polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols

polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols

Journal of Membrane Science 326 (2009) 222–233 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 326 (2009) 222–233

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols Yan Wang a , Suat Hong Goh a , Tai Shung Chung b,∗ , Peng Na b a b

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

a r t i c l e

i n f o

Article history: Received 19 August 2008 Received in revised form 1 October 2008 Accepted 3 October 2008 Available online 17 October 2008 Keywords: Pervaporation Dehydration Dual-layer hollow fiber Anti-swelling Polyamide-imide

a b s t r a c t To circumvent the common swelling and deteriorated performance of integral asymmetric hollow fiber membranes for pervaporation dehydration, we have developed novel polyamide-imide (PAI)/polyetherimide (PEI) hollow fiber membranes with synergized performance with the aid of duallayer spinning technology. Dehydration of C1–C4 alcohols has been conducted and the orders of their fluxes and permeances have been analyzed. The hollow fibers spun at 2 cm air gap and annealed at 75 ◦ C exhibit the highest pervaporation performance: separation factors for t-butanol/water and isobutanol/water binary systems are greater than 50,000 with flux more than 700 g/m2 h. A comparison with literature data shows that the newly developed membranes outperform most other polymeric membranes for the dehydration of IPA and butanols. The dual-layer hollow fiber membranes also exhibit good long-term stability up to 200 h. The superior performance can be attributed to (1) the balanced properties of PAI as the selective layer for dehydration pervaporation; (2) the low water uptake and less swelling characteristic of the PEI supporting layer; and (3) the desirable membrane morphology consisting of a fully porous inner layer, a porous interface, and an ultrathin dense-selective outer skin. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation (PV) is a promising alternative to conventional distillation technologies in separation of liquid mixtures for being economical, energy efficient and environmentally benign. Substantial progresses and exciting breakthroughs in pervaporation have been made by both academia and industries in past decades. Development of asymmetric hollow fibers for pervaporation is one of them because hollow fibers provide many advantages over flatsheet membranes such as larger surface area per volume ratio, self-supporting structure and a self-contained vacuum channel where a feed can be supplied from the shell side while vacuum is applied on the lumen side. However, it is very challenging to fabricate asymmetric hollow fiber membranes with an ultrathin defect-free selective layer which has superior pervaporation performance, meanwhile pos-

Abbreviations: PAI, polyamide-imide; PEI, poly ether imide; PV, pervaporation; PAA, poly(amic acid); PVA, poly(vinyl alcohol); PAN, poly(acryl nitrile); PI, polyimide; NMP, n-methyl pyrrolidone; MeOH, methanol; EtOH, ethanol; 1-PrOH, 1-propanol; IPA, isopropanol; 1-BuOH, 1-butanol; 2-BuOH, 2 -butanol; i-BuOH, isobutanol; t-BuOH, tert-butanol; FESEM, field emission scanning electron microscope. ∗ Corresponding author. Tel.: +65 6516 6645; fax: +65 6779 1936. E-mail address: [email protected] (T.S. Chung). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.10.005

sesses the ability to overcome the solvent-induced swelling. In the case of pervaporation dehydration, significant attention has been given to highly hydrophilic polymers, such as poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), sodium alginate, and chitosan, etc in the earlier stage [1]. These materials lack mechanical strength and stability in aqueous solutions due to the excessive swelling in water. To suppress the swelling, a common method is to cross-link the materials while sacrificing flux to some extents [2]. As a result, cross-linking modifications for the development of pervaporation membranes may not be desirable because the additional treatments incur extra costs and prolong production durations. Therefore, the cross-linking process may lower the competitiveness of pervaporation against other separation processes. Polyimides (PIs) are the emerging materials for pervaporation dehydration of alcohols [3–12] because of their superior thermal, chemical and mechanical stabilities and high selectivity towards water. Their flux is generally lower than the aforementioned materials. Although the degrees of swelling for PIs are not as high as the other polymer materials mentioned above, the swelling still reduces their pervaporation performance greatly [8,9]. To circumvent the above issues, one must choose new membrane materials with better solvent resistance and synergize them via molecular engineering with the aid of novel membrane fabrication. The aims of this study are (1) to fabricate dual-layer polyamideimide (PAI)/polyetherimide (PEI) hollow fiber membranes for

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Table 1 Previous studies of Torlon® polyamide-imide as pervaporation membrane. Membrane

Feed composition (alcohol/H2 O binary system)

Temperature (◦ C)

Flux (g/m2 h)

Torlon® 4000TF hollow fiber Torlon® /P84 (7:3) blended hollow fiber cross-linked with p-xylenediamine Torlon® 4000TF dense film thermal treated at 250 ◦ C, 2 h Torlon® 4000TF asymmetric membrane thermal treated at 200 ◦ C, 2 h Torlon® 4000TF hollow fiber, thermal treated at 265 ◦ C 30 min Torlon® 4000T dense film, thickness 16–19 ␮m Gelatin/Torlon® (14:86) blended dense film, thickness 14–18 ␮m PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C, 2 h

