US3162519A

US3162519A - Liquefaction of natural gas

Info

Publication number: US3162519A
Application number: US745390A
Authority: US
Inventors: Frank M Peters; Lury James De
Original assignee: Conch International Methane Ltd
Current assignee: Conch International Methane Ltd
Priority date: 1958-06-30
Filing date: 1958-06-30
Publication date: 1964-12-22
Anticipated expiration: 1981-12-22
Also published as: FR1227903A

Description

Dec. 22, 1964 Filed June 50, 1958 "31 1 www Bum 1am M w kw M N 5 1.. 9m QN Y B NW \QN ww @m w Qw 1 AW ATTORNEYS United States Patent 3,162,519 EQUEFACTION 0F NATURAL GAS Frank M. Peters, Mission, Kane, and .lames De Lory,

Kansas City, Mo, assignors, by inesne assignmentajo Conch International Methane Limited, Nassau, Bahomes, a corporation of the Bahamas Filed lune 3t), 1%8, Ser. No. 74.55%

11 Claims. (ill. 62-23) This invention relates to the liquefaction of a gas and, more particularly, to a method and apparatus for the liquefaction of natural gas which is normally composed mostly of methane but which may contain heavier hydrocarbons, such as ethane, propane, butane and the like, small amounts of aromatic hydrocarbons and variable amounts of non-hydrocarbons such as nitrogen, helium, carbon dioxide, hydrogen sulfide and the like. Illustration of this invention will hereafter be made with reference to the liquefaction of natural gas, but it will be understood that the concepts employed are also capable of application to other low boiling liquefiable gases, such as nitrogen, helium, air, oxygen and the like.

There are many purposes for which natural gas is desired to be reduced to a liquefied state. The main reason resides in the resultant reduction, at equivalent pressure, by about 3 in volume when reduced from the gaseous state to a liquefied state, thereby enabling storage and transportation in containers of more economical and practical design.

For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate under substantially constant high load factor. Gftentimes the flow capacity will exceed demand, while at other times the demand may exceed the capacity of the line. In order to shave oif the peaks where demand would exceed supply, it is desirable to store gas when the supply exceeds demand, whereby peaks in demand can be met by material in storage. For this purpose, it is desirable to provide for storage in a liquefied state and to vaporize liquid in amounts to meet demand.

Liquefaction of natural gas is of even greater importance in making it possible to transport the gas from a source of plentiful supply to a distant market where a deficiency exists, especially when the source of supply cannot be directly joined with the market by a pipeline or the like means for the transportation of the gaseous fuel in a gaseous state. By way of illustration, surplus natural gas is available in the Gulf States of the United States, in Venezuela, and in the Persian Gulf, while deficiencies exist in the northern parts of the United States, the European countries, and Japan, yet these sources of supply cannot be joined by pipeline with some of the markets. Ship transportation in the gaseous state would be uneconorrieal unless the gaseous materials were highly compressed and then the system would not be commercial because it would be impractical to provide containers or" suitable strength and capacity.

it has been determined that natural gas, when shipped from the United States or Venezuela in large volumes in liquefied state, can be made available in Great Britain, for example, at a price which is considerably less than locally manufactured gas. For shipment in large volume, it is desirable to house the liquefied natural gas in suitable insulated containers of large capacity at about atmospheric pressure, or preferably slightly above atmospheric, but not at such high pressures as would unduly limit the economical capacity of the tank. Depending upon the amount of higher boiling heavier hydrocarbons present in the natural gas, the liquefied natural gas will have a boiling point within the range of 240 F. to --258 F. at atmospheric pressure.

