ARTICLE pubs.acs.org/IECR
Development of Novel Multichannel Rectangular Membranes with Grooved Outer Selective Surface for Membrane Distillation May May Teoh, Na Peng, Tai-Shung Chung,* and Ley Ling Koo Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore ABSTRACT: In this study, poly(vinylidene fluoride) (PVDF) multichannel rectangular membranes were spun through the nonsolvent-induced phase separation method with the aid of a specially designed spinneret. This unique spinneret has an outer rectangular slit and seven inner needles arranged in series. The newly designed membranes have the combined advantages offered by (1) hollow fiber (i.e., high membrane area per volume ratio and easy assembly into membrane modules); (2) flat sheet membranes (i.e., greater mechanical durability and compressibility); and (3) woven or nonwoven spacers used in flat sheet modules (as mechanical supports). Microscopic views and scanning electron microscopy (SEM) microphotographs show an irregular inner contour align symmetrically in the hydrophobic PVDF membrane. Multichannel rectangular membranes with a grooved pattern have the following advantages: (1) easy handling, (2) can be acted as spacers to discrete membrane from attaching together, and (3) creating eddies flow at the membrane outer selective layer. Attempts are also made to understand the deformation of grooved outer surface and irregular inner contour. Apart from interesting membrane geometry, this study also explores the prospect of utilizing aforementioned membranes for seawater desalination via direct contact membrane distillation (DCMD). A promising distillated flux of 54.7 kg m2 s1, using a hot feed brine solution of 80 °C, is obtained through these newly spun multichannel rectangular membranes.
’ INTRODUCTION Hollow fibers (i.e., hollow fiber modules) and flat sheet asymmetric membranes (i.e., spiral wound modules) are two dominant membranes configurations used in water treatment and membrane distillation processes. Hollow fibers have a high membrane area per volume ratio and the advantage of easy assembly into membrane module.1 However, one of the major drawbacks of hollow fibers for membrane distillation (MD), microfiltration (MF), or ultrafiltration (UF) is the lower mechanical strength, compared to that of flat sheet membranes where a woven or nonwoven support layer is used. As a result, hollow fibers may deform or elongated easily during back washing and chemical cleaning. To overcome these problems, the multichannel rectangular membranes are designed to have the characteristics of greater mechanical durability by taking the strengths from flat sheet and hollow fiber membranes and wove and/or nonwoven fabrics, as well as the advantages of high surface area and easy assembly. Hallmark and co-workers from Cambridge24 pioneered the melt spinning of linear low-density polyethylene (LLDPE) “ribbon-like” microcapillary films (MCFs) embedded with a series of hollow capillaries. Different from their melt spinning process and end-use applications, Peng et al.,5 in our earlier work, designed novel multichannel spinnerets consisting of a rectangular slit for the polymer solution and seven injectors for the bore fluid. They fabricated rectangular polyacrylonitrile (PAN) ultrafiltration membranes via non-solvent-induced phase inversion method for water reuse and reported that the membrane geometry and microstructure are strongly related to spinning parameters, namely, (1) dope formulation, (2) bore fluid composition, (3) air-gap distance, and (4) external coagulants. In addition, they r 2011 American Chemical Society
found that the newly developed membrane has an interesting grooved outer contour. Compared to melt spinning, solution spinning of hollow fiber membranes via non-solvent-induced phase inversion is a morecomplicated process. The former does not experience sophisticated solvent-induced phase-inversion processes in both inner and outer surfaces, while the latter encounters both. However, both processes may develop spinning instabilities in longitudinal and transversal directions that lead to fiber breakup during production or defective products with nonuniform wall thickness, deformed cross section, or grooved inner surface.613 The purposes of this paper are (1) to design microporous PVDF multichannel rectangular membranes for the direct contact membrane distillation (DCMD) of seawater, and (2) to fundamentally understand the formation mechanisms of grooved outer layer and irregular inner contour deformations in rectangular membranes. In addition to having better mechanical strengths and easy assembly, the wavy contour of rectangular membranes may induce eddies flows and improve mass transfer and energy efficacy. Membrane distillation (MD) is an emerging technology for seawater desalination, because it requires modest energy if waste energy sources, low-cost solar, and geothermal energy are available.14 In addition, MD being a vaporization system, the overall performance is independent of the quality of feed seawater, which is unlikely in reverse osmosis (RO). Hence, it can be used for highsalinity water, even for salt crystallization. PVDF was specifically Received: June 17, 2011 Accepted: November 4, 2011 Revised: October 7, 2011 Published: November 04, 2011 14046
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lodinone (NMP, >99.5%), and nonsolvent ethylene glycol (EG, >99.5%) were purchased from Merck and Panreac, respectively. Sodium chloride (NaCl, 99.5%) was purchased from Merck and Milli-Q ultrapure water was produced in our laboratory with the resistivity of 18 MΩ cm. All chemicals were used as received. 2.2. Polymer Dope Preparation and PVDF Hollow Rectangular and Hollow Fiber Membranes Fabrication. The dry-jet wet phase inversion for the hollow fiber spinning process is schematically described elsewhere.20 To fabricate the hollow rectangular membrane, a novel spinneret with a multichannel of seven lumen holes has been specially designed and developed, as shown in Figure 1. First, The PVDF and Cloisute clay 20A hydrophobic particles were dried overnight at 100 ( 2 °C in a vacuum oven (2 mbar) to remove the moisture content before use. The PVDF resin and Cloisite clay particles were added into the NMP and EG mixture and stirred to become a homogeneous PVDF/ NMP/Cloisite clay/EG dope suspension. The formulated dope was poured into ISCO syringe pumps and degassed before spinning process. A mixture of NMP/water 50/50 wt % was employed as the bore fluid while tap water was utilized as the external coagulant. Water was selected because it is a strong nonsolvent that can induce an instantaneous precipitation and thus fix the membrane shape. In order to fabricate multichannel rectangular membranes, wet spinning was adopted where the nascent membranes entered the external coagulant bath without an air-gap and additional drawing. The as-spun membranes were then submerged in tap water at room temperature for 3 days to remove residual NMP and EG. To prepare dry samples for characterizations and module fabrication, the wet membranes were frozen in a freezer for 2 h, followed by freeze-drying (∼12 h) to prevent pore collapse. For comparison, hollow fiber membranes were also spun from similar conditions. The spinning conditions are tabulated in Table 1. 2.3. Membrane Characterizations. 2.3.1. Morphology Study. A stereozoom microscope (Olympus, Model SZ-1145) was used as the preliminary tool to observe the microscopic view of the nascent hollow rectangular membrane. The morphology of the resultant membranes was examined by field-emission scanning electron microscopy (FESEM) (JEOL, Model JSM-6700F) and scanning electron microscopy (SEM) (JEOL, Model JSM-5600LV). Samples were prepared in liquid nitrogen followed by platinum coating using a JEOL Model JFC-1100E ion-sputtering device.
selected due to its superior hydrophobicity, good solubility in common organic solvents, and easy fabrication during spinning. To design balanced physicochemical properties, hydrophobic cloisite clay particles were incorporated as the disperse phase into the PVDF membrane to enhance mechanical strength, reduce thermal expansion coefficients, and improve heat insulation.15,16 The discovery of rectangular membranes with a grooved outer contour was originally unexpected, because the flow channel for PVDF dope solution has a perfectly rectangular profile. Traditional approaches to create wavy contours with more surface area for mass transfer are to design spinnerets with corrugated patterns at the outer perimeter of the nozzle holes.1719 In this regard, we intend to investigate the formation mechanism of the grooved outer surface and examine the deformation of the inner contour. It is believed that spontaneous development of unique grooved outer surface in this work may provide new insights and advance the science and understanding of membrane formation. To the best of our knowledge, so far, there are only limited reports on the study of grooved outer contour and irregularity.5,6
2. EXPERIMENTAL SECTION 2.1. Materials. The Kureha poly(vinylidene fluoride) (PVDF) T#1300 resin (specific gravity = 1.77) was supplied by Kureha Corporation, Japan, while organophilic clay (Cloisite 20A, which is a natural montmorillonite modified with a dimethyl, dihydrogenated tallow quaternary ammonium salt) was purchased from Southern Clay (Gonzales, TX). The solvent N-methyl-2-pyrro-
Figure 1. (a) Schematic diagram and (b) spinneret of multichannel rectangular membrane.
