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CH3SiO2 composite hollow fiber membrane for membrane contactor application
Separation and Purification Technology 228 (2019) 115689
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Fabrication of hydrophobic PP/CH3SiO2 composite hollow fiber membrane for membrane contactor application Parya Amirabedia,b, Ali Akbaria,b, Reza Yegania,b, a b
T
⁎
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite polypropylene membrane Membrane contactor PP-CH3SiO2 CO2 absorption
Polymeric membrane wetting by liquid absorbent is one of the major challenges of the gas- liquid membrane contactors. In order to overcome this issue, composite polypropylene (PP) hollow fiber membranes were prepared by incorporation of methyl grafted silica nanoparticles (CH3-g-silica NPs) via two treatment approaches including blending the NPs with dope solution before membrane fabricating (approach 1), and also coating the NPs on the surface of the prepared membrane (approach 2). The fabricated membranes were characterized by using ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure, wetting resistance and gas permeation test. The obtained results from ATR-FTIR spectra confirmed the successful formation of the PP-CH3SiO2 composite hollow fiber membranes. It was seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. The fabricated membranes were also investigated for CO2 absorption process in gas- liquid membrane contactors. As a result, the gentle decrease in flux of composite membranes compare to the sharp flux drop of neat one confirmed the effective improvement in the non-wetting property of the membranes in the presence of inorganic particles.
1. Introduction It is vital to introduce devise techniques which would reduce acid gases like CO2 arising from the combustion of fossil fuels, present in the natural gas and industrial exhaust due to direct contribution in the greenhouse effects and climate change [1–4]. The most common process for removal of acid gases is an absorption into a solvent using conventional gas–liquid contactors such as packed towers, bubble columns, spray towers, etc. However, these conventional techniques are energy-consuming and not easy to operate because of some operating problems including flooding, foaming, entrainment and channeling [5,6]. A promising alternative technology which can overcome these disadvantages and possesses a high potential to replace conventional equipment is membrane contactor. This technology has numerous of benefits, such as flexibility of operation, high surface area per unit volume, being easy to scale up or down and predictable performance module [7–9]. Due to its excellent mass-transfer properties, membrane gas-absorption technology has been known as a technically viable option by the International Energy Agency's working group on CO2 removal [10]. However, the membrane contactor suffers from membrane wetting in a long-term operation, which will lead to a sharp increase in
⁎
membrane resistance against absorption process [2,11,12]. Mavroudi et al. investigated the wetting problem in membrane contactor systems and found that the membrane mass-transfer resistance accounted for 20–50% of the total mass-transfer resistance in the case of liquid-filled membrane pores [13]. It is well-known that the porous hollow fiber membrane is a core element in the membrane contactor device [14]. Accordingly, selecting an appropriate polymeric material is logically the first parameter to complete the requirements for membrane contactor processes. However, various experimental results revealed that polymeric membranes, utilized in the membrane contactors, are limited to short term and mild operating conditions, i.e. low alkalinity, acidity and low temperature applications. Accordingly, many studies have been carried out to improve the chemical, mechanical and thermal stabilities and non-wetting characteristics of conventional polymeric membranes to be used in membrane contactor applications [15]. According to the literature, addition of the inorganic fillers into polymeric membranes, known as “composite membranes”, seems to be a promising strategy to improve the membranes properties and consequently achieve a long-term stable operation [16–18]. Indeed, composite membranes combine the advantages of both polymeric and
Corresponding author. E-mail addresses: [email protected] (P. Amirabedi), [email protected] (A. Akbari), [email protected] (R. Yegani).
https://doi.org/10.1016/j.seppur.2019.115689 Received 12 November 2018; Received in revised form 3 May 2019; Accepted 10 June 2019 Available online 11 June 2019 1383-5866/ © 2019 Published by Elsevier B.V.
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methyltriethoxysilane (MTES ≥ 98%) and tetraethylorthosilicate (TEOS ≥ 98%), used for NPs synthesis, were purchased from Merck. Commercial grade of isotactic polypropylene (iPP, EPD60R, MFI = 0.35 g/10 min), used for membrane fabrication, provided from Arak Petrochemical Company of Iran. Mineral oil (MO) as a diluent, acetone as a extractor and Irganox 1010 as a heat stabilizer were purchased from Acros Organics, Merck and CibaCo., respectively. Sulfuric acid (98% H2SO4) and hydrogen peroxide (H2O2), used to prepare the Piranha solution, and Monoethanolamine (MEA) as a chemical absorbent were purchased from Merck.
inorganic membranes and exhibit interesting membrane properties such as considerable hydrophobicity, great chemical and thermal resistances and excellent adaptation to the severe operating conditions [16,19,20]. In general, the composite membranes can be produced by two methods, including blending the inorganic particles with dope solution before membrane fabricating, and also coating the inorganic particles on the surface of the prepared membrane [21]. In the case of composite membranes fabricated via first method, Rezaei et al. fabricated Polyvinylideneflouride (PVDF) hollow fiber mixed matrix membranes (MMMs) through wet phase inversion method using general MMT and Cloisite 15A as inorganic fillers to be used in a membrane contactor process [15]. They showed that the permeation flux and wetting resistance of the membranes in terms of contact angle and liquid entry pressure of water increased significantly by filler loading. Taghaddosi et al. prepared nanoclays embedded PP membranes by using thermally induced phase separation (TIPS) method to improve the anti-fouling properties of PP membrane [22]. The obtained results showed that incorporation of nanoclays into PP membranes could significantly mitigate fouling and improve the membranes performance. Among the inorganic fillers, silica NPs have received much attention due to their well-defined ordered structure, cost-effective production, high surface area and easy surface modification [23]. In the field of composite membranes prepared via coating the silica NPs on the membrane surface, Zhang et al. reported an appropriate method to form a highly hydrophobic Polyetherimide (PEI) membrane [21]. It was observed that incorporation of the fluorinated silica inorganic layer on the membrane surface offered high hydrophobicity and would protect the polymeric membrane from attacks of the chemical absorbents. Wang et al. fabricated PP composite membranes by loading SiO2 NPs and attaching 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) on the membrane surface to be used in vacuum membrane distillation (VMD) [24]. The resulted composite membranes showed good antiwetting ability and stable anti-fouling performance during the longterm operation. Xu et al. fabricated super-hydrophobic PP hollow fiber membrane by using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) for membrane distillation (MD) application [25]. There is a wealth of literature available on the utilization of PP composite membranes for wide application; however, there is a lack of literature reporting about application of PP composite membranes as a membrane contactor. PP has excellent characteristics such as high melting temperature, low production cost, excellent processability, widely adjustable mechanical properties, high resistance to water and chemical environments and so on [26]. Due to economic concerns, further expansion of the application area of PP composite membranes is highly desired, thus, in this work, two treatment approaches, described above, were employed fabricate PP composite membranes by incorporation of methyl grafted silica nanoparticles. In the present work, inorganic NPs were synthesized via sol-gel method. It should be noted that the influential parameters in fabrication of silica NPs via sol-gel method were optimized in our previous work using central composite design of the response surface method (RSM) [18], and the obtained results were used in this work. The fabricated membranes were characterized by various characterization analyses, including ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure and gas permeation test. Moreover, in order to investigate the wetting resistance of the neat and composite hollow fiber membranes, prepared membranes were tested in a gas-liquid hollow fiber membrane contactor for CO2 absorption.