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The present invention contemplates a novel method of liquefying gas normally available at at least a slightly elevated pressure, wherein the gas is first cooled to a liquefied state without substantially reducing the pressure of the natural gas stream, and then the liquefied gas stream is expanded to a pressure suitable for transportation of the liquefied gas. Vapors produced by expanding the liquefied gas to the transporting pressure, along with vapor boiling-oil of the receiving vessel, are added to a methane refrigeration cycle. In the methane refrigeration cycle, methane vapors are compressed and cooled, and at least a portion of the compressed vapors are expanded through a work-producing zone to produce a low-temperature refrigerant which in turn is used for liquefying the natural gas stream. The present invention also contemplates the use of a nitrogen stripping tower for removing nitrogen from the natural gas feed stream, wherein the removed nitrogen is also utilized in the liquefaction of the natural gas feed stream. Therefore, the refrigerants for cooling the natural gas stream to a liquefied state are obtained from the original natural gas feed stream, to greatly simplify the supply prob lems involved in operation of a commercial liquefaction system. All of the refrigerants used in a system practicing this invention may be obtained from the natural gas feed stream. This invention further contemplates the use of a portion of the methane refrigerant for maintaining the desired temperature in the nitrogen stripper, to utilize the maximum amount of refrigeration developed by the method, and to reduce energy requirements.

An important object of this invention is to provide a method of liquetying gas, wherein the refrigerants used in liquefying the gas are obtained from the gas feed stream.

Another object of this invention is to remove nitrogen from a natural gas feed stream during the liquefaction of the feed stream, and utilize the refrigeration of the removed nitrogen in the liquefaction of the feed stream.

Another object of this invention is to provide a novel expander cycle for methane vapors in a natural gas liquefaction system to provide refrigerants which are in turn used in the liquefaction of the gas.

A further object of this invention is to minimize the energy required in the liquefaction of a natural gas feed stream.

A still further object of this invention is to provide a method of liquefying gas which requires the use of a minimum amount of equipment, and wherein a system practicing the invention will have a long service life and will require a minimum of maintenance.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawing which illustrates this invention.

The single figure of the drawing is a flow diagram illustrating a preferred practice of the present invention.

The process will hereinafter be described in detail with reference to the liquefaction of natural gas at a source of supply, using an operative set of temperature and pressure conditions. We want it understood, however, that the conditions set forth are merely illustrative and may easily and properly be varied in consonance with the design and capacity of the apparatus, the character of the gas from the standpoint of composition, temperature, and pressure, and the conditions under which the liquefaction is carried out as influenced by the volume of material, types of refrigerants and the like, all within the scope of the invention. In the example, the gas to be liquefied will be a natural gas from which the moisture, acid gases such as carbon dioxide, hydrogen sulfide,

a and the like, will previously have been removed by pretreatment in the form of desiccators, amine extractors, and the like. In this typical example, a cleaned natural gas is used having about 73 mol. percent methane, about 12 mol. percent ethane, about 8 mol. percent propane, and about 2 mol. percent nitrogen, with the remainder being minor percentages of heavier hydrocarbons. It

will be understood that natural gas capable of being processed in accordance with the teachings of this invention may have up to 2\ 25 mol. percent heavier hydrocarbons, up to 20 mol. percent nitrogen, and up to 5 mol. percent carbon. dioxide or hydrogen sulfide, but usually the amount of methane will be from about 70 "to about 90 mol. percent of-the natural gas feed stream.

Referring to the drawing in detail, reference character 2 designates a line/leading from the source of supply (not shown) of the natural gas to be liquefied for conveying the natural gas in a feed or process stream to a heat exchanger 4. The natural gas will normally be available at a substantial pressure, such as 700 p.s.i.a, and the gas may be liquefied at whatever pressure it is available. However, it is preferred that a suitable compressor 6 be interposed in the gas feed line 2 upstream of as the temperature of the gas is reduced, the relative humidity is increased, raising the possibility of condensate plating out onto the'surfaces of the heat exchanger 4.

Therefore, it is preferred that the'natural gas feed stream be passed through the drier after the temperature has been reduced about 105 F. for a further removal of moisture from the gas. The drier may be of any desired construction and utilize any desired drying agent, such as, alumina or silica. The dried feed stream is fed back intothe heat exchanger through a line 34, for subsequent passage through the remainder of the heat exchangerto the outlet line .16.