IPA/H2 O 85/15 wt.% IPA/H2 O 85/15 wt.%

60 60

815 1000

IPA/H2 O 85/15 wt.% IPA/H2 O 85/15 wt.%

60 60

6.8 38

ethanol/H2 O 95/5 wt.% IPA/H2 O 85/15 wt.% IPA/H2 O 99.5/0.5 wt.%

60 82 82

15–21 100 2.4

IPA/H2 O 85/15 wt.%

60

765

pervaporation dehydration of a series of alcohols; (2) to conceptually demonstrate the possibility of synergism via molecular engineering of materials and membrane fabrication to overcome aforementioned swelling problem; (3) to fundamentally understand the dehydration and separation mechanisms in molecular level; and (4) to investigate their long-term performance and stability. Torlon® 4000T polyamide-imide is chosen as the outer layer material because it brings together both superior mechanical properties typically associated with polyamides, and high thermal stability, solvent resistance, gas permeability and permselectivity characteristics associated with PIs [9]. Previous studies on vapor permeation and pervaporation [9,11,13–15] have shown that PAI exhibits both solubility and diffusivity selectivity preferentially for water. However, all performance data published on Torlon® PAI as pervaporation membranes (as listed in Table 1) are not impressive. They have either low separation factor or low flux. One of the possible reasons may be the swelling problem when in contact with the feed liquid. PEI is selected as the inner supporting layer because it has long been studied as a membrane material for vapor permeation and pervaporation [3]. Similar to PAI, it also shows preferentially water selectivity over alcohols. Although its pervaporation performance is not very high, it has low water-adsorption. Especially, it does not suffer from swelling as severely as other polymers including other polyimides [9]. This makes it attractive to be an anti-swelling supporting layer in the design of dual-layer hollow fiber membranes. In addition, an important factor worthy of mention is that the adhesion between these two polyimides materials (PAI outer layer and

Separation factor 12 185 2973 2139 500–700 500 16700 1944

Reference [11] [11] [9] [9] [13] [14] [15] This study

PEI inner layer) is expected to be good because of their common imide structures. In this study, simultaneous co-extrusion through a triple-orifice spinneret was employed to fabricate the dual-layer hollow fiber because of its many unique advantages over traditional technologies for integrally skinned asymmetric membranes. Firstly, with the substrate being the major mechanical support, the thickness of the selective layer can be minimized without sacrificing the overall mechanical strength. Thus, it can be cost-saving since only a very small amount of expensive materials with superior performance is used as the thin selective layer, while a less expensive material is employed as the supporting layer. Secondly, the dual-layer structure allows the independent selection of materials for each layer, thus making possible the structural optimization. In addition, intermolecular diffusion and interactions at interface may result in surprising synergism with enhanced performance if a proper screening of material pair is conducted and optimal spinning conditions are found.

2. Experimental 2.1. Materials Torlon® 4000T-MV polyamide-imide and Ultem® 1010 polyetherimide were supplied by Solvay Advanced Polymers and GE plastics, respectively. Fig. 1 shows their chemical structure [9]. Polymers were dried overnight at 120 ◦ C under vacuum before

Fig. 1. Chemical structures of (A) Torlon® 4000T polyamide-imide and (B) Ultem® 1010 polyetherimide.

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Table 2 Spinning conditions of the PAI/PEI dual-layer hollow fiber membranes. Out-layer dope formulation

Torlon® (4000T-MV)/NMP (28/72 wt.%)

Inner-layer dope formulation Bore fluid External coagulant External coagulant Temperature Spinneret temperature Air gap (cm) Take up speed (m/min) Out-layer flow rate (ml/min) Inner-layer flow rate (ml/min) Bore fluid flow rate (ml/min)

Ultem® 1010/NMP (23/77 wt.%) NMP/DI water (90/10 wt.%) Tap water Ambient (23 ± 2 ◦ C) Ambient (23 ± 2 ◦ C) 4; 2; 0.5 Free fall, 6.37 m/min 0.2 0.8 0.3

use. N-methyl pyrrolidone (NMP), employed as the solvent to fabricate the hollow fiber membranes, was supplied by Merck with analytical grade and used as received. Methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), isopropanol (IPA), and four isomeric butanols (1-butanol (1-BuOH), 2-butanol (2-BuOH), iso-butanol (i-BuOH) and tert-butanol (t-BuOH)) of analytical grade were used to mix with deionized water to prepare the binary aqueous solutions with water concentration at 15 wt.%. 2.2. Spinning process and modules fabrication The schematic diagrams of single-layer and dual-layer hollow fiber spinning systems have been described elsewhere [10–12] and Table 2 lists the detailed spinning conditions for the dual-layer coextrusion process. The polymer solution was degassed 24 h before loading into a syringe pump (ISCO 1000), followed by overnight degassing after loading. A mixture of 90/10 (w/w) NMP/water was employed as the bore fluid in order to make a porous inner surface to minimize the substructure resistance. Tap water was used as the external coagulant. Both dope fluid and bore fluid were filtered through 15 ␮m sintered metal filters before spinning. Polymer solutions and bore fluid were extruded by three ISCO syringe pumps through a spinneret at room temperature with air gaps distances varying from 0.5 to 4 mm. The nascent fibers entered freely into the coagulation bath without additional drawing. The as-spun hollow fibers were stored in water for 2 days to remove residual solvents. They were then solvent-exchanged by methanol and hexane three times each for 30 min. The hollow fibers were dried in air naturally after solvent exchange. The single-layer PAI hollow fiber was fabricated using 28 wt.% NMP solution of Torlon® 4000T-MVwith 5 cm air gap and 20 m/min take-up speed at room temperature. The pervaporation module was prepared by loading 1 or 2 pieces of hollow fibers membranes into a polypropylene module with an effective length of around 15 cm except for long-term tests. Both ends were sealed by epoxy and cured for 48 h at ambient temperature. Thermal treatment was carried out before module fabrication if it was applied to hollow fibers. At least two modules were tested for each membrane sample. 2.3. Pervaporation study A laboratory scale pervaporation unit was employed and the details of the apparatus have been described elsewhere [16]. For all alcohols studied, a feed solution of alcohol/water (85/15 wt.%) was used. It was found that the feed composition varied less than 0.5 wt.% during the entire experiment and can therefore be considered constant. The operational temperature was 60 ◦ C. The feed flow rate was maintained at 0.5 l/min for each module. The permeate pressure was maintained less than 5 mbar by a vacuum pump. Retentate and permeate samples were collected after the membrane being conditioned for about 2 h.

