CO2 Removal by Membrane Contactor

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Gas/gas membrane contactors An emerging membrane unit operation Pingjiao Hao n , J.G. Wijmans, Jay Kniep, Richard W. Baker Membrane Technology and Research, Inc., 39630 Eureka Drive, Newark, CA 94560, USA article info Article history: Received 17 January 2014 Received in revised form 13 March 2014 Accepted 17 March 2014 Available online 21 March 2014 Keywords: Membrane contactors CO 2 capture Gas separation abstract Gas/gas membrane contactors are devices in which a feed gas and a sweep gas are circulated on either side of a membrane. The pressures of the two gas streams are often approximately equivalent, so the principal driving force for permeation is the concentration difference between the feed and sweep gas components. This type of contactor has been commercialized for energy recovery devices in air conditioning applications. More recently, these contactors have been suggested for use in carbon dioxide capture and sequestration. In this paper, the performance of an ideal contactor using perfectly selective membranes is examined. For this type of ideal contactor, analytical equations can be derived that allow the partial pressure proles within the contactor to be calculated. The effect of the sweep ratio (sweep ow rate/feed ow rate) and feed pressure on separation performance of the contactors has been calculated. In the nal section of the paper, the performance of contactors tted with membranes permeable to other components of the feed gas is described. & 2014 Elsevier B.V. All rights reserved. 1. Introduction This paper describes the operation of gas/gas membrane contactors. These are typically low-pressure devices in which a feed gas is circulated across one surface of a permeable membrane, and an approximately equal volume of another gas, at a similar pressure, is circulated countercurrently on the other side of the membrane. Permeation occurs because of the partial pressure difference between components in the feed and sweep gases. Most of this partial pressure difference occurs because of concentration rather than pressure differences across the membrane. This type of contactor has been commercialized for energy recovery devices in air conditioning and fuel cell humidity control applications [13]. More recently, these contactors have been suggested for use in carbon dioxide capture [4,5]. Most membrane gas separation processes do not use a perme- ate side sweep gas. In conventional processes, a feed gas mixture ows across the surface of a permselective membrane; a portion of the mixture permeates the membrane and is removed as lower pressure permeate gas. The remaining gas, depleted in the permeating components, is removed from the feed side of the membrane as a residue gas. One stream enters the membrane module (the feed), two streams leave (the residue and permeate). A few membrane processes have been developed in which a small ow of sweep gas is introduced on the permeate side of the membrane. This gas usually ows countercurrently to the incom- ing feed. Two streams enter the module (the feed and sweep) and two streams leave (the residue and permeate þ sweep). This type of sweep process has been used when the membrane selectivity is much higher than the pressure ratio across the membrane, and a small amount of an extremely permeable component must be removed. A well-known example is the removal of water vapor from compressed air, rst commercialized in the 1990s [6,7]. In this process, the membrane performance is pressure ratio limited, and the separation can be much improved by using a small fraction of dry gas as a permeate side sweep to reduce the partial pressure of the permeating component (water) on the permeate side of the membrane. In these applications, the volume of sweep gas used is only a few percent of the volume of the feed gas to be separated. A few years ago, Membrane Technology and Research, Inc. (MTR) proposed using a gas/gas membrane contactor to separate carbon dioxide (CO 2 ) from fossil fuel power plant ue gas [4,5]. If successfully developed, this could be an important membrane application. In this process, CO 2 -rich ue gas passes across the feed side of a membrane, while an approximately equal volume of air passes as a sweep gas on the membrane permeate side. In this way, the partial pressure of CO 2 on the permeate side is main- tained at a lower level than on the feed side. Because of this partial pressure difference, CO 2 passes from the ue gas into the sweep air stream. If membranes are used that are very permeable to CO 2 , Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2014.03.039 0376-7388/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ1 650 543 3344. E-mail address: [email protected] (P. Hao). Journal of Membrane Science 462 (2014) 131138

description

PENGHILANGAN CO2 DENGAN KONTAKTOR MEMBRAN

Transcript of CO2 Removal by Membrane Contactor

Page 1: CO2 Removal by Membrane Contactor

Gas/gas membrane contactors – An emerging membraneunit operation

Pingjiao Hao n, J.G. Wijmans, Jay Kniep, Richard W. BakerMembrane Technology and Research, Inc., 39630 Eureka Drive, Newark, CA 94560, USA

a r t i c l e i n f o

Article history:Received 17 January 2014Received in revised form13 March 2014Accepted 17 March 2014Available online 21 March 2014