The liquefied natural gas discharging from the heat exchanger 4 through the line 16 will be at about l65 F. and at a pressure of about 1490 p.s.i.a. 'It will thus be noted that the natural gas is only slightly reduced in pressure by passage through the heat exchanger 4 and V in being cooled to a liquefied state.

the heat exchanger 4 to compress the gas to a substantial 1 pressure, such as 1500 p.s.i.a. Also, it is preferred that a suitable heat exchanger 3 be interposed in the line 2 between the compressor 6 and the heat exchanger 4- for removingthe heat of compression from the natural gas feed stream by use of a readily available high temperature level refrigerant, such as water.

fed through the line 12 is at about 222 F., and the re-,

frigerant fed through the line leis at about -253 F. The three refrigerants are directed in parallel with the natural gas, such that the natural gas will be cooled upon passage through the heat exchanger 4, and the natural gas will be in a liquefied state upon discharging through the line 16 from the end of the heat exchanger 4 opposite feed line 2. It may also be noted that the heat exchanger 4 may beconstructed in two or more sections, although it is preferred toprovide one heat exchanger with three separate coils extending therethrough for passage of the three separate refrigerants in parallel with the natural gas being cooled. The refrigerants fed through the lineslll, 12, and 14 will be described in detail below.

In a preferred embodiment of this invention, the natural gas is withdrawn from the heat exchanger 4 through a line 18 after the gas has been cooled about 100 F. The cooled gas is fed through the line 18 into a knock-out drum 2t),wherein any condensates which may be present in the gas may be separated. The vapor collecting in the upper end of the knock-out drum 24) is withdrawn through aline 22 and then divided into separate streams through

lines

24 and 26. Only a minor portion of the gas or vapor. is fed through the line 24, and this gas is used to replenish the fuel gas supply in a line 28 which furnishes the fuel for various units of equipment used in a system practicing this invention. The major portion of the gas in the line 22 is directed through the line 26 into a suitable drier 30. It may also be noted that the condensates collecting in the lower end of the knockout drum 20 are discharged through a line 32 to combine with the gas in the line 26 being fed to the drier 3t).

As previously indicated, the original natural gas feed stream being fed to the heat exchanger 4 will ordinarily have been dried to a low relative humidity. However,

. Thus, the natural gas being fed to the heat exchanger 4 is preferably at about The liquefied natural gas is passed from the line 16 through the reboiler section 36 of a nitrogen stripper tower 38. I

The tower 33 may be of any suitable construction which has the reboiler 36 in the lower section thereof and a reflux condense-r 4% in the upper section thereof for maintaining the desired temperatures in the .tower and providing a removal of nitrogen from the feed stream, as will be described. The liquefied natural gas circulating through the reboiler 36 maintains the contents in the lower end portion of the tower 33 at a temperature of about 199 F., with a simultaneous cooling of the liquefied natural gas to a temperature of about 195 F. The cooled liquefied natural gas is discharged from the reboiler 36 through a line 42 back into the medial portionofthetower 38. However, an expansion valve 44 is-interposed in the line 42 to reduce the pressure of the liquefied natural gas entering the tower 38 to about.

80 to 85: p.s.i.a., with a resulting reduction in temperature v of the liquefied natural gas to about -203 F.

The expansion of the liquefied natural gas into the tower 38 provides a revapor-ization of a portion of the feed stream, with all of the nitrogen in the feed stream being included'in the vapor. The reflux condenser 40 1s mantained at a temperature below the temperature of the expanded feed stream entering the 'to'wer 38, and preferably at a temperatureof about 253 F., to facilitate the condensation of natural gas vapors flowing up- As previously indicated, the reboiler 36 is maintained at a temperature above the temperature of the expanded natural gas fed to the tower, such that the reboiler 36 will facilitate the vaporization of the nitrogen component'in the feed stream. The combination of the reboiler as and reflux condenser 40 provides nitrogen-enriched vapors in the upper end of the tower and substantially nitrogen-free liquid in the lower end of the tower. In the example used for illustration, the vapor in the upperend of the tower 38 will consistsof about mol. percent methane and about 25 mol. percent nitrogen; whereas, the liquid in the lower end of the tower 38 will consist only of methane and heavier hydrocarbons, with no nitrogen 1 being present.

