Long-Term Stability of Polytetrafluoroethylene (PTFE) Hollow Fiber

Dec 11, 2015 - Membrane wetting is the main obstacle for carbon dioxide (CO2) capture on long-term operation. In order to understand wetting, polytetr...
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Long-Term Stability of Polytetrafluoroethylene (PTFE) Hollow Fiber Membranes for CO2 Capture Hongyan Tang,†,‡ Yi Zhang,†,‡ Feng Wang,†,‡ Huapeng Zhang,†,‡ and Yuhai Guo*,†,‡ †

National Local Joint Engineering Laboratory of Textile Fiber Materials and Processing Technology, Zhejiang Sci-Tech University, Hangzhou, 310018, China ‡ The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China ABSTRACT: Membrane wetting is the main obstacle for carbon dioxide (CO2) capture on long-term operation. In order to understand wetting, polytetrafluoroethylene (PTFE) hollow fiber membranes were fabricated through paste extrusion, stretching, and sintering processes. CO2 capture performance over 180 days was performed. It indicates no obvious deterioration of CO2 flux over 180 days. Subsequently, further investigation was performed through immersion of PTFE hollow fiber membranes in different absorbent solutions. Results of Fourier transform infrared (FTIR) spectroscopy, energy-dispersive spectroscopy (EDS), and variations of membrane weight showed the PTFE polymer kept good stability in the absorbent solutions over 24 weeks. Scanning electron microscopy (SEM) images indicated the pore sizes of the PTFE hollow fiber membranes did not markedly enlarge during immersion, which was proved by pore size distributions. Contact angle measurements indicated no obvious reduction (8° reduction) in the membrane hydrophobicity over 24 weeks. Values of the contact angle relied on the surface tension values of the absorbent solutions. AFM images showed that immersion imposed a negative influence on the roughness values, which was the same order as the contact angle. The results suggested that none of the absorbent molecules entered the interior of the PTFE polymer. PTFE hollow fiber membranes may be a promising alternative for CO2 capture on long-term operation. membranes due to a plasticizing effect. Wang et al.18 presented that the pore structure and surface roughness of PP membranes was altered due to the chemical reaction between PP and diethanolamine (DEA). With regard to the wetting of PE, PVDF, and PTFE membranes, Sedghi et al.19 indicated a decrease in the hydrophobicity of PE membranes that was due to oxidative degradation. Khaisri et al.20−24 found that the flux of PVDF membranes decreased. Wang et al.25 further observed that the hydrophobicity of PVDF membranes decreased because of physical or chemical dissolution of the membrane surface. Noticeably, Nishikawa et al.26 found no wetting of PTFE hollow fiber membranes over 6600 h. Another important factor on membrane wetting is the property of liquid absorbents. Sodium hydroxide (NaOH) was once employed as the absorbent for CO 2 capture. 27 Monoethanolamine (MEA) was usually employed, because of high reactivity and low cost. Methyldiethanolamine (MDEA) was also used, because of its higher loading capacity and lower energy requirement for regeneration.22 The mixed absorbent may be more promising. Lu et al.28 used the mixture of MDEA and piperazine (PZ) as the absorbent for CO2 capture. Deionized water, NaOH, MEA, MDEA, and the mixture of MDEA and PZ (MDEA+PZ) were employed as absorbents in this study.

1. INTRODUCTION Carbon dioxide (CO2) is one of the main greenhouse gases. Recently, CO2 capture has been a focus of research.1 Common separation processes are adsorption, absorption, cryogenic distillation, and membrane techniques.2−4 The gas−liquid membrane contactor combines the advantages of conventional absorption and membrane separation, which is suggested to be a promising alternative.5−7 However, once the membrane pores are wetted by liquid absorbents, the absorption performance would decline. Wetting is mainly influenced by membrane properties, liquid absorbent properties, and the interactions between the membrane materials and the liquid absorbents.8 Polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) are the common membrane materials for CO2 capture. Recently, PP, PVDF, and PE membranes have attracted more attention for CO2 capture. However, membrane wettability is one of the main obstacles. All of the literature on membrane wetting is listed in Table 1. Wetting of PP membranes was studied by several researchers. Lv et al.2 found the strong reduction of contact angle of PP membranes during immersion in absorbent solutions. deMotigny et al.9−11 found that the flux of PP hollow fiber membranes declined due to the membrane wetting. FalkPedersen et al.12 observed that PP membranes coated with polydimethylsiloxane failed after 7 days. Yan et al.13 found no wetting of PP membranes in PG solutions over 40 h. As for the causes of wetting of PP membranes, Barbe et al.14−16 found that the wetting of PP membranes was due to pore intrusion by the liquid absorbent. Porcheron et al.17 studied the wetting of PP © 2015 American Chemical Society

