Sodium alginate–gelatin polyelectrolyte complex membranes with both high water vapor permeance and high permselectivity

Journal of Membrane Science 375 (2011) 304–312

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Sodium alginate–gelatin polyelectrolyte complex membranes with both high water vapor permeance and high permselectivity Yifan Li a,b , Huiping Jia a , Qinglai Cheng a , Fusheng Pan a , Zhongyi Jiang a,b,∗ a b

Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300160, China

a r t i c l e

i n f o

Article history: Received 22 February 2011 Received in revised form 28 March 2011 Accepted 29 March 2011 Available online 8 April 2011 Keywords: Polyelectrolyte complex membrane Sodium alginate Gelatin Propylene Dehydration

a b s t r a c t A series of sodium alginate–gelatin polyelectrolyte complex (PEC) membranes were prepared by physical blending and utilized for propylene dehydration. The chemical and physical structures of the membranes were characterized by scanning electron microscope (SEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and positron annihilation lifetime spectroscopy (PALS). To elucidate the structure–performance relationship, water vapor sorption and diffusion properties were investigated. For dehumidification tests, the water vapor permeance and water/propylene selectivity increased simultaneously with the gelatin content because of enhanced water uptake and reduced free volume cavity size. When gelatin content was 60 wt.%, the PEC membrane exhibited the highest permeance of 39.8 m3 (STP)/(m2 h bar) and an infinite permselectivity, which far outperformed the pure sodium alginate membrane and gelatin membrane. A tentative explanation for this interesting result was presented taking into account of the competition of sorption and diffusion. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Membranes with favorable water vapor transport properties have great application potentials in dehydration of hydrocarbon gases, dehumidification of compressed air, drying of flue gas [1,2], etc. Originally, polydimethylsiloxane and cellulose acetate are most widely used for air dehumidification, mainly due to their high permeabilities and moderate permselectivities [3]. However, in many cases, membranes with high water vapor selectivity are required to minimize the loss of valuable gases or the emission of hazard gases. For this purpose, hydrophilic-based polymers are usually chosen as the membrane bulk material, because water molecule can be preferentially and rapidly sorbed and transported, yielding an efficient separation [4]. Generally speaking, the hydrophilic polymer-based membrane materials can be classified into several types: hydrophilic polymers, hydrophilically modified polymers [4–8], block copolymers [1,9,10], and polymers blended with varieties of functional additives, such as inorganic salts [11], small organic molecules [10,12], and macromolecules [13]. It is widely accepted that the stronger the interaction between water and the membrane is, the higher the sorption selectivity will be, and the water sorbed in the poly-

∗ Corresponding author at: Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel.: +86 22 23500086; fax: +86 22 23500086. E-mail address: [email protected] (Z. Jiang). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.03.058

mer matrix can induce higher degree of membrane swelling, which makes the molecular diffusion easier to occur [4]. In terms of this viewpoint, polymers with large quantities of hydrophilic groups should be promising alternatives, especially for those derived from natural hydrophilic polymers, which usually possess excellent water solubility, good membrane forming property, abundant availability and non-toxicity. As a matter of fact, some of them, such as chitosan, alginate, sodium carboxymethyl cellulose, and hyaluronic acid, have been successfully applied to the fabrication of pervaporation (PV) membranes for solvent dehydration [14–16]. Meanwhile, sulfonated poly(ether ether ketone), sulfonated polyethersulfone, Pebax® 1074 membrane materials have been successfully utilized for gas dehydration [3] owing to the moderate swelling resistance of the membranes. It seems a crucial issue to achieve the balance between hydrophilicity and anti-swelling properties for gas dehydration membranes. Polyelectrolyte complex (PEC) membrane, which is both ionically cross-linked and highly hydrophilic [17], is a competitive candidate. Several types of PECs have been developed for the dehydration of alcohols with high PV performance especially for rapid water permeation, and their physical structures, in addition to water sorption and diffusion characteristics, can be easily tuned by changing the blend ratio of the concerned two polyelectrolytes [16–19]. Based on the identical solution–diffusion mechanism behind the two membrane processes, pervaporation and gas separation, it can be deduced that PEC membranes composed of natural hydrophilic polymers should have great potentials for gas dehydration.

