3    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  4    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  5    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  6    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  7    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  8    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  9    is a schematic diagram of a system for LNG production according to disclosed aspects. 
         FIG.  10    is a flowchart of a method according to aspects of the disclosure. 
         FIG.  11    is a flowchart of a method according to aspects of the disclosure. 
         FIG.  12    is a flowchart of a method according to aspects of the disclosure. 
     
    
    
     It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure. 
     DETAILED DESCRIPTION 
     To promote an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. For the sake of clarity, some features not relevant to the present disclosure may not be shown in the drawings. 
     At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims. 
     As one of ordinary skill would appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name only. The figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. When referring to the figures described herein, the same reference numerals may be referenced in multiple figures for the sake of simplicity. In the following description and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus, should be interpreted to mean “including, but not limited to.” 
     The articles “the,” “a” and “an” are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements. 
     As used herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure. The term “near” is intended to mean within 2%, or within 5%, or within 10%, of a number or amount. 
     As used herein, the term “ambient” refers to the atmospheric or aquatic environment where an apparatus is disposed. The term “at” or “near” “ambient temperature” as used herein refers to the temperature of the environment in which any physical or chemical event occurs plus or minus ten degrees, alternatively, five degrees, alternatively, three degrees, alternatively two degrees, and alternatively, one degree, unless otherwise specified. A typical range of ambient temperatures is between about 0° C. (32° F.) and about 40° C. (104° F.), though ambient temperatures could include temperatures that are higher or lower than this range. While it is possible in some specialized applications to prepare an environment with particular characteristics, such as within a building or other structure that has a controlled temperature and/or humidity, such an environment is considered to be “ambient” only where it is substantially larger than the volume of heat-sink material and substantially unaffected by operation of the apparatus. It is noted that this definition of an “ambient” environment does not require a static environment. Indeed, conditions of the environment may change as a result of numerous factors other than operation of the thermodynamic engine—the temperature, humidity, and other conditions may change as a result of regular diurnal cycles, as a result of changes in local weather patterns, and the like. 
     As used herein, the term “compression unit” means any one type or combination of similar or different types of compression equipment, and may include auxiliary equipment, known in the art for compressing a substance or mixture of substances. A “compression unit” may utilize one or more compression stages. Illustrative compressors may include, but are not limited to, positive displacement types, such as reciprocating and rotary compressors for example, and dynamic types, such as centrifugal and axial flow compressors, for example. 
     “Exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment or aspect described herein as “exemplary” is not to be construed as preferred or advantageous over other embodiments. 
     The term “gas” is used interchangeably with “vapor,” and is defined as a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term “liquid” means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state. 
     As used herein, “heat exchange area” means any one type or combination of similar or different types of equipment known in the art for facilitating heat transfer. Thus, a “heat exchange area” may be contained within a single piece of equipment, or it may comprise areas contained in a plurality of equipment pieces. Conversely, multiple heat exchange areas may be contained in a single piece of equipment. 
     A “hydrocarbon” is an organic compound that primarily includes the elements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any number of other elements can be present in small amounts. As used herein, hydrocarbons generally refer to components found in natural gas, oil, or chemical processing facilities. 
     As used herein, the terms “loop” and “cycle” are used interchangeably. 
     As used herein, “natural gas” means a gaseous feedstock suitable for manufacturing LNG, where the feedstock is a methane-rich gas. A “methane-rich gas” is a gas containing methane (CO as a major component, i.e., having a composition of at least 50% methane by weight. Natural gas may include gas obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). 
     The disclosed aspects provide a method for liquefying a feed gas stream, particularly one rich in methane. The method comprises: (a) providing the gas stream at a pressure less than 1,200 psia; (b) providing a compressed refrigerant with a pressure greater than or equal to 1,500 psia; (c) cooling the compressed refrigerant by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant; (d) expanding the compressed, cooled refrigerant in at least one work producing expander thereby producing an expanded, cooled refrigerant; (e) routing part or all of the expanded, cooled refrigerant to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing excessive heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; (f) combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant to form a cold primary refrigerant mixture; (g) passing the cold primary refrigerant mixture through a heat exchanger zone to form a warm refrigerant; (h) passing the gas stream through the heat exchanger zone to cool at least part of the gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and (i) compressing the warm refrigerant to produce the compressed refrigerant. 
