HYDROCARBON GAS PROCESSING

Abstract

A process is provided for treating a hydrocarbon gas stream by condensing a portion of the gas stream to produce a liquid stream, then through several steps subcooling a portion of the liquid stream to be introduced into a midpoint into a fractionation column. Unexpectedly, it has been found that the recompression and refrigeration power requirements are substantially reduced and the minimum approach to carbon dioxide freezing are increased when the liquid stream is introduced to a midpoint in the column when compared to introducing the liquid stream to a top position or a lower position in the fractionation column.

Claims

1. A process for separating a feed gas into a volatile residue gas fraction and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and C.sub.2 components together comprising the major portion of said feed gas, wherein a. said feed gas is cooled under pressure sufficiently to form a liquid stream under pressure and a vapor stream under pressure; b. said vapor stream is divided to form a first stream and a second stream; c. said first stream is expanded to a lower pressure to form an expanded first stream, whereupon said expanded first stream is supplied to a distillation column at a mid-column feed position; d. said second stream is cooled to substantial condensation and expanded to said lower pressure to form an expanded cooled second stream; e. said liquid stream is cooled to form a subcooled liquid stream, and said subcooled liquid stream is divided into a first portion and a second portion; f. said first portion is expanded to said lower pressure and heated to form a heated expanded first portion, thereby to supply at least a portion of said cooling in step e, whereupon said heated expanded first portion is supplied to said distillation column at a lower mid-column feed position; g. said second portion is expanded to said lower pressure to form an expanded second portion; h. said expanded cooled second stream is combined with said expanded second portion to form a combined stream, whereupon said combined stream is supplied to said distillation column at a top feed position; i. said combined stream, said expanded first stream, and said heated expanded first portion are fractionated in said distillation column at said lower pressure to form a distillation vapor stream and said relatively less volatile fraction, j. said distillation vapor stream is heated to form said volatile residue gas fraction, thereby to supply at least portion of said cooling in steps a and d; and k. the quantities and temperatures of said feed streams to said distillation column are effective to maintain the overhead temperature of said distillation column at a temperature whereby the major portion of said C.sub.2 components are recovered in said relatively less volatile fraction.

2. The process of claim 1 wherein a. said expanded first stream is combined with said expanded second portion to form said combined stream, whereupon said combined stream is supplied to said distillation column at said mid-column feed position; and b. said expanded cooled second stream is supplied to said distillation column at said top feed position.

3. The process of claim 1 wherein a. said expanded second portion is supplied to said distillation column at an upper mid-column feed position, said upper mid-column feed position being below said top feed position and above said mid-column feed position, and b. said expanded cooled second stream is supplied to said distillation column at said top feed position.

4. The process of claim 1 wherein (i) said distillation vapor stream is heated to form a heated distillation vapor stream, thereby to supply at least a portion of said cooling in steps a and d; (ii) said heated distillation vapor stream is compressed and cooled to form a cooled compressed distillation vapor stream; (iii) said cooled compressed distillation vapor stream is divided to form said volatile residue gas fraction and a recycle stream; (iv) said recycle stream is cooled to substantial condensation to form a cooled recycle stream, thereby to supply at least a portion of said heating in step (i); (v) said cooled recycle stream is expanded to said lower pressure to form an expanded cooled recycle stream, whereupon said expanded cooled recycle stream is supplied to said distillation column at said top feed position; and (vi) said combined stream is supplied to said distillation column at an upper mid-column feed position, said upper mid-column feed position being below said top feed position and above said mid-column feed position.

5. The process of claim 4 wherein a. said expanded first stream is combined with said expanded second portion to form said combined stream, whereupon said combined stream is supplied to said distillation column at said mid-column feed position; and b. said expanded cooled second stream is supplied to said distillation column at said upper mid-column feed position.

6. The process of claim 4 wherein a. said expanded second portion is supplied to said distillation column at an intermediate upper mid-column feed position, said intermediate upper mid-column feed position being below said upper mid-column feed position and above said mid-column feed position; and b. said expanded cooled second stream is supplied to said distillation column at said upper mid-column feed position.

7. The process of claim 1 wherein (i) said second portion is cooled under pressure; (ii) said cooled second portion is expanded to said lower pressure to form said expanded second portion; and (iii) said heating of said distillation vapor stream supplies at least a portion of said cooling in step (i).

8. The process of claim 2 wherein (i) said second portion is cooled under pressure; (ii) said cooled second portion is expanded to said lower pressure to form said expanded second portion; and (iii) said heating of said distillation vapor stream supplies at least a portion of said cooling in step (i).

9. The process of claim 3 wherein (i) said second portion is cooled under pressure; (ii) said cooled second portion is expanded to said lower pressure to form said expanded second portion; and (iii) said heating of said distillation vapor stream supplies at least a portion of said cooling in step (i).

