US3338061A

US3338061A - Low-temperature fractionation process

Info

Publication number: US3338061A
Application number: US389093A
Authority: US
Inventors: Leonard J Hvizdos; John A Pryor
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1964-08-12
Filing date: 1964-08-12
Publication date: 1967-08-29
Anticipated expiration: 1984-08-29

Description

' Aug. 29, 1967 I 1.. J.HVIZDOS ET AL 3,333,061

LOW-TEMPERATURE FRACTIONATION PROCESS Filed Aug. 12, 1964 MiW A TTOR NE Y GASEOUS N2 PRODUCT United States Patent 3,338,061 LOW-TEMPERATURE FRACTIONATION PROCESS Leonard J. Hvizdos, Allentown, and John A. Pryor, Emmaus, Pa., assignors to Air Products and Chemicals,

Inc., a corporation of Delaware Filed Aug. 12, 1964, Ser. No. 389,093 Claims. (Cl. 62-13) This invention relates to improvements in the separation of normally gaseous mixtures, more particularly, to the low-temperature separation of air into its components, especially relatively pure oxygen and high purity nitrogen.

In the low-temperature fractionation of air, a conventional method comprises the utilization of two columns, generally in superimposed relationship, wherein a high pressure column is used to produce a crude oxygenenriched mixture as bottoms and a nitrogen-enriched mixture as overhead, the bottoms fraction and overhead fraction being passed as feed and reflux, respectively, to alow pressure column.

In order to produce high purity nitrogen, a system is described in US. Patent No. 2,762,208 wherein the high pressure column is provided with additional plates, thereby forming a nitrogen-enriching section. Additionally, at the top of said column, the pressure is maintained at a high level in the column, and of most importance, substantially all of the overhead nitrogen vapor is condensed and then refluxed to the top of the same column, the patent indicating that it is preferred for only about 3% of the liquefied high purity nitrogen to be removed from the process. As the patent further states, the condensed nitrogen is returned to the top of the high pressure rectifying zone in order to reflux the rectification of air into a high purity nitrogen eflluent without extensive gas-liquid contact and controls. In other words, the patent operates at almost total reflux, thereby reducing the required number of theoretical plates to almost the theoretical minimum. On the other hand, the utilization of almost total reflux involves an additionalconsumption of energy as compared to normal operation, as well as a column of inordinately larger diameter to accommodate the relatively large volume of vapor throughput, and also equipment for pumping the reflux back to the top of the column.

An object of the present invention, therefore, is to provide an improved process for the production of oxygen, together with nitrogen of high purity.

Another object of this invention is to provide a process wherein the air, after being cooled in switching heat exchangers or the like, is further cooled by heat exchange with gaseous oxygen product, gaseous nitrogen from the high pressure column, and gaseous nitrogen from the low pressure column.

Still another object is to provide a novel process for the separation of a gaseous mixture into its components.

Upon further'study of the specification and the appended claims, other objects and advantages of this invention will become apparent.

To achieve the objects of this invention, there is provided a process for the low-temperature separation of a normally gaseous mixture into at least one enriched high boiling fraction and at least one enriched low boiling fraction, which process comprises the steps of:

(a) Fractionating the gaseous mixture in a main high pressure separation zone to obtain an overhead fraction and a bottoms fraction;

(b) Passing said bottoms fraction as feed to a main low pressure separation zone;

(c) Passing a portion of said overhead fraction to an auxiliary high pressure fractionation zone positioned away from the main high pressure separation zone and tractionating said overhead [fraction to produce an enriched low boiling overhead; and

(d) Passing at least a portion of said enriched low boiling overhead in indirect heat exchange relationship with said at least one enriched high boiling fraction, said fraction being withdrawn as liquid bottoms from the main low pressure separation zone, to condense said at least a portion of said enriched low boiling overhead, and to vaporize said at least one enriched high boiling fraction.

By passing only a portion of the overhead fraction. to an auxiliary high pressure fractionation zone, positioned away from the main high pressure fractionation zone, it is possible to produce enriched low boiling fraction economically. Furthermore, by condensing the enriched low boiling overhead with enriched high boiling fraction, both gaseous enriched high boiling fraction and liquid enriched low boiling fraction are obtained.

