Hydrocarbon Gas Processing

Abstract

A process and an apparatus are disclosed for a compact processing assembly to improve the recovery of C.sub.2 (or C.sub.3) and heavier hydrocarbon components from a hydrocarbon gas stream. The preferred method of separating a hydrocarbon gas stream generally includes producing at least a substantially condensed first stream and a cooled second stream, expanding both streams to lower pressure, and supplying the streams to a fractionation tower. In the process and apparatus disclosed, the tower overhead vapor is directed to an absorbing means and a heat and mass transfer means inside a processing assembly. A portion of the outlet vapor from the processing assembly is compressed to higher pressure, cooled and substantially condensed in a heat exchange means inside the processing assembly, then expanded to lower pressure and supplied, to the heat and mass transfer means to provide cooling. Condensed liquid from the absorbing means is fed to the tower.

Claims

1. In a process for the separation of a gas stream containing methane, C.sub.2 components, C.sub.3 components, and heavier hydrocarbon components into a volatile residue gas fraction and a relatively less volatile fraction containing a major portion of said C.sub.2 components, C.sub.3 components, and heavier hydrocarbon components or said C.sub.3 components and heavier hydrocarbon components, in which process (a) said gas stream is treated in one or more heat exchange steps and at least one division step to produce at least a first stream that has been cooled under pressure to condense substantially all of it, and at least a second stream that has been cooled under pressure; (b) said substantially condensed first stream is expanded to a lower pressure whereby it is further cooled, and thereafter supplied at a top feed position on a distillation column that produces at least an overhead vapor stream and a bottom liquid stream; (c) said cooled second stream is expanded to said lower pressure, and thereafter supplied to said distillation column at a mid-column feed position; and (d) at least said expanded further cooled first stream and said expanded second stream are fractionated in said distillation column at said lower pressure whereby the components of said relatively less volatile fraction are recovered in said bottom liquid stream and said volatile residue gas fraction is discharged as said overhead vapor stream; the improvement wherein (1) said overhead vapor stream is directed to an absorbing means housed in a processing assembly to be contacted with a condensed stream and thereby condense its less volatile components to form a partially rectified vapor stream; (2) said partially rectified vapor stream is collected from an upper region of said absorbing means and directed to a heat and mass transfer means housed in said processing assembly, whereby said partially rectified vapor stream is cooled while simultaneously condensing its less volatile components, thereby forming a further rectified vapor stream and said condensed stream, whereupon said condensed stream is directed to said absorbing means; (3) said further rectified vapor stream is directed to a heat exchange means housed in said processing assembly and heated, thereafter discharging said heated further rectified vapor stream from said processing assembly as an outlet vapor stream; (4) said outlet vapor stream is divided into a first portion and a second portion; (5) said first portion is compressed to higher pressure to form a compressed stream; (6) said compressed stream is directed to said heat exchange means and cooled to substantial condensation, thereby to supply at least a portion of the heating of step (3) and form a substantially condensed stream; (7) said substantially condensed stream is expanded to said lower pressure, whereby it is further cooled to form a flash, expanded stream; (8) said flash expanded stream is heated in said heat and mass transfer means, thereby to supply at least a portion of the cooling of step (2) and form a heated flash expanded stream; (9) said heated flash expanded stream is combined with said second portion to form said volatile residue gas fraction; (10) a distillation liquid stream is collected from a lower region of said absorbing means and combined, with said expanded further cooled first stream to form a combined feed stream, whereupon said combined feed stream is directed to said top feed position on said distillation column; (11) at least said combined feed stream and said expanded second stream are fractionated in said distillation column at said lower pressure whereby the components of said relatively less volatile traction are recovered in said bottom liquid stream; and (12) 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 portions of the components in said relatively less volatile fraction are recovered in said bottom liquid stream.

2. The process according to claim 1 wherein (1) said gas stream is cooled under pressure in said one or more heat exchange steps sufficiently to partially condense it; (2) said partially condensed gas stream is separated thereby to provide a vapor stream and at least one liquid stream; (3) said vapor stream is divided in said at least one division step to produce at least said first stream and said second stream; (4) said first stream is cooled under pressure in said one or more heat exchange steps to condense substantially all of it and thereby form said substantially condensed first stream; (5) at least a portion of said at least one liquid stream is expanded to said lower pressure, whereupon said expanded liquid stream is supplied to said distillation column at a lower mid-column feed position below said mid-column feed position; and (6) at least said combined feed stream, said expanded second stream, and said expanded liquid stream are fractionated in said distillation column at said lower pressure whereby the components of said relatively less volatile fraction are recovered in said bottom liquid stream.

3. The process according to claim 2 wherein (1) said vapor stream is divided in said at least one division step to produce at least a first vapor stream and said second stream; (2) said first vapor stream is combined with at least a portion of said at least one liquid stream to form said first stream; and (3) any remaining portion of said at least one liquid stream is expanded to said lower pressure, whereupon said expanded liquid stream is supplied to said distillation column at said lower mid-column feed position.

4. The process according to claim 1, 2, or 3 wherein (1) said heated flash expanded stream is combined with said overhead vapor stream to form a combined vapor stream; (2) said combined vapor stream is directed to said absorbing means to be contacted with said condensed stream and thereby form said partially rectified stream; and (3) said second portion is discharged as said volatile residue gas fraction.

5. The process according to claim 4 wherein (1) said heated flash expanded stream is directed to a separating means housed in said processing assembly and separated therein into a vapor fraction and a liquid fraction; (2) said vapor fraction is combined with said overhead vapor stream to form said combined vapor stream; (3) said liquid fraction is combined with said distillation liquid stream to form a combined liquid stream; and (4) said combined liquid stream is combined with said expanded further cooled first stream to form said combined feed stream.

6. The process according to claim 4 wherein (1) said overhead vapor stream is divided into said first portion and said second portion; (2) said second portion is combined with said heated flash expanded stream to form said combined vapor stream; and (3) said outlet vapor stream is discharged as said volatile residue gas fraction.

7. The process according to claim 5 wherein (1) said overhead vapor stream is divided into said first portion and said second portion; (2) said second portion is combined with said vapor fraction to form said combined vapor stream; and (3) said outlet vapor stream is discharged as said volatile residue gas fraction.

8. The process according to claim 1, 2, or 3 wherein said distillation liquid stream is pumped to higher pressure using a pumping means.

9. The process according to claim 4 wherein said distillation liquid stream is pumped to higher pressure using a pumping means.

10. The process according to claim 5 wherein said combined liquid stream is pumped to higher pressure using a pumping means.

11. The process according to claim 6 wherein said distillation liquid stream is pumped to higher pressure using a pumping means.

12. The process according to claim 7 wherein said combined liquid stream is pumped to higher pressure using a pumping means.

