Thin film composite membranes comprising of polyamide and polydopamine for dehydration of ethylene glycol by pervaporation

Journal of Membrane Science 493 (2015) 622–635

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Thin film composite membranes comprising of polyamide and polydopamine for dehydration of ethylene glycol by pervaporation Dihua Wu a, Jeff Martin a, Jennifer Du b, Yufeng Zhang b, Darren Lawless c, Xianshe Feng a,n a

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 State Key Lab of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin, China c Sheridan Institute of Technology, 1430 Trafalgar Road, Oakville, Ontario, Canada L6L 2L1 b

art ic l e i nf o

a b s t r a c t

Article history: Received 15 April 2015 Received in revised form 6 July 2015 Accepted 9 July 2015 Available online 11 July 2015

Thin film composite membranes comprising of a polyamide (PA) and polydopamine (PD) were prepared and studied for dehydration of ethylene glycol. The chemical composition, surface hydrophilicity and sorption uptake of permeant in the active skin layer of the membranes were characterized, and the effects of the number and sequence of the PD and PA sublayers in the membrane skin on the pervaporation performance were studied. It was shown that using 1 or 2 PD sublayers as a surface layer (i.e., on top of PA) or a transition layer between PA and the substrate would increase both permeation flux and selectivity for dehydration of ethylene glycol. Since inorganic salts are often present in spent glycol solutions in practical applications, the influence of inorganic salt in the feed on ethylene glycol dehydration was also studied using NaCl as a representative salt. The presence of NaCl in the feed solution enhanced the separation factor, while the permeation flux was reduced. Unlike pervaporative separation of binary water/ethylene glycol solutions where the separation factor was reduced at a higher temperature, the separation factor increased with an increase in temperature when NaCl was present in the feed. & 2015 Elsevier B.V. All rights reserved.

Keywords: Pervaporation Polyamide Polydopamine Ethylene glycol Dehydration

1. Introduction Polyamide-based thin film composite (TFC) membranes prepared by interfacial polymerization are generally used for reverse osmosis [1,2] and nanofiltration [3,4]. A great deal of research has been done recently to develop TFC membranes for pervaporation applications, especially for the dehydration of organic solvents. Current research efforts include synthesis and screening of suitable monomers for interfacial polymerization. For instance, Lai and co-workers [5] used ethylenediamine (EDA), m-phenylenediamine (MPD), piperaine (PIP) and 1,6-hexanediamine (HDA) as reactant amines for interfacial reaction with trimesoyl chloride (TMC) to produce membranes for isopropanol dehydration, and they reported that the EDA/TMC membrane showed the best pervaporation performance among the membranes produced with the various amines [5]. They also prepared polyamide TFC membranes by reacting 1,3-diaminopropane (DAPE), 1,3-cyclohexanediamine (CHDA) and MPD with TMC for use in dehydration of tetrahydrofuran, and the DAPE/TMC membrane was found to exhibit the desirable pervaporation performance [6]. In addition, n

Corresponding author. E-mail address: [email protected] (X. Feng).

http://dx.doi.org/10.1016/j.memsci.2015.07.016 0376-7388/& 2015 Elsevier B.V. All rights reserved.

novel amines (e.g., 2,2′-dimethylbenzidine hydrochloride (m-tolidine-H) [7]) and acyl chlorides (e.g., 5-nitrobenzene-1,3-dioyl dichloride and 5-tert-butylbenzene-1,3-dioyl dichloride [8]) have been synthesized to produce interfacially polymerized membranes for ethanol dehydration. Optimization of the membrane formation conditions during the procedure of interfacial polymerization is another subject of interest for TFC pervaporation membranes. The parameters commonly studied are the concentrations of amine and/or acyl chloride reactants, contact time between the two reactants [9–11], and annealing temperature and time [12,13]. In another approach, the surface coating method has also been used to fabricate membranes, and spin-coating is shown to be more favorable than a simple dip-coating for fabricating a thin layer with a dense structure [14]. To overcome the problems related to solvent-induced swelling of the membranes, inorganic components, cross-linkers or nanoparticles, may be introduced into the polyamide skin layer during interfacial polymerization. Chung and co-workers incorporated 3-glycidyloxypropyl trimethoxysilane [15] and nonafluoro hexylmethyl dichlorosilane [16] into the polyamide selective layer during interfacial polymerization. They also grafted in situ toluene 2,4-diisocyanate (TDI) [17] as a crosslinker into the polyamide to produce TFC membranes crosslinked with TDI. Fathizadeh et al. [18] incorporated NaX zeolite nanoparticles into the polyamide active layer to improve the

