Abstract:
A process for the recovery of propane and heavier hydrocarbon components from a hydrocarbon gas stream is disclosed. The stream is divided into first and second streams. The first stream is cooled to condense substantially all of it and is thereafter expanded to the pressure of the distillation column. After expansion, the cooled first stream is directed in heat exchange relation with a warmer distillation stream rising from fractionation stages of the distillation column. The warmed first stream is then supplied to the column at a first mid-column feed position. The second stream is expanded to the column pressure and is then supplied to the column at a second mid-column feed position. The distillation stream is cooled by the first stream sufficiently to partially condense it. The partially condensed distillation stream is then separated to provide volatile residue gas and a reflux stream. The reflux stream is supplied to the column at a top column feed position. The temperatures of the feeds to the column are effective to maintain the column overhead temperature at a temperature whereby the major portion of the C 3  + components is recovered. Alternatively, control means may be adapted so that the major portion of the C 2  + components is recovered.

Description:
BACKGROUND OF THE INVENTION 
     This invntion relates to a process for the separation of a gas containing hydrocarbons. 
     Propane and heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, i.e. methane and ethane together comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes, and the like as well as hydrogen, nitrogen, carbon dioxide and other gases. 
     The present invention is generally concerned with the recovery of propane and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 86.9% methane, 7.24% ethane and other C 2  components, 3.2% propane and other C 3  components, 0.34% isobutane, 1.12% normal butane, 0.19% iso-pentane, 0.24% normal pentane, 0.12% hexanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present. 
     The cryogenic expansion process is now the preferred process for the separation of ethane and heavier hydrocarbons from natural gas streams because it provides maximum simplicity, ease of start-up, operating flexibility, good efficiency and good reliability. The cryogenic expansion process is also preferred for the separation of propane and heavier hydrocarbons from natural gas streams while rejecting the ethane into the residue gas stream with the methane. In fact, it is quite common to see the same basic processing scheme used for either ethane recovery or propane recovery, with only the heat exchanger arrangement modified to accommodate the different operating temperatures within the process. U.S. Pat. Nos. 4,278,457, 4,251,249 and 4,617,039 describe relevant processes. 
     In recent years the fluctuations in both the demand for ethane as a liquid product and in the price of natural gas have created periods in which ethane was more valuable as a constituent of the residue gas streams from gas processing plants. This has resulted in the desire for gas processing facilities to maximize propane and heavier hydrocarbon recovery while, at the same time, maximizing the rejection of ethane into the residue gas stream. Although many variations of the turbo-expander process have been used in the past for propane recovery, they have usually been limited to propane recoveries in the mid-eighty percent to lower ninety percent range without excessive horsepower requirements for residue compression and/or external refrigeration. Although propane recoveries can be improved somewhat by allowing some of the ethane to be recovered in the liquid product, usually a significant percentage of the inlet ethane must leave in the liquid product to provide a small improvement in propane recovery. It is, therefore, desirable to have a process which is capable of recovering propane and heavier components from a gas stream in which only a minor amount of propane is lost to the residue gas while at the same time rejecting essentially all of the ethane. 
     In a typical cryogenic expansion process, the feed gas under pressure is cooled in one or more heat exchangers by cold streams from other parts of the process and/or by use of external sources of refrigeration such as a propane compression-refrigeration system. The cooled feed is then expanded to a lower pressure and fed to a distillation column which separates the desired product (as a bottom liquid product) from the residue gas which is discharged as column overhead vapor. It is the expansion of the cooled feed which provides the cryogenic temperatures required to achieve the desired product recoveries. 
     As the feed gas is cooled, liquids may be condensed, depending on the richness of the gas, and these liquids are typically collected in one or more separators. The liquids are then flashed to a lower pressure which results in further cooling and partial vaporization. The expanded liquid stream(s) may then flow directly to the distillation column (deethanizer) or may be used to provide cooling to the feed gas before flowing to the column. 
     If the feed gas is not totally condensed (usually it is not), the vapor remaining after cooling can be split into two or more parts. One portion of the vapor is passed through a work expansion machine or engine, or expansion valve, to a lower pressure. This results in further cooling of the gas and the formation of additional liquids. This stream then flows to the distillation column at a mid-column feed position. 