13. The process according to claim 8 wherein said pumping means is housed in said processing assembly.

14. The process according to claim 9 wherein said pumping means is housed in said processing assembly.

15. The process according to claim 10 wherein said pumping means is housed in said processing assembly.

16. The process according to claim 11 wherein said pumping means is housed in said processing assembly.

17. The process according to claim 12 wherein said pumping means is housed in said processing assembly.

18. In an apparatus for the separation of a gas stream containing methane, C.sub.2 components, C.sub.3 components, and heavier hydrocarbon components into a volatile residue gas fraction and a relatively less volatile fraction containing a major portion of said C.sub.2 components, C.sub.3 components, and heavier hydrocarbon components or said C.sub.3 components and heavier hydrocarbon components, in said apparatus there being (a) one or more heat exchange means and at least one dividing means to produce at least a first stream that has been cooled under pressure to condense substantially all of it, and at least a second stream that has been cooled under pressure; (b) a first expansion means connected to receive said substantially condensed first stream under pressure and expand it to a lower pressure, whereby said first stream is further cooled; (c) a distillation column connected to said first expansion means to receive said expanded further cooled first stream at a top feed position, with said distillation column producing at least an overhead vapor stream and a bottom liquid stream; (d) a second expansion means connected to receive said cooled second stream under pressure and expand it to said lower pressure; (e) said distillation column further connected to said second expansion means to receive said expanded second stream at a mid-column feed position; and (f) said distillation column adapted to fractionate at least said expanded further cooled first stream and said expanded second stream at said lower pressure whereby the components of said relatively less volatile fraction are recovered in said bottom liquid stream and said volatile residue gas fraction is discharged as said overhead vapor stream; the improvement wherein said apparatus includes (1) an absorbing means housed in a processing assembly and connected to said distillation column to receive said overhead vapor stream and contact it with a condensed stream, thereby condensing its less volatile components and forming a partially rectified vapor stream; (2) a heat and mass transfer means housed in said processing assembly and connected to said absorbing means to receive said partially rectified vapor stream from an upper region of said absorbing means, whereby said partially rectified vapor stream is cooled while simultaneously condensing its less volatile components, thereby forming a further rectified vapor stream and said condensed stream, said heat and mass transfer means being further connected to said absorbing means to direct said condensed stream to said absorbing means; (3) a second heat exchange means housed in said processing assembly and connected to said heat and mass transfer means to receive said further rectified vapor stream and heat it, thereafter discharging said heated further rectified vapor stream from said processing assembly as an outlet vapor stream; (4) a second dividing means connected to said processing assembly to receive said outlet vapor stream and divide it into a first portion and a second portion; (5) a compressing means connected to said second dividing means to receive said first portion and compress it to higher pressure, thereby forming a compressed stream; (6) said second heat exchange means further connected to said compressing means to receive said compressed stream and cool it to substantial condensation, thereby to supply at least a portion of the heating of step (3) and forming a substantially condensed stream; (7) a third expansion means connected to said second heat exchange means to receive said substantially condensed stream and expand it to said lower pressure, thereby forming a flash expanded stream; (8) said heat and mass transfer means further connected to said third expansion means to receive said flash expanded stream and heat it, thereby to supply the cooling of step (2) and forming a heated flash expanded stream; (9) a first combining means connected to said heat and mass transfer means and to said second dividing means to receive said heated flash expanded stream and said second portion and form said volatile residue gas fraction; (10) a second combining means connected to said absorbing means and to said first expansion means to receive a distillation liquid stream from a lower region of said absorbing means and said expanded further cooled first stream and form a combined feed stream, said second combining means being further connected to said distillation column to supply said combined feed stream at said top feed position of said distillation column; (11) said distillation column being adapted to fractionate at least said combined feed stream and said expanded second stream at said lower pressure whereby the components of said relatively less volatile fraction are recovered in said bottom liquid stream; and (12) control means adapted to regulate the quantities and temperatures of said feed streams to said distillation column to maintain the overhead temperature of said distillation column at a temperature whereby the major portions of the components in said relatively less volatile fraction are recovered in said bottom liquid stream.

19. The apparatus according to claim 18 wherein (1) said one or more heat exchange means is adapted to cool said gas stream under pressure sufficiently to partially condense it; (2) a feed separating means is connected to said one or more heat exchange means to receive said partially condensed gas stream and separate it into a vapor stream and at least one liquid stream; (3) said at least one dividing means is connected to said feed separating means and adapted to receive said vapor stream and divide it into at least said first stream and said second stream; (4) said one or more heat exchange means is connected to said at least one dividing means and adapted to receive said first stream and cool it sufficiently to substantially condense it, thereby forming said substantially condensed first stream; (5) said second expansion means is connected to said at least one dividing means and adapted to receive said second stream and expand it to said lower pressure, thereby forming said expanded second stream; (6) a fourth expansion means is connected to said feed separating means to receive at least a portion of said at least one liquid stream and expand it to said lower pressure, said fourth expansion means being further connected to said distillation column to supply said expanded liquid stream to said distillation column at a lower mid-column feed position below said mid-column feed position; and (7) said distillation column is adapted to fractionate at least said combined feed stream, said expanded second stream, and said expanded liquid stream at said lower pressure whereby the components of said relatively less volatile fraction are recovered in said bottom liquid stream.

20. The apparatus according to claim 19 wherein (1) said at least one dividing means is adapted to divide said vapor stream into at least a first vapor stream and said second stream; (2) a vapor-liquid combining means is connected to said at least one dividing means and to said feed separating means to receive said first vapor stream and at least a portion of said at least one liquid stream and form said first stream; (3) said one or more heat exchange means is connected to said vapor-liquid combining means and adapted to receive said first stream and cool it sufficiently to substantially condense it, thereby forming said substantially condensed first stream; and (4) said fourth expansion means is adapted to receive any remaining portion of said at least one liquid stream and expand it to said lower pressure, whereupon said expanded liquid stream is supplied to said distillation column at said lower mid-column feed position.

21. The apparatus according to claim 18, 19, or 20 wherein (1) said first combining means is adapted to be connected to said heat and mass transfer means and to said distillation column to receive said heated flash expanded stream and said overhead vapor stream and form a combined vapor stream; (2) said first combining means is further connected to said absorbing means to direct said combined vapor stream to said absorbing means, said absorbing means being adapted to contact said combined vapor stream with said condensed stream, thereby forming said partially rectified vapor stream; and (3) said second dividing means is adapted to discharge said second portion as said volatile residue gas fraction.

22. The apparatus according to claim 21 wherein (1) a separating means is housed in said processing assembly and connected to receive said heated flash expanded stream and separate it therein into a vapor fraction and a liquid fraction; (2) said first combining means is adapted to be connected to said separating means and to said distillation column to receive said vapor fraction and said overhead vapor stream and form said combined vapor stream; (3) a third combining means is connected to said absorbing means and to said separating means to receive said distillation liquid stream from said lower region of said absorbing means and said liquid fraction and form a combined liquid stream; and (4) said second combining means is adapted to be connected to said third combining means and to said first expansion means to receive said combined liquid stream and said expanded further cooled first stream and form said combined feed stream.

23. The apparatus according to claim 21 wherein (1) said second dividing means is adapted to be connected to said distillation column to receive said overhead vapor stream and divide it into said first portion and said second portion; (2) said first combining means is adapted to be connected to said heat and mass transfer means and to said second dividing means to receive said heated flash expanded stream and said second portion, thereby forming said combined vapor stream; and (3) said processing assembly is adapted to discharge said outlet vapor as said volatile residue gas fraction.

24. The apparatus according to claim 22 wherein (1) said second dividing means is adapted to be connected to said distillation column to receive said overhead vapor stream and divide it into said first portion and said second portion; (2) said first combining means is adapted to be connected to said separating means and to said second dividing means to receive said vapor fraction and said second portion, thereby forming said combined vapor stream; and (3) said processing assembly is adapted to discharge said outlet vapor as said volatile residue gas fraction.

25. The apparatus according to claim 18, 19, or 20 wherein (1) a pumping means is connected to said absorbing means to receive said distillation liquid stream from said lower region of said absorbing means and pump it to higher pressure, thereby forming a pumped distillation liquid stream; and (2) said second combining means is adapted to be connected to said pumping means and to said first expansion means to receive said pumped distillation liquid stream and said expanded further cooled first stream and form said combined feed stream.

26. The apparatus according to claim 21 wherein (1) a pumping means is connected to said absorbing means to receive said distillation liquid stream from said lower region of said absorbing means and pump it to higher pressure, thereby forming a pumped distillation liquid stream; and (2) said second combining means is adapted to be connected to said pumping means and to said first expansion means to receive said pumped distillation liquid stream and said expanded further cooled first stream and form said-combined feed stream.

27. The apparatus according to claim 22 wherein (1) a pumping means is connected to said third combining means to receive said combined liquid stream and pump it to higher pressure, thereby forming a pumped combined liquid stream; and (2) said second combining means is adapted to be connected to said pumping means and to said first expansion means to receive said pumped combined liquid stream and said expanded further cooled first stream and form said combined feed stream.

28. The apparatus according to claim 23 wherein (1) a pumping means is connected to said absorbing means to receive said distillation liquid stream from said lower region of said absorbing means and pump it to higher pressure, thereby forming a pumped distillation liquid stream; and (2) said second combining means is adapted to be connected to said pumping means and to said first expansion means to receive said pumped distillation liquid stream and said expanded further cooled first stream and form said combined feed stream.

29. The apparatus according to claim 24 wherein (1) a pumping means is connected to said third combining means to receive said combined liquid stream and pump it to higher pressure, thereby forming a pumped combined liquid stream; and (2) said second combining means is adapted to be connected to said pumping means and to said first expansion means to receive said pumped combined liquid, stream and said expanded further cooled first stream and form said combined feed stream.