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Among the natural hydrophilic polymers, sodium alginate (NaAlg), an anion electrolytic polysaccharide extracted from seaweeds [20,21], is one of the commonly used green polymers as membrane materials. NaAlg possesses not only strong hydrophilicity owing to the numerous of hydroxyl and carboxyl groups, but also large free volume between the chains because of the loose structure [21,22]. On the other hand, gelatin, a natural protein which is obtained from partially hydrolyzed collagen, is regarded as an attractive green polymer as well as a polyampholyte. Gelatin contains free carboxyl and amino groups on its backbone, and carries positive charges in aqueous solution (pI = 8.5). Gelatin has shown superior permselectivity toward water from aqueous organic mixtures in vapor permeation [23]. Furthermore, the flexible gelatin backbone is favorable for the compact and ordered chain packing, which ensures high permselectivity. The complementary properties in type of charge, hydrophilic groups and chain stiffness between these two highly hydrophilic materials may render the promising performance of NaAlg–gelatin PEC membrane for dehydration processes. However, despite many investigations on their performance in medicine, pharmacy and agriculture applications [24–26], rare reports have concerned about NaAlg–gelatin PEC as a separation membrane. The aim of this study is to exploit the potential use of NaAlg–gelatin PEC as gas dehydration membrane material and to probe the structure–performance relationship of the as-prepared membrane. A schematic representation of the NaAlg–gelatin interactions, mainly the ionic bond interaction and hydrogen bond interaction, is shown in Scheme 1. A series of composite membranes were prepared by coating the casting solutions with different ratios of NaAlg and gelatin onto the polysulfone (PSf) hollow fiber substrate. The chemical and physical structures, including interchain interaction, crystallinity, glass transition temperature and free volume property of the membranes, were systematically characterized. Moisture-containing propylene was selected as the feed gas, because propylene is an important raw material in petrochemical industry, and polymer-grade propylene has strict requirement for water content. Sorption and diffusion characteristics of the membranes with different blend ratios of gelatin to NaAlg were also discussed. The dehydration performance of the composite membranes was extensively studied, including the effects of membrane composition and operating temperature. 2. Experimental 2.1. Materials NaAlg was obtained from Shanghai Tianlian Fine Chemical Co., Ltd., China. Gelatin (type A, from porcine skin, Bloom 300) was purchased from Sigma–Aldrich, USA. Porous PSf hollow fiber membranes with molecular weight cut-off 30,000 were supplied by Tianjin Motian Membrane Engineering and Technology Co., Ltd., China. High-purity propylene and nitrogen (99.999+% pure) were used as feed and sweep gas, respectively. 2.2. Membrane preparation 2 wt.% NaAlg was prepared by dissolving 2 g NaAlg in 98 g distilled water with constant stirring for 1.5 h at room temperature. Gelatin solution was made by dissolving 2 g gelatin in 98 g distilled water, the dissolution was facilitated by heating the solution at 60 ◦ C for 1 h with mechanical stirring, and the resulting clear solution was cooled to 40 ◦ C. The as-prepared NaAlg and gelatin solutions were then mixed in different weight ratios of 90/10, 80/20, 70/30, 60/40, 50/50 and 40/60 at 40 ◦ C for 2 h. When the gelatin content was further increased, the mixed solution would