     In another aspect, a method is provided for liquefying a feed gas stream, comprising: (a) providing the feed gas stream at a pressure less than 1,200 psia; (b) compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; (c) cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream; (d) expanding the compressed, cooled gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream; (e) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; (f) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream; (g) expanding the compressed, cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; (h) routing part or all of the expanded, cooled refrigerant stream to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant stream with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing excessive heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; (i) combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant to form a cold primary refrigerant mixture; (j) passing the cold primary refrigerant mixture through a heat exchanger zone to form a warm refrigerant stream; (k) passing the chilled gas stream through the heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and (l) compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     In another aspect, a method is provided for liquefying a feed gas stream in a system having a first heat exchanger zone and a second heat exchanger zone, comprising: (a) providing the feed gas stream at a pressure less than 1,200 psia; (b) compressing the gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; (c) cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream; (d) expanding the compressed, cooled gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream; (e) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; (f) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream; (g) directing the compressed, cooled refrigerant stream to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream; (h) expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; (i) routing part or all of the expanded, cooled refrigerant stream to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant stream with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing excessive heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; (j) combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant stream to form a cold primary refrigerant mixture; (k) passing the cold primary refrigerant mixture through the first heat exchanger zone to form a warm refrigerant stream, whereby the warm refrigerant stream has a temperature that is cooler by at least 5° F. of the highest fluid temperature within the heat exchanger zone and whereby the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone; (l) passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and (m) compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     In still another aspect of the disclosure, a method of liquefying a feed gas stream is provided, comprising: (a) providing the feed gas stream at a pressure less than 1,200 psia; (b) providing a refrigerant stream at or near the same pressure of the feed gas stream; (c) mixing the feed gas stream with the refrigerant stream to form a second feed gas stream; (d) compressing the second feed gas stream to a pressure of at least 1,500 psia to form a compressed second feed gas stream; (e) cooling the compressed feed second gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled second feed gas stream; (f) directing the compressed, cooled second feed gas stream to a second heat exchanger zone to additionally cool the compressed, cooled second gas stream below ambient temperature to produce a compressed, additionally cooled second feed gas stream; (g) expanding the compressed, additionally cooled second feed gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the second feed gas stream was compressed, to thereby form an expanded, cooled second feed gas stream; (h) separating the expanded, cooled second feed gas stream into a first expanded refrigerant stream and a chilled gas stream; (i) expanding the first expanded refrigerant stream in at least one work producing expander, thereby producing a second expanded refrigerant stream; (j) routing part or all of the second expanded refrigerant stream to at least one separator, such as a separation vessel, and mixing the second expanded refrigerant stream with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing excessive heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; (k) combining the gaseous overhead refrigerant stream with the remaining second expanded refrigerant stream to form a cold primary refrigerant mixture; (l) passing the cold primary refrigerant mixture through a first heat exchanger zone to form a first warm refrigerant stream, whereby the first warm refrigerant stream has a temperature that is cooler by at least 5° F. than the highest fluid temperature within the first heat exchanger zone and whereby the heat exchanger type of the first heat exchanger zone is different from the heat exchanger type of the second heat exchanger zone; (m) passing the chilled gas stream through the first heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the second expanded refrigerant, thereby forming a liquefied gas stream; (n) directing the first warm refrigerant to the second heat exchanger zone to cool by indirect heat exchange the compressed, cooled second gas, thereby forming a second warm refrigerant; and (o) compressing the second warm refrigerant to produce the refrigerant stream. 
     Aspects of the disclosure may compress the gas stream to a pressure no greater than 1,600 psia and then cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water prior to directing the gas stream to the first heat exchanger zone. Aspects of the disclosure may cool the gas stream to a temperature below the ambient by indirect heat exchange within an external cooling unit prior to directing the gas stream to the first heat exchanger zone. Aspects of the disclosure may cool the compressed, cooled refrigerant to a temperature below the ambient temperature by indirect heat exchange with an external cooling unit prior to directing the compressed, cooled refrigerant to the at least one work producing expander or the second heat exchanger zone. These described additional steps may be employed singularly or in combination with each other. 
       FIG.  2    is a schematic diagram that illustrates a liquefaction system  200  according to an aspect of the disclosure. The liquefaction system  200  includes a primary cooling loop  202 , which may also be called an expander loop. The liquefaction system also includes a sub-cooling loop  204 , which is a closed refrigeration loop preferably charged with nitrogen as the sub-cooling refrigerant. Within the primary cooling loop  202 , a refrigerant stream  205  is directed to a heat exchanger zone  201  where it exchanges heat with a feed gas stream  206  to form a first warm refrigerant stream  208 . The first warm refrigerant stream  208  is compressed in one or more compression units  218 ,  220  to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed refrigerant stream  222 . The compressed refrigerant stream  222  is then cooled against an ambient cooling medium (air or water) in a cooler  224  to produce a compressed, cooled refrigerant stream  226 . Cooler  224  may be similar to cooler  112  as previously described. The compressed, cooled refrigerant stream  226  is near isentropically expanded in an expander  228  to produce an expanded, cooled refrigerant stream  230 . Expander  228  may be a work-expansion device, such as a gas expander, which produces work that may be extracted and used for compression. 
