METHOD AND APPARATUS FOR SEPARATING A FEED GAS CONTAINING AT LEAST 20 MOL % OF CO2 AND AT LEAST 20 MOL % OF METHANE, BY PARTIAL CONDENSATION AND/OR BY DISTILLATION

Abstract

The invention relates to a method for separating a feed gas containing at least 20 Mol % of CO2 and at least 20 Mol % of methane, by partial condensation and/or by distillation, the gas at a pressure of at least 40 bar abs, including expanding at least one portion of the feed gas in a turbine producing an expanded feed stream at a pressure of less than 90 bar abs, separating at least one portion of the expanded feed stream by partial condensation and/or by distillation thus obtaining a CO.sub.2-depleted gas and a CO.sub.2-enriched liquid, wherein the temperature of the expanded feed gas at the outlet of the turbine is below −56.6° C., and wherein the process does not use an external refrigeration source; and wherein the CO.sub.2-depleted gas is introduced into a supplementary separation step, in order to obtain a stream that is more depleted in CO.sub.2 and a CO.sub.2-rich stream.

Claims

1.-11. (canceled)

12. A process for separating a feed gas containing at least 20 mol % of CO.sub.2 and at least 20 mol % of methane, by partial condensation and/or by distillation, the gas being at a pressure of at least 40 bar abs, comprising: a) expanding at least one portion of the feed gas in a turbine producing an expanded feed stream at a pressure of less than 90 bar abs, b) separating at least one portion of the expanded feed stream by partial condensation and/or by distillation thus obtaining a CO.sub.2-depleted gas and a CO.sub.2-enriched liquid, wherein the temperature of the expanded feed gas at the outlet of the turbine is below −56.6° C., and wherein the process does not use an external refrigeration source; and wherein the CO.sub.2-depleted gas is introduced into a supplementary separation step, in order to obtain a stream that is more depleted in CO.sub.2 and a CO.sub.2-rich stream.

13. The process of claim 12, wherein at least one portion of the refrigeration energy is produced by the vaporization of the CO.sub.2-enriched liquid.

14. The process of claim 12, wherein at least one portion of the refrigeration energy is provided by expansion in one or more turbines of all or some of the CO.sub.2-depleted gas.

15. The process of claim 12, wherein the expanded feed stream contains a liquid fraction.

16. The process of claim 15, wherein the outlet temperature of the turbine is below the triple point of CO.sub.2.

17. The process of claim 16, wherein at least one portion of the gas phase of the expanded feed stream is compressed without first being heated upstream of the compression step.

18. The process of claim 12, wherein the expansion energy recovered in the turbine during step a) is used to pressurize one or more of the following gases: at least one portion of the CO.sub.2-enriched liquid after vaporization; at least one portion of the CO.sub.2-depleted gas; at least one portion of the stream that is more depleted in CO.sub.2; at least one portion of the CO.sub.2-rich stream.

19. The process of claim 12, wherein step b) further comprises at least one step of distillation with reboiling and wherein the overhead gas of the distillation column is recycled upstream of step b).

20. The process of claim 19, wherein the overhead gas of the distillation column is recycled upstream of step a).

21. The process of claim 20, wherein the overhead gas of the distillation column is compressed in the same compressor as the CO.sub.2-rich stream.

22. The process of claim 18, wherein the stream that is more depleted in CO.sub.2 is injected into a line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0039] FIG. 1 illustrates a process of separation by partial condensation and by distillation, in accordance with one embodiment of the present invention.

[0040] FIG. 2 illustrates a process without membrane recycle and/or column overhead recycle, in accordance with one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] In FIG. 1, a process of separation by partial condensation and by distillation is illustrated.

[0042] A natural gas stream 1, 2 at a pressure of at least 40 bar abs containing between 20 and 80 mol % of methane and between 20 and 80 mol % of carbon dioxide is cooled in an exchanger E-101 and then by a cooler in order to form a stream 3. This stream 3 is cooled in a brazed aluminum plate-fin heat exchanger BAHX in order to cool the gas 4 to a temperature of around −30° C.

