Pro

Highly Robust Thin-Film Composite Pressure Retarded Osmosis
(PRO) Hollow Fiber Membranes with High Power Densities for
Renewable Salinity-Gradient Energy Generation
Gang Han, Peng Wang, and Tai-Shung Chung*
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117602
*S Supporting Information
ABSTRACT: The practical application of pressure retarded osmosis (PRO)
technology for renewable blue energy (i.e., osmotic power generation) from
salinity gradient is being hindered by the absence of effective membranes.
Compared to flat-sheet membranes, membranes with a hollow fiber
configuration are of great interest due to their high packing density and
spacer-free module fabrication. However, the development of PRO hollow
fiber membranes is still in its infancy. This study aims to open up new
perspectives and design strategies to molecularly construct highly robust thin
film composite (TFC) PRO hollow fiber membranes with high power
densities. The newly developed TFC PRO membranes consist of a selective
polyamide skin formed on the lumen side of well-constructed Matrimid hollow
fiber supports via interfacial polymerization. For the first time, laboratory PRO
power generation tests demonstrate that the newly developed PRO hollow
fiber membranes can withstand trans-membrane pressures up to 16 bar and exhibit a peak power density as high as 14 W/m
2
using seawater brine (1.0 M NaCl) as the draw solution and deionized water as the feed. We believe that the developed TFC
PRO hollow fiber membranes have great potential for osmotic power harvesting.
1. INTRODUCTION
Renewable blue energy production has attracted rapid attention
due to the explosive increase in energy demand and the
depletion of fossil fuel resources in addition to the global trend
toward environmental sustainability.
1
Salinity-gradient energy
(i.e., osmotic power) generated from the mixing of solutions
with different salinities via pressure retarded osmosis (PRO)
represents a high potential source of renewable energy.
2−7
The
latest estimation of osmotic power generated from the mixing
of river water and seawater alone is approximately 1600 TW h
per year.
6,7
More energy can be generated when high salinity
reverse osmosis (RO) retentate is purposely mixed with
recycled water. Not only can this osmotic energy lower the
overall energy consumption for the RO process, but it can also
solve the disposal problem of RO retentate.
4,5
In a typical PRO process, water is osmotically drawn from a
low-salinity feed to a pressurized high-salinity solution across a
semipermeable membrane due to the water chemical potential
gradient. The continuous water influx into the high pressure
compartment provides the driving force to run the hydro-
turbine for electricity generation. Mathematically, the power
density is a product of the trans-membrane hydraulic pressure
and the water permeation flux across the membrane.
3,6
Based
on Statkraft’s analyses on commercially viable PRO processes,
the power density should be larger than 5 W/m
2
for flat
membranes in order to lower the capital cost and footprint
even for modest PRO plants.
6,7
However, the current PRO
technology is hindered by the absence of effective PRO
membranes with outstanding mechanical strength and power
density.
8−11
Traditional RO membranes for seawater desalination and
commercially available cellulose triacetate (CTA) forward
osmosis (FO) membranes all showed low power densities
less than 4.0 W/m
2
due to the severe concentration polarization
and relatively low intrinsic water permeability of CTA,
respectively.
8,11−13
Recently, some novel flat-sheet thin-film
composite (TFC) PRO membranes have been developed and
reported.
14−17
However, few studies have been devoted to the
engineering design of PRO hollow fiber membranes for
osmotic power generation. Compared with flat membranes,
membranes with a hollow fiber configuration are of great
interest because of their high packing density and ease of
module fabrication. Most importantly, hollow fiber modules
may not require spacers between the membranes.
18
Not only
could this minimize membrane deformation and structure
parameter enhancement owing to unavoidable spacer−
membrane interactions under high-pressure PRO operations,
but also eliminate the extra energy consumption for water
transport through the spacers.
18,19
Chou et al. reported one
TFC-PRO hollow fiber membrane, but their hollow fibers can
only withstand a hydraulic pressure of less than 9 bar.
