Zeolite ZSM5-Filled PVDF Hollow Fiber Mixed Matrix Membranes for

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Zeolite ZSM5-Filled PVDF Hollow Fiber Mixed Matrix Membranes for Efficient Carbon Dioxide Removal via Membrane Contactor Masood Rezaei Dasht Arzhandi, Ahmad Fauzi Ismail, Pei Sean Goh, Ihsan Wan Azelee, Mohammad Abbasgholipourghadim, Ghani Ur Rehman, and Takeshi Matsuura Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03117 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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Industrial & Engineering Chemistry Research

Zeolite ZSM5-Filled PVDF Hollow Fiber Mixed Matrix Membranes for Efficient Carbon Dioxide Removal via Membrane Contactor M. Rezaei-DashtArzhandi a,*, A. F. Ismail a,*, P. S. Abbasgholipourghadimb, Ghani Ur Rehmana, T. Matsuura a,c

Goha,

I.

Wan

Azeleea,

M.

a

Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia b Department of applied mechanics, Mechanical engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia c Industrial Membrane Research Institute, Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada

*Corresponding author email: [email protected], [email protected]

KEYWORDS: Carbon dioxide capture; Mixed matrix membrane; Polyvinylidene fluoride; ZSM5 zeolite; Membrane properties; Membrane contactor. ABSTRACT: ZSM5 zeolite-filled PVDF mixed matrix membranes were wet spun and used for CO2 absorption in contactor system. The properties of ZSM5 were analytically characterized. SEM images revealed the fully asymmetric structure of membranes, in which the creation of finger-like macrovoids was promoted with the increasing filler loading. A significant increase in gas permeance was observed, which was associated with the porosity increase of the membrane surface despite the decrease in surface pore size. The surface roughness, wettability resistance and mechanical stability of membranes were also considerably improved by filler loading. CO2 absorption test with water revealed higher CO2 flux of MMMs than that of the plain membrane. Peak absorption flux of 5.80 × 10-3 mol m-2 s-1 was achieved at liquid velocity of 1.2 m s-1 for 5 wt%ZSM5/PVDF membrane (MZ5), which was nearly 177% higher than neat PVDF and also surpassed that of several commercial and in-house made membranes. The mass transfer resistance of the MMMs was also considerably lower than that of PVDF.

1.0 Introduction Recently, porous hydrophobic membranes have attained great consideration for the capture of CO2, the primary greenhouse gas, via membrane contactor. The porous membrane offers a fixed and high gas-liquid contact area without dispersing one phase into another 1. The special feature of this membrane-based absorption technology is able to eliminate the shortcomings of conventional gas absorption devices. Although the membrane itself may impart additional resistance to the mass 2 transfer process , the overall mass transfer coefficient increases considerably unless the membrane becomes wetted. When the wetting occurs, the diffusion of gas takes place through stagnant liquid inside the wetted pores. As the diffusivity in the liquefied stage is 104 times less than

that of the gas phase, the mass transfer rate reduces considerably 3. Therefore, the membrane should have high hydrophobicity, wettability resistance, and excellent resistance towards any morphological changes upon exposure to chemicals and high temperature during the operation of contactor process to ensure the membrane pores are completely gas-filed 4. Furthermore, the membrane essentially requires being highly permeable to render high contact area among the gas and aqueous phases along with low mass transfer resistance. The wettability resistance of a porous membrane for a specific absorbent depends on the surface pore size, hydrophobicity, roughness and 1 chemical/thermal resistance . Therefore, as the first criterion of increasing wettability resistance of membranes used in gas-liquid contacting process is

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using hydrophobic fluoro-polymeric membrane materials (Polytetrafluoroethylene (PTFE), Polypropylene (PP), Polyethylene (PE) and Polyvinylidene fluoride (PVDF)). However, the current PP, PE, and PTFE membranes are usually fabricated via thermal or stretching methods providing a symmetrical structure with large pore 5 size and low porosity . In addition, PVDF, as the only polymer material that is soluble in common solvents, has difficulties of membrane fabrication. The PVDF has a small critical surface tension that restricts the easy exchange of the solvent and nonsolvent 6. Therefore, during the membrane fabrication via phase inversion method of this polymer, the slow solidification rate has unfavorably resulted in the formation of the membrane with relatively low surface porosity and mass transfer coefficient. Thus, modification of the hydrophobic PVDF membranes should be considered towards improving their morphology and properties in term of contact angle, wettability resistance and mass transfer rate. A number of methods were proposed to enhance the properties and wettability resistance of the membranes such as using salt additives or surface modifying macromolecules (SMM) into the spinning solutions. However, the requirements of the process could not be fully fulfilled. Salt additives are known to enhance the surface porosity and modify the morphology of the membrane but any improvement in the surface hydrophobicity can not be achieved 7. Rahbari-Sisakht, et al. 8 also used SMMs to increase the membrane hydrophobicity. As result, the surface hydrophobicity and absorption flux increased by blending SMM even though the membrane wettability did not change significantly due to the enlargement of the PVDF membrane pores. In addition, current polymeric membranes are practical only for mild operating conditions (e.g. low temperature, no chemical attack) and not able to maintain their absorption performance for a long time. Therefore, ceramic membranes which have high thermal and mechanical stability as well as very high porosity and permeability by controlling the sintering temperature are required under more demanding conditions. However, ceramic membranes are not free of drawbacks. They are expensive, fragile and their fabrication is difficult and time-consuming. Over the past few decades, polymer-inorganic composite membranes, more commonly known as mixed matrix membranes (MMMs), has provided solution to the aforementioned difficulties of polymeric and ceramic membranes have received much attention in various membrane applications.