2.2. Preparation of composite membrane In the present work, two treatment approaches including blending the inorganic particles with dope solution before membrane fabricating (approach 1) and also coating the inorganic particles on the surface of the prepared membrane (approach 2), were employed for fabrication of the PP-CH3SiO2 composite membranes. 2.2.1. Preparation of composite membrane with approach 1 In approach 1, at first, a sol solution was prepared while the molar ratios of TEOS:MeOH:H2O:NH4OH at 1:36:8:6.3 was kept constant, respectively, and vigorous stirring at room temperature according to the method explained in our previous work [18]. After 1 hr, a certain amount of MTES (molar ratio of MTES/TEOS = 3.774) was added into the reaction mixture. The mixture was then stirred vigorously for 12 hr. The obtained solution was centrifuged at 10,000 rpm for 10 min and then rinsed by DI water. Finally, the prepared particles were dried at 80 °C for 24 hr to obtain the methyl grafted silica NPs. After synthesis of the NPs, PP-CH3SiO2 composite membranes were fabricated by TIPS method, as described below. 0.5 g of modified silica NPs was dispersed into 75 g of MO using sonication by probe system (Sonopuls HD 3200, Bandelin) for 30 min. Then 24.5 g PP was added to the diluent-NPs suspension and melt-blended at 170 °C for 100 min in a sealed glass vessel. The homogenous dope solution was then degassed before spinning. In the spinning process, the bore fluids and dope solution were extruded through a spinneret and went through an air gap before immersing into a water bath. The nascent hollow fibers were collected at a certain take-up speed using a rotating drum and subsequently immersed in acetone for 24 hr to extract the diluent and achieve a desired porous membrane. The spinning conditions were summarized in Table 1. 2.2.2. Preparation of composite membrane via approach 2 In approach 2, at first, the neat PP hollow fiber membranes were fabricated by the spinning method described in 2.2.1. The casting solution composition; (PP/MO weight percent fraction), was kept constant at 25/75. Then, the fabricated membranes were treated by Piranha solution; (3:1 wt ratio of H2SO4:H2O2 solution), for 3 hr, in order to create a hydroxyl functional group, as a linkage agent between the synthesized inorganic particles and membrane surface. All samples Table 1 Spinning conditions for fabrication of PP hollow fiber membranes.
2. Material and methods 2.1. Materials Methanol (MeOH) (99%), ammonium hydroxide (NH4OH) (25%), 2
Spinning conditions
Remarks
Spinneret OD (mm) Spinneret ID (mm) Air gap (cm) Drum diameter (cm) Take up speed (m/s) Dope flow rate (ml/min) Bore fluid type Bore flow rate (ml/min) Bore fluid temperature (°C) Coagulant temperature (°C) Spinning temperature (°C)
1.1 0.65 1 14 0.147 4.0 Mineral Oil (MO) 2.0 170 25 170
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were washed with deionized water (DI) and then dried at room temperature. The detailed procedure of Piranha treatment method can be found in our previous published works [19,27]. After that, the certain amounts of TEOS, MeOH, NH4OH and H2O (similar to approach 1) were mixed for 1 hr to obtain silica colloid particles. Then the desired amount of MTES (molar ratio of MTES/ TEOS = 3.774) was added dropwise into the solution. The mixtures were continuously stirred for 12 hr at ambient temperature. In the following, Piranha-treated membranes were immersed in the mixture for 3 hr and then dried at 75 °C for 4 hr. Finally, all the prepared samples were washed with water under ultrasonication and dried prior to analysis.
rp,m =
2.3.6. Mechanical strength A tensile test machine (STM-5, SANTAM) was used to measure the mechanical properties of the neat and composite membranes. The initial length of each fiber was adjusted at 10 cm, and the stretch speed was kept constant at 10 mm/min. Three measurements were taken for each sample and the mean value was reported.
2.3.2. Membranes and NPs structure and morphology The morphology of the neat and composite hollow fiber membranes was investigated using field emission scanning electron microscopy (FESEM) device (LEO440-I). In order to investigate the cross section structure of the membranes, they were fractured in liquid nitrogen and coated with gold by sputtering before observation to make them conductive. The synthesized NPs were investigated by Philips BioTwin, the Netherlands transmission electron microscope (TEM).
2.3.7. Breakthrough pressure The wettability of a microporous membrane by a liquid absorbent can be evaluated by the breakthrough pressure value of the absorbent through the membrane pores. For small uniform pores with cylindrical geometry, the breakthrough pressure (ΔP) can be estimated using the Laplace–Young equation [30]:
Δp = −
2.3.3. Porosity Porosity of the obtained membranes was determined by impregnating the samples with isobutanol for 24 hr. Generally, the membrane porosity (ε) is defined as the pore volume divided by the total volume of the membrane using the following formula [28]:
(6)
2.3.8. Membrane contactor experiments The CO2 absorption experiment was carried out to evaluate the performance of the fabricated neat and composite hollow fiber membranes. Ten hollow fibers with an effective length of 18 cm were sealed in a cylindrical module. Table 2 gives the detailed information of the membrane contactor module. DI water and aqueous solution of MEA (30 wt%) were used as physical and chemical absorbents, respectively. For all experiments, the liquid absorbent flowed through the shell side and pure CO2 as well as mixed CO2 (CO2/CH4 20/80 vol%) gas flowed counter-currently through the lumen side of the hollow fiber membranes. The gas flow rates at the outlet and inlet of the contactor module were measured by soap film flow meters (HORIBA STEC- VP-3). A back-pressure regulator was used to adjust the gas-side pressure at 1 bar, where the liquid pressure was set at 0.5 bar higher than it to prevent the bubble formation in the liquid phase. Finally, for pure CO2, the absorbent flows concentrations of the inlet and outlet streams were measured by titration method as described in the literatures [32]. In order to analyze the mixed gas compositions, gas chromatography (GC) was employed. The CO2 removal efficiency and absorption
(1)
2.3.4. Effective surface porosity and mean pore size The mean pore size and effective surface porosity of the pure and composite membranes were obtained via gas permeation test. In this regard, three modules each containing one fiber with an effective length of 18 cm were prepared and the average value was recorded. Nitrogen gas was introduced to the shell side of the module and the permeation flux in lumen side was determined by using a soap bubble flow meter at various pressures. The gas permeation model was used to achieve the effective surface porosity and mean pore size as follow:
2 8RT 0.5 rp,m ξ 1 rp,m 2 ξ ⎛ ⎞ + Pm 3 ⎝ π M ⎠ RT lp 8μ RT lp
4σl cos θ d max
Here σ1is the liquid surface tension, θ is the contact angle between the liquid and the membrane surface, and dmax is the maximum pore diameter determined by using bubble point method [31].
where wib is the mass of the isobutanol -impregnated hollow fiber membrane (g), wd is the mass of the dry membrane (g), ρib is the isobutanol density and ρp is the density of the polymer (PP).
P = PP + PK =
(5)
2.3.5. Contact angle and AFM The contact angles of liquid absorbent (water and aqueous solution of MEA (30 wt%)) were measured by using contact angle goniometer (PGX, Thwing-Albert Instrument Co) to evaluate membrane hydrophobicity. All tests were conducted at least for 5 times on the different locations of each sample and the average value was reported. Also, the surface morphology of the pure and composite membranes was studied by using atomic force microscopy (AFM) (Nanosurf Mobile S. Samples).
2.3.1. ATR-FTIR Chemical structures of the samples before and after modification were analyzed by ATR-FTIR spectra using infrared spectroscopy device (BRUKER-TENSOR 27) in the range of 500–4000 cm−1.
(wib − wd)/ ρib × 100 (wib − wd)/ρib + (wd/ρp )
(4)
ξ 8μRTb = lp rp,m 2
2.3. Membrane characterizations
ε=
16 b 8RT 0.5 ⎛ ⎞ μ 3 a ⎝ πM ⎠
(2) (3)
P= B + S Pm
Table 2 Specification of the hollow fiber modules.