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

dehydration performance of TFC membranes. Besides developing new reactant monomers or tailoring interfacial polymerization conditions, an alternative approach is modifications of conventional polyamide TFC membranes for pervaporation applications. Albo et al. [19] treated commercial reverse osmosis membranes by subjecting to different solvent immersion and drying processes, and the membranes were evaluated for pervaporation dehydration of isopropanol. Xu et al. [20] assembled a thin layer of polyelectrolytes onto an interfacially polymerized polyamide membrane to take advantage of its charged surface for dehydration of ethylene glycol, and stability of the polyelectrolyte membranes was further improved by Zhang et al. [21]. In the present study, an attempt was made to modify polyamide nanofiltration membranes for pervaporation uses. Surface coating is a simple method for surface modification. Inspired by adhesive proteins secreted by mussels for attachment to wet surfaces [22], polydopamine has been used extensively in surface coatings due to its good adhesion to a broad range of surfaces and its good stability and durability in various solution environments (except at strong alkaline conditions (pH 4 13)) [23–25]. The good adhesive property results from its ability to self-polymerize at an oxidative and slightly basic pH conditions. In membrane fabrication, polydopamine can be used either to form a selective skin layer or merely for surface modifications. Composite membranes produced by simply coating polydopamine onto a substrate have been used for the dehumidification of propylene gas [26], separation of thiophene from octane by pervaporation [27] and salt removal from water by nanofiltration [28]. The fouling resistance of commercial reverse osmosis membranes [29,30] and the permeation flux of commercial ultrafiltration membranes [24] were reported to have been improved by surface coating of polydopamine. In this work, we report on TFC membranes comprising of polyamide and polydopamine for use in pervaporation. The polydopamine can be an outer surface layer if it is deposited onto a polyamide layer pre-formed by interfacial polymerization. It may also act as a gutter layer between the substrate and the polyamide if it is deposited prior to formation of the polyamide layer by interfacial polymerization. This approach has several potential advantages: (1) the process of interfacial polymerization for the polyamide layer formation has been studied in details in our previous study [3,4] and thus the properties of the polyamide layer can be easily tailored, (2) polyamide formation by interfacial polymerization and dopamine deposition by self-polymerization can be carried out independently, (3) the polydopamine deposition is easy to accomplish which does not need any catalyst, organic solvent or rigorous reaction conditions, (4) the catechol groups of dopamine can react with amines under oxidizing conditions via the Michael addition or Schiff base reactions [31,32], which will enhance the anchoring and adhesion between the polydopamine and polyamide sublayers, thereby improving the membrane stability. The effects of the number and sequence of the polydopamine depositions on the pervaporation performance of the resulting polyamide/polydopamine composite membranes were studied in this work. The separation performance of the formed membranes was evaluated for pervaporative dehydration of ethylene glycol. Ethylene glycol is commercially produced from hydrolysis of ethylene oxide in the presence of an excess amount of water (20–25 M ratio of water/ethylene glycol [33,34]). It is widely used as a raw material for polyester manufacture (99.9 wt% [35]), antifreeze in automobiles (95.0 wt% [35]), deicing agent for aircrafts and absorbent to scrub water vapor in natural gas industry, where it is of interest to separate water from the spent ethylene glycol for regeneration and reuse. Although ethylene glycol does not form an azeotrope with water, the separation of water from ethylene glycol by such thermal processes as multi-stage evaporation or

623

distillation is energy intensive because of its high boiling point (197.3 °C). From an energy consumption standpoint, pervaporation will compete favorably, especially at relatively low water concentrations in the feed. In this study, the effects of feed water concentration and temperature on the pervaporation performance were investigated. For gas processing and many other applications, the spent ethylene glycol often contains inorganic salts (e.g., 110– 800 mg/L of sodium [36]), and thus the pervaporative dehydration of ethylene glycol in the presence of inorganic salts was studied as well. For this purpose, NaCl was used as a representative salt in this study, and the influence of salt contents in the feed on the separation performance of the membrane was investigated.

2. Experimental 2.1. Materials Microporous polyethersulfone (PES) membrane (supplied by Sepro Membranes) with a molecular weight cut-off of 10,000 was used as a substrate. The substrate membrane had a water permeability of approximately 90 L/(m2 h bar). Branched polyethylenimine (PEI, Mn 10,000 and Mw 25,000), dopamine hydrochloride and tris(hydroxylmethyl)aminomethane (Tris) were purchased from Sigma-Aldrich. Trimesoyl chloride (TMC) and hexane were supplied by Alfa Aesar and Caledon Laboratories, respectively. Ethylene glycol was purchased from VWR International. All these chemicals were of reagent grades. Aqueous solutions of ethylene glycol used as feeds in the pervaporation experiments were prepared by blending ethylene glycol with de-ionized water at pre-determined concentrations. 2.2. Membrane preparation The composite membranes consisted of polyamide and polydopamine sublayers. The polyamide sublayer was formed by interfacial polymerization from polyethylenimine (PEI) and trimesoyl chloride (TMC) at a PEI concentration of 4.0 wt% and a TMC concentration of 0.8 wt%. The procedure of interfacial polymerization to form a polyamide layer has been described elsewhere [3]. A possible mechanism for oxidative self-polymerization of dopamine has been described elsewhere [24,28]. The polydopamine layer was formed by oxidative self-polymerization of dopamine using a 0.4 wt% dopamine solution freshly prepared by dissolving dopamine in a 15 mM Tris buffer solution (pH ¼ 8.8). If the polydopamine sublayer was to be formed on the top of the polyamide layer, a dopamine deposition time of 24 h was used; if the polydopamine layer was to be used as a gutter layer between the PES substrate and a polyamide layer, a deposition time 5 h was used. For both cases, the membrane was rinsed thoroughly with de-ionized after a polydopamine sublayer was formed, followed by heat treatment at 75 °C for 20 min. Fig. 1 illustrates the process of membrane synthesis with sequential deposition steps for polydopamine and polyamide formation by self-polymerization and interfacial polymerization, respectively. It should be mentioned that only the surface side of the PES was allowed to contact the deposition solutions so that the microporous substructure of the substrate would not be blocked by the macromolecules. For convenience, the polyamide and polydopamine sublayers are denoted as “PA” and “PD”, respectively. Based on the sequence and number of depositions in fabricating the membrane sublayers, the designations of the membranes used in this study are shown in Table 1.

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D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

Fig. 1. Schematic diagram showing the procedure to prepare thin film composite membrane [PD]2–[PA]–[PD]2 by polydopamine deposition and interfacial polymerization.

2.3. Membrane characterization The chemical structures of the membrane surface were examined using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Nicolet Aratar 370 FTIR spectrometer). For ATR-FTIR analysis of the membrane samples, ZnSe crystal at a 45° angle of incidence was used. The resolution of the apparatus was 4 cm  1, and a total of 32 scans were recorded during the IR test for each sample. The surface morphologies of the composite membranes were examined using a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM). Contact angle measurements were performed using a contact angle meter (Cam-plus Micro, Tantec Inc.). The sessile drop method was used to measure the contact angles of both de-ionized water and ethylene glycol (about 3 μl) on the surfaces of dried membrane samples at 25 °C. At least 10 measurements on different locations of a single membrane sample were performed, and the results presented were an average of the measured values. The sorption uptakes of pure water and pure ethylene glycol in the active layer of the composite membrane were measured to

study the effect of preferential sorption on the pervaporation performance. After drying in a vacuum oven at 80 °C for 1 day, the PES substrate (weight W1) and the composite membrane (weight W2) samples of the same area were immersed in the same liquid at room temperature to reach sorption equilibrium. Then the weights of the membrane samples (W3 for the substrate and W4 for the composite membrane) were determined quickly after gently blotting away the excess liquid on the surface. Since the weight of the dry skin layer (W2  W1) was very small and could not be accurately determined, the swelling degree of skin layer by the liquid sorbent, which was equal to [(W4  W3) (W2  W1)]/(W2  W1), was difficult to evaluate. It was thus decided to use the water to ethylene glycol sorption uptake ratio (mol/mol) [which is equal to (62/18)(W4  W3)water/(W4  W3)glycol] to measure the selective sorption of the two liquid in the membrane. The variation in water/ethylene glycol sorption uptake ratio in the porous substrate was found to be smaller than 3% by measuring 6 PES samples with the same area. Therefore, the liquid uptake in the porous substrate of the composite membrane was rightfully separated because it was the permeant sorption in the active skin layer of the

