Process for separating CO2 from CO2 containing gas stream

Abstract

The invention relates to gas separation, particularly to a process for separating CO.sub.2 from a CO.sub.2 containing gas stream with varying CO.sub.2 concentration using a membrane-based separation system. The process for separating CO.sub.2 comprises: (i) feeding the CO.sub.2 containing gas stream into a membrane-based separation system comprising one or more membrane stages in-line, each membrane stage producing a CO.sub.2-depleted retentate stream and a rich CO.sub.2 permeate stream, and (ii) recycling a part of the CO.sub.2-rich permeate stream from at least one membrane stage as a recycled stream, into a preceding feed stream of one or more membrane stages in the membrane-based separation system to adjust the CO.sub.2 concentration of the feed stream. The process could stabilize the CO.sub.2 concentration in the feed steam by recirculation of a fraction of the high purity CO.sub.2 permeate to the feed of the membrane system, which would result in a lower incremental auxiliary load than other options such as compression of the entire gas stream.

Claims

1. A process for separating CO.sub.2 from a CO.sub.2 containing gas stream with varying CO.sub.2 concentration comprising: (i) feeding the CO.sub.2 containing gas stream with varying CO.sub.2 concentration into a membrane-based separation system comprising one or more membrane stages in-line, each membrane stage producing a CO.sub.2-depleted retentate stream and a CO.sub.2-rich permeate stream, and (ii) recycling a part of the CO.sub.2-rich permeate stream of one or more membrane stages as a recycled stream, into a preceding feed stream of one or more membrane stages in the membrane-based separation system to adjust the CO.sub.2 concentration of the feed stream.

2. The process according to claim 1 wherein the flow rate of the recycled stream is adjusted to keep the CO.sub.2 concentration in the feed stream to one or more of stages of the membrane-based separation system steady.

3. The process according to claim 1 wherein the CO.sub.2 containing gas stream is a flue gas stream produced from power generation using fossil fuels.

4. The process according to claim 3, wherein the fossil fuel is coal.

5. The process according to claim 1 wherein using a part of the last CO.sub.2-rich permeate stream from the last membrane stage as the recycled stream.

6. The process according to claim 1 wherein recycling at least a part of the recycled stream into the first feed stream of the first membrane stage.

7. The process according to claim 6 wherein the CO.sub.2 concentration of the first feed stream is adjusted to 12 mol % or more by the recycled stream.

8. The process according to claim 7 wherein the CO.sub.2 concentration of the first feed stream is adjusted to 12 mol %-15 mol % by the recycled stream.

9. The process according to claim 1 wherein flow rate of the recycled stream is adjusted based on direct measurements of the CO.sub.2 concentration of the feed stream.

10. The process according to claim 1 where the flow rate of the recycled stream is adjusted based on upstream parameters in accordance with which the CO.sub.2 containing gas stream is generated including: operational parameters of the plant where the CO.sub.2 containing gas stream is generated, or operational parameters related to upstream emissions controls of the CO.sub.2 containing gas stream.

11. The process according to claim 1 wherein flow rate of the recycled stream is 70 mol % or less based on total content of the last CO.sub.2-rich permeate stream.

12. The process according to claim 11 wherein flow rate of the recycled stream is 25 mol %-70 mol % based on total content of the last CO.sub.2-rich permeate stream.

13. The process according to claim 12 wherein flow rate of the recycled stream is 30 mol %-65 mol % based on total content of the last CO.sub.2-rich permeate stream.

14. The process according to claim 1 wherein the recycled stream enables the total content of the feed stream to be increased by 15 mol % or less.

15. The process according to claim 1 wherein overall CO.sub.2 recovery rate is 50% or more.

16. The process according to claim 15 wherein the overall CO.sub.2 recovery rate is 60%-90%.

17. The process according to claim 1 wherein the concentration of CO.sub.2 in the last permeate is 60 mol %/o or above.

18. The process according to claim 17 wherein the concentration of CO.sub.2 in the last permeate is 90 mol % or above.

19. The process according to claim 17 wherein the concentration of CO.sub.2 in the last permeate stream is 95 mol % or above.

20. A membrane-based separation system comprising a membrane-based separation device comprising one or more membrane stages in-line, wherein at least one permeate line of the membrane stages is a multi-way line with at least one line as a recycle line connected to a preceding feed line of one or more membrane stages such that a part of rich CO.sub.2 permeate stream is recycled into the feed stream to adjusting the CO.sub.2 concentration of the feed stream.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0014] The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.

[0015] FIG. 1 shows a membrane-based separation system according to a preferred embodiment of the present invention.

[0016] FIG. 2 shows a membrane-based separation system according to another preferred embodiment of the present invention.

[0017] FIG. 3 shows the relationship between M1 and M2 needed to produce a final product stream with 95% CO.sub.2 concentration.

