US3358460A

US3358460A - Nitrogen liquefaction with plural work expansion of feed as refrigerant

Info

Publication number: US3358460A
Application number: US494177A
Authority: US
Inventors: Donald L Smith; John L Ferrell
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1965-10-08
Filing date: 1965-10-08
Publication date: 1967-12-19
Anticipated expiration: 1984-12-19
Also published as: GB1099669A; FR1517779A; DE1501697A1

Abstract

1,099,669. Gas liquefaction process. AIR REDUCTION CO. Oct. 7, 1966 [Oct. 8, 1965], No. 44923/66. Heading F4P. Liquefaction of a gas, e.g. effluent nitrogen from the low pressure column (103), Fig. 6 (not shown), of a two stage air rectification plant is effected by compression to not more than 2À5 times the critical pressure of the gas at 151, 153, 155, 157, 159, 163, 166 cooling in means including paths a, a<SP>1</SP> of a first heat exchanger 169, dividing the cooled gas at 186 into a minor portion which is cooled without liquefaction in path a of a second exchanger 192 and into a major portion which is work-expanded in turbines 188, 194, with intervening warming in a path a of exchanger 192 from which latter the cooled minor portion is passed through a line 203 and is throttle-expanded at 204, 209, 213 into a product liquid storage vessel 213; uncondensed vapour from the latter being forced by a pump 217 through paths c of exchangers 192, 169 and a line 221 to the compressor inlets where it is joined by the work expanded major portion. The compressed feed gas is withdrawn from path a of exchanger 169 cooled by external refrigeration means 175 and is then returned to path a of exchanger 169. Throttle valve 204 discharges into a separater 206 which also condenses oxygen. Vapour above liquid oxygen storage vessel 112 and a portion of condensed nitrogen is passed along a line 122 to provide additional reflux for the column (103).

Description

Dec. 19, 1967 D. L. SMITH ETAL 3,353,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANT Filed Oct. 8, 1965 s Sheets-Sheet 1 I so FIG.

' TEMPE/M TURE-ENTHALPY PLOT FOR COLD EXCHANGER x I60 I80ATM NITROGEN I COOLING CURVE Lu I40 it b Y L; I30 Q: l20 E I I HEA TING cum/.1:

IOO

l l l I l l l 240 200 I60 l so o ENTHALPY CHANGE TEMPERA TURE-ENTHALPV PLOT l 50 FOR COLD EXCHANGER 45 A TM NITROGEN a v COOL ING CURVE u Qzl b so 2 E12o H g1 10 u loo HEATING cum/E 2 .0 260 '30 |'2o 8'0 4'0 0 ENTHALPV CHANGE DONALD L. SMITH B ATTORNEY Dec. 19, 1967 D. L. SMITH ETAL 3,353,460

NITRQGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANT Filed Oct. 8, 1965 e Sheets-Sheet 2 7 FIG. 3

V TEMPERATURE-ENTHALPY PLOT FOR COLD EXCHANGER 45 ATM NITROGEN COOL ING CURVE ac Q5 E l 130- I E REHEATSECT/ON q Q 7 i' 120 I 3 8 6c LIL! RE LA 7'/ l/E POSITIONS OF ||O TURB/NES IN cow END E CYCLE V I HEAT/N6 CURVE i IOO- l I v l 1 I 240 200 I60 I20 80 4O 0 ENTHAL PV CHANGE DONALD L.$M/TH JOHN L FERRELL 5 $1M eh:

ATTORNE V Dec. 19, 1967 D. L. SMITH ETAL NITROGEN LIQUEFACTION wrrn PLURAL worm 6 Sheets-Sheet 5 EXPANSION OF FEED AS REFRIGERANT Filed Oct. 8, 1965 FIG. 4

TEMPERATURE -ENTHALPV PLOT FOR COLD EXCHANGER .35 ATM N/ 777065 N COOLING CURVE V \Q'IQ RELATIVE POSI T/ONS T OF TURBINES 11v SECTION CYCLE REHEA NO. I

REHEAT SECTION No.2

M m w M T m. m

OF TURBINE 5 IN C VCLE HEA T/NG CUR l/E [20 so ENT'HALPV CHANGE DONALD LSMITH Q 'NVENTORS JOHN L.FERRELL I ATTORNEY Dec. 19, 1967 D. L. SMITH ETAL 3,358,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION 0F FEED AS REFRIGERANT Filed (Jot. 8, 1965 6 Sheets-Sheet 4.

FIG. 5

N/ TROGEN REFRIGERATION CVCLE WITH DOUBLE EXPANSION 8 REHEAT 4 COMPRESSOR g? s 7 a b c REFRIGERATION l WARM UNIT 7 EXCHANGE? COMPRESSOR 33 1 /62 COLD /EXCHANGER EXPANDER Y 4/ COMPRESSOR L IOU/D PRODUCT DONALD L. SMITH 3 5i JOHN L. FERRELL ATTORNEY Dec. 19, 1967 D. SMITH ETAL NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIUERANT 6 Sheets-Sheet 5 Filed Oct. 8, 1965 mm W DONALD L. SMITH JOHN L .FERRELL b wvbw QK WX A TTORNE V Dec. 19, 1967 D. L. SMITH ETAL 3,358,460

NITROGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIGERANT Filed on. s, 1965 s Sheets-Sheet 6 2/6 LIQUID NITROGEN STORAGE REFR/GERAT/ON UN/T DONALD L SMITH JOHN L. FERRELL A NM-' ATTORNEY F IG. 7

United States Patent Ofifice 3,358,469 Patented Dec. 19, 1967 NI'IRGGEN LIQUEFACTION WITH PLURAL WORK EXPANSION OF FEED AS REFRIG- BRANT Donald L. Smith, Berkeley Hei hts, and John L. Ferrell, North Plat-infield, N.J., assignors to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Get. 8, 1%5, Ser. No. 494,177 11 Claims. (Cl. 62-9) This invention relates in general to the separation and liquefaction of low-boiling gases; and more particularly, to the liquefaction of nitrogen.

It is advantageous to operate liquefaction systems for low-boiling gases at relatively low head-pressures, say, of the order of 600 to 1,160 pounds per square inch absolute, since under such conditions an improved product may be realized while effecting substantial reductions in capital and maintenance expenditures. For example, centrifugal compression and expansion machinery, which is adapted for use in a relatively low pressure range, operates in such a manner that the product is substantially free of oil contamination. In addition, less expensive types of heat exchangers can be employed in the lower pressure ranges.

