Characterization of partial pore wetting in hollow fiber gas absorption membrane contactors: An EDX analysis approach

Accepted Manuscript Characterization of partial pore wetting in hollow fibergas absorption membrane contactors: An EDX analysisapproach S.A. Hashemifard, T. Matsuura, A.F. Ismail, M. Rezaei Dasht Arzhandi, D. Rana, G. Bakeri PII: DOI: Reference:

S1385-8947(15)01002-5 http://dx.doi.org/10.1016/j.cej.2015.07.036 CEJ 13934

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

21 October 2014 27 June 2015 11 July 2015

Please cite this article as: S.A. Hashemifard, T. Matsuura, A.F. Ismail, M. Rezaei Dasht Arzhandi, D. Rana, G. Bakeri, Characterization of partial pore wetting in hollow fibergas absorption membrane contactors: An EDX analysisapproach, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.07.036

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Characterization of partial pore wetting in hollow fibergas absorption membrane contactors: An EDX analysisapproach

S. A. Hashemifarda*, T. Matsuurab,c, A. F. Ismailb*, M. Rezaei Dasht Arzhandib, D. Ranac, G. Bakerid a

Chemical Engineering Department, Petroleum, Gas and Petrochemical Engineering Faculty, Persian Gulf University, Bushehr 75169-13817, Iran b Advanced Membrane Technology Research Centre (AMTEC), UniversitiTeknologi Malaysia, UTM Skudai, Johor DarulTa‘zim81310, Malaysia c Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada d Chemical Engineering Faculty, Babol Noshirvani University of Technology, Babol, P.O. Box 484, Iran

Abstract In this work a novel method to evaluate the partial pore wetting of gas absorption membranes is proposed. The method consists of two approaches, one by energy-dispersive X-ray spectrometry (EDX), and the other by calculating the mass transfer resistances from the gas and liquid phase in the pore, using the parameter obtained by He gas permeation experiments. For this purpose, hollow fibers were spun from polyetherimide (PEI) and polyethersulfone (PES). As well, the lumen of a PEI hollow fiber was coated withsilicone rubber. The hollow fibers were then characterized by He gas permeation experiments, critical entry pressure of water (CEPw), contact angle, scanning electron microscopy (SEM) and EDX. The hollow fibers were then subjected to CO2 gas absorption using aqueous NaCl solution as absorbent. After drying the hollow fiber was subjected to

the EDX analysis. The cross-sectional profile of Na and Cl content in the hollow fiber

revealed that the pores of the PES hollow fibers were partially wetted, particularly at the pore inlet. Calculation of mass transfer resistances, on the other hand, revealed that the resistance came from gas phase in most hollow fibers, except for the PES hollow fiber where the liquid phase contribution was significant. Silicon rubber coating of the PEI hollow fiber increased the surface hydrophobicity and reduced the pore wetting considerably. Thus, it was concluded that the proposed method can be used as a powerful tool to investigate the pore wetting for many membrane contactor applications.

Keywords: EDX, gas absorption, membrane contactor, partial pore wetting, wetting ratio.

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1. Introduction Much interest is shown nowadays to the membrane contactor due to its excellent gas separation performance and advantages, over the conventional scrubbing process in terms of capital investments, operational costs and energy saving [1]. Despite these advantages, the gas/liquid contactor has a serious problem of membrane wetting, i.e. the initial rapid mass transfer gradually decreases owing to the partial pore wetting.

It is also known that the impact of membrane pore wetting is more

pronounced for the membrane of higher porosity [2]. Thus, membrane wetting has become the main concern for a long-term operation of CO2 absorption and other industrial applications. Since the performance deterioration is believed to occur due to the slow gas diffusion in the liquid phase trapped in the pore [3], the penetration of liquid into the pore should be prevented [4.5]. Various physical and chemical techniques were applied for membrane surface modification to control membrane wetting, which include coating, grafting, plasma polymerization, and more recently incorporation of nanoparticles [6,7,8]. For example, a hydrophobic layer was coated on the surface of the hollow fiber membrane contactor. Despite a concern over the additional mass transfer resistance of the coated layer [9], Kreulenet al.[10] proved by that a thin silicone rubber layer coated on the inner surface of hollow fibers did not contribute any additional mass transfer resistance to the absorption of CO2 and N2O by aqueous NaOH solutionAs for the pore wetting, it was eliminated.completely. Different solvents were used to fabricate polymeric membranes and their effects on the key characterization parameters affecting the pore wetting such as the critical entry pressure of water, critical surface tension of the membrane and contact angle were studied by Dindore et al. [11].. To simulate CO2 absorption by water in microporous hollow fiber membrane contactors, Wang et al. developed a theoretical model for the non-wetted and wetted mode, respectively [12]. The performance deterioration that occurred in the wetted mode was due to the extra mass transfer resistance resulting from the liquid trapped in the pores. The aqueous solution of a new amino acid based salt was used for the absorbent by Yan et al.[13]. The new absorbent, aqueous potassium glycinate (PG), had not only desirable physical properties to prevent the wetting of commercial hollow fiber PP membranes but also suitable affinity towards CO2 in comparison to the conventional absorbents.Lu et al. [2] studied CO2 capturing by alkanolamine as an absorbent. Based on a series resistance model, in which the Laplace equation and the pore size distribution of membrane are combined, a mathematical model was developed. It was concluded that, under the same operating conditions, aqueous organic solutions wetted the hydrophobic membrane pores more easily than pure 2

water. It was also concluded that temperature had a substantial effect on the physical properties of both membranes and absorbents, e.g. viscosity, surface tension as well as contact angle. The capability of aqueous solutions of amines, either single or binary mixtures, for CO2 absorption was investigated by Rongwanget al.[14] under membrane wetting conditions. Sodium chloride as an inorganic salt and sodium glycinate (SG) as an organic salt were employed as additives. Employing mixed amines did not show any improvement in protecting the membrane against partial wetting. The addition of NaCl into a 0.25M monoethanoamine (MEA) solution reduced CO2 flux but partial wetting was suppressed. Faiza and Al-Marzouqi[15] developed a 2-dimentional model for CO2 separation from natural gas at high pressure. Although chemical absorbents such as aqueous MEA enhanced the CO2 absorption flux, the percentage of wetting increased, e.g. 1% wetting occurred when MEA solution was used. Nguyen et al. [16] reported on fabricating composite membranes as the contactor for CO2 separation via MEA. Poly(1-(trimethylsilyl)-1-propyne) (PTMSP) and Teflon AF2400 were selected as the dense top layer . They found that these two polymers withstood amine solutions for a prolonged contact time. Also, a mathematical model was developed for gas absorption membranes and validated with experimental results by Khaisriet al.[17]. PTFE hollow fiber membranes were employed in their experiments along with MEA solutions as the CO2 absorbent.They also investigated partial pore wetting to know its impact on the membrane mass transfer resistance. The maximum allowable wetting ratio was estimated to be 40%. Bougie and Iliuta[18] reported that absorption with a high surface tension solution is one of the key parameters to be considered to prevent the membrane from partial wetting. Recently, a comprehensive review on the wetting phenomenon was made by Mosadegh-Sedghiet al.[19]. They reported on the impact of membrane wetting on mass transfer coefficient and membrane contactor performance. They also discussed on the effect of membrane intrinsic properties such as hydrophobicity, pore size and porosity as well the effect of other parameters such as operational conditions, absorbent type and concentration on the wetting phenomenon. Various methods to prevent membrane wetting were shown in detail. The objective of this work is to present an experimental method to describe quantitatively the partial wetting phenomenon, since such a method has not been so far reported in the literature. To achieve this goal, an approach in which membrane contactor absorption experiments are combined with an EDX (energy-dispersive X-ray spectrometry) analysis is employed. To support the proposed technique, membranes with different degrees of hydrophobicity and different structural properties were fabricated and various characterizations were performed.

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2. Concept of the proposed approach for measuring partial wetting A vast number of researchers devoted their interests to the partial wetting of hollow fibers in membrane contactor processes [13-15,20].Fig. 1 demonstrates different gas/liquid contact modes and the concentration gradients of CO2 in the membrane structure for the gas absorption process, including: mode a) fully dry porous membrane contactor; mode b) partially wet porous membrane contactor; and mode c) fully wet porous membrane contactor. In all modes, the gas stream is assumed to be a pure gas. For example, CO2 in mode 'a' illustrates an ideal membrane which possesses fully solution-untouched pores. There is no barrier except the gas diffusion through the pores; hence, its absorption performance is the highest among all of the considered modes. Mode 'b' shows a membrane which is partially wetted. In such a mode, the absorbent solution can penetrate into the membrane pores to some extent, resulting in a substantial increase in mass transfer resistance against the CO2 pore diffusion due to slow diffusion of CO2 in the liquid phase. Mode ‘c’ displays a membrane which is fully wetted. This mode illustrates the highest possible membrane mass transfer resistance since the rate of diffusion of the absorbed molecules is slowed down from its full capacity. To track the extent of pore wetting experimentally, a novel approach is proposed. First, dilute NaCl solution is recirculated in the system for a predetermined period and then water is evaporated from the membrane. Thereafter, by applying EDX, one is able to know how far the pore wetting has advanced. Wetting normally starts from the skin of the lumen side and may progress across the membrane in a step wise manner. Therefore, it is expected to observe a step function for Na (or Cl) intensity versus the pore length, as shown in Fig. 2. The details of this technique will be discussed in the subsequent sections.

Fig. 1.a schematic of different gas/liquid contact modes and the concentration gradient of CO2 in the membranes structures, mode a) fully dry porous membrane contactor, mode b) partially wet porous membrane contactor, mode c) fully wet porous membrane contactor. (GB: gas bulk, GBL: gas boundary layer, LB: liquid bulk, and LBL: liquid boundary layer, and the colors represent the following media; yellow: gas stream, blue: liquid stream, and gray: porous membrane cross section)

Fig. 2.An ideal normal step function of probe element intensity versus wetting ratio

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Before proceeding to the experimental sections, a brief theoretical background is needed. According to the resistance-in-series model (Eq. (1)), the overall mass transfer resistance in a gas/liquid membrane contactor consists of three major resistances: liquid boundary layer, membrane, and gas boundary layer mass transfer resistance.

Hdi d 1 1 = + + i KOL k L k M dlm kG d o

(1)

whereKOL, kL, kM and kG are overall liquid, liquid boundary layer, membrane and gas boundary layer mass transfer coefficient (m/s), respectively, and di, do and dlm are hollow fiber inside, outside and log mean diameters (m), respectively. Therefore, the membrane mass transfer resistance can be written as:

RM ≡

Hdi d lm k M

(2)

Further, assuming the membrane pores are partially wetted and the mass transfer in the pore is a series combination of diffusion in the gas and liquid phase in the absence of convection [20], kM is given by: where DG and DL (m2/s) are the diffusion coefficients of gas and liquid respectively, τ is tortuosity factor, ε is the membrane surface porosity andlp is the pore length (m). Multiplying Eq. (3) by Hdi and dividing by dlm, yields:

RM = (1−η)RMD +ηRMW

(4)

Where η is the wetting ratio which is defined as the ratio of pore length wetted by liquid to the overall pore length and obviously 0<η<1 [21]. Here,

RMD , k MD , R MW

and

k MW

are gas filled (fully dry mode) and

liquid filled (fully wet mode) membrane mass transfer resistance and membrane mass transfer coefficient respectively and are written as follows:

RMD ≡

Hdi d lm k MD

(5) 5

RMW ≡

kMD ≡

kMW ≡

Hdi dlmkMW

(6)

(ε / l p ) DG

(7)

τ (ε / l p ) DL

(8)

τ

By rearranging and manipulating Eq. (4), the wetting ratio is obtainable by Eq. (9):

RM − RMD η= W RM − RMD

(9)

For more information on how to estimate the liquid and gas diffusion coefficients, the readers can refer to appendix A. It should be noted that Eqs. (1) and (2) are given for the case when the liquid flows in the lumen of the hollow fiber. When the liquid flows on the shell side, subscripts i and o should be exchanged. When the gas side is a single component, as in the present case, the resistance in the gas boundary layer can be ignored.

3. Experimental 3.1. Raw material The membranes were prepared from a commercial Ultem® 1000 polyetherimide (PEI) supplied by GE Plastic (USA) and polyethersulfone (PES) supplied by BASF Company (Germany). PEI and PES were chosen due to their different natural properties as probe polymers just to show the validation of the proposed technique, therefore PEI or PES were not proposed as promising polymers for fabricating membrane contactor. N-methyl-2-pyrrolidone (NMP) used as solvent was supplied by Merck. A rubbery silicone polymer (Sylgard 184) supplied by Dow Corning USA was used as the coating agent. PEI pellets were preconditioned in a vacuum oven at 80 °C for 72 h to remove trapped moisture. All materials were used as received. 6

3.2.Dope preparation To prepare the casting solution, the polymer was divided into three portions and they were added to the solvent consecutively with a time interval of 15 min. The polymer (12 or 15 wt.%) /solvent mixture was then stirred for 18 h at 60 °C to ensure complete dissolution of the polymer. The solution was further degassed via ultrasonication for 2 h and was left overnight before performing the spinning process.

3.3.Asymmetrichollow fiber membrane preparation Wet spinning technique, without any air gap, was employed for fabricating asymmetric PEI hollow fiber membrane for this study. The dope solution was loaded into a stainless steel reservoir and kept at room temperature during spinning. The spinneret used for spinning has a dimension of 1.25 mm for outer diameter and 0.55 mm for inner diameter. The details of the spinning conditions are summarized in Table 1. After completing the spinning, to ensure that all of the solvent in the membrane was removed, membranes were immersed in water for 3 days, followed by air-drying for 3 to 4 days at room temperature. The PES hollow fibers were spun under the same conditions from 15 wt.% PES solution. Table 1. Spinning condition for hollow fiber membrane fabrication.

3.4.Membrane coating Some of the PES hollow fibers were coated on the lumen side by using a rubbery silicone polymer (Dow Corning Sylgard 184) to make the internal surface of the hollow fibers more hydrophobic. In order to form the coating layer on the inner surface, the silicon rubber solution (0.2% w/w solution of silicone rubber in n-hexane) was fed into the lumen of the fibers. Then, the solution was discharged applying a nitrogen gas stream. The coated membranes were subsequently placed in an oven at ~60 °C for 24 h to ensure the curing of the coating layer before performing the gas permeation and gas absorption testing. The composition of the casting dope and the post treatment of the hollow fibers are summarized in Table 2.

Table 2.Code and specifications of the fabricated membranes

3.5.Gas permeation test

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Two to three hollow fibers were sealed with epoxy resin at one end and the other end was potted to a stainless steel fitting that was connected a stainless steel tube. The details of the gas permeation system are given elsewhere [6]. The feed gas (He 99.97 vol.%) was supplied to the lumen side of the hollow fibers and the pressure was controlled by a pressure regulator. The rate of gas permeation was measured by a soap bubble flow meter connected to a constant-pressure system. The permeation tests were carried out at 23 °C and the feed side pressure was varied from 25 kPa to 350 kPa gauge depending on the permeation rate of the membranes. Permeance was calculated by:

Ρ=

101325dV RTA∆p dt

(10)

where P is the gas permeance (mol m−2 Pa−1 s−1), R is the universal gas constant (8.314 J mol−1 K−1), T is the system absolute temperature (K), V is the volume of gas permeated through the membrane (m3, STP), A is the effective membrane area (m2), t is the permeation time (s) and ∆p is the transmembrane pressure drop. It is noted that, the constant 101325 is the absolute atmospheric pressure of the permeate side in Pa.

3.6. Measurement of Critical entry pressure of water and Contact angle The critical entry pressure of water (CEPw) is the minimum pressure required to let water penetrate through the membrane pores and it is also used as a measure of the resistance for membrane wetting.. Distilled water was pressurized into the lumen side of the dried hollow fiber membrane and the pressure was increased gradually at a step size of 50 kPa gauge. The pressure at which the first droplet of water appeared on the outer surface of the membrane was recorded as CEPw, which is the minimum pressure required to drive water through the largest membrane pores [2,6]. Contact angle measurement is a simple way to quantify the hydrophilicity/hydrophobicity of the membrane’s surface. The contact angle of the inner surface of the membranes was measured by a contact angle meter (model OCA20, DataPhysics, Germany) using distilled water as the probe liquid. At least 10 spots on the membrane surface were chosen for the contact angle measurement and the average values were reported.

3.7. Gas absorption test 8

To evaluate the performance of the hollow fiber membranes, the fibers were assembled in the membrane contactor module and CO2 absorption tests were conducted. The pure CO2 was running in the shell side and distilled water in the lumen side. Eight PEI hollow fiber membranes were assembled in a contactor module with an inner diameter of 1.57 cm. The effective length of the fibers in the module was 18 cm. The membrane contactor system is shown schematically in Fig. 3. A diaphragm pump was used to let distilled water flow at a constant pressure. The liquid pressure was adjusted so that it was 50kPa higher than the pressure of the CO2 gas to prevent undesired bubbling. The concentration of CO2 in water was measured by titration using a solution of 0.05 M NaOH, and Phenolphthalein was employed as the equivalent point indicator. In all the experiments, the gas flow rate was set constant at 1.0 dm3/min, and the liquid flow rate was changed in the range of 30-300 ml/min.

Fig. 3. Gas absorption (or NaCl aqueous solution circulation) rig; Orange lines: gas stream, blue lines: Liquid stream.

