Liquefaction Of Natural Gas Employing Multiple-component Refrigerants

Abstract

Natural gas is liquefied by heat exchange with a multicomponent refrigerant. The respective refrigerant components in the order of increasing volatility are heat exchanged serially with the natural gas stream at progressive points along zones of decreasing temperature.

Parent Case Text

This application is a continuation of application Ser. No. 722,135, filed Apr. 17, 1968 now abandoned, which is a continuation of application Ser. No. 468,008 filed June 29, 1965 now abandoned.

LIQUEFACTION OF NATURAL GAS

Description

The present invention 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 stream is heat exchanged with different refrigerants circulated in independent closed loops.

The above-indicated object, as well as other objects relating more particularly to specific structural and functional advantages, will become more fully apparent form the following description when taken with the accompanying drawing wherein the sole Figure is a simplified and schematic diagram illustrating the major flow circuits comprising the complete liquefaction facility or plant.

Referring first to the upper left-hand portion of the drawing, the natural gas feed is supplied to the liquefaction plant through a pipeline 10 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 This feed mixture is initially separated in a separator 12 from which the minor portion is withdrawn at the bottom as a liquid and pressurized by a liquid pump 14. The major portion is withdrawn in the gaseous phase from the top of the drum, compressed in compressor 16 and heat exchanged with cooling water in exchanger 18. The two portions of the feed are then joined and expanded into a flash drum 20 wherein a portion of the original gaseous feed is liquified 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.sub.6 hydrocarbons) is then hydrocarbons from the bottom of flash drum 20 and supplied through a line 22 to a conventional fractionation plant 23 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.sub.2 to C.sub.6 hydrocarbons is withdrawn from the top of flash drum 20 and passed through one or more conventional absorbers 24 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 26 and passed through line 28 to the lowermost heat exchanger coil 30 located in the bottom of main heat exchanger 32 wherein the stream is sufficiently cooled so that the water and most of the C.sub.6 and C.sub.5 hydrocarbons are condensed and separated out in the first stage 34 of a two-stage separator 36. The major portion of the stream is withdrawn in the gaseous phase through line 38 and is passed through one or more driers 40 wherein the remaining water is removed. After drying, the main stream flows through line 42 to intermediate heat exchange coils 44 wherein the stream is further cooled to form a second liquid fraction which is separated in the second stage 46 of separator 36; this fraction containing most of the C.sub.5 and C.sub.6 hydrocarbons. The major portion of the main stream remains in the gaseous phase and is conducted through lines 50 and 51 to low temperature coil 52 of the main exchanger 32 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 600 to 200 p.s.i.a. by passage through expansion valve 54 prior to passage through exchanger coil 56 wherein the LNG is subcooled. Thus, the provision of valve 54 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 58 after which the liquid is maintained at atmospheric pressure in storage tank 59.

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 18 and 26) is provided by main exchanger 32. This exchanger is an integral unit composed of a plurality of cylindrical shell segments 60, 62, 64, and 66 connected by a plurality of frustoconical transition sections 68, 70, and 72. In addition to the feed stream coils previously mentioned, the exchanger includes a plurality of refrigerant coils 74, 76, 78, and 80 as well as a plurality of refrigerant spray headers 82, 84, 86, 88, and 90.

The above-mentioned refrigerant coils and spray headers, together with a multistage refrigerant separator 92 and a refrigerant compressor 94, form the entire refrigeration system which will now be described in detail beginning with compressor inlet line 96 shown in the bottom right-hand corner of the drawing. Line 96 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 94 and cooled in interstage water cooler 97 so that a portion is condensed and then separated in separator 98. The condensate is withdrawn from the bottom of the separator and pumped directly into the first stage 99 of MCR separator 92. The gaseous fraction of the refrigerant is withdrawn from the top of separator 98, compressed in stage B, cooled in water cooler 100 and joined with the previously mentioned condensate which is supplied to separator 92 at a pressure in the order of 515 p.s.i.a. and at a temperature of 100.degree. F.

In the first stage 99 of MCR separator 92 the liquid fraction rich in C.sub.2 and heavier hydrocarbons is separated and supplied through line 101 and pressure reduction valve 102 to spray header 82 from which it is sprayed downwardly over the lower portions of coil 52 as well as coils 30, 44, and 74 whereby the liquid refrigerant is vaporized in cooling the fluids in the coils.

Referring back to separator 92, the gaseous fraction is withdrawn from stage 99 through line 103, cooled in coil 74, and returned to the second stage 104 of the separator wherein a second fraction of liquefied refrigerant is separated. This liquid fraction at a temperature in the order of 17.degree. F. is rich in the C.sub.1 to C.sub.4 hydrocarbons and is supplied through line 106 and pressure reduction valve 107 to spray header 84 which is positioned above coil 76 and at an intermediate point along coil 52. Thus, this second liquid fraction of the refrigerant is sprayed over coils 52 and 76 whereby the refrigerant is vaporized in cooling the fluids in these coils.