Table 1. Spinning Parameters of Various PVDF Multichannel Rectangular and Hollow Fiber Membranes Multichannel Rectangular Membranesa A dope solution composition (wt %)
B
B1
B2
Hollow Fibersb C
PVDF Kureha 1300/NMP/Cloisute clay 20A/EG
HF1
HF2
HF3
PVDF Kureha 1300/NMP/Cloisute clay 20A/EG
10/74.7/3.3/12
10/74.7/3.3/12 2
bore fluid composition (wt %) dope fluid flow rate (mL/min)
6
8
NMP/water 50/50 8
8
10
2
NMP/water 50/50 2
bore fluid flow rate (mL/min)
4
6
8
10
8
1.5
1.8
2.0
1.50
1.33
1.00
0.80
1.25
1.33
1.11
1.00
dope:bore fluids ratio external coagulant air gap distance (cm) post-treatment spinneret dimensionsc (cm)
water
water
0
0
store in tap water for 3 days, then freeze-dry
store in tap water for 3 days, then freeze-dry
W/H/ID/L: 11.35/2.05/1.05/5.25
OD/ID/L: 1.6/1.05/6.5
All the membranes were fabricated under free drawing spinning. b HF means hollow fiber. c W = width of membrane; H = height of membrane; ID = inner diameter; OD = outer diameter; and L = effective length of membrane. a
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2.3.2. Mechanical Property Tests. The mechanical properties of as-spun fibers were conducted by an Instron tensiometer (Model 5542, Instron Corp.). The fiber sample was clamped at both ends and pulled in tension at a constant elongation rate of 10 mm min1 with an initial gauge length of 25 mm. Tensile stresses at break, tensile strain, and Young’s modulus were obtained from the stressstrain curves. At least three readings were measured, and an average was obtained from the results. 2.3.3. Porosity Studies. The overall porosity of the hollow fiber membrane (ε) was estimated by the ratio of empty voids to the total volume of the membrane sample.21 The sample hollow fiber was first weighed with a beam balance, followed by immersing it in a 33% LIX54 kerosene solution for 10 days. An assumption was made where all the empty voids were filled with the liquid kerosene solution. The fully impregnated fiber then was removed from the kerosene and any excess kerosene in the lumen side and on the outer surface was removed using tissue paper (where the kerosene in the lumen side was absorbed by tissue paper via capillary force). 2.3.4. Contact Angle Measurements. The dynamic contact angle of the hollow fiber at 25 °C was measured using a Model KSV Sigma 701 tensiometer (KSV Instruments Limited, Finland). The hollow fiber was brought into contact with a reservoir of distilled water. The advancing contact angle was measured using the forces of interaction and the geometry of the solid/surface tension of the liquid with the aid of the computer software. Three readings were measured, and an average was obtained from the results. 2.4. Membrane Distillation Experiments. A laboratory-scale direct contact membrane distillation (DCMD) unit was employed, and details of the apparatus have been published elsewhere.15,22,23 The MD modules were fabricated and tested in model seawater (i.e., 3.5 wt % NaCl in water). The feed solution was circulated through the shell side of modules and purecold water was pumped through the lumen side of the fibers. The inlet temperature at the lumen side of the permeate was constantly kept at 17.5 ( 0.5 °C throughout the entire experiment, while the feed temperature was varied between 50 ( 0.5 °C to 80 ( 0.5 °C. In addition, the flow velocities of the feed and permeate were fixed at 1.1 ( 0.03 and 1.1 ( 0.03 m s1, respectively. The NaCl concentration of the feed solution and ionic conductivity of the permeate stream were determined by a conductivity meter (Lab 960, Schott Instruments). The separation factor (β) and vapor permeation flux, Jv (kg/m2 h) were determined using the equations given below: ! Cp β ¼ 1 100 ð1Þ Cf Jv ¼
Mw nAt
ð2Þ
Ahf ¼ πdo L
ð3Þ
Ahr ¼ WHL
ð4Þ
where Cp and Cf are NaCl concentrations in the bulk permeate and feed solutions, respectively; Mw represents the weight of the collected permeate (in kilograms); n refers to the number of hollow fibers; A is the effective membrane area, where Ahf and Ahr stand for the effective membrane areas of hollow fibers and rectangular
Figure 2. Cross sections of multichannel rectangular and hollow fiber membranes. Dope:bore fluids ratio = (A) 6:4, (B) 8:6, (C) 10:8, (B1) 8:8, (B2) 8:10; and (HF3) 2:2.