In some cases, moreover, it is desirable to produce gaseous enriched low boiling fraction which can be utilized, as such, in a nearby plant. Under such circumstances, it is desirable to produce as much gaseous enriched low boiling fraction as possible, and according to this invention, it is beneficial to produce the gaseous enriched low boiling fraction by employing another fractionation zone, which is auxiliary to the main low pressure fractionation zone. In this auxiliary low pressure zone, the resultant enriched liquid low boiling overhead from step (d) is employed as reflux liquid in direct countercurrent contact with a portion of overhead vapor from the main low pressure separation zone wherein an enriched overhead gaseous low boiling fraction is obtained. The bottoms fraction, comprising a less rich low boiling fraction, is then returned to the main low pressure column as reflux. By utilizing this modification, it is possible to produce more gaseous enriched low boiling product than would be possible by merely utilizing the gaseous enriched low boiling product produced in the auxiliary high pressure zone.

As specifically applied to the production of high purity oxygen and nitrogen, a system is provided wherein there is no attempt to obtain highly pure nitrogen in the high pressure column, but instead, impure nitrogen vapor is withdrawn from the top of the high pressure column, and is subjected to additional fractionation in an auxiliary high pressure column .for the production of highly pure nitrogen, i.e., no greater than about 50 ppm. 0 This pure nitrogen is then advantageously condensed by heat exchange with liquid oxygen product which is withdrawn from the bottom of the low pressure column. A portion of the resultant pure liquid nitrogen is employed as reflux for the auxiliary high pressure column. The remaining portion can then be used as product, or when an additional quantity of gaseous nitrogen product is desired, the pure liquid can be beneficially employed as reflux in a low pressure zone. This process is particularly advantageous for the production of gaseous oxygen which is used in steel refining operations.

Another feature of this invention comprises a novel and eflicient heat exchange technique wherein a portion of the incoming gaseous mixture is heat exchanged prior to fractionation with three streams, as follows: (1) gaseous enriched high boiling fraction, (2) gaseous portion of said overhead fraction from the high pressure separation zone, and (3) a gaseous portion of the overhead fraction from the low pressure separation zone.

It is believed that the preceding description of the invention is sufiicient to enable one skilled in the art to use it to its fullest extent. For purposes of illustration, however, reference is directed to the accompanying drawing which is a diagrammatic representation of a preferred embodiment of the present invention.

Refer-ring to the drawing in detail, 147,000 lbs/hr. of air enter the system at 10, are compressed in compressor 11 to about 87 p.s.i.a., and are thereupon sent through conduit 12 to the switching heat exchanger 13. The function of the switching heat exchanger is to cool the air to about 277 F., as well as to precipitate congealable impurities, such as CO When the streams are reversed, waste nit-rogen is passed through the conduit formerly containing the compressed air in order to sublime and remove the impurities therefrom. It is to be noted that there are other methods of removing the CO from the air, such as by a caustic wash, or by the utilization of regenerators instead of switching heat exchangers.

In the embodiment illustrated in the drawing, the air is passed through conduit 14, whereas the Waste nitrogen isv passed countercurrently thereto in conduit 15. The air is in heat exchange with gaseous nitrogen product in conduit 1-6 and gaseous oxygen in conduit 17. Additionally, some cooling is obtained from high pressure nitrogen in line 18 which is expanded in expander 19 to form additional cold waste nitrogen.

After the cooled and purified air leaves the switching heat exchanger, it is passed by conduit 20 to the bottom part of fractionating column 21. This bottom part 21H, hereinafter designated as the high pressure fractionation or separation zone or column, operates at about 85.0 p.s.i.a. The air is introduced below the plates in this high pressure column, but out of contact with the body of liquid in the bottom of the column. A minor portion of the air (about 2,400 lbs/hr.) is withdrawn from the bottom of the high pressure zone and is introduced by conduit 22 into three heat exchangers in parallel, namely, heat exchangers 23, 24 and 25 wherein the air is cooled by oxygen product in exchanger 23, by high pressure nitro gen in exchanger 24, and by waste, low pressure nitrogen in exchanger 25. This cooled minor air portion at 283 F. is then passed through conduit 26 and combined with the bottoms liquid from the high pressure fractionatin-g zone 21H. This bottoms liquid amounting to about 73,000 1bs./hr. at 279 F. is obtained via a conduit 27, and the combined portions from conduits 26 and 27 are passed via conduit 28 as feed to the low pressure column, but first through a heat exchanger which will be described in detail infra.

