PVP blends and their application in VMD

Journal of Membrane Science 364 (2010) 219–232

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Preparation of hollow fibre membranes from PVDF/PVP blends and their application in VMD S. Simone a,b , A. Figoli a,∗ , A. Criscuoli a,∗ , M.C. Carnevale b , A. Rosselli a , E. Drioli a,b a b

Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Via P. Bucci Cubo 17/C, 87030 Rende (CS), Italy Department of Chemical Engineering and Materials, University of Calabria, Via P. Bucci Cubo 42/A, 87030 Rende (CS), Italy

a r t i c l e

i n f o

Article history: Received 5 March 2010 Received in revised form 28 July 2010 Accepted 9 August 2010 Available online 14 August 2010 Keywords: Polyvinylidene fluoride (PVDF) Polyvinyl pyrrolidone (PVP) Hollow fibres Vacuum membrane distillation Desalination

a b s t r a c t Microporous hydrophobic poly(vinylidene fluoride) (PVDF) hollow fibres were prepared by the dry–wet spinning technique. N,N-Dimethylformamide (DMF) was used as solvent, while water and poly(vinyl pyrrolidone) (PVP) were used as pore forming additives. Mixtures of DMF or ethanol in water were employed as bore fluids. The influence of different parameters on the fibres characteristics was explored. Particular attention was focused on the PVP concentration and on the composition of the bore fluid. The obtained fibres exhibit good structure, excellent mechanical properties, high porosity (up to 80%) and an average pore size ranging from 0.12 to 0.27 ␮m. Fibres were tested in vacuum membrane distillation (VMD) configuration, using distilled water as feed. The effect of membrane properties on the water vapor fluxes was investigated. For the prepared membranes, the measured fluxes ranged between 3.5 and 18 kg/m2 h at 50 ◦ C and 20 mbar vacuum pressure. Some selected fibres were assessed for long term stability. Constant performances were observed up to 2 months. Furthermore, preliminary VMD tests on salty water were also performed, in order to verify fibres suitability for seawater desalination. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, it is widely recognized that, besides the global warming, water shortage is becoming a major emergency. In last years, membrane distillation (MD) was proposed as a valid alternative to traditional desalination techniques, such as multistage flash vaporization (MSFV), or coupled to reverse osmosis (RO) (integrated membrane systems), for its lower energy consumption and lower influence of osmotic pressure, respectively. Membrane distillation is a separation method in which a hydrophobic and microporous membrane is used with a liquid feed phase on one side of the membrane and a condensing, permeate phase on the other side. The driving force for transport is the partial pressure difference across the membrane. The principles of MD have been reviewed by many authors [1–4]. The possibility of using alternative energy sources, such as geothermal and solar energy, as well as to exploit low grade or waste energy, has been highlighted [5–7]. Although based on the same principle, MD processes can be divided in different categories, depending on the method applied for establishing the required driving force. Among the differ-

∗ Corresponding authors. Tel.: +39 0984 492027/492118; fax: +39 0984 402103. E-mail addresses: a.fi[email protected] (A. Figoli), [email protected] (A. Criscuoli). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.08.013

ent MD configurations, vacuum membrane distillation (VMD) has attracted increasing interest for various applications beside seawater desalination, i.e.: removal of volatile organic compounds (VOCs) from water, concentration of aqueous solutions, and separation of non-volatile components form water such as ions, colloids and macromolecules. The application areas range from environmental waste clean-up to food processing [8–18]. In VMD, the liquid feed is brought in contact with one side of a hydrophobic membrane, while vacuum is applied at the permeate side. Vapor condensation takes place outside of the module. VMD results in many advantages, with respect to conventional separation techniques, and, from an economic point of view, is comparable to alternative membrane processes, such as pervaporation [19]. With respect to other MD configurations, VMD allows to reach higher partial pressure gradients and, hence, higher fluxes and plant productivity. It performs better than DCMD in terms of transmembrane fluxes, energy consumption/permeate flow ratios and evaporation efficiency [20]. Recently, MD potentialities, in terms of productivity, costs and efficiency, have been examined in details [21]. Authors investigated the effect of different parameters on the MD performance. Membrane critical features resulted to be the thermal conductivity and the porosity. The heat lost by conduction through the membrane reduces the thermal gradient, which is the driving force of the process, thereby negatively affecting the transport.

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Table 1 Separation performance of PVDF flat sheet (FS) and hollow fibre (HF) membranes for VMD reported in the literature. MD process

Membrane

Membrane properties

Dope composition

J/◦ C

Ref

VMD/pure H2 O

FS

DMAC, H2 O

HF

1.39 kg/m2 h (feed 25 ◦ C) 0.5 kg/m2 h (feed 50 ◦ C)

[44]

VMD/pure H2 O

Pore size 0.22 ␮m Porosity 70.5% Avg. Pore size 0.031 ␮m Effective porosity 1516 m−1

DMAC, LiCl/H2 O

[46]

Abbreviations: DMAc: dimethylacetamide; FS: flat sheet; HF: hollow fibre; VMD: vacuum membrane distillation.

Membranes having higher porosity ensure better molecular diffusion and, therefore, higher flux. The effect of membrane thickness is more complex. Thinner membranes offer lower resistance to the transport, but, membranes of reduced thickness leads to higher heat losses by conduction. Different papers have pointed out that the search for new materials and the development of suitable membranes are of primary importance for the future commercialization of MD, in order to improve the process productivity and reduce its costs. The production of membranes suitable for MD has been discussed in different works. They can be divided in two main categories: (a) surface modification to turn hydrophilic flat sheet or hollow fibre membranes into hydrophobic ones and (b) preparation of flat membranes or fibres, from binary or ternary dopes, starting from a hydrophobic polymer, and using a suitable solvent and pore forming or other additives. Surface modifications of hydrophobic membranes, like coating, grafting or incorporation of macromolecules usually aim at the production of a membrane having reduced transport resistances. In fact, the hydrophilic part provides mechanical support with low resistance to transport, leading to high fluxes, while the modified surface ensures the required hydrophobicity [22–34]. In recent years, polyvinylidene fluoride (PVDF) became a popular membrane material due not only to its good hydrophobicity and excellent chemical stability, but also to the feasibility of forming hollow fibres and membranes via a phase inversion method; several examples of PVDF membrane preparation for various applications are already reported in literature [35–38]. There are different studies that have examined the possibility of producing PVDF flat sheet and hollow fibre membranes for application in MD [39–55]. Table 1 shows the VMD transmembrane fluxes reported in literature, with both PVDF flat sheet and hollow fibre membranes. In the recent papers [52–55], different approaches to improve PVDF membranes morphology and performances were proposed. Teoh and Chung [52] prepared hydrophobic polyvinylidene fluoride–polytetrafluoroethylene (PVDF–PTFE) hollow fibres showing improved hydrophobicity and macrovoid-free structure. Wang et al. [53] produced mixed matrix PVDF hollow fibre membranes by adding hydrophobic cloisite clay particles to the polymeric dope. These fibres have high porosity but a layer of

Table 3 Hollow fibre spinning base conditions. Spinning conditions Bore fluid Bore fluid injection rate (ml/min) Bore fluid temperature (◦ C) Coagulation bath Coagulation bath temperature (◦ C) Polymeric dope temperature (◦ C) Polymeric dope flow rate (g/min) Air gap (cm) Room temperature (◦ C)