membranes (given in units of m2), respectively; t represents the time interval (given in hours), do corresponds to the outer diameter of hollow fibers (given in meters), L indicates the effective length of membranes (given in meters), W is the width of membranes (given in meters), and H refers to the height of membranes (given in meters). All permeate fluxes obtained in this study were calculated using the membrane outer selective layer.
3. RESULTS AND DISCUSSION 3.1. Morphology of As-Spun Hollow Rectangular and Hollow Fiber Membranes. The fabrication of multichannel
rectangular membranes was first conducted using dry-jet wet phase inversion methods with a short air-gap distance. Since the nascent PVDF membrane is not only very elastic but also has a slow phase inversion rate, the extruded PVDF rectangular membrane tends to congregate together in the dry-jet wet spinning as a large-sized round fiber with few asymmetrical lumen holes at the center in order to minimize its surface energy. Hence, an instantaneous precipitation is needed and wet spinning is chosen to fabricate the multichannel PVDF membranes. 3.1.1. Deformation of Wavy Outer Surface. Figure 2 illustrates the microscopic views of both as-spun multichannel rectangular and hollow fiber membranes. All the multichannel rectangular membranes show a similar structure consisting of a grooved outer surface and irregular inner multichannel contours, despite any alteration of spinning parameters (i.e., polymer dope and bore flow rates). Interestingly, these membranes also reveal a symmetric pattern of irregularity, especially for membrane A, which is shown in Figure 2a. A conventional way of designing wavy patterns on membrane surface is by means of using spinnerets, which have corrugated patterns at the outer perimeter of nozzle holes.1719 Conversely, the wavy pattern that observed in our work is formed spontaneously by extruding from a perfectly shaped rectangular spinneret. The deformed fiber inner shape and uneven wall thickness have been reported by some membrane scientists.711 Bonyadi et al.7 proposed that the deformation of the inner contour is initiated by the mass transfer and hydrodynamic instability, while buckling instability induced by rapid and uneven shrinkage across the membrane is the core mechanism that facilitates the final deformation. Yin et al. modeled the formation of grooved shape in the lumen side of hollow fiber membranes and attributed the mechanism to the Marangoni instability.8 Generally, three possible 14048
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Figure 3. Proposed mechanism of grooved outer layer deformation.
Figure 4. Schematic diagram of elimination and reformation of irregular inner contour with increase in bore fluid flow rate for membrane B, B1, and B2 (dope flow rate = 6, 8, and 10 mL min1).
driving forces affect the geometry and morphology of the outer contour, namely, (1) Marangoni instability resulted from differences in surface tension and density between the spinning solution and the external coagulant, followed by mass transfer and hydrodynamic instability; (2) differences in local shrinkages due to uneven wall thickness, solvent exchange rates, and precipitation rates; and (3) buckling instability resulted from unbalanced stress and uneven shrinkages depending on nascent membrane’s material strengths and phase inversion rates. Figure 3 elucidates the proposed mechanisms of grooved outer surface deformation in multichannel rectangular membranes. Once the nascent membrane is extruded from the spinneret and enters the external coagulation bath, it tends to swell, because of the viscoelatic properties of the polymer solution, and rearrange the outer contour shape to reduce the surface energy. As a result, Marangoni instability is initiated because of the uneven contact area, the surface tension gradient, and the solvent exchange rate between the external coagulant and the dope along and across the membrane outer contour.
