Patent Publication Number: US-2009217701-A1

Title: Natural Gas Liquefaction Process for Ling

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/706,798, filed 9 Aug., 2005, and U.S. Provisional Application No. 60/795,101, filed 26 Apr. 2006. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention relate to a process for liquefaction of natural gas and other methane-rich gas streams, and more particularly to a process for producing liquefied natural gas (LNG). 
     BACKGROUND 
     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, 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 (which is called “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 may be 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 imposing logistics requirements. 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 has been of interest to process engineers. The expander system operates on the principle that the feed 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. Supplemental refrigeration is typically needed to fully liquefy the feed gas and this may be provided by a refrigerant system. The power obtained from the expansion is usually 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). 
     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 pre-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. Additional cooling may also be required after the expander cooling and may be provided by another external refrigerant system, such as nitrogen or a cold mixed refrigerant. 
     Accordingly, there is still a need for an expander cycle that eliminates the need for external refrigerants and has improved efficiency, at least comparable to that of technologies currently in use. 
     SUMMARY 
     Embodiments of the present invention provide a process for liquefying natural gas and other methane-rich gas streams to produce liquefied natural gas (LNG) and/or other liquefied methane-rich gases. The term natural gas as used in this specification, including the appended claims, means a gaseous feed stock suitable for manufacturing LNG. The natural gas could comprise gas obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). The composition of natural gas can vary significantly. As used herein, natural gas is a methane-rich gas containing methane (C 1 ) as a major component. 
     In one or more embodiments of the method for producing LNG herein, a first step is carried out in which a first fraction of the feed gas is withdrawn, compressed, cooled and expanded to a lower pressure to cool the withdrawn first fraction. The remaining fraction of the feed stream is cooled by indirect heat exchange with the expanded first fraction in a first heat exchange process. In a second step, involving a sub-cooling loop, a separate stream comprised of the flash vapor is compressed, cooled and expanded to a lower pressure providing another cold stream. This cold stream is used to cool the remaining feed gas stream in a second indirect heat exchange process, which constitutes the sub-cooling heat exchange process. The expanded stream exiting from the second heat exchange process is used for supplemental cooling in the first indirect heat exchange step. The remaining feed gas is subsequently expanded to a lower pressure, thereby partially liquefying this feed gas stream. The liquefied fraction of this stream is withdrawn from the process as LNG having a temperature corresponding to the bubble point pressure. The vapor fraction of this stream is returned to supplement the cooling provided in the indirect heat exchange steps. The warmed cooling gases from the various sources are compressed and recycled. 
     In one or more other embodiments according to the present invention, a process for liquefying a gas stream rich in methane is provided, said process comprising providing a gas stream rich in methane at a pressure less than 1,000 psia; providing a refrigerant at a pressure of less than 1,000 psia; compressing said refrigerant to a pressure greater than or equal to 1500 psia to provide a compressed refrigerant; cooling said compressed refrigerant by indirect heat exchange with a cooling fluid; expanding said compressed refrigerant to further cool said compressed refrigerant, thereby producing an expanded, cooled refrigerant; passing said expanded, cooled refrigerant to a heat exchange area; and passing said gas stream through said heat exchange area to cool at least part of said gas stream by indirect heat exchange with said expanded, cooled refrigerant, thereby forming a cooled gas stream. In one or more other specific embodiments, providing the refrigerant at a pressure of less than 1,000 psia comprises withdrawing a portion of the gas for use as the refrigerant. In other embodiments, the portion of the gas stream to be used as the refrigerant is withdrawn from the gas stream before the gas stream is passed to the heat exchange area. In still other embodiments, the process according to the present invention further comprises providing at least a portion of the refrigeration duty for the heat exchange area using a closed loop charged with flash vapor produced in the process for liquefying the gas stream rich in methane. Additional embodiments according to the present invention will be apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of one embodiment for producing LNG in accordance with the process of this invention. 
         FIG. 2  is a schematic flow diagram of a second embodiment for producing LNG that is similar to the process shown in  FIG. 1 , except that the gaseous refrigerant in the compressed, cooled and expanded loop is de-coupled from the feed gas and may therefore have a different composition than the feed gas. 
