A New-Generation Asymmetric Multi-Bore Hollow Fiber Membrane for

Subscriber access provided by MT ROYAL COLLEGE

Article

A New-Generation Asymmetric Multi-Bore Hollow Fiber Membrane for Sustainable Water Production via Vacuum Membrane Distillation Peng Wang, and Tai-Shung Chung Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es400356z • Publication Date (Web): 10 May 2013 Downloaded from http://pubs.acs.org on May 14, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Environmental Science & Technology

A New-Generation Asymmetric Multi-Bore Hollow Fiber Membrane for Sustainable Water Production via Vacuum Membrane Distillation Peng Wang,† Tai-Shung Chung*,† †

Department of Chemical &Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore Peng Wang,†Tai-Shung Chung*,†

†

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

ABSTRACT: Due to the growing demand for potable water, the capacities for wastewater reclamation and saline water desalination have been increasing. More concerns are raised on the poor efficiency of removing certain contaminants by the current water purification technologies. Recent studies demonstrated superior separation performance of the vacuum membrane distillation (VMD) technology for the rejection of trace contaminants such as boron, dye, endocrine-disruptive chemical and chloro-compound. However, the absence of suitable membranes with excellent wetting resistance and high permeation flux has severely hindered the 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

VMD application as an effective water production process. This work presents a new generation multi-bore hollow fiber (MBF) membrane with excellent mechanical durability developed for VMD. Its micro-morphology was uniquely designed with a tight surface and a fully porous matrix to maximize both high wetting resistance and permeation flux. Credit to the multi-bore configuration, a 65% improvement was obtained on the anti-wetting property. Using a synthetic seawater feed, the new membrane with optimized fabrication condition exhibits a high flux and the salt rejection is consistently greater than 99.99%. In addition, a comparison of 7-bore and 6bore MBF membranes was performed to investigate the optimum geometry design. The newly designed MBF membrane not only demonstrates its suitability for VMD but also makes VMD come true as an efficient process for water production.

■ ABSTRACT IMAGINE

Minimized diffusion resistance

Excellent antiwetting property

Easy module fabrication

Feed in interconnected globular pore structure

Tight but porous surface

Vacuum

Vacuum Feed in

Multi-bore configuration Larger fiber diameter

■ INTRODUCTION 2 ACS Paragon Plus Environment

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As the global urbanization and rapid population growth increase the demand for potable water, the interest in water production and reuse from wastewater reclamation and saline water desalination has been stimulated.1, 2 Membrane technologies such as ultra-filtration and reverse osmosis (RO) and membrane bioreactor (MBR) are widely used in this field. However, growing concerns are raised on poor removal efficiency of the trace amounts of harmful contaminants.3, 4 For example, boron containing-compounds are toxic contaminants which are commonly found in the wastewater or saline water.5 Unfortunately, the rejections of borate compounds in RO or electro-dialysis (ED) processes are only 30-50% owing to the complexity in boron Chemistry.5 As a result, the boron content in the purified water may exceed the boron standard (1.0 mg/L) set by the World Health organization (WHO).5 In order to mitigate this problem, extensive studies have been carried out to explore the alternative technologies for a better quality of potable water. As an emerging technology based on membrane technology but driven by thermal energy, membrane distillation (MD) demonstrates its superior performance in removal of boron, dye, endocrine-disruptive chemical and many other contaminant compounds.5-8 For example, the rejection of borate compounds in an MD process is >99%, while the removal efficiency of hormone is >99.5%.5-6 Despite the excellent removal efficiency, the MD technology has not progressed fully into commercialization. The major difficulty may arise from the lack of suitable membranes with both high anti-wetting property and high permeation flux.9-11

Vacuum membrane distillation (VMD) refers to the MD configuration where vacuum is applied in the permeate side to induce liquid vaporization and transportation. As compared with other MD configurations, the VMD configuration was demonstrated to exhibit a higher permeation flux and better theoretical thermal efficiency if a high efficiency external condenser is used.12, 13 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

