Thermally rearranged (TR) polybenzoxazole hollow fiber membranes for CO2 capture

Journal of Membrane Science 403–404 (2012) 169–178

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

Thermally rearranged (TR) polybenzoxazole hollow fiber membranes for CO2 capture Seungju Kim a , Sang Hoon Han a , Young Moo Lee a,b,∗ a b

School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea

a r t i c l e

i n f o

Article history: Received 14 December 2011 Received in revised form 16 February 2012 Accepted 21 February 2012 Available online 1 March 2012 Keywords: Thermally rearranged (TR) polymer Polybenzoxazole Hollow fiber membrane CO2 separation

a b s t r a c t Thermally rearranged polybenzoxazole (TR-PBO) hollow fiber membranes were prepared using a nonsolvent induced phase separation method intended to apply for CO2 separation from post-combustion flue gas. Asymmetric hollow fiber membranes were spun from a hydroxyl poly(amic acid) (HPAAc) precursor and subsequently converted to TR-PBO hollow fiber membranes after thermal treatment above 400 ◦ C. The skin structure and porous substructure in TR-PBO hollow fiber membranes were maintained even after thermal treatment at above glass transition temperature (Tg ) of precursor polymers. To fabricate defect-free hollow fiber membranes, the effects of dope composition and thermal rearrangement conditions were investigated. Gas permeation properties of TR-PBO hollow fiber membranes measured by constant pressure method with small laboratory scale test module revealed a high CO2 permeance (1940 GPU) with a CO2 /N2 selectivity of 13. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Membranes and membrane processes have been grown with advantages such as high energy efficiency, small foot-print, environmental sustainability, and so on [1]. Membranes and membrane processes have rapidly progressed since Loeb and Sourirajan developed cellulose acetate (CA) asymmetric membranes using a phase separation method to produce defect-free and high-flux membrane modules [2], resulting in a tremendous contribution to commercialization of membrane processes. Asymmetric membranes, which consist of a thin, effective skin layer supported by a porous sublayer, achieved high flux and good mechanical strength. Since the first commercial gas separation membrane modules were developed in the early 1980s by Monsanto Co. (and now Air Products and Chemicals, Inc.) [3], membrane gas separation processes have rapidly grown as a competitive separation technology to cryogenic distillation and adsorption. Gas separation membranes are used in many industrial applications such as hydrogen separation, nitrogen production, acid gas removal in natural gas, etc. [4]. For this gas separation technology, high pressure up to several tens of bars are required to separate gas mixtures through the membranes.

∗ Corresponding author at: School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Tel.: +82 2220 0525; fax: +82 2 2291 5982. E-mail addresses: [email protected], [email protected] (Y.M. Lee). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.02.041

For CO2 separation technology, membrane process has played a key role in natural gas sweetening and landfill gas upgrading. CA and polyimide (PI) membranes are two representative commercial membranes still used for this application despite of their low permeability even at high pressure [4–6]. Membrane-based CO2 separation from the post-combustion flue gas can be applied to carbon capture and storage (CCS) technology if the proper membrane materials and processes are developed. Flue gas from the coal-fired power plants is usually exhausted at lower temperature (under 50 ◦ C) and low pressure (under 1 bar). Therefore highly permeable membrane materials and high flux modules configuration are required when membranes are applied to CCS processes [7–11]. Otherwise, a high pressure operation for a large amount of CO2 from flue gas requires excessive energy costs and is not regarded as a competitive process to ammonia or amine processes. Recently, we reported on the thermally rearranged polymer membranes (TR-polymer membranes) with extraordinarily high permeability particularly for CO2 separation as potentially viable for flue gas and natural gas separation [12–18]. TR-polymers are aromatic polymers with heterocyclic rings, such as polybenzoxazoles (PBO) or polybenzothiazoles (PBT) structures prepared via thermal rearrangement process of polyimide with ortho-functional group (PIOFG) which were prepared from poly(amic acid). TRpolymers have good chemical and thermal stability. Precursors for TR-polymer have also good processability with excellent solubility in many organic solvents. Flat sheet TR-polymer membranes showed high CO2 permeability and separation properties of CO2 /N2 and CO2 /CH4 as evidenced by their narrow cavity size distribution

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and cavity size centered around 3.8 A˚ [12,16] when it was thermally treated at high temperature. In the present study, our objective is to fabricate TR-PBO hollow fiber membranes for the first time to prove our previously reported flat-sheet and dense TR-PBO membrane performances. TR-polymers do not have any processability because of insolubility due to thermal crosslinking after thermal rearrangement. However, its precursor polymer, hydroxyl poly(amic acid) (HPAAc), is soluble and can be spun into hollow fibers via non-solvent induced phase separation (NIPS) process. Highly permeable TR-PBO hollow fiber membranes can be obtained by thermal imidization followed by thermal rearrangement of hydroxyl polyimide (HPI). In fact, thermal imidization route provide the highest permeability of TR-PBO among other synthesis route [12,17]. Usually, polyimide asymmetric membranes were prepared from polyimide solution [19–22]. However, several papers have reported on polyimide asymmetric membranes prepared from PAA and thermally imidized after asymmetric membrane preparation [23–26]. To explore the appropriate meta-stable dope composition of hydrophilic prepolymer, diverse theoretical and empirical studies have been done to hollow fiber spinning. Based on the effect of dope composition on phase separation behavior, single gas separation performances of TR-PBO hollow fibers are measured by membrane module in order to determine the potential for flue gas application. 2. Experimental 2.1. Materials High purity 4,4 -hexafluoroisopropylidene diphthalic anhydride (6FDA) was purchased from Daikin Industries, Ltd. (Osaka, Japan) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF) was obtained from Central Glass Co. Ltd. (Tokyo, Japan). 6FDA and bisAPAF were dried at 160 ◦ C and 120 ◦ C each under vacuum conditions overnight to remove the absorbed water. N-Methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF), and glycerol were purchased from Aldrich Chemical Co. (Milwaukee, WI, USA) and they were used immediately, when opened, without further treatment. 2.2. Synthesis of a precursor polymer HPAAc as a precursor polymer of TR-PBO was prepared using conventional condensation polymerization of dianhydrides and diamines in polar solvent such as N-methyl-2-pyrrolidinone (NMP) as described in Fig. 1 [15]. Chemicals and glassware were put in a dried nitrogen purged glove box and synthesis was continued to prevent water vapor during the reaction. First, bisAPAF, as a diamine, was dissolved in a double layered three neck flask filled with NMP. After a diamine was dissolved completely, ethanol at −20 ◦ C was flowed to outer layer of a flask to decrease reaction temperature. Then 6FDA was added to a solution and stirred for 3 h for polymerization. An inert atmosphere and low temperature were kept to prevent depolymerization of HPAAc caused by water in the atmosphere and to remove heat exerted by exothermic reaction. Finally, a yellowish and viscous HPAAc solution was obtained. Polymers synthesized were used for viscosity measurement, phase diagram, and asymmetric membrane preparation. 2.3. Polymer characterization 2.3.1. Solubility parameters The solubility parameter of HPAAc was calculated based on group contribution procedures developed by Krevelen [27] and Fedors [28]. The solubility parameter (ı) of HPAAc in

