Integrated gas separation-turbine CO2 capture processes

Abstract

Sweep-based gas separation processes for reducing carbon dioxide emissions from gas-fired power plants. The invention involves at least two compression steps, a combustion step, a carbon dioxide capture step, a power generate step, and a sweep-based membrane separation step. One of the compression steps is used to produce a low-pressure, low-temperature compressed stream that is sent for treatment in the carbon dioxide capture step, thereby avoiding the need to expend large amounts of energy to cool an otherwise hot compressed stream from a typical compressor that produces a high-pressure stream, usually at 20-30 bar or more.

Claims

1. A process for controlling carbon dioxide exhaust from a combustion process, comprising: (a) compressing an oxygen-containing stream in a first compression step, thereby producing a first compressed gas stream; (b) routing at least a portion of the first compressed gas stream to a gas separation apparatus adapted to selectively remove carbon dioxide, thereby producing a carbon dioxide-enriched stream and a carbon dioxide-depleted stream; (c) compressing the carbon dioxide-depleted stream in a second compression step, thereby producing a second compressed gas stream; (d) combusting at least a portion of the second compressed gas stream with a gaseous fuel in a combustion apparatus, thereby producing a combusted gas stream; (e) routing the combusted gas stream as part of a working gas stream to a gas turbine apparatus mechanically coupled to an electricity generator, and operating the gas turbine apparatus, thereby generating electric power and producing a turbine exhaust stream; (f) passing at least a portion of the turbine exhaust stream to a membrane separation step, wherein the membrane separation step comprises: (i) providing a membrane having a feed side and a permeate side, and being selectively permeable to carbon dioxide over nitrogen and to carbon dioxide over oxygen, (ii) passing the first portion of the turbine exhaust stream across the feed side, (iii) passing air, oxygen-enriched air, or oxygen as a sweep stream across the permeate side, (iv) withdrawing from the feed side a residue stream that is depleted in carbon dioxide compared to the turbine exhaust stream, and (v) withdrawing from the permeate side a permeate stream comprising oxygen and carbon dioxide; and (g) passing the permeate stream to step (a) as at least a portion of the oxygen-containing gas.

2. The process of claim 1, wherein the gas separation apparatus is selected from the group consisting of absorption, adsorption, liquefaction, and membrane separation.

3. The process of claim 2, wherein the gas separation apparatus is a membrane separation apparatus.

4. The process of claim 3, wherein the membrane separation apparatus incorporates polymeric membranes.

5. The process of claim 1, further comprising the step of passing a second portion of the second compressed stream to step (e) as part of the working gas stream.

6. The process of claim 1 or 5, further comprising the step of: (h) passing a second portion of the turbine exhaust stream to step (a) as at least a portion of the oxygen-containing gas before carrying out step (f).

7. The process of claim 1, further comprising the step of routing at least a portion of the turbine exhaust stream to a heat recovery steam generator before carrying out step (f).

8. The process of claim 1, further comprising the step of cooling at least a portion of the turbine exhaust stream before carrying out step (f).

9. The process of claim 1, wherein the residue stream has a carbon dioxide concentration of less than 5 vol %.

10. The process of claim 1, wherein the first compressed gas stream is withdrawn from the first compression step at a pressure within the range of about 2 bar to about 10 bar.

11. The process of claim 1, wherein the second compressed gas stream is withdrawn from the second compression step at about 30 bar.

12. The process of claim 1, further comprising cooling the first compressed gas to a temperature of about 30-100° C. prior to step (b).

13. The process of claim 1, wherein the gaseous fuel comprises natural gas.

14. The process of claim 1, wherein a second portion of the first compressed stream is mixed with the carbon dioxide-depleted stream from step (b) prior to step (c).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of a flow scheme showing a basic embodiment of the invention having two compression steps with a gas separation unit integrated between the steps.

(2) FIG. 2 is an expanded view of the gas separation section of the invention, in which a carbon dioxide-selective membrane separation unit is used.

(3) FIG. 3 is a schematic drawing of a flow scheme showing an embodiment of the invention having three compression steps.

(4) FIG. 4 is a schematic drawing of a flow scheme showing a process using two compressors (not in accordance with the invention).

DETAILED DESCRIPTION OF THE INVENTION

(5) The term “gas” as used herein means a gas or a vapor.

(6) The terms “exhaust gas”, “flue gas” and “emissions stream” are used interchangeably herein.

(7) The terms “mol %” and “vol %” are used interchangeably herein.

