journal of membrane science · tions of all membranes were summarized in table 1. 2.3. membrane...

Design and fabrication of lotus-root-like multi-bore hollow fibermembrane for direct contact membrane distillation

Peng Wang, Tai-Shung Chung n

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

a r t i c l e i n f o

Article history:Received 11 May 2012Received in revised form29 July 2012Accepted 2 August 2012Available online 9 August 2012

Keywords:Membrane contactorDirect contact membrane distillationMulti-bore hollow fiberLotus root-like structurePVDF membraneMechanical strength

a b s t r a c t

Highly constrained by the requirements of high porosity and large pore sizes, traditional single-borehollow fiber membranes often suffer from easy breakage and performance instability during long-termoperations of membrane distillation (MD). In this work, a multi-bore PVDF hollow fiber (MBF)membrane with a lotus root-like geometry was designed and successfully fabricated via the speciallydesigned spinneret and optimized spinning conditions. Various effects of spinning parametersincluding bore flowrate, dope flowrate and take-up speed were investigated on the membrane macro-and micro-structure, mechanical properties and direct contact membrane distillation (DCMD) perfor-mance. The tensile strength characterizations have proven the excellent mechanical rigidity andelasticity of MBF membranes. Even for the MBF membrane with a thin wall of around 40 mm, themaximum load was as high as 2.4 N. Most importantly, the performance of the DCMD of the MBFmembrane was only slightly lower or even comparable to that of single-bore membranes. In addition,the MBF membranes exhibited superior stability in terms of vapor permeation flux and salt rejectionduring the continuous DCMD experiment with robust operational conditions. We believe this work mayhave profound implication to the development of mechanically durable membranes not only formembrane distillation MD but also for other membrane applications.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Membrane contactor is a powerful process to effectively pro-vide the contact and exchange of two different phases (liquid/liquid, liquid/vapor) [1]. Unlike the traditional membrane processsuch as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis(RO), the membrane in membrane contactors does not act as a sizediscrimination layer, but performs as a barrier and interfacebetween two phases. Membrane distillation (MD) is a special typeof membrane contactor which utilizes the temperature difference(i.e., temperature induced vapor pressure difference) as the drivingforce for vapor transport [2–4]. In an MD process, the feed solutionis always maintained at an elevated temperature (40–95 1C), hencethe volatile vapor could transport through the membrane pores,while any ion or liquid form is repelled by the hydrophobicmembrane barrier [5,6]. Specific to desalination application ofMD, the rejection is often as high as 99.9% since the salts inpretreated seawater are almost non-volatile [7].

In order to maximize the vapor diffusion while enhance thepermeation flux and thermal efficiency, the desired membranecontactor or MD membrane is preferably fabricated with a high

porosity of larger than 75% and pore size in the range of0.1–0.3 mm [8–10]. Due to the large pore size and high porosityin both bulk and surfaces, the membranes often suffer from weakmechanical properties in terms of tensile rigidity at both axial andradial directions. During an MD process, mechanical propertiesdiminish further due to the elevated operation temperature. Inthe past years, many researchers have made efforts to improvethe mechanical strength of MD membranes. For example, Wanget al., Teoh et al. and Su et al. have incorporated clay, polytetra-fluoroethylene (PTFE) particles or carbon nanotube (CNT) into thepolymer matrix to form a reinforced mixed matrix structure[11–13]. However, a further improvement is still needed in orderto ensure the long-term stability in robust operational conditionsand aggressive environments.

As shown in Fig. 1, lotus root in nature exhibits a uniquegeometry with regularly aligned empty channels along the axialdirection. The structure provides one of the best geometrieswhich ensure both large porosity and excellent mechanicalstrength. In the past years, works have been carried out to mimicthe lotus-root structure to obtain nano- or micro-structure withboth high mechanical strength and excellent diffusion or sorptionproperties [14]. The concept of lotus-root-like multi-bore hollowfiber (MBF) membranes has also been adopted for inorganic ceramicmembranes [15,16]. Compared with the traditional single-boreceramic fiber, the MBF ceramic membranes dramatically improve

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Journal of Membrane Science

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2012.08.003

n Corresponding author. Tel.: þ65 6516 6645; fax: þ65 6779 1936.E-mail address: [email protected] (T.-S. Chung).

Journal of Membrane Science 421–422 (2012) 361–374

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dx.doi.org/10.1016/j.memsci.2012.08.003



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tensile rigidity in both axial and radial directions [17]. The numberof bore channels usually varies from 7 to 37. Despite the greatsuccess of multi-bore concept in ceramic membranes, its applicationwas not extended to organic membranes due to the difficulty incontrolling the membrane micro- and macro-structure with themulti-bore configuration. In recent years, a seven-bore UF hollowfiber membrane named Multibores has been fabricated using aspecially modified polyethersulfone material (PESM) by inge GmbH(part of BASF). The outer diameter of the Multibores membraneranges from 4 mm to 6 mm, while the inner surface is the selectivelayer for all models. Later, Bu-Rashid and Czolkoss carried out aseries of pilot plant experiments to evaluate the feasibility of thismembrane as the pre-treatment for seawater reverse osmosis(SWRO) desalination [18]. Very promising results were obtained,where a 75% reduction in energy consumption was achieved ascompared with that of single-bore UF membranes. The MBFmembrane also showed high mechanical stability and good perfor-mance over a long testing period and frequent back flushing using afeed solution with high fouling potential. Beside the lotus rootround-shape MBF membrane, the MBF membrane works with asmulti-bore rectangular configuration were studied by two groups[19–21]. Rectangular MBF membranes also showed improvedmechanical strength and ease of module fabrication, but theprocessing stability at high take-up speeds and varied spinningparameters is still challenging.