REFERENCES IN THE DRAWINGS

[0018] 1: CO.sub.2 containing gas stream; 2: total CO.sub.2-depleted retentate line; 3: permeate line; 4: total recycle line; 5: CO.sub.2-rich gas product; 6: feed line; 7: membrane-based separation device; M: membrane stage; 2: the first retentate line; 2: the second retentate line; 3: the first permeate line; 3: the second permeate line; 4: the first recycle line; 4: the second recycle line; 6: the first feed line; 6: the second feed line; M1: the first membrane stage; M2: the second membrane stage.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Hereunder some embodiments of the present invention will be detailed. It should be appreciated that the embodiments described here are only provided to describe and explain the present invention, but shall not be deemed as constituting any limitation to the present invention.

[0020] The end points and any value in the ranges disclosed in the present invention are not limited to the exact ranges or values. Instead, those ranges or values shall be comprehended as encompassing values that are close to those ranges or values. For numeric ranges, the end points of the ranges, the end points of the ranges and the discrete point values, and the discrete point values can be combined to obtain one or more new numeric ranges, which shall be deemed as having been disclosed specifically in this document.

[0021] The present invention provides a process for separating CO.sub.2 from a CO.sub.2 containing gas stream with varying CO.sub.2 concentration comprising: [0022] (i) feeding the CO.sub.2 containing gas stream with varying CO.sub.2 concentration into a membrane-based separation system comprising one or more membrane stages in-line, each membrane stage would obtain a CO.sub.2-depleted retentate stream and a CO.sub.2-rich permeate stream, and [0023] (ii) recycling a part of the CO.sub.2-rich permeate stream from at least one membrane stage as a recycled stream, into preceding feed stream of one or more membrane stages in the membrane-based separation system to adjust the CO.sub.2 concentration of the feed stream.

[0024] According to the present invention, although the inventors do not wish the present invention to be bound by any theory, the CO.sub.2 concentration could be kept substantially steady by recirculation of a fraction of the high purity CO.sub.2 permeate to the feed of the membrane system, which could be verified using a mass balance of the gas stream flows. In this case, advantageously, the flow rate of the recycled stream is adjusted to keep the CO.sub.2 concentration in the feed stream to one or more of stages of the membrane-based separation system steady, preferably to keep the CO.sub.2 concentration in the feed stream to each stage of the membrane-based separation system steady.

[0025] According to the present invention, the membrane-based separation system comprises one or more membrane stages in-line. Such a system could comprise a single membrane stage (shown as in FIG. 1), two membrane stages (shown as in FIG. 2), or three or more membrane stages (not shown). The recycled stream may be a part of the CO.sub.2-rich permeate stream from at least one membrane stage, for example, in the case of a two stage membrane-based separation system, the recycled stream may be a part of CO.sub.2-rich permeate stream from the first membrane stage or a part of CO.sub.2-rich permeate stream from the second membrane stage, or both. The recycled stream from the first membrane stage and the recycled stream from the second membrane stage may be used separately, or in combination. However, it should be noted the the recycled stream from the first membrane stage has the same composition as the second feed stream of the second membrane stage (when there is no other recycled gas compensation) since they are both from the first CO.sub.2-rich permeate stream of the first membrane stage. Moreover, the CO.sub.2 concentration of the later permeate stream is higher than that of the preceding permeate stream, so the recycled stream from the later permeate has a higher CO.sub.2 concentration than that of the preceding permeate stream. Typically, the best embodiment is to use a part of the last CO.sub.2-rich permeate stream from the last membrane stage as the recycled stream, shown as in FIGS. 1 and 2. Use of part of the last CO.sub.2-rich permeate stream from the last membrane stage as the recycled stream simplifies the overall configuration.

[0026] In the present invention, terms such as preceding, later, first, last etc. depend on the position of the components. For example recycling a recycled stream into a preceding feed stream may mean that the inlet of the indicated permeate stream may be positioned in front of the formation site of the recycled stream (the formation site of the recycled stream can be the branch site on the permeate line to form the recycled stream).

[0027] According to the present invention, although the recycled stream can be introduced into any feed stream of the membrane stages if there is a fluctuation in the target feed stream, the preferred process is to recycle at least a part of the recycled stream into the first feed stream of the first membrane stage, more preferably only to recycle the recycled stream into the first feed stream of the first membrane stage, most preferably only to recycle the recycled stream from the last permeate stream of the last membrane stage into the first feed stream of the first membrane stage.

[0028] According to the present invention, the CO.sub.2 containing gas stream may be any type of CO.sub.2 containing gas, example of the CO.sub.2 containing gas stream may be CO.sub.2 containing industrial exhaust gas, such as a flue gas stream produced from power generation using fossil fuels (preferably the fossil fuel is coal), exhaust streams from industrial boilers, exhaust streams from coal-to-chemical processes, gas streams produced from natural gas recovery, etc., the present process is especially suitable for separating CO.sub.2 from a flue gas stream.

[0029] According to the present invention, the process of the present invention has no special limit on the CO.sub.2 concentration of the feed stream, the adjusted CO.sub.2 concentration of the feed stream may be reached any desired values. In a preferred embodiment, especially for treating flue gas streams from power generation using coal, the CO.sub.2 concentration of the first feed stream is adjusted to 12 mol % or more, more preferably to 12 mol %-15 mol %, most preferably to 13 mol %-14 mol % by the recycled stream.