Received: August 5, 2015 Revised: December 11, 2015 Published: December 11, 2015 492

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Energy & Fuels Table 1. Summary of the Studies on Membrane Wetting

a

membrane type

absorbenta

PP hollow fiber PP hollow fiber PP hollow fiber PP hollow fiber PP hollow fiber PP PES coated with PDMS PP coated with PDMS PP hollow fiber PP flat sheet PP hollow fiber PP hollow fiber PP hollow fiber PP hollow fiber PP hollow fiber PE hollow fiber PVDF hollow fiber solution PVDF hollow fiber PVDF hollow fiber PVDF hollow fiber PVDF hollow fiber PVDF hollow fiber PVDF hollow fiber PVDF hollow fiber PTFE PTFE hollow fiber PTFE hollow fiber

deionized water 30 wt % MDEA solution 30 wt % MEA solution 1 mol/L MDEA solution 0.5 mol/L MEA solution MEA solution MEA solution MEA solution PG solution water 20 wt % DEA 1 mol/L DEA solution water/2 mol/L DEA solution 1 mol/L DEA solution MEA solution 30 wt % MEA 1 mol/L MEA 1 mol/L MEA solution 2 mol/L NaOH solution 2 mol/L MEA solution MEA solution MEA+SG solution distilled water 1 mol/L DEA solution MEA solution 1 mol/L MEA solution MEA solution

operation time

immersion time

wettabilityb

ref

60 days 60 days 60 days

CA: from 126.1° to 100° CA: from 121.6° to 90.8° CA: from 121.6° to ∼92.5° yes yes yes yes yes no CA: from 105.0° to 90° CA: from 103.7° to 94.5° flux decreased by 37.1% yes flux decreased by 37.1% yes CA: from 99° ± 1° to 69° ± 1° flux decreased by 33% flux decreased by 40% flux decreased by 15% flux decreased by 43% yes no flux decreased by 25% flux decreased by 50.4% no no no

2 2 2 10 11 12 12 12 13 17 18 23 24 23 26 19 20 20 21 21 22 22 23 25 12 20 26

10 h 14 days some hours 6 days 7 days 40 h 8 days 10 days 30 days 4 days 30 days 6600 h 10 days 40 h 60 h 15 days 15 days 12 days 12 days 140 h 30 days 80 h 6600 h

PG = potassium glycinate; SG = sodium glycinate. bCA = contact angle.

measurement, atomic force microscopy (AFM), pore size distribution, and variations of membrane weight.

PTFE membranes attract more attention for CO2 capture due to the superior hydrophobicity, chemical resistance, and high mechanical strength. Our research group has been researching on preparation of the PTFE membranes and their applications.29−31 The impact of the absorbents on the PTFE membranes may induce changes in the membrane morphology and properties, such as chemical changes, pore structure, hydrophobicity, and surface roughness. Upon long-term operation, it would result in a decrease in the performance and efficiency. Therefore, the impact of the absorbents and inducing changes of the PTFE membranes may be the focus of the practical application of membrane contactors. Not much literature is available on this topic. The main objective of this study is to investigate long-term stability of PTFE hollow fiber membranes for CO2 capture. It examines the impact of the absorbents on the PTFE hollow fiber membranes and the inducing of changes in the PTFE hollow fiber membranes for CO2 capture on long-term operation. First, PTFE hollow fiber membranes were fabricated through paste extrusion, stretching, and sintering processes in this study. Subsequently, CO2 capture performance over 180 days was performed via a PTFE hollow fiber membrane contactor. Afterward, PTFE hollow fiber membranes were immersed in the absorbent solutions over 24 weeks to simulate exposure conditions used in practical membrane contactors. The chemical changes of PTFE hollow fiber membranes were characterized by attenuated total reflection−Fourier transformed infrared spectroscopy (ATR-FTIR) and energydispersive X-ray spectrometry (EDS). The morphologies, surface properties, and swelling were studied by field-emission scanning electron microscopy (FE-SEM), contact angle

