Preparation and characterization of hydrophobic PVDF membranes by ...

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May 10, 2013 - casting solutions. Solutions with low and high PVDF contents were co-cast as the upper layer and support layer of the membrane that was ...
J Polym Res (2013) 20:134 DOI 10.1007/s10965-013-0134-4

ORIGINAL PAPER

Preparation and characterization of hydrophobic PVDF membranes by vapor-induced phase separation and application in vacuum membrane distillation Hongwei Fan & Yuelian Peng & Zhehao Li & Ping Chen & Qi Jiang & Shaobin Wang Received: 25 October 2012 / Accepted: 26 March 2013 / Published online: 10 May 2013 # Springer Science+Business Media Dordrecht 2013

Abstract Hydrophobic symmetric flat-sheet membranes of polyvinylidene fluoride (PVDF) for use in vacuum membrane distillation (VMD) were successfully fabricated by the vaporinduced phase separation (VIPS) method using the doublelayer casting process. To avoid the delamination that often occurs in double-layered membranes, the same PVDF polymer was employed in both the upper layer and support layer casting solutions. Solutions with low and high PVDF contents were co-cast as the upper layer and support layer of the membrane that was formed. In the VIPS process, the low PVDF content solution favored the formation of a layer with a porous and hydrophobic surface, whereas the solution with a high PVDF concentration favored the formation of a layer with high mechanical strength. The effect of the vaporinduced time on the morphological properties of the membranes was studied. As the vapor-induced time was increased,

the cross-section of the membrane changed from an asymmetrical finger-like structure to a symmetrical sponge-like structure, and the surface of the membrane became rough and porous. The membrane subjected to the longer vaporinduced time also exhibited a higher permeating flux during the VMD process. The best PVDF membrane fabricated in this study had a mean radial pore size of 0.49 μm, and the rough upper surface produced a static contact angle of 145° with water. During the VMD process with a 3.5 wt.% sodium chloride (NaCl) aqueous solution, the best membrane that was fabricated produced a permeating flux of 22.4 kg m−2 h−1 and an NaCl rejection rate of 99.9 % at a feed temperature of 73 °C and a downstream pressure of 31.5 kPa. This performance is comparable to or superior to the performances of most of the flat-sheet PVDF membranes reported in the literature and a polytetrafluoroethylene membrane used in this study.

H. Fan : Y. Peng (*) Center of Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China e-mail: [email protected]

Keywords Double-layer casting process . PVDF . Membrane distillation . Desalination . Hydrophobic surface

Z. Li Changchun Gold Research Institute, Changchun 130012, People’s Republic of China P. Chen The Research Institute of Environmental Protection, North China Pharmaceutical Group Corporation, Shijiazhuang 050015, People’s Republic of China Q. Jiang National Major Science and Technology Program Management Office for Water Pollution Control and Treatment, MEP, Beijing 100029, People’s Republic of China S. Wang Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Nomenclature A Effective area of the membrane (m2) B Geometric factor Ba Gas permeance coefficient (mol m−2 S−1 Pa−1) Cf Concentration in the feed solution (g L−1) Cp Concentration in the permeate (g L−1) I0 Intercept defined in Eq. 2 (mol m−2 S−1 Pa−1) J Permeate flux (kg m−2 h−1) kB Boltzmann constant (J K−1) LP Effective pore length (m) LEPw Water entry pressure (Pa) M Molecular weight of gas (kg mol−1) P Total pressure (Pa) Pm Mean pressure (Pa) ΔP Transmembrane pressure (Pa)

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Pdownstream vap Pwater Q r R Rj S0 T t

Pressure at downstream (Pa) Vapor pressure of pure water (Pa) Total mass of the permeate (kg) Membrane pore radius (m) Gas constant (J mol−1 K−1) Rejection rate (%) Slope defined in Eq. 2 (mol m−2 S−1 Pa−2) Absolute temperature (K) Running time (h)

Greek letters αwater Activity of water δ Membrane thickness (m) λ Mean free path (m) γL Liquid surface tension (Pa m) ε Porosity (%) ε/LP Effective porosity (m−1) τ Tortuosity of membrane μ Gas viscosity (kg m−1 s−1) θ Liquid/solid contact angle (°) ρ Density (kg m−1 s−1) σ Collision diameter (m)