30. The apparatus according to claim 25 wherein said pumping means is housed in said processing assembly.

31. The apparatus according to claim 26 wherein said pumping means is housed in said processing assembly.

32. The apparatus according to claim 27 wherein said pumping means is housed in said processing assembly.

33. The apparatus according to claim 28 wherein said pumping means is housed in said processing assembly.

34. The apparatus according to claim 29 wherein said pumping means is housed in said processing assembly.

Description

[0017] For a better understanding of the present invention, reference is made to the following examples and drawings. Referring to the drawings:

[0018] FIGS. 1 and 2 are flow diagrams of prior art natural gas processing plants in accordance with U.S. Pat. No. 4,157,904 or 4,278,457;

[0019] FIGS. 3 and 4 are flow diagrams of natural gas processing plants adapted to use the process of co-pending application Ser. No. 14/462,056;

[0020] FIG. 5 is a flow diagram of a natural gas processing plant adapted to use the present invention; and

[0021] FIGS. 6 through 14 are flow diagrams illustrating alternative means of application of the present invention to a natural gas processing plant.

[0022] In the following explanation of the above figures, tables are provided summarizing flow rates calculated for representative process conditions. In the tables appearing herein, the values for flow rates (in moles per hour) have been rounded to the nearest whole number for convenience. The total stream rates shown in the tables include all non-hydrocarbon components and hence are generally larger than the sum of the stream flow rates for the hydrocarbon components. Temperatures indicated are approximate values rounded to the nearest degree. It should also be noted that the process design calculations performed for the purpose of comparing the processes depicted in the figures are based on the assumption of no heat leak from (or to) the surroundings to (or from) the process. The quality of commercially available insulating materials makes this a very reasonable assumption and one that is typically made by those skilled in the art.

[0023] For convenience, process parameters are reported in both the traditional British units and in the units of the Systme International d'Units (SI). The molar flow rates given in the tables may be interpreted as either pound moles per hour or kilogram moles per hour. The energy consumptions reported as horsepower (HP) and/or thousand British Thermal Units per hour (MBTU/Hr) correspond to the stated molar flow rates in pound moles per hour. The energy consumptions reported as kilowatts (kW) correspond to the stated molar flow rates in kilogram moles per hour.

DESCRIPTION OF THE PRIOR ART

[0024] FIG. 1 is a process flow diagram showing the design of a processing plant to recover C.sub.2+ components from natural gas using prior art according to U.S. Pat. No. 4,157,904 or U.S. Pat. No. 4,278,457. In this simulation of the process, inlet gas enters the plant at 91 F. [33 C.] and 1,000 psia [6,893 kPa(a)] 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.

[0025] The feed stream 31 is cooled in heat exchanger 111 by heat exchange with cool residue gas (stream 39a), demethanizer reboiler liquids at 27 F. [3 C.] (stream 41), and demethanizer side reboiler liquids at 74 F. [59 C.] (stream 40). (In some cases, the use of one or more supplemental external refrigeration streams may be advantageous as shown by the dashed line.) Stream 31a then enters separator 11 at 42 F. [41 C.] and 985 psia [6,789 kPa(a)] where the vapor (stream 32) is separated from the condensed liquid (stream 33).

[0026] The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37. The liquid (stream 33) from separator 11 is optionally divided into two streams, 35 and 38. (Stream 35 may contain from 0% to 100% of the separator liquid in stream 33. If stream 35 contains any portion of the separator liquid, then the process of FIG. 1 is according to U.S. Pat. No. 4,157,904. Otherwise, the process of FIG. 1 is according to U.S. Pat. No. 4,278,457.) For the process illustrated in FIG. 1, stream 35 contains 100% of the total separator liquid. Stream 34, containing about 31% of the total separator vapor, is combined with stream 35 and the combined stream 36 passes through heat exchanger 12 in heat exchange relation with the cold residue gas (stream 39) where it is cooled to substantial condensation. The resulting substantially condensed stream 36a at 141 F. [96 C.] is then flash expanded through expansion valve 13 to the operating pressure (approximately 322 psia [2,217 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 1, the expanded stream 36b leaving expansion valve 13 reaches a temperature of 147 F. [99 C.] and is supplied to separator section 17a in the upper region of fractionation to tower 17. The liquids separated therein become the top feed to demethanizing section 17b.

[0027] The remaining 69% of the vapor from separator 11 (stream 37) enters a work expansion machine 14 in which mechanical energy is extracted from this portion of the high pressure feed. The machine 14 expands the vapor substantially isentropically to the tower operating pressure, with the work expansion cooling the expanded stream 37a to a temperature of approximately 119 F. [84 C.]. The typical commercially available expanders are capable of recovering on the order of 80-85% of the work theoretically available in an ideal isentropic expansion. The work recovered is often used to drive a centrifugal compressor (such as item 1.5) that can be used to re-compress the residue gas (stream 39b), for example. The partially condensed expanded stream 37a is thereafter supplied as feed to fractionation tower 17 at an upper mid-column feed point. The remaining separator liquid in stream 38 (if any) is expanded to the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 38a before it is supplied to fractionation tower 17 at a lower mid-column feed point.

[0028] The demethanizer in tower 17 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 17a 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 17b is combined with the vapor portion of the top feed to form the cold demethanizer overhead vapor (stream 39) which exits the top of the tower. The lower, demethanizing section 17b contains the trays and/or packing and provides the necessary contact between the liquids falling downward and the vapors rising upward. The demethanizing section 17b also includes reboilers (such as the reboiler and the side reboiler described previously and supplemental reboiler 18) which heat and vaporize a portion of the liquids flowing down the column to provide the stripping vapors which flow up the column to strip the liquid product, stream 42, of methane and lighter components.

[0029] The liquid product stream 42 exits the bottom of the tower at 42 F. [6 C.], based on a typical specification of a methane to ethane ratio of 0.020:1 on a molar basis in the bottom product. The residue gas (demethanizer overhead vapor stream 39) passes countercurrently to the incoming feed gas in heat exchanger 12 where it is heated from 146 F. [99 C.] to 46 F. [43 C.] (stream 39a) and in heat exchanger 10 where it is heated to 85 F. [30 C.] (stream 39b). The residue gas is then re-compressed in two stages. The first stage is compressor 15 driven by expansion machine 14. The second stage is compressor 19 driven by a supplemental power source which compresses the residue gas (stream 39d) to sales line pressure. After cooling to 115 F. [46 C.] in discharge cooler 20, the residue gas product (stream 39e) flows to the sales gas pipeline at 1,020 psia [7,031 kPa(a)], sufficient to meet line requirements (usually on the order of the inlet pressure).

[0030] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 1 is set forth in the following table:

TABLE-US-00001 TABLE I (FIG. 1) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 18,236 1,593 407 100 20,491 33 947 260 153 99 1,470 34 5,609 490 125 31 6,303 36 6,556 750 278 130 7,773 37 12,627 1,103 282 69 14,188 39 19,149 146 7 0 19,382 42 34 1,707 553 199 2,579 Recoveries* Ethane 92.14% Propane 98.75% Butanes+ 99.78% Power Residue Gas Compression 12,012 HP [19,748 kW] *(Based on un-rounded flow rates)

[0031] FIG. 2 is a process flow diagram showing one manner in which the design of the processing plant in FIG. 1 can be adjusted to operate at a lower C.sub.2 component recovery level. This is a common requirement when the relative values of natural gas and liquid hydrocarbons are variable, causing recovery of the C.sub.2 components to be unprofitable at times. The process of FIG. 2 has been applied to the same feed gas composition and conditions as described previously for FIG. 1. However, in the simulation of the process of FIG. 2, the process operating conditions have been adjusted to reject nearly all of C.sub.2 components to the residue gas rather than recovering them in the bottom liquid product from the fractionation tower.