dispersion, polar, and hydrogen-bonding components was determined by group molar attraction constants based on chemical structure [27,28]. From the Flory–Huggins theory, polymer solvent interaction parameter (12 ) is used to explain the compatibility of the polymer and solvent. Polymer solvent interaction parameter (12 ) is defined as 2

12 = vs (ıs − ıp ) RT

(1)

where s means molar volume of solvent, ıs and ıp are the solubility parameters of solvent and polymer, respectively. Polymer solubility can be predicted with 12 parameter. From the criterion for Flory–Huggins theory, when 12 parameter is less than 0.5, polymer is clearly dissolved in a solvent [29–31]. Comparing 12 parameters between HPAAc and various solvents, solubility of HPAAc was determined from Eq. (1). Candidate solvents and non-solvents were estimated with solubility in HPAAc and physical properties of the solvents. In various organic solvents, NMP and THF were chosen as solvents while glycerol was used as a non-solvent in a dope solution to control the phase separation phenomena based on the solubility test results. NMP was used as a main solvent while THF is a volatile additive in dope solution. Because of its high vapor pressure (173 mbar) at room temperature, THF easily evaporates and forms dense skin layers. Glycerol, as non-solvent, should be miscible with a coagulant because a non-solvent is demixed from solution to coagulant during hollow fiber spinning. A non-solvent controls phase separation phenomena and determines hollow fiber morphology. 2.3.2. Phase diagram A binodal curve or cloud points of the dope solution on a ternary phase diagram was experimentally determined using titration method [32]. In a ternary system of HPAAc, NMP, and glycerol, a binodal curve can be basis to determine dope solution compositions. The enthalpy change (HM ) of polymer solution in solvent and non-solvent mixture is calculated to thermodynamically explain phase separation relationship determined as a function of solubility parameters and volume fractions [33,34]. HM = ϕs ϕp (ıs − ıp )

2

(2)

where ϕs , ϕp are the volume fraction of solvent and polymer, respectively. When H is as small as possible, the polymer will be dissolved in solvent. From Eq. (2), HM is estimated on a ternary phase diagram with solubility parameters of polymer (ıp ) and a mixture of solvent and non-solvent (ımix ) which is determined with volume fractions and solubility parameters of each solvents [35,36]. A binodal curve on the phase diagram is predicted based on Eqs. (1) and (2). Because a binodal curve is a boundary between miscible and immiscible regions, the enthalpy change for mixing (HM ) is at maximum. The volume fractions of polymer, solvent, and non-solvent on the binodal curve are calculated from the maximum enthalpy change (HM ) which is defined when 12 is 0.5. The volume fractions as cloud points are converted to weight fractions and plotted on a phase diagram. In a ternary system of HPAAc, NMP, and glycerol, binodal curve was prepared and compared with the experimental data. 2.3.3. Viscosity measurement The precursor polymer, HPAAc was prepared to measure the polymer viscosity at a different weight percentage at 25 ◦ C using the SV-100 Vibration Viscometer from A&D Company, Ltd. (Tokyo, Japan). A high polymer concentration HPAAc solution was prepared in NMP following the same procedure as in Section 2.2. After the termination of polymerization, solution viscosity was measured as

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171

Fig. 1. TR-PBO synthetic scheme.

a function of polymer concentration which ranged from a few wt% up to 30 wt%. 2.4. Preparation of TR-PBO asymmetric membranes and dope composition For hollow fiber spinning, dope solution composition is critical among the various spinning factors [19]. To obtain the proper dope composition, various asymmetric flat sheet membranes were

first prepared from various dope solutions prior to the actual hollow fiber spinning. Dope solution composition was selected with HPAAc, NMP, THF and glycerol based on the dope solution viscosity and a binodal curve at phase diagram. Asymmetric flat membranes were prepared by casting the dope solutions. Prepared dope solutions were cast on glass plate and immersed into a water coagulant. Asymmetric membranes were formed after phase separation. Retention time was chosen to be 10 s. During this retention time, a solvent in a cast solution on a glass plate was

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Fig. 2. Schematic drawing of the hollow fiber membrane spinning set-up.

evaporated from the surface of the casting solution, and, therefore, polymer concentration at the solution surface increased. A high polymer concentration region at the solution surface would help to form an effective skin layer on the asymmetric flat membranes or hollow fiber membranes. A skin layer is formed at the air-gap during hollow fiber spinning. Asymmetric membranes were immersed and kept in the water bath to remove residual solvents and dried in a nitrogen purged convection oven. Completely dried asymmetric membranes were thermally treated using thermal rearrangement protocol [15] to prepare TR-membranes. The morphology of asymmetric membranes prepared from various dope compositions was characterized by field emission scanning electron microscope (FE-SEM, JEOL JMS-6330F (Tokyo, Japan)).

