METHOD AND SYSTEM FOR RECOVERY OF METHANE FROM HYDROCARBON STREAMS

Abstract

The invention relates to a method for recovery of methane from hydrocarbon streams comprising the following steps: a. Introducing a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon free fluid, wherein in particular said at least one hydrocarbon free fluid is nitrogen, and at least one hydrocarbon fluid, into a demethanizer system (1); b. Separating said feed fluid stream (F) in the demethanizer system (1) into a carbon rich fraction (C), comprising hydrocarbons with a carbon content of C2 and higher, and a separation stream (S), comprising methane fluid and at least one hydrocarbon free fluid; c. Introducing said separation stream (S) into a hydrocarbon-free fluid separation system (2), in particular in a cryogenic hydrocarbon-free fluid separation system (2′), more particularly into a cryogenic nitrogen rejection system (2″); wherein said separation stream (S) is compressed by a compressor system (6) before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2), wherein said separation stream is compressed to a pressure of 12 bar to 80 bar; d. Separating said separation stream (S) in said free fluid separation system (2) into a methane stream (M) and a hydrocarbon-free fluid stream (HF) and a respective system for recovery of methane from hydrocarbon streams.

Claims

1. A method for recovery of methane from hydrocarbon streams comprising the following steps: a. Introducing a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon free fluid, wherein in particular said at least one hydrocarbon free fluid is nitrogen, and at least one hydrocarbon fluid, into a demethanizer system (1); b. Separating said feed fluid stream (F) in the demethanizer system (1) into a carbon rich fraction (C), comprising hydrocarbons with a carbon content of C.sub.2 and higher, and a separation stream (S,) comprising methane fluid and at least one hydrocarbon free fluid; c. Introducing said separation stream (S) into a hydrocarbon-free fluid separation system (2), in particular in a cryogenic hydrocarbon-free fluid separation system (2′), more particularly into a cryogenic nitrogen rejection system (2″); wherein said separation stream (S) is compressed by a compressor system (6) before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2), wherein said separation stream is compressed to a pressure of 25 bar to 80 bar; d. Separating said separation stream (S) in said free fluid separation system (2) into a methane stream (M) and a hydrocarbon-free fluid stream (HF).

2. The method according to claim 1, wherein said feed fluid stream (F) derives from a synthesis system (3), which uses methane as a reactant, in particular said synthesis system (3) is a system for oxidative coupling of methane.

3. The method according to any one of the claims 1 to 2, wherein said methane stream (M) is recycled and reused as a reactant, wherein in particular said methane stream (M) is transferred to said synthesis system (3).

4. The method according to any of the previous claims, wherein said separation stream (S) is compressed by said compressor system (6) to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 bar to 40 bar, particularly to a pressure of 30 bar, before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).

5. The method according to claim 1, wherein said carbon rich fraction (C) from the demethanizer system (1) is transferred to a C2-splitter (7), for separation and isolation of hydrocarbon compounds with different carbon contents of said carbon rich fraction (C) from each other.

6. The method according to any one of the claims 1 to 5, wherein at least parts of the feed fluid stream (F) are liquidized in a cooling system before the introduction into a demethanizer unit (10) of the demethanizer system (1).

7. The method according to any one of the claims 1 to 6, wherein said feed fluid stream (F) is separated in said cooling system into a liquid feed fluid stream and a gaseous feed fluid stream, wherein said liquid feed fluid stream is transferred to said demethanizer unit (10) and said gaseous feed fluid stream is transferred to an expander-booster system, in which said gaseous feed fluid stream is expanded to a lower pressure before introducing said gaseous feed fluid stream into said demethanizer unit (10).

8. The method according to any one of the claims 1 to 7, wherein said demethanizer system (1) system is operated at a pressure of 6 to 40 bar.

9. The method according to any one of the claims 1 to 7, wherein said demethanizer unit (10) of said demethanizer system (1) is operated at a pressure of 9 to 25 bar, in particular at a pressure of approximately 13 bar.

10. The method according to claim 1, wherein said separation stream (S) is introduced in at least one high pressure column (21), which is arranged in said hydrocarbon free fluid separation system (2), and in which said separation stream (S) is separated in a methane rich bottom liquid and an essentially pure hydrocarbon-free overhead, wherein said methane rich bottom liquid is transferred into at least one low pressure column (22), which is arranged in said hydrocarbon free fluid separation system (2), in which said methane rich bottom liquid is separated into a methane rich liquid and hydrocarbon-free gas.

11. The method according to claim 10, wherein said hydrocarbon free overhead from the high pressure column (21) is at least partially condensed on a heat exchanger (5) and said a methane rich liquid from the low pressure column (22) is at least partially vaporized on said heat exchanger (5), providing a liquid fraction and a methane gas fraction, wherein said heat exchanger (5) is situated between said high pressure column (21) and said low pressure column (22).

12. The method according to any one of the claims 10 to 11, wherein said high pressure column (21) is operated at a pressure of 6 to 40 bar, in particular at a pressure of approximately 20 bar, and at a temperature of −160 to −90° C., in particular at a temperature of approximately −140° C., and wherein said low pressure column (22) is operated at a pressure of 1 to 5 bar, in particular at a pressure of approximately 2 bar, and at a temperature of −220 to −180° C., in particular at a temperature of approximately −190° C.

13. A system for recovery of methane from hydrocarbon streams comprising a. a demethanizer system (1), which is designated to separate a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon-free fluid, wherein in particular said at least one hydrocarbon-free fluid is nitrogen, and at least one hydrocarbon fluid, into i. a carbon rich fraction (C), which comprises hydrocarbons with a carbon content of C.sub.2 and higher, and ii. a separation stream (S), which comprises methane fluid and at least one hydrocarbon free fluid, and b. a hydrocarbon-free fluid separation system (2), in particular a cryogenic hydrocarbon free fluid separation system (2′), more particularly a cryogenic nitrogen rejection system (2″), which is designated to separate said separation stream (S) into a methane stream (M) and a hydrocarbon free stream (HF), and c. a compressor system (6) that is configured to compress said separation stream (S) to a pressure of 25 bar to 80 bar before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).

14. The system according to claim 13, wherein the compressor system (6) is configured to compress said separation stream (S) to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 bar to 40 bar, particularly to a pressure of 30 bar, before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).

15. The system according to claim 13 or 14, wherein said system comprises a synthesis system (3), which uses methane as a reaction educt and provides said feed fluid stream (F), wherein in particular said synthesis system (3) is a system for oxidative coupling of methane.

Description

[0055] Further details and features of the invention are described in the following figures of two embodiments of the invention.

[0056] FIG. 1 shows a first embodiment of the invention comprising a demethaniser system 1 and a hydrocarbon-free fluid separation system 2; and

[0057] FIG. 2 shows a second embodiment of the invention comprising a demethaniser system 1, a cryogenic nitrogen rejection system 2″, and an OCM synthesis system 3.

[0058] FIG. 1 shows a system for recovery of methane from hydrocarbon streams comprising a demethaniser system 1 and a hydrocarbon-free fluid separation system 2.

[0059] A feed fluid stream F, comprising methane fluid, at least one hydrocarbon-free fluid, and at least one hydrocarbon fluid is introduced into a demethaniser unit 10 of the demethaniser system 1. The demethaniser unit 10 is operated at a pressure of 13 bar. Different pressures may be applied as necessary.

[0060] The demethaniser unit 10 comprises a temperature gradient with a temperature of −30° C. at the bottom of the demethaniser unit 10 and a temperature of approximately −150° C. at the top of the demethaniser unit 10. Thus the demethaniser unit 10 allows for a separation of the feed fluid stream F into a carbon-rich fraction C at the bottom of the demethaniser unit 10, and a separation stream S, which comprises methane fluid and at least one hydrocarbon-free fluid, in particular nitrogen, at the top of the demethaniser unit 10.

[0061] Optionally the feed fluid stream F may be cooled down with at least one cooling system (not depicted in the figure), wherein each separated liquid of each cooling step (liquid feed stream) is introduced into the demethaniser 10. A remaining gaseous feed stream may be transferred from the cooling systems into an expander boost system (not depicted in the figure), in which it is expanded to a lower pressure and subsequently introduced into the demethaniser unit 10.

[0062] The carbon-rich fraction C from the bottom of the demethaniser 10 is reboiled in a reboiler 4 in order to provide a carbon-rich fraction C, which is free of methane and hydrocarbon-free fluids like nitrogen. The carbon-rich fraction C is then transferred to a C2 splitter 7 for further separation in order to isolate the target product from the carbon-rich fraction C. For example, the target product is ethylene if the feed fluid stream F is derived from a synthesis system 3 (see FIG. 2), which applies the oxidative methane-coupling reaction (OCM).

[0063] The separation stream S is then transferred from the top of the demethaniser unit 10 to the hydrocarbon-free fluid separation unit 2. Optionally the separation stream S may be transferred—prior to the introduction to the hydrocarbon-free fluid separation system 2—into a second expander (not depicted), where it is expanded to approximately 4 bar, providing the chilling duty used in the demethaniser system. The work power of the first and the second expander can be recovered in order to recompress the separation stream S to approximately 6 bar, before it is introduced into the hydrocarbon-free fluid separation system 2.

