Process for removing CO2 from crude natural gas

Abstract

A method for treating a crude natural gas feed stream comprising methane and having a first carbon dioxide concentration, said method comprising the steps of: subjecting the crude natural gas feed stream to a separation process to provide: a purified natural gas stream having a second carbon dioxide content which is lower than the first carbon dioxide concentration in said crude natural gas stream; and, a carbon dioxide stream comprising carbon dioxide as the major component and methane; recovering the purified natural gas steam; optionally mixing the carbon dioxide stream with make-up methane and/or make-up air; passing the carbon dioxide stream and optional make-up methane or air through a heat exchanger to raise the temperature of the stream to the desired inlet temperature T.sub.1 of an oxidation reactor; optionally mixing the carbon dioxide stream with make-up methane and/or make-up air; passing the heated stream from step (d) and any optional make-up methane and/or air to the oxidation reactor containing an oxidation catalyst, where the methane is oxidised; removing a gas stream including the products of the oxidation reaction from the reactor, said gas stream being at an outlet temperature T.sub.2 which is higher than the inlet temperature T.sub.1; passing the gas stream removed in step (g) through the heat exchanger against the carbon dioxide stream from step (a) to allow the heat to be recovered from the gas stream removed in step (g) and utilised to heat the carbon dioxide stream in step (d); and measuring the outlet temperature T.sub.2 and controlling the inlet temperature T.sub.1 by adjusting the amount of make-up methane and/or air added in step (c) and/or step (e).

Claims

1. A method for treating a crude natural gas feed stream comprising methane and having a first carbon dioxide concentration, said method comprising: (a) subjecting the crude natural gas feed stream to a separation process to provide: a purified natural gas stream having a second carbon dioxide content which is lower than the first carbon dioxide concentration in said crude natural gas stream; and, a carbon dioxide stream comprising carbon dioxide as the major component and methane; (b) recovering the purified natural gas steam; (c) optionally mixing the carbon dioxide stream with make-up methane and/or make-up air; (d) passing the carbon dioxide stream and optional make-up methane or air through a heat exchanger to raise the temperature of the stream to an inlet temperature T.sub.1 of an oxidation reactor; (e) optionally mixing the carbon dioxide stream with make-up methane and/or make-up air; (f) passing the heated stream from step (d) and any optional make-up methane and/or air to the oxidation reactor containing an oxidation catalyst, where the methane is oxidised; (g) removing a gas stream including the products of the oxidation reaction from the reactor, said gas stream being at an outlet temperature T.sub.2 which is higher than the inlet temperature T.sub.1; (h) passing the gas stream removed in step (g) through the heat exchanger against the carbon dioxide stream from step (a) to allow the heat to be recovered from the gas stream removed in step (g) and utilised to heat the carbon dioxide stream in step (d); and (i) measuring the outlet temperature T.sub.2 and controlling the inlet temperature T.sub.1 by adjusting the amount of make-up methane and/or air added in step (c) and/or step (e).

2. The process according to claim 1, wherein the separation process is a membrane separation process.

3. The process according to claim 2, wherein the crude natural gas is treated in a single membrane separator and a retentate from the membrane separator is the purified natural gas stream and the permeate is the carbon dioxide stream comprising methane.

4. The process according to claim 2, wherein the crude natural gas is treated in a first membrane separator to form a retentate comprising the purified natural gas stream and a permeate which is passed to a second membrane separator, the permeate from the second membrane separator is the carbon dioxide stream comprising methane.

5. The process according to claim 4, wherein the permeate stream is compressed before being passed to the second membrane separator.

6. The process according to claim 2, wherein the crude natural gas is treated in first and second membrane separators in series to form a retentate comprising the purified natural gas stream, the permeate from one or both of the first and second membrane separators being passed to a third membrane separator, the permeate stream from the third membrane separator is the carbon dioxide stream comprising methane.

7. The process according to claim 6, wherein the permeate stream is compressed before being passed to the third membrane separator.

8. The process according to claim 1, wherein the reactor comprises two or more sub-reactors.

9. The process according to claim 8, wherein a product stream from a first sub-reactor is passed through a second heat exchanger before being passed to a second sub-reactor.

10. The process according to claim 9, wherein a product stream from a second sub-reactor is passed through one or both heat exchangers before being passed to a third sub-reactor.

11. The process according to claim 1, wherein the outlet temperature T.sub.2 is compared with a pre-determined temperature and the concentration of the methane and/or air is adjusted such that the temperature rise occasioned by the reaction in the oxidation reactor results in the outlet temperature T.sub.2 approaching the pre-determined temperature.

12. The process according to claim 1, wherein a portion of the carbon dioxide stream by-passes the heat exchanger in step (d).

13. The process according to claim 12, wherein make-up air is added to the by-pass stream.

14. The process according to claim 1, wherein the catalyst is iridium and platinum dispersed on a carrier.

15. The process according to claim 1, wherein T.sub.1 is at least about 300 C.

16. The process according to claim 1, wherein T.sub.2 is about 600 C. or below.

17. The process according to claim 1, wherein T.sub.1 is about 373 C. or above and T.sub.2 is about 580 C. or below.

18. The process according to claim 1, wherein the stream recovered from the reactor is passed through a steam to power generator.

19. The process according to claim 1, wherein the stream that has been passed through the heat exchanger in step (h) is passed through an expansion turbine before being vented.

