Thermally rearranged poly(benzoxazole-co-imide) hollow fiber membranes for CO2 capture

Journal of Membrane Science 498 (2016) 125–134

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

Thermally rearranged poly(benzoxazole-co-imide) hollow fiber membranes for CO2 capture Kyung Taek Woo 1, Jongmyeong Lee 1, Guangxi Dong, Ju Sung Kim, Yu Seong Do, Hye Jin Jo, Young Moo Lee n Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 13 August 2015 Received in revised form 5 October 2015 Accepted 6 October 2015 Available online 8 October 2015

Thermally rearranged poly(benzoxazole-co-imide) (TR-PBOI) hollow fiber membranes were fabricated from a hydroxyl polyimide-co-polyimide (HD5) precursor containing equal molar amounts of non-TRable DAM and TR-able HAB. A wide variety of spinning conditions were optimized in order to improve the gas permeation properties. A high bore flow rate (DI water) led to lowered gas permeation properties due to the generation of a dense, thick skin layer. The shear rate contributed significantly to manipulate the polymer chain packing density during spinning, therefore, CO2 permeance was critically enhanced in low shear rate. The addition of co-solvent (propionic acid) and pore forming agent (PEG 200) was shown to improve the gas permeation properties. The TR-PBOI hollow fiber membrane fabricated under optimal spinning conditions exhibited an excellent CO2 permeance of 560 GPU and CO2/N2 ideal selectivity of 16.8. A TR-PBOI hollow fiber module was successfully fabricated with an effective area of 106 cm2 for the mixed-gas permeation tests with a ternary gas mixture containing 14% CO2, 6% O2, and 80% N2. The results showed a permeate CO2 concentration around 50% and CO2 permeance of 400 GPU at a pressure ratio of 10. & 2015 Elsevier B.V. All rights reserved.

Keywords: TR-PBOI hollow fiber membranes Polyethylene glycol Co-solvent system Skin layer morphology Ternary mixed-gas measurement

1. Introduction The technology surrounding membrane gas separation has experienced steady growth during the past few decades, following the success of Permea’s PRISMs membrane system in industrialscale hydrogen recovery from ammonia purge gas [1,2]. Much effort has been dedicated to the commercialization of the membrane process in a variety of industrial-scale gas separation applications, including N2 production from air for the On-Board Inert Gas Generation System (OBIGGS), and natural gas sweetening process (CO2 separation from CH4) [3–6]. More recently, membrane gas separation was able to realize potential in carbon capture and storage, where the separation of CO2 from various sources has become a global concern to mitigate green-house induced climate change [3,4]. Despite the many advantages in this area of the membrane gas separation process, the majority of conventional polymeric membranes unfortunately do not have the capacity to effectively process large volumes of post-combustion flue gases at relatively low pressure [5–7]. In this regard, the focal point of research has been directed towards the synthesis of n

Corresponding author. E-mail address: [email protected] (Y.M. Lee). 1 These authors contributed equally.

http://dx.doi.org/10.1016/j.memsci.2015.10.015 0376-7388/& 2015 Elsevier B.V. All rights reserved.

highly-permeable polymers with large fractional free volume (FFV), such as thermally rearranged (TR) polymers, which deliver exceptional CO2 permeability, several orders of magnitude higher than conventional polymer materials [8–10]. TR-polymeric membranes were developed to enhance the gas permeation performance of these materials by investigating the effect of imidization methods [11,12], the use of cross-linked TRpolymers [13], polymer backbones with different monomers [14– 16], and the effect of the TR conversion [17]. In addition, fundamental studies on gas sorption [18,19], diffusion, permeation behaviors [12,20], the free volume [8,12,21] and cavity size [8,22] of the polymer matrix, and molecular dynamics simulations [23] have been performed to understand the relationship between the polymer structure and gas transport behavior. The fabrication of hollow fiber membranes using these TR-polymers was investigated, and laboratorial-scale TR hollow fiber modules (effective membrane area: 21.2 cm2) were prepared and tested using both single-gas and mixed-gas setups containing four components (CO2, N2, O2 and trace amount of H2O) [22,24,25]. Furthermore, improving the mechanical properties of the TR membranes is one of the most important factors to be considered for fabrication of industrial-size membrane modules. In this regard, the incorporation of non-TR-able moiety such as DAM to synthesize copolymers and the subsequent fabrication of TR-PBOI dense films using these polymers were studied in our previous work. The results

