A METHOD FOR REMOVING CO2 FROM A CO2-CONTAINING STREAM

Abstract

The present invention relates to a method for removing carbon dioxide (CO.sub.2) from a CO.sub.2-containing stream, the method at least comprising the steps of: a) providing a CO.sub.2-containing stream (10), preferably air wherein the CO.sub.2-containing stream (10) has a CO.sub.2 content in the range of from 10 to 1000 ppmv, preferably from 100 to 1000 ppmv; b) removing CO.sub.2 from the CO.sub.2-containing stream (10) provided in step a) in a first CO.sub.2 removal unit (2), thereby obtaining a first CO.sub.2-enriched stream (30) and a first CO.sub.2-depleted stream (20); c) liquefying the first CO.sub.2-enriched stream (30) obtained in step b) in a liquefaction unit (3); d) removing from the liquefaction unit (3) at least a liquefied CO.sub.2 stream (40) and a gaseous stream (15) containing at least nitrogen [N.sub.2 (g)], oxygen [O.sub.2 (g)] and CO.sub.2 (g).

Claims

1. A method for removing carbon dioxide (CO2) from a CO2-containing stream, the method at least comprising the steps of: a) providing a CO2-containing stream (10), preferably air, wherein the CO2-containing stream (10) has a CO2 content in the range of from 10 to 1000 ppmv, preferably from 100 to 1000 ppmv; b) removing CO2 from the CO2-containing stream (10) provided in step a) in a first CO2-removal unit (2), thereby obtaining a first CO2-enriched stream (30) and a first CO2-depleted stream (20); c) liquefying the first CO2-enriched stream (30) obtained in step b) in a liquefaction unit (3); d) removing from the liquefaction unit (3) at least a liquefied CO2 stream (40) and a gaseous stream (15) containing at least nitrogen [N2 (g)], oxygen [O2 (g)] and CO2 (g).

2. The method according to claim 1, wherein the first CO2-enriched stream (30) obtained in step b) has a CO2 content (excluding water) of at least 60 vol. %, preferably at least 80 vol. %, more preferably at least 90 vol. %.

3. The method according to claim 1, wherein the first CO2-enriched stream (30) obtained in step b) has a pressure of 0.5 to 1.5 bara, preferably from 0.9 to 1.1 bara.

4. The method according to claim 1, wherein at least a part of the gaseous stream removed (15) in step d) is combined with the CO2-containing (10) stream provided in step a).

5. The method according to claim 1, wherein at least a part of the gaseous stream (15) removed in step d) is separated in a second CO2 removal unit (7), thereby obtaining a second CO2-enriched stream (25) and a second CO2-depleted stream (80), wherein the second CO2-enriched stream (25) is combined with the first CO2-enriched stream (30).

6. The method according to claim 6, wherein the second CO2-enriched stream (25) has a CO2 content (excluding water) of at least 90 vol. %, preferably at least 95 vol. %.

7. The method according to claim 1, wherein at least a part of the gaseous stream (15) removed in step d) is used as a sweep gas in the CO2 removal unit (2) of step b).

8. The method according to claim 1, wherein the liquefied CO2 stream (40) removed in step d) is used in a conversion process, sequestration or transport, after optional storage and pumping.

Description

[0034] Hereinafter the present invention will be further illustrated by the following non-limiting drawings. Herein shows:

[0035] FIG. 1 schematically a flow scheme of a first embodiment of a method for removing CO.sub.2 from a CO.sub.2-containing stream according to the present invention; and

[0036] FIG. 2 schematically a flow scheme of a second embodiment of a method for removing CO.sub.2 from a CO.sub.2-containing stream according to the present invention; and

[0037] FIG. 3 schematically a flow scheme of a third embodiment of a method for removing CO.sub.2 from a CO.sub.2-containing stream according to the present invention.

[0038] For the purpose of this description, same reference numbers refer to same or similar components.

[0039] The flow scheme of FIG. 1, generally referred to with reference number 1, comprises a first CO.sub.2-removal unit 2, a CO.sub.2 liquefaction unit 3, a liquid CO.sub.2 storage unit 4, a CO.sub.2 pump 5, and a CO.sub.2 conversion unit 6.

[0040] During use of the flow scheme of FIG. 1, a CO.sub.2-containing stream 10 (preferably air) is provided.

[0041] In a first CO.sub.2-removal unit 2, e.g. in the form of a CO.sub.2 adsorption unit, CO.sub.2 is removed from the CO.sub.2-containing stream 10, thereby obtaining a first CO.sub.2-enriched stream 30 and a first CO.sub.2-depleted stream 20. In the embodiment of FIG. 1, steam stream 70 is used as a sweep gas (to desorb CO.sub.2 as adsorbed in the CO.sub.2 adsorption unit 2).

[0042] The first CO.sub.2-enriched stream 30 is then liquefied in the liquefaction unit 3. From the liquefaction unit 3 at least a liquefied CO.sub.2 stream 40 and a gaseous stream 15 are removed. The gaseous stream 15 contains at least N.sub.2 (g), oxygen O.sub.2 (g) and CO.sub.2 (g).