10. An apparatus for separating a feed gas into a volatile residue gas fraction and a relatively less volatile fraction, said feed gas containing hydrocarbons, methane and C.sub.2 components together comprising the major portion of said feed gas, said apparatus including a. a first heat exchange means to receive said feed gas under pressure and cool said feed gas sufficiently to partially condense it to form a partially condensed stream; b. a separation means connected to said first heat exchange means to receive said partially condensed stream under pressure and separate said partially condensed stream into a liquid stream and a vapor stream; c. a first dividing means connected to said separation means to receive said vapor stream and divide said vapor stream into a first stream and a second stream, d. a first expansion means connected to said first dividing means to receive said first stream and expand said first stream to a lower pressure to form an expanded first stream; e. a distillation column connected to receive said expanded first stream at a mid-column feed position, f. a second heat exchange means connected to said first dividing means to receive said second stream and cool said second stream to substantial condensation to form a cooled second stream; g. a second expansion means connected to said second heat exchange means to receive said cooled second stream and expand said cooled second stream to said lower pressure to form an expanded cooled second stream; h. a third heat exchange means connected to said separation means to receive said liquid stream and cool said liquid stream to form a subcooled liquid stream; i. a second dividing means connected to said third heat exchange means to receive said subcooled liquid stream and divide said subcooled liquid stream into a first portion and a second portion; j. a third expansion means connected to said second dividing means to receive said first portion and expand said first portion to said lower pressure to form an expanded first portion; k. said third heat exchange means further connected to said third expansion means to receive said expanded first portion and heat said expanded first portion to form a heated expanded first portion, thereby to supply at least a portion of said cooling of element h, said third heat exchange means further being connected to said distillation column connected to supply said heated expanded first portion to said distillation column at a lower mid-column feed position; l. a fourth expansion means connected to said second dividing means to receive said second portion and expand said second portion to said lower pressure to form an expanded second portion; m. a combining means connected to said second expansion means and said fourth expansion means to receive said expanded cooled second stream and said expanded second portion and combine said expanded cooled second stream and said expanded second portion to form a combined stream, said combining means being further connected to said distillation column to supply said combined stream to said distillation column at a top feed position; n. said distillation column being adapted to fractionate said combined stream, said expanded first stream, and said heated expanded first portion at said lower pressure to form a distillation vapor stream and said relatively less volatile fraction; o. said distillation column being further connected to said second heat exchange means and said second heat exchange means being further connected to said first heat exchange means to receive said distillation vapor stream and heat said distillation vapor stream to form said volatile residue gas fraction, thereby to supply at least a portion of said cooling of elements a and f; and p. a control means adapted to regulate the quantities and temperatures of said feed streams to said distillation column to maintain an overhead temperature of said distillation column at a temperature whereby the major portion of said C.sub.2 components are recovered in said relatively less volatile fraction.

11. The apparatus according to claim 10 wherein a. said combining means is adapted to be connected to said first expansion means and said fourth expansion means to receive said expanded first stream and said expanded second portion and combine said expanded first stream and said expanded second portion to form said combined stream, said combining means being further adapted to supply said combined stream to said distillation column at said mid-column feed position; and b. said second expansion means is adapted to be connected to said distillation column to supply said expanded cooled second stream to said distillation column at said top feed position.

12. The apparatus according to claim 10 wherein a. said fourth expansion means is adapted to be connected to said distillation column at an upper mid-column feed position, said upper mid-column feed position being below said top feed position and above said mid-column feed position, thereby to supply said expanded second portion to said distillation column at said upper mid-column feed position; and b. said second expansion means is adapted to be connected to said distillation column to supply said expanded cooled second stream to said distillation column at said top feed position.

13. The apparatus according to claim 10 wherein (i) said second heat exchange means and said first heat exchange means are adapted to heat said distillation vapor stream to form a heated distillation vapor stream, thereby to supply at least a portion of said cooling of elements a and f; (ii) a compressing and cooling means is connected to said first heat exchange means to receive said heated distillation vapor stream and compress to higher pressure and cool said heated distillation vapor stream to form a cooled compressed distillation vapor stream; (iii) a third dividing means is connected to said compressing and cooling means to receive said cooled compressed distillation vapor stream and divide said cooled compressed distillation vapor stream into said volatile residue gas fraction and a recycle stream; (iv) said first heat exchange means is further connected to said third dividing means and said second heat exchange means is further connected to said first heat exchange means to receive said recycle stream and cool said recycle stream to substantial condensation to form a cooled recycle stream, thereby to supply at least a portion of said heating of element (i); (v) a fifth expansion means is connected to said second heat exchange means to receive said cooled recycle stream and expand said cooled recycle stream to said lower pressure to form an expanded cooled recycle stream, said fifth expansion means being further connected to said distillation column to supply said expanded cooled recycle stream to said distillation column at said top feed position; and (vi) said combining means is adapted to be connected to said distillation column at an upper mid-column feed position, said upper mid-column feed position being below said top feed position and above said mid-column feed position, thereby to supply said combined stream to said distillation column at said upper mid-column feed position.

14. The apparatus according to claim 13 wherein a. said combining means is adapted to be connected to said first expansion means and said fourth expansion means to receive said expanded first stream and said expanded second portion and combine said expanded first stream and said expanded second portion to form said combined stream, said combining means being further adapted to supply said combined stream to said distillation column at said mid-column feed position; and b. said second expansion means is adapted to be connected to said distillation column to supply said expanded cooled second stream to said distillation column at said upper mid-column feed position.

15. The apparatus according to claim 13 wherein a. said fourth expansion means is adapted to be connected to said distillation column at an intermediate upper mid-column feed position, said intermediate upper mid-column feed position being below said upper mid-column feed position and above said mid-column feed position, thereby to supply said expanded second portion to said distillation column at said intermediate upper mid-column feed position; and b. said second expansion means is adapted to be connected to said distillation column to supply said expanded cooled second stream to said distillation column at said upper mid-column feed position.

