US3645106A

US3645106A - Process for liquefying natural gas employing a multicomponent refrigerant for obtaining low temperature cooling

Info

Publication number: US3645106A
Application number: US49622A
Authority: US
Inventors: Lee S Gaumer Jr; Charles L Newton
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1965-06-29
Filing date: 1970-06-25
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28
Also published as: FR1557019A; MY7000139A; GB1135871A; US3593535A

Abstract

Refrigeration for liquefying natural gas is provided by a closed cycle refrigeration system employing a multicomponent refrigerant. A stream of multicomponent refrigerant flowing in the closed refrigeration system is compressed and then successively fractionated by partial condensation in a plurality of steps to provide condensates at progressively decreasing temperature levels. The condensates are separated and introduced under reduced pressure into a common zone in heat exchange with the natural gas and vaporization of the condensates. A stream of multicomponent refrigerant including the resulting vapor from the condensates is withdrawn from the zone for recycle. The multicomponent refrigerant includes a mixture of components consisting essentially of nitrogen and a plurality of hydrocarbons. In one presently preferred variant, the refrigerant is a mixture of nitrogen and more than three hydrocarbons having molecular weights between the molecular weight of methane and the molecular weight of hexane. Two of the hydrocarbons are methane and ethane. Ethane is the component of greatest percentage, methane is the component of second greatest percentage, and nitrogen is present in a percentage substantially less than that of methane.

Description

Elite tes Gaumer, Jr. et al.

atent [54] PROCESS FOR LIQUEFYHNG NATURAL GAS EMPLOYING A MULTTCOMPONENT REFRHGERANT FOR OBTAINING LOW TEMPERATURE COOLING [72] Inventors: Lee S. Gaumer, Jr.; Charles L. Newton,

both of do Air Products & Chemicals, lnc., P.O. Box 538, Allentown, Pa. 18105 [22] Filed: June 25,1970

[21] Appl.No.: 49,622

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 882,781, Dec. 22, 1969, and a continuation-in-part of 3,571, Jan. 9, 1970, abandoned.

[52] US. Cl ..62/9, 62/11, 62/40 [51] Int. Cl ..F25j 1/00, F25j 3/06 [58] Field of Search ..62/9, 23, 24, 27, 28, 40

[56] References Cited UNITED STATES PATENTS 3,364,685 l/1968 Perret ..62/40 NATURAL GAS Primary Examiner-Norman Yudkoff Assistant ExaminerA. Purcell Attorney-Shanley and ONeil [5 7] ABSTRACT Refrigeration for liquefying natural gas is provided by a closed cycle refrigeration system employing a multicomponent refrigerant. A stream of multicomponent refrigerant flowing in the closed refrigeration system is compressed and then successively fractionated by partial condensation in a plurality of steps to provide condensates at progressively decreasing temperature levels. The condensates are separated and introduced under reduced pressure into a common zone in heat exchange with the natural gas and vaporization of the condensates. A stream of multicomponent refrigerant including the resulting vapor from the condensates is withdrawn from the zone for recycle. The multicomponent refrigerant includes a mixture of components consisting essentially of nitrogen and a plurality of hydrocarbons. In one presently preferred variant, the refrigerant is a mixture of nitrogen and more than three hydrocarbons having molecular weights between the molecular weight of methane and the molecular weight of hexane. Two of the hydrocarbons are methane and ethane. Ethane is the component of greatest percentage, methane is the com ponent of second greatest percentage, and nitrogen is present in a percentage substantially less than that of methane.

8 Claims, 2 Drawing Figures MULTl-COMPONENT REFRIGERANT PATENTEUFEB 29 m2 SHEET 1 BF 2 INVENTORS LEE 8. GAUMER,JR NEWTON CHARLES L.

REFRIGERANT MULTI-COMPONENT ATTORNEYS PROCESS FOR LIQUEFYING NATURAL GAS EMPLOYING A MULTICOMPONENT REFRIGERANT FOR OBTAINING LOW TEMPERATURE COOLING CROSS-REFERENCE TO RELATED APPLICATIONS The preset application is a continuation-in-part of out copending application Ser. No. 882,781, filed Dec. 22, 1969 for Liquefaction of Natural Gas, and out copending application Ser. No. 3,571, filed Jan. 9, 1970 for Multicomponent Refrigerator for Obtaining Low Temperature Cooling. Application Ser. No. 882,781 is a continuation of our abandoned application Ser. No. 722,136, filed Apr. 17, 1968 for Liquefaction of Natural Gas. Application Ser. No. 3,571 now abandoned is a continuation of our abandoned application Ser. No. 659,988, filed Aug. 11, 1967 for Multicomponent Refrigerant For Obtaining Low Temperature Cooling. Abandoned applications Ser. No. 659,988 and 722,135 were a continuation-in-part and a continuation, respectively, of our abandoned application Ser. No. 468,008, filed June 29, 1965 for Liquefaction of Natural Gas.

BACKGROUND OF THE INVENTION The present invention broadly relates to the liquefaction of low boiling point gases and, more particularly, to an improved method and apparatus particularly designed for liquefying natural gas with a substantial reduction in the cost of the liquefaction facility as compared to previous liquefaction cycles of the cascade type wherein the process steam is heat exchanged with different refrigerants circulated in independent closed loops. In one of its more specific variants, the invention relates to an improved process for liquefying natural gas wherein refrigeration is provided by a closed cycle refrigeration system employing a multicomponent refrigerant. The invention further relates to improvements in refrigerants of the multicomponent type, i.e., a mixture of component gases of different boiling points.