In the next stage, rapid precipitation and solidification occur at the outermost edge of the membrane, while delayed demixing takes place at the parts that are slightly further away from the bore fluid and external coagulant (as highlighted in Figure 3). Since those “elastic shells” are still in the liquid phase, while their outer surface is almost precipitating, it facilitates the occurrence of buckling to even internal and external stresses and reduces the overall energy.7 Moreover, because of the slow precipitation characteristics of PVDF dopes, the non-solvent-induced phase separation process during spinning provides sufficient time for solidification and eventually promotes the formation of a highly irregular outer layer. For the membrane’s outer selective layer, there are not significant changes in morphology, except that the degree of wavy becomes more profound with increases in the dope flow rate from 6 mL s1 to 8 and 10 mL s1, as illustrated in Figures 2ac. This is due to the fact that a higher dope flow rate increases the membrane wall thickness and prolongs the phase inversion process, thus facilitating sharp differences in the local shrinkage rates. As a result, the 14049
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Figure 5. Scanning electron microscopy (SEM) morphology of PVDF multichannel rectangular membranes (sample B).
Figure 6. SEM morphology of PVDF hollow fiber membranes (sample HF3).
edge of multichannel rectangular membranes shows a saw- or gear-type outer morphology, because of enhanced buckling. 3.1.2. Deformation of Fiber Inner Contour. Apart for the insufficient bore flow rate,10,11 the formation of irregular inner lumen holes is also due to hydrodynamic, elastic, and buckling instability, as discussed previously. As predicted, the deformation of the inner contour can be easily overcome by increasing bore flow rate, which, in turn, forms a greater circular lumen shape (as shown in Figure 2 (see panels B1 and B2). This is due to the fact that the bore fluid contains 50/50 (w/w) NMP/water and NMP is a solvent to PVDF. As a consequence, an increase in bore fluid volume leads to a faint reduction in the precipitation rate, so that it prevents the inner shape from immediate fixation. There is sufficient time for the lumen contour to further expand and accommodate the volume strain. Interestingly, a further increase in bore flow rate not only results in shrinkage of wall thickness between each lumen, but also the reformation of the inner contour toward an oval shape. Figure 4 illustrates the schematic diagram of elimination and reformation of irregular inner contour with an increase in bore
fluid flow rate. For membrane B (bore flow rate of 6 mL min1), the dominant instability force is in the horizontal direction and the ultimate irregular shape toward this pattern. Membrane B1 (8 mL min1) has the most normative spherical inner shape, because the dope/bore fluid ratio of 1 has approximately zero pressure difference to counterbalance the forces from both directions. However, the vertical motion becomes dominant when the bore fluid rate is increased to 10 mL min1. This is plausibly due to the fact that horizontal lumen expansion is restricted by the formation of membrane walls, because of the faster solidification rates between nozzle holes (left second row of Figure 4). Thus, the excess bore fluid is forced to move up or down and form a vertically oval lumen contour. 3.1.3. SEM Morphology. Figure 5 displays the SEM morphology of multichannel rectangular membranes B. From the enlarged images, the asymmetric PVDF membrane consists of three layers: a porous spongelike middle layer that is sandwiched between a porous selective layer and a porous substrate layer. Both of them are full of fingerlike macrovoids. Comparing macrovoid lengths and structure, the intrusion paths for the macrovoid formation in 14050
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Table 2. Characteristic Properties of Various Multichannel Rectangular and Hollow Fiber Membranes A
B
B1
B2
C
HF1
HF2
HF3
dope:bore flow rate (mL/min)
6:4
8:6
8:8
8:10
10:8
2:1.5
2:1.8
dope:bore flow ratio
1.50
1.33
1.00
0.80
1.25
1.33
1.11
1.00
220
180
150
thickness (μm) contact angle (deg) porosity (%)
89.2 ( 1.3
90.9 ( 0.5
90.7 ( 0.3
90.7 ( 1.5
89.4 ( 1.6
2:2
81.2 ( 0.8
81.8 ( 0.5
82.1 ( 1.0
89.9 ( 0.7
90.3 ( 0.1
90.7 ( 0.4 143 ( 4
strain at break (%)
97 ( 5
118 ( 10
113 ( 16
121 ( 12
146 ( 12
130 ( 16
126 (
tensile stress at break (MPa)
0.39 ( 0.05
0.42 ( 0.10
0.59 ( 0.06
0.52 ( 0.04
0.43 ( 0.15
0.72 ( 0.06
0.90 ( 0.04
0.93 ( 0.03
Young’s modulus (MPa) maximum load at break (N)
13.1 ( 1.9 2.23 ( 0.26
13.7 ( 1.4 2.27 ( 0.24
13.6 ( 2.1 2.59 ( 0.21
12.6 ( 2.9 2.87 ( 0.23
13.6 ( 3.9 2.52 ( 0.29
15.6 ( 1.6 0.54 ( 0.03
16.9 ( 0.56 ( 0.05
19.7 ( 2.5 0.52 ( 0.01
DCMD performance at 80 °C (kg/(m2 h))
51.79
54.73
52.99
53.57
49.63
