Preparation Science And Characterization Of Hollow Fiber Ceramic Membranes

Ceramic membranes are known to be superior to polymeric counterparts due to some special advantages, such as better thermal, chemical and mechanical resistances, narrow pore size distribution, controllable microstructure, long service life and little pollution to the environment, and could be used in very harsh environments. Actually, increasing attention has been paid to ceramic membranes in the past twenty years, and ceramic membranes are nowadays widely used in various fields, including petrochemical industry, chemical industry, metallurgical industry, food industry, environmental engineering, new energy resources and etc. Therefore, ceramic membranes and related separation technologies could play an important role in promoting energy saving, emission reduction and green production, and are very propitious to sustainable development of social economy. However, there are still many obstacles for the further development of traditional planar and tubular ceramic membranes, such as following:(1) low packing density, small active area/volume ratio and thus low separation efficiency; (2) long manufacturing time and complex process, leading to high cost; (3) single product and function. The existing porous ceramic membranes for separation are mainly made of Al2O3 material, and could not meet the requirements for some special applications. Ceramic hollow fiber membranes have recently attracted considerable attention, due to the high active area/volume ratio provided by its high packing density and less material consumption. With the application of ceramic membranes in hollow fiber configuration, the separation equipments can be miniaturized and simplified. Ceramic hollow fiber membranes are commonly fabricated by the phase inversion method in one step, and exhibit self-supported asymmetric structure and high permeability. The application of phase inversion method simplifies the fabrication process of ceramic membranes and could greatly reduce the production cost. Therefore, the development of hollow fiber ceramic membranes has the potential to eliminate the bottleneck problems, which has hindered the development of ceramic membrane technology for a long time.In order to advance the industrial applications of ceramic hollow fiber membranes, the research on the preparative technology of yttria-stabilized zirconia (YSZ) hollow fiber membranes by phase inversion was conducted in our work, and characterization technologies for the prepared membranes have also been developed (Chapter two). The cost-effective cordierite hollow fiber membrane with graded porous structure was fabricated successfully by phase inversion method and using coarse industrial cordierite powder as raw material (Chapter three). The low-cost mullite hollow fiber membrane with high porosity and high permeability was also developed by the combination of phase inversion method and in-situ reaction sintering technique (Chapter four) so as to obtain preferred microstructures for special applications. In the end, we have developed the micro-tubular ceramic membrane fuel cell (CMFC) with redox stable (La0.75Sr0.25)Cr0.5Mn0.5O3 (LSCM) and chemical stable (Pr0.5Nd0.5)0.7Sr0.3MnO3-δ(PNSM) as cathode, respectively, and the phase inversion technique was applied to fabricate the NiO-YSZ hollow fiber anode (Chapter five). The main achievements and innovations in this dissertation are summarized as follows:1. Research on the preparative technology of asymmetric YSZ hollow fiber membranesYSZ is one of the widely used materials for ceramic membranes because of its high mechanical strength, favorable chemical stability and competitive price, and can be used in liquid filtration with much better alkali durability than other ceramic membranes, e.g. Al2O3 membrane. However, pure YSZ composite membrane is rare at the market. This is mainly because of very high sintering temperature (≥1600℃) of YSZ membrane supports made of YSZ powders with an average particle size more than 10μm, and consequent high fabrication cost.Asymmetric YSZ hollow fiber membranes were prepared by the phase inversion method in one step in this work, based on the dry/wet spinning process. The influences of process parameters, including solid content of suspension, composition of internal and external coagulants, on the microstructure and properties of YSZ hollow fiber membranes were investigated systematically in order to effectively control the microstructure and properties for various applications. Results show that all these process parameters have significant influence on the kinetics conditions of phase inversion and thus the microstructure of the YSZ hollow fiber membranes.The increase of YSZ content in suspensions would increment the viscosity, and inhibits the phase inversion behavior. The prepared YSZ hollow fiber membrane shows a typical sandwich structure, i.e. sponge-like structure layer in the middle, and finger-like structure layer at the inner and outer regions, when the suspension has a solid content of 50 wt% and using water as the internal and external coagulants. As the solid content is increased up to 60%-65%, the membranes are composed of thick sponge-like