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O

F3C

HO

O

CF3

OH

N H

N H HOOC

O

F3C

CF3

H3C

O

N H

0.5

COOH

CH3

HOOC

CH3

COOH

N 0.5 H

Hydroxyl poly(amic acid) (HPAAc) O-xylene CF3

O

O

CF3

N

180 oC

N O

O

CF3

O

CH3 CF3

N

0.5

HO

O

O

O

OH

N

0.5

H3C

CH3

Hydroxyl polyimide-co-polyimide (HD5) Thermal rearrangement process F3C

CF3

O

O

N

N

O

0.5

F3 C

CF3

N O

O

CH3

N O

0.5

H3C

CH3

Thermally rearranged poly(benzoxazole-co-imide) (TR-PBOI) Fig. 1. The mechanism of thermal rearrangement reaction from HPAAc to TR-PBOI.

demonstrated that such a strategy substantially enhanced the mechanical properties of the TR polymeric dense films [26], and showed great potential to be adopted for TR-PBOI hollow fiber membrane fabrication in order to promote their mechanical properties. The primary strategy to improve the gas permeation properties of TR-PBOI hollow fiber membranes is to optimize spinning conditions, producing hollow fiber membranes with an ultra-thin defect-free skin layer [27–30]. The asymmetric membrane for gas separation usually consists of a dense skin layer to function as a selective barrier to separate different gas species, with a microporous layer underneath to provide mechanical support. In the case of flue gas CO2 capture, the micro-cavity size and the thickness of the dense skin layer are the key factors to determine the CO2 permeance as well as the CO2/N2 selectivity. The polymer solution with high polymer concentration is required in order to attain an enhanced polymer chain packing density in the selective skin layer. The use of polymeric additives, especially polyethylene glycol (PEG) with appropriate molecular weight, for pore formation exerts great influence on phase inversion. Increasing the molecular weight of PEG favors delayed de-mixing, due to lowered solubility with coagulant (water), as well as a propensity towards increased crystallization [31–33]. Other strategies have also been proposed to increase the FFV of the polymer matrix, thus enhancing the gas permeation properties. For instance, the use of Lewis acid–base complexes as co-solvents could increase the polymer molar volume, and subsequently increase the FFV and gas permeance [20,26,34,35]. The rheological factor should also be considered when optimizing spinning conditions. The high shear rate induced by the increased dope extrusion rate greatly affects not only the polymer chain packing density in the selective skin layer due to polymer chain entanglement and orientation in the air-gap, but also causes undesired high die swells in the nascent hollow fiber membranes. As a result, gas permeation properties are significantly decreased in the case of high shear rate [28]. In this study, TR-PBOI hollow fiber membranes were fabricated

using an in-house-synthesized polymer precursor containing both TR-able and non-TR-able segments. A comprehensive study was carried out to examine the effects of variations to the dope solution including viscosity, coagulation value, co-solvent, PEG additive, and dope rheological factors on the morphology and gas permeation performances (both single- and mixed-gas) of the hollow fiber membranes. Ternary mixed-gas was measured to showcase the feasibility of using TR-PBOI hollow fiber membranes in a flue gas CO2 capture process.

2. Experimental 2.1 Materials The monomers used for copolymer synthesis were 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) as dianhydride, purchased from Daikin Industries, Ltd. (Osaka, Japan), 3,3′-dihydroxyl-4-4′-diamino-biphenyl (HAB, Wakayama Seika Kogyo., Ltd., Wakayama, Japan) as TR-able diamine, and 2,4,6-trimethyl-mphenylene (DAM, Dottikon Exclusive Synthesis AG, Dottikon, Switzerland) as non-TR-able diamine. Other chemicals used in this study include N-methyl-2-pyrrolidinone (NMP, 99.8%, Daejung Chemicals & Metals, Siheung, Republic of Korea), propionic acid (PA, 99.0%, Junsei Chemical Co., Ltd., Tokyo, Japan), polyethylene glycol (PEG anhydrous, 99.8%, Sigma Aldrich Chemical Co., Ltd., Milwaukee, WI, USA), and o-xylene (99%, Daejung Chemicals & Metals). The gases used in the permeation tests including CO2 (þ99.995%), O2 (99.995%), N2 (þ 99.9992%), Ar (þ99.992%), and He (þ 99.9992%) were all purchased from Seoul Special Gas, Seoul, Republic of Korea.

2.2 Preparation and characterization of the polymer precursor and

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127

polymer solution

membrane

2.2.1 Synthesis of precursor The azeotropic imidization method reported by Han et al. [11] was employed for the synthesis of hydroxyl polyimide-co-polyimide (HD5) precursors [26], as shown in Fig. 1. The diamines (HAB and DAM) and dianhydride (6FDA) were dried at 40 and 100 °C, respectively, for 1 day under vacuum. The dried HAB and DAM monomers were added to the reactor filled with NMP at ambient temperature, and stirred vigorously for 3 h until the monomers were fully dissolved. Nitrogen was used as a purge gas to remove water vapor formed during polymerization. 6FDA was added to the solution at 0 °C, and stirred for 12 h for the synthesis of hydroxyl poly(amic acid) (HPAAc) as the precursor of HPI–PI (labeled as HD5 hereafter, with an equal molar ratio of HAB and DAM, as shown in Fig. 1). The molarity of 6FDA is equal to the sum of HAB and DAM molarities. O-xylene was added into the HPAAc solution as an azeotropic reagent at 180 °C using a MS-EB106 heating mantle (Misung Scientific. Co., Ltd., Yangju, Republic of Korea), and the solution was stirred for 24 h under nitrogen protection. During the reaction, HPAAc was converted to HD5. Upon the completion of azeotropic imidization, the solution underwent a precipitation process using de-ionized water to obtain the HD5 precursor powder. The HD5 powders were then kept in an oven at 60 °C under vacuum.

A dry-jet wet spinning technique was adopted to fabricate the TR-PBOI hollow fiber membrane, as shown in Fig. 2. The dope solution was extruded using a gear pump (GM-S series from Mitsubishi Company, Tokyo, Japan). The shear rate inside the spinneret was calculated based on the following equations:

2.2.2 Viscosity measurement The viscosity of the HD5 solution was measured using a SV-100 Vibration Viscometer (A&D Company, Ltd. Tokyo, Japan). The viscosities of the solution at various polymer concentrations (ranging from 6 to 28 wt% at 25 °C) and temperatures (from 25 to 70 °C at 25 wt% of polymer) were measured [22]. 2.2.3 Coagulation value measurement The coagulation value is defined as the amount of non-solvent required to precipitate 100 g polymer solution. The titration method was adopted to measure the coagulation value, in which 20 g of HD5 polymer solution containing polymer (HD5), solvent (NMP), and polymeric additives (PEG) was firstly prepared via vigorous stirring at 60 °C. De-ionized water as the non-solvent was titrated into the solution drop-wise until precipitation took place. The amount of de-ionized water was then measured to calculate the coagulation value [33,36,37]. 2.3 Fabrication and characterization of the TR-PBOI hollow fiber

λ=

1 (1−k 2)2 2 ln ( 1 ) k

A = 1 − k4 −

γ̇ =

(1)

(1−k 2)2 1

ln ( k )

(2)

⎛ R ⎞⎫ dV 4q ⎧ ⎛ r ‵ ⎞ = 3 ⎨ ⎜ ⎟ − λ2⎜ ⎟⎬ ⎝ ⎠ ⎝ r‵ ⎠⎭ dr R ·A ⎩ R

(3)

Here, R and r are the outer and inner radii of the spinneret, respectively. r′ is the sum of r and the wall thickness of the nozzle, and k is r’′ divided into R. q is the dope solution extrusion rate through the spinneret (from 1.05 to 3.00 ml/min in this study). The flow rate of the bore solution (de-ionized water) was controlled via a 307 HPLC pump (Gilson Company, Middleton, WI, USA). Both solutions were extruded through the spinneret (Φ0.44–0.2–0.1 from Kasen, Osaka, Japan) to form the nascent hollow fiber membrane. The nascent hollow fiber membrane passed through the air-gap (5 cm), the coagulation bath (tap water, 50–80 °C), and two godet bathes (tap water at 40 °C and 30 °C respectively). The final HD5 hollow fiber membranes were collected in a take-up drum bath filled with 25 °C tap water. The spun hollow fibers were kept in the washing bath, which was circulated with tap water for 3 days to remove residual solvent prior to the drying process in the oven with N2 protection [22,25]. The dried HD5 hollow fiber membranes (450 μm of outer diameter) underwent a thermal treatment process to be converted into TR-PBOI. Various thermal treatment temperatures were assessed (300, 375, 385, 400, and 450 °C for 1 h with 5 °C/min heating rate) to examine the effect of temperature on the TR conversion rate [26]. The morphology of TR-PBOI hollow fiber membranes was investigated via field-emission scanning electron microscope (FESEM, JEOL, JSM-6330F, Tokyo, Japan). The cross-sectional hollow fiber membrane samples were prepared using cryogenic fracturing in liquid nitrogen (þ 99.9997%, Seoul Special Gas). The SEM samples were placed in the oven at 100 °C for 12 h under vacuum to

Fig. 2. Schematic representation of the hollow fiber membrane spinning apparatus.

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K.T. Woo et al. / Journal of Membrane Science 498 (2016) 125–134

The mixed-gas composition was analyzed by using 490 micro Gas Chromatography (Agilent Technologies, Inc. Santa Clara, CA, and U.S.A.), and the CO2 permeance in the mixed-gas permeation test was calculated based on the following equation:

remove absorbed water molecules inside the samples. 2.4 Gas permeation measurement 2.4.1 Pure gas measurement The modules were produced by fabricated TR-PBOI hollow fiber membranes. The gases were fed into shell side in the module with the feed pressure from 1 to 6 bar at 30 °C. The gas permeance was calculated via the following equation:

P Q = l Δp·A

(4)

where P/l is the permeance with one unit of gas permeation unit (GPU) defined as 1 GPU ¼10  6 cm3(STP) cm  2 s  1 cm Hg  1. Q, Δp, and A are the volumetric gas flow rate in the permeate side (cm3 (STP)/s), pressure difference across the membrane (cm Hg), and the effective membrane area (cm2), respectively. The pure gas measurements were performed using N2 and CO2 from 1 to 6 bars at 30 °C with a shell side feed configuration. The ideal selectivity was calculated based on the following equation:

αA = B

( P l )A PA = PB ( P l )B

(5)

* PCO 2 l

=

Vt ·yCO

2

⎡ (p ·x ) − (p ·y )⎤·A ⎣ feed CO2 permeate CO2 ⎦

(7)

* /l is the CO2 permeance under mixed-gas conditions, Vt, where PCO 2 yCO and xCO2 are the total permeated gas flow rate (cm3 (STP)/s) 2 measured via bubble flow meter, the permeate and feed CO2 concentration (vol%) measured via micro GC, respectively. pfeed and ppermeate are the pressure difference between the feed and permeate side (cmHg), and A is the membrane effective area (cm2). The separation factor ( α A* ) was calculated using the following B

equation:

yA α A* = B

xA

yB xB

(8)

where xA and xB indicate the feed compositions of gases A and B, while yA and yB are the permeate compositions of gases A and B [22,39].

where α A is the ideal selectivity and (P/l)A and (P/l)B are the pure B

gas permeances of A and B, respectively [38].

3. Result and discussion

2.4.2 Mixed-gas measurement The mixed-gas was composed of 14 vol% CO2, 6 vol% O2, and 80 vol% N2. The feed gas flow rate was maintained at 6,000 ml/min (0.36 Nm3/h) using an M3030VA mass flow controller (Line Tech, Daejeon, Republic of Korea), and the feed pressure was controlled via a backpressure regulator (Tescom 44-1700). The schematic of the mixed-gas measurement instrument is shown in Fig. 3. The feed pressures were varied from 2 to 10 bar (absolute), while the permeate pressure was maintained at atmospheric pressure; the pressure ratio was calculated using the following equation:

3.1 Determination of the polymer concentration and dope temperature

φ=

Po Pl

(6)

where φ is the pressure ratio and Po, Pl are the feed and permeate pressures, respectively [39].

As shown in Fig. 4(a), increasing the polymer concentration from 6 wt% to 28 wt% at 25 °C led to an exponential increase in solution viscosity, from 500 to 120,000 cP. Similar to the approach described in our previous study, the critical concentration was found at 20.5 wt% (40,000 cP) [22]; however, such a polymer concentration, although suitable for casting dense films, is not sufficient for the fabrication of hollow fiber membranes. As many previous studies pointed out, the polymer concentration must be at least 25 wt% to ensure the formation of a defect-free skin layer [40,41]. It was found that at a polymer concentration of 25 wt%, the viscosity was too high to be spinnable (74,400 cP). As such, the effect of temperature on viscosity was also studied in this work, as

Fig. 3. Schematic representation of the mixed-gas permeation apparatus.

K.T. Woo et al. / Journal of Membrane Science 498 (2016) 125–134

129

120,000 70,000

Viscosity (cP)

Viscosity (cP)

100,000 80,000 60,000 40,000

60,000

50,000

40,000

20,000 30,000 0

6 8 10 12 14 16 18 20 22 24 26 28

70 65 60 55 50 45 40 35 30 25

Polymer concentration (wt%)

Dope solution temperature ( C)

o

Fig. 4. Effect of (a) polymer concentrations, and (b) solution temperatures on the viscosity of the polymer solution.

3.2 Fabrication of hollow fiber membranes 3.2.1 Effect of polymer additives on solution viscosity and coagulation value Polyethylene glycol (PEG) is commonly used for pore formation in polymeric membranes [31,32,37]. As shown in Fig. 5(a), with the fixed 25 wt% polymer concentration, increasing the PEG concentration from 0 to 25 wt% caused a decrease in the coagulation value and an increased solution viscosity, while the two profiles intersected at a PEG concentration of 15 wt%. Therefore, 15 wt% of PEG was determined to be the appropriate concentration of polymeric additives, as higher viscosity and lower coagulation

25

50,000

20

45,000

15

40,000

10

35,000

5

0

5

10

15

20

PEG concentration (wt%)

25

0

D o p e s o l u ti o n v i s c o s i ty (c P )

55,000

30,000

120,000

30 Viscosity Coagulation value

Coagulation value (g)

D o p e s o lu t io n v is c o s it y ( c P )

60,000

values were preferred. In terms of the effect of PEG molecular weight on the solution viscosity and coagulation value, it was found in Fig. 5(b) that decreasing the molecular weights caused a substantial decrease in both viscosity and coagulation value. A PEG with a higher molecular weight could hinder the polymer chain mobility and show a higher crystallization propensity, which in turn lowered the solvent exchange rate and caused a “delayed demixing”. The low-molecular-weight PEG exhibited a low coagulation value with low solution viscosity, which could lead to the socalled “instantaneous de-mixing” [42]. The TR-PBOI hollow fiber membranes were fabricated with various PEG concentrations and molecular weights. As shown in Fig. 6(a), increasing the PEG concentration led to an increased CO2 permeance and decreased CO2/N2 selectivity, which demonstrates the pore-forming functionality of the PEG additive. More specifically, it was found that when increasing the PEG concentration to over 15 wt%, the ideal selectivity of CO2/N2 dropped drastically from approximately 17 to approximately 5 with a PEG concentration of 25 wt%, suggesting the formation of defects on the membrane surface. As shown in Fig. 5(b), the molecular weight of PEG had great

35 Viscosity Coagulation value

100,000

80,000

30 The preferable viscosity for hollow fiber (50,000 cP)

25

60,000

20

40,000

15

20,000 100

1,000

10,000

PEG molecular weight (g/mol)

Fig. 5. Viscosity and coagulation value measurements as a function of (a) PEG concentration, and (b) PEG molecular weight.

10

Coagulation value (g)

shown in Fig. 4(b). The results revealed that increasing the temperature led to a decrease in viscosity; with 25 wt% polymer concentration, the temperature needed to be maintained above 55 °C to ensure that the viscosity was lower than the threshold value of 50,000 cP, which was the maximal spinnable viscosity. As such, the temperature was determined at 60 °C (41,000 cP) with 25 wt% polymer, taking the possible non-solvent effect into account.

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Polymeric additive - PEG 200

PEG concentration - 15 wt%

1,000

25 /l

20

600

15

400

10

200

5

800 20 600 15 400 10

84%

200

Selectivity (αCO2/N2)

800

Permeance (PCO2/l, GPU)

25

α

Selectivity (αCO2/N2)

Permeance (PCO2/l, GPU)

P

1000

5 0

0

5

10

15

20

25

0

0 100

PEG concentration (wt%)

1,000

10,000

PEG moledular weight (Da)

Fig. 6. The effects of (a) PEG concentration and (b) PEG molecular weight on CO2 permeance and CO2/N2 ideal selectivity.

3.2.2 Effect of the co-solvent The concept of using Lewis acid–base complexes as co-solvents was first introduced by Kesting et al. [34,35] for the enhancement of macromolecular packing density. The commonly used organic solvent, NMP, interacts with acidic co-solvents such as propionic acid by forming Lewis acid–base complexes. As shown in Table 1, it was found that by increasing the propionic acid ratio in the solvent system, the solvent molar volume was gradually improved due to the interaction between NMP and propionic acid. The equimolar co-solvent system showed a significantly larger solvent molar volume (167 ml/mol) than the mono-solvent free of propionic acid (94 ml/mol). Such an improvement in solvent molar volume could expand the polymer matrix volume and decrease the polymer chain packing density. TR-PBOI hollow fiber membranes with various molar ratios of co-solvents were prepared using the previously-determined optimal conditions (25 wt% HD5, 15 wt% PEG 200). The CO2/N2 pure Table 1 Solvent molar volume along with solvent composition. NMP/propionic acid (wt%/wt%)

Molar ratio of propionic acid

Solvent molar volume (ml/mol)

100/0 85/15 70/30 57/43

0 0.23 0.55 1.00

94 111 135 167

700

25 Equimolar ratio solvent

αCO2/N2

600

20

500

15

400

10

300

5

200

0

15

30

45

Selectivity (αCO2/N2)

PCO2 /l

Permeance (PCO2/l, GPU)

influence on the coagulation value and solution viscosity, which in turn greatly affected the gas permeation performance shown in Fig. 6(b). Significantly lower CO2 permeance and slightly higher CO2/N2 ideal selectivity were observed with increased PEG molecular weight. Özdemir et al. calculated the solubility of PEG in water [43], and found higher solubility of smaller molecular weight PEG in water. As a result, the use of low-molecular-weight PEG could lead to an instantaneous de-mixing, thus causing the formation of a thin skin layer and subsequently higher CO2 permeance. When choosing the PEG additive with higher molecular weight, a significantly higher polymer solution viscosity was observed and a higher crystallization propensity was expected, both of which induced the formation of a thick skin layer and lower CO2 permeance. Based on the observation from Fig. 6, 15 wt% PEG 200 was selected as the optimal additive concentration and molecular weight for the subsequent hollow fiber fabrication.

0

Propionic acid ratio (wt%) Fig. 7. CO2 permeance and CO2/N2 ideal selectivity as a function of molar ratio of propionic acid.

gas permeation results revealed that as a result of increasing the propionic acid ratio to 43 wt% (equimolar ratio with NMP), the CO2 permeance was 2.4 times higher than that of the membranes prepared without propionic acid, as shown in Fig. 7. This improvement in gas permeance is primarily ascribed to the decreased polymer chain packing density in the selective layer of TRPBOI hollow fiber membranes due to the expanded polymer matrix volume when using propionic acid as the co-solvent. It was also found that the ideal CO2/N2 selectivity only dropped by 19%, from 21 to 17. 3.2.3 Effect of dope and bore flow rates 3.2.3.1 Bore flow rate. In this study, the bore flow rates were increased from 1.0 to 2.5 ml/min with a fixed dope flow rate of 1.05 ml/min. Increasing the bore flow rate resulted in not only a higher bore flow speed, but also expanded inner and outer diameters in hollow fibers with decreased wall thickness due to the die swell in the nascent hollow fiber membrane (Table 2). The increased bore flow rate encouraged the solvent and non-solvent exchange rate in the bore side of the nascent hollow fiber

K.T. Woo et al. / Journal of Membrane Science 498 (2016) 125–134

Table 2 Dimensional properties of the TR-PBOI hollow fiber membrane fabricated under various bore flow rates.

131

Table 3 Dimensional properties of the TR-PBOI hollow fiber membranes fabricated under various dope flow rates.

Bore flow rate (ml/ min)

*Bore flow speed (m/ min)

Fiber outer diameter (μm)

Fiber inner diameter (μm)

Wall thickness (μm)

Dope flow rate (ml/ min)

Shear rate Fiber outer ( γ̇, s  1) diameter (μm)

1.0 1.5 2.0 2.5

127 191 255 318

450 72.7 457 73.4 464 74.6 471 73.9

324 72.2 337 72.0 348 72.7 357 71.8

637 1.3 607 2.2 587 3.1 577 1.8

1.05 1.50 2.25 3.00

639 913 1369 1826

*

450 7 2.7 463 7 3.1 4747 3.6 4887 2.1

Fiber inner diameter (μm)

Wall thickness (μm)

3247 2.2 3217 1.7 3147 2.9 3067 1.1

63 7 1.3 71 7 2.0 80 7 3.2 91 7 1.4

Bore flow speed ¼Bore flow rate/nozzle cross sectional area. -1

αCO2/N2

400

15

200

10

1.5

2.0

2.5

Permeance (PCO2/l, GPU)

α CO2/N2

Selectivity (αCO2/N2)

Permeance (PCO2/l, GPU)

PCO2/l

20

1.0

609

PCO2/l

600

0

800

25

20

400

15

200

10

1.0

membrane [44–46], and more importantly, enhanced the leaching of the pore-forming agent, PEG 200, into the bore solution. As a result, the higher bore flow rate not only led to a higher polymer concentration in the nascent hollow fiber membrane, but also limited the formation of pores due to the reduced amount of PEG 200, both of which induced the generation of a thicker skin layer and the reduction in CO2 permeance. As shown in Fig. 8, the highest CO2 permeance was achieved at the lowest bore flow rate (1.0 ml/min). 3.2.3.2 Dope flow rate. The dope flow rate was varied from 1.05 to 3.00 ml/min with a bore flow rate of 1.0 ml/min without any external force to pull the nascent hollow fiber in order to examine the effect of shear rate within the spinneret on membrane performance. As the shear rate ( γ ,̇ calculated using Eq. (3)) increased from 639 s  1 (1.05 ml/min) to 1826 s  1 (3.00 ml/min), the die swell of the nascent hollow fibers in the air-gap became more severe, since the dope solution with a high shear rate showed low shear viscosity (non-Newtonian fluid) [47]. The wall thickness of the hollow fiber increased to 91 μm, as shown in Table 3. The increase in shear stress improved molecular orientation before the extrusion of the dope solution from the spinneret, causing a denser skin layer and decreased CO2 permeance, which was in good agreement with previous studies [28,48–50]. As shown in Table 3 and Fig. 9, a dope flow rate of 1.05 ml/min led to a low shear rate of 639 s  1 while exhibiting the highest CO2 permeance.

1.5

2.0

2.5

3.0

5

Dope flow rate (ml/min)

Bore flow rate (ml/min) Fig. 8. Pure gas permeation properties of TR-PBOI hollow fiber membranes fabricated with various bore flow rates and a fixed dope flow rate of 1.05 ml/min.

25

600

0

5

1826

Selectivity (αCO2/N2)

Dope flow rate - 1.05ml/min

800

Shear rate (s ) 1215 1518 912

Fig. 9. Pure gas permeation properties of TR-PBOI hollow fiber membranes fabricated with various dope flow rates and a fixed bore flow rate of 1.0 ml/min.

3.2.4 Effect of the coagulation temperature As aforementioned, the phase inversion rate can be manipulated via dope and bore solution compositions and temperatures [22,27,28,51–55]. In the current study, the effect of the coagulation temperature on the liquid–liquid de-mixing rate between the dope solution and the coagulant medium (tap water) was assessed. As shown in Table 4, a clear trade-off relationship can be found between the CO2 permeance and CO2/N2 ideal selectivity along with the coagulation temperature. With an increase in coagulation bath temperature from 50 to 80 °C, the CO2 permeance increased from 330 to 862 GPU, while the CO2/N2 ideal selectivity dropped from 20 to 7.3. Increasing the coagulation bath temperature led to an instantaneous liquid–liquid de-mixing between the solvent and non-solvent, which led to the formation of a significantly thinner skin layer, thus contributing to higher permeance and lower selectivity [56–58]. A substantial drop in CO2/ N2 ideal selectivity was observed at coagulation temperatures Table 4 Gas permeation properties at various coagulation temperatures. Dope solution temperature (oC)

Coagulation temperature (oC)

Permeance ( PCO2 /l, GPU)

Selectivity ( α CO2 )

60

80 70 60 50

862 560 410 330

7.3 17 18 20

N2

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K.T. Woo et al. / Journal of Membrane Science 498 (2016) 125–134

Table 5 Gas permeation properties and TR conversion rate at various thermal treatment temperatures. Code

TR-300 TR-375 TR-385 TR-400 TR-450

TR conversion ratio (%)

0 36.6 43.5 48.6 99.7

Dense membranes [26]

Asymmetric hollow fiber membranes

Permeability PCO2 (barrer)

Selectivity α CO2

Permeance PCO2 /l (GPU)

Selectivity α CO2

26 105 N/A 196 747

52 23 N/A 21 20

120 560 448 253 97

29 17 17 20 28

N2

Effective skin layer thickness (μm)

N2

0.2 0.2 0.9 1.8 2.4

above 70 °C, suggesting the formation of defects on the membrane surface. As such, the coagulation temperature of 70 °C was considered the optimal temperature for the fabrication of TR-PBOI hollow fibers, as it offers a high CO2 permeance of 560 GPU while still delivering a reasonable CO2/N2 ideal selectivity of 17.

lower CO2 permeance and higher CO2 permeability were detected, providing critical information for the future development of TR hollow fiber membranes. The importance of thermal treatment protocol optimization should be highlighted, as the thickness of the skin layer was strongly susceptible to undesired densification.

3.3 Effect of the thermal treatment protocol

3.4 Ternary mixed-gas measurement

The effect of the thermal treatment protocol on the morphology and gas permeation performance of both the TR-PBOI dense film and hollow fiber membranes was assessed in this study, as shown in Table 5. In terms of the TR-PBOI dense film, it was found that the CO2 permeability increased along with thermal treatment temperature, while the CO2/N2 ideal selectivity remained nearly constant over the range of TR temperatures. More specifically, the CO2 permeability at the TR temperature of 450 °C (747 barrer) was 28.7 times higher than that at the TR temperature of 300 °C (26 barrer). It should also be noted that at 300 °C, the polymer did not undergo the thermal rearrangement reaction, therefore a significantly lower CO2 permeability was recorded. On the other hand, the highest CO2 permeance of the TR-PBOI hollow fiber membrane occurred at 375 °C. Further increasing the thermal treatment temperature led to a sharp decrease in CO2 permeance. Such a decrease in gas permeance can be ascribed to the HD5 polymer chain relaxation that took place at approximately 390 °C as previously reported [26], which caused densification of the polymer matrix and led to a substantial drop in CO2 permeance. The vast variations in gas permeation as a function of thermal treatment temperature over two membrane geometries suggests that the densification took place primarily in the outer-most region of the hollow fiber membrane. With a thermal treatment temperature of no more than 375 °C, the skin layer thickness appeared to remain at approximately 0.2 μm, whereas when treated at higher temperatures, a portion of the transition or support layer underwent densification and was converted into the dense skin layer, thus leading to a thicker skin layer (evidenced in Fig. 10). The SEM images of the cross-section, as well as the outer and inner surfaces of the TR-PBOI hollow fiber membranes showed no obvious differences when elevating the thermal treatment temperatures, and therefore are not presented in this work. As a result,

Mixed-gas permeation tests were carried out using TR-PBOI hollow fiber membranes with optimal pure gas permeation properties (CO2 permeance of 560 GPU and CO2/N2 ideal selectivity of 16.8). The membrane modules used for the mixed-gas permeation test contained 50 hollow fibers with an average effective membrane area of 106 cm2. Unlike most of the previous studies, which used a CO2/N2 binary gas mixture as a model flue gas, the mixed-gas in the current study was composed of 14 vol% of CO2, 6 vol% of O2, and 80 vol% of N2 to closely represent the typical post-combustion flue gas composition. As Huang et al. and Merkel et al. reported, pressure ratio is one of the key factors in designing a membrane gas separation process, and following this trend, the CO2 concentration in the permeate side exhibited little change from the pressure ratio tested [59,60]. The feed pressures were varied from 2 to 10 bar (absolute) with the permeate pressure set at atmosphere (pressure ratios from 2 to 10). As shown in Fig. 11(a), the permeate CO2 concentration experienced a substantial increase with pressure ratios less than 7, followed by a nearly constant permeate CO2 concentration (∼50 vol%) when the pressure ratios were above 7. Only a marginal increase in permeate O2 concentration was found when increasing pressure ratios, which can be ascribed to the low O2/N2 ideal selectivity of 3.43 offered by the TR-PBOI hollow fiber membrane. The permeate N2 concentration, on the other hand, decreased with the pressure ratio, and the permeate N2 concentration dropped to just above 40 vol% at a pressure ratio of 7. Up to 12% CO2 recovery was observed at a pressure ratio of 10, as shown in Fig. 11(a). The CO2 permeance in a ternary mixed-gas environment was calculated and is presented in Fig. 11(b) as a function of pressure ratio. The mixed-gas CO2 permeance was lower than the reported pure gas CO2 permeance (560 GPU), and the mixed-gas CO2 permeance sharply increased at the lower pressure ratio region, then

Effective skin layer thickness

0.2 μm TR-300

0.2 μm TR-375

1.8 μm

0.9 μm TR-385

TR-400

Fig. 10. Cross-section images of TR-PBOI hollow fiber membranes prepared under various thermal treatment temperatures.

2.4 μm TR -450

K.T. Woo et al. / Journal of Membrane Science 498 (2016) 125–134

12

N

CO recovery

60

9

40

6

3

20

0

2

3

4 5 6 7 8 Pressure ratio( )

9

10

0

CO2 permeance (P*CO2/l, GPU)

P*

O

80

10

450 /l

400

8

350

6

300

4

250

2 2

3

4

5

6

7

8

9

Separation factor ( *CO2/(N2+O2))

15

CO

CO2 recovery (%)

Permeate concentration (vol%)

100

133

10

Pressure ratio ( )

Fig. 11. The effect of pressure ratio on (a) permeate concentrations of CO2, O2 and N2, and CO2 recovery, and (b) CO2 permeance and separation factor.

reached a plateau of around 400 GPU when the pressure ratio was above 6. The lower CO2 permeance observed in the mixed-gas permeation test compared to that in the pure gas permeation test can be attributed to the competitive sorption effect, where the gas molecules were competing for the available sorption sites in the polymer matrix [61]. Fig. 11(b) also shows that the CO2/(O2 þ N2) * ) increased up to 6.0 along the presseparation factor ( α CO 2 O +N ( 2 2)

sure ratio.

4. Conclusions In this study, TR-able HAB and non-TR-able DAMs were used to synthesize TR-PBOI co-polymers. Various spinning conditions were optimized in order to produce TR-PBOI hollow fiber membranes with superior separation performance. The use of propionic acid as a co-solvent and PEG 200 as a pore former were also investigated, both of which greatly enhanced membrane gas permeation properties. In addition, the optimal dope and bore flow rates (1.05, and 1.0 ml/min, respectively) and coagulation bath temperature (70 °C) were identified. Thermal treatment temperature also influenced membrane morphology and gas permeation properties. The TR-PBOI hollow fiber membrane fabricated under the optimal conditions displayed an excellent CO2 permeance of 560 GPU and CO2/N2 ideal selectivity of 16.8. The mixed-gas permeation test was performed with a TR-PBOI hollow fiber module containing 50 fibers with an effective area of 106 cm2 with a ternary gas mixture (14 vol% of CO2, 6 vol% of O2, and 80 vol% N2). At a pressure ratio of 10, a permeate CO2 concentration of 50%, CO2 permeance of approximately 400 GPU, and separation factor of approximately 6.0 were achieved. However, the pressure ratio of 7 was favored, as it offered no significant difference in separation performance as that obtained with a pressure ratio of 10, while the energy consumption associated with the compression could be greatly reduced.

Acknowledgments This research was supported by the Korea–Italy Cooperation Program (Grant Number 2013K1A3A1A25037074) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning.

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