Gas liquefaction process with partial condensation of mixed refrigerant at intermediate temperatures

Abstract
Method of producing liquefied natural gas (LNG) whereby refrigeration for cooling and liquefaction is provided by a mixed refrigerant system precooled by another refrigeration system. At least one liquid stream is derived from the partial condensation and separation of the mixed refrigerant at a temperature higher than the lowest temperature provided by the precooling system when the mixed refrigerant is condensed at a final highest pressure. When the mixed refrigerant is condensed at a pressure lower than the final highest pressure, condensation is effected at a temperatures equal or higher than the lowest temperature provided by the precooling system. The mixed refrigerant liquid is used to provide refrigeration at a temperature lower than that provided by the precooling system.
Description




CROSS-REFERENCE TO RELATED APPLICATIONS




Not applicable.




STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT




Not applicable.




BACKGROUND OF THE INVENTION




The liquefaction of natural gas at remote sites, transportation of the liquefied natural gas (LNG) to population centers, and storage and vaporization of LNG for local consumption have been successfully practiced for many years around the world. LNG production sites typically are located on land at remote sites having docking facilities for large LNG tankers which transport the LNG to end users.




Numerous process cycles have been developed for LNG production to provide the large refrigeration requirements for liquefaction. Such cycles typically utilize combinations of single-component refrigeration systems using propane or single chlorofluorocarbon refrigerants operated in combination with one or more mixed refrigerant (MR) systems. Well-known mixed refrigerants typically comprise light hydrocarbons and optionally nitrogen, and utilize compositions tailored to the temperature and pressure levels of specific process steps. Dual mixed refrigerant cycles also have been utilized in which the first mixed refrigerant provides initial cooling at warmer temperatures and the second refrigerant provides further cooling at cooler temperatures.




U.S. Pat. No. 3,763,658 discloses a LNG production system which employs a first propane refrigeration circuit which precools a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed.




An alternative LNG production system, described in U.S. Pat. No. 4,065,278, uses a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature below the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This process differs from U.S. Pat. No. 3,763,658 cited above in that the distillation of the feed for heavy component removal occurs at a temperature lower than that provided by the first refrigeration circuit, and a pressure substantially lower than the feed pressure.




U.S. Pat. No. 4,404,008 discloses a LNG production system which employs a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid stream, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This prior art differs from U.S. Pat. No. 3,763,658 in that cooling and partial condensation of the mixed refrigerant of the second refrigeration circuit occurs between compression stages. The resulting liquid is then recombined with the resulting vapor stream at a temperature warmer than the lowest temperature of the first refrigeration circuit, and the combined mixed refrigerant stream is then further cooled by the first refrigeration circuit.




An alternative LNG production system is disclosed in U.S. Pat. No. 4,274,849 which system employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. In

FIG. 7

of this reference, the vapor resulting from the separation of the second refrigerant after precooling is further cooled to a temperature lower than that provided by the first refrigeration circuit and separated into liquid and vapor streams.




U.S. Pat. No. 4,539,028 describes a LNG production system which employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This patent differs from that of U.S. Pat. No. 4,274,849 described above by the fact that the second refrigerant is vaporized at two different pressures to provide refrigeration.




The state of the art as defined above describes the vaporization of subcooled mixed refrigerant streams to provide refrigeration for natural gas liquefaction wherein the subcooling is provided by a portion of the refrigeration generated by flashing and vaporizing of the subcooled mixed refrigerant streams. Refrigeration for cooling the mixed refrigerant streams and the natural gas feed is provided by the vaporization of mixed refrigerant streams in a main heat exchange zone. Cooling of the mixed refrigerant vapor during and/or after compression is provided by a separate refrigerant such as propane.




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 an additional vaporizing refrigerant stream in the main heat exchange zone. Various embodiments are described for the application of this improved refrigeration step which enhance liquefaction efficiency.




BRIEF SUMMARY OF THE INVENTION




The invention relates to a method for providing refrigeration for liquefying a feed gas which comprises:




(1) providing refrigeration from a first recirculating refrigeration circuit which provides refrigeration in a temperature range between a first temperature and a second temperature which is lower than the first temperature;




(2) providing refrigeration from a second recirculating refrigeration circuit in a temperature range between the second temperature and a third temperature which is lower than the second temperature, wherein the first refrigeration circuit provides refrigeration to the second refrigeration circuit in the temperature range between the first temperature and the second temperature;




(3) withdrawing a mixed refrigerant vapor from a main heat exchange zone in the second recirculating refrigeration circuit and compressing the mixed refrigerant vapor to a final highest pressure to yield a compressed mixed refrigerant vapor;




(4) partially condensing at least a portion of the mixed refrigerant vapor in the second recirculating refrigeration circuit and separating the resulting partially condensed mixed refrigerant into at least one liquid refrigerant stream and at least one vapor refrigerant stream; and




(5) subcooling the at least one liquid refrigerant stream to a temperature lower than the second temperature, reducing the pressure of the resulting subcooled liquid refrigerant stream, and vaporizing the resulting reduced-pressure refrigerant stream to provide at least a portion of the refrigeration for liquefying the feed gas between the second temperature and the third temperature.




The step of partially condensing the compressed mixed refrigerant vapor is effected at a pressure essentially equal to the final highest pressure.




The refrigeration for liquefying the feed gas between the second temperature and the third temperature can be provided by indirect heat exchange with a vaporizing mixed refrigerant in a main heat exchange zone. This vaporizing mixed refrigerant is provided by




(a) compressing the mixed refrigerant vapor to a first pressure;




(b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a first mixed refrigerant vapor fraction and a first mixed refrigerant liquid fraction;




(c) subcooling the first mixed refrigerant liquid fraction to provide a first subcooled mixed refrigerant liquid;




(d) reducing the pressure of the first subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide vaporizing mixed refrigerant for cooling and condensing the feed gas therein; and




(e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor for step (a).