2. MATERIALS AND METHODS 2.1. Materials. Sodium hydroxide (NaOH, >99%) was supplied by Hang Zhou Gaojing Fine Chemical Industry Co., Ltd. (Zhejiang, China). Methyldiethanolamine (MDEA, >99%), monoethanolamine (MEA, >99%) and piperazine (PZ, >99%) were supplied by Aladdin Industrial Corporation (Shanghai, China). Ammonium perfluorooctanoate (98%) was supplied by Juhua Co., Ltd. (Zhejiang, China). Isopar G was supplied by Exxon Mobil Co., Ltd. (USA). All reagents were used as received without further purification. 2.2. Fabrication of the PTFE Hollow Fiber Membranes. PTFE hollow fiber membranes were fabricated through paste extrusion, stretching, and sintering processes in this study. PTFE resins were supplied by Juhua Co., Ltd. Quzhou (Zhejiang, China). PTFE resins and 20 wt % lubricant (Isopar G) were mixed and aged at 30 °C for more than 24 h, and then preformed into a hollow cylinder (inner diameter/outer diameter is 6.0 mm/43.6 mm) at a pressure of 2 MPa, which underwent the paste extrusion process. As a result, the hollow tubes were produced. Subsequently, the hollow tubes were longitudinally stretched by rollers (the stretching ratio was 200%) in an oven at 310 °C. Finally, the sintering process were performed at 360 °C for 50 s, which are the conditions under which the PTFE hollow fiber membranes were produced. The characteristics of the PTFE hollow fiber membranes are listed in Table 2. 2.3. CO2 capture Performance on Long-Term Operation. Figure 1 is a schematic diagram of the experimental setup for CO2 capture via a PTFE hollow fiber membrane contactor through the membrane absorption process. Table 3 presents the characteristics of the contactor. The gas mixture of pure CO2 and the desiccated air with a volume ratio of 15:85 was used as the feed gas, which flowed in the inner side of PTFE hollow fiber membranes (the tube side of the contactor). The flow rate was recorded by a flow meter. The MEA 493

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Energy & Fuels Table 2. Characteristics of the PTFE Hollow Fiber Membranes characteristic

value

outer diameter inner diameter wall thickness porosity average pore diameter

1.6 mm 0.8 mm 0.45 mm 41.9% 0.31 μm

Table 3. Characteristics of the Contactor

(Q inC in − Q outCout) × 273.15 × 1000 22.4 × T × A

value 480 mm 25 mm 32 mm 390 mm 50 polypropylene

2.5.2. Field-Emission Scanning Electron Microscope (FE-SEM). PTFE hollow fiber membranes were frozen in liquid nitrogen, fractured to obtain fragments, and sputtered with platinum using a ion sputtering system (Hitachi, Model E-1010). The morphologies were examined by an FESEM system. The element concentrations were observed with an energy-dispersive X-ray spectrometry (EDS) system (Model EVO MA 25, Zeiss, Germany). 2.5.3. Contact Angle Measurement. The contact angle (CA) measurements were performed using a tensiometer (Model DCAT21, Dataphysics, Germany). Five specimens were tested for each sample. The reported CA data were the average of five values to ensure the reproducibility of the results. 2.5.4. Variations of Membrane Weight. Differential weight measurement was used to analyze variations of membrane weight in different absorbent solutions. The variation of membrane weight (VMW) is shown in eq 2, m − m0 VMW (%) = 1 × 100 m0 (2)

aqueous solution (1 mol/L) was employed as the liquid absorbent, which was pumped by a peristaltic pump (Model WL600, Changzhou Prefluid Co. Ltd., China) and flowed in the outer side of PTFE hollow fiber membranes (the shelf side of the contactor). CO2 in the gas mixture diffused through the membrane pores into the liquid absorbent and was absorbed. In each experiment, CO2 volume concentrations at the inlet and outlet were recorded by a CO2 gas analyzer, respectively. In this study, the CO2 flux (J) was used to indicate the CO2 capture performance, which can be estimated by eq 1:22,25 J=

characteristic the contactor length inner diameter outer diameter effective fiber length number of fibers potting material

(1)

where J is the CO2 flux. Cin and Cout are the CO2 volume concentrations at the inlet and outlet, respectively (each expressed as a percentage). Qin and Qout are the gas flow rates at the inlet and outlet, respectively (in units of m3/s), which were recorded by flow meters (Model LZB-6, Yuyao Auto-Instruments Company, China). T is the gas temperature (in Kelvin), and A is the inner surface area of hollow fiber membranes (in units of m2). 2.4. Membrane Immersion. In the experiments, NaOH, MDEA, MEA, and the mixture of MDEA and PZ (MDEA+PZ) were respectively dissolved in deionized water to prepare the aqueous solutions. Deionized water, 1 mol/L NaOH solution, 1 mol/L MEA solution, 1 mol/L MDEA solution, and the mixed solution (1 mol/L MDEA+PZ, where the molar ratio of MDEA and PZ is 4:1) were loaded into five conical flasks, respectively. The PTFE hollow fiber membranes were immersed into different absorbent solutions in the same depth of each conical flask at 25 °C over 24 weeks. Subsequently, all samples were removed and washed several times with deionized water. Then, all samples were dried under a vacuum at 70 °C for complete removal of the absorbent solutions on the fiber surface. Finally, all samples were placed in a desiccator for further characterization. 2.5. Membrane Characterization and Measurement. 2.5.1. Fourier Transform Infrared Spectroscopy (FTIR) Spectroscopy. The FTIR spectra were obtained using a spectrophotometer (Thermo Nicolet, Model 5700 FTIR) coupled with an attenuated total reflection (ATR) accessory. The wavenumber range was 400−4000 cm−1. Peak intensity was determined using Nicolet OMNIC software.