EtOH:H2 O 30:70; DMF:H2 O 15–35:85–65 7–20 50 H2 O 100% 20 85 12 25 25

nanoscale pores thus ensuring high fluxes, good thermal insulation and reduced risk of membrane pore wetting because of the high LEPw (water liquid entry pressure). Bonyadi and Chung [54] obtained highly porous and macrovoidfree PVDF hollow fibre membranes co-extruding the polymeric dope and the solvent by means of a triple spinneret. The solvent flow at the fibres outer surface prevented the formation of a dense skin, increased the outer surface porosity of the PVDF fibres and eliminated the formation of macrovoids. Finally, Hou et al. [55] proposed the synergic use of two pore forming agents (LiCl and PEG 1500) which led to fibres with high porosity and good hydrophobicity. The aim of this research work was the production of microporous hydrophobic PVDF hollow fibres suitable for VMD, by using water and poly(vinylpyrrolidone) as pore forming additives. Different works already reported about the use of water as pore forming additive for preparing both flat sheet and hollow fibre membranes for MD [44], sometimes in combination with other additives, such as LiCl [46]. PVP is widely used in preparing membranes and fibres to adjust pore size and pore size distribution, increase membrane permeability, produce hydrophilic membranes and prevent fouling. It is mainly combined in blends with polysulfone (PSU), polyethersulfone (PES) and polyvinylidene fluoride (PVDF) to prepare membranes for ultra- (UF) and microfiltration (MF) to be used in: biomedical applications (e.g. dialysis), water purification, waste

Table 2 PVDF dopes composition. Membrane dope

1

2

3

4

5

6

Polymer PVDF Solef 6020 (%)

20

20

20

20

20

20

Solvent DMF (%)

68

65

63

59

74

65

Additives PVP (K-17) (%) H2 O (%)

6 6

9 6

11 6

15 6

0 6

15 0

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Fig. 1. Simplified scheme of the VMD set-up.

water treatment and food processing (e.g. beer and wine). It is characterized by high biocompatibility, especially with blood, and very low toxicity. On the contrary, the use of PVP for preparing membranes to be applied in MD was never reported in the literature, due to its hydrophilic properties. In this work, the effect of the PVP percentage in the polymeric dope and of the bore fluid composition on fibres morphology and performance was examined. The produced fibres were tested in a VMD plant using distilled water as feed. Some selected fibres were assessed for long term stability; to this purpose, VMD tests on pure water were performed at different time intervals for a period of 2 months. Preliminary VMD runs using synthetic seawater as feed were also performed.

The polymeric dopes compositions used in hollow fibres preparation are listed in Table 2; spinning conditions for all experiments are listed in Table 3. 2.2. Fibres post-treatment The fibres were collected and washed with plenty of water to ensure complete coagulation and removal of the residual solvent. They were subsequently treated with a solution of sodium hypochlorite 4000 ppm buffered to pH 7, in order to remove PVP, as suggested in the literature [57–62]. Fibres were then washed thoroughly and placed in a solution of glycerol to 30% for 3–4 h. Finally, they were left to dry at room temperature. 2.3. SEM observations

2. Materials and methods 2.1. Fibre preparation For the preparation of the polymeric dopes, poly(vinylidene fluoride) (PVDF Solef® 6012, supplied by Solvay chemical company) was dissolved in N,N-dimethylformamide (DMF), adding water and poly(N-vinylpyrrolidone) (K-17) (Mw 7–11 kDa) as pore formers. The water amount was of 6% (w/w), while PVP was increased from 0 to 15% (w/w). In one experiment only PVP was added to the polymeric dope as pore former. The dope was kept under constant stirring at 85 ◦ C until the mixture was homogeneous. Afterwards, the stirring was stopped allowing the release, at a controlled temperature, of the gas bubbles incorporated during the mixing. The dope was, then, loaded in a stainless steel reservoir pressurized by pure nitrogen. The nitrogen pressure on the polymeric dope reservoir was adjusted to have a constant dope flow rate of around 12 g/min. Hollow fibres were spun using a spinneret an inner diameter (i.d.) of 600 ␮m and an outer diameter (o.d.) of 1600 ␮m. More details about the spinning set-up were reported elsewhere [56].

The membrane’s morphology was observed by using a scanning electron microscope (Quanta FENG 200, FEI Company). Fibres crosssections were prepared by freeze fracturing the samples in liquid nitrogen. The internal and external surfaces were also observed. 2.4. Mechanical properties The tensile strength of the fibres was measured by means of a ZWICK/ROELL Z 2.5 test unit. Each sample was stretched unidirectionally at a constant rate of 5 mm/min; the initial distance between the clamps was of 50 mm. Five specimens were tested for each sample. The breaking elongation and elastic or Young’s modulus were determined. 2.5. Bubble point and pore size distribution Fibres bubble point and pore size distribution were determined by using a PMI Capillary Flow porometer (Porous Materials Inc., USA), following the procedure described in literature [49]. Measurements were carried out on fibres not treated with glycerol,

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Table 4 Properties of the produced PVDF hollow fibres. Hollow fibre

Bore fluid composition

␧break (%)

Bubble point (bar)

Largest detected pore diameter (␮m)

Average pore size (␮m)

Porosity (%)

