【作者】 李伟；

【导师】 陈初升；

【作者基本信息】 中国科学技术大学 ， 材料学， 2009， 博士

【摘要】 基于氧离子电子混合传导的陶瓷氧分离膜有望将现有的氧气生产成本降低30%以上。氧分离膜技术实用化的主要障碍是缺乏氧渗透性能和稳定性均能满足要求的膜材料。本论文提出了突破这一障碍的新思路,即把稳定性好但氧渗透速率偏低的双相复合材料制成中空纤维膜,利用纤维膜单位面积氧渗透速率高、单位体积可填充的膜数量大的优点,从而大幅度提高膜组件和膜装置的制氧能力。第一章简要介绍了陶瓷氧分离膜的原理、背景和应用,重点综述了陶瓷中空纤维氧分离膜的研究进展、现状和主要问题。第二章研究了由氧离子导体Zr_0.84Y_0.16O_1.92（YSZ）和电子导体La_0.8Sr_0.2MnO_3-δ（LSM）构成的双相复合氧分离膜。采用相转化/烧结法将复合材料制成气密的中空纤维。所制得的纤维膜的外径为1.64 mm,壁厚为0.16 mm。中空纤维膜的热膨胀系数为11.1×10^-6 K^-1,三点支撑法测定的断裂强度为152±12 MPa。测量纤维膜的氧渗透速率时采用长度为57.0 mm的样品,其外壁与空气接触,用氦气或者CO₂作为吹扫气将渗透的氧携带出,用气相色谱分析。在950℃和He气吹扫速率30ml/min的条件下,中空纤维膜的氧渗透速率为2.1×10^-7mol·cm^-2·s^-1。采用二氧化碳替代氦气作为吹扫气,氧渗透速率没有下降。基于YSZ-LSM复合膜优异的耐CO₂侵蚀性能,我们采用该分离膜实验验证了富氧燃烧—CO₂捕获所需的O₂/CO₂混合气的制备新工艺,即在膜管的外侧施加高的氧分压（压缩空气）,通过调节管内CO₂吹扫气的速率,可以获得氧分压为0.2-0.3大气压的O₂/CO₂混合气。若采用该混合气作为含碳燃料的助燃剂,燃烧产物含高浓度的CO₂,可以方便地实现CO₂的捕获。与常见的单相钙钛矿型氧分离膜材料相比,YSZ-LSM复合氧分离膜的另一个重要特点是不含贵重和有毒的元素。综合考虑材料的氧渗透率和稳定性以及中空纤维膜的高填充密度,YSZ-LSM中空纤维膜的实用化前景良好。第三章研究了Ce_0.8Sm_0.2O_2-δ（SDC）和LSM双相复合氧分离膜,其中SDC作为氧离子导电相,其氧离子电导率在中温明显高于YSZ。采用相转化/烧结法将该复合材料制成气密的中空纤维膜。在air/He和air/CO₂梯度下,中空纤维膜在950℃时的氧渗透速率分别为3.2×10^-7mol·cm^-2·s^-1和3.0×10^-7mol·cm^-2·s^-1。经过700多个小时的测试,膜管的氧渗透速率只略有下降。SDC-LSM膜材料在二氧化碳中稳定存在,且具有较高氧渗透速率,可望用于制备富氧燃烧—CO₂捕获所需的O₂/CO₂混合气。采用活塞式流动模型和Wagner氧渗透理论模拟了双相复合中空纤维膜的氧渗透行为。该模拟方法可以用于膜管（组件）的氧气产能计算等。第四章研究了YSZ-La_0.8Sr_0.2Cr_0.5Mn_0.5O_3-δ（LSCM）双相复合氧分离膜。LSCM是一种新报道的固体氧化物燃料电池阳极材料,在还原性条件下能保持稳定。采用相转化/烧结法将该双相复合膜制成气密的中空纤维膜。所制得的纤维膜形貌均匀,膜体内部不含有手指状的大孔,只含有少量闭气孔。纤维膜具有优异的机械性能,其断裂强度高达279±5 MPa。在950℃和He吹扫速率30ml/min的条件下,中空纤维膜的氧渗透速率为3.3×10^-8mol·cm^-2·s^-1。改用同样流速的还原性气体CO作为吹扫气,经过250小时左右时间氧渗透速率达到稳定,为3.9×10^-7mol·cm^-2·s^-1。在air/CO梯度下经过600小时实验后,膜管仍然保持完好,没有出现裂纹。鉴于膜材料在大氧分压梯度下优异的稳定性,YSZ-LSCM中空纤维膜有希望用于膜反应器。第五章研究了SDC-LSCM双相复合氧分离膜。与前述几章不同,本研究没有采用SDC和LSCM粉体作为起始原料,而是采用金属氧化物和碳酸盐作为前驱物来制备浆料,挤出成型。这种改进的相转化法制备中空纤维膜的方法去掉了预先合成陶瓷粉这一步骤,将本来五步的工艺过程缩减为四步,并将成相和烧结在一步完成,缩短了制备时间,也减少了能耗,有利于降低制备成本。采用TGA/DTA研究了纤维膜坯体的热解行为,采用热膨胀仪研究了膜管的高温烧结过程。最终选定的热处理条件是:在N₂（+H₂4%）的气氛中,以2℃/min的速率升温将纤维膜坯体升至800℃,保温240min,除去有机物,升温至1350℃,保温300min,得到气密的陶瓷中空纤维膜。SEM和XRD分析表明烧结后的膜管由SDC和LSCM两相构成,不含其它杂相。在950℃和He吹扫速率30ml/min的条件下,中空纤维膜的氧渗透速率为1.4×10^-7mol·cm^-2·s^-1。改用同样流速的还原性气体CO吹扫时,氧渗透速率大幅增加至3.3×10^-6mol·cm^-2·s^-1。显然,从空气侧渗入的氧与CO发生反应,使得膜管内部的氧分压大幅度降低,增大了氧渗透的驱动力。XRD分析表明:氧渗透实验后膜管的相组成没有发生变化,但SEM分析发现LSCM相的品粒穿孔,几乎破碎成粉状,这可能表明其在还原性气氛中的稳定性不够高。第六章研究了SrCo_0.8Fe_0.2O_3-δ—SrZrO₃（10 mol%）(（SCF-SZ）复合膜。该复合膜为非对称型结构,其基底层为多孔的中空纤维,顶层为同质的致密氧分离膜。基底层采用相转化法制备,顶层膜则采用浸渍/烧结法制备。所制得的复合膜的外径为1.70 mm,壁厚为0.25 mm。在950℃和He气吹扫流速30 ml/min的条件下测得的氧渗透速率为1.0×10^-6mol·cm^-2·s^-1。我们还研究了以二氧化碳为吹扫气时SCF-SZ复合膜的透氧行为。当吹扫气中二氧化碳浓度低于40%时,SCF-SZ复合膜具有较高的透氧速率。采用活塞式流动模型和Wagner氧渗透理论模拟了中空纤维氧分离膜的氧渗透过程,得出的结果与所测实验数据符合较好。第七章总结了前述的研究工作,并展望了陶瓷中空纤维氧分离膜的实用化前景和面临的挑战。更多还原

【Abstract】 The ceramic membrane holds promise to reduce the oxygen production cost by 30%over the present cryogenic distillation process.The main barrier hindering the development of this novel technology is the lack of membrane materials exhibiting both high oxygen permeability and stability.The composite involving an oxygen ionic conducting phase and an electronic conducting phase exhibits improved stability over the single-phase mixed conducting material,but the oxygen permeability of the former is lower that of the