         FIG. 3  is a schematic flow diagram of a third embodiment for producing LNG in accordance with the process of this invention that uses a plurality of work expansion steps for improved efficiency. 
         FIG. 4  is a schematic flow diagram of a fourth embodiment for producing LNG in accordance with the process of this invention that uses a plurality of work expansion steps similar to  FIG. 3 , but also incorporates an additional expansion step as well as compression of the feed gas to improve performance of the expansion steps. 
         FIG. 5  is a schematic flow diagram of a fifth embodiment for producing LNG in accordance with the process of this invention that is similar to the embodiment shown in  FIG. 4 , but utilizes an additional side stream and expansion of process gas to provide sub-cooling. 
         FIG. 6  is another embodiment similar to the embodiments shown in  FIG. 1  and  FIG. 2  in which the refrigerant for the sub-cooling loop is cooled in the sub-cooling heat exchanger prior to expansion. 
         FIG. 7  is another embodiment in which the sub-cooling loop is coupled to the feed gas. 
         FIG. 8  is another embodiment showing an alternative arrangement for the sub-cooling loop. 
         FIG. 9  is a similar embodiment to that of  FIG. 8  but using split expanded streams through the sub-cooler wherein an expansion valve, Joules-Thompson valve, or similar expansion device is used for improved efficiency in the sub-cooler. 
         FIG. 10  is another embodiment in which a nitrogen rejection stage has been integrated for situations in which nitrogen rejection may be needed. 
         FIG. 11  is yet another embodiment in which the refrigerant for the sub-cooling loop is derived from the flash vapor from the nitrogen rejection unit and is therefore rich in nitrogen content. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide a process for natural gas liquefaction using primarily gas expanders and eliminating the need for external refrigerants. That is, in some embodiments disclosed herein, the feed gas itself (e.g., natural gas) is used as the refrigerant in all refrigeration cycles. Such refrigeration cycles do not require supplemental cooling using external refrigerants (i.e., refrigerants other than the feed gas itself or gas that is produced at or near the LNG process plant) as typical proposed gas expander cycles do, yet such refrigeration cycles have a higher efficiency. In one or more embodiments, cooling water or air are the only external sources of cooling fluids and are used for compressor inter-stage or after cooling. 
       FIG. 1  illustrates one embodiment of the present invention in which an expander loop  5  (i.e., an expander cycle) and a sub-cooling loop  6  are used. For clarity, expander loop  5  and sub-cooling loop  6  are shown with double-width lines in  FIG. 1 . In this specification and the appended claims, the terms “loop” and “cycle” are used interchangeably. In  FIG. 1 , feed gas stream  10  enters the liquefaction process at a pressure less than about 1200 psia, or less than about 1100 psia, or less than about 1000 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  10  will be about 800 psia. Feed gas stream  10  generally comprises natural gas that has been treated to remove contaminants using processes and equipment that are well known in the art. Before it is passed to a heat exchanger, a portion of feed gas stream  10  is withdrawn to form side stream  11 , thus providing, as will be apparent from the following discussion, a refrigerant at a pressure corresponding to the pressure of feed gas stream  10 , namely any of the above pressures, including a pressure of less than about 1000 psia. Thus, in the embodiment shown in  FIG. 1 , a portion of the feed gas stream is used as the refrigerant for expander loop  5 . Although the embodiment shown in  FIG. 1  utilizes a side stream that is withdrawn from feed gas stream  10  before feed gas stream  10  is passed to a heat exchanger, the side stream of feed gas to be used as the refrigerant in expander loop  5  may be withdrawn from the feed gas after the feed gas has been passed to a heat exchange area. Thus, in one or more embodiments, the present method is any of the other embodiments herein described, wherein the portion of the feed gas stream to be used as the refrigerant is withdrawn from the heat exchange area, expanded, and passed back to the heat exchange area to provide at least part of the refrigeration duty for the heat exchange area. 
     Side stream  11  is passed to compression unit  20  where it is compressed to a pressure greater than or equal to about 1500 psia, thus providing compressed refrigerant stream  12 . Alternatively, side stream  11  is compressed to a pressure greater than or equal to about 1600 psia, or greater than or equal to about 1700 psia, or greater than or equal to about 1800 psia, or greater than or equal to about 1900 psia, or greater than or equal to about 2000 psia, or greater than or equal to about 2500 psia, or greater than or equal to about 3000 psia, thus providing compressed refrigerant stream  12 . As used in this specification, including the appended claims, 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. 