Refrigeration systems employing multicomponent refrigerants are provided by the prior art. U.S. Pat. No. 2,041,725 of Podbielniak discloses a refrigeration system for obtaining a source of refrigeration at low temperature in which a multicomponent refrigerant is partially condensed at a plurality of decreasing temperature levels. The condensate at the lowest temperature provides the source of refrigeration and the condensates under reduced pressure are successively passed in countercurrent heat interchange with the refrigerant to effect the partial condensation steps. The Podbielniak patent teaches that it is only necessary that the refrigerant comprise a series of constituents having a range of condensation temperature such as mixtures of hydrocarbon gases including natural gas, refinery gas, coal gas and water gas, as well as chlorinated or fluorinated hydrocarbons. A process of A. P. Kleemenlro (Progress in Refrigeration Science and Technology, Volume I, pages 34-39, published in 1960 by Pergamon Press, Library of Congress Card No. 60-16886) embodies the multicomponent refrigeration teaching of the Podbielniak patent in a cycle for liquefying air or natural gas wherein the feed gas is passed continuously in countercurrent heat interchange with the condensates and their resultant vapors of a multicomponent refrigerant which is flowed in a closed cycle. Kleemenko recognizes that the efficiency of the process will depend upon the irreversibility of the heat interchange between the refrigerant and the fluid being cooled and teaches that small temperature differences between the refrigerant and the fluid may be obtained throughout the heat interchange by utilizing a multicomponent refrigerant. For that purpose, Kleemenko discloses an optimum refrigerant comprising a mixture of hydrocarbon gases consisting of 65 percent methane, 20 percent propane and percent butane while also disclosing mixtures of methane and propane and mixtures of methane, ethane and propane. It is also known in the art to employ natural gas as a multicomponent refrigerant in a process for liquefying natural gas.

Although a process embodying the Kleemenko teachings provides improved efficiency, it has been determined that potential advantages of multicomponent refrigeration systems in terms of overall efficiency cannot be achieved by utilizing multicomponent refrigerants of compositions taught by the prior art.

Accordingly, it is an object of the present invention to provide a multicomponent refrigerant of novel composition as disclosed hereinafter which makes it possible to obtain fully the potential advantages of the multicomponent refrigeration process in terms of overall efficiency and cost of capital equipment.

It is a further object to provide an improved process for liquefying natural gas wherein refrigeration is provided by a closed cycle refrigeration system employing a multicomponent refrigerant comprising nitrogen and a plurality of hydrocarbons including methane and ethane, wherein the ethane is the component of greatest percentage, the methane is the component of second greatest percentage, and nitrogen is present in a percentage substantially less than methane.

Other objects and features of the present invention will appear from the following detailed description considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating one presently preferred closed cycle refrigeration system employing a multicomponent refrigerant in accordance with the invention; and

FIG. 2 is a simplified schematic diagram illustrating the major flow circuits comprising a complete liquefaction facility or plant constructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION With reference to FIG. 1 of the drawings, the natural gas liquefaction cycle includes a heat exchange device 10 which is vertically disposed as illustrated with its warm end 11 being its lower end and its cold end 12 being its upper end. The heat exchange device includes an outer shell 13 defining an elongated shell space 14 or zone within which is positioned a plurality of coiled tubes or other means forming separate passageways for flow of fluids in out-of-contact heat interchange with fluid flowing through the shell space to provide an elongated heat exchange passageway 15, a first series of heat exchange passageways 16, 17, 18, and 119 and a second series of

heat exchange passageways

20, 21, 22, and 23. The passageway 15 extends throughout the length of the shell space 14 and is connected to a natural gas inlet conduit 24 at the warm end 111 and to a liquefied natural gas outlet conduit 25 at the cold end 12. The series of passageways 16, 17, 18, and 19 and the series of

passageways

20, 21, 22, and 23 are vertically positioned in the shell space with pairs of

passageways

16 and 20, 17 and 21, I8 and 22, and 19 and 23, disposed between similar temperature levels along the heat exchange zone. The heat exchange device also includes a plurality of liquid distributor means 26, 27, 28, 29, and 30, provided with

feed conduits

31, 32, 33, 34, and 35, respectively, located at spaced vertical positions in the shell space. The liquid distributor means 26, 27, 28, and 29 are located at or slightly above the pairs of

passageways

16 and 20, 17 and 21, 18 and 22, and 19 and 23, respectively, and function to distribute liquid directly onto respective pairs of passageways as well as directly onto the portion of the elongated passageway 15 adjacent respective pairs of passageways. The liquid distributor means 30 is located at the cold end 12 and functions to distribute liquid directly onto the cold end of the passageway 15.

The cycle also includes a compressor 36, an aftercooler 37 and a plurality of

phase separators

38, 39, 40, and 41. The inlet of the compressor is connected by conduit 42 to the shell space 14 at the warm end of the heat exchange device 10 and its outlet is connected by conduit 43 to passage 414 of the aftercooler in heat interchange with coolant fluid, such as water,

flowing through the shell space 45, and then by conduit 46 to the phase separator 38. The

phase separators

38, 39, 40, and 41 are provided with

vapor outlet conduits

47, 48, 49, and 50, respectively, and with

liquid outlet conduits

51, 52, S3, and 54, respectively; the

vapor outlet conduits

47, 48, 49, and 50 being respectively connected to the warm ends of passageways 16, 17, 18, and 19 and the

liquid outlet conduits

51, 52, 53, and 54 being respectively connected to the warm ends of

passageways

20, 21, 22, and 23. The

phase separators

39, 40 and 41 are also provided with

feed conduits

55, 56 and 57, respectively, and the feed conduits are connected to the cold ends of passageways 16, 17 and 18, respectively; the cold end of the passageway 19 being connected by conduit 58 and pressure reducing valve 59 to the feed conduit 35 for liquid distributor means 30. The liquid distributor means 26, 27, 28, and 29 are connected through their feed conduits to the cold ends of

passageways

20, 21, 22, and 23; in particular, the passageway 20 is connected by conduit 60 and pressure reducing valve 61 to the feed conduit 31, the passageway 21 is connected by conduit 62 and pressure reducing valve 63 to feed conduit 32, the passageway 22 is connected by conduit 64 and pressure reducing valve 65 to feed conduit 33, and the passageway 23 is connected by conduit 66 and pressure reducing valve 67 to feed conduit 34.