16. The apparatus according to claim 10 wherein (i) said second heat exchange means is adapted to have an additional cooling pass that is connected to said second dividing means to receive said second portion and cool said second portion under pressure to form a cooled second portion; (ii) said fourth expansion means is adapted to be connected to said second heat exchange means to receive said cooled second portion and expand said cooled second portion to said lower pressure to form said expanded second portion; and (iii) said heating of said distillation vapor stream in said second heat exchange means supplies at least a portion of said cooling of element (i).

17. The apparatus according to claim 1 wherein (i) said second heat exchange means is adapted to have an additional cooling pass that is connected to said second dividing means to receive said second portion and cool said second portion under pressure to form a cooled second portion; (ii) said fourth expansion means is adapted to be connected to said second heat exchange means to receive said cooled second portion and expand said cooled second portion to said lower pressure to form said expanded second portion, and (iii) said heating of said distillation vapor stream in said second heat exchange means supplies at least a portion of said cooling of element (i).

18. The apparatus according to claim 12 wherein (i) said second heat exchange means is adapted to have an additional cooling pass that is connected to said second dividing means to receive said second portion and cool said second portion under pressure to form a cooled second portion; (ii) said fourth expansion means is adapted to be connected to said second heat exchange means to receive said cooled second portion and expand said cooled second portion to said lower pressure to form said expanded second portion; and (iii) said heating of said distillation vapor stream in said second heat exchange means supplies at least a portion of said cooling of element (i).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 shows a typical prior art flow scheme of a process to treat a hydrocarbon stream that includes a split vapor configuration.

[0017] FIG. 2 shows a typical prior art flow scheme of a process to treat a hydrocarbon stream that includes a recycle split vapor configuration.

[0018] FIGS. 3, 5, and 7 show embodiments of the disclosure in which a subcooled liquid stream is added to a midpoint in a fractionation column.

[0019] FIGS. 4, 6, and 8 are concentration-temperature diagrams for carbon dioxide showing the effect of the prior art flow schemes and the present invention.

DETAILED DESCRIPTION

[0020] In accordance with the embodiments of the invention to be described herein, the hydrocarbon gas, under pressure, is cooled sufficiently to form a liquid portion, and the liquid portion is expanded to a lower pressure as in the conventional process. Expansion of the liquid portion vaporizes a portion of it and cools the remaining portion, which remains as a liquid. This expanded stream usually is supplied to a fractionation column where it is separated into a top fraction and a bottom fraction. In the present invention, the foregoing process is improved by subcooling the liquid portion and then dividing the subcooled liquid portion into first and second liquid portions. The first liquid portion is expanded to the lower pressure and then directed in heat exchange relation with the liquid portion of the feed stream to subcool the liquid portion. The subcooling of the liquid portion condensed from the feed gas under pressure lowers the temperature attained by the aforementioned second liquid portion after expansion. This expanded subcooled second liquid portion is added to the column at a midpoint instead of the upper portion as was done in the prior art. Midpoint of the distillation or fractionation column (terms used interchangeably within) means a point in the column in the middle 50% of the length of the column and preferably in the middle 33% of the column.

[0021] FIG. 1 shows a typical prior art flow scheme in which inlet gas enters the plant at 100° F. and 965 psia as stream 31. If the inlet gas contains a concentration of sulfur compounds which would prevent the product streams from meeting specifications, the sulfur compounds are removed by appropriate pretreatment of the feed gas (not illustrated). In addition, the feed stream is usually dehydrated to prevent hydrate (ice) formation under cryogenic conditions. Solid desiccant has typically been used for this purpose. Stream 31 is cooled to −29° F. by heat exchanger 10, which may be a series of coolers as known to one skilled in the art, to produce cooled stream 31a, which then enters separator 11 where vapor stream 32 is separated from liquid stream 33. Vapor stream 32 is divided into two streams 34 and 37. Stream 34, containing approximately 51% of the total vapor, is sent through heat exchanger 12 and cooled to −121° F. to become stream 34a which is flash expanded by expansion valve 27 down to the operating pressure (371 psia) of fractionation tower 17, cooling stream 34b to −137° F. before it is supplied to the tower as the top column feed. Fractionation tower 17 is a demethanizer column that is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. As is often the case in natural gas processing plants, the fractionation tower may consist of two sections. The upper section is a separator wherein the partially vaporized top feed is divided into its respective vapor and liquid portions, and wherein the vapor rising from the lower distillation or demethanizing section is combined with the vapor portion of the top feed to form the cold residue gas distillation stream 39 which exits the top of the tower at −126° F. The lower, demethanizing section contains the trays and/or packing and provides the necessary contact between the liquids falling downward and the vapors rising upward. The demethanizing section also includes reboilers which heat and vaporize a portion of the liquids flowing down the column to provide the stripping vapors which flow up the column.

[0022] The remaining portion of stream 32, stream 37, is expanded to the column operating pressure by expansion machine 14 to produce stream 37a at −93° F. that is sent to fractionation column 17 at a mid-column feed position. Liquid stream 33 is flash expanded to the column operating pressure by expansion valve 16 to form stream 33a at −75° F. which is supplied to fractionation column 17 at a lower mid-column feed position. A liquid stream 40 is withdrawn from fractionation column 17 to be heated in heat exchanger 10 and returned to fractionation column 17 in stream 40a to provide a portion of the reboiling heat to the column. Liquid stream 41 also is withdrawn from fractionation column 17 to be heated in heat exchanger 10 and returned to the fractionation column in stream 41a to provide another portion of the reboiling heat to the column. Some applications may also include supplemental reboiler 18 for fractionation column 17.