2
where P is the total gas permeance (mol/m Pas), PP and PK are the gas permeance by Poiseuille and Knudsen flow regimes respectively (mol/ m2Pas), R is the universal gas constant, T is the absolute temperature (K), M is the gas molecular weight (kg/mol), rp,m is the mean pore radius of the membrane (m), µ is the gas viscosity (Pa s), ξ is the porosity of the membrane surface, lp is the effective pore length (m) and Pm is the mean pressure (Pa). Finally the mean pore size and effective surface porosity can be obtained through Eqs. (4) and (5) by calculating the slope (S) and intercept (B) from linear plot between P and Pm [29]. 3
Parameter
Value
Fiber OD (mm) Fiber ID (mm) Effective fiber length (cm) Number of fibers Module ID (mm) Module length (cm)
0.7 0.5 18 10 10 24
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Fig. 1. Schematic figure of the experimental setup for CO2 absorption.
flux were calculated by Eqs. (7) and (8), respectively [9]:
ε=
J=
ΔCl,av =
(HCg,in − Cl,out )− (HCg,out − Cl,in)
Qg,in × Cg,in
(11)
g,out − Cl,in)
(7) where Cg, in and Cg, out are the inlet and outlet CO2 concentration in the gas-phase, respectively. In membrane contactor system, the Wilson plot is generally applied to determined mass transfer resistance [20,33]. In this case, the mass transfer resistance of the pure gas phase can be ignored while the mass transfer resistance of the liquid phase is proportional to the liquid velocity. The Wilson plot is drawn by 1 versusV−β , where V is the liquid
(Qg,in × Cg,in − Qg,out × Cg,out ) × 273.15 0.0224 × Tg × S
(HCg,in − Cl,out)
ln (HC
Qg,in × Cg,in − Qg,out × Cg,out
(8)
where ε is the CO2 removal efficiency (%); J is the flux of CO2 (mol/ m2s); Qg,in andQg,out are the gas flow rates at the inlet and the outlet, respectively (m3/s); Cg,in and Cg,out are the CO2 volumetric concentrations in the gas phase at the inlet and the outlet, respectively (%); Tg is the temperature of the feed gas (K) and S is the gas–liquid interfacial area (m2). Schematic figure of the setup is shown in Fig. 1. It should be mentioned that before taking the samples, all experiments were carried out for 30 min to achieve a steady state condition. The long-term absorption performances of the neat and composite PP hollow fiber membranes were examined by conducting the pure CO2 absorption experiments for 30 days. Furthermore, the chemical compatibility of the fabricated membranes and chemical absorbent was investigated by using FESEM images after immersing the membrane samples in the MEA solution for 90 days.
K ol
velocity and − β is the value which provides the best straight line. Accordingly, the mass transfer resistance of the membrane is given by the interception of the Wilson plot as follow:
1 Hd o = C1V−β + K ol kmdln
(12)
3. Results and discussion 3.1. ATR-FTIR
2.3.9. Wilson plot The mass transfer resistances of the gas and liquid phases inside a porous medium such as the membrane can be described by using resistance-in-series model which expressed by Eq. (9):
1 1 Hd o Hd o = + + K ol Kl Km dln K g di
ATR-FTIR spectra of the neat and composite membranes fabricated by both treatment approaches are shown in Fig. 2. For all samples, the absorption bands observed at 2700–2950 cm−1 were assigned to the CH stretching vibrations. Also, the bands observed at 1465 cm−1 and 1375 cm−1 were attributed to the bending vibrations of CeH bonds in CH2 and CH3 functional groups, respectively [34]. For composite membranes, the absorption peaks at 1100–1000 cm−1 were assigned to the SieOeSi asymmetric stretching vibrations, which confirm the formation of SiO2 network structure on the polymer membrane [35]. The broad peak observed at 3600–3100 cm−1 was assigned to the OeH functional group of the membranes surface [17]. The peaks appeared at 2973 and 1275 cm−1 are attributed to the absorption vibration of CeH bonds in methyl as well as SieCH3 groups, respectively [36]. Therefore, it can be concluded that the ATR-FTIR spectra confirmed the successfully formation of the PP-CH3SiO2 composite membranes. Moreover, it can be observed that the intensity of the SieCH3 bands in the composite membranes fabricated via approach 2 is higher than of the composite membranes fabricated via approach 1, which was related to the larger amounts of CH3-g-silica particles formed on the surface of the coated membranes compared to another composite membrane.
(9)
whereK ol is the overall liquid-phase mass transfer coefficient; kg, km and kl are the mass transfer coefficients of the gas phase, membrane and liquid phase, respectively; di, do and dln are the inner, outer and logarithmic mean diameters of the fibers, respectively and H represents Henry’s constant. Also, Kol can be obtained based on the experiment as follow:
K ol =
Q l (Cl,out − Cl,in) AT ΔCl,av
(10)
where Q l is the liquid flow rate, Cl,in and Cl,out are the inlet and outlet CO2 concentrations in the liquid-phase respectively, AT is the gas-liquid contact surface and ΔCl,av is the logarithmic mean concentration which expressed by Eq. (11): 4
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Fig. 2. ATR-FTIR spectra of the neat and composite PP hollow fiber membrane samples.
two treatment approaches. The results indicate that the composite membranes showed higher contact angle than the neat one. Actually, hydrophobic surface can be produced by either lowering the surface free energy or enhancing the surface roughness or a combination of both phenomena [21]. Therefore, increasing in contact angle is due to incorporation of methyl functional group with lower surface free energy (–Si–(CH3)3) [39] and also due to an increase in the surface roughness because of creating of the Si-O-Si groups on the membrane surface. Besides, it can be seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. By comparison three dimensional (3D) AFM images shown in Fig. 5, it can be observed that, the surface roughness of the sample fabricated by approach 2 was apparently higher than the other two samples. It should be noted that the obtained results by AFM analysis are in good agreement with the results of contact angle measurement. In fact, the large amounts of CH3-g-silica NPs on the surface of the composite membrane fabricated with approach 2 caused to a very rough surface which resulted in high contact angle for this membrane compared to other samples.
3.2. Membranes and NPs structure and morphology Fig. 3 shows the FESEM images captured from the cross-sections (1, 2) and surfaces (3) of the neat (a) and composite membranes fabricated with approaches 1 (b) and 2 (c), respectively. As shown in the figure, all membranes have cellular structures with spherical pores. From Fig. 3 (c2 and c3), it can also be observed that the inorganic NPs were formed on the surface as well as in the cross section of the hollow fiber membrane. By comparison of b2 and c2 samples, it can be seen that the number of pores as well as the mean pore size in sample (b2) is more than that of in sample (c2). The obtained results of mean pore size and porosity measurement shown in Table 3, confirmed our findings from the FESEM images. It can be observed that the mean pore size of the composite membranes fabricated with approach 1 is higher than that of fabricated with approach 2. Indeed, the heterogeneous nucleation effect of the inorganic NPs inside the dope solution caused to increase the membrane mean pore size and porosity. On the other hand, presence of methyl grafted silica NPs during phase separation resulted in greater nucleation rate and subsequently further pores [37]. Moreover, in order to obviously illustrate the structure of methyl grafted silica NPs, TEM image of synthesized NPs is shown in Fig. 4. It can be seen from this figure that the NPs are almost spherical with an average particle size of 20–40 nm. In addition, the core–shell structure for methyl grafted silica NPs is observed in Fig. 4, which silica is in the core and CH3 groups are in the shell.