In particular, fillers play an important role towards making the polymeric membranes more desirable while reducing the cost of ceramic membranes. It has been proven by many researchers that the MMMs have the potential of combining the benefits of polymeric and ceramic membranes such as high permeability, flexibility and mechanical/chemical 9 and thermal stability . Furthermore, the incorporated nanoparticles into the polymer matrix can function as membrane morphological modifiers besides modifying the membrane surface hydrophobicity/hydrophilicity. Generally, the selection of a proper inorganic material for the preparation of favorable organic/inorganic membrane for a specific application is mainly based on their particle size, the geometry, and orientation relative to diffusional direction, adhesion with the matrix and intrinsic hydrophobicity/hydrophilicity 10 . Most of the recent studies have focused on embedding isodimentional nanoparticles such as TiO2 11, SiO2 12, Al2O3 13 and clay type fillers 4, 9, 14, 15 towards the fabrication of membranes with favorable skin and support layers. Yang and Zhang 16 incorporated sphere shape TiO2 nanoparticles into polysulfone (PS) to fabricate a porous membrane for ultrafiltration application. The characterization results revealed that the membranes with the low loading of TiO2 had higher surface porosity, permeance, smaller mean pore size, and contact angle, which together resulted in higher process performance. Highly porous hydrophobic PVDF/Cloisite 15A hollow fiber membranes with high porosity were designed by Wang and Foo 17 to fulfill the requirements of membrane distillation. The fabricated membranes with nanoscale pore size rendered 100% salt rejection with no decay in the long-term filtration process. In our previous studies 6, 18, hydrophobic montmorillonite (MMT) nanoparticles were incorporated into PVDF polymer matrix to fabricate MMMs with improved surface and sublayer properties towards stabilizing high CO 2 absorption flux via a membrane contactor. The promising results indicated that hydrophobic inorganic particles can potentiality enhance the CO2 absorption performance of membranes. Further investigation on the effects of inorganic particles revealed that the particles with higher hydrophobicity and smaller particle size result in better membrane properties and performance 5. Porous zeolite is another attractive nanofiller option for non-porous polymeric membrane modification. Zeolites such as silicate-1 and NaX have been widely used in a variety of applications


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such as petrochemical cracking, water softening and purification and also in the fabrication of composite membranes to separate and remove gases or 19 solvents . Zeolites are commonly categorized into three groups regarding the Si/Al ratio in their framework, i.e. low silica (≈ 1), intermediate silica (between 1 and 10) and high silica (≥ 10) and the higher the silica in zeolite the higher surface 20 hydrophobicity . Considering the higher hydrophobicity and highly porous structure of high silica zeolite crystals versus lamellar structure of clay particles, high membrane gas permeance by incorporating zeolite nanoparticles into PVDF matrix in comparison with embedding clay type particles in our previous works can be expected. Consequently, a higher contactor process performance would be achieved.

least 18 h. The compositions of the spinning dopes are given in supporting information. The solutions were degassed and maintained for at least 2 h at ambient temperature. The viscosity of the provided spinning solutions was then evaluated by a digital viscometer (EW-98965-40, Cole-palmer, USA) before spinning. 2.2 Membrane fabrication Hollow fiber membranes were prepared by the phase inversion process induced by a nonsolvent. The description of the experimental setup and procedure are reported elsewhere 21. The spinning conditions are presented in supporting information. The as-spun fibers after collection were soaked in water for at least 3 days to remove the remaining solvent and LiCl. The hollow fiber membranes were then subjected to treatment by immersing them in methanol for 15 min methanol prior to membrane drying at ambient temperature. This minimizes the shrinkage and pore collapse of the membrane during the drying process.

Therefore, this work aims to develop the porous hydrophobic MMMs which embedded with high silica and defined pore structure zeolite nanoparticles in the PVDF matrix for membrane contactor application. The PVDF hollow fiber membranes were spun from spinning dopes containing different amounts of ZSM5 zeolite nanoparticles ranging from 0-5 wt%. The membranes were then characterized for their morphological structure, gas permeation, wettability resistance, and mechanical strength. Subsequently, the membrane performance was tested for CO2 absorption via a gas-liquid membrane contactor using water as absorbent. The CO2 mass transfer resistance was further split into several components to provide deeper insight into the effects of embedding zeolite particles on the membrane transport properties.

2.3 Scanning electron microscopy (SEM), Atomic force microscopy (AFM) and X-ray energydispersive spectrometry (EDX) examination SEM (TM 3000, Hitachi) was employed to characterize the morphology of the membranes. The hollow fibers were first dried then immersed in liquid nitrogen and fractured carefully. The SEM micrographs of the cross-section and external surface of the hollow fibers were examined at various magnifications. The membrane surfaces characteristics were further studied in terms of surface roughness by atomic force microscopy (AFM) (Model: Seiko SPA-300HV, Japan). The dispersion of the nanoparticles in the polymer matrix also was evaluated by EDX mapping analysis.

2.0 Experimental 2.1 Materials and spinning dope preparation ® PVDF (Kynar 740, Arkema Inc., PA, USA), 1Methyl-2-pyrrolidone (N-Methyl-2-pyrrolidone, NMP, 99.5%, Sigma–Aldrich®) and lithium chloride (LiCl, Sigma–Aldrich®) were used as the base polymer, solvent, and pore former, respectively. Zeolite ZSM5 with a high Si/Al ratio (>36), an estimated contact angle of 90° and particle dimensions ranging from 30 to more than 75 nm was purchased from ACS material, USA, and used as the inorganic filler. For coagulant purpose, tap water was used and distilled water was utilized as the absorbent. Methanol (Merck, GR grade, 99.9%) was also used for post-treating the fabricated membranes. The spinning dopes were prepared by stirring PVDF/LiCl/ZSM5 in NMP mixtures at 60 °C for at

2.4 Gas permeation test The nitrogen (N2) gas permeation test was conducted to examine gas permeability and measure the average pore size ( rP , m ) and surface 22

porosity of the membranes . The pore size and porosity are parameters that affect the mass transfer rate of porous membranes applied in contactor process. Permeation test modules were prepared by holding a sealed one end fiber and 10 cm length of with epoxy resin in a stainless steel tubing while the other end is kept open. N2 gas was forced to the lumen side from the open end side and the flow rate of gas exiting from the lumen side of the prepared module was determined by a soap bubble flow meter. A graphical representation of the gas permeation test is demonstrated in Figure 1. The



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transmembrane pressure difference was increased from 2 to 7 bars.

quantify the hydrophobicity/philicity of the membrane surface. The test was conducted at 10 numerous spots of the hollow fibers and then averaged. Liquid entry pressure of water (LEPw) was measured to evaluate the wettability resistance of the membranes. The LEPw is governed by the pore 27 size and hydrophobicity of the membrane . The distilled water was sent into the lumen side of the prepared modules (same as the module used for the gas permeation test) and the pressure was increased at 0.5 bar interval. By each pressure change, the experiment maintained at least for 10 min to achieve stability. The outer surface of the membranes was then carefully monitored to detect the first water droplet and report the pressure as LEPw where the first water droplet was observed. The mechanical stability of the prepared hollow fibers was obtained during gas permeation test. The upstream pressure was increased gradually at 0.5 bar interval until the membrane is collapsed and in turn, a sudden increase or decrease in the permeation rate is observed.

Figure 1. Schematic diagram of nitrogen gas permeation test. The gas that passes through the cylindrical and straight membrane pores is governed by Poiseuille and Knudsen diffusion processes, hence the gas 23 permeance can be obtained by Eqs. (1) and (2) . 2 2 8RT 0.5 rP, m  1 rP , m  P  PP  PK  ( )  p 3 M RT LP 8 RT LP

(1)

P  A  Bp

(2)

2.6 Evaluation of CO2 absorption flux and mass transfer resistance The CO2 absorption test was conducted to examine the performance of the prepared membranes. The hollow fibers with the effective length of 18 cm were placed in a stainless steel contactor module. Distilled water and pure CO2 flowed counter-currently on the lumen and shell side of the prepared modules, respectively. The liquid pressure was adjusted 1.5 bar, which is 0.5 bar upper than the gas side to avoid bubbling in the liquid side. The flow rate of water absorbent was slowly increased and the exit water containing absorbed CO2 was collected. Then, the concentration of CO2 was measured by chemical titration method using 0.02 M sodium hydroxide as a titrant and phenolphthalein as the indicator. The overall CO2 absorption flux (mol m -2 s-1) using the flow rate and the measured CO2 concentration can be determined as follow: Q (C out  C in ) KOL  L L Av L (5) ACL

where P is the total gas permeance, R is the universal gas constant, T is absolute temperature (K), M is molecular weight of gas, rP, m is the mean pore radius,  is the viscosity of the gas,



is

surface porosity, LP is effective pore length and p is mean pressure. Using the intercept (A) and the slope (B) of P versus p plot, the mean pore size, and the effective surface porosity can be calculated by Eqs. (3) and (4) 24-26. 0.5

16 B  8RT     3 A M   8 RTB  LP rP2, m

rP , m 

(3) (4)

where K OL is overall mass transfer coefficient, QL is the liquid flow rate, CL is CO2 concentration in liquid, A is the inner contact area (m 2). CLAv is the logarithmic mean of transmembrane concentration difference of solute gas, which shows the driving force of the gas transport in contactor process, can be determined by Eq. (6).

2.5 Evaluation of contact angle, liquid entry pressure of water (LEPw) and mechanical stability The measurement of contact angle was done by the sessile drop technique using a goniometer (model G1, Krüss GmbH, Hamburg, Germany) to


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CLAv 

( HCgin  CLin )  ( HCgout  CLout ) Ln(( HC  C ) / ( HC in g

in L

out g

K L di 3  3.673  1.623 Gz (8) D which is approximated by Gz  10  ShL  3.67 and ShL 

(6)

 C )) out L

Gz  20  ShL  1.62(Gz )1/3 , where D is the diffusivity of a solute gas in the liquid phase and Gz is Graetz number (VL di2 / DL) .

H indicates Henry’s constant in Eq. 6 and is equal to 0.831 for water at the temperature of 25°C 28. Typically, a feed gas in membrane contactor encounters three resistances (see Figure 2): diffusion to the membrane wall (gas phase boundary layer), diffusion through the membrane pores to the membrane-liquid interface (membrane resistance) and dissolution of the gas into the liquid (liquid boundary layer). Therefore, the gas and liquid resistance depend on the flow regimes in each respective phase, and the membrane resistance is influenced by the structure and wettability resistance of the membrane 29.

3.0 Results and discussion 3.1 Morphology study The viscous and thermodynamically stable dope was spun into hollow fibers through phase inversion process. It is then followed by immersion into a coagulation bath which has a high affinity with the solvent and no affinity with the polymer. Afterward, a mass transfer process during immersion was initiated and a continuous exchange of solvent and coagulant resulted in constituting the solid membrane. It is known that a variety of top layer structures, differing in pore size and porosity as well as sub-layer morphologies, for instance, finger-like or sponge-like can be formed by controlling the phase inversion process, which is strongly influenced by the thermodynamic stability of the spinning dope and the kinetics of the solvent/nonsolvent exchange 32. Therefore, the thermodynamic and diffusional (kinetic) parameters that govern the wet phase inversion process were taken into consideration to provide insights in the evolution of the system and the resultant membrane morphology with the presence of zeolite particles in the system. The polymer solution with low thermodynamic stability, which renders instantaneous phase inversion rate, tends to form a finger-like macrovoid structure, while the sponge-like structure in the membrane sublayer is created when the rate the phase inversion is slow 33. PVDF hollow fiber membranes were wet spun in which different contents of high silica zeolite ZSM5 were added to the solutions. To investigate the effect of embedding zeolite particles on the kinetic stability of the phase inversion process, the viscosity of the solutions containing different loadings of ZSM5 zeolite particles was measured. Table 1 reveals that the viscosity of spinning solutions increased with the addition of zeolite particles where the viscosity increased remarkably from MZ0 to MZ1 followed by considerably slow increase from MZ1 to MZ3 and MZ5. This trend can be ascribed to the viscosity enhancement by the PVDF/ZSM5 interaction, which was counterbalanced by the possible nanoparticle aggregation at high loading. As for the thermodynamic stability of the solutions, the incorporation of highly hydrophobic ZSM5 nanoparticles has lowered the

Figure 2 . Mass transfer resistance-in-series model for CO2 absorption via a contactor system. The overall resistance can be separated into the individual resistances using the resistance-in-series equation as follow3, 30: 1 1 Hdi Hdi    (7) KOL K L K m dlm K g do where

KL ,

Km ,

Kg

are the mass transfer

coefficients of liquid, membrane, and gas in the case of gas-filled pores, respectively. di, do, dlm are the inner, outer and logarithmic mean diameters of the hollow fibers, respectively. Various relations are applied to determine the individual liquid mass 31 transfer coefficient. Versteeg and Van Swaalj obtained K L from the Sherwood number (ShL) by Eq. (8) and assuming the liquid in a laminar flow regime:



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thermodynamic stability of the dopes and consequently promoted the phase inversion process and also the liquid-liquid phase separation. Therefore, the morphology and properties of the

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membranes were rendered by the combined effects of the embedded ZSM5 on the phase inversion and solution characteristics.

1 Table 1. Properties of the resultant PVDF membranes incorporated with ZSM5.

Solution viscosity (centipoise)

a

MZ0

MZ1

MZ3

MZ5

1250

3552

3911

4040

Permeance of N2 gas at 7 barg (10 mol/m s Pa)

0.67

1.08

1.72

3.34

Effective surface porosity (  / L P ) (m )

19

93

203

293

Mean pore size, rp,m (nm)

126

69

48

59

84

91

97

104

7.5

10

12

11

Collapsing pressure (10 Pa)

6.5

8.5

9.5

10

Mean roughness parameter (Ra) (µm)

19

38

44

48

-6

2

−1

Contact angle (°) 5

Liquid water entry pressure of water (10 Pa) 5

a

Measured in this work

The morphology of the membranes, as well as the adhesion between PVDF and ZSM5 nanoparticles, were investigated by the SEM images. Then the SEM images of MZ1 and MZ3 resembled that of MZ5, only the morphological images of MZ5 were used to denote all the MMMs as shown in Figure 3. As presented in Figure 3, the membrane cross-sections reveal a fully asymmetric structure of the membranes consisting of two distinguishable layers. The outer surface is a thin finger-like layer whereas the underneath substrate is a thick sponge-like layer. The formation of the asymmetric structure is typical for the membranes fabricated through phase 4, 34 inversion process . However, it was observed that the finger-like macrovoids extended into the sponge-like layer with the presence of zeolite nanoparticles. Considering the opposing effects of the increase in thermodynamic instability (thermodynamic effect) and the increase in viscosity (kinetic effect), the enhancement of finger-like microvoid formation by the incorporation of ZSM5 nanoparticles indicates that the thermodynamic effect dominates the formation of the cross-sectional structure. Despite the domination of the thermodynamic effect, the viscosity increased with the incorporation of nanoparticles also exhibited the kinetic effect on the membrane morphology. Due to the slow intrusion of nonsolvent water into the spinning dope when the viscosity increased, a sufficient amount of time was provided for the macrovoid growth, allowing the formation of large finger-like voids within the thick sponge-like layer as shown in

Figure 3 B1. The formed finger-like macrovoids are expected to enhance gas permeation rate of the MMMs. Similar results have been reported by Bakeri, et al. 35 for porous PEI membranes with various types of additives added to the polymer solutions. They found that the solution, containing water as nonsolvent additive, has the highest viscosity while its thermodynamic stability was the lowest in comparison with the other additives, resulted in the formation of the thick sponge-like layer and having the highest gas permeance. The top surface images are also shown in Figure 3. The surface seems rougher and more wrinkly upon the incorporation of ZSM5. Since the surface roughness change by the addition of filler was difficult to identify, AFM test was conducted and the results were demonstrated in Table 1. The mean surface roughness increased progressively by ZSM5 loading; more apparent at low loading. It has been reported by many researchers that embedding a small amount of nanoparticles into the soft polymer matrix increases the surface roughness of the membrane and in turn desirably enhances the hydrophobicity 18, 36. The effect of embedding ZSM nanoparticles of the membrane surface pore size was difficult to be judged by SEM images, hence the surface pore size and porosity evaluation was quantitatively made through the gas permeation test. Since the distribution of the nanoparticles in the polymer during the fabrication process is a crucial factor that affects the resultant mixed matrix membrane properties and performance, it was


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qualitatively investigated by EDX-SEM. The distribution of silica through the cross section and outer surface of the hollow fiber MMM is demonstrated in Figure 3C1 and Figure 3C2, respectively. The clearly observed high intensity of silica throughout the sponge-like and the formed

finger-like layers confirms the existence, well dispersion and no agglomeration of the zeolite particles. This has resulted in higher hydrophobicity and smaller pore size in the MMMs compared to the plain membrane.

1 (1)

(2)

(A)

(B)

(C)

Figure 3. SEM images: (A) neat PVDF (MZ0); (B) 5%ZSM5-filled PVDF (MZ5); (C) EDX of MZ5, (1) crosssection; (2) outer surface.

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3.2 Gas permeation test The membrane surface structural parameters, mean pore size and effective surface porosity, were calculated from the gas permeation test. The total gas permeance ( P ) versus mean pressure ( p ) was

with increasing the mean pressure, implying both Poiseuille and Knudsen flow regimes were responsible to the gas permeation through the PVDF membranes. Figure 4 reveals that the gas permeance enhanced when filler loading increased from 0 to 5 wt% and the MZ5 exhibited the highest permeance among the fabricated membranes.

plotted and the results are demonstrated in Figure 4. From the figure, the gas permeance enhanced

4.0 MZ0 Gas permeance (10 -6 mol m-2 s-1 Pa-1)

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3.5

MZ1

3.0

MZ3

2.5

MZ5

2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

Mean pressure (10 5 Pa) Figure 4. Measured N2 permeance versus mean pressure for PVDF hollow fiber membranes. The intercept and slope of the solid line of permeance versus mean pressure plot were employed in Eqs. (3) and (4) and the mean pore size and the effective surface porosity were calculated. The results are tabulated in Table 1 and show that the mean pore size of the MMMs is significantly smaller than that of neat PVDF membrane. The pore size decreased from MZ0 to MZ3 and then slightly increased to MZ5. As stated earlier, the addition of zeolite particles has thermodynamically reduced the stability of the spinning dopes, which enhanced the instantaneous demixing and reduced the available time for the growth of the surface pore in size. As a result, smaller membrane surface pores were created. On the other hand, the increased viscosity has also prevented the solvent from diffusing out rapidly from the surface, which consequently reduced the resistance against polymer concentration on the membrane surface around the pores 37. Thus, the size of the membrane surface pores was reduced in comparison with the membrane without zeolite nanoparticles. A slight increase of the surface pore size from MZ3 to ZM5

might be due to the particle agglomeration at high zeolite loading. The formation of voids around the filler aggregated parts has also been reported in 38 previous work . It should be noted that the mean pore size of the asymmetric membranes achieved in the gas permeation test does not have any noticeable physical meaning and only as a good criterion for comparing the membranes fabricated via the phase inversion method with different solution compositions and spinning conditions can 39 be used . The effective surface porosity of the membranes also increased progressively with the filler loading where the MZ5 exhibited the highest among all the MMMs. For example, the effective surface porosity -6 2 of 3.34 × 10 mol/m s Pa for MZ5 at 7 barg was almost 4 times greater than that of the neat PVDF membrane. Interestingly, the higher surface porosity of membranes containing ZSM5 zeolite particles than the membrane with no particle was despite their higher solution viscosity which confirms the dominating effect of the thermodynamic stability decrease rather than the


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kinetic effect on membrane surface porosity by ZSM5 loading. Similarly, Mansourizadeh and Ismail 37 reported a significant increase in the surface porosity as well as the finger-like formation of PVDF hollow fiber membranes by the addition of LiCl despite the considerable increase in the solution viscosity. As known, the porosity of the membrane skin layer is affected by the demixing time of the 40 spinning dope during the phase inversion process . The demixing time reduced as the thermodynamic stability of the solution decreased by the addition of ZSM5 particles. Consequently, the membranes with higher surface porosity were formed. In other words, the lower thermodynamic stability of the spinning dopes provided wider susceptible region to water penetration. As results, large pore numbers were created and in turn membrane surface with higher porosity compared to the membranes with no filler content were formed. Therefore, despite the reduced pore size of MMMs in comparison with the plain membrane, the increasing rate of the gas permeance was found to be related to the increased surface porosity of the membranes. In fact, the negative effect of the reduced pore size of MMMs on their gas permeation rate could be overtaken by the achieved higher surface porosity. In addition, it could be related to the enhanced finger-like macrovoids as was observed in the SEM cross section micrographs. Interestingly, the trend of increasing the surface porosity of MMMs was accompanied by reduction in the mean pore size (see Table 1). The occurrence is in contrast to the common trend of polymeric membranes, where the pore size generally increases in parallel with the increase in surface porosity 41. The phenomenon can desirably stabilize the contactor performance during long-term operations. In our earlier works 6, 18, MMM consisted of hydrophobic montmorillonite (MMT) nanoparticles were used for the absorption of CO2 via membrane contactor. The N2 gas permeance of the fabricated MMT-filled PVDF MMMs was higher than the neat membrane, but it was considerably lower than the ZSM5-filled PVDF MMMs at the same filler content. The phenomenon was most likely ascribed to the larger mean pore size and surface porosity of the membranes containing zeolite particles in comparison with the MMT. In addition, it might be related to the porous structure of ZSM5 zeolite crystals versus lamellar structure of MMT, which would render higher gas diffusion rate during gas 42 permeation test . A comprehensive comparative study on the effect of zeolite and MMT

nanoparticles considering their different crystal structure on the phase inversion behavior and membrane properties is in order for our future communications. 3.3 Measurement of membrane surfaces hydrophobicity, wettability resistance, and mechanical stability The wettability resistance of the MMMs was investigated by conducting contact angle and LEPw tests. The average contact angle between water and the solid membrane surface was used to evaluate the surface hydrophobicity of the membranes and the results are reported in Table 1. The surfaces of the fabricated hollow fiber MMMs were more hydrophobic than the neat PVDF hollow fiber in which the contact angle increased from 84° for the neat PVDF membrane (MZ0) to 104° for the membrane containing 5 wt% ZSM5 (MZ5). It was most likely attributed to the highly hydrophobic nature of the ZSM5 particles as well as the increased surface roughness as was confirmed by the AFM 43 test. Liu, et al. prepared ZIF-71 filled-polyetherblock-amide (PEBA) membranes for pervaporation application and observed higher surface contact angle of MMMs compared to the neat membrane. Similarly, the increased hydrophobicity was due to the nature of ZIF-71 and the increased membrane surface roughness. Although the effect of the surface roughness on the membrane surface contact angle seems controversial but according to the Wenzel equation ( cos m  r cosY where  m is the apparent contact angle,  Y is the Young contact angle and r is the surface roughness), the membrane surfaces become more hydrophobic 44 when the surface roughness increases . In fact, greater surface roughness enhances the effective solid surface area and in turn, increase the interfacial energy between the solid membrane surface and the liquid absorbent, hence enhances the surface hydrophobicity 45. The hydrophobicity of ZSM5-filled PVDF MMMs can also be related to their reduced surface pore size according to the classical Young and Wenzel equations; decreasing free energy of solid surfaces by increasing the contact area between the gas and solid surfaces lowers the free energy of solid surfaces, resulted in increasing surface hydrophobicity. Liquid entry pressure of water (LEPw), which is defined as the lowest pressure needed for water droplets to penetrate through the biggest membrane pore, is a key parameter in gas-liquid membrane contactor. The contactor operating liquid pressure should not exceed this minimum pressure to avoid water from entering the