Keywords:Membrane contactorsCO2 captureGas separation

a b s t r a c t

Gas/gas membrane contactors are devices in which a feed gas and a sweep gas are circulated on eitherside of a membrane. The pressures of the two gas streams are often approximately equivalent, so theprincipal driving force for permeation is the concentration difference between the feed and sweep gascomponents. This type of contactor has been commercialized for energy recovery devices in airconditioning applications. More recently, these contactors have been suggested for use in carbondioxide capture and sequestration. In this paper, the performance of an ideal contactor using perfectlyselective membranes is examined. For this type of ideal contactor, analytical equations can be derivedthat allow the partial pressure profiles within the contactor to be calculated. The effect of the sweep ratio(sweep flow rate/feed flow rate) and feed pressure on separation performance of the contactors has beencalculated. In the final section of the paper, the performance of contactors fitted with membranespermeable to other components of the feed gas is described.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

This paper describes the operation of gas/gas membranecontactors. These are typically low-pressure devices in which afeed gas is circulated across one surface of a permeable membrane,and an approximately equal volume of another gas, at a similarpressure, is circulated countercurrently on the other side of themembrane. Permeation occurs because of the partial pressuredifference between components in the feed and sweep gases. Mostof this partial pressure difference occurs because of concentrationrather than pressure differences across the membrane. This type ofcontactor has been commercialized for energy recovery devices inair conditioning and fuel cell humidity control applications [1–3].More recently, these contactors have been suggested for use incarbon dioxide capture [4,5].

Most membrane gas separation processes do not use a perme-ate side sweep gas. In conventional processes, a feed gas mixtureflows across the surface of a permselective membrane; a portion ofthe mixture permeates the membrane and is removed as lowerpressure permeate gas. The remaining gas, depleted in thepermeating components, is removed from the feed side of themembrane as a residue gas. One stream enters the membranemodule (the feed), two streams leave (the residue and permeate).

A few membrane processes have been developed in which asmall flow of sweep gas is introduced on the permeate side of themembrane. This gas usually flows countercurrently to the incom-ing feed. Two streams enter the module (the feed and sweep) andtwo streams leave (the residue and permeateþsweep). This typeof sweep process has been used when the membrane selectivity ismuch higher than the pressure ratio across the membrane, and asmall amount of an extremely permeable component must beremoved. A well-known example is the removal of water vaporfrom compressed air, first commercialized in the 1990s [6,7]. Inthis process, the membrane performance is pressure ratio limited,and the separation can be much improved by using a smallfraction of dry gas as a permeate side sweep to reduce the partialpressure of the permeating component (water) on the permeateside of the membrane. In these applications, the volume of sweepgas used is only a few percent of the volume of the feed gas to beseparated.

A few years ago, Membrane Technology and Research, Inc.(MTR) proposed using a gas/gas membrane contactor to separatecarbon dioxide (CO2) from fossil fuel power plant flue gas [4,5].If successfully developed, this could be an important membraneapplication. In this process, CO2-rich flue gas passes across the feedside of a membrane, while an approximately equal volume of airpasses as a sweep gas on the membrane permeate side. In thisway, the partial pressure of CO2 on the permeate side is main-tained at a lower level than on the feed side. Because of this partialpressure difference, CO2 passes from the flue gas into the sweepair stream. If membranes are used that are very permeable to CO2,

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/memsci

Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.03.0390376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ1 650 543 3344.E-mail address: [email protected] (P. Hao).

Journal of Membrane Science 462 (2014) 131–138

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but relatively impermeable to oxygen and nitrogen, very littlenitrogen passes from the flue gas into the sweep air and very littleoxygen passes from the air sweep into the flue gas. Because thefeed and permeate pressures are low, blowers are the onlycompression equipment needed to circulate the gases across themembrane. A useful separation is performed at a minimalenergy cost.

A block diagram illustrating the application of this device toCO2 capture in a coal-fired power plant is shown in Fig. 1. Airpassing countercurrently to the flue gas strips CO2 out of the gas.The CO2-laden air is then used to burn coal in the power plantboiler. This combustion process is used to produce steam to drive aturbine and make electricity; it also generates a CO2-enriched fluegas. A portion of this flue gas is separated as a CO2-enrichedstream by a conventional membrane unit. The remaining gaspasses across the feed surface of the membrane contactor andbecomes the CO2-depleted gas (2% CO2) that is discarded throughthe chimney. By using the membrane contactor, the CO2 concen-tration in the flue gas from the boiler can be enriched from thenormal concentration of 10–13% CO2 to 20% CO2 or more. Enhan-cing the CO2 concentration of the flue gas from the coal combustorincreases the efficiency of the selective CO2 purge step. A moreconcentrated CO2 purge is produced because only a portion of theCO2 must be removed in this step. The remaining CO2 is recycledby the membrane contactor. The use of a membrane contactorsignificantly reduces the cost of separating a CO2 concentratestream from the flue gas. This process is under development byMTR as a potential CO2 capture technology.