Referring now to the operation of the high pressure column, it is operated at as low a pressure as possible in order to separate the air into a crude nitrogen-fraction containing 16% oxygen as an impurity, preferably about 2.8%. In this connection, it is important to emphasize the fact that while it is technically feasible to increase the pressure and add additional plates to the top of the high pressure column to obtain a purer nitrogen fraction, this has been found to be economically undesirable in accordance with the teachings of this invention.

Vapor, at 286 F., from above the plates in the high pressure column is withdrawn in line 29 and divided at 30 into two portions. One portion of about 15,500 lbs/hr. is passed to heat exchanger 24 in heat exchange with air, as previously described.

The other portion of the gaseous high pressure nitrogen stream, about 40,000 lbs./hr., is introduced into the bottom of auxiliary high pressure fractionating column 32 via conduit 31. In this auxiliary column, the nitrogen is purified to a purity of less than 50 p.p.m. O to less than p.p.m. 0 depending upon the desired rate of production of pure N The overhead vapor emanating from column 32 (about 40,000 lbs/hr. at about 286 F. and 85 p.s.i.a.) is passed through conduit 33, and then is optionally divided into two portions at point 34. One of said portions, in the illustrated embodiment about 5,500 lbs./hr., is passed through line 35 into heat exchanger 36 in heat exchange with closed circuit nitrogen which is employed in a refrigeration cycle. The resultant condensed high purity nitrogen is withdrawn from heat exchanger 36 and is passed through line 37 to combine with additional high purity nitrogen at point 38. The other portion of the high purity nitrogen is passed into heat exchanger 39 and is condensed therein in heat exchange with high purity oxygen from the bottom of the low pressure column 21L which is superimposed above the high pressure column, in fractionating column 21. The main high and low pressure columns are connected through vaporizer-condenser 76, wherein all the re-boiler vapor for the main low pressure column is provided by the condensation of a portion of overhead vapor in the main high pressure column. In this way, there is limited reboil in 21L, with pure gaseous oxygen being produced outside the column, and not recycled as reboil vapor. The condensed high purity nitrogen from heat exchanger 39 is withdrawn and passed through line 40 to combine at point 38 With the fluid in conduit 37. It is to be further understood that the amount of pure N which is condensed by closed circuit nitrogen refrigeration is 0-100% of the contents of line 33, depending on the desired quantity of liquid oxygen as product. A particularly preferred embodiment of the invention, however, is to condense some of the pure N; with pure liquid oxygen from the main low pressure fractionating column, thereby effecting substantial savings in energy and equipment.

The combined portions in 37 and 40 then pass through line 41 and are split at point 42 wherein about 35,000 lbs/hr. are passed through line 43 as reflux for column 32. This amount of reflux yields a nitrogen purity of less than about 5 p.p.m. oxygen at a production rate of about 100,000 s.c.f./hr. of nitrogen. For higher production rates and correspondingly less purity, e.-g. less than about 50 p.p.m. at 200,000 s.c.f./-hr., the reflux ratio is correspondingly reduced.

The other portion of the purse liquid nitrogen is then passed through line 44 through heat exchanger 45 in heat exchanger relationship with high purity gaseous nitrogen. The cooled liquid nitrogen at about 82 p.s.i.a. and 312 F. is then passed through line 46 to pressure reducing valve 47. After its pressure is lowered to about 19 p.s.i.a., forming some gas phase, the resultant mixture at 316 F. is passed into phase separator 48.

The liquid which is collected at the bottom of the phase separator can then be deployed according to market conditions. It can be passed to liquid nitrogen storage in line 49, or instead, according to the preferred illustrated embodiment, it can be passed as reflux in line 50 to auxiliary low pressure column 51, Obviously, it is also possible to divide the liquid at point 52 so that some of it goes to liquid nitrogen storage and some of it is employed as reflux. This provides the process with substantial flexibility for responding to market conditions. The purpose of low pressure auxiliary column 51 is to provide additional pure nitrogen over and above the output in line 44 of the auxiliary high pressure column 32. This is accomplished by fractionating overhead nitrogen emanating from the low pressure column, said nitrogen being introduced into the auxiliary low pressure column via conduit 53. The amount of available nitrogen in line 53, about 7,200 lbs./hr., is limited, in the illustrated embodiment, by the necessary amount of scavenging gas in line 59. In a different modification wherein the air is cleaned by a different procedure, such as caustic scrubbing, this limitation would not be present.

