Hydrocarbon gas processing

• US 4,140,504 A
• Filed: 10/04/1976
• Issued: 02/20/1979
• Est. Priority Date: 08/09/1976
• Status: Expired due to Term

• Current Assignee: Elcor Corporation
• Original Assignee: The Ortloff Corporation
• Inventors: Wilkinson, John D., Campbell, Roy E.
• Primary Examiner(s): Yudkoff, Norman
• Application Number:
  US05/728,964
• Time in Patent Office: 869 Days
• Field of Search: 62/38, 62/39, 62/28, 62/27, 62/23
• US Class Current: 62/621
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2270/08 Internal refrigeration by flash gas recovery loop
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/60 Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more

Abstract

A process for separating hydrocarbon gases is described for the recovery of gases such as ethane and heavier hydrocarbons from natural gas streams or similar refinery or process streams. In the process described, the gas to be separated is cooled at a high pressure to produce partial condensation, and the vapor and liquid portions are separated. Liquid from the partial condensation is further cooled and then expanded to a lower pressure. At the lower pressure, the liquid is supplied to a distillation column, wherein it is separated into fractions. The vapor portion is work-expanded to the operating pressure of the distillation column and supplied to the distillation column below the feed point of the expanded liquid portion. The operating efficiency of the process is improved by turning back at least part of thevapor portion and warming it by heat exchange against incoming feed before it is work-expanded. By thus warming the vapor, more work and refrigeration can be recovered in the work expansion machine.

Claims

1. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense it and to form thereby a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means to receive at least some of said liquid portion, and to sub-cool it to a temperature below its bubble point;

(c) expansion means connected to said sub-cooling means (a) to receive the sub-cooled liquid portion and to expand it to a lower pressure;

(d) a fractionation column connected to receive at least a portion of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction;

(e) a second expansion means connected to said cooling means (a) to receive said feed gas vapor and to expand it to said lower pressure, said second expansion means being further connected to said fractionation column to supply at least a portion of the expanded feed gas vapor thereto as a feed,

the improvement comprising

(i) dividing means connected to said cooling means (a) to receive said feed gas vapor and to divide it into a first part and a second part;

(ii) means connecting said dividing means (i) to receive said first part of said feed gas vapor and supply it to said second expansion means (4) wherein said first part is expanded and supplied to said fractionation column;

(iii) heat exchange means connected to said dividing meas (i) to receive said second part of said feed gas vapor, said heat exchange means being further connected to receive a portion of said feed gas under pressure, thereby to direct said second part of said feed gas vapor into heat exchange relation with said feed gas under pressure to reheat said feed gas vapor;

(iv) expansion means connected to said heat exchange means (iii) to receive said reheated second part of said feed gas vapor and to expand it to said lower pressure while extracting work therefrom; and

(v) means connecting said expansion means (iv) to said fractionation column at a second feed point to supply said expanded second part to said fractionation column at said second feed point, said second feed point being at a lower column position than said first feed point.

2. The improvement according to claim 1 wherein said sub-cooling means (b) includes means to direct the liquid portion to be sub-cooled into heat exchange relation with at least a portion of said residue gas, thereby to cool said first part prior to expansion thereof.

3. The improvement according to claim 1 including a further heat exchange means connected between said expansion means (iv) and said fractionation column to receive expanded second part, said further heat exchange means being further connected to direct said expanded second part into heat exchange relation with at least a portion of said residue gas, thereby no further cool said expanded second part prior to supplying it to the fractionation column at said second feed point.

4. In a process for separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the cooled liquid portion is expanded in an expansion means to a lower pressure whereby a first part of said liquid portion vaporizes to cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, wherein

(a) said cooled liquid from step (2) is divided into a first part and a remaining part;

(b) said first part is expanded to said lower pressure, whereby a portion thereof vaporizes to cool the expanded first part;

(c) said expanded first part is directed into heat exchange relation with at least some of the liquid portion obtained in step (1), whereby said cooled liquid in step (2) is obtained;

(d) said remaining part is expanded to said lower pressure and at least some of the expanded remaining part is supplied to said demethanizer at the first feed point;

(e) at least some of said gas vapor from step (1) is reheated;

(f) thereafter said reheated feed gas vapor is expanded in a work-expansion machine; and

(g) the expanded, reheated portion from step (f) is supplied to said fractionation column at a second feed point, said second feed point being at a lower column position than said first feed point.

5. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;

(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;

(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction; and

(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising

(1) dividing means connected to receive at least part of sub-cooled liquid portion from said sub-cooling means (b) and to divide it into a first part and a remaining part;

(2) means connected to said dividing means (1) to receive said first part and to expand said first part to said lower pressure, whereby a portion thereof vaporizes to cool the expanded first part;

(3) means connected to said means (2) to receive expanded first part and to direct said first expanded part to said sub-cooling means (b), wherein it passes in heat exchange relation with the liquid portion to be sub-cooled;

(4) means connected to said dividing means (1) to receive said remaining part and to supply it to said first expansion means (c) wherein it is cooled, and from which at least a portion thereof is supplied to said fractionation column at said first feed point;

(5) heat exchange means connected to said cooling means (a) to receive at least a portion of said feed gas vapor, said heat exchange means being connected to reheat said portion of said feed gas vapor;

(6) a third expansion means connected to said heat exchange means (5) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom; and

(7) means connecting said third expansion means (6) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point.

6. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;

(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;

(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction;

(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising

(1) combining means connected to said cooling means (a) to combine a first portion of the feed gas vapor with said liquid portion to be sub-cooled prior to expansion thereof, to form thereby a combined stream;

(2) said sub-cooling means connected to cool at least one of the first portion of the feed gas vapor, the liquid portion to be sub-cooled and said combined stream, whereby said combined stream is cooled to a temperature below the bubble point of said liquid portion;

(3) means connected to supply the expanded combined stream at a temperature below the bubble point of said liquid portion to said first expansion means (c);

(4) heat exchange means connected to said cooling means (a) to receive a second portion of said feed gas vapor, said heat exchange means being connected to reheat said portion of said feed gas vapor;

(5) a third expansion means connected to said heat exchange means (4) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom; and

(6) means connecting said third expansion means (5) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point.

7. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;

(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;

(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction; and

(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising

(i) means connected to said cooling means (a) to receive said feed gas vapor and to divide said feed gas vapor into at least a first part and a second part;

(ii) heat exchange means connected to said dividing means (i) to receive the first part of said feed gas vapor, said heat exchange means being connected to reheat said portion of said feed gas vapor;

(iii) a third expansion means connected to said heat exchange means (ii) to receive said reheated part of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom;

(iv) means connecting said third expansion means (iii) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point;

(v) expansion means connected to said dividing means (i) to receive said second part of said feed gas vapor and to expand said second part to said lower pressure; and

(vi) means connecting said expansion means (v) to said fractionation column to supply the expanded second part of said feed gas vapor to said fractionation column at a feed point above said second feed point.

8. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;

(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;

(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction; and

(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising

(i) dividing means connected to said cooling means (a) to receive said feed gas vapor and to divide it into at least a first part and a second part;

(ii) heat exchange means connected to said dividing means (a) to receive the first part of said feed gas vapor, said heat exchange means being connected to reheat said first part of said feed gas vapor;

(iii) a third expansion means connected to said heat exchange means (ii) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom;

(iv) means connecting said third expansion means (iii) to said fractionation column (d) to supply the expanded reheated feed gas thereto at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point;

(v) heat exchange means connected to said dividing means (i) to receive said second part and to cool said second part;

(v) expansion means connected to said heat exchange means (v) to receive said cooled second part and to expand said cooled second part to said lower pressure; and

(vii) means connecting said expansion means (vi) to said fractionation column to supply said expanded second part to said fractionation column at a feed point above said second feed point.

9. In an apparatus for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, said apparatus having

(a) cooling means for cooling said feed gas under pressure to partially condense said gas sufficiently to form a liquid portion and a feed gas vapor;

(b) sub-cooling means connected to said cooling means (a) to receive at least some of said liquid portion and to sub-cool it to a temperature below its bubble point;

(c) a first expansion means connected to said sub-cooling means to receive the sub-cooled liquid portion and to expand it to a lower pressure, whereby a part of said liquid portion vaporizes to cool the expanded sub-cooled liquid portion;

(d) a fractionation column connected to said first expansion means to receive at least part of the expanded sub-cooled liquid portion at a first feed point and to separate said relatively less volatile fraction; and

(e) a second expansion means connected to said cooling means (a) to receive feed gas vapor therefrom and expand it to said lower pressure in a work-expansion engine, wherein work is extracted therefrom, said second expansion means being further connected to said fractionation column to supply at least part of the expanded feed gas vapor to said fractionation column,

the improvement comprising means for turning back at least a portion of said feed gas vapor prior to expansion thereof, said turn-back means comprising

(i) heat exchange means connected to said cooling means (a) to receive at least a portion of said feed gas vapor, said heat exchange means being connected to reheat said portion of said feed gas vapor;

(ii) a third expansion means connected to said heat exchange means (i) to receive said reheated portion of said feed gas vapor and to expand said reheated portion to said lower pressure while extracting work therefrom;

(iii) heat exchange means connected to said expansion means (ii) to receive said expanded reheated portion;

(iv) means further connecting said heat exchange means (iii) to receive at least a portion of residue gas and to direct said residue gas into heat exchange relation with said expanded reheated portion, whereby said expanded reheated portion is cooled; and

(v) means connecting said heat exchange means (iii) to said fractionation column to supply said cooled expanded reheated portion to said fractionation column at a second feed point, said second feed point being at a lower position on said fractionation column than said first feed point.

