Microporous polypropylene hollow fiber membranes

Journal of Membrane Science 214 (2003) 71–81

Microporous polypropylene hollow fiber membranes Part II. Pervaporation separation of water/ethanol mixtures by the poly(acrylic acid) grafted membranes Zhi-Kang Xu∗ , Qing-Wen Dai, Zhen-Mei Liu, Rui-Qiang Kou, You-Yi Xu Institute of Polymer Science, Zhejiang University, Hangzhou 310027, PR China Received 2 April 2002; received in revised form 20 October 2002; accepted 1 November 2002

Abstract Polypropylene hollow fiber membranes grafted with poly(acrylic acid) were used for water–ethanol separation by pervaporation. The effects of grafting degree of poly(acrylic acid) (PAA), the cross-linking degree of the grafted-PAA, the sodium counter-ions as well as the operation temperature and the ethanol concentration in feed mixtures on pervaporation properties were investigated. It was found that water-permselective pervaporation membranes could be prepared by the grafting polymerization of acrylic acid on microporous polypropylene hollow fiber membrane surface. The separation factor increased with the increase of grafting degree of PAA in the range of 20–70 wt.%. Incorporating counter-ions as well as multifunctional comonomer into the grafted chains could improve the selectivity but sacrificed the permeation flux. The separation factor of the counter-ion containing membrane decreased according to the sequence Al3+ > K+ > Ca2+ > Na+ > Li+ and the permeation flux increased following the sequence Al3+ < Ca2+ < K+ < Na+ < Li+ . Remarkable increase in permeation flux without decrease in separation factor was observed for Al3+ as counter-ion in this membrane. These results might be ascribed to the increase of packing density and the decrease of swelling degree of grafted layer on the membrane surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Water/ethanol separation; Pervaporation; Poly(acrylic acid)-grafted polypropylene hollow fiber membrane; Swelling degree; Sorption selectivity

1. Introduction Pervaporation technology represents one of the most effective and energy-saving means to separate azeotropic mixtures, close boiling point mixtures or isomers. Water/alcohol separation is a well-known example of pervaporation process in chemical industry. The membrane materials play a central role in pervaporation process, as the efficiency of the pervaporation process depends mainly on them. In the last ∗ Corresponding author. Tel.: +86-571-8795-1773. E-mail address: [email protected] (Z.-K. Xu).

decade, a series of membranes modified by polymer blending, copolymerizing, chemical grafting, plasma grafting and 60 Co ␥-ray irradiation grafting with hydrophilic polymers have been used for water/alcohol separation [1–12]. Most of them are usually dense membranes, which show high selectivity toward water/alcohol separation over a wide range of water concentration. In order to increase permeation flux without sacrificing the selectivity, composite pervaporation membranes with a dense hydrophilic skin on the top of porous-support have been considered. Recently, numerous dense-skin/porous-support composite membranes were prepared with different methods

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 5 3 6 - 7

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(including dip-coating, interfacial polymerization and self-assembly) and the pervaporation performance for water/alcohol separation were investigated [13–16]. Jiraratananon et al. [13] prepared composite hydrophilic pervaporation membranes by casting the solution of chitosan/hydroxylcellulose blend (3:1) on the porous cellulose acetate support, and the effect of operating conditions were studied. Krasemann and co-workers [14] fabricated composite membranes possessing ultrathin polyelectrolyte multilayer by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous-support, and the water/ethanol separation was described. Doguparthy [15] prepared a thin film on the surface of the microporous substrate (Anapore) by interfacial photopolymerization technique, and reported the pervaporation results of aqueous alcohol mixtures. Kim et al. [16] deposited polyimide dense-skin onto the porous polysulfone membranes by interfacial polymerization/thermal imidization method, and studied their dehydration performance for ethanol by pervaporation. Although, the skin layer can enhance permselectivity during the pervaporation separation process, peeling often occurs when drying these successive layers. Another disadvantage of the composite membranes prepared by deposition of hydrophilic polymer layer onto a porous-support is their solubility. The hydrophilic polymer layers are inevitably eroded by water in separation application. Such a drawback can be avoided by graft polymerization of hydrophilic polymer onto the porous membrane surface [17,18]. Microporous polypropylene membrane is well known for its extraordinary properties, including high void volumes, well-controlled porosity, chemical inertness, good mechanical strength and low cost. It is, therefore, suitable to be used as a support for the preparation of composite membranes [18–22]. Recently, composite membranes derived from microporous polypropylene hollow fiber membrane (MPPHFM) were prepared by polymerizing silicone skin on the outer surface [19,20] and by polymerizing acrylates in pores (pore-filling membranes) [21,22] with plasma. These composite membranes were successfully applied to remove volatile organic compounds (VOCs) from water by pervaporation process. In our previous papers, the MPPHFM was manufactured by a melt-spinning/cold-stretching (MSCA) technique and modified by the graft polymerization of acrylic

