Abstract:
The described invention relates to processes and systems for treating a gas stream, particularly one rich in methane for forming liquefied natural gas (LNG), the process including: (a) providing a gas stream; (b) providing a refrigerant; (c) compressing the refrigerant to provide a compressed refrigerant; (d) cooling the compressed refrigerant by indirect heat exchange with a cooling fluid; (e) expanding the refrigerant of (d) to cool the refrigerant, thereby producing an expanded, cooled refrigerant; (f) passing the expanded, cooled refrigerant to a first heat exchange area; (g) compressing the gas stream of (a) to a pressure of from greater than or equal to 1,000 psia to less than or equal to 4,500 psia; (h) cooling the compressed gas stream by indirect heat exchange with an external cooling fluid; and heat exchanging the compressed gas stream with the expanded, cooled refrigerant stream.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/US2008/008027, filed 26 Jun. 2008, which claims the benefit of U.S. Provisional Application No. 60/966,022, filed 24 Aug. 2007. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention relate generally to the liquefaction of gases, and more specifically liquefaction of natural gas, particularly the liquefaction of gases in remote locations. 
     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 cooling is typically needed to fully liquefy the feed gas and this may be provided by additional refrigerant systems, such as secondary cooling loops. 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. Though a typical expander cycle for making LNG can operate at the feed gas pressure, typically under about 5,516 kPa (800 psia), a high pressure primary cooling loop had been found to be particularly promising. See, for example, WO 2007/021351. It has also been discovered that adding external cooling to such a primary cooling loop provides additional advantages in many situations. See PCT/US08/02861. 
     Because expander cycles result in a high recycle gas stream flow rate and resulting high cooling load, introducing inefficiencies for the primary cooling (warm) stage, gas expander processes such as described above further cool the feed gas after it has been pre-cooled using a refrigerant in a secondary cooling unit. For example, U.S. Pat. No. 6,412,302 and U.S. Pat. No. 5,916,260 present expander cycles which describe the use of nitrogen as refrigerant in the sub-cooling loop. The primary (warm-end) expander cooling loop operates at low pressure and therefore limits the fraction of the feed gas cooling load provided by this primary loop. Consequently, a nitrogen (or nitrogen-rich) refrigerant is required in the sub-cooling loop. WO 2007/021351 (above) uses a portion of the flash gas derived from the feed gas in the final separation unit. Thus, generally, an element in expander cycle processes is the requirement for at least one second refrigeration cycle to sub-cool the feed gas before it enters the final expander for conversion of much, if not all, remaining gaseous feed to LNG. 
     Though this process performs comparably to alternative mixed external refrigerant LNG Production processes, including mixed expander-refrigerant processes, it has been of interest to improve the efficiency of the process of expander cycles for making LNG. In particular it has been of interest to use less fuel and reduce the power generation equipment required, especially for hard to reach locations, such as offshore or in environmentally severe onshore locations. 
     Other potentially relevant information may be found in International Publication No. WO2007/021351; Foglietta, J. H., et al., “Consider Dual Independent Expander Refrigeration for LNG Production New Methodology May Enable Reducing Cost to Produce Stranded Gas,” Hydrocarbon Processing, Gulf Publishing Co., vol. 83, no. 1, pp. 39-44 (January 2004); U.S. App. No. US2003/089125; U.S. Pat. No. 6,412,302; U.S. Pat. No. 3,162,519; U.S. Pat. No. 3,323,315; and German Pat. No. DE19517116. 
     SUMMARY OF THE INVENTION 
     The invention is a process for liquefying a gas stream, particularly one rich in methane, said process comprising: (a) providing said gas stream at a pressure of from 600 to 1,000 psia as a feed gas stream; (b) providing a refrigerant at a pressure of less than 1,000 psia; (c) compressing said refrigerant to a pressure greater than or equal to 1,500-5,000 psia to provide a compressed refrigerant; (d) cooling said compressed refrigerant by indirect heat exchange with a cooling fluid; (e) expanding the refrigerant of (d) to cool said refrigerant, thereby producing an expanded, cooled refrigerant at a pressure of from greater than or equal to 200 psia to less than or equal to 1,000 psia; (f) passing said expanded, cooled refrigerant to a first heat exchange area; (g) compressing the gas stream of (a) to a pressure of from greater than or equal to 1,000 psia to less than or equal to 4,500 psia; (h) cooling said compressed gas stream by indirect heat exchange with an external cooling fluid; and, (i) passing said compressed gas stream through the first heat exchange area to cool at least a part thereof by indirect heat exchange, thereby forming a compressed, further cooled gas stream. 
     In a preferred embodiment, the feed gas stream in (g) is compressed to 1,500 to 4,000 psia (10342 to 27579 kPa), more preferably 2,500 to 3,500 psia (17237 to 24132 kPa), for optimization of overall power requirements for the gas, methane-rich gas, or natural gas, liquefaction. 
     In another embodiment of the present invention a system for treating a gaseous feed stream is provided. The system includes: a gaseous feed stream; a first refrigeration loop having a refrigerant stream, a first compression unit, and a first cooler configured to produce a compressed, cooled refrigerant stream; a second compression unit configured to compress the gaseous feed stream to greater than 1,000 psia (8,274 kPa) to form a compressed gaseous feed stream; a second cooler configured to cool the compressed gaseous feed stream to form a compressed, cooled gaseous feed stream, wherein the second cooler utilizes an external cooling fluid; and a first heat exchange area configured to further cool the compressed, cooled gaseous feed stream at least partially by indirect heat exchange with the compressed, cooled refrigerant stream to produce a sub-cooled, compressed, cooled gaseous feed stream. 
    