[0032] In this simulation of the process, inlet gas enters the plant at 91 F. [33 C.] and 1,000 psia [6,893 kPa(a)] as stream 31 and is cooled in heat exchanger 10 by heat exchange with cool residue gas stream 39a and demethanizer side reboiler liquids at 68 F. [20 C.] (stream 40). (One consequence of operating the FIG. 2 process to reject nearly all of the C.sub.2 components to the residue gas is that the temperatures of the liquids flowing down fractionation tower 17 are much warmer, to the point, that side reboiler stream 40 is nearly as warm as the inlet gas and reboiler stream 41 can no longer be used to cool the inlet gas at all, so that nearly all of the column reboil heat must be supplied by supplemental reboiler 18.) Cooled stream 31a enters separator 11 at 9 F. [13 C. ] and 985 psia [6,789 kPa(a)] where the vapor (stream 32) is separated from any condensed liquid (stream 33). Under these conditions, however, no liquid is condensed.

[0033] The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37, and any liquid (stream 33) is optionally divided into two streams, 35 and 38. For the process illustrated in FIG. 2, stream 35 would contain 100% of the total separator liquid if any was formed. Stream 34, containing about 29% of the total separator vapor, is combined with any liquid in stream 35 and the combined stream 36 passes through heat exchanger 12 in heat exchange relation with the cold residue gas (stream 39) where it is cooled to substantial condensation. The resulting substantially condensed stream 36a at 91 F. [68 C.] is then flash expanded through expansion valve 13 to the operating pressure (approximately 323 psia [2,224 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 2, the expanded stream 36b leaving expansion valve 13 reaches a temperature of 142 F. [97 C.] and is supplied to fractionation tower 17 at the top feed point.

[0034] The remaining 71% of the vapor from separator 11 (stream 37) enters a work expansion machine 14 in which mechanical energy is extracted from this portion of the high pressure feed. The machine 14 expands the vapor substantially isentropically to the tower operating pressure, with the work expansion cooling the expanded stream 37a to a temperature of approximately 80 F. [62 C.] before it is supplied as feed to fractionation tower 17 at an upper mid-column teed point. The remaining separator liquid in stream 38 (if any) is expanded to the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 38a before it is supplied to fractionation tower 17 at a lower mid-column feed point.

[0035] Note that when fractionation tower 17 is operated to reject the C.sub.2 components to the residue gas product, as shown in FIG. 2, the column is typically referred to as a deethanizer and its lower section 17b is called a deethanizing section. The liquid product stream 42 exits the bottom of deethanizer 17 at 166 F. [75 C.], based on a typical specification of an ethane to propane ratio of 0.020:1 on a molar basis in the bottom product. The residue gas (deethanizer overhead vapor stream 39) passes countercurrently to the incoming feed gas in heat exchanger 12 where it is heated from 98 F. [72 C.] to 21 F. [29 C.] (stream 39a) and in heat exchanger 10 where it is heated to 85 F. [30 C.] (stream 39b) as it provides cooling as previously described. The residue gas is then re-compressed in two stages, compressor 15 driven by expansion machine 14 and compressor 19 driven by a supplemental power source. After stream 39d is cooled to 115 F. [46 C.] in discharge cooler 20, the residue gas product (stream 39e) flows to the sales gas pipeline at 1,020 psia [7,031 kPa(a)].

[0036] 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 [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 19,183 1,853 560 199 21,961 33 0 0 0 0 0 34 5,467 528 160 57 6,259 36 5,467 528 160 57 6,259 37 13,716 1,325 400 142 15,702 39 19,183 1,843 40 2 21,234 42 0 10 520 197 727 Recoveries* Propane 92.84% Butanes+ 98.90% Power Residue Gas Compression 12,012 HP [19,748 kW] *(Based on un-rounded flow rates)

[0037] Co-pending application Ser. No. 14/462,056 describes one means of improving the performance of the FIG. 2 process when rejecting nearly all of C.sub.2 components to the residue gas rather than recovering them in the bottom liquid product. FIG. 2 can be adapted to use this process as shown in FIG. 3. The operating conditions of the FIG. 3 process have been adjusted as shown to reduce the ethane content of the liquid product to the same level as that of the FIG. 2 process. The feed gas composition and conditions considered in the process presented in FIG. 3 are the same as those in FIG. 2. Accordingly, the FIG. 3 process can be compared with that of the FIG. 2 process.

[0038] Most of the process conditions shown for the FIG. 3 process are much the same as the corresponding process conditions for the FIG. 2 process. The main differences are the disposition of flash expanded substantially condensed stream 36b and column overhead vapor stream 39. In the FIG. 3 process, substantially condensed stream 36a is flash expanded through expansion valve 13 to slightly above the operating pressure (approximately 329 psia [2,271 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 3, the expanded stream 36b leaving expansion valve 13 reaches a temperature of 142 F. [97 C.] before it is directed into a heat and mass transfer means in rectifying section 117a of processing assembly 117. The heat and mass transfer means is configured to provide heat exchange between a combined vapor stream flowing upward through one pass of the heat and mass transfer means, and the flash expanded substantially condensed stream 36b flowing downward, so that the combined vapor stream is cooled while heating the expanded stream. As the combined vapor stream is cooled, a portion of it is condensed and falls downward while the remaining combined vapor stream continues flowing upward through the heat and mass transfer means. The heat and mass transfer means provides continuous contact between the condensed liquid and the combined vapor stream so that it also functions to provide mass transfer between the vapor and liquid phases, thereby providing rectification of the combined vapor stream. The condensed liquid, from the bottom of the heat and mass transfer means is directed to separator section 117b of processing assembly 117.

[0039] The flash expanded stream 36b is further vaporized as it provides cooling and partial condensation of the combined vapor stream, and exits the heat and mass transfer means in rectifying section 117a at 83 F. [64 C.]. The heated flash expanded stream discharges into separator section 117b of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with overhead vapor stream 39 to form the combined vapor stream that enters the heat and mass transfer means in rectifying section 117a as previously described, and the liquid phase combines with the condensed liquid from the bottom of the heat and mass transfer means to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 21 so that stream 154a at 81 F. [63 C.] can enter fractionation column 17 at the top feed point. The vapor remaining from the cooled combined vapor stream leaves the heat and mass transfer means in rectifying section 117a of processing assembly 117 at 103 F. [75 C.] as cold residue gas stream 153, which is then heated and compressed as described, previously for stream 39 in the FIG. 2 process.

[0040] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 3 is set forth in the following table:

TABLE-US-00003 TABLE III (FIG. 3) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 19,183 1,853 560 199 21,961 33 0 0 0 0 0 34 5,659 547 165 59 6,478 36 5,659 547 165 59 6,478 37 13,524 1,306 395 140 15,483 39 14,278 2,573 86 4 17,077 154 754 1,278 242 63 2,355 153 19,183 1,842 9 0 21,200 42 0 11 551 199 761 Recoveries* Propane 98.46% Butanes+ 99.98% Power Residue Gas Compression 12,012 HP [19,748 kW] *(Based on un-rounded flow rates)

[0041] A comparison of Tables II and III shows that, compared to the FIG. 2 process, the FIG. 3 process improves propane recovery from 92.84% to 98.46% and butane+ recovery from 98.90% to 99.98%. Comparison of Tables II and III further shows that these increased product yields were achieved without using additional power.

[0042] The process of co-pending application Ser. No. 14/462,056 can also be operated to recover the maximum amount of C.sub.2 components in the liquid product. The operating conditions of the FIG. 3 process can be altered as illustrated in FIG. 4 to increase the ethane content of the liquid product to the essentially the same level as that of the FIG. 1 process. The feed gas composition and conditions considered in the process presented in FIG. 4 are the same as those in FIG. 1. Accordingly, the FIG. 4 process can be compared with that of the FIG. 1 process.

[0043] Most of the process conditions shown for the FIG. 4 process are much the same as the corresponding process conditions for the FIG. 1 process. The main differences are again the disposition of flash expanded substantially condensed stream 36b and column overhead vapor stream 39. In the FIG. 4 process, substantially condensed stream 36a is flash expanded through expansion valve 13 to slightly above the operating pressure (approximately 326 psia [2,246 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 4, the expanded stream 36b leaving expansion valve 13 reaches a temperature of 147 F. [99 C.] before it is directed into the heat and mass transfer means in rectifying section 117a of processing assembly 117.