The flux was calculated by eq. (11) based on a simple mass balance of CO2 over the entire effective length of the membrane module [22]:

QL (CLout −CLin ) J= nπdL

(11)

Where J is the gas absorption flux (mol/m2s), QL is the liquid volumetric flow rate (m3/s), CLout and

CLin are the concentration of CO2 in the outlet and inlet solution (mol/m3) respectively, d is the inside fiber diameter (m), n is the number of hollow fibers and L is the length of the fibers (m). Also, the theoretical flux can be obtained by:

J = KOL(∆Clm)

(12)

whereKOL is the overall mass transfer coefficient (m/s) based on the liquid phase and (∆Clm) is a logmean concentration difference of CO2considering the change of CO2 concentration in water from the module inlet to the module outlet, which is given as[22]:

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( ∆C lm ) =

(C Lout ,i − C Lout ) − (C Lin ,i − C Lin )  C out ,i − C Lout   ln  L in ,i in   CL − CL 

(13)

Where CLout,i, CLin,iare the concentrations of CO2 in liquid which is in equilibrium with pure CO2 at the liquid/gas interface in the outlet, and inlet solution (mol/m3), respectively. Note that CLout,i and CLin,iare equal. The concentration of CO2 at the liquid/gas interface can be estimated by Henry's law given by Eq. (14):

CLi = HCGi

(14)

Where CiG is the concentration of CO2 at the liquid/gas interface for gas phase (mol/m3) and H is a dimensionless temperature-dependent Henry constant which is obtained by Eq. (15).

H =

0.461884 T h

(15)

And h is a Henry constant with dimension of pressure (MPa) for CO2 dissolved in water as well computed by [23]:

ln(h) = −6.8346+

1.2817×104 3.7668×106 2.9970×108 − + T T2 T3

(16)

The mass transfer coefficient of the liquid boundary layer for the lumen side is mainly determined by the system geometry and flow condition, for which many correlations are available to predict the mass transfer coefficient [9, 24]. An approximate solution for the differential equation driven from the continuity equation was proposed by Graetz for small values of Graetz number,

Gz(d2v/LDL), by which the average and the local Sherwood number, Sh (kL/dhDL), can be obtained as follows [25].

Sh=3.67, Gz<10

(17)

10

Where v is the liquid velocity in lumens (m/s), DL (m2/s) is the diffusion coefficient of CO2 in the liquid phase, kL is the liquid boundary layer mass transfer coefficient (m/s) and dh is the hydraulic diameter (m). Another solution was given by the Leveque equation [25]. The approximate solution proposed by Leveque to the system is based on the assumption that the concentration boundary layer is limited to a thin layer adjacent to the wall of the fiber. This assumption is valid for high mass velocities through relatively short fibers in laminar flow i.e. Gz exceeding 20 [26]. Many researchers have observed that for the flow of aqueous solutions at atmospheric pressures, a combination of Graetz and Leveque solution can be effective in predicting the lumen-side mass transfer coefficient. Kreulenet

al.[27] gave the generalized solution of the Graetz–Leveque equation by curve fitting of the models against experimental data as follows:

Sh = 3 3.673 + 1.623 Gz , 20 > Gz > 10

(18)

Equating Eq. (11) and (12), one can calculate the overall mass transfer coefficient [28]:

KOL =

QL (CLout − CLin ) nπdL(∆Clm )

(19)

3.8. NaCl-Circulation test

To investigate the extent of partial wetting in hollow fiber membrane contactors, the following experiments were carried out. An aqueous solution of 1.0 M NaCl was used as adsorbent in the absorption experiment. The NaCl solution was circulated in the system for a specific period of time, e.g. one hour, at a constant flow rate of 250 ml/min. In the meantime, carbon dioxide flowed on the shell side to maintain the pressure balance against the liquid in the lumen side. The gas flow rate was set constant at 1.0 dm3/min (see Fig. 3). After completing the circulation of the NaCl solution, nitrogen gas was blown through the lumen of the hollow fibers to drain the residual NaCl solution. The hollow fibers were taken out of the module carefully and dried in a vacuum oven at 60 °C for 24 h. To justify the proposed method, the level of pore wetting in the NaCl-circulation test should be the same as in the gas absorption test. It is already known that as the temperature increases the CO2 gas 11

becomes less soluble in aqueous solutions, however this is not our concern in the present study. Another factor that affects CO2 gas solubility in solutions is the presence of different salts in aqueous solutions which reduces the solubility of CO2 gas in these aqueous solutions in what is known as “salting-out effect”, which has been investigated by many workers [30,31,32]. Because of importance of this issue, the solubility of CO2 in aqueous solutions has been extensively studied over a broad range of salt concentration, temperature and pressure. The major impact of the salt in water is the change in the solution surface tension which in turn influences the extent of the pore wetting. According to Table 3, although, the concentration of CO2 changes inversely with increasing NaCl solution concentration, the change in the CO2 concentration with respect to pure water is negligible in the range of our experiment. As can be seen, the trend of the solution surface tension is ascending as a result of increasing NaCl concentration [33,34]. Also, NaCl concentration marginally influences the solution surface tension. In conclusion, it can be assumed that the NaCl-circulation and the CO2 absorption tests were performed under nearly identical conditions. Table 3. Concentration of CO2in aqueous NaCl solution and surface tension of aqueous NaCl solution as a function of NaCl concentration at 25 C and 1.0 bar.

3.9. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX)

Scanning electron microscope (SEM, Hitachi, TM-3000) was used to study the morphology of the membranes produced. After the membrane samples were fractured in liquid nitrogen, they were mounted on a stainless steel stand with carbon tape. To measure the atomic content of sodium (Na) and chlorine (Cl), inside the membrane pores to represent the extent of membrane partial wetting, X-ray energy-dispersive spectrometry (EDX) (Hitachi, TM 3000) was used.

4. Results and Discussion 4.1.Gas permeation results

Table 4 summarizes the structural parameters of the various hollow fiber membranes. According to the table, the results obtained by the conventional GPT model seem unreasonable, since pore sizes as high as 439 and 869 nm were recorded for PEI-12 and PEI-12-IC hollow fiber, respectively. Accordingly, the data obtained by the partial slip model [36], which seem to be more reliable, are used for further discussions, see appendix B. Comparing PEI-12 and PEI-12-IC hollow fibers, rp increased whereas, 12

ε/lp notably decreased from PEI-12 to PEI-12-IC.These observed trends can be explained by assuming

the penetration of low viscosity and low surface tension silicon rubber solution deep into a large number of small pores, consequently resulting in significant pore blocking. By changing the PEI concentration in the spinning dope from PEI-12 to PEI-15, ε/lp showed a large amount of decrease, which seemed natural. On the other hand, PES-15 hollow fiber displayed higher rp and ε/lp in comparison to PEI-15 even though they were spun from the spinning dope of the same polymer concentration. This can be attributed to the different properties of PES relative to PEI. Table 4. Prediction of pore size and effective surface porosity by conventional GPT model and Partial slip model, using heliumas probe gas at room temperature.