44.33
50.05
51.12
the outer layer are rather profound than those in the inner layer. This implies that the external coagulant (100 wt % water) induces greater convection and diffusion rates than the bore fluid (NMP/ water: 50/50 wt %). The aforementioned morphology could be also reproduced in the hollow fiber membrane, as illustrated in Figure 6. In addition, both multichannel rectangular and hollow fiber membranes show porous inner and outer surfaces, which are the desired morphology to reduce the transport resistance of water vapor across the membrane matrix. 3.2. Membrane Characterization. Table 2 summarizes the characteristics of multichannel rectangular and hollow fiber membranes obtained in terms of porosity, contact angle, and mechanical properties such as elongation at break, tensile at break, Young’s modulus, and load at break. All membranes have moderately high porosity (89%91%), despite the membrane configurations. It is also noted that rectangular membranes show similar mechanical properties to those of hollow fibers in terms of elongation at break, tensile at break, and Young’s modulus, because the same material is used, and tensile strength and moduli are calculated based on unit area. However, they have a much higher maximum load at break (i.e., a 34 fold increment). A higher working load is extremely important for membrane handling, module fabrication, and backwashing. 3.3. DCMD Performance. 3.3.1. Multichannel Hollow Rectangular Fibers. Figure 7 shows the distillated flux of multichannel rectangular PVDF membranes spun with different dope and bore fluid rates, as a function of feed temperature. As stated earlier, our first attempt is to produce this novel membrane structure and investigate its prospect on seawater desalination via DCMD. By varying the ratio of dope flow rate to bore flow rate, membrane B with a more-obvious wavy outer selective layer is produced and has a slight flux enhancement (of ∼5%) than membrane A. The enhancement may be attributed to the turbulence flow induced by a greater degree of wavy geometry for membrane B, which ultimately results in a higher water vapor convection rate from the hot feed saline solution. The details on turbulence flow induced by this novel structure will be discussed later in section 3.3.3. On the other hand, membrane C, with the highest dope and bore fluid flow rates, exhibited a reduction of ∼9% in the distillate flux, possibly due to (1) greater molecular orientation induced by shear stress within the spinneret that is frozen immediately during wet spinning;24 and (2) higher mass transfer resistance induced by a higher membrane wall thickness. 3.3.2. Effect of Bore Flow Rate. Since membrane B shows a more promising performance, the PVDF dope flow rate of 8 mL min1 was adopted for further investigating the effect of
Figure 7. Permeation flux obtained for PVDF multichannel rectangular membranes.
bore flow rate on flux performance. The DCMD performance of PVDF multichannel rectangular membranes with different bore fluid flow rates is illustrated in Figure 8a. Surprisingly, there is no obvious change in permeation flux as the bore flow rate increases from 6 mL min1 to 8 and 10 mL min1, respectively. Conventional wisdom believes that a reduction in membrane thickness can reduce the vapor transport resistance across the membrane matrix, shorten the diffusion pathway, and, consequently, increase flux.25 In addition, an increase in bore fluid rate is one of the easy ways to reduce membrane wall thickness. Therefore, conventional hollow fiber membranes were spun using a similar dope/bore fluid ratio for comparison. When the bore flow rate increases from 1.5 mL min1 to 1.8 and 2 mL min1, the overall membrane thickness is reduced dramatically, from 220 μm to 180 and 150 μm, respectively, as listed in Table 2. Figure 8b summarizes their desalination performance and shows that permeation flux increases (i.e., 12.9% and 15.3% increment for membranes B1 and B2, respectively) with an increase in bore flow rate. Clearly, the effect of increasing bore flow rate on DCMD performance is unnoticeable for multichannel rectangular membranes in our study. This is due to the fact that an increase in bore fluid rate only reduces the wall thickness among multiple bore fluid channels, but does not directly reduce the outer layer thickness, as illustrated in the top cross sections of Figure 4. 3.3.3. Effect of Feed and Permeate Flow Rates. Figure 9a displays the effect of brine feed linear velocity on flux across the rectangular membrane B, while Figure 9b shows the effect of lumen linear velocity on flux. The performance data of hollow fiber HF1 are included as a reference. Multichannel rectangular 14051
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Figure 8. Permeation flux obtained for PVDF membranes: (a) multichannel rectangular and (b) hollow fiber.