3.4. Mechanical strength Good mechanical stability is one of the important factors to be considered in the membrane contactor application. The mechanical stability of the neat and composite membranes was examined in the term of tensile modulus as well as stress and elongation at the break point. Fig. 6 shows the stress and elongation properties of the fabricated membranes at the break point. It can be seen that incorporation of the inorganic NPs on the surface as well as in the cross section of the composite membranes improved stress in comparison with the neat one. This reveals that the interfacial bonding between inorganic particles and polymer chain is good [40]. Moreover, from stress-elongation curves, the tensile modulus was
3.3. Contact angle and AFM Surface hydrophobicity of the membrane can be examined by contact angle measurement which is related to the surface free energy and roughness [38]. Fig. 5 shows the contact angle measurements of the outer surfaces of the neat and composite membranes fabricated with 5
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Fig. 3. FESEM images of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2, (1) & (2) Cross section and (3) Outer surface. Table 3 Characteristics of the neat and composite PP hollow fiber membranes. Membrane
Porosity (%)
Average pore size (μm)
Effective surface porosity (m−1)
Maximum pore size (μm)
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 2
45 49 42
0.140 0.154 0.120
248.004 273.521 240.712
0.180 0.187 0.150
CH4 (80%) was selected as a feed gas. Absorbent velocity is maybe the most important operating parameter in the gas–liquid membrane contactors because generally it has a noticeable influence on the mass transfer rate of CO2 [9]. The results of CO2 flux obtained at different liquid velocities were presented in Fig. 7. It was found that the increase in liquid velocity causes to increase in CO2 flux for both neat and composite membranes. On the other hand, by increasing the liquid velocity, the removal efficiency increased at the same time as shown in Fig. 8. It is due to increasing the mass transfer coefficient with liquid velocity [41]. It means that, by increasing the liquid flow rate the boundary layer thickness of the liquid phase in the sell side decreases, which leads to increase in the mass transfer diffusivity and decrease in the liquid phase resistance. It is also clearly observed that due to high porosity of the membrane fabricated with approach 1 (shown in Table 3), by increasing the liquid velocity from 0.01 (m/s) to 0.03 (m/s), the CO2 flux of this membrane is higher than the other one. However, at a velocity of more than 0.03 (m/s), the membrane fabricated with approach 2 shows higher CO2 flux. The reason for this can be attributed to the wetting phenomena of the membranes at
calculated and the obtained results were shown in Table 4. A high tensile modulus suggests a high rigidity of the membrane [21]. In this case, the membrane fabricated with approach 1 shows better tensile modulus compared to that fabricated with approach 2. This is because that the presence of NPs in the polymer matrix has a greater effect on the mechanical properties of the membranes than their presence on the membrane surface. It can also be seen that the elongation at break point decreased in composite membranes compared with the neat one. This is probably due to increasing the membrane rigidity in the presence of inorganic particles that caused in reduction of matrix deformation. The same result was reported by Ahsani et al. [19] for flat sheet PP membrane in the presence of SiO2 particles. 3.5. CO2 absorption in membrane contactor The effect of liquid velocity on the CO2 absorption performances of the neat and composite membranes was investigated in the membrane contactor system. In this case, a gas mixture containing CO2 (20%) and 6
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Fig. 6. Stress and elongation at the break point of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2. Table 4 Tensile modulus of the neat and composite PP hollow fiber membranes. Fig. 4. TEM images of synthesized NPs.
the higher liquid flow rate which is more intense for the membranes fabricated with approach 1 compared to the another one [42,43].
Membrane
Tensile modulus (MPa)
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 2
420.43 582.71 431.12
3.6. Membrane mass transfer resistance The Wilson plot was used to identify the membrane mass transfer resistance. Fig. 9 shows the Wilson plot for the neat and composite membranes. The power number of the liquid velocity was used to get the best straight line. Similar to the results obtained by Korminouri et al. [44], by increasing the absorbent velocity, the overall mass transfer resistance considerably decreased, confirming that the liquid phase resistance is prevailing in physical absorption, particularly at the lower liquid flow rates. The obtained Wilson equations and membrane resistances are shown in Table 5. The intercept of the Wilson plots at the y-axis gives the membrane’s mass transfer resistance. At the liquid velocity of 0.02 (m/s), the membrane mass transfer resistance contributed 24.58% of the total resistance for the composite membrane fabricated with approach 1, while its contribution decreased to 18.79% for the composite membrane fabricated with approach 2. On the other hand, the membrane with high hydrophobicity resulted in less mass transfer resistance contribution. Fig. 7. Effect of liquid velocity on the CO2 flux using MEA (30 wt%) as the liquid absorbent (gas flow rate: 50 ml/min; CO2 volume fraction in feed gas: 20 vol%).
3.7. Long term CO2 absorption and chemical stability of the membranes with MEA solution Chemical stability of the membrane material has a considerable effect on its long-term durability and stable performance. It is wellknown that any reaction between the solvent and membrane material
could possibly change the membrane matrix and surface morphology [7]. The impact of the operation time on the CO2 absorption
Fig. 5. Water contact angles and three dimensional (3D) AFM images of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2. 7
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Fig. 10. Long term CO2 flux of the PP hollow fiber membrane samples using MEA (30 wt%) as the liquid absorbent (feed gas: pure CO2; gas flow rate: 50 ml/ min).
Fig. 8. Effect of liquid velocity on the CO2 removal efficiency using MEA (30 wt %) as the liquid absorbent (gas flow rate: 50 ml/min; CO2 volume fraction in feed gas: 20 vol%).
the alkaline solution. However, CH3-g-silica NPs remained on the surface of composite membrane fabricated with approaches 2 during the immersion time and causes the surface of this membrane remains unchanged. In order to further investigate the morphology change of the membranes in the presence of chemical absorbent, contact angle and breakthrough pressure of the neat and composite membranes at different immersion times were investigated. The obtained results, shown in Figs. 12 and 13, reveal that an increase in the immersion time causes to decrease in the contact angle and consequently reduce the breakthrough pressure of the neat membrane. However, the composite membranes exhibited greater resistance against absorbent penetration and membrane wettability at a given operation condition during 30 days immersion time. It should be mentioned that for hydrophobic membranes, liquid absorbent cannot easily pass through the membrane pores. Therefore, higher pressure is required to push absorbent to enter the pore. In the other words, increase in breakthrough pressure of a membrane confirms the increase in the membrane hydrophobicity and enhanced resistance against absorbent penetration. Moreover, investigation the contact angle and breakthrough pressure of the composite membranes during immersion time reveals that the modification of membrane with coating method resulted in a membrane with more stable property during long-term process compared to another modification method.
Fig. 9. Wilson plot of the PP membrane samples (pure CO2–water system, gas flow rate: 50 ml/min). Table 5 Wilson equation and membrane resistance analysis at the liquid flow velocity of 0.02 m/s. Membrane
Equation
(Rm/Rt*)%
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 1
y = 3304.9x + 57648 y = 2461x + 28829
30.78% 24.58%
y = 1841.8x + 14693
18.79%
4. Conclusions In the present work, the hydrophobic property of the polypropylene hollow fiber membrane was improved by incorporation of methyl grafted silica NPs with two treatment approaches including blending (approach 1) as well as coating (approach 2). The obtained results from several structural and performance analyses confirmed the successful fabrication of the composite hollow fiber membranes with low wettability during CO2 absorption process. Comparison between two treatment approaches revealed that the coating method resulted in a membrane with more stable non-wetting property during long-term absorption process. Generally, as a comprehensive result it can be noted that the coating of nanoparticles on the membrane surface is more effective method than the blending method in order to modification of the membranes in terms of longterm chemical stability, hydrophobicity and CO2 flux to be used in a membrane contactor system.
Rm: Membrane resistance, Rt : Total resistance.
performances of the neat and composite membranes were investigated by running the absorption process with pure CO2 as the feed gas (described in Section 2.3.8) for 30 days. The obtained results are reported in Fig. 10. For the neat membranes, the CO2 flux was very slightly changed within the first 2 days, and then decreased, continuously. It is due to the penetration of absorbent liquid into the membrane pores and accordingly, caused the membrane contactor to operate under partial wetting mode. This trend can be attributed to the irreversible changes on the morphology of the membrane surface because of swelling phenomenon during a long-term membrane exposure to the MEA aqueous solution. This result was confirmed by the FESEM analysis which was showed in Fig. 11. It can be seen that the surface morphology of the neat membrane changed considerably during 90 days direct exposure to 8
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Fig. 11. FESEM images of the PP membrane samples immersing in MEA (30 wt%) solution for 90 days; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2.
[4] [5] [6]
[7] [8]
[9]
[10]
Fig. 12. MEA (30 wt%) solution contact angles of the neat and composite PP membrane samples at different immersion times in MEA (30 wt%) solution.
[11]
[12]
[13] [14]
[15]
[16]
[17]
[18]
[19]
Fig. 13. Breakthrough pressure of the neat and composite PP membrane samples at different immersion times in MEA (30 wt%) solution.
