Preparation And Performance Study Of Symmetrical Porous Self-supporting Dual-phase Mixed Conductor Oxygen Permeable Membran

The mixed conductor oxygen permeable membrane contains both oxygen ion and electron conductivity at high temperature,and the theoretical selectivity for O₂ is 100%.This kind of membrane material can separate the O₂ from the air efficiently,greatly reducing the operating cost and energy consumption,which has been widely used in the field of gas separation research.But there are limitations:the oxygen permeability and stability of the membrane are difficult to balance.Therefore,this paper prepared a60wt%Ce_0.9Gd_0.1O_2-δ-40wt%Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ（60CGO-40BSCF）dual-phase mixed conductor oxygen permeable membrane with good oxygen permeable performance and stability,in which CGO was used as the ion conductive phase,and BSCF was used as the electron conduction phase,oxygen ions and electrons transport efficiently in their own channels.Secondly,a porous symmetric supporting（PSS）ceramic oxygen-permeable membrane was successfully prepared by optimizing the membrane structure,which not only improved the surface exchange rate of O₂ through the membrane,but also shortened the transmission distance of O^2-inside the membrane,thus greatly improving the oxygen permeability of the membrane.The main research contents of this experiment are as follows:（1）A 60CGO-40BSCF dual-phase ceramic conductor ceramic oxygen permeable membrane with PSS structure was prepared by molten salt method,tape casting,high temperature lamination and high temperature sintering,and the porosity of the porous layer was 24.08%.PSS membrane structure is composed of a highly dense layer about 50μm in the middle and a porous support layer about 290μm on both sides.The main advantages of PSS membrane structure are as follows:（1）The highly dense ultra-thin dense layer shortens the ion diffusion distance of the membrane;（2）The porous layer on both sides of the middle compact layer expands the surface exchange area and provides more active sites.（3）The symmetrical structure can balance the inherent stress inside the membrane and increase the mechanical strength of the membrane itself.In order to compare the effect of the membrane structure,a monotonous dense membrane with a thickness of 510μm was prepared by the same method.（2）The effects of temperature,oxygen partial pressure,feed gas flow rate,sweep gas flow rate and CO₂-doped feed gas on the oxygen flux of the prepared 60CGO-40BSCF PSS ceramic permeable membrane were explored.Meanwhile,the stability of the membrane was tested in different conditions.At 975℃,the oxygen permeability of 60CGO-40BSCF PSS membrane with a thickness of 630μm is as high as 3.78 m L cm^-2 min^-1 when the flow rate of air at the feed side is 150 m L min^-1 and that of helium at the sweep gas is 50 m L min^-1.The oxygen flux of 510μm monotonous dense membrane was only 1.42 m L cm^-2min^-1.When the flow rate of feed gas（O₂/N₂）is 150 m L min^-1 and the oxygen partial pressure is 0.8 atm,the oxygen flux can reach a higher level of 8.67 m L cm^-2 min^-1.The present results show that the design of PSS membrane structure is an effective strategy for increasing oxygen permeation flux.In addition,the CO₂-tolerance of the membrane was tested over a longer period time.At 950℃,7%CO₂ was added into the feed gas,the oxygen permeability of the membrane decreased from 3.47 m L cm^-2 min^-1 to 3.20 m L cm^-2 min^-1,and remained stable at 3.20 m L cm^-2 min^-1 during 1200 min test.When CO₂ was removed,the oxygen permeability of the membrane returned to 3.47 m L cm^-2 min^-1,and the performance of the membrane in the CO₂ environment had no obvious attenuation,indicating that the prepared 60CGO-40BSCF PSS dual phase mixed conductor oxygen permeability membrane had a good CO₂-tolerance.Compared with ordinary disk membranes,this study greatly improved the oxygen permeability flux and CO₂-tolerance of the membranes,and verified the advanced structure of PSS membranes.（3）The simple application of 60CGO-40BSCF PSS permeable membrane reactor in partial oxidation of methane（POM）was investigated.Under high temperature conditions,the POM process is realized by the reaction of O₂ through the membrane reactor and sweep gas CH₄ under the action of catalyst.Under the conditions of 150 m L min^-1 air flow rate and 70 m L min^-1 sweep gas（CH₄,Ar）flow rate,at 975℃,the conversion rate of CH₄ and the selectivity of CO in this coupling reaction reach 92%and 99%respectively.In the study of the POM with 60CGO-40BSCF PSS membrane reactor,the conversion rate of CH₄ and the selectivity of CO have reached a high level.