In this paper, a gas/gas contactor of the type shown in Fig. 1 istreated as a unit operation. The impacts of different processparameters on unit performance are examined. The paper isdivided into two sections. In the first section, we will showcalculations for an ideal contactor; that is, a contactor fitted witha membrane permeable to one of the components of the feed gas(CO2), but impermeable to all other components. We will alsoassume the component to be removed is present at a lowconcentration (�1%) so that the volume change in the feed andsweep streams caused by permeation can be ignored. The proper-ties of a gas/gas contactor will then be illustrated using this idealdevice.

In the second section of the paper, we will illustrate thecomplications that result when real membranes with limitedselectivity are used and permeation of other components ispossible. The changes in performance that occur when permeationthrough the membrane causes volume changes to the feed andsweep flows will also be examined. This type of contactor is closerto the type that would be used for the application shown in Fig. 1.

2. Ideal contactor performance

The base case operating conditions for an ideal contactor thatforms the starting point of this analysis are shown in Fig. 2. Thebase case contactor is assumed to have an area of 5000 m2, andcontains a membrane having a CO2 permeance of 1000 gpu. Thefeed stream contains 1% CO2, and the sweep stream is purenitrogen. The feed and sweep streams are both at atmosphericpressure. The device achieves 80% CO2 removal from the feed tothe sweep gas when the feed and sweep flows are set at1 m3(STP)/s.

One concern with high permeance membranes of the typeshown in Fig. 2 is that concentration polarization effects may occurin stagnant boundary layers on either side of the membrane. Theproblem is expected to be most significant on the side in contactwith the microporous support layer of the composite membrane.This support creates a stagnant layer which is significantly thicker

than the gas boundary layers in the gas channels. The likelihood ofconcentration gradients forming in the stagnant layer can beestimated by calculating the Peclet number, Jvδ/D, where Jv isthe actual gas velocity or volume flux in the layer, δ is the stagnantlayer thickness and D is the gas diffusion coefficient in thestagnant layer gas at the stagnant layer pressure [8]. This dimen-sionless number represents the ratio of the convective transport Jvand the diffusive transport D/δ. When the Peclet number is large(JvZD/δ), the convective flux through the membrane cannot easilybe balanced by diffusion in the boundary layer, and concentrationgradients form in the boundary layers. When the Peclet number issmall (JvrD/δ), convection is easily balanced by diffusion in theboundary layer and significant concentration gradients do notform in the boundary layer. The CO2 permeance of the membranesshown in Fig. 2 is 1000 gpu (1000�10�6 cm3(STP)/cm2 s cmHg).Under typical operating conditions of the process in a power planttype of environment (1–2 bar feed, 1 bar permeate and �10% CO2

in the feed gas), the volume flux through the membrane is about1.5�10�2 cm3(STP)/cm2 s. Assuming the stagnant layer is atatmospheric pressure, the superficial velocity through the micro-porous support in the layer is 1.5�10�2 cm/s. The actual velocity,Jv, will be higher because of the effects of porosity and tortuosity;we will assume here that the actual velocity is about six timeshigher and equal to 1.0�10�1 cm/s. Assuming the microporoussupport layer that separates the selective membrane layer fromthe well-mixed counter-flowing gas is 200 μm thick (δ), andtaking the gas diffusion coefficient at atmospheric pressure to be�0.2 cm2/s, it follows that the permeate-side Peclet number Jvδ/Dis 1�10�2. A Peclet number this small implies that diffusion is

Fig. 1. Block diagram illustrating the use of a selective membrane contactor torecycle CO2 to the boiler of a coal power plant. In this way, the concentration of CO2

in the flue gas exiting the boiler increases from 10–13% to 20% at very little energycost [4].

Fig. 2. Base case conditions used in this paper for an ideal gas/gas membranecontactor.

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dominant over convection and that concentration gradients in thestagnant layer are small and can be ignored.

2.1. Factors affecting ideal contactor performance

One of the best ways to understand the influence of processoperating conditions on contactor performance is to calculate thepartial pressure difference between the gases on either side of themembrane. The equations required to make this possible arederived in an appendix to this paper. These equations have beenformulated into an Excel program that allows the partial pressureon the feed and permeate side of the membrane to be calculated atany point. This program is given in the supplemental material tothe paper.