[0030] According to the present invention, one significant aspect of the inventive concept is to use a recycle loop to dynamically compensate for changes in the CO.sub.2 concentration of the feed stream. In other words, the recycle rate is higher when the CO.sub.2 concentration is below a target level and the recycle rate is lowered when the CO.sub.2 concentration is closer to the target level. The primary focus of previous efforts has been to raise the overall concentration, without regard to transients in the CO.sub.2 concentration over time. Preferably, the flow rate of the recycled CO.sub.2-rich permeate stream is adjusted based on direct measurements of the CO.sub.2 concentration of the feed stream, or based on upstream parameters according to which the CO.sub.2 containing gas stream is generated including: operational parameters of the plant where the CO.sub.2 containing gas stream is generated (such as fuel feed rate, air flow rate, electrical output etc.), or operational parameters related to upstream emissions controls of the CO.sub.2 containing gas stream (such as CO.sub.2 components, chemical feed rates, water content etc.).

[0031] In a preferred embodiment of the present invention, the flow rate of the recycled stream is 70 mol % or less, preferably 25 mol %-70 mol %, more preferably 30 mol %-65 mol % of the total content of the last CO.sub.2-rich permeate stream.

[0032] According to the present invention, it is clear that the present process is to use permeate, rather than retentate stream, in the recycle loop. Previously known methods of recycle focus on recirculation of the retentate. The benefits of recycling permeate are not obvious when considering only the auxiliary loads, since recycling would increase the energy required per net unit of CO.sub.2 separated. However, the prior art does not consider the benefits in operational simplicity enabled by stabilizing the feed concentration in this manner. Moreover, the use of a permeate recycle to produce a steady concentration feed offers the potential for energy savings relative to alternate approaches of compensating for lower concentrations such as compression, since compression-based solutions would require compressing the entire gas stream. Finally, the use of the permeate means that the total volume of the feed stream is not significantly increased, such as the recycled stream enables the total content of the feed stream to be increased by 15 mol % or less, preferably by 10 mol % or less, more preferably by 3 mol %-8 mol %.

[0033] Together, these aspects of the present invention allow the possibility of achieving the performance benefits with a fraction of the incremental energy load of known approaches.

[0034] The process according to the present invention has no special limit to the membrane-based separation system, any common types of the membrane-based separation system capable of separating CO.sub.2 in the art can be used in the present process unless the membrane-based separation system comprises a recyle loop as described above. The membrane may be employed in plate and frame form, or may comprise spiral wound membranes, tubular membranes, hollow fiber membranes, or the like. It will be appreciated that, when membranes are used in tubular or hollow fiber form, a plurality of such membranes can conventionally be arranged in parallel bundle form. In such embodiments, the feed gas stream can be brought into contact with either the outer or shell side, or the inner or tube side of the membrane particles. Typical of the types of membrane materials utilized in membrane separation processes include organic polymeric materials or such organic polymeric materials mixed with inorganic materials such as fillers, reinforcements and the like. Organic polymers that may be considered for various practical commercial operations include such materials as polysulfones; polystyrenes; including such styrene-containing polymers as acrylonitrile, styrene copolymers, styrene-butadiene and styrene-vinylbenzyl halide copolymers; cellulosic polymers, such as cellulose acetate, cellulose acetate-butyrate, methyl or ethyl cellulose: polyamides and polyimides; polycarbonates; polyurethanes, polyesters, including polyacrylates, polyethylene: polypropylene; polyvinyl pyridines, and the like. Such polymers may be either substituted or unsubstituted, with typical substituents of such substituted polymers including halogens, such as chlorine, fluorine and bromine; hydroxyl groups: lower alkyl groups; lower alkoxy groups; monocyclic aryl; lower acyl groups, etc.

[0035] The process according to the present invention has no special limit to the CO.sub.2 recovery rate of the membrane system; the recovery rate can be any desired value. Preferably, the overall CO.sub.2 recovery rate is 50% or more, more preferably 60%-90%, such as 60%, 65%, 70%, 75%, 80%, 85%, or 90%. In addition, the concentration of CO.sub.2 in the permeate stream obtained by the present process may be any desired value. Preferably, the concentration of CO.sub.2 in the last permeate stream is 60 mol % or above, more preferably 80 mol % or above, further more preferably 90 mol % or above, most preferably 95 mol %/o or above. It would be appreciated that the overall CO.sub.2 recovery rate and CO.sub.2 concentration in the permeate stream obtained in the present process is comparable to processes known in the prior art.

[0036] In another aspect, the present invention provides a membrane-based separation device comprising one or more membrane stages in-line, wherein at least one permeate line of the membrane stages is a multi-way line with at least one line as a recycle line connected to preceding feed line of one or more membrane stages such that a part of CO.sub.2-rich permeate stream is recycled into the feed stream to adjust the CO.sub.2 concentration of the feed stream.

[0037] The membrane-based separation device herein can be used in the abovementioned process, thus the corresponding description would be included in the membrane-based separation device by reference.

[0038] As depicted above, the membrane-based separation device may just comprise a single membrane stage (shown as in FIG. 1), two membrane stages (shown as in FIG. 2), or three or more membrane stages (not shown). The device could use a normal device but with a novel recycle loop. The recycle loop can be formed in any structure as long as it provides a recycle stream derived from a permeate stream into a preceding feed stream, so to establish the present process.