The flux J was determined by the mass of permeate divided by the product of the interval time and membrane area. The mass of permeate was weighed using a Mettler Toledo balance. The separation factor ˛ is defined by the equation below: ˛=

yw,1 /yw,2 xw,1 /xw,2

(1)

where subscripts 1 and 2 refer to alcohol and water, respectively. yw and xw are the weight fractions of component in the permeate and feed, and were analyzed through a Hewlett-Packard GC 6890 with a HP-INNOWAX column (packed with cross-linked polyethylene glycol) and a TCD detector. The flux and separation factor were converted to permeance and selectivity by the following approach. The partial vapor pressure of each component at the feed side was calculated according to the following equation with the aid of the HYSYS DISTIL software (version 6.2) [17]: fi = xn,i i psat i

(2)

where xn,i is the mole fraction in the feed and  i is the activity coefficient calculated by the Wilson equation. The saturated vapor pressure pi sat is determined by the Antoine equation. Both  i and pi sat are calculated through software AspenTech DISTIL 6.2. For readers’ information, the Wilson equation is an empirical extension of the Flory–Huggins model, taking into account the effects of differing molecular sizes and intermolecular forces. It generally works well for many polar and nonpolar systems. However, a disadvantage of the Wilson equation is that it can only be used for miscible liquid systems. Nevertheless, in the absence of a well established model for water–butanol system, the Wilson equation may be still a good option for a given concentration range where the solution is nearly homogenous [18,19], which is what we did in this study. Except tert-butanol, other butanol isomers are generally not fully miscible with water at room temperature. However, at elevated temperatures, their miscibility with water is improved. We observed that, when the temperature is increased to 60 ◦ C, all four butanol/water mixtures (at 85/15 wt.%) became clear, uniform and transparent with little phase separation can be observed. Although it may not be suitable to use the same set of model parameters of the Wilson Equation for the whole concentration range, the equation may be still applicable as long as the solution under consideration is in a single phase. Thus, Eq. (3) was applied to butanol/water systems in this study. Based on the solution-diffusion mechanism, the basic transport equation for pervaporation can be written as [17] Ji =

P  i

l

(xn,i i psat − yn,i pp ) i

(3)

where Pi is the membrane permeability, a product of diffusivity and solubility coefficients, l is the membrane thickness, yn,i is the permeate mole fraction and pp is the permeate pressure. The term [Pi /l] is known as permeance that can be determined by rearranging the above equation if other terms are known. The ideal membrane selectivity ˇ is defined as the ratio of the permeability or the permeance of two components. 2.4. SEM characterization The morphology of the hollow fiber membranes was observed by using a JSM-6700F field emission scanning electron microscope (FESEM). The hollow fiber sample for SEM observation was prepared by fracturing the membrane strip in liquid nitrogen and then coated with platinum. The specimens of outer-layer inner surface and inner-layer outer surface were prepared according to a method mentioned by Li et al. [20]

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Fig. 2. FE-SEM images of the cross-section morphology of the dual-layer hollow fiber membranes.

2.5. The long-term stability study of the dual-layer hollow fibers

3. Results and discussion

Small modules with only 1 piece of hollow fiber and a much shorter length were prepared for the long-term performance study. A short sample length was chosen to minimize rapid variation of the feed composition as well as for easy sample collection over a long period of testing. The feed composition was analyzed daily to make sure the feed composition is constant. In case of a discrepancy larger than 0.5 wt.% (with the original 85 wt.% alcohol concentration) was detected, the circulation pump was stopped for a minute to allow the addition of a calculated amount of water (or alcohol) into the feed tank to adjust the composition back to around 85 wt.%. The samples were collected every 4–6 h generally. The pervaporation system was kept running during night with the aid of circulation and vacuum pumps.

3.1. The morphology of the dual-layer hollow fiber

2.6. The sorption tests of dual-layer hollow fibers The dual-layer hollow fibers were cut into about 5 cm length. The two ends of the pre-weighed hollow fibers were sealed with epoxy before thermal treatment under vacuum at 75 ◦ C for 2 h. The treated hollow fibers were weighted again and immersed in DI water, different pure alcohols and alcohol/water binary mixtures. The swollen samples were taken out at different time intervals, blotted between tissue papers, and then weighed in closed containers. It was repeated until the equilibrium of sorption was reached (when the membrane weight showed no significant change). The sorption degree of the hollow fiber SHF was calculated according to the following equation: Total weight gained = SHF × MHF + Sepoxy × Mepoxy

(4)

Here S and M refer to the degree of sorption and dry weight, respectively, while the subscript “HF” and “epoxy” refer to the tested hollow fiber and the epoxy used to seal it. Separated experiments were conducted to characterize the sorption characteristics of neat epoxy in various solutions so that the sorption effect of epoxy can be deducted from Eq. (4).

Fig. 2 shows the morphology of a dual-layer hollow fiber spun from 2 cm air gap studied by FESEM. From the cross-section image, the hollow fiber has a diameter of about 490 ␮m. The dual-layer wall thickness is about 108 ␮m and the outer layer thickness is about 14 ␮m. Both inner layer and outer layer have asymmetric cross-section morphology and good adhesion between them. No delamination can be observed on the interface even though a boundary between them can be clearly observed. The inner edge of the inner layer is very porous which is quite desirable since it does not constitute any substantial transport resistance. The outer edge of the outer layer reveals a dense and tight nodule structure at the top skin. The thickness of the dense selective layer can be visually estimated of approximately 140 nm from the SEM image at a high magnification (×50,000). Interfacial morphology is a unique feature in dual-layer composite membranes. A seamless interface may be obtained depending on the miscibility of both dopes and many other factors during phase inversion. If the solvents, non-solvents, polymers, and additives used in both dopes are thermodynamically compatible, both dopes may diffuse into each other before and during the phase separation because of chemical potential differences [20]. The mutual diffusion helps form a seamless interface. The formation of the desired interface between PEI layer and PAI layer in this study can be attributed to several factors. Since DSC characterization shows immiscibility between these two polymers, the causes responsible for the good adhesion at interface may arise from the following reasons: (1) they have similar chemical structure because both have imide groups; (2) inter-penetration occurs between these two dopes because of the interfacial energy-resulting convection/mass movement; and (3) the common solvent (NMP) used in the inner-layer and outerlayer spinning solutions. The NMP in the PEI solution can seep into the PAI layer during the spinning and thus enhance inter-diffusion. To further confirm our reasoning, the morphologies of the outer surfaces and inner surfaces of both outer layer and inner layer were investigated by SEM as shown in Fig. 3. Both the inner-layer’s outer

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Fig. 3. FE-SEM images showing the morphology of the outer surface and inner surface of both outer-layer and inner layer of the dual-layer hollow fiber membranes.

surface and outer-layer’s inner surface are fully porous. Only the outer layer has a dense-selective outer surface. The inner layer and outer layer toward the interface (Fig. 2D) have a fully porous structure interpenetrated into each other. This morphology makes permeate diffusion easy without much resistance. The inner-layer’s outer surface (Fig. 3B) is not as porous as the outer-layer’s inner surface (Fig. 3E) apparently under a low magnification. However, under a high magnification (i.e., the magnification is higher than 10 k), one can see this surface is also quite porous (Fig. 3C). In summary, only the outer layer has a dense-selective outer surface, and all the other layers and surface are porous. This is the desirable morphology required for pervaporation membranes with high performance. 3.2. The pervaporation performance of dual-layer PAI/PEI hollow fibers for isopropanol (IPA) dehydration Table 3 summarizes the pervaporation performance of duallayer PAI/PEI hollow fiber and single-layer PAI hollow fiber membranes for IPA dehydration. The dual-layer hollow fiber membrane has a much higher separation factor than that of the single-layer PAI hollow fiber membrane with comparable flux. The improvement may arise from three factors: (1) the low swelling characteristics of the PEI inner layer; (2) the porous and dry substructure; (3) the decoupling of shear and elongational stresses in the dual-layer hollow fiber fabrication. As revealed in our previous

sorption studies [9], PEI has a much lower water uptake compared with PAI (1.25 vs. 6.8 wt.%). PEI also has a higher contact angle than PAI (92.6◦ vs. 87.3◦ ). As a result, PEI is more hydrophobic than PAI, and it will not suffer from swelling as easily as PAI when in contact with feed liquid. The fully porous substructure morphology is also a plus because it helps keep membrane dry under vacuum during operation. Fig. 4 confirms our hypothesis that serious swelling and elongation can be observed in the single-layer PAI hollow fiber, while the dual-layer PAI/PEI hollow fiber remains its original straight shapes. Table 3 shows the pervaporation performance of dual-layer hollow fibers spun with different air gap distances for IPA and water separation. The effect of air gap on pervaporation performance of single-layer hollow fibers has been well studied [21,22]. Generally, an increase in air gap would result in hollow fibers with greater separation factor but lower flux. While a further increase in air gap may have an opposite effect, with decreasing separation factor and increasing flux. These two factors compete with each other and determine the final separation performance of asymmetric singlelayer hollow fiber membranes. However, the situation is different for asymmetric dual-layer hollow fibers. As shown in Table 3, the hollow fiber spun at 2-cm air gap has both the highest separation factor and flux, while the hollow fiber extruded at an air gap distance of 0.5 cm has both the lowest flux and separation factor. These interesting phenomena are due to the fact that, different from

Table 3 Pervaporation performance of the PAI/PEI dual-layer hollow fiber and PAI single-layer hollow fiber membranes. Membrane

Air gap

PAI/PEI dual-layer hollow fiber

4 cm 2 cm 0.5 cm

Single-layer PAI hollow fiber

5 cm a

1.8 cm a

Permeate (H2 O wt.%)

Total flux (g/m2 h)

Separation factor

HF-1 HF-2 HF-1 HF-2 HF-1 HF-2

99.32 99.18 99.38 99.56 98.79 99.11

624 670 786 758 581 523

838 668 974 1296 460 624

HF-1 HF-2 –

54.73 56.37 69.8

801 777 815

Data is from the Ref. [11]. The feed composition and the operational conditions are exactly same as those in this study.

6.84 7.28 12

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Fig. 4. Different swelling degrees observed before and after pervaporation test of (A) PAI single-layer hollow fiber membranes and (B) PAI/PEI dual-layer hollow fiber membranes.

Table 4 The effect of thermal treatment temperature on the pervaporation performance. Thermal treatment

Permeate (H2 O wt.%)

Total flux (g/m2 h)

Separation factor

no

HF-1 HF-2

99.38 99.56

786 758

974 1296

75 ◦ C

HF-1 HF-2

99.73 99.68

720 810

1998 1858

150 ◦ C

HF-1 HF-2

99.46 99.62

492 442

1012 1527

Table 5 Dehydration of isopropanol using polymeric membranes. Membrane

Feed composition (IPA wt.%)

Temperature (◦ C)

Flux (kg/m2 h)

Composite membrane of PVA-PSSA on PAN support, heat curing 120 ◦ C, 2 h, followed by ionic cross-link Chitosan cross-linked with glutaraldehyde Chitosan, cross-linked with glutaraldehyde and surface carboxylation by maleic anhydride Chitosan with polyethersulfone support, cross-linked by sulphuric acid 1 hr Chitosan, with polysulfone support binded with PVA PVA/chitosan (2:8), thermal cross-linked by glutaraldehyde PVA/chitosan (2:8), cross-linked by urea–formaldehyde in sulfuric acid (UFS) PVA/chitosan (6:4), cross-linked by urea–formaldehyde in sulfuric acid (UFS) PSSA-g-PTFE/chitosan composite membrane with GPTMS as cross-linker Na-Alg, cross-linked by glutaraldehyde + HCl Na-Alg/PVA (1:1), cross-linked by glutaraldehyde + HCl PVA, cross-linked by glutaraldehyde + HCl PVA-NaAlg/polysulfone composite hollow fiber P84 dense film, thermal treated at 250 ◦ C 24 h P84 asymmetric membrane, thermal treated at 250 ◦ C, 6h P84 asymmetric membrane, cross-linked by p-xylenediamine, 1 h P84 asymmetric membrane, cross-linked by p-xylenediamine 2 h, thermal treated at 200 ◦ C P84 asymmetric membrane PAN membranes doped with poly(acrylic acid) PVA cross-linked by citric acid polyelectrolyte complex membrane based on cellulose sulfate (SYMPLEX) PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C, 2 h