     The other portion of the vapor is cooled to substantial condensation by heat exchange with other process streams, e.g. the cold distillation column overhead. This substantially condensed stream is then expanded through an appropriate expansion device, typically an expansion valve. This results in cooling and partial vaporization of the stream. This stream, usually at a temperature below -120° F., is supplied as a top feed to the column. The vapor portion of this top feed is typically combined with the vapor rising from the column to form the residue gas stream. Alternatively, the cooled and expanded stream may be supplied to a separator to provide vapor and liquid streams. The vapor is combined with the column overhead and the liquid is supplied to the column as a top column feed. 
     In the ideal operation of such a separation process, the residue gas leaving the process will contain substantially all of the methane and C 2  components found in the feed gas and essentially none of the C 3  components and heavier hydrocarbon components. The bottom product leaving the deethanizer will contain substantially all of the C 3  components and heavier components and essentially no C 2  components and lighter components. 
     In practice, however, this situation is not obtained due to the fact that the deethanizer is operated basically as a stripping column. The residue gas product consists of the vapors leaving the top fractionation stage of the distillation column together with the vapors not subjected to any rectification. Substantial losses of propane occur because the top liquid feed contains considerable quantities of propane and the heavier components, resulting in corresponding (equilibrium) quantities of propane and heavier components in the vapor leaving the top fractionation stage of the deethanizer. The loss of these desirable components could be significantly reduced if the vapors could be brought into contact with a liquid (reflux), containing very little of the propane and heavier components, which is capable of absorbing propane and heavier hydrocarbons from the vapors. The present invention provides the means for accomplishing this objective and, therefore, significantly improving the recovery of propane. 
     In accordance with the present invention, it has been found that C 3  recoveries in excess of 99 percent can be maintained while providing essentially complete rejection of C 2  components to the residue gas stream. In addition, the present invention makes posiible essentially 100 percent propane recovery at reduced energy requirements, depending on the amount of ethane which is allowed to leave the process in the liquid product. Although applicable at lower pressures and warmer temperatures, the present invention is particularly advantageous when processing feed gases in the range of 600 to 1000 psia or higher under conditions requiring column overhead temperatures of -85° F. or colder. 
    
    
     For a better understanding of the present invention, reference is made to the following examples and drawings. Referring to the drawings: 
     FIG. 1 is a flow diagram of a cryogenic expansion natural gas processing plant of the prior art according to U.S. Pat. No. 4,278,457. 
     FIG. 2 is a flow diagram of a cryogenic expansion natural gas processing plant of another prior art design according to U.S. Pat. No. 4,251,249. 
     FIG. 3 is a flow diagram of a cryogenic expansion natural gas processing plant of another prior art process according to U.S. Pat. No. 4,617,039. 
     FIG. 4 is a flow diagram of a natural gas processing plant in accordance with the present invention. 
     FIG. 5 is a plot showing the relative propane recovery as a function of ethane rejection for the processes of FIGS. 1 through 4. 
     FIGS. 6 and 7 are flow diagrams of additional natural gas processing plants in accordance with the present invention. 
     FIGS. 8 and 9 are diagrams of alternate fractionating systems which may be employed in the process of the present invention. 
     FIG. 10 is a partial flow diagram showing a natural gas processing plant in accordance with the present invention for a richer gas stream. 
    
    
     In the following explanation of these figures, tables are provided summarizing flow rates calculated for representative process conditions. In the tables appearing herein, the values for flow rates (in pound 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 typically 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 above 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 used for minimizing heat loss/gain makes this a very reasonable assumption and one that is typically made by those skilled in the art. 
     DESCRIPTION OF PRIOR ART 
     Referring now to FIG. 1, in a simulation of the process according to U.S. Pat. No. 4,278,457, inlet gas enters the process at 120° F. and 935 psia as stream 10. If the inlet gas contains a concentration of sulfur compounds which would cause the product streams to not meet specifications, the sulfur compounds are removed by appropriate pretreatment of the feed (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. The feed stream is cooled in heat exchanger 11 by cool residue gas stream 27b. From heat exchanger 11, the partially cooled feed stream 10a at 34° F. enters a second heat exchanger 12 where it is cooled by heat exchange with an external propane refrigeration stream. The further cooled feed stream 10b exits heat exchanger 12 at 1° F. and is cooled to -16° F. (stream 10c) by residue gas (stream 27a) in heat exchanger 13. The partially condensed stream then flows to a vapor-liquid separator 14 at a pressure of 920 psia. Liquid from the separator, stream 16, is expanded in expansion valve 17 to the operating pressure (approximately 350 psia) of the distillation column, which in this instance is the deethanizing section 25 of fractionation tower 18. The flash expansion of stream 16 produces a cold expanded stream 16a at a temperature of -52° F., which is supplied to the distillation column as a lower mid-column feed. Depending on the quantity of liquid condensed and other process considerations, the expanded stream 16a could be used to provide a portion of the inlet gas cooling in an additional exchanger before flowinq to the deethanizer. 