structure at the outer side and finger-like structure at the inner side. Also, the increase of solid content obviously enhances the density of the sponge-like structure layer, and consequently results in higher bending strength but lower pure water permeability.The coagulation power of coagulant can be characterized by the difference of solubility parameters between coagulant and polymer. Larger difference means stronger coagulation power of coagulant. Strong coagulant would lead to rapid precipitation to form finger-like structure and dense skin layer. The addition of solvent NMP into the internal coagulant decreases its coagulation power and thus the precipitation rate in the inner region. This would lead to porous inner surface and facilitate the finger-like pores originated from the outer side to extend to the inner side. Moreover, higher content of NMP in internal coagulant contributes to higher porosity of the membrane and larger pore size of the outer skin layer, which accordingly increases the pure water permeability of the YSZ hollow fiber membrane. When the internal coagulant contains more than 90% NMP, a highly asymmetric structure can be obtained with large finger-like pores extending to the inner surface and the formation of highly porous inner surface. This special microstructure is beneficial for reducing the fluid resistance and increasing the permeability of porous hollow fiber ceramic membranes. The YSZ hollow fiber membrane shows an average pore size of 0.58μm when prepared with pure NMP as internal coagulant and sintered at 1320℃for 5h. The corresponding pure water flux could reach up to 16.34 m3/(m2·h·bar), which is about 2.91 times higher than that prepared using water as internal coagulant. The increase in porosity and the formation of large finger-like pores at higher NMP content would reduce the bending strength of the membranes.Highly permeable porous yttria-stabilized zirconia (YSZ) hollow fiber membranes can be successfully prepared as ethanol instead of water was used as the external coagulant. The prepared YSZ hollow fiber membranes show a special asymmetric structure with an outer-skinned separation layer, highly porous inner surface and sub-layer composed of long and large finger-like pores, and the inner surface shows higher porosity and larger mean pore size. The microstructure has significant effect on the fluid resistance and permeability of the prepared membranes. The YSZ hollow fiber membrane prepared with ethanol as the external coagulant shows much lower fluid resistance, compared with the one using water as the external coagulant. Results show that YSZ hollow fiber membranes with high permeability and high bending strength can be obtained using ethanol as the external coagulant and by controlling the sintering temperature. The outer-skinned YSZ hollow fiber membranes show a pure water flux of 2.27 to 4.30 m3·m-2·h-1·bar-1 and a bending strength of 154.5 to 201.7 MPa when sintered between 1350 and 1400℃, both of which are much higher than the tubular membrane.The present work shows that porous YSZ hollow fiber membranes with highly asymmetric microstructure and outer active layer could be obtained with 90-100vol% NMP as internal coagulant or ethanol as external coagulant. The prepared membranes are suitable to be used for microfiltration and as supports for composite membranes.2. Elaboration of asymmetric porous cordierite hollow fiber membrane for microfiltrationCordierite is one of the most popular ceramic materials for its interesting properties such as good thermal and chemical durability, low thermal expansion coefficient. This makes it particularly suitable to be applied at high temperature where a good thermal shock resistance is required.In our previous work, tubular cordierite membranes have been developed. The cordierite membranes were confirmed to be cost-effective since the material cost and the sintering temperature of cordierite ceramics are much lower than those of A12O3 and YSZ. However, to the best of our knowledge, little work has been done to develop cordierite hollow fiber membrane for microfiltration.Asymmetrical porous cordierite hollow fiber membranes have been successfully prepared by a combined phase inversion and sintering method, using industrial grade cordierite powders as raw material. The particle size of cordierite powders has significant influence on the microstructure of the hollow fiber membranes. Cordierite hollow fiber membranes with the inner macro-void structure and the outer thin sponge-like structure can be obtained when cordierite powder with larger particle size (d50=7.8μm) was used. This special structure is ideal for the actual application of cordierite hollow fiber membranes. But the hollow fiber derived from cordierite powder with smaller particle size (d50=1.6μm) shows large finger-like structure. The sintering temperature has significant effect on the microstructure, porosity and pore size distribution, gas permeability, bending strength and thermal expansion of the hollow fiber membranes prepared with cordierite powder of d50=7.8μm. The porous cordierite hollow fiber membrane shows a nitrogen flux of 745 m3/(m2-h-bar), bending strength of 76.5 MPa, and LTEC of 2.39×10-6℃-1 