0.089 0.045 0.061 0.065 0.063 0.063 0.04 0.08

104.64 137.42 149.70 155.31 162.22 157.02 172.34 170.48

148.59 171.31 126.13 120.05 119.90 164.54 175.66 145.47

2.11 2.47 2.55 3.67 2.34 2.79 2.24 2.76

0.29 0.25 0.24 0.17 0.27 0.22 0.28 0.23

0.14 0.16 0.15 0.16 0.17 0.14 0.16 0.16

76.61 75.03 77.86 78.03 75.83 76.50 74.5 77.72

± ± ± ± ± ± ± ±

0.56 0.41 2.02 1.57 2.45 1.04 1.31 1.78

± ± ± ± ± ±

0.068 0.04 0.026 0.07 0.08 0.052

88.90 97.78 92.78 73.54 103.32 120.75

142.48 155.84 142.65 134.18 129.30 174.47

1.29 1.42 1.26 1.25 1.57 2.39

0.48 0.44 0.49 0.50 0.40 0.26

0.15 0.14 0.16 0.15 0.16 0.16

80.87 78.64 80.95 79.09 80.62 76.84

± ± ± ± ± ±

0.73 1.5 0.36 1.78 1.27 1.85

0.201 0.174 0.175 0.190 0.211 0.208

± ± ± ± ± ±

0.074 0.09 0.043 0.07 0.064 0.063

103.68 119.68 100.20 100.64 118.28 127.01

147.75 186.67 206.75 197.76 150.82 180.42

2.12 2.17 2.12 2.59 2.60 1.99

0.29 0.29 0.29 0.24 0.24 0.31

0.17 0.15 0.14 0.15 0.18 0.16

82.54 77.04 76.86 82.07 81.5 81.5

± ± ± ± ± ±

0.54 0.35 0.82 0.12 0.08 0.14

0.014 0.033 0.034 0.042 0.026 0.033 0.028 0.023

0.186 0.174 0.249 0.202 0.131 0.178 0.139 0.139

± ± ± ± ± ± ± ±

0.072 0.086 0.067 0.088 0.071 0.077 0.079 0.088

125.82 140.75 128.38 159.87 168.80 125.77 148.98 199.23

230.11 220.68 167.51 196.21 194.09 183.93 216.99 192.82

2.56 2.60 3.12 2.47 2.55 2.56 4.10 4.19

0.24 0.24 0.20 0.25 0.24 0.24 0.15 0.15

0.21 0.22 0.15 0.16 0.20 0.15 0.13 0.13

83.27 81.23 80.56 78.09 78.03 78.85 78.45 77.54

± ± ± ± ± ± ± ±

2.15 3.13 1.67 0.76 1.64 0.47 0.86 1.12

± ± ± ± ± ± ±

0.01 0.038 0.024 0.012 0.013 0.032 0.035

0.161 0.17 0.165 0.152 0.202 0.203 0.159

± ± ± ± ± ± ±

0.029 0.068 0.058 0.024 0.061 0.087 0.085

116.72 87.59 120.98 129.48 119.45 80.48 86.58

130.41 184.99 233.78 175.76 112.07 152.90 96.92

3.15 2.98 3.45 3.05 2.75 2.55 3.15

0.20 0.21 0.18 0.20 0.23 0.24 0.20

0.15 0.14 0.12 0.16 0.15 0.15 0.16

73.62 71.21 70.55 74.22 75.17 71.2 73.82

± ± ± ± ± ± ±

1.67 0.56 1.04 1.54 0.78 2.04 1.57

± ± ± ± ± ± ±

0.013 0.013 0.012 0.019 0.014 0.012 0.017

0.065 0.058 0.027 0.073 0.073 0.090 0.081

± ± ± ± ± ± ±

0.024 0.048 0.033 0.049 0.046 0.024 0.052

84.02 86.55 83.54 93.52 95.50 87.70 89.43

382.70 412.25 256.65 411.70 391.79 282.20 412.34

1.15 1.28 2.15 1.65 2.05 1.75 1.85

0.54 0.49 0.29 0.38 0.30 0.35 0.34

0.23 0.23 0.25 0.26 0.27 0.26 0.25

82.79 83.59 82.31 77.76 77.43 77.56 80.81

± ± ± ± ± ± ±

2.38 1.08 1.6 1.6 1.36 1.97 2.48

Outer diameter (mm)

Inner diameter (mm)

Membrane thickness (mm)

(a) Dope 1 (PVP 6%, H2 O 6%) 1a EtOH 30% 1b EtOH 30% 1c DMF 15% 1d DMF 15% 1e DMF 25% 1f DMF 25% 1g DMF 35% 1h DMF 35%

10 14 14 10 10 14 14 10

1.502 1.711 1.712 1.76 1.67 1.568 1.714 1.69

± ± ± ± ± ± ± ±

0.053 0.035 0.030 0.045 0.033 0.037 0.023 0.05

1.124 1.335 1.371 1.37 1.34 1.187 1.346 1.38

± ± ± ± ± ± ± ±

0.036 0.01 0.031 0.02 0.03 0.026 0.017 0.03

0.189 0.188 0.170 0.194 0.166 0.190 0.184 0.157

± ± ± ± ± ± ± ±

(b) Dope 2 (PVP 9%, H2 O 6%) 2a EtOH 30% 2b EtOH 30% 2c DMF 25% 2d DMF 25% 2e DMF 35% 2f DMF 35%

14 17 17 14 14 17

1.678 1.73 1.76 1.899 1.775 1.83

± ± ± ± ± ±

0.056 0.02 0.015 0.029 0.041 0.039

1.338 1.33 1.35 1.519 1.435 1.5

± ± ± ± ± ±

0.013 0.02 0.011 0.041 0.039 0.013

0.170 0.198 0.203 0.190 0.170 0.163

(c) Dope 3 (PVP 11%, H2 O 6%) 3a EtOH 30% 3b EtOH 30% 3c DMF 25% 3d DMF 25% 3e DMF 35% 3f DMF 35%

14 17 17 14 14 17

1.64 1.694 1.73 1.721 1.661 1.67

± ± ± ± ± ±

0.047 0.012 0.01 0.041 0.026 0.02

1.238 1.347 1.387 1.341 1.24 1.25

± ± ± ± ± ±

0.027 0.078 0.033 0.029 0.038 0.043

(d) Dope 4 (PVP 15%, H2 O 6%) 4a EtOH 30% 4b EtOH 30% 4c DMF 25% 4d DMF 25% 4e DMF 25% 4f DMF 35% 4g DMF 35% 4h DMF 35%

10 14 7 14 20 7 14 20

1.602 1.694 1.597 1.69 1.759 1.798 1.767 1.767

± ± ± ± ± ± ± ±

0.058 0.053 0.033 0.046 0.045 0.044 0.051 0.065

1.230 1.347 1.098 1.287 1.497 1.443 1.490 1.490

± ± ± ± ± ± ± ±

(e) Dope 5 (PVP 0%, H2 O 6%) 5a EtOH 30% 5b EtOH 30% 5c DMF 25% 5d DMF 25% 5e DMF 35% 5f DMF 35% 5g EtOH 45%

10 14 14 10 10 14 10

1.47 1.6 1.61 1.544 1.642 1.69 1.557

± ± ± ± ± ± ±

0.019 0.03 0.034 0.012 0.048 0.055 0.05

1.15 1.26 1.28 1.24 1.24 1.285 1.4

(f) Dope 6 (PVP 15%, H2 O 0%) 6a EtOH 30% 6b DMF 25% 6c DMF 25% 6d DMF 25% 6e DMF 35% 6f DMF 35% 6g DMF 35%

10 10 14 20 20 14 10

1.531 1.467 1.577 1.674 1.554 1.509 1.454

± ± ± ± ± ± ±

0.011 0.035 0.021 0.03 0.032 0.012 0.035

1.401 1.350 1.522 1.528 1.408 1.329 1.293

S. Simone et al. / Journal of Membrane Science 364 (2010) 219–232

Emod (N/mm2 )

Bore fluid flow rate (ml/min)

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Fig. 2. Effect of the bore fluid on macrovoid formation in the microstructure of PVDF hollow fibres spun from a polymeric dope containing 6% of PVP (K-17) and H2 O (6%, v/v). Bore fluids: (a) EtOH 30% (v/v); (b) DMF 15% (v/v); (c) DMF 25% (v/v); (d) DMF 35% (v/v). Bore fluid injection rate: 14 ml/min.

using isopropanol (surface tension 21.7 dynes/cm) as wetting liquid. The bubble point and pore size distribution were determined processing data by the software Caprep (Porous Materials Inc., USA).

where w1 is the weight of the wet membrane; w2 is the weight of the dry membrane; Dk is the kerosene density (0.82 g/cm3 ) and Dp is the polymer density (1.78 g/cm3 , as reported in Solvay technical sheets).