where VMW is the variation of membrane weight, m0 the weight of the nonimmersed PTFE hollow membranes, and m1 the weight of PTFE hollow fiber membranes, which were dried after immersion. 2.5.5. Pore Size Distribution Analysis. The pore size distribution was investigated using a capillary flow porometer (Porometer 3GZH, Quantachrome Instruments, USA). A bundle of PTFE hollow fiber membranes fully wetted with GQ-16 liquid (Gaoqian Functional Materials Technology Co., Ltd, China) (surface tension = 16 dyn/cm, density = 1.87 g/mL) was mounted on the sample chamber and then the chamber was sealed. Subsequently, pure nitrogen was allowed to flow into the chamber gradually. The increased nitrogen pressure would first reach the point that overcame the capillary flow of the fluid within the largest pore. The pressure then increased continuously until all pores were empty of the fluid. The pore size distribution was calculated by using the computer software that was coupled to the capillary flow porometer. The measurements have been described in the literature.32 2.5.6. Atomic Force Microscopy (AFM). The surface morphology of the PTFE hollow fiber membranes was examined using a digital instrument (DI) (Nanoscope IIIa Multimode AFM in tapping mode). Images with an area of 30 μm × 30 μm were obtained. The root-meansquare of Z (standard deviation of height) values (Rms) and the mean

Figure 1. Experimental setup for CO2 capture. 494

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Energy & Fuels roughness (Ra), were used to express differences in the surface morphology. The above parameters were calculated according to the work of Chung et al.33

ooctanoate). It can be seen that two peaks at 2867 and 2960 cm−1 may be due to asymmetric and symmetric C−H stretching modes. Two peaks at 1363 and 1467 cm−1 may be ascribed to bending vibrations of C−H groups. As for the spectrum of ammonium perfluorooctanoate, the peak at 3457 cm−1 may belong to stretching vibrations of N−H bonds. The peaks at 1621 and 1751 cm−1 may belong to N−H bonds and CO bonds, respectively. Figure 3b shows the ATR-FTIR spectrum on the surface of the nonimmersed PTFE hollow fiber membranes. It can be seen that the intense peaks at 1200 and 1150 cm−1 were originated from the C−F groups. The stretching and sintering temperatures were >260 °C during fabrication of PTFE hollow fiber membranes. The amide groups were possibly produced due to dehydration of PFOA or ammonium perfluorooctanoate. Note that the PTFE hollow fiber membranes were washed with DI water several times and subsequently dried at 70 °C under vacuum, which may completely remove the absorbent solutions on the surface.36 Therefore, the peak at 3338 cm−1 was attributed to stretching vibrations of N−H bonds. The peak at 1650 cm−1 was attributed to stretching vibrations of CO bonds (amide I). The peak at 1540 cm−1 was attributed to bending vibrations of N−H groups (amide II). Furthermore, the peaks appeared at 2850 and 2920 cm−1, which may be due to asymmetric and symmetric C−H stretching modes. Two peaks at 1390 and 1459 cm−1 may be due to bending vibrations of C−H groups, which indicated the occurrence of alkyl groups. This may be due to the fact that the lubricant (Isopar G) was introduced during fabrication of the PTFE hollow fiber membranes. The ATR-FTIR spectra on the surface of the PTFE hollow fiber membranes over 9 weeks and 24 weeks are shown in Figure 4. It can be found almost no significant changes were observed in all the peaks of C−F bonds over 9 weeks and 24 weeks. However, large changes occurred in other peaks. The intensities of other peaks gradually decreased over 9 weeks in the five types of absorbent solutions. The peaks could hardly be observed over 9 weeks in NaOH, MDEA, MEA, and the mixed solutions. It may be due to the fact that the additives (PFOA or ammonium perfluorooctanoate) were water-soluble. The trend in Figure 4b was similar to that in Figure 4a. It indicated that the PTFE polymer exhibited good stability in the absorbent solutions during immersion over 24 weeks. The ATR-FTIR spectra on the cross section of the PTFE hollow fiber membranes immersed over 9 weeks and 24 weeks