However, many liquefaction systems employing headpressures within the range mentioned have the disadvantage of being thermodynamically inefiicient. A possible explanation for this is that in a system employing a relatively high head-pressure, the high pressure stream in the cold-le heat exchanger cools substantially as a straight-line function of temperature, whereas this is not true in a system employing a relatively low head-pressure. In the latter, the high-pressure stream undergoes a change in the cooling rate within the low temperature range which causes an iriiection in the cooling curve, thereby producing a substantial temperature spread with the warming curve of the low temperature returning stream. This discrepancy leads to the consumption of substantial amounts of power, making such a system inefiicient.

Accordingly, it is a principal object of this invention to improve the efiiciency of liquefaction systems for lowboiling gases; and more particularly, to provide a liquefaction cycle for low-boiling gases under substantially reduced head-pressures which is characterized by greater thermodynamic efficiency than heretofore achieved in the low-pressure range. An ancillary object of the invention is to provide a cycle for the liquefaction of low-boiling gases, more particularly nitrogen, which operates at increased eliiciency to produce a purer product, coupled with decreased maintenance and capital costs compared to similar prior art cycles.

These and other objects are realized in accordance with the present invention in a cycle for the liquefaction of low-boiling gases, more particularly, nitrogen, which contemplates the use of head-pressures within the range 600 to 1,100 pounds per square inch absolute, wherein a major portion of the pressurized stream is deployed to cool a minor portion thereof, without any substantial liquefaction occurring during this heat exchange. The salient feature of this arrangement is the work-expansion of the deployed major portion in two or more steps with one or more reheat steps intervening.

At head-pressures within the range, say, 600 to 1,100 pounds per square inch absolute, an arrangement of the type mentioned performs the function of approximating the shape of the low-pressure warming characteristic of the returning streams in the cold-leg heat exchanger to the inflected shape of the cooling characteristic of the high-pressure stream, thereby decreasing the temperature spread between the characteristics and increasing the thermodynamic efiiciency of the system.

For the purposes of illustration, the invention is described with reference to a system for air separation and the liquefaction of oxygen and nitrogen, in which the stream of nitrogen gas from the separator is first compressed through seven stages, then partially cooled, after which the initial stream is separated into two streams. The first stream, about two-thirds by volume, is workexpanded, then reheated, and again work-expanded. This expanded component combines with returning cold vapor, and then passes back through heat exchangers countercurrent to the second stream which consists of the remaining third of the initial stream. The second stream is cooled in the heat exchangers without liquefaction after which it passes through a throttle-valve for isenthalpic expansion, whereby a portion is converted to liquid which is separated out from the gaseous phase in -a vapor-liquid separator. This liquid is again isenthalpically expanded to a pressure slightly above atmospheric pressure, at which pressure the liquid fraction is again separated out and stored. The loW pressure vapor, including fiash vapor from the surface of the stored liquid, is returned through the heat exchangers in countercurrent with the high pressure streams from which heat is absorbed, to the first compression stage for recycling through the system.

Whereas in the particular embodiment disclosed, the nitrogen liquefaction system includes two work-expansion steps separated by a reheat step, it will be apparent to those skilled in the art that improved results might be obtained under certain circumstances by the inclusion of additional work expansion and reheat steps.

By way of further illustrating the present invention, a nitrogen liquefaction system is disclosed in combination with an air separation system, in such a manner that a portion of the liquid nitrogen product serves as a refrigerating reflux for one of the rectification columns of the air separation system, and also, as a refrigerant for condensing fiash vapor rising from the surface of the stored liquid oxygen.

Although the present invention is described herein with reference to a nitrogen liquefaction system, it will be understood by those skilled in the art that the principles disclosed are applicable to the liquefaction of any gas having a boiling point below about l12 E, including, for example, oxygen, ethane, ethylene, methane, argon, fluorine, carbon monoxide, neon, hydrogen and helium.

Other objects, features, and advantages of the system will be apparent from a detailed study of the specification hereinafter with reference to the attached drawings, in which:

FIGURES l, 2, 3, and 4 are graphs illustrating the theory of the present invention, showing temperature in degrees Kelvin plotted against enthalpy change for the cooling and heatin curves in the cold-leg exchanger of a nitrogen liquefaction system. FIGURE 1 illustrates the characteristics of a prior art system having a head-pressure of atmospheres; FIGURE 2 illustrates characteristics of a prior art system utilizing a reduced headpressure of 45 atmospheres; FIGURE 3 illustrates characteristics of an improved system in accordance with the present invention at 45 atmospheres head-pressure employing two worlr expansion steps; and FIGURE 4 shows the characteristics of a projected system at 35 atmospheres head-pressure employing three work expansion steps in accordance with the present invention;

FIGURE 5 shows a nitrogen liquefaction cycle in accordance with the present invention, in a block diagram schematic; and

FIGURES 6 and 7 show an over-all detailed schematic for a combined air separation and oxygen and nitrogen liquefaction system in accordance with the present invention, the two figures adapted to be fitted together in the manner indicated in FIGURE 8.

An objective of this liquefaction cycle is to minimize the power consumption requirements. In order to achieve low power consumption, the temperature difference between the countercurrent streams in the heat exchangers should be kept to a minimum.

In FIGURE 1 of the drawings, which shows temperature in degrees Kelvin plotted against change in enthalpy, in joules per gram-mol, for the cold exchanger in a nitrogen liquefaction system subject to 180 atmospheres head-pressure, the cooling curve 1 for the high pressure stream and the warming curve 2 for the low pressure return stream are both relatively straight line characteristics of approximately the same slope. Because of this fact, the temperature difierence AT between the cooling and warming streams remains substantially uniform throughout the exchanger, thus making a multi-step work expansion with intervening reheat of no particular advantage in such a system.

Let us refer, now, to FIGURE 2 of the drawings. This shows a plot of the same parameters for the cold exchanger in a nitrogen liquefaction system subject to a head pressure of 45 atmospheres. It is apparent that at the lower head-pressure the cooling curve 3 of the high pressure stream is substantially inflected, producing a substantial spread, or temperature difference AT, with the warming curve 4 at the lower end of the scale. As explained before, this spread in AT makes the system thermodynamically ineflicient, inasmuch as additional power must be consumed to make up this difference.

