GAS PROCESSING METHODOLOGY UTILIZING REFLUX AND ADDITIONALLY SYNTHESIZED STREAM OPTIMIZATION

Abstract

Gas processing methodology for high efficiency recovery of propane and/or ethane from a natural gas feed stream. The method is conducted without turboexpansion. A natural gas stream is processed to have gas and liquid portions. The gas portion is cooled and flows to a refluxed absorber column and the liquid portion flows to a lower pressure distillation column. Bottoms of the absorber column are depressurized directly into a lower pressure distillation column and the overhead vapor stream is used to cool the feed and/or reflux streams. The overhead vapour stream from the lower pressure distillation column split into at least two streams with one being depressurized into the absorber to provide reflux and the second passed into the absorber column to provide further reflux.

Claims

1. A method of processing natural gas to recover selected hydrocarbons contained therein, comprising: cooling and partially condensing a natural gas feed stream; separating said natural gas stream into a liquid stream and a gas stream; cooling and depressurizing said gas stream; passing said gas stream into a fractionator column and said liquid stream into a distillation column; recovering an overheads stream from said distillation column; treating said overheads stream to first recover cooling energy and then compress said overheads stream to form a plurality of overhead streams as reflux streams at different pressures; passing at least one of said overhead streams into said fractionator column as reflux and a second stream at a second point in said fractionator column to liquify hydrocarbon components present in said natural gas feed stream in said fractionator; depressurizing and passing a bottoms stream from said fractionator column stream directly into said distillation column; and collecting selected hydrocarbon components from said bottoms stream.

2. The method as set forth in claim 1, wherein said method is entirely conducted absent turboexpansion.

3. (canceled)

4. The method as set forth in claim 1, wherein said depressurizing and feeding said bottoms stream directly into said distillation column.

5. The method as set forth in claim 1, further including the step of varying fluid flow rate a at least between said fractionator column and said distillation column.

6. (canceled)

7. The method as set forth in claim 1, wherein said plurality of overheads streams comprise at least one liquid stream and at least one gas stream.

8. The method as set forth in claim 7, wherein said at least one gas stream is passed into said fractionator column at said second point.

9. A gas processing plant, comprising: a separator for receiving a cooled and partially condensed natural gas feed stream to separate said feed stream into a liquid stream and a gas stream; a fractionator column for receiving said gas stream and reflux streams produced from a distillation column; and a distillation column in fluid communication with said fractionator for producing an overheads stream, said overheads stream being utilized in said fractionator column as reflux streams to liquify hydrocarbons present in said feed stream, said fractionator column producing a bottoms stream which is depressurized into said distillation column to recover selected hydrocarbons in the absence of further unit operations.

10. The gas processing plant as set forth in claim 9, further including a network of heat exchangers positioned to recover and distribute heat from all streams of said streams during processing.

11. The gas processing plant as set forth in claim 9, wherein said overheads stream is compressed, cooled and divided into a plurality of overhead streams.

12. The gas processing plant as set forth in claim 11, wherein at least one of said overhead streams is compressed, cooled and depressurized for use in said fractionator column as a reflux stream.

13. The gas processing plant as set forth in claim 11, wherein said overhead streams are pressurized at a different pressure relative to one another.

14. The gas processing plant as set forth in claim 13, wherein said overhead streams may exist in a gas, partially condensed, or liquid state.

15. A method of processing natural gas with a fractionator column and distillation column in fluid communication to recover selected hydrocarbons contained therein, comprising a plurality of operations, comprising: a separation operation where a natural gas stream is processed to form a liquid stream and a gas stream which are separated; an overheads processing and distribution operation where an overheads stream is recovered from said distillation column and processed into a plurality of overhead streams distributed into predetermined locations in said fractionator column; a distillation operation in said fractionator column where said natural gas feed stream contacts said overhead streams to liquify hydrocarbon components present in said natural gas stream; and a bottoms recovery and processing operation where a bottoms stream is recovered from said fractionator column and processed by depressurization directly in said distillation column in the absence of turboexpansion to recover selected hydrocarbons.

16. The method as set forth in claim 15, wherein said method further includes heat exchange operations between each of the separation, overheads processing and distribution, refrigeration and bottoms recovery operations to recover and distribute heat between all streams associated with said operations.

17. The method as set forth in claim 15, wherein said distillation column is operated as a deethanizer.

18. The method as set forth in claim 15, wherein said distillation column is operated as a demethanizer.

19. The method as set forth in claim 15, wherein said operations are modifiable to recover ethane, propane, hydrocarbons, or a combination thereof.

20. The method as set forth in claim 15, wherein said separation operation includes cooling and partially condensing said natural gas feed stream.

