Dual-layer PVDF/PTFE composite hollow fibers with a thin macrovoid-free selective layer for water production via membrane distillation

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Abstract

In this study, the polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) composite is used to fabricate hollow fiber membranes for seawater desalination via direct contact membrane distillation (DCMD) application. The incorporation of PTFE particles in the formulated dope solution can efficiently suppress the formation of macrovoids and enhance the outer surface hydrophobicity. Dual-layer hollow fibers with a desirable macrovoid-free morphology and a relatively thin (13 ± 2 μm) outer-layer can be obtained via blending 30 wt% of PTFE particles in the outer-layer dope. The resultant dual-layer hollow fiber (DL-30) displays a moderately high contact angle of 114.5° and porosity of 81.5%. Compared to the single-layer hollow fiber with 30 wt% (SL-30) PFTE particles, the DL-30 fiber exhibits a flux enhancement of approximately 24% that is contributed to the reduction in inner-layer mass transfer resistance. Dual layer membrane configuration with a lower wall thickness as well as larger outer and inner diameters provides even higher water vapor transport is potentially suitable for desalination. Both single- and dual-layer PVDF/PTFE hollow fiber membranes reveal good long-term stability of up to 100 h of continuous testing. By utilizing the state-of-the-art dual-layer spinning technology, hollow fiber membranes with better performance (i.e. enhanced flux) and morphology (i.e. macrovoid-free) can be tailored.

Highlights

► State-of-the-art dual-layer spinning technology for PVDF/PTFE fiber. Direct contact membrane distillation application for seawater desalination. Dual-layer fiber is effective way in cost saving. ► Tailor-made thin wall outer layer enhances permeation flux. ► Hollow fibers reveal good long-term stability of up to 100 h.

Introduction

The growing demand for reliable and affordable water resources, coupled with the problem of surface and ground water pollution, results in the scarcity of clean water supply. It is anticipated that this global shortage in drinking water will exacerbate to an alarming rate by year 2025 [1]. In view of this pressing global concern, it is important to explore alternative water sources and seawater desalination is a feasible strategy for attaining water sustainability. With technological advancements and breakthroughs, purification tools with better cost effectiveness and energy efficiency have been introduced. Among many purification processes, the use of low-energy membrane separation technology appears to be one of the most promising solutions to curb the water crisis. Membrane distillation (MD), an emerging membrane process, has recently received worldwide attention as the increase in crude oil price has marginalized the cost effectiveness of reverse osmosis (RO). The feasibility of integrating MD with waste heat generated from other processes or renewable energy sources (e.g. solar and geothermal) potentially reap greater cost and energy savings, making the process more attractive than RO [2].

MD is based on the combination of diffusive and convective transports of vapor molecules driven by the imposed partial vapor pressure gradient across a membrane. Typically, a non-wetting microporous membrane serves as a barrier between two aqueous solutions. This implies that the hydrophobicity of the material is an essential factor to prevent the entry of liquid into membrane pores. Among the hydrophobic materials, it has been discovered that polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP) exhibit low surface energy and are preferred for use in MD [3], [4]. In some studies, commercially available hydrophobic membranes are employed for MD [5], [6], [7], [8]. However, these commercial membranes are typically designed for other applications and may not be suitable for MD. Other research works implement the post-treatment (e.g. plasma polymerization) on the commercial fibers which make them appropriate for MD processes [9], [10].

To better capitalize on the benefits of MD, modeling and optimization of membrane module design is an effect tool to enhance overall performance and bring MD closer to industrialization [11], [12]. On top of this, membrane scientists have explored and fabricated novel membranes for this application. Essentially, an ideal MD membrane should has (1) low heat conductivity to improve the thermal efficiency, (2) low vapor transport resistance to enhance the permeate flux, (3) anti-fouling characteristics and reasonable mechanical strength to withstand long term operation, and (4) good chemical resistance to withstand backwashing or cleaning solvents. PVDF is often chosen as the membrane material because it is a commercially available hydrophobic polymer and can be easily dissolved in common organic solvents used for membrane fabrication in addition to good mechanical properties and chemical resistance [13], [14], [15]. Tomaszewska [14] and Khayet et al. [15] fabricated flat sheet PVDF membranes by introducing additives in polymer dopes to increase the membrane pore size and porosity. Generally, hollow fiber membranes are preferred for industrial applications due to the higher membrane contact area and the self-supporting nature. Wang et al. [16] and Hou et al. [17] employed a similar approach to fabricate single-layer PVDF hollow fiber membranes and obtained considerable flux enhancement. In other studies, approaches such as decreasing the spinning dope concentration [18] and using a solvent-polymer dope co-extrusion method [19] have been investigated. The fabrication of single-layer PVDF composite hollow fibers for DCMD was reported by Teoh and Chung [20], Bonyadi et al. [21], and Wang et al. [22]. Bonyadi and Chung [23] have utilized the state-of-the-art dual-layer spinning technology to minimize the thickness of the hydrophobic functional layer by incorporating a hydrophilic support layer. The hydrophobic/hydrophilic dual-layer hollow fiber membrane configuration yields a higher permeation flux which is attributed to the reduction in the mass transport resistance across the membrane.

