Modelling the effect of the porous support on the flux through asymmetric oxygen gas separation membranes

Oxygen Transport Membranes (OTM) represent a new technology for energy-efficient oxygen generation which can be used in low-pollutant power plants and oxygen generators or membrane reactors in the chemical industry and health care. The two competing demands of low ionic resistance of the functional separation membrane and high mechanical stability lead to an asymmetric design comprising of a thin membrane layer and a thicker porous support. However, the overall membrane performance is strongly affected by the microstructure of this support layer which prevented the use of the full potential of such a design in the past. Please download the full abstract below

Simulation of Transport Processes through an Asymmetric Gas Separation Membrane

Oxygen gas separation membranes with mixed ionic and electronic conductivity (MIEC) havefound great prospects in membrane technology for oxygen separation from gas mixtures (e.g.air) under a partial pressure gradient as the driving force. The separation of oxygen usingmembranes is more energy efficient than traditional processes such as the cryogenic Lindeprocess or pressure swing adsorption (PSA). The inverse relationship between the membranethickness and flux underscores the need for a very thin membrane. Consequently, the low mechanicalstability of free-standing thin membranes motivated the processing of asymmetricmembranes, where the thin membranes are supported by a porous structure. Asymmetricmembranes provide a low ionic resistance of the functional separation layer together with ahigh mechanical stability. However, the microstructure of the porous support in the membraneassembly affects the overall flux significantly. Therefore, a porous support that provides therequired mechanical stability needed for the dense membrane, with little or no limiting effecton the overall flux is desired [...

Simulation of transport processes through an asymmetric gas separation membrane

Oxygen gas separation membranes with mixed ionic and electronic conductivity (MIEC) have found great prospects in membrane technology for oxygen separation from gas mixtures (e.g. air) under a partial pressure gradient as the driving force. The separation of oxygen using membranes is more energy efficient than traditional processes such as the cryogenic Linde process or pressure swing adsorption (PSA). The inverse relationship between the membrane thickness and flux underscores the need for a very thin membrane. Consequently, the low mechanical stability of free-standing thin membranes motivated the processing of asymmetric membranes, where the thin membranes are supported by a porous structure. Asymmetric membranes provide a low ionic resistance of the functional separation layer together with a high mechanical stability. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. Therefore, a porous support that provides the required mechanical stability needed for the dense membrane, with little or no limiting effect on the overall flux is desired. The gas flow through porous media including that of multiple species is often described by the Binary Friction Model (BFM) considering the binary diffusion, Knudsen diffusion, and viscous flow. Therefore, the transport through an asymmetric membrane was studied by applying the BFM for the support together with a modified Wagner equation for the dense membrane in one dimension. In addition, transport relevant parameters obtained from micro computed tomography data of asymmetric membranes manufactured from different processing routes were used. The effects of the geometrical parameters of the support’s microstructures on the overall flux through an asymmetric membrane were compared for different feed gases (oxygen and air) and flow configurations (3-end and 4-end mode, assembly orientations). The rate-limiting effect of the support with large pore diameters (> 35µm) for the 3-end mode transport process using oxygen as feed gas was less than 10% with respect to the flux of the isolated thin membrane. This was not the case for the 4-end mode irrespective of the feed gas, and for the 3-end mode with the support at the feed side using air as feed gas. This was attributed to the binary diffusion term in the BFM which is not affected by the change in pore size. Thin small-pored supports yield the same flux as thick large-pored supports considering a non-linear relationship between thickness and pore size. This can be used for the optimization of the support’s micro-structure with regards to mechanical strength and permeability. Elongated pores in the flow direction will be ideal for membrane reactors because of the mixed gas transport in the porous supports, whereas single gas transport through the porous support is significantly less influenced by the tortuosity. Computational fluid dynamics simulations were used to investigate the effect of pore geometry on viscous flow. The pore morphology (size, shape and orientation) and not just the pore opening diameter affects the in-pore velocity. Furthermore, a two-layer support system was designed for the optimization of the porous support with respect to mechanical strength and permeability. Comparable fluxes were obtained for the two-layer support system with respect to 90% membrane performance for single support layer system. Finally, two simplifications of the pressure profiles within the porous support (constant pressure according to the pressure at the free surface and the average at the surface and the interface) were compared to the exact numerical solution of the BFM, given that the exact numerical solution of the binary friction model is complicated and requires high computational efforts (most especially for implementations in three dimensional simulations). The simplification using a constant pressure equal to the gas pressure outside the support deviated from the exact solution under certain operating condition is ~ 3 times more than the average constant pressure simplification. The average constant configuration using still a constant pressure averaged be-tween the outside of the support and the support/membrane interface had no significant deviation with the exact solution. Therefore, this is a useful measure to reduce computational efforts when implementing the Binary Friction Model in computational fluid dynamics simulation

Modelling the Support Effect on the Flux Through an Asymmetric Oxygen Transport Gas Separation Membranes