This makes VMD attractive in many applications, i.e., concentration of RO brine, removal of heavy metal, purification of alcohols and e.t.c..5, 7, 14, 15 The higher flux of the VMD process arises from two factors; namely, 1) enhanced vapor transportation and 2) higher temperature polarization coefficient (TPC).The enhanced vapor transportation is caused by the vacuum applied at the permeate side. Thus, the diffusion of water vapor inside the pores can be significantly enhanced by the pressure-gradient driven Poiseuille flow, which commonly takes place in parallel with molecular and/or Knudsen diffusion.14, 16 On the other hand, the higher TPC indicates a higher effective driving force for the VMD process because there is a closer theoretical vapor pressure across the membrane. Since the water vapor is removed immediately at the VMD permeate side, the boundary layers for heat and mass transfer in the permeate side are significantly mitigated, as illustrated in FigureS1. In addition, because the membrane pores are vacuumed, the thermal conduction across the membrane matrix is minimized. As a result, not only can the feed temperature and the vapor pressure facing the membrane surface be maintained at higher values but also the VMD process has an overall higher thermal efficiency.

Like all MD processes, the essential requirements on VMD membranes are (1) excellent wetting resistance and (2) high permeation flux.17, 18 In order to maintain pores in a dry state, the transmembrane hydraulic pressure across the MD membrane should never exceed or even be close to the membrane’s liquid entry pressure (LEP).19 However, due to the applied vacuum, membranes for VMD require higher constraints on wetting resistance and radial-direction (from lumen to shell side) mechanical rigidity as compared with those for direct contact membrane distillation (DCMD) and other MD configurations.20 The suitable VMD membrane needs to be designed with smaller pores at the functional surface in order to generate a higher wetting resistance. As a 4 ACS Paragon Plus Environment

Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


result, Knudsen diffusion is the dominant diffusion mechanism at the functional surface.21 Based on the previous MD studies,11, 22 the LEP of a MD membrane is affected by membrane’s microstructure, mechanical strength and toughness, especially the anti-expansion ability of the liquid contacting surface at the feed side.23 For instance, the LEP values of flat asymmetric PVDF membranes cast on a nonwoven support exhibited almost twice of those without support.22 A similar phenomenon had also been found in our previous hollow fiber membranes.11 In this regard, hollow fiber membranes with multi-bore configurations are appealing as new VMD membranes.24, 25 Figure S2 illustrated in the schematic drawing of the multi-bore hollow fiber. This lotus-root-like geometry exhibits the regularly aligned empty channels in axial direction, which provides one of the best geometries which ensure both large porosity and high mechanical strength, particularly in radial direction.18,

20

However, their permeation properties must be

improved in order to increase the water productivity.

Therefore, the first objective of this work is to develop a new-generation high-performance VMD membrane with a multi-bore hollow fiber (MBF) configuration consisting of characteristics of superior mechanical durability and wetting resistance. We aim to design (1) the bulk matrix of the membrane to be fully porous and inter-connected so that it has a high permeation flux and a low thermal conductivity and (2) the surface facing the feed to be tight with a reduced pore size so that it can prevent the membrane from wetting. The fundamental science and engineering to bridge membrane morphology and VMD performance to spinning conditions such as bore fluid composition and dope composition be explored. Various membrane properties will be characterized in order to study the transport mechanism and membrane formation. Both MBF and traditional single-bore hollow fiber (SBF) membranes will be 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 19

prepared for comparison.

■ MATERIALS AND METHODS Materials. Polyvinylidene fluoride (PVDF) HSV#900 was purchased from Arkema Inc. Nmethyl-1-pyrrolidone (NMP, >99.5%), ethylene glycol (EG, >99.5%) and isopropanol (IPA,>99.5%) used in the MBF and single-bore hollow fiber (SBF) membrane fabrication were purchased from Merck, Germany. DI water from a Milli-Q (Millipore, USA) system was used in all experiments. Fabrication of MBF and SBF Hollow Fiber Membranes. The MBF hollow fiber membranes were prepared by a dry-jet wet spinning process employing through a specially designed sevenchannel spinneret. The SBF hollow fiber membrane with the same polymer solutions was also prepared with a traditional single-channel spinneret. A detailed description of the hollow fiber spinning process has been documented elsewhere.11 The spinning conditions, spinneret design and detailed spinning and post-treatment procedures used in this study are described in the Supporting Information (SI). Characterizations of MBF and SBF Hollow Fiber Membranes. Dynamic contact angle, θ, of the fabricated membranes was measured with a KSV Sigma 701 tensiometer (± 0.01°, KSV Instruments Ltd., Finland). Various tensile mechanical properties (i.e., the maximum load, maximum tensile stress, maximum tensile strain and Young’s modulus) of the fibers were measured by an Instron tensiometer (Model 5542, Instron Corp.). The porosity of the dried membranes was measured by a multi-pycnometer (Quantachrome MVP-D160-E). The average pore sizes of the membranes were measured by a CFP-1500 AE capillary flow porometer (PMI, USA). The details of the characterizations can be found in the SI or elsewhere.5, 11, 26 6 ACS Paragon Plus Environment

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Measurement of anti-wetting properties. The anti-wetting properties of the fabricated membranes were indicated by the LEP and burst pressure. These two parameters were determined using a laboratory fabricated set-up. The detailed drawing and procedures are documented in SI. VMD Water Production Processes. The VMD experiments were carried out to evaluate the permeation flux of the MBF membranes at different operation conditions. Prior to the test, the membrane modules were fabricated by assembling a predetermined number of fibers into a stainless-steel tube of 3/8 inch outer diameter, with both ends sealed by epoxy.

Figure 1.The schematic drawing of VMD set-up (feed at lumen side). Figure 1 shows the experimental apparatus for the characterization of VMD performance of hollow fiber membranes with the feed entering from the lumen side. A model seawater containing 3.5 wt% NaCl was used as the feed. The inlet temperature of the feed was maintained at the target value by a temperature circulator (F12, Julabo) with the aid of mechanical stirring. The feed was circulated by a rotary pump to the lumen/shell side of the membrane module. To 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

monitor the temperature variation during the test, two digital thermocouples with an accuracy of 0.1 °C were installed at the inlet and outlet of the feed stream. The permeate stream of VMD experiments was collected individually using cold traps immersed in liquid nitrogen, while the vacuum level was maintained by a vacuum pump (Edwards, RV5). The downstream pressure at the outlet of membrane module was maintained at 1500-2000 pa by a needle valve connected to the vacuum pump. A vacuum gauge (Vacuubrand, DRV2) was installed at the permeate side to monitor the downstream vacuum pressure. Prior to the sample collection, the VMD permeation system was conditioned for 0.5 h using the first cold trap. The sample was collected at 0.5 h interval by the second cold trap and weighed by an accurate beam balance (A&D, GR-200, 0.0001g). The sample was allowed to melt naturally and the salinity was measured by a conductivity meter Lab 960 meter (0-500 ms cm-1, ± 0.1 µs cm-1, SCHOOT instrument). For each sample, 3 measurements were carried out and an average was reported. The permeation flux for each feed temperature is calculated based on the outer surface of the membrane using Equation (1)27, while the permeation fluxes calculated based on the inner surface are included in the supporting figures as a reference:

Nw =

∆W Ao t

(1)

where Nw is the permeation flux, ∆W is the permeation weight collected over a predetermined time duration ( t ) and Ao is the effective permeation area calculated based on the outer diameter of hollow fibers.

8 ACS Paragon Plus Environment

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■RESULTS AND DISCUSSION Characterizations of membranes. Figure 2 displays the morphologies of membrane surface, cross-section, pores and globules of a typical MBF membrane with an ID of MBF1. The membrane has a lotus root-like structure composed of seven uniformly distributed inner bore channels. The average diameter of the inner bore channels is around 1.19 mm, while the outer diameter of the lotus root-like membrane is about 5.62 mm. Since pure water at 0 °C was used as the internal coagulant (bore fluid), the demixing process occurred instantaneously, which suppressed the PVDF nucleation and crystallization.28, 29 As a result, each bore channel consists of a tight inner surface full of small pores with high wetting resistance. In addition, a weak external coagulant consisting of IPA/water (60/40) was used to induce delayed demixing that facilitated both pore formation and PVDF crystallization.11 Hence, the outer surface and the outer cross-section area not only have a porous structure but also comprise interconnected globules. From enlarged cross-section images as shown in the upper right of Figure 2, the asymmetric membrane has finger-like macrovoids near the inner surface due to the rapid phase inversion and non-solvent (i.e., water) intrusion, while a sponge-like porous structure is formed close to the outer surface owing to the delayed demixing. The bulk porosity of the lotus root-like membrane is around 81%. The bottom row of Figure 2 also elucidates the detailed evolution of pore structure from the region underneath the inner surface to the middle of the cross-section and finally to the region close to the outer surface. The micro-morphology gradually transforms from a fully connected cellular structure to an interconnected globular structure and eventually to a porous structure full of tiny globules. Consistent with the previous discussion, the transition of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

surface morphology and pore structure across the membrane is caused by the distinguishing opposite effects of internal and external coagulants accompanying with the effect of PVDF crystallization.30, 31 Hence the cross-section near the outer surface not only has a porous structure but also contains different degrees of interconnectivity among globules. The interconnectivity might provide additional mechanical strength.32 Inner surface 1

Inner surface 2

Outer surface

Cross-section 1

Cross-section 2

Figure 2.The cross-section and surface morphologies of a typical MBF membrane (MBF1).

Table S2 summarizes the tensile properties, LEP, burst pressures of all MBF and SBF membranes as well as their dimensions and porosity. Depending on spinning conditions, the tightness of the inner surface and the overall mechanical strength, the LEP value can vary from


Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.28 to 4.7 bar. In the case of the MBF1 membrane, it has an impressive LEP as high as ~2.8 bar which well exceeds the operating pressure of the VMD process. In addition, it has a high burst pressure of ~4.28 bar coinciding with the excellent hoop mechanical strength of the multi-bore configuration. Comparison between multi-bore and single-bore hollow fiber geometries. Membranes with multi-bore configurations could provide better mechanical strength and wetting resistance as compared with the single-bore configuration.24, 33, 34 In the case where the feed is contacted with the inner surface, the effective contact area of the MBF membrane could be ~50% higher than the SBF membrane if both hollow fibers have a similar dimension. MBF1

SBF

Cross-section

Enlarged cross-section

Figure 3.The morphologies of MBF1 and SBF membranes spun with the same composition and temperature.

Figure 3 shows a comparison of MBF and SBF membrane morphologies spun from similar conditions, while Figure 4 illustrates their physicochemical properties including bulk porosity, mechanical strength, LEP, burst pressure and VMD fluxes. Since the same dope and coagulation



conditions were employed, the inner and outer surfaces morphologies are similar. The crosssections also show a similar structure with a layer of finger-like macrovoids close to the inner surface. The only morphological difference is that the MBF membrane has a slightly thicker wall than the SBF membrane. Owing to the similar micro-structure, the two membranes show a similar porosity and Young’s modulus. However, the MBF membrane has an enhanced tensile load and reversible maximum load due to its larger overall cross-section area. As the lotus-rootlike multi-bore structure could maximize the mechanical stability at the radial direction, the LEP and burst pressure of the MBF membrane are much higher than those of the SBF membrane. The LEP of the MBF membrane is about 65% higher than the SBF membrane, while the burst pressure improves 67%. Though the MBF membrane has a thicker membrane wall, the two membranes have comparable VMD permeation fluxes. This should be attributed to the larger effective area of the MBF inner surface as discussed earlier. It was noticed that the salinity of the produced water from MBF is ~ 1ppm, while that from the SBF is ~6ppm. Although both configurations produce the high quality drink water, yet the MBF membrane demonstrates its superiority in the anti-wetting property.