The nitrogen-enriched vapors in the upper end of the tower 38 are withdrawn. through the line 12 and fed to one of the sets of coils of the heat exchanger 4 to provide a portion of the cooling of the natural gas feed strearn, as previously described. The temperature of these ni trogen-enriched vapors will be about 222 F. upon entering the heat exchanger 4 and the vapors will be at a pressure of about 85 p.s.i.a. After passage through the heat exchanger 4, the nitrogen-enriched vapors will be at a temperature of about F. and are fed through the cooler Si) is interposed in the line 46 and is maintained at a temperature of about -253 F., as will be described, to subcool the liquefied natural gas to a temperature of about 246 F The pressure of the subcooled liquefied natural gas will be about 80 p.s.i.a. The liquefied natural gas flowing through the line 46 is then expanded through a suitable expansion valve 52 down to about atmospheric pressure, or slightly above, such as 17.7 p.s.i.a. The expansion valve 52 is operated by a suitable liquid level controller 54 mounted on the side of the. stripper 38 to control the liquid level in the stripper. By expanding the subcooled liquefied natural gas from about 80 p.s.i.a.

down to about 17.7 p.s.i.a., the liquefied natural gas is not cooled to any appreciable extent, with the temperature of the liquefied natural gas in the storage vessel 48 being about the same as the temperature of the liquefied natural gas discharging from the subcooler 50. It will also be noted that since the liquefied gas is subcooled, a minor amount of vapor will be flashed off by reducing the pressure to about 17.7 p.s.i.a.

As will be apparent to those skilled in the art, the storage vessel 43 cannot be perfectly insulated; there fore, at least a minor portion of the liquefied natural gas in the storage vessel will boil oif as a vapor with a temperature slightly higher than the temperature of the liquid in the storage vessel. In accordance with the present invention, this boil-oil vapor is withdrawn through a line 56 and added to methane refrigerant vapors being passed through a series of compressors 58 through 62. These methane refrigeration vapors may be obtained from the natural gas feed stream when the system is first started up, and the boil-01f vapors are used as make-up in the refrigeration cycle.

The compressors 58 through 62 progressively compress the methane refrigerant vapor until the pressure of the vapor is about 1500 p.s.i.a. Also, suitable watercooled heat exchangers or intercoolers 63 through 67 are interposed between the various compressors and at the discharge end of the compressor 62 to remove all, or substantialy all, of the heat of compression from the methane vapor.

In a typical installation, the compressor 53 Will increase the pressure of the methane vapor from about 16 p.si.a. to about 53 p.s.i.a., with an increase in temperature of the vapor from about 23 F. to about 223 F. The intercooler 63 reduces the temperature of this vapor to about 100 5., with a resulting pressure drop to about 48 p.s.i.a. The compressor 59 increases the pressure of the methane vapor to about 154 p.s.iia., with a resulting temperature rise to about 324 F. It may also be noted that the compressors 58 through 62 are preferably multi-stage compressors to facilitate the Withdrawal of vapor from the compression system at substantially any desired pressure. For example, a portion of the methane vapor is withdrawn from the compressor 59 at a pressure of about '70 p.s.i.a. and fed through a line 68 for use as make-up fuel in the line 28. The intercooler 64 cools the remaining compressed methane vapors to about 100 F, with the pressure of the vapors being reduced to 149 p.s.i.a. by passage through the cooler 64. The compressor 60 increases the pressure of the vapor to about 294 p.s.i.a., with a resulting temperature rise to about 165 F. The remaining intercoolers 65, 66, and 67, and the remaining compressors 61 and 62 operate in substantially the same manner to provide the methane refrigerant vapor discharging from the last cooler 67 with a pressure of about 1500 p.s.i.a. and a temperature of about 100 F.