Introduction Membrane distillation (MD) is a versatile and evolving physical separation technology that has been widely applied for many applications, including water desalination, water reuse, food processing, and the removal of volatile organic compounds from water [1–4]. MD, a promising desalination technology for recovering water from high-salinity solutions such as RO and ED brines, refers to the thermally driven transport of vapor through the pores of a hydrophobic microporous membrane, which involves simultaneous mass and heat transfer. The driving force in MD is the vapor pressure difference induced between both sides of the membrane [4]. This vapor pressure difference can be established utilizing various different configurations: (a) direct contact MD (DCMD); (b) air gap MD (AGMD); (c) sweeping gas MD (SGMD); or (d) vacuum MD (VMD) [5]. In the VMD configuration, because vacuum is applied at the permeating side, the permeating flux is higher for a larger vapor pressure difference across the membrane [6]. There is no boundary layer on the permeating side, and the heat transfer by conduction through the membrane is practically negligible [7–11]. VMD has therefore been a recent focus of research in the field of MD. Despite its great potential, MD has not been industrialized. One important reason for this is its relatively low flux compared to the reverse osmosis process. Flux enhancement of MD is the main focus of research of membrane scientists.

J Polym Res (2013) 20:134

Membrane performance is correlated with membrane structural parameters such as mean pore radius, thickness, pore radius distribution, porosity, and tortuosity. In the VMD configuration, to obtain a high flux, the membrane should be as thin as possible. The porosity and pore radius should be as large as possible, because these three parameters govern the resistance to mass transfer through the membrane. Special care must be taken in VMD to prevent the membrane from wetting, because the pressure difference between the interfaces is typically higher than the interfacial pressure of other MD configurations. For this reason, the trans-membrane hydrostatic pressure must be kept below the minimum water entry pressure of water (LEPw) for the membrane [12]. The Laplace (Cantor) equation [1] provides the relationship between the largest allowable pore radius (rmax) of the membrane, the liquid/solid contact angle (CA), and LEPw. The hydrophobicity of the membrane surface should be as high as possible. Unfortunately, there is no specific membrane for MD as yet. The porous membranes that have been used in MD are generally made of hydrophobic materials such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) [13]. The membranes that are available in capillary or flat-sheet forms are actually made for microfiltration. PTFE generally possesses a higher hydrophobicity than PVDF, and a porous PTFE membrane has a higher permeating flux than a PVDF membrane in MD. However, considering the difficulty involved in processing PTFE, PVDF is a more promising polymer for use in MD. Hydrophobic porous membranes can be prepared by different techniques depending on the properties of the materials considered [14]. The most common process is based on the phase inversion of a casting solution. Non-solventinduced phase separation (NIPS) is widely used in the membrane preparation process [4, 15, 16]. An asymmetric PVDF membrane with a dense skin layer and a finger-like void sublayer was obtained by wet casting [17, 18]. The disadvantage of using the membrane prepared with the above method is the low permeating flux in the VMD process, because the dense skin layer that is formed in the rapid mass exchange between solvent and nonsolvent increases the transfer resistance of the vapor. The vapor-induced phase separation (VIPS) method can produce a membrane with a porous top surface because the exchange between the nonsolvent and solvent is relatively slow during the film-forming process, and the pore size can be regulated effectively by adjusting the vapor-induced time (defined in the next paragraph) [19, 20]. This type of membrane often leads to a high permeating flux. VIPS was therefore chosen for the membrane preparation method in this work. A solution with a low polymer concentration is often favorable when attempting to form a thin membrane with a porous and hydrophobic surface via VIPS. The surface may even become superhydrophobic. However, the problem with a membrane fabricated with a solution with a very low polymer content is that it often becomes fragile and has low mechanical