     After exiting compression unit  20 , compressed refrigerant stream  12  is passed to cooler  30  where it is cooled by indirect heat exchange with a suitable cooling fluid to provide a compressed, cooled refrigerant. In one or more embodiments, cooler  30  is of the type that provides water or air as the cooling fluid, although any type of cooler can be used. The temperature of compressed refrigerant stream  12  as it emerges from cooler  30  depends on the ambient conditions and the cooling medium used and is typically from about 35° F. to about 105° F. Cooled compressed refrigerant stream  12  is then passed to expander  40  where it is expanded and consequently cooled to form expanded refrigerant stream  13 . In one or more embodiments, expander  40  is a work-expansion device, such as gas expander producing work that may be extracted and used for compression. 
     Expanded refrigerant stream  13  is passed to heat exchange area  50  to provide at least part of the refrigeration duty for heat exchange area  50 . As used in this specification, including the appended claims, the term “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. 
     Upon exiting heat exchange area  50 , expanded refrigerant stream  13  is fed to compression unit  60  for pressurization to form stream  14 , which is then joined with side stream  11 . It will be apparent that once expander loop  5  has been filled with feed gas from side stream  11 , only make-up feed gas to replace losses from leaks is required, the majority of the gas entering compressor unit  20  generally being provided by stream  14 . The portion of feed gas stream  10  that is not withdrawn as side stream  11  is passed to heat exchange area  50  where it is cooled, at least in part, by indirect heat exchange with expanded refrigerant stream  13 . After exiting heat exchange area  50 , feed gas stream  10  is passed to heat exchange area  55 . The principal function of heat exchange area  55  is to sub-cool the feed gas stream. Thus, in heat exchange area  55  feed gas stream  10  is sub-cooled by sub-cooling loop  6  (described below) to produce sub-cooled stream  10   a . Sub-cooled stream  10   a  is then expanded to a lower pressure in expander  70 , thereby partially liquefying sub-cooled stream  10   a  to form a liquid fraction and a remaining vapor fraction. Expander  70  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. Partially liquefied sub-cooled stream  10   a  is passed to surge tank  80  where the liquefied fraction  15  is withdrawn from the process as LNG having a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream  16  is used as fuel to power the compressor units and/or as a refrigerant in sub-cooling loop  6  as described below. Prior to being used as fuel, all or a portion of flash vapor stream  16  may optionally be passed from surge tank  80  to heat exchange areas  50  and  55  to supplement the cooling provided in such heat exchange areas. 
     Referring again to  FIG. 1 , a portion of flash vapor  16  is withdrawn through line  17  to fill sub-cooling loop  6 . Thus, a portion of the feed gas from feed gas stream  10  is withdrawn (in the form of flash gas from flash gas stream  16 ) for use as the refrigerant in sub-cooling loop  6 . It will again be apparent that once sub-cooling loop  6  is fully charged with flash gas, only make-up gas (i.e., additional flash vapor from line  17 ) to replace losses from leaks is required. In sub-cooling loop  6 , expanded stream  18  is discharged from expander  41  and drawn through heat exchange areas  55  and  50 . Expanded flash vapor stream  18  (the sub-cooling refrigerant stream) is then returned to compression unit  90  where it is re-compressed to a higher pressure and warmed. After exiting compression unit  90 , the re-compressed sub-cooling refrigerant stream is cooled in cooler  31 , which can be of the same type as cooler  30 , although any type of cooler may be used. After cooling, the re-compressed sub-cooling refrigerant stream is passed to heat exchange area  50  where it is further cooled by indirect heat exchange with expanded refrigerant stream  13 , sub-cooling refrigerant stream  18 , and, optionally, flash vapor stream  16 . After exiting heat exchange area  50 , the re-compressed and cooled sub-cooling refrigerant stream is expanded through expander  41  to provide a cooled stream which is then passed through heat exchange area  55  to sub-cool the portion of the feed gas stream to be finally expanded to produce LNG. The expanded sub-cooling refrigerant stream exiting from heat exchange area  55  is again passed through heat exchange area  50  to provide supplemental cooling before being re-compressed. In this manner the cycle in sub-cooling loop  6  is continuously repeated. Thus, in one or more embodiments, the present method is any of the other embodiments disclosed herein further comprising providing cooling using a closed loop (e.g., sub-cooling loop  6 ) charged with flash vapor resulting from the LNG production (e.g., flash vapor  16 ). 
     It will be apparent that in the embodiment illustrated in  FIG. 1  (and in the other embodiments described herein) that as feed gas stream  10  passes from one heat exchange area to another, the temperature of feed gas stream  10  will be reduced until ultimately a sub-cooled stream is produced. In addition, as side streams are taken from feed gas stream  10 , the mass flow rate of feed gas stream  10  will be reduced. Other modifications, such as compression, may also be made to feed gas stream  10 . While each such modification to feed gas stream  10  could be considered to produce a new and different stream, for clarity and ease of illustration, the feed gas stream will be referred to as feed gas stream  10  unless otherwise indicated, with the understanding that passage through heat exchange areas, the taking of side streams, and other modifications will produce temperature, pressure, and/or flow rate changes to feed gas stream  10 . 