96%

60

0.15

90% 90%

60 60

85%

Separation factor

Reference

13,900

[21]

0.197 0.211

196 366

[22] [22]

80

1.6

200

[23]

95% 90%

50 60

0.8 0.644

400 900,000

[24] [25]

90%

30

0.113

17,991

[26]

90%

30

0.214

6,419

[26]

90%

25

0.409

1,490

[27]

90% 90% 90% 85% 85% 85%

50 50 50 45 60 60

0.021 0.063 0.095 0.75 0.064 0.432

81.1 47.3 16.7 1,300 2,908 3,866

[28] [28] [28] [29] [7] [7]

85%

60

1.105

980

[6]

85%

60

0.335

1,733

[6]

85% 90% 90% 90%

60 80 30 25

2.578 0.3 0.053 0.2

16 10,000 291 3,000

85%

60

0.765

1,944

[6] [30] [31] [32] This work

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the single-layer hollow fiber spinning, the two layers in the duallayer spinning experience different shear and elongational stresses because they have quite different compositions and viscosities. Since the outer layer has a higher viscosity, it will bear more loads (i.e., elongational stresses). Thus it can be stretched, oriented, and become thinner simultaneously. This is the case for the hollow fiber spun at 2-cm air gap. Its dense-selective layer becomes thinner and oriented with greater selectivity and flux. However, for the hollow fiber extruded at 0.5-cm air gap, its dense-selective layer may be overstretched and its substructure may be tightened because of greater thinning in its diameter during the elongation. The former may create defects and reduce separation factor, while the latter may induce inward tightening, and thus substructure resistance is increased and flux is lowered. Table 4 shows the pervaporation performance of PAI/PEI dual-layer hollow fibers after thermal treatments at various temperatures. The flux declines with an increase in annealing temperature, while the separation factor increases initially and later decreases. The enhanced separation factor is due to defect reduction and/or the polymer chain rearrangement toward a denser and closer packing facilitated by the thermal-induced chain relaxation. However, the chain relaxation and rearrangement not only can eliminate the membrane defects on the dense-selective layer but also increase its thickness and substrate resistance. As a result, a further increase in thermal treatment temperature may lower both separation factor and flux. The dual-layer hollow fiber membranes thermally treated at 75 ◦ C for 2 h show the highest separation performance. The separation factor can reach about 2000 with a considerably high flux of about 765 g/m2 h for the IPA/water binary system. Table 5 compares the current work (i.e., the one after 2-hr thermal treatment at 75 ◦ C) with literature data for the dehydration of IPA [6,7,23–34], while Fig. 5 shows the comparison graphically. The PAI/PEI duallayer hollow fiber membrane has a much better performance than most other polymeric membranes. 3.3. The dehydration performance of C1–C4 alcohols Pervaporation dehydration of other C1–C4 alcohols has also been investigated by the newly developed PAI/PEI dual-layer hollow fiber membranes. Table 6 tabulates their performance. The newly developed membranes have impressive separation performance, especially for IPA and butanols aqueous systems. Table 7 and Fig. 6 show the comparison with literature data from polymeric membranes [5,23,31,32,34–41] for butanol dehydration. The dual-layer hollow fiber membrane developed in this study shows comparable or superior performance. For the dehydration of isobutanol and tert-butanol, our membrane outperforms others. The water concentration in the permeate can reach more than 99.99 wt.% with flux greater than 700 g/m2 h.

Fig. 5. Graphical representation of membrane performance data for isopropanol dehydration as presented in Table 5 (the number near the data point refers to the cited reference, while “*” represents the results in this study).

Table 6 shows the order of separation factor as follows: t-BuOH ≈ i-BuOH > IPA ≈ > 2-BuOH > 1-BuOH > 1-PrOH > EtOH > MeOH

(5)

The sequence of total flux is as follows: MeOH > 1-PrOH ≈ > 1-BuOH > IPA ≈ > 2-BuOH ≈ > i-BuOH ≈ > t-BuOH > EtOH

(6)

Generally, the order of separation factor of dehydrating water/alcohol mixtures may follow the order of the size of alcohol molecules while the flux may be in the opposite trend. However, the above experimental results show different trends. Previous studies [10,34,42] on pervaporation dehydration of different alcohols also reported a similar conclusion as ours. Qiao et al. [42] investigated PERVAP 2510 membranes for dehydration of isopropanol and butanols and found the total flux followed the order of 1BuOH > 2-BuOH > IPA > t-BuOH. A study by Jiang et al. [10] showed that 1-BuOH/water had a higher total flux but a lower separation factor than IPA/water. Scharnagl et al. [34] also reported that the total flux for alcohol/water separation followed the order of 1BuOH > EtOH > IPA while the separation factor obeyed an opposite order.

Table 6 Comparison of PAI/PEI dual-layer hollow fiber for pervaporation dehydration of various alcoholsa . Feed composition

Permeate (H2 O wt.%)

Total flux (g/m2 h)

MeOH/H2 O EtOH/H2 O 1-PrOH/H2 O IPA/H2 O 1-BuOH/H2 O 2-BuOH/H2 O i-BuOH/H2 O t-BuOH/H2 O

45.5 89.3 99.2 99.7 99.4 99.6 >99.99 >99.99

1033 659 862 765 846 744 730 721

a

Separation factor 4.71 50 753 1,944 1,174 1,801 >56,000 >56,000

All membranes are thermal treated at 75 ◦ C for 2 h.