J Polym Res (2013) 20:134

strength. For this reason, the VIPS process combined with a double-layer casting process was proposed as the approach to use to prepare the PVDF flat-sheet membranes for the VMD in this work. The double-layer casting process is used to prepare double membranes [21]. In the double-layer casting process, a coating solution layer is extruded simultaneously with a support solution layer (or the membrane layer). The cocast dual-layer solutions are then immersed in a coagulation bath for solidification. The final membranes are doublelayered, with different pore sizes in each layer. When the polymers in both layers are different, the phenomenon of delamination can easily occur. In this work, because the same PVDF polymer was employed in both solution layers, the phenomenon of delamination was circumvented. The co-cast dual-layer solutions were placed in saturated water vapor for a period of time (the vapor-induced time) before being immersed in the coagulation bath. Using this method, thin membranes with high strength and high hydrophobicity could be fabricated (more details are provided in “Casting solution preparation and co-casting”). The effect of the vaporinduced time on the morphological properties of the membranes was studied. The relative humidity (RH) of the air greatly influenced the hydrophobicity of the membrane surface in the VIPS process. The utilization of higher RH values meant that more water was needed at the film surface to equalize the increased chemical potential of water in the gaseous phase, meaning that the total amount of absorbed water increased, but not by enough to initiate phase separation. Hence, the film solution was in the metastable region in the phase diagram, which was rather close to the spinodal line [22]. When this solution was immersed in the water bath, spinodal decomposition occurred, and symmetrical membranes with bicontinuous structure and large pores in the top surface were obtained. Because of the crystalline nature of PVDF, a large quantity of spherical beads with diameters ranging from 0.3 μm to 1.0 μm adhered to the membrane surface. The bicontinuous structure and the presence of the spherical beads significantly improved the hydrophobicity of the PVDF membrane surface. The CA of water even increased to 150° [22]. In this study, the RH was maintained at 100 % in the device during membrane formation. The membranes prepared in this study were characterized in terms of nonwettability, thickness, porosity, pore size, and pore size distribution. VMD experiments were conducted using pure water and a 3.5 wt.% NaCl aqueous solution as feed. The effects of operating conditions such as the feed temperature and the vacuum on the permeating side on the flux were explored. By comparing with a flat-sheet PTFE membrane with a similar pore radius, thickness, and hydrophobicity to those of the flat-sheet PVDF membranes prepared in this study, the dependences of the VMD fluxes and NaCl rejection rates of the PVDF membranes on their

Page 3 of 15, 134

geometric properties were determined. The desalination performance of each membrane during 6 h of operation was monitored. The best-performing membrane prepared in this work was also successfully used in VMD and direct contact membrane distillation [23].

Experimental Materials The flat-sheet PTFE membrane, with a nominal pore radius of 0.2 μm, was provided by the Beijing Institute of Plastic Research (Beijing, China). The specifications of this membrane are shown in Table 1. PVDF, with an average molecular weight of 400,000 g/mol, was supplied by Shanghai 3 F New Material Co., Ltd. (Shanghai, China). Dimethylacetamide (DMAc, synthesis grade) supplied by Beijing Chemical Works (Beijing, China) was used as the solvent for the PVDF. Lithium chloride (LiCl, synthesis grade) was supplied by Beijing Yili Fine Chemicals Co. Ltd. (Beijing, China). Tianjin Fu Chen Chemical Reagent Factory (Tianjin, China) supplied the 1,2-propylene glycol (PG). Isopropyl alcohol (IPA, GR grade, Merck, Darmstadt, Germany) was used as a wetting liquid for the membranes. Ethanol (GR grade, Merck) was used for the solvent-exchange method. Casting solution preparation and co-casting PVDF was dried in an oven at 100 °C for over 48 h before use. Casting solution A (PVDF at 16 wt.%, LiCl at 4 wt.%, and DMAc at 80 wt.%) and solution B (PVDF at 10 wt.%, PG at 30 wt.%, and DMAc at 60 wt.%) were prepared. The two solutions were then stirred until they mixed well. Finally, the two solutions were placed into an oven at 65 °C and allowed to stand for de-aeration for approximately 12 h before casting. Two-bladed knives with blade thicknesses of 150 μm and 200 μm were used to cast the two de-aerated solutions onto a dry, clean glass plate. The polymer solutions used to obtain the support layer and the upper layer were referred to as solution A and solution B, respectively. Using the twobladed knife with a thickness of 150 μm, solution A was first cast on the dry clean glass plate to form a liquid film. Using the two-bladed knife with a thickness of 200 μm, solution B was then cast on the surface of the liquid film of A 1–3 s later [24, 25]. Solution B turned into a gel at room temperature, but the gel became clear again as the dissolution temperature was increased. The temperatures of the solutions and glass plates were therefore maintained at a certain value to ensure that the solutions were dissolved throughout the whole casting process. The cast films, together with the glass plates, were immediately placed into the saturated water vapor. After a period of time, the cast

134, Page 4 of 15 Table 1 A quantitative summary of the surface roughness

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Membrane

Root mean square roughness, RMS (nm)

Average roughness, Ra (nm)

Peak-to-valley height, Rz

MT0

77

59.4

599

0.199

3.44

MT1 MT2 MT5

129 155 305

105 120 239

967 1080 2880

0.41 −0.806 0.361

2.86 4 3.81

Skewness, Rsk

Kurtosis, Rku

films, together with the glass plates, were taken out and immersed in deionized water at 20 °C for solidification. The time period that the glass plates remained in the saturated water vapor is defined as the “vapor-induced time.” During solidification, the membranes spontaneously peeled off the glass plates. These membranes were subsequently transferred to pure water and maintained for approximately 24 h to remove the residual solvent and additives. Membranes MT0, MT1, MT2, and MT5 had vapor-induced times of 0 min, 1 min, 2 min, and 5 min, respectively. Due to the significant shrinkage observed for the PVDF membranes during the drying process, these membranes were immersed in ethanol for approximately 4 h before drying. In this process, the water in the membrane pores was replaced with ethanol, which has a lower surface tension. These membranes were subsequently dried at room temperature before the characterization tests.

roughness and the average (ave) roughness] was calculated using the PicoScan 5.3.3 Pclink software package.