[0023] The liquid product stream 42 exits the bottom of the tower at 59° F., with its temperature adjusted by control means 23 to meet a typical specification of a methane to ethane ratio of 0.05:1 on a molar basis in the bottom product. The stream is pumped to higher pressure (stream 42a) by pump 21 and warmed to 88° F. (stream 42b) in heat exchanger 10 as it provides cooling to stream 31. (The discharge pressure of the pump is usually set by the ultimate destination of the liquid product. Generally, the liquid product flows to storage and the pump discharge pressure is set so as to prevent any vaporization of stream 42a as it is warmed in heat exchanger 10.)

[0024] Residue gas stream 39 passes to heat exchanger 12 where it is heated to −34° F. (stream 39a), and to heat exchanger 10 where it is further heated to 88° F. (stream 39b). Stream 39b is passed to compressor 15 (driven by expansion machine 14) to produce partially compressed stream 39c, which is further compressed in compressor 19 to produce compressed stream 39d and cooled in cooler 20 to produce residue gas stream 39e at 140° F. and 765 psia.

[0025] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 1 is set forth in the following Table I

TABLE-US-00001 TABLE I (FIG. 1) Stream Flow Summary - Lb. Moles/Hr Stream Methane Ethane Propane Butanes.sup.+ Total 31 62,369 13,440 7,291 2,082 87,844 32 37,767  4,430 1,288 172 44,978 33 24,602  9,010 6,003 1,910 42,866 34 19,362  2,271 660 88 23,059 37 18,405  2,159 628 84 21,919 39 61,771  1,476 48 1 64,921 42 598 11,964 7,243 2,081 22,923 Recoveries* Ethane 89.02% Propane 99.34% Butanes.sup.+ 99.95% Power Residue Gas Compression 25,738 HP Refrigerant Compression 18,724 HP Total Compression 44,462 HP *(Based on un-rounded flow rates)

[0026] FIG. 2 shows another typical prior art flow scheme which adds a recycle stream to the split vapor process of FIG. 1. As in FIG. 1, inlet gas enters the plant at 100° F. and 965 psia as stream 31. Stream 31 is cooled to −30° F. in heat exchanger 10 and cooled stream 31a enters separator 11 where the vapor (stream 32) is separated from the condensed liquid (stream 33). The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37. Stream 34, containing about 36% of the total vapor, passes through heat exchanger 12 where it is cooled to substantial condensation. The resulting substantially condensed stream 34a at −135° F. is then flash expanded by expansion valve 27 to the operating pressure (approximately 351 psia) of fractionation tower 17, whereupon stream 34b is supplied to fractionation tower 17 at an upper mid-column feed position. The remaining portion of the vapor from separator 11 (stream 37) enters work expansion machine 14 and is expanded to the tower operating pressure, with the work expansion cooling the expanded stream 37a to about −97° F. before it is supplied to fractionation tower 17 at a mid-column feed position. Liquid stream 33 from separator 11 is flash expanded to the column operating pressure by expansion valve 16 to form stream 33a at −78° F. which is supplied to fractionation column 17 at a lower mid-column feed position.

[0027] The recompressed and cooled distillation stream 39e is divided into two streams. One portion, stream 153, is the volatile residue gas product. The other portion, recycle stream 151, flows to heat exchanger 22 where it is cooled to −29° F. by heat exchange with a portion (stream 156) of cool distillation stream 39a. The cooled recycle stream 151a then flows to exchanger 12 where it is cooled to −135° F. and substantially condensed by heat exchange with cold distillation stream 39 at −142° F. The substantially condensed stream 151b is then flash expanded by expansion valve 24 to the tower operating pressure, resulting in cooling of the total stream to −149° F. The expanded stream 151c is then supplied to fractionation tower 17 as the top column feed. The vapor portion of stream 151c combines with the vapors rising from the top fractionation stage of the column to form distillation stream 39, which is withdrawn from an upper region of the tower.

[0028] Liquid product stream 42 exits the bottom of the tower at 52° F., based on a methane to ethane ratio of 0.05:1 on a molar basis in the bottom product. It is pumped by demethanizer bottoms pump 21, and the pumped liquid product is then warmed to 90° F. as it provides cooling of stream 31 in exchanger 10 before flowing to storage.

[0029] The demethanizer overhead vapor (stream 39) passes countercurrently to the incoming feed gas and recycle streams in heat exchanger 12 where it is heated from −140° F. to −38° F. (stream 39a), and in heat exchanger 22 and heat exchanger 10 where it is heated to 100° F. (stream 39b). The distillation stream is then re-compressed by compressor 15 (driven by expansion machine 14) and compressor 19, then cooled to 140° F. in cooler 20 (stream 39e). Stream 39e is split into the residue gas product (stream 153) and recycle stream 151 as described earlier before residue gas stream 153 flows to the sales gas pipeline at 765 psia.