4.2.Wetability Resistance

Table 5 shows the contact angle and CEPw of the fabricated membranes. According to the YoungLaplace model [20], CEPw is supposed to be affected by contact angle and pore size. Considering Table 4, PEI-12 and PEI-15 membranes displayed contact angles of 78° and 82°, respectively. PEI-12IC, on the other hand, exhibited a much higher contact angle of 108o. These results indicate that even a thin layer of coated silicone rubber layer can increase the contact angle significantly. For this reason CEPw increased from 300 for PEI12 to 550 kPa for PEI-12-IC. Membrane PES-15 showed the largest rp and the lowest contact angle among the prepared hollow fibers. Consequently, CEPw of PES-15 was the lowest. The CEPws increased from 300 kPa for PEI-12 to 800 kPa for PEI-15, which is primarily due to the smaller pore size and the higher contact angle of PEI-15 than PEI-12. Table 5. Contact angle and CEPw of the coated and uncoated hollow fiber membranes.

4.3.Morphological study

Fig. 4 depicts the SEM spectra of the cross-section of the fabricated membranes. The yellow arrows show the direction of the EDX spectra to draw the cross-sectional profile of the Na and Cl atomic content. The SEM images of all the PEI hollow fibers look alike, while the PES hollow fiber possesses quite distinct morphology. The cross-section of the PEI membranes mostly shows a thick layer of fingerlike structure, which starts from the outer and inner skins and meet with each other at the midway, whereas the cross-section of the PES membrane is divided into three regions, including a vast area of the sponge-like structure in the middle which was sandwiched between two narrower fingerlike sections.

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Fig. 4. Cross section of a) PEI-12, b) PEI-12-IC, c) PEI-15, d) PES-15

4.4.Analysis of the EDX spectra

Fig. 5 illustrates the EDX spectra tracking the Na (the red bar lines) and Cl (the green bar lines) components that arise from the tiny crystals of NaCl formed inside the cross-section of the membranes.

Fig.5. EDX spectra of 800X a) PEI-12, b) PEI-12-IC, c) PEI-15, d) PES-15 (white lines: observed EDX trend, black lines: ideal EDX trend)

It is noted that the white lines are the observed EDX trend, while the black straight lines the idealized EDX trend. The latter lines were drawn as the best-fit, by eyes, to the EDX signals. According to Fig. 5a , an average intensity of nearly equal to 2 was observed for PEI-12. According to Fig. 5b, PEI-12-IC showed an average intensity of 1.0. The same value was obtained for membrane PEI-15 as shown in Fig. 5c. These results indicate that NaCl solution (as a representative of the system absorbent) penetration in the pore (and hence the partial pore wetting) was reduced by an increase in polymer concentration in the dope or by coating the lumen side of the hollow fiber with PDMS. In contrast to the PEI-membrane group, PES-15 (Fig. 5d) showed the highest intensity of Na and Cl atomic content. The intensity was more pronounced near the lumen side which was in direct contact with the NaCl solution. This revealed that pore wetting was more severe for PES-15 hollow fibers in comparison to the PEI-membrane group tested in the present study. Referring to Tables 4 and 5, the lowest contact angle, the largest pore size and effective porosity, and the lowest CEPw made PEI-15 the most susceptible to pore wetting. Accordingly, PES-15 exhibited a quite different profile for Na and Cl intensity. These findings were in good agreement with their corresponding absorption performance and the mass transfer resistance analysis which will be discussed in the following sections. It should be noted that the proposed method to investigate the partial pore wetting is still imperfect. Particularly, the formation mechanism of NaCl crystals inside the membrane has to be clarified in the future investigation. As illustrated in Fig. 2 , the profile of the probe elements intensity must be an ideal step function (the black solid lines), if the liquid enters into the pore as a plug flow, while what was observed in Fig. 5 was not an exact step function (the white solid lines). It is 14

postulated that this phenomena is due to the non-uniform distribution of the NaCl crystals in the membrane pores, which occurs during the vacuum drying stage. The saline solution penetrated into the pores to a specific length (η) as a plug flow. During the vacuum evaporation, NaCl started to crystallize, and since only a small amount of NaCl enters the pores, the crystal size is very small.. During the evaporation step, some of these crystals were likely forced by water vapor to the other edge of the pore , which resulted in unexpectedly flat EDX intensity profiles, as shown in Fig. 5. The probe migration could probably be avoided by choosing, as a probe, a low molecular weight water-miscible polymer such as polyvinylpyrrolidone (PVP) and poly(ethylene glycol) (PEG).

4.5.Analysis of membrane mass transfer resistance

The overall mass transfer resistance based on the liquid side should be proportional to a function of liquid velocity, v-α, where α is an empirical parameter and v is the liquid velocity [6,17,21]. A plot of 1/KOL versus v-α results in a straight line, which is known as the Wilson plot. In this work, α value in the range of 0.3-0.7 was chosen as the best linear fit and the results shown in Fig. 6. It should be noted that α equal to 0.33 has been recommended [6, 17]. Fig. 6. Wilson plot of the fabricated membranes, effect of silicon rubber lumen side coating on the MCs overall mass transfer resistances.

