US2996891A

US2996891A - Natural gas liquefaction cycle

Info

Publication number: US2996891A
Application number: US685580A
Authority: US
Inventors: Shao E Tung
Original assignee: Conch International Methane Ltd
Current assignee: Conch International Methane Ltd
Priority date: 1957-09-23
Filing date: 1957-09-23
Publication date: 1961-08-22
Anticipated expiration: 1978-08-22
Also published as: FR1203583A; NL112795C

Description

sHAo E. TUNG 2,996,891

4 Sheets-Sheet 1 Aug. 22, 1961 NATURAL GAS LIQUEFACTION CYCLE Filed sept. 23, 1957 INVENTOR.

.SHAO E. TUN 6 y M ATTORNEXS.

Aug. 22, 1961 sHAo E. TUNG 2,996,891

NATURAL GAS LIQUEFACTION CYCLE Filed Sept. 25, 195'.' 4 Sheets-Sheet 2 A TTO PNE V5,

Aug. 22, 1961 sHAo E. TUNG NATURAL GAS LIQUEFAcToN CYCLE 4 Sheets-Sheet 3 Filed Sept. 23, 1957 -lSO-l40 -IBO -120 -IlO -IOO -90 TEMPERATURE DEGREES FAHRENHEIT www@ 8765 0 O O O O 2 m 4 6 B Aug. 22, 1961 sHAo E. TUNG 2,996,891

NATURAL GAS LIQUEFACTION CYCLE:

Filed Sept. 25. 195'. 4 Sheets-Sheet 4 CH4 70% co2 30% p: 7|5ps|a FROM HIGH PRESSURE lSCRUBBING LIQUID IN COMBINED STREAM p= 673pso Z 2 SCRUBBERS f= 93 F SETTLER SCRUBBING LIQUID OUT BACK To HIGH PRESSURE coMBmED STR EAM JQQ, 5

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/Ao TUA/G v O IO 2O 30 40 50 60 70 BO 90 IOO ATTORNEYS- PHASE COMPOSITION MOL. FRACTION OF CARBON DIOXIDE United States Patent 2,996,891 NATURAL GAS LIQUEFACTION CYCLE ShaoE. Tung, Ponca City, Okla., assignor, by mesne assignments, to Conch International Methane Limited, Nassau, Bahamas, a corporation of the Bahamas Filed Sept. 23, 1957, Ser. No. 685,580 14 Claims. (Cl. 6212) This invention relates to the liquefaction and purification of natural gas and more specically to a process by which such liquefaction and purification can be accomplished at maximum eiiiciency to produce substantially pure liquid methane.

In gas liquefaction processes, the removal of acid gases (carbon dioxide and hydrogen sultide) and water vapor has always been considered essential; otherwise these constituents will solidify at low temperatures and then plug the liquefaction equipment `and render it inoperable. in the case of natural gas liquefactio-n, the installation for acid gas removal such as by the Girbotol process (aliphatic amine extraction) may well represent of the installation cost of rthe entire liquefaction plant. However, under properly chosen liquefaction conditions, a gas cleaning operation may be introduced into the liquefaction process itself and then the pre-liquefac tion gas cleanup step may be omitted.

One object of the invention is to provide a liquefaction process for producing substantially pure liquid methane from a stream of natural gas which contains one or more constituents such as heavier hydrocarbons, carbon dioxide, hydrogen sulfide, or nitrogen.

Another object of the invention is to liquefy natural gas streams containing impurities at a maximum ethciency and a minimum cost.

Other objects of this invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following descriptiony and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In said annexed drawings:

FIGS. l and 2 together comprise a flow diagram illustrating my new liquefaction process;

FIG. 3 is a graph showing the pressure-temperature relations of a liquid-vapor-solid carbon dioxide equilibrium in a CO2-CH4 system;

FIG. 4 is a graph showing the temperature-composition sections of a CO2-CH., system at 7.15 p.s.i.a.;

FIG. 5 shows the apparatus to be inserted into the liquefaction cycle when the CO2 content exceeds the tolerance limit; and

FIG. 6 is a graph which shows the temperature-composition section of a CO2-CH., system at 673 p.s.i.a.;

In my process the feed stream of well gas, substantially methane, driedl but not purifiedv of the iacid gases, is combined with the gas to be recycled and is thereafter sent through a series of heat exchangers to be cooled down to approximately minus 150 F. The pressure of the combined gas stream, however, is here maintained above the univariant vapor-liquid-solid equilibrium curve of the combined gas stream; therefore no solid will separate out in the heat exchangers as long as the content of carbon dioxide and/or hydrogen sulfide in this combined gas stream. is not excessive.