     The vapor stream 15 from separator 14 is divided into two branches 19 and 20. Following branch 19, which contains approximately 28 percent of vapor stream 15, the gas is cooled in heat exchanger 21 to -98° F. (stream 19a) at which temperature it is substantially condensed. The stream is then expanded in expansion valve 22. (While an expansion valve is usually preferred, an expansion machine could be substituted.) Upon expansion, the stream flashes to the operating pressure of the deethanizer (350 psia). At this pressure, the feed stream 19b is at a temperature of -142° F. and is supplied to the deethanizer as the top column feed. 
     Approximately 72 percent of the separator vapor, branch 20, is expanded in an expansion engine 23 to the deethanizer operating pressure of 350 psia. The expanded stream 20a reaches a temperature of -90° F. and is supplied to the deethanizer at a mid-column position. Typical commercially available expansion machines (turbo-expanders) are capable of recovering on the order of 80-85% of the work theoretically available in an ideal isentropic expansion. 
     The deethanizer in tower 18 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 tower consists of two sections. The upper section 24 is a separator wherein the partially vaporized top feed is divided into its respective liquid and vapor portions and wherein the vapor rising from the deethanizing or distillation section 25 is combined with the vapor portion of the top feed to form the cold residue gas stream 27 which exits the top of the tower. The lower, deethanizing section 25 contains trays and/or packing and provides the necessary contact between the liquids falling downward and the vapors rising upward. The deethanizing section also includes a reboiler 26 which heats and vaporizes a portion of the liquid at the bottom of the column to provide the stripping vapors which flow up the column to strip the product of methane and C 2  components. A typical specification for the bottom liquid product is to have an ethane to propane ratio of 0.03:1 on a molar basis. The liquid product stream 28 exits the bottom of tower 18 at 187° F. and is cooled to 120° F. (stream 28a) in exchanger 29 before flowing to storage. The residue gas stream 27 exits the top of the tower at -101° F. and enters heat exchanger 21 where it is warmed to -36° F. as it provides the cooling and substantial condensation of stream 19. The residue gas (stream 27a) then flows to heat exchanger 13 where it is warmed to -2° F. (stream 27b) followed by heat exchanger 11 where it is warmed to 117° F. as it provides cooling of the inlet gas stream 10. The warmed residue gas stream 27c is then partly re-compressed in the compressor 30 driven by the expansion turbine 23. The partly compressed stream 27d is then cooled to 120° F. in exchanger 31 (stream 27e) and then compressed to a pressure of 950 psia (stream 27f) in compressor 32 driven by an external power source. The stream is then cooled in exchanger 33 and exits the process at 120° F. as stream 27g. 
     A summary of stream flow rates and energy consumption for the process of FIG. 1 is set forth in the following table: 
     
                       TABLE I______________________________________(FIG. 1)Stream Flow Summary - Lb. Moles/Hr:Stream Methane  Ethane   Propane                           Butanes+ Total______________________________________10     5297     441      194    122      609415     5139     389      140    52       576016     158      52       54     70       33419     1441     109      39     15       161520     3698     280      101    37       414527     5297     436      11     0        578428     0        5        183    122      310Recoveries*Propane                 94.28%Butanes                 99.31%HorsepowerResidue Compression     3115Refrigeration Compression                   568Total                   3683______________________________________ *(Based on unrounded flow rates) 
    
     FIG. 2 represents an alternative prior art process in accordance with U.S. Pat. No. 4,251,249. The process of FIG. 2 is based on the same feed gas composition and conditions as described above for FIG. 1. In the simulation of this process, the inlet feed gas 10 is divided into two portions, 11 and 12 which are partially cooled in heat exchangers 13 and 14, respectively. The two portions recombine as stream 10a to form a partially cooled feed gas stream at -16° F. The partially cooled feed is then further cooled by means of external propane refrigeration in heat exchanger 15 to -37° F. (stream 10b). The further cooled stream then undergoes final cooling in heat exchanger 16 to a temperature of -45° F. (stream 10c) and is then supplied to a vapor-liquid separator 17 at a pressure of about 920 psia. Liquid stream 19 from separator 17 is flash expanded in expansion valve 20 to a pressure just above the operating pressure of the deethanizer in fractionation tower 27. In the process of FIG. 2, the deethanizer operates at about 353 psia. The flash expansion of stream 19 produces a cold, partially vaporized expanded stream 19a at a temperature of -90° F. This stream then flows to exchanger 16 where it is warmed and further vaporized (stream 19b) as it provides final cooling of feed gas stream 10b. From exchanger 16 the further vaporized stream 19b flows to exchanger 14 where it is heated to 104° F. as it provides cooling of stream 12. From exchanger 14 the heated stream 19c flows to the deethanizer section of the tower 27 at a lower mid-column feed position. 