     All or a portion of the expanded, cooled refrigerant stream  230  is directed to a separation vessel  232 . A make-up gas stream  234  is also directed to the separation vessel  232  and mixes therein with the expanded, cooled refrigerant stream  230 . The rate at which the make-up gas stream  234  is added to the separation vessel  232  will depend on the rate of loss of refrigerant due to such factors as leaks from equipment seals. The mixing conditions the make-up gas stream  234  by condensing heavy hydrocarbon components (e.g., C 2+  compounds) contained in the make-up gas stream  234 . The condensed components accumulate in the bottom of the separator and are periodically discharged as a separator bottom stream  236  to maintain a desired liquid level in the separation vessel  232 . The conditioned make-up gas stream, minus the condensed heavy hydrocarbon components, exits the separation vessel as a gaseous overhead refrigerant stream  238 . The gaseous overhead refrigerant stream  238  optionally mixes with a bypass stream  230   a  of the expanded, cooled refrigerant stream  230 , forming the refrigerant stream  205 . 
     The heat exchanger zone  201  may include a plurality of heat exchanger devices, and in the aspects shown in  FIG.  2   , the heat exchanger zone includes a main heat exchanger  240  and a sub-cooling heat exchanger  242 . The main heat exchanger  240  exchanges heat with the refrigerant stream  205 . These heat exchangers may be of a brazed aluminum heat exchanger type, a plate fin heat exchanger type, a spiral wound heat exchanger type, or a combination thereof. Within the sub-cooling loop  204 , an expanded sub-cooling refrigerant stream  244  (preferably comprising nitrogen) is discharged from an expander  246  and drawn through the sub-cooling heat exchanger  242  and the main heat exchanger  240 . Expanded sub-cooling refrigerant stream  244  is then sent to a compression unit  248  where it is re-compressed to a higher pressure and warmed. After exiting compression unit  248 , the re-compressed sub-cooling refrigerant stream  250  is cooled in a cooler  252 , which can be of the same type as cooler  224 , although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed through the main heat exchanger  240  where it is further cooled by indirect heat exchange with the refrigerant stream  205  and expanded sub-cooling refrigerant stream  244 . After exiting the heat exchange area  201 , the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander  246  to provide the expanded sub-cooling refrigerant stream  244  that is re-cycled through the heat exchanger zone as described herein. In this manner, the feed gas stream  206  is cooled, liquefied and sub-cooled in the heat exchanger zone  201  to produce a sub-cooled gas stream  254 . Sub-cooled gas stream  254  is then expanded to a lower pressure in an expander  256  to form a liquid fraction and a remaining vapor fraction. Expander  256  may be any pressure reducing device, including but not limited to a valve, control valve, Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and the like. The sub-cooled stream  254 , which is now at a lower pressure and partially liquefied, is passed to a surge tank  258  where the liquefied fraction  260  is withdrawn from the process as an LNG stream  262 . The remaining vapor fraction, which is withdrawn from the surge tank as a flash vapor stream  264 , may be used as fuel to power the compressor units. 
       FIG.  3    is a schematic diagram that illustrates a liquefaction system  300  according to another aspect of the disclosure. Liquefaction system  300  is similar to liquefaction system  200  and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  300  includes a primary cooling loop  302  and a sub-cooling loop  304 . The sub-cooling loop  304  is a closed refrigeration loop preferably charged with nitrogen as the sub-cooling refrigerant. Liquefaction system  300  also includes a heat exchanger zone  301 . Within the primary cooling loop  302 , a refrigerant stream  305  is directed to the heat exchanger zone  301  where it exchanges heat with a feed gas stream  306  to form a first warm refrigerant stream  308 . The first warm refrigerant stream  308  is compressed in one or more compression units  318 ,  320  to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed refrigerant stream  322 . The compressed refrigerant stream  322  is then cooled against an ambient cooling medium (air or water) in a cooler  324  to produce a compressed, cooled refrigerant stream  326 . Cooler  324  may be similar to cooler  112  as previously described. The compressed, cooled refrigerant stream  326  is near isentropically expanded in an expander  328  to produce an expanded, cooled refrigerant stream  330 . Expander  328  may be a work-expansion device, such as a gas expander, which produces work that may be extracted and used for compression. 
     In contrast with liquefaction system  200 , all of the expanded, cooled refrigerant stream  330  is directed to a separation vessel  332 . A make-up gas stream  334  is also directed to the separation vessel  332  and mixes therein with the expanded, cooled refrigerant stream  330 . The rate at which the make-up gas stream  334  is added to the separation vessel  332  will depend on the rate of loss of refrigerant due to such factors as leaks from equipment seals. The mixing conditions the make-up gas stream  334  by condensing heavy hydrocarbon components (e.g., C 2+  compounds) contained in the make-up gas stream  334 . The condensed components accumulate in the bottom of the separator and are periodically discharged as a separator bottom stream  336  to maintain a desired liquid level in the separation vessel  332 . The conditioned make-up gas stream, minus the condensed heavy hydrocarbon components, exits the separation vessel as a gaseous overhead refrigerant stream  338 . The gaseous overhead refrigerant stream  338  forms the refrigerant stream  305 . 
     The heat exchanger zone  301  may include a plurality of heat exchanger devices, and in the aspects shown in  FIG.  3   , the heat exchanger zone includes a main heat exchanger  340  and a sub-cooling heat exchanger  342 . The main heat exchanger  340  exchanges heat with the refrigerant stream  305 . These heat exchangers may be of a brazed aluminum heat exchanger type, a plate fin heat exchanger type, a spiral wound heat exchanger type, or a combination thereof. Within the sub-cooling loop  304 , an expanded sub-cooling refrigerant stream  344  (preferably comprising nitrogen) is discharged from an expander  346  and drawn through the sub-cooling heat exchanger  342  and the main heat exchanger  340 . Expanded sub-cooling refrigerant stream  344  is then sent to a compression unit  348  where it is re-compressed to a higher pressure and warmed. After exiting compression unit  348 , the re-compressed sub-cooling refrigerant stream  350  is cooled in a cooler  352 , which can be of the same type as cooler  324 , although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed through the main heat exchanger  340  where it is further cooled by indirect heat exchange with the refrigerant stream  305  and expanded sub-cooling refrigerant stream  344 . After exiting the heat exchange area  301 , the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander  346  to provide the expanded sub-cooling refrigerant stream  344  that is re-cycled through the heat exchanger zone as described herein. In this manner, the feed gas stream  306  is cooled, liquefied and sub-cooled in the heat exchanger zone  301  to produce a sub-cooled gas stream  354 . Sub-cooled gas stream  354  is then expanded to a lower pressure in an expander  356  to form a liquid fraction and a remaining vapor fraction. Expander  356  may be any pressure reducing device, including but not limited to a valve, control valve, Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and the like. The sub-cooled stream  354 , which is now at a lower pressure and partially liquefied, is passed to a surge tank  358  where the liquefied fraction  360  is withdrawn from the process as an LNG stream  362 . The remaining vapor fraction, which is withdrawn from the surge tank as a flash vapor stream  364 , may be used as fuel to power the compressor units. 