Membrane characterization

Here, ρm is the density of the membrane and ρpol is the density of the polymeric material. The membrane thickness was measured using an electronic outside micrometer (0–25 mm, 0.0001 mm, 908.750, Schut Geometrische Meettechniek bv, Groningen, The Netherlands). The thickness of each membrane was measured five times over different areas, and the average value was taken.

SEM and AFM characterization The surface and cross-section of each membrane were examined using a field emission scanning electron microscope (Quanta 200, FEI, Hillsboro, OR, USA). All the samples were coated with a thin layer of gold before the SEM observations. Cross-sections were prepared by fracturing the membranes in liquid nitrogen. To characterize the top surfaces of the membranes, an atomic force microscope (Pico Scan TM 2500, Agilent Technologies, Santa Clara, CA, USA) was used to capture the surface morphology. Small pieces approximately 0.5 cm×0.5 cm in area were cut from each membrane and fixed over a magnetic holder using double-sided adhesive tape. The scans were performed in air at room temperature. The images were scanned in tapping mode using a silicon cantilever (Olympus, Tokyo, Japan) with a nominal radius of 7 nm, a spring constant (ks) of 42 N m−1, and a resonance frequency of about 300 kHz. Scanning was performed at a speed of 0.6 lines/s, and a scan area of 20 μm×20 μm was selected randomly on the membrane surface for analysis. A sampling resolution of 256 points per line was used. The cantilever was tuned to a free air amplitude of 3 V. After scanning, the surface roughness [the root mean square (rms)

Membrane porosity and thickness The membrane porosity is defined as the volume of the pores in the membrane divided by the total volume of the membrane. This parameter can be determined by measuring the density of the polymer material in IPA, which penetrates into the pores of the membrane, and the density of the membrane in pure water, which does not enter the pores. In this method, a pycnometer (84032, Shanghai Huake Labware Co. Ltd., Shanghai, China) and a balance were employed. The porosity of the membrane can be calculated using the following equation, suggested by Smolder and Franken [26]: "¼1

ρm : ρpol

ð1Þ

Pore radius and pore radius distribution measurement The average pore radius of the flat-sheet PVDF membranes prepared in this work was determined by the following methods: the gas permeation method [27], the wet and dry flow method, and based on measurements from SEM images and TM-AFM images. The gas permeation test was performed prior to the measurement of LEPw. The experimental apparatus used for these measurements is shown in Fig. 1. The effective membrane area was 44.2×10−4 m2. The permeating flux of air through the dried membranes was measured at various trans-membrane pressures in the range 10–250 kPa at room temperature using a soap-bubble flow meter when the gas flow rate was low and a rotameter when the flow rate was high. The gas permeance coefficient, Ba, for a porous membrane contains both a diffusive term and a viscous term that

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Page 5 of 15, 134 2

P

average of all the measurements of the pores that appeared in the surface image. To obtain the mean pore radius, cross-sectional line profiles were selected that traversed the surface areas shown in the TM-AFM images. The diameters of the pores (i.e., low valleys) were measured using some line segments along the reference line. The horizontal distance between each of the line segments was taken as the diameter of the pore, and half the geometric value of the distance between these line segments was taken as the mean pore radius [29, 30].

5

3 4 2 1

P

3 3

6

1-Air compressor; 2-Pressure gauge; 3-Pressure regulator; 4-Ultrafiltration cup; 5-Soap froth flowmeter; 6-Glass tube float flowmeter