Total permeance (g/m2 h kPa)

Alcohol permeance (g/m2 h kPa)

Water permeance (g/m2 h kPa)

Gram-based selectivity

73.7 52.5 53.9 51.5 48.7 43.8 40.3 40.9

8.47 1.99 0.45 0.08 0.85 0.23 <0.0075 <0.0025

65.3 50.5 53.4 51.4 47.9 43.5 40.3 40.9

8 25 119 644 56 191 >5,365 >16,510

Mole-based selectivity 14 65 395 2,147 231 785 >22,056 >67,874

Y. Wang et al. / Journal of Membrane Science 326 (2009) 222–233

229

Table 7 Dehydration of butanols using polymeric membranes. Membrane

Feed composition (butanol wt.%)

Temperature (◦ C)

Flux (kg/m2 h)

Composite membrane of PVA–PSSA on PAN support, heat curing 120 ◦ C 2 h, followed by ionic cross-link PVA–NaAlg/polysulfone composite hollow fiber ceramic supported PVA hollow fiber cross-linked by maleic anhydride ceramic supported PVA hollow fiber cross-linked by maleic anhydride PVA cross-linked by citric acid Pervap® 2510 polyelectrolyte complex membrane based on cellulose sulfate (SYMPLEX) PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C 2 h PVA/nylon66 blend membrane cross-linked by glutaraldehyde PVA cross-linked by glutaraldehyde chitosan/hydroxyethylcellulose (7:3) blend membrane cross-linked by glutaraldehyde Pervap® 2510 PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C 2 h Composite membrane of PVA–PSSA on PAN support, heat curing 120 ◦ C 2 h, followed by ionic cross-link PVA cross-linked by citric acid Pervap® 2510 PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C 2 h Pervap® 2510 Pervap® 2510 Composite membrane of PVA–PSSA on PAN support, heat curing 120 ◦ C 2 h, followed by ionic cross-link PVA–NaAlg/polysulfone composite hollow fiber NaAlg/hydroxyethylcellulose blend membrane ionically cross-linked by phosphoric acid Matrimid dense film, thermal treated at 250 ◦ C 2 h Matrimid asymmetric membrane Matrimid/PBI (3.55% PBI) blend asymmetric membrane, thermal treated at 250 ◦ C 6 h Matrimid/PBI (3.55% PBI) blend dense film, thermal treated at 250 ◦ C 7 h PAI/PEI dual-layer hollow fiber, thermal treated at 75 ◦ C 2 h

1-BuOH 95%

60

0.23

1-BuOH 85% 1-BuOH 87%

45 70

1-BuOH 91%

Separation factor

Reference

18,600

[21]

1.2 3.9

100 16

[29] [33]

80

2.4

1,600

[33]

1-BuOH 90% 1-BuOH 85% 1-BuOH 90%

30 60 25

0.082 1.43 1.7

171 60 300

[31] [34] [32]

1-BuOH 85% 2-BuOH 85%

60 30

0.846 1.2

1,174 94

2-BuOH 85% 2-BuOH 85%

30 40

1 0.27

2-BuOH 87% 2-BuOH 85% i-BuOH 95%

60 60 60

1.35 0.744 0.2

272 1,801 32,800

[34] This study [21]

i-BuOH 90% i-BuOH 87% i-BuOH 85% t-BuOH 85% t-BuOH 87% t-BuOH 95%

30 60 60 60 60 60

0.052 2.25 0.73 0.78 1.35 0.21

351 376 >56,000 1,466 420 17,900

[31] [34] This study [37] [34] [21]

t-BuOH 85% t-BuOH 88%

45 30

0.75 0.197

620 3,237

[29] [38]

t-BuOH 85% t-BuOH 85% t-BuOH 85%

60 60 60

0.057 0.63 0.38

141 491 340

[5] [5] [39]

t-BuOH 85%

60

0.097

3,698

[39]

t-BuOH 85%

60

0.721

>56,000

78 650

This study [35] [35] [36]

This study

Fig. 6. Graphical representation of membrane performance data for the dehydration of (A) 1-butanol, (B) 2-butanol, (C) i-butanol and (D) t-butanol as presented in Table 7 (the number near the data point refers to the cited reference, while “*” represents the results in this study).

230

Y. Wang et al. / Journal of Membrane Science 326 (2009) 222–233

Table 8 List of activity coefficients and partial vapor pressures at feed side of water and alcohols at 60 ◦ C for different alcohol/water systems (water concentration = 15 wt.%). Feed

Water activity coefficient  2

MeOH/H2 O EtOH/H2 O 1-PrOH/H2 O IPA/H2 O 1-BuOH/H2 O 2-BuOH/H2 O i-BuOH/H2 O t-BuOH/H2 O

1.513 1.879 2.167 2.009 2.096 2.030 2.160 2.102

Water saturated pressure p2 sat (kPa)

Water partial vapor pressure f2 (kPa)

Alcohol activity coefficient  1

Alcohol saturated pressure p1 sat (kPa)

Alcohol partial vapor pressure f1 (kPa)

19.94

7.20 11.65 16.01 14.84 17.57 17.02 18.11 17.63

1.033 1.092 1.195 1.182 1.281 1.246 1.362 1.300

84.60 47.08 20.35 38.66 8.04 18.07 12.31 38.64

66.49 35.42 15.31 28.77 5.97 13.04 9.72 29.10

3.4. Correlations among flux, permeance and physicochemical properties of penetrants The causes for a low flux in the EtOH/water system and a higher flux in the 1-BuOH/water system than in the IPA/water system, as ranked in Eq. (6), may be explained by the definition of flux and permeance. According to Eq. (3), the flux is a product of permeance and driving force. Ji =

P  i

l

(xn,i i psat i

p

− yn,i p )