The results obtained using these equations, for the base caseprocess shown in Fig. 2, are given in Fig. 3 below. The feed gas at aconcentration of 1% CO2 enters on the feed side of the membraneat the left end of the module. An equal volume of nitrogen sweepgas enters on the permeate side of the membrane at the right endof the module. Because of the difference in partial pressure createdacross the membrane, CO2 permeates from the feed to thepermeate side. In the base case process illustrated in Fig. 3, theflows on either side of the membrane are equal. This means anincremental flow of CO2 from the feed to the permeate side of themembrane causes a decrease in the feed side CO2, while simulta-neously producing an equal increase in the CO2 concentration ofthe sweep gas on the other side of the membrane. The result(derived mathematically in Appendix 1) is that the partial pressureof CO2 decreases linearly on the feed side of the membrane andincreases linearly on the counter-flowing sweep gas side. Thedifference between the feed and sweep side partial pressure is thedriving force for CO2 permeation, and is equal at all points in themembrane module.

One of the most important parameters affecting the operationof a membrane contactor is the sweep ratio, defined as thevolumetric flow of the inlet sweep gas divided by the volumetricflow of the inlet feed gas (both as standard m3/s, not actual m3/s).

Sweep ratio¼ Inlet sweep flow rate ðm3ðSTPÞ=sÞInlet feed flow rate ðm3ðSTPÞ=sÞ ð1Þ

The definition of sweep ratio given in Eq. (1) is a reliable way ofcharacterizing the ideal membrane contactor shown in Fig. 2,where only a small fraction of the feed and sweep gas flowpermeate the membrane (the process stage-cut is small). In thesecond half of this paper, we will show a modified definition mightbe used when a significant volume flow takes place across themembrane.

Fig. 4 shows the calculated partial pressure profiles withinmodules when the sweep flow is increased or decreased toproduce a sweep ratio of 0.5 or 2.0. Consider first the case whenthe sweep ratio is 0.5, shown in Fig. 4(a). Because the volume ofsweep gas is half that of the feed, only half of the CO2 in the feedcan permeate, even when an infinitely permeable membrane isused. If more than half of the feed gas CO2 permeates themembrane, the sweep gas leaving the module would have ahigher concentration than the feed gas entering. CO2 would thenflow from the sweep to the feed. As Fig. 4(a) shows, a sweep ratioof 0.5 removes almost half of the CO2 from the feed, reducing theCO2 concentration of the feed to just a little over 0.5% CO2.Concurrently, the CO2 concentration in the sweep gas increases,reaching a little under 1.0% CO2 in the sweep gas leaving themodule. The driving force for CO2 permeation is the difference inCO2 partial pressure (concentration) across the membrane, and asthe figure shows, the driving force is highest at the incomingsweep end of the module. The bulk of CO2 permeation occurs atthe sweep input end.

Fig. 3. CO2 partial pressures on the feed and sweep sides of the ideal base case membrane contactor shown in Fig. 2. The driving force for permeation is constant because thetwo flow rates are equal.

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When the sweep ratio is 2.0, shown in Fig. 4(b), the situation isreversed. There is now more than enough sweep gas to remove allof the CO2 from the feed. With a membrane having the base caseproperties, the feed gas leaving the module contains less than 0.1%CO2. The permeated CO2 leaves with the sweep air at a concentra-tion of just under 0.5% CO2. The driving force is highest at theincoming feed end of the module and the bulk of CO2 permeationoccurs at the feed input end.

An alternative way to show the effect of sweep ratio on CO2

removal from the feed is shown in Fig. 5. The calculations shownin Fig. 5 and those reported elsewhere in this paper wereperformed using a computer process simulator (ChemCad 6.3,Chemstations, Inc., Austin, TX), enhanced with differential elementcode for the membrane separation step, written at MTR. In theFig. 5 calculation, the base case module is operated with varioussweep gas flow rates to change the sweep ratio. The fractionalremoval of CO2 from the feed gas is calculated at each sweep ratio.Plots determined this way are shown for membranes with differ-ent CO2 permeances. The base case membrane has a CO2 per-meance of 1000 gpu and the performance of this module (alreadyillustrated in Figs. 3 and 4) is shown on the line marked CO2

permeance¼1000 gpu. At a sweep ratio of 0.5, the fractionalremoval of this module is 49.6%; at a ratio of 1.0 (the base case),the removal is 80%; and at a sweep ratio of 2.0, the removalincreases to 93.4%.

Two limiting regions are shown in Fig. 5. The first limitingregion is to the left of the bold line that shows the fractional CO2

removal achieved by an infinitely permeable membrane. Thisboundary is defined by

Fractional removal¼ Sweep ratio ð2ÞIn pressure driven processes, two similar limiting regions are

also known, governed by the pressure ratio of the processes andcalled the pressure ratio limited region and the selectivity limited

region [9]. The sweep ratio, like the pressure ratio, provides a linkbetween the driving force, selectivity and separation.