Fig. 1 Schematic of the experimental system used for gas permeation tests

depends on the pressure, as expressed by the following equation:  0:5 4 2 r" Pm r2 " Ba ¼ þ ¼ I 0 þ S0 P m ; ð2Þ 3 pMRT Lp 8μRT Lp where R is the gas constant, T is the absolute temperature, M is the molecular weight of the gas, μ is the gas viscosity, Pm is the mean pressure within the membrane pore, r is the membrane pore radius, ε is the porosity, and LP is the effective pore length. By plotting the linear dependence between the permeance coefficient, Ba, and the mean pressure, Pm, the intercept I0 and the slope S0 can be determined, and the pore radius can then be calculated using the following equation [15]:    16 S0 8RT 0:5 r¼ μ: ð3Þ 3 I0 pM The wet and dry flow method was employed to determine the maximum pore radius, the mean pore radius, and the pore radius distribution of each membrane. The experimental apparatus used is shown in Fig. 1. In this experiment, IPA, which had a low surface tension value of 21.7 mN/m [28], was used as the wetting liquid, the gas permeation velocity was measured at room temperature, and the downstream pressure was maintained at atmospheric pressure. The range of pressures upstream depended on the membrane. The pore radius, r, can be calculated by the Laplace equation [1]: r¼

2g L ΔP

ð4Þ

Here, γL is the surface tension of the IPA. The ratio between the wet and dry flow rates can be plotted versus the pore radius. The mean pore radius and maximum pore radii can also be measured from the SEM images of the membrane surface. The pore radii that were determined were based on the

CA of water The CA of water was measured as an indicator of hydrophobicity. The CA of the membrane surface was assessed using an instrument that measured contact angles (DSA100, Krüss, Hamburg, Germany) equipped with a videocapturing system. The CA of static water was measured by the sessile drop method. A 5 μL drop was formed on the flat surface of the membrane using a syringe. The CA value of each membrane was measured five times at different locations, and the average was taken. LEPw measurement LEPw depends on the maximum pore radius and the hydrophobicity of the membrane surface. The apparatus used for this measurement has already been shown in the literature [31]. The membrane was placed in an ultrafiltration cup, the feed side of the upper chamber was filled with 0.5 L of pure water, and the lower chamber—the permeating side—was connected to a digital capillary flowmeter. Pressure was then applied to the water by an air compressor. The pressure was increased by 5 kPa each time until the water penetrated through the membrane and left the ultrafiltration cup. As soon as water started to flow, the pressure was recorded, and this value was considered the LEPw for the tested membrane. VMD process Figure 2 shows a schematic diagram of the VMD experimental apparatus. A flat-sheet VMD configuration with an area of 26.4×10−4 m2 was employed. The feed flow rate of 54 L/h was controlled by a peristaltic pump (BT300-1 J, Baoding Longer Precision Pump Co, Ltd., Hebei, China). While performing MD, the pressure reading was unclear for both the inlet and outlet pressure gauges because the feed flow rate (54 L/h) was rather low, so we regarded the inlet pressure as a constant pressure (1.01×105 Pa). The feed temperature was controlled by a thermostat water bath and varied over the range of 45–65 °C. The feed side temperature was measured by a sensor connected to a digital temperature gauge with an accuracy of ±0.1 °C (YF902C, Shenzhen Electronic

134, Page 6 of 15

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7

5

P

T

2

T

of the feed (Cf) was 78,000 μs cm−1. The NaCl rejection rate, Rj, was determined via the following equation:

P

4 8

Rj ¼

10 P

6 3

1

T

9

1-Vacuum pump; 2-Membranes; 3-Peristaltic pump; 4-Regulator; 5-Temperature gauge; 6-Feed liquid; 7-Vaccum gauge; 8-Chiller; 9-Filter flask; 10-Thermostat water bath

Cf  Cp  100%; Cf

ð5Þ

where Cf is the NaCl concentration in the feed solution and Cp is the NaCl concentration in the permeating solution.

Results and discussion Membrane morphology

Fig. 2 Schematic diagram of the experimental apparatus used for the VMD experiment

Technology Co., Ltd., Shenzhen, China). The detection points were located close to the module inlet and outlet (distance 5 min could not be assessed in the VMD process for two reasons: these membranes were too fragile, and their maximum pore sizes were too big. Their LEPw values were too low, possibly leading to leakage, even though the CA of water was greater than 150°. The LEPw value of the PTFE was 150 kPa because of the high hydrophobicity of the membrane surface and its appropriate maximum pore radius.