Since the downstream pressure pp is generally kept near 0, the flux is affected by the partial vapor pressure of each component at the feed side fi (equals to xn,i  i pi sat ), which includes (1) mole fraction of the component in the feed xn,i (fixed at alcohol/water 85/15 wt.% in this study), (2) the activity coefficient  i , and (3) partial saturated pressure pi sat . The values of  i and pi sat for different water/alcohol systems can be calculated by the software Aspen DISTIL 6.2 and their values are listed in Table 8. Because both water activity coefficient and partial water vapor pressure in the EtOH/water systems are much lower than those in the propanol/water and butanol/water systems, the former has a lower driving force of water across the membrane than the latter. This leads to a lower flux of the EtOH/water system. Similarly, because both water activity coefficient and partial water vapor pressure in the 1-butanol/water systems are higher than that in the IPA/water system, the total flux of the 1-BuOH/water system is higher than the IPA/water system. In order to investigate the causes of discrepancy in ranking the separation performance, we convert flux and separation factor to permeance and selectivity, respectively. It has been proved that the later two terms can significantly decouple the effect of operating conditions on performance evaluation, and may truly quantify the intrinsic contribution by the membrane to separation performance [36,43]. Table 6 displays the obtained permeance and selectivity, and their rankings are as follows:

Total permeance: MeOH > 1-PrOH ≈ > EtOH ≈ > IPA > 1-BuOH > 2-BuOH > t-BuOH ≈ > i-BuOH

(7)

Selectivity: t-BuOH ≈ > i-BuOH > IPA > 2-BuOH > 1-PrOH > 1-BuOH > EtOH > MeOH

(8)

The order of permeance for each individual penetrant in aqueous C1–C4 alcohol systems is as follows: Permeance of alcohols: MeOH > EtOH > 1-BuOH > 1-PrOH > 2-BuOH > IPA > i-BuOH ≈ > t-BuOH

(9)

Permeance of water in aqueous C1–C4 alcohol systems: MeOH > 1-PrOH > IPA ≈ > EtOH > 1-BuOH > 2-BuOH > t-BuOH ≈ > i-BuOH

(10)

Compared to the total flux ranking in Eq. (6), the ranking of permeance seems to be more straightforward. However, it is still very challenging to explain the specific ranking of each penetrant in the above equations. Table 6 suggests that the ranking of permeance of alcohols still does not follow the order of molecular size. The linearity of alcohol molecules also plays an important role because it correlates very well with the permeate results. In C3 and C4 alcohols, the more linear the molecular shape, the easier for them to pass through the membrane. This is especially true for the coupling dominated transport. Since 1-butanol and 2-butanol are of higher linearity than 1-propanol and isopropanol, respectively, their permeances are also enhanced accordingly. The ranking of water permeance is more complicated than the previous cases. The EtOH/water system has a lower water permeance than the two aqueous propanol isomer systems. Table 9

Table 9 The physicochemical properties of alcohols and water.

1-BuOH 2-BuOH i-BuOH t-BuOH 1-PrOH IPA EtOH MeOH Water a b

Boiling points (◦ C)

Density (g/cm3 ) [42]

Molecular weight (g/mol)

Molecular volume (Å3 )a

Radius of gyration (Å)b

Kinetic diameter (nm) [44]

Average dynamic cross-section [42]

ET (30) (kcal/mol) [45]

Solubility parameter (J/cm3 )1/2 [46]

117.8 94.0 107.9 82.4 97.2 82.3 78.3 64.7 100.0

0.810 0.808 0.802 0.781 0.803 0.785 0.789 0.792 1

74.1 74.1 74.1 74.1 60.1 60.1 46.1 32.0 18.0

151.8 152.1 153.3 157.4 124.1 127.0 96.8 67.1 29.9

3.251 3.203 3.332 3.067 2.825 2.807 2.259 1.552 0.615

0.505 0.504 0.504 0.506 0.469 0.470 0.430 0.380 0.296

1.22 1.48 1.79 2.04 1.18 1.38 1.05 0.92 –

49.7 47.1 48.6 43.3 50.7 48.4 51.9 55.4 63.1

23.1 22.2 22.7 21.8 24.5 23.5 26.5 29.6 47.8

The molecular volume is calculated by the molecular weight divided by the density and the Avogadro number [43]. The radii of gyration are calculated using AspenTech DISTIL (version 6.2).

Y. Wang et al. / Journal of Membrane Science 326 (2009) 222–233

231

Table 10 The solubility parameters of alcohols and water.

Water MeOH EtOH 1-PrOH IPA 1-BuOH 2-BuOH i-BuOH t-BuOH

ısp (MPa)1/2

ıd (MPa)1/2

ıp (MPa)1/2

ıh (MPa)1/2

ij

R (MPa)1/2

47.8 29.6 26.5 24.5 23.5 23.1 22.2 22.7 21.8

15.5 15.1 15.8 16.0 15.8 16.0 15.8 15.1 15.2

16.0 12.3 8.8 6.8 6.1 5.7 5.7 5.7 5.1

42.3 22.3 19.4 17.4 16.4 15.8 14.5 16.0 14.7

– 20.36 24.01 26.56 27.73 28.45 29.65 28.26 29.68

All ıd , ıp and ıh values are taken from Ref. [46].

compares the molecular properties of water and different alcohols, but the order of water permeance in these alcohol/water systems cannot be easily interpreted by the molecular properties of alcohols, empirical solvent polarity parameter ET (30) [47], solubility parameters [48], or the linearity effect. A previous study [42] attempted to correlate the alcohol flux to its coupling effect with water, and reported that closer solubility parameters and higher linearity may enhance the coupling effect. The “solubility parameter distance”, ij R2 , as defined by Eq. (11) [48] is therefore calculated: ij 2

2

2

R = 4(i ıd − j ıd ) + (i ıp − j ıp ) + (i ıh − j ıh )

2

(11)

where ıd , ıp and ıh are the dispersion cohesion, polar cohesion and hydrogen bonding cohesion parameter, respectively. Table 10 summarizes their values and the total cohesion solubility parameters ısp . However, since ethanol is neither the smallest “solubility parameter distance” with water (lager than methanol) nor the highest linearity (lower than 1-propanol and 1-butanol), the previous explanation of coupling effect may not be very applicable here. More fundamental studies are needed to reveal the mystery. Fig. 7 shows the sorption results of these dual-layer hollow fibers in different liquids. Traditionally, dense homogeneous membranes are often used in sorption studies where the weight gains are mainly due to penetrant sorption between interstitial space among polymeric chains [7–9,24,27]. However, the sorption results for asymmetric membranes are much complicated. This is due to the fact that an asymmetric membrane consists of a thin dense selective layer and a porous substructure. Both layers may adsorb liquids, and they have different weight gains via different sorption mechanisms. Dependent on membrane morphology, most weight gain may occur at the large pores of the substructure.