At least a portion of the refrigeration for the subcooling in step (c) can be provided by the vaporizing of the reduced pressure mixed refrigerant in the main heat exchange zone in step (d). At least a portion of the refrigeration for the subcooling in (c) can be provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone. The one or more additional refrigerant streams can comprise a single component refrigerant or a multicomponent refrigerant.




The method can further comprise partially condensing and separating the first mixed refrigerant vapor fraction to yield a second mixed refrigerant vapor and a second mixed refrigerant liquid, subcooling the second mixed refrigerant liquid by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting subcooled second mixed refrigerant liquid, and vaporizing the resulting reduced pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.




The method also can further comprise condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor, and vaporizing the resulting reduced-pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.




Typically, at least a portion of the refrigeration for the cooling and partial condensing in (b) can be provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone. At least one of the one or more additional refrigerant streams can comprise a single component refrigerant a multicomponent refrigerant.




A portion of the refrigeration for cooling the feed gas can be provided by indirect heat exchange with one or more additional refrigerant streams external of the main heat exchange zone. The one or more additional refrigerant streams can comprise a single component refrigerant or a multicomponent refrigerant.




The feed gas can comprise methane and one or more hydrocarbons heavier than methane, and in this case the method can further comprise:




(e) precooling the feed gas by indirect heat exchange with an additional refrigerant stream;




(f) introducing the resulting precooled feed gas into a scrub column with a lean scrub liquid enriched in hydrocarbons heavier than methane;




(g) withdrawing from the bottom of the scrub column a stream rich in hydrocarbons heavier than methane;




(h) withdrawing from the top of the scrub column an overhead stream containing methane and residual hydrocarbons heavier than methane;




(i) cooling the overhead stream in the main heat exchange zone to condense residual hydrocarbons heavier than methane;




(j) separating the resulting cooled overhead stream into a purified methane-enriched product and a stream enriched in hydrocarbons heavier than methane; and




(k) utilizing at least a portion of the stream enriched in hydrocarbons heavier than methane to provide the lean scrub liquid of (f).




The first mixed refrigerant vapor fraction can be compressed following separation in (b). The cooling and partially condensing of the resulting compressed first mixed refrigerant vapor in (b) can be effected by indirect heat exchange with a fluid at ambient temperature. A portion of the first mixed refrigerant liquid can be mixed with the first pressurized mixed refrigerant vapor.




Optionally, at least a portion of the first mixed refrigerant vapor in (b) can be further cooled, partially condensed, and separated into an additional mixed refrigerant liquid which is combined with the first pressurized mixed refrigerant liquid. A portion of the refrigeration for cooling and partially condensing the first mixed refrigerant vapor fraction can be provided by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone.




The first pressurized mixed refrigerant liquid after subcooling can be vaporized in the main heat exchange zone at a first pressure and the second pressurized mixed refrigerant liquid after subcooling can be vaporized in the main heat exchange zone at a second pressure. The method can further comprise condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor to the second pressure, and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.




The operation of the second recirculating refrigeration circuit can include




(a) compressing the mixed refrigerant vapor to a first pressure;




(b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a mixed refrigerant vapor fraction and a mixed refrigerant liquid fraction;




(c) subcooling the mixed refrigerant liquid fraction to provide a subcooled mixed refrigerant liquid;




(d) reducing the pressure of the subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide one of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein; and




(e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor in (a).




The refrigeration for subcooling the mixed refrigerant liquid fraction can be provided in part by indirect heat exchange with the resulting vaporizing reduced pressure refrigerant liquid in the main heat exchange zone and in part by indirect heat exchange with one or more portions of an additional refrigerant external to the main heat exchange zone.




The operation of the second recirculating refrigeration circuit can further comprise




(f) condensing and subcooling the mixed refrigerant vapor fraction to provide an additional subcooled mixed refrigerant liquid; and




(g) reducing the pressure of the additional subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure liquid in the main heat exchange zone to provide another of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein.




The refrigeration for condensing and subcooling the additional mixed refrigerant vapor can be provided in part by indirect heat exchange with the resulting vaporizing reduced pressure liquid in the main heat exchange zone and in part by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone.











BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS





FIG. 1

is a schematic flow diagram of a liquefaction process representative of the prior art.





FIG. 2

is a schematic flow diagram of an embodiment of the of the present invention in which compressed mixed refrigerant is partially condensed at an intermediate temperature following cooling in one stage of heat exchange with a second refrigerant.





FIG. 3

is a schematic flow diagram of another embodiment of the present invention in which compressed mixed refrigerant is partially condensed at an intermediate temperature following cooling in three stages of heat exchange with a second refrigerant and at an intermediate pressure below the final pressure of the compressed mixed refrigerant vapor.





FIG. 4

is a schematic flow diagram of another embodiment of the present invention in which intermediate mixed refrigerant vapor and liquid streams are further cooled in three stages of heat exchange with a second refrigerant.





FIG. 5

is a schematic flow diagram of another embodiment of the present invention in which compressed mixed refrigerant is partially condensed at an intermediate temperature following cooling in two stages of heat exchange with a second refrigerant.





FIG. 6

is a schematic flow diagram of another embodiment of the present invention in which intermediate mixed refrigerant vapor and liquid streams are further cooled in four stages of heat exchange with a second refrigerant.





FIG. 7

is a schematic flow diagram of another embodiment of the present invention in which the feed gas is precooled in three stages of heat exchange with a second refrigerant.





FIG. 8

is a schematic flow diagram of another embodiment of the present invention which utilizes two stages of partial condensation of the compressed mixed refrigerant to produce a combined liquid mixed refrigerant stream.





FIG. 9

is a schematic flow diagram of another embodiment of the present invention which utilizes two stages of partial condensation of the compressed mixed refrigerant to provide two subcooled liquid refrigerants to the main heat exchange zone.





