Patent Publication Number: US-RE39637-E

Title: Hybrid cycle for the production of liquefied natural gas

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The production of liquefied natural gas (LNG) is achieved by cooling and condensing a feed gas stream against multiple refrigerant streams provided by recirculating refrigeration systems. Cooling of the natural gas feed is accomplished by various cooling process cycles such as the well-known cascade cycle in which refrigeration is provided by three different refrigerant loops. One such cascade cycle uses methane, ethylene and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane pre-cooled, mixed refrigerant cycle in which a multicomponent refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant can contain hydrocarbons such as methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen. Versions of this efficient refrigeration system are used in many operating LNG plants around the world. 
     Another type of refrigeration process for natural gas liquefaction involves the use of a nitrogen expander cycle in which nitrogen gas is first compressed and cooled to ambient conditions with air or water cooling and then is further cooled by counter-current exchange with cold low-pressure nitrogen gas. The cooled nitrogen stream is then work expanded through a turbo-expander to produce a cold low pressure stream. The cold nitrogen gas is used to cool the natural gas feed and the high pressure nitrogen stream. The work produced by the nitrogen expansion can be used to drive a nitrogen booster compressor connected to the shaft of the expander. In this process, the cold expanded nitrogen is used to liquefy the natural gas and also to cool the compressed nitrogen gas in the same heat exchanger. The cooled pressurized nitrogen is further cooled in the work expansion step to provide the cold nitrogen refrigerant. 
     Refrigeration systems utilizing the expansion of nitrogen-containing refrigerant gas streams have been utilized for small liquefied natural gas (LNG) facilities typically used for peak shaving. Such systems are described in papers by K. Müller et al entitled “Natural Gas Liquefaction by an Expansion Turbine Mixture Cycle” in Chemical Economy &amp; Engineering Review, Vol. 8, No. 10 (No. 99), October 1976 and “The Liquefaction of Natural Gas in the Refrigeration Cycle with Expansion Turbine” in Erdöl und Kohie—Erdgas—Petrochemie Brennst-Chem Vol. 27, No. 7, 379-380 (July 1974). Another such system is described in an article entitled “SDG&amp;E: Experience Pays Off for Peak Shaving Pioneer” in Cryogenics &amp; Industrial Gases, September/October 1971, pp. 25-28. 
     U.S. Pat. No. 3,511,058 describes a LNG production system using a closed loop nitrogen refrigerator with a gas expander or reverse Brayton type cycle. In this process, liquid nitrogen is produced by means of a nitrogen refrigeration loop utilizing two turbo-expanders. The liquid nitrogen produced is further cooled by a dense fluid expander. The natural gas undergoes final cooling by boiling the liquid nitrogen produced from the nitrogen liquefier. Initial cooling of the natural gas is provided by a portion of the cold gaseous nitrogen discharged from the warmer of the two expanders in order to better match cooling curves in the warm end of the heat exchanger. This process is applicable to natural gas streams at sub-critical pressures since the gas is liquefied in a free-draining condenser attached to a phase separator drum. 
     U.S. Pat. No. 5,768,912 (equivalent to International Patent Publication WO 95/27179) discloses a natural gas liquefaction process which uses nitrogen in a closed loop Brayton type refrigeration cycle. The feed and the high pressure nitrogen can be pre-cooled using a small conventional refrigeration package employing propane, freon, or ammonia absorption cycles. This pre-cooling refrigeration system utilizes about 4% of total power consumed by the nitrogen refrigeration system. The natural gas is then liquefied and sub-cooled to −149° C. using a reverse Brayton or turbo-expander cycle employing two or three expanders arranged in series relative to the cooling natural gas. 
     A mixed refrigerant system for natural gas liquefaction is described in International Patent Publication WO 96/11370 in which the mixed refrigerant is compressed, partially condensed by an external cooling fluid, and separated into liquid and vapor phases. The resulting vapor is work expanded to provide refrigeration to the cold end of the process and the liquid is sub-cooled and vaporized to provide additional refrigeration. 
     International Patent Publication WO 97/13109 discloses a discloses a natural gas liquefaction process which uses nitrogen in a closed loop reverse Brayton-type refrigeration cycle. The natural gas at supercritical pressure is cooled against the nitrogen refrigerant, expanded isentropically, and stripped in a fractionating column to remove light components. 
     The liquefaction of natural gas is very energy-intensive. Improved efficiency of gas liquefaction processes is highly desirable and is the prime objective of new cycles being developed in the gas liquefaction art. The objective of the present invention, as described below and defined by the claims which follow, is to improve liquefaction efficiency by providing two integrated refrigeration systems wherein one of the systems utilizes one or more vaporizing refrigerant cycles to provide refrigeration down to about −100° C. and utilizes a gas expander cycle to provide refrigeration below about −100° C. Various embodiments are described for the application of this improved refrigeration system which enhance the improvements to liquefaction efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention relates to a method for the liquefaction of a feed gas, which method comprises providing at least a portion of the total refrigeration required to cool and condense the feed gas by utilizing (a) a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range; and (b) a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream. 
     The first recirculating refrigeration system may be operated by
         (1) compressing a first gaseous refrigerant;   (2) cooling and at least partially condensing the resulting compressed refrigerant;   (3) reducing the pressure of the resulting at least partially condensed compressed refrigerant;   (4) vaporizing the resulting reduced-pressure refrigerant to provide refrigeration in the first temperature range and yield a vaporized refrigerant; and   (5) recirculating the vaporized refrigerant to provide the first gaseous refrigerant of (1).
 