latter.The disadvantage of the dual-phase composite can be largely compensated by fabrication the membrane into the hollow fiber geometry.This is because that the hollow fiber has a small outer diameter,thus a large quantity of membranes can be packed in a module and that it usually has a small wall thickness and thus imposes less resistance to the permeation of oxygen.Based on these considerations,this dissertation is focused on the preparation and characterization of dual-phase composite hollow membranes.Chapter 1 presents an overview of the principle of oxygen permeation through the membrane and state-of-the-art membrane materials as well as the preparation of the membrane especially in hollow fiber geometry.The research needs in the oxygen separation membrane are identified,and the scope of the dissertation is described.In Chapter 2,dual phase composite of Zr_0.84Y_0.16O_1.92（YSZ） and La_0.8Sr_0.2MnO_3-δ（LSM） are explored for oxygen separation application,in which oxygen ions and electrons transport through YSZ and LSM phase respectively.The hollow fiber precursor was prepared by the phase-inversion process,and transformed to a gas-tight ceramic by sintering at 1350℃.The as-prepared fiber exhibited a thermal expansion coefficient of 11.1×10^-6 K^-1 and a three-point bending strength of 152±12 MPa.The oxygen permeability of the hollow fiber was measured by exposing its shell side to the ambient air and sweeping the tube side with high purity helium or CO₂ to carry away the permeated oxygen.An oxygen permeation flux of 2.1×10^-7 mol·cm^-2·s^-1 was obtained under air/He gradient at 950℃for a hollow fiber of length 57.00 mm and wall thickness 0.16 mm.The oxygen permeation flux remained almost unchanged when CO₂ was used as the sweep gas.The as-produced O₂/CO₂ mixture can be used as oxidant for combustion of fossil fuel;this oxyfuel combustion process produces a concentrated CO₂ stream and thus enables efficient CO₂ capture.The other important feature of the YSZ-LSM membrane is that unlike the single-phase perovskite-structured oxide membrane,the composite membrane does contain any toxic and expensive elements,which is also vital for practical application.Considering the satisfactory trade-off between the permeability and stability and the packing density of the hollow fiber,the YSZ-LSM hollow fiber is promising for oxygen production applications.In Chapter 3,dual-phase composite of Ce_0.8Sm_0.2O_2-δ（SDC） and LSM are investigated.In this composite,oxygen ions are transported through the SDC phase.The reason for choosing SDC is due to its higher oxygen ionic conductivity than that of YSZ.The SDC-LSM hollow fiber was prepared using the phase-inversion/sintering technique.A stable oxygen permeation rate of 3.2×10^-7 mol·cm^-2·s^-1 was measured under air/He gradient at 950℃,and 3.0×10^-7 mol·cm^-2·s^-1 under air/CO₂ gradient.The oxygen permeation rate was slightly lower than the value measured at the early stage of the measurement after 700 h.The oxygen permeability of the fiber did not degrade significantly under the given operation condition,thus the membrane is