In order to remedy this defect, and to provide a system which is more thermodynamically eflicient at the lower head-pressures, the present invention contemplates that the shape of the warming characteristic in the coldleg heat exchanger should be approximated to the inflected shape of the cooling curve. This is brought about by causing components of the warming stream to undergo a plurality of work expansion steps with intervening reheating.

For example, FIGURE 3 of the drawings shows curves plotted for a nitrogen liquefaction system similar to that represented in FIGURE 2, subject to 45 atmospheres head-pressure, but modified to include an auxiliary stream which undergoes two work expansion steps, in the positions indicated by 7 and 8, and an intervening reheat step in the cold-leg heat exchanger running countercurrent with the cooling stream. In this plot, the cooling curve 5 is inflected, and is similar to cooling curve 3 of FIGURE 2. The warming curve in this modified system comprises sections 6a, 6b, and 60, whose slopes tend to conform at the different levels to the slope of the cooling curve 5, thereby increasing the thermodynamic efficiency of the system.

FIGURE 4 of the drawings shows a nitrogen liquefaction system subject to a still lower head-pressure, 35 atmospheres, in which the same parameters are plotted, as in the foregoing figures. The'cooling curve 9 at this low head-pressure sustains an even greater inflection. In order to compensate for this in accordance with the present invention, three work expansion steps and two reheat sections are included as indicated schematically by the

expanders

11, 12, and 13, and

arrows

14 and 15. This produces an inflected warming curve a, 10b, 10c, 10d, and 10e which approximates the shape of cooling curve 9.

It will be understood that this approach can be applied to the use of even a larger number of work expansion steps and intervening reheat steps.

A nitrogen liquefaction system embodying the principles set forth in the foregoing paragraphs is shown in block diagram in FIGURE 5 of the drawings.

A stream of make-up gas, comprising nitrogen, say, 99.99% pure, at substantially atmospheric pressure and temperature, is brought into the system through conduit 20,'which flows into junction 21. This stream is compressed through a plurality of compression stages 22 to a pressure of, say, 435 pounds per square inch absolute. The stream then passes out through conduit 23 to a pair of centrifugal compression stages 24 and 26, connected by conduit 25, Where it is further compressed to a pressure of 670 pounds per square inch absolute, passing out through conduit 27.

The highly compressed stream then passes into channel a of warm-leg heat exchanger 28 where it is cooled to about -40 F. It is then withdrawn from an intermediate point in the heat exchanger through conduit 29, and passed through a refrigerator 31, where it is cooled, say, to -l0l F., before passing back into channel a of heat exchanger 28 through conduit 32.

The high pressure primary stream, cooled to about F., passes out of heat exchanger 28 to junction 33, where it is separated into two streams. The first, or major stream, comprising about two-thirds by volume, passes through conduit 34 to a work expansion stage 35, where it is expanded to a pressure of about pounds per square inch absolute, at a temperature of 236 F. The cooled, expanded major stream then passes out through conduit 36 and into channel d of cold-leg heat exchanger 37, where it is reheated to, say, 2l3 F. It then passes out through conduit 38 to a second work expansion stage 39, where it is expanded to a pressure of, say, 40 pounds per square inch absolute, and a temperature of 289 F.

This cooled, expanded medium-low pressure stream then flows into junction 42, where, joined by a medium low pressure stream from conduit 59, it flows through conduit 43 into channel b of heat exchanger 37, and through conduit 44 into channel b of'heat exchanger 28, where it absorbs heat from counterflowing high pressure cooling streams. It then leaves heat exchanger 28 at a temperature of about 85 F., and is returned through conduit 45 to the second stage of compressor 22 for recompression and recycling in the system.

Simultaneously, the remaining third of the high pressure stream flowing into junction 33 flows out through channel a of heat exchanger 37, Where it is cooled to a temperature of, say, --287 F. by counterflowing streams. It flows out of cold exchanger 37 through conduit 47, and into the throttle valve 48 where it is isenthalpically expanded to a pressure of, say, 40 pounds per square inch absolute, and cooled to a temperature of, say, 303 F. The stream then passes into a first separator 51 through conduit 49, where the liquid portion is separated out, and the vapor phase passes out of the top of the vessel through conduit 59 to join the medium-low pressure gas stream flowing into junction 42 for return through the heat exchangers. V The liquid 52 from the bottom of separator 51 passes out through conduit 53 and through a second isenthalpic expansion step in throttle valve 54, Where it is expanded to a pressure of, say, 19 pounds per square inch absolute at a temperature of, say, 3l6 F.' ThC fluid from this second expansion step flows into a second separator 55, in which the liquid 56 is collected in the bottom, from which the liquid product is withdrawn through outlet valve 57. The low pressure flash gas from this separator passes out through the top of the vessel through conduit 58, channel c of cold-leg heat exchanger 37, connecting conduit 62, channel 0 of heat exchanger 28, and conduit 63. The latter joins junction 21 at the inlet to the system, where the low pressure stream, warmed to about 85 F., and at a pressure of 15 pounds per square inch absolute, joins the incoming make-up stream for recompression and recycling through the system.

A combined air separation and liquefaction system will now be described in greater detail, which includes a nitrogen liquefaction cycle substantially similar to that described with reference to FIGURE 5, with several slight modifications whereby a portion of liquid nitrogen stream serves as refrigerating reflux in the low pressure column of the air separation system, and provides refrigeration for condensing flash vapor rising off of the liquid oxygen storage tank.

Referring now to FIGURES 6 and 7 of the drawings, which fit together in the manner indicated by FIGURE 8, there is shown, in detailed schematic, a combined air separation, oxygen and nitrogen liquefaction system, illustrative of the principles of the present invention.

For the purposes of the present illustration, a separation-liquefaction system will be described which is designed to produce liquid oxygen at the rate of 100 tons per day and liquid nitrogen'at the rate of 304 tons per day.

Referring to FIGURE 6 which shows the air separation unit, air flowing at the rate of about 8,000 standard cubic feet per minute, a temperature of 90 F., and at atmospheric pressure, is introduced through a conventional, non-lubricated type of fiberglas air filter 71 to the suction inlet of a conventional oil-free compressor 72, Where the air is compressed to about 110 pounds per square inch absolute. Compressor 72, in the present example, is a three-stage integral-gear centrifugal compressor with full cooling.