20
Received: March 30, 2013
Revised: June 3, 2013
Accepted: June 17, 2013
Published: June 17, 2013
Article
pubs.acs.org/est
© 2013 American Chemical Society 8070 dx.doi.org/10.1021/es4013917 | Environ. Sci. Technol. 2013, 47, 8070−8077
Theoretically, the preferable operating trans-membrane pres-
sure during the mixing of river water and seawater (or seawater
brine) in PRO is at about 13.5 bar (or higher) in order to
generate the maximal energy.
Therefore, the objectives of this work are to develop novel
TFC-PRO hollow fiber membranes with favorable robustness
and high power density for osmotic power production. By
effectively controlling the phase inversion during membrane
formation, the hollow fiber supports were designed to possess
different dimensions and morphologies. Polyamide selective
skin was formed on the lumen side of the hollow fibers via a
simple and highly reproducible interfacial polymerization. The
PRO performance of the newly developed TFC hollow fibers
was evaluated via a lab scale PRO setup to demonstrate their
potential for osmotic power generation.
2. EXPERIMENTAL METHODS
2.1. Fabrication of Highly Robust Hollow Fiber
Supports. The hollow fiber membrane supports were prepared
via a dry-jet wet phase inversion spinning process. As illustrated
in Figure 1, both dope-solvent coextrusion and dual-bath
coagulation technologies were explored during spinning to
effectively control the phase inversion during membrane
formation and obtain the desirable membrane structure and
morphology.
21−23
Table S1 summarizes the detailed spinning
conditions of all hollow fiber supports. The detailed procedures
for membrane post-treatment and module fabrication are
described in the Supporting Information (SI).
2.2. Interfacial Polymerization of TFC-PRO Hollow
Fiber Membranes. As shown in Figure S1, the polyamide
selective layer was formed on the inner surface of the fabricated
hollow fiber supports by interfacial polymerization between m-
phenylenediamine (MPD) and trimesoyl chloride (TMC). The
detailed specification of the experimental setup and preparation
steps is disclosed in the SI.
2.3. Membrane Characterizations. Membrane morphol-
ogy was observed by a field-emission scanning electron
microscope, while surface hydrophilicity was evaluated by
water contact angle measurements on the inner membrane
surface using a Contact Angle Geniometer.
21
Membrane
porosity of the hollow fiber support was measured using a
standard protocol described by Han et al. and Sukitpaneenit et
al.
17,21
Fiber mechanical properties were determined by an
Instron tensiometer at a constant elongation rate of 10 mm
Figure 1. Control of the phase inversion process with the aid of (a)
21
dope-solvent coextrusion technology employing a dual-layer spinneret; and
(b)
22
dual-bath coagulation technology using a single-layer spinneret.
Figure 2. SEM micrographs of different bulk and surface morphologies of HF-1 (left) and HF-3 (right) hollow fiber supports.
Environmental Science & Technology Article
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min
−1
. Pure water permeability (PWP) of the hollow fiber
supports was measured using a lab-scale filtration unit described
in SI and previously.
21,24
The pore size, pore size distribution,
and molecular weight cutoff (MWCO) of the hollow fiber
supports were measured by solute rejection experiments using
polyethylene glycol (PEG) as a neutral rejection probe as
described in SI.
25
Mass transport characteristics of the TFC-
PRO hollow fiber membranes, including the intrinsic water
permeability (A), the salt rejection rate (R), and the salt
permeability (B), were characterized by testing the membranes
under the RO mode via a lab scale circulating RO filtration
apparatus following the method described elsewhere.
17,21
2.4. PRO Tests. Figure S2 shows the schematic diagram of
the lab scale PRO setup for hollow fiber membrane tests. A
hydraulic pressure was applied on the draw solution and
increased to predetermined values. NaCl was used as the solute
to simulate the water sources of both draw and feed solutions.
The detailed experimental setup, operating conditions, and the
determination of water and salt flux and membrane structure
parameters are described in the SI and elsewhere.
21,26,27
3. RESULTS AND DISCUSSION
3.1. Characteristics of the Hollow Fiber Supports. The
newly developed TFC-PRO hollow fiber membranes are
designed with an inner polyamide selective layer and a porous
outer surface to reduce ICP. Figure 2 and Figure S3 show the
typical morphologies of the hollow fiber supports fabricated
from different protocols. The fibers were designed to have an
inner diameter ranging from 460 to 820 μm (Table S2).