The methane vapor is then divided in two separate streams by lines 70 and 72. In the example being taken for illustration, approximately twice as much of the methane vapor is directed through the line 70 as through the line 72. The methane vapor passing through the line 70 is chilled down to a temperature of about -l F. by passage through a propane chiller 74. Simultaneously,

the methane vapor passing through the line 72. is chilled down to about 30 F. by a propane chiller '76. As indicated, the refrigerant used in the chiller 74 and 76 is preferably propane which may be easily circulated through a closed propane refrigeration cycle (not shown) to maintain the temperature of the propane fed to the chiller 74 at a temperature of about 20 F. and the propane fed to the chiller 76 at a temperature of about 20 F. Al though the details of the propane refrigeration cycle form no part of the present invention, it should be noted that the propane refrigerant may be easily obtained from the natural gas feed stream, such that the refrigerant supply in a commercial installation will be substantially unlimited, and the supply problem for the installation will be minimized. However, any commercial refrigerant, such as Freon, may be used in the chillers 74 and 76 if desired.

The methane vapor passing through the line 70, after being chilled to l0 F., is expanded through any suitable device 73 which will form a Work-producing zone and derive work or energy from expansion of the vapor. For example, the device 78 may be a turbine, such that the energy taken from the shaft of the turbine will be derived from the expansion of the vapor through the turbine. It is preferred that the discharge pressure of the expander 73 be set at about 214.9 p.s.i.a., such that the temperature of the vapor discharging from the expander 73 will be at about 175 F. This expanded methane vapor is discharged from the expander 78 through the line lit to serve as the lower temperature refrigerant in the cooling of the natural gas feed stream passed through the heat exchanger 4, as previously described.

The refrigerant passed through the exchanger 4 from the line 10 will be heated to about 80 F. by heat exchange with the natural gas feed stream. This heated refrigerant is withdrawn from the heat exchanger 4 through a line 80 and directed into the compressor portion of the cycle at the compressor 60. It may be noted that this refrigerant will have been reduced slightly in pressure from 214.9 p.s.i.a. to about 212 p.s.i.a. by passage through the respective coils of the heat exchanger 4, such that the pressure is compatible with the pressure of the vapor at an intermediate stage of the compressor 60. It should also be noted that by expanding this portion of the vapor down to an intermediate pressure between the pressure of the methane vapor before and after the complete compression cycle, this portion of the vapor only needs to be rte-compressed partially through the compressor 60 and through the compressors 61 and 62 before being re-expanded and re-used.

The methane vapor chilled by the chiller 76 is in turn subdivided into two separate streams by a pair of

lines

82 and 34. In the example taken for illustration, about twice as much vapor is directed through the line 84 as is directed through the line 82. As previously noted, the methane vapor downstream of the chiller 76 will be at about 30 F. for passage through the

lines

82 and 84. That portion of the vapor passing through the line 32 is condensed by passage through a heat exchanger 86. The refrigerant for the heat exchanger or condenser 86 is obtained by expanding the portion of the methane vapor in the line 84 through a suitable expander 88. The expander 83 may take any desired form which will provide a work-producing zone for the expansion of the vapor therethrough. The discharge of the expander 88 is set at about 19 p.s.i.a., such that the expanding vapors will be decreased in temperature to about 253 F. The expanded and cooled methane vapors discharging from the expander 38 are directed through a line 90 to the con denser 86 for removing heat from the portion of the methane vapors directed through the line 82, such that the temperature of the expanded vapors downstream from the condenser 86 will be at about 15 F. and the ternperature of the condensed methane refrigerant discharging from the condenser 86Wi1l be at about 243 F.

The heated methane vapors discharging from the condenser 85 on through the line 96 are directed back to the line 56 for're cycling through the compressors 58 through The condensed methane refrigerant flowing through the line 82 from the condenser 86 is subdivided into two streams and directed through lines 92 and $4. In the example taken for illustration, approximately twice as much of the methane is directed through the line 94 as through the line 92. Each of the'lines 92 and 94 contains a suitable expansion valve 96 to reduce the pressure of the methane, refrigerant flowing through both of the-lines 92 and 94 to about 19 p.s.i.a., with a partial revaporization ofthe methane and a resulting temperature drop to about The methane refrigerant flowing through the line 92 is directed through the reflux condenser 40 in the upper sectionof'the tower 38 to maintain the. temperature of the condenser below the temperature of the expanded liquefied natural gas fed to the tower, as previously described. The methane refrigerant passing through the reflux condenser 46 Will pick up sufiicient heat to complete the revaporization thereof and is withdrawn through the line '14 to form the lowest temperature refrigerant directed through the heat exchanger 4. The methane refri erant directed through the line 94 is passed through the subcooler t? to subcool the liquefied natural gas discharging frornthe bottom of the tower 58, as previously described. This portion of the methane refrigerant willalso pick up sufiicient heat, by passage through the subcooler 56 to complete the revaporization thereof, and the vapor is then directed back to the line .114 to combine with thevapor discharging from the reflux condenser 49 and form the final refrigerant in the line 14. e 1