After compression, the air stream passes through aftercooler 73, where it is water cooled to a temperature of about 95 F., and thence through water separator 74, from which it flows through a refrigerator 76, consisting of a refrigeration circuit 75 which employs one ofthe fluorocarbon refrigerants. This unit operates to bring the temperature of the stream down to 40 F. The stream then passes through another water separator 77 to junction 78 where the stream passes through a pair of conventional drying chambers 79 and 81, where it is dried to a very low dew point.

The dried stream then passes at a pressure of 96 pounds per square inch absolute through conduit 82 to the channel a of a heat exchanger 83 where the temperature is reduced to l25 F. by counterflowing streams of air and nitrogen. In the present example, this heat exchanger is a conventional type, consisting of brazed aluminum cores.

Carbon dioxide impurity and any remaining water vapor are removed from the stream as it passes from the junction 84 through the dual-

bed absorption chambers

85 and 86, adapted for low temperature removal of carbon dioxide by means of absorbing materials such as the zeolites, known in the art as molecular sieves. The unit is adapted to be reactivated every 16 hours using lowpressure cycle nitrogen. The air stream then Passes at a pressure of about 90 pounds per square inch absolute through channel a of a second heat exchanger 3%, where the temperature is reduced to 287 F At the aforesaid temperature and at a pressure of about 89 pounds per square inch absolute, the stream enters into the bottom of the high pressure rectification column 91, which in the present illustration is a conventional type. The air is separated by distillation in column 91 into a bottom liquid product containing about 38% oxygen, which in the specification hereinafter will be referred to as rich liquid. Pure nitrogen gas at the top of column 91 passes out through the outlet pipe 113 and into the reboiler 106, Where it is condensed by heat exchange with evaporating liquid oxygen from the bottom of low pressure column 103.

About 60% of the pure liquid nitrogen from reboiler 106 passes back into the top of high pressure column 91 through inlet pipe 115, where it is used for reflux, while the remainder (3,400 standard cubic feet per minute) passes into coil 116 of the subcooler 95, which in the present example is a wound tube aluminum exchanger having three tube passes and a single shell pass. In subcooler 95 this liquid nitrogen is subcooled by cold lowpressure nitrogen gas passing in the outer shell, to a temperature of -305 F., at a pressure of pounds per square inch absolute, before it passes to junction 118, under control of valve 120, and through inlet 119 into the top of low-pressure rectification column 103, where it serves as reflux.

The rich liquid flows out of the bottom of the high pressure rectification column 91 through conduit 93 at the rate of 4,680 standard cubic feet per minute, at a temperature of -278 F., and a pressure of 89 pounds per square inch absolute, through coil 94 of subcooler 95, and through the junction 96 into the silica gel absorber 97-98, where hydrocarbon impurities, such as acetylene, are removed before it passes through valve 101 and through inlet 102 entering as a feed-stream into the low pressure column 103, at a point part-way up in the column.

The final separation of oxygen and nitrogen occurs in the low pressure column 103, where all of the air is separated into pure products except for a small control stream. The latter serves to stabilize operation of the column, passing out through outlet pipe 133 at the rate of 321 standard cubic feet per minute, a temperature of 307 F., and a pressure of 20 pounds per square inch absolute. This stream ultimately passes through heat exchanger column 89d, connecting pipe 134, and column d of heat exchanger 83, to outlet 135, where it is vented to the atmosphere at about 36 F. i

As previously pointed out, reflux for low pressure column 103 is supplied by condensed liquid nitrogen from high pressure column iil through valve 120. This is augmented for refrigeration purposes by a'stream of liquid nitrogen flowing from the nitrogen liquefaction unit through conduit 122 under control of valve 123. In the present example, this stream flows at the rate of 2,066 standard cubic feet per minute, at a temperature of 303 F., and a pressure of 40 pounds per square inch absolute. It will be apparent that

valves

120 and 123 will regulate the respective flows to provide the desired degree of refrigeration in the reflux stream.

Pure liquid oxygen flows out of the bottom of the low pressure column 103, through conduit 104 and into junction 105, where a portion passes through the column b of the reboiler 1136 where it is evaporated by heat exchange with condensing liquid nitrogen from the high pressure rectifying column 1, and passes back into low pressure column 103, through inlet 107 at the lower end.

Liquid oxygen product from the low pressure column 103, flowing at the rate of 1,676 standard cubic feet per minute, at a temperature of 289 F., and a pressure of 23 pounds per square inch absolute, is pumped out of junction by means of pump 169 through the coil 110 of subcooler 95, where it is reduced to a temperature of -299 F. at a pressure of 40 pounds per square inch absolute, and ultimately through conduit 111 to the liquid oxygen storage tank 112, shown in FEUURE 7.

Pure nitrogen from the top of low pressure column 103 passes out through vent 124, at the rate of 8,159 standard cubic feet per minute, a temperature of 3l6 F. and pressure of 19 pounds per square inch absolute. It fiows through the outer shell of subcooler $5, cooling the liquids passing through the inside coils, and then flows through conduit 126 at a temperature of 2 83 F. to channel b of heat exchanger 89, connecting conduit 127 and channel b of heat exchanger 83, where the temperature is raised to 36 F. at 15 pounds per square inch absolute pressure. From here it passes through conduit 1255 as the make-up stream for the nitrogen liquefaction cycle, to be described presently Pure oxygen vapor passes through conduit 130 at the lower end of low pressure column 103, at a temperature of 289 F. and a pressure of 23 pounds per square inch absolute, through channel 0 in each of

heat exchangers

89 and 83, and is vented into the atmosphere through vent 132 An auxiliary path is provided between the high pres- 7 sure column 91 and the low pressure column 103,

through outlet 136 at the lower end of column 91 so that a portion of the cooled air stream, about 25% or less, coming into the lower end of column 91 can be withdrawn if desired under control of valve 137, partially rewarmed in channel 2 of heat exchanger 89, expanded with the production of work in expander 139, and introduced into low pressure column 103 at a point below the middle, through inlet 142 under control of valve 141. This auxiliary path provides means, if desired, for saving power in the refiigeration system, at the expense of lower oxygen recovery and lower purity by the nitrogen product.