In this work, we aim to design and fabricate a lotus-root-likehydrophobic MBF membrane for MD applications. Since the spinningfundamentals and formation mechanism for MBF membranes have

never been reported, various effects of spinning parameters on themembrane macro- and micro- structure will be investigated andoptimized in details. Furthermore, the membrane properties of thenewly fabricated membranes will be evaluated from differentaspects including mechanical properties, water contact angle, directcontact membrane distillation (DCMD) performance and perfor-mance stability during the continuous operation.

2. Materials and methods

2.1. Materials

The working polymer, PVDF HSV900 resin (specific gravity: 1.76–1.79), was purchased from Arkema Inc. N-methyl-1-pyrrolidone(NMP, 499.5%), ethylene glycol (EG, 499.5%) and isopropanol(IPA, 499.5%) used in hollow fibers fabrication were purchased fromMerck. The de-ionized (DI) water used in DCMD tests was producedby a Milli-Q unit (MilliPore) with the resistivity of 18 MO cm.

2.2. Spinneret design and membrane fabrication

The lotus root-like MBF PVDF membranes were fabricated via adry-jet wet phase inversion spinning process by a specially designedseven needle spinneret. The details of spinning process have beendocumented elsewhere [5]. The schematic drawing of the spinneretwas shown in Fig. 2. Unlike the conventional single-bore spinneret,the MBF spinneret had seven needles, which distributed uniformly

Fig. 1. (a) The picture of a lotus-root and (b) The picture of multi-bore fiber produced in this work.

Fig. 2. The schematic design of a seven needle spinneret: (a) Side view and (b) Bottom view.

P. Wang, T.-S. Chung / Journal of Membrane Science 421–422 (2012) 361–374362

within the spinneret space. The spinneret dimension was adjustedso that the gap between adjacent needles as well as the gap betweenthe needle and the spinneret wall was equal to the needle diameter(1 mm). The 1 mm distance between channels was employed so toavoid the potential intra-bore crossing of nascent fibers resultingfrom the die swell phenomenon. During spinning, the dope solutionand bore fluid were extruded at specified flow rates through thespinneret by two ISCO syringe pumps (Teledyne, 500D). Afterentering the coagulation bath, the precipitated membranes werecollected by a take-up roller. To avoid the uneven distribution ofpolymer dope, the bore fluid and polymer dope were injected fromthe side and top of the spinneret, respectively. Propriety knowhowand experience were also applied to achieve a more uniformdistribution of dope solution at the spinneret outlet.

After spinning, the as-spun MBF membranes were immersedin tap water for a few days to complete solvent removal.Subsequently, the wet fibers were frozen in a refrigerator anddried for 12 h in a freeze drier (S61-Modulyo-D, Thermo ElectronCorp.). All the spinning conditions have been repeated for threetimes to ensure the reproducibility. The detailed spinning condi-tions of all membranes were summarized in Table 1.

2.3. Membrane characterizations

The rheological properties of the PVDF polymer dope werecharacterized using an ARES Rheometric Scientific Rheometer.The steady-state shear was measured by a 25 mm cone-platefixture in the range of 1–100 s"1.

Membrane morphology was observed by an optical microscope(microscope: Olympus, SZX16; digital camera: Olympus, CMAD3)and a scanning electron microscope (SEM; JEOL JSM-5600LV). TheSEM samples were prepared by immersing and fracturing the fiberin liquid nitrogen. Before testing, platinum was sputtered on thesamples by a JEOL JFC-1100E ion sputtering device.

Porosity of hollow fibers was obtained by Eq. (1).

e¼ 1"mf iber=rf iber

Vf iber"Vchannel

! "$ 100% ð1Þ

where Vfiber denotes the fiber volume, Vchannel is the inner channelvolume, mfiber represents the fiber weight and rfiber stands for thedensity of the fiber material. Vfiber was calculated from fiberdiameter and length. mfiber was measured by an accurate beambalance (A&D, GR-200). rfiber was determined by a multi pycn-ometer (Quantachrome MVP-D160-E). For each sample, 10 mea-surements were carried out.

The dynamic contact angle, y, of the fiber was measured witha KSV Sigma 701 tensiometer (70.011, KSV Instruments Ltd.,Finland) through a force tensiometry method at 25 1C. An averagevalue of ten fiber samples was reported to ensure the accuracy.

Fiber tensile mechanical properties such as the maximum load,maximum tensile stress, Young’s modulus and maximum tensilestrain were determined by an Instron tensiometer (Model 5542,Instron Corp.). A constant elongation rate of 10 mm min"1 with astarting gauge length of 50 mm was applied. For each spinningcondition, ten fiber samples were tested so as to ensure theaccuracy.

2.4. DCMD experiments

The DCMD experiments were carried out to evaluate thepermeation flux of the MBF at different conditions. Prior to thetest, the membrane modules were fabricated by assemblingpredetermined number of fibers into a plastic tube of 1/2 in.outer diameter, with both ends sealed by epoxy. The fiber numberwas adjusted to ensure a packing density of 1575%.