[20]
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10
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Fabrication of hydrophobic PP/CH3SiO2 composite hollow fiber membrane for membrane contactor application Parya Amirabedia,b, Ali Akbaria,b, Reza Yegania,b, a b
T
⁎
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite polypropylene membrane Membrane contactor PP-CH3SiO2 CO2 absorption
Polymeric membrane wetting by liquid absorbent is one of the major challenges of the gas- liquid membrane contactors. In order to overcome this issue, composite polypropylene (PP) hollow fiber membranes were prepared by incorporation of methyl grafted silica nanoparticles (CH3-g-silica NPs) via two treatment approaches including blending the NPs with dope solution before membrane fabricating (approach 1), and also coating the NPs on the surface of the prepared membrane (approach 2). The fabricated membranes were characterized by using ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure, wetting resistance and gas permeation test. The obtained results from ATR-FTIR spectra confirmed the successful formation of the PP-CH3SiO2 composite hollow fiber membranes. It was seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. The fabricated membranes were also investigated for CO2 absorption process in gas- liquid membrane contactors. As a result, the gentle decrease in flux of composite membranes compare to the sharp flux drop of neat one confirmed the effective improvement in the non-wetting property of the membranes in the presence of inorganic particles.
1. Introduction It is vital to introduce devise techniques which would reduce acid gases like CO2 arising from the combustion of fossil fuels, present in the natural gas and industrial exhaust due to direct contribution in the greenhouse effects and climate change [1–4]. The most common process for removal of acid gases is an absorption into a solvent using conventional gas–liquid contactors such as packed towers, bubble columns, spray towers, etc. However, these conventional techniques are energy-consuming and not easy to operate because of some operating problems including flooding, foaming, entrainment and channeling [5,6]. A promising alternative technology which can overcome these disadvantages and possesses a high potential to replace conventional equipment is membrane contactor. This technology has numerous of benefits, such as flexibility of operation, high surface area per unit volume, being easy to scale up or down and predictable performance module [7–9]. Due to its excellent mass-transfer properties, membrane gas-absorption technology has been known as a technically viable option by the International Energy Agency's working group on CO2 removal [10]. However, the membrane contactor suffers from membrane wetting in a long-term operation, which will lead to a sharp increase in
⁎
membrane resistance against absorption process [2,11,12]. Mavroudi et al. investigated the wetting problem in membrane contactor systems and found that the membrane mass-transfer resistance accounted for 20–50% of the total mass-transfer resistance in the case of liquid-filled membrane pores [13]. It is well-known that the porous hollow fiber membrane is a core element in the membrane contactor device [14]. Accordingly, selecting an appropriate polymeric material is logically the first parameter to complete the requirements for membrane contactor processes. However, various experimental results revealed that polymeric membranes, utilized in the membrane contactors, are limited to short term and mild operating conditions, i.e. low alkalinity, acidity and low temperature applications. Accordingly, many studies have been carried out to improve the chemical, mechanical and thermal stabilities and non-wetting characteristics of conventional polymeric membranes to be used in membrane contactor applications [15]. According to the literature, addition of the inorganic fillers into polymeric membranes, known as “composite membranes”, seems to be a promising strategy to improve the membranes properties and consequently achieve a long-term stable operation [16–18]. Indeed, composite membranes combine the advantages of both polymeric and
Corresponding author. E-mail addresses: [email protected] (P. Amirabedi), [email protected] (A. Akbari), [email protected] (R. Yegani).
https://doi.org/10.1016/j.seppur.2019.115689 Received 12 November 2018; Received in revised form 3 May 2019; Accepted 10 June 2019 Available online 11 June 2019 1383-5866/ © 2019 Published by Elsevier B.V.
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methyltriethoxysilane (MTES ≥ 98%) and tetraethylorthosilicate (TEOS ≥ 98%), used for NPs synthesis, were purchased from Merck. Commercial grade of isotactic polypropylene (iPP, EPD60R, MFI = 0.35 g/10 min), used for membrane fabrication, provided from Arak Petrochemical Company of Iran. Mineral oil (MO) as a diluent, acetone as a extractor and Irganox 1010 as a heat stabilizer were purchased from Acros Organics, Merck and CibaCo., respectively. Sulfuric acid (98% H2SO4) and hydrogen peroxide (H2O2), used to prepare the Piranha solution, and Monoethanolamine (MEA) as a chemical absorbent were purchased from Merck.
inorganic membranes and exhibit interesting membrane properties such as considerable hydrophobicity, great chemical and thermal resistances and excellent adaptation to the severe operating conditions [16,19,20]. In general, the composite membranes can be produced by two methods, including blending the inorganic particles with dope solution before membrane fabricating, and also coating the inorganic particles on the surface of the prepared membrane [21]. In the case of composite membranes fabricated via first method, Rezaei et al. fabricated Polyvinylideneflouride (PVDF) hollow fiber mixed matrix membranes (MMMs) through wet phase inversion method using general MMT and Cloisite 15A as inorganic fillers to be used in a membrane contactor process [15]. They showed that the permeation flux and wetting resistance of the membranes in terms of contact angle and liquid entry pressure of water increased significantly by filler loading. Taghaddosi et al. prepared nanoclays embedded PP membranes by using thermally induced phase separation (TIPS) method to improve the anti-fouling properties of PP membrane [22]. The obtained results showed that incorporation of nanoclays into PP membranes could significantly mitigate fouling and improve the membranes performance. Among the inorganic fillers, silica NPs have received much attention due to their well-defined ordered structure, cost-effective production, high surface area and easy surface modification [23]. In the field of composite membranes prepared via coating the silica NPs on the membrane surface, Zhang et al. reported an appropriate method to form a highly hydrophobic Polyetherimide (PEI) membrane [21]. It was observed that incorporation of the fluorinated silica inorganic layer on the membrane surface offered high hydrophobicity and would protect the polymeric membrane from attacks of the chemical absorbents. Wang et al. fabricated PP composite membranes by loading SiO2 NPs and attaching 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) on the membrane surface to be used in vacuum membrane distillation (VMD) [24]. The resulted composite membranes showed good antiwetting ability and stable anti-fouling performance during the longterm operation. Xu et al. fabricated super-hydrophobic PP hollow fiber membrane by using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) for membrane distillation (MD) application [25]. There is a wealth of literature available on the utilization of PP composite membranes for wide application; however, there is a lack of literature reporting about application of PP composite membranes as a membrane contactor. PP has excellent characteristics such as high melting temperature, low production cost, excellent processability, widely adjustable mechanical properties, high resistance to water and chemical environments and so on [26]. Due to economic concerns, further expansion of the application area of PP composite membranes is highly desired, thus, in this work, two treatment approaches, described above, were employed fabricate PP composite membranes by incorporation of methyl grafted silica nanoparticles. In the present work, inorganic NPs were synthesized via sol-gel method. It should be noted that the influential parameters in fabrication of silica NPs via sol-gel method were optimized in our previous work using central composite design of the response surface method (RSM) [18], and the obtained results were used in this work. The fabricated membranes were characterized by various characterization analyses, including ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure and gas permeation test. Moreover, in order to investigate the wetting resistance of the neat and composite hollow fiber membranes, prepared membranes were tested in a gas-liquid hollow fiber membrane contactor for CO2 absorption.
