Analysis of flux reduction behaviors of PRO hollow fiber membranes: Experiments, mechanisms, and implications
Introduction
Osmotic energy has received growing attention worldwide as an alternative energy. Pressure retarded osmosis (PRO) is a process able to harvest salinity gradient energy by employing a semipermeable membrane between a low salinity feed solution (referred to as “the feed” thereafter) and a high salinity draw solution [1], [2], [3], [4], [5], [6], [7]. Comparing to many fossil fuel based energy sources, PRO is a green and sustainable technology friendly to environments because no greenhouse gas and chemicals are released during the harvest of osmotic power.
In spite of the aforementioned advantages over other traditional energy sources, harvesting osmotic energy from PRO processes is still in an infant stage because power density and water flux are severely reduced by many limiting factors, such as internal concentration polarization (ICP), external concentration polarization (ECP), reverse salt permeation, and membrane fouling [7], [8], [9], [10], [11], [12], [13], [14], [15]. Even though many breakthroughs have been achieved in recent years on PRO membranes with impressive mechanical strength and power density [7], [8], [14], [16], [17], [18], [19], [20], [21], [22], [23], membrane performance deteriorates sharply in real applications [8], [15]. As a result, the net energy drops to a level only marginally higher or even lower than the economically feasible value. Therefore, it is of great importance to investigate the underlying mechanisms responsible for the significant water flux reduction in order to overcome the limitations and design innovative PRO membranes for real practices.
In general, ICP coupled with undesirable reverse salt flux accounts for the water flux decline in PRO processes when the feed pair contains negligible fouling tendency [9], [14], [16], [24]. Prior works have indicated a direct link between intrinsic membrane properties such as water permeability (A), reverse salt permeability (B), structural parameter (S) and operation conditions (for example, operation pressure ∆P) and external performance indexes such as water flux (Jw), reverse salt flux (Js), and power density. Yip et al. used flat sheet membranes and studied the combined effects of A and B with a relatively small structural parameter [16]. It was found that membranes with a high A value and a moderate B value are highly favorable to achieve outstanding PRO performance, while membranes with a high A value but a very low selectivity (i.e., high B value as well) would suffer from a severe reverse salt leakage, leading to a significantly reduced water flux and projected peak power density at high operation pressures. Zhang and Chung studied the instant and accumulative effects of reverse salt permeation on water flux and power density using 1 M NaCl and DI water as the feed pair [14]. Three types of inner-selective thin-film composite (TFC) hollow fiber membranes with different combinations of A, B and S values were used. It was found that a low B value is crucial to suppress the severe flux decline in order to harvest a high power density, while a high B value not only causes an instant drop of water flux but also leads to a significant salt build-up in the feed, resulting in a further rapid flux decline along membrane modules. Sivertsen et al. developed a unique iso-watt diagram to link A, B, and S directly with peak power density, enabling an efficient way to compare different membranes [24]. She et al. employed HTI CTA flat sheet membranes and specific reverse solute flux (i.e., Js/Jw, the ratio of reverse salt flux Js to volumetric water flux Jw) to investigate the effects of various operation parameters in PRO [9]. They found that the significantly reduced water flux was mainly caused by the enhanced ICP due to the severe reverse salt permeation. To date, although most prior works have observed direct links among the intrinsic membrane properties, operation conditions and external performance, the work to systematically elaborate the complicated interactions among them is still absent. By choosing two TFC hollow fiber membranes recently developed in our group, an inner-selective TFC membrane [8] and an outer-selective TFC membrane [25] (Fig. 1), this work aims to in-depth bridge the intrinsic membrane properties, operation conditions and external performance by elucidating the interactions among some internal factors and illustrating their effects on the external performance. The internal factors include the surface salinity of the selective layer at the feed side (CF,m) and its ICP contribution and Js contribution, while the external performance comprises water flux (Jw), reverse salt flux (Js), and power density.