The natural gas to be liquefied, previously purified and freed of moisture, enters the cycle under pressure at ambient temperature through conduit 24 and upon flowing through the passageway 15 is gradually cooled to liquefaction temperature and is withdrawn from the cycle by conduit 25 totally in liquid phase and preferably subcooled for subsequent pressure reduction as required for storage or other use. If desired, the natural gas feed before leaving the passageway 15 may be reduced in pressure to an intermediate superatmospheric pressure and flowed at such pressure through the cold end portion of passageway 15 located above the liquid distributor means 29. in situations where the natural gas includes high boiling point components which are not desired in the liquid product, the natural gas feed, after initial cooling to effect liquefaction of undesirable high boiling point components, may be withdrawn from the passageway 15 by conduit 68 and fed to a phase separator 69 from which the liquefied high boiling point components are withdrawn by conduit 70 and the remaining unliquefied natural gas returned by conduit 71 to the passageway 15 for continued flow through the heat exchange device 10. If desired, the natural gas may be withdrawn from the passageway 15 at more than one temperature level to effect plural separation of undesired high boiling point components.

Refrigeration for the process is provided by a closed refrigeration system employing a multicomponent refrigerant, i.e., a refrigerant comprising a mixture of elemental gases having different boiling points. The multicomponent refrigerant is successively fractionated by partial condensation in a plurality of steps to provide liquids at progressively decreasing temperature levels and such liquids are introduced under reduced pressure into the zone of the heat exchange device at corresponding temperature levels to provide the refrigeration required to effect liquefaction and subcooling of the natural gas feed as well as the refrigeration required to effect the fractional condensation of the multicomponent refrigerant. The multicomponent refrigerant is delivered by the compressor 36 under superatmospheric pressure and the heat interchange in the aftercooler 37 effects the first fractional condensation step at the highest temperature level, the resulting liquid vapor mixture being separated in the phase separator 38 to provide the first condensate 72. The second fractional condensation step is effected by passing unliquefied refrigerant from the phase separator 38 through the passageway 16 to effect its partial condensation followed by separation in the phase separator 39 to provide the second condensate 73. In a similar manner, third and fourth fractional condensation steps are performed by passing unliquefied refrigerant withdrawn from the

phase separators

39 and 60 through the passageways l7 and 18, respectively, to effect partial liquefaction with the respective respective liquefied portions being separated in

phase separators

40 and 41 providing a third condensate 74 and the fourth condensate 75. The unliquefied refrigerant withdrawn from the phase separator 41 by conduit 50 is cooled upon flowing through the passageway 19 to effect at least its partial condensation and, in some operations, the fluid leaving the passageway 19 by conduit 58 may be totally in liquid phase. In either event, liquid in the conduit 58 comprises a fifth condensate. The latter condensate and the

condensates

72, 73, 74, and 75 have different compositions of the components that comprise the multicomponent refrigerant fed to the compressor 36, the first condensate 72 being rich in high boiling point components, the last condensate in conduit 58 being rich in low boiling point components and the

intermediate condensates

73, 74, and 75 having decreasing high boiling point components and increasing low boiling point components in the order named. The condensates are formed under uniform pressure, except for pressure drops resulting from friction of the conduits; however, the exact composition of each condensate will depend upon the composition of the multicomponent refrigerant fed to the inlet of compressor 36 and upon the temperature of the liquid-vapor mixtures leaving the passageways 16, 17, 18, and 19. In any event, however, the bubble point temperature of the

condensates

72, 73, 74, and 75 and the condensate in conduit 58 will progressively decrease in the order named which is the order of progressively increasing composition of low boiling components of the multicomponent refrigerant. Again, depending upon the components and their percentage composition of the multicomponent refrigerant fed to the compressor 36, the composition of the condensates will vary not only with respect to percentage composition of components but also with respect to presence of components. For example, the condensate in conduit 58 having the lowest bubble point temperature will contain a high percentage of low boiling point components and will not contain the highest boiling point components. It is to be understood that the number of fractional condensation steps and hence the number of condensates of the multicomponent refrigerant may vary. As a specific example, the cubiole shown in the drawing may be modified to include three phase separators, such as

phase separators

38, 39 and 41 with passageways 18 and 22 removed and the

passageways

16, 17, 19, 20, 21, and 23 extended as required to provide heat exchange passageways extending substantially throughout the zone between the cold end of the passageway 19 and the warm end of the passageway 16.

The condensates are reduced in pressure by

valves

61, 63, 65, 67, and 59 to pressures existing in the shell space 14 in the regions of respective liquid distributor means 26, 27, 28, 29, and 30. Such pressures will correspond, except for variations due to the pressure gradient along the heat exchange device 10, to the inlet pressure of the compressor 36 which is preferably above atmospheric pressure. The

condensates

72, 73, 74, and 75, prior to pressure reduction, are subcooled upon flowing through

passageways

20, 21, 22, and 23, respectively; however, in some cycles, the subcooling step may be employed. The condensates under reduced pressure are fed by

conduits

31, 32, 33, 34, and 35 to respective liquid distributor means 26, 27, 28, 29, and 30, each located in the shell space 14 where the temperature therein corresponds substantially to the temperature of the respective condensates under reduced pressure. The condensate from the distributor means 30 is vaporized upon heat interchange with relatively warm fluid flowing through the upper part, as viewed in the drawing, of the passageway 15 and the condensates from the distributor means 26, 27, 28, and 29 are vaporized by heat interchange with relatively warm fluid in the passageway 15 and by heat interchange with relatively warm fluids flowing through the pairs of

passageways

16 and 20, 17 and 21, 18 and 22, and 19 and 23, respectively. The condensates, being mixtures of components ofdifferent boiling points, will vaporize within a range of increasing temperatures and the vapor from all of the condensates will flow through the shell space 14, in a direction from the cold end 12 to the warm end 11, and leave the heat exchange device through the compressor feed conduit 42. it will thus be appreciated that the refrigerant, in liquid and vapor phases, flows through the vapor space 14 in countercurrent heat interchange with the natural gas feed in the passageway 15 and with portions of the refrigerant in

passageways

16, 17, 18, 19, 20, 21, 22, and 2.3 to effect cooling, liquefaction and subcooling of the natural gas feed and the plural fractional condensation steps to provide the condensates and to effect subcooling of the condensates.