At least a portion of the cooling in (2) may be provided by indirect heat exchange with one or more additional vaporizing refrigerant streams provided by a third recirculating  refrigeration circuit  system. The third recirculating  refrigeration circuit  system may utilize a single component refrigerant or alternatively may utilize a mixed refrigerant comprising two or more components.
       

     In an alternative embodiment, the invention relates to a method for the liquefaction of a feed gas which comprises providing at least a portion of the total refrigeration required to cool and condense the feed gas by utilizing (a) a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range; and (b) a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream. The second recirculating refrigeration system may be operated by
         (1) compressing a second gaseous refrigerant to provide the pressurized gaseous refrigerant in (b);   (2) cooling the pressurized gaseous refrigerant to yield a cooled gaseous refrigerant;   (3) work expanding the cooled gaseous refrigerant to provide the cold refrigerant in (b);   (4) warming the cold refrigerant to provide refrigeration in the second temperature range; and   (5) recirculating the resulting warmed refrigerant to provide the second gaseous refrigerant of (1).
 
At least a portion of the cooling in (2) may be provided by indirect heat exchange with one or more additional vaporizing refrigerants provided by a third recirculating  refrigeration circuit  system.
       

     The third recirculating  refrigeration circuit  system may utilize a single component refrigerant or a mixed refrigerant which comprises two or more components. 
     In another alternative embodiment, the invention relates to a method for the liquefaction of a feed gas which comprises providing at least a portion of the total refrigeration required to cool and condense the feed gas by utilizing (a) a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range;  and (b) a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream. The first refrigerant system may be operated by
         (1) compressing a first gaseous refrigerant;   (2) cooling and partially condensing the resulting compressed refrigerant to yield a vapor refrigerant fraction and a liquid refrigerant fraction;   (3) further cooling and reducing the pressure of the liquid refrigerant fraction, and vaporizing the resulting liquid refrigerant fraction to provide refrigeration in the first temperature range and yield a first vaporized refrigerant;   (4) cooling and condensing the vapor refrigerant fraction, reducing the pressure of at least a portion of the resulting liquid, and vaporizing the resulting liquid refrigerant fraction to provide additional refrigeration in the first temperature range and yield a second vaporized refrigerant; and   (5) combining the first and second vaporized refrigerants to provide the first gaseous refrigerant of (1).
 
The V  vaporization of the resulting liquid in (4) may be effected at a pressure lower than the vaporization of the resulting liquid refrigerant fraction in (3), and the second vaporized refrigerant may be compressed before combining with the first vaporized refrigerant.
       

     In a further alternative embodiment, the invention relates to a method for the liquefaction of a feed gas which comprises providing at least a portion of the total refrigeration required to cool and condense the feed gas by utilizing (a) a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range; and (b) a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream. The second recirculating refrigeration system may be operated by
         (1) compressing a second gaseous refrigerant to provide the pressurized gaseous refrigerant in (b);   (2) cooling the pressurized gaseous refrigerant to yield a cooled gaseous refrigerant;   (3) work expanding the cooled gaseous refrigerant to provide the cold refrigerant in (b);   (4) warming the cold refrigerant to provide refrigeration in the second temperature range; and   (5) recirculating the resulting warmed refrigerant to provide the second gaseous refrigerant of (1).
 
The feed gas may be natural gas, the resulting liquefied natural gas stream may be flashed to lower pressure to yield a light flash vapor and a final liquid product, and the light flash vapor may be used to provide the second gaseous refrigerant in the second refrigerant circuit  refrigeration system.
       

     Another embodiment of the invention relates to a method for the liquefaction of a feed gas which comprises providing at least a portion of the total refrigeration required to cool and condense the feed gas by utilizing ( a )  a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range, and  ( b )  a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream. The second refrigeration system is operated by            (   1   )  compressing a second gaseous refrigerant to provide the pressurized gaseous refrigerant in  ( b );   (   2   )  cooling the pressurized gaseous refrigerant to yield a cooled gaseous refrigerant;      (   3   )  work expanding the cooled gaseous refrigerant to provide the cold refrigerant in  ( b );   (   4   )  warming the cold refrigerant to provide refrigeration in the second temperature range; and      (   5   )  recirculating the resulting warmed refrigerant to provide the second gaseous refrigerant of  (   1   ).
 
 At least a portion of the pressurized gaseous refrigerant in  (   2   )  is entirely cooled separately from cooling of the feed gas. All of the pressurized gaseous refrigerant may be cooled separately from cooling of the feed gas.  
       
       A portion of the pressurized gaseous refrigerant may be cooled by indirect heat exchange with the at least one recirculating refrigeration circuit of  ( a ).  The first refrigeration system may comprise a mixed component, pure component, and/or a cascaded vapor recompression refrigeration system.    
     
       Another embodiment of the invention includes an apparatus for the liquefaction of a feed gas comprising  
         
         
           
             ( a )  a first refrigeration system comprising at least one recirculating refrigeration circuit, wherein the first refrigeration system utilizes two or more refrigerant components and provides refrigeration in a first temperature range, wherein at least a portion of the first temperature range is between − 40 ° C. and − 100 ° C.; and    
             ( b )  a second refrigeration system which provides refrigeration in a second temperature range by work expanding a pressurized gaseous refrigerant stream, wherein at least a portion of the second temperature range is below − 100 ° C.  
 
 The first refrigeration system comprises  
 
             (   1   )  compression means for comprising a first gaseous refrigerant;    
             (   2   )  heat exchange means for cooling and at least partially condensing the resulting compressed refrigerant;    
             (   3   )  means for reducing the pressure of the resulting at least partially condensed compressed refrigerant;    
             (   4   )  heat exchange means for vaporizing the resulting reduced - pressure refrigerant to provide refrigeration in the first temperature range and yield a vaporized refrigerant; and    
             (   5   )  means for recirculating the vaporized refrigerant to provide the first gaseous refrigerant of  (   1   ).
 
 The apparatus may comprise additional heat exchange means to provide at least a portion of the cooling of  (   2   )  by indirect heat exchange with one or more additional vaporizing refrigerant streams and a third refrigeration system to provide the one or more additional vaporizing refrigerant streams.  
 
           
         
       
    
     
       In this embodiment, the second refrigeration system may comprise  
         
         
           
             (   6   )  compression means for compressing a second gaseous refrigerant to provide the pressurized gaseous refrigerant;    
             (   7   )  heat exchange means for cooling the pressurized gaseous refrigerant to yield a cooled gaseous refrigerant;    
             (   8   )  expansion means for work expanding the cooled gaseous refrigerant to provide the cold refrigerant;    
             (   9   )  heat exchange means for warming the cold refrigerant to provide refrigeration in the second temperature range; and    
             (   10   )  means for recirculating the resulting warmed refrigerant to provide the second gaseous refrigerant of  (   6   ).
 
 At least one of the heat exchange means in the first and second refrigeration systems of this embodiment may comprise a wound coil heat exchanger.  
 
           
         
       
    
     
       A final embodiment of the invention relates to an apparatus for the liquefaction of a feed gas comprising  
         
         
           
             ( a )  a first refrigeration system comprising at least one recirculating refrigeration circuit utilizing two or more refrigerant components and providing refrigeration in a first temperature range; and    
             ( b )  a second refrigeration system which provides refrigeration in a second temperature range having a lowest temperature less than the lowest temperature in the first temperature range.  
 
 The second refrigeration system comprises  
 
             (   1   )  compression means for compressing the second gaseous refrigerant to provide the pressurized gaseous refrigerant;    
             (   2   )  heat exchange means for entirely cooling at least a portion of the pressurized gaseous refrigerant separately from cooling of the feed gas to yield at least a portion of the cooled gaseous refrigerant;    
             (   3   )  expansion means for work expanding the cooled gaseous refrigerant to provide the cold refrigerant;    
             (   4   )  heat exchange means for warming the cold refrigerant to provide refrigeration in the second temperature range; and    
             (   5   )  means for recirculating the resulting warmed refrigerant to provide the second gaseous refrigerant of  (   1   ).
 
 In this embodiment, the heat exchange means of  (   2   )  may cool all of the pressurized gaseous refrigerant separately from cooling of the feed gas. The first refrigeration system may comprise  
 
             ( A )  compression means for compressing the first gaseous refrigerant;    
             ( B )  heat exchange means for cooling and at least partially condensing the resulting compressed refrigerant;    
             ( C )  pressure reducing means for reducing the pressure of the resulting at least partially condensed compressed refrigerant;    
             ( D )  heat exchange means for vaporizing the resulting reduced - pressure refrigerant to provide refrigeration in the first temperature range and yield the vaporized refrigerant; and    
             ( E )  means for recirculating the vaporized refrigerant to provide the first gaseous refrigerant of  ( A ). 
           
         
       