    
     
       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 where the feed gas stream  10  is compressed in accordance with the invention prior to being cooled by the primary cooling loop  5  which optionally may use a portion of the feed gas  11 , before the compression, as the primary cooling loop  5  refrigerant, and a portion of the expanded, cooled feed gas  10   d  is used as a refrigerant in a secondary cooling loop  6 . 
         FIG. 2  is a preferred embodiment where the secondary cooling loop  6  is a closed loop using nitrogen gas, or a nitrogen-rich gas, or a portion of the flash gas  17  from a gas-liquid separation unit  80 . 
         FIG. 3  represents the respective cooling curves for heat exchanger  50  at conventional low feed gas pressure ( FIG. 3A ) and the invention process elevated feed gas pressure ( FIG. 3B ). 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide increased efficiencies by taking advantage of elevating the pressure of the feed gas stream for subsequent heat exchange cooling in both a primary cooling loop and one or more secondary cooling loops. Additional benefit or improvement of the elevated pressure results when a portion of the cooled, elevated feed pressure stream is extracted and used as the refrigerant in a sub-cooling loop. In the prior art, the feed gas is provided typically at a pressure less than about 800 psia (5516 kPa). To enhance cooling the feed gas may be combined with one or more cooling streams of the secondary cooling loops, particularly where such cooling stream, or streams, consists of recycled feed gas or fractions or portions thereof. However, in doing so, the feed stream and provided cooling stream must typically be at the same pressure so as to allow piping, joints and flanges to be economically sized and constructed with characteristics suitable to the larger volume feed gas stream and to minimize the number of streams passing through each heat exchange area. Operating the primary heat exchange at this low pressure limits the thermodynamic performance since an ideal matching of the cooling curve of the feed gas to the warming curve of the primary refrigerant cannot be achieved. Further, since the pressure of the primary refrigerant stream is fixed by the primary heat exchanger cold end temperature, the refrigerant stream condition cannot be changed to better match the cooling curve of the feed stream. 
     The improved embodiments of the present invention involve operating the feed gas and/or the secondary cooling stream at elevated pressures and employing heat exchangers capable of high-pressure operation (e.g., printed circuit heat exchangers manufactured by the Heatric Company, now part of Meggitt Ltd. (UK)). Operation at the elevated pressures allows reduction of the refrigeration load, or cooling requirement, in the primary heat exchange unit and allows a better match of the composite cooling curves in it. As shown below in data Table 1 the cooling load for the feed gas stream  10   b  from the inlet to exchanger  50  to the exchanger  55  outlet at  10   d  is reduced by 16% as the pressure is increased from 1,000 psia (6895 kPa) to 3,000 psia (20,684 kPa). As noted, operating at high pressure allows a shift of the cooling load from the high pressure primary cooling loop  5  to the ambient cooling units  35  and  37  that require no compression. Further, as shown in  FIGS. 3A and 3B , the cooling curves are better matched at the higher pressure 3000 psia (20684 kPa) in  FIG. 3B  and pinched at the lower pressure of 800 psia (5516 kPa) in  FIG. 3A  for cooling the feed gas stream  10   b  in exchanger  50  to provide cooled stream  10   c . This results in significant improvement in the overall performance of the process of WO 2007/021351. 
       FIG. 1  illustrates one embodiment of the present invention in which a high pressure primary expander loop  5  (i.e., an expander cycle) and a sub-cooling loop  6  are used. 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 1,200 psia (8274 kPa), or less than about 1,100 psia (7584 kPa), or less than about 1,000 psia (6895 kPa), or less than about 900 psia (6205 kPa), or less than about 800 psia (5516 kPa), or less than about 700 psia (4826 kPa), or less than about 600 psia (4137 kPa). Typically, the pressure of feed gas stream  10  will be about 800 psia (5516 kPa). 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. Optionally, after being passed through an external refrigerant cooling unit  35 , typically at ambient cooling temperature, 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 1,200 psia (8274 kPa). 