Referring now to FIGURE 7, this shows a schematic diagram in accordance with the present invention of a nitrogen liquefaction circuit to which the make-up stream of gas is furnished through conduit 128. In the present example, the following is a typical analysis of the impurity content of the nitrogen make-up stream:

In the example under description, which anticipates a liquid nitrogen output of 304 tons per day, the stream of nitrogen flows from conduit 128 into the junction 129 at the rate of 8,159 standard cubic feet per minute, a temperature of 36 F., and a pressure of about 15 pounds per square inch absolute.

The valve 149 can be utilized to control the rate of flow of gas from the junction 129. In the present example, gas flows at the rate 7,998 standard cubic feet per minute from junction 129 intojuuction 150, at a temperature of 36 F., and a pressure of 15 pounds per square inch absolute. The low pressure nitrogen stream returning through conduit 221 flows into junction 150 at the rate of 637 standard cubic feet per minute, a temperature of 85 F., and a pressure of 15 pounds per square inch absolute.

The low pressure stream flows out of junction 150 to the first compression stage 151 at the rate of 8,535 standard cubic feet per minute, at a temperature of 50 F., and pressure of 15 pounds per square inch absolute, which is raised to approximately 32 pounds per square inch absolute in the first stage. The stream from the first stage is cooled down in after-cooler 152, and flows into junction 201, where it is joined by the medium pressure return stream flowing through conduit 199 at the rate 25,665 standard cubic feet per minute, at a temperature approximating 85 F., and pressure of 32 pounds per square inch absolute.

The fully compressed nitrogen then flows through the rate of 34,200 standard cubic feet per minute through four additional stages of compression, 153, 155, 157, and 159, each followed by a respective after-

cooler

154, 156,

, 158, or'160. The stream emerges from the fifth compression stage and its after-cooler into conduit 162 at a pressure of 430 pounds per square inch absolute and a temperature of 95 F.

The

compressors

151, 153, 155, 157, and 159 are conventional types which are operated in series fiom a common 10,000 horsepower motor, and controlled by means of suction-throttling and discharge pressure control. Reverse rotation is prevented by a check valve installed in the discharge line. The intercoolers and

aftercoolers

152, 154, 156, 158, and 160 are conventional types of water coolers.

The partially compressed stream passes through conduit 162 to a series-connected pair of centrifugal compressors, of conventional form, in which the stream is compressed in two steps to 670 pounds per square'inch absolute, at a temperature of F. Each of these compressors is followed by a conventional after-cooler, 164 and 167 respectively. Moreover, the centrifugal compressors are respectively coupled to a pair of expansion turbines 194 and 188, which are mounted on the same shaft, in such a manner that the compressor wheels serve as a brake for the turbines, absorbing the work of the expanding gas.

The fully compressed nitrogen then flows through the conduit 168 into channel a of heat exchanger 169 for cooling by the outgoing low pressure and medium pressure nitrogen recycle streams. This heat exchanger is a brazed aluminum type comprising five cores in parallel flow.

At the point in channel a of exchanger 169 where the high pressure nitrogen stream reaches 40 F., the total stream is withdrawn from the heat exchanger 169 through conduit 170 and passed into a conventional refrigeration unit where it is cooled from -40 F. to 101 F.

In the present example, the refrigeration system 175 comprises two different refrigeration cycles operating in cascade to cool the high pressure nitrogen stream. The

first of these refrigeration cycles'employs dichlorodifiuoromethane CCI F known by the trade-name Freon 12; and the second cycle employs monochlorotrifluoromethane, known by the trade-name Freon 13. (Both of these are registered tradenames of E. I. du Pont de Nemours and Company.) The nitrogen stream emerging at -40 F. from channel a of heat exchanger 169 flows through conduit 170 under a pressure of 669 pounds per square inch absolute, at the rate of 34,200 standard cubic feet per minute into the refrigeration unit 175.

After the cooled nitrogen stream hasreturned from refrigeration unit 175' to channel a of heat exchanger 169 at a temperature of 101 F., it is further cooled by the counter-flowing fluids in channels b and 0 thereof to -130 F. at which temperature and a pressure of 663 pounds per square inch absolute it leaves the heat exchanger 169 and flows into junction 186. At this point approximately 70% of the high pressure nitrogen stream (24,200 standard cubic feet per minute) is drawn off into conduit 187. This major portion passes into turboexpander 188, where it is expanded with the production of work to a pressure of 147 pounds per square inch absolute, and is thereby cooled to a temperature of -236 F. As pointed out previously turboexpander 188, and its companion, turboexpander 194, used in the next stage, are of the radial inflow type. These turboexpanders are mounted on the same shaft with and drive the respective centrifugal compressors 166 and 163; they are lubricated by a common system. Variable inlet nozzles control the flow through the turboexpanders which regulates the power available to compress the cycled nitrogen.

The stream, expanded to 147 pounds per square inch absolute, passes out of turboexpander 188 through conduit 189 and is reheated in channel d of heat exchanger 192 to a temperature of 213 F. The stream is then again expanded with the performance of work through a second stage comprising the turboexpander 194 from a pressure of 145 to a pressure of 35 pounds per square inch absolute and is cooled to a temperature of 289 F. The medium-low pressure stream passes out of turboexpander 194 through conduit 195 and into junction 196, where it re-enters channel b of heat exchanger 192 through conduit 197, in combination with the return medium-low pressure flow through conduit 210. The combined streams pass through conduit 197 into channel b of heat exchanger 192, and through conduit 198 into channel [7 of heat exchanger 169, where they are heated up to a temperature of 85 F. Flowing at the rate of 25,665 standard cubic feet per minute, the stream then passes through conduit 199 and into the suction end of the second compression stage, through junction 201, at a pressure of 32 pounds per square inch absolute for recycling in the liquefaction system.

The approximately 30% of the high pressure stream remaining at junction 186, where the major portion was diverted through the turboexpanders, passes through channel a of heat exchanger 192, where it is cooled to a saturated fluid of a single phase at about -269 F. This product stream, flowing at the rate of approximately 10,000 standard cubic feet per minute in conduit 203, and at a pressure of 660 pounds per square inch absolute, is throttled (isenthalpically expanded) to 19 pounds per square inch absolute in two steps. The stream is first expanded through Joule-Thompson expansion valve 204, which is a conventional cryogenic throttle-valve, to approximately 40 pounds per square inch absolute, at a temperature 303 F. This fluid passes into vessel 206 atop the liquid oxygen storage tank 112. Vessel 206, which is a conventional type of liquid separator, serves as a liquid-vapor separator as Well as a condenser for flash oxygen vapor from the tank, which flows back into the oxygen tank through receptacle 206a.