     The vapor stream 18 from separator 17 is expanded in expansion machine 21 to the deethanizer operating pressure. The expanded stream 18a reaches a temperature of -116° F. upon expansion and enters an expander outlet separator 22. Liquid stream 24 from separator 22 flows to the distillation section of the fractionation tower at an upper mid-column feed position. Vapor stream 23 from expander separator 22 flows to reflux condenser 28 located internally in the upper part of the fractionation tower. The cold expander outlet vapor stream 23 provides cooling and partial condensation of the vapor flowing upward from the top-most fractionation stage of the distillation column. The liquids resulting from this partial condensation fall downward as reflux to the deethanizer. As a result of providing this cooling and partial condensation, the expander outlet vapor stream is warmed to a temperature of -27° F. (stream 23a). 
     The deethanizer overhead vapor stream 25 exits from the top of the column at a temperature of -57° F. and combines with the warmed expander outlet separator vapor stream 23a to form the cold residue gas stream 30 at a temperature of -34° F. The liquid product stream 26 exits the bottom of tower 27 at a temperature of 188° F. and is cooled to 120° F. in exchanger 29 before leaving the process. The deethanizer reboiler 35 heats and partially vaporizes a portion of the liquid flowing down the column to help strip the product of ethane. 
     The cold residue gas stream 30 at -34° F. enters heat exchanger 13 where it is warmed to 115° F. as it provides cooling of inlet gas stream 11. The warmed residue gas stream 30a is then partly compressed in the compressor 31 driven by the expansion machine 21. The partly re-compressed stream 30b is then cooled to -120° F. in exchanger 32 (stream 30c) and then compressed to 950 psia (stream 30d) in compressor 33 driven by an external power source. The compressed stream 30d is then cooled to 120° F. in exchanger 34 and exits the process as stream 30e. 
     A summary of stream flow rates and energy consumption for the process of FIG. 2 is set forth in the following table: 
     
                       TABLE II______________________________________(FIG. 2)Stream Flow Summary - Lb. Moles/hr:Stream Methane  Ethane   Propane                           Butanes+ Total______________________________________10     5297     441      194    122      609418     4788     308      89     25       524819     509      133      105    97       84623     4484     154      11     0        468624     304      154      78     25       56226     0        5        183    122      31030     5297     436      11     0        5784Recoveries*Propane                 94.36%Butanes                 100.00%HorsepowerResidue Compression     2975Refrigeration Compression                   706                   3681______________________________________ *(Based on unrounded flow rates) 
    
     FIG. 3 represents an alternative prior art process in accordance with U.S. Pat. No. 4,617,039. The process of FIG. 3 is based on the same feed gas composition and conditions as described above for FIGS. 1 and 2. In the simulation of this process, the inlet feed gas 10 is partially cooled in exchanger 11 to a temperature of -13° F. (stream 10a). The partially cooled stream is then further cooled by means of external propane refrigeration in heat exchanger 12 to -33° F. (stream 10b). The further cooled stream then undergoes final cooling in heat exchanger 13 to a temperature of -41° F. (stream 10c) and is then supplied to a vapor-liquid separator 14 at a pressure of about 920 psia. Liquid stream 16 from the separator 14 is flash expanded in expansion valve 17 to a pressure about 10 psi above the operating pressure of deethanizer 27. In the process of FIG. 3, the deethanizer operates at about 350 psia. The flash expansion of stream 16 produces a cold, partially vaporized expanded stream 16a at a temperature of -84° F. This stream then flows to exchanger 13 where it is warmed and further vaporized as it provides a portion of the final cooling of feed gas stream 10b. The further vaporized stream 16b then flows to exchanger 11 where it is heated to 101° F. as it provides cooling of stream 10. From exchanger 11 the heated stream 16c flows to deethanizer 27 at a mid-column feed position. 