     The refrigerant for the primary expander loop  5  may be any suitable gas component, preferably one available at the processing facility, and most preferably, as shown, is a portion of the methane-rich feed gas stream  10 . Thus, in the embodiment shown in  FIG. 1 , a portion of the feed gas stream  10  is used as the refrigerant for expander loop  5 . 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 compressor, the side stream  11  of feed gas to be used as the refrigerant in expander loop  5  may be withdrawn from the feed gas stream  10  before the feed gas stream  10   a  has been passed to the initial cooling unit  35 . 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  11  to be used as the refrigerant is withdrawn prior to the heat exchange area  50 , compressed, cooled and expanded, and passed back to the heat exchange area  50  to provide at least part of the refrigeration duty for that heat exchange area  50 . 
     Thus side stream  11  is passed to compression unit  20  where it is compressed to a pressure greater than or equal to about 1,500 psia (10,342 kPa), thus providing a compressed refrigerant stream  12 . Alternatively, side stream  11  is compressed to a pressure greater than or equal to about 1,600 psia (11,032 kPa), or greater than or equal to about 1,700 psia (11,721 kPa), or greater than or equal to about 1,800 psia (12,411 kPa), or greater than or equal to about 1,900 psia (13,100 kPa), or greater than or equal to about 2,000 psia (13,789 kPa), or greater than or equal to about 2,500 psia (17,237 kPa), or greater than or equal to about 3,000 psia (20,684 kPa), 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 ambient air or water to provide a compressed, cooled refrigerant  12   a . The temperature of the compressed refrigerant stream  12   a  as it emerges from cooler  30  depends on the ambient conditions and the cooling medium used and is typically from about 35° F. (1.7° C.) to about 105° F. (40.6° C.). Where the ambient temperature is in excess of 50° F. (10° C.), more preferably in excess of 60° F. (15.6° C.), or most preferably in excess of 70° F. (21.1° C.), the stream  12   a  is optionally passed through a supplemental cooling unit (not shown), operating with external coolant fluids, such that the compressed refrigerant stream  12   a  exits said cooling unit at a temperature that is cooler than the ambient temperature. The external refrigerant cooled compressed refrigerant stream  12   a  is then expanded in a turbine expander  40  before being passed to heat exchange area  50 . Depending on the temperature and pressure of compressed refrigerant stream  12   a , expanded stream  13  may have a pressure from about 100 psia (689 kPa) to about 1,000 psia (6895 kPa) and a temperature from about −100° F. (−73° C.) to about −180° F. (−118° C.). In an illustrative example, stream  13  will have a pressure of about 302 psia (2082 kPa) and a temperature of −162° F. (−108° C.). The power generated by the turbine expander  40  is used to offset the power required to re-compress the refrigerant in loop  5  in compressor units  60  and  20 . The power generated by the turbine expander  40  (and, any of the turbine expanders to be used) may be in the form of electric power where it is coupled to a generator, or mechanical power through a direct mechanical coupling to a compressor unit. 
     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   a  is fed to compression unit  60  for pressurization to form stream  13   b , 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  13   b . 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  and becomes a cooled fluid stream that may comprise liquefied gas, cooled gas, and/or two-phase fluid. 
     Thus the portion of feed gas stream  10  not withdrawn as side stream  11  is passed to a compressor, such as a turbine compressor  25 , and then subjected to optional cooling with one or more external refrigerant units  37  to remove at least a portion of the heat of compression. There the feed gas stream  10   a  is compressed to a pressure greater than or equal to about 1,000 psia (6895 kPa), thus providing a compressed feed gas stream  10   b . Alternatively, side stream  10   a  is compressed to a pressure greater than or equal to about 1,500 psia (10342 kPa), or greater than or equal to about 2,000 psia (13789 kPa), or greater than or equal to about 2,500 psia (17237 kPa), thus providing compressed feed gas stream  10   b . The pressure need not exceed 4,500 psia (31026 kPa), as noted earlier, and preferably not exceed 3,500 psia (24132 kPa). Compressed feed gas stream  10   b  then enters heat exchange area  50  where cooling is provided by streams from primary cooling loop  5 , secondary cooling loop  6 , optionally, as shown, with flash gas stream  16 . 