An automatic controlled DCMD set-up is shown in Fig. 3.A 3.5 wt% NaCl solution and DI water were employed as the feedand permeate solutions, respectively. The inlet temperatures ofthe two streams were maintained by temperature circulators(F12, Julabo; RT7, Thermal Scientific). The feed solution wascirculated by a centrifugal pump to the shell side of the mem-brane module when a rotary pump circulated the permeatesolution to the lumen side of the membrane module. The linearvelocities of feed and permeate solutions were kept at 0.4 and0.8 m s"1, respectively. Four digital thermal couples with accu-racy of 0.1 1C were installed at the inlets and outlets of feed and

Table 1Spinning conditions of MBF membranes: (a) MBF-1–MBF-6, (b) MBF-7–MBF-12.

Membrane ID MBF-1 MBF-2 MBF-3 MBF-4 MBF-5 MBF-6a

(a)Dope composition (wt%) PVDF HSV900/NMP/EG:15/77/8 (viscosity: 26.570.9 pa-s @10 s"1)Dope flow rate (ml min"1) 14Bore fluid composition (NMP/water wt%) 70/30Bore flow rate (ml min"1) 7 9 11 5 3 7Take up speed (m/min) Free fallExternal coagulant (wt%) IPA/water: 50/50Air gap (cm) 3Temperature (1C) 25–29Humidity 65%–75%

Membrane ID MBF-7b MBF-8 MBF-9 MB2F-10 MBF-11 MBF-12 SBF-1

(b)Dope composition (wt%) PVDF HSV#900/NMP/EG:15/77/8 (viscosity: 26.570.9 pa-s @10 s"1)Dope flow rate (ml min"1) 14 10 6 14 14 14 2.5Bore fluid composition (NMP/water wt%) 70/30Bore flow rate (ml min"1) 7 3.5Take up speed (m/min) Free fall Free fall Free fall 5 7.5 10 Free fallExternal coagulant (wt%) IPA/water: 50/50Air gap (cm) 3Temperature (1C) 25–29Humidity 65%–75%

a MBF6 was spun using conditions of MBF-1 but immediately after spinning of MCF-6.b MBF-7 was spun using conditions of MBF-1 but with a lower polymer dope pressure.

P. Wang, T.-S. Chung / Journal of Membrane Science 421–422 (2012) 361–374 363

permeate streams, respectively. The conductivity variations offeed and permeated outlets were recorded by two conductivitytransducers with different measuring ranges.

The permeation flux for each feed temperature is calculatedbased on the outer surface of the membrane using Eq. (2):

Nw ¼DWAot

ð2Þ

where Nw is the permeation flux, DW is the permeation weightcollected over a predetermined time duration (t) and Ao is theeffective permeation area calculated based on the outer diameterof the hollow fiber.

The energy efficiency (EE) for each feed temperature iscalculated using Eq. (3) [22,23]:

EE¼NwlmAo

mpCp,pðTp,out"Tp,inÞð3Þ

where lm is the latent heat of water vaporization evaluated at theaverage membrane temperature, mp is the mass flow rate of thepermeate solution, Cp,p is the average specific heat capacity of thepermeate solution, Tp,in and Tp,out are inlet and outlet tempera-tures of the bulk permeate solution, respectively.

2.5. Continuous DCMD experiments

The continuous performance of DCMD for desalination wasconducted with fiber MBF-1 using a similar piece of equipmentfor short-term DCMD experiments. In order to test the membraneperformance in much robust conditions, the inlet temperature ofthe feed solution was varied at 50 1C or 70 1C, while onetemperature was switched to the other about every 12 h. Theinlet temperature of the permeate solution was always main-tained at 15.5 1C. Feed and permeate flow rates of 1.8 l/min and0.2 l/min were employed. The produced water from the permeatetank was recycled to the feed tank to maintain the salinity of thefeed solution. The salt rejection (R) was calculated with Eq. (4)[5,24]:

R¼ 1"cp

cf

! "$ 100% ð4Þ

where cf and cp are the NaCl concentrations of feed and permeatesolutions, respectively.

3. Results and discussion

3.1. Membrane morphology

3.1.1. Morphology of MBF membraneFig. 4 displays the schematic design, cross-section structure

and surface morphology of membrane MBF-1. From the micro-scopy image, it can be seen that the lotus root-like novelmorphology with seven uniformly distributed bore channels hasbeen attained. The average diameter of the inner bore channel isaround 1.15 mm, while the outer fiber diameter is about4.61 mm. The distance between the bore channels is around230 mm, which is similar to the gap between bore channels andfiber outer wall. Hence, the size of bore channels and membranewall thickness are comparable with the typical single bore hollowfiber for MD [4,11,12,25]. This coincidence ensures fibers withhigh permeation performance and strong mechanical durability.

From the enlarged cross-section images, a macrovoid-free struc-ture with globule-like open cell pores is formed across the entiremembrane wall owing to the use of a viscous HSV900 PVDF solutionand weaker coagulants during phase inversion. A high porosity ofaround 75.9% is obtained because of the addition of EG, a pore-forming agent, in the polymer dope. The membrane cross-sectionshows a typical globule-like structure. When a delayed liquid–liquiddemixing is induced by weaker coagulants, crystallization (also referto as solid–liquid demixing) of PVDF dominates the phase inversionprocess and semi-crystalline PVDF globules can be formed [10,26,27].