[0044] The flash expanded stream 36b is further vaporized as it provides cooling and partial condensation of the combined vapor stream, and exits the heat and mass transfer means in rectifying section 117a at 147 F. [99 C.]. (Note that the temperature of stream 36b does not change as it is heated, due to the pressure drop through the heat and mass transfer means and the resulting vaporization of some of the liquid methane contained in the stream.) The heated flash expanded stream discharges into separator section 117b of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with overhead vapor stream 39 to form the combined vapor stream that enters the heat and mass transfer means in rectifying section 117a as previously described, and the liquid phase combines with the condensed liquid from the bottom of the heat and mass transfer means to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 21 so that stream 154a at 146 F. [99 C.] can enter fractionation column 17 at the top feed point. The vapor remaining from the cooled combined vapor stream leaves the heat and mass transfer means in rectifying section 117a or processing assembly 117 at 147 F. [99 C.] as cold residue gas stream 153, which is then heated and compressed as described previously for stream 39 in the FIG. 1 process.

[0045] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 4 is set forth in the following table:

TABLE-US-00004 TABLE IV (FIG. 4) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 18,361 1,620 419 105 20,661 33 822 233 141 94 1,300 34 5,640 498 129 32 6,346 36 6,462 731 270 126 7,646 37 12,721 1,122 290 73 14,315 39 18,937 145 7 0 19,157 154 6,250 732 270 126 7,423 153 19,149 144 7 0 19,380 42 34 1,709 553 199 2,581 Recoveries* Ethane 92.21% Propane 98.77% Butanes+ 99.79% Power Residue Gas Compression 12,010 HP [19,744 kW] *(Based on un-rounded flow rates)

[0046] A comparison of Tables I and IV shows that, compared to the FIG. 1 process, the FIG. 4 process does not offer any significant improvement when operated to recover the maximum amount of C.sub.2 components. To understand this, it is instructive to compare the FIG. 1 process (operating to recover the maximum amount of C.sub.2 components) with the FIG. 2 process (operating to recover the minimum amount of C.sub.2 components), particularly with respect to the temperatures of the top feed (stream 36b) and the overhead vapor (stream 39) of fractionation column 17.

[0047] When the processing plant is operated as shown in FIG. 2 to reject the C.sub.2 components to the residue gas (overhead vapor stream 39), the overhead temperature of fractionation column 17 is relatively warm, 98 F. [72 C.], because the C.sub.2 components and heavier components in stream 39 raise its dewpoint temperature. This results in a large temperature difference between the column overhead vapor (stream 39) and the top column feed (stream 36b), which enters the column at 142 F. [97 C.]. This differential provides the temperature driving force that allows the heat and mass transfer means in rectifying section 117a of processing assembly 117 added in the FIG. 3 process to condense the heavier components in the combined vapor stream rising from separator section 117b, thereby rectifying the vapor stream and capturing the desired C.sub.3+ components in stream 154 so that they can be recovered in bottom product stream 42 from column 17.

[0048] Contrast this now with streams 36b and 39 of FIG. 1 when the processing plant is operated to recover the C.sub.2 components. The overhead temperature of fractionation column 17 is much colder because the dewpoint temperature of stream 39 is so much lower. Consequently, the column overhead temperature (146 F. [99 C.] for stream 39) is almost the same as the top column feed temperature (147 F. [99 C.] for stream 36b), meaning that there is essentially no temperature driving force for the heat and mass transfer means in rectifying section 117a of processing assembly 117 added in the FIG. 4 process. Without any driving force, there is no condensation of the heavier components from the combined vapor stream rising from separator section 117b, so no rectification can take place and there is no improvement in the recovery of C.sub.2 components between the FIG. 1 process and the FIG. 4 process. The process of co-pending application Ser. No. 14/462,056 has no means for creating any temperature driving force for rectifying section 117a when the operating conditions of the processing plant are adjusted to recover the maximum amount of C.sub.2 components.

DESCRIPTION OF THE INVENTION

EXAMPLE 1

[0049] In those cases where it is desirable to maximize the recovery of C.sub.2 components in the liquid product (as in the FIG. 1 prior art process described previously, for instance), the present invention offers significant efficiency advantages over the prior art processes depicted in FIGS. 1 and 4. FIG. 5 illustrates a flow diagram of the FIG. 1 prior an process that has been adapted to use the present invention. The operating conditions of the FIG. 5 process have been adjusted as shown to increase the ethane-content of the liquid product above the level that is possible with the FIGS. 1 and 4 prior art processes. The feed gas composition and conditions considered in the process presented in FIG. 5 are the same as those in FIGS. 1 and 4. Accordingly, the FIG. 5 process can be compared with that of the FIGS. 1 and 4 processes to illustrate the advantages of the present invention.

[0050] Most of the process conditions shown for the FIG. 5 process are much the same as the corresponding process conditions for the FIG. 1 process. The main difference is the disposition of column overhead vapor stream 39. In the FIG. 5 process, stream 39 is divided into two streams, stream 151 and stream 152, whereupon stream 151 is compressed from the operating pressure (approximately 330 psia [2,273 kPa(a)]) of fractionation tower 17 to approximately 496 psia [3,421 kPa(a)] by reflux compressor 22. Compressed stream 151a at 95 F. [70 C.] is then directed into a heat exchange means in cooling section 117a of processing assembly 117. This heat exchange means may be comprised of a fin and tube type heat exchanger, a plate type heat, exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat exchange means is configured to provide heat exchange between stream 151a flowing through one pass of the heat exchange means and a further rectified vapor stream arising from rectifying section 117b of processing assembly 117, so that stream 151a is cooled to substantial condensation (stream 151b) while heating the further rectified vapor stream.

[0051] Substantially condensed stream 151b at 135 F. [93 C.] is then flash expanded through expansion valve 23 to slightly above the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 5, the expanded stream 151c leaving expansion valve 23 reaches a temperature of 154 F. [104 C.] before it is directed into a heat and mass transfer means in rectifying section 117b of processing assembly 117. This heat and mass transfer means may also be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat and mass transfer means is configured to provide heat exchange between a partially rectified vapor stream arising from absorbing section 117c of processing assembly 117 that is flowing upward through one pass of the heat and mass transfer means, and the flash expanded substantially condensed stream 151c flowing downward, so that the partially rectified vapor stream is cooled while heating the expanded stream. As the partially rectified vapor stream is cooled, a portion of it is condensed and falls downward while the remaining vapor continues flowing upward through the heat and mass transfer means. The heat and mass transfer means provides continuous contact between the condensed liquid and the partially rectified vapor stream so that it also functions to provide mass transfer between the vapor and liquid phases, thereby providing further rectification of the partially rectified vapor stream to form the further rectified vapor stream. This further rectified vapor stream arising from the heat and mass transfer means is then directed to cooling section 117a of processing assembly 117. The condensed liquid from the bottom of the heat and mass transfer means is directed to absorbing section 117c of processing assembly 117.

[0052] The flash expanded stream 151c is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b at 153 F. [103 C.]. The heated flash expanded stream discharges into separator section 117d of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with the remaining portion (stream 152) of overhead vapor stream 39 to form a combined vapor stream that enters a mass transfer means in absorbing section 117c of processing assembly 117. This mass transfer means may consist of a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing, but could also be comprised of a non-heat transfer zone in a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The mass transfer means is configured to provide contact between the cold condensed liquid, leaving the bottom of the heat and mass transfer means in rectifying section 117b and the combined vapor stream arising from separator section 117d. As the combined vapor stream, rises upward through absorbing section 117c, it is contacted with the cold liquid falling downward to condense and absorb C.sub.2 components, C.sub.3 components, and heavier components from the combined vapor stream. The resulting partially rectified vapor stream is then directed to the heat, arid mass transfer means in rectifying section 117b of processing assembly 117 for further rectification as previously described.

[0053] The liquid phase (if any) from the heated flash expanded stream leaving rectifying section 117b of processing assembly 117 that is separated in separator section 117d combines with the distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c of processing assembly 117 to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 21 so that stream 154a at 148 F. [100 C.] can join with flash expanded stream 36b to form combined feed stream 155, which then enters fractionation column 17 at the top feed point at 145 F. [98 C.].

[0054] The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b of processing assembly 117 at 154 F. [103 C.] and enters the heat exchange means in cooling section 117a of processing assembly 117. The vapor is heated to 124 F. [87 C.] as it provides cooling to stream 151a as described previously. The heated vapor is then discharged from processing assembly 117 as cool residue gas stream 153, which is heated and compressed as described previously for stream 39 in the FIG. 1 process.