(8) The invention is a process involving membrane-based gas separation and power generation, specifically for controlling carbon dioxide emissions from gas-fired power plants, including traditional plants, combined cycle plants incorporating HRSG, and IGCC plants. The process includes multiple compression steps, a combustion step, and an expansion/electricity generation step, as in traditional power plants. The process also includes a sweep-driven membrane separation step and a carbon dioxide removal or capture step. Besides generating electric power, the process yields two gas streams: a vent or flue gas stream of low carbon dioxide concentration that can be sent to the power plant stack, and a carbon dioxide product stream of high concentration that can be sent for purification and/or sequestration.

(9) A simple flow scheme for a basic embodiment of a gas separation and power generation process in accordance with the invention is shown in FIG. 1. It will be appreciated by those of skill in the art that FIG. 1 and the other figures showing process schemes herein are very simple block diagrams, intended to make clear the key unit operations of the processes of the invention, and that actual process trains may include additional steps of a standard type, such as heating, chilling, compressing, condensing, pumping, monitoring of pressures, temperatures, flows, and the like. It will also be appreciated by those of skill in the art that the unit operations may themselves be performed as multiple steps or in a train of multiple pieces of equipment.

(10) Turning back to FIG. 1, air, oxygen-enriched air, or oxygen is introduced into the processes as stream 130 and flows as a sweep stream across the permeate side of the sweep-driven membrane separation unit, 127, discussed in more detail below. The permeate stream, 131, comprises both the sweep gas and carbon dioxide that has permeated the membranes, 128, and preferably has a carbon dioxide content of at least about 10 vol %, more preferably at least about 15 vol %, and most preferably at least about 20 vol %. Stream 131 passes, with optional addition of a portion of turbine exhaust stream 120 and/or make-up air stream 132, as air intake stream 135, to compression step, 101.

(11) The first compression step is carried out in one or multiple compression units, and produces compressed stream, 102, at a modest pressure in the region of about 2 to 10 bar.

(12) Typically, stream 102 is hot, at a temperature of about 150-200° C. Depending on the operating temperature of the separation equipment, stream 102 may be cooled by heat exchange, recuperation, or otherwise in optional cooling step, 103, to produce cooled stream 104. Stream 104 is preferably cooled to a temperature of about 30-100° C. Water condensed as a result of the cooling may be removed as stream 105.

(13) Compressed stream 102 (or cooled stream 104) is directed to a gas separation step, 106, where carbon dioxide is captured and removed from the process via stream 107.

(14) Various considerations affect the choice of technology and operating methodology for step 106. In steady state, the mass of carbon dioxide removed from the process in streams 107 and 129 equals the mass of carbon dioxide generated by combustion. Preferably, at least 50%, and more preferably at least 80% or 90% of the generated carbon dioxide should be captured into stream 107.

(15) Nevertheless, very high levels of removal of carbon dioxide from the feed inlet gas streams 102 or 104 by gas separation are not required, because the off-gas, stream 108, is not vented to the atmosphere, but is eventually directed to sweep-based membrane separation step 127. The sweep-based membrane separation step recycles carbon dioxide in stream 131, so that the carbon dioxide concentration in stream 102/104 tends to be relatively high, such as 15 vol %, 20 vol % or more. Only a portion of this recirculating carbon dioxide needs to be removed into stream 107 to achieve the target high levels of carbon dioxide capture. This is a significant advantage of the process, as step 106 can then be operated using relatively low-cost, low-energy options.

(16) Step 106 can be carried out by means of any technology or combination of technologies that can create a concentrated carbon dioxide stream from stream 102 or 104. Representative methods that may be used include, but are not limited to, physical or chemical sorption, membrane separation, compression/low temperature condensation, and adsorption. All of these are well known in the art as they relate to carbon dioxide removal from gas mixtures of various types. However, based on the considerations discussed above, the preferred technologies are absorption and membrane separation.

(17) Step 106 produces a concentrated carbon dioxide stream, 107, which is withdrawn from the process. In addition to meeting the specified preferred capture targets, this stream has a relatively high carbon dioxide concentration, and preferably contains greater than about 60 or 70 vol % carbon dioxide. Most preferably, this stream contains at least about 80 vol % carbon dioxide. Thus, unusually, the process achieves in one stream both high levels of carbon dioxide capture and high carbon dioxide concentration.

(18) After withdrawal from the process, stream 107 may pass to any desired destination. The high concentration facilitates liquefaction, transport, pipelining, injection and other forms of sequestration.

(19) The off-gas stream, 108, from the carbon dioxide removal or capture step still contains carbon dioxide, but at a lower concentration than the compressed gas stream, 102/104. Typically, but not necessarily, this concentration is at least about 5 vol %, and can be up to about 10 vol % or even more.