Fig. 5 displays the cross-section structure and surface morphologyof membrane SBF-1, which was spun from the same dope composi-tion and coagulants. The bore and dope flowrates were also adjustedto allow the fiber to have a similar wall dimension with fiber MBF-1.While similar morphologies are obtained for the cross-section micro-structure and outer surface, the inner surface exhibits distinguishedstructure. SBF-1 exhibits a typical porous inner surface of PVDFmembranes; while a loosely packed fiber-like network structure isformed on the top of the inner surface for MBF-1. From theperspective of heat and mass transfers, this structure may promoteturbulence flow near the boundary layer, which suppresses tempera-ture and concentration polarizations and hence, enhancing theeffective driving force. To the best of our knowledge, this uniquemembrane morphology and its formation mechanism have neverbeen reported in the literature. It is hypothesized that the bigdifference in hydraulic pressure between the spinning dope and borefluid can be account for the formation of the loosely fiber-likenetwork. Owing to the special seven-needle spinneret design and

Feed in

Permeate in

Module configuration

Cold permeate side Hot feed side

Fig. 3. The DCMD set-up and membrane module configuration.


viscous characteristics of the PVDF dope, the pressure differencebetween the dope and bore fluid is around 480 psi (33.1 bar), whichis much higher than that of typical single-bore hollow fiber spinningprocesses. Since the phase inversion of PVDF polymer takes place at amuch slower rate than other glassy polymer solutions, a liquid–liquiddemixing starts at the inner surface and gradually proceeds to thecross-section of the nascent fiber. Under this circumstance, the dopewith a high pressure will facilitate the diffusion of the un-solidifiedpolymer solution into the bore fluid in order to release pressure.Thereafter, the diffused polymer solution forms a loosely fiber-likenetwork on top of the inner surface. Interestingly, the direction ofthese loosely fibers follows the spinning direction, which is also therelative flowing direction to the bore fluid. To verify this hypothesis,the fiber MBF-1 was re-fabricated with shorter tubing and a largerinner diameter to connect the ISCO pump and the spinneret in orderto lower the dope pressure to 208 psi (14.1 bar). Fig. 6 illustrates thephase inversion mechanism and a comparison of membrane innersurfaces. A distinct fiber morphology and lower fiber coverage can beobserved for fibers spun at a lower dope pressure. This experimentverifies the important role of dope pressure in the formation of thesesurface fibers. More details will be discussed in later sections.

3.1.2. Effect of bore flow rateFig. 7 shows the cross-section morphology of MBF membranes

with different bore flow rates. With an increase in bore fluid flow

rate from 7 to 11 ml min"1, the average bore diameter increasesfrom 1.15 mm to 1.44 mm. The relative size of the center borechannel is also getting larger than those of the surrounding sixchannels. In order to investigate the causes, a series of spinningexperiments were carried out by varying bore flow rate from7 ml min"1 to 3 ml min"1 then increasing to 7 ml min"1 again.Fig. 8 illustrates the evolution of the center bore channel. Thecenter bore channel apparently disappears at the bore flow rate of3 ml min"1 but reappears after increasing the flow rate back tothe original value. This finding excludes the possibility of needleblockage but suggests the non-uniform distribution of bore fluidwithin the spinneret needles to be one of the possible causes.As shown in Fig. 2, the needles are all connected to a bore fluidvessel inside the spinneret. Since the bore-fluid flow rates at the7 needle outlets are similar at low Reynolds numbers when noresistance exists as reported by previous studies on the two-phase distributor header [28,29] and verified in our lab bymeasuring individual bore-fluid flow rate without any dope flow,the diminish of the center bore channel at low bore fluid ratesshould be attributed to the uneven stress and slow phase inver-sion at the center. As illustrated in Fig. 8(e) and (f), a radialdirection stress exists under gravitational and elongational forcesowing to (1) the membrane gravitational mass, (2) the stretchinduced by the collection drum, and (3) the non-uniform phaseinversion across the membrane cross-section [30–32]. Sincephase inversion takes place faster at the outer contour, the area

Inner surface

Enlarged Inner surface

Cross-section

Enlarged cross-section

Outer surface

Fiber: MBF-1

Bore:70%NMP/30%water

Bore flowrate: 7 ml/min

Dope flowrate: 14 ml/min

Fig. 4. The cross-section and surface morphology of MBF membranes MBF-1.


Inner surfaceCross-section Outer surface

Fig. 5. The cross-section and surface morphology of SBF membranes SBF-1.

spinning direction

Fig. 6. Proposed mechanism for the fiber-like inner surface structure (a) inner surfaces of MBF membranes spun with different polymer dope pressures: (b) 493 psi and(c) 208 psi. Blue arrow represents the spinning direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

7 ml/min 9 ml/min 11 ml/min

Fig. 7. MBF membranes spun with different bore flowrates: (a) 7 m/min, (b) 9 m/min and (c) 11 m/min.


of the center bore channel would have a relatively delayeddemixing. On the other hand, the elongational stress may inducea radial stress toward the fiber center [33]. As a result, more borefluid will be squeezed into the surrounding bore-fluid needles,while less into the center bore-fluid needle if the overall borefluid flow rate is reduced.

3.1.3. Effect of dope flow rateFig. 9 shows the cross-section and inner surface morphologies

of MBF membranes spun with dope flow rates of 14, 10 and6 ml min"1, respectively. As the dope flow rate decreases, the boresize and uniform seven-channel structure are well maintained,while the fiber wall thickness reduces from 230 mm to 56 mm andthe outer diameter of the fiber also declines from 4.61 mm to3.17 mm. In addition, the gradual disappearance of the looselyfiber-like network on the inner surface is observed. Consistent withthe discussion in Section 3.1.1, the disappearance of this fiber-likestructure in Fig. 8(c) may be resulted from a lower dope pressurebecause of a small dope flow rate of 6 ml min"1.