acid [23]. In present paper, an attempt has been conducted to separate water/ethanol mixtures using the PAA-grafted MPPHFM. 2. Experimental 2.1. Materials The preparation and characterization of microporous polypropylene hollow fiber membranes grafted with poly(acrylic acid) were described in our previous paper [23]. The inner and outer diameters of the original MPHFM were 271 and 311 ␮m, with porosity of 50%, average pore size of 0.056 ␮m and N2 permeability of 9.0 × 10−2 cm3 s cmHg. Ethanol, alkaline hydroxide and inorganic halides (CaCl2 and AlCl3 ) were commercial product and used without further purification. De-ionized water with a conductivity of 18 s/cm was used for permeation measurements. The alkaline hydroxide was dissolved in water/ethanol solution (50/50 in volume) with 10% concentration. 2.2. Sample preparation To obtain the acid form of PAA-grafted microporous polypropylene hollow fiber membrane samples, the materials were treated with HCl solution, and then thoroughly washed with de-ionized water. The alkaline acrylate form membranes were prepared by immersing the acid form membranes in 10% alkaline hydroxide (or CaCl2 and AlCl3 ) water/ethanol solution to change all –COOH groups into –COOM groups, followed by the washing and vacuum drying steps. The dried membrane samples were stored in a desiccator over 4 Å molecular sieve until their use. 2.3. Swelling measurements The swelling degree of the PAA-grafted MPPHFM was measured in aqueous ethanol solution with various compositions. The dry acid membrane and/or sodium acrylate membrane was first weighed (Wdry ) and then immersed in the water/ethanol mixture for 24 h at 20 ◦ C. The fully swollen membranes were blotted between tissue papers to remove the excess solution and weighted (Wwet ). The swelling degree was calculated


by following equation: swelling degree (%) =

Wwet − Wdry × 100 Wdry

For the composition determination of the sorption solution in the swollen membrane, the fully swollen sample was placed in the tube 1 of a twin tube set-up. The tube 2 was cooled in liquid nitrogen. Then, the system was evacuated with a vacuum pump while tube 1 was heated to 75 ◦ C for 2 h. The composition of the liquid condensed in tube 2 was analyzed by a HP Gas Chromatograph with a flame ionization detector. The sorption selectivity was calculated by: YH2 O /YEtOH sorption selectivity = XH2 O /XEtOH

where Xi and Yi were the weight fractions of species i in the feed and tube 2, respectively. 2.4. Pervaporation experiments A traditional vacuum-pervaporation apparatus was used in this study. Ten pieces of 20 cm long PAA-grafted MPPHFM was used for the pervaporation measurement. The effective membrane area in contact with feed liquid was 20 cm2 . The downstream pressure was maintained about 3–5 mmHg by a vacuum regulator. The permeate was condensed and collected in a cold trap immersed in liquid nitrogen. The composition of the feed and the permeate was analyzed by a HP Gas Chromatograph with a flame ionization detector. After weighing and composition analysis, the collected permeate was returned to the feed tank in order to maintain the feed concentration. The effect of operating temperature on pervaporation performance was carried out at a series of constant temperature, ranging from 20 to 60 ◦ C, regulated by a thermostatic bath. The permeation flux (Jr ) was calculated from the following equation: Jr =

G S t

where G is the weight of permeate, S the effective membrane area (m2 ) and t is the measuring time.

