Analysis of oxygen exchange-limited transport and chemical stresses in perovskite-type hollow fibers

The effects of oxygen exchange kinetics at the oxygen-lean side of the perovskite-type hollow fiber membrane for oxygen separation from air, on chemical stresses, as well as those from bulk oxygen transport, are studied. Closed-form analytical solutions for oxygen permeation flux at steady state in the fiber lumen, and for the distribution of oxygen non-stoichiometry and chemical stresses in a transient state in the hollow fiber membrane have been obtained. Different operating modes of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ hollow fiber membrane module to produce oxygen from air at 1173 K have been considered including sweep-gas operation, high-pressure operation and vacuum operation. Values of the chemical surface exchange coefficient in the broad range of oxygen partial pressure and the chemical diffusion coefficient have been determined from the permeation measurements. Values of the oxygen non-stoichiometry on both sides of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ hollow fiber at equilibrium with the surrounded atmosphere have been obtained from the experimental data for different operating modes of membrane module. Chemical stresses in the hollow fibers of the module with three operating modes have been calculated. Practical recommendations on how to improve the performance and operating conditions, as well as, to avoid membrane break have been formulated. Investigations have shown that influence of oxygen surface exchange kinetics on chemical stresses increases strongly with decreasing the hollow fiber membrane thickness.