(20) Stream 108 (or stream 136) is sent to a second compression step, 109. The second compression step is carried out in one or multiple compressors, and produces second compressed stream, 109, at a pressure of about 20 bar, 30 bar, or even higher. Although the first and second compression steps in FIG. 1 are shown using two separate compressors, the compression steps may be carried out using a single compression train or apparatus, which has been modified to allow a portion of the compressed gas to be introduced or removed from the compression apparatus at an intermediate stage in the train/apparatus.

(21) Optionally, it may be preferred that a portion of first compressed stream, 134, bypasses cooling step 103 and gas separation step 106, and is mixed with membrane residue stream 108 to form air intake stream 136 before entering second compression step 109. In a membrane gas separation process where carbon dioxide removal is only 40-70% of the carbon dioxide in the gas, the bypass is closed. In an amine process where carbon dioxide removal is about 90%, then the bypass is partially open and only a bit of the first compressed stream goes to the separation unit.

(22) Second compressed stream 110 is introduced with fuel stream 111 into combustion step or zone 112. Natural gas, other methane-containing gas, syngas, hydrogen, or any other fuel capable of burning in air may be used. Combustion produces a hot, high-pressure gas, stream 113.

(23) In a traditional gas-fired combustion process, the exhaust gas from the combustor typically contains about 4 or 5 vol % carbon dioxide. In our process, carbon dioxide is recycled via streams 131/133/135, as discussed in more detail below. As a result, the concentration of carbon dioxide in stream 113 is higher than in a traditional natural gas-fired plant, and is frequently as high as at least about 10 vol %, or even at least 15 vol %, 20 vol % or more.

(24) Stream 113 is then sent as a working gas stream, 115, to gas turbine section, 116. Optionally, a portion of the second compressed stream, 114, may be mixed with stream 113 to form the working gas stream, 115, before being sent to the gas turbine section, 116. This section contains one or more commonly multiple gas turbines, which are coupled by means of a shaft, 117, to compressor(s) 101 and 109, and to electricity generator, 118. The working gas drives the gas turbines, which in turn drive the generator and produce electric power.

(25) The low-pressure exhaust gas from the turbines, stream 119, is still hot, and is optionally and preferably directed to a heat recovery steam generator, 121. This section includes a boiler that produces steam, 122, which can be directed to a steam turbine (not shown). Gas exiting the steam generator, stream 123, is routed as feed gas to sweep-based membrane separation step, 127. If it is necessary to cool the turbine exhaust gas before passing it to the membrane unit, this may be done by heat exchange or otherwise in a cooling step, 124. Any condensed water may be removed as stream 125. After passing through optional HRSG, 121, an optional cooling step, or both, the turbine exhaust stream now passes as feed stream, 126, to sweep-based membrane separation step 127.

(26) Step 127 is carried out using membranes that are selective in favor of carbon dioxide over oxygen and nitrogen. It is preferred that the membranes provide a carbon dioxide/nitrogen selectivity of at least about 10, and most preferably at least about 20 under the operating conditions of the process. A carbon dioxide/oxygen selectivity of at least 10 or 20 is also preferred. A carbon dioxide permeance of at least about 300 gpu, more preferably at least about 500 gpu and most preferably at least about 1,000 gpu is desirable. The permeance does not affect the separation performance, but the higher the permeance, the less membrane area will be required to perform the same separation.

(27) Any membrane with suitable performance properties may be used. Many polymeric materials, especially elastomeric materials, are very permeable to carbon dioxide. Preferred membranes for separating carbon dioxide from nitrogen or other inert gases have a selective layer based on a polyether. A number of such membranes are known to have high carbon dioxide/nitrogen selectivity, such as 30, 40, 50 or above. A representative preferred material for the selective layer is Pebax®, a polyamide-polyether block copolymer material described in detail in U.S. Pat. No. 4,963,165.

(28) The membrane may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art. If elastomeric membranes are used, the preferred form is a composite membrane including a microporous support layer for mechanical strength and a rubbery coating layer that is responsible for the separation properties.

(29) The membranes may be manufactured as flat sheets or as fibers and housed in any convenient module form, including spiral-wound modules, plate-and-frame modules and potted hollow-fiber modules. The making of all these types of membranes and modules is well known in the art. We prefer to use flat-sheet membranes in spiral-wound modules.

(30) Step 127 may be carried out in a single bank of membrane modules or an array of modules. A single unit or stage containing one or a bank of membrane modules is adequate for many applications. If the residue stream requires further purification, it may be passed to a second bank of membrane modules for a second processing step. If the permeate stream requires further concentration, it may be passed to a second bank of membrane modules for a second-stage treatment. Such multi-stage or multi-step processes, and variants thereof, will be familiar to those of skill in the art, who will appreciate that the membrane separation step may be configured in many possible ways, including single-stage, multistage, multistep, or more complicated arrays of two or more units in serial or cascade arrangements.