MT0

0.8

Fwet/Fdry

Membrane

350

ΔF/Δr (106m2h-1)

Table 3 The mean pore radii r1, r2, r3, and r4 of the prepared PVDF membranes and the PTFE membrane as measured by four different methods (wet and dry flow method, gas permeation test, direct calculation from the SEM images of the membrane surface, as well as analysis of surface MT-AFM images)

MT1 MT2

0.6

MT5

0.4

PTFE

0.2 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

r (µm) Fig. 7 Pore radius distributions of the PVDF membranes and the PTFE membrane prepared by (1) gas permeation tests and (2) the dry and wet flow method

Pure-water VMD experiments In this work, the VMD fluxes of the membranes were assessed using pure water as the feed solution. The effect of the feed temperature and the pressure on the flux in the permeating side was studied. In the MD process, the feed temperature is the main operational parameter that significantly affects the MD flux,

280

LEPw, (kPa)

250 220 190 160 130 100

MT0

MT1

MT2

MT5

PTFE

ID of the Membrane Fig. 6 The lines obtained from Eq. 2 for MT0, MT1, MT2, MT3, MT4, MT5, and PTFE

Fig. 8 LEPw values of the prepared PVDF membranes and the PTFE membrane

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Fig. 9 Variation of the VMD flux with feed temperature. Vacuum in the permeating side, 70 kPa; feed flow, 54 L/h

Fig. 10 VMD flux versus rε/Lp for MT0, MT1, MT2, and MT5. Feed temperature, 73 °C; feed flow, 54 L/h; vacuum in the permeating side, 70 kPa

MD Flux, J, (kg m-2h-1)

due to the exponential increase in the vapor pressure with temperature. This type of dependence has been thoroughly investigated in a number of studies [29, 37, 38]. As shown in Fig. 9, all of the PVDF membranes and the PTFE membrane exhibited exponential increases in the VMD flux with increasing feed temperature. The flux of the PVDF membrane with the longer vapor-induced time was larger. It is known that, at least to a first approximation, VMD flux is proportional to εr/(δτ) [8], which is equal to rε/Lp, where τ is the tortuosity of the membrane. When MT5 was compared with MT2, the two membranes were found to have similar membrane thicknesses (Table 2) and tortuosities (Fig. 3) because they had similar symmetrical structures. MT5 had a larger flux due to its larger pore size (Table 3) and porosity or effective porosity (Table 2). Upon comparing MT0, MT1, and MT2, the effective porosity and pore size increased greatly with increasing vapor-induced time, which resulted in an increase in flux. The VMD flux is plotted in Fig. 10 versus the product of the pore radius and the effective porosity obtained in the gas permeation test. However, the fluxes of all of the PVDF membranes prepared in this work were lower than the flux of the PTFE membrane under the same operating conditions. For example, the largest flux of MT5, 23.6 kg m −2 h −1 , was lower than 27.1 kg m−2 h−1, the flux of the PTFE membrane when the

temperature of the feed solution was 73 °C, because the latter had a lower membrane thickness and a higher porosity, effectively decreasing the vapor transfer resistance and increasing the vaporization area. In the VMD process, the driving force for permeation is the vapor pressure difference across the membrane. Most of the research published in this area studied the VMD process when the vacuum applied on the permeating side was very high or nearly close to atmospheric pressure. Under these conditions, the pressure on the permeating side was considered the vapor pressure, and the driving force for permeation was the pressure difference between the liquid saturation vapor pressure upstream and the pressure on the permeating side. However, in this work, the vacuum applied on the permeating side was relatively low (in the range of 20 to 70 kPa). Under these conditions, the VMD process could also be realized because the vapor pressure corresponding to the temperature measured on the permeating side was always lower than the saturation vapor pressure at the upstream membrane surface. The variation of the VMD flux with the vacuum applied on the permeating side is shown in Fig. 11. In Fig. 11, the fluxes of all of the membranes are seen to increase fairly linearly with increasing vacuum on the permeating side, because the vapor difference across the membrane increased with decreasing vacuum pressure on the permeating side, and the driving force increased. The PVDF membranes with long vapor-induced times had the largest fluxes because the mean pore radius and the effective porosity increased as the vapor-induced time was increased. Similar trends were observed experimentally in previous studies [39–41]. The dusty gas model is generally applied to describe mass transfer across the membrane. This model involves Knudsen diffusion, molecular diffusion, surface diffusion, Poiseuille flow, or combinations of the above. However, surface diffusion is always neglected in membrane distillation. The mass transfer mechanism can be determined via the ratio of the mean free path of the transported gas molecule (λ) through the membrane pores to the mean pore radius of the membrane (r) [1, 14, 41]. Most of the

26 y = 2.3989 x + 13.5755 R² = 0.9328

24

22 20 18 16 14 1

1.5

2

2.5

3

( 10-3

3.5

r ) Lp

4

4.5

5

134, Page 12 of 15

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MD Flux, J, (kg m-2h-1)

MD Flux, J, (kg m-2h-1)