FIG. 10

is a schematic flow diagram of another embodiment of the present invention which utilizes two stages of partial condensation of the compressed mixed refrigerant, the second stage of which utilizes refrigeration provided by mixed refrigerant in the main heat exchange zone,





FIG. 11

is a schematic flow diagram of another embodiment of the present invention in which the mixed refrigerant is vaporized at two different pressures in the main heat exchange zone.





FIG. 12

is a schematic flow diagram of another embodiment of the present invention in which precooling is provided by a mixed refrigerant circuit.





FIG. 13

is a schematic flow diagram of another embodiment of the present invention in which precooling is provided by a mixed refrigerant circuit with two refrigerant pressure levels.





FIG. 14

is a schematic flow diagram of another embodiment of the present invention which utilizes a single stage of mixed refrigerant partial condensation.











DETAILED DESCRIPTION OF THE INVENTION




The current invention provides an efficient process for the liquefaction of a gas stream, and is particularly applicable to the liquefaction of natural gas. The invention utilizes a mixed refrigerant system in which the mixed refrigerant after compression is precooled by a second refrigerant system, and at least one liquid stream is derived from the partial condensation and separation of the compressed mixed refrigerant. When the partial condensation step is effected at a pressure less than the final highest pressure of the compressed mixed refrigerant, condensation is carried out at a temperature equal to or higher than the lowest temperature provided by the second refrigerant system. When the partial condensation is effected at a pressure essentially equal to the final highest pressure of the compressed mixed refrigerant, condensation is carried out at a temperature above the lowest temperature provided by the second refrigerant system.




The mixed refrigerant is a multicomponent fluid mixture typically containing one or more hydrocarbons selected from methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen.




The precooling system generally cools the mixed refrigerant to temperatures below ambient. Although there is no limitation to the lowest temperature achieved by the precooling system in the present invention, it has been found for liquefied natural gas (LNG) production that the lowest precooling temperature should generally be between about 0° C. and about −75° C., and preferably between about −20° C. and about −45° C. The lowest precooling temperature depends on the natural gas composition and LNG product requirements. The precooling system can form a cascade of heat exchangers each employing a single component refrigerant selected from C


2


-C


5


hydrocarbons or C


1


-C


4


halocarbons. If desired, the cooling system can employ a mixed refrigerant comprising various hydrocarbons. One embodiment of the invention utilizes a propane precooled mixed refrigerant system with mixed refrigerant liquid derived after the first stage of propane cooling of the mixed refrigerant, resulting in power savings or increased production over a standard propane precooled mixed refrigerant cycle. Several embodiments are described including the application of the invention to dual mixed refrigerant cycles.




The invention may utilize any of a wide variety of heat exchange devices in the refrigeration circuits including plate-fin, wound coil, shell and tube, and kettle type heat exchangers, or combinations of heat exchanger types depending on specific applications. The invention is applicable to the liquefaction of any suitable gas stream, but is described below as a process for the liquefaction of natural gas. The invention is independent of the number and arrangement of the heat exchangers utilized in the claimed process.




In the present disclosure, the term “heat exchange zone” defines a heat exchanger or combination of heat exchangers in which refrigeration is provided by one or more refrigerant streams to cool one or more process streams within a given temperature range. A heat exchanger is a vessel containing any heat exchange device; such devices can include plates and fins, wound coils, tube bundles, and other known heat transfer means. The term “main heat exchange zone” defines the zone in which refrigeration is provided from the second recirculating refrigeration circuit in a temperature range between the second temperature and the third temperature for cooling and liquefying the feed gas. In the embodiments described below, the main heat exchange zone is a heat exchanger or group of heat exchangers in which refrigeration is provided by the vaporization of a recirculating mixed refrigerant to cool and liquefy the feed gas between the second temperature and the third temperature.




A representative gas liquefaction process according to the prior art is illustrated in FIG.


1


. Natural gas


100


is first cleaned and dried in a pretreatment section


102


for the removal of acid gases such as CO


2


and H


2


S along with other contaminants such as mercury. Pre-treated gas


104


then enters first stage propane exchanger


106


and is cooled therein to a typical intermediate temperature of about 8° C. The stream is further cooled in second stage propane exchanger


108


to a typical temperature of about −15° C., and the resulting further cooled stream


110


enters scrub column


112


. In the scrub column, heavier components of the feed, typically pentane and heavier, are removed as stream


116


from the bottom of the scrub column. The scrub column condenser is refrigerated by propane exchanger


114


. Propane exchangers


106


,


108


, and


114


employ vaporizing propane to provide refrigeration by indirect heat exchange.




Natural gas stream


118


after heavy component removal is at a typical temperature of about −35° C. Stream


118


is further cooled in cooling circuit


120


in the first zone of main heat exchanger


122


to a typical temperature of about −100° C. by a boiling mixed refrigerant stream supplied via line


124


. The resulting cooled feed gas stream is flashed across valve


126


and is further cooled in cooling circuit


128


in a second zone of main exchanger


122


by boiling mixed refrigerant stream supplied via line


130


. The resulting liquefied stream


132


may be flashed across valve


134


to yield final LNG product stream


136


at a typical temperature of −166° C. If necessary, stream


132


or stream


136


can be processed further for the removal of residual contaminants such as nitrogen.




Vaporizing refrigerant streams


124


and


130


flow downward through heat exchanger


122


, and combined mixed refrigerant vapor stream


138


is withdrawn therefrom. Mixed refrigerant vapor stream


138


is compressed to a typical pressure of 50 bara in multi-stage compressor


140


, is cooled against an ambient heat sink in exchanger


142


, and is further cooled and partially condensed against vaporizing propane in heat exchangers


144


,


146


, and


148


to yield two-phase mixed refrigerant stream


150


at a typical temperature of −35° C.