2.5. Preparation of TR-PBO hollow fiber membranes 2.5.1. Hollow fiber spinning After preparing a dope solution, a non-solvent induced phase separation (NIPS) process was applied to prepare hollow fiber membranes via a so-called dry-jet wet spinning process [37]. Details of the hollow fiber spinning set-up are described in Fig. 2. Prior experience suggests to us the correct spinning conditions such as solution flow rate, air-gap height, and so on. The dope solution was determined from FE-SEM images in Section 2.4 and extruded at a rate of 3.0 ml/min. The air gap height was set at 10 cm and tap water was used as a coagulant. In the air-gap, volatile solvents, added intentionally in the dope solution, were partially evaporated, so that polymer concentration at the surface of the dope solution is locally increased. When a dope solution with a high polymer concentration at the surface is immersed, it easily forms an effective dense skin layer. In the coagulation bath, the dope solution became a hollow fiber membrane by phase separation, and the residual solvents were submerged in the washing bath. After take-up of the fiber, hollow fiber membranes were kept in a separate water bath to remove any residual solvent. Appropriate washing time was determined by thermogravimetric analysis of HPAAc hollow fibers after washing. After drying hollow fibers contain less than 0.9 wt% residual solvents. Residual solvents in hollow fiber are detected in a temperature range of 150–220 ◦ C considering the boiling point of NMP (204.3 ◦ C) by thermogravimetric analysis.

2.5.2. Thermal rearrangement process After HPAAc hollow fibers were spun, washed and dried, thermal imidization and thermal rearrangement processes were followed to prepare TR-PBO hollow fiber membranes with the same thermal treatment reported in our previous studies [12,17]. Dried HPAAc hollow fiber membranes were thermally imidized at 300 ◦ C [38]. Converted hydroxyl polyimide (HPI) hollow fibers were thermally rearranged at various thermal treatment conditions around 450 ◦ C to convert to TR-PBO hollow fibers. HPAAc hollow fiber membranes were heated from ambient temperature to 300 ◦ C at a ramp rate of 5 ◦ C/min and held for 1 h for complete imidization. Then, temperature were increased to the thermal rearrangement temperature at a ramp rate of 5 ◦ C/min and held for the desired time. A thermal rearrangement process was performed in a tubular muffle furnace in argon atmosphere to prevent oxidation of membranes. The oxidation effect during high temperature treatment of polymeric materials is known to be quite severe especially for preparation of carbon molecular sieve membranes. Only a few ppm oxygen contents in inert gas can affect membrane performance by activation of carbon structure during carbonization [39]. The thermal rearrangement process is not a carbonization process and it finishes before the thermal decomposition temperature [12], however, a high purity argon gas with oxygen content less than 3 ppm was used. 2.6. Hollow fiber membrane characterization 2.6.1. Membrane morphology Morphology of the asymmetric flat-sheet membranes and hollow fiber membranes was observed using JEOL JMS-6330F (Tokyo, Japan) scanning electron microscopy (FE-SEM) after platinum coating using Cressington sputter coater 108 (Watford, the U.K.). 2.6.2. Gas permeance measurement Thermally rearranged hollow fiber membranes were bundled with five fibers and small-scale test module was prepared with epoxy sealant. The permeance is defined as thickness irrelevant permeability, P/L and considered as a unit of GPU (1 GPU = 10−6 cm3 (STP) cm−2 s−1 cm Hg−1 ). P Q Q = = L Ap nDlp

(3)

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173

Fig. 3. A solubility parameter diagram, showing the soluble region of hydroxyl poly(amic acid).

where P is the intrinsic permeability of the membrane materials, L is the thickness of the effective skin layer of the hollow fibers, Q is the flow rate of pure gas, A is the surface area of the hollow fibers, which can be obtained with an outer diameter of hollow fiber (D), the effective length of hollow fiber (l), and the number of hollow fibers in module (n), and p is the gas pressure ratio. The ideal separation factor of an asymmetric membrane is described as the ratio of pure gas permeance of targeted components [40]. ˛A/B =

(P/L)A (P/L)B

(4)

The permeance of six representative gases such as H2 , He, O2 , N2 , CH4 , and CO2 were measured with pressure range from 1 atm to 6 atm at room temperature. The gas flow rate linearly increased as the pressure increased, so gas permeance, which is normalized with pressure, was almost the same by various feed pressures. Average gas permeances of hollow fiber membranes measured at various pressures were estimated. 3. Results and discussion 3.1. Physical properties of a precursor polymer 3.1.1. Solubility parameters Hansen’s solubility parameters of HPAAc and common solvents were determined in dispersion, polar, and hydrogen bonding components as shown in Fig. 3 and Table 1. Solubility of HPAAc in organic solvents was measured, indicating that most common solvents can dissolve HPAAc. HPAAc has a large solubility region, predicted from Eq. (1), as can be seen in Fig. 3, and thus only a few solvents can be used as non-solvents, such as glycerol and water (see Fig. 3). Non-solvent additives in a dope solution can control the phase separation phenomena and thus is expected to control the thickness of the skin layer and membrane morphology. Although alkane solvents such as n-hexane or n-heptane do not dissolve polymers, they cannot be used as non-solvent additives in dope solution because non-solvents should be miscible with solvent and coagulant [41]. Polymer–solvent interaction parameter (12 ) of HPAAc with various solvents were calculated from Eq. (1) and summarized in Table 2. As can be seen, solubility between a polymer and solvents agreed well with 12 parameter when the value is less than 0.5 [30] with the exception of THF and acetone. Thus, the polymer–solvent interaction theory matched well with 12 parameter and experimental solubility data. Among various solvents, NMP and THF were