20. The process according to claim 1, wherein T.sub.1 is at least about 350 C.

Description

(1) The present invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 is a schematic illustration of a prior art one-stage carbon dioxide removal process;

(3) FIG. 2 is a schematic illustration of a prior art two-stage carbon dioxide removal process;

(4) FIG. 3 is a schematic representation of an alternative prior art carbon dioxide removal process;

(5) FIG. 4 is a schematic illustration of a flow sheet according to one aspect of the present invention;

(6) FIG. 5 is a schematic illustration of a flow sheet according to a second aspect of the present invention;

(7) FIG. 6 is a schematic illustration of a flow sheet according to the second aspect of the present invention in combination with the carbon dioxide removal plant shown in FIG. 2; and

(8) FIG. 7 is a schematic illustration of a flow sheet according to a third aspect of the present invention.

(9) It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, flow control dampers, duct work, flame arresters, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

(10) As illustrated in FIG. 4, a carbon dioxide stream recovered from a crude natural gas feed separation process (not shown) is fed in line 21 using a fan 22 where it is mixed with optional make-up methane, generally in the form of a methane-rich gas, in line 23 and/or optional make-up air in line 24, supplied by fan 25. The gas is then fed to heat exchanger 26, where it is heated to a temperature T.sub.1 and then fed in line 27 to the oxidation reactor 28 which comprises the catalyst. In the oxidation reactor the methane is converted to carbon dioxide and water. The gas stream, which will have been heated during the exothermic reaction to the temperature T.sub.2, is then removed from the reactor in line 29, where it is passed through the heat exchanger 26 against the incoming feed in line 21 such that it is cooled and the feed stream is heated. The cooled product gas is then removed in line 30.

(11) A controller 31 monitors the temperatures T.sub.1 and T.sub.2 measured in detectors 32 and 33 and controls the make-up methane and make-up air supplies as appropriate.

(12) In the alternative arrangement illustrated in FIG. 5, a carbon dioxide stream recovered from a crude natural gas feed separation process (not shown) is fed to the process in line 41 using the main fan 42 and is then passed to the heat exchanger 43, where it is heated. A portion of the crude natural gas feed may be bypassed around the heat exchanger 43 in line 44 and then mixed with the heated gas. A valve 45 controls the bypass. Make-up air may be added into the bypass stream in line 46 using fan 47. At start up, methane may be added in line 58 having been passed through the start-up burner 51. It is also possible to add methane via line 58 which bypasses the start-up burner during normal operation.

(13) The stream is then fed in line 48 to the reactor 49, where reaction occurs. The gas stream is then removed from the reactor 49 in line 52 and passed through the heat exchanger 43, where it is cooled while heating the incoming gas. The cool gas is then released in line 53.

(14) A controller 54 monitors the temperatures T.sub.1 and T.sub.2 measured in detectors 55 and 56 and exit methane analyser 57. The controller then adjusts the amount of waste gas bypassing the heat exchanger 43 using valve 45 and the amount of make-up air added in line 46.

(15) FIG. 6 shows the schematic illustration of FIG. 5 in combination with the carbon dioxide removal plant of FIG. 2. A crude natural gas feed stream is fed in line 60 to a first membrane separator 61. In one arrangement, the stream may have a concentration of 10% carbon dioxide. A retentate comprising a purified natural gas stream having a lower carbon dioxide concentration is recovered in line 62. This stream may have a carbon dioxide concentration reduced to about 2%.

(16) The permeate from the first membrane separator 61 will have a higher carbon dioxide concentration than in the feed to the membrane, generally about 44%. This permeate stream 63 is passed to a second membrane separator 65 via a compressor 64. Further carbon dioxide is removed from the stream 63 in the second membrane separator 65.

(17) The retentate from the second membrane separator 65 is fed back to the first membrane separator 61. The permeate 67 from the second membrane separator 65 contains a high concentration of carbon dioxide, generally about 86%. This permeate is then passed in line 41 to the process illustrated in FIG. 5.

(18) As illustrated in FIG. 6 optionally the stream recovered from the reactor 49 in line 52 is passed to a steam to power generator 68 before being passed to the heat exchanger 43.

(19) An alternative arrangement is illustrated in FIG. 7. In this arrangement, the purified gas stream in line 70 is passed from the membrane separation stage, not show, to a first heat exchanger 72. During start-up, the purified gas stream may be passed through a start-up heater such as a start-up burner 71. In the first heat exchanger 72 the stream is heated to a suitable inlet temperature before it is passed to a first sub-reactor 73 which comprises the catalyst. where a portion of the methane is oxidised to produce carbon dioxide and water. This oxidation increases the temperature of the stream. Make-up air may be fed in line 80 into the stream before it is passed to the first heat exchanger 72. The amount of make-up air added may be varied to control the degree of oxidation which takes places in the first sub-reactor.

(20) The stream recovered from the first sub-reactor 73 is passed to a second heat exchanger 74 where it is cooled before being passed to a second sub-reactor 75. Make-up air may be added to the stream in line 80 before it is added to the second sub-reactor 75. Further oxidation of the methane in the gas stream occurs in the second reactor 75. Again, the degree of oxidation may be controlled by the amount of make-up air added.

(21) The exhaust gas from the second reactor 75 are recovered in line 76 and passed through the first heat exchanger 72 where it is cooled while heating the stream fed in line 70 to the first sub-reactor 73.

(22) The cooled exhaust gas is then passed in line 77 to the second heat exchanger 74, where they are heated by the hot gases exiting the first sub-reactor 73. After being heated, the gas is passed to a third sub-reactor 79 where further oxidation occurs. Make-up air may be added to the third sub-reactor 79 to cool and dilute the hot gas from the second heat exchanger 75.

(23) The exhaust gas from the third sub-reactor 79 are recovered in line 81 and are then passed to an expansion turbine 82 before being vented in line 83. Power generated in the expansion turbine may be utilised in the process of the invention.