By maintaining the proper pressure the gaseous stream can be cooled down to minus 50 F. with no formation of solid particles by heat exchange with external refrigeration and cold expanded gas to be later recycled.

Following the liquefaction at elevated pressures the stream may be reduced to a liquid by throttling the stream and withdrawing the gas subsequently evaporated. During this throttling process the temperature of the liquid stream will be substantially reduced and the acid gases will freeze. The acid gases may be separated from the methane stream during this throttling process as solid particles.

When hydrocarbons heavier than methane are also present in the natural gas stream, since substantially pure methane is desired, a pressure below that of the cricondenbar point of the combined stream (combined feed stream and the recycle stream) but above the univariant vapor-liquid-solid equilibrium curve of that same stream may be chosen so that heavy hydrocarbons will condense during the heat exchanging process and can be separated from the methane, but at the same time no solid heavy hydrocarbons will deposit in the exchanger wall.

Since it is advantageous to remove heavy hydrocarbons as separate streams, heavy hydrocarbon separators may be included at appropriate points in the refrigeration stage to separate the condensed liquid from Ithe gaseous stream. While the gas rich in methane continues its path in the major liquefaction cycle, the separated liquid may be throttled down as a separate stream and the resultant liquid rich in heavy hydrocarbon recovered as a byproduct by draining it from a separator following the final throttling.

The invention comprises the method of liquefying and purifying a stream of natural gas which contains a suibstantial proportion of methane with portions of gases having a higher solidilication temperature than methane which comprises the following steps: reducing the temperature of the stream of gas while maintaining the pressure on such stream above the maximum value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream; then throttling the cold stream to effect liquefaction and thereby also further reduce the temperature of the stream Ito cause solidiiication of constituents of such stream other than methane, and separating such solidified constituents from the remaining liquid stream to leave a liquid now more largely methane.

In a typical liquefaction system embodying my invention, referring now more particularly to FIGS. 1 and 2, well gas is iirst cooled by a water cooler 1, and then, if necessary, reduced in pressure by a turbo-expander 2 to a pressure slightly above the maximum value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream. As long as the content of carbon dioxide and/or hydrogen sulfide is not excessive, if the pressure on `the stream is maintained above this maximum value of this curve, no solid will form as the temperature of the stream is lowered. The maximumr value of the univariant vapor-liquid-solid equilibrium curve of a combined carbon dioxide and methane stream is 7'10 p.s.i.a. as shown in FIG. 3.

The principle that no solid will separate out in the heat exchangers can be illustrated by taking a simple case in which the gas to be liquefied contains only methane and carbon dioxide. At 715 p.s.i.a. no solid particles of carbon dioxide will form when the content of carbon dioxide is less than about 1.3% (based upon data of Donnelly and Katz, Industrial Eng. Chem. 46, 511 (1954)) as is shown in FIG. 4 which illustrates the temperaturecomposition sections of a 715 p.s.i.a. isobar of a methanecarbon dioxide mixture. This maximum tolerable carbon dioxide gas content may be somewhat modified by the presence of other constituents, such as hydrogen sultide, ethane, propane, etc.

Following any reduction of pressure in the turbo-expander 2, water is removed from the gas in a drier 3. The dried stream of gas is then combined with gas at the same pressure which is to be recycled, and this combined gas stream is cooled by refrigerating it in

heat exchangers

4, and 6 with cold expanded gas which is subsequently to be recycled and by means of additional external refrigeration. This external refrigeration may be by successive refrigerating cycles, such as `a propane cycle 7 cooling the stream in

heat exchangers

4 and 28 and an ethylene cycle 8 cooling the stream in heat exchanger 5.

When hydrocarbons heavier than methane are present in the well gas they will liquefy when the stream is cooled. If after the first step in the refrigeration stage a liquid separator 9 is inserted in the system, the condensate of the heavier hydrocarbons may be collected in the separator and withdrawn from the gaseous stream.

For example, if the well gas is 52.5% methane and 47.5% heavier hydrocarbons tiowing at the rate of 1748 lbs/min. and is combined with gas to be recycled which is 95.8% methane and 4.2% heavier hydrocarbons llowing at 692 lbs/min., when these streams are combined and cooled to minus 30 F. at 720 p.s.i.a. a liquid will be formed containing 43% methane and 57% heavier hydrocarbons which when separated from the main gaseous stream fiows at 1630 lbs/min. while the remaining gaseous stream will contain 98% methane and only 2% heavier hyd-rocarbons flowing at 810 lbs/min.

