Effect of Asymmetric Membrane Structure on Hydrogen Transport Resistance and Performance of a Catalytic Membrane Reactor for Ethanol Steam Reforming

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Model-Based Performance Analysis of Membrane Reactor with Ethanol Steam Reforming over a Monolith

Membrane reactors (MR) with an appropriate catalyst are considered to be an innovative and intensified technology for converting a fuel into the hydrogen-rich gas with the simultaneous recovery of high-quality hydrogen. Characteristics of an asymmetric membrane disk module consisting of a gas-tight nanocomposite functional coating (Ni + Cu/Nd_5.5WO_11.25-δ mixed proton-electron conducting nanocomposite) deposited on a gas-permeable functionally graded substrate has previously been extensively studied at lab-scale using MRs, containing the catalyst in a packed bed and in the form of a monolith. The catalytic monolith consisted of a FeCrAl substrate with a washcoat and an Ni + Ru/Pr_0.35Ce_0.35Zr_0.35O₂ active component. It has been shown that the driving potential for hydrogen permeation across the same membrane in a monolithic catalyst –assisted MR is greater compared to the packed bed catalyst. This paper presents results of the study where a one-dimensional isothermal model was used to interrelate catalytic and permeation phenomena in a MR with ethanol steam reforming over the monolith, operating at atmospheric pressure and in the temperature range of 700–900 °C. The developed mathematical reaction–transport model for the constituent layers of the catalyst-asymmetric membrane assembly together with a Sieverts’ equation for the functional dense layer, taking also into account the effect of boundary layers, was implemented in a COMSOL Multiphysics environment. Good agreement with the experimental data of the lab-scale MR with reasonable parameters values is provided. In numerical experiments, concentration profiles along the reactor axis were obtained, showing the effect of the emerging concentration gradient in the boundary layer adjacent to the membrane. Studies have shown that a MR with a catalytic monolith along with appropriate organization of a stagnant feed flow between the monolith and the membrane surface may enhance production and flux of hydrogen, as well as the efficiency characteristics of the reactor compared to a reactor with packed beds.

An Experimental Performance Study of a Catalytic Membrane Reactor for Ethanol Steam Reforming over a Metal Honeycomb Catalyst

The present study deals with the combination of ethanol steam reforming over a monolithic catalyst and hydrogen separation by membrane in a lab-scale catalytic membrane reactor (CMR). The catalyst was comprised of honeycomb thin-walled Fechralloy substrate loaded with Ni + Ru/Pr_0.35Ce_0.35Zr_0.35O₂ active component. The asymmetric supported membrane consisted of a thin Ni-Cu alloy–Nd tungstate nanocomposite dense permselective layer deposited on a hierarchically structured asymmetric support. It has been shown that the monolithic catalyst-assisted CMR is capable of increasing the driving potential for hydrogen permeation through the same membrane as compared with that of the packed bed catalyst by increasing the retentate hydrogen concentration. Important operating parameters responsible for the low carbon deposition rate as well as the amount of hydrogen produced from 1 mol of ethanol, such as the temperature range of 700–900 °C, the water/ethanol molar ratio of 4 in the feed, have been determined. Regarding the choice of the reagent concentration (ethanol and steam in Ar), its magnitude may directly interfere with the effectiveness of the reaction-separation process in the CMR.