305

become muddy and lose the film forming ability. Next, the porous PSf hollow fibers were dipped into the degassed membrane casting solution for 10 min at 40 ◦ C, and hung up to dry. After 2 h, another dipping process was repeated and membranes were hung up upside down to dry for additional 24 h. Finally, four of these hollow fibers were packed into a self-made membrane module and the effective membrane area was 10.83 ± 0.25 cm2 . The residual mixed solutions after coating were casted on glass plates homogeneously to form flat membranes via solvent evaporation. To ensure reproducibility, all the membranes above were repeated at least three times. In order to compare with PEC membranes, pure NaAlg and pure gelatin membranes were prepared as well. For simplicity, these composite hollow fiber membranes were designated as NaAlg/PSf, NaAlg–gelatin (X)/PSf and gelatin/PSf, and their corresponding separating layers are designated as NaAlg, NaAlg–gelatin (X) and gelatin, respectively, where X represents the percentage mass ratio of gelatin to NaAlg–gelatin PEC. 2.3. Membrane characterization The morphology of the composite membrane was investigated by Philips XL-30E scanning electron microscope (SEM) instrument. The membranes were freeze-fractured in liquid nitrogen and then sputtered with gold before SEM analysis. Flat-sheet membranes as a substitution for selective skin layers of hollow fibers were used to characterize the chemical and physical structures because the latter were too thin to be operated. The Fourier transform infrared (FT-IR) spectra (4000–680 cm−1 ) of the membranes were recorded on a Nicolet MAGNA-IR 560 Spectrometer. The crystalline structures of specimens were tested using X-ray diffraction (XRD) in the range of 3–40◦ at the speed of 2◦ /min (Rigaku D/max 2500 v/pc, CuK 40 kV, 200 mA). The glass transition temperature (Tg ) was measured with a differential scanning calorimetry (DSC) module. The samples (5–10 mg) were heated under a nitrogen atmosphere and scanned from 0 ◦ C to 150 ◦ C with a heating rate of 10 ◦ C/min. All the specimens were stored 24 h in a vacuum oven at 25 ◦ C before analysis. Free volume properties were characterized by positron annihilation lifetime spectroscopy (PALS) with an EG&G ORTEC fast–fast system with 181 ps of time resolution. The 22 Na positron source was sandwiched between two 1 mm thick samples. LT-v9 program was applied to analyze the spectrum. The integral statistics for each spectrum was equal to 1.5 million coincidences. Tao [27] and Eldrup [28] proposed a preferential localization of ortho-positronium (oPs) into the free volume cavity on the assumption of spherical potential well and the lifetime of the o-Ps ( i ) is directly related to the free volume radius (ri ). Supposing that the annihilation rate of the o-Ps inside the electron layer of thickness r at the internal surface of VFi is 2 ns−1 , a semi-empirical can be obtained as follows: i =



1 ri + 1− 2 ri + r

 1  2

sin

 2r −1 i ri + r

(1)

where  i ( 3 or  4 ) is the o-Ps lifetime (ns); ri (r3 or r4 ) is the radius of the free volume element; and r is the thickness of electron layer, which is approximately 0.1656 nm [29]. The volume of the equivalent sphere VFi can be easily calculated by Eq. (2). VFi =

4 3 r 3 i

(2)

Hence, the apparent fractional free volume f could be computed from the following equation: f = f3 + f4 = VF3 I3 + VF4 I4

(3)

where fi and Ii are the apparent fractional free volume and intensity of the o-Ps component i, respectively [12].

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Scheme 1. Schematic diagram of the NaAlg–gelatin interactions.

2.4. Sorption experiments Flat membranes, initially dried in vacuum oven for 48 h, were weighed (m0 ) and exposed in pure propylene gas or feed gas (moisture content 0.13 ± 0.03 wt.%) at 350 kPa and 25 ◦ C for 24 h. The samples were re-weighted (mt ) as they had reached sorption equilibrium. The concentration of absorbed water in the membrane was calculated by Eq. (4): mt − m0 water uptake = m0

(4)

2.5. Gas permeation experiments The gas separation apparatus (described in our previous work [12]) was used to study the separation performance of the composite hollow fiber membranes for the propylene/water vapor mixture. The feed gas was fed into the shell side under pressure, and the sweep gas flowed through the inside of the fibers. A feed pressure of 350 kPa was applied to the shell side of the fibers, and the permeate side was swept by nitrogen with a constant pressure of 180 kPa. The flowrates of feed and sweep gases were 200 and 100 cm3 (STP)/min, respectively. Water concentration was 0.13 ± 0.03 wt.% in the feed; the operation temperature, controlled by a water jacket around the module, was maintained at 25 ◦ C. The compositions of feed and permeate were measured by a gas chromatography (Agilent 6820, USA) equipped with a TCD detector and a 30 m capillary column. Permeance and permselectivity were introduced to evaluate the separation properties of membranes. As a defect-free composite hollow fiber membrane, the functional layer that plays a role of separating is the outer one, while the inner porous substructure only provides mechanical support with no selective effect. We present “(P/l)i ” to characterize the permeability of component i for the hollow fiber membrane for the purpose of avoiding the requirement to measure the thickness (l) of the selective skin layer which is difficult in carrying out. The formula is as follows:

P  l

i

=

Qi pi A

(5)

where Qi is volumetric permeation rate of gas i at standard temperature and pressure, pi is the transmembrane pressure difference, and A is the membrane active surface area. The unit of permeance in this study can be either m3 (STP)/(m2 h bar) or gas permeation

unit (GPU, 1 GPU = 10−6 cm3 (STP)/(cm2 s cm Hg). The permselectivity, ˛i/j , corresponding to the permeance ratio of components i and j, is calculated through Eq. (6). ˛i/j =

(P/l)i (P/l)j

(6)