Two-phase mixed refrigerant stream


150


is separated in separator


152


to yield vapor stream


154


and liquid stream


156


which flow into heat exchanger


122


. Liquid stream


156


is subcooled in cooling circuit


158


and flashed across valve


160


to provide a vaporizing refrigerant stream via line


124


. Vapor stream


154


is condensed and subcooled in cooling circuits


162


and


164


, and is flashed across valve


166


to provide the vaporizing mixed refrigerant stream via line


130


.




A preferred embodiment of the present invention is illustrated in FIG.


2


. Natural gas feed stream


118


, after heavy component removal and cooling to about −35° C., is provided as described above with respect to FIG.


1


. Stream


118


is cooled further in cooling circuit


219


in the lower zone of heat exchanger


220


to a typical temperature of about −100° C. by indirect heat exchange with a first vaporizing mixed refrigerant introduced via lines


222


and


224


. Heat exchanger


222


is the main heat exchange zone earlier defined wherein refrigeration is provided by one or more refrigerant streams to cool a process stream within a given temperature range. The gas stream is further cooled to a typical temperature of about −130° C. in cooling circuit


225


in the middle zone of heat exchanger


220


by indirect heat exchange with a second vaporizing mixed refrigerant introduced via lines


226


and


227


. The resulting stream then is further cooled to a typical temperature of about −166° C. in cooling circuit


228


in the upper zone of heat exchanger


220


by indirect heat exchange with a third vaporizing mixed refrigerant introduced via lines


230


and


231


. Final LNG product is withdrawn as stream


232


and sent to a storage tank or to further processing if required.




In the process of

FIG. 2

, when very low levels of heavy components are required in the final LNG product, any suitable modification to scrub column


110


can be made. For example, a heavier component such as butane may be used as the wash liquid.




Refrigeration to cool and condense natural gas stream


118


from about −35° C. to a final LNG product temperature of about −166° C. is provided at least in part by a mixed refrigerant circuit utilizing a preferred feature of the present invention. Combined vaporized mixed refrigerant stream


233


is withdrawn from the bottom of heat exchanger


220


and compressed in multistage compressor


234


to a typical pressure of about 50 bara. Compressed refrigerant


235


is then cooled against an ambient heat sink in exchanger


236


to about 30° C. Initially cooled high pressure mixed refrigerant stream


237


is further cooled and partially condensed in first stage propane exchanger


238


at a temperature of approximately 8° C. The partially condensed stream flows into separator


240


where it is separated into vapor stream


242


and liquid stream


244


. Vapor stream


242


is further cooled in propane exchanger


246


to a temperature of approximately −15° C. and is further cooled in propane exchanger


248


to about −35° C. Liquid stream


244


is further cooled in propane exchanger


250


to a temperature of approximately −15° C. and is further cooled in propane exchanger


252


to about −35° C. to provide subcooled refrigerant liquid stream


262


.




After separation in separator


240


, a portion of liquid stream


244


may be blended with the vapor at any point before, during, or after the cooling steps as represented by optional streams


254


,


256


, and


266


. The resulting two-phase refrigerant stream


260


is then separated into liquid and vapor streams


268


and


270


in separator


272


. Optionally, a portion of subcooled liquid stream


262


as stream


258


may be blended with saturated liquid stream


268


to yield liquid refrigerant stream


274


.




Three mixed refrigerant streams enter the warm end of heat exchanger


220


at a typical temperature of about −35° C.: heavy liquid stream


262


, lighter liquid stream


274


, and vapor stream


270


. Stream


262


is further subcooled in cooling circuit


275


to a temperature of about −100° C. and is reduced in pressure adiabatically across Joule-Thomson throttling valve


276


to a pressure of about 3 bara. The reduced-pressure refrigerant is introduced into exchanger


220


via lines


222


and


224


to provide refrigeration as earlier described. If desired, the refrigerant stream may be reduced in pressure by work expansion using a turboexpander or expansion engine in place of throttling valve


276


. Liquid refrigerant stream


274


is subcooled in cooling circuit


278


to a temperature of about −130° C. and is reduced in pressure adiabatically across Joule-Thomson throttling valve


280


to a pressure of about 3 bara. The reduced-pressure refrigerant is introduced into exchanger


220


via lines


226


and


227


to provide refrigeration therein as earlier described. If desired, the refrigerant stream may be reduced in pressure by work expansion using a turboexpander or expansion engine in place of throttling valve


280


.




Refrigerant vapor stream


270


is liquefied and subcooled in cooling circuit


282


to a temperature of about −166° C. and is reduced in pressure adiabatically across Joule-Thomson throttling valve


284


to a pressure of about 3 bara. The reduced-pressure refrigerant is introduced into exchanger


220


via lines


230


and


231


to provide refrigeration therein as earlier described. If desired, the refrigerant stream may be reduced in pressure by work expansion using a turboexpander or expansion engine in place of throttling valve


284


.




In the process of

FIG. 2

, some heat exchangers may be combined into one heat exchanger if desired. For example, heat exchangers


246


and


250


could be combined, or heat exchangers


246


and


248


could be combined.




While the preferred embodiment in

FIG. 2

is described using typical temperatures and pressures of various streams, these pressures and temperatures are not intended to be limiting and may vary widely depending on design and operating conditions. For example, the pressure of the high pressure mixed refrigerant may be any suitable pressure and not necessarily 50 bara, and the pressure of the low pressure pressure mixed refrigerant stream


233


could be any suitable pressure between 1 bara and 25 bara. Similarly, the typical temperatures given above in describing the process may vary and will depend on specific design and operating conditions.




Thus an important feature of the present invention is the generation of additional subcooled liquid refrigerant stream


262


, which is further subcooled and vaporized to provide refrigeration in the bottom section of heat exchanger


220


. The use of this additional refrigerant stream results in power savings by reducing the total amount of required subcooling of liquid streams. Utilization of liquid refrigerant stream


262


, which contains heavier hydrocarbon components, provides a thermodynamically preferred composition for vaporization in the bottom or warm zone of heat exchanger


220


. The condensation and separation of heavier refrigerant stream


262


results in a higher concentration of lighter components in liquid refrigerant stream


274


, which is more appropriate for providing refrigeration in the middle zone of heat exchanger


220


. The use of optimum compositions of refrigerant streams


262


and


274


yields better cooling curves and improved efficiency in heat exchanger


220


.




Another embodiment of the invention is illustrated in FIG.


3


. In this embodiment, three stages of propane precooling are provided by exchangers


300


,


302


, and


304


between the compression stages of compressor


306


. After the final stage of propane precooling, partially condensed stream


308


is separated into vapor stream


310


and liquid stream


362


. Vapor stream


310


is further compressed to the final high pressure in an additional stage or stages in compressor


306


, and optionally is further cooled in propane precooling exchanger


312


. Liquid stream


362


is subcooled, reduced in pressure adiabatically across throttling valve


376


, and introduced into heat exchanger


320


via line


322


to provide refrigeration as earlier described with reference to FIG.


2


. If desired, the pressure of stream


378


could be reduced by work expansion using a turboexpander or expansion engine in place of throttling valve


376


.




Another embodiment of the invention is illustrated in FIG.


4


. In this embodiment, four stages of propane precooling are employed for feed precooling and pretreatment, shown as earlier-described feed heat exchangers


106


,


108


,


114


, and additional exchanger


401


, respectively. Additional propane refrigeration also is used for cooling the mixed refrigerant circuit, wherein exchangers


402


and


403


are used with previously-described exchangers


246


,


248


,


250


, and


252


. The additional exchangers add some complication but improve the efficiency of the liquefaction process.




Another embodiment of the invention is illustrated in

FIG. 5

wherein the first separator


540


is located after the second stage of propane precooling


500


rather than after the first stage of propane precooling as in the embodiment of FIG.


2


.

FIG. 6

shows another optional embodiment wherein the first separator


640


is located immediately after ambient cooler


164


rather than after the first stage of propane precooling in the embodiment of FIG.


2


. In the embodiment of

FIG. 6

, all propane cooling is carried out after separator


640


.





FIG. 7

illustrates another embodiment of the invention in which all stages of feed precooling occur in propane exchangers


706


,


708


, and


714


prior to scrub column


710


. Refrigeration for the overhead condenser of the scrub column is provided by cooling overhead stream


716


in cooling circuit


718


in the warmest zone of heat exchanger


720


. Cooled and partially condensed overhead stream


722


is returned to scrub column separator


724


. This embodiment is useful when very low levels of heavy components are required in the final LNG product.




Another embodiment is illustrated in

FIG. 8

wherein an additional mixed refrigerant liquid stream


802


is generated before the final propane precooling stage by means of additional separator


801


. All or a portion of additional liquid stream


802


may be mixed with the first liquid generated after subcooling to the same temperature, and optionally a portion as stream


803


may be combined with the vapor from separator


801


.





FIG. 9

illustrates another embodiment of the invention in which a second additional liquid stream


901


is generated before the final propane stage by means of additional separator


900


. In this embodiment, second additional liquid stream


901


generated is not mixed with the first liquid generated as was the case in the above embodiment of

FIG. 8

, but instead is subcooled and introduced into exchanger


920


as a liquid feed which is subcooled and expanded through throttling valve


903


. The use of this additional liquid requires additional heat exchanger


902


as shown in FIG.


9


. This embodiment differs from other embodiments in that brazed aluminum heat exchangers can be used in main heat exchange zone


920


as shown in

FIG. 9

, rather than the wound coil heat exchangers widely used in gas liquefaction processes. However, any suitable type of heat exchanger can be used for any embodiment of the present invention.




Another optional embodiment of the invention is given in FIG.


10


. In this embodiment, the second phase separator


1000


is located at a colder temperature than that provided by the final propane precooling stage


148


. Two phase stream


1060


enters exchanger


1020


directly and is cooled in the warmest heat exchange zone of the exchanger before being separated.





FIG. 11

discloses another feature of the invention wherein the mixed refrigerant streams are vaporized at two different pressures. Streams


1168


and


1170


are liquefied, subcooled, reduced in pressure, and vaporized at a low pressure in exchanger


1102


. Vaporized mixed refrigerant stream


1104


may be fed cold directly to compressor


1136


, or may be warmed in exchanger


1100


before being fed to compressor


1136


. Liquid refrigerant stream


1162


is further subcooled, reduced in pressure to a pressure above the pressure in exchanger


1102


, vaporized in exchanger


1100


, and returned as stream


1106


to compressor


1136


between compression stages as shown.




The mixed refrigerant utilized for gas liquefaction may be precooled by another mixed refrigerant rather than by propane as described above. In this embodiment as shown in

FIG. 12

, liquid refrigerant stream


1202


is obtained from the partial condensation of a precooling mixed refrigerant between compression stages in compressor


1204


. This liquid is then subcooled in exchanger


1200


, withdrawn at an intermediate location, flashed across throttling valve


1206


, and vaporized to provide the refrigeration to the warm zone of heat exchanger


1200


. Vapor


1210


from exchanger


1200


is compressed in compressor


1204


, cooled against an ambient temperature heat sink, and introduced to exchanger


1200


as stream


1212


. Stream


1212


is cooled and subcooled in exchanger


1200


, withdrawn at the cold end of


1200


, flashed across throttling valve


1208


, and vaporized to provide the refrigeration to the cold zone of exchanger


1200


.




Compressed mixed refrigerant stream


1214


is cooled and partially condensed in the bottom portion of heat exchanger


1200


, and then is separated in separator


1288


. The resulting liquid stream


1244


is then subcooled in the upper end of exchanger


1200


, the resulting subcooled stream


1162


is further subcooled in the bottom section of exchanger


1220


, reduced in pressure adiabatically across throttling valve


1276


, introduced via line


1222


into exchanger


1220


, and vaporized to provide refrigeration therein. Vapor from separator


1288


is cooled in the top section of exchanger


1200


to provide two-phase refrigerant stream


1260


, which is separated in separator


1262


and utilized in exchanger


1220


as earlier described.