[0039] In a preferred embodiment shown in FIG. 1, the membrane-based separation system comprises a membrane-based separation device 7 with one membrane stage M, a feed line 6 connected with a CO.sub.2 containing gas stream inlet of the membrane-based separation device 7, a retentate line 2 connected with a CO.sub.2-depleted retentate stream outlet of the membrane-based separation device 7, and a permeate line 3 connected with a CO.sub.2-rich permeate stream outlet of the membrane-based separation device 7, while the permeate line 3 is a two-way line with one line as a recycle line 4 connected to the feeding line 6 such that a part of the CO.sub.2-rich permeate stream as recycled stream is recycled into the feed stream to adjusting the CO.sub.2 concentration of the feed stream. By using the system shown in FIG. 1, the CO.sub.2 containing gas stream 1 would combine with the recycled stream from recycle line 4 to form the feed stream, and the combination of the CO.sub.2 containing gas stream 1 and the recycled stream that is the feed stream would be fed into the membrane stage M through feed line 6. The CO.sub.2 in the feed stream would selectively permeate through the membrane disposed in the membrane stage M to form an CO.sub.2-rich gas. The CO.sub.2-rich gas would pass through a CO.sub.2-rich permeate stream outlet of the membrane-based separation device 7 into the permeate line 3 to obtain a permeate stream. Part of the permeate stream is diverted into recycle line 4 to be used as the recycle stream, while the rest of the permeate stream would be removed from the membrane-based separation system to be a CO.sub.2-rich gas product 5.

[0040] In another preferred embodiment shown in FIG. 2, the membrane-based separation system comprises a membrane-based separation device 7 with two membrane stages, i.e. a first membrane stage M1 and a second membrane stage M2, a first feed line 6 connected with a CO.sub.2 containing gas stream inlet of the first membrane stage M1, a first retentate line 2 connected with a CO.sub.2-depleted retentate stream outlet of the first membrane stage M1, a first permeate line 3 connected with a CO.sub.2-rich permeate stream outlet of the first membrane stage M1, the downstream line of line 3 is defined as a second feed line 6 connected with a gas inlet of the second membrane stage M2, a second retentate line 2 connected with a CO.sub.2-depleted retentate stream outlet of the second membrane stage M2 and a second permeate line 3 connected with a CO.sub.2-rich permeate stream outlet of the second membrane stage M2, wherein the second permeate line 3 is a two-way line with one line as a total recycle line 4 and another line to output CO.sub.2-rich gas product 5. Advantageously, the total recycle line 4 is divided into two lines, one is a first recycle line 4 connected with the first feed line 6 such that a part of the CO.sub.2-rich permeate stream provided by the second membrane stage M2 is recycled into the first feed stream (initial feed steam) to adjust the CO.sub.2 concentration of the first feed stream, and another one is a second recycle line 4 connected with the second feed line 6 such that a part of the CO.sub.2-rich permeate stream provided by the second membrane stage M2 is recycled into the feed stream of stage M2 to adjust the CO.sub.2 concentration of the this feed stream. Otherwise, the first retentate line 2 and second retentate line 2 would joined together to form a total retentate line 2 through which the total CO.sub.2-depleted retentate stream is output the membrane-based separation system. By using the system shown in FIG. 2, the CO.sub.2 containing gas stream 1 would combine with a recycled stream from the first recycle line 4 to form a first feed stream (initial feed stream), and the combination of the CO.sub.2 containing gas stream 1 and the recycled stream would be fed into the first membrane stage M1 through the first feed line 6, then CO.sub.2 in the feed stream would pass through the membrane disposed in the stage M1 to form a CO.sub.2-rich gas, the retentate stream would flow into the first retentate line 2, while the CO.sub.2-rich gas would pass through an CO-rich permeate stream outlet of the stage M1 into the first permeate line 3 to obtain a permeate stream, the permeate stream would combine with a recycle stream from the second recycle line 4 to form a feed stream into the second membrane stage M2 through the second feed line 6 which actually is the downstream line of the line 3, then CO.sub.2 in the feed stream would selectively pass through the membrane disposed in the stage M2 to form a CO.sub.2-rich gas, the retentate stream would flow into the second retentate line 2 to combine with the retentate stream from line 2, and the combined stream would be output through the line 2, while the CO.sub.2-rich gas would pass through an CO.sub.2-rich permeate stream outlet of the stage M2 into the second permeate line 3 to obtain a permeate stream, a part of the permeate stream would flow into recycle line 4 used as the recycle stream separately into recycle line 4 and line 4, the rest of the permeate stream would be removed from the membrane-based separation system to be a CO.sub.2-rich gas product 5.

[0041] Hereunder the present invention will be detailed in embodiments.

Example 1

[0042] A preferred embodiment using a single stage membrane system is shown in FIG. 1. The performance is summarized in Table 1. Calculations were performed to determine the mass flows in each stream, the overall CO.sub.2 recovery in the membrane stage, the overall CO.sub.2 capture rate, the recycle rate required to stabilize the feed flow at the set point (in this case 13.5 mol %), the increase in flow rate to the membrane stage, and the required CO.sub.2/N.sub.2 selectivity to produce a 95 mol % CO.sub.2 permeate stream. To simplify the calculation, the feed stream is assumed to be a mixture of CO.sub.2 and N.sub.2 at temperature and pressure consistent with a power plant flue gas stream.