Table 1 Designation of membranes based on the sequence and number of the depositions. Number of depositions Membrane designation Description 0 1 2 3 4 5

PES [PA] [PD] [PA]–[PD] [PA]–[PD]2 [PD]–[PA]–[PD]2 [PD]2–[PA]–[PD]2

PES substrate One ply of polyamide formed on the substrate One ply of polydopamine formed on the substrate One ply of polyamide and one ply of polydopamine formed on the substrate sequentially One ply of polyamide and two plies of polydopamine formed on the substrate sequentially One ply of polydopamine, one ply of polyamide and two plies of polydopamine formed on the substrate sequentially Two plies of polydopamine, one ply of polyamide and two plies of polydopamine formed on the substrate sequentially

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

membrane that was relevant to pervaporation. Note the molar sorption uptake ratio was used as it characterizes the solubility selectivity pertaining to the membrane permeability.

0

3

4

5

6

wt% H 2 O in Permeate

90

The experimental set up and procedure for pervaporation have been described before [21]. The membrane was mounted in the membrane cell with an effective permeation area of 21.2 cm2. The feed solution was pumped from the feed tank to the membrane surface, and the retentate was circulated back to the feed tank. The permeate side was evacuated, and the permeate pressure was maintained below 1.7 kPa absolute. The permeate vapor was condensed and collected in a cold trap immersed in liquid nitrogen. At a given temperature and feed composition, the permeation flux (J) and separation factor (α) were measured:

Q A Δt

2

100

2.4. Pervaporation

J=

1

625

80

70 Membranes: 60

[PD]

1: [PA] 2: [PA]-[PD] 3: [PA]-[PD]2

membrane

50

(1)

4: [PD]-[PA]-[PD] 2 5: [PD]2-[PA]-[PD]2

Y /(1 − Y ) X /(1 − X )

40

(2)

where Q is the mass of the permeate sample collected over a time interval △t, A is the effective area of the membrane for permeation, and X and Y are the mass fractions of water in the feed and permeate, respectively. The permeate composition was determined with a refractometer. The partial permeation fluxes of water and ethylene glycol can be calculated readily from the total permeation flux and the permeate composition. The permeation was considered to have reached steady state when the permeation flux and permeate composition became constant, and the steady state of permeation was generally attained within 3 h after a pervaporation run was initiated. The effects of feed water concentration (0.5–20 wt%) and operating temperature (25–55 °C) on the membrane performance were studied. The operating temperature was controlled using a thermal bath. In addition, the influence of inorganic salt present in the feed solution on the performance of pervaporative dehydration of ethylene glycol was also investigated. In the latter case, after a pervaporation run was completed, the membrane was thoroughly washed by circulating pure water on the feed side for 2 h, followed by pervaporation of pure water experiment at room temperature for 3 h to wash away any salt from the membrane. The pervaporation data reported were an average value of at least two measurements, and the experimental errors in the flux and permeate composition measurements were within 5%.

3. Results and discussion 3.1. Importance of polydopamine sublayer to pervaporation performance Fig. 2 shows the water concentration in the permeate and the total permeation flux of the thin film composite membranes comprising of a polyamide and polydopamine sublayers for the separation of water from ethylene glycol at 38 °C at a feed water concentration of 9.5 wt%. The membrane [PA] exhibited a total permeation flux of 100 g/(m2.h) and a permeate water concentration of 51 wt%. This membrane was prepared under conditions appropriate for nanofiltration [3,4] and its skin layer was not dense enough to yield a high selectivity in pervaporation. The pervaporation performance of the [PD] membrane is also shown in Fig. 2, and this membrane exhibited a much higher flux of 430 g/ (m2 h) and an improved permeate water concentration of 63 wt%.

Total Permeation Flux (g/m 2 .h)

α=

500 480 460 440 420 400

[PD] membrane

260 240 220 200 180 160 140 120 100 80 0

1

2

3

4

5

6

Number of Layers Deposited Fig. 2. Effects of number and sequence of PA and PD sublayers deposited in the membrane (as shown in Table 1) on (a) water concentration in permeate and (b) total permeation flux. Feed composition: 9.5 wt% waterþ 90.5 wt% ethylene glycol, temperature: 38 °C.

Interestingly, adding an additional polydopamine sublayer on the membrane surface improved the membrane selectivity substantially. For instance, when the [PA] membrane was deposited with one sublayer of polydopamine (i.e., membrane [PA]–[PD]), the water content in the permeate increased to 85 wt%; the permeate water concentration was further increased when 2 sublayers of polydopamine were deposited on the surface of the polyamide membrane. It is now clear that the either PA or PD sublayer alone is incapable of yielding a good selectivity for pervaporation, and the selectivity of the membrane is mainly derived from the polydopamine sublayer. On the other hand, the substrate used for preparing the polyamide nanofiltration membranes was a microporous PES ultrafiltration membrane, and the multi-sublayers in the membrane skin were not really a distinct layer-overlayer structure, rather there would be significant interpenetrations

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

0

1

2

3

4

5

6

80

70

Contact Angle ( o )

between the sublayers. It was thus thought that if the big pores on the PES substrate could be decreased by depositing a layer of polydopamine as a gutter layer between the substrate and the interfacially formed polyamide sublayer, a further improvement in the pervaporation performance might be achieved. This was found to be indeed the case, as demonstrated by membranes [PD]–[PA]– [PD]2 and [PD]2–[PA]–[PD]2 which showed a permeate water concentration of 92 wt% and 96 wt%, respectively [see Fig. 2(a)]. It may be pointed out that due to its rigid supramolecular structure [23,24,26,30], while polydopamine can be hydrated in aqueous solutions (which is desirable for pervaporative dehydration of solvents), polydopamine films may crack upon drying under high internal stresses [37]. Therefore, instead of forming a thicker [PD] layer on top of a [PA] sublayer, the [PA] sublayer was sandwiched by the [PD] sublayers in the above membranes in anticipation that this would improve the membrane stability. It is interesting to note from Fig. 2(b) that the permeation flux increased when the polydopamine sublayer was incorporated into the membrane either as a gutter layer for polyamide formation or as an outer surface layer. Based on the solution-diffusion model, the permeability of a component (i.e., water or ethylene glycol) is affected by both the solubility in the membrane and the molecular diffusion through the membrane. Each polydopamine sublayer deposited onto the membrane will increase the diffusional path for the permeant. However, as shown in Fig. 3(a), the contact angles of water and ethylene glycol on the membrane surface decreased by the deposited polydopamine sublayer, indicating improved affinity between the membrane surface and the permeant, which favors the sorption of the permeant onto the membrane. Thus, the increased diffusion path may be compensated by the enhanced solubility for permeation in the membrane. Similar phenomena were also observed by Xu et al. [20] when they assembled fewer than 3 polyelectrolytes double layers onto an interfacially polymerized polyamide membrane. For a given membrane, ethylene glycol had a lower contact angle on the membrane surface than water [see Fig. 3(a)]. This is not surprising in view of its strong hydrogen bonding due to the hydrophilic –OH groups. Similar results have been reported for poly(N,N-dimethylaminoethyl methacrylate) membranes [38]. However, the sorption uptake into the membrane matrix, which is a bulk property of the membrane-permeant system, is affected not only by the affinity between the membrane surface and the permeant, the molecular size and shape are also important [39]. In addition, unlike water which has no intramolecular hydrogen bonding, there is a competition between intermolecular and intramolecular interactions (hydrogen bonds) in ethylene glycol. The hydrophobic alkyl chain of ethylene glycol disfavors the solubility of ethylene glycol in the membrane. The sorption uptake data in Fig. 3(b) shows that increasing polydopmine depositions in the membrane led to an increased sorption selectivity of water over ethylene glycol. On the other hand, as one may expect that from a diffusion point of view, the membrane also favored the permeation of water over ethylene glycol because of the larger molecular size and intermolecular hydrogen bonding that would affect molecular motion of ethylene glycol. Fig. 4(a) shows that the partial fluxes of water and ethylene glycol in the [PA] membrane are very close; with an increase in the [PD] sublayers incorporated into the membrane, water flux became substantially higher than the ethylene glycol flux. The corresponding separation factor of the membranes for dehydration of ethylene glycol is shown in Fig. 4(b). Clearly, the [PA] membrane was not selective enough for pervaporation uses, and an increase in the membrane permselectivity was achieved by the deposition of the [PD] sublayers either as an outer surface layer or as a gutter layer between the substrate and the [PA] sublayer. At a feed water concentration of 9.5 wt%, a separation factor of

60

50

Water

40 Membranes: 1: [PA] 2: [PA]-[PD] 3: [PA]-[PD]2

30

Ethylene Glycol

4: [PD]-[PA]-[PD]2 5: [PD]2 -[PA]-[PD]2

20

60 55 Water/glycol soption uptake ratio (mol/mol)

626

50 45 40 35 30 25 20 15 10 5 0 0

1

2

3

4

5

6

Number of Layers Deposited Fig. 3. Effects of number and sequence of PA and PD sublayers deposited in the membrane (as shown in Table 1) on (a) contact angle and (b) water/ethylene glycol sorption uptake ratio.

220 was obtained with membrane [PD]2–[PA]–[PD]2. This membrane was selected for further studies in the following to evaluate the effects of feed water concentration, operating temperature and NaCl contents in the feed on the membrane performance for pervaporative dehydration of ethylene glycol. It may be pointed out that this proof-of-concept study was aimed at demonstrating the feasibility of using [PA]/[PD] composite membranes for pervaporation applications by simply depositing polydopamine sublayers before and/or after a polyamide sublayer having structures applicable to nanofiltration was formed. The membrane fabrication conditions (e.g., the concentrations of the reactants in selfpolymerization of polydopamine and interfacial-polymerization of polyamide, deposition time of the reactants, and the numbers of polydopamine and polyamide sublayers) were not optimized in this study and the separation performance data presented here do

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

627

280 1651

1170

1505 1554

1358

240

(d) [PD]2-[PA]-[PD]

1170

1505

200 1651

1: [PA] 2: [PA]-[PD] 3: [PA]-[PD]2

160

Absorbance

Partial Permeation Flux (g/m2 .h)

3310

Water

Membranes:

4: [PD]-[PA]-[PD] 2 5: [PD]2 -[PA]-[PD]2

120

1554

1365

3310

(c) [PD]2-[PA] 1170

1505 1365

1640 3310

80

(b) [PD]

Ethylene

40

1664

Glycol

(a) PES Substrate

0

4000 300

3000

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) Fig. 5. ATR-FTIR spectra of (a) PES substrate and thin film composite membranes: (b) [PD], (c) [PD]2–[PA] and (d) [PD]2–[PA]–[PD]. The rectangular insets in the figure show the colors of the membrane surfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

250

Separation Factor

3500

200

150

100

50

0 0

1

2

3

4

5

6

Number of Layers Deposited Fig. 4. Effects of number and sequence of PA and PD sublayers deposited in the membrane (as shown in Table 1) on (a) partial fluxes of water and ethylene glycol and (b) separation factor. Feed composition: 9.5 wt% water þ90.5 wt% ethylene glycol, temperature: 38 °C.