50

5.0 Maximum loa d

5

0

Pressure (bar)

Maximum reversible load

10

LEP

4.0

Burst pressure

SBF

Membrane geometry

40 30

3.0 2.0

20

1.0

10

0.0 MBF-3

Permeation flux (LMH)

15

Load (N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

MBF-3

SBF

Membrane geometry

SBF

MBF

0 40

50

60

70

Feed inlet temperature (°C)

80

Figure 4: A) Mechanical strength, B) LEP & burst pressure and C) VMD permeation fluxes of MBF1 and SBF membranes.


Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Optimization of fabrication parameters. In order to further optimize the pore size of the inner channel surface, firstly the bore fluid composition was varied during spinning in order to induce different solvent strengths and inner surface morphologies. Bore fluid temperature Water (0 ºC) MBF1

Water (40 ºC) MBF2

Bore fluid composition NMP/Water (0 ºC, 50/50) MBF3

NMP/Water (0 ºC, 70/30) Nil

Cross-section

Enlarged cross-section

Nil

Inner surface

Nil

Figure 5: The cross-section and inner surface morphologies of MBF membranes spun from different bore fluid compositions and temperatures. The left two columns of Figure 5 display the effects of bore fluid temperature (0 and 40 ºC water) while the right two columns show the effects of bore chemistry (50/50 and 70/30 NMP/water at 0 ºC) on cross-section and inner surface morphologies. Opposite transitions of the amounts of pores in the inner surface and macrovoids in the cross-section can be observed with a decrease in bore fluid strength (the ability to induce the phase inversion). The average pore sizes for MBF1 to MBF3 are 104 nm, 130 nm and 236 nm, respectively. Water at 0 ºC is the strongest bore fluid that results in the MBF membrane with large macrovoids and very tight surface pores,



while water at 40 ºC brings about a similar macrovoid structure but an inner surface with slightly larger pores. The mechanisms of pore and macrovoid formation for these two membranes follow the typical solvent-nonsolvent instantaneous de-mixing.35 On the other hand; a slightly weaker bore fluid of 50/50 NMP/water at 0 °C produces the MBF membrane with obviously larger surface pores and much smaller macrovoids.36 A further decrease in bore fluid strength to 70/30 NMP/water of 0 °C causes an irregular membrane geometry, which cannot be used for

6

30 LEP

Burst pressure

Strength (Mpa)

5 4 3 2 1 0

25

Maximum tensile strength Young's modulus

20 15 10 5 0

water (0 °C)

Bore fluid

water (40 °C)

50/50 NMP/water (0 composition °C)

water (0 °C)

water (40 °C)

Bore fluid

50/50 NMP/water (0 composition °C)


membrane characterizations and performance tests.

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

50 40 30 20 water (0 °C) water (40 °C)

10

50/50 NMP/water (0 °C)

0 40

50

60

70

80


Figure 6: A) Mechanical strength, B) LEP & burst pressure and C) VMD permeation fluxes of MBF membranes fabricated from different bore fluid compositions and temperatures. A comparison of membrane properties including bulk porosity, mechanical strength, LEP, burst pressure and VMD flux as a function of bore fluid chemistry and temperature is illustrated in Figure 6. Owing to the differences in macrovoids, the MBF membranes spun from stronger bore fluids show higher porosity. Interestingly, the membrane spun from the bore fluid of pure water at 40 °C has the lowest tensile strength possibly due to large macrovoids and weak interconnected microstructure. The MBF membrane spun from the bore fluid of 50/50 NMP/water at 0 °C shows the highest tensile strength probably because of the almost macrovoidfree cross-section morphology and better interconnected microstructure.37 Unlike tensile strength 14 ACS Paragon Plus Environment