(31) Stream 126 flows across the feed side of the membranes, and sweep gas stream, 130, of air, oxygen-enriched air or oxygen flows across the permeate side. The gas flow pattern within the membrane modules should preferably, although not necessarily, be such that flow on the permeate side is at least partly or substantially countercurrent to flow on the feed side.

(32) In membrane gas separation processes, the driving force for transmembrane permeation is supplied by lowering the partial pressure of the desired permeant on the permeate side to a level below its partial pressure on the feed side. The use of the sweep gas stream 130 maintains a low carbon dioxide partial pressure on the permeate side, thereby providing driving force.

(33) The partial pressure of carbon dioxide on the permeate side may be controlled by adjusting the flow rate of the sweep stream. High sweep flow rates will achieve maximum carbon dioxide removal from the membrane feed gas, but a comparatively carbon dioxide dilute permeate stream (that is, comparatively low carbon dioxide enrichment in the sweep gas exiting the modules). Low sweep flow rates will achieve high concentrations of carbon dioxide in the permeate, but relatively low levels of carbon dioxide removal from the feed.

(34) Typically and preferably, the flow rate of the sweep stream should be between about 50% and 200% of the flow rate of the membrane feed stream, and most preferably between about 80% and 120%. Often a ratio of about 1:1 is convenient and appropriate.

(35) The total gas pressures on each side of the membrane may be the same or different, and each may be above or below atmospheric pressure. If the pressures are about the same, the entire driving force is provided by the sweep mode operation. Optionally, stream 126 may be supplied to the membrane unit at slightly elevated pressure, by which we mean at a pressure of a few bar, such as 2 bar, 3 bar or 5 bar. If this requires recompression of stream 126, a portion of the energy used for the compressors may be recovered by expanding the residue stream, 129, in a turbine.

(36) The membrane separation step divides stream 126 into residue stream 129, which is depleted in carbon dioxide, and permeate/sweep stream 131. The residue stream forms the treated flue gas produced by the process, and is usually discharged to the environment via the power plant stack. The carbon dioxide content of this stream is preferably less than about 5 vol %; more preferably less than about 2 vol %, and most preferably no greater than about 1 vol %.

(37) The permeate/sweep stream, 131, preferably containing at least 10 vol % carbon dioxide, and more preferably at least about 15 vol % carbon dioxide, is withdrawn from the membrane unit and is passed to the first compression unit, 101, to form at least part of the air, oxygen-enriched air or oxygen feed.

(38) Optionally, turbine exhaust stream 119 may be split into a second portion, and the second portion, indicated by dashed line 120, may bypass the sweep-based membrane separation and be sent with stream 131 as stream 133 to the first compression unit, 101, as at least part of the air, oxygen-enriched air or oxygen feed.

(39) FIG. 2 shows a representative example using membrane separation for the carbon dioxide removal step, 105, with heat integration used to cool in the incoming feed stream. Like elements are numbered as in FIG. 1.

(40) Referring to FIG. 2, first compressed stream 102 is passed through cooling step 103, as shown in FIG. 1, in this case carried out in two heat exchange steps, 103a and 103b. In step 103a, stream 102 is run against membrane residue stream 108, with stream 108 entering the heat exchanger as indicated as position A and exiting at position B. Heated stream 108 is then directed to second compression step 109 as described above for FIG. 1.

(41) In step 103b, additional cooling of stream 102 is provided before it passes as cooled stream 104 to the membrane step, 105, containing membranes, 235.

(42) An alternative embodiment of the invention is shown in FIG. 3. Air, oxygen-enriched air, or oxygen is introduced into the processes as stream 327 and flows as a sweep stream across the permeate side of the sweep-driven membrane separation unit, 324, discussed in more detail below. The permeate stream, 328, comprises both the sweep gas and carbon dioxide that has permeated the membranes, 325, and preferably has a carbon dioxide content of at least about 10 vol %, more preferably at least about 15 vol %, and most preferably at least about 20 vol %. Stream 328 passes, with optional addition of stream 331 and make-up air stream 332, as air intake stream 330, to first compression step, 301.

(43) The first compression step is carried out in one or multiple compressors, and produces first compressed stream, 302, at a typical pressure of a few tens of bar, such as 20 bar or 30 bar. Stream 302 is introduced with a fuel stream, 304, into combustion step or zone, 303. Natural gas, other methane-containing gas, syngas, hydrogen, or any other fuel capable of burning in air may be used. Combustion produces hot, high-pressure gas stream 313.