not represent the best membrane performance that could be obtained. 3.2. Morphology and chemical composition of the membrane surface The chemical composition of the membrane surface was analyzed by ATR-FTIR. Fig. 5 shows the ATR-FTIR spectra of the pristine PES substrate and the thin film composite membranes comprising of polydopamine and polyamide sublayers. Compared to the PES substrate, there are several new peaks on the ATR-FTIR spectra for [PD] membrane: 3310 cm  1 (N–H/O–H stretching), 1640 cm  1 (overlap of C ¼C resonance vibration in aromatic ring and N–H bending vibration), 1505 cm  1 (N–H scissoring), 1365 cm  1 (phenolic O–H bending) and 1170 cm  1 (phenolic C–O stretching). These new absorption peaks confirm the existence of polydopamine layer on the PES substrate membrane. For

membrane [PD]2–[PA], the peaks at 1651 cm  1 and 1545 cm  1 are characteristic of amide-I (C ¼ O stretching) band and amide-II (N– H) band of the amide groups (–CONH–). The peak at around 1650 cm  1 becomes broader for membrane [PD]2–[PA]–[PD] since it overlaps with the characteristic peaks of C ¼C resonance vibration, N–H bending vibration and C ¼O stretching vibration. The band intensities at 3310 cm  1 and 1650 cm  1 increased with an increase in the polydopamine sublayers deposited. The colors of the membrane surfaces are also shown in Fig. 5. The PES substrate was white, and the membrane surface became brown when deposited with one sublayer of polydopamine, and the color turned to be darker with additional polydopamine depositions. All these results confirm the deposition of self-polymerized polydopamine and interfacially-polymerized polyamide; they also confirm that the composite membranes do not have a distinct layer-over-layer structure and there is interpenetration between the neighboring sublayers. The surface morphologies of the membranes were examined using FE-SEM. Fig. 6 shows the surface images of the PES substrate and the thin film composite membranes with polydopamine/ polyamide sublayers. Unlike the PES substrate with a smooth surface (Fig. 6(a)), relatively small patch-like and large fractal-like aggregated structures appeard on the surface of membrane [PD] (Fig. 6(b)), indicating that the deposition of polydopamine was not evenly distributed on the membrane surface and one layer of polydopamine deposition was not dense enough to fully cover the substrate surface. After a polydopamine sublayer and a polyamide sublayer were deposited on the membrane surface, the membrane (i.e., [PD]2–[PA]) showed a denser surface (Fig. 6(c)), and the substrate was mostly covered by these depositions. Depositing an additional polydopamine sublayer made the membrane surface denser (Fig. 6(d)). For comparison, the surface image of membrane [PA] (formed from 4.0 wt% PEI and 0.8 wt% TMC) is shown in Fig. 6 (e). Clearly, the deposition of polydopamine onto the polyamide sublayer resulted in a denser and more compact skin layer in the thin film composite membrane.

628

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

Fig. 6. Surface images (20,000  ) of (a) PES substrate and thin film composite membranes: (b) [PD], (c) [PD]2–[PA], (d) [PD]2–[PA]–[PD] and (e) [PA].

3.3. Effects of feed concentration on pervaporative dehydration of ethylene glycol To investigate the influence of water concentration in the feed on the performance for dehydration of ethylene glycol using membrane [PD]2–[PA]–[PD]2, pervaporation experiments were carried out at 38 °C in the range of feed water concentrations from 0.5 to 20 wt%. This concentration range is of industrial interest, particularly for regeneration of ethylene glycol related to natural gas dehydration by ethylene glycol. Fig. 7 shows the water concentration in the permeate and the total permeation flux as a function of feed water concentration. At a feed water

concentration of 0.5 wt%, the water concentration in the permeate was 81 wt%. The total permeation flux increased almost linearly with an increase in the feed water concentration. The latter trend was also observed in other studies on hydrophilic composite membranes for dehydration of ethylene glycol [20,40]. For comparison with conventional distillation, the vapor-liquid equilibrium (VLE) data for ethylene glycol/water mixtures [41] are also plotted in Fig. 7(a). It is clear that pervaporative separation with membrane [PD]2–[PA]–[PD]2 was more selective than distillation for dehydration of ethylene glycol, especially at relatively low feed water concentrations. This makes pervaporation advantageous in terms of energy consumption since it can be operated at a lower

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

100

20 VLE

18 Flux of Ethylene Glycol (g/m 2 .h)

80

wt% H O in Permeate

629

60

40

20

0

16 14 12 10 8 6 4

500 450

2

Total Flux (g/m .h)

400

0

350 300 250

1200

200 150

1100

100

1000

50

900 0

2

4

6

8

10

12

14

16

18

20

Water Concentration in Feed (wt%) Fig. 7. Effects of feed water concentration on (a) water concentration in permeate and (b) total permeation flux. Membrane [PD]2–[PA]–[PD]2, temperature: 38 °C.

temperature than flash operation and only a small fraction of the feed (mainly water) needs to permeate through the membrane. Fig. 8 shows the flux of ethylene glycol and the separation factor at different feed water concentrations. The permeation flux of ethylene glycol increased slightly when the water concentration in the feed increased from 0.5 to 4.0 wt%, and then a further increase in the feed water concentration had little impact on the permeation flux of ethylene glycol. The permeation flux of water (not shown in this figure), which was very close to the total permeation flux shown in Fig. 7(b), was affected significantly by the feed concentration over the concentration range studied. In the binary feed mixtures of water and ethylene glycol, an increase in water content means a decrease in the content of ethylene glycol, and thus the driving force for water permeation increases, whereas the opposite holds for the permeation of ethylene glycol. On the other hand, an increased feed water concentration will make the membrane more swollen, which tends to facilitate the permeation of both permeants, resulting in a lower separation factor. There exhibits a trade-off relationship between the permeation flux and separation factor. At 0.5 wt% water in feed, the separation factor was considerably high (i.e, 992), and it decreased to 388 when the feed water concentration increased to 2.4 wt%. Above 4.1 wt% water in the feed, a further increase in the feed water content did not decrease the separation factor significantly. 3.4. Effect of operating temperature on water/ethylene glycol separation Operating temperature is an important parameter in pervaporation since it influences the solubility and diffusivity of the permeating species in the membrane as well as the driving force for permeation. Fig. 9(a) shows the partial permeation fluxes of