3. RESULTS AND DISCUSSION 3.1. Long-Term Performance of CO2 Capture. Figure 2 presents long-term performance for CO2 capture via the PTFE

Figure 2. Long-term performance for CO2 capture over 180 days (flow rate = 1680 mL/min, gas flow rate = 0.09 m3/h, T = 298 K).

hollow fiber membrane contactor over 180 days. No obvious deterioration of CO2 flux can be found throughout the entire operation period. The CO2 flux was maintained almost constant, with a slight reduction, which indicated the superior hydrophobicity of PTFE hollow fiber membranes upon longterm operation. Afterward, the impact of the absorbents on the PTFE hollow fiber membranes and inducing changes in the PTFE hollow fiber membranes for CO2 capture on long-term operation were investigated. 3.2. Impact of the Absorbents on the PTFE Hollow Fiber Membranes. The results of the lubricant (isopar G) by an elemental analyzer (Vario Micro Cube, Elementar Analysensysteme GmbH) were 84.97% C and 15.03% H. Perfluorooctanoic acid (PFOA) or ammonium perfluorooctanoate was necessary during the processing of the PTFE polymer.34,35 Figure 3a shows the ATR-FTIR spectra of the additives (the lubricant (isopar G) and ammonium perfluor-

Figure 3. ATR-FTIR spectra of (a) the additives and (b) the surface of the nonimmersed PTFE hollow fiber membranes. 495

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Figure 4. ATR-FTIR spectra on the surface of the PTFE hollow fiber membranes immersed over (a) 9 weeks and (b) 24 weeks.

Figure 5. ATR-FTIR spectra on the cross section of the PTFE hollow fiber membranes immersed over (a) 9 weeks and (b) 24 weeks.

are shown in Figure 5, which is similar to Figure 4. The intensities of other peaks on the cross section gradually decreased after immersion. Furthermore, no new peaks appeared in the cross section. Comparison of Figures 5a and Figure 5b shows that the intensities of the peaks of the additives and the lubricant on the cross section gradually decreased during immersion from 9 weeks to 24 weeks. If the MEA molecules entered the interior of the PTFE polymer during immersion, two characteristic peaks of the primary amine (MEA) may appear at ∼3338 cm−1, which would be different from the single peak of the amide. The obvious peak of the hydroxy group may also appear at ∼3300 cm−1. The peak of C−N bond of the tertiary amine may appear at 1020−1250 cm−1 when immersed in MDEA solutions. The peak of the hydroxy group may also appear. As for immersion in the mixed solution, the peaks of the secondary amine, the tertiary amine, and the hydroxyl group may all appear. However, it can be found that none of these peaks appeared in Figure 5. Therefore, the absorbent molecules did not enter the interior of the PTFE polymer during immersion. It may be due to its extraordinary microstructure. As a linear fluoropolymer, the PTFE polymer (−CF2−CF2−) has the spiral molecular structure, which is such that the carbon backbone is totally covered by inert F atoms. Both C−C bonds and C−F bonds are extremely strong (C−C, 607 kJ/mol; C−F, 552 kJ/ mol). The size of the F atom allows for continuous coverage around the C−C bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. Moreover, the PTFE polymer is highly crystalline (the

crystallinity is typically 92%−98%). It is insoluble in almost all solvents, even at increased temperature. Only a few chemicals, such as molten alkali metals, can attack PTFE molecules.34,35 Therefore, it is very difficult for the absorbent molecules to enter the interior of the PTFE polymer. Analysis of the elemental concentration variations on the surface of the PTFE hollow fiber membranes was performed using the FESEM-EDS technique. Figure 6a shows that the surface of nonimmersed PTFE hollow fiber membranes was comprised of carbon, nitrogen, oxygen, and fluorine (the H atom is unmeasured). The carbon and fluorine concentrations were 40.92% and 52.64%, respectively, which were greatly different from the ideal concentrations of the PTFE polymer (25% and 75%, respectively). This was indicative of the occurrence of amide groups and alky groups, because of the introduction of the additives. It is in accordance with the result of Figure 3. It can also be shown from Figure 6b that the carbon and fluorine concentrations immersed in NaOH, MEA, MDEA, and mixed solutions over 24 weeks approached the ideal concentrations of the PTFE polymer. Furthermore, the concentration of nitrogen was zero, except for that which was immersed in DI water over 9 weeks. The concentration of oxygen decreased during immersion, from 9 weeks to 24 weeks. It indicated that the concentrations of the additives on the surface of the PTFE hollow fiber membranes decreased during 496