(44) In a traditional gas-fired combustion process, the exhaust gas from the combustor typically contains about 4 or 5 vol % carbon dioxide. In our process, carbon dioxide is recycled via stream 328/330, as discussed in more detail below. As a result, the concentration of carbon dioxide in stream 313 is higher than in a traditional nature gas-fired plant, and is frequently as high as at least about 10 vol %, or even at least 15 vol %, 20 vol %, or more.

(45) A portion of the turbine exhaust stream, 329, is sent to a second compression step, 305. The second compression step is carried out in one or multiple compressors, and produces second compressed stream, 306, at a typical pressure between 2-10 bar, preferably about 5 bar, more preferably about 2 bar. Stream 306 is directed to a gas separation step, 310, where carbon dioxide is captured and removed from the process via stream 311. Depending on the operating temperature of the separation equipment, stream 306 may be cooled by heat exchange or otherwise in optional cooling step, 307, to produce cooled stream 309. Water condensed as a result of the cooling may be removed as stream 308.

(46) Various considerations affect the choice of technology and operating methodology for step 310. In steady state, the mass of carbon dioxide removed from the process in streams 311 and 326 equals the mass of carbon dioxide generated by combustion. Preferably, at least 50%, and more preferably at least 80% or 90% of the generated carbon dioxide should be captured into stream 311.

(47) Nevertheless, very high levels of removal of carbon dioxide from the feed inlet gas streams 306 or 309 by gas separation are not required, because the off-gas, stream 312, is not vented to the atmosphere, but is directed to sweep-based membrane separation step 324. The sweep-based membrane separation step recycles carbon dioxide in stream 330, so that the carbon dioxide concentration in stream 309 tends to be relatively high, such as 15 vol %, 20 vol % or more. Only a portion of this recirculating carbon dioxide needs to be removed into stream 311 to achieve the target high levels of carbon dioxide capture. This is a significant advantage of the process, as step 310 can then be operated using relatively low-cost, low-energy options.

(48) Step 310 can be carried out by means of any technology or combination of technologies that can create a concentrated carbon dioxide stream from stream 306 or 309. Representative methods that may be used include, but are not limited to, physical or chemical sorption, membrane separation, compression/low temperature condensation, and adsorption. All of these are well known in the art as they relate to carbon dioxide removal from gas mixtures of various types. However, based on the considerations discussed above, the preferred technologies are absorption and membrane separation.

(49) Step 310 produces a concentrated carbon dioxide stream, 311, which is withdrawn from the process. In addition to meeting the specified preferred capture targets, this stream has a relatively high carbon dioxide concentration, and preferably contains greater than 60 or 70 vol % carbon dioxide. Most preferably, this stream contains at least about 80 vol % carbon dioxide. Thus, unusually, the process achieves in one stream both high levels of carbon dioxide capture and high carbon dioxide concentration.

(50) After withdrawal from the process, stream 311 may pass to any desired destination. The high concentration facilitates liquefaction, transport, pipelining, injection and other forms of sequestration.

(51) The off-gas stream, 312, from the carbon dioxide removal or capture step still contains carbon dioxide, but at a lower concentration than the compressed gas stream, 306/309. Typically, but not necessarily, this concentration is at least about 5 vol %, and can be up to about 10 vol % or even more.

(52) Stream 312 is sent as an air intake stream to a third compression step, 314. Optionally, it may be preferred that a portion of second compressed stream, 333, bypasses cooling step 307 and gas separation step 310, and is mixed with membrane residue stream 312 to form air intake stream 335 before entering second compression step 314. In a membrane gas separation process where carbon dioxide removal is only 40-70% of the carbon dioxide in the gas, the bypass is closed. In an amine process where carbon dioxide removal is about 90%, then the bypass is partially open and only a bit of the first compressed stream goes to the separation unit.

(53) The third compression step is carried out in one or multiple compressors, and produces third compressed stream, 315, at a typical pressure of a few tens of bar, such as 20 bar or 30 bar.

(54) Stream 315 is combined with combusted gas stream 313, and to produce a working gas stream, 316, that is introduced into gas turbine section, 317. This section contains one or more gas turbines, which are coupled by means of shaft, 318, to compressor(s) 301, 305, and 314 and to electricity generator, 319. The working gas drives the gas turbines, which in turn drive the generator and produce electric power.

(55) The low-pressure exhaust gas from the turbines, stream 317, is still hot, and is optionally and preferably directed to a heat recovery steam generator, 321. This section includes a boiler that produces steam, stream 322, which can be directed to a steam turbine (not shown). Gas exiting the steam generator, stream 323, is routed as feed gas to sweep-based membrane separation step, 324. If it is necessary to cool the turbine exhaust gas before passing it to the membrane unit, this may be done by heat exchange or otherwise in a cooling step (not shown). After passing through optional HRSG, 321, an optional cooling step, or both, the turbine exhaust stream now passes as feed stream, 323, to sweep-based membrane separation step 324.

