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Development of polymeric membranes for membrane distillation and processes for desalination

Development of polymeric membranes for membrane distillation and processes for desalination
Other Titles
담수화를 위한 막 증류법용 고분자 분리막 및 시스템 개발
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The purpose of this dissertation is to understand and develop the emerging desalination technology, membrane distillation (MD). Although the MD process has been researched during the last five decades, however, until now, large-scale of implementation is constrained due to several challenges. Herein, to overcome the challenges and strengthen the opportunities for large-scale implementation of the MD process, the set of researches were conducted through this dissertation. The dissertation consists of 5 chapters including the main text and conclusions as follows. In Chapter 1, as an introduction of this thesis, challenges and opportunities of the MD process were introduced, and the direction and goals of this dissertation were presented. The MD process is a thermal-driven desalination process using microporous and hydrophobic membrane through which only vapor can pass. Because non-volatile ions are not able to pass through the membrane, the MD process can exhibit 100 % of salt rejection, theoretically. Moreover, the MD process operates at relatively low temperature, low pressure, and is less sensitive to feed concentration. However, until now, the MD process exhibits lower water flux and energy efficiency than those of other desalination techniques due to some challenges. One of main challenges which constrains the large-scale implementation of the MD process is the lack of the optimal membrane for the MD process. Ideally, an MD membrane would have a low resistance to the mass transport of the water vapor through the membrane while having a high resistance to conductive heat transfer. However, until now, hydrophobic membrane for microfiltration, rather than specifically designed for MD are often used in the MD studies. As a result, the development and optimization of membranes for the MD process is required. In addition, one of main opportunities of the MD process is treating highly concentrated feed solution. The treatment of high concentrated solution is a challenging task that is attracting increased interest due to the high costs and environmental risks associated with disposal of high concentrated solution. As a result, the MD process is able to be utilized not only for desalination but also for valuable resources recovery (e.g., lithium, rubidium, and so on) and the protein and inorganic salts crystallization. In Chapter 2, the key parameters of the MD process were introduced and classified from the characteristics of membranes for the MD process to materials for the MD process. To make the membranes with low resistance for water vapor transport and high resistance for heat transport, several important properties were defined and summarized. Moreover, membrane materials determine the interaction between the membrane and water molecules, hydrophobic property of membrane is influenced by the membrane material itself. Various kinds of polymeric membranes have been intensively studied for the MD process due to their good processability and intrinsic hydrophobicity of the polymer itself. In this chapter, polymeric membranes for the MD process was summarized and classified. In Chapter 3, based on the optimal membrane characteristics and materials discussed in Chapter 2, enhanced, hydrophobic, fluorine-containing thermally rearranged nanofiber membranes (TR-NFMs) were developed. In our previous work, TR-NFM was firstly introduced as one of the promising candidates for MD application. However, nascent TR-NFM is susceptible to fluctuation of operating conditions by insufficient liquid entry pressure (with water, LEPw) of TR-NFM. Herein, we introduced the fluorine-containing TR-NFMs (F-TR-NFMs) for MD application. As fluorine atoms exhibit the outstanding hydrophobic behavior due to its low packing density inducing weaker van der Waals interactions with water, F-TR-NFMs showed enhanced hydrophobic properties such as high water contact angle (143 o) and LEPw (1.3 bar). Moreover, The developed F-TR-NFM exhibited outstanding MD performance (114.8 kg m-2 hr-1 of water flux and > 99.99 % of salt rejection at 80 oC and 20 oC of feed and permeate temperatures, respectively) and excellent energy efficiency (52.1 % at 50 oC and 20 oC of feed and permeate temperatures, respectively). In Chapter 4, the MD process for treating high concentrated solution was covered. In particular, the MD process was utilized for lithium recovery process and compare it to the conventional process, solar evaporation. Because the conventional solar evaporation takes long processing time, requires a large footprint, and is a climate-dependent process, the novel membrane-based lithium recovery process was based on the combination of MD and nanofiltration (NF) to concentrate a brine solution containing lithium and to remove undesirable divalent ions such as Ca2+ and Mg2+, respectively. The proposed membrane-based process was demonstrated to concentrate 100 ppm lithium-containing brine to 1,200 ppm within several days and exhibited up to 60 times higher water flux (22.5 L m-2 h-1) than that of solar evaporation (0.37 L m-2 h-1 at 30 oC and 0.56 L m-2 h-1 at 50 oC). Lastly in Chapter 5, the further interesting research points which remained to develop the MD process to commercial-scale implementation were introduced as a future study.
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