     After exiting heat exchange area  50 , feed gas stream  10   c  is optionally passed to heat exchange area  55  for further cooling. 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   c  is preferably sub-cooled by a sub-cooling loop  6  (described hereinafter) to produce sub-cooled fluid stream  10   d . Sub-cooled fluid stream  10   d  is then expanded to a lower pressure in expander  45 , thereby cooling further said stream. A portion of fluid stream  10   d  is taken off for use as the loop  6  refrigerant stream  14 . The portion of fluid stream  10   d  not taken off forms stream  10   e  which is optionally passed to an expander  70  to additionally cool sub-cooled fluid stream  10   e  to form principally 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. The largely liquefied sub-cooled stream  10   e  is passed to a separator, e.g., surge tank  80  where the liquefied portion  15  is withdrawn from the process as LNG having a temperature corresponding to the bubble point pressure. The remaining vapor portion (flash vapor) stream  16  is used as fuel to power the compressor units and may be optionally used as a refrigerant in sub-cooling loop  6 , as illustrated in  FIG. 1 . So, 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 those heat exchange areas. The flash vapor stream  16  may also be used as the refrigerant, or to supplement the refrigerant, in refrigeration loop  5 , not shown. 
     The refrigerant stream  14  of sub-cooling loop  6  is led through heat exchange area  55  to provide part of the heat removal duty and exits as stream  14   a , which in turn is provided to heat exchange area  50  for further heat removal duty. The thus warmed stream exits as stream  14   b  which is compressed in compressor unit  90 , and then cooled in cooling unit  31 , which can be an ambient temperature air or water external refrigerant cooler, or may comprise any other external refrigerant unit(s). This compressed, cooled stream  14   b  is then added to feed gas stream  10   a , thus completing loop  6 . 
     Referring now to  FIG. 2 , sub-cooling loop  6  is a closed loop utilizing nitrogen, or nitrogen-containing gas as refrigerant stream  14 . Stream  14  can typically be provided from bottled sources, or from other contiguous air separation and treatment processes, and will be provided typically at a temperature of about 60° F. (15.6° C.) to about 95° F. (35° C.) and a pressure of about 800 psia (5516 kPa) to about 2,500 psia (17237 kPa). Gaseous stream  14   d  is provided to expander  41  and exits expander  41  as gaseous stream  14  typically having a temperature from about −220° F. (−140° C.) to about −260° F. (−162° C.) (e.g. about −242° F. (−52° C.)) and a pressure of about 50 psia (345 kPa) to about 550 psia (3792 kPa). Stream  14  can be provided to heat exchange areas  55  and  50  as illustrated. The warmed stream  14   b , after passing through the exchange areas, is then compressed in compression unit  90  and cooled in external refrigerant cooling unit  31 , which can be of the same type as ambient temperature cooler  37 , so as to be approximately at the original temperature and pressure of stream  14   s  for merging with or comprising stream  14   c . After cooling, the re-compressed sub-cooling refrigerant stream  14   b  becomes stream  14   c , and 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  14   a , and, optionally, flash vapor stream  16   a  before returning to expander  41  as stream  14   d.    
     Alternatively, in  FIG. 2 , 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  after liquefaction is withdrawn (in the form of flash gas from flash gas stream  16 ) for use as the refrigerant by providing into the secondary expansion cooling loop, e.g., 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 gas from line  17 ) to replace losses from leaks is required. In sub-cooling loop  6 , stream  14  is drawn through heat exchange areas  55  to become stream  14   a  and  50  to become stream  14   b . The sub-cooling refrigerant stream  14   b  (the flash vapor stream) is then returned to compression unit  90  where it is re-compressed to a higher pressure and is warmed further. After exiting compression unit  90 , the re-compressed sub-cooling refrigerant stream  14   b  is cooled in one or more external refrigerant cooling units (e.g., an ambient temperature cooler  31 , as above). 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  14   a , 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 ). 