[0043] It should be noted, in the following description, stream 2 to stream 4 and stream 6 are corresponding to the streams in the relevant lines shown in FIG. 1 such as stream 2 is the stream in line 2.

[0044] Inlet feed CO.sub.2 concentration refers to the CO.sub.2 concentration of the feed stream without combining with a recycle stream.

[0045] CO.sub.2 recovery by the membrane stage refers to percentage ratio of the CO.sub.2 amount passing through the membrane disposed in the membrane stage to the total CO.sub.2 amount in the feed stream into the membrane stage.

[0046] Overall CO.sub.2 capture rate means percentage ratio of the CO.sub.2 amount in the rich CO.sub.2 product (product 5) to the CO.sub.2 amount of the CO.sub.2 containing gas stream (stream 1).

[0047] Recycle rate required refers to percentage ratio of the flow rate of the recycle stream to the flow rate of the relevant permeate stream.

[0048] Increase in flow rate of feed refers to percentage ratio of flow rate of the recycle stream combining with the feed stream to the flow rate of the feed stream (stream 1) without combining with a recycle stream.

TABLE-US-00001 TABLE 1 Summary table for Example 1. Increase in Inlet feed CO.sub.2 CO.sub.2 recovery by Overall CO.sub.2 Recycle rate flow rate of Required concentration the membrane stage capture rate required feed CO.sub.2/N.sub.2 [% mol] [%] [%] [%] [%] selectivity* Case 1A 13.5%.sup. 90% 90% 0% 0% 121 10% 90% 90% 0% 0% 170 7% 90% 90% 0% 0% 250 Case 1B 10% 70% 70% 0% 0% 170 7% 50% 50% 0% 0% 250 Case 2 10% 93% 90% 31% 4% 122 7% 95% 90% 54% 8% 122 Case 3 10% 90% 86% 31% 4% 122 7% 90% 79% 59% 8% 120 Case 4 10% 75% 65% 38% 4% 122 7% 83% 65% 62% 8% 122 *For 95 mol % purity CO.sub.2 product

[0049] Case 1A is a baseline configuration for a coal-fired power plant producing a constant volume of flue gas with three levels of CO.sub.2. The flue gas is sent to a membrane system which is operated in a manner that captures 90% of the CO.sub.2. The CO.sub.2/N.sub.2 membrane selectivities required to achieve a 95% pure CO.sub.2 permeate stream are listed. Note that the membrane selectivity requirement needed to achieve a given purity level increases as the feed CO.sub.2 concentration decreases. This is a comparative baseline for assessing the performance of the other cases. The permeate is not recycled. The molar mass flow rates for the streams are listed in Table 2.

TABLE-US-00002 TABLE 2 CO.sub.2 N.sub.2 Flow concen- concen- [kg .Math. tration tration CO.sub.2 flow N.sub.2 flow Stream mol/hr] [% mol] [% mol] [kg .Math. mol/hr] [kg .Math. mol/hr] 13.5% feed 1 74091 13.5%.sup. 87% 10002 64089 2 64612 2% 98% 1000 63612 3 9479 95% 5% 9002 477 4 0 0% 0% 0 0 5 9479 95% 5% 9002 477 6 74091 14% 87% 10002 64089 10% feed 1 74091 10% 90% 7409 66682 2 67070 1% 99% 741 66329 3 7021 95% 5% 6668 353 4 0 0% 0% 0 0 5 7021 95% 5% 6668 353 6 74091 10% 90% 7409 66682 7% feed 1 74091 7% 93% 5186 68905 2 69175 1% 99% 519 68657 3 4916 95% 5% 4668 248 4 0 0% 0% 0 0 5 4916 95% 5% 4668 248 6 74091 7% 93% 5186 68905

[0050] Case 1B is a second comparative example for the same power plant configuration as Case 1A, producing a constant volume of flue gas with two reduced levels of CO.sub.2. The flue gas is sent to a membrane system which is operated at approximately the same conditions for all three levels of CO.sub.2 concentration. Under these conditions, the membrane system is expected to perform at lower total recovery for lower CO.sub.2 concentrations, due to the lower driving force for separation. Assuming the membrane system separation efficiency scales with the CO.sub.2 concentration, the effectiveness drops from 90% with 13.5% CO.sub.2 in the flue gas to about 70% and 50% recovery for flue gas with 10% and 7% CO.sub.2, respectively. In these cases, no additional measures are taken to improve the membrane-based separation system and the overall CO.sub.2 capture rate is reduced from 90% to 70% and 50%, respectively. As with Case 1A, the permeate is not recycled. The molar mass flow rates for the streams are listed in Table 3.