3.1.4. Effect of take-up speedHigh speed spinning is always preferred in industries because it

not only increases throughput but also lower production costs.However, various membrane properties including inner surfacemorphology, cross-section structure and mechanical strength willbe altered with high collection speeds [20,34,35]. Fig. 10 shows thecross-section and inner surface morphologies of the MBF mem-branes spun with free fall and take-up speeds of 5, 7.5 and10 m min"1. The outer diameter of MBF membrane decreases from

4.61 mm to 1.31 mm when the take-up speed changes from free fallto 10 m min"1, while the average diameter of the fiber bore channeldecreases from 1.15 mm to 0.38 mm. Meanwhile, the lotus-rootshape structure gradually becomes a wheel-like structure as thetake-up speed increases, as illustrated by Fig. 10(e) and (f).

Compared with the sample spun from free fall, the wheel-likestructure consists of a round-shape small center with a reduceddiameter and shell-shape surrounding channels with a uniformwall thickness. With a more uniform wall thickness, the wheel-like structure could provide a better balance of vapor transportefficiency, liquid wetting resistance and mechanical properties.The relationship between the morphology of the central borechannel and take up speed agrees with the Bonyadi and Mackley’sobservation on rectangular multi-bore membranes. In their stu-dies, the round shape of the center bore channel was wellmaintained at higher air gap or take-up speeds, while those attwo sides of rectangular membranes gradually changed from around shape to an oval shape [21]. It is believed that thetransition of the membrane geometry is closely related to thestresses resulting from the high take-up speed and differentspeeds of phase inversion processes across the membrane. Sincea relatively rapid phase inversion occurs at the outer skin because50/50 IPA/water is used as the outer coagulant and a relativelyslow phase inversion at the bore center because 70/30 NMP/wateris employed as the inner coagulant, a high take-up speed wouldinduce different degrees of orientation and inward stress betweenthe center and surrounding areas. As a consequence, a slow phaseinversion may take place at the fiber center that results in a loosemorphology and small center bore channel.

7 ml/min 5 ml/min

3 ml/min 7 ml/min

Fig. 8. Evolution of cross-section morphology with different bore flow rates: (a) 7 m/min, (b) 5 ml/min, (c) 3 m/min, (d) 7 m/min, (e) and (f): Stress direction of thepolymer solution for one-needle and seven-needle spinnerets.


From the SEM image of enlarged cross-section structure, agradual change of micro-structure from a globule shape to a cellularshape can be easily observed. The transition is owing to the semi-crystalline characteristics of the PVDF polymer. Two types ofmembrane structures and formation mechanisms may take placesimultaneously during the phase inversion; namely, semi-crystallineglobule resulting from solid–liquid demixing and amorphous cellu-lar structure resulting from liquid–liquid demixing [10,20,24]. Inaddition, the phase inversion speed of PVDF hollow fiber duringspinning can be enhanced by the increasing take-up speed [10].When the collection speed of hollow fiber spinning is increased, thenascent fiber elongates and the fiber dimension reduces rapidlyduring the air gap region. This could enhance the speed of phaseinversion [26,36], favor the liquid–liquid demixing mechanism andresult in a structure without much globule.

Besides the structural change in the cross-section, there is also amorphological evolution of MBF membranes on the inner surfacenano-fiber structure. As shown in Fig. 10, the quantity of nano-fibersis decreasing as the take-up speed increases, while the length of asingle nano-fiber is also larger. The decrease of nano-fiber quantity ismainly caused by the faster phase inversion speed as evidenced bythe transformation of membrane structure from a globular shape intoa cellular shape. This faster phase inversion could impede theconvection of polymer solution which forms nano-fibers [37].In addition to the change in phase inversion speed, the slightly higherbore fluid pressure observed during the experiment may also con-tribute to this morphological evolution. Due to the rapid elongation ofthe nascent hollow fiber, several shorter nano-fibers may merge intoone, which results in the increased length of a single nano-fiber.

3.2. Membrane characterizations

Table 2 summarizes the results for various membrane char-acterizations. High contact angles ranges 85–901 are obtained for

all MBF membranes due to the higher fluorine content and highmolecular weight of HSV900 PVDF. The variation of contact angleis minor because the same external coagulant, 50/50 IPA/water, isemployed. The membrane hydrophobicity could be furtherimproved via polymer blending of super-hydrophobic PTFEparticles or fluorine-containing silica [10,38,39].

Similar with contact angle, bulk porosities around 75–80% areobtained for most of MBF membranes. However, the porosity ofMBF spun at higher take-up speeds is slightly lower than thefibers spun at the free fall speed. This may be related to differentglobule structures in the membrane cross-section.