Besides ICP and reverse salt permeation, scaling is another limiting factor responsible for the flux reduction when the feed pair contains scaling precursors [8], [11], [12], [15]. For example, scaling precursors such as Ca2+, Sand are commonly present in wastewater retentate from wastewater reclamation plants [8], [11], [15]. Zhang et al. observed that gypsum precursors such as and can cause pore clogging inside the membrane porous support due to the severe ICP of the precursors [11]. They pointed out that the initial rapid flux reduction was due to the significant ICP at high water fluxes, while the subsequent slower flux decline was attributed to the weakened ICP at lower fluxes. They also observed a limiting flux phenomenon similar to the flux behaviors in RO and nanofiltration (NF) processes [26], [27]. To date, although prior works have found that scaling may result in a drastic flux reduction [8], [11], [12], [15], the relationship between pore clogging and flux reduction phenomena still remains unclear. This work aims to elaborate this relationship by employing the advanced nucleation theory, simulation, and experimental studies.
To investigate the underlying mechanisms responsible for the flux reduction, both the inner- and outer-selective TFC hollow fiber membranes will be firstly employed to examine the interactions among the internal factors and their effects on the external performance of PRO processes. Then the outer-selective TFC hollow fiber membrane will be used in the gypsum scaling study to elucidate the scaling kinetics, especially the relationship between the pore blocking and the flux reduction behavior. Finally, implications of this study will be discussed to provide useful insights to design more suitable TFC hollow fiber membranes with enhanced water flux so that PRO process may become more promising in the future.
Section snippets
Materials
Materials used for hollow fiber spinning such as Polyethersulfone (PES), N-methyl-2-pyrrolidone (NMP) et al. were the same as some prior works [14], [20], [25]. Materials for the formation of TFC selective layers such as 1,3,5- benzenetricarboxylic acid chloride (TMC), M-phenylenediamine (MPD), hexane and sodium dodecyl sulfate (SDS) can be found in some prior works [8], [25]. Sodium sulfate (Na2SO4, >99% from Sigma-Aldrich) and calcium chloride (CaCl2, 98% from Sigma-Aldrich) were employed as
Theoretical simulation: mass transport across the membrane in PRO processes
A schematic diagram of the salt concentration profile across a TFC membrane under PRO is illustrated in Fig. 2. It has been shown by some prior works that the model for a radial geometry converges with the model developed for a flat geometry when the fiber radius increases [24], [30]. Furthermore, the model for a flat geometry can be employed for hollow fiber membranes if an equivalent thickness of the hollow fibers is used for the determination of the structural parameter [30]. Based on the
Comparison between inner- and outer-selective TFC hollow fiber membranes under a fixed bulk salinity gradient
To study the relationship among the internal factors and their effects on the external performance, a 0.81 M NaCl aqueous solution was used as the draw solution, which has the same osmolality as the first-stage RO brine of a local RO plant. A 0.011 M NaCl aqueous solution was used as the feed, which has the same osmolality as the wastewater retentate obtained from a local wastewater plant [8].
Fig. 3(1a)–(2f) shows various internal factors (Js, Js contribution, CF,m, ICP contribution, Δπeff, Js/Jw
TFC hollow fiber membranes
This study may provide useful insights to design suitable membranes for PRO applications. Since inner- and outer-selective TFC hollow fiber membranes deform differently under high-pressure PRO operations, different design strategies may be adopted for their membrane fabrication. Although the outer-selective TFC hollow fiber membrane benefits from its low salt permeability, it suffers from (1) low water flux due to its relatively low water permeability and large structural parameter and (2)
Conclusion
By using both inner- and outer-selective PRO membranes, we have studied the relationship among the internal factors and their effects on the external performance of PRO processes. As elucidated in Fig. 11, both the water flux and the reverse salt flux are ultimately linked to the same internal factor: the salinity gradient across the selective layer (∆Cm). With negligible ECP, ∆Cm is determined by the surface salinity on the feed side (CF,m). CF,m comprises two terms with two associated
Acknowledgments
This research study is supported by the National Research Foundation, Prime Minister’s Office, Republic of Singapore, under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB. This research work was funded by the project entitled “Membrane development for osmotic power generation, Part 2, Module fabrication and system integration” (1102-IRIS-11-02) and NUS Grant Number of R-279-000-382-279.
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These authors have contributed equally to this study.