    
       In this embodiment, at least a portion of the cooling in the heat exchanger of  (   2   )  may be provided by indirect heat exchange by warming the cold refrigerant in  (   4   ).  At least one of the heat exchange means of the first and second refrigeration systems may comprise a wound coil heat exchanger.   
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of a preferred embodiment of the present invention. 
         FIG. 2  is a schematic flow diagram of another embodiment of the present invention which utilizes an alternative method for pre-cooling the recirculating refrigerant in the gas expander refrigeration cycle. 
         FIG. 3  is a schematic flow diagram of another embodiment of the present invention which utilizes product flash gas as the refrigerant in the gas expander refrigeration cycle. 
         FIG. 4  is a schematic flow diagram of another embodiment of the present invention which utilizes an additional refrigeration system to pre-cool the feed gas, the compressed refrigerant in the vapor recompression refrigeration cycle, and the compressed refrigerant in the gas expander refrigeration cycle. 
         FIG. 5  is a schematic flow diagram of another embodiment of the present invention which utilizes an additional liquid mixed refrigerant stream in the vapor recompression refrigeration cycle. 
         FIG. 6  is a schematic flow diagram of another embodiment of the present invention in which heat exchange among the feed gas and two refrigeration systems is consolidated into a minimum number of heat exchange zones. 
         FIG. 7  is a schematic flow diagram of another embodiment of the present invention which utilizes an additional vapor recompression refrigeration system. 
         FIG. 8  is a schematic flow diagram of another embodiment of the present invention which utilizes a cascade refrigeration cycle to precool the feed gas. 
         FIG. 9  is a schematic flow diagram of another embodiment of the present invention which utilizes expander work to provide a portion of the compression work in the gas expander refrigeration cycle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Most LNG production plants today utilize refrigeration produced by compressing a gas to a high pressure, liquefying the gas against a cooling source, expanding the resulting liquid to a low pressure, and vaporizing the resulting liquid to provide the refrigeration. Vaporized refrigerant is recompressed and utilized again in the recirculating refrigeration circuit. This type of refrigeration process can utilize a multi-component mixed refrigerant or a cascaded single component refrigerant cycle for cooling, and is defined generically herein as a vaporizing refrigerant cycle or as a vapor recompression cycle. This type of cycle is very efficient at providing cooling at near ambient temperatures. In this case, refrigerant fluids are available which will condense at a pressure well below the refrigerant critical pressure while rejecting heat to an ambient temperature heat sink, and will also boil at a pressure above atmospheric while absorbing heat from the refrigeration load. 
     As the required refrigeration temperature decreases in a single component vapor compression refrigeration system, a particular refrigerant which boils above atmospheric pressure at a temperature low enough to provide the required refrigeration will be too volatile to condense against an ambient temperature heat sink because the refrigerant critical temperature is below ambient temperature. In this situation, cascade cycles can be employed. For example, a two-fluid cascade can be utilized in which a heavier fluid provides the warmer refrigeration while a lighter fluid provides the colder refrigeration. Rather than rejecting heat to an ambient temperature, however, the light fluid rejects heat to the boiling heavier fluid while itself condensing. Very low temperatures can be reached by cascading multiple fluids in this manner. 
     A multi-component refrigeration (MCR) cycle can be considered as a type of cascade cycle in which the heaviest components of the refrigerant mixture condense against the ambient temperature heat sink and boil at low pressure while condensing the next lighter component which itself will boil to provide condensing to the still lighter component, and so on, until the desired temperature is reached. The main advantage of a multi-component system over a cascaded system is that the compression and heat exchange equipment is greatly simplified. The multi-component system requires a single compressor and heat exchanger, while the cascade system requires multiple compressors and heat exchangers. 
     Both of these cycles become less efficient as the temperature of the refrigeration load decreases because of the necessity to cascade multiple fluids. To provide the temperatures (typically −220° F. to −270° F.) required for LNG production, multiple steps involving multiple components are employed. In each step there are thermodynamic losses associated with the boiling/condensing heat transfer across a finite temperature difference, and with each additional step these losses increase. 
     Another type of industrially important refrigeration cycle is the gas expander cycle. In this cycle the working fluid is compressed, cooled sensibly (without phase change), work expanded as a vapor in a turbine, and warmed while providing cooling to the refrigeration load. This cycle is also defined as a gas expander cycle. Very low temperatures can be obtained relatively efficiently with this type of cycle using a single recirculating cooling loop. In this type of cycle, the working fluid typically does not undergo phase change, so heat is absorbed as the fluid is warmed sensibly. In some cases, however, the working fluid can undergo a small degree of phase change during work expansion. 
     The gas expander cycle efficiently provides refrigeration to fluids which are also cooling over a temperature range, and is particularly useful in providing for very low temperature refrigeration such as that required in producing liquid nitrogen and hydrogen. 
     A disadvantage of the gas expander refrigeration cycle, however, is that it is relatively inefficient at providing warm refrigeration. The net work required for a gas expander cycle refrigerator is equal to the difference between the compressor work and the expander work, while the work for a cascade or single component refrigeration cycle is simply the compressor work. In the gas expander cycle, expansion work can easily be 50% or more of the compressor work when providing warm refrigeration. The problem with the gas expander cycle in providing warm refrigeration is that any inefficiency in the compressor system is multiplied. 