FIG. 13

illustrates a modification to the embodiment of

FIG. 12

wherein the precooling mixed refrigerant is vaporized at two different pressures in exchangers


1300


and


1302


. The first separation of the cold mixed refrigerant in separator


1388


occurs after cooling in precooling exchanger


1300


. The resulting liquid stream


1344


is then subcooled before being reduced in pressure adiabatically across throttling valve


1376


and introduced to exchanger


1320


as stream


1322


to provide refrigeration by vaporization therein.




A final embodiment of the invention is illustrated in

FIG. 14

, which is a simplified version of the embodiment of FIG.


2


. In this embodiment, the flowsheet is simplified by eliminating the separation of stream


160


just prior to heat exchanger


220


of FIG.


2


. In

FIG. 14

, the two heat exchange zones in exchanger


1420


replace the three heat exchange zones of heat exchanger


220


of FIG.


2


. Stream


1460


is liquefied and subcooled in exchanger


1420


, subcooled stream


1486


is reduced in pressure adiabatically across throttling valve


1484


to a pressure of about 3 bara, and is introduced as stream


1430


into the cold end of exchanger


1420


where it vaporizes to provide refrigeration. If desired, the pressure of stream


1486


could be reduced by work expansion in a turboexpander or expansion engine.




The embodiments described above utilize an important common feature of the present invention wherein at least one intermediate liquid stream is derived from the partial condensation and separation of the mixed refrigerant at a temperature equal to or greater than the lowest temperature achievable by cooling against the first recirculating refrigeration circuit. The intermediate liquid stream is used to provide refrigeration at a temperature lower than that provided by the precooling system.




The condensation temperature at which the intermediate stream is obtained can be varied as required; in the embodiment of

FIG. 6

this condensation is effected at ambient temperature in heat exchanger


164


, while in the embodiment of

FIG. 3

the condensation is effected at the lowest propane precooling temperature in heat exchanger


304


at a pressure lower than the final highest pressure of the compressed mixed refrigerant vapor from compressor


306


. Condensation is effected at temperatures between these extremes in the embodiments of

FIGS. 2

,


4


, and


5


.




The embodiments described above can be summarized in generic process terms as follows. The invention is basically a method for providing refrigeration to liquefy a feed gas which comprises several general steps. Refrigeration is provided by a first recirculating refrigeration circuit which provides refrigeration in a temperature range between a first temperature and a second temperature which is lower than the first temperature, and is described as precooling refrigeration. The second temperature is typically the lowest temperature to which a process stream can be cooled by indirect heat exchange with the refrigerant in the first refrigeration circuit. For example, if the first refrigeration circuit uses propane, the lowest temperature to which a process stream can be cooled is about −35° C., and this is typical of the second temperature.




Additional refrigeration is provided by a second recirculating refrigeration circuit in a temperature range between the second temperature and a third temperature which is lower than the second temperature. The first refrigeration circuit provides at least a portion of the refrigeration to the second refrigeration circuit in the temperature range between the first temperature and the second temperature, and also may provide refrigeration to precool the feed gas.




The first refrigeration circuit, which may utilize a single component or multiple components as described above, provides refrigeration at several temperature levels depending upon the pressure at which the refrigerant is vaporized. This first refrigeration circuit provides refrigeration for precooling the feed gas in exchangers


106


,


108


,


114


,


401


,


706


,


708


,


714


,


1200


,


1300


, and


1302


as described above. The first refrigeration circuit also provides refrigeration to cool the second refrigerant circuit in exchangers


238


,


246


,


248


,


250


,


252


,


300


,


302


,


304


,


312


,


402


,


403


, and


500


as described above.




The second refrigerant circuit, as exemplified in the preferred embodiment of

FIG. 2

, typically comprises refrigerant line


233


, compressor


234


, separator


240


, the several cooling exchangers which provide cooling from the first refrigerant circuit, refrigerant lines


260


,


262


,


270


, and


274


, separator


272


, subcooling circuits


275


,


278


, and


282


, throttling valves


276


,


280


, and


284


, and refrigerant lines


222


,


224


,


226


,


227


,


230


, and


231


. Similar components are utilized in similar fashion in the embodiments of

FIGS. 4-13

. The second refrigerant circuit in the embodiment of

FIG. 14

includes features of

FIG. 2

but without separator


272


, refrigerant line


274


, subcooling circuit


278


, refrigerant lines


226


and


227


, and throttling valve


280


.




When the mixed refrigerant vapor is compressed to a final highest pressure in multistage compressor


234


of

FIG. 2

(and similarly in the embodiments of FIGS.


4


-


13


), the compressed vapor is partially condensed and separated at temperatures greater than the lowest temperature provided by refrigerant from the first refrigerant circuit. At least one of the mixed refrigerant vapor and liquid streams produced in the condensation/separation step is further cooled by refrigerant from the first refrigerant circuit to the lowest temperature possible using the first refrigerant. Such additional cooling can be provided by exchangers


246


,


248


,


250


, and


252


of FIG.


2


.




When the mixed refrigerant vapor is initially compressed to a pressure less than the final highest pressure, as in the embodiment of

FIG. 3

, condensation of the compressed mixed refrigerant vapor stream is effected between the stages of compressor


306


at a temperature equal to or higher than the lowest temperature achievable by cooling with refrigeration from the first refrigeration circuit, i.e., the second temperature. The separated vapor in line


310


is further compressed in a final stage of compressor


306


. If no additional cooling is provided from the first refrigeration circuit in exchanger


312


, condensation and separation of stream


308


could be carried out above the second temperature. If additional cooling is provided in exchanger


312


, condensation and separation of stream


308


could be carried out at or above the second temperature.




The liquid refrigerant stream generated as described above, which is at or above the second temperature, is subcooled against vaporizing mixed refrigerant in the main heat exchanger, reduced in pressure, and vaporized in the main exchanger to provide refrigeration between the second temperature and the third temperature.