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decreased during immersion. Accordingly, the carbon and fluorine concentrations varied during the earlier immersion time. Slight variations occurred after the fourth week. Finally, the carbon and fluorine concentrations were close to the ideal concentrations of the PTFE polymer, which indicated that the concentrations of the additives in the PTFE hollow fiber membranes decreased to almost zero over 24 weeks. The chemical compositions of the following absorbents were determined: water (H 2 O), MEA (C 2 H 7 NO), MDEA (C5H13NO2), and the mixture of MDEA+PZ (C5H13NO2 and C4H10N2). Figure 8 presents the elemental concentration

Figure 6. Elemental concentrations on the surface of the PTFE hollow fiber membranes immersed over (a) 9 weeks and (b) 24 weeks.

immersion. This may be due to the fact that most of the additives were dissolved in the absorbent solutions. Figure 7 shows the variations in the elemental concentrations on the surface of the PTFE hollow fiber membranes immersed in the mixed solution over time. It can be found that the concentration of nitrogen quickly decreased to zero over 1 week. The concentration of oxygen quickly decreased to be almost zero; this observation indicated that the concentrations of the additives in the PTFE hollow fiber membranes quickly Figure 8. Elemental concentrations on the cross section of the PTFE hollow fiber membranes immersed over (a) 9 weeks and (b) 24 weeks.

variations on the cross section of the PTFE hollow fiber membranes. The concentration of nitrogen was zero, except for that immersed in DI water. The concentration of oxygen decreased during immersion in all absorbent solutions. Almost no changes (especially, no increase) occurred in the oxygen concentration during immersion from 9 weeks to 24 weeks. There is no elemental sodium on the cross section immersed in NaOH solution over 9 and 24 weeks. In combination with Figures 6−8, it can be concluded that none of the absorbent molecules entered into the interior of the PTFE polymer. Figure 9 shows variations of membrane weight (VMW) over time. It can be found that all VMW values except for that in DI water were below 0%. Furthermore, the VMW continuously decreased during immersion. It may be due to the dissolution of the remained additives in the absorbent solutions, which induce a decrease in VMW. It is consistent with the results of

Figure 7. Elemental concentrations on the surface of the PTFE hollow fiber membranes immersed in the mixed solution over time. 497

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surface resulted from a higher reduction ratio than that on the inner surface of the PTFE hollow tube.30 Figure 11 illustrates the SEM images of the PTFE hollow fiber membranes immersed in different absorbent solutions over 24 weeks. It can be found that the surface morphologies did not exhibit great changes during immersion. Overall, the pore sizes were enlarged after immersion in the absorbent solutions. The pore size distributions of the PTFE hollow fiber membranes immersed over 24 weeks are presented in Figure 12. The corresponding data are listed in Table 4. It can be found that the pore size distribution profiles became flattened. The largest average pore diameter was observed in the PTFE hollow fiber membranes immersed in the mixed solution, followed by the MDEA solution, the MEA solution, the NaOH solution. The corresponding values of surface tension are listed in Table 5. It indicated that smaller surface tension values would induce larger changes in pore size. This observation was consistent with the results of Barbe et al.14 and Kamo et al.36 The changes of the surface morphologies of the PTFE hollow fiber membranes were less than those of the PP hollow fiber membranes,38 which may be due to the fact that the PTFE polymer has an extraordinary microstructure. 3.4. Surface Properties of the PTFE Hollow Fiber Membranes. Figure 13 shows values of contact angle of the PTFE hollow fiber membranes immersed over 9 weeks and 24 weeks. It can be seen that the PTFE hollow fiber membranes after immersion were still hydrophobic. The contact angle on the outer surface slightly decreased during immersion in the absorbent solutions (from CA = 123.8° to CA = 115.8°). The decrease in contact angle on the inner surface was larger than that on the outer surface, which may be due to the asymmetric structure.30,31 As shown in Figure 8, the pore sizes on the inner surface were larger than those on the outer surface. It can be concluded from the results of section 3.1 and 3.2 that none of the absorbent molecules entered the interior of the PTFE polymer during immersion. The absorbent molecules may diffuse into the pores. It is much easier for the absorbent molecules to diffuse into the larger pores during immersion. It may be obviously observed that the order of the additives immersed in the absorbent solutions, ranked based on the contact angle, was as follows: deionized water > NaOH > MEA > MDEA > mixed solution (MDEA+PZ). This was exactly the same order as the surface tension values of these solutions (as

Figure 9. Variations of membrane weight (VMW) over time.