       FIG. 2  illustrates another embodiment of the present invention that is similar to the embodiment shown in  FIG. 1 , except that expander loop  5  has been replaced with expander loop  7 . The other items in  FIG. 2  have been previously described above. Expander loop  7  is shown with double-width lines in  FIG. 2  for clarity. Expander loop  7  utilizes substantially the same equipment as expander loop  5  (for example, compressor  20 , cooler  30 , and expander  40 , all of which have been described above). The gaseous refrigerant in expander loop  7  however, is de-coupled from the feed gas and may therefore have a different composition than the feed gas. That is, expander loop  7  is essentially a closed loop and is not connected to feed gas stream  10 . The refrigerant for expander loop  7  is therefore not necessarily the feed gas, although it may be. Expander loop  7  may be charged with any suitable refrigerant gas that is produced at or near the LNG process plant in which expander loop  7  is utilized. For example, the refrigerant gas used to charge expander loop  7  could be a feed gas, such as natural gas, that has only been partially treated to remove contaminants. 
     Like expander loop  5 , expander loop  7  is a high pressure gas loop. Stream  12   a  exits compression unit  20  at a pressure greater than or equal to about 1500 psia, or greater than or equal to about 1600 psia, or greater than or equal to about 1700 psia, or greater than or equal to about 1800 psia, or greater than or equal to about 1900 psia, or greater than or equal to about 2000 psia, or greater than or equal to about 2500 psia, or greater than or equal to about 3000 psia. The temperature of compressed refrigerant stream  12   a  as it emerges from cooler  30  depends on the ambient conditions and the cooling medium used and is typically about from about 35° F. to about 105° F. Cooled compressed refrigerant stream  12   a  is then passed to expander  40  where it is expanded and further cooled to form expanded refrigerant stream  13   a . Expanded refrigerant stream  13   a  is passed to heat exchange area  50  to provide at least part of the refrigeration duty for heat exchange area  50 , where feed gas stream  10  is at least partially cooled by indirect heat exchange with expanded refrigerant stream  13   a . Upon exiting heat exchange area  50 , expanded refrigerant stream  13   a  is returned to compression unit  20  for re-compression. In any of the embodiments described herein, expander loops  5  and  7  may be used interchangeably. For example, in an embodiment utilizing expander loop  5 , expander loop  7  may be substituted for expander loop  5 . 
       FIG. 3  shows another embodiment for producing LNG in accordance with the process of the invention. The process illustrated in  FIG. 3  utilizes a plurality of work expansion cycles to provide supplemental cooling for the feed gas and other streams. The use of such work expansion cycles results in overall improved efficiency for the liquefaction process. Referring to  FIG. 3 , feed gas stream  10  again enters the liquefaction process at the pressures described above. In the particular embodiment shown in  FIG. 3 , side stream  11  is fed to expander loop  5  in the manner previously described, but it will be apparent that closed expander loop  7  could be utilized in the place of expander loop  5 , in which case side stream  11  would not be necessary. Expander loop  5  operates in the same manner as described above for the embodiment shown in  FIG. 1 , except that expanded refrigerant stream  13  is passed through heat exchange area  56 , described in detail below, to provide at least a part of the refrigeration duty for heat exchange area  56 . 
     The portion of feed gas stream  10  that is not withdrawn as side stream  11  is passed to heat exchange area  56  where it is cooled, at least in part, by indirect heat exchange with expanded refrigerant stream  13  and other streams described below. After exiting heat exchange area  56 , feed gas stream  10  is passed through heat exchange areas  57  and  58  where it is further cooled by indirect heat exchange with additional streams described below. In the present embodiment, first and second work expansion cycles are utilized for improved efficiency as follows: before feed gas stream  10  enters heat exchange area  57 , side stream  11   b  is taken from feed gas stream  10 . After feed gas stream  10  exits heat exchange area  57 , but before it enters heat exchange area  58 , side stream  11   c  is taken from feed gas stream  10 . Thus, side streams  11   b  and  11   c  are taken from feed gas stream  10  at different stages of feed gas stream cooling. That is, each side stream is withdrawn from the feed gas stream at a different point on the cooling curve of the feed gas such that each successively withdrawn side stream has a lower initial temperature than the previously withdrawn side stream. 