Some of the carbon dioxide in the system will dissolve in the heavier hydrocarbons fro mthe system at the proper temperature. The carbon dioxide that remains in the gas stream will not either liquefy or solidify at this stage unless the carbon dioxide content is really excessive (see FIG. 4).

After the refrigerating step the separate streams of methane and heavier hydrocarbons are then independently progressively reduced to substantially atmospheric pressure either in a single or in multiple stages to lower pressure levels. A multiple stage reduction may be accomplished by means of a series of throttle valves, 10, 11 and 12 in the methane stream and 13, 14, and 15 in the heavier hydrocarbon stream. The solid carbon dioxide formed during throttling will probably not plug up a throttle valve of special streamline design because of the turbulent condition from the reduction of pressure. However, as a precaution, a heating jacket is provided for each throttle valve so that a slight amount of heat may be added to loosen up any solid deposit if it does occur.

The liquid that evaporates into gas in these throttling processes is separated from the resultant liquid in

separators

16, 17` and 18 in the methane stream and 19, 20 and 21 in the heavier hydrocarbon stream and enters the heat exchanging process with the high pressure stream in

heat exchangers

4, 5 and 6 either independently or with other equi-pressure recycle streams combined at equitemperature points. Then before being recycled the separate streams of expanded gas are recompressed to the pressure of the well gas stream. This is done by irst compressing in compressor 22 and cooling in water cooler 23 the gas passed oi from the liquid after the last throttling process to the pressure of the stream before the last throttling process. This compressed gas is then combined with the gas passed off after the second last throttling process and this combined stream compressed to the pressure of the well `gas stream and cooled in compressor 24 and cooler 25. The gas passed off after the first throttling process in the heavier hydrocarbon stream is recompressed to the well gas pressure in compressor 26, cooled in cooler 27, and then combined with the rest of the gas recompressed for recycling. This combined compressed stream that is to be recycled may initially be cooled by heat exchange with the expanded gas in heat exchanger 28.

The carbon dioxide in the system will form solid particles when the methane stream is throttled and may conveniently be separated from the liquefaction system during the throttling process.

Two alternate liquid settlers may be provided to each CII separator to settle out the solid from the liquid before it passes on to the next throttling stage. As the specific gravity of carbon dioxide solid (1.5 gm./ml. at minus 69.9 F.) is greater than that of methane liquid (.42 gm./ ml. at minus 259 F.), settling is easy to achieve and is an appropriate process for phase separation. After enough solid is settled in the settler, the liquid may be drained oli. and the solid may be removed as liquid by supplying heat to a steam coil provided for each settler. In using the liquid settlers a slight rise is provided to each of the entrance pipes of the throttle valves to prevent it from being plugged by the solid deposition.

Filtration devices may be used instead of a settler if desired.

The gas withdrawn from each of the

separators

16, 17 and 18 in the throttling process of the methane stream will be practically free from solid contamination because of the liquid-scrubbing action occurring during the throttle process. As a precaution, however, two gas scrubbers, which may also serve as liquid lters, may be provided for each separator. Thus, the gas withdrawn from each separator before being passed in heat exchange with the high pressure stream is passed respectively through scrubbing

towers

29, 30 and '31 in counter-current iiow with the liquid withdrawn from the same separator. The liquid after passing through the scrubbing towers is returned to the main stream to continue the throttling process. The two scrubbing towers provided for each separator may be used alternatively and the deposited solid may be removed in liquid form by applying heat to the scrubbing tower by steam coils. Then the carbon dioxide may be collected in the

receivers

32, 33 and 34.

The liquid withdrawn from the final methane separator 18 is the desired methane product, substantially pure and approximately at atmospheric pressure. In liquid form at atmospheric pressure methane occupies about one six hundredth of the space it does in gaseous form. This liquid methane is sent to storage tanks or transportation vessels to be shipped to points of use.

The liquid heavy hydrocarbon by-product may be drawn off as liquid or it may be used to provide additional refrigeration by passing it through the

heat exchangers

4, 5, 6 and 28 before withdrawing it from the system.