A similar cross-section structure consisting of small elongated
and tear-shape macrovoids in the middle of the cross-section is
observed for all hollow fibers. This is mainly due to an
instantaneous de-mixing induced by the non-solvent-rich (70
wt % water) bore fluid. A thick open-cell sponge-like layer
(larger than 40 μm) was observed underneath the membrane
inner surface for all three fibers, which is critical for interfacial
polymerization to form a robust TFC-PRO membrane.
28
Credit to the fast phase inversion induced by the water-rich
bore fluid, all as-spun fibers have smooth inner surfaces with
uniformly distributed small pores. This help form a less
defective polyamide layer with a high water permeability and
salt rejection during interfacial polymerization.
16,17,28
Interestingly, the outer surfaces and cross-section morphol-
ogies underneath the outer surfaces of the hollow fibers are
varied with different fabrication methods. HF-1 and HF-2
membranes fabricated via the dope-solvent coextrusion
technology exhibit a quite similar outer surface with fully
open porous rough morphology. This is due to the fact that a
pure NMP solvent was fed at the outer channel of the tri-orifice
spinneret during spinning that not only reduces the polymer
concentration at the outer surface of the nascent fiber, but also
delays the phase inversion in the air-gap region prior to
entering the water coagulant (Figure 1). However, HF-3
membrane fabricated from a dual-bath coagulation technology
exhibit a porous outer surface with disconnected big pores. The
isopropyl alcohol (IPA) rich coagulant in the first bath reduces
the solvent−nonsolvent demixing rate, which provides time for
the nuclei formation and surface pore formation. However, the
IPA/water mixture has stronger coagulation strength than the
NMP used in the dope-solvent coextrusion process. Thus, a
relative thick outer surface with discontinuous big pores was
formed.
23
All aforementioned fibers have porous cross-section
morphologies that facilitate water and salt transportation in the
support and reduce the ICP effects; however, the HF-3
membrane is expected to possess higher membrane strength
due to its connected outer surface.
Table S2 summarizes the basic characteristics of the as-spun
hollow fiber supports including the mean pore size, pure water
permeability (PWP), molecular-weight cutoff (MWCO),
porosity, and water contact angle. Figure S4 shows that all
spun hollow fibers possess a small mean pore size less than 7
nm. This pore feature is favorable for the formation of a
continuous and homogeneous polyamide layer in the
subsequent interfacial polymerization.
21,27
The HF-2 mem-
brane has a smaller pore size distribution than HF-1 mainly due
to the effect of gravity elongation because the former has a
larger air gap distance during spinning than the latter. The
PWP of each developed hollow fiber support is higher than 245
L/(m
2
·bar·h) mainly due to its high overall porosity (72.4−
79.0%).
In order to investigate the stability of the hollow fiber
supports in high pressure PRO tests, the evolution of their
PWP as a function of ΔP was explored using the PRO setup. As
shown in Figure 3a, the normalized PWP of each developed
hollow fiber support decreases with an increase in ΔP due to
membrane compaction, but relatively slightly.
16,17
However,
HF-2 and HF-3 exhibit much smaller reductions compared to
HF-1 at equal pressure. This may be due to the fact that HF-2
has a smaller fiber dimension and HF-3 has a continuous outer
surface morphology. As a result, they have better membrane
mechanical strength and anticompaction ability.
Interestingly, it is worth noting that the PWP suddenly
changed to increase when the hydraulic pressure is beyond a
Figure 3. (a) Variations of the normalized pure water permeability
(PWP) as a function of hydraulic pressure; and (b) the “critical
pressure” of the hollow fiber supports (the pressure represented by the
filled symbol in (a)).
Environmental Science & Technology Article
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certain value. This is possibly due to the microstructural
changes caused by the greatly increased membrane surface area
and reduced thickness induced by the high pressure in the
lumen side. In this study, the pressure at which the PWP begins
to increase is defined as “critical pressure” to quantitatively
characterize the hollow fiber membrane overall robustness. It is
believed that the polyamide selective layer of the TFC hollow
fibers with an inner selective layer will form significant defects
when the applied hydraulic pressure is larger than the defined
“critical pressure”. The newly developed hollow fiber supports
show excellent “critical pressures”, particularly for HF-2 and
HF-3. They are 13 and 16 bar, respectively (Figure 3(b)). To
the best of our knowledge, these values are superior to other
reported PRO hollow fibers in terms of “critical pressure” or
burst pressure.
5,8,9,15,20
In order to investigate the sources of membrane robustness,
the mechanical properties of the newly developed hollow fiber
supports was measured in terms of Young’s modulus, tensile
strength, and elongation at break, as summarized in Table S3.
Since the hollow fiber membrane under high pressure PRO
processes will be subjected to several forces together such as
compaction, expansion, bending, and shear forces, membrane
toughness was also estimated by integrating the stress−strain
curve. Interestingly, HF-3 shows the highest membrane
toughness, followed by HF-2 and then HF-1. This toughness
order seems to be consistent with their critical pressures and
PWP stability under PRO tests. In summary, the newly
developed hollow fiber membrane supports show desirable
characteristics of being supports for TFC-PRO membranes.
3.2. Characteristics of TFC-PRO Hollow Fiber Mem-
branes. A selective polyamide skin was formed on the inner
surface of the newly developed hollow fiber supports via
interfacial polymerization as depicted in Figure S1. Figure 4
shows the surface and cross-sectional morphologies of the
fabricated TFC-PRO hollow fiber membranes. A typical thin
layer of “ridge-and-valley” morphology has been attached onto
the inner surfaces of the supports. The estimated thickness of
the selective polyamide layer varies from 100 to 200 nm. The
water contact angle of the inner surface is lowered from about
80.5° to about 48.6° after interfacial polymerization. This
increase in surface hydrophilicity also confirms the successful
formation of the polyamide selective skin. The hydrophilic
thinner polyamide layer is crucial to achieve high water
permeation and potentially high power generating efficiency.
Table 1 summarizes the intrinsic transport properties of the
developed TFC-PRO hollow fiber membranes in terms of pure
water permeability (A), salt permeability coefficient (B), and
membrane structure parameter (S). TFC-HF1 and TFC-HF2
membranes made of hollow fiber supports from the dope-
solvent coextrusion technology exhibit a water permeability of
1.40 and 1.70 L/(m
2
·bar·hr) with relatively small salt
permeability of 0.13 and 0.41 L m
−2
h
−1
, respectively. The
TFC-HF3 membrane made of a support from the dual-bath
coagulation technology show the highest water permeability of
1.90 L/(m
2
·bar·hr) with a salt permeability of 0.48 L m
−2
h
−1
.
Compared with the commercial HTI FO membranes and some
reported TFC membranes, the newly developed TFC-PRO
hollow fiber membranes possess much higher water perme-
ability.
3,8,9
The FO performance (ΔP = 0) of the TFC-PRO hollow
fibers was evaluated using deionized water as the feed and 1 M
NaCl as the draw solution under the PRO mode. As
summarized in Table S4, a water flux as high as 30−38 LMH
with a salt reverse flux of 8−10 gMH can be achieved.
Comparing with other reported FO membranes, the newly
developed TFC-PRO hollow fiber membranes possess
remarkably high water fluxes and relatively low salt
leakage.
4,5,8,9
This superior performance may be attributed to
the inherent characteristics of the hollow fiber membrane
configuration in addition to the well-structured polyamide
selective layer. Since membrane structural parameter (S) plays a
determining role on membranes’ PRO performance,
13,18
Table
1 indicates that these TFC-PRO membranes have reasonably
small S in the range of 776−987 μm. The TFC-HF3 membrane
made of a support from the dual-bath coagulation exhibits a
relatively smaller structure parameter, and the order of structure
Figure 4. Typical morphology of TFC-PRO hollow fiber membranes.