     The objective of the present invention is to exploit the benefits of the gas expander cycle in providing cold refrigeration while utilizing the benefits of pure or multicomponent vapor recompression refrigeration cycles in providing warm refrigeration, and applying this combination of refrigeration cycles to gas liquefaction. This combination refrigeration cycle is particularly useful in the liquefaction of natural gas. 
     According to the invention, mixed component, pure component, and/or cascaded vapor recompression refrigeration systems are used to provide a portion of the refrigeration needed for gas liquefaction at temperatures below about −40° C. and down to about −100° C. The residual refrigeration in the coldest temperature range below about −100° C. is provided by work expansion of a refrigerant gas. The recirculation circuit of the refrigerant gas stream used for work expansion is physically independent from but thermally integrated with the recirculation circuit or circuits of the pure or mixed component vapor recompression cycle or cycles. More than 5% and usually more than 10% of the total refrigeration power required for liquefaction of the feed gas can be consumed by the pure or mixed component vapor recompression cycle or cycles. The invention can be implemented in the design of a new liquefaction plant or can be utilized as a retrofit or expansion of an existing plant by adding the gas expander cooling circuit to the existing plant refrigeration system. 
     The pure or mixed component vapor recompression working fluid or fluids generally comprise one or more components chosen from nitrogen, hydrocarbons having one or more carbon atoms, and halocarbons having one or more carbon atoms. Typical hydrocarbon refrigerants include methane, ethane, propane, i-butane, butane, and i-pentane. Representative halocarbon refrigerants include R22, R23, R32, R134a, and R410a. The gas stream to be work expanded in the gas expander cycle can be a pure component or a mixture of components; examples include a pure nitrogen stream or a mixture of nitrogen with other gases such as methane. 
     The method of providing refrigeration using a mixed component circuit includes compressing a mixed component stream and cooling the compressed stream using an external cooling fluid such as air, cooling water, or another process stream. A portion of the compressed mixed refrigerant stream is liquefied after external cooling. At least a portion of the compressed and cooled mixed refrigerant stream is further cooled in a heat exchanger and then reduced in pressure and vaporized by heat exchange against the gas stream being liquefied. The evaporated and warmed mixed refrigerant steam is then recirculated and compressed as described above. 
     The method of providing refrigeration using a pure component circuit consists of compressing a pure component stream and cooling it using an external cooling fluid, such as air, cooling water, another pure component stream. A portion of the refrigerant stream is liquefied after external cooling. At least a portion of the compressed and liquefied refrigerant is then reduced in pressure and vaporized by heat exchange against the gas stream being liquefied or against another refrigerant stream being cooled. The resulting vaporized refrigerant steam is then compressed and recirculated as described above. 
     According to the invention, the pure or mixed component vapor recompression cycle or cycles preferably provide refrigeration to temperature levels below about −40° C., preferably below about −60° C., and down to about −100° C., but do not provide the total refrigeration needed for liquefying the feed gas. These cycles typically may consume more than 5%, and usually more than 10%, of the total refrigeration power requirement for liquefaction of the feed gas. In the liquefaction of natural gas, pure or mixed component vapor recompression cycle or cycles typically can consume greater than 30% of the total power requirement required to liquefy the feed gas. In this application, the natural gas preferred is cooled to temperatures well below −40° C., and preferably below −60° C., by the pure or mixed component vapor recompression cycle or cycles. 
     The method of providing refrigeration in the gas expander cycle includes compressing a gas stream, cooling the compressed gas stream using an external cooling fluid, further cooling at least a portion of the cooled compressed gas stream, expanding at least a portion of the further cooled stream in an expander to produce work, warming the expanded stream by heat exchange against the stream to be liquefied, and recirculating the warmed gas stream for further compression. This cycle provides refrigeration at temperature levels below the temperature levels of the refrigeration provided by the pure or mixed refrigerant vapor recompression cycle. 
     In a preferred mode, the pure or mixed component vapor recompression cycle or cycles provide a portion of the cooling to the compressed gas stream prior to its expansion in an expander. In an alternative mode, the gas stream may be expanded in more than one expander. Any known expander arrangement to liquefy a gas stream may be used. The invention may utilize any of a wide variety of heat exchange devices in the refrigeration circuits including plate-fin, wound coil, and shell and tube type heat exchangers, or combinations thereof, depending on the specific application. The invention is independent of the number and arrangement of the heat exchangers utilized in the claimed process. 
     A preferred embodiment of the invention illustrated in FIG.  1 . The process can be used to liquefy any feed gas stream, and preferably is used to liquefy natural gas as described below to illustrate the process. Natural gas is first cleaned and dried in pretreatment section  172  for the removal of acid gases such as CO 2  and H 2 S along with other contaminants such as mercury. Pre-treated gas steam  stream  100  enters heat exchanger  106 , is cooled to a typical intermediate temperature of approximately −30° C., and cooled stream  102  flows into scrub column  108 . The cooling in heat exchanger  106  is effected by the warming of mixed refrigerant stream  125  in the interior  109  of heat exchanger  106 . The mixed refrigerant typically contains one or more hydrocarbons selected from methane, ethane, propane, i-butane, butane, and possibly i-pentane. Additionally, the refrigerant may contain other components such as nitrogen. In scrub column  108 , the heavier components of the natural gas feed, for example pentane and heavier components, are removed. In the present examples the scrub column is shown with only a stripping section. In other instances a rectifying section with a condenser can be employed for removal of heavy contaminants such as benzene to very low levels. When very low levels of heavy components are required in the final LNG product, any suitable modification to scrub column  110    108  can be made. For example, a heavier component such as butane may be used as the wash liquid. 