Page 15 of 19

measured along the axial direction, the radial direction LEP and burst pressure mainly follow the tightness of inner-surface pore size. The tighter surface due to the smaller pore size exhibits the higher LEP and burst pressure. On the other hand, flux increases with pore size. The MBF membrane spun from the bore fluid of 50/50 NMP/water at 0 ºC has the highest VMD flux, while MBF membranes spun from water at both 0 °C and 40 °C show similar but lower VMD fluxes. 5

80%

Porosity

78%

B

Porosity

Pressure (bar)

A

76% 74% 72% 7-bore

7-bore

3 2 1 7-bore

6-bore

C

Maximum tensile strength Young's modulus

30 20 10 0 7-bore

6-bore

Number of center channels


40

LEP Burst pressure

D

Numbers of bore channels 50

6-bore

4

0

70%

Strength (Mpa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6-bore

Number of bore channels

45

E

35 25 15

6-bore 7-bore

5 40

60

80


Figure 7: A) Microscopic images, B) Bulk porosity, C) Mechanical strength, D) LEP & burst pressure and E) VMD permeation fluxes of MBF membranes fabricated from spinnerets with 7 and 6 needles. Figure 7 A) illustrates the microscopic images of MBF membranes fabricated from spinnerets with 7 and 6 needles. The center of the 7-bore MBF is occupied by a bore channel while that of 6-bore MBF is occupied by the PVDF polymer. Figure 7 B) displays various membrane properties for MBFs with 7 and 6 bore channels. A decrease in bulk porosity of the 6-bore MBF



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

is probably due to the fact the fact that the center PVDF portion has a lower porosity. As a result, it has a higher Young’s modulus.11 Improvements were also noticed on its LEP and burst pressure. Depending on different feed temperatures employed, the fluxes of the 7-bore MBF are 10-15% higher than those of the 6-bore MBF. It is clear that the center bore channel in the 7-bore MBF contribute towards the total water production. Even though the diffusion pathway in this channel is longer, the diffusion resistance in the membrane matrix may be less in a VMD process due to the combined Poiseuille flow and molecular diffusion.20,

28

However, considering the

minor difference in their membrane structures; it is difficult to conclude how much the center channel contributes to the total flux. ■ ASSOCIATED CONTENT Supporting Information Materials; Procedures for hollow fiber spinning, membrane post-treatment, and module fabrication; protocols for various characterizations of hollow fiber membranes; additional SEM images of the membranes. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Tel: (65) 6516-6645; Fax: 65-6779-1936 ■ ACKNOWLEDGMENTS The authors would like to acknowledge Agency for Science Technology and Research (A*STAR) and National University of Singapore for funding the research through the project 16 ACS Paragon Plus Environment

Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


‘Development of Hybrid Desalination Processes using Cold Energy from LNG Re-gasification’ (grant number: R-279-000-291-305). The authors also appreciate Dr. K. Y. Wang, Mr. Y.K. Ong, Miss F. Edwie, Dr. P. Sukitpaneenit and Mr. Y.H. Sim for their valuable suggestions and comments. ■REFERENCES 1.

2.

3.

4.

5.

6.

7.

8.

9. 10. 11.

12. 13.