[0030] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 2 is set forth in the following table:

TABLE-US-00002 TABLE II (FIG. 2) Stream Flow Summary - Lb. Moles/Hr Stream Methane Ethane Propane Butanes.sup.+ Total 31 62,369 13,440 7,291 2,082 87,844 32 36,952  4,279 1,237 165 43,919 33 25,417  9,161 6,054 1,917 43,925 34 13,230  1,532 443 59 15,724 37 23,722  2,747 794 106 28,195 39 73,688 802 0 0 76,330 151 11,957 130 0 0 12,386 153 61,731 672 0 0 63,944 42 638 12,768 7,291 2,082 23,900 Recoveries* Ethane  95.00% Propane 100.00% Butanes.sup.+ 100.00% Power Residue Gas Compression 33,523 HP Refrigerant Compression 16,422 HP Total Compression 49,945 HP *(Based on un-rounded flow rates)

[0031] FIG. 3 shows a flow scheme of the present invention in which a cooled liquid is introduced into a middle section of the fractionation column. The difference from the prior art flow scheme of FIG. 1 is that liquid stream 33 is first cooled in heat exchanger 28 to produce subcooled stream 33a. A portion of this stream, stream 158, is expanded to the column operating pressure by expansion valve 16 to produce stream 158a that enters heat exchanger 28 to be used as the cooling medium, and the resulting heated stream 158b is then supplied to fractionation column 17 at a lower mid-column feed position. The remaining portion of stream 33a, stream 159, is flash expanded to the column operating pressure by expansion valve 29 and then supplied to fractionation column 17 at an upper mid-column feed position.

[0032] As in FIG. 1, inlet gas enters the plant at 100° F. and 965 psia as stream 31. Stream 31 is cooled to −22° F. in heat exchanger 10 and cooled stream 31a enters separator 11 where the vapor (stream 32) is separated from the condensed liquid (stream 33). The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37. Stream 34, containing about 47% of the total vapor, passes through heat exchanger 12 where it is cooled to substantial condensation. The resulting substantially condensed stream 34a at −115° F. is then flash expanded by expansion valve 27 to the operating pressure (approximately 430 psia) of fractionation tower 17, whereupon stream 34b is supplied to fractionation tower 17 at a top column feed position. The remaining portion of the vapor from separator 11 (stream 37) enters work expansion machine 14 and is expanded to the tower operating pressure, with the work expansion cooling the expanded stream 37a to about −77° F. before it is supplied to fractionation tower 17 at a mid-column feed position.

[0033] The separator liquid (stream 33) is cooled to −92° F. in heat exchanger 28, and cooled liquid stream 33a is then divided into two streams, stream 158 and stream 159. Stream 158 is expanded to slightly above the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 158a to −97° F. before it is heated as it supplies the cooling in heat exchanger 28. The warmed stream 158b at −27° F. is then supplied to fractionation tower 17 at a lower mid-column feed position.

[0034] The remaining portion of cooled liquid stream 33a, stream 159, is flash expanded to the operating pressure of demethanizer 17 by expansion valve 29. A portion of the stream is vaporized, further cooling stream 159a to −98° F. before it is supplied to fractionation tower 17 at a mid-column feed position. The cold liquid in stream 159a serves as reflux to absorb and condense the C.sub.2 components, C.sub.3 components, and heavier components rising in the upper region of demethanizer 17.

[0035] Liquid product stream 42 exits the bottom of the tower at 72° F. based on a typical specification of a methane to ethane ratio of 0.05:1 on a molar basis in the bottom product. It is pumped to a pressure of approximately 515 psia in demethanizer bottoms pump 21, and the pumped liquid product is then warmed to 77T as it provides cooling of stream 31 in exchanger 10 before flowing to storage.

[0036] The residue gas (demethanizer overhead vapor stream 39) passes countercurrently to the incoming feed gas in heat exchanger 12 where it is heated from −120° F. to −27° F. (stream 39a), and in heat exchanger 10 where it is heated to 81° F. (stream 39b). Stream 39b is then re-compressed by compressor 15 (driven by expansion machine 14) and compressor 19, then cooled to 140° F. in cooler 20 before residue gas stream 39e flows to the sales gas pipeline at 765 psia.

[0037] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 3 is set forth in the following Table III:

TABLE-US-00003 TABLE III (FIG. 3) Stream Flow Summary - Lb. Moles/Hr Stream Methane Ethane Propane Butanes.sup.+ Total 31 62,369 13,440 7,291 2,082 87,844 32 42,686  5,460 1,662 227 51,573 33 19,683  7,980 5,629 1,855 36,271 34 19,849  2,539 773 106 23,981 37 22,837  2,921 889 121 27,592 158 11,396  4,620 3,259 1,074 21,000 159  8,287  3,360 2,370 781 15,271 39 61,771  1,473 67 2 64,732 42 598 11,967 7,224 2,080 23,112 Recoveries* Ethane 89.04% Propane 99.08% Butanes.sup.+ 99.91% Power Residue Gas Compression 18,554 HP Refrigerant Compression 21,634 HP Total Compression 40,188 HP *(Based on un-rounded flow rates)

[0038] A comparison of Tables I and III shows that, compared to the prior art, the present invention maintains essentially the same ethane recovery (89.02% versus 89.04%), propane recovery (99.34% versus 99.08%), and butanes+recovery (99.95% versus 99.91%). However, comparison of Tables I and III further shows that these yields were achieved with substantially lower power requirements than those of the prior art process. The total power requirement of the present invention is 10% lower than that of the FIG. 1 process.

[0039] The key feature of the present invention is the supplemental rectification provided by subcooled liquid stream 159a, which reduces the amount of C.sub.2 components, C.sub.3 components, and C.sub.4+ components contained in the vapors rising in the upper region of fractionation column 17. Whereas all the liquid (stream 33) from separator 11 is supplied below the feed (stream 37a) from work expansion machine 14 in the FIG. 1 process, the embodiment of the present invention in FIG. 3 is able to cool more than 40% of this liquid and use it as reflux (stream 159a) above stream 37a. This allows operating separator 11 at a warmer temperature, so that there is a corresponding increase in the flow rate of stream 37 to work expansion machine 14. This in turn provides a two-fold improvement in the process efficiency. First, with more flow to expansion machine 14, the increase in power recovery increases the refrigeration generated by the process. Second, the greater power recovery means more power available to compressor 15, reducing the external power consumption of compressor 19.