     Bottoms product  110  of the scrub column then enters fractionation section  112  where the heavy components are recovered as stream  114 . The propane and lighter components in stream  118  pass through heat exchanger  106 , where the stream is cooled to about −30° C., and recombined with the overhead product of the scrub column to form purified feed stream  120 . Stream  120  is then further cooled in heat exchanger  122    106  to a typical temperature of about −100° C. by warming mixed refrigerant stream  124    125 . The resulting cooled stream  126  is then further cooled to a temperature of about −166° C. in heat exchanger  128 . Refrigeration for cooling in heat exchanger  128  is provided by cold refrigerant fluid stream  130  from turbo-expander  166 . This fluid, preferably nitrogen, is predominantly vapor containing less than 20% liquid and is at a typical pressure of about 11 bara (all pressures herein are absolute pressures) and a typical temperature of about −168° C. Further cooled stream  132  can be flashed adiabatically to a pressure of about 1.05 bara across throttling valve  134 . Alternatively, pressure of further cooled stream  132  could be reduced across a work expander. The liquefied gas then flows into separator or storage tank  136  and the final LNG product is withdrawn as stream  142 . In some cases, depending on the natural gas composition and the temperature exiting heat exchanger  128 , a significant quantity of light gas is evolved as stream  138  after the flash across valve  134 . This gas can be warmed in heat exchangers  128  and  150  and compressed to a pressure sufficient for use as fuel gas in the LNG facility. 
     Refrigeration to cool the natural gas from ambient temperature to a temperature of about −100° C. is provided by a mufti  multi-component refrigeration loop as mentioned above. Stream  146  is the high pressure mixed refrigerant which enters heat exchanger  106  at ambient temperature and a typical pressure of about 38 bara. The refrigerant is cooled to a temperature of about −100° C. in heat exchangers  exchanger  106  and  122  , exiting as stream  148 . Stream  148  is divided into two portions in this embodiment. A smaller portion, typically about 4%, is reduced in pressure adiabatically to about 10 bara and is introduced as stream  149  into heat exchanger  150  to provide supplemental refrigeration as described below. The major portion of the refrigerant as stream  124    125  is also reduced in pressure adiabatically to a typical pressure of about 10 bara and is introduced to the cold end of heat exchanger  106 . The refrigerant flows downward and vaporizes in interior  109  of heat exchanger  106  and leaves at slightly below ambient temperature as stream  152 . Stream  152  is then re-combined with minor stream  154  which was vaporized and warmed to near ambient temperature in heat exchanger  150 . The combined low pressure stream  156  is then compressed in multi-stage intercooled compressor  158  back to the final pressure of about 38 bara. Liquid can be formed in the intercooler of the compressor, and this liquid is separated and recombined with the main stream  160  exiting final stage of compression. The combined stream is then cooled back to ambient temperature to yield stream  146 . 
     Final cooling of the natural gas from about −100° C. to about −166° C. is accomplished using a gas expander cycle employing nitrogen as the working fluid. High pressure nitrogen stream  162  enters heat exchanger  150  typically at ambient temperature and a pressure of about 67 bara, and is then cooled to a temperature of about −100° C. in heat exchanger  150 . Cooled vapor stream  164  is substantially isentropically work expanded in turbo-expander  132 , typically exiting at a pressure of about 11 bara and a temperature of about −168° C. Ideally the exit pressure is at or slightly below the dewpoint pressure of the nitrogen at a temperature cold enough to effect the cooling of the LNG to the desired temperature. Expanded nitrogen stream  130  is then warmed to near ambient temperature in heat exchangers  128  and  150 . Supplemental refrigeration is provided to heat exchanger  150  by a small steam  149  of the mixed refrigerant, as described earlier, and this is done to reduce the irreversibility in the process by causing the cooling curves heat exchanger  150  to be more closely aligned. From heat exchanger  150 , warmed low pressure nitrogen stream  170  is compressed in multistage compressor  168  back to a high pressure of about 67 bara. 
     As mentioned above, this gas expander cycle can be implemented as a retrofit or expansion of an existing mixed refrigerant LNG plant. 
     An alternative embodiment of the invention is illustrated in FIG.  2 . Instead of the wound coil heat exchangers  106  and  128  shown in  FIG. 1 , this alternative utilizes plate and fin heat exchangers  206 ,  222 , and  228  along with plate and fin heat exchanger  250 . In this embodiment, the irreversibility in the warm nitrogen heat exchanger  250  is reduced by decreasing the flow of the cooling streams rather than by increasing the flow of warming streams. In either case the effect is similar and the cooling curves heat exchanger  250  become more closely aligned. In the embodiment of  FIG. 2 , a small portion of the warm high pressure nitrogen as stream  262  is cooled in heat exchangers  206  and  222  to a temperature of about −100° C., exiting as stream  202 . Stream  202  is then re-combined with the main high pressure nitrogen flow and expanded in work expander  232 . 