     Side stream  11   b , which is part of the first work expansion cycle, is passed to expander  42  where it is expanded and consequently cooled to form expanded stream  13   b . Expanded stream  13   b  is passed through heat exchange areas  56  and  57  to provide at least part of the refrigeration duty for heat exchange areas  56  and  57 . Similarly, side stream  11   c , which is part of the second work expansion cycle, is passed to expander  43  where it is expanded and consequently cooled to form expanded stream  13   c . Expanded stream  13   c  is then passed through heat exchange areas  56 ,  57 , and  58  to provide at least part of the refrigeration duty for heat exchange areas  56 ,  57 , and  58 . Accordingly, feed gas stream  10  is also cooled in heat exchange areas  56  and  57  by indirect heat exchange with expanded streams  13   b  and  13   c . In heat exchange area  58  feed gas stream  10  is also cooled by additional indirect heat exchange with expanded stream  13   c.    
     Upon exiting heat exchange area  56 , expanded streams  13   b  and  13   c  are passed to compression units  61  and  62 , respectively, where they are re-compressed and combined to form stream  14   a . Stream  14   a  is cooled by cooler  32  prior to being re-combined with feed gas stream  10 . Cooler  32  can be the same type of cooler or cooler types as coolers  30  and  31 . Expanders  42  and  43  are work expansion devices of the type well know to those of skill in the art. Illustrative, non-limiting examples of suitable work expansion devices include liquid expanders and hydraulic turbines. Thus, in the embodiment shown in  FIG. 3 , the feed gas stream is further cooled using a plurality of work expansion devices. It will be apparent to those of ordinary skill in the art that additional work expansion cycles can be added to the embodiment illustrated in  FIG. 3 , or that a single work expansion cycle could be employed. Generally, therefore, one or more work expansion devices may be employed in the manner described above. Each of the work expansion devices expands a portion of the feed gas stream and thereby cools such portion, wherein each of the portions of the feed gas stream expanded in the work expansion devices is withdrawn from the feed gas stream at a different stage of feed gas stream cooling (i.e., at a different feed gas stream temperature). 
     In one or more other embodiments according to the present invention, the work expansion devices are utilized by withdrawing one or more side streams from the feed gas stream; passing said one or more side streams to one or more work expansion devices; expanding said one of more side streams to expand and cool said one or more side streams, thereby forming one or more expanded, cooled side streams; passing said one or more expanded, cooled side streams to at least one heat exchange area; passing said gas stream through said at least one heat exchange area; and at least partially cooling said gas stream by indirect heat exchange with said one or more expanded, cooled side streams. 
     Referring again to  FIG. 3 , feed gas stream  10 , after being cooled in heat exchange areas  56 ,  57 , and  58 , is then passed to heat exchange area  59  where it is further cooled to produce sub-cooled stream  10   a . The principal function of heat exchange area  59  is to sub-cool feed gas stream  10 . Sub-cooled stream  10   a  is then expanded to a lower pressure in expander  85 , thereby partially liquefying sub-cooled stream  10   a  to form a liquid fraction and a remaining vapor fraction. Expander  85  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. Partially liquefied sub-cooled stream  10   a  is passed to surge tank  80  where the liquefied fraction  15  is withdrawn from the process as LNG having a temperature corresponding to the bubble point pressure. The remaining vapor fraction (flash vapor) stream  16  is used as fuel to power the compressor units and/or as a refrigerant in sub-cooling loop  8  in a manner substantially the same as previously described for sub-cooling loop  6 . As can be seen from  FIG. 3 , sub-cooling loop  8  is similar to sub-cooling loop  6 , except that sub-cooling loop  8  supplies cooling to four heat exchange areas (heat exchange areas  56 ,  57 ,  58 , and  59 ). 
       FIG. 4  illustrates yet another embodiment of the present invention. The embodiment shown in  FIG. 4  is substantially the same as the embodiment shown in  FIG. 3 , except that compression unit  25  and expander  35  have been added. Expander  35  may be any type of liquid expander or hydraulic turbine. Expander  35  is placed between heat exchange areas  58  and  59  such that feed gas stream  10  flows from heat exchange area  58  into expander  35  where it is expanded, and consequently cooled to produce expanded feed gas stream  10   b . Stream  10   b  then is passed to heat exchange area  59  where it is sub-cooled to produce sub-cooled stream  10   c . By expanding and consequently cooling feed gas stream  10  in expander  35  to produce stream  10   b , the overall cooling load on sub-cooling loop  8  is advantageously reduced. Thus, in one or more embodiments, the present method is any of the other embodiments disclosed herein further comprising expanding at least a portion of the cooled feed gas stream to produce a cooled, expanded feed gas stream (e.g., stream  10   b ); and further cooling the cooled, expanded feed gas stream by indirect heat exchange with a closed loop (e.g., sub-cooling loop  6  or  8 ) charged with flash vapor resulting from the LNG production (e.g., flash vapor  16 ). 