25 MT0

20

MT1 MT2

15

MT5

10 PTFE

5 0

0

10

20

30

40

50

60

70

Fig. 11 Variation of the VMD flux with the downstream vacuum value. Feed temperature, 65 °C; feed flow, 54 L/h

published research on the VMD process ignores molecular diffusion because a high vacuum was applied continuously on the permeating side [14, 41, 42] and the air trapped in the membrane pores was removed. However, the molecular diffusion should not be ignored when the vacuum applied on the permeating side is low. Under these conditions, molecular diffusion should be considered during the process of mass transfer. For the binary mixture of water vapor and air, the mean molecular free path (λm) is evaluated at the average membrane temperature (Tm) [43]: ð6Þ

where kB is the Boltzmann constant, P is the absolute pressure (Pa), σw and σa are the collision diameters for water vapor (2.61×10−10 m) and air (3.711×10−10 m), respectively, and Mw and Ma are the molecular weights of water and air, respectively. In this work, low vacuum values of 20–70 kPa were applied on the permeating side. Molecular diffusion should not be ignored during the process of mass transfer. At feed temperatures of 45–75 °C, the mean free path of water vapor

MD Flux, J, (kg m-2h-1)

30

25

Pure water 3.5 wt% NaCl

20 15

10 5 0 MT0

MT1

MT2

MT5

24 21 MT5

18

PTFE

15 1

2

3

4

5

6

7

Time (h)

Vacuum in permeate side, (kPa)

1 λm ¼ σ þσ 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; pð w 2 a Þ P 1 þ ðMw =Ma Þ

27

0

80

kB Tm

30

PTFE

ID of the Membrane Fig. 12 Permeating fluxes of PVDF and PTFE membranes in VMD desalination. Feed temperature, 73 °C; vacuum on the permeating side, 70 kPa; feed flow, 54 L/h.

Fig. 13 Flux vs. operating time in VMD. Hot feed solution: 3.5 wt.% NaCl, 73 °C, 54 L/h. Vacuum on the permeating side, 70 kPa.

in air calculated by Eq. 6 was between 0.18 μm and 0.33 μm, close to values of 0.17–0.49 μm, the mean pore radii of the membranes MT0, MT1, MT2, MT5, as well as the PTFE membrane. Therefore, the combination of Knudsen diffusion, molecular diffusion, and Poiseuille flow of the vapor through the membrane pores was responsible for the mass transport in VMD. The details of the mass-transfer simulation will be reported in later research. Saline-water VMD experiments The conductivity of the distillate cold water (Cp) was less than 40 μs cm−1 during the VMD process, indicating that the NaCl rejection was higher than 99.9 % for all of the tested membranes. Figure 12 shows the VMD permeating fluxes of all of the membranes used in desalination. When a 3.5 wt.% NaCl aqueous solution was used as the feed instead of pure water, the permeate fluxes of all the tested membranes decreased. However, a drop in the flux when using the salt solution as the feed was expected because the water vapor pressure at the membrane surface lowered when the water activity in the salt solution decreased and the driving force for vapor transport across the membrane divap minished. The driving force is Pwater awater  Pdownstream , where α water is the activity of water (98.2 % for a vap 3.5 wt.% NaCl solution [1]); Pwater is the vapor pressure of pure water. The theoretical decrease in driving force is 4.7 % when temperature polarization is considered. The fluxes actually decreased by 4–8 % for all of the tested membranes compared to when pure water was used as the feed, indicating that when the concentration in the feed solution was low, the concentration at the membrane is nearly the same as that in the bulk, and it has no influence on the process [44]. Therefore, it can be argued that the concentration polarization had little effect on the MD flux at the low feed concentration [45]. Figure 12 shows that, due to its high hydrophobicity, thinness, and unique membrane structure, the flux of the PTFE membrane was the highest, which is consistent with the experimental results given in [46].

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Table 4 Comparison of the membrane performances of various commercial flat-sheet membranes reported in the literature with that of a novel membrane fabricated in the present work Membrane type

Reference

MD type

Solution

Permeating flux (kg m−2 h−1)

PVDF SMM/PS (M1) SMM/PEI (M12) SMM/PEI (M2) PVDF PVA/PEG/PVDF SMM/PES iPP PVDF MT5

[16] [27] [47] [13] [15] [33] [48] [49] This work

DCMD DCMD DCMD DCMD VMD DCMD VMD VMD VMD

Tf =55 °C; Tp =25 °C; 0.3 M NaCl Tf =50 °C; Tp =40 °C; 0.5 M NaCl Tf =50 °C; Tp =40 °C; 0.5 M NaCl Tf =65 °C; Tp =15 °C; 0.5 M NaCl Tf =25 °C; Pp =1666.5 Pa; distilled water as feed Tf =70 °C; Tp =22 °C; 3.5 wt.% NaCl Tf =26 °C; Pp =400 Pa; distilled water as feed Tf =70 °C; Pp =3 kPa; 0.5 M NaCl Tf =73 °C; Pp =31 kPa; 3.5 wt.% NaCl