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membrane pores so that the pores can remain completely gas-filled. All the MMMs exhibited higher LEPw then the neat PVDF hollow fiber membrane, suggesting that MMM hollow fibers are more favorable for the long-term gas absorption via membrane contactor (see Table 1). The LEPw depends on the membrane surface contact angle and surface pore properties such as size, number, and distribution. This is in agreement with the 5, 35 Young-Laplace equation where the membrane surfaces having high surface contact angle and well distributed small size pores render high LEPw. However, a trade-off between contact angle and the mentioned pore characteristics seems necessary. Many previous studies have reported that the LEPw of the most hydrophobic PTFE polymeric material applied in membrane contactor is lower than that of 41, 46 less hydrophobic PVDF , which is most likely related to the smaller pore size of the asymmetric PVDF membranes compared to that of symmetric PTFE. The PTFE membranes are typically fabricated using thermal or stretching method due to their weak solubility in solvents at normal temperature. These methods normally provide a with large surface pore symmetric structure hence resulted in the lower LEPw 34, 47. However, small pore size together with a sufficient surface hydrophobicity also can not fully warranty a high LEPw. Bakeri, Matsuura and Ismail 35 reported lower LEPw of polyetherimide (PEI) membrane (NMP+acetic acid) with smaller pore size (122 nm) due to low pore size distribution than the PEI membrane (NMP+ethanol) having a pore size of 140 nm. The results of the LEPw test in our earlier study also revealed that there is a threshold for the surface porosity since the membranes with the smallest pore size possessed the lowest contact angles and LEPws due to their significantly high surface porosity up to a threshold value 5. Conclusively, a high LEPw for the membranes can not be achieved unless the adequate value for contact angle simultaneously with sufficiently well distributed small size surface pores and pore number are obtained. In other words, the LEPw of a membrane above a sufficient degree of the surface contact angle is not solely affected by the pore size, and the other pore properties such as pore size distribution and pore number also should be considered. As mentioned previously, the LEPw increased by embedding ZSM nanoparticles to the PVDF polymer solutions which was in agreement with the contact angle and gas permeation test results. The higher hydrophobicity that accompanied with pore size decrease could maximize the LEPw value, confirming the achievement of a good trade-off

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between the increased surface hydrophobicity and surface pore properties by introducing ZSM5 nanoparticles into PVDF solutions. The MZ5 MMM hollow fiber with the highest surface hydrophobicity, relatively small pore size, and adequate surface porosity exhibited the highest LEPw among all the fabricated membranes. Sufficiently high mechanical stability is required for the hollow fiber membranes to prevent membrane structural deformation during operation and ensure the membrane's operational viability. The collapsing pressure as a criterion of membrane mechanical stability was measured and the results are demonstrated in Table 1. The collapsing pressure increased by incorporating ZSM5 zeolite filler into the PVDF membrane. In fact, the zeolite particles could act as a crosslinking/bridging point in the MMMs and increased the rigidity of polymeric chains. Hence, the mechanical stability could be improved. Briefly, the MMMs embedded with ZSM5 particles possessed higher wettability resistance and were mechanically robust. These desired features are difficult to obtain from neat polymeric materials without expensive surface modifications. The higher mechanical stability of membranes may reduce the interaction between membrane and absorbent to some extent and results in less membrane structural changes during prolonged and harsh contactor operating conditions 4, 48. It was proved in our previous study where the PVDF membranes with embedded hydrophobic MMT were exposed to hot water in a closed contactor loop that the fabricated MMMs possess significantly higher properties stability in comparison with the plain PVDF membrane 48. 3.4 Evaluation of CO2 gas absorption flux and mass transfer resistance Short-term physical CO2 absorption experiment as a common test in the gas-liquid contacting process for performance examination of the fabricated membranes was performed 2. Distilled water was supplied as absorbent in the lumen side counter-current with pure CO2 as solute gas on the shell side since the high LEPw was obtained for the membranes. As known, the CO2 flux and mass transfer rate of a membrane in gas-liquid contacting processes are influenced by the liquid and gas side mass transfer resistances as well as the resistance of the membrane itself 49. Therefore, following the CO2 absorption test via contactor system, the mentioned mass transfer resistances were quantitatively determined to better understand the CO 2 flux evolution upon the addition of ZSM5 nanoparticles into the PVDF. Eq. (5) was used to obtain the


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case, since the liquid flowed in the lumen side of the hollow fibers, a low resistance owing to high liquid 41 turbulency was obtained . It should be mentioned that the liquid phase resistance often controls the mass transfer process in contactor system in the case of physical absorption, hence increasing liquid 51 velocity can favorably enhance the CO 2 flux . As for the effects of the embedded zeolite particles into the polymer solutions, the impregnated membranes with the ZSM5 exhibited an obviously higher CO 2 absorption flux than the neat PVDF membrane (see Figure 5). The flux progressively increased by ZSM5 loading and the MZ5 MMM experienced the highest -3 -1 -1 absorption flux of 5.80 × 10 mol m s at the liquid -1 velocity of 1.2 m s , which was almost 177% higher than that of the plain PVDF hollow fiber at the same liquid velocity. Referring to liquid mass transfer resistance of the membranes in Table 2, it can be detected that all the membranes have similar and constant resistance, hence implied that the variation of CO2 flux in Figure 5 is most likely ascribed to the difference in the mass transfer resistance which is dictated by the membrane properties 41.

overall mass transfer coefficient ( K OL ) and CO2 absorption flux. Afterward, the total mass transfer resistance ( 1 / K OL ), liquid side ( 1 / K L ) and mass transfer resistance of the membrane ( Hdi / K m dlm ) were determined by Eqs. (7) and (8). It should be noted that the mass transfer resistance of the gas on the shell side was neglected since the pure CO2 was applied as the solute gas. Absorption of pure CO2 in water in a continuous mode was performed and the effect of liquid velocity on the CO2 absorption flux of the membranes was investigated. The results are depicted in Figure 5. All the membranes show a similar trend, in which the increase in liquid velocity has resulted in the increase of CO2 absorption flux. This is due to reducing liquid boundary layer resistance to some extent, which consequently reduced the mass transfer resistance at the liquid side when the absorbent velocity was increased. In fact, increasing liquid velocity prevented CO2 saturation in the lumen side resulted 50 in CO2 flux enhancement . The liquid side mass transfer resistance by referring to Eq. 8 depends mostly on flow regime, liquid velocity as well as the length and diameter of the hollow fibers 3. In our 0.7

MZ0 CO2 absorption flux (10-2 mol m-2 s-1)