Fig. 4. Typical viscosity of hydroxyl poly(amic acid) in NMP at 25 ◦ C as a function of polymer concentration.

selected as solvents here and glycerol as a non-solvent as explained in Section 2.3.1. 3.1.2. Viscosity Fig. 4 shows viscosity of HPAAc as a function of polymer concentration in NMP. The viscosity of the polymer solution increased with a polymer concentration where viscosity drastically increased above 20 wt% polymer concentration. Viscosity of the dope solution is an important factor for hollow fiber spinning. A dope solution with too low of a viscosity cannot produce good asymmetric membranes because of low polymer content in the dope solution. Otherwise, a highly viscous dope solution would form an uneven membrane surface and the resulting hollow fiber membranes are usually kinked and defected during spinning due to high shear stress. Usually, dope solution viscosity for hollow fiber spinning is required to be minimum ranged to about a few thousand centipoises (cP) [42], but for gas separation applications, it is required to be optimum and ranged above ten thousand centipoises (cP) at room temperature. Based on viscosity data, polymer concentration in dope solution was determined between 20 wt% and 30 wt%. 3.1.3. Phase separation behavior Phase separation behavior of HPAAc in a ternary system of polymer, solvent, and non-solvent was observed using NMP and glycerol as a solvent and non-solvent, respectively. Fig. 5 shows a phase diagram and binodal curves of the ternary system, both estimated experimentally and theoretically. In the meantime, the theoretical binodal curve was calculated using the equation of the free energy of mixing in Section 2.3.2 which matched well with experimental data. HPAAc has a large soluble region and strong interaction between the polymer and NMP, solvent. Non-solvent power of glycerol is not strong. Therefore, the binodal curve is located away from polymer–solvent axis, meaning that the mixing enthalpy of HPAAc and glycerol is not high. Therefore, HPAAc is soluble whenever a strong solvent such as NMP is introduced as a mixture. 3.2. Effects of additives in dope solution Various dope solutions were prepared to fabricate asymmetric flat-sheet membranes. Details of 10 dope solution compositions

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Table 1 Solubility parameters of HPAAc and typical solvents [35,36].

Hydroxyl poly(amic acid) NMP DMF THF Acetone Ethanol n-Hexane Glycerol Water

ıD (cal1/2 /cm3/2 )

ıP (cal1/2 /cm3/2 )

ıH (cal1/2 /cm3/2 )

ı (cal1/2 /cm3/2 )

9.1 8.8 8.5 8.2 7.6 7.7 7.2 8.5 7.6

5.5 6.0 6.7 2.8 5.1 4.3 0 5.9 7.8

5.4 3.5 5.5 3.9 3.4 9.5 0 14.3 20.7

11.9 11.2 12.1 9.5 9.8 13.0 7.3 17.7 23.4

Table 2 Physical properties of common organic solvents and interaction parameter of solvents with HPAAc and experimental solubility.

NMP DMF THF Acetone Ethanol n-Hexane Glycerol Water a

Molar mass (g/mol)

Density (g/ml)

Molar volume (ml/mol)

12 parameter (with HPAAc)

Solubilitya

99.13 73.09 72.11 58.08 46.07 86.18 92.09 18.01

1.032 0.944 0.889 0.791 0.789 0.655 1.261 1.000

96.06 77.43 81.11 73.43 58.39 131.57 73.03 18.01

0.08 0.01 0.79 0.55 0.12 4.70 4.15 4.02

O O O O O X X X

O: soluble; X: insoluble.

Fig. 5. Binodal curve in the ternary phase diagram of hydroxyl poly(amic acid), NMP and glycerol. Blue line is theoretical biodal curves determined by Eqs. (3) and (4), whereas red dots represent experimental data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

are described in Table 3. HPAAc, NMP and glycerol were selected as a ternary system whereas a small amount of THF was added as volatile solvent. Dope solution compositions are determined based on phase separation behavior as described in Fig. 5. Hollow fiber membranes should have stable morphology both in the supporting layer and in the effective skin layer. A porous supporting layer should be stable and strong enough to withstand Table 3 Ten representative dope solution compositions in wt%. Sample #