Lattemann, S.; Kennedy, M. D.; Schippers, J. C.; Amy, G., Chapter 2: Global Desalination Situation. In Sustainable water for the future-Water recycle versus desalination, Escobar, I.; Schafer, A. I., Eds. Elsevier: 2010; Vol. 2, pp 7-39. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marĩas, B. J.; Mayes, A. M., Science and technology for water purification in the coming decades. Nature 2008, 452, (7185), 301-310. Sinha, S.; Amy, G.; Yoon, Y.; Her, N., Arsenic removal from water using various adsorbents: Magnetic ion exchange resins, hydrous ion oxideparticles, granular ferric hydroxide, activated alumina, sulfur modified iron, and iron oxide-coated microsand. Environmental Engineering Research 2011, 16, (3), 165-173. Peng, W.; Escobar, I. C., Rejection efficiency of water quality parameters by reverse osmosis and nanofiltration membranes. Environmental Science and Technology 2003, 37, (19), 4435-4441. Criscuoli, A.; Rossi, E.; Cofone, F.; Drioli, E., Boron removal by membrane contactors: The water that purifies water. Clean Technologies and Environmental Policy 2010, 12, (1), 53-61. Cartinella, J. L.; Cath, T. Y.; Flynn, M. T.; Miller, G. C.; Hunter, K. W.; Childress, A. E., Removal of natural steroid hormones from wastewater using membrane contactor processes. Environmental Science and Technology 2006, 40, (23), 7381-7386. Criscuoli, A.; Zhong, J.; Figoli, A.; Carnevale, M. C.; Huang, R.; Drioli, E., Treatment of dye solutions by vacuum membrane distillation. Water Research 2008, 42, (20), 50315037. Ge, Q.; Wang, P.; Wan, C.; Chung, T. S., Polyelectrolyte-promoted Forward OsmosisMembrane Distillation (FO-MD) hybrid process for dye wastewater treatment. Environmental Science and Technology 2012, 46, (11), 6236-6243. Khayet, M., Membranes and theoretical modeling of membrane distillation: A review. Advances in Colloid and Interface Science 2011, 164, (1-2), 56-88. Tomaszewska, M., Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation. Desalination 1996, 104, (1-2), 1-11. Wang, P.; Teoh, M. M.; Chung, T. S., Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance. Water Research 2011, 45, (17), 5489-5500. Li, B.; Sirkar, K. K., Novel membrane and device for vacuum membrane distillationbased desalination process. Journal of Membrane Science 2005, 257, (1-2), 60-75. Khayet, M.; Matsuura, T., Pervaporation and vacuum membrane distillation processes: 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14.

15.

16.

17. 18. 19.

20. 21.

22.

23.

24.

25.

26.

27.

28.

29.