2.6. Fibres porosity

2.7. CAM measurements

Membrane porosity was determined by gravimetric method [43], measuring the weight of liquid (here kerosene, as suggested in literature, Refs. [51,53,54]) contained in the membrane pores. The porosity of the hollow fibre membranes (ε) can be calculated using the following formula [43]:

The hydrophobicity of the produced fibres was assessed by measurements of the contact angle to water. In particular, the dynamic contact angle of the PVDF hollow fibre membranes was measured by using a tensiometer (DCAT11 Tensiometer, Dataphysics, Germany) as suggested in the literature [49,53]. A fibre glued to the holder was hung from the arm of an electro-balance, and then put, by cyclic immersions, into DI water. The contact angle was calculated from the wetting force based on the Wihelmy method.

 ε=

(w1 − w2)/Dk (w1 − w2)/Dk + w2/Dp

 × 100%

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Fig. 3. Effect of the PVP percentage on the microstructure of PVDF hollow fibres. H2 O (6%, v/v); PVP (K-17) percentage: (a) 9% (w/w); (b) 11% (w/w); (c) 15% (w/w). Bore fluid: DMF 35% (v/v) and injection rate: 14 ml/min.

2.8. VMD tests 2.8.1. Membrane modules preparation Membrane modules were prepared by inserting three hollow fibres in a glass device, sealing both extremities with epoxy glue. In order to remove glycerol from fibres, before each test, membrane modules were washed with double distilled water at 40 ◦ C and 1 bar of pressure for 3 h. Modules were completely dried by sending air overnight before being connected to the VMD plant.

VMD experiments on pure water were performed over a period of almost 2 months in order to test the stability of produced hollow fibres. VMD tests were also performed feeding a salty solution, prepared in lab and simulating seawater, having the following composition: NaCl 23.27 g/l, Na2 SO4 3.99 g/l, MgCl2 ·6H2 O 11.28 g/l, KBr 0.095 g/l, CaCl2 ·2H2 O 1.47 g/l, NaHCO3 0.1932 g/l, KCl 0.6635 g/l, Na2 CO3 ·H2 O 0.0072 g/l. 3. Results and discussion

2.8.2. VMD plant The VMD set-up used in this study was already described elsewhere [20]. Fig. 1 shows a simplified scheme. The feed was recirculated inside the hollow fibres, while vacuum was applied at the shell side. VMD tests were performed using double distilled water as feed at: feed temperature 50 ◦ C; feed flow rate 32 l/h; vacuum pressure 20 mbar. The distillate flux, J (kg/m2 h), was calculated according to the following equation: J=

Md A.t

where Md is the mass of the permeate; A is the active membrane surface; t is the time interval. A was calculated taking into account the number of membranes inserted in the module, their length and internal diameter.

In this work, an extensive study of how the addition of PVP to the polymeric dope affects fibres properties and, then, their performance in VMD, was carried out. The effect of the bore fluid composition was also considered. The main features of the produced PVDF hollow fibres are reported in Table 4a–f. 3.1. Morphology examination by SEM Fibres morphology was investigated using a scanning electron microscope (SEM). Looking at the images of fibres cross-sections, presented in Figs. 2–4, it can be seen that fibres show a sponge-like structure, with some small macrovoids on the surfaces. The obtained structures are related to the mechanisms of the coagulation process. As reported in the literature, during the hollow fibre coagulation, the bore fluid causes the polymer precipita-

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225

Fig. 4. Effect of the bore fluid composition and PVP amount on the microstructure of PVDF hollow fibres. All pictures were taken at 1000× magnification grade. H2 O (6%, v/v); PVP (K-17) percentage: (a) 0% (w/w); (b) 6% (w/w); (c) 9% (w/w); (d) 11% (w/w); (e) 15% (w/w). Bore fluid injection rate: 14 ml/min.

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Fig. 5. Fibres cross-sections observed a high magnification (10,000×). H2 O (6%, v/v); PVP (K-17) percentage: (a) 6% (w/w); (b) 9% (w/w); (c) 11% (w/w); (d) 15% (w/w). Bore fluid injection rate: 14 ml/min.

tion and formation of the inner surface; while, the formation of the outer surface starts during fibre falling along the air gap and ends in the coagulation bath. The composition of both coagulation media (internal and external), the air gap, the presence of hydrophilic additives and the temperature are all affecting fibre’s final structure, since they influence the solvent/non-solvent exchange velocity, that is the main variable during the hollow fibre coagulation. In our case, the morphology of the produced PVDF hollow fibres are discussed taking into account the effect of the different concentrations of PVP in the polymeric dope and of the bore fluid composition.

Regarding the fibres outer layer, when the nascent fibre reaches the water bath, it undergoes to solidification in a short time-period. This causes a limited growth of the polymer-lean phase, resulting in small macrovoids near the outer edge. The presence of finger-like structures at the outer surface is modulated by the PVP percentage. Cavities of bigger size can be observed, at the outer surface by increasing the PVP percentage from 6 to 9% (Figs. 2d and 3a). Further increase of PVP percentage to 11 and 15% leads to a reduction or even to a elimination of fingers formation (Fig. 3b and c). Our findings are in agreement with literature. It is reported that the addition of PVP, which is frequently used as pore former additive [63–65], could induce the formation of finger-like structures and macrovoids

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Fig. 6. Effect of the PVP percentage on the mechanical properties of PVDF hollow fibres. Results obtained for three different bore fluids with the same injection rate (14 ml/min).

both at the outer and the inner surfaces [66]. However, PVP effect on membrane morphology is strongly influenced by its concentration. At low concentration, the size of cavities is larger, but it is reported to reduce when the concentration of PVP is increased. In fact, on one side, PVP increases the dope thermodynamic instability, accelerating the solvent and non-solvent diffusion rates during the phase inversion process. Big cavities and macrovoids develop as a consequence of the rapid demixing. On the other hand, PVP is compatible with PVDF and increases the dope viscosity. At higher PVP concentration, the solvent/non-solvent exchange can be hindered. This counterbalances the thermodynamic effect of PVP, avoiding the formation of macrovoids, as also reported in literature [67,68]. Comparing fibres internal surfaces reported in Figs. 2d and 3a–c it can be also noticed how the increase of PVP concentration leads to a higher surface porosity of the fibres internal layer. In order to study the effect of the bore fluid, aqueous solutions of ethanol or DMF at different concentrations (from 15 to 45%) have been tested. As it results from the SEM pictures presented in Fig. 2, some macrovoids at the inner surface are formed when using the bore fluid having the highest water percentage (DMF 15%, Fig. 2b). Some macrovoids can be still detected when using DMF at 25% or EtOH at 30%, but they are completely eliminated when using DMF 35% as bore fluid. Internal surfaces look porous in all cases. For a better understanding of the effect of the bore fluid composition on hollow fibres morphology, SEM pictures of the cross-sections are compared in Fig. 4. For PVP 0% in the dope, there are macrovoids at both fibres surfaces. They reduce, but not disappear, only when using DMF 35% as bore fluid. For PVP 6-9-11% there are finger-like macrovoids mainly at the fibres outer surfaces. Some finger-like structures can be detected at the fibres inner layer when using DMF 15% (see Fig. 2b) or DMF 25% as bore fluid. For PVP 15% there are only small macrovoids at the fibres outer surface, for all bore fluids. These results can be discussed referring to literature, where water and mixtures of water and various alcohols or solvents are used as bore fluids [36,69,35]. In general, it is known that large voids are formed when using pure water as the internal coagulant, while the void formation can be eliminated by using aqueous mixtures of alcohols or solvents. Furthermore, Sukitpaneenit and Chung [70] recently reported in details the effect of different non-solvents on PVDF hollow fibres morphology and properties. Since PVDF is a semi-crystalline polymer, both liquid/liquid and solid/liquid demixing can take place during membrane coagulation, depending on the non-solvent

227

Fig. 7. Effect of the PVP percentage on the porosity of PVDF hollow fibres. Results obtained for three different bore fluids with the same injection rate (14 ml/min).

strength. Water is a strong coagulant; hence, precipitation is fast, by liquid/liquid demixing, resulting in finger-like macrovoids and a small portion of interconnected cellular type structure. When using “softer” coagulants, as methanol, ethanol or isopropanol, it can be clearly noticed that macrovoids disappear while membrane structure shifts to globule type. The observed spherical globules are made of semi-crystalline PVDF. They grow in a process of solid/liquid demixing, that can take place only when coagulation is delayed, leaving enough time to induce crystallization. Looking at fibres cross-sections at very high magnification (Fig. 5) it can be seen that, for all bore fluids, fibres structure is of cellular type. This means that the water percentage in the bore fluid is enough high to ensure liquid/liquid demixing in all cases and that, even if coagulation is delayed, there is insufficient time to induce crystallization. It can be concluded that, in our case, the bore fluid composition affects mainly macrovoid formation at the fibres inner layer; however, as observed in the SEM pictures, at high PVP concentration, this effect is less evident. The influence of both PVP and bore fluid on the mechanical properties of the produced fibres is discussed in more details in next section. 3.2. Mechanical tests The analysis of mechanical properties led to interesting results that are summarized in Table 4a–f. In particular, the values of Young’s modulus reached peaks of 160–170 N/mm2 .

Fig. 8. VMD flux vs. εrp /ı. Results obtained for three different bore fluids at the same injection rate (14 ml/min). For each dataset the H2 O percentage in the dope is 6%.

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S. Simone et al. / Journal of Membrane Science 364 (2010) 219–232 Table 5 Selection of results of VMD tests performed on some fibres in order to analyse the effect of porosity and thickness. Feed double distilled water; Tfeed 50 ◦ C; vacuum pressure 20 mbar. Fibre

PVP (%)

Bore fluid

Flux (VMD)kg/(m2 h)

1b 1f 1g

6

ETOH 30%, 14 ml/min DMF 25%, 14 ml/min DMF 35%, 14 ml/min

8.50 10.3 9.80

2a 2b 2c 2d 2e 2f

9

ETOH 30%, 14 ml/min ETOH 30%, 17 ml/min DMF 25%, 17 ml/min DMF 25%, 14 ml/min DMF35%, 14 ml/min DMF35%, 17 ml/min

17.0 16.7 18.5 12.8 17.9 13.2

3a 3b 3c 3d 3e 3f

11

ETOH 30%, 14 ml/min ETOH 30%, 17 ml/min DMF 25%, 17 ml/min DMF 25%, 14 ml/min DMF 35%, 14 ml/min DMF 35%, 17 ml/min

16.7 15.1 15.2 14.1 14.0 15.3

4b 4d 4g

15

ETOH 30%, 14 ml/min DMF 25%, 14 ml/min DMF 35%, 14 ml/min

Water leakage 12.3 15.3

5b 5c 5f

0 0 0

ETOH 30%, 14 ml/min DMF 25%, 14 ml/min DMF 35%, 14 ml/min

3.5 5.7 4.3

6a 6c

15 15

ETOH 30%, 10 ml/min DMF 25%, 14 ml/min

10 10.9

The mechanical properties of a membrane are strongly connected to its structure. Membranes having a good sponge-like structure should be stronger; while, the presence of macrovoids reduces membranes mechanical strength [71]. It was already reported in the literature that the addition of PVP could improve the fibres mechanical strength [56]. The formation of macrovoids is linked to the PVP percentage in the dope and to the bore fluid composition. In particular, as already explained in Section 3.1, the percentage of PVP strongly affects the size of cavities developed at the fibres outer surface. The plots presented in Fig. 6 compare the Young’s Modulus values of fibres spun from polymeric dopes containing increasing PVP percentages, but coagulated with the same bore fluid. It can be noticed that, when PVP percentage increases from 0 to 6%, there is an evident increase of the Young’s Modulus. This is connected to the increase of the total percentage of solids in the polymeric dope, which, in turn, affects the polymeric dope viscosity and, hence, fibres mechanical strength. Then, when PVP percentage reaches 9%, the Young’s Modulus drops from 137.42 to 88.90 N/mm2 , in the case of fibres produced using EtOH 30%, from 157.02 to 73.54 N/mm2 when DMF 25% is used as internal coagulant, and from 172.34 to 103.32 N/mm2 for fibres coagulated with DMF 35%. Further increase of the PVP concentration improves fibres mechanical properties. This can be explained considering the PVP effect on fibres structure. While, for small amounts, the increase in PVP concentration induces the formation of large cavities and macrovoids, higher concentrations promote the formation of sponge-like structures, thus improving also fibres mechanical properties. This is also completely in agreement to what observed with SEM pictures. The elongation-at-break percentages show a similar behaviour, as also reported in Fig. 6. In this case, very good results were obtained for the highest PVP concentration: εbreak reaches 230.11%. Regarding the effect of the bore fluid, it is very difficult to find a common trend among all the produced fibres. Sukitpaneenit and Chung [70] evidenced that a delayed demixing during fibres coagulation can suppress macrovoid formation but, if the microstructure shifts to globular, polymer crystallites are loosely packed, with subsequent reduction of the Young’s modulus.

As already discussed in Section 3.1, in our case, water percentage in the bore fluid is so high that fibres structure is always of cellular type, and, therefore, no PVDF crystallization-solid/liquid demixing takes place during coagulation. In our case, fibres mechanical properties are strongly connected to the size of macrovoids, which are only moderately affected by the bore fluid composition. Therefore, it can be concluded that, in our case, the effect of PVP percentage in the dope on fibres properties prevails with respect to that of the bore fluid composition. The same happens for the membrane porosity. 3.3. Porosity Experimental results show that fibres porosity ranges from 71 to 83%, as it is shown in Table 4a–f. As already discussed, porosity is a crucial parameter if fibres are designed to be used in MD. In Fig. 7 the dependence of the fibres porosity on the PVP concentration is illustrated, taking into account three different bore fluids (EtOH 30%, DMF 25%, and DMF 35%). PVP is a hydrophilic additive that is combined with the polymer to promote the formation of porous structures. Therefore, in general, membrane porosity should increase by increasing the percentage of PVP added to the polymeric dope. In fact, fibres porosity clearly increases for PVP percentage moving from 0 to 9% in all cases. A further increase of the PVP percentage brings about to a slight reduction of the porosity. This effect can be explained taken into account that fibres porosity is measured on the basis of the void fraction. Membranes having finger-like structures and macrovoids could have higher void fractions with respect to spongiform membranes. As observed in section 3.2, the increase of PVP percentage from 9 to 15% reduces the sizes of cavities at the outer surface, thus reducing the membrane void fraction. 3.4. Bubble point and pore size distribution From Table 4a–f, it can be found that the maximum diameter of the membrane pores ranges from 0.15 ␮m (Fibre 4h) to 0.54 ␮m (Fibre 6a), as obtained from bubble point data. The membrane aver-

S. Simone et al. / Journal of Membrane Science 364 (2010) 219–232

Fig. 9. VMD flux dependence on time (days) and module storage. Fibre 4d.

Fig. 10. VMD fluxes measured during tests performed on Fibre 1f feeding distilled water and synthetic seawater.

Fig. 11. VMD fluxes measured during tests performed on Fibre 4d feeding distilled water and synthetic seawater.

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230

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age pore size falls in the range 0.13 ␮m (Fibre 4h)–0.27 ␮m (Fibre 6e). In general, for each fibre, the pore size distribution is very narrow. The most dispersed one is that of Fibre 2d (Diameter at maximum pore size distribution: 0.15 ␮m, largest detected pore diameter: 0.50 ␮m.). 3.5. CAM measurement All PVDF fibres batch were screened by using double distilled water, in order to evaluate their hydrophobic character and, hence, their applicability in MD. Contact angle values ranging from 82 ± 1◦ (for fibres spun form dope 1) to 91 ± 1◦ (for fibres spun from dope 4) were obtained. Therefore, the contact angle to water does not decrease when increasing the PVP percentage in the dope. 3.6. VMD tests on distilled water The PVDF hollow fibres produced were tested in a VMD plant. At the beginning, experiments were carried out using double distilled water as the feed. This allowed to assess membrane hydrophobicity and performance. Comparing the results listed in Table 5 with those reported in Table 1, for VMD, it can be seen that, in general, transmembrane flux values comparable to or even better than those reported in literature were obtained. In fact, VMD flux fluctuates around 15 kg/h m2 . The obtained results confirm that PVP and water play a synergic action in enhancing fibres performances. In fact, the fluxes obtained when working with fibres from experiment 5 (H2 O 6%, PVP 0% in the dope) are the lowest observed. On the other hand, fluxes obtained when working with fibres from experiment 6 (H2 O 0%, PVP 15% in the dope) are generally lower than those obtained when using fibres containing both pore forming additives. Only in one case (Fibre 4b), a liquid water leakage was observed. By repeating VMD tests on Fibre 4b several times, some water droplets were noticed at the shell side. This is probably due to some defects in the fibre rather than hydrophobicity loss. VMD fluxes are strongly affected by fibres porosity and thickness. In general, membranes should ensure good molecular diffusion (high porosity, reduced thickness) minimizing the heat loss (high void fraction, higher thickness). From Table 5, the worst performances were observed for fibres having reduced thickness and low porosity (Fibres 5b, c and f, VMD flux 3.5, 5.7 and 4.3 kg/m2 h respectively). On the other hand, the best performances were obtained working with fibres having high porosity and thickness around 0.17 mm (Fibre 2e, VMD flux 17.9 kg/m2 h). Fig. 8 reports the flux vs. the ratio: εrp /ı, which takes into account the membrane porosity, thickness and pore size, all properties that are the result of an interplay between the PVP percentage and the bore fluid type. The flux increases with this ratio and a somehow linear trend is registered. 3.7. Long term performances In order to evaluate the hollow fibre stability with time, a membrane module (Fibre 4d) was tested at different time intervals up to 2 months. VMD flux dependence on time is reported in Fig. 9. On the x-axis the time (days) and the module storage way (dried or with water inside the fibres) are indicated. Looking at the plot, it can be noticed that the performance is stable over 15 days. At day 23 the flux seems to drop, but after 4 measurements, the initial performance is restored. At the end of the test, a slight reduction (around 7%) of the flux is observed. However, it can be concluded membrane performances are quite stable over time. Moreover, no membrane damage and/or leakage were detected. These results are quite interesting, but, it has to be pointed out that they refer to tests carried out with pure water as

feed. As further analysis, the long term performance of the prepared membranes with scaling water as feed should be also evaluated. 3.8. Tests on synthetic seawater VMD experiments on simulated seawater were carried out using two different types of PVDF hollow fibres (Fibre 1f, PVP 6%, bore fluid DMF 25%, injection rate 14 ml/min; Fibre 4d, PVP 15%, bore fluid 25%, injection rate 14 ml/min). The experimental tests were performed in sequence: a first test with distilled water, then different tests on seawater (without any distilled water flushing in between), then, another test with distilled water and so on. As it can be seen from Figs. 10 and 11, VMD flux measured during tests performed on salty water is only slightly lower than that measured when feeding double distilled water, but it is constant with time. Moreover, pure water fluxes measured after experiments on simulated seawater are similar to that measured at the beginning of the experiment. For all tests, the salt rejection was around 99.98%. 4. Conclusions In this work, microporous hydrophobic PVDF hollow fibres were produced by the phase inversion method, by using H2 O and PVP as pore forming additives. This is the first time that PVP is used as pore forming additive for preparing membranes designed for MD. The PVP effect on fibres structure can be resumed as follows: for small amounts, the increase in PVP concentration induces the formation of large cavities and macrovoids, increasing the membrane void fraction and reducing its strength; higher concentrations promote the formation of sponge-like structures, thus improving also fibres mechanical properties, but slightly reducing the porosity. Referring the effect of the bore fluid, it was found an influence on macrovoids formation at the fibres inner surface. However, its effect is negligible with respect to that of PVP. The water flux values obtained in VMD tests made on distilled water as feed, were comparable to or even better than those reported in the literature. The produced fibres were assessed for long time stability in prolonged VMD runs, showing good performance up to 2 months. Some preliminary VMD tests on simulated seawater were also performed, in order to test if fibres could be applied in desalination. A slight reduction of the transmembrane flux was observed, but the flux was constant with time. On the other hand, when working with pure water, the initial performances were restored. Acknowledgements The authors acknowledge the financial support of the European Commission within the 6th Framework Program for the grant to the Membrane-Based Desalination: An Integrated Approach project (acronym MEDINA). Project no.: 036997. The authors are grateful to Dr. M. Davoli (Department of Earth Science, University of Calabria, Via P. Bucci, 87030 Rende (CS), Italy) for the use of SEM microscope. The authors are grateful to Prof. R. Wang (SMTC, Singapore) for the dynamic contact angle measurements. References [1] K.W. Lawson, D.R. Lloyd, Membrane distillation, Journal of Membrane Science 124 (1997) 1–25. [2] E. Curcio, E. Drioli, Membrane distillation and related operations—a review, Separation & Purification Reviews 34 (2005) 35–86. [3] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, Journal of Membrane Science 285 (2006) 4–29.