Polymer

NMP

THF

Glycerol

TR-21-1 TR-21-2 TR-21-3 TR-21-4 TR-25-1 TR-25-2 TR-28-1 TR-28-2 TR-30-1 TR-30-2

21 21 21 21 25 25 28 28 30 30

58 58 58 58 60 55 57 55 57.5 55

0 5 10.5 21 7.5 10 7.5 8.5 6.25 7.5

21 16 10.5 0 7.5 10 7.5 8.5 6.25 7.5

feed gas pressure and must not block feed flow. The skin layer is an actual separating layer and should be as thin as possible without defects, because only a few defects can cause a significant selectivity drop [43]. Thickness of skin layer affects gas flux or permeance of membrane and is directly related to membrane performances. Fig. 6 shows cross-sectional images of TR-PBO asymmetric membranes named as TR-21-1 to TR-30-2 in Table 3. First of all, the effect of additives in a dope solution on membrane morphology was studied. From images TR-21-1 to TR-21-4, the ratio of THF and glycerol additives was controlled to investigate the crosssectional morphology of final asymmetric TR-PBO membranes. THF is a volatile solvent which helps to form an effective skin layer on the surface at the air-gap region. Glycerol, as a non-solvent additive, controls the phase separation properties and causes delayed or instant demixing [44]. We can clearly see that without the addition of THF (see image TR-21-1), the skin layer was not formed during phase separation. On the other hand, without glycerol (see image TR-21-4), membrane morphology was quite unstable. An appropriate amount of THF and glycerol is necessary to form stable membrane morphology with a suitable skin layer. Secondly, the effects of polymer concentration were characterized. From images TR-25-1 to TR-30-2, polymer concentration in dope solution was varied while the ratio of additives remained fixed. As the polymer concentration increased, the skin layer became thicker and membrane morphology was more stable. However, a too thick skin layer was observed from image TR-30-2. From this study, the dope solution composition for hollow fiber spinning was determined from the dope composition TR-30-1. Hollow fiber membranes prepared from the same dope composition may not have the same morphology as asymmetric flat membranes. However, the morphology of flat-sheet membranes can at least be reflected to prepare hollow fiber membranes to minimize the time for hollow fiber spinning of newly synthesized polymers. 3.3. TR-PBO hollow fiber membranes TR-PBO hollow fiber membranes were prepared from HPAAc precursor by thermal rearrangement. As shown in Fig. 7, HPAAc hollow fibers are white colored asymmetric fibers and, after thermal imidization, they were converted to yellowish colored hydroxyl

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175

Fig. 6. FE-SEM images of 10 representative asymmetric flat sheet TR-PBO membranes.

polyimide hollow fibers. Finally, dark brown colored TR-PBO hollow fiber membranes were fabricated after thermal conversion above 450 ◦ C. Spun HPAAc hollow fibers were characterized by thermogravimetric analysis to investigate the amount of residual solvent in hollow fiber membranes and to determine appropriate washing time (data not shown here). The residual solvent is mainly NMP with a boiling point of about 200 ◦ C. After 9 h of washing, residual solvent was almost removed. The washing time of HPAAc hollow fibers should be determined carefully because it affects HPAAc hollow fiber morphology and structure. HPAAc contains carboxylic acid group and hydroxyl group. They easily underwent hydrolysis in water because they tended to form hydrogen bonding with water molecules and break the polymer backbone [45]. Therefore, when HPAAc is in water for long time, its structure might be destroyed. After 36 h washing in a water bath, HPAAc hydrolysis can be recognized by a color change. White colored HPAAc hollow fiber membranes became greenish and became more brittle. Otherwise, the residual solvent in the hollow fibers ruin the membrane morphology. Residual NMP, stuck in asymmetric structure, remained during drying because of boiling point difference of water and NMP. It would dissolve fibers again after drying, so any residual solvent should be removed completely. HPAAc hollow fibers were washed for 9 h.

The cross-sectional morphology of TR-PBO hollow fibers was investigated before and after thermal treatment using FE-SEM. Usually, thermal treatment of thermoplastic polymers above the glass transition temperature (Tg ) causes the collapse of porous substructure in polymeric asymmetric membranes because the glassy polymer phase changes to a rubbery phase over Tg [46,47]. When a porous sub-structure is changed to a rubbery state, the viscous flow of polymer chains occur and they are congealed together. However, for TR-PBO hollow fiber membranes, a porous sub-structure remained even when the treatment temperature was over Tg . HPAAc has high Tg over 300 ◦ C. During the thermal imidization process at 300 ◦ C, HPAAc are thermally crosslinked. In thermal cyclization of HPAAc to HPI, thermal crosslinking of HPAAc generated H2 O causing insolubility of HPI in most organic solvents [17]. Note that the thermally crosslinked HPI can overcome the collapse of a porous substructure during the thermal rearrangement process. Fig. 8 shows cross-sectional images of TR-PBO hollow fiber membranes. Prepared hollow fibers have a mainly sponge-like substructure without macrovoids. It would resist high pressure from feed gases during membrane processes. From image (c), a thin skin layer of about 1–1.5 ␮m was observed on the shell side of hollow fibers. Actual thickness of the skin layer is calculated by comparing it with gas permeation data as described in the following section

Fig. 7. Hollow fiber membranes made of (a) hydroxyl poly(amic acid), (b) hydroxyl polyimide, and (c) TR-PBO.

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Fig. 8. Cross-sectional images of TR-PBO hollow fiber membranes (a) overall hollow fiber, (b) morphology at wall, and (c) outer skin layer.

which is comparable with skin thickness observed from FE-SEM images. 3.4. Single gas permeation of TR-PBO hollow fiber membranes Gas permeation properties of hollow fiber membranes are usually defined as a Gas Permeation Unit (GPU) unit which is normalized by membrane thickness. Because the actual thickness of the skin layer cannot be measured accurately, a GPU unit is usually used for asymmetric membranes or hollow fiber membranes, and membrane module performances. GPU unit is usually compared with Barrer unit which is used for gas permeability of dense membrane or film as membrane material performance. Compared with the scales of GPU and Barrer units, the thickness of skin layer in asymmetric membranes can be calculated from GPU and Barrer values. Tables 4 and 5 describe the gas permeation properties of TR-PBO dense membranes and hollow fiber membranes prepared at various thermal treatment conditions. As described in Table 5, the gas permeances of TR hollow fiber membranes were measured in various pressures from 1 atm to 6 atm and they were pressure independent. Usually high pressure condensable feed gases such as CO2 cause membrane plasticization [17,48], as a result, gas permeability increases with feed pressure. However, TR-PBO membranes show excellent resistance to plasticization [12]. Moreover, TR-PBO hollow fiber membranes shows very high gas permeance and flux even at low pressure at 1–2 atm, indicating that they are suitable for post-combustion CCS process. TR-PBO hollow fiber membranes showed high CO2 permeance of 1938 GPU with CO2 /N2 and CO2 /CH4 selectivity of 13 and