2.2. Preparation of composite membrane In the present work, two treatment approaches including blending the inorganic particles with dope solution before membrane fabricating (approach 1) and also coating the inorganic particles on the surface of the prepared membrane (approach 2), were employed for fabrication of the PP-CH3SiO2 composite membranes. 2.2.1. Preparation of composite membrane with approach 1 In approach 1, at first, a sol solution was prepared while the molar ratios of TEOS:MeOH:H2O:NH4OH at 1:36:8:6.3 was kept constant, respectively, and vigorous stirring at room temperature according to the method explained in our previous work [18]. After 1 hr, a certain amount of MTES (molar ratio of MTES/TEOS = 3.774) was added into the reaction mixture. The mixture was then stirred vigorously for 12 hr. The obtained solution was centrifuged at 10,000 rpm for 10 min and then rinsed by DI water. Finally, the prepared particles were dried at 80 °C for 24 hr to obtain the methyl grafted silica NPs. After synthesis of the NPs, PP-CH3SiO2 composite membranes were fabricated by TIPS method, as described below. 0.5 g of modified silica NPs was dispersed into 75 g of MO using sonication by probe system (Sonopuls HD 3200, Bandelin) for 30 min. Then 24.5 g PP was added to the diluent-NPs suspension and melt-blended at 170 °C for 100 min in a sealed glass vessel. The homogenous dope solution was then degassed before spinning. In the spinning process, the bore fluids and dope solution were extruded through a spinneret and went through an air gap before immersing into a water bath. The nascent hollow fibers were collected at a certain take-up speed using a rotating drum and subsequently immersed in acetone for 24 hr to extract the diluent and achieve a desired porous membrane. The spinning conditions were summarized in Table 1. 2.2.2. Preparation of composite membrane via approach 2 In approach 2, at first, the neat PP hollow fiber membranes were fabricated by the spinning method described in 2.2.1. The casting solution composition; (PP/MO weight percent fraction), was kept constant at 25/75. Then, the fabricated membranes were treated by Piranha solution; (3:1 wt ratio of H2SO4:H2O2 solution), for 3 hr, in order to create a hydroxyl functional group, as a linkage agent between the synthesized inorganic particles and membrane surface. All samples Table 1 Spinning conditions for fabrication of PP hollow fiber membranes.
2. Material and methods 2.1. Materials Methanol (MeOH) (99%), ammonium hydroxide (NH4OH) (25%), 2
Spinning conditions
Remarks
Spinneret OD (mm) Spinneret ID (mm) Air gap (cm) Drum diameter (cm) Take up speed (m/s) Dope flow rate (ml/min) Bore fluid type Bore flow rate (ml/min) Bore fluid temperature (°C) Coagulant temperature (°C) Spinning temperature (°C)
1.1 0.65 1 14 0.147 4.0 Mineral Oil (MO) 2.0 170 25 170
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were washed with deionized water (DI) and then dried at room temperature. The detailed procedure of Piranha treatment method can be found in our previous published works [19,27]. After that, the certain amounts of TEOS, MeOH, NH4OH and H2O (similar to approach 1) were mixed for 1 hr to obtain silica colloid particles. Then the desired amount of MTES (molar ratio of MTES/ TEOS = 3.774) was added dropwise into the solution. The mixtures were continuously stirred for 12 hr at ambient temperature. In the following, Piranha-treated membranes were immersed in the mixture for 3 hr and then dried at 75 °C for 4 hr. Finally, all the prepared samples were washed with water under ultrasonication and dried prior to analysis.
rp,m =
2.3.6. Mechanical strength A tensile test machine (STM-5, SANTAM) was used to measure the mechanical properties of the neat and composite membranes. The initial length of each fiber was adjusted at 10 cm, and the stretch speed was kept constant at 10 mm/min. Three measurements were taken for each sample and the mean value was reported.
2.3.2. Membranes and NPs structure and morphology The morphology of the neat and composite hollow fiber membranes was investigated using field emission scanning electron microscopy (FESEM) device (LEO440-I). In order to investigate the cross section structure of the membranes, they were fractured in liquid nitrogen and coated with gold by sputtering before observation to make them conductive. The synthesized NPs were investigated by Philips BioTwin, the Netherlands transmission electron microscope (TEM).
2.3.7. Breakthrough pressure The wettability of a microporous membrane by a liquid absorbent can be evaluated by the breakthrough pressure value of the absorbent through the membrane pores. For small uniform pores with cylindrical geometry, the breakthrough pressure (ΔP) can be estimated using the Laplace–Young equation [30]:
Δp = −
2.3.3. Porosity Porosity of the obtained membranes was determined by impregnating the samples with isobutanol for 24 hr. Generally, the membrane porosity (ε) is defined as the pore volume divided by the total volume of the membrane using the following formula [28]:
(6)
2.3.8. Membrane contactor experiments The CO2 absorption experiment was carried out to evaluate the performance of the fabricated neat and composite hollow fiber membranes. Ten hollow fibers with an effective length of 18 cm were sealed in a cylindrical module. Table 2 gives the detailed information of the membrane contactor module. DI water and aqueous solution of MEA (30 wt%) were used as physical and chemical absorbents, respectively. For all experiments, the liquid absorbent flowed through the shell side and pure CO2 as well as mixed CO2 (CO2/CH4 20/80 vol%) gas flowed counter-currently through the lumen side of the hollow fiber membranes. The gas flow rates at the outlet and inlet of the contactor module were measured by soap film flow meters (HORIBA STEC- VP-3). A back-pressure regulator was used to adjust the gas-side pressure at 1 bar, where the liquid pressure was set at 0.5 bar higher than it to prevent the bubble formation in the liquid phase. Finally, for pure CO2, the absorbent flows concentrations of the inlet and outlet streams were measured by titration method as described in the literatures [32]. In order to analyze the mixed gas compositions, gas chromatography (GC) was employed. The CO2 removal efficiency and absorption
(1)
2.3.4. Effective surface porosity and mean pore size The mean pore size and effective surface porosity of the pure and composite membranes were obtained via gas permeation test. In this regard, three modules each containing one fiber with an effective length of 18 cm were prepared and the average value was recorded. Nitrogen gas was introduced to the shell side of the module and the permeation flux in lumen side was determined by using a soap bubble flow meter at various pressures. The gas permeation model was used to achieve the effective surface porosity and mean pore size as follow:
2 8RT 0.5 rp,m ξ 1 rp,m 2 ξ ⎛ ⎞ + Pm 3 ⎝ π M ⎠ RT lp 8μ RT lp
4σl cos θ d max
Here σ1is the liquid surface tension, θ is the contact angle between the liquid and the membrane surface, and dmax is the maximum pore diameter determined by using bubble point method [31].
where wib is the mass of the isobutanol -impregnated hollow fiber membrane (g), wd is the mass of the dry membrane (g), ρib is the isobutanol density and ρp is the density of the polymer (PP).
P = PP + PK =
(5)
2.3.5. Contact angle and AFM The contact angles of liquid absorbent (water and aqueous solution of MEA (30 wt%)) were measured by using contact angle goniometer (PGX, Thwing-Albert Instrument Co) to evaluate membrane hydrophobicity. All tests were conducted at least for 5 times on the different locations of each sample and the average value was reported. Also, the surface morphology of the pure and composite membranes was studied by using atomic force microscopy (AFM) (Nanosurf Mobile S. Samples).
2.3.1. ATR-FTIR Chemical structures of the samples before and after modification were analyzed by ATR-FTIR spectra using infrared spectroscopy device (BRUKER-TENSOR 27) in the range of 500–4000 cm−1.
(wib − wd)/ ρib × 100 (wib − wd)/ρib + (wd/ρp )
(4)
ξ 8μRTb = lp rp,m 2
2.3. Membrane characterizations
ε=
16 b 8RT 0.5 ⎛ ⎞ μ 3 a ⎝ πM ⎠
(2) (3)
P= B + S Pm
Table 2 Specification of the hollow fiber modules.