The boundary of the sweep ratio limited region can be derivedfrom simple mass balance considerations. As the curves in Fig. 5show, at a sweep ratio of 0.5, all membranes with a permeance

Fig. 4. CO2 concentration profiles in the base case membrane module shown in Fig. 2, except that the sweep flow rate, and thus the sweep ratio, is changed. (a) Sweep flowrate 0.5 m3/s, sweep ratio 0.5: most CO2 permeation occurs at the incoming sweep end of the module. (b) Sweep flow rate 2.0 m3/s, sweep ratio 2.0: most CO2 permeationoccurs at the incoming feed end of the module.

Fig. 5. Effect of sweep ratio on CO2 removal from the feed, calculated formembranes of different CO2 permeance. The feed flow rate is maintained constantat 1 m3/s, while the sweep flow rate is changed to adjust the sweep ratio. The basecase membrane module (Fig. 2) is shown as a dot on the 1000 gpu membrane line.Two limiting regions are shown: one is in the region below a sweep ratio of 1.0,where the CO2 removal is limited at least in part by the sweep ratio, and the otheris a region at higher sweep ratios where the CO2 removal is limited at least in partby the inability of the membranes to permeate sufficient CO2.

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above �1000 gpu produce essentially the same fractional CO2

removal; that is, very close to 0.5. Increasing the membranepermeance above 1000 gpu does not produce a higher CO2

removal. The contactor performance is limited by the sweep ratio.This limit occurs at all sweep ratios below 1.0. In this region, themaximum fractional removal is set by the sweep ratio, no matterhow high the membrane permeance.

A second limiting region is also shown in Fig. 5. This region, onthe right-hand side of the figure, occurs when low membranepermeance limits the removal of CO2. Performance in this region isbest explained by calculating the partial pressure on the feed andsweep side of the membrane. This calculation is shown in Fig. 6,for a membrane with a permeance of 200 gpu. The sweep ratio isset at 100. Because of the very high sweep ratio, the partialpressure of CO2 on the sweep side of the module is almost zeroat all points along the module. The driving force for permeation istherefore at its maximum value and cannot be further improvedby increasing the sweep flow rate. The fractional removal of CO2

from the feed is then also at its maximum value, set by thepermeation rate of CO2 through the membrane. In the exampleillustrated, a membrane with a permeance of 200 gpu achieves alimiting maximum removal (xmax) of �55%.

The limiting (maximum possible) removal of CO2 at high sweepratios is linked to the contactor membrane permeance, area andthe operating conditions of the device. In Appendix 2 at the end ofthis paper, it is shown that the term xmax is given by theexpression:

xmax ¼ 1�exp�ðPCO2=lÞpftA

Ff

!ð3Þ

where ðPCO2=lÞ is the membrane permeance, ptt is the total feedpressure of the contactor, A is the contactor surface area and Ff isthe feed volume flow rate (STP) to the contactor.

One final way to illustrate this interaction of sweep ratio andpermeance is to replot the data in Fig. 5 as a plot of permeanceagainst sweep ratio for various levels of CO2 removal. Twoscenarios are shown in Fig. 7, one for 80% CO2 removal and theother for 50% removal. On the line marked 80% removal, the basecase is marked as a solid point (black circle) at a permeance of1000 gpu and a sweep ratio of 1.0. Increasing the permeance of themembrane used in the base case device above 1000 gpu allowslower sweep ratios to be used while still reaching the target of 80%removal. However, as the sweep ratio approaches 0.8, the

membrane permeance required increases asymptotically. In thisregion, the separation reaches the sweep ratio limit set by Eq. (2).Similarly at high sweep ratios, the membrane permeance requiredto achieve 80% CO2 removal asymptotically approaches a limitingvalue of 480 gpu. The plot for 50% CO2 removal has the same form.The sweep ratio limit is at a ratio of 0.5, and the limitingpermeance is at 195 gpu.

Thus far, the discussion of contactor performance has beenlimited to units operating with equal pressures on either side ofthe membrane. The driving force for permeation in these units isonly due to the concentration differences between components ofthe feed and sweep gases. However, the concentration drivingforces can be enhanced by creating a pressure difference across themembrane. This effect is illustrated in Fig. 8, which compares thepartial pressure driving force profiles for the base case shown inFig. 3, and the same device operating at a feed pressure of 2 bar.