21. The method as set forth in claim 15, wherein said bottoms recovery and processing operation includes introducing said bottoms stream at a pressure of between 500 kPag and 2000 kPag into said distillation column relative to the pressure of between 2000 and 7000 kPag prior to introduction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 is a process flow diagram in accordance with a first embodiment of the present invention;

[0049] FIG. 2 is a process flow diagram in accordance with a second embodiment of the present invention;

[0050] FIG. 3 is a is a process flow diagram in accordance with a third embodiment of the present invention;

[0051] FIG. 4 is a is a process flow diagram in accordance with a fourth embodiment of the present invention; and

[0052] FIG. 5 is a process flow diagram in accordance with a fifth embodiment of the present invention,

[0053] Similar numerals used in the figures denote similar elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] Referring now to FIG. 1, shown is a process flow diagram for a first embodiment of the present invention for propane recovery and ethane rejection.

[0055] A natural gas feed stream 10 at a pressure above, for example, approximately 5000 kPag flows through heat exchanger 12 cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 leaving exchanger 12 is then passed through a chiller 18, which partially condenses the natural gas stream. The chiller 18 may operate at, for example, approximately −40° C. and may use common refrigerants such as propane, or ammonia. Other suitable refrigerant examples are well within the purview of one skilled in the art.

[0056] The partially condensed natural gas stream 20 is then separated in separator 22 and the gas portion 24 leaving separator 22 is further cooled by heat exchange at 26 with a sales gas stream 28 and a reflux stream 30, discussed in greater detail herein after. The gas stream 32 is then depressurized across a valve 34 entering, as stream 36, to the bottom of a refluxed fractionator tower 38 at, for example, approximately 3500 kPag. The gas flows upwardly through tower 38 where it is contacted with the downwardly flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of tower 38 as illustrated and are depressurized across a valve 42 directly into the top of a second distillation column 44, operating in the example as a deethanizer at approximately 1000 kPag. The deethanizer (second distillation column 44) is also fed the condensed liquids 46 from the separator 22. Liquids 46 are depressurized via valve 48 into the deethanizer 44.

[0057] Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce a low ethane content C3+ liquid product 52. The deethanizer reboiler circuit can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.

[0058] The deethanizer overheads 54 flow to a heat exchanger 56, which is used to further cool the stream after compression in compressor 58 to a pressure above the fractionator operating pressure followed by cooling in an air exchanger 60. The overheads stream 62 is then chilled and partially condensed in a chiller 64, and further condensed in an exchanger 66 with the sales gas, prior to being separated in separator 68.

[0059] The liquid stream 70 from the separator 68 is used as a medium quality source of reflux, with a composition that is approximately half methane and half ethane. The medium quality reflux stream 70 is fed into a mid-point of the fractionator tower 38 as illustrated in the Figure.

[0060] The gas stream 72 from the separator 68 can optionally be split into two streams 30 (referenced herein previously) and 74, with the ratio of the split determining the degree of propane recovery versus the refrigeration and reflux compression energy requirement. For the highest propane recovery, all of the gas stream from separator 68 can be used to generate high quality reflux liquids. The stream 30 is used to cool the feed to the fractionator 38 in a heat exchanger 76. The stream 74 then flows to another heat exchanger 78 which is used to further cool the stream after compression in compressor 80 and cooling in an air exchanger 82.

[0061] The stream 84 is compressed to approximately 7000 kPag, and then chilled in chiller 86 and cooled with heat exchanger 88 with the sales gas stream 90 from fractionator tower 38. Depressurization of this stream 92 is achieved by valve 94 to the fractionator tower 38 and this depressurization condenses a portion of the stream into high quality liquefied natural gas reflux, composed mainly of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas into the liquid phase. The fractionator overheads 90 have propane and heavier components effectively absorbed by the reflux and this exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers 88, 66, and 12 and then the sales gas exits the process for further compression, if required.

[0062] Turning now to FIG. 2, shown is a process flow diagram in another embodiment of the invention for ethane recovery.

[0063] In this embodiment, natural gas stream 10 at a pressure above approximately 7000 kPag flows through a heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process, generally denoted by numeral 14. The natural gas stream 16 is then passed through a series of heat exchangers and a chiller, 18, 100 and 102, respectively, which partially condenses the natural gas stream. The chiller 100 may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia. The partially condensed natural gas stream 109 is then separated in separator 22 and the gas portion 24 is then depressurized across a valve 34, entering the bottom of fractionator tower 38 at approximately 3500 kPag.