14, respectively. Selectivity of CO2 /N2 was somewhat lower than the value for dense membrane (15 for CO2 /N2 and 28 for CO2 /CH4 [16]) meaning that we need to adjust several parameters for further optimization. The gas permeance of hollow fiber membranes in Table 5 was compared with gas permeability of a dense membrane described in Table 4 with the same thermal treatment conditions. Thus, the skin layer thickness in TR-PBO hollow fiber membranes can be roughly calculated to be 1.5–2.0 ␮m which is comparable with skin layer thickness observed by FE-SEM images in Fig. 8. During the thermal rearrangement process, precursor polymer hydroxyl polyimide is converted to polybenzoxazole, and gas permeability of the latter increases dramatically over the precursor membrane. The same phenomena were observed in TR-PBO hollow fiber membranes. As reported in our previous papers [15,16], precursor HPI polymer are thermally treated at 450 ◦ C for 1 h to prepare TR-PBO. However, 1 h treatments at high temperatures may not be economically or industrially feasible. Therefore, the thermal conversion from imide to benzoxazole rearrangement process was controlled using different thermal rearrangement conditions such as temperature and time. High temperatures and prolonged time usually produce high imide-to-benzoxazole conversion membranes [49]. At the same temperature conditions with shorter times, such as 10 min rather than 1 h, as described in Table 6, the gas permeance of hollow fiber membranes decreased about 50% because of low conversion from imide to benzoxazole. Its performance is similar to gas permeance of 1 h treated hollow fiber at 400 ◦ C described in Table 5. From FT-IR measurement, it is calculated to have about 86.3% conversion according to our

Table 4 Single gas permeability of TR-PBO series dense membrane film [15]. Gas permeability (Barrera )

PIOFG-1 (300 ◦ C) 350 ◦ C treated 400 ◦ C treated TR-PBO a

Selectivity (˛)

He

H2

O2

N2

CH4

CO2

O2 /N2

CO2 /N2

CO2 /CH4

62 89 354 1589

35 61 372 2856

2.6 5.5 59.8 776

0.4 0.8 11.6 155

0.08 0.9 4.9 80.8

10 24 297 3575

6.5 6.9 5.2 5.0

25 30 26 23

125 26 61 44

1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cm Hg−1 .

Table 5 Single gas permeance of TR-PBO series hollow fiber membranes. Gas permeance (GPUa )

◦

PIOFG-1 (300 C) 350 ◦ C treated 400 ◦ C treated TR-PBO a

Selectivity (˛)

He

H2

O2

N2

CH4

CO2

O2 /N2

CO2 /N2

CO2 /CH4

124 250 750 1002

105 246 980 1550

7.8 25.5 121.2 324.4

3.5 12.3 60.4 145.6

0.8 8.4 40.7 137.1

31 158 730 1938

2.3 2.1 2.0 2.2

8.8 13 12 13

37 19 18 14

1 GPU = 10−6 cm3 (STP) cm−2 s−1 cm Hg−1 .

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177

Table 6 Gas permeance of TR-PBO hollow fiber membranes as a function of thermal treatment conditions. Thermal treatment

450 ◦ C for 10 min 450 ◦ C for 60 min 500 ◦ C for 10 min

Gas permeance (GPU)

Selectivity (˛)

He

H2

O2

N2

CH4

CO2

O2 /N2

CO2 /N2

CO2 /CH4

737 1002 1298

956 1550 1996

251 324 498

75 146 119

57 137 105

934 1938 2326

3.3 2.2 4.2

12 13 20

16 14 22

Table 7 Membrane module performances of TR-PBO and other membranes [50–53]. Gas permeance (GPU)

TR-PBO Cardo polyimide [51] PolarisTM [52] PDMC polyimide [53] 6FDA-2,6 DAT polyimide [50]

Selectivity (˛)

He

H2

O2

N2

CH4

CO2

O2 /N2

CO2 /N2

CO2 /CH4

1001 n/a n/a n/a n/a

1550 n/a n/a n/a n/a

324 n/a n/a n/a 20.3

146 25 20 n/a 3.5

137 n/a n/a 6.7 3

1938 1000 1000 203 125

2.2 n/a n/a n/a 5.8

13 40 50 n/a 35

14 n/a n/a 30 42

previous studies. Gas permeance of hollow fibers treated at 500 ◦ C for 10 min increased to similar or even higher levels to TR-PBO and their selectivity even improved. TR-PBO hollow fiber membranes decomposed and partially became carbon membranes above 500 ◦ C treatment. Actually, after 500 ◦ C treatment, hollow fibers became more brittle than 450 ◦ C treated hollow fibers. Table 7 shows membrane module performances of TR-PBO and is compared with those in the published literature. So far membrane modules for CO2 separation are reported with high gas permeance around 1000 GPU. Polyimide membrane module performances are in the order of a few hundred GPU, but with high selectivity. Flat-sheet TR-PBO membranes showed high permeability, which is about two to three orders of magnitude higher than common membrane materials such as polyimide and cellulose acetate. Therefore, TR-PBO hollow fiber membranes can achieve much higher permeance by lowering the effective skin layer thickness. For CCS study, it is important to prepare highly permeable membrane modules with reasonable selectivity [54]. Ongoing and future studies will focus on the development of hollow fiber membrane modules with even higher permeance and selectivity, efficient module design, as well as the actual mixed gas tests for flue gas application.