The gaseous fraction in stage 104 is withdrawn through line 108, cooled in coil 76, and returned to the third stage 110 of the separator wherein a third fraction of liquefied refrigerant is separated. This liquid fraction at a temperature in the order of minus 71.degree. F. is rich in the C.sub.1 to C.sub.3 hydrocarbons and is supplied through line 112 and pressure reduction valve 113 to spray header 86 which is positioned above coil 78 and at a point near the upper portion of coil 52. Thus, this fraction of the liquid refrigerant is sprayed over coil 78 and the intermediate portion of coil 52 whereby the refrigerant is vaporized in cooling the fluids in these coils.

The gaseous fraction in stage 110 of the separator is withdrawn through line 114, cooled in coil 78 and returned to the fourth state 116 of the separator wherein a fourth liquid fraction is separated. This liquid fraction at a temperature in the order of minus 140.degree. F. is rich in nitrogen and the C.sub.1 to C.sub.2 hydrocarbons and is supplied through line 118 and pressure reduction valve 119 to spray header 88 which is positioned above coils 52 and 80. Thus, the fourth liquid fraction is vaporized in cooling the feed in the upper end of coil 52 as well as the last remaining fraction of the refrigerant which is supplied to coil 80 through line 120 from stage 116. This last fraction of the refrigerant rich in nitrogen and methane is liquified in passing through coil 80 from which it exits at a temperature in the order of minus 206.degree. F. and is reduced in pressure by passage through expansion valve 122 whereby its temperature drops to minus 259.degree. F. Thereafter, it is supplied to spray header 90 which is positioned above subcooling coil 56 so that the last refrigerant fraction is vaporized in subcooling the feed stream in coil 56 to a temperature in the order of minus 258.degree. 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 a specific temperature level. For example, the temperature levels in the vicinity of the spray headers 82, 84, 86, 88, and 90 may be in the order of 9.degree. F., minus 79.degree. F., minus 146.degree. F., minus 222.degree. F., and minus 259.degree. F., respectively. At the same time, the temperatures of the MCR fractions downstream of pressure reduction valves 102, 107, 113, 119, and 122 are in the order of 27.degree. F., minus 66.degree. F., minus 142.degree. F., minus 220.degree. F., and minus 259.degree. F.

After being vaporized in heat exchanger 32, each of the MCR fractions are recombined, withdrawn through line 124 at a pressure in the order of 41 p.s.i.a. and recycled back to compressor 94 along with a small amount of makeup refrigerant which is supplied through line 126. 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 128 through control valve 129. That is, the liquid fractions in stages 34 and 46 of feed separator 36 are withdrawn through lines 130 and 132, dried in drier 134, and supplied to the previously mentioned fractionation plant 23 through line 136. This plant is conventional in that it consists of a plurality of fractionation columns which separate the natural gas feed from line 136 into the components. Thus, predetermined amounts of ethane, propane, butane, pentane, and hexane are withdrawn through lines 138, 140, 142, 144, and 146 as determined by control valves 148 while methane is added to makeup line 126 through branch line 150 and control valve 152. In order to maintain a desired heating value of the LNG, controlled amounts of the C.sub.1 to C.sub.5 hydrocarbons are withdrawn from the fractionation plant through line 154 and added to main process stream in line 50.

From the foregoing description of the liquefaction plant 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 the 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 54 reduces the undesired flash losses of the liquefied product while the utilization of interstage phase separator 98 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 following claims including all patentable equivalents thereof.

Claims

We claim:

1. A method of liquefying and subcooling a natural gas stream comprising the steps of:

a. compressing said natural gas to a first relatively high pressure,

b. flowing said compressed natural gas through a confined passageway of an elongated heat exchange zone, said confined passageway having an inlet and an outlet,

c. reducing the pressure of said compressed natural gas in said confined passageway intermediate said inlet and said outlet,

d. compressing a multicomponent refrigerant having a plurality of components of different normal boiling points,

e. forming a plurality of liquid fractions of said multicomponent refrigerant by successively partially liquefying and fractionating said multicomponent refrigerant in heat exchange with said liquid fractions after expansion thereof,

f. liquefying said natural gas in said confined passageway by successive indirect heat exchange with said liquid fractions after expansion thereof, and

g. subcooling said liquified natural gas in said confined passageway after reducing the pressure thereof to a second lower pressure by indirect heat exchange and vaporization of at least one of said liquid fractions.