Table 2 tabulates the mechanical properties of the MBF andSBF membranes including maximum load, maximum tensilestrength, maximum extension and Young’s modulus. A compar-ison is made between MBF membranes and previous reportedrectangular PVDF and traditional single-bore hollow fiber PVDFmembrane SBF-1 [19]. The MBF-1 membrane has a higher max-imum load, maximum tensile strength and Young’s modulus(7.28 N, 0.92 MPa and 28.9 MPa) than the rectangular hollowfiber (2.87 N, 0.52 MPa and 12.6 MPa) and single-bore hollowfiber SBF-1. Fig. 11 compares the maximum load and Young’smodulus as functions of bore-fluid flow rate, dope flow rate andtake-up speed. As shown in Fig. 11(a), the effects of bore flow rateare minor for both maximum load and Young’s modulus. This isprobably due to the similar membrane cross-section geometryand cross-section area. As the bore-fluid flow rate increases, theinner and outer diameters of MBF membranes increase simulta-neously. Fig. 11(b) illustrates the effect of dope flow rate. Due to asimilar micro-structure in the membrane cross-section, theYoung’s modulus does not vary much with dope flow rate becauseof comparable tensile rigidity. However, the maximum load isincreased with an increase in dope flow rate as the membranewall becomes thick. Unlike bore flow rate and dope low rate, take-upspeed has greater effects on both properties. As illustrated in

Cross-section:

Enlarged cross-section:

Inner surface:

Fig. 9. The cross-section and inner-surface morphologies of MBF membranes spun with different dope flowrates: (a) 14 ml/min, (b) 10 ml/min and (c) 6 ml/min.


Fig. 11(c), the Young’s modulus improves rapidly, while themaximum load decreases slightly with an enhanced take-upspeed. These phenomena may be attributed to the evolution ofmembrane micro-structure and chain orientation. As mentionedpreviously, there is a transition from globule to cellular structurefor a higher take-up speed. This inter-connected cellular structurecould give much higher tensile rigidity [10,38]. However, theincreases of Young’s modulus and maximum tensile stress cannotcompensate the reduction of membrane cross-section area. Overalla reduction of maximum load is still observed.

3.3. DCMD performance

3.3.1. Comparison between MBF and SBFThe permeation fluxes and energy efficiency (EE) values of the

newly developed MBF membranes and SBF-1 are shown in Fig. 12.Fig. 12(a) compares the performance of MBF and SBF membrane

with comparable wall dimension. It should be noted that all thepermeation fluxes are calculated based on the outer surface of themembrane, which is the interface of feed solution and vaporphase. The surface area of vapor/permeate liquid is slightlydifferent. The DCMD performance of MBF is slightly lower thanthe single-bore hollow fibers with similar spinning conditions andaverage wall thickness. For instance, the permeation flux ofsingle-bore PVDF fiber SBF-1 using a model seawater feed at80 1C is around 36 LMH, which is similar with other PVDF hollowfiber membranes [4,12,40–42], but the value for the MBF-1 isaround 28 LMH. The decline in permeation flux is mainly causedby the reduction in effective area for vapor transport andcondensation. The finding is consistent with the experimentalresults on rectangular membranes [19]. Compared with thetraditional single-bore fiber, polymers between bore channelsact as the membrane matrix to improve the mechanical rigidityfor both axial and radial directions. Hence a smaller area can be

Cross-section:

Enlarged cross-section:

Inner surface:

Fig. 10. Morphologies of MBF membranes spun with different take-up speeds: (a) free fall, (b) 5 m/min, (c) 7.5 m/min, (d) 10 m/min, (e) and (f): Schematic illustration ofgeometry change with increasing take-up speed. The blue arrow represents the spinning direction. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)


utilized for vapor condensation. Besides, this polymer matrix alsoreduces the overall thermal resistance, hence a slightly lower EE isobserved too [39,43]. Nevertheless, the rejection of NaCl salt isclose to 100% for all the MBF membranes, where the salinity ofthe permeate water is always lower than 1 ppm.

3.3.2. Effect of bore flow rate and dope flow rateThe permeation fluxes and energy efficiency (EE) values of the

newly developed MBF membranes are shown in Fig. 12(b) and (c)as functions of bore flow rate and dope flowrate, respectively. Withdifferent bore or dope flow rates, a slight improvement on EE valuecan be observed with a larger bore-fluid flow rate, while a biggerimprovement on flux for a reduced dope flow rate. The variations ofDCMD permeation flux and EE are controlled by membrane geo-metry and membrane wall thickness. For membrane MBF-9 thepermeation flux is as high as 67.9 LMH at a 90 1C feed solution,which is even higher than most of single-bore fibers. The high

performance is mainly due to its small wall thickness of around40 mm spun at a reduced dope flow rate. According to the heat andmass transfer simulation or semi-empirical correlations reportedpreviously, the optimum membrane thickness of a DCMD mem-brane to achieve the highest water flux is in the range of 10–40 mmdepending on membrane structure parameters [9,44,45]. This MBF-9 falls within the optimum range of wall thickness that not onlyprovides a shorter pathway for vapor transport but also maintainsthe membrane thermal resistance and effective driving force. On theother hand, it is hard for single-bore hollow fiber membranes toenjoy these two benefits simultaneously. Most single-bore hollowfiber membranes are usually fabricated with a wall thickness greaterthan 100 mm [4,13,36,37,44,46]. Any reduction in membrane thick-ness could result in fiber collapse and ease damage. For MBFmembranes, even though the fiber wall is thinner, the maximummechanical load of this membrane is still as high as 2.4 N. Therefore,the MBF membrane has enough strength and resistance against anydamage from flow turbulence and instability.

Table 2Characteristic properties of MBF membranes: (a) MBF-1–MBF-6, (b) MBF-7–MBF-12 and SBF-1.

