US3792590A

US3792590A - Liquefaction of natural gas

Info

Publication number: US3792590A
Application number: US00099973A
Authority: US
Inventors: A Lofredo; D Biava
Original assignee: Airco Inc
Current assignee: Airco Inc
Priority date: 1970-12-21
Filing date: 1970-12-21
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

A process and system for purifying and liquefying a natural gas stream. The stream initially is passed through driers and filters. The stream is then split and a minor fraction is passed through CO2 adsorbers and enters the refrigeration cycle for liquefaction. The major fraction of said split stream is workexpanded and passed through a heat exchanger wherein it is utilized for high level refrigeration. The said fraction is then sent to the distribution pipeline and not further used in the cycle. The remaining refrigeration is effected by a nitrogen cycle, which provides low level refrigeration, as well as part of the high level refrigeration requirements.

Description

United States Patent [191 v [11] 3,7925% Lofiredo et al. Feb. w, 1974* LIQUEFACTION OF NATURAL GAS 2,677,438 5/1954 Reid 62/18 2,799,364 7/1957 [75] Inventors: Antony Loiredo, Springfield; 3 078 634 2/1963 Domenic! Biava both of 3,271,965 9/1966 Maher 62/40 Somerset, NJ.

[73] Assignee: Airco, Inc., New York, NY. Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. Purcell [22] plied 1970 Attorney, Agent, or Firm-Roger M. Rathbun; Ed- [21] App]. No.: 99,973 mund W. Bopp; H. Hume Mathews Related US. Application Data I [63] Continuation-impart of Ser. No. 843,427, July 22, [57] ABSTRACT A process and system for purifying and liquefying a natural gas stream. The stream. initially is passed LS. through driers and filterS -The strewn is then Split and I f r d th h d b d 51 int. Cl. FZSj 1/00, F25j 1/02, F25j 3/06 a m passe mug CO2 a sot em enters the refrigeration cycle for liquefaction. The major fraction of said split stream is work-expanded and passed through a heat exchanger wherein it is utilized for high level refrigeration. The said fraction is [58] Field of Search..... 62/9, ll, l7, 18, 23, 24, 27, 62/28, 30, 38, 40, 50, 54; 55/62, 73, 74, 80

[56] References Cited then sent to the distribution pipeline and not further UNITED STATES PATENTS used in the cycle. The remaining refrigeration is ef- 3,302,416 2/1967 Proctor 62/55 fected by a nitrogen cycle, which provides low level 3,312,073 4/1967 Jackson 62/38 refrigeration, as well as part of the: high level refrigera- 3,l82,461 5/1965 Johanson 62/38 [ion requirements,

3,360,944 l/l968 Knapp t 62/38 3,194,025 7/1965 Grossman 62/40 10 Claims, 2 Drawing; Figures PATENTED FEB l 9 l974 SHEEI 1 OF 2 QEE qmmqowQw 8 M0 ER bvm MW V L It I LIQUEFACTION or NATURAL GAS BACKGROUND OF INV ENTION This application is a continuation-in-part of our co pending application, Ser. No. 843,427, filed July 22, l969 and entitled Liquefaction of Natural Gas.

This invention relates generally to processes and apparatus for the liquefaction of natural gas, and more specifically relates to a process and apparatus wherein both a nitrogen cycle and a let-down cycle are utilized to produce the refrigeration effecting'liquefaction.

The usage of natural gas as a fuel in both commercial and household applications has increased tremendously in recent years and an ever growing network of natural gas pipelines is spreading throughout the country. In order to anticipate seasonal loads (winter months), unexpected loads, and to insure continuity of service, it hasbeen found necessary to store natural gas at various locations throughout the country. The gas is stored in liquid form since this is the most economical way of storing a large quantity of gas. When a peak demand occurs, the liquid storage is drawn down and vaporized and supplied to the user to supplement the gas which is normally supplied from a gas distribution sys tem. LNG plants have also been installed at points of shipment (i.e., seaports, etc.) since the gas is usually shipped in liquid form.

In our aforesaid copending patent application Ser. No. 843,427, assigned to the assignee of the instant application, there is disclosed a process for liquefaction of natural gas which constitutes a major step forward in the art. A plant built according to the teaching of said application utilizes centrifugal machinery at high efficiencies and eliminates the need for cold blowers and cold pumps for the transfer of LNG to storage. The design contains a unique arrangement for the cleanup of the gas that is to be liquefied and provides for the control of composition of the LNG in storage and eliminates the high boilers in storage. In addition, natural gas is used in an efficient manner to regenerate adsorb ers and then to drive a compressor.