In addition to the fabrication of MD membranes, works are in progress to develop membranes with high wetting resistance and long time stability. Wetting is undesirable and usually occurs when the applied pressure exceeds the liquid entry pressure (LEP). A membrane with a high LEP value can be designed by enhancing the contact angle of the material (higher than 90°) and by minimizing the dimension of the largest pore size (0.1–0.5 μm) [24]. Two approaches to increase the hydrophobicity of the membrane are (1) polymer blending [18], [20] and (2) chemical surface modification [9], [10], [25], [26]. In MD applications for desalination, highly porous and macrovoid-free hollow fiber membranes are desirable in order to obtain a high permeation flux and to achieve long term stability. A typical dual-layer hollow fiber membrane consists of a thin asymmetric separating outer-layer and a microporous supporting inner-layer [27], [28]. This unique dual-layer hollow fiber membrane configuration with thin sponge-like outer selective layer allows the maximum utilization of an expensive material and provides greater cost savings. Thus, the objective of this work is to fabricate high performance hydrophobic PVDF/PTFE composite hollow fiber membranes with desirable porosity, thickness and morphology via the state-of-the-art dual-layer spinning technology.

To better understand the roles of the PVDF/PTFE outer selective layer for MD applications, a series of polymer dope solutions with different PTFE particle loadings are investigated. PTFE particles (<1 μm) are specifically chosen as the disperse phase because of their superior physicochemical properties (i.e. super-hydrophobic in nature) and excellent stability in many organic and inorganic solvents. Single-layer fibers are also spun for comparison. The pure water production of PVDF/PTFE composite hollow fibers via the DCMD process, and their long-term desalination performance and stability are highlighted. To the best of our knowledge, so far there is only one report using co-extruded dual-layer hollow fiber membranes for MD studies [23].

Section snippets

Materials

The copolymer polyvinylidene fluoride (PVDF) T #2300 was kindly provided by Kureha Corporation and PTFE particles (<1 μm) were purchased from Sigma–Aldrich. The solvent N-methyl-2-pyrrolodinone (NMP, >99.5%), non-solvent ethylene glycol (EG, >99.5%) and external coagulant isopropanol (IPA) were purchased from Merck, Panreac and Fisher Scientific, respectively. Sodium chloride (NaCl, 99.5%) was purchased from Merck and Milli-Q ultra-pure water was produced in our laboratory with the resistivity

Characterization

The contact angle and porosity of as-spun hollow fiber membranes are summarized in Table 3. The porosity study was conducted and all fibers show a comparatively high membrane void fraction between 80.5 and 88.4%. On the other hand, for single-layer hollow fibers, the incorporation of PTFE particles increases the advancing contact angle from 85.7 ± 4.5° to 108 ± 3.3°. A similar trend of increment in contact angle can be observed for the dual-layer fiber, as tabulated in Table 3. Although the

Conclusions

The potential of using dual-layer PVDF/PTFE composite hollow fiber membranes for desalination via DCMD has been demonstrated in this work. FESEM characterization confirms that a thin macrovoid-free outer-selective layer of 13 μm can be achieved by incorporating 30 wt% PTFE loading in the outer dope solution. Compared to the single-layer PVDF/PTFE configuration, the dual-layer fibers posses enhanced desalination performance that is contributed to the reduction in inner-layer mass transfer

Acknowledgements

The authors would like to thank the Agency for Science, Technology and Research (A*STAR) and National University of Singapore (NUS) for funding this research with grant numbers R-279-000-291-305 and R-279-000-291-331. Thanks are also due to Dr. K.Y. Wang and Miss B.T. Low from National University of Singapore who have contributed useful comments and suggestions to this study. Special thanks are dedicated to Kureha Corporation for the provision of PVDF T #2300 polymer resin.

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