N2 separation

Journal of Membrane Science 573 (2019) 476–484

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Simultaneous effects of temperature and vacuum and feed pressures on facilitated transport membrane for CO2/N2 separation

T

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Yang Han, Dongzhu Wu1, W.S. Winston Ho

William G. Lowrie Department of Chemical and Biomolecular Engineering and Department of Materials Science and Engineering, The Ohio State University, 151 West Woodruff Avenue, Columbus, OH 43210-1350, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Facilitated transport membrane Carbon capture Vacuum operation Carrier saturation

The facilitated transport of CO2 through an amine-containing polymeric membrane on a nanoporous PES substrate was studied at large feed-to-permeate pressure differentials that are relevant to post-combustion carbon capture. With the selective layer reinforced mechanically by incorporation of carbon nanotubes, the carrier saturation of amino groups by excessive CO2 was observed at high feed or permeate vacuum pressure. Nearly complete carrier saturation was observed at 7 atm feed pressure or a permeate vacuum near ambient pressure. At relatively low vacuum pressures, the vacuum degree and operating temperature affected the CO2 transport simultaneously. A vacuum pressure significantly lower than the water saturation pressure at certain temperature resulted in the dehydration of the selective layer, thereby a reduced CO2 permeance. This dehydration was mitigated at a moderate vacuum pressure. At the moderate vacuum pressure, the nanoporous PES substrate controlled the water permeation, leading to a sufficiently hydrated selective layer with high CO2 permeance and CO2/N2 selectivity. Overall, the membrane performed well with 0.3–0.6 atm vacuum pressures at 67 °C and a feed pressure of 4 atm, achieving the best membrane performance with a CO2 permeance of 1451 GPU and a CO2/N2 selectivity of 165.

1. Introduction Polymeric membranes containing amino groups can give highly selective separations of CO2 from hydrogen, flue gases and fuel gases [1–5]. These separations are CO2-selective because the amine can serve as a carrier to facilitate the CO2 transport. A carrier reacts with CO2 on one side of the membrane to form carbamate or bicarbonate [5]. This reaction product then diffuses across the membrane and releases the CO2 to the other side. Unlike CO2, other nonreactive species, e.g., N2 and H2, are largely rejected due to the absence of this carrier-mediated diffusion. Therefore, these facilitated transport membranes (FTMs) yield separations that are often more selective than membranes based on size or condensability discrimination, i.e., the solution-diffusion mechanism [6]. The high CO2/N2 selectivities exhibited by aminecontaining FTMs are of special interest for decarbonizing coal or natural gas derived flue gases, where the limited separation driving force requires a considerable CO2/N2 selectivity to achieve high CO2 recovery and purity simultaneously [7]. Most of the successes in the construction of amine-containing FTMs are demonstrated by polyamines, such as polyethylenimine (PEI) [8]

and polyvinylamine (PVAm) [9]. Often, small molecule amines are blended and stabilized within the polyamine matrix to increase the amino group content [1–3,5,10–12]. Such polymeric materials possess excellent processability. In most cases, water is a good solvent, and an ultrathin selective layer, e.g., 100 nm, can be coated on a proper porous substrate in flat-sheet [13,14] or hollow-fiber configuration [15]. Despite of the substantial research efforts in the material development, the application of FTMs in post-combustion carbon capture still faces several engineering challenges. Firstly, an actual separation modality in carbon capture involves a large feed-to-permeate pressure differential. Moderate flue gas compression as well as a permeate vacuum is required by most membrane processes, especially for the final CO2 enrichment [16–18]. However, the FTMs are often tested at nearzero pressure differential, where the mechanical integrity of the polyamine matrix is not rigorously characterized. Secondly, the CO2 partial pressures of the feed and permeate streams affect the amine-CO2 reaction equilibrium. The amine carriers tend to be saturated at a high CO2 partial pressure, resulting in a reduced CO2 permeance with increasing CO2 partial pressure [19]. Thirdly, as a consequence of the feed compression, the flue gas temperature rises, which is detrimental

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Corresponding author. E-mail address: [email protected] (W.S.W. Ho). 1 Current address: Liquid Microcontamination Control (LMC) Membrane R&D Group, Entegris, 9 Crosby Drive, Bedford, MA 01730-1401, USA. https://doi.org/10.1016/j.memsci.2018.12.028 Received 12 September 2018; Received in revised form 3 December 2018; Accepted 9 December 2018 Available online 11 December 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

Journal of Membrane Science 573 (2019) 476–484

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Pi0 p ∆p R r T νM

Nomenclature

B0 Ep J Ji K0 k0 k1 l M P

Pi

geometrical parameter contributing to viscous flow activation energy of permeation permeation flux of a pure gas through the membrane or PES substrate permeation flux of species i through the membrane geometrical parameter contributing to Knudsen diffusion shape factor shape factor effective thickness of membrane or PES substrate molecular weight permeability of a pure gas through the membrane or PES substrate permeability of species i

preexponential factor in Arrhenius relation the average of feed and permeate pressures pressure differential ideal gas constant pore radius temperature molecular speed

Greek letters

δ ε μ τ

shape factor porosity viscosity tortuosity

IL) under the commercial name Polymin® VX. The product was a waterin-water (w/w) emulsion with protonated PVAm suspended by a proprietary salt via salt bridging interaction. To purify the polymer, six times of reverse osmosis (RO) water by volume was added under mixing to break the emulsion. The polymer was then precipitated by pouring the clear solution into 4 times of ethanol by volume. After drying in a vacuum oven (60 °C, 29.9 inHg) for 16 h, the polymer was dissolved in RO water to form a 2.2 wt% solution. The polymer was deprotonated by Purolite® A600OH strong base anion-exchange resin (Purolite Corporation, Bala Cynwyd, PA) with a final pH of 11.7. After removing the resin by vacuum filtration, the PVAm solution was ready for use. The purified polymer was a poly(N-vinylformamide-co-vinylamine) (PNVF-co-VAm) random copolymer, which had a MW ca. 2000 kDa with an amino/amide molar ratio of ca. 15/85. The structure of this copolymer at a pH of 11.7 is shown in Fig. 1. The protonation-deprotonation equilibrium of vinyl amine depends on the pH [22].

for most polymers due to the reduced CO2/N2 solubility selectivities [20]. Typically, an intensive cooling is needed before passing the compressed flue gas to the membrane, which incurs undesirable energy penalty. This issue does not affect amine-containing FTMs significantly as a moderately high temperature also enhances the kinetics of the amine-CO2 reaction. However, an optimization on the operating temperature is necessary. This paper serves as a continuous investigation of a nanotube-reinforced FTM reported in our previous work [21]. The membrane comprised PVAm with amino groups covalently bound to the polymer backbone as the fixed-site carrier and an aminoacid salt, synthesized by deprotonating sarcosine with 2-(1-piperazinyl)ethylamine, as the mobile carrier. Multi-walled carbon nanotubes wrapped by a copolymer poly(1-vinylpyrrolidone-co-vinyl acetate) were dispersed in the selective layer as reinforcement nanofillers to sustain a large pressure differential. This membrane was further tested at conditions that are relevant to post-combustion carbon capture. The CO2 transport behavior was systematically studied to characterize the simultaneous effects of temperature and vacuum and feed pressures on membrane performance.

2.2. Polymer wrapping and dispersion of MWNTs The hydrophobic interaction between the hydrophobic MWNTs and amphiphilic PVP-co-VAc was utilized to assist the dispersion of MWNTs in water. The MWNTs powder was firstly suspended in 1 wt% SDS aqueous solution at a MWNTs concentration of 100 µg/mL in a 45-mL conical centrifuge tube. This suspension was placed in an ice bath and sonicated by a 1/8″ Microtip probe (Branson Sonifier® SFX 150, Danbury, CT, 150 W output power) at 70% amplitude for 30 min. PVPco-VAc was added in the dispersion to result in a 1 wt% solution, which was incubated at 50 °C for 16 h. The resultant mixture was centrifuged at 20,000 ×g for 1 h (Eppendorf 5806, Westbury, NY), forming a gelatinous pellet and a supernatant containing the excess polymer and SDS. The pellet was collected and dispersed in RO water at a concentration of 0.1 wt% by the sonication probe for 10 min, then centrifuged again. This centrifugation-dispersion procedure was repeated 5 times to remove the residual polymer and SDS. After the last centrifugation, the polymer-wrapped MWNTs could form stable dispersion up to 1 wt%. It is denoted as PVP-co-PVAc/MWNTs hereafter. A thermogravimetric analysis in our previous work revealed that this copolymer could wrap the MWNTs in a triple helix configuration and accounted for 24 wt% of the total mass of the polymer-wrapped MWNTs;

2. Experimental 2.1. Materials 2-(1-piperazinyl)ethylamine (PZEA, 99%), sarcosine (Sar, 98%), and sodium dodecyl sulfate (SDS, 99%) were purchased from SigmaAldrich (Milwaukee, WI). Ethanol (85%) was purchased from Fisher Scientific Inc. (Pittsburgh, PA). Poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP-co-VAc, 7:3 mol ratio of VP: VAc, 50% in ethanol) was purchased from VWR International (Radnor, PA). Graphistrength® C100 multi-walled carbon nanotubes (MWNTs, 0.1–10 µm length, 10–15 nm diameter, 5–12 walls) were provided by Arkema Inc. (Philadelphia, PA) as highly entangled powder. The powder comprised 90 wt% MWNTs with balance of Fe2O3 and Al2O3 as impurities. All the chemicals were used as received without further purification. For gas permeation measurement, pre-purified CO2 and argon were purchased from Praxair Inc. (Danbury, CT). Polyvinylamine (PVAm) was kindly provided by BASF (Vandalia,

Fig. 1. Chemical structures of PNVF-co-VAm as the fixedsite carrier and PZEA-Sar as the mobile carrier. In PNVFco-VAm, m:n:o = 85:12:3 at a pH of 11.7.

477

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2.5. Membrane characterization

the elimination of the hydrophobic surface of the MWNTs also enhanced their dispersity in PNVF-co-VAm [21].

The mixed-gas transport properties of the composite membrane were measured by a gas permeation unit shown in Fig. 2, which allowed for both sweep and vacuum modes on the permeate side. The synthesized membrane was loaded into a stainless-steel rectangular permeation cell inside a temperature-controlled oven (Bemco Inc. Simi Valley, CA) with an effective area of 2.7 cm2. The membrane was supported by a sintered stainless-steel plate with an average pore size of 100 µm. A 100-sccm dry feed gas containing 20% CO2 and 80% N2 was used, which was formulated by mixing the CO2 and N2 streams controlled by two mass flow controllers (MFC1 and MFC2, Alicat Scientific Inc., Tucson, AZ), respectively. When a sweep gas was used on the permeate side, a third mass flow controller (MFC3, Alicat Scientific Inc., Tucson, AZ) was employed to pass a 30-sccm dry argon, which was directed by Valve 1 to the permeate side to form a countercurrent flow pattern. The feed and sweep gases were fully saturated with water vapor by bubbling through 100 mL water in each of two 500 mL stainless-steel humidifiers (Swagelok, Westerville, OH) packed with 60 vol% Raschig rings, respectively. The humidifier temperature was controlled at 57 °C, which is the typical flue gas temperature leaving the flue gas desulfurization (FGD) unit [24]. The feed water content was calibrated by cooling the humidified feed gas at 0 °C and weighing the condensed water cumulated in a period of ca. 24 h, which corresponded to a water partial pressure of 17.2 ± 0.1 kPa at 57 °C. The oven temperature, however, was controlled in the range of 57–87 °C with an accuracy of ± 0.1 °C. Before reaching the permeation cell, the feed and sweep streams passed through two dry pressure vessels packed with Raschig rings, respectively, to remove any entrained water droplets. The feed and sweep pressures were controlled at 1–7 atm (abs) and 1 psig, respectively, by two pressure regulators, PC1 and PC2. After the moisture was knocked out by condensers at room temperature, the outlet gases were sent to an Agilent 6890 N gas chromatograph (GC, Agilent Technologies, Palo Alto, CA) for composition analysis. The GC was equipped with thermal conductivity detectors and a SUPELCO Carboxen® 1004 micropacked GC column (Sigma-Aldrich, St. Louis, MO). For the vacuum mode, the permeate side of the permeation cell was connected to an Ebara MD1 vacuum diaphragm pump (Ebara Technologies, Inc., Sacramento, CA). The permeate pressure was controlled precisely at 0.1–0.9 atm by a vacuum regulator (VC, Alicat Scientific Inc., Tucson, AZ). Before the permeate stream entered the vacuum pump, it passed through a 500 mL stainless-steel water knockout vessel that was cooled by a chiller (Fisher Scientific, Hampton, NH) at 0 °C to remove the moisture. The 30-sccm dry argon was directed by Valve 1 to carry the vacuum pump discharge to the GC for composition analysis. The membrane separation performance was characterized by CO2 permeance and ideal CO2/N2 selectivity. For the sweep mode, the

2.3. Mobile carrier synthesis The aminoacid salt mobile carrier was synthesized by reacting a base, i.e., PZEA, with an aminoacid, i.e., Sar. The stoichiometric amount of sarcosine was added slowly in a 24 wt% PZEA aqueous solution under vigorous mixing protected by a N2 shower. The solution was mixed at room temperature for 2 h before future use. The structures of the aminoacid salt, denoted as PZEA-Sar, is also shown in Fig. 1.

2.4. Membrane synthesis The MWNTs-reinforced composite membrane was synthesized by the following steps. Firstly, the purified PNVF-co-VAm solution was concentrated to 4 wt% by evaporating water under N2 purge at 50 °C. The PVP-co-VAc/MWNTs dispersion with a concentration of 1 wt% was added dropwise to the concentrated PNVF-co-VAm solution by a 10-µL glass capillary tube under vigorous agitation, aiming for 3 wt% MWNT loading in the final total solid of the coating solution. The mixture was transferred to a 15-mL conical centrifuge tube, in which it was homogenized by the 1/8″ Microtip sonication probe with a 50% amplitude for 20 min, cooled by an ice bath. A calculated amount of PZEA-Sar mobile carrier solution was incorporated in the dispersion to form the coating solution, aiming for a PZEA-Sar/PNVF-co-VAm ratio of 85/15 (wt./wt.). After centrifugation at 8000 ×g for 3 min to remove any air bubbles, the coating solution was coated on a nanoporous polyethersulfone (PES) substrate by a GARDCO adjustable micrometer film applicator (Paul N. Gardner Company, Pompano Beach, FL) with a controlled gap setting. The PES substrate was synthesized in house with an average surface pore radius of 15 nm and a surface porosity of ca. 13%. The PES layer had a thickness of 20 µm and contained 0.1 wt% PVP (360 kDa MW) to provide a slightly hydrophilic interface for the adhesion of the highly hydrophilic selective layer. The PES layer was supported by a 100-µm thick polyethylene terephthalate (PET) nonwoven fabric. The detailed composition and fabrication method of the PES substrate was reported in our previous work [23]. The addition of PVP, along with the PET non-woven fabric, affected the permeation of condensable gas species in the PES substrate, which will be discussed in Section 3.3. The coating solution had a viscosity of 1400 ± 32 cp at room temperature. The highly viscous coating solution allowed for the fabrication of a 170-nm thick selective layer via the knife coating technique. The membrane was dried in a fume hood at room temperature for at least 6 h before testing.

Fig. 2. Mixed-gas permeation unit for sweep and vacuum operations. 478

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8RT 1/2

( )

calculation method involving logarithmic-mean driving force for a countercurrent flow pattern has been described in our previous work [21]. For the vacuum mode, a crossflow permeation was assumed, and the permeance and selectivity were determined through the model developed by Shindo et al. [25]. The surface and cross-sectional morphologies of the membranes were observed by scanning electron microscope (SEM) using FEI Nova Nano SEM 400 (FEI Company, Hillsboro, OR). ImageJ, an open source software developed by National Institutes of Health, was used to analyze the SEM images.

speed, which equals to πM with M as the molecular weight. The two constants B0 [cm3 (STP)/(cm2 cmHg)] and K 0 [cm3 (STP)/(cm3 cmHg)], which account for the viscous flow and Knudsen diffusion, respectively, are given by

B0 = K0 =

ε 2

τ

(2)

δ r k1 lRT

(3)

These constants are partly determined by the specific geometry of the porous media, including the porosity ε , the tortuosity τ, the pore radius r , the effective thickness l , and the shape factors k 0 and δ / k1. For all porous membranes, k 0 = 2.5 and δ / k1 = 0.8 [27]. Therefore, the average pore radius and the effective porosity can be estimated by

2.6. Transport of water vapor, CO2 and N2 in PES substrate The transport of water vapor in the PES substrate was measured by an experimental apparatus shown in Fig. 3. 200 mL of water was injected in the feed humidifier with its temperature controlled at 57 °C. After the water vapor was formed in the feed humidifier, it passed to the permeation cell and permeated through the PES substrate under a given pressure differential. The permeation cell was placed in the oven at 57 °C. Both the feed and permeate sides were operated under vacuum, but two different vacuum controllers, VC1 and VC2, were used to control the vacuum pressures on the two sides, respectively. Typically, the feed pressure was varied in the range of 0.08–0.17 atm, while the permeate pressure was fixed at 0.02 atm. The retained and permeated water vapors were collected by their respective water knockout vessels that were chilled at 0 °C. At each pressure differential, the system was operated for 12 h before weighing the collected water for water vapor permeance calculation. After each run, the remaining water in the feed humidifier was also drained and weighed to verify the water balance. The transport of the non-condensable species CO2 and N2 in the PES substrate was measured in a similar fashion by the unit depicted in Fig. 2. The feed and permeate pressures were controlled by the two back pressure regulators PC1 and PC2, respectively. The feed pressure was varied in the range of 2–6 atm, whereas the pressure differential was kept at ca. 0.7 atm. The permeation rate was measured by a digital bubble flowmeter (Model 520, Fisher Scientific, Pittsburgh, PA), which had an accuracy of ± 0.2%. The dusty gas model was used in this study to characterize the effective pore size of the PES substrate. For single gas permeation, the permeance through a porous media is [26]

P JRT B 4 = = 0 p + K 0 νM ℓ Δp μ 3

ε r2 τ 2 k 0 lRT

r=

2B0 K0

(4)

K 02

ε = (lRT ) τ2 1.6B0

(5)

A further separation of ε and q is possible if the thickness l is known. However, the thickness of the PES substrate is not well-defined due to its asymmetric morphology. Consequently, an alternative measure can be formulated as

lτ 2 ε

=

1.6B0 , K 02 RT

which is the effective flow length

of gas through the PES substrate. 3. Results and discussion

3.1. Mechanical integrity of the membrane under large pressure differential The membrane reinforced by 3 wt% PVP-co-PVAc/MWNTs was tested at 57 °C with 7 atm feed pressure and 0.2 atm permeate vacuum to evaluate the mechanical integrity of the synthesized membrane under the large pressure differential. The cross-section SEM images before and after the test are shown in Fig. 4. The untested membrane had a distinct selective layer of 160–170 nm thickness on top of the nanoporous PES substrate. As reported in our previous work, the glass transition temperature of PNVF-co-VAm was in the range of 50–60 °C; without the MWNTs, the rubbery selective layer sank into the pores of the PES substrate under vacuum suction [21]. The dispersed MWNTs could form a network structure and their high flexural modulus refrained the deflection of the selective layer into the substrate pores. Consequently, the selective layer thickness did not change within the experimental error before and after applying the large pressure differential. The enhanced mechanical properties by carbon nanotubes have been previously reported for polymeric membranes subject to feed compression [28–30]. This work extended the application of MWNTs to membranes under vacuum operation. The stiffness of the reinforced

(1)

where P/ l [cm3 (STP)/(cm2 s cmHg)] is the permeance, J [cm3 (STP)/ (cm2 s)] is the flux, R [cm3 cmHg/(K cm3 (STP))] is the ideal gas constant, T [K] is the absolute temperature, ∆p [cmHg] is the pressure differential, μ [cmHg s] is the viscosity, p [cmHg] is the average of the feed and permeate pressures, and νM [cm/s] is the mean molecular

Fig. 3. Gas permeation unit for low-pressure water vapor transport measurement. 479

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Fig. 4. SEM images of membrane cross-sections. (a) and (b) are cross-sections of the membrane before and after vacuum test, respectively.

Fig. 5. Effects of feed pressure on (a) CO2 permeance and CO2/N2 selectivity and (b) CO2 and N2 fluxes at 57 °C with a vacuum of 0.2 atm on the permeate side.

Fig. 7. Effects of operating temperature and permeate vacuum on CO2 permeance with a feed pressure of 4 atm. Fig. 6. CO2 permeances and CO2/N2 selectivities at various feed pressures but at the constant CO2 partial pressure of 0.166 atm at 57 °C and 0.2 atm vacuum.

cmHg) with a CO2/N2 selectivity of 129 ± 5. With a 0.2 atm vacuum pulled on the permeate side, a CO2 permeance of 733 ± 7 GPU with a CO2/N2 selectivity of 122 ± 7 was observed, which practically reproduced the results by using a sweep gas. Based on this observation, the change of separation performance at different feed and permeate pressures in the following sections was not a consequence of the

selective layer was further substantiated by measuring the mixed-gas transport properties with sweep gas and a 0.2 atm vacuum, respectively. With the sweep gas of argon, the membrane demonstrated a CO2 permeance of 739 ± 6 GPU (1 GPU = 10−6 cm3(STP) cm−2 s−1 480

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Fig. 8. Gas permeances as a function of the average feed and permeate pressure p at 57 °C: (a) CO2 and N2 in PES-I and (b) water vapor in PES-I and PES-II. PES-I and PES-II contained 0.1 and 0.25 wt% PVP as the hydrophilic additive, respectively. Table 1 Geometry parameters of PES-I substrate estimated by the dusty gas model. Model parameter –13

3

CO2 −2

−1

B0 × 10 (cm (STP) cm s cmHg−2) K 0 × 10−7 (cm3 (STP) cm−3 s−1 cmHg−2) r (nm) ε /(τ 2l) × 1010 (cm−1)

N2

H2O

SEM measured

2.974

2.987

0.074 –

3.717

3.704

0.133 –

16.0 2.67

16.1 2.64

11.1 15.3 0.136 –

Fig. 10. CO2 and N2 permeances as a function of reciprocal of absolute temperature. Data correspond to the highest CO2 permeance at each temperature in Fig. 7.

Fig. 9. Gas-phase equivalent water partial pressure profiles in the composite membrane at 57 °C with a permeate vacuum of 0.2–0.5 atm.

mechanical integrity of the composite membrane. 3.2. Effect of feed pressure Fig. 5 illustrates the separation performance at 57 °C with a feed pressure of 1–7 atm (abs.) and a vacuum of 0.2 atm on the permeate side. The feed gas was saturated with water vapor at 57 °C (0.171 atm water partial pressure). As shown in Fig. 5(a), the CO2 permeance reduced from 970 to 730 GPU with increasing feed pressure from 1 to 7 atm. However, a nearly constant N2 permeance of 5.9 ± 0.1 GPU was observed, resulting in a proportional reduction in the ideal CO2/N2 selectivity. The CO2 and N2 fluxes at different feed pressures are depicted in Fig. 5(b). Due to the high vacuum degree on the permeate side and the relatively high CO2/N2 selectivities, the permeation driving forces for CO2 and N2 were almost proportional to the feed pressure. As

Fig. 11. Membrane stability in the presence of 3 ppm SO2 and 7% O2 at 67 °C with 4 atm feed pressure and 0.3 atm vacuum.

seen, the CO2 flux was not linear with the feed pressure but exhibited a saturation behavior. The N2 flux, however, increased linearly with the feed pressure. 481

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or secondary amine (e.g., Sar) consumes 2 mol of amine to convert 1 mol of CO2 into carbamate ion [38].

It is known that the CO2 permeance of the FTM depends on the feed CO2 partial pressure, i.e., at a higher CO2 partial pressure, the carrier saturation phenomenon occurred [31]. The initial non-linear increase of CO2 flux in Fig. 5(b) indicated a reducing amount of unreacted amines in the membrane. When nearly all the amine carriers had reacted with CO2 molecules, i.e., a fully carrier-saturated membrane, the CO2 flux increased in a much slower fashion with further increasing driving force. A transition to a flux plateau can be used to identify the saturation pressure when the Fickian diffusion of the reactive permeant is absent; otherwise, the flux increases linearly but very slowly with the driving force at a complete carrier saturation [32]. Based on the trend in Fig. 5(b), we hypothesize that the complete carrier saturation occurred at a feed pressure not too much higher than 7 atm. The selective layer material in this study had an amino group concentration of 0.01 mol/(cm3 polymer). Rigorously speaking, the amino groups in PNVF-co-PVAm and PZEA-Sar are not sterically hindered. Therefore, it is conservatively assumed that 1 mol of amine reacts with 0.5 mol of CO2 [33]. This leads to an estimated CO2 solubility of 82.1 cm3 (STP)/ (atm cm3 polymer), which is at least one-order-of-magnitude higher than most amorphous polymers and is in the same order-of-magnitude as polar physical solvents [34,35]. In order to further verify that the reduced CO2 permeance at high feed pressure was not caused by the polymer densification upon feed compression, the membrane was tested with a feed pressure of 1–7 atm but the feed CO2 concentration was adjusted to maintain a constant CO2 partial pressure of 0.166 atm (the CO2 partial pressure at 1 atm feed in Fig. 5(a)). As shown in Fig. 6, the CO2 permeance as well as the CO2/N2 selectivity nearly maintained the same at different feed pressures. This confirmed that the carrier saturation phenomenon was the dominant factor.

CO2 + 2RNH2 ⇌RHNCOO− + RNH+3

(6)

In the presence of water as well as a less basic environment, the carbamate ion can be hydrolyzed to form a bicarbonate ion and regenerate an amine, resulting in an overall 1 mol of amine for 1 mol of CO2 [39].

RHNCOO− + H2 O ⇌ HCO−3 + RNH2

(7)

Additionally, water molecules can form strong hydrogen bonds to polyamines, which increases the interchain spacing and leads to a highly swollen polymer matrix [40]. The reduced water retention in the polymer matrix under low vacuum pressures restricted the diffusion of CO2 and its reaction products, resulting in reduced CO2 permeances. 3.3.2. Substrate-controlled water permeation At a downstream pressure higher than the water saturation pressure, the membrane dehydration was mitigated. However, even for certain vacuum pressure range in the middle region of Fig. 7, the permeate stream was not fully saturated with water. For instance, at 57 °C and 0.2 atm vacuum, a H2O/CO2 selectivity of ca. 1 was obtained. Since the permeation rate of N2 was about two-orders-of-magnitude lower than those of CO2 and H2O, the crossflow pattern resulted in a water partial pressure of ca. 0.1 atm on the permeate side (58.1% relative humidity). Under this condition, the membrane showed a CO2 permeance of 910 GPU. This result, however, was very close to that measured with a fully saturated argon sweep at 57 °C. This observation was perplexing. It is well documented that an insufficiently hydrated sweep gas reduces the CO2 permeance of amine-containing polymeric membranes [1,41]. However, the selective layer of the membrane in this work was at least 100 times thinner than those of the previously studied FTMs. The high CO2 permeance at 57 °C and 0.2 atm vacuum indicated that the amine-containing selective layer itself was adequately hydrated, and the majority of the water activity gradient existed in the PES substrate. In order to further understand how the morphology of the PES substrate affected its transport properties, the gas flows through the PES substrate containing 0.1 wt% PVP (denoted as PES-I) were characterized by the dusty gas model at 57 °C. Fig. 8(a) shows a linear dependence of pure CO2 and N2 permeances on the average feed and permeate pressure p . The geometry parameters fitted by the dusty gas model for PES-I are summarized in Table 1. As seen, the geometry parameters B0 and K 0 obtained from these two non-condensable gases are consistent. The dusty gas model predicts an average pore radius of 16.0 ± 0.1 nm; this value agrees well with that characterized by SEM, for which the measurement is described in the Supporting Information. The water vapor transport through the PES-I substrate, however, behaved quite differently from the non-condensable gases. As shown in Fig. 8(b), the water vapor permeance of PES-I was at least one-order-ofmagnitude lower than those of CO2 and N2. The dusty gas model predicts an average pore radius of 11.1 nm, indicating the adsorption of condensable vapor on the pore surface. This adsorption layer reduced the water vapor permeance due to the less available pore space. We hypnotize that the addition of PVP increased the hydrophilicity of the pore surface [23], thereby promoting the adsorption of the condensable species. This assumption was further evidenced by measuring the water vapor permeance through an even more hydrophilic PES substrate containing 0.25 wt% PVP, denoted as PES-II in Fig. 8(b). PES-II exhibited a lower water vapor permeance than that of PES-I, and the permeance increased significantly with increasing p . This trend resembles the vapor permeation through a porous Vycor glass membrane reported by Lee and Kwang [42], where capillary condensation occurs in the hydrophilic pores and the permeance of the capillary condensate is pressure dependent. Another insight of the dusty gas model is to compare the ε /(τ 2l)

3.3. Simultaneous effects of operating temperature and vacuum pressure So far, the membrane was tested at 57 °C, the typical flue gas temperature after the FGD [24]. However, a higher operating temperature may be accessible since most membrane processes for post-combustion carbon capture require a mild feed compression in the range of 1.5–4 atm [7,17,36]. The flue gas temperature elevates when it is compressed. For instance, the flue gas temperature increases from 57 to ca. 100 °C if it is compressed to 1.5 atm. Fig. 7 shows the CO2 permeances at different temperatures (see also Fig. 10) under various vacuum pressures. A constant feed pressure of 4 atm was used; before passing to the membrane, the feed was fully saturated with water vapor at the corresponding test temperature. The vacuum pressure on the permeate side was varied in the range of 0.1–0.9 atm. The general trend was that the CO2 permeance increased with increasing temperature due to enhanced reaction kinetics. However, the relationship between permeance and temperature was more complicated than an Arrhenius form [37], since the vacuum pressure also played a role. For each temperature, the relationship between permeance and vacuum pressure resembled a bell curve. Three regions could be distinguished. In the left region, the CO2 permeance reduced sharply if the vacuum pressure was significantly below the saturation water vapor pressure at a given temperature, which was indicated by the red dashed line. In the middle region, the permeance only weakly depended on the vacuum pressure. In the right region, the permeance reduced gradually with increasing vacuum pressure. The simultaneous effects of temperature and vacuum pressure in these three regions will be discussed individually in the following sections. 3.3.1. Vacuum-induced dehydration of selective layer In the left region of Fig. 7, the sharp permeance reduction incurred when the vacuum pressure was just below the water saturation pressure at that temperature. In this case, the low vacuum dehydrated the selective layer, and the lack of moisture adversely affected the amine-CO2 reaction. The reaction between CO2 and a primary (e.g., PNVF-co-VAm) 482

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values for water vapor and the non-condensable gases. As discussed in Section 2.6, the reciprocal of this value represents the effective flow length through the PES substrate. A smaller ε /(τ 2l) value corresponds to a thicker media that contributes to the mass transfer resistance. As seen in Table 1, water vapor exhibited a considerable longer flow length through the PES substrate. Since the PES substrate comprised a 20-µm PES layer on top of a 100-µm non-woven fabric, this observation suggested that the non-woven fabric contributed to the mass transfer resistance to the water vapor permeation, but not much to CO2 and N2. To confirm this, the water vapor permeances were measured for the composite membrane, the PES substrate, and the non-woven fabric, respectively, at feed and permeate water partial pressures of 0.17 and 0.1 atm, respectively. At 57 °C, the measured water permeances were 921, 971, and 2219 GPU, respectively, for the three media above. For pure-gas permeation data, a resistance-in-series model can be used to analyze the permeance of each layer [26]. A description on the resistance-in-series model can be found in the Supporting Information. The calculation predicts that the water permeances of the selective layer, nanoporous PES layer, and non-woven fabric as 17,886, 1726, and 2219 GPU, respectively, by which the non-woven fabric contributes to 42% of the total resistance for water permeation. Furthermore, the water partial pressure profiles in the composite membrane can be calculated, and the results for 57 °C and a vacuum of 0.2–0.5 atm are shown in Fig. 9. As seen, the gas-phase equivalent water partial pressure in the selective layer only reduces from 0.172 to 0.168 atm at a vacuum of 0.2 atm. Therefore, the selective layer was sufficiently hydrated for the amine-CO2 reaction. This analysis shows that the substrate was the controlling step for water transport through the composite membrane. Similar observations have been reported in membranes tested by sweep gas [43,44].

selectivity increased from 153 to 165 when the temperature increased from 57° to 67°C, then reduced to 119 and 75 at 77 and 87 °C, respectively. Overall, the best membrane performance was achieved at a temperature of 67 °C, with a CO2 permeance of 1451 GPU and a CO2/N2 selectivity of 165. This membrane performance is exceptionally attractive for CO2 capture from flue gas since it can meet the stringent DOE capture target of $40/tonne CO2 captured set for 2025 [49]. 3.4. Membrane stability Fig. 11 shows the membrane stability with a simulated flue gas at 67 °C, 4 atm feed pressure and 0.3 atm vacuum. The simulated flue gas comprised 17.1% CO2, 68.5% N2, 7.4% H2O, 7% O2 and 3 ppm SO2. In comparison with the CO2 permeance tested without the SO2, the membrane showed a slightly permeance reduction of 20 GPU. This reduction stemmed from the physical sorption of SO2 into the aminecontaining selective layer, as evidenced by the in-situ FTIR study by Wu et al. [50]. However, the CO2/N2 separation was stable for a course of 120 h as shown in this figure, indicating promises of the membrane for post-combustion carbon captures at elevated temperatures. Under this condition, the permeate comprised 75.9% CO2, 3.2% N2, 0.1% O2, and 20.8% water vapor, corresponding to H2O and O2 permeances of 925 and 0.82 GPU, respectively. This led to a CO2 purity of 95.8% on dry basis. 4. Conclusions The CO2/N2 separation properties of an amine-containing FTM were studied at large feed-to-permeate pressure differentials that are relevant to post-combustion carbon capture. With the amine-containing selective layer reinforced mechanically by the incorporation of MWNTs, no membrane compaction or selective layer penetration was observed at a feed pressure up to 7 atm and a permeate vacuum of 0.2 atm. The CO2 flux was not linear with the feed pressure but exhibited a saturation behavior. Therefore, the CO2 permeance reduced with increasing feed pressure. A similar carrier saturation phenomenon was also observed when the downstream vacuum pressure was high, where the increased CO2 partial pressure at the downstream membrane surface retarded the dissociation of the amine-CO2 reaction products. At relatively low downstream vacuum pressures, the vacuum pressure and operating temperature played simultaneous roles in the CO2 transport. For a vacuum pressure significantly lower than the corresponding water saturation pressure at that temperature, the CO2 permeance reduced due to the dehydration of the selective layer. For a moderate vacuum pressure, however, the nanoporous PES substrate controlled the water permeation, leading to a sufficiently hydrated selective layer with high CO2 permeance and CO2/N2 selectivity. Overall, the membrane performed well with 0.3–0.6 atm vacuum pressures at 67 °C and a feed pressure of 4 atm, achieving the best performance with a CO2 permeance of 1451 GPU and a CO2/N2 selectivity of 165. The membrane also demonstrated a 120-h stable performance at 67 °C with a simulated flue gas containing 3 ppm SO2 and 7% O2.

3.3.3. Reverse reaction resistance at high downstream pressure In the right region of Fig. 7, the downstream interfacial pressure of CO2 increased when the vacuum pressure approached to the ambient pressure. The increased CO2 interfacial pressure might cause the reduced CO2 permeance in two possible ways. First, a hydrodynamic boundary layer might form on the permeate side of the membrane due to the low permeation rate (Reynolds number Re < 2.6 when permeate vacuum is larger than 0.5 atm). But, this hypothesis was excluded because the N2 permeance did not vary with vacuum pressure. Secondly, the high CO2 interfacial pressure slowed down the dissociation of the CO2-carrier reaction products (the reverse reaction in Eq. (7)). In this case, the permeation of CO2 through the membrane was retarded, owing to the saturation of the amine carriers. The reverse reaction resistances have been reported in Ag+-containing FTM for olefin/paraffin separation [45] and facilitated O2 transport through liquid membrane containing hemoglobin [46]. 3.3.4. Effect of temperature As discussed in the previous sections, the developed membrane was effective only for a certain vacuum pressure range at each temperature. For instance, at 67 °C, a vacuum below 0.3 atm or higher than 0.6 atm would be unfavorable. At the effective vacuum pressures, the temperature dependences of gas permeance actually followed the Arrhenius P

P0

Acknowledgements

Ep

relation li = li exp (− RT ) , where Pi0 is a preexponential factor and Ep is the activation energy of permeation. The highest CO2 and N2 permeances for each temperature are depicted in the Arrhenius plots as shown in Fig. 10. Provided that the selective layer was sufficiently hydrated, the Ep value for CO2 was 38.7 kJ/mol, which was of the same order of magnitude as typical rubbery polymers with moderate fractional free volume [47,48]. The Ep value for N2 was as high as 62.7 kJ/ mol, which was attributed to the low N2 solubility and the lack of reactive diffusion pathway. A consequence of the higher activation energy for N2 was that the N2 permeance increased faster than the CO2 permeance with increasing temperature. For this reason, the CO2/N2

We would like to thank José D. Figueroa and David A. Lang of National Energy Technology Laboratory for providing helpful discussion and input. We would also like to thank BASF (Charlotte, NC) and Purolite Corporation (Bala Cynwyd, PA) for their free samples of Polymin® VX and Purolite® A600OH, respectively. We would like to gratefully acknowledge the U.S. Department of Energy/National Energy Technology Laboratory (DE-FE0026919) and the Ohio Development Services Agency (OOE-CDO-D-13-05 and OER-CDO-D-15-09) for their financial support of this work. This work was partly supported by the Department of Energy under Award Number DE-FE0026919 with substantial involvement of the National Energy Technology Laboratory, 483

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Pittsburgh, PA, USA.

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