[0064] The hydrocarbon-free fluid separation system 2 comprises a high-pressure column 21 and a low-pressure column 22, which are interconnected with a heat exchanger 5 situated between the high-pressure column 21 and the low-pressure column 22. Before the separation stream S is introduced into the bottom of the high-pressure column 21 it may be cooled down by, for example, a plate fin heat exchanger.

[0065] Alternatively, the high-pressure column 21 and the low-pressure column 22 can be constructed as separate columns.

[0066] In the high-pressure column 21 the separation stream S is separated into a methane-rich bottom liquid at the bottom of the high pressure column 21 and a gaseous stream, comprising essentially pure hydrocarbon-free overhead product, in particular an essentially pure nitrogen overhead product. The pressure at the bottom of the high-pressure column 21 is approximately 20 bar, and the temperature is approximately −140° C. The bottom liquid from the bottom of the high pressure column 21 is transferred to the mid-section of the upper low-pressure column 22.

[0067] Optionally the bottom liquid may be sub-cooled in a reflex cooler to approximately −160° C. before it is transported to the mid-section of the low-pressure column 22. The low-pressure column 22 operates at a pressure of 2 bar, which allows for a further separation of hydrocarbon-free gas, in particular nitrogen, and methane, due to their physical properties.

[0068] Columns 21 and 22 are connected by an integrated heat exchanger 5. In this heat exchanger 5 the overhead vapour from the high-pressure column 22 will be condensed while simultaneously the bottom liquids from the lower-pressure column 22 will be partially vaporised. The low-pressure hydrocarbon-free gas, in particular nitrogen, and the methane can be used to cool the inlet streams of both columns. The use of the high-pressure column 21, the low-pressure column 22 and the integrated exchanger 5 allows for the separation and isolation of a hydrocarbon-free gas HF, in particular nitrogen, and methane M in a high purity. Alternatively, the high-pressure column 21, the low-pressure column 22 and the integrated exchanger 5 may be separate units.

[0069] The hydrocarbon-free product HF, in particular nitrogen, can be sent to the atmosphere, while the isolated methane M can be recycled and introduced into a reaction process, which uses methane as a reactant. Alternatively, hydrocarbon-free product HF and the isolated may be further processed before being sent to the atmosphere or being recycled and introduced into a reaction process.

[0070] FIG. 2 shows a system for recovery of methane from hydrocarbon streams comprising a demethaniser system 1, a cryogenic nitrogen rejection system 2″ and a synthesis system 3, which uses methane in an OCM reaction.

[0071] Concerning the description and the features of functions or applications with the same numbering or letter, reference is made to the description of FIG. 1. The system for recovery of methane from hydrocarbon streams is essentially the same as in FIG. 1.

[0072] The two main differences are that the feed fluid stream F derives from a synthesis system 3 which uses methane as a reactant in an OCM reaction. Thus, the separation stream S comprises essentially methane and nitrogen. Another difference is that, before the separation stream S is transferred from the demethaniser system 1 and introduced into the cryogenic nitrogen rejection system 2″, the separation stream S is compressed with a compression system 6 to approximately 25 bar to 80 bar, preferably to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 to 40 bar, in particular to 30 bar. The other pressure ranges stated above may also be used.

[0073] As discussed previously, a cryogenic nitrogen rejection system 2″ provides a very good separation and isolation of nitrogen and methane, if it is operated with a high pressure.

[0074] Conversely the demethaniser system 1 is preferably operated at a lower pressure in order to minimise product losses concerning the main product ethylene (derived from the OCM reaction). Thus, the use of a compressor system 6, in order to provide a separation stream S with a higher pressure compared to the situation in the demethanizer system 1, compensates for these deficiencies.

[0075] The use of the feed fluid stream F derived from an OCM reaction, the separation of the feed fluid stream F in a demethaniser unit 10 into a carbon-rich fraction C and a separation stream S, the compression of said separation stream S, the subsequent separation of the compressed separation stream S in a cryogenic nitrogen rejection system 2″ into essentially pure nitrogen and essentially pure methane, and the recycling and re-use of the thus separated methane in the aforementioned OCM reaction allows for an efficient and economically effective use of the important reactant methane.

LIST OF REFERENCES

[0076]

TABLE-US-00001 demethaniser system 1 demethaniser unit 10 hydrocarbon-free fluid separation system 2 cryogenic hydrocarbon-free fluid separation system 2′ cryogenic nitrogen rejection system 2″ high-pressure column 21 low-pressure column 22 synthesis system 3 Reboiler 4 heat exchanger 5 compression system 6 C2 splitter 7 hydrocarbon-free fluid stream HF methane stream M feed fluid stream F separation stream S carbon rich fraction C