4. Conclusions This study demonstrated that TR-PBO hollow fiber membranes were successfully prepared using non-solvent induced phase separation from HPAAc precursor. Before spinning the hollow fiber membranes, asymmetric flat sheet membranes were fabricated to determine dope solution composition. TR-PBO asymmetric membranes were first prepared from quaternary dopes of HPAAc, NMP, THF, and glycerol in different composition. Using the information on asymmetric membrane, HPAAc hollow fiber membranes were fabricated and thermally rearranged to TR-PBO hollow fiber membranes. Spinning parameters such as dope composition, temperature, flow rate, etc. were investigated in the present study. Six representative single gas permeance of hollow fiber membranes were compared with the gas permeability of a dense membrane. TR-PBO hollow fibers with up to 2 ␮m thin effective skin layer can be prepared with CO2 permeance of around 2000 GPU and selectivity of CO2 /N2 about 13. Further studies on increasing CO2 permeance by reducing the effective skin layer thickness will be carried out as well as a new module design, followed by actual flue

gas testing of TR-PBO membrane modules for preliminary carbon capture and sequestration purpose. Acknowledgements This work was financially supported by the Carbon Dioxide Reduction and Sequestration Center, one of the 21st Century Frontier R&D Programs funded by the Ministry of Education, Science and Technology in Korea. YML appreciates support from WCU (World Class University) program, National Research Foundation (NRF) of the Korean Ministry of Science and Technology (No. R31-2008-00010092-0), which we gratefully acknowledge. References [1] R.W. Baker, Membrane Technology and Applications, John Wiley & Sons Ltd., England, 2004. [2] S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, in: Saline Water Conversion II, American Chemical Society, 1963, pp. 117–132. [3] J.M.S. Henis, M.K. Tripodi, Multicomponent Membranes for Gas Separation, M. Company, USA, 1980. [4] R.W. Baker, Future directions of membrane gas separation technology, Industrial & Engineering Chemistry Research 41 (2002) 1393–1411. [5] R.W. Spillman, Economics of gas separation membranes, Chemical Engineering Progress 85 (1989) 41–62. [6] B.D. Bhide, A. Voskericyan, S.A. Stern, Hybrid processes for the removal of acid gases from natural gas, Journal of Membrane Science 140 (1998) 27–49. [7] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: a review/state of the art, Industrial & Engineering Chemistry Research 48 (2009) 4638–4663. [8] T.C. Merkel, M. Zhou, R.W. Baker, Carbon dioxide capture with membranes at an IGCC power plant, Journal of Membrane Science 389 (2012) 441–450. [9] C.A. Scholes, J. Bacus, G.Q. Chen, W.X. Tao, G. Li, A. Qader, G.W. Stevens, S.E. Kentish, Pilot plant performance of rubbery polymeric membranes for carbon dioxide separation from syngas, Journal of Membrane Science 389 (2012) 470–477. [10] C.E. Powell, G.G. Qiao, Polymeric CO2 /N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases, Journal of Membrane Science 279 (2006) 1–49. [11] T.C. Merkel, H. Lin, X. Wei, R. Baker, Power plant post-combustion carbon dioxide capture: an opportunity for membranes, Journal of Membrane Science 359 (2010) 126–139. [12] H.B. Park, C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E. Van Wagner, B.D. Freeman, D.J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 318 (2007) 254–258. [13] J.I. Choi, C.H. Jung, S.H. Han, H.B. Park, Y.M. Lee, Thermally rearranged (TR) poly(benzoxazole-co-pyrrolone) membranes tuned for high gas permeability and selectivity, Journal of Membrane Science 349 (2010) 358–368. [14] C.H. Jung, J.E. Lee, S.H. Han, H.B. Park, Y.M. Lee, Highly permeable and selective poly(benzoxazole-co-imide) membranes for gas separation, Journal of Membrane Science 350 (2010) 301–309. [15] S.H. Han, J.E. Lee, K.-J. Lee, H.B. Park, Y.M. Lee, Highly gas permeable and microporous polybenzimidazole membrane by thermal rearrangement, Journal of Membrane Science 357 (2010) 143–151.

178

S. Kim et al. / Journal of Membrane Science 403–404 (2012) 169–178

[16] H.B. Park, S.H. Han, C.H. Jung, Y.M. Lee, A.J. Hill, Thermally rearranged (TR) polymer membranes for CO2 separation, Journal of Membrane Science 359 (2010) 11–24. [17] S.H. Han, N. Misdan, S. Kim, C.M. Doherty, A.J. Hill, Y.M. Lee, Thermally rearranged (TR) polybenzoxazole: effects of diverse imidization routes on physical properties and gas transport behaviors, Macromolecules 43 (2010) 7657–7667. [18] M. Calle, Y.M. Lee, Thermally rearranged (TR) poly(ether-benzoxazole) membranes for gas separation, Macromolecules 44 (2011) 1156–1165. [19] D.T. Clausi, W.J. Koros, Formation of defect-free polyimide hollow fiber membranes for gas separations, Journal of Membrane Science 167 (2000) 79–89. [20] H. Kawakami, K. Nakajima, H. Shimizu, S. Nagaoka, Gas permeation stability of asymmetric polyimide membrane with thin skin layer: effect of polyimide structure, Journal of Membrane Science 212 (2003) 195–203. [21] M. Das, W.J. Koros, Performance of 6FDA–6FpDA polyimide for propylene/propane separations, Journal of Membrane Science 365 (2010) 399–408. [22] M. Niwa, H. Kawakami, T. Kanamori, T. Shinbo, A. Kaito, S. Nagaoka, Surface orientation effect of asymmetric polyimide hollow fibers on their gas transport properties, Journal of Membrane Science 230 (2004) 141–148. [23] K.-Y. Chun, S.-H. Jang, H.-S. Kim, Y.-W. Kim, H.-S. Han, Y.-i. Joe, Effects of solvent on the pore formation in asymmetric 6FDA–4,4 ODA polyimide membrane: terms of thermodynamics, precipitation kinetics, and physical factors, Journal of Membrane Science 169 (2000) 197–214. [24] H.J. Lee, J. Won, H.C. Park, H. Lee, Y.S. Kang, Effect of poly(amic acid) imidization on solution characteristics and membrane morphology, Journal of Membrane Science 178 (2000) 35–41. [25] H.J. Lee, J. Won, H. Lee, Y.S. Kang, Solution properties of poly(amic acid)–NMP containing LiCl and their effects on membrane morphologies, Journal of Membrane Science 196 (2002) 267–277. [26] J. Huang, R.J. Cranford, T. Matsuura, C. Roy, Sorption and transport behavior of water vapor in dense and asymmetric polyimide membranes, Journal of Membrane Science 241 (2004) 187–196. [27] D.W.v. Krevelen, P.J. Hoftyzer, Properties of Polymers: Their Estimation and Correlation with Chemical Structure, Elsevier, Amsterdam, 1976. [28] R.F. Fedors, A method for estimating both the solubility parameters and molar volumes of liquids, Polymer Engineering & Science 14 (1974) 147–154. [29] J. Brandrup, E.H. Immerhut, Polymer Handbook, John Wiley & Sons, Inc., New Jersey, 1989. [30] H.R. Allcock, F.W. Lampe, J.E. Mark, Contemporary Polymer Chemistry, Pearson Education, Inc., New Jersey, 2003. [31] G. Ovejero, P. Pérez, M.D. Romero, I. Guzmán, E. Díez, Solubility and Flory Huggins parameters of SBES poly(styrene-b-butene/ethylene-b-styrene) triblock copolymer, determined by intrinsic viscosity, European Polymer Journal 43 (2007) 1444–1449. [32] P.S.T. Machado, A.C. Habert, C.P. Borges, Membrane formation mechanism based on precipitation kinetics and membrane morphology: flat and hollow fiber polysulfone membranes, Journal of Membrane Science 155 (1999) 171–183. [33] L.H. Sperling, Introduction to Physical Polymer Science, John Wiley & Sons, Inc., New Jersey, 2006. [34] I. Teraoka, Polymer Solutions: An Introduction to Physical Properties, John Wiley & Sons, Inc., New Jersey, 2006. [35] A.F.M. Barton, Solubility parameters, Chemical Reviews 75 (1975) 731–753. [36] C.M. Hansen, Hansen Solubility Parameters: A User’s Handbook, CRC Press, Florida, 2007.