2
where P is the total gas permeance (mol/m Pas), PP and PK are the gas permeance by Poiseuille and Knudsen flow regimes respectively (mol/ m2Pas), R is the universal gas constant, T is the absolute temperature (K), M is the gas molecular weight (kg/mol), rp,m is the mean pore radius of the membrane (m), µ is the gas viscosity (Pa s), ξ is the porosity of the membrane surface, lp is the effective pore length (m) and Pm is the mean pressure (Pa). Finally the mean pore size and effective surface porosity can be obtained through Eqs. (4) and (5) by calculating the slope (S) and intercept (B) from linear plot between P and Pm [29]. 3
Parameter
Value
Fiber OD (mm) Fiber ID (mm) Effective fiber length (cm) Number of fibers Module ID (mm) Module length (cm)
0.7 0.5 18 10 10 24
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Fig. 1. Schematic figure of the experimental setup for CO2 absorption.
flux were calculated by Eqs. (7) and (8), respectively [9]:
ε=
J=
ΔCl,av =
(HCg,in − Cl,out )− (HCg,out − Cl,in)
Qg,in × Cg,in
(11)
g,out − Cl,in)
(7) where Cg, in and Cg, out are the inlet and outlet CO2 concentration in the gas-phase, respectively. In membrane contactor system, the Wilson plot is generally applied to determined mass transfer resistance [20,33]. In this case, the mass transfer resistance of the pure gas phase can be ignored while the mass transfer resistance of the liquid phase is proportional to the liquid velocity. The Wilson plot is drawn by 1 versusV−β , where V is the liquid
(Qg,in × Cg,in − Qg,out × Cg,out ) × 273.15 0.0224 × Tg × S
(HCg,in − Cl,out)
ln (HC
Qg,in × Cg,in − Qg,out × Cg,out
(8)
where ε is the CO2 removal efficiency (%); J is the flux of CO2 (mol/ m2s); Qg,in andQg,out are the gas flow rates at the inlet and the outlet, respectively (m3/s); Cg,in and Cg,out are the CO2 volumetric concentrations in the gas phase at the inlet and the outlet, respectively (%); Tg is the temperature of the feed gas (K) and S is the gas–liquid interfacial area (m2). Schematic figure of the setup is shown in Fig. 1. It should be mentioned that before taking the samples, all experiments were carried out for 30 min to achieve a steady state condition. The long-term absorption performances of the neat and composite PP hollow fiber membranes were examined by conducting the pure CO2 absorption experiments for 30 days. Furthermore, the chemical compatibility of the fabricated membranes and chemical absorbent was investigated by using FESEM images after immersing the membrane samples in the MEA solution for 90 days.
K ol
velocity and − β is the value which provides the best straight line. Accordingly, the mass transfer resistance of the membrane is given by the interception of the Wilson plot as follow:
1 Hd o = C1V−β + K ol kmdln
(12)
3. Results and discussion 3.1. ATR-FTIR
2.3.9. Wilson plot The mass transfer resistances of the gas and liquid phases inside a porous medium such as the membrane can be described by using resistance-in-series model which expressed by Eq. (9):
1 1 Hd o Hd o = + + K ol Kl Km dln K g di
ATR-FTIR spectra of the neat and composite membranes fabricated by both treatment approaches are shown in Fig. 2. For all samples, the absorption bands observed at 2700–2950 cm−1 were assigned to the CH stretching vibrations. Also, the bands observed at 1465 cm−1 and 1375 cm−1 were attributed to the bending vibrations of CeH bonds in CH2 and CH3 functional groups, respectively [34]. For composite membranes, the absorption peaks at 1100–1000 cm−1 were assigned to the SieOeSi asymmetric stretching vibrations, which confirm the formation of SiO2 network structure on the polymer membrane [35]. The broad peak observed at 3600–3100 cm−1 was assigned to the OeH functional group of the membranes surface [17]. The peaks appeared at 2973 and 1275 cm−1 are attributed to the absorption vibration of CeH bonds in methyl as well as SieCH3 groups, respectively [36]. Therefore, it can be concluded that the ATR-FTIR spectra confirmed the successfully formation of the PP-CH3SiO2 composite membranes. Moreover, it can be observed that the intensity of the SieCH3 bands in the composite membranes fabricated via approach 2 is higher than of the composite membranes fabricated via approach 1, which was related to the larger amounts of CH3-g-silica particles formed on the surface of the coated membranes compared to another composite membrane.
(9)
whereK ol is the overall liquid-phase mass transfer coefficient; kg, km and kl are the mass transfer coefficients of the gas phase, membrane and liquid phase, respectively; di, do and dln are the inner, outer and logarithmic mean diameters of the fibers, respectively and H represents Henry’s constant. Also, Kol can be obtained based on the experiment as follow:
K ol =
Q l (Cl,out − Cl,in) AT ΔCl,av
(10)
where Q l is the liquid flow rate, Cl,in and Cl,out are the inlet and outlet CO2 concentrations in the liquid-phase respectively, AT is the gas-liquid contact surface and ΔCl,av is the logarithmic mean concentration which expressed by Eq. (11): 4
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Fig. 2. ATR-FTIR spectra of the neat and composite PP hollow fiber membrane samples.
two treatment approaches. The results indicate that the composite membranes showed higher contact angle than the neat one. Actually, hydrophobic surface can be produced by either lowering the surface free energy or enhancing the surface roughness or a combination of both phenomena [21]. Therefore, increasing in contact angle is due to incorporation of methyl functional group with lower surface free energy (–Si–(CH3)3) [39] and also due to an increase in the surface roughness because of creating of the Si-O-Si groups on the membrane surface. Besides, it can be seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. By comparison three dimensional (3D) AFM images shown in Fig. 5, it can be observed that, the surface roughness of the sample fabricated by approach 2 was apparently higher than the other two samples. It should be noted that the obtained results by AFM analysis are in good agreement with the results of contact angle measurement. In fact, the large amounts of CH3-g-silica NPs on the surface of the composite membrane fabricated with approach 2 caused to a very rough surface which resulted in high contact angle for this membrane compared to other samples.
3.2. Membranes and NPs structure and morphology Fig. 3 shows the FESEM images captured from the cross-sections (1, 2) and surfaces (3) of the neat (a) and composite membranes fabricated with approaches 1 (b) and 2 (c), respectively. As shown in the figure, all membranes have cellular structures with spherical pores. From Fig. 3 (c2 and c3), it can also be observed that the inorganic NPs were formed on the surface as well as in the cross section of the hollow fiber membrane. By comparison of b2 and c2 samples, it can be seen that the number of pores as well as the mean pore size in sample (b2) is more than that of in sample (c2). The obtained results of mean pore size and porosity measurement shown in Table 3, confirmed our findings from the FESEM images. It can be observed that the mean pore size of the composite membranes fabricated with approach 1 is higher than that of fabricated with approach 2. Indeed, the heterogeneous nucleation effect of the inorganic NPs inside the dope solution caused to increase the membrane mean pore size and porosity. On the other hand, presence of methyl grafted silica NPs during phase separation resulted in greater nucleation rate and subsequently further pores [37]. Moreover, in order to obviously illustrate the structure of methyl grafted silica NPs, TEM image of synthesized NPs is shown in Fig. 4. It can be seen from this figure that the NPs are almost spherical with an average particle size of 20–40 nm. In addition, the core–shell structure for methyl grafted silica NPs is observed in Fig. 4, which silica is in the core and CH3 groups are in the shell.