The individual mass transfer resistances associated with the different fabricated membranes can be estimated. Hence, an attempt was made to split the overall mass transfer resistance into individual mass transfer resistances as follows. Ignoring the contribution of gas phase resistance, and applying the resistance-in-series model, i.e. Eq. (1): (20)

ROL = RL + RM

Where ROL and RL are defined as 1/KOL and 1/kL, respectively. KOL (and ROL) is obtained from the experiments using Eq. (19). RM was already obtained from the intercept of the Wilson plot (Fig. 6). Hence, by subtracting RM from ROL, one can obtain RL at each liquid velocity. The results for a liquid velocity of 3.0 m/s are typically shown in Fig. 7 and in the first 3 columns of Table 6. It is believed that this method is more accurate than the conventional method in which correlations between dimensionless numbers are used.. 15

Fig. 7. Absorption based mass transfer resistance vs. different fabricated membranes at liquid velocity of 3.0 m/s.

From Fig. 7, it is found that RL contributes to ROL much more than RM for hollow fibers PEI-12, PEI-12-IC and PEI-15, whereas the opposite is the case for PES-15. Moreover, RL’s are more or less very similar except for PEI-15. This is because RL does not depend on the hollow fiber very much due to similar lumen side diameter and fluid dynamics of the liquid phase on the lumen side of all hollow fibers. This large RM value of PES-15 is ascribed to the resistance contributed from the liquid phase in the pores of PES-15.

The membrane mass transfer resistance for gas filled,

RMD ,and liquid filled, RMw , pores can also

be calculated using the ε/lp values (Table 4) obtained from helium gas permeation experiments in Eqns. (5) to (8), assuming τ ≈ 1. Then, η can be calculated using Eq. (9) and RM values given in Fig. 7. All RM,

RMD , RMW and η values thus obtained are listed in Table 6. ( RMD is also given in Fig. 7 as Rm

(dry).)

Also, based on the definition of wetting ratio, η can be estimated according to the point numbers on the x-axis of the EDX spectra, Fig. 5. Interestingly, the two approaches (EDX results and mass transfer resistance analysis) were in close agreement. According to either approaches, Fig. 5 and Table 6, within the time frame of the experiment, i.e. one h, all the membranes did not experience any significant pore wetting, with the exception of membrane PES-15. As can be seen from Table 6, RM and

RMD agree with each other in orders of magnitude except for PES-15 for which RM is two orders D

of magnitude higher than RM . Moreover, η of PES-15 is much higher than other hollow fibers, which is also in good agreement with the EDX results which showed the highest salt penetration into PES-15. Hence, it can be firmly concluded that the pore wetting was not significant for the prepared hollow fibers except for PES-15. In conclusion, the EDX results and gas absorption/permeation results displayed the same trend for membrane partial wetting.

16

Table 6. Comparison between membrane mass transfer resistances based on gas absorption and gas permeation results, v≈ ≈3.0 m/s, DL=1.89×10-9.

4.6.Membrane contactor performance

Fig. 8 shows the CO2 absorption fluxes of the fabricated membranes. The different trend in the fluxes observed in Fig. 8 is postulated to be due to three major reasons. The first reason is the nature of the polymer, which is exhibited by its degree of hydrophobicity, which in turn is quantified by contact angle. The second reason is the membrane structural parameters, i.e. effective porosity and mean pore size, and the last one is the properties of the gas and liquid in contact, i.e. the Henry constant, gas and liquid diffusion coefficients and the surface tension of the adsorbent. All of these factors can affect the wetting ratio as well. Within the aforementioned factors, since the gas and liquid applied here were the same for all the membranes, the most influential factors were the contact angle and the structural parameters. From Fig. 8 the descending order in the fluxes is PEI-12 > PEI-12-IC > PEI-15 ≥ PES-15. Although the absorption flux is affected by the above factors, none of these parameters showed the same descending (or ascending order) as that of the absorption flux. For example, between PEI-12 and PEI-12-IC, contact angle increased (Table 5) while the effective porosity, ε/lp, decreased (Table 4) considerably from PEI-12 to PEI-12-IC, indicating that the effective porosity is the dominant factor governing the fluxes of these two hollow fibers. On the other hand, between PEI-15 and PES-15, even though the pore size and effective porosity of PES-15 are much larger than PEI-15, the flux of PES-15 is slightly less than that of PEI-15. This is because of the lower contact angle and CEPw values of PES-15, which caused the severest pore wetting of PES-15 hollow fiber. As a result, PES-15 hollow fiber showed the lowest fluxes among all the hollow fibers tested.

Fig. 8. CO2 absorption flux vs. liquid velocity through lumen, for membranes PEI-12, PEI-12-IC, PEI-15 and PES-15.

5. Conclusion

In this study, evaluation of partial pore wetting of the gas absorption membranes was attempted by a newly proposed method. The method consists of two approaches, one based on EDX analysis and the other calculation of mass transfer resistance distribution between the gas and liquid phase in the membrane pore. In the latter approach, the effective porosities obtained from the He gas permeation test were used. It was found that mass transfer resistance in the pore was contributed mainly from the gas phase for most of the fabricated hollow fibers, except for PES-15 in which the contribution from the liquid phase was much more than that from the gas phase. These results were supported further by 17

the EDX analysis. Thus, the same conclusion could be obtained from these two different approaches, i.e. no serious pore wetting had taken place in the hollow fibers except for PES-15 within a limited period of 1 h, confirming the usefulness of both approaches. It can be further concluded that the newly proposed method is a powerful tool to investigate the pore wetting and its effect in various membrane contactor applications.

Acknowledgements

The authors gratefully acknowledge Advanced Membrane Technology Research Centre (AMTEC) of Universiti Teknologi Malaysiafor their financial support.