10. In a process for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and to form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the liquid portion is expanded to a lower pressure, whereby a first part of the liquid portion vaporizes to cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point, wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column, the improvement comprising

(a) dividing said feed gas vapor into a first part and a second part;

(b) combining said first part of the feed gas vapor with the liquid portion of said feed gas to form thereby a combined stream;

(c) cooling at least one of said first part of the feed gas vapor, liquid portion of the feed gas, and said combined stream, whereby the combined stream is cooled to a temperature below the bubbled point of said liquid portion;

(d) expanding said combined stream and supplying the expanded combined stream to said distillation column at said first feed point;

(e) directing at least a portion of said feed gas vapor into heat exchange relation with said feed gas under pressure, whereby said feed gas vapor is reheated;

(f) thereafter expanding the reheated second part of the feed gas vapor to said lower pressure while extracting work therefrom; and

(g) supplying said expanded second part to said fractionation column at a second feed point, said second feed point being at a lower position on the fractionation column than said first feed point.

11. In a process for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and to form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is subcooled to a temperature below its bubble point;

(3) the subcooled liquid portion is expanded to a lower pressure, whereby a first part of the liquid portion vaporizes to further cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point, wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column, the improvement comprising

(a) dividing said feed gas vapor into a first part and a second part;

(b) reheating said first part of the feed gas vapor by directing it into heat exchange relation with said feed gas under pressure;

(c) expanding said reheated feed gas vapor to said lower pressure;

(d) supplying the reheated first part after expansion thereof to said fractionation column at a second feed point, said second feed point being below said first feed point; and

(e) expanding the second part of said feed gas vapor to said lower pressure and supplying said expanded second part of the feed gas vapor to said fractionation column at a feed point above said second feed point.

12. In a process for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and to form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the liquid portion is expanded to a lower pressure, whereby a first part of the liquid portion vaporizes to cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point, wherein said relatively less volatile fraction is separated;

(59 said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column, the improvement comprising

(a) dividing said first feed gas vapor into a first part and a second part;

(b) directing said first part into heat exchange relation with said feed gas under pressure, whereby said first part of the said feed gas vapor is reheated;

(c) expanding said first part of the feed gas vapor and thereafter suppying the expanded first part to said fractionation column at a second feed point, said second feed point being at a lower column position than said first feed point;

(d) cooling said second part of the feed gas vapor;

(e) expanding the second part of said feed gas vapor to said lower pressure; and

(f) supplying said expanded second part to said fractionation column at a feed point above said second feed point.

13. In a process for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and to form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the liquid portion is expanded to a lower pressure, whereby a first part of the liquid portion vaporizes to cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point, wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column,

the improvement comprising

(a) reheating at least some of the feed gas vapor prior to expansion thereof;

(b) expanding said reheated feed gas vapor in a work-expansion machine to said lower pressure, whereby work is extracted therefrom;

(c) directing said expanded feed gas vapor portion into heat exchange relation with residue gas from said fractionation column, wherein the expanded feed gas vapor is further cooled; and

(d) thereafter supplying said further cooled expanded feed gas vapor to said fractionation column at a second feed point, said second feed point being at a lower column position than said first feed point.

14. In a process for the separation of a feed gas into a volatile residue gas and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and ethane together comprising a major portion of the feed gas, wherein

(1) said feed gas under pressure is cooled to partially condense said gas and to form thereby a liquid portion and a feed gas vapor;

(2) at least some of the liquid portion thereby obtained is cooled to a temperature below its bubble point;

(3) the liquid portion is expanded to a lower pressure, whereby a first part of the liquid portion vaporizes to cool the expanded liquid portion;

(4) at least part of said expanded liquid portion is thereafter supplied to a fractionation column at a first feed point, wherein said relatively less volatile fraction is separated;

(5) said feed gas vapor is expanded to said lower pressure in a work-expansion machine, wherein work is extracted therefrom; and

(6) at least part of the expanded feed gas vapor is supplied to said fractionation column,

the improvement comprising

(a) dividing said feed gas vapor into a first part and a second part;

(b) expanding said first part of the feed gas vapor to said lower pressure and supplying the expanded first part to said fractionation column;

(c) directing the second part of said feed gas vapor into heat exchange relation with said feed gas under pressure, whereby said second part of the feed gas vapor is reheated;

(d) expanding the second part of said feed gas vapor to said lower pressure in a work-expansion machine, thereby extracting work therefrom; and

(e) thereafter supplying said expanded second part to said fractionation column at a second feed point, said second feed point being at a lower column position than said first feed point.

15. The improvement according to claim 14, wherein said liquid portion is subcooled by directing it into heat exchange relation with at least a portion of said residue gas from said fractionation column.

16. The improvement according to claim 14, wherein said expanded second part of the feed gas vapor is directed into heat exchange relation with at least a portion of the residue gas, thereby to further cool said expanded second part prior to supplying it to the fractionation column at the second feed point.