(56) Step 324 is carried out using membranes, 325, that are selective in favor of carbon dioxide over oxygen and nitrogen. It is preferred that the membranes provide a carbon dioxide/nitrogen selectivity of at least about 10, and most preferably at least about 20 under the operating conditions of the process. A carbon dioxide/oxygen selectivity of at least 10 or 20 is also preferred. A carbon dioxide permeance of at least about 300 gpu, more preferably at least about 500 gpu and most preferably at least about 1,000 gpu is desirable. The permeance does not affect the separation performance, but the higher the permeance, the less membrane area will be required to perform the same separation.

(57) Any membrane with suitable performance properties may be used. Many polymeric materials, especially elastomeric materials, are very permeable to carbon dioxide. Preferred membranes for separating carbon dioxide from nitrogen or other inert gases have a selective layer based on a polyether. A number of such membranes are known to have high carbon dioxide/nitrogen selectivity, such as 30, 40, 50 or above. A representative preferred material for the selective layer is Pebax®, a polyamide-polyether block copolymer material described in detail in U.S. Pat. No. 4,963,165.

(58) The membrane may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art. If elastomeric membranes are used, the preferred form is a composite membrane including a microporous support layer for mechanical strength and a rubbery coating layer that is responsible for the separation properties.

(59) The membranes may be manufactured as flat sheets or as fibers and housed in any convenient module form, including spiral-wound modules, plate-and-frame modules and potted hollow-fiber modules. The making of all these types of membranes and modules is well known in the art. We prefer to use flat-sheet membranes in spiral-wound modules.

(60) Step 324 may be carried out in a single bank of membrane modules or an array of modules. A single unit or stage containing one or a bank of membrane modules is adequate for many applications. If the residue stream requires further purification, it may be passed to a second bank of membrane modules for a second processing step. If the permeate stream requires further concentration, it may be passed to a second bank of membrane modules for a second-stage treatment. Such multi-stage or multi-step processes, and variants thereof, will be familiar to those of skill in the art, who will appreciate that the membrane separation step may be configured in many possible ways, including single-stage, multistage, multistep, or more complicated arrays of two or more units in serial or cascade arrangements.

(61) Stream 323 flows across the feed side of the membranes, and sweep gas stream, 327, of air, oxygen-enriched air or oxygen flows across the permeate side. The gas flow pattern within the membrane modules should preferably, although not necessarily, be such that flow on the permeate side is at least partly or substantially countercurrent to flow on the feed side.

(62) In membrane gas separation processes, the driving force for transmembrane permeation is supplied by lowering the partial pressure of the desired permeant on the permeate side to a level below its partial pressure on the feed side. The use of the sweep gas stream 327 maintains a low carbon dioxide partial pressure on the permeate side, thereby providing driving force.

(63) The partial pressure of carbon dioxide on the permeate side may be controlled by adjusting the flow rate of the sweep stream. High sweep flow rates will achieve maximum carbon dioxide removal from the membrane feed gas, but a comparatively carbon dioxide dilute permeate stream (that is, comparatively low carbon dioxide enrichment in the sweep gas exiting the modules). Low sweep flow rates will achieve high concentrations of carbon dioxide in the permeate, but relatively low levels of carbon dioxide removal from the feed.

(64) Typically and preferably, the flow rate of the sweep stream should be between about 50% and 200% of the flow rate of the membrane feed stream, and most preferably between about 80% and 120%. Often a ratio of about 1:1 is convenient and appropriate.

(65) The total gas pressures on each side of the membrane may be the same or different, and each may be above or below atmospheric pressure. If the pressures are about the same, the entire driving force is provided by the sweep mode operation. Optionally, stream 323 may be supplied to the membrane unit at slightly elevated pressure, by which we mean at a pressure of a few bar, such as 2 bar, 3 bar or 5 bar. If this requires recompression of stream 323, a portion of the energy used for the compressors may be recovered by expanding the residue stream, 326, in a turbine.

(66) The membrane separation step divides stream 323 into residue stream, 326, depleted in carbon dioxide, and permeate/sweep stream, 328. The residue stream forms the treated flue gas produced by the process, and is usually discharged to the environment via the power plant stack. The carbon dioxide content of this stream is preferably less than about 5 vol %; more preferably less than about 2 vol %, and most preferably no greater than about 1 vol %.