sintered at 1360℃for 2h, and the probable pore size of its outer separation layer is about 0.38μm. This work demonstrates that the graded porous ceramic hollow fiber membrane for micro-filtration can be prepared using industrial grade cordierite powder with large particle size. The prepared cordierite hollow fiber membrane will be particularly ideal for the treatment of hot waste gas at high temperatures and the treatment of waste water, etc.3. Research on the formation of highly permeable mullite hollow fiber membraneMullite is an important material for thermal shock resistant porous ceramic at elevated temperatures, because of its favorable properties, such as high creep resistance, high strength and fracture toughness at high temperature, low thermal expansion and excellent corrosion resistance. In particular, the formation of acicular mullite grains could increase the porosity, mechanical strength and thermal shock resistance of porous mullite ceramic. Therefore, in this work, highly permeable asymmetric mullite hollow fiber membranes with different microstructure were prepared by the combination of phase inversion and in-situ reaction sintering technique. The natural mineral kaolin and Al(OH)3 were used as the main raw materials, and AIF3 and V2O5 as additives. The mechanism and reaction process for the formation of the special acicular mullite structure were investigated in detail. The hollow fiber membranes sintered at 1400℃for 2.5h in closed crucible shows a nearly pure mullite phase, and consists of two different layers:the outer thin layer composed of prismatic mullite grains, and the inner thick and highly porous layer composed of inter-connected acicular mullite grains with aspect ratio more than 25. For the membrane sintered at the same temperature without closed crucible, the main crystalline phase is mullite accompanied by a small quantity of corundum. The synthesized mullite is in irregular form and no acicular crystal was observed. In this case, the prepared mullite hollow fiber membrane shows a three-layered graded porous structure with decreasing pore size towards the outer side, which is resulted from different precipitation rate. EDS analysis show that large acicular mullite grains exhibit a non-uniform composition across the grain with Si-rich core(Al/Si=2.38) and Al-rich rims (Al/Si=3.47), which corresponds to 66-74 wt% Al2O3.The formation of inter-connected acicular mullite with high aspect ratio could significantly increase the porosity and gas permeability of the hollow fiber membrane. The membranes sintered at 1400℃for 2.5h with and without closed crucible exhibit a high porosity of 68.4% and 53.6%, and high nitrogen flux 1.82×104 m3·m-2·h-1 and 1.75×103 m3·m-2·h-1, respectively, which are much higher than those for commonly- used tubular ceramic membranes. Results show that the acicular mullite hollow fiber membrane sintered in closed crucible is ideal to be applied in the treatment of dust-containing waste gas at elevated temperature and used as membrane contactor for chemical reactions, etc., and that the mullite membrane sintered without crucible can be used for the treatment of waste water and gas in large scale and as support for composite hollow fiber membranes. The preparation method developed in this work could reduce the manufacturing cost of ceramic membrane drastically, and facilitate the formation of mullite hollow fiber membranes with different microstructure to meet requirements for various applications.4. Research on the hollow fiber CMFCHollow fiber (micro-tubular) CMFC possesses many desirable advantages over conventional planar and tubular systems, including high mechanical strength, high electrode area per unit volume and high volume current density, a superior tolerance for thermal stress, more facile sealing, high feasibility for rapid start-up and shut-down operations, as well as rapid response to load variation, and becomes a new development tendency of SOFC. The micro-tubular anodes were often prepared by a traditional plastic extrusion process. The anode derived from this method usually has symmetric structure with large wall thickness which results in a large resistance in the anode side during cell operations.In this research, the phase inversion method was applied to prepare the NiO/YSZ hollow fiber anode, and thin YSZ electrolyte membrane (10μm) was then deposited on the pre-sintered anode by a vacuum-assisted dip-coating technique. Based on this, we developed the micro-tubular CMFCs based on the redox stabe LSCM cathode and chemical stable PNSM cathode, respectively. The former shows peak power densities of 513,408 and 278 mW/cm2 at 850,800 and 750℃, respectively, while the latter exhibits peak power densities of 459,325 and 172 mW/cm2 at 800,700 and 600℃, respectively. In view of the small diameter of the prepared single cells (≤1.30 mm), the resultant hollow fiber CMFC stacks would have high electrode/volume ratio and thus high power output, and could meet the practical requirements of IT-CMFC based on YSZ electrolyte. The anode-supported hollow fiber CMFC is an excellent candidate for smaller scale applications such as auxiliary power units for automobile and power sources for portable wireless devices, etc.