promising for production of O₂/CO₂ required for combustion of fossil fuels with integrated CO₂ capture.It was also found that oxygen permeation through the hollow fiber can be well described by the Wagner equation and assuming that the gas flow in the core of the fiber conforms to the plug flow model. The oxygen production capacity for a membrane unit can be accessed through modeling.Chapter 4 presents a study on dual-phase composite of YSZ and La_0.8Sr_0.2Cr_0.5Mn_0.5O_3-δ （LSCM）.LSCM,as a potential anodic material for solid oxide fuel cells,has been reported to be stable under reducing conditions,thus the composite membrane of YSZ-LSCM is expected to be stable under a large oxygen gradient,i.e.,with one side of the membrane exposed to air and the other side to reducing atmosphere.The composite was fabricated into hollow fibers using phase-inversion/sintering process.The as-prepared fiber shows a three-point bending strength of 279±5 MPa.A stable oxygen permeation rate of 3.3×10^-8 mol·cm^-2·s^-1 was observed under air/He gradient at 950℃,and 3.9×10^-7 mol·cm^-2·s^-1 under air/CO gradient.The membrane was found to remain stable under stringent condition for over 600 h,showing that it is promising for chemical reactor application.Chapter 5 describes a study on SDC-LSCM composite membrane.The composite was fabricated into hollow fibers by an improved phase inversion/sintering process.Instead of using SDC and LSCM as starting materials,individual metal oxides and carbonates were used,thus reducing the number of preparation steps and costs.The thermal decomposition behaviour of the hollow fiber precursor was analyzed using TGA/DTA,and its densification process was investigated using dilatometer.The hollow fiber precursor was converted to a gas-tight ceramic by sintering at 1350℃in the atmosphere of N₂ containing 4%H₂.An appreciable oxygen permeation flux of 1.4×10^-7 mol·cm^-2·s^-1 was observed for the fiber under air/He gradient at 950℃,and a much larger flux(3.3×10^-6 mol·cm^-2·s^-1) under a large oxygen gradient（air/CO）. Examination on the membrane after oxygen permeation measurement shows that the LSCM phase of the composite has been degraded by CO,indicating that it may not possess sufficient stability under highly reducing atmosphere（CO）.In Chapter 6,a hollow fiber membrane of SrCo_0.8Fe_0.2O_3-δ in composite with SrZrO₃（10 mol%） is investigated.The hollow fiber was prepared using the phase-inversion/sintering method. The as-prepared hollow fiber had a dimension of 0.25 mm in thickness,1.70 mm in outer diameter. An oxygen flux as large as 1.0×10^-6 mol·cm^-2·s^-1 was obtained under the air/helium gradient at 950℃.The permeation flux increased with temperature as expected,and the apparent activation energy was calculated to be 35.3±1.6 kJ/mol in the temperature range of 850-950℃.A plug-flow model in combination with the Wagner theory was used to simulate the oxygen permeation process.The simulation result is in a fair agreement with the measured permeation data.In the last chapter,the summary of this dissertation is presented,and future research need indentified.更多还原