FTIR and EDS. However, the PTFE polymer has an extraordinary microstructure (as previously mentioned). It is very difficult for the absorbent molecules to enter into the interior of the PTFE polymer. Therefore, none of the absorbent molecules entered the interior the PTFE polymer, which was shown as the results of FTIR and EDS. Furthermore, when the PTFE hollow fiber membranes were immersed in the absorbent solutions, the absorbent molecules may diffuse into the pores, and they may be adsorbed onto the surface of the PTFE hollow fiber membranes. The absorbent molecules that were adsorbed on the surface may be removed when the PTFE hollow fiber membranes were washed several times, because of the weak adsorption.36 Consequently, the weights of the immersed PTFE hollow fiber membranes were less than that of nonimmersed PTFE hollow fiber membranes. 3.3. Membrane Surface Morphology Analysis. Figure 10 shows SEM images of the nonimmersed membrane surface. It can be observed that the PTFE hollow fiber membranes have the microstructures of nodes interconnected by fibrils. The formation mechanism of porous structure was studied by Kitamura et al.37 It was noticed that the pore sizes on the inner surface were larger than those on the outer surface. It may be due to the fact that the higher extrusion pressure on the outer

Figure 10. Scanning electron microscopy (SEM) images of the nonimmersed PTFE hollow fiber membranes. 498

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Figure 11. SEM images of the PTFE hollow fiber membranes immersed in different absorbent solutions over 24 weeks. 499

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Figure 12. Pore size distributions of the PTFE hollow fiber membranes immersed in different absorbent solutions over 24 weeks.

Table 4. Pore Sizes of the PTFE Hollow Fiber Membranes Immersed over 24 Weeks absorbent solution

average pore diameter (μm)

maximum pore diameter (μm)

0.31 0.32 0.34

0.55 0.60 0.67

0.36 0.36

0.69 0.70

0.38

0.72

nonimmersion deionized water 1 mol/L NaOH solution 1 mol/L MEA solution 1 mol/L MDEA solution 1 mol/L mixed solutiona a

Figure 13. Values of the contact angle (CA) of the PTFE hollow fiber membranes immersed over (a) 9 weeks and (b) 24 weeks.

MDEA+PZ; the molar ratio of MDEA and PZ is 4:1.

Table 5. Surface Tension Values of Different Absorbent Solutions after 24 Weeks

a

absorbent solution

surface tension (mN/m)

deionized water 1 mol/L NaOH solution 1 mol/L MEA solution 1 mol/L MDEA solution 1 mol/L mixed solutiona

73.08 66.99 51.01 50.49 47.43

MDEA+PZ; the molar ratio of MDEA and PZ is 4:1.

shown in Table 5). In other words, the contact angle was dependent on the surface tension values of the absorbent solutions. It may be due to that the absorbent solution with a lower surface tension diffused more easily into the pores. This was consistent with the literature.2,6 CA values in the mixed solution over time are shown in Figure 14. It can be found that the contact angle continuously decreased over time. The CA value on the outer surface decreased from 123.8° to 115.8° when the PTFE hollow fiber membranes were immersed over 24 weeks. Only 8° reduction existed in the contact angle, which indicated that the surface hydrophobicity did not markedly decrease during immersion. Further immersion would result in a slow decrease in CA from the ninth week, and then a stable state was attained, which was consistent with the result that the CO2 flux decreased until a stable state was attained.16

Figure 14. CA values on the outer surface in the mixed solution over time.

In a word, a slight reduction in CA happened to the PTFE hollow fiber membranes during immersion. It may be due to the extraordinary microstructure of the PTFE polymer. In other words, the absorbent solutions had a slight impact on the surface properties of the PTFE hollow fiber membranes. Nishikawa et al.26 employed the PTFE hollow fiber membranes for CO2 capture over 6600 h, in which the membranes were still hydrophobic. In contrast, it can be concluded from Table 1 that other membranes were largely influenced by the absorbent solutions. It is believed that the PTFE hollow fiber membranes are superior to other membranes upon long-term operation, 500

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Energy & Fuels

Figure 15. Atomic force microscopy (AFM) images on the inner surface of the PTFE hollow fiber membranes immersed over 24 weeks (including nonimmersion).