[0043] The liquefied CO.sub.2 stream 40 is used in a CO.sub.2 conversion process performed in the CO.sub.2 conversion unit 6, after optional storage in liquid CO.sub.2 storage unit 4 (from which it is pumped as liquid stream 50,60 by CO.sub.2 pump 5 to the CO.sub.2 conversion unit 6). The CO.sub.2 conversion process in the CO.sub.2 conversion unit 6 along with any other reactants (shown as stream 85) as required by the conversion process) results in a products stream 90.

[0044] Instead of converting the liquefied CO.sub.2 stream 40, it can also be used for sequestration or transport (e.g. by pipeline or ship).

[0045] As shown in the embodiment of FIG. 1, at least a part of the gaseous stream 15 is combined with the CO.sub.2-containing stream 10. This surprisingly allows that a higher CO.sub.2 recovery from the CO.sub.2-containing stream 10 is achieved, without adding complexity to the system.

[0046] FIG. 2 and FIG. 3 show schematically alternative embodiments of methods for removing CO.sub.2 from a CO.sub.2-containing stream according to the present invention. It goes without saying that the embodiments of FIGS. 1-3 may be combined in any way.

[0047] In the embodiment of FIG. 2, a second CO.sub.2 removal unit 7 is present. At least a part of the gaseous stream 15 is separated in the second CO.sub.2 removal unit 7, thereby obtaining a second CO.sub.2-enriched stream 25 and a second CO.sub.2-depleted stream 80, wherein the second CO.sub.2-enriched stream is combined with the first CO.sub.2-enriched stream 30.

[0048] In the embodiment of FIG. 3, at least a part of the gaseous stream 15 is used as a sweep gas in the CO.sub.2 removal unit 2.

EXAMPLES

Example 1

[0049] The flow scheme of FIG. 1 was used for illustrating an exemplary method according to the present invention. The compositions and conditions of the streams in the various flow lines are provided in Table 1 below.

[0050] The values in Table 1 were calculated using a model generated with commercially available UniSim software, whilst using standard thermodynamic fluid packages with settings such that CO.sub.2 removal processes and CO.sub.2 liquefaction processes are simulated.

[0051] The obtained CO.sub.2 content of the first CO.sub.2-enriched stream 30 was 90 vol. % and of the liquefied CO.sub.2 stream 40 was 99.7 vol. %.

[0052] The total CO.sub.2 recovery was defined as the ratio of moles of CO.sub.2 in the liquefied CO.sub.2 stream 40 to the moles of CO.sub.2 in the CO.sub.2-containing stream 10; a total CO.sub.2 recovery of 58% was obtained.

TABLE-US-00001 TABLE 1 stream 10 15 20 30 40 50 60 T [? C.] 20 40 20 20 ?31 ?31 ?31 p [bara] 1.0 15 1.0 1.0 15 15 60 Molar flow 4.3 ? 79 4.3 ? 179 100 100 100 [kg .Math. mol/hr] 10.sup.5 10.sup.5 CO.sub.2 [vol. %] 0.04 77.9 0.02 90.0 99.7 99.7 99.7 O.sub.2 [vol. %] 20.64 6.5 20.64 3.0 0.2 0.2 0.2 N.sub.2 [vol. %] 76.92 15.6 76.92 7.0 0.1 0.1 0.1 H.sub.2O [vol. %] 1.48 1.48 Ar [vol. %] 0.92 0.92

Example 2 (Comparative)

[0053] For comparison with FIG. 1, and using the same UniSim software, the method of FIG. 1 but without recycling stream 15 to stream 10 was simulated. The compositions and conditions of the streams in the various flow lines are provided in Table 2 below. The total CO.sub.2 recovery in this case was 42%.

TABLE-US-00002 TABLE 2 stream 10 15 20 30 40 50 60 T [? C.] 20 40 20 20 ?31 ?31 ?31 p [bara] 1.0 15 1.0 1.0 15 15 60 Molar flow 5.9 ? 79 5.9 ? 179 100 100 100 [kg .Math. mol/hr] 10.sup.5 10.sup.5 CO.sub.2 [vol. %] 0.04 77.9 0.01 90.0 99.7 99.7 99.7 O.sub.2 [vol. %] 20.64 6.5 20.64 3.0 0.2 0.2 0.2 N.sub.2 [vol. %] 76.92 15.6 76.94 7.0 0.1 0.1 0.1 H.sub.2O [vol. %] 1.48 1.48 Ar [vol. %] 0.92 0.92

[0054] As can be seen from Tables 1 and 2, the exemplary method of FIG. 1 (with recycle) has a higher CO.sub.2 recovery (58%) than the one without recycle (viz. 42%). Comparison of flow rates of stream 10 in Tables 1 and 2 indicate that recycling the gaseous stream 15 (containing at least N.sub.2 (g), O.sub.2 (g) and CO.sub.2 (g)) as removed from the liquefaction unit 3 to the feed stream 10 reduces the total flow of the CO.sub.2-containing stream 10, thereby increasing efficiency of the present invention.

DISCUSSION

[0055] As can be seen from the above Figures and Examples, the method according to the present invention allows for a surprisingly simple and effective way of increasing CO.sub.2 recovery and purity from a CO.sub.2-containing stream, without adding complexity to the system. According to the present invention, CO.sub.2 concentrations of at least 90 vol. % can be achieved, and even as high as above 99 vol. %.

[0056] The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention.