## 3 Results and Discussion
Figure 4e,f also shows the hydrophobicity with contact angles both at about 137\({}^{\circ}\). Furthermore, it has been reported that the FAS modification only reduces the pore size of the fiber slightly for several nanometers. After characterization, the as-prepared hydrophobic fiber modules were used for MD. VMD was used because of its high permeate flux. The 3.5 wt% NaCl aqueous solution was circulated at the shell side and the permeate water was collected at the tube side at 0.03 bar every 30 min. As shown in Figure 5, the permeate flux of the fiber sintered at 1400 \({}^{\circ}\)C is twice as high compared to that sintered at 1500 \({}^{\circ}\)C, and this result is owing to the higher average pore size and porosity of the former. The salt rejections, which were detected by measuring conductivity, are both greater than 99.9%. In addition, the feeding temperature dependence of the permeate flux is investigated with the as-prepared fiber sintered at 1400 \({}^{\circ}\)C. As shown in Figure 6, the permeate flux increases with the temperature of feed solution in the range of 50 \({}^{\circ}\)C to 70 \({}^{\circ}\)C, that is expected from the relation between temperature and  
Figure 3: Pore size distribution, as measured by a mercury porosimeter. vapor pressure which is the driving force in VMD. The high permeate flux of 60 Lm\({}^{-2}\)h\({}^{-1}\) is obtained at a feed temperature of 70 \({}^{\circ}\)C and the salt rejections under three of the feed temperatures are all greater than 99.9% over a period of 4.5 h. These results demonstrate that the as-prepared hydrophobic fiber is eminently suitable for VMD. A list of the permeate flux values for ceramic membranes published recently is summarized in Table 2. The permeate flux depends principally on the feed solution and the configuration. A variety of configurations including direct contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), sweeping-gas membrane distillation (SGMD) and vacuum membrane distillation (VMD) have been used for MD. They can be distinguished by the driving method inclusive of a condensing fluid contacting with the membrane (DCMD), an air gap between the condensing surface and membrane (AGMD), a sweeping gas (SWMD) and a vacuum condition at the permeate side. The choice of configuration used for MD is dependent on the feeding composition, volatility, flux and operation. DCMD is easy to operate and needs the least equipment. VMD generally exhibits a high permeate flux due to the driving from vacuum. SGMD is usually used to remove a volatile organic from an aqueous solution. AGMD is widely used due to its characteristic of collecting separately at permeate side. All of them can be applied to seawater desalination. For the purpose of comparison, the permeate flux is plotted as a function of feed solution temperature in Figure 7. It is seen that the permeate flux value is much greater than those in the published data using a ceramic membrane with various configurations. The greater permeate flux shown in this study is attributed to the appropriate pore size (Figure 3), high porosity and the symmetrical finger-like structure (Figure 4e), which provides macro voids that enhance the mass transfer. On the other hand, it is believed that high contact angle, well permeation and the appropriate LEP\({}_{\rm w}\) value are indispensable to MD. Furthermore, checking the durability of the as-prepared hollow fiber over longer periods will be the subject of future work. Figure 4: Pure water permeation for as-prepared alumina hollow membrane: before hydrophobization/ sintered at (**a**) 1400 \({}^{\circ}\)C, (**b**) 1500 \({}^{\circ}\)C; after hydrophobization/sintered at (**c**) 1400 \({}^{\circ}\)C, (**d**) 1500 \({}^{\circ}\)C; contact angle after hydrophobization/sintered at (**e**) 1400 \({}^{\circ}\)C, (**f**) 1500 \({}^{\circ}\)C.  
Figure 5: The membrane distillation (MD) permeate flux of as-prepared hydrophobic hollow fiber sintered at different temperatures in vacuum membrane distillation (VMD).