       FIG. 3  illustrates another alternate embodiment of the invention. In this embodiment, the working fluid for the gas expander refrigeration loop is a hydrocarbon-nitrogen mixture from the light vapor stream  300  evolved by flashing the liquefied gas from heat exchanger  128  across valve  134 . This vapor is then combined with the fluid exiting turbo-expander  132 , warmed in heat exchangers  128  and  150 , and compressed in compressor  368 . The gas exiting compressor  368  is then cooled in heat exchanger  308 . The bulk of the gas exiting  308  is passed into heat exchanger  150  and small portion  304 , equal in flow to the flow of flash gas stream  300 , is withdrawn from the circuit for as fuel gas for the LNG facility. In this embodiment, the functions of fuel gas compressor  140  and recycle compressor  168  of  FIG. 1  are combined in compressor  368 . It is also possible to withdraw stream  304  from an interstage location of recycle compressor  368 . 
     An alternate embodiment is illustrated in  FIG. 4  in which another refrigerant (for example propane) is used to pre-cool the feed, nitrogen, and mixed refrigerant streams in heat exchangers  402 ,  401 , and  400  respectively before introduction into heat exchangers  106  and  150 . In this embodiment, three levels of pre-cooling are used in heat exchangers  402 ,  401 , and  400 , although any number of levels can be used as required. In this case, returning refrigerant fluids  156  and  170  are compressed cold, at an inlet temperature slightly below that provided by the pre-cooling refrigerant. This arrangement could be implemented as a retrofit or expansion of an existing propane pre-cooled mixed refrigerant LNG plant. 
       FIG. 5  shows another embodiment of the invention in which high pressure mixed refrigerant stream  146  is separated into liquid and vapor sub-streams  500  and  501 . Vapor stream  501  is cooled to about −100° C., substantially liquefied, reduced to a low pressure of about 3 bars, and used as stream  503  to provide refrigeration. Liquid stream  500  is cooled to about −30° C., is reduced to an intermediate pressure of about 9 bara, and used as stream  502  to provide refrigeration. A minor portion of cooled vapor stream  505  is used as stream  504  to provide supplemental refrigeration to heat exchangers  150  as earlier described. 
     The two vaporized low pressure mixed refrigerant return streams are combined to form stream  506 , which is then compressed cold at a temperature of about −30° C. to an intermediate pressure of about 9 bara and combined with vaporized intermediate pressure stream  507 . The resulting mixture is then further compressed to a final pressure of about 50 bara. In this embodiment, liquid is formed in the intercooler of the compressor, and this liquid is recombined with the main flow  160  exiting the final compression stage. 
     Optionally, compressed nitrogen stream  510  could be cooled before entering heat exchanger  150  by utilizing subcooled refrigerant liquid stream  511  (not shown). A portion of stream  511  could be reduced in pressure and vaporized to cool stream  510  by indirect heat exchange, and the resulting vapor would be returned to the refrigerant compressor. Alternatively, stream  510  could be cooled with other process streams in the heat exchanger cooled by vaporizing refrigerant stream  502 . 
     Another embodiment is shown in  FIG. 6  in which heat exchangers  122 ,  106  and  150  of  FIG. 1  are combined functionally into heat exchangers  600  and  601  to yield an equipment simplification. Note that a balancing stream such as stream  168  of  FIG. 1  is no longer required. In this embodiment, the vaporizing mixed refrigerant circuit and the gas expander refrigeration circuit provide in heat exchanger  601  a portion of the total refrigeration required to liquefy the feed gas. These two refrigeration circuits also provide in heat exchanger  600  another portion of the total refrigeration required to liquefy the feed gas. The remainder of the total refrigeration required to liquefy the feed gas is provided in heat exchanger  128 . 
       FIG. 7  presents an embodiment of the invention in which two separate mixed refrigerant loops are employed before final cooling by the gas expander refrigeration loop. The first refrigeration loop employing compressor  701  and pressure reduction device  703  provides primary cooling to a temperature of about −30° C. A second refrigeration loop employing compressor  702  and expansion devices  704  and  705  is used to provide further cooling to a temperature of about −100° C. This arrangement could be implemented as a retrofit or expansion of an existing dual mixed refrigerant LNG plant. 
       FIG. 8  presents an embodiment of the invention in which a two-fluid cascade cycle is used to provide precooling prior to final cooling by the gas expander refrigeration cycle. 
       FIG. 9  illustrates the use of expander  800  to drive the final compressor stage of the compressor for the gas expander refrigeration circuit. Alternatively, work generated by expander  800  can be used to compress other process streams. For example, a portion or all of this work could be used to compress the feed gas in line  900 . In another option, a portion or all of the work from expander  800  could be used for a portion of the work required by mixed refrigerant compressor  958 . 
     The invention described above in the embodiments illustrated by  FIGS. 1-9  can utilize any of a wide variety of heat exchange devices in the refrigeration circuits including wound coil, plate-fin, shell and tube, and kettle type heat exchangers. Combinations of these types of heat exchangers can be used depending upon specific applications. For example in  FIG. 2 , all four heat exchangers  106    206 ,  122    222 ,  128    228 , and  150    250  can be wound coil exchangers. Alternatively, heat exchangers  106    206 ,  122    222 ,  128   and  228  can be wound coil exchangers and heat exchanger  150    250  can be a plate and fin type exchanger as utilized in FIG.  1 . 
     In the preferred embodiment of the invention, the majority of the refrigeration in the temperature range of about −40° C. to about −100° C. is provided by indirect heat exchange with at least one vaporizing refrigerant in a recirculating refrigeration circuit. Some of the refrigeration in this temperature range also can be provided by the work expansion of a pressurized gaseous refrigerant. 
     EXAMPLE 
     Referring to  FIG. 1 , natural gas is cleaned and dried in pretreatment section  172  for the removal of acid gases such as CO 2  and H 2 S along with other contaminants such as mercury. Pretreated feed gas  100  has a flow rate of 24,431 kg-mole/hr, a pressure of 66.5 bara, and a temperature of 32° C. The molar composition of the stream is as follows: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Feed Gas Composition 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Mole Fraction 
               