The impure bottoms liquid (about 4,800 lbs./hr. at

about 4% O in auixilary low pressure column 51 is then passed as reflux to the top of the main low pressure column via line 54. The pure gaseous nitrogen from column 51 (about 7,000 lbs/hr.) and phase separator 48 (about -200 lbs/hr.) is then passed through line 55 to heat exchanger 45 where it is warmed by heat exchange relationship with the pure liquid nitrogen; and then the resultant warm pure gaseous nitrogen is passed through line 56 into conduit 16 of the switching heat exchanger 13 where it is further warmed. The resultant warmed gaseous pure nitrogen product is withdrawn in line 57 and can either be stored, or continuously consumed, in accordance with consumer demand.

Returning now to below the main high pressure column 21H, the combined portions in line 28 (about 75,000 lbs/hr. at 280 F. and about 85 p.i.s.a.) are passed to heat exchanger 58 wherein they are heat exchanged with overhead impure gaseous nitrogen from the low pres sure column, said impure nitrogen being withdrawn from conduit 59. After this heat exchange step, the combined portions are then introduced into the feed point of the low pressure column via conduit 60.

In order to obtain additional reflux for the main low pressure column, impure, liquid nitrogen (about 13,000 lbs./hr. at 287 F. and about 85 p.s.i.a.) is withdrawn from the high pressure column at line 61 and is also passed through heat exchanger 58 in heat exchange with with the overhead gaseous nitrogen stream from the low pressure column. The cooled impure liquid nitrogen is then withdrawn from heat exchanger 58 in line 62 and is passed through pressure reducing valve 63 so that it can then be set as reflux at about 19 p.s.i.a. and 316 F. to the top of the low pressure column 21L via line 64.

'For even more reflux, the impure liquid nitrogen which is less pure (about 2.8% oxygen) than the nitrogen present in line 61, is withdrawn from the bottom of the auxiliary high pressure column 32. It is also passed through heat exchanger 58 via conduit 65 and then passed through pressure reducing valve 66 and combined with the liquid nitrogen from the top of the high pressure column at point 67, both streams going through conduit 64 to the top of the low pressure column.

According to the preferred embodiment of this invention, oxygen is produced in the low pressure column with a minimum purity of 99.5%. This liquid oxygen which is withdrawn from the low pressure column in line 68 (about 32,000 lbs/hr. at 22 p.s.i.a. and 290 F.) is first passed through hydrocarbon adsorber 75, then passed through heat exchanger 39 where it is vaporized, thereby condensing a portion of the overhead nitrogen from auxiliary high pressure column 32. The oxygen is not completely vaporized in one pass through the heat exchanger 39. so it is withdrawn from said heat exchanger and passed into phase separator 70 via line 69. Liquid oxygen is collected at the bottom of the phase separator and a portion can be sent to liquid oxygen storage, if desired, or it can be recycled via line 71 through heat exchanger 39, thereby completing a thermal syphon cycle. In the preferred illustrated embodiment, about 3,100 lbs./hr. are recovered as lox.

The gaseous oxygen product is withdrawn from the top of phase separator 70 and is passed through heat exchanger 23 via conduit 72. The warmed oxygen stream is then passed from heat exchanger 23 into heat exchanger 13, specifically into conduit 17 where it is further warmed, and is withdrawn as the warmed gaseous oxygen product. It is of additional interest to note that the gaseous oxygen is not returned to the main low pressure column 21L, and that a pressure leg is suflicient driving force to enable the liquid oxygen to flow through the hydrocarbon adsorber.

Lastly, the impure nitrogen overhead from the top of the main low pressure column, after having been warmed in heat exchanger 58, is thereupon further warmed in heat exchanger 25 via conduit 73. The warmed impure gaseous nitrogen is then withdrawn from heat exchanger 25 and is passed, via conduit 74, into the switching heat exchanger 13, specifically, in the illustrated embodiment, into conduit 15 where it sublimes and purges the impurities. The contaminated nitrogen is then Withdrawn and passed to waste.