6.7 8.3 12.6 14 18 23.4 1.9 24.8 22.4

However, even though the fluxes of the membranes prepared in this work were lower than that of the PTFE membrane flux under the same operating conditions, MT5 still exhibited a high flux of 22.4 kg m−2 h−1 and showed good desalination performance in VMD.

performances reported in other published reports. In this work, the VMD flux was assessed at a low vacuum on the permeating side. Future work will further investigate the heat and mass transfer mechanisms that occur across the membrane in the VMD process under low vacuum.

Effect of test time on the VMD performance during desalination

Conclusions

During a 6-h test of membrane performance, distilled water was returned to the feed tank to maintain a constant NaCl concentration in the feed and thus eliminate the negative effects caused by concentration polarization near the membrane surface. The feed temperature and the vacuum on the permeating side were kept at 73 °C and 70 kPa, respectively. Figure 13 shows the variations in the VMD fluxes of MT5 and the PTFE membrane with operational time. The permeating flux decreased by only 4.8 % for MT5 and by 3.9 % for the PTFE membrane after a period of 6 h, but the NaCl rejection rates were still over 99.9 %, indicating that there was hardly any wetting during the 6 h test in VMD. This lack of wetting was due to the hydrophobicity of the membrane surface. Figure 6 shows that the water CAs of MT5 and the PTFE membrane surface were as high as 145° and 143°, respectively: high enough to prevent hot water from entering the membrane pores. Moreover, the values of LEPw measured for the two membranes were 120 kPa and 150 kPa, respectively: high enough for the VMD process. Comparison with other MD membranes Table 4 illustrates a comparison of flat-sheet membranes used in the MD desalination process; the data were obtained in the present work or reported in the literature. The newly developed laboratory-fabricated PVDF hydrophobic flatsheet membranes reported in the present work have performances that are comparable to or even better than the

A VIPS method in combination with a double-layer casting process was explored as a method of preparing flat-sheet PVDF membranes. To avoid the delamination that widely occurs in double-layer membranes, the same polymer (PVDF) was employed in both the upper layer and support layer solutions. Low- and high-concentration PVDF solutions were co-cast as the upper layer and support layer, respectively, of the formed membrane. The low-concentration PVDF solution favored the formation of a layer with a porous and hydrophobic surface in the VIPS process. The high-concentration PVDF solution favored the formation of a layer with high mechanical strength. A series of PVDF hydrophobic flat-sheet membranes with mean pore radii ranging from 0.17 to 0.49 μm were fabricated effectively and used in VMD. The effects of the vapor-induced time on the structure and geometric properties of the membranes were studied. With increasing vapor-induced time, the crosssections of the membranes changed from having asymmetrical finger-like structure to symmetrical sponge-like structure, and the surface of the membrane became both rough and porous. The best PVDF membrane had a thickness of 81.6 μm, a porosity of 78.6 %, and a static CA of 145° with water at the upper surface. When the VMD process was implemented with a 3.5 wt.% NaCl aqueous solution, the best fabricated membrane produced a permeating flux of 22.4 kg m−2 h−1 and an NaCl rejection rate of 99.9 % at a feed temperature of 73 °C and with a pressure of 31.5 kPa (the vacuum was 70 kPa) applied on the permeating side. This performance is comparable or superior to those of most of the flat-sheet PVDF membranes reported in the

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literature. When a flat-sheet PTFE membrane with a similar pore radius, thickness, and hydrophobicity to the best flat-sheet PVDF membrane prepared in this study was compared to that best PVDF membrane, the two membranes showed the same NaCl rejection rate and similar permeating flux values. Considering the difficulty involved in processing PTFE, the combination of the VIPS method with a double-layer casting process explored in this study is a promising method of fabricating porous and hydrophobic PVDF membranes for use in MD. Hardly any wetting was observed after 6 h of operation, indicating that the best flat-sheet PVDF membrane fabricated in this study possesses good VMD desalination performance. Acknowledgements The authors would like to thank Professor Liu Zhongzhou of the Research Center for Eco-Environmental Science, the Chinese Academy of Sciences, for his valuable discussions and suggestions. The authors also thank the National Natural Science Foundation of China for financial support (21176008).

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