3. Results and discussion 3.1. Membrane characterization 3.1.1. SEM SEM images of the cross-section morphologies of NaAlg–gelatin (X)/PSf composite hollow fiber membranes are presented in Fig. 1. The inner and outer diameters of the hollow fibers are around 130 and 340 ␮m (Fig. 1a), respectively. For NaAlg–gelatin (10)/PSf (Fig. 1c), NaAlg–gelatin (30)/PSf (Fig. 1d), NaAlg–gelatin (50)/PSf (Fig. 1e), dense and homogenous skin layers with a thickness about 1.5 ␮m are uniformly and tightly coated onto PSf substrate, by the comparison of the view of PSf hollow fiber (Fig. 1b). This indicates that the thickness of active layer was independent of the gelatin content of casting solution, on condition that the overall concentration of each casting solution remains 2 wt.%. 3.1.2. FT-IR Fig. 2 shows the FT-IR spectra of pure NaAlg, pure gelatin and NaAlg–gelatin(X) membranes in the range of 1800–700 cm−1 (to give a more clear picture of the effective wavelength shift). The characteristic absorption bands at 1593 and 1411 cm−1 presented in the FT-IR spectrum of NaAlg membrane correspond to –COO− asymmetric and symmetric stretching peaks, respectively [24]. In addition, the bands or shoulders around 1310 (C–O stretching), 1088 (C–O stretching), 1031 (CO–C stretching), and 946 cm−1 (C–O stretching) are attributed to its saccharide structure [30,31]. The gelatin spectrum is characterized by strong bands at 1637 cm−1 and 1554 cm−1 , attributed to amide carbonyl (C O and C–N stretching vibration) and bending vibration of –NH. The bands from 1225 cm−1 to 1040 cm−1 are the characteristic peaks of amino and alkyl chains [32,33]. From the FT-IR spectra of NaAlg and NaAlg–gelatin membranes in Fig. 2, it is found that carboxyl group band (1593 cm−1 ) in NaAlg shifts toward higher wavenumbers (1602, 1608, and 1622 cm−1

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Fig. 1. SEM micographs of cross-section views on composite membranes: (a) and (b) PSf hollow fiber; (c) NaAlg–gelatin (10)/PSf; (d) NaAlg–gelatin (30)/PSf; (e) NaAlg–gelatin (50)/PSf.

for NaAlg–gelatin (10), NaAlg–gelatin (30), and NaAlg–gelatin (50), respectively). Meanwhile, the intensities of bands at 1593 and 1411 cm−1 of pure NaAlg gradually decrease with increasing gelatin content. Comparing the pure gelatin with the PEC membranes, the characteristic peaks at 1637 cm−1 and 1554 cm−1 of gelatin shifted to lower wavenumbers. All of those changes imply the formation of strong intermolecular interactions including hydrogen-bonding and electrostatic attractions between NaAlg and gelatin chains [24,34]. 3.1.3. XRD The membranes were subjected to XRD analyses to evaluate the influence of physical blending on crystalline structures of the polymers as shown in Fig. 3. The diffractogram of a NaAlg presents a broad peak at 2 = 13◦ , indicating its amorphous nature [8]. Two typical peaks: a sharp peak at around 2 = 7.9◦ and a relative broad peak at around 2 = 18◦ are observed for gelatin [35]. Compared with pure NaAlg, the peaks at 2 = 13◦ in NaAlg–gelatin membranes shift toward higher degree, and the intensity is gradually enhanced as the gelatin content increases. Likewise, the characteristic sharp

peaks at around 2 = 7.9◦ in NaAlg–gelatin membranes shift toward higher degree in comparison with pure gelatin membrane. Such results are attributed to the strong interactions between NaAlg and gelatin, especially the electrostatic attraction caused by ionic crosslinking. The weight-averaged crystallinity is calculated following the method in the previous literature [13]. The results are shown in Table 1. The crystallinity of the membranes increases with the increase of gelatin content, which indicates that the incorporation

Table 1 The crystallinity of pure NaAlg, NaAlg–gelatin (X) and pure gelatin membranes. Membrane

Overall crystallinity (%)

Pure NaAlg NaAlg–gelatin (10) NaAlg–gelatin (30) NaAlg–gelatin (50) Pure gelatin

–a 4.42 9.22 20.54 32.44

a

NaAlg is amorphous polymer and its crystallinity can be ignored.

