Oxygen flux and process analysis of hydrogen separation from water through mixed conducting membrane
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0885-A11-05.1
Oxygen flux and process analysis of hydrogen separation from water through mixed conducting membrane Annamalai Karthikeyan, Hengdong Cui, Srikanth Gopalan and Uday B. Pal Department of Manufacturing Engineering, Boston University 15 Saint Mary’s Street, Brookline, MA 02446 USA
ABSTRACT Hydrogen synthesis and segregation from water splitting with simultaneous partial oxidation of methane can be achieved using MIEC membranes that conduct oxygen ions and electrons. The process offers hydrocarbon free hydrogen production on the feed side of the membrane (steam side) and syn-gas on the permeate side (methane side). A composite MIECs system comprising GdxCe1-xO2-x/2 (GDC) and GdxSr1-xTi1-yAlyO3 (GSTA) has been used and the hydrogen flux has been estimated. The oxygen diffusion coeffcient for oxygen transport through the bulk of the membrane and surface exchange coefficient of oxygen at the solid/gas interface were obtained using electrical conductivity relaxation (ECR) experiments and the hydrogen generation flux was measured for membranes of different thicknesses, with and without surface exchange catalysts for various experimental conditions.
INTRODUCTION Dense MIEC membranes that are stable in high steam content and very low oxygen partial pressures around 10-10 to 10-20 atm are very attractive for hydrogen generation process. A novel process to generate and separate hydrogen is schematically illustrated in Figure 1 [1]. This process involves reacting methane with steam in a membrane reactor to form syn-gas on one side of the membrane and yield hydrocarbon free hydrogen on the other side of the membrane. H2O + 2e- = O2- + H2 H2O
H2, H2O
K 1ex ~
MIEC membrane ( D )
CH4
e- O2-
2 K ex
CO, H2
2-
CH4+O =CO+2H2 Figure 1. Schematic of the oxygen separation process from water using MIEC membranes resulting in syn-gas and hydrogen production. A new dual-phase composite MIEC membrane system comprising Gd0.2Ce0.8O1.9 (GDC), an oxygen ion conductor, and Gd0.08Sr0.88 Ti0.95Al0.05O3±δ (GSTA) an electronic conductor has been developed [2]. This composite membrane posseses the requisite stability under high steam and
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low oxygen partial pressure and is very suitable for the hydrogen separation process depicted in Figure 1. The parameters which characterize the oxygen permeation (hydrogen generation) ~
through this membrane are the chemical diffusion coefficient of oxygen ( D cm2/s) and surface exchange coefficient of oxygen ( K ex cm/s). Surface exchange properties at the feed side (steam or higher pO2 side) may be quite different than that of the permeate side (methane or lower pO2 ~
2 2 ; where K 1ex and K ex are side) and hence the flux in general is controlled by D , K 1ex and K ex surface exchange coefficients on the feed and permeate side. These parameters can be obtained with a reasonable accuracy using electrical conductivity relaxation (ECR) experiments. The flux of hydrogen production rate J(H2) (note that 2J(O2) = J(H2)) was measured under different conditions and the results were
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