According to the general scheme of the aforementioned copending application, liquefaction of a natural gas stream is brought about primarily through use of a nitrogen cycle. Now, however, in accordance with the present invention, it has been found that important advantages and excellent results are obtained in a liquefaction system which, while incorporating the advantageous clean-up and boil'off features of the system taught in said copending application, makes use of a let-down cycle and a nitrogen cycle, to provide desired refrigeration.

In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method and system for liquefaction of natural gas which advantageously utilizes flow and pressure of the incoming gas to, in part, provide the refrigeration used for liquefaction and to furnish additional power to the nitrogen refrigeration cycle.

It is a further object of the present invention, to provide a method and system for liquefaction of natural gas wherein both a let-down cycle and a nitrogen cycle act in combination to effect required refrigeration.

It is another object of the invention to provide a method and system for liquefaction of natural gas which effects efficient refrigeration via use of both nitrogen and let-down cycles and which, as well, utilizes the natural gas to regenerate adsorbers in an economical fashion and to drive the gas turbine for the main compressor of the refrigeration cycle.

It is still an additional object of the invention to provide method and system for liquefaction of natural gas, which utilizes both a nitrogen and a let-down cycle for effecting refrigeration and wherein at least a part of the nitrogen refrigerant is used to maintain a constant LNG composition in the storage tank by liquefying the vapors which are generated in said tank.

SUMMARY OF INVENTION Now in accordance with the present invention, the foregoing objects and others as will become apparent in the course of the ensuing specification are achieved in a process and system according to which a natural gas stream is initially passed through driers and filters and then split. A minor fraction of the split stream is passed through CO adsorbers and enters the refrigeration cycle for liquefaction. The major fraction of said split stream is work-expanded and passed through a heat exchanger wherein its reduced temperature is utilized for high level refrigeration. The said major fraction is then sent to the distribution pipeline and not further used in the cycle. The remaining refrigeration is effected by a nitrogen cycle, which provides both low level refrigeration and part of the high level refrigeration. The system includes a clean-up cycle for incoming gas in which the carbon dioxide adsorption units are regenerated and cooled by circulating natural gas at op,- erating pressure. The system further includes utilization of liquid nitrogen produced in the nitrogen cycle for purposes of condensing the boil-off in the LNG storage tank, whereby to maintain constant purity in the tank.

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated by way of example in the drawings appended hereto, in which:

FIG. 1 and 2, when taken together, show a schematic flow diagram of a liquefied natural gas plant according to one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT.

In the description that ensues, various system parameters such as pressures, temperatures, flow rates, etc., may from time to time be specifically set forth. In this connection it will be understood that such recitations are in no way intended to delimit the invention, but rather are supplied for the purpose of providing concrete illustration of the invention and thereby assisting the reader in gaining an understanding of such invention. 7

Referring to FIG. 1, natural gas enters the system 1 through conduit 3. The feed gas generally comes from a distribution network or it may come directly from a natural gas wellhead or other source. The inlet temperature, pressure and flow of the feed gas remain relatively constant and the particular system is designed acv cordingly. In a typical instance the infeed gas is at approximately 200psig and F and. flows at the rate of about 27 MM SCFD. The system would, of course, be readily adapted to accommodate other inlet pressures, temperatures and flows. Y

A typical natural gas feedstream contains methane, nitrogen, ethane and other hydrocarbons down to pentanes in varying proportions. A representative composition treated by the present invention may, for example, have the following major constituents in mol percent: N at 0.53 percent, methane at 94.5 percent, ethane at 3.19 percent, propane at 0.53 percent, CO at 0.98 percent and remaining heavier hydrocarbons and other impurities (odorant, H O, etc.).

The feed stream (a very small part of which is diverted into conduit 7 for purposes to be subsequently described) is passed through separator 5 which removes the entrained liquids (water, oils, odorants, sulphurs, etc.). The feed gas is then dried to approximately lF dew point in the desiccant drier 9, which is of the molecular sieve bed variety. Drier 9 is one of a pair of such driers 9, 9a, which serve to further remove moisture, heavy hydrocarbons, odorants, sulphurs and sulphur compounds from the feed gas stream. As shown in FIG. 1, drier 9 is on stream while drier 9a is being regenerated. The regeneration process will be described in detail hereinbelow. The feed stream then proceeds through a filter 11 which removes any adsorber particles which may have been entrained in the gas stream as it passes through the bed of drier 9.