     EXAMPLES 
     The below presented tables and description depict performance curves and comparisons developed using an Aspen HYSYS® (version 2006) process simulator, a computer aided design program from Aspen Technology, Inc., of Cambridge Mass. The enthalpy values are calculated using the HYSYS process simulator. The enthalpy values are negative because of the enthalpy reference basis used by HYSYS. In HYSYS, this enthalpy reference basis is the heat of formation at 25° C. and 1 atm (ideal gas). 
     Table 1 illustrates the cooling load reduction for expander loop  5  and subcooling loop  6  when the cooling loads are compared from operating the feed gas at 1,000 psia (6895 kPa) versus 3,000 psia (20684 kPa), as discussed above. 
     Tables 2 and 3 below illustrate flow rate, pressures, and power consumption data using the invention process where the feed gas pressure at the entry to the primary heat exchange (e.g.,  50 ) was varied from 1,000 psia (6895 kPa) to 5,000 psia (34474 kPa) while keeping the temperature at the cold end of the primary heat exchanger  50  (at  10   c ) constant. The feed gas rate is kept constant and just enough fuel (for the embodiments in  FIG. 1  or  FIG. 2 ) is separated to provide a fuel source for power production. The feed gas used in this illustrative case is predominantly methane (e.g., about 96%) with about 4% nitrogen. A nitrogen rejection unit (not shown) for the LNG withdrawn from separation unit  80  will be typically in use. 
     The data of Table 2 and Table 3 illustrate the benefits of the invention on process performance. The flow rate through the primary loop  5  decreases monotonically as the pressure of the feed gas stream  10   b  to the heat exchange unit is elevated. This results in a reduction in the primary loop compression horsepower requirement. However, this reduction is partially offset by the increased compression requirement for both the feed gas  10   a  and the sub-cooling loop refrigerant in loop  6 , to the elevated pressure. Consequently, the total horsepower (representing the installed compression power) and the net horsepower for the cycle (representing the installed turbine power) do not track the monotonic decrease in the primary loop power requirement. As the pressure of the feed gas increases, the contribution of the feed gas compression to the total compression power requirements becomes increasingly significant, eventually becoming the dominant incremental contributor so as to increase unacceptably the total compression power requirements. On the other hand, at lower feed gas pressures, the composite effect of the increased cooling requirement and the heat exchange inefficiency result in a high compression requirement in primary loop  5 . As a consequence the total power requirement is higher. Accordingly optimum performance has been found unexpectedly to be in the ranges described and claimed in this application. 
     Further, as shown in Table 2 (below), the refrigerant flow rate through the primary loop  5  is reduced by more than a factor of two as the heat exchange pressure is increased from 1,000 psia (6895 kPa) to 5,000 (34474 kPa) psia. Table 3 shows a similar trend. The reduced flow rate enables the use of compact equipment that is particularly attractive for offshore gas processing applications. 
     The performance benefits of the invention, as shown by the data in Tables 2 and 3, show that the optimum performance was attained when the primary heat exchanger  50  was operated at a feed gas pressure between 2,000 psia (13789 kPa) and 4,000 psia (27579 kPa). However, there can be variations in the optimal heat exchange unit or feed gas pressure for a given process configuration, based on feed gas composition, feed gas supply pressure prior to compression, refrigerant composition, and the refrigerant pressure in loop  5 , all of which can be determined empirically by those skilled in the art and informed by the description above. For the illustrative example provided, the optimum mode (least total compression power) was determined to be operation at about 2,750 psia (18961 kPa). The primary loop operating pressure for this illustrative example was fixed at 3,000 psia (20684 kPa). 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Cooling Load Reduction Using High Pressure 
               