The efficiency of a multicomponent refrigeration cycle as described above depends upon a large number of parameters which are complexly interrelated; certain of such parameters depend directly while others depend indirectly on the composition of the refrigerant entering the suction inlet of the compressor 36 through the conduit 42; such refrigerant is the multicomponent refrigerant of the process and the term multicomponent refrigerant used herein and in the appended claims refers to the composition of the mixture of gases as exists when compressed from the relatively low vaporization pressure to the relatively high condensation pressure. In order to cool the liquefied natural gas to desired subcooled temperature, it is necessary that the condensate discharged from the liquid distributor means 30 be at a sufficiently lower temperature. Thus, the multicomponent refrigerant must include a component having a boiling point temperature below the subcooled temperature of the feed mixture and, in addition, the quantity of such component in the multicomponent refrigerant must be such as to provide the proper composition in the condensate from the liquid distributor 30 after the preceding fractional condensation steps, to attain the required low temperature. Also, the composition of the

condensates

72, 73, 74, and 75 will depend upon the composition of the multicomponent refrigerant, the exhaust pressure of the compressor 36 and the temperature at which the fractional condensation occurs, and the latter temperatures will likewise depend upon the composition of the multiple component refrigerant, and also upon the inlet pressure of the compressor 36, that is, the vaporization pressure of the condensates, and a relationship involving the quantity of available refrigeration utilized in cooling the feed stream. Furthermore, insofar as compression efficiency is concerned, the inlet pressure and exhaust pressure of the compressor 36 may be advantageously set at specific superatmospheric pressures. in addition, the overall efficiency of the process depends upon the heat interchange efficiency between the natural gas feed flowing through the passageway 15 and the multicomponent refrigerant flowing in the shell space 14 in countercurrent heat interchange therewith. in order to obtain optimum heat exchange efficiency, it is necessary to establish and maintain between the natural gas feed and the multicomponent refrigerant a specific temperature difference proportional to the absolute temperature level. The temperature difference between the countercurrently flowing fluids in the passageway 15 and the shell space 14 will depend upon a number of factors which include the composition of the multicomponent refrigerant, that is, the specific components of the refrigerant and the percentage composition of such components.

it has been determined that a natural gas liquefaction process using a multicomponent refrigeration as described above may be operated with optimum efficiency but employing a multicomponent refrigerant consisting of specific components in specific percentage relationships by satisfying the optimum requirements of parameters of the process including optimum heat interchange between the natural gas feed and the multicomponent refrigerant. in accordance with the principles of the present invention, a novel multicomponent refrigerant which achieves the foregoing results is broadly characterized to include the following:

1. A mixture of component gases having different boiling point temperatures including nitrogen, methane and hydrocarbons having two or more carbon atoms.

2. The component having the third lowest boiling point temperature comprising the component of greatest percentage of the mixture.

3. The component having the second lowest boiling point temperature comprising the component of second greatest percentage of the mixture.

The foregoing broad characteristics and other distinguishing characteristics of the multicomponent refrigerants provided by the present invention have been embodied in the actual design of multicomponent refrigeration systems for effecting liquefaction of natural gas of varying compositions as demonstrated by the specific examples appearing hereinafter.

FIG. 2 is a simplified schematic diagram illustrating the major flow circuits comprising a complete liquefaction facility or plant. Referring first to the upper left-hand portion of FIG. 2 of the drawings, the natural gas feed is supplied to the liquefaction plant through a pipeline as a two-phase mixture having a major portion in the gaseous phase and a minor portion in the liquid phase. This feed mixture is initially separated in a separator 112 from which the minor portion is withdrawn at the bottom as a liquid and pressurized by a liquid pump 114. The major portion is withdrawn in the gaseous phase from the top of the drum, compressed in compressor 1 16 and heat exchanged with cooling water in exchanger 118. The two portions of the feed are then joined and expanded into a flash drum wherein a portion of the original gaseous feed is liquefied and mixes with that portion of the feed which was initially in the liquid phase. The total liquid comprising most of the heavy hydrocarbons (i.e., heavier than C hydrocarbons) is then withdrawn from the bottom of flash drum 120 and supplied through a line 122 to a conventional fractionation plant 123, the general operation of which will be described hereinafter although the particular details of the plant form no part of the present invention. The gaseous fraction of the feed comprising most of the methane, nitrogen and the C to C hydrocarbons is withdrawn from the top of flash drum 120 and passed through one or more conventional absorbers 124 which remove impurities such as hydrogen sulfide and carbon dioxide. The gaseous feed is then slightly cooled by heat exchange with cooling water in exchanger 126 and passed through line 128 to the lowermost heat exchanger coil 130 located in the bottom of main heat exchanger 132 wherein the stream is sufficiently cooled so that the water and most of the C and C hydrocarbons are condensed and separated out in the first stage 134 of a two-stage separator 136. The major portion of the stream is withdrawn in the gaseous phase through line 138 and is passed through one or more driers 141) wherein the remaining water is removed. After drying, the main stream flows through line 142 to intermediate heat exchange coils 144 wherein the stream is further cooled to form a second liquid fraction which is separated in the second stage 146 of separator 136; this fraction containing most of the C and C hydrocarbons. The major portion of the main stream remains in the gaseous phase and is conducted through lines 150 and 151 to low temperature coil 152 of the main exchanger 132 wherein the feed stream is totally liquefied. In order to prevent vapor losses during subsequent expansion of the LNG to atmospheric pressure, the pressure of the liquefied natural gas is reduced from 660 to 206 p.s.i.a. by passage through expansion valve 15d prior to passage through exchanger coil 156 wherein the LNG is subcooled. Thus, the provision of valve 1554 enables the enthalpy of the LNG to be reduced so that the liquid is not vaporized during the final pressure letdown in passing through expansion valve 158 after which the liquid is maintained at atmospheric pressure in storage tank 159.