       FIG.  4    is a schematic diagram that illustrates a liquefaction system  400  according to another aspect of the disclosure. Liquefaction system  400  is similar to liquefaction system  200 , and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  400  includes a primary cooling loop  402  and a sub-cooling loop  404 . Liquefaction system  400  includes first and second heat exchanger zones  401 ,  410 . Within the first heat exchanger zone  401 , the first warm refrigerant stream  405  is used to liquefy the feed gas stream  406 . One or more heat exchangers  410   a  within the second heat exchanger zone  410  uses all or a portion of the first warm refrigerant stream  408  to cool a compressed, cooled refrigerant stream  426 , thereby forming a second warm refrigerant stream  409 . The first heat exchanger zone  401  may be physically separate from the second heat exchanger zone  410 . Additionally, the heat exchangers of the first heat exchanger zone may be of a different type(s) from the heat exchangers of the second heat exchanger zone. Both heat exchanger zones may comprise multiple heat exchangers. 
     The first warm refrigerant stream  405  has a temperature that is cooler by at least 5° F., or more preferably, cooler by at least 10° F., or more preferably, cooler by at least 15° F., than the highest fluid temperature within the first heat exchanger zone  401 . The second warm refrigerant stream  409  may be compressed in one or more compressors  418 ,  420  to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to thereby form a compressed refrigerant stream  422 . The compressed refrigerant stream  422  is then cooled against an ambient cooling medium (air or water) in a cooler  424  to produce the compressed, cooled refrigerant stream  426  that is directed to the second heat exchanger zone  410  to form a compressed, additionally cooled refrigerant stream  429 . The compressed, additionally cooled refrigerant stream  429  is near isentropically expanded in an expander  428  to produce the expanded, cooled refrigerant stream  430 . All or a portion of the expanded, cooled refrigerant stream  430  is directed to a separation vessel  432  where it is mixed with a make-up gas stream  434  as previously described with respect to  FIG.  2   . The rate at which the make-up gas stream  434  is added to the separation vessel  432  will depend on the rate of loss of refrigerant due to such factors as leaks from equipment seals. The conditioned make-up gas stream, minus the condensed heavy hydrocarbon components, exits the separation vessel as a gaseous overhead refrigerant stream  438 . The gaseous overhead refrigerant stream  438  optionally mixes with a bypass stream  430   a  of the expanded, cooled refrigerant stream  430 , forming the warm refrigerant stream  405 . 
       FIG.  5    is a schematic diagram that illustrates a liquefaction system  500  according to another aspect of the disclosure. Liquefaction system  500  is similar to liquefaction systems  200  and  300  and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  500  includes a primary cooling loop  502  and a sub-cooling loop  504 . Liquefaction system  500  also includes a heat exchanger zone  501 . 
     Liquefaction system  500  stream includes the additional steps of compressing the feed gas stream  506  in a compressor  566  and then, using a cooler  568 , cooling the compressed feed gas  567  with ambient air or water to produce a cooled, compressed feed gas stream  570 . Feed gas compression may be used to improve the overall efficiency of the liquefaction process and increase LNG production. 
       FIG.  6    is a schematic diagram that illustrates a liquefaction system  600  according to still another aspect of the disclosure. Liquefaction system  600  is similar to liquefaction systems  200  and  300  and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  600  includes a primary cooling loop  602  and a sub-cooling loop  604 . Liquefaction system  600  also includes a heat exchanger zone  601 . Liquefaction system  600  includes the additional step of chilling, in an external cooling unit  665 , the feed gas stream  606  to a temperature below the ambient temperature to produce a chilled gas stream  667 . The chilled gas stream  667  is then directed to the first heat exchanger zone  601  as previously described. Chilling the feed gas as shown in  FIG.  6    may be used to improve the overall efficiency of the liquefaction process and increase LNG production. 
       FIG.  7    is a schematic diagram that illustrates a liquefaction system  700  according to another aspect of the disclosure. Liquefaction system  700  is similar to liquefaction system  200  and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  700  includes a primary cooling loop  702  and a sub-cooling loop  704 . Liquefaction system  700  also includes first and second heat exchanger zones  701 ,  710 . Liquefaction system  700  includes an external cooling unit  774  that chills the compressed, cooled refrigerant  726  in the primary cooling loop  702  to a temperature below the ambient temperature, to thereby produce a compressed, chilled refrigerant  776 . The compressed, chilled refrigerant  776  is then directed to the second heat exchanger zone  710  as previously described. Using an external cooling unit to further cool the compressed, cool refrigerant may be used to improve the overall efficiency of the process and increase LNG production. 
       FIG.  8    is a schematic diagram that illustrates a liquefaction system  800  according to another aspect of the disclosure. Liquefaction system  800  is similar to liquefaction system  400  and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  800  includes a primary cooling loop  802  and a sub-cooling loop  804 . Liquefaction system  800  also includes first and second heat exchanger zones  801 ,  810 . In liquefaction system  800 , the feed gas stream  806  is compressed in a compressor  880  to a pressure of at least 1,500 psia, thereby forming a compressed gas stream  881 . Using an external cooling unit  882 , the compressed gas stream  881  is cooled by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream  883 . The compressed, cooled gas stream  883  is expanded in at least one work producing expander  884  to a pressure that is less than 2,000 psia but no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream  886 . The chilled gas stream  886  is then directed to the first heat exchanger zone  801  where a primary cooling refrigerant and a sub-cooling refrigerant are used to liquefy the chilled gas stream as previously described. 
       FIG.  9    is a schematic diagram that illustrates a liquefaction system  900  according to yet another aspect of the disclosure. Liquefaction system  900  contains similar structure and components with previously disclosed liquefaction systems and for the sake of brevity similarly depicted or numbered components may not be further described. Liquefaction system  900  includes a primary cooling loop  902  and a sub-cooling loop  904 . Liquefaction system  900  also includes first and second heat exchanger zones  901 ,  910 . In liquefaction system  900 , the feed gas stream  906  is mixed with a refrigerant stream  907  to produce a second feed gas stream  906   a . Using a compressor  960 , the second feed gas stream  906   a  is compressed to a pressure greater than 1,500 psia, or more preferably, to a pressure of approximately 3,000 psia, to form a compressed second gas stream  961 . Using an external cooling unit  962 , the compressed second gas stream  961  is then cooled against an ambient cooling medium (air or water) to produce a compressed, cooled second gas stream  963 . The compressed, cooled second gas stream  963  is directed to the second heat exchanger zone  910  where it exchanges heat with a first warm refrigerant stream  908 , to produce a compressed, additionally cooled second gas stream  913  and a second warm refrigerant stream  909 . 
     The compressed, additionally cooled second gas stream  913  is expanded in at least one work producing expander  926  to a pressure that is less than 2,000 psia, but no greater than the pressure to which the second gas stream  906   a  was compressed, to thereby form an expanded, cooled second gas stream  980 . The expanded, cooled second gas stream  980  is separated into a first expanded refrigerant stream  905  and a chilled feed gas stream  906   b . The first expanded refrigerant stream  905  may be near isentropically expanded using an expander  982  to form a second expanded refrigerant stream  905   a , which is directed to a separation vessel  932 . A make-up gas stream  934  is also directed to the separation vessel  932  and mixes therein with the expanded, cooled refrigerant stream  930 . The rate at which the make-up gas stream  934  is added to the separation vessel  932  will depend on the rate of loss of refrigerant due to such factors as leaks from equipment seals. The mixing conditions the make-up gas stream  934  by condensing heavy hydrocarbon components (e.g., C 2+  compounds) contained in the make-up gas stream  934 . The condensed components accumulate in the bottom of the separator and are periodically discharged as a separator bottom stream  936  to maintain a desired liquid level in the separation vessel  932 . The conditioned make-up gas stream, minus the condensed heavy hydrocarbon components, exits the separation vessel as a gaseous overhead refrigerant stream  938 , which is directed to the first heat exchanger zone  901 . The chilled feed gas stream  906   b  is directed to the first heat exchanger zone  901  where a primary cooling refrigerant (i.e., the gaseous overhead refrigerant stream  938 ) and a sub-cooling refrigerant (from the sub-cooling loop  904 ) are used to liquefy and sub-cool the chilled feed gas stream  906   b  to produce a sub-cooled gas stream  948 , which is processed as previously described to form LNG. The sub-cooling loop  904  may be a closed refrigeration loop, preferably charged with nitrogen as the sub-cooling refrigerant. After exchanging heat with the chilled feed gas stream  906   b , the gaseous overhead refrigerant stream  938  forms the first warm refrigerant stream  908 . The first warm refrigerant stream  908  may have a temperature that is cooler by at least 5° F., or more preferably, cooler by at least 10° F., or more preferably, cooler by at least 15° F., than the highest fluid temperature within the first heat exchanger zone  901 . The second warm refrigerant stream  909  is compressed in one or more compressors  918  and then cooled with an ambient cooling medium in an external cooling device  924  to produce the refrigerant stream  907 . 
     Aspects of the disclosure illustrated in  FIG.  9    demonstrate that the primary refrigerant stream may comprise part of the feed gas stream, which in a preferred aspect may be primarily or nearly all methane. Indeed, it may be advantageous for the refrigerant in the primary cooling loop of all the disclosed aspects (i.e.,  FIGS.  2  through  9   ) be comprised of at least 85% methane, or at least 90% methane, or at least 95% methane, or greater than 95% methane. This is because methane may be readily available in various parts of the disclosed processes, and the use of methane may eliminate the need to transport refrigerants to remote LNG processing locations. As a non-limiting example, the refrigerant in the primary cooling loop  202  in  FIG.  2    may be taken through line  206   a  of the feed gas stream  206  if the feed gas is high enough in methane to meet the compositions as described above. Make-up gas may be taken from the sub-cooled gas stream  254  during normal operations. Alternatively, part or all of a boil-off gas stream  259  from an LNG storage tank  257  may be used to supply refrigerant for the primary cooling loop  202 . Furthermore, if the feed gas stream is sufficiently low in nitrogen, part or all of the end flash gas stream  264  (which would then be low in nitrogen) may be used to supply refrigerant for the primary cooling loop  202 . Lastly, any combination of line  206   a , boil-off gas stream  259 , and end flash gas stream  264  may be used to provide or even occasionally replenish the refrigerant in the primary cooling loop  202 . 
       FIG.  10    is a flowchart of a method  1000  for liquefying a feed gas stream rich in methane, where the method comprises the following steps:  1002 , providing the feed gas stream at a pressure less than 1,200 psia;  1004 , providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia;  1006 , cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream;  1008 , expanding the compressed, cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream;  1010 , mixing part or all of the expanded, cooled refrigerant stream with a make-up refrigerant stream in a separator, thereby condensing heavy hydrocarbon components from the make-up refrigerant stream and forming a gaseous expanded, cooled refrigerant stream;  1012 , passing the gaseous expanded, cooled refrigerant stream through a heat exchanger zone to form a warm refrigerant stream;  1014 , passing the feed gas stream through the heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream; and  1016 , compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
       FIG.  11    is a flowchart of a method  1100  for liquefying a feed gas stream rich in methane in a system having a first heat exchanger zone and a second heat exchanger zone, where the method comprises the following steps:  1102 , providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia;  1104 , cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream;  1106 , directing the compressed, cooled refrigerant stream to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream;  1108 , expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream;  1110 , routing part or all of the expanded, cooled refrigerant stream to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant stream with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream;  1112 , combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant stream to form a cold primary refrigerant mixture;  1114 , passing the cold primary refrigerant mixture through the first heat exchanger zone to form a warm refrigerant stream, whereby the warm refrigerant stream has a temperature that is cooler by at least 5° F. of the highest fluid temperature within the heat exchanger zone, and wherein a heat exchanger type of the first heat exchanger zone is different from a heat exchanger type of the second heat exchanger zone;  1116 , passing the feed gas stream through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and  1118 , compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
       FIG.  12    is a method  1200  for liquefying a feed gas stream rich in methane, where the method comprises the following steps:  1202 , providing the feed gas stream at a pressure less than 1,200 psia;  1204 , compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream;  1206 , cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream;  1208 , expanding the compressed, cooled gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream;  1210 , providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia;  1212 , cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream;  1214 , expanding the compressed, cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream;  1216 , routing part or all of the expanded, cooled refrigerant stream to at least one separator, and mixing said expanded, cooled refrigerant stream therein with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream;  1218 , combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant to form a cold primary refrigerant mixture;  1220 , passing the cold primary refrigerant mixture through a heat exchanger zone to form a warm refrigerant stream;  1222 , passing the chilled gas stream through the heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and  1224 , compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     The steps depicted in  FIGS.  10 - 12    are provided for illustrative purposes only and a particular step may not be required to perform the disclosed methodology. Moreover,  FIGS.  10 - 12    may not illustrate all the steps that may be performed. The claims, and only the claims, define the disclosed system and methodology. 
     Aspects of the disclosure have several advantages over the known liquefaction processes, in which feed gas must be consistently sufficiently lean to be used as make-up gas in the primary refrigerant loop. BOG, which is rich in lighter components such as nitrogen, is required as a reliable make-up gas source. But using BOG as make-up gas negatively impacts the effectiveness of the primary loop refrigerant, either by demanding higher power consumption or requiring a larger main cryogenic heat exchanger. In addition, BOG composition is very sensitive to variation in the composition of light ends (e.g., nitrogen, hydrogen, helium) in the feed gas, thereby potentially adversely impacting process stability. The disclosed aspects enable the primary refrigerant make-up gas to comprise feed gas having a wide range of compositions, from lean to rich. Taking liquefaction system  300  as an example, the size of the main cryogenic heat exchanger can be reduced 10-16% and thermal efficiency can be improved up to about 1%, when compared to a similar system using BOG as the primary refrigerant make-up gas. Such size reductions of the main cryogenic heat exchanger, which typically is one of the largest and heaviest component or vessel in an LNG liquefaction system, may greatly reduce the size and cost of LNG liquefaction plants. Additionally, the disclosed aspects offer flexibility in tuning light (e.g., N 2 ) and heavy (e.g., C 2+ ) contents for the primary refrigerant loop that could potentially dynamically match incoming feed from gas wells, thereby optimizing energy use or production rate. For example, the make-up gas streams could be from feed gas, N 2 , and LPG product streams. Their relative rates could be tuned for optimization purposes illustrated above. 
     Aspects of the disclosure may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible aspects, as any number of variations can be envisioned from the description above. 
     1. A method for liquefying a feed gas stream rich in methane, comprising: 
     (a) providing the feed gas stream at a pressure less than 1,200 psia; 
     (b) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; 
     (c) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water, to produce a compressed, cooled refrigerant stream; 
     (d) expanding the compressed, cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 
     (e) mixing part or all of the expanded, cooled refrigerant stream with a make-up refrigerant stream in a separator, thereby condensing heavy hydrocarbon components from the make-up refrigerant stream and forming a gaseous expanded, cooled refrigerant stream; 
     (f) passing the gaseous expanded, cooled refrigerant stream through a heat exchanger zone to form a warm refrigerant stream; 
     (g) passing the feed gas stream through the heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the expanded, cooled refrigerant stream, thereby forming a liquefied gas stream; and 
     (i) compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     2. The method of paragraph 1, further comprising: 
     controlling a flow rate of the make-up gas stream into the separator to maintain at least one pressure at a suction side of a compressor at a target value. 
     3. The method of paragraph 1 or paragraph 2, further comprising: 
     collecting the condensed heavy hydrocarbon components in the separator; and 
     discharging the condensed heavy hydrocarbon components to maintain a desired liquid level in the separator. 
     4. The method of any one of paragraphs 1-3, further comprising: 
     further cooling the liquefied gas stream within the heat exchanger zone using a sub-cooling refrigeration cycle, to thereby form a sub-cooled gas stream. 
     5. The method of paragraph 4, further comprising: 
     expanding the sub-cooled gas stream to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream. 
     6. The method of paragraph 5, wherein the sub-cooled gas stream is expanded within a hydraulic turbine.
 