Membrane ID MBF-1 MBF-2 MBF-3 MBF-4 MBF-5 MBF-6

OD/ID (mm/mm) 4610/1178 4968/1335 5162/1437 4322/1094 3431/957 4597/1126Porosity (%) 75.970.3 76.270.2 76.170.3 – – –Contact angle (1) 85.376.2 85.672.3 85.174.7 – – –Maximum load (N) 7.2870.47 7.3770.94 7.2270.65 – – –Maximum tensile stress (MPa) 0.9270.06 1.0270.13 0.9870.09 – – –Young’s modulus (MPa) 28.975.7 28.976.0 29.773.9 – – –Maximum tensile strain (%) 56.373.9 46.972.6 49.778.0 – – –

Membrane ID MBF-7 MBF-8 MBF-9 MBF-10 MBF-11 MBF-12 SBF-1

OD/ID (mm/mm) 4711/1152 3362/1004 3173/944 1803/478 1468/382 1306/355 1554/1135Porosity (%) – 75.670.1 76.070.5 69.670.4 68.570.4 67.370.2 77.170.5Contact angle (1) – 89.574.5 89.071.6 88.973.7 89.374.2 87.573.1 89.173.3Maximum load (N) – 3.1870.12 2.4370.18 1.4570.03 1.2870.03 1.2270.05 0.8270.06Maximum tensile stress (MPa) – 0.9770.05 0.9170.07 1.0570.02 1.7170.05 2.2570.08 0.9170.07Young’s modulus (MPa) – 31.1712.1 31.576.7 34.672.9 56.278.1 79.977.3 27.976.7Maximum tensile strain (%) – 0.5970.98 0.5171.04 46.176.2 47.773.1 50.676.3 41.376.6

30

40

50

60

70

80

90

1.00

1.20

1.40

1.60

4 6 8 10

Youn

g'a

mod

ulus

(Mpa

)

Max

imu

load

(N)

Take up speed (m/min)

0

10

20

30

40

50

2.00

3.00

4.00

5.00

6.00

7.00

8.00

5 10 15

Youn

g'a

mod

ulus

(Mpa

)

Max

imu

load

(N)

Dope flowrate (ml/min)

0

10

20

30

40

5.00

6.00

7.00

8.00

9.00

6 8 10 12

Youn

g'a

mod

ulus

(Mpa

)

Max

imu

load

(N)

Bore flowrate (ml/min)

Fig. 11. Maximum load and Young’s modulus of MBF membranes with different spinning conditions: (a) Bore fluid flowrate, (b) Dope fluid flowrate and (c) take-up speed.


0

10

20

30

40

50

50 60 70 80 90

Perm

eatio

n flu

x (L

MH)

Feed inlet temperature (°C)

MBF-1 MBF-2 MBF-3

0.0

0.2

0.4

0.6

0.8

50 60 70 80 90

EE

Feed Inlet Temperature (°C)

MBF-1 MBF-2 MBF-3

0

10

20

30

40

50

60

70

50 60 70 80 90

Perm

eatio

n flu

x (L

MH)


MBF-1 MBF-8 MBF-9

0.0

0.2

0.4

0.6

0.8

50 60 70 80 90

EE


MBF-1 MBF-8 MBF-9

0

10

20

30

40

50

50 60 70 80 90

Perm

eatio

n flu

x (L

MH)


MBF-1 MBF-10

MBF-11 MBF-12

0.0

0.2

0.4

0.6

0.8

50 60 70 80 90

EE


MBF-1 MBF-10

MBF-11 MBF-12

0

10

20

30

40

50

50 60 70 80 90

Perm

eatio

n flu

x (L

MH)


MBF-1 SBF-1

0.0

0.2

0.4

0.6

0.8

50 60 70 80 90

EE

Feed Inlet Temperature (°C)

MBF-1 SBF-1

Fig. 12. The DCMD permeation flux and EE of MBF membranes spun with different conditions (a) MBF-1 and SBF-1, (b) bore flowrate, (c) dope flowrate and (d) take-up speed.


3.3.3. Effect of take-up speedThe permeation fluxes and EE obtained for MBF membranes

with different take-up speed are shown in Fig. 12(d). An improve-ment in the DCMD performance can be seen with MBF mem-branes spun with a higher take-up speed. The enhancement ofpermeation flux is mainly contributed by two factors; namely,(1) decrease of vapor transport resistance with thinner and amore uniform fiber wall and (2) a reduced temperature polariza-tion with a higher heat transfer coefficients.

With a higher take-up speed and draw ratio, the diameter andwall thickness of the spun MBF membranes decrease simulta-neously. Furthermore, a membrane wall with a more uniformwall thickness is obtained owing to the take-up stretching. Thedifferent geometry could reduce the vapor transport resistanceacross the membrane. In addition, the wheel-shape channel couldprovide a higher fraction of effective inner surface for vaporcondensation.

Beside the reduced vapor transport resistance, the highertemperature polarization coefficient plays another important rolein enhancing DCMD performance. As the same linear velocity ofpermeate solution is applied for all membrane modules, the heattransfer coefficient (hp) is higher for fibers with a smaller innerchannel size (refer to Eq. (5)). For example, the hp value increasesfrom 2260 to 6726 W 1C"1 m"2 when the take-up speedincreases from free fall to 10 m min"1. Hence, the temperaturedifference between two membrane surfaces will be higher, whichprovides a higher driving force.

For laminar flow at the tube side:

Nup ¼hpDi

kp¼ 4:36þ

0:036Re0:8p Prp

0:33ðDi=LÞ

1þ0:0012 Prp=DiL

# $h i0:8ð5Þ

Rep ¼rpvpDi

mpð5aÞ

Prp ¼Cp,pmp

kpð5bÞ

where Nu is the Nusselt number, k is the thermal conductivityvalues, Re is the Reynolds numbers, Pr is the Prandtl numbers,p denotes the permeate side, L is the module length and Di is theequivalent inner diameter of the MBF membrane.