S. Simone et al. / Journal of Membrane Science 364 (2010) 219–232 [4] E. Drioli, A. Criscuoli, E. Curcio, Membrane Contactors: Fundamentals, Applications and Potentialities, Membrane Science and Technology Series 11, Elsevier, Amsterdam, Boston, 2006. [5] C. Cabassud, D. Wirth, Membrane distillation for water desalination: how to chose an appropriate membrane, Desalination 157 (2003) 307–314. [6] Y. Xu, B.K. Zhu, Y.Y. Xu, Pilot test of vacuum membrane distillation for seawater desalination on a ship, Desalination 189 (2006) 165–169. [7] J. Koschikowski, M. Wieghaus, M. Rommel, Solar thermal-driven desalination plants based on membrane distillation, Desalination 156 (2003) 295–304. [8] F. Banat, S. Al-Asheh, M. Qtaishat, Treatment of waters colored with methylene blue dye by vacuum membrane distillation, Desalination 174 (2005) 87–96. [9] A. Criscuoli, J. Zhong, A. Figoli, M.C. Carnevale, R. Huang, E. Drioli, Treatment of dye solutions by vacuum membrane distillation, Water Research 42 (2008) 5031–5037. [10] F.A. Banat, J. Simandl, Removal of benzene traces from contaminated water by vacuum membrane distillation, Chemical Engineering Science 51 (1996) 1257–1265. [11] M.S. EL-Bourawi, M. Khayet, R. Ma, Z. Ding, Z. Li, X. Zhang, Application of vacuum membrane distillation for ammonia removal, Journal of Membrane Science 301 (2007) 200–209. [12] T. Mohammadi, M. Akbarabadi, Separation of ethylene glycol solution by vacuum membrane distillation (VMD), Desalination 181 (2005) 35–41. [13] H.J. Huang, S. Ramaswamy, U.W. Tschirner, B.V. Ramarao, A review of separation technologies in current and future biorefineries, Separation and Purification Technology 62 (2008) 1–21. [14] S. Al-Asheha, F. Banat, M. Qtaishat, M. Al-Khateeb, Concentration of sucrose solutions via vacuum membrane distillation, Desalination 195 (2006) 60–68. [15] T. Mohammadi, O. Bakhteyari, Concentration of l-lysine monohydrochloride (l-lysine–HCl) syrup using vacuum membrane distillation, Desalination 200 (2006) 591–594. [16] M.A. Izquierdo-Gil, J. Abildskov, G. Jonsson, The use of VMD data/model to test different thermodynamic models for vapor–liquid equilibrium, Journal of Membrane Science 239 (2004) 227–241. [17] V. Soni, J. Abildskov, G. Jonsson, R. Gani, Modeling and analysis of vacuum membrane distillation for the recovery of volatile aroma compounds from black currant juice, Journal of Membrane Science 320 (2008) 442–455. [18] Z.P. Zhao, F.W. Ma, W.F. Liu, D.Z. Liu, Concentration of ginseng extracts aqueous solution by vacuum membrane distillation. 1. Effects of operating conditions, Desalination 234 (2008) 152–157. [19] F. Banat, F.A. Al-Rub, K. Bani-Melhem, Desalination by vacuum membrane distillation: sensitivity analysis, Separation and Purification Technology 33 (2003) 75–87. [20] A. Criscuoli, M.C. Carnevale, E. Drioli, Evaluation of energy requirements in membrane distillation, Chemical Engineering and Processing 47 (2008) 1098–1105. [21] S. Al-Obaidani, E. Curcio, F. Macedonio, G. Di Profio, H. Al-Hinai, E. Drioli, Potential of membrane distillation in seawater desalination: thermal efficiency, sensitivity study and cost estimation, Journal of Membrane Science 323 (2008) 85–98. [22] Y Wu, Y. Kong, X. Lin, W. Liu, J. Xu, Surface-modified hydrophilic membranes in membrane distillation, Journal of Membrane Science 72 (1992) 189–196. [23] Y. Kong, X. Lin, Y. Wu, J. Chen, J. Xu, Plasma polymerization of octafluorocyclobutane and hydrophobic microporous composite membranes for membrane distillation, The Journal of Applied Polymer Science 46 (1992) 191–199. [24] M. Khayet, T. Matsuura, Application of surface modifying macromolecules for the preparation of membranes for membrane distillation, Desalination 158 (2003) 51–56. [25] M. Khayet, J.I. Mengual, T. Matsuura, Porous hydrophobic/hydrophilic composite membranes: application in desalination using direct contact membrane distillation, Journal of Membrane Science 252 (2005) 101–113. [26] S.R. Krajewski, W. Kujawski, M. Bukowska, C. Picard, A. Larbot, Application of fluoroalkylsilanes (FAS) grafted ceramic membranes in membrane distillation process of NaCl solutions, Journal of Membrane Science 281 (2006) 253–259. [27] B. Li, K.K. Sirkar, Novel membrane and device for vacuum membrane distillation-based desalination process, Journal of Membrane Science 257 (2005) 60–75. [28] J. Xua, M. Furuswa, A. Itoa, Air-sweep vacuum membrane distillation using fine silicone, rubber, hollow-fibre membranes, Desalination 191 (2006) 223–231. [29] Z Jin, D.L. Yang, S.H. Zhang, X.G. Jian, Removal of 2,4-dichlorophenol from wasterwater by vacuum membrane distillation using hydrophobic PPESK hollow fibre membrane, Chinese Chemical Letters 18 (2007) 1543–1547. [30] Z. Jin, D.L. Yang, S.H. Zhang, X.G. Jian, Hydrophobic modification of poly(phthalazinone ether sulfoneketone) hollow fibre membrane for vacuum membrane distillation, Chinese Chemical Letters 19 (2008) 367–370. [31] Z. Jin, D.L. Yang, S.H. Zhang, X.G. Jian, Hydrophobic modification of poly(phthalazinone ether sulfone ketone) hollow fibre membrane for vacuum membrane distillation, Journal of Membrane Science 310 (2008) 20–27. [32] Z.D. Hendren, J. Brant, M.R. Wiesnera, Surface modification of nanostructured ceramic membranes for direct contact membrane distillation, Journal of Membrane Science 331 (2009) 1–10. [33] M. Qtaishat, D. Rana, M. Khayet, T. Matsuura, Preparation and characterization of novel hydrophobic/hydrophilic polyetherimide composite membranes for desalination by direct contact membrane distillation, Journal of Membrane Science 327 (2009) 264–273. [34] M. Qtaishat, M. Khayet, T. Matsuura, Novel porous composite hydrophobic/hydrophilic polysulfone membranes for desalination by direct con-

[35]

[36]