" "The refrigerant fed to the heat exchanger 4- through the line 14 is increased in temperature to about 80 F. by

heat exchange with the natural gas feed stream, with a a resulting decrease in temperature of the natural gas feed stream until the natural gas is liquefied, as previously described. This last-mentioned refrigerant is withdrawn from the respective coils of the heat exchanger 4, fed through a line 98 and then line 56 for re-cycle through the compressors 53 through 62. Thus, all of the methane refrigerant is re-cycled through the compressors to form a closed methane refrigeration cycle. 7

From the foregoing it will be apparent that the present invention provides a novel method of liquefying natural gas, wherein the necessary refrigerants are obtained from the natural gas feed stream and need not be acquired from an outside source. Any nitrogen which maybe present in the natural gas feed stream is efficiently removed and [then usedas arrefrigerant in the liquefaction of the feed stream; whereupon the removed nitrogen may be used in a fuel for various units of equipment required in a system practicing the invention. It will be further apparent that the natural gas feed stream is first liquefied at an elevated pressure to take advantage of using refrigerants at relatively high temperature levels, with the liquefied natural gas being subsequently expanded into a suitable storage vessel. 7 The boil-01f from the storage vessel is added to a methane refrigeration cycle wherein the methane is compressed and cooled, and at least a portion of the compressed methane is expanded through a workrangement of steps and procedures as, heretofore set forth a in the specification and shown in the drawing, it being understood that changes may be made in the precise em,- bodiment disclosed without departing from thespirit and scope of the invention, as defined in thefollowing claims. For example, the preferred method involves the withdrawal of the natural gas feed stream from an intermediate portion of the heat exchanger 4 for make-up fuel and for passage'o'f the feed stream through the drier 30 to lower the relative humidity of the stream and prevent plat ing of condensates on any of the coils orexposed surfaces of the heat exchanger 4. However, if suflicient fuel is available from the nitrogen stripper 38 and the boil-off vapor,the knock-out drum Zilmay be eliminated and the natural gas feed stream passed directly from the line 18 through the drier 30.. Also, that portion of the methane refrigerant vapor passed through the line 82 andcondensed by the condenser -may be taken from the line 70 rather than the line "72 if desired. In fact, this portion of the methane vapor will then be at lower temperature to facilitate condensation thereof by passage through the condenser 85, although the refrigerationiload of the sepa-' rate propane refrigeration cycle will be increased by the necessity of'increasing the amount of methane vapor directed through the lower temperature chiller 74. We claim: a 1. In the method of liquefying natural gas available in a stream at a pressure above atmospheric, the steps of: V (a) cooling the natural gas stream to a liquid without a substantial reduction in the pressure of the stream, -.(b) expanding the liquefied gas into a storage vessel,

(0) compressing and cooling methane vapor, a portion of which is recovered from the vapor zation of liquid gas contained in the storage vessel, 7 (d) expanding at least a portion of the cornpressed methane vapor through a work producing zone to produce refrigerant having a temperature below the initial temperature of the natural gas stream, and

(e) passing said refrigerantin heat exchange relation with the natural gas stream to obtain at least a portion of the cooling called for in step (A); characterized further in dividing the compressed and cooled'methane vapor into first, second, and third separate streams; expanding the first separate stream through relation with the naturalgas stream to obtain a portion producing zone to form a refrigerant for heat exchange With the natural gas feed stream. Also, in the preferred embodiment, the compressed methane vapor is initially subdivided into two separate streams, with one of the streams being passed through a Work-producingzone for the formation of a relatively high temperature level refrigeraut used-in cooling the natural gas feed stream, and the other methanervapor stream being in turn subdivided for a partial passage through a work-producing zone in the formation of a lowtemperature level refrigerant for cooling and liquefaction of the natural gas feed stream,

Changes may be made in the combination and arof the cooling :called for in step (a); expanding the second separate stream through a work-producing zone to a temperature below the condensation temperature of the third separate stream; passing the expanded second separate stream in heat-exchange relation with the third separate stream tocondense the third'separate stream; expanding the condensed third separate stream to form a second refrigerant having a temperature below the condensation temperature of the natural gas stream, then passing the second refrigerant in heat-exchange relation with the natural gas stream to complete the cooling calledfor in step (a).

2. The method defined in claim 1 characterizedfurther in removing moisture from the natural gas stream at an intermediate stage of the cooling thereof by said firs and second refrigerants. I

3." The method defined in claim 1 characterized further in" that said finst and second refrigerants, after passage in heat-exchange relation with the natural gas, and said second separate stream, after passage thereof in heatexchange relation with the third separate stream, are recompresse'dand re-cycled throughthe steps set forth in claim 4. 1

4. The method defined in claim 1 characterized fur-' ther in that the methane vapor is compressed and cooled in stages before division into said separatestreams, and

said first refrigerant is re-cycled to an intermediate stage of compression after passage in heat-exchange relation with the natural gas stream.

5. In a method of liquefying natural gas containing nitrogen and available in a stream at a temperature above atmospheric, the steps of:

(a) cooling the natural gas stream to a liquid without substantially reducing the pressure of the stream,

(1)) expanding the liquefied gas stream into a nitrogen stripping tower maintained at such temperatures as to provide nitrogen-enriched vapors in the upper end of the tower and substantially nitrogen-free liquid in the lower end of the tower,

(c) passing the nitrogen-enriched vapors in heat-exchange relation with the natural gas stream to obtain a portion of the cooling called for in step (a),

(at) expanding the nitrogen-free liquefied gas into a storage vessel,

(e) compressing and cooling methane vapor, at portion of which is recovered from the vaporization of liquid gas contained in the storage vessel,

(f) expanding at least a portion of the compressed methane vapor through a work-producing zone to produce a refrigerant, and

(g) passing said refrigerant in heat-exchange relation with the natural gas stream to obtain an additional portion of the cooling called for in step (a);

characterized further in removing moisture from the natural gas stream after the stream has been cooled about 100 F.

6. In a method of liquefying natural gas containing nitrogen and available in a stream at a temperature above atmospheric, the steps of:

(b) expanding the liquefied gas stream into a nitrogen stripping tower maintained at such temperatures as to provide nitrogen-enriched vapors in the upper end of the tower and substantially nitrogen-f ee liquid in the lower end of the tower,

(d) expanding the nitrogen-free liquefied gas into a storage vessel,

(e) compressing and cooling methane vapor, a portion of which is recovered from the vaporization of liquid gas contained in the storage vessel,

(7) expanding at least a portion of the compressed methane vapor through a work-producing zone to produce a refrigerant, and

characterized further in that the methane vapor is compressed and cooled in stages; the compressed methane vapor is subdivided into a first, a second, and a third separate stream; expanding the first separate stream through a work-producing zone to an intermediate pressure between the pressure of the methane vapor before and after compression to form a first refrigerant; passing the first refrigerant in heat-exchange relation with the natural gas stream to obtain a portion of the cooling called for in step (a); expanding the second separate stream through a work-producing zone to a temperature level below the condensation temperature of the third separate stream; passing the expanded second separate stream in heat-exchange relation with the third separate stream to condense the third separate stream; expanding the condensed third separate stream to produce a second refrigerant having a temperature level below the condensation temperature of the natural gas stream; then passing the second refrigerant in heat-exchange relation with the natural gas stream to complete the cooling called for in step (a).

7. The method defined in claim 6 characterized further in re-cycling the first refrigerant to an intermediate state of compression of the methane vapor.

8. The method defined in claim 6 characterized further in that the nitrogen stripping tower has a reflux condenser in the upper section thereof maintained at a temperature below the temperature of the expanded liquefied 1 natural gas and a reboiler in the lower section thereof maintained at a temperature slightly above the temperature of the expanded liquefied natural gas, and said reboiler is maintained at said temperature by passing the liquefied natural gas stream therethrough prior to expension of the stream into the tower.

9. The method defined in claim 8 characterized further in that the reflux condenser is maintained at the same temperature by passing a portion of the second refrigerant therethrough prior to passage of the second refrigerant in heat-exchange relation with the natural gas stream.

10. The method defined in claim 9 characterized further in passing the remainder of the second refrigerant in heat-exchange relation with the liquefied natural gas Withdrawn from the lower end of the tower before passage of the second refrigerant in heat-exchange relation with the natural gas stream to subcool the liquefied natural gas prior to expansion thereof into the storagevessel.

11. In the method of liquefying natural gas available in a stream at a pressure above atmospheric, the steps of:

(a) cooling the natural gas stream to a liquid without a substantial reduction in the pressure of the stream,

(12) expanding the liquefied gas into a storage vessel,

(c) compressing and cooling methane vapor, a portion of which is recovered from the vaporization of liquid gas contained in the storage vessel,

(d) expanding at least a portion of the compressed methane vapor through a work-producing zone to produce refrigerant having a temperature below the initial temperature of the natural gas stream, and

(e) passing said refrigerant in heat exchange relation with the natural gas stream to obtain at least a portion of the cooling called for in step (:1);

characterized further in condensing another portion of the compressed methane vapor, expanding said condensed methane vapor to form a second refrigerant having a temperature below the condensation temperature of the natural gas stream at the initial pressure of the stream, and passing said second refrigerant in heat-exchange relation with the natural gas stream to obtain an additional portion of the cooling called for in step (a).

References Cited in the file of this patent UNITED STATES PATENTS Knapp Nov. 8,

Claims

1. IN THE METHOD OF LIQUEFYING NATURAL GAS AVAILABLE IN A STREAM AT A PRESSURE ABOVE ATMOSPHERIC, THE STEPS OF; (A) COOLING THE NATURAL GAS STREAM TO A LIQUID WITHOUT A SUBSTANTIAL REDUCTION IN THE PRESSURE OF THE STREAM, (B) EXPANDING THE LIQUEFIED GAS INTO A STORAGE VESSEL, (C) COMPRESSING AND COOLING METHANE VAPOR, A PORTION OF WHICH IS RECOVERED FROM THE VAPORIZATION OF LIQUID GAS CONTAINED IN THE STORAGE VESSEL, (D) EXPANDING AT LEAST A PORTION OF THE COMPRESSED METHANE VAPOR THROUGH A WORK PRODUCING ZONE TO PRODUCE REFRIGERANT HAVING A TEMPERATURE BELOW THE INITIAL TEMPERATURE OF THE NATURAL GAS STREAM, AND (E) PASSING SAID REFRIGERANT IN HEAT EXCHANGE RELATION WITH THE NATURAL GAS STREAM TO OBTAIN AT LEAST A PORTION OF THE COOLING CALLED FOR IN STEP (A); CHARACTERIZED FURTHER IN DIVIDING THE COMPRESSED AND COOLED METHANE VAPOR INTO FIRST, SECOND, AND THIRD SEPARATE STREAMS; EXPANDING THE FIRST SEPARATE STREAM THROUGH A WORK-PRODUCING ZONE TO FORM A FIRST REFRIGERANT HAVING A TEMPERATURE BELOW THE INITIAL TEMPERAURE OF THE NATURAL GAS STREAM; PASSING SAID FIRST REFRIGERANT IN HEAT-EXCHANGE RELATION WITH THE NATURAL GAS STREAM TO OBTAIN A PORTION OF THE COOLING CALLED FOR IN STEP (A); EXPANDING THE SECOND SEPARATE STREAM THROUGH A WORK-PRODUCING ZONE TO A TEMPERATURE BELOW THE CONDENSATION TEMPERATURE OF THE THIRD SEPARATE STREAM; PASSING THE EXPANDED SECOND SEPA-