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1554

41.20 ◦ C, 41.77 ◦ C and 43.51 ◦ C, respectively, lying between those of pure components. Such results may indicate that the PEC membranes are miscible, and no phase separation occurs. Fox’s equation is usually employed to predicate the Tg of blend membrane without considering the interaction between the two component polymers, which is expressed by Eq. (7):

Gelatin

1637

NaAlg-gelatin(50)

Transmittance (%)

1622 NaAlg-gelatin(30) NaAlg-gelatin(10) 1608 NaAlg

1 ω1 ω2 = + Tg Tg1 Tg2

1602

(7)

1411 1593

1800

1600

1400

1200

1000

800

-1

Wavelength (cm )

Intensity

Fig. 2. FT-IR spectra of pure NaAlg, NaAlg–gelatin (X) and pure gelatin.

where Tg , Tg1 and Tg2 are glass transition temperatures of the blend, component 1 and component 2, respectively, ω1 and ω2 are the mass fractions of component 1 and component 2 in the PEC membrane [36]. For both of the PEC membranes, the experimental Tg values are higher than the calculated ones (37.12 ◦ C, 39.22 ◦ C and 41.56 ◦ C for NaAlg–gelatin (10), NaAlg–gelatin (30) and NaAlg–gelatin (50), respectively). The result is attributed to intense intermolecular hydrogen-bonding and electrostatic attraction, confining the flexibility and free mobility of polymer chains, as verified by FT-IR and XRD analyses. Moreover, the detectable Tg indicates that the ionic complexation degree of the PEC membrane is not very strong [37], in good accordance with FT-IR and XRD results.

Gelatin NaAlg-gelatin(50) NaAlg-gelatin(30) NaAlg-gelatin(10) NaAlg

10

20

2θ (deg)

30

40

Fig. 3. XRD patterns of pure NaAlg, NaAlg–gelatin (X) and pure gelatin.

of gelatin alters the loose packing of the NaAlg polymer chains and leads to higher packing efficiency and more regular structure [24]. 3.1.4. DSC The glass transition temperature (Tg ) of membrane was obtained by DSC technique, which was commonly utilized to investigate the miscibility of polymer blends. As thermograms of the specimens illustrated in Fig. 4, it is observed that NaAlg–gelatin (10), NaAlg–gelatin (30) and NaAlg–gelatin (50) show a single Tg at

NaAlg

Heat flow (mW/mg) endo

36.16

NaAlg-gelatin(10) NaAlg-gelatin(30)

41.20

NaAlg-gelatin(50)

41.77

Gelatin

43.51 48.85

20

40

60

80

o

Temperature ( C) Fig. 4. DSC curves of pure NaAlg, NaAlg–gelatin (X) and pure gelatin.

3.1.5. PALS Free volume is defined as the fraction of volume which is not occupied by the electronic clouds of the polymer. It is approved that free volume plays an important role in permeating and diffusing of low molecular weight gases [38]. Both the number and the size of the free volume cavity are considered to have consequential impact on membrane gas separation performance. Hence, free volume elements are investigated by PALS, which is considered as the most direct method to probe the properties of free volume in polymer [29,39]. The PALS results are tabulated in Table 2. From the last column of this table, we can conclude that the overall apparent fractional free volume (FFV) decreases with the increment of gelatin. This result is consistent with DSC curves (the Tg values of NaAlg–gelatin (X) rising with increasing gelatin content), which suggests that the mobility and flexibility of blend molecular chains is suppressed. The free volume data in Table 2 is further divided into two types of cavities having radii in the ranges 0.213–0.233 nm from  3 , network cavities, and 0.302–0.320 nm from  4 , aggregate cavities [40]. The sizes of network cavities and aggregate cavities are both decreased when gelatin content increases, indicating that the polymer chains become more compact due to the strong interactions between the two polymers. In the case of NaAlg–gelatin (X), the intermolecular of hydrogen-bonding and electrostatic attraction can be related to hydroxyl and carboxyl groups of NaAlg with the amino and carboxyl groups of gelatin. Furthermore, with the increase of gelatin amount, the number of network cavities increases while the number of aggregate cavity displays an opposite tendency, which hints that the larger cavities split into more but smaller ones. The overall outcome of the opposite changes of the amount and size of free volume cavities is the decrease of the apparent FFV in total (f) and that in the form of aggregate cavities (f4 ), as well as the increase of the apparent FFV in the form of network cavities (f3 ). Considering the radius of propylene molecule (0.235 nm) is usually in the range of network cavity size and smaller than the radii of aggregate cavities in this study, the increase of f3 and the decrease of f4 can both promote the preferential diffusion of water. However, the decrease of f indicates that the diffusion routes for penetrates will become less.