2. A method of liquefying and subcooling a methane-rich stream comprising:

a. compressing said methane-rich stream to a first pressure and passing said stream through a first heat exchange zone,

b. expanding said stream to a lower pressure after passage through said first heat exchange zone,

c. passing said expanded stream through a second heat exchange zone,

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

e. partially condensing said multicomponent refrigerant to form a first vapor fraction and a first liquid fraction, expanding said first liquid fraction and passing some of said expanded first liquid fraction in countercurrent heat exchange with said multicomponent refrigerant undergoing partial condensation,

f. passing another portion of said expanded first liquid fraction through said first heat exchange zone to condense said methane-rich stream,

g. partially condensing said first vapor fraction to form a second liquid fraction, expanding said second liquid fraction and passing some of said expanded second liquid fraction in countercurrent heat exchange with said first vapor fraction undergoing partial condensation, and

h. passing another portion of said second expanded liquid fraction through said second heat exchange zone to subcool said condensed methane-rich stream.

3. A system for liquefying a methane-rich stream comprising:

a. means forming an elongated, vertically extending heat exchanger having a warm end and a cold end,

b. first fluid passage means extending through said exchanger from said warm end to said cold end,

c. first compressor means for compressing said methane-rich stream and passing said stream through said first fluid passage means from said warm end to said cold end,

d. second compressor means for compressing a multicomponent refrigerant comprising a plurality of components having different boiling points,

e. a series of phase separators, each of said separators having an inlet, a liquid fraction outlet and a vapor fraction outlet,

f. passage means connecting said second compressor means to the first separator of said series,

g. a series of refrigerant passage means located in said exchanger and positioned along the vertical length thereof,

h. passage means connecting said vapor fraction outlet of each separator to one of said refrigerant passage means,

i. passage means connecting each of said refrigerant passage means to the inlet of the next separator for progressive partial condensation of said multicomponent refrigerant to form a plurality of liquid refrigerant fractions,

j. means for spraying each of said liquid refrigerant fractions in countercurrent heat exchange with said first fluid passage means for progressively vaporizing each liquid fraction and liquefying said methane-rich stream, and

k. closed cycle passage means for collecting all of said vaporized liquid fractions at the warm end of said exchanger and returning them to said second compressor means.

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

a. first compressor means for compressing a multicomponent refrigerant comprising a plurality of components having different boiling points,

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 of decreasing temperatures,

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

d. multistage heat exchanger means for passing each of said expanded liquid condensates in countercurrent heat exchange with said feed stream at progressively lower temperature stages for liquefying said feed stream, said multistage heat exchanger means including a final lowest temperature stage, at least said lowest temperature stage comprising a vertically extending coil through which said feed stream is passed upwardly, and means for spraying said lowest temperature liquid condensate downwardly over said coil.

5. A method of liquefying a compressed natural gas stream comprising the steps of:

a. precooling and partially condensing said compressed natural gas stream to form a first liquid fraction and a first vapor fraction,

b. separating said vapor and liquid fractions,

c. passing said first vapor fraction through a confined passageway of a heat exchange zone from the warm end thereof to the cold end thereof,

d. passing said first liquid fraction through independent fractionation means to fractionate said liquid into a plurality of substantially pure hydrocarbon components,

e. combining selected amounts of each of said plurality of hydrocarbon components to maintain a multicomponent refrigerant of selected composition,

f. maintaining independently of said natural gas stream and said fractionation means a separate source of a nonhydrocarbon component having a boiling point substantially below that of methane,

g. introducing selected amounts of said nonhydrocarbon component from said separate source into said multicomponent refrigerant so as to substantially reduce the boiling point of said multicomponent refrigerant,

h. forming a series of multicomponent liquids fractions of said multicomponent refrigerant at progressively lower temperatures by successive partial liquefaction of said multicomponent refrigerant in heat exchange with expanded liquid fractions thereof in closed cycle passage means, the lowest temperature multicomponent liquid fraction so formed being rich in said nonhydrocarbon component having a boiling point substantially below methane,

i. introducing said multicomponent liquid fractions, including said lowest temperature liquid fraction, into said heat exchange zone at a plurality of locations intermediate said warm and cold ends thereof and flowing each of said multicomponent liquid fractions in countercurrent heat exchange with said first vapor fraction in said confined passageway,

j. progressively vaporizing each of said multicomponent liquid fractions in countercurrent heat exchange with said first vapor fraction so as to liquefy said first vapor fraction,

k. withdrawing all of said vaporized multicomponent fractions from said heat exchange zone, and

l. recombining all of said withdrawn vaporized multicomponent fractions to reconstitute the major portion of said multicomponent refrigerant.