(67) The permeate/sweep stream, 328, preferably containing at least 10 vol % carbon dioxide, and more preferably at least about 15 vol % carbon dioxide, is withdrawn from the membrane unit and is passed to the compression unit to form at least part of the air, oxygen-enriched air or oxygen feed.

(68) Turbine exhaust stream 320/323 is split into a second portion, stream 329, which bypasses the sweep-based membrane separation and is sent to the second compression unit, 305.

(69) Optionally, turbine exhaust stream 320/323 may be split into a third portion, indicated by dashed line 331, which may bypass the sweep-based membrane separation and be sent with stream 328 to the first compression unit 301 as at least part of the air, oxygen-enriched air or oxygen feed.

(70) The invention is now illustrated in further detail by specific examples. These examples are intended to further clarify the invention, and are not intended to limit the scope in any way.

EXAMPLES

(71) All calculations were performed with a modeling program, ChemCad 6.3 (ChemStations, Inc., Houston, Tex.), containing code for the membrane operation developed by MTR's engineering group, For the calculations, all compressors and vacuum pumps were assumed to be 85% efficient. In each case, the calculation was normalized to a combustion process producing 1 ton/hour of carbon dioxide.

(72) It was further assumed that a membrane separation unit was used as the carbon capture unit.

Example 1: Molten Salt Membranes Used for Gas Separation Step, Two Compressor Loops (not in Accordance with the Invention)

(73) As a comparative example, a computer calculation was performed to model the performance of the process with the design shown in FIG. 4. Each compressor was assumed to deliver a compressed gas at 30 bara. The ratio of turbine exhaust gas directed to the sweep unit to gas directed to compression step 401b was set at 3:1. Molten salt membranes were assumed to be used for step 412. The permeate side of the membranes was assumed to be at 2 bara.

(74) The results of the calculation are shown in Table 1.

(75) TABLE-US-00001 TABLE 1 Stream 416 428 406 445 444 419 425 413 429 Molar 22 302 401 119 123 521 357 25 259 flow (kmol/ h) Temp 38 15 25 25 375 634 25 374 11 (° C.) Pressure 30 1 1 2 30 1 2 2 1 (bara) Component (vol %) Oxygen 0 20.7 15 4.5 4.5 4.2 4.5 1.1 7.2 Nitro- 1.6 77.3 62.1 69.6 68.8 63.6 70.0 8.6 90.3 gen Carbon 1.0 0 20.4 23.5 24.3 21.4 23.5 90.3 0.8 dioxide Meth- 93.1 0 0 0 0 0 0 0 0 ane Water 0 1.0 1.8 1.5 1.5 10.0 1.5 0 0.5 Argon 0 1.0 0.7 0.8 0.8 0.8 0.8 0 1.1 Ethane 3.2 0 0 0 0 0 0 0 0 Propane 0.7 0 0 0 0 0 0 0 0 n- 0.4 0 0 0 0 0 0 0 0 butane

(76) The process produces a stack gas containing 0.8 vol % carbon dioxide, and a concentrated product stream containing about 90 vol % carbon dioxide. The process requires a membrane area of about 74 m.sup.2 for the molten salt membranes, which removes 82% of the carbon dioxide in gas stream 444, and a membrane area of about 1,430 m.sup.2 for the sweep-based unit.

Example 2: Embodiment of FIG. 1, Feed Gas Pressure at 5 Bar for the Gas Separation Step

(77) A calculation was performed to model the performance of the process of the invention shown in FIG. 1 where the feed gas to the gas separation step, 106, is compressed to a pressure of 5 bar by compression step 101.

(78) For the calculation, the feed gas stream 104 was calculated to have a flow rate of 16,557 kg/hour and contain nitrogen, oxygen, carbon dioxide and water. It was also calculated that the molar compositions were approximately as follows:

(79) TABLE-US-00002 Nitrogen: 74.4% Oxygen 14.4% Carbon dioxide: 10.4% Water: 0.8%

(80) It was assumed that a portion of the exhaust gas 119 was used as an internal recycle as stream 120.

(81) The results of the calculations are shown in Table 2.

(82) TABLE-US-00003 TABLE 2 Stream 104 107 108 111 119 120 129 130 135 Total Mass 16,557 1,018 15,539 371 15,527 3,494 9,642 11,050 15,539 Flow (kg/h) Temp (° C.) 30 30 30 30 35 35 30 30 30 Pressure (bara) 5.0 0.2 5.0 30.0 1.1 1.1 1.1 0.9 1.0 Component (mol %) Nitrogen 74.4 17.0 77.3 0.5 77.1 77.1 88.6 79.0 71.6 Oxygen 14.4 6.5 14.8 0.0 6.0 6.0 9.8 21.0 13.8 Carbon 10.4 68.8 7.4 0.1 11.8 11.8 1.3 0.0 10.0 Dioxide Water 0.8 7.7 0.5 0.0 5.1 5.1 0.3 0.0 4.6 Methane 0.0 0.0 0.0 98.2 0.0 0.0 0.0 0.0 0.0 C.sub.2, Hydrocarbons 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.0

(83) The process produces a stack gas containing 1.3% carbon dioxide, and a concentrated product stream containing about 69% carbon dioxide. The process achieves a carbon dioxide recovery of 80%. The membrane area used for step 106 was 198 m.sup.2 and the membrane area required for step 127 was 10,000 m.sup.2.