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Table 2 Free volume properties of pure NaAlg, NaAlg–grelatin (X) and pure gelatin membranes. Membranes

I3 (%)

 3 (ns)

r3 (nm)

I4 (%)

 4 (ns)

r4 (nm)

f3

f4

f

Pure NaAlg NaAlg–gelatin (10) NaAlg–gelatin (20) NaAlg–gelatin (30) NaAlg–gelatin (40) NaAlg–gelatin (50) NaAlg–gelatin (60) Pure gelatin

7.40 9.30 11.50 12.40 14.70 15.80 17.50 16.50

1.65 1.47 1.43 1.36 1.33 1.31 1.30 1.34

0.250 0.230 0.226 0.217 0.212 0.211 0.209 0.214

9.98 9.90 9.12 7.60 6.10 5.60 3.20 4.80

2.94 2.86 2.72 2.53 2.46 2.41 2.38 2.34

0.359 0.353 0.343 0.329 0.323 0.320 0.317 0.314

0.487 0.476 0.553 0.532 0.593 0.622 0.668 0.677

1.931 1.827 1.546 1.135 0.864 0.768 0.429 0.625

2.418 2.303 2.099 1.667 1.457 1.390 1.097 1.302

the hypothesis that the concentration profile is linear along the diffusion length:

3.2. Sorption–diffusion properties of membranes The permeation process is supposed to involve three consecutive steps: sorption, diffusion and desorption [41]. Generally, the third step has a negligible effect on mass transfer due to low concentration of the penetrants in the permeate. Herein, sorption process and diffusion process were studied individually. 3.2.1. Sorption of water vapor/propylene The effect of gelatin content on gas sorption selectivity of the prepared membranes is studied by sorption experiment. Since it is difficult for propylene molecules to be sorbed by hydrophilic membrane due to their lower polarity and larger size, the sorption uptake is extremely low (less than 0.2 mg gas/g membrane). Thus, we only show the results of water vapor sorption because the propylene sorption amount can be ignored comparing with water vapor sorption capability (more than 5 mg gas/g membrane). The high sorption selectivity should be attributed to the strong hydrophilic nature of NaAlg and gelatin resulting from the large amount of hydroxyl and carboxylic groups. According to Fig. 5, the amount of water vapor sorption in membrane dramatically increased with increasing gelatin content, and pure gelatin membrane has the largest water uptake. For NaAlg–gelatin (X), with the increase of gelatin content in PEC membrane, the enhanced membrane hydrophilicity promotes water molecules to be preferentially sorbed. 3.2.2. Diffusion coefficient of water vapor in membrane According to Fick’s law, the flux of water through membrane can be expressed by Eq. (8): J = −D

dc dx

(8)

Jl c

(9)

where J is the mass flux of water vapor permeation, l is membrane thickness of the active layer of the composite hollow fiber membranes, c is the difference in weight fraction of water on membrane surface between feed side and permeate. The effect of gelatin content on concentration-averaged diffusion coefficient is presented in Fig. 5. In comparison with sorption test results, water vapor diffusion coefficient of NaAlg–gelatin (X) does not change significantly when X is within 50. Concretely speaking, it slightly increases with the addition of gelatin before 50 wt.%, and then sharply decreases. This result seems not in accordance with the free volume data calculated by PALS results, because it is the polymer–water interaction and the free volume that determine the water vapor diffusivity together [5]. On the one hand, the sorbed water in the membrane can induce slight membrane swelling, which promotes the water diffusivity. Moreover, the water-sorption ability of gelatin is stronger than that of NaAlg. This implies the increase of gelatin content can enhance the polymer–water interaction, while the water–water interaction is weakened. Thus the opportunity of water molecules to forming large clusters becomes less [4], which is also beneficial to the diffusion of water. On the other hand, the decrease of free volume cavity size may hinder water diffusion. When gelatin content is less than 50 wt.%, the free volume cavity is large enough for a water molecule to easily diffuse through, and the polymer–water interaction remains the deciding factor. However, when gelatin content is beyond 50 wt.%, the free volume cavity becomes narrower and narrower, so that the free volume properties absolutely dominate the water diffusivity. Furthermore, as the gelatin content increases, the crystallinity of the PEC membrane becomes higher and higher, which may have negative effect on the connectivity of the free volume cavities.