Upon leaving filter 11, the stream is split into two parts which proceed respectively through the two

conduits

13 and 15. The major portion of the dried gas is directed toward an expander 17' (FIG. 2) by flow control valve 19. The remaining gas goes through a second molecular sieve bed CO adsorber 21 for removal of carbon dioxide. After this final purification and passage through a filter 22 a small'portion is diverted into conduit 23 to the cooling loop purge gas and the remaining gas is sent via conduit 25 toward heat exchanger 29 (FIG. 2) and ultimate liquefaction.

The

carbon dioxide adsorbers

21a and 21b, not on stream, are regenerated and cooled by circulating natural gas at approximately operating feed gas pressure. Regeneration flow is circulated by blower 31 and heated in exchanger 37 to about 550F by exhaust gas proceeding from gas turbine driven recycle compressor 33 via conduit 35. The said flow is then passed through the adsorption unit being regenerated (such as adsorber 21b), cooled to allowable blower inlet temperature in exchanger 37 and recirculated. A similar loop is provided by blower 39 and heat exchanger 41 for cooling the carbon dioxide adsorber 21a after regeneration. The three

adsorbers

21, 21a, and 21b are arranged so that at a given period of time one unit is in active service, one is in regeneration and the third is in I a cool-down period. The flows for these three operations are sequenced by an automatic timer controlling pneumatic valves in a manifolded system. These latter items are not explicitly shown in the present drawings but are standard in the art and well understood by those skilled in flow control techniques.

effected for the off-stream drier. Driers 9 and 9a are arranged so that at a given period of time one unit is in active service and the other is regenerating or cooling. The flows for these operations are sequenced by an automatic timer controlling pneumatic valves in a manifolded system. Such sequencing and flow'controlling means are not explicitly shown in the present drawings as they are standard in the art and well understood by those skilled in flow control techniques.

The purge stream is subsequently vented into the fuel gas system for the gas turbine driver 93 of recycle compressor 33 via the valves 54 and 55.

Referring now especially to FIG. 2, it is seen that the dry, purified approximately 200 psig natural gas passing through conduit 25 (FIG. 1) is conveyed to heat exchanger 29 and there sent through pass 53 where such gas is cooled by nitrogen and expanded natural gas from its original (ambient) temperature to approximately l20F. At this pressure and temperature approximately 1.0 percent of the feed gas will separate as liquid in heavy ends separator 55. This liquid contains essentially all of the pentanes and heavier components and a majority of the butanes included in the design feed gas specification. Heat input of the LNG to storage can thereby be controlled by the amount of forecooling provided to the natural gas through exchanger 29.

The liquid accumulated in heavy ends separator 55 is passed through conduit 57 and valve 59, mixed with cold expanded gas from high level expander 17 and vaporized and warmed in exchange pass 61 against incoming let-down gas in pass 63, incoming feed gas in pass 53 and incoming nitrogen in pass 119. The mixture is warmed to near ambient temperature and sent via conduit 65 to the distribution pipeline.

The gas phase from separator 55 is passed via conduit 67 to heat exchanger 71 and conveyed through exchange pass 69 wherein it is liquefied and subcooled to about -2 60F. The condensate is then passed through conduit 73, expanded to tank pressure through eductor 75 and sent to LNG storage tank 77 with no flash. Flow control valve 79 maintains the desired liquefaction rate for the system.

High level refrigeration is provided by expanding natural gas to approximately 75 psia and 65F in high level expander 17. The gas, as has been previously mentioned, is supplied from conduit 13; a portion is passed through conduit 81 and temperature control valve 83; the bulk, however, is cooled in pass 63 prior to expansion. The expanded gas is mixed with heavy ends from separator 55 and warmed to near ambient temperature as described above.

Low level refrigeration is provided by a closed loop nitrogen expansion cycle. In general the working pressure levels selected in this nitrogen cycle represent an optimization of the'required cycle thermodynamics and enable use of standard hardware with operating simplicity and high reliability.