[0055] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 5 is set forth in the following table:

TABLE-US-00005 TABLE V (FIG. 5) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 18,897 1,757 492 139 21,448 33 286 96 68 60 513 34 5,340 496 139 39 6,061 36 5,626 592 207 99 6,574 37 13,557 1,261 353 100 15,387 39 20,465 180 7 0 20,763 151 2,922 26 1 0 2,965 152 17,543 154 6 0 17,798 154 1,318 128 7 0 1,470 155 6,944 720 214 99 8,044 153 19,147 52 0 0 19,293 42 36 1,801 560 199 2,668 Recoveries* Ethane 97.22% Propane 100.00% Butanes+ 100.00% Power Residue Gas Compression 11,655 HP [19,161 kW] Reflux Compression 357 HP [587 kW] Total Compression 12,012 HP [19,748 kW] *(Based on un-rounded flow rates)

[0056] A comparison of Tables I and V shows that, compared to the prior art of FIG. 1, the present invention improves ethane recovery from 92.14% to 97.22%, propane recovery from 98.75% to 100.00%, and butane+ recovery from 99.78% to 100.00%. A comparison of Tables IV and V shows similar improvements for the present invention over the prior art of FIG. 4. The economic impact of these improved recoveries is significant. Using an average incremental value $0.10/gallon [24.2/m.sup.3] for hydrocarbon liquids compared to the corresponding hydrocarbon gases, the improved recoveries represent more than US $910.000 [825,000] of additional annual revenue for the plant operator. Comparison of Tables I, IV, and V further shows that these increased product yields were achieved using essentially the same power as the prior art. In terms of the recovery efficiency (defined by the quantity of C.sub.2 components and heavier components recovered per unit of power), the present invention represents more than a 4% improvement over the prior art of the FIGS. 1 and 4 processes.

[0057] The dramatic improvement in recovery efficiency provided by the present invention over that of the prior art of the FIG. 1 process is primarily due to the additional cooling of the column overhead vapor provided by flash expanded stream 151c in rectifying section 117b of processing assembly 117. The prior art of the FIG. 1 process has only the flash expanded stream 36b at 147 F. [99 C.] to cool the column vapor, limiting the overhead temperature of column 17 to this value or wanner. This results in significant amounts of the desired C.sub.2 components and heavier components leaving column 17 in overhead vapor stream 39 rather than being recovered in bottom liquid product stream 42. Contrast this to the significantly colder 154 F. [104 C.] temperature of stream 151c in the FIG. 5 embodiment of the present invention, which is thereby able to condense most of the desired C.sub.2 components and heavier components from column overhead vapor stream 39. Note that the concentration of C.sub.2 components in stream 39 (0.87 mol %) of the FIG. 5 embodiment of the present invention (which is about the same as the concentration of C.sub.2 components in stream 39 of the prior art process in FIG. 1) is reduced to 0.27 mol % in stream 153 leaving processing assembly 117 by the additional cooling afforded by stream 151c of the present invention.

[0058] An additional advantage of the present invention over that of the prior art of the FIG. 1 process is the indirect cooling of the column vapor provided by flash expanded stream 151c in rectifying section 117b of processing assembly 117, rather than the direct-contact cooling that characterizes stream 36b in the prior art process of FIG. 1. Although stream 36b is relatively cold, it is not an ideal reflux stream because it contains significant concentrations of the C.sub.2 components and C.sub.3+ components that column 17 is supposed to capture, resulting in losses of these desirable components due to equilibrium effects at the top of column 17 for the prior art process of FIG. 1. For the FIG. 5 embodiment of the present invention, however, there are no equilibrium effects to overcome because there is no direct contact between flash expanded stream 151c and the partially rectified vapor stream that is further rectified in rectifying section 117b.

[0059] The present invention has the further advantage over that of the prior art of the FIG. 1 process of using the heat and mass transfer means in rectifying section 117b to simultaneously cool the partially rectified vapor stream and condense the heavier hydrocarbon components from it, providing more efficient rectification than using reflux in a conventional distillation column. As a result, more of the C.sub.2 components and heavier hydrocarbon components can be removed from the partially rectified vapor stream using the refrigeration available in expanded stream 151c than is possible using conventional mass transfer equipment and conventional heat transfer equipment. The rectification provided by the heat and mass transfer means in rectifying section 117b is further enhanced by the partial rectification accomplished by the mass transfer means in absorbing section 117c of processing assembly 117. The combined vapor stream from separator section 117d is contacted by the condensed liquid leaving the bottom of the heat and mass transfer means in rectifying section 117b, thereby condensing and absorbing some of the C.sub.2 components and nearly all of the C.sub.3+ components in the combined vapor stream to reduce the quantity that must be condensed and captured in rectifying section 117b.

[0060] The present invention offers two other advantages over the prior art in addition to the increase in processing efficiency. First, the compact arrangement of processing assembly 117 of the present invention replaces two separate equipment items in the prior art of co-pending application Ser. No. 12/869,139 (the third pass in heat exchanger 12 and the upper absorbing section in the top of distillation column 17 in FIG. 2 of application Ser. No. 12/869,139) with a single equipment item (processing assembly 117 in FIG. 5 of the present invention). This reduces the plot space requirements and eliminates some of the interconnecting piping, reducing the capital cost of modifying a process plant to use the present invention. Second, reduction of the amount of interconnecting piping means that a processing plant modified to use the present invention has fewer flanged connections compared to the prior art of co-pending application Ser. No. 12/869,139, reducing the number of potential leak sources in the plant. Hydrocarbons are volatile organic compounds (VOCs), some of which are classified as greenhouse gases and some of which may be precursors to atmospheric ozone formation, which means the present invention reduces the potential for atmospheric releases that may damage the environment.

[0061] One additional advantage of the present invention is how easily it can be incorporated into an existing gas processing plant to effect the superior performance described above. As shown in FIG. 5, only two connections (commonly referred to as tie-ins) to the existing plant are needed: for flash expanded substantially condensed stream 36b (to connect with stream 154a to form combined feed stream 155), and for column overhead vapor stream 39 (represented by the dashed line between stream 39 and stream 153 that is removed from service). The existing plant can continue to operate while the new processing assembly 117 is installed near fractionation tower 17, with just a short plant shutdown when installation is complete to make the new tie-ins to these two existing lines. The plant can then be restarted, with all of the existing equipment remaining in service and operating exactly as before, except that the product recovery is now higher with no increase in the total compression power.

[0062] Although the prior art of the FIG. 4 process can also be easily incorporated into an existing gas processing plant, it cannot provide the same improvement in recovery efficiency that the present invention does. There are two primary reasons for this. The first is the lack of additional cooling for the column vapor, since the prior art of the FIG. 4 process is also limited by the temperature of flash expanded stream 36 as was the case for the prior art of the FIG. 1 process. The second is that all of the rectification in processing assembly 117 of the FIG. 4 prior art process must be provided by its rectifying section 117a, because it lacks the absorbing section 117c in processing assembly 117 of the FIG. 5 embodiment of the present invention which provides partial rectification of the column vapor and reduces the load on its rectifying section 117b.

EXAMPLE 2

[0063] FIG. 6 illustrates a flow diagram of the FIG. 1 prior art process that has been adapted to use another embodiment of the present invention. The operating conditions of the FIG. 6 process have been adjusted as shown to increase the ethane content of the liquid product above the level that is possible with the FIGS. 1 and 4 prior art processes. The feed gas composition and conditions considered in the process presented in FIG. 6 are the same as those in FIGS. 1 and 4. Accordingly, the FIG. 6 process can be compared with that of the FIGS. 1 and 4 processes to illustrate the advantages of the present invention, and can likewise be compared to the embodiment displayed in FIG. 5.

[0064] Most of the process conditions shown for the FIG. 6 embodiment of the present invention are much the same as the corresponding process conditions for the FIG. 5 embodiment of the present invention. The main difference is the source of the gas (stream 151) supplied to reflux compressor 22. In the FIG. 6 embodiment, outlet vapor stream 153 from processing assembly 117 is divided into two streams, stream 151 and stream 152. Stream 152 is the cool residue gas, which is heated and compressed as described previously for stream 39 in the FIG. 1 process.