The nitrogen flash gas passing out of the top of separator 2G6, moves through conduit 210 to junction 196, flowing at the rate of 1,465 standard cubic feet per minute, at a temperature of -303 F., and a pressure of 36 pounds per square inch absolute. The combined low pressure stream then passes through channel b of each of the exchangers 192 and 169, joining the suction portion of the second compression stage at 85 F., as previously described.

The liquid fraction of nitrogen passes out of separator 206 through conduit 207, and into junction 121, flowing at the rate of 8,535 standard cubic feet per minute, at a temperature of 303 F., and pressure of 40 pounds per square inch absolute. Part of the liquid nitrogen serves as refrigerating reflux for the low pressure column 103 of the air separation system, passing through conduit 122 at the rate 2,066 standard cubic feet per minute. The remainder (flowing at the rate of 6,469 standard cubic feet per minute) passes through conduit 298 to a second Joule-Thompson expansion valve 209, where it is again throttled (isenthalpically expanded) to a pressure of 19 pounds per square inch absolute, and a temperature of 3l6 F. It flows into a second separator 211 at the rate of 6,469 standard cubic feet per minute, where the liquid nitrogen separates out and is expanded into the storage tank 214 at a pressure of 15 pounds per square inch absolute through valve 213. Both the liquid oxygen storage tank 112 and liquid nitrogen storage tank 214, are conventional insulated types of a form well-known in the art.

Saturated nitrogen vapor flows out of separator 211 at the rate of 430 standard cubic feet per minute, a temperature of --316 F., and a pressure of 19 pounds per square inch absolute, through conduit 215 to junction 218 where it is joined by flash nitrogen vapor from stor age tank 214, pumped through conduit 216 by a conventional cold blower pump 217, at a rate of 207 standard cubic feet per minute, a temperature of 303 F., and a pressure of 19 pounds per square inch absolute. The combined low pressure stream passes through conduit 219 and channel of heat exchanger 192. It then passes through conduit 220 to channel c of heat exchanger 169, where it emerges at a temperature of 85 F, flowing through conduit 227 into junction 150 at a rate of 637 standard cubic feet per minute and at a pressure of 15 pounds per square inch absolute, through which it returns to the first compression stage for recycling in the nitrogen liquefaction system.

It will be apparent to those skilled in the art that this invention is not limited to the specific embodiments disclosed by way of illustration; but rather, that the scope of the invention is defined in terms of the appended claims.

We claim:

1. In a cycle for the liquefaction of low boiling gas which comprises the steps of:

compressing a primary stream including the feed stream of said gas to a head pressure not exceeding about two and one-half times the critical pressure of said cooling said compressed primary stream through a first cooling step in a first heat exchanger,

further cooling at least a portion of said compressed primary stream through a second cooling step in a second heat exchanger, without substantial liquefaction thereof, isenthalpically expanding said compressed portion after said second cooling step to a low pressure to form a liquid fraction and a remaining vapor fraction, and

returning the remaining low pressure vapor fraction of said portion at said low pressure for recompression and recycling through a path including said heat exchangers in countercurrent with streams being cooled including said compressed portion;

the improvement which comprises:

separating said compressed primary stream before said second cooling step into a major portion and a minor portion, whereby the compressed portion further cooled in said second heat exchanger is said minor portion,

withdrawing and expanding said major portion through a plurality of work expansion steps wherein said major portion is cooled and expanded to a medium-low pressure without liquefaction,

each of said work expansion steps being separated by an intervening reheat step wherein the stream comprising said major portion is warmed up in said second heat exchanger in countercurrent with streams being cooled including said compressed minor portion,

and after said final work expansion step, returning said major portion at a medium-low pressure without liquefaction through said heat exchangers in countercurrent with streams being cooled including said compressed minor portion, for recompression and recycling.

2. In a cycle for the liquefaction of low boiling gas in accordance with claim 1, expanding said major portion through two work expansion steps separated by one intervening reheat step wherein the stream comprising said major portion is warmed in countercurrent with said minor portion being cooled.

3. A cycle for the liquefaction of low boiling gas in accordance with claim 1 wherein said step of compressing said primary stream of gas takes place in a plurality of stages, wherein said low pressure vapor "naction is returned to the initial compression stage for recompression and recycling, and wherein said medium-low pressure major portion is returned to a later compression stage for recompression and recycling.

4. In a cycle for the liquefaction of low boiling gas which comprises compressing a primary stream of said gas to a head pressure not exceeding about two and one-half times the critical pressure of said gas,

cooling said compressed primary stream through a first cooling step, further cooling at least a portion of said compressed primary stream through a second cooling step in heat exchanger means, wherein an inflected cooling characteristic is produced in said heat exchanger,

isenthalpically expanding said compressed and further cooled portion to a low pressure to form a liquid fraction and a remaining vapor fraction,

and returning the remaining low pressure vapor fraction of said portion for recompression and recycling through a path including said heat exchanger means withdrawing and expanding said major portion through a plurality of work expansion steps, cooling and expanding said major portion to a medium-low pressure Without liquefaction, each of said work expansion steps being separated by an intervening reheat step wherein the stream comprising said major portion is warmed up in said heat exchanger means in countercurrent with streams being cooled including said compressed minor portion, and following said final work expansion step, returning said major portion at medium-low pressure without liquefaction through said heat exchanger means in countercurrent with streams being cooled including said minor portion for recompression and recycling, thereby producing a composite warming characteristic which approximates the inflected shape of the cooling characteristic of the stream comprising said compressed portion. 5. A cycle for liquefying nitrogen gas which comprises the steps of compressing a primary stream of nitrogen including a feed stream through a plurality of compression stages to an elevated pressure within the range 40 to 75 atmospheres, cooling said compressed primary stream by means including a first heat exchanger to a temperature within the range -l20 F. to 130 F., subsequently separating said compressed primary stream into a major stream consisting of between 65 and 75% by volume of said primary stream, and a minor stream consisting of the remainder, further cooling said compressed minor stream through a second heat exchanger, isenthalpically expanding said compressed minor stream to a low pressure to form a liquid fraction and a remaining vapor traction, returning the remaining low pressure vapor fraction through said heat exchangers in countercurrent with streams being cooled including said compressed minor stream for recompression and recycling, expanding said compressed major stream through a plurality of work expansion steps wherein said major stream is cooled and expanded to a medium-low pressure Without liquefaction, each of said work expansion steps being separated by a reheat step wherein said major stream is warmed up in said second heat exchanger in countercurrent with said cooling compressed minor stream, and after said final work expansion step returning said major stream at a medium-low pressure without liquefaction for recompression and recycling through a path including said heat exchangers in countercurrent with streams being cooled including said compressed minor stream. 6. A cycle for the liquefaction of nitrogen in accordance with claim wherein said major stream is expanded through a first work expansion step to a pressure less than a quarter of said elevated pressure and a temperature within the range 235 F. to 245 F., said major stream is subsequently reheated in said second heat exchanger in counter-current with said compressed minor portion to a temperature within the range -210 F. to 220 F., and said major stream is finally expanded in a second work expansion, step to a pressure slightly in excess of 2.5 atmospheres and cooled to a temperature within the range -285 F. to 305 F.,