[0064] The gas flows upwards through the refluxed fractionator tower 38 where it is contacted with the downwards flowing reflux liquids. The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38 and are depressurized across a valve 42 directly into the top of second distillation column 44 operating as a demethanizer at approximately 1000 kPag. The demethanizer 44 is also fed the condensed liquids 46 from the separator 22 which are depressurized across a valve 48 into the demethanizer.

[0065] Distillation of these liquids occurs in the demethanizer 44 with the bottoms product reboiled in a reboiler circuit 50 to produce a low methane content C2+ liquid product 106. The demethanizer reboiler circuit 50 can be heated by heat exchange with the feed gas stream, another process stream of appropriate temperature, or a heat medium stream. The demethanizer overheads 34 flows to a series of heat exchangers 102, 18 which are used to cool the feed gas stream.

[0066] The demethanizer overheads then flow to a compressor 58 which compresses the gas stream 106 to a pressure above the fractionator operating pressure, and then the gas is cooled in an air exchanger 60. This gas is then split into two streams 108, 110 with the ratio of the split determining the degree of ethane recovery versus the refrigeration and reflux compression energy requirement.

[0067] One of these two streams 108 can be chilled and depressurized into a mid-point of the fractionator tower 38. The other of these two split streams, can be further compressed to approximately 9000 kPag in compressor 110, cooled with an air cooler 112, chilled with chiller 114, cooled by heat exchangers 116, 118 with the fractionator tower overheads 120 and depressurized across a valve 122 into the fractionator tower 38 to generate high quality reflux liquids.

[0068] Depressurization of this stream 124 to the fractionator tower 38 pressure condenses a portion of the stream into high quality liquefied natural gas reflux, consisting primarily of liquid methane. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the ethane and heavier components from the upwards flowing natural gas, into the liquid phase. The fractionator overheads 120 have had ethane and heavier components effectively absorbed by the reflux, and exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux stream, and inlet natural gas in heat exchangers (118, 116, 12), and then the sales gas exits the process for further compression, if required.

[0069] FIG. 3 illustrates yet another variation of the present invention for switchable ethane recovery.

[0070] The process may be arranged so that the ethane recovery can be switched on and off. The figure illustrates the ethane rejection mode. Generally speaking, the process is very similar to that discussed regarding FIG. 1, with the exception of the following:

[0071] The feed gas pressure for high ethane recovery is desirably above approximately 7000 kPag. The distillation column 44 operates as a demethanizer as opposed to a deethanizer. Overheads stream 54 from the demethanizer 44 is redirected from the exchanger 56 to the exchanger 76.

[0072] The gaseous stream 72 from the separator 68 is redirected from exchanger 76 to exchanger 78.

[0073] Stream 74 from the exchanger 76 is redirected from the exchanger 76 to the exchanger 56. The demethanizer reboiler 44 can instead be heated using a heat medium, or by heat exchange with process streams such as the feed gas stream 16.

[0074] Turning now to FIG. 4, shown is a propane recovery, ethane rejection process flow diagram.

[0075] Natural gas stream 10 at a pressure above approximately 5000 kPag flows through heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process at 14.

[0076] The natural gas stream 16 is then passed through a heat exchanger 126 which partially condenses the natural gas stream. The partially condensed natural gas stream 128 is then separated in separator 22 and the gas portion 24 is further cooled in a chiller 27. The chiller 27 may operate at approximately −40° C., and may use common refrigerants such as propane, or ammonia.

[0077] The partially condensed gas stream is then depressurized across valve 34, entering the bottom of a fractionator tower 38 at approximately 3500 kPag. The gas flows upwardly through the refluxed fractionator tower 38 where it is contacted with the downwardly flowing reflux liquids.

[0078] The fractionator tower 38 bottoms liquid 40 exit the bottom of the tower 38 and are depressurized across valve 42 directly into the top of a second distillation column 44 operating as a deethanizer at approximately 1000 kPag.

[0079] The deethanizer 44 is also fed the condensed liquids 46 from separator 22, which are depressurized across valve 48 into the deethanizer 44.

[0080] Distillation of these liquids occurs in the deethanizer 44 with the bottoms product reboiled in circuit 50 to produce a liquid product 52, in this example, a low ethane content C3+.

[0081] The deethanizer reboiler circuit 50 can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.

[0082] The deethanizer overheads 54 flow to heat exchanger 126 which uses the overheads to cool the feed gas. The overheads then flow to another heat exchanger 56 which is used to further cool the stream after compression with compressor 58 to a pressure above the fractionator operating pressure, and cooling in air exchanger 60.

[0083] The overheads stream is then split into two streams 108 and 110 each to be used as a source of reflux in the absorber.