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0.6

MZ1

0.5

MZ3 MZ5

0.4 0.3 0.2 0.1 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Absorbent liquid velocity (m s-1) Figure 5. Effect of liquid velocity on the CO 2 absorption flux of the fabricated hollow fiber membranes (pure CO2 in shell side, distilled water in lumen side). The mass transfer resistance was calculated at -1 Vliquid = 0.5 m s and the results tabulated in Table 4 indicated that the membrane resistance was significantly decreased by increasing ZSM5 loading from MZ0 to MZ5. It was associated with the

improvement of the membrane properties by introducing zeolite nanoparticles into the spinning solutions in accordance to the SEM micrographs, gas permeation, and contact angle test results. The mentioned characterization results revealed the



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formation of larger finger-like pores, the decrease in pore size, the increase in surface porosity and the increase in surface hydrophobicity by the ZSM5 particle incorporation, respectively. In other words, an improvement in the finger-like pores formation and surface porosity enhanced the contact area between gas and liquid, while the smaller pore size and higher contact angle prevented easy penetration of the liquid into the membrane pores. Therefore, the mass transfer resistance of the membranes could be minimized and the MMMs could experience higher performance than the plain PVDF. Generally, the mass transfer resistance of a membrane is influenced by the pore size, porosity, tortuosity of the sublayer as well as the 52 configuration of the contactor system . High porosity, small pore size, low tortuosity by the formation of longer finger-like macrovoids and high wettability resistance as well as counter-current mode operation of the liquid and gas flow minimized the resistance of the membranes containing zeolite filler. It is noteworthy, however, that the reducing pore size of the membrane surface detrimentally reduces the effective contact area between the gas and liquid phases, and consequently decline the CO2 flux. On the other hand, it would positively increase the wettability resistance of the membranes and stabilize the high performance of membranes as discussed earlier. The negative effect resulted from the reduced pore size of the fabricated MMMs on the absorption flux could be counterbalanced by increasing the number of those small membrane surface pores. The statement could be proved where the MZ1 and MZ3 interestingly exhibited nearly the same CO2 flux and mass transfer resistance despite the fact that the surface porosity and contact angle of MZ3 were significantly higher than those of MZ1. In fact, the high surface porosity of MZ3 as the absorption flux enhancement factor was counterbalanced by its smaller pore size, hence the

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short-term contactor performance of MZ3 was not too dissimilar to MZ1, which possessed low porosity and large pore size. Although large pore size renders a high effective contact area between gas and liquid and increases the short-term performance, it increases the possibility of membrane pore wetting and consequently increases the mass transfer resistance. Therefore, the MZ3 may be a more suitable candidate for a long-term contactor operation compared to the MZ1. It should be mentioned that the mass transfer resistance increase by the membrane pore wetting is a longterm effect, which will be investigated thoroughly in our future study. In summary, the MZ5 showed the highest absorption flux and lowest mass transfer resistance among the membranes with/without ZSM5 filler owing to its superior surface porosity, permeance, surface contact angle and wettability resistance as well as its sufficiently small surface pore size. Table 2: Overall mass transfer resistance (1/KOL), liquid side mass transfer resistance (1/KL) and membrane mass transfer resistance (Rm) of PVDF membranes at Vliquid = 0.3 (m s−1). Rm ( Hdi /K mdlm ) 1/KOL 1/KL MZ0

35,484

12,332

23,152

MZ1

22,997

12,090

10,907

MZ3

21,529

12,254

9,275

MZ5

19,080

12,111

6,969

To evaluate the competitiveness of this MMM, the performance, mass transfer resistance and some properties of MZ5 were compared with several commercial and in-house made membranes reported in the open literature (see Table 3).

1 Table 3. Comparison of the flux and properties of MZ5 MMM with other in-house made and commercial membranes. -1 a Membrane Average pore size LEPw Absorption flux 1/Ko (s m ) Polymer Ref. −2 −1 (nm) (mol m s ) No. (bar) type #1

20

NA

25366

PVDF

41

-3

-4

20833

PP

53

-3

54869

PVDF

54

-4

54681

PTFE

52

-4

3440

PVDF

6

-3

19080

PVDF

This

9.4 × 10

#2

40

NA

1.0 × 10

#3

2.33

5.33

2.2 × 10

#4

20000

NA

4.3 × 10

M5

21

11

9.8 × 10

MZ5

59

11

3.4 × 10


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work a

Achieved at Vliquid = 0.5 m s NA, not available.

-1

the MZ5. As well, it can be the reason why the membrane #3 possessed remarkably higher mass transfer resistance against the absorption of CO2 than the MZ5 MMM since the membrane #3 is more susceptible to pore wetting and resistance increase. The higher mass transfer resistance of the membrane #3 also could be related to the very small pore size of this membrane, which restricts the easy diffusion of CO2 into the membrane pores. Khaisri, deMontigny, Tontiwachwuthikul and Jiraratananon 52 studied the membrane resistance and absorption performance of commercial PTFE, PP and PVDF membranes for CO2 absorption. Experimental results revealed higher CO2 flux of PTFE than the other membranes in both physical and chemical absorption. The reported physical CO 2 flux of PTFE was 4.3 × 10-4 mol m-2 s-1 at Vliquid = 0.01 m s-1 (obtained by interpolation), which is significantly lower than that of MZ5 at the same operating conditions. There were no reported data about the wettability resistance of the PTFE membrane, but a low LEPw and consequently a high membrane resistance with large pore size of this membrane (20000 nm as tabulated in Table 3) seemed inevitable. Expectedly, the MZ5 MMM fabricated in this work exhibited higher absorption flux than the PVDF membranes impregnated with MMT clay 6, 18 nanoparticles reported in our earlier works . The absorption flux of 5 wt% MMT-filled PVDF MMM (M5) was 9.8 × 10-4 mol m-2 s-1 at the water flow rate of 60 ml min-1, which is 246% lower than that of MZ5 at the same operating conditions. It was most likely related to the superior properties of MZ5 MMM such as higher surface porosity and permeance as well as larger pore size in comparison with the M5. The diffusion in the membrane pores governs the mass transfer mechanism and is known to be greater for the membranes with greater surface porosity and pore size. As a result, these desired properties have rendered greater interfacial area between gas and liquid, and led to higher CO2 6 absorption performance . The findings of this study showed that zeolite filled-PVDF MMMs with sufficiently small surface pore size, highly porous substrate, and surface hydrophobicity hold great potential in gas-liquid contacting process based on their excellent CO2 absorption performance.