Separation Factor

0

800 700 600 500 400 300 200 100 0 0

2

4

6

8

10

12

14

16

18

20

Water Concentration in Feed (wt%) Fig. 8. Effects of feed water concentration on (a) permeation flux of ethylene glycol and (b) separation factor. Membrane [PD]2–[PA]–[PD]2, temperature: 38 °C.

water and ethylene glycol through membrane [PD]2–[PA]–[PD]2 at various temperatures in the range 25–55 °C. Both permeation fluxes of water and ethylene glycol increased with an increase in the temperature. Generally, at a higher temperature, the permeating molecules are more energetic and the thermal motion of the polymer chains is enhanced, resulting in an increased diffusivity of the permeant in the membrane. In addition, for a given feed composition, the vapor pressures of water and ethylene glycol will both increase with an increase in temperature, which increases the driving force for the mass transport through the membrane. All these effects lead to an enhanced permeation flux. The temperature dependence of the permeation flux normally follows an Arrhenius type of relationship

J = J0 exp( − EJ /RT )

(3)

where EJ is the apparent activation energy for permeation that measures the overall temperature dependence of permeation flux.

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100

1000

100

Water

10

Feed Water wt% 0.48 1.14 4.34 Permeance (mol/m 2 .h.kPa)

Partial Permeation Flux (g/m 2 .h)

Feed Water wt% 0.48 1.14 4.34

Water

10

Ethylene Glycol Ethylene Glycol

1

1 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45

1000/T (1/K)

1800

Fig. 10. Effects of temperature on permeance of water and ethylene glycol through membrane [PD]2–[PA]–[PD]2 at different feed water concentrations.

1600

Separation Factor

1400 1200 1000 800 600 400 200 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45

1000/T (1/K) Fig. 9. Effects of temperature on (a) permeation fluxes of water and ethylene glycol and (b) separation factor for separation of water from ethylene glycol using membrane [PD]2–[PA]–[PD]2 at different feed water concentrations.

Table 2 Activation energies of permeation of water and ethylene glycol based on permeation flux (EJ) and membrane permeance (EP) at different feed water concentrations. Feed water conc. (wt%)

0.48 1.14 4.34

Water EJ (kJ/mol)

EP (kJ/mol)

Ethylene glycol EJ (kJ/mol) EP (kJ/mol)

15.9 22.5 29.2

 28.6  25.9  15.5

28.6 28.7 32.8

 39.7  40.3  31.0

The apparent activation energies for the permeation of water and ethylene glycol at different feed water concentrations are presented in Table 2. At a given feed water concentration, the apparent activation energy (EJ) for ethylene glycol permeation is greater than the apparent activation energy for water permeation. This is understandable because (1) ethylene glycol has a larger molecular size and will need to overcome a higher energy barrier in order to diffuse through the membrane, and (2) ethylene glycol has a higher molar heat of evaporation than water, and thus an

increase in the temperature will increase the vapor pressure of ethylene glycol more significantly than water vapor pressure. Since the temperature affects the permeation flux of ethylene glycol more significantly than water flux, the separation factor tends to decrease with an increase in temperature, as shown in Fig. 9(b). The above apparent activation energy based on permeation flux characterizes the overall influence of temperature on the permeation flux, including the effects of temperature on the driving force for mass transport. In order to evaluate the effects of temperature on the permeability of the membrane, the membrane permeance was estimated in analog to gas permeation using permeation flux normalized by the driving force for permeation, which also follows an Arrhenius type of temperature dependence

(Pi/l) =

Ji pis xi γi − pip yi

⎛ E ⎞ = (Pio/l)exp⎜− Pi ⎟ ⎝ RT ⎠

(4)

where (Pi/l) is the permeance of the membrane to permeant i, ps is the saturated vapor pressure of pure component i that can be calculated from the Antoine equation [42], γ is the activity coefficient in the liquid phase which can be calculated by the Wilson equation [43], pp is the permeate pressure, x and y are the mole fractions in the feed and permeate respectively, EP is the activation energy based on membrane permeability, and subscript i represents component i. Fig. 10 shows the membrane permeance as a function of the reciprocal of temperature, and the EP values for water and ethylene glycol are shown in Table 2. It is shown that the temperature has a negative impact on the membrane permeability for both water and ethylene glycol. Based on the solutiondiffusion model, the membrane permeability is equal to the product of the solubility and diffusivity of the permeant in the membrane. As a first approximation [44], EP is determined by the activation energy for diffusion (ED) and the heat of sorption (ΔHS) (i.e., EP ¼ED þ ΔHS). The diffusion is a thermally activated process that requires energy to occur and thus ED 4 0, whereas the sorption process is often exothermic (i.e., ΔHS o0). The negative EP values for the permeation of water and ethylene glycol suggest that the effect of temperature in the exothermic sorption process overweighs that in the diffusion process. That is to say, with an increase in temperature, the increased diffusivity is insufficient to compensate the reduced solubility, resulting in a decrease in the

D. Wu et al. / Journal of Membrane Science 493 (2015) 622–635

631

1000

100

Feed Water wt% (salt-free basis) 1.1 5.0 8.6 12.0

wt% H2 O in Permeate

90 85 80 75 70

Partial Permeation Flux (g/m 2 .h)

95 100 Water

10

1

65 0.1

60

Ethylene Glycol

Feed Water wt% (salt-free basis) 1.1 5.0 8.6 12.0

1800

400

1600 1400 1200

300 Separation Factor

Total Permeation Flux (g/m 2 .h)

350

250 200 150

1000 800 600 500 450 400 350

100

300

50

250 200

0

0

0

1

2

3

4

5

6

7

8

9

10

NaCl Molality in Feed × 103 Fig. 11. Effects of NaCl molality in the feed on (a) water concentration in permeate and (b) total permeation flux. Membrane [PD]2–[PA]–[PD]2, temperature: 38 °C.