[0084] Reflux stream 108 is cooled by heat exchange 126 with the deethanizer overheads stream, then chilled with chiller 132, partially condensed and depressurized across valve 134 into a mid point of the fractionator tower 38 as a source of medium quality reflux.

[0085] The other split reflux stream 110 is further compressed at 111, cooled with air exchanger 112, chilled at 114, and further cooled and condensed by heat exchange at 116 with the sales gas. Depressurization of this stream across valve 122 to the fractionator tower 38 pressure condenses a portion of the stream into high quality reflux. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwardly flowing natural gas, into the liquid phase.

[0086] The fractionator overheads 120 at this pint in the process have had propane and heavier components effectively absorbed by the reflux and the exit the fractionator 38 as sales gas.

[0087] The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers and then the sales gas exits the process for further compression, if required.

[0088] FIG. 5 is a process flow diagram depicting propane recovery with ethane rejection).

[0089] Similar to the previous embodiments, natural gas stream 10 at a pressure above approximately 5000 kPag flows through heat exchanger 12, cooling the gas by exchanging heat with the cold sales gas leaving the process at 14.

[0090] The natural gas stream 16 is then passed through heat exchanger 126, which partially condenses the natural gas stream, The partially condensed natural gas stream 128 is then separated with separator 22 with gas portion 24 further cooled in chiller 27.

[0091] Chiller 27 may operate at approximately −40° C.

[0092] The partially condensed gas stream (9) is further cooled by heat exchange with the sales gas stream as shown and depressurized across valve 34, entering the bottom of a fractionator tower 38 at a pressure of approximately 3500 kPag.

[0093] The gas flows upwardly through the refluxed fractionator tower 38 where it is contacted with the downwardly flowing reflux liquids.

[0094] The fractionator tower 38 bottoms liquids 40 exit the bottom of the tower 38. and are depressurized across valve 42 directly into the top of distillation column 44, operating as a deethanizer at approximately 1000 kPag.

[0095] The deethanizer 44 is also fed the condensed liquids 46 from the separator 22, which are depressurized across valve 48 into the deethanizer 44. Distillation of these liquids occurs in the deethanizer 44, with the bottoms product reboiled in circuit 50 to produce liquid product 52 which in this example is a low ethane content C3+ product.

[0096] The deethanizer reboiler circuit 50 can be heated using a heat medium, or by heat exchange with a hot compressor discharge stream.

[0097] The deethanizer overheads 54 flow to heat exchanger 126 which uses the overheads to cool the feed gas. The overheads then flow to another heat exchanger 56, which is used to further cool the stream after compression at 58 to a pressure above the fractionator operating pressure, and cooling in air exchanger 60.

[0098] The overheads stream is then split into two streams 108 and 110 each to be used as a source of reflux in the absorber.

[0099] Reflux stream 108 is cooled by heat exchanger 56 with the deethanizer overheads stream, then chilled at 132, further cooled and condensed by heat exchange at 138 with the sales gas, and depressurized across valve 134 into a mid point of the fractionator tower 38 as a source of medium quality reflux.

[0100] The other split reflux stream 110 is further compressed at 111, cooled with air exchanger 112, chilled at 114 and further cooled and condensed by heat exchange with the sales gas.

[0101] Depressurization of this stream across a valve 122 to the fractionator tower pressure condenses a portion of the stream into high quality reflux. This reflux stream flows downwardly through the fractionator tower 38, effectively absorbing the propane and heavier components from the upwards flowing natural gas, into the liquid phase.

[0102] The fractionator overheads 120 at this point have had propane and heavier components effectively absorbed by the reflux, and the exits the fractionator 38 as sales gas. The sales gas is used to cool the reflux streams, fractionator feed, and inlet natural gas in heat exchangers and then the sales gas exits the process for further compression, if required.

[0103] These processes can economically achieve higher ethane and/or propane recoveries than the traditional turbo-expander processes with lower plant inlet pressures. Generally, depending upon gas composition, it is desired to have a plant inlet pressure of approximately 7000 kPa to achieve 99% ethane recovery. When inlet pressures are higher, energy, capital cost, and operating cost can be saved by having lower refrigerant temperatures than −40° C. Depending upon markets conditions for ethane sales and energy purchase, some scenarios may favor lower ethane recovery for reasons of economics. Lower ethane recovery can be easily achieved with warmer refrigerant temperatures, but raising the demethanizer pressure and/or the gas fractionator pressure can also be considered.

[0104] Similarly, an inlet pressure of approximately 5000 kPa is generally desired for 99% propane recovery, but with higher inlet pressures, lower refrigerant temperatures can save capital cost, energy cost, and operating cost and still achieve an economical 99% propane recovery.