Atchariyawut, Feng, Wang, Jiraratananon and 41 Liang studied the effect of various additives on the CO2 absorption flux of the PVDF hollow fiber membranes and compared the results with the PVDF membrane supplied from Tianjin Motian Membrane Eng. & Tech. Co. They found that the PVDF membrane containing 3 wt% phosphorous acid as additive (membrane #1 in Table 3) has the highest performance and the least resistance among the other membranes. The absorption flux of the -1 -4 membrane #1 at Vliquid = 0.3 m s was 9.4 × 10 mol -2 -1 m s , approximately 261% lower than the flux of MZ5 at the same operating conditions, which was 3.4 × 10-3 mol m-2 s-1. The higher absorption flux of the MZ5 compared to the membrane #1 was most likely related to the improved surface contact angle, wettability resistance as well as the porosity by the addition of zeolite nanoparticles into the PVDF dopes. The overall mass transfer resistance of MZ5 was also found to be 25% lower than that of membrane #1. Wang, Zhang, Feron and Liang 53 studied CO 2 absorption flux of a commercial membrane contactor, Celgard MiniModule® 0.75X5, containing PP hollow fiber membranes (membrane #2 in Table 3). Pure CO2 was supplied to the shell side while distilled water to the lumen side. The absorption flux of the commercial PP membrane contactor at the liquid velocity of 0.3 m s -1 was 1.0 × 10-3 mol m-2 s-1, which was considerably lower (240%) than the flux of MZ5 at the same liquid velocity. The mass transfer resistance of the membrane #2 was also slightly higher than the MZ5 fabricated in this work. Mansourizadeh, Ismail and 54 Matsuura provided a solution containing 17 wt% polymer, 5 wt% LiCl·H 2O and 78 wt% NMP to fabricate PVDF membrane by the wet spinning process for CO2 absorption (membrane #3). The addition of LiCl·H 2O improved the structure of the membrane and in turn increased the absorption flux. An absorption flux of 2.2 × 10 -3 mol m-2 s-1 obtained at the volumetric liquid flow rate flowed in the lumen side of 100 ml min-1 was almost 54% lower than the flux of MZ5 at the same liquid flow rate. It was most likely related to the lower LEPw of membrane #3 compared to MZ5 (see Table 3). In fact, the addition of LiCl·H 2O to the PVDF solutions did not impart any significant effect on the membrane surface contact angle and wettability resistance. This might be the reason for the poorer performance of membrane #3 in comparison with



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4.0 Conclusion Tailor-made asymmetric PVDF hollow fiber membranes embedded with highly hydrophobic ZSM5 zeolite particles were prepared by wet phase inversion method for CO2 absorption in a contactor system. The effects of loadings of zeolite were investigated by characterizing membranes in terms of morphology, pore size, porosity, permeance, wettability resistance, mechanical stability and mass transfer resistance prior to the physical absorption of CO2. The following conclusions were drawn from the experimental results. 1. The addition of small amounts of ZSM5 zeolite particles lowered the thermodynamic stability of the spinning solutions, resulted in the formation of longer finger-like macrovoids and increased surface porosity compared to the plain PVDF. Moreover, the thermodynamic destabilizing effect of the embedded zeolite particles induced the instantaneous demixing and reduced the time available for pore growth, resulting in the formation of smaller membrane surface pores. 2. The gas permeation rate of the fabricated MMMs was considerably higher than that of the neat membrane.

3. The contact angle and surface roughness consequently led wettability resistance of the membranes to increase progressively upon the incorporation of highly hydrophobic zeolite particles. In addition, the mechanical stability of the membrane was also improved. 4. The CO2 absorption flux of the membranes significantly increased by incorporating zeolite particles into PVDF matrix and MZ5 with superior surface porosity, permeance, surface contact angle and wettability resistance, as well as its sufficiently small surface pore size, exhibited the highest absorption flux and the lowest mass transfer resistance. 5. The absorption performance of MZ5 was superior over several commercial and in-house made membranes. Funding Sources The authors would like to gratefully thank for the research grant funded by Universiti Teknologi Malaysia (UTM) under Pos Doc RU Grant (Vot No. Q.J130000.21A2.03E01 and R.J090301.7809.4J195) and Flagship Grant (Vot No. Q.J130000.2446.03G43)

ABBREVIATIONS A CL

contact area (m2) solute gas concentration in liquid (mol m -3)

Cg

solute gas concentration in gas (mol m -3)

CLAv di do dlm Gz H K OL

logarithmic mean of the difference in the concentration of solute gas in liquid phase (mol m-3) inner diameter of hollow fiber (m) outer diameter of hollow fiber (m) log mean diameter (m) Graetz number, dimensionless Henry’s constant -1 overall mass transfer coefficient (m s ) -1

KL

liquid side mass transfer coefficient (m s )

Kg

gas side mass transfer coefficient (m s -1)

Km

P PP

membrane mass transfer coefficient (m s ) hollow fiber membrane length (m) effective pore length (m) molecular weight (g mol-1) pressure (pa) mean pressure (Pa) −2 −1 −1 total gas permeance (mol m Pa s ) -2 -1 -1 gas permeance by Poiseuille flow regime (mol m Pa s )

PK

gas permeance by Knudsen flow regime (mol m -2 Pa-1 s- 1)

L LP M p p

Page 14 of 17

-1


Page 15 of 17

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QL

liquid flow rate (m-1)

rP, m

mean pore radius (m) universal gas constant (8.314 J mol-1 K-1) Sherwood number, dimensionless Graetz number temperature (K) liquid velocity in lumen side (m s -1) effective surface porosity contact angle of liquid and surface gas viscosity (Pa s)

R ShL

Gz T Vliquid

 θ 

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1 GRAPHICAL ABSTRACT MZ5

2


Zeolite ZSM5-Filled PVDF Hollow Fiber Mixed Matrix Membranes for

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