Table 1. Transport Properties and Structural Parameters of
TFC-PRO Hollow Fiber Membranes
membrane
water
permeability,
A [L/(m
2
·
bar·hr)]
salt
rejection
(200 ppm
@ 1 bar)
salt
permeability,
B
(L m
−2
h
−1
)
K
m
(×10
5
s m
−1
)
S
(×10
−4
m)
TFC-HF1 1.40 89.20% 0.13 6.82 9.87
TFC-HF2 1.70 88.50% 0.41 5.04 7.45
TFC-HF3 1.90 87.80% 0.48 5.24 7.76
Environmental Science & Technology Article
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parameter of these TFC-PRO membranes follows: TFC-HF2 ≈
TFC-HF3 < TFC-HF1.
3.3. Implications of Developed TFC-PRO Hollow
Fibers for Osmotic Power Generation. The applicability
and power output of the newly developed TFC-PRO hollow
fibers were evaluated using different synthetic brine as draw
solutions and several water sources as feed solutions. Table S5
lists the details of synthetic solutions for PRO tests where NaCl
is the model solute. Figure 5(a) compares the water flux (J
w
) as
a function of hydraulic pressure difference (ΔP) across the
TFC-PRO hollow fiber membranes using 1.0 M NaCl synthetic
brine as the draw solution and deionized water as the feed. As
ΔP rapidly increases, J
w
exhibits a nearly linear decrease due to
the reduced driving force and membrane compaction. At the
same ΔP, TFC-HF3 and TFC-HF2 show substantially higher
water fluxes than the TFC-HF1 membrane. This could be
attributed to their higher water permeability (A) and smaller
membrane structure parameter (S) (Table 1).
Figure 5. Water flux and power density of the developed TFC-PRO hollow fiber membranes with seawater brine (1 M NaCl) as draw solution, and
fresh water as feed solution.
Figure 6. Schematic of membrane properties variations of the TFC-PRO hollow fiber membranes during PRO operations.
Environmental Science & Technology Article
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An interesting phenomenon was observed that the water flux
suddenly begins to increase when ΔP reaches a certain value,
which is contrary to the theoretical prediction. This may be due
to the unique hollow fiber configuration and the property
changes in the polyamide selective layer induced by the high
pressure water flow in the lumen side. As illustrated in Figure
6(a), the hollow fiber support and polyamide layer will be
expanded by the increased ΔP in the lumen side because of the
highly porous and polymeric nature, resulting in an enlarged
surface area and reduced thickness. Therefore, the effective
channel dimension and length for water transport through the
TFC layer may be slightly enlarged and shortened respectively
due to the stretching and thinning of the polyamide layer.
However, the polyamide selective layer may experience
irreversible changes, and minor defects are formed before
physically breaking away from the hollow fiber support when
ΔP is beyond a certain value, as illustrated in Figure 6(b). If we
define the “critical pressure” for TFC-PRO hollow fiber
membranes with an inner polyamide selective layer as the
pressure (ΔP) at which the water flux begins to increase
sharply. It is interesting to note that this “critical pressure” is
quite similar and closely related to the afore-defined “critical
pressure” for their corresponding hollow fiber supports (Figure
3), such as 16 bar for TFC-HF3, 12 bar for TFC-HF2, and 10
bar for TFC-HF1 membranes. In addition, the TFC-PRO
hollow fibers with an inner polyamide selective layer could be
prestabilized using a pressure below the “critical pressure”. As
shown in Figure 5(c), after being stabilized by rapidly
increasing ΔP to 16 bar, the TFC-HF3 membrane shows a
higher and stabilized water flux as confirmed by the hysteresis
tests.
Figure 5(b) and (d) plot power density (W) as a function of
ΔP; the newly developed TFC-PRO hollow fibers can
withstand a ΔP as high as 10−16 bar with a stable power
density up to 6−14 W/m
2
when using 1 M NaCl synthetic
brine as the draw solution and fresh water as the feed.
