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