membrane permeability or permeance. Therefore, it may be inferred that the observed increase in permeation flux with an increase in temperature was mainly caused by the increased driving force for permeation. Fig. 10 also shows that at a given temperature, the water permeability in the membrane is higher than that of ethylene glycol. This may be attributed to the smaller size of water molecules, which diffuses through the membrane more easily. Similar observations were also obtained by Wang et al. who used polybenzimidazole/polyetherimide membranes for the dehydration of ethylene glycol [45]. In addition, an increase in the feed water concentration tended to increase the permeability of ethylene glycol, while the opposite was found true for water. This is similar to results observed with other hydrophilic membranes for dehydration of ethylene glycol [40]. A possible explanation is that the membrane will become increasingly swollen at a higher feed water concentration, which enhances the membrane permeability; however, the water molecules tend to form clusters at

1

2

3

4

5

6

7

8

9

10

3

NaCl Molality in Feed 10

Fig. 12. Effects of NaCl molality in the feed on (a) permeation fluxes of water and ethylene glycol and (b) separation factor for separation of water from ethylene glycol using membrane [PD]2–[PA]–[PD]2, temperature: 38 °C.

higher concentrations, making them more difficult to diffuse in the membrane [46]. It may be mentioned that phenomenologically the overall separation achieved in pervaporation may be considered to be the product of separation due to selective evaporation of the liquid feed and the separation achieved by the membrane permselectivity [47]. Compared to the overall separation factor (i.e., 357– 1600, in the ranges of temperatures and feed water concentrations studied), the water/ethylene glycol permeability ratio of the membrane (which is 1.2–5.4) is rather low. This indicates that the selective evaporation contributes to the overall separation more significantly than the permselectivity of the membrane, which is understandable in view of the large difference in the volatilities of the two permeants. However, it does not mean the membrane permselectivity is unimportant because it is the product of the two that determines the overall separation in pervaporation. 3.5. Effect of NaCl contents in feed on pervaporative dehydration of

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Table 3 Activity coefficient γ for water and ethylene glycol in the presence of NaCl estimated using Aspen Plus. Feed water concentration: 1.14 wt% (salt-free basis). Feed NaCl molality (  103)

Temperature (°C)

Activity coefficient (γ) Water Ethylene glycol

0

25 30 35 45 55

0.814 0.822 0.830 0.844 0.858

1.00 1.00 1.00 1.00 1.00

25 30 35 45 55

0.673 0.675 0.678 0.682 0.690

1.12 1.13 1.13 1.14 1.15

25 30 35 45 55

0.598 0.601 0.605 0.612 0.626

1.29 1.29 1.30 1.31 1.34

100 95

2.56

5.13

wt% H2 O in Permeate

90

80 75 70 65 60

1000

Feed NaCl molality 0 2.564 5.128

1600

Feed NaCl molality 0 2.564 5.128

1500 1400 1300

100

Water 10 Ethylene

Separation Factor

Partial Permeation Flux (g/m 2 .h)

85

1200 1100 1000 900 800

Glycol

700 600 1 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45

1000/T (1/K)

Table 4 Apparent activation energy for permeation of water and ethylene glycol at different feed NaCl concentrations. Feed water concentration 1.14 wt% (salt-free basis).

0 2.56 5.13

20

25

30

35

40

45

50

55

60

Temperature (ć)

Fig. 13. Effects of temperature on permeation fluxes of water and ethylene glycol through membrane [PD]2–[PA]–[PD]2 at different feed NaCl concentrations, feed water concentration: 1.14 wt% (salt-free basis).

Feed NaCl molality (  103)

500

EJ (kJ/mol) Water

Ethylene glycol

22.5 29.3 33.6

28.7 18.3 16.5

ethylene glycol Many chemical processes and gas processing operations generate waste streams containing mixed organic/inorganic solutes, which influence the separation performance. To date, there have been only a few studies addressing the effects of salts in the feed on the membrane performance for the dehydration of organic solvents [48–51]. In the present study, the effects of NaCl present

Fig. 14. Effects of temperature on (a) water concentration in permeate and (b) separation factor for separation of water from ethylene glycol using membrane [PD]2–[PA]–[PD]2 at different feed NaCl concentrations, feed water concentration: 1.14 wt% (salt-free basis).

in the feed on the performance of membrane [PD]2–[PA]–[PD]2 for dehydration of ethylene glycol in were studied. For convenience of discussion, the salt concentrations in the aqueous ethylene glycol feed solutions were expressed in terms of molality (i.e., the number of moles of NaCl per kg of the water/ethylene glycol solvent), while the water and ethylene glycol concentrations in the feed mixtures were still in wt% but on a salt-free basis. Fig. 11 shows the water concentration in the permeate and the total permeation flux as a function of NaCl content in the feed. The presence of NaCl in the feed mixture was shown to increase the water concentration in the permeate, and the increase in the permeate water concentration was more significant at lower feed water concentrations. For example, increasing the molal concentration of NaCl in the feed to 8.5  10  3 (i.e., 0.05 wt%) the water content in the permeate increased from 88.0 wt% to 95.4 wt% at a feed water concentration of 1.1 wt%, while there is only a slight increase in the permeate water concentration (from

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633

Table 5 A comparison of membranes performance for dehydration of ethylene glycol by pervaporation. Membrane

Feed water content (wt%)

Temperature (°C)

Flux (g/m2 h)

Separation factor

Reference

SPE PES–PVA PVA/PAA PAAM/PVA CS–PES CS/PAA CS/PAA PVA (heat treatment) PVA/GPTMS/TEOS PVA/silica (basic) PVA/silica (acidic) MA crosslinked CA/CS PA-polyelectrolytes PA-polyelectrolytes PES–PD/PA/PD PES–PD/PA/PD PES–PD/PA/PD

5 17.5 20 20 10 20 20 20 20 20 20 3.2 3 10 19.2 9.5 2.4

80 80 30 30 35 70 70 70 70 70 70 30 40 22 38 38 38

454 383 480 140 300 165 216 145 60 67 84 85 400 12 429 250 81

44.5 231 196 96 105 258 105 54 714 311 44 302 340 415 196 220 388

[52] [53] [54] [54] [55] [56] [57] [58] [58] [59] [59] [60] [20] [21] This work This work This work

SPE: sulfonated polyethylene, PES: polyethersulfone, PVA: polyvinyl alcohol, PAA: poly acrylic acid, PAAM: polyacrylamide, CS: chitosan, GPTMS: γ-glycidyloxypropyltrimethoxysilane, TEOS: tetraethoxysilane, MA: maleic anhydride, CA: calcium alginate, PA: polyamide, PD: polydopamine.