               
                   
                   
               
               
                   
                 Nitrogen 
                 0.009 
               
               
                   
                 Methane 
                 0.9378 
               
               
                   
                 Ethane 
                 0.031 
               
               
                   
                 Propane 
                 0.013 
               
               
                   
                 i-Butane 
                 0.003 
               
               
                   
                 Butane 
                 0.004 
               
               
                   
                 i-Pentane 
                 0.0008 
               
               
                   
                 Pentane 
                 0.0005 
               
               
                   
                 Hexane 
                 0.001 
               
               
                   
                 Heptane 
                 0.0006 
               
               
                   
                   
               
            
           
         
       
     
     Pre-treated gas  100  enter  enters first heat exchanger  106  and is cooled to a temperature of −31° C. before entering scrub column  108  as stream  102 . The cooling is effected by the warming of mixed refrigerant stream  109    125 , which has a flow of 554,425 kg-mole/hr and the following composition: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Mixed Refrigerant Composition 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Mole Fraction 
               
               
                   
                   
               
               
                   
                 Nitrogen 
                 0.014 
               
               
                   
                 Methane 
                 0.343 
               
               
                   
                 Ethane 
                 0.395 
               
               
                   
                 Propane 
                 0.006 
               
               
                   
                 i-Butane 
                 0.090 
               
               
                   
                 Butane 
                 0.151 
               
               
                   
                   
               
            
           
         
       
     
     In scrub column  108 , pentane and heavier components of the feed are removed. Bottoms product  110  of the scrub column enters fractionation section  112  where the heavy components are recovered as stream  114  and the propane and lighter components in stream  118  are recycled to heat exchanger  106 , cooled to −31° C., and recombined with the overhead product of the scrub column to form stream  120 . The flow rate of stream  120  is 24,339 kg-mole/hr. 
     Stream  120  is further cooled in heat exchanger  122    106  to a temperature of −102.4° C. by warming mixed refrigerant stream  124    125  which enters heat exchanger  122    106  at a temperature of −104.0° C. The resulting stream  128    126  is then further cooled to a temperature of −165.7° C. in heat exchanger  128 . Refrigeration for cooling in heat exchanger  128  is provided by pure nitrogen stream  130  exiting turbo-expander  166  at −168.0° C. with a liquid fraction of 2.0%. The resulting LNG stream  132  is then flashed adiabatically to its bubble point pressure of 1.05 bara across valve  134 . The LNG then enters separator  136  with the final LNG product exiting as stream  142 . In this example, no light gas  138  is evolved after the flash across valve  134 , and flash gas recovery compressor  140  is not required. 
     Refrigeration to cool the natural gas from ambient temperature to a temperature of −102.4° C. is provided by a multi-component refrigeration loop as mentioned above. Stream  146  is the high pressure mixed refrigerant which enters heat exchanger  106  at a temperature of 32° C. and a pressure of 38.6 bara. It is then cooled to a temperature of −102.4° C. in heat exchangers  exchanger  106  and  122  , exiting as stream  148  at a pressure of 34.5 bara. Stream  148  is then divided into two portions. A smaller portion, 4.1%, is reduced in pressure adiabatically to 9.8 bara and introduced as stream  149  into heat exchanger  150  to provide supplemental refrigeration. The major portion  124   of the mixed refrigerant is also flashed adiabatically to a pressure of 9.8 bara and introduced as stream  124    125  into the cold end of heat exchanger  122    106 . Stream  124    125  is warmed and vaporized in heat exchangers  122  and  exchanger  106 , finally exiting heat exchanger  106  at 29° C. and 9.3 bara as stream  152 . Stream  152  is then recombined with the minor portion of the mixed refrigerant as stream  154  which has been vaporized and warmed to 29° C. in heat exchanger  150 . The combined low pressure stream  156  is then compressed in 2-stage intercooled compressor  158  to the final pressure of 34.5 bara. Liquid is formed in the intercooler of the compressor, and this liquid is recombined with the main flow  160  exiting the final compressor stage. The liquid flow is 4440 kg-mole/hr. 
     Final cooling of the natural gas from −102.4° C. to −165.7° C. is accomplished using a closed loop gas expander type cycle employing nitrogen as the working fluid. The high pressure nitrogen stream  162  enters heat exchanger  150  at 32° C. and a pressure of about 67.1 bara and a flow rate of 40,352 kg-mole/hr, and is then cooled to a temperature of −102.4° C. in heat exchanger  150 . The vapor stream  164  is substantially isentropically work-expanded in turbo-expander  166 , exiting at −168.0° C. with a liquid fraction of 2.0%. The expanded nitrogen is then warmed to 29° C. in heat exchangers  128  and  150 . Supplemental refrigeration is provided to heat exchanger  150  by stream  149 . From heat exchanger  150 , the warmed low pressure nitrogen is compressed in three-stage centrifugal compressor  168  from 10.5 bara back to 67.1 bara. In this illustrative Example, 65% of the total refrigeration power required to liquefy pretreated feed gas  100  is consumed by the recirculating refrigeration circuit in which refrigerant stream  146  is vaporized in heat exchangers  106  and  150  and the resulting vaporized refrigerant stream  156  is compressed in compressor  158 . 
     Thus the present invention offers an improved refrigeration process for gas liquefaction which utilizes one or more vaporizing refrigerant cycles to provide refrigeration below about −40° C. and down to about −100° C., and utilizes a gas expander cycle to provide refrigeration below about −100° C. The gas expander cycle also may provide some of the refrigeration in the range of about −40° C. to about −100° C. Each of these two types of refrigerant systems is utilized in an optimum temperature range which maximizes the efficiency of the particular system. Typically, a significant fraction of the total refrigeration power required to liquefy the feed gas (more than 5% and usually more than 10% of the total) can be consumed by the vaporizing refrigerant cycle or cycles. The invention can be implemented in the design of a new liquefaction plant or can be utilized as a retrofit or expansion of an existing plant by adding gas expander refrigeration circuit to the existing plant refrigeration system. 
     The essential characteristics of the present invention are described completely in the foregoing disclosure. One skilled in the art can understand the invention and make various modifications without departing from the basic spirit of the invention, and without deviating from the scope and equivalents of the claims which follow.