     The vapor stream 15 from separator 14 is expanded in expansion machine 18 to a pressure about 5 psi below the operating pressure of the deethanizer. The expanded stream 15a reaches a temperature of -113° F., at which temperature it is partially condensed, and then flows to the lower feed position of absorber/separator 19. The liquid portion of the expanded stream commingles with liquids falling downward from the upper section of the absorber/separator and the combined liquid stream 21 exits the bottom of absorber/separator 19 This stream is then supplied as top feed (stream 21a) to deethanizer 27 at a temperature of -117° F. via pump 22. The vapor portion of the expanded stream flows upward through the fractionation section of absorber/separator 19. 
     The overhead vapor from absorber/separator 19 (stream 20) is the cold residue gas stream. This cold stream passes in heat exchange relation with the overhead vapor stream from the deethanizer (stream 23) in heat exchanger 27. The deethanizer overhead vapor stream 23 exits the top of the column at a temperature of -34° F. and a pressure of 350 psia. The cold residue gas stream 20 is warmed to approximately -37° F. (stream 20a as it provides cooling and partial condensation of the deethanizer overhead. The partially condensed deethanizer overhead stream 23a then flows as top feed to absorber/separator 19 at a temperature of -89° F. The liquid portion of this stream 23a flows downward onto the top fractionation stage of the absorber/separator while the vapor portion combines with the vapor rising upward from the fractionation section and the combined stream exits the top of the absorber/separator as cold residue gas (stream 20). 
     The liquid product stream 24 exits the bottom of the deethanizer at a temperature of 186° F. and is cooled to 120° F. (stream 24a) in exchanger 26 before leaving the process. The deethanizer reboiler 32 heats and partially vaporizes a portion of the liquid flowing down the column to strip the product of ethane. 
     The residue exits exchanger 27 at a temperature of -37° F. and flows through exchangers 13 and 11 where it is warmed to a temperature of 117° F. The warmed residue gas stream 20c is then partly compressed in compressor 28 driven by the expansion machine 18. The partly re-compressed stream 20d, now at a pressure of about 414 psia, is cooled to 120° F. (stream 20e) in exchanger 29 and then compressed to 950 psia (stream 20f) in compressor 30 driven by an external power source. The compressed stream 20f is then cooled to 120° F. in exchanger 31 and exits the process as stream 20g. 
     A summary of stream flow rates and energy consumption for the process for FIG. 3 is set forth in the following table: 
     
                       TABLE III______________________________________(FIG. 3)Stream Flow Summary - Lb. Moles/hr:Stream Methane  Ethane   Propane                           Butanes+                                   Total______________________________________10     5297     441      194    122     609415     4878     325      97     29      536716     419      116      97     93      72720     5297     435      3      0       577521     745      470      114    30      136223     1164     580      20     1       177024     0        6        191    122     319Recoveries*Propane                 98.41%Butanes                 99.96%HorsepowerResidue Compression     3066Refrigeration Compression                    612Total                   3678______________________________________ *(Based on unrounded flow rates) 
    
     DESCRIPTION OF THE INVENTION 
     FIG. 4 illustrates a flow diagram of a process in accordance with the present invention. The feed gas composition and conditions considered in the process of FIG. 4 are the same as those in FIGS. 1 through 3. Accordingly, the process for FIG. 4 and flow conditions can be compared with the processes of FIGS. 1 through 3 to illustrate the advantages of the present invention. 