Figure 9. (a) Permeation flux versus flow rate of hot brine feed solution, 3.5 wt % NaCl, 60.1 ( 0.2 °C; cold distillate water: 17.2 ( 0.2 °C, 1.14 ( 0.02 m s1 and, (b) permeation flux vs flow rate of cold distillate water, 17.2 ( 0.3 °C; hot brine feed solution: 3.5 wt % NaCl, 60.1 ( 0.2 °C, 1.15 ( 0.03 m s1.
Figure 10. Proposed transport mechanism of water vapor for both multichannel rectangular and typical hollow fiber membranes: (a) hot brine feed and (b) cold distillate permeate solutions.
membranes show a higher increase in flux with increasing feed linear velocity than that of hollow fibers. The proposed transport mechanisms of water vapor across the membrane matrix for both membranes are illustrated in Figure 10a. It clearly shows that rectangular membranes possess lower surface areas and effective
evaporation paths than hollow fibers, because hollow fibers are individually separated and spread out efficiently in a membrane module. Therefore, the flux enhancement in rectangular membranes is attributed to the provocation of turbulences and formation of eddies, leading to an increase in momentum convection 14052
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Industrial & Engineering Chemistry Research at the grooved outer surface of hot feed brine. In addition, this phenomenon also may enhance fluid mixing on the surface and plausibly reduce temperature and concentration polarizations. Apart of enhancement in fluid mechanics, this wary geometry could be serviced as a sieve spacer and efficiently prevents the cluster agglomeration in the membrane. On the other hand, the lumen linear velocity has a converse DCMD result for the multichannel rectangular membranes, as shown in Figure 9b. An attempt to estimate the surface area for effective condensation or vapor transport of rectangular membranes was performed based on the following assumptions: (1) two lumen holes at the membrane’s edge have ∼75% condensation capability, (2) middle five contours are presumed to be ∼50%, and (3) blind spot at the membrane region between each inner contour do not involved in the diffusion path (Figure 10b). The calculated total condensation area is reduced by ∼42%, compared to seven single hollow fibers. Therefore, the performance of rectangular membranes with a lower condensation area may reach a plateau relatively fast and has a less flux improvement by increasing the lumen linear velocity. In view of membrane configuration, multichannel rectangular membranes with grooved outer surface are apparently a desirable candidate for promoting the turbulent flow and result in a greater performance than hollow fibers when increasing brine linear velocity in the shell side.
4. CONCLUSIONS Unique multichannel rectangular membranes with a grooved outer surface for seawater desalination via direct contact membrane distillation (DCMD) have been demonstrated. The following conclusions can be drawn from our study: (1) Hydrodynamic instability is the onset of instability grooved outer shape formation. However, solidification-induced shrinkage coupling with buckling instability is the core factor that magnifies and facilitates the final deformation. The degree of wavy pattern increases proportionally with enhanced polymer dope flow rate. (2) Adopting a proper combination of dope and bore flow rates can efficiently eliminate the inner shape irregularity. It is found that a dope/bore fluids ratio of 1 has the highest probability to form a normative spherical inner contour. (3) For hollow fibers, the effect of increasing bore flow rate can be noticed prominently, in terms of reduced membrane wall thickness and enhanced DCMD performance. Interestingly, for multichannel rectangular membranes, the effect of increasing bore flow rate on flux is almost unnoticed. This is due to the fact that an increase in bore fluid rate only reduces the wall thickness among the multiple channels, but does not directly reduce the outer layer thickness. (4) Multichannel rectangular membranes demonstrate a relatively high distillated flux of 54.7 kg m2 s1, using a hot feed brine solution of 80 °C. In addition, rectangular membranes exhibit a greater flux enhancement with an increase in brine linear velocity, compared to hollow fibers, because of the provocation of turbulences and the formation of eddies. The newly designed configuration may be used as spacers and prevent them from attaching to each other.