[0043] The principle is to cool the stream before a turbine T-100 to around −30° C., then to expand the gas 4 in a turbine typically up to between 35 and 50 bar, at most up to 90 bar. This expansion may in particular make it possible to achieve temperatures below the temperature of the triple point of CO.sub.2. This is because the CO.sub.2—CH.sub.4 mixture only freezes at lower temperatures. The lower this temperature, the more effective the process in terms of CO.sub.2 yield. A typical operation point is around −60° C. while the triple point of CO.sub.2 has a temperature of −56.6° C.

[0044] A two-phase stream 5 leaves the turbine T-100 and is sent to a phase separator V-100. The gas 6 from the separator V-100 is heated in the exchanger BAHX to form a stream 14. The stream 14 is heated in a heat exchanger E-103 and the exchanger E-101 before being sent, as stream 20, to a permeation unit M-1. A methane-enriched stream 21 leaves the unit M-1 as non-permeate and is compressed by compressors to serve as produced methane stream 23. The permeate 22 is heated in the exchanger E-103, is compressed by the compressors C-100A and sent to be mixed with the stream 16 in order to form a stream 25 sent to the compressors C-100B.

[0045] The liquid 7 is expanded in a valve and is then sent directly to the column K-100. The overhead gas 8 from the column K-100 is then recompressed in the compressors C-100B and sent back to the feed stream 1 in order to form the stream 2 at the inlet pressure. This layout makes it possible to limit the number of pieces of equipment and also to increase the CO.sub.2 yield without introducing an external refrigeration cycle. A valve may make it possible to reduce the pressure of the stream 7 and of the column if necessary (for example to improve the purity of the CO.sub.2 drawn off from the bottom of the column as stream 9). The stream 9 is divided into two portions 10, 11. One portion 10 is divided into two portions 12, 13.

[0046] The portion 12 is heated, or even vaporized, in the exchanger BAHX in order to form a stream 15, then divided into two factions. The fraction 19 is sent back to the bottom of the column K-100 as gaseous reboiling stream.

[0047] The portion 13 is cooled in the exchanger BAHX, is expanded in a valve V, then is heated at a pressure lower than that of the stream 12, or even is vaporized at a pressure lower than that of the stream 12, in the exchanger BAHX in order to form a stream 17. Next, the stream 17 is compressed in one stage of a compressor C-200, mixed with the stream 18, compressed in two other stages of the compressor C-200, condensed, mixed with the stream 11, then pumped in a pump P-200 in order to form a pure carbon dioxide stream 27.

[0048] The fraction 18 is sent to the compressor C-200. The portion 11 is pressurized by a pump P-100.

[0049] It is possible to envisage this layout without the membrane recycle (6, 14, M-1) and/or column overhead recycle (8; 16, C-100B) as illustrated in FIG. 2. Here, the permeate 22 from the membrane M-1 is cooled in the exchanger E-103 then vented to the air as stream 24. The overhead gas 8 from the column is heated in the exchanger BAHX then vented to the air as stream 16.

[0050] It is obvious that the layout as described above is only one example of an application of the invention presented and that many variations thereof are possible. It could in particular be advantageous in certain cases to include a second, or even a third partial condensation step. In this case, the turbine could for example be placed at the gas outlet of a first separator vessel.

[0051] The presence of the permeation unit M-1 or of another unit for separating the stream 20 is not essential.

[0052] The exchanger could also be physically divided into several exchangers that carry out for example different functions and that may optionally use different technologies (a BAHX-type plate exchanger and a shell and tube exchanger for example).

[0053] One conventional method for achieving a cold point temperature of −60° C. in the process would consist in using an external refrigeration cycle with a refrigerant other than CO.sub.2 (for example a refrigerant consisting of a mixture of hydrocarbons).

[0054] In all these layouts, in order to increase the efficiency of the membranes M-1, these membranes could be used at cryogenic temperature (typically below −20° C.) instead of being used at a hot temperature.

[0055] For a feed stream 1 with a medium pressure (40-70 bar) it may be useful to begin with a compression of this stream that enables: [0056] a condensation of the water upstream of potential dryers; [0057] a pressure to be achieved that is sufficient to produce refrigeration using a cryogenic turbine without lowering the separation pressure.

[0058] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.