Page 18 of 19

Modeling and experiments. Aiche Journal 2004, 50, (8), 1697-1712. Simone, S.; Figoli, A.; Criscuoli, A.; Carnevale, M. C.; Rosselli, A.; Drioli, E., Preparation of hollow fibre membranes from PVDF/PVP blends and their application in VMD. Journal of Membrane Science 2010, 364, (1-2), 219-232. Camacho, L. M.; Dumée, L.; Zhang, J.; Li, J.-d. D.; Mikel; Gomez, J.; Gray, S., Advances in Membrane Distillation for Water Desalination and Purification Applications." Water 5, no. 1: 94-196. Water 2013, 5, (1), 94-196. Yang, X.; Yu, H.; Wang, R.; Fane, A. G., Optimization of microstructured hollow fiber design for membrane distillation applications using CFD modeling. Journal of Membrane Science 2012, 421-422, 258-270. Gryta, M.; Tomaszewska, M., Heat transport in the membrane distillation process. Journal of Membrane Science 1998, 144, (1-2), 211-222. Gryta, M.; Tomaszewska, M.; Morawski, A. W., Membrane distillation with laminar flow. Separation and Purification Technology 1997, 11, (2), 93-101. Edwie, F.; Teoh, M. M.; Chung, T. S., Effects of additives on dual-layer hydrophobichydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance. Chemical Engineering Science 2012, 68, 567-578. Fan, H.; Peng, Y., Application of PVDF membranes in desalination and comparison of the VMD and DCMD processes. Chemical Engineering Science 2012, 79, 94-102. Zhang, J.; Li, J. D.; Duke, M.; Xie, Z.; Gray, S., Performance of asymmetric hollow fibre membranes in membrane distillation under various configurations and vacuum enhancement. Journal of Membrane Science 2010, 362, (1-2), 517-528. Khayet, M.; Matsuura, T., Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation. Industrial and Engineering Chemistry Research 2001, 40, (24), 5710-5718. Garcia-Payo, M. C.; Izquierdo-Gil, M. A.; Fernandez-Pineda, C., Wetting study of hydrophobic membranes via liquid entry pressure measurements with aqueous alcohol solutions. Journal of Colloid and Interface Science 2000, 230, (2), 420-431. Wang, P.; Chung, T. S., Design and fabrication of lotus-root-like multi-bore hollow fiber membrane for direct contact membrane distillation. Journal of Membrane Science 2012, 421-422, 361-374. Teoh, M. M.; Peng, N.; Chung, T. S.; Koo, L. L., Development of novel multichannel rectangular membranes with grooved outer selective surface for membrane distillation. Industrial and Engineering Chemistry Research 2011, 50, (24), 14046-14054. Sun, S. P.; Hatton, T. A.; Chung, T. S., Hyperbranched polyethyleneimine induced crosslinking of polyamide-imide nanofiltration hollow fiber membranes for effective removal of ciprofloxacin. Environmental Science and Technology 2011, 45, (9), 4003-4009. Maab, H.; Francis, L.; Al-saadi, A.; Aubry, C.; Ghaffour, N.; Amy, G.; Nunes, S. P., Synthesis and fabrication of nanostructured hydrophobic polyazole membranes for lowenergy water recovery. Journal of Membrane Science 2012, 423-424, 11-19. Teoh, M. M.; Chung, T. S., Membrane distillation with hydrophobic macrovoid-free PVDF-PTFE hollow fiber membranes. Separation and Purification Technology 2009, 66, (2), 229-236. Li, C. L.; Wang, D. M.; Deratani, A.; Quémener, D.; Bouyer, D.; Lai, J. Y., Insight into the preparation of poly(vinylidene fluoride) membranes by vapor-induced phase separation. Journal of Membrane Science 2010, 361, (1-2), 154-166. 18 ACS Paragon Plus Environment

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Kong, J.; Li, K., Preparation of PVDF hollow-fiber membranes via immersion precipitation. Journal of Applied Polymer Science 2001, 81, (7), 1643-1653. 31. Sukitpaneenit, P.; Chung, T. S., Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology. Journal of Membrane Science 2009, 340, (1-2), 192-205. 32. Liu, F.; Hashim, N. A.; Liu, Y.; Abed, M. R. M.; Li, K., Progress in the production and modification of PVDF membranes. Journal of Membrane Science 2011, 375, (1-2), 1-27. 33. Phattaranawik, J.; Fane, A. G.; Pasquier, A. C. S.; Bing, W.; Wong, F. S., Experimental study and design of a submerged membrane distillation bioreactor. Chemical Engineering and Technology 2009, 32, (1), 38-44. 34. Peng, N.; Teoh, M. M.; Chung, T. S.; Koo, L. L., Novel rectangular membranes with multiple hollow holes for ultrafiltration. Journal of Membrane Science 2011, 372, (1-2), 20-28. 35. Chung, T. S.; Kafchinski, E. R., The effects of spinning conditions on asymmetric 6FDA/6FDAM polyimide hollow fibers for air separation. Journal of Applied Polymer Science 1997, 65, (8), 1555-1569. 36. Wang, P.; Chung, T. S., A conceptual demonstration of freeze desalination-membrane distillation (FD-MD) hybrid desalination process utilizing liquefied natural gas (LNG) cold energy. Water Research 2012, 46, (13), 4037-4052. 37. Widjojo, N.; Chung, T. S., Thickness and air gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes. Industrial & Engineering Chemistry Research 2006, 45, (22), 7618-7626.


A New-Generation Asymmetric Multi-Bore Hollow Fiber Membrane for

Recommend Documents