             
          
           
               
                   
                   
                 Total 
                 % Feed 
                 % Feed Load 
               
               
                   
                 Stream Condition 
                 Cooling 
                 Load from 
                 from 
               
             
          
           
               
                   
                   
                   
                 Enthalpy 
                 Load 
                 Expander 
                 Ambient 
               
               
                 Stream 
                 Press. 
                 Temp. 
                 (BTU/lb)/ 
                 (BTU/lb)/ 
                 Cooling 
                 Cooling 
               
               
                 definition 
                 (psia/kPa) 
                 (° F./° C.) 
                 (kJ/kg) 
                 (kJ/kg) 
                 Loops 
                 (Water/Air) 
               
               
                   
               
             
          
           
               
                 Inlet Feed 
                 1000/6895 
                 95/35 
                 −1879/−4371 
                 321/747 
                   
                   
               
               
                 Gas (stream 
               
               
                 10) 
               
               
                 Exchanger 50 
                 1000/6895 
                   60/15.6 
                 −1901/−4422 
                 299/696 
                 93 
                 7 
               
               
                 Inlet (stream 
               
               
                 10b) (low 
               
               
                 pressure) 
               
               
                 Exchanger 
                  3000/20684 
                   60/15.6 
                 −1949/−4536 
                 251/582 
                 78 
                 22 
               
               
                 Inlet (stream 
               
               
                 10b) (elevated 
               
               
                 pressure) 
               
             
          
           
               
                 Exchanger 55 Outlet 
                 −240/−151 
                 −2200/−5118 
                   
                   
                   
               
               
                 stream 10d 
               
               
                   
               
             
          
         
       
     
     The foregoing application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such obvious modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Example Case: Natural Gas 1 using feed gas as sub-cooling loop refrigerant (FIG. 1 Configuration) 
               
             
          
           
               
                   
                 Primary Loop 
                 Subcool 
                 Primary Loop 
                 Subcool Loop 
                 Feed Gas 
                 Total 
                   
                 Net 
               
               
                 Feed 
                 Flow 
                 Loop Flow 
                 Compression 
                 Compression 
                 Compression 
                 Compression 
                 Expander 
                 Compression 
               
               
                 Pressure 
                 Mmscfd/ 
                 Mmscfd/ 
                 Power 
                 Power 
                 Power 
                 Power 
                 Power 
                 Power 
               
               
                 Psia/kPa 
                 kg-mole/hr 
                 kg-mole/hr 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
               
               
                   
               
               
                 5000/34474 
                  950/47334 
                 212.1/10564 
                 120.8/90  
                 62.1/46.3 
                 66.8/49.8 
                 267.4/199.4 
                 53.30/39.7 
                 214.1/159.7 
               
               
                 4500/31026 
                  977/48669 
                 216.8/10798 
                 124.2/93  
                 61.5/45.9 
                 61.0/45.5 
                 264.4/197.2 
                 53.16/39.6 
                 211.2/157.5 
               
               
                 4000/27579 
                 1010/50303 
                 222.5/11082 
                 128.3/96  
                 61.0/45.5 
                 54.8/40.9 
                 261.9/195.3 
                 53.23/39.7 
                 208.7/155.6 
               
               
                 3500/24132 
                 1052/52394 
                 229.3/11420 
                 133.8/100 
                 60.5/45.1 
                 48.2/35.9 
                 260.0/193.9 
                 53.73/40.1 
                 206.3/153.8 
               