It is worth noting that along with the higher heat transfercoefficient, the hydraulic pressure built up within the lumen sideof membranes is increasing exponentially. From take-up speed offree fall to 10 m min"1, the pressure built up increases fromaround 2.0 psi (0.14 bar) to 11.5 psi (0.80 bar). This should be dueto the reverse relationship between the pressure built up and thesquare of bore diameter (D2

i ). Specific to the laminar flow region, a50% reduction in inner diameter would result in an about fourtimes increase in pressure built up [47]. This implies the impor-tance of a suitable bore diameter so as to balance the permeationperformance and hydraulic pressure built up.

3.4. Continuous DCMD experiments

The continuous DCMD desalination performance of the fabri-cated MBF membrane (MBF-1) is conducted for 300 h. In order totest the membrane performance in a more robust environment,the inlet temperature of feed solution is switching between 50 1Cand 70 1C about every 12 h. The permeation flux is shown inFig. 13(a), while salt rejection and permeate salinity are illu-strated in Fig. 13(b). Though different temperatures are applied,the permeation flux and salt rejection are very stable duringthe whole test. The permeation fluxes are about 7.370.5 LMHand 15.870.6 LMH for feed inlet temperatures of 50 1C and70 1C, respectively. The salt rejection and permeate salinityfor the whole experiment are 99.9977870.00009% and0.7870.03 ppm, respectively. The salinity of permeate streamis better than the standard of DI water ('10 ppm). Furthermore,

0

5

10

15

20

0 50 100 150 200 250 300

Perm

eatio

n flu

x (L

MH)

Time (h)

50 C

70 C

0

0.2

0.4

0.6

0.8

1

99.997%

99.998%

99.999%

100.000%

0 50 100 150 200 250 300

Salt

reje

ctio

n

Perm

eate

sal

inity

(ppm

)

Salt rejection Permeate salinity

Time (h)

Fig. 13. Continuous DCMD desalination experiments for the MBF membrane (MBF-1). (a) permeation flux and (b) salt rejection and permeate salinity.


the salt rejections for two feed temperatures are similar. Thisindicates the superior stability of MBF membranes at both lowand high temperatures.

4. Conclusions

In this work, we have studied the fabrication and performanceof the multi-bore fiber (MBF) membranes with a lotus root-likegeometry for the DCMD application. MBF membranes with auniformly distributed bore structure have been successfullyfabricated with optimized dope formulation and spinning condi-tions. The following conclusions are drawn from this study:

1) The uniform seven-bore geometry has been well maintainedfor MBF membranes spun with different bore flow rates, dopeflow rates and take-up speeds. A layer of fiber-like network isformed on top of the membrane inner surface. This may becaused by the high pressure difference between dope and borefluid. Meanwhile, the relative size of the center bore channeltends to decrease with reducing bore flowrate. This is probablyattributed to the non-uniform distribution of bore fluid flow-rates among the spinneret needles. It is also noticed that undertake-up drum stretching, the lotus-root structure graduallytransforms into the wheel structure as the take-up speedincreases.

2) The mechanical properties of MBF membranes are much betterthan the single bore and rectangular MBF membranes. MBFmembranes spun from a lower dope flowrate show a similarYoung’s modulus but a reduced maximum load. An increase inYoung’s modulus and a decrease in maximum load areobserved for membranes spun from a higher take-up speed.This observation is closely related to the phase inversionmechanism and resultant globule structure at the membranecross-section.

3) The DCMD performance of MBF membranes is only slightlylower or even comparable than the traditional single-borehollow fiber owing to the compensation of effective vaporcondensation area. This performance gap can be improved bylowering the dope flowrate or increasing the take-up speed.

4) The 300 h continuous DCMD experiment has shown superiorlong-term operation stability of MBF membranes. The permea-tion flux and rejection higher than 99.9977% have been wellmaintained even under robust operation conditions.

Acknowledgements

The authors would like to acknowledge Agency for ScienceTechnology and Research (AnSTAR) and National University ofSingapore for funding the research through the project ‘Develop-ment of Hybrid Desalination Processes using Cold Energy fromLNG Re-gasification’ (grant number: R-279-000-291–305). Theauthors also appreciate Dr. K. Y. Wang and Miss F. Edwie for theirvaluable suggestions and comments.

Nomenclature

Ao outer surface areas of MBF membranes (m2)Cp,p specific heat capacity of permeate solution

(J 1C"1 kg"1)cf,cp NaCl concentration of feed and permeate solutions

(%)Di Equivalent inner diameter of MBF membrane (m)

EE energy efficiency of DCMD experimenthp convective heat transfer coefficients of feed and

permeate solutions, respectively (W 1C"1 m"2)kp thermal conductivity of permeate solution (kg s"1)mp mass flowrate of permeate solution (kg s"1)Nup Nusselt numbers of permeate solutionNw permeate flux calculated based on outer surface

area (l m"2 h"1)Prp Prandtl numbers of permeate solutionR salt rejection of MBF membrane in DCMD

experimentRep Reynolds number of permeate solutionTp,in,Tp,out inlet and outlet temperatures of permeate solu-

tion, respectively (1C)vp superficial velocity of permeate solution (m s"1)

Greek symbols

e membrane bulk porositylm latent heat of water vaporization at average mem-

brane temperature (J kg"1)mp viscosity of permeate solution (kg m"1 s"1)rp densitiy of permeate solutions (kg m"3)

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