Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance
Graphical abstract
Highlights
► We carry out morphological design to enhance membrane distillation performance. ► Dual-layer fiber with macrovoid inner- and sponge-like outer-layer is designed. ► A superior flux of 98.6 L m2 h−1 is obtained during the DCMD experiments. ► Mathematical modeling is conducted to reveal the mechanism of flux enhancement.
Introduction
Water, a necessity to daily life, is currently in great deficiency because of the rapid growth of global population and the accelerated industrialization and urbanization (Escobar, 2010). To mitigate the water crisis, the desalination and wastewater reclamation processes have been studied extensively in past years (Shannon et al., 2008). Among the developed technologies, membrane distillation (MD) shows a promising perspective credited to its large membrane contact area, high salt rejection, small foot print and mild operation conditions (Song et al., 2008). Furthermore, as compared with other desalination technologies like multi-stage flash distillation (MSFD) and reverse osmosis (RO), MD is capable of integrating with various renewable energy sources such as solar energy, geothermal energy and waste heat source (Curcio and Drioli, 2005). However, at present, the application of MD process is still challenging in the real desalination industry application, mainly due to the low permeation flux, high risk of membrane wetting and long term operation instability (Khayet, 2011).
MD is a thermally driven process based on the principle of vapor–liquid equilibrium and coupled heat and mass transfer. During MD process, the volatile compounds in the hot feed solution evaporate at the feed/membrane interface, diffuse through the membrane pores and condense into liquid via various approaches (Suárez et al., 2010). The non-volatile compounds are left in the feed solution. In direct contact membrane distillation (DCMD), the most widely MD configuration in desalination application, the preheated brine feed and cooled permeate are brought in direct contact with two surfaces of micro-porous membrane, respectively. Owing to the hydrophobic characteristic, MD membrane acts as barrier between two phases. Even though in MD process, membrane does not directly contribute the selectivity, its structure, material and geometrical properties play an important role in the production rate, mechanical property and wetting resistance (Bonyadi et al., 2007). As a result, properties of MD membranes have been investigated by many researchers in recent years.
As a coupled mass and heat transfer process, the effective driving force of DCMD process is the vapor pressure difference between the feed and permeate interfaces. Unlike other membrane separation processes, improvement of permeation flux for a DCMD membrane can be ascribed to two possible mechanisms: namely, improvement of mass transfer coefficient and enhancement of effective driving force. Via the first mechanism, DCMD membrane structure should be tailored in an attempt to reduce the mass transfer resistance. This can be attained by fabricating membranes with large mean pore size and porosity, open-cell pore structure, thin functional layer and small tortuosity. Through the second mechanism, the thermal conductivity of the membrane needs to be minimized so as to reduce the temperature polarization and achieve a higher driving force. It can be reduced by increasing the membrane porosity, since the conductivity of filled air in membrane pores is much lower than the polymer matrix (Wang et al., 2009). Recently, by fabricating the single-layer hollow fiber membranes with finger-like macrovoids, Wang et al. reported that the fabrication of membrane with macrovoids is an effective way for flux enhancement attributed to high porosity and low tortuosity which allows fast diffusion of water vapor (Wang et al., 2009). A remarkably high permeation flux of 79 kg m−2 h−1 was achieved at 80 °C feed. Apart from improving permeation flux, the macrovoid structure also has potential on energy recovery improvement ascribed to its excellent thermal insulation because of air-filled macro pores. However, the impact of the macrovoid structure on membrane wetting and long-term stability are still concerned (Gryta and Barancewicz, 2010, Wang et al., 2009).
During the continuous DCMD operation, there is a risk of membrane pore wetting by vapor condensation and liquid penetration. At a worst situation, entirely wetted pores would result in the loss of membrane selectivity when the feed and permeate solutions at both streams are in direct contact (Tomaszewska, 1999). Therefore, wetting of membrane pores should always be avoided. To improve the wetting resistance, a membrane of high hydrophobicity, small pore size and high tortuosity is highly desired (Phattaranawik et al., 2003). Qtaishat et al. modified the polyetherimide (PEI) asymmetric flat membrane by using fluorinated surface modifying macromolecules (SMM) to increase the membrane hydrophobicity (Qtaishat et al., 2009). The contact angle of their modified membrane increased by more than 10° and the wetting resistance was also found to be improved greatly. On top of material hydrophobicity, a membrane matrix with a fully sponge-like structure can amplify the wetting resistance because of narrow pore size distribution and higher tortuosity (Khayet, 2011). Teoh and Chung (2009) have produced composite polyvinylidene fluoride polytetrafluoroethylene (PVDF/PTFE) hollow fiber membranes with a fully sponge-like structure. The resultant fiber membranes showed a higher resistance to pore wetting and stable performance in long-term tests (Gryta and Barancewicz, 2010, Teoh and Chung, 2009).
On one hand, membranes with smaller tortuosity, higher porosity, larger pore size, interconnected pore structure and macrovoid structure may improve permeation flux. On the other hand, larger tortuosity, smaller porosity and pore size as well as sponge-like structure are desired for high wetting resistance. The trade-off relationship makes it difficult for traditional single-layer MD membranes to achieve both high permeation flux and wetting resistance in a single configuration. Hence, dual-layer hollow fiber membranes comprising dual morphologies offer the opportunity to optimize these two properties. The objective of this study is to elucidate a novel approach to enhance the permeation flux and energy efficiency (EE) without compromising the wetting resistance via morphological architecture. The inner-layer is designed to be full of finger-like macrovoids whereas the outer-layer comprises sponge-like structure. We also aim to measure liquid entry pressure (LEP) using a homemade set-up as a quantitative indication of membrane wetting resistance. In addition, a mathematical model will be established based on the DCMD permeation flux and operation properties to obtain and analyze the essential transportation parameters including temperature profiles and apparent diffusivity. This information could provide the implicit understanding of vapor permeation mechanism and relative contributions of aforementioned two possible mechanisms for flux enhancement through these newly developed MD membranes. Overall, this work may provide profound implications for the strategies to enhance MD performances and attempts to make the application of this process more practical.
Section snippets
Materials
The working polymer, Kureha PVDF#1300 resin, was kindly provided by Kureha Corp. N-methyl-1-pyrrolidone (NMP, >99.5%), ethylene glycol (EG, >99.5%) and isopropanol (IPA, >99.5%) used in the hollow fiber fabrication were purchased from Merck. The hydrophobic clay particles Closite 20A were purchased from Southern Clay (Gonzales) while PTFE particles (<1 μm) were supplied by Sigma–Aldrich. The ultra-pure water used in DCMD tests was produced by a Milli-Q unit (MilliPore) with the resistivity of
Hollow fiber membrane morphology
Fig. 3 displays the morphology of dual-layer hollow fiber membrane D3. It can be seen that a novel morphology consisting of an inner-layer full of finger-like macrovoids and an outer-layer made of sponge-like structure has been attained. From the enlarged images, large macrovoids of regular finger-like shapes fully occupy the entire porous substrate layer, whilst open cell sponge-like pores form at the outer selective layer. As PVDF was used as the organic phase for the inner- and outer-layer
Conclusions
We have demonstrated that morphological architecture of PVDF dual-layer hollow fiber membranes can be utilized to enhance the DCMD performance. The dual-layer hollow fiber with a fully finger-like inner-layer and a totally sponge-like outer-layer is a preferred structure. The resultant dual-layer membranes show enhanced MD performance with minimum sacrifice of wetting resistance. The aforementioned morphology can be manipulated and tailor made via careful manipulations of inner- and outer-dope
Acknowledgments
The authors would like to acknowledge A*STAR and National University of Singapore for funding the research through the grant number R-279-000-291-305. The authors also appreciate Kureha Corp., Japan for the provision of the Kureha PVDF resin. Special thanks are due to Prof. E.L. Cussler (University of Minnesota), Dr. K. Y. Wang, Dr. N. Widjojo, Dr. J. C. Su, Miss H. Wang and Mr. Y. H. Sim for their valuable suggestions.
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