because of the excellent hydrophobicity and chemical resistance. The surface hydrophobicity may decrease when the hydrophilic absorbent molecules diffused into the membrane pores. As the immersion time increased, more-absorbent molecules would diffuse into the membrane pores. Therefore, the differences of the absorbent concentrations between the membrane pores and the absorbent solutions would decrease, which meant a reduction in the driving force for diffusion, according to Fick’s theory of molecular diffusion. Consequently, the reduction in CA slowed. In addition, no significant reduction in CA occurred. It also showed that none of the absorbent molecules entered into the interior of the PTFE polymer. The AFM images on the inner surface of the PTFE hollow fiber membranes immersed over 24 weeks are presented in Figure 15. The corresponding data are listed in Table 6. It can be found that the nonimmersed PTFE hollow fiber membranes had the highest Rms and Ra values. The surface roughness values decreased after immersion in all absorbent solutions. It showed that immersion imposed a negative influence on the roughness values. As previously mentioned, none of the absorbent molecules entered into the interior of the PTFE polymer during immersion. However, the absorbent molecules may diffuse into the membrane pores, which would induce that the

Table 6. Surface Roughness on the Inner Surface of the PTFE Hollow Fiber Membranes Immersed over 24 Weeks AFM Parameter

nonimmersion deionized water 1 mol/L NaOH solution 1 mol/L MEA solution 1 mol/L MDEA solution 1 mol/L mixed solutiona a

root-mean-square of Z, Rms (nm)

mean roughness, Ra (nm)

251.0 217.1 206.2

191.1 166.2 153.3

142.1 105.1

106.3 74.8

94.2

69.6

MDEA+PZ; the molar ratio of MDEA and PZ is 4:1.

pore size distribution profiles became flattened (shown in Figures 11 and 12). It would further induce less surface roughness. Generally, the contact angle can be calculated using Young’s equation, as shown in eq 3: γsg − γsl cos θ0 = γgl (3) 501

DOI: 10.1021/acs.energyfuels.5b01789 Energy Fuels 2016, 30, 492−503

Article

Energy & Fuels where θ0 is the contact angle on a smooth surface, γsg the solid/ gas interfacial free energy, γsl is the solid/liquid interfacial free energy, and γgl the gas/liquid interfacial free energy. If a water droplet is placed on a rough surface, the contact angle should be predicted by the Wenzel equation, as shown in eq 4:39,40 cos θ = R f cos θ0

Ministry of Major Science & Technology of Zhejiang Province in China (Grant No. 2013C01055), the Training Plan for “521” Talents of Zhejiang Sci-Tech University and Zhejiang Key Discipline Fund of Universities for support of this program.

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where θ is the contact angle on a rough surface and θ0 is the contact angle on a smooth surface. Rf is the nondimensional factor, which is calculated in eq 5: Rf =

ASL AF

(5)

where ASL is the surface area and AF is the flat projected area. The Wenzel model indicates that a hydrophobic surface (θ > 90°) will become more hydrophobic when the surface roughness increases. Therefore, the hydrophobicity order of the PTFE hollow fiber membranes immersed over 24 weeks, would be as follows: nonimmersion > deionized water > NaOH solution> MEA solution > MDEA solution > the mixed solution. This was the same order as the contact angle that was discussed in Figure 13.

4. CONCLUSIONS In order to better understand wetting of the PTFE hollow fiber membranes for CO2 capture upon long-term operation, PTFE hollow fiber membranes were fabricated through paste extrusion, stretching, and sintering processes. CO2 capture performance upon long-term operation over 180 days was performed, which was further investigated through immersion of the PTFE hollow fiber membranes in different absorbent solutions. It indicated no obvious deterioration of CO2 flux upon longterm operation over 180 days. Fourier transform infrared (FTIR) spectroscopy and energy-dispersive spectroscopy (EDS) analyses showed the PTFE polymer maintained good stability in the absorbent solutions over 24 weeks. Variations of membrane weight were below 0%, except in deionized water. The pore sizes of the PTFE hollow fiber membranes did not markedly enlarge after immersion. The surface hydrophobicity did not markedly decrease during immersion. A reduction in contact angle (CA) of only 8° was observed over 24 weeks. CA values relied on the surface tension values of the absorbent solutions. Immersion imposed a negative influence upon the roughness values. It may be due to the fact that the absorbent molecules diffused into the membrane pores and were adsorbed onto the surface of the PTFE hollow fiber membranes, while not entering into the interior of the PTFE polymer, because of its extraordinary microstructure. PTFE hollow fiber membranes may be a promising alternative for CO2 capture upon long-term operation.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-057186843622. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the Natural Science Foundation of Zhejiang Province in China (Grant No. LY15B060010), 502

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DOI: 10.1021/acs.energyfuels.5b01789 Energy Fuels 2016, 30, 492−503