     In the simulation of the process of FIG. 4, inlet gas enters the process at 120° F. and 935 psia as stream 10. The feed is cooled in heat exchanger 11 by cool residue gas stream 29b. From heat exchanger 11, the partially cooled feed stream 10a at 36° F. is further cooled to -5° F. in heat exchanger 12 by external propane refrigeration at -2° F. This further cooled stream 10b is then cooled to -13° F. (stream 10c) by residue gas stream 29a in heat exchanger 13. The partially condensed stream 10c then enters vapor-liquid separator 14 at a pressure of 920 psia. Liquid stream 16 from separator 14 is expanded in expansion valve 17 to the operating pressure of the distillation column 24. In the process of FIG. 4 the column operates at 350 psia. The flash expansion of condensed stream 16 produces a cold expanded stream 16a at a temperature of -47°  F. which is supplied to the column as a partially condensed feed at a lower mid-column feed position. 
     The vapor stream 15 from seprrator 14 is divided into gaseous first and second streams, 19 and 20. Following branch 19, approximately 29 percent of stream 15 is cooled in heat exchanger 21 to -104° F. (stream 19a) at which temperature the stream is substantially condensed. The substantially condensed stream 19a is then expanded in expansion valve 22 and supplied to heat exchanger 23. The flash expansion of stream 19a to a lower pressure results in a cold flash expanded stream 19b at a temperature of -142° F. This stream is warmed and partially vaporized in heat exchanger 23 as it provides cooling and partial condensation of the distillation stream 25 rising from the fractionation stages of column 24. The warmed stream 19c at a temperature of -93° F. is then supplied to the column at an upper mid-column feed position. Stream 25 is cooled to a temperature of -107° F. (stream 25a) by heat exchange with stream 19b. This partially condensed stream 25a is supplied to separator 26 operating at about 345 psia. Liquid stream 27 from separator 26 is returned to the column 24 as reflux stream 27a at a top column feed position above the upper mid-column feed position by means of a reflux pump 28. The vapor stream 29 from separator 26 is the cold volatile residue gas stream. 
     When the distillation column forms the lower portion of a fractionation tower, heat exchanger 23 may be located inside the tower above column 24 as shown in FIG. 8. This eliminates the need for separator 26 and pump 28 because the distillation stream is then both cooled and separated in the tower above the fractionation stages of the column. Alternatively and as depicted in FIG. 9, use of a dephlegmator in place of heat exchanger 23 eliminates the separator and pump and also provides concurrent fractionation stages to replace those in the upper section of the deethanizer column. If the dephlegmator is positioned in a plant at grade level, it is connected to a vapor/liquid separator and liquid collected in the separator is pumped to the top of the distillation column. The decision as to whether to include the heat exchanger inside the column or to use the dephlegmator usually depends on plant size and heat exchanger surface area requirements. 
     Returning to gaseous second stream 20, the remaining portion of vapor stream 15 is expanded in work expansion machine 18 to the lower, operating pressure of the column and is thereafter supplied to the column 24 at a mid-column feed position. Expansion of stream 20 results in a cold expanded stream 20a at a temperature of -86° F. 
     The liquid product stream 30 exits the bottom of column 24 at a temperature of 186° F. and is cooled to -120° F. (stream 30a) by exchanger 32 before flowing to storage. The cold residue gas stream 29 flows to heat exchanger 21 where it is partially warmed to -32° F. (stream 29a) as it provides cooling and substantial condensation of stream 19. The partially warmed stream 29a then flows to heat exchanger 13 where it is further warmed to 2° F. as it provides cooling of inlet gas stream 10b. The further warmed residue gas stream 29b is then warmed to 117° F. in heat exchanger 11 as it provides cooling of inlet gas stream 10. The warmed residue gas stream 29c, now at about 330 psia, is partly re-compressed in compressor 33 driven by the expansion machine 18. The partly re-compressed residue gas stream 29d at about 404 psia is cooled to 120° F. (stream 29e) in exchanger 34, compressed to 950 psia (stream 29f) in compressor 35 driven by an external power source, cooled to 120° F. (stream 29g) in exchanger 36 and then exits the process. 