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

231

tact membrane distillation, Journal of Membrane Science 341 (2009) 139–148. S.P. Deshmukh, K. Li, Effect of ethanol composition in water coagulation bath on morphology of PVDF hollow fibre membranes, Journal of Membrane Science 150 (1998) 75–85. D. Wang, K. Li, W.K. Teo, Preparation and characterization of Polyvinylidene fluoride (PVDF) hollow fibre membranes, Journal of Membrane Science 163 (1999) 211–220. J. Kong, K. Li, Preparation of PVDF hollow-fibre membranes, Journal of Applied Polymer Science 81 (2001) 1643–1653. M. Khayet, C.Y. Feng, K.C. Khulbe, T. Matsuura, Preparation and characterization of polyvinylidene fluoride hollow fibre membranes for ultrafiltration, Polymer 43 (2002) 3879–3890. Y. Fujii, S. Kigoshi, H. Iwatani, M. Aoyama, Selectivity and characteristics of direct contact membrane distillation type experiments. I. Permeability and selectivity through dried hydrophobic fine porous membranes, Journal of Membrane Science 72 (1992) 53–72. Y. Fujii, S. Kigoshi, H. Iwatani, M. Aoyama, Y. Fusaoka, Selectivity and characteristics of direct contact membrane distillation type experiments. II. Membrane treatment and selectivity increase, Journal of Membrane Science 72 (1992) 73–89. ˜ J.I. Mengual, Characterization of membrane disJ.M. Ortiz de Zárate, L. Pena, tillation membranes prepared by phase inversion, Desalination 100 (1995) 139–148. M. Tomaszewska, Preparation and properties of flat-sheet membranes from polyvinylidene fluoride for membrane distillation, Desalination 104 (1996) 1–11. C.S. Feng, B. Shi, G. Li, Y. Wu, Preparation and properties of microporous membrane from poly(vinylidene fluoride-cotetrafluoroethylene) (F2.4) for membrane distillation, Journal of Membrane Science 237 (2004) 15–24. M. Khayet, T. Matsuura, Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation, Industrial & Engineering Chemistry Research 40 (2001) 5710–5718. M. Khayet, K.C. Khulbe, T. Matsuura, Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process, Journal of Membrane Science 238 (2004) 199–211. B. Wu, K. Li, W.K. Teo, Preparation and characterization of poly(vinylidene fluoride) hollow fibre membranes for vacuum membrane distillation, Journal of Applied Polymer Science 106 (2007) 1482–1495. B. Wu, X. Tan, K. Li, W.K. Teo, Removal of 1,1,1-trichloroethane from water using a polyvinylidene fluoride hollow fibre membrane module: vacuum membrane distillation operation, Separation and Purification Technology 52 (2006) 301–309. C. Feng, K.C. Khulbe, T. Matsuura, R. Gopal, S. Kaur, S. Ramakrishna, M. Khayet, Production of drinking water from saline water by air-gap membrane distillation using polyvinylidene fluoride nanofiber membrane, Journal of Membrane Science 311 (2008) 1–6. K. Wang, T. Chung, M. Gryta, Hydrophobic PVDF hollow fibre membranes with narrow pore size distribution and ultra-thin skin for the freshwater production through membrane distillation, Chemical Engineering Science 63 (2008) 2587–2594. C. Kuo, H. Lin, H. Tsai, D. Wang, J. Lai, Fabrication of a high hydrophobic PVDF membrane via nonsolvent induced phase separation, Desalination 233 (2008) 40–47. S. Bonyadi, T.S. Chung, Flux enhancement in membrane distillation by fabrication of dual layer hydrophilic–hydrophobic hollow fibre membranes, Journal of Membrane Science 306 (2007) 134–146. M. Teoh, T. Chung, Membrane distillation with hydrophobic macrovoid-free PVDF–PTFE hollow fibre membranes, Separation and Purification Technology 66 (2009) 229–236. K.Y. Wang, S.W. Foo, T. Chung, Mixed matrix PVDF hollow fibre membranes with nanoscale pores for desalination through direct contact membrane distillation, Industrial & Enginnering Chemistry Research 48 (2009) 4474–4483. S. Bonyadi, T. Chung, Highly porous and macrovoid-free PVDF hollow fibre membranes for membrane distillation by a solvent-dope solution co-extrusion approach, Journal of Membrane Science 331 (2009) 66–74. D. Hou, J. Wang, D. Qu, Z. Luan, X. Ren, Fabrication and characterization of hydrophobic PVDF hollow fibre membranes for desalination through direct contact membrane distillation, Separation and Purification Technology 69 (2009) 78–86. F. Tasselli, J.C. Jansen, F. Sidari, E. Drioli, Morphology and transport property control of modified poly(ether ether ketone) (PEEKWC) hollow fibre membranes prepared from PEEKWC/PVP blends: influence of the relative humidity in the air gap, Journal of Membrane Science 255 (2005) 13–22. J.J. Qin, Y.M. Cao, Y.Q. Li, Y. Li, M.H. Oo, H. Lee, Hollow fibre ultrafiltration membranes made from blends of PAN and PVP, Separation and Purification Technology 36 (2004) 149–155. B. Jung, J.K. Yoon, B. Kim, H.W. Rhee, Effect of molecular weight of polymeric additives on formation, permeation properties and hypochlorite treatment of asymmetric polyacrylonitrile membranes, Journal of Membrane Science 243 (2004) 45–57. J.J Qin, M.H. Oo, Y. Li, Development of high flux polyethersulfone hollow fibre ultrafiltration membranes from a low critical solution temperature dope via hypochlorite treatment, Journal of Membrane Science 247 (2005) 137–142.

232

S. Simone et al. / Journal of Membrane Science 364 (2010) 219–232

[60] L.S. Wan, Z.K. Xu, Z.G. Wang, Leaching of PVP from polyacrylonitrile/PVP blending membranes: a comparative study of asymmetric and dense membranes, Journal of Polymer Science Part B: Polymer Physics 44 (2006) 1490–1498. [61] S. Rouaix, C. Causserand, P. Aimar, Experimental study of the effects of hypochlorite on polysulfone membrane properties, Journal of Membrane Science 277 (2006) 137–147. [62] I.M. Wienk, E.E.B. Meuleman, Z. Borneman, T. van den Boomgaard, C.A. Smolders, Chemical treatment of a polymer blend: mechanism of the reaction of hypochlorite with poly(vinyl pyrrolidone), Journal of Polymer Science Part A: Polymer Chemistry 33 (1995) 49–54. [63] R.M. Boom, I.M. Wienk, T. van den Boomgaard, C.A. Smolders, Microstructures in phase inversion membranes. Part 2. The role of a polymeric additive, Journal of Membrane Science 73 (1992) 277–292. [64] R.M. Boom, T. van den Boomgaard, C.A. Smolders, Mass transfer and thermodynamics during immersion precipitation for a two-polymer system: evaluation with the system PES–PVP–NMP–water, Journal of Membrane Science 90 (1994) 231–249. [65] M.J. Han, S.T. Nam, Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane, Journal of Membrane Science 202 (2002) 55–61.

[66] C.A. Smolders, A.J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phaseinversion membranes. Part 1. Formation of macrovoids, Journal of Membrane Science 73 (1992) 259–275. [67] N. Chen, L. Hong, Surface phase morphology and composition of the casting films of PVDF–PVP blend, Polymer 43 (2002) 1429–1436. [68] K.W. Lee, B.K. Seo, S.T. Nam, M.J. Han, Trade-off between thermodynamic enhancement and kinetic hindrance during phase inversion in the preparation of polysulfone membranes, Desalination 159 (2003) 289–296. [69] A. Bottino, G. Capannelli, S. Munari, Effect of coagulation medium on properties of sulphonated polyvinylene fluoride membranes, Journal of Applied Polymer Science 30 (1985) 3009–3022. [70] P. Sukitpaneenit, T.S. Chung, Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology, Journal of Membrane Science 340 (2009) 192–205. [71] N. Peng, T.S. Chung, K.Y. Wang, Macrovoid evolution and critical factors to form macrovoid-freee hollow fibre membranes, Journal of Membrane Science 61 (2008) 363–372.