TABLE-US-00003 TABLE 3 CO.sub.2 N.sub.2 concen- concen- Flow tration tration CO.sub.2 flow N.sub.2 flow Stream [kg .Math. mol/hr] [% mol] [% mol] [kg .Math. mol/hr] [kg .Math. mol/hr] 10% feed 1 74091 10% 90% 7409 66682 2 68864 4% 96% 2445 66419 3 5227 95% 5% 4964 263 4 0 0% 0% 0 0 5 5227 95% 5% 4964 263 6 74091 10% 90% 7409 66682 7% feed 1 74091 7% 93% 5186 68905 2 71251 3% 97% 2489 68761 3 2840 95% 5% 2697 143 4 0 0% 0% 0 0 5 2840 95% 5% 2697 143 6 74091 7% 93% 5186 68905

[0051] Case 2 is a configuration where recycle streams are used for the cases of flue gas with 10% and 7% CO.sub.2 concentration to boost the effective concentration of CO.sub.2 in stream 6 to 13.5%. The membrane-based separation system still maintains an overall 90% recovery of CO.sub.2. The recycle rates needed to stabilize the inlet CO.sub.2 concentration of the feed stream 6 to membrane stage M when the CO.sub.2 concentration in overall inlet stream 1 drops to the 10% and 7% levels are about 31% and 54% of the permeate stream, respectively. Given the concentrated nature of the recycle stream, the flow of Stream 6 is only increased about 4% and 8%, respectively, relative to the flue gas feed Stream 1. The use of the recycle reduces the CO.sub.2/N.sub.2 selectivities required to achieve the target purity to levels comparable to that needed for a feed with 13.5% CO.sub.2 concentration. However, to maintain an overall CO.sub.2 capture rate of 90%, the membrane system must recover a higher fraction of the CO.sub.2 from stream 6 to close the mass balance. The required recovery of the CO.sub.2 by the membrane system increases from 90% to 93% and 95%, respectively. This indicates that the membrane stage M must be operated more aggressively to maintain the same overall CO.sub.2 capture rate. In membrane systems, higher stage cuts are generally accompanied by increased membrane area requirements and capital costs. In this case, a beneficial trade-off is a relaxed membrane selectivity requirement. The molar mass flow rates for the streams are listed in Table 4.

TABLE-US-00004 TABLE 4 CO.sub.2 N.sub.2 concen- concen- Flow tration tration CO.sub.2 flow N.sub.2 flow Stream [kg .Math. mol/hr] [% mol] [% mol] [kg .Math. mol/hr] [kg .Math. mol/hr] 10% feed 1 74091 10% 90% 7409 66682 2 67072 1% 99% 741 66331 3 10154 95% 5% 9646 508 4 3135 95% 5% 2978 157 5 7019 95% 5% 6668 351 6 77226 13% 87% 10387 66839 7% feed 1 74091 7% 93% 5186 68905 2 69178 1% 99% 519 68659 3 10774 95% 5% 10236 539 4 5861 95% 5% 5568 293 5 4913 95% 5% 4668 246 6 79952 13% 87% 10754 69198

[0052] Case 3 is the same configuration as Case 2. The difference is that the membrane system is operated to recover 90% of the CO.sub.2 fed to it. The overall CO.sub.2 capture rate is allowed to drop to satisfy the mass balance. As with Case 2, recycle streams are added to the cases of flue gas with 10% and 7% CO.sub.2 to boost the effective concentration of CO.sub.2 in stream 6 to 13.5%. The recycle rates needed to stabilize the inlet CO.sub.2 concentration of the feed stream 6 to membrane stage M when the CO.sub.2 concentration in overall inlet stream 1 drops to the 10% and 7% levels are about 31% and 59% of the permeate stream, respectively. Given the concentrated nature of the recycle stream, the flow of Stream 6 is only increased about 4% and 8% relative to the flue gas feed Stream 1. As with Case 2, the use of the recycle reduced the CO.sub.2/N.sub.2 selectivities required to achieve the target purity to levels comparable to that needed for a feed with 13.5% CO.sub.2 concentration. With a membrane system recovery of 90%, the total CO.sub.2 capture rates are 86 and 79%, respectively. In this scenario, lower levels of CO.sub.2 capture are attained. However, these levels of capture are higher than the membrane-based system recovery levels estimated in comparative example 1B. The molar mass flow rates for the streams are listed in Table 5.

TABLE-US-00005 TABLE 5 CO.sub.2 N.sub.2 concen- concen- Flow tration tration CO.sub.2 flow N.sub.2 flow Stream [kg .Math. mol/hr] [% mol] [% mol] [kg .Math. mol/hr] [kg .Math. mol/hr] 10% feed 1 74091 10% 90% 7409 66682 2 67385 1% 98% 741 66347 3 9840 95% 5% 9348 492 4 3135 95% 5% 2978 157 5 6706 95% 5% 6370 335 6 77226 13% 87% 10387 66839 7% feed 1 74091 7% 93% 5186 68905 2 69785 1% 98% 519 68689 3 10380 95% 5% 9861 519 4 6074 95% 5% 5770 304 5 4306 95% 5% 4091 215 6 80165 14% 86% 10956 69208