The initial nitrogen charge and subsequent nitrogen make-up enters the system from conduit 85 and is passed by control valve 87 to the closed loop. The nitrogen then proceeds via conduit 89 to the compressor section 91 of gas turbine driven recycle compressor 33. As was described in connection with FIG.1, purge gas from the gas conditioning section of system provides the majority of the fuel required for the gas turbine driver 93 of recycle compressor 33; supplemental fuel is derived from the gas flowing through conduit 7. The nitrogen is intercooled during compression by an intercooler 95 associated with compression section 91. The nitrogen is then cooled in aftercooler 97, split into

conduits

99 and 101, and compressed to about 620 psia by

compressors

103 and 105. The

compressors

103 and 105 mentioned are seen to be driven respectively by process expanders 107 and 17; expander 107 and compressor 103 together comprise the low level exp/comp unit generally designated as 109; similarly, exander 17 and compressor 105 together comprise the high level exp/comp unit generally designated as 111.

The high pressure nitrogen in conduit 113 is again cooled in aftercooler 115, and then conveyed by conduit 117 to exchanger 29. There the nitrogen proceeds through pass 119 where it is forecooled by expanded natural gas and separated heavy ends in pass 61 and by cold return nitrogen gas in pass 121. The nitrogen then flows via conduit 123 to pass 125 of exchanger 71. The flow is splitintermediate such pass with the bulk of the flow being sent through conduit 127 to low level exp/comp unit 109 where in expander 107 the nitrogen is workexpanded to approximately 80 psia and 265F. The expanded nitrogen then flows through pass 129, there providing the majority of the refrigeration for liquefying and subcooling the remaining natural gas passing through pass 69 and liquefying the minor stream of nitrogen passing through pass 125. The cold nitrogen then passes upwardly through pass 121 of exchanger 29, there being warmed to near ambient temperature and supplementing the high level refrigeration provided by natural gas flowing through pass 61. The nitrogen flow through expander 107 is controlled by adjusting variable inlet guide vanes with a suitable control means 131. Therefore, with a constant amount of let-down natural gas available for expander 17, the liquefaction capacity and heat input of the LNG can be controlled by adjusting such means 131.

That portion of the high pressure nitrogen entering pass 125 which is not split into conduit 127, is cooled and liquefied and'used for recondensing LNG tank vapors in boil-off condenser 133, which vapors reach condenser 133 via conduit 78 and valve 80. More specifically the nitrogen passes through conduit 135'to ni trogen separator 137. A level of nitrogen is maintained in separator 137 by a level controller 147 which controls valve 145. Liquid nitrogen passes through conduit 139, exchange pass 143 and return pass 141a. Nitrogen gas is returned via conduit 149 and joins the low temperature gas coming from the expander 107..

Make-up nitrogen to the refrigeration cycle is controlled by the position of valve 87. A pressure controller 87a senses the pressure in the high pressure line 117 and when the line pressure drops below that desired, additional nitrogen is added to the pressure suction to recycle compressor 33. Generally nitrogen is stored in liquid form and vaporized as necessary for supply to line 85.

It will be readily apparent that the process and system described in detail above is especially suited for situations wherein only a selected portion of the feed gas is liquefied. The LNG which is stored in the storage tank 77 may be withdrawn in liquid form. To ship liquefied natural gas by truck, etc. to remote locations, it is thus only necessary to withdraw the liquid through conduit A trailer loading pump 153 may assist in transfer ring the liquid to trailer station 155.

During times when the liquefier is not in operation, the vapor which is in the tank above the liquid is withdrawn through conduit 78. To bring the gas up to proper pressure for entrance into the distribution network, it is passed through a heater 157 and then compressed in boil-off compressor 159', it is then cooled in after-cooler 161 and passed through valve 163 to the distribution network. Certain compressor designs will handle low temperature gas and if such designs are chosen, then heater 157 may be dispensed with. For peak demand requirements, liquid may be withdrawn through conduit 165 and pumped by LNG liquid sendout pump 167 to vaporizing unit 169. The vaporized product may then be forwarded to the distribution network as desired.

Although the invention has been described primarily with respect to the liquefaction of natural gas and is especially suited therefor, it may also be used for the liquefaction of other gases. The temperatures, pressures, flow rates, etc. would under such circumstances be appropriately adjusted to render the system compatable for processing of the gas in question. 7

The unique combination of a letdown cycle and a nitrogen cycle provides for a most economical liquefier coming feed gas. If less energy is available, the let-down cycle may only provide a minor part of the refrigeration. As shown in the drawing, the let-down expander is compressor loaded to provide a part of the compressor work of the nitrogen refrigeration cycle. If conditions permit the letdown expander could be generator loaded. In the cycle shown, letdown provides a substantial portion of the nitrogen cycle horsepower require- 7 ment.