7. The method of any one of paragraphs 4-6, wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant.
 
8. The method of any one of paragraphs 1-7, further comprising:
 
     prior to directing the feed gas stream to the heat exchanger zone, compressing the feed gas stream to a pressure no greater 1,600 psia, and then cooling it by indirect heat exchange with an ambient temperature air or water. 
     9. The method of any one of paragraphs 1-8, wherein the feed gas stream is cooled to a temperature below an ambient temperature by indirect heat exchange within an external cooling unit prior to directing the feed gas stream to the heat exchanger zone.
 
10. The method of any one of paragraphs 1-9, wherein the compressed, cooled refrigerant stream is cooled to a temperature below the ambient temperature by indirect heat exchange within an external cooling unit prior to expanding the compressed, cooled refrigerant stream in the at least one work producing expander.
 
11. The method of any one of paragraphs 1-10, wherein the make-up gas stream comprises a portion of the feed gas stream, a boil-off gas obtained from the liquefied gas stream, or any combination thereof.
 
12. The method of any one of paragraphs 1-11, wherein the make-up gas stream comprises a mixture of methane with at least one component having a molecular weight heavier or lighter than methane.
 
13. The method of paragraph 12, wherein the make-up gas stream comprises methane and one or more of nitrogen and liquefied petroleum gas.
 
14. A method for liquefying a feed gas stream rich in methane in a system having a first heat exchanger zone and a second heat exchanger zone, comprising:
 
     (a) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; 
     (b) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream; 
     (c) directing the compressed, cooled refrigerant stream to the second heat exchanger zone to additionally cool the compressed, cooled refrigerant stream below ambient temperature to produce a compressed, additionally cooled refrigerant stream; 
     (d) expanding the compressed, additionally cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 
     (e) routing part or all of the expanded, cooled refrigerant stream to at least one separator, such as a separation vessel, and mixing said expanded, cooled refrigerant stream with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; 
     (f) combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant stream to form a cold primary refrigerant mixture; 
     (g) passing the cold primary refrigerant mixture through the first heat exchanger zone to form a warm refrigerant stream, whereby the warm refrigerant stream has a temperature that is cooler by at least 5° F. of the highest fluid temperature within the heat exchanger zone, and wherein a heat exchanger type of the first heat exchanger zone is different from a heat exchanger type of the second heat exchanger zone; 
     (h) passing the feed gas stream through the first heat exchanger zone to cool at least part of the feed gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and 
     (i) compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     15. The method of paragraph 14, further comprising: 
     controlling a flow rate of the make-up gas stream into the separator to maintain at least one pressure at a suction side of a compressor at a target value. 
     16. The method of paragraph 14 or paragraph 15, further comprising: 
     collecting the condensed heavy hydrocarbon components in the separator; and 
     discharging the condensed heavy hydrocarbon components to maintain a desired liquid level in the separator. 
     17. The method of any one of paragraphs 14-16, further comprising: 
     further cooling the liquefied gas stream within the first heat exchanger zone using a sub-cooling refrigeration cycle, to thereby form a sub-cooled gas stream. 
     18. The method of paragraph 17, further comprising: 
     expanding the sub-cooled gas stream to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream. 
     19. The method of paragraph 18, wherein the sub-cooled gas stream is expanded within a hydraulic turbine.
 