[0053] Case 4 is a configuration operated at an overall CO.sub.2 capture rate of 65%. This level of CO.sub.2 capture could allow a coal-fired power plant to approach a natural gas-fired combined cycle plant in terms of specific CO.sub.2 emissions per unit of energy generated. As with cases 2 and 3, recycle streams are added to the cases of flue gas with 10% and 7% CO.sub.2 to boost the effective concentration of CO.sub.2 in stream 6 to 13.5%. The recycle rates needed to stabilize the inlet CO.sub.2 concentration of the feed stream 6 to membrane stage M when the CO.sub.2 concentration in overall inlet stream 1 drops to the 10% and 7% levels are about 38% and 62% of the permeate stream, respectively. Given the concentrated nature of the recycle stream, the flow of Stream 6 is only increased about 4% and 8% relative to the flue gas feed Stream 1. The required membrane system recovery rate needed to achieve 65% capture rate is 75% and 83% recovery of CO.sub.2 from the feed streams containing 10% and 7% CO.sub.2, respectively. As with Cases 2 and 3, the use of the recycle reduced the CO.sub.2/N.sub.2 selectivities required to achieve the target purity to levels comparable to that needed for a feed with 13.5% CO.sub.2 concentration. The molar mass flow rates for the streams are listed in Table 6.

TABLE-US-00006 TABLE 6 CO.sub.2 N.sub.2 concen- concen- Flow tration tration CO.sub.2 flow N.sub.2 flow Stream [kg .Math. mol/hr] [% mol] [% mol] [kg .Math. mol/hr] [kg .Math. mol/hr] 10% feed 1 74091 10% 90% 7409 66682 2 69022 4% 96% 2593 66428 3 8204 95% 5% 7794 410 4 3135 95% 5% 2978 157 5 5069 95% 5% 4816 253 6 77226 13% 87% 10387 66839 7% feed 1 74091 7% 93% 5186 68905 2 70542 3% 97% 1815 68727 3 9410 95% 5% 8939 470 4 5861 95% 5% 5568 293 5 3549 95% 5% 3371 177 6 79952 13% 87% 10754 69198

Example 2

[0054] A second preferred embodiment is shown in FIG. 2. This embodiment illustrates the invention when applied to multi-stage systems. In this example, two stages are used. Cascades of membranes are known in the prior art for gas separations, but the use of a recycle of the permeate has not been described for the purpose of stabilizing the inlet feed concentration. The introduction of multiple stages opens up a degree of freedom in that the permeate can be recycled to any given stage, or a combination thereof. The following cases illustrates the trade-offs for a two stage idealized system, where the permeate is recycled to the first stage or the second stage. Mass balances are used to illustrate the key relationships, and one skilled in the art could use more detailed modeling to optimize configurations in this manner. The flow rates are normalized to 100 mol/s total feed to simplify the presentation of the results.

[0055] Calculations were performed to estimate the membrane stage recoveries and selectivities (designated M1 for stage 1 and M2 for stage 2) needed to meet overall separation system performance requirements. For cases 5 to 7 below, the targets are 65% recovery of the input CO.sub.2 at a purity of 95%. To simplify the calculation, the feed stream is assumed to be a mixture of CO.sub.2 and N.sub.2 at temperature and pressure consistent with a power plant flue gas stream. For illustrative purposes, the calculations were performed assuming feed CO.sub.2 concentrations of 14%, 10% and 8%. The results are summarized in Table 7.

[0056] It should be noted, in the following description, stream 2 to stream 4, stream 2 to stream 4, stream 6 and stream 6, and streams 2 and 4 are corresponding to the streams in the relevant lines shown in FIG. 2 such as stream 2 is the stream in line 2. The corresponding term definition can be the same as described in example 1.

TABLE-US-00007 TABLE 7 Overall Recycle Recycle CO.sub.2 CO.sub.2 CO.sub.2 rate Increase in Required rate Increase in Required Inlet feed CO.sub.2 recovery recovery capture required flow rate of CO.sub.2/N.sub.2 required flow rate of CO.sub.2/N.sub.2 concentration in M1 in M2 rate to M1 feed to M1 selectivity to M2 feed to M2 selectivity [% mol] [%] [%] [%] [%] [%] for M1 [%] [%] for M2 Case 5 14% 90% 72% 65% 0% 0% 10 0% 0% 11 10% 90% 72% 65% 0% 0% 10 0% 0% 16 8% 90% 72% 65% 0% 0% 10 0% 0% 20 Case 6 10% 90% 85% 65% 45% 6% 10 0% 0% 7

[0057] Case 5 is a baseline case for a two stage membrane system. The flue gas is sent to a membrane system which is operated in a manner that separates 90% of the CO.sub.2. No recycle is used. Table 8 shows stream compositions for a system fed by mass flows typical for a coal-fired power plant producing a constant volume of flue gas with three levels of CO.sub.2. In all three examples the system is operated with a M1 CO.sub.2 recovery of 90% and M2 CO.sub.2 recovery of 72%. This gives an overall CO.sub.2 capture rate of 65%. It should be noted that other combinations of M1 and M2 recovery can be used to achieve the same overall capture rate. The values selected for these examples are for illustrative purposes to show the degree of recycle required to satisfy the mass balances needed to achieve stabilization of the inlet CO.sub.2 concentration.