prior to returning without liquefaction through said heat exchangers. 7. In a cycle for the liquefaction of nitrogen in accordance with claim 5 wherein said step of compressing said primary stream takes place in a plurality of stages, wherein said low pressure vapor fraction is returned to the initial compression stage for recompression and recycling, and wherein said medium-low pressure major portion is returned to a later compression stage for recompression and recycling.

8. A cycle for the liquefaction of nitrogen in accordance with claim 5 wherein said compressed primary stream is initially cooled in said first heat exchanger to a temperature within the range -35 F. to 45 F.,

said compressed cooled primary stream is then withdrawn laterally from said heat exchanger and cooled by external refrigeration means to a temperature within the range F. to F.,

and said further cooled primary stream is again passed into said first heat exchanger where it is cooled to a temperature within the range l20 F. to F. by heat exchange with counterflowing streams in said heat exchanger.

9. In an air separation and liquefaction cycle which comprises in combination the steps of:

removing impurities from a feed-stream of said air including carbon dioxide and water,

compressing said feed-stream to an elevated pressure,

cooling said feed-stream to approximately its satura tion temperature by heat exchange with outgoing gas streams, separating said feed-stream by distillation in a high pressure rectification column into a bottom product called rich liquid containing substantially more than 20% by volume of oxygen and an overhead high purity nitrogen product,

subcooling and further separating said rich liquid by distillation in a low pressure rectification column into a high purity liquid oxygen product and an overhead high purity nitrogen product,

condensing the nitrogen product of said high pressure rectification column by reboiling said liquid oxygen 7 product from the bottom of said low pressure rectification column,

utilizing a major portion of said condensed liquid nitrogen product as reflux for said high pressure rectification column,

subcooling and utilizing the remainder of said condensed liquid nitrogen product for reflux in said low pressure rectification column,

storing the liquid oxygen product of said low pressure rectification column in an insulated storage vessel,

passing a stream of high purity nitrogen gas overhead from said low pressure rectification column through subcooling means for concurrent'streams of rich liquid oxygen and liquid nitrogen,

utilizing said stream of high purity nitrogen as a feedstream for a liquefaction cycle operating in conjunction with said air separation cycle,

compressing and cooling nitrogen feed-stream through a first cooling step in a first heat exchanger,

further cooling at least a portion of said compressed nitrogen feed-stream through a second cooling step in a second heat exchanger without substantial lique: faction thereof,

isenthalpieally expanding said compressed nitrogen portion after said second cooling step to a low pressure to form a liquid fraction and a remaining vapor fraction, and

separating and returning the remaining low pressure vapor fraction of said portion at said low pressure for recompression and recycling through a path in:

cluding said heat exchangers in countercurrent with streams being cooled including the compressed nitrogen feed-stream;

the improvement which comprises:

separating said compressed nitrogen feed-stream before said second cooling step in said nitrogen liquefaction cycle into a major portion and a minor portion whereby the compressed portion further cooled in said heat exchanger is said minor portion,

each of said work expansion steps being separated by an intervening reheat step, wherein the stream comprising said major portion is warmed up in said second heat exchanger in countercurrent with streams being cooled including said compressed minor portion,

after said final work expansion step returning said major portion at a medium-low pressure without liquefaction through said heat exchangers in countercurrent with streams being cooled including said minor portion for recompression and recycling,

utilizing a heat exchange between oxygen vapor rising off of said insulated storage vessel in said air separation system and said isenthalpically expanded nitrogen portion for separating liquid and vapor in said portion and for recondensing said oxygen vapor to liquid,

and utilizing at least :a portion of the liquid fraction of said nitrogen to supply refrigerating reflux to said low pressure rectification column in said air separation cycle.

10. An air separation cycle in combination with oxygen and nitrogen liquefaction cycles which comprises the steps of:

compressing a feed-stream of said air,

cooling said compressed stream of air through a path including heat exchanger means,

passing said cooled compressed stream of air into a high pressure rectification column for separation into rich liquid in the bottom of said column comprising substantially more than 20% by volume of oxygen, and nitrogen gas in the top of said high pressure column,

further cooling said rich liquid derived from the bottom of said high pressure rectification column in a subcooler in a heat exchange with counterflowing cold nitrogen vapor,

introducing said cooled rich liquid part-way up in a second low pressure rectification column,

passing the nitrogen vapor from the top of said high pressure rectification column through a condenser for condensation in a heat exchange with counterfiowing liquid oxygen, returning one portion of said condensed liquid nitrogen to the top of said high pressure column for refluxing,

and passing another portion of said liquid nitrogen through said subcooler in countercurrent with cooled nitrogen vapor and into the top of said low pressure rectification column for refluxing,

passing purified liquid oxygen out through the bottom of said low pressure column,

vaporizing part of said liquid oxygen in said condenser in a heat exchange with cold nitrogen vapor from the top of said high pressure rectification column and passing said oxygen vapor into said low pressure column,

pumping part of said liquid oxygen through said subcooler for further cooling and into an insulated receptacle for storage,

returning a control stream of cold low pressure nitrogen from near the top of said low pressure rectification column through said heat exchanger means in countercurrent with high pressure cooling streams, and ultimately releasing said stream to the atmosphere,

returning a stream of cold oxygen vapor from near the bottom of said low pressure rectification column through said heat exchanger means and releasing said stream into the atmosphere,

returning a stream of purified cold nitrogen from the top of said low pressure rectification column, and through said subcooler and heat exchanger means to said nitrogen liquefaction cycle as the make-up stream therefor,