Whereas the preferred embodiment of the invention is related to the separation of air, it is to be understood that this invention is not limited thereto. As one skilled in the art will appreciate, the principles of this invention can be applied to the separation of other gaseous mixtures, particularly cryogenic fluids, such as helium-con- 5 taining gases and mixtures of hydrocarbons.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

1. A process for the low-temperature separation of a vnormally gaseous mixture into at least one enriched high boiling fraction and at least one enriched low boiling fraction, which process comprises the steps of:

a (a) fractionating the gaseous mixture in a main high pressure separation zone to obtain an overhead fraction and a bottoms fraction;

(b) passing said bottoms fraction as feed to a main low pressure separation zone to obtain overhead vapor and an enriched high boiling bottoms fraction;

(c) passing a portion of said overhead fraction to an auxiliary high pressure fractionation zone positioned away from the main high pressure separation zone and fractionating said overhead fraction to produce an enriched low boiling overhead; and

(d) passing at least a portion of said enriched low boiling overhead in heat exchange relationship with enriched high boiling bottoms fraction withdrawn from the main low pressure separation zone, to condense said enriched low boiling overhead, thereby producing liquefied enriched low boiling overhead, and to vaporize at least a portion of the enriched high boiling bottoms fraction,

. 2. The process of claim 1, further comprising the steps preceding step (a) of:

(1) cooling and cleaning the gaseous mixture in a switching heat exchange zone,

(2) passing the cooled and cleansed gaseous mixture to the main high pressure separation zone,

(3) Withdrawing a minor portion of the cooled and cleansed gaseous mixture from the main high pressure separation zone, and

(4) passing said minor portion. in indirect heat exchange relationship with:

(A) the gaseous enriched high boiling fraction,

(B) a gaseous portion of said overhead fraction fromthe main high pressure separation zone,

and

(C) a gaseous portion of said overhead fraction from the main low pressure separation zone.

3. The process of claim 1, further comprising the step of returning a portion of the liquefied enriched liquid low boiling overhead as reflux to said auxiliary high pressure fractionation zone.

4. Process of claim 1 comprising the further step of withdrawing as product the enriched high boiling bottoms fraction following the step (d).

5. The process of claim 1, comprising the further step of passing a portion of the liquefied enriched liquid low boiling overhead to an auxiliary low pressure fractionation zone positioned away from said main low pressure separation zone, and employing liquefied enriched low boiling point overhead as reflux in direct countercurrent contact with a portion of the overhead vapor from said main low pressure separation zone to produce an enriched gaseous low boiling overhead fraction and a less rich liquid low boiling bottoms fraction.

6. The process of claim 5, further comprising the step of returning a portion of the liquefied enriched liquid low boiling overhead as reflux to said auxiliary high pressure fractionation zone.

7. I11 a low-temperature process for the separation of air into oxygen and nitrogen, which process comprises the steps of cooling and cleaning the air, fractionating the cooled and cleansed air in a main high pressure separation zone to obtain a bottoms fraction enriched in oxygen and an overhead fraction enriched in nitrogen, and passing said bottoms fraction as feed to a main low pressure separation zone to provide overhead vapor and liquid oxygen bottoms product, the improvement which comprises:

(a) fractionating a portion of said overhead fraction in an auxiliary high pressure fractionation zone positioned away from the main high pressure separation zone, said fractionating being sufficient to form an overhead stream of purified nitrogen and a less pure bottoms stream,

(b) reducing the pressure of the less pure bottoms stream, and

(c) passing said less pure bottoms stream under reduced pressure as reflux to said main low pressure separation zone.

8. The process of claim 7, comprising a further step of (d) passing a portion of said overhead stream of purified nitrogen in indirect heat exchange relationship with liquid oxygen bottoms product withdrawn from the main low pressure separation zone, whereby said portion of the overhead stream of purified nitrogen is condensed, and the liquid oxygen is at least partially made gaseous.

9. The process of claim 8, further comprising the step of passing said liquid oxygen bottoms product through a hydrocarbon absorption zone, before said liquid oxygen bottoms product is heat exchanged with said overhead stream of purified nitrogen. 1

10. The process of claim 8, further comprising the step of returning a portion of the condensed purified nitrogen as reflux to said auxiliary high pressure fractionation zone.

11. The process of claim 8, further comprising the step of (e') passing a part of the condensed purified nitrogen stream, as reflux, to an auxiliary low pressure fractionation zone positioned away from said main low pressure separation zone, in direct countercurrent contact with overhead vapor withdrawn from said main low pressure searation zone, to produce high purity nitrogen gas.