     Continuing to refer to  FIG. 4 , compression unit  25  is utilized to increase the pressure of feed gas stream  10  prior to entry into the liquefaction process. Thus, feed gas stream  10  is passed to compression unit  25  where it is compressed to a pressure above the feed gas supply pressure or, in one or more other embodiments, to a pressure greater than about 1200 psia. Alternatively, feed gas stream  10  is compressed to a pressure greater than or equal to about 1300 psia, or greater than or equal to about 1400 psia, or greater than or equal to about 1500 psia, or greater than or equal to about 1600 psia, or greater than or equal to about 1700 psia, or greater than or equal to about 1800 psia, or greater than or equal to about 1900 psia, or greater than or equal to about 2000 psia, or greater than or equal to about 2500 psia, or greater than or equal to about 3000 psia. After compression, feed gas stream  10  is passed to cooler  33  where it is cooled prior to being passed to heat exchange area  56 . It will be appreciated that to the extent compression unit  25  is used to compress feed gas stream  10  (and, hence, side stream  11 ) to a lower pressure than that desired for compressed refrigerant stream  12 , compression unit  20  may be used to boost the pressure. 
     The compression of feed gas stream  10  as described above provides three benefits. First, by increasing the pressure of the feed gas stream, the pressures of side streams  11   b  and  11   c  are also increased, with the result that the cooling performance of work expansion devices  42  and  43  is enhanced. Second, the heat transfer coefficient in the heat exchange areas is improved. Thus, in one or more embodiments, the process for producing LNG described herein is carried out according to any of the other embodiments describe herein wherein the feed gas is compressed to the pressures described above prior to entry into a heat exchange area. In still other embodiments, the present method comprises providing supplemental cooling for the feed gas stream from a plurality of work expansion devices, each of the work expansion devices expanding a portion of the feed gas stream and thereby cooling the portion to form one or more expanded, cooled side streams, wherein each of the portions of the feed gas stream expanded in the work expansion devices is withdrawn from the feed gas stream at a different stage of feed gas stream cooling (i.e., at a different feed gas stream temperature); and cooling said feed gas stream by indirect heat exchange with said one or more expanded, cooled side streams. 
     In still other embodiments, each of the above-described portions of feed gas has a pressure, prior to expansion, greater than about 1200 psia, or greater than or equal to about 1300 psia, or greater than or equal to about 1400 psia, or greater than or equal to about 1500 psia, or greater than or equal to about 1600 psia, or greater than or equal to about 1700 psia, or greater than or equal to about 1800 psia, or greater than or equal to about 1900 psia, or greater than or equal to about 2000 psia, or greater than or equal to about 2500 psia, or greater than or equal to about 3000 psia. In yet other embodiments, the present method is any of the other embodiments described herein further comprising compressing the feed gas stream to any of the pressures described above to produce a pressurized feed gas stream; feeding the pressurized feed gas stream to a work expansion device, or to a plurality of work expansion devices; expanding the compressed feed gas stream through the work expansion device, or through a plurality of work expansion devices, to provide supplemental cooling for the feed gas stream. 
     A third benefit obtained by compression the feed gas stream as described above is that the cooling capacity of expander  35  is improved, with the result that expander  35  is able to even further reduce the cooling load on sub-cooling loop  8 . It will be appreciated that compression unit  25  and/or expander  35  could also be advantageously added to other embodiments described herein to provide similar reductions in the cooling load on the sub-cooling loops utilized in those embodiments or other improvements in cooling, and that compression unit  25  and expander  35  may be used independently of each other in any embodiment herein. Moreover, it will also be appreciated that the cooling capacity of expander  35  (or the work expansion devices  42  and  43 ) will be improved, even without compression of the feed stream, to the extent the feed stream is supplied at a pressure above the bubble point pressure of the LNG. For example, if the feed gas is supplied at any of the pressures described above resulting from compression of the feed gas, the benefit of such pressure will obviously be obtainable without additional compression. Therefore, in interpreting this specification, including the appended claims, the use of work expansion devices and/or expander  35  to expand streams having pressures above about 1200 psia should not be construed as requiring the use or presence of compression unit  25  or of any other compressor or compression step. 