8

3.3. Separation performance of membrane

20

6

15 4 10 2

5

0

0

20

40

60

80

100

-8

2

cm /s)

25

Diffusion coefficient (10

Water uptake (mg water/g membrane)

where D, c and x are the diffusion coefficient, the concentration and the distance from the surface of the membrane, respectively. The concentration-averaged diffusion coefficient (D) is obtained on

D=

0

Gelatin content in membrane (wt.%) Fig. 5. Effect of gelatin content on water uptake and water diffusion coefficient of NaAlg–gelatin (X) at 298 K.

3.3.1. The effect of gelatin content on separation performance Table 3 shows the effect of gelatin content on separation performance of the membranes (the water vapor permeance of the support layer was 8.3 m3 (STP)/(m2 h bar)). An “anti-tradeoff” phenomenon can be observed, that is, the permeance and permselectivity of the PEC membranes increase dramatically with the increment of gelatin content. This is not surprising because the socalled “tradeoff” relationship between permeability and selectivity [5] may not exist for gas dehydration membranes, and many highly selective polymers also have high water permeability [3]. However, it is interesting that the PEC membranes exhibit much better separation performance than both pure NaAlg and gelatin membranes. In this way, the in-depth discussion of the structure–performance is conducted as follows.

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Table 3 Effect of the gelatin content on separation performance of the composite membranes. H2 O permeancea

NaAlg/PSf NaAlg–gelatin (10)/PSf NaAlg–gelatin (20)/PSf NaAlg–gelatin (30)/PSf NaAlg–gelatin (40)/PSf NaAlg–gelatin (50)/PSf NaAlg–gelatin (60)/PSf Gelatin/PSf

3.04 8.11 12.7 18.1 21.8 31.2 39.8 2.76

± ± ± ± ± ± ± ±

0.17 0.53 1.0 1.4 1.5 2.1 2.3 0.26

4.0 H2 O/C3 H6 selectivity 54 ± 5 544 ± 47 3968 ± 284 8812 ± 568 27,470 ± 1895 ∞b ∞b 96,808 ± 5677

Lg Pw (GPU)

Membrane

4.4

3.6 3.2 2.8

a

Permeance unit: m3 (STP)/(m2 h bar). Permselectivity of NaAlg–gelatin (50)/PSf and NaAlg–gelatin (60)/PSf are considered as infinity due to no propylene is detected by chromatography whose detection limit of propylene content is 1 ppm.

2.4

b

The effects of gelatin content on sorption and diffusion properties have been discussed in Section 3.2, from which we have known that the weight gain trends and diffusion trends are not exactly identically. The most interesting point occurs when gelatin content is 50 wt.%, and the diffusion trends are reversed. This may arise from the competition of two factors: polymer–water interaction and the free volume. Similarly, another pair of competing factors, sorption and diffusion, determines the water permeance and permselectivity. When gelatin content exceeds 50 wt.%, a significant decrease of diffusion coefficient can be observed, but the permeance and permselectivity remain the increasing trends. This is mainly because that it is the sorption process that dominates the preferential permeation of water, when the diffusion resistance of water molecules is low. However, as the diffusion resistance becomes greater and greater, the dominating process may switch from sorption to diffusion. That is to say, the relatively larger water uptake does not necessarily endow higher water permeance, because the mobility of water molecules is strictly restrained. Based on the above facts, it can be assumed that if the PEC membrane with gelatin content beyond 60 wt.% can be acquired, such as NaAlg–gelatin (70) and NaAlg–gelatin (80), there should be a maximum point of water permeance, like that of water diffusivity. However, such membranes cannot be successfully prepared due to the occurrence of precipitation in the casting solutions, and no more detailed information based on the gelatin-dominating NaAlg–gelatin PEC membranes is presented. 3.3.2. The effect of operation temperature on separation performance Temperature plays an important role in membrane gas separation performance as it is a variable affecting sorption and diffusion processes. Generally, sorption is an exothermic process and diffusion is an endothermic process, hence, the former decreases while the latter increases with temperature [42]. NaAlg–gelatin (60)/PSf, exhibiting the most competitive permeance and permselectivity, is especially chosen to study the separation performance of PECs membrane at different temperatures. The permselectivity of NaAlg–gelatin (60)/PSf maintains infinity at all studied temperatures. Previous study [43] has suggested that variation of water permeance (Pw ) with temperature can be expressed by Arrhenius equation: Pw = P0 exp