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The separation factor (αH2 O/EtOH ) was calculated by: αH2 O/EtOH =

YH2 O /YEtOH XH2 O /XEtOH

where Xi and Yi were the weight fractions of species i in the feed and permeate, respectively. All the measurements for each data were repeated three times, and the mean values were reported here. 3. Results and discussion 3.1. Effect of PAA grafting degree on pervaporation properties Hydrophilic polymer such as PAA, PVA and chitosan were used as pervaporation membranes for water/alcohol separation [4,5,8,10,11]. However, the abundant hydrophilic moiety in these materials would induce an excessive swelling during the pervaporation process, which reduced the diffusion selectivity of permeate. Therefore, it is important to balance the hydrophilic moiety with the hydrophobic moiety in membrane materials when preparing high performance membrane [24]. The microporous polypropylene hollow fiber membrane is usually not suitable for water/ethanol separation by pervaporation. However, after modified with acrylic acid by grafting polymerization, a thin layer of poly(acrylic acid) can be immobilized on the surface of the MPHFM. As can be seen from Fig. 1, with the grafting degree of PAA being up to 20 wt.%, the membrane changed its microporous structure to a dense-skin/microporous-layer structure which could be used for water/ethanol separation by pervaporation. As the increase of grafting degree, the grafted layer became denser, especially for the sodium acrylate form membrane, the effect of which would be discussed in detail later. Figs. 2 and 3 show the effect of grafting degree of PAA on separation factor (αH2 O/EtOH ) and permeation flux (Jr ) for the feed mixtures with 30, 50, 70 wt.% ethanol, respectively. As can be seen from these figures, the αH2 O/EtOH increased while the Jr decreased with the increase of the grafting degree of PAA from 20 to 70 wt.%. However, morphology changes, which

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Fig. 1. Scanning electron microscopy pictures of outer surface for the microporous polypropylene hollow fiber membranes: (a) original membrane; (b) grafted membrane with 20 wt.% PAA; (c) acid form of grafted membrane with 70 wt.% PAA; (d) sodium acrylate form of grafted membrane with 70 wt.% PAA.

resulted in the lowing of the selectivity, occurred for the MPPHFM when the grafting degree of PAA exceeded 70 wt.%. Because in our situation, higher grafting degree can only be obtained using higher monomer concentration. The high reactivity of acrylic acid might cause gelling effect or propagation in the bulk layer of the substrate, and then destroyed the membrane structure.

3.2. Effect of feed temperature Figs. 4 and 5 show the effect of feed temperature on permeation flux (Jr ) and separation factor (αH2 O/EtOH ) with different ethanol concentrations in feed mixtures. As expected, the separation factor decreased, while the permeation flux increased slightly with the increase of feed temperature.


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Fig. 2. Effect of PAA grafting degree on separation factor at 24 ◦ C.

It is well known that temperature affect the transport properties of components to be separated through membrane. Both the mass transfer coefficient of components in feed and the sorption of components into the membrane increase with the operation temperature. In addition, higher feed temperature caused an increase in grafting chain mobility and swelling. By all the given reasons, the permeation flux increased and the separation factor decreased with the feed temperature. The activation energy of MPPHFM grafted

with 33 wt.% of PAA for the permeation of 30, 50 and 70 wt.% ethanol aqueous solution are ca. 1.59, 1.59 and 1.97 kcal/mol, respectively. 3.3. Effect of sodium counter-ion Much attention has been paid to the water/ethanol separation by pervaporation through ionic membranes [9,25,26]. It has been reported that poly(acrylic acid)-contained membranes with both high selectivity

Fig. 3. Effect of PAA grafting degree on permeation flux at 24 ◦ C.