       FIG. 5  is a schematic flow diagram of a fifth embodiment for producing LNG in accordance with the process of this invention that is similar to the embodiment shown in  FIG. 4 , but utilizes yet another expansion step to provide sub-cooling. Referring to  FIG. 5 , it will be seen that sub-cooling loop  8  is not present in the embodiment shown in  FIG. 5 . Instead, side stream  11   d  is taken from stream  10   b  and passed to expansion device  105  where it is expanded and consequently cooled to form expanded stream  13   d . Expansion device  105  is a work-producing expander, many types of which are readily available. Illustrative, non-limiting examples of such devices include liquid expanders and hydraulic turbines. Expanded stream  13   d  is passed through heat exchange areas  59 ,  58 ,  57 , and  56  to provide at least part of the refrigeration duty for those heat exchange areas. As can be seen from  FIG. 5 , stream  10   b  is also cooled by indirect heat exchange with expanded stream  13   d , as well as by the flash vapor stream  16 . Thus, in one or more embodiments, the inventive process further comprises expanding at least a portion of the cooled gas stream (feed gas stream  10 ) in expander  35  before the final heat exchange step (for example, prior to heat exchange area  59 ) to produce an expanded, cooled gas stream (for example, stream  10   b ); passing a portion of said expanded, cooled gas stream to a work-producing expander; further expanding said expanded, cooled gas stream in said work-producing expander; and passing the stream emerging from said work-producing expander (for example, stream  13   d ) to a heat exchange area to further cool said expanded, cooled gas stream by indirect heat exchange in said heat exchange area. 
     Upon exiting heat exchange area  56 , expanded stream  13   d  is passed to compression unit  95  where it is re-compressed and combined with the streams emerging from compression units  61  and  62  to form part of stream  14   a , which is cooled and then re-cycled to feed stream  10  as before. 
     A further embodiment shown in  FIG. 6  is similar to the embodiment shown in  FIG. 1  and described above, except that sub-cooling loop  6  has been modified such that after exiting heat exchange area  50 , the re-compressed and cooled sub-cooling refrigerant stream is further cooled in heat exchange area  55  prior to being expanded through expander  41 . This embodiment is favorable where a cooling fluid is used that does not present much condensation after expander  41 . 
       FIG. 7  depicts another embodiment in which sub-cooling loop  6   a  uses a portion of feed gas  10 . The portion of feed gas  10  is re-pressurized in compressor  25  and cooled in cooler  33  from 201, in the same fashion as in  FIG. 4 . 
       FIG. 8  is another embodiment similar to  FIG. 7  showing an alternative arrangement for the sub-cooling loop  6 . Depending on the composition of feed gas  10 , an additional compressor (not shown) may be used to prevent condensation in the sub-cooling loop or to ensure adequate line pressures. 
       FIG. 9  depicts an embodiment for use with certain feed gas  10  compositions and/or pressures. To better match the cooling curve of the feed gas  10  being cooled for LNG collection, to the cooling curve of that portion of feed gas  10  being used for cooling in sub-cooling heat exchange area  55 , it may be necessary to further expand a split of the portion of the refrigerant gas going to the sub-cooling loop  6 . This is accomplished using an expansion valve  82  or other expander (e.g., a Joules-Thompson valve) to provide supplemental cooling in sub-cooling loop  6 . 
       FIG. 10  represents another embodiment showing the integration of a nitrogen rejection stage using distillation column  81  or equivalent device, for the case where nitrogen rejection is needed, based on feed gas  10  composition. This may be needed to meet the nitrogen specification of product LNG for transmission and end use. 
       FIG. 11  represents another embodiment showing the integration of a nitrogen rejection unit, where the flash vapor from the nitrogen rejection unit is used as refrigerant for the sub-cooling loop. The resulting refrigerant is therefore rich in nitrogen. 
     EXAMPLE 
     A hypothetical mass and energy balance was carried out to illustrate the embodiment shown in  FIG. 4 , and the results are shown in the Table below. The data were obtained using a commercially available process simulation program called HYSYS™ (available from Hyprotech Ltd. of Calgary, Canada); however, other commercially available process simulation programs can be used to develop the data, including for example HYSIM™, PROII™, and ASPEN PLUS™, which are familiar to those of ordinary skill in the art. This example assumed that feed gas stream  10  had the following composition in mole percent: C 1 : 90.25%; C 2 : 5.70%; C 3 : 0.01%; N 2 : 4.0%; He: 0.04%. The data presented in the Table are offered to provide a better understanding of the embodiment shown in  FIG. 4 , but the invention is not to be construed as unnecessarily limited thereto. The temperatures, pressures, and flow rates can have many variations in view of the teachings herein. The specific temperature, pressure, and flow rate calculated for state points 201 through 214 (at the locations shown in  FIG. 4 ) are set forth in the Table. 
     In one embodiment of the inventive method, by controlling the temperature of the stream emerging from the final heat exchange area, the volume of flash vapor stream  16  is controlled to match the fuel requirements of the compression units and other equipment. For example, referring to  FIG. 4 , the temperature at state point 207 can be controlled to produce more or less flash vapor (stream  16 ) depending on the fuel requirements. Higher temperatures at state point 207 will result in the production of more flash vapor (and hence more available fuel), and vice-versa. Alternatively, the temperature may be adjusted such that the flash vapor flow rate is higher than the fuel requirement, in which case the excess flow above the fuel flow requirement may be recycled after compression and cooling. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                   
                 Temperature 
                 Pressure 
                 Flow 
               