While the present invention has been particularly described in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention may be made by those skilled in the art, which variations are yet within the proper scope of the instant teaching. Accordingly the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.

We claim:

1. A system for the low temperature liquefaction of natural gas and for the storage of the liquefied product, comprising, means for splitting a stream of natural gas into at least first and second fractions upstream of a heat exchanger means having a high and low level heat exchanger means; means cooling at least part of the said first fraction in the high level heat exchanger means, then directing the first fraction to expansion turbine means for work-expanding said first fraction to cool said first fraction and then returning said first fraction for heat exchange to said high level heat exchanger means, said high level exchanger means also reducing the temperature of said second fraction by heat exchange with said cooled first fraction, said low level heat exchanger means further reducing the temperature of said second fraction to at least liquefaction temperature by heat exchange with a low temperature nitrogen stream; conduit means for directing said second fraction through said high and low level heat exchanger means; A

a closed nitrogen refrigeration cycle having passes through said high and low level heat exchanger means, said nitrogen cycle including a source of gaseous nitrogen, means for compressing said nitrogen driven at least in part by the power developed at said turbine means, means for passing said compressed nitrogen through said high level heat exchanger means in countercurrent relationship with said cooled first fraction of natural gas to precool said nitrogen, means to expand at least part of saidpre-cooled nitrogen to further cool said nitrogen and means to pass said further cooled nitrogen successively through said low and high level heat exchanger means in countercurrent relationship to the said second fraction;

LNG storage means, means for conveying the liquefied product from said low level exchanger means to said storage means; and means to selectively withdraw LNG from said storage means and furnish product to gas distribution means.

2. A system according to claim 1, further including conduit means for conducting said first fraction from said high level heat exchanger means to said distribution means.

3. A system according to claim 2, further including drier means for removing moisture from said stream of natural gas before the stream is split.

4. A system according to claim 3, further including carbon dioxide adsorber means positioned to remove carbon dioxide from said second fraction of said natural gas prior to liquefaction.

5. A system according to claim 4, wherein said carbon dioxide adsorber means and said drier means are respectively members of groups of such units, said sys tem including means for activating one adsorber means and one drier means at a time, said system further including means for regenerating the members of said groups with a flow of warm natural gas and for cooling said members following regeneration with a flow of cool natural gas and means for withdrawing a portion of said second fraction of natural gas for use in purging said regeneration and cooling flows to prevent an excessive build-up of contaminants.

6. A system according to claim 5, further including boil-off condenser means for receiving boil-off vapors from said LNG storage means and condensing said vapors, means for returning the condensate to said storage means to maintain the constituency at said storage, means for forming liquid nitrogen insaid low level heat exchanger means, conduit means directing said liquid nitrogen to said condenser means to effect cooling thereat and means to return the nitrogen after passage through said condenser means to said low level exchanger means.

7. A process for liquefying natural gas, comprising splitting a natural gas stream into at least first and second fractions upstream of a heat exchanger means including a high level heat exchanger means and a low level heat exchanger means; removing moisture and carbon dioxide from at least said second fraction by passing said fraction through adsorption units; regenerating said adsorption units with warm and then cool streams of natural gas, purging said regenerating streams with a natural gas stream to prevent an excessive build-up of contaminants; cooling at least part of said first fraction in said high level heat exchanger means, then directing the first fraction to expansion turbine means for work-expanding said first fraction to cool said first fraction and then returning said first fraction for heat exchange to said high level heat exchanger means; reducing the temperature-of said second fraction by heat exchange with said cooled first fraction at said high level heat exchange means; further reducing the temperature of said second fraction to at least liquefaction temperature by heat exchange with a closed nitrogen refrigeration cycle; using the power developed in said expansion of said first fraction to drive a compressor in said nitrogen cycle, engine-expanding a nitrogen stream in said cycle to produce low temperature nitrogen gas, using said low temperature nitrogen gas to liquefy said second fraction reducing the pressure of said liquefied stream to a desired storage pressure and directing the same to a storage facility; and condensing the boil-off from said storage facility byheat exchange with a stream of liquid nitrogen obtain from said nitrogen cycle.

8. A process as defined in claim 7, further comprising sensing the pressure in said nitrogen cycle and supplying additional nitrogen to said cycle to maintain desired cycle operation.