[0040] A further advantage of the present invention is a reduced likelihood of carbon dioxide freezing. FIG. 4 is a graph of the relation between carbon dioxide concentration and temperature. Line 71 represents the equilibrium conditions for solid and liquid carbon dioxide in methane. (The liquid-solid equilibrium line in this graph is based on the data given in FIG. 16-33 on page 16-24 of the Engineering Data Book, Twelfth Edition, published in 2004 by the Gas Processors Suppliers Association.) A liquid temperature on or to the right of line 71, or a carbon dioxide concentration on or above this line, signifies a freezing condition. Because of the variations which normally occur in gas processing facilities (e.g., feed gas composition, conditions, and flow rate), it is usually desired to design a demethanizer with a considerable safety factor between the expected operating conditions and the freezing conditions. (Experience has shown that the conditions of the liquids on the fractionation stages of a demethanizer, rather than the conditions of the vapors, govern the allowable operating conditions in most demethanizers. For this reason, the corresponding vapor-solid equilibrium line is not shown in FIG. 4.)

[0041] Also plotted in FIG. 4 is a line representing the conditions for the liquids on the fractionation stages of demethanizer 17 in the FIG. 1 process (line 72). As can be seen, a portion of this operating line lies relatively close to the liquid-solid equilibrium line, indicating that the FIG. 1 process could easily encounter carbon dioxide freezing problems during variations in operating conditions. Increasing the operating margin would require removal of at least some of the carbon dioxide from the feed gas. This would, of course, substantially increase capital cost.

[0042] Line 73 in FIG. 4 represents the conditions for the liquids on the fractionation stages of demethanizer 17 in the present invention as depicted in FIG. 3. In contrast to the FIG. 1 process, there is a minimum safety factor of 2.0 between the anticipated operating conditions and the freezing conditions for the FIG. 3 process. That is, it would require a 100 percent increase in the carbon dioxide content of the liquids to cause freezing. Thus, the present invention could tolerate a 100% higher concentration of carbon dioxide in its feed gas than the FIG. 1 process could tolerate without risk of freezing. Further, whereas the FIG. 1 process cannot be operated to achieve more than the recovery levels given in Table I because of freezing, the present invention could in fact be operated at even higher recovery levels than those given in Table III without risk of freezing.

[0043] The shift in the operating conditions of the FIG. 3 demethanizer as indicated by line 73 in FIG. 4 can be understood by comparing the distinguishing features of the present invention to the prior art process of FIG. 1. While the shape of the operating line for the FIG. 1 process (line 72) is similar to the shape of the operating line for the present invention (line 73), there are two key differences. One difference is that the operating temperatures of the critical upper fractionation stages in the demethanizer in the FIG. 3 process are warmer than those of the corresponding fractionation stages in the demethanizer in the FIG. 1 process, effectively shifting the operating line of the FIG. 3 process away from the liquid-solid equilibrium line. The warmer temperatures of the fractionation stages in the FIG. 3 demethanizer are partly the result of operating the tower at higher pressure than the FIG. 1 process.

[0044] The more significant difference between the two operating lines in FIG. 4, however, is the much lower concentrations of carbon dioxide in the liquids on the fractionation stages of demethanizer 17 in the FIG. 3 process compared to those of demethanizer 17 in the FIG. 1 process. One of the inherent features in the operation of a demethanizer column to recover C.sub.2 components is that the column must fractionate between the methane that is to leave the tower in its overhead product (vapor stream 39) and the C.sub.2 components that are to leave the tower in its bottom product (liquid stream 42). However, the relative volatility of carbon dioxide lies between that of methane and C.sub.2 components, causing the carbon dioxide to appear in both terminal streams. Further, carbon dioxide and ethane form an azeotrope, resulting in a tendency for carbon dioxide to accumulate in the intermediate fractionation stages of the column and thereby cause large concentrations of carbon dioxide to develop in the tower liquids.

[0045] It is well known that adding a third component is often an effective means for “breaking” an azeotrope. As noted in U.S. Pat. No. 4,318,723, C.sub.3-C.sub.6 alkane hydrocarbons, particularly n-butane, are effective in modifying the behavior of carbon dioxide in hydrocarbon mixtures. Experience has shown that the composition of the upper mid-column feed (i.e., stream 159a in FIG. 3) to demethanizers of this type has significant impact on the composition of the liquids on the crucial fractionation stages in the upper section of the demethanizer. Examining this stream in Table III, note that the C.sub.3+ component and C.sub.4+ component concentrations for the FIG. 3 process are greater than 20% and 5%, respectively. The net impact of this is to “break” the azeotrope and reduce the carbon dioxide concentrations in the column liquids accordingly. A further impact of the higher concentrations of C.sub.3+ components and C.sub.4+ components in the liquids on the fractionation stages of demethanizer 17 in the FIG. 3 process is to raise the bubble point temperatures of the tray liquids, adding to the favorable shift of operating line 73 for the FIG. 3 process away from the liquid-solid equilibrium line in FIG. 4.