From the foregoing description of the main process stream, it is apparent that all of the refrigeration required to liquefy and subcool the feed stream (except for the very small amount of refrigeration provided by water coolers 118 and 126) is provided by main exchanger 132. This exchanger is an integral unit composed of a plurality of

cylindrical shell segments

166, 162, 164 and 166 connected by a plurality of frustoconical thin-H: mun

transition sections

168, 170, and 172. in addition to the feed stream coils previously mentioned, the exchanger includes a plurality of

refrigerant coils

174, 176, 178, and 180 as well as a plurality of refrigerant spray headers 182, 18 3, 186, 188, and 190.

The above-mentioned refrigerant coils and spray headers, together with a multistage refrigerant separator 192 and a refrigerant compressor 194-, form the entire refrigeration system which will now be described in detail beginning with compressor inlet line 196 shown in the bottom right-hand corner of FIG. 2 of the drawings. Line 196 contains a single gaseous refrigerant which is a mixture of a plurality of component gases hereinafter referred to as a multicomponent refrigerant (MCR). For example, a preferred multicomponent refrigerant consists by volume of 31 parts methane, 35 parts ethane, 7 parts propane, 14 parts butane, 4 parts pentane, 3 parts hexane, and 6 parts nitrogen. This multicomponent refrigerant is compressed in stage A of compressor 194 and cooled in interstage water cooler 197 so that a portion is condensed and then separated in separator 198, The condensate is withdrawn from the bottom of the separator and pumped directly into the first stage 199 of MCR separator 192. The gaseous fraction of the refrigerant is withdrawn from the top of separator 198, compressed in stage B, cooled in water cooler 200 and joined with the previously mentioned condensate which is supplied to separator 1.92 at a pressure in the order of l 5 p.s.i.a. and at a temperature of 100 F.

in the first stage 199 of MCR separator 192, the liquid fraction rich in C and heavier hydrocarbons is separated and supplied through line 201 and pressure reduction valve 202 to spray header 182, from which it is sprayed downwardly over the lower portions of coil 152 as well as

coils

130, 144, and 174 whereby the liquid refrigerant is vaporized in cooling the fluids in the coils.

Referring back to separator 192, the gaseous fraction is withdrawn from stage 199 through line 203, cooled in coil 174 and returned to the second stage 204 of the separator wherein a second fraction of liquefied refrigerant is separated. This liquid fraction at a temperature in the order of 17 F. is rich in the C, to C hydrocarbons and is supplied through line and pressure reduction valve 207 to spray header 184 which is positioned above coil 176 and at an intermediate point along coil 152. Thus, this second liquid fraction of the refrigerant is sprayed over

coils

152 and 176 whereby the refrigerant is vaporized in cooling the fluids in these coils.

The gaseous fraction in stage 204 is withdrawn through line 208, cooled in coil 176, and returned to the third stage 210 of the separator wherein a third fraction of liquefied refrigerant is separated. This liquid fraction at a temperature in the order of minus 71 F. is rich in the C, to C hydrocarbons and is supplied through line 212 and pressure reduction valve 213 to spray header 186 which is positioned above coil 178 and at a point near the upper portion of coil 152. Thus, this fraction of the liquid refrigerant is sprayed over coil 178 and the intermediate portion of coil 152 whereby the refrigerant is vaporized in cooling the fluids in these coils.

The gaseous fraction in stage 210 of the separator is withdrawn through line 214, cooled in coil 178 and returned to the fourth stage 216 of the separator wherein a fourth liquid fraction is separated. This liquid fraction at a temperature in the order of minus 140 F. is rich in nitrogen and the C, to C hydrocarbons and is supplied through line 218 and pressure reduction valve 219 to spray header 188 which is positioned above

coils

152 and 180. Thus, the fourth liquid fraction is vaporized in cooling the feed in the upper end of coil 152 as well as the last remaining fraction of the refrigerant which is supplied to coil 160 through line 220 from stage 216. This last fraction of the refrigerant rich in nitrogen and methane is liquefied in passing through coil 180 from which it exits at a temperature in the order of minus 206 F. and is reduced in pressure by passage through expansion valve 222 whereby its temperature drops to minus 259 F. Thereafter, it is supplied to spray header 1% which is positioned above subcooling coil 156 so that the last refrigerant fraction is vaporized in sub cooling the feed stream in coil 156 to a temperature in the order of minus 258 F.

From the foregoing description, it is apparent that each of the liquid fractions of the multicomponent refrigerant is vaporized by heat exchange with the feed stream and high pressure refrigerant fractions at specific temperature levels. For example, the temperature levels in the vicinity of

spray headers

182, 184, 186, 188, and may be in the order of 9 F., minus 79 F., minus 146 F., minus 228 F., and minus 259 F., respectively. At the same time, the temperatures of the MCR fractions downstream of

pressure reduction valves

202, 207, 213, 219, and 222 are in the order of 27 F., minus 66 F., minus 142 F., minus 220 F., and minus 259 F.