EXAMPLE




The preferred embodiment of the invention was simulated by performing heat and material balances for liquefying natural gas. Referring to

FIG. 2

natural gas


100


is first cleaned and dried in pretreatment section


102


for the removal of acid gases such as CO


2


and H


2


S along with other contaminants such as mercury. Pretreated feed gas


104


has a flow rate of 30,611 kg-mole/hr, a pressure of 66.5 bara, and a temperature of 32° C. (89.6° F.) with a molar composition as follows:












TABLE 1









Feed Gas Composition, Mole Fraction


























Nitrogen




0.009







Methane




0.8774







Ethane




0.066







Propane




0.026







i-Butane




0.007







Butane




0.008







i-Pentane




0.002







Pentane




0.002







Hexane




0.001







Heptane




0.001















Pre-treated gas


104


enters first exchanger


106


and is cooled to a temperature of 9.3° C. by propane boiling at 5.9 bara. The feed is further cooled to −14.1° C. in exchanger


108


by propane boiling at 2.8 bara before entering scrub column


110


as stream


112


. The overhead condenser


114


of the scrub column operates at −37° C. and is refrigerated by propane boiling at 1.17 bara. In scrub column


110


the pentane and heavier components of the feed are removed.




Natural gas stream


118


, after heavy component removal and cooling to −37° C., is then further cooled in cooling circuit


219


in the first zone of main heat exchanger


220


to a temperature of −94° C. by boiling mixed refrigerant. The vaporized mixed refrigerant stream


233


has a flow of 42,052 kg-mole/hr and the following composition:












TABLE 2









Mixed Refrigerant Composition (Mole Fraction)


























Nitrogen




0.092







Methane




0.397







Ethane




0.355







Propane




0.127







i-Butane




0.014







Butane




0.014















The resulting feed gas is then further cooled in cooling circuit


225


to a temperature of about −128° C. in the second zone of exchanger


220


by boiling mixed refrigerant stream via lines


226


and


227


. The resulting gas stream is further cooled in cooling circuit


228


to a temperature of −163° C. in a third zone of exchanger


220


by boiling mixed refrigerant stream introduced via lines


230


and


231


. The resulting further cooled LNG stream


232


is then sent to a storage tank.




Refrigeration to cool the natural gas stream


118


from −37° C. to a temperature of −163° C. is provided by a mixed component refrigeration circuit. Stream


235


is the high pressure mixed refrigerant exiting multistage compressor


234


at a pressure of 51 bara. It is then cooled to 32° C. against cooling water in exchanger


236


. High pressure mixed refrigerant stream


237


enters first stage propane exchanger


238


, is cooled to a temperature of 9.3° C. by propane boiling at 5.9 bara, and flows into separator


240


where it is separated into vapor and liquid streams


242


and


244


respectively. Vapor stream


242


is further cooled in propane exchanger


246


to a temperature of −14.1° C. by propane boiling at 2.8 bara followed by propane exchanger


248


where it is further cooled to −37° C. by propane boiling at 1.17 bara. Liquid stream


244


at a flow rate of 9240 kg-mole/hr is further cooled in propane exchanger


250


to a temperature of −14.1° C. by propane boiling at 2.8 bara followed by propane exchanger


252


where it is further cooled to −37° C. by propane boiling at 1.17 bara.




The resulting cooled vapor stream


260


is then separated at −37° C. into liquid and vapor streams


268


and


270


respectively in separator


272


. Liquid stream


268


has a flow rate of 17,400 kg-mole/hr.




Subcooled liquid stream


262


is further subcooled to a temperature of −94° C. in cooling circuit


275


and is reduced in pressure adiabatically across throttling valve


276


to a pressure of about 3 bara and introduced to exchanger


220


via lines


222


and


224


. Liquid stream


274


is subcooled to a temperature of −128° C. in cooling circuit


278


and is reduced in pressure adiabatically across throttling valve


280


to a pressure of about 3 bara and introduced to exchanger


220


via lines


226


and


227


. Vapor stream


270


is liquefied and subcooled to a temperature of −163° C. in cooling circuit


282


, is reduced in pressure adiabatically across throttling valve


284


to a pressure of about 3 bara, and is introduced to the cold end exchanger


220


via lines


230


and


231


.




The present invention in its broadest embodiment thus offers an improvement to the gas liquefaction art by generating at least one intermediate liquid stream derived from the partial condensation and separation of the mixed refrigerant at a temperature warmer than the lowest temperature provided by the precooling system or at a pressure lower than the final highest pressure of the mixed refrigerant circuit. This intermediate liquid mixed refrigerant stream is used at least in part to provide additional refrigeration at a temperature lower than that provided by the precooling system, and this additional refrigeration may be used in the main heat exchanger. The present invention is a more efficient process which provides increased LNG production for a given compression power compared with prior art processes.




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.



