Investigation Of High Stable YSZ-LSCF Dual-Phase Hollow Fiber Membrane For Oxygen Separation And Membrane Reactor Applications

The ceramic oxygen-permeable membrane is a new technology being studied, which can reduce the oxygen production cost by more than30%over the present cryogenic distillation process. It has many potential applications, especially when used as membrane reactor coupled with downstream oxygen-consuming reactions. When involving membrane reactor applications, the membrane material need to possess not only high oxygen permeability but also good stability under the oxidizing atmosphere and strong reducing atmosphere. The existing material cannot simultaneously meet these requirements. In this thesis, we have explored a dual-phase composite which shows outstanding stability. When fabricated it into hollow fiber geometry, the oxygen permeation performance can largely enhanced. This is because that the hollow fiber has a thinner wall thickness and thus imposes less resistance to the permeation of oxygen. Besides, the hollow fiber has a small outer diameter, thus a large quantity of membranes can be packed in a module. Based on these advantages, the ceramic oxygen-permeable hollow fiber is promising in membrane reactor applications.Chapter1introduces the background and typical applications of ceramic oxygen-permeable membranes. The concepts and theories of oxygen permeation for dense ceramic membranes are reviewed, and the research progress of the hollow fiber membrane is introduced as well.In Chapter2, dual phase composite of Zr0.84Y0.16O1.92(YSZ) and La0.8Sr0.2Cr0.5Fe0.5O3(LSCF) are explored for oxygen separation application. The hollow fiber precursor is prepared by phase inversion/sintering technology. After sintering in air at1430℃for10hours, the hollow fiber membrane turns to be gas-tight and shows an asymmetric structure with out diameter of1.82mm, wall thickness of0.34mm. The oxygen permeability of the YSZ-LSCF hollow fiber is measured with its shell side to the ambient air and feed the core side with pure CO. The oxygen permeation flux can reach as high as3.8ml cm-2min-1with a4.46cm-long hollow fiber when CO feed rate was30ml/min at950℃, and remain almost unchanged in more than500h continuous measurements. Almost same value of oxygen pemieation rate was obtained after cooling down to room temperature and then reheating up to950℃, indicating good heat-shock-resistance of the fiber. SEM observation reveals that the500h oxygen permeation test under the stringent conditions did not cause significant change to the microstructure of the membrane. CO oxidation reaction in the membrane mode had a fast kinetics and can be simulated using the plug flow model. The calculated outlet CO and CO2concentration for various CO feed rate are in good agreement with the measured ones, verifying the validity of the simulation method. The simulation makes it possible to determine the axial profiles of CO and CO2concentration and oxygen partial pressure (at the lumen side) and corresponding oxygen permeation rate which are hardly possible to measure experimentally.In Chapter3, a partial oxidation of methane (POM) membrane reactor is built using the YSZ-LSCF hollow fiber membrane coated with Ru/LSCF as catalyst. The influence of the weight percentage of Ru in the catalyst to the catalytic activity is studied. When the weight percentage of Ru in the catalyst increased to33%, the membrane reactor shows outstanding POM performances. With a5.77cm-long hollow fiber coated with5μm Ru(33%wt)/LSCF catalyst (-7.6mg Ru/cm2inner surface area), an oxygen permeation flux as high as17.1ml/min (7.9ml/min/cm2) is observed when CH4feed rate reach over16ml/min at950℃. At a higher CH4feed rate of31.6ml/min, the CH4feed rate/oxygen permeation flux ratio becomes near the stoichiometric ratio of the POM reaction. CH4conversion over90%, CO selectivity near100%and H2selectivity over93%can be observed. The formation rate of the syngas (CO+H2) is over83.8ml/min and the H2/CO ratio in the syngas is near1.85, which is suitable for the Fischer-Tropsch synthesis. The membrane reactor gives a stable performance in more than600h continuous measurements. The O/C ratio in the syngas is high than1, which located in the theoretical no carbon deposition area of the Ni-based anode of solid oxide fuel cell (SOFC). So the effluent syngas is a suitable fuel for SOFC. A disk SOFC can stably run on the fuel produced from methane by the membrane reactor even at low current density of200mA/cm2. The output voltage remains almost unchanged in30h measurement. After800h experiment under POM condition, the YSZ-LSCF hollow fiber remains intact but loss of Ru/LSCF catalyst is observed. The adherence of the catalyst to the membrane need to be improved.In Chapter4, Ni/LSCF is used as POM catalyst instead of Ru/LSCF since Ru is a noble metal. The influence of the composition of the catalyst slurry to the adherence of the catalyst layer is studied. When Na2SiO4·9H2O is added into the catalyst slurry and H2O is used as solvent, the adherence of the Ni/LSCF catalyst to the YSZ-LSCF hollow fiber membrane can be improved, which is due to the Si sol formed after Na2SiO4·9H2O dissolved in water. The Si sol acts as a binder. The adherence of the Ni/LSCF catalyst layer can significantly affect the POM performance of the YSZ-LSCF catalytic hollow fiber membrane. In long-term stability experiments, the CH4conversion decrease rate is4.29%/100h for catalyst with bad adherence (without Na2SiO4·9H2O) and9.18%/1000h for catalyst with good adherence (with Na2SiO·9H2O), respectively. When catalyst with good adherence is used, a8.30cm-long hollow fiber membrane reactor shows good POM performance, CO selectivity over95%, H2selectivity over92%, H2/CO ratio near1.75. The CO and H2formation rate are32.7ml/min and57.2ml/min respectively and oxygen permeation rate is7.1ml/min/cm2, showing Ni/LSCF is also a promising POM catalyst.Chapter5presents a study on CH4combustion membrane reactor which composed of YSZ-LSCF hollow fiber and PdO/LSCF catalyst. At CH4feed rate of5.65ml/min and temperature of950℃, CH4can combust into CO2and H2O in the membrane reactor with CH4conversion over99.5%and CO2selectivity over90%. The oxygen permeation flux is4.57ml/min/cm2. All these data is stable in near200h measurement. The phase composition and microstructure of the used membrane remains almost unchanged. This membrane reactor holds applications in CH4catalytic combustion at low temperature for heat and CO2capture.In Chapter6, a new type of hollow fiber membrane integration module is put forward and its perfonnance in oxygen separation is investigated. The dual phase hollow fiber, consisting of Zr0.84Y0.16O1.92(YSZ) and La0.8Sr0.2MnO3-δ(LSM), is fonned by the phase inversion method. Then five neck-by-neck fibers are co-sintered together to form a plate module at1350℃. The perfomiances of the module for oxygen separation have been studied at different temperatures, feed gases and flow rates. An oxygen permeation flux of1.52ml/min is obtained under air/He(100ml/min) gradient at950℃(Plate length:280mm Hollow fiber O.D.:1.76mm Wall thickness:0.24mm). When sweep gas switched from He to CO2, the oxygen permeation flux almost remain unchanged. The hollow fiber membrane plate shows better mechanical performance than one single fiber. Based on this kind of integration way, further experiments have been done on co-sintering several plates to a block.Chapter7summarizes the research conducted in this thesis, and presents recommendations for further research.