96.8 wt% to 97.4 wt%) when the feed contained 12.0 wt% of water. However, the total permeation flux was reduced by NaCl in the feed mixture. The permeation fluxes of water and ethylene glycol at different feed NaCl concentrations are shown in Fig. 12(a), and the corresponding separation factor for water removal from ethylene glycol is shown in Fig. 12(b). The partial fluxes of both water and ethylene glycol decreased with an increase in the NaCl content in the feed, and the reduction in the permeation flux is more drastic for ethylene glycol than for water, especially at lower feed water concentrations. Similar observations can be made from the work of Heisler et al. [48] and Misra et al. [49] who used cellulose films to separate water from ethanol and methanol, respectively. The interactions between the components in the feed affect the pervaporation performance. For binary mixtures of ethylene glycol/water, the presence of water enhanced the permeation of ethylene glycol due to the increased swelling of the membrane. However, the presence of NaCl in the feed solution will make the situation much different. On the one hand, the strong polar–polar interactions between NaCl and water will decrease the activity of water and make it less volatile, resulting in a decrease in the permeation flux of water. On the other hand, the solvation of the salt in the solution reduces the amount of water available to swell the membrane, and thus the facilitating effect of water on the permeation of ethylene glycol due to membrane swelling will be depressed. In principle, the salt will also weakens the interactions between ethylene glycol and water due to reduced amount of water available to hydrogen bond with ethylene glycol, which tends to increase the activity of ethylene glycol. However, the latter effect is unlikely to be very significant because ethylene glycol is the major component in the feed solution, unless the salt content is sufficiently high. This is supported by the activity coefficients estimated using Aspen Plus shown in Table 3. During the pervaporation process, although the NaCl molecules may diffuse under a concentration gradient into the wet membrane to a certain depth along with the permeation of water and ethylene glycol near the membrane surface on the feed side, no NaCl was detected in the permeate collected. This is expected because NaCl does not evaporate and membrane surface on the permeate side is dry due to the vacuum applied, making it difficult for NaCl to penetrate the entire thickness of the membrane. There was no flux decline observed during a 15-h pervaporation run for a feed solution containing 5 wt% of H2O and 8.5  10  3 molality of NaCl at 38 °C, which confirmed that NaCl did not block the

membrane for solvent permeation. In addition, there was no degradation in the separation performance of the membrane over a prolonged period of 7 weeks under various operating conditions (e.g., NaCl content, feed H2O content and temperature), which indicates the good membrane stability. The effects of temperature on the membrane performance for ethylene glycol dehydration in the presence of NaCl were also investigated. Fig. 13 shows the logarithmic fluxes of water and ethylene glycol through membrane [PD]2–[PA]–[PD]2 as a function of the reciprocal of temperature at different NaCl concentrations in the feed. Here the feed water concentration was fixed at 1.14 wt% (salt-free basis). Both the permeation fluxes of water and ethylene glycol increased with an increase in the temperature. While the temperature dependence of water flux was more significant at a higher NaCl content in the feed, the opposite was true for the flux of ethylene glycol. Their apparent activation energies EJ for permeation are presented in Table 4. The permeate water concentration and the water/ethylene glycol separation factor are presented in Fig. 14, which shows that the membrane permselectivity for water removal from ethylene glycol was enhanced by the presence of the salt. The [PD]2–[PA]–[PD]2 membrane was found to be stable. There was no noticeable change in the membrane performance after pervaporation tests with various feed mixtures at different temperatures for a prolonged period of experiments. For example, this membrane showed a total permention flux of 53.7 g/(m2.h) and a water/ethylene glycol separation factor of 662 for a feed containing 1.14 wt% water (salt-free basis). After extensive pervaporation tests with various feed mixtures (e.g., binary ethylene glycol/water solutions and ternary ethylene glycol/water/NaCl solutions with different compositions) at different temperatures (25–55 °C) for 3.5 months, the membrane maintained essentially the same pervaporation performance, with a total permeation flux of 57.8 g/ (m2 h) and a separation factor of 640 for a feed containing 1.25 wt% water (salt-free basis). A comparison of the pervaporation performance of the [PD]2–[PA]–[PD]2 membrane with other membranes reported in the literature is presented in Table 5. It can be noted that the [PD]2–[PA]–[PD]2 membrane showed a comparable flux and good selectivity for removing water from ethylene glycol. It is feasible to fabricate such pervaporation membranes by simply depositing polydopamine sublayers before and/ or after a polyamide sublayer.

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4. Conclusions Thin film composite membranes comprising of polydopamine (PD) and polyamide (PA) were prepared for pervaporative dehydration of ethylene glycol. The effects of the number and sequence of PA and PD sublayer depositions on the pervaporation performance were studied. The membrane performance was investigated at different temperatures and feed water concentrations, with and without the presence of NaCl in the feed solution, and the following conclusions can be drawn:

[11]

(1) Incorporation of the polydopamine sublayers into the membrane, either as an outer layer (i.e., on top of the polyamide) or as a gutter layer (i.e., between the polyamide and the substrate) increased both permeability and selectivity of the membrane. The [PD]2–[PA]–[PD]2 membrane showed a total permeation flux of 81 g/(m2 h) and a separation factor of 390 at 38 °C for a feed solution containing 2.4 wt% water. (2) An increase in temperature increased the permeation flux, which was mainly attributed to the increased driving force for permeation. (3) The presence of inorganic salt NaCl in the feed solution decreased the permeation flux of ethylene glycol more significant than that of water, resulting in an enhanced separation factor for dehydration of ethylene glycol. (4) For pervaporation of binary water/ethylene glycol solutions, the separation factor was lowered at a higher temperature. However, when NaCl was present in the feed, the separation factor increased with an increase in temperature.

[14]

[12]

[13]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22] [23]

Acknowledgments [24]

Research support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.

[25]

[26]

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