9. A system in accordance with claim 1, further including means for diverting at least a portion of said natural gas stream upstream of said heat exchanger means, and work expanding said portion at a second turbine means; and wherein said means compressing said nitrogen is driven at least in part by the power developed at said second turbine means.

10. A method in accordance with claim 7, further including diverting at least a portion of said natural gas stream upstream'of said heat exchanger means, and work expanding said portion at a second turbine means; and wherein said means compressing said nitrogen in said nitrogen cycle is driven at least in part by the power developed at said second turbine means.

Claims

1. A system for the low temperature liquefaction of natural gas and for the storage of the liquefied product, comprising, means for splitting a stream of natural gas into at least first and second fractions upstream of a heat exchanger means having a high and low level heat exchanger means; means cooling at least part of the said first fraction in the high level heat exchanger means, then directing the first fraction to expansion turbine means for work-expanding said first fraction to cool said first fraction and then returning said first fraction for heat exchange to said high level heat exchanger means, said high level exchanger means also reducing the temperature of said second fraction by heat exchange with said cooled first fraction, said low level heat exchanger means further reducing the temperature of said second fraction to at least liquefaction temperature by heat exchange with a low temperature nitrogen stream; conduit means for directing said second fraction through said high and low level heat exchanger means; a closed nitrogen refrigeration cycle having passes through said high and low level heat exchanger means, said nitrogen cycle including a source of gaseous nitrogen, means for compressing said nitrogen driven at least in part by the power developed at said turbine means, means for passing said compressed nitrogen through said high level heat exchanger means in countercurrent relationship with said cooled first fraction of natural gas to pre-cool said nitrogen, means to expand at least part of said pre-cooled nitrogen to further cool said nitrogen and means to pass said further cooled nitrogen successively through said low and high level heat exchanger means in countercurrent relationship to the said second fraction; LNG storage means, means for conveying the liquefied product from said low level exchanger means to said storage means; and means to selectively withdraw LNG from said storage means and furnish product to gas distribution means.

5. A system according to claim 4, wherein said carbon dioxide adsorber means and said drier means are respectively members of groups of such units, said system including means for activating one adsorber means and one drier means at a time, said system fUrther including means for regenerating the members of said groups with a flow of warm natural gas and for cooling said members following regeneration with a flow of cool natural gas and means for withdrawing a portion of said second fraction of natural gas for use in purging said regeneration and cooling flows to prevent an excessive build-up of contaminants.

6. A system according to claim 5, further including boil-off condenser means for receiving boil-off vapors from said LNG storage means and condensing said vapors, means for returning the condensate to said storage means to maintain the constituency at said storage, means for forming liquid nitrogen in said low level heat exchanger means, conduit means directing said liquid nitrogen to said condenser means to effect cooling thereat and means to return the nitrogen after passage through said condenser means to said low level exchanger means.

7. A process for liquefying natural gas, comprising splitting a natural gas stream into at least first and second fractions upstream of a heat exchanger means including a high level heat exchanger means and a low level heat exchanger means; removing moisture and carbon dioxide from at least said second fraction by passing said fraction through adsorption units; regenerating said adsorption units with warm and then cool streams of natural gas, purging said regenerating streams with a natural gas stream to prevent an excessive build-up of contaminants; cooling at least part of said first fraction in said high level heat exchanger means, then directing the first fraction to expansion turbine means for work-expanding said first fraction to cool said first fraction and then returning said first fraction for heat exchange to said high level heat exchanger means; reducing the temperature of said second fraction by heat exchange with said cooled first fraction at said high level heat exchange means; further reducing the temperature of said second fraction to at least liquefaction temperature by heat exchange with a closed nitrogen refrigeration cycle; using the power developed in said expansion of said first fraction to drive a compressor in said nitrogen cycle, engine-expanding a nitrogen stream in said cycle to produce low temperature nitrogen gas, using said low temperature nitrogen gas to liquefy said second fraction reducing the pressure of said liquefied stream to a desired storage pressure and directing the same to a storage facility; and condensing the boil-off from said storage facility by heat exchange with a stream of liquid nitrogen obtain from said nitrogen cycle.

10. A method in accordance with claim 7, further including diverting at least a portion of said natural gas stream upstream of said heat exchanger means, and work expanding said portion at a second turbine means; and wherein said means compressing said nitrogen in said nitrogen cycle is driven at least in part by the power developed at said second turbine means.