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: (65) 6516-6645. Fax: 65-6779-1936. E-mail: chencts@ nus.edu.sg.
’ ACKNOWLEDGMENT The authors would like to thank the Agency for Science, Technology and Research (A*STAR) and National University of Singapore (NUS) for funding this research (through Grant No. R-279-000-291-305). Thanks are also due to Dr. K. Y. Wang, who has contributed useful comments and suggestions to this study. Special thanks are dedicated to Kureha Corporation for the provision of PVDF T#1300 polymer resin. ’ REFERENCES (1) Baker, R. W. Membrane Technology and Applications, 2nd ed.; John Wiley & Sons, Ltd.: Chichester, U.K., 2004. (2) Hallmark, B.; Gadala-Maria, F.; Mackley, M. R. The melt processing of polymer microcapillary film (MCF). J. Non-Newtonian Fluid Mech. 2005, 128, 83. (3) Hallmark, B. The experimental observation and numerical modeling of cast film deformation using novel capillary markers. Polym. Eng. Sci. 2008, 48, 37. (4) Hornung, C. H.; Hallmark, B.; Hesketh, R. P.; Markley, M. R. The fluid flow and heat transfer performance of thermoplastic microcapillary films. J. Micromech. Microeng. 2006, 16, 434. (5) Peng, N.; Teoh, M. M.; Koo, L. L.; Chung, T.-S. Novel rectangular membranes with multiple hollow holes for ultrafiltration. J. Membr. Sci. 2010accepted. (6) Widjojo, N.; Chung, T.-S. Thickness and air gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes. Ind. Eng. Chem. Res. 2006, 45, 7618. (7) Bonyadi, S.; Chung, T.-S.; Krantz, W. B. Investigation of corrugation phenomenon in the inner contour of hollow fibers during the non-solvent induced phase-separation process. J. Membr. Sci. 2007, 299, 200. (8) Yin, J.; Coutris, N.; Huang, Y. Role of Marangoni instability in fabrication of axially and internally grooved hollow fiber membranes. Langmuir 2010, 26, 16991. (9) Shi, L.; Wang, R.; Cao, Y. M. Effect of the rheology of poly(vinylidene fluoride-co-hexafluropropylene)(PVDFHFP) dope solutions on the formation of microporous hollow fibers used as membrane contactors. J. Membr. Sci. 2007, 305, 215. (10) Roesink, H. D. W. The influence of spinning conditions on the morphology of microporous capillary membranes, Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1989; Chapter 3. (11) Santoso, Y. E.; Chung, T.-S.; Wang, K. Y.; Weber, M. The investigation of irregular inner skin morphology of hollow fiber membranes at high-speed spinning and the solutions to overcome it. J. Membr. Sci. 2006, 282, 383. (12) Petrie, C. J. S.; Denn, M. M. Instabilities in polymer processing. AIChE J. 1976, 22, 209. (13) Piau, J. M.; Kissi, N. E.; Tremblay, B. Influence of upstream instabilities and wall slip on melt fracture and sharkskin phenomena during silicones extrusion through orifice dies. J. Non-Newtonian Fluid. Mech. 1990, 34, 145. (14) Burgoyne, A.; Vahdati, M. M. Review. Direct contact membrane distillation. Sep. Sci. Technol. 2000, 35, 1257. (15) Wang, K. Y.; Foo, S. W.; Chung, T.-S. Mixed matrix PVDF hollow fiber membranes with nanoscale pores for desalination through direct contact membrane distillation. Ind. Eng. Chem. Res. 2009, 48, 4474. (16) Monticelli, O.; Bottino, A.; Scandale, I.; Capannelli, G.; Russo, S. Preparation and properties of polysulfone-clay composite membranes. J. Appl. Polym. Sci. 2007, 103, 3737. 14053
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