12. The process of claim 11 wherein only a portion of the overhead vapor from said main low pressure separation zone is fractionated in the auxiliary low pressure fractionation zone.

, 13. The process of claim 11, further comprising the steps preceding step (a) of:

(1) passing the cooled and cleansed air to the bottom of the main high pressure separation zone,

(2) withdrawing a minor portion of the cooled and cleansed air from the bottom of the main high pressure separation zone, and

(3) passing said minor portion in indirect heat exchange relationship with (A) a gaseous oxygen product,

(B) a portion of said overhead fraction enriched in nitrogen, from the main high pressure separation zone, and

(C) a second portion of overhead vapor from said main low pressure separation zone.

14. In a low-temperature process for the separation of air into oxygen .and nitrogen, which process comprises the steps of cooling and cleaning the air, fractionating the cooled and cleansed air in a main high pressure separation zone to obtain a bottoms fraction enriched in oxygen and an overhead fraction enriched in nitrogen, and passing said bottoms fraction as feed to a main low pressure separation zone to obtain liquid oxygen product as bottoms of the main low pressure separation zone, the improvement which comprises:

(a) fractionating a portion of said overhead fraction in an auxiliary high pressure fractionation zone, said fractionating being sufiicient to form an overhead stream of purified nitrogen and a less pure bottoms stream, and

(b) passing a portion of said overhead stream of purified nitrogen in indirect heat exchange relationship with liquid oxygen product bottoms withdrawn from the main low pressure separation zone, whereby said portion of the overhead stream of purified nitrogen is condensed, and the liquid oxygen is made gaseous.

15. The process of claim 14 wherein the oxygen after step (b) is withdrawn as product, and wherein all reboil vapor in said main low pressure separation zone is obtained as a result of heat transferred from condensation of a portion of said overhead fraction enriched in nitrogen obtained in said main high pressure separation zone, said condensation being conducted in a condensingvaporizing zone communicating between the main high and low pressure separation zones.

References Cited UNITED STATES PATENTS 3,062,016 11/ 1962 Dennis et al 62-22 3,113,854 12/1963 Bernstein 62-29 X 3,127,260 3/ 1964 Smith 62-29 X 3,216,206 11/ 196-5 Kessler 6213 3,260,056 7/ 1966 Becker 62-13 FOREIGN PATENTS 136,027 1/ 1950 Australia.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,338,061 August 29, 1967 Leonard J. Hvizdos et al.

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 4, line 37, for "purse" read pure line 39, for "exchanger" read exchange line 50, for "51," read 51. line 68, for "auixilary" read auxiliary column 5, line 8, for "p.i.s.a." read p.s.i.a. line 16, for "impure," read impure line 20, strike out "with"; line 24, for "set" read sent column 7, line 32, for

' "absorption" read adsorption line 45, for "searation" read separation Signed and sealed this 5th day of November 1968.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims

1. A PROCESS FOR THE LOW-TEMPERATURE SEPARATION OF A NORMALLY GASEOUS MIXTURE INTO AT LEAST ONE ENRICHED HIGH BOILING FRACTION AND AT LEAST ONE ENRICHED LOW BOILING FRACTION, WHICH PROCESS COMPRISES THE STEPS OF: (A) FRACTIONATING THE GASEOUS MIXTURE IN A MAIN HIGH PRESSURE SEPARATION ZONE TO OBTAIN AN OVERHEAD FRACTION AND A BOTTOMS FRACTION; (B) PASSING SAID BOTTOMS FRACTION AS FEED TO A MAIN LOW PRESSURE SEPARATION ZONE TO OBTAIN OVERHEAD VAPOR AND AN ENRICHED HIGH BOILING BOTTOMS FRACTION; (C) PASSING A PORTION OF SAID OVERHEAD FRACTION TO AN AUXILIARY HIGH PRESSURE FRACTIONATION ZONE POSITIONED AWAY FROM THE MAIN HIGH PRESSURE SEPARATION ZONE AND FRACTIONATING SAID OVERHEAD FRACTION TO PRODUCE AN ENRICHED LOW BOILING OVERHEAD; AND (D) PASSING AT LEAST A PORTION OF SAID ENRICHED LOW BOILING OVERHEAD IN HEAT EXCHANGE RELATIONSHIP WITH ENRICHED HIGH BOILING BOTTOMS FRACTION WITHDRAWN