Example 3: Embodiment of FIG. 1, Feed Gas Pressure at 2 Bar for the Gas Separation Step

(84) A calculation was performed to model the performance of the process of the invention shown in FIG. 1 where the feed gas to the gas separation step, 106, is compressed to a pressure of 2 bar by compression step 101.

(85) For the calculation, the feed gas stream 104 was calculated to have a flow rate of 16,354 kg/hour and contain nitrogen, oxygen, carbon dioxide and water. It was also calculated that the molar compositions were approximately as follows:

(86) TABLE-US-00004 Nitrogen: 70.9% Oxygen 14.8% Carbon dioxide: 12.2% Water: 2.1%

(87) It was assumed that a portion of the exhaust gas 119 was used as an internal recycle as stream 120.

(88) The results of the calculations are shown in Table 3.

(89) TABLE-US-00005 TABLE 3 Stream 104 107 108 111 119 120 129 130 135 Total Mass 16,354 811 15,543 372 15,414 3,237 9,560 10,742 16,593 Flow (kg/h) Temp (° C.) 30 30 30 30 35 35 30 30 43 Pressure (bara) 2.0 0.2 2.0 1.0 1.1 1.0 1.1 0.9 1.0 Component (mol %) Nitrogen 70.9 17.6 73.1 0.5 73.9 73.9 87.4 79.0 69.2 Oxygen 14.8 7.1 15.1 0.0 6.3 6.3 9.3 21.0 14.4 Carbon 12.2 62.0 10.2 0.1 14.7 14.7 2.8 0.0 12.0 Dioxide Water 2.1 13.3 1.6 0.0 5.1 5.1 0.5 0.0 4.4 Methane 0.0 0.0 0.0 98.2 0.0 0.0 0.0 0.0 0.0 C.sub.2, Hydrocarbons 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.0

(90) The process produces a stack gas containing about 3% carbon dioxide, and a concentrated product stream containing about 62% carbon dioxide. The process achieves a carbon dioxide recovery of 60%. The membrane area used for step 106 was 456 m.sup.2 and the membrane area required for step 127 was 6,000 m.sup.2.

Example 4: Embodiment of FIG. 3, Three Compressor Loops

(91) A calculation was performed to model the performance of the process of the invention using three compressors in accordance with the design shown in FIG. 3. The feed gas to the gas separation step, 310, was compressed to a pressure of 2 bar by compression step 305.

(92) For the calculation, the feed gas stream 309 was calculated to have a flow rate of 16,354 kg/hour and contain nitrogen, oxygen, carbon dioxide and water. It was also calculated that the molar compositions were approximately as follows:

(93) TABLE-US-00006 Nitrogen: 70.9% Oxygen 14.8% Carbon dioxide: 12.2% Water: 2.1%

(94) It was assumed that a portion of the exhaust gas 323 was used as an internal recycle as stream, 331.

(95) The results of the calculations are shown in Table 4.

(96) TABLE-US-00007 TABLE 4 Stream 309 311 312 320 329 331 326 327 330 Total Mass 6,920 744 6,176 15,175 6,980 819 5,978 7,111 6,980 Flow (kg/h) Temp (° C.) 30 30 30 33 33 35 30 30 32 Pressure (bara) 2.0 0.2 2.0 1.1 1.1 1.1 1.1 0.9 1.0 Component (mol %) Nitrogen 80.9 18.4 86.7 79.7 79.7 79.7 92.1 79.0 79.7 Oxygen 1.1 0.5 1.2 1.1 1.1 1.1 2.9 21.0 1.1 Carbon 15.9 69.8 10.9 15.7 15.7 15.7 4.5 0.0 15.7 Dioxide Water 2.1 11.3 1.2 3.5 3.5 3.5 0.5 0.0 3.5

(97) The process produces a stack gas containing about 4.5% carbon dioxide, and a concentrated product stream containing about 70% carbon dioxide. The process also achieves a carbon dioxide recovery of 60%. The membrane area used for step 310 was 360 m.sup.2 and the membrane area required for step 324 was 2,000 m.sup.2.