When the pressure is equal on either side of the membrane(1 bar/1 bar) [Fig. 8(a)], the partial pressure driving force feed-to-sweep is uniform across the module at 0.002 bar. However, whenthe same volume of feed gas is compressed to 2 bar [Fig. 8(b)], thepartial pressure driving force is significantly higher. At the feed endof the module, the driving force (feed-to-sweep) increases fivefoldto 0.011 bar, and then decreases steadily as feed CO2 concentrationfalls. The net result is to increase CO2 removal from 80% to 95%.

2.2. Non-ideal contactors

In the description of contactor performance given thus far, wehave made two significant assumptions. First, the membranes arepermeable to one component and impermeable to all others.Second, the volume of gas permeating the membrane is smallcompared to the feed flow. These assumptions are realistic forsome contactor applications. For example, in many dehydrationapplications, the permeability of water can be several-hundred-fold higher than that of the other components in the feed. Also, thefeed often contains only 1–2% water, so the volume flow throughthe membrane is small. However, in other applications, includingCO2 separation from flue gas, the simplifying assumptions are nolonger valid, so the separation performance will deviate from theideal contactor behavior described thus far. The consequences ofthese effects are described below.

For this section of the paper, we will use a new base casecontactor operating under the conditions shown in Fig. 9. Compar-ing this case to the ideal base case (Fig. 2):

Fig. 6. Feed and sweep concentration profiles within the ideal contactor (Fig. 2),operated with membranes having a permeance of 200 gpu. The sweep ratio is fixedat 100. Under these conditions, the exiting sweep gas contains very little CO2

(o0.01%) and the driving force for CO2 permeation reaches the maximumpossible value.

Fig. 7. Contactor membrane permeances required to achieve 50% and 80% CO2

removal as a function of sweep ratio for the ideal 5000 m2 contactor illustrated inFig. 2.

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� The concentration of CO2 in the feed gas has been increasedtenfold.

� The sweep gas is air containing 21% oxygen.� The membrane still has a CO2 permeance of 1000 gpu, but is

now also permeable to nitrogen (25 gpu) and oxygen (50 gpu).

The permeances of these membranes are comparable to mem-branes currently available for CO2 separations at industrial operat-ing conditions.

Comparing the new base case contactor performance assump-tions in Fig. 9 with the ideal contactor in Fig. 2 shows three maindifferences. First, the definition of the sweep ratio used in Eq. (1)for the ideal contactor no longer reflects the reality of the newcontactor. Using the Eq. (1) definition, the contactor has a sweepratio of 1.0 [sweep flow rate 1 m3(STP/s) and feed flow 1 m3(STP/s)], but at the feed end of the contactor, the sweep-to-feed ratio is1.08 (m3/s)/1.0 (m3/s) or 1.08, while at the sweep end of thecontactor the ratio is 1.0 (m3/s)/0.92 (m3/s), or 1.09. Defining thesweep ratio as the average of the inlet and exit sides of the sweepratio is clearly a better method to use than the ratio of the inletsweep and feed flows. Second, because of the volumeflow through the membrane, 80% CO2 removal from the feedmeans the residue gas concentration is 2.1% CO2. The membranearea required to achieve the same fractional removal is thenslightly less, at 4800 m2 rather than 5000 m2. Finally, some oxygenfrom the sweep side permeates into the feed side and somenitrogen from the feed permeates to the sweep. This effect isdescribed below.

3. The effect of membrane selectivity

The calculations for the ideal contactor described in the firstsection of this paper assumed the contactor membrane waspermeable to CO2 and impermeable to all other gases. Fig. 9 showswhat might be expected when a membrane with more realisticproperties is used. The membrane still achieves the requiredremoval of CO2 from the feed, but a significant amount of nitrogenalso permeates with the CO2 to the sweep, and some oxygen backpermeates from the sweep into the feed. A consequence of theseadditional flows is that when this contactor is used in the CO2

separation scheme illustrated in Fig. 1, the oxygen concentration inthe air sweep sent to the coal boiler decreases from 21% toapproximately 18%. Fortunately, an oxygen level of 18% in theboiler air stream will still provide efficient combustion, althoughminor changes to the boiler burner may be required [10].

Fig. 10 shows a plot of the oxygen concentration in the sweepgas leaving the contactor on its way to the boiler as a function offeed gas pressure. The performance profiles of several differenthypothetical membranes are shown. The CO2 flue gas feed pres-sure is varied from a pressure of 1 bar (the base case shown inFig. 9) to a feed pressure of 3 bar. As described earlier in the

Fig. 8. Effect of increasing the feed side pressure on the partial pressure profiles through the base case membrane contactor. Increasing the feed pressure from 1 bar [Figs. 8(a)] to 2 bar [Fig. 8(b)] increases the driving force (partial pressure difference, feed-to-sweep) fivefold at the feed end of the module. The fractional CO2 removal achievedwith the same size module then increases from 80% to 95%.