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Fig. 4. Influence of ethanol concentration in feed on separation factor for the membrane grafted with 70 wt.% PAA at different temperature.

and high permeation flux to water can be obtained when counter-ion such as Li+ , Na+ and K+ was incorporated into the membranes [25,26]. In this work, the acid form membrane was also changed into a

Fig. 5. Relationship between permeation flux and feed temperature for the membrane grafted with 70 wt.% PAA. (䊏) EtOH/ H2 O = 30/70; (䉱) EtOH/H2 O = 50/50; (䊉) EtOH/H2 O = 70/30.

sodium acrylate form membrane by immersing the acid membrane in 10% NaOH water/ethanol solution to change all –COOH groups into –COONa groups. The effects of the sodium counter-ions on the pervaporation properties are shown in Figs. 6 and 7. Fig. 6 illustrates the influence of the ethanol concentration in feed mixtures on the water concentration in permeate. It can be seen that both the acid and acrylate membranes were selective to water in the investigated ethanol concentration range. However, the sodium acrylate membrane showed better selectivity to water than the acid membrane. This result might be due to the fact that the interaction between the Na+ ion and the pendent carboxyl group of poly(acrylic acid) increased the packing density of the grafted-PAA layer. As to Chen et al. [9], a lower packing density of membrane usually leads to decrease diffusion selectivity in pervaporation process. On the other hand, mostly water-permselective pervaporation membranes focus on separation based on solubility selectivity rather than mobility selectivity. Thus the sorption selectivity and swelling properties were measured to further illustrate this point later. As can be seen from Fig. 1, compared with the acid membrane with 70 wt.% of grafted-PAA (Fig. 1c), denser surface layer was observed by SEM for the corresponding sodium acrylate membrane (Fig. 1d).


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Fig. 6. Relationships between water concentrations in permeate and ethanol concentration in feed mixture for the membranes grafted with 70 wt.% PAA at 24 ◦ C.

Fig. 7 shows the effect of sodium counter-ion on permeation flux. Compared with the slight decrease for the acid membrane, it was found that the permeation flux for sodium acrylate membrane decreased sharply with the increase of ethanol concentration in feed mixtures. Furthermore, the permeation flux of the sodium acrylate membrane was larger than that of

the acid membrane at low ethanol concentration but the value of the former was less than the latter when ethanol concentration in feed mixtures was higher than 35%. To demonstrate these results, swelling degree and sorption measurements in aqueous ethanol solution were conducted. Fig. 8 indicates that the swelling degree of the sodium acrylate membrane

Fig. 7. Effect of ethanol concentration in feed on permeation flux for the membranes grafted with 70 wt.% PAA at 24 ◦ C.

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Fig. 8. Effect of ethanol concentration in feed on swelling degree for the membranes grafted with 70 wt.% PAA at 20 ◦ C.

was almost two times of the acid membrane. Increasing the ethanol concentration decreased the swelling degree for both membranes. In a general case of increasing feed ethanol concentration, the permeation flux decreases and the separation factor increases because of a reduced swelling of water in hydrophilic polymer membranes. With a decreased swelling or sorption of water in polymer membranes, water flux through the membrane decreases [9,27]. Such water flux decreasing was more drastically for the sodium acrylate membrane than the acid membrane because

the former membrane had denser grafted layer on the surface and the decreased swelling degree had more serious influence on the former membrane than the latter membrane at similar condition. Fig. 9 illustrates the relationship between ethanol concentration in feed mixtures and sorption selectivity of the membranes. The data indicates that the sorption selectivity increased slightly with increasing ethanol concentration. Corresponding to the swelling degree, the sorption selectivity of the sodium acrylate membrane was higher than that of the acid membrane,

Fig. 9. Effect of ethanol concentration in feed mixture on sorption selectivity for membranes grafted with 70 wt.% PAA at 20 ◦ C.


especially at higher ethanol concentration, indicating that the higher water permselectivity for the sodium acrylate membrane than the acid membrane was mainly derived from the sorption selectivity. As suggested by Ping et al. [28], (1) the affinity of the ion pairs to water was higher than to ethanol and (2) the dissociated ions (COO− and M+ ) exhibited stronger interaction to water than the non-dissociated ions-pairs. As a weak acid, the acrylic acid in the H-form membranes remained essentially non-ionized in the mixture and even in pure water (Ka = 5.6×10−5 at 25 ◦ C), while for the Na-form membrane, the dissociated ions were much more, thus led to the stronger interaction to water, which means higher sorption selectivity for the Na-form membranes. On the other hand, as a common consideration, higher packing density could result in higher diffusion selectivity and lower flux in pervaporation process, and the SEM photograph had shown denser surface layer for the Na-form membranes, which suggested higher diffusion selectivity. According to the solution-diffusion mechanism, the separation factor (α) can be defined as: α = αD α S where α D and α S are the diffusion selectivity and the sorption selectivity, respectively. For the Na-form membranes, both sorption selectivity and diffusion selectivity would result in higher water permselectivity.