Claims
  • 1. A method for providing refrigeration for liquefying a feed gas which comprises:(1) providing refrigeration from a first recirculating refrigeration circuit which provides refrigeration in a temperature range between a first temperature and a second temperature which is lower than the first temperature; (2) providing refrigeration from a second recirculating refrigeration circuit in a temperature range between the second temperature and a third temperature which is lower than the second temperature, wherein the first refrigeration circuit provides refrigeration to the second refrigeration circuit in the temperature range between the first temperature and the second temperature; (3) withdrawing a mixed refrigerant vapor from a main heat exchange zone in the second recirculating refrigeration circuit and compressing the mixed refrigerant vapor to a final highest pressure to yield a compressed mixed refrigerant vapor; (4) partially condensing at least a portion of the compressed mixed refrigerant vapor in the second recirculating refrigeration circuit and separating the resulting partially condensed mixed refrigerant into at least one liquid refrigerant stream and at least one vapor refrigerant stream; and (5) subcooling the at least one liquid refrigerant stream to a temperature lower than the second temperature, reducing the pressure of the resulting subcooled liquid refrigerant stream, and vaporizing the resulting reduced-pressure refrigerant stream to provide at least a portion of the refrigeration for liquefying the feed gas between the second temperature and the third temperature; whereinthe step of partially condensing the compressed mixed refrigerant vapor is effected at a pressure essentially equal to the final highest pressure.
  • 2. The method of claim 1 wherein refrigeration for liquefying the feed gas between the second temperature and the third temperature is provided by indirect heat exchange with a vaporizing mixed refrigerant in the main heat exchange zone, and wherein the vaporizing mixed refrigerant is provided by(a) compressing the mixed refrigerant vapor to a first pressure; (b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a first mixed refrigerant vapor fraction and a first mixed refrigerant liquid fraction; (c) subcooling the first mixed refrigerant liquid fraction to provide a first subcooled mixed refrigerant liquid; (d) reducing the pressure of the first subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide vaporizing mixed refrigerant for cooling and condensing the feed gas therein; and (e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor for step (a).
  • 3. The method of claim 2 wherein at least a portion of the refrigeration for the subcooling in step (c) is provided by the vaporizing of the reduced pressure mixed refrigerant in the main heat exchange zone in step (d).
  • 4. The method of claim 2 wherein at least a portion of the refrigeration for the subcooling in (c) is provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone.
  • 5. The method of claim 4 wherein the one or more additional refrigerant streams comprises a single component refrigerant.
  • 6. The method of claim 4 wherein the one or more additional refrigerant streams comprises a multicomponent refrigerant.
  • 7. The method of claim 2 which further comprises partially condensing and separating the first mixed refrigerant vapor fraction to yield a second mixed refrigerant vapor and a second mixed refrigerant liquid, subcooling the second mixed refrigerant liquid by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting subcooled second mixed refrigerant liquid, and vaporizing the resulting reduced pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
  • 8. The method of claim 7 which further comprises condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor, and vaporizing the resulting reduced-pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
  • 9. The method of claim 7 wherein a portion of the refrigeration for cooling and partially condensing the first mixed refrigerant vapor fraction is provided by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone.
  • 10. The method of claim 7 whereinthe first pressurized mixed refrigerant liquid after subcooling is vaporized in the main heat exchange zone at a first pressure; and the second pressurized mixed refrigerant liquid after subcooling is vaporized in the main heat exchange zone at a second pressure.
  • 11. The method of claim 10 which further comprises condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor to the second pressure, and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
  • 12. The method of claim 2 wherein at least a portion of the refrigeration for the cooling and partial condensing in (b) is provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone.
  • 13. The method of claim 12 wherein at least one of the one or more additional refrigerant streams comprises a single component refrigerant.
  • 14. The method of claim 12 wherein at least one of the one or more additional refrigerant streams comprises a multicomponent refrigerant.
  • 15. The method of claim 2 wherein a portion of the refrigeration for cooling the feed gas is provided by indirect heat exchange with one or more additional refrigerant streams external of the main heat exchange zone.
  • 16. The method of claim 15 wherein the one or more additional refrigerant streams comprises a single component refrigerant.
  • 17. The method of claim 15 wherein the one or more additional refrigerant streams comprises a multicomponent refrigerant.
  • 18. The method of claim 2 wherein the feed gas comprises methane and one or more hydrocarbons heavier than methane, and wherein the method further comprises:(e) precooling the feed gas by indirect heat exchange with an additional refrigerant stream; (f) introducing the resulting precooled feed gas into a scrub column with a lean scrub liquid enriched in hydrocarbons heavier than methane; (g) withdrawing from the bottom of the scrub column a stream rich in hydrocarbons heavier than methane; (h) withdrawing from the top of the scrub column an overhead stream containing methane and residual hydrocarbons heavier than methane; (i) cooling the overhead stream in the main heat exchange zone to condense residual hydrocarbons heavier than methane; (j) separating the resulting cooled overhead stream into a purified methane-enriched product and a stream enriched in hydrocarbons heavier than methane; and (k) utilizing at least a portion of the stream enriched in hydrocarbons heavier than methane to provide the lean scrub liquid of (f).
  • 19. The method of claim 2 wherein the cooling and partially condensing of the resulting compressed first mixed refrigerant vapor in (b) is effected by indirect heat exchange with a fluid at ambient temperature.
  • 20. The method of claim 2 wherein a portion of the first mixed refrigerant liquid is mixed with the first pressurized mixed refrigerant vapor.
  • 21. The method of claim 2 wherein further cooling, partially condensing, and separating of at least a portion of the first mixed refrigerant vapor in (b) yields an additional mixed refrigerant liquid which is combined with the first pressurized mixed refrigerant liquid.
  • 22. The method of claim 1 wherein the operation of the second recirculating refrigeration circuit includes(a) compressing the mixed refrigerant vapor to a first pressure; (b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a mixed refrigerant vapor fraction and a mixed refrigerant liquid fraction; (c) subcooling the mixed refrigerant liquid fraction to provide a subcooled mixed refrigerant liquid; (d) reducing the pressure of the subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide one of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein; and (e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor in (a); wherein the refrigeration for subcooling the mixed refrigerant liquid fraction is provided in part by indirect heat exchange with the resulting vaporizing reduced pressure refrigerant liquid in the main heat exchange zone and in part by indirect heat exchange with one or more portions of an additional refrigerant external to the main heat exchange zone.
  • 23. The method of claim 23 which further comprises(f) condensing and subcooling the mixed refrigerant vapor fraction to provide an additional subcooled mixed refrigerant liquid; and (g) reducing the pressure of the additional subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure liquid in the main heat exchange zone to provide another of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein; wherein the refrigeration for condensing and subcooling the additional mixed refrigerant vapor is provided in part by indirect heat exchange with the resulting vaporizing reduced pressure liquid in the main heat exchange zone and in part by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone.
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