Fig. 9. Non-ideal base case membrane contactor.

Fig. 10. Calculated oxygen concentration of sweep gas in a non-ideal membranecontactor used to remove 80% CO2 from flue gas with a countercurrent air sweepstream. The performance obtained with different hypothetical membranes isplotted as a function of the flue gas feed pressure. The sweep ratio is maintainedat 1.0 in all the calculations shown.

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discussion of Fig. 8, increasing the feed pressure has a verysignificant impact on the driving force for CO2 permeation andcontactor performance. This means the membrane area requiredto perform the target separation decreases from 4800 m2 at a feedgas pressure of 1 bar (the base case) to 850 m2 at a feed gaspressure of 3 bar.

The top line in Fig. 10 is a reference line showing the normaloxygen concentration of air (21%). The next line down, markedCO2¼1000 gpu, N2¼0 gpu, O2¼0 gpu, is the calculated performanceof a contactor fitted with a membrane that permeates CO2, but isimpermeable to nitrogen and oxygen. The oxygen concentration inthe sweep air leaving a contactor fitted with this membrane is 19.3%.This decrease in oxygen concentration from 21% to 19.3% is due to thedilution effect of CO2 permeating the membrane into the sweep gas,thus increasing its volume. The next line in Fig. 10, markedCO2¼1000 gpu, N2¼25 gpu, O2¼0 gpu, shows the dilution effectdue to permeation of both CO2 and nitrogen from the feed into thesweep gas. The nitrogen contribution to dilution increases slightly asthe feed pressure increases, reflecting the effect of pressure on thedriving force for nitrogen permeation. Finally, the bottom line inFig. 10, marked CO2¼1000 gpu, N2¼25 gpu, O2¼50 gpu, shows thesweep gas concentration when the base case membrane from Fig. 9is used. The difference between this line and the one immediatelyabove it is the contribution of oxygen loss from the sweep gas to thefeed. This loss is small compared to the combined dilution effects ofCO2 and nitrogen, and decreases as the feed pressure increasesbecause the membrane area needed to perform the target separationdecreases with increasing feed pressure.

4. Conclusions

In this paper, we have shown the effect of operating parameterson the performance of a gas/gas membrane contactor used for CO2

removal. These devices are not in common use today, but couldfind future use in CO2 capture processes. The most importantoperating parameters affecting the gas/gas contactor performanceare the sweep gas-to-feed gas volume ratio, the relative pressuresof the feed and sweep gases, and the permeance and selectivity ofthe membranes used.

Acknowledgments

This work was performed as part of a research programsupported by the U.S. Department of Energy Project no. DE-FE-0007553.

Appendix 1

Equations used to calculate the partial pressure differencebetween the gases on either side of a gas/gas membrane contactor(Fig. A1).

Starting from the expression for gas permeation of individualcomponents1:

Ji ¼Pi

ℓðpfi �psi Þ ðA1Þ

we obtain the following equation for the partial pressure gradi-ents:

dJidа

¼ Pi

dpfidа

�dpsidа

!ðA2Þ

The partial pressure gradients can also be obtained from themass balance in each differential element:

Jida¼ �Ffpftdpfi or

dpfidа

¼ � JipftFf

ðA3Þ

Jida¼ �Fspsdpsi or

dpsidа

¼ � JipstFs

ðA4Þ

Combining Eqs. (A2)–(A4) gives

dJidа

¼ Pi

ℓJi

pstFs�pftFf

!ðA5Þ

Integrating Eq. (A5) over the membrane area, the expression forthe permeate flux as a function of the membrane area then gives

Ji;a ¼ Ji;0ePi=ℓððpst=FsÞ� ðpft =Ff ÞÞa ¼ Ji;0e

ba ðA6Þwith

b¼ Pi

ℓpstFs�pftFf

!ðA7Þ

Inserting Eq. (A6) into Eqs. (A3) and (A4) and integrating overthe membrane area then give the following expression for thepartial pressures in the two exit streams:

pfi;A ¼ pfi;0þ Ji;0pftFf

1�ebA

bðA8Þ

psi;0 ¼psi;A�pfi;0Pi=ℓpst=Fsðð1�ebAÞ=bÞ1�ðPi=ℓÞpst=Fsðð1�ebA=bÞ ðA9Þ

Combining with following equation,

Ji;0 ¼Pi

ℓðpfi;0�psi;oÞ ðA10Þ

we obtain for the partial pressure in the sweep outlet stream:

psi;0 ¼psi;A�pfi;0Pi=ℓpst=Fsðð1�ebAÞ=bÞ1�ðPi=ℓÞpst=Fsðð1�ebAÞ=bÞ ðA11Þ

and for the partial pressure in the feed outlet stream:

pfi;A ¼ pi;0f �pft=Ff

pst=Fsðpsi;0�psi;AÞ ðA12Þ

An Excel file based on these equations, which calculates thepressure profiles and local permeate fluxes, is attached in thesupplementary material to this paper. The graphics in the file willauto-adjust if the inputs are changed.