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A separation factor of about 80 was reported by Hirotsu [18] for microporous polypropylene sheet membrane grafted by plasma-polymerized acrylic acid. Compared with the plasma grafting polymerized sheet membrane, the hollow fiber membranes studied in this work showed considerably low selectivity toward water. This result could be ascribed to the linear characteristics and the low packing density of the grafted chain by free radical polymerization. For plasma polymerization, the grafting chains were normally cross-linked and the separation factor of the resultant composite membrane changed remarkably depending on the grafting polymerization. 3.4. Effect of cross-linking degree As mentioned above, a lower packing density of membrane usually leads to decrease diffusion selectivity in pervaporation process, as a result of which the permselectivity will vary accordingly. Cross-linking the grafting chain can also be used to increase the packing density of the membrane surface. Therefore, a certain amount of multifunctional cross-linker (divinylbenzene, DVB) was added to the grafting reaction to form a cross-linked thin layer on the microporous membrane surface. The effects of the cross-linking degree on separation factor and permeation flux are shown in Fig. 10.

Fig. 10. Effect of cross-linking degree on separation factor and permeation flux for the membrane with 33 wt.% grafting degree at 24 ◦ C and 90% ethanol in feed.

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Fig. 11. Effect of counter-ions on the pervaporation performance of membrane with 46 wt.% PAA/DVB at 24 ◦ C and 90% ethanol in feed.

It can be seen that, as the cross-linking degree increased, the separation factor increased while the permeation flux declined. Incorporating counter-ions into the PAA-grafted MPPHFM with partial cross-linkage could further improve the separation performance for these membranes. As shown in Fig. 11, both permeation flux and separation factor for the ionized membranes were larger than those of acid membrane, with the exception of Li+ as counter-ion. The separation factor of the counter-ion containing membrane decreased according to the sequence Al3+ > K+ > Ca2+ > Na+ > Li+ and the permeation flux increased following the sequence Al3+ < Ca2+ < K+ < Na+ < Li+ . Similar phenomenon for K+ , Na+ and Li+ as counter-ions was reported in literature [25]. A very interesting result is that, compared with the acid membrane, remarkable increase in permeation flux without decrease in separation factor was observed for Al3+ as counter-ion in this membrane. However, the reason for this is not clear. On the other hand, the increase in separation factor by cross-linking the grafting layer was not as dramatic as expected. This could be ascribed to the hydrophobic characteristics of DVB. One can envisage that multifunctional monomer with hydrophilic characteristics will be suitable for the preparation of PAA-grafted microporous polypropylene hollow fiber membranes possessing both higher separation factor and higher permeation flux for water/ethanol

separation by pervaporation process. Such work is conducting in our lab.

4. Conclusions Microporous polypropylene hollow fiber membranes modified by graft polymerization of acrylic acid can be used as water-permselective pervaporation membranes for water/ethanol separation. The separation factor increases with the increase of grafting degree of PAA in the range of 20–70 wt.%. Incorporating different counter-ions as well as multifunctional comonomer into the grafted chain has obvious influence on the pervaporation properties. Remarkable increase in permeation flux without decrease in separation factor can be reached by incorporating Al3+ counter-ion into the membrane. Compared with the plasma grafting polymerized sheet membranes [18], the hollow fiber membranes studied in this work show relative low selectivity toward water. This result could be ascribed to the linear characteristics and the low packing density of the grafted chain by free radical polymerization. Using hydrophilic multifunctional comonomer to cross-link the grafted polymer will be suitable for the preparation of PAA-grafted microporous polypropylene hollow fiber membranes with both higher separation factor and higher permeation flux for water/ethanol separation by pervaporation process.


Acknowledgements The authors are grateful to the National Natural Science Foundation of China for financial support (Grant no. 20074033).

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