 −E  p

RT

(10)

where P0 , Ep , R, T are the pre-exponential factor, apparent activation energy of water permeation, molar gas constant and operation temperature (T) in Kelvin, respectively. The logarithmic of water permeance is plotted as a function of reciprocal temperature is shown in Fig. 6. As can be seen, the functional graph exhibits linear, indicating that temperature

3.1

3.2

3.3

-1

1000/T (K ) Fig. 6. Effect of operation temperature on water permeance of NaAlg–gelatin (60)/PSf composite membrane.

dependence of water permeance agrees well with the Arrhenius trend. Furthermore, water permeability decreases with temperature raising, hinting that water sorption process in membrane is obviously affected by temperature. On the basis of sorption–diffusion mechanism, both sorption and diffusion processes play decisive roles on water transport through a polymer membrane, and the apparent activation energy of water permeation can be written by Eq. (11): Ep = Hs + Ed

(11)

where Hs and Ed are the apparent activation energy for the heat of sorption and diffusion process, respectively. From Eq. (10) and the slopes of the straight lines shown in Fig. 6, the apparent activation energy of water permeation (Ep ) in NaAlg–gelatin (60)/PSf is obtained (−7.21 kJ/mol). The result indicates that separation performance is sensitive to temperature variation and the heat generated in water sorption surpasses energy requirement in water diffusion. 4. Conclusions A series of NaAlg–gelatin (X)/PSf composite hollow fiber membranes were prepared for propylene dehydration. Effect of blend ratio of gelatin to NaAlg on membrane structure and performance was investigated. With the increase of the gelatin content, the water sorption ability of membranes was promoted due to the enhanced hydrophilicity, which is more favorable for the transport of water molecules in membranes. Accordingly, the permeance and permselectivity of membranes increased with the increment of gelatin content, which indicated that sorption dominated the permeation process. On the other hand, water diffusion coefficient firstly increased slightly and then decreased significantly with the gelatin content increasing, as a result of the competition of the increase of swelling degree, caused by polymer–water interaction, and the decrease of free volume. Besides, the increasing amount of free volume cavity and the decreasing free volume cavity size characterized by PALS led to the increasing trends of permselectivity, especially when the gelatin content is 50 wt.% and 60 wt.%. It was assumed that the diffusion process would become the governing factor at higher gelatin content, and this might explain why the separation performance of PEC membranes was much better than pure gelatin membrane. Among all the prepared membranes, NaAlg–gelatin (60)/PSf exhibited most competitive separation performance (permeance reached 39.8 m3 (STP)/(m2 h bar) or 1.47 × 104 GPU, permselectivity achieved infinity for 0.13 wt.% water in feed at 298 K), which was much superior to pure NaAlg membrane or pure gelatin mem-

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brane, and as far as we know, this may be the highest water vapor permeance for gas dehydration under the comparable operating conditions. Acknowledgements We gratefully acknowledge financial supports from the Petrochina Research Program, National Basic Research Program of China (2009CB623404, 2009CB724700), the Program of Introducing Talents of Discipline to Universities (No.: B06006), and State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University (No.: 201013).

Nomenclature A c c ¯ D, D Ed Ep f Hs I J l m0 , mt (P/l) P0 Pw p Q r, r T, Tg VF X x

effective membrane area (m2 ) penetrant concentration (g/m3 ) the difference of water weight fraction between the feed side and the permeate side (g/m3 ) diffusion coefficient and concentration-averaged diffusion coefficient (m2 /s) heat of the diffusion process of water (kJ/mol) apparent activation energy of water permeation (kJ/mol) apparent fractional free volume heat of the sorption process of water (kJ/mol) intensity of ortho-positronium component water permeation flux (g/(m2 s Pa)) thickness of separating layer (m) initial and final weights of the film (g) permeance (m3 (STP)/(m2 h bar) or GPU) pre-exponential factor (GPU) water permeance (GPU) transmembrane pressure difference (Pa) volumetric flowrate (m3 (STP)/h) free volume radius (nm) and electron layer thickness (nm) temperature (K) and glass transition temperature (◦ C) volume of equivalent sphere (nm3 ) the percentage mass ratio of gelatin to NaAlg–gelatin PEC distance from the surface of the membrane (m)

Greek letters ˛ permselectivity  the diffraction angle  lifetime of o-Ps (ns) Subscripts i, j component 3, 4 positron or positronium lifetime components

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