3.4. Mechanical strength Good mechanical stability is one of the important factors to be considered in the membrane contactor application. The mechanical stability of the neat and composite membranes was examined in the term of tensile modulus as well as stress and elongation at the break point. Fig. 6 shows the stress and elongation properties of the fabricated membranes at the break point. It can be seen that incorporation of the inorganic NPs on the surface as well as in the cross section of the composite membranes improved stress in comparison with the neat one. This reveals that the interfacial bonding between inorganic particles and polymer chain is good [40]. Moreover, from stress-elongation curves, the tensile modulus was
3.3. Contact angle and AFM Surface hydrophobicity of the membrane can be examined by contact angle measurement which is related to the surface free energy and roughness [38]. Fig. 5 shows the contact angle measurements of the outer surfaces of the neat and composite membranes fabricated with 5
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Fig. 3. FESEM images of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2, (1) & (2) Cross section and (3) Outer surface. Table 3 Characteristics of the neat and composite PP hollow fiber membranes. Membrane
Porosity (%)
Average pore size (μm)
Effective surface porosity (m−1)
Maximum pore size (μm)
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 2
45 49 42
0.140 0.154 0.120
248.004 273.521 240.712
0.180 0.187 0.150
CH4 (80%) was selected as a feed gas. Absorbent velocity is maybe the most important operating parameter in the gas–liquid membrane contactors because generally it has a noticeable influence on the mass transfer rate of CO2 [9]. The results of CO2 flux obtained at different liquid velocities were presented in Fig. 7. It was found that the increase in liquid velocity causes to increase in CO2 flux for both neat and composite membranes. On the other hand, by increasing the liquid velocity, the removal efficiency increased at the same time as shown in Fig. 8. It is due to increasing the mass transfer coefficient with liquid velocity [41]. It means that, by increasing the liquid flow rate the boundary layer thickness of the liquid phase in the sell side decreases, which leads to increase in the mass transfer diffusivity and decrease in the liquid phase resistance. It is also clearly observed that due to high porosity of the membrane fabricated with approach 1 (shown in Table 3), by increasing the liquid velocity from 0.01 (m/s) to 0.03 (m/s), the CO2 flux of this membrane is higher than the other one. However, at a velocity of more than 0.03 (m/s), the membrane fabricated with approach 2 shows higher CO2 flux. The reason for this can be attributed to the wetting phenomena of the membranes at
calculated and the obtained results were shown in Table 4. A high tensile modulus suggests a high rigidity of the membrane [21]. In this case, the membrane fabricated with approach 1 shows better tensile modulus compared to that fabricated with approach 2. This is because that the presence of NPs in the polymer matrix has a greater effect on the mechanical properties of the membranes than their presence on the membrane surface. It can also be seen that the elongation at break point decreased in composite membranes compared with the neat one. This is probably due to increasing the membrane rigidity in the presence of inorganic particles that caused in reduction of matrix deformation. The same result was reported by Ahsani et al. [19] for flat sheet PP membrane in the presence of SiO2 particles. 3.5. CO2 absorption in membrane contactor The effect of liquid velocity on the CO2 absorption performances of the neat and composite membranes was investigated in the membrane contactor system. In this case, a gas mixture containing CO2 (20%) and 6
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Fig. 6. Stress and elongation at the break point of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2. Table 4 Tensile modulus of the neat and composite PP hollow fiber membranes. Fig. 4. TEM images of synthesized NPs.
the higher liquid flow rate which is more intense for the membranes fabricated with approach 1 compared to the another one [42,43].
Membrane
Tensile modulus (MPa)
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 2
420.43 582.71 431.12
3.6. Membrane mass transfer resistance The Wilson plot was used to identify the membrane mass transfer resistance. Fig. 9 shows the Wilson plot for the neat and composite membranes. The power number of the liquid velocity was used to get the best straight line. Similar to the results obtained by Korminouri et al. [44], by increasing the absorbent velocity, the overall mass transfer resistance considerably decreased, confirming that the liquid phase resistance is prevailing in physical absorption, particularly at the lower liquid flow rates. The obtained Wilson equations and membrane resistances are shown in Table 5. The intercept of the Wilson plots at the y-axis gives the membrane’s mass transfer resistance. At the liquid velocity of 0.02 (m/s), the membrane mass transfer resistance contributed 24.58% of the total resistance for the composite membrane fabricated with approach 1, while its contribution decreased to 18.79% for the composite membrane fabricated with approach 2. On the other hand, the membrane with high hydrophobicity resulted in less mass transfer resistance contribution. Fig. 7. Effect of liquid velocity on the CO2 flux using MEA (30 wt%) as the liquid absorbent (gas flow rate: 50 ml/min; CO2 volume fraction in feed gas: 20 vol%).
3.7. Long term CO2 absorption and chemical stability of the membranes with MEA solution Chemical stability of the membrane material has a considerable effect on its long-term durability and stable performance. It is wellknown that any reaction between the solvent and membrane material
could possibly change the membrane matrix and surface morphology [7]. The impact of the operation time on the CO2 absorption
Fig. 5. Water contact angles and three dimensional (3D) AFM images of the PP membrane samples; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2. 7
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Fig. 10. Long term CO2 flux of the PP hollow fiber membrane samples using MEA (30 wt%) as the liquid absorbent (feed gas: pure CO2; gas flow rate: 50 ml/ min).
Fig. 8. Effect of liquid velocity on the CO2 removal efficiency using MEA (30 wt %) as the liquid absorbent (gas flow rate: 50 ml/min; CO2 volume fraction in feed gas: 20 vol%).
the alkaline solution. However, CH3-g-silica NPs remained on the surface of composite membrane fabricated with approaches 2 during the immersion time and causes the surface of this membrane remains unchanged. In order to further investigate the morphology change of the membranes in the presence of chemical absorbent, contact angle and breakthrough pressure of the neat and composite membranes at different immersion times were investigated. The obtained results, shown in Figs. 12 and 13, reveal that an increase in the immersion time causes to decrease in the contact angle and consequently reduce the breakthrough pressure of the neat membrane. However, the composite membranes exhibited greater resistance against absorbent penetration and membrane wettability at a given operation condition during 30 days immersion time. It should be mentioned that for hydrophobic membranes, liquid absorbent cannot easily pass through the membrane pores. Therefore, higher pressure is required to push absorbent to enter the pore. In the other words, increase in breakthrough pressure of a membrane confirms the increase in the membrane hydrophobicity and enhanced resistance against absorbent penetration. Moreover, investigation the contact angle and breakthrough pressure of the composite membranes during immersion time reveals that the modification of membrane with coating method resulted in a membrane with more stable property during long-term process compared to another modification method.
Fig. 9. Wilson plot of the PP membrane samples (pure CO2–water system, gas flow rate: 50 ml/min). Table 5 Wilson equation and membrane resistance analysis at the liquid flow velocity of 0.02 m/s. Membrane
Equation
(Rm/Rt*)%
Neat membrane Composite membrane fabricated with approach 1 Composite membrane fabricated with approach 1
y = 3304.9x + 57648 y = 2461x + 28829
30.78% 24.58%
y = 1841.8x + 14693
18.79%
4. Conclusions In the present work, the hydrophobic property of the polypropylene hollow fiber membrane was improved by incorporation of methyl grafted silica NPs with two treatment approaches including blending (approach 1) as well as coating (approach 2). The obtained results from several structural and performance analyses confirmed the successful fabrication of the composite hollow fiber membranes with low wettability during CO2 absorption process. Comparison between two treatment approaches revealed that the coating method resulted in a membrane with more stable non-wetting property during long-term absorption process. Generally, as a comprehensive result it can be noted that the coating of nanoparticles on the membrane surface is more effective method than the blending method in order to modification of the membranes in terms of longterm chemical stability, hydrophobicity and CO2 flux to be used in a membrane contactor system.
Rm: Membrane resistance, Rt : Total resistance.
performances of the neat and composite membranes were investigated by running the absorption process with pure CO2 as the feed gas (described in Section 2.3.8) for 30 days. The obtained results are reported in Fig. 10. For the neat membranes, the CO2 flux was very slightly changed within the first 2 days, and then decreased, continuously. It is due to the penetration of absorbent liquid into the membrane pores and accordingly, caused the membrane contactor to operate under partial wetting mode. This trend can be attributed to the irreversible changes on the morphology of the membrane surface because of swelling phenomenon during a long-term membrane exposure to the MEA aqueous solution. This result was confirmed by the FESEM analysis which was showed in Fig. 11. It can be seen that the surface morphology of the neat membrane changed considerably during 90 days direct exposure to 8
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Fig. 11. FESEM images of the PP membrane samples immersing in MEA (30 wt%) solution for 90 days; (a) neat membrane, (b) composite membrane fabricated with approach 1, (c) composite membrane fabricated with approach 2.
[4] [5] [6]
[7] [8]
[9]
[10]
Fig. 12. MEA (30 wt%) solution contact angles of the neat and composite PP membrane samples at different immersion times in MEA (30 wt%) solution.
[11]
[12]
[13] [14]
[15]
[16]
[17]
[18]
[19]
Fig. 13. Breakthrough pressure of the neat and composite PP membrane samples at different immersion times in MEA (30 wt%) solution.
[20]
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