TABLE-US-00008 TABLE 8 CO.sub.2 N.sub.2 concen- concen- Flow .Math. total tration tration Flow CO.sub.2 Flow N.sub.2 Stream [mol/s] [% mol] [% mol] [mol/s] [mol/s] 14% feed 1.sup. 100 14% 86% 14 86 6 100 14% 86% 14 86 2 70 1% 99% 0.7 70 3 30 45% 55% 13 16 .sup.6 30 45% 55% 13 16 .sup.2 16 4% 96% 0.7 16 2.sup. 87 2% 98% 1.4 85 .sup.3 13 95% 5% 13 1 .sup.4 0 / / 0 0 5.sup. 13 95% 5% 13 1 4 0 / / 0 0 10% feed 1.sup. 100 10% 90% 10 90 6 100 10% 90% 10 90 2 83 1% 99% 1.0 82 3 17 53% 47% 9.0 8.1 .sup.6 17 53% 47% 9.0 8.1 .sup.2 10 25% 75% 2.5 7.7 2.sup. 93 4% 96% 3.5 90 .sup.3 7 95% 5% 6.5 0.4 .sup.4 0 / / 0 0 5.sup. 7 95% 5% 6.5 0.4 4 0 / / 0 0 8% feed 1.sup. 100 8% 92% 8.0 92 6 100 8% 92% 8.0 92 2 85 1% 99% 0.8 84 3 15 47% 53% 7.2 8.3 .sup.6 15 47% 53% 7.2 8.3 .sup.2 10 20% 80% 2.0 8.0 2.sup. 95 3% 97% 2.8 92 .sup.3 5 95% 5% 5.2 0.3 .sup.4 0 / / 0 0 5.sup. 5 95% 5% 5.2 0.3 4 0 / / 0 0

[0058] The CO.sub.2/N.sub.2 selectivities of the two stages are coupled. In general, there is a trade-off between M1 and M2, where the required M2 decreases as M1 increases. FIG. 3 shows the relationship between M1 and M2 needed to produce a final product stream with 95% CO.sub.2 concentration. As the CO.sub.2 concentration in the feed drops from 14% to 10% or to 8%, the membrane stage CO.sub.2/N.sub.2 selectivity requirements become more stringent. Table 8 lists one set of M1 and M2 CO.sub.2/N.sub.2 selectivities needed to satisfy the mass balance. Other combinations are possible, as shown in FIG. 3.

[0059] Case 6 is a configuration where the permeate stream from M2 is recycled to the feed of M1. A mass balance showing flows for the case of a 10% CO.sub.2 feed, with 65% CO.sub.2 capture rate and a 95% CO.sub.2 product stream is shown in Table 9. This level of CO.sub.2 capture could allow a coal-fired power plant to approach a natural gas-fired combined cycle plant in terms of specific CO.sub.2 emissions per unit of energy generated. In this example, an inlet concentration set point of 14% mol CO.sub.2 was selected. A recycle stream is added to the case of flue gas with 10% CO.sub.2 to boost the effective concentration of CO.sub.2 in stream 6 to the setpoint value of 14%. The recycle rates needed to stabilize the inlet CO.sub.2 concentration of the feed stream 6 to membrane stage M1 when the CO.sub.2 concentration in overall inlet stream 1 drops to the 10%-level is about 45% of the permeate stream. Given the concentrated nature of the recycle stream, the flow of Stream 6 is only increased about 6% relative to the flue gas feed Stream 1. One set of required M1 and M2 selectivities needed to achieve 65% capture rate are 10 and 7. Compared to Case 5, the selectivities needed in the Case 6 example are significantly lower than those required to achieve a comparable level of purity using a single stage system. However, the membrane stages M1 and M2 must be operated more aggressively to maintain the same overall CO.sub.2 capture rates.

TABLE-US-00009 TABLE 9 CO.sub.2 N.sub.2 concen- concen- 10% feed Flow .Math. total tration tration Flow CO.sub.2 Flow N.sub.2 Stream [mol/s] [% mol] [% mol] [mol/s] [mol/s] 1.sup. 100 10% 90% 10 90 6 106 14% 86% 15 91 2 84 2% 98% 1.5 82 3 22 62% 38% 14 8.2 .sup.6 22 62% 38% 14 8.2 .sup.2 9 22% 78% 2.0 7.2 2.sup. 93 4% 96% 3.5 90 .sup.3 12 92% 8% 11.5 1.0 .sup.4 0 / / 0 0 5.sup. 7 95% 5% 6 0.4 4 6 89% 11% 5.0 0.6

[0060] While some preferred embodiments of the present invention are described above, the present invention is not limited to the details in those embodiments. Those skilled in the art can make modifications and variations to the technical scheme of the present invention, without departing from the spirit of the present invention. However, all these modifications and variations shall be deemed as falling into the protected scope of the present invention.

[0061] In addition, it should be appreciated that the technical features described in the above embodiments can be combined in any appropriate manner, provided that there is no conflict among the technical features in the combination. To avoid unnecessary iteration, such possible combinations are not described here in the present invention.

[0062] Moreover, different embodiments of the present invention can be combined freely as required, as long as the combinations don't deviate from the ideal and spirit of the present invention. However, such combinations shall also be deemed as falling into the scope disclosed in the present invention.