               
                 3000/20684 
                 1103/54934 
                 237.6/11834 
                 140.3/105 
                 59.8/44.6 
                 40.9/30.5 
                 258.7/192.9 
                 54.53/40.7 
                 204.2/152.2 
               
               
                 2500/17237 
                 1180/58769 
                 247.9/12347 
                 149.9/112 
                 60.0/44.7 
                 32.9/24.5 
                 260.5/194.3 
                 56.42/42.1 
                 204.1/152.2 
               
               
                 2000/13789 
                 1298/64646 
                 261.1/13004 
                 164.2/122 
                 60.1/44.8 
                 23.8/17.8 
                 265.9/198.3 
                 60.01/44.7 
                 205.9/153.5 
               
               
                 1500/10342 
                 1550/77197 
                 279.1/13900 
                 193.3/144 
                 59.9/44.7 
                 13.2/9.9  
                 284.1/211.9 
                 69.19/51.6 
                 214.9/160.3 
               
               
                 1250/8618  
                 1728/86062 
                 291.0/14493 
                 213.4/159 
                 59.7/44.5 
                 7.0/5.2 
                 297.8/222.1 
                 75.95/56.6 
                 221.9/165.4 
               
               
                 1000/6895  
                  2112/105187 
                 306.3/15255 
                 255.1/190 
                 58.7/43.8 
                 0.0/0.0 
                 331.5/247.2 
                 91.34/68.1 
                 240.2/179.1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Example Case: Natural Gas 2 using nitrogen as sub-cooling loop refrigerant (FIG. 2 Configuration) 
               
             
          
           
               
                   
                 Primary Loop 
                 Subcool 
                 Primary Loop 
                 Subcool Loop 
                 Feed Gas 
                 Total 
                   
                 Net 
               
               
                 Feed 
                 Flow 
                 Loop Flow 
                 Compression 
                 Compression 
                 Compression 
                 Compression 
                 Expander 
                 Compression 
               
               
                 Pressure 
                 Mmscfd/ 
                 mmscfd/ 
                 Power 
                 Power 
                 Power 
                 Power 
                 Power 
                 Power 
               
               
                 psia/kPa 
                 Kg-mole/hr 
                 kg-mole/hr 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
                 khp/MW 
               
               
                   
               
               
                 5000/34474 
                 1417/70573 
                 1061/52843 
                 198/148 
                  93.9/70.0 
                 110.3/82.3  
                 424/316 
                 94.2/70.3 
                 329.8/245.9 
               
               
                 4500/31026 
                 1448/72117 
                 1075/53540 
                 203/151 
                  95.4/71.2 
                 100.6/75.0  
                 420/313 
                 94.3/70.3 
                 326.0/243.1 
               
               
                 4000/27579 
                 1487/74059 
                 1092/54387 
                 208/155 
                  97.3/72.5 
                 90.4/67.4 
                 418/311 
                 94.8/70.7 
                 322.7/240.6 
               
               
                 3500/24132 
                 1534/76400 
                 1112/55383 
                 215/160 
                  99.5/74.2 
                 79.4/59.2 
                 415/310 
                 95.6/71.3 
                 319.6/238.3 
               
               
                 3000/20684 
                 1592/79289 
                 1135/56528 
                 223/166 
                 102.2/76.2 
                 67.4/50.3 
                 414/309 
                 97.0/72.3 
                 317.0/236.4 
               
               
                 2500/17237 
                 1675/83423 
                 1163/57923 
                 234/175 
                 105.5/78.7 
                 54.1/40.4 
                 416/310 
                 99.5/74.2 
                 316.0/235.6 
               
               
                 2000/13789 
                 1799/89598 
                 1199/59716 
                 251/187 
                 109.6/81.7 
                 39.2/29.2 
                 421/314 
                 104.0/77.6  
                 316.9/236.3 
               
               
                 1500/10342 
                  2010/100107 
                 1247/62106 
                 277/207 
                 115.4/86.1 
                 21.7/16.2 
                 436/325 
                 112.4/83.8  
                 323.4/241.2 
               
               
                 1000/6895  
                  2487/123864 
                 1313/65393 
                 334/249 
                 123.7/92.2 
                 0.0/0.0 
                 479/357 
                 132.8/99.0  
                 346.1/258.1