[0065] Stream 151 is compressed from the operating pressure (approximately 330 psia [2,275 kPa(a)]) of fractionation tower 17 to approximately 494 psia [3,405 kPa(a)] by reflux compressor 22. Compressed stream 151a at 70 F. [57 C.] is then directed into the heat exchange means in cooling section 117a of processing assembly 117 and cooled to substantial condensation (stream 151b) while heating the further rectified vapor stream.

[0066] Substantially condensed stream 151b at 149 F. [101 C.] is flash expanded through expansion valve 23 to slightly above the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 6, the expanded stream 151c leaving expansion valve 23 reaches a temperature of 155 F. [104 C.] before it is directed into the heat and mass transfer means in rectifying section 117b of processing assembly 117.

[0067] The flash expanded stream 151c is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b at 152 F. [102 C.]. The heated flash expanded stream discharges into separator section 117d of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with overhead vapor stream 39 to form the combined vapor stream that, enters the mass transfer means in absorbing section 117c of processing assembly 117.

[0068] The liquid phase (if any) from the heated flash expanded stream leaving rectifying section 117b of processing assembly 117 that is separated in separator section 117d combines with the distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c of processing assembly 117 to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 21 so that stream 154a at 146 F. [99 C.] can join with flash expanded stream 36b to form combined feed stream 155, which then enters fractionation column 17 at the top feed point at 145 F. [98 C.].

[0069] The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b of processing assembly 117 at 154 F. [103 C.] and enters the heat exchange means in cooling section 117a. The vapor is heated to 127 F. [88 C.] as it provides cooling to stream 151a as described previously, and is then discharged from processing assembly 117 as outlet vapor stream 153.

[0070] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 6 is set forth in the following table:

TABLE-US-00006 TABLE VI (FIG. 6) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 18,906 1,760 494 140 21,461 33 277 93 66 59 500 34 5,417 504 142 40 6,149 36 5,694 597 208 99 6,649 37 13,489 1,256 352 100 15,312 39 20,206 183 7 0 20,509 151 2,397 7 0 0 2,416 153 21,544 58 0 0 21,711 154 1,059 132 7 0 1,214 155 6,753 729 215 99 7,863 152 19,147 51 0 0 19,295 42 36 1,802 560 199 2,666 Recoveries* Ethane 97.23% Propane 100.00% Butanes+ 100.00% Power Residue Gas Compression 11,657 HP [19,164 kW] Reflux Compression 357 HP [587 kW] Total Compression 12,014 HP [19,751 kW] *(Based on un-rounded flow rates)

[0071] A comparison of Tables V and VI shows that the FIG. 6 embodiment of the present invention has essentially the same performance as the FIG. 5 embodiment, meaning that it has the same advantages as the FIG. 5 embodiment compared to the prior art of the FIGS. 1 and 4 processes. The choice of whether to take the gas for reflux compressor 22 from the column overhead vapor stream 39 as in the FIG. 5 embodiment or from the rectified outlet vapor stream as in the FIG. 6 embodiment will generally depend on factors such as the feed gas composition and the desired recovery level for the C.sub.2 components.

EXAMPLE 3

[0072] FIG. 7 illustrates a flow diagram of the FIG. 1 prior art process that has been adapted to use another embodiment of the present invention. The operating conditions of the FIG. 7 process have been adjusted as shown to increase the ethane content of the liquid product above the level that is possible with the FIGS. 1 and 4 prior art processes. The feed gas composition and conditions considered in the process presented in FIG. 7 are the same as those in FIGS. 1 and 4. Accordingly, the FIG. 7 process can be compared with that of the FIGS. 1 and 4 processes to illustrate the advantages of the present invention, and can likewise be compared to the embodiments displayed in FIGS. 5 and 6.

[0073] Most of the process conditions shown for the FIG. 7 embodiment of the

[0074] present invention are much the same as the corresponding process conditions for the FIG. 6 embodiment of the present invention. The main difference is the disposition of the flash expanded stream (stream 151c) after it has been heated in rectifying section 117b of processing assembly 117.

[0075] In the FIG. 7 embodiment, residue gas stream 153 from processing assembly 117 is divided into two streams, stream 151 and stream 152. Stream 151 is compressed from the operating pressure (approximately 331 psia [2,279 kPa(a)]) of fractionation tower 17 to approximately 495 psia [3,410 kPa(a)] by reflux compressor 22. Compressed stream 151a at 68 F. [55 C.] is then directed into the heat exchange means in cooling section 117a of processing assembly 117 and cooled to substantial condensation (stream 151b) while heating the further rectified vapor stream.

[0076] Substantially condensed stream 151b at 140 F. [96 C.] is flash expanded through expansion valve 23 to slightly above the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 7, the expanded stream 151c leaving expansion valve 23 reaches a temperature of 155 F. [104 C.] before it is directed into the heat and mass transfer means in rectifying section 117b of processing assembly 117. The flash expanded stream 151c is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, then exits the heat and mass transfer means in rectifying section 117b at 151 F. [101 C.] and is discharged from processing assembly 117 as stream 151d.

[0077] Overhead vapor stream 39 is directed to the mass transfer means in absorbing section 117c of processing assembly 117. As the vapor stream rises upward through absorbing section 117c, it is contacted with the cold liquid tailing downward to condense and absorb C.sub.2 components, C.sub.3 components, and heavier components from the vapor stream to form the partially rectified vapor stream.

[0078] The distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c is discharged from the bottom of processing assembly 117 and pumped to higher pressure by pump 21 so that stream 154a at 146 F. [99 C.] can join with flash expanded stream 36b to form combined feed stream 155, which then enters fractionation column 17 at the top feed point at 145 F. [98 C.].

[0079] The further rectified vapor stream leaving the heat and mass transfer means in rectifying section 117b of processing assembly 117 enters the heat exchange means in cooling section 117a at 153 F. [103 C.]. The vapor is heated to 125 F. [87 C.] as it provides cooling to stream 151a as described previously, and is then discharged from processing assembly 117 as residue gas stream 153. Residue gas stream 153 is divided into streams 151 and 152 as described previously, whereupon stream 152 is recombined with heated flash expanded stream 131d to form stream 153a at 129 F. [89 C.]. Stream 153a is the cool residue gas, which is heated and compressed as described previously for stream 39 in the FIG. 1 process.

[0080] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 7 is set forth in the following table:

TABLE-US-00007 TABLE VII (FIG. 7) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 18,917 1,763 496 141 21,481 33 266 90 64 58 480 34 5,550 517 146 41 6,303 36 5,816 607 210 99 6,783 37 13,367 1,246 350 100 15,178 39 20,069 183 7 0 20,369 151 2,196 7 0 0 2,416 152 16,751 51 0 0 16,886 154 922 125 7 0 1,067 155 6,738 732 217 99 7,850 153 19,147 58 0 0 19,302 42 36 1,795 560 199 2,659 Recoveries* Ethane 96.88% Propane 100.00% Butanes+ 100.00% Power Residue Gas Compression 11,651 HP [19,154 kW] Reflux Compression 360 HP [592 kW] Total Compression 12,011 HP [19,746 kW] *(Based on un-rounded flow rates)

[0081] A comparison of Tables V, VI, and VII shows that the FIG. 7 embodiment of the present invention has almost the same performance as the FIGS. 5 and 6 embodiments, meaning that it has the same advantages as the FIGS. 5 and 6 embodiments compared to the prior art of the FIGS. 1 and 4 processes. Although the ethane recovery for the FIG. 7 embodiment is not quite as high as for the FIGS. 5 and 6 embodiments, less vapor flows through processing assembly 117 for the FIG. 7 embodiment. The reduction in the size of the assembly may reduce the capital cost enough to justify the slightly lower recovery of the FIG. 7 embodiment of the present invention. The choice of which embodiment is best for a given application will generally depend on factors such as the feed gas composition, and the desired recovery level for the C.sub.2 components.

EXAMPLE 4

[0082] The present invention also offers advantages when product economies favor rejecting the C.sub.2 components to the residue gas product. The present invention can be easily reconfigured to operate in a manner similar to that of co-pending application Ser. No. 14/462,056 as shown in FIG. 8. The operating conditions of the FIG. 5 embodiment of the present invention can be altered as illustrated in FIG. 8 to reduce the ethane content of the liquid product to the same level as that of the FIG. 3 prior art process. The feed gas composition and conditions considered in the process presented in FIG. 8 are the same as those in FIG. 3. Accordingly, the FIG. 8 process can be compared with that of the FIG. 3 process to further illustrate the advantages of the present invention.