Fig. 7. The sorption results of the dual-layer hollow fiber in DI-water, different alcohols and alcohol/water binary mixtures (85/15 alcohol/water).

Fig. 7 shows the sequence of weight gain (in g/g membrane) of dual-layer hollow fibers in different solutions as follows: Weight gain (g/g membrane) in alcohol/water mixtures (85/15 in wt.%): EtOH/H2 O > 1-BuOH/H2 O > IPA/H2 O > t-BuOH/H2 O

(12)

Weight gain (g/g membrane) in pure liquids: MeOH > 1BuOH > EtOH > H2 O > IPA > t-BuOH

(13)

The above order corresponds somewhat consistently with the flux order for alcohol/water separation (as shown Eq. (6)) but not all. Both 1-butanol aqueous solution in Eq. (12) and 1-butanol in Eq. (13) show abnormal high sorption, which may be due to the favorable coupling effect between 1-butanol and water. The pure water shows a lower weight gain than methanol, ethanol and 1butanol because the porous membrane structure may be filled with alcohols easier than water. 3.5. Long-term performance of the dual-layer hollow fiber in pervaporation tests Figs. 8 and 9 show the long-term stability of the PAI/PEI duallayer hollow fiber membranes for the dehydration of four aqueous alcohols (i.e., ethanol, isopropanol, 1-butanol and tert-butanol) up to 200 h. The water concentrations in the permeate and the total flux remain almost constant during the entire testing duration. No

Fig. 8. Long-term pervaporation performance (permeate composition) of PAI/PEI dual-layer hollow fiber for various alcohols’ dehydration. (B is the magnified part of A at the water concentration range 99.0–100.0 wt.%.)

232

Y. Wang et al. / Journal of Membrane Science 326 (2009) 222–233

May May, Dr. Jiang Lanying, Mr. Zheng Zhong Zhou, Mr. Sina Bonadi and Dr. Qiao Xiangyi for their kind help with this work.

Nomenclature ET (30) fi J l pp pi sat Pi [Pi /l] ij R2 xw,i yw,i Fig. 9. Long-term pervaporation performance (total flux) of PAI/PEI dual-layer hollow fiber for various alcohols’ dehydration.

obvious swelling of the hollow fiber was found. Here the separation performance is expressed in terms of water concentration in the permeate because the separation factors for IPA/water and butanol/water systems are very high (the water concentration in the permeate can reach more than 99 wt.%) thus their exact values cannot be obtained easily. Fig. 8B shows a magnified portion of Fig. 8A and indicates the water concentration in the permeate follows the order of t-BuOH > IPA > 1-BuOH > EtOH. Fig. 9 displays their fluxes that follow the order of 1-BuOH > IPA ≈ 1-BuOH > EtOH. The orders are consistent with the results obtained in the previous section. Clearly, the combination of Torlon® PAI as the outer selective layer and Ultem® PEI as the inner supporting layer with the aid of dual-layer membrane fabrication can synergize their performance for the dehydration of C2–C4 alcohols effectively. 4. Conclusion Novel PAI/PEI dual-layer hollow fiber membranes have been fabricated with the aid of dual-layer spinning technology and tested for pervaporation dehydration of C1–C4 alcohols. Compared to the single-layer PAI hollow fiber, the dual-layer hollow fiber membranes possess a much improved dehydration performance. The superior pervaporation performance can be attributed to (1) the balanced properties of PAI as the selective layer for dehydration pervaporation; (2) the low water uptake and less swelling characteristic of the PEI supporting layer; and (3) the unique membrane structure consisting of a fully porous inner layer, a porous interface, and an ultrathin dense-selective outer skin via molecular engineering of morphology and dual-layer co-extrusion. Dehydration of C1–C4 alcohols has been conducted and the orders of their fluxes and permeances have been analyzed. The newly developed PAI/PEI dual-layer hollow fiber membranes outperform most other polymeric membranes for the dehydration of IPA and butanols. In addition, the dual-layer hollow fiber membranes exhibit good long-term stability up to 200 h. Acknowledgements The authors would like to thank A-Star (R-398-000-044-305) and NUS (R-279-000-249-646) for funding this research work. We want to thank Prof. Xianshe Feng in University of Waterloo and Prof. Takeshi Matsuura in University of Ottawa for their valuable suggestions. Thanks also due to Dr. Li Yi, Dr. Wang Kaiyu, Dr. Teoh

xn,i yn,i

empirical solvent polarity parameter (k cal/mol) partial vapor pressure of component i at the feed side (kPa) permeate flux (g/m2 h) membrane selective layer thickness (␮m) permeate pressure (kPa) saturated vapor pressure (kPa) the membrane permeability (g/m h kPa) permeance (g/m2 h kPa) solubility parameter distance (MPa)1/2 weight fraction of component i in the feed (wt.%) weight fraction of component i in the permeate (wt.%) mole fraction of component i in the feed (mol.%) mole fraction of component i in the permeate (mol.%)

Greek symbols ˛ separation factor ˇ ideal membrane selectivity dispersion solubility parameter (MPa)1/2 ıd ıh hydrogen-bonding solubility parameter (MPa)1/2 ıp polar force solubility parameter (MPa)1/2 ısp total solubility parameter (MPa)1/2 activity coefficient i

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