Should in rare occasions the carbon dioxide content exceed the tolerance limit (13%) my liquefaction cycle can still be operated if some additional equipment is provided. This additional equipment needed is shown in FIG. 5. For example, in a case in which the combined gas stream contains 30% carbon dioxide, the high pressure gas stream can be cooled down to minus F. without any solid carbon dioxide being separated out. This is shown in FIG. 4. In this case, gas may be cooled down to a temperature whereby, after throttling down to 673 p.s.i.a., it will attain a temperature of minus 93 F., as indicated in FIG. 5. After this throttling step in throttle valve 35, most of the carbon dioxide will separate as solid carbon dioxide, and the carbon dioxide content in the `gaseous phase will be reduced to about 8% (FIG. 6). The solid is settled out in settler 36 and the gas is then scrubbed clean by appropriate scrubbing liquid in scrubber 37, compressed back to 715 p.s.i.a. in compressor 38, and sent back to the main gas line at the equi-temperature point for subsequent cooling and throttling as indicated in FIGS. 1 and 2.

The scrubbing liquid used in scrubber 37 may be recycling liquid methane from the lirst separator 16, may be heavy hydrocarbon condensed out in the liquefaction process, or may be some externally provided hydrocarbon liquid in an extra recycling cycle. The solid carbon dioxide in the scrubbing tower 37 or in the solid settler 36 may be removed by heat supplied to steam coils provided for such purpose.

No phase diagram is available for hydrogen sulfidemethane system in the high pressure and low temperature region; however as hydrogen sulde does not condense out as solid as easily as `carbon dioxide, it may be estimated that no separation of hydrogen vsulfide as solid will occur in the heat exchanger walls as long as its concentration is not excessive, say, not over 13% in the Well gas stream.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims, or the equivalent of such, be employed. Cricondentherm point as used herein is described in the paper Behavior of Hydrocarbon Mixtures Illustrated by.,a Simple Case, presented at the fourteenth annual meeting of the American Petroleum Institute at Chicago, Illinois, on October 26, 1933.

I therefore particularly point out and distinctly claim as my invention:

1. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with heavier hydrocarbons and acid gases (carbon dioxide and hydrogen suliide) which comprises the following stages:

loweringfthe temperature of said stream of natural gas while maintaining the pressure of such stream above the maximum value. of the univariant vapor-liquid-solid equilibrium curve of the combined lgas stream;

separating from the stream heavier hydrocarbons whichV have been liquefied;

then reducing the temperature of Ithe gas stream to a level above the level at which an acid gas will solidify;

then reducing theY pressure on the main gas stream (methane and acid gases) to effect liquefaction of a portion of the stream and thereby also further reduce the temperature of the stream sufficiently to cause solidification of the acid gases; and

separating such solidified acid gases from the liquid stream.

2. The method of liquefying' and purifying a stream of natural gas which contains a substantial portion of methane with a portion of heavier hydrocarbons and portion of acid gases above the tolerance limit which comprises the following stages:

lowering the temperature of 'said stream of natural gas to reduce the Iheavier hydrocarbons to a liquid state at a pressure above the maximum value of the univariant vapor-liquid solid equilibrium curve of the combined stream;

separating from the main gas stream the heavier hydrocarbons after they have been thus liquefied;

further lowering the tempera-ture of the main gas stream to a predetermined temperature at which, when the stream is throttled, solid acid gases will form and may be separated from the stream leaving the acid gas content of the stream below the tolerance limit;

throttling the main gas stream until the required amount of acid gases solidiiies;

separating from said stream the solidified acid gases;

recompressing said stream above the maximum value of the unvariant vapor-liquid-solid equilibrium curve of the combined gas stream;

then reducing the temperature of the gas stream to a level above the level at which an acid gas will solidify;

then reducing the pressure on such liquefied stream of methane and acid gases to effect liquefaction and thereby also further reduce the temperature of the stream suiiiciently to cause solidiiication of the acid gases; and

separating such solidified acid gases from the liquid stream.

3. The method of liquefying and purifying a stream of natural gas which contains a substantial portion methane, heavier hydrocarbons and acid gases (carbon dioxide and hydrogen sulde) which comprises the following stages:

lowering the temperature of said stream of natural gas While maintaining the pressure on such stream above the maximum value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream;

separating from the stream heavier hydrocarbons which have been thus liquefied;

then reducing the pressure on the liquid stream of heavier hydrocarbons and the main gas stream to atmospheric pressure to effect the partial liquefaction of the gas stream and partial vaporization of the liquefied stream of heavier hydrocarbons and also further reduction in the temperatures of the streams, the reduction of the temeprature of the main stream being sufficient to cause solidification of the acid gases therein;

separating the vapors from the vaporized heavier hydrocarbon stream and from the partially liquefied gas stream;

separating such solidified acid gases from the main liquid stream;

recycling the vapors separated from the stream of heavier hydrocarbons and the gas stream; and

passing such vapors in indirect heat exchange relation with said stream of natural gas in the initial stage of temperature reduction.

4. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane and portions of heavier hydrocarbons and acid gases (carbon dioxide and hydrogen sulfide) which comprises the following stages:

lowering the temperature of said stream of natural gas while maintaining the pressure on such stream above the maximum value of the univariant vapor-liquid-solid equilibrium curve ofthe combined gas. stream;

separating from the stream the heavier hydrocarbons which have become liquefied;

then `progressively reducing the pressure on the main gas stream to effect liquefaction and subsequent vaporization of some of the liquefied portion upon further reduction in pressure and thereby also further reduce the temperature of such stream to cause solidification of the acid gases;

withdrawing such vapors as are released in response to reduction in pressure;

collecting the liquefied portion of the main gas stream in steps as said stream is progressively reduced in pressure;

passing at least a portion of the liquid portion collected in each step in counter-current flow in an acid gas scrubbing tower with the port-ion vaporized in the same step to remove from the gas and the liquid the acid gases which have solidified due to the reduction in pressure and temperature ofthe stream; and

reducing the pressure of the scrubbed liquid portion in a subsequent step.

5. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with portions of heavier hydrocarbons, and acid gases (carbon dioxide and hydrogen sulfide) which comprises the following stages:

lowering the temperature of said stream of natural gas while maintaining the pressure on such stream above the univariant vapor-liquid-solid equilibrium curve of the combined gas stream;

separating from the stream the heavier hydrocarbons after they have been thus liquefied;

then progressively reducing the pressure of the main stream to effect liquefaction of a portion of the stream which is largely methane with subsequent vaporization of some of the liquefied methane upon subsequent reduction in pressure and thereby reduce the temperature of the stream to cause solidification of the acid gases;

collecting the liquid methane in steps as the pressure on the stream is progressively reduced;

passing at least a portion of such liquid methane collected in each step in counter-current flow in an acid gas scrubbing tower with the portion vaporized in the same step to remove from the gas and the liquid the acid gases which have solidified due to the reduction in the pressure and the temperature of the stream;

reducing the pressure of the scrubbed liquid methane in a subsequent pressure reduction step;

then passing such vaporized portions in heat exchange relation with said stream of natural gas in the initial stage of temperature reduction; and

recycling the vapors.

6. The method of liquefying and purifying a stream of natural gas containing a substantial portion of methane with a portion of acid gas which comprises the following stages:

then reducing the pressure on such cold stream of natural gas to effect liquefaction of a portion of the methane and further reduce the temperature of the stream to cause solidification of the acid gases; and

separating such solidified acid gases while they are in the solid state from the remaining liquid stream.

7. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with a portion of acid gases above the tolerance limit which comprises the following stages:

lowering the temperature of said stream of natural gas at a pressure above the maximum value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream without formation of solid acid gases to a predetermined temperature at which when throttled solid acid gases will form and may be separated leaving the acid gas content below the tolerance limit;

throttling said natural gas stream until the required amount of acid gases solidifies;

separating from the natural gas stream the solidified acid gases;

recompressing the methane stream above the maximum value of said curve of the combined stream;

then reducing the pressure on such stream of natural gas to effect liquefaction of a portion of the methane and thereby further reduce the temperature of the stream to cause solidification of the remaining acid gases; and

separating such solidified acid gases from the remaining liquid methane stream.

8. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with a portion of -acid gases which comprises the following stages:

then reducing the pressure on such stream of natural gas to effect liquefaction of a portion of the methane thereby to produce a wet gas containing a mixture of liquid and vaporized methane with concurrent further reduction in the temperature of the stream to cause solidification of the acid gases;

withdrawing such vaporized methane;

separating such solidified acid gases from the liquid methane stream; and

passing such vapors in indirect heat exchange relation with said stream of natural gas in the initial stage of temperature reduction, `and recycling the vapors.

9. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with portions of acid gases which comprises the following stages:

then progressively reducing the pressure in stepwise fashion on the liquefied stream of natural gas to effect liquefaction of a portion of the natural gas to produce a wet gas composed of liquid and vaporized natural gas with corresponding reduction in the temperature of the stream to cause solidification of the acid gases;

collecting the liquefied portion in steps as the pressure on the stream is progressively reduced;

passing at least a portion of such liquid natural gas collected in each step in counter-current fiow in an acid gas scrubbing tower with the portion vaporized in the same step to remove from the gas and the liquid the acid gases which have solidified due to the reduction in the pressure and the temperature of the stream;

returning the liquid natural gas for further reduction of pressure in a subsequent pressure reduction step;

passing the vaporized portions of the natural gas in heat exchange relation with the stream of natural gas in the initial stage of temperature reduction; and recycling the vapors.

l0. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with portions of acid gases which comprises the following stages:

then progressively reducing the pressure on the stream of natural gas to effect liquefaction of a portion of the methane to produce a wet gas containing the liquefied methane in admixture with vaporized natural gas with concurrent reduction in the temperature of the stream to cause solidification of the acid gases;

separating such solidified acid gases from the liquid methane stream;

then passing such vaporized portions in indirect heat exchange relation with said stream of natural gas in the initial stage of temperature reduction; and

recycling the vapors.

l1. The method of liquefying and purifying a stream of natural gas which contains a substantial portion of methane with portions of acid gases which comprises the following stages:

then progressively reducing the pressure on the liquefied stream of natural gas to effect liquefaction of a portion of the methane to produce a wet gas containing the liquefied methane in admixture with vaporized natural gas with concurrent reduction in the temperature of the stream to cause solidification of the acid gases;

separating the vaporized portion of the natural gas from the liquefied methane stream;

separating such solidified acid gases from the liquid methane stream;

then passing vapors in indirect heat exchange relation with said stream of natural gas in the initial stage of temperature reduction; and

recycling the vapors.

12. The method of liquefying and purifying a stream comprising hydrocarbon gases which contains a plurality of gases which when liquefied have different solidification temperatures which comprises the following steps:

reducing the temperature of the stream of gas while maintaining the pressure on such liquefied stream above 9 the maximum value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream;

then reducing the pressure on such stream to effect liquefaction of a portion of the constituents having the lowest soliditication temperature and thereby also further reduce the temperature of the stream to cause solidication of constituents of such stream whose solidification temperatures are relatively high; and

separating such solidified constituents while they are in the solid state from the remaining liquid stream to leave a liquid now more pure in constituents with lower solidification temperatures.

13. The method of claim 12 wherein the constituent with lowest solidiiication temperature is methane.

14. The method of liquefying a stream of natural gas which contains a substantial portion of methane with portions of heavier hydrocarbons and nitrogen which comprises the following stages:

lowering the temperature of said stream of natural gas while maintaining the pressure of such stream above the maximum Value of the univariant vapor-liquid-solid equilibrium curve of the combined gas stream to effect condensation of the heavier hydrocarbons contained in the gas stream;

separating the gaseous portion from the liquefied portion containing the heavier hydrocarbons;

progressively reducing the pressure on the separated liquefied stream containing the heavier hydrocarbons to about atmospheric pressure with corresponding release of vapors formed mostly of methane and some of the heavier hydrocarbons Iand with corresponding reduction in temperature;

progressively reducing the gas stream remaining after separation of the liqueed portion containing the heavier hydrocarbons stepwise to about atmospheric pressure to achieve liquefaction of the main gas stream and vaporization of portions of the liquefied stream as it is subsequently reduced in pressure;

separating the dry gas from the liqueiied gas in the rst pressure reduction step subsequent to liquefaction of the main gas stream to remove the major portion of the nitrogen lfrom the system;

separating the Vaporized portions released from the liquefied portions of the main gas stream upon subsequent expansions;

passing such separated vaporized portions in heat exchange relation with the stream of natural gas in the initial stage of temperature reduction;

venting from the cycle that portion of the vaporized gas from the first stage of pressure reduction for removal of nitrogen from the system; and

recycling all of the remaining portions of the vaporized gas through the liquefaction steps.

References Cited in the lile of this patent UNITED STATES PATENTS 668,197 Le Sueur Feb. 19, 1901 2,022,165 Twomey Nov. 26, 1935 2,500,129 Laverty et al. Mar. 7, 1950 2,777,299 Skaperdas Ian. 15, 1957 2,826,266 Hachmuth et al Mar. 11, 1958 2,863,296 Newton Dec. 9, 1958