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Abstract This study deals with the modelling of the mechanical behaviour accounting for the
expansion induced by the oxygen diffusion in MIECs membrane during semi-permeation transient
stage. A dedicated model of chemical expansion and its numerical implementation is used to study
the relationship between the mechanical stress and the oxygen flux. The impact of the ratio between
oxygen bulk diffusion and surface exchange kinetics on mechanical stress in transient stage is
discussed. At last, the need of a compromise between the oxygen flux performance and the
mechanical reliability is underlined.
Introduction
Oxygen separation from air using ceramic mixed ionic electronics conductors (MIECs) appears as a
pure oxygen source for industrial application. The membrane can be set-up in a reactor for partial
oxidation of methane (POM) to synthesis gas (H
2
/CO) without contaminating the reaction gas
mixture with N
2
. This energy-saving technology avoids the cryogenic air separation process to
produce pure oxygen [1]. The semi-permeability of oxygen in MIECs occurs at high temperature
(873 - 1173 K) and under a gradient of chemical potential between oxidizing side and reducing side.
Oxygen ions diffuse through the membrane while electrons move in the opposite direction. The
oxygen diffusion goes with a local oxygen stoechiometry change associate to a change in the
valence of the dopant which induces a “chemical expansion” phenomenon [2].
In the POM process, the oxygen partial pressure between each side of the membrane may vary from
oxidizing atmosphere (air) to reducing atmosphere (CH
4
) in a range of more than 20 orders of
magnitude of the oxygen partial pressure. Such oxygen activity gradient induces a chemical
expansion gradient in the thickness of the membrane which leads to chemo-mechanical stresses [2].
At membrane reactor scale, these chemo-mechanical stresses affect the mechanical reliability of the
whole structure and are identified as a cause of failure [3].
This paper proposed a macroscopic modelling of the oxygen diffusion and induced chemical
expansion dedicated to the mechanical stress predictions. In the first part, the equations of the model
are presented. The second parts deals with results produced by this model and focus on the relation
between the stresses and the oxygen flux in transient stage. The impact of the ratio of oxygen
surface exchange to bulk diffusion is especially regarded.
Advances in Science and Technology Vol. 65 (2010) pp 232-237
© (2010) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AST.65.232
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the
publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 194.167.30.129-30/09/10,18:01:28)