in said nitrogen liquefaction cycle compressing said nitrogen stream through a plurality of compression stages terminating in at least one centrifugal compressor stage,

passing said compressed nitrogen make-up stream into a first heat exchanger for cooling to a temperature within the range 35" F. to 40 F.,

withdrawing said compressed nitrogen stream laterally from said first heat exchanger and passing it through refrigeration means comprising at least one external refrigerating cycle, wherein said nitrogen make-up stream is cooled to a temperature within the range F. to F.,

again passing said compressed cooled nitrogen stream into said first heat exchanger for cooling to a temperature within the range F. to F.,

separating said compressed nitrogen stream into a major stream consisting of 65% to 75% by volume of the flow of said compressed nitrogen stream, and a minor stream consisting of the remainder,

withdrawing said major compressed nitrogen stream through a first expansion turbine wherein said stream is work-expanded to a pressure within the range to pounds per square inch absolute and cooled to a temperature within the range 235 F. to 245 F.,

reheating said major stream to a temperature within the range -2l0 F. to 220 F. in a second heat exchanger in countercurrent with said compressed minor stream,

re-expanding said major stream through a second expansion turbine wherein said stream is workexpanded to a medium-low pressure within the range 30 to 40 pounds per square inch absolute, and cooled to a temperature within the range 285 F. to 305 F.,

returning said medium-low pressure stream through said heat exchangers to a stage following the initial stage of said compression for recompression and recycling,

passing said minor compressed nitrogen stream through said second heat exchanger in countercurrent with said low pressure returning major stream and thereby cooling said minor stream to a temperature within the range 260 F. to 270" F. at a pressure within the range 650 to 11,000;

expanding said cooled compressed minor stream in a first isenthalpic expansion step through a throttle valve to a pressure slightly above two atmospheres, thereby to convert a fraction of said stream to liquid,

separating out the nitrogen liquid fraction from the nitrogen vapor fraction in a heat exchange with flash oxygen vapor in condensing means communicating with said insulated receptacle for storing liquid oxygen from said air separation system,

eturning the medium-low pressure vapor fraction from said first isenthalpic expansion step through said heat exchanger, together with the medium-low ressure stream from said expansion turbine, to a stage following the initial stage of said compressor for recompression and recycling,

making a portion of said nitrogen liquid fraction available for use as a reflux in the top of the low-pressure rectification column in said air separation system,

passing the remainder of the nitrogen liquid fraction resulting from said first isenthalpic expansion step into a valve for a second isenthalpic expansion step to a pressure slightly above atmospheric pressure and a temperature below the liquefaction temperature of nitrogen,

separating off the low pressure saturated nitrogen vapor fraction from the final liquid nitrogen fraction resulting from said second isenthalpic expansion step inanother separating means, which includes flash nitrogen vapor from liquid nitrogen stored in said cycle,

passing said low pressure nitrogen vapor through said heat exchanger in countercurrent with cooling high pressure streams to the initial compression stage of said nitrogen liquefaction cycle for recompression and recycling,

References Cited UNITED STATES PATENTS.

Dennis 629 Koehn et a1. 629 Shaievitz et al 62--29 X Smith 6213 X Geist et al; 6229 Dennis v 6239 X NORMAN YUDKOFF, Primary Examiner.

20 W. PRETKA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,358,460 December 19, 1967 Donald L. Smith et 8.1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7

lines

56 and 57 for "The fully compressed nitrogen then flows through the rate of" read The augmented stream flows out of junction 201 at the rate of column 15, line 2, for "as a reflux" read 3 reflux Signed and sealed this 10th day of June 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. WI LL I AM E Attesting Officer Commissioner of Patents SCHUYLER, JR.

Claims

1. IN A CYCLE FOR THE LIQUEFACTION OF LOW BOILING GAS WHICH COMPRISES THE STEPS OF: COMPRESSING A PRIMARY STREAM INCLUDING THE FEED STREAM OF SAID GAS TO A HEAD PRESSURE NOT EXCEEDING ABOUT TWO AND ONE-HALF TIMES THE CRITICAL PRESSURE OF SAID GAS, COOLING SAID COMPRESSED PRIMARY STREAM THROUGH A FIRST COOLING STEP IN A FIRST HEAT EXCHANGER, FURTHER COOLING AT LEAST A PORTION OF SAID COMPRESSED PRIMARY STREAM THROUGH A SECOND COOLING STEP IN A SECOND HEAT EACHANGER, WITHOUT SUBSTANTIAL LIQUEFACTION THEREOF, ISENTHALPICALLY EXPANDING SAID COMPRESSED PORTION AFTER SAID SECOND COOLING STEP TO A LOW PRESSURE TO FORM A LIQUID FRACTION AND A REMAINING VAPOR FRACTION, AND RETURNING THE REMAINING LOW PRESSURE VAPOR FRACTION OF SAID PORTION AT SAID LOW PRESSURE FOR RECOMPRESSION AND RECYCLING THROUGH A PATH INCLUDING SAID HEAT EXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED PORTION; THE IMPROVEMENT WHICH COMPRISES: SEPARATING SAID COMPRESSED PRIMARY STREAM BEFORE SAID SEOND COOLING STEP INTO A MAJOR PORTION AND A MINOR PORTION, WHEREBY THE COMPRESSED PORTION FURTHER COOLED IN SAID SECOND HEAT EXCHANGER IS SAID MINOR PORTION, WITHDRAWING AND EXPANDING SAID MAJOR PORTION THROUGH A PLURALITY OF WORK EXPANSION STEPS WHEREIN SAID MAJOR PORTION IS COOLED AND EXPANDED TO A MEDIUM-LOW PRESSURE WITHOUT LIQUEFACTION, EACH OF SAID WORK EXPANSION STEPS BEING SEPARATED BY AN INTERVENING REHEAT STEP WHEREIN THE STREAM COMPRISING SAID MAJOR PORTION IS WARMED UP IN SAID SECOND HEAT EXCHANGER IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED MINOR PORTION, AND AFTER SAID FINAL WORK EXPANSION STEP, RETURNING SAID MAJOR PORTION AT A MEDIUM-LOW PRESSURE WITHOUT LIQUEFACTION THROUGH SAID HEAT EXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED MINOR PORTION, FOR RECOMPRESSION AND RECYCLING.