[0046] In the present invention, the second cooled portion of the “auto-cooled” liquid (stream 159) is flash expanded and fed to an upper mid-column feed position or an intermediate upper mid-column feed position. This supplemental reflux stream feeds the column at or above the feed location of expanded stream 37a. In some embodiments, the subcooled liquid portion that is feeding the fractionation column may be mixed with expanded subcooled stream 34b with similar results. In other embodiments, the subcooled liquid portion can be combined with stream 37a downstream of work expansion machine 14 prior to feeding the column. Combining these two streams is particularly beneficial when the “auto-cooled” liquid stream is colder than the stream leaving the expander. In that case, additional heavier hydrocarbons are condensed from the vapor fraction leaving the expander. These additional liquids subsequently flow downward in the distillation column, thereby requiring less reflux flow to be provided by the upper reflux stream. The decrease in the flow rates of the upper reflux stream reduces power consumption for a specific hydrocarbon recovery level.

[0047] Another embodiment of the present invention is shown in FIG. 5. The processing conditions are much the same as for the FIG. 3 embodiment, except that the subcooled liquid portion (stream 159) is further cooled before being flash expanded to the tower pressure. In this example, stream 159 is cooled from −93° F. to −116° F. in heat exchanger 12, and stream 159a is then flash expanded by expansion valve 29 to form stream 159b at −114° F. that is thereafter supplied to fractionation column 17 at a mid-column feed position.

[0048] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 5 is set forth in the following Table IV:

TABLE-US-00004 TABLE IV (FIG. 5) Stream Flow Summary - Lb. Moles/Hr Stream Methane Ethane Propane Butanes.sup.+ Total 31 62,369 13,440 7,291 2,082 87,844 32 42,686  5,460 1,662 227 51,573 33 19,683  7,980 5,629 1,855 36,271 34 17,741  2,269 691 94 21,434 37 24,945  3,191 971 133 30,139 158 11,390  4,618 3,257 1,073 20,990 159  8,293  3,362 2,372 782 15,281 39 61,771  1,473 66 2 64,714 42 598 11,967 7,225 2,080 23,130 Recoveries* Ethane 89.04% Propane 99.10% Butanes.sup.+ 99.91% Power Residue Gas Compression 18,652 HP Refrigerant Compression 21,304 HP Total Compression 39,956 HP *(Based on un-rounded flow rates)

[0049] A comparison of Tables III and IV shows that this embodiment of the present invention achieves essentially the same ethane recovery (89.04% versus 89.04%), propane recovery (99.08% versus 99.10%), and butanes+recovery (99.91% versus 99.91%). Comparison of Tables III and IV further shows that these yields were achieved with still lower power requirements, dropping the total power requirement by an additional 0.5% relative to the FIG. 1 prior art process. This is due to the improved rectification provided in the upper region of fractionation column 17 by the colder liquid (stream 159b). This allows a reduction in the reflux (stream 34b) to the top of the column and a corresponding increase in the flow to work expansion machine 14, generating more power and providing more refrigeration to the process, thereby reducing the total compression power.

[0050] More importantly, the colder liquid feeding the upper mid-column region of fractionation tower 17 serves to further suppress accumulation of carbon dioxide in the upper stages of the column. FIG. 6 is another graph of the relation between carbon dioxide concentration and temperature, with line 71 as before representing the equilibrium conditions for solid and liquid carbon dioxide in methane and line 73 representing the conditions for the liquids on the fractionation stages of demethanizer 17 in the FIG. 3 embodiment of the present invention. Line 74 represents the conditions for the liquids on the fractionation stages of demethanizer 17 in this FIG. 5 embodiment of the present invention, and it lies below line 73. This means this embodiment could tolerate still higher concentrations of carbon dioxide in its feed gas than the embodiment in FIG. 3 without risk of freezing.

[0051] FIG. 7 is an example of how the present invention can be used to improve the prior art of the process in FIG. 2. As in FIG. 2, inlet gas enters the plant at 100° F. and 965 psia as stream 31. Stream 31 is cooled to −20° F. in heat exchanger 10 and cooled stream 31a enters separator 11 where the vapor (stream 32) is separated from the condensed liquid (stream 33). The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37. Stream 34, containing about 32% of the total vapor, passes through heat exchanger 12 where it is cooled to substantial condensation. The resulting substantially condensed stream 34a at −131° F. is then flash expanded by expansion valve 27 to the operating pressure (approximately 390 psia) of fractionation tower 17, whereupon stream 34b is supplied to fractionation tower 17 at an upper mid-column feed position. The remaining portion of the vapor from separator 11 (stream 37) enters work expansion machine 14 and is expanded to the tower operating pressure, with the work expansion cooling the expanded stream 37a to about −81° F. before it is supplied to fractionation tower 17 at a mid-column feed position.

[0052] The separator liquid (stream 33) is cooled to −98° F. in heat exchanger 28, and cooled liquid stream 33a is then divided into two streams, stream 158 and stream 159. Stream 158 is expanded to slightly above the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 158a to −103° F. before it is heated as it supplies the cooling in heat exchanger 28. The warmed stream 158b at −25° F. is then supplied to fractionation tower 17 at a lower mid-column feed position.

[0053] The remaining portion of cooled liquid stream 33a, stream 159, is flash expanded to the operating pressure of demethanizer 17 by expansion valve 29. A portion of the stream is vaporized, further cooling stream 159a to −104° F. before it is supplied to fractionation tower 17 at a mid-column feed position. The cold liquid in stream 159a serves as reflux to absorb and condense the C.sub.2 components, C.sub.3 components, and heavier components rising in the upper region of demethanizer 17.