After being vaporized in heat exchanger 132, each of the MCR fractions are recombined, withdrawn through line 224 at a pressure in the order of 41 p.s.i.a. and recycled back to compressor 194 along with a small amount of makeup refrigerant which is supplied through line 226. This makeup refrigerant, as well as the original charge of refrigerant, is obtained from the feed stream except for the nitrogen which is supplied from an air separation plant 228 through control valve 229. That is, the liquid fractions in

stages

134 and 146 of feed separator 136 are withdrawn through

lines

230 and 232, dried in drier 234, and supplied to the previously mentioned fractionation plant 123 through line 236. This plant is conventional in that it consists of a plurality of fractionation columns which separate the natural gas feed from line 236 into the components. Thus, predetermined amounts of ethane, propane, butane, pentane, and hexane are withdrawn through

lines

238, 240, 242, 244, and 246 as determined by control valves 248 while methane is added to makeup line 226 through branch line 250 and control valve 252. in order to maintain a desired heating value of the LNG, controlled amounts of the C, to C hydrocarbons are withdrawn from the fractionation plant through line 254 and added to main process stream in line 150.

From the foregoing description of the liquefaction plant of FIG. 2 of the drawings, it will be apparent that significant economies in initial capital investment are possible due to the fact that the utilization of a single refrigerant requires a single compressor as opposed to the utilization of separate refrigerants in cascade wherein each refrigerant requires a separate compressor. In addition, the effective utilization of more than three hydrocarbons plus nitrogen substantially reduces the compression horsepower since closer matching of the cooling curves is possible at each temperature level in exchanger. Furthermore, substantial functional as well as economic advantages are obtained from the utilization of a one-piece, multitemperature level exchanger as opposed to a plurality of individual exchangers operating over individual temperature ranges which cannot be matched so exactly to the optimum cooling curve of the feed stream. Significant advantages in the cost and ease of fabrication also flow from the combination of utilizing more than three hydrocarbons as an MCR refrigerant in an integral exchanger in that at least the majority if not all of the spray headers may be physically positioned between adjacent MCR coils as opposed to their physical location intermediate the inlet and outlets of the coils. Lastly, the utilization of pressure reduction valve 154 reduces the undesired flash losses of the liquefied product while the utilization of interstage phase separator 198 decreases operating horsepower requirements. Of course, it is to be understood that the foregoing description is intended to be illustrative rather than exhaustive of the invention and that the latter is not to be limited other than as expressly set forth in the claims including all patentable equivalents thereof.

The following specific examples further illustrates the invention.

lllllllfi llllfll ExA'MTi Lm A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen l.l2 Methane 70.02 C, 15.23 c, 8.06 10 c, 3.80 c, L28 u 0.49

The natural gas feed, freed of hydrogen sulfide, carbon dioxide and water is introduced into the process at about 100 F. and under a pressure of about 620 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicom' ponent refrigerant fractionated by partial condensation in three successive steps. After initial cooling to effect liquefac' tion of high boiling point components and after separation of such components, natural gas feed of the following composition is flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 1.15

Methane 71.03

Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average mol'ecular weight of 34.94:

Component Mol Percent by Volume Nitrogen 0.8l

Methane 88.20

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water is introduced into the process at about F. and under a pressure of about 620 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. After initial cooling to effect liquefaction of high boiling point components and after separation of such components, natural gas feed of the following composition is flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 0.83 Methane 9015 C, 4.19 2.74 c, 1.67 C, 0.32 u 0.10

Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about -262.5 F. The multicom ponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 33.42:

Component Mol Percent by Volume Nitrogen 4.85 Methane 32.50 C, 315.50 c, 6.55 C, 8.50 C, l0.80 C 0.30

EXAMPLE III A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.7] Methane 89.l3 C, 5.68 c 2.46 C, 1.1 1 C 0.49 C, 0.42

The natural gas feed, free of hydrogen sulfide and carbon dioxide, is introduced into the process at about 100 F. and under a pressure of about 650 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. After initial cooling to effect liquefaction of high boiling point components and after separation of such components, natural gas feed of the following composition if flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 0,74 Methane 93.2l 5.19 C; 0.72 C4 0.10 5. 0.04

Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 264 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 42 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 34.28:

Component Mol Percent by Volume Nitrogen 4.40 Methane 27.40 c, 41.00 C, 5.50 c, 12.50 c, 9.20

EXAMPLE IV A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.43 Methane 99.49 c, 0.06 c 0.01 C C,. C 0.01

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about 60 F. and under a pressure of about 600 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. The natural gas feed flows from the zone totally in liquid phase under a pressure of about 550 p.s.i.a. and at a temperature of about 263 F. The multicomponent refrigerant entered the suction of the compressor at about 50 p.s.i.a. and was discharged from the compressor at about 465 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 34.49:

Component Mol Percent by Volume Nitrogen 5.42 Methane 22.50 C, 41.70 C 11.92 C. 12.43 C 6.03

EXAMPLE V A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 1.18 Methane 66.98 C, 17.22 c, 8.94 4.03 c, 1.27 c. 0.38

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about 100 F. and under a pressure of about 615 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. After initial cooling to effect liquefaction of high boiling point components and after separation of such components, natural gas feed of the following composition is flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 1.20 Methane 68.34

Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 34.73:

undo

Component Mol Percent by Volume Nitrogen 4.85 Methane 27.80 C, 38.00 c 6.00 C, 14.50 C, 8.55 C 0.30

EXAMPLE VI A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component M01 Percent by Volume Nitrogen 0.36 Methane 99.54 C, 0.10

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about F. and under a pressure of about 615 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 3 l .03:

Component Mol Percent by Volume Nitrogen 5.0 Methane 32.6 C, 43.11 C, 2.0 C 1 1.0 c, 5.0

EXAMPLE Vll A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.36 Methane 99.54 C, 0.10

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about 100 F. and under a pressure of about 620 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. Before leaving the heat exchange zone, the natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 33.17:

Component Mol Percent by Volume Nitrogen 5.5 Methane 24.0 C, 43.5 C, 12.0 C, 10.0 C 50 EXAMPLE Vlll A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about 100 F. and under a pressure of about 615 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. The natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 34.45:

Component Mol Percent by Volume Nitrogen 7.0 Methane 29.5 C: 37.5 C; 3.6 C, 9.0 C, l3.4

EXAMPLE IX A multicomponent refrigeration process of the type described herein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.11 Methane 90.33 1 5.44 C 2.03 C, 1.30 c, 0.46 C, 0.33

The natural gas feed, free of hydrogen sulfide, carbon dioxide and water, is introduced into the process at about 100 F. and under a pressure of about 620 p.s.i.a. in a heat exchange zone in countercurrent heat interchange with a multicomponent refrigerant fractionated by partial condensation in three successive steps. The natural gas feed was reduced in pressure to about 150 p.s.i.a. and flowed from the zone totally in liquid phase at a temperature of about 263 F. for subsequent storage under a pressure of about 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponent refrigerant entered the suction of the compressor at about 40 p.s.i.a. and was discharged from the compressor at about 450 p.s.i.a. The multicomponent refrigerant entering the compressor consisted of the following mixture having an average molecular weight of 34.03:

From the foregoing examples, it will be appreciated that each multicomponent refrigerant disclosed comprises a mixture of component gases including nitrogen, methane and hydrocarbons having two or more carbon atoms; that nitrogen is the component of lowest boiling point temperature; that the hydrocarbon having two carbon atoms, the component having the third lowest boiling point temperature, comprises the component of greatest percentage of the mixture and that methane, the component having the second lowest boiling point temperature, comprises the component of second greatest percentage of the mixture. It will be further appreciated from the foregoing examples that, in multicomponent refrigerants including hydrocarbons having three, four and five carbon atoms, the component comprising the third greatest percentage of the mixture is selected from the group consisting of a hydrocarbon having three carbon atoms, a hydrocarbon having four carbon atoms and a hydrocarbon having five carbon atoms. It has been determined that, by using multicomponent refrigerants of compositions provided by the present invention, natural gas may be liquefied with a higher overall efficiency as compared to use of multicomponent refrigerants of compositions provided by the prior art. It is believed that the increased efficiency results at least in part by the provision of a multicomponent refrigerant in which the component having the third lowest boiling point temperature comprises the component of greatest percentage of the mixture; such component being less than 50 percent of the mixture and preferably within a range of about 35 percent to 45 percent of the mixture. Another characteristic of the multicomponent refrigerants provided by the present invention not found in prior multicomponent refrigerants is the feature that the components of the greatest percentage and the second greatest percentage of the mixture together make up more than 50 percent of the mixture, preferably falling within a range of about 64 percent to about 77 percent of the mixture. The percentage composition of the methane and hydrocarbon having two carbon atoms in multicomponent refrigerants including heavier hydrocarbons provides mixtures 0 having average molecular weights which fall within a range of about 31 to about 35. In this regard, it will be appreciated that natural gas, a multicomponent refrigerant provided by the prior art, usually has an average molecular weight of 20 or less but in any event does not exceed 25. Of the natural gas feeds in the examples, only the natural gas feeds of Examples 1 and V have average molecular weights in excess of 20, Le, 23.23 and 23.77, respectively. Also, the multicomponent refrigerant suggested by Kleemenko, i.e., 65 percent methane, 20 percent propane and 15 percent butane, has an average molecular weight of 27.96. Furthermore, it will be appreciated from the foregoing examples that, in addition to the multicomponent refrigerants having the foregoing characteristics, the present invention also provides multicomponent refrigerants having additional characteristics depending upon the component which makes up the third greatest percentage of the mixture. When the component of the third greatest percentage of the mixture comprises a hydrocarbon having three carbon atoms, a hydrocarbon having four carbon atoms comprises the component of fourth greatest percentage of the mixture and nitrogen comprises the component of fifth greatest percentage of the mixture. Also, when the component of the third greatest percentage of the mixture is a hydrocarbon having four carbon atoms, the component of the fourth greatest percentage of the mixture is selected from a group consisting of a hydrocarbon having three carbon atoms and a hydrocarbon having five carbon atoms; when the component of the fourth greatest percentage is a hydrocarbon having three carbon atoms, the component of the fifth greatest percentage comprises a hydrocarbon having five carbon atoms whereas, when the component of the fourth greatest percentage of the mixture is a hydrocarbon having five carbon atoms, the component of the fifth greatest percentage of the mixture is selected from a group consisting of nitrogen and a hydrocarbon having three carbon atoms. ln addition, when the component of the third greatest percentage is a hydrocarbon having five carbon atoms, the component of the fourth greatest percentage of the mixture is a hydrocarbon having four carbon atoms and the component of the fifth greatest percentage of the mixture is selected from a group consisting of nitrogen and a hydrocarbon having three carbon atoms.

There is thus provided by the present invention multicomponent refrigerants of novel composition adapted particularly for use in liquefying natural gas of varying composition. While the multicomponent refrigerants vary with respect to the percentage of the components, each embodies the discovery that the component of greatest percentage of the mixture comprises the component having the third lowest boiling point, Le, a hydrocarbon having two carbon atoms, e.g., ethane and/or ethylene, and that the component of next greatest percentage of the mixture comprises methane, the component having the second lowest boiling point. From the foregoing examples, it will be appreciated that, in addition to the variations discussed above, the percentage of the mixture comprising the hydrocarbon having two carbon atoms may vary within the range of about 35 percent to about 45 percent and that percentage of the methane in the mixture may vary within the range of about 22 percent to 36 percent. Reference therefore will be had to the appended claims for a definition of the limits of the invention.

We claim:

1. A method of liquefying a feed stream composed primarily of methane comprising:

a. compressing a multicomponent refrigerant comprising a plurality of individual hydrocarbon components having different boiling points,

b. said plurality of hydrocarbon components including a C hydrocarbon selected from the group of ethane and ethylene as the component of greatest percentage, and methane as the component of second greatest percentage,

c. said multicomponent refrigerant further including nitrogen comprising at least 4 percent by volume of the multicomponent mixture.

d. progressively condensing and phase separating said compressed multicomponent refrigerant into a plurality of liquid condensates and vapor portions of progressively colder temperatures,

. expanding each of said progressively colder condensates,

vaporizing each of said progressively colder condensates in heat exchange with portions of said feed stream and separated vapor portions of said multicomponent refrigerant so as to progressively condense said multicomponent refrigerant according to step (d) while progressively liquefying said feed stream, said vaporizations of said progressively colder condensates against said feed stream portions and said multicomponent vapor portions occurring at the same vaporization pressure for each condensate,

g. withdrawing said vaporized condensates from said heat exchanger means, and

h. recycling said withdrawn vaporized condensates as said multicomponent refrigerant in a closed cycle to a compressor for compressing the same in accordance with step a.

2. The method as claimed in claim 1 wherein the C hydrocarbon is present in an amount constituting at least 35 percent by volume of said multicomponent refrigerant.

3. The method as claimed in claim 1 wherein the average molecular weight of the multicomponent refrigerant is equal to or greater than 30.

4. A system for liquefying a feed stream composed primarily of methane comprising:

a. compressor means for compressing a multicomponent refrigerant comprising a plurality of hydrocarbons including a C hydrocarbon selected from the group of ethane and ethylene as the component of greatest percentage,

and further including substantially more than 3 percent by volume of a nonhydrocarbon component having a normal boiling point substantially below that of methane,

b. a plurality of heat exchange means and phase separator means connected in series for progressively condensing and separating said compressed multicomponent refrigerant into a plurality of liquid condensates and vapor portions decreasing temperatures,

c. expansion means for expanding each of said liquid condensates,

d. multistage heat exchanger means for vaporizing each of said expanded liquid condensates at the same vaporization pressure in each stage thereof, each of said heat exchanger stages including a shell portion and heat exchange passage means within each respective shell portion for passing one of said expanded liquid condensates in heat exchange relationship with both said feed stream and at least one of said vapor portions separated from said unexpanded condensate of said multicomponent refrigerant and undergoing condensation, and

. closed cycle means for withdrawing all of said vaporized condensates from said heat exchanger means and passing the same as said multicomponent refrigerant to said compressor means.

5. The system as claimed in claim 4 wherein the C hydrocarbon is present in an amount constituting at least 35 percent by volume of said multicomponent refrigerant.

6. The system as claimed in claim 4 wherein the average molecular weight of the multicomponent refrigerant is equal to or greater than 30.

7. The system as claimed in claim 4 wherein the combined percentage of the nonhydrocarbon component plus the C and heavier hydrocarbon components is greater than 11 percent but less than 21 percent by volume.

8. The method as claimed in claim 1 wherein the combined percentage of the nitrogen component plus the C and heavier hydrocarbon components is greater than 11 percent but less than 21 percent by volume.

2%? TED STATES PATENT o TtT cRTTTTcE tcTTN Patent No. 3.6%,106 a d February 29, 1972 Inventm-( Lee S. Gaumer, Jr. and Charles L. Newton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Line 9 of the Abstract after "gas" insert --to be liquefied to effect eooling; and liquefaction of the natural gas-- In the Specification:

Column 1, line 7, "preset" should read -present--;

' and "out" should read --our--.

Column 1, line 9, "out" should read --our-- Column 1, line 11, "Refrigerator" should read --Refrigerant-- Column 1, line 13, "Seru No 722,136" should read Ser, No. 722,135-- 1 Column 5, line 6 4, "but should read --by-- Signed and sealed this 26th day of September 1972.

(SEAL) Attest:

EDWARD M..FLETCHER,JR. ROBERT GOTTSCHALK ilttesting; Officer Commissioner of Patents-

Claims

2. The method as claimed in claim 1 wherein the C2 hydrocarbon is present in an amount constituting at least 35 percent by volume of said multicomponent refrigerant.
3. The method as claimed in claim 1 wherein the average molecular weight of the multicomponent refrigerant is equal to or greater than 30.
4. A system for liquefying a feed stream composed primarily of methane comprising: a. compressor means for compressing a multicomponent refrigerant comprising a plurality of hydrocarbons including a C2 hydrocarbon selected from the group of ethane and ethylene as the component of greatest percentage, and further including substantially more than 3 percent by volume of a nonhydrocarbon component having a normal boiling point substantially below that of methane, b. a plurAlity of heat exchange means and phase separator means connected in series for progressively condensing and separating said compressed multicomponent refrigerant into a plurality of liquid condensates and vapor portions decreasing temperatures, c. expansion means for expanding each of said liquid condensates, d. multistage heat exchanger means for vaporizing each of said expanded liquid condensates at the same vaporization pressure in each stage thereof, each of said heat exchanger stages including a shell portion and heat exchange passage means within each respective shell portion for passing one of said expanded liquid condensates in heat exchange relationship with both said feed stream and at least one of said vapor portions separated from said unexpanded condensate of said multicomponent refrigerant and undergoing condensation, and e. closed cycle means for withdrawing all of said vaporized condensates from said heat exchanger means and passing the same as said multicomponent refrigerant to said compressor means.
5. The system as claimed in claim 4 wherein the C2 hydrocarbon is present in an amount constituting at least 35 percent by volume of said multicomponent refrigerant.
6. The system as claimed in claim 4 wherein the average molecular weight of the multicomponent refrigerant is equal to or greater than 30.
7. The system as claimed in claim 4 wherein the combined percentage of the nonhydrocarbon component plus the C5 and heavier hydrocarbon components is greater than 11 percent but less than 21 percent by volume.
8. The method as claimed in claim 1 wherein the combined percentage of the nitrogen component plus the C5 and heavier hydrocarbon components is greater than 11 percent but less than 21 percent by volume.