Oxygen Transport Membranes (OTM) display a new technology for energy-efficient oxygen generation which can be used in low-pollutant power plants and oxygen generators or membrane reactors in the chemical industry and health care. Low ionic resistance of the membrane and high mechanical stability typically demands the usage of an asymmetric design comprising a thin functional membrane and a thicker porous support. The overall membrane performance is strongly affected by the microstructure of this porous structural layer. The effect of the support on the flux performance has been thus studied applying the Binary Friction Model (BFM, including binary and Knudsen diffusion and viscous flow) for the support together with a modified Wagner equation for the dense membrane. The parameters describing the tape-cast porous medium were obtained by numerical diffusion and flow simulations based on micro computed tomography (µCT) data. Using different flow conditions (3-end, 4-end) and oxygen as feed gas, the effect of the support thickness, pore diameter, position (either on the feed or permeate side) of the support on the flux were investigated. Knudsen diffusion was found to dominate the transport process for small pore sizes (~2µm) in particular for the 3-end mode with the support on the permeate side being most pore size sensitive, whereas for the other configurations the viscous flow was of higher significance. For typical membrane assembly geometry with a membrane thickness of 20 µm and a support thickness of 0.9 mm, the flux became membrane limited starting from a pore size of approx. 5 µm

Comparison of the Simplification of the Pressure Profiles Solving the Binary Friction Model for Asymmetric Membranes

The gas flow through porous media including that of multiple species is frequently described by the binary friction model (BFM) considering the binary diffusion, Knudsen diffusion, and viscous flow. Therefore, a numerical simulation was performed on a microporous support of an asymmetric oxygen transport membrane. As its exact numerical solution is complicated and not always possible, the results of two different levels of simplification of the pressure profiles within the porous support are compared to the exact numerical solution. The simplification using a constant pressure equal to the gas pressure outside the support leads to a deviation by up to 0.45 mL·min−1·cm−2 from the exact solution under certain operating condition. A different simplification using a constant pressure averaged between the outside of the support and the support/membrane interface reduces this deviation to zero. Therefore, this is a useful measure to reduce computational efforts when implementing the Binary Friction Model in computational fluid dynamics simulations

The effect of two different support microstructure of an asymmetric membrane with comparable porosities on flux

Oxygen transport membranes (OTM) display a new technology for the generation of energy-efficient oxygen. These membranes can be used in low-pollutant power plants and oxygen generators or membrane reactors in the chemical industry and health care. Research studies over the years have found that the thinner the dense membrane, the higher the observed flux, but the lower the mechanical stability. This motivated the state of the art processing of an asymmetric membrane; whereby the thin dense membrane is supported by a porous structure. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. To study and optimize this effect, tape cast and freeze cast Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3– (BSCFZ) asymmetric membranes having comparable support porosities but different pore architecture were processed. Permeation measurements showed that the flux from the two membranes yielded comparable flux, which is not in agreement to literature.A computer tomography of the membranes was acquired to understand, simulate and optimize the porous support. This effect was simulated by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane, using transport relevant parameters obtained from computer tomography data of the freeze cast, and tape cast support using Geodict software

Optimization of the porous support of an asymmetric oxygen transport membrane by numerical modelling

Asymmetric oxygen transport membranes (OTM) provide a low ionic resistance of the functional separation layer together with a high mechanical stability. Hence, they are promising candidates for high-permeation in a variety of high-temperature applications for the separation of oxygen from gas mixtures. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly [1].In this work, the optimization of the porous support was studied by simulating numerically the effect of geometrical changes (pore size, pore geometry, substrate thicknesses) of the support on the overall flux, using different flow conditions (3-end, 4-end), and assembly orientation [2]. These effects were studied by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane using transport relevant parameters obtained from micro computed tomography data of a BSCF-Z support. Additionally, the effect of the support geometry and the depth of travel of the sweep gas on the permeated flux were investigated by computational fluid dynamics using Ansys Fluent. From the CFD simulation, u-shaped pores are more desirable for inverse tape cast porous support and enables quick removal of the permeated gas. Supports with elongated pores would be ideal for 4-end mode (binary diffusion limited configurations/gas mixtures e.g. membrane reactors) transport, while for oxygen generation from air (3-end), supports with either compressed or elongated pores are comparable (rel. difference < ~7%). A relationship between the opposing factors substrate thickness and pore size was developed that ensures a given flux. This can be used to optimize support’s microstructure with regards to mechanical strength and permeability. [1] P. Niehoff, et al. Oxygen transport through supported Ba0.5Sr0.5Co0.8Fe0.2O3–d membranes, Sep.Purif Technol, 121(2014)60-67.[2] U. Unije, et al. Simulation of the effect of the porous support on flux through an asymmetric oxygen transport membrane, J.Membrane Sci., 524(2017)334-343

Performance study of asymmetric oxygen transport membranes with vertically channelled pores by phase inversion tape casting

Asymmetric oxygen transport membranes have been extensively studied revealing the importance of the support porosity. In this study, asymmetric membranes with vertically channelled pores were prepared from Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ (BSCF-Z) by phase inversion tape casting. The rate-limiting effects of the membrane layer thickness, the support structure, and the activation layers were thoroughly analysed. This entails comparing the permeation rate of the samples with the membrane layer on different sides of the channelled supports. Pore tortuosity and computed permeability were evaluated from 3D-X-ray computed tomography and compared to previously reported asymmetric membranes prepared by tape-casting and freeze-drying. Due to a lower pore tortuosity, the channelled supports have an advantage in gas transport over the tape cast and freeze cast supports. However, this is compensated by a thicker membrane layer (∼54 μm) resulting in similar performance compared to the thinner membrane layers (∼20 μm) with freeze cast and tape cast supports