     A summary of stream flow rates and energy consumption for the process of FIG. 4 is set forth in the following table: 
     
                       TABLE IV______________________________________(FIG. 4)Stream Flow Summary - Lb. Moles/Hr:Stream Methane  Ethane   Propane                           Butanes+ Total______________________________________10     5297     441      194    122      609415     5161     396      146    56       579916     136      45       48     66       29519     1497     115      42     16       168220     3664     281      104    40       411729     5297     435      1      0        577330     0        6        193    122      321Recoveries*Propane                 99.68%Butanes                 100.00%HorsepowerResidue Compression     3164Refrigeration Compression                   514                   3678______________________________________ *(Bsed on unrounded flow rates) 
    
     The improvement of the present invention can be seen by comparing the propane recovery levels in Tables I through IV. The present invention offers more than 5 percentage points improvement in propane recovery for the same horsepower (utility) consumption as the prior art processes of FIGS. 1 and 2 and more than 1.25 percentage points improvement compared to the FIG. 3 prior art process. A one percent increase in propane recovery can mean substantial economic advantages for a gas processor during the life of a plant. 
     As an alternate to the higher C 3  component recovery (at constant utility consumption) disclosed for FIG. 4 above, the operating conditions of the FIG. 4 process can be adjusted to obtain a propane recovery level equal to the FIG. 1 or FIG. 2 process at significantly reduced horsepower requirements. As an example, the operating pressure of the deethanizer in FIG. 4 can be increased to about 385 psia. This results in somewhat warmer temperatures in and around the deethanizer. The vapor liquid separator 14 operates at a temperature of -13° F. with 29 percent of the separator vapor 15 flowing in stream 19 to heat exchanger 21. The substantially condensed stream 19a exits heat exchanger 21 at -96° F. and is flash expanded via expansion valve 22 to 390 psia. The temperature of flash expanded stream 19b in this case is -136° F. This stream is then heated to -81° F. in heat exchanger 23 as it provides cooling and partial condensation of the distillation stream 25 before being supplied to the deethanizer. 
     Because of the higher operating pressure of the distillation column, the expansion engine 18 outlet stream 20a and expansion valve 17 outlet stream 16a are both warmer. In this example the temperatures of these streams are -81° F. and -44° F., respectively. 
     The cold residue gas stream 29 exits the vaporliquid separator 26 at a temperature of -99° F. and a pressure of 380 psia. This stream is heated in exchangers 21, 13 and 11 before being compressed as discussed previously. Because the pressure of the residue gas leaving the column is higher, less residue compression horsepower is required. The liquid product stream 30 exits the bottom of the column at -197° F. and is cooled to 120° F. (stream 30a) in exchanger 32. 
     A summary of stream flow rates and energy consumption for the alternate processing conditions of FIG. 4 is set forth in the following table: 
     
                       TABLE V______________________________________(Alternate FIG. 4 Operating Conditions)Stream Flow Summary - Lb. Moles/Hr:Stream Methane  Ethane   Propane                           Butanes+ Total______________________________________10     5297     441      194    122      609415     5161     396      146    56       579816     136      45       48     66       29619     1497     115      42     16       168120     3664     281      104    40       411729     5297     436      11     0        578330     0        5        183    122      311Recoveries*Propane                 94.29%Butanes                 100.00%HorsepowerResidue Compression     2826Refrigeration Compression                   500                   3326______________________________________ *(Based on unrounded flow rates)   On a constant recovery basis, therefore, the present invention provides almost a 10 percent reduction in energy (horsepower) consumption compared to the prior art processes of FIGS. 1 and 2. 
    
     The advantages of the present invention are further illustrated in the graph shown in FIG. 5. This graph indicates the relationship between the quantity of ethane rejected to the residue gas (abscissa) as a percent of the amount in the feed and the propane recovery (ordinate) for the processes of FIGS. 1 through 4. These plots are based on the same feed composition and conditions as used for the process comparisons given above and are based on a constant horsepower utilization of about 3678 horsepower, except as noted for individual points on the graph. 
     Line 1 on the graph corresponds to the process of FIG. 1 and shows that as the quantity of ethane rejected to the residue gas decreases from about 99 percent to 50 percent, the propane recovery increases from 94.3 percent to 97.8 percent. Line 2 corresponds to the process of FIG. 2 and shows that for the same range of ethane rejection, propane recovery increases from 94.3 percent to about 96.2 percent. Line 3 corresponds to the process of FIG. 3 and shows a propane recovery increase from 98.4 percent to 99.4 percent for the same ethane rejection range. Line 4 corresponds to the process of the present invention. This line shows that at an ethane rejection to the residue gas of 90 percent, essentially 100 percent propane recovery is achieved. Thereafter, as ethane rejection decreases, it is possible to maintain 100 percent propane recovery at reduced horsepower requirements. At 80 percent ethane rejection the horsepower requirement has dropped to 3392. At 50 percent ethane rejection the value is 3118 horsepower, more than 15 percent lower than for the other three processes. 