Appendix 2

The limiting (maximum possible) removal of CO2 is linked tothe contactor membrane permeance, membrane area and the

Fig. A1. Model for the countercurrent sweep module. Pressure drops in the feedand sweep channels are ignored. The permeating compound is assumed to bepresent at a low concentration, which means that the feed and sweep flow ratescan be assumed to be constant throughout the module.

1 In the equations that follow the superscripts f and s represent the feed andsweep side of the membrane, and the terms 0 and A represents positions along themembrane module from the feed entrance (0) to the feed exit (A).

P. Hao et al. / Journal of Membrane Science 462 (2014) 131–138 137

Page 8: CO2 Removal by Membrane Contactor

operating conditions of the device. The CO2 removal (x) is the ratioof the amount of CO2 permeating the membrane divided by theamount of the CO2 that enters the contactor; that is,

x¼R A0 JCO2

dAFf nCO2

ðA13Þ

where JCO2is the flux through the membrane at any point in the

contactor area A. Ff is the volumetric feed flow of gas into thecontactor at standard temperature and pressure conditions (STP),and nCO2 is the molar fraction of CO2 in the feed gas.

The maximum CO2 removed (xmax) is obtained at infinite sweepratio, in which case the membrane flux at any point is proportionalto membrane permeance ðPCO2=lÞ and the feed side CO2 partialpressure pfCO2

(the permeate side CO2 partial pressure is close tozero and can be ignored). Eq. (A13) can then be written2

xmax ¼R 0A ðPCO2=lÞpfCO2

dA

Ff nCO2

ðA14Þ

and since

nCO2 ¼pfCO2 ;0

pftðA15Þ

where pft is the total pressure on the feed side of the module,

xmax ¼pft ðPCO2=lÞ

R 0A pfCO2

dA

Ff pCO2 ;0f

ðA16Þ

The integral of the partial pressure driving forceR 0A pfCO2

dA hasthe familiar form for the log mean and Eq. (A16) can become

xmax ¼pft ðPCO2=lÞAFf p

fCO2 ;0

UpfCO2 ;0

�pfCO2 ;A

lnðpfCO2 ;0=ppfCO2 ;A

ÞðA17Þ

Because the fractional removal (xmax) can be also be written as

xmax ¼pfCO2 ;0

�pfCO2 ;A

pfCO2 ;0

ðA18Þ

Combining Eqs. (A17) and (A18) gives

lnpfCO2 ;0

pfCO2 ;A

0@

1A¼ PCO2=l

� �pftA

Ff¼ � lnð1�xmaxÞ ðA19Þ

which can be rearranged to

xmax ¼ 1�exp�ðPCO2=lÞUpft UA

Ff

!ðA20Þ

This expression shows the dependence of the limiting value forCO2 removal on the permeance, pressure, area and feed flow rateof an ideal contactor.

Nomenclature

A area of the membrane contactor, m2

Ff volumetric feed flow of gas into the contactor atstandard temperature and pressure conditions,m3(STP)/s

Fs volumetric sweep flow of gas into the contactor atstandard temperature and pressure conditions,m3(STP)/s

Jv actual gas velocity through the membrane support,cm3/cm2 s

JCO2carbon dioxide flux through the membrane at anypoint in the contactor, cm3(STP)/cm2 s

i ith component in the feed or sweep side.nCO2 molar fraction of CO2 in the feed gasðPCO2=lÞ CO2 permeance, gpu [1�10�6 cm3(STP)/

(cm2 s cmHg)]pfCO2 ;0

CO2 partial pressure at the feed entrance of themodule (point 0), cmHg

pfCO2 ;ACO2 partial pressure at the residue exit end of themodule (point A), cmHg

pft total feed pressure, cmHgxmax maximum possible fractional removal of a compo-

nent from the feed gas, that is, the ratio of theamount of a gas permeating the membrane dividedby the amount of the gas that enters the contactor.For this paper, xmax is discussed and calculated onlyfor CO2.

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P. Hao et al. / Journal of Membrane Science 462 (2014) 131–138138