20. The method of any one of paragraphs 17-19, wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant.
 
21. The method of any one of paragraphs 14-20, further comprising:
 
     prior to directing the feed gas stream to the heat exchanger zone, compressing the feed gas stream to a pressure no greater 1,600 psia, cooling the feed gas stream by indirect heat exchange with an ambient temperature air or water, and then expanding the feed gas stream in a work-producing expander. 
     22. The method of any one of paragraphs 14-21, wherein the feed gas stream is cooled to a temperature below an ambient temperature by indirect heat exchange within an external cooling unit prior to directing the feed gas stream to the heat exchanger zone.
 
23. The method of any one of paragraphs 14-22, wherein the compressed, cooled refrigerant stream is cooled to a temperature below the ambient temperature by indirect heat exchange within an external cooling unit prior to expanding the compressed, cooled refrigerant stream in the at least one work producing expander.
 
24. The method of any one of paragraphs 14-23, wherein the make-up gas stream comprises a portion of the feed gas stream, a boil-off gas obtained from the liquefied gas stream, or any combination thereof.
 
25. The method of any one of paragraphs 14-24, wherein the make-up gas stream comprises a mixture of methane with at least one component having a molecular weight heavier or lighter than methane.
 
26. The method of paragraph 25, wherein the make-up gas stream comprises methane and one or more of nitrogen and liquefied petroleum gas.
 
27. A method for liquefying a feed gas stream rich in methane, comprising:
 
     (a) providing the feed gas stream at a pressure less than 1,200 psia; 
     (b) compressing the feed gas stream to a pressure of at least 1,500 psia to form a compressed gas stream; 
     (c) cooling the compressed gas stream by indirect heat exchange with an ambient temperature air or water to form a compressed, cooled gas stream; 
     (d) expanding the compressed, cooled gas stream in at least one work producing expander to a pressure that is less than 2,000 psia and no greater than the pressure to which the gas stream was compressed, to thereby form a chilled gas stream; 
     (e) providing a compressed refrigerant stream with a pressure greater than or equal to 1,500 psia; 
     (f) cooling the compressed refrigerant stream by indirect heat exchange with an ambient temperature air or water to produce a compressed, cooled refrigerant stream; 
     (g) expanding the compressed, cooled refrigerant stream in at least one work producing expander, thereby producing an expanded, cooled refrigerant stream; 
     (h) routing part or all of the expanded, cooled refrigerant stream to at least one separator, and mixing said expanded, cooled refrigerant stream therein with a make-up refrigerant gas stream, to thereby condition the make-up refrigerant gas stream by condensing heavy hydrocarbon components therefrom and producing a gaseous overhead refrigerant stream; 
     (i) combining the gaseous overhead refrigerant stream with the remaining expanded, cooled refrigerant to form a cold primary refrigerant mixture; 
     (j) passing the cold primary refrigerant mixture through a heat exchanger zone to form a warm refrigerant stream; 
     (k) passing the chilled gas stream through the heat exchanger zone to cool at least part of the chilled gas stream by indirect heat exchange with the cold primary refrigerant mixture, thereby forming a liquefied gas stream; and 
     (l) compressing the warm refrigerant stream to produce the compressed refrigerant stream. 
     28. The method of paragraph 27, further comprising: 
     controlling a flow rate of the make-up gas stream into the separator to maintain at least one pressure at a suction side of a compressor at a target value. 
     29. The method of paragraph 27 or paragraph 28, further comprising: 
     collecting the condensed heavy hydrocarbon components in the separator; and 
     discharging the condensed heavy hydrocarbon components to maintain a desired liquid level in the separator. 
     30. The method of any one of paragraphs 27-29, further comprising: 
     further cooling the liquefied gas stream within the first heat exchanger zone using a sub-cooling refrigeration cycle, to thereby form a sub-cooled gas stream. 
     31. The method of paragraph 30, further comprising: 
     expanding the sub-cooled gas stream to a pressure greater than or equal to 50 psia and less than or equal to 450 psia, to produce an expanded, sub-cooled gas stream. 
     32. The method of paragraph 31, wherein the sub-cooled gas stream is expanded within a hydraulic turbine.
 
33. The method of any one of paragraphs 30-32, wherein the sub-cooling refrigeration cycle comprises a closed loop gas phase refrigeration cycle using nitrogen gas as a refrigerant.
 
34. The method of any one of paragraphs 27-33, wherein the compressed, cooled refrigerant stream is cooled to a temperature below the ambient temperature by indirect heat exchange within an external cooling unit prior to expanding the compressed, cooled refrigerant stream in the at least one work producing expander.
 
35. The method of any one of paragraphs 27-34, wherein the make-up gas stream comprises a portion of the feed gas stream, a boil-off gas obtained from the liquefied gas stream, or any combination thereof.
 
36. The method of any one of paragraphs 27-35, wherein the make-up gas stream comprises a mixture of methane with at least one component having a molecular weight heavier or lighter than methane.
 
37. The method of paragraph 36, wherein the make-up gas stream comprises methane and one or more of nitrogen and liquefied petroleum gas.
 
     It should be understood that the numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.