Appendix A: Predicting the diffusion coefficients The liquid diffusion coefficient of CO2 in water, DL (m2/s) is given by an equation proposed by Frank et al. as follows [29]:

 − 16900 DL = 1.81× 10−6 exp   RT 

(A1)

Further assuming that the gas diffusion is primarily contributed by Knudsen and molecular diffusion, DG is given by [35]:

1 1 1 = + D G DK DM

(A2)

The molecular diffusion coefficient (DM) can be easily calculated by using the Chapman and Cowling [28] equation:

DM =

3 1 1000RT 8 σ 2n πM

(A3)

with

18

n=

6.02×1026 ρ g

(A4)

M

and

ρg =

pM 1000 ZRT

(A5)

where σ is the molecular size which is equal to 3.3×10-10 m for CO2, n is the number density (1/m3), p is the average pressure between upstream and downstream (Pa), ρg is the gas density (kg/m3), Z is the z-factor which approaches unity for an ideal gas, and R is a universal gas constant (8.314 Pa m3/mol K). The Knudsen diffusion coefficient (DK) is calculated by [35]:

2 8000RT DK = rp 3 πM

(A6)

Appendix B:Pore size and Effective porosity The results of the gas permeation testing were used to calculate the pore size and effective porosity. In the gas permeation test, it was assumed that the pores are cylindrical and straight and gas flows through the pores. The conventional GPT model along with the partial slip model [36] was applied to determine the mean pore size and effective porosity. In the conventional GPT model, it is assumed that the Knudsen and viscous flow contribute equally to the total permeance, while neglecting the slip flow contribution. The applicable equation is, therefore [37]: 0.5  rp2 p  ε 1  2rp  8000RT   Ρ = ΡK + ΡV =    +   l RT  3  πM  8 µ   p

(B1)

Where P , PKand PV are the total permeance, the Knudsen permance and the viscous permeance, respectively (mol/m2Pas), ε is the membrane surface porosity, lp is the pore length (m) and the ratio ε/lp is called effective porosity.M is the molecular weight (kg/kmol) and µ is the gas viscosity (Pa.s). From 19

a linear plot of J versus P, the intercept, I, and slope, S, are calculated. Based on Eq. (B1), the pore radius rp and the effective porosity, ε/lp of the membrane are obtained as follows:

 16   S  8000 RT  rp =   µ     3   I  πM 

0.5

(B2)

and

ε / lp =

8µRTS rp2

(B3)

The mean pore size, measured by the conventional gas permeation test is correlated to the pore size obtained by other methods such as solute rejection and atomic force microscopy (AFM) [38] and can be used as a criterion for comparing the different membranes. In order to more realistically achieve the membranes structural parameters, the partial slip model also was applied. This model demonstrates the nonlinear trend of the data especially in the range of transition flow between the free molecular regime and the viscous flow regime. The required equationsare as follows:

0.5   1000πRT 0.5 rp2 p  ε 1  2rp  8000RT  Ρ=   + (1 − φ)ψrp   +  φ RT  3  πM  8µ  l p   8M 

φ=

1

(B4)

(B5)

1 1+ kn

and

kn =

λ

(B6)

2rp

where φ is called the wall-molecule collision probability function which shows the nonlinearity of the gas permeance versus average pressure, due to the pressure dependency of the slope, and ψis a factor 20

which shows the extent of the slip flow regime, and is supposed to be 0.0 <ψ< 1.0. Here, ψ has been assumed to be 0.5. Kn is the Knudsen number and λis the gas mean free path (m) given by the kinetic theory of gases:

λ=

1 k BT 2 2 πσ P

(B7)

Where kB is the Boltzmann constant (equal to 1.38×10−23 J/K), σ is the collision diameter (m) and P is the system mean pressure (Pa) [39]. The details for the derivation of these equations and the simple algorithm utilized to solve rp and ε/lp were presented elsewhere [36]. Nomenclature

A

effective membrane area (m2)

CG

concentration of CO2 in the bulk of gas phase (mol/m3)

CL

concentration of CO2 in the bulk of solution (mol/m3)

CiG

concentration of CO2 at the liquid/gas interface for gas phase (mol/m3)

CLin

inlet concentration of CO2 in solution (mol/m3)

CLin,i

inlet concentration of CO2 in liquid/gas interface (mol/m3)

CLout

outlet concentration of CO2 in solution (mol/m3)

CLout,i

outlet concentration of CO2 in liquid/gas interface (mol/m3)

(∆Clm)

log-mean concentration difference

d

inside or outside fiber diameter (m)

dh

hydraulic diameter (m)

di, do and dlm

hollow fiber inside, outside and log mean diameters (m)

DG

diffusion coefficient of gas (m2/s)

DK

Knudsen diffusion coefficient (m2/s)

DL

diffusion coefficient of CO2 in liquid phase (m2/s)

DM

molecular diffusion (m2/s)

Gz

Graetz number 21

H

dimensionless temperature-dependent Henry constant

h

Henry constant (MPa)

I

intercept

J

absorption flux (mol/m2s)

kB

Boltzmann constant (equal to 1.38×10−23 J/K)

kG

gas boundary layer mass transfer coefficient (m/s)

kL

liquid boundary layer mass transfer coefficient (m/s)

kM

membrane mass transfer coefficient (m/s)

kMD

gas filled membrane mass transfer coefficient (m/s)

kMW

liquid filled membrane mass transfer coefficient (m/s)

Kn

Knudsen number

KOL

overall mass transfer coefficient based on the liquid phase (m/s)

L

length of the fibers (m)

lp

pore length (m)

M

molecular weight (kg/kmol)

n

number of hollow fibers, gas number density (1/m3)

p

system mean pressure (Pa)

P

total permeance (mol/m2Pas)

PK

Knudsen permeance (mol/m2Pas)

PV

viscous permeance (mol/m2Pas)

∆p

transmembrane pressure drop

QL

liquid volumetric flow rate (m3/s)

R

universal gas constant (8.314 J mol−1 K−1)

Re

shell side Reynolds number

RL

liquid boundary layer mass transfer resistance (s/m)

RM

membrane mass transfer resistance (s/m)

ROL

overall membrane mass transfer resistance (s/m)

RMD

gas filled membrane mass transfer resistance (s/m)

RMW

liquid filled membrane mass transfer resistance (s/m)

rp

pore radius

S

slope 22

Sc

liquid Schmidt number

Sh

Sherwood number

t

permeation time (s)

T

system absolute temperature (K)

V

volume of gas permeated through the membrane (m3, STP)

Z

z-factor

α

fitting constant of Wilson plot

ε

membrane surface porosity

ε/lp

effective porosity

φ

wall-molecule collision probability function

η

wetting ratio

λ

gas mean free path (m)

µ

gas viscosity (Pa.s).