[0083] When operating die present invention in this manner, many of the process conditions shown for the FIG. 8 process are much the same as the corresponding process conditions for the FIG. 3 process, although most of the process configuration is like the FIG. 5 embodiment of the present invention. The main difference relative to the FIG. 5 embodiment is that the flash expanded stream 151b directed to the heat and mass transfer means in rectifying section 117b of processing assembly 117 for FIG. 8 originates from cooled combined stream 36a, rather than from column overhead vapor stream 39 as in FIG. 5. As such, reflux compressor 22 is not needed and can be taken out of service (as indicated by the dashed lines), reducing the power requirements when operating in this manner.

[0084] For the operating conditions shown in FIG. 8, combined stream 36 is cooled to 62 F. [52 C.] in heat exchanger 12 by heat exchange with cool residue gas stream 153. The partially condensed combined stream 36a becomes stream 151 and is directed to the heat, exchange means in cooling section 117a in processing assembly 117 where it is further cooled to substantial condensation (stream 151a) while heating the further rectified vapor stream.

[0085] Substantially condensed stream 151a at 97 F. [71 C.] is flash expanded through expansion valve 23 to slightly above the operating pressure (approximately 344 psia [2,375 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 8, the expanded stream 151b leaving expansion valve 23 reaches a temperature of 140 F. [96 C.] before it is directed into the heat and mass transfer means in rectifying section 117b of processing assembly 117.

[0086] The flash expanded stream 151b is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b at 83 F. [64 C.]. The heated flash expanded stream discharges into separator section 117d of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with overhead vapor stream 39 to form the combined vapor stream that, enters the mass transfer means in absorbing section 117c of processing assembly 117.

[0087] The liquid phase (if any) from the heated flash expanded stream leaving rectifying section 117b of processing assembly 117 that is separated in separator section 117d combines with the distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c of processing assembly 117 to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 21 so that stream 154a at 76 F. [60 C.] can enter fractionation column 17 at the top feed point.

[0088] The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b of processing assembly 117 at 103 F. [75 C.] and enters the heat exchange means in cooling section 117a. The vapor is heated to 69 F. [56 C.] as it provides cooling to stream 151 as described previously. The heated vapor is then discharged from processing assembly 117 as cool residue gas stream 153, which is heated and compressed as described previously for stream 39 in the FIG. 2 process.

[0089] A summary of stream flow rates and energy consumption for the process illustrated in FIG. 8 is set forth in the following table:

TABLE-US-00008 TABLE VIII (FIG. 8) Stream Flow Summary - Lb. Moles/Hr [kg moles/Hr] Stream Methane Ethane Propane Butanes+ Total 31 19,183 1,853 560 199 21,961 32 19,183 1,853 560 199 21,961 33 0 0 0 0 0 34 5,947 574 174 62 6,808 36/151 5,947 574 174 62 6,808 37 13,236 1,279 386 137 15,153 39 14,032 2,616 95 4 16,881 154 796 1,348 268 66 2,498 153 19,183 1,842 1 0 21,191 42 0 11 559 199 770 Recoveries* Ethane 0.60% Propane 99.91% Butanes+ 100.00% Power Residue Gas Compression 11,656 HP [19,162 kW] *(Based on un-rounded flow rates)

[0090] A comparison of Tables III and VIII shows that, compared to the prior art, the FIG. 8 process improves propane recovery from 98.46% to 99.91% and butane+ recovery from 99.98% to 100.00%. Comparison of Tables III and VIII further shows that these increased product yields were achieved using about 3% less power than the prior art. In terms of the recovery efficiency (defined by the quantity of C.sub.3 components and heavier components recovered per unit of power), the FIG. 8 process represents more than a 4% improvement over the prior art of the FIG. 3 process. The economic impact of these improved recoveries and reduced power consumption is significant. Using an average incremental value $0.69/gallon [165/m.sup.3] for hydrocarbon liquids compared to the corresponding hydrocarbon gases and a value of $3.00/MMBTU [2.58/GJ] for fuel gas, the improved recoveries and reduced power represent more than US $590,000 [530,000] of additional annual revenue for the plant operator.

[0091] The superior performance of the FIG. 8 process compared to the prior art of the FIG. 3 process is due to two key additions to its processing assembly 117 compared to processing assembly 117 in the FIG. 3 process. The first is cooling section 117a which allows further cooling of stream 36a leaving heat exchanger 12, reducing the amount of flashing across expansion valve 23 so that there is more liquid in the flash expanded stream supplied to rectifying section 117b in the FIG. 8 process than to rectifying section 117a in the FIG. 3 process. This in turn provides more cooling of the partially rectified vapor stream in the heat and mass transfer means in rectifying section 117b as the liquid in the flash expanded stream is vaporized, which allows it to condense more of the heavier components from the partially rectified vapor stream and thereby more completely rectify the stream.

[0092] The second key addition is absorbing section 117c which provides partial rectification of the combined vapor stream arising from separator section 117d. Contacting the combined vapor stream with the cold condensed liquid leaving the bottom of the heat and mass transfer means in rectifying section 117b condenses and absorbs C.sub.3 components and heavier components from the combined vapor stream, before the resulting partially rectified vapor stream enters the heat and mass transfer means in rectifying section 117b. This reduces the load on rectifying section 117b and allows a greater degree of rectification in this section of processing assembly 117.

[0093] The net effect of these two additions is to allow more effective rectification of column overhead vapor stream 39 in processing assembly 117 of the FIG. 8 process, which also allows deethanizer column 17 to operate at a higher pressure. The more effective rectification provides higher product recoveries and the higher column pressure reduces the residue gas compression power, increasing the recovery efficiency of the FIG. 8 process by more than 4% compared to the prior art of the FIG. 3 process. The FIGS. 6 and 7 embodiments of the present invention can likewise be easily reconfigured to operate in this same fashion, so that all of these embodiments allow the plant operator to recover C.sub.2 components in the bottom liquid product when product prices are high or to reject C.sub.2 components to the residue gas product when product prices are low, thereby maximizing the revenue for the plant as economic conditions change.

Other Embodiments

[0094] Some circumstances may favor also mounting the liquid pump inside the processing assembly to further reduce the number of equipment items and the plot space requirements. Such embodiments are shown in FIGS. 9, 10, and 11, with pump 121 mounted inside processing assembly 117 as shown to send the combined liquid stream from separator section 117d via conduit 154 to combine with stream 36b and form combined feed stream 155 that is supplied as the top feed to column 17. The pump and its driver may both be mounted inside the processing assembly if a submerged pump or canned motor pump is used, or just the pump itself may be mounted inside the processing assembly (using a magnetically-coupled drive for the pump, for instance). For either option, the potential for atmospheric releases of hydrocarbons that may damage the environment is reduced still further.

[0095] Some circumstances may favor locating the processing assembly at a higher elevation than the top feed point on fractionation column 17. In such cases, it may be possible for combined liquid stream 154 to flow by gravity head and combine with stream 36b so that the resulting combined feed stream 155 then flows to the top feed point on fractionation column 17 as shown in FIGS. 12, 13, and 14, eliminating the need for pump 21/121 shown in the FIGS. 5 through 11 embodiments.

[0096] Depending on the feed gas composition, the desired recovery level for the C.sub.2 components or the C.sub.3 components, and other factors, it may be desirable to completely vaporize flash expanded stream 151c in the heat and mass transfer means in rectifying section 117b of processing assembly 117 in the FIGS. 5, 6, 9, 10. 12, and 13 embodiments of the present invention. In such cases, processing assembly 117 may not require separator section 117d.

[0097] The present invention provides improved recovery of C.sub.2 components, C.sub.3 components, and heavier hydrocarbon components per amount of utility consumption required to operate the process. An improvement in utility consumption required for operating the process may appear in the form of reduced power requirements for compression or re-compression, reduced power requirements for external refrigeration, reduced energy requirements for supplemental heating, or a combination thereof.

[0098] While there have been described what are believed to be preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto, e.g. to adapt the invention to various conditions, types of feed, or other requirements without departing from the spirit of the present invention as defined by the following claims.