Particularly, TFC-HF3 membrane exhibits the highest W of 14
W/m
2
at 16 bar, which is attributed to its superior mechanical
properties (Table S3) and highest water permeability (Table 1
and Table S4). In addition, the power output is very stable and
repeatable confirmed by the 1.5 hysteresis tests under a
hydraulic pressure varying from 0 to 16 bar. To the best of our
knowledge, this PRO performance (i.e., operating pressure of
16 bar and power density of 14 W/m
2
) outperform all other
PRO hollow fibers and most flat-sheet membranes reported in
the literature.
8,9,14−17
The prestabilized TFC-HF3 PRO membrane was further
evaluated using synthetic river water (10 mM NaCl) and
wastewater (40 mM NaCl) as feeds. As shown in Figure 7, a
slightly reduction in power density is observed with an increase
in feedwater salinity. For example, power density drops from
14.0 W/m
2
to 11.5 W/m
2
and 9.5 W/m
2
at 16 bar when
replacing fresh water by synthetic river water and wastewater,
respectively. This reduction is caused by the combinative effects
of reduced osmotic driving force and enhanced ICP effects.
However, the obtained power density is still superior to most
Figure 7. Power density of the TFC-HF3 PRO hollow fiber membranes with seawater brine (1 M NaCl) as draw solution, and river water and
wastewater brine as feed solutions.
Environmental Science & Technology Article
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other reported values as compared in Table 2. Clearly, the
newly developed TFC-PRO hollow fiber membranes possess
encouraging power density which is much higher than the
required value of 5 W/m
2
estimated by Statkraft to make PRO
commercially viable.
32−34
Future works will focus on
membrane module design together with membrane fouling
and long-term behavior.
■
ASSOCIATED CONTENT
*S Supporting Information
PRO process; materials; hollow fiber spinning (Table S1) and
post-treatment and module fabrication; interfacial polymer-
ization (Figure S1); pressure retarded osmosis setup (Figure
S2) and operating conditions and performance evaluation;
membrane characterization; additional SEM images (Figure S3)
of the supports; protocols for physical and mass transport
characterizations of membranes; pore size distribution profiles
(Figure S4 and Table S2); membrane mechanical strength
(Table S3); forward osmosis performance of the hollow fibers
(Table S4); and synthetic feed solutions for PRO tests (Table
S5).This material is available free of charge via the Internet at
http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +65-65166645; fax: +65-67791936; E-mail: chencts@nus.
edu.sg.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This research grant is supported by the Singapore National
Research Foundation under its Environmental & Water
Technologies Strategic Research Programme and administered
by the Environment & Water Industry Programme Office
(EWI) of the PUB under the project entitled “Membrane
development for osmotic power generation, Part 1. Materials
development and membrane fabrication” (1102-IRIS-11-01)
and NUS grant number of R-279-000-381-279. Special thanks
are due to Dr. Panu Sukitpaneenit, Dr. Sui Zhang, Miss Xue Li,
and Miss Xiuzhu Fu for their valuable suggestions and kind
help.
■
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Table 2. Comparison of the PRO Membrane Performance
salty water fresh water
operation pressure, P
(bar)
power density,
W (W/m
2
) membrane reference
Seawater brine (1.0 M
NaCl)
Tap water 12 8.9 TFC-hollow fiber
membrane
Current work
Tap water 16 14.0
River water (10 mM NaCl) 16 11.0
Waste water brine (40 mM
NaCl)
16 9.2
Seawater brine (1.0 M
NaCl)
River water (10 mM NaCl) 8.4 11 TFC-hollow fiber
membrane
20
Waste water brine (40 mM
NaCl)
5.1 6.2
Seawater (3.5 wt % NaCl) DI water 12 2.85 TFC flat sheet membrane 16
10 2.6 TFC flat sheet membrane 15
60 g/L NaCl (1.03 M NaCl) DI water 9.72 5.06 HTI flat sheet membrane 29
π = 101.3 bar Water 19.25 1.56 FRL composite membrane 26
π = 25.3 bar Water 12.16 0.35 Permasep B-10 membranes 30
π = 78.0 bar Water 40.53 3.12 12
π = 81.1 bar Water with 0.2% formaldehyde 40.53 3.27 31
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