               
                   
                 State Point 
                 (deg. F.) 
                 (psia) 
                 (lb-mole/hr) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 201 
                 262 
                 985 
                 3.35 × 10 5   
               
               
                   
                 202 
                 100 
                 1500 
                 1.08 × 10 6   
               
               
                   
                 203 
                 −36 
                 1480 
                 4.85 × 10 5   
               
               
                   
                 204 
                 −130 
                 1470 
                 3.35 × 10 5   
               
               
                   
                 205 
                 −213 
                 1460 
                 3.35 × 10 5   
               
               
                   
                 206 
                 −229 
                 48 
                 3.35 × 10 5   
               
               
                   
                 207 
                 −236 
                 42 
                 3.35 × 10 5   
               
               
                   
                 208 
                 −254 
                 18 
                 3.35 × 10 5   
               
               
                   
                 209 
                 −217 
                 71 
                 3.12 × 10 5   
               
               
                   
                 210 
                 −140 
                 420 
                 2.29 × 10 4   
               
               
                   
                 211 
                 100 
                 126 
                 2.57 × 10 4   
               
               
                   
                 212 
                 −240 
                 44 
                 2.57 × 10 4   
               
               
                   
                 213 
                 100 
                 3000 
                 8.57 × 10 5   
               
               
                   
                 214 
                 −40 
                 895 
                 8.57 × 10 5   
               
               
                   
                   
               
            
           
         
       
     
     A person skilled in the art, particularly one having the benefit of the teachings herein, will recognize many modifications and variations to the specific embodiments disclosed above. For example, features shown in one embodiment may be added to other embodiments to form additional embodiments. Thus, the specifically disclosed embodiments and example should not be used to limit or restrict the scope of the invention, which is to be determined by the claims that follow.