[37] S. Bonyadi, T.S. Chung, W.B. Krantz, Investigation of corrugation phenomenon in the inner contour of hollow fibers during the non-solvent induced phaseseparation process, Journal of Membrane Science 299 (2007) 200–210. [38] N. Takahashi, D.Y. Yoon, W. Parrish, Molecular order in condensed states of semiflexible poly(amic acid) and polyimide, Macromolecules 17 (1984) 2583–2588. [39] M. Kiyono, P.J. Williams, W.J. Koros, Effect of pyrolysis atmosphere on separation performance of carbon molecular sieve membranes, Journal of Membrane Science 359 (2010) 2–10. [40] T.S. Chung, S.K. Teoh, X. Hu, Formation of ultrathin high-performance polyethersulfone hollow-fiber membranes, Journal of Membrane Science 133 (1997) 161–175. [41] A.J. Reuvers, J.W.A. van den Berg, C.A. Smolders, Formation of membranes by means of immersion precipitation: Part I. A model to describe mass transfer during immersion precipitation, Journal of Membrane Science 34 (1987) 45–65. [42] S.P. Deshmukh, K. Li, Effect of ethanol composition in water coagulation bath on morphology of PVDF hollow fibre membranes, Journal of Membrane Science 150 (1998) 75–85. [43] N. Peng, T.S. Chung, The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon® hollow fiber membranes, Journal of Membrane Science 310 (2008) 455–465. [44] H. Bokhorst, F.W. Altena, C.A. Smolders, Formation of asymmetric cellulose acetate membranes, Desalination 38 (1981) 349–360. [45] C.-P. Yang, S.-H. Hsiao, Effects of various factors on the formation of high molecular weight polyamic acid, Journal of Applied Polymer Science 30 (1985) 2883–2905. [46] Y. Kusuki, H. Shimazaki, N. Tanihara, S. Nakanishi, T. Yoshinaga, Gas permeation properties and characterization of asymmetric carbon membranes prepared by pyrolyzing asymmetric polyimide hollow fiber membrane, Journal of Membrane Science 134 (1997) 245–253. [47] N. Tanihara, H. Shimazaki, Y. Hirayama, S. Nakanishi, T. Yoshinaga, Y. Kusuki, Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber, Journal of Membrane Science 160 (1999) 179–186. [48] J.S. Lee, W. Madden, W.J. Koros, Antiplasticization and plasticization of Matrimid® asymmetric hollow fiber membranes—Part A. Experimental, Journal of Membrane Science 350 (2010) 232–241. [49] Y. Chan, S.H. Han, Y.M. Lee, Conversion study of thermally rearranged polybenzoxazole membranes, Journal of Thermal Analysis and Calorimetry, submitted for publication. [50] J. Ren, T.-S. Chung, D. Li, R. Wang, Y. Liu, Development of asymmetric 6FDA2,6 DAT hollow fiber membranes for CO2 /CH4 separation: 1. The influence of dope composition and rheology on membrane morphology and separation performance, Journal of Membrane Science 207 (2002) 227–240. [51] S. Kazama, CO2 separation with molecular gate membrane, in: GCEP Energy Workshops, Stanford, 2004. [52] R. Baker, CO2 separation and sequestration from flue gas with membranes, in: Post-Combustion CO2 Capture Workshop, Talloires, France, 2010. [53] I.C. Omole, R.T. Adams, S.J. Miller, W.J. Koros, Effects of CO2 on a high performance hollow-fiber membrane for natural gas purification, Industrial & Engineering Chemistry Research 49 (2010) 4887–4896. [54] M.T. Ho, G.W. Allinson, D.E. Wiley, Reducing the cost of CO2 capture from flue gases using membrane technology, Industrial & Engineering Chemistry Research 47 (2008) 1562–1568.