[0054] The recompressed and cooled distillation stream 39e is divided into two streams. One portion, stream 153, is the volatile residue gas product. The other portion, recycle stream 151, flows to heat exchanger 22 where it is cooled to −21° F. by heat exchange with a portion (stream 156) of cool distillation stream 39a. The cooled recycle stream 151a then flows to exchanger 12 where it is cooled to −131° F. and substantially condensed by heat exchange with cold distillation stream 39. The substantially condensed stream 151b is then flash expanded by expansion valve 24 to the tower operating pressure, resulting in cooling of the total stream to −144° F. The expanded stream 151c is then supplied to fractionation tower 17 as the top column feed. The vapor portion of stream 151c combines with the vapors rising from the top fractionation stage of the column to form distillation stream 39, which is withdrawn from an upper region of the tower at −136° F.

[0055] Liquid product stream 42 exits the bottom of the tower at 60° F. based on a typical specification of a methane to ethane ratio of 0.05:1 on a molar basis in the bottom product. It is pumped to a pressure of approximately 515 psia in demethanizer bottoms pump 21, and the pumped liquid product is then warmed to 86° F. as it provides cooling of stream 31 in exchanger 10 before flowing to storage.

[0056] The demethanizer overhead vapor (stream 39) passes countercurrently to the incoming feed gas and recycle streams in heat exchanger 12 where it is heated to −29° F. (stream 39a), and in heat exchanger 22 and heat exchanger 10 where it is heated to 95° F. (stream 39b). The distillation stream is then re-compressed by compressor 15 (driven by expansion machine 14) and compressor 19, then cooled to 140° F. in cooler 20 (stream 39e). Stream 39e is split into the residue gas product (stream 153) and recycle stream 151 as described earlier before residue gas stream 153 flows to the sales gas pipeline at 765 psia.

[0057] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 7 is set forth in the following Table V.

TABLE-US-00005 TABLE V (FIG. 7) Stream Flow Summary - Lb. Moles/Hr Stream Methane Ethane Propane Butanes.sup.+ Total 31 62,369 13,440 7,291 2,082 87,844 32 43,883  5,746 1,775 245 53,241 33 18,486  7,694 5,516 1,837 34,603 34 13,902  1,820 562 78 16,866 37 29,981  3,926 1,213 167 36,375 158 10,699  4,453 3,193 1,063 20,027 159  7,787  3,241 2,323 774 14,576 39 74,150 808 1 0 76,574 151 12,419 135 0 0 12,825 153 61,731 673 1 0 63,749 42 638 12,767 7,290 2,082 24,095 Recoveries* Ethane  95.00% Propane  99.99% Butanes.sup.+ 100.00% Power Residue Gas Compression 26,744 HP Refrigerant Compression 17,139 HP Total Compression 43,883 HP *(Based on un-rounded flow rates)

[0058] A comparison of Tables II and V shows that, compared to the prior art, the present invention maintains essentially the same ethane recovery (95.00% versus 95.00%), propane recovery (100.00% versus 99.99%), and butanes+recovery (100.00% versus 100.00%). However, comparison of Tables II and V further shows that these yields were achieved with substantially lower power requirements than those of the prior art process. The total power requirement of the present invention is 12% lower than that of the FIG. 2 process.

[0059] As with the FIG. 3 and FIG. 4 embodiments of the present invention, the supplemental rectification provided by subcooled liquid stream 159a reduces the amount of C.sub.2 components, C.sub.3 components, and C.sub.4+ components contained in the vapors rising in the upper region of fractionation column 17. This embodiment of the present invention is able to cool more than 40% of the liquid from separator 11 and use it as reflux (stream 159a) above stream 37a. This allows operating separator 11 at a warmer temperature, so that there is a corresponding increase in the flow rate of stream 37 to work expansion machine 14. This in turn increases the refrigeration generated by the process and reduces the external power consumption of compressor 19.

[0060] This embodiment of the present invention also reduces the likelihood of carbon dioxide freezing compared to the prior art of FIG. 2. FIG. 8 is another graph of the relation between carbon dioxide concentration and temperature, with line 71 as before representing the equilibrium conditions for solid and liquid carbon dioxide in methane and line 75 representing the conditions for the liquids on the fractionation stages of demethanizer 17 in the prior art process of FIG. 2. As can be seen, a portion of this operating line lies above the liquid-solid equilibrium line, indicating that the prior art FIG. 2 process cannot be operated at these conditions without encountering carbon dioxide freezing problems. As a result, it is not possible to use the FIG. 2 process under these conditions, so the prior art FIG. 2 process cannot actually achieve the recovery efficiencies stated in Table II in practice without removal of at least some of the carbon dioxide from the feed gas. This would, of course, substantially increase capital cost.

[0061] Line 76 in FIG. 8 represents the conditions for the liquids on the fractionation stages of demethanizer 17 in the FIG. 7 embodiment of the present invention, and shows a safety factor of 1.1 between the anticipated operating conditions and the carbon dioxide freezing conditions. Thus, this embodiment of the present invention could tolerate an increase of 10% in the concentration of carbon dioxide without risk of freezing. In practice, this improvement in the freezing safety factor could be used to advantage by operating the demethanizer at lower pressure (i.e., with colder temperatures on the fractionation stages) to raise the C.sub.2+ component recovery levels still higher without encountering freezing problems.