ρL

liquid density (kg/m3)

ρG

gas density (kg/m3)

σ

collision diameter (m)

σ

molecular size(m)

τ

tortuosity

v

liquid velocity in lumen (m/s),

ψ

slip flow regime factor

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26

Fig. 1.a schematic of different gas/liquid contact modes and the concentration gradient of CO2 in the membranes structures, mode a) fully dry porous membrane contactor, mode b) partially wet porous membrane contactor, mode c) fully wet porous membrane contactor. (GB: gas bulk, GBL: gas boundary layer, LB: liquid bulk, and LBL: liquid boundary layer, and the colors represent the following media; yellow: gas stream, blue: liquid stream, and gray: porous membrane cross section)

Fig. 2.An ideal normal step function of probe element intensity versus wetting ratio

27

Distilled water(or NaCl aqueous solution) reservoir Sample collection

Fig. 3. Gas absorption (or NaCl aqueous solution circulation) rig; Orange lines: gas stream, blue lines: Liquid stream.

28

a

b

c

d

Fig. 4. Cross section of a) PEI-12, b) PEI-12-IC, c) PEI-15, d) PES-15

a

b

c

d

Fig.5. EDX spectra of 800X a) PEI-12, b) PEI-12-IC, c) PEI-15, d) PES-15 (white lines: observed EDX trend, black lines: ideal EDX trend)

29

Overal mass transfer resistance (s/m)

60000 PEI PEI-12

50000

PEI-12-IC

40000

PEI-15 PES-15

30000 20000 10000 0 0

0.25

0.5

0.75

1

1.25

V-αα Fig. 6. Wilson plot of the fabricated membranes, effect of silicon rubber lumen side coating on the MCs overall mass transfer resistances.

Membrane mass transfer resistance (s/m)

45000 40000

ROL

35000

RL

30000

Rm

25000

Rm (dry)

20000 15000 10000 5000 0 PEI-12

PEI-12-IC

PEI-15

PES-15 15

Membrane Fig. 7. Absorption based mass transfer resistance vs. different fabricated membranes at liquid velocity of ~3.0 m/s.

30

Absorption flux of CO2 (mol/s.m2)

3.50E-03 PEI-12 3.00E-03 PEI-12-IC

2.50E-03

PEI-15

2.00E-03

PES-15

1.50E-03 1.00E-03 5.00E-04 0.00E+00 0.00

1.00

2.00

3.00

4.00

5.00

Liquid velocity (m/s) Fig. 8. CO2 absorption flux vs. liquid velocity through lumen, for membranes PEI-12, PEI-12-IC, PEI-15 and PES-15 at room temperature.

31

Table 1. Spinning condition for hollow fiber membrane fabrication. Spinning conditions Polymer concentration Bore fluid Dope flow rate Bore flow rate External coagulant Bore fluid temperature (C) External coagulant temperature (C)

Remarks 12 and 15 wt.% PEI distilled water 4.0 cm3/min 1.7 cm3/min water 23 ºC 23 ºC

Table 2.Code and specifications of the fabricated membranes Membrane Code

Polymer concentration, wt.%

PEI-12 PEI-12-IC* PEI-15 PES-15 * Silicon rubber inside coated

Hollow fiber inside diameter (nm)

Polymer

407 ~407 443 410

PEI PEI PEI PES

12 12 15 15

Table 3. Concentration of CO2in aqueous NaCl solution and surface tension of aqueous NaCl solution as a function of NaCl concentration at 25 C and 1.0 bar. Concentration of aqueous NaCl solution Molarity Wt%

Concentration of CO2 in aqueous NaCl solution [32] Molarity ppm

Surface tension of aqueous NaCl solution [34] mN/m

0.0*

0.0*

0.033

1450

72.5

0.5

2.93

0.030

1320

73.57

1.0

5.85

0.028

1200

74.62

*

pure water

32

Table 4. Prediction of pore size and effective surface porosity by conventional GPT model and Partial slip model, using heliumas probe gas at room temperature. Membrane PEI-12 PEI-12-IC PEI-15 PES-15

Conv. GPT model rp(nm) ε/lp(m-1) 439 10.5 869 0.66 276 2.05

372

25.7

Partial slip model, ψ=0.5 rp(nm) ε/lp(m-1) 193 66.5 290 9.94 148 9.7

313

62.3

Table 5. Contact angle and CEPw of the coated and uncoated hollow fiber membranes at room temperature.

Membrane code Dense PDMS PEI-12 PEI-12-IC PEI-15 PES-15 + From reference [40]

Contact angle(°) 106+ 78±3.4 108±4.5 82±1.4 75±2.3

33

CEPw (kPa) 300±45 550±60 800±75 200±45

Table 6. Comparison between membrane mass transfer resistances based on gas absorption and gas permeation results, v≈3.0 m/s, DL=1.89×10-9.

Membrane code PEI-12 PEI-12-IC PEI-15 PES-15

Gas absorption (Wilson plot) results

Percentage of wetting ratio(η)

Gas permeation Results

ROL (s/m)

RL (s/m)

RM (s/m)

rp (nm)

17000 23000 35000 43000

15379 20066 31043 19924

1621 2934 3957 23076

193 290 148 313

(m )

-5

DG (10 m2/s)

RMD (s/m)

RMW (106 s/m)

66.5 9.94 9.7 62.3

1.20 1.31 1.12 1.33

411 2522 3031 397

2.62 17.50 17.93 2.79

ε/lp -1

34

Mass transfer resistance approach 0.0463 0.00235 0.00516 0.812

EDX approach Negligible Negligible Negligible ~1.5

Research highlights:

1234-

Analysis of partial pore wetting was attempted by using EDX results. Partial pore wetting was characterized by calculation of mass transfer resistances. There was a direct correlation between the two approaches. The newly proposed method is a powerful tool to investigate the pore wetting.

35

Graphical abstract

Different gas/liquid contact modes, mode a) fully dry porous membrane contactor, mode b) partially wet porous membrane contactor, mode c) fully wet porous membrane contactor. (yellow: gas stream, blue: liquid stream, and gray: porous membrane cross section)

An ideal step function of probe element intensity versus wetting ratio

36