     It can be seen from FIG. 5 that incorporating the split flow reflux system of the present invention into the design of an NGL recovery plant provides considerable operating flexibility to respond to changes in the market for ethane. Any level of ethane rejection to the residue can be achieved while maintaining high propane recovery. This allows the plant operator to maximize operating income as the incremental value of ethane as a liquid (the gross selling price of ethane as a liquid less its value on a BTU basis as a constituent of the residue gas) changes. 
     At the same time, a process with the split flow reflux system can also be operated to attain relatively high ethane recoveries. As the ethane recovery is increased by reducing the temperature at the bottom of the column, the temperature difference between the flash expanded stream (stream 19b in FIG. 4) and the deethanizer overhead stream (stream 25 in FIG. 4) decreases. As this temperature difference decreases, less cooling and condensation of the column overhead stream occurs resulting in less warming of the flash expanded stream and a colder temperature for this stream entering the column. The process of the present invention provides a means of obtaining maximum propane recovery at any given level of ethane rejection to the residue gas. If maximizing ethane recovery is desired, use of the process disclosed in co-pending application No. 194,822 should be considered. 
     In instances where the inlet gas is richer than that heretofore described, an embodiment of the invention such as that depicted in FIG. 10 may be employed. Condensed stream 16 flows through exchanger 40 where it is subcooled by heat exchange with the cooled stream 39a from expansion valve 17. The subcooled liquid is then divided into two portions. The first portion (stream 39) flows through expansion valve 17 where it undergoes expansion for flash vaporization as the pressure is reduced to about the pressure of the distillation column. The cold stream 39a from expansion valve 17 then flows through exchanger 40 where it is used to subcool the liquids from separator 14. From exchanger 40 the stream 39b flows to distillation column 24 as a lower mid-column feed. The second liquid portion 37, still at high pressure, is (1) combined with portion 19 of the vapor stream from separator 14 or (2) combined with substantially condensed stream 19a or (3) expanded in expansion valve 38 and thereafter either supplied to the distillation column 24 at an upper mid-column feed position or combined with expanded stream 19b. Alternatively, portions of stream 37 may follow any or all of the flow paths heretofore described and depicted in FIG. 10. 
     In accordance with this invention, the splitting of the vapor feed may be accomplished in several ways. In the process of FIG. 4, the splitting of the vapor occurs following cooling and separation of any liquids which may have been formed. However, the splitting of the vapor may be accomplished prior to any cooling of the gas as shown in FIG. 6 or after the cooling of the gas and prior to any separation stages as shown in FIG. 7. In some embodiments, vapor splitting may be effected in a separator. Alternatively, the separator 14 in the processes shown in FIGS. 6 and 7 may be unnecessary if the inlet gas is relatively lean. Where appropriate, the second stream 15 depicted in FIG. 7 may be cooled after division of the inlet stream and prior to expansion of the second stream. 
     It will also be recognized that the relative amount of feed flowing in each branch of the split vapor feed will depend on several factors, including feed gas pressure, feed gas composition, the amount of heat which can economically be extracted from the feed and the quantity of horsepower available. More feed to the top of the column may increase recovery while decreasing power recovered from the expander thereby increasing the recompression horsepower requirements. Increasing feed lower in the column reduces the horsepower consumption but may also reduce product recovery. The first (upper mid-column), second (mid-column) and third (lower mid-column) feed positions depicted are the preferred feed locations for the process operating under the conditions described. However, the relative locations of the mid-column feeds may vary depending on inlet composition and other factors such as desired recovery levels and amount of liquid formed during inlet gas cooling. Moreover, two or more of the feed streams, or portions thereof, may be combined depending on the relative temperatures and quantities of the individual streams, and the combined stream(s) fed mid-column. The streams may be combined before or after expansion and/or cooling. For example, all or a part of stream 16 in FIG. 7 may be combined with stream 19 and the combined stream cooled in exchanger 21 and expanded in valve 22. FIG. 4 is the preferred embodiment for the composition and pressure conditions shown. Although individual stream expansion is depicted in particular expansion devices, alternative expansion means may be employed where appropriate. For example, conditions may warrant work expansion of the minor portion of the stream. 
     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.