Preparation, Characterization And Application Of Co₂-tolerant Mixed Ionic-Electronic Conducting Hollow Fiber Membrane

Dense ceramic oxygen permeable membranes （OPM）, which shows a mixed conducting ofelectronic and oxygen ionic, has gained much attention in the oxygen production from air orother oxygen containing atmosphere, pure oxygen production and as membrane reactor for thechemical upgrading cleanly, efficiently and economically. For practical applications, a denseceramic OPM has to show considerable high oxygen permeability under operation conditionsand high stability under harsh atmospheres, especially in CO₂-containing atmosphere orreducing atmosphere. However, the membrane materials of perovskite always containalkaline-earth metals on A site, which tend to react readily with CO₂and can’t sustain theiroxygen transporting property. In this paper,（Pr0.9La0.1）2（Ni0.74Cu0.21Ga0.05）O4+δ（PLNCG）, analkaline-earth metal-free mixed conductor of electronic and oxygen ionic with K₂NiF₄structure, is chosen to study. Considering several advantages of hollow fiber membrane, suchas the large membrane area per unit volume for oxygen permeation, high oxygen permeationflux resulted from thin walls and easy assembly for large-scale module fabrications, PLNCGhollow fiber ceramic membrane was prepared by the phase inversion technique and aimed tosupply oxygen for methane combustion and partial oxidation in such an OPM reactor.Firstly, we developed U-shaped hollow fiber membrane based on PLNCG for the first time,which can solve the sealing problem and avoid the breakage of the membrane due to theexpansion or shrinkage at varying temperatures. The oxygen permeation flux through U-shaped PLNCG hollow fiber membrane under air/He gradient was investigated. A steadyoxygen permeation flux of1.0mL/（min.cm2） was obtained at975oC during about320h. Theresults indicate that the U-shaped PLNCG hollow fiber membrane exhibits good oxygenpermeability and stability.Secondly, the chemical stability and phase structure stability of PLNCG powder in CO₂were analyzed through in situ-XRD, which showed good stability. The chemical stability andoxygen permeation flux through the PLNCG hollow fiber membrane under CO₂atmosphereare also studied. Compared with the oxygen permeation flux under air/He gradient, the mostserious decrease of oxygen permeation flux is only6%when pure CO₂is used as the sweepgas on the core side. When the pure CO₂is used as the sweep gas and even10%CO₂wasadded in the feed air simultaneously, a steady oxygen permeation flux of0.9mL/（min.cm2） isobtained at975oC and no decline of the oxygen permeation flux is observed during more than310h. XRD, SEM and EDS characterizations indicate that the spent membrane still maintains the perfect K₂NiF₄-type phase structure and intact microstructure without carbonate. All ofthese results demonstrate that the PLNCG hollow fiber membrane is a promising stablemembrane under CO₂-containing atmosphere and has a great potential for the practicalapplication.Thirdly, the CO₂capture for power station based on the oxy-fuel combustion using such anovel CO₂-stable OPM based on PLNCG in hollow fiber geometry was explored for the firsttime. During the450h operation,100%CH4conversion and100%CO₂selectivity areobtained steadily, which shows an excellent reaction performance. XRD, SEM and EDScharacterizations indicate that the spent membrane still maintains the perfect K₂NiF₄-typephase structure and intact microstructure without carbonate. To the best of our knowledge,this is the first report on the oxy-fuel combustion of methane in a ceramic hollow fibermembrane reactor using the recycled CO₂diluted methane as the feed gas instead of inert gasfor dilution.Finally, the U-shaped PLNCG hollow fiber membrane was applied in POM successfully.The oxygen permeation flux arrives approximately10.5mL/（min.cm2） and the CO selectivityis higher than99.5%while the CH4conversion is around97.0%during the140h steadyoperation, which exhibits a long lifetime among various hollow fiber membrane reactors inPOM. From XRD, SEM and EDS characterizations, the spent membrane still maintains thedense microstructure and K₂NiF₄-type phase structure, which indicates that the U-shapedCO₂-stable PLNCG hollow fiber membrane reactor can be steadily operated for POM withgood reaction performance.